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   4947篇
  免费   99篇
  国内免费   7篇
电工技术   35篇
综合类   10篇
化学工业   948篇
金属工艺   58篇
机械仪表   62篇
建筑科学   362篇
矿业工程   9篇
能源动力   171篇
轻工业   857篇
水利工程   43篇
石油天然气   9篇
无线电   216篇
一般工业技术   922篇
冶金工业   579篇
原子能技术   23篇
自动化技术   749篇
  2024年   47篇
  2023年   56篇
  2022年   30篇
  2021年   73篇
  2020年   72篇
  2019年   92篇
  2018年   165篇
  2017年   168篇
  2016年   170篇
  2015年   166篇
  2014年   178篇
  2013年   320篇
  2012年   247篇
  2011年   313篇
  2010年   220篇
  2009年   170篇
  2008年   219篇
  2007年   222篇
  2006年   216篇
  2005年   137篇
  2004年   123篇
  2003年   144篇
  2002年   120篇
  2001年   98篇
  2000年   91篇
  1999年   87篇
  1998年   186篇
  1997年   118篇
  1996年   94篇
  1995年   77篇
  1994年   56篇
  1993年   53篇
  1992年   34篇
  1991年   40篇
  1990年   28篇
  1989年   22篇
  1988年   15篇
  1987年   15篇
  1986年   19篇
  1985年   29篇
  1984年   38篇
  1983年   29篇
  1982年   15篇
  1981年   28篇
  1980年   29篇
  1979年   20篇
  1977年   25篇
  1976年   36篇
  1974年   15篇
  1973年   18篇
排序方式: 共有5053条查询结果,搜索用时 0 毫秒
61.
Vitamin D exists in 2 forms that are important regarding vitamin D status and supply in cattle: vitamin D2 (D2) and vitamin D3 (D3). To become physiologically active, both D2 and D3 must undergo 25-hydroxylation in the liver. The resulting 25-hydroxyvitamin D2 [25(OH)D2] and 25-hydroxyvitamin D3 [25(OH)D3] are measured as indicators of the physiological vitamin D status of cattle. The study used 14 Danish Holstein cows housed without access to sunlight. The cows were orally administered 250 mg (1.0 × 107 IU) of D2 and D3 in a cross-over design with 2 treatment groups and 2 study periods, rendering 4 treatments when carryover effects were taken into account: D2 given first, D2 given last after D3, D3 given first, and D3 given last after D2. Two weeks elapsed between the treatment in the first study period and the treatment in the second study period. Blood samples were collected 0, 3, 6, 14, 17, 20, 23, 26, 40, 48, 70, 94, 166, and 214 h after providing the oral bolus of vitamin to the cows. Comparisons between plasma levels of the metabolites D2, D3, 25(OH)D2, and 25(OH)D3 over time were made by comparing areas under the plasma concentration curves. Oral administration of D3 increased plasma D3 (182.6 ± 17.1 ng/mL; mean ± SEM) and 25(OH)D3 (103.5 ± 10.0 ng/mL) more efficiently than oral administration of D2 increased plasma D2 (49.1 ± 32.6 ng/mL) and 25(OH)D2 (27.9 ± 2.1 ng/mL). The D3 given after an oral dose of D2 was less efficient for increasing plasma concentrations of 25(OH)D3 (61.2 ± 12.0 ng/mL) compared with D3 given without previous D2 administration (103.5 ± 10.0 ng/mL), whereas the plasma concentrations of D3 itself were the same when given first (182.6 ± 17.1 ng/mL) as when given after D2 (200.0 ± 123.9 ng/mL). The same occurred for plasma concentrations of D2 metabolites both if D2 was given first (49.1 ± 32.6 ng/mL) and after D3 (54.7 ± 7.7 ng/mL). In conclusion, D3 given after D2 is less efficient at increasing the plasma status of 25(OH)D3 than D3 given without previous D2 administration.  相似文献   
62.
The purpose of this study was to investigate the imputation error and loss of reliability of direct genomic values (DGV) or genomically enhanced breeding values (GEBV) when using genotypes imputed from a 3,000-marker single nucleotide polymorphism (SNP) panel to a 50,000-marker SNP panel. Data consisted of genotypes of 15,966 European Holstein bulls from the combined EuroGenomics reference population. Genotypes with the low-density chip were created by erasing markers from 50,000-marker data. The studies were performed in the Nordic countries (Denmark, Finland, and Sweden) using a BLUP model for prediction of DGV and in France using a genomic marker-assisted selection approach for prediction of GEBV. Imputation in both studies was done using a combination of the DAGPHASE 1.1 and Beagle 2.1.3 software. Traits considered were protein yield, fertility, somatic cell count, and udder depth. Imputation of missing markers and prediction of breeding values were performed using 2 different reference populations in each country: either a national reference population or a combined EuroGenomics reference population. Validation for accuracy of imputation and genomic prediction was done based on national test data. Mean imputation error rates when using national reference animals was 5.5 and 3.9% in the Nordic countries and France, respectively, whereas imputation based on the EuroGenomics reference data set gave mean error rates of 4.0 and 2.1%, respectively. Prediction of GEBV based on genotypes imputed with a national reference data set gave an absolute loss of 0.05 in mean reliability of GEBV in the French study, whereas a loss of 0.03 was obtained for reliability of DGV in the Nordic study. When genotypes were imputed using the EuroGenomics reference, a loss of 0.02 in mean reliability of GEBV was detected in the French study, and a loss of 0.06 was observed for the mean reliability of DGV in the Nordic study. Consequently, the reliability of DGV using the imputed SNP data was 0.38 based on national reference data, and 0.48 based on EuroGenomics reference data in the Nordic validation, and the reliability of GEBV using the imputed SNP data was 0.41 based on national reference data, and 0.44 based on EuroGenomics reference data in the French validation.  相似文献   
63.
64.
Estrogenicity of river water is highly variable and it is difficult to obtain an average measure of the estrogenicity. Consequently it is difficult to tie the estrogenic effects observed in fish to their level of exposure to estrogens. To get a better handle on average estrogenic exposure we tested a recently developed passive sampling system (polar organic chemical integrative sampler, POCIS). In addition, we investigated the bioaccumulation of estrogens in caged brown trout and measured plasma vitellogenin in males as a bioindicator of estrogenic effects. We developed a mini-caging method to suit the hydrological conditions in small rivers and to improve upon the often poor survival of salmonids in caging trials. POCISs were positioned upstream and downstream of 5 sewage treatment works' discharges and left on site for 3 weeks (as were the caged fish), during which period 3 water grab samples were taken at each site. Concentrations of estrogens were determined using a yeast-based reporter gene assay and chemical analysis. Results from grab sampling, passive sampling, and bioaccumulation were correlated; however, plasma vitellogenin concentrations were elevated at only 1 of 5 sites. POCISs provide an integrated and biologically meaningful measure of estrogenicity in thatthey accumulate estrogens in a pattern similar to that of brown trout. Mini-caging appears a significant methodological advance; no fish were lost, moreover, all fish survived in excellent health.  相似文献   
65.
The application of cold atmospheric pressure plasma for decontamination of a sliced ready-to-eat (RTE) meat product (bresaola) inoculated with Listeria innocua was investigated. Inoculated samples were treated at 15.5, 31, and 62 W for 2–60 s inside sealed linear-low-density-polyethylene bags containing 30% oxygen and 70% argon. Treatments resulted in a reduction of L. innocua ranging from 0.8 ± 0.4 to 1.6 ± 0.5 log cfu/g with no significant effects of time and intensity while multiple treatments at 15.5 and 62 W of 20 s with a 10 min interval increased reduction of L. innocua with increasing number of treatments. Concentrations of thiobarbituric acid reactive substances (TBARS) increased with power, treatments and storage time and were significantly higher than those of control samples after 1 and 14 days of storage at 5 °C. However, the levels were low (from 0.1 to 0.4 mg/kg) and beneath the sensory threshold level. Surface colour changes included loss of redness of ∼40% and 70% after 1 and 14 days of storage, respectively, regardless of plasma treatment. The results indicate that plasma may be applicable in surface decontamination of pre-packed RTE food products. However, oxidation may constitute an issue in some products.  相似文献   
66.
The use of a wide variety of antimicrobials in human and veterinary medicine, including aquaculture, has led to the emergence of antibiotic resistant pathogens. In the present study, bacteria from water, sediments, and fish were collected from fish farms in Pakistan and Tanzania with no recorded history of antibiotic use. The isolates were screened for the presence of resistance genes against various antimicrobials used in aquaculture and animal husbandry. Resistant isolates selected by disk diffusion and genotyped by Southern hybridization were further screened by polymerase chain reaction (PCR) and amplicon sequencing. The prominent resistance genes identified encoded tetracycline [tetA(A) and tetA(G)], trimethoprim [dfrA1, dfrA5, dfrA7, dfrA12, and dfrA15], amoxicillin [bla(TEM)], streptomycin [strA-strB], chloramphenicol [cat-1], and erythromycin resistance [mefA]. The int1 gene was found in more than 30% of the bacterial isolates in association with gene cassettes. MAR indices ranged from 0.2 to 1. The bla(NDM-1) gene was not identified in ertapenem resistant isolates. It is hypothesized that integrated fish farming practices utilizing domestic farm and poultry waste along with antibiotic residues from animal husbandry may have contributed to a pool of resistance genes in the aquaculture systems studied.  相似文献   
67.
A new extrudate bulk density model framework was developed, which is taking into account the melt rheological effects based on mixing principle and non-Newtonian power law theory for a twin-screw extrusion cooking process. The main ingredients in the extrusion process investigation were wheat, fish meal and soybean protein. The average absolute deviation (AAD) of the model prediction for extrudate bulk density is 5.3 % for the investigated process. The prediction results demonstrate that the proposed equation can be used to model the extrudate bulk density in the twin-screw extruder extrusion cooking process.  相似文献   
68.
To assess emissions of dioxins (chlorinated dibenzodioxins and dibenzofurans) and PCB from uncontrolled domestic combustion of waste ("backyard burning"), test combustions in barrels and open fires were monitored. The waste fuels used were garden waste, paper, paper and plastic packaging, refuse-derived fuel (RDF), PVC, and electronic scrap. Combustions including PVC and electronic scrap emitted several orders of magnitude more dioxins than the other waste fuels. Emissions from the other fuels had considerable variations, but the levels were difficult to relate to waste composition. Emission factors of PCDD/F and PCB from the backyard burning ranged from 2.2 to 13 000 ng (WHO-TEQ)/kg. The levels found in ash usually were less than 5% of the total. For assessment of total emissions of dioxins and PCB from backyard burning of low and moderately contaminated wastes, an emission factor range of 4-72 ng (WHO-TEQ)/kg is suggested. These figures implythat combusting waste in the backyard could contribute substantially to total emissions, even if the amounts of fuel involved are equivalent to just a few tenths of a percent of the amounts combusted in municipal waste incinerators.  相似文献   
69.
70.
The article describes the chronology of events of the recent EHEC O104:H4 outbreak from the perspective of food inspection in Schleswig-Holstein until the National Task Force EHEC was founded. Taking the district of Schleswig-Flensburg as an example, the collaboration and information exchange between health and food control authorities is described. The Ministry of Agriculture, Environment and Rural Areas of Schleswig-Holstein (MLUR) received the first information about clusters of disorders with symptoms of bloody diarrhoea and HUS from the districts around Hamburg and the city of Hamburg on 19 May 2011. On the same day, the veterinary services and the service for consumer protection in the district of Schleswig-Flensburg were informed about a situation comparable to the one in the environment of Hamburg. Due to this situation, official procedures were initiated to discover the origins of this food-associated outbreak - both at the state and county level and in collaboration with the health authorities. On 1 June 2011, an initial telephone conference was held between the responsible ministries and the food inspection and health authorities of the counties and cities in order to coordinate a joint action. On 3 June 2011, the National Task Force EHEC was founded at the Federal Office for Consumer Protection and Food Safety (BVL) in Berlin. By founding the National Task Force EHEC, the coordination of the clearing process was passed over to the BVL two weeks after the public health authorities had knowledge of accumulated and cross-border cases. On 7 June 2011 an employee of the district Schleswig-Flensburg was sent to the Task Force EHEC in Berlin by the MLUR. The lesson to be learned as a result of the EHEC O104:H4 outbreak is that among the ??dangerous foods?? there are more plant-based foods than often assumed. In the future this must be much more taken into account.  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