全文获取类型
收费全文 | 54439篇 |
免费 | 4451篇 |
国内免费 | 4篇 |
出版年
2023年 | 257篇 |
2021年 | 468篇 |
2020年 | 384篇 |
2019年 | 383篇 |
2018年 | 977篇 |
2017年 | 1025篇 |
2016年 | 1162篇 |
2015年 | 1261篇 |
2014年 | 1406篇 |
2013年 | 2300篇 |
2012年 | 3579篇 |
2011年 | 3849篇 |
2010年 | 2036篇 |
2009年 | 1464篇 |
2008年 | 3153篇 |
2007年 | 3301篇 |
2006年 | 3022篇 |
2005年 | 2718篇 |
2004年 | 2595篇 |
2003年 | 2487篇 |
2002年 | 2442篇 |
2001年 | 1746篇 |
2000年 | 1957篇 |
1999年 | 1142篇 |
1998年 | 540篇 |
1997年 | 438篇 |
1996年 | 522篇 |
1995年 | 447篇 |
1994年 | 471篇 |
1993年 | 432篇 |
1992年 | 551篇 |
1991年 | 489篇 |
1990年 | 435篇 |
1989年 | 419篇 |
1988年 | 408篇 |
1987年 | 392篇 |
1986年 | 337篇 |
1985年 | 435篇 |
1984年 | 502篇 |
1983年 | 432篇 |
1982年 | 385篇 |
1981年 | 367篇 |
1980年 | 313篇 |
1979年 | 344篇 |
1978年 | 338篇 |
1977年 | 316篇 |
1976年 | 297篇 |
1975年 | 339篇 |
1974年 | 299篇 |
1973年 | 262篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Gabriel Bek? Charles J. Weschler Sarka Langer Michael Callesen J?rn Toftum Geo Clausen 《PloS one》2013,8(4)
Total daily intakes of diethyl phthalate (DEP), di(n-butyl) phthalate (DnBP), di(isobutyl) phthalate (DiBP), butyl benzyl phthalate (BBzP) and di(2-ethylhexyl) phthalate (DEHP) were calculated from phthalate metabolite levels measured in the urine of 431 Danish children between 3 and 6 years of age. For each child the intake attributable to exposures in the indoor environment via dust ingestion, inhalation and dermal absorption were estimated from the phthalate levels in the dust collected from the child’s home and daycare center. Based on the urine samples, DEHP had the highest total daily intake (median: 4.42 µg/d/kg-bw) and BBzP the lowest (median: 0.49 µg/d/kg-bw). For DEP, DnBP and DiBP, exposures to air and dust in the indoor environment accounted for approximately 100%, 15% and 50% of the total intake, respectively, with dermal absorption from the gas-phase being the major exposure pathway. More than 90% of the total intake of BBzP and DEHP came from sources other than indoor air and dust. Daily intake of DnBP and DiBP from all exposure pathways, based on levels of metabolites in urine samples, exceeded the Tolerable Daily Intake (TDI) for 22 and 23 children, respectively. Indoor exposures resulted in an average daily DiBP intake that exceeded the TDI for 14 children. Using the concept of relative cumulative Tolerable Daily Intake (TDIcum), which is applicable for phthalates that have established TDIs based on the same health endpoint, we examined the cumulative total exposure to DnBP, DiBP and DEHP from all pathways; it exceeded the tolerable levels for 30% of the children. From the three indoor pathways alone, several children had a cumulative intake that exceeded TDIcum. Exposures to phthalates present in the air and dust indoors meaningfully contribute to a child’s total intake of certain phthalates. Such exposures, by themselves, may lead to intakes exceeding current limit values. 相似文献
72.
73.
74.
75.
Continuous coenzyme dependent stereoselective synthesis of sulcatol by alcohol dehydrogenase 总被引:2,自引:0,他引:2
Summary 6-methyl-5-hepten-2-one was reduced to sulcatol ((+)-6-methyl-5-hepten-2-ol) by using alcohol dehydrogenase fromThermoanaerobium brockii in a continuous process. The cofactor NADP(H) was retained by a charged UF-membrane and regenerated by oxidation of isopropanol to acetone. Use of native NADP in a charged UF-membrane reactor proved to be superior to use of PEG coupled NADP in a uncharged UF-membrane reactor. 相似文献
76.
V Bocci K von Bremen F Corradeschi E Luzzi L Paulesu 《Journal of biological regulators and homeostatic agents》1991,5(4):121-124
No one has ever doubted that maternal milk, in comparison to formula milk, has a far superior nutritional value. Colostrum has a well acknowledge crucial value for the survival of the animal species that cannot receive immunoglobulins through the placenta. Until recently the presence of cytokines in colostrum was unsuspected but it has been now clarified that normally there are at least four cytokines, namely interleukin 1 and 6, tumor necrosis factor and interferon gamma, that may exert an important immunostimulatory role particularly on the oropharyngeal-associated lymphoid tissue. As a corollary, physiological concentration of cytokines administered per os may exert a useful adjuvant activity in aged or immunodeficient people. 相似文献
77.
78.
79.
Summary Saruplase — a recombinant single-chain urokinase-type plasminogen activator was identified immunohistochemically in normal rat tissue after intravenous administration by means of a polyclonal antibody. For this purpose, rat tissues were fixed in various ways (liquid nitrogen, ethanol, formaldehyd solution). Saruplase could be detected by the PAP method, streptavidinbiotin system and indirect immunofluorescence in the kidney (proximal tubule), liver (hepatocytes, Kupffer cells) and spleen (reticular cells). Saruplase was not localized in the rat endothelium. It is discussed that the ratspecific receptors for urokinase-type plasminogen activator on endothelial cells cannot bind Saruplase due to the extreme species specificity. 相似文献
80.
1. Ecosystem processes depend on the biomass of the involved organisms, but their functional diversity may play an additional role. In particular, the exclusion of key functional groups through habitat disturbance may lead to the breakdown of ecosystem functions. Dung removal is an important process contributing to nutrient cycling and thus productivity in grazed ecosystems. 2. This study investigated the role of different functional groups of dung beetles in dung removal in different habitats within a wood-pasture in two different seasons. An experimental setting with 12 blocks and 108 dung pads was used to investigate short-term dung removal over 1 week of exposure. 3. Dung removal was most strongly affected by habitat type, with almost 40% lower levels in grassland than in adjacent forest and forest gaps. Of all assemblage characteristics, total biomass of tunneller species was the strongest predictor of dung removal, whereas functional diversity showed no significant effect. In accordance with the dung removal pattern at habitat type level, densities of large tunnellers were suppressed in grassland compared with forest. 4. It is concluded that dung removal is habitat-specific and large tunnellers play a disproportionate role in this important ecosystem function in temperate forests. 相似文献