全文获取类型
收费全文 | 16800篇 |
免费 | 34篇 |
国内免费 | 37篇 |
专业分类
系统科学 | 71篇 |
丛书文集 | 263篇 |
教育与普及 | 30篇 |
理论与方法论 | 71篇 |
现状及发展 | 7993篇 |
研究方法 | 693篇 |
综合类 | 7589篇 |
自然研究 | 161篇 |
出版年
2013年 | 76篇 |
2012年 | 185篇 |
2011年 | 364篇 |
2008年 | 222篇 |
2007年 | 241篇 |
2006年 | 279篇 |
2005年 | 270篇 |
2004年 | 376篇 |
2003年 | 254篇 |
2002年 | 254篇 |
2001年 | 479篇 |
2000年 | 442篇 |
1999年 | 324篇 |
1994年 | 74篇 |
1992年 | 286篇 |
1991年 | 221篇 |
1990年 | 238篇 |
1989年 | 235篇 |
1988年 | 202篇 |
1987年 | 234篇 |
1986年 | 278篇 |
1985年 | 335篇 |
1984年 | 230篇 |
1983年 | 226篇 |
1982年 | 205篇 |
1981年 | 217篇 |
1980年 | 224篇 |
1979年 | 563篇 |
1978年 | 475篇 |
1977年 | 437篇 |
1976年 | 341篇 |
1975年 | 404篇 |
1974年 | 564篇 |
1973年 | 453篇 |
1972年 | 472篇 |
1971年 | 553篇 |
1970年 | 696篇 |
1969年 | 538篇 |
1968年 | 539篇 |
1967年 | 536篇 |
1966年 | 430篇 |
1965年 | 371篇 |
1964年 | 121篇 |
1959年 | 193篇 |
1958年 | 330篇 |
1957年 | 247篇 |
1956年 | 190篇 |
1955年 | 192篇 |
1954年 | 190篇 |
1948年 | 127篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Fracture toughness is very important when applying Damage Tolerance Design and Assessment Techniques. The traditional testing approach for obtaining fracture toughness values is costly and time consuming. In order to estimate the fracture toughness of ductile metals, the fracture mechanics theory, materials plastic deformation theory and materials constructive relationships are employed here. A series of formulae and a theoretical approach are presented to calculate fracture toughness values of different materials in the plane stress and plane strain conditions. Compared with test results, evaluated values have a good agreement. 相似文献
142.
Selective functionalization of mesoporous silica nanospheres (MSNs) is crucial for nanoengineering of MSNs.
Herein, we have combined “surface-protected etching strategy” and “cationic surfactant assisted etching
strategy” to prepare functionalized MSNs with externally attached amino groups. The externally attached NH2
groups endow the catalysts with excellent catalytic performance for nitroaldol reaction between nitromethane
and benzaldehyde. In addition, those NH2-MSNs can also be used to support gold nanoparticles, which display
very good catalytic performance for reduction of 4-nitrophenol. It can be envisioned that the synthesis protocol
developed in this work could also be extended to nanoengineered MSNs, which provides opportunities for nanoreactors
design. 相似文献
143.
Jialu Chen Jemima J.Davies Alister S.Goodfellow Sarah M.D.Hall Helena G.Lancaster Xinyu Liu Cameron J.Rhodes Wuzong Zhou 《自然科学进展(英文版)》2021,31(1):141-151
Dendritic silver and copper crystals were produced via Galvanic replacement reactions on zinc and aluminum plates, respectively. The growth orientations of these metals were determined using electron microscopy. The results showed that a fast crystal growth associated with a high concentration of metal cations led to kinetically controlled growth along the <112> axes of the cubic close-packed structures. However, a slow growth rate resulted in thermodynamically controlled growth along the ... 相似文献
144.
Minimum information about a microarray experiment (MIAME)-toward standards for microarray data. 总被引:36,自引:0,他引:36
A Brazma P Hingamp J Quackenbush G Sherlock P Spellman C Stoeckert J Aach W Ansorge C A Ball H C Causton T Gaasterland P Glenisson F C Holstege I F Kim V Markowitz J C Matese H Parkinson A Robinson U Sarkans S Schulze-Kremer J Stewart R Taylor J Vilo M Vingron 《Nature genetics》2001,29(4):365-371
Microarray analysis has become a widely used tool for the generation of gene expression data on a genomic scale. Although many significant results have been derived from microarray studies, one limitation has been the lack of standards for presenting and exchanging such data. Here we present a proposal, the Minimum Information About a Microarray Experiment (MIAME), that describes the minimum information required to ensure that microarray data can be easily interpreted and that results derived from its analysis can be independently verified. The ultimate goal of this work is to establish a standard for recording and reporting microarray-based gene expression data, which will in turn facilitate the establishment of databases and public repositories and enable the development of data analysis tools. With respect to MIAME, we concentrate on defining the content and structure of the necessary information rather than the technical format for capturing it. 相似文献
145.
Chiang C Jacobsen JC Ernst C Hanscom C Heilbut A Blumenthal I Mills RE Kirby A Lindgren AM Rudiger SR McLaughlan CJ Bawden CS Reid SJ Faull RL Snell RG Hall IM Shen Y Ohsumi TK Borowsky ML Daly MJ Lee C Morton CC MacDonald ME Gusella JF Talkowski ME 《Nature genetics》2012,44(4):390-7, S1
We defined the genetic landscape of balanced chromosomal rearrangements at nucleotide resolution by sequencing 141 breakpoints from cytogenetically interpreted translocations and inversions. We confirm that the recently described phenomenon of 'chromothripsis' (massive chromosomal shattering and reorganization) is not unique to cancer cells but also occurs in the germline, where it can resolve to a relatively balanced state with frequent inversions. We detected a high incidence of complex rearrangements (19.2%) and substantially less reliance on microhomology (31%) than previously observed in benign copy-number variants (CNVs). We compared these results to experimentally generated DNA breakage-repair by sequencing seven transgenic animals, revealing extensive rearrangement of the transgene and host genome with similar complexity to human germline alterations. Inversion was the most common rearrangement, suggesting that a combined mechanism involving template switching and non-homologous repair mediates the formation of balanced complex rearrangements that are viable, stably replicated and transmitted unaltered to subsequent generations. 相似文献
146.
147.
148.
Purification and cloning of amyloid precursor protein beta-secretase from human brain 总被引:40,自引:0,他引:40
Sinha S Anderson JP Barbour R Basi GS Caccavello R Davis D Doan M Dovey HF Frigon N Hong J Jacobson-Croak K Jewett N Keim P Knops J Lieberburg I Power M Tan H Tatsuno G Tung J Schenk D Seubert P Suomensaari SM Wang S Walker D Zhao J McConlogue L John V 《Nature》1999,402(6761):537-540
Proteolytic processing of the amyloid precursor protein (APP) generates amyloid beta (Abeta) peptide, which is thought to be causal for the pathology and subsequent cognitive decline in Alzheimer's disease. Cleavage by beta-secretase at the amino terminus of the Abeta peptide sequence, between residues 671 and 672 of APP, leads to the generation and extracellular release of beta-cleaved soluble APP, and a corresponding cell-associated carboxy-terminal fragment. Cleavage of the C-terminal fragment by gamma-secretase(s) leads to the formation of Abeta. The pathogenic mutation K670M671-->N670L671 at the beta-secretase cleavage site in APP, which was discovered in a Swedish family with familial Alzheimer's disease, leads to increased beta-secretase cleavage of the mutant substrate. Here we describe a membrane-bound enzyme activity that cleaves full-length APP at the beta-secretase cleavage site, and find it to be the predominant beta-cleavage activity in human brain. We have purified this enzyme activity to homogeneity from human brain using a new substrate analogue inhibitor of the enzyme activity, and show that the purified enzyme has all the properties predicted for beta-secretase. Cloning and expression of the enzyme reveals that human brain beta-secretase is a new membrane-bound aspartic proteinase. 相似文献
149.
Nanostructured materials have drawn considerable attention because they are promising candidates for nextgeneration electronic and photonic devices with low power consumption[1-5]. A number of methods, such as laser ablation[6], template-induced growth[7], arc discharge [8], vapor transport [9], and molecular-beam epitaxy[10] have been developed to synthesize Si, Ge, MgO,SnO2, GaN, and Ga2O3 nanowires or nanorods[11-15]. 相似文献
150.