Past, present, and the future of the research and commercialization of CVD diamond in China

Abstract It has been half of a century since the publication of the early reports about CVD diamond films in the world in the early 1970’s. The reports for meaningful laboratory growth of diamond films with much higher growth rate and higher quality could be found in the early 1980’s, under the so-called “Diamond Fever” initiated all over the world. In less than 10 years later, CVD diamond research had started in China as “863 Plan” (High Technology Research and Development Plan in China), a newly launched program in 1987. 35 years later, it is very interesting to explore what really happened to the CVD diamond in China. As a multi-functional material with a vast combination of extraordinary electrical, mechanical, thermal, optical, acoustic, and electro-chemistry properties, the CVD diamond has wide applications potentially in the field of multidiscipline high technologies. Therefore, this article aims to provide a general review on the CVD diamond by presenting a clearer picture about the history, the research status and its development, particularly the commercialization in China. Finally, the general trend in the near future is discussed.


Brief history
Looking back into the history of the CVD diamond research in China, the name of a Chinese scientist, academician of CAS, Prof. Chen Nanxian, must not be forgotten. He was the head of the Specialist Group of the Functional Materials and a member of the Committee of the Field of New Materials of the "863 Plan" (High Technology Research and Development Plan in China, launched in March 1986). It was Prof Chen who suggested to the Committee that CVD diamond films should be included in the first batch of the research programs, in a meeting organized by the committee in 1987. His suggestion was supported by 17 papers he brought back from the USA where he spent more than 3 years as a visiting scholar in the Pen State University, and a message that CVD diamond films were definitely included in the "Plan of Star War" of the US president Ronald Reagan. Soon after I became the head of the Office of the Specialist Group of Functional Materials, and these papers (most of which were reviews about CVD diamond films) were handed to me. I was deeply attracted to this subject and had devoted my whole life working in this area.
At a demonstration meeting organized by the Specialist Group of Functional Materials of the "863 Plan" in Beijing in 1987, it was amazing that among the nearly 20 attendees, only Professor Jin Zengsun from Jilin University had ever seen CVD diamond films. Prof. Jin was a visiting scholar in the Research Institute of Inorganic Materials in Japan where a few top scientists were working in the field of CVD diamond films. Soon after the demonstration meeting, four groups, namely, Jilin University, Sichuan University, Beijing Research Institute of Synthetic Crystals, and Beijing Institute of Science and Engineering (Beijing University of Science and Engineering) were chosen to conduct the first research projects under The CVD Diamond Films Special Subject. Prof. Jin was appointed as the head of the Specialist Subject. Considering the fact that CVD diamond film research was actually non existent in China at that time, the Specialist Group decided that the target for the first year was the preparation for CVD diamond films, no matter whatever methods were used. All the four members in the Special Subject were required to submit one diamond film sample to the Office of the Specialist Group of Functional Materials within one year, then an open exhibition and competition would be organized. All the samples would be sent to the Physics Institute of CAS by the Office of the Specialist Group for evaluation by SEM, Raman, and X-Ray diffraction. Besides, those who were not the 863 members were also allowed to join in the competition. One year later, the exhibition and competition were Beside the "863 Plan", the Natural Science Foundation of China (NSFC) also started the financial support for CVD diamond proposals almost at the same time, which had been continued up to now, and very likely, will be extended to the near future.
Later on, in order to celebrate the tenth anniversary of the "863 Plan", Key Technology Projects were organized with the aim to speed up the processes for commercialization in the fields of high technologies. Very luckily, a project named "Key Technology for the Preparation and Commercialization of CVD Diamond Films" was ranked amongst the list of the Key Technology Projects of the "863 Plan", which was launched in the second half of 1994 and terminated at the end of 1995, took one and a half year in total. Jilin University, Beijing Research Institute of Synthetic Crystals, and University of Science and Technology Beijing (USTB, in cooperation with the Laser Institute of the Academy of Sciences of Hebei Province) were included in the Key Technology Project, in which the Jilin University Group was required to develop the technology leading to the thermal and electronic applications of CVD diamond films, the Beijing Institute of Synthetic Crystals group was aiming at the development for mechanical applications, whilst the USTB group should accomplish the design and construction of a high power DC Arc Plasma Jet CVD system for large area high quality diamond film deposition at high speed. The evaluation meeting of this Key Project was organized in February 1996 in University of Science and Technology Beijing. 7 academicians of the CAS and CAE attended the meeting, including these famous Chinese scientists in the field of materials science and engineering, Prof. Si Changxu (CAS and CAE), Prof. Lin Lanying (CAS and CAE), Prof. Jiang Minghua (CAE), Prof. Wei Shoukun (CAS and CAE), Prof. Chen Nanxian (CAS), Prof. Wang Zhanguo (CAS), and Prof. Chen Xianlin (CAE). They highly commented on the progresses achieved. Particularly, they commented that the successful development of the proto-type 100 kW high power DC Arc Plasma Jet which had demonstrated the capability to grow uniform diamond films with an area of Φ110mm with a growth rate as high as 30-40µm/h, "had its own originality", "the system was one of the three high power DC Arc Plasma Jet CVD system with a similar power class in the world", "had laid down the foundation not only for the mass production of CVD diamond films for thermal and mechanical applications, but also for the research and development of optical applications". [1]. Besides, a proto-type 1000 carat diamond tool production line for wire drawing dies and cutting tools was demonstrated with hot filament CVD equipment. A DC Hot Cathode CVD diamond film deposition equipment had been built, demonstrating the capability for high thermal conductivity diamond film deposition with a growth rate of 20 µm/h, by which a proto-type heat sink for high power laser diode arrays had been fabricated, which showed the potential for industrial applications. [1]. Soon after, the evaluation meeting was reported by the CCTV Evening News. However, it was very sad and unfortunate that 5 of the 7 academicians attended the evaluation meeting passed away. In the Exhibition for the Achievements of the Tenth Anniversary of the "863 Plan", the progresses were displayed prominently (Figure 1).
In the next Five-Year Plan (the Nineth Five-Year Plan, [1996][1997][1998][1999][2000], it was the "Good Days" for the R&D of CVD diamond films in China. New "863" Key Project on CVD diamond films named "CVD Diamond Films and Applications" was launched with the same three attendees as the Key Project in the Eighth Five-Year Plan. The targets set by the "863" Specialist Group were: (1) to set up a demonstration production line for the CVD diamond film tool (mechanical) applications, and to realize an output value of ten million RMB annually (for Beijing Institute of Synthetic Crystals); (2) to demonstrate the thermal and electronic applications for CVD diamond films (for Jilin University); (3) and to develop the key technologies for the preparation, processing, and application of large area high optical quality freestanding diamond films (for USTB, and the Laser Institute of the Academy of Sciences of Hebei Province). Besides the Key Project, the "863 Plan" Diamond Film Special Subject was continued and further expanded in the 9 th Five-Year Plan, in which about 20 research groups were included, with a variety of research topics in the thermal, electrical (electronic), mechanical, optical, detectors and sensors, electrochemistry, acoustic, and so on, as well as in the R&D of the deposition technics, and characterization of diamond films etc. Besides, the NSFC also increased its financial support for CVD diamond films. A typical example was that, a proposal on heteroepitaxial growth of diamond film on single crystal silicon by Prof. Lin Zhangda of the Physics Institute of CAS was approved as a "Key Project" by the NSFC. This was because the reviewers of the NSFC believed it was possible to grow diamond film hetero-epitaxially on single crystal silicon simply by the single crystal silicon substrate preprepared with a designed misorientation angle. Another signal event was that a company named "Beijing Tiandi (TD) Orient Diamond Tech. Co. Ltd. " was established by Beijing Research Institute of Synthetic Crystals and the Nanjing Tiandi Group Co. Ltd. This was the first diamond film enterprise in China, marking the beginning of the era of commercialization of CVD diamond films in China. Another historical event was the holding of an international diamond film symposium in Beijing in 1999 where 83 papers, including 30 orals and 53 postal, particularly 11 high level invited papers were presented. This was the first diamond film symposium held in China. Proceeding of this symposium was published in Dimond and Related Materials as a special issue [2]. DeBeers Diamond Industry (UK) Ltd. was one of the main sponsors of the symposium.
However, a "cold winter" approached later! In the 10 th Five-Year Plan (2001)(2002)(2003)(2004)(2005), Key Project in diamond films no longer existed in the "863 Plan", whilst the Diamond Film Special Subject was also dismissed. A few remaining research groups were admitted into another Special Subject (Functional Thin Films and Coatings). At the same time, the NSFC also reduced its support on the CVD diamond films. The extreme sudden difficulties forced quite a few research groups to leave or to change their research directions to nano diamond, DLC, Graphene, nano carbon tube, and so on. However, it is worth pointing out that the "low tide" not only happened in China but also all over the world, just a few years before China. The "low tide" in the R&D in CVD diamond films was caused by the worldwide "Diamond Fever" which had awoken the excessive expectations for this multi-functional material, particularly, in the fond dream for the so-called "ultimate semiconductor" to come true soon. However, the reality was that, extreme difficulties in the N-type doping and the large-area heteroepitaxy arose in the 90's of the twentieth century, and are not solved completely even now! The fond dream thus had been smashed, and hence the "cold tide" came. Nevertheless, soon after, scientists as well as the funding organizations realized the truth that even if the active semi-conductor devices were facing serious technical problems which sooner or later would be overcome completely, the research for the inactive devices, e.g. detectors and sensors, composite electronic devices (SOD, GAN-ON-DIAMOND, etc.), devices based on semi-conductive surfaces (H-terminated or O-terminated) were still open. Besides, other applications in the area of thermal, optical, mechanical, acoustic, electrochemistry, etc., were not affected at all. Thus, the weather gradually became warmer and warmer. This was also the real situation for the R&D of CVD diamond films in China afterwards. In this rather long period (approximately from 2001-2015), the status of CVD diamond films in China could be roughly summarized as following. With its great efforts to improve the production technologies as well as the management, Beijing Tiandi (TD) Orient Diamond Tech. Co. Ltd. had passed its first historical Profit or Loss Equilibrium Point, and eventually became profitable. A commercial type 30 KW DC Arc Plasma CVD system had been developed. It was more stable, easier to operate and cheaper than the 100 KW DC Arc Jet. And it was capable of mass production of thick freestanding diamond wafers of Φ60mm in diameter for mechanical, thermal and optical applications at a growth rate of 10-30 µm/h depending on the quality required. Therefore, it soon became the main power in the commercialization of CVD diamond films in China. Based on the 30 KW DC Arc Plasma Jet technology, a new company named"Hebei Plasma Diamond Technology Co. Ltd. " was co-established by the Laser Institute of Hebei Academy of Sciences and the USTB in 2009. The annual output of this company had expanded very rapidly, increasing more than 10 times from 2009 to 2021. It sold thick freestanding diamond film products for mechanical, thermal, optical and acoustic products to Europe, USA and Asia, and become the world main supplier for freestanding diamond film products. Another successful company named "Shanghai Jiaoyou" was also established by Shanghai Jiaotong University in this period. It sold diamond film coated dies for drawing small to medium size metal (mostly copper) tubes. Special Hot Filament CVD equipment and the corresponding production technologies had been developed for this particular purpose. Its annual output had exceeded 30 M just a few years ago. With the investment by the government gradually increasing, quite a few advanced microwave plasma CVD diamond film deposition systems had been imported for the university laboratories and the government institutions. Another event was that advanced microwave plasma CVD system based on modelling started [3,4] to develop in China. Besides, government funding also increased gradually, including that from the NSFC and that from the Ministry of Science and Technology (MSTC, the "863 Plan" was one of the many government plans funded by it), which guaranteed the continuity of the R&D of CVD diamond films. Research activities in this period were more focused on the fundamental research on electronic, e.g. diamond film detectors and sensors, composite electron devices (SOD and GAN-ON-DIAMOND), surface based (H-terminated and O-terminated) devices, thermal and optical applications, etc.
In the period from 2016 up to now, the hottest news was the boosting of the "Laboratory Grown Diamonds". Since the ISO claimed that the phrase "Laboratory Grown Diamond" and "Synthetic Diamond" were synonyms in 2015, and the FTC (Federal Trade Committee) recognized the laboratory grown diamond as "real" diamond in 2018 [5], numerous companies had emerged all over the China. Thousands of microwave plasma CVD diamond deposition systems had been imported from abroad or been built in house. Within just a few years, China has become the largest producer of laboratory grown diamond (HPHT and CVD) in the world. This rather dazzling situation will be discussed later in more details. In the meantime, with the continuous increases in the national GDP, funding in the R&D of CVD diamond films also increased rapidly. Consequently, some money burning projects, for example, key technologies leading to the industrial application of diamond semiconductors, large size optical and microwave (Gyrotron) windows, diamond film thermal management for heavy loading electronic systems, diamond chip for quantum applications, etc., had been arranged. Good Days for the R&D of the CVD diamond films came back again!

Diamond film deposition equipment
Up to now, Hot Filament CVD remains one of the most important techniques for CVD diamond film deposition in China. This technique was developed from 1990 to 2010 by the Beijing Research Institute of Synthetic Crystals as well as many others, and is still used in the mass production of the thick freestanding diamond films for mechanical and thermal applications. For this particular purpose, an improved Hot Filament CVD technique, i.e. the EACVD (Electron Assisted Hot Filament CVD) had been established [6,7], the technique of the "closely packed filament multi arrays", and the "short filament to substrate distance" had been adopted. However, it is the most important and dominant technique for the preparation of large area low-cost diamond thin film (coatings) material and products. In fact, it is the most promising technique for the preparation of the BDD (Boron Doped Diamond) films for a variety of "functional" applications. Tribological applications, e.g. diamond thin film coated cutting tools, wire drawing dies (for drawing small to medium diameter metal wires and tubes) also mainly rely on this technique. However, specially designed equipment for this particular purpose must be used. Several industrial type diamond film coating systems based on the Hot Filament CVD technique, like Semecon, the Balzers, the SP3, and the Neocoat etc. had been imported. Some Chinese companies and institutions are trying to develop their own systems. It should be pointed out that, some of the imported coaters, like that from the SP3 are also capable of the preparation of thick freestanding diamond film products. However, those imported diamond film coaters are not suitable for the production of the small to medium diameter diamond film coated metal tube drawing dies. The Chinese company "Shanghai Jiaoyou", was credited for developing the industrial type Hot Filament CVD coaters specially designed for mass production of the diamond film coated small to medium diameter metal tube drawing dies. To the best of my knowledge, it may be the only one available in the world. The author of the present article is, however, not very sure if there may be any such diamond film coaters elsewhere.
At present, MPCVD is definitely the most widely used diamond film deposition technique in China. It is not only because of its capability for the preparation of the highest quality diamond films to be used for electronic, thermal, optical, and quantum technology, but also for its flexibility both for laboratory research as well as for mass industrial production requirement. MPCVD is also suitable for both thin and thick diamond films. It is now the only technique most suitable for the mass industrial production of the "Laboratory Grown Diamonds". Unbelievably, just in the past few years, China has become the biggest supplier for the Laboratory Grown Diamonds in the world jewel market with more than 50% are Chinese made (HPHT and CVD). Thousands of MWCVD diamond deposition units have emerged, in which quite a lot (might be 50% or more) are imported from abroad, of which the MWCVD systems from Plassys (France), iPlas (Germany), and Seki (Japan) being the most popular ones. However, those imported MPCVD diamond deposition systems are quite expensive. Therefore, cheaper homemade MWCVD diamond deposition systems were developed, and entered the Chinese market very fast. Right now, more than ten Chinese manufacturers are involved, in which the Uniplasma (Shenzhen), DMT-Diamond (Xian), Three Three Zero (Chengdu), Heuray Microwave (Chengdu), and Tanfangcheng (Carbon Equations, Shanxi) are the main suppliers. Besides, some of the Laboratory Grown Diamond manufacturers also develop their own MWCVD reactors. The information in the operational MWCVD units in China is rather confusing. There are quite different types of machines, from the early circular column resonance cavity type to the much more modernized types similar to that of the iPlas and the Seki machines working busily for growing laboratory diamonds. The output power level of the most MWCVD systems is 6 kW (2475 MHz). However, more and more high power MWCVD CVD diamond deposition systems (75 kW, 915 MHz) have been imported. Besides, 75 kW, 915 MHz high power MWCVD systems have been developed in China too. One of the general trends is to develop the "Single Crystal Diamond Growth" type MWCVD system both in China and abroad, which will be specially designed and fabricated for the mass production of the Laboratory Grown Diamond.
At present, DC Arc Plasma Jet CVD diamond film deposition technique with arc rotation and gas recycling is the main technique for mass industrial production of large area, high quality thick freestanding diamond film products in China. DC Arc Plasma Jet CVD diamond film deposition technique was originated in 1980s in Japan, and held the world record of the diamond film deposition rate as high as 930 µm/h [8,9]. However, the deposition area was very small (about 1 mm diameter only), which was not very useful for actual applications. And the gas consumption was huge (the Jet was supersonic). For this reason, great efforts had been devoted to the development of the DC Arc Jet capable for large area high quality diamond film deposition at high growth rate by numerous researchers all over the world [10]. Before the successful development of the 100 kW high power DC Arc Jet in China [11,12], Norton company had claimed the successes in the development of the high-power DC Arc Plasma Jet based on the technology of "Magnetic Mixing and Arc Expansion", which was capable for preparation of high-quality diamond film wafers with a thickness of 2 mm, and a maximum diameter of 150 mm (six inches) [13][14][15]. However, the Norton Company (USA) went bankrupt in the first decade of the twenty first century. And it was very strange that with the disappearance of this famous company, its technology had also gone away noiseless. Even now, nobody knows what the "Magnetic Mixing and Arc Expansion" really means. Another 100 kW class high power DC Arc Plasma Jet (USA) mentioned in the evaluation report [1,16] was not successful at all. Because the large area diamond film deposition was realized by the scanning of the specimen stage related to the torch nozzle, it was impossible to fabricate high quality thick freestanding diamond films. Right now, the homemade 100 kW high power DC Arc Plasma Jet has become the only one left and is still in normal operation in USTB. At present, the DC Arc Plasma Jet technology is fully mature in China. A series of different power class DC Arc Jet have been developed for different application purposes. These are fully capable for the mass production of the mechanical, thermal, and optical grade (but not the electronic grade presently) diamond film wafers with a maximum diameter of 150 mm, and a thickness of 3 mm. It is very different from the Hot Filament CVD and the MWCVD, China is the only country that has established a dominating production in DC Arc Plasma Jet, and the Chinese company Hebei Plasma Diamond Technology Co. Ltd is now the main supplier for the freestanding diamond film products in the world. Another difference is that most of the Arc Jets in China are highly concentrated in the Hebei Plasma Diamond and the USTB. The former is focused exclusively on commercial as well as the technology and products development, whilst the latter is mainly focused on the research and development for the high technology applications, particularly in the field of the thermal management and the optical windows.
Another technique for diamond film deposition worthy of mentioning is the DC Hot Cathode Plasma CVD (or PACVD (Plasma Assisted CVD)). It was established in China in the 1990s by Jilin University [17,18]. This technique is capable of large area diamond film deposition with a reasonably high quality at high growth rate. This technology is fully developed both in China and in Korea. Korea is now actually the leading country both in the deposition area as well as the film quality [19].
The Combustion Flame CVD diamond film deposition technique is now nearly not in use in China or elsewhere. This is simply due to the extreme difficulty for large area uniform diamond film deposition, and the huge amount of gas consumption.

Mechanical (tribological) applications
Diamond film is ideal for mechanical (tribological) applications because of the combination of its extraordinary properties of the highest hardness (80-100GPa), the highest thermal conductivity (20 W/cm.K at room temperature), and the very low friction coefficient (0.1 in air, 0.001 in water), together with a reasonably high fracture strength (400-1000MPa) and fracture toughness (6-10MPam 1/2 ).

Thick freestanding diamond film tool
products. The first CVD diamond film enterprise in China, the "Beijing Tiandi (TD) Orient Diamond Tech. Co. Ltd. " was jointly established by the Beijing Research Institute of Synthetic Crystal and the Nanjing Tiandi Group Co. Ltd in the 1990s. It used the Hot Filament CVD technology and aimed at the mass production of the thick freestanding diamond film tool products. Typical products included the wire drawing dies, dressing logs, cutting tool blanks, etc. This company had successfully passed its first critical profit-loss equilibrium point in the first decade of the twenty first century, and become profitable latter on. However, the Nanjing TD Group Co. Ltd. had withdrawn its investment later on as Beijing Research Institute of Synthetic Crystal took back control, and continued the normal operation up to now. Using the same Hot Filament CVD technology, the Beijing Worldia Diamond Tools Co., Ltd. is producing the similar thick diamond film products. Now the maximum size of the freestanding diamond film wafer has been increased to 200 mm in diameter ( Figure 2).
With the establishment of the Hebei Plasma Diamond Technology Co. Ltd. in 2009, the DC Arc Plasma Jet CVD entered the thick freestanding diamond film product market. Because of the advantages in the higher fracture strength and higher wear ratio (as defined by the ratio of the wear of the diamond film sample to the wear of the SiC sand wheel after the "Sand Wheel Test"), freestanding diamond tools produced by DC Arc Plasma Jet are more welcome in Europe, USA and China. The annual output of this company has increased more than ten times since the year of its founding up to now. At present, it is the main supplier of thick freestanding diamond film product in the world. Figure 3 is a glance of one of the three workshops for mass production of freestanding diamond film wafers of the Hebei Plasma Diamond, showing the industrial Arc Jets with the capability for production of the freestanding diamond films wafers from Φ60mm to Φ150mm in diameter, and a thickness of from 0.3 mm to 3 mm. Besides the mechanical grade diamond films, they are also used for the production of the thermal grade and the optical grade diamond wafers. In the early stage, the main product was the dressing logs and the wire drawing dies. However, with the progress of the laser processing technology, now the diamond film cutting tool blanks have become more and more acceptable, and the market share of the diamond cutting tool blanks has increased year by year. Besides the higher fracture strength, higher fracture toughness and higher wear ratio, another reason of the better performance of the DC Arc Jet freestanding diamond film tool products might be its higher thermal conductivity. MWCVD is apparently not suitable for the industrial production of the freestanding diamond film tool applications, simply due to economic considerations.

Thin diamond film tool products.
From the very beginning of the R&D of the CVD diamond films in 1980s, diamond thin film coated cutting tools, particularly, diamond coated cemented tungsten carbide drills and endmills had been expected to have a huge market al.l over the world because of its potential use in cutting various kinds of nonferrous metals (particularly Aluminum-Silicon alloys), polymers and composites, inorganic minerals and crystals, glasses, almost everything except iron and steels. In the early 1990s, it was predicted that the world market in diamond film coated cutting tools might be higher than 1.5 billion US dollars by the year of 1995 [21]. However, 30 years has passed, this prediction is still not realized. Yet, up to now, great success has been achieved, thousands of patents and papers have been published. Adhesion deterioration due to the cobalt in the cemented tungsten carbide was no longer a serious problem since many years ago. And a series of industrial coaters (like that from the SP3, Semecon, Balzers, etc.) were in the market for a long time, which were specially designed and fabricated for the mass production (hundreds of drills and endmills per run), and had also been imported in China. Diamond thin film coated tools are certainly in the market, however, it is definitely much less than the $1.5 billion predicted. This might be due to the fact that the consistency is still a technical problem difficult to be solved (just imaging how difficult it would be, to guarantee every drill (or endmill) out of the hundreds should be in the same good quality). It might also be due to the recent progress in the laser precision processing technology, which made possible the precision fabrication of the complex shaped polycrystalline-diamond composite drills and endmills (now, it is possible to fabricate micro-drills with a diameter less than 1 mm by laser precision processing), thus greatly increased their competition power, and has become more advantageous compared to the diamond thin film coated counterparts.
However, it is different in the case of the small to medium diameter (0.8-35mm) metal wire and tube drawing dies, where the thin diamond film coated cemented tungsten carbide dies are more advantageous than the single crystal diamond (in the low bound), or the polycrystalline diamond composite (for larger diameters). At present, diamond thin film coated small to medium diameter metal wire and tube drawing dies are in the market. Special type of Hot Filament CVD reactors suitable for coating thin diamond film coatings onto the interior surface of the small to medium diameter cemented tungsten carbide dies in the industrial scale has been developed by the Shanghai Jiaoyou Diamond Coating Co. Ltd. [22,23], which are now widely used for the copper tube drawing with much better efficiency (with a tool life about 7-13 times longer than that of the cemented tungsten carbide dies) and better surface finishing (see Figure 4). Besides, similar Hot Filament type coaters are also used for drawing the high strength steel wires and tubes [24]. It must be pointed out that, although there were a few reports on diamond thin film coatings in the interior [25] or in the upper part [26] of the cemented tungsten carbide dies in the literature quite a long time ago in abroad, there is no information available in concerning the industrial applications of the thin diamond film coated metal wire and tube drawing cemented tungsten carbide dies.
Very recently, extra-large diameter diamond thin film coated tube drawing dies with a maximum diameter of 200 mm has been put into the Chinese market by the Hebei Plasma Diamond Technology [27][28][29]. It is unimaginable that these 200 mm diameter thin diamond film coated ultra-large metal tube drawing dies are produced by the Arc Rotating DC Arc Plasma Jet with Gas Recycling! At present, it is possible to supply diamond thin film coated cemented tungsten carbide tube drawing dies with a diameter from 8 mm to 200 mm (see Figure 5), whilst those made by the Hot Filament CVD are 0.8 mm-35mm only. Besides, the end users seem to be happier with the DC Arcjet dies, as the tool life was reported to be 30 times longer than the uncoated cemented tungsten carbide dies, and 5 times longer than the polycrystalline diamond composite one, when used for the diameter holding and the tube straightening of the large size stainless steel tubes. The better performance of the DC Arcjet dies might be due to the extraordinary high atomic hydrogen concentration produced by the very high gas temperature, which would lead to a better diamond film quality. It might be also due to the fact that the outward diffusion of the cobalt inside the cemented tungsten carbide was depressed efficiently in the process of diamond film deposition by DC Arcjet. Simply because the growth rate was higher than that of the Hot filament CVD, there was no sufficient time for cobalt outward diffusion [30]. This newly emerged technology is still under improvement.

Boron doped diamond (BDD) films and its functional applications.
Boron doped diamond (BDD) films appeared quite a long time ago in the early 1990s in China. Diborane is the ideal boron source for the preparation of BDD films. However, it is seldom used in China due to the safety issues [31,32]. Diborane is under extremely strict control, because of its serious toxicity, flammability and explosibility. Trimethoxyboron (dissolved in acetone) had been the widely used less toxic boron source for the preparation of the BDD films [33]. However, it is still toxic to certain extent and corrosive to the pipeline system. Therefore, more nontoxic solid boron source, e.g. boric anhydride (B 2 O 3 ) and elementary boron, are being used [33]. Recently a mixture of graphite powder and boron powder is also used, which was reported to have the advantage of better uniformity and controllability of the boron concentration in the BDD films [33]. At present, Hot Filament CVD is the most widely used technique for the preparation of BDD films in the industrial scale (see Figure 2). However, microwave plasma CVD is also widely used, particularly for the laboratory research and development. Recently, DC Arc Plasma Jet is also used for both laboratory work as well as for the industrial mass production of BDD films [28,34]. Right now, the Hebei Plasma Diamond is selling thick freestanding BDD diamond films to the domestic customers with a diameter of 100 mm and a thickness of 0.5-2mm. The substrate materials usually used are titanium, niobium, tantalum and single crystal silicon [33]. However, way back in 2001, Shafer et al. reported a BDD film with diameter of 200 mm and maximum area of 0.5 m 2 [35]. However, it was special to use thick freestanding diamond plate as the substrate material for the preparation of BDD films, which is in fact a process of epitaxial diamond growth. The advantage of using free standing diamond plate as the substrate for the preparation of BDD films is most likely due to the high thermal conductivity, which was reported by the USTB to be as high as 17 W/cm.K [36]. Obviously, this is beneficial for certain functional applications where extreme heat dissipation is needed.
Industrial waste water treatment has been the most hopeful functional applications for BDD film electrodes from the very beginning of its history.
However, due to the difficulties in treating huge quantity of water body and the relative high cost of the BDD electrode material, equipment, and electricity, its application is now limited. This applies particularly to certain pollutants that are very difficult to remove (degradation), and the waste water body is not very large. The current research is focused on the surface microstructure design and fabrication, and the surface decoration of the BDD film electrodes, the degradation of various kinds of difficulties to remove pollutants, and the R&D of commercial type equipment and technologies [33]. There are successful commercial cases in China in environment disinfection and sterilization, which are of . left: the diamond thin film coated tube drawing die and the copper tube drawn by it; Right: copper tube drawing using diamond film coated cemented tungsten carbide die at work [22]. particular social importance to meet the urgent needs to cope with the Covid-19 pandemic [33]. There are also successful cases in waste water treatment based on the BDD film electrochemistry technology. However, the scale is still not very large. Because of the limitations, it is a good idea to develop certain kind of combined processes, like the combining of the BDD electrochemistry degradation, the sponge iron electrochemical reduction, and the per-ferrate chemical oxidation to form a multicell installation for the treatment of the industrial waste water [37]. A similar idea to combine the BDD electrode, the metallic electrode, and the graphene electrode to form a synergistic "three dimensional" device for more efficient environment disinfection and sterilization is also patented recently [38].
Progress in the fields of the bio-medical applications is also attractive. A series of successful cases on the BDD based bio-sensors and the drug detection and trace analysis for medical treatment can be found in the Chinese journals [39][40][41]. Besides, application on the BDD based electronic devices [42,43], the attenuation total reflection wafer [44], the improvement in diamond film tool life [45], and the development of the super capacitors [33,46] are also reported.

Thermal management applications
The advantage for using diamond film as the heat dissipating material is of course due to the extraordinary high thermal conductivity (20 W/cm.K at room temperature, almost 5 times to that of Cu) , and the highly insulative property (resistivity as high as 10 16 Ω.cm). This prompted a long list of potential applications waiting to be realized, from the heat spreader (heat sink) of the high-power laser diode arrays and the packaging of the high-power electronic devices, to the thermal management of the high power electronic and the electron-optic systems. However, the progress was limited to a large extent by the very high cost. Just imagine that a diamond heat sink is more expensive than the high-performance CPU in your desktop computer! Therefore, it is reasonable to think that the diamond heat spreading material may most suitably to be used in the cases which are of great social, economic or military importance, and when there are no other alternative materials or technologies for replacement.
An excellent example is the high-power electronic system in satellite, where the available space is extremely limited, and the heat dissipation is very difficult. Recently, high performance thermal grade freestanding diamond plates have been successfully used in the Chinese Big Dipper navigation satellites as the heat spreader for the high power space phased array antenna ( Figure 6). The heat spreaders were fabricated with the high-power DC Arc Plasma Jet by USTB with the collaboration of Hebei Plasma Diamond) ( Figure 6). Mirotrenches were fabricated by laser cutting on the back side of the diamond plate to form the passages for the cooling fluid. Boarding test in the orbit showed that the cooling ability was as high as 500 W/cm 2 , which was the same as that of the ground test. Now, there are four Big Dipper navigation satellites with diamond heat spreader cooling space phased array antenna currently functioning in space [47].
There are also other successful cases using CVD diamond as the heat dissipation material in China. This has increased rapidly the volume of sale of the Hebei Plasma Diamond thermal grade diamond film products in the recent years. More than half of the customers are from the domestic electronic enterprises. The product production technologies including the design of the thermal management technical route, the metallization (if needed), and the fabrication of the micro passages etc. are all done by themselves, since there are no major technical difficulties left after the intensive R&D in the past 20 years both in China and abroad.
At present, almost all the commercial thermal grade freestanding diamond film products are produced by DC Arc Plasma Jet. And the Hebei Plasma diamond is the main supplier in the world. Other companies, like Henan Fei Meng Diamond Co. Ltd., are also producing free standing thermal grade diamond film products on a smaller scale, all using DC Arc Plasma Jet technology from the USTB. The main reason why DC Arc Plasma Jet is superior in the mass production of thermal grade freestanding diamond films is due to its capability to supply very high concentration of atomic hydrogen over a large area substrate at a very high gas temperature (6000 °C at the nozzle of the plasma torch) heated by the arc discharge. Moreover, the special design of the highpower large orifice plasma torch with arc root rotation and gas recycling [11,12] also facilitate the process. Even the thermal conductivity of the tool grade freestanding diamond films by DC Arc Plasma Jet is also higher than 10 W/cm.K (this is probably one of the reasons why DC Arc Plasma Jet diamond tools are superior to that of the Hot Filament). Presently, different thermal grades (12-14 W/cm.K, 15-17 W/cm.K, 17-20 W/cm.K) with a maximum size of 150 mm in diameter and 2 mm in thickness can be supplied by the Hebei Plasma Diamond. Very recently 7 inch diameter thermal grade diamond wafer with a thermal conductivity as high as 17 W/cm.K and 2 mm in thickness was prepared by USTB using an improved high power DC Arc Plasma Jet with a newly designed double magnetic field controlled plasma torch [48]. It was reported that an 8 inch thermal grade freestanding diamond film with a thickness of 0.6 mm was prepared by PACVD (Hot Cathode Plasma CVD) in Korea. However, the thermal conductivity was only 10 W/(cm·K) [19]. MWCVD is not commercially competitive simply due to the higher cost.
Besides the relatively high cost, another obstacle for the use of the diamond film as the heat spreader of the high-power electronic devices is the large mismatch in the thermal expansion coefficient between the diamond and the substrate of the electronic devices. Thermal expansion coefficient of the diamond is too low (about 1x10 −6°C−1 ), which is many times lower than all the substrate materials for the high-power electronic devices. Therefore, a kind of diamond/metal composites is being developed which are much cheaper than CVD diamond. The thermal expansion coefficient can be adjusted to meet the application requirement [49][50][51].

Optical applications 2.1.4.1. Optical grade diamond films.
Diamond is transparent to the electromagnetic wave from the ultraviolet (0.22 µm) to the far infrared (mm waves). There are no other absorption peaks in the whole transmission spectrum except the intrinsic absorption peak at 2.79-5.04 µm, which is quite small (12 cm −1 ) [52]. However, CVD diamond films always contain certain amounts of impurities (mainly nitrogen) which result in the increase in the absorption and become less transparent or even dark. Optical grade diamond film usually means the high-quality diamond films which contain very little impurities (less than 5 ppm), and are fully transparent (colorless). The first optical grade freestanding diamond film in China was prepared by DC Arc Plasma Jet in 1997 which was only about 30 mm in diameter and 0.3 mm in thickness (with both sides polished. See Figure 7). This small piece of transparent freestanding diamond film had been presented to the chief scientist of the Specialist Committee of the "863 Plan", Prof. Shi Likai, and was rewarded by him a prize of 100,000 RMB on site as extra funding to the USTB group at that year [53]. This was an appreciable amount of money at that time. At present, preparation of freestanding optical grade diamond films is no longer a problem. 6 inches fully transparent freestanding diamond films with a thickness of 1.6 mm have been prepared both by the MWCVD and by the DC Arc Plasma Jet (see Figure 8) [54]. This is because both the MWCVD and the DC Arc Plasma Jet are able to supply the extremely high concentration of atomic hydrogen needed for the deposition of high optical quality diamond, whilst the Hot Filament CVD is not suitable, because the temperature of the hot filament is too low (1800-2400°C) to generate sufficiently high concentration of atomic hydrogen. It is many orders of magnitude lower than that produced by the MWCVD (working at high chamber pressure and high power) and that by the high-power DC Arc Plasma Jet). The present quality level of the optical grade diamond films in China is as following: Maximum size: 150 mm in diameter with a thickness of 2 mm maximum (polished); Impurity level (N 2 ): 0.1 ppm for that by the MWCVD and the DC Arc Plasma Jet; Raman spectrum: very sharp diamond characteristic peak at 1332.5 cm −1 , and a FWHM less than 4 cm −1 . No any other peaks in the whole spectrum; Transmission: fully transparent and colorless, about 70% transmission at 8-14µm; Fracture strength: higher than 300 MPa as measured by three-point bending [55]; Fracture toughness: 8-11MPam 1/2 as measured by three-point bending with pre-cracked specimen [56]; Dielectric loss (Tan δ): 1x10 −4 at 138 GHz for that by DC Arc Plasma Jet, 2.7x10 −6 at 133 GHz for that by MWCVD [57]; Thermal shock: no cracking when quenched from 1100 °C into the water at room temperature (diamond film specimen was sealed in quartz tube filled with Ar to avoid oxidation) [58]; Sand erosion rate (134 m/s, 180mesh (85 µm) erodent) [59]: Diamond film: 1.2x10 −3 -1.6x10 −3 mg/g (SiC powder); ZnS: 47000x10 −3 mg/g (Al 2 O 3 powder); Oxidation resistance: less than 5% loss in transmission at 8-14µm after an exposure of 180 s at 780 °C in laboratory air [60].

Diamond film optical widows.
Advantage of the diamond film optical window relies on the combination of the excellent optical transmission in the wide spectra  range from the visible to the far infrared (mm waves), the highest thermal conductivity, the very low thermal expansion coefficient (close to that of the quartz), and the relatively high strength and toughness (as compared to other ceramic materials). It was expected to be used as the ideal windows for the high-power CO 2 lasers, as well as the domes and windows for the high-speed guided missiles.
There are now no major technical difficulties for the R&D of the high-power CO 2 laser windows. However, due to the very rapid advancements in the high-power laser technologies in recent years, CO 2 laser is no longer the main power of the industrial lasers. Therefore, it is not very much of concern for the CO 2 laser windows at present. Nevertheless, it must be pointed out that although the commercial CO 2 diamond window had been in the market outside China for a long time (since the first decade of the twenty first century), diamond film CO 2 laser windows are not fully commercialized in China even now. The situation seems to be that in the past the technologies for the preparation and processing of the CVD diamond optical windows were not sufficiently mature. However, when the technologies are ready, there is no longer the need.
Diamond domes and windows for high-speed guided missiles have been of great concern from the very beginning of the history of the CVD diamond films in abroad [61][62][63]. The related preparation and processing technologies had reached the state very close to the practical applications in the period from late 1990s to the 2010s [64]. This is because diamond is an ideal multi-spectrum optical window material which is capable of working in the LWIR (Long Wave Infrared, 8-12µm) and the millimeter wave (Radar Wave) range simultaneously. Most of all, it is most robust amongst all the IR window materials. In fact, it is the only one that can be used in the case of hypersonic missiles. The stagnant temperature for a hypersonic missile flying at 5-7 Ma in the dense atmosphere would reach 3000 o K, which would induce a serious thermal shock that no window material except diamond could survive. Zinc sulfide (ZnS) is also an excellent multi-spectrum window material which could be used in the 3-5µm MWIR (Medium Wave Infrared) and 8-12µm LWIR (Long Wave Infrared) as well as in the Radar wave spectra ranges. Unfortunately, due to the extremely serious thermal shock, ZnS dome and windows would be damaged within 0.4 s at 4 Ma in the dense atmosphere [65].
However, it is extremely difficult to prepare and process diamond missile dome and windows. Firstly, the preparation of the crack-free large size thick freestanding optical grade diamond film is not easy. Due to the severe mismatch between the thermal expansion coefficient of the diamond film and the substrate, very often the grown diamond film was cracked into pieces immediately after the deposition run. However, this is essentially solved by USTB that use the graphite to replace the Mo as the substrate. Since the thermal expansion coefficient of the graphite is much smaller than the Mo, the thermal stress will be greatly reduced. A Ti thin interlayer is then applied onto the graphite substrate which was carbonized and became TiC at the very early stage of the diamond deposition. Because the adhesion between the diamond film and the TiC layer is much weaker than the adhesion between the TiC and the graphite substrate, the diamond film would detach off the SiC interlayer first during the cooling down process after the deposition, and very likely at certain temperature much higher than the ambient. Hence the thermal stresses was efficiently released before the building up [66]. Precision processing of diamond optical windows and domes is another major difficulty. The present level in China for the precision processing of CVD diamond window is: surface roughness Ra -about 1 nm; thickness tolerance -±5µm (for 1.5 mm thick window); flatness -one fringe at 633 nm [67], which are within the requirements for the application of optical windows and domes [64]. R&D in the optical windows and domes are in progress in China. At present, USTB is leading in this application field, whilst the Hebei Plasma Diamond has been selling optical grade diamond materials for a few years. However, the market scale is small.
Recently, the need for the diamond Gyrotron window has also been placed on the schedule. A joint group including the USTB, the Hebei Plasma Diamond, the Harbin Engineering University and the Northwest University of Electronic Science and Engineering, funded by the Ministry of Science and Technology are working together on the R&D of the practical Gyrotron windows for the ITER program in China. This is a real challenge since a very low dielectric loss (less than 10 −5 ), very high processing accuracy (the flatness within one fringe at 633 nm), and the extremely high vacuum tightness (less than 10 −9 Pa.l/s vacuum leakage) are required [64].
Progress in the application of the X-Ray windows are also reported in China. Diamond film had been proved suitable for its use as the side-view output window of the miniature X-Ray tubes [68]. Diamond film window had also been used successfully as the soft X-Ray window in the Shanghai Synchrotron Radiation Source [69].

Diamond thin film optical coatings.
Diamond thin film coated optical windows can be used in extremely hostile environment (high temperature, highly erosive, corrosive, or radiative environments) where other substrate window materials could not withstand. The transmission of the diamond thin film coated window depends not only on the optical quality of the thin diamond film and the substrate, but also to a greater importance on the surface roughness Ra and thickness of the thin film coating, and should be calculated by the theory of the thin film optics [70]. Besides, adhesion of the thin diamond film coating on the substrate surface is of vital importance, but unfortunately, due to the very low thermal expansion coefficient of the diamond, the adhesion is usually poor. And a buffer layer is usually needed. Typical example is the R&D of the diamond thin film coated ZnS windows which were of great concern both in and outside of China [71][72][73]. However, due the extreme difficulties encountered, it was not successful. Nevertheless, there were a few successful cases reported for other substrate materials [69,74]. The needs for diamond thin film coated optical windows is apparent while the market is rather small.

Diamond Raman laser.
Diamond is a Raman crystal with the combination of the excellent properties of the largest Raman shift (1332.5 cm −1 ), the widest transmission spectrum from the ultraviolet (0.22 µm) to the far infrared (mm waves), the highest thermal conductivity, and the very low thermal expansion coefficient. Therefore, it is an ideal material for the high power, narrow line-width, wide wavelength range tunable Raman lasers, which could be used in very important high technology, such as the quantum computing, the detection of the gravitational waves, the bio-detectors, and the laser radar, etc. In recent years, great progress has been made outside of China [75][76][77]. However, similar work has also been carried out in China, and notable progresses have been reported [78][79][80].

Electronic applications
Electronic application is the most expected and also the most disappointed application area in the entire history of the CVD diamond. This is because of the excellent electronic properties which no other semiconductors could compete with, and the extreme difficulties in the N-type doping and the large area heteroepitaxy which could not be easily solved in a short time span. In the past 20 years or so in abroad, although efforts have been continuously made and notable progress has been achieved, unfortunately, the present level of the N-type doping and the large area heteroepitaxy is still distant from the target. Nevertheless, great progress towards the practical or commercial applications has been achieved in the diamond passive electronics, e.g. diamond detectors and sensors, SOD and GaN-on-Diamond devices, and the surface terminated electronic devices, etc. The situation in China is more or less the same as that abroad, but lags slightly behind.

N-type doping.
P-type doping is easy and the present level of boron doping is more or less close to the requirement of the practical applications [81].
However, the N-type doping is still not very good. Present work on the N-type doping in China is focused on the Phosphorous, Nitrogen, and Sulfur dopants. Both in situ doping in the chemical vapor deposition process and the ion implantation are used. However, the resultant carrier concentration and the mobility are not sufficiently high, which are not good enough for the practical applications [81]. Besides, potassium was also used as a N-type dopant [82]. Co-doping by boron and sulfur [83], nitrogen and boron [84] were reported. Intensive research on diamond film doing is being carried out continuously.

Large area heteroepitaxy.
The first report on diamond film heteroepitaxy in China was by Jilin University in 1994 [85], in which c-BN (001) and (111) single crystals were used as the substrate, and the experiment was carried out by HFCVD. Quite a few reports were followed in the 1990s [86][87][88]. Afterwards, there was a long period of inactivity for over 10 years. However, it comes back again in the recent years, with the distinct difference that the main substrate material has been changed from silicon to iridium (or iridium coatings), and that the used CVD equipment have been changed from the HFCVD mainly to the MWCVD [89][90][91]. The present level of the electronic properties of the heteroepitaxial diamond films abroad are very close to the high-quality HPHT or CVD single crystals. The FWHM of the diamond (400) X-Ray rocking curve was reported to be 0.064°. The dislocation density was as low as 5x10 6 /cm 2 and the maximum size was 92 mm in diameter, which are much better than the highest quality polycrystalline diamond films. However, it's still not so good when compared to the high-quality HPHT or CVD single crystal diamond. Therefore, at present, it is still considered not suitable for the high-power electronic devices yet [90]. For this reason, and also because the recent progress in the growth of large size high quality single crystal diamond, the main efforts for the ultimate diamond semiconductors have been focused on the R&D of the large size high quality single crystal diamond substrate. This will be discussed later. However, intense activities in the R&D on diamond heteroepitaxy are still being carried out continuously both in and outside China. [92,93]. Now it is fully matured and is in practical applications worldwide. Thanks to the very high thermal conductivity of the diamond, SOD devices can be used in the higher temperature and hostile radiation environment. In real production processes, diamond film was deposited on top of the silicon substrate first. After the diamond film deposition, silicon substrate was polished to a thin layer of the pre-designed thickness to form the SOD structure, which was then used as the substrate for the microfabrication of the SOD devices. As compared to the SOD devices, GaN-on-Diamond (Gallium Nitride (GaN) on Diamond) devices can be used at much higher power output and at higher frequencies (because GaN is a wide-bandgap semiconductor). Therefore, it is of great interest in China and abroad in recent years. However, it is much more difficult to prepare simply due to the active chemical properties of the GaN which would react with the hydrogen atoms in the diamond film deposition atmosphere and would result in the severe etching of the GaN. There are four technical roads which could lead to the solution: (1) the direct CVD of the diamond films on the back of GaN wafers by using protective interlayers; (2) the bonding of High Electron Mobility Transistors (HEMTs) and diamond substrates; (3) the direct epitaxy of the GaN layers on diamond substrates; (4) the diamond capping of passivated GaN HEMTs [94]. At present, the fabrication of the GaN-on-Diamond wafers by direct diamond CVD on the back of the GaN wafers has been quite successful abroad. And commercial RF power amplifiers fabricated on GaN-on-diamond wafers are available in the market. The bonding of the GaN HEMTs and the diamond substrates and the capping of the GaN HEMTs have not only made notable progress but also raised interest from the electronic industries. However, the epitaxial growth of the GaN layer on diamond substrate is still under intense R&D [94]. The current research in China is still active and is mainly focused on the preparation of the GaN-on-Diamond structure and the fabrication of the GaN-on-Diamond devices [95][96][97][98]. It is expected that the practical devices would be commercialized in the near future. It is worth noting that Chinese electronic industries have been deeply involved in the R&D on the GaN-on-Diamond technologies and will continue to be actively involved in the coming years.

Terminated surface-based diamond devices.
At present, major progress in the terminated surfacebased diamond devices has been achieved in China. H-terminated diamond surface was usually used. However, other types of terminated diamond surfaces, such as O-terminated and F-terminated diamond surfaces were also employed, for the fabrication of the surface terminated diamond devices. Several kinds of devices have been successfully fabricated, including the dual-termination (O-terminated and F-terminated) Schottky barrier diode [99], H-terminated surface diamond MESFET [100], H-terminated diamond MOSFET [101], H-terminated diamond MISFET [102] etc. They are expected to be used as the high power and high frequency electronic devices in the near future. However, there is still a notable gap in performance compared to the theoretical prediction. Same applies to the SOD and GaN-on-Diamond which the Chinese Electronic Industries have been deeply involved.

Diamond detectors.
R&D of the diamondbased detector can be traced back to the 2010s in China [103,104]. However, it was primitive and the research level was rather low (charge collection distance (CCD) was 9 µm and 12.6 µm, and the charge collection efficiency was 46% and 60% respectively). Nevertheless, promising advances could be seen in recent years. At present both polycrystalline and single crystal diamond are used. The present research is focused on the R&D of the violet [105,106], the violetblue [107], the sun-blind violet [108], the α − particle [109] and the neutron [110] detectors. Single crystal diamond neutron detectors were successfully tested as the beam monitor for the Spallation Neutron Source of China (SNSC), showing the apparent pulse resolution in the flight-in-time spectrum and the high reliability against the fluctuations of the detector. They were considered satisfactory for the practical use as the beam monitor of the SNSC [110]. Single crystal diamond neutron detectors had also been successfully tested in the Dubna Nuclear Center in Russia, where the packaged single crystal and polycrystalline diamond film detectors were irradiated by the ultrahigh power IBR-2MW neutron line at JINR for 17 days, to an accumulated fluence as high as 3.3x10 17 n/cm 2 which was higher than the designed accumulation fluence in ten operation years in the Hadron Collider in Europe [111,112]. The normalized remaining signal response was 10% and was higher than that of the polycrystalline diamond detectors (2%) [113]. Single crystal diamond detectors had been irradiated in ultrahigh power proton beam line for 47 h in the high luminance proton beam cyclotron accelerator in the Chinese Institute of Atomic Energy (CIAE), to an accumulated fluence as high as 1.7x10 17 p/cm 2 . The normalized remaining signal response was 5.6% [114], which also met the requirement of ten years operation of the Hadron Collider. The above two cases were supported by an international cooperation program of the Ministry of Science and Technology of China, with an aim to provide an alternative technical route for the upgrading of the Liquid Ar Forward Calorimeter for the upgrading of the Hadron Collider [111]. Up to now, almost all the experimental results both in and outside of China have shown that the performance of the single crystal diamond detector is superior over the polycrystalline diamond counterparts. This is not surprising since the single crystal diamond contains no grain boundaries and much fewer crystal defects and impurities.

Quantum applications.
Quantum technology has been considered as one of the most important and top urgent technologies to develop at present and in the future in most of the advanced countries in the world. Compared to the existing counterpart technologies, quantum computing and quantum communication have already been demonstrated to have unimaginable supremacy. However, diamond is also one of the most important quantum materials, because it has been shown that the NV color centers and the SiV color centers in the extremely high-quality diamond have ideal quantum properties. They are very stable and easy to be manipulated (even at room temperatures). In recent years, major progress has been achieved in China. Present researches are focused on the fabrication, identification, characterization and manipulation of the NV (NV 0 and NV -) color centers [115,116], SiV color centers [117] and Ge color centers [118]; application of the NV centers in the paramagnetic resonance spectroscopy [119], quantum information processing and metrology [120], quantum sensing [121]; and quantum computing [122,123]. Nevertheless, in practical applications, extremely pure diamond is absolutely needed which is defined as the quantum grade diamond and contains very little impurities (mainly nitrogen) as low as 0.1 ppb and a dislocation density ≤ 3/cm 2 only [124]. Apparently, this is incredibly difficult to prepare. At present, Element Six is probably still the only supplier for quantum grade diamond in the world.

Acoustic application
At present, diamond-based SAW (Surface Attenuation Wave) device is no longer in the main stream of the R&D of the SAW devices and is not of concern. Diamond membranes are used in the high-quality loudspeakers both in and outside of China. However, the market is very small. There is no new publication on diamond loudspeakers in China since 2010.

Laboratory grown diamond
As mentioned previously, progress in the laboratory grown diamond in China is amazing and unimaginable. According to the Annual report 2021 of the CarbonTech [125], the world output of the laboratory grown diamond in 2020 was estimated as 6-7 million carats, of which about 3 M was from China (about 50%), whilst 1.5 M from India (20%) and 1 M from USA 10%). However, within the 3 million carats of the laboratory grown diamond in China, the output of the CVD grown diamond was negligible. Almost all the 3 M was from the HPHT technique, which was made of 90% of the world HPHT laboratory grown diamond [125]. Nevertheless, according to the latest cyber source report (unconfirmed) [126], the world output of the laboratory grown diamond in 2022 is estimated to be 23 million carats, of which 14 M would come from China. This would double compare with that of the previous year (2021) and would amount to 60.8% of the world yield of the laboratory grown diamond. Meanwhile, the CVD laboratory grown diamond would be 1.5 M carats, which would be considered as a big burst out! Technically, the emergence of the MWCVD diamond film deposition systems working at high pressure and high power in the late 1990s had laid down the foundation for the rapid advancement in the R&D of the large size high quality single crystal diamonds in the first decade of the twenty first century [127,128]. Soon after, commercial CVD diamonds appeared in the jewel market in 2012 [127]. Single crystal CVD diamond companies in Singapore and USA had begun selling CVD diamond jewelry in the world market. However, the size of the CVD diamonds was small (0.3-0.7 carat). In the same time period, numerous companies and organizations for the production of the CVD single crystal diamond had emerged in China. In 2015, a 5.09 carat CVD diamond of color J, and clarity VS2 only was sent to IGI for examination. In the same year, ISO claimed that the phrase "Laboratory Grown Diamond" and "Synthetic Diamond" were synonymous. Then the phrase "synthetic diamond" was replaced by "Laboratory Grown Diamond" (LGD). In 2018, FTC (Federal Trade Committee) recognized that the laboratory grown diamond belonged to the diamond categories. This meant that the laboratory grown diamond had already been accepted by the consumers. On 17 March 2020, the IGI claimed that it has received a 7.06 carats high quality laboratory grown colorless diamond, with color F, clarity VVS2 and cut 3EX. This diamond was from the Hangzhou Chao Ran Diamond Technology Ltd. China and was considered as the biggest high-quality laboratory grown diamond in the world [129]. It indicated the maturity of the large carat laboratory grown diamond. However, only 4 months later, this world record was broken again by another Chinese company, the Shanghai Zhenshi Technology Ltd. It sent a 10.57 carat colorless laboratory grown diamond with F color, VVS2 clarity and 3EX cut to the IGI for examination on 10 July 2020. As claimed by the IGI, the as-grown blank weighed 45.152 carat. Then the 10.57 carat diamond became the world largest high-quality laboratory grown diamond ( Figure 9) [130]. At present, the HPHT LGD in China has remained as the world first position for three years, whilst the CVD LGD in China is also moving fast. It is estimated that the MWCVD systems for the growth of the LGD in China will reach 2500-3000 units by the end of this year [125]. However, this number is almost the same as predicated in India in 2022 [128]. Since the predicted annual output for India in 2022 is 6 M carat, the estimated output of 1.5 M carat in 2022 in China [126] is apparently underestimated. The major difference between the HPHT and the CVD LGD is that: the CVD LGD is usually of larger size and of better quality as compared to the HPHT LGD. However, the growth of the large carat CVD LGD is not easy. Up to now, only a few companies are capable of producing it. Both the HPHT and the CVD techniques are now nearly fully mature. The future of the LGD will be determined by the market, i.e. the extent of the acceptance of the consumers. However, it is reported that at present 80% of the LGD were consumed in US market and that only about 10% were sold in China [126]. Therefore, the future of the laboratory gown diamond (LGD) will be very bright.
As discussed in the previous sections, the electronic properties of the single crystal diamond are much better than the polycrystalline diamond. Therefore, it is now considered as the suitable candidate for the R&D of the "Ultimate Semiconductor". This will be discussed in details in next section. Single crystal diamond detectors have already been successfully used as the beam monitor of the Chinese Spallation Neutron Source [110], and also been tested for the irradiation damage limit at a proton fluence as high as 1.7x10 17 p/cm 2 in the high luminance proton beam cyclotron accelerator in the Chinese Institute of Atomic Energy (CIAE) [114]. High purity single crystal diamond has been used for the R&D in quantum applications [116]. Very recently, large size CVD single crystal diamond has also been successfully used as the soft X-ray window for small angle X-ray scattering in situ loading test in the Shanghai Synchrotron Radiation Source [131]. Diamond Anvil Cell (DAC) is the only instrument which can be used at high pressures in the million atmospheric pressure range at a temperature up to 2000 °C. At the same time, multi-inspection techniques including the optical microscopy, photo-spectroscopy, Raman spectroscopy, and X-Ray diffraction, etc. can all be employed. These facilities are indispensable in the research in ultrahigh pressure physics, chemistry, geology, material synthesis, etc. In this particular application field, high quality and large size single crystal diamond is absolutely needed. DAC is widely used in China [132][133][134]. However, in the past most of the DACs were fabricated with natural diamonds which were gradually replaced by the CVD single crystal diamond in the recent years. However, DAC neutron scattering at ultrahigh pressures is not very successful up to now because of the large scattering angle and low intensity scattered beam particularly with the low atomic number elements such as hydrogen, deuterium and tritium which can be clearly distinguished by the neutron scattering but not by the X-Ray diffraction. Same applies to nitrogen, helium, lithium, beryllium, boron etc. In the case of neutron scattering, much larger size diamond single crystals with high fracture strength and high fracture toughness are needed, but it is difficult to fabricate them.

Ultimate diamond semiconductor
The urgent need for the electronics which can be operated at high power, high frequency and high temperature has hastened the rapid progresses in the third-generation wide bandgap semiconductors. Due to the serious difficulties met in the R&D in the diamond electronics, and that the achievements up to now are still not sufficient to meet the technical requirements, SiC has become the main stream in the development of the high performance wide bandgap electronics. This is particularly true for the development of the high-power electronics, since SiC technology is much maturer than diamond. At present, SiC high power electronics have already been successfully used in the railway power station [135] and automotive power systems [136] in Japan with a pronounced saving of power losses of 20-30% as well as 20% smaller and 15% lighter compared to the Si power electronics. However, as shown in Figure 10 [137], there is still opportunity of developing the next generation (ultimate) diamond power electronics. Because of the superior electronic properties of the diamond over SiC (like high breakdown field, high thermal conductivity, high mobility and low dielectric constant), diamond power electronics can be operated at much higher current and voltage, i.e. at much higher power densities, where SiC and other semiconductors cannot be used. Another advantage of the diamond power electronics is its capability to be used at higher frequencies than SiC, which is of importance to increase efficiency and to reduce the volume and weight of the power module [137]. Increase in the "end use efficiency" is now considered as a very important factor for realizing the target of CO 2 emission reduction worldwide. It was predicted that the increase in the "end use efficiency" may be responsible for the 49% of the target of the planed global CO 2 emission reduction by 2030, which is three times of the renewable energy (17%)! However, it is still very difficult to develop next generation diamond power electronics. At least, three challenging technical requirements must be met, i.e. large size high quality single crystal diamond wafer, low defect (dislocation) density, and low resistivity. In addition, another drawback is the deep accepter (B: 0.37 eV) and donor (P: 0.57 eV) level, which will result in insufficient carrier density for large current operation. However, it was predicted that at higher operation temperature (say 200-250 °C, the self-heating temperature of the diamond power device) the conduction loss would be reduced by increasing the thermally activated carriers with a reasonably high mobility (say 240cm 2 /Vs at 250 °C) [138]. This is advantageous to develop a light weight diamond power device without cooling system. The suggested target for the development of next generation diamond power electronics is listed in Table 1 [137]. Large size high quality single crystal wafer is considered as the first step toward the final goal. At present, the large size heteroepitaxial diamond growth and the mosaic single crystal diamond homoepitaxial growth are the main technical routes. The main drawback of the former is the high dislocation density (10 6 /cm 2 -10 7 /cm 2 at present) which is very difficult to be further decreased by several orders of magnitude in a short time period, whilst the main drawback of the later is the boundary area between the original single crystal diamond plates where a distinct lattice misorientation always exists and is very difficult to be completely eliminated [137]. At present, intensive works towards the target of full realization of the next-generation (ultimate) diamond electronics are being carried out.
In recent years, the development of the next generation (ultimate) diamond semiconductor devices is also of great concern in China. This was indicated by the international proseminars continuously organized in 2018 and 2019 in Xian [139,140]. Current research activities are focused on the mosaic growth of large size single crystal diamond wafers [141][142][143], large size heteroepitaxial growth of high-quality diamond films [89][90][91], and the technical road that leads to the reduction of dislocation density of the epitaxial grown diamond wafer [144,145].

A brief summary and the perspective to the near future
Mechanical applications and laboratory grown diamond are two most successful application areas of the CVD diamond in China.
At present, CVD freestanding thick diamond film products are dominated by China. However, the output value is still not very big. This is because the market is limited by the specialty of the applications (dressing logs and cutting tool blanks). Another reason is that the present products are of the primitive type -CVD diamond raw materials. It is suggested that the output value may be greatly increased by the development of diamond component, i.e. the diamond dressers and diamond cutting tools etc. Some high value tools such as the diamond surgery knives and diamond dental tools are also suggested. As to the thin film diamond tool applications, it is suggested popularizing (spreading) the existing technology first, and at the same time, upgrading the applications from diamond coated metal tube (and wire) drawing dies to the sealing components and the wear and abrasion resistant parts to be used in the  ultra-hazardous environments, e.g. high temperature, highly corrosive and erosive, high radiative etc., and particularly with heavy loading and working at high speed! Diamond is most advantageous in the application area of wear and abrasion in the hazardous environments where the potential market is huge and almost unlimited. The future of the thin film diamond tool applications will be very bright. Diamond heat spreaders have been successfully used in the Big Dipper navigation satellites. Thermal grade diamond film materials are now commercially available in the Chinese market. However, the market size is still not very big. The main obstacle is the high price of the diamond materials, which substantially lowers the ratio of the performance to price and makes the end users shrink back at the sight. However, there is an urgent need in the cheaper diamond heat spreaders for the rapid development in the 5 G communication high power base station electronics, the high-power electronics and optoelectronics for civilian and the military uses. Therefore, it is necessary for the material providers to lower the price, either by further improving diamond film deposition and processing technologies, or by grading the thermal grade diamond film products. For the end users, it is wise to choose the correct grade material for the diamond heat spreaders to lower the cost. However, efficient intercommunications between the materials provider and the end user are absolutely needed. DC Arc Plasma Jet is advantageous in the mass production of the thermal grade freestanding diamond wafers. However, it is capable for the production of different quality grade diamond materials from the cheaper tool grade to the expensive optical grade diamond wafers. The thermal conductivity of the thermal grade diamond material by DC Arc Plasma Jet overlaps with the tool grade at the low end (8-10 W/cm.K, light brown to yellowish) and the optical grade at the upper end (≥17 W/ cm.K, light yellow to colorless). Therefore, the right choice of the proper thermal grade diamond materials will be an effective way to lower the cost. It is strongly suggested that the diamond film material providers should build up a close tie with the end users forwardly. In one word, the need for the diamond heat spreaders is urgent and is a rigid demanding. If the cost of the thermal grade diamond materials could be decreased, the future of the thermal applications of the CVD diamond film will be very bright.
The present quality level of the optical grade diamond films in China is sufficient to meet the requirement for the optical applications. Research programs on diamond film windows and domes are being carried out, and are making continuous progresses. Technical problems of the precision processing, particularly of the geometric tolerance of the diamond film optical component, must be further improved. Degradation in optical quality of the very thick diamond film (higher than 2 mm thickness, as polished) is inevitable. However, in most cases, 1-2mm thickness is enough for the diamond film optical components to withstand the dynamic atmospheric pressure or to keep vacuum tightness. It is suggested that the end users should properly reduce the safety coefficient in the design of the diamond optical component, by which the provider could provide the optical diamond component with much higher quality. Since the thermal emission is highly dependent on the quality (defect density) of the diamond film material used, this may be very important for those to be used at hyper sonic flight in dense atmospheric environment where the thermal emission of the diamond component might be a serious problem due to the atmospheric dynamic heating. At present, optical diamond film products are commercially available in the Chinese market. However, the market scale is small. In the near future, practical use of CVD diamond film window and domes will be realized, which may play an important role in the relevant application field, but the market may not be very large.
BBD (Boron Doped Diamond) film electrode has been successfully used in the R&D and prototype equipment for the electro-chemical treatment of the industrial and civilian waste waters which are difficult to be treated by conventional methods, disinfection and sterilization of the anti-coronavirus environments, biosensors and detectors for medical applications, and so on. However, up to now, there is no major industrial scale application market in China.
Diamond detectors have already been successfully tested in the Chinese Spallation Neutron Source, the High Luminance Proton Beam Cyclotron Accelerator in the Chinese Institute of Atomic Energy (CIAE), as well as the IBR-2MW Neutron Line at JINR, Dubna Nuclear Center in Russia. In the near future, it is highly likely to meet the requirement for the outer space exploration programs, the ITER (International Thermal Nuclear Fusion Electricity Generation Program) project, and the R&D in high energy particle physics programs in China.
Progress in quantum application is obvious. However, the preparation of the "quantum grade" diamond film (or diamond single crystal) is extremely difficult. It requires the substantial technical advances in the ultrapure gas sources, near zero vacuum leakage MWCVD diamond deposition system, as well as the optimized diamond deposition technology.
SAW devices are no longer in the main stream in the R&D of high-performance SAW devices. The market for diamond film loudspeakers is very small.
The extremely successful commercialization of the Laboratory Grown Diamond (LGD) is the most significant achievement in the R&D of the CVD diamond films in China. It is amazing that the estimated MWCVD reactors for the growth of the LGD may reach 2500 to 3000 sets by the end of this year, which may be compatible to the predicted value for India who was the largest manufacturer of the CVD LGD in the world in the past few years. However, considering that the annual output of CVD LGD was almost neglected only in one and half years ago in 2020, this is really an unimaginable big leap forward! Accordingly, it is reasonable to guess that the annual output in CVD LGD might very likely surpass that of the Idea next year. Nevertheless, it must be pointed out that the information available right now is somehow ambiguous with some uncertainties. Particularly, there is a controversial argument that the crisis of the LGD (HPHT and CVD) redundant capacity in China will appear soon [126], which is based on the fact that at present most of the LGD are produced in China (60%) while most of the LGD are sold in USA (80%) and that the market size in China is only 10% while most of the large carat LGD are polished in India. And the further argument is that the capacity of the LGD production in China has been expanding madly, doubling every year, whilst the retail sale market abroad and in China does not change very much. Anyhow, this argument is somewhat reasonable. The future of the laboratory grown diamond depends on the Chinese market, i.e. the willingness of the Chinese customers to buy the product.
Progress in the R&D of CVD diamond films in the application areas of electronics are obvious and significant in China, but it still lags behind the achievements outside China. SOD devices have already been used in practical applications. GaN-on-Diamond devices will be used in the near future, which will be very helpful to promote the application of the GaN devices further into the areas of higher power, higher frequency, and higher temperatures (or in highly radiative environment). The R&D of the surface terminated diamond devices will continue. There is an urgent need in the R&D of the third-generation wide bandgap semiconductors. However, it is the SiC, not diamond, that has been placed in the main stream as the SiC technology is much more advanced. Nevertheless, there is still the need for the diamond as the next generation (ultimate) semiconductor, since the electronic properties of the diamond completely surpass that of the SiC. The development of the high-power electronic devices which can be used at very high power density and at high frequencies than any other semiconductors is of great technical and economic significance. Intense R&D for the ultimate diamond semiconductor has already begun both in and outside of China.