全文获取类型
收费全文 | 11866篇 |
免费 | 1845篇 |
国内免费 | 1794篇 |
专业分类
化学 | 12798篇 |
晶体学 | 543篇 |
力学 | 67篇 |
综合类 | 54篇 |
数学 | 111篇 |
物理学 | 1932篇 |
出版年
2024年 | 31篇 |
2023年 | 227篇 |
2022年 | 280篇 |
2021年 | 339篇 |
2020年 | 539篇 |
2019年 | 462篇 |
2018年 | 309篇 |
2017年 | 439篇 |
2016年 | 585篇 |
2015年 | 570篇 |
2014年 | 618篇 |
2013年 | 887篇 |
2012年 | 815篇 |
2011年 | 889篇 |
2010年 | 688篇 |
2009年 | 782篇 |
2008年 | 822篇 |
2007年 | 873篇 |
2006年 | 708篇 |
2005年 | 669篇 |
2004年 | 706篇 |
2003年 | 622篇 |
2002年 | 449篇 |
2001年 | 328篇 |
2000年 | 258篇 |
1999年 | 269篇 |
1998年 | 226篇 |
1997年 | 173篇 |
1996年 | 154篇 |
1995年 | 170篇 |
1994年 | 94篇 |
1993年 | 67篇 |
1992年 | 70篇 |
1991年 | 46篇 |
1990年 | 38篇 |
1989年 | 22篇 |
1988年 | 21篇 |
1987年 | 17篇 |
1986年 | 21篇 |
1985年 | 21篇 |
1983年 | 13篇 |
1982年 | 17篇 |
1981年 | 18篇 |
1980年 | 14篇 |
1979年 | 18篇 |
1978年 | 15篇 |
1976年 | 12篇 |
1975年 | 16篇 |
1974年 | 15篇 |
1971年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 20 毫秒
81.
82.
Luigi Angiolini Daniele Caretti Elisabetta Salatelli Monique Biesemans 《Journal of organometallic chemistry》2006,691(9):1965-1972
Starting from 4-isopropylbenzoic acid, three new triorganotin carboxylates bearing methyl, butyl and phenyl substituents at tin, respectively, were prepared and fully characterized by spectroscopic and thermal techniques, with particular regard to the coordination number of tin atom, in solution as well as in the solid state. The triorganotin compounds, tested as transesterification catalysts in the reaction between ethyl acetate and primary, secondary or tertiary alcohol, respectively, displayed, as expected, a strong decrease of activity on passing from the primary to the tertiary alcohol reactant. Different activities by the tin carboxylates were also observed in the reaction between primary alcohol and ethyl acetate. The reaction mechanism, as elucidated by Sn NMR, involves coordination of both ester substrate and alcohol reactant to the triorganotin compound, the reaction conversion appearing related not only to the Lewis acidity of the tin atom, but also to the nature of the reactants. Preliminary catalytic tests were also carried out in the reaction between glyceryl tridodecanoate (as a model of natural triglyceride) and ethanol, mimicking the preparation of biodiesel fuel. Although in this case lower conversions were obtained with respect to the reactions on ethyl acetate, the catalytic activity of organotin derivatives appears considerable. 相似文献
83.
Chiral diphosphine ligands based on camphor: synthesis and applications in asymmetric hydrogenations
The synthesis of a novel class of atropisomer chiral diphosphine ligands with a bornene framework is described. The new ligands showed in Rh catalyzed asymmetric hydrogenation of α- and β-enamides very high ee’s (more than 99%). 相似文献
84.
85.
An attempt is made to apply dielectric theories of interfacial polarization to observations of dielectric relaxations for W/O emulsions. Approximate formulas for disperse systems in a W/O type were derived from the two theories: one proposed by Maxwell and Wagner for dilute disperse systems of spherical particles, and the other developed by Hanai for concentrated disperse systems. Dielectric measurements were carried out on concentrated W/O emulsions prepared from kerosene and distilled water or KCl aqueous solutions by minimal use of emulsifiers. Marked dielectric relaxations were observed with the emulsions, the dielectric parameters having been determined to characterize the relaxation data. Phase parameters such as relative permittivity, electric conductivity and volume fraction of the disperse phase were evaluated from the dielectric parameters by use of the approximate formulas of the respective theories. The phase parameters evaluated and the frequency dependence of complex permittivity of the W/O emulsions deduced from the theory for concentrated disperse systems are in excellent agreement with the observed data in comparison to that for dilute disperse systems. It is concluded that the dielectric relaxations due to the interfacial polarization of disperse systems of spheres are explained satisfactorily by the theory for concentrated disperse systems. 相似文献
86.
87.
首次研究了碳化钨负载S2 O2 -8 ZrO2 (PSZ)固体超强酸催化剂 (WC PSZ)上正戊烷的反应情况及影响催化剂活性的各种因素 ,并用GC MS ,XRD ,BET等手段分析了正戊烷反应产物和催化剂的物理化学性质等 .结果表明 :WC PSZ对正戊烷具有异构化和裂解的双重催化作用 .PSZ在负载适量碳化钨后对正戊烷反应的活性和选择性显著提高 ,显示出优于Pt PSZ催化剂的效果 ,碳化钨的负载量为 2 0 %的效果最佳 .适当的焙烧、活化和反应温度等条件是取得良好反应效果的关键 相似文献
88.
The aim of this research work was to clarify the role of the mixed emulsifier in the structure formation and water binding
mode in the case of o/w creams prepared with different surfactants. The swelling behavior of mixed emulsifiers was examined
by means of direct investigation methods such as transmission electron microscopy (TEM) and X-ray diffraction. The detailed
structure image of the creams was created with the help of the latter. The influence of the structure of the hydrophilic gel
phase, and the structural changes during storage were studied with rheological methods. On the basis of the results, it can
be stated that the investigated creams had different structures from those mentioned in the literature: surfactant did not
create a mixed bilayer with the structure to furnish fatty amphiphile; instead, micelles were formed. These results correlated
well with the results of the rheological tests. 相似文献
89.
《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(1):303-313
The reaction of electron‐deficient alkenes with alkoxyalkynyl Fischer carbene complexes (FCCs) represents a straightforward route to a new type of captodative (donor–acceptor) alkynylcyclopropanes, which have been prepared in moderate to high yields and in a diastereoselective manner. Some studies regarding the employment of additives to facilitate the recovery of the metal moiety after the reaction are also described. Finally, the first example of a cyclopropanation reaction through employing Fischer carbene complexes under microwave irradiation is presented; this method proved to be an advantageous alternative to the thermal reaction. 相似文献
90.
黄永平 《影像科学与光化学》2007,25(2)
以稀土氧化物为原料,用溶胶-凝胶法制备前驱液,加入适量的聚乙烯醇做成膜物质,用浸渍拉提法在石英玻璃表面上得到均匀的薄膜,然后经过适当的干燥和热处理得到Y2O3∶Eu3+发光薄膜.讨论了Eu3+的掺杂浓度和热处理温度对薄膜发光性能的影响.试验表明:Eu3+的最佳掺杂浓度为8%(摩尔分数),薄膜的发光性能随热处理温度提高而增强,当热处理温度达到700℃后,薄膜的发光性能基本上稳定.同时用原子力显微镜和X射线衍射分析了薄膜的表面形貌和结构. 相似文献