全文获取类型
收费全文 | 27062篇 |
免费 | 1180篇 |
国内免费 | 5141篇 |
专业分类
化学 | 30891篇 |
晶体学 | 313篇 |
力学 | 61篇 |
综合类 | 255篇 |
数学 | 18篇 |
物理学 | 1845篇 |
出版年
2024年 | 42篇 |
2023年 | 274篇 |
2022年 | 813篇 |
2021年 | 761篇 |
2020年 | 765篇 |
2019年 | 793篇 |
2018年 | 684篇 |
2017年 | 865篇 |
2016年 | 1005篇 |
2015年 | 932篇 |
2014年 | 1112篇 |
2013年 | 2507篇 |
2012年 | 1867篇 |
2011年 | 1494篇 |
2010年 | 1315篇 |
2009年 | 1544篇 |
2008年 | 1637篇 |
2007年 | 1808篇 |
2006年 | 1707篇 |
2005年 | 1618篇 |
2004年 | 1450篇 |
2003年 | 1184篇 |
2002年 | 1025篇 |
2001年 | 746篇 |
2000年 | 729篇 |
1999年 | 623篇 |
1998年 | 496篇 |
1997年 | 524篇 |
1996年 | 471篇 |
1995年 | 422篇 |
1994年 | 366篇 |
1993年 | 320篇 |
1992年 | 306篇 |
1991年 | 223篇 |
1990年 | 165篇 |
1989年 | 178篇 |
1988年 | 129篇 |
1987年 | 91篇 |
1986年 | 64篇 |
1985年 | 52篇 |
1984年 | 59篇 |
1983年 | 29篇 |
1982年 | 47篇 |
1981年 | 28篇 |
1980年 | 31篇 |
1979年 | 21篇 |
1978年 | 23篇 |
1977年 | 6篇 |
1975年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 5 毫秒
961.
962.
Hidalgo C Rios C Hidalgo M Salvadó V Sancho JV Hernández F 《Journal of chromatography. A》2004,1035(1):153-157
An existing method for the determination of glyphosate and its main metabolite aminomethylphosphonic acid (AMPA) in water has been improved. It is based on precolumn derivatization with the fluorescent reagent 9-fluorenylmethylcloroformate (FMOC) followed by large-volume injection in a coupled-column LC system using fluorescence detection (LC-LC-FD). The derivatization step was slightly modified by changing parameters such as volume and/or concentration of sample and reagents to decrease the limits of quantification (LOQ) of glyphosate and AMPA to 0.1 microg/l. Additionally, the use of Amberlite IRA-900 for preconcentration of glyphosate, prior to the derivatization step, was investigated; the LOQ of glyphosate was lowered to 0.02 microg/l. Drinking, surface and ground water spiked with glyphosate and AMPA at 0.1-10 microg/l concentrations were analysed by the improved LC-LC-FD method. Recoveries were 87-106% with relative standard deviations lower than 8%. Drinking and ground water spiked with glyphosate at 0.02 and 0.1 microg/l were analysed after preconcentration on the anion-exchange resin with satisfactory recoveries (94-105%) and precision (better than 8%). 相似文献
963.
Alkylsulfonates from methanesulfonic acid to decanesulfonic acid were separated by isotachophoresis with conductivity detection in a common electrolyte system at pH 4.8. The electrolyte system consisted of 10 mM HCl buffered with epsilon-aminocaproic acid (pH 4.8) and 0.1% methylhydroxyethylcellulose (MHEC) acting as the leading electrolyte. The terminating electrolyte was 20 mM caproic acid also containing 0.05% MHEC. Current settings of 250 microA for the first and 50 microA for the second capillary were applied. On one hand, the method was applied to the determination of the content of methanesulfonate as the salt forming agent (mesilate) in a recently registered drug substance. The results obtained by ITP were compared with an orthogonal titration method. On the other hand, due to the column-coupling configuration of the electrophoretic instrument, the method could be extended to the trace determination in the ppm range in order to monitor methanesulfonic acid as an impurity in a drug substance. The validation confirmed the linearity of the method between 1 and 10 mg/l, limits of detection and quantification below 1 mg/l, recovery rates from 92.4 to 95.4%, and repeatability with a R.S.D. of 3.8% (six runs with a 4 mg/l spiked sample). Finally, three batches of a newly produced drug substance could be checked for methanesulfonic acid giving results of below 0.0014% (concentration related to the drug substance). 相似文献
964.
A simple, efficient, and general method has been developed for the synthesis of bis-(α-hydroxyalkyl)phosphinic acids from hypophosphorous acid using microwave irradiation. Bis-(α-hydroxyalkyl)phosphinic acids were obtained in high yield under mild conditions by reaction of hypophosphorous acid with aldehydes under microwave irradiation. 相似文献
965.
M. Patri Varsha R. Hande Swati Phadnis B. Somaiah Suhasini Roychoudhury P. C. Deb 《先进技术聚合物》2004,15(5):270-274
A Novel solid polymer electrolyte (SPE) membrane containing both ? COOH and ? SO3H group has been prepared by simultaneous method of radiation grafting of acrylic acid onto FEP followed by sulfonation. The presence of weakly acidic acrylic acid controls the swelling in water while ? SO3H group provides conductivity due to its strongly ionic characteristic. FEP‐g‐acrylic acid and its sulfonated derivatives were characterized by their properties. While the mechanical properties decreased, other properties such as ion exchange capacity (IEC), water uptake and ionic conductivity increased with increase in graft content. These properties further changed on sulfonation. Acrylic acid being weakly acidic in nature, conductivity values of the grafted membrane were quite low. However, introduction of strong ? SO3H group resulted in conductivity closer to Nafion 117. Few sulfonated membranes have been tested with respect to H2/O2 fuel cell performance. Short‐term fuel cell test for 100 hr gave a stable performance. These membranes are less expensive compared to Nafion. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
966.
Liquid chromatographic comparisons for enantiomer resolution of α-amino acids and chiral primary amino compounds were made using chiral stationary phases (CSPs) prepared by covalently bonding (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TA) of the same chiral selector. The resolution of all α-amino acids on CSP 1 developed in our group was found to be better than that on CSP 2 reported by Machida et al. All α-amino acids examined in this study were well enantioseparated on CSP 1 (α=1.22–2.47), while four analytes were not resolved or all the other analytes were poorly resolved on CSP 2 than on CSP 1. However, in resolving the primary amino compounds without a carbonyl group, CSP 1 was comparable with CSP 2. Although (+)-18-C-6-TA of the same chiral selector was used to prepare CSP 1 and CSP 2, this study showed that different connecting methods for the CSPs might influence their ability to resolve the analytes depending on their structures related to the chiral recognition mechanism. 相似文献
967.
A chemiluminescence method for the determination of folic acid by the sodium hypochlorite–folic acid–semicarbazide hydrochloride system with a new flow injection technique has been established. The new method can perform simple, sensitive and rapid determinations of folic acid. The response to the concentration of folic acid, in the range of 1.0×10−75.0×10−5 g/ml, is linear. The relative standard deviation of the method is 2.3% (Cs=4.0×10−6 g/ml, n=11). The detection limit is 2.7×10−8 g/ml. This method is suitable for automatic and continuous analysis, and has been successfully tested for the determination of folic acid in a folic acid tablet. 相似文献
968.
A. N. Danilenko V. S. Romanova E. F. Kuleshova Z. N. Parnes E. E. Braudo 《Russian Chemical Bulletin》1998,47(11):2134-2136
The concentration dependences of heat capacities of aqueous solutions of several amino acid and peptide derivatives of fullerene
were measured by scanning differential calorimetry at 298 K. The heat capacities for the arginine, alanylalanine, and glycylvaline
derivatives dissolved in water depend slightly on concentration. The concentration dependences of the heat capacities of aqueous
solutions of the serine and alanine derivatives display extrema. The calculated contributions of hydration to the heat capacities
of the dissolved fullerene derivatives have both positive and negative signs.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2202–2204, November, 1998. 相似文献
969.
970.
Styrene (St)-acrylic acid (AA) copolymer plates were synthesized in the frames made of glass, aluminum (Al) and poly(difluoroethylene) (PDFE). Surface properties of the prepared copolymer plates were characterized with regard to critical surface tension (
C
), chemical components by IR analysis and surface energies (total energy,
S
; dispersion force components,
S
d
; polar component,
S
p
; hydrogen bonding component,
S
h
) and the following results were obtained.The
C
values of the copolymer plates increased with AA content and also depended on the sort of the used frame. The increasing order of the
C
values of the copolymer plates corresponded to those of the used frames, namely, PDFE frame < Al frame < Glass frame.The prepared copolymer plates with low AA contents (ca. 10 mol%) were enriched in the AA moiety in the surface layer regardless of the kind of the used frame.The total and the individual components of the surface free energies of the copolymer plates were largely affected by the property of the used frame. The glass frame gave the plate with higher
S
h
values suggesting preferential orientation of the polar site of AA component. 相似文献