Synthesis of Cubic Boron Nitride by Decomposition of Magnesium Boron Nitride

Cubic boron nitride ( c -BN) was synthesized by the decomposition of Mg₃BN₃ under high pressure and high temperature. The minimum pressure for c -BN synthesis was 4 GPa, which was 1 GPa lower than that of the conventional catalytic process. The decomposition of Mg₃BN₃ was observed only when H₂O was added. Therefore, the reaction was as follows: Mg₃BN₃+ 3H₂O = 3MgO + c -BN + 2NH₃. The c -BN crystals obtained were tetrahedron in shape and about 10 μ m in diameter.     

Preparation of Nanostructured Hexagonal Boron Nitride Powder

In this communication, we describe an inexpensive and feasible method for the preparation of hexagonal boron nitride (h–BN) nanorods in the absence of metal catalyst. Tertiary calcium phosphate (Ca₃(PO₄)₂) and ammonium biborate hydrate (NH₄HB₄O₇·3H₂O) were selected as starting materials where calcium phosphate was used as a diluting agent to prevent the formation of bulk B₂O₃ during the thermolysis of ammonium biborate hydrate. The mixture was nitrided at 900°C in the flowing ammonia and was transformed into h–BN nanorods after subsequent crystallization. After crystallization at 1650°C for 2 h, the unique microstructure of h–BN nanorods was observed.     

前驱物法低温合成六方氮化硼

本文以三聚氰胺和硼酸为原料,先采用湿化学法合成棒状前驱物,然后将其在空气气氛中高温培烧制得六方氮化硼.实验考察了反应原料配比,反应物浓度,高温培烧的时间及温度对产物的影响.采用IR、化学分析、XRD、粒度分析和SEM等方法对前驱物及产物进行了表征,确定了前驱物及产物的组成、物相、粒度分布及形貌.研究结果表明:合成前驱物的适宜原料配比是C3N6H6∶H3BO3=1∶2,浓度为0.4 mol/L,合成的前驱物是分子式为C3N6H6·2H3BO3的棒状超分子加合物;在温度950℃,空气气氛中培烧6h能得到晶型良好、平均粒径为15 μm的六方氮化硼粉体.     

Machinability of Silicon Nitride/Boron Nitride Nanocomposites

The machinability and deformation mechanism of Si₃N₄/BN nanocomposites were investigated in the present work. The fracture strength of Si₃N₄/BN microcomposites remarkably decreased with increased hexagonal graphitic boron nitride ( h -BN) content, although machinability was somewhat improved. However, the nanocomposites fabricated using the chemical method simultaneously had high fracture strength and good machinability. Hertzian contact tests were performed to clarify the deformation behavior by mechanical shock. As a result of this test, the damage of the monolithic Si₃N₄ and Si₃N₄/BN microcomposites indicated a classical Hertzian cone fracture and many large cracks, whereas the damage observed in the nanocomposites appeared to be quasi-plastic deformation.     

Shock-Induced Transformations in Hexagonal Boron Nitride by High-Velocity Thermal Spray

Shock synthesis of cubic BN ( c -BN) was accomplished using high-velocity thermal spray. Al-8Si-20BN (hexagonal BN, h -BN) composite powders were injected into a high-energy flame where the particles partially melted and accelerated to impact on steel substrates. The shock wave generated by the sudden impact of the droplets propagated through the underlying deposit, which experienced a polymorphic transition to high-pressure forms. Transmission electron microscopy revealed that the deposits contained platelike c -BN embedded in h -BN. The c -BN formed with a specific orientation relation, with (0001) h -BN parallel to (111) c -BN and [11–20] h -BN nearly parallel to [2–20] c -BN. As a result of shock propagation, a number of half-Frank loops with Burgers vector ½[0001] were introduced and thin (∼3 nm) layers of amorphous BN ( a -BN), parallel to the (0002) plane, were formed. Significant shearing of the (10–10) plane with respect to the c -axis, delamination, and kink bands were also observed in most of the h -BN grains. The Hugoniot pressure calculation suggested that the impact pressure was sufficient to trigger the nucleation of c -BN.     

Replacement of Flake Graphite in Aluminacarbon Refractories by Hexagonal Boron Nitride

Three kinds of composite alumina refractories were prepared with tabular alumina （3-1 and 1-0 mm） as aggregates,tabular alumina powder,α-Al2 O3 micropowder,and Si powder as matrix,adding 3 mass％ hexagonal boron nitride （h-BN）,3 mass％ flake graphite and 10 mass％ flake graphite,respectively.Cold physical properties,hot modulus of rupture,oxidation resistance,thermal shock resistance and slag corrosion resistance of the specimens were compared.The results show that：（1） physical properties and hot modulus of rupture of Al2 O3-h-BN refractories are slightly different from those of low carbon Al2 O3-C refractories,but better than those of traditional Al2 O3-C refractories with 10 mass％ graphite ; （2） Al2 O3-h-BN refractories have better thermal shock resistance and oxidation resistance than the carbon containing refractories,while similar slag resistance with low carbon Al2 O3-C refractories ; （3） h-BN can replace flake graphite as a starting material for the preparation of composite alumina refractories,considering the overall properties of the material.     

Chemical-Vapor-Infiltrated Silicon Nitride, Boron Nitride, and Silicon Carbide Matrix Composites

Composites of carbon/chemical-vapor-deposited (CVD) Si₃N₄, carbon/CVD BN, mullite/CVD SiC, and SiC yarn/CVD SiC were prepared to determine if there were inherent toughness in these systems. The matrices were deposited at high enough temperatures to ensure that they were crystalline, which should make them more stable at high temperatures. The fiber-matrix bonding in the C/Si₃N₄ composite appeared to be too strong; the layers of BN in the matrix of the C/BN were too weakly bonded; and the mullite/SiC composite was not as tough as the SiC/SiC composites. Only the SiC yarn/CVD SiC composite exhibited both strength and toughness.     

Electrical Resistivity and Microwave Transmission of Hexagonal Boron Nitride

The dc conductivity of hexagonal boron nitride (BN) and BN-containing composites was measured as a function of temperature up to 2400°C. The results confirm that at high temperatures BN is an intrinsic semiconductor with an energy gap of 0.99 ± 0.06 aJ (6.2 ± 0.4 eV) at T = 0 K. Extrapolated values for the resistivity of BN in the range 2600° to 3000°C are used to analyze the absorption, reflectivity, and transmissivity of a BN window when subjected to microwave radiation under atmospheric reentry conditions. It appears that the transmissivity is of the order of 1 to 10% at these temperatures due mainly to the high conductivity in a very thin, very hot surface layer. The transmissivity can be improved by using a composite made of boron nitride and silica.     

Fracture Behavior of Multilayer Silicon Nitride/Boron Nitride Ceramics

The fracture behavior of multilayer Si₃N₄/BN ceramics in bending has been studied. The materials were prepared by a process of tape casting, coating, laminating, and hot pressing. The Si₃N₄ layers were separated by thin, weak BN interlayers. Crack patterns in bending bars were examined with a scanning electron microscope. The weak layers deflected cracks in bending and thus prevented catastrophic failure. In one well-aligned multilayer ceramic A, a main crack propagated through the specimen although along a zigzag path. A second multilayer ceramic B was made to simulate a wood grain structure. Its failure was dominated by shear cracking along the weak BN layers. Besides crack deflection, interlock bridging between toothlike layers in the wake of the main crack appeared also to contribute to toughening.     

Fabrication and Microstructure of Silicon Nitride/Boron Nitride Nanocomposites

A chemical process for fabrication of Si₃N₄/BN nanocomposite was devised to improve the mechanical properties. Si₃N₄/BN nanocomposites containing 0 to 30 vol% hexagonal BN ( h -BN) were successfully fabricated by hot-pressing α-Si₃N₄ powders, on which turbostratic BN ( t -BN) with a disordered layer structure was partly coated. The t -BN coating on α-Si₃N₄ particles was prepared by reducing and heating α-Si₃N₄ particles covered with a mixture of boric acid and urea. TEM observations of this nanocomposite revealed that the nanosized hexagonal BN ( h -BN) particles were homogeneously dispersed within Si₃N₄ grains as well as at grain boundaries. As expected from the rules of composites, Young's modulus of both micro- and nanocomposites decreased with an increase in h -BN content, while the fracture strength of the nanocomposites prepared in this work was significantly improved, compared with the conventional microcomposites.     

Sintering of the Mechanochemically Activated Powders of Hexagonal Boron Nitride

Pressureless sintering of hexagonal boron nitride (BN) was performed using a powder activated by mechano-chemical treatments. Physical properties of the sintered BN bodies depend on the type of starting powder and the conditions of the treatments. The BN body, which was obtained at 2000°C using an appropriate activated powder, was 99 wt% pure and was excellent in mechanical and physical properties, in spite of its low density (1.64 g/cm³).     

化学气相沉积六方氮化硼涂层的制备及应用

六方氮化硼(h-BN)涂层是一种性能优异的功能陶瓷材料,介绍了化学气相沉积( CVD)六方氮化硼涂层的制备工艺,综述了h-BN涂层的优异性能和应用现状,并对其研究发展趋势进行了展望.认为先驱体性能存在缺陷、沉积机理复杂、工艺可控性差、生产成本高是目前CVD制备h-BN涂层存在的主要工艺问题,指出今后还需在新型先驱体的研发和使用、沉积机理的深入探究、工艺优化和放大等方面开展深入研究,以实现h-BN涂层的大规模工业化生产和应用.     

利用真空氮化炉制备六方氮化硼粉体工艺研究与表征

本研究用一种操作简单、适合规模化、低成本的方法制备六方氮化硼粉末.研究了硼、氮元素的物质的量比值,烧成制度,助熔剂的添加量对产物物相成份和颗粒形貌的影响,并对酸洗和碱洗对产物中杂质的去除效果进行了比较.用X-射线衍射仪(XRD)、扫描电镜(SEM)对制备的产物样品进行了分析表征,发现用该工艺制得的六方氮化硼纯度可达95.6%,去杂后纯度可达99%以上,晶粒直径约为0.2～0.5 μm,厚度在60～70 nm,晶粒尺寸比较均匀.     

热压烧结高纯hBN陶瓷材料及其致密性研究

本文研究了B2O3和BN粉料的热行为,分析了未处理的BN陶瓷的物相特性,采用除杂处理后的BN原料,热压烧结制备了高纯BN陶瓷材料,分析了影响陶瓷材料弯曲强度和致密性的主要因素,探讨了BN晶粒的定向性与热压工艺之间的关系.     

六方BN微颗粒表面化学镀镍

研究了以肼为还原剂、氨为配合剂的六方BN微颗粒表面化学镀纯镍反应新体系. 热力学计算表明，此镀镍体系是可行的. 实验发现，新的镀液体系比传统酒石酸盐镀液体系具有更好的热稳定性和化学稳定性. 镀层的XRD分析表明，不同体系镀层相结构相同. SEM观测和EDS分析显示，新体系镀层更为致密.     

Mechanical Properties of Three-Layered Monolithic Silicon Nitride–Fibrous Silicon Nitride/Boron Nitride Monolith

A three-layered composite, composed of a strong outer layer (monolithic S₃N₄) and a tough inner layer (fibrous Si₃N₄/BN monolith), was fabricated by hot-pressing. For the inner layer, a Si₃N₄–polymer fiber made by extrusion was coated by dipping it into a 20 wt% BN-containing slurry. The three-layered composite exhibited excellent mechanical properties, including high strength, work of fracture, and crack resistance, because of the combination of a strong outer layer and a tough inner layer. In other words, the strong outer layer withheld the applied stress, while the tough inner layer promoted crack interactions through the weak BN cell boundaries. Also, the residual thermal stress on the surface due to the anisotropy in the coefficient of thermal expansion of BN affected a median/radial crack generation after indentation.     

氮化硅低温转化合成碳化硅晶须研究

对氮化硅转化法制备碳化硅晶须的反应过程进行了热力学分析;采用氮化硅为硅源,石墨、活性炭和炭黑为碳源,氧化硼作为催化剂,利用氮化硅转化法分别在1500 ℃、1550 ℃、1600 ℃合成碳化硅晶须,通过X射线衍射和扫描电子显微镜分析合成晶须的特征.结果表明:合成反应在1450℃以上可以发生,且随着温度的升高,平衡常数急剧增加,SiC晶须直径变大;以活性炭和炭黑等较高活性的碳源代替石墨可以提高晶须的质量和生成量,通过对晶须合成过程的分析,推测晶须的生长属于螺旋位错生长机理.     

Formation of Silicon Nitride Film by Plasma Decomposition of Methylsilazane

Si₃N₄ thin films were deposited on silicon wafer or α-Al₂O₃wafer substrates from methylsilazane in a microwave nitrogen-hydrogen-mixture plasma at substrate temperatures of 590 to 1000 K. The deposits were identified by IR spectroscopy and XPS as approximately stoichiometric Si₃N₄, and they contained small amounts of impurities. Decomposition of the methylsilazane in the plasma may be one of the reasons for the lower deposition temperature of the Si₃N₄.     

Hot Isostatic Pressing of Silicon Nitride with Boron Nitride, Boron Carbide, and Carbon Additions

Si₃ N₄ test bars containing additions of BN, B₄C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room-temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si₃ N₄ sintered with Y₂O₃, Y₂O₃ and Al₂O₃, or ZrO₂. The 1370°C strengths were less than those obtained for additions of Y₂O₃ or ZrO₂ but greater than those obtained from a combination of Y₂O₃ and Al₂O₃. Scanning electron microscope fractography indicated that, as with other types of Si₃N₄, roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si₃N₄ having an amorphous grainboundary phase. The primary advantage of non-oxide additions appears to be in facilitating specimen removal from the Ta cladding.