全文获取类型
收费全文 | 251726篇 |
免费 | 4342篇 |
国内免费 | 1032篇 |
专业分类
电工技术 | 4329篇 |
综合类 | 203篇 |
化学工业 | 37029篇 |
金属工艺 | 8218篇 |
机械仪表 | 7267篇 |
建筑科学 | 6807篇 |
矿业工程 | 543篇 |
能源动力 | 7239篇 |
轻工业 | 28828篇 |
水利工程 | 2119篇 |
石油天然气 | 1852篇 |
武器工业 | 5篇 |
无线电 | 33388篇 |
一般工业技术 | 46230篇 |
冶金工业 | 46474篇 |
原子能技术 | 3366篇 |
自动化技术 | 23203篇 |
出版年
2021年 | 1818篇 |
2020年 | 1347篇 |
2019年 | 1672篇 |
2018年 | 2580篇 |
2017年 | 2597篇 |
2016年 | 2894篇 |
2015年 | 2180篇 |
2014年 | 3683篇 |
2013年 | 11974篇 |
2012年 | 6784篇 |
2011年 | 9695篇 |
2010年 | 7470篇 |
2009年 | 8422篇 |
2008年 | 8941篇 |
2007年 | 8949篇 |
2006年 | 8298篇 |
2005年 | 7508篇 |
2004年 | 7226篇 |
2003年 | 7110篇 |
2002年 | 6609篇 |
2001年 | 7028篇 |
2000年 | 6328篇 |
1999年 | 6825篇 |
1998年 | 16385篇 |
1997年 | 11374篇 |
1996年 | 8586篇 |
1995年 | 6548篇 |
1994年 | 5710篇 |
1993年 | 5567篇 |
1992年 | 3997篇 |
1991年 | 3774篇 |
1990年 | 3553篇 |
1989年 | 3369篇 |
1988年 | 3277篇 |
1987年 | 2591篇 |
1986年 | 2486篇 |
1985年 | 3156篇 |
1984年 | 2797篇 |
1983年 | 2560篇 |
1982年 | 2346篇 |
1981年 | 2390篇 |
1980年 | 2200篇 |
1979年 | 2048篇 |
1978年 | 1880篇 |
1977年 | 2177篇 |
1976年 | 2725篇 |
1975年 | 1584篇 |
1974年 | 1489篇 |
1973年 | 1558篇 |
1972年 | 1144篇 |
排序方式: 共有10000条查询结果,搜索用时 984 毫秒
251.
Investigation into polishing process of CVD diamond films 总被引:1,自引:0,他引:1
A new technique used for polishing chemical vapor deposition (CVD) diamond films has been investigated, by which rough polishing of the CVD diamond films can be achieved efficiently. A CVD diamond film is coated with a thin layer of electrically conductive material in advance, and then electro-discharge machining (EDM) is used to machine the coated surface. As a result, peaks on the surface of the diamond film are removed rapidly. During machining, graphitization of diamond enables the EDM process to continue. The single pulse discharge shows that the material of the coated layer evidently affects removal behavior of the CVD diamond films. Compared with the machining of ordinary metal materials, the process of EDM CVD diamond films possesses a quite different characteristic. The removal mechanism of the CVD diamond films is discussed. 相似文献
252.
Younan N.H. Cox B.I. Taylor C.D. Prather W.D. 《Electromagnetic Compatibility, IEEE Transactions on》1994,36(4):394-398
Efficient numerical solution techniques have been developed and used to examine the electromagnetic fields that can be developed in the working volume of the CW Ellipticus antenna operated at frequencies from 100 kHz to 1 GHz. An exponentially tapered transition section is designed to obtain the desired illumination pattern in the working volume. The input transition section is needed for impedance matching and to drive efficiently the Ellipticus antenna. A parametric study is performed to ascertain the performance of the Ellipticus antenna for frequencies up to 1 GHz 相似文献
253.
254.
Oleuropein Inhibits LDL Oxidation Induced by Cooking Oil Frying By-products and Platelet Aggregation Induced by Platelet-Activating Factor 总被引:1,自引:0,他引:1
Oleuropein, a polyphenolic glucoside, constituent of olive fruit and oil, has been previously shown to exhibit, in vitro, potent properties as a biological antioxidant. In the present paper, two new properties are demonstrated in vitro as well, concerning its vascular protection activity, against the atheromatic lesion formation and its blood antithrombotic effect. Firstly, it was found that the oxidation of low-density lipoproteins (LDL), induced by different quantities of total polar materials (TPM), was inhibited by 10 μ M (final concentration) of oleuropein, revealing approx. 50% protection. Secondly, platelet-rich human plasma aggregation, induced by either the platelet-activating factor or adenosine diphospate or arachidonic acid, was inhibited by oleuropein. The 10 μ M (final concentration) of oleuropein was proven to be the most effective among the tested concentrations against LDL oxidation, while an IC50=0.41 mM was calculated for its inhibitory activity against platelet aggregation induced by 0.7 μ M platelet-activating factor. 相似文献
255.
McDonald K.C. Zimmermann R. Kimball J.S. 《Geoscience and Remote Sensing, IEEE Transactions on》2002,40(9):2063-2082
Spatial and temporal variations in vegetation dielectric properties strongly influence the microwave backscatter characteristics of forested landscapes. This paper examines the relationship between xylem tissue dielectric constant, xylem sap flux density, and xylem sap chemical composition as measured in the stems of two Norway Spruce (Picea abies [L.] Karst.) trees in the Fichtelgebirge region of Northern Bavaria, Germany. Dielectric constant and xylem sap flux were monitored continuously from June through October 1995, at several heights along the tree trunks. At the end of the measurement series, each tree was harvested, and its xylem sap extracted and analyzed to determine the concentrations of amino acids and cations. Results show that the sap flux density was correlated with vapor pressure deficit (VPD) at all heights in the stem. In contrast, the xylem tissue dielectric constant is influenced by VPD but can exhibit a significant temporal lag relative to changes in VPD. This lag varies with position along the tree trunk. The temporal variability of the dielectric constant is compared with both trees at several positions along the tree trunks. Results of xylem sap chemical analysis are presented. We show that spatial and temporal variability in the xylem tissue dielectric constant is influenced not only by water content, but by variations in xylem sap chemistry as well. This has important implications for microwave remote sensing of forested landscapes, as useful information may be acquired regarding stand physiology and water relations and where variations in dielectric properties within individual trees and across geographic areas can be significant error sources for forest inventory mapping. 相似文献
256.
MB Heyman JM LaBerge KA Somberg P Rosenthal C Mudge EJ Ring JD Snyder 《Canadian Metallurgical Quarterly》1997,131(6):914-919
The transjugular intrahepatic portosystemic shunt procedure is an accepted treatment for adults with complications of portal hypertension. We performed a retrospective review of all pediatric TIPS placements performed at the University of California, San Francisco between 1990 and 1996. Twelve procedures were attempted in nine children, with a mean age (+/- SD) of 9.4 +/- 3.9 years (range, 5 to 15 years) and a mean weight of 31 +/- 18 kg (range, 16 to 70 kg). The indications for TIPS placement were portal hypertension complicated by chronic variceal hemorrhage not controlled with sclerotherapy (n = 7) and hypersplenism with thrombocytopenia (n = 2). TIPS placement was successfully completed initially in seven of nine (78%) patients. Unfavorable vascular anatomy was the cause of failure in two cases. The seven patients who underwent successful TIPS placement were followed up for an average of 136 days (range, 1 to 800 days); two still have patent shunts, three underwent liver transplantation, one had a splenorenal shunt after stenosis, and one died of underlying liver disease. Variceal bleeding was controlled in four of five patients who successfully underwent TIPS placement. Shunt occlusion occurred in four patients; patency was restored by transjugular shunt revision in three, and a splenorenal shunt was performed in one. 相似文献
257.
C Harro GL Braden AB Morris GS Lipkowitz RL Madden 《Canadian Metallurgical Quarterly》1997,24(5):955-957
Nontuberculous mycobacteria are increasingly recognized as important pathogens in peritonitis associated with continuous ambulatory peritoneal dialysis (CAPD). Mycobacterium gordonae rarely causes human infection and is the least likely mycobacterium to produce clinical infection in CAPD patients. We describe a patient with persistent M. gordonae peritonitis acquired while undergoing CAPD. During 18 months of treatment, clinical improvement occurred but a microbiological cure could not be achieved. Principles of therapy for mycobacterial peritonitis developing during CAPD are reviewed, and potential explanations for our patient's failure to respond to therapy are discussed. 相似文献
258.
A modification to the `shape-invariant' sinusoidal speech model is proposed, whereby the phases of the component sinewaves used for the excitation are made to add coherently at each glottal closure. Applied to pitch and time-scale modification, higher quality synthetic speech is produced when large changes are required 相似文献
259.
Parametric DC measurements on pseudomorphic AlGaAs/InGaAs modulation-doped field-effect transistors (MODFETs) were carried out over the 300-405 K temperature range. A gradual channel device model was developed to simulate the temperature dependent behavior and assist in the interpretation of the characteristics. The simulations are shown to provide good predictive ability and confirm the physical reasons why the zero temperature coefficient point of a MODFET occurs only for gate bias voltages below the threshold voltage 相似文献
260.