全文获取类型
收费全文 | 243604篇 |
免费 | 3208篇 |
国内免费 | 1460篇 |
专业分类
化学 | 138016篇 |
晶体学 | 3336篇 |
力学 | 9424篇 |
综合类 | 61篇 |
数学 | 28537篇 |
物理学 | 68898篇 |
出版年
2020年 | 2034篇 |
2019年 | 2194篇 |
2018年 | 2695篇 |
2017年 | 2714篇 |
2016年 | 4104篇 |
2015年 | 2847篇 |
2014年 | 3988篇 |
2013年 | 10100篇 |
2012年 | 8718篇 |
2011年 | 10709篇 |
2010年 | 7309篇 |
2009年 | 7094篇 |
2008年 | 9917篇 |
2007年 | 10079篇 |
2006年 | 9512篇 |
2005年 | 8797篇 |
2004年 | 7833篇 |
2003年 | 6881篇 |
2002年 | 6742篇 |
2001年 | 7033篇 |
2000年 | 5346篇 |
1999年 | 3906篇 |
1998年 | 3292篇 |
1997年 | 3289篇 |
1996年 | 3349篇 |
1995年 | 2853篇 |
1994年 | 2975篇 |
1993年 | 2815篇 |
1992年 | 3050篇 |
1991年 | 3115篇 |
1990年 | 2895篇 |
1989年 | 2837篇 |
1988年 | 2780篇 |
1987年 | 2676篇 |
1986年 | 2716篇 |
1985年 | 3577篇 |
1984年 | 3681篇 |
1983年 | 3009篇 |
1982年 | 3398篇 |
1981年 | 3126篇 |
1980年 | 2899篇 |
1979年 | 3098篇 |
1978年 | 3294篇 |
1977年 | 3306篇 |
1976年 | 3315篇 |
1975年 | 3020篇 |
1974年 | 3133篇 |
1973年 | 3174篇 |
1972年 | 2433篇 |
1971年 | 1929篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
The behaviour of the oxinates of niobium, tantalum and associated metals in the infra-red region was studied and a method developed for the determination of niobium and tantalum. Vanadium caused no interference, but other heavy metals, such as molybdenum, manganese and cobalt, which interfered were removed by preliminary treatment when the method was applied to the determination of niobium and tantalum in steels. 相似文献
982.
983.
A simple and sensitive method for determination of benzodiazepines in plasma has been developed using high-performance liquid chromatography in a reverse-phase mode. The method is illustrated by application to plasma samples containing diazepam and N-desmethyldiazepam at concentrations which would be encountered during therapy, with limits of detection of 10 ng/ml and 2 ng/ml for diazepam and N-desmethyldiazepam, respectively. 相似文献
984.
L. D. Smirnov L. G. Stolyarova L. V. Shirokova V. P. Lezina 《Russian Chemical Bulletin》1977,26(2):408-410
Conclusions The aminomethylation, azo-coupling, and nitration of 3-hydroxyisonicotinic acid and its ester are directed to the 2 position, whereas their iodination proceeds with the formation of the 2-iodo and 2,6-diiodo derivatives.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 452–454, February, 1977. 相似文献
985.
The purpose of this communication is to describe the preparation and some properties of the first two synthetic peptides containing D - and L -γ-carboxyglutamic acid. Use was made of N-protected γ,γ′-di-t-butyl-γ-carboxyglutamic acids (D , L , and DL ) described earlier [1 a]. Preliminary 1H-NMR. data (360 MHz) indicate a restricted rotation of the Gla side chain in the free amino acid as well as in the C-terminal Gla of Gla-Gla in H2O solution at acid pH. The proton dissociation from Gla and Gla-Gla was studied by potentiometric titration and NMR. methods. The pH titration in the presence of Ca2+ ions shows that Gla-Gla has a much higher association constant for this cation than Gla. It is almost as great as that of prothrombin (pCa2+ = 3.2 vs. 3.5). 相似文献
986.
Exposure of ent-kaura-2,16-dien-19-ol (1) or its succinate (2) to resuspended mycelia of G. fujikuroi has produced a complex mixture of acids which after methylation gave the esters of two C19 (24) and (30) and five C20 gibberellins (4, 11, 20, 32 and 33). The triester (32) and the lactone ester (24) have been prepared before from the esters of gibberellin A13 (8) and gibberellin A4 (26) respectively. The structures of the other metabolites were assigned on spectroscopic data and by chemical transformations. Thus the lactone diester (4) has been converted to the known keto triester (6). The epoxide (11) has been related to gibberellin A14 (14) and the aldehyde (33) has been related to gibberellin A13 trimethyl ester (8) by way of the triol (34). Selective de-epoxidation of the 16,17-epoxy function in diepoxides has provided a route from the dienes (20 and 24) to the epoxides (11 and 30) respectively, but not from the ester of gibberellin A5 (23) to that of gibberellin A6 (29). On the other hand the latter can be obtained by epoxidation of gibberellin A5 methyl ester trifluoroacetate. Backfeeding experiments carried out with the epoxy diacid (12), the diene diacid (21) and the derived diol (39) indicate pathways connecting the various metabolites. The natural gibberellins A5 and A6 were shown to be formed in some of the backfeeding experiments. 相似文献
987.
Y. K. Agrawal A. L. Shashimohan A. B. Biswas 《Journal of Thermal Analysis and Calorimetry》1975,7(3):635-641
Thermogravimetry (TG), differential thermal analysis (DTA) and X-ray diffraction studies of antimony(III) oxide, (Sb2O3), in air, nitrogen and argon atmospheres have been made. In air Sb2O3 becomes oxidized to Sb2O4 above 510°. The oxidation reaction proceeds in two stages as revealed by the TG and DTA curves. The behaviour of Sb2O3 is similar in both N2 and Ar. Sb2O3 remains unaffected up to 430°, above which there is a slow, and continuous mass loss up to 550°. Above 550° Sb2O3 volatilizes resulting in an enormous weight loss. X-ray studies of the sublimate and the residue indicate the former to be the cubic form of Sb2O3 (Senarmontite) while the residue is the orthorhombic (Valentinite) structure. From the DTA curves in air, N2 and Ar, the transition temperature for the cubic to the orthorhombic modification has been estimated to be around 610°. 相似文献
988.
N. V. Bondarenko A. L. Shinkarenko G. I. Gerashchenko 《Chemistry of Natural Compounds》1973,6(4):449-451
Conclusions 1. On the basis of literature data on the structure of theVeratrum alkaloids, a definite law for the arrangement of the acyl radicals in these ester alkaloids and an interrelationship between the melting point and the structure of the substituent in the C7 position, and between the melting point and the types of amino alcohols upon which the ester alkaloids are based, has been found.2. The features mentioned may be used in studying the structure of theVeratrum ester alkaloids according to a scheme which we propose.Khimiya Prirodnykh Soedinenii, Vol. 6, No. 4, pp. 440–443, 1970 相似文献
989.
M. L. Mandich K. D. Rinnen 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):147-149
Absorption spectra have been recorded for gas phase, size-selected neutral silicon clusters using resonant one- and two-color photodissociation spectroscopy. We now have spectra between 0.94 – 5.58 eV for clusters containing up to 70 atoms. Starting at ~15 atoms, the spectra are all amazingly identical. Comparisons of the silicon cluster spectra to those of various forms of bulk silicon show that the cluster spectra have much in common with the spectrum of the most stable diamond structure of bulk crystalline silicon. 相似文献
990.
The products of the reaction of OClO with NO3 were investigated between 220 and 298 K using a flow reactor and infrared, visible, and ultraviolet analysis. At temperatures below 250 K new infrared and ultraviolet absorption features were observed and assigned to the novel compound chloryl nitrate (O2ClONO2). Additionally, ClO and NO2 were observed as reaction products, indicating the existence of a second reaction channel. O2ClONO2 formation predominates at temperatures below 230 K. The reaction rate constant at 220 K is estimated to be on the order of 10(-14) cm3 molecule-1 s-1 in 1-5 Torr of helium. These observations suggest that O2ClONO2 may exist in the terrestrial stratosphere. 相似文献