首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   146206篇
  免费   12804篇
  国内免费   7611篇
化学   72595篇
晶体学   1386篇
力学   10974篇
综合类   322篇
数学   39411篇
物理学   41933篇
  2024年   174篇
  2023年   1545篇
  2022年   2310篇
  2021年   2650篇
  2020年   3084篇
  2019年   2850篇
  2018年   12219篇
  2017年   11884篇
  2016年   9133篇
  2015年   4123篇
  2014年   4242篇
  2013年   5445篇
  2012年   10167篇
  2011年   16748篇
  2010年   9864篇
  2009年   9986篇
  2008年   10892篇
  2007年   12548篇
  2006年   3954篇
  2005年   4385篇
  2004年   3893篇
  2003年   3819篇
  2002年   2724篇
  2001年   1704篇
  2000年   1639篇
  1999年   1683篇
  1998年   1494篇
  1997年   1434篇
  1996年   1485篇
  1995年   1166篇
  1994年   995篇
  1993年   855篇
  1992年   727篇
  1991年   640篇
  1990年   551篇
  1989年   444篇
  1988年   393篇
  1987年   331篇
  1986年   306篇
  1985年   250篇
  1984年   186篇
  1983年   138篇
  1982年   121篇
  1981年   105篇
  1980年   111篇
  1979年   76篇
  1978年   64篇
  1977年   58篇
  1976年   46篇
  1973年   53篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The Titius–Bode law for planetary distances is reviewed. A model describing the basic features of this rule in the “quantum-like” language of a wave equation is proposed. Some considerations about the ’t Hooft idea on the quantum behavior of deterministic systems with dissipation are discussed.  相似文献   
992.
Poly(3-hydroxybutyrate), PHB has been structurally modified through reaction with maleic anhydride, MA. Transesterification reaction was carried out fixing the PHB and MA and besides time and temperature the concentration of the triethylamine (used as catalyst) was changed. Glass transition, melting and crystallization temperature obtained from DSC curves and thermal degradation temperatures obtained from TG traces were used to evaluate the influence of the reaction conditions on the modification of PHB according to factorial design. On the base of the results the optimum conditions are to perform the PHB modification reaction with MA reaction at 110°C for 1 h with 5% v/v triethylamine.  相似文献   
993.
 We study the metrical theory of fibred systems, in particular, in the case of continued fraction mixing systems. We get the limit distribution of the largest value of a continued fraction mixing stationary stochastic process with infinite expectation and some related results. These are analogous to J. Galambos, W. Philipp, and H. G. Diamond–J. D. Vaaler theorems for the regular continued fractions. As an application, we see that these theorems hold for Jacobi-Perron algorithm. Received September 30, 2001; in revised form January 8, 2002  相似文献   
994.
Decisions during the reliability growth development process of engineering equipment involve trade-offs between cost and risk. However slight, there exists a chance an item of equipment will not function as planned during its specified life. Consequently the producer can incur a financial penalty. To date, reliability growth research has focussed on the development of models to estimate the rate of failure from test data. Such models are used to support decisions about the effectiveness of options to improve reliability. The extension of reliability growth models to incorporate financial costs associated with ‘unreliability’ is much neglected. In this paper, we extend a Bayesian reliability growth model to include cost analysis. The rationale of the stochastic process underpinning the growth model and the cost structures are described. The ways in which this model can be used to support cost–benefit analysis during product development are discussed and illustrated through a simple case.  相似文献   
995.
In the title centrosymmetric binuclear complex, [Cd2(C17H11N3O)4(H2O)2](ClO4)4, the CdII ion assumes a distorted octahedral geometry. There are π–π stacking interactions between the pyridine and 1,10‐phenanthroline ring systems of adjacent ligands at the same CdII centre. Intermolecular hydrogen bonds between the coordinated aqua ligand and the O atom of a keto group connect adjacent complex cations into extended chains. Hydrogen bonds also exist between the complex cations and the perchlorate anions. Compared with the fluorescence spectrum of the organic ligand, the complex displays strong fluorescent emission and an ipsochromic shift of the emission peaks, which may be attributed to the structural character.  相似文献   
996.
Saint-Venant’s Problem, Almansi–Michell Problems, Meshfree Methods, Piezoelectricity. We present a semi-analytical method for analyzing prismatic nonhomogeneous piezoelectric cylinders with arbitrary cross-sectional geometry. The prescribed loads considered in this study include axial forces, torques, moments, and voltage resultants prescribed at the cylinder’s ends, as well as body forces, lateral surface shears, voltages, and pressures as long as they can be represented by a power series in the axial coordinate. This problem can be considered as an extension of Saint-Venant and Almansi–Michell problems for elastic bodies to piezoelectric bodies. In this computationally efficient method, the cross-sectional plane is discretized with a meshfree approach, and the solution is obtained analytically with respect to the axial coordinate. A number of examples are provided to demonstrate the veracity and utility of the proposed method.  相似文献   
997.
The shock structure problem is one of the classical problems of fluid mechanics and at least for non-reacting dilute gases it has been considered essentially solved. Here we present a few recent findings, to show that this is not the case. There are still new physical effects to be discovered provided that the numerical technique is general enough to not rule them out a priori. While the results have been obtained for dense fluids, some of the effects might also be observable for shocks in dilute gases.  相似文献   
998.
It is shown that time-dependent temperatures in a transient, conductive system can be approximately modeled by a fractional-order differential equation, the order of which depends on the Biot number. This approximation is particularly suitable for complex shapes for which a first-principles approach is too difficult or computationally time-consuming. Analytical solutions of these equations can be written in terms of the Mittag-Leffler function. The approximation is especially useful if a suitable fractional-order controller is to be designed for the system.  相似文献   
999.
The purpose of this paper is to introduce and study the concepts of discrete semi-stability and geometric semi-stability for distributions with support inZ +. We offer several properties, including characterizations, of discrete semi-stable distributions. We establish that these distributions posses the property of infinite divisibility and that their probability generating functions admit canonical representations that are analogous to those of their continuous counterparts. Properties of discrete geometric semi-stable distributions are deduced from the results obtained for discrete semi-stability. Several limit theorems are established and some examples are constructed.  相似文献   
1000.
Diethoxyphosphinyl acetic acid hydrazide is a unique reagent that provides a convenient and efficient process to prepare fused [5,5]-, [5,6]-, and [5,7]-3-[(E)-2-(arylvinyl)]-1,2,4-triazoles from aldehydes and alkoxyimines. The process involves three steps without isolation of any intermediates to afford 1,2,4-triazoles in modest to excellent overall yield.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号