全文获取类型
收费全文 | 614824篇 |
免费 | 6881篇 |
国内免费 | 1573篇 |
专业分类
电工技术 | 11057篇 |
综合类 | 465篇 |
化学工业 | 92241篇 |
金属工艺 | 31097篇 |
机械仪表 | 21800篇 |
建筑科学 | 12424篇 |
矿业工程 | 5247篇 |
能源动力 | 14715篇 |
轻工业 | 36557篇 |
水利工程 | 7837篇 |
石油天然气 | 17277篇 |
武器工业 | 36篇 |
无线电 | 65858篇 |
一般工业技术 | 136022篇 |
冶金工业 | 100517篇 |
原子能技术 | 16740篇 |
自动化技术 | 53388篇 |
出版年
2021年 | 5544篇 |
2020年 | 4276篇 |
2019年 | 5433篇 |
2018年 | 13987篇 |
2017年 | 14252篇 |
2016年 | 13313篇 |
2015年 | 6506篇 |
2014年 | 10639篇 |
2013年 | 26923篇 |
2012年 | 17562篇 |
2011年 | 25406篇 |
2010年 | 21153篇 |
2009年 | 22342篇 |
2008年 | 23110篇 |
2007年 | 23771篇 |
2006年 | 16749篇 |
2005年 | 16457篇 |
2004年 | 15302篇 |
2003年 | 15070篇 |
2002年 | 13818篇 |
2001年 | 13255篇 |
2000年 | 12709篇 |
1999年 | 12593篇 |
1998年 | 29754篇 |
1997年 | 21098篇 |
1996年 | 16122篇 |
1995年 | 12235篇 |
1994年 | 10924篇 |
1993年 | 11052篇 |
1992年 | 8382篇 |
1991年 | 8062篇 |
1990年 | 8128篇 |
1989年 | 7686篇 |
1988年 | 7374篇 |
1987年 | 6733篇 |
1986年 | 6520篇 |
1985年 | 7222篇 |
1984年 | 6781篇 |
1983年 | 6241篇 |
1982年 | 5790篇 |
1981年 | 5908篇 |
1980年 | 5647篇 |
1979年 | 5771篇 |
1978年 | 5879篇 |
1977年 | 6356篇 |
1976年 | 7794篇 |
1975年 | 5206篇 |
1974年 | 5117篇 |
1973年 | 5218篇 |
1972年 | 4546篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Dr. P. S. Dineva-Vladikova Dr. P. Borejko Prof. L. Hadjikov Prof. F. Ziegler 《Acta Mechanica》1996,115(1-4):203-211
Summary The two-dimensional problem of wave propagation in an elastic half-space is studied by the DBIEM (Direct Boundary Integral Equation Method) combined with the finite difference procedure applied to the time variable. The present hybrid formulation employs the fundamental solution depending neither on the frequency nor on the time variable. Time records of surface responses of the half-space are computed and compared with those obtained by the numerical evaluation of exact analytical solutions to this problem. 相似文献
992.
993.
This article describes a study of delamination growth along 0 °/0 °, 0 °/ 5 °, ± 5 °, and 0 °/90 ° interfaces sandwiched between unidirectional carbon fibre/epoxy composite material. Relationships between damage criticality, growth rate and acoustic emission activity for delamination growth have been studied and the transferability of results from laboratory coupons to composite structural elements has been examined. Two types of coupon tests, conventional delamination beams and rigidly loaded single edge notched strips, have been compared for different mode ratios. Comparative tests have been made on buckling-induced delamination in plates. A graphite crack gauge has been used to measure delamination length and growth rate, ranging from 0.05–2000 m/s. Damage growth was also followed using visual, ultrasonic C-scan, X-ray radiography, macro-video and acoustic emission measurements. Empirical evaluations of interlaminar toughness for delamination beams are made using the Irwin-Kies relation. Unstable growth is analysed using elasto-dynamic moving finite elements. Bucklinginduced delamination is analysed using plate/shell FE methods with growth/remeshing algorithms. 相似文献
994.
O. V. Denisenko 《Measurement Techniques》1996,39(11):1092-1094
A method is considered for increasing the immunity from interference of broadband systems for measuring and transmitting information.
It is based on utilizing nonlinear methods to process spread-spectrum signals. The possibility is investigated of using a
quasioptimal receiving device which can be implemented in practice. It is shown that the proposed method makes it possible
to solve the rejection problem at the physical level.
Translated from Izmeritel'naya Tekhnika, No. 11, pp. 13–15, November, 1996. 相似文献
995.
We consider a class of linear infinite-dimensional systems subject to boundary uncertainties, which are linear and time-invariant. Using a deterministic approach to the design of stabilizing feedback controls, uniform exponential stability of the zero state is achieved. Lyapunov techniques are combined with Datko or Ichikawa theory. Three illustrative examples are presented. 相似文献
996.
Nitric oxide reduction and carbon monoxide oxidation over carbon-supported copper-chromium catalysts 总被引:1,自引:0,他引:1
S. Stegenga R. van Soest F. Kapteijn J. A. Moulijn 《Applied catalysis. B, Environmental》1993,2(4):257-275
Carbon supported copper-chromium catalysts are shown to be very active for both the reduction of nitric oxide with carbon monoxide and the oxidation of carbon monoxide with oxygen. Mixed copper-chromium oxide active phases have good activity in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases. The influence of several catalyst variables has been investigated. The activity per volume of catalyst increases with increasing loading, while the intrinsic activity shows a maximum around C/M=100−50. An optimum catalyst for nitric oxide reduction and carbon monoxide oxidation has a copper/chromium ratio of 2/1. The apparent activation energy for the carbon monoxide oxidation over carbon supported copper-chromium catalysts is 77 kJ/mol, suggesting that the Cu---O bond rupture is the rate-limiting process. The reduction of nitric oxide takes place at higher temperatures. Since all catalysts have a low selectivity for molecular nitrogen formation at lower temperatures, the dissociation of nitric oxide is probably rate determining, resulting in a slightly reduced catalyst system. In an excess of carbon monoxide the reaction is first-order in nitric oxide and zero-order in carbon monoxide. Moisture inhibits the reaction by reversible competitive adsorption, whereas carbon dioxide does not. Oxygen completely inhibits the reduction of nitric oxide due to the more rapid reoxidation of the catalytic sites compared to nitric oxide. Therefore, the reduction of nitric oxide takes place only when all oxygen has been converted and, hence, is shifted to higher temperatures. As a possible consequence, the production of nitrous oxide is reduced. Nitric oxide and molecular oxygen react preferentially with carbon monoxide, so, in an excess of oxidizing component, gasification of the carbon support occurs at higher temperatures after carbon monoxide has been completely consumed. 相似文献
997.
A 4*4 directional coupler switch matrix is developed which uses, for the first time, the quantum confined Stark effect of InGaAlAs/InAlAs multiquantum well structures. The rearrangeable nonblocking 4*4 network with six 2*2 switches is shown to be perfectly functional with switching voltages between 5 and 6 V and crosstalk below -17 dB in all the operation states.<> 相似文献
998.
The noise characteristics of semiconductor laser amplifiers (SLAs) in the Ga1-xInxAs/GaInAsP/InP strained quantum well (QW) system are theoretically calculated and analyzed using density-matrix theory and taking into account the effects of band mixing on both the valence subbands and the transition dipole moments. The numerical results show that a reduced noise figure can be obtained in both tensile and compressively strained QW structures due to the increase in differential gain and the decrease in transparent carrier density. From a comparison among compressively strained (x=0.70), unstrained (x=0.53), and tensile strained (x=0.40) QW SLAs at a fixed carrier density and optical confinement factor, it is found that the noise figure of the tensile strained QW reaches its lowest value of 3.4 dB at average input optical power of -20 dB 相似文献
999.
R.N. ARNOLD K.K. SCHELLER S.C. ARP S.N. WILLIAMS D.M. SCHAEFER 《Journal of food science》1993,58(1):28-33
The effect of long-term feeding (252 days) of three supplemental levels of α-tocopheryl acetate (actual 0, 360 and 1290 IU/head/day) on meat quality was evaluated in Holstein and beef breed steers. Tissue vitamin E concentrations were increased by each increment of supplementation. The color display life of fresh beef under simulated retail conditions was extended 2 to 5 days by vitamin E and lipid oxidation was markedly reduced. Microbial population was not affected by level of supplementation. No deterioration in sensory quality occurred for steaks from supplemented steers that were displayed until the time steaks from unsupplemented steers discolored. 相似文献
1000.
S Harino M Motokura N Nishikawa M Fukuda A Sasaoka JE Grunwald 《Canadian Metallurgical Quarterly》1994,117(3):302-307
We studied the ocular findings of two adult patients with the Eisenmenger's syndrome who had atrial septal defects that were diagnosed before the age of 10 years but not operated on and pulmonary hypertension. Both eyes of these patients showed microaneurysms, multiple small blot hemorrhages, or capillary dilation in the temporal peripheral fundus. Multiple microaneurysms and retinal collaterals were confirmed by fluorescein angiography. One of the patients developed bilateral rubeosis iridis with slow progression. These retinal lesions and the rubeosis iridis are probably related to chronic ocular ischemia caused by chronic systemic hypoxia. 相似文献