全文获取类型
收费全文 | 18384篇 |
免费 | 3363篇 |
国内免费 | 2502篇 |
专业分类
化学 | 9533篇 |
晶体学 | 202篇 |
力学 | 1905篇 |
综合类 | 330篇 |
数学 | 1956篇 |
物理学 | 10323篇 |
出版年
2024年 | 44篇 |
2023年 | 196篇 |
2022年 | 781篇 |
2021年 | 707篇 |
2020年 | 725篇 |
2019年 | 631篇 |
2018年 | 585篇 |
2017年 | 782篇 |
2016年 | 866篇 |
2015年 | 650篇 |
2014年 | 993篇 |
2013年 | 1508篇 |
2012年 | 1143篇 |
2011年 | 1235篇 |
2010年 | 1023篇 |
2009年 | 1166篇 |
2008年 | 1170篇 |
2007年 | 1089篇 |
2006年 | 1127篇 |
2005年 | 907篇 |
2004年 | 897篇 |
2003年 | 772篇 |
2002年 | 638篇 |
2001年 | 612篇 |
2000年 | 556篇 |
1999年 | 413篇 |
1998年 | 441篇 |
1997年 | 357篇 |
1996年 | 324篇 |
1995年 | 282篇 |
1994年 | 253篇 |
1993年 | 233篇 |
1992年 | 195篇 |
1991年 | 153篇 |
1990年 | 129篇 |
1989年 | 105篇 |
1988年 | 121篇 |
1987年 | 87篇 |
1986年 | 61篇 |
1985年 | 51篇 |
1984年 | 54篇 |
1983年 | 21篇 |
1982年 | 38篇 |
1981年 | 22篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 21篇 |
1977年 | 9篇 |
1973年 | 15篇 |
1957年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Bridgette Yani Ansie Harding 《International Journal of Mathematical Education in Science & Technology》2019,50(7):1037-1049
ABSTRACTThis paper focuses on the students who are registered in the University of Pretoria’s academic development programme, named the Four-year Programme (FYP). The programme was introduced as a gateway for students who are underprepared but have the potential to succeed and then continue their studies into the mainstream science programmes. Our research focuses on measuring the change in the academic maturity of these students. In the theoretical framework that we developed, academic maturity is subdivided into two components namely non-subject based maturity and subject based maturity (mathematical maturity). This paper focuses on measuring non-subject based academic maturity. The survey used for this purpose is called the Student Academic Readiness Survey (STARS), taken at the beginning of the year and after the first semester respectively. The results of the surveys are compared to measure the change in students’ views. Results show that in all constructs there is a surprising decline in students’ perceptions regarding their own abilities over the first semester at university. We use the Dunning–Kruger effect to explain this unexpected decline, in that students seem to develop a more realistic view of their own maturity, which in itself can be seen as a growth in academic maturity. 相似文献
2.
Alexander Zeller 《Journal of organometallic chemistry》2006,691(21):4379-4385
The ethylene polymerization reaction of a neutral nickel catalyst was studied by DFT calculations at the Becke3LYP/6-31G(d) level of theory. As in related cases a β-agostic bond stabilizes the nickel alkyl ground states. Transition states for the insertion of the olefin show a distinct α-agostic interaction, which has not been observed for late metal polymerization catalysts before. An ethylene-alkyl complex was identified as the resting state of the reaction. The overall barrier height of the reaction amounts to 17.54 kcal/mol, which slightly increases to 17.60 kcal/mol for the polymerization of deuterated ethylene. Therefore, a small positive kinetic isotope effect (kH/kD = 1.09) can be calculated, which is caused by the α-agostic interaction in the transition state. A comparison to other late metal based polymerization systems reveals that the ethylene coordination step of highly active catalysts is significantly lower in energy compared to catalysts which are only moderately active. 相似文献
3.
M. Georgelin A. Pocheau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):169-174
The polycrystalline perovskitelike manganese oxides La1-xAxMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation
state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates
for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie
temperature.
Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998 相似文献
4.
5.
6.
Stress-Strain State in the Zone of Load Transfer in a Composite Specimen under Uniaxial Tension 总被引:2,自引:2,他引:0
Kulakov V. L. Tarnopol'skii Yu. M. Arnautov A. K. Rytter J. 《Mechanics of Composite Materials》2004,40(2):91-100
The stress-strain state in the zone of load transfer in a uniaxially stretched specimen made of a unidirectional epoxy carbon-fiber-reinforced plastic (CFRP) is investigated. A parametric analysis of the influence of geometric and mechanical characteristics of the specimen on its stress-strain state is performed by means of finite-element modeling. The parameters allowing us to significantly reduce the dangerous concentration of transverse and tangential stresses are revealed. The mechanical tensile characteristics of a high-strength pultruded unidirectional CFRP are determined experimentally, and the size effect of its strength is estimated. 相似文献
7.
David Morris 《Applied biochemistry and biotechnology》2004,113(1-3):5-12
This article addresses two questions: Has the effectiveness of the US governments federal research and development (R&;D) spending suffered from the post-1980 strategic change from freely shared and publicly owned to privately owned scientific advances? What criteria would a federal R&;D program use to design a strategy that most effectively enhances the well-being of farmers and rural communities? Several studies found that the pre-1980 US Department of Agriculture research strategy was very effective. No comparable studies have analyzed the comparative effectiveness of the post-1980 strategy of restricting access to the results of public research. Recent experience and several analytical studies suggest that to significantly enhance the health of rural economies from an expanded use of plant matter as an industrial material, federal policy should channel scientific and engineering research into small- and medium-sized production and processing technologies and should encourage farmer-owned, value-added enterprises. 相似文献
8.
Accelerated phase transformations and chemical reactions of metastable aluminas and kaolinite, doped with Cu2
+, Mn3
+/Mn2
+ and Fe3
+/Fe2
+ ions, are accompanied with accelerated decrease of surface area and pore volume values. The phenomena in metal ion doped
samples are explained by a catalytic mechanism, in terms of the Jahn-Teller effect.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
9.
Anti-BZ-Structure in Effect Algebras 总被引:1,自引:0,他引:1
The definitions of sharply approximating effect algebras, anti-BZ-effect algebras, central approximating effect algebras, and S-anti-BZ-effect algebras are given, the relationships between sharply approximating effect algebras and anti-BZ-effect algebras, between central approximating effect algebras and anti-BZ-effect algebras are established, and the set of anti-BZ-sharp elements in S-anti-BZ-effect algebras is proved to be an orthomodular lattice. 相似文献
10.
Most of the methods used in the ARCH literature for selecting the appropriate model are based on evaluating the ability of the models to describe the data. An alternative model selection approach is examined based on the evaluation of the predictability of the models in terms of standardized prediction errors. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献