全文获取类型
收费全文 | 5032篇 |
免费 | 116篇 |
国内免费 | 8篇 |
专业分类
化学 | 2957篇 |
晶体学 | 125篇 |
力学 | 167篇 |
数学 | 391篇 |
物理学 | 1516篇 |
出版年
2023年 | 34篇 |
2022年 | 113篇 |
2021年 | 108篇 |
2020年 | 114篇 |
2019年 | 123篇 |
2018年 | 120篇 |
2017年 | 159篇 |
2016年 | 190篇 |
2015年 | 135篇 |
2014年 | 182篇 |
2013年 | 390篇 |
2012年 | 298篇 |
2011年 | 312篇 |
2010年 | 212篇 |
2009年 | 188篇 |
2008年 | 247篇 |
2007年 | 212篇 |
2006年 | 146篇 |
2005年 | 147篇 |
2004年 | 115篇 |
2003年 | 100篇 |
2002年 | 108篇 |
2001年 | 59篇 |
2000年 | 54篇 |
1999年 | 35篇 |
1998年 | 32篇 |
1997年 | 38篇 |
1996年 | 44篇 |
1995年 | 29篇 |
1994年 | 33篇 |
1993年 | 41篇 |
1992年 | 52篇 |
1991年 | 36篇 |
1990年 | 40篇 |
1989年 | 49篇 |
1988年 | 44篇 |
1987年 | 42篇 |
1986年 | 32篇 |
1985年 | 61篇 |
1984年 | 57篇 |
1983年 | 40篇 |
1982年 | 44篇 |
1981年 | 51篇 |
1980年 | 45篇 |
1979年 | 46篇 |
1978年 | 43篇 |
1977年 | 51篇 |
1976年 | 54篇 |
1975年 | 44篇 |
1974年 | 38篇 |
排序方式: 共有5156条查询结果,搜索用时 15 毫秒
71.
The instability of the plane interface between two uniform, superposed, electrically conducting and counter-streaming fluids through a porous medium is considered in the presence of a horizontal magnetic field. In the absence of surface tension, perturbations transverse to the direction of streaming are found to be unaffected by the presence of streaming if perturbations in the direction of streaming are ignored. For perturbations in all other directions there exists instability for a certain wavenumber range. The instability of this system is postponed by the presence of magnetic field. The magnetic field and surface tension are able to suppress this Kelvin-Helmholtz instability for small wavelength perturbations and the medium porosity reduces the stability range given in terms of a difference between the streaming velocities and the Alfvén velocity.This research forms a part of the research project awarded to the first author (R.C.S.) by the University Grants Commission. 相似文献
72.
The thermosolutal instability of a plasma in porous medium is considered in the presence of finite Larmor radius effect. The finite Larmor radius, stable solute gradient and magnetic field introduce oscillatory modes in the systems which were nonexistent in their absence. For stationary convection, the finite Larmor radius and stable solute gradient have stabilizing effects on the thermosolutal instability in porous medium. In presence of finite Larmor radius effect, the medium permeability has a destabilizing (or stabilizing) effect and the magnetic field has a stabilizing (or destabilizing) effect under certain condition whereas in the absence of finite Larmor radius effect, the medium permeability and the magnetic field have destabilizing and stabilizing effects, respectively, on thermosolutal instability of a plasma in porous medium. The sufficient conditions for nonexistence of overstability are obtained.The financial assistance to Mr. Sunil in the form of Senior Research Fellowship of the Council of Scientific and Industrial Research (CSIR), New Delhi is gratefully acknowledged. 相似文献
73.
Recent X-ray diffraction studies on α-quartz (SiO2) by Kingmaet al [1], have shown the occurrence of a reversible, crystalline-to-crystalline, phase transition just prior to amorphization
at ≈ 21 GPa. This precursor transition has also been confirmed by our recent molecular dynamics simulation study [2]. In order
to investigate the possibility of a similar behaviour in other isostructural compounds, which also undergo pressure induced
amorphization, α-GeO2 and α-AlPO4 (berlinite form) were studied using energy dispersive X-ray diffraction. In either of these materials, no such phase transition
is detected prior to amorphization. The onset of amorphization and its reversal is found to be time dependent in GeO2. 相似文献
74.
Ramesh Sharma 《Journal of Geometry》2003,78(1-2):156-167
Contact hypersurfaces of a Kaehler manifold have been
characterized and classified, assuming the second fundamental form to be
Codazzi (in particular, parallel). We have also discussed the special
cases when the ambient space is a (i) Calabi-Yau manifold and (ii) a
complex space-form. 相似文献
75.
Fulton R Jensen T Johnson DR Kagan H Kass R Morrow F Whitmore J Wilson P Bortoletto D Chen W Dominick J McIlwain RL Miller DH Ng CR Schaffner SF Shibata EI Shipsey IP Yao W Battle M Sparks K Thorndike EH Wang C Alam MS Kim IJ Li WC Romero V Sun CR Wang P Zoeller MM Goldberg M Haupt T Horwitz N Jain V Mestayer MD Moneti GC Rozen Y Rubin P Sharma V Skwarnicki T Thulasidas M Zhu G Csorna SE Letson T Alexander J Artuso M Bebek C Berkelman K Browder T Cassel DG Cheu E Coffman DM Crawford G DeWire JW 《Physical review D: Particles and fields》1991,43(3):651-663
76.
R. Singh R. M. Misra D. C. Gupta D. D. Shukla M. N. Sharma M. P. Madan 《Zeitschrift für Physik B Condensed Matter》1985,58(2):83-89
The parameterC
1=[(1/K
T
)/P]
T
, which describes the pressure variation of the compressibility, has been examined correlating the thermodynamical and interatomic potential approaches employing fewer approximations than has been usual heretofore. General expressions have been derived forC
1 by including the thermal correction terms, which have generally been ignored in previous studies concerning thermal properties of ionic crystals. The parameterC
1 has also been related to the Grüneisen parameter, , using a relation given earlier. The applicability of the derived equations is investigated and discussed for alkali halides employing few realistic potential forms. A good general accord is found with the available experimental data, which exhibits an essential improvement over other theoretical determinations. 相似文献
77.
A 1Σ u + -X 1Σ g + emission in Na2 is observed following excitation ofB 1π u by various lines of an argon ion laser. The excitation energy ofB 1π u is collisionally transferred to the (2)1Σ g + which then radiatively populates theA 1Σ u + state. The Na vapour is contained in a stainless steel crossed heat pipe with Ar buffer gas and temperature around 600°C. For all laser lines except 4579 Å, the coarse features ofA-X emission are independent of the laser wavelength. However, at high resolution the finer differences between different laser line excitation are explained. Variousv′-v″ transitions in this emission are identified. Computer simulation is presented to help explain some features of this emission. 相似文献
78.
An explicit expression has been derived for the volume dependence of electron-phonon coupling strength () by using a recently proposed quadratic form of the screened pseudopotential and by considering the variation of Fermi momentum with volume. Variation of Coulomb pseudopotential (
*) has also been explicitly considered by varying the Fermi momentum. Dependence of transition temperatureT
c
and the logarithm of the effective interaction strength (N
0
V) on the volume has been studied for eight nontransition metallic superconductors. A linear drop inT
c
for small values of (–/
0) as well as the calculated values of= InN
0
V/ In, exhibits better matching with the experimental data, as compared to the earlier theoretical results. 相似文献
79.
Theoretical and experimental study of the surface plasmon–polariton and guided wave plasmon polariton modes is presented for the Sapphire/Ag/Polycarbonate/Air structure. Theoretical results are obtained by solving complex multilayer eigenvalue equations as well as the reflectivity equation for this structure. It is proposed that the mode attenuation can be significantly reduced by inserting a low index dielectric buffer between the metal and the guiding dielectric layer. The dispersion and attenuation curves are generated. Both the surface plasmon and guided wave plasmon polariton modes are studied experimentally. The experimental values of the effective refractive indices agree well with the theoretical values. The electric field profiles are generated and used to examine the nature of modes. After optimization of various parameters the condition for low loss single mode guiding is obtained for the proposed structure. Effect of metal thickness on surface plasmon mode is also discussed. It is inferred that in a properly optimized plasmonic waveguide, the losses can be reduced by a factor of 4. 相似文献
80.
Sweta Sharma 《光谱学快报》2013,46(8):520-528
ABSTRACTThe present paper deals with the investigation of the changes in chemical contents of the leaf of wheat seedling stressed by excess manganese using attenuated total reflectance Fourier transform infrared spectroscopy (FTIR) technique. The infrared spectra of the leaves of control and manganese-treated wheat plants have been recorded in the spectral region 485–4000?cm?1 at a resolution of 4?cm?1. The recorded spectrum has been analyzed with the help of curve-fitting method for the quantitative estimation of chemical contents and conformational changes. The study indicated changes arising in the polysaccharide, lignin, amino acid, secondary structure of protein, and lipid in the leaves of control and manganese-treated wheat plants. Manganese treatment increased the amount of cellulose, lignin, and amide II till 200?μM concentration, while a decrease was observed at 1000?µM concentration. Manganese induced conformational changes in the secondary structure of protein which was indicated by shifting of the bands to the higher wavenumber and change in the β sheet/α helix ratio. In addition, manganese stress decreased lipid content in the leaves of the wheat seedlings. The study demonstrates the potential of attenuated total reflectance FTIR for the non-invasive and rapid monitoring of the plants stressed with heavy metals. 相似文献