全文获取类型
收费全文 | 76515篇 |
免费 | 5571篇 |
国内免费 | 4963篇 |
专业分类
87049篇 |
出版年
2024年 | 161篇 |
2023年 | 1051篇 |
2022年 | 2445篇 |
2021年 | 4119篇 |
2020年 | 2648篇 |
2019年 | 3272篇 |
2018年 | 3208篇 |
2017年 | 2321篇 |
2016年 | 3277篇 |
2015年 | 4852篇 |
2014年 | 5586篇 |
2013年 | 6014篇 |
2012年 | 7048篇 |
2011年 | 6179篇 |
2010年 | 3726篇 |
2009年 | 3347篇 |
2008年 | 3725篇 |
2007年 | 3360篇 |
2006年 | 2906篇 |
2005年 | 2380篇 |
2004年 | 1959篇 |
2003年 | 1654篇 |
2002年 | 1399篇 |
2001年 | 1230篇 |
2000年 | 1218篇 |
1999年 | 1121篇 |
1998年 | 661篇 |
1997年 | 655篇 |
1996年 | 666篇 |
1995年 | 616篇 |
1994年 | 543篇 |
1993年 | 376篇 |
1992年 | 568篇 |
1991年 | 435篇 |
1990年 | 406篇 |
1989年 | 282篇 |
1988年 | 244篇 |
1987年 | 234篇 |
1986年 | 166篇 |
1985年 | 193篇 |
1984年 | 109篇 |
1983年 | 118篇 |
1982年 | 71篇 |
1981年 | 58篇 |
1980年 | 37篇 |
1979年 | 61篇 |
1977年 | 30篇 |
1974年 | 38篇 |
1973年 | 34篇 |
1972年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
971.
土壤水分含量的理论分析及预测模型 总被引:5,自引:0,他引:5
本文应用物理学中的电介质和电磁理论,分析和研究土壤的组成成分,得到了反映土壤水分含量的理论表达式,并在自行研制的测试仪器上,对相关变量进行测量,由此建立了土壤水分含量的预测模型,统计检验和国代结果显示了理论模型和预测模型的合理性. 相似文献
972.
Yuzhong Zhang Xiulan Chen Baicheng Zhou Chengkui Zeng Jie Liu Dongxia Shi Shijin Pang 《中国科学C辑(英文版)》1999,42(1):74-79
Phycobilisomes (PBS) were isolated from blue-green algaSpirulina platensis. Scanning tunneling microscope was used to investigate the three-dimensional structure of PBS deposited on freshly cleaved highly oriented pyrolytic graphite (HOPG) in ambient condition at room temperature. The results showed that the rods of PBS radiated from the core to different directions in the space other than arrayed in one plane, which was different from the typical hemi-discoidal model structure. The diameter of PBS was up to 70 nm, and the rod was approximately 50 nm in length. Similar results were observed in Langmuir-Blodgett (LB) film of PBS. The dissociated PBS could reaggregate into rod-like structures and easily form two-dimensional membrane while being absorbed on HOPG, however, no intact PBS was observed. The filling-space model structure of PBS inSpirulina platensis with STM from three-dimensional real space at nanometer scale was found, which showed that this new structural model of PBS surely exists in blue-green algae and red algae. The function of this structural model of PBS was also discussed. 相似文献
973.
974.
975.
The relationship between human fluid intelligence and social-emotional abilities has been a topic of considerable interest. The current study investigated whether adolescents with different intellectual levels had different automatic neural processing of facial expressions. Two groups of adolescent males were enrolled: a high IQ group and an average IQ group. Age and parental socioeconomic status were matched between the two groups. Participants counted the numbers of the central cross changes while paired facial expressions were presented bilaterally in an oddball paradigm. There were two experimental conditions: a happy condition, in which neutral expressions were standard stimuli (p = 0.8) and happy expressions were deviant stimuli (p = 0.2), and a fearful condition, in which neutral expressions were standard stimuli (p = 0.8) and fearful expressions were deviant stimuli (p = 0.2). Participants were required to concentrate on the primary task of counting the central cross changes and to ignore the expressions to ensure that facial expression processing was automatic. Event-related potentials (ERPs) were obtained during the tasks. The visual mismatch negativity (vMMN) components were analyzed to index the automatic neural processing of facial expressions. For the early vMMN (50–130 ms), the high IQ group showed more negative vMMN amplitudes than the average IQ group in the happy condition. For the late vMMN (320–450 ms), the high IQ group had greater vMMN responses than the average IQ group over frontal and occipito-temporal areas in the fearful condition, and the average IQ group evoked larger vMMN amplitudes than the high IQ group over occipito-temporal areas in the happy condition. The present study elucidated the close relationships between fluid intelligence and pre-attentive change detection on social-emotional information. 相似文献
976.
Yan Liu Kathleen DeBoer David M. de Kretser Liza O’Donnell Anne E. O’Connor D. Jo Merriner Hidenobu Okuda Belinda Whittle David A. Jans Athina Efthymiadis Robert I. McLachlan Christopher J. Ormandy Chris C. Goodnow Duangporn Jamsai Moira K. O’Bryan 《PLoS genetics》2015,11(3)
Male infertility affects at least 5% of reproductive age males. The most common pathology is a complex presentation of decreased sperm output and abnormal sperm shape and motility referred to as oligoasthenoteratospermia (OAT). For the majority of OAT men a precise diagnosis cannot be provided. Here we demonstrate that leucine-rich repeats and guanylate kinase-domain containing isoform 1 (LRGUK-1) is required for multiple aspects of sperm assembly, including acrosome attachment, sperm head shaping and the initiation of the axoneme growth to form the core of the sperm tail. Specifically, LRGUK-1 is required for basal body attachment to the plasma membrane, the appropriate formation of the sub-distal appendages, the extension of axoneme microtubules and for microtubule movement and organisation within the manchette. Manchette dysfunction leads to abnormal sperm head shaping. Several of these functions may be achieved in association with the LRGUK-1 binding partner HOOK2. Collectively, these data establish LRGUK-1 as a major determinant of microtubule structure within the male germ line. 相似文献
977.
Erbao Liu Xiaoli Liu Siyuan Zeng Kaiming Zhao Changfeng Zhu Yang Liu Manamik Caleb Breria Baojuan Zhang Delin Hong 《PloS one》2015,10(3)
Detecting quantity trait locus (QTLs) and elite alleles that are associated with grain-filling rate (GFR) in rice is essential for promoting the utilization of hybrid japonica rice and improving rice yield. Ninety-five varieties including 58 landraces and 37 elite varieties from the core germplasm collection were genotyped with 263 simple sequence repeat (SSR) markers. The GFR of the 95 varieties was evaluated at five stages, 7, 14, 21, 28 and 35 days after flowering (DAF) both in 2011 and 2012. We found abundant phenotypic and genetic diversity in the studied population. A population structure analysis identified seven subpopulations. A linkage disequilibrium (LD) analysis indicated that the levels of LD ranged from 60.3 cM to 84.8 cM and artificial selection had enhanced the LD. A time-course association analysis detected 31 marker-GFR associations involving 24 SSR markers located on chromosomes 1, 2, 3, 4, 5, 6, 8, 9, 11 and 12 of rice at five stages. The elite alleles for high GFR at each stage were detected. Fifteen excellent parental combinations were predicted, and the best parental combination ‘Nannongjing62401×Laolaihong’ could theoretically increase 4.086 mg grain-1 d-1 at the five stages. Our results demonstrate that the time-course association mapping for GFR in rice could detect elite alleles at different filling stages and that these elite alleles could be used to improve the GFR via pyramiding breeding. 相似文献
978.
979.
为了考察过量表达苹果酸酶对于E.coli NZN111(ldhA::Kan pfl::Cam)厌氧发酵产丁二酸的影响, 将连接有苹果酸酶基因sfcA的表达载体pTrc99a-sfcA转化进NZN111中, 构建了重组NZN111(pTrc99a-sfcA)。0.5 mmol/L IPTG诱导8 h后, 测定的苹果酸酶比酶活为30.67 u/mg, 比受体菌提高了140倍。采用两阶段发酵模式, 结果表明: 过量表达的苹果酸酶在NZN111体内催化了从丙酮酸到苹果酸的逆向反应, 丁二酸是发酵过程中积累的主要有机酸, 且当加入0.7 mmol/L IPTG诱导, 初始葡萄糖糖浓度为18.5 g/L时, 选择对数生长期后期的菌种以10%的接种量转入厌氧发酵, 发酵结束时发酵液中丁二酸的浓度为12.84 g/L, 对葡萄糖的收率为69.43%, 乙酸为0.58 g/L, 二者浓度比为22:1, 没有检测到甲酸和乳酸。构建的菌种具有高产丁二酸和副产物极少的优点, 在同类菌种中处于先进水平。 相似文献
980.
The analysis of genetic diversity and differentiation of six Chinese cattle populations using microsatellite markers 总被引:1,自引:0,他引:1
Yongjiang Mao Hong Chang Zhangping Yang Liu Zhang Ming Xu Guobin Chang Wei Sun Guangming Song Dejun Ji 《遗传学报》2008,35(1):25-32
A total of 321 individuals from six cattle populations of four species in a bovine subfamily in China were studied using 12 pairs of microsatellite markers. The genetic diversities within and between populations were calculated. The phylogenetic trees were constructed by (δμ)^2 and DA distances, and the divergence times between populations were estimated by (δμ)^2. Altogether, 144 microsatellite alleles were detected including 24 private alleles and nine shared alleles. Chinese Holstein had the largest number of private alleles (10), whereas Bohai black and Buffalo had the smallest number of private alleles (2). Chinese Holstein showed the highest genetic variability. Its observed number of alleles (Na), mean effective number of alleles (MNA), and mean heterozygosity (He) were 7.7500, 4.9722, and 0.7719, respectively, whereas, the Buffalo and Yak showed low genetic variability. In the phylogenetic trees, Luxi and Holstein grouped first, followed by Bohai and Minnan. Yak branched next and buffalo emerged as the most divergent population from other cattle populations. Luxi and Bohai were estimated to have diverged 0.039-0.105 million years ago (MYA), however, buffalo and Holstein diverged 0.501-1.337 MYA. The divergence time of Yak versus Minnan, Holstein and buffalo was 0.136-0.363, 0.273-0.729, and 0.326-0.600 MYA, respectively. 相似文献