Effect of spark plasma sintering (SPS) on the thermoelectric properties of magnesium ferrite
Mater Renew Sustain Energy (2017) 6:2
DOI 10.1007/s40243-016-0086-9
ORIGINAL PAPER
Effect of spark plasma sintering (SPS) on the thermoelectric
properties of magnesium ferrite
Ryosuke S. S. Maki1,2 • Seiji Mitani2,3 • Takao Mori1,2
Received: 8 September 2016 / Accepted: 4 December 2016 / Published online: 16 December 2016
Ó The Author(s) 2016. This article is published with open access at Springerlink.com
Abstract Magnesium ferrite MgFe2O4 was synthesized
with two different methods, spark plasma sintering (SPS)
and conventional solid-state reaction sintering (SSRS), and
thermoelectric properties were investigated. SPS processing was found to yield two attractive features: SPS at
900 °C enabled retaining the submicron particle size of
0.3–0.5 lm from ball-milling, leading to lower thermal
conductivity, 3 W/mK@300 K. 1200 °C SPS sintering led
to the same sample grain size of 1.0–3.0 lm as SSRS, but
still exhibited significantly lower thermal conductivity of
4.3 W/mK@300 K compared to the SSRS sample with
14 W/mK@300 K, which exhibited neck formation
between particles. Furthermore, while the finer
microstructuring led to a reduction in the thermal conductivity, the resistivity of SPS MgFe2O4 showed little
dependence on the particle size at expected thermoelectric
working temperatures above 523 K, which indicates success to some degree of phonon selective scattering due to
differences in mean-free-paths of electrons and phonons.
As a process, SPS samples are found to exhibit four- to
sevenfold enhancement of ZT compared to the conventional SSRS sample. While the maximum ZT in the present
samples is relatively low, taking a value of 0.07 for the SPS
1200 °C sintered sample, the processing insights may be
utilized for similar systems.
& Takao Mori
Keywords Thermoelectric properties Spinel
Magnesium ferrite (MgFe2O4) Spark plasma sintering
(SPS) Induction furnace (IF)
Introduction
Thermoelectric materials have attracted interest because of
the potential large benefits of solid-state conversion of
waste heat to electricity [1–4]. Some impactful high-temperature thermoelectric applications have recently been
reviewed [5]. For very high temperatures, applicable
materials will naturally be limited to refractory materials
with good heat resistance, like borides [6–8] and oxides
[9–12], etc. The material system of spinel-type oxide exists
for a wide range of constituent elements, and various
chemical and physical properties have been studied
[13–17]. Our initial motivation to focus particularly on the
spinel-type magnesium ferrites is because its magnetic
properties have been extensively studied [13, 14], and we
are interested in the link between magnetism (magnetic
semiconductors) and thermoelectric properties due to our
previous work in chalcopyrite, where thermoelectric
enhancement was indicated [18–20]. It is also a system
where the crystal structure is well-characterized in a wide
temperature range, and a system with good thermal stability and sintering characteristics, and in the present work
we tried to learn more about processing effects on its
thermoelectric properties.
The performance of thermoelectric materials is gauged
by the figure of merit:
1
National Institute for Materials Science (NIMS), MANA,
Namiki 1-1, Tsukuba 305-0044, Japan
2
Graduate School of Pure and Applied Sciences, University of
Tsukuba, 1-1-1 Tennodai, Ibaraki 305-8573, Japan
ZT ¼ a2 r T=j;
3
National Institute for Materials Science (NIMS), Sengen 1-21, Tsukuba 305-0047, Japan
where T is the temperature, a is the Seebeck coefficient, r
is the electrical conductivity, and j is the thermal
ð1Þ
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Mater Renew Sustain Energy (2017) 6:2
conductivity. The denominator a2r is the power factor.
There are some difficulties to straightforwardly obtain high
values of ZT because of the typical tradeoff between Seebeck coefficient and electrical conductivity, and conducting electricity well but not heat is also somewhat
paradoxical.
In regards to the latter problem, different methods of
nanostructuring have been shown to be quite effective to
lower the lattice thermal conductivity and enhance ZT
[21–28]. Nanostructuring of oxides has been actively carried out [29–31]. Finding ways to enhance the power factor
is still a large challenge [32]. Various band engineering
methods have been proposed, such as utilization of confinement effects [33], resonance levels [34], modulation
doping [35, 36], etc. As a more phenomenological and
easily accessible enhancement effect compared to band
engineering, large enhancements in the power factor have
been reported for nanocomposites [37–41], with mechanisms not fully elucidated. Recently, enhancement has also
been proposed for magnetic semiconductors [18–20].
In this work, we have investigated the effect on the
thermoelectric properties of magnetic magnesium ferrites
synthesized with different conditions to add more insight
into this field.
Experiment
In situ reactive synthesis of MgFe2O4 was performed using
spark plasma sintering (SPS, FUJI DENPA KOKI SPS1080). The starting materials MgO (99.9%, Wako Pure
Chemical Industries Ltd.) and Fe2O3 (99.9%, Wako Pure
Chemical Industries Ltd.) were mixed with a nominal
composition of MgFe2O4 using planetary ball-milling with
chromium iron media. The powder mixtures were pulverized for 3 h in helium atmosphere. In order to measure
thermoelectric properties, samples were prepared by
pressing at 60 MPa for 3 min at 900–1200 °C in argon
atmosphere using SPS. Prior to the sintering, the powder
mixtures were put into graphite dies with diameters of
10 mm, and with graphite foil inserted between the sample
and the die/punches. The heating rate for the SPS experiments in the graphite die was limited to 100 °C min-1. The
die was wrapped in carbon felt when heating up to
1200 °C. For comparison of the thermoelectric properties
of the sample prepared with SPS and standard ceramic
procedure, a conventional oxide sample was prepared with
solid-state reaction sintering (SSRS) at 1200 °C for 12 h in
air using a green pellet uniaxially pressed at 7.4 MPa for
1 min. In addition, we prepared the oxygen defect-rich
sample by heating a green pellet set into a carbon crucible
at 1200 °C for 12 h with an induction furnace (IF) in order
to examine the effect of oxygen vacancies on
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thermoelectric properties. Then, the sample was pulverized
in an agate mortar and pressed at 80 MPa for 5 min at
900 °C in argon atmosphere with SPS.
The constitution phases in the pulverized samples were
analyzed by X-ray powder diffraction measurement with
CuKa radiation (RIGAKU RINT-ULTIMA 3) operated at
40 kV and 40 mA. The detailed crystal structure and site
occupancies at cation sites were determined by means of
Rietveld refinement using Rietan-FP software [42].
Microstructural observation was carried out by a scanning
electrical microscope (JEOL SM-67F, operated at 10 kV).
Resistivity and thermoelectric power were measured (...truncated)
