全文获取类型
收费全文 | 1918篇 |
免费 | 75篇 |
国内免费 | 21篇 |
专业分类
化学 | 1276篇 |
晶体学 | 4篇 |
力学 | 91篇 |
数学 | 194篇 |
物理学 | 449篇 |
出版年
2022年 | 27篇 |
2021年 | 55篇 |
2020年 | 42篇 |
2019年 | 31篇 |
2018年 | 38篇 |
2017年 | 17篇 |
2016年 | 57篇 |
2015年 | 44篇 |
2014年 | 52篇 |
2013年 | 91篇 |
2012年 | 66篇 |
2011年 | 98篇 |
2010年 | 78篇 |
2009年 | 60篇 |
2008年 | 103篇 |
2007年 | 87篇 |
2006年 | 96篇 |
2005年 | 80篇 |
2004年 | 64篇 |
2003年 | 77篇 |
2002年 | 57篇 |
2001年 | 56篇 |
2000年 | 55篇 |
1999年 | 48篇 |
1998年 | 20篇 |
1997年 | 20篇 |
1996年 | 29篇 |
1995年 | 27篇 |
1994年 | 30篇 |
1993年 | 33篇 |
1992年 | 25篇 |
1991年 | 19篇 |
1990年 | 13篇 |
1989年 | 19篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 24篇 |
1984年 | 23篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1980年 | 14篇 |
1979年 | 11篇 |
1978年 | 12篇 |
1976年 | 17篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1973年 | 10篇 |
1972年 | 9篇 |
排序方式: 共有2014条查询结果,搜索用时 375 毫秒
41.
J. J. La Brecque 《Journal of Radioanalytical and Nuclear Chemistry》1978,43(1):253-262
A rapid method for the determination of titanium in geochemical samples using a55Fe radioisotope as the extraction source coupled to a Si(Li) detector and a PDP-11.05 computer is described. The relative
standard deviation for a thin sample about 100 mg in weight is less than 4% and it is achieved in a fluorescent time of 100
sec. Data are given for the use of two different internal standards, KBrO3 and K2Cr2O7, the latter being more precise. The values for five international reference standards are in agreement for this method with
the reported values. Finally, besides being rapid, no hydrofluoric acid dissolution of fusion is necessary as in atomic absorption
and classical methods.
This paper was presented at the XX Colloquium Spectroscopium Internationale, 8 Aug.–7. Sept. 1977, Prague, Czechoslovakia. 相似文献
42.
Antonino Arcoria Emanuele Maccarone Gaetano A. Tomaselli Rosario Calí Salvatore Currieri 《Journal of heterocyclic chemistry》1975,12(2):333-335
The second order rate constants k2 and the activation parameters for the reaction of 2-thiophenesulfonyl chloride with aniline together with solution enthalpies of the reactants have been measured in methanol, ethanol, 2-propanol, acetonitrile and acetone. The reaction rates are slower in dipolar aprotic solvents than in protic ones due to a remarkable activation negative entropy. The rate constants k2 are correlated with empirical solvent polarity parameters. The data seem in accord with a SAN reaction mechanism. 相似文献
43.
Alessandro D'Aprano Camillo La Mesa Noemi Proietti Bianca Sesta Sonia Tatone 《Journal of solution chemistry》1994,23(12):1331-1346
The interactions between octyl--D-glucoside and glycine in water have been investigated by surface tension, viscosity, and density measurements. The results show that the -amino acid causes an unexpected lowering of the critical micellar concentration of octyl--D-glucoside. Such a finding has been interpreted in temss of dipole-dipole interactions between the hydrophilic site of the surfactant and the peptidic cosluttes. From three to seven amino acid molecules have been estimated to be coordinated with each glucoside unity in the micellar state. The research has been extended to glycine oligopeptides and L-lysine. The latter compound has effects similar to those observed with glycine whereas diglycine and triglycine show weaker effects on the micellization process. 相似文献
44.
Electroanalytical methods based on biosensor technology and differential pH measurement for the determination of l-lactic and l-malic acids in wine were developed. Lactic acid was determined according to two procedures consisting of the use of two lactate electrochemical biosensors, one based on a Clark O(2) probe and the other based on the H(2)O(2) probe. The two probes were assembled employing a polymeric membrane where the enzyme lactate oxidase was previously immobilized. Both sensors exhibited high storage and operational stabilities, and good reproducibility when used in wine matrix. Measurement of lactate was carried out using two different flow-through cells assembled with the lactate probes. The low detection limit of these probes and the relatively high concentration of lactate in wine samples required a serial dilution in the range 1:100 to 1:200, thus eliminating all potential electrochemical or enzyme interferences present in the sample. Malic acid was determined using a differential commercially available pH-meter and an enzymatic procedure. For this analysis, we employed the malic enzyme (EC 1.1.1.40) and the NADP(+) cofactor. The pH variation due to the action of this enzyme on malic acid was found to be proportional to the malic acid present in the sample. Experimental parameters such as pH, temperature, and co-factor concentration were optimized, resulting in malate determination in less than 1 min with good reproducibility. Various samples of wine and wine musts were assayed for lactate with the two biosensor procedures, and for malate with differential pH-metry. The results when compared with those obtained with the commonly used spectrophotometric procedure correlated well. 相似文献
45.
46.
Lazarus EA Navratil GA Greenfield CM Strait EJ Austin ME Burrell KH Casper TA Baker DR DeBoo JC Doyle EJ Durst R Ferron JR Forest CB Gohil P Groebner RJ Heidbrink WW Hong R Houlberg WA Howald AW Hsieh C Hyatt AW Jackson GL Kim J Lao LL Lasnier CJ Leonard AW Lohr J La Haye RJ Maingi R Miller RL Murakami M Osborne TH Perkins LJ Petty CC Rettig CL Rhodes TL Rice BW Sabbagh SA Schissel DP Scoville JT Snider RT Staebler GM Stallard BW Stambaugh RD St John HE Stockdale RE Taylor PL Thomas DM 《Physical review letters》1996,77(13):2714-2717
47.
Fustero S Navarro A Díaz D de La Torre MG Asensio A Sanz F González ML 《The Journal of organic chemistry》1996,61(25):8849-8859
Reaction of azaenolates of 2-alkyl-oxa(thia)zolines 6 with imidoyl chlorides 7 as electrophiles to furnish masked N-substituted beta-enamino acid derivatives 1-2 in 70-90% yield is described. Alternative routes are discussed. Compounds 1-2 generally appear in one tautomeric form, imino or enamino, depending on the nature of the imidoyl chloride. The configuration of the enamino moiety (Z) and the conformation (s-cis) of compounds 1-2 obtained were established by an NMR study and unequivocally set by nuclear Overhauser effect difference experiments. An X-ray structure of compound 1e is also reported, showing a strong intramolecular NH.N hydrogen bond. Ab initio calculations (HF/3-21G and HF/3-21+G) have been carried out on several representative examples (1e, 1p, and 1l) in an attempt to support and provide the correct geometry of these derivatives. Structural considerations among the possible isomers of compounds 1 are discussed. From these studies it was concluded that the theoretical calculations agree with the experimental results. In addition, a very simple one-pot procedure for the preparation of masked N-substituted alpha-alkylated beta-enamino acid derivatives 2 from 6, 7, and different alkyl halides (R(3)Y) is described. 相似文献
48.
Anhydrites(II)- CaSO4 were prepared from two waste gypsums (PRECHEZA Perov, FOSFA Potorná). The samples of anhydrites(II) with sulphate activators were tested on spillage, beginning and the end of setting time, bending and pressure strength. Both sodium and potassium sulphates accelerated setting of anhydrite, more impressively that prepared from Potorná gypsum. The addition of Na2SO4 influenced the strength of this anhydrite very favourably. On the contrary K2SO4 influenced favourably the strength of Perov anhydrite. Results confirm the necessity to evaluate properties of such materials with respect to their origin. 相似文献
49.
50.