Role of boron carbide in carbothermic formation of hexagonal boron nitride

Formation of hexagonal boron nitride by carbothermic reduction of boric oxide under nitrogen atmosphere at 1500 °C was investigated. Experiments were performed for durations in the range of 15 min to 3 h. Reaction products were subjected to powder X-ray diffraction analysis, chemical analysis and were examined by scanning electron microscope. Formation of hexagonal boron nitride was found to be complete in 3 h with most forming in the initial 2 h. Boron carbide was found to exist in the reaction products of the experiments in which hexagonal boron nitride formation was not complete. The aim of this study was to investigate the role of boron carbide in the carbothermic production of hexagonal boron nitride. For this purpose, conversion reaction of boron carbide into hexagonal boron nitride was studied. Boron carbide used in these experiments was produced in the same conditions that hexagonal boron nitride was formed, but under argon atmosphere. It was found that formation of hexagonal boron nitride from boron carbide—boric oxide mixtures was slower than activated carbon—boric oxide mixtures. It was concluded that boron carbide is not a necessary intermediate product in the carbothermic production of hexagonal boron nitride.     

固相反应法合成碳化硼纳米粉体

以六方氮化硼和炭黑(或石墨)为原料, 采用固相反应法合成了碳化硼粉体. 碳源、反应气氛和温度对粉体合成产生重要影响. 以炭黑为碳源, 在1900℃真空下保温5 h, 得到了平均粒径约为100 nm的碳化硼纳米粉体. 与商业粉体相比, 合成的粉体具有较好的烧结活性. 在2000℃/30 MPa/1 h条件下烧结, 样品的相对密度达到97.9%(商业粉体样品为93.1%), 这可归结于合成的粉体具有细小的粒径、低的氧含量和一定程度的孪晶结构.     

Catalyst-free synthesis of carbon and boron nitride nanoflakes using RF-magnetron sputtering

Catalyst-free boron nitride (BN) and carbon (C) nanoflakes have been produced by direct radio frequency (RF)-magnetron sputtering on molybdenum and tungsten substrates at or above temperatures of 1000 °C and 800 °C, respectively. Selected-area electron diffraction (SAED) shows that the films are polycrystalline and contain disordered graphite and hexagonal BN. Transmission electron microscopy (TEM) reveals curved or twisted flakes up to several hundred nanometres in length. High resolution transmission electron microscopy (HRTEM) confirms the nanoflake structure to be turbostratic, which is intermediate between an amorphous phase and the ordered layered phases of hexagonal BN or graphite.     

Chemical states of carbon in amorphous boron carbide thin films deposited by radio frequency magnetron sputtering

Boron carbide thin films were deposited by radio frequency (RF) magnetron sputtering and characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high resolution transmission electron microscopy. The results reveal that the structure of thin films deposited at substrate temperatures lower than 350 °C is amorphous. We found that there are four chemical states for carbon in amorphous boron carbide thin films deposited by RF magnetron sputtering. One is the segregated carbon in form of the graphitic inclusions in the thin film identified by Raman spectroscopy and Raman mapping using two strong peaks at ~ 1360 cm^− 1 and ~ 1590 cm^− 1, but the XPS results show that the graphitic inclusions do not connect to the substrate directly. On the surface the carbon forms C=O bonds characterized by the peak of C1s core level at 285.0 eV besides B-C bonds in the boron carbide with the peak of C1s being at 282.8 eV. The detailed analysis of B-C bonds in the boron carbide shows that there are two states for carbon atoms in B-C bonds: in the C-B-C models with C1s peak at 282.3 eV and in the icosahedra with C1s peak at 283.3 eV.     

Low-temperature synthesis of boron carbide powder from condensed boric acid-glycerin product

Crystalline boron carbide (B₄C) powder was synthesized by the carbothermal reduction of a condensed product formed from boric acid (H₃BO₃) and glycerin (C₃H₈O₃). The condensed product was prepared by dehydration after directly mixing equimolar amounts of H₃BO₃ and glycerin, which was followed by pyrolysis in air to obtain a precursor powder from which the excess carbon had been eliminated. The prepared precursor powder had a bicontinuous boron oxide (B₂O₃)/carbon network structure. Crystalline B₄C powder without residual carbon was successfully synthesized from this precursor powder by heating at 1250 °C for 5 h in an Ar flow.     

Synthesis of boron carbide powder from polyvinyl borate precursor

Polyvinyl borate (PVBO) was prepared by the condensation of poly(vinyl alcohol) (PVA) and boric acid, and used as a precursor for boron carbide. Boron carbide powder was synthesized by the pyrolysis of the PVBO precursor in air at 600 °C for 2 h, followed by heat treatment in Ar flow at 1300 °C for 5 h, which is a relatively low temperature compared with conventional carbothermal methods. Pyrolysis of the PVBO precursor resulted in submicron-size particles of B₂O₃ dispersed in a carbon matrix. In addition, the pyrolysis temperature governed the carbon content in the pyrolyzed product of the PVBO precursor, which led to the synthesis of crystalline boron carbide powder with little free carbon.     

One simple synthesis route to whisker-like nanocrystalline boron nitride by the reaction of NaBH4 and NaN3

Nanocrystalline boron nitride (BN) was synthesized via a simple route by the reaction of sodium borohydride with sodium azide in an autoclave at 600 °C. X-ray powder diffraction pattern indicated that the product was hexagonal BN, and the cell constant was a = 2.495 Å, c = 6.687 Å. Transmission electron microscopy image showed that it consisted of whisker-like particles with an average size of 200 nm × 20 nm. The product was also studied by FT-IR, XPS and TGA. It has good thermal stability and oxidation resistance in high temperature.     

New synthetic route for nanocrystalline boron nitride powder

Nanocrystalline hexagonal boron nitride powder (h-BN) was synthesized by sol-gel polycondensation of resorcinol and formaldehyde in the presence of boric acid followed by freeze drying. Pyrolysis and subsequent heat treatment of these cryogels resulted in formation of boron nitride powder. Characterization by nitrogen adsorption showed that precomposite cryogels and the BN powders were micro and mesoporous materials with high surface areas. Materials have been analyzed by means of X-ray diffraction, Raman scattering and electron microscopy methods.     

Preparation of carbon nanotube by rotary CVD on Ni nano-particle precipitated cBN using nickelocene as a precursor

Ni nano-particles were deposited on hexagonal boron nitride (hBN) and cubic boron nitride (cBN) powders by rotary chemical vapor deposition (RCVD) using nickelocene as a precursor. Ni nano-particles precipitated on hBN and cBN powders were about 20 nm and 10 to 50 nm in diameter, respectively. Carbon nanotubes (CNTs) were grown from relatively large Ni particles about 50 nm in diameter on cBN powder, whereas carbon layers surrounded Ni nano-particles on the surface of hBN powders.     

Variation of residual stress in cubic boron nitride film caused by hydrogen addition during unbalanced magnetron sputtering

The effect of hydrogen on compressive residual stress of cubic boron nitride (cBN) was investigated. The deposition was performed by unbalanced magnetron sputtering of a hexagonal boron nitride (hBN) target connected to radio-frequency electric power of 400 W. Up to 2 sccm of hydrogen was added to a gas mixture of argon and nitrogen flowing at 9 and 1 sccm, respectively. The compressive stress rapidly decreased from 10.5 GPa to 3 GPa, with increasing hydrogen flow up to 1.0 sccm. The cBN fraction in these films, however, remained over 60%, with only a trivial decrease with increasing hydrogen. This reduction was discussed in terms of the relation between the penetration probabilities of hydrogen and argon ions into the film, which was main origin of compressive residual stress of the hBN layer.     

Effect of the radio-frequency power on the dielectric properties of hydrogen-containing boron carbon nitride films deposited by plasma-assisted chemical vapor deposition using tris(dimethylamino)boron gas

We investigated the properties of boron carbon nitride film containing hydrogen (BCNH film) deposited using tris(dimethylamino)boron as the source gas. The dielectric constant (k) of BCNH film decreases with decreasing radio-frequency plasma power used for deposition, and can be as low as 1.8 at 10 W. Thermal desorption spectroscopy analysis shows that the film contains a large amount of hydrogen. Fourier transform infrared spectroscopy shows an absorption band at 2960 cm−1, attributed to the asymmetrical stretching mode of C-H in the methyl group. It is thought that increasing the number of C-H bonds, which have a low polarizability, can achieve a lower k value.     

Electroextraction of boron from boron carbide scrap

Studies were carried out to extract elemental boron from boron carbide scrap. The physicochemical nature of boron obtained through this process was examined by characterizing its chemical purity, specific surface area, size distribution of particles and X-ray crystallite size. The microstructural characteristics of the extracted boron powder were analyzed by using scanning electron microscopy and transmission electron microscopy. Raman spectroscopic examination of boron powder was also carried out to determine its crystalline form. Oxygen and carbon were found to be the major impurities in boron. Boron powder of purity ~ 92 wt. % could be produced by the electroextraction process developed in this study. Optimized method could be used for the recovery of enriched boron (¹⁰B > 20 at. %) from boron carbide scrap generated during the production of boron carbide.     

Graphite and boron carbide composites made by hot-pressing

Composites consisting of graphite and boron carbide were made by hot-pressing mixed powders of coke carbon and boron carbide. The change of relative density, mechanical strength and electrical resistivity of the composites and the X-ray parameters of coke carbon were investigated with increase of boron carbide content and hot-pressing temperature. From these experiments, it was found that boron carbide powder has a remarkable effect on sintering and graphitization of coke carbon powder above the hot-pressing temperature of 2000° C. At 2200° C, electrical resistivity of the composite and d(002) spacing of coke carbon once showed minimum values at about 5 to 10 wt% boron carbide and then increased. The strength of the composite increased with increase of boron carbide content. It was considered that some boron from boron carbide began to diffuse substitutionally into the graphite structure above 2000° C and densification and graphitization were promoted with the diffusion of boron. Improvements could be made to the mechanical strength, density, oxidation resistance and manufacturing methods by comparing with the properties and processes of conventional graphites.     

Synthesis of multiwall boron nitride nanotubes dependent on crystallographic structure of boron

Synthesis and growth of multiwall boron nitride nanotubes (BNNTs) under the B and ZrO₂ seed system in the milling–annealing process were investigated. BNNTs were synthesized by annealing a mechanically activated boron powder under nitrogen environment. We explored the aspects of the mechanical activation energy transferred to milled crystalline boron powder producing structural disorder and borothermal reaction of the ZrO₂ seed particles on the synthesis of BNNTs during annealing. Under these circumstances, the chemical reaction of amorphous boron coated on the seed nanoparticles with nitrogen synthesizing amorphous BN could be enhanced. It was found that amorphous BN was crystallized to the layer structure and then grown to multiwall BNNTs during annealing. Especially, bamboo-type multiwall BNNTs were mostly produced and grown to the tail-side of the nanotube not to the round head-side. Open gaps with ∼0.3 nm of the bamboo side walls of BNNTs were also observed. Based on these understandings, it might be possible to produce bamboo-type multiwall BNNTs by optimization of the structure and shape of boron coat on the seed nanoparticles.     

Microwave synthesis of Fe-doped β-rhombohedral boron

Iron-doped β-rhombohedral boron was synthesized by 28 GHz microwave irradiation on a powder mixture of iron and β-boron. β-Boron strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to promote a reaction with iron dopant. The powder mixture was heated to 1800°C within 2 min by microwave irradiation, resulting in the formation of β-rhombohedral boron interstitially doped with iron. The reaction proceeded rapidly without accompanying grain growth. The XRD analysis and the electrical conductivity measurements revealed successful incorporation of iron into two doping sites of β-boron.     

Atomic-scale study of the role of carbon on boron clustering

Boron (BF₂, 20 keV, 3.14/cm²) and carbon (13 keV, 10¹⁵/cm²) implanted silicon annealed at 800 °C during 30 min or at 1000 °C during 10 s has been investigated using a laser-assisted wide-angle tomographic atom probe (LaWaTAP) instrument. Boron-silicon clusters containing ~ 1.3 at.% of boron atoms have been observed in boron implanted silicon with a concentration exceeding the solubility limit. Often identified as BICs, they are interpreted as a metastable phase. Furthermore, addition of carbon clearly reduced the clustering of boron. This was interpreted as a diminution of boron diffusion or as an increase of the solubility limit of boron. Carbon-silicon clusters containing ~ 1.5 at.% of carbon atoms were observed, maybe the precursors of the SiC phase.     

Convenient synthesis of Fe-filled boron nitride nanotubes by SHS method

Fe-filled boron nitride (BN) nanotubes with high purity and good yield were conveniently synthesized by a novel ball-milling and self-propagation high-temperature synthesis (SHS) method at a low temperature (700 °C). The as-prepared product was characterized by XRD, FTIR, SEM, TEM and HRTEM. The results of XRD, FTIR and HRTEM reflect that the product is a hexagonal BN nanotube filled with Fe. The results of SEM and TEM reveal that the Fe-filled BN nanotubes have a diameter of 20-150 nm with the wall-thickness of about 20 nm and the length of more than 5 μm. The possible growth mechanism was also discussed.     

Structural evolution of boron nitride films grown on diamond buffer-layers

Boron nitride films on diamond buffer layers of varying grain size, surface roughness and crystallinity are deposited by the reaction of B₂H₆ and NH₃ in a mixture of H₂ and Ar via microwave plasma-assisted chemical vapor deposition. Various forms of boron nitride, including amorphous α-BN, hexagonal h-BN, turbostratic t-BN, rhombohedral r-BN, explosion E-BN, wurzitic w-BN and cubic c-BN, are detected in the BN films grown on different diamond buffer layers at varying distances from the interface of diamond and BN layers. The c-BN content in the BN films is inversely proportional to the surface roughness of the diamond buffer layers. Cubic boron nitride can directly grow on smooth nanocrystalline diamond films, while precursor layers consisting of various sp²-bonded BN phases are formed prior to the growth of c-BN film on rough microcrystalline diamond films.     

The effect of copper on the crystallization of hexagonal boron nitride

The crystallization process of hexagonal boron nitride in the presence of copper has been investigated. The positive effect of copper on the crystallinity of boron nitride was observed in the three studied systems of: nitrided boron, nitrided boron–carbon, and previously prepared turbostratic boron nitride. However, the presence of copper hindered the formation of boron carbonitride and produced graphite and boron nitride phases separately. Poor crystallinity was found as a conditio sine qua non for the existence of such a compound. Well-crystallized boron nitride had a very low spacing parameter 0.3328 nm and a regular hexagonal shape.     

Spark plasma sintering of Co-WC cubic boron nitride composites

25 vol.% cubic boron nitride (cBN) added tungsten carbide (WC) powders containing 6 wt.% Co (WC-6Co) were densified by spark plasma sintering (SPS) technique under different experimental conditions and the effect of cBN addition on the microstructure, mechanical properties and thermal conductivity were investigated. Over 99.5% theoretical density was achieved for WC-6Co-cBN composites sintered at 1300 °C, under 75 MPa pressure for 7.5 min. Under these conditions, cBN → hBN phase transformation was not observed.