Received: 24 Aug 2018
Revised: 27 Sep 2018
Accepted: 08 Oct 2018
Published online: 08 Oct 2018
Yining Feng,1 Evan Witkoske,2 Elizabeth S Bell,3 Yang Wang,4 Athanasios Tzempelikos,5 Ian T. Ferguson6 and Na Lu1,2,4,*
1Lyles School of Civil Engineering, Birck Nanotechnology Center, Sustainable Materials and Renewable Technology (SMART) Lab, Purdue University, West Lafayette, IN 47906 USA
2School of Electrical and Computer Engineering, Birck Nanotechnology Center, Sustainable Materials and Renewable Technology (SMART) Lab, Purdue University, West Lafayette, IN 47906 USA
3Davidson School of Chemical Engineering, Sustainable Materials and Renewable Technology (SMART) Lab, Purdue University, West Lafayette, IN 47906 USA
4School of Materials Engineering, Birck Nanotechnology Center, Sustainable Materials and Renewable Technology (SMART) Lab, Purdue University, West Lafayette, IN 47906 USA
5Lyles School of Civil Engineering, Center for High Performance Buildings, Ray W. Herrick Laboratories, Purdue University, West Lafayette, IN 47906, USA
6Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla MO 65409, USA
Metal oxides and nitrides are widely used in many applications as a result of their high mechanical, chemical, and electrical properties. In high temperature thermoelectric applications, oxides and nitrides exhibit high thermopower and thermal stability. Moreover, most oxides and nitrides are consisted of earth abundant elements, which are non-toxic, cost effective and easy for large-scale synthesis. In this article, we reviewed the recent advances of metal oxides and nitrides and their applications in thermoelectrics. The materials that are examined include both p-type semiconductors (e.g. NaxCoO2, Ca3Co4O9, GaN) and n-type semiconductors (e.g. ZnO-based, SrTiO3, InGaN, InN). This study is focused on the temperature dependent thermoelectric transport properties of oxides and nitrides aiming for reaching a high power factor.
Table of Content
A comprehensive study focused on the temperature dependent thermoelectric transport properties of oxides and nitrides aiming for reaching a high power factor.
Keywords: Oxides; Nitrides; Thermoelectric; Figure of Merit; Power Generation
It is evident that thermoelectric (TE) devices are the important advancement in the renewable energy field. They are able to directly convert heat into electricity via the Seebeck effect, whichcould take advantages of the waste heat generated by various processes.1–3 Factories, homes and even the human body can be used as heat sources for TE devices. However, the biggest challenge standing in the way of TE devices is the suitable materials with high power factor, an indicator of a large voltage with high current will be generated using TE devices. So far TE devices have mainly been limited to niche applications such as power generation space and medical devices, where the benefits of a stable energy source are worth the high cost and low efficiency of typical TE materials.4–6 There are hopes of using TE devices for broader applications including electricity production in rural areas, enhancement of solar panels, energy recovery from the exhaust pipes of cars, and wearable flexible devices.7–9 However, the use of TE devices will not become widespread until more efficient, environmentally friendly, and cost effective TE materials are produced.
There are several factors to determine the quality of materials for TE applications, which have been combined into one parameter called the figure of merit, zT. , where S is the Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity, which includes both electron and lattice thermal conductivity.5,10 A high Seebeck coefficient, high electrical conductivity, and low thermal conductivity are ideal for TE materials. This combination is challenging because electrical and themal conductivity often go hand in hand, as stated in the Wiedemann-Franz law, so improving one parameter often simutantlously improves another property.1,10 Ideal TE materials should scatter the movement of phonons, the major heat-carrying component of a material, while allowing charge carriers, electrons and holes, to flow freely.11
For TE power generation, it is important to look at power factor (PF), which is defined as S2σ .12PF indicates the voltage output under a certain temperature difference. It is urgent to search for high PF TE materials as most of current TE materials have achieved the high zT value owing to their low thermal conductivity, which does not ensure that it will be able to produce a large amount of power. Therefore increasing the power factor should be a major goal in the development of new TE materials for power generation, but making sure the figure of merit remains fairly high is still necessary.13
To this end, we focused on reviewing the recent advancement in n- and p-type oxide and nitride materials, particularly ZnO-based alloy,14NaxCoO2,15 Ca3Co4O9,16 SrTiO3,17 InN,18 InGaN,19 AlGaN20 and GaN,21 since these materials typically have large PF. The temperature dependent TE transport properties of these materials are summarized and analyzed.
There is a promising opportunity for metal oxides used as TE materials because of their non-toxicity, cost effective, and high thermal stability compared to conventional TE materials.11,22Also, by altering their crystal structures and chemical compositions, electronic properties can be manipulated from insulator to metallic conductor behavior, which leads to a higher power factor.23Waste heat sources are often free and unlimited, so materials with high power factor are more important than figure of merit for many TE power generation applications.5 The following sections review the TE properties of the most common n- and p-type of metal oxides materials.
2.1 N-type Oxides
The direct band gap of Zinc oxide (ZnO) is reported as 3.37 eV, which also shows large exciton binding energy of 60 meV.24 It is a promising candidate for n-type TE material used in high temperature energy harvesting.25–27 The study showed the PF of bulk ZnO to be about 0.75×10-4 W/mK2 at carrier concentration (n~10-17 cm-3) at room temperature. The high PF can be attributed to the high crystal quality, which resulted in a favorable Seebeck coefficient (~478 µV/K).28
Although pure ZnO already has high Seebeck coefficient, significant enhancement of TE power factor and Seebeck coefficient is reported with Ga29 and In-Al30 doping. As shown in Figure 1, the maximum power factor value of 12.5×10-4 W/mK2 at 1273 K observed in Zn0.985Ga0.015O yields zTmax of 0.25. Sean et al30 reported that (ZnO)Al0.03In0.02 exhibited the best TE properties with a PF of 22.1×10−4 W/mK2 at 975 K. From this study, an electrical conductivity of 5.88×102 S/cm and Seebeck coefficient of −220 μV/K has been reported. The PF is three times greater than that of the film without In dopants.
Figure 1. Temperature dependence of TE properties of ZnO
P-type doping of ZnO has been found to be very difficult. Ideally, group-I (Li and Na), group V (N, P, As, Sb and Bi), and group IB (Cu and Ag) elements could be p-type dopants. However, it is still very challenging due to the stability and reliability of p-type ZnO.31
Strontium titanate (SrTiO3)-based perovskite oxide materials have shown potential applications as n-type TE materials at low temperature.32 Nag et al. reported that at high doping concentration (n~1021 cm-3) and its high electron mobility (10 cm2V-1s-1 to 100 cm2V-1s-1) lead to good electrical conductivity.33 Also, the study showed the large effective mass (m*∼2-16 m0)34,35 caused by the material’s d-band nature and conduction band degeneracy.36,37 Additionally, introducing oxygen vacancies or substitutional doping of Sr2+ or Ti4+ sites with higher valence elements will change the electrical conductivity of SrTiO3 from insulating to metallic behavior.35,38,39
Figure 2 shows that Sr0.95La0.05TiO3 reaches a maximum PF of 28×10-4 W/mK2 at 320 K and carrier density of 0.2-2×1021 cm-3,37 a value comparable to the PF of the most commonly used low temperature TE material, Bi2Te3. This performance arises from the high Seebeck coefficient (~350 µV/K), which is due to high degeneracy of the conduction band and the high energy-dependent scattering rate. The rare earth elements Sm, Dy and Y have smaller ionic radii+, which reduce lattice parameter.40 The La and Dy co-doped La0.08Dy0.12Sr0.8TiO3 has a lower PF owing to the decreased electrical conductivity. The decrease in conductivity which comes from the reduced carrier mobility due to the formation of the second phase (Dy2Ti2O7) during the doping mechanism.41
Figure 2. Temperature dependence of TE properties of SrxTiO3
2.2 P-type oxides
NaxCoO2 has a hexagonal layered crystal structure formed by alternating sodium ion (Na+) and CoO2 planes along the c axis. The structure of this material gives phonons and electrons different transport paths.42 Electrons and holes pass through the CoO2 layer during p-type conduction, whereas the disordered Na+ layer provides the channel for phonon movement. Thus, this type of layered structure will simultaneously provide high electrical conductivity and low thermal conductivity, which is referred to as “phonon glass electron crystal” behavior,33 an ideal combination for TE applications. Therefore, it has been recognized that p-type alkali cobalt oxides as highly promising oxide TE materials.43 The study reported polycrystalline Na0.85CoO2exhibited a high PF of 14×10-4 W/mK2 at 300 K.15,44
The effects of different dopants and doping levels have been widely investigated for NaxCoO2. Figure 3 shows a summary of the effects of various dopants on PF. Ag doping can increase both the electrical conductivity and Seebeck coefficient of NaxCoO2, resulting in a high PF value. It was expected the increase in electrical conductivity since Ag is a metal-phase dopant. However, it is not clear that the mechanism of the enhancement of the Seebeck coefficient. One possibility could be caused by the uniformity of Ag doping throughout the sample.45With 10% Ag doping, NaxCoO2 reached a PF of 18.92×10-4 W/mK2 at ~ 1100 K with a doping concentration of ~1021 cm-3.46
Figure 3. Temperature dependence of TE properties of NaxCoO2
Ca3Co4O9 is also a promising p-type oxide because of good electrical properties.47,48 Ca3Co4O9 has stacked crystal structure that CoO2 and Ca2CoO3 layers alternating along the c axis. The CoO2 planes are mainly responsible for electrical conduction, and the Ca2CoO3 interlayers transfer heat by phonons. Un-doped polycrystalline Ca3Co4O9 shows a room temperature Seebeck coefficient of 150 μV/K, electrical conductivity of 80 S/cm, and PF of 1.5×10-4 W/mK2.49 However, using nobles metals such as Ag as a dopant at the Ca cationic atom site can both increase thermopower and electrical transport properties, which lead to increased PF values.50 This can be attributed to more improvement for the Fermi level (EF) than the valence band energy (EV) of the crystal materials, which resulting from substituting Ag+ for Ca2+ in Ca3-xAgxCo4O9 (0 < x < 0.3).51 For TE materials, the Seebeck coefficient is proportional to the difference between the Fermi level and valence band energy (EF - EV), which indicates that Ag doping in Ca3Co4O9 can increase the Seebeck coefficient. The PF of Ca3Co4O9 is much smaller than that of NaxCoO2 at 300K, but Ca3Co4O9 is being more commonly used in TE applications. It is more stable with different compositional changes.52
Figure 4 shows the effects of doping Ca3Co4O9 with different transition metal elements. The PF increases with temperature for all of the dopants.53,54 Substituting transition elements (i.e. Fe, Bi, Mn, Ba, Ga) for Ca or Co is also effective in increasing the PF of Ca3Co4O9. The Fe-doped Ca3Co4O9 has a significant increase in PF from 2.3×10-4 W/mK2 to 6.10×10-4 W/mK2 at ~1000 K as shown in figure. This is due to the substitution of Fe ions for Co ions in the CoO2 layers changes the electronic structure and increases the electronic correlations. Thus, Fe is an effective dopant, which increases both Seebeck coefficient and electrical conductivity.54
Figure 4. Temperature dependence of TE properties of Ca3Co4O9
III-nitrides are used in a wide variety of applications, such as light-emitting diodes (LEDs), optical and electronic devices, and ultraviolet (UV) detectors.22 These materials are considered for TE applications for a variety of reasons, including low material cost, high thermal stability, high mechanical strength, and radiation hardness.55,56 III-nitrides also generally can be alloyed or nano-structured during the fabrication process, which allows the ability to tune TE properties.56–58 Due to the wide bandgap property in many N based materials, III-Nitrides can maintain a high Seebeck coefficient at high temperatures without the excitation of minority carriers and the loss of the n- or p- type character of the materials.22,59 With this array of unique properties and fabrication possibilities, III-nitrides offer a promising solution for developing viable TE devices that would operate at high temperatures, where most current TE materials are limited to less than 800 K.60 The synthesis methods and quality of nitride-based materials are continually improving due to their usefulness in a variety of applications.22 The following sections review the TE properties of the most common n- and p-types of III-nitride materials and their notable alloys.
3.1 N-type Nitrides
Indium nitride (InN) can be considered a third-generation semiconductor material which has only recently gained increased interests. InN has a band gap of 0.7 eV,61 which displays good electrical conductivity even at intrinsic carrier concentrations. Similar to GaN, InN has a wurtzite crystal structure with lattice constants 3.574Å and 5.704 Å for a and c, which are higher than GaN due to indium’s larger atomic mass.62
A reasonable estimate of the maximum Seebeck at 900K can be obtained using InN’s band gap of 0.7 eV, yielding a value of around 390 μV/K.63Figure 5 shows electrical conductivity versus temperature for three Fermi level values of 0.108 eV, 0.025 eV and 0.011 eV corresponding to carrier concentrations of 6x1018 cm-3, 9x1017 cm-3 and 4x1017 cm-3, respectively. Due to the low effective mass of 0.05 m0 of this narrow band gap material,64 we expect this material to have a large electrical conductivity. The lattice conductivity has not been extensively studied, however due to its larger discrepancy between constituent elements, one would expect its thermal conductivity below that of GaN. We do observe a reduction in lattice thermal conductivity from InN at room temperature, however the reduction is only on the order of 3-4 times that of GaN.65In Figure 6, the zT is shown for bulk InN as well as for InN nanowires with varying diameters. The largest zT value of 1.6 is obtained for the 6 nm nanowire at 1000K, which is much larger than the value at room temperature for this same structure. This shows indium nitride’s potential for high temperature TE applications through the use of nanostructuring, which reduces dimensionality to increase the overall figure of merit.66
Figure 5. Temperature dependence of electrical conductivity of InN63
Figure 6. Temperature dependence of TE properties of InN65
Indium gallium nitride (InGaN) is a ternary alloy semiconductor that has the same wurtzite crystal structure as GaN and InN. InGaN combines the properties of InN and GaN, arising from the ability to substitute In atoms in place of Ga atoms, or vice versa. InN has a band gap of 0.7 eV,but upon replacing a fraction (x) of Ga atoms with indium atoms, In1-xGaxN has a band gap that varies between 3.4 eV and 0.7 eV. This change in band gap will directly affect the Seebeck coefficient and therefore the overall TE performance of the materials. The effect on electrical conductivity is not obvious, however the thermal conductivity will decrease due to the addition of an element with a different mass.
Figure 7 shows the TE transport properties versus temperature, assuming a carrier concentration of 1.1x1019 cm-3 and a 0.17 InN fraction.67 The power factor increases with the temperature increases due to the enhanced Seebeck coefficient. However, both electrical and thermal conductivity drop as temperature is increased. The trend for Seebeck coefficient versus temperature is common for most materials. Also, the reductions in both electrical and thermal conductivity due to the increase of phonon scattering as the temperature is raised.At a temperature of 875K, zT attains a maximum of 0.34, showing InGaN’s applicability for high temperature TE applications.
Figure 7. Temperature dependence of TE properties of InGaN
The high temperature TE properties of AlGaN have been described in several studies to date. Szteinet al.68reporteda simulationworkonInGaNTE propertiesbyusingCallaway’sapproximation in tandemwith a virtual crystalextensiondevelopedby B. Abeles,69 as shown in Figure 8. As the temperature increases, as one would normally expect, the thermal conductivity,