全文获取类型
收费全文 | 39673篇 |
免费 | 6492篇 |
国内免费 | 4040篇 |
专业分类
化学 | 27897篇 |
晶体学 | 442篇 |
力学 | 2309篇 |
综合类 | 253篇 |
数学 | 4289篇 |
物理学 | 15015篇 |
出版年
2024年 | 72篇 |
2023年 | 771篇 |
2022年 | 839篇 |
2021年 | 1330篇 |
2020年 | 1527篇 |
2019年 | 1501篇 |
2018年 | 1375篇 |
2017年 | 1243篇 |
2016年 | 1907篇 |
2015年 | 1776篇 |
2014年 | 2117篇 |
2013年 | 2889篇 |
2012年 | 3464篇 |
2011年 | 3663篇 |
2010年 | 2396篇 |
2009年 | 2302篇 |
2008年 | 2606篇 |
2007年 | 2344篇 |
2006年 | 2157篇 |
2005年 | 1729篇 |
2004年 | 1359篇 |
2003年 | 1061篇 |
2002年 | 1058篇 |
2001年 | 786篇 |
2000年 | 720篇 |
1999年 | 860篇 |
1998年 | 693篇 |
1997年 | 705篇 |
1996年 | 727篇 |
1995年 | 597篇 |
1994年 | 572篇 |
1993年 | 471篇 |
1992年 | 385篇 |
1991年 | 384篇 |
1990年 | 291篇 |
1989年 | 225篇 |
1988年 | 200篇 |
1987年 | 166篇 |
1986年 | 162篇 |
1985年 | 138篇 |
1984年 | 114篇 |
1983年 | 78篇 |
1982年 | 70篇 |
1981年 | 51篇 |
1980年 | 56篇 |
1979年 | 27篇 |
1978年 | 23篇 |
1976年 | 21篇 |
1975年 | 21篇 |
1973年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Synthesis and characterization of thermally crosslinked polyimide with second-order nonlinear optical chromophore 总被引:2,自引:0,他引:2
Incomparisonwithinorganicnonlinearoptic(NLO)materials,organicpolymericNLOmaterialshavemanyadvantages,suchasliablemoleculardesignandoptimization,largernonresonanceNLOcoefficiency,lowerdielectricconstant,goodworkingability,opticaltransparencyandresistancea… 相似文献
993.
994.
本文研究了熔融制样时熔融温度、熔融时间和脱模剂的加入量对分析结果的影响。研究结果表明,随着熔融温度的升高和熔融时间的加长,分析结果的总值将随之增大。相反,脱模剂量的增加会使分析结果降低。通过对熔融样品时产生的升华物的研究,发现在熔触过程中,四硼酸锂比样品以更大的比例逸出熔融体,从而造成了样品在分析圆片中的相对浓缩。而且在高温熔融时,钾和钠比样品中的其他元素例如硅、铝、铁、钛、钙、镁等更易于逸失。制样条件的不同引起样品和熔剂逸失的比例会有变化,它直接影响测定的结果,这证明了在X射线荧光光谱分析中保持制样条件一致的重要性。 相似文献
995.
996.
Relationship between microhardness and fluorine contents on tooth enamel determined by PIGE analysis
D. S. Ma D. I. Paik D. Y. Park H. S. Moon Y. I. Chang J. B. Kim 《Journal of Radioanalytical and Nuclear Chemistry》1997,217(2):225-227
The remineralization effect of fluoride has been measured by surface microhardness on tooth enamel. The purpose of this study was to investigate the relationship between microhardness and fluorine concentration on tooth enamel. Twelve sound bovine enamel specimens were prepared and immersed in 0.05% NaF solution for 1, 3, 6, 24 and 36 hours, respectively. The concentration of fluorine in specimens were measured by PIGE analysis and surface microhardness of each specimen was measured by surface microhardness tester. Fluorine concentration was increased by immersing time. There was no change in microhardness of each specimen by fluorine content. The results of this study suggest that there was no relationship between the fluorine concentration and surface microhardness in sound tooth enamel. PIGE analysis can be used effectively to assess the remineralization effect of fluorine content in tooth enamel. 相似文献
997.
Mechanism of TiO2-assisted photocatalytic degradation of dyes under visible irradiation: photoelectrocatalytic study by TiO2-film electrodes 总被引:3,自引:0,他引:3
Photoelectrocatalytic degradation of various dyes under visible light irradiation with a TiO(2) nanoparticles electrode has been investigated to reveal the mechanism for TiO(2)-assisted photocatalytic degradation of dyes. The degradation of both cationic and anionic dyes at different biases, including the change in the degradation rate of the dyes and the photocurrent change with the bias potential, the degraded intermediates, the voltage-induced adsorption of dyes, the accumulation of electrons in the TiO(2) electrode, the effect of various additives such as benzoquinone (BQ) and N,N-dimethyl aniline (DMA), and the formation of active oxygen species such as O(2)(*-) and H(2)O(2) were examined by UV-visible spectroscopy, HPLC, TOC, and spin-trap ESR spectrometry. It was found that the dyes could controllably interact with the TiO(2) surface by external bias changes and charging of dyes. The cationic dyes such as RhB and MG underwent efficient mineralization at negative bias, but the N-dealkylation process predominated at positive bias under visible light irradiation. The discolorations of the anionic dyes SRB and AR could not be accelerated significantly at either negative or positive bias. At a negative bias of -0.6 V vs SCE, O(2)(*-) and dye(*+) were formed simultaneously at the electrode/electrolyte interface during degradation of cationic RhB. In the case of anionic dyes, however, it is impossible for the O(2)(*-) and dye cationic radical to coexist at the electrode/electrolyte surface. Experimental results imply both the superoxide anionic radical and the dye cationic radical are essential to the mineralization of the dyes under visible light-induced photocatalytic conditions. 相似文献
998.
Hongjian Lu 《Tetrahedron letters》2005,46(36):5983-5985
N-Acyltriazenes serve as a tin-free and initiator-free source for amidyl radicals. Thermal decomposition of N,N′-diaryl-N-(4-pentenoyl)triazenes in refluxing toluene led to the formation of monocyclic and tricyclic lactams in satisfactory yields via 5-exo amidyl radical cyclization. 相似文献
999.
1000.
枸椽酸锗能明显提高小鼠血清溶血素抗体水平,增强二硝基氯苯(DNCB)致小鼠迟发型皮肤过敏反应。增加小鼠免疫器官胸腺和脾脏的重量,表明枸椽酸锗能增强小鼠体液免疫和细胞免疫水平。 相似文献