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961.
962.
Deterioration of concrete or reinforcing steel through excessive contaminant concentration is often the result of repeated
wetting and drying cycles. At each cycle, the absorption of water carries new contaminants into the unsaturated concrete.
Nuclear Magnetic Resonance (NMR) is used with large concrete samples to observe the shape of the wetting profile during a
simple one-dimensional wetting process. The absorption of water by dry concrete is modelled by a nonlinear diffusion equation
with the unsaturated hydraulic diffusivity being a strongly nonlinear function of the moisture content. Exponential and power
functions are used for the hydraulic diffusivity and corresponding solutions of the diffusion equation adequately predict
the shape of the experimental wetting profile.
The shape parameters, describing the wetting profile, vary little between different blends and are relatively insensitive
to subsequent re-wetting experiments allowing universal parameters to be suggested for these concretes.
Résumé La détérioration du béton ou du béton armé à travers la concentration excessive de contaminant est souvent le résultat de cycles humides et secs répétés. A chaque cycle, l'absorption de l'eau amène de nouveaux contaminants à l'intérieur du béton non saturé en eau. Pour observer le profil du processus d'humidification, on peut utiliser la Résonance Magnétique Nucléaire (RMN) sur de grands échantillons de béton dans le cadre d'une simple étude unidimensionnelle de ce profil. Il est démontré que l'absorption d'eau par du béton sec peut être précisément modélisée par une équation de diffusion non linéaire où la diffusivité hydraulique non saturée est fonction hautement non linéaire de l'humidité du contenant. Des fonctions exponentielles et puissances peuvent définir la diffusivité hydraulique. Les solutions résultant de l'équation modélisant cette diffusion permettent de prédire avec précision la forme du profil de l'humidification expérimentale. Les paramètres de forme qui décrivent le profil de l'humidification varient peu entre les différents mélanges de béton et sont relativement insensibles à des expériences de réhumidification ultérieures. Cela permet de suggérer que ces bétons suivent des paramètres d'humidification universels.相似文献
963.
R. Meyer Hilde Hardtdegen R. Carius D. Grützmacher M. Stollenwerk P. Balk A. Kux B. Meyer 《Journal of Electronic Materials》1992,21(3):293-298
This paper presents a study of the structural and optical properties of strained GaInAs/ InP multiple quantum well (MQW) structures
fabricated by LP-MOVPE. The composition of the Ga
x
In1−x
As films ranged fromx = 0.17 tox = 1.0 and was determined by sputtered neutral mass spectrometry (SNMS) on thick layers. The structures of the MQW samples
with well widths from 1.5 to 5 nm were investigated by high resolution x-ray diffraction (HR-XRD). Simulations of the diffraction
patterns showed that transition layers of approximately 2 monolayer (ML) thickness with high lattice mismatch exist at the
interfaces. Photoluminescence (PL) measurements indicate well widths of a multiple of a monolayer with local variations of
one monolayer. The PL peak energies vary smoothly with the Ga concentration. These results were confirmed by optical absorption
measurements. 相似文献
964.
Microwave transistor amplifier combinations that have controlled frequency response over a specified bandwidth were designed. Theoretical analysis of such an amplifier with an arbitrary number of sections is presented. The response of the amplifier is controlled by tapering the frequency selectivity or Q of each section of the amplifier. To verify the theory, a three-section amplifier with maximally flat time delay response was designed, constructed, and evaluated. Existing traveling-wave amplifiers were modeled as lossy transmission lines. Although the amplifiers were relatively broadband, a prescribed frequency response was not achieved, and each transistor did not receive an equal portion of the signal power. Resistive elements were required for impedance matching at the input and output. The design seeks to improve on previous techniques by trading bandwidth for controlled gain. By making the transmission line that connects the amplifier sections nonuniform, the frequency response was controlled over the design bandwidth. The designs can easily be implemented using familiar components 相似文献
965.
S. R. Syrtsov V. N. Shut D. A. Il'yushchenko E. L. Gavrilenko 《Journal of Engineering Physics and Thermophysics》2002,75(5):1229-1233
The influence of the parameters of posistor elements and the conditions of their heat exchange on the electrical and dynamic characteristics of a posistor block have been investigated. Good agreement between calculation results and experimental data has been established. 相似文献
966.
967.
Rumpel D. Krost G. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1992,80(5):758-764
Applying expert systems (ESs) to power system operation means embedding them in an existing environment which consists of computational facilities (SCADA, EMS) on one side, and the operating staff on the other side. An interface between process data and ES data is required to achieve this goal. For an unambiguous and clear information exchange with and good acceptance by the operators, an approximate natural language dialogue of the ES is desirable. Several approaches to these problems are described 相似文献
968.
D Pang 《Canadian Metallurgical Quarterly》1995,6(2):243-257
The embryogenesis, closure technique, and preoperative preparation of open myelomeningocele are described in this article. Both early and late complications of myelomeningocele closure are discussed with respect to predisposing factors, diagnosis, treatment, and prevention. These complications include worsened neurological level, wound dehiscence, wound infection, cerebrospinal fluid leak, postoperative ileus, symptomatic Chiari malformation, shunt infection, necrotizing enterocolitis, and problems related to kyphectomy. 相似文献
969.
970.
Student O. Z. Rusyn B. P. Kysil' B. V. Kobasyar M. I. Stakhiv T. P. Markov A. D. 《Materials Science》2003,39(1):17-24
We give a quantitative estimate of changes in the structure of 15Kh2MFA steel after its long-term exposure to the combined action of high temperature, tensile stresses, and hydrogen as the factors of degradation of this steel under conditions of oil hydrocracking. Under laboratory conditions, we study the effect of high-temperature hydrogen degradation of 15Kh2MFA heat-resistant steel on changes in the quantitative characteristics of its structural elements, in particular, in the dimension and average distance between carbides as well as in their orientation from one grain to another. Algorithms for the solution of the formulated problems are described, and the possibilities of proposed approaches for the quantitative automatic processing of metallographic images are shown. 相似文献