全文获取类型
收费全文 | 1411篇 |
免费 | 36篇 |
国内免费 | 13篇 |
专业分类
化学 | 1002篇 |
晶体学 | 13篇 |
力学 | 11篇 |
数学 | 116篇 |
物理学 | 318篇 |
出版年
2022年 | 10篇 |
2021年 | 12篇 |
2020年 | 12篇 |
2019年 | 23篇 |
2018年 | 11篇 |
2017年 | 12篇 |
2016年 | 25篇 |
2015年 | 16篇 |
2014年 | 32篇 |
2013年 | 74篇 |
2012年 | 70篇 |
2011年 | 81篇 |
2010年 | 36篇 |
2009年 | 40篇 |
2008年 | 70篇 |
2007年 | 72篇 |
2006年 | 80篇 |
2005年 | 93篇 |
2004年 | 70篇 |
2003年 | 77篇 |
2002年 | 53篇 |
2001年 | 32篇 |
2000年 | 40篇 |
1999年 | 20篇 |
1998年 | 23篇 |
1997年 | 19篇 |
1996年 | 16篇 |
1995年 | 11篇 |
1994年 | 14篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 14篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 24篇 |
1984年 | 16篇 |
1983年 | 13篇 |
1982年 | 14篇 |
1981年 | 18篇 |
1980年 | 13篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1977年 | 10篇 |
1976年 | 6篇 |
1975年 | 10篇 |
1974年 | 13篇 |
1973年 | 13篇 |
1972年 | 11篇 |
排序方式: 共有1460条查询结果,搜索用时 15 毫秒
101.
Lembehsterols A and B,novel sulfated sterols inhibiting thymidine phosphorylase,from the marine sponge Petrosia strongylata 总被引:1,自引:0,他引:1
Aoki S Naka Y Itoh T Furukawa T Rachmat R Akiyama S Kobayashi M 《Chemical & pharmaceutical bulletin》2002,50(6):827-830
Lembehsterols A (1) and B (2), two novel sulfated sterols, were isolated from the marine sponge Petrosia strongylata. Both sterols showed inhibitory activity against thymidine phosphorylase, which is an enzyme related to angiogenesis in solid tumors. The structures of these sulfated sterols were established on the basis of chemical and physicochemical evidence. 相似文献
102.
103.
Ken Ono 《Compositio Mathematica》1999,119(1):1-11
Let D denote the fundamental discriminant of a real quadratic field, and let h(D) denote its associated class number. If p is prime, then the Cohen and Lenstra Heuristics give a probability that ph(D). If pgt;3 is prime, then subject to a mild condition, we show that
This condition holds for each 3<p<5000. 相似文献
104.
105.
The reaction between a hydroquinone aqueous solution and manganese dioxide has been investigated by electron paramagnetic resonance spectroscopy. A very stable p-benzosemiquinone anion is formed when an aqueous solution of hydroquinone is passed through a column of manganese dioxide. The kinetics for the reaction between hydroquinone and manganese dioxide follow the equation where A0 is the initial concentration of hydroquinone, C is the concentration of p-benzosemiquinone anions, W is the amount of manganese dioxide, F is the feed rate of a hydroquinone aqueous solution, and k10 and k20 are constants; k10 = 3.1 × 10?2 (cm3/g MnO2· min), k20 = 3.2 (cm3/g MnO2·min). A mechanism for the reaction is proposed such that Mn(IV) in manganese dioxide is reduced to Mn (II) with hydroquinone from which the semiquinone anion is formed via a neutral semiquinone. The formation of semiquinone anions from several substituted hydroquinones is also discussed. 相似文献
106.
107.
Nakayama M Goto S Uchimoto Y Wakihara M Kitajima Y Miyanaga T Watanabe I 《The journal of physical chemistry. B》2005,109(22):11197-11203
Changes in the electronic structure of olivine Li(1-x)CoPO(4), 4.8 V positive electrode material for lithium ion batteries, were investigated using the X-ray absorption spectroscopy (XAS) technique. The threshold energy in the Co K-edge increased with electrochemical Li removal, indicating the oxidation of cobalt ions due to charge compensation. Moreover, P and O K-edge XAS showed a slight shift in threshold energy with Li removal. Although it is generally believed that the electrons of PO(4) polyanion do not contribute to the oxidation process, present experimental results indicate changes in the electronic structure around PO(4) units. Such results would be interpreted by the idea of the hybridization effect between the Co 3d and O 2p orbitals and of the polarization effect introduced by Li ions. 相似文献
108.
Kaihara S Osanai Y Nishikawa K Toshima K Doi Y Matsumura S 《Macromolecular bioscience》2005,5(7):644-652
The enzymatic transformation into an oligomer was carried out with the objective of developing the chemical recycling of bacterial polyesters. Poly(R-3-hydroxyalkanoate)s (PHAs), such as poly[(R-3-hydroxybutyrate)-co-12%(R-3-hydroxyhexanoate)] and poly[(R-3-hydroxybutyrate)-co-12%(R-3-hydroxyvalerate)], were degraded by granulated Candida antarctica lipase B immobilized on hydrophilic silica (lipase GCA) in a diluted organic solvent at 70 degrees C. The degradation products were cyclic oligomers having a molecular weight of a few hundreds. The obtained cyclic oligomer was readily repolymerized by the same lipase (lipase GCA) to produce the corresponding polyester in a concentrated solution. The cyclic oligomer was copolymerized with epsilon-caprolactone using lipase to produce the corresponding terpolymers having an Mw of 21,000. This is the first example of the enzymatic chemical recycling of bacterial PHAs using lipase. Poly(R-3-hydroxybutyrate) [P(3HB)] was also degraded into the linear-type R-3HB monomer to trimer by P(3HB)-depolymerase (PHBDP) in phosphate buffer at 37 degrees C. The degradation using PHBDP required a longer reaction time compared with the lipase-catalyzed degradation in organic solvent. The monomer composition of the oligomer depended on the origin of the PHBDP. The R-3HB monomer was predominately produced by PHBDP from Pseudomonas stutzeri, while the R-3HB dimer was produced by PHBDP from Alcaligenes faecalis T1. Repolymerization of these oligomers by lipase in concentrated organic solvent produced a relatively low-molecular-weight P(3HB) (e.g., Mw=2,000). Degradation of P(3HB) by lipase in organic solvent into repolymerizable cyclic oligomer and degradation of P(3HB) by PHBDP in buffer into hydroxy acid type R-3HB dimer. 相似文献
109.
Subwavelength optical imaging through a metallic nanorod array 总被引:1,自引:0,他引:1
We propose a subwavelength imaging system without a lens or a mirror but with an array of metallic nanorods. The near-field components of dipole sources were plasmonically transferred through the rod array to reproduce the source distribution in the other side. We calculated the field distribution at the different planes of imaging process using the finite-difference time-domain algorithm and found that the spatial resolution was 40 nm given by the rod size and spacing. A typical configuration is a hexagonal arrangement of silver rods of 50 nm height and 20 nm diameter. We also show that the image formation highly depends on the coherence and the polarization of the source distribution and the source-array distance. 相似文献
110.
The third-harmonic-generation (THG) spectrum was measured for a NiBr-chain compound, which is a one-dimensional Mott insulator, in a reflection configuration. A sharp peak and a shoulder structure in the THG spectrum are attributed to three-photon resonance to an exciton and a continuum, respectively. The band-edge energy, the exciton binding energy, and the spectral weights for the exciton and the continuum were determined from comparative studies of linear absorption, THG, and electroreflectance spectra. The excitonic effect is more pronounced in the NiCl chain than in the NiBr chain. 相似文献