全文获取类型
收费全文 | 15585篇 |
免费 | 2547篇 |
国内免费 | 2121篇 |
学科分类
数理化 | 20253篇 |
出版年
2024年 | 38篇 |
2023年 | 314篇 |
2022年 | 455篇 |
2021年 | 535篇 |
2020年 | 634篇 |
2019年 | 618篇 |
2018年 | 527篇 |
2017年 | 518篇 |
2016年 | 791篇 |
2015年 | 740篇 |
2014年 | 950篇 |
2013年 | 1161篇 |
2012年 | 1431篇 |
2011年 | 1508篇 |
2010年 | 988篇 |
2009年 | 939篇 |
2008年 | 1089篇 |
2007年 | 884篇 |
2006年 | 814篇 |
2005年 | 746篇 |
2004年 | 605篇 |
2003年 | 481篇 |
2002年 | 451篇 |
2001年 | 382篇 |
2000年 | 285篇 |
1999年 | 339篇 |
1998年 | 260篇 |
1997年 | 224篇 |
1996年 | 241篇 |
1995年 | 229篇 |
1994年 | 198篇 |
1993年 | 138篇 |
1992年 | 136篇 |
1991年 | 122篇 |
1990年 | 117篇 |
1989年 | 78篇 |
1988年 | 58篇 |
1987年 | 52篇 |
1986年 | 43篇 |
1985年 | 35篇 |
1984年 | 22篇 |
1983年 | 21篇 |
1982年 | 11篇 |
1981年 | 15篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1957年 | 3篇 |
1928年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
This paper presents a multiobjective hybrid metaheuristic approach for an intelligent spatial zoning model in order to draw territory line for geographical or spatial zone for the purpose of space control. The model employs a Geographic Information System (GIS) and uses multiobjective combinatorial optimization techniques as its components. The proposed hybrid metaheuristic consists of the symbiosis between tabu search and scatter search method and it is used heuristically to generate non-dominated alternatives. The approach works with a set of current solution, which through manipulation of weights are optimized towards the non-dominated frontier while at the same time, seek to disperse over the frontier by a strategic oscillation concept. The general procedure and its algorithms are given as well as its implementation in the GIS environment. The computation has resulted in tremendous improvements in spatial zoning. 相似文献
42.
Ching Sum Hui Lillian Sze Man Lam Chun Yin Wai Kin Chan Aleksandra B. Djurii 《Journal of polymer science. Part A, Polymer chemistry》2003,41(11):1708-1715
Polyurethanes incorporated with rhenium diimine complexes were synthesized. The polymers exhibited interesting morphologies and solution properties. Results from gel permeation chromatography suggested the formation of polymer aggregates in solutions. The polymers could act as probes for metal ions. The addition of metal ions to polymer solutions led to significant changes in the electronic absorption properties of the polymer solutions. This was attributed to the interactions between the polyether moieties and metal ions. The metal complexes could also act as efficient photosensitizers. After doping with charge‐transport viologens, the photoconductivity of the polymers was greatly enhanced. The experimental quantum efficiency was simulated with Onsager's theory. The thermalization distances and the primary yields were typically 12–14 Å and 10?3, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1708–1715, 2003 相似文献
43.
44.
45.
46.
E. V. McCloskey A. Dey R. M. Parr N. Aras A. Balogh J. Bostock A. Borell S. Krishnan G. Lobo L. L. Qin Y. Zhang S. Cvijetic V. Zaichick M. Lim-Abraham K. Bose S. Wynchank G. V. Iyengar 《Journal of Radioanalytical and Nuclear Chemistry》2004,259(2):341-345
In 1994, the International Atomic Energy Agency (IAEA) initiated a 5-year Co-ordinated Research Project (CRP) to determine
geographical and racial differences in peak bone mineral density (BMD) in men and women aged 15-49 years. The study demonstrates
that there are distinct global differences in BMD at the hip and spine in both men and women approximating to one population
standard deviation between populations with the highest and lowest BMD. These differences persist following adjustments for
age, sex and body size. Such information is valuable in understanding the reasons for global differences in fracture rate
and predicting future trends in fracture incidence.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
47.
Zhaoqiang Lu Lin Cheng Jun Li Kai Zhang Song Yi Jingui Qin 《Journal of polymer science. Part A, Polymer chemistry》2004,42(4):925-932
The synthesis and structural characterization of a series of novel, fluorinated poly(phthalazinone ether)s containing perfluorophenylene moieties are described. The monomers, 4‐(4′‐hydroxyaryl)phthalazin‐1(2H)‐ones ( 2a – 2d ), were conveniently and efficiently synthesized from phenols and phthalic anhydride in two steps via 2‐(4′‐hydroxybenzoyl)benzoic acids, which were first obtained by the Friedel–Crafts reaction in good yields and with high stereoselectivity and were then converted into 2a – 2d by fusion with hydrazine. All the polymers were prepared by nucleophilic aromatic substitution (SNAr) polycondensation between the compounds perfluorobiphenyl and 4‐(4′‐hydroxyaryl)phthalazin‐1(2H)‐ones ( 2a ‐ 2d ). The resulting fluorinated polymers were readily soluble in common organic solvents (e.g., CHCl3, tetrahydrofuran, dimethylformamide, dimethyl sulfoxide, N‐methylpyrrolidone, etc.) at room temperature. Their weight‐average molecular weights and the polydispersities ranged from (7.96–18.25) × 103 to 1.31–2.71, respectively. Their glass‐transition temperatures varied from 213 to 263 °C. They were all stable up to 390 °C both in air and in argon. The 5% weight‐loss temperatures of these polymers in air and argon ranged from 393–487 to 437–509 °C, respectively. Wide‐angle X‐ray diffraction studies indicated they were all amorphous and could be attributed to the presence of kink nonplanar moiety, phenyl phthalazinone along the polymer backbone. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 925–932, 2004 相似文献
48.
49.
Qing An WU Zhen Yuan XU Mei ZHENG Dan Qian XU Jun XU Yin Chu SHENCollege of Chemical Engineering Materials Zhejiang University of Technology Hangzhou 《中国化学快报》2004,(7)
Six new 13-O-acylavermectin Bl aglycones(3-8) were synthesized from avermectin B1 aglycone and their bioactivities were evaluated against Spodoptera exigua, Spodoptera eridania, Tetranychus urticae and Aphis fabae. 相似文献
50.
Zhaoyang Yin 《Monatshefte für Mathematik》2004,141(4):337-344
We prove under quite general assumptions the existence of a bounded positive solution to the semilinear Schrödinger equation in a two-dimensional exterior domain. 相似文献