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941.
A simple weak avalanche model valid in a wide range of voltages and currents, is presented. The proposed model is derived by using the base-collector depletion capacitance for predicting the avalanche current. The model needs only one additional transistor parameter; the extraction method and temperature dependence of this parameter are discussed. The decrease in avalanche current for high collector current densities may originate from internal device heating, a voltage drop in the epilayer, or mobile carriers in the depleted part. From experimental results it is concluded that, below a critical hot-carrier current, the decrease in avalanche current due to mobile carriers is negligible 相似文献
942.
The synthesis of thermal-shock-resistant materials from the system Ta2O5WO3 was investigated. Ta2WO8 had a very low unit-cell thermal expansion coefficient (+0.5 X 10–6° C–1). Ta30W2O81 also had a relatively low coefficient (+4.0 X 10–6 ° C–1) and a thermal durability over 1600° C. The thermal expansion curves of these polycrystalline ceramics were lowered because of microcracks caused by the large thermal expansion anisotropy of the crystal axes and were accompanied by hysteresis loops. The densification of Ta2WO8 ceramic was promoted by the addition of some metal oxides, and the strong ceramic of Ta30W2O81 was obtained by controlling grain growth. 相似文献
943.
944.
The microstructure of a series of Ni-B alloys (15 at%3B phase (a=0.44 nm,b=0.52 nm,c=0.66 nm) is expected. High-resolution electron microscopy images of Ni-25 at% B rapidly quenched indicate that a two-phase structure is observed. A metastable phase is observed with a periodicity of 1.0 nm. HREM images and EELS analyses led us to propose the structure of this metastable phase. This phase (Ni5B2) is correlated with the monoclinic Hägg carbide Fe5C2. Depending on the velocity of the substrate and on the boron concentration, a variety of microstructures has been observed from a faulted to a semi-amorphous structure. The structures are related to the solidification behaviour and the heat flow in the undercooled melt. Particular attention was paid to the eutectic Ni-Ni3B composition. 相似文献
945.
P. Papet N. Le Bars J. F. Baumard A. Lecomte A. Dauger 《Journal of Materials Science》1989,24(11):3850-3854
Transparent yttria-stabilized zirconia monolithic gels were synthesized under controlled conditions for hydrolysis and chemical polymerization. The influence of acetylacetone on the previous chemical reactions was explored, and a plausible explanation was proposed concerning the outstanding role of this additive during the gelation process. Small-angle X-ray scattering was used to estimate the size of the polymeric species and to investigate the influence of acetylacetone on the structure of the gels prepared. 相似文献
946.
Grain-boundary resistivity in yttria-containing tetragonal zirconia polycrystalline (Y-TZP) materials dominates the total resistivity. Impedance measurements combined with microstructural studies suggest that post-sintering heat treatments (in particular the cooling rate) influence the location of the grain-boundary phase which, in turn, has a significant effect on the grain-boundary resistivity. Higher cooling rates from the sintering temperature lead to reduction in the grain-boundary resistivity. In both alumina-containing and relatively pure tetragonal zirconia polycrystals, post-sintering heat treatments have a less conspicuous effect. The activation energy associated with the grain-boundary resistivity was independent of the post-sintering heat treatments but was 25 to 30 kJ mol–1 higher than that for the oxygen-ion conduction within the grains at low temperatures. 相似文献
947.
948.
949.
J. Botsis 《Journal of Materials Science》1989,24(6):2018-2024
Damage analysis of a crack layer in polystyrene is carried out by employing optical microscopy and principles of quantitative
steteology. The results show that, within the quasistatic phase of crack layer propagation, the average crazing density, along
the trailing edge of the active zone, is constant. This is consistent with a self-similarity hypothesis of damage evolution
employed by the crack layer theory. The average crazing densities within the active zone and along its trailing edge are found
to be practically equal. A layer of constant crazing density, adjacent to the crack planes, accompanies the crack during its
quasi-static growth. This suggests that: (1) a certain level of crazing density should be reached, around the crack tip, prior
to crack advance; (2) the specific energy, associated with this ‘core’ of damage, could be considered as a Griffith's type
energy. The results are in favour of certain hypotheses adopted by the crack layer theory. 相似文献
950.
J. M. Richards M. I. Darby R. Baggott G. H. Wosteim Holm B. Yates G. Dorey L. N. Phillips 《Journal of Materials Science》1989,24(2):584-589
The effects of defects on the mechanical properties of carbon fibre reinforced polyethersulphone laminates have been measured. The defects studied were cut fibre plies, omission of polymer films and local delamination produced by the inclusion of foreign matter. Of these it was found that only cut plies had a significant detrimental effect on the strength of a laminate. For specimens with two cut plies, the failure stress, tensile, flexural and compressive in the remaining continuous plies was the same as in the defect-free material, provided that the cut plies were widely separated. However the failure stresses were 15–18% lower in the continuous plies in the specimens containing two cut plies which were more closely spaced and in specimens containing four cut plies. 相似文献