全文获取类型
收费全文 | 3178篇 |
免费 | 82篇 |
国内免费 | 17篇 |
专业分类
化学 | 1923篇 |
晶体学 | 15篇 |
力学 | 64篇 |
数学 | 488篇 |
物理学 | 486篇 |
无线电 | 301篇 |
出版年
2022年 | 33篇 |
2021年 | 29篇 |
2020年 | 40篇 |
2019年 | 40篇 |
2018年 | 46篇 |
2017年 | 46篇 |
2016年 | 62篇 |
2015年 | 54篇 |
2014年 | 93篇 |
2013年 | 158篇 |
2012年 | 185篇 |
2011年 | 164篇 |
2010年 | 110篇 |
2009年 | 95篇 |
2008年 | 203篇 |
2007年 | 184篇 |
2006年 | 172篇 |
2005年 | 194篇 |
2004年 | 138篇 |
2003年 | 108篇 |
2002年 | 102篇 |
2001年 | 54篇 |
2000年 | 37篇 |
1999年 | 20篇 |
1998年 | 27篇 |
1997年 | 37篇 |
1996年 | 50篇 |
1995年 | 27篇 |
1994年 | 30篇 |
1993年 | 31篇 |
1992年 | 32篇 |
1991年 | 22篇 |
1990年 | 24篇 |
1989年 | 22篇 |
1988年 | 28篇 |
1987年 | 24篇 |
1985年 | 40篇 |
1984年 | 45篇 |
1983年 | 35篇 |
1982年 | 44篇 |
1981年 | 37篇 |
1980年 | 37篇 |
1979年 | 25篇 |
1978年 | 37篇 |
1977年 | 29篇 |
1976年 | 30篇 |
1975年 | 17篇 |
1974年 | 18篇 |
1973年 | 39篇 |
1972年 | 18篇 |
排序方式: 共有3277条查询结果,搜索用时 31 毫秒
121.
The aim of this work was to develop alternative peptide‐loaded microspheres using liposphere formulation—a lipid based microdispersion system. This formulation represents a new type of lipid or polymer‐based encapsulation system developed for parenteral and topical drug delivery of bioactive compounds. Our strategy was to utilize the liposphere formulation to improve the entrapment efficiency and release profile of triptorelin and leuprolide [luteinizing hormone–releasing hormone (LHRH) analogues] in vitro. Peptides (2% w/w) were loaded into lipospheres contained of polylactic acid (PLA) or poly(lactic‐co‐glycolic acid) (PLGA) with several types of phospholipids. The effects of polymer and phospholipid type and concentration, method of preparation and solvents on the liposphere characteristics, particle size, surface and bulk structure, drug diffusion rate, and erosion rate of the polymeric matrix were studied. The use of L ‐PLA (Mw = 2000) and hydrogenated soybean phosphatidylcholine (HSPC) with phospholipid–polymer ratio of 1 : 6 w/w, was the most efficient composition that formed lipospheres of particle size in the range of 10 µm with most of the phospholipid embedded on the particles surface. In a typical procedure, peptides were dissolved in N‐methyl‐2‐pyrrolidone (NMP), and dispersed in a solution of polymer and phospholipids in a mixture of NMP and chloroform with the use of 0.1% poly(vinyl alcohol) (PVA) as the emulsified aqueous medium. Uniform microspheres were prepared after solution was mixed at 2000 rpm at room temperature for 30 min. Using this formulation, the entrapment efficiency of LHRH analogues in lipospheres was up to 80%, and the peptides were released for more than 30 days. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
122.
Jay K. Kochi 《Angewandte Chemie (International ed. in English)》1988,27(10):1227-1266
The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer-sphere and inner-sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D⊕, A?], for which the free energy change can be evaluated from the charge-transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge-transfer ion pairs [D⊕, A?] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge-transfer bands of various EDA complexes with a 25-ps laser pulse. Such studies confirm the radical ion pair [Arene⊕, NO2] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron-transfer mechanism between arenes and the nitryl cation (NO) electrophile. 相似文献
123.
The formula of a new compound isolated in the LaOsO system has been established by means of crystal structure determination. There are two La3Os2O10 units in a face-centered monoclinic unit cell (S.G. ); a = 7.911(2) Å, b = 7.963(2) Å, c = 6.966(2)Å, β = 115.76(2)°;. For 1082 intensities, collected on an automated single-crystal diffractometer, the final R value was 0.025 after absorption corrections. The structure consists of isolated Os2O10 clusters composed of two edge-shared OsO6 octahedra. These dimeric units are connected together by two types of La3+ ions in eightfold coordination. In view of the OsOs distance inside the pair (2.462 Å), La3Os2O10 provides an example of metal-metal bonding involving a transition metal in a half-integral formal oxidation state of 5.5. 相似文献
124.
125.
126.
127.
Hsiu‐Fu Hsu Corresponding author Shih‐Jay Chien Hsiu‐Hui Chen Chien‐Hsing Chen Li‐Ya Huang Chung‐Hao Kuo 《Liquid crystals》2013,40(6):683-689
Liquid crystalline α,α′‐bis(4‐alkoxyphenylethynyl)oligothiophenes (bi‐ and ter‐thiophene) have been synthesized and their mesogenic behaviour and optical properties investigated. They all exhibited a nematic mesophase, and compounds with long alkoxy chains also showed lamellar phases. Increasing the number of thiophene units increased both the transition temperatures and the mesophase ranges. As for their optical properties, incorporating more thiophene units results in red‐shifted absorption and emission spectra, slightly enhanced quantum efficiency, and a larger Stoke's shift. Most importantly, in terms of the absorption and emission maxima, the incorporation of one 4‐alkoxyphenylethynyl moiety was found to be equivalent to adding one thiophene ring. 相似文献
128.
Oxazolidin-2-ones and thiazolidin-2-one are conveniently prepared by condensation of L-serine, L-threonine and L-cysteine, respectively with triphosgene. The corresponding methyl esters may be subsequently obtained by quenching the reaction mixture with methanol, without prior need for the isolation of the free acids. An improved procedure for preparation of triphosgene using an internal cooling system is described. 相似文献
129.
We have reported the synthesis and structural elucidation of a series of hexahydroimidazo [4,5-d] imidazole-2,5-diones and hydantoins from glyoxal and substituted ureas.1,2 The general equation of this condensation reaction and a few representative compounds: those reported, current and hypothetical, are depicted in Scheme 1. 相似文献
130.