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   569337篇
  免费   6676篇
  国内免费   2723篇
电工技术   9741篇
综合类   3075篇
化学工业   82071篇
金属工艺   25069篇
机械仪表   17375篇
建筑科学   13926篇
矿业工程   2534篇
能源动力   13470篇
轻工业   46614篇
水利工程   5856篇
石油天然气   7348篇
武器工业   46篇
无线电   68407篇
一般工业技术   109718篇
冶金工业   98576篇
原子能技术   10954篇
自动化技术   63956篇
  2021年   3683篇
  2019年   3315篇
  2018年   19341篇
  2017年   18274篇
  2016年   15514篇
  2015年   4765篇
  2014年   7255篇
  2013年   21653篇
  2012年   14607篇
  2011年   25214篇
  2010年   20634篇
  2009年   19896篇
  2008年   21802篇
  2007年   22846篇
  2006年   13851篇
  2005年   13807篇
  2004年   13516篇
  2003年   13333篇
  2002年   12519篇
  2001年   12271篇
  2000年   11533篇
  1999年   12320篇
  1998年   31021篇
  1997年   21881篇
  1996年   16781篇
  1995年   12636篇
  1994年   11140篇
  1993年   11055篇
  1992年   7942篇
  1991年   7690篇
  1990年   7197篇
  1989年   7083篇
  1988年   6788篇
  1987年   5947篇
  1986年   5904篇
  1985年   6715篇
  1984年   6135篇
  1983年   5644篇
  1982年   5285篇
  1981年   5319篇
  1980年   5159篇
  1979年   4840篇
  1978年   4877篇
  1977年   5708篇
  1976年   7931篇
  1975年   4119篇
  1974年   4000篇
  1973年   4022篇
  1972年   3338篇
  1971年   2966篇
排序方式: 共有10000条查询结果,搜索用时 6 毫秒
1.
2.
ABSTRACT

An in situ hot press bonding technology has been developed to clad aluminium on magnesium. Followed by regular hot rolling, magnesium sheets, covered by ductile and corrosion-resistant aluminium without detectable oxides in the interface, are produced. The new technology requires no welding, vacuum, protective atmosphere or barrier layer, and it makes good interfacial strength and rollability. Aluminium–magnesium intermetallic phases are formed along the clad–core interface at elevated temperatures. They are not detrimental under compression but may cause clad-core delamination in tensile strain. However, the tensile failure is more dependent on the formability of magnesium core than on the strength of interface.  相似文献   
3.
A tunable, passively Q-switched thulium doped fluoride fibre (TDFF) laser using a reduced-graphene oxide-silver (rGO-Ag) thin film as a saturable absorber (SA) for S band operation is proposed and its efficacy demonstrated. Over a pump power range of 91.4?mW up to 158.6?mW, passively generated Q-switched pulses are observed with repetition rates from 20 to 34.5?kHz and pulse widths from 3.1 to 7.1?µs. The highest pulse energy observed is 101.2?nJ with a signal to noise ratio of ~42?dB. The proposed laser has a tuning range ~52?nm from 1458 to 1510?nm with a tunable bandpass filter (TBPF) introduced into the cavity.  相似文献   
4.
During approximate 773 K aging treatment of 100Mn13 steel, degenerate pearlite will occur and evolve into lamellar pearlite during growth process. The microstructures of degenerate pearlite and its evolutionary lamellar pearlite are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that after 748 K, 773 K and 798 K aging, degenerate pearlites occur at grain boundary. At growth front of degenerate pearlite forming at 773 K and 798 K, pearlite presents a morphology of short lamellae of carbide and ferrite, indicating a trend of developing into lamellar pearlite. The higher the temperature is, the more obvious the trend is, and even a conventional lamellar pearlite has developed. However, there is no morphological evolution for degenerate pearlite forming at 748 K aging. Besides, the constituents of degenerate pearlite is identified as M23C6 and ferrite, and Kurdjumov-Sachs orientation relationship exists between them, (01 )α//( 1 )M23C6, [111]α//[110]M23C6. This orientation relationship maintains in morphological evolution from degenerate pearlite to lamellar pearlite.  相似文献   
5.
Abstract

In the present work, we compare the structure and transport properties of carbon nanohorns (CNHs) synthesized by arc evaporation of graphite alone and with the addition of some portion of toluene. The materials have been investigated using transmission electron microscopy, Raman and infrared spectroscopies, thermogravimetric and BET analyses. The addition of a small amount of toluene during the evaporation of graphitic rod increases the length of CNHs, affects their hierarchical arrangement in aggregates and results in surface functionalization. All these features significantly enhance the conductivity of CNHs obtained with toluene additive in comparison with the pristine CNHs.  相似文献   
6.
The electrochemical behavior of aluminum in tannin from Acacia mearnsii bark was evaluated by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) in media under three different pH conditions: acid, alkaline, and neutral. A relationship among pH, polymerization grade (PG), zeta potential, surface, and electrochemical properties was observed in the inhibition performance of the tannin. At high pH, the oligomer structure of the tannin was affected, resulting in a low PG (4) and high zeta potential (−75 mV), and consequently, the inhibition efficiency decreased (68%) in comparison with that in acid (99%) and neutral media (96%). The values obtained indicate a physisorption mechanism for the aluminum corrosion inhibition in the studied conditions.  相似文献   
7.
8.
Pinate  S.  Eriksson  F.  Leisner  P.  Zanella  C. 《Journal of Materials Science》2021,56(33):18463-18476

This study analysed the influence of the codeposition of SiC particles with different sizes: 50 nm, 500 nm and 5 μm, and the type of bath agitation (stirring or ultrasonic) on the electrocrystallisation of nickel coatings. The composites matrix microstructure was analysed by means of SEM, EBSD and XRD, to evaluate the grain size, crystal orientation, and internal stresses and was benchmarked against pure nickel samples electrodeposited in equivalent conditions. The codeposition of nano- and microsize particles with an approximate content of 0.8 and 4 vol.%, respectively, caused only a minor grain refinement and did not vary the dominant?<?100?>?crystal orientation observed in pure Ni. The internal stress was, however, increased by particles codeposition, up to 104 MPa by nanoparticles and 57 MPa by microparticles, compared to the values observed in pure nickel (41 MPa). The higher codeposition rate (11 vol.%) obtained by the addition of submicron-size particles caused a change in the grain growth from columnar to equiaxial, resulting in deposits with a fully random crystal orientation and pronounced grain refinement. The internal stress was also increased by 800% compared to pure nickel. The ultrasound (US) agitation during the deposition caused grain refinement and a selective particle inclusion prompting a decrease in the content of the particles with the larger particles. The deposits produced under US agitation showed an increase in the internal stresses, with double values compared to stirring. The increase in the deposits microhardness, from 280 HV in pure Ni to 560 HV in Ni/SiC submicron-US, was linked to the microstructural changes and particles content.

Graphical abstract
  相似文献   
9.

In this paper, a novel compact semi-circular slot (SCS) 2 × 2 MIMO antenna is presented for 5G NR sub-6 GHz applications with high isolation. The proposed antenna consists of a semi-circular slot in ground plane, U-shaped stub, and 50-ohm microstrip feed line. The novelty of this paper are the Semi-Circular Slot acts a radiator, the port isolation  is enhanced using a simple conductor strip as a neutralization line, very compact in size, low ECC, and good impedance matching. The overall size of the proposed SCS MIMO antenna is 16 mm x 21 mm, and FR4 substrate is used with thickness of 1.6 mm. The two SCS antenna elements are separated by edge-to-edge distance of 1mm (\(=0.019\lambda _{0}\)). The proposed compact MIMO antenna design is simulated using Ansys HFSS. To validate SCS MIMO antenna, a prototype was fabricated and tested. The measured results are attained at 5.5 GHz with isolation greater than 25dB, impedance bandwidth (S11\(<-10\) dB) covers from 5.10 GHz to 5.80 GHz with return loss of ? 39.5 dB. The MIMO antenna parameters, ECC, CCL, TARC, and MEG are studied, and the values are obtained within acceptable limits. The measured and simulated antenna results are almost similar. This compact MIMO antenna is suitable for 5G communications in sub-6 GHz wifi-5 band applications.

  相似文献   
10.
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