Enhancing the thermoelectric power factor of nanostructured ZnCo2O4by Bi substitution

Article abstract of DOI:10.1039/d0ra01542c

Bi_xZnCo_2-xO₄(0 =x= 0.2) nanoparticles with differentxvalues have been prepared by the sol-gel method; the structural, morphological, thermal and thermoelectric properties of the prepared nanomaterials are investigated. XRD analysis confirms that Bi is completely dissolved in the ZnCo₂O₄lattice till thexvalues of =0.1 and the secondary phase of Bi₂O₃is formed at higherxvalue (x> 0.1). The synthesized nanomaterials are densified and the thermoelectric properties are studied as a function of temperature. The electrical resistivity of the Bi_xZnCo_2-xO₄decreased withxvalue and it fell to 4 × 10^-2O m for the sample withxvalue = 0.1. The Seebeck coefficient value increased with the increase of Bi substitution till thexvalue of 0.1 and decreased for the sample with higher Bi content (x= 0.2) as the resistivity of the sample increased due to secondary phase formation. With the optimum Seebeck coefficient and electrical resistivity, Bi_0.1ZnCo_1.9O₄shows the high-power factor (a²s_{550 K}) of 2.3 μW K^-2m^-1and figure of merit of 9.5 × 10^-4at 668 K respectively, compared with other samples. The experimental results reveal that Bi substitution at the Co site is a promising approach to improve the thermoelectric properties of ZnCo₂O₄.

Full text of DOI:10.1039/d0ra01542c

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Enhancing the thermoelectric power factor of
nanostructured ZnCo O by Bi substitution
2
4
Cite this: RSC Adv., 2020, 10, 18769
a
a
b
a
c
A. S. Alagar Nedunchezhian, D. Sidharth, R. Rajkumar, N. Yalini Devi, K. Maeda,
a
c
b
a
M. Arivanandhan, * K. Fujiwara, G. Anbalagan and R. Jayavel
x 4
Bi ZnCo2ꢀxO (0 # x # 0.2) nanoparticles with different x values have been prepared by the sol–gel
method; the structural, morphological, thermal and thermoelectric properties of the prepared
nanomaterials are investigated. XRD analysis confirms that Bi is completely dissolved in the ZnCo
lattice till the x values of #0.1 and the secondary phase of Bi is formed at higher x value (x > 0.1). The
synthesized nanomaterials are densified and the thermoelectric properties are studied as a function of
2 4
O
2 3
O
ꢀ
2
temperature. The electrical resistivity of the Bi
x
ZnCo2ꢀx
O
4
decreased with x value and it fell to 4 ꢁ 10
U m for the sample with x value # 0.1. The Seebeck coefficient value increased with the increase of Bi
substitution till the x value of 0.1 and decreased for the sample with higher Bi content (x # 0.2) as the
resistivity of the sample increased due to secondary phase formation. With the optimum Seebeck
Received 18th February 2020
Accepted 11th May 2020
m
2
ꢀ2
coefficient and electrical resistivity, Bi0.1ZnCo1.9
O
4
shows the high-power factor (a s550 K) of 2.3 mW K
at 668 K respectively, compared with other samples. The
experimental results reveal that Bi substitution at the Co site is a promising approach to improve the
thermoelectric properties of ZnCo
ꢀ1
ꢀ4
and figure of merit of 9.5 ꢁ 10
DOI: 10.1039/d0ra01542c
rsc.li/rsc-advances
2 4
O .
6,7
attention in recent decades. Although the metal-oxide based
thermoelectric materials possess lower efficiency than interme-
Introduction
Enormous amounts of waste heat are liberated from automo- tallic compound, oxide thermoelectric material has been widely
biles, industries, etc.; the liberated waste heat can be efficiently studied due to their attractive properties such as high durability
recycled into electric energy by thermoelectric technology. Ther- at higher temperature, non-toxic, lower cost and environmentally
8
–12
moelectric is a greener form of energy conversion technology, as friendly.
The layered materials and single-crystal oxide mate-
, CaCo , CaCoO and BiSr with rock
The power generation of a thermoelectric salt structures have been extensively studied. Recent reports
it can generate electricity directly from waste heat without rials such as NaCo
O
O
O
2 4
2
4
4
9
3
1–4
13–18
moving parts.
material is governed by the dimensionless gure of merit, on nanostructured cobalt oxide and mixed valence spinel oxide
2
S s
show promising results and opened the path for the current
ZT ¼
T, where S is Seebeck coefficient, s is electrical
research. Bi substitution in Co paved a novel way for
3 4
O
k
conductivity, k is thermal conductivity, and T is temperature. To improving the thermoelectric properties of oxide thermoelec-
19
obtain high thermoelectric performance the material should trics ZnCo O is the mixed valent ternary metal oxide with cubic
2
4
possess low thermal conductivity (k), high electrical conductivity structure and it belongs to Fd 3ꢀ m space group. ZnCo O is a P-type
2
4
(
s) and large thermopower (S); it can be achieved by nano- material with the divalent Zn ions at the tetrahedral site and the
20–24
structuring, band engineering, and heavy element doping. At trivalent Co ions at the octahedral site.
The ZnCo O has been
2 4
present available conventional thermoelectric materials are extensively studied for supercapacitor and photocatalytic appli-
5
heavy metal-based intermetallic compounds. However, the use cations. However, the thermoelectric properties of nano-
of the metallic materials is limited due to their low operating structured ZnCo O are not reported so far. Therefore in the
2
4
temperature, oxidation at atmosphere, environmental issue of present work, pure and bismuth substituted ZnCo O4 (Bix-
2
heavy element, high toxicity and lower abundance of materials. ZnCo2 O (0 # x # 0.2)) were synthesized and the effect Bi
ꢀx
4
Owing to many drawbacks in using intermetallic compounds, substitution on structural, morphological and thermoelectric
metal oxide-based thermoelectric materials have gained much properties of ZnCo O were investigated.
2
4
a
Centre for Nanoscience and Technology, Anna University, Chennai 600025, India. Experimental
E-mail: arivucz@gmail.com
b
x 4
Bi ZnCo2ꢀxO (0 # x # 0.2) nanomaterials with different x
values (x ¼ 0, 0.025, 0.05, 0.1, 0.2) are prepared by sol–gel
Department of Nuclear Physics, University of Madras, Chennai 600025, India
c
Institute for Materials Research, Tohoku University, Sendai, Japan
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Fig. 1 Photograph of pellets used for measuring (a) electrical resistivity
(
b) Seebeck coefficient and the arrow indicate the pressing direction.
technique. Cobalt acetate, bismuth acetate, zinc acetate, and
ethanol were purchased from Sigma Aldrich, India and used as
the precursors without further purication. The appropriate
ratio of the precursors were taken and stirred in the oil bath at
ꢂ
9
0 C for 3 h. The obtained gel was aged and dried in a hot air
ꢂ
oven at 150 C for 12 h. The obtained samples were and
ꢂ
annealed in furnace at 400 C for 2 h. The obtained powder was
densied by applying pressure of 200 bar in circular and rect-
ꢂ
angular shaped dies. The pellets were sintered at 650 C for 2 h.
The prepared pellets are shown in Fig. 1a with the dimension of
1
mm thickness and 16 mm diameter was used for Hall
measurements and the rectangular-shaped pellets shown in
Fig. 1b with the dimension of 5 mm width, 4 mm thickness, and
16 mm length was used for Seebeck coefficient and thermal
conductivity measurements. The silver paste was used as the
metallic contact for Hall effect measurements.
The structural analysis was carried out using Rigaku miniex
ꢂ
diffractometer, Japan in the 2q range between 20–80 with Cu-
ꢁ
Ka (l ¼ 1.5406 A) source. The morphological and elemental
mapping analysis was performed using SEM – Tescan Vega,
Czech Republic and FE-SEM – Quanta-250FEG, USA, respec- Fig. 2 (a) XRD patterns of Bi ZnCo O4 (0 # x # 0.2) nanomaterials
x
2ꢀx
tively. The temperature stability of the material was analyzed with different x values, (b) shifting of XRD peaks at 36.8 degree towards
lower angle side, (c) the variation of lattice parameter of Bi
0 # x # 0.2) as a function of Bi content and the inset show unit cell
diagram of ZnCo , (d) the crystallite size of Bi ZnCo2ꢀx (0 # x #
.2) as a function of Bi substitution.
x
ZnCo2ꢀxO
4
using TG & DTA (EXTAR-6300, Japan). The electrical transport
properties of the sample were measured using the Hall effect
measurement system, (MMR Technologies, USA) as a function
of temperature from 330–550 K under vacuum condition. The
Seebeck coefficient of the samples was measured using indig-
enously fabricated Seebeck measurement instrument as
a function of temperature. The thermal conductivity of the
samples are measured using thermal conductivity measure-
ment system, (Marine India, India).
(
O
2 4
x
O
4
0
formed along with ZnCo O The mixed-phase formation of
2
4.
Bi O and ZnCo O at higher Bi substitution (x $ 0.2) is due to
2
3
2 4
the low solid solubility limit of bismuth in cobalt oxide lattice.
The lattice parameter of ZnCo O is calculated using the
2 4
d
formula, a ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi and the variation of lattice
2
2
2
ðh þ k þ l Þ
Result
Structural analysis of Bi
27
parameter with Bi content is shown in Fig. 2c. The lattice
parameter is calculated for the (311) diffraction plane and the
lattice parameter increases gradually with the Bi content and
indicates the solid solubility of Bi in ZnCo O lattice as it
x
4
ZnCo2ꢀxO (0 # x # 0.2)
The XRD patterns of Bi ZnCo O (0 # x # 0.2) nanomaterials
x
2ꢀx
4
2
4
with different x values are shown in Fig. 2a. The diffraction
peaks of ZnCo nanomaterials are well matched with the
JCPDS card no 23-1390 and conrm the formation of cubic
follows Vegard's law. The inset shows the unit cell diagram of
ZnCo which is drawn using the vesta soware. The crystallite
sizes of Bi ZnCo2ꢀx (0 # x # 0.2) samples were calculated
using Debye Scherrer equation along (311) plane. Fig. 2d shows
the variation of crystallite size of Bi ZnCo2ꢀx (0 # x # 0.2)
2 4
O
2 4
O
2
5,26
x
O
4
ꢀ
structured ZnCo
The XRD results conforms the formation of pure phase of
ZnCo nanomaterials without any impurity phase in the pure
sample. Moreover, the XRD patterns of Bi substitution ZnCo O
2 4
O nanomaterials with Fd3m space group.
x
O
4
2 4
O
samples as the function of Bi substitution. The crystallite size
2
4
decrease with the Bi substitution as it promotes shape anisot-
nanomaterials show the pure phase of ZnCo O until the
bismuth substitution of 0.1. When the bismuth substitution
increased above 0.1 of x value, secondary phase of Bi are
2
4
27
ropy and it induce variation in oxygen stoichiometry.
2 3
O
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in Fig. 5–7 and the measured composition of the samples are
shown in Table 1. Homogeneous distribution of Zn, Co, Bi, O
are observed in all the samples. The substitution of Bi is clearly
observed from Fig. 6 and 7 and the homogeneous distribution
of Bi is also clearly evident from the gures. Table 1 clearly
indicates that the Bi substitution in the Co site resulted the
variation in the Bi and Co content as the Bi composition
increased and the Co composition decreased with x values.
Thermal stability analysis of ZnCo
The thermal stability of ZnCo
2
O
4
and Bi0.2ZnCo1.8
O
4
2
O
4
and Bi0.2ZnCo1.8
O
4
nano-
materials was studied using TG & DTA analysis and Fig. 8 shows
the TG curves of the samples. The pure ZnCo nanomaterials
2 4 4
Fig. 3 FE-SEM images of (a) ZnCo O (b) Bi0.025ZnCo1.975O (c)
2 4
O
Bi0.1ZnCo1.9
4 4
O (d) Bi0.2ZnCo1.8O .
show relatively low weight loss of 3.6%. The weight of ZnCo
2
O
4
ꢂ
sample decreases gradually till 400 C which is attributed to
dehydration of water and removal of unreacted residuals from
Morphological analysis of Bi ZnCo O (0 # x # 0.2)
x
2ꢀx
4
nanomaterials
ꢂ
the sample. The pure sample is stable till 700 C as there is no
FE-SEM images of Bi ZnCo O (0 # x # 0.2) with different x further weight loss. The Bi0.2ZnCo1.8O4 nanomaterials show
x
2ꢀx
4
values are shown in Fig. 3. The pure ZnCo O sample exhibits a higher weight loss of 10% which is more than three times
2
4
the spherical morphology with a particle size 50 nm. As the Bi compared to the pure sample. The higher amount of weight loss
substitution increases the morphology of the nanomaterials in Bi0.2ZnCo1.8O nanomaterials is observed due to the presence
4
changes from spherical to ake structures. The sample with of lattice strain and secondary phase formation. The
high Bi content shows the ake-like morphology. The particles
are highly agglomerated due to the heat treatment at the
ꢂ
calcination temperature of 400 C.
Fig. 4 shows the morphological analysis of pelletized Bix-
ZnCo2 O (0 # x # 0.2) samples. The SEM reveals that the
ꢀx
4
morphology of the particles is not affected by the high-
temperature treatment as it exhibits the same morphology of
as-synthesized samples, however, the size of the particles is
relatively larger due to high-temperature treatment of the pellets.
Elemental analysis of Bi
nanomaterials
x 4
ZnCo2ꢀxO (0 # x # 0.2)
SEM images and the respective elemental mappings of
ZnCo and Bi0.2ZnCo1.8 pellets are shown
2
O
4
, Bi0.1ZnCo1.9
O
4
O
4
Fig. 5 (a) SEM image and (b–d) elemental mapping of O, Co, and Zn of
ZnCo nanomaterials.
2 4
O
2 4 4
Fig. 4 SEM images of (a) ZnCo O (b) Bi0.025ZnCo1.975O (c) Bi0.1-
ZnCo1.9 (d) Bi0.2ZnCo1.8 pellets and the insets show the respec- Fig. 6 (a) SEM image and (b–e) elemental mapping of O, Co, Zn and Bi
O
4
O
4
tive pellets used for the morphological analysis.
4
of Bi0.1ZnCo1.9O nanomaterials.
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Fig. 7 (a) SEM image and (b–e) elemental mapping of O, Co, Zn and Bi
of Bi0.2ZnCo1.8O nanomaterials.
4
Fig. 9 Electrical resistivity of Bi
of temperature.
x
ZnCo2ꢀx
O
4
(0 # x # 0.2) as a function
x
4
Table 1 Composition of Zn, Co, Bi and O of Bi Zn1ꢀxO samples
Electrical transport properties of Bi
nanomaterials
x 4
ZnCo2ꢀxO (0 # x # 0.2)
Composition of elements (atomic%)
Samples
ZnCo
Bi0.1ZnCo1.9
Bi0.2ZnCo1.8
Zinc
Cobalt
Bismuth
Oxygen
The variation of electrical resistivity the Bi
.2) nanomaterials as a function of temperature is shown in
Fig. 9. All the samples exhibit semiconducting behavior as the
x
4
ZnCo2ꢀxO (0 # x #
0
2
O
4
41.53%
40.07%
39.31%
38.09%
32.52%
26.98%
—
9.04%
18.91%
20.36%
18.30%
O
O
4
4
14.78% resistivity decreases with increase in temperature. The Bi-free
2
ZnCo O sample exhibit the high resistivity of r
308 K
2.9 ꢁ 10
2
4
U m at low temperature which is comparable with the report
ꢀ1
values and decreased (r
1.5 ꢁ 10 U m) at high tempera-
6
68 K
Bi0.2ZnCo1.8O nanomaterials show three stage of weight loss
and the rst stage of weight loss of 2% was observed till 230 C
weight loss of 2% till 230 C is attributed to the dehydration of
water. The second stage of weight loss of 2% is observed
29
4
2 4
ture. The Bi-free ZnCo O nanomaterials exhibit the high
ꢂ
resistivity at all the measured range of temperatures compared
to Bi substituted samples. As the Bi content increases the
resistivity of the nanomaterials decreased until the x values of
ꢂ
ꢂ
ꢂ
between 230 C to 350 C which are due to removal of unreacted
#
4
0.1. The Bi0.1ZnCo1.9O nanomaterials exhibit the low resis-
residuals. The Bi0.2ZnCo1.8
O
4
nanomaterials are stable till
tivity in all range of temperature compared with the other
ꢂ
ꢂ
ꢂ
600 C as no weight loss is observed between 350 C to 600 C.
samples. Bi0.1ZnCo1.9O exhibits low-temperature resistivity of
4
ꢂ
1
The major weight loss of 5.5% is observed above 600 C as the
sample loses its stability due to the phase transformation of
3
1
.1 ꢁ 10 U m at 308 K and high-temperature resistivity of 4.5 ꢁ
ꢀ2
0
U m at 668 K which is three orders lower than that of Bi-
secondary phases. The ZnCo
2
O
4
and Bi0.2ZnCo1.8
O
4
nano-
free sample. As the Bi content increases beyond 0.1 of x value
the resistivity increased. For example, Bi0.2ZnCo1.8 nano-
materials exhibit the low-temperature resistivity of 9.3 U m at
ꢂ
materials were stable till 600 C (ref. 28) and they can be used
for thermoelectric application in the intermediate temperature
range.
O
4
ꢀ2
3
08 K and high-temperature resistivity of 7.9 ꢁ 10 U m at 668
K. The higher resistivity of the sample with high Bi content (x ¼
.2) is possibly due to the formation of secondary phase of Bi O
0
2
3
as evident from XRD (Fig. 2a). Fig. 10 shows the carrier
concentration of the samples as a function of Bi content at 330
K. The pure sample shows the carrier concentration of 8.6 ꢁ
1
5
ꢀ3
10
cm
and the carrier concentration of the samples
1
6
ꢀ3
increased till the x ¼ 0.1 (1.2 ꢁ 10 cm ) and slightly
1
6
ꢀ3
decreased (1.1 ꢁ 10 cm ) for the sample with x ¼ 0.2. The
3
+
substitution of heterovalent Bi at the Co site increases the
carrier concentration due to charge compensation and the
slight drop in the carrier density is possibly due to localization
30
of charge carriers in the nanostructured samples.
Seebeck coefficient of Bi
x
ZnCo2ꢀx
O
4
(0 # x # 0.2)
nanomaterials
The Seebeck coefficient value of Bi ZnCo2ꢀxO (0 # x # 0.2)
x
4
Fig. 8 Thermal analysis of ZnCo
2
O
4
and Bi0.2ZnCo1.8
O
4
.
nanomaterials with the function of temperature is shown in
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Fig. 12 (a) Power factor of Bi
thermal conductivity of pure and Bi0.1ZnCo1.9
x
ZnCo2ꢀx
O
4
(0 # x # 0.2) samples (b)
O samples and (c) ZT of
4
x
4
Fig. 10 Carrier concentration of Bi ZnCo2ꢀxO (0 # x # 0.2) at 330 K
as a function of Bi content.
pure and Bi0.1ZnCo1.9 samples as a function of temperature.
O
4
Fig. 11. The positive values of measured Seebeck coefficient power factor of the samples with temperature are shown in
indicate the P-type nature of the prepared materials. The Bi-free Fig. 12a. As shown in Fig. 11a, the power factor of the samples
2
ZnCo O sample shows the high Seebeck coefficient compared increased with temperature. The power factor (a s668 K) of pure
2
4
ꢀ5
ꢀ2
ꢀ1
with the Bi substituted samples. The pure sample shows the low- sample is relatively low (0.9 ꢁ 10 mW K
m
) and increased
ꢀ
1
temperature Seebeck coefficient of (a328 K) 50 mV K and the with increasing Bi content until the x value of 0.1. For instance,
ꢀ
1
high-temperature Seebeck coefficient of (a668 K) 392 mV K . The at 668 K, the Bi0.1ZnCo1.9
O
4
sample exhibits the power factor of
which is relatively higher than that of other
decreases with the Bi substitution until the x value of 0.1. Typi- samples. At the same temperature, the power factor is slightly
ꢀ2
ꢀ1
Seebeck coefficient value of Bi ZnCo2ꢀxO
x 4
(0 # x # 0.2) gradually 2.3 mW K
m
ꢀ
2
ꢀ1
cally Bi0.1ZnCo1.9
O
4
sample shows the high-temperature Seebeck decreased to 1 mW K
m
for the sample with higher Bi
). The monotonic linear increase in
coefficient (a328 K) of 94 mV K at low temperature. The Seebeck power factor is due to decrease in resistivity (r). However, the
coefficient of Bi0.2ZnCo1.8O4 slightly increased for all the power factor decreased for Bi0.2ZnCo1.8 due to the secondary
temperatures compared to Bi0.1ZnCo1.9O due to the high resis- phase formation of Bi . The obtained power factor (2.3 mW
is relatively higher than that of Bi
ꢀ
1
coefficient of (a668 K) 221 mV K and it exhibits the Seebeck content (Bi0.2ZnCo1.8
O
4
ꢀ
1
O
4
2 3
O
4
ꢀ2
ꢀ1
tivity resulted from secondary phase formation.
K
m
) of Bi0.1ZnCo1.9
O
4
1
9,31,32
substituted Co O and other reported values.
3
4
The temperature dependent thermal conductivity of pure
and Bi0.1ZnCo1.9 are shown in Fig. 11b. As can be seen from
Fig. 12b, the thermal conductivity of Bi0.1ZnCo1.9 is relatively
low especially at high temperatures possibly due to sheet like
structures as observed in the FE-SEM images (Fig. 3) which may
Power factor of Bi ZnCo O (0 # x # 0.2) nanomaterials
x
2ꢀx
4
O
4
The power factors of the samples are calculated from the See-
beck coefficient and electrical resistivity. The variations of the
O
4
enhanced the phonon scattering. The pure ZnCo
2
O
4
and Bi0.1
-
ZnCo1.9
O
4
ꢀ
shows the total thermal conductivity of 2.42 and
1
ꢀ1
2.39 W K
m
at 308 K and the thermal conductivity of the
ꢀ1
ꢀ1
sample decreases to 1.67 and 1.59 W K
68 K. The gure of merit, ZT was calculated from the obtained
resistivity, Seebeck coefficient and thermal conductivity. The ZT
of pure ZnCo and Bi0.1ZnCo1.9 samples as a function of
temperature is shown in Fig. 12c. The Bi0.1ZnCo1.9 sample
m
respectively, at
6
2
O
4
O
4
O
4
ꢀ4
shows the high ZT of 9.5 ꢁ 10 at 668 K. The experimental
results demonstrate that substitution of Bi up to 0.1 value of x is
benecial for the improving the thermoelectric properties of
ZnCo O nanomaterials.
2
4
Discussion
As shown in XRD (Fig. 2a) pure phase of ZnCo
till the x value of 0.1 and the mixed phase of ZnCo
2
O
4
was formed
2 3
and Bi O
x
4
Fig. 11 Seebeck coefficients of Bi ZnCo2ꢀxO (0 # x # 0.2) samples as
a function of temperature.
O
2
4
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were formed in the samples with higher Bi content (x ¼ 0.2).
Conflicts of interest
The excess phase of Bi O was formed due to the high Bi content
2
3
which exceeds the solid solubility limit of Bi in ZnCo O lattice. There is no conict of interest to declare.
2
4
The Bi rich precipitates were clearly observed in the EDX
mapping of the samples with higher Bi content (Fig. 6). The
electrical resistivity of the samples decreased with Bi content up
to the x values of 0.1 and the resistivity of the samples with high
Bi content (x ¼ 0.2) increased. The decrease in resistivity of
Acknowledgements
The work was nancially supported by Department of Science
and Technology (DST)-SERB, India under Early Career Research
x
4
Bi ZnCo2ꢀxO is mainly attributed due to the mixed valence
(
(
ECR) award (ECR/2015/000575) and Extra Mural Research
EMR) funding (EMR/2016/007203). The author (A. S. Alagar
state of Bi and Co. The increase in resistivity at the higher Bi
substitution are probably due to the formation of secondary
phase of Bi O The secondary phase creates the interfaces
Nedunchezhian) thanks DST-SERB for Junior Research Fellow-
ship (2016–2019). The work is partially supported by Tohoku
University, Japan under GIMRT collaborative project (19K0512).
2
3.
between two different materials, probably the carriers may trap
at the interfaces, which in turn increases the resistivity. The low
x
4
Seebeck coefficient values of Bi ZnCo2ꢀxO (x < 0.2) were mainly
due to low electrical resistivity of the samples. The relatively
high Seebeck coefficient of sample with high Bi content (x ¼ 0.2)
is attributed due to high electrical resistivity of the sample.
Furthermore, it is noteworthy that the high Seebeck coefficient
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33,34
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-
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4
with the other samples due to scattering of phonons at the
35–38
interfaces of different phases of materials.
The ZT of Bi0.1-
ZnCo1.9 samples are relatively higher than that of Bi free
sample and the high ZT is mainly due to high power factor. The
experimental results revealed that the Bi substitution in
2 4
ZnCo O has the benecial effect and it is further expected to
O
4
6 K. Koumoto, Y. Wang, R. Zhang, A. Kosuga and
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increase by optimizing the composition and nanostructuring.
2
010, 2, 152–158.
0 D. J. Hagen, T. S. Tripathi and M. Karppinen, Dalton Trans.,
017, 46, 4796–4805.
1
Conclusion
2
Bi ZnCo O (0 # x # 0.2) nanomaterials have been synthe- 11 H. Alam and S. Ramakrishna, Nano Energy, 2013, 2, 190–212.
x
2ꢀx
4
sized by the sol–gel method. XRD analysis conrms that Bi is 12 J. He, Y. Liu and R. Funahashi, J. Mater. Res., 2011, 26, 1762–
completely dissolved in the ZnCo till the x value of #0.1 and 1772.
the secondary phase of Bi is formed at higher x value (x # 13 M. Saxena and T. Maiti, Dalton Trans., 2017, 46, 5872–5879.
.2). The thermal analysis reveals that the synthesized nano- 14 W. Koshibae, K. Tsutsui and S. Maekawa, Phys. Rev. B:
2 4
O
2 3
O
0
ꢂ
materials are stable between 400–600 C and it can be used for
Condens. Matter Mater. Phys., 2000, 62, 6869–6872.
intermediate temperature applications. The electrical resistivity 15 M. Karppinen, H. Fjellvag, T. Konno, Y. Morita,
of the nanomaterials increased till the x value of 0.1 and then
the electrical resistivity decreased in the sample with higher Bi
T. Motohashi and H. Yamauchi, Chem. Mater., 2004, 16,
2790–2793.
content (x ¼ 0.2) due to secondary phase formation. The See- 16 I. Terasaki, M. Iwakawa, T. Nakano, A. Tsukudaa and
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electrical resistivity till the Bi substitution of x ¼ 0.1. The 17 M. Ito and D. Furumoto, J. Alloys Compd., 2008, 450, 517–
2
ꢀ2
Bi0.1ZnCo1.9
m
O
4
shows high power factor of a s668 K – 2.3 mW K
520.
ꢀ
1
ꢀ2
due to low resistivity of (r668 K) 3 ꢁ 10 U m, and it shows 18 S. R. Hostlera, Y. Qiao Qu, M. T. Demkoa, A. R. Abramsona,
ꢀ
4
the high gure of merit of 9.5 ꢁ 10 which is relatively higher
X. Qiub and C. Burdab, Superlattices Microstruct., 2008, 43,
195–207.
than the reported values for similar oxide materials. The
experimental results show signicant improvement in the 19 A. S. Alagar Nedunchezhian, D. Sidharth, N. Yalini Devi,
thermoelectric power factor of Bi ZnCo O4 (0 # x # 0.2)
nanomaterials and it can be effectively used for intermediate
waste heat harvesting.
R. Rajkumar, P. Rajasekaran, M. Arivanandhan,
G. Anbalagan and R. Jayavel, Ceram. Int., 2019, 45, 6782–
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18774 | RSC Adv., 2020, 10, 18769–18775
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