全文获取类型
收费全文 | 5962篇 |
免费 | 396篇 |
国内免费 | 511篇 |
专业分类
化学 | 1716篇 |
晶体学 | 30篇 |
力学 | 176篇 |
综合类 | 57篇 |
数学 | 1576篇 |
物理学 | 1291篇 |
综合类 | 2023篇 |
出版年
2024年 | 3篇 |
2023年 | 64篇 |
2022年 | 117篇 |
2021年 | 185篇 |
2020年 | 217篇 |
2019年 | 225篇 |
2018年 | 215篇 |
2017年 | 207篇 |
2016年 | 210篇 |
2015年 | 263篇 |
2014年 | 380篇 |
2013年 | 413篇 |
2012年 | 651篇 |
2011年 | 335篇 |
2010年 | 258篇 |
2009年 | 285篇 |
2008年 | 273篇 |
2007年 | 252篇 |
2006年 | 206篇 |
2005年 | 190篇 |
2004年 | 212篇 |
2003年 | 198篇 |
2002年 | 198篇 |
2001年 | 177篇 |
2000年 | 201篇 |
1999年 | 174篇 |
1998年 | 131篇 |
1997年 | 109篇 |
1996年 | 89篇 |
1995年 | 66篇 |
1994年 | 57篇 |
1993年 | 37篇 |
1992年 | 43篇 |
1991年 | 27篇 |
1990年 | 29篇 |
1989年 | 22篇 |
1988年 | 17篇 |
1987年 | 20篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 10篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 9篇 |
1979年 | 11篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1973年 | 5篇 |
1959年 | 2篇 |
排序方式: 共有6869条查询结果,搜索用时 15 毫秒
81.
I. E. Filatov G. L. Rusinov O. N. Chupakhin X. Solans M. Font-Bardia M. Font-Altaba 《Russian Chemical Bulletin》1994,43(7):1214-1219
The reaction of 2-chloro-3,5-dinitropyridine with two equivalents of KN3 in the presence of ROH results in stable Meisenheimer-type -adducts of 4,6-dinitrotetra-zolo[1,5-a]pyridine with RO– anions (R = H, Alk, Ph). The mechanism of -complex formation was suggested. The structure of the -adduct with R = Me was established by IR and NMR spectroscopy and by X-ray diffraction analysis.For the preliminary communication, see Ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1278–1283, July, 1994.This work was supported by the Russian Foundation for Basic Research (project code 93-03-5329). 相似文献
82.
We consider variational problems involving nonlocal free energy functionals that arise from Gibbs measures with Kac potentials and are related to the characterization of the optimal (i.e., typical) shape of an interface under given constraints on the magnetization profile. 相似文献
83.
The Modified Barrier Functions (MBF) have elements of both Classical Lagrangians (CL) and Classical Barrier Functions (CBF). The MBF methods find an unconstrained minimizer of some smooth barrier function in primal space and then update the Lagrange multipliers, while the barrier parameter either remains fixed or can be updated at each step. The numerical realization of the MBF method leads to the Newton MBF method, where the primal minimizer is found by using Newton's method. This minimizer is then used to update the Lagrange multipliers. In this paper, we examine the Newton MBF method for the Quadratic Programming (QP) problem. It will be shown that under standard second-order optimality conditions, there is a ball around the primal solution and a cut cone in the dual space such that for a set of Lagrange multipliers in this cut cone, the method converges quadratically to the primal minimizer from any point in the aforementioned ball, and continues, to do so after each Lagrange multiplier update. The Lagrange multipliers remain within the cut cone and converge linearly to their optimal values. Any point in this ball will be called a hot start. Starting at such a hot start, at mostO(In In
-1) Newton steps are sufficient to perform the primal minimization which is necessary for the Lagrange multiplier update. Here, >0 is the desired accuracy. Because of the linear convergence of the Lagrange multipliers, this means that onlyO(In
-1)O(In In
-1) Newton steps are required to reach an -approximation to the solution from any hot start. In order to reach the hot start, one has to perform
Newton steps, wherem characterizes the size of the problem andC>0 is the condition number of the QP problem. This condition number will be characterized explicitly in terms of key parameters of the QP problem, which in turn depend on the input data and the size of the problem.Partially supported by NASA Grant NAG3-1397 and National Science Foundation Grant DMS-9403218. 相似文献
84.
Muhammad El-Taha 《Queueing Systems》1996,22(1-2):47-63
An input-output processZ = {Z(t), t 0} is said to be-rate stable ifZ(t) = o((t)) for some non-negative function(t). We prove that the processZ is -rate stable under weak conditions that include the assumption that input satisfies a linear burstiness condition and Z is asymptotically average stable. In many cases of interest, the conditions for-rate-stability can be verified from input data. For example, using input information, we establish-rate stability of the workload for multiserver queues, an ATM multiplexer, and-rate stability of queue-length processes for infinite server queues. 相似文献
85.
本文合成了通式为REL3(RE3+=Pr3+、Sm3+、Tb3+,,L=对茴香酸根)的二元配合物.用元素分析、电导、IR、UV、TGDTA等对其结构和性质进行了表征.该类配合物属桥式双齿配位 相似文献
86.
K. Bogdanowicz-Szwed M. Krasodomska M. Lipowska B. Rys A. Skonecka 《Monatshefte für Chemie / Chemical Monthly》1993,124(6-7):721-731
Summary The tandem Michael addition-cyclization of 2-oxo-cycloalkane carbothioic acid anilides1–3 to benzylidenemalononitrile4 yielded spiroannulated pyridines5–7. Reaction of acrylonitrile with2 and3 gave 2,2-disubstituted Michael adducts14,15, whereas with1 led to 2,2,5-tri(2-cyanoethyl)-cyclopentanone11.
Synthese von Pyridinderivaten durch Reaktionen von ,-ungesättigten Nitrilen mit 2-Oxo-cycloalkano-thiokohlensäure-aniliden
Zusammenfassung Die Michael Tandem-Addition-Cyclisierung von 2-Oxo-cycloalkano-thiokohlensäure-aniliden1–3 mit Benzylidenmalononitril4 ergab die spiroannelierten Pyridine5–7. Reaktion von Acrylnitril mit2 und3 ergab die 2,2-disubstituierten Michael-Addukte14,15, wohingegen mit1 2,2,5-Tri(2-cyanethyl)-cyclopentanon11 erhalten wurde.相似文献
87.
Yoshitsugu Hasegawa Suguru Kontani Isao Tomita 《Journal of inclusion phenomena and macrocyclic chemistry》1993,16(4):329-337
Intercalation of alkylalcohols into -zirconium phosphate was investigated at 25°C and/or under reflux.n-Alcohols having two to five carbons and 2-propanol were taken up at 25°C. These alcohols, andn-hexanol and heptanol, also intercalated under reflux.n-Alcohols having eight to eighteen carbon atoms intercalated when a stepwise method was employed. 2-Butanol and tertiary amylalcohols intercalated under reflux using a butanol intercalate as a starting material. Ethanol-to-butanol intercalates were unstable at room temperature, losing alcohols and changing to -zirconium phosphate. The particle size of the -zirconium phosphate did not affect the intercalation of alcohols. 相似文献
88.
The essential features (geometries of the minima and of the saddle points, energy barriers) of the potential energy surface of the four hydrides YXH4 mentioned in the title have been determined with two basis sets, of minimal and DZ quality respectively. The importance of the different extent of the deformation of the XH4 group in the different structures of the four hydrides is brought out and discussed. The aspects of charge distribution and bonding are examined drawing on population analysis, comparison of the electrostatic molecular potentials and decomposition of the interaction energy (this last referred to the Y+ + XH
4
–
YXH4 process). The capability of XH3 in effecting the etherolytic disruption of the Y-H bond is finally brought out. 相似文献
89.
A computational study is made of the effect of basis set upon the energy, properties and inversion barrier of the phosphine molecule. The calculations are performed at both the SCF and CI level. The flexibility of the double zeta basis is discussed in the light of the results. 相似文献
90.
我们已对O(~3p)原子和酮类分子化学反应速率常数进行了系列测定,分析讨论了羰基对O(~3p)抽提氢原子反应速率的影响,关于O(~3p)与醛类分子反应速率的测定也有报道。本文用流动微波放电-化学发光光子计数方法测定O(~3p)原子与丙醛和正丁醛化学反应速率常数及其与温度的关系,并由动力学数据对醛基中C—H键的离解能进行了讨论。 相似文献