首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   178515篇
  免费   2559篇
  国内免费   1080篇
化学   96928篇
晶体学   2517篇
力学   7918篇
综合类   47篇
数学   18721篇
物理学   56023篇
  2021年   1276篇
  2020年   1402篇
  2019年   1367篇
  2018年   1332篇
  2017年   1351篇
  2016年   2633篇
  2015年   2222篇
  2014年   2797篇
  2013年   8179篇
  2012年   6777篇
  2011年   8529篇
  2010年   5256篇
  2009年   5408篇
  2008年   7753篇
  2007年   7707篇
  2006年   7403篇
  2005年   6876篇
  2004年   6120篇
  2003年   5279篇
  2002年   5163篇
  2001年   6162篇
  2000年   4638篇
  1999年   3564篇
  1998年   2694篇
  1997年   2676篇
  1996年   2639篇
  1995年   2331篇
  1994年   2228篇
  1993年   2083篇
  1992年   2389篇
  1991年   2342篇
  1990年   2061篇
  1989年   2041篇
  1988年   2049篇
  1987年   1981篇
  1986年   1884篇
  1985年   2803篇
  1984年   2786篇
  1983年   2225篇
  1982年   2422篇
  1981年   2286篇
  1980年   2241篇
  1979年   2208篇
  1978年   2267篇
  1977年   2212篇
  1976年   2152篇
  1975年   2131篇
  1974年   2040篇
  1973年   2118篇
  1972年   1205篇
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
801.
The photoelectron spectrum of HCP has been obtained, Ionization potentials are found at 10.79 ± 0.01 eV (X2 Π) and 12.86 ± 0.01 eV (A 2 Σ). By comparison with HCP, a new interpretation of the first photoelectron band of HCN has been proposed which involves a Renner-Teller interaction in the Σ+ vibronic states of the molecular ion.  相似文献   
802.
Many unexplained phenomena associated with the radioactive halos in very old minerals are described in the literature. In this paper these phenomena are enumerated. The new hypothesis that a primordial superheavy element (SHE) survived in the earth"s crust can help to solve the problems. According to this hypothesis, the SHE decays by spontaneous fission into fragments of very different mass. The heavy fragments, assumed to lie in the region of the natural decay series, exhibit many indications for a SHE. Additionally, the alpha decay of SHE may occur. Applying the methods of mass spectrometry and X-ray fluorescence can probably prove the existence of SHE.  相似文献   
803.
804.
805.
806.
807.
A three-organism food chain within a rock pool at Rosedale, NSW, Australia, was investigated with respect to arsenic compounds by high performance liquid chromatography – hydraulic high pressure nebulization – inductively coupled plasma mass spectrometry (HPLC-HHPN-ICP-MS). Total arsenic concentration was determined in the seaweed Hormosira banksii (27.2 μg/g dry mass), in the gastropod Austrocochlea constricta (74.4 μg/g dry mass), which consumes the seaweed, and in the gastropod Morula marginalba (233 μg/g dry mass), which eats Austrocochlea constricta. The major arsenic compounds in the seaweed were (2′R)-dimethyl[1-O-(2′,3′-dihydroxypropyl)-5-deoxy-β-d-ribofuranos-5-yl]arsine oxide and an unidentified compound. The herbivorous gastropod Austrocochlea constricta transformed most of the arsenic taken up with the seaweed to arsenobetaine. Traces of arsenite, arsenate, dimethylarsinic acid, arsenocholine, the tetramethylarsonium cation, and several unknown arsenic compounds were detected. Arsenobetaine accounted for 95% of the arsenic in the carnivorous gastropod Morula marginalba. In Morula marginalba the concentration of arsenocholine was higher, and the concentrations of the minor arsenic compounds lower than in the herbivorous gastropod Austrocochlea constricta.  相似文献   
808.
Fe-oxide species in Fe/ZSM-5 (prepared by chemical vapor deposition of FeCl3)--active in N2O decomposition--react with zeolite protons during high temperature calcination to give highly active cationic Fe species, this transformation being reversible upon exposure to water vapor at lower temperature.  相似文献   
809.
The Galactosidase repressor (GalR) of Escherichia coli modulates the expression of the gal operon by binding to two DNA operators, OE and O1. The OE and O1 elements are 16 bp pallindromic DNA sequences, differing in four of the base pairs. OE and O1 DNA fragments, both free and complexed with repressor, were analyzed by "quantitative gel electrophoresis". By the criteria of that method, applied to the linear Ferguson plots of both DNA fragments and the linear ranges of those of the DNA-GalR complexes, it was shown that the apparent size of DNA increases upon repressor binding. Moreover, this size increase is greater for the complex with the O1 operator than for the complex with the OE operator in the case that GalR is located in the center of a 155 bp DNA fragment. This is not the case when GalR is located in a peripheral position. By contrast with their size differences, the centrally located GalR-O1 and GalR-OE complexes appear to possess indistinguishable net surface charge densities as judged from the intercepts with the mobility axis. The larger size of the complex with centrally located O1 fragment, as compared with that bearing the OE fragment, is interpreted as being due to bending of the DNA-protein complex, since an authentically bent fragment of a plasmid with bent upstream activator sequence also exhibits a larger slope of the Ferguson plot, and thus the larger size, than predicted on the basis of its DNA chain length (bp).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
810.
The reductive alkylation of amine with carbonyl compounds usingsodium hydrogen telluride has been investigated in recent years.Application of this method to the synthesis of N-alkyl derivativesof biologically important amino acids and their esters is described.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号