全文获取类型
收费全文 | 304955篇 |
免费 | 1581篇 |
国内免费 | 648篇 |
专业分类
化学 | 146358篇 |
晶体学 | 4470篇 |
力学 | 15078篇 |
综合类 | 6篇 |
数学 | 46490篇 |
物理学 | 94782篇 |
出版年
2020年 | 1970篇 |
2019年 | 2265篇 |
2018年 | 10953篇 |
2017年 | 11674篇 |
2016年 | 7997篇 |
2015年 | 3110篇 |
2014年 | 3931篇 |
2013年 | 9369篇 |
2012年 | 10876篇 |
2011年 | 19878篇 |
2010年 | 12514篇 |
2009年 | 12559篇 |
2008年 | 17831篇 |
2007年 | 21218篇 |
2006年 | 7820篇 |
2005年 | 12974篇 |
2004年 | 9268篇 |
2003年 | 8885篇 |
2002年 | 6852篇 |
2001年 | 6097篇 |
2000年 | 4994篇 |
1999年 | 3509篇 |
1998年 | 3054篇 |
1997年 | 3052篇 |
1996年 | 2992篇 |
1995年 | 2427篇 |
1994年 | 2503篇 |
1993年 | 2530篇 |
1992年 | 2666篇 |
1991年 | 2730篇 |
1990年 | 2634篇 |
1989年 | 2617篇 |
1988年 | 2557篇 |
1987年 | 2525篇 |
1986年 | 2436篇 |
1985年 | 3121篇 |
1984年 | 3299篇 |
1983年 | 2652篇 |
1982年 | 2919篇 |
1981年 | 2797篇 |
1980年 | 2592篇 |
1979年 | 2825篇 |
1978年 | 2953篇 |
1977年 | 2889篇 |
1976年 | 2897篇 |
1975年 | 2739篇 |
1974年 | 2714篇 |
1973年 | 2900篇 |
1972年 | 1912篇 |
1967年 | 1868篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The singular-perturbation expansion of the plasma cold-fluid equations for crossed fields in a planar geometry is presented. The general expansion is carried out to third order. Various instabilities that occur in the first, second, and third orders are discussed. 相似文献
72.
Periodic sound-pressure time functions with frequency components below 50 Hz were used to measure within their period (a) the temporal course of masking, called a masking-period pattern (MPP), and (b) the temporal course of suppression of delayed evoked otoacoustic emissions, called a suppression-period pattern (SPP). Three different time functions were used: an alternating Gaussian impulse, its first integral, and its second integral. In each case, the course of the SPP is a mirror image of that of the MPP: Small masking corresponds to small suppression, while strong masking coincides with almost total suppression. Since otoacoustic emissions are assumed to have their origin in the inner ear, it can be argued that simultaneous masking, an effect including central processing, is very strongly based on peripheral processes located in the cochlea. Both MPP and SPP are closely related to the second derivative of the sound-pressure time function. 相似文献
73.
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
74.
75.
76.
The effect of hydrogen on the photoluminescence and planar conductivity of GaAs/InGaAs quantum-well heterostructures with an island Pd layer at the anodically oxidized surface was studied. Unlike continuous deposited Pd layers, island layers do not cause the formation of defects in the GaAs surface region and yet the Pd layer maintains high catalytic activity with respect to hydrogen. It is found that the thermal treatment of such a structure in a hydrogen atmosphere causes atomic-hydrogen passivation of the defects in quantum wells. Studies of the characteristics of planar photoresistors with an island Pd layer acting as hydrogen sensors show that their hydrogen detectivity is approximately two orders of magnitude higher than that of diode structures with continuous Pd layers. 相似文献
77.
E. G. Tarakanova 《Journal of Applied Spectroscopy》1991,54(4):357-362
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 4, pp. 581–587, April, 1991. 相似文献
78.
P. E. Cladis 《Journal of statistical physics》1991,62(5-6):899-925
We first discuss nonlinear aspects of phase transition theory applied to a particular liquid crystal phase transition. A simple derivation is given to show how two coupled Goldstone modes (one appearing as gauge fluctuations of the ordered phase) can force a phase transition, against all expectations, to take place discontinuously (theory of Halperin, Lubensky, and Ma)-but the discontinuity may be immeasurably small. Then, we describe a new dynamical test of phase transition order, developed by Cladiset al., that turns out to be more sensitive than x-ray diffraction and adiabatic calorimetry. Quantitative data found by this new method are in excellent agreement with the measurements of adiabatic calorimetry and x-ray diffraction as well as expectations implicit in the predictions of HLM.This is the text of an after-banquet talk given at the CNLS Workshop on the Dynamics of Concentrated Systems. 相似文献
79.
E. V. Gromachevskaya A. G. Sakhabutdinov V. G. Kul'nevich I. S. Arustamova R. B. Valeev B. A. Bazhenov 《Chemistry of Heterocyclic Compounds》1991,27(5):502-503
It was shown that the reaction of o-hyroxyphenyldiphenylmethanol with benzaldehyde labeled with 17O and 18O isotopes proceeds in such a way that oxygen atoms of the starting phenoloalcohol are retained.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 632–633, May, 1991. 相似文献
80.
M. B. Wright 《The Journal of the Operational Research Society》1989,40(2):187-192
This paper addresses a problem common to all railway networks. Given a fixed train timetable and locomotives (or other forms of traction) of various types, each train must be allocated a locomotive. This paper examines the use of stochastic algorithms for such a problem. Two types of algorithm are used—a simple ‘local improvement’ method, performed successively from randomly chosen starting points, and a ‘simulated annealing’ approach. Both are found to give considerably better results than a deterministic method in current use, and the annealing approach is probably the better stochastic method. 相似文献