全文获取类型
收费全文 | 1619篇 |
免费 | 168篇 |
国内免费 | 475篇 |
专业分类
2262篇 |
出版年
2024年 | 14篇 |
2023年 | 38篇 |
2022年 | 66篇 |
2021年 | 62篇 |
2020年 | 69篇 |
2019年 | 76篇 |
2018年 | 65篇 |
2017年 | 65篇 |
2016年 | 88篇 |
2015年 | 61篇 |
2014年 | 99篇 |
2013年 | 106篇 |
2012年 | 82篇 |
2011年 | 120篇 |
2010年 | 85篇 |
2009年 | 110篇 |
2008年 | 103篇 |
2007年 | 106篇 |
2006年 | 116篇 |
2005年 | 89篇 |
2004年 | 82篇 |
2003年 | 64篇 |
2002年 | 47篇 |
2001年 | 51篇 |
2000年 | 44篇 |
1999年 | 35篇 |
1998年 | 24篇 |
1997年 | 31篇 |
1996年 | 17篇 |
1995年 | 20篇 |
1994年 | 17篇 |
1993年 | 18篇 |
1992年 | 8篇 |
1991年 | 24篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 15篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 13篇 |
1983年 | 12篇 |
1982年 | 16篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1978年 | 5篇 |
1977年 | 5篇 |
1974年 | 4篇 |
1972年 | 4篇 |
排序方式: 共有2262条查询结果,搜索用时 0 毫秒
61.
瑞香狼毒是分布在青海省高寒草甸的主要毒害草之一,近年来其迅速蔓延对当地畜牧业危害严重并使草地生态系统日趋退化.在海北州祁连县选取狼毒分布的典型退化草甸,采用2012—2014年狼毒盛花期获取的实测光谱数据,分析狼毒与牧草的光谱差异性.结果表明: 在350~900 nm的可见光 近红外波段,狼毒顶花的光谱反射特征明显异于狼毒叶片和同期牧草等绿色背景,顶花与绿色背景的光谱反射率差异主要体现在红谷和蓝谷.随着盖度的增加,狼毒群落光谱反射率整体升高,在近红外反射峰处狼毒群落与牧草群落光谱反射率具有最大差值,且不同盖度狼毒群落之间的差异性最明显.顶花与绿色背景以及狼毒群落与牧草群落的一阶导数光谱差异均体现在黄边幅值和蓝边幅值.狼毒群落盖度与光谱特征参量的线性回归分析表明,红谷与狼毒群落盖度的相关性最好(R2=0.94),反演狼毒群落盖度的精度最高.盛花期区分狼毒与牧草的主要光谱特征参量为红谷、蓝谷与近红外反射峰,其对应的红、蓝及近红外波段的组合可用于构建狼毒提取的敏感指数. 相似文献
62.
棕色固氮菌(Azotobacter vinelandii Lipmann)突变种UW3能在无Mo的无氨培养基中固氮生长,低浓度的Mn对UW3突变种生长有促进作用,从在Mn中生长的UW3菌体中分离得到的部分纯固氮酶组分Ⅰ蛋白含量有Mn和Fe原子(Fe/Mo/Mn为10.41:0.19:1.00)并有OP MoFe蛋白一半的还原乙炔和质子的活性。这种蛋白的吸收光谱和圆二色谱与MoFe蛋白存在明显的差异,含Mn蛋白的亚基分子量都与MoFe蛋白的α亚基相近。初步结果表明,这种含Mn蛋白可胡是一种固氮酶组分Ⅰ蛋白。 相似文献
63.
Treichel H Mazutti MA Maugeri Filho F Rodrigues MI 《Bioprocess and biosystems engineering》2009,32(4):425-433
The present work aimed to study the viability of the use of sugarcane molasses and corn steep liquor (CSL) in a sequential
inulinase production performing an up-stream pretreatment of these agroindustrial residues. A sequential strategy was used
applying three central composite rotatable designs (CCRDs) to optimise medium composition, followed by a down-stream step.
The medium containing 150 g L−1 molasses, 50 g L−1 CSL and 6 g L−1 yeast extract, yielded a maximum inulinase production of 1,294 ± 7 U mL−1, after 72 h of fermentation. A down-stream evaluation was carried out using an expanded bed of Streamline DAE resin (Pharmacia),
with and without the up-stream treatment. The results showed that the enzyme could not be recovered from the non-pretreated
medium, whereas a yield of 91% was obtained in the adsorption stage from the medium prepared with the up-stream treatment,
showing the viability of producing the enzyme inulinase from agroindustrial residues using the integrated process. 相似文献
64.
种子脱水耐性及其与种子类型和发育阶段的相关性 总被引:7,自引:0,他引:7
种子脱水耐性是种子发育过程中获得的综合特性,是判断种子贮藏持性的一个重要依据。当种子获得脱水耐性时,生理,形态和结构会发生相应的变化,包括糖,蛋白质,脂类和抗氧化系统等保护性物质的合成,各种保护性物质不是单独作用的,而是协同调节种子的脱水耐性。不同的植物种子,其脱水耐性不同,并且随着种子的发育而变化。关于种子脱水耐性的获得,主要有2种观点,一种认为是数量性状,另一种认为是突变性状。种子库收集种子保存时,适时采集和适度脱水才能有效地延长种子的贮藏寿命。 相似文献
65.
In this paper, we report the results of a systematic attempt to relate the intrinsic plasma membrane fluidity of three different
cell lines to their electroporation behaviour, which consists of reversible and irreversible electroporation. Apart from electroporation
behaviour of given cell lines the time course required for membrane resealing was determined in order to distinguish the effect
of resealing time from the cell’s ability to survive given electric pulse parameters. Reversible, irreversible electroporation
and membrane resealing were then related to cell membrane fluidity as determined by electron paramagnetic resonance spectroscopy
and computer characterization of membrane domains. We found that cell membrane fluidity does not have significant effect on
reversible electroporation although there is a tendency for the voltage required for reversible electroporation to increase
with increased membrane fluidity. Cell membrane fluidity, however, may affect irreversible electroporation. Nevertheless,
this effect, if present, is masked with different time courses of membrane resealing found for the different cell lines studied.
The time course of cell membrane resealing itself could be related to the cell’s ability to survive. 相似文献
66.
67.
In a failure time analysis, we sometimes observe additional study subjects who enter during the study period. These late entries are treated as left-truncated data in the statistical literature. However, with real data, there is a substantial possibility that the delayed entries may have extremely different hazards compared to the other standard subjects. We focus on a situation in which such entry bias might arise in the analysis of survival data. The purpose of the present article is to develop an appropriate methodology for making inference about data including late entries. We construct a model that includes parameters for the effect of delayed entry bias having no specification for the distribution of entry time. We also discuss likelihood inference based on this model and derive the asymptotic behavior of estimates. A simulation study is conducted for a finite sample size in order to compare the analysis results using our method with those using the standard method, where independence between entry time and failure time is assumed. We apply this method to mortality analysis among atomic bomb survivors defined in a geographical study region. 相似文献
68.
69.
Roger Scholz Falk Hoffmann Sandra von Sachsen Welf-Guntram Drossel Carsten Klöhn Christian Voigt 《Journal of biomechanics》2013
In total hip arthroplasty and particularly in revision surgery, computer assisted pre-operative prediction of the best possible anchorage strategy for implant fixation would be a great help to the surgeon. Computer simulation relies on validated numerical models. In the current study, three density–elasticity relationships (No. 1–3) from the literature for inhomogeneous material parameter assignment from CT data in automated finite element (FE) modeling of long bones were evaluated for their suitability for FE modeling of human pelvic bone. Numerical modal analysis was conducted on 10 FE models of hemipelvic bone specimens and compared to the gold standard provided by experimental modal analysis results from a previous in-vitro study on the same specimens. Overall, calculated resonance frequencies came out lower than measured values. Magnitude of mean relative deviation of numerical resonance frequencies with regard to measured values is lowest for the density–elasticity relationship No. 3 (−15.9%) and considerably higher for both density–elasticity relationships No. 1 (−41.1%) and No. 2 (−45.0%). Mean MAC values over all specimens amount to 77.8% (No. 1), 78.5% (No. 2), and 83.0% (No. 3). MAC results show, that mode shapes are only slightly influenced by material distribution. Calculated resonance frequencies are generally lower than measured values, which indicates, that numerical models lack stiffness. Even when using the best suited (No. 3) out of three investigated density–elasticity relationships, in FE modeling of pelvic bone a considerable underestimation of model stiffness has to be taken into account. 相似文献
70.
磷对铝氟交互处理下茶树主要生理生化特性的影响 总被引:1,自引:0,他引:1
通过白茶和智仁早茶在铝氟交互处理下添加外源磷的水培试验,研究了不同浓度磷(0,0.2,1.0 mmol/L)对茶树主要化学成分及抗氧化酶的影响.结果表明:(1)当施入0.2 mmol/L磷时,茶叶中的蛋白质、叶绿素和茶多酚含量及SOD、POD和CAT活性均比无磷组高,且在5.0 mmol/L铝和1.0 mmol/L氟处理下,均达到最高值(叶绿素除外),表明适量外源磷的施加对茶树的生理生化特性起促进作用,并能有效缓解铝和氟毒所致的氧化损伤,从而抑制了脂质过氧化作用;(2)随着磷浓度(1.0 mmol/L)增加,茶叶中的各指标值均比无磷组和对照低,出现最低值,表明磷对铝氟胁迫下茶树的缓解效应有限,超过一定的浓度,则表现出明显的抑制作用;(3) 在外源磷的施加下,白茶的可溶性蛋白、茶多酚和叶绿素含量及SOD、POD和CAT活性均比智仁早茶的变化幅度大.因此,外源磷对茶树幼苗生理生化作用具有一定的调控效应. 相似文献