全文获取类型
收费全文 | 215631篇 |
免费 | 3098篇 |
国内免费 | 526篇 |
专业分类
化学 | 116984篇 |
晶体学 | 3240篇 |
力学 | 8580篇 |
综合类 | 3篇 |
数学 | 26581篇 |
物理学 | 63867篇 |
出版年
2021年 | 1677篇 |
2020年 | 2016篇 |
2019年 | 2131篇 |
2018年 | 2910篇 |
2017年 | 2820篇 |
2016年 | 4576篇 |
2015年 | 3178篇 |
2014年 | 4419篇 |
2013年 | 10337篇 |
2012年 | 8862篇 |
2011年 | 10077篇 |
2010年 | 6907篇 |
2009年 | 6520篇 |
2008年 | 9218篇 |
2007年 | 9152篇 |
2006年 | 8465篇 |
2005年 | 7837篇 |
2004年 | 7093篇 |
2003年 | 6173篇 |
2002年 | 5840篇 |
2001年 | 5981篇 |
2000年 | 4521篇 |
1999年 | 3419篇 |
1998年 | 2831篇 |
1997年 | 2703篇 |
1996年 | 2652篇 |
1995年 | 2370篇 |
1994年 | 2405篇 |
1993年 | 2310篇 |
1992年 | 2634篇 |
1991年 | 2517篇 |
1990年 | 2461篇 |
1989年 | 2384篇 |
1988年 | 2349篇 |
1987年 | 2341篇 |
1986年 | 2212篇 |
1985年 | 2964篇 |
1984年 | 2995篇 |
1983年 | 2587篇 |
1982年 | 2778篇 |
1981年 | 2586篇 |
1980年 | 2533篇 |
1979年 | 2661篇 |
1978年 | 2683篇 |
1977年 | 2566篇 |
1976年 | 2669篇 |
1975年 | 2568篇 |
1974年 | 2531篇 |
1973年 | 2696篇 |
1972年 | 1715篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
An emission spectrographic method is described for determination of 20 elements present as impurities in telluric acid. Samples are excited in a 12-A. d.c. arc with the use of NaCl-KCl as a spectrographic buffer and germanium as internal standard. The limit of determination lies between 10(-5) and 10(-2)%. The precision of the method calculated as relative standard deviation is 10-25%. 相似文献
852.
A. B. Sannigrahi 《International journal of quantum chemistry》1972,6(2):281-287
A simple semi-theoretical expression for β, the core resonance integral as used in the semi-empirical π-èlectron calculations has been proposed. β's for some standard C ? C and C ? 0 lengths have been evaluated with the proposed formula and the calculated values are compared with those obtained by other semi-theoretical and empirical methods. Electronic spectra of ethylene, formaldehyde, trans-butadiene and benzene have been calculated by a Pariser-Parr-like semi-empirical method with the values of β thus obtained. The calculated spectra are found to be in good agreement with the observed ones. 相似文献
853.
With an excess of dithizone over tellurium, the extraction of Te(IV) from 1 M perchloric acid solutions into a carbon tetrachloride solution (o) of dithixone follows the relation When the acidity is varied, again with a sufficiently large excess of dithissone, the following relation seems to be approached; (μ=1.0). 相似文献
854.
Bourosh P. N. Koropchanu E. B. Bologa O. A. Gdaniec M. Simonov Yu. A. Gerbeleu N. V. 《Russian Journal of Coordination Chemistry》2004,30(6):375-381
Anhydrous compound [Co(NioxH)2(Thio)2]2[SiF6] (I) was synthesized. Its structure was determined by X-ray diffraction analysis. The structures and character of intramolecular and intermolecular hydrogen bonds of complex Iand the previously studied [Co(NioxH)2(Thio)2]2[SiF6] · 3H2O complex were compared. 相似文献
855.
A new, highly sensitive, fast responding and stable potentiometric biosensor for creatinine determination is developed. The biosensor is based on an ammonium ion-selective electrode. Creatinine deiminase (EC 3.5.4.21) is chemically immobilized on the surface of the polymeric ion-sensitive membrane in the form of monomolecular layer using a simple, one-step carbodiimide covalent attachment method. The resulting enzyme electrodes are useful for measurement under flow injection analysis (FIA) conditions. The biosensors exhibit excellent operational and storage stability. The enzyme electrodes retain over 70% of initial sensitivity after ten weeks of work under FIA conditions. The storage stability at 4 °C is longer than half a year without loss of sensitivity. Under optimized conditions near 30 samples per hour can be analyzed and the determination range (0.02-20.0 mmol l−1) fully covers creatinine concentrations important from clinical and biomedical point of view. The simple biosensor/FIA system has been successfully used for determination of creatinine in urine, serum and posthemodialysate samples. 相似文献
856.
The oxidation of iron(II) with tert-butyl hydroperoxide was investigated in the absence of oxygen in water, methanol, and the dichloromethane—methanol solvent
mixture (φr = 2:1). The oxidation rate depends on solvent polarity; measured in the presence of SCN− at constant 0.8 mmol dm−3 HCl, the rate constant increases with the polarity decrease passing from water and methanol to the dichloromethane—methanol
solvent mixture. Further, in non-aqueous solutions at this acid concentration the rate constant was higher than the rate constant
in the presence of Cl− only. The oxidation rate measured in the [FeCl]2+ complex in dichloromethane—methanol was slow in acidic medium and increased by decreasing the acid concentration. Approaching
the physiological pH conditions the rate constant attained the value of an order of magnitude of 103 dm3 mol−1 s−1, while very little alteration of stoichiometry of the oxidation reaction was observed. The rate constant measured in the
presence of Cl− strongly depends on electrolyte concentration at concentrations less than 0.5 mmol dm−3 HCl, both in MeOH and the solvent mixture. Based on these results, a possible mechanism of the influence of solvent, acidity,
and ligand type on the rate constant is discussed. We assume that the oxidation proceeds by an inner-sphere mechanism considering
that the breakdown of the successor inner-sphere complex forming reactive alkoxyl radicals is probably the rate-limiting step.
Presented at the 20th International Conference on the Coordination and Bioinorganic Chemistry organized by the Slovak Chemical
Society, Slovak University of Technology, Comenius University, and the Slovak Academy of Sciences, Smolenice Castle, 5–10
June 2005. 相似文献
857.
[structure: see text]. The solution structure of (+)-discodermolide (1) has been determined via 1- and 2-D NMR techniques in conjunction with Monte Carlo conformational analysis. Taken together, the results demonstrate that in solution (+)-discodermolide occupies a helical conformation remarkably similar to the solid state conformation. 相似文献
858.
P. Hoffmann G. Paller B. Thybusch U. Stingl 《Fresenius' Journal of Analytical Chemistry》1991,339(4):230-234
Summary Instrumental neutron activation analysis (INAA), atomic absorption spectrometry (AAS), and X-ray fluorescence analysis (XRF) were tested for the determination of the abrasion effect in plastics processing equipment. As optimal solution measurement of Fe by XRF in cylindrical samples was found. 相似文献
859.
Aspects of the use of lipophilic gels in manual sample preparation procedures are reviewed. Neutral gels with a controlled hydrophobicity are used for sorbent extraction of non-polar and medium polarity compounds from biological fluids. Acidic amphiphilic compounds can be extracted as ion-pairs with decyltrimethylammonium ions. Solvent or detergent extracts of tissues or faeces can be mixed with hydrophobic gels for transfer of analytes from a solvent to a gel phase, permitting subsequent sample preparation in gel bed systems. Hydrophobic gels, alkyl-bonded silica and polystyrene matrices can be used in series for extraction of compounds with a wide range of polarities. Group fractionations are performed on neutral and ion-exchanging lipophilic gels to yield fractions of neutral, basic and acidic metabolites within selected polarity ranges. Selective isolation of phenolic acids on a strong anion exchanger, of ethynylic steroids on a strong cation exchanger in silver form and of oximes of ketonic steroids on a strong cation exchanger in hydrogen form is possible. A computerized system for automatic sample preparation is also described. It consists of an extraction bed, a cation-exchange column and an anion-exchange column. The pumps and switching valves are arranged so that the columns can operate in series or parallel for isolation of neutral, basic and acidic metabolites of amphiphilic compounds and for regeneration of the column beds. Fractions can be collected, or the effluent from the column beds can be diluted with water to permit sorption on a solid phase. The applicability of the automated method to the analysis of bile acids and metabolites of mono(2-ethylhexyl) phthalate is demonstrated. 相似文献
860.
Molybdenum-95 NMR spectra have been measured for a selection of molybdenum carbonyl compounds and Mo(Σ-C5H5)(η-C5H5)(NO)(S2CNMe2). A chemical shift range of more than 1500 ppm is found. The chemical shifts and linewidths are discussed. 相似文献