全文获取类型
收费全文 | 10684篇 |
免费 | 391篇 |
国内免费 | 86篇 |
专业分类
化学 | 7765篇 |
晶体学 | 52篇 |
力学 | 257篇 |
数学 | 1592篇 |
物理学 | 1495篇 |
出版年
2023年 | 43篇 |
2022年 | 72篇 |
2021年 | 149篇 |
2020年 | 174篇 |
2019年 | 163篇 |
2018年 | 127篇 |
2017年 | 106篇 |
2016年 | 273篇 |
2015年 | 229篇 |
2014年 | 276篇 |
2013年 | 603篇 |
2012年 | 676篇 |
2011年 | 842篇 |
2010年 | 357篇 |
2009年 | 287篇 |
2008年 | 635篇 |
2007年 | 717篇 |
2006年 | 742篇 |
2005年 | 621篇 |
2004年 | 536篇 |
2003年 | 453篇 |
2002年 | 429篇 |
2001年 | 154篇 |
2000年 | 103篇 |
1999年 | 93篇 |
1998年 | 89篇 |
1997年 | 114篇 |
1996年 | 121篇 |
1995年 | 90篇 |
1994年 | 83篇 |
1993年 | 101篇 |
1992年 | 78篇 |
1991年 | 61篇 |
1990年 | 86篇 |
1989年 | 52篇 |
1988年 | 66篇 |
1987年 | 65篇 |
1986年 | 75篇 |
1985年 | 122篇 |
1984年 | 119篇 |
1983年 | 72篇 |
1982年 | 104篇 |
1981年 | 103篇 |
1980年 | 100篇 |
1979年 | 72篇 |
1978年 | 90篇 |
1977年 | 69篇 |
1976年 | 65篇 |
1975年 | 59篇 |
1973年 | 63篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Synthesis of Highly Substituted γ‐Butyrolactones by a Gold‐Catalyzed Cascade Reaction of Benzyl Esters 下载免费PDF全文
Maria Camila Blanco Jaimes Alexander Ahrens Daniel Pflästerer Dr. Matthias Rudolph Prof. Dr. A. Stephen K. Hashmi 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(1):427-433
Easily accessible benzylic esters of 3‐butynoic acids in a gold‐catalyzed cyclization/rearrangement cascade reaction provided 3‐propargyl γ‐butyrolactones with the alkene and the carbonyl group not being conjugated. Crossover experiments showed that the formation of the new C?C bond is an intermolecular process. Initially propargylic–benzylic esters were used, but alkyl‐substituted benzylic esters worked equally well. In the case of the propargylic–benzylic products, a simple treatment of the products with aluminum oxide initiated a twofold tautomerization to the allenyl‐substituted γ‐butyrolactones with conjugation of the carbonyl group, the olefin, and the allene. The synthetic sequence can be conducted stepwise or as a one‐pot cascade reaction with similar yields. Even in the presence of the gold catalyst the new allene remains intact. 相似文献
2.
Giorgia Scetta Jianzhu Ju Nathan Selles Patrick Heuillet Matteo Ciccotti Costantino Creton 《Journal of polymer science. Part A, Polymer chemistry》2021,59(8):685-696
We investigate the cyclic mechanical behavior in uniaxial tension of three different commercial thermoplastic polyurethane elastomers (TPU) often considered as a sustainable replacement for common filled elastomers. All TPU have similar hard segment contents and linear moduli but sensibly different large strain properties as shown by X-ray analysis. Despite these differences, we found a stiffening effect after conditioning in step cyclic loading which greatly differs from the common softening (also referred as Mullins effect) observed in chemically crosslinked filled rubbers. We propose that this self-reinforcement is related to the fragmentation of hard domains, naturally present in TPU, in smaller but more numerous sub-units that may act as new physical crosslinking points. The proposed stiffening mechanism is not dissimilar to the strain-induced crystallization observed in stretched natural rubber, but it presents a persistent nature. In particular, it may cause a local reinforcement where an inhomogeneous strain field is present, as is the case of a crack propagating in cyclic fatigue, providing a potential explanation for the well-known toughness and wear resistance of TPU. 相似文献
3.
4.
Kounta Moussa Dawson Nathan J. 《Methodology and Computing in Applied Probability》2021,23(3):1155-1172
Methodology and Computing in Applied Probability - The problem of optimally controlling one-dimensional diffusion processes until they enter a given stopping set is extended to include Markov... 相似文献
5.
Dr. Wan‐Ling Liu Ni‐Shin Yang Ya‐Ting Chen Stephen Lirio Cheng‐You Wu Prof. Chia‐Her Lin Prof. Hsi‐Ya Huang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(1):115-119
A green and sustainable strategy synthesizes clinical medicine warfarin anticoagulant by using lipase‐supported metal–organic framework (MOF) bioreactors (see scheme). These findings may be beneficial for future studies in the industrial production of chemical, pharmaceutical, and agrochemical precursors. 相似文献
6.
The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness. Methods by which the intrinsic molecular strength of the constituents of a given polymeric material might be switched are therefore potentially useful both for applications in which triggered property changes are desirable, and as tests of molecular theories for bulk behaviors. Here we report that the sequential oxidation of sulfide containing polyesters (PE-S) to the corresponding sulfoxide (PE-SO) and then sulfone (PE-SO2) first weakens (sulfoxide), and then enhances (sulfone), the effective mechanical integrity of the polymer backbone; PE-S ∼ PE-SO2 > PE-SO. The relative mechanical strength as a function of oxidation state is revealed through the use of gem-dichlorocyclopropane nonscissile mechanophores as an internal standard, and the observed order agrees well with the reported bond dissociation energies of C–S bonds in each species and with the results of CoGEF modeling.The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness. 相似文献
7.
8.
Stephen C. Littlechild 《The Journal of the Operational Research Society》1996,47(5):601-611
In this Blackett Lecture, Professor Littlechild, Director General of Electricity Supply in the UK's Office of Electricity Regulation, renews acquaintance with some models he has developed over the years, introduces some models he has recently met, and discusses models that it would seem to be worthwhile developing. He concludes that for all concerned in the electricity industry, success depends on ever improving decision-making, in which OR has a vital role. 相似文献
9.
The dependence of the beam propagation factor (M
2 parameter) with the absorbed pump power in the case of monolithic microchip laser under face-cooled configuration is extensively
studied. Our investigations show that the M
2 parameter is related to the absorbed pump power through two parameters (α and β) whose values depend on the laser material properties and laser configuration. We have shown that one parameter arises due
to the oscillation of higher order modes in the microchip cavity and the other parameter accounts for the spherical aberration
associated with the thermal lens induced by the pump beam. Such dependency of M
2 parameter with the absorbed pump power is experimentally verified for a face-cooled monolithic microchip laser based on Nd3+ -doped GdVO4 crystal and the values of α and β parameters were estimated from the experimentally measured data points. 相似文献
10.
Let V be a hypersurface with an isolated singularity at the origin in ? n+1. It is a natural question to ask when V is defined by weighted homogeneous polynomial or homogeneous polynomial up to biholomorphic change of coordinates. In 1971, a beautiful theorem of Saito gives a necessary and sufficient condition for V to be defined by a weighted homogeneous polynomial. For a two-dimensional isolated hypersurface signularity V, Xu and Yau found a coordinate free characterization for V to be defined by a homogeneous polynomial. Recently Lin and Yau gave necessary and sufficient conditions for a 3-dimensional isolated hypersurface singularity with geometric genus bigger than zero to be defined by a homogeneous polynomial. The purpose of this paper is to prove that Lin-Yau’s theorem remains true for singularities with geometric genus equal to zero. 相似文献