首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3679篇
  免费   199篇
  国内免费   17篇
化学   2799篇
晶体学   41篇
力学   116篇
数学   233篇
物理学   706篇
  2024年   3篇
  2023年   16篇
  2022年   74篇
  2021年   85篇
  2020年   74篇
  2019年   69篇
  2018年   54篇
  2017年   47篇
  2016年   129篇
  2015年   114篇
  2014年   125篇
  2013年   216篇
  2012年   292篇
  2011年   328篇
  2010年   226篇
  2009年   182篇
  2008年   280篇
  2007年   249篇
  2006年   202篇
  2005年   201篇
  2004年   145篇
  2003年   123篇
  2002年   132篇
  2001年   79篇
  2000年   55篇
  1999年   57篇
  1998年   32篇
  1997年   27篇
  1996年   35篇
  1995年   34篇
  1994年   28篇
  1993年   26篇
  1992年   15篇
  1991年   15篇
  1990年   14篇
  1989年   10篇
  1988年   9篇
  1987年   7篇
  1986年   7篇
  1985年   8篇
  1984年   5篇
  1983年   5篇
  1982年   11篇
  1981年   6篇
  1980年   5篇
  1979年   6篇
  1978年   9篇
  1977年   7篇
  1976年   4篇
  1974年   5篇
排序方式: 共有3895条查询结果,搜索用时 15 毫秒
51.
Imide-siloxane block copolymer/silica hybrid membranes with covalent bonds were prepared via sol–gel reaction. The structural informations of these hybrid membranes were obtained by using Fourier transform-infrared spectrometry (FT-IR), 29Si nuclear magnetic resonance (29Si NMR), XPS and thermogravimetric analysis (TGA). The gas separation properties of the hybrid membranes were also investigated in terms of organosiloxane (PDMS) or silica content at various temperatures. In the hybrids, the addition of PDMS phase increased the permeabilities of gases such as He, CO2, O2, and N2, indicating that the gas transport occurred mainly through rubbery organic matrix. Meanwhile, the PDMS phase contributed the decreased gas selectivities to nitrogen but the reduction in selectivities was very small in comparison with other siloxane containing polymeric membranes. This might be due to the restriction of chain mobility by the existence of inorganic component such as silica network in the hybrids. Additionally, the increase of silica content in these hybrid membranes considerably retarded the falling-off of gas selectivity at elevated temperature. The increase of silica content in hybrid membranes resulted in well-formed silica networks and hence these inorganic components restricted the plasticization of organic matrix by the thermal segmental motion of organic components, leading to preventing the large decrease of the gas selectivity.  相似文献   
52.
53.
A series of novel 1,3‐altemate calix[4]arene azacrowns having mono and bis crown ethers on the lower rim of the calix[4]arene framework were synthesized. Solid‐state structures confirmed the three dimensional conformation of compounds 1–3.  相似文献   
54.
55.
Polypyrrole (PPy) nanotubes were readily fabricated through chemical oxidation polymerization in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse (water-in-oil) emulsions. The reverse cylindrical micelle phase was characterized, and the key factors affecting the formation of PPy nanotubes were systematically inspected. AOT reverse cylindrical micelles were prepared via a cooperative interaction between an aqueous FeCl3 solution and AOT in an apolar solvent. In the H2O/FeCl3/AOT/apolar solvent system, the aqueous FeCl3 solution played a role in increasing the ionic strength and decreasing the second critical micelle concentration of AOT. As a result, AOT reverse cylindrical micelles could be spontaneously formed in an apolar solvent. In addition, iron cations were adsorbed to the anionic AOT headgroups that were capable of extracting metal cations from the aqueous core. Under these conditions, the addition of pyrrole monomer resulted in the chemical oxidation polymerization of the corresponding monomer at the surface of AOT reverse cylindrical micelles, followed by the formation of tubular PPy nanostructures. In a typical composition (74.0 wt % hexane, 22.4 wt % AOT, and 3.6 wt % aqueous FeCl3 solution at 15 degrees C), the average diameter of PPy nanotubes was approximately 94 nm and their length was more than 2 mum. The PPy nanotube dimensions were affected by synthetic variables such as the weight ratio of aqueous FeCl3 solution/AOT, type of apolar solvent, and reaction temperature. Moreover, the relationship between the diameter and the conductivity of the nanotubes was investigated.  相似文献   
56.
A silica monomer-estrone complex (EstSi) having a thermally cleavable urethane bond and a cross-linkable triethoxysilane group was synthesized. From EstSi and TEOS, spherical silica particles with sizes of 1.5-3 mum were prepared. The template molecules were removed from the silica matrix by heating at 180 degrees C in DMSO in the presence of water, generating a cavity with an amino group. The control silica particles that had the same sizes and shapes were obtained with aminopropyl triethoxysilane and TEOS. When ethylene glycol was added in place of H2O, an ethyl alcoholic group was formed in the cavity. Their recognition ability and specific binding for estrone were characterized by uptake experiments. The estrone-imprinted silica particles showed a much higher recognition ability than the control silica particles and higher selectivity for estrone than testosterone propionate.  相似文献   
57.
58.
We study compact connected surfaces inm-dimensional Euclidean spaceE m (3 m 5) with a point through which every geodesic is aW-curve regarded as a curve in Em.  相似文献   
59.
The total synthesis of a new platelet aggregation-inhibiting gamma-lactam PI-091 (1) gave a 1:1 diastereomeric mixture at the gamma-ketal carbon. The high-yielding aldol reaction of an appropriately protected 1,3,4-trihydroxy-4-methyldecan-2-one 42, prepared from D-glucose, with the kinetically generated enolate of 3-methyl-2-butanone provided 43. The resulting diastereomeric mixture of the aldol adduct 43 was converted to a 2,4-alkylated furan 45 via an intramolecular ketalization followed by dehydration. The addition of a singlet oxygen to the alpha-trimethylsilylated furan 48derived from 45 under photochemical conditions efficiently provided an alpha,gamma-dialkylated gamma-hydroxy gamma-lactone 47. The transformation of methyl ketal 52 prepared from 47 into gamma-hydroxy gamma-lactam 53 was achieved by exposure to liquid ammonia in MeOH. The total synthesis of 1 was achieved from 52 through the Dess-Martin periodinane oxidation of the secondary hydroxy group in the side chain. The present total synthesis revealed that the stereogenic carbon center in the side chain in natural 1 is S.  相似文献   
60.
Previously overlooked condensed matter effects (CME) can significantly influence nuclear fusion rates in both laboratory and astrophysical environments. In dense plasmas, the ensemble of fusing particles has a significant exchange of kinetic and potential energies. Thus, there are diminished effective flux velocities resulting in a significant selective reduction of fusion rates. Our CME predictions are testable in laboratory experiments and have broad-ranging implications on the fusion rates for stellar media in general. By calculating reaction rates forp(p, e + v e ) D and7Be(p, )8B in the sun, we show that CME help to solve the solar neutrino problem.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号