全文获取类型
收费全文 | 9022篇 |
免费 | 1246篇 |
国内免费 | 867篇 |
专业分类
化学 | 2976篇 |
晶体学 | 41篇 |
力学 | 2367篇 |
综合类 | 181篇 |
数学 | 1792篇 |
物理学 | 3778篇 |
出版年
2024年 | 22篇 |
2023年 | 92篇 |
2022年 | 246篇 |
2021年 | 276篇 |
2020年 | 324篇 |
2019年 | 267篇 |
2018年 | 290篇 |
2017年 | 341篇 |
2016年 | 380篇 |
2015年 | 316篇 |
2014年 | 337篇 |
2013年 | 776篇 |
2012年 | 437篇 |
2011年 | 465篇 |
2010年 | 349篇 |
2009年 | 421篇 |
2008年 | 438篇 |
2007年 | 448篇 |
2006年 | 517篇 |
2005年 | 449篇 |
2004年 | 396篇 |
2003年 | 406篇 |
2002年 | 388篇 |
2001年 | 381篇 |
2000年 | 355篇 |
1999年 | 284篇 |
1998年 | 225篇 |
1997年 | 203篇 |
1996年 | 181篇 |
1995年 | 171篇 |
1994年 | 136篇 |
1993年 | 111篇 |
1992年 | 107篇 |
1991年 | 81篇 |
1990年 | 77篇 |
1989年 | 55篇 |
1988年 | 61篇 |
1987年 | 41篇 |
1986年 | 29篇 |
1985年 | 43篇 |
1984年 | 42篇 |
1983年 | 15篇 |
1982年 | 35篇 |
1981年 | 18篇 |
1980年 | 10篇 |
1979年 | 12篇 |
1978年 | 17篇 |
1977年 | 13篇 |
1976年 | 13篇 |
1957年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
Dr. Carla Carrera Dr. Eleonora Cavallari Dr. Giuseppe Digilio Dr. Oksana Bondar Prof. Silvio Aime Dr. Francesca Reineri 《Chemphyschem》2021,22(11):1042-1048
An efficient synthesis of vinyl-[1-13C]pyruvate has been reported, from which 13C hyperpolarized (HP) ethyl-[1-13C]pyruvate has been obtained by means of ParaHydrogen Induced Polarization (PHIP). Due to the intrinsic lability of pyruvate, which leads quickly to degradation of the reaction mixture even under mild reaction conditions, the vinyl-ester has been synthesized through the intermediacy of a more stable ketal derivative. 13C and 1H hyperpolarizations of ethyl-[1-13C]pyruvate, hydrogenated using ParaHydrogen, have been compared to those observed on the more widely used allyl-derivative. It has been demonstrated that the spin order transfer from ParaHydrogen protons to 13C, is more efficient on the ethyl than on the allyl-esterdue to the larger J-couplings involved. The main requirements needed for the biological application of this HP product have been met, i. e. an aqueous solution of the product at high concentration (40 mM) with a good 13C polarization level (4.8 %) has been obtained. The in vitro metabolic transformation of the HP ethyl-[1-13C]pyruvate, catalyzed by an esterase, has been observed. This substrate appears to be a good candidate for in vivo metabolic investigations using PHIP hyperpolarized probes. 相似文献
3.
The structure and rotational barrier for the mesityl-silicon bond of 2,2-dimesityl-1,1,1,3,3,3-hexamethyltrisilane have been investigated by 1H- and 13C-variable temperature nuclear magnetic resonance (NMR) as well as by density functional theory structural calculations. The calculations show that the lowest energy structure has C2 symmetry with nonequivalent ortho methyl groups, consistent with the crystal structure and solution NMR. The nonequivalent ortho methyl groups exchange through a Cs transition state with a calculated relative free energy of 11.0 kcal mol−1. The barrier for this rotation found by dynamic NMR is 13.4 ± 0.2 kcal mol−1 at 298 K. 相似文献
4.
Dr. Liat Avram Dr. Václav Havel Ronit Shusterman-Krush Dr. Mark A. Iron Dr. Moritz Zaiss Prof. Vladimír Šindelář Dr. Amnon Bar-Shir 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(7):1687-1690
The accumulated knowledge regarding molecular architectures is based on established, reliable, and accessible analytical tools that provide robust structural and functional information on assemblies. However, both the dynamicity and low population of noncovalently interacting moieties within studied molecular systems limit the efficiency and accuracy of traditional methods. Herein, the use of a saturation transfer-based NMR approach to study the dynamic binding characteristics of an anion to a series of synthetic receptors derived from bambusuril macrocycles is demonstrated. The exchange rates of BF4− are mediated by the side chains on the receptor (100 s−1<kex<5000 s−1), which play a critical role in receptor-anion binding dynamics. The signal amplification obtained with this approach allows for the identification of different types of intermolecular interactions between the receptor and the anion, something that could not have been detected by techniques hitherto used to study molecular assemblies. These findings, which are supported by a computational molecular dynamic study, demonstrate the uniqueness and added value of this NMR method. 相似文献
5.
In this paper, we consider a viscoelastic wave equation of variable coefficients in the presence of past history with nonlinear damping and delay in the internal feedback and dynamic boundary conditions. Under suitable assumptions, we establish an explicit and general decay rate result without imposing restrictive assumption on the behavior of the relaxation function at infinity by Riemannian geometry method and Lyapunov functional method. 相似文献
6.
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
7.
We consider a resource management problem in which the management objective is to minimize fluctuations in resource stocks. Stabilizing management policies constitute the designing of memoryless state feedback control strategies for a discrete time resource model which contains unknown but bounded fluctuations. We also show that the problem of maximizing sustainable yield in an uncertain fishery can be considered as the problem of stabilizing of the stock level.The paper corresponds to an invited talk at the 14th Symposium on Operations Research, Ulm, September 6–8, 1989.The support by Yrjö Jahnsson Foundation is gratefully acknowledged.This work was supported by NSF and AFOSR under grant ECS 8602524. 相似文献
8.
9.
Franz H. Asthalter T. Dommach M. Ehnes A. Messel K. Sergueev I. 《Hyperfine Interactions》2002,141(1-4):131-134
The present status of the new nuclear resonance beamline PETRA 1 at HASYLAB, DESY, Hamburg is described. Besides an overview
of the experimental setup some examples of recent experiments are given. Those cover the main applications, i.e., inelastic
scattering from iron alloys and quasielastic scattering from glass-forming liquids.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
Papert's (1978) appeal to reconsider the power and possibilities of the aesthetic in mathematics learning is often ignored in mathematics education research. This paper begins with the premise, put forth by Dewey (1934), that the aesthetic structures many dimensions of inquiry and experience. In the same way that using particular paintings, musical compositions, or even everyday experiences has been instrumental to attempts by philosophers to understand the aesthetic dimensions of meaning and experience in artistic domains, I propose that analysing a particular encounter with mathematics may help reveal the nature and role of the often nebulous responses of elegance, beauty, and `fit' to which mathematicians lay claim in their mathematical activity. To achieve this, I draw on and adapt the defining features of the aesthetic character of experience set forth by the aesthetician Beardsley (1982). This, in turn, sheds light on the role thataesthetics can play in mathematical inquiry and experience, and provides initial categories and conjectures that can be used to investigate the potential roles of aesthetics in mathematics learning contexts.This revised version was published online in September 2005 with corrections to the Cover Date. 相似文献