全文获取类型
收费全文 | 16075篇 |
免费 | 2494篇 |
国内免费 | 1914篇 |
专业分类
化学 | 12093篇 |
晶体学 | 167篇 |
力学 | 871篇 |
综合类 | 148篇 |
数学 | 2100篇 |
物理学 | 5104篇 |
出版年
2024年 | 30篇 |
2023年 | 334篇 |
2022年 | 419篇 |
2021年 | 562篇 |
2020年 | 760篇 |
2019年 | 708篇 |
2018年 | 603篇 |
2017年 | 548篇 |
2016年 | 733篇 |
2015年 | 797篇 |
2014年 | 968篇 |
2013年 | 1286篇 |
2012年 | 1456篇 |
2011年 | 1503篇 |
2010年 | 1081篇 |
2009年 | 986篇 |
2008年 | 1047篇 |
2007年 | 945篇 |
2006年 | 896篇 |
2005年 | 666篇 |
2004年 | 532篇 |
2003年 | 418篇 |
2002年 | 478篇 |
2001年 | 314篇 |
2000年 | 317篇 |
1999年 | 301篇 |
1998年 | 232篇 |
1997年 | 213篇 |
1996年 | 218篇 |
1995年 | 150篇 |
1994年 | 139篇 |
1993年 | 112篇 |
1992年 | 91篇 |
1991年 | 87篇 |
1990年 | 71篇 |
1989年 | 67篇 |
1988年 | 66篇 |
1987年 | 41篇 |
1986年 | 38篇 |
1985年 | 42篇 |
1984年 | 28篇 |
1983年 | 21篇 |
1982年 | 14篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 11篇 |
1978年 | 22篇 |
1977年 | 16篇 |
1976年 | 13篇 |
1975年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 25 毫秒
41.
Mid-infrared generation based on a periodically poled LiNbO3 optical parametric oscillator 下载免费PDF全文
We report a nanosecond Nd:YVO_4-pumped optical parametric oscillator (OPO) based on periodically poled LiNbO_3 (PPLN). Tuning is achieved in this experiment by varying the temperature and period of the PPLN. The design of double-pass singly resonant oscillator (DSRO) and confocal cavity enables the OPO threshold to be lowered considerably, resulting in a simple, compact, all-solid-state configuration with the mid-infrared idler powers of up to 466mW at 3.41μm. 相似文献
42.
Pengbo Jiang Tiexin Cheng Yan Liu Xianghao Cui Yingli Bi Kaiji Zhen 《Reaction Kinetics and Catalysis Letters》2004,82(1):49-56
The novel phase transfer catalysts S-8 [4-(dimethyloctylammonium) propansultan] and DB-X [1,4-bis(triethylmethylammonium)benzene dibromide] were synthesized and employed for high conversion synthesis of dichlorocyclopropane from various olefins.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
43.
A signed graph has a plus or minus sign on each edge. A simple cycle is positive or negative depending on whether it contains an even or odd number of negative edges, respectively. We consider embeddings of a signed graph in the projective plane for which a simple cycle is essential if and only if it is negative. We characterize those signed graphs that have such a projective-planar embedding. Our characterization is in terms of a related signed graph formed by considering the theta subgraphs in the given graph. 相似文献
44.
Mingbao Zhang Raymond Reeves Cheng Bi Robert Dally Gaetan Ladouceur William Bullock Jefferson Chin 《Tetrahedron letters》2004,45(27):5229-5231
Three novel, optically active, 6-substituted 2-(aminomethyl)chromans were synthesized from readily available chroman 2-carboxylic acid precursors. These chroman-containing primary amines are useful building blocks for the synthesis of chroman-derived pharmaceutical agents. 相似文献
45.
The basic features of a chemical measurement 总被引:2,自引:0,他引:2
46.
In the present work, a new method was established by applying solid-phase extraction (SPE) to preconcentrate and separate polymeric aluminum (Al) and using ICP-AES to determine the polymeric Al, the total monomeric Al, and the total Al in soil extracts, respectively. A modified resin was prepared with impregnated 8-hydroxyquinoline-5-sulfoxinate (HQS) on the anion-exchange resin. It has good recognition ability for Al fractions, compared to the commonly used cation ion-exchange resin, which has a better ability to adsorb cations and a weak ability to recognize detailed Al species. The optimum conditions for Al fractionation sorption, elution and separation and the interference of foreign ions were studied with the prepared resin by continuous column and batch procedures. Monomeric Al was bound to Pyrocathecol Violet (PCV) at pH 6.2, whereas the polymeric Al species did not react with PCV for at least 15 min. Because a stable complex of Al-PCV was not absorbed on the HQS modified resin, the polymeric Al could be preconcentrated on-line by the HQS-modified resin. The adsorbed polymeric Al was eluted with 3 mL of 3 mol L(-1) of HCl, and then detected by ICP-AES. The method has been applied to directly determine polymeric Al in soil extracts with high selectivity as well as a high preconcentration factor. It gives a limit of detection of 0.6 ng mL(-1) with a relative standard deviation of less than 5.7% (n = 5, 0.24 microg mL(-1) Al). 相似文献
47.
The adsorption of human serum albumin onto hydroxyapatite-modified silver electrodes has been in situ investigated by utilizing the piezoelectric quartz crystal impedance technique. The changes of equivalent circuit parameters were used to interpret the adsorption process. A kinetic model of two consecutive steps was derived to describe the process and compared with a first-order kinetic model by using residual analysis. The experimental data of frequency shift fitted to the model and kinetics parameters, k1, k2, psi1, psi2 and qr, were obtained. All fitted results were in reasonable agreement with the corresponding experimental results. Two adsorption constants (7.19 kJ mol(-1) and 22.89 kJ mol(-1)) were calculated according to the Arrhenius formula. 相似文献
48.
49.
半自由声场环境下的声源重建和声场预测研究对声全息技术走向实际应用具有非常重要的意义.在提出基于分布源边界点法的半自由声场全息重建和预测方法的基础上,对此展开了实验研究.并将重建和预测的结果与常规方法重建和预测的结果进行了比较和讨论,说明了重建预测过程中反射声压的影响和考虑反射声压的必要性,证明了所提出方法在解决半自由声场环境下存在地面反射时的声源重建和声场预测时的有效性和准确性.还提出了采用奇异值截断滤波和Tikhonov正则化方法来削弱测量误差的影响,从而进一步优化了重建结果,提高了全息成像的可信度.
关键词:
声全息
半自由场
边界点
声辐射
反射声 相似文献
50.
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