全文获取类型
收费全文 | 23051篇 |
免费 | 2179篇 |
国内免费 | 6356篇 |
专业分类
化学 | 28906篇 |
晶体学 | 381篇 |
力学 | 155篇 |
综合类 | 185篇 |
数学 | 101篇 |
物理学 | 1858篇 |
出版年
2024年 | 21篇 |
2023年 | 202篇 |
2022年 | 333篇 |
2021年 | 632篇 |
2020年 | 828篇 |
2019年 | 691篇 |
2018年 | 710篇 |
2017年 | 779篇 |
2016年 | 1108篇 |
2015年 | 1077篇 |
2014年 | 1358篇 |
2013年 | 2312篇 |
2012年 | 1600篇 |
2011年 | 1652篇 |
2010年 | 1395篇 |
2009年 | 1597篇 |
2008年 | 1646篇 |
2007年 | 1727篇 |
2006年 | 1753篇 |
2005年 | 1658篇 |
2004年 | 1595篇 |
2003年 | 1357篇 |
2002年 | 889篇 |
2001年 | 605篇 |
2000年 | 516篇 |
1999年 | 526篇 |
1998年 | 482篇 |
1997年 | 436篇 |
1996年 | 344篇 |
1995年 | 332篇 |
1994年 | 290篇 |
1993年 | 280篇 |
1992年 | 265篇 |
1991年 | 139篇 |
1990年 | 96篇 |
1989年 | 90篇 |
1988年 | 73篇 |
1987年 | 41篇 |
1986年 | 29篇 |
1985年 | 37篇 |
1984年 | 29篇 |
1983年 | 10篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 8篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1968年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
N.M. Shash F.E. Salman M.G. El-Sharawy M.K. El-Mansy 《Journal of Physics and Chemistry of Solids》2004,65(5):881-889
Vitrification suppression in the (V2O5)1−x (P2O5)x glasses where x=0.10, 0.15, 0.20, and 0.25 was controlled by changing the rate of quenching glasses. The structure variations occurring in the glasses were detected by differential thermal analysis and optical microscope. The results implied the separation and growth of V2O5 orthorhombic microcrystal in the samples with x=0.10 and 0.15 whereas other samples did not illustrate remarkable changes in their microstructure. However, in temperature range between 300 and 473 K a semiconducting behavior for all samples appears during the study of electrical conductivity-temperature dependence. A decrease in conductivity values accompanied with some variations in activation energies by reducing quenching rate was observed. The conductivity results suggested that the conduction occurs by the phonon assisted hopping of a small polaron between V4+ and V5+ states at relatively higher temperature range above θD/2. Whereas at relatively low temperatures the conduction may occur by electron jumping between filled and empty states at Fermi level in the disordered matrix besides polaronic conduction. Reasonable values for the density of localized states, carrier concentration and carrier mobility were estimated and discussed. Also, dielectric constant and dielectric loss were studied as a function of frequency at different temperatures confirming the structure variations in the glass system. 相似文献
82.
Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V1−xMox)2−δO3. The oxidation effects were monitored from the rate of paramagnetic V4+ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V4+ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V4+ content. 相似文献
83.
Maculalactones A, B and C from the marine cyanobacterium Kyrtuthrix maculans are amongst the only compounds based on the tribenzylbutyrolactone skeleton known in nature and (+) maculalactone A from the natural source possesses significant biological activity against various marine herbivores and marine settlers. We now report a concise synthesis of racemic maculalactone A in five steps from inexpensive starting materials. Maculalactones B and C were synthesized by a minor modification to this procedure, and the synthetic design also permitted an asymmetric synthesis of maculalactone A to be achieved in around 85% ee. The (+) and (−) enantiomers of maculalactone A were assigned, respectively, to the S and R configurations on the basis of the chiral selectivity expected for catecholborane reduction of an unsymmetrical ketone in the presence of Corey's oxazoborolidine catalyst. Surprisingly, it appeared that natural (+) maculalactone A was biosynthesized in K. maculans in a partially racemic form, comprising ca. 90-95% of the (S) enantiomer and 5-10% of its (R) enantiomer. Coincidentally therefore, the percentage enantiomeric excess of the product obtained from asymmetric synthesis almost exactly matched that found in nature. 相似文献
84.
The synthesis of enantiopure bis-THF is described, starting from d-mannitol. Bis-THF is used as chiral ligand for organolithium reagents in four different reactions. The enantioselectivity provided by this ligand is moderate, and the asymmetric induction is in line with the expected model. 相似文献
85.
The thermal migration of cyclic and acyclic organoboranes were studied. In most cases, a stereoselective 1,2-dyotropic migration was observed, allowing the stereocontrol of three contiguous chiral centers. Scope and limitations of this thermal migration are presented. 相似文献
86.
选用新方法合成了2-碘—N-甲基苯磺酰胺,通过^1H NMR,^13C NMR,^13C DEPT NMR(Distortionless Enhancement by Polarization Transfer NMR)和MS确证的其结构。 相似文献
87.
88.
Saeed Balalaie Elahe Kowsari Mehri S. Hashtroudi 《Monatshefte für Chemie / Chemical Monthly》2003,134(3):453-456
Summary. Three component condensation of alkylacetoacetates, primary amines, and alkyl cyanoacetates catalyzed by solid supports under
microwave irradiation gave N-alkyl 3-cyano-6-hydroxy-2(1H)-pyridinones with high yields. Upon carrying out the reaction under the same condition on acidic alumina, zeolite HY, silica
gel, and montmorillonite K-10, the best yields were achieved by silica gel.
Corresponding author. E-mail: balalaie@yahoo.com
Received August 28, 2002; accepted September 3, 2002 相似文献
89.
A convenient and large-scale preparation of retinoic acid 1 from β-ionone in five steps with 38% overall yield is described.The key steps are the epoxidization of 2 with a new methylated agent and the condensation 4 with tetraethyl methylenediphophonate in one-pot procedure to prepare 6. 相似文献
90.
MAO Shao-Yu a② KANG You-Juna MI Jin-Xiaoa② LI Man-Rongb WEI Zan-Bina WU Xue-Yanb ZHAO Jing-Taib a 《结构化学》2006,25(2):173-179
1 INTRODUCTION Considerable research effort on the heavy metal fluoride glasses like barium, zirconium fluoride and a series of rare-earth fluorozirconate compounds du- ring the last two decades was initiated by the broad optical transmission window of these glasses and thereby the potential for ultralow-loss optical fi- bers[1~10]. The fluorozirconate of alkali metals was first reported in 1938[11], and most of the sodium and potassium fluorozirconates were reported amongthe 1940s and 19… 相似文献