全文获取类型
收费全文 | 11167篇 |
免费 | 985篇 |
国内免费 | 1142篇 |
专业分类
化学 | 9022篇 |
晶体学 | 150篇 |
力学 | 68篇 |
综合类 | 32篇 |
数学 | 2957篇 |
物理学 | 1065篇 |
出版年
2024年 | 14篇 |
2023年 | 115篇 |
2022年 | 126篇 |
2021年 | 172篇 |
2020年 | 274篇 |
2019年 | 326篇 |
2018年 | 253篇 |
2017年 | 277篇 |
2016年 | 316篇 |
2015年 | 409篇 |
2014年 | 509篇 |
2013年 | 987篇 |
2012年 | 698篇 |
2011年 | 641篇 |
2010年 | 568篇 |
2009年 | 636篇 |
2008年 | 738篇 |
2007年 | 683篇 |
2006年 | 548篇 |
2005年 | 553篇 |
2004年 | 512篇 |
2003年 | 447篇 |
2002年 | 647篇 |
2001年 | 337篇 |
2000年 | 326篇 |
1999年 | 272篇 |
1998年 | 258篇 |
1997年 | 211篇 |
1996年 | 174篇 |
1995年 | 140篇 |
1994年 | 144篇 |
1993年 | 110篇 |
1992年 | 100篇 |
1991年 | 75篇 |
1990年 | 81篇 |
1989年 | 86篇 |
1988年 | 60篇 |
1987年 | 46篇 |
1986年 | 44篇 |
1985年 | 53篇 |
1984年 | 48篇 |
1983年 | 33篇 |
1982年 | 36篇 |
1981年 | 33篇 |
1980年 | 35篇 |
1979年 | 31篇 |
1978年 | 21篇 |
1974年 | 13篇 |
1973年 | 16篇 |
1971年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
在微波放电系统中,对NH_3-F-F_2-CF_3I体系进行研究,结果表明,向IF(X)传能的诸多媒介中,N_2(A)及N(~2D)起着主要作用,并且这一结论在经微波激发后的N_2与CF_3I的直接反应中得到了进一步证实。 相似文献
62.
Georg Süss-Fink Ulf Bodensieck Lisa Hoferkamp Gerd Rheinwald Helen Stoeckli-Evans 《Journal of Cluster Science》1992,3(4):469-478
The reaction of trirutheniumdodecacarbonyl with various thioureas was found to give, depending upon the substituents and the reaction conditions, a large variety of tri-, tetra-, penta-, and hexanuclear ruthenium clusters. The ligand systems observed in the products are derived from the thioureas employed; the fragmentation of the thiourea molecules involve N-H, C-S, C-H, and C-N activation processes. 相似文献
63.
64.
Noora Virtanen Ville Nevalainen Taru Lehtinen Satu Mikkola 《Journal of Physical Organic Chemistry》2007,20(1):72-82
Transesterification of a phosphodiester bond of RNA models has been studied in various buffer solutions, under neutral and slightly alkaline conditions in H2O and D2O. The results show that imidazole is the only buffer system where a clear buffer catalysis on the cleavage of a phosphodiester bond is observed. The rate enhancement in sulphonic acid buffers is smaller, and a sulphonate base, particularly, is inactive as a catalyst. The rate‐enhancing effect of imidazole is, however, catalytic, and the catalytic inactivity of sulphonate buffers can be attributed to their structure and/or charge. The catalysis by imidazole is a complex system which, in addition to first‐order reactions, involves a process that shows a second‐order dependence in imidazole concentration. The latter reaction becomes significant in acidic imidazole buffers (pH < pKa), as the buffer concentration increases. The kinetic solvent deuterium isotope effect kH/kD, referring to first‐order catalysis by imidazole base, is 2.3 ± 0.3. That referring to second‐order catalysis is most probably much larger, but an accurate value could not be obtained. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
65.
Sheng‐Huei Hsiao Chien‐Wei Chen Guey‐Sheng Liou 《Journal of polymer science. Part A, Polymer chemistry》2004,42(13):3302-3313
Two new diamines, 2,4‐diaminotriphenylamine ( 3 ) and N‐(2,4‐diaminophenyl)carbazole ( 4 ), were synthesized via the cesium fluoride‐mediated aromatic substitution reactions of 1‐fluoro‐2,4‐dinitrobenzene with diphenylamine and carbazole, followed by palladium‐catalyzed hydrazine reduction. Amorphous and soluble aramids having pendent diphenylamino and carbazolyl groups were prepared by the phosphorylation polycondensation of aromatic dicarboxylic acids with diamines 3 and 4 , respectively. The aramids derived from diamine 3 had sufficiently high molecular weights to permit the casting of flexible and tough films. They exhibited excellent mechanical properties and moderately high softening temperatures in the 221–298 °C range. However, the reactions of diamine 4 with aromatic diacids gave relatively lower molecular weights products that could not afford flexible films. For a comparative purpose, the parent aramids derived from m‐phenylenediamine and aromatic diacids were also prepared and characterized. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3302–3313, 2004 相似文献
66.
Perch Melik-Adamyan 《Complex Analysis and Operator Theory》2007,1(3):423-438
An analog of the Kreĭn–Saakyan formula is derived for any pair of relatively prime self-adjoint extensions of a minimal symmetric
canonical differential operator. This allows us to deduce a trace formula in the matrix case.
I am grateful to Sh. Saakyan for his interest in this work and lively discussion.
Received: December 8, 2006. Accepted: December 30, 2006. 相似文献
67.
68.
S. -S. Chow 《Numerische Mathematik》1989,54(4):373-393
Summary In this paper we shall consider the application of the finite element method to a class of second order elliptic boundary value problems of divergence form and with gradient nonlinearity in the principal coefficient, and the derivation of error estimates for the finite element approximations. Such problems arise in many practical situations — for example, in shock-free airfoil design, seepage through coarse grained porous media, and in some glaciological problems. By making use of certain properties of the nonlinear coefficients, we shall demonstrate that the variational formulations associated with these boundary value problems are well-posed. We shall also prove that the abstract operators accompanying such problems satisfy certain continuity and monotonicity inequalities. With the aid of these inequalities and some standard results from approximation theory, we show how one may derive error estimates for the finite element approximations in the energy norm. 相似文献
69.
70.
The oxidation behaviour of LnSiAlON (Ln=Y, La) glasses was studied at different temperatures (990-1150 °C) and under different water vapour pressures (360-2690 Pa). These results were also compared with those obtained under O2, N2/H2O or O2/H2O mixtures. When glasses are treated under a N2/H2O mixture, optical and SEM observations show porous scales. Transformations of the reaction rate data and a kinetic model show that there is only one limiting process occurring during oxidation. This rate limiting step is the progress of the chemical reaction at the internal interface. Determination of the pressure law dependence and thermodynamics calculations of water vapour molecules dissociation at the investigated temperatures allow us to suggest that the mechanism of oxidation corresponds to decomposition of water molecules on the oxynitride glass surface. 相似文献