S. Yannacopoulos

Specific heat capacity of lentil seeds by differential scanning calorimetry

ABSTRACT Differential scanning calorimetry (DSC) was used to determine the specific heat of lentil seed. Accurate measurement with this method requires a good seal condition of the sample pan and relatively thin sample specimen. The value of specific heat of lentil seeds ranged from 0.81 to 2.2 kJ/kgK. Specific heat increased quadratically with moisture content over the range from 2.1% to 25.8% (w.b.) and linearly with temperature varying from 10° C to 80° C. The classical compartment model was modified to accommodate the variation of specific heat with temperature and moisture content. KEYWORDS. Specific heat. Physical properties. Differential scanning calorimetry. Lentils.

Thermal Fatigue of Composites: Ultrasonic and SEM Evaluations

Results are presented on the evaluation of thermal fatigue in three fiber reinforced polymer composites, using ultrasonic techniques and scanning electron microscopy. The composites examined were (a) continuous carbon fibers in a vinylester matrix (b) continuous aramid fibers in a vinylester matrix and (c) randomly oriented aramid fibers in a polyphenylene matrix. Specimens of these composites were subjected to thermal fatigue by thermal cycling from [minus]25 C to 75 C. Changes in ultrasonic attenuation and velocity were monitored during thermal cycling, and scanning electron microscopy was used to qualitatively evaluate any damage. It was observed that ultrasonic attenuation is sensitive to thermal fatigue, increasing with increasing number of thermal cycles. SEM evaluations showed that the primary damage due to thermal fatigue is due to fiber-matrix debonding.

Glass transformation phenomena in bulk and film amorphous selenium via DSC heating and cooling scans

Recently there has been an emphasis on the importance of using cooling scans in DSC experiments in studying the glass transformation kinetics of glasses. The physical interpretation of the apparent activation energy from DSC heating scans has been questioned as not being meaningful. The present paper reports glass transition temperature ( T g ) measurements derived from Differential Scanning Calorimetry (DSC) experiments on bulk and film amorphous selenium samples subjected to heating, at constant rate r , and cooling, at constant rate q , scans. Film samples were prepared by thermal evaporation techniques in vacuum. It is shown that for both bulk and film forms of a –Se, within experimental errors, log( r / T 2 gm ) vs 1/ T gm plot where T gm is the peak glass transformation temperature, and log( r ) vs 1/ T gh plot, where T gh is the glass transition onset temperature from DSC heating scans, are parallel to the log( q ) vs 1/ T gc plot where T gc is the glass transition temperature from cooling scans. Within the Hutchinson and Kovacs formulation of the glass transformation phenomenon, the results imply that the structural contribution to the mean retardation time, τ, is negligible in comparison with the temperature dependent part. The mean structural relaxation time for both bulk and film forms was found to exhibit a typical Vogel-Tammann-Fulcher type of temperature dependence. Furthermore, the structural relaxation rate was observed to be inversely proportional to the viscosity, η( T ), implying that the mean structural retardation time is proportional to the viscosity, τ ∼ η. The results also confirm that the earlier studies of glass transformation kinetics in a –Se utilizing only DSC heating scans remain meaningful.

Drift mobility relaxation in a‐Se

Time‐of‐flight (TOF) hole drift mobility (μd) measurements have been carried out on vacuum deposited and identically aged a‐Se photoreceptor type films over a temperature range encompassing the glass transformation region to study the nature of mobility controlling shallow traps in this elemental chalcogenide semiconductor. Differential scanning calorimetry (DSC) experiments using both heating and cooling scans have also been carried out on the same films to correlate the enthalpy relaxation phenomenon with the TOF drift mobility dependence on temperature. By considering the heating rate dependence of the minimum peak in the μd vs T behavior and the heating and cooling rate dependence of the DSC glass transition temperature, it is shown that the mean retardation times, τμ and τH, associated with the relaxation of the shallow traps and the enthalpy, respectively, have similar temperature dependencies, essentially Vogel–Tammann–Fulcher type, with negligible structural contribution. Correlation is also estab...

Thermal, mechanical and electrical relaxation phenomena in a-Se films

Abstract Time-of-Flight (TOF) hole drift mobility ( μ d ), Differential Scanning Calorimeter (DSC) and Thermomicrohardness (TμH) measurements have been carried out on vacuum deposited and identically aged a-Se photoreceptor type films over a temperature range encompassing the glass transformation region to study the nature of structural relaxations in this elemental chalcogenide semiconductor.

Mechanical properties of the semiconducting glass a-Se in the Tg region via thermomicrohardness measurements

Abstract Behavior of the mechanical properties of the elemental semiconducting chalcogenide glass a-Se was investigated using a novel thermoanalysis technique based on microhardness measurements in which the Vickers microhardness, HV(T), is monitored as a function of temperature through a phase transformation with the heating rate, r, as a parameter (the method has been termed thermomicrohardness analysis; TμHA). The experiments were carried out over a temperature range encompassing the glass transformation to examine the changes in the mechanical properties in this temperature region. It is observed that the temperature dependence of the mechanical property Vickers microhardness follows the general shape of the shear and five-second elastic moduli previously reported. It is found that an Itoh-Shishokin plot of log Hv vs. T exhibits a “hardness transition” temperature, Tg, in the glass transformation region. It is shown that the heating rate and aging dependence of Tg is very similar to the glass transition temperature behavior reported from DSC experiments. Two plausible interpretations are discussed which include the onset of viscous flow-dominated indentations in the Tg region, and the observed change in the microhardness being a possible manifestation of a relaxation process such as that of the shear modulus or the density in the Tg region. For the latter interpretation, the semilogarithmic plot of the heating rate, r, against the reciprocal of Tg indicates a Vogel-Tammann-Fulcher type of behavior for the mean structural retardation time which was found to correlate remarkably well with the viscosity-temperature data of Cukierman and Uhlmann [11] over the same temperature range indicating that the relaxation time is proportional to the viscosity: τ ∼ η.

Charge trapping studies in a-Se films via interrupted field TOF technique

Abstract We report on Interrupted Field Time-of-Flight (IFTOF) experiments on a-Se1−xAsx films which involve the interruption of the applied bias in the conventional TOF measurement for an interruption time ti. The electronic technique developed allows a displacement current free, and thus accurate measurement of the fractional recovered photocurrent J(ti), as a function of the interruption time ti. The IFTOF technique thus provides a means of studying charge trapping on a time scale far beyond the transit time, and probes localized states with trapping and release times comparable to the interruption time. Using IFTOF techniques, charge trapping and release phenomena in a-Se and a-Se 0.997As0.003 photoreceptor type films were investigated.

Thermomechanical properties of amorphous semiconductor photoreceptor films of As2Se3

Abstract Thermal and mechanical properties of vapor deposited amorphous As2Se3 photoreceptor type films have been investigated using Thermomicrohardness (TμH), Thermomechanical (TM) and Differential Scanning Calometry (DSC) experiments over a temperature range encompassing the glass transformation region.

Method and apparatus for thermal microhardness analysis

A method and apparatus are described to carry out microhardness measurements as a function of temperature on amorphous solids to investigate the glass transformation. The Vickers hardness number Hv of a typical well characterised inorganic glassy polymer a-Se has been monitored as a function of temperature with the heating rate kept constant during the measurements. In the glass transformation region, HV(T) was observed to go through a sharp fall. An empirical glass transition temperature, TG, was defined at the inflection point of HV(T) against T behaviour which depended on the heating rate, r. It is shown that the TG against r data can be used to study the structural relaxation processes which control the mechanical properties in the TG region. The activation energy for the structural relaxation process in a-Se, for example, was determined to be 2.77 eV over the temperature range 36-50 degrees C. The apparatus consisted of modifying a commercially available microhardness test instrument using a microcomputer and a data acquisition system, with an appropriate software and algorithm, to control the rate of heating of the sample.

Charge transport in halogenated amorphous Se1−xTex photoreceptor films

Abstract The present paper examines the interpretation of Time-of-Flight experiments on halogenated a-Se 1−x Te x photoreceptor type films. The first possible interpretation involves Te or Cl introduced shallow defects which results in a distinct feature on the density of localized states peaking at around ∼0.43 eV above E v . It is assumed that the the conductivity mobility remains to represent microscopic transport via extended states as in pure a-Se. The second possibility is that Te and Cl additions essentially alter the native defect concentrations so that the energy location of the shallow hole traps at ∼0.29eV above E v remains reasonably unaffected but their density and energy width is modified. In addition, the increase in the disorder causes the microscopic mobility to become thermally activated due to hopping in the band tail states with a hop activation energy of ∼0.14eV. The latter model is essentially a trap controlled hopping transport similar to that for doped a-As 2 Se 3 . The two models discussed for hole transport may also be applied to electron transport in Cl free a-Se 1−x Te x . It is suggested that the Te addition modifies the already present defect population leaving their energy location below E c unchanged so that the observed increase in the mobility activation energy of ∼0.15eV with Te addition corresponds to the top activation energy of electrons in the band tail localized states.