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排序方式: 共有168条查询结果,搜索用时 15 毫秒
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Shoko Yoshikawa Toshitaka Ota Robert Newnham Ahmed Amin 《Journal of the American Ceramic Society》1990,73(2):263-267
Composite materials composed of randomly dispersed semiconducting ceramic particles in an insulating polymer matrix show a pronounced change in resistivity with pressure. Different amounts of iron oxide (Fe3 O4 ) powder and antimony-doped tin oxide (SnO2 :Sb) powder were dispersed in an epoxy polymer matrix to form pressure-sensitive composites. In each family of materials, an insulator-to-semiconductor transition is observed in agreement with percolation theory. Composites within a certain range of filler content showed substantial piezoresistive effect under both uniaxial and hydrostatic pressure in which sensitivity is controlled by the choice of filler material and the volume fraction. The effect of temperature on the piezoresistance effect was also examined. Piezoresistors made from Fe3 O4 composites showed larger temperature changes than those filled with Sb-doped SnO2 . 相似文献
3.
Barium Metaplumbate Ceramic Electrodes for Ceramic Capacitors 总被引:2,自引:0,他引:2
Frank P. Skeele Robert E. Newnham Leslie E. Cross 《Journal of the American Ceramic Society》1988,71(5):263-C
A novel approach to reduce electrode costs in ceramic capacitors is presented. Barium metaplumbate (BaPbO3 ) was fired onto sintered commercial (Ba, Sr)TiO3 disk capacitors as external electrodes. Samples were compared to disk capacitors of the same composition with electrodes made of sputtered gold and air-dried silver. The ceramic electrode led to substantial changes in the insulation resistance and dc breakdown strengths, but other properties remained the same. The feasibility of using BaPbO3 electrodes strongly depends on chemical reactions between the electrode and dielectric. 相似文献
4.
A quantitative general effective media (GEM) equation is used to describe quantitatively the resistivity of an Fe3O4-epoxy composite system over a large range of volume fractions in terms of the resistivities of each component and two percolation morphology parameters. One parameter is the critical (percolation) volume fraction, c, and the other is an exponent,t. Preliminary models, also based on the GEM equation, are used to describe the positive temperature coefficient of resistivity (PTC) and the piezoresistivity (uniaxial pressure) of the composite when the composition is near the percolation threshold. 相似文献
5.
Smart Electroceramics 总被引:8,自引:0,他引:8
"Smart" materials have the ability to perform both sensing and actuating functions. Passively smart materials respond to external change in a useful manner without assistance, whereas actively smart materials have a feedback loop which allows them to both recognize the change and initiate an appropriate response through an actuator circuit. Many smart materials are analogous to biological systems: piezoelectric hydrophones are similar in mechanism to the "ears" by which a fish senses vibrations. Piezoelectrics with electromechanical coupling, shape-memory materials that can "remember" their original shape, electrorheological fluids with adjustable viscosities, and chemical sensors which act as synthetic equivalents to the human nose are examples of smart electroceramics. "Very smart" materials, in addition to sensing and actuating, have the ability to "learn" by altering their property coefficients in response to the environment. Integration of these different technologies into compact, multifunction packages is the ultimate goal of research in the area of smart materials. 相似文献
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Marina Villegas Amador C. Caballero Carlos Moure Pedro Durán José F. Fernández Robert E. Newnham 《Journal of the American Ceramic Society》2000,83(1):141-46
Pb(Zn1/3 Nb2/3 )O3 -based ceramics have been prepared by two different processing methods: conventional (PZN-C) and reaction-sintering (PZN-RS). The conventionally prepared PZN-based ceramics densified at lower temperatures (950°C) than the reaction-sintered samples (1100°C), but the perovskite/pyrochlore ratio was always higher in PZN-RS. The presence of a substantial amount of pyrochlore phase in PZN-C ceramics caused a decrease in the electrical properties. The maximum dielectric constant values in PZN-C ceramics were 10%–15% lower than those of PZN-RS, despite a similar average grain size, 7 ± 0.2 μm. The temperature of the maximum of the dielectric constant ( T max ) was lower than that expected from the mixing rule because of the possible formation of Ba–Nb clusters. The higher chemical homogeneity in PZN-RS ceramics is the main reason for the higher dielectric constant, T max and electromechanical response, as well as for the lower difference between T max and the depolarization temperature ( T d ) and the lower diffusiveness parameter (δ). 相似文献
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9.
Bassarello C Cimino P Bifulco G Boger DL Smith JA Chazin WJ Gomez-Paloma L 《Chembiochem : a European journal of chemical biology》2003,4(11):1188-1193
We report the NMR solution structure of (+)-CPI-indole (CPI, 1,2,8,8a-tetrahydrocyclopropa[c]pyrrolo[3,2-e]indol-4(5H)-one), an agent belonging to the CC-1065/duocarmycin family of antitumor compounds. This (+)-CPI-indole structure is covalently bound to d(G(1)ACTAATTGTC(11))-d(G(12)TCAATTAGTC(22)), a synthetic DNA duplex containing a high-affinity binding site. The three-dimensional structure has been determined by several cycles of restrained molecular dynamics calculations with a total of 563 NMR-derived constraints, both in vacuo and by using the generalized Born solvent continuum model. In-depth analysis of the structure of this ligand-DNA complex led to a detailed knowledge of the bound state conformation of the CPI-indole, the most simplified agent related to CC-1065 and duocarmycins, the parent members of a family of extremely potent antitumor compounds. Comparison of the CPI-indole bound conformation with those previously found for (+)-duocarmycin SA (DSA), its unnatural enantiomer (-)-DSA, and the demethoxylated analogue (+)-DSI in their DNA complexes provided additional evidence of the tight correlation between the catalytic effect exerted by DNA on the alkylation reaction and the extent of angular twist between the two planar heteroaromatic subunits of these agents. Additionally, comparison of the structural features of the DNA-bound state of a "naked" ligand, such as CPI-indole, with those of various other duocarmycin agents provided useful information for the interpretation of the observed effects on chemical reactivity of the different substitution patterns at the hemispheres of these types of complex. 相似文献
10.
We have constructed a coolable spectroscopic cell for characterizing the physical and chemical properties of simulated atmospheric aerosol particles. The cell is designed for experiments in which the refractive indices, freezing temperatures, and the phase and chemical composition of a wide range of aerosol types are measured. The relatively large volume (0.075 m(3)) of the cell reduces wall-aerosol interactions and allows the aerosol residence time to exceed 2 h. The cell has been optically interfaced to Fourier-transform spectrometers to record broadband infrared, visible, and ultraviolet extinction spectra of aerosol particles and gas-phase components over a range of temperatures (180-300 K). The data generated with the cell have applications in remote sensing, radiative transfer models, heterogeneous atmospheric chemistry, and pollution studies. 相似文献