K. N. Madhusoodanan

Optical absorption and thermal diffusivity in GexTe100−x glasses by the photoacoustic technique

Abstract Abstract The variation in the optical energy gap with composition in the GexTe100-x system (15≤x≤28) of glasses in bulk form has been studied using photoacoustic techniques. It is found that the optical energy gap increases with increasing composition factor x, with the rate of increase showing a slowing-down trend for compositions with x > 20. The composition dependence of the thermal diffusivity of these samples has also been measured. The thermal diffusivity is found to peak around the composition corresponding to x = 20. The observed phenomena are explained on the basis of chemical bond formation in this system of glasses. The change in the short-range order and increase in the number of Te-Te bonds with decreasing x are interpreted as being responsible for the behaviour exhibited by these materials.

Photoacoustic measurements of the thermal conductivity of some bulk polymer samples

Thermal parameters such as thermal diffusivity and thermal conductivity of the polymers nylon 66, cellulose acetate, polyacetal, Teflon, and polystyrene have been determined using the photoacoustic technique. Samples having thickness of the order of 25–50 μm have been specially prepared from bulk solid samples for measurements. The experimental method involves the determination of the characteristic frequency fc by measuring the variation of the amplitude and phase of the photoacoustic signal as a function of the chopping frequency for a monochromatic incident beam. Front surface illumination geometry has been used in the measurements. The measured thermal properties agree with the existing values in literature.

Characteristics of Photoconductivity in Amorphous GexSb10Se90—x Thin Films

Electrical transport in amorphous semiconducting thin films with the general formula Ge x Sb 10 Se 90-x (10 < x < 32.5) has been investigated in the dark as well as under illumination. The results are found to be consistent with the ABFH model for photoconductivity. Due to the change in the nature of transitions between localized states, the Ge-Sb-Se system is found to exhibit a photoconductivity maximum when the temperature is increased. The composition with x=25 shows minimum values in photoconductivity and dark conductivity owing to its structure. The observed composition dependence of the transport behaviour and activation energy are discussed in terms of the chemically ordered network model for disordered materials.

Photoacoustic investigation of the optical absorption and thermal diffusivity in SixTe100−x glasses

The photoacoustic technique is used to determine the optical energy gap E0 of bulk SixTe100−x glasses in the glass-forming region 10 ≤ x ≤ 28. The thermal diffusivity α of these samples has also been measured. The variation of E0 and α with x is reported. It is found that E0 increases with x nearly linearly with a sharp decrease in the rate of increase beyond x = 20. The thermal diffusivity also increases with x up to x = 20 but decreases for compositions with higher values of x. The observed behaviour is explained on the basis of a chemical bond approach. It is accounted for in terms of the increase in the number of Te---Te bonds and formation of SiTe4 tetrahedra with an increase in the chalcogen content.

Composition dependence of the energy gap and thermal diffusivity in bulk As-Se glasses

We have measured the composition dependence of the optical energy gap and thermal diffusivity in bulk AsxSe1−x (0.10⩽x⩽0.50) glasses using photoacoustic technique. The energy gap shows a threshold minimum value and thermal diffusivity has a threshold maximum value at the stoichiometric composition As2Se3 corresponding tox=0.40. The decrease in energy gap is explained on the basis of chemical bonding. It is argued that the threshold percolation of rigidity in the random network is responsible for the peaking of the thermal diffusivity at the stoichiometric composition.

Thermal diffusion near glass transition in Ge-Se glasses measured by photoacoustics

The glass transition in GexSe1−x) (0·1 ⩽x ⩽ 0·25) glasses has been investigated using the photoacoustic (PA) technique. It is found that the PA amplitude and phase undergo anomalous changes at the glass transition temperatureTg. The amplitude has critical minimum and phase has maximum values atTg. The variation of the thermal diffusivity, determined by measuring the frequency dependence of the PA amplitude and phase, with temperature shows sharp decrease near the glass transition temperature. The temperature dependence of the optical energy gap also has been measured and it shows a decrease with temperature for all compositions, the rate of decrease being higher for temperatures greater thanTg.

Temperature Dependence of Optical Energy Gap in ASx Se1-x and Gex Se1-x Glasses

We have recorded the PA spectra of ASxSe1−x (0.10 ≤ x ≤ 0.40) and GexSe1−x (0.10 ≤ x ≤ 0.25) glasses in the absorption edge region at various fixed temperatures using a high temperature gas-microphone PA spectrometer. The spectra have been recorded for temperatures above glass transition temperature Tg. It is observed that the exponential edge region of the spectrum broadens as the temperature is increased and the rate of this broadening is more for temperatures larger than Tg. The optical energy gap Eg of the samples have been determined from the PA spectra. It is found that Eg decreases with increase in temperature and the rate of this decrease is larger for temperatures T > Tg.

Photoacoustic Study of the Glass Transition and Crystallization in Bulk GexTe1-x and SixTe1-x Glasses

We have investigated the thermal events like glass transition and crystallization in GexTe1-x (0.15 ≤ x ≤ 0.28) and SixTe1-x (0.15≤x≤0.25) glasses using photoacoustic (PA) technique. The composition dependence of glass transition temperature and crystallization temperature show anomalous changes at x = 0.20.