Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB₂ powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB₂ formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB₂ powder with an average grain size of 15m was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB₂ powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB₂ powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.     

Synthesis of boron carbide powder from hexagonal boron nitride

We report the synthesis of boron carbide powder via the reaction of hexagonal boron nitride with carbon black. The reaction between hexagonal boron nitride and carbon black completed at 1900 °C for 5 h in vacuum. The particle sizes of the synthesized boron carbide powder were about 100 nm from transmission electron microscopy. The possible reaction mechanism was that hexagonal boron nitride decomposed into elemental boron and nitrogen even when there was no carbon at a relatively low rate, and introduction of carbon into hexagonal boron nitride powder facilitated the decomposition process; the boron from the decomposition of boron nitride reacted with carbon to form boron carbide.     

B的计算纯度对TiB2-Cu复合材料的影响

用SHS工艺制备了TiB2-Cu复合材料，研究了B的4组计算纯度90％、92%、95％和100％对复合材料的相对密度、相组成、微观组织、弯曲强度和断裂韧性的影响．结果发现TiB2-Cu复合材料的综合性能在B计算纯度为92％时最好，而并非实际纯度90％．基于B与粉末中的杂质O反应生成B2O2而挥发掉的原理，得出产生这种结果的原因主要是：B的计算纯度越低，表明B的实际含量越高，但同时相应的杂质含量也越高，这样由于杂质的挥发而产生的孔隙也越高；一小部分Ti在高温下固溶于基体Cu中而消耗Ti，使B剩余，所以在计算配比时，B的计算纯度应大于实际纯度，但是考虑到两种原因的综合作用，应存在一个最佳的计算纯度。     

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.     

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.     

超细高能燃料无定形硼粉的自蔓延制备与表征

采用自蔓延高温合成(SHS)法,用高活性金属Mg粉还原B2O3粉,通过镁热还原反应B2O3 3Mg→2B 3MgO制备出超细高能燃料无定形硼粉,研究镁热还原工艺对还原产物的影响.采用X射线衍射仪对盐酸溶浸后的还原产物进行物相分析.结果表明:还原过程中生成的酸不溶物MgB6、BxO、Mg2SiO4、FeB49是影响硼粉纯度的主要杂质.利用费歇尔粒度测试仪(F.S.S.S)检测无定形硼粉的平均粒径和比表面积.通过优化制备工艺,在B2O3/Mg(质量分数,%)为3.0、反应体系温度为850℃的条件下,可制备出纯度高于94%、比表面平均粒径为0.5～0.7μm、比表面积达4～6m2/g的超细无定形硼粉.     

Synthesis of nanocrystalline SnO2 powder by amorphous citrate route

Nanocrystalline SnO₂ has been synthesized by liquid mix technique using citric acid as the complexing agent. The tin oxide powder obtained at different calcination temperatures (773–1223 K) is characterized using powder X-ray diffraction (XRD), SEM, TEM, TG-DTG and UV spectroscopic techniques. The material obtained is nanocrystalline, having particle size in the range of 10–14 nm. The technique is cost-effective and yields the desired product at temperatures as low as 773 K.     

Formation of willemite from powder mixture with TiO2 addition

The formation of willemite, Zn₂SiO₄, from stoichiometric oxide powder mixtures either with 3.2 mol % TiO₂ additive (ZST) or without TiO₂ addition (ZS) and from flux-added glaze powder (G) were studied in the temperature range of 650 to 1515° C. Activation energy of willemite formation were 167, 226 and 188 kJ mol^–1 for G, ZS and ZST specimens respectively. TiO₂ addition not only reduced the activation energy of willemite formation, but was also found to stabilize the metastable phase,-Zn₂SiO₄, to room temperature after cooling from melts. Annealing of the melted ZnO-SiO₂-TiO₂ composition resulted in the transformation of -Zn₂SiO₄ to -Zn₂SiO₄. It is suggested that the substitution of a Si⁺⁴-site by a Ti⁺⁴ ion contributes to the stabilization of the -phase, the larger d-spacing, and smaller c/a ratio of willemite transformed.     

Synthesis and characterization of bulk amorphous/amorphous composite alloys through powder metallurgy route

In the present study, amorphous Ni₆₀Nb₂₀Zr₂₀ and Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders were synthesized separately using a mechanical alloying (MA) technique. The dual-amorphous-phased (Ti₅₀Cu₂₈Ni₁₅Sn₇)_100−x (Ni₆₀Nb₂₀Zr₂₀) _x (x = 0, 10, 20, and 30 vol%) powders were prepared by mixing the corresponding amorphous powders. The dual-amorphous-phased powders were then consolidated into bulk amorphous/amorphous composite (BA/AC) alloy discs. The amorphization status of as-prepared powders and bulk BA/AC composite discs was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructure of the BA/AC discs showed that the Ni₆₀Nb₂₀Zr₂₀ phase is distributed homogeneously within the Ti₅₀Cu₂₈Ni₁₅Sn₇ matrix. The (Ti₅₀Cu₂₈Ni₁₅Sn₇)₇₀(Ni₆₀Nb₂₀Zr₂₀)₃₀ BA/AC disc exhibited a relative density of 96.6% and its Vickers microhardness was 726 kg/mm².     

Development and characterisation of SiAlON composites with TiB2, TiN, TiC and TiCN

30 vol% of TiB₂, TiCN, TiN or TiC was added to a sialon matrix with an X-phase sialon (Si₁₂Al₁₈O₃₉N₈) and an Al₂O₃–Si₃N₄ (77/23 wt%) starting powder composition and hot pressed at 1650°C in vacuum. The microstructures of the obtained composites were characterised by means of X-ray diffraction and electron microscopy, and the mechanical properties; E-modulus, hardness, bending strength and fracture toughness were measured and evaluated.Fully dense composites with an X-phase sialon or a polyphase Al₂O₃–-sialon–X-sialon matrix with 30 vol% of TiB₂, TiN and TiCN were obtained. TiC, added as a dispersed phase, however reacts with the nitrogen from the Si₃N₄ during liquid phase sintering, with the formation of TiC_1–x N _x , SiC and a changed sialon matrix composition. In the case of the X-phase sialon starting composition, a mullite matrix is obtained after sintering. The microstructural observations with respect to the sialon-TiC composites are found to be in agreement with the thermodynamic calculations.     

Structure of amorphous boron

The short range order structure of amorphous boron is studied. Three sample materials of different appearances, film, whisker and fibre, are prepared by chemical vapour deposition by the reduction of BCl₃ with hydrogen. The reduced radial distribution functions (reduced RDFs) derived from their X-ray or electron diffraction patterns are found to be almost identical among the three samples. The reduced RDFs are compared with those calculated for the four crystalline modifications of boron, i.e.-rhombohedral,-tetragonal,-tetragonal and-rhombohedral boron. The reduced RDFs of amorphous boron are found to be similar to those of the-tetragonal and-rhombohedral boron. From the similarity of physical and chemical properties and co-existence in deposition, the short range order structure of amorphous boron is shown to be closer to that of the-rhombohedral modification.     

Synthesis and characterization of amorphous SiO2 nanowires derived from a polymeric precursor

Bulk quantity and long-scale amorphous SiO2 nanowires have been prepared simply by direct pyrolysis of an iron-modified perhydropolysilazane (Fe-PHPS) at 1300 degrees C in the flowing Ar atmosphere. Scanning electronic microscope (SEM) study shows that most SiO2 nanowires have smooth surface and they consist of straight and smoothly curved parts, with diameters in the range of 50 to 200 nm and lengths up to 200 microm. Beside this normal form of SiO2 nanowires via the typical vapor-liquid-solid (VLS) mechanism, four other different helical forms of nanowires, named spring-shaped nanowires, twinborn nanospring, fishbone-shaped nanowires and braided-like helical SiO, nanowires, are also observed. The formation of these nanowires is proposed to be the modulated contact angle anisotropy (CAA) mechanism based on the VLS mechanism.     

TiN as a high temperature diffusion barrier for arsenic and boron

The use of TiN thin films as high temperature diffusion barrier layers for arsenic and boron was investigated. The TiN films are formed by reactive evaporation at room temperature and then annealed at a higher temperature. TiN and TiN/TiSi₂ films are placed between heavily doped polycrystalline silicon films and single-crystal silicon substrates of opposite doping polarity for secondary ion mass spectrometry analysis and electrical measurements of Schottky and ohmic contacts respectively. The results indicate that TiN is a good diffusion barrier for arsenic at 900°C. The effectiveness of this property is degraded as the temperature exceeds 900°C and it becomes ineffective at 1000°C. TiN is a better diffusion barrier for boron than for arsenic. It allows limited diffusion of boron at temperatures of up to 1000°C. The TiN/TiSi₂ composite forms good ohmic contacts when the substrates are heavily doped. The ohmic contacts can survive after annealing at temperatures of up to 1000°C. It also forms good Schottky contacts when the substrates are lightly doped. The Schottky contacts can survive after annealing at temperatures of up to 950°C in one case and of up to 1000°C in another case.     

Synthesis and characterization of boron suboxide powder produced at ambient pressure condition

Boron suboxide (B₆O)?based materials have shown good properties which make them good candidates for cutting tool and other applications where abrasive wear resistance is important. The objective of this study was to optimize the production of B₆O at ambient pressure by varying reaction temperature and time. B₆O powders were produced from the reaction between boric acid and amorphous boron powders at the reaction temperatures between 300°C and 1400°C for 6 h. The powders produced were characterized in terms of particle size, phase analysis and composition, product yield, as well as morphology. Increase in temperature increased both the yield as well as the particle size of the produced powders. X-ray diffraction (XRD) pattern obtained also showed improved crystallinity of the produced powder as the temperature increases. Scanning electron microscopy (SEM) images of the synthesized powder at higher temperature clearly showed improved crystallinity (star-like crystals) as the reaction temperature was increased. The B₆O powders synthesized at 1300°C for 6 h had the optimum yield of over 95%.     

Fabrication of TiB2 particulate reinforced magnesium matrix composites by powder metallurgy

Magnesium metal matrix composites (MMCs) reinforced with 10, 20 and 30 vol.% TiB₂ particulates, respectively, were fabricated by powder metallurgy. The microstructure, porosity, hardness and abrasive wear behavior of the composites were evaluated. Microstructural characterization of Mg MMCs showed generally uniform reinforcement distribution. As compared with pure Mg, the hardness (HB) values of Mg MMCs reinforced with 10, 20 and 30 vol.% TiB₂ particulates were increased by 41%, 106% and 181%, respectively. The abrasive wear tests showed that the wear resistance of Mg MMCs is increased with the increasing of the reinforcement volume fraction. This was due to the strong particulate-matrix bonding and high hardness of the TiB₂ particulate.     

偏压对磁控溅射制备Ni掺杂TiB2基涂层结构及力学性能的影响

采用双靶非反应磁控溅射,通过改变基底偏压,制备了一系列Ni掺杂TiB2基的涂层.通过X射线能谱仪确定其成分,利用X射线衍射、扫描电镜对涂层的结构进行分析,并通过纳米压痕、维氏压痕、划痕以及摩擦磨损分别对涂层的硬度、模量、断裂韧性、膜基结合力和摩擦学性能进行了表征.结果表明:此工艺下制备的TiB2-Ni涂层中均存在六方相的TiB2结构,并且生长结构非常致密,无明显的柱状生长结构,表面粗糙度低;硬度均大于40 GPa;涂层均具有较好的断裂韧性;且随着偏压增大断裂韧性和结合力都有所提高;同时所制备涂层摩擦系数均在0.5～0.6,磨损率在同一数量级.     

High pressure synthesis of cubic boron nitride from amorphous state

Cubic-zincblende-type boron nitride was synthesized from amorphous state at pressures higher than 6 GPa and at temperatures higher than 800°C, without any planned addition of catalysts. Also, wurtzite type boron nitride was formed although the amount was small.