全文获取类型
收费全文 | 184413篇 |
免费 | 1670篇 |
国内免费 | 512篇 |
专业分类
化学 | 97772篇 |
晶体学 | 2612篇 |
力学 | 8099篇 |
综合类 | 9篇 |
数学 | 18694篇 |
物理学 | 59409篇 |
出版年
2020年 | 1390篇 |
2019年 | 1450篇 |
2018年 | 1937篇 |
2017年 | 1861篇 |
2016年 | 3069篇 |
2015年 | 1906篇 |
2014年 | 3039篇 |
2013年 | 7704篇 |
2012年 | 6176篇 |
2011年 | 7785篇 |
2010年 | 5387篇 |
2009年 | 5432篇 |
2008年 | 6969篇 |
2007年 | 7030篇 |
2006年 | 6566篇 |
2005年 | 5975篇 |
2004年 | 5352篇 |
2003年 | 4692篇 |
2002年 | 4688篇 |
2001年 | 5490篇 |
2000年 | 4149篇 |
1999年 | 3272篇 |
1998年 | 2768篇 |
1997年 | 2707篇 |
1996年 | 2424篇 |
1995年 | 2312篇 |
1994年 | 2283篇 |
1993年 | 2079篇 |
1992年 | 2411篇 |
1991年 | 2529篇 |
1990年 | 2311篇 |
1989年 | 2274篇 |
1988年 | 2255篇 |
1987年 | 2148篇 |
1986年 | 2020篇 |
1985年 | 2587篇 |
1984年 | 2756篇 |
1983年 | 2211篇 |
1982年 | 2416篇 |
1981年 | 2311篇 |
1980年 | 2172篇 |
1979年 | 2388篇 |
1978年 | 2540篇 |
1977年 | 2421篇 |
1976年 | 2457篇 |
1975年 | 2328篇 |
1974年 | 2360篇 |
1973年 | 2367篇 |
1972年 | 1607篇 |
1971年 | 1426篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
Under idealized conditions, electroconvection in a nematic liquid crystal sets in at a well-defined threshold of the driving ac voltage. Fluctuating convection rolls of a small amplitude below that threshold have been observed recently. The measurement of the amplitude of these fluctuations is described in detail in this paper. It is based on averaging the light intensity signals using the structure function and a quantitative analysis of the light deflection. 相似文献
32.
33.
34.
William F. Bowlin 《The Journal of the Operational Research Society》1987,38(2):127-135
The objective of this research is to evaluate a new methodology, data envelopment analysis (DEA), as a tool for measuring and evaluating the operational efficiency of US Air Force organizations. This study involves the application of DEA to locate possible inefficiencies in the performance of US Air Force real-property maintenance activities. Dynamic behaviour, as exhibited via window analyses, is reviewed along with static counterparts. The testing was done in close coordination with Air Force officials, who reviewed the results for accuracy, validity and relevance. We conclude that this type of efficiency analysis does have value for the Air Force, where it can serve as a guide to auditors, budget programmers, managers and others in measuring, evaluating and enhancing operational efficiency. 相似文献
35.
F. G. Gerasimato 《Journal of Mathematical Sciences》1991,57(5):3381-3383
We propose a simplified method of determining the integral characteristics of the opening of a parachute which makes it possible to compute the maximal stress on a parachute and the trajectory parameters of the motion of the load in the region of the opening of the parachute, and to determine the size of the portion of the canopy at the time of maximal stress.Translated fromDinamicheskie Sistemy, No. 6, 1987, pp. 38–41. 相似文献
36.
We consider the problem of contact of two elastic wedges and assume that only the vertices of the wedges touch before loading. After loading, the edges of both wedges come in contact near their common vertex. We reduce the constructed system of dual integral equations to a Fredholm integral equation of the second kind with difference kernel given on the semiaxis. We analytically solve the Fredholm equation by reducing it to the boundary-value Riemann problem for analytic functions. We obtain an analytic expression for contact stresses. 相似文献
37.
Afraimovich É. L. Vodyannikov V. V. Voyeikov S. V. Perevalova N. P. Yakovets A. F. 《Radiophysics and Quantum Electronics》2002,45(10):741-757
Using the COPHASE method and the GPS interferometry method for travelling ionospheric disturbances, we analyze in detail the spatio-temporal properties of travelling wave packets (TWP) of total electron content (TEC) disturbances. The analysis is performed on the example of a clearest TWP manifestation observed in California, USA, in October 18, 2001, using the GLOBDET technique, developed at the Institute of Solar-Terrestrial Physics of the Siberian Branch of RAS for global detection and monitoring of natural and technogenic ionospheric disturbances on the basis of TEC variations retrieved from the global network of GPS receivers. In the time domain, TWPs are quasi-periodic TEC oscillations of duration about 1 h, period of 10–20 min, and amplitude exceeding that of the background TEC fluctuations by at least one order of magnitude. The velocity and direction of TWP motion are similar to those of mid-latitude mesoscale travelling ionospheric disturbances, as obtained earlier from the analysis of phase parameters of HF radio signals and the signals of geostationary satellites and discrete space radio sources. 相似文献
38.
High-temperature series expansions of the susceptibility and second moment to 15th order are calculated for zero external field on the linear chain (LC), plane square (PSQ), simple cubic (SC), and body-centered cubic (BCC) lattices. Checks for specific models against pertinent work in the literature are detailed. 相似文献
39.
A Modified Quasi-Newton Method for Structured Optimization with Partial Information on the Hessian 总被引:2,自引:0,他引:2
This paper develops a modified quasi-Newton method for structured unconstrained optimization with partial information on the
Hessian, based on a better approximation to the Hessian in current search direction. The new approximation is decided by both
function values and gradients at the last two iterations unlike the original one which only uses the gradients at the last
two iterations. The modified method owns local and superlinear convergence. Numerical experiments show that the proposed method
is encouraging comparing with the methods proposed in [4] for structured unconstrained optimization
Presented at the 6th International Conference on Optimization: Techniques and Applications, Ballarat, Australia, December
9–11, 2004 相似文献
40.
The effects of surface preparation and illumination on electric parameters of Au/InSb/InP(100) Schottky diode were investigated, in the later diode InSb forms a fine restructuration layer allowing to block In atoms migration to surface. In order to study the electric characteristics under illumination, we make use of an He-Ne laser of 1 mW power and 632.8 nm wavelength. The current-voltage I(VG), the capacitance-voltage C(VG) measurements were plotted and analysed. The saturation current Is, the serial resistance Rs and the mean ideality factor n are, respectively, equal to 2.03 × 10−5 A, 85 Ω, 1.7 under dark and to 3.97 × 10−5 A, 67 Ω, 1.59 under illumination. The analysis of I(VG) and C(VG) characteristics allows us to determine the mean interfacial state density Nss and the transmission coefficient θn equal, respectively, to 4.33 × 1012 eV−1 cm−2, 4.08 × 10−3 under dark and 3.79 × 1012 eV−1 cm−2 and 5.65 × 10−3 under illumination. The deep discrete donor levels presence in the semiconductor bulk under dark and under illumination are responsible for the non-linearity of the C−2(VG) characteristic. 相似文献