全文获取类型
收费全文 | 13493篇 |
免费 | 2497篇 |
国内免费 | 1668篇 |
专业分类
化学 | 9448篇 |
晶体学 | 143篇 |
力学 | 805篇 |
综合类 | 105篇 |
数学 | 1343篇 |
物理学 | 5814篇 |
出版年
2024年 | 61篇 |
2023年 | 308篇 |
2022年 | 502篇 |
2021年 | 599篇 |
2020年 | 590篇 |
2019年 | 543篇 |
2018年 | 526篇 |
2017年 | 485篇 |
2016年 | 730篇 |
2015年 | 686篇 |
2014年 | 866篇 |
2013年 | 1075篇 |
2012年 | 1288篇 |
2011年 | 1236篇 |
2010年 | 875篇 |
2009年 | 860篇 |
2008年 | 935篇 |
2007年 | 739篇 |
2006年 | 692篇 |
2005年 | 581篇 |
2004年 | 451篇 |
2003年 | 336篇 |
2002年 | 335篇 |
2001年 | 268篇 |
2000年 | 300篇 |
1999年 | 268篇 |
1998年 | 198篇 |
1997年 | 200篇 |
1996年 | 193篇 |
1995年 | 163篇 |
1994年 | 129篇 |
1993年 | 107篇 |
1992年 | 111篇 |
1991年 | 65篇 |
1990年 | 80篇 |
1989年 | 56篇 |
1988年 | 29篇 |
1987年 | 29篇 |
1986年 | 42篇 |
1985年 | 46篇 |
1984年 | 15篇 |
1983年 | 17篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 6篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1966年 | 2篇 |
1957年 | 2篇 |
1911年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Theoretical prediction of the host–guest interactions between novel photoresponsive nanorings and C60: A strategy for facile encapsulation and release of fullerene 下载免费PDF全文
Kun Yuan Jing‐Shuang Dang Yi‐Jun Guo Xiang Zhao 《Journal of computational chemistry》2015,36(8):518-528
A series of photoresponsive‐group‐containing nanorings hosts with 12~14 Å in diameter is designed by introducing different number of azo groups as the structural composition units. And the host–guest interactions between fullerene C60 and those nanoring hosts were investigated theoretically at M06‐2X/6‐31G(d)//M06‐L/MIDI! and wB97X‐D/6‐31G(d) levels. Analysis on geometrical characteristics and host–guest binding energies revealed that the designed nanoring molecule (labeled as 7 ) which is composed by seven azo groups and seven phenyls is the most feasible host for encapsulation of C60 guest among all candidates. Moreover, inferring from the simulated UV‐vis‐NIR spectroscopy, the C60 guest could be facilely released from the cavity of the host 7 via configuration transformation between trans‐form and cis‐form of the host under the 563 nm photoirradiation. Additionally, the frontier orbital features, weak interaction regions, infrared, and NMR spectra of the C60@7 host–guest complex have also been investigated theoretically. © 2015 Wiley Periodicals, Inc. 相似文献
962.
Determination of hexavalent chromium in traditional Chinese medicines by high‐performance liquid chromatography with inductively coupled plasma mass spectrometry 下载免费PDF全文
Peng Li Li‐min Li Jing Xia Shuai Cao Xin Hu Hong‐Zhen Lian Shen Ji 《Journal of separation science》2015,38(23):4043-4047
An analytical method that combined high‐performance liquid chromatography with inductively coupled plasma mass spectrometry has been developed for the determination of hexavalent chromium in traditional Chinese medicines. Hexavalent chromium was extracted using the alkaline solution. The parameters such as the concentration of alkaline and the extraction temperature have been optimized to minimize the interconversion between trivalent chromium and hexavalent chromium. The extracted hexavalent chromium was separated on a weak anion exchange column in isocratic mode, followed by inductively coupled plasma mass spectrometry determination. To obtain a better chromatographic resolution and sensitivity, 75 mM NH4NO3 at pH 7 was selected as the mobile phase. The linearity of the proposed method was investigated in the range of 0.2–5.0 μg L?1 (r2 = 0.9999) for hexavalent chromium. The limits of detection and quantitation are 0.1 and 0.3 μg L?1, respectively. The developed method was successfully applied to the determination of hexavalent chromium in Chloriti lapis and Lumbricus with satisfactory recoveries of 95.8–112.8%. 相似文献
963.
Effects of β‐nucleating agent and crystallization conditions on the crystallization behavior and polymorphic composition of isotactic polypropylene/multi‐walled carbon nanotubes composites 下载免费PDF全文
Jian Kang Jinghan He Zhengfang Chen Feng Yang Jinyao Chen Ya Cao Ming Xiang 《先进技术聚合物》2015,26(1):32-40
In this study, the effects of crystallization conditions (cooling rate and end temperature of cooling) on crystallization behavior and polymorphic composition of isotactic polypropylene/multi‐walled carbon nanotubes (iPP/MWCNTs) composites nucleated with different concentrations of β‐nucleating agent (tradename TMB‐5) were investigated by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD) and scanning electronic microscopy (SEM). The results of DSC, WAXD and SEM revealed that the addition of MWCNTs and TMB‐5 evidently elevates crystallization temperatures and significantly decreases the crystal sizes of iPP. Because of the competition between α‐nucleation (provided by MWCNTs) and β‐nucleation (induced by TMB‐5), the β‐phase crystallization takes place only when 0.15 wt% and higher concentration of TMB‐5 is added. Non‐isothermal crystallization kinetics study showed that the crystallization activation energy ΔE of β‐nucleated iPP/MWCNTs composites is obviously higher than that of pure iPP, which slightly increases with the increase of TMB‐5 concentration, accompanying with the transition of its polymorphic crystallization behavior. The results of non‐isothermal crystallization and melting behavior suggested that the cooling rate and end temperature of cooling (Tend) are important factors in determining the proportion and thermal stability of β‐phase: Lower cooling rate favors the formation of less amount of β‐phase with higher thermal stability, while higher cooling rate encourages the formation of higher proportion of β‐phase with lower thermal stability. The Tend = 100°C can eliminate the β–α recrystallization during the subsequent heating and therefore enhance the thermal stability of the β‐phase. By properly selecting TMB‐5 concentration, cooling rate and Tend, high β‐phase proportion of 88.9% of the sample was obtained. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
964.
Theoretical study on the degradation mechanism of methamidophos and chloramine phosphorus with OH radicals 下载免费PDF全文
Shengmin Sun Kun Zhang Yang Lu Ping Liu Hui Zhang 《International journal of quantum chemistry》2015,115(17):1187-1193
Theoretical investigation on the gas‐phase degradation reaction mechanism of methamidophos (MAP) and chloramine phosphorus (CHP) with OH radicals is performed. The equilibrium geometries and the harmonic vibration frequencies of the stationary points are obtained at M06‐2x/6‐31+G(d,p) level, and the higher‐level energetic information is further refined at M06‐2x/6–311++G(3df,2p) level. The rate constants for the 14 reaction channels are calculated by the improved canonical variational transition state theory with small‐curvature tunneling correction over the temperature range 200–2000 K. The three‐parameter expressions of k1(T) = 1.53 × 10?19T2.74exp(?1005.12/T), k2(T) = 1.36 × 10?20T3.02exp(?1259.56/T) are given. The total rate constants of all reaction channels of MAP with OH radicals are in good agreement with the available experimental data. Our results indicate that the H‐abstraction reactions on methyl are the major channels for the reaction of MAP and CHP with OH radicals. © 2015 Wiley Periodicals, Inc. 相似文献
965.
A Facile One‐Step Synthesis of 2‐Arylindazoles via Reductive Cyclization of N‐(2‐nitroarylidene)amines 下载免费PDF全文
Wei Lin Minghua Hu Xian Feng Chengpao Cao Zhibin Huang Daqing Shi 《Journal of heterocyclic chemistry》2015,52(4):1170-1174
A mild and efficient synthesis of 2‐arylindazole derivatives via the reductive cyclization of nitro‐aryl substrates mediated by a low‐valent titanium reagent (TiCl4/Sm/Et3N) has been developed. The attractive features of the current method include an N–N bond formation and the selective reduction of the C = N bond and nitro group, both of which were easily achieved in one‐pot by controlling the pH of the reaction mixture. 相似文献
966.
1-环丙基-6-氟-7-(4-酰基-1-哌嗪基)-1,4-二氢-4-氧代喹啉-3-羧酸的合成及其抗菌活性研究 总被引:2,自引:0,他引:2
以1-环丙基-6-氟-7-氯-1, 4-二氢-4-氧代喹啉-3-羧酸为原料经过两步反应合成出了16个新的1-环丙基-6-氟-7-(4-酰基-1-哌嗪基)-1, 4-二氢-4-氧代喹啉-3-羧酸, 并利用IR, 1H NMR, 13C NMR, MS谱及元素分析对其结构进行了表征. 初步体外抗菌活性研究表明, 大部分目标化合物对大肠杆菌有一定的抑菌活性, MIC值在0.312~20 g/mL之间, 但活性低于对照物环丙沙星和氧氟沙星; 目标化合物对绿脓杆菌无抑菌活性, 最低抑菌浓度(MIC)均大于20 g/mL. 相似文献
967.
968.
在B3LYP/6-311++g**、MP2/6-311++g(3df,3pd)及MP2/aug-cc-pvtz水平上分别求得H3SiH…MeHn(Me=Na,Mg,Be;n=1或2)复合物势能面上的3个稳定构型,探讨了以Si-H为电子供体的红移反向氢键相互作用(IHB).经MP2/6-311++g(3df,3pd)水平的计算,在3个复合物中,含基组重叠误差(BSSE)校正的单体间相互作用能分别为-5.98、-8.65和-3.96kJ.mol-1,与MP2/aug-cc-pvtz水平下计算得到的-6.18、-9.12和-4.28kJ·mol-1接近,可见3个反向氢键复合物的相对稳定性顺序为:SiH4...MgH2SiH4...NaHSiH4...BeH2.NBO分析及对相关原子化学位移的计算表明,在复合物中,电子流向总体表现为SiH4→MeHn(n=1或2),且直接参与反向氢键形成的H3的化学位移向低场移动.与传统氢键相比,这里Si1-H3既是氢键供体,又是电子供体,从而形成反向氢键相互作用.另外,采用分子中原子理论(AIM)分别对各复合物中相关键鞍点处的电子密度拓扑性质进行了分析,结果表明3个复合物中均存在以静电性质为主的分子间反向氢键弱相互作用. 相似文献
969.
Fan Xia Dr. Xu Hou Liuxuan Cao Lin Wang Jianming Xue Prof. Guangzhao Zhang Prof. Yanlin Song Prof. Daoben Zhu Prof. Yugang Wang Prof. Lei Jiang Prof. 《Chemphyschem》2010,11(4):859-864
Herein we demonstrate a fully abiotic smart single‐nanopore device that rectifies ionic current in response to the temperature. The temperature‐responsive nanopore ionic rectifier can be switched between a rectifying state below 34 °C and a non‐rectifying state above 38 °C actuated by the phase transition of the poly(N‐isopropylacrylamide) [PNIPAM] brushes. On the rectifying state, the rectifying efficiency can be enhanced by the dehydration of the attached PNIPAM brushes below the LCST. When the PNIPAM brushes have sufficiently collapsed, the nanopore switches to the non‐rectifying state. The concept of the temperature‐responsive current rectification in chemically‐modified nanopores paves a new way for controlling the preferential direction of the ion transport in nanofluidics by modulating the temperature, which has the potential to build novel nanomachines with smart fluidic communication functions for future lab‐on‐chip devices. 相似文献
970.
Three new dammarane‐type triterpene saponins, 1 – 3 , together with three known compounds, 4 – 6 , were isolated from the aerial parts of Gynostemma pentaphyllum (Thunb.) Makino . By means of chemical and spectroscopic methods, their structures were established as (20S)‐3β,20,21‐trihydroxydammara‐23,25‐diene 3‐O‐[α‐L ‐rhamnopyranosyl‐(1→2)] [β‐D ‐xylopyranosyl‐(1→3)]‐β‐D ‐glucopyranosyl‐21‐O‐β‐D ‐glucopyranoside ( 1 ), (20R,23R)‐3β,20‐dihydroxy‐19‐oxodammar‐24‐en‐21‐oic acid 21,23‐lactone 3‐O‐[α‐L ‐rhamnopyranosyl‐(1→2)] [β‐D ‐xylopyranosyl‐(1→3)]‐α‐L ‐arabinopyranoside ( 2 ), and (21S,23S)‐3β,20ξ,21,26‐tetrahydroxy‐19‐oxo‐21,23‐epoxydammar‐24‐ene 3‐O‐[α‐L ‐rhamnopyranosyl‐(1→2)] [β‐D ‐xylopyranosyl‐(1→3)]‐α‐L ‐arabinopyranoside ( 3 ). 相似文献