全文获取类型
收费全文 | 3709篇 |
免费 | 1099篇 |
国内免费 | 941篇 |
专业分类
化学 | 5123篇 |
晶体学 | 82篇 |
力学 | 128篇 |
综合类 | 32篇 |
数学 | 109篇 |
物理学 | 275篇 |
出版年
2024年 | 14篇 |
2023年 | 91篇 |
2022年 | 169篇 |
2021年 | 211篇 |
2020年 | 453篇 |
2019年 | 288篇 |
2018年 | 289篇 |
2017年 | 221篇 |
2016年 | 380篇 |
2015年 | 356篇 |
2014年 | 331篇 |
2013年 | 470篇 |
2012年 | 323篇 |
2011年 | 292篇 |
2010年 | 233篇 |
2009年 | 196篇 |
2008年 | 218篇 |
2007年 | 185篇 |
2006年 | 203篇 |
2005年 | 162篇 |
2004年 | 132篇 |
2003年 | 142篇 |
2002年 | 85篇 |
2001年 | 72篇 |
2000年 | 34篇 |
1999年 | 33篇 |
1998年 | 21篇 |
1997年 | 36篇 |
1996年 | 15篇 |
1995年 | 10篇 |
1994年 | 16篇 |
1993年 | 14篇 |
1992年 | 11篇 |
1991年 | 4篇 |
1990年 | 8篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 2篇 |
1986年 | 7篇 |
1985年 | 5篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 3篇 |
排序方式: 共有5749条查询结果,搜索用时 359 毫秒
21.
A. V. Fionov M. V. Burova E. A. Tveritinova I. V. Krylova 《Russian Chemical Bulletin》2006,55(4):639-644
Donor-acceptor properties of alumina modified with lithium cations were studied by ESR of paramagnetic complexes of adsorbed
anthraquinone. The results were compared with the data on negative ion emission (exoemission) accompanying the decomposition
of isopropyl alcohol in the adsorption layer. The data on the activity measured by the pulse microcatalytic technique in isopropyl
alcohol decomposition are discussed. Small additives of lithium were found to promote catalytic activity of the samples. The
role of acid and basic sites in isopropyl alcohol decomposition was discussed.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 616–621, April, 2006. 相似文献
22.
23.
J?rg Libuda 《Chemphyschem》2004,5(5):625-631
Reaction kinetics on nanometer-scale particles are different from those on extended surfaces of bulk materials. This fact has been utilized for a long time to empirically maximize the performance of heterogeneous catalysts, but the understanding of the underlying effects is poor at the microscopic level. Modern molecular beam-based methods, however, allow us to derive very detailed kinetic information on catalytically active surfaces. In combination with structurally highly controlled model catalysts, microscopic insights into the activity and selectivity of specific reaction centers on catalyst nanoparticles can be obtained. This combined approach is illustrated through simple model reactions. 相似文献
24.
CoOx改性TiO2光催化剂的制备、优化及其光催化分解水析氢性能研究 总被引:4,自引:0,他引:4
利用热分解法制备了不同掺杂量的CoOx-TiO2系列光催化剂, 并优化了制备方法。在用XRD、TEM和XPS等技术对催化剂表征的基础上,探讨了乙醇作为电子给体时CoOx改性对P25-TiO2光催化分解水析氢性能的影响。本文还利用连续瞬态电流-时间响应和循环伏安法等电化学方法,考察了CoOx-TiO2改性系列光催化剂光照条件下电流响应强度、起始光电响应特性差别及光催化析氢电位变化等光电化学性能。光催化性能实验结果表明,适当掺杂CoOx(110~400 ℃焙烧时最佳掺杂量约为0.4wt% Co,500 ℃焙烧时最佳掺杂量约为0.6~0.8wt% Co)能够显著地提高TiO2析氢光催化性能,产氢速率提高2个数量级。另外在适当温度下的热处理,能够改善催化剂光催化性能(最佳处理温度为300~400 ℃)。光电化学性能评价结果表明,适当掺杂CoOx和适当温度下焙烧能在催化剂表面形成较多的析氢活性物种——含钴复合物,能有效增强催化剂光电流响应强度,并使得析氢光还原电位向有利于析氢方向位移。高含量掺杂CoOx会在催化剂表面形成某种碳化物物种,大量含钴复合物活性中心的出现会降低光吸收效率,增加TiO2表面光生电子-空穴复合率而使催化剂失去析氢光催化性能。 相似文献
25.
本文综述了半导体氧化物光催化裂解水制氢的反应机理,以及近年来半导体光催化裂解水制氢的研究进展。讨论了各种因素对材料光催化性能的影响以及改性方法,并对今后的研究方向提出了一些建议。 相似文献
26.
27.
28.
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
29.
The reaction of diethyl 3-ethoxycarbonyl-2-methyl-2-propenylphosphonate (1a) with 3-methylbutanal (2) in heterogeneous MOH (solid)-benzene systems in the presence of 5–10 mol.% of benzyltriethylammonium chloride (BTEAC) gives the reaction product (3) with a higher, for M=K, or lower, for M=Li, ratio of 2E,4E and 2Z,4E-stereoisomers than that observed in the absence of BTEAC. Tetrabutylammonium bromide (TBAB) as a catalyst of the reaction1a + 2 3 in the system KOH (solid)-wet benzene leads to a higher [2E,4E-3][2Z,4E-3] ratio than BTEAC; this ratio grows from 4456 without TBAB to 8020 at 100 mol.% of TBAB. In the latter case the stereochemical outcome of the reaction is similar to that obtained when tetrabutylammonium hydroxide in dry benzene is used as the deprotonating agent. The corresponding diisopropyl phosphonate (1b) and 3,7-dimethyloctanal (4a) interact in the system KOH (solid)-wet benzene-TBAB to give hydroprene (5) containing 88 % of the 2E,4E-isomer (5a) while in the case of 1 equiv. of [(n-Bu4)N]OH in dry benzene the content of5a is 92 %. Diisopropyl 3-isopropoxycarbonyl-2-methyl-2-propenylphosphonate (1c) and 7-methoxy-3,7-dimethyloctanal (4b) under either of these conditions afford methoprene (6) containing 93 % of the 2E,4E-isomer.Part 6, see ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1094–1098, June, 1993. 相似文献
30.