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   4713篇
  免费   256篇
  国内免费   290篇
  2023年   64篇
  2022年   79篇
  2021年   112篇
  2020年   131篇
  2019年   144篇
  2018年   91篇
  2017年   106篇
  2016年   121篇
  2015年   137篇
  2014年   234篇
  2013年   326篇
  2012年   191篇
  2011年   242篇
  2010年   159篇
  2009年   183篇
  2008年   243篇
  2007年   236篇
  2006年   208篇
  2005年   184篇
  2004年   185篇
  2003年   163篇
  2002年   134篇
  2001年   127篇
  2000年   122篇
  1999年   124篇
  1998年   82篇
  1997年   70篇
  1996年   75篇
  1995年   83篇
  1994年   77篇
  1993年   67篇
  1992年   57篇
  1991年   54篇
  1990年   43篇
  1989年   47篇
  1988年   44篇
  1987年   45篇
  1986年   50篇
  1985年   52篇
  1984年   34篇
  1983年   35篇
  1982年   42篇
  1981年   26篇
  1980年   42篇
  1979年   32篇
  1978年   35篇
  1977年   21篇
  1976年   20篇
  1975年   24篇
  1972年   16篇
排序方式: 共有5259条查询结果,搜索用时 515 毫秒
991.
Copper(II) chromate and dichromate complexes with 2,2'-bipyridyl and 1,10-phenathroline were tested for their mutagenic activity in the standard Ames test. All of six tested complexes exhibited markedly lower mutagenic activity than the reference compounds--potassium dichromate and sodium chromate. The blockage of Cr(VI) reduction capability in the presence of the complex Cu2+ ion and the competition between copper and chromium ions in the interaction with cellular components are discussed in the light of the results of our previous chemical study.  相似文献   
992.
 本文报道了凤眼莲对无锡电影胶片厂含银废水的净化生产性应用试验结果:(1)污水停留时间为49h时,氧化沟出水中的银净化率为98.0%到100%(未检出);(2)COD的去除率为54.58%;(3)混浊度的去除率为68.9%;(4)NH4+–N和PO43–的去除率分别为45.55%和34.3%。该法已被该厂采纳应用,并取代了原来的活性炭吸附和O3氧化三级处理,年节约运转费用12.5万元。  相似文献   
993.
本文对49例慢性子宫内膜炎、慢性宫颈炎等妇科疾病的病理切片,用金黄色葡萄球菌细菌型及L型的特异性杭体作酶免疫组化染色(PAP法)证明在金黄色葡萄球菌引起的各种妇科感染中,细菌型及L型混合感染占60%,单独细菌型感染占28.9%,单独L型感染占11.1%。本研究证明了金葡菌变异为L型后,有特异性杭原出现,并证明细菌L型的特异性抗原有一定的免疫学诊断价值。  相似文献   
994.
In the Vecht river plain, the vegetation diversity has been diminished during recent decades, due to hydrological changes and resulting eutrophication. Different options are discussed for hydrological management aimed at abating the eutrophication and at stimulating the characteristic mesotraphent succession phases of the fen vegetation. The ecological impact of the management plans are estimated with the hydro-ecological model ICHORS; this model takes into account the relations between the non-biotic environment and the presence of hydrophytes and phreatophytes. The impact of the hydrological options is assessed, after rearrangement of the predicted values of the species to values of the required fen vegetation. These results are incorporated in the management options with the water balances of the polders. Information on the costs of the options is used as a basis for a cost-benefit analysis. Technical hydrological solutions such as purification of the supplied water might contribute to the eutrophication abatement, but they are on their own unlikely to restore the ecosystems. Contrastingly, options involving the restoration of the original hydrology would probably lead to the characteristic mesotraphent fen vegetation once more, and the financial implications of this are likely to be the more favourable in the long-term.  相似文献   
995.
In order to search for radical scavengers which could be used as raw materials for cosmetics, phenyl propanoids (eugenol, isoeugenol, dehydrodieugenol, dehydrodieugenol B and coniferyl aldehyde) were examined for their hydroxyl radical (· OH) scavenging ability. A Fenton system was used to produce -OH. In order to see scavenging by these phenyl propanoids, competition reactions between a spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), and these phenyl propanoids for -OH were studied. The relative yield of the spin adduct of -OH (DMPO-OH) was measured by electron spin resonance spectroscopy. The approximate rate constants of the reactions between these phenyl propanoids and -OH estimated by measuring the reduced height of the ESR signals of DMPO-OH were found to be at least in the order of 109 M-1 s-1 (diffusion-controlled). Also, using the TBA tests, the reactions between ·OH and several compounds reactive with ·OH were investigated in the presence of the phenyl propanoids and it was found that the phenyl propanoids compete with such reactive compounds for ·OH. These results indicate that these phenyl propanoids can be used as antioxidants for skin damage perhaps caused by -OH generated by UV-light.  相似文献   
996.
Summary Molecular markers are currently being developed for Betula alleghaniensis Britton using random amplified polymorphic DNA (RAPD). Arbitrarily designed 11-mer primers were tested on three intraspecific controlled crosses for which more than 15 full-sibs were available. Using two of these primers, we were able to genetically characterize a total of nine polymorphic RAPD markers. Segregation of these markers was consistent with a biparental diploid mode of inheritance, and all appeared dominant. RAPDs were valuable in detecting contaminants and, therefore, in assessing the validity of controlled crosses. Limitations of the technique are discussed in relation to the determination of parental genotypes and construction of linkage maps for hardwood species.  相似文献   
997.
The mouselpr gene, which is an autosomal recessive gene causing autoimmune disease with features of human systemic lupus erythematosus and eventually death from severe immune-complex glomerulonephritis, has been mapped on chromosome 19. To determine its exact chromosomal location, a three-point backcross was carried out by mating (MRL/MpJ-lpr/lpr × MOL-MIT)F1 × MRL/MpJ-lpr/lpr using the genesLy-44 (lymphocyte differentiation antigen-44) andTdt (terminal deoxynucleotidyl transferase) as markers. The following order of genes is proposed, with the distances between genes given in parentheses: centromere-Ly-44 (19.3 cM)-lpr (6.1 cM)-Tdt-telomere. TheLy-44 a andTdt a alleles are found in all laboratory strains and in the wild Western European subspecies,domesticus andbrevirostris. In contrast, theLy-44 b andTdt b alleles are found in some Asian subspecies, Chinese mice of wild origin,yamashinai andmolossinus. Furthermore the thirdTdt allele,Tdt c , is detected incastaneus.Some of the data in this study were previously presented at the 4th Mouse Gene Mapping Workshop, Annapolis, Maryland, in November 1990.  相似文献   
998.
999.
Sequential Monte Carlo p-values   总被引:4,自引:0,他引:4  
BESAG  JULIAN; CLIFFORD  PETER 《Biometrika》1991,78(2):301-304
  相似文献   
1000.
Capillary gas chromatography indicated the presence of (Z)-7-dodecenyl alcohol and (Z)-7-dodecenyl acetate in the abdominal tip extracts of female Cornutiplusia circumflexa (L.) (Lepidoptera: Noctuidae: Plusiinae) moths. Gas chromatography combined with mass spectrometry and dimethyl disulfide derivatization confirmed the structure of these two principal pheromone components. In a flight-tunnel bioassay, a 5:1 blend of alcohol and acetate elicited the complete courtship sequence by males. The response was comparable to that evoked by a live virgin female, but males spent significantly longer time at calling females than at the synthetic source. The 5:1 blend was also attractive to male C. circumflexa in the field, as indicated by catches in traps baited with this mixture. (Z)-7-dodecenyl alcohol alone was slightly attractive whereas (Z)-7-dodecenyl acetate alone was completely inactive. C. circumflexa is the first reported Plusiinae species which utilizes (Z)-7-dodecenyl alcohol as the major pheromone component.  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