碳还原锆英石的Si-C-O系中C和CO优势图

对联还原锆英石中SiO2进行了温度、配碳量、时间和粒度的实验研究，得到相应的脱硅率的影响规律．建立了Si-C－O系中C－CO型优势图，讨论了SiO分压和StO2活度等对脱硅率的影响，得到温度与StO2最低含量的理论关系．配碳量为7．5％时的脱硅率达到极大值97．22％，在1873～2273K温度范围内的化学反应表现活化能为282．0kJ／mol，反应的主要限制环节是锆英石的热分解     

铅丹-硅系延期药贮存中的化学反应机理

基于铅丹和硅的物理化学特性,对铅丹-硅系延期药在常温封闭贮存下发生自发氧化还原反应的可能性及其反应机理进行了研究。给出了药剂贮存中可能发生的化学反应方程式,计算了各反应的主要热力学参数ΔrGθm、ΔrHθm、ΔrSθm,通过对热力学参数的分析,讨论了各反应的自发性。结合固体化学和薄膜生长理论,建立了药剂氧化还原反应的扩散模型和硅粉表面的氧化模型。结果表明,单一Pb3O4在贮存中不会自发分解,但是,当Pb3O4与Si混合后贮存时,两者发生氧化还原反应,部分反应产物PbO会驻留在SiO2的网络结构中。     

复杂反应体系的化学平衡分析

本文提出了一种新的通用化学平衡计算方法.该方法以反应程度和组分摩尔数同为迭代变量,用松弛法估计初值,Newton-Raphson法加速收敛,具有良好的收敛稳定性.同时用新算法分析了甲烷蒸汽转化中的结焦和Ti(C.N)的化学气相沉积,并对NH_3-CO_2-SO_2-H_2S-H_2O-CH_4-C_2H_6体系进行了热力学计算.所有考证的实例从零初值都得到了正确的结果.     

含多个化学反应体系热力学中的元素组成法

When species in the solution undergo multiple chemical reactions, the solution may be treated as a solution of all species actually present or as a hypothetical solution composed of elemental species. Based on the fundamental thermodynamic principle, the relationships of mole numbers, molar fractions, thermodynamic properties, partial molar properties, potential and fugacity between the hypothetical solution of elemental species and the equilibrated solution of actual species were derived. The hypothetical elemental solution provides a way of reducing the dimensionality of problem, simplifying the analysis and visualizing the phase behavior.     

SiC晶须在SiO2-C-N2系统中的合成

对SiO2-C-N2系统中的主要化学反应和SiC晶须在该系统中的合成条件进行了热力学分析,采用SiO2微粉为硅源、石墨、活性炭和碳黑为碳源,氧化硼为催化剂,分别在1 500℃、1 550℃和1 600℃利用碳热还原法合成碳化硅晶须,通过x射线衍射、扫描电子显微镜和电子探针分析合成晶须的特征.结果表明:在氮气气氛下利用碳热还原反应合成SiC晶须的温度在1 450℃以上,且随着温度的升高,SiC晶须的生成量增多,晶须直径变大;以炭黑和活性炭等较高活性的碳源代替石墨可以使反应速度加快,但合成的SiC晶须较粗甚至生成SiC颗粒;杂质含量较多会使得SiC晶须生成数量降低,同时晶须出现弯曲现象.     

低品位复杂难选菱铁矿氧化焙烧热力学及实验研究

氧化焙烧是解决低品位复杂难选菱铁矿的有效前处理工艺,氧化焙烧参数对焙烧效果影响很大.采用热力学计算软件FactSage对西南地区某低品位复杂难选菱铁矿进行氧化焙烧的热力学模拟计算,分析研究了氧化焙烧过程的特点与规律,得到了适宜的氧化焙烧条件.计算结果表明,适宜的氧化焙烧温度为1000～1050℃,焙烧后矿石品位提高了6.09%,达到37.28%,脱硫率为99.99%.计算结果与实验结果一致.     

合成过程中的化学反应

在当今科学水平不断提高的社会生活中,无时无刻不在体验着化学带来的新发现与创造,而合成过程所产生的化学反应也是多种多样。本文主要探讨合成过程中主要的化学反应类型及注意事项,旨在提高原料的利用率,减少环境污染,并降低危险事故的发生率。     

超临界流体中的化学反应

超临界流体中的化学反应可分为两大类 ,即超临界流体作为反应介质的反应和超临界流体作为反应原料的反应 ,分别介绍了其研究进展 ,着重介绍了第一类反应中的酶催化反应、超临界水氧化、高分子合成     

PX氧化反应体系的热力学平衡计算

全面考察了PX氧化反应体系热力学平衡规律,建立了针对该体系完整的热力学平衡数据库。总结归纳了已发表的有关该体系的热力学平衡数据,实验测定了部分相平衡数据,回归得到各物质间的作用参数。提出的热力学模型及参数很好地与文献数据、实验数据以及现场数据吻合,为PX氧化过程的热力学平衡计算提供了有力的依据。     

反应烧结SiC材料的高温氧化行为研究

研究了反应烧结SiC材料在 110 0℃空气中的高温氧化行为。结果表明 :反应烧结SiC在110 0℃的氧化动力学曲线符合抛物线规律 ;材料的氧化受O2 和CO在玻璃态硅酸盐中的扩散所控制 ;材料中的杂质元素降低了SiO2 氧化膜的粘度 ,促进了O2 和CO在氧化膜中的扩散     

Oxidation Behavior of Si-C-O Fibers (Nicalon) under Oxygen Partial Pressures from 102 to 105 Pa at 1773 k

Polycarbosilane-derived SiC fibers (Nicalon) were oxidized at 1773 K under oxygen partial pressures from 10² to 10⁵ Pa. The effect of oxygen partial pressure on the oxidation behavior of the Nicalon fibers was investigated by examining mass change, surface composition, crystal phase, morphology, and tensile strength. The Nicalon fibers were passively oxidized under oxygen partial pressures of >2.5 ×10² Pa and actively oxidized under an oxygen partial pressure of 10² Pa. Under oxygen partial pressures from 2.5 × 10² to 10³ Pa, active oxidation occurred at the earliest stage of oxidation, resulting in the formation of both a silica film and a carbon intermediate layer. Although the unoxidized core retained considerable levels of strength under the passive-oxidation condition, fiber strength was lost under the active-oxidation condition.     

Preparation of reaction bonded silicon carbide (RBSC) using boron carbide as an alternative source of carbon

RBSC composites are fully dense materials fabricated by infiltration of compacted mixtures of silicon carbide and carbon by molten silicon. Free carbon is usually added in the form of an organic resin that undergoes subsequent pyrolysis. The environmentally unfriendly pyrolysis process and the presence of residual silicon are serious drawbacks of this process. The study describes an alternative approach that minimizes the residual silicon fraction by making use of a multimodal particle size distribution, in order to increase the green density of the preforms prior infiltration. The addition of boron carbide provides an alternative source of carbon, thereby eliminating the need for pyrolized organic compounds. The residual silicon fraction in the RBSC composites, prepared according to the novel processing route, is significantly reduced. Their mechanical properties, in particular the specific flexural strength is by 15% higher than the value reported for RBSC composites prepared by the conventional approach.     

Modeling of Laser-Induced Chemical Vapor Deposition of Silicon Carbide Rods from Tetramethylsilane

Finite-difference fluid-dynamics modeling has been used to predict deposition rates, fractional amounts of phases, and deposition morphology for the codeposition of silicon carbide and pyrolitic carbon from tetramethylsilane via laser-induced chemical vapor deposition (LCVD). Calculated results agree fairly well with rod deposition experiments. The morphologic features of rods that have been grown using LCVD are examined and explained using the results of the finite-difference calculations.     

Toward understanding the reaction between silicon carbide and iridium in a broad temperature range

The reaction between iridium and SiC in the 1000°C–1900°C temperature range was studied in details. The rate of this reaction was found to depend not only on temperature, but also on the grain sizes of the initial reagents, oxygen impurities in SiC, as well as the Ir: SiC ratio. The use of fine-grained initial reagents accelerates the reaction, whereas oxygen impurities in SiC powders slow it down. For the Ir: SiC ratio = 1:1, the IrSi silicide phase became dominant at 1400°C and remained the main phase at temperatures up to 1900°C. For the 3:1 ratio, Ir₂Si was the main phase at 1900°C. It was suggested that stabilization of this phase is due to the quenching effect. No Si-rich silicide phases were detected in the 1000°C–1900°C temperature range. The coefficients of thermal expansion of silicide phases were determined by high-temperature X-ray diffraction analysis.     

Improvement of toughness of reaction bonded silicon carbide composites reinforced by surface-modified SiC whiskers

To improve the reliability, especially the toughness, of the reaction bonded silicon carbide (RBSC) ceramics, silicon carbide whiskers coated with pyrolytic carbon layer (PyC-SiC_w) by chemical vapor deposition (CVD) were introduced into the RBSC ceramics to fabricate the SiC_w/RBSC composites in this study. The microstructures and properties of the PyC-SiC_w/RBSC composites under different mass fraction of nano carbon black and PyC-SiC_w were investigated methodically. As a result, a bending strength of 550 MPa was achieved for the composites with 25 wt% nano carbon black, and the residual silicon decreased to 11.01 vol% from 26.58 vol% compared with the composite of 15 vol% nano carbon black. The fracture toughness of the composites reinforced with 10 wt% PyC-SiC_w, reached a high value of 5.28 MPa m^1/2, which increased by 39% compared to the RBSC composites with 10 wt% SiC_w. The residual Si in the composites deceased below to 7 vol%, resulting from the combined actively reaction of nano carbon black and PyC with more Si. SEM and TEM results illustrated that the SiC_w were protected by PyC coating. A thin SiC layer formed of outer surface of whiskers can provide a suitable whisker-matrix interface, which is in favor of crack deflection, SiC_w bridging and pullout to improve the bending strength and toughness of the SiC_w/RBSC composites.     

Chemical Vapor Deposited Sic Matrix Composites

Composites of Sic yarnlchemical vapor deposited Sic matrix and multitei chemical vapor deposited Sic matrix were prepared using methyldichloro-silane and applying a thermal gradient in a low-pressure reactor environment. The preparation parameters were selected from an X-ray study where the pressure and temperature were varied. The Sic-reinforced composite had a higher strength (450 versus 120 MPu) and exhibited more fiber pullout than the mullite-reinforced composite.     

碳化硅冶炼炉逸出气体的组成及理化性质研究

利用自行发明的多热源碳化硅冶炼炉，以煤（或焦炭）和石英砂为原料合成SiC，同时伴生大量副产品可燃性气体，研究了煤种和冶炼工艺对燃气的组成及酸碱性的影响，并对经济效益进行分析。结果表明，燃气中以CO气体为主，平均含量可达70％，CO＋H2平均含量达85％，CO＋H2＋CH4平均含量达90％，使用无烟煤、烟煤焦炭或焙烧料法可提高合成气中CO含量，若燃气用于火力发电可节约25．8％的电费成本。     

Robocasting of reaction bonded silicon carbide structures

A novel shaping method for the fabrication of reaction bonded silicon carbide structures was investigated in this work. A paste consisting of silicon carbide as inert filler and carbon powder was developed and printed by robocasting technology. Layer by layer deposition of the ceramic paste facilitates the printing of complex shaped structures. Different structures such as lattices, hollow cylinders, bending bars and gyroids were printed using nozzles with diameter of 0.5 mm and 1.5 mm. After pyrolysis at 700 °C and further heat treatment at 1850 °C the samples were infiltrated using the liquid silicon infiltration technique to obtain dense near-net shape RBSC structures. The robocasted structures showed a hardness of approximately 20 GPa, a thermal conductivity of ～112 W/m*K, Young’s modulus of ～356 GPa, flexural strength of ～224 MPa and an amount of residual silicon of approximately 23%. These measured properties are comparable with those of traditionally fabricated RBSC.     

Experimental and numerical analysis of the complex permittivity of open-cell ceramic foams

Open-cell ceramic foams are promising materials in the field of microwave heating. They can be manufactured from susceptor materials and can, therefore, be used as selective heating elements. In this study, the complex permittivities of ceramic foam materials, including silicon-infiltrated silicon carbide (SiSiC), pressureless sintered silicon carbide (SSiC), silicate-bonded silicon carbide (SBSiC), and cordierite were determined. The dielectric properties of the foams were determined by the cavity perturbation technique using a TE₁₀₄ WR340 waveguide resonator at 2.45 GHz. Samples were preheated in a tubular furnace, enabling temperature-dependent permittivity measurements up to 200 °C. The effective dielectric constant and effective loss factor were found to depend on the porosity and material composition of the foam. The SiSiC material had a higher effective dielectric constant than the SSiC and SBSiC ceramics. The effective dielectric constant of the foams showed a trend of gradual increase with increasing temperature. Some selected dielectric mixing relations were then applied to describe the effective permittivity of the foams and compare them with predictions from finite element simulations performed using the CST Studio Suite. The foam morphologies and simple block inclusions were used in the simulations.