ReviewMicrowave versus conventional sintering: A review of fundamentals, advantages and applications
Introduction
Powder metallurgy is a process whereby a material powder is compacted as a green body and sintered to a net shape at elevated temperatures about 0.6–0.8Tm. There are challenging demands from the PM industry for new and improved sintering process with finer microstructures and enhanced physical and mechanical properties. This is where the microwave technology is found to be advantageous.
Microwave energy is a form of electromagnetic energy with the frequency range of 300 MHz to 300 GHz. Microwave heating is a process in which the materials couple with microwaves, absorb the electromagnetic energy volumetrically, and transform into heat. This is different from conventional methods where heat is transferred between objects by the mechanisms of conduction, radiation and convection. In conventional heating, the material's surface is first heated followed by the heat moving inward. This means that there is a temperature gradient from the surface to the inside. However, microwave heating generates heat within the material first and then heats the entire volume [1]. This heating mechanism is advantageous due to the following facts: enhanced diffusion processes, reduced energy consumption, very rapid heating rates and considerably reduced processing times, decreased sintering temperatures, improved physical and mechanical properties, simplicity, unique properties, and lower environmental hazards. These are features that have not been observed in conventional processes [1], [2], [3], [4], [5].
Microwave energy has been in use for variety of applications for over 50 years. These applications include communication, food processing, wood drying, rubber vulcanization, medical therapy, polymers, etc. Use of microwave technology in material science and processing is not rather new. The areas where this technology has been applied include: process control, drying of ceramic sanitary wares, calcination, and decomposition of gaseous species by microwave plasma, powder synthesis, and sintering [6], [7]. Microwave processing of materials was mostly limited until 2000 to ceramics, semiconductors, inorganic and polymeric materials. There was a misconception between researchers that all metals reflect microwave or cause plasma formation, and hence cannot be heated, except exhibiting surface heating due to limited penetration of the microwave radiation. The researchers did not notice that this relation is valid only for sintered or bulk metals at room temperature, and not for powdered metals and/or at higher temperatures [6]. Now it has been found that the microwave sintering can also be applied as efficiently and effectively to powdered metals as to many ceramics. This paper compares advantages of microwave sintering against conventional sintering and presents some applications confirming its advantages.
Section snippets
Theoretical aspects of microwave sintering
Microwave energy is a form of electromagnetic energy with the frequency range of 300 MHz to 300 GHz and the corresponding wavelengths are between 1 mm and 1 m. The frequency and wavelength range of microwaves are shown in Fig. 1. Microwaves have longer wavelengths and lower available energy quanta than other forms of electromagnetic energy such as visible, ultraviolet or infrared light. The first microwaves application came to the extensive use in communication such as radar, television and
Application of microwave sintering in engineering materials
Microwave energy has been in use for variety of applications for over 50 years. These applications include communication, food processing, wood drying, rubber vulcanization, medical therapy, polymers, etc. Use of microwave technology in material science and processing is not rather new. The areas where it has been applied include: process control, drying of ceramic sanitary wares, calcination, and decomposition of gaseous species by microwave plasma, powder synthesis, and sintering [6], [7].
Conclusion
Microwave sintering has achieved worldwide acceptance due to its significant advantages against conventional sintering methods. A summary of these are:
- (1)
Sintering materials with microwave consumes much lower energy than conventional sintering.
- (2)
Diffusion process intensifies by using microwave due to its enhanced mechanism.
- (3)
Higher heating rates can be attained and thus the sintering time reduces by using microwave sintering.
- (4)
Generally higher density and better grain distribution can be achieved
Acknowledgement
The authors would like to thank the office of gifted students at Semnan University for its financial support.
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