J. J. Alvarado-Gil

Band‐gap shift in CdS semiconductor by photoacoustic spectroscopy: Evidence of a cubic to hexagonal lattice transition

The band‐gap energies of the CdS semiconductor are obtained by a photoacoustic spectroscopy (PAS) technique over a range of temperature of thermal annealing (TTA), in which the evolution of the sample structure is characterized by x‐ray diffraction patterns. The PAS experiment gives a set of data for the band‐gap shift in the region of the fundamental absorption edge. With increasing TTA the band‐gap shift increases up to a critical TTA when its slope decreases in a roughly symmetrical way. It is suggested that at this temperature a cubic to hexagonal‐lattice transition occurs.

Influence of thermal annealings in different atmospheres on the band‐gap shift and resistivity of CdS thin films

We study by photoacoustic spectroscopy the band‐gap shift effect of CdS films. The CdS films were grown by chemical bath deposition and exposed to different annealing atmospheres over a range of temperature in which the sample structure changes. We show the band‐gap evolution and resistivity as a function of temperature of thermal annealing and determine the process that produces the best combination of high band‐gap energy and low resistivity.

On the thermal conductivity of particulate nanocomposites

The modified effective medium approximation model proposed by Minnich and Chen [Appl. Phys. Lett. 91, 073105 (2007)] for the thermal conductivity of nanocomposites is extended for spheroidal inclusions. It is shown that the dependence of the thermal conductivity of nanocomposites on the shape and size of particle inclusions can be described by: (1) the collision cross-section per unit volume of the particles and (2) the mean distance that the energy carriers (electrons or phonons) can travel inside the particles. The predictions of this analytical approach are in good agreement with available data obtained through numerical calculations of the Boltzmann equation.

The influence of slaked lime content on the processing conditions of cooked maize tortillas: changes of thermal, structural and rheological properties

Basic data on thermal, structural and rheological properties of cooked maize tortillas were investigated, with the aim of understanding the role of the slaked lime [Ca(OH)2] that is incorporated during the alkaline cooking process. The changes in the thermal parameters, such as thermal conductivity, thermal diffusivity and specific heat, measured by photoacoustic techniques, as well as the changes in texture and X-ray patterns are presented as a function of the Ca(OH)2 concentration. The results show strong evidence that there is a threshold concentration of Ca(OH)2 of 0.25%, above and below which there are different behaviour patterns of the measured properties of the tortillas.

Photoacoustic determination of non-radiative carrier lifetimes

From photoacoustic (PA) experiments we determine the nonradiative carrier lifetime in direct band-gap semiconductors. We use the Rosencwaig and Gerscho model to calculate the PA signal in semiconductors taking into account the distinction between non-radiative and radiative carrier lifetimes. We have assumed that for our high quality crystalline samples, the main contribution to the non-radiative processes comes from CHCC and CHSH Auger recombination for n and p-type materials, respectively. For GaAs, InSb and GaSb samples, the experimental data obtained by means of an open photoacoustic cell were fitted to the theoretical model and we show that the values we determined for the non-radiative recombination lifetime agree well with those reported in the literature.

Photosynthetic O2 evolution in maize inbreds and their hybrids can be differentiated by open photoacoustic cell technique

Abstract Photosynthetic efficiency is considered one of the traits potentially suitable to differentiate hybrids from their inbred lines. Previous evaluations concerning photosynthetic efficiencies of maize (Zea mays L.) plants in the field have shown inconsistent and contradictory data. In this work, we attempted to study photosynthetic O2 evolution through photoacoustic spectroscopy in intact undetached leaves of dark-adapted seedlings of inbreds and their hybrids. The results indicate that photosynthetic efficiencies of inbreds and hybrids can be differentiated by photoacoustic measurements of oxygen evolution, providing a parameter that might prove useful in evaluating plant genotypes.

The effect of the electron-phonon coupling on the effective thermal conductivity of metal-nonmetal multilayers

The metal-nonmetal interface plays a critical role in modern electronic and energy conversion devices. For example, metal-nonmetal multilayered structures have recently been proposed as promising materials for solid-state thermionic devices, which could potentially achieve an efficiency that might not be feasible for metals or semiconductors alone. In this work, the effective thermal conductivity of a metal-nonmetal multilayered system (superlattices) is studied using the two-temperature model of heat conduction. By defining the total interfacial thermal resistance, which strongly depends on the electron-phonon coupling factor, it is shown that the thermal conductivity of the system has a simple interpretation as the sum of thermal resistances in series. The role of the electron-phonon coupling and the phonon-phonon interfacial thermal resistance on the total interfacial thermal resistance is discussed. The derived analytical expressions show that the effective thermal conductivity of the multilayered str...

On the thermal characterization of two-layer systems by means of the photoacoustic effect

In this work, the problem of the thermal characterization of two-layer systems by means of the photoacoustic technique is discussed. For a two-layer system under rear-side illumination conditions, we have applied the Rosencwaig and Gersho model for calculating the pressure fluctuation in the photoacoustic gas chamber. The limiting cases in which both layers are thermally thin, thermally thick and one layer is thermally thin and the other is thermally thick are discussed. When both layers are thermally thin, a consistent equation for the heat capacity is obtained and an effective thermal diffusivity equation is derived when both layers are thermally thick. In order to test our theoretical results, we apply them to two-layer systems consisting of AlGaAs layers of different Al concentrations, grown by liquid phase epitaxy on GaAs substrates. The results of our measurements are in good agreement with the theoretical predictions. Our results show the general character of the expression for the effective thermal diffusivity of two-layer systems reported by Mansanares et al (1990 Phys. Rev. B 42 4477).

A crowding factor model for the thermal conductivity of particulate composites at non-dilute limit

The effective thermal conductivity models for particulate composites are usually restricted to the dilute limit, with volumetric concentrations of particles typically less than 15%. By considering the particle interactions through a crowding factor, in this work, a new formula is developed to describe the thermal conductivity of composites with a dielectric matrix, for volume fractions of particles up to their maximum packing fraction. The crowding factor model is then applied to analyze two particulate composites with dielectric or metallic particles, where the effects of both interfacial thermal resistance and the electron-phonon coupling are taken into account. It is shown that the predictions of the proposed crowding factor model are larger than the ones predicted by the dilute-limit models, for composites with high volume fractions of particles, due to the particle interactions. The proposed crowding factor model extends the applicability of various thermal conductivity models for composites reported...

Photothermal radiometry monitoring of light curing in resins

Real time measurement of thermal diffusivity during the evolution of the light curing process in dental resins is reported using photothermal radiometry. The curing is induced by a non-modulated blue light beam, and at the same time, a modulated red laser beam is sent onto the sample, generating a train of thermal waves that produce modulated infrared radiation. The monitoring of this radiation permits to follow the time evolution of the process. The methodology is applied to two different commercially available light curing resin-based composites. In all cases thermal diffusivity follows a first order kinetics with similar stabilization characteristic times. Analysis of this kinetics permits to exhibit the close relationship of increase in thermal diffusivity with the decrease in monomer concentration and extension of the polymerization in the resin, induced by the curing light. It is also shown that the configuration in which the resin is illuminated by the modulated laser can be the basis for the development of an in situ technique for the determination of the degree of curing.