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   3477篇
  免费   128篇
电工技术   26篇
综合类   1篇
化学工业   689篇
金属工艺   32篇
机械仪表   29篇
建筑科学   169篇
矿业工程   4篇
能源动力   49篇
轻工业   522篇
水利工程   44篇
石油天然气   13篇
无线电   132篇
一般工业技术   415篇
冶金工业   1137篇
原子能技术   9篇
自动化技术   334篇
  2023年   26篇
  2022年   68篇
  2021年   112篇
  2020年   59篇
  2019年   70篇
  2018年   91篇
  2017年   76篇
  2016年   101篇
  2015年   54篇
  2014年   101篇
  2013年   211篇
  2012年   167篇
  2011年   226篇
  2010年   172篇
  2009年   172篇
  2008年   178篇
  2007年   181篇
  2006年   147篇
  2005年   145篇
  2004年   93篇
  2003年   91篇
  2002年   98篇
  2001年   65篇
  2000年   60篇
  1999年   65篇
  1998年   54篇
  1997年   47篇
  1996年   59篇
  1995年   44篇
  1994年   51篇
  1993年   46篇
  1992年   40篇
  1991年   16篇
  1990年   35篇
  1989年   38篇
  1988年   36篇
  1987年   24篇
  1986年   29篇
  1985年   27篇
  1984年   25篇
  1983年   20篇
  1982年   19篇
  1981年   22篇
  1980年   12篇
  1979年   17篇
  1978年   16篇
  1976年   9篇
  1975年   14篇
  1974年   10篇
  1969年   8篇
排序方式: 共有3605条查询结果,搜索用时 15 毫秒
31.
Consumption of 100% fruit juice remains controversial for its potential adverse impact on weight and displacement of essential foods in the diets of children. A systematic review of the literature published from 1995–2013 was conducted using the PubMed database to evaluate associations between intake of 100% fruit juice and weight/adiposity and nutrient intake/adequacy among children of 1 to 18 years of age. Weight status outcome measures included body mass index (BMI), BMI z-score, ponderal index, obesity, weight gain, adiposity measures, and body composition. Nutrient outcome measures included intake and adequacy of shortfall nutrients. Data extraction and analysis was conducted according to the Academy of Nutrition and Dietetics Evidence Analysis Process. Twenty-two studies on weight status provided evidence that did not support an association between 100% fruit juice consumption and weight/adiposity in children after controlling for energy intake. Limited evidence from eight studies suggests that children consuming 100% fruit juice have higher intake and adequacy of dietary fiber, vitamin C, magnesium, and potassium. Differences in methodology and study designs preclude causal determination of 100% fruit juice as sole influencer of weight status or nutrient intake/adequacy of shortfall nutrients. In context of a healthy dietary pattern, evidence suggests that consumption of 100% fruit juice may provide beneficial nutrients without contributing to pediatric obesity.  相似文献   
32.
Lactobacillus delbrueckii subsp. bulgaricus was dried under vacuum at different temperatures and its preservation evaluated analyzing the evolution of three parameters throughout the process: lag time, percentage of membrane damage and zeta potential. Microorganisms were dehydrated at 30, 45 and 70 degrees C in a vacuum centrifuge for different times. The aw achieved for each time of drying was correlated with the cell recovery at all the temperatures assayed. The recovery of microorganisms was evaluated by means of: a) kinetics of growth in milk after drying, as a measure of the global damage; b) quantification of the membrane damage using the fluorescent dyes SYTO 9 and PI; c) determination of changes in the superficial charges (zeta potential) as measured of the increase in the hydrophobic residues exposed in the bacterial surface after dehydration. These changes correlate well with the bacterial damage occurred during the dehydration process. The Page's equation allowed fitting of aw and time of drying, thus making possible the determination of the appropriate dehydration conditions (time-temperature ratios) for which no cell damage occurs. The evaluation of three parameters (lag time, percentage of membrane damage and zeta potential) allowed us to conclude that at the lowest temperature of dehydration, the first target of damage is the cell membrane. However, this damage is not decisive for the bacterial recovery after rehydration, as are the increase in the lag time and the changes in the zeta potential, as was observed for L. bulgaricus dehydrated at 45 and 70 degrees C for larger times.  相似文献   
33.
The NIEHS funds several basic and applied research programs, many of which also require research translation or outreach. This paper reports on a project by the Brown University Superfund Basic Research Program (SBRP), in which outreach and research translation teams collaborated with state regulatory agency personnel and community activists on a legislative initiative to mitigate the financial impacts of living in a contaminated community. The Environmentally Compromised Home Ownership (ECHO) program makes home equity loans of up to $25,000 available to qualified applicants. This collaboration provides a case study in community engagement and demonstrates how research translation and outreach activities that are clearly differentiated yet well-integrated can improve a suite of basic and applied research. Although engaging diverse constituencies can be difficult community-engaged translation and outreach have the potential to make research findings more useful to communities, address some of the social impacts of contamination, and empower stakeholders to pursue their individual and collectively held goals for remediation. The NIEHS has recently renewed its commitment to community-engaged research and advocacy, making this an optimal time to reflect on how basic research programs that engage stakeholders through research translation and outreach can add value to the overall research enterprise.  相似文献   
34.
Beef from cattle less than 30 months old at the point of slaughter are generally sold in meat markets. Though the age of the animal can generally be estimated by dentition, this method cannot be applied to a carcass or a piece of meat from which the teeth have been removed. Since gas chromatography of volatile organic compounds has been used to classify biological samples, this technique was used for classifying meat obtained from cattle aged over 30 months (OTM) at point of slaughter. Volatile organic compounds of fresh meat (M. longissimus dorsi) samples obtained from cattle of different age and dentition (two, four, and six teeth) at point of slaughter were analyzed using gas chromatography/mass spectrum (GC/MS). Six volatile organic compounds were selected (hexanoic acid, benzoic acid, octanoid acid, tridecane, tetradecane, and 4-formylbenzoic acid) and confirmed by using chemical standards and Kovat’s indexes. The chromatogram data were used to train two classifiers, one based on the partial least square (PLS) method and another based on support vector machine (SVM), but only SVM showed high accuracy. Data exploration based on the principal component analysis allowed for comprehension of the data distribution, proposing a new version of PLS in a parallel configuration. This modified version of the PLS algorithm showed better accuracy than the SVM classifier. In conclusion, volatile organic compounds analyzed by GC/MS–solid-phase microextraction can be used to recognize OTM beef.  相似文献   
35.
Organic production benefits from fair competition and sustained consumer confidence. The latter can only be assured by paper trailing and verification assessments. Traditional analytical strategies for guaranteeing quality and uncovering adulteration have relied on the determination of the amount of a marker compound or compounds in a material and a subsequent comparison of the value(s) obtained with those established for equivalent material. Since it is unlikely to find a unique marker that allows discrimination between organic and conventional produce, selective fingerprinting (profiling) combined with chemometrics is a more promising approach. In this study, carotenoid High Performance Liquid Chromatography–Diode Array Detection profiling combined with k-nearest neighbour classification chemometrics was used to predict the production system (organic vs. non-organic) of chicken eggs. A proof-of-concept set (training set for the model) was composed of eggs from 24 organic farms, 12 free range, and 12 barn farms in the Netherlands. The identities of organic, free range, and barn eggs were predicted with success rates of 100%, 100%, and 84%, respectively. The identity of eggs determined using an additional market test set with eggs from 12 organic, 12 free range, and 12 barn farms originating from the Netherlands and New Zealand resulted in correct classifications for 35 of the 36 samples. The results of this study indicate that this fingerprint approach is a promising tool for analytical verification of the production system of organic eggs.  相似文献   
36.
The 2015 to 2020 Dietary Guidelines for Americans (DGAs) recognize the role of 100% fruit juice in health and in helping people meet daily fruit recommendations and state that 100% fruit juice is a nutrient‐dense beverage that should be a primary choice, along with water and low‐fat/fat‐free milk. The DGAs note that children are consuming 100% fruit juice within recommendations (that is, 120 to 180 mL/d for children aged 1 to 6 y and 236 to 355 mL/d for children aged 7 to 18 y). Evidence shows that compared to nonconsumers, those who consume 100% fruit juice come closer to meeting daily fruit needs and have better diet quality. In children, 100% fruit juice is associated with increased intakes of nutrients such as vitamin C, folate, and potassium. When consumed within the DGA recommendations, 100% fruit juice is not associated with overweight/obesity or childhood dental caries and does not compromise fiber intake. Preliminary data suggest that polyphenols in some 100% fruit juices may inhibit absorption of naturally occurring sugars. Given its role in promoting health and in helping people meet fruit needs, experts participating in a roundtable discussion agreed that there is no science‐based reason to restrict access to 100% fruit juice in public health nutrition policy and programs such as the Special Supplemental Nutrition Program for Women, Infants, and Children (WIC). Reducing or eliminating 100% fruit juice could lead to unintended consequences such as reduced daily fruit intake and increased consumption of less nutritious beverages (for example, sugar‐sweetened beverages).  相似文献   
37.
A novel methodology “fractal browning indicator” (FBI) is presented, that describes the enzymatic browning kinetic based on the use of irregular color patterns from banana slice images. It uses the fractal Fourier texture image value in a selected area, to calculate a fractal dimension (FD), which represents the complexity of color distribution. During the procedure, colors from digital images were first transformed to L*a*b* space color using a transformation function (quadratic model), in order to derivate three color channels, lightness (L*), redness (a*), and yellowness (b*). In the results, lightness and yellowness parameters decreased during the browning kinetic, when their respective FD values increased, indicating major color distribution complexity in a selected area analyzed during the kinetic. The redness color (a*) did not show any statistical variation. The empirical power law model was suitable to correlate enzymatic browning kinetic data both for FBI and for the traditional method (when an L* mean was used). However, enzymatic browning rates using the FBI method, were between 8.5 and 35 times higher than rates calculated with the traditional method.  相似文献   
38.
Administered a structured questionnaire to a cross-sectional sample of 1,034 mothers and a welfare sample of 1,000 mothers of 6-18 yr olds in New York City. One or more psychiatrists rated each of the children on a 5-point scale of total psychiatric impairment. Factor analysis of the questionnaire items of child behavior yielded 18 factors. The 7 factors with the highest correlations with total psychiatric impairment were selected for the inventory, each represented by 5 items. The correlation of the total 35-item score and total psychiatric impairment was .82. The screening inventory significantly and substantially improved over chance in assigning children to impairment statuses in the cross-sectional sample and the sample used for cross-validation, the welfare sample. Correlations of the total score with child's treatment status and a direct clinical examination rating were expectedly low. In its present form, the instrument is recommended only for comparisons of different child and adolescent groups. (25 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
39.
Extrusion‐cooked blends of cassava starch (CS) and wheat gluten (WG) were studied. The data were analysed using response surface methodology. The results showed the formation of new structures characterised by lower radial expansion (RE) and specific volume (SV) when gluten was added to CS. WG content was the most important variable affecting RE, water absorption index (WAI), water solubility index (WSI), SV and difference in colour (ΔE) of the extruded products, but feed moisture content and barrel temperature also influenced RE. WAI was affected by a significant interaction between WG content and barrel temperature. The extruded products had lower WAI values and higher WSI values than WG‐free products. Heat, high shear rate and high pressure during extrusion cooking caused mechanical destruction or denaturation of the WG and gelatinisation and dextrinisation of the starch components. The extruded blends could have diverse applications. © 2002 Society of Chemical Industry  相似文献   
40.
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