全文获取类型
收费全文 | 4697篇 |
免费 | 705篇 |
国内免费 | 862篇 |
专业分类
化学 | 3041篇 |
晶体学 | 111篇 |
力学 | 137篇 |
综合类 | 27篇 |
数学 | 40篇 |
物理学 | 1087篇 |
综合类 | 1821篇 |
出版年
2024年 | 8篇 |
2023年 | 32篇 |
2022年 | 102篇 |
2021年 | 108篇 |
2020年 | 139篇 |
2019年 | 131篇 |
2018年 | 115篇 |
2017年 | 149篇 |
2016年 | 201篇 |
2015年 | 197篇 |
2014年 | 240篇 |
2013年 | 432篇 |
2012年 | 361篇 |
2011年 | 331篇 |
2010年 | 293篇 |
2009年 | 259篇 |
2008年 | 282篇 |
2007年 | 315篇 |
2006年 | 314篇 |
2005年 | 267篇 |
2004年 | 272篇 |
2003年 | 266篇 |
2002年 | 183篇 |
2001年 | 155篇 |
2000年 | 130篇 |
1999年 | 131篇 |
1998年 | 104篇 |
1997年 | 101篇 |
1996年 | 90篇 |
1995年 | 101篇 |
1994年 | 81篇 |
1993年 | 61篇 |
1992年 | 67篇 |
1991年 | 46篇 |
1990年 | 46篇 |
1989年 | 36篇 |
1988年 | 30篇 |
1987年 | 21篇 |
1986年 | 23篇 |
1985年 | 9篇 |
1984年 | 9篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1973年 | 4篇 |
1972年 | 1篇 |
1955年 | 5篇 |
排序方式: 共有6264条查询结果,搜索用时 906 毫秒
61.
A titanium dioxide sol with narrow particle size distribution was synthesized using TiCl4 as the starting material. The sol was prepared by a process where HCl was added to a gel of hydrated titanium oxide to dissolve it. The resulting aqueous titanic acid solution was heated to form titanium dioxide sol. The effects of preparation parameters were investigated. TiCl4 was slowly added to distilled water at 5°C. Aqueous solution of sodium hydroxide was added to adjust the pH of the system to 8–12. After aging for a period of time, the peptized sol was filtered and sufficiently washed. The filtered cake was repulped in water. Hydrochloric acid was slowly added to the solution with stirring. After condensation reaction and crystallization, a transparent sol with suspended TiO2 was formed. XRD results show that the crystalline phase was anatase. The suspended TiO2 particles were rhombus primary particles with the major axis ca. 20 nm and the minor axis ca. 5 nm. The TiO2 particles prepared at pH 8 had the largest surface area of 141 cm3/g and it was microporous. The compositions of the solution which yielded the smallest suspended TiO2 particles were TiO2:HCl (35% HCl) = 1:1 (molar ratio), concentration of TiO2 = 10%. Hydroxypropyl cellulose with viscosity of 150–400 cps was added as a dispersant. The sol was excellent in dispersibility and long-term stability. Transparent thin films could be obtained through dip-coating glass substrate in the sol. The dip-coating on glass can be less than three times to have one monolayer TiO2. The transparent TiO2 thin film had strong hydrophilicity after being illuminated by UV light. 相似文献
62.
Wataru Uchikawa 《Tetrahedron letters》2004,45(49):9037-9040
ω-Vinylimines reacted with a Ti(O-i-Pr)4/2i-PrMgX reagent to generate the corresponding azatitanacyclopentanes in quantitative yield, which in turn reacted with H2O, I2 and O2 to give 2-methyl-, 2-iodomethyl-, 2-hydroxymethyl-1-aminocyclic compounds, respectively. The azatitanacyclopentanes thus generated reacted with formaldehyde to afford the corresponding 2,3-annulated pyrrolidines in good yield. 相似文献
63.
超声波作用下羰基化合物的还原偶联反应 总被引:5,自引:0,他引:5
超声波在一些有机反应中有明显的促进作用「“,Luche等认为超声波的作用有两种主要类型[’j.一类是涉及电子转移的自由基或离子一自由基型反应,由于超声波的空化作用所产生的局部高温高压可以促进电子转移,从而明显地加快反应速度;对大多数离子型反应,超声波只能起到机械的物理作用.我们曾报道偕二卤化合物在Li、Na、Mg等金属作用下产生卡宾的反应是单电子转移过程,该反应可由于超声辐射而大大加速[‘];而在芳基叔脚的断裂反应中,超声波可以促进三苯基俄与金属馊反应生成二苯基脚负离子,但对它进一步的亲核取代反应却没有明… 相似文献
64.
《Mendeleev Communications》2020,30(2):192-194
- Download : Download high-res image (135KB)
- Download : Download full-size image
65.
简要总结了我们在C=C及C=O双键低温加氢双金属催化剂方面的最新研究成果. 首先, 我们以环己烯加氢为探针反应, 证明了平行使用多种研究手段的重要性, 包括单晶表面的基础研究与DFT计算, 多晶表面的合成与表征, 负载型催化剂的制备与性能测试等. 其次, 总结了双金属催化剂在其他加氢反应, 如丙烯醛C=O双键的选择性加氢, 苯的低温加氢, 以及乙炔的选择性加氢等反应中的应用. 最后, 讨论了利用金属碳化物代替贵金属Pt以减少双金属催化剂中Pt用量的可能性. 相似文献
66.
简要总结了我们在C=C及C=O双键低温加氢双金属催化剂方面的最新研究成果. 首先, 我们以环己烯加氢为探针反应, 证明了平行使用多种研究手段的重要性, 包括单晶表面的基础研究与DFT计算, 多晶表面的合成与表征, 负载型催化剂的制备与性能测试等. 其次, 总结了双金属催化剂在其他加氢反应, 如丙烯醛C=O双键的选择性加氢, 苯的低温加氢, 以及乙炔的选择性加氢等反应中的应用. 最后, 讨论了利用金属碳化物代替贵金属Pt以减少双金属催化剂中Pt用量的可能性. 相似文献
67.
Thermal decomposition kinetics of titanium hydride and Al alloy melt foaming process 总被引:3,自引:0,他引:3
YANG Donghui* HE Deping*& YANG Shangrun* . Department of Materials Science Engineering Southeast University Nanjing China . Department of Chemistry Nanjing University Nanjing China 《中国科学B辑(英文版)》2004,47(6):512-520
Metal foams, now one of research foci, are a newclass of materials with low densities and novel physi-cal, mechanical, thermal, electrical and acoustic prop-erties[1—8]. Demands from high-tech make Al alloyfoam, which has much higher specific strength than ofpure Al foam, the new development focus[9—11]. Melt foaming process is one of the approaches tofabricate Al foam and Al alloy foam and their porestructure (pore diameter and porosity) has close rela-tionship with the thermal decompo… 相似文献
68.
Chaozhang Huang 《Talanta》2007,73(2):274-281
Mesoporous titanium dioxide as a novel solid-phase extraction material for flow injection micro-column preconcentration on-line coupled with ICP-OES determination of trace metals (Co, Cd, Cr, Cu, Mn, Ni, V, Ce, Dy, Eu, La and Yb) in environmental samples was described. Possessing a high adsorption capacity towards the metal ions, mesoporous titanium dioxide has found to be of great potential as an adsorbent for the preconcentration of trace metal ions in samples with complicated matrix. The experimental parameters including pH, sample flow rate, volume, elution and interfering ions on the recovery of the target analytes were investigated, and the optimal experimental conditions were established. Under the optimized operating conditions, a preconcentration time of 90 s and elution time of 18 s with enrichment factor of 10 and sampling frequency of 20 h−1 were obtained. The detection limits of this method for the target elements were between 0.03 and 0.36 μg L−1, and the relative standard deviations (R.S.D.s) were found to be less than 6.0% (n =7, c =5 ng mL−1). The proposed method was validated using a certified reference material, and has been successfully applied for the determination of the afore mentioned trace metals in natural water samples and coal fly ash with satisfactory results. 相似文献
69.
70.
Rüdiger Beckhaus 《Angewandte Chemie (International ed. in English)》1997,36(7):686-713
The relation of thermodynamic stability and kinetic lability of σ-organometallic compounds of transition metals, together with an improved understanding of the subtle interactions between central metal, ligands, and substrates, has increased the chemist's ability to plan organometallic syntheses. This article presents new results on intermediary and isolable synthetic building blocks incorporating metal–ligand multiple bonds of electron-deficient transition metals; the main emphasis will be placed on compounds with titanium–carbon double bonds. This particular class of compounds is mainly generated by H-transfer reactions starting from readily accessible alkyl and alkenyl derivatives. The preparative use of [L2Ti(CHR2)R′] derivatives as sources for [L2Ti?CR2] intermediates will be discussed, as well as the nature of these intermediates. Application of the same approach to vinyltitanium compounds [L2Ti(CH?CH2)R] opens up an access to a short-lived metallaallene derivative [L2Ti?C?CH2] of an electron-deficient transition metal. The reactivity of these synthetic building blocks is mainly characterized by the nucleophilic properties of the α-C atoms as well as by the spatial orientation of the π-bonding planes. Numerous cycloaddition products with unsaturated substrates could be isolated and characterized for the first time by using [L2Ti?C?CH2] intermediates. Hence it is possible to compare the properties of a multitude of metallacyclic ring systems with those obtained from “Tebbe–Grubbs chemistry”, and in this context, the dependence of the properties of metallacyclic four-membered rings on the substitution pattern is discussed. This class of compounds includes the metallaoxetanes, which have been obtained for the first time by the cycloaddition of the [CpTi?C?CH2] intermediate with cumulenes and metal carbonyls. The differing cycloreversion behavior of these metallaoxetanes enables the differentiation of species exhibiting classical and nonclassical reactivity. The number and position of the exocyclic double bonds are the determining factors of the reactivity of the formed metallacycles. The discussion of the products obtained from titanium methylene and vinylidene building blocks is an up-to-date report on the formation and applications of carbene complexes and carbene intermediates of group 4 metals. 相似文献