全文获取类型
收费全文 | 86281篇 |
免费 | 25632篇 |
国内免费 | 5669篇 |
专业分类
化学 | 85849篇 |
晶体学 | 659篇 |
力学 | 4085篇 |
综合类 | 328篇 |
数学 | 7042篇 |
物理学 | 19619篇 |
出版年
2024年 | 266篇 |
2023年 | 4738篇 |
2022年 | 3322篇 |
2021年 | 4241篇 |
2020年 | 6468篇 |
2019年 | 4202篇 |
2018年 | 3910篇 |
2017年 | 2253篇 |
2016年 | 7678篇 |
2015年 | 7703篇 |
2014年 | 7619篇 |
2013年 | 8546篇 |
2012年 | 7586篇 |
2011年 | 5672篇 |
2010年 | 6476篇 |
2009年 | 6231篇 |
2008年 | 4275篇 |
2007年 | 3573篇 |
2006年 | 2730篇 |
2005年 | 2321篇 |
2004年 | 1771篇 |
2003年 | 1421篇 |
2002年 | 1287篇 |
2001年 | 1041篇 |
2000年 | 909篇 |
1999年 | 1059篇 |
1998年 | 867篇 |
1997年 | 849篇 |
1996年 | 840篇 |
1995年 | 834篇 |
1994年 | 677篇 |
1993年 | 704篇 |
1992年 | 495篇 |
1991年 | 448篇 |
1990年 | 366篇 |
1989年 | 284篇 |
1988年 | 253篇 |
1987年 | 223篇 |
1986年 | 201篇 |
1985年 | 185篇 |
1984年 | 144篇 |
1983年 | 157篇 |
1980年 | 132篇 |
1977年 | 180篇 |
1976年 | 201篇 |
1975年 | 206篇 |
1974年 | 202篇 |
1972年 | 171篇 |
1970年 | 218篇 |
1968年 | 138篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Gudiksen KL Gitlin I Yang J Urbach AR Moustakas DT Whitesides GM 《Journal of the American Chemical Society》2005,127(13):4707-4714
This study compares the folding of two polypeptides--bovine carbonic anhydrase (BCA) and peracetylated BCA (BCA-Ac(18))--having the same sequence of amino acids but differing by 18 formal units of charge, from a solution containing denaturing concentrations of sodium dodecyl sulfate (SDS). Acetylation of BCA with acetic anhydride converts all 18 lysine-epsilon-NH(3)(+) groups to lysine-epsilon-NHCOCH(3) groups and generates BCA-Ac(18). Both BCA and BCA-Ac(18) are catalytically active, and circular dichroism spectroscopy (CD) suggests that they have similar secondary and tertiary structures. SDS at concentrations above approximately 10 mM denatured both proteins. When the SDS was removed by dialysis, both proteins were regenerated in native form. This study suggests that large differences in the net charge of the polypeptide have no significant influence on the structure, the ability to refold, or the rate of refolding of this protein from solutions containing SDS. This study reinforces the idea that charged residues on the surface of BCA do not guide protein folding and raises the broader question of why proteins have charged residues on their surface, outside of the region of the active site. 相似文献
962.
利用傅立叶变换红外(FT-IR)和拉曼(FT-Raman)光谱研究了高浓度磷脂酰胆碱(PC)与牛血清白蛋白(BSA)的相互作用,及Eu3+对该作用的影响.FT-IR结果显示,PC/BSA混合体系中二者的相互作用主要发生在PC头部极性基团,且这一作用随BSA含量的增加而增强,作用后蛋白质二级结构中α螺旋的比例有所增加. FT-Raman光谱说明PC与BSA的相互作用影响磷脂CH链的排列有序程度. PC/BSA/Eu3+体系的红外光谱显示, Eu3+与PC的磷氧键发生了强相互作用,并使蛋白α螺旋的比例进一步增加. 相似文献
963.
采用动态TG-DTG,研究了铁催化剂的制备方法、前驱化合物的 种类及 热处理对其在碳-氧反应中催化活性的影响。观察到在干混法制备催化剂时,所用铁盐分解温度越低活性越高。此外还发现,热处理可明显提高由分解温度高的含氧铁盐制备的 催化剂的活性,但对于不含氧铁盐和分解温度低的含氧铁盐无影响。 相似文献
964.
Heung Joon Yang Carole-Ann Cole Nobuo Monji Allan S. Hoffman 《Journal of polymer science. Part A, Polymer chemistry》1990,28(1):219-226
N-isopropylacrylamide and N-acryloxysuccinimide have been copolymerized in various mixtures of terrahydrofuran and toluene using azobisisobutyronitrile as initiator. Polymerization has been conducted for 24 h at 50°C under a slightly positive pressure of nitrogen. The copolymers were assayed for active ester content by measuring the UV absorbance (259 nm) of N-hydroxysuccinimide anion, generated by reacting the copolymers with N-isopropylamine in dimethylformamide and dissolving the resulting mixture in 0.1M HEPES buffer, pH 7.5. The molecular weight and its distribution have been estimated by gel permeation chromatography. The active ester content was found to be equivalent to the comonomer feed ratio, and the major factor controlling the molecular weight was the ratio of tetrahydrofuran to toluene. Thus, the number of active esters per polymer chain could be controlled by adjustment of the comonomer feed ratio and the ratio of tetrahydrofuran to toluene. Monomer reactivity ratios for copolymerization of N-isopropylacrylamide with N-acryloxysuccinimide were also estimated. These copolymers are useful for immobilizing binding ligands such as antibodies for subsequent thermally induced precipitation immunoassays and bioseparation processes. 相似文献
965.
3,4—亚甲二氧基苯甲酸锡酯的合成及表征 总被引:3,自引:0,他引:3
3,4-亚甲二氧基苯甲酸及其取代物与三苯基氢氧化锡或二丁基化锡反应合成了5个锡酯,通过元素分析,IR和^1HNMR对其进行了表征。 相似文献
966.
The layered perovskites Sr2TiO4 and Sr2SnO4 are more active and selective for the oxidative coupling of methane at 1073 K than the corresponding perovskites SrTiO3 and SrSnO3. The role of the surface oxygen species for this reaction is discussed. 相似文献
967.
基于Mn(Ⅱ)对试剂2-(8'-羟基喹啉-5'-磺基-7'-偶氮基)-1-羟基-8-氨基一萘二磺酸(以下简称HQSAH)分解反应的催化作用,提出了锰的荧光催化动力学分析新方法,其λex/em=230/415nm,适宜酸度范围为pH11.0~12.0,Mn(Ⅱ)含量在0~0.08μg/mL呈线性关系,该法用于分析铸造铝合金中的痕量锰,效果良好。还初步探讨了反应机制,确定了反应动力学方程,测定了反应速率和活化能。 相似文献
968.
969.
Thermal analysis supported by kinetic calculations was applied exhaustively to these compounds. Under dynamic conditions, tetrafluorometallates(III), which as intermediates, could not be isolated, for the first three compounds. In each case, the final step was the pure metal(III) fluoride. Ammonium hexafluorochromate(III) decomposed directly to the pure chromium(III) fluoride. The decomposition rate of all compounds slowed down towards the end, probably for kinetic reasons. Polymorphic transitions of ammonium hexafluorotitanate(III) were observed at 35 and 100°C. Ammoniumhexafluoroscandate(III) underwent polymorphic transition at 47°C. The decomposition patterns for all these compounds were similar. Conditions for the preparation of pure ammonium tetrafluorometallates(III) of Sc, Ti and V are described. 相似文献
970.
The fluorimetric determination of mercury ions with o-vanillin-8-aminoquinoline (OVAQ) in aqueous solutions was investigated. Hg(II) could react with the fluorescent reagent OVAQ
(λex/em = 278/314 nm) to form a nonfluorescent complex in an ethanol-water medium of pH 6.00. The linear range of the proposed method
was from 2.5 to 80 μg/L, and the detection limit was 0.80 μg/L. The interferences of 24 foreign ions were also studied. The
method was successfully applied to the determination of Hg(II) in sludge.
The text was submitted by the authors in English. 相似文献