全文获取类型
收费全文 | 669篇 |
免费 | 24篇 |
国内免费 | 21篇 |
专业分类
化学 | 407篇 |
晶体学 | 1篇 |
力学 | 11篇 |
数学 | 168篇 |
物理学 | 127篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 3篇 |
2020年 | 8篇 |
2019年 | 13篇 |
2018年 | 11篇 |
2017年 | 8篇 |
2016年 | 23篇 |
2015年 | 21篇 |
2014年 | 25篇 |
2013年 | 35篇 |
2012年 | 47篇 |
2011年 | 44篇 |
2010年 | 28篇 |
2009年 | 31篇 |
2008年 | 51篇 |
2007年 | 55篇 |
2006年 | 57篇 |
2005年 | 39篇 |
2004年 | 42篇 |
2003年 | 29篇 |
2002年 | 32篇 |
2001年 | 8篇 |
2000年 | 1篇 |
1999年 | 13篇 |
1998年 | 4篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 6篇 |
1994年 | 4篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1991年 | 6篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1975年 | 1篇 |
排序方式: 共有714条查询结果,搜索用时 62 毫秒
71.
Deng S Sriwichai S Taranekar P Krueger G Mays JW Advincula RC 《Chemical communications (Cambridge, England)》2011,47(31):8805-8807
A series of thiophene dendrons and dendrimers with peripheral functional groups were designed and synthesized. Two methodologies using thiophene dendrons and dendrons as synthetic building blocks, namely, (1) periphery functionalization; (2) a combination of focal and periphery functionalization have been demonstrated. 相似文献
72.
Let R be an isolated hypersurface singularity, and let M and N be finitely generated R-modules. As R is a hypersurface, the torsion modules of M against N are eventually periodic of period two (i.e., for i?0). Since R has only an isolated singularity, these torsion modules are of finite length for i?0. The theta invariant of the pair (M,N) is defined by Hochster to be for i?0. H. Dao has conjectured that the theta invariant is zero for all pairs (M,N) when R has even dimension and contains a field. This paper proves this conjecture under the additional assumption that R is graded with its irrelevant maximal ideal giving the isolated singularity. We also give a careful analysis of the theta pairing when the dimension of R is odd, and relate it to a classical pairing on the smooth variety Proj(R). 相似文献
73.
74.
Gary D Josephsen Kelly A Josephsen Greg J Beilman Jodie H Taylor Kristine E Muiler 《Microscopy and microanalysis》2005,11(6):500-505
This is a report of the adaptation of microwave processing in the preparation of liver biopsies for transmission electron microscopy (TEM) to examine ultrastructural damage of mitochondria in the setting of metabolic stress. Hemorrhagic shock was induced in pigs via 35% total blood volume bleed and a 90-min period of shock followed by resuscitation. Hepatic biopsies were collected before shock and after resuscitation. Following collection, biopsies were processed for TEM by a rapid method involving microwave irradiation (Giberson, 2001). Samples pre- and postshock of each of two animals were viewed and scored using the mitochondrial ultrastructure scoring system (Crouser et al., 2002), a system used to quantify the severity of ultrastructural damage during shock. Results showed evidence of increased ultrastructural damage in the postshock samples, which scored 4.00 and 3.42, versus their preshock controls, which scored 1.18 and 1.27. The results of this analysis were similar to those obtained in another model of shock (Crouser et al., 2002). However, the amount of time used to process the samples was significantly shortened with methods involving microwave irradiation. 相似文献
75.
76.
77.
Fred L. Tobiason Greg H. Cain Steve M. Gregory 《Journal of polymer science. Part A, Polymer chemistry》1978,16(3):659-666
Rotational isomeric-state chain-configurational calculations have been applied to the novolac phenol–formaldehyde structure. Steric interference allows the chain to be considered with a twofold potential energy barrier model. Computations that fit the observed dipole-moment data over a range of molecular weights indicate that the conformational angle is near ±80°, with the g±g= states on the average being 155 cal/mole below the g±g± states. The limiting dipole-moment ratio is computed to be 1.47, compared to the experimental value of 1.48. A negative temperature coefficient agrees in sign with the experimental value, and the characteristic ratio of the end-to-end molecular dimensions is calculated to be 3.27, as compared to the experimental value of 4.76. 相似文献
78.
79.
Alan R. Fersht Jian-Ping Shi Anthony J. Wilkinson David M. Blow Paul Carter Mary M. Y. Waye Greg P. Winter 《Angewandte Chemie (International ed. in English)》1984,23(7):467-473
Structure-activity relationships of enzymes can now be analyzed for the first time by the systematic alteration of protein structure. Recent developments in the chemical synthesis of DNA fragments and recombinant DNA technology enable the facile modification of proteins by highly specific mutagenesis of their genes. Kinetic analysis of the mutant enzymes combined with high-resolution structural data from protein X-ray crystallography allow direct measurements on the relationships between structure and function. In particular, the strength and nature of enzyme-substrate interactions and their detailed roles in catalysis and specificity can now be studied. We have developed such analysis of enzyme structure-function by site-directed mutagenesis of the tyrosyl-tRNA synthetase from Bacillus stearothermophilus, concentrating so far on the subtle role of hydrogen bonding in both substrate specificity and catalysis. We find that the energetics of tyrosine and ATP binding must be analyzed in terms of an exchange reaction with solvent water. Based on this idea and structural data, we have engineered an enzyme of improved enzyme-substrate affinity, and there thus appear to be real prospects of engineering proteins of new specificities, activities, and structural properties. We are also using protein engineering to gather direct information on the nature of enzyme catalysis. For example, we find the catalysis of formation of Tyr-AMP from Tyr and ATP is due largely to electrostatic and hydrogen bonding interactions that are stronger in the transition state than in the ground state—a “strain” mechanism rather than acid-base or covalent catalysis. 相似文献
80.
In Oman and Salminen [19], the authors introduce and study residually small rings, defined as follows: an infinite commutative ring R with identity is residually smallif for every r∈R?{0}, there exists an ideal Ir of R such that r?Ir and |R∕Ir|<|R|. The purpose of this note is to extend our study. In particular, we continue our investigation of residually small rings and then generalize this notion to modules. 相似文献