全文获取类型
收费全文 | 147005篇 |
免费 | 1464篇 |
国内免费 | 397篇 |
专业分类
化学 | 80307篇 |
晶体学 | 2389篇 |
力学 | 6090篇 |
综合类 | 3篇 |
数学 | 13582篇 |
物理学 | 46495篇 |
出版年
2021年 | 1035篇 |
2020年 | 1191篇 |
2019年 | 1256篇 |
2018年 | 1642篇 |
2017年 | 1693篇 |
2016年 | 2550篇 |
2015年 | 1570篇 |
2014年 | 2350篇 |
2013年 | 6042篇 |
2012年 | 4853篇 |
2011年 | 5943篇 |
2010年 | 4191篇 |
2009年 | 4211篇 |
2008年 | 5732篇 |
2007年 | 5510篇 |
2006年 | 5388篇 |
2005年 | 4957篇 |
2004年 | 4447篇 |
2003年 | 4125篇 |
2002年 | 4007篇 |
2001年 | 5548篇 |
2000年 | 3970篇 |
1999年 | 3072篇 |
1998年 | 2111篇 |
1997年 | 2217篇 |
1996年 | 2074篇 |
1995年 | 1919篇 |
1994年 | 1917篇 |
1993年 | 1730篇 |
1992年 | 2217篇 |
1991年 | 2302篇 |
1990年 | 2135篇 |
1989年 | 2103篇 |
1988年 | 2076篇 |
1987年 | 2085篇 |
1986年 | 1910篇 |
1985年 | 2370篇 |
1984年 | 2393篇 |
1983年 | 1909篇 |
1982年 | 1913篇 |
1981年 | 1753篇 |
1980年 | 1740篇 |
1979年 | 2027篇 |
1978年 | 2100篇 |
1977年 | 2046篇 |
1976年 | 1976篇 |
1975年 | 1862篇 |
1974年 | 1842篇 |
1973年 | 1874篇 |
1972年 | 1236篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Betancourt L Takao T Hernandez L Padron G Shimonishi Y 《Journal of mass spectrometry : JMS》1999,34(3):169-174
High-resolution matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was used to characterize the primary structure of the levansucrase (EC 2.4.1.10) secreted by Acetobacter diazotropicus SRT4. The technique permitted not only the reading frame of this enzyme, the amino acid sequence of which was deduced from DNA, but also the elucidation of an N-terminal blocking group and the position of a disulfide bridge between Cys309 and Cys365 among the three Cys residues. A free cysteine (Cys127) was identified by modifying an intact molecule with a sulfhydryl reagent, 5-(octyldithio)-2-nitrobenzoic acid, under non-reducing conditions. In addition, the enzyme obtained by site-directed mutagenesis at Asp279 to Asn279 was also identified by the above methods. Post-source decay analysis of the tryptic peptide containing the mutation site unequivocally revealed an Asn residue at position 279. 相似文献
982.
J. Martinez Salazar R. K. Bayer T. A. Ezquerra F. J. Baltá Calleja 《Colloid and polymer science》1989,267(5):409-413
The morphology and structure of high molecular weight linear polyethylene (M
w 450000) filled with carbon black and processed using molds that introduce an elongational flow component during injection molding has been examined using electron microscopy and x-ray diffraction techniques. The study of fracture surfaces reveals the display of shish-kebabs oriented along the injection direction with segregated longitudinal channels of carbon black particles. Molecular and lamellar changes in orientation are, furthermore, studied across the thickness of the moldings. It is shown that addition of carbon black particles to injection-molded polyethylene induces significant changes in lamellar orientation. Thus, while lamellar overgrowth proceeds perpendicular to the fiber axes within carbon free channels, lamellae grow randomly within carbon-enriched regions where flow is less pronounced. 相似文献
983.
984.
G. Merle J. P. Pascault Q. T. Pham C. Pillot R. Salle J. Gol I. Rashkov I. Panayotov D. Gurard A. Hrold 《Journal of polymer science. Part A, Polymer chemistry》1977,15(9):2067-2074
In the present work, we use the binary insertion compound LiC12 to polymerize styrene, methyl methacrylate, butadiene, isoprene, and to copolymerize isoprene and styrene in various hydrocarbon solvents (aromatics and aliphatic) and etheral solvents. We show that the styrene polymerization in aromatic solvents gives better yields than in the etheral solvents, the polymer being atactic. Methyl methacrylate does not polymerize in toluene but does so completely in DME. More generally, the yields of polymerization are better with KC37 than with LiC12 because of the different capacities of the monomer to get into the carbon layers. The polymerization of dienes with LiC12 shows that the microstructures of the polymer obtained in π-or n-donor solvents are similar to the ones obtained by homogenous polymerization with Li cation in such solvents. However, for isoprene in cyclohexane, the results are different. The isoprene styrene copolymers are statistical ones and the mean length of styrene blocks is less than 5. The monomer interaction with the insertion compound and the growing chain geometry between the carbon layers are the facts which control either the stereospecificity of the polymerization or the selectivity of the copolymerization. 相似文献
985.
986.
Conclusions The complexes of Co, Ni, Fe, and Cu with dimethyl- and diphenylglyoxime inhibit the initiated oxidation of cumene by reacting with the peroxide radicals.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1342–1344, June, 1971. 相似文献
987.
988.
In this paper, the NO-to-NO
2
conversion in various gaseous mixtures is experimentally investigated. Streamer coronas are produced with a dc-superimposed high-frequency ac power supply (10–60 kHz). According to NO
x
removal experiments in N
2
+NO
x
and N
2
+O
2
+NO
x
gaseous mixtures, it is supposed that the reverse reaction NO
2
+ONO+O
2
may not only limit NO
2
production in N
2
+NO
x
mixtures, but also increase the energy cost for NO removal. Oxygen could significantly suppress reduction reactions and enhance oxidation processes. The reduction reactions, such as N+NON
2
+O, induce negligible NO removal provided the O
2
concentration is larger than 3.6%. With adding H
2
O into the reactor, the produced NO
2
per unit removed NO can be significantly reduced due to NO
2
oxidation. NH
3
injection could also significantly decrease the produced NO
2
via NH and NH
2
- related reduction reactions. Almost 100% of NO
2
can be removed in gaseous mixtures of N
2
+O
2
+H
2
O+NO
2
with negligible NO production. 相似文献
989.
S. D. Mikhano N. S. Kulachkina T. M. Filippova I. G. Suchkova V. M. Berezovskii 《Chemistry of Natural Compounds》1981,17(2):176-178
(±)-Biotin has been synthesized from a mixture of cis- and allo-trisubstituted dehydrothiophanes by a scheme providing for the easy elimination of the accompanying allo-substituted thiophane in the form of an intermediate and not as the final isomeric allo-biotin. The imidazoline ring of biotin is formed from a dialkoxy-carbonyldiaminothiophane under the conditions of partial hydrolysis of one of the protective groups with the elimination of phosgene from the scheme of synthesis.All-Union Scientific-Research Vitamin Institute, Moscow. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 220–223, March–April, 1981. 相似文献
990.