Research Article
Research Article
Thermal and thermoelectric properties of metal-doped zinc oxide ceramics
expand article infoAliaksei V. Pashkevich§, Alexander K. Fedotov, Eugen N. Poddenezhny|, Liudmila A. Bliznyuk, Vladimir V. Khovaylo#, Vera V. Fedotova, Andrei A. Kharchenko
‡ Research Institute for Nuclear Problems of Belarusian State University, Minsk, Belarus
§ Belarusian State University, Minsk, Belarus
| Sukhoi State Technical University of Gomel, Gomel, Belarus
¶ Scientific-Practical Materials Research Centre of the National Academy of Sciences of Belarus, Minsk, Belarus
# National University of Science and Technology “MISIS”, Moscow, Russia
Open Access


The thermal, electrical and thermoelectric properties of ZnO–MexOy ceramics with 1 ≤ x, y ≤ 3, where Me = Al, Co, Fe, Ni, Ti, have been studied. The specimens have been synthesized using the ceramic sintering technology from two or more oxides in an open atmosphere with annealing temperature and time variation. The structural and phase data on the ceramics have shown that post-synthesis addition of MexOy doping powders to wurtzite-structured ZnO powder causes Znx ()yO4 spinel-like second phase precipitation and a 4-fold growth of ceramics porosity. Room temperature heat conductivity studies have testified to predominant lattice contribution. A decrease in the heat conductivity upon doping proves to be caused by phonon scattering intensification due to the following factors: size factor upon zinc ion substitution in the ZnO lattice (wurtzite) by MexOy doping oxide metal ions; defect formation, i.e., point defects, grain boundaries (microstructure refinement); porosity growth (density decline); secondary phase particle nucleation (Znx ()yO4 spinel-like ones). The above listed factors entailed by zinc ion substitution for metal ions (Co, Al, Ti, Ni, Fe) increase the figure-of-merit ZT by four orders of magnitude (due to a decrease in the electrical resistivity and heat conductivity coupled with a moderate thermo-emf decline). The decrease in the electrical resistivity originates from a more homogeneous distribution of doping metal ions in the wurtzite lattice upon longer annealing which increases the number of donor centers.


ceramics, zinc oxide, density, heat conductivity, phonon scattering, thermoelectric efficiency

1. Introduction

Zinc oxide possesses a unique combination of physical properties and can be produced by a wide variety of methods. This has made it the subject of close attention of researchers for many decades. ZnO based ceramics find increasingly wide application in gages and various electric transducers [1, 2], gas sensors [3], high-power electronics where heat removal is of great importance [4] and other domains. Investigations aimed at improving the properties and broadening the applications of these materials are currently focused on the search for various combinations of doping elements, such as transition metals (TM). Ceramics have a number of advantages over single crystals, polycrystalline ingots and thin films as well as other ZnO based materials requiring more expensive technologies. Ceramic technologies provide items of any shapes and sizes having various morphologies of structural inclusions (grains and phases). This provides for efficient control of the functionality of ceramics by varying the temperature, atmosphere and time of synthesis and post-synthesis heat treatment as well as doping element type added to initial powder mixtures [5–10]. However, to attain a composition with required combinations of properties one should identify correlations between the structure (phase and chemical composition), morphology (grain shape, porosity, size distribution, etc.), electrical and thermal properties of zinc oxide based composite ceramics and search for more resource-saving technologies [5–7, 11, 12].

In this work we studied the abovementioned relations favorable for ZnO based ceramic application as n-type conductivity thermoelectric materials that can be quite promising due to their high electron mobility, thermal stability and corrosion resistance as well as low health and environment hazard. However, the heat conductivity of undoped ZnO is so high that its dimensionless figure-of-merit ZT turns out to be far lower than required for industrial application. The figure-of-merit can be calculated using the following formula:

ZT=S2Tκ1+κeρ; (1)

κ = κl + κe = λCpd0, (2)

where Z is the efficiency, T is the absolute temperature, S is the thermo-emf coefficient, ρ is the electrical resistivity, κl is the lattice heat conductivity coefficient, κe is the electron conductivity coefficient, λ is the temperature conductivity coefficient, Cp is the isobaric heat capacity and d0 is the density of specimens.

Therefore thermoelectric applications require the heat conductivity (it consists of two contributions, i.e., the lattice one κl and the electron one κe) and electrical resistivity ρ of the material be greatly reduced without reducing S.

From this standpoint, doping is one of the key tools to tune the κ, S and ρ parameters for increasing the thermoelectric efficiency ZT in Eq. (1). Since the direct measurement of the heat conductivity coefficient κ is quite a labor-consuming task, it is often assessed with a simpler technique, i.e., laser flash temperature conductivity coefficient measurement.

Literary sources contain a large number of ZnO based material doping options for microstructure controlling aimed at matching the thermoelectric and transport properties. There are data [13–19] on the effect of doping elements such as Al, Bi, Co, Fe, Ni, Ga, Mn, Sb, Sn and pairwise combinations thereof on the thermoelectric and transport properties of these materials. The cited works contain helpful information on the relation between the structure and various properties obtained as a result of doping. However, there is no systematic study of the correlation between the thermoelectric and thermal properties, as well as the microstructure, phase composition and morphology (porosity, grain size) in films and ceramics [20, 21]. Multiple experimental and theoretical works showed that there are two efficient methods to reduce the heat conductivity: formation of substitution alloys and nanostructuring (e.g. by reducing grain sizes, introducing pores and nanosized phase inclusions) [22].

Presented below are experimental data on ceramics formed from zinc oxide and metal oxides such as ZnO–MexOy with 1 ≤ x, y ≤ 3, where Me = Al, Co, Fe, Ni, Ti. The aim of the experiments was to study the effect of ceramic composition on the matching of thermoelectric, thermal and electrical properties.

2. Specimens and methods

ZnO based specimens were synthesized using the ceramic sintering technology from two or more oxides in an open atmosphere. The initial components for the charge were ZnO and MexOy powder mixtures of the following compositions: 10 wt.% CoO, 3–5 wt.% Al2O3, 10 wt.% TiO2. The charge composition for the test ceramic specimens was designed based on the formula (ZnO)100-z(MexOy)z, where z = 3÷10 wt.% is the metal oxide powder weight percentage in the test specimens. Most of the specimens were in the form of compacted tablets fabricated by single-step calcination, while the (ZnO)90(СоО)10-2 specimens were sintered in two step sintering process after compaction as described earlier [23, 24]. Details on the compositions and annealing modes for the specimens are summarized in Table 1. Specimens 3 and 5 differ in the origin of the raw ZnO powders.

Table 1.

Test specimen composition and synthesis mode notations

# Specimen Preliminary/final sintering temperature, K Preliminary/final sintering time, h
1 ZnO 1373 2
2 (ZnO)90(CoO)10-2* 1173/1473 2/2
3 (ZnO)97(Al2O3)3 1473 3
4 (ZnO)95(Al2O3)5 1473 3
5 (ZnO)97(Al2O3)3 1473 3
6 (ZnO)96.5(Al2O3)3(NiO)0.5 1473 3
7 (ZnO)96.5(Al2O3)3(Fe2O3)0.5 1473 3
8 (ZnO)96.5(Al2O3)3(Fe3O4–SiO2)0.5 1473 3
9 (ZnO)90(TiO2)10 1473 3

The matrix phase structure and the phase composition of the synthesized specimens were studied using X-ray diffraction (XRD) at room temperature on a DRON-3 M diffractometer with CuKα radiation. The data were processed with the Math!(3.14 Build 238) and FullProf software by XRD pattern deconvolution into integral intensities. The following ceramic phase identification crystallographic database cards were used:

– ZnO #96-900-4179;

– ZnFe2O4 #96-900-5108;

– ZnCo2O4 #96-591-0137 (or Сo3O4 #96-900-5888);

– Zn2TiO4 #96-900-1693;

– ZnAl2O4 #96-900-6202.

The FullProf software uses Rietveld (full-profile) analysis designed for neutron and X-ray diffraction studies [25]. The structural morphology and the chemical composition of the synthesized ceramics were studied under TescanVega 3LMU and LEO 1455 VP scanning electron microscopes (SEM) with EDX attachments for determining component concentrations in the composites. Details of operation and results provided by the method were described earlier [26].

For room temperature measurements of the electrical resistivity, thermo-emf coefficient S and density, the synthesized ceramic tablets were cut into rectangular specimens 2–3 mm in width and 7–10 mm in length. Indium was applied to the butt-ends of the specimens with an ultrasonic soldering for more homogeneous current distribution by the specimen and thermal resistivity reduction. The Seebeck coefficient S and the electrical resistivity were measured at room temperature using measuring system with a moving copper-tip as gradient heater, Agilent 34410A and Agilent 34411A multimeters. The butt-end temperature of the specimen placed in the metering system was monitored with RT-100M platinum thermometers. The heat conductivity λ was measured for 8.8 × 1.5 mm2 ceramic specimens in the T = 300÷573 K range by the laser flash method with LFA 467 (Netzch, Germany) and TC-1000 (Ulvac-Riko, Japan) analyzers.

3. Structure and phase composition of composite ceramics

The XRD and EDX data (Fig. 1, Table 2) suggest that CoO, Al2O3 and TiO2 additions to ZnO cause the formation of not only the ZnO wurtzite tetragonal phase (Specimen 1, see Fig. 1 a and Fig. 1, f), but also the second Znx (Mе)yO4 cubic phases (ZnCo2O4, ZnAl2O4, Zn2TiO4 and ZnFe2O4 for Co, Al, Ti and Fe doping, respectively), where x = 1÷2 and y = 1÷2 (Specimens 2–9, see Fig. 1 a and Fig. 1 b–e) [27]. The X-ray peaks for the two-stage annealed specimen containing 10 wt.% CoO (Specimen 2, see Fig. 1 a) suggest a relatively low content of the ZnCo2O4 (or Сo3O4) phase provided that most Co ions have substituted Zn ones in the ZnO lattice [22].

Figure 1.

(a) X-ray patterns for ZnO based ceramic, (b) EDX data for element distributions (c–f) for (ZnO)96.5(Al2O3)3(Fe3O4–SiO2)0.5: (1) ZnO; (2) (ZnO)90(CoO)10-2; (3) ZnO)97(Al2O3)3; (4) ZnO)95(Al2O3)5; (5) ZnO)97(Al2O3)3; (6) (ZnO)96.5(Al2O3)3(NiO)0.5; (7) (ZnO)96.5(Al2O3)3(Fe2O3)0.5; (8) ZnO)96.5(Al2O3)3(Fe3O4–SiO2)0.5; (9) (ZnO)90(TiO2)10

Table 2.

Calculated parameters of ZnO based ceramics

# Specimen a/с (for ZnO) (nm) B (for Znx (Mе)yO4) (nm) d 1 (kg/m3) d 2 (kg/m3) d 3 (kg/m3) d 0 (kg/m3) Π (%)
1 ZnO 0.324719 / 0.519646 5667 5036 11
2 (ZnO)90(CoO)10-2 0.324949 / 0.519748 0.807038 (or 0.86969 for Сo3O4) 5658 3120 5633 4450 21
3 (ZnO)97(Al2O3)3 0.324770 / 0.519884 0.807548 5663 2307 5562 4200 24
4 (ZnO)95(Al2O3)5 0.324964 / 0.520207 0.808657 5653 2298 5485 3200 42
5 (ZnO)97(Al2O3)3 0.324563 / 0.519568 0.807476 5674 2308 5573 4780 14
6 (ZnO)96.5(Al2O3)3(NiO)0.5 0.324813 / 0.519993 0.808001 5660 2303 5531 4260 23
7 (ZnO)96.5(Al2O3)3(Fe2O3)0.5 0.325265 / 0.520535 0.810126 5639 2285 5510 4300 22
8 (ZnO)96.5(Al2O3)3(Fe3O4–SiO2)0.5 0.325199 / 0.520596 0.808865 5640 2296 5512 4100 26
9 (ZnO)90(TiO2)10 0.325040 / 0.520823 0.846505 5643 2649 5344 5240 2

The X-ray patterns of the specimens shown in Fig. 1 a suggest that additions of 3–5 wt.% Al2O3 (Specimens 3–8) and 10 wt.% TiO2 (Specimen 9) to zinc oxide ceramics cause Al and Ti incorporation into the ZnO wurtzite phase lattice (Fig. 1 b, f) and the formation of the Znx (Mе)yO4 : ZnAl2O4 cubic phases for the Al doped specimens [28] (Specimens 3–8, see Fig. 1 d, e and Fig. 2 h) and the Zn2TiO4 cubic phases for the Ti doped specimen (Specimen 9) [18]. For Specimens 6–8, addition of 0.5 wt.% NiO [29], Fe2O3 and Fe3O4–SiO2 also causes the formation of Znx(Mе)yO4 phases in small quantities, e.g., ZnFe2O4 [14, 20, 21] for the (ZnO)96.5(Al2O3)3(Fe3O4–SiO2)0,5 specimen (Specimen 8) which is shown in Fig. 1 c.

Figure 2.

SEM images of ceramics: (a) ZnO, (b) (ZnO)90(CoO)10-2, (c) (ZnO)90(TiO2)10, (d) (ZnO)97(Al2O3)3 (Specimen 5), (e) (ZnO)96.5(Al2O3)3(Fe2O3)0.5, (f) (ZnO)96.5(Al2O3)3(Fe3O4-SiO2)0.5, (g) (ZnO)95(Al2O3)5 and (h) EDX aluminum distribution for (ZnO)96.5(Al2O3)3(NiO)0.5

The XRD-determined lattice parameters allowed evaluating the X-ray densities of the material d1, d2 and d3 (in kg/m3) using the following relationships:

– for the ZnO phase:

d1=N1A1V1=N1A1/6a234C=2N1A133a2c; (3)

– for the Znx(Mе)yO4 phase:

d2=N2A2V2=N2A2b3; (4)

– for the total of the ZnO + Znx (Mе)yO4 phases:

d3=100-z100d1+z100d2=100-z100N1A1V1+z100N2A2V2=100-z1002N1A133a2c+z100N2A2b3.. (5)



is the hexagonal unit cell volume, N1 = 6 is the number of atoms per hexagonal unit cell, N2 = 4 is the number of atoms per cubic unit cell, Ai is the mass of one atom in a.m.u (1 a.m.u. = 1.66 ∙ 10-24 g), А1 = 81 (for ZnO), А2 = 242 for Zn2TiO4, 183 for ZnAl2O4 and 247 for ZnCo2O4; a and c are the ZnO lattice parameters (wurtzite), b is the Znx(Mе)yOzlattice parameter (spinel), and z = 3–10 wt.% is the impurity weight percentage in ceramics except (ZnO)90(CoO)10-2 (Specimen 2) for which z was accepted as unity in Eq. (5) due to a high CoO solubility in wurtzite.

The X-ray densities d1, d2 and d3 calculated using Eqs. (3)–(5) are summarized in Table 2. It can be seen from Table 2 that the X-ray density d1 of the ZnO phase itself (i.e., without Znx(Mе)yO4) in Specimens 2–9 depends but slightly on the type and quantity of doping impurity. However, the X-ray density of the final ceramic d3 containing metal oxides (Specimens 2–9) decreases as compared with d1. This can be accounted for by the fact that the overwhelming majority of Al2O3 and TiO2 weight is spent for the formation of the ZnAl2O4 and Zn2TiO4 cubic spinels the X-ray density d2 of which is lower than that of hexagonal ZnOd1.

The difference between the X-ray densities d1 (ZnO), d3 (ZnO + Znx(Mе)yO4) and d0 of the final specimens (Table 2) as determined by weighing is caused by pores in the ceramics. This allowed calculating the porosity of the specimens using Eq. (6) for ZnO (Specimen 1) and Eq. (7) for Specimens 2–9:

Π=1-d0d1·100%;; (6)

Π=1-d0d3·100%;; (7)

As can be seen From Table 2, MexOy oxide addition to ZnO powder increases the porosity of the ceramics by up to 42% which is probably caused by presence of the additional Znx (Mе)yO4 spinel phase (Specimen 4), as well as by the differences between the raw powders and the annealing modes for undoped and doped zinc oxide.

SEM data are shown in Fig. 2. The doped (ZnO)90(CoO)10-2 two-stage annealed specimen exhibits a decline in grain size (Fig. 2 b) as compared with the undoped zinc oxide specimen (Fig. 2 a). We showed earlier that grain sizes in one-stage annealed ceramics with iron oxide additions were also greater than those in similar powders after two-stage annealing [20, 21]. We therefore attribute the smaller grain size in the two-stage annealed ceramics to an increase in the number of recrystallization centers at granule boundaries of tablet specimens as a result of the second grinding. It also follows from Fig. 2 that less dense specimens have larger grains.

4. Thermal properties of composite ceramics

The thermo-emf coefficient S and the electrical resistivity ρ of the test specimens were measured at room temperature (300 K), and the temperature conductivity λ(Т) was measured at 300–600 K. The λ(Т) curves are shown in Fig. 3 a. These curves can be described by a hyperbolic law such as

λ(T) ~ TB, (8)

where the exponent В, according to Table 3, is in the В ≈ 0.90–1.50 range. The B parameter was determined from the slope of the lg λ vs lg Т lines in Fig. 3 b. As can be seen from Fig. 3 a, the temperature conductivity at 300 K decreased as a result of ceramic doping somewhat greater than at high temperatures. This can be accounted for by a decrease in the lattice contribution to the temperature conductivity with an increase in temperature [14, 18, 22, 28, 29].

Equation (2) suggests that to determine the temperature dependence of the temperature conductivity κ(T) of the specimens it is required to know their heat capacity Cp and density d0. The temperature dependence of the isobaric specific heat Cp (Т) in the 300–600 K range for the test specimens was evaluated using the Neuman–Kopp method [30]. Cp (Т) functions for different phases burrowed from literature [31] are shown in Fig. 4 a. For this method the specific heat of doped ceramics is approximated as the sum of the specific heat of the constituent phases taking into account their weight fractions in the (ZnO)100-z(MexOy)z ceramic where z = 3÷10 is the weight fraction of metal oxide powders in the test specimens in wt.%. As can be seen from Fig. 4 a, the isobaric specific heat Cp of the phases and the ceramics varies in accordance with the following law:

Cp (T) ~ TA, (9)

where A ≈ 0.20–0.25. It was accepted for the calculations that almost all the added metal oxide is spent for the formation of the Znx(Mе)yO4 spinel the fraction of which cannot exceed 10% of the total weight of ceramics. The presence of Znx (Mе)yO4 increases the specific heat by not more than 4% (see Fig. 4 a) and only slightly affects the behavior of the temperature conductivity upon doping. The positions of the points in Fig. 4 b suggest a nearly linear pattern of the heat conductivity as a function of density except for the titanium doped specimen (ZnO)90(TiO2)10 having one of the lowest temperature conductivities.

The experimental densities d0 (see Fig. 4 b) and the λ(Т) curves of the studied specimens, as well as their isobaric specific heat Cp (see Fig. 4 a) allow evaluating the temperature dependences of the heat conductivity κ(Т) for the specimens using Eq. (2). As can be seen from Fig. 5 a, the κ(Т) function can be described as

κ(T) ~ TC, (10)

where the C values as determined from the slopes of the lg κ vs lg Т lines in Fig. 5 b are summarized in Table 3.

Potential electron contribution κe to the full heat conductivity was determined using the Wiedemann–Franz method [29, 32]:

κe=LTρ, (11)

in which the Lorenz number was calculated through the thermo-emf coefficient S using the formula

L=1.5+exp-|S|116·10-8, (12)

obtained by solving Boltzmann transfer equations [32]. The components’ contributions to the full heat conductivity summarized in Table 3 suggest that the lattice component has the predominant contribution to Eq. (2), i.e., κ ≈ κl, since the electron contribution κe evaluated using Eqs. (11) and (12) is incomparably small. Therefore the decline of the ionic conductivity with an increase in temperature and the difference in the behavior of the κ(Т) curves at high and low temperatures should only be accounted for by the contribution of the phonon spectrum.

Comparison of the temperature dependences of the phonon heat conductivity (Eq. (10)) and the temperature conductivity (Eq. (8)) shown in Figs 5 a and 3 a suggests that the difference in their exponents B – C = A exactly equals the exponent of the temperature dependence of the isobaric heat capacity (Eq. (9). The behavior of the temperature dependence of the heat conductivity is in agreement with well-known literary data [4, 29, 33]: the decline of κ(Т) is attributed to the growth of phonon-phonon scattering with an increase in temperature. It should be noted that the heat conductivity and the temperature conductivity behave quite similarly.

The difference between the heat conductivity of raw zinc oxide and those of doped ceramics (as well as many other semiconductors) is typically associated with the wide gap between the optical and the acoustic branches in the phonon dispersion law. The latter fact may significantly distort the energy and quasi-impulse conservation laws for three-phonon scattering and increase the phonon lifetime, which can be potentially coupled with anharmonicity (the Gruneisen parameter) and large phonon group velocities (due to stronger interatomic bonds) [4].

By and large, the decline in the heat conductivity seen from Fig. 5 a upon metal oxide addition to ZnO based ceramics can be accounted for by the following four factors:

– greater phonon scattering at point defects formed due to zinc ion substitution for metal ions in the ZnO lattice;

– stronger phonon scattering at grain boundaries due to microstructure refinement (greater total grain boundary area) [14];

– growth of the porosity of ceramics as a result of doping [33];

– precipitation of the Znx (Mе)yO4 phase with a layered spinel-like structure causing additional phonon scattering.

Figure 3.

Temperature conductivity λ of test ceramics as a function of temperature in (a) linear and (b) double logarithmic scales: (1) ZnO; (2) (ZnO)90(CoO)10-2; (3) (ZnO)97(Al2O3)3; (4) (ZnO)95(Al2O3)5; (5) (ZnO)97(Al2O3)3; (6) (ZnO)96.5(Al2O3)3(NiO)0.5; (7) (ZnO)96.5(Al2O3)3(Fe2O3)0.5; (8) ZnO)96.5(Al2O3)3(Fe3O4-SiO2)0.5; (9) (ZnO)90(TiO2)10

Figure 4.

(a) Temperature dependences of isobaric specific heat Cp of total materials in percents [31] and (b) heat conductivity κ as a function of density d0 for test ceramics at T = 300 K in a linear scale. b: (1) ZnO; (2) (ZnO)90(CoO)10-2; (3) (ZnO)97(Al2O3)3; (4) (ZnO)95(Al2O3)5; (5) (ZnO)97(Al2O3)3; (6) (ZnO)96.5(Al2O3)3(NiO)0.5; (7) (ZnO)96.5(Al2O3)3(Fe2O3)0.5; (8) ZnO)96.5(Al2O3)3(Fe3O4-SiO2)0.5; (9) (ZnO)90(TiO2)10

Table 3.

Experimental temperature conductivity λ, exponents in Eqs. (8)–(10), electron κe and lattice κl ≈ κ contributions to heat conductivity κ for ceramic specimens at 300 K

# Speimen λ (10-6 m2/s) A B C κe (W/(m ∙ K)) κl (W/(m ∙ K))
1 ZnO 16.8 0.23 1.50 1.27 1.19 ∙ 10-7 43.05
2 (ZnO)90(СоО)10-2 6.94 0.20 (0.23) 1.14 0.93 2.21 ∙ 10-7 16.36
3 (ZnO)97(Al2O3)3 11.1 0.25 1.33 1.09 2.59 ∙ 10-5 23.98
4 (ZnO)95(Al2O3)5 7.60 0.26 1.20 0.96 1.46 ∙ 10-4 12.66
5 (ZnO)97(Al2O3)3 12.3 0.25 1.41 1.16 4.59 ∙ 10-5 30.27
6 (ZnO)97(Al2O3)3(NiO)0.5 6.79 0.25 0.90 0.65 1.56 ∙ 10-3 14.91
7 (ZnO)97(Al2O3)3(Fe2O3)0.5 7.76 0.25 1.35 1.08 8.56 ∙ 10-7 17.18
8 (ZnO)97(Al2O3)3(Fe3O4-SiO2)0.5 9.92 0.25 1.39 1.11 1.16 ∙ 10-6 20.95
9 (ZnO)90(TiO2)10 7.46 0.24 1.32 1.08 2.31 ∙ 10-4 20.05
Figure 5.

Heat conductivity κ of test ceramics as a function of temperature in (a) linear and (b) double logarithmic scales: (1) ZnO; (2) (ZnO)90(CoO)10-2; (3) (ZnO)97(Al2O3)3; (4) (ZnO)95(Al2O3)5; (5) (ZnO)97(Al2O3)3; (6) (ZnO)96.5(Al2O3)3(NiO)0.5; (7) (ZnO)96.5(Al2O3)3(Fe2O3)0.5; (8) (ZnO)96.5(Al2O3)3(Fe3O4-SiO2)0.5; (9) (ZnO)90(TiO2)10

5. Thermoelectric efficiency of composite ceramics

The power factor PF [29]

PF=S2ρ and the dimensionless figure-of-art ZT in Eq. (1) [32] were calculated on the basis of experimentally measured S and ρ at 300 K which are summarized in Table 4.

Table 4.

Thermoelectric, electrical and thermal parameters of ZnO based ceramics at 300 K

# Specimen ρ (Ohm ∙ m) S (mV/K) P (W/(m ∙ K2)) κe (W/(m ∙ K)) ZT
1 ZnO 3.67 ∙ 101 385 4.04 ∙ 10-9 43.05 2.81 ∙ 10-8
ZnO (ann)* 3.80 ∙ 101 435 4.99 ∙ 10-9 3.47 ∙ 10-8
2 (ZnO)90(СоО)10-2 5.10 ∙ 100 580 1.65 ∙ 10-8 16.36 3.02 ∙ 10-7
3 (ZnO)97(Al2O3)3 1.90 ∙ 10-1 221 2.57 ∙ 10-7 23.98 3.20 ∙ 10-6
4 (ZnO)95(Al2O3)5 3.19 ∙ 10-2 346 3.75 ∙ 10-6 12.66 8.88 ∙ 10-5
5 (ZnO)97(Al2O3)3 1.07 ∙ 10-1 229 4.90 ∙ 10-7 30.27 4.86 ∙ 10-6
6 (ZnO)97(Al2O3)3(NiO)0.5 2.46 ∙ 100 224 2.04 ∙ 10-8 14.91 4.10 ∙ 10-7
(ZnO)97(Al2O3)3(NiO)0.5 (ann) 3.06 ∙ 10-3 278 2.53 ∙ 10-5 5.08 ∙ 10-4
7 (ZnO)97(Al2O3)3(Fe2O3)0.5 5.46 ∙ 100 330 2.00 ∙ 10-8 17.18 3.48 ∙ 10-7
8 (ZnO)97(Al2O3)3(Fe3O4–SiO2)0.5 4.14 ∙ 100 276 1.84 ∙ 10-8 20.95 2.64 ∙ 10-7
9 (ZnO)90(TiO2)10 2.30 ∙ 10-2 162 1.14 ∙ 10-6 20.05 1.46 ∙ 10-5

It follows from Table 4 that addition of metals to ZnO ceramics increases the figure-of-art (by almost four orders of magnitude for Specimen 6) in comparison with that for undoped ZnO ceramics [8, 9] (see also [14]). This is caused by the decrease in the electrical resistivity by four orders of magnitude (which can be attributed to extrinsic conductivity caused by the formation of small donor centers having a low ionization energy [20]), as well as by the decline in the heat conductivity as described herein. Noteworthy, the greatest augment of the figure-of-art is observed for the specimen doped with Al and Ni (Specimen 6) and subjected to high-temperature measurements that are in fact an additional anneal. The figure-of-art of ZnO ceramics differs from that of the same ceramic composition without additional annealing (Specimen 6) by only three orders of magnitude due to a lower electrical resistivity (Table 4). For undoped zinc oxide ceramic (Specimen 1), additional annealing did not increase the figure-of-art by orders of magnitude. This is because additional annealing leads to a more homogeneous distribution of doping metal ions in the wurtzite lattice thus entailing an increase in the number of donor centers that are favorable for electrical conductivity [20].

6. Conclusion

The thermal, electrical and thermoelectric properties of ceramics ZnO–MexOy with 1 ≤ x, y ≤ 3, where Me = Al, Co, Fe, Ni, Ti, were studied. X-ray diffraction data suggest that addition of MexOy doping powders to ZnO powder with a wurtzite structure causes spinel-like Znx(Mе)yO4 secondary phase precipitations and a four-fold growth of the porosity after synthesis. Lattice heat conductivity is predominant in the ceramics at room temperature but its contribution decreases with an increase in temperature. The decline in the heat conductivity upon doping is favored by an increase in phonon scattering at grain boundaries, at point defects forming due to zinc ion substitution for doping metal ions, at pores and at additional Znx(Mе)yO4 phase precipitates. In the Al, Co, Fe, Ni and Ti doped ceramics, the decline in the heat conductivity and the decrease in the electrical resistivity by orders of magnitude combined with a moderate change of the termo-emf coefficient provide for a growth of the figure-of-art by four orders of magnitude. Longer annealing reduces the electrical resistivity due to a more homogeneous distribution of doping metal ions in the wurtzite lattice which is favorable for the growth of donor centers.


This research was funded by the State program of scientific research “PhysMatTech, New Materials and Technologies” (Belarus) under grant number 1.15.1.


  • 1. Ponja S.D., Sathasivam S., Parkin I.P., Carmalt C.J. Highly conductive and transparent gallium doped zinc oxide thin films via chemical vapor deposition. Scientific Reports. 2020; 10(1): 638.
  • 2. Lee Y.-P., Lin Ch-Ch., Hsiao Ch-Ch., Chou P.-A., Cheng Y.-Y., Hsieh Ch-Ch., Dai Ch-A. Nanopiezoelectric devices for energy generation based on ZnO nanorods / flexible-conjugated copolymer hybrids using all wet-coating processes. Micromachines. 2020; 11(1): 14.
  • 4. Wu X., Lee J., Varshney V., Wohlwend J.L., Roy A.K., Luo T. Thermal conductivity of wurtzite zinc-oxide from first-principles lattice dynamics – a comparative study with gallium nitride. Scientific Reports. 2016; 6(1): 22504.
  • 6. Winarski D. Synthesis and characterization of transparent conductive zinc oxide thin films by sol-gel spin coating method. Thesis diss. of master science. Graduate College of Bowling Green State University; 2015.
  • 7. Chen H., Zheng L., Zeng J., Li G., Effect of Sr doping on nonlinear current-voltage properties of ZnO-based ceramics. Journal of Electronic Materials. 2021; 50(7): 4096–4103.
  • 8. Mohammed M.A., Izman S., Alias M.N., Rajoo S., Uday M.B., Obayes N.H., Omar M.F. A review of thermoelectric ZnO nanostructured ceramics for energy recovery. International Journal of Engineering & Technology. 2018; 7(2.29): 27–30.
  • 9. Colder H., Guilmeau E., Harnois C., Marinel S., Retoux R., Savary E. Preparation of Ni-doped ZnO ceramics for thermoelectric applications. Journal of the European Ceramic Society. 2011; 31(15): 2957–2963.
  • 10. Jeong A., Suekuni K., Ohtakia M., Jang B.-K. Thermoelectric properties of In- and Ga-doped spark plasma sintered ZnO ceramics. Ceramics International. 2021; 47(17): 23927–23934.
  • 11. Levinson L.M., Hirano S. Ceramic transactions. In: Procced. of Inter. symposium. Vol. 41. Grain boundaries and interfacial phenomena in electronic ceramics. Westerville: American Ceramic Society; 1994.
  • 12. Li J., Yang S., Pu Y., Zhu D. Effects of pre-calcination and sintering temperature on the microstructure and electrical properties of ZnO-based varistor ceramics. Materials Science in Semiconductor Processing. 2021; 123(6): 105529. 2020.105529
  • 13. Liang X. Thermoelectric transport properties of naturally nanostructured Ga–ZnO ceramics: Effect of point defect and interfaces. Journal of the European Ceramic Society. 2016; 36(7): 1643–1650.
  • 14. Liang X. Thermoelectric transport properties of Fe-enriched ZnO with high- temperature nanostructure refinement. ACS Applied Materials & Interfaces. 2015; 7(15): 7927–7937.
  • 15. Walia S., Balendhran S., Nili H., Zhuiykov S., Rosengarten G., Wang Q.H., Bhaskaran M., Sriram S., Strano M.S., Kalantar-zadeh K. Transition metal oxides – thermoelectric properties. Progress in Materials Science. 2013; 58(8): 1443–1489.
  • 16. Li P., Zhang H., Gao C., Jiang G., Li Z. Electrical property of Al/La/Cu modified ZnO-based negative temperature coefficient (NTC) ceramics with high ageing stability. Journal of Materials Science: Materials in Electronics. 2019; 30(21): 19598–19608.
  • 17. Pullar R.C., Piccirilloa C., Novais R.M., Quarta A., Bettini S., Iafisco M. A sustainable multi-function biomorphic material for pollution remediation or UV absorption: aerosol assisted preparation of highly porous ZnO-based materials from cork templates. Journal of Environmental Chemical Engineering. 2019; 7(2): 102936.
  • 18. Sun Q., Li G., Tian T., Zeng J., Zhao K., Zheng L., Barre M., Dittmer J., Gouttenoire F., Rousseau A., Kassiba A.H. Co-doping effects of (Al, Ti, Mg) on the microstructure and electrical behavior of ZnO-based ceramics. Journal of the American Ceramic Society. 2020; 103(5): 3194–3204.
  • 19. Vu D.V., Le D.H., Nguyen C.X., Trinh T.Q. Comparison of structural and electric properties of ZnO-based n-type thin films with different dopants for thermoelectric applications. Journal of Sol-Gel Science and Technology. 2019; 91(1): 146–153.
  • 20. Pashkevich A.V., Fedotov A.K., Poddenezhny E.N., Bliznyuk L.A., Fedotova J.A., Basov N.A., Kharchanka A.A., Zukowski P., Koltunowicz T.N., Korolik O.V., Fedotova V.V. Structure, electric and thermoelectric properties of binary ZnO-based ceramics doped with Fe and Co. Journal of Alloys and Compounds. 2022; 895: 162621.
  • 21. Pashkevich A.V., Bliznyuk L.A., Fedotov A.K., Khovaylo V.V., Fedotova V.V., Kharchanka A.A. Thermal and thermoelectric properties of ceramics based on zinc oxide alloyed with iron. Zhurnal Belorusskogo gosudarstvennogo universiteta. Fizika. 2022; (3): 56–67. (In Russ.).
  • 22. Wu Z.-H., Xie H.-Q., Zhai Y.-B. Preparation and thermoelectric properties of Co-doped ZnO synthesized by sol-gel. Journal of Nanoscience and Nanotechnology. 2015; 15(4): 3147–3150.
  • 24. Gorokhova E.I., Anan’eva G.V., Eron’ko S.B., Oreshchenko E.A., Rodnyi P.A., Chernenko K.A., Khodyuk I.V., Lokshin E.P., Kunshina G.B., Gromov O.G., Lott K.P. Structural, optical, and scintillation characteristics of ZnO ceramics. Journal of Optical Technology.2011; 78(11): 733–760.
  • 25. Krzhizhanovskaya M.G., Firsova V.A., Bubnova R.S. Application of the Rietveld method for solving problems of powder diffractometry. St. Petersburg: Sankt-Peterburgskii universitet; 2016. 67 p. (In Russ.)
  • 26. Zeer G.M., Fomenko O.Yu., Ledyaeva O.N. Application of scanning electron microscopy in material science. Zhurnal Sibirskogo federal'nogo universiteta. Seriya: Khimiya = Journal of Siberian Federal University. Chemistry. 2009; 4(2): 287–293. (In Russ.)
  • 28. Cheng H., Xu X.J., Hng H.H., Ma J. Characterization of Al-doped ZnO thermoelectric materials prepared by RF plasma powder processing and hot press sintering. Ceramics International. 2009; 35(8): 3067–3072.
  • 29. Chernyshova E., Serhiienko I., Kolesnikov E., Voronin A., Zheleznyy M., Fedotov A., Khovaylo V. Influence of NiO nanoparticles on the thermoelectric propertiesof (ZnO)1-x(NiO)x composites. Nanobiotechnology Reports. 2021; 16(3): 381–386.
  • 30. Adun H., Kavaz D., Wole-Osho I., Dagbasi M. Synthesis of Fe3O4–Al2O3–ZnO / water ternary hybrid nanofluid: Investigating the effects of temperature, volume concentration and mixture ratio on specific heat capacity, and development of hybrid machine learning for prediction. Journal of Energy Storage. 2021; 41(13-14): 102947.
  • 31. Barin I. Thermochemical data of pure substances. Weinheim, Federal Republic of Germany; N.Y., USA: VCH; 1995. 2003 p.
  • 32. Kim H.-S., Gibbs Z.M., Tang Y., Wang H., Snyder G.J. Characterization of Lorenz number with Seebeck coefficient measurement. APL Materials. 2015; 3(4): 041506.
login to comment