全文获取类型
收费全文 | 5038篇 |
免费 | 15篇 |
国内免费 | 20篇 |
专业分类
系统科学 | 11篇 |
丛书文集 | 18篇 |
教育与普及 | 4篇 |
理论与方法论 | 2篇 |
现状及发展 | 2210篇 |
研究方法 | 261篇 |
综合类 | 2539篇 |
自然研究 | 28篇 |
出版年
2013年 | 52篇 |
2012年 | 100篇 |
2011年 | 117篇 |
2010年 | 26篇 |
2008年 | 86篇 |
2007年 | 111篇 |
2006年 | 98篇 |
2005年 | 98篇 |
2004年 | 119篇 |
2003年 | 80篇 |
2002年 | 89篇 |
2001年 | 181篇 |
2000年 | 149篇 |
1999年 | 118篇 |
1992年 | 112篇 |
1991年 | 72篇 |
1990年 | 75篇 |
1989年 | 65篇 |
1988年 | 69篇 |
1987年 | 78篇 |
1986年 | 56篇 |
1985年 | 96篇 |
1984年 | 91篇 |
1983年 | 78篇 |
1982年 | 62篇 |
1981年 | 54篇 |
1980年 | 68篇 |
1979年 | 174篇 |
1978年 | 141篇 |
1977年 | 111篇 |
1976年 | 116篇 |
1975年 | 123篇 |
1974年 | 147篇 |
1973年 | 119篇 |
1972年 | 123篇 |
1971年 | 149篇 |
1970年 | 198篇 |
1969年 | 142篇 |
1968年 | 150篇 |
1967年 | 127篇 |
1966年 | 137篇 |
1965年 | 104篇 |
1964年 | 39篇 |
1959年 | 39篇 |
1958年 | 68篇 |
1957年 | 47篇 |
1956年 | 38篇 |
1955年 | 36篇 |
1954年 | 46篇 |
1948年 | 28篇 |
排序方式: 共有5073条查询结果,搜索用时 15 毫秒
381.
Hon WC Wilson MI Harlos K Claridge TD Schofield CJ Pugh CW Maxwell PH Ratcliffe PJ Stuart DI Jones EY 《Nature》2002,417(6892):975-978
382.
HT Mebrahtu IV Borzenets DE Liu H Zheng YV Bomze AI Smirnov HU Baranger G Finkelstein 《Nature》2012,488(7409):61-64
A Luttinger liquid is an interacting one-dimensional electronic system, quite distinct from the 'conventional' Fermi liquids formed by interacting electrons in two and three dimensions. Some of the most striking properties of Luttinger liquids are revealed in the process of electron tunnelling. For example, as a function of the applied bias voltage or temperature, the tunnelling current exhibits a non-trivial power-law suppression. (There is no such suppression in a conventional Fermi liquid.) Here, using a carbon nanotube connected to resistive leads, we create a system that emulates tunnelling in a Luttinger liquid, by controlling the interaction of the tunnelling electron with its environment. We further replace a single tunnelling barrier with a double-barrier, resonant-level structure and investigate resonant tunnelling between Luttinger liquids. At low temperatures, we observe perfect transparency of the resonant level embedded in the interacting environment, and the width of the resonance tends to zero. We argue that this behaviour results from many-body physics of interacting electrons, and signals the presence of a quantum phase transition. Given that many parameters, including the interaction strength, can be precisely controlled in our samples, this is an attractive model system for studying quantum critical phenomena in general, with wide-reaching implications for understanding quantum phase transitions in more complex systems, such as cold atoms and strongly correlated bulk materials. 相似文献
383.
Recognition of haemagglutinins on virus-infected cells by NKp46 activates lysis by human NK cells 总被引:41,自引:0,他引:41
Mandelboim O Lieberman N Lev M Paul L Arnon TI Bushkin Y Davis DM Strominger JL Yewdell JW Porgador A 《Nature》2001,409(6823):1055-1060
Natural killer (NK) cells destroy virus-infected and tumour cells, apparently without the need for previous antigen stimulation. In part, target cells are recognized by their diminished expression of major histocompatibility complex (MHC) class I molecules, which normally interact with inhibitory receptors on the NK cell surface. NK cells also express triggering receptors that are specific for non-MHC ligands; but the nature of the ligands recognized on target cells is undefined. NKp46 is thought to be the main activating receptor for human NK cells. Here we show that a soluble NKp46-immunoglobulin fusion protein binds to both the haemagglutinin of influenza virus and the haemagglutinin-neuraminidase of parainfluenza virus. In a substantial subset of NK cells, recognition by NKp46 is required to lyse cells expressing the corresponding viral glycoproteins. The binding requires the sialylation of NKp46 oligosaccharides, which is consistent with the known sialic binding capacity of the viral glycoproteins. These findings indicate how NKp46-expressing NK cells may recognize target cells infected by influenza or parainfluenza without the decreased expression of target-cell MHC class I protein. 相似文献
384.
Early association of electrocardiogram alteration with infarct size and cardiac function after myocardial infarction 总被引:7,自引:0,他引:7
OBJECTIVE: Myocardial infarction (MI) is the main cause of heart failure, but the relationship between the extent of MI and cardiac function has not been clearly determined. The present study was undertaken to investigate early changes in the electrocardiogram associated with infarct size and cardiac function after MI. METHODS: MI was induced by ligating the left anterior descending coronary artery in rats. Electrocardiograms, echocardiographs and hemodynamic parameters were assessed and myocardial infarct size was measured from mid-transverse sections stained with Masson's trichrome. RESULTS: The sum of pathological Q wave amplitudes was strongly correlated with myocardial infarct size (r = 0.920, P < 0.0001), left ventricular ejection fraction (r = -0.868, P < 0.0001) and left ventricular end diastolic pressure (r = 0.835, P < 0.0004). Furthermore, there was close relationship between MI size and cardiac function as assessed by left ventricular ejection fraction (r = -0.913, P < 0.0001) and left ventricular end diastolic pressure (r = 0.893, P < 0.0001). CONCLUSION: The sum of pathological Q wave amplitudes after MI can be used to estimate the extent of MI as well as cardiac function. 相似文献
385.
386.
Alafara A. Baba Kuranga I. Ayinla Folahan A. Adekola Rafiu B. Bale Malay K. Ghosh Abdul G. F. Alabi Abdul R. Sheik Ismael O. Folorunso 《矿物冶金与材料学报》2013,20(11):1021-1028
The dissolution kinetics of a Nigerian chalcopyrite ore in hydrochloric acid was studied in this article. Acid concentration, reaction temperature, and ore particle size were chosen as experimental parameters. The chemical and morphological studies of the ore before and after leaching at optimal conditions were carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is revealed that increasing the acid concentration and system temperature and decreasing the ore particle size greatly enhances the dissolution rate. The dissolution kinetics was found to follow the shrinking core model for the diffusion control mechanism where the activation energy (Ea) of 32.92 kJ·mol?1 was obtained for the process and supported by morphological changes at a higher dissolution of 91.33%. 相似文献
387.
Marco G. Alves Luís Rato Rui A. Carvalho Paula I. Moreira Sílvia Socorro Pedro F. Oliveira 《Cellular and molecular life sciences : CMLS》2013,70(5):777-793
Hormonal regulation is essential to spermatogenesis. Sertoli cells (SCs) have functions that reach far beyond the physical support of germ cells, as they are responsible for creating the adequate ionic and metabolic environment for germ cell development. Thus, much attention has been given to the metabolic functioning of SCs. During spermatogenesis, germ cells are provided with suitable metabolic substrates, in a set of events mediated by SCs. Multiple signaling cascades regulate SC function and several of these signaling pathways are hormone-dependent and cell-specific. Within the seminiferous tubules, only SCs possess receptors for some hormones rendering them major targets for the hormonal signaling that regulates spermatogenesis. Although the mechanisms by which SCs fulfill their own and germ cells metabolic needs are mostly studied in vitro, SC metabolism is unquestionably a regulation point for germ cell development and the hormonal control of these processes is required for a normal spermatogenesis. 相似文献
388.
Igh-V or closely linked gene(s) control immunological memory to a thymus-independent antigen 总被引:1,自引:0,他引:1
Mice mount a normal primary antibody response on stimulation with the thymic-independent antigen trinitrophenylated lipopolysaccharide (TNP-LPS). Although we have previously reported the generation of functional B-memory lymphocytes to TNP-LPS, this memory response was only observed in few mouse strains. Here we have used congeneic mouse strains in an attempt to locate the genetic regions involved in the memory response. We show that genes of the major histocompatibility complex (MHC) do not have a critical role but that genes coding for the variable region of immunoglobulin heavy chains or gene(s) closely linked to them are required for memory cell induction by TNP-LPS. 相似文献
389.
Genome sequencing in microfabricated high-density picolitre reactors 总被引:21,自引:0,他引:21
Margulies M Egholm M Altman WE Attiya S Bader JS Bemben LA Berka J Braverman MS Chen YJ Chen Z Dewell SB Du L Fierro JM Gomes XV Godwin BC He W Helgesen S Ho CH Ho CH Irzyk GP Jando SC Alenquer ML Jarvie TP Jirage KB Kim JB Knight JR Lanza JR Leamon JH Lefkowitz SM Lei M Li J Lohman KL Lu H Makhijani VB McDade KE McKenna MP Myers EW Nickerson E Nobile JR Plant R Puc BP Ronan MT Roth GT Sarkis GJ Simons JF Simpson JW Srinivasan M Tartaro KR Tomasz A Vogt KA Volkmer GA Wang SH Wang Y Weiner MP 《Nature》2005,437(7057):376-380
The proliferation of large-scale DNA-sequencing projects in recent years has driven a search for alternative methods to reduce time and cost. Here we describe a scalable, highly parallel sequencing system with raw throughput significantly greater than that of state-of-the-art capillary electrophoresis instruments. The apparatus uses a novel fibre-optic slide of individual wells and is able to sequence 25 million bases, at 99% or better accuracy, in one four-hour run. To achieve an approximately 100-fold increase in throughput over current Sanger sequencing technology, we have developed an emulsion method for DNA amplification and an instrument for sequencing by synthesis using a pyrosequencing protocol optimized for solid support and picolitre-scale volumes. Here we show the utility, throughput, accuracy and robustness of this system by shotgun sequencing and de novo assembly of the Mycoplasma genitalium genome with 96% coverage at 99.96% accuracy in one run of the machine. 相似文献
390.