全文获取类型
收费全文 | 215532篇 |
免费 | 2446篇 |
国内免费 | 378篇 |
专业分类
电工技术 | 3182篇 |
综合类 | 754篇 |
化学工业 | 32853篇 |
金属工艺 | 10125篇 |
机械仪表 | 6800篇 |
建筑科学 | 4114篇 |
矿业工程 | 2289篇 |
能源动力 | 3419篇 |
轻工业 | 11936篇 |
水利工程 | 3396篇 |
石油天然气 | 8779篇 |
武器工业 | 18篇 |
无线电 | 16577篇 |
一般工业技术 | 42450篇 |
冶金工业 | 47796篇 |
原子能技术 | 7157篇 |
自动化技术 | 16711篇 |
出版年
2021年 | 1995篇 |
2019年 | 1863篇 |
2018年 | 3709篇 |
2017年 | 4055篇 |
2016年 | 4696篇 |
2015年 | 2647篇 |
2014年 | 3645篇 |
2013年 | 7300篇 |
2012年 | 6763篇 |
2011年 | 8257篇 |
2010年 | 5431篇 |
2009年 | 6035篇 |
2008年 | 5852篇 |
2007年 | 5742篇 |
2006年 | 4928篇 |
2005年 | 7238篇 |
2004年 | 6288篇 |
2003年 | 5653篇 |
2002年 | 4397篇 |
2001年 | 4265篇 |
2000年 | 3712篇 |
1999年 | 3857篇 |
1998年 | 12743篇 |
1997年 | 8671篇 |
1996年 | 6187篇 |
1995年 | 4307篇 |
1994年 | 3617篇 |
1993年 | 3744篇 |
1992年 | 2496篇 |
1991年 | 2597篇 |
1990年 | 2703篇 |
1989年 | 2584篇 |
1988年 | 2589篇 |
1987年 | 2395篇 |
1986年 | 2424篇 |
1985年 | 2453篇 |
1984年 | 2286篇 |
1983年 | 2225篇 |
1982年 | 2112篇 |
1981年 | 2368篇 |
1980年 | 2245篇 |
1979年 | 2328篇 |
1978年 | 2520篇 |
1977年 | 3013篇 |
1976年 | 4338篇 |
1975年 | 2334篇 |
1974年 | 2342篇 |
1973年 | 2385篇 |
1972年 | 2194篇 |
1971年 | 1956篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
Fahmy H.I. Develekos G. Douligeris C. 《Selected Areas in Communications, IEEE Journal on》1997,15(2):226-237
Communication network design is becoming increasingly complex, involving making networks more usable, affordable, and reliable. To help with this, we have proposed an expert network designer (END) for configuring, modeling, simulating, and evaluating large structured computer networks, employing artificial intelligence, knowledge representation, and network simulation tools. We present a neural network/knowledge acquisition machine-learning approach to improve the END's efficiency in solving the network design problem and to extend its scope to acquire new networking technologies, learn new network design techniques, and update the specifications of existing technologies 相似文献
72.
73.
C.H. Chu C.I. Hung Y.H. Wang M.P. Houng 《Photonics Technology Letters, IEEE》1997,9(9):1262-1264
We present a theoretical model for the dark current of bound-to-continuum quantum-well infrared photodetectors (QWIPs), by considering the field-induced mixing effect, tunneling rate and phonon scattering rate between bound and continuum states. Using this model, we can see clearly how these mechanisms significantly influence the Fermi levels of bound and continuum electrons, and thus, the dark current. Nonlinear temperature dependence of the dark current at low temperature is predicted and discussed in detail. The simulated dark currents exhibit good agreement with the experimental results, without use of parameter fitting techniques. 相似文献
74.
Yu. N. Denisyuk N. M. Ganzherli I. M. Maurer S. A. Pisarevskaya 《Technical Physics Letters》1997,23(4):279-280
A thick-layer light-sensitive material consisting of glycerin-containing bichromated gelatin has been prepared and used to
record real-time volume holograms with 0.44 μm helium-cadmium laser radiation. The thickness of the layers is 400–600 μm.
The holographic sensitivity of the material is 1 J/cm2. The highest diffraction efficiency of holograms recorded using a symmetric system with parallel beams is 32%.
Pis’ma Zh. Tekh. Fiz. 23, 62–65 (April 12, 1997) 相似文献
75.
76.
Space diversity reception and forward-error correction coding are powerful techniques to combat multipath fading encountered in mobile radio communications. In this paper, we analyze the performance of a discrete-time switched diversity system using trellis-coded modulation multiple phase-shift keying (TCM-MPSK) on slow, nonselective correlated Nakagami (1960) fading channels. Analytical upper bounds using the transfer function bounding technique are obtained and illustrated by several numerical examples. A simple integral expression for calculating the exact pairwise error probability is presented. The use of optimum adaptive and fixed switching thresholds is considered. Monte Carlo simulation results, which are more indicative of the exact system performance, are also given 相似文献
77.
T Mukasa Y Khoroku T Tsukahara MY Momoi I Kimura T Momoi 《Canadian Metallurgical Quarterly》1997,232(1):192-197
P19 EC cells undergoes apoptosis during neuronal differentiation induced by retinoic acid. Two CPP32-like proteases, CPP32 and Mch-3, are expressed in untreated and retinoic acid-treated P19 EC cells. CPP32-like activity is remarkably increased in apoptosis during neuronal differentiation of P19 EC cells. Inhibition of CPP32-like proteases prevents apoptosis, suggesting that activation of CPP32-like proteases play central roles in the apoptosis during neuronal differentiation of P19 EC cells. Wortmannin, PI-3K inhibitor, enhances the CPP32-like activity of the retinoic acid-treated P19 EC cells. PI-3K may be involved in the apoptosis during neuronal differentiation as negative regulator. 相似文献
78.
The sudden onset of hypertension mandates investigation for secondary causes. We report the case of a young man with a very rare systemic fungal disease that included massive unilateral perirenal adenopathy. Treatment was associated with the abrupt onset of severe hypertension. Imaging studies suggested progressive constriction of one kidney with treatment, presumably as a result of healing adenopathy. We suggest that this case is a new example of hypertension secondary to constrictive perinephritis (Page kidney). 相似文献
79.
80.
A Sawicka-Grzelak F Meisel-Miko?ajczyk SM S?otwińska M Wierzbicka 《Canadian Metallurgical Quarterly》1997,49(1-2):95-100
The aim of study was the evaluation of periodontal pockets microflora in patients with advanced periodontitis. From each subject 16-20 samples were taken using paper points. Pooled sample after 60 s. mixing was serially diluted in reduced BHI. For total cell counts and for the isolation of black pigmented anaerobes Brucella agar supplemented with 5% sheep blood, hemin, menadione, with and without Kanamycin-Vancomycin mixture and BM agar plates were used. For isolation of A. actinomycetemcomitans TSBV agar plates were used. Cultures were incubated in anaerobic chamber at 37 degrees C for 7 days and TSBV agar plates in an atmosphere of 95% air-5% CO2 at 37 degrees C for 5 days. Microorganisms were identified by Gram staining, colony morphology, fluorescence in UV-light, haemagglutination of 3% sheep erythrocytes, fermentation of sugars, production of indole, urease (API 20A), specific enzymes (Rapid ID 32A). Twenty seven subjects with clinically recognized periodontitis were examined. Microorganisms important in periodontitis were isolated from periodontal pockets of almost all examined subjects. The number of bacteria obtained from the sample of one patient ranged from 1 x 10(4) CFU/ml to 3,6 x 10(6) CFU/ml. Porphyromonas gingivalis was identified in the samples taken from 17 patients, Prevotella intermedia-19, Actinobacillus actinomycetemcomitans -11, Fusobacterium nucleatum-9, Peptostreptococcus spp.-22. 相似文献