全文获取类型
收费全文 | 886449篇 |
免费 | 8311篇 |
国内免费 | 2637篇 |
专业分类
化学 | 461249篇 |
晶体学 | 12717篇 |
力学 | 42794篇 |
综合类 | 32篇 |
数学 | 118591篇 |
物理学 | 262014篇 |
出版年
2021年 | 6999篇 |
2020年 | 7814篇 |
2019年 | 8593篇 |
2018年 | 11071篇 |
2017年 | 11079篇 |
2016年 | 16438篇 |
2015年 | 9925篇 |
2014年 | 15665篇 |
2013年 | 39256篇 |
2012年 | 29708篇 |
2011年 | 35825篇 |
2010年 | 25501篇 |
2009年 | 25308篇 |
2008年 | 33492篇 |
2007年 | 33553篇 |
2006年 | 31328篇 |
2005年 | 28022篇 |
2004年 | 25773篇 |
2003年 | 23162篇 |
2002年 | 22976篇 |
2001年 | 24341篇 |
2000年 | 18996篇 |
1999年 | 14986篇 |
1998年 | 12795篇 |
1997年 | 12592篇 |
1996年 | 12163篇 |
1995年 | 10986篇 |
1994年 | 10927篇 |
1993年 | 10431篇 |
1992年 | 11333篇 |
1991年 | 11642篇 |
1990年 | 11151篇 |
1989年 | 10814篇 |
1988年 | 10908篇 |
1987年 | 10596篇 |
1986年 | 10101篇 |
1985年 | 13467篇 |
1984年 | 14037篇 |
1983年 | 11749篇 |
1982年 | 12442篇 |
1981年 | 12005篇 |
1980年 | 11536篇 |
1979年 | 12024篇 |
1978年 | 12526篇 |
1977年 | 12413篇 |
1976年 | 12365篇 |
1975年 | 11766篇 |
1974年 | 11580篇 |
1973年 | 11994篇 |
1972年 | 8327篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
G. N. Kozhemyakin D. V. Lutskiy M. A. Rom P. V. Mateychenko 《Crystallography Reports》2008,53(7):1267-1271
The growth conditions and structural quality of Sb-Bi gradient single crystals with Bi content from 2 to 18 at %, grown by
the Czochralski method with solid phase feed, are investigated. Bi distribution in the crystals along their pulling direction
are studied by electron probe microanalysis and the change in the interplanar spacing is analyzed by double-crystal X-ray
diffraction. It is established that the pulling rate and feed mass affect the Bi distribution in Sb-Bi single crystals. 相似文献
992.
993.
High quality factor of dynamic structures at micro and nano scale is exploited in various applications of micro electro-mechanical
systems (MEMS) and nano electro-mechanical system. The quality factor of such devices can be very high in vacuum. However,
when vacuum is not desirable or not possible, the tiny structures must vibrate in air or some other gas at pressure levels
that may vary from atmospheric to low vacuum. The interaction of the surrounding fluid with the vibrating structure leads
to dissipation, thus bringing down the quality factor. Depending on the ambient fluid pressure or the gap between the vibrating
and the fixed structure, the fluid motion can range from continuum flow to molecular flow giving a wide range of dissipation.
The relevant fluid flow characteristics are determined by the Knudsen number which is the ratio of the mean free path of the
gas molecule to the characteristic flow length of the device. This number is very small for continuum flow and reasonably
big for molecular flow. In this paper, we study the effect of fluid pressure on the quality factor by carrying out experiments
on a MEMS device that consists of a double gimbaled torsional resonator. Such devices are commonly used in optical cross-connects
and switches. We only vary fluid pressure to make the Knudsen number go through the entire range of continuum flow, slip flow,
transition flow, and molecular flow. We experimentally determine the quality factor of the torsional resonator at different
air pressures ranging from 760 Torr to 0.001 Torr. The variation of this pressure over six orders of magnitude ensures required
rarefaction to range over all flow conditions. Finally, we get the variation of quality factor with pressure. The result indicates
that the quality factor, Q, follows a power law, Q ∝P
–r
, with different values of the exponent r in different flow regimes. In the second part of the paper, we propose the use of effective viscosity for considering velocity
slip conditions in solving Navier–Stokes equation numerically. This concept is validated with analytical results for a simple
case and then compared with the experimental results presented in this paper. The study shows that the effective viscosity
concept can be used effectively even for the molecular regime if the air-gap to length ratio is sufficiently small (h
0/L<0.01). As this ratio increases, the range of validity decreases. 相似文献
994.
Methods for accelerometer unit calibration are usually based on sufficiently strict requirements on the knowledge of the gravity orientation with respect to the unit (of the order of fractions of an arc minute). But it is not always possible to know the orientation with such high accuracy. In the present paper, we consider an approach to accelerometer unit calibration for whose implementation it suffices to have rough angular information (of the order of tens of arc minutes). We use a guaranteeing approach to calculate optimal schemes calibration experiments and propose an iterative scheme of calibration. 相似文献
995.
A new technique has been developed to compute mean and fluctuating concentrations in complex turbulent flows (tidal current near a coast and deep ocean). An initial distribution of material is discretized into any small clouds which are advected by a combination of the mean flow and large scale turbulence. The turbulence can be simulated either by kinematic simulation (KS) or direct numerical simulation. The clouds also diffuse relative to their centroids; the statistics for this are obtained from a separate calculation of the growth of individual clouds in small scale turbulence, generated by KS. The ensemble of discrete clouds is periodically re-discretized, to limit the size of the small clouds and prevent overlapping. The model is illustrated with simulations of dispersion in uniform flow, and the results are compared with analytic, steady state solutions. The aim of this study is to understand how pollutants disperses in a turbulent flow through a numerical simulation of fluid particle motion in a random flow field generated by Fourier modes. Although this homogeneous turbulent is rather a “simple” flow, it represents a building block toward understanding pollutant dispersion in more complex flow. The results presented here are preliminary in nature, but we expect that similar qualitative results should be observed in a genuine turbulent flow. 相似文献
996.
V.A. Lubarda 《International Journal of Solids and Structures》2008,45(6):1536-1545
A modified energy momentum tensor, in the presence of body forces, is introduced and used to construct the nonconserved J, M, and L integrals, and to derive the energetic forces associated with a defect motion within the material. The J integral is then applied to evaluate the Peach–Koehler force on an inclined edge dislocation within a large block due to its own weight. The equilibrium position of the dislocation is determined for different boundary conditions of interest in geomechanics. 相似文献
997.
Three-dimensional laminar forced convection including steady-periodic transition is investigated up to periodic-chaotic transition
in the fully developed region of coolant passages with staggered arrays of pin fins. Comparative examples concern overall
pressure losses and heat transfer characteristics of circular, square and elliptical pins made of nickel and copper. In the
numerical model, transient conjugate heat transfer is assumed and space periodicities in pressure, velocity components and
temperatures are taken into account. In the range of operative conditions investigated, overall friction factors increase
almost linearly with the Reynolds number, while the increase of overall Nusselt numbers with the Reynolds number is characterized
by two slope changes connected with the onset of streamwise vortices, and the shedding of transverse vortices, respectively.
The use of copper, instead of nickel, increases the overall Nusselt number with all shapes, but is particularly beneficial
to the elliptical section. Square pins are characterized by the highest values of friction factors, but are also the best
performers as far as convection enhancing is concerned. The reverse is true for the elliptical pins which are characterized
by the lowest values of friction factors, but are the worst performers as far as convection enhancing is concerned. On the
basis of overall performances, the elliptical pins made of copper are the best choice, at least in the upper range of Reynolds
numbers investigated. 相似文献
998.
Adsorption of the copper ions from aqueous solutions, benzene, and water on the active carbon obtained by chemical activation
from walnut shells was studied. The active carbon was additionally oxidized with hydrogen peroxide and then impregnated with
nitrogen-containing compounds. As a result of impregnation with melamine, a secondary porosity is formed within the oxidized
active carbon, leading to an increase in the specific surface of the sorbent. A modified carbon surface is heterogeneous,
and the carbons themselves exhibit catalytic activity in the oxidation of luminol and decomposition of hydrogen peroxide. 相似文献
999.
1000.
A. I. Kolesnikov I. A. Kaplunov I. V. Talyzin S. A. Tret’yakov O. V. Gritsunova E. Yu. Vorontsova 《Crystallography Reports》2008,53(7):1144-1149
A computer simulation and measurements of the light transmittance of germanium and paratellurite crystals of different thickness
were used to show that, at scattering probabilities of photons comparable to their absorption probabilities, the standard
methods for calculating light extinction coefficients on the basis of the Bouguer law lead to rough errors in estimation of
the optical quality of a material. 相似文献