全文获取类型
收费全文 | 588958篇 |
免费 | 6655篇 |
国内免费 | 2320篇 |
专业分类
化学 | 313538篇 |
晶体学 | 8853篇 |
力学 | 26505篇 |
综合类 | 15篇 |
数学 | 71639篇 |
物理学 | 177383篇 |
出版年
2020年 | 4238篇 |
2019年 | 4468篇 |
2018年 | 5140篇 |
2017年 | 4995篇 |
2016年 | 8712篇 |
2015年 | 6403篇 |
2014年 | 9307篇 |
2013年 | 26491篇 |
2012年 | 20142篇 |
2011年 | 25042篇 |
2010年 | 16436篇 |
2009年 | 16418篇 |
2008年 | 22976篇 |
2007年 | 23212篇 |
2006年 | 22261篇 |
2005年 | 20221篇 |
2004年 | 18379篇 |
2003年 | 16198篇 |
2002年 | 15996篇 |
2001年 | 17720篇 |
2000年 | 13686篇 |
1999年 | 10867篇 |
1998年 | 8899篇 |
1997年 | 8758篇 |
1996年 | 8617篇 |
1995年 | 7888篇 |
1994年 | 7500篇 |
1993年 | 7393篇 |
1992年 | 7968篇 |
1991年 | 7938篇 |
1990年 | 7359篇 |
1989年 | 7214篇 |
1988年 | 7541篇 |
1987年 | 7071篇 |
1986年 | 6760篇 |
1985年 | 9650篇 |
1984年 | 9776篇 |
1983年 | 8109篇 |
1982年 | 8589篇 |
1981年 | 8440篇 |
1980年 | 8163篇 |
1979年 | 8417篇 |
1978年 | 8481篇 |
1977年 | 8452篇 |
1976年 | 8414篇 |
1975年 | 8047篇 |
1974年 | 7922篇 |
1973年 | 8164篇 |
1972年 | 5230篇 |
1971年 | 3999篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
92.
C. Ghezzi E. Gombia R. Mosca M. Pillan 《Applied Physics A: Materials Science & Processing》1989,48(5):457-463
Capacitance DLTS measurements have been performed in VPE GaAs MESFETs prepared on Bridgman Cr-doped and LEC undoped semi-insulating substrates. A band of electron traps not intrinsically related to the VPE growth process and accumulating near the metal (gate) — semiconductor interface was detected in all the samples. Deeper regions into the channel were free from any detectable trap. Near pinch-off conditions, a positive capacitance signal was found to dominate the DLTS spectra only in the case of samples prepared on Cr-doped substrates. The hypothesis of this positive transient being related to changes in the occupation of surface states in the ungated surface access regions has been checked by comparing experimental and calculated dependencies of the signal amplitude on reverse gate voltage. Unexplained discrepancies, together with the absence of positive signal in MESFETs prepared on LEC undoped substrates, suggest the possibility of hole emission from hole traps within the bulk of the device. 相似文献
93.
Bernard Chazelle Herbert Edelsbrunner Leonidas J. Guibas 《Discrete and Computational Geometry》1989,4(1):139-181
This paper investigates the combinatorial and computational aspects of certain extremal geometric problems in two and three dimensions. Specifically, we examine the problem of intersecting a convex subdivision with a line in order to maximize the number of intersections. A similar problem is to maximize the number of intersected facets in a cross-section of a three-dimensional convex polytope. Related problems concern maximum chains in certain families of posets defined over the regions of a convex subdivision. In most cases we are able to prove sharp bounds on the asymptotic behavior of the corresponding extremal functions. We also describe polynomial algorithms for all the problems discussed.Bernard Chazelle wishes to acknowledge the National Science Foundation for supporting this research in part under Grant No. MCS83-03925. Herbert Edelsbrunner is pleased to acknowledge the support of Amoco Fnd. Fac. Dev. Comput. Sci. 1-6-44862. 相似文献
94.
When solving large complex optimization problems, the user is faced with three major problems. These are (i) the cost in human time in obtaining accurate expressions for the derivatives involved; (ii) the need to store second derivative information; and (iii), of lessening importance, the time taken to solve the problem on the computer. For many problems, a significant part of the latter can be attributed to solving Newton-like equations. In the algorithm described, the equations are solved using a conjugate direction method that only needs the Hessian at the current point when it is multiplied by a trial vector. In this paper, we present a method that finds this product using automatic differentiation while only requiring vector storage. The method takes advantage of any sparsity in the Hessian matrix and computes exact derivatives. It avoids the complexity of symbolic differentiation, the inaccuracy of numerical differentiation, the labor of finding analytic derivatives, and the need for matrix store. When far from a minimum, an accurate solution to the Newton equations is not justified, so an approximate solution is obtained by using a version of Dembo and Steihaug's truncated Newton algorithm (Ref. 1).This paper was presented at the SIAM National Meeting, Boston, Massachusetts, 1986. 相似文献
95.
P. M. Gandini A. Zucco 《Abhandlungen aus dem Mathematischen Seminar der Universit?t Hamburg》1989,59(1):15-21
Some well-known theorems on typical properties of real-valued continuous functions defined on [0, 1] are improved using the
notion of porosity. 相似文献
96.
William J. Reed 《Natural Resource Modeling》1989,3(4):463-480
It is assumed that the probability of destruction of a biological asset by natural hazards can be reduced through investment in protection. Specifically a model, in which the hazard rate depends on both the age of the asset and the accumulated invested protection capital, is assumed. The protection capital depreciates through time and its effectiveness in reducing the hazard rate is subject to diminishing returns. It is shown how the investment schedule to maximize the expected net present value of the asset can be determined using the methods of deterministic optimal control, with the survival probability regarded as a state variable. The optimal investment pattern involves “bang-bang-singular” control. A numerical scheme for determining jointly the optimal investment policy and the optimal harvest (or replacement) age is outlined and a numerical example involving forest fire protection is given. 相似文献
97.
98.
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
99.
Partially supported by the general research fund at the University of Kansas 相似文献
100.
Dr. H. Schmieg Prof. Dr. P. Vielsack 《Archive of Applied Mechanics (Ingenieur Archiv)》1987,57(6):446-458
Übersicht Betrachtet wird ein zwangserregtes Zweikörpersystem mit wechselnden Bindungen infolge trockener Reibung. Stationäre Bewegungen werden als Grenzfall instationärer Einschwingvorgänge berechnet. Abhängig von den Systemparametern ergeben sich drei typische Bewegungsformen. Ihnen entsprechen dauernde Haftzustände, wechselnde Haft-Gleitzustände oder dauernde Gleitzustände an der Berührfläche beider Körper.
Intermittant constraints in a two-body-system with dry friction
Summary An externally excited two-body-system with intermittant constraints due to dry friction is considered. Stationary motions are calculated as limit cases of instationary transients. Depending on the parameters of the system, three typical modes are of interest. These correspond to permanent sticking, slipstick behaviour, or to permanent slipping in the contact surface of the bodies, respectively.相似文献