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991.
This paper is concerned with the generalized job shop scheduling problem with due dates wherein the objective is to minimize total job tardiness. An efficient heuristic algorithm called the revised exchange heuristic algorithm (REHA) is presented for solving this problem. It has been shown that the algorithm can be completed in polynomial time. Results, generated over a range of shop sizes with different due date tightness levels, indicate that the proposed technique is capable of yielding notable reductions in total tardiness (over initial schedules) for practical size problems. This suggests that this approach is an efficient scheduling option for this class of complex optimization problems. 相似文献
992.
Cao Zhiqiang Jin Junze Lin Dong Zheng Xianshu Youngwhan Cho Youngjoo Oh Jaedong Shim 《Journal of Materials Science》1998,33(9):2313-2317
About 26 kinds of simple and complex binary eutectics were frozen directionally under the condition of electromagnetic stirring. It was found that anomalous eutectics including most metals/non-metals and some metal/intermetallic eutectics, can be separated on macroscopic scale by flow caused by electromagnetic stirring. Moreover, the eutectics in which one of their component phases possess an entropy of solution over 23 J mol-1K-1 showed notable separation. In addition, process factors also affect the formation of a separated eutectic. © 1998 Chapman & Hall 相似文献
993.
RH Johnson H Hu ST Haworth PS Cho CA Dawson JH Linehan 《Canadian Metallurgical Quarterly》1998,43(4):929-940
Detailed morphometric knowledge of the microvascular network is needed for studies relating structure to haemodynamic function in organs like the lung. Clinical volumetric CT is limited to millimetre-order spatial resolution. Since evidence suggests that small arterioles (50 to 300 micrometres) dominate pulmonary haemodynamics, we built a micro-CT scanner, capable of imaging excised lungs in 3D with 100 microm resolution, for basic physiology research. The scanner incorporates a micro-focal (3 microm) x-ray source, an xyz theta stage and a CCD-coupled image intensifier detector. We imaged phantoms and contrast-enhanced rat lungs, reconstructing the data with either the Feldkamp or the circle-and-line cone-beam reconstruction algorithm. We present reconstructions using 180 views over 360 degrees for the circular trajectory, augmented with views from a linear scan for the circle-and-line algorithm. Especially for platelike features perpendicular to the rotation axis and remote from the midplane, the circle-and-line algorithm produces superior reconstructions compared with Feldkamp's algorithm. We conclude that the use of nonplanar source trajectories to perform micro-CT on contrast-enhanced, excised lungs can provide data useful for morphometric analysis of vascular trees, currently down to the 130 microm level. 相似文献
994.
Huliehel F.A. Wei Tang Lee F.C. Cho B.H. 《Power Electronics, IEEE Transactions on》1995,10(5):597-604
A current-mode control technique, quasi-charge control (QCC), is studied. A small-signal model is developed for QCC. The model is derived for converters operating in the continuous conduction mode. The model is accurate up to half of the switching frequency. The proposed small-signal model is used to assess the dynamics of pulse-width-modulated (PWM) power converters with QCC, and to compare their performance to the current-injected control (CIC) and charge control (CC) schemes. As in CC, the QCC converges to CIC at light loads close to the boundary between continuous and discontinuous conduction modes. The dynamics of power converters with QCC or CC are more sensitive to load change and less sensitive to line change than those of power converters with CIC, especially the current loop dynamics. However, unlike in the case of CC, the dependency on the load/line can be controlled when the QCC is designed. Control design guidelines for DC-DC power converters and power factor correction rectifiers are presented, and experimental measurements are performed to verify the proposed model 相似文献
995.
The squeeze casting of an Al-4.5 wt pct Cu alloy was carried out to investigate the conditions for the formation and the prevention
of macrosegregation. The effects of the process parameters, applied pressure, die temperature, pouring temperature, delay
time, and humidity on the formation of macrosegregation were investigated in correlation with the evolution of macrostructure
and shrinkage defects. Two critical applied pressures were defined: one is the critical applied pressure, P
SC
, under which shrinkage defects form, and the other is the critical applied pressure, P
MS
, above which macrosegregates form in the squeeze castings. A quantitative diagram describing the optimum process conditions
was proposed for obtaining sound squeeze castings. It was found that the pouring temperature, the die temperature, the delay
time, and the humidity are closely related to the two critical applied pressures P
SC
and P
MS
, in different manners. It was concluded that sound castings without macrosegregation and shrinkage defects can only be obtained
when the applied pressure is in the range of P
SC
<P<P
MS
. 相似文献
996.
997.
A new semiconductor laser, the field-effect semiconductor (FES) laser, in which the both sides of the active region are surrounded by the burying layers with the reverse-biased p-n junctions is proposed. The proposed FES laser is aQ -switching device based on the direct control of laser beam intensity by the internal loss modulation through the field effect. The structure design and the operating principles of the device are discussed. Numerical solution shows that a pulsewidth shorter than 30 ps can be obtained. 相似文献
998.
The composition of MBE-grown GaAs1?xSbx (x<0.15) is investigated as a function of growth temperature for constant As and Sb flux and as a function of Sb effusion cell temperature for a fixed growth temperature. Under conditions of constant As and Sb flux, x remains fairly constant for growth temperatures of 480?540°C, but decreases rapidly with increasing temperature in the range 540?640°C. As a function of Sb effusion cell temperature, the Sb mole fraction is shown to increase slightly slower than the Sb vapour pressure. 相似文献
999.
The construction of an image function from the physically measured projections of some object is discussed with reference to the least squares optimum filters, originally derived to enhance the signal-to-noise ratio in communications theory. The computerized processing associated with tomography is generalized so as to include 3 distinct parts: the construction of an image from the projection information, the restoration of a blurred, noisy image, degraded by a known space-invariant impulse response, and the further enhancement of the image, e.g. by edge sharpening. In conjuction with given versions of the popular convolution algorithm, not to be confused with filtering by a 2-dimensional convolution, we consider the conditions under which a concurrent construction, restoration, and enhancement are possible. Execution speeds are compared. ** Department of Electrical Science and Engineering, School of Engineering, University of California, Los Angeles, California 90024 相似文献
1000.