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   495613篇
  免费   4989篇
  国内免费   1873篇
电工技术   7651篇
综合类   2934篇
化学工业   72572篇
金属工艺   23355篇
机械仪表   16076篇
建筑科学   11074篇
矿业工程   3669篇
能源动力   10374篇
轻工业   33117篇
水利工程   6514篇
石油天然气   13139篇
武器工业   54篇
无线电   53888篇
一般工业技术   102733篇
冶金工业   76246篇
原子能技术   12795篇
自动化技术   56284篇
  2021年   3419篇
  2019年   3313篇
  2018年   19917篇
  2017年   18978篇
  2016年   16169篇
  2015年   4443篇
  2014年   6892篇
  2013年   17342篇
  2012年   13525篇
  2011年   23130篇
  2010年   19030篇
  2009年   18063篇
  2008年   19601篇
  2007年   20534篇
  2006年   11315篇
  2005年   11514篇
  2004年   11211篇
  2003年   10962篇
  2002年   10040篇
  2001年   9734篇
  2000年   9356篇
  1999年   9440篇
  1998年   22186篇
  1997年   15868篇
  1996年   12159篇
  1995年   9330篇
  1994年   8195篇
  1993年   8346篇
  1992年   6266篇
  1991年   6116篇
  1990年   6027篇
  1989年   5914篇
  1988年   5657篇
  1987年   5197篇
  1986年   5213篇
  1985年   5602篇
  1984年   5226篇
  1983年   4963篇
  1982年   4582篇
  1981年   4791篇
  1980年   4701篇
  1979年   4609篇
  1978年   4803篇
  1977年   5098篇
  1976年   6634篇
  1975年   4199篇
  1974年   4216篇
  1973年   4269篇
  1972年   3716篇
  1971年   3293篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
The paper discusses optimal control of ecosystem state for observation of a generalized monotonically varying random parameter. A model is proposed for the controlled vector random process, whose components are assigned acceptable ranges and the observations are performed discretely. __________ Translated from Khimicheskoe i Neftegazovoe Mashinostroenie, No. 6, pp. 40–44, June, 2006.  相似文献   
12.
Trust is the key element of commerce, both for traditional models and — more recently — for eCommerce. Unlike its traditional counterpart, eCommerce relies on trust being created and maintained with the help of digital devices and services. Convergence brings significant changes to the way people interact through digital media by offering ubiquitous and seamless communication. This has a significant impact on the way trust is established and maintained, with consequent impact upon eCommerce. This paper identifies a number of the most important challenges to trust in eCommerce as well as trust-related aspects of key enablers of eCommerce. From this perspective, the paper studies an impact of convergence on trust within eCommerce to identify several positive relationships.  相似文献   
13.
An inventory analysis of rural pollution loads in China.   总被引:1,自引:0,他引:1  
Rural pollution has attracted increasing attention over the past decade for its important consequences on surface and groundwater quality. This is particularly so in China due to the wide and effective control of industrial and municipal wastewater discharges over the past decade. Based on the discussion of rural activities in China, this paper develops a new method of inventory analysis through the definition of elementary unit (EU). This inventory analysis approach is then applied to estimate the total rural pollution loads of COD, TN and TP, as well as their spatial distributions in China. Further analysis is also conducted to investigate the correlations between rural pollution loads and agricultural production outputs, as well as population. It is found that China's agriculture is developing largely at the cost of environment quality, and a high population, as well as the increasing demand for agricultural products, is one of the dominant driving forces. The constructed EKC, which describes the relationship between rural pollution loads and agricultural income, also reveals that current agricultural development in China is mostly far from de-locked from the environmental problem.  相似文献   
14.
It was found that vanadyl porphyrin complexes synthesized from petroleum metal porphyrin concentrates stimulated epoxidation during the olefin oxygenation process. The yields of obtained oxiranes turned out to be 38–75%, depending on the olefin structure. An epoxidation mechanism that suggests the formation of a protonated dioxygen adduct as an intermediate during oxygenation of olefins in the presence of vanadyl porphyrin complexes was proposed. An analogy is drawn between the epoxide formation reaction upon the catalytic oxygenation of olefins and the Prilezhaev reaction.  相似文献   
15.
The Magnitogorsk Metallurgical Combine has conducted a study of the effect of technological factors on the hydrogen content of chromium-nickel-molybdenum steel after vacuum degassing. It was established that the most important factor is the hydrogen content of the steel before the degassing operation. The study also determined the effects of the circulation coefficient, the duration of the degassing operation, and the vacuum used in the treatment. __________ Translated from Metallurg, No. 7, pp. 68–69, July, 2006.  相似文献   
16.
By measuring the total energy flow from an optical device, we can develop new design strategies for thermal stabilization. Here we present a comprehensive model for heat exchange between a semiconductor laser diode and its environment that includes the mechanisms of conduction, convection, and radiation. We perform quantitative measurements of these processes for several devices, deriving parameters such as a laser's heat transfer coefficient, and then demonstrate the feasibility of thermal probing for the nondestructive wafer-scale characterization of optical devices.  相似文献   
17.
Deposition of Ag films by direct liquid injection-metal organic chemical vapor deposition (DLI-MOCVD) was chosen because this preparation method allows precise control of precursor flow and prevents early decomposition of the precursor as compared to the bubbler-delivery. Silver(I)-2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedionato-triethylphosphine [Ag(fod)(PEt3)] as the precursor for Ag CVD was studied, which is liquid at 30 °C. Ag films were grown on different substrates of SiO2/Si and TiN/Si. Argon and nitrogen/hydrogen carrier gas was used in a cold wall reactor at a pressure of 50–500 Pa with deposition temperature ranging between 220 °C and 350 °C. Ag films deposited on a TiN/Si diffusion barrier layer have favorable properties over films deposited on SiO2/Si substrate. At lower temperature (220 °C), film growth is essentially reaction-limited on SiO2 substrate. Significant dependence of the surface morphology on the deposition conditions exists in our experiments. According to XPS analysis pure Ag films are deposited by DLI-MOCVD at 250 °C by using argon as carrier gas.  相似文献   
18.
The effect of crystal orientation on the photogeneration of free charge carriers was studied for C60 single crystals in a weak magnetic field. The photoconductivity sharply depends on the orientation of magnetic field with respect to the crystallographic directions, showing a 5–8% increase for seven axes of the C60 crystal.  相似文献   
19.
We propose a standardization procedure that provides a convenient, quantitative and reproducible laboratory-based method for measuring the state of polarization (SOP) fluctuations produced by polarization varying devices. This method is based on the SOP distributions generated by commercial polarization scramblers. We show that these devices generate distributions of the maximum change of the SOP (in a given sample time) that follow Rayleigh statistics, which scale linearly with scrambling frequency and the sample time. We use this procedure to measure the SOP fluctuations in a short length of coiled fiber subject to mechanical perturbations.  相似文献   
20.
Ferrite particles coated with biocompatible phases can be used for hyperthermia treatment of cancer. We have synthesized substituted calcium hexaferrite, which is not stable on its own but is stabilized with small substitution of La. Hexaferrite of chemical composition (CaO)0.75(La203)0.20(Fe2O3)6 was prepared using citrate gel method. Hydroxyapatite was prepared by precipitating it from aqueous solution of Ca(NO3)2 and (NH4)2HPO4 maintaining pH above 11. Four different methods were used for coating of hydroxyapatite on ferrite particles. SEM with EDX and X-ray diffraction analysis shows clear evidence of coating of hydroxyapatite on ferrite particles. These coated ferrite particles exhibited coercive field up to 2 kOe, which could be made useful for hysteresis heating in hyperthermia. Studies by culturing BHK-21 cells and WBC over the samples show evidence of biocompatibility. SEM micrographs and cell counts give clear indication of cell growth on the surface of the sample. Finally coated ferrite particle was implanted in Kasaulli mouse to test its biocompatibility. The magnetic properties and biocompatibility studies show that these hydroxyapatite coated ferrites could be useful for hyperthermia.  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