全文获取类型
收费全文 | 1272篇 |
免费 | 21篇 |
国内免费 | 6篇 |
专业分类
化学 | 1037篇 |
晶体学 | 8篇 |
力学 | 8篇 |
数学 | 142篇 |
物理学 | 104篇 |
出版年
2023年 | 16篇 |
2022年 | 50篇 |
2021年 | 57篇 |
2020年 | 33篇 |
2019年 | 22篇 |
2018年 | 25篇 |
2017年 | 19篇 |
2016年 | 22篇 |
2015年 | 35篇 |
2014年 | 30篇 |
2013年 | 69篇 |
2012年 | 63篇 |
2011年 | 75篇 |
2010年 | 55篇 |
2009年 | 41篇 |
2008年 | 80篇 |
2007年 | 74篇 |
2006年 | 79篇 |
2005年 | 63篇 |
2004年 | 55篇 |
2003年 | 60篇 |
2002年 | 49篇 |
2001年 | 10篇 |
2000年 | 13篇 |
1999年 | 12篇 |
1998年 | 10篇 |
1997年 | 10篇 |
1996年 | 14篇 |
1995年 | 12篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 14篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 11篇 |
1981年 | 8篇 |
1980年 | 7篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1975年 | 3篇 |
1974年 | 4篇 |
1972年 | 4篇 |
1959年 | 2篇 |
1940年 | 2篇 |
排序方式: 共有1299条查询结果,搜索用时 15 毫秒
11.
Carla M. Bossu Edilene C. Ferreira Fernanda S. Chaves Eveline A. Menezes Ana Rita A. Nogueira 《Microchemical Journal》2006,84(1-2):88-92
A flow injection system was proposed to evaluate the transient product of a colorimetric reaction between hydrolysable tannin and potassium iodate (KIO3) solution. The system optimization was accomplished by using statistical methods based on experimental design. Flow rate of KIO3 solution, sample volume, carrier flow rate, and reaction coil were the selected factors for evaluation. On screening step, complete factorial 24 was used and two levels for each selected factor were studied. For the optimization phase, a centered face composite design 22 + star was employed to evaluate sample volume and flow rate of KIO3 solution, which were the factors identified in the screening phase as having more influence on the absorbance signal. After optimization, the proposed system was compared with batch determination. Some characteristics, such as analytical frequency, reagent consumption and chemical residues generation presented better results by the use of the proposed system if compared with batch method. The system presented good repeatability with standard deviation lower than 3%, for n = 10, linearity (R2 = 0.9974) for tannic acid standard, analytical frequency of 15 injections h− 1 and limit of quantification of 24 mg L− 1 of tannic acid. Good results were obtained when the proposed system was applied to hydrolysable tannin determination in Stryphnodendron barbatimão, Eucalyptus citriodora and Phyllanthus niruri, samples of plants commonly used in popular medicine. 相似文献
12.
13.
Mixed acetals 7 of benzeneseleninylacetaldehyde, prepared by a simple 2-step procedure from mono- and bicyclic allylic alcohols 5 , undergo benzeneselenenic acid elimination to transient ketene acetals 8 which afford γ, δ-unsaturated esters 9 via the ester Claisen rearrangement (Scheme 2). Under the same conditions selenoxide 7h derived from benzyl alcohol 5h is converted back to benzyl alcohol with the concomitant formation of ethylphenylselenoacetate 12 . 相似文献
14.
Berisio R Granata V Vitagliano L Zagari A 《Journal of the American Chemical Society》2004,126(37):11402-11403
The analysis of factors contributing to the stability of proteins is a subject of intense debate. Particularly challenging is the study of structural proteins, since their function is their structure. Among these is collagen, the key structural component of bones, skin, cartilage, tendons, and other connecting tissues. It is well established that the collagen triple helix is characterized by the presence of hydroxyproline, whose content modulates triple helix thermal stability according to the requirement of the host organism. Because of the complexity and the fibrous nature of collagen, data on the stability and structure of this protein have been mainly obtained by the use of collagen-like polypeptides. On the basis of CD characterization of collagen-like polypeptides we here show that the presence of Hyp at the X position of repeating triplets Hyp-Hyp-Gly stabilizes the triple helix significantly. This extra-stabilization has been ascribed, by using molecular modeling, to the formation of a hydrogen bond between Hyp residues belonging to the X and the Y positions of adjacent chains. This communication also provides a comprehensive interpretation of the ensemble of available data on polypeptides containing proline derivatives. 相似文献
15.
Muzzalupo R Trombino S Iemma F Puoci F La Mesa C Picci N 《Colloids and surfaces. B, Biointerfaces》2005,46(2):78-83
Vesicular formulations (liposomes and niosomes) play an increasingly important role since they can be used as drug delivery and targeting systems. We described the formation of two niosomal systems based on synthetic bolaform surfactants (4,7,10,13-pentaoxa-16-aza-cyclooctadecane)-hexadecanedioc acid diamide (BD-16) and alpha,omega-(4,7,10,13-pentaoxa-16-aza-cyclooctadecane)-hexadecane (BC-16). Systems containing BD-16 or BC-16 and different amount of cholesterol (CH) were prepared by aqueous dispersion of films, followed by examination of methylene blue (MB) entrapment, particle size and morphology. Indeed, we also studied the hydration in the distilled water and physiological solution, in order to investigate the complexing ability on vesicle formation. The results obtained in this study show a high encapsulation capacity and this ability and the size depends on cholesterol content. 相似文献
16.
Milk samples can be efficiently digested using a focused microwave oven, however the conventional procedure of addition of concentrated acids to the liquid sample leads to digestates with elevated acidity and residual carbon concentrations. In this work a focused microwave oven was applied for acid digestion of bovine milk samples using a conventional and an alternative procedure based on gradual sample addition to hot and concentrated acids. A two-level 23 full factorial design experiment with eight runs was carried out to evaluate the optimum experimental conditions for reducing both the residual carbon and the final acidity of digestates. The three studied parameters were: temperature of the digestion medium for sample addition, addition of sulfuric acid before the sample or during the first step, and number of aliquots of the sample gradually added. The best conditions were attained by adding small aliquots of milk (ten-fold a volume of 0.5 ml added during 5.0 min) to a digestion mixture containing 3.0 ml nitric acid plus 1.0 ml sulfuric acid heated at 105 °C. It was demonstrated that the digestion efficiency of the alternative procedure was better than the conventional procedure, i.e. 98 and 80%, respectively. The alternative procedure was applied for determination of Ba, Ca, Cu, K, Mg, Na, P, and Zn in whole and non-fat bovine milk. The accuracy was proved using two certified reference materials (whole and non-fat milk powder). 相似文献
17.
The oxidation of 2,6-di-tert-butylphenol with tert-butylhydroperoxide (ButO2H) has been studied using polymer (XAD4) anchored salicylaldoxime, 1,3-propylene-bis-salicylaldimine and o-phenylene-bis-salicylaldimine complexes of molybdenum and vanadium in acetonitrile. The predominant products formed in the oxidation reactions were 2,6-di-tert-butylbenzoquinone (BQ) and 3,3′-5,5′-tetra-tert-butyldiphenoquinone (dPQ), whereas with some only 2,6-di-tert-butylbenzoquinone was formed. This is the first reported use of polymer anchored molybdenyl and vanadyl complexes in selective oxidation of 2,6-di-tert-butylphenol. Solvent plays an important role in this reaction. The effects of varying the ligand, metal and the support on the catalytic activity in the oxidation of 2,6-di-tert-butylphenol have been studied. With polymer anchored MoO2(salpen), 81% of 2,6-di-tert-butylbenzoquinone was formed from 2,6-di-tert-butylphenol. 相似文献
18.
19.
Boris S. Krumgalz Rita Pogorelsky Kenneth S. Pitzer 《Journal of solution chemistry》1995,24(10):1025-1038
The ion interaction approach developed by Pitzer was used for the prediction of volumetric properties of mixed electrolyte solutions at 25°C based on parameters calculated from experimental data for single-solute electrolyte solutions. Such an approach was shown to be especially effective for application to the calculation of volumetric properties of natural hypersaline brines and of industrial electrolyte solutions of large complexity. The use of the latest recommended sets of volumetric ion interaction parameters for single electrolyte solutions and symmetrical mixing parameters for Na–K–Cl ion combinations considerably improved the precision of the density calculations of highly concentrated mixed electrolyte solutions and of various natural waters. 相似文献
20.
Studies aimed at a comparison of chemical, biomimetic (Gif system GoAgg(III))and enzymatic (CHMO) transformations of natural (+)-10beta,14-dihydroxy-allo-aromadendrane have led to preparation of an eight-member sesquiterpene lactone. 相似文献