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   15638篇
  免费   3205篇
  国内免费   2712篇
化学   9333篇
晶体学   170篇
力学   1440篇
综合类   145篇
数学   1362篇
物理学   9105篇
  2024年   33篇
  2023年   207篇
  2022年   350篇
  2021年   522篇
  2020年   722篇
  2019年   550篇
  2018年   544篇
  2017年   608篇
  2016年   714篇
  2015年   649篇
  2014年   918篇
  2013年   1403篇
  2012年   971篇
  2011年   1063篇
  2010年   925篇
  2009年   1124篇
  2008年   1131篇
  2007年   1162篇
  2006年   1118篇
  2005年   872篇
  2004年   796篇
  2003年   740篇
  2002年   602篇
  2001年   528篇
  2000年   468篇
  1999年   419篇
  1998年   350篇
  1997年   280篇
  1996年   249篇
  1995年   233篇
  1994年   197篇
  1993年   149篇
  1992年   124篇
  1991年   129篇
  1990年   85篇
  1989年   84篇
  1988年   75篇
  1987年   65篇
  1986年   60篇
  1985年   58篇
  1984年   41篇
  1983年   22篇
  1982年   36篇
  1981年   36篇
  1980年   29篇
  1979年   28篇
  1978年   12篇
  1977年   21篇
  1976年   12篇
  1974年   10篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
走滑式断层地震的折迭突变模型   总被引:1,自引:1,他引:0  
利用过势函数微分形式的途径,计入远场外力功的影响,对走滑式断层地震机制进行分析.研究表明:突变理论中折迭突变模型展示的性状,与走滑式断层地震的主要特征之间一一对应.折迭突变可对包括震前、震后围岩-断层系统稳定性描述在内的断层失稳起、终点位置、断层失稳错距等作出描述.给出3种性状可相互印证的断层失稳围岩弹性能释放量图解.走滑式断层地震强度与围岩压力及围岩断层的刚度比等有关,围岩压力大、刚度比小、主切应力轴与发震断层面夹角大,则地震强度大.  相似文献   
992.
从基于等效夹层壳思想的双层网格扁壳,非线性弯曲理论的变分方程出发,利用坐标变换方法和驻值余能原理,导出双层网格圆底扁球壳,在均布压力作用下的轴对称大挠度方程和边界条件.采用修正迭代法,求得了两类边界条件下双层网格圆底扁球壳的非线性载荷-位移关系式和临界屈曲载荷的解析表达式,并讨论了几何参数对临界屈曲载荷的影响.  相似文献   
993.
The spectra of Rydberg hydrogen atom in magnetic fields have been calculated using linear variational method with B-splines basis functions [Acta Phys. Sin. 55 (2006) 3380]. Based on these calculations we have done some statistics analysis about the high Rydberg energy levels. The nearest-neighbor energy spacing distribution and the 3-statistics have been shown about diamagnetic Rydberg hydrogen atom with the magnetic field being 0.6 T and 6 T. The phenomena of multiply crossing, multiply anti-crossing, and the mixed of crossing and anti-crossing of energy levels have appeared in this paper. For both cases, in range of lower energy, the energy 1evel statistics properties close to Poisson distribution. With the increasing of the energy, the energy level statistics properties are away to Poisson distribution and tend to Wigner distribution step by step.  相似文献   
994.
Ashcroft's local empty core (EMC) model pseudopotential in the second-orderperturbation theory is used to study the electron dispersion relation, theFermi energy, and deviation in the Fermi energy from free electron value forthe liquid alkali metals and their equiatomic binary alloys for the firsttime. In the present computation, the use of pseudo-alloy-atom model (PAA)is proposed and found successful. The influence of the six different formsof the local field correction functions proposed by Hartree (H), Vashishta-Singwi (VS), Taylor (T), Ichimaru-Utsumi (IU), Farid et al. (F), andSarkar et al. (S) on the aforesaid electronic properties is examined explicitly,which reflects the varying effects of screening. The depth of the negativehump in the electron dispersion of liquid alkalis decreases in the orderLi →K, except for Rb and Cs, it increases. The results of alloys are in predictive nature.  相似文献   
995.
A Brownian microscopic heat engine with a particle hopping on a one-dimensional lattice driven by a discrete and periodic temperature field in a periodic sawtooth potential is investigated. In order to clarify the underlying physical pictures of the heat engine, the heat flow via the potential energy and the kinetic energy of the particles are considered simultaneously. Based on describing the jumps among the three states, the expressions of theefficiency and power output of the heat engine are derived analytically. The general performance characteristic curves are plotted by numerical calculation. It is found that the power output-efficiency curve is a loop-shaped one, which is similar to one for a real irreversible heat engine. The influence of the ratioof the temperature of the hot and cold reservoirs and the sawtooth potential on the maximum efficiency and power output is analyzed forsome given parameters. When the heat flows via the kinetic energy isneglected, the power output-efficiency curve is an open-shaped one,which is similar to one for an endroeversible heat engine.  相似文献   
996.
基于材料的热力学关系和能量守恒定律,给出弹性体形变过程中红外辐射温度改变量和辐出度变化的物理计算方程.并结合有限元方法进行物理建模,对光弹材料三点弯载荷实验过程中的红外辐射出射度的分布进行数值计算和模拟.结果与实验结果吻合较好,表明模型及其相关理论是合理的,可以用来量化揭示实验中的红外辐射特征.从而得到一种能够利用计算机量化分析固体材料载荷过程中红外辐射特征机理的计算方法.  相似文献   
997.
CdS thin films were prepared by chemical-bath-deposited method and the effect of tempera-ture and time on the properties of CdS thin films was studied. Independent of the deposited temperature, the growth was mainly controlled by the ion-by-ion growth mechanism at the beginning of the film deposition, then the cluster-by-cluster mechanism came to be domi-nant. The growth rate increased faster with the increasing of temperature until the thickness reached the limitation, then thickness instead become thinner. The scanning electron micro-scope results revealed that the morphology of the CdS film changed from pinholes to rough,inhomogeneous surface with increasing deposition time and deposition temperature. The X-ray diffraction results showed the film structure was a mixture of two phases: hexagonal and cubic, and it was very important to controll deposition time to the film's crystal phase. All films in depth of approximate 100 nm existed above 65% transmittance, the absorption edge became 〝red-shift〞 with temperature rising. At 60 and 70 oC, with 20 min deposited-time, the energy band gap was more than 2.42 eV and decreased with time, while at 80 and 90 oC, the energy band gap was less than 2.42 eV and increased little when the time changed from 10 min to 15 min at 80 oC.  相似文献   
998.
欧利辉  陈胜利 《电化学》2011,17(2):155-160
应用密度泛函理论(DFT)反应能计算及最小能量路径分析研究了CO2在气相和电化学环境中于Cu(111)单晶表面的还原过程。气相中,CO2还原为碳氢化合物的反应路径可能为:CO2(g) + H* → COOH* → (CO +OH)* → CHO*;CHO + H* → CH2O* → (CH2 + O)*;CH2* + 2H* → CH4或2CH2* → C2H4。整个反应由CO2(g) + H* → COOH* → (CO +OH)*,(CO + H)* → CHO*和CH2O* → (CH2 + O)*等几个步骤联合控制。在-0.50V (vs RHE) 以正的电势下,CO2在Cu(111)表面电化学还原主要形成HCOO-和CO吸附物;随着电势逐渐负移,CO2加氢解离形成CO的反应越来越容易,CO成为主要产物;随电势进一步变负,形成碳氢化合物的趋势逐渐变强。与CO2的气相化学还原不同的是,电化学环境下CO质子化形成的CHO中间体倾向于解离形成CH,而在气相中CHO中间体则倾向于进一步质子化形成CH2O中间体。  相似文献   
999.
用同位旋相关的量子分子动力学(IQMD)模型计算了入射能量在30~200 MeV/u内, 不同中质比的反应系统(76Zn+76Zn, 76Ge+76Ge,76Se+76Se和76Kr+76Kr)碰撞产生的原子核阻止和反应产物能谱。 发现原子核阻止和不同方向的动能比值有很强的关联性,且可以用垂直方向和平行方向的碎片总动能比值的平方根近似地代替原子核阻止,用来提取同位旋相关介质中的核子-核子碰撞截面的信息。在核反应实验中,很容易测量到该物理量, 它是一个较好的探针。 The collision of different neutron/proton ratio, intermediate mass reaction systems(76Zn+76Zn, 76Ge+76Ge,76Se+76Se and 76Kr+76Kr) at the beam energy of 30~200 MeV/u are studied by using isospin dependent quantum molecular dynamics(IQMD) model. It is found that there exists a strong relationship between nuclear stopping and the ratio of different direction kinetic energy, and nuclear stopping can be replaced by the square root of the ratio of perpendicular and parallel total kinetic energy of fragments, and it can be used to extract information of isospin dependent in medium nucleon nucleon cross section. This physical quantity can be easily measured in nuclear reaction experiments, and it is a better probe.  相似文献   
1000.
在修改的Fisher模型框架下, 利用同质异位素产额比和结合能方程中对称能系数与温度比率的关联, 可以提取对称能系数与温度的比值。 利用3种不同的近似方法得到了重离子核反应产生丰中子余核的对称能系数与温度比(asym/T),并研究了相关物理量对asym/T的影响。结果表明, 库仑能对提取丰中子余核的asym/T影响较小,且参考核的选取对asym/T的提取也有一定的影响。 In the framework of the modified Fisher model, correlations between the symmetry energy coefficient in semi classical mass formula to temperature (asym/T) and the isobaric yield ratio in Heavy ion collisions are established. asym/Tof neutron rich fragments are extracted using these correlations. It is found that the Coulomb energy term has little effect on asym/T of fragments, while the reference isobars adopted have great influence on the extracted a sym/T of fragments.  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