1. Introduction
As the consumption of electrical energy in the world increases and fossil fuel supply decreases, there is a need for new, sustainable energy sources and different technologies in energy conversion. One method is based on thermoelectric (TE) effects and it implies direct conversion of waste heat into electric energy using TE moduli.
The efficiency of thermoelectric materials is determined by the dimensionless figure of merit,
ZT:
where
S,
T,
ρ,
κ are the Seebeck coefficient (thermopower), temperature, electrical resistivity and thermal conductivity, respectively [
1]. Total thermal conductivity consists of electron (
κe) and phonon parts (
κph). A good TE material with high
ZT also requires high
S, low
ρ and low
κ. In semiconductors,
S and
ρ are inversely proportional, so it is difficult to control them simultaneously. Various attempts have been made to improve the figure of merit of TE materials: by doping [
2], decreasing of
κph by introducing nanoscale preparation techniques [
3] or choosing a material with atoms that induce anharmonic vibrations of chemical bonds [
4]. Moreover, it can be achieved in layered oxides since their intrinsic crystal structure enables phonon scattering [
5]. High
S together with low
ρ in Na
xCo
2O
4, as its essential properties, makes it a promising p-type material for potential TE application. The other advantage of this material is that it is non-toxic with high thermal and chemical stability unlike common thermoelectric materials such as Bi
2Te
3, Sb
2Te
3 and PbTe, which need to be protected from surface oxidation at high temperatures.
NaCo
2O
4 belongs to the alkali ternary oxide group, A
xMO
2 (A = Na, K; M = Cr, Mn, Co, etc.), with a hexagonal layered structure (
P63/
mmc space group symmetry) [
6]. It consists of conductive edge–sharing 2D triangle CoO
2 sheets and insulating Na layers, alternately stacked along the
c-axis [
6,
7]. Electrons in the CoO
2 layer are localized due to the strong electron correlation, while Na layers serve as regions for phonon scattering. Each of these layers has its function which can be independently controlled [
6]. There are three types of the crystal structure of sodium cobaltite, depending on the sodium stoichiometry: P2, γ-Na
xCo
2O
4 (1.0 ≤ x ≤ 1.4), P3, δ-Na
xCo
2O
4 (1.1 ≤ x ≤ 1.2) and O3, α-Na
xCo
2O
4 (1.8 ≤ x ≤ 2.0) [
8]. The best thermoelectric performance shows the P2 structure [
9].
The thermoelectric properties of polycrystalline NaCo
2O
4 can be further improved by introducing dopants in place of Na or Co. Na-site doping with high valence ions decreases carrier density [
10], while Co-site doping affects physical properties through changes in the band structure and transport mechanism [
11]. Cu-doping of Na
xCo
2O
4 was previously reported by Park and coworkers [
12], but secondary phases were present in all samples. Thus, one of the main challenges in obtaining pure NCO is the synthesis process and Na evaporation, which changes the stoichiometry of the obtained ceramic [
13]. Regarding the practical application of NCO in TE generators at high temperatures, it is necessary to investigate its thermoelectric properties at high temperatures and mechanical properties in general. Some authors investigated the influence of different weight% of Ag on fractural strength of Ca
3Co
4O
9 samples [
14]. They reported that Ag filled the intergranular holes and drastically enhanced mechanical properties, providing plastic regions which prevented crack spreading [
14]. Unlike structural, microstructural and thermoelectric properties of sodium cobaltite, which were the subject of many investigations [
12,
15,
16], there are scarce data about its mechanical properties.
The aim of this work was to investigate the influence of low dopant concentrations and different syntheses methods on thermoelectric and mechanical properties of ceramic NaCo
2−xCu
xO
4 (x = 0, 0.01, 0.03, 0.05). To reduce synthesis time to obtain polycrystalline samples, lower sintering temperature, prevent Na evaporation and improve mixing of the precursors, a citric acid complex method was applied. Alternatively, a mechanochemically assisted solid-state reaction method was used as another approach for comparison. The material’s mechanical behavior and its potential for construction of TE modulus were estimated. For that purpose indentation testing was carried out to determine the hardness and elastic modulus of materials, and force-displacement curves were recorded. In this way, high-temperature thermoelectric and mechanical properties reported in this work, together with already published low-temperature TE performance [
13], provide a more complete understanding of Cu-doped NaCo
2O
4 behavior.
3. Results
The X-ray powder diffraction patterns of the sintered samples are displayed in
Figure 1. MASSR ceramic samples (
Figure 1a) contained only the diffraction peaks indexed to the single-phase γ-NaCo
2O
4 (JCPDF card No. 73-0133, space group
P63/
mmc). On the other hand, the CAC ceramic sample with x = 0.05 (
Figure 1b) showed two reflections at approximately 45° and 49° 2
θ originating from the secondary phase CuO (JCPDF card No. 89-2529 and space group
C2/
c).
The lattice parameters
a and
c of both types of the samples were calculated according to the XRD patterns in the computer programme LSUCRI, as was explained in detail in our previous work [
13]. The parameters
a and
c are similar to the standard values of
a = 2.843 Å and
c = 10.881 Å, reported in JCPDF card No. 73-0133 [
8]. Both parameters were higher for the MASSR than for the CAC samples for the same Cu amount. This was the result of higher concentration of defects (dislocations and point defects) in the crystal lattice of the MASSR samples, induced by mechanical activation of the solids [
18].
SEM micrographs (
Figure 2) of both types of samples showed uniformed microstructure with layered, plate-like grains, which increased with increasing Cu concentration. MASSR grains were larger than CAC grains, with lengths ranging from 8–13 μm for the MASSR and 4 up to 11 μm for the CAC samples. The densities of the MASSR samples were between 88% and 91% of the theoretical value, and approximately 92–93% for the CAC samples. The lowest density (88%) was calculated for NCO5-CAC. Moreover, the SEM analysis indicated the highest porosity of this sample. In samples NCO5-MASSR and NCO5-CAC, white precipitates were noticed and EDX analysis confirmed a higher Cu amount in these spots, although secondary phases were not detected in the XRD of the NCO5-MASSR sample [
13].
The electrical resistivity, the thermal conductivity and the Seebeck coefficient of all samples were measured in the temperature range 320–830 K. This is a sequel of our previous research, where the thermoelectric properties were presented in low temperature regions, below room temperature [
13]. Since the thermoelectric parameters (i.e.,
ρ,
S,
κ and
ZT) were measured by the large Δ
T method and thus determined in the temperature gradient, their values are graphically represented according to the temperature on the hot side of the sample and the temperature difference between the hot and cold sides of the sample at this temperature of hot side.
The electrical resistivity of NaCo
2−xCu
xO
4 (x = 0, 0.01, 0.03 and 0.05) is shown in
Figure 3.
In the measured temperature range, measured
ρ of all samples implied a metal-insulator transition. Metallic behavior of NaCo
2O
4 originates from low spin Co
3+ and Co
4+ ions. Namely, it is assumed that in stoichiometric NaCo
2O
4 there are equal amounts of Co
4+ and Co
3+ ions. These ions can occupy low spin state, with electronic configurations, t
2g5 (t
2g6); intermediate, t
2g4e
g1 (t
2g5e
g1); or high spin state, t
2g3e
g2 (t
2g4e
g2). It is assumed that Co ions occupy low spin state. As a result, the t
2g band splits into a
1g and e’
g bands. The e
g bands of all CoO
6 octahedra, regardless of whether they contain Co
4+ or Co
3+ ions, overlap and form wide e’
g-block bands, responsible for metallicity [
19,
20]. The CAC samples showed smaller resistivity, and among them, the lowest value was obtained for NCO5-CAC. The resistivities of the samples did not follow monotonous behavior of dependencies. This can be attributed to a complex influence of dopant concentration on variety of material properties, such as structure (symetry), microstructure (porosity), phase purity (NCO5-CAC contained traces of CuO) and even Co
3+/Co
4+ ratio. Some of these properties can have the opposite effect on the resistivity or thermal conductivity of the samples. Furthermore, different behavior between CAC and MASSR samples can be explained by difference in their microstructure and chemical homogeneity. Larger and flatter grains observed in MASSR samples make them more prone to the anisotropy of the electrical conductivity. Similar behavior was observed by Seetawan et al. [
21] and Terasaki et al. [
2], where the resistivity curves of NCO samples also showed non-monotonous dependence with the concentration of Ag and Cu, respectively. In general, the electrical conductivity of Na
xCo
2O
4-based systems consists of an ionic and electronic component. The ionic part originates from Na
+-ions and it is several orders of magnitude lower than the electronic, so the measured conductivity has an electronic character [
12]. In the temperature range between 2 and 300 K, CAC samples showed metallic behaviour [
13]. Therefore, the mechanism of the electrical conductivity can be explained in the following way. The valence band (VB) and conductive band (CB) consist of Na 3s, Co 3d and O 2p orbitals and transition between VB and CB occurs because of the maximum contribution of 2p and 3d states in VB and CB [
22]. Low spin Co
3+ (3d
6) and Co
4+ (3d
5) ions formed partially filled 3d bands in the CoO
2 sheets. Overlapping 3d orbital from Co
3+ and/or Co
4+-ions and 2p orbital from O
2−-ion created weak hybridization between these orbitals, and formed a broad Co-O-Co band, located between the valent and conduction band. The transition between valence and conduction band occurred through the Co-O-Co band and in low temperature regions this mechanism of conduction dominated. As the temperature increased, the carriers from Co-O-Co passed in the conduction band; their number also increased with increasing temperature and caused a moderate decrease of the resistivity. Doping with Cu
2+ in place of Co
3+/
4+ increases the number of oxygen vacancies; this can be expressed according to the following equation with Kröger–Vink notation:
With respect to these equations, the conductivity of the samples should increase with Cu concentration. However, since the other part of the conductivity contribution is related to Co3+/Co4+ distribution as the intrinsic source of charge carriers, it can be assumed that some charge carrier recombination takes place and reflects on the resistivity trend.
The thermal conductivity of the samples is presented in
Figure 4. In this range,
κ has a parabolic shape. Minimum values for the MASSR samples were between 1.35 W/m K and 1.57 W/m K, and for the CAC samples between 1.18 W/m K and 1.58 W/m K, indicating that preparation methods had a weak influence on
κ. In both cases, maximum
κ was obtained for the samples containing 3 mol% of Cu, and the minimum
κ was obtained for undoped samples. Lower thermal conductivity of the CAC samples is the consequence of enhanced phonon scattering, due to the fine microstructure of the samples, which enables precursor preparation by CAC [
7]. The NCO1-MASSR sample possesses smaller grains with irregular forms compared to NCO5-MASSR, which may reflect on its lower thermal conductivity.
The electron thermal conductivity can be calculated with the Wiedemann–Franz law:
where
L0 is the Lorenz number (
). Its values were one order of magnitude lower than total thermal conductivity (
Figure 5), so the measured
κ came from the lattice [
5].
The Seebeck coefficient was positive in the whole temperature range (
Figure 6), indicating that major conductivity carriers were holes and increased with increasing temperature [
12].
The CAC samples showed larger values compared with the MASSR and in both cases the doped samples showed higher
S compared with the undoped ones. The highest value was found in samples with 3 mol% of Cu (145 μV/K for NCO3-CAC and 110 μV/K for NCO3-MASSR). High
S was the consequence of the high electron correlation present in this type of compound [
2,
5]. As we have already emphasized, the CAC method enabled better homogenization of the constituents during the synthesis, resulting in homogeneous precursor powders, higher density and smaller grains of the sintered ceramics. Bearing in mind the above properties, better results of TE measurements were confirmed in the CAC samples. Moreover,
S of undoped samples (111 μV/K for NCO-CAC and 100 μV/K for NCO-MASSR at T
HOT = 830 K, Δ
T = 473 K) was comparable with already reported values [
21,
23,
24,
25].
The figure of merit was calculated based on the obtained values for the electrical resistivity, the Seebeck coefficient and the thermal conductivity and its temperature dependence are presented in
Figure 7.
Taking into account both synthesis methods, the highest
ZT was obtained for the sample NCO5-CAC (0.061 at Δ
T = 473 K), mainly due to high
S and low
ρ, which is in accordance with
ZT = 0.07, obtained for Na
0.75CoO
2 epitaxial film [
25]. This value was more than three times larger than
ZT for the undoped CAC sample and 1.7 times larger than the highest value obtained for the MASSR sample (
ZT(NCO3-MASSR) = 0.036 at Δ
T = 473 K). Although the sample NCO3-MASSR showed increased
κ, it also showed enhanced
S, which is a more dominant characteristic because of squared
S in the equation for
ZT. The difference between the minimum and maximum
κ for MASSR and CAC samples did not have much of an impact on the final result of the figure of merit.
Within mechanical properties, Young’s modulus of elasticity and hardness of both types of samples were determined. The technique involved the recording of force and displacement when an indentation was made. While nanoindentation tests were performed, the indentation tip was pressed on the surface of the sample and load-displacement curves were recorded. The average values of the mechanical properties obtained for NaCo
2−xCu
xO
4 (x = 0, 0.01, 0.03 and 0.05) ceramics are given in
Table 2.
The hardness of the CAC samples was higher compared to the MASSR samples and it could be related to the homogeneous microstructure, higher density of these samples and higher concentration of microstructural defects in the MASSR samples. The NCO5-MASSR sample possesses the lowest density and the most porous microstructure, and therefore a significant reduction of the Young’s modulus and the hardness was obtained for this sample. Moreover, the modulus and the hardness of the NCO1-CAC sample, which were estimated to be 65.2 GPa and 1.41 GPa, respectively, were almost 2 and 1.5 times, respectively, larger than the one reported for Bi
2Te
3, common-type TE material [
26]. The reason for increased hardness can be explained by strong cohesion forces and increased number of grain boundaries, which hinder crack propagation [
27].Furthermore, the absence of secondary phases, which can hinder crack propagation, can be attributed to enhanced hardness. Uniform values of the Young’s modulus for CAC samples reflected on the properties of bulk material and highlighted the advantage of the citric acid method for obtaining homogeneous microstructures. Load-displacement curves for the MASSR and CAC ceramic samples are presented in
Figure 8 and
Figure 9. Different shapes of the curves for the same sample indicated some inhomogeneity of the sample (
Figure 8). Smaller deviations of the curves for CAC samples (
Figure 9) pointed to a higher degree of homogeneity. Lower depths of impression came as a result of increased hardness. Taking into account obtained results, the highest figure of merit and the best mechanical properties exhibited the NCO3-CAC sample, and can be considered for practical application.
Having in mind all the results presented in this work, it is apparent that the synthesis procedure notably affects the phase purity, microstructure, thermoelectric and also mechanical properties of NCO ceramics.