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(1998)Journal ArticleThe first results obtained using a SOI device for microdosimetry applications are presented. Microbeam and broadbeam spectroscopy methods are used for determining minority carrier lifetime and radiation damage constants. A spectroscopy model is presented which includes the majority of effects that impact spectral resolution. Charge collection statistics were found to substantially affect spectral resolution. Lateral diffusion effects significantly complicate charge collection
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(1996)Journal ArticleThe passivated emitter, rear locally diffused (PERL) cells, fabricated in our laboratory, reach an efficiency of 24.0%, the highest value for any silicon-based solar cell under terrestrial illumination. In an attempt to improve the rear surface passivation, which is usually obtained by a thermally grown oxide, we add a floating (i.e., noncontacted) p-n junction at the rear surface, resulting in the passivated emitter, rear floating p-n junction (PERF) cell design. Although these cells exhibit record 1-sun open-circuit voltages of up to 720 mV, their efficiency is degraded by nonlinearities ("shoulders") in the logarithmic I-V curves. In order to understand and manipulate such nonlinearities, this paper presents a detailed investigation of the internal operation of PERF cells by means of numerical modelling based on experimentally determined device parameters. From the model, we derive design rules for optimum cell performance and develop a generalized argumentation that is suitable to compare the passivation properties of different surface structures. For example, the oxidized rear surface of the PERL cell is treated as an electrostatically induced floating junction in this approach and analogies to the diffused floating p-n junction are drawn. Our simulations indicate that optimum rear surface passivation can be obtained in three different ways. (i) The floating junction of the PERF cell should be very lightly doped, resulting in a sheet resistivity of 5000 Omega/[D'Alembertian], and losses due to shunt leaking paths between the p-n junction and the rear metal contacts must be avoided. (ii) The rear surface of the PERL cell should be passivated by chemical vapor deposition of a silicon nitride film containing a larger positive interface charge density than exists in thermally grown oxides. (iii) An external gate can be added at the rear with low leakage currents and gate voltages of around 15 V.
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(1998)Journal ArticleGrowth under elevated [CO2] promoted spring frost damage in field grown seedlings of snow gum (Eucalyptus pauciflora Sieb. ex Spreng.), one of the most frost tolerant of eucalypts. Freezing began in the leaf midvein, consistent with it being a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO2] (– 5.7 oC versus – 4.3 oC), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO2 fertilization and may affect predictions of vegetation change based on increasing temperature.
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Freezing-Induced Hydration Forces Between Phospatidylcholine Bilayers-The Effect of Osmotic Pressure(1997)Journal ArticleQuantitative nuclear magnetic resonance was used to measure the freezing behaviour of lamellar phases of phosphatidylcholine in water and in solutions of sorbitol. Both solute and solvent were deuterated in different series of experiments to allow the calculation of the partitioning of solute and solvent molecules between the lamellar phase and unfrozen bulk solution. Sorbitol, as well as water, was found to redistribute between these phases as a function of temperature. The results show a strong, repulsive, interlamellar force which decreases approximately exponentially with hydration. Compared to measurements on lipid/water systems and solute/water systems, the hydration of the lamellar phase containing solutes is slightly less than the sum of the hydrations of lipid and solute at any given chemical potential of water. For a lamellar phase with a given quantity of lipid, interlamellar sorbitol and water, reduction of chemical potential of water is greater than that due to lipid acting alone plus that due to solute acting alone.
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(1997)Journal ArticleThe acoustic impedance Z(f) of the flute has a range of around 70 dB. We report the use of a novel impedance spectrometer to measure the acoustic impedance of the flute with a resolution of +/- 1 1 Hz, a dynamic range of over 80 dB and which makes the measurement in 1 second. We report Z(f) for the standard fingerings over the range C4 to C7, and for selected non-standard fingerings as well. These results explain some of the idiosyncracies of the instrument well known to flutists, and in some cases suggest ways in which these may be overcome.
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(1997)Journal ArticleAcoustic resonances of the vocal tract give rise to formants (broad bands of acoustic power) in the speech signal when the vocal tract is excited by a periodic signal from the vocal folds. This paper reports a novel instrument which uses a real-time, non-invasive technique to measure these resonances accurately during phonation. A broadband acoustic current source is located just outside the mouth of the subject and the resulting acoustic pressure is measured near the lips. The contribution of the speech signal in the pressure spectrum is then digitally suppressed and the resonances are calculated from the input impedance of the vocal tract as a function of frequency. The external excitation signal has a much smaller harmonic spacing than the periodic signal from the vocal folds and consequently the resonances are determined much more accurately due to the closer sampling. This is particularly important for higher pitched voices and we demonstrate that this technique can be markedly superior to the curve-fitting technique of linear prediction. The superior frequency resolution of this instrument which results from external vocal tract excitation can provide the precise, stable, effective, articulatory feedback considered essential for some language-learning and speech therapy applications.
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(1995)Journal ArticleThis paper examines changes to the abrasive wear resistance of aluminium-based composites when heat-treated to different ageing conditions. The composites studied were the age-hardenable aluminium alloys 2014 Al or 6061 Al reinforced with 3 μm or 20 μm SiC particles. The materials were aged at temperatures between 50°C and 250°C, and changes to the wear resistance were measured using a pin-on-drum machine. When aged at the lowest temperatures (between ≈50–150°C), transmission electron microscopy revealed the presence of the solute clusters (e.g. GP zones) and small coherent precipitates in the aluminium alloy matrices, and these were easily sheared by mobile dislocations. Consequently, the hardness and abrasive wear resistance of these under-aged composites were measured to be relatively low. Raising the ageing temperature to ≈200°C increased the hardness and abrasion resistance of the composites to the peak-aged condition, and this was associated with the precipitation of small intermetallic compounds which were incoherent with the crystal structures of the aluminium alloy matrices. At 250°C the composites were over-aged, and this resulted in a reduction in hardness and wear resistance due to the coarsening of the intermetallic precipitates. The composites containing 20 μm SiC particles were slightly more wear resistant than those containing 3 μm SiC.
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(1996)Journal ArticleThe effect of angular and spherical shaped Al2O3 particulates on the precipitation behaviour of 6061 aluminium-matrix composites has been studied using microhardness testing, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The evolution of precipitates at each stage of precipitation corresponding to the DSC-peak was monitored through careful TEM observations. Both the formation and dissolution behaviour of the precipitates in the ageing process in the composites were compared with the unreinforced 6061 matrix alloy. Although an overall accelerated ageing response in the composites was reported, not all stages in the ageing process were influenced by the addition of particulates. However, the precipitation sequence was not altered by the presence of the particulates. The degree of acceleration and the relative proportion of the phase/phases was found to depend on reinforcement parameters such as size, shape and volume fraction. In the composites, higher dislocation densities were observed in the immediate vicinity of the ceramic particles which may have formed due to the large difference in the coefficients of thermal expansion (CTE) between the ceramic particles and the matrix. These CTE-dislocation effects influence the kinetics of precipitation.