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   1874篇
  免费   40篇
  国内免费   6篇
电工技术   12篇
综合类   1篇
化学工业   204篇
金属工艺   22篇
机械仪表   42篇
建筑科学   87篇
矿业工程   6篇
能源动力   55篇
轻工业   188篇
水利工程   14篇
石油天然气   4篇
无线电   198篇
一般工业技术   263篇
冶金工业   625篇
原子能技术   26篇
自动化技术   173篇
  2023年   15篇
  2021年   17篇
  2020年   25篇
  2019年   28篇
  2018年   21篇
  2017年   22篇
  2016年   29篇
  2015年   14篇
  2014年   32篇
  2013年   73篇
  2012年   53篇
  2011年   78篇
  2010年   54篇
  2009年   36篇
  2008年   55篇
  2007年   68篇
  2006年   62篇
  2005年   51篇
  2004年   42篇
  2003年   46篇
  2002年   47篇
  2001年   44篇
  2000年   32篇
  1999年   45篇
  1998年   153篇
  1997年   116篇
  1996年   70篇
  1995年   42篇
  1994年   43篇
  1993年   30篇
  1992年   22篇
  1991年   20篇
  1990年   20篇
  1989年   17篇
  1988年   19篇
  1987年   15篇
  1986年   22篇
  1985年   24篇
  1984年   27篇
  1983年   13篇
  1982年   19篇
  1981年   18篇
  1980年   24篇
  1979年   13篇
  1978年   13篇
  1977年   33篇
  1976年   42篇
  1975年   14篇
  1974年   18篇
  1972年   11篇
排序方式: 共有1920条查询结果,搜索用时 15 毫秒
1.
Energy retrofits can reduce air exchange, raising the concern of whether indoor radon and moisture levels could increase. This pre/post-intervention study explored whether simple radon interventions implemented in conjunction with energy retrofits can prevent increases in radon and moisture levels. Treatment homes (n = 98) were matched with control (no energy retrofits or radon intervention) homes (n = 12). Control homes were matched by geographic location and foundation type. t-tests were used to determine whether post-energy retrofit radon and moisture level changes in treatment homes significantly differed from those in control homes. The radon interventions succeeded in preventing statistically significant increases in first floor radon using arithmetic (p = 0.749) and geometric means (p = 0.120). In basements, arithmetic (p = 0.060) and geometric (p = 0.092) mean radon levels statistically significantly increased, consistent with previous studies which found that basement radon levels may increase even if first floor levels remain unchanged. Changes in infiltration were related to changes in radon (p = 0.057 in basements; p = 0.066 on first floors). Only 58% of the change in infiltration was due to air sealing, with the rest due to weather changes. There was no statistically significant association between air sealing itself and radon levels on the first floor (p = 0.664). Moisture levels also did not significantly increase.  相似文献   
2.
One of the conventional ways to improve the mechanical behavior of soils is to mix them with cementing agents such as cement, lime and fly ash. Recently, introduction to alternative materials or sub-products that can be adopted to improve the soil strength is of paramount importance. Therefore, the present study aims to investigate the effects of porosity (η), dry unit weight (γd) of molding, cement content (C) and porosity/volumetric cement content ratio (η/Civ) or void/cement ratio on the unconfined compressive strength (qu or UCS) of silty soil–roof tile waste (RT) mixtures. Soil samples are molded into four different dry unit weights (i.e. 13 kN/m3, 13.67 kN/m3, 14.33 kN/m3 and 15 kN/m3) using 3%, 6% and 9% cement and 5%, 15% and 30% RT. The results show that with the addition of cement, the strength of the RT–soil mixtures increases in a linear manner. On the other hand, the addition of RT decreases qu of the samples at a constant percentage of cement, and the decrease in porosity can increase qu. A dosage equation is derived from the experimental data using the porosity/volumetric cement content ratio (η/Civ) where the control variables are the moisture content, crushed tile content, cement content and porosity.  相似文献   
3.
4.
Diffuser plates in exhaust system manifolds are designed to provide uniform flow pattern within the manifold for maximum utilization of the catalytic converter substrate during high-temperature applications. In this paper, failure analysis of a diffuser which survived only 20% duration of a manifold crack test and various design optimization studies of the diffuser plate using computer-aided engineering (CAE) analyses are presented. During the manifold crack test, the failure occurred at the inner and outer periphery of the diffuser. Metallurgical failure analysis coupled with CAE thermal fatigue analysis of the component concluded that thermal fatigue was the root cause of the failure. The new recommended robust design showed considerable improvement in the thermal durability of the diffuser plate assembly.  相似文献   
5.
6.
Models to estimate economic impacts of disasters have recently been augmented to include resilience. However, most research has incorporated only a limited set of resilience tactics and has not estimated their individual loss reduction effect. We present a comprehensive framework for estimating the relative effects of a broad set of post-disaster resilience tactics. Our methodological innovation is illustrated by adapting the TERM multi-regional CGE model for a seaport disruption, distinguishing inherent resilience working through the price system from adaptive resilience and other inherent tactics to cope with input shortages. We also overcome a path-dependency problem in the modelling process.  相似文献   
7.
Biodiesel has provided an eco-friendly solution to fuel crisis, as it is renewable, biodegradable and a non-toxic fuel that can be easily produced through enzymatic transesterification of vegetable oils and animal fats. Enzymatic production of biodiesel has many advantages over the conventional methods as high yields can be obtained at low reaction temperatures with easy recovery of glycerol. Microbial lipases are powerful biocatalysts for industrial applications including biodiesel production at lower costs due to its potential in hydrolyzing waste industrial materials. Among them, lipases from yeasts, Candida antarctica, Candida rugosa, Cryptococcus sp., Trichosporon asahii and Yarrowia lipolytica are known to catalyze such reactions. Moreover, stepwise addition of methanol in a three step, two step and single step reactions have been developed using yeast lipases to minimize the inhibitory effects of methanol. The latest trend in biodiesel production is the use of whole-cell as biocatalysts, since the process requires no downstream processing of the enzyme. Synthesis of value added products from the byproduct glycerol further reduces the production cost of biodiesel. This review aims at compiling the information on various yeast lipase catalyzed transesterification reactions for greener production of biodiesel.  相似文献   
8.
The economic viability of new energy technologies is held as a central tenet to their future deployment; conventional wisdom posits economically rational decision-makers will readily invest in proven low-risk and affordable technologies. But what happens when this is not true. This paper examines the non-financial barriers facing economically viable Combined Heat and Power (CHP) projects. CHP is a mature and lower carbon technology that efficiently uses waste heat from thermal electricity generation; CHP can also provide flexibility services to help integrate variable renewable resources. CHP is low risk and many industrialized countries, particularly those in colder climates in Northern Europe and Russia, generate as much as 50% of their electricity and heat needs from CHP, but United States deployment remains low and investment hurdle rates high. While lower U.S. energy costs make some projects un-economic, many economically-viable CHP projects are stalled or killed by non-financial barriers. To better understand why financially viable CHP projects are not getting built, developers, owners and operators, regulators, and other stakeholders of this technology were interviewed and three major barriers emerged a) the business model of the electrical utility b) negative subjective impressions and c) challenges in allocating the risks and benefits.  相似文献   
9.
10.
Lanthanide (Ln)-dependent methanol dehydrogenases (MDHs) have recently been shown to be widespread in methylotrophic bacteria. Along with the core MDH protein, XoxF, these systems contain two other proteins, XoxG (a c-type cytochrome) and XoxJ (a periplasmic binding protein of unknown function), about which little is known. In this work, we have biochemically and structurally characterized these proteins from the methyltroph Methylobacterium extorquens AM1. In contrast to results obtained in an artificial assay system, assays of XoxFs metallated with LaIII, CeIII, and NdIII using their physiological electron acceptor, XoxG, display Ln-independent activities, but the Km for XoxG markedly increases from La to Nd. This result suggests that XoxG′s redox properties are tuned specifically for lighter Lns in XoxF, an interpretation supported by the unusually low reduction potential of XoxG (+172 mV). The X-ray crystal structure of XoxG provides a structural basis for this reduction potential and insight into the XoxG–XoxF interaction. Finally, the X-ray crystal structure of XoxJ reveals a large hydrophobic cleft and suggests a role in the activation of XoxF. These studies enrich our understanding of the underlying chemical principles that enable the activity of XoxF with multiple lanthanides in vitro and in vivo.  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