Heat Pulse Technique Research Articles

Propagation of acoustic phonons across the interfaces in CdTe and Si/CVD-diamond and quasi-two-dimensional phonon wind in CdTe/ZnTe quantum wells

Using a heat-pulse technique we investigated propagation of acoustic phonons in twinned CdTe, ZnTe crystals, in the vicinity of CdTe/ZnTe quantum well, and Si/CVD-diamond structures. Phonon mean free paths in CdTe were estimated. We studied the effect of phonon wind on the luminescence of narrow CdTe quantum well and observed an increase in the total luminescence intensity and the change of the luminescence band shape. We believe that this effect is due to a quasi-two-dimensional lateral propagation of subsurface and interface acoustic phonons in the ZnTe/CdTe structure. Preliminary data obtained on the Si/CVD-diamond interfaces suggest they are strongly damaged. To reduce the thermal boundary resistance of the interface between the material studied and the thin-film phonon detector we developed a buried bolometer based on a graphitized layer produced by ion implantation in diamond.

Studies of canopy structure and water use of apple trees on three rootstocks

Abstract Canopy structure and water use was studied on full size, fruit bearing apple (Malus domestica, Golden delicious variety) orchards on three rootstocks: M9, MM106 and the local Hashabi. Structure was measured by the gap fraction inversion method and crop water use (sap flow) by the heat pulse technique. The results showed that leaf area index (LAI) was not significantly different for each rootstock even though measured leaf area (LA) per tree differed greatly. Results also showed LAI measured destructively differed by less than 1 from the gap fraction inversion measurements. No significant differences in LAI between the two orchards (M9, MM106) were found in either measurement method, indicating good relative accuracy of the gap fraction inversion method. Water use of the M9 orchard was much less than that of MM106 and Hashabi orchards, and measured water use was not well correlated with water application based on a graduated class A pan crop factor for the three orchards. The dwarfing rootstock, M9, had the lowest canopy conductance, and was found to have lower sunlit leaf conductance in the porometer measurements, so it is in a condition of water stress. However, the M9 orchard received enough irrigation, therefore, water stress in M9 orchard was probably not caused by insufficient irrigation, but by high resistance to water transport in the stem or root.

Measurement of microscopic growth rates in float zone silicon crystals

The microsegregation of doped silicon crystals grown by the float zone technique was investigated by using a heat pulse technique. The heat pulses (3–10 Hz) generated finely spaced periodic time markers which overlay the dopant inhomogeneities caused by convective instabilities in the melt. The microscopic growth rate was determined by measuring the distance of the markers. For a flow regime governed by time-dependent thermocapillary convection, the average growth rate of 2 mm/min fluctuated from 1.27 to 2.63 mm/min with fluctuation frequencies between 0.01 and 0.15 Hz. The microscopic growth rate becomes constant when applying a static axial magnetic field of 500 mT. The impact of concentration fluctuations within the solute boundary layer ahead of the crystal-melt interface was observed qualitatively by comparing the interface profile given by the pulse markings with the contours of the convectively induced striations.

Measuring peat moisture content using the dual-probe heat pulse technique

The dual-probe heat pulse (DPHP) technique for measuring soil volumetric moisture content (Θv) is evaluated for use in peat soils with very high organic matter contents. The method has a greater sensitivity in peat soils compared with mineral soils and excellent resolution is possible, even at moisture contents as high as 90% by volume. Advantages of the DPHP technique are that sensors are simple to construct from inexpensive parts and calibration is not required since the method is based on a physical model of radial heat flow in soil. A multiplexer method was developed to allow multiple probes to be deployed in the field. DPHP measurements of Θv for small peat samples compared closely to reference measurements made using the gravimetric method, and in the field were similar to results obtained using a time domain reflectometry (TDR) method. Peat soils display a high level of spatial variation in Θv at the scales of both DPHP and TDR probes, so that multiple probes of each type are required for adequate spatial sampling of Θv. Rapid changes in peat moisture content were recorded following rainfall infiltration events yet moisture storage did not remain elevated following rainfall, even for peat that was very dry. wetlands, hydrology, soil moisture.

On Measuring Soil Thermal Properties with a Dual-Probe Heat-Pulse Technique

On Measuring Soil Thermal Properties with a Dual-Probe Heat-Pulse Technique

Point defect scattering of THz phonons in sapphire determined by heat pulse back-scattering

Using a nanosecond heat pulse technique, we determined the point defect scattering rate for THz phonons due to oxygen isotopes in sapphire. We measured the generator power level at which phonon back-scattering could just be observed in a 2 mm thick pure crystal, corresponding to an elastic scattering rate for fast transverse phonons of 3 × 106s−1. To obtain the equivalent phonon frequency the Planck distribution of the emitted heat pulses was convolved with the ν4 dependence of point defect scattering, assuming the continuum model of lattice vibrations. This gave a dominant frequency for the scattered phonons of 2.5 THz. The residual elastic scattering rate due to oxygen isotopes at this frequency was calculated to be 1.6 × 106s−1 Despite the closeness of the numerical agreement, the observed power dependence of the scattered flux gave an apparent frequency variation closer to ν than to ν4. We believe this is due to the effect at higher frequencies of phonon down-conversion and the breakdown of the continuum model.

Water use, yield and fruit quality of lysimeter-grown apple trees: responses to deficit irrigation and to crop load

Effective irrigation scheduling for maximum crop yield and quality in a sustainable environment requires a good understanding of crop water use in relation to crop load and soil water availability. This study determined tree water use (TWU), fruit growth, yield, and quality of 'Braeburn' apple grown in lysimeters in response to two irrigation regimes and two crop loads in the growing season of 1997/1998. The irrigation regimes tested were control level of irrigation (CI) and deficit irrigation (DI) and the crop loads were commercial crop load (CCL) and light crop load (LCL) which was equivalent to 60% of the CCL. Tree water use was measured using two methods: soil-water balance through lysimetry and by a heat-pulse technique. There was no significant interaction between irrigation and crop load treatments and therefore only results for the main effects, irrigation and crop load, are presented. DI and LCL reduced TWU. Higher stomatal conductance (gs) was responsible for the higher TWU in CI but not in CCL. Increased mean fruit weight but decreased gross yield was observed in LCL. DI reduced mean fruit weight but had no significant effect on gross yield. Fruit quality, in terms of flesh firmness, total soluble solids, and dry matter concentration, was improved in DI both at harvest and after 12 weeks of cold storage but was, generally, not affected by crop load.

THEORETICAL ANALYSIS OF InP CRYSTAL GROWTH EXPERIMENT PERFORMED ON-BOARD RUSSIAN FOTON-11 SATELLITE

Traveling heater method (THM) experiments on InP crystal growth from indium solution (SKAT-FD) were performed during the Russian FOTON-11 mission in October 1997. The aim of these experiments was to ascertain the effectiveness of controlling heat and mass transfer in the liquid phase by applying a rotating magnetic field (RMF). As an effort to understand the parameters influencing the crystal growth a mathematical model was developed for numerical simulation of these experiments. The SKAT-FD crystals were grown with RMF at an induction of 1 and 2 mT. As the calculations show, such magnetic fields influence remarkably the hydrodynamics of the melt, and consequently, the growth process. The change of the magnetic induction value (or switching off the RMF) led to an abrupt change of the growth rate, measured from artificial dopant striations produced in the growing crystals by the heat-pulse technique.

Determination of transpiration in irrigated grapevines: comparison of the heat-pulse technique with gravimetric and micrometeorological methods

The use of a heat-pulse technique to monitor sap flow from which transpiration can be deduced was evaluated in ungrafted grapevines (Vitis vinifera L., cv. Sultana) under glasshouse and field conditions. There was a significant degree of agreement between daily transpiration deduced from heat-pulse velocity (T hp) and that determined directly by gravimetry in the glasshouse and by calibration using the Penman-Monteith equation in the field. Comparison throughout the growing season of T hp to transpiration calculated with the full Penman-Monteith equation produced a high coefficient of determination (r 2=0.69). A similar comparison of T hp with transpiration calculated with only the aerodynamic component of the Penman-Monteith equation produced a non-linear relationship, due to the equation over-estimating transpiration relative to T hp at high vapour pressure deficits (i.e. above 2.5 kPa). Values for total seasonal transpiration measured with heat-pulse sensors or calculated with either the full Penman-Monteith equation or with only the modified aerodynamic component of the equation were within 10% of each other. Transpiration (T hp) in grapevines with an adequate supply of soil water was shown to be coupled to ambient air conditions.

Transpiration of cottonwood/willow forest estimated from sap flux

Cottonwood/willow forests in the American Southwest consist of discrete, even-aged vegetation patches arranged in narrow strips along active and abandoned stream channels of alluvial flood plains. We used the heat-pulse velocity technique in this study to estimate transpiration in 12 such forest patches along a perennially flowing reach of the San Pedro River in southeastern Arizona, USA during five periods from April to October 1997. Transpiration per unit sapwood area was consistently higher for the larger cottonwood trees found on outer secondary channels compared to that of smaller cottonwood trees along the active channel, but statistically significant differences were found only in August and October. Conversely, transpiration per unit sapwood area in willow was markedly higher for trees along the primary channel than for those few larger trees that were sampled on the outer margins of the forest. Average daily transpiration at the canopy scale among the patches in July was 4.8±0.7 mm per day and ranged from 5.7±0.6 mm per day in young forest patches adjacent to the primary stream channel to 3.1±0.6 mm per day in more successionally advanced patches on secondary channels. Differences in our estimates of transpiration between forest patches along primary and secondary stream channels were related to differences in the ratio of sapwood area to ground area of the forest patches, and leaf area index. Estimates of transpiration from this forest type, and projections of transpiration and groundwater flux over larger areas on the San Pedro River, should take into account structural variation in these forests that relate to population dynamics of dominant trees.

Unsteady-state heart transfer in stoichiometric cadmium telluride at low temperatures and laser excitation

The heat-pulse technique was employed to study the heat transfer in laser-pulse excitation in a high-purity grain-oriented CdTe growth by multiple sublimation. Comparison of the heat pulses obtained in the experiment with the results of calculations by the Monte Carlo method allowed one to evaluate the mean free path of the acoustic phonons in this material. It is proposed on the basis of the results obtained that the scattering by planar defects (grain boundaries and twinning planes) is the main mechanism of phonon scattering at low temperatures.

Factors controlling transpiration of mature field-grown tea and its relationship with yield

The objective of this experiment was to determine the factors influencing the transpiration rates of mature, clonal tea ( Camellia sinensis L.) and estimate its transpiration efficiency. The heat pulse technique was used to measure transpiration rates of tea plants growing in the field as part of extensive canopies at Talawakelle, Sri Lanka during the period between 1 January and 19 February 1997. Irrigation and shading treatments were used to determine the influence of soil water content ( S) and irradiance on transpiration rate. The transpiration rate declined only slightly when S decreased from field capacity (44%) to 33%. However, when S declined below 33%, the transpiration rate showed a rapid decline from 1.6 to 0.7 l per plant per day at 15% S. When S was near field capacity, maximum transpiration rates of 0.53–0.93 l plant −1 h −1 occurred between 1000 and 1500 h. The corresponding maxima when the S was near permanent wilting point (i.e. at −1.5 MPa matric potential) were 0.27–0.53 l plant −1 h −1. Transpiration decreased linearly with decreasing irradiance throughout the range of radiation levels tested (i.e. from 100 to 15% of full sunlight) at a rate of 0.031 l per plant per day per % reduction in solar irradiance. The daily transpiration rates of tea plants (0.42–1.07 l per plant per day) under the natural shade of Grevillea robusta were considerably lower than the value of tea plants in the open, 3.511 l per plant per day. Spraying of an antitranspirant, Kaolin, decreased canopy temperature by 2–4°C and especially around mid-day. Kaolin also decreased transpiration slightly during the period between 1000 and 1500 h. Transpiration efficiency ( T E) was 9.637 kg ha −1 (made tea) mm −1 of water transpired. The relationship between total dry matter yield and the ratio between transpiration and mean saturation vapour pressure deficit also was linear with a proportionality constant of 6.9 g kg −1 kPa.

Response of a Fermi gas in a disordered system to a phonon flux

Phonon-induced conductivity (PIC) in a strongly elastically scattered two-dimensional electron gas in $\ensuremath{\delta}$-doped GaAs was studied using the heat-pulse technique. The results of phonoconductivity behavior with carrier temperature as well as magnetic field are presented. It was established that PIC is due to the influence of phonon flux on the temperature-dependent quantum corrections to conductivity through carrier heating. It was shown that the value of PIC is determined both by the absorbed nonequilibrium phonon power and the energy losses to lattice. The value of phonon flux energy absorbed by carriers was estimated from experiment.

Photo-enhanced response of hopping bolometers for detection of particles and non-equilibrium phonons

Using a heat pulse technique we have observed that the sensitivity of a GaAs:Zn bolometer operating in the hopping regime is significantly enhanced by illumination with white light. However, the enhancement cannot be understood simply in terms of the steady-state photoconductive change in resistivity varying as T 1/4 exp(T 0/T) 1/4 with temperature. We propose that the primary consequence of illumination is the population of excited states of the Zn acceptors between which non-equilibrium, and therefore higher frequency, phonon-assisted tunnelling is more effective than normal hopping.

Small-scale liquid mixing in a bioreactor column with xanthan-gum simulated filamentous media

Using the heat pulse technique, the local mean flow liquid velocity and the mixing conditions for twophase flow in the riser of an airlift bioreactor have been measured and analysed. Xanthan-gum solutions were used as the physical model to some filamentous broths reported in the literature. A two-fold decrease of liquid velocity and diffusional mixing regime are predicted for the course of a fermentation process proceeding in a non-Newtonian biomass growth circulation system.

Determining Soil Water Flux and Pore Water Velocity by a Heat Pulse Technique

A method is presented for measuring soil water flux density (J) with a thermo‐TDR (time domain reflectometry) probe. Constant heat input during a small time interval (15 s) is used to emit a heat pulse from a line heat source. Asymmetry in the thermal field near the heat source is quantified by computing the maximum dimensionless temperature difference (MDTD) between upstream and downstream locations. Heat transfer theory was used to relate MDTD to J A thermo‐TDR probe was used to obtain measurements of MDTD in water‐saturated soil materials of different textures (sand, sandy loam, and clay loam) with imposed water flux densities ranging from 1.16 × 10−5 to 6.31 × 10−5 m3 m−2 s−1 A nearly linear relationship between measured MDTDs and fluxes was observed for all soil materials. Measured and predicted MDTDs agreed well for flow experiments in sand. Greater discrepancies were observed for flow experiments in sandy loam and clay loam. Despite the lack of universal agreement between measured and predicted MDTDs, the experimental results indicate that the proposed method may provide a useful means of measuring J The method presented herein improves upon earlier methods by reducing distortion of the water flow field and minimizing heat‐induced soil water redistribution. Because the thermo‐TDR probe can be used to make TDR‐based measurements of volumetric water content (θ), the proposed method also may permit measurement of pore water velocity (J/θ).

Seasonal Water Acquisition and Redistribution in the Australian Woody Phreatophyte, Banksia prionotes

Sap flows in the xylem of plant roots in response to gradients in water potential, either between soil and atmosphere (transpiration) or soil layers of different moisture content (termed hydraulic redistribution). The latter has the potential to influence water budgets and species interactions, but we lack information for all but a few plant communities. We combined heat pulse measurements of sap flow with dye and isotope tracing techniques to gauge the movement of xylem sap within, and exudation from, roots of Banksia prionotes (Lindley). We demonstrated ‘ hydraulic lift’ during the dry season and provide some evidence that extremely dry soils limit hydraulic lift. In addition we report difficulties posed by spiralled xylem tissue in roots for the application of heat pulse techniques.

Energy relaxation by hot carriers in wurtzite GaN epilayers

In this paper, we report hot carrier energy relaxation processes studied by acoustic phonon emission in wurtzite GaN epilayers, using the heat pulse technique. In this method, the carriers were heated up by means of short (≈ 10 ns) voltage pulses and emitted phonons were detected by Al bolometers biased at their superconducting transition. Obtained phonon signals indicate that the optical phonon emission threshold has not been reached and longitudinal acoustic and transverse acoustic modes can be clearly resolved. This paper specifically concentrates on the electron temperature dependence of the energy relaxation rates and compares the experimental results with the existing theory.

Tree water use and rainfall partitioning in a mature poplar-pasture system.

Traditionally, poplars (Populus) have been planted to control erosion on New Zealand's hill-slopes, because of their capacity to dry out and bind together the soil, by reducing effective rainfall and increasing evapotranspiration and soil strength. However, the effect of widely spaced poplars on the partitioning of soil water and rainfall has not been reported. This study determined rainfall partitioning for 18 mid-spring days in a mature P. deltoides (Bart. ex Marsh, Clone I78)-pasture association (37 stems per hectare, unevenly spaced at 16.4 +/- 0.4 m) and compared it with a traditional open pasture system in grazed areas of a hill environment. Tree transpiration was measured by the heat pulse technique. A time-driven mathematical model was used to set a zero offset, adjust anomalous values and describe simultaneous sap velocity time courses of trees. The model showed that daylight sap flow velocities can be represented with a nonlinear Beta function (R(2) > 0.98), and differences in the parameters representing the initiation, duration and conformation of the sap velocity can be tested statistically to discern tree transpiration differences during the day. Evapotranspiration was greater for the poplar-pasture association than for the open pasture (2.7-3.0 versus 2.2 mm day(-1)). The tree canopy alone contributed 0.92 mm day(-1) as transpiration and 1.37 mm day(-1) as interception, whereas evapotranspiration of the pasture understory was only 0.4-0.6 mm day(-1). Despite the higher water use of the poplar-pasture association, soil water in the 0-300 mm soil stratum was higher than, or similar to, that of the open pasture. Tree shading decreased evapotranspiration and pasture accumulation under the trees.

The effect of root restriction on the incidence of blossom-end rot in bell pepper (Capsicum annuum L.)

SummaryWater and calcium uptake are important factors affecting the incidence of fruit blossom-end rot (BER) in tomato and pepper. In the present study an attempt was made to manipulate these factors by severe root pruning and to examine the effect on BER in greenhouse-grown bell pepper (Capsicum annuum L., cv. Mazurka). Pepper plants were transplanted, with the root system split into four separated compartments, each containing a single root quarter. Removal of half, or three quarters, of the root from fruit-bearing plants significantly reduced fruit BER incidence compared with plants with intact roots, especially in fruits which were at the rapid expansion stage at the beginning of the treatments. Removal of three quarters of the root reduced midday leaf water potential, stomatal conductance, and plant height. The number and weight of fruits were not affected by these treatments. Root pruning caused only a slight reduction in stem sap flow, as measured by the heat pulse technique. Calcium concentrations in the distal part of fruits from quarter-root-plants were higher than in fruits from non-pruned plants, whereas magnesium and potassium concentrations were not affected. On the other hand, in the leaves, calcium, magnesium and potassium concentrations were all reduced by root pruning. The K/Ca ratio decreased in the blossom-end of the fruits and increased in the leaves of root-pruned compared with control plants. The results suggest that root pruning did not affect the total uptake of calcium, apparently driven by transpiration, but did enhance calcium partitioning to the developing fruit. Root pruning also affected calcium distribution within the fruit and therefore attenuated BER incidence.