全文获取类型
收费全文 | 42843篇 |
免费 | 7379篇 |
国内免费 | 4922篇 |
专业分类
化学 | 30418篇 |
晶体学 | 507篇 |
力学 | 2523篇 |
综合类 | 259篇 |
数学 | 4561篇 |
物理学 | 16876篇 |
出版年
2024年 | 175篇 |
2023年 | 852篇 |
2022年 | 1555篇 |
2021年 | 1691篇 |
2020年 | 1771篇 |
2019年 | 1773篇 |
2018年 | 1535篇 |
2017年 | 1405篇 |
2016年 | 2097篇 |
2015年 | 2048篇 |
2014年 | 2632篇 |
2013年 | 3258篇 |
2012年 | 3855篇 |
2011年 | 3994篇 |
2010年 | 2673篇 |
2009年 | 2623篇 |
2008年 | 2932篇 |
2007年 | 2533篇 |
2006年 | 2361篇 |
2005年 | 1851篇 |
2004年 | 1408篇 |
2003年 | 1120篇 |
2002年 | 1138篇 |
2001年 | 918篇 |
2000年 | 772篇 |
1999年 | 843篇 |
1998年 | 693篇 |
1997年 | 657篇 |
1996年 | 683篇 |
1995年 | 593篇 |
1994年 | 446篇 |
1993年 | 386篇 |
1992年 | 362篇 |
1991年 | 303篇 |
1990年 | 265篇 |
1989年 | 213篇 |
1988年 | 130篇 |
1987年 | 117篇 |
1986年 | 137篇 |
1985年 | 95篇 |
1984年 | 56篇 |
1983年 | 66篇 |
1982年 | 42篇 |
1981年 | 32篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1974年 | 4篇 |
1972年 | 4篇 |
1971年 | 3篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Liang Zhao 《Tetrahedron letters》2006,47(19):3291-3294
Allylmagnesium, allylindium, and allylbismuth generally showed a preference for axial addition to cyclohexenones. Allylmagnesium was the most stereoselective. Reactions with an α-methylated enone (carvone) were the most selective, except that allylbismuth was unreactive with this substrate. 相似文献
92.
93.
ZnO naorods on ZnO-coated seed substrates were fabricated by solution chemical method from Zn(NO3)2/NaOH under assisted electrical field. The working mechanism of electrical field was analyzed and the factors affecting the rod growth such as potential, precursor concentration and growth temperature were elucidated. The structural and optical properties are characterized by SEM, TEM, XRD, HRTEM and UV-vis. The results indicated that the nanorods have wurtzite structure without electrical field and are primarily of zincite structure under electrical field; when the electrical field is 1.1-1.3 V, not only the elevation of ion diffusion and adsorption lower the crystallite/solution interfacial energy and then the crystal nucleation barrier by increasing charge intensity, but also the production of H+ through oxidation of OH− increases properly the degree of solution supersaturation near the substrate, and thus lowers the activation energy. Both the two processes do favor to rod growth. With increasing precursor concentration in this system, the average diameter and length of ZnO nanorods increase, leading to decreasing of optical transmittance. The maximum rod growth rate at given concentration of Zn2+ occurs at a specific temperature. 相似文献
94.
Anchoring Properties of Nematic Liquid Crystals Studied Using the Attenuated Total Reflection Method
By using the attenuated total reflection method associated with the excitation of surface plasmons, the tilt angle of the liquid crystal director and its gradient at the surface are measured in a planar nematic cell as a function of the applied voltage. The surface anchoring anisotropy δπ of the liquid crystal and the surface elastic constant ks, are found to be δπ = 0.288 erg/cm and ks, = 9·12 × 10-11 erg, respectively, when the boundary condition suggested by Barbero et al is used. The theoretical and experimental values obtained with this boundary condition and that of Mada are discussed. The results show that the boundary condition proposed by Barbero et al is in better agreement with the experiment. 相似文献
95.
Zhao Yang YIN 《数学学报(英文版)》2006,22(3):819-826
Sufficient conditions for the existence of positive solutions to a class of quasilinear elliptic equations in two-dimensional exterior domains are given. 相似文献
96.
97.
本文研究了未知分布的逼近问题,利用随机加权法,给出了有Edgeworth展式的一类(未知)分布的模拟分布,证明了在一定条件下,模拟分布与未知分布的逼近精度达到O(n^-1√lnlnn),称之为随机加权逼近的重对数律。 相似文献
98.
基于超分子结构共掺杂纳米复合薄膜的制备与荧光特性 总被引:5,自引:0,他引:5
为改善功能分了的特性,提出一种基于金属纳米粒子-偶氮染料复合物共掺杂超分子结构功能材料的设计新方法.并依照此方法制备出复合材料,观测了其显微结构,测量了其紫外-可见光吸收,研究了该超分子结构复合体系的荧光特性.实验发现,由于金属银纳米粒子的掺杂,使得超分子结构复合体系中功能分子甲基橙在溶液态体系的荧光强度增强近5倍,而在两种不同结构(共混结构和包覆结构)的薄膜态超分子结构体系中,其荧光强度分别被猝灭15%和20%.研究结果表明,复合膜中采用超分子结构完全能够改善功能分子的特性. 相似文献
99.
Y Watanabe M Suda Y Matsumoto K Takayama M Matsumoto W Zhao 《Chemical & pharmaceutical bulletin》1991,39(9):2391-2394
The dissolution behavior of the aspirin enteric granule prepared using acylglycerols, glyceryl monostearate (GMS) and glyceryl trilaurate (GTL), was investigated in vitro and in human subjects in a fasting or non-fasting state. Aspirin was slowly released from the granule in vitro at pH 1.2. No acceleration of the aspirin dissolution rate in the medium without lipase and cholic acid was observed when the pH level of the medium increased to a neutral region (pH 6.4). However, the dissolution of aspirin was significantly increased by increasing the concentrations of lipase and cholic acid in the medium. Lipase appears to play an essential role in the dissolution process of aspirin granules. In human subjects, the average levels of the cumulative amount of total salicylate excreted in a urine-time curve, and the mean residence time (MRT) obtained after oral administration of a granule in the fasting state were markedly delayed in comparison with the results observed using an aqueous solution and a crystalline form of aspirin. In comparing the fasting condition with the non-fasting condition (after food ingestion), no significant difference was recognized in the total amount of salicylate excreted in urine to an infinite time (Ae(infinity)), whether the MRT was obtained by granule, crystalline form or aqueous solution. It can be concluded that aspirin granule prepared by GMS and GTL has a property of pancreatic lipase-sensitive dissolution, and its bioavailability is unaffected by food intake. 相似文献
100.
通过对Eu2-xPbxRu2O7(x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0和1.8)系列样品的结构、电阻和磁化率的观测,结果发现,随着Pb替代浓度x值的增加,样品的电阻率逐渐减小,系统在x=0.8附近发生了金属-绝缘体(M-I)相变;Ru4+的局域磁矩及其自旋玻璃冻结温度TG也随之降低. 在该体系中,Pb2+对Eu3+的部分替代使样品中载流子浓度增加,Pb的6p能带与Ru 4d电子的T2g能带混合,能带得以拓宽,Ru 4d电子的巡游性增强,导致该体系物性的系列变化. 相似文献