全文获取类型
收费全文 | 589506篇 |
免费 | 7542篇 |
国内免费 | 1856篇 |
专业分类
电工技术 | 10626篇 |
综合类 | 478篇 |
化学工业 | 89726篇 |
金属工艺 | 23402篇 |
机械仪表 | 17913篇 |
建筑科学 | 14933篇 |
矿业工程 | 2975篇 |
能源动力 | 14923篇 |
轻工业 | 53193篇 |
水利工程 | 5955篇 |
石油天然气 | 9460篇 |
武器工业 | 35篇 |
无线电 | 72203篇 |
一般工业技术 | 115398篇 |
冶金工业 | 105380篇 |
原子能技术 | 11653篇 |
自动化技术 | 50651篇 |
出版年
2021年 | 4496篇 |
2019年 | 4219篇 |
2018年 | 7169篇 |
2017年 | 7284篇 |
2016年 | 7800篇 |
2015年 | 5397篇 |
2014年 | 9014篇 |
2013年 | 26588篇 |
2012年 | 14945篇 |
2011年 | 20607篇 |
2010年 | 16171篇 |
2009年 | 18352篇 |
2008年 | 19305篇 |
2007年 | 19600篇 |
2006年 | 17297篇 |
2005年 | 16083篇 |
2004年 | 15282篇 |
2003年 | 14912篇 |
2002年 | 14431篇 |
2001年 | 14661篇 |
2000年 | 13637篇 |
1999年 | 14159篇 |
1998年 | 32454篇 |
1997年 | 23535篇 |
1996年 | 18460篇 |
1995年 | 14383篇 |
1994年 | 12758篇 |
1993年 | 12618篇 |
1992年 | 9633篇 |
1991年 | 9285篇 |
1990年 | 8868篇 |
1989年 | 8594篇 |
1988年 | 8358篇 |
1987年 | 7137篇 |
1986年 | 7140篇 |
1985年 | 8483篇 |
1984年 | 7973篇 |
1983年 | 7098篇 |
1982年 | 6554篇 |
1981年 | 6481篇 |
1980年 | 6340篇 |
1979年 | 6228篇 |
1978年 | 5954篇 |
1977年 | 6849篇 |
1976年 | 8746篇 |
1975年 | 5149篇 |
1974年 | 4921篇 |
1973年 | 4912篇 |
1972年 | 4141篇 |
1971年 | 3658篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Yuan Y. Jambunathan R. Singh J. Bhattacharya P. 《Quantum Electronics, IEEE Journal of》1997,33(3):408-415
The performance characteristics of a coupled cavity InGaAsP-InP MQW laser/active waveguide made by one-step epitaxy and well-controlled reactive ion etching (RIE) have been theoretically analyzed and experimentally determined. A theoretical model based on a finite-difference time-domain (FDTD) technique was used to simulate the propagation of an optical wave launched in the coupled system and determine the reflectivity of the facets created by RIE. The calculated effective reflectivity of the coupling region consisting of two facets and an air gap is between 0.45 and 0.55, which is in good agreement with the experimentally measured value of 0.5. The reflectivity of a single etched mirror derived from this value is estimated to be 0.3. A 120-μm-long monolithically integrated active waveguide when biased as a modulator and excited by the laser shows a maximum extinction ratio of 8 dB and a modulation bandwidth ⩾14 GHz at a dc bias of -0.5 V with a bias swing of 2 V 相似文献
992.
Improved modulation techniques for PWM-VSI drives 总被引:2,自引:0,他引:2
Blaabjerg F. Pedersen J.K. Thoegersen P. 《Industrial Electronics, IEEE Transactions on》1997,44(1):87-95
PWM-VSI based AC motor drives have two main problems. The inverter is nonlinear which causes instability problems in some specific working points of the AC machine and it emits acoustic noise due to the switching frequency. Nonlinearities like dead-time in the inverter, load dependent DC-link voltage ripple and the voltage drop across the switches are modeled and compensated by improved modulation techniques in order to obtain an almost ideal inverter. Different feedback and feedforward techniques are proposed. The acoustic noise is reduced by using a random modulation strategy. Measurements show a significant improvement by using feedforward and feedback techniques for linearizing the inverter. An improvement in reduction of the acoustic noise emission is also achieved by using random modulation. It is concluded that a combination of a random modulation strategy and feedforward/feedback techniques gives an almost ideal AC motor drive system 相似文献
993.
L. Verger J. P. Bonnefoy F. Glasser P. Ouvrier-Buffet 《Journal of Electronic Materials》1997,26(6):738-744
There has been considerable recent progress in II-VI semiconductor material and in methods for improving performance of the
associated radiation detectors. New high resistivity CdZnTe material, new contact technologies, new detector structures, new
electronic correction methods have opened the field of nuclear and x-ray imaging for industrial and medical applications.
The purpose of this paper is to review new developments in several of these fields. In addition, we will present some recent
results at LETI concerning first the CdTe 2-D imaging system (20 × 30 mm2 with 400 × 600 pixels) for dental radiology and second the CdZnTe fast pulse correction method applied to a 5 × 5 × 5 mm3 CdZnTe detector (energy resolution = 5% for detection efficiency of 85% at 122 keV) for medical imaging. 相似文献
994.
Kinema is a fermented soya bean food of Nepal and the hilly regions of North-eastern States of India. Generally, the fermentation is dominated by Bacillus spp. that often cause alkalinity and desirable stickiness in the product. The present study was undertaken in a limited number of commercial (market) kinema samples to test for the presence of foodborne pathogens and their properties. Bacillus cereus was present in numbers exceeding 104 cfu/g product in five of the tested 15 market samples. Enterobacteriaceae and coliform bacteria exceeded 105 cfu/g in 10 of the 15 samples. Escherichia coli exceeding 105 cfu/g was found in two samples. Staphylococcus aureus was not detected in any of the tested samples. Of 31 isolated typical and atypical strains of B. cereus, 18 representative strains were tested qualitatively for the ability to produce diarrhoeal type enterotoxin (BCET) using an Oxoid BCET-RPLA test kit. Overall, BCET was formed by 12 strains in BHIG (brain heart infusion broth +1% glucose), by seven strains on sterilized cooked rice, and by five strains on sterilized cooked soya beans. Semi-quantitative tests on BCET revealed that levels exceeding 256 ng/g soya beans, produced by single pure culture inoculation with the isolated B. cereus strains, were reduced to ≤ 8 ng/g by frying kinema in oil, a common procedure when making kinema curry. It was also shown in a mixed pure culture experiment that a kinema strain B. Subtilis DK-W1, is able to suppress growth and BCET formation by a selected toxin producing strain (BC7-5) of B. cereus. It is concluded that the traditional way of making kinema and its culinary use in curries is safe. However, for novel applications of kinema, safety precautions are advisable. 相似文献
995.
The measurement of the accumulated phase error of phase-locked loops (PLLs) in microprocessor systems is discussed. A system which creates controlled power supply noise and measures the PLL response is described. Examples of the use of this technique are shown for a PLL used in a 400 MHz microprocessor 相似文献
996.
997.
998.
Experience with electrical diagnostic and high-potential tests, performed at the terminals of unshielded cables in older nuclear and thermal plants is presented. The three basic types of cable-low- and high-voltage power cables, control cables and instrumentation cables-that are important to the reliable operation of a generating station and large industrial plants are described, and their aging mechanisms are indicated. Dielectric testing of unshielded cables, the effect of grounding, field testing of station cables, and insulation characterization at low frequencies are discussed 相似文献
999.
The object-oriented approach to integrating knowledge data, and models is examined. Under the object-oriented paradigm, everything is an object, which is itself an encapsulation of data and methods to manipulate and access the data within the object. Thus knowledge, data, and models can be defined as special kinds of objects. Active knowledge/data language (KDL), a functional object-oriented database system is considered, covering the active-KDL-type system and functions, and query-driven simulation 相似文献
1000.
Sverak J.G. Wang W. Gervais Y. Do X.-D. Mukhedkar D. 《Power Delivery, IEEE Transactions on》1992,7(3):1196-1206
An improved multivariable, probabilistic method for the safety analysis of power grounding systems is presented. The application of this method is demonstrated in four examples. The first three examples illustrate the practical evaluation of one new and two existing substation grounding grids when the substation is connected to an overhead distribution system. The fourth example ascertains the relative impact of a difference between the S curve of IEC Publication 479 and Dalziel's equation for the allowable body current used in ANSI/IEEE Std 80-1986 on the validity of an exposure model based on the requirements of Std 80. In this example, the statistical data on ground faults have been taken from actual system records. Further possible refinements of the exposure model are discussed 相似文献