Mn by, ZT=S2?T/?, which depends on the Seebeck

Mn substituted FeVSb half-Heusler alloys were
synthesized by mechanical alloying process and consolidated by vacuum hot
pressing. Microstructure and phase transformation of all the studied samples
were examined by XRD and SEM. Thermoelectric properties such as Seebeck
coefficient, electrical conductivity and thermal conductivity has been
investigated for all the samples in the moderate temperature range. The
negative value of Seebeck coefficient and Hall coefficient confirms the
presence of n-type of conductivity. The Seebeck coefficient increased with
increasing the doping elements, but the electrical conductivity decreased due
to decreasing carrier concentration. The thermal conductivity found in this experiment
was quite high due to the presence of undesired elements which was confirmed by
EDS. The maximum value of the dimensionless figure of merit was achieved by
relatively high value of Seebeck coefficient with a significantly higher value
of electrical conductivity. The resultant high ZT was observed
for Fe0.996Mn0.004VSb at 468K.

 

Keywords: Thermoelectric,
Mechanical Alloying, Half-Heusler, Doping

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1. INTRODUCTION

 

The demand for alternative energy
is escalating at an alarming rate because of the limited resource of the
energetic materials and fossil fuels. Few researchers have addressed the issue and
the search for the alternative energy has become the topmost priority for the
energy scientists. A few of the research groups are working on it to sustain
the energy conversation. Thermoelectric materials can be a suitable choice to
overcome this problem due to its high efficiency to transfer waste heat into
electrical energy 1, 2. The ability of a given material to efficiently
produce thermoelectric power is related to its figure of merit given by,
ZT=S2?T/?, which depends on the Seebeck coefficient S, thermal conductivity ?, electrical conductivity ?, and temperature T,
respectively 3.          

 

In recent times, half-Heusler (HH) intermetallics are
the group of thermoelectric materials that have gained extreme research
interest on the basis of their thermoelectric power 4. The most notable
feature of HH alloys for selecting as an efficient thermoelectric material is its
room temperature Seebeck coefficient of approximately 100µVK-1.
Besides, It has very large electrical conductivity range of 1000~10000
Scm-1 5. Researchers investigated several HH alloys and among them
ZnNiSn was the most interested one to be noticed 6. Bhattacharya et. al. and Pope et. al. reported that the Ti-doped ZnNiSn produced maximum ZT value
of 0.45 with high power factor, though its thermal conductivity is relatively
high 7. The remarkable research work was done with multicomponent doping for
Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01
with ZT?0.7 at 800K 8. So far, the maximum dimensionless
figure of merit ZT ? 1.4 was reported by Sakurada et. al. and Shutoh et. al.
for n-type (Zr0.5Hf0.5)0.5Ti0.5NiSn1-ySby
at 700K 9. Sb-doping on MNiSn HH alloys also produced effective ZT
10. The high ZT of 0.81 at 1025K and 0.78 at 1070K was
noticed for p-type Hf0.75Zr0.25NiSn0.975Sb0.025
11. It might be due to the maximum Seebeck coefficient at
high temperature leading to large power factor. HH alloys have acquired much
attention to use as a suitable candidate in the thermoelectric device because
of their easy synthesis, their reasonable cost ingredients and have the
stability to retain high melting point in the range of 1200-1500K 12, 13. FeVSb
based HH has relatively large power factor and the narrow band gap in the range
0.1~0.4ev 14. Doping in this system might shift the Fermi level to the
conduction band leading to optimize the electronic structure. Considering all
the material properties mentioned above, we chose HH alloys to investigate the
thermoelectric performance as a function of temperature.  

 

Various techniques have been proposed for synthesize of FeVSb
HH alloys. Among them induction melting (IM), levitation melting (LM), arc
melting (AM), spark plasma sintering (SPS), mechanical alloying (MA) process and
so far are very common 15. We have adopted MA for this experiment because it
is well established, a unique and controlled processing method that may produce
extremely fine microstructure 16. The key advantage of this process is to
create metallic substances with ultrafine structures that are difficult to
produce by a typical metallurgical process like forging and casting 17, 18. Beside,
Sb has a relatively low melting point that may undergo sublimation at a very
high temperature and the formation of homogeneous single phase would be very
difficult. Addressing all these problems we applied MA, followed by vacuum hot
pressing (VHP) to prepare various compositions of FeVSb compound 19. The
phenomenal effects of Mn-doped FeVSb HH matrix as a function of temperature
were investigated in this study.

 

2. EXPERIMENTS

 

Fe1-xMnxVSb
(x=0.000~0.008) was synthesized from the significant amount of powder mixtures
of Fe (99.9%, 325 mesh), Mn (99.9%, 325 mesh), V (99.9%, 325 mesh) and Sb (99.9%,
325 mesh). The mixture was then chose to undergo a MA process in a high energy
vibrator mill (Spex mill 8000D, K & Us Equipment, Inc., USA) for 10 hours
with the speed of 1650 rpm. For the milling purpose, we chose stainless steel
(SLS) vial to carry out the MA process. We have used 5 mm diameter of Zirconia
balls and the process was continued with keeping the ball-to-powder ratio 6?1.
After MA, the alloying powders were then consolidated by a vacuum hot pressing
(VHP) under the pressure of 70 MPa at 1073K for 2 hours. Every process from
weighing to MA was done under inert atmosphere.

 

X-ray diffraction (XRD; Bruker AXS
ADVANCE D-8) was used to observe phase transformation of the MA powders and the
VHPed samples by using Cu K? radiation. Microstructure was investigated by using
scanning electron microscopy (SEM, Quanta-400). Thermoelectric properties of
the VHPed samples in terms of Seebeck coefficient and electrical conductivity
were measured in the 4-probe methods (ZEM-3, Ulvac-Riko, Japan) at 300~973 K.
The hot pressed samples were cut by a diamond edge precision saw to get
different dimensions, such as 3×3×10 mm3 for Seebeck and electrical
conductivity and 10?×1 mm for thermal conductivity. The thermal conductivity of
the VHPed samples was calculated using the equation ?=D×Cp×d, where D is the density,
Cp is the heat capacity, and d is the thermal diffusivity. Thermal conductivity
and heat capacity were evaluated by a laser flash instrument (TC-9000,
Ulvac-Riko, Japan). Density of the samples was calculated by Archimedes principle.
The Hall coefficient, carrier mobility, and carrier concentration were measured
by van der paw method (Keithley 7065) in a constant magnetic field (1 T) and
electric current (50 mA) at 300 K.The X-ray diffraction pattern of the mechanically
alloyed powders of Fe1-xMnxVSb (x=0.000~0.008) revealed
that HH phases seemed to be formed after 8 hours of milling and peak broadening
took place with the milling time. The MA process was stopped at 10 hours of
milling. Further alloying might proceed in hot consolidation 20. 

Fig. 1(a) showed that the phase transformation
progressed with respect to the milling time. It represented the progressive
phase formation of all the samples. During MA, lattice distortion was also induced
by the collision of balls to the powder mixtures during milling 21. Peak
shifting from smaller to greater angle in the XRD was found as shown in figure
1(c). This shifting might be caused by the change of lattice parameter or
shrinkage of the unit cell volume. Substitution of the large atomic radius of
Mn compared to Fe might also reduce the lattice structure. The XRD pattern of Fe1-xMnxVSb
(x=0.000~0.008) samples showed a nearly single phase with a minor second phase
in XRD (Fig. 1(b)). The second phase was detected as Sb2Fe. Figure 2
showed the SEM images of VHPed samples of Fe1-xMnxVSb
(x=0.000~0.008) consolidated at 1073K. In order to investigate the
microstructure, we have analyzed SEM image. We couldn’t find the grain
structure because it might form ultrafine microstructure which was very
difficult to analysis in SEM 22.Temperature dependence of the thermoelectric properties
in terms of Seebeck coefficient, electrical conductivity and thermal
conductivity were presented here by Fig. 4(a-c). Seebeck coefficient of Fe1-xMnxVSb
(x=0.000~0.008) HH systems were increased with various Mn-doping states except
for x=0.008. With the increase of temperature, the Seebeck coefficient
decreased for all the samples. It might be due to the reduced value of Hall
mobility because at high temperature electron scattering took place which
reduced the mobility. In addition, the stainless steel (SLS) composition such
as Cr, Ni from the vial might be entered into the resultant HH system and made
the system unstable. Substitution of Fe with Ni and V with Cr might play an
important role for this effect 23. We have analyzed EDS in order to identify
the unwanted elements mixed up during the milling process. It was revealed that
both the Ni and Cr were existed in the system which was confirmed by EDS as shown
in Fig. 3 and Table 1. The elemental powders which were used for the experiment
vulnerable to oxidation. During oxidation, some of the oxygen concentrated to
the surface of the alloy and form acceptor state. It decreased the power factor,
leading to the decrease in Seebeck coefficient 24.The electrical conductivity
of Fe1-xMnxVSb (x=0.000~0.008) HH systems showed combined
behavior as indicated by Fig. 4(b). Conductivity was decreased for most of the
samples except x=0.008 up to 550 K, indicated that they behaved semi-metals in
this temperature range 25. This might be due to the degeneracy of
conductivity. After passed this temperature range, the electrical conductivity was
increased for all the samples. It means, they behaved metallic in this
temperature zone. With the increasing temperature, the alloys formed a
localized state by the thermal promotion of electron to the conduction band. Hence,
the electrical conductivity was increased. This increasing could be attributed
to the fact that, at high temperature, the Fermi level jumps to the conduction
band 26. This non-degenerate behavior explained in terms of the carrier
concentration as well. This could be supported by the data given in Table 2.Fig. 4(c) represented the temperature dependence of the
thermal conductivities of Fe1-xMnxVSb
(x=0.000~0.008) samples measured in the temperature range of 300~973 K. Thermal
conductivities of the systems were
relatively very high as seen in the Fig. 4(c). Thermal conductivities of all
the samples decreased up to the temperature range of 300~780 K on doping and
then increased with the rise of temperature. It means, when lattice vibration
took place at this temperature zone, it decreased the mean free path of
molecules and electrons.  As a result,
the flow of the electron might be hindered, leading to reduce thermal conductivity.
The thermal conductivity might be influenced by the presence of trace elements
from the SLS vial leading to increase the thermal expansion coefficient of the
system during alloying process and suppressed the void 27. This might be the
key reason for the higher thermal conductivity of this experiment. This
anomalous behavior of the thermal conductivity due to the incorporation of
undesired elements might be overcome by the utilization of Zirconium vial.  Fig. 4(d) showed the temperature dependence of the
dimensionless figure of merit (ZT = S2?T/?). The ZT values of the
samples were calculated using the above equation. The ZT value increased with
increasing temperature and showed a peak value at 468 K. The highest ZT value
obtained here relatively low for analogous studies. The maximum ZT value found
in this experiment was 0.054 for x=0.004. Possibly, high thermal conductivity
and low Seebeck coefficient played the main role for lower ZT. n-type Fe1-xMnxVSb
(x=0.000~0.008) HH compounds were prepared by MA process, followed by VHP and
their thermoelectric and transport properties were examined. The Hall
coefficient and the temperature dependence of the Seebeck coefficient demonstrated
the n-type conduction. Electrical conductivity showed bipolar nature in the doping
state and increased at high temperature. Temperature dependence of the thermal
conductivity showed increasing value for all the samples except for x=0.008. In
this study, the ZT was affected by the participation of undesired trace elements
from the SLS vial. The maximum ZTmax obtained for the systems was
0.054 at 468 K, which revealed a moderate temperature dependent HH alloy.
Although the ZT value obtained from the system was very low, thermoelectric
performance might be improved by multicomponent element doping with proper
handling and processing. It is expected that further substitution on V and Sb
site may produce high ZT in the presence of Mn-doped FeVSb system. Thermoelectric
properties of the current experiment may be enhanced by optimizing Mn-rich
nano-inclusions and improving of the microstructure. It may reduce the thermal
conductivity of our desired system.