全文获取类型
收费全文 | 289786篇 |
免费 | 3157篇 |
国内免费 | 820篇 |
专业分类
电工技术 | 5064篇 |
综合类 | 166篇 |
化学工业 | 45994篇 |
金属工艺 | 11926篇 |
机械仪表 | 8654篇 |
建筑科学 | 6922篇 |
矿业工程 | 1835篇 |
能源动力 | 6799篇 |
轻工业 | 26079篇 |
水利工程 | 3199篇 |
石油天然气 | 7095篇 |
武器工业 | 16篇 |
无线电 | 31654篇 |
一般工业技术 | 57364篇 |
冶金工业 | 51501篇 |
原子能技术 | 7545篇 |
自动化技术 | 21950篇 |
出版年
2021年 | 2167篇 |
2018年 | 3732篇 |
2017年 | 3681篇 |
2016年 | 3967篇 |
2015年 | 2538篇 |
2014年 | 4317篇 |
2013年 | 12138篇 |
2012年 | 6958篇 |
2011年 | 9398篇 |
2010年 | 7687篇 |
2009年 | 8760篇 |
2008年 | 9052篇 |
2007年 | 8919篇 |
2006年 | 7938篇 |
2005年 | 7395篇 |
2004年 | 6896篇 |
2003年 | 6606篇 |
2002年 | 6689篇 |
2001年 | 6544篇 |
2000年 | 6207篇 |
1999年 | 6265篇 |
1998年 | 14813篇 |
1997年 | 11076篇 |
1996年 | 8601篇 |
1995年 | 6555篇 |
1994年 | 5936篇 |
1993年 | 5803篇 |
1992年 | 4492篇 |
1991年 | 4452篇 |
1990年 | 4295篇 |
1989年 | 4310篇 |
1988年 | 4261篇 |
1987年 | 3602篇 |
1986年 | 3596篇 |
1985年 | 4166篇 |
1984年 | 3968篇 |
1983年 | 3621篇 |
1982年 | 3426篇 |
1981年 | 3561篇 |
1980年 | 3426篇 |
1979年 | 3367篇 |
1978年 | 3445篇 |
1977年 | 3926篇 |
1976年 | 5043篇 |
1975年 | 3159篇 |
1974年 | 3009篇 |
1973年 | 3026篇 |
1972年 | 2651篇 |
1971年 | 2473篇 |
1970年 | 2106篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
971.
Collins C.E. Pollard R.D. Miles R.E. Dildine R.G. 《IEEE transactions on instrumentation and measurement》1996,45(3):721-727
A new method for determining the single-sideband noise figure of a device from double-sideband measurements is proposed which eliminates the need for tunable filters and hence greatly reduces the complexity and cost of performing accurate noise figure measurements. This paper compares results obtained to those obtained using a traditional single-sideband measurement system with a tuneable filter and provides a full error analysis of the new technique. The method also offers considerable possibilities for future use at higher (microwave) frequencies where YIG filters and image reject mixers are not readily available 相似文献
972.
The growth structure of MgF2 and NdF3 films grown on polished CaF2(111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF2 and NdF3 films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K. 相似文献
973.
This paper addresses the aging behaviour of NiCr/CuNiMn/NiCr triple layers on Al2O3 ceramics at temperatures up to 200°C for film thicknesses d0.5 μm. Investigations of the film structure and the increase of resistance and its temperature coefficient during the annealing process and studies of the dependence of this aging drift on both the film thickness and the storage temperature have been carried out. Furthermore, the film stress and the effect of substrate bending on resistance have been measured. The results can be explained by the irregular film structure (columns and small bridges between them), which causes stress and current concentrations as well as local creeping, cracking and oxidation processes in the micro-bridges. They are compared with such for structurally homogeneous films on silicon wafers. 相似文献
974.
Nomograms are given for claculating the dielectric constant and dielectric loss of polar liquids by measuring the standing
wave ratio for an infinitely thick layer of fluid and the layer thickness at which wave reflection is minimum.
Translated from Izmeritel'naya Tekhnika, No. 2, pp. 46–47, February, 1996. 相似文献
975.
The whole history of failure of a rectangular panel with two symmetrical notches and a central crack subjected to a progressively increasing tension load normal to the crack plane is studied. The material of the panel exhibits substantial plastic deformation prior to fracture. An elastic-plastic analysis of the plate is first performed based on finite elements. The results of stress analysis are coupled with the strain energy density theory to determine the critical load for crack initiation and the history of stable crack growth up to the point of instability. At instability the crack runs fast through the elastic material bypassing the plastic zone near the plate boundary. The crack deviates from its initial direction and is curved even though the plate is subjected to opening-mode loading. Results for crack trajectories are given for various initial crack lengths and notch radii of the plate.Presented at Fourth Greek National Congress on Mechanics, 26–29 June 1995, held at Xanthi, Greece. 相似文献
976.
A generalized mathematical model of the interaction between hydrogen and structural defects in metals is analyzed. The temporal dependence of hydrogen permeation flow is examined for three typical cases. The approximations to the general equation for hydrogen permeation flow obtained enable one to determine the interaction from experimental data. The extreme values of permeation flow are analyzed. A method for determining the kinetic parameters of interaction for a given permeation flow is suggested. The conclusion is made that, within the framework of the generalized model, it is possible to establish the mechanism of hydrogen trapping.Dnepropetrovsk Institute of Chemical Engineering, Dnepropetrovsk. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 25–29, September–October, 1993. 相似文献
977.
Kovacs G.T.A. Storment C.W. Halks-Miller M. Belczynski C.R. Jr. Santina C.C.D. Lewis E.R. Maluf N.I. 《IEEE transactions on bio-medical engineering》1994,41(6):567-577
A new process for the fabrication of regeneration microelectrode arrays for peripheral and cranial nerve applications is presented. This type of array is implanted between the severed ends of nerves, the axons of which regenerate through via holes in the silicon and are thereafter held fixed with respect to the microelectrodes. The process described is designed for compatibility with industry-standard CMOS or BiCMOS processes (it does not involve high-temperature process steps nor heavily-doped etch-stop layers), and provides a thin membrane for the via holes, surrounded by a thick silicon supporting rim. Many basic questions remain regarding the optimum via hole and microelectrode geometries in terms of both biological and electrical performance of the implants, and therefore passive versions were fabricated as tools for addressing these issues in on-going work. Versions of the devices were implanted in the rat peroneal nerve and in the frog auditory nerve. In both cases, regeneration was verified histologically and it was observed that the regenerated nerves had reorganized into microfascicles containing both myelinated and unmyelinated axons and corresponding to the grid pattern of the via holes. These microelectrode arrays were shown to allow the recording of action potential signals in both the peripheral and cranial nerve settings, from several microelectrodes in parallel 相似文献
978.
Airborne scatterometers: investigating ocean backscatter under low and high-wind conditions 总被引:1,自引:0,他引:1
Carswell J.R. Carson S.C. McIntosh R.E. Li F.K. Neumann G. McLaughlin D.J. Wilkerson J.C. Black P.G. Nghiem S.V. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1994,82(12):1835-1860
Attempting to understand and predict weather on a local and global basis has challenged both the scientific and engineering communities. One key parameter in understanding the weather is the ocean surface wind vector because of its role in the energy exchange at the air-sea surface. scatterometers, radars that measure the reflectivity of a target offer a tool with which to remotely monitor these winds from tower-, aircraft-, and satellite-based platforms. This paper introduces three current airborne scatterometer systems, and presents data collected by these instruments under low-, moderate-, and high-wind conditions. The paper focuses on airborne scatterometers because of their ability to resolve submesoscale variations in wind fields. Discrepancies between existing theory and the observations are noted and the concerns in measuring low-wind speeds discussed. Finally, the application of using this technology for estimating the surface-wind vector during a hurricane is demonstrated 相似文献
979.
Maxwell G.D. Kashyap R. Sherlock G. Collins J.V. Ainslie B.J. 《Electronics letters》1994,30(18):1486-1487
The authors report the first demonstration of a semiconductor external cavity waveguide laser, modulated at 2.5 Gbit/s over 100 km of standard optical fibre using a UV written grating in a planar silica waveguide as the feedback element 相似文献
980.
P Cruz R Almeida M Gon?alves G Marote J Silva C Cavaleiro V Jacinto 《Canadian Metallurgical Quarterly》1994,7(5):277-280
Ocular paresis is a usual diagnosis in neuro-ophthalmologic consultation, and its aetiology is indeed a challenge. The authors studied 3400 records concerning this consultation, between 1982 and the third quarter of 1993. During this period 221 cases with ocular paresis were selected. During this retrospective study we point out the following items: the cranial nerve affected; the neuro-ophthalmologic semiology; the aetiology and clinical evolution, according to the age groups. From 221 cases selected, 111 were paresis from the VIth pair (50.2%), 88 from the IIIrd pair (39.8%), 14 multiple paresis (6.4%) and 8 paresis from the IVth pair (3.6%). The most frequent complaint was diplopia (> 90%). As far as the aetiological diagnosis is concerned, this was easier to establish in patients > 50 years of age. In this age group the most usual aetiology was vascular and traumatic pathologies. In younger patients the most frequent pathologies were traumatic and tumoral. The prognostic was better in the vascular group, the paresis recovery being > 50% in all other pathologies, except the tumoral one. 相似文献