全文获取类型
收费全文 | 314760篇 |
免费 | 4718篇 |
国内免费 | 1091篇 |
专业分类
电工技术 | 4717篇 |
综合类 | 2657篇 |
化学工业 | 47685篇 |
金属工艺 | 15608篇 |
机械仪表 | 10593篇 |
建筑科学 | 6880篇 |
矿业工程 | 2855篇 |
能源动力 | 6196篇 |
轻工业 | 20264篇 |
水利工程 | 4341篇 |
石油天然气 | 8178篇 |
武器工业 | 32篇 |
无线电 | 31994篇 |
一般工业技术 | 65688篇 |
冶金工业 | 43594篇 |
原子能技术 | 7021篇 |
自动化技术 | 42266篇 |
出版年
2021年 | 2748篇 |
2020年 | 2080篇 |
2019年 | 2456篇 |
2018年 | 18537篇 |
2017年 | 17431篇 |
2016年 | 14541篇 |
2015年 | 3609篇 |
2014年 | 4935篇 |
2013年 | 10671篇 |
2012年 | 10055篇 |
2011年 | 17585篇 |
2010年 | 14947篇 |
2009年 | 12862篇 |
2008年 | 14330篇 |
2007年 | 15075篇 |
2006年 | 6488篇 |
2005年 | 6919篇 |
2004年 | 6186篇 |
2003年 | 6102篇 |
2002年 | 5273篇 |
2001年 | 4883篇 |
2000年 | 4656篇 |
1999年 | 4665篇 |
1998年 | 12289篇 |
1997年 | 8462篇 |
1996年 | 6417篇 |
1995年 | 4681篇 |
1994年 | 4051篇 |
1993年 | 4247篇 |
1992年 | 3156篇 |
1991年 | 3184篇 |
1990年 | 3037篇 |
1989年 | 3001篇 |
1988年 | 3013篇 |
1987年 | 2661篇 |
1986年 | 2698篇 |
1985年 | 3059篇 |
1984年 | 2858篇 |
1983年 | 2712篇 |
1982年 | 2423篇 |
1981年 | 2436篇 |
1980年 | 2509篇 |
1979年 | 2468篇 |
1978年 | 2485篇 |
1977年 | 2699篇 |
1976年 | 3329篇 |
1975年 | 2270篇 |
1974年 | 2148篇 |
1973年 | 2276篇 |
1972年 | 1981篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
993.
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
994.
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
995.
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
996.
997.
A. A. Khrushchinskii S. A. Kuten’ N. M. Budevich V. F. Minenko O. M. Zhukova N. K. Luk’yanov 《Journal of Engineering Physics and Thermophysics》2007,80(6):1170-1177
Measurements of the beta-activity of milk, serving as the main source of information on the radioactive contamination of the
environment by the iodine isotope 131I, carried out on a DP-100 radiometer in the early post-Chernobyl period (1986) in Belarus, have been mathematically simulated.
The results obtained allow the conclusion that the indicated measurements should be analyzed again with consideration for
all of the nuclides present in milk.
__________
Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 6, pp. 102–108, November–December, 2007. 相似文献
998.
V. Sh. Shagapov S. A. Lepikhin M. N. Galimzyanov 《Journal of Engineering Physics and Thermophysics》2007,80(6):1206-1209
A steady flow of a bubble gas-liquid mixture in a nozzle having a circular cross section has been investigated. The possibility
of realization of superhigh temperatures and pressures in the gas phase of the mixture in the region near the smallest cross
section of the nozzle has been analyzed. The influence of the initial radius of the flow and the volume content of bubbles,
determining the volumetric rate of flow of the liquid supplied to the nozzle, on the pattern of the flow has been considered.
__________
Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 6, pp. 134–137, November–December, 2007. 相似文献
999.
V. A. Sychevskii N. V. Pavlyukevich L. L. Vasil'ev S. Rozin 《Journal of Engineering Physics and Thermophysics》2003,76(1):18-27
A problem of the Stefan type which describes heat and mass transfer in a porous body with the double phase transition liquid–ice–vapor has been formulated and solved. 相似文献
1000.
Analytical expressions for the shape of a charged drop of an incompressible liquid nonlinearly oscillating upon multimode
initial deformation have been obtained for the first time to within the third order of smallness. The second-order corrections
to oscillation frequencies are calculated depending on the spectrum of modes determining the initial deformation. The third-order
calculations show that the fundamental mode amplitude may increase due to the energy exchange with higher modes in a large
number of possible four-mode resonance interactions. 相似文献