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981.
Woolard D.L. Buot F.A. Rhodes D.L. Xiaojia Lu Perlman B.S. 《Electron Devices, IEEE Transactions on》1996,43(2):332-341
The intrinsically fast process of resonant tunneling through double barrier heterostructures along with the existence of negative differential resistance in the current-voltage characteristic of these structures has led to their implementation as sources for high frequency electromagnetic energy. While sources based upon resonant tunneling diodes (RTDs) have produced frequency of oscillations up to 712 GHz, only microwatt levels of performance has been achieved above 100 GHz. Since stability criteria plays critical role in determining the deliverable power of any oscillator, a physically accurate equivalent-circuit model for the RTD is extremely important for optimizing the dynamics of the device-cavity package. This study identifies a distinctly new equivalent circuit model for characterizing the modes of oscillation in RTD-based sources. Specifically, in order to exhibit the fundamental self-oscillations and the overall I-V characteristics (plateau structure and hysteresis) observed experimentally, an accurate circuit model of the RTD must incorporate: (i) a quantum-well inductance which directly chokes the nonlinear conductance and, (ii) a nonlinear access resistance, associated with the accumulation of charge in the injection region of the double barriers, with a nonlocal dependence on the bias across the double barrier structure 相似文献
982.
F Murad 《Canadian Metallurgical Quarterly》1996,276(14):1189-1192
Understanding of the formation and biological actions of nitric oxide (NO) has grown extensively during the past 2 decades. Through our discoveries of the biological effects of NO and nitrovasodilators on cyclic guanosine monophosphate (GMP) and our discoveries of the biochemical mechanisms of NO and regulation of NO synthase in numerous cell types and tissues, the complexity of the signal transduction cascade and the extensive function-regulating interactions of the NO molecule have been increasingly demonstrated. Although the diverse roles of this unique molecule continue to unfold, NO can be recognized as an intracellular second messenger, a local substance for regulation of neighboring cells, a neurotransmitter in central and peripheral neurons, and perhaps a hormone that can act at distant sites and has been shown to have beneficial or deleterious biological effects, depending on its concentration, the system, and the cellular environment. 相似文献
983.
F. J. Balta Calleja L. Giri T. Asano T. Mieno A. Sakurai M. Ohnuma C. Sawatari 《Journal of Materials Science》1996,31(19):5153-5157
The microhardness of films of fullerene-polyethylene composites prepared by gelation from semidilute solution, using ultrahigh molecular weight polyethylene (PE) (6×106), has been determined. The composite materials were characterized by optical microscopy and X-ray diffraction techniques. The microhardness of the films is shown to increase notably with the concentration of fullerene particles within the films. In addition, a substantial hardening of the composites is obtained after annealing the materials at high temperatures (T
a=130 °C) and long annealing times (t
a=105s). The hardening of the composites with annealing temperature has been identified with the thickening of the PE crystalline lamellae. Comparison of X-ray scattering data and the microhardness values upon annealing leads to the conclusion of phase separation of C60 molecules from the polyethylene crystals within the material. The temperature dependence is discussed in terms of the independent contribution of the PE matrix of the C60 aggregates to the hardness value. 相似文献
984.
A general numerical model is described for the dissolution kinetics of spherical particles in binary systems for any combination of first order reactions at the particle-matrix interface and long distance diffusion within the matrix. The model is applicable to both finite and infinite media and handles both complete and partial particle dissolution. It is shown that interfacial reactions can have a strong effect on the dissolution kinetics, the solute concentration at the particle-matrix interface and the solute concentration profile in the matrix. 相似文献
985.
C Serveau L Juliano P Bernard T Moreau R Mayer F Gauthier 《Canadian Metallurgical Quarterly》1994,76(2):153-158
A discontinuous borate/formate buffer system is presented for horizontal polyacrylamide gel electrophoresis of DNA fragments. The resolution potential of the system could be altered by changing the total monomer concentration (5-9%T), the concentration of the crosslinker piperazine diacrylamide (2-5%CPDA), as well as the concentration of formate in the gel (40-120 mM), the leading ion of the buffer system. The separation of DNA fragments would be improved by increasing the migration distance from 22 to 28 cm. This discontinuous polyacrylamide gel electrophoresis system proved highly reproducible. 相似文献
986.
Román Gómez Gonz ález Cosío 《International Planning Studies》1998,3(3):367-389
The concept of capacity building for environmental protection and management refers to the process by which the abilities of societies to identify and solve environmental problems are developed. This work assumes a social constructivist perspective to the process of capacity building for environmental governance and thus depicts it as critically dependent on broader processes of political and socioeconomic modernization. In an attempt to depict such a process, a heuristic device is put forward as a model of capacity analysis. A theoretical discourse and new institutionalist perspectives are used to test such a model on a theoretical plane. Some preliminary conclusions are drawn from this theoretical exercise. 相似文献
987.
Pentacene-based organic thin-film transistors 总被引:7,自引:0,他引:7
Yen-Yi Lin Gundlach D.I. Nelson S.F. Jackson T.N. 《Electron Devices, IEEE Transactions on》1997,44(8):1325-1331
Organic thin-film transistors using the fused-ring polycyclic aromatic hydrocarbon pentacene as the active electronic material have shown mobility as large as 0.7 cm2/V-s and on/off current ratio larger than 108; both values are comparable to hydrogenated amorphous silicon devices. On the other hand, these and most other organic TFT's have an undesirably large subthreshold slope. We show here that the large subthreshold slope typically observed is not an intrinsic property of the organic semiconducting material and that devices with subthreshold slope similar to amorphous silicon devices are possible 相似文献
988.
Polymorphous low-grade adenocarcinoma is an unusual tumor that has been described as occurring in the minor salivary glands, particularly of the palate. To our knowledge, there is only one previous report that has documented the occurrence of this type of lesion in the major salivary glands. We describe an elderly patient with polymorphous low-grade adenocarcinoma arising in the parotid gland that was originally diagnosed as carcinoma ex pleomorphic adenoma. 相似文献
989.
990.
The use of periodic inspection methods for early detection of motor deterioration is addressed. Two alternatives to traditional procedures that are appropriate for detecting stator-winding deterioration in three-phase induction motors of the sizes commonly found in the process industries are presented. The use of an effective-negative-sequence impedance as a predictor of incipient failure is recommended 相似文献