全文获取类型
收费全文 | 80049篇 |
免费 | 2651篇 |
国内免费 | 1069篇 |
专业分类
化学 | 43988篇 |
晶体学 | 779篇 |
力学 | 2867篇 |
综合类 | 507篇 |
数学 | 15060篇 |
物理学 | 20568篇 |
出版年
2023年 | 906篇 |
2022年 | 1813篇 |
2021年 | 2084篇 |
2020年 | 2598篇 |
2019年 | 1653篇 |
2018年 | 2266篇 |
2017年 | 2758篇 |
2016年 | 2814篇 |
2015年 | 2908篇 |
2014年 | 3838篇 |
2013年 | 5754篇 |
2012年 | 4235篇 |
2011年 | 4206篇 |
2010年 | 3668篇 |
2009年 | 3481篇 |
2008年 | 2945篇 |
2007年 | 3379篇 |
2006年 | 3016篇 |
2005年 | 2395篇 |
2004年 | 1934篇 |
2003年 | 1758篇 |
2002年 | 1523篇 |
2001年 | 1844篇 |
2000年 | 1046篇 |
1999年 | 1472篇 |
1998年 | 1057篇 |
1997年 | 848篇 |
1996年 | 695篇 |
1995年 | 566篇 |
1994年 | 514篇 |
1993年 | 447篇 |
1992年 | 516篇 |
1991年 | 431篇 |
1990年 | 431篇 |
1989年 | 470篇 |
1988年 | 1368篇 |
1987年 | 2204篇 |
1986年 | 2128篇 |
1985年 | 643篇 |
1984年 | 550篇 |
1983年 | 337篇 |
1982年 | 487篇 |
1981年 | 571篇 |
1980年 | 440篇 |
1979年 | 383篇 |
1978年 | 422篇 |
1977年 | 336篇 |
1976年 | 403篇 |
1975年 | 243篇 |
1973年 | 227篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
火焰原子吸收光谱法测定生物样品中锌、铜和锰 总被引:1,自引:1,他引:0
生物样品经消化处理后,用火焰原子吸收光谱法测定样品中锌、铜和锰的含量,平均加标回收率为97.3%-102.4%,相对标准偏差(RSD)为0.65 %-1.40%.锌、铜和锰在给定范围内呈现良好的线性关系,相关系数分别为0.9994、0.9994和0.9983,方法简便,快速,结果可靠. 相似文献
72.
An Nd:YAG laser-pumped periodically poled RbTiOAsO4 (PPRTA) optical parametric oscillator (OPO) near degeneracy is reported. At a pump power of 3.7 W, a net conversion efficiency of 43% was obtained. These crystals were found to be phase-matchable for the higher order quasi-phase-matched second-harmonic generation (SHG) in the visible range. Detailed analyses of the phase-matching properties for these processes were made by using the published Sellmeier and thermo-optic dispersion formulas. 相似文献
73.
Bingxin LiuYoshio Tatamitani Jun ShimadaTeruhiko Ogata 《Journal of Molecular Spectroscopy》2002,211(1):99-106
The microwave spectrum of the 35Cl and 37Cl isotopic species of 1-chloro-1,1,2-trifluoroethane (HCFC-133b) has been investigated in the frequency region 10 to 50 GHz using a Stark modulation microwave spectrometer. A pulsed jet Fourier transform microwave spectrometer was also used for the measurement of hyperfine splittings. A least-squares analysis of the observed b-type Q- and R-branch transition frequencies gave rotational and centrifugal distortion constants and components of the chlorine nuclear quadrupole coupling constant tensors in the principal axes system as follows: A=4625.161 (3) MHz, B=2004.127 (2) MHz, C=1875.813 (2) MHz, ΔJ=0.144 (9) kHz, ΔJK=1.0748 (8) kHz, ΔK=1.57 (1) kHz, δJ=0.01376 (4) kHz, δK=−0.146 (4) kHz, χaa=−57.958 (10) MHz, χbb=21.231 (11) MHz, and χcc=36.727 (11) MHz for 35ClCF2CH2F species, and A=4607.684 (6) MHz, B=1960.565 (2) MHz, C=1834.823 (2) MHz, ΔJ=0.106 (7) kHz, ΔJK=1.022 (3) kHz, ΔK=1.48 (1) kHz, δJ=0.0142 (2) kHz, δK=−0.18 (2) kHz, χaa=−46.268 (11) MHz, χbb=17.319 (13) MHz, and χcc=28.950 (13) MHz for 37ClCF2CH2F species. The structural parameters are calculated from the observed six rotational constants by assuming the partial structure of ab initio calculation. The electronic properties of the C-Cl bond are evaluated from the observed nuclear quadrupole constants of chlorine. These molecular properties are compared with those of other related molecules. 相似文献
74.
A.R. DegheidyM.T. Attia M. Sallah 《Journal of Quantitative Spectroscopy & Radiative Transfer》2002,74(3):285-297
The time-independent radiative transfer problem in a scattering and absorbing planar random medium with general boundary conditions and internal energy source is considered. The medium is assumed to consist of two randomly mixed immiscible fluids, with the mixing statistics described as a two-state homogeneous Markov process. The problem is solved in terms of the solution of the corresponding free-source problem with simple boundary conditions which is solved using Pomraning-Eddington approximation in the deterministic case. A formalism, developed to treat radiative transfer in statistical mixtures, is used to obtain the ensemble-averaged solution. The average partial heat fluxes are calculated in terms of the albedoes of the source-free problem. Results are obtained for isotropic and anisotropic scattering for specular and diffused reflecting boundaries. 相似文献
75.
Dikandé A. M. 《The European Physical Journal B - Condensed Matter and Complex Systems》2004,42(2):247-253
Small and large-amplitude elastic deformations of the armchair structure of single-walled carbon nanotubes are investigated with emphasis on the cylindrical geometry. As starting model, we consider a discrete one-dimensional lattice of atoms interacting via a Lennard-Jones type two-body potential. In an expansion scheme using cylindrical coordinates where radial displacements are assumed negligible compared to the angular motions, a sine-lattice Hamiltonian is derived. In the limit of small-amplitude angular displacements, the dispersion spectrum of acoustic phonons is derived and the associate characteristic frequency is given as a function of parameters of the model. In the large-amplitude regime, lattice vibrations give rise to kink-type deformations which move undergoing lattice dispersion and lattice discreteness effects. The dispersion law of the kink motion is obtained and shown to lower the effect of lattice discreteness, giving rise to a vanishing Peierls stress for kink sizes of the order of a few lattice spacings. Implications of the coupling of two armchair structures on the stability of vibrational modes of an individual armchair nanotube are also discussed. A gap of forbidden modes is predicted in the phonon spectrum while the energy needed to create a kink deformation in individual nanotubes is shifted in the presence of a wall-to-wall interaction.Received: 2 August 2004, Published online: 14 December 2004PACS:
81.07.De Nanotubes - 62.30. + d Mechanical and elastic waves-vibrations - 63.22. + m Phonons in low-dimensional nanoscale materials - 63.20.Ry Anharmonic lattices modes 相似文献
76.
Shinji Takesue 《Journal of statistical physics》1989,56(3-4):371-402
This is the first part of a series devoted to the study of thermodynamic behavior of large dynamical systems with the use of a family of fully-discrete and conservative models named elementary reversible cellular automata (ERCAs). In this paper, basic properties such as conservation laws and phase space structure are investigated in preparation for the later studies. ERCAs are a family of one-dimensional reversible cellular automata having two Boolean variables on each site. Reflection and Boolean conjugation symmetries divide them into 88 equivalence classes. For each rule, additive conserved quantities written in a certain form are regarded as a kind of energy, if they exist. By the aid of the discreteness of the variables, every ERCA satisfies the Liouville theorem or the preservation of phase space volume. Thus, if an energy exists in the above sense, statistical mechanics of the model can formally be constructed. If a locally defined quantity is conserved, however, it prevents the realization of statistical mechanics. The existence of such a quantity is examined for each class and a number of rules which have at least one energy but no local conservation laws are selected as hopeful candidates for the realization of thermodynamic behavior. In addition, the phase space structure of ERCAs is analyzed by enumerating cycles exactly in the phase space for systems of comparatively small sizes. As a result, it is revealed that a finite ERCA is not ergodic, that is, a large number of orbits coexist on an energy surface. It is argued that this fact does not necessarily mean the failure of thermodynamic behavior on the basis of an analogy with the ergodic nature of infinite systems. 相似文献
77.
《ournal of non Newtonian Fluid Mechanics》2006,136(1):38-49
We have investigated the orientation state of a dilute fiber suspension flow in a planar contraction at high Reynolds numbers in turbulent flow. High speed imaging is used to directly measure the orientation distribution function at different downstream positions along the contraction centerline. The results from the direct measurement of the orientation distribution are used to evaluate the existing closure models. The results show that the fitted orthotropic and natural closure approximations give almost identical results with the best agreement to the orientation distribution in the contraction flow considered here. 相似文献
78.
《Electrochemistry communications》2007,9(7):1783-1787
Au nanoparticles dispersed over a self-organized nanotubular TiO2 matrix can be used as a highly efficient catalyst system for the electrochemical oxygen reduction reaction in aqueous solutions. For the same loading of Au nanoparticles, the nanotubular support provides a manifold increase in the reaction rate in comparison with a flat TiO2 support, or a pure Au sheet electrode. 相似文献
79.
Hiroshi Maruyama Tadao Ishii Naruhiko Bamba Hironobu Maeda Akihisa Koizumi Yuzo Yoshikawa Hitoshi Yamazaki 《Physica C: Superconductivity and its Applications》1989,160(5-6):524-532
The temperature dependence of the extended X-ray absorption fine structure (EXAFS) is studied in the high Tc superconductors, YBa2Cu3O7−δ. The measurements were done at the Cu K-edge for samples of two orthorhombic phases (Tc≈90 K and ≈58 K, respectively) and a nonsuperconducting tetragonal phase. Interatomic distances and mean square relative displacements σ2 for Cu-O bonds are determined by the least squares refinement. The results indicate that values of σ2 increase near Tc for both the orthorhombic samples. It is concluded that this anomalous behavior related to Tc is caused by an anomalous vibration of oxygen atoms in the Ba-O layer. Changes in the Cu-O distances from 300 to 20 K are not found. 相似文献
80.
Trond Stølen Gustavsen 《Compositio Mathematica》2003,138(2):199-231
We relate the equisingular deformation theory of plane curve singularities and sandwiched surface singularities. We show the existence of a smooth map between the two corresponding deformation functors and study the kernel of this map. In particular we show that the map is an isomorphism when a certain invariant is large enough. 相似文献