全文获取类型
收费全文 | 2429篇 |
免费 | 104篇 |
国内免费 | 9篇 |
专业分类
化学 | 2000篇 |
晶体学 | 12篇 |
力学 | 27篇 |
数学 | 234篇 |
物理学 | 269篇 |
出版年
2023年 | 24篇 |
2022年 | 47篇 |
2021年 | 69篇 |
2020年 | 48篇 |
2019年 | 60篇 |
2018年 | 40篇 |
2017年 | 38篇 |
2016年 | 81篇 |
2015年 | 86篇 |
2014年 | 82篇 |
2013年 | 122篇 |
2012年 | 150篇 |
2011年 | 206篇 |
2010年 | 122篇 |
2009年 | 113篇 |
2008年 | 157篇 |
2007年 | 155篇 |
2006年 | 118篇 |
2005年 | 123篇 |
2004年 | 106篇 |
2003年 | 95篇 |
2002年 | 75篇 |
2001年 | 38篇 |
2000年 | 24篇 |
1999年 | 34篇 |
1998年 | 17篇 |
1997年 | 13篇 |
1996年 | 25篇 |
1995年 | 25篇 |
1994年 | 25篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 15篇 |
1990年 | 21篇 |
1989年 | 16篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 15篇 |
1984年 | 9篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 5篇 |
1980年 | 9篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 5篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 4篇 |
排序方式: 共有2542条查询结果,搜索用时 18 毫秒
1.
2.
3.
Mary E. Neubert DAVID G. ABDALLAH Jr Sandra S. Keast Julie M. Kim Soonnam Lee Ryan M. Stayshich Margaret E. Walsh Rolfe G. Petschek Shin-Tson Wu 《Liquid crystals》2003,30(6):711-731
New diphenyldiacetylenes of the type with A, B = H and/or F; m = 0, 1; n = 1-4; and X = C n H 2n + 1 , F, CF 3 or CN were synthesized and their mesomorphic properties determined by hot stage polarizing microscopy and DSC. When m = 0, all of these compounds showed only a nematic phase except when X = CF 3 when both nematic and smectic A phases were seen. Both clearing and melting temperatures were higher than those reported for substitution with the corresponding alkyl chains but the much larger increase in clearing temperatures produced considerably wider nematic phases. Eutectic mixtures of a few of these olefins yielded nematic materials also having much wider temperature ranges and higher clearing temperatures than the eutectic mixtures of the alkyl compounds, while retaining their high birefringence and low viscosities. Such materials are of interest for beam-steering devices.
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
Four of the diacetylenes with m = 1 ( A, B = H) were also prepared ( X = C 6 H 13 , F, n = 2, 3). When X was C 6 H 13 ( n = 2), the nematic range was smaller in the 2- than in the 1-olefin but wider than in the alkyl series. When X = F, either no nematic phase or a monotropic one was observed, whereas the 1-olefins gave a much wider nematic phase. Both transition temperatures were lower than those for the corresponding 1-olefin and alkyl analogues. The compound with X = C 6 H 13 and n = 2 had a melting temperature below room temperature. 相似文献
4.
Zero-schemes on smooth complex projective varieties, forcing all elements of ample and free linear systems to be reducible, are studied. Relationships among the minimal length of such zero-schemes, the positivity of the line bundle associated with the linear system, and the dimension of the variety are established. Bad linear spaces are also investigated. 相似文献
5.
6.
7.
Giuseppe Bartoli Sandra Giuli Enrico Marcantoni Massimo Massaccesi 《Tetrahedron letters》2006,47(37):6501-6504
The arylidene malonates with two different geminal carboxylate functions, a suitable class of substrates of several synthetic and pharmacological studies, are easily available through Knoevenagel condensation of ethyl tert-butyl malonate and different aromatic aldehydes. The results have increased the potentialities of CeCl3·7H2O-NaI system as a type of water-tolerant green Lewis acid promoter for carbon-carbon bond forming procedures. 相似文献
8.
9.
A simple method has been developed for the simultaneous determination of N-butylscopolamine bromide and oxazepam in pharmaceutical formulations using first-order digital derivative spectrophotometry. Acetonitrile was selected as the solvent in which both compounds showed well-defined bands. Both analytes showed good stability in this solvent when solutions of the analytes were exposed to light and temperatures between 20 degrees and 80 degrees C. The simultaneous determination of both drugs was performed by the zero-crossing method at 226.0 and 257.0 nm for N-butylscopolamine and oxazepam, respectively. The linear range of determination was found to be 2.5 x 10(-7) to 8.0 x 10(-5) mol/L for N-butylscopolamine and 7.1 x 10(-8) to 8.0 x 10(-5) mol/L for oxazepam. A very good level of repeatability (relative standard deviation) of 0.2% was observed for N-butylscopolamine and oxazepam. The ingredients commonly found in pharmaceutical formulations do not interfere. The proposed method was applied to the determination of these drugs in pharmaceutical formulations (capsules). 相似文献
10.
Merchant KA Noid WG Akiyama R Finkelstein IJ Goun A McClain BL Loring RF Fayer MD 《Journal of the American Chemical Society》2003,125(45):13804-13818
Spectrally resolved infrared stimulated vibrational echo data were obtained for sperm whale carbonmonoxymyoglobin (MbCO) at 300 K. The measured dephasing dynamics of the CO ligand are in agreement with dephasing dynamics calculated with molecular dynamics (MD) simulations for MbCO with the residue histidine-64 (His64) having its imidazole epsilon nitrogen protonated (N(epsilon)-H). The two conformational substate structures B(epsilon) and R(epsilon) observed in the MD simulations are assigned to the spectroscopic A(1) and A(3) conformational substates of MbCO, respectively, based on the agreement between the experimentally measured and calculated dephasing dynamics for these substates. In the A(1) substate, the N(epsilon)-H proton and N(delta) of His64 are approximately equidistant from the CO ligand, while in the A(3) substate, the N(epsilon)-H of His64 is oriented toward the CO, and the N(delta) is on the surface of the protein. The MD simulations show that dynamics of His64 represent the major source of vibrational dephasing of the CO ligand in the A(3) state on both femtosecond and picosecond time scales. Dephasing in the A(1) state is controlled by His64 on femtosecond time scales, and by the rest of the protein and the water solvent on longer time scales. 相似文献