全文获取类型
收费全文 | 1950篇 |
免费 | 36篇 |
国内免费 | 11篇 |
专业分类
化学 | 1236篇 |
晶体学 | 20篇 |
力学 | 79篇 |
数学 | 277篇 |
物理学 | 385篇 |
出版年
2023年 | 13篇 |
2022年 | 16篇 |
2021年 | 38篇 |
2020年 | 27篇 |
2019年 | 26篇 |
2018年 | 29篇 |
2017年 | 23篇 |
2016年 | 34篇 |
2015年 | 27篇 |
2014年 | 48篇 |
2013年 | 108篇 |
2012年 | 91篇 |
2011年 | 120篇 |
2010年 | 73篇 |
2009年 | 71篇 |
2008年 | 114篇 |
2007年 | 107篇 |
2006年 | 111篇 |
2005年 | 93篇 |
2004年 | 91篇 |
2003年 | 68篇 |
2002年 | 80篇 |
2001年 | 34篇 |
2000年 | 35篇 |
1999年 | 22篇 |
1998年 | 22篇 |
1997年 | 24篇 |
1996年 | 34篇 |
1995年 | 27篇 |
1994年 | 25篇 |
1993年 | 25篇 |
1992年 | 9篇 |
1991年 | 16篇 |
1990年 | 21篇 |
1989年 | 27篇 |
1988年 | 20篇 |
1987年 | 19篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 24篇 |
1983年 | 20篇 |
1982年 | 15篇 |
1981年 | 16篇 |
1980年 | 17篇 |
1979年 | 15篇 |
1978年 | 19篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1975年 | 8篇 |
1973年 | 7篇 |
排序方式: 共有1997条查询结果,搜索用时 15 毫秒
971.
Marco Frasconi Gabriele Favero Cristina Tortolini Franco Mazzei 《Electroanalysis》2009,21(21):2378-2386
Heme peroxidase are ubiquitous enzymes catalyzing the oxidation of a broad range of substrates by hydrogen peroxide. In this paper the bioelectrochemical characterization of horseradish peroxidase (HRP) and soybean peroxidase (SBP), belonging to class III of the plant peroxidase superfamily, was studied. The homogeneous reactions between peroxidases and some common redox mediators in the presence of hydrogen peroxide have been carried out by cyclic voltammetry. The electrochemical characterization of the reactions involving enzyme, substrate and mediators concentrations allowed us to calculate the kinetic parameters for the substrate–enzyme reaction (KMS) and for the redox mediator–enzyme reaction (KMM). A full characterization of the direct electron transfer kinetic parameters between the electrode and enzyme active site was also performed by opportunely modeling data obtained from cyclic voltammetry and square wave voltammetry experiments. The experimental data obtained with immobilized peroxidases show enhanced direct electron transfer and excellent electrocatalytical performance for H2O2. Despite the structural similarities and common catalytic cycle, HRP and SBP exhibit differences in their substrate affinity and catalytic efficiency. Basing on our results, it can be concluded that peroxidase from soybean represents an interesting alternative to the classical and largely employed one obtained from horseradish as biorecognition element of electrochemical mediated biosensors. 相似文献
972.
973.
Naturally occurring (1S,2R,3R,5R,7aR)-1,2-dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine [(+)-hyacinthacine A6, 2] together with unnatural (1S,2R,3R,7aS)-1,2-dihydroxy-3-hydroxymethylpyrrolizidine [(+)-7a-epi-hyacinthacine A1, 3] and (1S,2R,3R,5S,7aS)-1,2-dihydroxy-3-hydroxymethyl-5-methylpyrrolizidine [(+)-5,7a-diepi-hyacinthacine A6, 4] have been synthesized from a DALDP derivative [5, (2R,3S,4R,5R)-3,4-dibenzyloxy-2′-O-tert-butyldiphenylsilyl-2,5-bis(hydroxymethyl)pyrrolidine], as the homochiral starting material. The synthetic process employed took advantages of Wittig methodology followed by internal lactamization, in the case of (+)-7a-epi-hyacinthacine A1 (3), and reductive amination for (+)-hyacinthacine A6 (2) and (+)-5,7a-diepi-hyacinthacine A6 (4). 相似文献
974.
Blanca Ayuso de Dios Franco Brezzi Oto Havle L. Donatella Marini 《Numerical Methods for Partial Differential Equations》2012,28(5):1440-1465
We discuss the optimality in L2 of a variant of the Incomplete Discontinuous Galerkin Interior Penalty method (IIPG) for second order linear elliptic problems. We prove optimal estimate, in two and three dimensions, for the lowest order case under suitable regularity assumptions on the data and on the mesh. We also provide numerical evidence, in one dimension, of the necessity of the regularity assumptions. © 2011 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2012 相似文献
975.
Nimisha Patel Suzanne Franco Yoko Miura Brian Boyd 《School science and mathematics》2012,112(5):300-309
This paper examines professional development workshops focused on Connected Math, a particular curriculum utilized or being considered by the middle‐school mathematics teachers involved in the study. The hope was that as teachers better understood the curriculum used in their classrooms, i.e., Connected Math, they would simultaneously deepen their own understanding of the corresponding mathematics content. By focusing on the curriculum materials and the student thought process, teachers would be better able to recognize and examine common student misunderstandings of mathematical content and develop pedagogically sound practices, thus improving their own pedagogical content knowledge. Pre‐ and post‐mathematics content knowledge assessments indicated that engaging middle‐school teachers in the curriculum materials using pedagogy that can be used with their middle‐school students not only solidified teachers' familiarity with such strategies, but also contributed to their understanding of the mathematics content. 相似文献
976.
da Silva DA Alves VG Franco DM Ribeiro LC de Souza MC Kato L de Carvalho JE Kohn LK de Oliveira CM da Silva CC 《Natural product research》2012,26(4):364-369
Luehea candicans Mart. et Zucc. (Tiliaceae) is known as 'a?oita-cavalo' and is one of the most important medicinal plants found in the Brazilian cerrado. The crude methanolic extracts of the branches and leaves and their fractions were evaluated using the following cancer cell lines: MCF-7 (breast), NCI-ADR (breast expressing the multidrug resistance phenotype), NCI-460 (lung), UACC-62 (melanoma), 786-0 (kidney), OVCAR (ovarian), PCO-3 (prostate), HT-29 (colon) and K-562 (leukaemia). The crude methanolic extracts from the branches (B) and leaves (L) were able to inhibit the growth of the K-562 and 786-0 cell lines in a dose-dependent manner, with GI(50) values of 8.1 and 5.4 μg mL(-1), respectively. The hexane (L1), chloroform (L2) and methanol (L4) fractions derived from extract L showed a high selectivity and pronounced cytostatic activity against 786-0 (GI(50) ~ 40 μg mL(-1)). A significant amount of lupeol was isolated from fraction L2. The chloroform (B2) and methanol (B3) fractions derived from extract (B) exhibited less selectivity, showing the highest cytostatic activity against K-562, NCI-ADR, OVCAR, MCF-7 and NCI-460 cells, with GI(50) values between 27 and 40 μg mL(-1). Lupeol, betulin, a mixture of steroids, (-)-epicatechin, vitexin and liriodendrin were isolated from these active fractions. 相似文献
977.
M Natali S Berardi A Sartorel M Bonchio S Campagna F Scandola 《Chemical communications (Cambridge, England)》2012,48(70):8808-8810
Water oxidation catalysts: evolution of [Co(4)(H(2)O)(2)(α-PW(9)O(34))(2)](10-) to catalytically active species is assessed by laser flash photolysis in sacrificial photocatalytic cycles with Ru(bpy)(3)(2+) as a photosensitizer. 相似文献
978.
Consider a system of independent random walks in the discrete torus with creation-annihilation of particles and possible explosion of the total number of particles in finite time. Rescaling space and rates for diffusion/creation/annihilation of particles, we obtain a strong law of large numbers for the density of particles in the supremum norm. The limiting object is a classical solution to the semilinear heat equation ? t u=? xx u+f(u). If f(u)=u p , 1<p??3, we also obtain a law of large numbers for the explosion time. 相似文献
979.
da Silva JC Bispo GL Pavanelli SP Afonso RJ Augusti R 《Rapid communications in mass spectrometry : RCM》2012,26(11):1305-1310
980.
E Aprea F Morisco F Biasioli P Vitaglione L Cappellin C Soukoulis V Lembo F Gasperi G D'Argenio V Fogliano N Caporaso 《Journal of mass spectrometry : JMS》2012,47(9):1098-1103
Breath testing has been largely used as a diagnostic tool, but the difficulties in data interpretation and sample collection have limited its application. We developed a fast (< 20?s), on-line, non-invasive method for the collection and analysis of exhaled breath in awake rats based on proton transfer reaction time of flight mass spectrometry (PTR-ToF-MS) and applied it to investigate possible relationships between pathologies induced by dietary regime and breath composition. As a case study, we investigated rats with dietary induced non-alcoholic steatohepatitis (NASH) and modifications induced by coffee addition to the diet. We considered two different diets (standard and high fat) complemented with two different drinking possibilities (water or decaffeinated coffee) for a total of four groups with four rats each. Several spectrometric peaks were reliable markers for both dietary fat content and coffee supplementation. The high resolution and accuracy of PTR-ToF-MS allowed the identification of related compounds such as methanol, dimethyl sulphide, dimethyl sulphone and ammonia. In conclusion, the rapid and minimally invasive breath analysis of awake rats permitted the identification of markers related to diet and specific pathologic conditions and provided a useful tool for broader metabolic investigations. Copyright ? 2012 John Wiley & Sons, Ltd. 相似文献