全文获取类型
收费全文 | 5475篇 |
免费 | 619篇 |
专业分类
系统科学 | 1073篇 |
丛书文集 | 1篇 |
理论与方法论 | 275篇 |
现状及发展 | 890篇 |
研究方法 | 5篇 |
综合类 | 3849篇 |
自然研究 | 1篇 |
出版年
2020年 | 1篇 |
2018年 | 727篇 |
2017年 | 733篇 |
2016年 | 431篇 |
2015年 | 32篇 |
2014年 | 3篇 |
2013年 | 3篇 |
2012年 | 261篇 |
2011年 | 960篇 |
2010年 | 815篇 |
2009年 | 455篇 |
2008年 | 507篇 |
2007年 | 763篇 |
2006年 | 4篇 |
2005年 | 44篇 |
2004年 | 123篇 |
2003年 | 150篇 |
2002年 | 57篇 |
2000年 | 1篇 |
1994年 | 2篇 |
1991年 | 3篇 |
1989年 | 2篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1967年 | 1篇 |
1966年 | 2篇 |
1955年 | 1篇 |
排序方式: 共有6094条查询结果,搜索用时 78 毫秒
951.
We propose a secure digital holographic system with signal and reference waves dually encrypted. Two random phase masks are used to encrypt the images in the input and the Fourier planes. The reference beam is phase encoded by another random phase mask. The encrypted image and the key are recorded by a CCD camera. The data can be processed or transferred directly by computer. We theoretically and experimentally demonstrate encryption and decryption of multiple images and the results show a high quality and good fault tolerance. 相似文献
952.
Mitochondrial disease currently received an increasing concern. However, the case-control design commonly adopted in this field is vulnerable to genetic background, population stratification and poor data quality. Although the phylogenetic analysis could help solve part of these problems, it has not received adequate attention. This paper is a review of this method as well as its application in mitochondrial disease study. 相似文献
953.
Molten-salt synthesis of tungsten oxide nanotubes: Morphological and gas sensitivity 总被引:1,自引:0,他引:1
KANG MingYang CAO ChuanBaot XU XingYan LIAN Bo 《科学通报(英文版)》2008,53(3):335-338
A very simple molten-salt method was used to synthesize WO3 nanotubes at moderate temperature (600 ). The outer and inner diameters of the WO3 nanotubes were about 100 nm and 30 nm,with a length of dozens of micrometers. TEM and XRD analyses showed that the WO3 nanotubes were single crystal, and had monoclinic structure with the axes preferentially aligned along the [011] direction. The photoluminescence and the gas sensitivity of the nanotubes had been tested, showing that the products very well responded to acetone at 340℃. 相似文献
954.
Salinity is one of the most severe environmental factors that may impair crop productivity. A proteomic study based on two-dimensional gel electrophoresis is performed in order to analyze the long-term salinity stress response of Thellungiella halophila, an Arabidopsis-related halophyte. Four-week-old seedlings are exposed to long-term salinity treatment. The total crude proteins are extracted from leaf blades, separated by 2-DE, stained with Coomassie Brilliant Blue, and differentially displayed spots are identified by MALDI-TOF MS or QTOF MS/MS. Among 900 protein spots reproducibly detected on each gel, 30 spots exhibit significant change and some of them are identified. The identified proteins include not only some previously characterized stress-responsive proteins such as TIR-NBS-LRR class disease resistance protein, ferritin-1, and pathogenesis-related protein 5, but also some proteins related to energy pathway, metabolism, RNA processing and protein degradation, as well as proteins with unknown functions. The possible functions of these proteins in salinity tolerance of T. halophila are discussed and it is suggested that the long-term salinity tolerance of T. halophila is achieved, at least partly, by enhancing defense system, adjusting energy and metabolic pathway and maintaining RNA structure. 相似文献
955.
We explore nitric oxide (NO) effect on K^+in, channels in Arabidopsis guard cells. We observed NO inhibited K^+in, currents when Ca^2+ chelator EGTA (Ethylene glycol-bis(2-aminoethylether)-N,N,N′,N;tetraacetic acid) was not added in the pipette solution; K^+in currents were not sensitive to NO when cytosolic Ca^2+ was chelated by EGTA. NO inhibited the Arabidopsis stomatal opening, but when EGTA was added in the bath solution, inhibition effect of NO on stomatal opening vanished. Thus, it implies that NO elevates cytosolic Ca^2+ by activating plasma membrane Ca^2+ channels firstly, then inactivates K^+in, chartnels, resulting in stomatal opening suppressed subsequently. 相似文献
956.
The mobilization of rat’s mesenchymal stem cells into peripheral blood by LiCL and its potency differentiation 总被引:2,自引:0,他引:2
DENG Jun ZOU ZhongMin ZHOU TaoLi AI GuoPing WANG JunPing DONG ShiWu SU YongPing↑ Institute of Combined Injury State Key Laboratory of Trauma Burns Combined Injury Department of Preventive Medicine Third Military Medical University Chongqing China Department of Pathogenic Biology Third Military Medical University Chongqing China 《科学通报(英文版)》2008,53(17):2632-2638
Mesenchymal stem cells (MSCs) are multipotent cells, which can differentiate into different tissues. It is still controversial whether MSCs can be isolated from adult peripheral blood (PB) under normal conditions and whether they can be mobilized in a way similar to that of hematopoieUc stem cells (HSCs). In this study, rat MSCs circulating in the PB under normal conditions can be isolated and cultured, and MSCs from PB and BM can be mobilized by LiCL. The mobilized MSCs can be induced to differentiate into neuron by such factors as β-mercaptoethanol and supernatants of nerve cell cultures. The present study provides a broader perspective on the repair of neural trauma. 相似文献
957.
Novel protein detection method based on proximity-dependent polymerase reaction and aptamers 总被引:1,自引:0,他引:1
Yang XiaoHai Wang Lei Wang KeMin Tan WeiHong Tang HongXing Meng XiangXian Guo QiuPing 《科学通报(英文版)》2008,53(2):204-208
In recent years, specific detection of proteins is one of the hot issues about aptamers in proteomics. Here we reported a simple, sensitive and specific proximity-dependent protein assay with dual DNA aptamers. Thrombin was used as the model protein, and two aptamer probes with complementary sequence at 3′-end were designed for the two distinct epitopes of the protein. Association of the two aptamers with thrombin resulted in stable hybrids due to the proximity of 3′-end, then polymerase reaction was induced. The amount of obtained dsDNA was indicated using the fluorescence dye Sybr Green I. The results showed that the initial velocity of polymerase reaction had a positive correlation with concentration of thrombin. The advantages of this dual-aptamer-based approach included simple and flexible design of aptamer probes, high selectivity and high sensitivity. The detection limit was 6.9 pmol/L. 相似文献
958.
XiuChun Yang ZhiHui Li WeiJie Li JingXian Xu ZhiWei Dong ShiXiong Qian 《科学通报(英文版)》2008,53(5):695-699
Ag nanoparticles embedded in soda-lime silicate glass were fabricated by ion-exchange and subse-quently annealing method. Z-scan technique, femtosecond time-resolved optical Kerr effect (OKE) technique and femtosecond pump-probe experiment were used to investigate the effects of laser wavelength and laser pulse duration as well as annealing temperature on the third-order optical nonlinearity and ultrafast dynamics of the composites. It was found that the third-order susceptibility of Ag nanoparticles composite glass measured by 400 nm pulse source is larger than that measured by 800 nm pulse source due to an enhancement effect of local field near surface plasmon resonance of Ag nanoparticles in silicate glass. The third-order optical nonlinearity measured by ns laser source is about two orders of magnitude larger than that measured from fs pulse. The annealing temperature has an important effect on the third-order optical nonlinearity and ultrafast dynamics of the composites. Third-order nonlinear susceptibility up to 10^-l0 esu and fast relaxation process up to 0.2 ps have been obtained in Ag nanoparticles doped glass. 相似文献
959.
Grenvillian orogeny in the Southern Cathaysia Block: Constraints from U-Pb ages and Lu-Hf isotopes in zircon from metamorphic basement 总被引:3,自引:1,他引:2
LiJuan Wang JinHai Yu S.Y. O’Reilly W.L. Griffin Tao Sun ZhenYang Wei ShaoYong Jiang LiangShu Shu 《科学通报(英文版)》2008,53(19):3037-3050
Metamorphic basement rocks in the Cathaysia Block are composed mainly of meta-sediments with different ages. New zircon U-Pb geochronological results from the meta-sedimentary rocks exposed in the Zengcheng and Hezi areas, southern Cathaysia Block, show that they consist dominantly of early Neoproterozoic (1.0-0.9 Ga) materials with minor Paleo- to Mesoproterozoic and late Neoproterozoic (0.8-0.6 Ga) components, suggesting that the detritus mostly come from a Grenvillian orogen. The youngest detrital zircon ages place a constraint on the deposition time of these sediments in Late Neoproterozoic. Zircon Hf isotopic compositions indicate that the Grenvillian zircons were derived from the reworking of Mesoproterozoic arc magmatic rocks and Paleoproterozoic continental crust, implying an arc-continent collisional setting. Single-peak age spectra and the presence of abundant euhedral Grenvillian zircons suggest that the sedimentary provenance is not far away from the sample location. Thus, the Grenvillian orogen probably preexisted along the southern margin of the Cathaysia Block, or very close to the south. Similarity in the ages of Grenvillian orogeny and the influence of the assembly of Gondwana in South China with India and East Antarctic are discussed, with suggestion that South China was more likely linked with the India-East Antarctica continents in Early Neoproterozoic rather than between western Laurentia and eastern Australia. 相似文献
960.
This paper develops a general hypercube-based key predistribution scheme for establishing pairwise keys between sensor nodes using polynomials, which is parameterized by the dimension of hypercube and the Hamming distance threshold variables. The scheme addresses the weaknesses of existing key predistribution schemes, which have either worse security or lower efficiency. It exhibits a nice property--when the Hamming distance between any two neighboring sensor nodes is less than the pre-defined threshold, the pairwise key can be established directly. Extensive performance and security analysis shows that by increasing Hamming distance threshold value, we can trade off the resilience against node capture attack for higher probability of direct pairwise key establishment, so as to save the energy consumption which is the most important issue for sensor networks. 相似文献