High-quality MgB2nanocrystals synthesized by using modified amorphous nano-boron powders: Study of defect structures and superconductivity properties

Nano sized magnesium diboride (MgB2) samples were synthesized using various high-quality nano-B precursor powders. The microscopic defect structures of MgB2 samples were systematically investigated using X-ray powder diffraction, Raman, resistivity measurements and electron paramagnetic resonance spectroscopy. A significant deviation in the critical temperature Tc was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra. Scanning electron microscopy analysis demonstrate uniform and ultrafine morphology for the modified MgB2. Defect center in particular Mg vacancies influence the connectivity and the conductivity properties which are crucial for the superconductivity applications.Nano sized magnesium diboride (MgB2) samples were synthesized using various high-quality nano-B precursor powders. The microscopic defect structures of MgB2 samples were systematically investigated using X-ray powder diffraction, Raman, resistivity measurements and electron paramagnetic resonance spectroscopy. A significant deviation in the critical temperature Tc was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra. Scanning electron microscopy analysis demonstrate uniform and ultrafine morphology for the modified MgB2. Defect center in particular Mg vacancies influence the connectivity and the conductivity properties which are crucial for the superconductivity applications.


INTRODUCTION
Extended studies have been performed on improvement of the synthesis process of magnesium diboride (MgB 2 ) since its discovery in 2001. 1 The present study investigates the choice of boron precursors which is of prime importance for the functionality of MgB 2 materials and which in turn is governed by the particle size. Actually, there is a linear size correlation between the particles of the reactant boron powders and the final product MgB 2 . 2 This means that a significant average particle size reduction for MgB 2 can be obtained only by using boron particles of submicron or nano size. The most commonly used precursor boron powders are commercially available, with the purity grades of 86-88%-wt (B-86), 90-92%-wt (B-90) and 95-97%-wt. (B-95), respectively. The powders are produced via Moissan process followed by a purification step after which the level of the purity can be increased to a maximum of 97%-wt. [3][4][5] The so obtained materials are semi-crystalline with a particle size range of ca. 0.8-1.2 µm. 6 Extended studies in the last decade have revealed that the crucial factor for fabricating high performance MgB 2 powders is not the purity grade of the precursor boron but its amorphousness and the particle size, allowing a fast reaction at moderate temperatures. [6][7][8] Until now, these criteria are best met by the commercially available nano-size boron powder (nano-B, Pavezyum Kimya) which is accessible via "on demand" production and "in situ" pyrolysis of diborane gas. 6,9 The product is fully amorphous, exhibiting a particle size within the range of 50 to 300 nm and a purity grade of >98.5%-wt. 6,10 One reason for the efficiency of this material is that MgB 2 powders synthesized from nano-B can be also in nano-size, wires and bulks depending on the synthesis technique. This leads to a better grain connectivity and ARTICLE scitation.org/journal/adv enhancement of Jc. [11][12][13] Furthermore, the comparatively poor crystallinity of the so-prepared powders cause structural point defects which in turn act as pinning centers improving the superconducting performance as it was reported in literature. 2,14,15 Despite its superior features, nano-B is due to the high processing costs still about 8-10 times more expensive than the other commercial powders obtained from magnesiothermic reaction based Moissan process. 16 In the last two years we have been focused on new synthesis methods and routes to improve the chemical and morphological properties of the commercial semi-crystalline B-86/-90/-95 series with the target of obtaining a cost-effective powder, revealing similar features as nano-B. Very recently, utilizing different inorganic salt mixtures in various ratios as flux, we were able to synthesize two different modified powders M-B-86 and M-B-95 which show superior properties to the commercial B-86 and B-95 regarding the amorphousness, particle size and surface activity (BET), as well as defects and chemical reactivity. 17 It is worthy to note that there are alternative techniques to obtain pure boron powder effectively: via-gas and magnesiothermic reaction (mentioned above). Via-gas technique uses the hydrogen gas to reduce gaseous boron trichloride (BCl 3 ), whereas magnesiothermic reaction uses a combination of heat and magnesium to reduce the boron trioxide (B 2 O 3 ) to elemental B. Such techniques revealed not only complete removal of MgO from Mg-reduced boron providing clean sources of B but also T C of 39 K. [18][19][20] To test modified M-B-86 and M-B-95 powders suitability as precursor, MgB 2 samples were synthesized and characterized by means of powder x-ray diffraction (PXRD), scanning electron microscopy (SEM), transport measurements, Raman and EPR spectroscopy. In the following, we will discuss and comment on the results in context with the previously reported data for MgB 2 specimen prepared from the unmodified commercial powders B-86 and B-95. 2,10,21

EXPERIMENTAL Synthesis
The two different boron powders M-B-86 and M-B-95 were prepared to modify the magnesiothermal self-propagating high temperature synthesis (SHS) process by addition of a suitable salt mixture (NaCl/CaCl 2 ; 30%-wt) which acted as a flux. The major advantage of this step is the reduction of the reaction temperature and the particle size.
The product obtained after the reaction contains beside the primary phase B and the flux, MgO and magnesium borates as major impurities. The removal of the byproducts and the salt mixture was performed by hot HCl leaching, followed by filtration, washing and drying. The purity of the boron powder at this stage is 85.9%-wt (M-B-86), comprising 10-12%-wt Mg, as well as small amounts of boron oxide and sub-oxides as further impurities.

Characterization methods
All samples were analyzed by powder x-ray diffraction (PXRD). PXRDs were recorded with a BRUKER D2 Phaser diffractometer (CuK alpha radiation with LYNXEYETM detector). SEM images of powders were taken with Zeiss Ultra Plus FE-SEM at 2 kV accelerating voltage. CIF files for Raman analysis were done using nearly 5mg of sample in sealed Pyrex tube (∅=4mm) in the range of 200-1400 cm -1 with a BURKER RFS 100/S spectrometer (Nd:YAG-Laser, 1064 nm, 200 mW). Transport measurements of the pressed samples were carried out with the four-point method in a physical property measurement system (PPMS) at external magnetic fields up to 9 T. The critical temperature Tc was determined in the heating-up resistance curve at 90% of the normal-state resistance at 40 K. The samples were specially treated for electrical measurements as follows: From powders, disk-shaped bulk samples (∅ 10mm, height ≈ 1.5) were obtained in a vacuum chamber by hot-pressing under Ar atmosphere (500 mbar) applying a pressure of 640 MPa at 700 ○ C during 10 min. X-band (9.86 GHz) continuous-wave-EPR measurements were performed with a Bruker EMX spectrometer. We have used a rectangular TE102 (X-band) resonator from Bruker. The offset in the magnetic field and the exact g-factors in X-band measurements were determined with a polycrystalline DPPH (2-diphenyl-1-picrylhydrazyl) reference sample with well-known g-factor (g=2.0036). For cooling (to liquid helium temperatures) an Oxford CF-935 cryostat was used. The temperature was regulated by a temperature controller (Oxford ITC-503). The EPR spectral analysis has been performed using the WINEPR program from Bruker. DLS analysis revealed that for M-B-86 particle size are <500 nm while particle size in as-received B-86 are <1400 nm. Same is valid for M-B-85 particle size which are <400 nm whereas in as-received B-95 particle size are <1000 nm. Worth mentioning are the results of the BET measurements which show a 6-8 fold increase of the surface activity for the modified powders (BET(gm  Fig. 3(b) and 3(d). Poor crystallinity might be related to higher amorphous content of the particles with a mean size of 50-250 nm. Furthermore, the Fig. 3(b) and (d) reveals that the particles of the M-MgB 2 -86 and M-MgB 2 -95 are more uniform and smaller compared to those of MgB 2 -86, MgB 2 -95 in Fig. 3(a)  and 3(c). The reason of the particle sizes difference in these MgB 2 samples might be related to particle size of the starting B powders. 22 25 Moreover, the shape of resistivity curve for all samples at temperature higher than 37 K (above the superconducting transition) exhibit a metallic behavior. [26][27][28] However that maximum resistivity at room temperature ρ 300  Resistivity in MgB 2 will be effected by electron scattering due to defects, impurities and phonons. [29][30][31] All will be caused in increment in electron scattering and reduce their mobility. Therefore higher resistivity in M-MgB 2 -86 might originate from lower purity in M-B-86 which is used to synthesize this compound.   Since MgB 2 is a phonon-mediated superconductor, electronphonon interaction plays an important role in the pairing mechanism that is responsible for MgB 2 superconductivity. Therefore it is important to apply Raman spectroscopy to understand the Raman active vibration modes, thus defect structures. It is well known that for the P 6mmm space group to which MgB 2 belongs, only the E 2g in-plane B stretching mode is Raman active. This mode is highly depending on the morphology, sample size, temperature and the defect structures. Here in Fig. 5. E 2g appeared at 591 cm -1 with anomalously large Lorentzian linewidth for all samples. Since E2g is the only and dominant Raman mode we do not see much difference between the samples. This is closely related to the detection limit of Raman spectroscopy where we cannot get further information about the anharmonicity. Such phonon anomaly including anharmonicity and multi-phonon contributions can be closely relate with the point defects. Further elaborate this approach EPR spectroscopy is one of the important techniques for understanding defect structures since EPR has extensively sensitive and high detection limit for paramagnetic defects.

ARTICLE
scitation.org/journal/adv EPR does not only work very well on the detection of paramagnetically active point defects but also one may obtain reliable correlation to the electronic and structural properties of MgB 2 . Up to now, very limited studies have been reported for investigating the defect structures of MgB 2 by the aid of EPR. 34,35 Spectral changes in EPR signal mostly attributed to the changes in local crystalline field symmetry around weakly localized conduction electrons or holes. Conceivably possible defect centers in MgB 2 could be the Mg vacancies (V Mg ), oxygen vacancies (V O ), boron vacancies (V B ) and the interstitials (Oi or Bi). In particular effect of Mg vacancy in MgB 2 on structural and superconducting properties has been discussed controversially. [36][37][38] Although controversy exists, in those works together with the reduction in Tc and coexistence of superconductive and non-superconductive electronic phases has been correlated with the poor and rich regions of Mg vacancies. There are two main sources for oxygen related defect centers: oxygen vacancies may be formed in the minor boron oxide or magnesium oxide phases, persistent in both the amorphous B powder and the grain boundaries of the MgB 2 samples. 38 In this study highest grade B powders were used that's why the only source for Vo is the formation of MgO secondary phase which is also observed in PXRD in Fig. 2. In Fig. 6 we present the temperature dependent EPR data of three MgB 2 samples. Here we present the most interesting temperature region (30-60 K) for MgB 2 material. At paramagnetic defect state (normal state), according to their g-factors (close to free-electron g-factor: 2.0023) such defect signals can be attributed to V O either on the grain boundaries or on the surface of MgB 2 since MgO is present as a secondary phases. By reducing the temperature down to Tc (∼39 K) both paramagnetic defect states and non-resonant superconductive states coexist. This feature occurs slightly at different temperatures from sample to sample. But it is common feature for all samples. Below a certain temperature normal states does not exist anymore and the EPR spectra ends up with the huge superconductive state where defect signal smears out and at g∼2 region only cavity/resonator signal left with very low signal-to-noise. Here it is worth to note that, as it is depicted in resistivity measurements the metallic character of the samples has been observed in EPR measurements as well. This can be understood from the typical Dysonian 2,39 lineshape of EPR lines indicating more metallic character, thus high concentration of conduction electrons. Such conduction electrons play crucial role in the superconductive properties of MgB 2 material. For instance, the upper critical field decreases with increase of defects density and there is a strong coupling between the conduction electrons and the Raman active E 2g phonon, in which neighboring boron atoms move in opposite directions within the plane. When we compare each sample M-MgB2-95 is quite interestingly shows highest Tc where superconducting and paramagnetic states both exist at 35 K. In other two samples superconductive state starts at lower temperatures 34.5 and 33.5 K for M-MgB 2 -86 and MgB 2 -95, respectively. Furthermore, spin counting procedure were applied where one can quantitatively determine the absolute concentration of defect centers, in other words, unpaired electrons. By the aid of, thanks to EPR, it is possible to determine the concentration of EPR at 60 and 34.5 K which are listed in Table II. Here it is important to note that previously we investigated Al doped MgB 2 by Raman and EPR spectroscopy to check the effect of  EPR results is presented here give further understanding of the competing effects between defect states at the temperatures below and above superconducting states MgB2. It is clearly visible that below critical temperature the superconducting phase become dominant and the defect centers quenched. This shows that above Tc the defect centers play crucial role for critical temperature thus critical current density Jc.

CONCLUSIONS
The results in this work showed the importance of defect structures in MgB 2 and their roles and effects on the superconductive properties. The quality of starting materials in particular B powder precursors are highly important to control the defects thus the superconductivity and size. Here we have selected three different sample systems with different grades and they all showed not only different electronic properties such as defect distributions in MgB 2 nanocrystal lattice but also significant changes in Tc and resistivity values. Nowadays to have high technological application of MgB 2 materials for instance nanowires for high energy applications it is necessary to produce well defined MgB 2 products with high quality. From this point of view present work may serve superconductive research and development engineers in a sense that intrinsic defects should be controlled for high performance and extensive functionality.

SUPPLEMENTARY MATERIAL
See supplementary material for detailed description of spin counting procedure.

ACKNOWLEDGMENTS
We thank Baris Yagcioglu and Kamil Kiraz for their support to collect SEM data.