全文获取类型
收费全文 | 99908篇 |
免费 | 12596篇 |
国内免费 | 7509篇 |
专业分类
化学 | 60111篇 |
晶体学 | 910篇 |
力学 | 6029篇 |
综合类 | 323篇 |
数学 | 17051篇 |
物理学 | 35589篇 |
出版年
2024年 | 175篇 |
2023年 | 1541篇 |
2022年 | 2274篇 |
2021年 | 2650篇 |
2020年 | 3084篇 |
2019年 | 2856篇 |
2018年 | 3598篇 |
2017年 | 3649篇 |
2016年 | 4122篇 |
2015年 | 3840篇 |
2014年 | 4396篇 |
2013年 | 5764篇 |
2012年 | 9640篇 |
2011年 | 8961篇 |
2010年 | 6243篇 |
2009年 | 5758篇 |
2008年 | 5109篇 |
2007年 | 4678篇 |
2006年 | 4323篇 |
2005年 | 7450篇 |
2004年 | 6252篇 |
2003年 | 4142篇 |
2002年 | 2206篇 |
2001年 | 1751篇 |
2000年 | 1446篇 |
1999年 | 1702篇 |
1998年 | 1411篇 |
1997年 | 1361篇 |
1996年 | 1332篇 |
1995年 | 1090篇 |
1994年 | 962篇 |
1993年 | 784篇 |
1992年 | 844篇 |
1991年 | 732篇 |
1990年 | 637篇 |
1989年 | 492篇 |
1988年 | 434篇 |
1987年 | 328篇 |
1986年 | 291篇 |
1985年 | 240篇 |
1984年 | 171篇 |
1983年 | 127篇 |
1982年 | 98篇 |
1981年 | 85篇 |
1980年 | 77篇 |
1979年 | 61篇 |
1977年 | 58篇 |
1976年 | 109篇 |
1975年 | 60篇 |
1973年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
961.
962.
Protein tyrosine phosphatase 1B (PTP1B) functions by removing the phosphoryl group from tyrosinephosphorylated proteins in
insulin signaling and metabolism. The regeneration of the active site involves a sulphenylamide intermediate derived from
the intrastrand cross-linking between the catalytic serine and the neighboring backbone nitrogen. Two mechanisms have been
proposed for the formation of the sulphenylamide intermediate and the subsequent reactivation of the catalytic site. In the
current work, the proposed mechanisms have been investigated by the use of density functional theory calculations. Our results
suggest that these two mechanisms have similar overall energy barriers and that the preferred route will be determined by
the availability of hydrogen peroxide or other oxidizing reagents. 相似文献
963.
心脑血管病患者头发微量元素铜、锌含量调查 总被引:1,自引:0,他引:1
调查了心脑血管病患者头发微量元素铜、锌含量,发现患者发现Cu、Zn含量普通遇低于正常值。 相似文献
964.
我们曾报道了一类新型双羟基键合固定相(GDSB)的合成,研究表明这类固定相具有键合相浓度高,热和化学稳定性好的特点,可用于性质相似的有机化合物的分离。 本文以GDSB为色谱柱填料,含Cu~(2+)离子和烷基磺酸盐的HAc-NaAc缓冲溶液作流动 相似文献
965.
Rongbiao?WangEmail author Herbert?Schmiedel Bernd-Reiner?Paulke 《Colloid and polymer science》2004,283(1):91-97
Isothermal titration calorimetry (ITC) measurements of the mixture of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) with negatively charged, hairy copolymer latices (poly-(2,3-epoxypropylmethacrylate-co-methacrylic acid) in different ratio) at high water excess indicate a monomer adsorption mechanism of CTAB by the polymer particles. The number of adsorbed CTAB molecules at saturation corresponds approximately to the number of negative elementary charges bound at the surface of the latices. The mixing enthalpy is the sum of demicellization and sorption enthalpies. At 25 °C for CTAB the demicellization enthalpy amounts to 10 kJ/mol, whereas the adsorption enthalpy varies from –7 kJ/mol (surface charge density of the latices
=–0.37 C/m2) to +3 kJ/mol (=–0.085 C/m2). The hydrodynamic radius
RH of the latex particles upon titration of cationic detergent and salt (NaBr) decreases by about 2 nm until the onset of aggregation near the isoelectric point. Titration of nonionic or anionic detergents has much less influence on the hydrodynamic radius and produces no measurable adsorption heat. The results are consistent within a model of latex particles with extended negatively charged polymer chains interacting predominantly via Coulombic forces with detergents. 相似文献
966.
本文叙述氨基酸及其对映体的全氟酰基烷基脂(O-异丙基、-异丁基、-正丁基;N(O,S)-三氟乙酰基,TFA、-七氟丁酰基,HFB)在Chirasil-Val上的洗脱特性。随烷基取代增大,保留时间增加;用HFB代替TFA,对映体的分离因子降低。此外,DL-氨基酸与另丁醇衍生,生成的对映及非对映衍生物在Chirasil-Val上可拆分成四个色谱峰。 相似文献
967.
Jia Rui Wang Chu Ting Yang Lei Liu Qing Xiang Guo Department of Chemistry University of Science Technology of China Hefei China 《中国化学快报》2007,18(2):133-136
Aerobic oxidation of electron-rich benzylic and phenyl allylic alcohols was achieved with high yields with only 0.1 mol.% of Pd(OAc)2 catalyst in the absence of any ligand. This procedure was expected to be valuable for realistic industrial-scale applications from both economic as well as environmental points of view. 相似文献
968.
Study of urinary nucleosides as biological marker in cancer patients analyzed by micellar electrokinetic capillary chromatography 总被引:10,自引:0,他引:10
Thirteen normal and modified nucleosides, primarily degradation products of transfer ribonucleic acid (tRNA), were evaluated as potential tumor markers for cancer patients. Their urinary concentrations were determined by means of micellar electrokinetic capillary chromatography (MEKC) in the urine from 54 healthy adults and 70 cancer patients, then quantitatively expressed as a function of creatinine excretion. It was found that urinary nucleosides for cancer patients were on the average significantly higher than those for healthy controls, however, no significant differences were found between male and female or between different ages. Based on 13 urinary nucleoside concentrations, principal component analysis (PCA) could be used to classify 72% of cancer patients from the healthy controls. The present study shows that the precise measurement of urinary nucleosides by MEKC in combining with PCA technique may provide a clinically useful approach for diagnosis of cancer. 相似文献
969.
N-Methylpyrrolidine in cefepime for injection was determined by capillary electrophoresis with indirect UV detection. Best results were achieved with background electrolyte consisting of 10 mM creatinine adjusted to pH 3.8 with formic acid and an applied voltage of 30 kV in a bare fused-silica capillary. Indirect UV detection was performed at a wavelength of 225 nm. The application of a small amount of inlet pressure during the separation assisted the attainment of a stable baseline. The optimized method was validated regarding selectivity, linearity, accuracy, precision, ruggedness, repeatability and detection limits. Careful control of capillary conditioning enabled migration time precision values of <0.2% RSD. The use of an internal standard enabled precision values of <1% RSD to be obtained for peak area ratios. 相似文献
970.
Veronica?StigssonEmail author G?ran?Kloow Ulf?Germg?rd Niclas?Andersson 《Cellulose (London, England)》2005,12(4):395-401
Controlling the reduction in molecular weight of the cellulose chains is essential in the production of carboxymethyl cellulose
(CMC). Such a reduction can be achieved by the addition of cobalt during the process of cobalt(II) ions, which act as a catalyst
for oxidative cleavage, and the influence thereof has been studied under a variety of conditions. This study has resulted
in a model that summarises the effects of the added amount of cobalt, the time for the cobalt reaction, the temperature in
the mercerisation stage of the CMC-manufacturing process and finally the effect of the temperature in the etherification stage.
It is shown that it is important for cobalt to be present during the mercerisation stage in order to achieve the desired viscosity. 相似文献