全文获取类型
收费全文 | 229649篇 |
免费 | 5345篇 |
国内免费 | 836篇 |
专业分类
电工技术 | 3827篇 |
综合类 | 345篇 |
化学工业 | 37753篇 |
金属工艺 | 8831篇 |
机械仪表 | 6257篇 |
建筑科学 | 6113篇 |
矿业工程 | 1019篇 |
能源动力 | 6038篇 |
轻工业 | 24489篇 |
水利工程 | 2223篇 |
石油天然气 | 3363篇 |
武器工业 | 13篇 |
无线电 | 22870篇 |
一般工业技术 | 41619篇 |
冶金工业 | 45195篇 |
原子能技术 | 4562篇 |
自动化技术 | 21313篇 |
出版年
2021年 | 2649篇 |
2020年 | 1977篇 |
2019年 | 2487篇 |
2018年 | 3882篇 |
2017年 | 3898篇 |
2016年 | 4031篇 |
2015年 | 2974篇 |
2014年 | 4533篇 |
2013年 | 11293篇 |
2012年 | 7330篇 |
2011年 | 9314篇 |
2010年 | 7339篇 |
2009年 | 8062篇 |
2008年 | 7907篇 |
2007年 | 7756篇 |
2006年 | 6368篇 |
2005年 | 5995篇 |
2004年 | 5811篇 |
2003年 | 5394篇 |
2002年 | 5309篇 |
2001年 | 5105篇 |
2000年 | 4717篇 |
1999年 | 4858篇 |
1998年 | 14017篇 |
1997年 | 9854篇 |
1996年 | 7292篇 |
1995年 | 5320篇 |
1994年 | 4412篇 |
1993年 | 4477篇 |
1992年 | 2960篇 |
1991年 | 2955篇 |
1990年 | 2730篇 |
1989年 | 2749篇 |
1988年 | 2685篇 |
1987年 | 2386篇 |
1986年 | 2320篇 |
1985年 | 2649篇 |
1984年 | 2405篇 |
1983年 | 2170篇 |
1982年 | 2103篇 |
1981年 | 2178篇 |
1980年 | 2018篇 |
1979年 | 1956篇 |
1978年 | 1910篇 |
1977年 | 2473篇 |
1976年 | 3446篇 |
1975年 | 1670篇 |
1974年 | 1628篇 |
1973年 | 1711篇 |
1972年 | 1392篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
V. F. Shevchenko A. A. Petrov V. G. Petrov 《Journal of Infrared, Millimeter and Terahertz Waves》1993,14(9):1755-1768
A basic principle of pulse radar reflectometry is considered in this paper. A numerical analysis is performed in order to study errors appearing due to the expansion of a microwave pulse reflected from the plasma. A block diagram of the basic pulse radar scheme and its overall performance is presented. The first experimental results obtained on T11-M tokamak are discussed. An improved pulse radar scheme with cross-detection is considered. The results of bench test experiments and future applications of this scheme are discussed. 相似文献
992.
The GaSb layers investigated were grown directly on GaAs substrates by molecular beam epitaxy (MBE) using SnTe source as the
n-type dopant. By using admittance spectroscopy, a dominant deep level with the activation energy of 0.23-0.26 eV was observed
and its concentration was affected by the Sb4/Ga flux ratio in the MBE growth. A lowest deep-level concentration together with a highest mobility was obtained for GaSb
grown at 550°C under a Sb4/Ga beam equivalent pressure (BEP) ratio around 7, which should correspond to the lowest ratio to maintain a Sb-stabilized
surface reconstruction. In the Hall measurement, an analysis of the temperature-dependent mobility shows that the ionized
impurity concentration increases proportionally with the sample’s donor concentration, suggesting that the ionized impurity
was introduced by an SnTe source. In addition, optical properties of an undoped p-, a lightly and heavily SnTe-doped GaSb
layers were studied by comparing their photoluminescence spectra at 4.5K. 相似文献
993.
We investigate the error probability bit error rate (BER) of minimum shift keying (MSK) modulation with differential detection in a two-path fading channel without noise (error floor). We develop a new method for the computation of the BER: we show that errors occur if the phasors of the instantaneous impulse response fall into certain regions of the complex plane; then we average over the statistics of the phasors to arrive at the mean BER. With this method, we derive analytical expressions for the BER for arbitrary amplitude statistics of the paths. For the special case of two Rayleigh-fading paths with small delay, we find that the BER is proportional to the square of the mean delay spread (normalized to the bit length) if we sample between the two pulses. This proves the qualitative behavior of previous estimates, but our results allow also a more exact quantitative formulation. The quadratic dependence of the BER on the delay spread breaks down if we have one Rayleigh-fading and one Rician-fading path. We find that the bit combinations 1-11 and -11-1 do not lead to errors in the two-path model. However, additional Monte Carlo simulations show that these bit combinations do lead to errors in a three-path model 相似文献
994.
L Pezard JL Nandrino B Renault F el Massioui JF Allilaire J Müller F Varela J Martinerie 《Canadian Metallurgical Quarterly》1996,39(12):991-999
Mathematical models are helpful in the understanding of diseases through the use of dynamical indicators. A previous study has shown that brain activity can be characterized by a decrease of dynamical complexity in depressive subjects. The present paper confirms and extends these conclusions through the use of recent methodological advances: first episode and recurrent patients strongly differ in their dynamical response to therapeutic interventions. These results emphasize the need for clinical follow-ups to avoid recurrence and the necessity of specific therapeutic intervention in the case of recurrent patients. 相似文献
995.
BACKGROUND: A diaphragmatic phonomyogram (PMG) evoked by maximal phrenic nerve stimulation at end expiratory lung volume (FRC) has been previously described as a good index of changes in diaphragmatic contractility with fatigue. A study was undertaken to assess whether this conclusion could be extended to different lung volumes. METHODS: Diaphragmatic compound motor action potentials (CMAPs) were recorded on each side of the chest by the means of surface electrodes placed over the eight intercostal spaces in five healthy subjects. Diaphragmatic PMGs from both sides were recorded with condenser microphones fixed to the skin close to the CMAP recording electrodes. Oesophageal and gastric balloon tipped catheters were employed to measure transdiaphragmatic pressure twitches (TwPDI) which served as the standard measure of changes in diaphragmatic contractility. PMG and TwPDI responses were compared at different lung volumes over inspiratory capacity both before and after fatiguing inspiratory resistive loading. RESULTS: No consistent relationship was found in different subjects or on different days in the same subject between PMG and lung volume or between PMG and TwPDI. However, the PMG:CMAP ratio from both sides at any given lung volume decreased after fatigue in roughly the same proportion as the TwPDI. CONCLUSIONS: These results show that, although PMG can detect changes in diaphragmatic contractility caused by fatigue in normal subjects, lung volume changes need to be controlled and each subject should serve as his or her own control. 相似文献
996.
EJ Velthuis-te Wierik H van den Berg JA Weststrate KH van het Hof C de Graaf 《Canadian Metallurgical Quarterly》1996,50(4):214-219
Plasma levels of fibrinogen, factor VIIc and prothrombin fragment F1 + 2, a marker of thrombin generation in vivo, were studied in 68 subjects with serum total cholesterol (TC) levels between 135 and 349 mg/dl but without clinical evidence of cardiovascular disease and other atherosclerotic risk factors. F1 + 2 plasma levels were directly correlated with TC (p < 0.0004), low-density lipoprotein cholesterol (LDL-C; p < 0.0018) and factor VIIc (p < 0.024). Thirty-five subjects with TC greater than 249 mg/dl (median value of the whole group) showed higher levels of F1 + 2 (p < 0.0001) and fibrinogen (p < 0.0015) than those with TC lower than 249 mg/dl. In subjects with TC > 249 mg/dl and F1 + 2 > 1.2 nM (median value of the whole group), a cholesterol-lowering drug (simvastatin) was able to reduce F1 + 2 (p < 0.009) as well as TC and LDL-C. This study shows a relationship between serum cholesterol and the rate of thrombin generation supporting the hypothesis that a hypercoagulable state may occur in hypercholesterolemic subjects before the onset of clinical evidence of atherosclerotic cardiovascular disease. 相似文献
997.
Experiments were carried out in a two pack system; polyvinyl butyral, zinc tetroxychromate in one pack, and phosphoric acid in the other with an isopropanol solvent in both packs. This study examines the functions of the phosphoric acid and the chromate containing pigment separately by utilising the feature that the alcohol dispersed components have sufficient conductivity to carry out potential-time measurements in the wet components before and during mixing. 相似文献
998.
R Gouider E LeGuern J Emile S Tardieu F Cabon M Samid J Weissenbach Y Agid P Bouche A Brice 《Canadian Metallurgical Quarterly》1994,44(12):2250-2252
Fistulas secondary to Crohn's disease occur in about 33% of patients. A colobronchial fistula complicating Crohn's disease is very rare, having been reported only twice previously. We present an unusual fistula secondary to Crohn's colitis that originated from the splenic flexure and crossed the diaphragm to involve the bronchial tree. 相似文献
999.
1000.
Cadiou J.F. Guena J. Penard E. Legaud P. Minot C. Palmier J.F. Le Person H. Harmand J.E. 《Electronics letters》1994,30(20):1690-1691
For the first time, a direct optically injection-locked oscillator based on the Esaki-Tsu effect in superlattices is reported. The microwave output power is -7 dBm at 20 GHz with an optical input power of -10 dBm at a wavelength of 1.3 μm 相似文献