摘要:

Linear shrinkage profiles of an unconstrained gold thick film material and a low ‐tempearture cofireable glass‐ceramic (LTCC) green tape were measured using a noncontact optical technique. A laser beam scans across a sample, at various times during the sintering process. The unconstrained sintering kinetics of the gold film were found to differ significantly from those of the LTCC tape. The densification of the gold film was nearly completed before the LTCC began to densify. The development of camber (warpage) during consintering of a gold/LTCC composite structure was monitored and recorded with a video camera. This camber development is analysed based on the viscous constitutive relations for porous based on the viscous constitutive relations for porous sintering bodies. The mismatched sintering kinetics of the two materials lead to the development of in‐plane stresses in the two constituents of the composite structure as one material being constrained from composite structure as one material being constrained of the composite structure as one material being constrained from sintering by the other. The resulting camber during the consintering process is explained by the development of these str

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08311.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Tempertures are calculated in a ceramic material exposed to microwaves. The method entails calculation of electromagnetic fields by integral formulation and subsequent solution of the heat conduction equation for temperature in a ceramic piece. The solution of the equation is numerical and the parameters used are estimates for properties of SiC. The results include a case where the complex dielectric constant is varied with temperature The computed results demonstrate that SiC can be heated to high temperatures (1000‐1500 K) and that both the temperature and the temperature gradient can be controlled by varying the power density of the micro waves and the external cooling. The results also exhibit high sensitivity of temperatures to the dimensions of the material and the orientation in which microwaves impinge on the ceramic bod

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08312.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A mathematical model, in which mass fluxes are expressed according to the “dusty gas” model, is proposed to described densification of a ceramic perform by chemical vapor infiltration (CVI) Local temperatures previously computed for a perform subjected to microwaves are input to the mass transport model. The model is solved numerically for deposition of SiC from trichloromethylsilane in a porous SiC perform. The computed results clearly demonstrate an advantage in employing, microwaves heating in CVI, Also, greater infilatration is achieved in a perform of greater pore size and at lower temperature. However for successful infiltration, diffusion/chemical‐reaction‐controlled CVI is shown to impose limits on the prefo

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08313.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Rate laws for the chemical vapor deposition of Si3N4from SiF4and NH3are obtained by fitting the results of pareametric reactor experiments with a one‐dimensional steady‐state model for the reactor. The model includes axial mass transport by both convection and multicomponent diffusion, and allowance is made for the use of an expressions with adjustable constants are used to account for deposition on both crystalline and amorphous surfaces, as well as the heterogeneous decomposition of NH3. In addition, there are mechanisms that determine the actual degree of surface crystallinity at any location. Optimum values for the rate constants are found by searching for the best overall fit to the experimental deposition rate and crystallinity data. It is shown that the model, with simple second‐order expressions for the deposition rates, is quite successful in reproducing the experimentally observed effects of temperature, flow rate, reactant mole ratio, and axial pos

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08314.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

The wear characteristics of an yttria‐stabilized zirconia (Y‐TZP) are represented by a set of three‐dimensional wear maps under dry and lubricated conditions. Water, paraffin oil, and a formulated oil were used as lubricants. Different wear regions were identified as function of load, speed, and lubricants environment. Sudden increases in wear, identified as wear transitions, were found at certain loads and speeds. The onset of wear transitions was moderated by the presence of a lubricant. Below the wear transitions, the wear was mild and the wear mechanism was predominantly plastic deformation and microfracture. Above the wear transisitons, the wear was severe, dominated by brittle fracuture and third‐body abrasion. Diferent fluids had different effects on wear. Water had a deleterious effect on wear of this material. The presence of oil lubricants effectively reduced friction and moderated wear. Under high load andd high speed, additional stress induced by a thermal gradient within a small area contributed significantly to wear. A critical velocity model was found to describe the locations of the wear transition zones succe

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08315.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

X‐ray diffraction patterns show that most samples of Y1‐xPrxBa2Cu4O8examined in the present study contained a single YBa2Cu4O8(1‐2‐4) superconductive phase for x0. The room‐temperature resistivity changes linearly until x=0.7 and increases abruptly at x=‐0.75. The critical concentration, xcr, thus was estimated to be 0.7. The effective magnetic moments of Pr in Y1‐xPrxBa2Cu4O8were 3.63., 3.35, and 3.23, μBfor x=0.2, 0.4 and 0.6, respectively. In the R0.8Pr0.2Ba2Cu4O8system, the depression ofTcweakly depends on the ionic radius of rare‐earth elements. Similarities and differences between Y1‐xPrxBa2Cu4O8and Y1‐xPrx‐Ba2Cu3O7‐yalso were noted a

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08316.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

An experimental study has been conducted to examine the cyclic fatigue crack growth characteristics in 1200oC air of a MoSi2‐50 mol% Wsi2alloy the unreinforced condition and with 30 vol% SiC particles. For comparison purposes, crack growth experiments under sustained loads were also carried out in the silicide‐matrix composite. Particular attention is devoted to developing an understanding of the micromechanisms of subcritical crack‐tip damage. The results indicate that enhanced viscous flow of glass films along interfaces and grain boundaries imparts pronounced levels of subcritical crack growth in the composite material; the composite exhibits a higher fatigue fracture threshold and a more extended range of stable fracture than the unreinforced alloy. The effects of glass phase in influencing fatigue crack growth in the silicide‐based material are compared to the influence ofin situ‐formed and preexisting glass films on high‐temperature cyclic fatigue crack growth in ceramics and ceramic composites. The paper concludes with a comparison of present results with the high‐temperature damage tolerance of a variety of intermetallic alloys and cera

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08317.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Young's modulus of CVD and SiC coatings was measuredin situ(on the substrate) using strain gage/flexure and dynamic resonance techniques. The average Young's moduli of CVD SiC from these techniques were 446 and 415 Gpa, respectively, and were considered to be in good agreement. The determination of a representative coating thickness and cross section is critical in order to accurately estimate Young's modulus of coatings.

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08318.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

Silicon nitride materials typically reveal thin amorphous intergranular films along grain boundaries, with only the exception of special boundaries. It is known that such grainboundary films strongly affect the high‐temperature properties of the bulk material. High‐resolution electron microscopy (HREM) was used to study these amorphous films in different Si3N4ceramics. The observed film thicknesses at grain boundaries in these materials varied between 5and 15 Å. It was shown that the grain‐boundary film thickness strongly depends on film chemistry. Careful inspections of film‐thickness measurements across grain boundaries in a given material suggest that the film widths vary on the order of 1 Å. Therefore, a quantitative evaluation should allow for the determination of the standard deviation of the film thickness. The amorphous film widths along grain boundaries in four materials were measured over the entire length (up to 1 μm) of the grain boundary between two triple points. Forty to fifty data points were evaluated for each boundary, giving a Gaussian‐like distribution of the film thickness around a median value, which corresponded well with the film width measured from single HREM micrographs. The accuracy achieved by the statistical method was bette

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08319.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

A method was proposed for calculation of the effect of elastic strain on spinodal decomposition in the tetragonal system. An effective free energy for the composition field was derived by eliminating elastic fields which are coherently induced by composition inhomogeneities It was shown that anisotropic long‐range interactions between composition fields play a major role in determining both the domainmorphology and domain growth law of spinodal decomposition with the coherence of the lattice. Computers simulations were performed on the basis of a two‐dimensional model by taking into account those long‐range interactions. The results demonstrated the appearance of lamella structure and its coarsening in the late stage of the phase separation The calculation for the TiO2‐SnO2system showed slow coarsening due to the anisotropic elastic long ‐range interactions The asymptotic growth of the lamella size was described by λα tn, wh

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1993.tb08320.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY