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   3520篇
  免费   149篇
电工技术   26篇
综合类   1篇
化学工业   677篇
金属工艺   50篇
机械仪表   33篇
建筑科学   172篇
矿业工程   4篇
能源动力   60篇
轻工业   510篇
水利工程   42篇
石油天然气   12篇
无线电   150篇
一般工业技术   441篇
冶金工业   1152篇
原子能技术   8篇
自动化技术   331篇
  2023年   23篇
  2022年   41篇
  2021年   113篇
  2020年   60篇
  2019年   69篇
  2018年   90篇
  2017年   75篇
  2016年   99篇
  2015年   55篇
  2014年   97篇
  2013年   208篇
  2012年   165篇
  2011年   231篇
  2010年   173篇
  2009年   177篇
  2008年   184篇
  2007年   181篇
  2006年   149篇
  2005年   146篇
  2004年   95篇
  2003年   93篇
  2002年   104篇
  2001年   68篇
  2000年   68篇
  1999年   66篇
  1998年   54篇
  1997年   51篇
  1996年   68篇
  1995年   47篇
  1994年   54篇
  1993年   49篇
  1992年   43篇
  1991年   17篇
  1990年   37篇
  1989年   38篇
  1988年   36篇
  1987年   28篇
  1986年   30篇
  1985年   28篇
  1984年   30篇
  1983年   22篇
  1982年   22篇
  1981年   23篇
  1980年   14篇
  1979年   17篇
  1978年   17篇
  1977年   10篇
  1976年   11篇
  1975年   14篇
  1974年   14篇
排序方式: 共有3669条查询结果,搜索用时 46 毫秒
991.
This research investigates the kinetic parameters associated with the ozonation of 2,3-, 2,4-, and 2,5-dichlorophenols (DCPs) in aqueous solutions at 25°C. The overall ozonation kinetics of each DCP was found to be first order with respect to each reactant concentration (i.e., DCP and ozone). Among the three isomers studied, the ozonation of 2,5-DCP was the fastest with the overall rate constant varying from 68,500 to 7,750,000 M?1 s?1 within a pH range of 2.1 to 5.0. The overall rate constant increased with pH because of the increase in the degree of dissociation of the DCP molecules to dichlorophenoxide ions. The electrophilic addition of ozone at the ortho and para positions of the molecular and ionic forms of the DCPs was postulated to be the likely mechanism for the degradation of the DCPs to chlorobenzenediols, monochlorophenols, chlorocyclohexadienediones, ethylacetate, heptanone, carboxylic acids, and other by-products that were detected using gas chromatograph∕mass spectrometry analysis.  相似文献   
992.
Recent work suggests that closely related cerambycid species often share pheromone components, or even produce pheromone blends of identical composition. However, little is known of the pheromones of species in the subfamily Prioninae. During field bioassays in California, males of three species in the prionine genus Tragosoma were attracted to 2,3-hexanediols, common components of male-produced aggregation pheromones of beetles in the subfamily Cerambycinae. We report here that the female-produced sex pheromone of Tragosoma depsarium “sp. nov. Laplante” is (2R,3R)-2,3-hexanediol, and provide evidence from field bioassays and electroantennography that the female-produced pheromone of both Tragosoma pilosicorne Casey and T. depsariumharrisi” LeConte may be (2S,3R)-2,3-hexanediol. Sexual dimorphism in the sculpting of the prothorax suggests that the pheromone glands are located in the prothorax of females. This is the second sex attractant pheromone structure identified from the subfamily Prioninae, and our results provide further evidence of pheromonal parsimony within the Cerambycidae, in this case extending across both subfamily and gender lines.  相似文献   
993.
This paper introduces a new variation on the capacitive feed probe for patch antennas on thick substrates. It consists of a small circular probe-fed capacitor patch that is situated next to the resonant patch. This configuration can bring about significant savings in terms of manufacturing cost, but also lends itself to a very efficient full-wave analysis. As such, the main focus of this paper is a spectral-domain moment-method formulation, which was specifically developed for the analysis of large, but finite, arrays of these antenna elements. Entire-domain and subdomain basis functions are combined in an efficient way to minimize the computational requirements, most notably computer memory. It is shown that, for general antenna array configurations, memory savings of more than 1000 times can be achieved when compared with typical commercial software packages where only subdomain basis functions are used. A number of numerical and experimental results are also included in order to verify the spectral-domain moment-method formulation and to illustrate various applications of the new antenna element.  相似文献   
994.
A new class of chiral polyurethanes containing amido linkages in the polymer backbone have been synthesized by reacting toluene diisocyanate with isosorbide (IS) chiral moiety and the chromophores [N,N′-ethane-1,2-diyl bis(6-hydroxy hexanamide), N,N′-butane-1,4-diyl bis(6-hydroxy hexanamide) and N,N′-hexane-1,6-diyl bis(6-hydroxy hexanamide)]. The corresponding chromophores were obtained by the aminolysis of ε-caprolactone by using the diamines, diaminoethane, diaminobutane and diaminohexane, respectively. All the polymers were synthesized according to the symmetry conditions so as to obtain the non-centrosymmetric environment. A series of polyurethanes were synthesized by varying the chiral–chromophore composition. The polyurethanes developed were characterized by optical and thermal methods.  相似文献   
995.
Examines responses from 349 psychologists of the American Psychological Association (APA) Division of Psychotherapy who completed a questionnaire regarding the prevalence of treating fellow psychotherapists, the type of psychotherapist-patients they treated, and the stressors and satisfactions of conducting such work. Three fourths of respondents related that they had treated mental health professionals and professionals-in-training over the past 3 years, and that this population constituted 3% to 796 of their caseloads. Nearly all of the therapist-patients were self-referred, and 85% received individual therapy. The psychotherapist-patients tended to be psychologists (37%) or social workers (29%) of diverse theoretical traditions, with the exception of biologically oriented clinicians. Satisfactions clustered around pleasures inherent in doing meaningful work, acknowledgment by peers, and the opportunity to help therapist-patients enhance their own clinical effectiveness. Predominant stressors concerned evaluation anxieties in treating colleagues and in the related belief that colleagues are more challenging and resistant to change. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
996.
In order to sustain the fast‐expanding beer industry, companies need to attract new female consumers. The main objective of this study was to identify the extrinsic and intrinsic attributes that drive female consumers' purchase of beer. A literature review and focus group (n = 6) were conducted, and six attributes were identified as purchase drivers of beer. These attributes included flavour, appearance (colour), packaging, brand, production methods and beer style. These attributes were used to design a choice‐based conjoint analysis survey. The survey was administered to 277 females (aged 35.09 ± 15.2) residing in Nova Scotia (Canada). The results indicated that sweetness has a positive effect on liking. Bitterness has a strong negative effect on liking of beer. Black colour, stouts and macro‐brewed beers also had negative effects on liking of beer products. The consumer clusters showed that generally all of the consumer groups liked sweet beers with low bitterness.  相似文献   
997.
The paper analyzes the Energy Policy Act's goals and proposed action in Title VIII, the section dedicated to hydrogen. It then assesses whether or not the resources dedicated to this cause are sufficient to overcome the obstacles facing the commercial use of hydrogen and fuel cells.  相似文献   
998.
The experimental drying behaviour of clingstone peach halves dehydrated without sulphites was investigated. Fruit treatments such as peeling and peeling followed by blanching led to greatly increased drying rates in comparison to unpeeled halves during isothermal dehydration. Superficial product temperature during dehydration was also affected by fruit treatment. Drying rates during isothermal dehydration were found to increase as linear air velocity increased from 2 to 4 m.s-1. Fruit size had a pronounced effect on drying rates, with halves obtained from the smallest fruit (54 mm.), showing the highest rates. Experimental drying studies were  相似文献   
999.
Neurophysiological studies show that the firing of place and head-direction (HD) cells in rats can become anchored to features of the perceptible environment, suggesting that those features partially specify the rat's position and heading. In contrast, behavioral studies suggest that disoriented rats and human children rely exclusively on the shape of their surroundings, ignoring much of the information to which place and HD cells respond. This difference is explored in the current study by investigating young children's ability to locate objects in a square chamber after disorientation. Children 18–24 months old used a distinctive geometric cue but not a distinctively colored wall to locate the object, even after they were familiarized with the colored wall. Results suggest that the spatial representations underlying reorientation and object localization are common to humans and other mammals. Together with the neurophysiological findings, these experiments raise questions for the hypothesis that hippocampal place and HD cells serve as a general orientation device for target localization. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
1000.
This installment of Building Security In presents metrics that can help quantify the impact that process changes in one life-cycle phase have on other phases.  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