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   6269篇
  免费   95篇
  国内免费   8篇
电工技术   77篇
综合类   5篇
化学工业   1644篇
金属工艺   72篇
机械仪表   168篇
建筑科学   225篇
矿业工程   10篇
能源动力   266篇
轻工业   996篇
水利工程   72篇
石油天然气   28篇
无线电   427篇
一般工业技术   965篇
冶金工业   268篇
原子能技术   30篇
自动化技术   1119篇
  2024年   91篇
  2023年   70篇
  2022年   209篇
  2021年   286篇
  2020年   200篇
  2019年   230篇
  2018年   236篇
  2017年   212篇
  2016年   276篇
  2015年   208篇
  2014年   283篇
  2013年   484篇
  2012年   390篇
  2011年   457篇
  2010年   303篇
  2009年   340篇
  2008年   310篇
  2007年   270篇
  2006年   220篇
  2005年   177篇
  2004年   155篇
  2003年   120篇
  2002年   118篇
  2001年   59篇
  2000年   59篇
  1999年   46篇
  1998年   79篇
  1997年   58篇
  1996年   53篇
  1995年   48篇
  1994年   41篇
  1993年   38篇
  1992年   28篇
  1991年   22篇
  1990年   22篇
  1989年   18篇
  1988年   20篇
  1987年   18篇
  1986年   8篇
  1985年   11篇
  1984年   19篇
  1983年   10篇
  1982年   12篇
  1981年   6篇
  1980年   11篇
  1979年   5篇
  1977年   8篇
  1976年   9篇
  1975年   7篇
  1974年   5篇
排序方式: 共有6372条查询结果,搜索用时 0 毫秒
61.
The main objective of this study was to evaluate the risk factors for late embryonic loss (LEL) in supplemented grazing dairy cows. Additional objectives were to assess the incidence of LEL and its association with the reproductive performance of cows. A data set containing productive, reproductive, and health records of 13,551 lactations was used. A retrospective case-control study involving 631 cows with LEL (cases) and 2,524 controls (4 controls per case within each study year) was run. A case of LEL was defined when the embryo had no heartbeat or there was evidence of detached membranes or floating structures including embryo remnants by ultrasonography (US) at 28 to 42 d post-artificial insemination (AI), whereas a non-case was defined as a cow diagnosed with positive pregnancy by US 28 to 42 d post-AI and reconfirmed as pregnant 90 ± 7 d post-AI. Four controls per case were randomly selected from the non-cases with a temporal matching criterion (±3 d around the date of the fecundating AI of the case). Multivariable logistic models were offered with the following predictors: year of LEL (2011 through 2015), season of LEL (summer vs. fall vs. winter vs. spring), parity (1 vs. 2 vs. ≥3), uterine disease (UD), non-uterine disease (NUD), body condition score at parturition, body condition score at 28 to 42 d post-AI (BCS-LEL), days in milk (DIM), and daily milk yield (MY). Statistical significance was set at P < 0.05 and a tendency was set at P ≤ 0.10. We found that 4.7, 22, and 23% of cows had LEL, UD, and NUD, respectively. Cases tended to have higher daily MY than controls (32.5 vs. 31.8 kg); also, cases had much longer calving to pregnancy interval (226 vs. 118 d), lower hazard of pregnancy [hazard ratio = 0.39, 95% confidence interval (CI) = 0.35–0.43], and higher odds for non-pregnancy [odds ratio (OR) = 2.89, 95% CI = 2.37–3.54] than controls. We found that the odds for LEL increased with parity number (OR = 2.48, 95% CI = 1.99–3.08 for parity ≥3) and with BCS-LEL <2.50 (OR = 1.81, 95% CI = 1.33–2.47). Conversely, the odds for LEL decreased with BCS-LEL >3.00 (OR = 0.70, 95% CI = 0.53–0.91). The odds for LEL increased with UD (OR = 1.23, 95% CI = 1.01–1.49), NUD (OR = 1.24, 95% CI = 1.01–1.54), DIM (OR = 1.03, 95% CI = 1.00–1.05), and daily MY (OR = 1.14, 95% CI = 1.04–1.25) in univariable models only. Finally, the odds for LEL were not associated with year, season, DIM, and body condition score at parturition. In conclusion, LEL is associated with extended calving to pregnancy interval, and among its risk factors are parity number and BCS-LEL.  相似文献   
62.
Chemical composition and fatty acid profiles were determined in Caiman yacare meat originating from the neck and tail cuts of animals raised in the wild (n = 6) or in captivity (n = 6), slaughtered at a live weight of approximately 6 kg. All experimental methods were approved by the appropriate environmental protection agencies. Most chemical components were affected by the origin-cut interaction, with the tail cut of wild animals having the highest amount of intramuscular fat (19.2% of DM) and the lowest of moisture (71.8%) and protein (77.0% of DM). Yacare meat had low amounts of SFA (35.1%), which were similar (P > 0.05) in the cuts and origins studied. The total amount of PUFA was higher (P < 0.05) in wild (31.0%) than in captive animals (23.6%), and n − 3 fatty acids had means of about 5% and 2% for the same groups, respectively (P < 0.05). In general, the FA profile of intramuscular fat in yacare meat had a desirable PUFA/SFA ratio above 0.4.  相似文献   
63.
Low fat dry fermented sausages were manufactured using controlled ripening conditions and a slow fermented process. The effect of fat content and ripening time on the chemical, colour, texture parameters and sensory acceptability was studied. The fat reduction in slow fermented sausages produced an increase in the pH decline during the first stage of the process that was favoured by the higher water content of the low fat sausages. Fat reduction did not affect the external appearance and there was an absence of defects but lower fat content resulted in lower sausage lightness. The sausage texture in low fat sausages caused an increase in chewiness and at longer ripening times, an increase in hardness. The sensory acceptability of the fermented sausages analyzed by internal preference mapping depended on the different preference patterns of consumers. A group of consumers preferred sausages with high and medium fat content and high ripening time. The second group of consumers preferred sausages with low ripening time regardless of fat content except for the appearance, for which these consumers preferred sausages of high ripening time. Finally, the limit to produce high acceptability low fat fermented sausages was 16% fat content in the raw mixture that is half the usual content of dry fermented sausages.  相似文献   
64.
We present characterisation data for two Spanish autochthonous grapevines, Bobal and Crujidera, in comparison with the well-known cultivars Tempranillo and Cabernet Sauvignon. Microsatellite markers were used for the molecular characterisation of Crujidera grapevines. Leaf macronutrient contents of the four cultivars were evaluated, as well as their changes at different vine developmental stages, and veraison was seen as the most suitable time to evaluate the nutritional status. Quantitative changes in some physiological parameters and the phenolic composition of the four grape varieties were measured during the last month of ripening. Polyphenols, tannins and anthocyanins increased with grape maturation, although the accumulation of these phenolic compounds and their patterns of evolution varied considerably with the cultivar. The biosynthetic potential of these grapes to produce resveratrol largely depended on the grape variety, with a remarkably high content found in Bobal berry skins.  相似文献   
65.
Two trials were conducted to determine the sugars, fructans and starch in oat plants (Avena sativa L) and the changes occurring in these carbohydrate fractions during field drying in the haymaking process. The composition of oat herbage in non-structural carbohydrates varied markedly between years; the contents of sugars, fractals and starch in both leaf blade and non-leaf blade materials being nether in 1990–1991 than in 1991–1992. Field drying produced a significant decrease (P < 0.05) in the amount of soluble carbohydrates. For the whole aerial plant parts, the decline in the fructans concentration ranged between 42.8 and 38.2% and that of sugars between 16.5 and 5.8%. The total losses of sugars and fractions in hay, expressed as losses of dry matter, ranged from 88 to 63 g kg?1 depending on the year.  相似文献   
66.
The fatty acid composition of the seed oils of six pomegranate (Punica granatum L) cultivars was qualitatively and quantitatively determined by gas chromatography. The seeds contained oil in the range 51-152 mg kg?1 dry matter. Intervarietal differences in fatty acid composition were demonstrated (fatty acid esters as % (w/w) total fatty acid esters. Sour varieties had the highest while sour-sweet varieties had the lowest oil content. Eleven fatty acids were identified. In all varieties, the predominant fatty acids were linoleic (25.2-38.6%) and oleic acid (24.8-35.5%) followed by palmitic (18.2-22.6%), stearic (6.9-10.4%) and linolenic acid (0.6-9.9%). To a lesser extent arachidic (1.1-3.4%) and palmitoleic acid (0.2-2.7%) were also found in all varieties. Lauric, myristic, behenic and lignoceric acids were rarely detected. As far as we know linolenic (18:3), arachidic (20:0), behenic (22:0) or lignoceric (24:0) acids have not been previously reported in the seed oils of edible pomegranate varieties. Lignoceric acid has not previously been found in pomegranate seed oil.  相似文献   
67.
The study for the elaboration and certification of four olive oil reference materials covering all the physical–chemical characteristics, defined in Commission Regulation (EC) 1989/2003, is reported herein. The different steps of the process, preparation of reference materials, homogeneity and stability studies, and characterization study have been carried out for 14 laboratories from five European countries. The certificate of analysis for these certified reference materials (CRMs) provide assigned values for concentrations of more than 50 parameters or characteristics of olive oils in compliance with the European legislation about the labeling of food products. The certified values and its corresponding uncertainties were obtained applying robust statistics to the raw data provided by the laboratories taking part in the interlaboratory study. The certificate is valid for a maximum period of 18 months from the elaboration date. These four CRMs are intended for use as a primary material for quality control and for validation of analytical methods for measurements in authentication and adulteration problems of olive oils.  相似文献   
68.
In this work a novel amperometric biosensor for fructose determination in solutions was developed. The device was constructed by the incorporation of a tetrathiofulvalene-tetracyanoquinodimethane organic conducting salt and fructose dehydrogenase enzyme, include in a polymeric matrix of epoxy resin and graphite powder. Because of the electrocatalytic function of the salt, the direct transfer of the electron between the reduced prosthetic group (PQQH2) of the enzyme and the transducing material, was verified at a low working potential (150 mV vs. Ag/AgCl), where the interfering reactions were minimized. The response time at 90% of the steady state value was less than 20 s. The current response was directly proportional to the D-fructose concentration from 0.01 to 0.3 mmol/l with a detection limit of 0.005 mmol/l (signal/noise of 3) and a sensitivity of 1.9985 μA/mmol. The biosensor sensitivity diminishes when its surface is not polished between successive determinations, and remains constant (rsd=1.85, n=10) when the surface is polished between determinations. The effects of temperature and pH on the biosensor response were studied and analyzed; also the properties of the enzyme (Km ap, I max, Q10) were determinate in this work. The biosensor was used to determine fructose in high fructose syrups and there were not significant differences between these results and those obtained by HPLC (p≤0.05). During 4 months, in intermittent determinations the biosensor kept 100% of its original sensitivity and after 18 months stored at 4°C, it only lost 32% of its sensitivity. The simplicity, low working potential, high stability and good performance of this biosensor shows a great potential for its use in the fructose determination.  相似文献   
69.
The effect of adding sucrose (5–25% w/w) and aspartame (0.04–0.16% w/w) on the compression resistance of three hydrocolloid gelled systems: κ‐carrageenan, gellan gum and κ‐carrageenan/locust bean gum at three different concentrations (0.3, 0.75 and 1.2% w/w) was studied. Sucrose addition increased true rupture stress in the three‐gelled systems, this effect being stronger in gellan gels. The deformability modulus increased with sucrose concentration in gellan gels, but not in the other systems. Rupture stress and deformability modulus increased with the addition of sucrose only in the harder gels (0.75 and 1.2% w/w). The effect of sucrose addition on the true rupture strain was significant but, in general, not important, mainly for lower gum concentrations. Aspartame addition did not affect the compression parameters.  相似文献   
70.
Concerns regarding the presence of drug residues in foods include allergic reactions, toxicity, technological problems in fermented products and the development of antibiotic resistance in human pathogens. The analysis of antimicrobial residues in foods is generally carried out, in a first step, through microbiological screening tests. These tests commonly use Geobacillus stearothermophilus as target specie but show a low ability to detect quinolones. The goal of our study was to evaluate the performance of a new microbiological test (Equinox) for detection of quinolone residues in muscle. The kit contains an ampoule with a standardized number of freeze-dried Escherichia coli and must be diluted with a specific detection medium containing a redox indicator. Microbial growth will modify the redox potential of the medium being observed through a colour change (from blue to brown/orange). Equinox limits of detection for most of tested quinolones (enrofloxacin, norfloxacin, sarafloxacin, marbofloxacin, ciprofloxacin, danofloxacin and difloxacin) were below or around maximum residue limit (MRL) UE and CCβ values obtained corresponded with the determined sensitivities. In contrast, flumequine could not be detected at MRL UE levels. Moreover, Equinox displayed a low sensitivity to other antimicrobials. Sensitivity data obtained in vitro were consistent when testing incurred muscle samples. Matrix constituents, test batch and animal species did not affect the performance of the test. Equinox could be easily automated enabling a large numbers of simultaneous analysis, and a photometric reading can be applied for a precise interpretation. The results obtained in this study prove that Equinox is a useful tool when screening for quinolone residues or can be combined with other methods for screening of unknown samples.  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