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   8616篇
  免费   1419篇
  国内免费   598篇
化学   3424篇
晶体学   89篇
力学   1721篇
综合类   71篇
数学   301篇
物理学   5027篇
  2024年   19篇
  2023年   71篇
  2022年   217篇
  2021年   193篇
  2020年   231篇
  2019年   234篇
  2018年   197篇
  2017年   298篇
  2016年   373篇
  2015年   399篇
  2014年   439篇
  2013年   1131篇
  2012年   562篇
  2011年   604篇
  2010年   432篇
  2009年   476篇
  2008年   440篇
  2007年   495篇
  2006年   431篇
  2005年   410篇
  2004年   393篇
  2003年   304篇
  2002年   269篇
  2001年   241篇
  2000年   206篇
  1999年   154篇
  1998年   185篇
  1997年   167篇
  1996年   143篇
  1995年   131篇
  1994年   112篇
  1993年   125篇
  1992年   99篇
  1991年   72篇
  1990年   55篇
  1989年   45篇
  1988年   58篇
  1987年   32篇
  1986年   34篇
  1985年   27篇
  1984年   27篇
  1983年   5篇
  1982年   22篇
  1981年   20篇
  1980年   6篇
  1979年   20篇
  1978年   4篇
  1976年   4篇
  1973年   5篇
  1972年   5篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Abstract

A new solvothermal route for the synthesis of nitrides is proposed using liquid NH3 as solvent in supercritical conditions, Such a preparation method was applied to the synthesis of GaN using gallium metal as starting material.

GaN is a wide band-gap semi-conductor (3.4eV). It is a very attractive nitride due to its various applications in micro- and opto-electronics [1,2]. Consequently, many research groups are interested in synthesising GaN. Two methods have been principally developed:

(i) synthesis of thin films by epitaxy [3,4]

(ii) synthesis of bulk GaN by high pressure method [5,6].

The new proposed process leads to fine microcrystallites of GaN with the wurtzite-type structure. The chemical purity can be optimised versus the synthesis mechanism. The size and shape of the crystallites would be influenced by the nature of the nitriding additive and the thermodynamical conditions (pressure and temperature) used for the synthesis.  相似文献   
92.
Abstract

Raman phonon spectra of 9, 10-dinitroanthracene have been recorded in the pressure range 0-6GPa. No phase transition is detected up to the maximum pressure studied. Quasi Harmonic Lattice Dynamics calculations, based on an atom-atom potential previously modeled on homologous 9,10-disubstituted anthracenes, have been performed. The optimized potential was used to calculate the equilibrium geometry and the lattice phonon frequencies as a function of pressure. The calculated structure at ambient conditions closely resembles the experimental one. The calculated phonon frequencies show a good agreement with the experimental values at all pressures measured.  相似文献   
93.
Abstract

The changes in the characteristics of vibrational and optical properties of the conducting form of polypyrrole under pressure is studied.  相似文献   
94.
Abstract

The effect of high temperature and high hydrostatic pressure on the residual hydrolytic activity of a commercial Rhizomucor miehei lipase has been studied. Inactivation at high temperature and/or high pressure was carried out. Under non-denaturing pressure conditions, results showed that pressurisation protects enzymes against thermal deactivation. This is in accordance with previous results obtained with enzymes from mesophilic sources, such as invertase and galactosidases.  相似文献   
95.
Abstract

The crystal structure of Nd2CuO4 has been studied by neutron diffraction at pressure up to 5 Gpa. The volume compressibility value was determined as 5·6·10?3/Gpa. The decrease of positional parameter of neodymium at high pressure has been observed. This structural change is explained by pressure induced neodymium ions charge increase.  相似文献   
96.
Abstract

Although potentially extremely important for understanding the high-pressure microscopic behaviour of materials, over the years the area of high-pressure EXAFS in particular using diamond anvil cells has proved to be technically difficult. This has significantly hampered its development. The interference of X-ray dimaction from the diamonds in the diamond anvil cell with the absorption signal has proved to be a challenging problem to tackle, restricting the use of high-pressure EXAFS to energies below about 11 key Below 11 keV however the technique is also limited due to absorption of incident X-rays by the diamonds making it virtually impossible to conduct X-ray absorption experiments below about 9keV In this paper we describe in detail the methodology for scanriirig high-pressure EXAFS in diamond anvil cells and examine the origins of the associated problems and ways of dealing with them. We also demonstrate that it is possible to extend the useful range of studied absorption edges from 7keV up to at least 30keV This brings about new opportunities for high pressure EXAFS using diamond anvil cells.  相似文献   
97.
A new variant of the method of finding the equation of state in the Mie–Grüneisen form is presented. It is based only on high-pressure isotherms of solids. Using this procedure, the semiempirical equation of state and shock adiabats of solids may be found at high pressures and high temperatures. The method is tested on periclase MgO within the range of shock pressures up to 300–500 GPa.  相似文献   
98.
Abstract

The elaboration of the theoretical experimental model of the absorption process under a pressure of 30 MPa and moderate temperatures included the experimental research of methan and argon absorption from the five-component mixture H2-N2-CH4-Ar-NH3 by liquid ammonia on a pilot plant and carrying out the computerized research of process statics.  相似文献   
99.
Naringinase, an enzyme complex that expresses α -l-rhamnosidase and β -d-glucosidase activities in native state, can be used to deglycosylate natural glycosides. The selective inactivation of one of these activities will allow the biosynthesis of different bioactive compounds in a simple, effective and cheap way. In this work, pressure and temperature were the tools used to selectively inactivate the activities expressed by naringinase. The main goal was the identification of pressure–temperature conditions to acquire conditions for the maximization of enzymatic hydrolysis of substrates with different numbers of glycosidic residues. α -l-Rhamnosidase was 32-fold more resistant against inactivation at 250 MPa than at atmospheric pressure. The best pressure condition to reduce β -d-glucosidase inactivation at 75°C was 173 MPa, while in the case of α -l-rhamnosidase inactivation at 85°C, it was above 250 MPa. Moreover, a selective inactivation of β -d-glucosidase activity of naringinase was attained, allowing an easy and cheap method with which to produce prunin and other expensive glycosides. The present work highlights the effect of high pressure on enzyme protection against thermal inactivation, demonstrating its potential as a powerful tool in biosynthesis.  相似文献   
100.
We describe here a simple set-up for X-ray diffraction under high pressure using a diamond anvil cell employing a sealed tube. The set-up works in angular dispersive geometry and is built using rather common components that are available in a research laboratory. We show that using this set-up one can routinely acquire good-quality data for the determination of structural properties under pressure and the equations of state. Technical details are presented along with examples of experimental results for sodium chloride and LiV2O4.  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