全文获取类型
收费全文 | 270086篇 |
免费 | 2492篇 |
国内免费 | 615篇 |
专业分类
电工技术 | 4909篇 |
综合类 | 225篇 |
化学工业 | 39731篇 |
金属工艺 | 16240篇 |
机械仪表 | 11519篇 |
建筑科学 | 5149篇 |
矿业工程 | 2764篇 |
能源动力 | 5198篇 |
轻工业 | 14007篇 |
水利工程 | 3840篇 |
石油天然气 | 8661篇 |
武器工业 | 22篇 |
无线电 | 27449篇 |
一般工业技术 | 61906篇 |
冶金工业 | 39227篇 |
原子能技术 | 7439篇 |
自动化技术 | 24907篇 |
出版年
2021年 | 2050篇 |
2019年 | 1998篇 |
2018年 | 10922篇 |
2017年 | 11412篇 |
2016年 | 8224篇 |
2015年 | 2681篇 |
2014年 | 3865篇 |
2013年 | 8789篇 |
2012年 | 7929篇 |
2011年 | 14435篇 |
2010年 | 12845篇 |
2009年 | 12325篇 |
2008年 | 12414篇 |
2007年 | 14060篇 |
2006年 | 5633篇 |
2005年 | 8138篇 |
2004年 | 6394篇 |
2003年 | 6063篇 |
2002年 | 4974篇 |
2001年 | 4535篇 |
2000年 | 4317篇 |
1999年 | 4215篇 |
1998年 | 10190篇 |
1997年 | 7184篇 |
1996年 | 5511篇 |
1995年 | 4142篇 |
1994年 | 3626篇 |
1993年 | 3865篇 |
1992年 | 3000篇 |
1991年 | 3017篇 |
1990年 | 2890篇 |
1989年 | 2853篇 |
1988年 | 2872篇 |
1987年 | 2510篇 |
1986年 | 2568篇 |
1985年 | 2854篇 |
1984年 | 2687篇 |
1983年 | 2554篇 |
1982年 | 2300篇 |
1981年 | 2269篇 |
1980年 | 2345篇 |
1979年 | 2359篇 |
1978年 | 2382篇 |
1977年 | 2487篇 |
1976年 | 2876篇 |
1975年 | 2188篇 |
1974年 | 2068篇 |
1973年 | 2166篇 |
1972年 | 1940篇 |
1971年 | 1755篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Bing Q. Han Farghalli A. Mohamed Enrique J. Lavernia 《Metallurgical and Materials Transactions A》2003,34(1):71-83
In the present study, the mechanical properties of Fe processed via severe plastic deformation (equal-channel angular pressing (ECAP)) at room temperature were investigated for the first time.
The grain size of annealed Fe, with an initial grain size of about 200 μm, was reduced drastically during ECAP. After eight passes, the grain size reaches 200 to 400 nm, as documented by means of
transmission electron microscopy (TEM). The value of microhardness during pressing increases 3 times over that of the starting
material after the first pass and increases slightly during subsequent pressing for higher-purity Fe. Examination of the value
of microhardness after eight passes as a function of post-ECAP annealing temperature shows a transition from recovery to recrystallization,
an observation that resembles the behavior reported for heavily deformed metals and alloys. The tensile and compression behaviors
were examined. In tension, a drop in the engineering stress-engineering strain curve beyond maximum load was observed both
in the annealed Fe and the ECAP Fe. This drop is related to the neck deformation. The fracture surface, examined by scanning
electron microscopy (SEM), shows vein patterns, which is different from the dimples found on the fracture surface of annealed
Fe. In compression, an initial strain-hardening region followed by a no-strain-hardening region was observed in the ECAP Fe.
The yield strength in tension of the ECAP Fe was observed to be higher than that in compression. The strengthening mechanisms
and softening behavior are discussed. 相似文献
993.
Er. V. Kal’yanov 《Technical Physics Letters》2007,33(12):1015-1017
A mathematical model of a new controllable autooscilatory chaotic system based on inductively coupled Chua’s oscillators is described. Numerical simulations show that, using a chaotizing feedback algorithm, chaotic oscillations in this system can be excited in regimes where only regular oscillations are generated otherwise. 相似文献
994.
In the late eighties and up to the beginning of nineties computation of turbulent flows is mostly dominated by RANS (Reynolds
Averaged Navier-Stokes Simulation) type modelling. During the last few years URANS (Unsteady RANS) and LES (Large Eddy Simulation)
type of approaches have been attempted with some success. Yet, there have been many difficulties when LES is applied to practical
engineering problems and to high Reynolds number flows as energy dissipating eddies become really small and mesh resolution
required for a reasonably resolved LES approaches that of DNS (Direct Numerical Simulation). An alternative solution suggested
was to combine RANS and LES, which in general referred to as Hybrid LES. There have been many proposals for combining RANS
and LES in different ways. In this article, some of the issues involved in performing hybrid LES reported in the recent literature
is briefly reviewed. 相似文献
995.
A. M. Skorobogatov A. V. Tvardovskii 《Journal of Engineering Physics and Thermophysics》2007,80(3):634-637
A model for calculating of the change in the basic thermodynamic characteristics of a sorbent (internal energy and entropy)
with the phenomenological-thermodynamics method has been developed. Calculations for the systems polycarbonate-carbon dioxide
and polymethylmethacrylate-carbon dioxide have been carried out.
__________
Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 3, pp. 198–200, May–June, 2007. 相似文献
996.
A. D. Bulanov P. G. Sennikov V. A. Krylov T. G. Sorochkina L. A. Chuprov O. Yu. Chernova O. Yu. Troshin 《Inorganic Materials》2007,43(4):364-368
Using gas chromatography and high-resolution Fourier-transform IR spectroscopy, we have determined the concentrations of C1–C4 hydrocarbon impurities in isotopically unmodified silicon tetrafluoride before and after fine purification and in 28Si-enriched SiF4. The concentrations of C1–C4 hydrocarbon impurities in silicon tetrafluoride for SiH4 synthesis have been shown to correlate with those in the synthesized silane. 相似文献
997.
D. L. Kuznetsov E. V. Kol’man Yu. S. Surkov V. V. Uvarin I. E. Filatov 《Technical Physics Letters》2007,33(7):604-606
The process of methane conversion in a mixture modeling the natural gas under the action of a periodic nanosecond pulsed electron beam and an electron-beam-induced non-self-sustained discharge has been numerically simulated and studied in experiment. The possibility of methane conversion under the action of electric pulses without significant heating of the processed gas mixture is demonstrated for the first time. The main conversion products in experiment were hydrogen, ethylene, and acetylene. 相似文献
998.
Radio-over-fibre (ROF) has received increasing attention for its ability to enable broadband wireless access. This fibre-based wireless access scheme meets the demand for broadband service by integrating the high capacity of optical networks with the flexibility of radio networks (the optical and wireless channels are concatenated with one another). There are, however, impairments that come with this appealing technology. The nonlinear distortion of the optical link and the multipath dispersion of the wireless channel are two of the major factors. In order to limit the effects of these distortions, estimation, and subsequently equalisation, of the concatenated fibre- wireless channel needs to be done. An estimation algorithm for the fibre-wireless uplink in a multiuser code division multiple access (CDMA) environment is presented using pseudonoise training sequences. It has already been shown by Fernando et al. (2001) that identification of the fibre- wireless uplink is possible in a single user CDMA environment. However, the more difficult task of identification in a multiuser spread spectrum environment, which is more realistic, is shown. In the multiuser case, the cumulative effect of multiuser interference, multipath dispersion, nonlinear distortion and noise should all be handled together which makes it more challenging. Numerical evaluations of the developed algorithm show that a good estimation of both the linear and nonlinear systems is possible in the presence of 16 independent users and an signal- to-noise ratio (SNR) of 22 dB. The estimation accuracy increases with the length of the PN sequence. 相似文献
999.
Peter Su Adrian Gerlich Motomichi Yamamoto Thomas H. North 《Journal of Materials Science》2007,42(24):9954-9965
The formation of local melted films during friction stir spot welding of as-cast AZ91D and thixomolded AZ91 material is investigated.
The average temperatures close to the tip of the rotating pin vary from 438 to 454 °C during the dwell period in friction
stir spot welding. These measured temperature values are higher than the melting temperature of α-Mg + Mg17Al12 eutectic (437 °C). It is suggested that the temperature in the stir zone during the dwell period is determined by the relative
proportions of α-Mg and (α-Mg + Mg17Al12) eutectic material, which are incorporated during friction stir spot welding. Based on the stir zone temperature measurements
and a detailed examination of material located at the root of the pin thread it is suggested that material is moved downwards
via the pin thread and into the stir zone during the dwell period in friction stir spot welding. Evidence of local melted
film formation is observed in the stir zone of AZ91 spot welds. It is suggested that melted films are retained since their
dissolution rate is much slower in the high temperature stir zone than it is when melted films is formed in the stir zone
during Al 7075-T6 friction stir spot welding. The spontaneous melting temperature, solute diffusion rate and the thermodynamic
driving force for droplet dissolution are much higher during Al 7075-T6 friction stir spot welding. 相似文献
1000.