首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   5026篇
  免费   682篇
  国内免费   957篇
化学   5252篇
晶体学   62篇
力学   202篇
综合类   22篇
数学   190篇
物理学   937篇
  2024年   13篇
  2023年   68篇
  2022年   189篇
  2021年   193篇
  2020年   216篇
  2019年   196篇
  2018年   184篇
  2017年   221篇
  2016年   325篇
  2015年   266篇
  2014年   323篇
  2013年   642篇
  2012年   375篇
  2011年   394篇
  2010年   328篇
  2009年   366篇
  2008年   333篇
  2007年   330篇
  2006年   289篇
  2005年   227篇
  2004年   196篇
  2003年   166篇
  2002年   115篇
  2001年   77篇
  2000年   74篇
  1999年   45篇
  1998年   57篇
  1997年   47篇
  1996年   45篇
  1995年   52篇
  1994年   44篇
  1993年   36篇
  1992年   34篇
  1991年   21篇
  1990年   18篇
  1989年   16篇
  1988年   13篇
  1987年   12篇
  1986年   8篇
  1985年   15篇
  1984年   11篇
  1982年   10篇
  1981年   5篇
  1980年   8篇
  1979年   12篇
  1978年   12篇
  1977年   10篇
  1976年   8篇
  1974年   6篇
  1973年   6篇
排序方式: 共有6665条查询结果,搜索用时 15 毫秒
101.
研究了以Bro¨nsted酸性离子液体1-正丁基-3-甲基咪唑磷酸二氢盐(BMImH2PO4)为催化剂,在不使用其它有机溶剂且无需脱水的条件下,于室温下进行醛(酮)与二元醇的缩合反应,得到了由中至高的转化率和高选择性.产物1,3-二口恶戊烷可以和反应物自动分层,后处理操作简单.过量的醇与离子液体可以多次重复使用,且醛(酮)的转化率无明显降低.  相似文献   
102.
103.
潘旭戴松元  王孔嘉 《中国化学》2007,25(11):1601-1603
For the fist time the preparation of a mostly solid-state high efficiem electro-conductive material comprising 1-methyl-3-propylimidazolium iodide (MPII), benzimidazole (BI), iodine and lithium iodide was reported. In this electrolyte, BI acts as not only additives but also gelators. With its significant electrochemical properties, an overall efficiency of 3.07% was achieved under AM 1.5 (100 mW/cm^2).  相似文献   
104.
Transport numbers for oxygen ions and protons are measured by an emf method in the system CaTi1?x FexO3?δ (x = 0.1–0.5) in the oxidizing and reducing atmospheres in the temperature interval 973–1173 K. It is shown that the compounds under study are mixed ion-electron conductors at small iron concentrations and electron conductors, at large iron contents. The proton conductivity in the compounds is very poor and does not exceed 0.5% in air. On the basis of the temperature dependences of transport numbers for ions and linear expansion, it is established that the CaTi0.9Fe0.1O3?δ system has a phase transition of a second order in a reducing environment at 1020–1050 K. The total and partial electron conductivities of CaTi0.9Fe0.1O3?δ are studied as a function of the partial pressure of oxygen at 1173 K. The nature of electroconduction in CaTi1?x FexO3?δ is discussed.  相似文献   
105.
The first report of the use of task-specific ionic liquid as synthetic equivalent of ionic liquid-phase matrice for the preparation of a small library of 4-thiazolidinones is reported in this paper. The starting (ethyleneglycol)ionic liquid-phase is functionalized in good yields with 4-(formylphenoxy)butyric acid by using usual esterification reaction conditions (DCC/DMAP as catalyst). The synthesis of the ionic liquid-phase bound 4-thiazolidinones was performed by a one-pot three-component condensation under microwave dielectric heating. The final cleavage under microwave/catalysis strategy provides the expected 4-thiazolidinones in high purity after flash-chromatography purification. According to the ionic liquid-phase organic synthesis (IoLiPOS) methodology, it was found that optimized reaction conditions were performed by standard analytical methods (NMR, TLC). The 1H, 13C NMR spectrum of some representative 4-thiazolidinones and ionic liquid-phase bound benzaldehyde are also reported.  相似文献   
106.
In this work we address the optimization of mixed conductivity in fluorite compounds based on zirconia. Phase relations of the new systems YbO1.5-NbO2.5-ZrO2, and CaO-NbO2.5-ZrO2 are presented. The limit of the cubic defect fluorite phase in YbO1.5-NbO2.5-ZrO2 closely resembles that of the system YO1.5-NbO2.5-ZrO2, whilst in CaO-NbO2.5-ZrO2 is narrow extending to include composition Ca0.255Nb0.15Zr0.595O1.82 at 1500°C. The influence of dopant ion size, charge and composition on ionic conduction is assessed and parallels are drawn with the systems YO1.5-NbO2.5-ZrO2 and YO1.5-TiO2-ZrO2. Comparison of these results with published data on the Ti containing systems CaO-TiO2-ZrO2, GdO1.5-TiO2-ZrO2 shows that the highest mixed conducting compositions can only be offered in the system YO1.5-TiO2-ZrO2 out of all the systems here studied.  相似文献   
107.
离子液体BMIBF4性质的研究   总被引:1,自引:0,他引:1  
用最大气泡法和韦氏天平法, 在278.15~343.15 K范围内测定了离子液体BMIBF4的表面张力和密度; 讨论了这个离子液体的体积性质和表面性质; 根据离子个头大又极不对称的特点, 提出了离子液体的空隙模型. 根据空隙模型计算的离子液体恒压热膨胀系数与实验值相比, 偏差在10%左右.  相似文献   
108.
王文昭  方唯硕 《有机化学》2007,27(12):1453-1462
综述了近8年来液相平行合成和组合合成中应用的不同技术, 包括可溶性载体、氟合成技术、离子液体、固相试剂树脂以及低聚乙烯二醇(OEG)衍生物的应用等几方面内容. 论述了它们的基本原理以及相关的应用实例, 并着重强调了目标化合物的分离纯化方法.  相似文献   
109.
Complex formation constants were determined potentiometrically (by a ISE-H+, glass electrode) in the systems, M2+ – Lz – H+ [M2+ = (C2H5)2Sn2+, Lz = malonate, glycinate and ethylenediamine] at t = 25 C and 0.1 mol-L−1I/ ≤ 1 mol-L−1 in NaClaq (0.1 mol-L−1I ≤ 0.75 mol-L−1 for the ethylenediamine system). Thermodynamic values of formation constants, at infinite dilution, are [± 95% confidence interval, Tβpqr refer to the equilibrium, pM2+ + qLz + rH+ = MpLqHr(2+z+r)]: for malonate, log10 Tβ110 = (5.47 ± 0.10); for glycinate, log10 Tβ110 = (9.54 ± 0.08), log10 Tβ111 = (12.97 ± 0.10); and for ethylenediamine, log10 Tβ110 = (10.47 ± 0.10), log10 Tβ120 = (16.17 ± 0.12) and log10 Tβ111 = (15.46 ± 0.10). The dependence on ionic strength of the formation constants was modeled by a simple Debye–Hückel type equation and by the SIT approach. By analyzing the stability of the species in the three different systems we found a simple additivity rule that can be expressed by the relationship: log10 K = 6.46 nN + 3.96 nO − 0.60 (nN2+ nO2), with a mean deviation, ε(log10 K) = 0.15 (K = equilibrium constant for the interaction of the organometal cation with the unprotonated or protonated ligand, nN = number of amino groups and nO = number of carboxylic groups of the ligand(s) involved in the formation reaction of complex species).  相似文献   
110.
The semiconductive properties of a series of pyrenyl polyenes of the type R-(CH=CH)n-R, are studied as a function of the adsorption of different amount of a vapour. With regard to the compensation temperature (T 0) the compounds are divided into two groups. For compounds which have odd number of double bonds,T 0 is infinite and in compounds having even number double bonds,T 0 is finite.T 0 for II2 is higher while that of II4 is lower than the experimental temperature. DifferentT 0 for the compounds having odd or even number of double bonds suggests a physical basis for the compensation rule, which we believe, is related to the molecular and crystalline structure of the compounds  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号