全文获取类型
收费全文 | 13497篇 |
免费 | 477篇 |
国内免费 | 69篇 |
专业分类
电工技术 | 266篇 |
综合类 | 20篇 |
化学工业 | 2439篇 |
金属工艺 | 379篇 |
机械仪表 | 390篇 |
建筑科学 | 235篇 |
矿业工程 | 73篇 |
能源动力 | 746篇 |
轻工业 | 1205篇 |
水利工程 | 195篇 |
石油天然气 | 84篇 |
无线电 | 1723篇 |
一般工业技术 | 2952篇 |
冶金工业 | 1573篇 |
原子能技术 | 201篇 |
自动化技术 | 1562篇 |
出版年
2024年 | 47篇 |
2023年 | 197篇 |
2022年 | 541篇 |
2021年 | 547篇 |
2020年 | 445篇 |
2019年 | 451篇 |
2018年 | 603篇 |
2017年 | 548篇 |
2016年 | 510篇 |
2015年 | 304篇 |
2014年 | 477篇 |
2013年 | 936篇 |
2012年 | 549篇 |
2011年 | 727篇 |
2010年 | 584篇 |
2009年 | 555篇 |
2008年 | 502篇 |
2007年 | 414篇 |
2006年 | 356篇 |
2005年 | 280篇 |
2004年 | 256篇 |
2003年 | 230篇 |
2002年 | 187篇 |
2001年 | 154篇 |
2000年 | 151篇 |
1999年 | 162篇 |
1998年 | 360篇 |
1997年 | 237篇 |
1996年 | 257篇 |
1995年 | 223篇 |
1994年 | 189篇 |
1993年 | 177篇 |
1992年 | 148篇 |
1991年 | 158篇 |
1990年 | 116篇 |
1989年 | 111篇 |
1988年 | 136篇 |
1987年 | 113篇 |
1986年 | 91篇 |
1985年 | 116篇 |
1984年 | 97篇 |
1983年 | 115篇 |
1982年 | 100篇 |
1981年 | 90篇 |
1980年 | 81篇 |
1979年 | 55篇 |
1978年 | 46篇 |
1977年 | 58篇 |
1976年 | 77篇 |
1975年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
The compositions (1 −x)Ag2SO4−(x)BaSO4, wherex=0·01 to 0·6, were prepared by slow cooling of the melt. The extent of the solid solubility of Ba2+ in Ag2SO4 was determined by X-ray powder diffraction and scanning electron microscopy. The bulk conductivity of each sample was obtained
using a detailed impedance analysis. The partial substitution of Ba2+ results in the enhancement of conductivity in compliance with the classical aliovalent doping theory. A simplistic model
based on lattice distortion (expansion) due to partial substitution of Ag+ by the bigger Ba2+ has been considered to explain enhanced conductivity. Beyond solid-solubility limit (5·27 mole%) the BaSO4-dispersed Ag2SO4 conductivity follows the usual trend seen in binary systems. An increase in conductivity in this case is discussed in the
light of interfacial reactions and surface defect chemistry. The maximum conductivity in 20 mole% BaSO4 dispersed Ag2SO4 is due to percolation threshold. 相似文献
82.
Naber J.F. Singh H.P. Tanis W.J. Koshar A.J. Segalla G.L. 《Solid-State Circuits, IEEE Journal of》1992,27(10):1327-1331
An indirect, phase-locked-loop (PLL), GaAs-enhanced frequency synthesizer with 700 ns loop settling time has been developed. The two-chip GaAs insertion reduced the size of an existing synthesizer from 90 in3 to only 30 in3. The 6.0 in×5.5 in.×0.9 in module contains a 400 gate GaAs programmable divider and a sample and hold (S/H) which improved the settling time by 77% and reduced the size by 67% over the current state-of-the-art synthesizer. Furthermore, the divider reduced power dissipation by 9.7 W and the S/H reduced power dissipation by 1.3 W 相似文献
83.
84.
A new seismic support device and its application in piping systems is described. The device, E-BAR (patented), can be cost effectively used for snubber replacement programs, mitigation of hydraulic transients, pipe whip and as a thermal stop. The device has pre-set gaps to allow free thermal movement. During a seismic or other dynamic load event, if the pipe movement exceeds the gap dimension, the device acts as an elastic or elastic-plastic restraint. The device also has a unique design feature for not exceeding the restraint force beyond a specified limit design value. To analyze piping systems with gap supports having elastic-plastic characteristics, modal analysis procedures for both response spectrum and time history methods are developed. The comparison of responses obtained from the procedures with nonlinear time history analysis and test results available in the literature shows excellent correlation. A pilot program conducted for snubber replacement with E-BARs demonstrates that the limit force feature of E-BAR makes them very attractive for snubber replacement. This is because a particular E-BAR with a specified limit design force can be selected, such that, the E-BAR replacing the snubber does not require any modifications be made to the existing support steel and hardware. 相似文献
85.
Xie K. Zhao J.H. Flemish J.R. Burke T. Buchwald W.R. Lorenzo G. Singh H. 《Electron Device Letters, IEEE》1996,17(3):142-144
A 6H-SiC thyristor has been fabricated and characterized. A forward breakover voltage close to 100 V and a pulse switched current density of 5200 A/cm2 have been demonstrated. The thyristor is shown to operate under pulse gate triggering for turn-on and turn-off, with a rise time of 43 ns and a fall time of less than 100 ns. The forward breakover voltage is found to decrease by only 4% when the operating temperature is increased from room temperature to 300°C. It is found that anode ohmic contact resistance dominates the device forward drop at high current densities 相似文献
86.
87.
RJ Wilkinson K Hasl?v R Rappuoli F Giovannoni PR Narayanan CR Desai HM Vordermeier J Paulsen G Pasvol J Ivanyi M Singh 《Canadian Metallurgical Quarterly》1997,35(3):553-557
The diagnosis of infection caused by Mycobacterium tuberculosis is of increased public health concern following increases in the number of cases in developed countries and major increases in developing countries associated with the spread of human immunodeficiency virus (HIV) infection. The specificity of purified protein derivative skin testing for the detection of infection is compromised by exposure to environmental mycobacteria. Examination of sputum detects the most infectious patients, but not those with extrapulmonary disease. The 38-kDa antigen of M. tuberculosis contains two M. tuberculosis-specific B-cell epitopes. We overexpressed the gene for this antigen in Escherichia coli and evaluated the recombinant product in in vitro assays of T-cell function and as a target for the antibody response in humans. The sensitivity and specificity of the antigen as a skin test reagent were also assessed in outbred guinea pigs. We found that 69% of healthy sensitized humans recognize the antigen in vitro, as manifested by both cell proliferation and the production of gamma interferon. Untreated patients initially have a lower frequency of response (38%); this recovers to 72% during therapy. A total of 292 patients (20 with HIV coinfection) and 58 controls were examined for production of antibody to the 38-kDa antigen by using a commercially available kit. The sensitivity of the test in comparison with that of culture was 72.6%, and the specificity was 94.9%. The antigen was also tested for its ability to induce skin reactions in outbred guinea pigs sensitized by various mycobacterial species. The antigen provoked significant skin reactions in M. tuberculosis-, M. bovis BCG-, and M. intracellulare-sensitized animals. The significance of these findings and the usefulness of this antigen in immunodiagnosis are discussed. 相似文献
88.
89.
A two-dimensional cross-section finite difference model is presented to simulate density dependent leachate migration in leaky aquifers. Unlike existing models, a new approach is adopted to couple the groundwater-flow equation and the hydrodynamic dispersion equation with the elimination of the intermediate step of calculating velocities. The concept of the reference density is employed, permitting increased accuracy (over pressure-based models) in the representation of the transport process. The model is then used to study the effect of several hydraulic and transport parameters on the flow pattern and plume migration which are found to be very sensitive to most of these parameters. Equiconcentration and equipotential lines are overlapped to provide a better understanding of the coupling effect. 相似文献
90.
The effect of interface-roughness-related disorder on the electronic and optoelectronic properties of a quantum wire structure are studied. It is seen that the disorder causes strong localization in the quasi-one-dimensional system. While the electronic states are seriously perturbed, the density of states is not affected drastically. Optoelectronic properties as reflected in the interband transition related phenomenon are not found to suffer significant deterioration as a result of the disorder. However, the results suggest that intraband relaxation processes may be seriously affected because of electron (hole) states being localized in different regions of the wire 相似文献