Synthesis of cubic boron nitride with boron nitride powder formed from triammoniadecaborane

Cubic BN was synthesized under high temperature and pressure conditions from BN powder formed by reaction of triammoniadecaborane (TAD) with ammonia. The BN powder formed from TAD and ammonia had a low degree of ordering. The crystal lattice of the BN powder increased in regularity with increasing synthesis temperature and time for the reaction of TAD with ammonia. The conversion yield of cubic BN at 1300 °C and 6.5 GPa in the presence of AIN increased with decreasing of reaction temperature of TAD and ammonia from 1000–700 °C. Cubic BN decreased in yield with increasing reaction time of TAD and ammonia at 800 °C. BN powder pre-heat treated at 1550 °C had a crystallite size,L _c, of 22 nm, and was converted to cubic BN in a 43% yield at 1300 °C and 6.5 GPa for 10 min. The activation energy for cubic BN synthesis from BN powder-20 mol% AIN was 97 kJ mol^–1, when the starting BN was synthesized at 800 °C. The conversion yield of cubic BN from the disordered BN-20 mol% AIN was 100% after heat treatment at 1300 °C and 6.5 GPa for 20 min.     

Shock compressibility and spallation strength of cubic modification of polycrystalline boron nitride

Methods of shock-wave physics are used to perform experimental investigations of the elastoplastic and strength properties of cubic modification of polycrystalline boron nitride. The samples are prepared by pressing to a pressure of 7–8 GPa at a temperature of 1700–1800 °C and rank just a little below diamond in hardness. The measurements are performed under conditions of the samples being acted upon by plane shock waves with the pressure behind the front of up to 85 GPa and duration of ～10^?6 s. It is demonstrated that a two-wave configuration consisting of an elastic precursor and a plastic compression wave is formed at the amplitude of shock compression of over 60 GPa. The dynamic yield strength, determined by the amplitude of elastic precursor, depends on the structure of samples and varies in the range from 31 to 49 GPa. The dynamic strength, measured under conditions of pulsed tension in expansion waves, is likewise defined by the structure of samples and, in the elastic region of deformation, lies in the range from 0.7 to 1.6 GPa.     

Study of polycrystalline sintered compacts of diamond

Study of polycrystalline sintered compacts of diamond has been made using powder X-ray diffraction, scanning electron microscopy (SEM) and energy- dispersive X-ray analysis (EDX). Various crystalline phases formed at high temperature and high pressure, microstructure, particle-size distribution of diamond and binder and concentration of different elements in the sintered diamond compacts were determined.     

Characterization of wurtzitic boron nitride compacts

Characterization of sintered polycrystalline wurtzitic boron nitride compacts was carried out regarding the different crystalline phases that are formed at high temperature and high pressure, composition, particle size distribution of BN and binder and hardness. Wurtzitic boron nitride, cubic boron nitride, TiC/TiN solid solution, TiB and TiB₂ were the crystalline phases that have been observed in sintered wurtzitic boron nitride compacts. The particle size distribution of BN and the binder was found to be comparable (1 to 5m), with about 80% of the particles lying between 2 and 3m. Weight percentages of different elements present in these compacts were determined. The average Knoop hardness values under 500 g load were measured, and the variation of hardness as a function of position on the specimen surface was studied.     

Synthesis and characterization of boron nitride nanoropes

Nanocrystalline hexagonal boron nitride (h-BN) nanoropes with diameters of 60–150 nm and with lengths up to several micrometers have been successfully synthesized by reacting KBH₄ and NH₄Cl using CoCl₂ as the catalyst. The products were characterized by X-ray powder diffraction (XRD), Fourier transformation infrared spectroscopy (FTIR), energy dispersive spectrum (EDS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cobalt chloride (CoCl₂) played a key role in the formation of the rope-like nanostructure.     

Effect of notch root radius on fracture toughness of polycrystalline cubic boron nitride

The fracture toughness of five grades of polycrystalline cubic boron nitride (PCBN) has been determined using Single Edge V-Notched Beam specimens. Both coarse and fine grade materials were considered, containing CBN grain sizes of between 1 μm and 22 μm. The influence of notch root radius on the measured fracture toughness was examined. The notch root radius was found to have a major effect for materials with smaller CBN grain sizes while only a small effect was noted for the material with large CBN grain sizes. A simple analytical model was developed to explain the effect of the notch root radius on the fracture toughness and was found to agree well with experiment for all the materials tested. It was shown that the effect of notch root radius is directly linked to the size of the CBN grain. It is proposed that this effect results from the interaction between the microstructure and the stress field around the notch tip.     

High-pressure sintering of cubic boron nitride

Cubic boron nitride (cBN) powder was compacted at high pressure and high temperature using a solid solution of titanium carbide and titanium nitride (TiC _x N_1–x ) as binding material in the presence of a small amount of aluminium. Different compositions of (TiC _x N_1–x ), 0<x<1 were used as binders. The weight percentage of cBN, TiC _x N_1–x and aluminium were optimized and found to be critical; any marked deviation from these optimized values deteriorated the quality of compacts. Various high-pressure sintering parameters such as pressure, temperature and sintering time, etc, were optimized for this binder. The compacts were also characterized using X-ray diffraction, scanning electron microscopy and microhardness measurements. The microstructural and X-ray diffraction observations indicated no marked changes in the compacts as the value ofx in TiC _x N_1–x was varied, but the microhardness was found to depend on the value ofx.     

Shock compaction of cubic boron nitride powders

C-BN powders with different grain sizes were dynamically compacted by explosive shock loading using approximate peak pressures from 33 to 77G Pa. The density and the microhardness of the resulting c-BN compacts were strongly dependent upon the grain size of the c-BN powders used as the starting materials. The best c-BN compacts, with 98% of the theoretical density and microhardness of 51.3G Pa, were obtained from the coarse c-BN powder (40 to 60m). In the compacted fine c-BN powder (2 to 4m) conversion of the c-BN to low density forms of BN at a residual temperature degraded the interparticle bonding significantly. X-ray line-broadening analysis of the compacted c-BN powders indicated that the residual lattice strain increased with the increase in grain size of the starting powder, while the crystallite size was independent of the grain size.     

Special features of the applications of cutting tools from polycrystalline cubic boron nitride with protective coatings

The results of the investigations on the efficiency of cutting tools from polycrystalline superhard materials based on cubic boron nitride with a protective coating. A hypothesis is proposed for the increase of the cutting tool life in turning hardened steel due to a decrease of the temperature in the cutting zone and a complex of physico-mechanical properties of the protective coating required for this is analyzed. A protective coating of boron nitride is considered in the amorphous state that plays a role of a solid lubricant in the zone of the tool contact with cutting chips, decreases the temperature in the contact zone through the reduction of the contact length and cutting force, ensures an increase of the tool life and reliability, especially at the stage of the run-in.     

Synthesis of cubic boron nitride from rhombohedral form under high static pressure

The cubic, zincblende-type boron nitride (z-BN) has been synthesized from the rhombohedral form (r-BN) under high static pressures greater than 6 GPa without any planned addition of catalysts. The process of forming z-BN has been delineated from isobaric and isothermal series of data. At 6GPa, r-BN begins conversion to the graphite-type form (g-BN) upon heating to 600 °C. This conversion terminates at 1200 °C forming single-phase g-BN, which in turn transforms into z-BN at temperatures higher than 1300 °C. The appearance of z-BN occurs at lower temperatures when the pressure is raised to 7 or 8 GPa. At pressures beyond 10 GPa the wurtzite-type form (w-BN) is observed between 400 and 1200 °C, whereas z-BN is formed above 1000 °C. The boundary of pressure-temperature conditions for synthesizing z-BN from r-BN runs through 6GPa and 1300 °C, and is located near to the lowest bound hitherto known for non-catalytic z-BN synthesis from g-BN.     

Intrinsic metallic and semiconducting cubic boron nitride nanofilms

We show by density functional theory calculations with both hybrid and semilocal functionals that cubic boron nitride (111) nanofilms are intrinsically metallic and even turn into semiconductors once the thickness is less than 0.69 nm, which is in sharp contrast to the known insulating nature of boron nitride materials. The exceptional metallic or semiconducting band gap is due to a combined effect of thickness-dependent inbuilt electric polarization and labile near-gap states unique in the polar nanofilms. The band gap and dipole moment of the nanofilms can be further significantly tuned by applying an in-plane strain. These distinguished features of the boron nitride nanofilms are robust to surface passivation and can be enhanced by hybridizing with diamond films, thereby opening an exciting prospect of using the versatile cubic nanofilms in future electronic and piezoelectric devices.     

Synthesis of boron nitride

The authors examine the boric oxide—ammonia route with special stress on the yield and composition of the intermediate addition compound (BN) _x (B₂O₃) _y (NH₃) _z . It has been concluded that B₂O₃ and NH₃ present in the addition compound formed between 350°C and 900°C cannot be further reacted to convert the B₂O₃ into BN and the BN yield remains at around 66%. A formula (BN)_12·7(B₂O₃)_7·5NH₃ has been suggested for the addition compound.     

立方氮化硼薄膜表面的XPS研究

研究立方氮化硼薄膜表面的性质对于研究立方氮化硼薄膜的成核机理和应用,具有重要的价值.本文用XPS对立方氮化硼薄膜表面进行研究,并对有关问题进行了讨论.XPS分析表明,立方氮化硼薄膜表面除了B、N外,还含有C和O.从XPS谱图计算得到含有立方相的氮化硼薄膜的N/B为0.90,较接近于氮化硼的理想化学配比11;不含立方相的氮化硼薄膜的N/B为0.86,离氮化硼的理想化学配比11较远.计算表明立方氮化硼薄膜的顶层六角相的厚度约为0.8nm.     

Synthesis of boron nitride nanotubes employing mechanothermal process and its characterization

Multi-walled boron nitride (BN) nanotubes having cylindrical structure were synthesized employing the mechanothermal process. In this process hexagonal boron nitride powder (hBN) was first ball milled for 50–100 h using a high-energy ball mill and the ball-milled samples were annealed in N₂ atmosphere for about 10 h in the temperature range of 950–1300 °C. The BN nanotubes exhibited a well-crystallized hexagonal structure with about 25–40 nm in diameter and up to 1 μm length. These BN nanotubes were well characterized using various techniques, such as, XRD, SEM, TEM and Raman Spectroscopy.     

Infrared optical constants of polycrystalline boron nitride: comment

In contrast to statements in a recent article by M. Khelkhal and F. Herlemont [Appl. Opt. 32, 57-59 (1993)], it is pointed out that well-documented IR data exist for boron nitride in both the cubic and hexagonal phases. Furthermore, the existence of a strong reststrahlen band causes a significant dispersion over the wavelength region of 1-10 μm.     

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.     

Large scale synthesis of single-crystal and polycrystalline boron nitride nanosheets

Boron nitride nanosheets (BNNSs) have an identical crystal structure and similar lattice parameter to those of graphene sheets. However, growing quality BNNSs consisting of only several atomic layers remains a challenge. Here, we report on the synthesis of BNNSs at a temperature of 350 °C using a CO₂ pulsed laser plasma deposition (CO₂-PLD) technique by irradiating a pyrolytic hexagonal boron nitride (h-BN) target. The deposition was performed either in vacuum at a pressure of 0.2 Pa, for which we obtained polycrystalline BN, or in hydrogen (H₂) atmosphere at a pressure of 26 Pa for which we obtained single-crystal BNNSs. The presence of H₂ seems to minimize the side effects of sputtering and the material shows higher purity and better crystallinity. High resolution transmission electron microscopy (HRTEM) showed the sheets to be mostly defect-free and to have the characteristic honeycomb structure of six-membered B₃-N₃ hexagon. HRTEM, electron diffraction, X-ray diffraction, Raman scattering, and Fourier transform infrared spectroscopy clearly identified h-BN.