摘要:

Silicon‐based ceramics and composites are prime candidates for heat engine and heat exchanger structural components. In such applications these materials are exposed to combustion gases and deposit‐forming corrodents. In this paper combustion environments are defined for various applications. These environments lead to five main types of corrosive degradation: passive oxidation, deposit‐induced corrosion, active oxidation, scale/substrate interactions, and scale volatility. Each of these is discussed in detail. The key issues in oxidation mechanisms of high‐purity silicon carbide (SiC) and silicon nitride (Si3N4) in pure oxygen are discussed. The complicating factors due to the actual combustion environment and commercial materials are discussed. These discussions include secondary elements in the ceramics; additional oxidants, such as water and carbon dioxide (CO2); combustion environment impurities; long‐term oxidation effects; and thermal cycling. Active oxidation is expected in a limited number of combustion situations, and the active‐to‐passive transition is discussed. At high temperatures the limiting factors are scale melting, scale volatility, and scale/substrate interactions. Deposit‐induced corrosion is discussed, primarily for sodium sulfate (Na2SO4), but also for vanadate and oxide‐slag deposits as well. In applying ceramics in combustion environments it is essential to be aware of these corrosion routes and how they affect the performan

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03684.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The relationship between a composite and its constituents with respect to high‐temperature deformation behavior was investigated using the Al2O3‐Y3Al5O12(YAG) eutectic system. The eutectic is essentially a composite with sapphire and YAG as the two phases with a volume fraction of YAG of 0.45. The deformation behavior of the eutectic and those of single‐crystal sapphire and YAG were studied under identical conditions, using constant‐strain‐rate compression tests in air at 1530°C. The composite was also studied at 1650°C and compared with existing data on sapphire and YAG. The stronger YAG phase was found to reinforce the sapphire matrix at higher strain rates, but the composite crept faster than sapphire at lower strain rates. It is suggested using a simple semiempirical analysis that diffusional relaxation at the YAG‐sapphire interface boundaries may cause the inferior creep behavior at lower

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03685.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Time‐dependent crack bridging forces were measured at temperatures from 1150° to 1275°C in a vitreous bonded alumina using the rear‐notched, short double cantilever beam technique. Glassy ligaments were observed to bridge the crack even after the crack entered the rear notch. The crack bridging force exhibited both a temperature and displacement rate dependency. A model was developed that describes this temperature and time dependence of the bridging forces that result from the viscous glass br

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03686.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Silicon tetrachloride is a major product of the pyrolysis of dichlorodimethylsilane in hydrogen. A model is developed for the deposition of silicon from the reduction of silicon tetrachloride with hydrogen in a tubular reactor at temperatures from 700° to 1100°C and 1.013 × 105Pa (1 atm) pressure. Concentrations of silicon tetrachloride varied from 1 to 9 vol%. Gas chromatography was used to determine the volatile products of reaction, and gravimetric analysis was used to determine the total silicon deposition on the tube. The model, based on the experimental data, assumes the following reversible chemical reactions:The rate constants derived from a nonlinear regression analysis are report

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03687.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A model is developed for the deposition of silicon carbide from the pyrolysis of dichlorodimethylsiane in hydrogen, in a tubular reactor at temperatures from 700° to 1100°C and 1.013 × 105Pa (1 atm) pressure. Concentration of dichlorodimethylsilane varied from 2 to 8 vol%. Gas chromatography was used to determine the volatile products of reaction, and gravimetric analysis was used to determine the total silicon and silicon carbide deposition on the tube. The model developed based on the experimental data that assumes the following chemical reactions:The rate constants derived from a nonlinear regression analysis are report

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03688.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Powder compacts of alkoxide‐derived fine silica powders were consolidated into a highly dense and uniform structure by ultra‐high‐pressure cold isostatic pressing of granules with controlled structure. The diameters of spherical and nearly monosized amorphous silica particles, prepared from metal alkoxide, were successfully controlled in the range of 9 to 760 nm by varying the concentration of ammonia. Close‐packed granules of these powders were produced by spray drying. These powders were isostatically pressed up to 1 GPa at room temperature. Although the average particle diameter was less than 100 nm, the maximum relative density of the compacts was more than 78% of theoretical density. The optimum particle size to obtain highly dense compacts was in the range of 30 to 300 nm at 1 GPa. Furthermore, the ratio of mode pore diameter in these compacts to particle diameter was less than 0.155, which corresponded to the minimum ratio of calculated three‐particle pore channel radii for hexagonal close packing. Viscous deformation of particles under ultra‐high isostatic pressure played an important role in the densification of t

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03689.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The effects of sintering time and rate of cooling on the microstructure and current/voltage characteristics of a ZnO varistor material have been investigated. The results provide information about the development of microstructure during sintering and have been interpreted in terms of the role which microstructure plays in the determination of electrical properties. A continuous network of intergranular Bi2O3, which lies mainly along the triple junctions between ZnO grains, can give a substantial contribution to the leakage current. The conductivity of this skeleton depends upon its internal microstructure.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03690.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The suspension hydration of coarse‐grained (0.07 m2.g−1) tricalcium silicate (C3S) in relatively large amounts of water (1 g per 500 mL) was studied as a function of time (up to 48 h), temperature (10°, 25°, and 40°C), and pH (8, 10, 11, 11.5, 12). The pH of the solution was maintained with dilute HCl, and both solid and solution samples were withdrawn periodically for analysis. At earliest times, the microstructure of the hydration products suggests that a fine‐grained, highly conformable hydrate forms on the C3S grains as early as 2 min. Once formed, the growing hydrate layer imparts anywhere from 0.5 to 6 h of diffusion control to the dissolution reaction. The parabolic rate constants (kps) calculated for this period increase with increasing temperature and acidity of the solution. Similarly, activation energies calculated using the Arrhenius equation range from 2.0 kJ·mol−1at pH 10 to 20.6 kJ·mol−1at pH 12. Values less than 21 kJ·mol−1are generally indicative of diffusion control in aqueous systems. Depending on the pOH of the solution, the initial calcium silicate hydrate (C─S─H) was observed either to persist throughout the 48‐h experiments (aOH<10−2.5) or to act as a nucleation site for a second, less soluble, highly reticulated C─S─H hydrate (aOH≥ 10−2.5). The chemical composition of the solution phase and the characteristics of the evolving microstructure over the course of the reaction lead to the conclusion that under high‐pH conditions (aOH≥ 10−2.5), the reticulated phase which is precipitating is identical to the Type I/II C─S─H observed in non‐pH‐controlled experiments. Experiments confirm and strengthen the hypothesis that the system CaO–SiO2–H2O contains at least two distinct hydrated phases and that the activity of the hydroxide ion plays a dominant role in determining the nature and

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03691.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Ethanolic silicon suspensions, with and without a polyethoxylated amine of low molecular weight, were studied by rheological, adsorption, electrophoretic, and sedimentation methods. Pellets were pressure‐cast and nitrided to form reaction‐bonded silicon nitride. Density and binding strength in the green state relate well to rheological behavior and other collodial aspects of the suspensions used, particularly the additive's role and distribution. Density and degree of nitridation in the final state are not importantly affected by the additive's use. Its greatest benefit is to modify the binding strength in the green state. The mode by which this small molecule affects the processing of silicon consists of adsorption, combined with an increased electrostatic interparticle repulsion which increases the suspension viscosity and that of undried pellets. Although the improved binding strength is accompanied by decreased green and nitrided densities, high degrees of conversion to silicon nitride are still achie

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03692.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The kinetics of anisotropic β‐Si3N4grain growth in silicon nitride ceramics were studied. Specimens were sintered at temperatures ranging from 1600° to 1900°C under 10 atm of nitrogen pressure for various lengths of time. The results demonstrate that the grain growth behavior of β‐Si3N4grains follows the empirical growth lawDn– D0n=kt, with the exponents equaling 3 and 5 for length [001] and width [210]directions, respectively. Activation energies for grain growth were 686 kJ/mol for length and 772 kJ/mol for width. These differences in growth rate constants and exponents for length and width directions are responsible for the anisotropy of β‐Si3N4growth during isothermal grain growth. The resultant aspect ratio of these elongated grains increases with sintering temperat

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb03693.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY