全文获取类型
收费全文 | 9362篇 |
免费 | 1067篇 |
国内免费 | 844篇 |
专业分类
化学 | 2999篇 |
晶体学 | 125篇 |
力学 | 1109篇 |
综合类 | 197篇 |
数学 | 3400篇 |
物理学 | 3443篇 |
出版年
2024年 | 19篇 |
2023年 | 76篇 |
2022年 | 173篇 |
2021年 | 217篇 |
2020年 | 241篇 |
2019年 | 221篇 |
2018年 | 215篇 |
2017年 | 316篇 |
2016年 | 329篇 |
2015年 | 274篇 |
2014年 | 456篇 |
2013年 | 742篇 |
2012年 | 470篇 |
2011年 | 543篇 |
2010年 | 454篇 |
2009年 | 601篇 |
2008年 | 592篇 |
2007年 | 587篇 |
2006年 | 570篇 |
2005年 | 502篇 |
2004年 | 400篇 |
2003年 | 430篇 |
2002年 | 346篇 |
2001年 | 292篇 |
2000年 | 281篇 |
1999年 | 250篇 |
1998年 | 187篇 |
1997年 | 204篇 |
1996年 | 171篇 |
1995年 | 180篇 |
1994年 | 110篇 |
1993年 | 96篇 |
1992年 | 97篇 |
1991年 | 69篇 |
1990年 | 73篇 |
1989年 | 58篇 |
1988年 | 57篇 |
1987年 | 47篇 |
1986年 | 39篇 |
1985年 | 45篇 |
1984年 | 51篇 |
1983年 | 14篇 |
1982年 | 34篇 |
1981年 | 26篇 |
1980年 | 16篇 |
1979年 | 25篇 |
1978年 | 10篇 |
1977年 | 16篇 |
1975年 | 9篇 |
1974年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
NH chemical shift temperature coefficients have been measured in a large series of N-substituted-3-piperidinethiopropionamides in which the NN distances are short but of varied length, as well as in a couple of the corresponding amides and in some simpler amides and thioamides. Geometries are calculated by means of ab initio DFT methods. The N-substituted-3-piperidinethiopropionamides show in most cases strong intramolecular N–HN hydrogen bonds according to IR spectra and ab initio calculations. For compounds with rather short NN distances the S=C–N–H moiety is non-planar. Dihedral angles as small as 160° are found. The NH chemical shift coefficients measured in non-polar solvents in all the N-substituted-3-piperidinethiopropionamides are more negative (−8 to −17 ppb/K) than in non-hydrogen bonded thioamides. For the latter in non-polar solvents like CDCl3 and toluene the temperature coefficients are as small as −1 to −4 ppb/K. The large negative effects can be related not only to the non-planarity of the thioamide group in a way that the more pronounced the non-planarity the more negative the temperature coefficients, but also to strong hydrogen bonding and the fact that the acceptor is a nitrogen. For similar amides with non-planar amide groups and nitrogen acceptor large negative temperature coefficients are likewise seen. In polar solvents like DMF the effects in simple thioamides are uniform and close to −6 ppb/K, whereas in the more complex compound like 4p(t) the temperature coefficient is close to 0. An essential feature of measuring temperature coefficients of compounds without strong intramolecular hydrogen bonds in non-polar solvents and at low temperatures is to keep the concentration low enough to avoid dimerisation. 相似文献
22.
Hidetaka Tobita 《Journal of Polymer Science.Polymer Physics》2004,42(14):2801-2812
The matrix formula developed in the context of heterochain theory, M?w = M?wp + WF ( I ? M )?1 S , was applied to describe the molecular weight development during free‐radical multicomponent polymerization. All of the required probabilistic parameters are expressed in terms of the kinetic‐rate constants and the various concentrations associated with them. In free‐radical polymerization, the number of heterochain types, N, needs to be extrapolated to infinity, and such extrapolation is conducted with only three different N values. This matrix formula can be used as a benchmark test if other approximate approaches can give reasonable estimates of the weight‐average molecular weights. The moment equations with the average pseudo‐kinetic‐rate constants for branching and crosslinking reactions may provide poor estimates when the copolymer composition drift during polymerization is very significant. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2801–2812, 2004 相似文献
23.
24.
以三能级V型系统为例研究修饰态布居的选择性激发对无反转激光增益的作用. 当非 相干驱动场的频谱宽度远小于驱动场产生的修饰态能级的间距时,非相干驱动场只将一个修 饰态的布居抽运至激发态. 借助原子的衰减通道,系统中形成单向布居转移通道,从而建立 修饰态布居的选择性激发. 利用修饰态布居的选择性激发,可以摆脱裸态共振无反转激光的 三个限制: (1) 不再要求辅助的低频驱动跃迁比高频激光跃迁具有更高的衰减速率;(2) 显 著降低非相干激发速率的阈值;(3) 无反转激光的线性增益不再反比于相干驱动场的强
关键词:
修饰态布居的选择性激发
无反转激光增益
原子衰减速率
非相干激发阈值速率 相似文献
25.
26.
27.
Introduction In the previous studies on the oxidation reaction,peroxydisulfate was widely used as an oxidizingagent.1-5 One of the advantages of this oxidant lies in itsstability in a wide range of pH values. The reaction be-tween Fe(CN)5L3- (L=N-aromatic heterocyclic li-gands) and S2O8 2- has been proved to proceed throughan outer-sphere electron transfer mechanism.5 For a re-action [(Eq. (1)] to be under an outer-sphere mechanismthe steps involved are the formation of a reactant … 相似文献
28.
In this paper, we prove convergence rates for spherical spline Hermite interpolation on the sphere Sd−1 via an error estimate given in a technical report by Luo and Levesley. The functionals in the Hermite interpolation are either point evaluations of pseudodifferential operators or rotational differential operators, the desirable feature of these operators being that they map polynomials to polynomials. Convergence rates for certain derivatives are given in terms of maximum point separation. 相似文献
29.
DEEB MARJI KHAMIS ABBAS 《Journal of inclusion phenomena and macrocyclic chemistry》1997,27(4):269-277
The effect of 18-crown-6 and kryptofix (2,2,1) on the oxidation of triethylamine byaqueous ferricyanide ion has been studied at pH 4, 7, and 9.The macrocycle retarded the normal oxidation process at all pH values,the effect depending on the macrocyclic ligand concentration Kryptofix (2,2,1) had a greater effect than 18-crown-6 at all pHvalues in this study. For 18-crown-6, retardation decreases in the order pH 9 > 4 > 7 while forkryptofix it follows the order pH 9 > 7 > 4. 相似文献
30.
报道了KDP晶体中的三倍频产生的实验研究,计算了比较了直接三倍频过程和级联过程对KDP晶体三阶有效非线性系数的贡献,并探讨了KDP作为非线性晶体材料三阶非线性系数测量基准的可行性。 相似文献