全文获取类型
收费全文 | 1994篇 |
免费 | 45篇 |
国内免费 | 4篇 |
专业分类
化学 | 978篇 |
晶体学 | 69篇 |
力学 | 64篇 |
数学 | 251篇 |
物理学 | 413篇 |
无线电 | 268篇 |
出版年
2023年 | 12篇 |
2022年 | 26篇 |
2021年 | 18篇 |
2020年 | 21篇 |
2019年 | 18篇 |
2018年 | 33篇 |
2017年 | 29篇 |
2016年 | 35篇 |
2015年 | 35篇 |
2014年 | 54篇 |
2013年 | 139篇 |
2012年 | 97篇 |
2011年 | 118篇 |
2010年 | 74篇 |
2009年 | 62篇 |
2008年 | 114篇 |
2007年 | 107篇 |
2006年 | 105篇 |
2005年 | 85篇 |
2004年 | 58篇 |
2003年 | 59篇 |
2002年 | 48篇 |
2001年 | 37篇 |
2000年 | 35篇 |
1999年 | 22篇 |
1998年 | 20篇 |
1997年 | 28篇 |
1996年 | 19篇 |
1995年 | 22篇 |
1994年 | 20篇 |
1993年 | 22篇 |
1992年 | 22篇 |
1989年 | 15篇 |
1988年 | 17篇 |
1987年 | 22篇 |
1986年 | 23篇 |
1985年 | 19篇 |
1984年 | 27篇 |
1983年 | 21篇 |
1982年 | 31篇 |
1981年 | 16篇 |
1980年 | 17篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 26篇 |
1976年 | 17篇 |
1975年 | 23篇 |
1974年 | 18篇 |
1973年 | 22篇 |
1971年 | 12篇 |
排序方式: 共有2043条查询结果,搜索用时 15 毫秒
11.
R. Krishna Mohan Rao Kallury Pramod V. Upadhyaya Tumbalam G. Surendra Nath Vankipuram R. Srinivasan 《Journal of mass spectrometry : JMS》1977,12(5):307-312
The mass spectra of eight 1,2,4-triazole derivaties have been recorded and found tao reveal extensive hydrogen and skeletal migrations. The structures of the fragments have been confirmed by deuterium labelling and exact mass measurement. The compounds revealed striking differences in their spectra depending on the nature of the substituents. 相似文献
12.
A primal transportation algorithm is devised via post-optimization on the costs of a modified problem. The procedure involves altering the costs corresponding to the basic cells of the initial (primal feasible) solution so that it is dual feasible as well. The altered costs are then successively restored to their true values with appropriate changes in the optimal solution by the application of cell or area cost operators discussed elsewhere. The cell cost operator algorithm converges to optimum within (2T – 1) steps for primal nondegenerate transportation problems and [(2T + 1) min (m, n)] – 1 steps for primal degenerate transportation problems, whereT is the sum of the (integer) warehouse availabilities (also the sum of the (integer) market requirements) andm andn denote the number of warehouses and markets respectively. For the area cost operator algorithm the corresponding bounds on the number of steps areT and (T + 1) min (m, n) respectively.This report was prepared as part of the activities of the Management Sciences Research Group, Carnegie—Mellon University, under Contract N00014-67-A-0314-0007 NR 047-048 with the U.S. Office of Naval Research. 相似文献
13.
Damodaran S 《Analytical and bioanalytical chemistry》2003,376(2):182-188
A new circular dichroism (CD) spectroscopy technique for studying conformational changes in proteins in situ at the air-water interface is described. By using this technique, conformations of four proteins, viz., beta-casein, bovine serum albumin (BSA), lysozyme, and fibrinogen in the adsorbed state at the air-water interface have been studied. beta-Casein, which is predominantly in a disordered state in solution, assumes a beta-sheet conformation at the air-water interface. On the other hand, lysozyme and fibrinogen, which are alpha+beta-type proteins in solution, become beta-type proteins by completely transforming their alpha-helix structure into beta-sheets. Bovine serum albumin, which is an alpha-type protein in solution, loses its alpha-helix and becomes a disordered protein at the air-water interface. The results indicated that during unfolding and film formation at the interface, structural changes in proteins, regardless of their initial native state, follow the course of increasing beta-sheet and disordered structure and decreasing alpha-helix content. Although this seems to be the general trend, the exceptional case of BSA suggests, however, that this is not universal. 相似文献
14.
R. H. Dekker B. N. Srinivasan J. R. Huber ‡ K. Weiss 《Photochemistry and photobiology》1973,18(6):457-466
Abstract— The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Λmax 420 nm and 550 nm, times; = 9 μs) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with Λmax 440 nm, which appears after decay of the triplet state, and whose second-order decay is pH-dependent, is postulated to be due to the semiquinone radical (AH2 *) and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH*), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second-order decay of the flavin radicals in oxygen-containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines. 相似文献
15.
The reaction of acidhydrazides with α-substituted carbonyl compounds in the presence of metal acetates gives substituted 1,2,4-triazines. These cyclisations could be effected without any added acetate by refluxing in dimethyl formamide, pyridine/acetic acid or dimethyl sulfoxide Sixty five 3,5,6-tri-3,6-di and 6 monosubstituted-1,2,4-triazines (in 50–90% yields) with a wide variation in the C3 substituent (alkyl, aryl or heteryl) and the C6-substituent (aryl or heteryl) are reported. The mechanistic path followed in the formation of these compounds is discussed. 相似文献
16.
A hydrothermal reaction of a mixture of ZnCO3, phosphoric acid, 1, 10‐phenanthroline in H2O gave rise to large plates of a new zinc phosphate, [(C12H8N2Zn)2(HPO4)(H2PO4)2], I . The structure consists of ZnO3N2 distorted trigonal‐bipyramidal and PO4 tetrahedral units linked through their vertices to give rise to a zero‐dimensional molecular solid (monomer). The structure of the monomer appears to be similar to the secondary building unit (SBU) 4 = 1, commonly found in many fibrous zeolites. To our knowledge, this is the first time this building unit has been isolated. The structure, with a unique composition, is stabilized by hydrogen bond interactions between the terminal —OH groups forms a one‐dimensional molecular wire and also by strong π…π interactions between the 1, 10‐phenanthroline units. Photoluminescence studies show that there is a ligand‐to‐metal charge transfer (LMCT). Crystal data: orthorhombic, space group = Fdd2 (no. 43), a = 40.4669(1), b = 7.4733(2), c = 17.4425(5)Å, V = 5274.9(2)Å3, Z = 8. 相似文献
17.
18.
19.
Aryl/alkyl cyanides were quickly converted into the corresponding esters in the presence of iron(III) chloride in refluxing alcohols with very good yields. 相似文献
20.
Woldemedhin M. T. Srinivasan J. Kelly R. G. 《Journal of Solid State Electrochemistry》2021,25(10):2699-2700
Journal of Solid State Electrochemistry - 相似文献