全文获取类型
收费全文 | 25023篇 |
免费 | 1002篇 |
国内免费 | 4700篇 |
专业分类
化学 | 28379篇 |
晶体学 | 296篇 |
力学 | 53篇 |
综合类 | 231篇 |
数学 | 17篇 |
物理学 | 1749篇 |
出版年
2024年 | 41篇 |
2023年 | 263篇 |
2022年 | 778篇 |
2021年 | 727篇 |
2020年 | 724篇 |
2019年 | 748篇 |
2018年 | 638篇 |
2017年 | 790篇 |
2016年 | 938篇 |
2015年 | 877篇 |
2014年 | 1022篇 |
2013年 | 2249篇 |
2012年 | 1768篇 |
2011年 | 1383篇 |
2010年 | 1206篇 |
2009年 | 1436篇 |
2008年 | 1513篇 |
2007年 | 1665篇 |
2006年 | 1572篇 |
2005年 | 1464篇 |
2004年 | 1332篇 |
2003年 | 1066篇 |
2002年 | 930篇 |
2001年 | 677篇 |
2000年 | 664篇 |
1999年 | 542篇 |
1998年 | 438篇 |
1997年 | 466篇 |
1996年 | 422篇 |
1995年 | 387篇 |
1994年 | 335篇 |
1993年 | 287篇 |
1992年 | 279篇 |
1991年 | 203篇 |
1990年 | 158篇 |
1989年 | 172篇 |
1988年 | 122篇 |
1987年 | 84篇 |
1986年 | 59篇 |
1985年 | 48篇 |
1984年 | 55篇 |
1983年 | 29篇 |
1982年 | 42篇 |
1981年 | 28篇 |
1980年 | 22篇 |
1979年 | 20篇 |
1978年 | 22篇 |
1977年 | 5篇 |
1973年 | 5篇 |
1968年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
A novel mixed sol method was developed for the preparation of supported catalysts. Analyses by means of XRD and BET show that a 40%AgBiVMoO/γ-Al2O3 catalyst prepared by this method possessed high specific surface area and high dispersion of the active phase. As a result, high acrylic acid selectivity of 8.5% was obtained when the catalyst was used in the reaction of propane partial oxidation to acrylic acid in a fixed-bed reactor. 相似文献
22.
23.
树脂吸附法处理水杨酸甲酯生产废水的研究 总被引:7,自引:0,他引:7
采用吸附树脂NDA—99处理水杨酸甲酯生产废水,结果表明该树脂时废水中的5磺基水杨酸及水杨酸均具有良好的吸附—脱附性能.废水经预处理和吸附处理后,CODCr由57000-59000mg/L降至6300mg/L左右,去除率接近89%.用IBV8%NaOH 3BVH2O作脱附剂,在温度为60℃、流量为IBV/h的条件下,脱附率接近100%,树脂可重复使用.高浓度脱附液经酸化、浓缩、冷却结晶,可回收5—磺基水杨酸,回收率为95%左右。纯度为78%。 相似文献
24.
The thermodynamic quantities associated with ionization of the N1 and N9 protons of adenine have been calorimetrically determined as a function of temperature. The H values for proton dissociation of these groups, with pK values of 4.19 and 9.92, were found to be 5.1 and 9.1 kcal/mole, respectively, at 25°C, =0.025. The C
p
values for proton dissociation of these groups were estimated to be –11 and –17 cal/mole-deg. These results indicate that the large heat capacity changes observed during conformational transitions of polynucleotides are not the result of ionization of the bases. 相似文献
25.
The synthesis of a biotinated amphiphile assembled from d-(+)-biotin, ethylene diamine as spacer, galactaric acid and 1-dodecylamine was achieved in six steps. The key step was the synthesis of a bisacetonide protected galactaric ester, the structure of which was determined by X-ray analysis. Aminolysis, spacer attachment, coupling with biotin and deprotection led to the amphiphilic galactaramide. 相似文献
26.
Michael SchwarzPeter Winterhalter 《Tetrahedron letters》2003,44(41):7583-7587
Anthocyanins, isolated from natural sources by countercurrent chromatography, were reacted with cinnamic acids bearing at least one electron-donating substituent at the para-position. The resulting pyranoanthocyanins obtained by this simple one-step reaction were much less susceptible to pH shifts and retained their original colour over a wide pH-range. Through reaction with p-dimethylamino cinnamic acid, synthetic malvidin- and cyanidin-based anthocyanins with a unique violet hue were prepared. 相似文献
27.
In distinction to Extended Hückel Theory which predicts as the most stable conformation of free zwitterionic GABA a totally extended form, PCILO and SCF ab initio studies show that the intrinsically preferred conformation of the isolated molecule is a highly folded one, resulting from strong interactions between the two charged ends. Computations are also carried out for hydrated GABA in the supermolecule approach allowing moreover for the flexibility of binding of some of the water molecules of the first hydration shell. They predict the coexistence in solution of a large number of conformations showing different degrees of folding (or extension), a result confirmed by recent NMR studies. This and a number of similar results show that we have to adapt our thinking on the role of conformations in pharmacological activity to this situation, which was frequently obscured by the more abundant results of X-ray crystallography yielding a single conformation. 相似文献
28.
《Tetrahedron》2004,60(22):4781-4787
In addition to the known compounds of the type of nonactic and homononactic acids and their lactones, dilactones and tetralactones, five new compounds, namely homononactyl-nonactoate, a dilactone consisting of nonactic and homononactic acids and three cyclic trimers with nonactic and homononactic acids, were isolated from a strain of Streptomyces globisporus. Their structures, including the absolute configurations of the hydroxyl and methyl groups, were determined by extensive spectroscopic techniques such as UV, IR, MS, 1D and 2D NMR. 相似文献
29.
As an extension of previous reports, the barium carbonate procedure has been optimized in detail for the preparation of apolar columns. The aim was to produce optimum overall column characteristics, and to maintain them unchanged under the prolonged influence of the highest possible temperature. The main parameters under optimization were glass variety, leaching of glass surface with aqueous HCl, and amount of barium carbonate deposited, while deactivation and coating were kept constant. The basic column characteristics were adsorption properties and thermostability of deactivation, acid/base behaviour and separation efficiency. They were determined by a new, quantitative testing procedure. Intense leaching was able to eliminate almost totally the differences between glass varieties and to create a well-defined glass surface. While untreated glass, leached glass, and barium carbonate treated glass showed specific weak points in the respective column quality, the combination of leaching and barium carbonate treatment yielded the highest and most stable quality. Some technical modifications of the preparation procedure are described, including deactivation in the gas phase, and use of pentane as a solvent for static coating. 相似文献
30.
本文报导的方法是在南京大学高鸿教授研究的K_3Fe(CN)_6示波极谱滴定法(在PH8的K_2HPO_4—KH_2PO_4—KBr底液中,用0.04660mol/LK_3Fe(CN)_6标准溶液对纯抗坏血酸测定)的基础上,考虑到L—抗坏血酸(以下简称V_e)在碱性、中性介质中稳定性差,如用于V_e含量很微的果蔬测定,对结果影响大。本文改为H_2C_2O_4—KBr的酸性介质作底液,用 相似文献