全文获取类型
收费全文 | 287篇 |
免费 | 12篇 |
专业分类
化学 | 212篇 |
力学 | 3篇 |
数学 | 36篇 |
物理学 | 48篇 |
出版年
2022年 | 1篇 |
2021年 | 5篇 |
2020年 | 8篇 |
2019年 | 9篇 |
2018年 | 1篇 |
2017年 | 6篇 |
2016年 | 6篇 |
2015年 | 8篇 |
2014年 | 7篇 |
2013年 | 11篇 |
2012年 | 21篇 |
2011年 | 26篇 |
2010年 | 12篇 |
2009年 | 3篇 |
2008年 | 16篇 |
2007年 | 25篇 |
2006年 | 12篇 |
2005年 | 22篇 |
2004年 | 12篇 |
2003年 | 13篇 |
2002年 | 17篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1999年 | 3篇 |
1998年 | 1篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1992年 | 3篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1981年 | 4篇 |
1980年 | 3篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
1967年 | 1篇 |
1963年 | 1篇 |
1961年 | 2篇 |
1958年 | 2篇 |
1914年 | 2篇 |
排序方式: 共有299条查询结果,搜索用时 15 毫秒
161.
Stacey RJ 《Analytical and bioanalytical chemistry》2011,401(6):1749-1759
Residues from medicine containers in the collections of the British Museum have been investigated as part of a wider programme
of scientific work on Roman surgical instruments. The cylindrical bronze containers are often described as instrument cases,
but some contain materia medica, ranging from extensive extant remains of ancient preparations to possible minor deposits on the interior surfaces of the
containers. Samples from seven residues have been analysed by gas chromatography–mass spectrometry (GC-MS) to identify lipid,
resin and carbohydrate components and by X-ray fluorescence and Raman spectroscopy to characterise inorganic materials. The
results have provided evidence for ointments and powders or pills consistent with a medical purpose. The ingredients identified
include beeswax, fat, conifer resin and gum-derived sugars, plus elemental carbon and lead and zinc salts. Particularly significant
were the varied compositions of residues from four sections of a multi-compartment container. In one of these compartments,
the beeswax seems to have been prepared as the ‘Punic wax’ described by Pliny. Experimental preparation of Punic wax following
Pliny’s method was undertaken in the laboratory and the product analysed to compare with the ointment residues. This paper
discusses the GC-MS results of both the experimental material and the archaeological residues and their significance for the
interpretation of the past intended applications of the medicines and the use of the containers. 相似文献
162.
Stacey D. Haskins David G. Kelly Ron D. Weir 《Journal of Radioanalytical and Nuclear Chemistry》2011,287(2):471-478
Instrumental neutron activation analysis (INAA) was used to determine trace elemental contamination in bird feathers. Primary
feathers from twelve mallard (Anas platyrhynchos) ducks, migrating through the Thousand Islands region of Ontario, Canada, were analyzed for selenium, mercury, chromium,
arsenic and antimony. Certified reference materials were used to assess the quality of the analytical procedure. Quantification
of chemical elements was performed using Ortec Gamma Vision software. Five chemical elements were quantified, with corresponding
analytical uncertainties of less than 20%. Results indicated the presence of As (max = 0.13 mg kg−1), Cr (max = 2.6 mg kg−1), Hg (max = 7.7 mg kg−1), Sb (max = 0.31 mg kg−1) and Se (max = 1.31 mg kg−1). To assess the validity of using INAA as a quantitative analytical technique for feather samples, two standard reference
materials were examined and mercury results were compared to those obtained from both direct mercury analysis (DMA) and cold
vapour atomic absorption spectroscopy (CVAAS). Several CVAAS results differed significantly from the INAA results; in many
instances CVAAS appeared to under-report when compared to INAA, with relative percent difference values as high as 126%. Conversely,
results obtained using DMA compared favourably with INAA. For all samples, RPD values were within 30%. This is the first study
to use INAA to examine feather contamination in Canadian migratory waterfowl and the first to corroborate INAA feather results
by comparing them to those obtained using CVAAS and DMA. 相似文献
163.
The 200.1 MHz 1H NMR spectra of 5-methyl-5-phenylhydantoin, 1, have been studied in CD3CN solution at ambient temperatures with the achiral shift reagent, tris (6, 6, 7, 7, 8, 8, 8-heptafluoro-2,2-dimethyl-3,5-octanedionato) europium (III), 2, and the chiral reagent, tris[3-(heptafluoropropylhydroxymethylene)-(+)-camphorato] europium (III), 3. Although 1 cannot chelate the LSRs, use of sufficiently high LSR:1 molar ratios served to compensate for competitive binding of LSR by the polar solvent, and permitted substantial lanthanide-induced shifts to be observed with 2 or 3. With 3, significant enantiomeric shift differences were produced for the ortho aryl protons and for the CH3 signals. The ortho proton signal appears to offer excellent potential for direct determination of enantiomeric excess of 1. These results demonstrate the utility of LSRs 2 or 3 even in a polar solvent with a nonchelating substrate. 相似文献
164.
165.
166.
167.
Zachariah J. Berkson Ming‐Feng Hsieh Stef Smeets David Gajan Alicia Lund Anne Lesage Dan Xie Stacey I. Zones Lynne B. McCusker Christian Baerlocher Bradley F. Chmelka 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(19):6321-6325
The adsorption and reaction properties of heterogeneous zeolite catalysts (e.g. for catalytic cracking of petroleum, partial oxidation of natural gas) depend strongly on the types and distributions of Al heteroatoms in the aluminosilicate frameworks. The origins of these properties have been challenging to discern, owing in part to the structural complexity of aluminosilicate zeolites. Herein, combined solid‐state NMR and synchrotron X‐ray powder diffraction analyses show the Al atoms locate preferentially in certain framework sites in the zeolite catalyst Al‐SSZ‐70. Through‐covalent‐bond 2D 27Al{29Si} J‐correlation NMR spectra allow distinct framework Al sites to be identified and their relative occupancies quantified. The analyses show that 94 % of the Al atoms are located at the surfaces of the large‐pore interlayer channels of Al‐SSZ‐70, while only 6 % are in the sub‐nm intralayer channels. The selective siting of Al atoms accounts for the reaction properties of catalysts derived from SSZ‐70. 相似文献
168.
169.
B. Niczyporuk T. Zeludziewicz K. W. Chen R. Hartung G. Folger B. Lurz H. Vogel U. Volland H. Wegener J. K. Bienlein R. Graumann J. Krüger M. Leißner M. Schmitz F. H. Heimlich R. Nernst A. Schwarz U. Strohbusch H. -J. Trost P. Zschorsch A. Engler R. W. Kraemer F. Messing C. Rippich B. Stacey S. Youssef A. Fridman G. Alexander A. Av-Shalom G. Bella Y. Gnat J. Grunhaus E. Hörber W. Langguth M. Scheer LENA Collaboration 《Zeitschrift fur Physik C Particles and Fields》1981,9(1):1-8
Charged hadron production ine + e ? annihilation is studied in the 7 to 10 GeV CM energy region and at the Υ (9.46) and Υ′ (10.01) resonances with the LENA detector at DORIS. The statistical moments of the charged multiplicities are studied. The data show KNO scaling behaviour and suggest the presence of long range correlations. An average charged multiplicityrise of Δn(Υ)=0.55±0.19 and Δn(Υ′)=1.26±0.29 over the continuum is observed for the Υ and Υ′ direct decays. The jet structure of the Υ and Υ′ direct decays is investigated using the charged particles. The polar angular distributions of the jet axis behave like 1+α(T) cos2θ with 〈α(T)〉Υ=0.7±0.3 and 〈α(T)〉Υ′=0.6±0.4. The 〈α(T)〉Υ value is in agreement with the QCD vector gluon assignment and excludes scalar gluons by more than four standard deviations. 相似文献
170.
[reaction: see text] The synthesis of stereochemically defined tri- and penta-heterocyclic ring systems 9 and 28, respectively, via the metathesis reaction of substituted oxanorbornanes derived from 3 is described. 相似文献