全文获取类型
收费全文 | 2645篇 |
免费 | 147篇 |
国内免费 | 530篇 |
专业分类
化学 | 3056篇 |
晶体学 | 27篇 |
力学 | 11篇 |
综合类 | 21篇 |
数学 | 1篇 |
物理学 | 206篇 |
出版年
2023年 | 16篇 |
2022年 | 26篇 |
2021年 | 37篇 |
2020年 | 52篇 |
2019年 | 62篇 |
2018年 | 56篇 |
2017年 | 62篇 |
2016年 | 81篇 |
2015年 | 87篇 |
2014年 | 81篇 |
2013年 | 306篇 |
2012年 | 144篇 |
2011年 | 146篇 |
2010年 | 134篇 |
2009年 | 136篇 |
2008年 | 135篇 |
2007年 | 198篇 |
2006年 | 165篇 |
2005年 | 156篇 |
2004年 | 169篇 |
2003年 | 142篇 |
2002年 | 110篇 |
2001年 | 114篇 |
2000年 | 111篇 |
1999年 | 105篇 |
1998年 | 71篇 |
1997年 | 63篇 |
1996年 | 52篇 |
1995年 | 42篇 |
1994年 | 50篇 |
1993年 | 55篇 |
1992年 | 38篇 |
1991年 | 28篇 |
1990年 | 15篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 7篇 |
1986年 | 11篇 |
1985年 | 3篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 7篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有3322条查询结果,搜索用时 77 毫秒
51.
Ryashentseva M. A. Egorova E. V. Trusov A. I. Antonyuk S. N. 《Russian Chemical Bulletin》2002,51(9):1698-1701
The activity and selectivity of mono- and bimetallic catalysts containing copper and rhenium on sibunite were studied in the decomposition of methanol to methyl formate (MF), water, H2, CO, and CO2at 200—400 °C. Methane is also formed on rhenium-containing catalysts at 300—400 °C. The dehydrogenating activity and selectivity to form MF are higher on the copper-sibunite catalysts than on the rhenium-sibunite samples. The introduction of 0.25% Re into the 4% copper-containing catalyst enhances its total activity and stability. 相似文献
52.
Noëlle Martine Gosselet Horacio NaranjoEstelle Renard Catherine AmielBernard Sebille 《European Polymer Journal》2002,38(4):649-654
An acrylic polymer with pendent adamantyl groups was synthesized and its properties in an aqueous solution with a β-cyclodextrin (βCD) epichlorhydrin polymer examined. Viscosity properties of precursor and modified polymers show differences at low concentrations, but not at higher concentration probably due to very important hydrogen bonds which prevent the formation of intermolecular hydrophobic bonds. The association of both complementary polymers through the inclusion of adamantyl groups is evidenced by phases separation occurrence. Phase diagrams were established at two different concentrations of polymers. We have shown a maximal association of both polymers at these two concentrations, for the same ratio βCD moles/adamantyl groups: 2.4. Salt addition favors this association and displaces the two phases zone to smaller concentrations of modified polymer. Further, 4-nitrophenol can be extracted by the concentrated phase resulting from mixture of solutions of guest and host polymers, pointing out the availability of the associated phase to trap organic molecules. 相似文献
53.
以糠酸、甲醇、氢气为原料,采用连续流动固定床微反应器,Pd-Ni/γ-Al2O3为催化剂,使糠酸一步加氢甲酯化生成α-四氢糠酸甲酯.研究了反应的温度,压力,气、液体流速,进料流量等因素对催化反应的影响.结果表明:在1.5MPa,250℃,氢气空速3300 h-1,液体空速3.0 h-1(氢油比为50)时,糠酸转化率为96.6%,四氢糠酸甲酯的选择性97.2.0%,产率94.0%.催化剂稳定性较好,连续运转280小时后未见活性下降.该反应体系活性高,选择性好,反应压力低,催化剂性能稳定,操作简单,产物易分离. 相似文献
54.
55.
56.
环丙基甲基酮是合成环丙氟哌酸类广谱抗菌药物和抗艾滋特效药依法韦仑的重要中间体,在医药、化工及农林等领域均有广泛应用。环丙基甲基酮的合成方法大致分为4类,分别以酮类、酯类、醇类及呋喃有机物为原料,在一定条件下生成环丙基甲基酮粗品,经萃取及精馏得到环丙基甲基酮纯品。本文综述了环丙基甲基酮的合成研究进展。 相似文献
57.
Aza Diels—Alder reactions of methyl trifluoropyruvate sulfonyl- and phosphorylimines with 1,3-dienes
2-Trifluoromethyl-substituted 1,2,3,6-tetrahydro-2-pyridyl carboxylates were synthesized in one step by the aza Diels—Alder reactions of methyl trifluoropyruvate sulfonyl- and phosphorylimines with 1,3-dienes. 相似文献
58.
59.
K. Sekine 《Colloid and polymer science》1987,265(12):1054-1060
Dielectric measurements were carried out on aqueous suspensions of poly(methyl methacrylate) microcapsules charged with KCl solutions to examine the KCl permeability of the microcapsules. The specimens exhibited three kinds of dielectric relaxation. Two kinds of dielectric relaxation were observed immediately after washing the specimens with distilled water. These overlapped each other when the specimen was left standing in the measuring cell for a long time. Their relaxation frequencies were affected by the KCl concentration of the suspending medium in which the specimen was kept before washing. Another relaxation was observed distinctly when the spepcimens were washed thoroughly with distilled water. Its relaxation frequency was affected by the KCl concentration of the charged solution. These results were explained by assuming that the specimens were mixtures of KCl-permeable and KCl-impermeable capsules. The theoretical analysis procedure was devised in the light of dielectric theory of interfacial polarization to estimate the parameters which characterize the structure of the specimens. 相似文献
60.
Acyclic nitrones react with dimethyl acetylenedicarboxylate (DMAD) to give stable isoxazolines, from which the ones that contain electron-donating aromatic rings at the C3 position (R1) were shown to undergo unprecedented fragmentation at room temperature, giving the R1-aldehyde and inseparable product mixtures, probably due to the formation of highly reactive species such as iminocarbenes. Attempts to convert the isoxazolines to the corresponding stable azomethine ylides, by refluxing in toluene, again led to the same product mixtures as above (e.g., the room temperature decomposition). Isoxazolines when reacted with methoxide at room temperature afforded highly functionalised diastereomeric mixtures. Also, isoxazolines, when reacted with propylamine, gave the corresponding amides regioselectively, all of which were more stable than the parent isoxazolines. 相似文献