全文获取类型
收费全文 | 7859篇 |
免费 | 114篇 |
国内免费 | 120篇 |
专业分类
化学 | 387篇 |
力学 | 2篇 |
综合类 | 53篇 |
数学 | 48篇 |
物理学 | 37篇 |
综合类 | 7566篇 |
出版年
2024年 | 3篇 |
2023年 | 20篇 |
2022年 | 61篇 |
2021年 | 44篇 |
2020年 | 42篇 |
2019年 | 35篇 |
2018年 | 33篇 |
2017年 | 42篇 |
2016年 | 60篇 |
2015年 | 127篇 |
2014年 | 406篇 |
2013年 | 292篇 |
2012年 | 541篇 |
2011年 | 582篇 |
2010年 | 478篇 |
2009年 | 476篇 |
2008年 | 619篇 |
2007年 | 698篇 |
2006年 | 618篇 |
2005年 | 557篇 |
2004年 | 470篇 |
2003年 | 387篇 |
2002年 | 330篇 |
2001年 | 285篇 |
2000年 | 184篇 |
1999年 | 133篇 |
1998年 | 82篇 |
1997年 | 86篇 |
1996年 | 67篇 |
1995年 | 68篇 |
1994年 | 48篇 |
1993年 | 51篇 |
1992年 | 36篇 |
1991年 | 33篇 |
1990年 | 33篇 |
1989年 | 32篇 |
1988年 | 18篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
排序方式: 共有8093条查询结果,搜索用时 15 毫秒
81.
谷建军 《北京联合大学学报(自然科学版)》1998,(4)
运用文化语言学的理论,从汉语词汇的意义、结构和交际三个方面进行论证,提出词汇与文化具有“融合关系”、“渗透关系”和“制约关系”。这些关系说明文化对词汇的理解、运用和规范有着很大的影响,重视文化因素对全面灵活地理解词汇的意义、深入细致地了解词汇的结构、准确扎实地掌握词汇的运用技巧大有裨益。每一语言系统中有相当一部分词汇的内涵是以民族文化为依托,文化是这些词义产生、发展、衍变的关键。在汉语词汇系统中,表义的结构形式反映了汉民族的思维方式、特点,是它们形成了汉语词汇的构成规则。传统的伦理等级观念、约定俗成的民族表达习惯附加给词汇各种文化色彩的语用意义,这些多维的复合性文化使开放的词汇系统变得有序可循。 相似文献
82.
胡玉荣 《北京联合大学学报(自然科学版)》1998,(4)
中华民族有着五千年的光辉灿烂文化,从而铸就了优良的道德传统。中华民族传统美德不仅在历史上而且在现代都对中国广大人民群众思想的形成产生着重要影响。应认真研究传统美德的内涵及特征,继承和弘扬传统美德的意义,以及如何继承和弘扬传统美德。应营造社会大环境,加强道德教育,振兴中华民族精神,弘扬传统美德,发展社会主义共产主义道德。 相似文献
83.
从几个籼稻品种不同发育阶段的幼穗分离出密度平均为22×105个/mL~46×105个/mL的原生质体。将这些原生质体包埋在藻酸钠颗粒里并分别在粳稻愈伤组织滋养下培养和没有愈伤组织滋养。在这两种培养条件下原生质体稳定,分裂频率分别达到484%和388%,群体存活率达到53%和47%,愈伤组织绿苗分化率达到716%和623%。这两种方法优于常用的琼脂糖包埋法和琼脂糖颗粒法。 相似文献
84.
风信子子房的组织培养 总被引:1,自引:0,他引:1
通过组织培养技术可以从风信子(HyacinthusorientalisL.)子房诱导出完整植株.在我们的研究中,MS+BA5mg/L+NAA1mg/L较适于诱导愈伤组织和芽.适于芽生长的培养基是MS+BA2mg/L+NAA2mg/L.MS(不加任何外源激素)培养基适于诱导生根 相似文献
85.
A novel alkaloid with an unprecedented ring system consisting of a dodecahydro-7-oxa-9a-aza-benzo[a]azulene ring (1), has been isolated from cultures of Clitocybe concava (Basidiomycetae). Its structure and relative stereochemistry were elucidated on the basis of spectroscopic data. Concavine presents a weak antibacterial activity. 相似文献
86.
Brian A. Naughton Benson Sibanda Jory P. Weintraub Julia San Román Vafa Kamali 《Applied biochemistry and biotechnology》1995,54(1-3):65-91
A method of coculturing adult rat hepatic parenchymal cells (PC) and stromal cells in a three-dimensional framework of nylon
filtration screens or biodegradable polymer meshes was developed in our laboratory. Rat liver stroma, which includes vascular
and bile duct endothelial cells, fat-storing cells, fibroblasts, and Kupffer cells, were isolated by gradient centrifugation
afterin situ liver perfusion and expanded in monolayer culture prior to seeding onto nylon screens or bioresorbable polyglycolic acid
(PGA) polymers oriented into a felt-like construct. A second inoculum of freshly isolated PC was applied after the stromal
cells became established. Histological analyses revealed that PC proliferation occurred until all available space for expansion
within the template was exhausted. These cells retained their rounded morphology, and after 4–5 wk 7–9 “layers” of PC filled
the 140-μm deep template. Dioxin-inducible cytochrome P450 activity was detected for up to 58 d in culture, and albumin, fibrinogen,
transferrin, and soluble fibronectin were detected in the medium by enzyme-linked immunosorbent assay (ELISA) for 48 d in
vitro. Immunohistochemical analysis of sections through the cultures confirmed the presence of these proteins as well as cytokeratin
at the cellular level; the extracellular matrix stained for both collagen type III and laminin. Long-term PC proliferation
and function were enhanced by the presence of stromal cells as well as by a meshwork template whose geometry allows the interaction
of PC with stroma and matrix on several different planes. To permit transplantation, co-cultures of hepatic PC and stromal
cells were established on PGA felt constructs instead of nylon screens. After 24 d in vitro, these constructs were grafted
into sites in the mesentery, omentum, and sub-cutaneous tissues of adult Long-Evans rats. The growth of hepatocytes after
30 din situ was evident by histological analysis; grafts of co-cultures regenerated a liver-like architecture consisting of sinusoids
and putative biliary structures. In addition, PC at these extrahepatic graft sites were positive for albumin, transferrin,
and fibrinogen synthesis by immunohistochemistry. Graft survival was enhanced when recipients were subjected to 40% hepatectomy.
Hepatic PC:stromal cell cocultures may prove useful in the restoration of liver function either by direct transplantation
using PGA or similar templates, or as extracorporeal devices, using nylon screens. 相似文献
87.
Guillermo R. Castro Luis M. Ducrey Santopietro Faustino Siñeriz 《Applied biochemistry and biotechnology》1992,37(3):227-233
Within the frame of a screening program aimed at the isolation of amylolytic sporeformers, one strain with high amylolytic
activity designated MIR-23 was selected. The microbial characterization was carried out by morphological and biochemical tests
and, by means of statistical treatment, was identified asBacillus polymyxa. The organism could grow in acidic conditions (pH 5.0) on a starch medium and produce α-amylase, pullulanase, and α-glucosidase.
Batch cultures showed the highest enzyme activities in the stationary phase. Pullulanase activity exhibited an optimal temperature
of 52–57°C at pH 4.5–5.5. These properties would allow its use in the saccharification processes in the starch industries. 相似文献
88.
Assessment of various carbon sources and nutrient feeding strategies for Panax ginseng cell culture 总被引:1,自引:0,他引:1
Ginseng (root of Panax ginseng C. A. Meyer) cells were cultivated on medium supplemented with various carbohydrates including sucrose, glucose, and fructose,
at initial concentrations ranging from 10 to 110 g/L. Sucrose was shown to be the superior carbon source to the monosaccharides
for ginseng cell growth and the optimal concentration was between 30 and 50 g/L. An increase in the initial concentration
within this range increased the maximum cell density and growth index significantly, whereas much higher concentrations inhibited
cell growth. Feeding of sucrose and some other medium components during the growth (fed-batch mode) was more effective in
enhancing the cell growth and biomass productivity, increasing the growth index by more than 60–70% and biomass productivity
by more than 50%. 相似文献
89.
Kin-Ho Law Pui-Ling Chan Wai-Sum Lau Yin-Chung Cheng Yun-Chung Leung Wai-Hung Lo Hugh Lawford Hoi-Fu Yu 《Applied biochemistry and biotechnology》2004,114(1-3):361-372
Plastic wastes constitute a worldwide environmental problem, and the demand for biodegradable plastics has become high. One
of the most important characteristics of microbial polyesters is that they are thermoplastic with environmentally degradable
properties. In this study, pUC 19/PHA was cloned and transformed into three different Escherichia coli strains. Among the three strains that were successfully expressed in the production of polyhydroxyalkanoates (PHA), E. coli HMS174 had the highest yield in the production of poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (P[HB-HV]). The cell dry
weight and PHA content of recombinant HMS174 reached as high as 10.27 g/L and 43% (w/w), respectively, in fed-batch fermentor
culture. The copolymer of PHA, P(HB-HV), was found in the cells, and the biopolymers accumulated were identified and analyzed
by gas chromatography, proton nuclear magnetic resonance spectroscopy, and differential scanning calorimetry. We demonstrated
clearly that the E. coli host for PHA production has to be carefully selected to obtain a high yield. The results obtained indicated that a superior
E. coli with high PHA production can be constructed with a desirable ratio of P(HB-HV), which has potential applications in industry
and medicine. 相似文献
90.
Methane production from synthesis gas using a mixed culture ofR. rubrum M. barkeri,and M. formicicum
Klasson K. T. Cowger J. P. Ko C. W. Vega J. L. Clausen E. C. Gaddy J. L. 《Applied biochemistry and biotechnology》1990,(1):317-328
The components of synthesis gas, CO, H2, and CO2, may be converted into CH4 biologically through either acetate or H2/CO2 as intermediates. Of these two routes, conversion through H2/CO2 is preferred. This paper presents results of mixed-culture studies employing the photosynthetic bacteriumR. rubrum for converting CO to CO2 and H2 by the water gas shift reaction and two methanogens,M. formicicum andM. barkeri, for converting CO2 and H2 into CH4. Results are presented for triculture operation in two types of reactors, the packed bubble column and the trickle-bed reactor. 相似文献