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   4999篇
  免费   29篇
  国内免费   2篇
电工技术   15篇
综合类   1篇
化学工业   106篇
金属工艺   20篇
机械仪表   12篇
建筑科学   10篇
矿业工程   1篇
能源动力   21篇
轻工业   99篇
水利工程   11篇
石油天然气   3篇
无线电   20篇
一般工业技术   80篇
冶金工业   4555篇
原子能技术   7篇
自动化技术   69篇
  2023年   3篇
  2022年   12篇
  2021年   24篇
  2020年   15篇
  2019年   22篇
  2018年   18篇
  2017年   22篇
  2016年   12篇
  2015年   11篇
  2014年   20篇
  2013年   36篇
  2012年   23篇
  2011年   39篇
  2010年   25篇
  2009年   21篇
  2008年   26篇
  2007年   19篇
  2006年   21篇
  2005年   17篇
  2004年   13篇
  2003年   17篇
  2002年   15篇
  2001年   4篇
  2000年   7篇
  1999年   158篇
  1998年   1371篇
  1997年   745篇
  1996年   516篇
  1995年   268篇
  1994年   240篇
  1993年   255篇
  1992年   38篇
  1991年   75篇
  1990年   62篇
  1989年   86篇
  1988年   74篇
  1987年   64篇
  1986年   69篇
  1985年   54篇
  1984年   1篇
  1983年   4篇
  1982年   16篇
  1981年   28篇
  1980年   50篇
  1979年   3篇
  1978年   11篇
  1977年   121篇
  1976年   271篇
  1975年   5篇
  1955年   3篇
排序方式: 共有5030条查询结果,搜索用时 15 毫秒
981.
Anthocyanins bioavailability is a major issue regarding their biological effects and remains unclear due to few data available on this matter. This work aimed to evaluate the absorption of anthocyanins at the intestine using Caco‐2 cells. Anthocyanin extract, rich in malvidin‐3‐glucoside, was obtained from red grape skins and tested on Caco‐2 cells. The absorption of anthocyanins, in absence or presence of 1% ethanol, was detected by HPLC/DAD/LC‐MS. Our results showed that this transport was significantly increased in the presence of ethanol especially after 60 min of incubation. In addition, cells that were pretreated for 96 h with anthocyanins (200 μg/mL) showed an increase of their own transport (about 50% increase). Expression of glucose transporters sodium‐dependent glucose transporter 1, facilitative glucose transporters 5, and facilitative glucose transporters 2 was assessed by RT‐PCR. It was found that facilitative glucose transporters 2 expression was increased (60%) in Caco‐2 cells pretreated with anthocyanins, by comparison with controls. When the effect of anthocyanin extract on 3H‐2‐deoxy‐D ‐glucose uptake was tested, an inhibitory effect was observed (about 60% decrease). However, the malvidin aglycone was tested and had no effect. In conclusion, anthocyanins could be absorbed through Caco‐2 cells, and can interfere with their own transport and also with glucose intestinal uptake.  相似文献   
982.
The objectives were to evaluate the effects of a culture of Saccharomyces cerevisiae (YC) on lactation performance of cows fed diets differing in starch content. Fifty-six Holstein cows at 42 d postpartum were blocked by parity and milk production and randomly assigned to 1 of 4 treatments, low starch (23% diet DM) and no YC (LS-control), low starch and 15 g/d of YC (LS-YC), high starch (29% diet DM) and no YC (HS-control), and high starch and 15 g/d of YC (HS-YC). The experiment lasted 14 wk. Blood was sampled twice weekly during the first 5 wk in the experiment. Feeding behavior was evaluated in 2 consecutive days when cows were 33 d in the experiment. On d 92 in the experiment, cows were challenged with 3 kg of corn grain DM immediately before the morning feeding. Blood was sampled in the first 12 h after the challenge. Rumen fluid was collected 5 h after the challenge, and pH, ammonia N, short-chain fatty acids, and lactate concentrations were quantified. Lactation performance was measured daily before and after the challenge. Supplementation with YC increased yields of 3.5% fat-corrected milk and energy-corrected milk by 2.2 and 2.0 kg/d, and the increments were observed in both low- and high-starch diets. Feeding HS tended to decrease milk fat content (LS = 3.88 vs. HS = 3.73%), but increased concentration (LS = 2.87 vs. HS = 3.00%) and yield (LS = 1.11 vs. HS = 1.20 kg/d) of milk true protein. Feeding YC increased yields of fat and true protein in milk by 100 and 60 g/d. Energy balance, body weight, and feed efficiency did not differ with treatments. Feeding HS reduced eating time (LS = 177 vs. HS = 159 min/12 h) and intermeal interval (LS = 103 vs. HS = 82 min), but tended to increase eating rate (LS = 139 vs. HS = 150 g/min). Interactions were detected between level of starch and YC for ruminating time, meal duration, and meal size because within LS, feeding YC increased ruminating time 23 min/12 h, but reduced meal duration 6 min/meal and meal size 0.7 kg/meal. Concentrations of glucose in plasma increased (LS = 62.1 vs. HS = 63.8 mg/dL), whereas those of urea N decreased (LS = 10.1 vs. HS = 9.4 mg/dL) with feeding HS compared with LS in the first 5 wk in the experiment, and the same responses were observed after the challenge with corn grain. After the challenge, rumen pH was less and short-chain fatty acid concentrations were greater in cows fed HS compared with those fed LS; however, supplementing YC to high-starch diets increased rumen pH (HS-control = 5.72 vs. HS-YC = 6.12) and reduced concentrations of lactate in rumen fluid (HS-control = 7.72 vs. HS-YC = 1.33 mM) and haptoglobin in plasma 28%. Feeding YC improved lactation performance irrespective of the level of dietary starch and reduced the risk of subacute rumen acidosis induced by a grain challenge when cows were fed a high-starch ration.  相似文献   
983.
The consumption of diet products has increased greatly in recent years. The objectives of the study were to develop a bittersweet chocolate added inulin and stevias with different rebaudioside A contents (60%, 80%, and 97%). Five chocolate samples were formulated with different sucrose concentrations to determine the ideal sucrose concentration for bittersweet chocolate. The use of just‐about‐right scale identified an ideal sucrose concentration of 47.5% (w/w). The sweetness equivalence in sugar‐free bittersweet chocolates was determined by the time–intensity method by 14 selected and trained judges. The data collected during each session of sensory evaluation furnished the following parameters in relation to the sweet stimulus: Imax (maximum intensity recorded), Timax (time at which the maximum intensity was recorded), Area (area of time × intensity curve), and Ttot (total duration time of the stimulus). The time–intensity analysis indicated that the percentages of rebaudioside A did not interfere with the sweetness intensity of the sweetener stevia in bittersweet chocolate and there was no significant difference in the concentrations tested (0.16%, 0.22%, 0.27%) of each stevia, in relation to the parameters evaluated. In addition, the reduction in fat content did not alter the perception of the sweetness intensity of the samples. These results showed important information to research and development of chocolate products. Therefore, the use of the lowest stevia concentration tested (0.16%) is the most indicated for use, since this quantity was sufficient to reach the ideal sweetness of the product, so there was no point in adding more.  相似文献   
984.
985.
In make-to-order (MTO)/engineer-to-order (ETO) business environments multiple customer-oriented projects compete for and share resources through interdependent engineering and production activities. Deep knowledge of critical dimensions that affect performance is key in this context. For this, we propose a set of determinants – workload, complexity, outsourcing, design reuse, project type, and knowledge/experience with technology, that impact performance. These determinants are input to an extended hybrid simulation model using system dynamics (SD), discrete event simulation (DES) and agent-based simulation (ABS) that tackles the needs imposed by activities of very different nature, as the project development and manufacturing/assembly operations. The hybrid model is applied to the case of an advanced manufacturing company. Through Monte Carlo sampling, the influence of different combinations of determinants in the performance variability is assessed. A correlation analysis shows evidence of association between all performance determinants and the project time and cost, while no evidence of association between the design reuse and project type determinants and the manufacturing and assembly time.  相似文献   
986.
The main advantage of the TIG with flux process (A-TIG welding) is the possibility of obtaining greater penetration of the weld bead while employing the same welding parameters as conventional TIG welding. Various studies have shown the influence of active fluxes on the geometric characteristics of stainless steel austenitic welds. However, little is known about the influence of this process on the geometric and metallurgical characteristics of the weld beads in ferritic stainless steels. In this work, different types of flux are applied when welding ferritic stainless steel with the objective of verifying possible influences on the weld bead's profile, on its visual appearance, on the microstructure, on the hardness of the welded zone and on the impact resistance (Charpy test). The bead-on-plate welds were produced without using any addition metal. Six types of flux were used – one being an oxide produced in a laboratory (TiO2) and five commercial fluxes. The results showed that use of the flux allows an increase in penetration with significant changes in the appearance of the weld bead. It was also confirmed that the microstructure and the hardness of the weld bead for the steel studied were not affected by the type of flux used, with the microstructure analysed under an optical microscope. The steel in the study showed a high degree of fragility at ambient temperature.  相似文献   
987.
The aim of this work was to determine vitamin contents in spinach produced by different cultivation type (organic and traditional), harvesting period and after cooking in water. The determination was carried out by High Performance Liquid Chromatography. There was no significant difference in folate contents between spinach cultivated by traditional and organic method and there was also no significant difference between the values obtained at different periods of the year. Folate levels determined as 5-methylTHF and 5-formylTHF varied from 226 to 527 microg/100 g and 4.6 to 10 microg/100 g, respectively. Cooking in water resulted in approximately 74% of losses of 5-methylTHF and 56% of 5-formylTHF. The mainly losses occurred by leaching.  相似文献   
988.
The discrete element method, DEM, has been used in fracture studies of non-homogeneous continuous media adopting circular or spherical particles. A 2D circular rigid DEM formulation developed with the purpose of modelling concrete is described and evaluated in uniaxial tensile and compression tests. According to this model, the aggregate can be modelled either as a rigid macro-particle or as a deformable group of particles. The inter-particle contacts can either be assumed as brittle or follow a given bilinear softening curve. It is shown that aggregate deformability, together with the consideration of pure friction contacts working under compression, increases the fracture energy in compression, leading to a better agreement with concrete tests. The softening contact model, by adding a higher capability of load redistribution, is shown to give a better agreement than the brittle model under tensile loading. The recognized crack mechanisms of the brittle model (tensile splitting, branching, bridging) are also present with softening.  相似文献   
989.
The aim of the study was to evaluate the overall hygienic–sanitary conditions of university hospitals foodservices and assess food safety knowledge and self‐reported practices of food handlers. A self‐administered questionnaire based on sanitary standards for foodservices was given to 123 food handlers from three public hospitals. Microbiological analysis of food handling surfaces and food hygiene inspections were also performed. Food handlers' self‐reported knowledge and practices reached satisfactory levels (77.2% and 78.1%, respectively), and showed a significant positive intercorrelation (p < .05). Food hygiene inspections in the hospitals foodservices presented a 60.9% average of adequacy and microbiological analysis results showed that 66.7% of the evaluated surfaces presented significant contamination by mesophilic bacteria. The presence of coliforms at 45°C above the safety limit was identified in 47.9% of the surfaces. The results showed a need for motivating hospital food handlers to practice the safe handling knowledge that they have demonstrated in theory.  相似文献   
990.
The aim of this study was to gain information on quality traits, mainly bacterial and somatic cell counts of bulk milk, produced by small- and medium-scale producers in a semiarid northeastern region of Brazil and to identify and characterize possible risk factors associated with those quality traits. A cross-sectional study was performed on 50 farms. Bulk milk samples were collected for bacterial and somatic cell counts. Additionally, information about farm demographics, general management practices, hygiene, and milking procedures was also obtained. Multivariable analysis using logistic regression was performed with predictors previously identified by univariate analysis using a Fisher's Exact test. Aerobic mesophilic bacteria counts varied from 3.59 log to 6.95 log cfu/mL, with geometric mean of 5.27 log cfu/mL. Mean total coliform count was 3.27 log (1.52 log to 5.89 log) most probable number (MPN)/mL, whereas mean thermotolerant coliforms was 2.38 log (1.48 log to 4.75 log) MPN/mL. A high positive correlation was observed between aerobic mesophilic bacteria and coliform counts. Although most farms met the standard for the current regulations for total bacteria (88%) and somatic cell counts (94%), nearly half of the producers (46%) would have problems in achieving the 2012 threshold limit for total bacteria count if no improvement in milk quality occurs. Mean value for staphylococci was 3.99 log (2.31 log to 6.24 log) cfu/mL, and Staphylococcus aureus was detected in 33 (66%) farms. Premilking teat-end wash procedure (odds ratio = 0.191) and postmilking teat dip (odds ratio = 0.67) were associated with lower aerobic mesophilic bacteria and Staphylococcus aureus counts in bulk milk, respectively. Considering that the farm characteristics in this study are representative of the semiarid northeastern region, these findings encourage further investigations for supporting intervention measures intended to improve the quality of milk produced by smallholders.  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