全文获取类型
收费全文 | 7047篇 |
免费 | 185篇 |
专业分类
电工技术 | 97篇 |
综合类 | 2篇 |
化学工业 | 989篇 |
金属工艺 | 100篇 |
机械仪表 | 106篇 |
建筑科学 | 237篇 |
矿业工程 | 22篇 |
能源动力 | 138篇 |
轻工业 | 582篇 |
水利工程 | 77篇 |
石油天然气 | 17篇 |
无线电 | 734篇 |
一般工业技术 | 1020篇 |
冶金工业 | 2145篇 |
原子能技术 | 44篇 |
自动化技术 | 922篇 |
出版年
2022年 | 58篇 |
2021年 | 96篇 |
2020年 | 68篇 |
2019年 | 117篇 |
2018年 | 125篇 |
2017年 | 85篇 |
2016年 | 101篇 |
2015年 | 85篇 |
2014年 | 144篇 |
2013年 | 315篇 |
2012年 | 221篇 |
2011年 | 277篇 |
2010年 | 207篇 |
2009年 | 193篇 |
2008年 | 271篇 |
2007年 | 249篇 |
2006年 | 236篇 |
2005年 | 193篇 |
2004年 | 160篇 |
2003年 | 149篇 |
2002年 | 151篇 |
2001年 | 143篇 |
2000年 | 113篇 |
1999年 | 148篇 |
1998年 | 501篇 |
1997年 | 330篇 |
1996年 | 219篇 |
1995年 | 195篇 |
1994年 | 182篇 |
1993年 | 179篇 |
1992年 | 99篇 |
1991年 | 76篇 |
1990年 | 100篇 |
1989年 | 100篇 |
1988年 | 76篇 |
1987年 | 79篇 |
1986年 | 69篇 |
1985年 | 87篇 |
1984年 | 80篇 |
1983年 | 66篇 |
1982年 | 54篇 |
1981年 | 50篇 |
1980年 | 54篇 |
1979年 | 61篇 |
1978年 | 56篇 |
1977年 | 77篇 |
1976年 | 158篇 |
1975年 | 51篇 |
1974年 | 38篇 |
1973年 | 38篇 |
排序方式: 共有7232条查询结果,搜索用时 15 毫秒
831.
Our objective was to evaluate growth performance, nutrient utilization, and health of calves supplemented with condensed whey solubles (CWS). Twenty-four Holstein calves (2 d old) in hutches were used in a 12-wk randomized complete block design study. Calves were blocked by birth date and sex. Treatments were (1) control (CON) with no supplement and (2) 50 g/d of CWS. Preweaning CWS was fed with milk and postweaning CWS was top-dressed on starter pellets. Calves were fed 2.83 L of pasteurized milk 2×/d during wk 1 to 5, 1×/d in wk 6, and weaned at d 42. Calves had ad libitum access to starter pellets and water. Individual intakes of milk and starter pellets were measured daily. Fecal scores (0 = firm, 3 = watery) and respiratory scores (healthy ≤3, sick ≥5), calculated from the sum of scores for rectal temperature, cough, ocular, and nasal discharge, were recorded daily. Body weights (BW), frame growth, and jugular blood samples were taken once every week at 3 h after the morning feeding. Fecal grab samples were collected in wk 12 for analysis of apparent total-tract digestibility. Data were analyzed using MIXED procedures of SAS 9.4 (SAS Institute Inc., Cary, NC) with repeated measures. Total dry matter intake was greater in calves fed CWS compared with CON. Calves fed CWS tended to have greater BW during the postweaning period. Gain:feed, average daily gains, withers heights, and concentrations of plasma urea nitrogen were not different. Concentrations of plasma β-hydroxybutyrate were greater in calves fed CWS compared with CON. Serum glucose concentrations were greater in calves fed CON during the preweaning stage, but similar concentrations were found between treatments during the postweaning stage. Rumen pH, ammonia-N, and total volatile fatty acids were similar between treatments. The acetate:propionate ratio was greater in calves fed CWS. The apparent total-tract digestion of nutrients was similar; however, CON tended to have greater digestion of neutral detergent fiber. Fecal scores were not different, but a treatment by week interaction was observed. Supplementing CWS improved starter intake, postweaning BW, β-hydroxybutyrate, fecal scores, and maintained frame growth. 相似文献
832.
Dry cow therapy, administered at the end of lactation, is aimed at eliminating current and preventing future intramammary (IMM) bacterial infections and typically involves intramammary administration of antibiotics. Certified organic dairies in the United States are restricted from using antibiotics and must consider an alternative therapy or no dry cow therapy. The current study compared 2 herbal products to conventional dry cow therapy and no treatment for a total of 5 treatments over 2 trials. Trial 1 was conducted over 3 yr on 1 research farm and trial 2 included 4 commercial farms plus the research herd over 2 yr. Treatments included (1) a conventional IMM antibiotic and internal teat sealant (penicillin-dihydrostreptomycin and bismuth subnitrate; CON); (2) an herbal IMM product purported to act as a teat sealant (Cinnatube, New AgriTech Enterprises, Locke, NY; CIN); (3) an herbal IMM product (Phyto-Mast, Bovinity Health LLC, Narvon, PA; P-M); (4) Phyto-Mast and Cinnatube (PC); or (5) no dry cow therapy (NT). Each treatment group was balanced by breed, lactation number, due date, herd, and year. However, the CON treatment was used only in the research herd because of the intent to avoid antibiotic usage on the other 4 farms. Comparisons among treatments included the difference between pre- and posttreatment 305-d mature equivalent milk production (trial 1), somatic cell score change from dry-off to freshening at the cow and quarter levels (trials 1 and 2), and milk microbiology change over the dry period (trial 2). We detected no significant differences among treatments for milk yield differences between the lactation following treatment and the lactation preceding treatment. Changes in somatic cell score from one lactation to the next also did not differ significantly among treatments in either trial. Cure rates were not significantly different among treatments; only 19.6% of all quarters were infected at dry off. The proportion of quarters with new infections at 3 to 5 d postcalving did not significantly differ among treatments, except between CIN and NT. Percentages (least squares means ± standard error) of quarters with new infections were 24 ± 21% for CON, 15 ± 7% for CIN, 30 ± 10% for P-M, 32 ± 11% for PC, and 35 ± 11% for NT. The efficacy of the herbal products was similar to that of conventional therapy, and the herbal products had no apparent adverse effects. 相似文献
833.
Jennifer K Fowler Susan A Anderson Janet H Geggie 《Revue canadienne de la pratique et de la recherche en diététique》2004,65(3):118-121
The benefits of nutrition assessment and support of the high-risk infant are well established. The premature infant remains vulnerable for poor growth and developmental disabilities, thus requiring consistent monitoring, intervention, and follow-up care. The purpose of this study was to determine the registered dietitian's role in neonatal/perinatal follow-up programs. A survey was sent to the 26 follow-up programs in Canada. The questionnaire response rate was 81%. Registered dietitians were involved in 67% of these programs. Of these dietitians, 43% were assigned to neonatal/perinatal follow-up programs while 57% were involved only by consult. The average time that assigned registered dietitians devoted to programs was 0.35 full-time equivalents. Over 80% of the dietitians did ongoing development, evaluation, and modification of nutrition care plans; 71% screened new patients for nutritional risk, and 100% instructed patient families and developed teaching materials. The study findings will assist program planners who wish to establish a dietitian position in a neonatal/ perinatal follow-up program. For registered dietitians already working in such programs, the results may provide some guidance on role definition and expansion. 相似文献
834.
Organochlorine compounds in Lake Superior: chiral polychlorinated biphenyls and biotransformation in the aquatic food web 总被引:1,自引:0,他引:1
Wong CS Mabury SA Whittle DM Backus SM Teixeira C DeVault DS Bronte CR Muir DC 《Environmental science & technology》2004,38(1):84-92
The enantiomeric composition of seven chiral PCB congeners was measured in the Lake Superior aquatic food web sampled in 1998, to determine the extent of enantioselective biotransformation in aquatic biota. All chiral PCB congeners studied (CBs 91, 95, 136, 149, 174, 176, and 183) biomagnified in the Lake Superior aquatic food web, based on biomagnification and food web magnification factors greater than unity. PCB atropisomers were racemic in phytoplankton and zooplankton, suggesting no biotransformation potential toward PCBs for these low trophic level organisms. However, Diporeia and mysids had significantly nonracemic residues for most chiral congeners studied. This observation suggests that these macrozooplankton can stereoselectively metabolize chiral congeners. Alternatively, macrozooplankton obtained nonracemic residues from feeding on organic-rich suspended particles and sediments, which would imply that stereoselective microbial PCB biotransformation may be occurring in Lake Superior sediments at PCB concentrations far lower than that previously associated with such activity. Widely nonracemic PCB residues in forage fish (lake herring, rainbow smelt, and slimy sculpin) and lake trout suggest a combination of both in vivo biotransformation and uptake of nonracemic residues from prey for these species. Minimum biotransformation rates, calculated from enantiomer mass balances between predators and prey, suggest metabolic half-lives on the order of 8 yr for CB 136 in lake trout and 2.6 yr for CB 95 in sculpins. This result suggests that significant biotransformation may occur for metabolizable PCB congeners over the lifespan of these biota. This study highlights the potential of chiral analysis to study biotransformation processes in food webs. 相似文献
835.
Tom S. Edrington J. Armando Garcia Buitrago G. Robert Hagevoort G.H. Loneragan D.M. Bricta-Harhay Todd R. Callaway Robin C. Anderson David J. Nisbet 《Journal of dairy science》2018,101(10):9266-9274
The objective of the current research was to determine if pasteurization of nonsaleable waste milk influences fecal Salmonella concentrations and prevalence, or antimicrobial susceptibility and serotype of the cultured isolates. Holstein dairy calves (n = 211) were housed on a single commercial dairy in the southwestern United States and randomly allotted to be fed either pasteurized (PWM; n = 128 calves) or nonpasteurized waste milk (NPWM; n = 83 calves). Fecal samples were collected via rectal palpation or from freshly voided, undisturbed fecal pats, weekly during the first 4 wk of the animal's life and then again at weaning. Eight total collections were made and 1,117 fecal samples cultured for Salmonella. One isolate from each culture-positive fecal sample was preserved for antimicrobial susceptibility screening and serotyping. Sixty-nine percent of the fecal samples were culture positive for Salmonella with no difference due to treatment (67.7 and 69% Salmonella positive for PWM and NPWM treatments, respectively). Few fecal samples (178/1,117; 15.9%) contained Salmonella concentrations above the limit of detection (~1 cfu/g of feces) with concentrations ranging from 1.0 to 6.46 cfu (log10)/g of feces. Concentration was not affected by treatment. Seventeen different serotypes were identified, the majority of which were Montevideo and Anatum. A greater percentage of Typhimurium (87 vs. 13%), Muenchen (88 vs. 12%), and Derby (91 vs. 9%) were recovered from calves fed PWM compared with NPWM-fed calves. Conversely, Newport (12.5 vs. 86%), Bredeney (22.2 vs. 77.8%), and Muenster (12.5 vs. 87.5%) were lower in PWM compared with NPWM treatments. The majority (66.7%) of isolates were susceptible to all of the antibiotics examined. Results from this one commercial dairy suggest that milkborne Salmonella is not an important vector of transmission in dairy neonates, nor does pasteurization of waste milk influence fecal shedding of this pathogen. Caution should be used, however, when extrapolating results to other farms as Salmonella contamination of milk on farm is well documented. The potential benefits of pasteurization in disease prevention outweigh the potential risks of feeding a nonpasteurized product and warrants incorporation into any calf-rearing program using nonsaleable waste milk for feeding young dairy neonates. 相似文献
836.
Raisin Consumption by Humans: Effects on Glycemia and Insulinemia and Cardiovascular Risk Factors 总被引:1,自引:0,他引:1
Raisins are widely recommended as good foods and preferred snacks because of their nutrient content. They are rich in dietary fiber, potassium, and many health‐promoting phytonutrients and antioxidants. Raisins have a high dietary fiber score. Laboratory studies document that raisins rank in the upper quartile of foods for antioxidant content. Clinical research related to the potential health benefits of raisins has intensified over the past 10 y. Much of this research has focused on blood glucose and insulin responses to raisins compared to other foods or snacks. Meal studies indicate that raisins have a low to moderate glycemic index and a low insulinemic index. Longer term studies suggest that customary intake of raisins is associated with lower postprandial blood glucose values than with equicaloric control foods. Cross‐sectional studies suggest that higher levels of dried fruit consumption are associated with lower systolic and diastolic blood pressure than seen with minimal dried fruit use. One clinical study documents that raisin intake over 12 wk is accompanied by a significant reduction in blood pressure. Preliminary results indicate that raisins are associated with increased satiety and decreased food consumption over an 8‐h period, but the potential benefits of raisins for weight loss have not been assessed. Raisin intake is accompanied by reductions in serum low‐density lipoprotein cholesterol, triglycerides, and oxidized low‐density cholesterol values. Thus, raisins have the potential to significantly reduce the risk for developing diabetes or cardiovascular disease. 相似文献
837.
On the basis of these facts about freshwater fish and invertebrates: (i) the Na+ turnover is a physiological process associated with the gill membranes; (ii) the key mechanism of acute silver toxicity consists of reduction in Na+ uptake by blockade of gill Na+,K+-ATPase; (iii) the mass-specific surface area of the gills depends on animal body mass; and (iv) the gill surface is also the major site of Na+ loss by diffusion, we hypothesized that whole body Na+ uptake rate (i.e., turnover rate) and secondarily body mass would be good predictors of acute silver toxicity. Results obtained from toxicological (LC50 of AgNO3) and physiological (22Na uptake rate) tests performed on juvenile fish (rainbow trout, Oncorhynchus mykiss), early juvenile and adult crayfish (Cambarusdiogenes diogenes), and neonate and adult daphnids (Daphnia magna) in moderately hard water of constant quality support the above hypothesis. Therefore, sensitivity to AgNO3, in terms of either total measured silver or free Ag+, was reliably predicted from the whole body Na+ uptake rate in animals with body mass ranging over 6 orders of magnitude (from micrograms to grams). A positive log-log correlation between acute AgNO3 toxicity and body mass of the same species was also observed. Furthermore, the whole body Na+ uptake rate was inversely related to body mass in unexposed animals. The combination of these last two results explains why the small animals in this study were more sensitive to Ag+ than the larger ones. Taken together, these results clearly point out the possibility of incorporating the Na+ uptake rate into the current version of the Biotic Ligand Model to improve the predictive capacity of this model. In the absence of information on Na+ uptake rate, then body mass may serve as a surrogate. 相似文献
838.
Ogliari PJ de Andrade DF Pacheco JA Franchin PR Batista CR 《Journal of food protection》2007,70(8):1933-1936
The main goal of the present study was to discuss the application of the McNemar test to the comparison of proportions in dependent samples. Data were analyzed from studies conducted to verify the suitability of replacing a conventional method with a new one for identifying the presence of Salmonella. It is shown that, in most situations, the McNemar test does not provide all the elements required by the microbiologist to make a final decision and that appropriate functions of the proportions need to be considered. Sample sizes suitable to guarantee a test with a high power in the detection of significant differences regarding the problem studied are obtained by simulation. Examples of functions that are of great value to the microbiologist are presented. 相似文献
839.
A wide range of literature streams and methods were examined for this research, including sustainability, integrated resource planning, and construction of portfolios of electricity generation technologies. The research then focused on current and emerging HPSTs (hydropower generation and storage technologies), and technical, economic, social, and environmental sustainability objectives associated with those technologies in the PNW (Pacific Northwest) region of the United States. Candidate technologies obtained from the literature were examined using the Delphi Method, and then rated according to their perceived impacts using the AHP (Analytical Hierarchy Process). GP (Goal Programming) was then used to determine an optimal mix of technologies to achieve sustainability objectives, using these weightings and assumptions related to specific scenarios regarding technology development, adoption, and availability. This research is important because few previous studies have systematically considered multiple objectives and criteria from multiple stakeholder experts for creating portfolios of sustainable electricity generation technologies. Previous research has also not comprehensively investigated the manner in which changing scenarios of technology development and availability rates may lead to various technological, economic, environmental, and social sustainability impacts with regard to planning of regional electrical generation and storage systems. 相似文献
840.
Michael R. A. Morgan Ruth McNerney Henry W.-S. Chan Peter H. Anderson 《Journal of the science of food and agriculture》1986,37(5):475-480
An indirect, double antibody enzyme-linked immunosorbent assay (ELISA) has been validated for application to pig kidney. The specificity of the anti-ochratoxin A antiserum employed is such that minimal sample preparation is required prior to assay, with consequent beneficial effects on sample throughput. Kidneys (303) obtained in the UK as being unsuitable for human consumption were examined for ochratoxin A content by ELISA. Of these, 191 (63%) contained no detectable toxin (the detection limit was 0.5 ng g?1). Only eight samples (2.6%) contained more than 5 ng g?1 ochratoxin A, and only two were above 10 ng g?1 (at 11.5 and 12.4 ng g?1). The significance of these findings is discussed. 相似文献