全文获取类型
收费全文 | 3518篇 |
免费 | 249篇 |
国内免费 | 772篇 |
专业分类
化学 | 1758篇 |
晶体学 | 7篇 |
力学 | 291篇 |
综合类 | 14篇 |
数学 | 123篇 |
物理学 | 211篇 |
综合类 | 2135篇 |
出版年
2024年 | 24篇 |
2023年 | 49篇 |
2022年 | 56篇 |
2021年 | 80篇 |
2020年 | 97篇 |
2019年 | 84篇 |
2018年 | 92篇 |
2017年 | 100篇 |
2016年 | 132篇 |
2015年 | 132篇 |
2014年 | 166篇 |
2013年 | 207篇 |
2012年 | 232篇 |
2011年 | 270篇 |
2010年 | 210篇 |
2009年 | 213篇 |
2008年 | 241篇 |
2007年 | 285篇 |
2006年 | 242篇 |
2005年 | 242篇 |
2004年 | 205篇 |
2003年 | 201篇 |
2002年 | 151篇 |
2001年 | 149篇 |
2000年 | 118篇 |
1999年 | 102篇 |
1998年 | 63篇 |
1997年 | 72篇 |
1996年 | 52篇 |
1995年 | 60篇 |
1994年 | 39篇 |
1993年 | 39篇 |
1992年 | 26篇 |
1991年 | 32篇 |
1990年 | 17篇 |
1989年 | 24篇 |
1988年 | 13篇 |
1987年 | 7篇 |
1986年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1973年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有4539条查询结果,搜索用时 15 毫秒
991.
Treatment of lithio derivativ
e of novel PEG-supported a-phenylselenopropionate with aldehydes, followed by oxidation-elimination with 30% hydrogen peroxide, formed Baylis-Hillman products, which were then reacted with sodium arylsulfinate. The resulting sulfonylated products were cleaved from the PEG efficiently affording methyl (2Z)-2-arylsulfonylmethyl-2-alkenoates in good yields and high purities. 相似文献
992.
993.
In recent years, original hybrid assemblies composed of a particle core surrounded by a lipid shell emerged as promising entities for various biotechnological applications. Their broadened bio-potentialities, ranging from model membrane systems or biomolecule screening supports, to substance delivery reservoirs or therapeutic vectors, are furthered by their versatility of composition due to the possible wide variation in the particle nature and size, as well as in the lipid formulation. The synthesis, the characteristics, and the uses of these Lipid/Particle assemblies encountered in the literature so far are reviewed, and classified according to the spherical core size in order to highlight general trends. Moreover, several criteria are particularly discussed: i) the interactions involved between the particles and the lipids, and implicitly the assembly elaboration mechanism, ii) the most suited techniques for an accurate characterization of the entities from structural and physicochemical points of view, and iii) the remarkable properties of the solid-supported lipid membrane obtained. 相似文献
994.
Fabrication of Nitrogen‐Doped Mesoporous‐Carbon‐Coated Palladium Nanoparticles: An Intriguing Electrocatalyst for Methanol and Formic Acid Oxidation 下载免费PDF全文
Chaiti Ray Soumen Dutta Ramkrishna Sahoo Anindita Roy Prof. Dr. Yuichi Negishi Prof. Dr. Tarasankar Pal 《化学:亚洲杂志》2016,11(10):1588-1596
Inspired by the attractive catalytic properties of palladium and the inert nature of carbon supports in catalysis, a concise and simple methodology for in situ nitrogen‐doped mesoporous‐carbon‐supported palladium nanoparticles (Pd/N‐C) has been developed by carbonizing a palladium dimethylglyoximate complex. The as‐synthesized Pd/N‐C has been exfoliated as a fuel cell catalyst by studying the electro‐oxidation of methanol and formic acid. The material synthesized at 400 °C,namely, Pd/N‐C‐400,exhibitssuperior mass activity and stability among catalysts synthesized under different carbonization temperaturesbetween300 and 500 °C. The unique 1D porous structure in Pd/N‐C‐400 helps better electron transport at the electrode surface, which eventually leads to about five times better catalytic activity and about two times higher stability than that of commercial Pd/C. Thus, our designed sacrificial metal–organic templatedirected pathway becomes a promising technique for Pd/N‐C synthesis with superior catalytic performances. 相似文献
995.
Dr. Laia Francàs Dr. Craig Richmond Pablo Garrido‐Barros Dr. Nora Planas Dr. Stephan Roeser Dr. Jordi Benet‐Buchholz Dr. Lluís Escriche Dr. Xavier Sala Prof. Dr. Antoni Llobet 《Chemistry (Weinheim an der Bergstrasse, Germany)》2016,22(15):5261-5268
Three distinct functionalisation strategies have been applied to the in,in‐[{RuII(trpy)}2(μ‐bpp)(H2O)2]3+ (trpy=2,2′:6′,2′′‐terpyridine, bpp=bis(pyridine)pyrazolate) water‐oxidation catalyst framework to form new derivatives that can adsorb onto titania substrates. Modifications included the addition of sulfonate, carboxylate, and phosphonate anchoring groups to the terpyridine and bis(pyridyl)pyrazolate ligands. The complexes were characterised in solution by using 1D NMR, 2D NMR, and UV/Vis spectroscopic analysis and electrochemical techniques. The complexes were then anchored on TiO2‐coated fluorinated tin oxide (FTO) films, and the reactivity of these new materials as water‐oxidation catalysts was tested electrochemically through controlled‐potential electrolysis (CPE) with oxygen evolution detected by headspace analysis with a Clark electrode. The results obtained highlight the importance of the catalyst orientation with respect to the titania surface in regard to its capacity to catalytically oxidize water to dioxygen. 相似文献
996.
Ma. Joaquina Caballero Inmaculada Suarez Baudilio Coto Rafael Van Grieken Benjamín Monrabal 《Macromolecular Symposia》2007,257(1):122-130
Summary: The incorporation of comonomer molecules in the backbone of a homopolymer can influence the final properties of the material, decreasing its crystallinity and the melting and glass transition temperatures, and increasing its impact resistance and transparency. In the present work, ten ethylene/propylene copolymers have been synthesized using a supported metallocene catalytic system covering the whole composition range. Any desired composition was obtained by controlling the feed composition during the reaction. These synthesized copolymers have been characterized by different techniques in order to study the effect of the comonomer incorporation onto their final properties. When the comonomer content is low, the behaviour of the copolymer is similar to that of the corresponding homopolymer. Nevertheless, if the comonomer content increases, the copolymer becomes more amorphous (low crystallization temperature and soft XRD signals) and easily deformable, reaching a behaviour close to that corresponding to an elastomeric material. In order to corroborate these results the samples have been characterized by TREF and GPC-MALS. TREF analysis showed that copolymers containing less than 10% and more than 80% of ethylene are semicrystalline, with elution temperatures typical of this kind of polymers. Molecular weights are higher for homopolymers and they decrease as the comonomer concentration increases, whereas the polydispersity index keeps almost constant at the expected value for this kind of samples. 相似文献
997.
998.
Cationic Silica‐Supported N‐Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis 下载免费PDF全文
Margherita Pucino Dr. Victor Mougel Roman Schowner Dr. Alexey Fedorov Prof. Dr. Michael R. Buchmeiser Prof. Dr. Christophe Copéret 《Angewandte Chemie (International ed. in English)》2016,55(13):4300-4302
Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ‐donating N‐heterocyclic carbene ligands with weak σ‐donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well‐defined silica‐supported catalysts, [(≡SiO)W(=O)(=CHCMe2Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe2Ph)(IMes)+][B(ArF)4?] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazol‐2‐ylidene, B(ArF)4=B(3,5‐(CF3)2C6H3)4] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene. 相似文献
999.
Bergwerff JA van de Water LG Visser T de Peinder P Leliveld BR de Jong KP Weckhuysen BM 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(16):4591-4601
The physicochemical processes that occur during the preparation of CoMo-Al2O3 hydrodesulfurization catalyst bodies have been investigated. To this end, the distribution of Mo and Co complexes, after impregnation of gamma-Al2O3 pellets with different CoMoP solutions (i.e., solutions containing Co, Mo, and phosphate), was monitored by Raman and UV-visible-NIR microspectroscopy. From the speciation of the different complexes over the catalyst bodies, insight was obtained into the interaction of the different components in the impregnation solution with the Al2O3 surface. It is shown that, after impregnation with a solution containing H2PMo11CoO40(5-), the reaction of phosphate with the Al2O3 leads to the disintegration of this complex. The consecutive independent transport of Co2+ complexes (fast) and Mo6+ complexes (slow) through the pores of the Al2O3 is envisaged. By the addition of extra phosphate and citrate to the impregnation solution, the formation of the desired heteropolyanion can be achieved inside the pellets. Ultimately, the H2PMo11CoO40(5-) distribution could be controlled by varying the aging time applied after impregnation. The power of a combination of spatially resolved spectroscopic techniques to monitor the preparation of supported catalyst bodies is illustrated. 相似文献
1000.
高分子载体试剂PSSF催化合成1,2异丙叉甘油的研究 总被引:4,自引:0,他引:4
1,2-异丙叉甘油,无色透明液体,bp19—190.5℃,n_(D4)~(25)·1.4326,d~(25)1.0626,性稳定,是重要的有机溶剂,增塑剂及合成类脂等的中间体。近也用于合成医用快速止血剂及组织粘结剂。其合成除个别以甘露醇为原料外,一般都以甘油和丙酮在强酸催化下缩合成。这些方法的共同缺点是反应时间较长,后处理繁琐,特别是用强酸作催化剂,对设备要求严格,不利于工业生产。由于这一化合物的应用广泛,再加此缩合反应本身是常用的保护1,2-二醇或羰基的重要手段,所经至今还在不断地探索。本文报导了一种制取这一化合物较为理想的高分子载体lewis酸催化剂PSSF及其使用的最佳条件。不仅避免了强酸催化剂的缺点,而且使收率大为提高。 相似文献