全文获取类型
收费全文 | 14729篇 |
免费 | 627篇 |
国内免费 | 42篇 |
专业分类
化学 | 10041篇 |
晶体学 | 231篇 |
力学 | 465篇 |
综合类 | 1篇 |
数学 | 1118篇 |
物理学 | 3542篇 |
出版年
2024年 | 65篇 |
2023年 | 169篇 |
2022年 | 441篇 |
2021年 | 428篇 |
2020年 | 467篇 |
2019年 | 525篇 |
2018年 | 489篇 |
2017年 | 466篇 |
2016年 | 673篇 |
2015年 | 477篇 |
2014年 | 766篇 |
2013年 | 1271篇 |
2012年 | 1120篇 |
2011年 | 1152篇 |
2010年 | 762篇 |
2009年 | 598篇 |
2008年 | 722篇 |
2007年 | 723篇 |
2006年 | 568篇 |
2005年 | 491篇 |
2004年 | 380篇 |
2003年 | 313篇 |
2002年 | 253篇 |
2001年 | 156篇 |
2000年 | 130篇 |
1999年 | 99篇 |
1998年 | 71篇 |
1997年 | 99篇 |
1996年 | 97篇 |
1995年 | 73篇 |
1994年 | 68篇 |
1993年 | 94篇 |
1992年 | 97篇 |
1991年 | 77篇 |
1990年 | 68篇 |
1989年 | 71篇 |
1988年 | 50篇 |
1987年 | 45篇 |
1986年 | 42篇 |
1985年 | 64篇 |
1984年 | 62篇 |
1983年 | 53篇 |
1982年 | 50篇 |
1981年 | 38篇 |
1980年 | 48篇 |
1979年 | 62篇 |
1978年 | 49篇 |
1977年 | 53篇 |
1976年 | 41篇 |
1975年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Simplified expressions for one-centre electron interaction integrals in general and Slater-Condon parameters as well as repulsion
integrals of the type (aa|aa) in particular have been obtained over Slater-type atomic orbitals. Results calculated using these expressions are in agreement
with those due to other authors. 相似文献
992.
The rectangular dielectric waveguide is the most commonly used structure in integrated optics, especially in semi-conductor
diode lasers. Demands for new applications such as high-speed data backplanes in integrated electronics, waveguide filters,
optical multiplexers and optical switches are driving technology toward better materials and processing techniques for planar
waveguide structures. The infinite slab and circular waveguides that we know are not practical for use on a substrate because
the slab waveguide has no lateral confinement and the circular fiber is not compatible with the planar processing technology
being used to make planar structures. The rectangular waveguide is the natural structure. In this review, we have discussed
several analytical methods for analyzing the mode structure of rectangular structures, beginning with a wave analysis based
on the pioneering work of Marcatili. We study three basic techniques with examples to compare their performance levels. These
are the analytical approach developed by Marcatili, the perturbation techniques, which improve on the analytical solutions
and the effective index method with examples.
T Srinivas received the B.Sc. (Hon.) degree from Nehru Science College, Hydrabad and M.E. (Int.) and Ph.D. dgrees from the Indian Institute
of Science, Bangalore, India. He was a Postdoctoral Research Fellow at Toyohashi University of Technology, Japan from 1992
to 1996. He is currently an Associate Professor with the Department of Electrical Communication Engineering, Indian Institute
of Science. His areas of interests are optical communication networks, integrated optics, micro-opto-electrical-mechanical
systems (MOEMS) and fiberoptic sensors 相似文献
993.
A generalized single‐particle model for the prediction of combustion dynamics of a porous coal char in a fluidized bed is analyzed in the present work using a volume reaction model (VRM). A fully transient nonisothermal model involving both heterogeneous and homogeneous chemical reactions, multicomponent mass transfer, heat transfer with intraparticle resistances, as well as char structure evolution is developed. The model takes into account convection and diffusion inside the particle pores, as well as in the boundary layer. By addressing the Stefan flow originated due to nonequimolar mass transfer and chemical reactions, this work enables a more realistic analysis of the combustion process. The model, characterized by a set of partial differential equations coupled with nonlinear boundary conditions, is solved numerically using the implicit finite volume method (FVM) with a FORTRAN code developed in‐house. The use of a FVM for solving such an elaborate char combustion model, based on the VRM, was not reported earlier. Experiments consisting of fluidized‐bed combustion of a single char particle were carried out to determine the internal surface area of a partially burned char particle and to enable model validation. Predicted results are found to compare well with the reported experimental results for porous coal char combustion. The effects of various parameters (i.e., bulk temperature and initial particle radius) are examined on the dynamics of combustion of coal char. The phenomena of ignition and extinction are also investigated. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 299–315, 2010 相似文献
994.
Periodic orbits belonging to the stromgren families A, B and C around the collinear libration points in the restricted three
body problem have been studied when the smaller primary is a triaxial rigid body and more massive body is a source of radiation
pressure. These families are determined in three different cases: (i) classical case, (ii) when bigger primary is a source
of radiation pressure, (iii) when smaller primary is a triaxial rigid body and bigger primary is a source of radiation pressure.
The Liapunov stability of each periodic solution has also been examined.
相似文献
995.
Nd2O3-SiO2 binary oxide was prepared by solgel technique using tetra-ethoxysilane and neodymium nitrate as precursor materials and HCl as a catalyst. The prepared samples were subjected to heat treatment in the temperature range from 600 to 1100 °C for different time duration. Characterization of heat treated samples was carried out by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The effect of sintering temperature and time on structural changes of Nd-doped silica has been discussed. The sample sintered at 1100 °C for 12 h shows the formation of monoclinic Nd2O3 nanocrystallites in silica matrix with average grain size ∼18 nm. 相似文献
996.
Rakesh Kumar Singh P. Senthilkumaran Kehar Singh 《Optics and Lasers in Engineering》2009,47(7-8):831-841
Tight focusing of linearly and circularly polarized vortex beams is studied in the presence of third-order spherical aberration, using vectorial Debye–Wolf integral. Results for total intensity distribution are presented for both polarizations. In addition, results for x-, y-, and z-polarization components are presented for the circularly polarized beam. Generation of longitudinal optical vortex in the tightly focused left circularly polarized beam has also been demonstrated by plotting its phase distribution. Compensation for the effect of spherical aberration has been studied in the presence of defocusing. Effect of aberration on the dark core of a tightly focused azimuthally polarized beam is also investigated. 相似文献
997.
S. V. Singh A. N. Thakur O. P. Singh S. C. Kumar Aqil Ahmad 《Indian Journal of Physics》2009,83(3):375-381
PbSrWO4 and PbBaWO4 have been synthesised by the solid state reaction technique XRD patterns show them to be tetragonal. Dielectric constant
(K/) and Dielectric loss (K
//) of PbSrWO4 and PbBaWO4 have been measured at 1 kHz in the temperature range of 300 to 1050 K. The log K/ vs T as well as log K
//
vs T plot of PbSrWO4 and PbBaWO4 shows rapid increase of dielectric constant above 590 K and 640 K, respectively. 相似文献
998.
999.
The synthesis of a unique tetrasaccharide linked to the serine 61 of human clotting factor IX through an α-l-fucose residue has been achieved for the first time in excellent yield. All glycosylation and protecting group manipulation steps are high yielding and reproducible for a scale-up preparation. A sequential glycosylation strategy has been used to assemble suitably protected monosaccharide synthons for the preparation of the target tetrasaccharide. 相似文献
1000.
Separation of lignosulfonate from its aqueous solution using supported liquid membrane 总被引:2,自引:1,他引:1
Kabita Chakrabarty Prabirkumar Saha Aloke Kumar Ghoshal 《Journal of membrane science》2009,340(1-2):84-91
This paper presents an experimental and theoretical study on facilitated transport of lignosulfonate (LS) through a flat sheet supported liquid membrane using trioctylamine (TOA) as carrier and dichloroethane as diluent. The studies were carried out with various support materials and operating conditions (viz. carrier concentration, strip phase concentration, salt concentration, etc.) and their effects on the transport of LS. The results were analyzed to identify a suitable combination of support and operating condition that would yield best performance of the supported liquid membrane (SLM) in terms of fast and efficient transport of LS. The stability of the SLM was assessed in terms of loss of liquid from the pores of membrane support. The SLM is found to be stable till 10 h. Co-transport mechanism has been adopted in this work by using NaOH as the strip phase. It was observed that extraction of LS is increased with increase in concentration of NaOH up to a limiting value of 0.5 M NaOH. Difference of salt concentration between feed and strip phase considerably affect the separation process. The diffusional resistances of organic membrane (Δorg) and aqueous solution (Δaq) calculated from the permeation model, which is again a combination of three unique mechanisms viz., diffusion through a feed aqueous layer, a fast interfacial chemical reaction, and diffusion of carrier–complex through the organic membrane, are found to be 609.9 and 176.6 s cm−1, respectively. The values of the diffusion coefficient in the membrane (Dorg) and in the bulk organic phase (Dcomplex) are 1.67×10−9 and 6.68 × 10−8 m2s−1, respectively. The extraction of LS is about 90%. Nearly 43% of LS can be recovered at optimum condition. 相似文献