全文获取类型
收费全文 | 233847篇 |
免费 | 2274篇 |
国内免费 | 803篇 |
专业分类
化学 | 117517篇 |
晶体学 | 4001篇 |
力学 | 10837篇 |
综合类 | 1篇 |
数学 | 25613篇 |
物理学 | 78955篇 |
出版年
2020年 | 1811篇 |
2019年 | 2058篇 |
2018年 | 2728篇 |
2017年 | 2661篇 |
2016年 | 3944篇 |
2015年 | 2407篇 |
2014年 | 3980篇 |
2013年 | 10111篇 |
2012年 | 7576篇 |
2011年 | 9459篇 |
2010年 | 6716篇 |
2009年 | 6666篇 |
2008年 | 8757篇 |
2007年 | 8783篇 |
2006年 | 8312篇 |
2005年 | 7534篇 |
2004年 | 6956篇 |
2003年 | 6137篇 |
2002年 | 6082篇 |
2001年 | 7107篇 |
2000年 | 5392篇 |
1999年 | 4250篇 |
1998年 | 3569篇 |
1997年 | 3480篇 |
1996年 | 3314篇 |
1995年 | 3023篇 |
1994年 | 2962篇 |
1993年 | 3062篇 |
1992年 | 3169篇 |
1991年 | 3358篇 |
1990年 | 3103篇 |
1989年 | 3046篇 |
1988年 | 3081篇 |
1987年 | 2966篇 |
1986年 | 2808篇 |
1985年 | 3823篇 |
1984年 | 4024篇 |
1983年 | 3218篇 |
1982年 | 3428篇 |
1981年 | 3347篇 |
1980年 | 3248篇 |
1979年 | 3343篇 |
1978年 | 3444篇 |
1977年 | 3357篇 |
1976年 | 3460篇 |
1975年 | 3151篇 |
1974年 | 3153篇 |
1973年 | 3199篇 |
1972年 | 2241篇 |
1971年 | 1850篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
Jerry P. Suits 《School science and mathematics》2004,104(6):248-257
A laboratory practical examination was used to compare the investigative skills developed in two different types of general‐chemistry laboratory courses. Science and engineering majors (SEM) in the control group used a traditional verification approach (SEM‐Ctrl), whereas those in the treatment group learned from an innovative, inquiry‐based approach (SEM‐Trt). A scoring rubric was developed from their examination sheets to assess six component investigative skills. Results indicated that SEM students in the SEM‐Trt group scored significantly higher than those in SEM‐Ctrl for all six skills. Furthermore, nursing and applied science majors (NonSEM) in the inquiry‐based group (NonSEM‐Trt) wrote significantly better discussions than did SEM students in SEM‐Ctrl group. Overall, competency at the mid‐range level of laboratory skills was attained by most SEM‐Trt students (72.5%) but by only 30.5% of SEM‐Ctrl and 28.6% of NonSEM‐Trt students. Apparently, during the semester students in the SEM‐Trt group were able to use the inquiry‐based method effectively to combine chemical tasks with writing tasks and postlaboratory discussions. One implication of this study for science instructors is that practical examinations can provide useful feedback regarding the quality of the laboratory experience. Another implication is that this study provides evidence for the use of the innovative inquiry‐based laboratory approach to support student learning of high‐level investigative skills. However, students' requisite background knowledge must match the level of these skills. 相似文献
282.
Tracing is a method of assigning flows in an electricity network to particular generators and loads, assuming perfect mixing at each node. It can be used to assign costs to transmission users. We show that the resulting allocation is equal to the Shapley value of an equivalent co-operative game. 相似文献
283.
D. P. Zhelobenko 《Acta Appl Math》2004,81(1):347-354
A special class of associative algebras is introduced and studied. A criteria of semisimplicity for suitable category of modules is obtained. 相似文献
284.
D. P. Zhelobenko 《Acta Appl Math》2004,81(1):355-383
An algebraic approach to harmonic analysis on reductive Lie groups is proposed. The case of complex semisimple Lie groups is considered in details. Some problems for real reductive Lie groups are discussed. 相似文献
285.
Using the result by D. Gessler, we show that any invariant variational bivector (resp., variational 2-form) on an evolution equation with nondegenerate right-hand side is Hamiltonian (resp., symplectic). 相似文献
286.
S. Aravinda N. Shamala Rituparna S. Roy P Balaram 《Journal of Chemical Sciences》2003,115(5-6):373-400
An overview of the use of non-protein amino acids in the design of conformationally well-defined peptides, based on work from
the author’s laboratory, is discussed. The crystal structures of several designed oligopeptides illustrate the useα-aminoisobutyric acid (Aib) in the construction of helices, D-amino acids in the design of helix termination segments andDPro-Xxx segments for nucleating ofβ-hairpin structures.β- andγ-amino acid residues have been used to expand the range of designed polypeptide structures.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
287.
Received: 14 September 2001 / in final form: 28 April 2002 // Published online: 20 March 2003 相似文献
288.
I. P. Kuranova K. M. Polyakov E. A. Smirnova W. E. Höhne V. S. Lamzin R. Meijer 《Crystallography Reports》2003,48(6):953-958
Crystals of Saccharomyces cerevisiae inorganic pyrophosphatase suitable for X-ray diffraction study were grown by cocrystallization of the enzyme with cobalt chloride and imidodiphosphate. Saccharomyces cerevisiae is a metal-dependent enzyme which catalyzes hydrolysis of inorganic pyrophosphate to orthophosphate. The three-dimensional structure of this enzyme was solved by the molecular-replacement method and refined at 1.8 Å resolution to an R factor of 19.5%. Cobalt and phosphate ions were revealed in the active centers of both identical subunits (A and B) of the pyrophosphatase molecule. In subunit B, a water molecule was found between two cobalt ions. It is believed that this water molecule acts as an attacking nucleophile in the enzymatic cleavage of the pyrophosphate bond. It was demonstrated that cobalt ions and a phosphate group occupy only part of the potential binding sites (two chemically identical and crystallographically independent subunits have different binding sites). The arrangement of ligands and the structure of the nucleophile-binding site are discussed in relation to the mechanism of action of the enzyme and the nature of the metal activator. 相似文献
289.
The operating principles, design, and characteristics of an ion source with a cold magnetron cathode and magnetic plasma compression are described. The source is intended for the injector of a linear proton accelerator. 相似文献
290.
L. M. Vasilyak M. N. Vasil’ev S. P. Vetchinin D. N. Polyakov V. E. Fortov 《Journal of Experimental and Theoretical Physics》2003,96(3):440-443
The action of an electron beam on ordered dust structures in glow and low-pressure RF discharges was studied experimentally. The electron beam produces destruction and dynamic displacement of the dust structure. In the center of a dust structure, an electron beam with a low electron energy (tens of eV) at currents up to 1 mA caused structural disordering and “melting” in the region of its action but did not excite external crystal regions. Local action of an electron beam with a high electron energy (25 keV) and a beam current above 10 mA caused deformation of the whole dust structure and shifted it in the horizontal direction so that it was carried away from the RF discharge zone. The effect of dust structure displacements can be used to locally remove particles from a plasma. 相似文献