全文获取类型
收费全文 | 4033篇 |
免费 | 474篇 |
国内免费 | 446篇 |
专业分类
化学 | 2109篇 |
晶体学 | 31篇 |
力学 | 689篇 |
综合类 | 44篇 |
数学 | 360篇 |
物理学 | 1720篇 |
出版年
2023年 | 38篇 |
2022年 | 65篇 |
2021年 | 78篇 |
2020年 | 79篇 |
2019年 | 76篇 |
2018年 | 92篇 |
2017年 | 139篇 |
2016年 | 116篇 |
2015年 | 116篇 |
2014年 | 187篇 |
2013年 | 315篇 |
2012年 | 199篇 |
2011年 | 251篇 |
2010年 | 207篇 |
2009年 | 242篇 |
2008年 | 222篇 |
2007年 | 273篇 |
2006年 | 278篇 |
2005年 | 231篇 |
2004年 | 232篇 |
2003年 | 209篇 |
2002年 | 168篇 |
2001年 | 115篇 |
2000年 | 130篇 |
1999年 | 148篇 |
1998年 | 112篇 |
1997年 | 110篇 |
1996年 | 73篇 |
1995年 | 65篇 |
1994年 | 54篇 |
1993年 | 53篇 |
1992年 | 49篇 |
1991年 | 31篇 |
1990年 | 25篇 |
1989年 | 29篇 |
1988年 | 11篇 |
1987年 | 20篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 20篇 |
1983年 | 5篇 |
1982年 | 13篇 |
1981年 | 13篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 2篇 |
1976年 | 8篇 |
1959年 | 1篇 |
1957年 | 1篇 |
排序方式: 共有4953条查询结果,搜索用时 734 毫秒
31.
旋转带电体磁矩计算的若干法则与算例 总被引:2,自引:0,他引:2
本文在文献 [1]、[2 ]等的基础上给出关于旋转带电体的磁矩计算的若干法则 ,均以定理形式表达 ,并列表枚举其相关算例 相似文献
32.
The rotational spectra of nine isotopomers of dimethyl diselenide, CH3SeSeCH3, have been measured with a molecular-beam Fourier transform microwave spectrometer. The spectra were complex due to the presence of many isotopomers in natural abundance and the splitting caused by the interactions with two methyl internal rotors. The spectra were assigned and fit to experimental precision to an effective rotational Hamiltonian for molecules with two periodic internal motions. The spectra of the symmetric isotopomers are consistent with a C2 equilibrium structure. The rotational constants were used to determine the rs structure of the C-Se-Se-C frame with the results r(SeSe)=2.306(3) Å, r(SeC)=1.954(6) Å, ?(CSeSe)=99.8(2)°, ?(CSeSeC)=85.2(1)°. A barrier to internal rotation of the methyl groups of 395 ± 2 cm−1 was derived from the internal rotation splittings. 相似文献
33.
Tjalling J. Ypma 《Numerical Linear Algebra with Applications》1995,2(6):533-541
An algorithm for pre- or post-multiplication of a matrix by a plane rotation, using only three vector saxpy operations instead of the four vector operations usually considered necessary, is described. No auxiliary storage for overwriting is required. The method is shown to be numerically stable. 相似文献
34.
M. Stenger S. Baumhoff J.-Y. Prieur J. Joffrin J.-P. Contour 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(2):177-182
We present measurements of the attenuation and phase velocity of surface acoustic waves in thin YBa2Cu3O films as a function of temperature, in magnetic fields up to 3.6 T applied parallel to the c-axis of the films. We have observed anomalies in both, the attenuation and the phase velocity in the vicinity of the superconducting
critical temperature which do not depend on the magnetic field. Possible origins of these anomalies, observed, to our knowledge,
for the first time in YBa2Cu3O thin films, are discussed and compared to bulk acoustic wave experiments. We present a kind of feedback technique for surface
acoustic waves which improves the sensitivity of this type of measurement. The actual sensitivity limits are mentioned.
Received: 7 August 1997 / Revised: 7 November 1997 / Accepted: 17 November 1997 相似文献
35.
S.F. Lienin R. Brüschweiler R.R. Ernst 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,131(2):184-190
The viscosity-dependent retarding effect of a polymeric solvent on the rotation of small solute molecules is investigated by13C NMR relaxation measurements. It is found that the relaxation data of 1,3-dibromoadamantane in highly viscous polymeric chlorotrifluoroethene can be explained neither by isotropic nor by realistic anisotropic tumbling in a single environment. The experimental data are rationalized in terms of fast exchange between at least two environments with correlation times differing by up to two orders of magnitude. The study shows that a uniform retardation of molecular tumbling by a polymeric solvent, desirable for shifting the NMR observation window in studies of intramolecular mobility, is not always feasible. 相似文献
36.
Asymptotic expansions for the exponential growth rate, known as the Lyapunov exponent, and rotation numbers for two coupled oscillators driven by real noise are constructed. Such systems arise naturally in the investigation of the stability of steady-state motions of nonlinear dynamical systems and in parametrically excited linear mechanical systems. Almost-sure stability or instability of dynamical systems depends on the sign of the maximal Lyapunov exponent. Stability conditions are obtained under various assumptions on the infinitesimal generator associated with real noise provided that the natural frequencies are noncommensurable. The results presented here for the case of the infinitesimal generator having a simple zero eigenvalue agree with recent results obtained by stochastic averaging, where approximate ItÔ equations in amplitudes and phases are obtained in the sense of weak convergence.Dedicated to Thomas K. Caughey on the occasion of his 65th birthday. 相似文献
37.
We show that the composition of not only two SO(3,1) boosts, but also that of two SO(n,1) boosts for anyn 2, is basically an SO(2,1) problem and hence can be analysed completely using SL(2,R) matrices. By computing the expression for the Thomas/Wigner angle directly using SL(2,R) matrices we show that this approach results in considerable economy of algebra. 相似文献
38.
The study of the partitioning mechanism of methyl orange in an aqueous two-phase system 总被引:2,自引:0,他引:2
An aqueous two-phase system of dodecyl triethylammonium bromide (C12NE, cationic surfactant) and sodium dodecyl sulfate (SDS, anionic surfactant) mixture is proposed for the extraction of some dyes and porphyrin compounds. Transparent two phase-systems are formed when the surfactant concentrations and C12NE/SDS ratios are in certain regions. In this study, the aqueous two phase-systems were prepared by mixing 0.1 mol l−1 C12NE and SDS with a molar ratio of 1.7:1.0. The results showed that negatively charged chlorophyll (sodium copper chlorophyllin) and positively charged dye (methyl violet) were efficiently extracted into the upper phase. The negatively charged methyl orange (pH>7) was moved into the upper phase mostly while amphoteric methyl orange (pH<3) was distributed in the two phases uniformly. Except for hydrophobic force, charge interaction between solute and surfactant also play an important role in the extraction process. 相似文献
39.
用膨胀计法研究了西氯苯醌(CA)、苯醌(BQ)与受阻胺二异丙胺(DIPA)、2,2,6,6-四甲基-4-羟基哌啶(TMP)形成的电荷转移络合物(CTC)对偶氮二异丁腈(AIBN)引发的甲基丙烯酸甲酯(MMA)自由基聚合阻聚效应。结果表明,CA-DIPA、CA-TMP、BQ-DIPA和BQ-TMP对MMA的自由基聚合均产生较单独的醌或胺更显著的阻聚能力。其中以CA-DIPA阻聚效果更佳。比较了MMA在苯或乙腈中的溶液聚合,得类似结果。初步讨论了受阻胺-醌电荷转移络合物对MMA自由基聚合的阻聚反应机理。 相似文献
40.
甲基橙褪色动力学光度法测定槲皮素的研究 总被引:1,自引:0,他引:1
基于稀H2SO4介质中,槲皮素对KBrO3氧化甲基橙褪色反应的明显催化作用,建立了测定痕量槲皮素的动力学光度法。在选定的条件下,方法的线性范围为0~0.05 mg/L,表观摩尔吸光系数为4.52×106L.mol-1.cm-1,检出限为0.0012 mg/L。利用此法测定了银杏叶和中药制剂慢严舒柠中的槲皮素,加标回收率为95%~106.7%,相对标准偏差小于3.3%。 相似文献