Citation
Haydari, Mehdi
(2004)
Development and Applications Of Photoflash-Pvdf Technique in Thermal Diffusitivity Measurement at Low Temperatures.
Doctoral thesis, Universiti Putra Malaysia.
Abstract
The photoflash technique is developed and used for measuring thermal
diffusivity of various types of material, at temperature range from -77K to ambient
temperature. It uses a cheap and simple camera flash and polyvinlidene difluoride
(PVDF) film as signal generating source and detector, respectively.
The theoretical signal was derived based on the square pulse approximation
of the camera flash that replaced Dirac-6 function approximation employed in other
studies. Comparative studies on these two different approximations have been
performed on SiCIB4C composites. Although the camera flash temporal shape is
closer to square pulse, Dirac-6 function approximation is still valid for the limited case of PVDF signal that is significantly longer than camera flash pulse duration.
The square wave approximation model was further used in determining the
thermal diffusivity of superconductors, semiconductors, magnetoresistances, carbon
nanotubes, ceramics, composites, polymers and porous samples.
The thermal diffusivity for SiC/B4C composites and Sic doped with A1
decreased with increasing temperature. This suggests that thermal diffusivity is
basically influenced by phonon interaction within lattice that determines the phonon
mean free path.
In case of superconducting materials, thermal diffusivity measurements were
carried for BSCCO, doped with Samarium (Sm) at Bi, Sr and Cu sites and sintered
for 24,48 and 100hrs respectively. The results were explained in terms of electronphonon
and phonon-lattice interactions in association with the sample grain size.
The magneto-resistive of LCMO doped with Er at La site was also studied in
this study. Thermal diffusivity measurements revealed that the transition from
metallic to insulator and from insulator to semiconductor behavior in the materials,
were closely matched to the results obtained from electrical resistivity measurement
of other researchers.
The thermal diffusivity of carbon nanotubes (CNTs) decreased when the
temperature was increased from low to room temperature. Besides, there were also double slope phenomena in the way the thermal diffusivity changed with
composition of CNT in the range of temperature covered in the measurement.
In the case of polymers of Emeraldine Base (EB) and Emeraldine Salt (ES),
the thermal diffusivity changed with temperature as in other insulating materials.
Finally, the effect of porosity on thermal diffusivity was studied using Nickel
Copper Zinc Ferrite samples. The thermal diffusivity decreased with increasing
porosity of the sample. The results also showed that porosity has a greater effect on
thermal conductivity of the material than its thermal capacity.
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