全文获取类型
收费全文 | 6553篇 |
免费 | 283篇 |
国内免费 | 41篇 |
专业分类
化学 | 4739篇 |
晶体学 | 14篇 |
力学 | 197篇 |
数学 | 1148篇 |
物理学 | 779篇 |
出版年
2023年 | 41篇 |
2022年 | 154篇 |
2021年 | 219篇 |
2020年 | 160篇 |
2019年 | 154篇 |
2018年 | 141篇 |
2017年 | 104篇 |
2016年 | 288篇 |
2015年 | 227篇 |
2014年 | 286篇 |
2013年 | 412篇 |
2012年 | 463篇 |
2011年 | 531篇 |
2010年 | 269篇 |
2009年 | 312篇 |
2008年 | 444篇 |
2007年 | 415篇 |
2006年 | 351篇 |
2005年 | 324篇 |
2004年 | 299篇 |
2003年 | 222篇 |
2002年 | 160篇 |
2001年 | 67篇 |
2000年 | 48篇 |
1999年 | 52篇 |
1998年 | 54篇 |
1997年 | 48篇 |
1996年 | 64篇 |
1995年 | 35篇 |
1994年 | 34篇 |
1993年 | 19篇 |
1992年 | 33篇 |
1991年 | 26篇 |
1990年 | 26篇 |
1989年 | 23篇 |
1988年 | 23篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 39篇 |
1984年 | 30篇 |
1983年 | 37篇 |
1982年 | 32篇 |
1981年 | 19篇 |
1980年 | 21篇 |
1979年 | 15篇 |
1978年 | 22篇 |
1977年 | 15篇 |
1976年 | 14篇 |
1974年 | 12篇 |
1972年 | 5篇 |
排序方式: 共有6877条查询结果,搜索用时 296 毫秒
101.
Huenerbein A Sipoli Marques MA Pereira Ados S de Aquino Neto FR 《Journal of chromatography. A》2003,985(1-2):375-386
The Medical Commission of the International Olympic Committee forbids the use of anabolic androgenic steroids and beta2-agonists to improve athletic performance. In this work we have selected examples of anabolic androgenic compounds and their metabolites to evaluate the GC-MS analysis of some trimethylsilyl derivatives. The aim is to set the best GC conditions to improve the detection within the whole range of analyte elution temperatures. The initial column temperature was changed to 105 or 140 degrees C followed by 40 degrees C min(-1) to 200 degrees C and then 15 degrees C min(-1) to 300 degrees C. Using 140 degrees C as the initial oven temperature it was possible to obtain narrower initial analyte distributions for the compounds that elutes at the beginning of the chromatogram as clenbuterol, mabuterol, epimethylenediol and norandrosterone, without loss of derivatized metabolites signal. Later. eluting analytes, such as the stanozolol metabolites, furazabol and oxandrolone were not affected. Temperatures below 140 degrees C. resulted in partial derivatization for some analytes mainly stanozolol related structures. Therefore evaluation of derivatization conditions as occurring in three steps, the vial, vaporization chamber and capillary column, was thoroughly assessed. The new program temperature improves the signal-to-noise ratio for some compounds and shows adequate resolution for endogenous compounds. Some of the difficult key separations necessary for doping control enforcement were also obtained with the proposed method. 相似文献
102.
Chabinyc ML Salleo A Wu Y Liu P Ong BS Heeney M McCulloch I 《Journal of the American Chemical Society》2004,126(43):13928-13929
A method for the fabrication of polymeric thin-film transistors (TFTs) by lamination is described. Poly(dimethylsiloxane) stamps were used to delaminate thin films of semiconducting polymers from silicon wafers coated with a self-assembled monolayer (SAM) formed from octyltrichlorosilane. These supported films were laminated onto electrode structures to form coplanar TFTs. The fabrication process was used to make TFTs with poly(3-hexylthiophene), P3HT, and poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene], PQT-12. TFTs, where these polymers were laminated onto gate dielectrics coated with SAMs from octyltrichlorosilane, had effective field-effect mobilities of 0.03 and 0.005 cm2/(V s), respectively. TFTs where PQT-12 was laminated onto gate dielectrics that were not coated with a SAM also had mobility of 0.03 cm2/(V s). In contrast, TFTs fabricated by spin-coating PQT-12 onto the same structure had mobilities ranging from 10-3 to 10-4 cm2/(V s). These results suggest that the lower mobilities of polymer TFTs made with hydrophilic gate dielectrics are caused by molecular ordering in the semiconducting film rather than electronic effects of dipolar groups at the interface. 相似文献
103.
Marta Sánchez-Cabezudo Margarita G. Prolongo Catalina Salom Rosa M. Masegosa 《Journal of Thermal Analysis and Calorimetry》2006,86(3):699-705
The cure kinetics
and morphology of diglycidyl ether of bisphenol A (DGEBA) modified with polyvinyl
acetate (PVAc) using diaminodiphenylmethane (DDM) as hardener were investigated
through differential scanning calorimetry (DSC) and environmental scanning
electron microscopy (ESEM). Isothermal curing measurements were carried out
at 150, 120 and 80°C. The kinetic parameters were obtained using the general
autocatalytic chemically controlled model. The comparison of the kinetic data
indicates that the presence of PVAc does not change the autocatalytic nature
of the cure reaction. Two T
g’s
were observed in the fully cured samples of the modified systems. ESEM micrographies
confirm the biphasic morphology. 相似文献
104.
Alberto Bolognesi Chiara Botta Clara Mercogliano Marco Marinelli W. Porzio Luigi Angiolini Elisabetta Salatelli 《先进技术聚合物》2003,14(8):537-543
The degree of orientation of thin films of eight different polymeric structures, belonging to the class of soluble polyalkylthiophenes (PATs) has been studied. Thin films of the polymers, obtained by spin coating onto glass substrates, were oriented by means of the rubbing technique. The degree of orientation is related to the regioregularity of the system: highly regioregular polymers can be oriented, while regiorandom materials are not oriented. The degree of orientation can be improved by thermal annealing of the oriented films. As a result of annealing, which was performed at different temperatures according to the thermal behavior of the polymers, it was possible to increase the polarization ratio for poly(3-decylthiophene) up to 12–13 as detected from the UV-vis spectra in polarized light. Moreover in the investigated PATs, both the key role of molecular weight and its distribution for achieving a high orientation degree has been assessed. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
105.
106.
Synthesis of 1-(4-substituted)benzyl-6-hydroxyisoquinolines with potential activity on NA+,K+-atpase
Alberto Cerri Paola Mauri Marina Mauro Piero Melloni 《Journal of heterocyclic chemistry》1993,30(6):1581-1591
The synthesis of 1-(4-substituted)benzyl-6-hydroxyisoquinolines, to be evaluated in the displacement of the specific 3H-ouabain binding to Na+,K+-ATPase, is described. The key step involved a cyclization to the isoquinoline ring under Pictet-Gams conditions which was best performed with the 6-hydroxy group protected as the benzyl ether. When an unsaturated ester group was present in position 4 of the 1-benzyl group, this was best introduced before the cyclization step, since the IIeck reaction on 1-(4-bromobenzyl)-6-hydroxyisoquinoline ( 8 ) with acrylic acid derivatives was not successful in all cases. 相似文献
107.
Several on-column sample preconcentration modes--large-volume sample stacking using the EOF pump (LVSEP), LVSEP with anion-selective exhaustive injection (LVSEP-ASEI) and field-amplified sample injection with sample matrix removal using the electroosmotic flow (EOF) pump (FAEP)--were used to analyze some nonsteroidal anti-inflammatory drugs (NSAIDs) by capillary electrophoresis, and then compared. Methanol was the background electrolyte solvent to suppress the EOF. The effect of the type and length of the solvent plug, and the sample injection time were investigated in FAEP to determine the conditions that provided the best response. LVSEP, LVSEP-ASEI, and FAEP improved the sensitivity of the peak area by 100-, 1200-, and 1800-fold, respectively. The methodology developed, in combination with solid-phase extraction (SPE), was applied to the analysis of water samples. 相似文献
108.
Pietro Schenone Alberto Bargagna Gaetano Bignardi Filippo Evangelisti 《Journal of heterocyclic chemistry》1976,13(5):1105-1107
The reaction of 2-carbonyl-1,3-dithiane, a sulfene prepared in situ from 2-chloroearbonyl-1,3-dithiane and triethylamine, with N,N-disubstituted 2-aminomethylvnecycloalkanones gave the 1,4-cycloadducts, namely N,N-disubstituted 4′-aminospiro[1,3-dithiane-2,3′-(5′,6′-poly-methylene-3′,4′-dihydro-α-pyrones)]. 相似文献
109.
Aringhieri Roberto Bigharaz Sara Duma Davide Guastalla Alberto 《Central European Journal of Operations Research》2022,30(1):189-211
Central European Journal of Operations Research - Disaster management generally includes the post-disaster stage, which consists of the actions taken in response to the disaster damages. These... 相似文献
110.
Hereditarily finite sets and hypersets are characterized both as algorithmic data structures and by means of a first-order axiomatization which, despite being rather weak, suffices to make the following two problems decidable:
- (1) Establishing whether a conjunction r of formulae of the form: ? y1 ?? ym((y1?W1& ?&ym ?Wm) ←q), with q quantifier-free and involving only the relators =, ? and propositional connectives, and each yi distinct from all wj's, is satisfiable.
- (2) Establishing whether a formula of the form ? y q, q quantifier-free, is satisfiable.