全文获取类型
收费全文 | 601848篇 |
免费 | 7197篇 |
国内免费 | 1972篇 |
专业分类
电工技术 | 11362篇 |
综合类 | 625篇 |
化学工业 | 88521篇 |
金属工艺 | 23878篇 |
机械仪表 | 19513篇 |
建筑科学 | 13379篇 |
矿业工程 | 2980篇 |
能源动力 | 16282篇 |
轻工业 | 45983篇 |
水利工程 | 6187篇 |
石油天然气 | 10602篇 |
武器工业 | 80篇 |
无线电 | 74611篇 |
一般工业技术 | 119500篇 |
冶金工业 | 112727篇 |
原子能技术 | 12854篇 |
自动化技术 | 51933篇 |
出版年
2021年 | 5301篇 |
2020年 | 4024篇 |
2019年 | 5142篇 |
2018年 | 8777篇 |
2017年 | 8855篇 |
2016年 | 9420篇 |
2015年 | 5992篇 |
2014年 | 9935篇 |
2013年 | 28414篇 |
2012年 | 15972篇 |
2011年 | 21699篇 |
2010年 | 17273篇 |
2009年 | 19502篇 |
2008年 | 20319篇 |
2007年 | 20072篇 |
2006年 | 18072篇 |
2005年 | 16209篇 |
2004年 | 15496篇 |
2003年 | 15408篇 |
2002年 | 14581篇 |
2001年 | 14444篇 |
2000年 | 13703篇 |
1999年 | 14284篇 |
1998年 | 35390篇 |
1997年 | 24673篇 |
1996年 | 19209篇 |
1995年 | 14504篇 |
1994年 | 12825篇 |
1993年 | 12807篇 |
1992年 | 9572篇 |
1991年 | 8992篇 |
1990年 | 8811篇 |
1989年 | 8299篇 |
1988年 | 7961篇 |
1987年 | 7020篇 |
1986年 | 6849篇 |
1985年 | 7771篇 |
1984年 | 7162篇 |
1983年 | 6438篇 |
1982年 | 5995篇 |
1981年 | 6159篇 |
1980年 | 5731篇 |
1979年 | 5545篇 |
1978年 | 5395篇 |
1977年 | 6250篇 |
1976年 | 7896篇 |
1975年 | 4794篇 |
1974年 | 4617篇 |
1973年 | 4673篇 |
1972年 | 3818篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
The objective of this study is to determine the economic and operational impact on energy cost of incorporating large photovoltaic (PV) and wind energy conversion systems (WECS) into the electric utility generation mix. In most cases, PV and WECS power outputs are subtracted from the utility load with the expectation that conventional generation would meet the residual load. This approach is valid for small penetration levels and/or for PV and WECS facilities connected near load centers, However, several constraints such as thermal generation characteristics, fuel supply and delivery, spinning reserve requirements, and hydro availability are not adequately represented in this process. To determine the optimal value of large-scale PV and WECS applications, a new methodology that would take into account the aforementioned constraints as well as a more global penetration is developed. Results indicate that while high hydro availability increases PV penetration levels, high ramping rates can also significantly increase penetration levels 相似文献
72.
Auld B.A. Chimenti D.E. Shull P.J. 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1996,43(2):319-325
The transverse resonance approach to guided wave analysis is applied to shear horizontal (SH) wave propagation in periodically layered composites. It is found for SH waves that at high values of the guided wavevector β, the wave energy is trapped in the slower of the two media and propagates accordingly at the slower wavespeed. At low values of β, however, the modes demonstrate a clustering behavior, indicative of the underlying Floquet wave structure. The number of modes in a cluster is observed to correlate with the number of unit cells in the layered plate. New physical insights into the behavior of these systems are obtained by analyzing the partial waves of the guided SH modes in terms of Floquet waves. We show that the fast and slow shear waves in the periodically layered composite play an analogous role to the longitudinal and shear partial waves comprising Lamb waves in a homogeneous plate 相似文献
73.
A 3-wave longitudinal design was used to examine the relationships among emotional restraint, peer drug associations, and gateway drug use in a sample of 1,256 middle school students. Structural equation modeling was used to compare 3 models: (1) One model viewed drug use as a consequence of emotional restraint and peer variables; (2) 1 viewed drug use as a cause of restraint and peer variables; and (3) 1 included reciprocal effects. All 3 models fit the data fairly well. However, the reciprocal model fit the data significantly better than either of the others. Within this model, low emotional restraint was significantly related to subsequent increases in gateway drug use among boys. In contrast, peer drug models and peer pressure were not related to subsequent changes in gateway drug use. Changes in peer drug models were, however, predicted by previous levels of gateway drug use. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
74.
75.
76.
Thanapalan Murugesan G. S. Venkat Rathnam S. Panduranga Rao P. Gangadhar Rao 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1995,63(3):290-294
In the present study of gas–liquid contactors, mean residence/contact time was calculated from knowledge of superficial velocity and the gas phase hold-up, for various gas rates and impeller geometry and speeds, and compared with values obtained from RTD measurements. A new correlation, involving Flow Number, Froude Number, system geometry and the physical properties, is proposed. This uses the authors data and those available in literature. 相似文献
77.
78.
79.
A method is described for experimentally determining the dose rate created in building bricks by incorporated natural radionuclides.
It was established using the thermoluminescence dosimetry method that the measured dose rate depends on the detector thickness,
the mass of the ceramic product investigated, and the irradiation geometry. The contributions to the measured dose of weakly
penetrating α and β radiation and hard ψ radiation are separated, the ratio between them depending on the experimental conditions.
Translated from Izmeritel'naya Tekhnika, No. 7, pp. 62–66, July, 1996. 相似文献
80.
This article describes the phenomenon of morphological instability in solid-solid phase transformations during thin-film interdiffusion,
specifically related to the initial stages of precipitation when phase growth occurs along the interface between thin films.
The experimental observations that revealed this effect will be presented, and a working hypothesis will be discussed. Experimental
observations suggest that the ledge mechanism of growth is present in this system but does not inhibit the formation of interfacial
instabilities. It is proposed that morphological stability for solid-phase growth occurring during thin-film interdiffusion
can be treated by the inclusion of a solute-source term into the two-dimensional perturbation approach generally used to study
unstable growth morphologies. Experimental observations that provide qualitative and semiquantitative support for the solute-source
model are also presented. 相似文献