全文获取类型
收费全文 | 7237篇 |
免费 | 668篇 |
国内免费 | 3690篇 |
专业分类
化学 | 10811篇 |
晶体学 | 43篇 |
力学 | 21篇 |
综合类 | 96篇 |
数学 | 8篇 |
物理学 | 616篇 |
出版年
2024年 | 9篇 |
2023年 | 126篇 |
2022年 | 210篇 |
2021年 | 275篇 |
2020年 | 384篇 |
2019年 | 396篇 |
2018年 | 348篇 |
2017年 | 344篇 |
2016年 | 350篇 |
2015年 | 328篇 |
2014年 | 457篇 |
2013年 | 652篇 |
2012年 | 452篇 |
2011年 | 525篇 |
2010年 | 444篇 |
2009年 | 457篇 |
2008年 | 546篇 |
2007年 | 540篇 |
2006年 | 487篇 |
2005年 | 483篇 |
2004年 | 510篇 |
2003年 | 423篇 |
2002年 | 339篇 |
2001年 | 330篇 |
2000年 | 328篇 |
1999年 | 233篇 |
1998年 | 233篇 |
1997年 | 249篇 |
1996年 | 184篇 |
1995年 | 201篇 |
1994年 | 164篇 |
1993年 | 201篇 |
1992年 | 121篇 |
1991年 | 66篇 |
1990年 | 66篇 |
1989年 | 45篇 |
1988年 | 32篇 |
1987年 | 17篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 6篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1972年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 24 毫秒
991.
Yi Yan Jiuyang Zhang Perry Wilbon Yali Qiao Chuanbing Tang 《Macromolecular rapid communications》2014,35(21):1840-1845
In the last decades, metallopolymers have received great attention due to their various applications in the fields of materials and chemistry. In this article, a neutral 18‐electron exo‐substituted η4‐cyclopentadiene CpCo(I) unit‐containing polymer is prepared in a controlled/“living” fashion by combining facile click chemistry and ring‐opening metathesis polymerization (ROMP). This Co(I)‐containing polymer is further used as a heterogeneous macromolecular catalyst for atom transfer radical polymerization (ATRP) of methyl methacrylate and styrene.
992.
Kohei Tada Kohei Sakata Satoru Yamada Kazuyuki Okazaki Yasutaka Kitagawa Takashi Kawakami 《Molecular physics》2014,112(3-4):365-378
Residual chlorines, which originate from HAuCl4, enhance the aggregation of gold (Au) nanoparticles and clusters, preventing the generation of highly active supported Au catalysts. However, the detailed mechanism of residual-chlorine-promoted aggregation of Au is unknown. Herein to investigate this mechanism, density functional theory (DFT) calculations of Au and Cl adsorption onto a reduced rutile TiO2 (110) surface were performed using a generalised gradient approximation Perdew, Burke, and Ernzerhof formula (GGA–PBE) functional and plane-wave basis. Although both Au and Cl atoms prefer to mono-absorb onto oxygen defect sites, Cl atoms have a stronger absorption onto a reduced TiO2 (110) surface, abbreviated as rTiO2 (110) in the following, than Au atoms. Additionally, co-adsorption of a Cl atom and a Au atom or Au nanorod onto a rTiO2 surface was investigated; Cl adsorption onto an oxygen defect site weakens the interaction between a Au atom or Au nanorod and rTiO2 (110) surface. The calculation results suggest that the depletion of interaction between Au and rTiO2 surface is due to strong interaction between Cl atoms at oxygen defect sites and neighbouring bridging oxygen (OB) atoms. 相似文献
993.
《Ultrasonics sonochemistry》2014,21(3):1155-1165
The bimetallic Pd–Cu nanocatalysts with different Pd loadings and ultrasonic irradiation times were sonochemically synthesized and their activities toward CO oxidation were investigated. XRD, FESEM, TEM, BET, FTIR and TG-DTG techniques were employed in nanocatalysts characterization. XRD data confirmed formation of CuAl2O4 spinel with an average crystallite size of 4.9 nm. FESEM images revealed more uniform pattern and also fewer agglomerations were observed by increasing ultrasonic irradiation time. In agreement with FESEM result, TEM images depicted nanoparticles and uniform dispersion of active phase over alumina. BET surface analysis showed that increasing the Pd loading has no significant effect on surface area; whereas by increasing irradiation time the surface area increases slightly. Catalytic performance tests of synthesized samples showed that Pd(1.5%)–Cu(20%)/Al2O3 with 95 min ultrasonic irradiation time had the best activity over the course of reaction. In addition, increasing CO at feed composition revealed that among synthesized nanocatalysts with 0.5%, 1% and 1.5% of Pd, synthesized sample with 1.5% of Pd had the best low-temperature activity. 相似文献
994.
《Ultrasonics sonochemistry》2014,21(5):1752-1762
The aim of this study is to investigate modified TiO2 doped with C4H4O6HK as heterogeneous solid base catalyst for transesterification of non-edible, Silybum marianum oil to biodiesel using methanol under ultrasonication. Upon screening the catalytic performance of modified TiO2 doped with different K-compounds, 0.7 C4H4O6HK doped on TiO2 was selected. The preparation of the catalyst was done using incipient wetness impregnation method. Having doped modified TiO2 with C4H4O6HK, followed by impregnation, drying and calcination at 600 °C for 6 h, the catalyst was characterized by XRD, FTIR, SEM, BET, TGA, UV and the Hammett indicators. The yield of the biodiesel was proportional to the catalyst basicity. The catalyst had granular and porous structures with high basicity and superior performance. Combined conditions of 16:1 molar ratio of methanol to oil, 5 wt.% catalyst amount, 60 °C reaction temperature and 30 min reaction time was enough for maximum yield of 90.1%. The catalyst maintained sustained activity after five cycles of use. The oxidative stability which was the main problem of the biodiesel was improved from 2.0 h to 3.2 h after 30 days using ascorbic acid as antioxidant. The other properties including the flash point, cetane number and the cold flow ones were however, comparable to international standards. The study indicated that Ti-0.7-600-6 is an efficient, economical and environmentally, friendly catalyst under ultrasonication for producing biodiesel from S. marianum oil with a substantial yield. 相似文献
995.
Synthesis of multi-walled carbon nanotubes using CoMnMgO catalysts through catalytic chemical vapor deposition 下载免费PDF全文
The Co Mg O and Co Mn Mg O catalysts are prepared by a co-precipitation method and used as the catalysts for the synthesis of carbon nanotubes(CNTs) through the catalytic chemical vapor deposition(CCVD). The effects of Mn addition on the carbon yield and structure are investigated. The catalysts are characterized by temperature programmed reduction(TPR) and X-ray diffraction(XRD) techniques, and the synthesized carbon materials are characterized by transmission electron microscopy(TEM) and thermo gravimetric analysis(TG). TEM measurement indicates that the catalyst Co Mg O enclosed completely in the produced graphite layer results in the deactivation of the catalyst. TG results suggest that the Co Mn Mg O catalyst has a higher selectivity for CNTs than Co Mg O. Meanwhile, different diameters of CNTs are synthesized by Co Mn Mg O catalysts with various amounts of Co content, and the results show that the addition of Mn avoids forming the enclosed catalyst, prevents the formation of amorphous carbon, subsequently promotes the growth of CNTs, and the catalyst with decreased Co content is favorable for the synthesis of CNTs with a narrow diameter distribution.The Co Mn Mg O catalyst with 40% Co content has superior catalytic activity for the growth of carbon nanotubes. 相似文献
996.
最近的理论研究筛选出CuCs掺杂Ag基催化剂是一种高效的乙烯环氧化催化剂[ACS Catal. 11,3371 (2021)]. 然而,该工作是基于研究表面建模预测Ag基催化剂的性能,在实际反应过程中,Ag基催化剂是颗粒状的. 本文结合密度函数理论、Wulff构造理论和微观动力学分析来研究Ag基催化剂在颗粒模型上的催化性能. 研究表明,CuCs掺杂Ag基催化剂在选择性和活性方面都优于纯Ag基催化剂,这一点通过实验得到了证明. 进一步地表征分析发现,CuCs掺杂能促进颗粒的生长以及颗粒的分散,从而形成富含晶界的Ag颗粒. 此外,CuCs促进了催化剂表面亲电氧的形成,这均有利于环氧乙烷的形成和解吸. 本工作为理论与实验相结合的催化剂设计提供了一个案例研究. 相似文献
997.
Simple catalytic systems for the Suzuki—Miyaura reaction of aryl chlorides under microwave conditions were tested. Microwave
irradiation facilitated the reaction course. The catalyst, base, and solvent effect were studied and two reaction systems
offered reasonable to high yields within a short time. 相似文献
998.
999.
Ana R. Rodríguez 《Tetrahedron letters》2005,46(21):3623-3627
The first total synthesis of 7(S),17(S)-Resolvin D5, a lipid mediator derived from docosahexaenoic acid, has been achieved. The chiral centers were generated via a Co-salen hydrolytic kinetic resolution of a terminal epoxide with >99% ee. Key steps include Takai olefination, Pd0/CuI coupling and simultaneous deprotection and ester cleavage with lipase from Candida rugosa. 相似文献
1000.
Tao JIANG ? Hong Xia CHEN Ying Nan NING Wei CHEN School of Chemistry & Chemical Daqing Petroleum Institute Daqing SINOPEC Beijing Research Institute of Chemical Industry Beijing 《中国化学快报》2006,(3)
Ethylene trimerization has been attracted much attention because of their high selectivity towards 1-hexene1-6. The common belief is that ethylene tetramerization is improbable. This is because if ethylene tetramerization proceeds via the same mechanism o… 相似文献