全文获取类型
收费全文 | 2354篇 |
免费 | 291篇 |
国内免费 | 234篇 |
专业分类
化学 | 729篇 |
晶体学 | 17篇 |
力学 | 287篇 |
综合类 | 31篇 |
数学 | 618篇 |
物理学 | 1197篇 |
出版年
2024年 | 3篇 |
2023年 | 16篇 |
2022年 | 41篇 |
2021年 | 46篇 |
2020年 | 68篇 |
2019年 | 58篇 |
2018年 | 41篇 |
2017年 | 72篇 |
2016年 | 83篇 |
2015年 | 66篇 |
2014年 | 126篇 |
2013年 | 186篇 |
2012年 | 140篇 |
2011年 | 161篇 |
2010年 | 127篇 |
2009年 | 158篇 |
2008年 | 181篇 |
2007年 | 137篇 |
2006年 | 132篇 |
2005年 | 126篇 |
2004年 | 95篇 |
2003年 | 81篇 |
2002年 | 85篇 |
2001年 | 84篇 |
2000年 | 62篇 |
1999年 | 57篇 |
1998年 | 57篇 |
1997年 | 49篇 |
1996年 | 71篇 |
1995年 | 29篇 |
1994年 | 35篇 |
1993年 | 29篇 |
1992年 | 34篇 |
1991年 | 26篇 |
1990年 | 14篇 |
1989年 | 24篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 11篇 |
1983年 | 3篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1976年 | 2篇 |
1973年 | 1篇 |
1970年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有2879条查询结果,搜索用时 10 毫秒
61.
设计了一款长度为265 mm的飞秒条纹管。采用短磁聚焦透镜和行波偏转板,并将行波偏转板置于磁透镜之前以提高偏转灵敏度。采用蒙特卡罗方法对阴极表面理想点和阴极狭缝发射的光电子初始参量进行了模拟抽样,用CST软件的Particle Tracking模块模拟跟踪了光电子的运行轨迹,统计分析了光电子在最佳像面上的位置分布和渡越时间,给出了光电子在最佳像面上的点扩展函数和调制传递函数。计算结果显示,所设计的条纹管阴极有效尺寸达到6 mm,放大率为2.4~2.5,动态空间分辨力大于55 lp/mm。经保守估算,条纹管的时间分辨力有望达到245 fs。 相似文献
62.
Jing-Yuan Chang Yen-Jin Pan Pei-Yu Huang Yi-Ting Sun Chen-Hsu Yu Zhi-Jun Ning Shou-Ling Huang Shing-Jong Huang Richard P. Cheng 《Molecules (Basel, Switzerland)》2022,27(13)
The β-sheet is one of the common protein secondary structures, and the aberrant aggregation of β-sheets is implicated in various neurodegenerative diseases. Cross-strand interactions are an important determinant of β-sheet stability. Accordingly, both diagonal and lateral cross-strand interactions have been studied. Surprisingly, diagonal cross-strand ion-pairing interactions have yet to be investigated. Herein, we present a systematic study on the effects of charged amino acid side-chain length on a diagonal ion-pairing interaction between carboxylate- and ammonium-containing residues in a β-hairpin. To this end, 2D-NMR was used to investigate the conformation of the peptides. The fraction folded population and the folding free energy were derived from the chemical shift data. The fraction folded population for these peptides with potential diagonal ion pairs was mostly lower compared to the corresponding peptide with a potential lateral ion pair. The diagonal ion-pairing interaction energy was derived using double mutant cycle analysis. The Asp2-Dab9 (Asp: one methylene; Dab: two methylenes) interaction was the most stabilizing (−0.79 ± 0.14 kcal/mol), most likely representing an optimal balance between the entropic penalty to enable the ion-pairing interaction and the number of side-chain conformations that can accommodate the interaction. These results should be useful for designing β-sheet containing molecular entities for various applications. 相似文献
63.
Lei Sun Tian Du Cheng Wang Prof. Dongling Geng Dr. Lin Li Prof. Yang Han Prof. Yunfeng Deng 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(69):17437-17443
Low-bandgap organic semiconductors have attracted much attention for their multiple applications in optoelectronics. However, the realization of narrow bandgap is challenging particularly for small molecules. Herein, we have synthesized four quinoidal compounds, i. e., QSN3 , QSN4 , QSN5 and QSN6 , with electron rich S,N-heteroacene as the quinoidal core and indandione as the end-groups. The optical bandgap of the quinoidal compounds is systematically decreased with the extension of quinoidal skeleton, while maintaining stable closed-shell ground state. QSN6 absorbs an intense absorption in the first and second near-infrared region in the solid state, and has extremely low optical bandgap of 0.74 eV. Cyclic voltammetry analyses reveal that the lowest unoccupied molecular orbital (LUMO) energy levels of the four quinoidal compounds all lie below −4.1 eV, resulting in good electron-transporting characteristics in organic thin-film transistors. These results demonstrated that the combination of π-extended quinoidal core and end-groups in quinoidal compounds is an effective strategy for the synthesis of low-bandgap small molecules with good stability. 相似文献
64.
自由基聚合的模块化教学及难点剖析 总被引:1,自引:0,他引:1
自由基聚合作为高分子化学的重点内容,内容丰富,难点较多。本文拟对作者在教学过程中总结的模块化教学方法进行介绍,并尝试对自由基聚合的动力学链长、聚合度及链转移这一难点内容进行剖析。 相似文献
65.
通过对不同情况下参数测量误差导致的到靶激光平均功率密度变化的统计,得到了大气参数精度对激光传输计算结果的影响。风速和气溶胶吸收系数测量误差主要来自热晕效应的影响,热晕越强,对计算结果的影响就越大。强热晕时,风速±(5%~8%)的误差可导致计算结果约±10%的误差;气溶胶吸收权重±(10%~20%)的误差可导致计算结果约 10%的误差。强湍流弱热晕时,大气相干长度±(5%~7%)的误差可导致计算结果约±10%的误差。 相似文献
66.
研究了相对论速调管放大器(RKA)输入腔和中间腔之间的高阶杂模振荡问题。通过模式分析得知杂模在谐振腔内为TM11模式,而在漂移管中表现为TE11模式,针对该模式能在漂移管中传输的特性,利用漂移管内壁涂覆吸波材料吸收杂模功率的方法进行抑制。通过3维粒子模拟程序,分析了吸波材料的电导率及涂覆长度对抑制杂模增长率的影响。利用模拟分析得到的结果,对漂移管中涂覆吸波材料的RKA输入腔及中间腔结构进行了3维模拟研究,结果显示:合适的吸波材料的引入能够很好地抑制RKA输入腔和中间腔之间的杂模振荡。 相似文献
67.
68.
69.
Charlotte Duval Cécile Nouvel Jean‐Luc Six 《Journal of polymer science. Part A, Polymer chemistry》2014,52(8):1130-1138
Poly(d,l ‐lactide‐co‐glycolide) (PLGA) copolyesters are commonly used in biomedical applications. Researches were carried out on nontoxic or low‐toxic catalysts that are enough efficient to provide short polymerization times, adequate microstructure chains and similar properties than the commercial PLGA materials. In this study, PLGA were synthesized by ring‐opening copolymerization (ROP) using three different catalysts. Stannous octoate is the first catalyst we used, as it is very efficient, even its toxicity is still on debate. Two others low‐toxic catalysts [zinc lactate and bismuth subsalicylate (BiSS)] were also evaluated. The comparison of these ROP was realized in terms of kinetics and control of the polymerization. Then, the influence of the catalyst on the PLGA microstructure chains is reported. Finally, abiotic hydrolytic degradation rate is studied. Results described in this article show that BiSS is one very attractive catalyst to produce low toxic PLGA for biomedical applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1130–1138 相似文献
70.
Matthew Thompson Carolyn Carkner Adrian Bailey Nicholas J. Mosey Nadia Kapernaum 《Liquid crystals》2013,40(9):1246-1260
Two sterically equivalent series of phenoxy-terminated 5-alkoxy-2-(4-alkoxyphenyl)pyrimidine liquid crystals were synthesised, and their mesogenic properties were characterised by polarised optical microscopy and differential scanning calorimetry (DSC). The phenoxy end-group causes a significant increase in melting point and inhibits – at least partially – the mesomorphism of these materials relative to the parent isomers; in most cases, the broad enantiotropic SmC phase formed by the parent isomers is suppressed by the addition of the phenoxy end-group. However, detailed analyses by small-angle X-ray scattering and monodomain 2D X-ray scattering suggest that these compounds form a SmA phase with a partially intercalated bilayer structure in which the phenoxy end-groups are nanosegregated. Such an intercalated bilayer structure might enable the tuning of smectogenic properties by appropriate substitution of the phenoxy end-groups. 相似文献