全文获取类型
收费全文 | 32138篇 |
免费 | 1279篇 |
国内免费 | 5050篇 |
专业分类
化学 | 28378篇 |
晶体学 | 296篇 |
力学 | 53篇 |
综合类 | 233篇 |
数学 | 17篇 |
物理学 | 1748篇 |
综合类 | 7742篇 |
出版年
2024年 | 52篇 |
2023年 | 291篇 |
2022年 | 830篇 |
2021年 | 806篇 |
2020年 | 785篇 |
2019年 | 817篇 |
2018年 | 711篇 |
2017年 | 884篇 |
2016年 | 1042篇 |
2015年 | 1018篇 |
2014年 | 1263篇 |
2013年 | 2469篇 |
2012年 | 2122篇 |
2011年 | 1728篇 |
2010年 | 1443篇 |
2009年 | 1726篇 |
2008年 | 1830篇 |
2007年 | 2087篇 |
2006年 | 2028篇 |
2005年 | 1866篇 |
2004年 | 1713篇 |
2003年 | 1422篇 |
2002年 | 1261篇 |
2001年 | 1000篇 |
2000年 | 968篇 |
1999年 | 794篇 |
1998年 | 659篇 |
1997年 | 664篇 |
1996年 | 641篇 |
1995年 | 566篇 |
1994年 | 542篇 |
1993年 | 430篇 |
1992年 | 409篇 |
1991年 | 331篇 |
1990年 | 274篇 |
1989年 | 266篇 |
1988年 | 217篇 |
1987年 | 125篇 |
1986年 | 82篇 |
1985年 | 50篇 |
1984年 | 56篇 |
1983年 | 29篇 |
1982年 | 42篇 |
1981年 | 28篇 |
1980年 | 22篇 |
1979年 | 20篇 |
1978年 | 22篇 |
1977年 | 5篇 |
1973年 | 5篇 |
1968年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
O. Slisenko E. Lebedev P. Pissis A. Spanoudaki E. Kontou O. Grigoryeva 《Journal of Thermal Analysis and Calorimetry》2006,84(1):15-19
The structure-property relationships of thermoplastic
polymer blends based on poly(ether-urethane) ionomer (PEUI) and ion-containing
styrene-acrylic acid copolymer (S-co-AA(K))
have been investigated by using DMTA, DSC and TGA, as well as tensile tests.
Convergence of the glass transition temperature (Tg)
values of the PEUI and the S-co-AA(K) components
in the blends studied, as compared to the individual polymers, was found and
explained by improving compatibility of the components due to increasing effective
density of physical networks formed by ion-dipole and ion-ion interactions
of ionic groups of the components. Character of E'=f(T) and E'=f(T) dependencies confirms the increase of the effective
density of physical networks in the compositions studied compared to individual
PEUI and S-co-AA(K). Improvement of end-use
properties, i.e. thermal stability and tensile properties has been found for
the PEUI/S-co-AA(K) compositions with lower
content of S-co-AA(K) (i.e. <10 mass%)
and explained by formation of additional network of intermolecular ionic bonds
between the functional groups of PEUI and S-co-AA(K). 相似文献
112.
P. V. Katsikopoulos C. D. Papaspyrides 《Journal of polymer science. Part A, Polymer chemistry》1994,32(3):451-456
Loss of diamine and distinct transition phenomena from the solid to the melt state restrain solid-state polymerization (SSP) of nylon salts from industrial application. To depress these phenomena temperatures well below the melting point of the starting material are employed, resulting however in low reaction rates. The need to use catalysts arises and accordingly in this study hexamethylenediammonium adipate (HMA) particles were polymerized containing a strictly controlled amount of uniformly distributed catalyst. The catalysts included boric, sulphuric, and phosphoric acids with boric acid being the most effective in accelerating the reaction while eliminating the tendency to agglomeration. In general, catalyst concentration up to a critical value plays a considerable role in the overall behavior. Furthermore, in agreement with non catalytic processes, the reaction temperature remains an influential parameter. © 1994 John Wiley & Sons, Inc. 相似文献
113.
E. V. Gert 《Cellulose (London, England)》1996,3(1):217-228
For preparation of various modified forms of microcrystalline cellulose, an optional combination is proposed of hydrolytic activity with other properties of nitric acid such as, adduct forming ability (manifested as Knecht compound formation), oxidizing, bleaching and nitrating. The manifestation of a chosen acid function is reached by variation of the reaction conditions: concentration, temperature, time, etc. The sole-reagent continuous procedure involving several operations made in a predetermined sequence allows the use of a more simple technological scheme and saves energy, time and materials. The features of transformation of fibrous and microcrystalline forms of native cellulose into cellulose-II through the Knecht compound are discussed. Purely acidic transformation schemes of native cellulose into microcrystalline cellulose hydrate and amorphous low-nitrate powder cellulose according to the above scheme are considered. Morphological features as well as distinguishing properties — enhanced hydrophilicity, sorption activity and degree of whiteness — of the prepared materials are pointed out. The possibility of obtaining uniformly oxidized forms of powder cellulose according to this scheme and the outlook for the use of nitrogen(IV) oxide as a multifunctional reagent are discussed. 相似文献
114.
A sequential injection UV method was developed to determine benzophenone-4 (BZ4) and phenylbenzimidazole sulphonic acid (PBS) simultaneously, these being the most commonly used UV-filters in aqueous formulations used as sunscreen sprays. The selective elution of both was performed by on-line solid-phase extraction, by retention on a SAX microcolumn and separation by varying the pH of elution. The sensitivity obtained was 0.042±0.001 ml μg−1 for PBS and 0.0159±0.0003 ml μg−1 for BZ4. The limit of detection was 1.6 μg ml−1 for PBS and 0.6 μg ml−1 for BZ4. The R.S.D. of the results was 1-6% for PBS and 1-12% for BZ4. The method was validated using commercial sunscreen formulations with concentrations determined by a liquid chromatographic procedure. The two procedures gave comparable results. Automation of the method means the amount of reagents used and residues generated are decreased. The system allows the required analysis sequence to be programmed using suitable software. 相似文献
115.
Isabelle FavierElisabet Duñach 《Tetrahedron》2003,59(10):1823-1830
Mandelic acid and some aryl substituted derivatives were oxidised under molecular oxygen and a catalytic amount of Bi(0). The corresponding aldehydes and/or the carboxylic acids were obtained selectively depending on the nature of the substituent. Aldehydes and α-keto acids were oxidised under the same Bi(0)/O2 system and α-keto acids were proposed as intermediates in the formation of benzoic acid derivatives. 相似文献
116.
Ulrich Jordis 《Monatshefte für Chemie / Chemical Monthly》1986,117(11):1339-1342
5-Amino-benzo-1,3-dithiol, available by reduction of 5-amino-benzo-1,3-dithiol-2-thion, was elaborated into dithio-oxolinic Acid (6 a). 相似文献
117.
Markku J. Oila 《Tetrahedron》2005,61(45):10748-10756
A novel supported amino alcohol linker was synthesized and utilized for attachment of picolinic acid derivatives onto different supports. When the resin bound molecule was further activated, the PyOX-moiety could be constructed reliably in enantiopure form. Furthermore, an efficient Pd-catalyzed modification of a picolinic acid derivative is presented. 相似文献
118.
Frontera A Saczewski F Gdaniec M Dziemidowicz-Borys E Kurland A Deyà PM Quiñonero D Garau C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(22):6560-6567
Several structures of pi complexes of isocyanuric acid and of several thio derivatives with anions have been computed by using high level ab initio calculations. The nature of the complexes has been studied by means of the method of molecular interaction potential with polarization (MIPp) and Bader's theory of atoms-in-molecules. These molecules form favorable complexes with anions and can be used as binding units for building receptors for the molecular recognition of anions. In several cases, the anion-pi interaction has been demonstrated experimentally by means of X-ray crystallography. 相似文献
119.
Post-column oxidative derivatization for the liquid chromatographic determination of phenothiazines 总被引:2,自引:0,他引:2
A first post-column chemical derivatization method for the liquid chromatographic determination of phenothiazines is presented. Peroxyacetic acid is introduced as a derivatizing agent for phenothiazines, yielding the colored radical cations or fluorescent sulfoxides, depending on reaction conditions. Both reaction products were successfully employed for the detection of the phenothiazines after their liquid chromatographic separation. The fluorescence spectroscopic detection of the sulfoxides proved to be the more robust and sensitive method. Limits of detection ranged from 4 nM for triflupromazine and trimeprazine to 300 nM for phenothiazine for the fluorescence spectroscopic detection of the sulfoxide and from 0.3 μM for phenothiazine and triflupromazine to 2 μM for trifluperazine for the UV–Vis spectroscopic detection of the radical cation. The calibration functions for the fluorimetric sulfoxide determination ranged from two to more than three decades, starting at the limit of quantification. 相似文献
120.
Changes of the rheological properties of hyaluronic acid (sodium-magnesium salt) solutions after exposure to UV radiation
indicate a vigorous decrease in their viscosity, but its still strong shear rate dependence. Whereas the presence of the singlet
oxygen sensitizer (anthracene-1-sulphonic acid) brings about a loss of shear dependence; the studied solutions show newtonian
behavior. 相似文献