Publication: A study on low temperature synthesis of silicon carbide thermionic cathode and its electron emission analysis for thermionic energy converter
Date
2020-07-01
Authors
Leong, Thye Jien
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Abstract
Energy conversion of renewable and clean energy such as thermal energy is
often related to solar cell and photovoltaic cell. These are the rising technology devices
that generate electrical energy for the use in industrial and residential sectors. In order
to seek for an alternative approach in meeting the growing energy demand and
reducing the dependency on conventional energy resources such as biomass energy,
another direct thermal energy to electricity conversion device namely Thermionic
Energy Converter (TEC) is being studied. However, the technology involved in this
device has one major limitation which is the unavailability of low cost cathode
materials with low work function yet easily-obtained. This drawback has motivated
the study of cathode material in term of lowering the work function by surface
nanostructuring the thermionic cathode for TEC in the mentioned research direction.
By using atmospheric pressure chemical vapor deposition (APCVD) method, a low
temperature (600°C) synthesis thermionic cathode was developed, as validated by the
literature (Xi et al., 2006). In this approach, silicon carbide (SiC) nanostructures were
synthesized on the silicon substrate. This method was optimized based on two
parameters which are controlling the thickness of catalyst (Magnesium) and the
amount of precursors (silicon tetrachloride and 2-ethoxyethanol) used in the
experiments. Some material characterization techniques, such as SEM, EDX, and
FESEM were performed to the thermionic cathode to support the proposed synthesis
method. Besides, a new in-house built TEC system that is equipped with a vacuum
chamber, CO2 laser heating system, turbomolecular pump, and digital nanoammeter
was developed, which can manipulate temperature of the thermionic cathode. The
electron emission analysis of thermionic cathode was performed at different
temperature ranges where a copper plate (4.7eV) was used as the anode. Based on the
results, a total reduction of ~0.36 eV to the work function of the p-type silicon (111)
with ~4.84 eV is achieved by the as-synthesized SiC nanowires thermionic cathode
using M200SC3 - ~4.48 eV sample. Lastly, the Schottky effect is proven showing an
enhancement to the current density of the SiC nanowires thermionic cathode by
compromising a small deviation of the work function of ~0.02 eV. These findings
prove that the presence of SiC nanostructures on the thermionic cathode has achieved
higher current density, 87 nA cm-2 with the presence of applied voltage at 4000 V/cm
and 71.8 nA cm-2 at zero field effect, as compared to 14.4 nA cm-2 for the control
sample in this TEC study.