全文获取类型
收费全文 | 103690篇 |
免费 | 18749篇 |
国内免费 | 24043篇 |
专业分类
化学 | 75318篇 |
晶体学 | 2721篇 |
力学 | 7093篇 |
综合类 | 2158篇 |
数学 | 14048篇 |
物理学 | 45144篇 |
出版年
2024年 | 319篇 |
2023年 | 1672篇 |
2022年 | 3944篇 |
2021年 | 4066篇 |
2020年 | 4142篇 |
2019年 | 3944篇 |
2018年 | 3528篇 |
2017年 | 4268篇 |
2016年 | 4453篇 |
2015年 | 5248篇 |
2014年 | 6329篇 |
2013年 | 8037篇 |
2012年 | 8907篇 |
2011年 | 9228篇 |
2010年 | 7803篇 |
2009年 | 7752篇 |
2008年 | 8499篇 |
2007年 | 7672篇 |
2006年 | 7131篇 |
2005年 | 6165篇 |
2004年 | 4775篇 |
2003年 | 3504篇 |
2002年 | 3422篇 |
2001年 | 3236篇 |
2000年 | 3231篇 |
1999年 | 2361篇 |
1998年 | 1598篇 |
1997年 | 1345篇 |
1996年 | 1322篇 |
1995年 | 1225篇 |
1994年 | 1096篇 |
1993年 | 984篇 |
1992年 | 840篇 |
1991年 | 685篇 |
1990年 | 636篇 |
1989年 | 546篇 |
1988年 | 387篇 |
1987年 | 361篇 |
1986年 | 312篇 |
1985年 | 263篇 |
1984年 | 226篇 |
1983年 | 187篇 |
1982年 | 146篇 |
1981年 | 134篇 |
1980年 | 96篇 |
1979年 | 97篇 |
1978年 | 34篇 |
1977年 | 39篇 |
1976年 | 36篇 |
1975年 | 28篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Nonlinear Dynamics - Global dynamics of a flexible asymmetrical rotor resting on vibrating supports is investigated. Hamilton’s principle is used to derive the partial differential governing... 相似文献
42.
43.
Two new homosecoiridoids, named loniaceticiridoside (1) and lonimalondialiridoside (2), were isolated from an aqueous extract of the flower buds of Lonicera japonica. Their structures including the absolute configuration were determined by extensive spectroscopic studies, especially by 2D NMR and CD data analysis. A proposed biosynthetic pathway and preliminary investigations of the biological activity of compounds 1 and 2 are also discussed. 相似文献
44.
对石英音叉增强型光声光谱(QEPAS)系统中常用的石英音叉进行了有限元模态计算,获得石英音叉前6阶振型与模态频率,认知了第4阶对称摆动振型为有效振动,利用单因素法分析了石英音叉的音臂长度l1、音臂宽度w1、音臂厚度t、音臂切角θ、音臂圆孔直径d及音臂圆孔高度h对低阶有效共振频率(Fre)的影响,敏感度依次为: l1> w1>d>θ>t>h,考虑实际设计情形,筛选出了l1,w1,d与h四个石英音叉设计变量,采用Box-Behnken实验设计方案与RSM(response surface methodology)方法,以Fre为函数目标,建立l1,w1,d与h的二次回归响应面模型,得到了参数之间的交互作用,利用Design-Expert软件对响应面模型进行设计参数反求,结果表明,在15 000 Hz≤Fre≤25 000 Hz计算区域内误差较小,基本满足QEPAS系统的计算需求,所提出的研究与设计方法具有一定通用性,可为QEPAS系统中石英音叉结构参数设计提供参考。 相似文献
45.
Ortiz Adolfo Yang Jianhua Coccolo Mattia Seoane Jesús M. Sanjuán Miguel A. F. 《Nonlinear dynamics》2020,102(4):2323-2337
Nonlinear Dynamics - The main purpose of this paper is to study both the underdamped and the overdamped dynamics of the nonlinear Helmholtz oscillator with a fractional-order damping. For that... 相似文献
46.
47.
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
48.
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
49.
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
50.
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