A New Route to High Yield Mesostructure Boron Carbide Powder from SiC/Si3N4 Whiskers

Boric acid/Mg (magnesiothermic or metal sintering aid)/C (activated carbon)/N₂ or Ar (atmosphere)/additives (mesoporous SiO₂ or mesoporous SiC or SiC/Si₃N₄ whiskers) systems were used in the one‐step synthesis of mesostructured B₄C (221.04 m²/g). In this study, a mixture of the active precursors was allowed to react via a self‐sustaining reaction (high‐energy ball milling process). Also, the properties of the samples prepared using powdered activated carbon (PAC) and SiC/Si₃N₄ whiskers (concentration in the range 5–10 wt%) as sources of carbon were investigated. X‐ray diffraction results proved the presence of crystalline boron carbide in the peak positions of B₄C (B₁₂C₃). The advantage of the present route for yielding mesostructured B₄C powder seems to be limited by the growth of carbide crystals. This restriction is believed to be imposed by a lack of whisker additives around the pores where B₄C crystals grow. The results also show that the best mesoporous additive for the synthesis of nanoscale boron carbide is mesoporous SiC. The effect of the concentration of CO (reduction of α‐Fe₂O₃ to Fe by CO) on the B₄C synthesis suggests that, in addition to the concentration of CO, the pressure of the N₂ atmosphere is an important factor in the synthesis of mesostructured B₄C.     

Chemical Vapor Deposition of Silicon Carbide and Silicon Nitride—Chemistry's Contribution to Modern Silicon Ceramics

Pure silicon carbide and silicon nitride have valuable properties in bulk pore-free form; however, their industrial exploitation has hardly been possible so far. Neither compound can be melted or sintered in pure form; hot pressing or sintering at normal pressure requires the presence of additives; and the reaction-sintering process in which only Si and C or Si and N are employed as additives affords porous materials.–The novel process of chemical vapor deposition has partly overcome the drawbacks of the previous methods. In the new process SiC is produced, e.g., by pyrolysis of CH₃SiCl₃, and Si₃N₄ by reaction of SiCl₄ with NH₃. This technique can also be used for pore filling in objects made of SiC and Si₃N₄ (gas phase impregnation) and for producing extremely fine SiC and Si₃N₄ (gas phase impregnation) and for producing extremely fine SiC and Si₃N₄ powder and SiC monofilaments suitable as components for SiC composites. Moreover, gas phase impregnation can also give fiber composites.     

碳化物修饰的有序介孔炭的制备及其催化肼分解性能

 以间苯二酚和甲醛为炭源, F127 (EO₁₀₆PO₇₀EO₁₀₆) 为结构导向剂, 在酸性水/乙醇溶液中引入 (NH₄)₆Mo₇O₂₄•4H₂O 或 (NH₄)₂WO₄ 溶液, 经静置自组装形成凝胶, 再于 N₂ 中焙烧即合成出金属碳化物修饰的有序介孔炭材料. 结果表明, 金属离子的种类和用量对碳化物的分散度和介孔炭的有序度影响很大. 通过控制金属离子的用量可制备出粒径为 3~5 nm 且高度分散在介孔炭骨架中的碳化物粒子. 与分步浸渍法相比, 一步法制备的碳化物具有更高的分散度和催化肼分解活性.     

Boron-bearing species in ceramic matrix composites for long-term aerospace applications

Boron-bearing refractory species are introduced in non-oxide ceramic matrix fibrous composites (such as SiC/SiC composites) to improve their oxidation resistance under load at high temperatures with a view to applications in the aerospace field. B-doped pyrocarbon and hex-BN have been successfully used as interphase (instead of pure pyrocarbon) either as homogeneous or multilayered fiber coatings, to arrest and deflect matrix cracks formed under load (mechanical fuse function) and to give toughness to the materials. A self-healing multilayered matrix is designed and used in a model composite, which combines B-doped pyrocarbon mechanical fuse layers and B- and Si-bearing compound (namely B₄C and SiC) layers forming B₂O₃-based fluid healing phases when exposed to an oxidizing atmosphere. All the materials are deposited by chemical vapor infiltration. Lifetimes under tensile loading of several hundreds hours at high temperatures are reported.     

Template‐Synthesized Porous Silicon Carbide as an Effective Host for Zeolite Catalysts

A facile method has been developed for the fabrication of porous silicon carbide (SiC) by means of sintering a mixture of SiC powder and carbon pellets at a relatively lower temperature, that is, 1450 °C, in air. The pore density and the total pore volume of the resulting porous SiC could be tuned by changing the initial SiC/C weight ratio. The structure evolution and the associated property changes during the preparation were examined through X‐ray diffraction, scanning electron microscopy, thermogravimetric analysis, ²⁹Si magic‐angle spinning (MAS) NMR spectroscopy, and mercury‐intrusion porosimetry analyses. Silica and SiO_xC_y ceramics formed in situ during the calcination process acted as binders of the porous SiC grains. The porous SiC can be used as a host for the growth of ZSM‐5 zeolite crystals to form the ZSM‐5/porous‐SiC composite material. After loading another catalytic active component of molybdenum, a novel catalytic material, Mo‐ZSM‐5/porous‐SiC, was obtained, which exhibited improved catalytic activity in the methane dehydroaromatization reaction.     

Effect of Modifying Additives on the Sintering and Properties of SiC/SiC<Subscript>w</Subscript> Ceramic Composite Material

Spark plasma sintering and hot compaction methods were used to obtain experimental samples of a composite material of the SiC?SiC_w system with various modifying additives (AlN, B₄C, HfB₂, Y₂O₃, Al₂O₃, Si₃N₄). The effect of the modifying additives on the sintering process, physicomechanical, and thermal properties of the ceramic composite material was examined. The introduction of the modifying additives lowered the sintering temperature of silicon carbide produced by the hot compaction method by 200°C and that formed with spark plasma spark sintering by 300?450°C as compared with the sintering temperature of silicon carbide without additives.     

The morphology of ceramic phases in BxC-SiC-Si infiltrated composites

The present communication is concerned with the effect of the carbon source on the morphology of reaction bonded boron carbide (B₄C). Molten silicon reacts strongly and rapidly with free carbon to form large, faceted, regular polygon-shaped SiC particles, usually embedded in residual silicon pools. In the absence of free carbon, the formation of SiC relies on carbon that originates from within the boron carbide particles. Examination of the reaction bonded boron carbide revealed a core-rim microstructure consisting of boron carbide particles surrounded by secondary boron carbide containing some dissolved silicon. This microstructure is generated as the outcome of a dissolution-precipitation process. In the course of the infiltration process molten Si dissolves some boron carbide until its saturation with B and C. Subsequently, precipitation of secondary boron carbide enriched with boron and silicon takes place. In parallel, elongated, strongly twinned, faceted SiC particles are generated by rapid growth along preferred crystallographic directions. This sequence of events is supported by X-ray diffraction and microcompositional analysis and well accounted for by the thermodynamic analysis of the ternary B-C-Si system.     

Bioactive ceramics in the CaO-Al2O3-P2O5-ZnO system prepared by the sol-gel technology

The sol-gel method was applied to the synthesis of porous bioactive glass-ceramic materials in the Ca-Al₂O₃-P₂O₅-ZnO system when compositions were chosen in the glass formation range. Aluminium sol and soluble inorganic salts of the other components have been used as starting materials. The sol-gel transition was achieved by controlling the pH value of the medium. After drying the gel products were thermally treated up to 800°C, Phase formation was studied by X-ray diffraction, IR spectroscopy and electron microscopy.It was found that the main crystalline phases in the glass ceramics were C(PO₃)₂ and Ca₂P₂O₇. The amorphous powder was sintered to form ceramic materials with average pore size distribution of 150–200 µm by isostatic pressing and heat treatment at 500–600°C. This glass ceramic is more bioactive than hydroxylapatite in the initial stage after implantation.     

中空Ta_2O_5/TiO_2复合光催化剂的可控氧化制备及性能

以钛粉、钽粉为原料,炭黑作为反应性模板,通过熔盐法在炭黑表面原位生长了TaTiC_2纳米碳化物涂层,并以所得TaTiC_2/C复合物为碳化物前驱体,再经可控氧化制备出中空Ta_2O_5/TiO_2复合光催化剂。采用X射线粉末衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、紫外-可见(UV-Vis)漫反射(DRS)及N2物理吸附等手段对所制备的光催化剂进行形貌、显微结构及孔结构表征。以高压汞灯为紫外光源,以亚甲基蓝为目标降解物,通过光催化降解实验评价中空Ta_2O_5/TiO_2复合光催化剂的光催化活性。结果表明,熔盐法生长碳化物涂层厚度均匀(20~30 nm),碳化物主要以TaTiC_2晶相存在且具有纳米级的颗粒尺寸。中空Ta_2O_5/TiO_2复合光催化剂同时具有200 nm左右的中空大孔结构及壳层10 nm左右的介孔结构。中空大孔和介孔的存在提高了所制备催化剂对亚甲基蓝的吸附能力。此外,TiO_2与Ta2O5通过电子能带结构的耦合,有效提高了光生电子和空穴的分离效率,从而显著提高了光催化活性。nTi∶nTa=2.5∶1.5时,相应的中空Ta_2O_5/TiO_2复合光催化剂表现出最佳的光催化活性,对亚甲基蓝的紫外光催化降解率高达97%。     

中空Ta2O5/TiO2复合光催化剂的可控氧化制备及性能

以钛粉、钽粉为原料,炭黑作为反应性模板,通过熔盐法在炭黑表面原位生长了TaTiC₂纳米碳化物涂层,并以所得TaTiC₂/C复合物为碳化物前驱体,再经可控氧化制备出中空Ta₂O₅/TiO₂复合光催化剂。采用X射线粉末衍射（XRD）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）、紫外-可见（UV-Vis）漫反射（DRS）及N₂物理吸附等手段对所制备的光催化剂进行形貌、显微结构及孔结构表征。以高压汞灯为紫外光源,以亚甲基蓝为目标降解物,通过光催化降解实验评价中空Ta₂O₅/TiO₂复合光催化剂的光催化活性。结果表明,熔盐法生长碳化物涂层厚度均匀（20~30 nm）,碳化物主要以TaTiC₂晶相存在且具有纳米级的颗粒尺寸。中空Ta₂O₅/TiO₂复合光催化剂同时具有200 nm左右的中空大孔结构及壳层10 nm左右的介孔结构。中空大孔和介孔的存在提高了所制备催化剂对亚甲基蓝的吸附能力。此外,TiO₂与Ta₂O₅通过电子能带结构的耦合,有效提高了光生电子和空穴的分离效率,从而显著提高了光催化活性。n_Ti∶n_Ta=2.5∶1.5时,相应的中空Ta₂O₅/TiO₂复合光催化剂表现出最佳的光催化活性,对亚甲基蓝的紫外光催化降解率高达97%。     

Crystal-chemistry of mullite-type aluminoborates Al18B4O33 and Al5BO9: A stoichiometry puzzle

Orthorhombic Al₂O₃-rich aluminoborate is an important ceramic material for which two slightly different compositions have been assumed: Al₅BO₉ (5Al₂O₃:B₂O₃) and Al₁₈B₄O₃₃ (9Al₂O₃:2B₂O₃). The formula Al₁₈B₄O₃₃ (=Al_4.91B_1.09O₉) was derived from results of chemical analyses when crystal structure data were not yet available. Subsequent structural investigations indicated Al₅BO₉ composition. Nevertheless, Al₁₈B₄O₃₃ was still accepted as the correct stoichiometry assuming that additional B replaces 9% Al.Powder samples of both compositions and ones with excess boron were prepared by solid state reactions between α-Al₂O₃ and B₂O₃/H₃BO₃ at temperatures above 1100 °C and single-crystals were grown from flux at 1100 and 1550 °C. Products were investigated by single-crystal and powder XRD, ¹¹B and ²⁷Al solid-state MAS-NMR, Raman and FTIR spectroscopy as well as Laser-ablation ICP-MS. No indication of the predicted 9% B→Al substitution was found. LA ICP-MS indicated 12.36(27) wt% B₂O₃ corresponding to Al_4.97B_1.03O₉. Hence, the suggested Al₁₈B₄O₃₃ stoichiometry can be excluded for all synthesized samples. A very low amount of Al vacancies at a five-fold coordinated site are likely, charge balanced by an additional nearby three-fold coordinated B site. All evidences indicate that the title compound should be reported as Al_5−xB_1+xO₉ with x<0.038(6), which is close to Al₅BO₉.     

Chemical etching of hot‐pressed p‐type polycrystalline SiC surfaces by HF/K2S2O8 solutions

In this work, an experimental study on the etching of p‐type hot‐pressed silicon carbide (SiC) was carried out in HF/K₂S₂O₈ solutions. The SiC wafers used in this work were p‐type polycrystalline materials, supplied by Goodfellow, with an acceptor concentration of 2.31 × 10¹² cm^?3. The SiC substrate was a hot‐pressed material, the latter realized from a mixture of 1 part of SiO₂ with 3 parts of C (carbon) ‘1SiO₂ + 3C’ heated in an oven at 2500 °C. In order to facilitate the chemical etching of the SiC substrate, a thin aluminium film was deposited on the SiC substrate. The morphology of the etched surface was examined with varying K₂S₂O₈ concentration. The surfaces of the etched samples were analysed using secondary ions mass spectrometry (SIMS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT‐IR) and photoluminescence (PL). The surface morphology of the samples etched in HF/K₂S₂O₈ is shown to depend on the solution composition. The investigation of the effect of the HF/K₂S₂O₈ solution on SiC samples shows that as K₂S₂O₈ concentration increases, the chemical etching reveals defects with random geometry. Finally, chemical etching of p‐type SiC induces a decrease in the PL intensity, which indicates clearly the defects on the polycrystalline SiC surface. In addition, the result is very interesting, as to date no chemical etching solution at low temperature (<100 °C) has been developed for SiC. Finally, we have proposed a dissolution mechanism of SiC in 2HF/1K₂S₂O₈ solutions. Copyright © 2007 John Wiley & Sons, Ltd.     

CoAl2O4/蜂窝陶瓷催化剂的制备及其催化臭氧化性能

采用涂覆法制备了CoAl₂O₄/蜂窝陶瓷催化剂。利用X射线衍射、N₂吸附-脱附和透射电镜等方法对所制备的催化剂进行了表征,并分析了其催化臭氧化降解对苯二酚的效能。结果表明,CoAl₂O₄/蜂窝陶瓷的晶相属于典型的尖晶石结构,具有较大的比表面积、孔容和孔径,分别达到77 m²·g^-1、0.001 7 cm³·g^-1和3.9 nm。CoAl₂O₄/蜂窝陶瓷催化臭氧化对苯二酚的去除率高达81.2%,COD去除率可达47.7%。在叔丁醇存在的条件下,对苯二酚的去除率显著下降,说明CoAl₂O₄/蜂窝陶瓷催化臭氧化遵循羟基自由基机理。     

CoAl_2O_4/蜂窝陶瓷催化剂的制备及其催化臭氧化性能

采用涂覆法制备了CoAl_2O_4/蜂窝陶瓷催化剂。利用X射线衍射、N_2吸附-脱附和透射电镜等方法对所制备的催化剂进行了表征,并分析了其催化臭氧化降解对苯二酚的效能。结果表明,CoAl_2O_4/蜂窝陶瓷的晶相属于典型的尖晶石结构,具有较大的比表面积、孔容和孔径,分别达到77 m~2·g~(-1)、0.001 7 cm~3·g~(-1)和3.9 nm。CoAl_2O_4/蜂窝陶瓷催化臭氧化对苯二酚的去除率高达81.2%,COD去除率可达47.7%。在叔丁醇存在的条件下,对苯二酚的去除率显著下降,说明CoAl_2O_4/蜂窝陶瓷催化臭氧化遵循羟基自由基机理。     

High performance anode-supported intermediate temperature solid oxide fuel cells (IT-SOFCs) with La0.8Sr0.2Ga0.8Mg0.2O3−δ electrolyte films prepared by electrophoretic deposition

Remarkable power density was obtained for anode-supported solid oxide fuel cells (SOFCs) based on La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ (LSGM) electrolyte films, fabricated following an original procedure that allowed avoiding undesired reactions between LSGM and electrode materials, especially Ni. Electrophoretic deposition (EPD) was used for the fabrication of 30 μm-thick electrolyte films. Anode supports were made of La_0.4Ce_0.6O_2−x (LDC). The LSGM powder was deposited by EPD on an LDC green tape-cast membrane added with carbon powder, both as pore former and substrate conductivity booster. A subsequent co-firing step at 1490 °C produced dense electrolyte films on porous LDC skeletons. Then, a La_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (LSFC) cathode was applied by slurry-coating and calcined at 1100 °C. Finally, the porous LDC layer was impregnated with molten Ni nitrate to obtain, after calcination at 900 °C, a composite NiO–LDC anode. Maximum power densities of 780, 450, 275, 175, and 100 mW/cm² at 700, 650, 600, 550, and 500 °C, respectively, were obtained using H₂ as fuel and air as oxidant, demonstrating the success of the processing strategy. As a comparison, electrolyte-supported SOFCs made of the same materials were tested, showing a maximum power density of 150 mW/cm² at 700 °C, more than 5 times smaller than the anode-supported counterpart.     

Vanadium- and molybdenum-containing compositions prepared by mechanochemical and following thermal treatments of V2O5/(NH4)2Mo2O7 (V/Mo = 0.7/0.3)

X-ray powder diffraction, DTA, FT-IR spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), nitrogen adsorption, and mercury porometry were used to characterize samples prepared as a result of mechanochemical treatment (MCT) of a V₂O₅/(NH₄)₂Mo₂O₇ (V/Mo = 0.7/0.3) composition in water, ethanol, and air, as well as after calcining them at temperatures from the range 300–700°C. The MCT of nonporous powders in water yields porous materials with definite meso- and macropore sizes. Heat treatment in air at 300–450°C enhances the formation of a molybdenum substitutional solid solution in V₂O₅ and conserves rather high values of specific surface areas and pore volumes. An increase in heat treatment temperature is accompanied by the degradation of the solid solution and the formation of a V₂MoO₈ phase.     

Solid State Reactions in the Al‐Si‐C System

The study is aimed to prevent the formation of the aluminium carbide compound Al₄C₃ that negatively affects Al‐Si‐C based materials. The reaction products of elementary aluminium, silicon and graphite as well as aluminium with either β‐SiC or α‐SiC without and with graphite at temperatures 1200°‐2500 °C under different atmospheres and reaction times were characterized using powder X‐ray diffraction and scanning electron microscopy (SEM) with an energy dispersive X‐ray (EDX) analysis. The results of the powder diffraction study show that under the conditions (1450 °C; 8 h; vacuum) the formation of Al₄C₃ could be prevented. The reaction products at those conditions consist of the ternary compound Al₄SiC₄ besides SiC and residual carbon. The ternary aluminium silicon carbide Al₄SiC₄ crystallizes in a hexagonal crystal system with unit cell dimensions a = 327.64(4) pm, b = 2171.2(6) pm and space group P6₃mc (no. 186). The crystal structure of Al₄SiC₄ is isostructural with Al₅C₃N and consists of layers of Al₄C₃ and SiC.     

Synthesis and structural study of the new rare earth magnesium borates LnMgB5O10 (Ln = La, …, Er)

To obtain rare earth luminescent materials with weak concentration quenching, the B₂O₃-rich portion of the ternary diagram Ln₂O₃MgOB₂O₃ (Ln = rare earth) has been investigated. A ternary phase of composition LnMgB₅O₁₀ has been found for Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er. These compounds all crystallize in the monoclinic space group $\text{P2}{}_1\text{c}$. The structure has been determined on a LaMgB₅O₁₀ crystal. A full-matrix least-squares refinement leads to R = 0.039. The structure can be described as being made of (B₅O₁₀^5?)_n two-dimensional layers linked together by the lanthanum and magnesium ions. The rare earth atom coordination polyhedra form isolated chains. These borates are isostructural with some rare earth cobalt borates.     

Electrophoretic deposition of Al2O3/ZrO2 layer with controllable thickness in ethanol medium

ZrO₂ toughened Al₂O₃ (Al₂O₃/ZrO₂) ceramic layers with required thickness were prepared by electrophoretic deposition (EPD) method using ethanol suspensions with stabilizing agent of polyethyleneimine (PEI) under constant-voltage mode in this paper. The deposition of Al₂O₃/ZrO₂ ceramic powders occurred on the titanium alloy cathode. A stable suspension with 1wt% PEI in ethanol at pH 5 was prepared in terms of the zeta potential and sedimentation of the suspension. The effects of the suspension concentration, applied voltage, deposition time and processing method of titanium alloy cathode on the coating thickness and morphology were investigated. The deposition layers on titanium alloys with smooth surfaces and thickness of 0.35?C1.2 mm could be obtained by adjusting the aforementioned parameters. In addition, after being sintered at 1500°C for 3 h in air atmosphere, ZrO₂ toughened Al₂O₃ ceramic layers became smooth and dense.     

Investigation into the Structural,Chemical and High Mechanical Reforms in B4C with Graphene Composite Material Substitution for Potential Shielding Frame Applications

In this work, boron carbide and graphene nanoparticle composite material (B₄C–G) was investigated using an experimental approach. The composite material prepared with the two-step stir casting method showed significant hardness and high melting point attributes. Scanning electron microscopy (SEM), along with energy dispersive X-ray spectroscopy (EDS) analysis, indicated 83.65%, 17.32%, and 97.00% of boron carbide + 0% graphene nanoparticles chemical compositions for the C-atom, Al-atom, and B₄C in the compound studied, respectively. The physical properties of all samples’ B₄C–G like density and melting point were 2.4 g/cm³ density and 2450 °C, respectively, while the grain size of B₄C–G was in the range of 0.8 ± 0.2 µm. XRD, FTIR, and Raman spectroscopic analysis was also performed to investigate the chemical compositions of the B₄C–G composite. The molding press composite machine was a fabrication procedure that resulted in the formation of outstanding materials by utilizing the sintering process, including heating and pressing the materials. For mechanical properties, high fracture toughness and tensile strength of B₄C–G composites were analyzed according to ASTM standard designs. The detailed analysis has shown that with 6% graphene content in B₄C, the composite material portrays a high strength of 134 MPa and outstanding hardness properties. Based on these findings, it is suggested that the composite materials studied exhibit novel features suitable for use in the application of shielding frames.