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971.
The effect of lanthanum substitution on the structural, dielectric and piezoelectric properties of lead calcium titanate (PCT) ceramics is studied. Samples with compositional formula Pb0.76? x La x Ca0.24Ti0.98Mn0.02O3 (PLCT) (x?=?0–0.08, in steps of 0.02) were prepared by the conventional dry ceramic technique. Phase and structural analysis was carried out using XRD and SEM respectively. X-ray analysis confirmed the formation of single-phase compound with tetragonal crystal structure. Dielectric properties were studied in detail as a function of frequency and temperature. The Curie temperature (T C) and tetragonality (c/a) was found to decrease with increase in lanthanum content. The Curie temperature was also confirmed from the thermal-expansion behaviour of the sintered samples. Piezoelectric coefficients (d 33, d 31, g 33, g 31, g h, d h, k t and k p) were measured at room temperature. From this study it is found that La3+ substitution helps in the improvement of electro-mechanical anisotropy. 相似文献
972.
973.
Thermal post-buckling paths of homogeneous, isotropic, square plate configurations resting on elastic foundation (Winkler type) subjected to biaxial compressive thermal loads are expressed as simple closed-form solutions by using the Rayleigh–Ritz method based on coupled displacement fields. Geometric non-linearity of von-Karman type is considered. The in-plane displacement field variations used in the formulation of Rayleigh–Ritz method are derived by using the governing in-plane static differential equations of the plate which in turn simplifies the difficulty of assuming an in-plane displacement field variations of the square plate. Accuracy and robustness of the proposed closed-form solutions are demonstrated by using the non-linear finite element formulation results which are obtained from an equilibrium path approach. 相似文献
974.
Presented herein is the prediction of buckling behavior of size-dependent microbeams made of functionally graded materials (FGMs) including thermal environment effect. To this purpose, strain gradient elasticity theory is incorporated into the classical third-order shear deformation beam theory to develop a non-classical beam model which contains three additional internal material length scale parameters to consider the effects of size dependencies. The higher-order governing differential equations are derived on the basis of Hamilton’s principle. Afterward, the size-dependent differential equations and related boundary conditions are discretized along with commonly used end supports by employing generalized differential quadrature (GDQ) method. A parametric study is carried out to demonstrate the influences of the dimensionless length scale parameter, material property gradient index, temperature change, length-to-thickness aspect ratio and end supports on the buckling characteristics of FGM microbeams. It is revealed that temperature change plays more important role in the buckling behavior of FGM microbeams with higher values of dimensionless length scale parameter. 相似文献
975.
N. V. Petrova I. N. Yakovkin 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(1):17-24
The associative desorption of hydrogen from the Ru(10
0) surface has been
studied on the atomic level by means of density-functional calculations with
various exchange-correlation functionals and kinetic Monte Carlo
simulations. The simulations reproduce forming structures of the layers and
main features of TPD spectra for a wide range of hydrogen coverages. In
particular, it has been shown that the decrease of binding energies with
coverage due to lateral repulsion is correlated with the appearance of
low-temperature peaks in the spectra. 相似文献
976.
S. Matsunaga S. Tamaki 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(4):417-424
The theory of the pre-melting phenomena in ionic crystals on the basis of the concept of the hetero phase fluctuation has
been applied to KCl and AgCl crystal. The large scale molecular dynamics simulations (MD) in KCl and AgCl crystals are also
performed to examine the ionic configuration in premelting region in the vicinity of their melting points. The size of the
liquid like clusters are estimated by the theory and MD. The structural features of liquid like clusters are discussed by
MD results using the Lindemann instability condition. The ionic conductivities in the pre-melting region are also discussed
on the same theoretical basis. 相似文献
977.
A. I. Karasevskii V. V. Lubashenko 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(3):375-383
We demonstrate that the discrete character of the vibrational spectrum of a small crystal accounts for size dependence of
its thermodynamic properties and melting temperature. Using a self-consistent statistical method [Phys. Rev. B 66, 054302 (2002)] we derive the Gibbs free energy of free nanocrystalline plates and calculate the thermodynamic parameters
as functions of plate thickness for Cu. 相似文献
978.
W. Wang X. Chen Q. Cai G. Mo L. S. Jiang K. Zhang Z. J. Chen Z. H. Wu W. Pan 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,65(1):57-64
Poly(vinylpyrrolidone) (PVP)-coated platinum (Pt) nanoparticles were prepared in methanol-water reduction method. In situ
small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) techniques were used to probe the size change of particles
and crystallites with temperature. Tangent-by-tangent (TBT) method of SAXS data analysis was improved and used to get the
particle size distribution (PSD) from SAXS intensity. Scherrer’s equation was used to derive the crystallite size from XRD
pattern. Combining SAXS and XRD results, a step-like characteristic of the Pt nanoparticle growth has been found. Three stages
(diffusion, aggregation, and agglomeration) can be used to describe the growth of the Pt nanoparticles and nanocrystallites.
Aggregation was found to be the main growth mode of the Pt nanoparticles during heating. The maximum growth rates of Pt nanoparticles
and Pt nanocrystallites, as well as the maximum aggregation degree of Pt nanocrystallites were found, respectively, at 250
°C, 350 °C and 300 °C. These results are helpful to understanding the growth mode of nanoparticles, as well as controlling
the nanoparticle size. 相似文献
979.
《Tetrahedron》2019,75(35):130487
We designed and synthesized photochromic diarylethenes for a write-by-light/erase-by-heat recording system. The introduction of polar substituents at both sides of the diarylethene maintained the photocyclization and photocycloreversion reactivities, but significantly affected the thermal cycloreversion reactivity. The introduction of electron-withdrawing substituents accelerated the thermal reaction and the introduction of electron-donating substituents suppressed the thermal reaction. The rate constants of the thermal reaction were well correlated with Brown-Okamoto's substituent constant σ+ that is a modified value of Hammett's substituent constant σ. The large rate constants are ascribed to the lower activation energy for the thermal reaction. These results provide new knowledge for the molecular design of diarylethenes for a write-by-light/erase-by-heat recording system. 相似文献
980.
Ivan Gospodinov Johannes Singer Thomas M. Klaptke Jrg Stierstorfer 《无机化学与普通化学杂志》2019,645(21):1247-1254
In the present studies, the synthesis of new energetic materials based on the pyridazine scaffold and their characterization is the main subject. For this purpose, desired 3,5‐dimethoxy‐4,6‐dinitropyridazine‐1‐oxide ( 7 ) was synthesized in the first instance. The persubstituted pyridazine precursor laid the groundwork for further preparative modification. The targeted functionalization through the regioselective introduction of various smaller amine nucleophiles such as methylamine or 2‐aminoethanol gave several new energetic materials. Among them are 3,5‐bis(methylamino)‐4,6‐dinitropyridazine‐1‐oxide ( 8 ), 3,5‐bis(methylnitramino)‐4,6‐dinitropyridazine‐1‐oxide ( 9 ), 3,5‐bis(dimethylamino)‐4,6‐dinitropyridazine‐1‐oxide ( 10 ), and 3,5‐bis((2‐hydroxyethyl)amino)‐4,6‐dinitropyridazine‐1‐oxide ( 11 ). With the aim of increasing the detonation performance, compound 8 was additionally nitrated and 3,5‐bis(methylnitramino)‐4,6‐dinitropyridazine‐1‐oxide ( 9 ) was obtained. These new energetic materials were characterized and identified by multinuclear NMR (1H, 13C, 14N, 15N) and IR spectroscopy, elemental analysis and mass spectrometry. In addition, their sensitivities toward impact, friction and electrostatic discharge were thoroughly examined. Furthermore, obtained single‐crystals of the substances were characterized by low‐temperature single‐crystal X‐ray diffraction. 相似文献