全文获取类型
收费全文 | 278284篇 |
免费 | 3338篇 |
国内免费 | 1304篇 |
专业分类
电工技术 | 4802篇 |
综合类 | 520篇 |
化学工业 | 40316篇 |
金属工艺 | 11132篇 |
机械仪表 | 8421篇 |
建筑科学 | 6442篇 |
矿业工程 | 1576篇 |
能源动力 | 6969篇 |
轻工业 | 22525篇 |
水利工程 | 3038篇 |
石油天然气 | 5782篇 |
武器工业 | 43篇 |
无线电 | 33505篇 |
一般工业技术 | 56513篇 |
冶金工业 | 50977篇 |
原子能技术 | 7494篇 |
自动化技术 | 22871篇 |
出版年
2021年 | 2156篇 |
2019年 | 2067篇 |
2018年 | 3579篇 |
2017年 | 3673篇 |
2016年 | 3906篇 |
2015年 | 2499篇 |
2014年 | 4346篇 |
2013年 | 11977篇 |
2012年 | 7075篇 |
2011年 | 9712篇 |
2010年 | 7542篇 |
2009年 | 8760篇 |
2008年 | 9106篇 |
2007年 | 9161篇 |
2006年 | 7818篇 |
2005年 | 7272篇 |
2004年 | 7217篇 |
2003年 | 6979篇 |
2002年 | 6728篇 |
2001年 | 6850篇 |
2000年 | 6485篇 |
1999年 | 6770篇 |
1998年 | 16309篇 |
1997年 | 11589篇 |
1996年 | 8926篇 |
1995年 | 6849篇 |
1994年 | 6024篇 |
1993年 | 6059篇 |
1992年 | 4403篇 |
1991年 | 4182篇 |
1990年 | 4076篇 |
1989年 | 4061篇 |
1988年 | 3753篇 |
1987年 | 3390篇 |
1986年 | 3399篇 |
1985年 | 3743篇 |
1984年 | 3469篇 |
1983年 | 3230篇 |
1982年 | 3028篇 |
1981年 | 3043篇 |
1980年 | 3033篇 |
1979年 | 2832篇 |
1978年 | 2874篇 |
1977年 | 3146篇 |
1976年 | 4101篇 |
1975年 | 2423篇 |
1974年 | 2383篇 |
1973年 | 2399篇 |
1972年 | 2020篇 |
1971年 | 1784篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A simple measurement method for the absolute Raman scattering cross-section of an optical fibre is presented. This method does not require a high power pump laser and it measures the fibre directly. It is also shown that the calculation of the cross-section depends on the fibre structure, such as its refractive index profile and the differential attenuation between the pump wavelength and Raman wavelength.<> 相似文献
942.
WO3 semiconductor particles, useful in solar energy conversion processes, were doped with transition metal ions, Ti(III), V(IV),
Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Ru(III) by a high-temperature sintering technique. The method
of preparation of these photocatalysts is described in detail. The structural changes effected during sintering were investigated
by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM). The XRD analysis indicated that the monoclinic crystal
structure of WO3 was not altered during sintering. SEM studies showed that the sizes of the particles ranged from 1 to 10 μm and the crystallinity
was increased due to doping. The dopants were found to be mostly distributed on the surface of WO3 particles. 相似文献
943.
A predictive analogy of turbulent mass transfer across wavy, sheared air-water interface, in deep water bodies is proposed. The theory is based on the formulation of a theoretical eddy diffusivity profile in the surface region, and it applies to the non-breaking wave regime. The predictions for a water-side friction velocity range of 0.5–6 cm/s are in excellent agreement with laboratory data from wind-wave facilities with an average error of 15.4%. 相似文献
944.
Accuracy and repeatability of complexometric methods for measuring calcium in cheese are limited by titration endpoints that are difficult to recognize. Much of this difficulty results from turbidity during titration. An alternative procedure is described in which 2 to 3 g of cheese are ashed, dissolved in dilute acid, and added calcium chloride is back titrated with ethylenediaminetetraacetic acid using hydroxy naphthol blue as indicator. Samples remain free of turbidity, and the titration endpoint is recognized easily.Recovery of calcium from solutions containing magnesium and phosphate was 100.9 ± .3%, indicating that a portion of sample magnesium (about 12%) also was measured. Error caused by magnesium recovery represented less than 1% of true calcium content.Analyses of 10 Cheddar cheese samples by the proposed method and by atomic emission differed by an average of .010% calcium. Mean values of the 10 samples did not differ significantly (P>.05) between methods. Duplicate analyses of 67 Cheddar cheese samples differed by an average of .008 ± .006% calcium. Similar repeatability was achieved by several inexperienced analysts. The method is applied readily to other dairy products. 相似文献
945.
The effect of electrical stimulation (ES) of beef carcasses at 450 V on the total extractable activity and subcellular distribution of the mitochondrial enzymes lipoamide dehydrogenase, citrate synthase and beta-hydroxyacyl-CoA-dehydrogenase in skeletal muscle (activities in the supernatant of a phosphate buffer extract and in muscle press juice) was studied. There was no influence of ES on the total activity and the subcellular distribution of these enzymes in the muscle tissue stored at +2 degrees C for 7 days nor did ES influence the extent of the release of the three enzymes from the mitochondria into the sarcoplasm by freezing (-20 degrees C) and thawing. From these results it can be concluded that ES does not result in an appreciable disintegration of the inner membrane of muscle mitochondria. 相似文献
946.
On-body communications around the human torso are considered, using ultra-wideband transmissions. The velocity of the diffracted wave propagating around the body is extracted from frequency- domain spatial correlation analyses. 相似文献
947.
Spataro A. Deval Y. Begueret J.-B. Fouillat P. Belot D. 《Solid-State Circuits, IEEE Journal of》2002,37(3):336-341
In this paper, we present a waveform converter implemented on a 0.25-μm CMOS technology using a dedicated design methodology (Delay Oriented Design). The circuit converts a square wave signal in both in-phase and quadrature-phase sinusoidal differential outputs. It also multiplies the frequency by seven. The output frequency range of this converter extends from 1.05 GHz up to 2.17 GHz. This converter is dedicated for the design of a third-generation mobile phone synthesizer using a double-loop architecture. For an output frequency of 2 GHz, the measured phase noise at 10-kHz offset from the carrier is -97 dBc/Hz. The circuit consumes 50 mW from a 2.5-V supply 相似文献
948.
Like other flexible objects, the public key infrastructure sacrifices some utility in trying to be all things to all people. Mainly, PKI's generic, all-purpose identity certificates fall short of what the marketplace demands, forcing vendors to develop more economically efficient, useful, and imaginative business models. Thus, we must adapt the PKI design to the real world rather than trying to constrain the real world to match the PKI. A variety of alternative approaches, ranging from simple workarounds to designing the application to sidestep PKI's shortcomings entirely, can help solve the problems inherent in the standard X.509 model.
Despite an original design that failed to address the marketplace's needs, the use of innovative public key infrastructure models can make the technology meet today's requirements. 相似文献
949.
The gates of GaAs MESFETS rely on the formation of a Schottky diode between the semiconductor surface and a deposited metal and so are extremely sensitive to the nature of this interface. For this reason an investigation of surface damage and contamination by the various processing stages involved in IC fabrication has been undertaken. This paper outlines the results obtained from processes involved in activation of ion implantation through annealing with a dielectric encapsulant, wet chemical etching, and metallization. Methods for removing or minimizing the effects of processing damage or contamination are also investigated. 相似文献
950.
Adaptive smoothing: a general tool for early vision 总被引:18,自引:0,他引:18
Saint-Marc P. Chen J.-S. Medioni G. 《IEEE transactions on pattern analysis and machine intelligence》1991,13(6):514-529
A method to smooth a signal while preserving discontinuities is presented. This is achieved by repeatedly convolving the signal with a very small averaging mask weighted by a measure of the signal continuity at each point. Edge detection can be performed after a few iterations, and features extracted from the smoothed signal are correctly localized (hence, no tracking is needed). This last property allows the derivation of a scale-space representation of a signal using the adaptive smoothing parameter k as the scale dimension. The relation of this process to anisotropic diffusion is shown. A scheme to preserve higher-order discontinuities and results on range images is proposed. Different implementations of adaptive smoothing are presented, first on a serial machine, for which a multigrid algorithm is proposed to speed up the smoothing effect, then on a single instruction multiple data (SIMD) parallel machine such as the Connection Machine. Various applications of adaptive smoothing such as edge detection, range image feature extraction, corner detection, and stereo matching are discussed 相似文献