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   1402篇
  免费   230篇
  国内免费   294篇
化学   1280篇
晶体学   11篇
力学   49篇
综合类   18篇
数学   180篇
物理学   388篇
  2024年   2篇
  2023年   19篇
  2022年   27篇
  2021年   37篇
  2020年   58篇
  2019年   47篇
  2018年   44篇
  2017年   43篇
  2016年   60篇
  2015年   69篇
  2014年   82篇
  2013年   91篇
  2012年   93篇
  2011年   105篇
  2010年   90篇
  2009年   86篇
  2008年   90篇
  2007年   68篇
  2006年   90篇
  2005年   73篇
  2004年   67篇
  2003年   68篇
  2002年   97篇
  2001年   102篇
  2000年   67篇
  1999年   51篇
  1998年   38篇
  1997年   16篇
  1996年   9篇
  1995年   16篇
  1994年   13篇
  1993年   14篇
  1992年   9篇
  1991年   17篇
  1990年   6篇
  1989年   2篇
  1988年   3篇
  1987年   2篇
  1986年   3篇
  1985年   2篇
  1984年   4篇
  1982年   2篇
  1974年   2篇
  1973年   4篇
  1969年   4篇
  1968年   7篇
  1967年   3篇
  1966年   5篇
  1962年   2篇
  1903年   2篇
排序方式: 共有1926条查询结果,搜索用时 15 毫秒
131.
Chromatographic (GC/flame ionization detection, GC/MS) and statistical analyses were applied to the study of essential oils and extracts obtained from flowers, leaves, and stems of Lippia origanoides plants, growing wild in different Colombian regions. Retention indices, mass spectra, and standard substances were used in the identification of 139 substances detected in these essential oils and extracts. Principal component analysis allowed L. origanoides classification into three chemotypes, characterized according to their essential oil major components. α‐ and β‐phellandrenes, p‐cymene, and limonene distinguished chemotype A; carvacrol and thymol were the distinctive major components of chemotypes B and C, respectively. Pinocembrin (5,7‐dihydroxyflavanone) was found in L. origanoides chemotype A supercritical fluid (CO2) extract at a concentration of 0.83±0.03 mg/g of dry plant material, which makes this plant an interesting source of an important bioactive flavanone with diverse potential applications in cosmetic, food, and pharmaceutical products.  相似文献   
132.
本文讨论了含割点$u$的连通图G,其中$G-u$含路、圈或$D_{n}$分支时图$G$的伴随多项式的最小实根的变化情况.得到一些新的序关系,这推广了文[10-13]中有关图的伴随多项式最小根的一些结果.  相似文献   
133.
Ruipeng Guo 《Optik》2011,122(21):1890-1894
An experimental investigation of a modified Beckmann-Kirchhoff scattering theory applied in an in-process optical measurement of surface quality is described. The proposed theory describes the scattered light intensity distribution from a surface with the additional layers, and can be employed to analyze the surface characteristics in in-process measurement. Based on light scattering principle and machine vision method, the surface roughness is extracted to testify the correction of the modified Beckmann-Kirchhoff scattering theory. The experimental apparatus consists of a collimated laser diode, a beam splitter, a screen, a measuring lens and a camera. Test specimens with different surface roughness are studied. The results obtained demonstrate the feasibility of in-process optical measurement of surface quality using the modified model.  相似文献   
134.
郑琳  周敏  赵建军  成昭华  张向群  邢茹  张雪峰  鲁毅 《中国物理 B》2011,20(8):87501-087501
The magnetic and electrical properties of nonmagnetic Ga +3 ion substitution for Mn site are investigated in the bilayer manganite La 1.2 Sr 1.8 Mn 2 O 7.When the Mn is substituted by Ga,the ferromagnetic property obviously weakens,the magnetic transition temperature decreases and a spin-glass behaviour occurs at low temperature.Meanwhile,doping causes the resistivity to dramatically increase,the metal-insulator transition temperature to disappear,and a greater magneto-resistance effect to occur at low temperature.These effects result from the fact that Ga substitution dilutes the magnetic active Mn-O-Mn network and weakens the double exchange interaction,and further suppresses ferromagnetic ordering and metallic conduction.  相似文献   
135.
We introduce a method to study anisotropic flow parameter v<,n> as a collective probe to Quark Gluon Plasma in relativistic heavy ion collisions. The emphasis is put on the use of the Fourier expansion of initial spatial azimuthal distributions of participant nucleons in the overlapped region. The coefficients ε<,n> of Fourier expansion are called the spatial anisotropy parameter for the n-th harmonic. We propose that collective dynamics can be studied by v<,n>/ε<,n>. In this paper, we will discuss in particular the second (n=2) and the fourth (n=4) harmonics.  相似文献   
136.
The present work reports on novel four-layer thermally driven piezoresistive cantilevers implemented in one- and two-dimensional arrays for parallel proximity scanning. There, the heater (metallic meander), the piezoresistive deflection sensor, and the metal actuation film with significantly higher thermal expansion coefficient make up separate layers. Actuation efficiency and cross-talk of the novel cantilever design are studied and compared with two recent designs: thin metallic film and ion-implanted heater. The novel actuator, integrated on a 240 μm long and 3 μm thick silicon cantilever and supplied by V dc=1 V enables deflections up to 5 μm of the AFM-tip with an actuation efficiency of about 170 nm/mW and suppressed cross-talk between actuator and sensor.  相似文献   
137.
The extended boundary condition method, also called the null-field method, provides a semianalytic solution to the problem of electromagnetic scattering by a particle by constructing a transition matrix (T-matrix) that links the scattered field to the incident field. This approach requires the computation of specific integrals over the particle surface, which are typically evaluated numerically. We introduce here a new set of simplified expressions for these integrals in the commonly studied case of axisymmetric particles. Simplifications are obtained using the differentiation properties of the radial functions (spherical Bessel) and angular functions (associated Legendre functions) and integrations by parts. The resulting simplified expressions not only lead to faster computations, but also reduce the risks of loss of precision and provide a simpler framework for further analytical work.  相似文献   
138.
In this paper, we propose an independence test between two time series which is based on permutations. The proposed test can be carried out by means of different common statistics such as Pearson’s chi-square or the likelihood ratio. We also point out why an exact test is necessary. Simulated and real data (return exchange rates between several currencies) reveal the capacity of this test to detect linear and nonlinear dependences.  相似文献   
139.
Automatic presentations, also called FA-presentations, were introduced to extend finite model theory to infinite structures whilst retaining the solubility of fundamental decision problems. This paper studies FA-presentable algebras. First, an example is given to show that the class of finitely generated FA-presentable algebras is not closed under forming finitely generated subalgebras, even within the class of algebras with only unary operations. In contrast, a finitely generated subalgebra of an FA-presentable algebra with a single unary operation is itself FA-presentable. Furthermore, it is proven that the class of unary FA-presentable algebras is closed under forming finitely generated subalgebras and that the membership problem for such subalgebras is decidable.  相似文献   
140.
针对犹豫模糊信息下的多属性决策问题,提出了余弦优化投影方法,将投影夹角定义为决策者的风险偏好,并设置了态度参数用于调节决策者的心理变化,通过灵活描述决策者的保守心理,改进了投影法仅考虑投影长度的不足,使决策者可通过投影长度和投影夹角对备选方案进行综合判断。算例验证了方法的合理性和可行性。  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