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1

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New model for nearly constant dielectric loss in conductive systems: Temperature and concentration dependencies (2002)

Macdonald, J. Ross

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 116 (2002), S. 3401-3409

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: By appropriate fitting of conductive-system frequency-response data for two different ionic materials over ranges of temperature and ionic concentration, it is shown how dispersion associated entirely with ionic motion and that leading to nearly constant dielectric loss (NCL) can be unambiguously distinguished and separated. The latter is clearly associated with polarization of the bulk material, and in the limit of zero mobile-ion concentration NCL appears to approach zero, yielding only a bulk dielectric constant, cursive-epsilonD∞0, one that is frequency-independent over the usual immittance-spectroscopy experimental range. For nonzero ionic concentration, however, dielectric NCL appears and can be represented by a small-exponent constant phase element (CPE) complex power law in frequency. This part of the full response may be modeled either by a CPE that includes all bulk dielectric dispersion or, more plausibly, by cursive-epsilonD∞0 and a CPE representing only incremental bulk dispersion associated with coupling between ionic motion and bulk polarization. In this case, interestingly, precise power-law dependencies of various dielectric parameters on ionic concentration are established but need theoretical explanation. Fitting of the ionic part of the total dispersion with three different Kohlrausch–Williams–Watts models leads to dependencies of their different β-shape parameters and dielectric quantities on temperature and on ionic concentration and strongly suggests that the widely used original-modulus-formalism dispersion fitting model is incorrect and should be replaced by a corrected version. © 2002 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1434953

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Nearly constant loss or constant loss in ionically conducting glasses: A physically realizable approach (2001)

Macdonald, J. Ross

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 115 (2001), S. 6192-6199

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Conductivity exhibiting power-law frequency response with an exponent of unity leads to frequency-independent dielectric loss. Such constant-loss (CL) behavior is not physically realizable over a nonzero frequency range, and approximate expressions that have been used to represent it are inconsistent with the Kronig–Kramers relations. Response models are proposed and investigated that do satisfy these relations and can lead to very close approximation to CL over many frequency decades, as often observed at low temperatures in ionic conductors such as glasses. Apparent CL response is shown to arise from the series connection of a constant-phase complex-power-law element (CPE), with exponent δ (0〈δ(very-much-less-than)1), and a frequency-independent dielectric constant, cursive-epsilonU. Two physically disparate situations can lead to such a series connection. The first involves bulk CPE response in series with an electrode-related, double-layer blocking capacitance involving a dielectric constant cursive-epsilonS. Then, apparent CL behavior may be associated with localized ionic motion in the bulk of the material. The second (mirror-image) situation involves CPE response associated with ionic motion in or at an electrode in series with a capacitance such as the bulk high-frequency-limiting total dielectric constant cursive-epsilon∞ or the pure-dielectric quantity cursive-epsilonD∞. The present model is used to simultaneously fit both the real and imaginary parts of data derived from measurements on a sodium-trisilicate glass at 122 K. This data set exhibits power-law nearly constant loss for cursive-epsilon′(ω) and apparent CL for cursive-epsilon″(ω). The magnitude of the CL closely satisfies a simple equation involving only δ and cursive-epsilonU. Further, for the electrode-power-law situation, estimated values of limiting-high-frequency dielectric constants turn out to be more consistent with bulk values established at much higher temperatures where nearly constant loss is no longer a dominant part of the response. Data at −0.5°C are also analyzed with a more complicated composite model, one that is a generalization of both of the above approaches, and nearly constant loss bulk, not electrode, power-law effects in both cursive-epsilon′(ω) and cursive-epsilon″(ω) are isolated and quantified. For this data set it is shown that electrode effects are important at both ends of the frequency range.© 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1398299

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3

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Scaling and modeling in the analysis of dispersive relaxation of ionic materials (2001)

Macdonald, J. Ross

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 90 (2001), S. 153-161

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ISSN: 1089-7550

Source: AIP Digital Archive

Topics: Physics

Notes: Problems with scaling of conductive-system experimental Mdat″(ω) and σdat′(ω) data are considered and resolved by dispersive-relaxation-model fitting and comparison. Scaling is attempted for both synthetic and experimental M″(ω) data sets. A crucial element in all experimental frequency-response data is the influence of the high-frequency-limiting dipolar-and-vibronic dielectric constant cursive-epsilonD∞, often designated cursive-epsilon∞, and not related to ionic transport. It is shown that cursive-epsilonD∞ precludes scaling of Mdat″(ω) for ionic materials when the mobile-charge concentration varies. When the effects of cursive-epsilonD∞ are properly removed from the data, however, such scaling is viable. Only the σ′(ω) and cursive-epsilon″(ω) parts of immittance response are uninfluenced by cursive-epsilonD∞. Thus, scaling is possible for experimental σ′(ω) data sets under concentration variation if the shape parameter of a well-fitting model remains constant and if any parts of the response not associated with bulk ionic transport are eliminated. Comparison between the predictions of the original-modulus-formalism (OMF) response model of 1972–1973 and a corrected version of it that takes proper account of cursive-epsilonD∞, the corrected modulus formalism (CMF), demonstrates that the role played by cursive-epsilonD∞ (or cursive-epsilon∞) in the OMF is incorrect. Detailed fitting of data for three different ionic glasses using a Kohlrausch–Williams–Watts response model, the KWW1, for OMF and CMF analysis clearly demonstrates that the OMF leads to inconsistent shape-parameter (β1) estimates and the CMF does not. The CMF KWW1 model is shown to subsume, correct, and generalize the recent disparate scaling/fitting approaches of Sidebottom, León, Roling, and Ngai. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1374480

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Neurobiology Cannabinoids act backwards (2001)

Vaughan, Christopher W. ; Christie, MacDonald J.

[s.l.] : Nature Publishing Group

Nature 410 (2001), S. 527-530

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] Preparations from the plant Cannabis sativa have been used since antiquity, not only for their intoxicating effects, but also to treat a number of ailments. The main active component of these preparations, Δ9-tetrahydrocannabinol, produces most of its effects on the central ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/35069167

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Nitrogen input together with ecosystem nitrogen enrichment predict nitrate leaching from European forests (2002)

MacDonald, J. A. ; Dise, N. B. ; Matzner, E. ; [et al.]

Oxford, UK : Blackwell Science Ltd

Global change biology 8 (2002), S. 0

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ISSN: 1365-2486

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography

Notes: The IFEF database (Indicators of Forest Ecosystem Functioning), consisting of nitrogen deposition, nitrate leaching fluxes, and soil and ecosystem characteristics, is analysed to evaluate the C/N ratio in the organic horizon as an indicator of nitrate leaching. One hundred and eighty one forests are examined, from countries across Europe ranging from boreal to Mediterranean regions, encompassing broadleaf and coniferous sites and plot and catchment studies. N input in throughfall ranges from less than 1 kg N ha−1 y−1 in northern Norway and Finland to greater than 60 kg N ha−1 y−1 in the Netherlands and Czech Republic. The amount of NO3– leached covers a smaller range, between 1 and 40 kg N ha−1 y−1. Nitrate leaching is strongly dependent on the amount of nitrogen deposited in throughfall (N input) and simply adding the C/N ratio in the organic horizon to a regression equation does not improve this relationship. However, when the data are stratified based on C/N ratios less than or equal to 25 and greater than 25, highly significant relationships (P 〈 0.05) are observed between N input and NO3– leached. The slope of the relationship for those sites where C/N ratio is ≤ 25 (′nitrogen enriched′ sites) is twice that for those sites where C/N ratio is 〉 25. These empirical relationships may be used to identify which forested ecosystems are likely to show elevated rates of nitrate leaching under predicted future nitrogen deposition scenarios. Elevated NO3– leaching also shows a relationship with soil pH, with high rates of NO3– leaching only observed at sites with a pH 〈 4.5 and N inputs 〉 30 kg N ha−1 y−1. Tree age and species have no significant impact on the ecosystem response to N input at a regional scale.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-2486.2002.00532.x

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6

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Oxidation of atmospheric methane in Northern European soils, comparison with other ecosystems, and uncertainties in the global terrestrial sink (2000)

Smith, K. A. ; Dobbie, K. E. ; Ball, B. C. ; [et al.]

Oxford, UK : Blackwell Science Ltd

Global change biology 6 (2000), S. 0

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ISSN: 1365-2486

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology , Energy, Environment Protection, Nuclear Power Engineering , Geography

Notes: This paper reports the range and statistical distribution of oxidation rates of atmospheric CH4 in soils found in Northern Europe in an international study, and compares them with published data for various other ecosystems. It reassesses the size, and the uncertainty in, the global terrestrial CH4 sink, and examines the effect of land-use change and other factors on the oxidation rate.Only soils with a very high water table were sources of CH4; all others were sinks. Oxidation rates varied from 1 to nearly 200 μg CH4 m−2 h−1; annual rates for sites measured for ≥1 y were 0.1–9.1 kg CH4 ha−1 y−1, with a log-normal distribution (log-mean ≈ 1.6 kg CH4 ha−1 y−1). Conversion of natural soils to agriculture reduced oxidation rates by two-thirds –- closely similar to results reported for other regions. N inputs also decreased oxidation rates. Full recovery of rates after these disturbances takes 〉 100 y. Soil bulk density, water content and gas diffusivity had major impacts on oxidation rates. Trends were similar to those derived from other published work. Increasing acidity reduced oxidation, partially but not wholly explained by poor diffusion through litter layers which did not themselves contribute to the oxidation. The effect of temperature was small, attributed to substrate limitation and low atmospheric concentration.Analysis of all available data for CH4 oxidation rates in situ showed similar log-normal distributions to those obtained for our results, with generally little difference between different natural ecosystems, or between short-and longer-term studies. The overall global terrestrial sink was estimated at 29 Tg CH4 y−1, close to the current IPCC assessment, but with a much wider uncertainty range (7 to 〉 100 Tg CH4 y−1). Little or no information is available for many major ecosystems; these should receive high priority in future research.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1365-2486.2000.00356.x

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Initial stages of molecular-beam epitaxy growth of GaN on 6H-SiC(0001) (2001)

Lu, J. ; Haworth, L. ; Westwood, D. I. ; [et al.]

Woodbury, NY : American Institute of Physics (AIP)

Applied Physics Letters 78 (2001), S. 1080-1082

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ISSN: 1077-3118

Source: AIP Digital Archive

Topics: Physics

Notes: We studied the atomic H etching of 6H-SiC substrates and the initial stages of GaN/6H-SiC molecular-beam epitaxy growth. Atomic H etched 6H-SiC(0001)Si and (0001¯)C surfaces show a ((square root of 3)×(square root of 3))−R30° and a (1×1) reconstruction respectively, with 0.7±0.2 monolayers of remnant O on both surfaces. GaN/6H-SiC(0001)Si growth is initiated by the formation of islands that develop into flat-top terraces through coalescence. Growth steps of one or integer numbers of the GaN atomic bilayer height are observed. GaN grown on 6H-SiC(0001¯)C is rougher with islands of irregular shape. X-ray photoemission spectroscopy studies show that Si 2p and C 1s photoelectron inelastic mean free paths in GaN are 22±1 and 20±1 Å, respectively. © 2001 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.1350430

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The effect of exercise and captivity on energy partitioning, reproductive maturation and fertilization success in adult sockeye salmon (2004)

Patterson, D. A. ; Macdonald, J. S. ; Hinch, S. G. ; [et al.]

Oxford, UK; Malden, USA : Blackwell Science Ltd/Inc.

Journal of fish biology 64 (2004), S. 0

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ISSN: 1095-8649

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Biology

Notes: Releasing a population of adult Fraser River sockeye salmon Oncorhynchus nerka from an energetically more expensive non-feeding natural migration (1089 km, natal groups) and allowing them to sexually mature in either a moderate flow (captive exercise group) or a no flow (captive non-exercise group) environment resulted in significant differences in body energy densities (MJ kg−1) among groups (ranked: initial natural migrator 〉 non-exercise 〉 exercise 〉 natal arrivals). Similarly, per cent body lipids were significantly lower in exercise females compared with non-exercise females, experimentally confirming the central role lipid catabolism has in powering swimming. Although restricting exercise did result in greater body energy reserves at spawning, this did not result in a reallocation of energy to reproduction, as fecundity, egg size and gonad composition were unchanged among the groups. Furthermore, non-exercise females had delayed maturity, lower egg deposition rates, and were more likely to die prior to egg ovulation than exercise females and natal spawners. Eggs from captive exercise adult females were more likely to survive to the eyed stage than eggs from captive non-exercise females, but both captive groups had significantly lower egg fertilization success than natal spawners suggesting that confinement stress played a role in some of results presented. There were no significant differences in plasma levels of glucose, lactate, cortisol and reproductive hormones among the groups, but correlations among acute and chronic indicators of stress were significant among individuals. These results indicate that exercise during the late stages of maturation, as might occur normally, may be required for optimizing reproductive maturation and maximizing reproductive success.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1111/j.1095-8649.2004.0370.x

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Discovery and characterization of a family of insecticidal neurotoxins with a rare vicinal disulfide bridge (2000)

Wang, Xiu-hong ; Connor, Mark ; Smith, Ross ; [et al.]

[s.l.] : Nature America Inc.

Nature structural biology 7 (2000), S. 505-513

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ISSN: 1072-8368

Source: Nature Archives 1869 - 2009

Topics: Biology , Medicine

Notes: [Auszug] We have isolated a family of insect-selective neurotoxins from the venom of the Australian funnel-web spider that appear to be good candidates for biopesticide engineering. These peptides, which we have named the Janus-faced atracotoxins (J-ACTXs), each contain 36 or 37 residues, with four ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/nsb0600_505

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