全文获取类型
收费全文 | 197866篇 |
免费 | 2116篇 |
国内免费 | 633篇 |
专业分类
电工技术 | 4063篇 |
综合类 | 123篇 |
化学工业 | 26702篇 |
金属工艺 | 7508篇 |
机械仪表 | 5461篇 |
建筑科学 | 4039篇 |
矿业工程 | 747篇 |
能源动力 | 4660篇 |
轻工业 | 14381篇 |
水利工程 | 1695篇 |
石油天然气 | 2842篇 |
武器工业 | 7篇 |
无线电 | 27305篇 |
一般工业技术 | 37395篇 |
冶金工业 | 45400篇 |
原子能技术 | 3566篇 |
自动化技术 | 14721篇 |
出版年
2021年 | 1246篇 |
2019年 | 1282篇 |
2018年 | 2126篇 |
2017年 | 2109篇 |
2016年 | 2176篇 |
2015年 | 1490篇 |
2014年 | 2622篇 |
2013年 | 7950篇 |
2012年 | 4462篇 |
2011年 | 6110篇 |
2010年 | 4932篇 |
2009年 | 5773篇 |
2008年 | 6150篇 |
2007年 | 6196篇 |
2006年 | 5638篇 |
2005年 | 5306篇 |
2004年 | 5252篇 |
2003年 | 5092篇 |
2002年 | 4900篇 |
2001年 | 5307篇 |
2000年 | 4916篇 |
1999年 | 5437篇 |
1998年 | 15622篇 |
1997年 | 10312篇 |
1996年 | 7931篇 |
1995年 | 5783篇 |
1994年 | 5013篇 |
1993年 | 5022篇 |
1992年 | 3387篇 |
1991年 | 3260篇 |
1990年 | 3216篇 |
1989年 | 3041篇 |
1988年 | 2785篇 |
1987年 | 2234篇 |
1986年 | 2302篇 |
1985年 | 2607篇 |
1984年 | 2320篇 |
1983年 | 2063篇 |
1982年 | 1903篇 |
1981年 | 2043篇 |
1980年 | 1790篇 |
1979年 | 1659篇 |
1978年 | 1656篇 |
1977年 | 2022篇 |
1976年 | 2703篇 |
1975年 | 1422篇 |
1974年 | 1369篇 |
1973年 | 1316篇 |
1972年 | 1124篇 |
1971年 | 956篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
It is shown that direct compression grade tricalcium phosphate yields linear Heckel plots only if the particle density used is that obtained by liquid pycnometry or mercury porosimetry. If the true density is used (i.e. that obtained by gas displacement), then linearization is not obtained in a pressure range where bonding evidently occurs. It is shown that the pore volume in the range of diameters from 0.5 to 2 μm decreases as tableting pressure increases, and that bonding is associated with the loss of pores of this size. 相似文献
72.
73.
Tipton C.W. Bayne S.B. Griffin T.E. Scozzie C.J. Geil B. Agarwal A.K. Richmond J. 《Electron Device Letters, IEEE》2002,23(4):194-196
This paper reports on the first demonstration of a half-bridge power inverter constructed from silicon carbide gate turn-off thyristors (GTOs) operated in the conventional GTO mode. This circuit was characterized with input bus voltages of up to 600 VDC and 2 A (peak current density of 540 A/cm2) with resistive loads using a pulse-width modulated switching frequency of 2 kHz. We discuss the implications of the thyristor's electrical characteristics and the circuit topology on the overall operation of the half-bridge circuit. This work has determined the conservative critical rate of rise value of the off-state voltage to be 200 V/μs in these devices 相似文献
74.
75.
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
76.
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
77.
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
78.
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
79.
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
80.
In this paper, an alternative method of connecting fixed-speed wind turbines, with active pitch mechanisms or variable-slip generators, is proposed and evaluated. Instead of using the conventional soft starter, external resistors are used. The method was evaluated via laboratory tests on a 15-kW machine and theoretically on a 1-MW wind turbine. The proposed method causes smaller grid power quality impact, especially on inductive grids, compared with soft starters. This means that it will be possible to install constant-speed wind turbines to weaker grids using this method. 相似文献