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   1819篇
  免费   55篇
  国内免费   5篇
电工技术   31篇
综合类   2篇
化学工业   280篇
金属工艺   25篇
机械仪表   62篇
建筑科学   92篇
矿业工程   6篇
能源动力   77篇
轻工业   117篇
水利工程   25篇
石油天然气   13篇
无线电   145篇
一般工业技术   314篇
冶金工业   394篇
原子能技术   4篇
自动化技术   292篇
  2023年   10篇
  2022年   26篇
  2021年   23篇
  2020年   17篇
  2019年   32篇
  2018年   29篇
  2017年   29篇
  2016年   40篇
  2015年   27篇
  2014年   43篇
  2013年   135篇
  2012年   75篇
  2011年   100篇
  2010年   100篇
  2009年   80篇
  2008年   85篇
  2007年   91篇
  2006年   93篇
  2005年   86篇
  2004年   52篇
  2003年   51篇
  2002年   41篇
  2001年   26篇
  2000年   22篇
  1999年   30篇
  1998年   41篇
  1997年   40篇
  1996年   27篇
  1995年   25篇
  1994年   20篇
  1993年   26篇
  1992年   25篇
  1991年   9篇
  1990年   21篇
  1989年   20篇
  1988年   20篇
  1987年   15篇
  1986年   17篇
  1985年   23篇
  1984年   25篇
  1983年   22篇
  1982年   9篇
  1981年   15篇
  1980年   10篇
  1979年   12篇
  1977年   11篇
  1976年   11篇
  1975年   16篇
  1970年   8篇
  1969年   8篇
排序方式: 共有1879条查询结果,搜索用时 15 毫秒
81.
Obesity, and resultant health hazards which include diabetes, cardiovascular disease and metabolic syndrome, are worldwide medical problems. Control of diet and exercise are cornerstones of the management of excess weight. Foods with a low glycemic index may reduce the risk of diabetes and heart disease as well as their complications. As an alternative to a low glycemic index diet, there is a growing body of research into products that slow the absorption of carbohydrates through the inhibition of enzymes responsible for their digestion. These products include alpha-amylase and glucosidase inhibitors. The common white bean (Phaseolus vulgaris) produces an alpha-amylase inhibitor, which has been characterized and tested in numerous clinical studies. A specific and proprietary product named Phase 2® Carb Controller (Pharmachem Laboratories, Kearny, NJ) has demonstrated the ability to cause weight loss with doses of 500 to 3000 mg per day, in either a single dose or in divided doses. Clinical studies also show that Phase 2 has the ability to reduce the post-prandial spike in blood glucose levels. Experiments conducted incorporating Phase 2 into food and beverage products have found that it can be integrated into various products without losing activity or altering the appearance, texture or taste of the food. There have been no serious side effects reported following consumption of Phase 2. Gastro-intestinal side effects are rare and diminish upon extended use of the product. In summary, Phase 2 has the potential to induce weight loss and reduce spikes in blood sugar caused by carbohydrates through its alpha-amylase inhibiting activity.  相似文献   
82.
To date, the price of electricity to commercial or business energy consumers has generally increased at greater rates in the areas of Texas where retail competition has been introduced than in areas that do not enjoy competition. Trends in commercial competitive prices have largely mirrored trends in residential prices. Market restructuring has tended to increase the sensitivity of retail electricity prices to changes in the price of natural gas, the marginal fuel used for generation in Texas. Consequently, the rapid increases in the commodity price of natural gas following restructuring led to increases in competitive electric rates which exceeded the increases in areas not exposed to restructuring, where the fuel component of electric rates tend to reflect a weighted average of the utilities’ fuel costs. There is some evidence that pricing behavior by competitive retailers changed when the retailers affiliated with the incumbent utilities were permitted some pricing flexibility, resulting in a reduction in prices.  相似文献   
83.
84.
Well‐defined and uniform pore structure catalysts were used to study the intraparticle diffusion of fractionated Saudi vacuum residue under hydrodesulfurization (HDS) reaction conditions. HDS rates of residue oil cuts with different molecular weights are determined as functions of pore size, temperature, and pressure in a trickle‐bed reactor. Credible intrinsic and bulk diffusivities of organosulfur compounds in residue oil were obtained for the first time, from the apparent and intrinsic reaction kinetic constants. Intrinsic diffusivities ranged from 2 × 10?7 to 8 × 10?7 cm2/s for the residual oil molecules; diffusivity decreases with increasing molecular weight of the residual oil. The intrinsic diffusivity for molecular weights ~1000 Daltons increases with pore size for pores <70 nm, but is nearly independent of pore size for pores >70 nm. The diffusivity dependences on pore size and molecular weight suggest that the onset of restricted diffusion occurs for ratios of molecular diameter to pore diameter of ~0.04. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3267–3275, 2014  相似文献   
85.
The currently prevalent machine performance degradation assessment techniques involve estimating a machine’s current condition based upon the recognition of indications of failure features,which entail complete data collected in different conditions.However,failure data are always hard to acquire,thus making those techniques hard to be applied.In this paper,a novel method which does not need failure history data is introduced.Wavelet packet decomposition(WPD) is used to extract features from raw signals,principal component analysis(PCA) is utilized to reduce feature dimensions,and Gaussian mixture model(GMM) is then applied to approximate the feature space distributions.Single-channel confidence value(SCV) is calculated by the overlap between GMM of the monitoring condition and that of the normal condition,which can indicate the performance of single-channel.Furthermore,multi-channel confidence value(MCV),which can be deemed as the overall performance index of multi-channel,is calculated via logistic regression(LR) and that the task of decision-level sensor fusion is also completed.Both SCV and MCV can serve as the basis on which proactive maintenance measures can be taken,thus preventing machine breakdown.The method has been adopted to assess the performance of the turbine of a centrifugal compressor in a factory of Petro-China,and the result shows that it can effectively complete this task.The proposed method has engineering significance for machine performance degradation assessment.  相似文献   
86.
The contribution of fecal pollution from dogs in urbanized areas can be significant and is an often underestimated problem. Microbial source tracking methods (MST) utilizing quantitative PCR of dog-associated gene sequences encoding 16S rRNA of Bacteroidales are a useful tool to estimate these contributions. However, data about the performance of available assays are scarce. The results of a multi-laboratory study testing two assays for the determination of dog-associated Bacteroidales (DogBact and BacCan-UCD) on 64 single and mixed fecal source samples created from pooled fecal samples collected in California are presented here. Standardization of qPCR data treatment lowered inter-laboratory variability of sensitivity and specificity results. Both assays exhibited 100% sensitivity. Normalization methods are presented that eliminated random and confirmed non-target responses. The combination of standardized qPCR data treatment, use of normalization via a non-target specific Bacteroidales assay (GenBac3), and application of threshold criteria improved the calculated specificity significantly for both assays. Such measures would reasonably improve MST data interpretation not only for canine-associated assays, but for all qPCR assays used in identifying and monitoring fecal pollution in the environment.  相似文献   
87.
有限区间内设备顺序预防性维护策略研究   总被引:10,自引:0,他引:10  
为了克服等周期维护模型忽略设备可靠性要求,以及长期运行期望维护周期模型忽略设备特定阶段维护需求的缺点,通过引入基于役龄递减因子和故障率递增因子的混合式故障率演化规则,建立了有限区间内基于设备可靠性的顺序预防性维护优化模型,并以威布尔分布为例,利用仿真方法对其进行了优化。仿真结果显示,该模型可使设备的维护周期呈递减之势,反映了设备在某一特定阶段的可靠性需求。  相似文献   
88.
89.
A flexible endoscope could reach the potential surgical site via a single small incision on the patient or even through natural orifices, making it a very promising platform for surgical procedures. However, endoscopic surgery has strict spatial constraints on both tool-channel size and surgical site volume. It is therefore very challenging to deploy and control dexterous robotic instruments to conduct surgical procedures endoscopically. Pioneering endoscopic surgical robots have already been introduced, but the performance is limited by the flexible neck of the robot that passes through the endoscope tool channel. In this article we present a series of new developments to improve the performance of the robot: a force transmission model to address flexibility, elongation study for precise position control, and tissue property modeling for haptic feedback. Validation experiment results are presented for each sector. An integrated control architecture of the robot system is given in the end.  相似文献   
90.
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