摘要:

Grüneisen's constant, γ, is ideally temperature‐independent on constant‐volume lines; a similar constant, λ, which is ideally temperature‐independent on isobaric lines, has been found. The comparison between γ and λ shows that both expressions contain the ratio of the volumetric thermal expansion coefficient to the specific heat capacity; γ has an additional temperature dependence due to the thermal part of the compressibility, whereas λ has only a slight thermal component due to the volume per unit mass. λ has been chosen so that λ=γ at the point where the temperature and pressure are both zero. For most materials it is expected that λ≥γ with a different temperature dependence. It is shown empirically for a selection of ceramics that λ is nearly temperature‐independent and has a val

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04431.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

SiC‐platelet‐reinforced Al2O3‐matrix composites were made by three different forming techniques, i.e., slip casting, tape casting, and dry compaction of a granulated powder. All samples were densified with hot pressing at 1650°C and 25 MPa for 0.5 h. The orientation of SiC platelets in the composites was studied before and after hot pressing using optical microscopy and a pole figure X‐ray device. X‐ray diffraction of the (0006) plane of silicon carbide (6H) was used to analyze the degree of preferred orientation. It was found that both tape casting and die pressing could give rise to preferred orientation in green bodies with the faces of SiC platelets parallel to the tape faces or perpendicular to the pressing direction, respectively. The preferred orientation in die‐pressed samples also showed an increase with the increase of the compaction stress; however, this reached a saturation level at about 70 MPa in a similar way to the green density. Samples formed by slip casting gave a platelet orientation close to a random one in the green body. After hot pressing, preferred orientation was observed in both slip‐cast and tape‐cast samples with the faces of SiC platelets perpendicular to the direction of hot pressing. The effect of platelet size on the orientation was also investigated. The preferred orientation in platelet composites was found to yield higher toughness than

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04432.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Although copper‐bonded alumina substrate has been used for some power electronic devices, its use has been rather limited because of lack of information on the thermally induced failure of the substrate. The phenomenology of thermal cracking was experimentally studied in this work. Interface debonding occurred at the side edge of copper chips and grew inwardly along the interface. Subsequently, the interface crack deviated into the alumina after propagating up to approximately 200 μm. The curving of the interface crack was caused by a mixed Mode I and Mode II loading near the edge. The cracking was widespread, whereby small cracks coalesced with one another to form a long, shallow crack along the perimeter of the copper chips. The cracking sensitivity was influenced by such design factors as edge sharpness, nickel plating, and copper backing. Annealing the substrate above 700 K before testing promoted the cracking considerably. The substrate cracking is discussed in terms of stress conditions near the ed

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04433.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Push‐out experiments on sapphire fibers in both glass and γ‐TiAl matrices show a sinusoidal modulation in the load–displacement trace that is identical in wavelength to the surface roughness of the sapphire fibers. A model, presented in a previous paper, is used to describe the push‐out behavior, and is shown to agree with experiment. The results quantitatively validate the key role of fiber surface roughness during fiber

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04434.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Nonohmic behavior is obtained for polycrystalline ZnO with copper as the only additive in the range 0.3x1 wt%. The effect of copper on the microstructure and electrical behavior of ZnO:Cu ceramics is investigated. The leakage current decreases and the breakdown electric field increases as the copper concentration increases. The large apparent dielectric constant of ZnO:Cu ceramic (k>>kZnO,kZnOis the dielectric constant of pure ZnO) is attributed to the grain boundary barrier layer effect. A Schottky barrier height of 0.27–0.46 eV is obtained for various copper‐added samples, depending on sintering temperatu

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04435.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Al2O3–ZrO2–SiC whisker composites were prepared by surface‐induced coating of the precursor for the ZrO2phase on the kinetically stable colloid particles of Al2O3and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO2and SiC whisker phases throughout the Al2O3matrix. The fracture toughness values of the Al2O3–15 vol% ZrO2–20 vol% SiC whisker composites (∼12 MPa.m1/2) are substantially greater than those of comparable Al2O3–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot‐pre

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04436.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

The oxidation process of Si2N2O, prepared by a hot isostatic pressing technique, has been studied by the thermogravimetric method. The oxidation has been performed in oxygen for 20 h in the temperature range 1300° to 1600°C, producing oxide scales of amorphous SiO2and α‐cristobalite. The weight gain forT1350°C does not begin to follow a parabolic rate law, until a certain time,t0. TheA0parameter in the parabolic rate law, (Δw/A0)2=Kpt+B, represents the cross section area,A, through which the oxygen diffuses; in the derivation of this lawAis assumed to be constant during the experiment. If crystallization occurs during the oxidation process,Awill decrease with time. A function,A(t), describing the time dependence, has been developed and incorporated into the parabolic rate law, yielding a new rate law, which reads ΔW/A0=aarctan √bt+c√t. This new rate law is valid in the time intervaltt0, the oxidation process follows the equation (Δw/A0)2=K°pt+B0. The relation of the latter equation to the common parabolic rate law is described. All of the oxidation curves are described by these equations. The activation energy of the oxygen diffusion (and of the oxidation (Kp)) is found to be 245 ± 25 kJ/mol, which is consistent with literature values reported for ox

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04437.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

BaTiO3ceramics with grain sizes from 0.6 to 60 μm and relative densities of 89% to 92% were prepared by hot forging and conventional sintering from very pure‐oxalate‐derived powder. The aging of both the dielectric constant and the dielectric loss was examined at weak and strong fields with respect to grain size and frequency. It was concluded that the main aging mechanism is the aging of hysteretic domain wall motion for coarse‐grained ceramics. At grain size of less than 1 μm, the lack of frequency andEACdependence, along with a lower aging rate, suggests that domain motions or hysteretic domain wall motions are restricted in finegrained ceramic BaTiO3and contribute little to th

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04438.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

BaTiO3ceramics with grain sizes of 0.6 to 6 mm and relative densities of 89% to 92% were prepared by hot forging and conventional sintering from high‐purity oxalate‐derived powder. The aging of both the dielectric constant and the dielectric loss was examined with respect to grain size, temperature, and frequency. It is suggested that the main aging mechanism in coarse‐grained BaTiO3is the aging of hysteretic domain wall motion. The dielectric aging is a thermally activated process and shows two aging rates when observed at a temperature of 8°C for coarse‐grained

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04439.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

Adding aluminum titanate to alumina can result in dramatic improvements in toughness andR‐curve properties. However, the improved toughness is offset by a significant reduction in strength at small flaw sizes. This problem can be overcome through the use of a laminated composite construction. By placing a thin layer of high‐strength material on the surface of a high‐toughness body, the toughness and flaw tolerance of the body material can be maintained without sacrificing small flaw strength. In this study, alumina + 20 vol% aluminum titanate (AAT20) was used for both the surface layer and the bulk material. The surface material was a homogeneous, fine‐grained mixture of the two phases, while the bulk was an inhomogeneous mixture having a bimodal grain structure. In monolithic form, the homogeneous AAT20 displays a nearlyP−1/3indentation strength response, and the inhomogeneous material displays a flat strength response, indicative ofR‐curve behavior. The trilayer material shows a composite indentation strength response, with high strength throughout the entire range of starting flaw sizes. A method for predetermining the optimum surface layer thickness is presented. The processing and mechanical properties of these materials will b

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1992.tb04440.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY