Optimized weighted mobility induced high thermoelectric performance of ZnO-based multilayered thin films

As a transparent thermoelectric oxide, gallium-doped zinc oxide (GZO) has the potential to power wearable or portable electronics and may be used in the integrated circuits industry for chip cooling. Constructing ZnO–GZO interfaces has been proposed as an effective strategy for improving thermoelectric performance of GZO thin films. However, without the aid of band structure calculation for multilayered films, it is hard to directly elucidate the underlying mechanisms of carrier transport. Weighted mobility is an indicator that reveals the inherent electronic transport properties like carrier scattering, electronic band structure, and so on. Thus, to further investigate the effects of ZnO–GZO interfaces on electrical properties of GZO thin films, the structures containing different numbers of ZnO–GZO interfaces were designed and the correlations among numbers of ZnO–GZO interfaces, weighted mobility, and electrical properties were explored. It was found that with more ZnO–GZO interfaces, the weighted mobility increased, and the power factor values also improved as well. Consequently, an enhanced power factor value reached 439 μW m⁻¹ K⁻² at 623 K. This work demonstrated the beneficial effects of multiple interfaces on the improvements of electrical transport performance through analyzing weighted mobility, which laid a foundation for further optimization of thermoelectric performance.     

High thermoelectric performance of high-mobility Ga-doped ZnO films via homogenous interface design

Ga-doped ZnO (GZO) thin films grown on sapphire substrates have been widely investigated as a promising transparent thermoelectric (TE) material. However, due to the large lattice mismatch and thermal expansion between the sapphire substrate and GZO film, strain-induced lattice distortion impedes the transport of electrons, leading to low carrier mobility. In this study, ZnO homo-buffer layers with different thicknesses were inserted between sapphire substrates and GZO films, and their effect on the TE properties was investigated. A thin ZnO interlayer (10 nm) effectively reduced the lattice mismatch of the GZO film and improved the carrier mobility, which contributed to the large enhancement in the electrical conductivity. Simultaneously, energy filtering occurred at the interface between GZO and ZnO, resulting in a relatively high density of states (DOS) effective mass and maintaining a high Seebeck coefficient compared to that of the unbuffered GZO films. Consequently, the GZO film with a 10 nm thick ZnO buffer layer possessed a high power factor value of 449 μW m⁻¹ K⁻² at 623 K. This study provides a facile and effective method for optimizing the TE performance of oxide thin films by synergistically improving their carrier mobility and enhancing their effective mass.     

Studies of thermoelectric transport properties of atomic layer deposited gallium-doped ZnO

The thermoelectric transport properties of atomic layer deposited (ALD) gallium doped zinc oxide (GZO) thin films were investigated to identify their potential as a thermoelectric material. The overall thermoelectric properties, such as the Seebeck coefficient and electrical conductivity, were probed as a function of Ga concentration in ZnO. The doping concentration was tuned by varying the ALD cycle ratio of zinc oxide and gallium oxide. The GZO was deposited at 250 °C and the doping concentration was modified from 1% to 10%. Sufficient thermoelectric properties appeared at a doping concentration of 1%. The crystallinity and electronic state, such as the effective mass, were investigated to determine the enhancement of the thermoelectric properties. The efficient Ga doping of GZO showed a Seebeck coefficient of 60 μV/K and an electrical conductivity of 1808.32 S/cm, with a maximum power factor of 0.66 mW/mK².     

Enhancement of thermoelectric performance of N-type Bi2Te3 based thin films via in situ annealing during magnetron sputtering

In this work, n-type Bi₂Te₃ based thin films were prepared in 300 °C via DC magnetron sputtering, and influences of sputtering power and annealing time on thermoelectric properties of films were investigated. The raise of sputtering power brings about the improvement of deposited rate and enhancement of grain size. Taking the consideration that the large-sized grains are to phonon scattering, we determine the medial power of 30 W as the basic technical parameters for the purpose of further optimizing performance through an in situ annealing process. Subsequently, thin-film treated by in situ annealing process acts out an obvious reduction in electrical conductivity attributed to the decrease in carrier concentration. Especially, the film annealed for 40 min shows an enhancement in the Seebeck coefficient and leads to a maximum power factor 0.82 m W m⁻¹ K⁻² at 543 K.     

Revealing the high sensitivity in the metal toinsulator transition properties of the pulsed laser deposited VO2 thin films

Vanadium dioxide (VO₂) is known as a typical 3d-orbital transition metal oxide exhibiting the metal-to-insulator-transition (MIT) property near room temperature. However, their electronic applications have been challenged by the quality and uniformity of VO₂ thin films. In this work, we demonstrate the high sensitivity in the valence charge of vanadium and the MIT properties of the VO₂ thin films to the deposition temperature. This observation indicates the necessity to eliminate the inhomogeneity in the temperature distribution of substrate during the vacuum-deposition process of VO₂. In addition, a high thermoelectric power factor (PF, e.g., exceeding 1 μWcm⁻¹K⁻²) was achieved in the metallic phase of the VO₂ thin films and this value is comparable to typical organic or oxide thermoelectric materials. We believe this high PF enriches the potential functionality in thermoelectric energy conversions beyond the existing electronic applications of the current vacuum-grown VO₂ thin films.     

Thermoelectric properties of layered oxyselenides with 3d transition metal ions

Layered oxychalcogenides have received extensive attention in the fields of magnetism, superconductivity, lithium battery, and luminescence due to their unique electronic, magnetic properties, and layered structure, among which layered oxyselenides have excellent and promising thermoelectric performance, such as BiCuSeO and Bi₂LnO₄Cu₂Se₂. Here, we successfully synthesized Sr₂MO₂Cu₂Se₂ (M = Co, Ni, Zn) and Sr₂FeO₃CuSe and investigated the thermoelectric properties at a wide temperature range (298–923 K). They have a relatively high Seebeck coefficient (>300 μV K⁻¹) in medium to high temperature range and possess a low thermal conductivity. The power factor and ZT reach 65 μW m⁻¹ K⁻¹ and 0.07 at 923 K for intrinsic Sr₂NiO₂Cu₂Se₂, and a higher performance is expected to be achieved by strategies like carrier concentration optimization and band structure engineering.     

Thermal conductivity of yttria-stabilized zirconia thin films grown by plasma-enhanced atomic layer deposition

Zirconia doped with yttrium, widely known as yttria-stabilized zirconia (YSZ), has found recent applications in advanced electronic and energy devices, particularly when deposited in thin film form by atomic layer deposition (ALD). Although ample studies reported the thermal conductivity of YSZ films and coatings, these data were typically limited to Y₂O₃ concentrations around 8 mol% and thicknesses greater than 1 μm, which were primarily targeted for thermal barrier coating applications. Here, we present the first experimental report of the thermal conductivity of YSZ thin films (∼50 nm), deposited by plasma-enhanced ALD (PEALD), with variable Y₂O₃ content (0–36.9 mol%). Time-domain thermoreflectance measures the effective thermal conductivity of the film and its interfaces, independently confirmed with frequency-domain thermoreflectance. The effective thermal conductivity decreases from 1.85 to 1.22 W m⁻¹ K⁻¹ with increasing Y₂O₃ doping concentration from 0 to 7.7 mol%, predominantly due to increased phonon scattering by oxygen vacancies, and exhibits relatively weak concentration dependence above 7.7 mol%. The effective thermal conductivities of our PEALD YSZ films are higher by ∼15%–128% than those reported previously for thermal ALD YSZ films with similar composition. We attribute this to the relatively larger grain sizes (∼23–27 nm) of our films.     

Influence of a Ni interlayer on the optical and electrical properties of trilayer GZO/Ni/GZO films

Trilayer GZO/Ni/GZO films were deposited onto polycarbonate (PC) substrates with RF and DC magnetron sputtering, and then the influence of a Ni interlayer on the optical and electrical properties of the films was investigated. A 2-nm-thick Ni interlayer decreased the resistivity to 6.4×10⁻⁴ Ω cm and influenced the optical transmittance.Although optical transmittance deteriorated with Ni insertion, the films showed a relatively high optical transmittance of 74.5% in the visible wavelength region. The figure of merit (FOM) of a GZO single layer film was 1.2×10⁻⁴ Ω⁻¹, while that of the GZO/Ni/GZO films reached a maximum of 8.2×10⁻⁴ Ω⁻¹.Since a higher FOM results in higher quality transparent-conductive oxide (TCO) films, it is concluded that GZO films with a 2 nm Ni interlayer have better optoelectrical performance than single-layer GZO films.     

Enhanced thermoelectric properties of carbon fiber reinforced cement composites

Thermoelectric properties of carbon fiber reinforced cement composites (CFRCs) have attracted relevant interest in recent years, due to their fascinating ability for harvesting ambient energy in urban areas and roads, and to the widespread use of cement-based materials in modern society. The enhanced effect of the thin pyrolytic carbon layer (formed at the carbon fiber/cement interface) on transport and thermoelectric properties of CFRCs has been studied. It has been demonstrated that it can enhance the electrical conduction and Seebeck coefficient of CFRCs greatly, resulting in higher power factor 2.08 µW m⁻¹ K⁻² and higher thermoelectric figure of merit 3.11×10⁻³, compared to those reported in the literature and comparable to oxide thermoelectric materials. All CFRCs with pyrolytic carbon layer, exhibit typical semiconductor behavior with activation energy of electrical conduction of 0.228-0.407 eV together with a high Seebeck coefficient. The calculation through Mott’s formula indicates the charge carrier density of CFRCs (10¹⁴–10¹⁶ cm⁻³) to be much smaller than that of typical thermoelectric materials and to increase with the carbon layer thickness. CFRCs thermal conductivity is dominated by phonon thermal conductivity, which is kept at a low level by high density of micro/nano-sized defects in the cement matrix that scatter phonons and shorten their mean free path. The appropriate carrier density and mobility induced by the amorphous structure of pyrolytic carbon is primarily responsible for the high thermoelectric figure of merit.     

Enhanced thermoelectric performance of p-type sintered BiSbTe-based composites with AgSbTe2 addition

Bismuth telluride-based materials have been widely used in the field of thermoelectric cooling near room temperature. However, the material utilization and device conversion efficiency were limited by the low thermoelectric performance and poor mechanical properties of commercial zone-melting materials. With an aim to optimize the comprehensive properties, we prepared the composite samples of Bi_0.48Sb_1.52Te₃ (BST)-x wt% AgSbTe₂ (x = 0, 0.05, 0.1, 0.2) via the hot pressing method. It was found that the AgSbTe₂ addition can effectively increase the carrier concentration and improve the power factor to 46 μW cm^?1 K^?2 at 300 K. Due to the introduction of dislocations, stress and Te inhomogeneities, the lattice thermal conductivity of the composite was significantly reduced to 0.69 W m^?1 K^?1 at 325 K. As a result, a maximum ZT of 1.15 at 325 K is obtained for the x = 0.1 sample. Interestingly, BST-0.1 wt% AgSbTe₂ exhibits roughly isotropic thermoelectric performance perpendicular to and parallel to the pressing direction. Our study suggests that the BST-AgSbTe₂ composite is very promising for the application of thermoelectric refrigeration near room temperature.     

Functionalized few-layered graphene nanoplatelets for superior thermal management in heat transfer nanofluids

The superior thermal conductivity and lightweight of graphene flakes make them materials of choice for advanced heat transfer applications, especially for transport of electricity from sustainable power stations such as concentrating solar power plants. In view of the excellent thermal conductivity of graphene or graphene-like nanomaterials (3000–5000 W m⁻¹ K⁻¹), their dispersion into conventional host fluids such as water (0.613 W m⁻¹ K⁻¹) or ethylene glycol (0.25 W m⁻¹ K⁻¹) can significantly improve fluid heat transfer characteristics. The two-dimensional structure and high surface area as well as the cost-efficient carbon-based material make graphene nanoplatelets (GNPs) suitable for large-scale applications in colloidal thermal conductive fluids. For an efficient dispersion of GNPs in base fluids, intrinsically hydrophobic GNPs were acid treated to obtain highly concentrated (4 wt.%) graphene-based nanofluids. Investigations on various GNP sizes and reaction parameters showed significant influences on the resulting thermal conductivity values of the nanofluid. After 14 h measurements in a dormant system, the most efficient nanofluid reached a thermal conductivity of 0.586 W m⁻¹ K⁻¹ (the base fluid of 0.391 W m⁻¹ K⁻¹) and a low viscosity of 6.39 cP resulting in an overall efficiency improvement of 77%, when compared to the base fluid without particles.     

Preparation of ultralow thermal conductivity (Mg0.5Ca0.3Ba0.2) (AlSi)2O8 ceramics

A type of nonequimolar multicomponent ceramic solid solution (Mg_0.5Ca_0.3Ba_0.2) (AlSi)₂O₈ with a low thermal conductivity was prepared through solid-state synthesis. Results show that the (Mg_0.5Ca_0.3Ba_0.2) (AlSi)₂O₈ solid solution exhibits excellent high-temperature stability and an ultralow thermal conductivity (.3676 W m⁻¹ K⁻¹), far lower than widely used 3YSZ (2.9 W m⁻¹ K⁻¹), La₃NbO₇ (1.5 W m⁻¹ K⁻¹), and Gd₂Zr₂O₇ (1.28 W m⁻¹ K⁻¹). Furthermore, the Young modulus of the final product is 64.56 GPa. Therefore, the proposed ceramic solid solution provides a new research direction for ultralow thermal conductivity materials and has a practical application value for the field of wall thermal insulation.     

(001)-oriented Cu2-ySe thin films with tunable thermoelectric performances grown by pulsed laser deposition

Highly (001)-oriented Cu_2-ySe thin films with tunable thermoelectric performances have been grown by pulsed laser deposition. By using targets with different Cu/Se ratios that further determines the copper deficiency of as-grown films, the carrier concentrations of as-grown films are tuned within a broad range from 10¹⁸ to 10²¹ cm⁻³. The optimum performance is observed at carrier concentration ~1.58×10²⁰ cm⁻³. The distinct properties of Cu_2-ySe thin films with nearly ideal chemical stoichiometric ratio are observed. In addition, a weak change in the electrical transport during the second-order phase transition was observed in the thin films due to the anisotropic structure of the Cu_2-ySe.     

Enhanced thermoelectric properties and electronic structures of p-type BiCu1−xAgxSeO ceramics

The thermoelectric properties and electronic structures were investigated on p-type BiCu_1-xAg_xSeO (x=0, 0.02, 0.05, 0.08) ceramics prepared using a two-step solid state reaction followed by inductively hot pressing. All the samples consist of single BiCuSeO phase with lamella structure and no preferential orientation exists in the crystallites. Upon replacing Cu⁺ by Ag⁺, maximum values of electrical conductivity of 36.6 S cm⁻¹ and Seebeck coefficient of 350 μV K⁻¹ are obtained in BiCu_0.98Ag_0.02SeO and BiCu_0.92Ag_0.08SeO, respectively. Nevertheless, a maximum power factor of 3.67 μW cm⁻¹K⁻² is achieved for BiCu_0.95Ag_0.05SeO at 750 K owing to the moderate electrical conductivity and Seebeck coefficient. Simultaneously, this oxyselenide exhibits a thermal conductivity as low as 0.38 W m⁻¹ K⁻¹ and a high ZT value of 0.72 at 750 K, which is nearly 1.85 times as large as that of the pristine BiCuSeO. The enhancement of thermoelectric performance is mainly attributed to the increased density of states near the Fermi level as indicated by the calculated results.     

Enhanced thermoelectric performance of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) with long‐term humidity stability via sequential treatment with trifluoroacetic acid

This paper reports a range of effective sequential chemical processes to enhance the thermoelectric performance of conducting poly(3,4‐ethylenedioxythiophene) films doped with poly(styrene sulfonate) anions (PEDOT:PSS). The electrical conductivity of the PEDOT:PSS films was significantly increased from 0.33 to 3748 S cm^?1 after a series of sequential treatments with trifluoroacetic acid (TFA) while the Seebeck coefficient and thermal conductivity were slightly reduced from 17.5 ± 1.2 to 16.0 ± 1.1 μV K^?1 and 0.537 to 0.415 W m^–1 K^?1 for the pristine film and treated film, respectively, leading to a significant improvement in power factor up to 97.1 ± 5.4 μW m^–1 K^?2. More importantly, around 80% of the electrical conductivity and Seebeck coefficient was retained after 20 days for these TFA‐treated PEDOT:PSS films, revealing the potential for real thermoelectric applications. © 2019 Society of Chemical Industry     

Effect of moisture on the thermoelectric properties in expanded graphite/carbon fiber cement composites

A kind of dry mixing and pressing process was introduced to prepare expanded graphite/carbon fiber cement composites (EG-CFRC). Significant effect of moisture on the thermoelectric properties of EG-CFRC was observed. The higher the moisture content is, the greater the absolute Seebeck coefficient. The maximum of absolute Seebeck coefficient 11.59 μV/°C was obtained with moisture of 14.98% at 33 °C. Simultaneously, the maximum of electrical conductivity 0.78 S cm⁻¹ was got with moisture of 11.44%. Furthermore, the largest power factor 7.85×10⁻⁴ µW m⁻¹ K⁻² was calculated at 33 °C with moisture of 11.44%. The carrier scattering, polarization effects and high density defects interface of EG-CFRC are attributed to the enhancement of thermoelectric properties in the case of higher moisture content.     

Low thermal conductivity of SrTiO3−LaTiO3 and SrTiO3−SrNbO3 thermoelectric oxide solid solutions

Electron-doped SrTiO₃ has been attracting attention as oxide thermoelectric materials, which can convert wasted heat into electricity. The power factor of the electron-doped SrTiO₃, including SrTiO₃-LaTiO₃ and SrTiO₃-SrNbO₃ solid solutions, has been clarified. However, their thermal conductivity (κ) has not been clearly identified thus far. Only a high κ (>12 W m⁻¹ K⁻¹) has been assumed from the electron contribution based on Wiedemann–Franz law. Here, we show that the κ of the electron-doped SrTiO₃ is lower than the assumed κ, and its highest ZT exceeded 0.1 at room temperature. The κ slightly decreased with the carrier concentration (n) when n is below 4 × 10²¹ cm⁻³. In the case of SrTiO₃-SrNbO₃ solid solutions, an upturn in κ was observed when n exceeds 4 × 10²¹ cm⁻³ due to the contribution of conduction electron to the κ. On the other hand, κ decreased in the case of SrTiO₃-LaTiO₃ solid solutions probably due to the lattice distortion, which scatters both electrons and phonons. The highest ZT was 0.11 around n = 1 × 10²¹ cm⁻³. These findings would be useful for the future design of electron-doped SrTiO₃-based thermoelectric materials.     

Sol–Gel‐Derived High‐Performance Stacked Transparent Conductive Oxide Thin Films

Single‐ and multi‐layer transparent conductive oxide (TCO) thin films exhibiting high performance, good packing density and low surface/interface roughness are deposited on silica glass substrates by the sol–gel method. The crystal and microstructural properties of the TCO thin films are evaluated as an alternate to films prepared by ultra‐high vacuum deposition. Tin‐doped indium oxide (ITO) thin films produced using a two‐step drying process showed low surface roughness because of dense packing structure not only horizontal but also vertical directions. As a result, electrical conductivity, carrier concentration, carrier mobility, and optical transmittance of 2.3 × 10³ S/cm, 8 × 10²⁰ cm^?3, 18 cm²/Vs, and over 98% at 500 nm, respectively, were achieved. A multilayer ZnO/ITO stacked structure was also fabricated using the sol–gel process. Our findings suggest that solution‐based methods show promise as an alternative to existing ultra‐high vacuum methods to fabricate TCO thin films.     

Synergistically optimizing thermoelectric performance of ZnO ceramics by interfacial band alignment and self-doping defects

ZnO is a promising thermoelectric ceramic material due to non-toxicity and abundance in resources. However, its thermoelectric performance is limited by the intrinsic low carrier concentration and high thermal conductivity. In this work, we synthesized the (1 ? x)ZnO/xZnS (x = 0–0.05) powders by a two-step solution method followed by microwave sintering in an oxygen-deficient environment at 1000 ℃, and then produced the self-doped ZnO ceramics with ZnO/ZnS interfaces. The electrical and thermal properties was investigated from room temperature to 900 K. The ZnO/ZnS interface and self-doping significantly increased the electrical properties of ZnO ceramics, the electrical conductivity (σ) and Seebeck coefficient (α) increased simultaneously with temperature for (1 ? x)ZnO/xZnS (x > 0), and the highest power factor (PF, 3675 µW·m^?1·K^?2) was obtained from 0.98ZnO/0.02ZnS at 900 K. At the same time, the ZnO/ZnS interfaces and self-doped defects greatly reduced the lattice thermal conductivity. Finally, the highest ZT value of 0.94 has been reached in 0.95ZnO/0.05ZnS at 900 K.     

SPS-assisted synthesis of InGaO3(ZnO)m ceramics,and influence of m on the band gap and the thermal conductivity

In this study, we report on the use of a two-stage annealing treatment at 1100°C coupled to reactive Spark Plasma Sintering to reduce the synthesis temperature of InGaO₃(ZnO)_m (m = 1 to 9) dense polycrystalline pellets below 1200°C, in order to suppress the volatilization of ZnO and get a better control of the crystalline quality of the pellets. We show that using this treatment, dense single-phase pellets can be prepared with randomly oriented grains. Besides, we evidence a monotonic evolution of the band gap in the series from 3.27 eV in InGaO₃(ZnO) to 3.02 eV in InGaO₃(ZnO)₉, as well as a non-monotonic evolution of the lattice thermal conductivity that reaches a minimum for InGaO₃(ZnO)₃, lower than 2 W m⁻¹ K⁻¹ above 350°C. Last, we propose a procedure for the high-temperature measurement of the thermal diffusivity of oxides by the laser flash method to avoid possible reactions between the measured material and the graphite spray.