首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   355037篇
  免费   4761篇
  国内免费   1215篇
电工技术   6398篇
综合类   243篇
化学工业   53879篇
金属工艺   14858篇
机械仪表   10506篇
建筑科学   8822篇
矿业工程   2063篇
能源动力   9297篇
轻工业   31373篇
水利工程   3757篇
石油天然气   6656篇
武器工业   10篇
无线电   40362篇
一般工业技术   69003篇
冶金工业   66763篇
原子能技术   8533篇
自动化技术   28490篇
  2021年   2753篇
  2019年   2630篇
  2018年   4384篇
  2017年   4444篇
  2016年   4721篇
  2015年   3199篇
  2014年   5450篇
  2013年   15828篇
  2012年   8991篇
  2011年   12545篇
  2010年   9975篇
  2009年   11537篇
  2008年   11596篇
  2007年   11627篇
  2006年   9946篇
  2005年   9277篇
  2004年   8902篇
  2003年   8495篇
  2002年   8157篇
  2001年   8263篇
  2000年   8027篇
  1999年   8309篇
  1998年   20448篇
  1997年   14638篇
  1996年   11362篇
  1995年   8462篇
  1994年   7481篇
  1993年   7250篇
  1992年   5498篇
  1991年   5335篇
  1990年   5029篇
  1989年   5123篇
  1988年   4934篇
  1987年   4281篇
  1986年   4127篇
  1985年   4860篇
  1984年   4458篇
  1983年   4156篇
  1982年   3776篇
  1981年   3924篇
  1980年   3652篇
  1979年   3692篇
  1978年   3684篇
  1977年   4216篇
  1976年   5617篇
  1975年   3215篇
  1974年   3125篇
  1973年   3188篇
  1972年   2648篇
  1971年   2469篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
4.
In this study, dilute chemical bath deposition technique has been used to deposit CdZnS thin films on soda-lime glass substrates. The structural, morphological, optoelectronic properties of as-grown films have been investigated as a function of different Zn2+ precursor concentrations. The X-ray diffractogram of CdS thin-film reveals a peak corresponding to (002) plane with wurtzite structure, and the peak shift has been observed with the increase of the Zn2+ concentration upon formation of CdZnS thin film. From morphological studies, it has been revealed that the diluted chemical bath deposition technique provides homogeneous distribution of film on the substrate even at a lower concentration of Zn2+. Optical characterization has shown that the transparency of the film is influenced by Zn2+ concentration and when the Zn2+ concentration is varied from 0 M to 0.0256 M, bandgap values of resulting films range from 2.42 eV to 3.90 eV while. Furthermore, electrical properties have shown that with increasing zinc concentration the resistivity of the film increases. Finally, numerical simulation validates and suggests that CdZnS buffer layer with composition of 0.0032 M Zn2+ concentration would be a promising candidate in CIGS solar cell.  相似文献   
5.
Journal of Chemical Ecology - Biocontrol agents such as parasitic wasps use long-range volatiles and host-associated cues from lower trophic levels to find their hosts. However, this chemical...  相似文献   
6.
A set of novel hydrazone derivatives were synthesized and analyzed for their biological activities. The compounds were tested for their inhibitory effect on the phosphorylating activity of the protein kinase CK2, and their antioxidant activity was also determined in three commonly used assays. The hydrazones were evaluated for their radical scavenging against the DPPH, ABTS and peroxyl radicals. Several compounds have been identified as good antioxidants as well as potent protein kinase CK2 inhibitors. Most hydrazones containing a 4-N(CH3)2 residue or perfluorinated phenyl rings showed high activity in the radical-scavenging assays and possess nanomolar IC50 values in the kinase assays.  相似文献   
7.
Journal of Communications Technology and Electronics - A new design of wavelets based on the convolution of a compactly supported function with a rectangular pulse is proposed and theoretically...  相似文献   
8.
Journal of Communications Technology and Electronics - A statistical study of the effectiveness of the non-threshold search procedure for a noise-like phase-shift keyed signal by the delay time is...  相似文献   
9.
Chemical engineering systems often involve a functional porous medium, such as in catalyzed reactive flows, fluid purifiers, and chromatographic separations. Ideally, the flow rates throughout the porous medium are uniform, and all portions of the medium contribute efficiently to its function. The permeability is a property of a porous medium that depends on pore geometry and relates flow rate to pressure drop. Additive manufacturing techniques raise the possibilities that permeability can be arbitrarily specified in three dimensions, and that a broader range of permeabilities can be achieved than by traditional manufacturing methods. Using numerical optimization methods, we show that designs with spatially varying permeability can achieve greater flow uniformity than designs with uniform permeability. We consider geometries involving hemispherical regions that distribute flow, as in many glass chromatography columns. By several measures, significant improvements in flow uniformity can be obtained by modifying permeability only near the inlet and outlet.  相似文献   
10.
We considered the magnetohydrodynamic (MHD) free convective flow of an incompressible electrically conducting viscous fluid past an infinite vertical permeable porous plate with a uniform transverse magnetic field, heat source and chemical reaction in a rotating frame taking Hall current effects into account. The momentum equations for the fluid flow during absorbent medium are controlled by the Brinkman model. Through the undisturbed state, both the plate and fluid are in a rigid body rotation by the uniform angular velocity perpendicular to an infinite vertical plate. The perpendicular surface is subject to the homogeneous invariable suction at a right angle to it and the heat on the surface varies about a non-zero unvarying average whereas the warmth of complimentary flow is invariable. The systematic solutions of the velocity, temperature, and concentration distributions are acquired systematically by utilizing the perturbation method. The velocity expressions consist of steady-state and fluctuating situations. It is revealed that the steady part of the velocity field has a three-layer characteristic while the oscillatory part of the fluid field exhibits a multi-layer characteristic. The influence of various governing flow parameters on the velocity, temperature, and concentration are analyzed graphically. We also discuss computational results for the skin friction, Nusselt number, and Sherwood number in the tabular forms.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号