全文获取类型
收费全文 | 197837篇 |
免费 | 2258篇 |
国内免费 | 632篇 |
专业分类
电工技术 | 4061篇 |
综合类 | 123篇 |
化学工业 | 26713篇 |
金属工艺 | 7509篇 |
机械仪表 | 5461篇 |
建筑科学 | 4037篇 |
矿业工程 | 747篇 |
能源动力 | 4657篇 |
轻工业 | 14383篇 |
水利工程 | 1695篇 |
石油天然气 | 2843篇 |
武器工业 | 7篇 |
无线电 | 27301篇 |
一般工业技术 | 37389篇 |
冶金工业 | 45527篇 |
原子能技术 | 3560篇 |
自动化技术 | 14714篇 |
出版年
2021年 | 1244篇 |
2019年 | 1281篇 |
2018年 | 2125篇 |
2017年 | 2105篇 |
2016年 | 2171篇 |
2015年 | 1490篇 |
2014年 | 2619篇 |
2013年 | 7952篇 |
2012年 | 4463篇 |
2011年 | 6111篇 |
2010年 | 4931篇 |
2009年 | 5774篇 |
2008年 | 6146篇 |
2007年 | 6199篇 |
2006年 | 5637篇 |
2005年 | 5304篇 |
2004年 | 5253篇 |
2003年 | 5092篇 |
2002年 | 4901篇 |
2001年 | 5309篇 |
2000年 | 4916篇 |
1999年 | 5441篇 |
1998年 | 15655篇 |
1997年 | 10329篇 |
1996年 | 7941篇 |
1995年 | 5789篇 |
1994年 | 5022篇 |
1993年 | 5030篇 |
1992年 | 3386篇 |
1991年 | 3260篇 |
1990年 | 3217篇 |
1989年 | 3043篇 |
1988年 | 2786篇 |
1987年 | 2235篇 |
1986年 | 2302篇 |
1985年 | 2607篇 |
1984年 | 2313篇 |
1983年 | 2067篇 |
1982年 | 1903篇 |
1981年 | 2043篇 |
1980年 | 1792篇 |
1979年 | 1662篇 |
1978年 | 1660篇 |
1977年 | 2026篇 |
1976年 | 2711篇 |
1975年 | 1424篇 |
1974年 | 1369篇 |
1973年 | 1319篇 |
1972年 | 1127篇 |
1971年 | 959篇 |
排序方式: 共有10000条查询结果,搜索用时 19 毫秒
241.
ABSTRACT: Physical aging of amorphous anhydrous fructose at temperature 5 °C and at 22 °C was studied using differential scanning calorimetry (DSC). The dynamic glass transition temperature, Tg0 for unaged samples was 16 °C and 13.3 °C for heating rate of 10 °C/min and 1 °C/min, respectively. The fictive temperature, Tf0 for unaged samples calculated by Richardson and Savill method was 12 °C, which is close to the dynamic value obtained from the lower DSC heating rate. The fictive temperature Tf of the aged fructose glasses at temperatures both below and above the transition region was fitted well by a non-exponential decay function (Williams-Watts form). Aging above the transition region (22 °C) for 18 d increased both the dynamic glass transition temperature Tg and the fictive temperature Tf . However, aging below the transition region (5 °C) for 1 d increased the dynamic glass transition temperature Tg but decreased the fictive temperature Tf . 相似文献
242.
1-GHz-spaced 16-channel arrayed-waveguide grating for a wavelength reference standard in DWDM network systems 总被引:3,自引:0,他引:3
We fabricated a 1-GHz-spaced 16-channel arrayed-waveguide grating (AWG) by using a new AWG configuration where the path of each arrayed waveguide winds backward and forward across a 4-in diameter wafer without crossing any other waveguides. The ultra-narrow (< 1 GHz) and stable transmission bands of this AWG can be used to construct a wavelength reference standard covering the S, C, and L bands in the dense wavelength-division-multiplexing network systems whose frequency deviation is /spl plusmn/160 MHz. 相似文献
243.
In this paper, the influence of viewing distance on subjective assessment of the impairment in video sequences is investigated. Subjective tests using the double-stimulus impairment scale variant II (DSIS II) method have been conducted at viewing distances of 5H (where H stands for the screen height) and 3H, respectively. Several statistical measures have been used to analyze the influence, including correlations and ANOVA (analysis of variance) tests. The results reveal that there is a very high correlation between the subjective scores, the variances are similar under the two viewing distances, the means of subjective data at these two viewing distances are the same, and there is no interaction between the viewing distance and the other two factors, i.e., the codec system and the source sequence. Throughout the tests, there is no evidence that a closer viewing distance such as 3H will vary the subjective test result statistically significantly. 相似文献
244.
245.
Chen H.H. Huang T.S. 《IEEE transactions on pattern analysis and machine intelligence》1991,13(9):872-878
The specific problem addressed by the authors is how to detect the true match of a fourth point from among candidate matches in a situation in which three points have already been matched. The two sets of points to be matched are both subject to measurement errors. The depth error is more dominant than errors in the other two coordinates; however, the exact statistical distribution of the measurement errors is not known. The authors present a new method for solving the problem. The method is based on the technique of motion analysis using orthographic views. It discards the noisy z (depth) coordinates and uses only the x and y coordinates of the points to verify the match. The effect of depth errors on the motion estimate is completely prevented. Results show that this method is substantially more effective than previous methods that use all three coordinates 相似文献
246.
The void fraction in a three-component randomly packed bed was calculated from the authors' model, and the calculated values were compared with published experimental data for spherical and irregularly shaped particles and with results from computer simulations. Results from the model were in good agreement with simulated and published experimental data. 相似文献
247.
The strong interaction of electrons with the flat surfaces of small crystals has been investigated by high resolution CTEM and STEM instruments. When cubic crystals of MgO smoke with edges 20–300 nm are oriented so that the ?001? or ?011? zone axis is parallel to the optical axis, then two kinds of external fringes are observed at (100) surfaces. One kind is parallel to the surface, having spacings up to 0.4 nm. These are caused by interference among the electron channelled along the surface. Fresnel-diffracted ones and the remnant of the incident beam. Fringes of the other kind, which appear as fine structure in the first kind of fringes, are perpendicular to the crystal edge. When an electron beam is parallel to the ?011? axis, the second kind of fringe, whose spacing is 0.3 nm corresponding to d011, shows the difference of the surface potential between magnesium atoms and oxygen atoms. Selected area diffraction patterns and microdiffraction patterns also show the same periodicities as in the two kinds of fringes. Simulated images, using the scattering amplitudes for ions, are compared with observations. 相似文献
248.
A theoretical expression is derived for the far fields of semiconductor lasers with coated facets. It is shown that the far fields are different for uncoated, antireflection-coated and high-reflection-coated lasers. The correction factor for the far fields is shown to depend on the transmission coefficient of the facets. 相似文献
249.
Asymptotic stability of finite-dimensional linear continuous-time periodic (FDLCP) systems is studied by harmonic analysis. It is first shown that stability can be examined with what we call the harmonic Lyapunov equation. Another necessary and sufficient stability criterion is developed via this generalized Lyapunov equation, which reduces the stability test into that of an approximate FDLCP model whose transition matrix can be determined explicitly. By extending the Gerschgorin theorem to linear operators on the linear space l2, yet another disc-group criterion is derived, which is only sufficient. Stability of the lossy Mathieu equation is analyzed as a numerical example to illustrate the results 相似文献
250.
Field R. D. Hartwig K. T. Necker C. T. Bingert J. F. Agnew S. R. 《Metallurgical and Materials Transactions A》2002,33(13):965-972
The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions
have been successfully completed on Ni-canned billets of Be at approximately 425°C. No cracking was observed in the billets,
and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed
for a singlepass extrusion, including evidence of <c> and <c+a> dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation
processing technique and the underlying physical processes which sustain the deformation.
S.R. AGNEW, formerly with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献