全文获取类型
收费全文 | 7166篇 |
免费 | 1429篇 |
国内免费 | 760篇 |
专业分类
化学 | 5844篇 |
晶体学 | 41篇 |
力学 | 173篇 |
综合类 | 75篇 |
数学 | 2158篇 |
物理学 | 1064篇 |
出版年
2024年 | 21篇 |
2023年 | 118篇 |
2022年 | 176篇 |
2021年 | 259篇 |
2020年 | 407篇 |
2019年 | 302篇 |
2018年 | 320篇 |
2017年 | 269篇 |
2016年 | 408篇 |
2015年 | 401篇 |
2014年 | 503篇 |
2013年 | 714篇 |
2012年 | 461篇 |
2011年 | 462篇 |
2010年 | 384篇 |
2009年 | 443篇 |
2008年 | 506篇 |
2007年 | 422篇 |
2006年 | 389篇 |
2005年 | 335篇 |
2004年 | 307篇 |
2003年 | 257篇 |
2002年 | 206篇 |
2001年 | 162篇 |
2000年 | 173篇 |
1999年 | 168篇 |
1998年 | 127篇 |
1997年 | 122篇 |
1996年 | 116篇 |
1995年 | 83篇 |
1994年 | 57篇 |
1993年 | 46篇 |
1992年 | 42篇 |
1991年 | 28篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 16篇 |
1985年 | 10篇 |
1984年 | 14篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 10篇 |
1980年 | 8篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1974年 | 2篇 |
排序方式: 共有9355条查询结果,搜索用时 328 毫秒
171.
Despite the popularity of boron and silicon allylation reagents in stereocontrolled synthesis, they suffer from a number of inherent limitations that have slowed down their development as synthetic tools for nucleophilic additions to carbonyl compounds and imine derivatives. These limitations are the low reactivity and diastereoselectivity of allyl trialkylsilane reagents, and the lack of catalytic systems for the activation and substoichiometric control of enantioselectivity in the additions of allyl boron reagents. To develop more efficient and general methods for the control of absolute stereochemistry in the resulting homoallylic alcohols, new approaches aimed at solving the problem of activation of allylic boron and silicon reagents are needed. This Minireview describes a number of recent approaches that have been devised to address this problem. 相似文献
172.
Jian-binSong Min-qiaoRen Qing-yongChen Shu-yunWang Qing-xiangZhao Hong-fangZhang 莫志深 《高分子科学》2004,(5):491-496
Based on the X-ray scattering intensity theory and using the approximate expression for the atomic scattering factor, the correction factors for three crystalline peaks and an amorphous peak of Nylon 1212 were calculated and the formula of degree of crystallinity of Nylon 1212 was derived by a graphic multipeak resolution method. The degree of crystallinity calculated from the WAXD method is compatible with those obtained by density and calorimetry methods. 相似文献
173.
Rama Acharyya Shie-Ming Peng Ren-Zhang Wang Samaresh Bhattacharya 《Journal of organometallic chemistry》2005,690(17):3908-3917
Reaction of 2-(2′,6′-dimethylphenylazo)-4-methylphenol with [Ir(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3) affords an organoiridium complex 5, where the 2-(2′,6′-dimethylphenylazo)-4-methylphenol is coordinated to iridium, via C-H activation of a methyl group, as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. A similar reaction carried out in toluene affords complex 5 along with a similar complex 7, where a chloride is coordinated to iridium instead of the hydride. Reaction of 2-(2′-methylphenylazo)-4-methylphenol with [Ir(PPh3)3Cl] in refluxing ethanol in the presence of a base (NEt3) affords an organoiridium complex 12, where the 2-(2′-methylphenylazo)-4-methylphenol is coordinated to iridium, via C-H activation at the ortho position of the phenyl group in the 2′-methylphenylazo fragment, as a dianionic tridentate C,N,O-donor. Two triphenylphosphines and a hydride are also coordinated to the metal center. A similar reaction carried out in toluene affords a complex 12 along with a similar complex 13, where a chloride is coordinated to iridium instead of the hydride. Structures of complexes 5, 12 and 13 have been determined by X-ray crystallography. In all these complexes, the two triphenylphosphines are trans. All these complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows an Ir(III)-Ir(IV) oxidation within 0.60-0.73 V vs. SCE, followed by an oxidation of the coordinated 2-(arylazo)phenolate ligand within 1.08-1.39 V vs. SCE. A reduction of the coordinated 2-(arylazo)phenolate ligand is observed within −1.10 to −1.26 V vs. SCE. 相似文献
174.
部分水解聚丙烯酰胺在多孔介质中的静态吸附研究——水解度对吸附量的影响 总被引:8,自引:0,他引:8
部分水解聚丙烯酰胺在多孔介质中的静态吸附研究———水解度对吸附量的影响杨继萍李惠生黄鹏程(北京理工大学化工与材料学院北京100081)(北京航空航天大学材料科学与工程系北京100083)关键词部分水解聚丙烯酰胺,水解度,多孔介质,静态吸附聚合物作为... 相似文献
175.
A. K. Galwey 《Journal of Thermal Analysis and Calorimetry》2006,86(1):267-286
This critical
survey argues that the theory, conventionally used to interpret kinetic data
measured for thermal reactions of initially solid reactants, is not always
suitable for elucidating reaction chemistry and mechanisms or for identifying
reactivity controls. Studies of solid-state decompositions published before
the 1960s usually portrayed the reaction rate as determined by Arrhenius type
models closely related to those formulated for homogeneous rate processes,
though scientific justifications for these parallels remained incompletely
established. Since the 1960s, when thermal analysis techniques were developed,
studies of solid-state decompositions contributed to establishment of the
new experimental techniques, but research interest became redirected towards
increasing the capabilities of automated equipment to collect, to store and
later to analyze rate changes for selected reactions. Subsequently, much less
attention has been directed towards chemical features of the rate processes
studied, which have included a range of reactants that is much more diverse
than the simple solid-state reactions with which early thermokinetic studies
were principally concerned. Moreover, the theory applied to these various
reactants does not recognize the possible complexities of behaviour that may
include mechanisms involving melting and/or concurrent/consecutive reactions,
etc. The situation that has arisen following, and attributable to, the eclipse
of solid-state decomposition studies by thermal analysis, is presented here
and the consequences critically discussed in a historical context. It is concluded
that methods currently used for kinetic and mechanistic investigations of
all types of thermal reactions indiscriminately considered by the same, but
inadequate theory, are unsatisfactory. Urgent and fundamental reappraisal
of the theoretical foundations of thermokinetic chemical studies is now necessary
and overdue.
While there are important, but hitherto unrecognized,
delusions in thermokinetic methods and theories, an alternative theoretical
explanation that accounts for many physical and chemical features of crystolysis
reactions has been proposed. However, this novel but general model for the
thermal behaviour and properties of solids has similarly remained ignored
by the thermoanalytical community. The objective of this article is to emphasize
the now pressing necessity for an open debate between these unreconciled opinions
of different groups of researchers. The ethos of science is that disagreement
between rival theories can be resolved by experiment and/or discussion, which
may also strengthen the foundations of the subject in the process. As pointed
out below, during recent years there has been no movement towards attempting
to resolve some fundamental differences of opinion in a field that lacks an
adequate theory. This should be unacceptable to all concerned. Here some criticisms
are made of specific features of the alternative reaction models available
with the stated intention of provoking a debate that might lead to identification
of the significant differences between the currently irreconciled views. This
could, of course, attract the displeasure of both sides, who will probably
criticise me severely. Because I intend to retire completely from this field
soon, it does not matter to me if I am considered to be ‘wrong’,
if it contributes to us all eventually agreeing to get the science ‘right’. 相似文献
176.
177.
178.
Substituted 5-aryl-3-ethylidene-3H-pyrrol-2-ones were synthesized by the reaction of the corresponding 4-aryl-4-oxobutanoic acids with ketones in the presence
of aminating agents. The conditions of this reaction were developed with the use of both the conventional condensation technique
and microwave activation. The structures of the reaction products were con-firmed by elemental analysis, IR spectroscopy,
and 1H NMR spectroscopy.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 693–696, April, 2006. 相似文献
179.
碳氟醇与阴离子表面活性剂的相互作用 总被引:2,自引:0,他引:2
测定了不同比例的C_(10)_H_(21)SO_4Na-C_3F_7CH_2OH、C_7F_(15)COONa-C_3H_7CH_2OH混合水溶液的表面张力,加入C_3F_7CH_2OH可增加阴离子表面活性剂的表面活性;在表面层中,C_3F_7CH_2OH与C_(10)H_(21)SO_4Na间分子相互作用比C_3F_7CH_2OH-C_7F_(15)COONa体系弱;这是由于CF链与CH链间“互疏”作用的结果;随着C_3F_7CH_2OH浓度增加,对C_(10)H_(21)SO_4Na胶团反离子结合度也随之增加。 相似文献
180.