全文获取类型
收费全文 | 63823篇 |
免费 | 10757篇 |
国内免费 | 7956篇 |
专业分类
化学 | 46316篇 |
晶体学 | 874篇 |
力学 | 3680篇 |
综合类 | 468篇 |
数学 | 7064篇 |
物理学 | 24134篇 |
出版年
2024年 | 230篇 |
2023年 | 1351篇 |
2022年 | 2552篇 |
2021年 | 2578篇 |
2020年 | 2786篇 |
2019年 | 2698篇 |
2018年 | 2303篇 |
2017年 | 2264篇 |
2016年 | 3140篇 |
2015年 | 3210篇 |
2014年 | 3818篇 |
2013年 | 4846篇 |
2012年 | 5798篇 |
2011年 | 5755篇 |
2010年 | 4125篇 |
2009年 | 3838篇 |
2008年 | 4067篇 |
2007年 | 3578篇 |
2006年 | 3286篇 |
2005年 | 2863篇 |
2004年 | 2234篇 |
2003年 | 1823篇 |
2002年 | 1740篇 |
2001年 | 1449篇 |
2000年 | 1182篇 |
1999年 | 1270篇 |
1998年 | 1004篇 |
1997年 | 896篇 |
1996年 | 914篇 |
1995年 | 807篇 |
1994年 | 707篇 |
1993年 | 598篇 |
1992年 | 482篇 |
1991年 | 459篇 |
1990年 | 369篇 |
1989年 | 269篇 |
1988年 | 199篇 |
1987年 | 181篇 |
1986年 | 178篇 |
1985年 | 154篇 |
1984年 | 94篇 |
1983年 | 103篇 |
1982年 | 56篇 |
1981年 | 40篇 |
1980年 | 31篇 |
1979年 | 20篇 |
1976年 | 20篇 |
1975年 | 21篇 |
1972年 | 19篇 |
1957年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Two novel 2′-hydroxychalcone derivatives (i.e., M1 and M2) are explored in this work. We mainly focus on investigating the effects of photoexcitation on hydrogen bonds and on the excited-state intramolecular proton transfer (ESIPT) process. On the basis of calculations of electrostatic potential surface and intramolecular interactions, we verify the formation of hydrogen bond O1 H2···O3 in both S0 and S1 states. Exploring the ultraviolet–visible spectra in the liquid phase, our simulated results reappear in the experimental phenomenon. Analyzing molecular geometry and infrared stretching vibrational spectra, we confirm O1 H2···O3 is strengthened for both M1 and M2 in the S1 state. We further confirm that charge redistribution facilitates ESIPT tendency. Constructing potential energy curves, we find the ultrafast ESIPT behavior for M1, which is because of the deficiency of side hydroxyl moiety comparing with M2. This work makes a reasonable affiliation of the ESIPT mechanism for M1 and M2. We wish this paper could facilitate understanding these two novel systems and promote their applications. 相似文献
62.
The γcmc values of CTAB-SDS decrease from 63.67 mN/m at 10‡C to 36.38 mN/m at 90‡C, slightly lower than those of either CTAB or SDS.
Correspondingly, the CMC of CTAB-SDS decreases almost by half. The increase of surface activity of CTAB-SDS can be attributed
to the relatively weak electrostatic interaction at high temperature, which is supported by the increase of solubility of
CTAB-SDS with rise in temperature. Catalytic effect on oxidation of toluene derivatives with potassium permanganate follows
the order CTAB-SDS > SDS > CTAB. This is not caused by the dissociative effect of CTAB-SDS with low surface activity at low
temperature, as seen from the fact that almost all oxidative products can be retrieved for different toluene derivatives and
surfactants by mimicking the conditions of reaction. In the emulsifications of toluene derivatives at 90‡C, the time that
turbid water layers of surfactant solutions take to become clear is the same as that of the catalytic effect on oxidation
of toluene derivatives. Thus, it can be inferred that surfactants can improve the oxidation yields of toluene derivatives
by increasing the contact between two reacting phases. 相似文献
63.
64.
65.
In this paper, we study the existence of the uniformly minimum risk equivariant (UMRE) estimators of parameters in a class
of normal linear models, which include the normal variance components model, the growth curve model, the extended growth curve
model, and the seemingly unrelated regression equations model, and so on. The necessary and sufficient conditions are given
for the existence of UMRE estimators of the estimable linear functions of regression coefficients, the covariance matrixV and (trV)α, where α > 0 is known, in the models under an affine group of transformations for quadratic losses and matrix losses, respectively.
Under the (extended) growth curve model and the seemingly unrelated regression equations model, the conclusions given in literature
for estimating regression coefficients can be derived by applying the general results in this paper, and the sufficient conditions
for non-existence of UMRE estimators ofV and tr(V) are expanded to be necessary and sufficient conditions. In addition, the necessary and sufficient conditions that there
exist UMRE estimators of parameters in the variance components model are obtained for the first time. 相似文献
66.
电路过渡过程所列方程是微分方程,本文中采用的是方框图模型分析法,即将微分方程的复杂示解分解成最基本的加(减)、乘(除)、积分(微分)、增益等运算,采用VB设计用户界面产进行计算,并给出了一算例。 相似文献
67.
Boundedness of commutators on homogeneous Herz spaces 总被引:9,自引:0,他引:9
The boundedness on homogeneous Herz spaces is established for a large class of linear commutators generated by BMO(R
n
) functions and linear operators of rough kernels which include the Calderón-Zygmund operators and the Ricci-Stein oRfiUatory
singular integrals with rough kernels.
Project supponed in pan by the National h’atural Science Foundation of China (Grant No. 19131080) and the NEDF of China. 相似文献
68.
杨晓光 《应用数学学报(英文版)》1997,13(4):337-341
1.IntroductionIntillspaperweanalyzetheconvergenceonmultiplicativeiterativealgorithmsfortheIninimizationofadiffcrentiablefunctiondefinedonthepositiveorthantofR".ThealgorithmissllggestedbyEggermolltl'],andisrelatedtotheEM[2](Expextation--Maximization)algoritllnlforPositronemissiontonlography[']andimagereconstructi..14].Wecollsidertheproblenl"linf(x)s.t.x20.Themultiplicativeiterativealgorithmshavethel'orlniforj=l,2,',n,withAhdeterminedthroughalinesearch.Whilelusem[5]establishedanelegantconv… 相似文献
69.
Ma Jiangle Yang Hong-shen Zhang Zhongxi Lu Zhong-zuo 《International Journal of Infrared and Millimeter Waves》1993,14(5):1069-1076
A simple method is introduced to improve the analysing accuracy of circular groove guide. With this method, the characteristic equation of circular groove guide is derived and its solution is given and discussed.The Project Supported by National Natural Science Foundation of P.R.China 相似文献
70.
采用密度泛涵理论(density functional theory,DFT)中的广义梯度近似(generally gradient approximation,GGA)对富勒烯C72和内掺金属La富勒烯La2@C72三种同分异构体的几何结构和电子结构进行研究.发现在C72的三种同分异构体中,满足独立五边形规则(isolated-pentagon-rule,IPR)的C72(D6d)结构最为稳定;在La2@C72三种同分异构体中,有着两对两两相邻五元环(twofused-pentagon)的La2@C72(#10611)结构最为稳定,而满足IPR的La2@C72(D6d)的结构变成了最不稳定结构.从能级图和态密度图分析得知,笼子稳定性的变化与La原子的原子轨道与C72原子轨道之间的杂化有关.Mulliken电荷分析得知,La2@C72(#10611)的两个La原子共转移了约3个电子给C72,并且,它们几乎分布在整个C笼上,形成的电子结构为La23+@C3-72.净自旋分析得知,La2@C72(#10611)中La原子磁性完全淬灭. 相似文献