Synthesis,Characterization And Properties Of Boron Carbon Nitride Compounds

The synthesis, characterization and properties of the new super-hard materials have been one of important subjects in material science and condense matter physics. While the traditional synthetic methods all are formulated simply by experience, the one of main characterizations of material science in the modern times is the comprehensive application including solid physics, quantum chemistry theory and computer technology which can process manual design ideal composition, structure and good properties materials in the molecular and atomic level, and then instruct the preparation of new materials. Theoretical calculation predicts that B-C-N materials may be a high-temperature semiconductor or new super-hard material, with good mechanical, electrical and optical properties possessing a full potential applicable prospect. At present, the BCN compounds with different compositions and structures were prepared by using high temperature high pressure, mechanical milling, vapor deposition and chemical methods. But most of the compounds were disorder layer graphite structure, the use of the XRD method is limited under this condition. The crystal structures of BCN are mainly the hexagonal and cubic. Up to now, the single crystal with enough large size and bulk polycrystalline are not obtained yet. Therefore the research about the electrical, optical and mechanical properties of the BCN compounds is seldom, even there are no reliable XRD pattern and many other reliable data for characterizing the BCN compounds. So there exist many problems for the researching of BCN as a new material. Searching the methods on how to synthesize the single crystal film, or single phase bulk of compound with enough size is one of the most important aspects at present. In this paper, using the graphite and h-BN as starting materials, the formation, structure and phase transition of BCN compounds were investigated by high energy mechanical milling and high temperature high pressure technology. The amorphous BCN were prepared after milling time of 120h with mol ratio 1:1 of graphite and h-BN mixtures. This amorphous BCN possess semiconductor properties with the narrow energy gap from room temperature to 523K, and behave semimetal properties ranging of 523～873K. The XPS results reveal that there are not only B-N and C-C bonds but also B-C and C-N bonds for the samples after milled 120h. The result shows that it is not physical mix with amorphous BN and amorphous graphite and the B-C-N amorphous compound have been synthesized after milled 120 hours. The chemical composition is as B0.28C0.39N0.33 detected by XPS. Because graphite and h-BN show strongly chemical inertia, General, it is difficult to synthesis BCN compound through solid-solid reaction. The result of amorphous BCN prepared by high energy mechanical milling result indicate that the fine processing of the particles of the mixtures occur between graphite and h-BN by high energy mechanical milling and transform to amorphous BN and amorphous C. When the above particles reach atomic level, the amorphous BN and amorphous C possessed more not saturation suspension bonds and higher chemical activation properties than the rough particles before milling, so the diffusion rate between atoms become increase significantly in the milling process under high pressure high temperature operation in local area, which make the reactive rate among the B, C and N atoms increasing and the formation of C-N and C-B bonds easier. The single phase hexagonal BCN was obtained firstly staring materials of this amorphous BCN annealed 45min. under 4.0GPa, 1473K with crystal lattice a=0.2505nm, c=0.6664nm. This indicate that the high pressure impel the crystallization of the amorphous BCN. The infrared spectroscopy results indicate that there exist B-N, C-N, B-C and C-C bonds in the hexagonal BCN. This reveals that the hexagonal BCN compounds have been formed among the elements of B, C and N with atomic level. There are four Raman spectra neared 1328, 1358, 1528 and 1614cm-1 in the single phase hexagonal BCN. The peaks of it are different from the Raman spectra of the graphite and h-BN. The present of strong peaks neared 1330 and 1614cm-1 are assigned to characteristic peaks of the h-BCN.Annealing 45min. under 4.5GPa and 1600K, the amorphous BCN crystallized to the hexagonal BCN I phase with crystal constant a1=0.2551nm,c1=0.6716nm and the hexagonal BCN II phase with crystal constant a2=1.2360nm,c2=0.8570nm, respectively. And the hexagonal BCN I phase is similar to the hexagonal BCN annealing 45 min. under 4.0GPa and 1473K, but the hexagonal BCN II phase haven't been reported up to now. The Raman spectra measurement was employed to analysis the new phase BCN at room temperature(298K). The whole crystallized sample possessed four peaks located at 1279,1368,1398 and 1627cm-1. And the red shift occurred with the decreasing temperature in the temperature range of 298～93K. The peak at 1627cm-1 is observed ripped to two peaks in the low temperature and the peak intensity become stronger. While the intensity of the peaks located at 1262, 1368, and 1398cm-1 become weaker then all the peaks located at 1262, 1352, 1368, 1588 and 1614cm-1peaks. Which the 1262 and 1614cm-1 are consistent with the characteristic peaks of single phase BCN so can be inferred that result from the hexagonal BCN I phase, while the other three peaks resulted from hexagonal BCN II phase. In the high temperature range, the intensity of the peak at 1279 and 1627cm-1 decreasing and vanish with increasing temperature, while the intensity of the peak at 1368 and 1398cm-1 increasing with increasing temperature. Therefore, we can infer that there occur to phase transition from the hexagonal BCN I phase to hexagonal BCN II phase. And the four Raman peaks at room temperature change to a broad peak with increasing temperature. This illuminated that with temperature increasing the thermal vibration prick up and the symmetry of the molecular structure was destroyed then the atoms located at out-of-order states. To our knowledge, the formational conditions and conductive properties of amorous B-C-N were investigated firstly with different mass ratios between Boracic acid (H3BO3) and melamine (C3N6H6) under 10-3 Pa pressure by using the cooperation of the chemical solid state reaction and thermal treatment technology. The high pressure properties were also investigated. The experimental result indicates that the amorous BCN with high temperature semiconductor properties were obtained under 10-3 Pa pressure and in the heat treatment of 1273K in the mass ratios of H3BO3 to C3N6H6 of 1:2-1:4. It was found that the conductivity of this amorphous BCN is zero below 890K, showing insulting properties, but larger than zero above 890K, behaving amorous semiconductor properties. This amorphous B-C-N preparedbehave the different relationships between conductivity and temperature in the temperature range of 913～963 and 963～1083K. The conductive activation energy is 0.26～0.34 and 1.02～1.10eV, respectively. It is inferred that the conduction of the amorphous BCN comes mainly from contribution of defect localized state electrons in the range of 913～963K and of extended state electrons in the range of 963-1083K. The experimental results reveal that the conductivity of amorphous BCN is related to the C concentration. The conductive activation energy decreases with increasing C concentration. The XPS result indicates that the C concentration in the amorous BCN was most and the conductive activation energy was lowest in the mass ratios of H3BO3 to C3N6H6 of 1:3. This amorphous BCN transforms into single phase of hexagonal BCN (h-BCN) crystal by annealing at 1473K for 45min. under 4.0GPa. In order to clarify the bond state of B-C,C-C,B-N and N-C in the h-BCN, Raman spectrum (RS) and FTIR of the h-BCN were employed. The measuring results reveal that the hexagonal BCN is formed with atomic level chemical combination. The Raman peaks at wave number of 1330 and 1617cm-1 in the Raman spectra of hexagonal BCN were consistent with the hexagonal BCN prepared by milling above. Therefore, the peaks located at 1330 and 1617cm-1 are assigned to characteristic peaks of the h-BCN. In summary, the experiment results of the amorphous BCN, single phase hexagonal BCN and the hexagonal BCN new phase were synthesized successfully and the new phase BCN with the coexist of two different hexagonal phases indicate that it is an effective technological route to prepare the crystal and amorphous BCN bulk by the cooperation of the milling and high pressure phase. The FTIR and Raman spectra of the single phase hexagonal BCN prepared by different raw materials and different methods were consistent basically. The peaks of FTIR and Raman spectra on the single phase hexagonal BCN can be considered as characteristic data which are not only applicable to analysis and characterize the single phase hexagonal BCN but the new phase BCN.