全文获取类型
收费全文 | 19626篇 |
免费 | 629篇 |
国内免费 | 113篇 |
专业分类
化学 | 14074篇 |
晶体学 | 189篇 |
力学 | 378篇 |
综合类 | 1篇 |
数学 | 2955篇 |
物理学 | 2771篇 |
出版年
2022年 | 139篇 |
2021年 | 197篇 |
2020年 | 294篇 |
2019年 | 290篇 |
2018年 | 186篇 |
2017年 | 181篇 |
2016年 | 406篇 |
2015年 | 357篇 |
2014年 | 451篇 |
2013年 | 929篇 |
2012年 | 977篇 |
2011年 | 1268篇 |
2010年 | 619篇 |
2009年 | 524篇 |
2008年 | 1014篇 |
2007年 | 1092篇 |
2006年 | 1073篇 |
2005年 | 1038篇 |
2004年 | 881篇 |
2003年 | 745篇 |
2002年 | 732篇 |
2001年 | 250篇 |
2000年 | 254篇 |
1999年 | 230篇 |
1998年 | 223篇 |
1997年 | 259篇 |
1996年 | 284篇 |
1995年 | 227篇 |
1994年 | 212篇 |
1993年 | 244篇 |
1992年 | 203篇 |
1991年 | 185篇 |
1990年 | 163篇 |
1989年 | 141篇 |
1988年 | 140篇 |
1987年 | 158篇 |
1986年 | 135篇 |
1985年 | 202篇 |
1984年 | 249篇 |
1983年 | 178篇 |
1982年 | 220篇 |
1981年 | 241篇 |
1980年 | 180篇 |
1979年 | 181篇 |
1978年 | 214篇 |
1977年 | 159篇 |
1976年 | 157篇 |
1975年 | 150篇 |
1974年 | 165篇 |
1973年 | 140篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Paul Mandel 《Contemporary Physics》2013,54(5):235-246
The historical development that led to the modern concept of amplification and losing without inversion is reviewed. The two basic mechanisms for amplification without inversion and their theoretical formulation are discussed. Finally, the three experiments that confirm the occurrence of these effects are described. 相似文献
952.
953.
954.
955.
Daniel S. Covita Dimitrios F. Anagnostopoulos Hermann Fuhrmann Hubert Gorke Detlev Gotta Alexander Gruber Albert Hirtl Tomoichi Ishiwatari Paul Indelicato Thomas S. Jensen Eric-Olivier Le Bigot Valeri E. Markushin Michael Nekipelov Vladimir N. Pomerantsev Vladimir P. Popov Joaquim M. F. dos Santos Philipp Schmid Leopold M. Simons Marian Theisen Martino Trassinelli Joao F. C. A. Veloso Johann Zmeskal 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2018,72(4):72
956.
957.
Sodium borohydride, lithium borohydride and borane. THF reduce carbonyl groups (esters, amide) in 1-substituted 2-nitroimidazoIes in high yield without reduction of the nitroheterocycle. 相似文献
958.
Abstract A comparison and evaluation is made of recent proposals for multivariate matched sampling in observational studies, where the following three questions are answered: (1) Algorithms: In current statistical practice, matched samples are formed using “nearest available” matching, a greedy algorithm. Greedy matching does not minimize the total distance within matched pairs, though good algorithms exist for optimal matching that do minimize the total distance. How much better is optimal matching than greedy matching? We find that optimal matching is sometimes noticeably better than greedy matching in the sense of producing closely matched pairs, sometimes only marginally better, but it is no better than greedy matching in the sense of producing balanced matched samples. (2) Structures: In common practice, treated units are matched to one control, called pair matching or 1–1 matching, or treated units are matched to two controls, called 1–2 matching, and so on. It is known, however, that the optimal structure is a full matching in which a treated unit may have one or more controls or a control may have one or more treated units. Optimal 1 — k matching is compared to optimal full matching, finding that optimal full matching is often much better. (3) Distances: Matching involves defining a distance between covariate vectors, and several such distances exist. Three recent proposals are compared. Practical advice is summarized in a final section. 相似文献
959.
Michael A. Beckett Paul Owen Gary C. Strickland K. Sukumar Varma 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):509-512
The interaction of benzyl alcohol with trimethoxyboroxine has been investigated by hydrogen-1 and carbon-13 NMR and IR spectroscopy. These techniques indicate formation of a weak Lewis acid/base adduct, additionally stabilized by a hydrogen-bonding interaction, which undergoes rapid ligand exchange at room temperature. The system has been modelled by the synthesis and characterization of a series of amine-triarylboroxine adducts. These adducts rapidly undergo ligand dissociation-recombination in solution with ΔG+ of ca. 40-50 kJ/mol. 相似文献
960.
Jared M. Schwartz Oluwadamilola Phillips Anthony Engler Alexandra Sutlief Jihyun Lee Paul A. Kohl 《Journal of polymer science. Part A, Polymer chemistry》2017,55(7):1166-1172
Low ceiling temperature, thermodynamically unstable polymers have been troublesome to synthesize and keep stable during storage. In this study, stable poly(phthalaldehyde) has been synthesized with BF3‐OEt2 catalyst. The role of BF3 in the polymerization is described. The interaction of BF3 with the monomer is described and used to maximize the yield and molecular weight of poly(phthalaldehyde). Various Lewis acids were used to investigate the effect of catalyst acidity on poly(phthalaldehyde) chain growth. In situ nuclear magnetic resonance was used to identify possible interactions formed between BF3 and phthalaldehyde monomer and polymer. The molecular weight of the polymer tracks with polymerization yield. The ambient temperature stability of poly(phthalaldehyde) was investigated and the storage life of the polymer has been improved. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 55, 1166–1172 相似文献