Citation
Jibrin, Sani
(2015)
Effects of electric field on the thermal diffusivity of conductors in threelayer solid configurations.
Doctoral thesis, Universiti Putra Malaysia.
Abstract
In order to extract the thermal properties of thin films from their thermal responses
an analytical model is developed by solving the heat diffusion equation. In this study,
a novel mathematical theory essential for the experimental determination of thermal
diffusivity of conductors in threelayer solid configurations by the Converging Thermal
Wave Technique is developed. This is achieved by expressing the hyperbolic
Laplace associated solution of the derived equation in negative exponential form.
Binomial series expansion is then used to simplify the solution and hence, Laplace
conversion tables, in place of the tedious integral inversion technique are finally used
to retrieve the temporal temperature profile for the threelayer solid sample in real
space and time domains. The equation is plotted using Mathematica Software and
its accuracy is checked by performing sensitivity analysis on all the physical parameters
contained in the derived equation. The result of the sensitivity analysis shows
that the mathematical model is sensitive to all relevant parameters needed for the
evaluation of the thermal diffusivity of the sample.
Using the Converging Thermal Wave Technique, three different sets of thermal diffusivity
experiments are performed on previously prepared samples. The first set of
experiment comprising four threelayer samples is performed to test the mathematical
theory and calibrate the measuring scheme and apparatuses. Results obtained
in this series of experiments show that the mathematical model and the measuring
scheme adopted for the thermal diffusivity evaluation of the samples are accurate to
within about 5% error.The second set of experiments were performed on different threelayer solid samples
when direct electric current pass across the samples. Results obtained here
indicate that the thermal diffusivity value of the metal foils in threelayer solid configurations
increase with the potential difference applied across the metal foils. It
is also observed in this series of experiments that the temperature of the metal foils
increases slightly as a result of the Joule heating effects.
When different threelayer solids are placed in a uniform static electric field and are
being charged either positive or negative as the PD is varied from 1.00 V to 10.00
V in a step of 1 V each, the effect of electrostatic field on the thermal diffusivity
of metal is investigated. At both positive and negative static charge on the sample,
ten different temperature signal readings are obtained as the potential difference of
the DC power source is varied. In this way, the free electrons in the conductor are
made to move either towards or away from the temperature signal detection point
respectively and hence the effect of the flow of free electrons in an open circuitry is
established. It is found out that free electrons flowing both normal and parallel to
thermal dissipation path in a metal affect its thermal diffusivity significantly. When
electrons flow towards the thermal dissipation path due to electrostatic repulsion,
the thermal diffusivity value of the metal is observed to increase and when the direction
of electrons flow and thermal dissipation path due to electrostatic attraction are
opposite one another, the thermal diffusivity value for the sample decreases.
The last phenomenon is understood to be the results of the fact that electrons in
a metal behave like a collection of gas particles, ’FermiGas’ such that they move
about through the metal unhindered and unaffected by the potentials of the ion core
and hence remained un scattered for quite a long time. Within the limit of the room
temperature at which the experiment is carried out, scattering of electrons in the
metal by phonons is negligible. Similarly, for metals free from point defects, impurity
and imperfections scattering by point defects and crystal imperfection is also
ruled out. Hence, the applied potential difference accelerates the electrons towards
or away from the temperature probe. When the electrons move towards the probe
the thermal diffusivity value for the metal increases, and decrease when the electrons
flow in the opposite direction. We also note that all these happened with no
additional heat in the solid as the phenomenon occur in an open circuitry such that
the Joule heating effects of flowing electrons are eliminated.
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