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.     

Effect of point defects on the thermal conductivity of Sc2O3-Y2O3 co-stabilized tetragonal ZrO2 ceramic materials

In this paper, the reduction mechanism in thermal conductivity of a series of Sc₂O₃-Y₂O₃ co-stabilized tetragonal ZrO₂ ceramics is systematically discussed. The thermal conductivity is approximately 20–28% lower than that of 6–8 wt.% yttria-stabilized zirconia (YSZ). A phonon scattering model, on account of the influence of oxygen vacancy variation and cation mass fluctuation, is optimized and utilized to depict the thermal conductivity of these materials. For the samples with the same amount of oxygen vacancy, Sc³⁺ is more effective in lowering thermal conductivity than Y³⁺ due to the large mass difference with Zr⁴⁺, as evidenced by the scattering model and phonon vibrational density of states. The experimental and calculation results suggest that this optimized model is proved to be more effective in predicting the thermal conductivity of binary or multiple rare earth oxides co-doped tetragonal ZrO₂ and guiding the compositional design of thermal barrier materials.     

Composition-structure-property synergistically tailoring of Zr-Y-Ta-O oxides as candidate abradable seal coatings materials

To explore novel abradable seal coatings (ASCs) materials with high working temperatures and excellent performance, we reported the multicomponent Zr-Y-Ta-O oxides with improved properties in ZrO₂-YO_1.5-TaO_2.5 ternary diagram. Six Zr-Y-Ta-O oxides with different components were successfully synthesized by a solid-state reaction, and we elucidated their composition-structure-property relationship. The origins of glass-like thermal conductivity and high toughness were discussed from structural characteristics, and wear resistance was measured by nanoindentation. The high concentration of oxygen vacancies, effective inhibitions of high-temperature thermal radiation, and mitigation of phonon speed simultaneously contributed to the low and glass-like thermal conductivity (1.04–1.43 W·m⁻¹·K⁻¹, 900 ºC). Furthermore, the high thermal expansion coefficients and excellent phase stability indicated that their working temperatures were as high as 1400 °C. The studied Zr-Y-Ta-O oxides can be applied as candidate high-temperature ASCs materials with improved performance.     

Ferroelectric-thermoelectricity and Mott transition of ferroelectric oxides with high electronic conductivity

This paper reviews ferroelectric oxides in the unusual condition where the concentration of electronic carriers is close to a metal–insulator transition; in certain structures and compositions these materials have properties of interest for oxide based thermoelectric applications. In relaxor ferroelectrics, nanopolar regions associated with intrinsic localized phonon modes provide glass-like phonon characteristics due to the large levels of phonon scattering. The (Sr_1?xBa_x)Nb₂O_6?δ relaxor ferroelectric single crystals have a high thermoelectric power factor, S²σ ～ 40 μW/cm K² at 277 °C along the c-axis, which is competitive with the best thermoelectrics.In the heavily reduced, nonstoichiometric n-type perovskite BaTiO_3?δ and tungsten bronze (Sr_1?xBa_x)Nb₂O_6?δ, it is shown that metallic-like conductivity occurs in the paraelectric phase and the onset of ferroelectricity stabilizes semiconducting character. Both the phase transition temperature dependence on the carrier concentration and evidence for polarization coupling to the conductivity mechanism will be discussed.     

Effect of high-content Yttria on the thermal expansion behaviour and ionic conductivity of a stabilised cubic Hafnia

The needs for space propulsion thruster induce the development of new designs and material compositions able to withstand 3000 K of flame combustion temperature. Cubic-stabilised hafnia appears as one of the most promising candidates to protect refractory materials in such conditions. Here, the influence of dopant content on the thermal expansion (473−1823 K) and ionic conductivity (600−1150 K) in highly doped-hafnia (12−33 mol% Y₂O₃) with disordered cubic systems is reported. The composition and the homogeneity of the samples were carefully checked using crystallographic, chemical and spectroscopy analyses. Finally, the study of thermal and oxygen conductivity properties highlighted their dependence on the amount of dopant. The average thermal expansion coefficient was lowered from 11.3 to 10.9 10⁻⁶/K and the ionic conductivity decreased by two decades with 33 mol% of Y₂O₃ by using the optimised substitution ratio. Interactions and local ordering of oxygen vacancies can explain this behaviour.     

Theoretical predictions on intrinsic lattice thermal conductivity of ZrB2

As a most important thermal management material, high thermal conductivity of ZrB₂ is expected. However, the reported values of thermal conductivity κ of ZrB₂ are quite scattering, and no consensus has been reached. The contribution from lattice separated by Wiedemann-Franz law is low and the relationship between electron and phonon contributions is still blurry. To explore the intrinsic κ of ZrB₂, in this work, two approaches, i.e. analytical Debye-Callaway model and iterative solution to the Boltzmann transport equation (BTE), are used to simulate the temperature-dependent theoretical lattice κ of ZrB₂. Our work demonstrates that the lattice thermal conductivity of ZrB₂ has been underestimated. The intrinsic lattice thermal conductivity of ZrB₂ is estimated to be 91 and 88 W m^?1 K^?1 at 300 K, by two different models, respectively. The effects of low lying optical phonon modes and grain boundary on the thermal conductivity of ZrB₂ are discussed. The thermal conductivity of ZrB₂ is controllable by designing effective grain size and microstructure. By casting light on the micro mechanism on lattice heat conduction of ZrB₂, our work will be constructive to the application of ZrB₂ as thermal management material.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Thermoelectric properties of Mo-doped bulk In2O3 and prediction of its maximum ZT

In a search for new thermoelectric materials, indium oxide (In₂O₃) was selected as a candidate for high-temperature thermoelectric oxide materials due to its intrinsically low thermal conductivity (<2 W/mK) and ZT values around 0.05. However, low electrical conductivity is a factor limiting the thermoelectric performance of this oxide, and was addressed in this study by Mo doping. It was found that Mo is soluble in In₂O₃ but forms secondary phases at a fraction near x = 0.06 and higher. Mo was found to be unsuitable for heavy n-type doping necessary to improve the thermoelectric performance of the oxide to the desired level (ZT = 1). However, the experimental data enabled us to analyze the electrical conductivity behavior and the Seebeck coefficient of doped In₂O₃ with different carrier concentrations, predicting a theoretically achievable maximum power factor value of 1.77 × 10^?3 W/mK² at an optimum carrier concentration. This estimation predicts the highest ZT value of 0.75 at 1073 K, assuming the lattice thermal conductivity value remaining at an amorphous level.     

Thermal conduction mechanism of ferroelastic Zr-Y-Yb-Ta-Nb-O high-entropy oxides with glass-like thermal conductivity

Ferroelastic high-entropy oxides (HEOs) with high fracture toughness and low thermal conductivity are promising topcoat materials for next-generation thermal barrier coatings (TBCs). However, phonon transmission characteristics in the ferroelastic HEOs are less well understood due to their complicated and highly-disordered structure. Here we prepared two types of ferroelastic Zr-Y-Yb-Ta-Nb-O HEOs and investigated their thermal conduction behaviors at 100–900°C. Hybrid Monte Carlo and molecular dynamic simulations were employed to understand the thermal conduction mechanism in this high-entropy system. Results show the HEOs present glass-like, low thermal conductivity (< 1.30 W m^–1 K^–1 at 100°C), which is attributed to the diffused lattice vibration mode with random polarization distribution. Dominant delocalized phonon modes in the HEOs are beneficial to strengthening phonon scattering and consequently decrease the thermal conductivity significantly. Lower thermal conductivity can be achieved by tailoring the composition and crystal structure of the HEOs to increase the contribution from diffused lattice vibration and “diffusion”. This work provides a fundamental insight into the thermal conduction mechanisms of HEOs, which may facilitate materials design of low thermal conductivity materials for TBCs applications.     

Structural and Thermo‐Mechanical Properties of Nd: Y2O3 Transparent Ceramics

Various content of neodymia Nd: Y₂O₃ (Nd: 0.5–5.0 at.%) transparent ceramics were fabricated by vacuum sintering. The prepared Nd: Y₂O₃ ceramics exhibit high transmittance (~80%) at the wavelength of 1100 nm. It is found that the increase in Nd concentration enhances the grain size growth, while decreases the phonon energy, which is benefit for improving both the luminescence quantum and up‐conversion efficiency. The thermal conductivity and thermal expansion coefficient of the transparent 1.0 at.% Nd: Y₂O₃ ceramic is 5.51 W·(m·K)^?1 and 8.11 × 10^?6 K^?1, respectively. The hardness and the fracture toughness of the transparent ceramic is 9.18 GPa and 1.03 Mpa·m^1/2, respectively. The results indicate that the Nd: Y₂O₃ transparent ceramic is a potential candidate material for laser.     

Synthesis and thermoelectric performance of Li-doped NiO ceramics

Ni_1?xLi_xO (x = 0, 0.03, 0.06, 0.09) powders were prepared by sol–gel method combined with sintering procedure using Ni(CH₃COO)₂·4H₂O and citric acid as the raw materials and alcohol as solvent. The crystal structures of the samples were investigated by X-ray diffraction and Raman spectroscopy. The thermoelectric properties, such as the electrical conductivity, the Seebeck coefficient and the thermal conductivity were measured. The results showed that all the samples are p-type semiconductors. The electrical conductivity increases with the increase of the temperature, which indicates that the substitution of Li⁺ for Ni²⁺ can increase the concentrations and mobility of the carriers. The thermal conductivity decreases remarkably with the increase of the Li doping content, which indicates that Li doping can enhance the scattering of phonon. However, the Seebeck coefficient will decline with the increase of the Li doping content. As results of the increase of electrical conductivity and reduction of thermal conductivity, Li doping can increase the figure of merit (ZT) of NiO, the ZT value reach 0.049 at 770 K for Ni_1?xLi_xO with x = 0.06.     

Effects of Y2O3 doping on the phase composition,chemical diffusion coefficient and electrochemical properties of CaHfO3 proton conductor

In order to further understand the effect of Y₂O₃ doping on the electrical conductivity of CaHf_1−xY_xO_3−δ, which was prepared by conventional solid-state reaction. The electrical conductivity of CaHf_1−xY_xO_3−δ was measured by two-terminal AC method in an oxygen-rich atmosphere, hydrogen-rich atmosphere and water vapor-rich atmosphere at the temperature between 973 and 1373 K. The test results show that the conductivity of CaHf_1−xY_xO_3−δ first increases and then decreases with x from 0.08 to 0.20, and its total conductivity and conductivity activation energy are 3.88 × 10⁻⁷ to 5.34 × 10⁻⁵S/m and 0.76–0.92eV respectively. Combined with the test results of the H/D isotope effect, it is found that protons are the main conductive carriers in the three different atmosphere at temperatures range of 973–1173 K. In addition, in the temperatures range of 1273–1373 K, the positive holes are the main conductive carriers in the oxygen rich atmosphere, and the vacancies participates in the conductive process as the main conductive carriers in the water vapor rich atmosphere. The chemical diffusion coefficients of CaHf_1−xY_xO_3−δ is 3.9 × 10⁻⁶ to 2.4 × 10⁻⁵ cm²/s in the temperature range of 973–1373 K. According to the test results of electromotive force, the theoretical electromotive force is consistent with the measured electromotive force. The proton transfer number of CaHf_1−xY_xO_3−δ exceeded 97% in hydrogen atmosphere at temperatures from 973 to 1173 K. In sum, these findings of CaHf_1−xY_xO_3−δ can be used as alternative materials for hydrogen sensor electrolytes.     

Phonon engineering in tuning the thermal conductivity of alkaline-earth hexaborides

Controlling the thermal conductivity of alkaline-earth hexaborides is of great importance for their applications as thermoelectric materials and ultrahigh temperature thermal insulation materials. However, no consensus has been reached on intrinsic κ, and the mechanism behind thermal reduction due to metal element incorporation is still blurry. In this work, the intrinsic thermal conductivity of three alkaline-earth hexaborides has been investigated theoretically. The simulated thermal conductivity of three borides shows good agreement with experiments. By examining mode contributions to total thermal conductivity, important role of low-lying optical modes in shaping the thermal transport property of hexaborides through controlling the interaction between acoustic and optical modes is uncovered. Our results demonstrated that tuning the thermal conductivity relies on shortening of mean free path of contributive phonon either by grain boundary scattering or by phonon-phonon interaction. The choice criterion on the doping atom to efficiently decrease thermal conductivity of alkaline-earth hexaborides is proposed.     

Low Thermal Conductivity of SnO2‐Doped Y2O3‐Stabilized ZrO2: Effect of the Lattice Tetragonal Distortion

Considering the phonon scattering effect and the stability of t′ zirconia, Sn⁴⁺ ion is recognized as an appropriate dopant to achieve the best combination of thermal insulating capability and durability of yttria‐stabilized zirconia thermal barrier coatings (TBCs). In this research, unusual lattice expansion and strong structural disordering were observed in a series of SnO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds, which are caused by the tetragonal distortion of oxygen coordination. Phonon scattering due to the structural disordering rather than point defects of Sn⁴⁺ substitutions predominates in reducing the thermal conductivity. However, deterioration of the thermal properties was observed at high doping content, which may be attributed to the t‐m phase transformation during the measurements. Considering the structure stability and thermal properties, SnO₂‐doped Y₂O₃‐stabilized ZrO₂ compounds can be promising candidates for TBCs.     

Thermoelectric response of oxygen nonstoichiometric YBaCo2O5+δ cobaltites synthesized via non-ion selective EDTA-citrate-metal complexing

The physical transport properties of layered 112-type YBaCo₂O_5+δ cobaltites, synthesized via non-ion selective EDTA-citrate-metal complexing, were systematically investigated by means of electrical and thermal conductivity as well as through thermopower measurements. The transport properties were shown to be markedly dependent on the oxygen content δ. The temperature dependence of the thermopower, S(T), exhibits p-type conductivity in the temperature range 60–320 K. In the low-temperature antiferromagnetic region, S(T) exhibits an unusual behavior (broad peak), which can be explained by the electron magnon scattering mechanism. The temperature dependence of the thermal conductivity exhibits a glass-like behavior, and no thermal conductivity peak can be seen at low temperatures for any of the samples. The absence of the phonon peak at low temperatures is associated with the oxygen nonstoichiometry, which brings about strong lattice disorder. The calculated figure of merit (ZT) shows values (∼10⁻⁵ at room temperature) too small for applications, as expected from 112-type cobaltite systems.     

Effect of CaF2 on sintering behavior and thermal shock resistance of Y2O3 materials

Y₂O₃ materials have become a popular candidate for preparing refractory crucibles for ultra-pure high-temperature alloy melting in recent years. However, its difficulty in sintering and poor thermal shock resistance limited its industrial application. The effect of CaF₂ on the densification microstructure, mechanical properties, and thermal shock resistance of Y₂O₃ materials was investigated in this paper. The main purpose of this study was to optimize the amount of CaF₂ added in the preparation of Y₂O₃ materials to improve its thermal shock resistance and get better mechanical properties. The mechanism of the densification process of CaF₂-doped Y₂O₃ materials was analyzed by phase analysis and microstructure. The results showed that successive doping of large Ca²⁺ ions caused more lattice distortion in the Y₂O₃ materials, and the diffusion rate of Y³⁺ was increased, thus enhanced grain boundary diffusion and promoted sintering densification in the Y₂O₃ materials. Meanwhile, the addition of CaF₂ also significantly reduced the apparent porosity and enhanced the mechanical properties of the materials. The improvement of these properties was attributed to the increased relative density of CaF₂-doped Y₂O₃ materials and the high sintering activity of CaF₂. In addition, crack deflections effectively improved the thermal shock resistance of the materials. The residual flexural strength ratio of Y₂O₃ materials doped with 1 wt % CaF₂ was increased by 21.2% after thermal shock test compared with undoped specimens.     

Tuning stoichiometry of high-entropy oxides for tailorable thermal expansion coefficients and low thermal conductivity

Thermal barrier coating materials with proper thermal expansion coefficient (TEC), low thermal conductivity, and good high-temperature stability are of great significance for their applications in next-generation turbine engines. Herein, we report a new class of high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x with different Ce⁴⁺ contents synthesized by a solid-state reaction method. They exhibit different crystal structures at different Ce⁴⁺ content, including a bixbyite single phase without Ce⁴⁺ doping (x = 0), bixbyite-fluorite dual-phase in the RE₂O₃-rich region (0 < x < 2), and fluorite single phase in the stoichiometric (x = 2) and CeO₂-rich region (x > 2). The high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x exhibit tailorable TECs at a large range of 9.04 × 10^–6–13.12 × 10^–6 °C^–1 and engineered low thermal conductivity of 1.79–2.63 W·m^–1·K^–1. They also possess good sintering resistance and high-temperature phase stability. These results reveal that the high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x are promising candidates for thermal barrier coating materials as well as thermally insulating materials and refractories.     

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Effects of Ho3+ concentration on the fabrication and properties of Ho: Y2O3-MgO nanocomposite for Mid-infrared laser applications

Infrared transparent Ho: Y₂O₃-MgO nanocomposite ceramics with a volume ratio of 50:50 (RE₂O₃: MgO) were prepared by combining sol-gel powder synthesis and hot-pressing sintering techniques. In order to obtain Ho: Y₂O₃-MgO nanocomposite ceramics with fine grain size, dense microstructure and homogeneous phase domains, the effect of sintering temperature and Ho³⁺ doping concentration were studied. Transmittance and SEM measurement revealed that the grain size of 3 at.% Ho: Y₂O₃-MgO ceramic sintered at 1250 °C is 141 nm, and the transmission is up to 85.2% at 5 μm. The detailed spectroscopic investigation of x at.% Ho: Y₂O₃-MgO (x = 1, 3, 5, 7, 9, 15) ceramics was performed. The nanocomposites exhibited photoluminescence properties similar to that of Ho: Y₂O₃ crystals and ceramics. In addition, the thermal conductivity of 3 at.% Ho: Y₂O₃-MgO ceramic is 13.04 W/m·K, which is superior to that of Ho:Y₂O₃ ceramics. The high transmission, excellent thermal conductivity, and outstanding optical characteristics indicated that Ho: Y₂O₃-MgO ceramics is a promising material for efficient infrared solid-state laser.