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   4403篇
  免费   161篇
  国内免费   5篇
电工技术   21篇
综合类   8篇
化学工业   1189篇
金属工艺   44篇
机械仪表   47篇
建筑科学   186篇
矿业工程   13篇
能源动力   67篇
轻工业   727篇
水利工程   27篇
石油天然气   9篇
无线电   122篇
一般工业技术   486篇
冶金工业   1084篇
原子能技术   24篇
自动化技术   515篇
  2023年   27篇
  2022年   189篇
  2021年   204篇
  2020年   82篇
  2019年   98篇
  2018年   108篇
  2017年   84篇
  2016年   100篇
  2015年   94篇
  2014年   134篇
  2013年   259篇
  2012年   173篇
  2011年   225篇
  2010年   190篇
  2009年   155篇
  2008年   212篇
  2007年   192篇
  2006年   148篇
  2005年   134篇
  2004年   119篇
  2003年   96篇
  2002年   120篇
  2001年   72篇
  2000年   66篇
  1999年   85篇
  1998年   79篇
  1997年   52篇
  1996年   73篇
  1995年   50篇
  1994年   55篇
  1993年   54篇
  1992年   61篇
  1990年   49篇
  1989年   69篇
  1988年   45篇
  1987年   51篇
  1986年   55篇
  1985年   53篇
  1984年   58篇
  1983年   45篇
  1982年   30篇
  1981年   47篇
  1980年   26篇
  1979年   23篇
  1978年   22篇
  1977年   22篇
  1976年   30篇
  1975年   26篇
  1973年   16篇
  1970年   17篇
排序方式: 共有4569条查询结果,搜索用时 13 毫秒
21.
Colonies ofNasutitermes costalis (Holmgren) andN. ephratae (Holmgren) were collected from five locations in Trinidad. Cuticular hydrocarbons were characterized by gas chromatography-electron impact mass spectrometry and quantified by capillary gas chromatography. Sixteen major components were identified; all but one component (12, 16-dimethyltriacontane) were common to both species. The methyl-branched hydrocarbons were predominant inN. costalis, while the majority of the hydrocarbon components inN. ephratae weren-alkanes. One hydrocarbon (11,15-dimethylheptacosane) was found in abundance in samples ofN. ephratae from Trinidad but was not previously reported from collections of this species in Panama. In addition to the morphology of the soldiers and alates and the architecture of the arboreal nests,N. costalis andN. ephratae from Trinidad can easily be separated by chromatograms of the hydrocarbons.N. costalis has an enormous 13,17-dimethylhentriacontane peak (mean = 42.4% of total hydrocarbon). InN. ephratae this peak is much smaller and the 12,16-dimethyltriacontane peak is completely missing.N. costalis from Trinidad andN. corniger from Panama appear to have cuticular hydrocarbon profiles that are more similar to one another than are those ofN. ephratae from Trinidad and Panama.Isoptera: Termitidae: Nasutitermitinae.  相似文献   
22.
Human mucus proteinase inhibitor (MPI) consists of 107 aminoacids arranged in two domains showing high homology to eachother. This protein is an inhibitor of different serine proteinasesincluding trypsin, chymotrypsin, leukocyte elastase and cathepsinG. On the basis of sequence comparisons it has been suggestedthat the first domain inhibits trypsin, whereas the second onewas thought to be active against chymotrypsin and elastase.To prove the location of the different inhibitory activitiesgene fragments for both domains have been cloned separatelyand expressed in Escherichia coli. Inhibition assays with theisolated recombinant domains showed that the second domain isactive against chymotrypsin, neutrophil elastase and trypsin,whereas for the first domain only a weak activity against trypsincould be detected. These results suggest that the inhibitoryactivities of the native molecule towards these three proteinasesare all located in the second domain.  相似文献   
23.
Zusammenfassung Kommerzielle Weizenkleien des Handels (9 Muster) enthielten zwischen 32 and 46 mg/g TS Phytinsäure, das entspricht einer 3- bis 5 fachen Anreicherung gegenüber Ganzkornweizen, bei Ballaststoffgehalten von 32 bis 47% FS. Weizenkeime des Handels (4 Muster) besaßen 15 bis 18 mg/g TS Phytinsäure bei Ballaststoffgehalten von 9–10% FS. Der Phytinsäuregehalt ließ sich auf unter 1% des Ausgangswertes reduzieren, wenn Weizenkleien und -keime mit Natriumcitratpuffer pH 5,25 1/2 h bei 40 °C extrahiert und mit Wasser nachgewaschen wurden. Der dabei entstehende erhebliche Substanzverlust von 34–47% bei Weizenkleien and 60–73% bei Weizenkeimen ging mit einer Erhöhung der Ballaststoffmenge (phytatfreie Weizenkleie: 55–65% FS, phytatfreie Weizenkeime: 18–26%) einher. Mit dieser sehr einfachen, schnellen Extraktion lassen sich also Weizenkleien praktisch phytinfrei herstellen, so daß mögliche negative ernährungsphysiologische Effekte bei zusätzlicher Anreicherung von Ballaststoffen ausgeschaltet werden können.
Phytic acid in wheat bran and germ products. How to remove phytic acid from these products
Summary Commercial wheat bran (nine samples) and germ products (four samples) purchased from local shops were analysed for phytic acid, dietary fibre and ash contents. Phytic acid in wheat brans ranged over 32 to 46 mg/g dry mass and in wheat germ products over 15 to 18 mg/g dry mass, the dietary fibre contents ranged over 32 to 47% and 9 to 10%, respectively. The phytic acid level can be reduced to less than 1 % of the original content by extraction with sodium citrate buffer pH 5.25 for 0.5 h at 40 °C, followed by washing with water. As a consequence, 34–47% and 60–73% of the soluble material was lost from bran and germ products, respectively; coincidently the dietary fibre content increased to 55–65% in wheat brans and 18 to 26% in wheat germ products. Such a simple and fast method enables wheat brans to be produced practically free of phytic acid, resulting in the removal of possible detrimental effects of reduced mineral availibility by increasing dietary fibre levels.


Veroffentl.-Nr. 5732 der Bundesforschungsanstalt für Getreideund Kartoffelverarbeitung, Detmold  相似文献   
24.
25.
The extent to which nanoscale‐engineered systems cross intact human skin and can exert pharmacological effects in viable epidermis is controversial. This research seeks to develop a new lipid‐based nanosome that enables the effective delivery of siRNA into human skin. The major finding is that an ultraflexible siRNA‐containing nanosome—prepared using DOTAP, cholesterol, sodium cholate, and 30% ethanol—penetrates into the epidermis of freshly excised intact human skin and is able to enter into the keratinocytes. The nanosomes, called surfactant‐ethanol‐cholesterol‐osomes (SECosomes), show excellent size, surface charge, morphology, deformability, transfection efficiency, stability, and skin penetration capacity after complexation with siRNA. Importantly, these nanosomes have ideal characteristics for siRNA encapsulation, in that the siRNA is stable for at least 4 weeks, they enable highly efficient transfection of in vitro cultured cells, and are shown to transport siRNA delivery through intact human skin where changes in the keratinocyte cell state are demonstrated. It is concluded that increasing flexibility in nanosomes greatly enhances their ability to cross the intact human epidermal membrane and to unload their payload into targeted epidermal cells.  相似文献   
26.
Tailored health communication (THC) is any combination of information and behavior change strategies intended to reach one specific person based on information unique to that person, related to the outcome of interest, and derived from an individual assessment. THCs have been studied as a means to facilitate behavior change by influencing some key intermediate steps that precede the behavioral outcome. These include the extent to which people attend to communications, think about them, find them relevant and salient, and intend to take action. Evidence shows that THCs have achieved modest success in changing a number of cancer-related behaviors, including smoking, diet, exercise, and cancer screening. However, it is likely that THCs could be more effective if they were developed with a greater understanding of message effects and what we refer to as the behavioral pathway. Instead of using unidimensional approaches to influence behavior change, a message effects approach would help researchers identify key leverage points for impact on such intermediate outcomes as persuasion and yielding. Such a strategy also might be used to determine when THCs are the preferred approach and when generic, targeted, or combinations of THCs and targeted communications might be appropriate. Viewing THCs from the perspective of the behavioral pathway might indicate use of different messages, sources, and formats to influence different people at different points on the pathway. We provide a brief history of THCs and suggest how integrating a broader perspective of health behavior and health communication theories could enrich THCs.  相似文献   
27.
The web has made easily available an enormous amount of information in digital form and has facilitated the exchange of such information. In this environment, access control is a primary concern. The key issue is how to trade-off between maximizing the sharing of information and enforcing a controlled access to web data. In this paper we start by outlining which are the main access control requirements of web data. Then, we review researches carried on in the field, by mainly focusing on xml. Finally, we discuss policy languages for the semantic web, and outline which are the main research directions in this field.  相似文献   
28.
In the pregnant rat, killed at about mid gestation and 1 h after injection of tritiated thymidine, 40% of the cells in the epithelium lining the uterine lumen at the implantation site were labelled. Between implantation sites fewer than 20% of the surface epithelial cells were labelled. A series of rats was given tritiated thymidine on day 12 of pregnancy and killed at intervals in the next 30 h. A percentage labelled mitoses analysis of the epithelium between implantation sites (interconceptual) and within the implantation site (conceptual) showed that cells in either region spent 7 h in DNA synthesis and 1.5 h in the G2 + ½ mitosis phases. The epithelial cells in the conceptual region spent 1.5 h in the G1 + ½ mitosis phases whereas cells in interconceptual regions spent at least 11.5 h in these phases. The average cycle times of cells in conceptual regions was 10 h: in interconceptual regions minimum cycle time was 20 h and the average appeared to be considerably longer. The grain count of the epithelial cells in the conceptual region was rapidly reduced during the 30 h after injection of tritiated thymidine suggesting successive rounds of cell division. In contrast the grain count distribution of cells in interconceptual regions changed only slowly during this time. The percentage of labelled epithelial cells was determined in the animals killed up to 30 h after injection of tritiated thymidine. In both conceptual and interconceptual regions these percentages increased initially as labelled cells produced labelled progeny. In the conceptual region the increase was not maintained after 7 h as cells initially in G1 divided to give unlabelled progeny. In the interconceptual region the increase in the percentage of labelled cells continued for 14 h; thereafter the percentage did not significantly alter. The interpretation of these results is discussed in relation to the differences in the kinetic characteristics of the epithelial cells in the two regions and in relation to the morphology of the epithelium lining the uterus during pregnancy.  相似文献   
29.
The aim of this paper is to analyze how the uncertainties in modelling the geometry and the material properties of a human bone affect the predictions of a finite-element model derived from computed tomography (CT) data. A sensitivity analysis, based on a Monte Carlo method, was performed using three femur models generated from in vivo CT datasets, each subjected to two different loading conditions. The geometry, the density and the mechanical properties of the bone tissue were considered as random input variables. Finite-element results typically used in biomechanics research were considered as statistical output variables, and their sensitivity to the inputs variability assessed. The results showed that it is not possible to define a priori the influence of the errors related to the geometry definition process and to the material assignment process on the finite-element analysis results. The errors in the geometric representation of the bone are always the dominant variables for the stresses, as was expected. However, for all the variables, the results seemed to be dependent on the loading condition and to vary from subject to subject. The most interesting result is, however, that using the proposed method to build a finite-element model of a femur from a CT dataset of the quality typically achievable in the clinical practice, the coefficients of variation of the output variables never exceed the 9%. The presented method is hence robust enough to be used for investigating the mechanical behavior of bones with subject-specific finite-element models derived from CT data taken in vivo.  相似文献   
30.
A content analysis was conducted to examine the portrayal of social aggression in the 50 most popular television programs among 2‐ to 11‐year‐old children. Results revealed that 92% of the programs in the sample contained some social aggression. On average, there were 14 different incidents of social aggression per hour in these shows, or one every 4 minutes. Compared to the portrayals of physical aggression, social aggression was more likely to be enacted by an attractive perpetrator, to be featured in a humorous context, and neither rewarded or punished. In these ways, social aggression on television poses more of a risk for imitation and learning than do portrayals of physical aggression. Findings are discussed in terms of social cognitive theory.  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