全文获取类型
收费全文 | 602765篇 |
免费 | 8057篇 |
国内免费 | 1948篇 |
专业分类
电工技术 | 11320篇 |
综合类 | 657篇 |
化学工业 | 89755篇 |
金属工艺 | 21803篇 |
机械仪表 | 17990篇 |
建筑科学 | 15938篇 |
矿业工程 | 1813篇 |
能源动力 | 15726篇 |
轻工业 | 57956篇 |
水利工程 | 5211篇 |
石油天然气 | 5497篇 |
武器工业 | 81篇 |
无线电 | 78165篇 |
一般工业技术 | 112350篇 |
冶金工业 | 112666篇 |
原子能技术 | 9688篇 |
自动化技术 | 56154篇 |
出版年
2021年 | 4315篇 |
2019年 | 4088篇 |
2018年 | 6618篇 |
2017年 | 6762篇 |
2016年 | 7194篇 |
2015年 | 5236篇 |
2014年 | 8770篇 |
2013年 | 27640篇 |
2012年 | 14967篇 |
2011年 | 21010篇 |
2010年 | 16356篇 |
2009年 | 18631篇 |
2008年 | 19975篇 |
2007年 | 20277篇 |
2006年 | 18264篇 |
2005年 | 16796篇 |
2004年 | 16295篇 |
2003年 | 15827篇 |
2002年 | 15362篇 |
2001年 | 15322篇 |
2000年 | 14403篇 |
1999年 | 15082篇 |
1998年 | 34648篇 |
1997年 | 25146篇 |
1996年 | 19856篇 |
1995年 | 15501篇 |
1994年 | 13796篇 |
1993年 | 13337篇 |
1992年 | 10248篇 |
1991年 | 9703篇 |
1990年 | 9155篇 |
1989年 | 8778篇 |
1988年 | 8390篇 |
1987年 | 7258篇 |
1986年 | 7109篇 |
1985年 | 8581篇 |
1984年 | 8014篇 |
1983年 | 7105篇 |
1982年 | 6605篇 |
1981年 | 6669篇 |
1980年 | 6258篇 |
1979年 | 6051篇 |
1978年 | 5680篇 |
1977年 | 6846篇 |
1976年 | 8934篇 |
1975年 | 4933篇 |
1974年 | 4806篇 |
1973年 | 4731篇 |
1972年 | 3772篇 |
1971年 | 3405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.
An exact and simple method to evaluate the frequency reuse efficiency for the reverse link of cellular CDMA systems over a non-uniform traffic distribution is presented. This method is based on a traffic redistribution and decomposition process and on standard curves with which partial reuse efficiencies are obtained and conveniently added to give the final parameter 相似文献
996.
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
997.
998.
999.
BD Shenstone A Mahmoud R Woodward D Elvins R Palmer EF Ring AK Bhalla 《Canadian Metallurgical Quarterly》1994,33(6):541-545
A prospective longitudinal study of patients with early RA was performed to examine the influence of disease duration, disease activity and physical activity on bone loss. Sixty-seven patients with non-steroid treated RA of less than 5 yr duration, including 16 patients with disease duration less than 6 months, had BMD measurements of the femoral neck and the lumbar spine over a 12-month period using dual energy X-ray absorptiometry. The BMD changes were compared with values from 72 control patients and were also correlated with serial measurements of disease activity (measured by the Stoke Index) and disability [measured by the Health Assessment Questionnaire (HAQ) score], at 3-monthly intervals over the 12-month period. No significant differences in BMD changes were found between RA patients and controls overall. Patients with disease duration of less than 6 months had significantly greater loss of BMD at the femoral neck (-3.9%, S.E.M. 1.5) than the remainder of the cohort (-0.2%, S.E.M. 0.7) (P = 0.02) and controls (-0.8%, S.E.M. 0.6). Lumbar spine BMD changes correlated with the initial Stoke Index (Rs-0.373, P = 0.01) but not mean Stoke Indices. There was no correlation of BMD changes with age or HAQ scores. These findings suggest that significant bone loss occurs within the first few months of disease in patients with RA. 相似文献
1000.
B Conrad E Weidmann G Trucco WA Rudert R Behboo C Ricordi H Rodriquez-Rilo D Finegold M Trucco 《Canadian Metallurgical Quarterly》1994,371(6495):351-355
Insulin-dependent diabetes mellitus (IDDM) is a T-cell-mediated autoimmune disease whose onset is believed to be triggered by unknown environmental factors acting on a predisposing genetic background. Islet-infiltrating T (IIT) cells from two IDDM patients, who had died at the onset of the disease from brain swelling as a complication of ketoacidosis, were analysed. The results provided evidence for the involvement of a pancreatic islet cell membrane-bound superantigen as a diabetes aetiopathogenetic factor. There was a selective expansion of a T-cell receptor (TCR) variable segment of the beta-chain (V beta 7) in these IIT cells in association with unselected V alpha-chain segments; extensive junctional diversity of the TCR V beta 7 chains; and evidence of positive selection, after exposure to diabetic islet cell membrane preparations, of V beta 7+ T-cell clones among peripheral blood lymphocytes from non-diabetic individuals. 相似文献