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   243610篇
  免费   2948篇
  国内免费   1243篇
电工技术   4832篇
综合类   265篇
化学工业   35446篇
金属工艺   10861篇
机械仪表   7068篇
建筑科学   6247篇
矿业工程   481篇
能源动力   6275篇
轻工业   24197篇
水利工程   1836篇
石油天然气   1336篇
武器工业   3篇
无线电   31039篇
一般工业技术   44886篇
冶金工业   50111篇
原子能技术   3860篇
自动化技术   19058篇
  2021年   1566篇
  2019年   1424篇
  2018年   2321篇
  2017年   2223篇
  2016年   2444篇
  2015年   1907篇
  2014年   3159篇
  2013年   10816篇
  2012年   5449篇
  2011年   7789篇
  2010年   6286篇
  2009年   6967篇
  2008年   7560篇
  2007年   7664篇
  2006年   7037篇
  2005年   6522篇
  2004年   6247篇
  2003年   6268篇
  2002年   6243篇
  2001年   6371篇
  2000年   5804篇
  1999年   6421篇
  1998年   16560篇
  1997年   11601篇
  1996年   8952篇
  1995年   6692篇
  1994年   5911篇
  1993年   5729篇
  1992年   4053篇
  1991年   3995篇
  1990年   3582篇
  1989年   3504篇
  1988年   3443篇
  1987年   2908篇
  1986年   2843篇
  1985年   3414篇
  1984年   3051篇
  1983年   2744篇
  1982年   2547篇
  1981年   2572篇
  1980年   2430篇
  1979年   2272篇
  1978年   2235篇
  1977年   2809篇
  1976年   4171篇
  1975年   1900篇
  1974年   1799篇
  1973年   1784篇
  1972年   1465篇
  1971年   1322篇
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
81.
82.
83.
Data clustering consists of a group of procedures used to collect similar entries or data points within a set into clusters. No existing clustering technique considers entries sequentially in time. In some cases, it is desirable to generate clusters that represent a segment of a time-ordered data set. For these purposes, an order-specific clustering algorithm is proposed. The proposed algorithm employs representative load curves to describe the clusters it generates. The capabilities of the order-specific clustering algorithm are demonstrated on a case study using electricity demand data for the province of Ontario, Canada. Two different applications of the clustering algorithm on this data set are given to demonstrate the effect of error threshold values on the formation of clusters. An analysis of the error for each of these clustering applications is presented.  相似文献   
84.
Hot filament and microwave plasma CVD micro- nanocrystalline diamond films are analysed by visible and ultra-violet excitation source Raman spectroscopy. The sample grain size varies from 20 nm to 2 μm. The hydrogen concentration in samples is measured by SIMS and compared to the grain size, and to the ratio of sp2 carbon bonds determined by Raman spectroscopy from the 1332 cm 1 diamond peak and the sp2 1550 cm 1 G band. Hydrogen concentration appears to be proportional to the sp2 bonds ratio. The 3000 cm 1 CHx stretching mode band intensity observed on the Raman spectra is decreasing with the G band intensity. Thermal annealing modifies the sp2 phase structure and concentration, as hydrogen outdiffuses.  相似文献   
85.
A novel integrated vacuum field emission (VFE) differential amplifier (diff-amp) utilizing carbon nanotube (CNT) emitters has been developed. A dual-mask microfabrication process was employed to achieve a VFE diff-amp by integrating identical CNT VFE transistors with built-in split gates and integrated anodes. The identical pair of triode amplifiers was well-matched in their device characteristics. The measured ac small-signal characteristics of the diff-amp showed a common-mode-rejection ratio (CMRR) of ~ 320 (~ 50 dB). The proposed analytical model of the CMRR was verified to be in good agreement with the experimental data. The successful implementation of the CNT diff-amp demonstrates a new way to achieve temperature and radiation tolerant VFE integrated microelectronics.  相似文献   
86.
87.
This paper presents results of a research project, in which a process-dependent real-time control (RTC) strategy for a sequencing batch reactor plant was realised in full-scale. The cycle controller is based on NH4 analysers, NO3 probes, TSS probes and sludge level probes. With this new RTC strategy it was possible to increase the treatment capacity by 50%. By implementation of the new controller the TN, TP and NH4-N treatment efficiency could be improved significantly, too. The treatment efficiency concerning COD is comparable.  相似文献   
88.
We present closed-form expressions for the average bit error probability (ABEP) of BPSK, QPSK and M-QAM of an amplify-and-forward average power scaling dual-hop relay transmission, over non-identical Nakagami-m fading channels, with integer values of m. Additionally, we evaluate in closed-form the ABEP under sufficiently large signal-to-noise ratio for the source-relay link, valid for arbitrary rn. Numerical and simulation results show the validity of the proposed mathematical analysis and point out the effect of the two hops unbalanced fading conditions on the error performance.  相似文献   
89.
The castings produced by the evaporative pattern casting (EPC) process have blow holes. The blow holes in EPC castings are because of the non-escape of the gas produced as a result of burning of polystyrene pattern in the sand mold. To overcome the problem of blow holes, the EPC process is combined with the vacuum (V)- process. The vacuum applied to EPC mold draws the decomposed gases and improves the casting quality produced by the EPC process. The developed hybrid process has been termed as the vacuum assisted evaporative pattern casting (VAEPC) process. The objective of this paper to investigates the effect of process parameters, i.e, degree of vacuum, pouring temperature, grainfineness number, amplitude of vibration and time of vibration on the impact strength of Al-7% Si alloy castings in VAEPC process. In order to evaluate the effect of selected process parameters, the response surface methodology (RSM) is used to formulate a mathematical model which correlates the independent process parameters with the desired impact strength. The central composite rotatable design has been used to conduct the experiments. The results indicate that the impact strength decreases with increases in the grainfineness number and pouring temperature. Whereas, it has an inverse relationship with amplitude of vibration, time of vibration and degree of vacuum. The best value of impact strength (2.34 N/mm2) has been obtained at 400 mm Hg degree of vacuum imposed, 650°C as pouring temperature, 60 as sand grainfineness number, 460 μm as amplitude of vibration, and 70 s as time of vibration.  相似文献   
90.
By applying a combination of characterisation tools, changes in structural and superconducting properties with nominal Mg non‐stoichiometry in MgxB2 are found. The non‐stoichiometry produces enhanced in‐field critical current densities (Jc's) and upper critical field / irreversibility field (Hc2/Hirr(T)) values. Upper critical fields of ~ 21 T (4.2 K) were obtained in nominal Mg‐deficient samples compared to ~ 17 T (4.2 K) for near‐stoichiometric samples.  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