全文获取类型
收费全文 | 850篇 |
免费 | 26篇 |
国内免费 | 4篇 |
学科分类
数理化 | 880篇 |
出版年
2023年 | 7篇 |
2022年 | 9篇 |
2021年 | 10篇 |
2019年 | 10篇 |
2018年 | 7篇 |
2017年 | 6篇 |
2016年 | 18篇 |
2015年 | 17篇 |
2014年 | 24篇 |
2013年 | 41篇 |
2012年 | 40篇 |
2011年 | 66篇 |
2010年 | 28篇 |
2009年 | 22篇 |
2008年 | 58篇 |
2007年 | 50篇 |
2006年 | 69篇 |
2005年 | 44篇 |
2004年 | 42篇 |
2003年 | 39篇 |
2002年 | 40篇 |
2001年 | 5篇 |
2000年 | 10篇 |
1999年 | 8篇 |
1998年 | 10篇 |
1997年 | 12篇 |
1996年 | 18篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 14篇 |
1992年 | 8篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 4篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1982年 | 5篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1973年 | 3篇 |
1969年 | 2篇 |
排序方式: 共有880条查询结果,搜索用时 375 毫秒
71.
72.
Stephanie L. Gantt Nancy B. Valentine Adam J. Saenz Mark T. Kingsley Karen L. Wahl 《Journal of the American Society for Mass Spectrometry》1999,10(11):1131-1137
A method to aid in the analysis of bacterial samples of unknown concentration by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is demonstrated. It is shown that in MALDI analysis of bacteria, the intensities of resulting peaks in spectra are sensitive to the microbial concentration. At the high and low ends of the concentration range, no signal can be obtained, leaving very concentrated or very dilute samples indistinguishable. The addition of cytochrome c as an internal control allows the differentiation of these concentrated and dilute samples. The presence of the internal control causes only a 20% to 30% decrease in signal intensity when the bacterial concentration is optimum. However, the signal quality is improved when the internal control is added to some low concentrations of bacteria. 相似文献
73.
A new National Science Foundation supported curriculum, Interactions in Physical Science?, was evaluated on students’ conceptual change in the twelve concept areas of the national physical science content standard (B) for grades 5–8. Eighth grade students (N=66) were evaluated pre and post on a 31‐item multiple‐choice test of conceptual understanding developed by the Harvard ‐Smithsonian Center for Astrophysics. Significant student gains (p<. 05, t‐test, two‐tailed) occurred in all concept areas in the category of properties and changes in properties of matter; for the force concept areas in the category motions and forces; and for the heat transfer and light interactions areas in the category of transfer of energy. Two of the six concept areas in the category of transfer of energy, chemical and nuclear reactions and the sun as a major source of energy, were not addressed in this study. Significant learning gains as item percent correct were typically close to 20%, though effect sizes were small to medium in magnitude (d = 0.3–0.6). Implications of the study for conceptual change curriculum and teaching are discussed. 相似文献
74.
Nancy V. Phillips 《The Journal of the Operational Research Society》1987,38(9):797-802
This paper presents a weighting function for aggregating pre-emptive criteria into a representative composite in multicriteria assignment problems. The result is based on taking advantage of the bipartite graph structure of such problems. An application is presented which involves assigning faculty and personnel to parking lots at a large university. 相似文献
75.
Coordinating Theories of Learning to Account for Second-Grade Children's Arithmetical Understandings
This article coordinates social constructivism and socioculturalism orientations to explain 2nd-grade children's reasoning with 2-digit quantities. From a social constructivist position, we illustrate how the classroom teacher and the students constituted what counted as an acceptable mathematical explanation. As children offered informal and conventional ways of interpreting problem situations, they were expected to reason with quantities in sensible ways. From a sociocultural position, we explain how the teacher's and students' contributions were situated within the mathematical ways of knowing constituted by the community at large. Particular children's contributions were clarified in terms of the ways in which they participated in socially organized activities. By coordinating these lenses, we argue the local classroom mathematical practices constrained and enabled the mathematical practices of the wider society. 相似文献
76.
Nancy M. Edwards Donatella Peressini James E. Dexter Steven J. Mulvaney 《Rheologica Acta》2001,40(2):142-153
Linear viscoelastic properties (LVP) were determined for five durum wheat doughs and five common wheat doughs (representing
four different classes of Canadian common wheat) of different strength using creep testing. A creep time of 10,000 s was sufficient
to reach a state of steady state flow for all of the doughs. Creep compliances were analyzed in terms of a Burgers model.
For the durum doughs, the entire elastic compliance curve was shifted to higher values as the strength of the dough (as measured
by extensigraph) decreased, while the steady state viscosity increased with strength. For common wheat doughs, the elastic
compliance curves were steeper and the steady state viscosities were lower than for durum doughs of comparable extensigraph
strength. The retardation strengths associated with a maximum in the retardation spectra were lower for the stronger durum
doughs than for common wheat doughs of comparable strength. Differences in the LVP between durum and common wheat doughs of
similar extensigraph strength were interpreted in the context of physical gels with crosslinks and entanglements, whose contributions
to material properties are difficult to distinguish in short-time creep or dynamic measurements. The increased extensibility
of common wheat doughs relative to durum doughs of comparable extensigraph strength was attributed to a higher molecular weight
fraction in the polypeptide chains, similar in some respects to end-linked bimodal polymer networks. The idea of considering
these doughs as physical gels was supported by their stress relaxation behavior.
Received: 6 February 2000 Accepted: 22 August 2000 相似文献
77.
Herbert L. Holland Peter R. Andreana Reza Salehzadeh-Asl Aaron van Vliet Nancy J. Ihasz Frances M. Brown 《Monatshefte für Chemie / Chemical Monthly》2000,40(5):667-672
Biotransformation of a series of racemic N-benzoyl α-amino acids by the fungus Beauveria bassiana ATCC 7159 results in isolation of the corresponding D-amino acid benzamides in high enantiomeric purity and yield. 相似文献
78.
79.
NirmlaDevi Thakur Vilas G. Gaikar Debasis Sen Subhasish Mazumder Nancy S. Pandita 《Analytical letters》2017,50(4):690-711
A green method using Juglans regia bark extract was used to synthesize silver nanoparticles at room temperature with monitoring by absorption spectroscopy. The size and shape of the synthesized nanoparticles were characterized by infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, high-resolution transmission electron microscopy, and small-angle X-ray scattering. The average particle size was from 10 to 30?nm. Gas chromatography–mass spectrometry (GC–MS) was used for the separation, identification, and quantification of components of the plant extracts. A possible mechanism for the synthesis of nanoparticles was elucidated based on the GC–MS results. The synthesized silver nanoparticles showed effective inhibition against Streptococcus mutans, which is the main causative agent for dental caries. The nanoparticles also showed promising antibiofilm activity by inhibiting the glucosyltransferase enzyme. 相似文献
80.
Sara Luisa Rodríguez De Luna Luis Ángel GarzaSylvain Bernès Perla ElizondoBlanca Nájera Nancy Pérez 《Polyhedron》2010
A series of mononuclear complexes based on lanthanide ions has been synthesized and X-ray characterized. The compounds [LnIIIL2(NO3)3(H2O)2] (Ln = La, Ce, Pr, Nd, Sm, Gd and Tm; L = 2,6-bis(2-formylphenoxymethyl)pyridine) are found to be isomorphous and isostructural. Ligand L systematically coordinates through one carbonyl functionality, and the resulting complexes are placed on a twofold axis in crystals belonging to C2/c space-group. Emission spectra for Ln = La, Pr, Nd revealed a correlation between the Ln–O coordination bond length and the photoluminescent properties of the complexes, in line with a Förster–Dexter mechanism for intramolecular energy transfer. Ligand L is therefore a suitable sensitizer for lanthanide ions. 相似文献