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991.
992.
基于ARM和CPLD的NMR谱仪前放控制设计 总被引:1,自引:1,他引:0
提出了一种基于ARM(Advanced RISC Machine,先进精简指令计算机)和CPLD(Complex Programmable Logic Device, 复杂可编程逻辑器件)的NMR(Nuclear Magnetic Resonance,核磁共振)谱仪前放控制系统设计方案. 该控制系统以ARM为控制核心、 CPLD为控制命令执行单元, 控制前置放大器和混频器的工作状态. 本电路包括以下主要功能:1) 前置放大器收发切换的门控信号电平转换;2) 氘梯度匀场时身频通道自动切换;3) 探头调谐的信息显示;4) 锁前放和混频器的配置. 该设计方案具有系统运行速度高、 控制灵活和成本低的优点,并且可以有效减小系统噪声,降低控制部件对前置放大器和混频器的干扰. 相似文献
993.
高温高压生长宝石级金刚石单晶的表面特征研究 总被引:1,自引:1,他引:0
本文利用高温高压温度梯度法,NiMnCo合金作为触媒,分别采用籽晶{100}和{111}作为生长面,合成了Ib型宝石级金刚石单晶,对其表面特征进行了分析和讨论.结果发现,宝石级金刚石单晶的表面特征不具有唯一性,多数情况下,晶体{111}面明显较{100}面平整,而且{100}面生长台阶的棱角不清晰,经常会出现经触媒融融过的痕迹,并且这种现象的出现跟籽晶生长面不同和合成温度条件高低无关;{111}面有时也会出现明显的生长台阶,棱角清晰,并且形状较为规则.宝石级金刚石晶体表面特征的不唯一性说明晶体表面特征对生长条件稳定性有更高的要求. 相似文献
994.
BEHAVIOR OF AIR-ENTRAINED CONCRETE AFTER FREEZE-THAW CYCLES 总被引:2,自引:0,他引:2
Huaishuai Shang Yupu Song Jinping Ou 《Acta Mechanica Solida Sinica》2009,22(3):261-266
The experimental study of air-entrained concrete specimens subjected to different cycles of freeze-thaw was completed. The dynamic modulus of elasticity, weight loss, the cubic compressive strength, compressive strength, tensile strength and cleavage strength of air-entrained concrete were measured after 0, 100, 200, 300, 400 cycles of freeze-thaw. The experimental results showed that the dynamic modulus of elasticity and strength decreased as the freeze-thaw was repeated. The influences of freeze-thaw cycles on the mechanical properties, the dynamic modulus of elasticity and weight loss were analyzed according to the experimental results. It can serve as a reference for the maintenance, design and the life prediction of dams, hydraulic structures, offshore structures, concrete roads and bridges in northern cold regions. 相似文献
995.
This work studies the response of bodies governed by dipolar gradient elasticity to concentrated loads. Two-dimensional configurations
in the form of either a half-space (Flamant–Boussinesq type problem) or a full-space (Kelvin type problem) are treated and
the concentrated loads are taken as line forces. Our main concern is to determine possible deviations from the predictions
of plane-strain/plane-stress classical linear elastostatics when a more refined theory is employed to attack the problems.
Of special importance is the behavior of the new solutions near to the point of application of the loads where pathological
singularities and discontinuities exist in the classical solutions. The use of the theory of gradient elasticity is intended
here to model material microstructure and incorporate size effects into stress analysis in a manner that the classical theory
cannot afford. A simple but yet rigorous version of the generalized elasticity theories of Toupin (Arch. Ration. Mech. Anal.
11:385–414, 1962) and Mindlin (Arch. Ration. Mech. Anal. 16:51–78, 1964) is employed that involves an isotropic linear response and only one material constant (the so-called gradient coefficient)
additional to the standard Lamé constants (Georgiadis et al., J. Elast. 74:17–45, 2004). This theory, which can be viewed as a first-step extension of the classical elasticity theory, assumes a strain-energy
density function, which besides its dependence upon the standard strain terms, depends also on strain gradients. The solution
method is based on integral transforms and is exact. The present results show departure from the ones of the classical elasticity
solutions (Flamant–Boussinesq and Kelvin plane-strain solutions). Indeed, continuous and bounded displacements are predicted
at the points of application of the loads. Such a behavior of the displacement fields is, of course, more natural than the
singular behavior present in the classical solutions.
相似文献
996.
This paper examines the combination of radial deformation with torsion for a circular cylindrical tube composed of a transversely
isotropic hyperelastic material subject to finite deformation swelling. The stored energy function involves separate matrix
and fiber contributions such that the fiber contribution is minimized when the fiber direction is at a natural length. This
natural length is not affected by the swelling. Hence swelling preferentially expands directions that are orthogonal to the
fiber. The swelling itself is described via a swelling field that prescribes the local free volume at each location in the
body. Such a treatment is a relatively simple generalization of the conventional incompressible theory. The direction of transverse
isotropy associated with the fiber reinforcement is described by a helical winding about the tube axis. The swelling induced
preferential expansion orthogonal to this direction induces the torsional aspect of the deformation. For a specific class
of strain energy functions we find that the twist increases with swelling and approaches a limiting asymptotic value as the
swelling becomes large. The fibers reorient such that fibers at the inner portion of the tube assume a more circumferential
orientation whereas, at least for small and moderate swelling, the fibers in the outer portion of the tube assume a more axial
orientation. For large swelling the fibers in the outer portion of the tube reorient beyond the axial orientation, and so
are described by helices with orientation in the opposite sense to that in the reference configuration.
相似文献
997.
Consider an incompressible, hyperelastic material occupying the unit ball B⊂ℝ
n
in its reference state. Suppose that the deformation u:B→ℝ
n
is specified on the boundary by
where λ>1 is a given constant.
In this paper, isoperimetric arguments are used to prove that the radial deformation, producing a spherical cavity, is the
energy minimiser in a general class of isochoric mappings that are discontinuous at the centre of the ball and produce a (possibly
non-symmetric) cavity in the deformed body. This result has implications for the study of cavitation in certain polymers.
This work was supported in part by the National Science Foundation under Grant No. DMS–0405646. 相似文献
998.
The present work concerns with the investigation of the interior transmission problem, which is naturally associated to the
inverse elastic scattering problem of determining the support of an isotropic homogeneous penetrable body from a knowledge
of the time harmonic incident plane waves and the far-field patterns of the corresponding scattered wave-fields. Our approach
combines a boundary integral formulation of the problem and a compact perturbation argument to establish the discreteness
of the set of transmission eigenvalues and the well-posedness of the interior transmission problem under the most general
assumptions on the elastic parameters of the underlying media.
相似文献
999.
V形切口应力强度因子的一种边界元分析方法 总被引:1,自引:0,他引:1
将V形切口结构分成围绕切口尖端的小扇形和剩余结构两部分. 尖端处扇形域应力场表示成关于尖端距离$\rho$的渐近级数展开式,从线弹性理论方程推导出了一组分析平面V形切口奇异性的常微分方程特征值问题,通过求解特征方程,得到前若干个奇性指数和相应的特征向量. 再将切口尖端的位移和应力表示为有限个奇性阶和特征向量的组合. 然后用边界元法分析挖去小扇形后的剩余结构. 将位移和应力的线性组合与边界积分方程联立,求解获得切口根部区域的应力场、应力幅值系数和整体结构的位移和应力. 从而准确计算出平面V形切口的奇异应力场和应力强度因子. 相似文献
1000.
An analysis for vibration of non-homogenous visco-elastic rectangular plate of linearly varying thickness subjected to thermal
gradient has been discussed in the present investigation. For visco-elastic, the basic elastic and viscous elements are combined.
We have taken Kelvin model for visco-elasticity that is the combination of the elastic and viscous elements in parallel. Here
the elastic element means the spring and the viscous element means the dashpot. The governing differential equation of motion
has been solved by Galerkin’s technique. Deflection, time period and logarithmic decrement at different points for the first
two modes of vibration are calculated for various values of thermal gradients, non homogeneity constant, taper constant and
aspect ratio for non-homogenous visco-elastic rectangular plate which is clamped on two parallel edges and simply supported
on remaining two edges. Comparison studies have been carried out with homogeneous visco-elastic rectangular plate to establish
the accuracy and versatility. 相似文献