全文获取类型
收费全文 | 16391篇 |
免费 | 2560篇 |
国内免费 | 1966篇 |
专业分类
化学 | 7687篇 |
晶体学 | 524篇 |
力学 | 2049篇 |
综合类 | 224篇 |
数学 | 2264篇 |
物理学 | 8169篇 |
出版年
2024年 | 25篇 |
2023年 | 126篇 |
2022年 | 337篇 |
2021年 | 421篇 |
2020年 | 399篇 |
2019年 | 380篇 |
2018年 | 415篇 |
2017年 | 578篇 |
2016年 | 685篇 |
2015年 | 540篇 |
2014年 | 802篇 |
2013年 | 1359篇 |
2012年 | 1047篇 |
2011年 | 1019篇 |
2010年 | 933篇 |
2009年 | 1074篇 |
2008年 | 1003篇 |
2007年 | 1105篇 |
2006年 | 932篇 |
2005年 | 914篇 |
2004年 | 847篇 |
2003年 | 720篇 |
2002年 | 650篇 |
2001年 | 540篇 |
2000年 | 521篇 |
1999年 | 490篇 |
1998年 | 410篇 |
1997年 | 366篇 |
1996年 | 307篇 |
1995年 | 262篇 |
1994年 | 231篇 |
1993年 | 237篇 |
1992年 | 230篇 |
1991年 | 131篇 |
1990年 | 122篇 |
1989年 | 104篇 |
1988年 | 128篇 |
1987年 | 90篇 |
1986年 | 70篇 |
1985年 | 44篇 |
1984年 | 43篇 |
1983年 | 19篇 |
1982年 | 43篇 |
1981年 | 42篇 |
1980年 | 25篇 |
1979年 | 45篇 |
1978年 | 36篇 |
1977年 | 20篇 |
1976年 | 21篇 |
1974年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
The present study is concerned with the determination of the optimal shape for a package containing multiple heating elements.
The optimization tool has been developed based on the inverse heat transfer (IHT) approach, incorporating a direct problem
solver, a numerical grid generator, a direct-differentiation sensitivity analyzer, and the conjugate gradient method. Shape
design that leads to a specified outer surface temperature distribution is predicted by the approach. In this study, the effects
of internal heat generation on optimal shapes of the packagings have also been evaluated. Several practical cases with various
imbedded heating elements and thermal conditions are studied. Results show that the approach provides an efficient computer-aided
design scheme for the shape profile determination. 相似文献
152.
This paper presents the mathematical approach for the abnormal multiplication of plankton. An abnormal multiplication can be expressed as an unstable problem and the stability of the system is investigated by introducing eigenvalues of a mathematical equation. The stability of the system can be judged by an eigenvalue based on the Lyapunov's stability theory. In this paper, the Arnoldi‐QR method is used to obtain eigenvalues and eigenvectors of the system. The mode superposition method is employed to create spatial distribution needed to analyse the stability. To obtain the objective eigenvalue, the parameter identification technique is employed. The finite element method is used for the discretization in space. Lake Kasumigaura, which is located in Ibaraki Prefecture in Japan, is selected and actual data in 1975, 1976, 1991 and 2000 are used in order to investigate the stability of the specified lake in Japan. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
153.
The combination of the optical diagnostics and image processing technique has produced various methods of practical measurement in engineering. The measurement of size and velocity of droplets in fuel spray is one typical example. In general, the spray is a complicated and three-dimensional two-phase flow; therefore multi-dimensional field measurement of the spray field is indispensable. In the present paper, a technique of simultaneous measurement of droplet size and three-components of velocity is proposed and applied to spray issuing from the swirl nozzle as a practical application. The focused image of glare points is captured by a stereoscopic arrangement, and droplet size and the three-components of velocity are evaluated from the doublet image. Fundamental optical arrangements to capture the image of the glare points with a stereo view are established, and the feasibility and the accuracy of the technique are confirmed. © 2004 The Optical Society of Japan 相似文献
154.
The patterns of absorption and reflection of an ultrashort laser pulse by a plasma with a solid-state density, which are realized in the mode of the normal skin effect, are studied. It is shown that a decrease in low-power pulse duration shorter than the period corresponding to the fundamental frequency of radiation leads to an increase in the absorption coefficient. If the pulse power is so high as to provide electron heating during a time shorter than the reciprocal fundamental frequency, nonlinear suppression of absorption takes place and the spectrum of the reflected radiation contains odd harmonics of the fundamental frequency. 相似文献
155.
基于半导体材料的新型光纤温度传感器 总被引:1,自引:0,他引:1
提出利用全反射定律以半导体材料作为敏感器件的光纤温度传感器,着重阐述半导体材料的温度敏感特性及传感原理,介绍传感器构造,并给出初步实验方案。 相似文献
156.
157.
The optical nonlinearities of Cu(mpo)2 [formula = C10H8N2CuO2S2] complex have firstly been investigated by using the Z-scan technique with a nanosecond-pulsed Nd:YAG laser at its second harmonic (532 nm) radiation. The nonlinear optical absorption and refraction coefficients of Cu(mpo)2 have been measured with the different on-axial peak irradiances I
0 at the waist ranging from 0.48 to 5.66 GW/cm2. The nonlinear transmittance characteristics exhibit the near resonant two-photon absorption (TPA) at 532 nm and are explained by a population redistribution model. The nonlinear absorption originates from the near resonant TPA while the mechanism of the nonlinear refraction is the near resonant TPA transition enhancement. The linear increasing dependences of the optical nonlinearities on the incident irradiance arise from the population redistribution due to the near resonant TPA. 相似文献
158.
J. Haslinger T. Kozubek K. Kunisch G. Peichl 《Journal of Mathematical Analysis and Applications》2004,290(2):665-685
This contribution combines a shape optimization approach to free boundary value problems of Bernoulli type with an embedding domain technique. A theoretical framework is developed which allows to prove continuous dependence of the primal and dual variables in the resulting saddle point problems with respect to the domain. This ensures the existence of a solution of a related shape optimization problem in a sufficiently large class of admissible domains. 相似文献
159.
陈化温度和Fe/Zr配比对SO42-/Fe2O3-ZrO2固体超强酸结构与性能的影响 总被引:6,自引:0,他引:6
用低温陈化法制备了SO42 -/Fe2 O3 ZrO2 (简称SFZ)固体超强酸催化剂 ,用红外光谱 (IR)和X光衍射(XRD)对其结构进行了表征 ,并考察了它对合成癸二酸二正丁酯的催化性能 .IR谱显示 ,低温陈化的SFZ样品在10 70cm-1处吸收峰远强于常温陈化样品 .XRD分析则显示 ,在焙烧温度为 6 5 0℃、Fe/Zr为 2∶1时 ,低温陈化的样品出现了亚稳态的ZrO2 四方晶相 .该样品在催化酯化反应中使产率达 90 %以上 ,高于常温陈化样品的 30 % .研究结果表明 :在其他条件不变时 ,低温陈化所出现的亚稳态的ZrO2 四方晶相是表面酸性和催化活性增加的微观原因 相似文献
160.
Huang H. X. Liang Z. A. Pardalos P. M. 《Journal of Optimization Theory and Applications》2004,120(1):53-71
A flow search approach is presented in this paper. In the approach, each iterative process involves a subproblem, whose variables are the stepsize parameters. Every feasible solution of the subproblem corresponds to some serial search stages, the stepsize parameters in different search stages may interact mutually, and their optimal values are determined by evaluating the total effect of the interaction. The main idea of the flow search approach is illustrated via the minimization of a convex quadratic function. Based on the flow search approach, some properties of the m-step linear conjugate gradient algorithm are analyzed and new bounds on its convergence rate are also presented. Theoretical and numerical results indicate that the new bounds are better than the well-known ones. 相似文献