全文获取类型
收费全文 | 953篇 |
免费 | 44篇 |
国内免费 | 11篇 |
专业分类
电工技术 | 12篇 |
综合类 | 3篇 |
化学工业 | 103篇 |
金属工艺 | 11篇 |
机械仪表 | 40篇 |
建筑科学 | 47篇 |
能源动力 | 20篇 |
轻工业 | 75篇 |
水利工程 | 8篇 |
石油天然气 | 2篇 |
无线电 | 164篇 |
一般工业技术 | 186篇 |
冶金工业 | 200篇 |
原子能技术 | 4篇 |
自动化技术 | 133篇 |
出版年
2023年 | 4篇 |
2022年 | 8篇 |
2021年 | 7篇 |
2020年 | 7篇 |
2019年 | 5篇 |
2018年 | 18篇 |
2017年 | 14篇 |
2016年 | 14篇 |
2015年 | 12篇 |
2014年 | 25篇 |
2013年 | 56篇 |
2012年 | 30篇 |
2011年 | 41篇 |
2010年 | 33篇 |
2009年 | 33篇 |
2008年 | 34篇 |
2007年 | 47篇 |
2006年 | 43篇 |
2005年 | 24篇 |
2004年 | 39篇 |
2003年 | 36篇 |
2002年 | 38篇 |
2001年 | 24篇 |
2000年 | 23篇 |
1999年 | 32篇 |
1998年 | 72篇 |
1997年 | 58篇 |
1996年 | 36篇 |
1995年 | 24篇 |
1994年 | 17篇 |
1993年 | 19篇 |
1992年 | 17篇 |
1991年 | 17篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1988年 | 9篇 |
1987年 | 3篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 10篇 |
1973年 | 2篇 |
1970年 | 2篇 |
排序方式: 共有1008条查询结果,搜索用时 15 毫秒
51.
Z. L. Du P. C. W. Fung J. C. L. Chow Y. Y. Luo Q. Y. Li 《Journal of Superconductivity and Novel Magnetism》1996,9(1):43-48
We report a simple and safe technology where rather pure Hg-1201 phase superconducting bulk samples can be fabricated. The pellet was heated at about 700°C for only 30 min in open air under atmospheric pressure. We have found that a dehydration process (heating for 20 h at 250°C) is a crucial procedure for success in forming the Hg-1201 phase. 相似文献
52.
AC magnetic susceptibility of Rb3C60 superconductor as a function of temperature was measured in an applied static magnetic field perpendicular to the AC magnetic field. The peaks in real part of AC susceptibility curves located in the transitional temperature indicate that there may exist the differential paramagnetic effect (T) = dM/dH > 0 in the specimen. The amplitude of the peak and the temperature of diamagnetic onset are proportional to intensity of the applied field, and upon cooling the peaks occur before the transition temperature of zero field. Through discussing and comparing our experimental result with those reported previously, we have put forward a new opinion on producing condition of the differential paramagnetic effect (DPE). 相似文献
53.
This communication provides a comparison between the backscattering coefficients computed using Gaussian versus non-Gaussian surface statistics. The computations are performed for a class of surface height distributions and surface correlation functions. Results indicate that the coherent component of the scattering coefficient is strongly dependent upon the surface height distribution. The noncoherent component also depends on the surface height distribution and the surface root mean square (rms) slope when the rms surface height is large relative to the incident wavelength. On the other hand, when the surface rms height is moderate or small, the noncoherent component is sensitive mainly to the surface height correlation function. 相似文献
54.
Wan-Chi Siu Yui-Lam Chan Kai-Tat Fung 《Multimedia, IEEE Transactions on》2007,9(6):1093-1102
Only a limited number of methods have been proposed to realize heterogeneous transcoding, for example from MPEG-2 to H.263, or from H.264 to H.263. The major difficulties of transcoding a B-picture to a P-picture are that the incoming discrete cosine transform (DCT) coefficients of the B-frame are prediction errors arising from both forward and backward predictions, whilst the prediction errors in the DCT domain arising from the prediction using the previous frame alone are not available. The required new prediction errors need to be re-estimated in the pixel domain. This process involves highly complex computation and introduces re-encoding errors. We propose a new approach to convert a B-picture into a P-picture by making use of some properties of motion compensation in the DCT domain and the direct addition of DCT coefficients. We derive a set of equations and formulate the problem of how to obtain the DCT coefficients. One difficulty is that the last P-frame inside a GOP with an IBBP structure, for example, needs to be transcoded to become the last P-frame in the IPPP structure, and it has to be linked to the previous reconstructed P-frame instead of to the I-frame. We increased the speed of the transcoding process by making use of the motion activity which is expressed in terms of the correlation between pictures. The whole transcoding process is done in the transform domain, hence re-encoding errors are completely avoided. Results from our experimental work show that the proposed video transcoder not only achieves a speed-up of two to six times that of the conventional video transcoder, but it also substantially improves the quality of the video. 相似文献
55.
M. B. Sobnack F. V. Kusmartsev J. C. H. Fung 《Journal of Low Temperature Physics》2007,148(5-6):869-873
We study the nucleation of vortices in a thin (thickness ≪ penetration depth) mesoscopic superconducting disc in an applied magnetic field perpendicular to the disc (i.e., parallel
to the axis of the disc). We write down an expression for the free energy of the system with an arbitrary number of vortices
and anti-vortices at finite (non-zero) temperatures. For a given applied field, we minimize the free energy to find the optimal
position of the vortices and anti-vortices (the configuration which minimizes the energy). We show that, whereas at zero temperature
anti-vortices do not nucleate, anti-vortices do penetrate the disc at finite temperatures. We also calculate the magnetization
of the disc as a function of the applied field and hence determine the different configurations possible in which a fixed
number of fluxoids can penetrate the disc. 相似文献
56.
The nonlinear behavior of laminated plates in a general state of non-uniform initial stress was studied at large vibration amplitudes. The nonlinear governing equations of this study were derived using a higher-order theory approach. The results were compared with the Mindlin plate theory’s results. The results showed that the higher-order shear deformation terms had a significant influence on the plate in a large amplitude vibration when the thickness ratio decreases and the plate was stacked with less layers. In addition, the effect of Young’s modulus in the thickness direction on the frequency ratio was significant for the two-layered plate. However, the results of the four-layered plates were not affected too much. 相似文献
57.
S. G. Beaven S. P. Gogineni S. Tjuatja A. K. Fung 《International journal of remote sensing》2013,34(12):2483-2503
Interpretation of Synthetic Aperture Radar (SAR) images of sea ice is complex because of the natural variability of sea ice and sensor-induced effects, such as speckle. Most of the research on SAR image interpretation has focused on the winter months and algorithms were developed to classify sea ice successfully under cold conditions. However, interpretation of SAR images during the seasonal transitions has proved difficult due to rapidly changing weather conditions. In this paper we address the application of SAR during the transition from summer to the fall freeze-up. This period is important because it signals the start of significant new ice growth, which affects the air-ocean heat exchange and injects brine into the upper layers of the ocean. We have interpreted SAR images of the sea ice in terms of the basic ice characteristics by using shipborne radar measurements of sea ice during the freeze-up and models derived from these measurements. We have shown that the model-based approach is effective in interpreting SAR images during this seasonal transition. This work also provides the physical mechanisms responsible for the large increase in backscatter observed at the end of the summer melt season. 相似文献
58.
Normalized difference vegetation index (NDVI) derived from SPOT HRV multispectral data was used to study the changing environmental quality of Hong Kong from 1987, 1991 and 1993 to 1995. Conventional change detection techniques such as image differencing or principal components analysis helped to highlight salient changes. These techniques, however, were less effective in identifying subtle changes, in particular the amount and quality of green space. Integrating the mean NDVI values at the Tertiary Planning Unit (TPU) level with census and land-cover data showed that the NDVI values were related to woodland, tall scrubland and high-density urban areas. It was also related to the level of crowding as depicted from a factor analysis of census data. Tracing the changing pattern of mean NDVI values revealed that areas with continuous increases in NDVI values are scattered around old urban districts experiencing improved landscaping. Areas of continuous decrease in NDVI values covered a large part of rural New Territories and western Hong Kong Island revealing the urban expansion process. This provided valuable information for the assessment of environmental quality for planning and management of the environment. 相似文献
59.
Kochy Fung 《Aerosol science and technology》2013,47(1):122-127
A selective thermal oxidation method was developed for the speciation of carbonaceous aerosols collected on filters into organic carbon (OC) and elemental carbon (EC). The technique is based on studying the thermal oxidation of microcrystalline graphite by MnO2as well as various organic compounds. The procedure uses a modified Dohrmann DC-52 carbon analyzer with a flame ionization detector to detect the CO2resulting from the oxidation as methane after catalytic conversion. The results led to the selection of 525 °C as the optimal temperature for the oxidation of OC while leaving EC intact. After the organic oxidation, the sample is heated at 850° C, at which EC is oxidized rapidly and completely by MnO2. Carbonates that may be present in either the particles or the filter medium are removed by acidification and heating to ~ 120°C prior to performing the organic and EC measurements. Analysis of split ambient particulate samples in which the OC levels had been reduced by solvent extraction produced EC results statistically the same as the original untreated samples. These results suggest that the speciation is not sensitive to the level of organics in the sample. During the Carbonaceous Species Methods Comparison Study (CSMCS) in which the participants analyzed 20 blind samples, with four being triplicates, this technique yielded results in good agreement with the average results of the participants, with coefficients of variation (CV) derived from the triplicate analysis being 2.1%, 2.6%, and 8.1%, respectively, for total, organic, and elemental carbon. 相似文献
60.
Insertion of an insulating paper between point-plane electrodes normally increases the total breakdown potential threshold, except when its position is next to the plane. A series of DC [both (+) and (-)] experiments was designed and conducted to examine this latter distinction. It was found that visual ionic spots on the paper surface are a dominant prebreakdown phenomenon. Local failure at one of such discrete spots ultimately brings about a streamer sparkover in air at a sufficiently high potential. This paper describes the variability and other characteristics of the pattern in a two-dimensional spot distribution. Related experiments reveal that, with double paper layers already in the barrier, the pattern of spots becomes less dense visually but the breakdown potential would increase for the case of (+) point but would decrease further for the case of (-) point if a third sheet is introduced. A sharply defined “dark” band on the double-layer side in which glow spots are absent was always observed to clearly bridge the separate domains of, respectively, single-layer and double-layer spot distributions. Also related were the mechanical agitations of the paper accompanying all phases of the different discharge activities 相似文献