全文获取类型
收费全文 | 11265篇 |
免费 | 929篇 |
国内免费 | 1291篇 |
专业分类
化学 | 9175篇 |
晶体学 | 160篇 |
力学 | 67篇 |
综合类 | 38篇 |
数学 | 2958篇 |
物理学 | 1087篇 |
出版年
2024年 | 23篇 |
2023年 | 120篇 |
2022年 | 139篇 |
2021年 | 167篇 |
2020年 | 278篇 |
2019年 | 322篇 |
2018年 | 232篇 |
2017年 | 267篇 |
2016年 | 292篇 |
2015年 | 417篇 |
2014年 | 520篇 |
2013年 | 992篇 |
2012年 | 683篇 |
2011年 | 648篇 |
2010年 | 585篇 |
2009年 | 651篇 |
2008年 | 753篇 |
2007年 | 677篇 |
2006年 | 576篇 |
2005年 | 547篇 |
2004年 | 519篇 |
2003年 | 473篇 |
2002年 | 643篇 |
2001年 | 342篇 |
2000年 | 345篇 |
1999年 | 278篇 |
1998年 | 264篇 |
1997年 | 214篇 |
1996年 | 178篇 |
1995年 | 145篇 |
1994年 | 148篇 |
1993年 | 120篇 |
1992年 | 105篇 |
1991年 | 84篇 |
1990年 | 87篇 |
1989年 | 95篇 |
1988年 | 73篇 |
1987年 | 53篇 |
1986年 | 48篇 |
1985年 | 52篇 |
1984年 | 47篇 |
1983年 | 33篇 |
1982年 | 36篇 |
1981年 | 34篇 |
1980年 | 35篇 |
1979年 | 31篇 |
1978年 | 21篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1971年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
利用方势阱模型对InxGa1-xN/GaN MQWs结构的光特性进行了量子力学定性理论分析.并在MO源流量恒定条件下,在570℃~640℃范围内进行了不同生长温度的多量子阱制备实验,对InxGa1-xN制备过程中的In组份掺入效率的温度依赖关系进行研究.通过对制备样品的PL谱测量分析,得到了587℃~600℃的In组份最佳掺入温度区间. 相似文献
992.
Sergey I. Vdovenko Igor I. Gerus Valery P. Kukhar 《Journal of Physical Organic Chemistry》2007,20(3):190-200
The kinetics of the reaction of β‐substituted β‐alkoxyvinyl trifluoromethyl ketones R1O‐CR2?CH‐COCF3 ( 1a – e ) [( 1a ), R1?C2H5, R2?H; ( 1b ), R1?R2?CH3; ( 1c ), R1?C2H5, R2?C6H5; ( 1d ), R1?C2H5, R2?V?pNO2C6H4; ( 1e ), R1?C2H5, R2?C(CH3)3] with four aliphatic amines ( 2a – d ) [( 2a ), (C2H5)2NH; ( 2b ), (i‐C3H7)2NH; ( 2c ), (CH2)5NH; ( 2d ), O(CH2CH2)2NH] was studied in two aprotic solvents, hexane and acetonitrile. The least reactive stereoisomeric form of ( 1a – d ) was the most populated ( E‐s‐Z‐o‐Z ) form, whereas in ( 1e ), the more reactive form ( Z‐s‐Z‐o‐Z ) dominated. The reactions studied proceeded via common transition state formation whose decomposition occurred by ‘uncatalyzed’ and/or ‘catalyzed’ route. Shielding of the reaction centre by bulky β‐substituents lowered abruptly both k′ (‘uncatalyzed’ rate constant) and k″ (‘catalyzed’ rate constant) of this reaction. Bulky amines reduced k″ to a greater extent than k′ as a result of an additional steric retardation to the approach of the bulky amine to its ammonium ion in the transition state. An increase in the electron‐withdrawing ability of the β‐substituent increased ‘uncatalyzed’ k′ due to the acceleration of the initial nucleophile attack (k1) and ‘uncatalyzed’ decomposition of transition state (k2) via promoting electrophilic assistance (through transition state 8 ). The amine basicity determined the route of the reaction: the higher amine basicity, the higher k3/k2 ratio (a measure of the ‘catalyzed’ route contribution as compared to the ‘uncatalyzed’ process) was. ‘Uncatalyzed’ route predominated for all reactions; however in polar acetonitrile the contribution of the ‘catalyzed’ route was significant for amines with high pKa and small bulk. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
993.
Olivier Lagrille Jacques Taillades Laurent Boiteau Auguste Commeyras 《Journal of Physical Organic Chemistry》2007,20(4):271-284
Nitrosation of N‐carbamoylamino acids (CAA) by gaseous NO + O2, an interesting synthetic pathway to amino acid N‐carboxyanhydrides (NCA), alternative to the phosgene route, was investigated on N‐carbamoyl‐valine either in acetonitrile suspension or solventless conditions, and compared to the classical nitrosating system NaNO2 + CF3COOH (TFA), the latter being quite less efficient in terms of either rate, stoichiometric demand, or further tractability of the product. The rate and efficiency of the NO + O2 reaction mainly depends on the O2/NO ratio. Evaluation of the contribution of various nitrosating species (N2O3, N2O4, HNO2) through stoichiometric balance showed the reaction to be effected mostly by N2O3 for O2/NO ratios below 0.3, and by N2O4 for O2/NO ratios above 0.4. The relative contribution of (subsequently formed) HNO2 always remains minor. Differential scanning calorimetry (DSC) monitoring of the reaction in the solid phase by either HNO2 (from NaNO2 + TFA), gaseous N2O4 or gaseous N2O3, provides the associated rate constants (ca. 0.1, 2 and 108 s?1 at 25°C, respectively), showing that N2O3 is by far the most reactive of these nitrosating species. From the DSC measurement, the latent heat of fusion of N2O3, 2.74 kJ · mol?1 at ?105 °C is also obtained for the first time. The kinetics was investigated under solventless conditions at 0°C, by either quenching experiments or less tedious, rough calorimetric techniques. Auto‐accelerated, parabolic‐shaped kinetics was observed in the first half of the reaction course, together with substantial heat release (temperature increase of ca. 20°C within 1–2 min in a 20‐mg sample), followed by pseudo‐zero‐order kinetics after a sudden, important decrease in apparent rate. This kinetic break is possibly due to the transition between the initial solid‐gas system and a solid‐liquid‐gas system resulting from water formation. Overall rate constants increased with parameters such as the specific surface of the solid, the O2/NO ratio, or the presence of moisture (or equivalently the hydrophilicity of the involved CAA), however without precise relationship, while the last two parameters may directly correlate to the increasing acidity of the medium. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
994.
A direct kinetic study is reported for the electrophilic amination of substituted phenylmagnesium bromides with N,N‐dimethyl O‐(mesitylenesulfonyl)hydroxylamine in THF. Rate data, Hammett relationship, and activation entropy are consistent with a SN2 displacement involving the attack of carbanions to sp3N in the amination reagent (AR). Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
995.
996.
土壤的光谱特征及氮含量的预测研究 总被引:13,自引:9,他引:13
应用近红外光谱分析技术(NIR)测定土壤参数具有快速、方便的特点.文章分析了不同含水率、不同颗粒大小的土壤样本在不同测试角、不同测试高度对土壤光谱的影响,并得到了不同含水率和不同粒径土壤的含氮量预测模型.研究了这些因素对含氮率测量的影响,分析了NIR技术在田间实地应用预测的可能性.研究表明,光谱仪在距土壤高度为100 mm,测试角为45°时,具有最大的吸光度.土壤粒径和含水率这2个参数明显影响,当粒径在0.5~5 mm变化时,含氮量预测相关系数r为0.81左右,当土壤粒径在<0.25和>5 mm模型的预测能力变差.当土壤样品呈天然潮湿状态时,氮的预测结果较差.而样品干燥以后,预测相关系数较高.为土壤原位光谱测试提供了依据. 相似文献
997.
Yang Su Meihan Dan Xin Xiao Xiaohui Wang Wangqing Zhang 《Journal of polymer science. Part A, Polymer chemistry》2013,51(20):4399-4412
The multi‐thermo‐responsive block copolymer of poly[2‐(2‐methoxyethoxy)ethyl methacrylate]‐block‐poly[N‐(4‐vinylbenzyl)‐N,N‐diethylamine] (PMEO2MA‐b‐PVEA) displaying phase transition at both the lower critical solution temperature (LCST) and the upper critical solution temperature (UCST) in the alcohol/water mixture is synthesized by reversible addition‐fragmentation chain transfer polymerization. The poly[2‐(2‐methoxyethoxy)ethyl methacrylate] (PMEO2MA) block exhibits the UCST phase transition in alcohol and the LCST phase transition in water, while the poly[N‐(4‐vinylbenzyl)‐N,N‐diethylamine] (PVEA) block shows the UCST phase transition in isopropanol and the LCST phase transition in the alcohol/water mixture. Both the polymer molecular weight and the co‐solvent/nonsolvent exert great influence on the LCST or UCST of the block copolymer. By adjusting the solvent character including the water content and the temperature, the block copolymer undergoes multiphase transition at LCST or UCST, and various block copolymer morphologies including inverted micelles, core‐corona micelles, and corona‐collapsed micelles are prepared. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4399–4412 相似文献
998.
999.
1000.
《Journal of polymer science. Part A, Polymer chemistry》2018,56(4):365-375
In this report, we synthesized poly(N‐vinyl imidazole) (PNVI) grafted silica nanoparticles (SiNP) by using RAFT polymerization through grafting‐from approach to demonstrate that the self‐assembled structure of SiNP is the key diving force in improving physical properties of SiNP based nanocomposites. In a multistep synthetic process, well‐defined PNVI chains with tunable molecular weights and surface chain densities were grown from the RAFT agent anchored SiNP surface using N‐vinyl imidazole (NVI) as a monomer. Spectroscopic and thermal analysis confirmed surface grafting of PNVI on SiNP surface and the amount of grafted PNVI chins were also quantified. The mean diameter of the PNVI grafted SiNP (PNVI‐g‐SiNP) particles altered between 50 and 100 nm with the variation of PNVI chain lengths. The present approach is metal‐catalyst free, straight forward, and provides PNVI functionalized SiNP in a simple manner in comparison to the reported methods. Further, these PNVI‐g‐SiNP particles were used as a nanofiller to prepare nanocomposites with Poly(4,4′‐diphenylether‐5,5′‐bibenzimidazole) (OPBI). These nanocomposites displayed significantly higher mechanical, proton conductivity and less acid leaching properties than the pristine OPBI. The anisotropic self‐assembled ordered structure formation of nanofillers in the nanocomposites believed to be the driving force for the enhanced physical properties. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 365–375 相似文献