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   23185篇
  免费   332篇
  国内免费   122篇
电工技术   497篇
综合类   64篇
化学工业   3165篇
金属工艺   939篇
机械仪表   657篇
建筑科学   545篇
矿业工程   67篇
能源动力   599篇
轻工业   1832篇
水利工程   212篇
石油天然气   179篇
武器工业   1篇
无线电   2587篇
一般工业技术   4067篇
冶金工业   6133篇
原子能技术   419篇
自动化技术   1676篇
  2021年   149篇
  2020年   114篇
  2019年   157篇
  2018年   229篇
  2017年   229篇
  2016年   255篇
  2015年   177篇
  2014年   296篇
  2013年   819篇
  2012年   507篇
  2011年   774篇
  2010年   558篇
  2009年   621篇
  2008年   715篇
  2007年   745篇
  2006年   629篇
  2005年   718篇
  2004年   587篇
  2003年   634篇
  2002年   675篇
  2001年   670篇
  2000年   599篇
  1999年   614篇
  1998年   2159篇
  1997年   1403篇
  1996年   1043篇
  1995年   718篇
  1994年   618篇
  1993年   642篇
  1992年   358篇
  1991年   344篇
  1990年   341篇
  1989年   343篇
  1988年   292篇
  1987年   267篇
  1986年   245篇
  1985年   275篇
  1984年   215篇
  1983年   200篇
  1982年   185篇
  1981年   217篇
  1980年   203篇
  1979年   164篇
  1978年   152篇
  1977年   277篇
  1976年   385篇
  1975年   127篇
  1974年   109篇
  1973年   117篇
  1972年   90篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The aim of this study was to determine the role of cold condensation and fractionation on the occurrence of organochlorine contaminants (OCs) in avian marine top predators along a latitudinal gradient. We measured 24 polychlorinated biphenyl (PCB) congeners and six pesticide OCs in blood of great black-backed gulls (Larus marinus) from the Norwegian Coast (58 degrees N-70 degrees N) and glaucous gulls (Larus hyperboreus) from Bjornoya in the Norwegian Arctic (74 degrees N). Glaucous gulls had up to 3 times higher sigmaOC concentrations compared to the great black-backed gulls, and a OC pattern dominated largely by persistent and low volatile compounds such as highly chlorinated PCBs and metabolites such as oxychlordane. This was not consistent with cold condensation and fractionation theory, but probably related to diet and elevated biomagnification. Among great black-backed gulls, however, there were indications of both cold condensation and fractionation. Higher and lower chlorinated PCBs had highest absolute concentrations in the south and in the north, respectively, except for one location at an intermediate latitude, where concentrations of most OCs exceeded all other locations. In terms of proportional contribution to sigmaOC (pattern), relatively volatile OCs such as HCB, oxychlordane and tri- to penta- PCB congeners were more important at northern latitudes, while hexa- to nona-PCBs made up a larger proportion of sigmaOC in the south. The results thus showed that differences in global distribution of compounds with different physicochemical properties could be detected in avian top predators such as large gulls, even if biomagnification and biotransformation influence both the absolute concentrations and the patterns of OCs.  相似文献   
992.
Identification of anthocyanins in the wine made of the Croatian autochthonous grape variety of Babić (Vitis vinifera L.) was carried out and their profile was determined by means of high-performance liquid chromatography with diode array and mass spectrometric detection. Dependence of anthocyanins content and profile on maceration treatment conditions was investigated. Statistically significant differences of anthocyanins concentration in wines Babić produced by various maceration treatments were confirmed by the use of multivariate analysis of variances. The investigation results indicated that the maceration temperature exerts higher influence on anthocyanins concentration than the duration of maceration. In addition, on the basis of anthocyanin composition and using different multivariate statistical analyses, differentiation of wines Babić according to maceration treatments was procured.  相似文献   
993.
Zusammenfassung. Seit Jahren ist bekannt, dass eine Vielzahl von Substanzen hormonartige Wirkungen besitzen, wobei die estrogene Wirksamkeit im Mittelpunkt des Interesses steht. Eine potentielle Gesundheitsgef?hrdung wird zwar kontrovers diskutiert, es bestehen jedoch durchaus Hinweise, die Einflüsse auf die Gesundheit und Entwicklung des Menschen durch hormonell wirksame Stoffe vermuten lassen. Da derartige Substanzen auch in Lebensmitteln enthalten sein k?nnen, wurde im Lebensmittelinstitut Braunschweig ein biologisches, zellgestütztes Testsystem (E-Screen) zum Nachweis estrogener Wirksamkeit etabliert. In vorliegendem Artikel werden die bisherigen Untersuchungen dargestellt. Neben 17 lebensmittelrelevanten Einzelsubstanzen (Herbizide, Fungizide, Insektizide) wurden 7 Trinkwasserproben sowie 37 Mineral- und Tafelwasserproben in den E-Screen eingesetzt. In weiterführenden Untersuchungen wurden 12 Quellwasserproben sowie 10 dazugeh?rige Mineralwasserproben mit dem Bioassay analysiert. Von den untersuchten Einzelsubstanzen erwiesen sich die Herbizide Nitrofen und Pendimethalin sowie das Fungizid Fenarimol als partielle Agonisten, das Insektizid Chlorpyriphos und das Fungizid Propiconazol sind schwache Agonisten. Alle untersuchten Trinkw?sser zeigten ein negatives Ergebnis. Von den 37 Mineral- und Tafelw?ssern wurden 8 Proben als estrogen wirksam eingestuft. In weiterführenden Untersuchungen zeigte sich, dass teilweise schon die Quellw?sser einen positiven Befund im E-Screen aufwiesen, sodass ein übergang estrogen wirksamer Substanzen von der Verpackung in das Wasser eher unwahrscheinlich ist. Bei einer Mineralwasserprobe jedoch wurde nach einer Lagerdauer von 6,5 Monaten eine Erh?hung der estrogenen Potenz festgestellt, was für einen übertritt entsprechender Stoffe von der Verpackung in das Wasser spricht. In weiterführenden physikalisch-chemischen Untersuchungen muss gekl?rt werden, welche Substanzen für die estrogene Wirksamkeit der W?sser verantwortlich sind.

Eingegangen: 24. Mai 2006  相似文献   
994.
The antioxidant activities of peanut (Arachis hypogaea L.) hull extracts were evaluated after far-infrared (FIR) radiation or heat treatment. Peanut hulls in petri dishes were FIR-irradiated or heat-treated (150 °C) for 5, 10, 15, 20, 40 or 60 min. The water extracts (300 mg/10 mL) of peanut hulls (WEPH) were prepared and their total phenol contents (TPC), radical scavenging activity (RSA), and reducing power were determined. The antioxidant activities of WEPH increased as the time of heating or FIR-radiation increased. When peanut hulls were FIR-irradiated at 150 °C for 60 min, the values of TPC, RSA, and reducing power of WEPH increased from 72.9 to 141.6 μM, 2.34% to 48.83%, and 0.473 to 0.910, respectively, compared to the untreated controls. Heat treatment of peanut hull under the same conditions (150 °C for 60 min) also increased the TPC, RSA, and reducing power of WEPH from 72.9 to 90.3 μM, 1.90% to 23.69%, and from 0.471 to 0.718, respectively. The result indicated that FIR-radiation or heat treatment on peanut hulls increased the antioxidant activities of WEPH.  相似文献   
995.
精细粗梳工艺生产高档纱   总被引:3,自引:0,他引:3  
AIF-Nr.14827N是德国联邦工业合作研究会(AIF)的一项研究课题,旨在研发出一种既能提高短纤维排除率又能减少棉结的粗梳工艺,可在生产线密度20tex以上的纱线时省去精梳工艺。  相似文献   
996.
997.
Data on health, management, and housing from birth to first calving were collected for 2,126 heifers on semi-monthly visits made by project veterinarians to 107 dairy herds from southwest Sweden. Additional data were obtained from the official milk- and health-recording program. Factors associated with incidence of veterinarian-treated clinical mastitis (VTCM) in the period 7 d before (d −7) to 30 d after first calving and of elevated cow composite somatic cell count (SCC, ≥200,000 cells/mL) at first test milking after first calving, respectively, were investigated using a 2-level (animal and herd) logistic regression analysis after initial screening by univariate analyses. The incidence risk of VTCM during the complete first lactation (305 d) was 10.8%. Ten percent of the diseased animals had more than 1 case of VTCM and 51% of total cases occurred from −7 to 30 d postcalving. The incidence rate of VTCM during the complete first 305-d lactation was 1.13 cases per 100 cow-mo. In total, 18.1% of the animals had elevated SCC at first test milking (mean 21 d) after calving. Veterinarian-treated clinical mastitis at −7 to 30 d postcalving was associated with higher overall incidence of mastitis in the herd and with reproductive disorders (i.e., retained placenta, endometritis, pyometra, dystocia, or twin birth). The risk of elevated SCC increased with increasing percentage of cows in the herd that, some time during the year, had had an increased udder disease score (chronically increased SCC). Other factors associated with increased risk of elevated SCC were increasing amounts of concentrates fed to 11- to 16-mo-old heifers, moving to confined housing the day of calving instead of earlier, and use of restraint measures at milking. In addition, growth rate from birth to weaning, and several feed-related variables (e.g., amount of concentrates and type of roughage given) were associated with VTCM at −7 to 30 d post-calving or elevated SCC at first test milking in the univariate analyses.  相似文献   
998.
The objective was to study, by simulation, whether survival analysis results in a more precise genetic evaluation for mastitis in dairy cattle than cross-sectional linear models and threshold models by using observation periods for mastitis of 2 lengths (the first 150 d of lactation, and the full lactation, respectively). True breeding values for mastitis liability on the underlying scale were simulated for daughters of 400 sires (average daughter group size, 60 or 150), and the possible event of a mastitis case within lactation for each cow was created. For the linear models and the threshold models, mastitis was defined as a binary trait within either the first 150 d of lactation or the full lactation. For the survival analysis, mastitis was defined as the number of days from calving to either the first case of mastitis (uncensored record) or to the day of censoring (i.e., day of culling, lactation d 150 or day of next calving; censored record). Cows could be culled early in lactation (within 10 d after calving) for calving-related reasons or later on because of infertility. The correlation between sire true breeding values for mastitis liability and sire predicted breeding values was greater when using the full lactation data (0.76) than when using data from the first 150 d (0.70) with an average of 150 daughters per sire. The corresponding results were 0.60 and 0.53, respectively, with an average of 60 daughters per sire. Under these simulated conditions, the method used had no effect on accuracy. The higher accuracy of sire breeding values can be translated into a greater genetic gain, unless counteracted by a longer generation interval.  相似文献   
999.
The objective of this study was to determine the effects of feeding an increased amount of extruded flaxseed with high proportions of n-3 fatty acids (FA) to transition dairy cows on performance, energy balance, and FA composition in plasma, adipose tissue, and milk fat. Multiparous Israeli-Holstein dry cows (n = 44) at 256 d of pregnancy were assigned to 2 treatments: (1) control cows were fed prepartum a dry-cow diet and postpartum a lactating-cow diet that consisted of 5.8% ether extracts; and (2) extruded flaxseed (EF) cows were supplemented prepartum with 1 kg of extruded flaxseed (7.9% dry matter)/cow per d, and postpartum were fed a diet containing 9.2% of the same supplement. The EF supplement was fed until 100 d in milk. On average, each pre- and postpartum EF cow consumed 160.9 and 376.2 g of C18:3n-3/d, respectively. Postpartum dry matter intake was 3.8% higher in the EF cows. Milk production was 6.4% higher and fat content was 0.4% U lower in the EF group than in the controls, with no differences in fat and protein yields. Energy balance in the EF cows was more positive than in the controls; however, no differences were observed in concentrations of nonesterified fatty acids and glucose in plasma. Compared with controls, EF cows had greater proportions of C18:3n-3 in plasma and adipose tissue. The proportion of n-3 FA in milk fat was 3.7-fold higher in the EF cows, and the n-6:n-3 ratio was decreased from 8.3 in controls to 2.3 in the EF cows. Within-group tests revealed that the C18:3n-3 content in milk fat in the EF cows was negatively correlated with milk fat percentage (r = –0.91) and yield (r = –0.89). However, no decrease in de novo synthesis of less than 16-carbon FA was found in the EF group, whereas C16:0 yields were markedly decreased. It appears that the enrichment of C18:3n-3 in milk fat was limited to approximately 2%, and the potential for increasing this n-3 FA in milk is higher for cows with lower milk fat contents. In conclusion, feeding increased amounts of C18:3n-3 during the transition period enhanced dry matter intake postpartum, increased milk production, decreased milk fat content, and improved energy balance. Increased amounts of EF considerably influenced the FA profile of plasma, adipose tissue, and milk fat. However, the extent of C18:3n-3 enrichment in milk fat was limited and was negatively correlated with milk fat content and yield.  相似文献   
1000.
实施HACCP的必要基础程序(待续)   总被引:6,自引:0,他引:6  
为推动我国HACCP的实施,从GMP、化学品控制、清洗和消毒、微生物控制、水的安全、空气的安全、卫生设计、预防性维护、产品溯源和回收、虫害控制、接收、贮存和运输控制、供应商控制、食品安全培训、设备的校准、消费者食品安全的投诉及审核和监督程序16个方面阐述了必要基础程序所包含的具体内容,尤其是对生产控制中容易忽视的过敏原问题进行了较为详细的阐述。为食品生产企业建立食品安全保障体系提出了一些建议,可作为卫生监督员的现场审核的参考。  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