Formation of Cubic Boron Nitride from Rhombohedral Boron Nitride by Explosive Shock Compression

When rhombohedral BN, which has a layer structure with a three-layered stacking sequence, was shock-compressed at 40, 60, and 100 GPa, it was converted to cubic BN. Hexagonal BN was converted to wurtzite-type BN under the same conditions. These results indicate that the transformation proceeds by a diffusionless mechanism in which the stacking sequence of the BN layers in the starting materials is retained during the process.     

The Effect of Stoichiometry on Mechanical Properties of Boron Carbide

The mechanical properties of chemically vapor-deposited boron carbides (B₄C) with varied B/C ratios were investigated as a function of composition. The maximum hardness, H, and fracture toughness, K_1c, were observed at an almost stoichiometric composition. For nonstoichiometric B₄C (B/C>4), H and K_1c decreased with increasing B content, suggesting that excess B diminishes the bond strength in the B₄C structure. The decrease in H and K_1C at B /C <4 was attributed to free C in the microstructure.     

Sintering and Mechanical Properties of Mullite-Reinforced Boron Carbide Matrix Composite

In order to improve the mechanical properties of boron carbide (B₄C) ceramic, a mullite-reinforced B₄C matrix ceramic with complete densification was fabricated via hot pressing for the first time. The dense sintering mechanism of mullite-reinforced B₄C ceramic was discussed through the phase and element analysis. A new dense sintering mechanism was found in which the diffusion of Si in mullite through the B₄C matrix enhances the sintering of mullite-reinforced B₄C ceramics effectively. The mechanical properties and microstructure of the composite ceramics were investigated in contrast with monolithic B₄C and one kind of commercial B₄C ceramic. The flexural strength and fractural toughness of B₄C with 3 vol% mullite addition reached 560 MPa and 3.33 MPa·m^1/2, which is 154% and 96% higher than that of monolithic B₄C, respectively.     

质量法测定铝基碳化硼材料中碳化硼的含量

采用HCl溶解质量法测定铝基碳化硼材料中碳化硼的含量。对称样量、水浴温度等实验条件进行了优化,并验证了方法的精密度和准确度。结果表明,质量法测定铝基碳化硼材料中碳化硼的含量,简单、快速,精密度高,结果准确可靠,可用于批量样品测试。     

Anisotropy of Mechanical Properties in a Hot‐Pressed Boron Carbide

Effects of microstructure and material properties on the mechanical behavior of hot‐pressed boron carbide are presented. The microstructure and intrinsic microstructural inhomogeneities have been characterized using scanning electron microscopy characterization techniques (SEM/EDS/EBSD). In situ mechanical characterizations of the boron carbide microstructure and its larger inhomogeneities have been performed by nanoindentation. Macroscopic dynamic and quasi‐static compressive responses have been studied in two characteristic orientations (parallel and perpendicular to the hot‐pressing direction) using a modified compression Kolsky bar setup (strain rates of /s) and standard MTS test machine (strain rates of /s). The microstructure characterization showed that boron carbide has a fine‐grained microstructure with a complex superposition of nonmetallic inclusions, such as free carbon, AlN, and BN. Nanoindentation tests conducted in three principal planes of the plate revealed an anisotropy of the mechanical properties. The compression tests revealed that the strength of this hot‐pressed boron carbide is orientation dependent. Detailed SEM analysis indicated transgranular fracture and microcracking originating at large carbon inclusions. Influences of microstructural anisotropy on the mechanical response of the material are discussed.     

碳化硼微粉不同检测方法对比分析

通过对F标准碳化硼微粉不同检测方法的对比分析,得出激光粒度分析仪与沉降管、电阻颗粒计数器检测结果之间的线性关系,其结果对生产具有指导意义.     

Electrical Properties of Boron Nitride Matrix Composites: I, Analysis of McLachlan Equation and Modeling of the Conductivity of Boron Nitride–Boron Carbide and Boron Nitride–Silicon Carbide Composites

The McLachlan equation, which incorporates both effective medium models and percolation, was used to predict the volume fraction–conductivity relationships of insulator–conductor composites, and results were compared with experimental data. Two composite systems were investigated (BN–B₄C and BN–SiC). Both systems are anisotropic, because of the orientation of BN platelets perpendicular to the hot-pressing direction. For BN–B₄C composites, with increasing B₄C content, the ac and dc conductivities are relatively constant to ∼40% B₄C (the critical volume fraction). At this composition, the conductivity suddenly increases to a value closer to that of B₄C and then resumes a gradual increase. Little difference is seen for measurements made perpendicular or parallel to the hot-pressing direction, i.e., perpendicular or parallel to the BN platelets. Similar results are found for the BN–SiC composites, except that the critical volume fraction is ∼20% SiC in this case. The experimental curves are in good agreement with those predicted by the McLachlan equation. The parameters s and t of the McLachlan equation relate to the morphology of the phases present in the microstructure. The critical volume fraction relates to the connectivity of the phases in the composites.     

Fe3O4纳米晶表面裹负SiO2的制备研究

纳米材料又称超微颗粒材料,是由纳米粒子组成,具有纳米尺寸效应。纳米材料用于化学反应会呈现出不同寻常的反应性能。磁性纳米材料作为一种新型的纳米材料,具有不同于常规材料的独特效应,如量子尺寸效应、表面效应、小尺寸效应、宏观量子隧道效应及顺磁效应等,这些效应使磁性纳米粒子具有不同于常规材料的光、电、声、热、磁敏感特性。近年来有关磁性纳米材料的研究备受瞩目。特别是Fe3O4纳米晶,由于其优异的磁性和表面活性及其在磁流体、微波吸收材料、水处理、催化、生物医药、生物分离等方面的应用前景,正在成为众多领域研究的热点。基于Fe3O4纳米晶的磁性纳米催化剂兼有了磁性纳米材料的所有独特性能,将其应用于催化领域,会呈现出常规催化材料所不具备的催化性能。目前液相制备Fe3O4纳米晶的液相方法主要有沉淀法、水热法、溶胶-凝胶法、微乳液法、微波超声法等。这几种方法制得的Fe3O4纳米晶有较大的差异,往往因其在不同领域的应用而采用不同的方法制备Fe3O4纳米晶。结合前人研究成果,笔者采用共沉淀法在无氮气保护的条件下制备出了粒径分布在15 nm±4 nm之间的Fe3O4纳米晶。考察了n(Fe^2+)/n(Fe^3+)、晶化时间、晶化温度、pH值对Fe3O4纳米颗粒粒径分布的影响,并在Fe3O4纳米晶表面裹负SiO2,提高了其抗氧化性能并增强其表面修饰性能,为进一步表面裹负金属活性组分制备磁性纳米催化剂打下基础。     

Pressureless Sintering of Boron Carbide

B₄C powder compacts were sintered using a graphite dilatometer in flowing He under constant heating rates. Densification started at 1800°C. The rate of densification increased rapidly in the range 1870°–2010°C, which was attributed to direct B₄C–B₄C contact between particles permitted via volatilization of B₂O₃ particle coatings. Limited particle coarsening, attributed to the presence or evolution of the oxide coatings, occurred in the range 1870°–1950°C. In the temperature range 2010°–2140°C, densification continued at a slower rate while particles simultaneously coarsened by evaporation–condensation of B₄C. Above 2140°C, rapid densification ensued, which was interpreted to be the result of the formation of a eutectic grain boundary liquid, or activated sintering facilitated by nonstoichiometric volatilization of B₄C, leaving carbon behind. Rapid heating through temperature ranges in which coarsening occurred fostered increased densities. Carbon doping (3 wt%) in the form of phenolic resin resulted in more dense sintered compacts. Carbon reacted with B₂O₃ to form B₄C and CO gas, thereby extracting the B₂O₃ coatings, permitting sintering to start at ∼1350°C.     

Effect of Amount of Boron Doping on Compression Deformation of Fine-Grained Silicon Carbide at Elevated Temperature

The effect of the amount of boron doping in the range of 0 to 1.0 wt% on the high-temperature deformation of fine-grained β-silicon carbide (SiC) was investigated by compression testing. Flow stress at the same grain size increased as the amount of boron doping decreased. The stress exponent increased from 1.3 to 3.4 as the amount of boron doping decreased. The strain rates of undoped SiC were ∼2 orders of magnitude lower than those of 1.0-wt%-boron-doped SiC of the same grain size. The apparent activation energies of SiC doped with 1.0 wt% boron and of undoped SiC were 771 ± 12 and 884 ± 80 kJ/mol, respectively. These results suggest that the actual contribution of grain-boundary diffusion to the accommodation process of grain-boundary sliding decreased as the amount of boron doping decreased. Consequently, the apparent contribution of the dislocation glide increased.     

无压烧结碳化硼的研究进展

碳化硼陶瓷具有高硬度、高熔点和低密度的特点,是优异的结构陶瓷,在民用、宇航和军事等领域都得到了重要应用.综述了无压烧结碳化硼陶瓷的国内外研究进展,阐述了不同的烧结助剂、烧结温度和颗粒尺寸等因素对碳化硼陶瓷性能的影响.     

碳化硼材料的制备技术

本文综述了国内外碳化硼粉末和碳化硼陶瓷制备技术的研究现状与进展情况,重点介绍了碳管炉、电弧炉碳热还原法、自蔓延高温合成法、激光诱导化学气相沉积法、溶胶凝胶碳热还原法合成碳化硼粉末以及热压、热等静压、无压烧结、放电等离子烧结和反应烧结制备碳化硼陶瓷的研究进展。     

碳化硼的研究进展

碳化硼是高性能陶瓷材料中的一种重要原料,包含诸多的优良性能,除了高硬度、低密度等性能外,它还具备高化学稳定性和中子吸收截面及热电性能等特性,在国防军事设备、功能陶瓷、热电元件等诸多领域具有十分广泛的应用。本文重点介绍了碳化硼的相关性质、研究进展和应用现状。详细地介绍了碳化硼的制备方法,如电弧炉碳热还原法、自蔓延高温法、化学气相沉积法、溶胶-凝胶法等方法,并分析了它们的优缺点。     

无压烧结碳化硼的研究进展

碳化硼陶瓷具有高硬度、高熔点和低密度的特点,是优异的结构陶瓷,在民用、航空和军事等领域都得到了重要应用。本文综述了无压烧结碳化硼陶瓷的国内外研究进展,阐述了不同的烧结助剂、烧结温度和颗粒尺寸等因素对碳化硼陶瓷性能的影响。     

Hot-Pressing of Silicon Carbide with 1% Boron Carbide Addition

The densification behavior of pressure sintered cubic silicon carbide containing 1 wt% boron carbide was studied as a function of temperature (1750° to 1950°C). Specimens of theoretical density were obtained at 1950° with a pressure of 3000 psi. Experimental results showed that densification proceeded by a plastic flow mechanism. Interpretation of the data in terms of Murray's equation yields an activation energy of (11S ± 18) kcal/mol. At 1850C and above, tabular grains of 6H and 2H SiC were observed in a matrix of fine grains of 3C SiC.     

反应烧结碳化硼/碳化硅凝胶注模成形工艺的研究

以碳化硼颗粒为增强相,采用凝胶注模成形工艺制备反应烧结B_4C/SiC复合材料。通过对低粘度、高固相含量碳化硼、炭黑和碳化硅浆料的制备技术以及凝胶注模成形工艺参数的研究,制备出了结构均匀、致密度高的反应烧结碳化硼/碳化硅复合陶瓷。并分析了碳化硼、炭黑和碳化硅料浆制备过程中不同碳化硅颗粒级配、碳化硼含量和碳化硼颗粒大小、球磨时间、料浆pH值、固相含量对料浆粘度的影响。     

中国卫生陶瓷行业的发展与努力方向

经过30多年的迅猛发展,中国卫生陶瓷产量已多年位居世界第一,成为名符其实的世界卫生陶瓷生产大国。笔者介绍了中国卫生陶瓷行业由最初引进国外先进技术和设备到消化吸收,科技创新,逐步形成完整的工业体系的发展历程;指出了中国卫生陶瓷目前尚存在的不足;提出了中国卫生陶瓷行业会今后努力的方向。     

Effect of Alumina on the Structure and Mechanical Properties of Spark Plasma Sintered Boron Carbide

Uniform densification of relatively thick (~7 mm) consolidated boron carbide plates at relatively low temperatures (e.g. 1800°C) and low facture toughness are two of the primary challenges for further development of boron carbide applications. This work reports that these two challenges can be overcome simultaneously by adding 5 wt% alumina as a sintering aid. Nearly fully dense (97%), fine grained boron carbide (B₄C) samples were produced using spark plasma sintering at 1700°C and above in the B₄C‐5 wt% Al₂O₃ system. The alumina and boron carbide matrix reacted to form an Al₅O₆BO₃ (a mullite‐like phase) during sintering. The Al₅O₆BO₃ phase facilitated uniform densification via liquid phase sintering. This secondary phase is dispersed throughout the intergranular pores, providing obstacles for crack propagation and resulting in tougher boron carbide ceramics.     

Rate‐Dependent Mechanical Behavior and Amorphization of Ultrafine‐Grained Boron Carbide

An investigation into mechanical properties and amorphization behavior of ultrafine‐grained (0.3 μm) boron carbide (B₄C) is conducted and compared to a baseline coarse‐grained (10 μm) boron carbide. Static and dynamic uniaxial compressive strength, and static and dynamic Vickers indentation hardness were determined, and Raman spectroscopy was then conducted on indented regions to quantify and compare the intensity of amorphization. In relation to coarse‐grained B₄C the ultrafine‐grained material exhibited, on average, a 33% higher static compressive strength, 20% higher dynamic compressive strength, 10% higher static Vickers hardness, and 23% higher dynamic Vickers hardness. In addition, there was an 18% reduction in indentation‐induced radial crack length in ultrafine‐grained B₄C, which corresponded to an increase in estimated fracture toughness. Although traditional coarse‐grained B₄C exhibits an 8.6% decrease in hardness from the static to dynamic regimes, ultrafine‐grained B₄C showed only negligible change under similar conditions, suggesting a reduced propensity for amorphization. Raman spectroscopic analysis confirmed this result by revealing significantly lower amorphization intensity in ultrafine‐B₄C compared to coarse‐grained B₄C. These results may have significant positive implications in the implementation of ultrafine‐grained boron carbide as a material for improved performance in impact and other high‐pressure applications.     

硼酐碳热法合成碳化硼的研究

用Ｘ－射线衍射分析方法，研究了温度及反应物中Ｂ２Ｏ３摩尔量对碳热合成Ｂ４Ｃ粉末中残留含量的影响，同时探讨其反应机理，实验表明，碳化硼粉末中的残留碳含量不仅取决于反应而且与Ｂ２Ｏ３的摩尔量有关。在碳管炉内合成Ｂ４Ｃ的反应以液固反应为主，在反应时间一定条件下Ｂ２Ｏ３摩尔量为５－６ｍｏｌ的反应物料，在１６５０℃Ａｒ气氛中反应，其残留碳含量最低，其相对量为１０％左右。