全文获取类型
收费全文 | 89653篇 |
免费 | 11420篇 |
国内免费 | 4648篇 |
专业分类
电工技术 | 12608篇 |
技术理论 | 13篇 |
综合类 | 6117篇 |
化学工业 | 9173篇 |
金属工艺 | 12928篇 |
机械仪表 | 4989篇 |
建筑科学 | 10607篇 |
矿业工程 | 2396篇 |
能源动力 | 12901篇 |
轻工业 | 2210篇 |
水利工程 | 1501篇 |
石油天然气 | 3380篇 |
武器工业 | 759篇 |
无线电 | 5749篇 |
一般工业技术 | 9468篇 |
冶金工业 | 4517篇 |
原子能技术 | 1326篇 |
自动化技术 | 5079篇 |
出版年
2024年 | 581篇 |
2023年 | 1897篇 |
2022年 | 3043篇 |
2021年 | 3281篇 |
2020年 | 3567篇 |
2019年 | 3265篇 |
2018年 | 2836篇 |
2017年 | 3550篇 |
2016年 | 3921篇 |
2015年 | 3724篇 |
2014年 | 6230篇 |
2013年 | 6206篇 |
2012年 | 6471篇 |
2011年 | 7168篇 |
2010年 | 5508篇 |
2009年 | 5486篇 |
2008年 | 4913篇 |
2007年 | 5701篇 |
2006年 | 4815篇 |
2005年 | 3711篇 |
2004年 | 3263篇 |
2003年 | 2736篇 |
2002年 | 2410篇 |
2001年 | 2077篇 |
2000年 | 1776篇 |
1999年 | 1395篇 |
1998年 | 1120篇 |
1997年 | 918篇 |
1996年 | 836篇 |
1995年 | 613篇 |
1994年 | 525篇 |
1993年 | 387篇 |
1992年 | 310篇 |
1991年 | 277篇 |
1990年 | 231篇 |
1989年 | 197篇 |
1988年 | 142篇 |
1987年 | 108篇 |
1986年 | 69篇 |
1985年 | 96篇 |
1984年 | 83篇 |
1983年 | 52篇 |
1982年 | 71篇 |
1981年 | 34篇 |
1980年 | 41篇 |
1979年 | 25篇 |
1978年 | 13篇 |
1977年 | 11篇 |
1959年 | 5篇 |
1951年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The magnetic properties of polyethersulfone-matrix composites with 3-19 vol.% polycrystalline nickel filaments (0.4 (im diam)
were investigated. These filaments were found to exhibit hysteresis energy loss 10800 J/m3 of nickel and coercive force 16.9 kA/m, compared to corresponding values of 4930 J/m3 and 4.7 kA/m for 2 μ.m diam polycrystalline nickel fibers, 1020 J/m3 and 0.5 kA/m for 20 μm diam polycrystalline nickel fibers, and 1280 J/m3 and 2.3 kA/m for solid polycrystalline nickel. 相似文献
72.
73.
Air cleaning as a means of mitigating the risks arising from exposure to indoor radon progeny has been evaluated in a single-family house in the north eastem US. using an automated, semi-continuous activity-weighted size distribution measurement system. The measurements included radon concentration, condensation nuclei count, and activity-weighted size distribution of radon decay products. Measurements were made in the house with and without an operating air filtration system and with various particle sources common to normal indoor activities operating. Aerosols were generated by running water in a shower, candle burning, cigarette smoking, vacuuming, opening doors, and cooking. Using a room model, the changes in attachment rates, average attachment diameters, and deposition rates of the unattached fraction with and without the air cleaning system were calculated. In the presence of active aerosol sources, the air filtration unit typically reduced the concentration of particles within the hour following the end of particle generation. After candle burning, cigarette smoking, and vacuuming in the bedroom, the reductions of PAEC by air filtration are about 60% with the air filtration system operating in the bedroom. During cooking in the kitchen, the reductions of PAEC in the bedroom with the air filtration system were about 40%. However, for all cases the dose reductions were smaller than the particle and PAEC reductions. For those particles that were generated within the bedroom, there was a 20% to 50% reduction in dose. In the case of cooking where the door was open and particles infiltrated from the rest of the house, the dose reduction was only 5% on average and appears to be insignificant. Thus, the dose reductions were h e r than the reductions in activity concentration, but there were no cases where the estimated dose actually increased. 相似文献
74.
75.
含钇奥氏体基堆焊焊条堆焊层研究 总被引:1,自引:0,他引:1
针对高温、磨损、热疲劳工况下服役零件的失效,研制了一种含稀土元素钇的奥氏体基沉淀强化型堆焊焊条,并对堆焊层组织与性能进行了较系统的研究。 相似文献
76.
77.
热力系统节能技术改造方案介绍 总被引:3,自引:0,他引:3
针对热电厂热力系统存在的浪费能源和设计不合理现象作了浅析 ,介绍了几种对热力系统进行改造的方案 ,并对取得的经济效益进行了计算。 相似文献
78.
79.
The energies of various steps on the As-terminated GaAs(001)-2 × 4 surface are evaluated using a novel, approximate method of “linear combination of structural motifs”. It is based on the observation that previous total energy minimizations of semiconductor surfaces produced invariably equilibrium structures made of the same recurring local structural motifs, e.g. tetrahedral fourfold Ga, pyramidal threefold As, etc. Furthermore, such surface structures were found to obey consistently the octet rules as applied to the local motifs. We thus express the total energy of a given semiconductor surface as a sum of (i) the energies M of the local structural motifs appearing in the surface under consideration and (ii) an electrostatic term representing the Madelung energy of point charges resulting from application of the octet rule. The motif energies are derived from a set of pseudopotential total energy calculations for flat GaAs(001) surfaces and for point defects in bulk GaAs. This set of parameters suffices to reproduce the energies of other (001) surfaces, calculated using the same pseudopotential total energy approach. Application to GaAs(001)-2 × 4 surfaces with steps reveals the following. (i) “Primitive steps”, defined solely according to their geometries (i.e. step heights, widths and orientations) are often unstable. (ii) Additional, non-geometric factors beyond step geometries such as addition of surface adatoms, creation of vacancies and atomic rebonding at step edges are important to lower step energies. So is step-step interaction. (iii) The formation of steps is generally endothermic. (iv) The formation of steps with edges parallel to the direction of surface As dimers (A steps) is energetically favored over the formation of steps whose edges are perpendicular to the As dimers (B steps). 相似文献
80.
Wrinkling is a well known phenomenon experimented by tension membranes in Civil Engineering applications. This paper will
present an efficient numerical technique for the computational simulation of such wrinkles in a prestressed membrane. In particular,
the relaxed energy approach (Pipkin in IMA J Appl Math 36:85–99, 1986) is particularized for prestressed membranes (Gil in
Textile composites and inflatable structures, CIMNE, 2003) undergoing moderate strains. Wrinkling conditions in terms of the
Euler-Lagrange finite deformation tensor along principal directions will be obtained. This will provide a framework to describe
properly the initial instant when wrinkles start to be encountered in a prestressed Saint Venant–Kirchhoff hyperelastic membrane.
Subsequently, a modified Helmholtz’s free energy functional will be introduced with the purpose of describing the modified
constitutive behaviour of the continuum after the onset of wrinkling. Consistent derivations of the stress tensor as well
as the constitutive tensor will de depicted. The results will be particularized for membranes and cables in a Finite Element
discretization basis. Some numerical examples will prove the accuracy and robustness of the described algorithm. 相似文献