C3N4 as a precursor for the synthesis of NbC, TaC and WC nanoparticles

While there already exit some routes to prepare carbides, highly efficient and facile routes are still desired to meet the increasing demand on carbides. By a facile solid-state reaction process using graphite-like phase of C₃N₄ (g-C₃N₄) as the carbonizing reagent, we synthesized three technologically important carbides including cubic NbC and TaC, and hexagonal WC nanoparticles at relatively low temperature (1150 °C). The products were characterized by power X-ray diffraction (XRD), field-emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The results show that g-C₃N₄ is a highly efficient carbonizing reagent and the oxides Nb₂O₅, Ta₂O₅ and WO₃ are completely converted into the corresponding carbides at 1150 °C, which is significantly lower than that reported for the commercial preparation of the carbides, typically >1600 °C. The NbC, TaC and WC nanoparticles are found to have an average particle size of 4, 35 and 60 nm, respectively. An important feature of this solid-state reaction process is that g-C₃N₄ plays double roles as both efficiently reducing and carbonizing reagent.     

Mechanochemical and volume combustion synthesis of ZrB2

Formation of ZrB₂ by volume combustion synthesis (VCS) and mechanochemical process (MCP) from ZrO₂-Mg-B₂O₃ was studied. Production of ZrB₂ by VCS in air occurred with the formation of side products, Zr₂ON₂ and Mg₃B₂O₆ in addition to MgO and ZrB₂. Zr₂ON₂ formation was prevented by conducting VCS experiments under argon. Wet ball milling was applied to the VCS products before leaching for easier removal of Mg₃B₂O₆ phase. MgO and Mg₃B₂O₆ were removed from wet ball-milled products by leaching in 5 M HCl for 2.5 h. In MCP, 30-hour ball milling was found to be sufficient for the formation of ZrB₂ with no minor phase formation. Leaching of MCP products in 1 M HCl for 30 min was sufficient to remove MgO. Complete conversion of ZrO₂ to ZrB₂ did not take place in both production methods, even with excess amounts of Mg and B₂O₃. Therefore, formed ZrB₂ contained residual ZrO₂.     

Mechanochemical synthesis of zirconia nanoparticles: Formation mechanism and phase transformation

Studies on the synthesis and formation mechanism of zirconia nanopowder prepared by mechanochemical technique have been carried out by means of X-ray powder diffraction (XRPD), differential thermal and thermogravimetric analysis (DSC-TG), Fourier transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM). A dry powder mixture of anhydrous ZrCl₄ and CaO was milled in stoichiometric ratio to produce ZrO₂. The milling resulted in the formation of crystalline CaO and partially amorphous ZrCl₄ while there was no sign of chemical reactions during milling. Heating the as-milled powder resulted in the formation of amorphous zirconium hydroxide (ZrOH)₄) and calcium chloride (CaCl₂(2H₂O)). According to our results, ZrCl₄ hydrolyses during the heating stage while CaO adsorbs the produced HCl forming CaClOH and CaCl₂ subsequently. Heat treatment of Zr(OH)₄ resulted in the production of zirconia (ZrO₂). Based on Rietveld refinement it has been shown that an average critical crystallite size around 46 nm exists above which tetragonal zirconia (t-ZrO₂) transforms to monoclinic zirconia (m-ZrO₂).     

钽铌电子材料新进展

通过降低杂质含量、改善物理性能等新技术以及Ta2O5钠还原制取高比电容钽粉、TaCl5低温钠还原制取纳米级钽粉的新工艺方法,生产了性能优良的高比容钽粉,并研究了其微观结构;同时开发了制造高比电容铌粉、一氧化铌粉的新技术,制得了高性能的电容器级铌粉和一氧化铌粉,为铌电容器作为一种新类型电容器产业参与竞争提供了优质的基础材料.     

Phase evolution of tantalum nitride and tantalum carbide films with PBII parameters

Tantalum nitride and tantalum carbide films were fabricated using magnetron sputtering of tantalum followed by nitrogen and carbon plasma-based ion implantation (N-PBII and C-PBII). The phase evolution and morphology of the films were studied using glancing angle X-ray diffraction (GXRD) and transmission electron microscopy (TEM). It was found that the main phase in the tantalum nitride films was crystalline TaN_0.1 whose grain size increases with increasing implantation voltage and phase content increases with increasing implantation dose. In the tantalum carbide film, the main phase was Ta₂C. TaC phase also appeared as the implantation dose increased. XRD results from various glancing angles show that the phases with high nitrogen or carbon content, Ta₄N₅ and TaC, are present in the surface of the films. X-ray photoelectron spectra (XPS) from the tantalum carbide film reveal that the surface carbon content is higher than that of the inner film.     

Oxidation of hafnium carbide and hafnium carbide with additions of tantalum and praseodymium

The oxidation behavior of HfC, HfC-25 wt. % TaC, and HfC-7 wt.% PrC₂ has been studied between 1200–2200° C. Parabolic growth of the oxide layer has been observed for both HfC and HfC-TaC over the entire temperature range. A break in the temperature dependence of the oxidation kinetics occurs around 1600°C. At lower temperatures, the kinetics are limited by gaseous diffusion via pores in the oxide. Above 1800°C, gaseous diffusion through pores becomes less important as scale-growth kinetics are dominated by bulk (ambipolar) diffusion of oxygen and electrons through the oxide.     

难熔金属碳化物Cr_3C_2、VC、WC、TaC、NbC及Ti(C、N)粉中硫的测定

研究了燃烧──—电导法测定金属陶瓷添加剂Cr3C2；VC；WC；TaC；NbC及Ti（C、N）粉中硫的测定条件，建立了分析方法．方法相对标准偏差5％左右，仪器的灵敏度为3X10－6，最低检测限为1ppm．     

Oxidation rates of niobium and tantalum alloys at low pressures

Niobium and tantalum alloys have excellent properties for use in high-temperature, space-power applications, but must be protected from oxidation that would result from exposure to air in ground-evaluation tests. The oxygenuptake/oxidation rates of three alloys, Nb-1Zr, PWC-11, and ASTAR-811C were measured at oxygen partial pressure of 10^–6 and 10^–7 torr at temperatures up to 1350 K. No visible oxide film was observed, and the oxidation rate was found to be linearly proportional to pressure and exponentially proportional temperature. A thin molybdenum coating on Nb–1Zr was a barrier to lowpressure oxidation at 773 K.     

Layered niobium carbide enabling excellent kinetics and cycling stability of Li-Mg-B-H hydrogen storage material Layered niobium carbide enabling excellent kinetics

The Li-Mg-B-H composite(2LiBH₄+MgH₂)has a high capacity of 11.4 wt% as a hydrogen storage material.However,the slow kinetics and poor cycling stability severely restrict its practical applications.In this work,a layered Nb₂C MXene was first synthesized and then introduced to tailor the kinetics and cycling stability of the Li-Mg-B-H composite.The milled 2LiH+MgB₂ composites were initially hydrogenated to obtain the 2LiBH₄+MgH₂ com...     

Preparation of niobium borides NbB and NbB2 by self-propagating combustion synthesis

Preparation of niobium borides NbB and NbB₂ was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of the sample green density, preheating temperature and starting stoichiometry on combustion characteristics, as well as on the composition of final products were studied. Experimental evidence indicates the self-sustained reaction zone propagating along a spiral trajectory for the reactant compacts without prior heating or preheated at 100 °C. The increase of initial sample temperature to 200 and 300 °C by prior heating brings about a planar flame-front propagating in a steady mode. As the preheating temperature or sample green density increased, the combustion temperature was found to increase and the propagation rate of combustion wave was correspondingly enhanced. According to the temperature dependence of combustion wave velocity, the activation energies associated with the Nb + B and Nb + 2B reactions were determined to be 151.8 and 132.4 kJ/mol, respectively. As indicated by the XRD analysis, the final composition of burned products was essentially governed by the starting stoichiometry of reactant compacts. Synthesized products composed of a single boride phase NbB and a small amount of unreacted Nb were obtained from the reactant compacts of Nb:B = 1:1. In addition, the SHS reaction of powder compacts with an initial composition Nb:B = 1:2 yielded niobium diboride NbB₂ as the dominant phase, along with another boride phase Nb₃B₄ in a minor quantity.     

含TaC和VC抑制剂的WC-10%Co纳米粉末的烧结性能(英文)

采用高能球磨法,在球料比为15时合成了WC-10%Co（质量分数）纳米复合材料。为抑制烧结过程中WC颗粒的长大,在粉末中添加TaC和VC作抑制剂。结果表明:由于抑制剂有效减小了WC晶粒尺寸,添加TaC能增加硬质合金的韧性,添加VC增加其硬度;对同时添加0.6%TaC和0.7%VC对材料的物理、力学性能的影响也进行研究。结果表明:WC-10%Co-0.6%TaC-0.7%VC 烧结样品的硬度达到了HV₃01787,比没有抑制剂的样品高24%;断裂韧性达到8.7MPa·m1/2,比没有抑制剂的样品高35%;0.6%TaC和0.7%VC共同抑制了晶粒的长大,烧结后晶粒尺寸为3.8μm。     

Direct solid-state synthesis of tungsten carbide nanoparticles from mechanically activated tungsten oxide and graphite

Solid-state carbothermic reduction of tungsten oxide (WO₃) to nano-sized tungsten carbide (WC) particles was achieved by calcining mechanically activated mixtures of WO₃ and graphite at 1215 °C under vacuum condition. By experiments and thermodynamic calculations, the intermediate phases, WO_2.72, WO₂ and metallic tungsten (W), were observed at 741 °C, which decomposed to synthesize the final product (WC). Homogeneity increase and associated decrease in the diffusion path by mechanical milling and formation of these intermediates are mainly responsible for the successful production of WC. The process indicates that solid-state synthesis of WC nanoparticles directly from as-milled mixtures of tungsten oxide and graphite powder is possible.     

Effects of additives on synthesis of vanadium carbide (V8C7) nanopowders by thermal processing of the precursor

Vanadium carbide (V₈C₇) nanopowders can be synthesized by thermal processing of the precursor of ammonium vanadate (NH₄VO₃) and nanometer carbon black. Effects of additives (CaF₂, CeCl₃·7H₂O and LaCl₃·7H₂O) on the phase composition and microstructure of the synthesized powders were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that additives (CaF₂, CeCl₃·7H₂O and LaCl₃·7H₂O) can accelerate the solid state reaction during synthesis of V₈C₇, and these additives play a vital role in determining the phase composition and microstructure. V₈C₇ powders with basically single phase can be synthesized at 1100 °C for 0.5 h with the addition of additives (CaF₂, CeCl₃·7H₂O and LaCl₃·7H₂O), and the powders show good dispersion and are mainly composed of uniformly-sized spherical particles with a mean diameter of 50 nm. Further experiment shows that V₈C₇ powders can be prepared at 950 °C for 1 h with 1.0 wt% CaF₂ as additive.