全文获取类型
收费全文 | 269812篇 |
免费 | 19800篇 |
国内免费 | 10731篇 |
专业分类
化学 | 160303篇 |
晶体学 | 3577篇 |
力学 | 14148篇 |
综合类 | 537篇 |
数学 | 29430篇 |
物理学 | 92348篇 |
出版年
2023年 | 2386篇 |
2022年 | 3666篇 |
2021年 | 4590篇 |
2020年 | 5256篇 |
2019年 | 5129篇 |
2018年 | 4537篇 |
2017年 | 4291篇 |
2016年 | 7262篇 |
2015年 | 6548篇 |
2014年 | 8170篇 |
2013年 | 15104篇 |
2012年 | 15224篇 |
2011年 | 17348篇 |
2010年 | 11121篇 |
2009年 | 10980篇 |
2008年 | 13600篇 |
2007年 | 13006篇 |
2006年 | 12217篇 |
2005年 | 10890篇 |
2004年 | 9053篇 |
2003年 | 7565篇 |
2002年 | 7253篇 |
2001年 | 7806篇 |
2000年 | 6198篇 |
1999年 | 5447篇 |
1998年 | 4419篇 |
1997年 | 4279篇 |
1996年 | 4404篇 |
1995年 | 3740篇 |
1994年 | 3596篇 |
1993年 | 3148篇 |
1992年 | 3379篇 |
1991年 | 3255篇 |
1990年 | 2786篇 |
1989年 | 2547篇 |
1988年 | 2475篇 |
1987年 | 2320篇 |
1986年 | 2232篇 |
1985年 | 3073篇 |
1984年 | 2967篇 |
1983年 | 2351篇 |
1982年 | 2513篇 |
1981年 | 2369篇 |
1980年 | 2292篇 |
1979年 | 2205篇 |
1978年 | 2265篇 |
1977年 | 2217篇 |
1976年 | 2156篇 |
1975年 | 2139篇 |
1973年 | 2120篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
992.
A finite element solution of the Navier-Stokes equations for steady flow under the magnetic effect through a double-branched two-dimensional section of a three-dimensional model of the canine aorta is discussed. The numerical scheme involves transforming the physical co-ordinates to a curvilinear boundary-fitted co-ordinate system. The shear stress at the wall is calculated for a Reynolds number of 1000 with the branch-to-main aortic flow rate ratio as a parameter. The results are compared with earlier works involving experimental data and found to be in reasonable qualitative agreement. The steady flow, shear stress and branch flow under the effect of a magnetic field have been discussed in detail. 相似文献
993.
Runge–Kutta based convolution quadrature methods for abstract, well-posed, linear, and homogeneous Volterra equations, non
necessarily of sectorial type, are developed. A general representation of the numerical solution in terms of the continuous
one is given. The error and stability analysis is based on this representation, which, for the particular case of the backward
Euler method, also shows that the numerical solution inherits some interesting qualitative properties, such as positivity,
of the exact solution. Numerical illustrations are provided. 相似文献
994.
In this paper, we study the homogenous quotient modules of the Hardy module on the bidisk. The essential normality of the
homogenous quotient modules is completely characterized. We also describe the essential spectrum for a general quotient module.
The paper also considers K-homology invariant defined in the case of the homogenous quotient modules on the bidisk.
This work is partially supported by the National Natural Science Foundation of China (Grant No. 10525106), the Young Teacher
Fund, the National Key Basic Research Project of China (Grant No. 2006CB805905) and the Specialized Research for the Doctoral
Program 相似文献
995.
1689-nm diode lasers used in medical apparatus have been fabricated and characterized. The lasers had pnpn InP current confinement structure, and the active region consisted of 5 pairs of InGaAs quantum wells and InGaAsP barriers. 相似文献
996.
N. Nishi J. Nishijo K. Judai C. Okabe O. Oishi 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,43(1-3):287-290
UV photoexcitation of (t-butylethynyl copper)24
cluster films induces segregation of the crystals into metallic and organic
phases and leads to evolve the metallic sheets sandwiched by organic
polymers. The growth of the metallic crystals in the plane of the
photo-electromagnetic field is attributed due to plasmon-plasmon interaction
among nanoparticles embedded in dielectric polymer matrices. The surface
enhanced photochemical reaction of residual cluster molecules on the photon
incident direction is expected to take an important role for joining the
metal particles to produce a metallic sheet. We can apply this phenomenon
for photolithographic copper pattern generation on a flexible base plate. 相似文献
997.
C. von Borczyskowski F. Cichos J. Martin J. Schuster A. Issac J. Brabandt 《The European physical journal. Special topics》2007,144(1):13-25
Optical spectroscopy in combination with wide field or confocal optical
microscopy enables the investigation of single quantum objects such as
organic molecules, II/VI semiconductor quantum dots and silicon nanocrystals.
They all have fluctuations of luminescence intensities on time scales longer
than μs in common. A comparison reveals that despite the large differences
of the nature of the respective quantum objects, the intensity fluctuations
are related to a slow ionisation process followed
by a trapping of the photoejected charge in the non-conducing environment.
Detailed aspects of the dynamics are controlled by the dielectric properties
of the matrix. 相似文献
998.
Alpeshkumar K. Malde Santosh A. Khedkar Evans C. Coutinho 《Journal of Physical Organic Chemistry》2007,20(2):151-160
Modification of peptides to produce peptidomimetics is of great interest, with the aim of designing potent, selective, and metabolically stable analogs having certain conformational properties. Organoboranes have been reported in the literature with a wide range of therapeutic applications. One of the therapeutically important class of molecules is amine‐carboxyboranes derived from amino acids by replacement of the Cα atom of an amino acid/peptide by boron. The conformational preferences of these peptides, with respect to backbone ω, ?, and ψ torsion angles, have been investigated by theoretical calculations. The amide bond in these molecules has the same geometry in the ground and transition states as the natural peptides. However, the boron isosteres of glycine and alanine peptides are less structured than their natural derivatives, but are distinguished by structures with a positive value for the ? angle, which is normally disfavored for natural peptides. This property could be used to build peptides with a geometry not usually seen in natural peptides. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
999.
The specific heat and related thermophysical properties of liquid Fe77.5Cu13Mo9.5 monotectic alloy were investigated by an electromagnetic levitation drop calorimeter over a wide temperature range from 1482 to 1818 K. A maximum undercooling of 221 K (0.13 Tm) was achieved and the specific heat was determined as 44.71 J·mol-1·K-1. The excess specific heat, enthalpy change, entropy change and Gibbs free energy difference of this alloy were calculated on the basis of experimental results. It was found that the calculated results by traditional estimating methods can only describe the solidification process under low undercooling conditions. Only the experimental results can reflect the reality under high undercooling conditions. Meanwhile, the thermal diffusivity, thermal conductivity, and sound speed were derived from the present experimental results. Furthermore, the solidified microstructural morphology was examined, which consists of (Fe) and (Cu) phases. The calculated interface energy was applied to exploring the correlation between competitive nucleation and solidification microstructure within monotectic alloy. 相似文献
1000.
Numerical studies were conducted to investigate the natural convection heat transfer around a uniformly heated thin plate
with arbitrary inclination in an infinite space. The numerical approach was based on the finite volume technique with a nonstaggered
grid arrangement. For handling the pressure–velocity coupling the SIMPLE-algorithm was used. QUICK scheme and first order
upwind scheme were employed for discretization of the momentum and energy convective terms respectively. Plate width and heating
rate were used to vary the modified Rayleigh number over the range of 4.8×106 to 1.87×108. Local and average heat transfer characteristics were compared with regarding to the inclination angle. The empirical expressions
for local and average Nusselt number were correlated. It has been found that for inclination angle less than 10∘, the flow and heat transfer characteristics are complicated and the average Nusselt number can not be correlated by one equation
while for inclination angle larger than 10∘, the average Nusselt number can be correlated into an elegant correlation.
Received on 18 April 2001 / Published online: 29 November 2001 相似文献