全文获取类型
收费全文 | 62084篇 |
免费 | 5504篇 |
国内免费 | 9543篇 |
专业分类
化学 | 55335篇 |
晶体学 | 1771篇 |
力学 | 936篇 |
综合类 | 359篇 |
数学 | 5757篇 |
物理学 | 12973篇 |
出版年
2024年 | 74篇 |
2023年 | 575篇 |
2022年 | 802篇 |
2021年 | 1316篇 |
2020年 | 1611篇 |
2019年 | 2583篇 |
2018年 | 1595篇 |
2017年 | 2636篇 |
2016年 | 2141篇 |
2015年 | 2097篇 |
2014年 | 2756篇 |
2013年 | 5316篇 |
2012年 | 3708篇 |
2011年 | 4181篇 |
2010年 | 3359篇 |
2009年 | 3854篇 |
2008年 | 4193篇 |
2007年 | 4134篇 |
2006年 | 3811篇 |
2005年 | 3507篇 |
2004年 | 3301篇 |
2003年 | 2716篇 |
2002年 | 2559篇 |
2001年 | 1976篇 |
2000年 | 1916篇 |
1999年 | 1506篇 |
1998年 | 1348篇 |
1997年 | 1159篇 |
1996年 | 958篇 |
1995年 | 929篇 |
1994年 | 779篇 |
1993年 | 666篇 |
1992年 | 575篇 |
1991年 | 406篇 |
1990年 | 278篇 |
1989年 | 278篇 |
1988年 | 228篇 |
1987年 | 148篇 |
1986年 | 142篇 |
1985年 | 148篇 |
1984年 | 123篇 |
1983年 | 72篇 |
1982年 | 105篇 |
1981年 | 112篇 |
1980年 | 94篇 |
1979年 | 108篇 |
1978年 | 69篇 |
1977年 | 51篇 |
1976年 | 38篇 |
1973年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Debendra K. Mohapatra Dhananjoy Mondal Mukund S. Chorghade Mukund K. Gurjar 《Tetrahedron letters》2006,47(52):9215-9219
The non-proteinogenic amino acid 3-hydroxy-4-methylproline (HMP) is an active constituent of some potent antimicrobials including echinocandins, nostopeptins, pneumocandins, sporiofungin and mulundocandins. A synthesis has been achieved in 10 steps with 29% overall yield; the Evans’ aldol reaction using Crimmins’ modified method was pivotal to the success of the strategy. 相似文献
52.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
53.
A Volterra type integral equation in a Hilbert space with an additional linear operator L and a spectral parameter depending
on time is considered. If the parameter does not belong to the spectrum of L unconditional solvability of the considered problem
is proved. In the case where the initial value of the parameter coincides with some isolated point of the spectrum of the
operator L sufficient conditions for solvability are established. The obtained results are applied to the partial integral
equations associated with a contact problem of the theory of elasticity. 相似文献
54.
M.F. Al-Kuhaili 《Journal of luminescence》2006,117(2):209-216
Thin films of sodium salicylate were deposited by spin coating from a solution prepared by dissolving sodium salicylate in methanol. The films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, UV-visible spectroscopy and spectrofluorometry. The films were crystalline with a strong blue fluorescence peak at an emission wavelength of 419.3 nm. The influence of solution concentration, spin speed and annealing temperature on the fluorescence intensity was studied. Optimum results were obtained with a solution whose concentration was 2.5 M. It was found that lower spin speed resulted in higher fluorescence intensity. Moreover, the fluorescence intensity decreased as the annealing temperature was increased. The durability of the films over a period of 30 days was also investigated, and films annealed at higher temperatures were found to be less degraded with time. 相似文献
55.
56.
Srinivasan G. 《物理学报》2006,55(5):2548-2552
讨论了Ni0.8Zn0.2Fe2O4 (NZFO)与锆钛酸铅(PZT)的双层膜结构样品的磁电(ME)效应.NZFO粉料由溶胶-凝胶法制成,再经900℃热压,并高温烧结.在该双层膜中测量到了很强的磁电相互作用.发现横向的磁电效应比纵向效应大一个数量级,并且随NZFO烧结温度的提高而增加.当烧结温度从950℃上升到1380℃时,横向ME电压系数(αE)的最大值变化范围为25.6 mV Am-2≤αE≤199.6 mV Am-2.理论分析显示NZFO-PZT双层膜样品中ME效应源于NZFO与PZT之间相对良好的磁电耦合.
关键词:
镍铁氧体
PZT
热压法
ME效应 相似文献
57.
Steven D. Bull Simon W. Epstein Nadeam Mujtaba Edward D. Savory Juan A. Tamayo 《Tetrahedron》2006,62(33):7911-7925
Enolates of (S)-N,N′-bis-(p-methoxybenzyl)-3-iso-propylpiperazine-2,5-dione exhibit high levels of enantiodiscrimination in alkylations with (RS)-1-aryl-1-bromoethanes and (RS)-2-bromoesters, affording substituted diketopiperazines containing two new stereogenic centres in high de. Deprotection and hydrolysis of the resultant substituted diketopiperazines provides a route to the asymmetric synthesis of homochiral methyl 2-amino-3-aryl-butanoates and 3-methyl-aspartates in high de and ee. 相似文献
58.
59.
Let G=(V(G),E(G)) be a graph. A (n,G, λ)‐GD is a partition of the edges of λKn into subgraphs (G‐blocks), each of which is isomorphic to G. The (n,G,λ)‐GD is named as graph design for G or G‐decomposition. The large set of (n,G,λ)‐GD is denoted by (n,G,λ)‐LGD. In this work, we obtain the existence spectrum of (n,P3,λ)‐LGD. © 2002 Wiley Periodicals, Inc. J Combin Designs 10: 151–159, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jcd.10008 相似文献
60.
A. P. Vink E. van der Kolk P. Dorenbos C. W. E. van Eijk 《Optics Communications》2002,210(3-6):277-284
The optical properties of Ce3+ in CaSO4, SrSO4 and BaSO4 are reported. The Ce3+ ion shows 4f05d1→2F5/2,2F7/2 luminescence in all three sulphates. Co-doping with Na+ does not change the local surrounding of the Ce3+ ion, but enhances the amount of Ce3+ ions built in. Under optical excitation, besides the typical Ce3+ doublet emission in the ultraviolet spectral region, band emission around 445 nm was observed. This band emission was not assigned to emission from a Ce3+ centre, but to emission from an impurity-trapped exciton. Under X-ray excitation, both Ce3+ emission and an emission band around 380 nm was observed. This band was assigned to emission from a self-trapped exciton. 相似文献