全文获取类型
收费全文 | 572213篇 |
免费 | 26545篇 |
国内免费 | 12935篇 |
专业分类
电工技术 | 22156篇 |
技术理论 | 56篇 |
综合类 | 19934篇 |
化学工业 | 95578篇 |
金属工艺 | 27825篇 |
机械仪表 | 28512篇 |
建筑科学 | 31134篇 |
矿业工程 | 12347篇 |
能源动力 | 14869篇 |
轻工业 | 43289篇 |
水利工程 | 7719篇 |
石油天然气 | 28062篇 |
武器工业 | 2631篇 |
无线电 | 61301篇 |
一般工业技术 | 83993篇 |
冶金工业 | 63496篇 |
原子能技术 | 9146篇 |
自动化技术 | 59645篇 |
出版年
2023年 | 5519篇 |
2022年 | 10156篇 |
2021年 | 14614篇 |
2020年 | 11254篇 |
2019年 | 9990篇 |
2018年 | 12640篇 |
2017年 | 13763篇 |
2016年 | 12925篇 |
2015年 | 14411篇 |
2014年 | 19700篇 |
2013年 | 29282篇 |
2012年 | 26202篇 |
2011年 | 30620篇 |
2010年 | 25674篇 |
2009年 | 25484篇 |
2008年 | 25459篇 |
2007年 | 25003篇 |
2006年 | 25082篇 |
2005年 | 22088篇 |
2004年 | 16076篇 |
2003年 | 14658篇 |
2002年 | 13736篇 |
2001年 | 12981篇 |
2000年 | 13330篇 |
1999年 | 14893篇 |
1998年 | 20626篇 |
1997年 | 15611篇 |
1996年 | 13160篇 |
1995年 | 10615篇 |
1994年 | 9072篇 |
1993年 | 7659篇 |
1992年 | 6137篇 |
1991年 | 5324篇 |
1990年 | 5075篇 |
1989年 | 4690篇 |
1988年 | 4310篇 |
1987年 | 3566篇 |
1986年 | 3460篇 |
1985年 | 3707篇 |
1984年 | 3454篇 |
1983年 | 3090篇 |
1982年 | 2928篇 |
1981年 | 2945篇 |
1980年 | 2843篇 |
1979年 | 2689篇 |
1978年 | 2712篇 |
1977年 | 2983篇 |
1976年 | 3856篇 |
1975年 | 2387篇 |
1973年 | 2383篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
A study was undertaken to determine fertility in a group of females who as children had been operated on for appendicitis. The 134 women operated on for appendicitis were reviewed. Their ages ranged from 2-18 years at the time of appendectomy. Our data show that perforated appendicitis before puberty has little if any role in the etiology of tubal infertility. 相似文献
102.
对菱镁矿在固体燃料燃烧过程中的固硫特性进行了理论研究,所得结论对进一步将菱镁矿作为固体燃料燃烧过程中的固硫剂具有非常重要的意义。 相似文献
103.
Neviani A. Meneghesso G. Zanoni E. Hafizi M. Canali C. 《Electron Device Letters, IEEE》1997,18(12):619-621
Impact ionization is a major limiting factor to the maximum operating voltage of InGaAs-based, high-speed transistors. In this work, data on the positive temperature dependence of the electron impact ionization coefficient αn in In0.53Ga0.47As at medium-low electric fields are reported for the first time. The increase of αn with temperature is opposite to the behavior normally observed in most semiconductors. This anomalous behavior implies the onset of a positive feedback between power dissipation and avalanche generation which may adversely affect the power handling capability of In0.53Ga 0.47As-based devices, and which should be taken into account in device thermal modeling. In the experimental procedure, based on the measurement of the multiplication factor M-1 in npn In0.53Ga 0.47As/InP Heterojunction Bipolar Transistors (HBT), particular care has been taken in order to rule out possible spurious, temperature-dependent contributions to the measured multiplication current 相似文献
104.
This paper presents the design criteria, procedure, and implementation of a soft-switched power-factor-correction (PFC) circuit based on the extended-period quasi-resonant (EPQR) principles. All power electronic devices including switches and diodes in the circuit are fully soft switched. The design method is demonstrated in a prototype circuit. The operating principles are confirmed with computer simulation and experimental results. A comparison of the EP-QR operation and zero-voltage-transition (ZVT) pulse-width modulation (PWM) method 相似文献
105.
In the present study, attempts are made to extend the application of the mechanical model for the fatigue crack initiation (FCI) and the FCI life formula of metallic notched elements in laboratory air to those in the corrosive environment. The test results and analysis of the corrosion FCI (CFCI) life of aluminum alloys and Ti---6A1---4V show that the expression of the CFCI life obtained by modifying the FCI life formula in laboratory air can give a good fit to the test results of the CFCI life. The salt water (3.5% NaCl) environment has no effects on the CFCI resistant coefficient compared with the FCI resistant coefficient in laboratory air. However, 3.5% NaCl environment greatly decreases the CFCI threshold of aluminum alloy, but has little effect on the CFCI threshold of Ti---6A1---4V. The loading frequency ranging from 1 Hz to 10 Hz has no appreciable effect on the CFCI life, and thus, the CFCI threshold of aluminum alloys investigated. Hence, the expression for the CFCI life of metallic notched elements proposed in this study is a better one, which reveals a correlation between the CFCI life and the governing parameters, such as, the geometry of the notched elements, the nominal stress range, the stress ratio, the tensile properties and the CFCI threshold. However, this new expression of the CFCI life needs to be verified by more test results. 相似文献
106.
Douay M. Xie W.X. Taunay T. Bernage P. Niay P. Cordier P. Poumellec B. Dong L. Bayon J.F. Poignant H. Delevaque E. 《Lightwave Technology, Journal of》1997,15(8):1329-1342
A comprehensive survey of photosensitivity in silica glasses and optical fiber is reviewed. Recent work on understanding the mechanisms contributing to germanium or aluminum doped fiber photosensitivity is discussed within the framework of photoelastic densification models 相似文献
107.
108.
X. Wang S. B. Lambert 《Fatigue & Fracture of Engineering Materials & Structures》1997,20(12):1637-1655
Abstract— A Fourier series approach is proposed to calculate stress intensity factors using weight functions for semi-elliptical surface cracks in flat plates subjected to two-dimensional stress distributions. The weight functions were derived from reference stress intensity factors obtained by three-dimensional finite element analyses. The close form weight functions derived are suitable for the calculation of stress intensity factors for semi-elliptical surface cracks in flat plates under two-dimensional stress distributions with the crack aspect ratio in the range of 0.1 ≤ a/c ≤ 1 and relative depth in the range of 0 ≤ a/t ≤ 0.8. Solutions were verified using several two-dimensional non-linear stress distributions; the maximum difference being 6%. 相似文献
109.
Somatic hypermutation of the immunoglobulin variable genes during germinal reactions might permit the expansion of B-cell clones with unwanted (e.g. autoreactive) specificity. Here, Ernst Lindhout and colleagues propose three antigen-specific checkpoints that ensure the appropriate antigen specificity of activated B cells is maintained by regulating the activation, selection and further differentiation of B cells. 相似文献
110.
OBJECTIVE AND DESIGN: We reported previously that the betamethasone derivative betamethasone dipropionate behaves as an anti-glucocorticoid in rat endotoxin-induced uveitis (EIU). In the present study, we produced EIU in guinea pigs and investigated the effects of betamethasone dipropionate on the EIU. MATERIAL: Male Hartley guinea pigs were used. TREATMENT: Glucocorticoids were instilled into the eye. METHOD: To elicit EIU, lipopolysaccharide (LPS) was injected into the anterior chamber of the eye. Cell numbers in the aqueous humor after LPS injection were determined by flow cytometry. Prostaglandin E2 (PGE2) production after LPS injection into the anterior chamber was also examined. RESULTS: Intracameral injection of LPS (1 microgram/eye) induced cell infiltration into the anterior chamber and PGE2 production. Betamethasone dipropionate inhibited cell infiltration and PGE2 production more strongly than betamethasone. These results suggest that betamethasone dipropionate is a potent glucocorticoid in guinea pigs. CONCLUSIONS: Structure-activity relationships of glucocorticoids in the guinea pig EIL model may differ from those in the rat EIU model. 相似文献