全文获取类型
收费全文 | 322209篇 |
免费 | 3699篇 |
国内免费 | 1252篇 |
专业分类
化学 | 149355篇 |
晶体学 | 4181篇 |
力学 | 11754篇 |
综合类 | 12篇 |
数学 | 28779篇 |
物理学 | 84123篇 |
无线电 | 48956篇 |
出版年
2018年 | 1881篇 |
2017年 | 1819篇 |
2016年 | 3766篇 |
2015年 | 2941篇 |
2014年 | 4001篇 |
2013年 | 12581篇 |
2012年 | 9515篇 |
2011年 | 12024篇 |
2010年 | 7828篇 |
2009年 | 8373篇 |
2008年 | 12055篇 |
2007年 | 12646篇 |
2006年 | 12326篇 |
2005年 | 11748篇 |
2004年 | 10579篇 |
2003年 | 9388篇 |
2002年 | 9149篇 |
2001年 | 10531篇 |
2000年 | 8446篇 |
1999年 | 6887篇 |
1998年 | 5761篇 |
1997年 | 5706篇 |
1996年 | 5508篇 |
1995年 | 5209篇 |
1994年 | 4940篇 |
1993年 | 4869篇 |
1992年 | 5267篇 |
1991年 | 5180篇 |
1990年 | 4744篇 |
1989年 | 4655篇 |
1988年 | 4489篇 |
1987年 | 3921篇 |
1986年 | 3713篇 |
1985年 | 5130篇 |
1984年 | 5143篇 |
1983年 | 4281篇 |
1982年 | 4685篇 |
1981年 | 4416篇 |
1980年 | 4396篇 |
1979年 | 4286篇 |
1978年 | 4466篇 |
1977年 | 4319篇 |
1976年 | 4274篇 |
1975年 | 4190篇 |
1974年 | 3918篇 |
1973年 | 4269篇 |
1972年 | 2587篇 |
1971年 | 1953篇 |
1968年 | 1828篇 |
1967年 | 1856篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
An annular slot-coupled dielectric resonator antenna is investigated experimentally. As compared with the previous rectangular-slot version, the new configuration offers a much wider bandwidth of 18%. The return loss, radiation patterns, and antenna gain of the configuration have been measured and are discussed 相似文献
992.
The operation and management of the Kansas University Packet Switch Network (KUPSN),which provides interactive and file transfer services to its users, is discussed, focussing on the problems encountered and solutions developed. Areas of network management of particular concern are identified, and tools developed to deal with them are described. Management system integration and the evolution of problem-handling procedures are discussed 相似文献
993.
P. Sharrock MA Msc MBA C. Eng MInstMC MIEE 《e & i Elektrotechnik und Informationstechnik》1997,114(5):245-248
This paper introduces LonWorks technology, intrinsic safety and how the two are combined in the IS-78 physical channel which enables networks to be taken into hazardous areas. The capabilities of the physical channel are described, together with some applications in which it has been used. 相似文献
994.
Paul C. Bressloff 《Physica D: Nonlinear Phenomena》1997,110(3-4):195-208
We prove the existence of spatially localized ground states of the diffusive Haken model. This model describes a self-organizing network whose elements are arranged on a d-dimensional lattice with short-range diffusive coupling. The network evolves according to a competitive gradient dynamics in which the effects of diffusion are counteracted by a localizing potential that incorporates an additional global coupling term. In the absence of diffusive coupling, the ground states of the system are strictly localized, i.e. only one lattice site is excited. For sufficiently small non-zero diffusive coupling , it is shown analytically that localized ground states persist in the network with the excitations exponentially decaying in space. Numerical results establish that localization occurs for arbitrary values of in one dimension but vanishes beyond a critical coupling c(d), when d> 1. The one-dimensional localized states are interpreted in terms of instanton solutions of a continuum version of the model. 相似文献
995.
A. Edwards Mulpuri V. Rao B. Molnar A. E. Wickenden W. Holland P. H. Chi 《Journal of Electronic Materials》1997,26(3):334-339
Doping by ion implantation using Si, O, Mg, and Ca has been studied in single crystal semi-insulating and n-type GaN grown
on a-sapphire substrates. The n-and p-type dopants used in this study are Si and O; Mg and Ca, respectively. Room temperature
activation of Si and O donors has been achieved after 1150°C annealing for 120 s. The activation of Mg and Ca acceptors is
too low to measure at both room temperature and 300°C. Using higher doses to achieve a measurable p-type conduction increases
the amount of damage created by the implantation. Rutherford back scattering measurements on this material indicate that the
damage is still present even after the maximum possible heat treatment. Secondary ion mass spectrometry measurements have
indicated a redistribution in the measured profiles of Mg due to annealing. 相似文献
996.
A novel directional-coupler optical switch based on the concept of the field-induced guide is demonstrated for the first time. The device exhibits a good switching characteristic between two output ends with a crosstalk of -10 dB at a 1.15 mu m wavelength for a device with an 800 mu m coupling length.<> 相似文献
997.
Il Nuovo Cimento D - The applicability of the Kosterlitz-Thouless theory to HTSC is examined. The model initially presented in its general lines for bidimensional systems is then applied to... 相似文献
998.
The design, fabrication and characterisation of GaAs Schottky-barrier photodiodes with evaporated, free-standing-metal airbridges is reported. The photodiodes were fabricated using all dry-etching techniques. Anisotropic chemically assisted ion beam etching was used to etch vertical sidewall mesas, and isotropic reactive ion etching was used to etch a lateral tunnel. A free-standing-metal airbridge created by the lateral tunnel etch results in isolation of the active area at the same time providing free-standing-metal interconnection to the contact pad.<> 相似文献
999.
S. Coffa D. C. Jacobson J. M. Poate F. Priolo 《Applied Physics A: Materials Science & Processing》1992,54(6):481-484
We have investigated ion-beam-enhanced diffusion of Au in undoped and B doped amorphous Si. The diffusion coefficients depend linearly on ion flux and exibit an Arrhenius-like temperature dependence with an activation energy of 0.37 eV in the temperature range 200–350° C. Moreover the diffusivity is enhanced by a factor of 5 by B-doping at a concentration of 1×1020 atoms/cm3. A similar enhancement is observed in thermal diffusion of Au which has an activation energy of 1.5 eV. On the basis of these results a model for the ion-beam-enhanced diffusion of Au is proposed where the high density of defects present in amorphous Si act as traps for the fast moving interstitial Au atoms. The effectiveness of this trapping process can be changed by the high concentration of mobile defects generated by the beam and also by a change in the charge state of the traps induced by the presence of B. 相似文献
1000.
Summary Glass formation in nature and materials science is reviewed and the recent recognition of polymorphism within the glassy state,
polyamorphism, is discussed. The process by which the glassy state originates during the continuous cooling or viscous slowdown
process, is examined and the three canonical characteristics of relaxing liquids are correlated through the fragility. The
conversion of strong liquids to fragile liquids by pressure-induced coordination number increases is discussed, and then it
is shown that for the same type of system it is possible to have the same conversion accomplished via a first-order transition
within the liquid state. The systems in which this can happen are of the same type which exhibit polyamorphism, and the whole
phenomenology can be accounted for by a recent simple modification of the van der Waals model for tetrahedrally bonded liquids.
The concept of complex amorphous systems which can lose a significant number of degrees of freedom through weak first-order
transitions is then used to discuss the relation between native and denatured hydrated proteins, since the latter have much
in common with plasticized chain polymer systems. Finally, we close the circle by taking a short-time-scale phenomenon given
much attention by protein physicists,viz., the onset of an anomaly in the Debye-Waller factor with increasing temperature, and showing that for a wide variety of liquids,
including computer-simulated strong and fragile ionic liquids, this phenomenon is closely correlated with the experimental
glass transition temperature. This implies that the latter owes its origin to the onset of strong anharmonicity in certain
components of the vibrational density of states (evidently related to the boson peak) which then permits the system to gain
access to its configurational degrees of freedom. The more anharmonic these vibrational components, the closer to the Kauzmann
temperature will commence the exploration of configuration space and, for a given configurational microstate degeneracy, the
more fragile the liquid will be.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献