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   2615篇
  免费   61篇
  国内免费   504篇
化学   2819篇
晶体学   17篇
力学   8篇
综合类   14篇
数学   2篇
物理学   320篇
  2024年   1篇
  2023年   25篇
  2022年   45篇
  2021年   32篇
  2020年   45篇
  2019年   53篇
  2018年   73篇
  2017年   78篇
  2016年   66篇
  2015年   70篇
  2014年   112篇
  2013年   252篇
  2012年   152篇
  2011年   140篇
  2010年   123篇
  2009年   160篇
  2008年   156篇
  2007年   168篇
  2006年   155篇
  2005年   174篇
  2004年   162篇
  2003年   117篇
  2002年   91篇
  2001年   75篇
  2000年   79篇
  1999年   72篇
  1998年   59篇
  1997年   69篇
  1996年   47篇
  1995年   56篇
  1994年   48篇
  1993年   36篇
  1992年   49篇
  1991年   27篇
  1990年   25篇
  1989年   24篇
  1988年   11篇
  1987年   12篇
  1986年   7篇
  1985年   4篇
  1984年   8篇
  1983年   2篇
  1982年   6篇
  1981年   3篇
  1980年   1篇
  1979年   6篇
  1978年   1篇
  1977年   2篇
  1966年   1篇
排序方式: 共有3180条查询结果,搜索用时 0 毫秒
971.
Two syntheses of 7E,9Z-dodecadienyl acetate from 1,3-butadiyne were carried out using either 2E,4Z-heptadienyl acetate or 1-bromo-3E,5Z-octadiene as the key intermediates. The latter underwent organocopper cross-coupling with the respective complementary Grignard reagents (prepared from the corresponding 1-tert-butoxy--chlorohydrins) as alkylating agents.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 766–768, April, 1993.  相似文献   
972.
    
The kinetics of Tl(III) acetate oxidation of semicarbazones of benzaldehyde and substituted benzaldehydes have been investigated in aqueous (90%) acetic acid. Electron-releasing substituents in the phenyl ring enhance the rate of the reaction, whereas electron-withdrawing substituents retard the rate. The rho value of −1 favours electron-deficient transition state. To account for the major product of the reaction,viz respective benzaldehyde, a suitable scheme involving N-thallated intermediate has been proposed  相似文献   
973.
In this article we determine the miscibility of azobenzene derivative (poly(4‐(N‐(2‐methacryloyloxyethyl)‐N‐ethylamino)‐4′‐nitroazobenzene)90‐co‐(methyl methacrylate)10)/poly(vinyl acetate) (PVAc) and azobenzene derivative/poly(vinyl chloride) (PVC) blends using Fourier Transform infrared (FT‐IR) spectroscopy. With this method we can clearly identify the exact interactions responsible for miscibility. In the azobenzene derivative 50:50PVAc blend new peaks were evident at 2960, 2890, 1237 and 959 cm?1, these peaks depict miscible interactions. These wavenumbers indicate that the miscible interactions occurring are from the C? H stretching band, the vinyl acetate C?O, conjugated to the ester carbonyl, the cis‐transformation N?N stretch frequency and the acetate ester weak doublet. The azobenzene derivative 80:20PVC blend display peaks identical in profile to the blend homopolymers, indicating no miscible interactions. However, this could be due to overlapping of peaks within the same wavenumber region, making resolution difficult. This research demonstrates FT‐IR can deduce favorable interactions for miscibility and therefore numerous miscible blends can successfully be calculated if possessing the same groups responsible for miscibility. This paves the way for a new generation of designer optical materials with the desired properties. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   
974.
制备了交联聚4-乙烯基吡啶及碘甲烷,溴乙烷季铵化的聚4-乙烯基吡啶负载钯催化剂,考察了它们催化丙烯酸甲酯加氢反应的性能。对碘甲烷季铵化聚4-乙烯基吡啶负载钯催化剂,其催化加氢活性随载体季铵化程度的增加而减小,并且当载体季铵化程度低于80%时,其催化加氢活性比聚4-乙烯基吡啶负载催化剂高。实验结果表明催化剂制备条件对催化性能有很大影响。  相似文献   
975.
(4E)-5-(3-Chlorophenyl)-N-(4-chlorophenyl)-2-diazo-3-oxopent-4-enoic acid amide (5) was synthesized from p-chloroaniline to N-(4-chlorophenyl)-2-diazo-3-oxo-butyramide (4) with 3-chlorobenzaldehyde. The yielded product 5 was investigated with X-ray crystallographic, NMR, MS, and IR techniques. Compound 5 (C17H11Cl2N3O2, Formula wt = 360.19), crystallizes in the monoclinic space group P21/c with unit cell parameters a = 10.516(2), b = 17.996(4), c = 8.902(2) Å, α = 90.00, β = 105.36(3), γ = 90.00°. V = 1624.5(6) Å3, Z = 4, D x = 1.473 Mg m?3. The final R was 0.0511.  相似文献   
976.
溶胶-凝胶法制备纳米ZnO薄膜的结构和光学性质   总被引:1,自引:2,他引:1  
采用溶胶-凝胶方法制备纳米ZnO薄膜。X射线衍射(XRD)结果表明,薄膜具有六角纤锌矿多晶结构,且随水解时间的增加,粒子尺寸逐渐增大。室温下,观察到近带边紫外发射和较强的可见区(2.45eV)发射。红外吸收光谱研究表明ZnO薄膜表面存在单齿、双齿和桥状结构的醋酸锌副产物,阐述了不同结构醋酸锌副产物对ZnO薄膜可见区发光性质的影响。  相似文献   
977.
2-Acetoxy-3-(3-methoxy-4-acetoxy-5-chlorophenyl)-propenoic acid 2 was synthesized from the azlactone of 5-chlorovanillin and its structure confirmed by X-ray crystallography and nuclear magnetic resonance methods. Compound 2 crystallized with a molecule of acetic acid in the P-1(#2) space group (Z = 2) and with cell dimensions a = 6.303(2), b = 9.779(1), c = 15.528(3) Å, = 101.46(2), = 100.71(2) and = 90.21(2)°. This study reveals the formation of an -acetoxy propenoic acid with a trans extended side acid side chain conformation. The 1H and 13C NMR spectral values of 2 also show the existence of the enolic ester in solution.  相似文献   
978.
Commercially available cellulose (Avicell PH101) was successfully acylated under homogeneous solution conditions by the following procedure: 2.0 g of cellulose were stirred with 75 mL of N,N‐dimethylacetamide for 1 h at 150°C, 3.5 g of LiCl were added, the temperature was raised to 170°C, ca. 18.5 mL of the solvent were distilled and the suspension was cooled to room temperature and stirred overnight. The temperature of the clear cellulose solution was raised to 110°C, kept at that temperature for 1 h, an acid anhydride was added and the solution stirred at 110°C for additional 4 h. Acetates, propionates, butyrates, and acetate/propionate mixed ester were prepared with excellent control of the degree of substitution, DS, 1 to 3 for acetates, 2 and 3 for propionates and butyrates, and 3 for acetate/propionate. The degree of polymerization of cellulose is negligibly affected under these reaction conditions. The distribution of the acetyl moiety among the three OH groups of the anhydroglucose unit shows a preference for the C6 position. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1357–1363, 1999  相似文献   
979.
Critical temperatures for polystyrene/methyl acetate (PS/MA) and polystyrene/ethyl formate (PS/EF) solutions were measured at positive (PS/MA, PS/EF) and negative (PS/MA) pressure. The results confirm that solvent quality is sensitive to pressure; some solvents, designated Θ, at Pnominal ∼ 0, undergo a Θ-to-poor transition at negative pressure, and others, nominally designated “poor,” show a poor-to-Θ transition at positive pressure. Thus, any dichotomous division into sets of “poor” and “Θ” solvents is inaccurate, unless it accounts for the effects of pressure and other variables on solvent quality. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1251–1259, 1997  相似文献   
980.
A new method of synthesis of acetic acid in water has been developed from the carboxylation of methane with carbon monoxide using lanthanide catalysts. Ytterbium(III) acetate has been found to be the most active catalyst among the compounds of the lanthanide series in the carboxylation reaction of methane with carbon monoxide. Sodium hypochlorite or hydrogen peroxide was used as the oxidant in this reaction. Sodium hypochlorite exhibited more favorable activity than hydrogen peroxide in the reaction. The catalytic activity was improved by the addition of transition-metal salts such as manganese(II) acetate. The best result has been found at a ratio of manganese(II) acetate to ytterbium(III) acetate of 1:10. The optimum reaction conditions (reaction temperature, 40 °C; time, 20 h; methane, 20 atm; carbon monoxide, 5 atm) have been obtained. © 1998 John Wiley & Sons, Ltd.  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