Low Thermal Conductivity of Rare‐Earth Zirconate‐Stannate Solid Solutions (Yb2Zr2O7)1−x(Ln2Sn2O7)x (Ln = Nd,Sm)

Dense monoliths of rare‐earth zirconate‐stannate solid solutions (Yb₂Zr₂O₇)_1?x(Ln₂Sn₂O₇)_x (Ln = Nd, Sm) were prepared by solid‐state reaction. Characterized by XRD, Raman, SEM, and TEM, a double‐phase structure of Yb₂Zr₂O₇‐rich fluorite and Nd₂Sn₂O₇‐rich pyrochlore was observed in the specimens of x = 0.4 and 0.5 of (Yb₂Zr₂O₇)_1?x(Nd₂Sn₂O₇)_x series while complete solid solutions were formed within the whole composition range of (Yb₂Zr₂O₇)_1?x(Sm₂Sn₂O₇)_x series. Except for the defect phonon scattering, lattice softening caused by order–disorder phase transformation between pyrochlore and fluorite structures also plays an important role in minimizing the thermal conductivity. Low thermal conductivity with positive temperature dependence is achieved in both the series. Considering the structure stability and low thermal conductivity, rare‐earth zirconate‐stannate solid solutions may be promising materials for thermal insulating applications, such as thermal barrier coatings.     

Annealing temperature-dependent structural and electrical properties of (Ta2O5)1-x - (TiO2)x thin films,x ≤ 0.11

(Ta₂O₅)_1-x- (TiO₂)_x (TTO_x) thin films, with x = 0, 0.03, 0.06, 0.08, and 0.11, were deposited using magnetron direct current (DC) sputtering method onto the P/boron-silicon (1 0 0) substrates by varying areas of Tantalum and Titanium metallic targets, in oxygen environment at ambient temperature. The as-deposited thin films were annealed at temperatures ranging from 500 to 800 °C. Generally, the formation of the Ta₂O₅ structure was observed from the X-ray diffraction measurements of the annealed films. The capacitance of prepared metal– oxide– semiconductor (MOS) structures of Ag/TTO_x/p-Si was measured at 1 MHz. The dielectric constant of the deposited films was observed altering with varying composition and annealing temperature, showing the highest value 71, at 1 MHz, for the TTO_x films, x = 0.06, annealed at 700 °C. With increasing annealing temperature, from 700 to 800 °C, the leakage current density was observed, generally decreasing, from 10^?5 to 10^?8 A cm^?2, for the prepared compositions. Among the prepared compositions, films with x = 0.06, annealed at 800 °C, having the observed value of dielectric constant 48, at 1 MHz; and the leakage current density 2.7 × 10^?8 A cm^?2, at the electric field of 3.5 × 10⁵ V cm^?1, show preferred potential as a dielectric for high-density silicon memory devices.     

Fabrication of tunable bandgap epitaxial β-(AlxGa1−x)2O3 films using a spin-coating method

β-(Al_xGa_1−x)₂O₃ films have several critical properties of interest to the research community, including a wide bandgap that may be used in the development of new electronic, optoelectronic, and photonic devices. Here we demonstrate the first time fabricated metal-alkoxide-based spin-coated single-phase epitaxial β-(Al_xGa_1−x)₂O₃ films on c-sapphire substrates with $(\overline{2}01)$ orientation and good crystallinity that is comparable to the films fabricated using other film deposition techniques, such as molecular beam epitaxy and chemical vapor deposition. Using this technique, we generated films with broad Al compositions (x) of 0.3, 0.5, and 0.7 with bandgap energies of 5.15, 5.56, and 6.16 eV, respectively, estimated from the X-ray photoelectron spectroscopy inelastic energy-loss spectra. Photoluminescence emission spectra in the ultraviolet and visible (blue) wavelength range highlighted several intrinsic defects in the film structure that functioned as luminescence centers, including self-trapped exciton and recombining donor-to-acceptor band. Detailed analysis of the structural and optical properties of β-(Al_xGa_1−x)₂O₃ epitaxial films revealed that this low-cost and scalable solution-deposition approach coupled with a spin-coating technique could be used to fabricate β-(Al_xGa_1−x)₂O₃ films with tunable properties.     

CaO-MgO-Al2O3-SiO2 corrosion behavior of air-plasma-sprayed (LaxYb1−x)2Zr2O7

The CaO-MgO-Al₂O₃-SiO₂ (CMAS) corrosion of thermal barrier coatings (TBCs) is a crucial problem for the lifetime of blades and vanes of jet engine and gas turbine at high operating temperature. Although many new alternative materials for TBCs have been developed in recent years, their application is limited by the CMAS corrosion. On the other hand, the composition difference of CMAS between regions makes solving this problem very difficult. Therefore, in this study, the yearly composition changes of sand-dust around Beijing area were investigated. The high-temperature corrosion behavior of air-plasma-sprayed 8YSZ and newly developed (La_xYb_1−x)₂Zr₂O₇ TBCs by the representative sand-dust of Beijing was investigated. In comparison, a universally used CaO-riched composition of simulated silicate deposit was also adopted for the TBCs corrosion test. It is found that the (La_xYb_1−x)₂Zr₂O₇ TBCs performs much better anti-corrosion behavior than that of 8YSZ, both by Beijing sand-dust and simulated one. Particularly, Yb₂Zr₂O₇ TBCs appear to be the optimum material against silicate deposits attack. The mechanism of silicate deposits corrosion has also been discussed.     

Study on La2(Hf1-xTix)2O7 (x ≤ 0.20) pyrochlores for potential thermal/environmental barrier coating applications

Lanthanum hafnate (La₂Hf₂O₇) with a pyrochlore structure has excellent high temperature stability and low thermal conductivity, which is promising for thermal/environmental barrier coatings (T/EBCs) applications. To reduce its thermal expansion coefficient (TEC) so as to better match SiC_f/SiC composites, a smaller tetravalent dopant Ti⁴⁺ has been introduced in the Hf-sites to form La₂(Hf_1-xTi_x)₂O₇ (x ≤ 0.20). The phase composition and microstructure confirms that La₂(Hf_1-xTi_x)₂O₇ solid solutions possess a pure pyrochlore structure. With an increase of x, their TECs are decreasing consistently, whilst their thermal conductivities of La₂(Hf_1-xTi_x)₂O₇ are slightly increasing at high temperature but still much lower than those of meta-stable yttria partially stabilized zirconia, both of which are attributing to an increase of elastic modulus after Ti⁴⁺ doping on Hf-sites. The extremely excellent high temperature stability, relatively low thermal conductivities and low TECs suggest that La₂(Hf_1-xTi_x)₂O₇ is a prospective candidate material for T/EBC applications.     

Growth and electrical properties of high-Curie point rhombohedral Mn-Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 thin films

High-quality ternary relaxor ferroelectric (100)-oriented Mn-doped 0.36Pb(In_1/2Nb_1/2)O₃-0.36Pb(Mg_1/3Nb_2/3)O₃-0.28PbTiO₃ (Mn-PIMNT) thin films were grown on SrRuO₃-buffered SrTiO₃ single-crystal substrate in a wide deposition temperature range of 550-620°C using the pulsed laser deposition method. The phase structure, ferroelectric, dielectric, piezoelectric properties, and nanoscale domain evolution were studied. Under the deposition temperature of 620°C, the ferroelectric hysteresis loops and current-voltage curves showed that the film owned significantly enhanced remnant ferroelectric polarization of 34.5 μC/cm² and low leakage current density of 2.7 × 10⁻¹⁰ A/cm². Moreover fingerprint-type nanosized domain patterns with polydomain structures and well-defined macroscopic piezoelectric properties with a high normalized strain constant of 40 pm/V was obtained. Under in situ DC electric field, the domain evolution was investigated and 180° domain reversal was observed through piezoelectric force microscope. These global electrical properties make the current Mn-PIMNT thin films very promising in piezoelectric MEMS applications.     

Phase evolution and thermophysical properties of high-entropy RE2(Y0.2Yb0.2Nb0.2Ta0.2Ce0.2)2O7 oxides

Pursuing novel thermal barrier–coating materials with lower thermal conductivity and high-temperature stability can simultaneously improve the working efficiency and service temperature of a gas turbine. In this study, a series of high-entropy RE₂(Y_0.2Yb_0.2Nb_0.2Ta_0.2Ce_0.2)₂O₇ (RE = La, Nd, Sm, Gd, Dy, and Er) oxides were prepared though solid-state reaction. Through tuning the rare-earth cations, an order–disorder transition occurs from certain partially ordered weberite structure (C222₁) to disordered defective fluorite structure (Fm$\overline{3}$m). All the high-entropy RE₂(Y_0.2Yb_0.2Nb_0.2Ta_0.2Ce_0.2)₂O₇ oxides possess low thermal conductivity in the range of 0.91–1.34 W m⁻¹ K⁻¹ at room temperature, which can be attributed to increased lattice anharmonicity and disorder, resulting in additional phonon scattering. Herein, we proved that the incorporation of heterovalent cations at B-sites in high-entropy A₂B₂O₇ crystals is an effective strategy to reduce the thermal conductivity without compromising the decrease of oxygen vacancy. Moreover, the high-entropy RE₂(Y_0.2Yb_0.2Nb_0.2Ta_0.2Ce_0.2)₂O₇ oxides show the relatively higher thermal expansion coefficients of 10.3–10.7 × 10⁻⁶°C⁻¹ and excellent phase stability at elevated temperatures.     

Preparation and electrical properties of Sm-doped Bi2Ti2O7 thin films prepared on Pt (111) substrates

Crack-free Sm-doped Bi₂Ti₂O₇ (Sm:BTO) thin films with strong (111) orientation have been prepared on Pt (111) substrates using a chemical solution deposition (CSD) method. The structural properties and crystallizations were studied by X-ray diffraction. The surface morphology and quality were examined using atomic force microscopy (AFM). The insulating and dielectric properties were also evaluated at room temperature. The results demonstrate that the Sm:BTO films exhibit improved electrical performances as compared to the pure Bi₂Ti₂O₇ thin films and suggest a strong potential for utilization in microelectronics devices.     

The dependence of lattice thermal conductivity on phonon modes in pyrochlore-related Ln2Sn2O7 (Ln = La,Gd)

Rare-earth pyrochlore materials are promising thermal barrier coatings materials and fundamental understanding of their thermal transport is crucial for further improving its performance. In this work, using density functional theory (DFT) method, we calculated the intrinsic lattice thermal conductivities of Ln₂Sn₂O₇ (Ln = La, Gd) and conducted a comprehensive analysis on the mode thermal conductivity, relaxation time, Grüneisen parameters, group velocity, and specific heat, respectively. It is shown that in pyrochlore-type materials the number of the optical phonons is much larger than that of the acoustic phonon, and the thermal conductivity of acoustic phonons are suppressed, both of which increase the contribution ratio of optical phonons. Especially, through cumulative analysis, we found that the contribution of optical phonons is significant: the ratio of optical contribution is more than 50% and 64% in La₂Sn₂O₇ and Gd₂Sn₂O₇. This work provides a comprehensive picture illustrating the significant role of the optical phonons in the lattice thermal conduction in rare-earth pyrochlore materials, and points out an avenue to obtain low thermal conductivity in complex structural thermal insulation materials.     

ZnO:Al thin films from (Al2O3)x(ZnO)(1-x) powder targets by magnetron sputtering

In the present work we prepared Aluminum doped Zinc Oxide (AZO) thin films from powder targets. Various concentrations (W/W percentages) of Al₂O₃ such as1%, 2%, 3%, 4%, 5%, 6%, 7% and 8% were mixed in ZnO powder and made in the form of a 3 inch disc target. These ceramic targets are sputtered in RF magnetron sputtering unit for the deposition of AZO thin films. Optical and electrical properties are analyzed to get an optimized percentage of mixing for achieving high transparency and low resistivity. At Al₂O₃ percentage of 3% there is a considerable decrement in the resistivity, and at 7% there is a considerable decrease in the optical transmittance. Mobility and carrier concentration are increasing with Al₂O₃ percentage. Bandgap of the films is observed to be decreasing with increasing the Al₂O₃ percentage.     

Structure and properties of Hf-O-N films prepared by high-rate reactive HiPIMS with smoothly controlled composition

Hafnium oxynitride ceramics were prepared in the form of thin films by high-power impulse magnetron sputtering of Hf in various Ar+O₂+N₂ gas mixtures. Smooth composition control was achieved by maximizing the degree of dissociation in plasma, suppressing the importance of the difference between reactivities of undissociated O₂ and N₂. The application potential of the films was further enhanced by extremely high deposition rates (e.g. 230 nm/min for stoichiometric HfO₂; achieved by feedback pulsed reactive gas flow control), low deposition temperatures (<140 °C) and not using any substrate bias. We focus on the relationships between elemental composition, phase structure, and optical, electrical, mechanical and hydrophobic properties of the materials. We quantify the evolution of smoothly controlled film properties along the transition from an oxide to a nitride, such as increasing extinction coefficient, decreasing electrical resistivity, increasing hardness or increasing water droplet contact angle.     

Controlled synthesis of Mg(OH)2 thin films by chemical solution deposition and their thermal transformation to MgO thin films

The chemical solution deposition of Mg(OH)₂ thin films on glass substrates and their transformation to MgO by annealing in air is presented. The chemical solution deposition consists of a chemical reaction employing an aqueous solution composed of magnesium sulfate, triethanolamine, ammonium hydroxide, and ammonium chloride. The as-deposited films were annealed at different temperatures ranging from 325 to 500?°C to identify the Mg(OH)₂-to-MgO transition temperature, which resulted to be around 375?°C. Annealing the as-deposited Mg(OH)₂ films at 500?°C results in homogeneous MgO thin films. The properties of the Mg(OH)₂ and MgO thin films were analyzed by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, UV–Vis spectroscopy, and by circular transmission line model. Results by X-ray diffraction show that the as-deposited thin films have a brucite structure (Mg(OH)₂), that transforms into the periclase phase (MgO) after annealing at 500?°C. For the as-deposited Mg(OH)₂ thin film, a nanowall surface morphology is found; this morphology is maintained after the annealing to obtain MgO, which occurred with the evident formation of pores on the nanowall surface. The assessed chemical composition from X-ray photoelectron spectroscopy yields Mg_0.36O_0.64 (O/Mg ratio of 1.8) for the as-deposited Mg(OH)₂ film, where the expected stoichiometric composition is Mg_0.33O_0.67 (O/Mg ratio of 2.0); the same assessment yields Mg_0.60O_0.40 (O/Mg ratio of 0.7) for the annealed thin film, which indicates the obtainment of a MgO material with oxygen vacancies, given the deviation from the stoichiometric composition of Mg_0.50O_0.50 (O/Mg ratio of 1.0). These results confirm the deposition of Mg(OH)₂ films and the obtainment of MgO after the heat-treatment. The energy band gap of the films is found to be 4.64 and 5.10?eV for the as-deposited and the film annealed at 500?°C, respectively. The resistivity of both Mg(OH)₂ and MgO thin films lies around 10⁸?Ω·cm.     

Thermal treatment effect on the magnetoelectric properties of (Ni0.5Zn0.5)Fe2O4/Pt/Pb(Zr0.3Ti0.7)O3 heterostructured thin films

Magnetoelectric (ME) property modulation in heterostructured (Ni_0.5Zn_0.5)Fe₂O₄/Pt/Pb(Zr_0.3Ti_0.7)O₃ (NZFO/Pt/PZT) thin films on platinized Si substrate by thermal annealing condition variation was studied. In an attempt to prevent interfacial reaction between NZFO and PZT layers during high temperature annealing, thin Pt layer was deposited which can serve as inter-diffusion barrier as well as electrode. The ferroelectric, magnetic, and ME properties of the heterostructured film were noticeably modulated due to microstructural evolution and clamping relaxation developed during thermal annealing process. Room temperature ME voltage coefficient of the heterostructured thin films was enhanced with increasing annealing temperature and reached to 29 mV/cm·Oe when annealed at 650 °C.     

Structural,optical and morphological evolution of Ga2O3/Al2O3 (0001) films grown at various temperatures by pulsed laser deposition

This study systematically investigated the structural, optical, and morphological evolution of Gallium oxide (Ga₂O₃) films deposited at different substrate temperatures on Al₂O₃(0001) using pulsed laser deposition (PLD). The thickness of the Ga₂O₃ films was standardized in order to eliminate its effect on the film properties. The effect of substrate temperature from room temperature to 600 °C on the film's transmittance, crystalline structure, chemical composition and surface morphology, was explored. The plasma species generated during the deposition of the PLD process were monitored and analyzed employing in situ optical emission spectroscopy. The deposition rate of the films decreased with increasing substrate temperature. X-ray photoelectron spectroscopy was used to detect both Ga³⁺ and Ga ⁺ oxidation states in all prepared films, which indicated substoichiometric Ga₂O₃ films deficient in oxygen. The percentage of non-lattice oxygen decreased with increasing substrate temperature. At optimal condition, mono-crystaline β-Ga₂O₃ was produced with a high visible and near-infrared transmittance, large grain size and smooth surface, which is suitable for the application in high-performance power electric devices and photoelectronic devices.     

Microstructure of mayenite 12CaO·7Al2O3 and electron emission characteristics

Electron emission characteristic, electrical conductivity of polycrystalline mayenite (12CaO·7Al₂O₃) electride, formation of [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ framework as a function of phase content, and microstructure have been investigated. The mayenite microstructure was investigated using high-resolution transmission microscopy which revealed the type cage structure of 12CaO·7Al₂O₃ partially filled by extra-framework oxygen ions. Incorporation of electrons by means of carbon ion template 12CaO·7Al₂O₃ produces complex structure, and an incomplete ion template 12CaO·7Al₂O₃ structure consisting of mixture of a [Ca₂₄Al₂₈O₆₄]⁴⁺(e⁻)₄ and [Ca₂₄Al₂₈O₆₄]⁴⁺(O²⁻)₂ framework had a direct effect on the electron emission. Surface chemistry and stability of the 12CaO·7Al₂O₃ electride have been studied using x-ray photoelectron spectroscopy. The work function of phase pure 12CaO·7Al₂O₃ electride was determined from direct thermionic emission data and compared to the measurement from ultraviolet photoelectron spectroscopy (UPS). Depending on the extent of ion template of 12CaO·7Al₂O₃ structure, a work function of 0.9–1.2 eV and 2.1–2.4 eV has been measured and thermionic emission initiating at 600°C.     

Characterization of single crystal β-Ga2O3 films grown on SrTiO3 (100) substrates by MOCVD

Gallium oxide (Ga₂O₃) films have been deposited on SrTiO₃ (100) substrates by using the metal-organic chemical vapor deposition (MOCVD) method. Post-deposition annealing was performed at different temperatures. XRD θ-2θ scans displayed that the annealing at 1000 °C leaded to β phase Ga₂O₃ film with the best crystalline quality. Microstructural and chemical composition analyses revealed that this film was single crystal β-Ga₂O₃ with stoichiometric ratio. The epitaxial relationships were clearly determined as β-Ga₂O₃ (100) || SrTiO₃ (100) with β-Ga₂O₃ [001] || SrTiO₃ <011>. All of the prepared Ga₂O₃ samples have average transmittances in the visible range of more than 70%.     

Preparation of Y1−x Yb x Ba2Cu3O7−y superconducting films by chemical vapor deposition

High Tc Y_1−x Yb _x Ba₂Cu₃O_7−y films were prepared on SrTiO₃(100) substrates by chemical vapor deposition method. Yb₁Ba₂Cu₃O_7−y films were obtained at higher oxygen partial pressure compared with Y₁Ba₂Cu₃O_7−y films at the same deposition temperature. Tc,o (R=0) decreased about 1.5 K when Y was fully substituted with Yb. The caxis lattice parameter of Y_1−x Yb _x Ba₂Cu₃O_7−y films also decreased as the amount of Yb(x) increased.     

Influence of post-deposition annealing on electrical and optical properties of (Ta2O5)1-x - (TiO2)x thin films,x ≤ 0.08

Tantalum (Ta) and titanium (Ti) metal targets were direct current (DC) magnetron sputtered in the oxygen environment by varying its relative areas to deposit (Ta₂O₅)_1-x- (TiO₂)_x (TTO_x) thin films, with x = 0, 0.03, 0.06, and 0.08, onto the boron-doped p-silicon (1 0 0) and optically polished quartz substrates, at room temperature; and were annealed at 500, 600, 700, and 800 °C, for 1.5 h. The thin films annealed at and above 600 °C show the Ta₂O₅ structure. The leakage current density and capacitance-voltage (C–V) characteristics were measured for TTO_x, x ≤ 0.08, assisted Ag/TTO_x/p-Si metal– oxide– semiconductor (MOS) structures. The leakage current density was found minimum, for the films annealed at 800 °C, for all the prepared TTO_x films, x ≤ 0.08. The minimum leakage current density 1.6 × 10^?8 A/cm², at 3.5 × 10⁵ V/cm electric field, was observed for x = 0.03, annealed at 800 °C, among the prepared compositions. The prepared TTO_0.03 films, annealed at 700 °C show maximum dielectric constant 39, at 1 MHz. The optical parameters, viz., refractive index (n), extinction coefficient (k), and optical band gap (E_g) of the films, with x = 0.03, prepared on quartz substrates, were determined from their optical transmittance plots. The values of n and k of the crystalline films were observed increasing from 2.123 to 2.143, and 0.099 to 0.130, respectively, at 550 nm wavelength; and E_g decreasing from 3.95 to 3.89 eV with the increasing annealing temperature, from 600 to 800 °C. Ohmic emission, in the lower electric field; Schottky and space-charge- limited current conduction mechanisms, in the intermediate to higher electric fields, were generally envisaged from the current-voltage characteristics in the prepared Ag/TTO_0.03/p-Si structures.     

High temperature failure modes of In2O3 thin films and improved thermal stability using Al2O3/ZrO2 protective layers

The limiting temperature of an In₂O₃ thin film sensor is much lower than its melting point. Herein, the failure modes of In₂O₃ thin films at high temperatures, including sublimation and changes in composition, have been studied. The edge and surface layer sublimation rates increased dramatically at 1350 °C, indicating that it is the limiting temperature of no-protection In₂O₃ films. In addition, oxygen atoms will escape from In₂O₃ thin films at high temperatures, forming oxygen vacancies. As the main current carrier type in In₂O₃, the increasing number of oxygen vacancies affects the resistance of In₂O₃ thin film sensors. To solve these problems and promote the high temperature performance of In₂O₃ thin films, protection methods based on Al₂O₃ and ZrO₂ layers have been investigated. The ZrO₂ protective layer alleviated the serious considerable sublimation of In₂O₃ thin films at high temperatures, and the Al₂O₃ protective layer was beneficial for reduction the escape of oxygen atoms. Finally, different protection layers were evaluated by in-situ resistivity measurements of In₂O₃ thin films at high temperatures. The resistance of the In₂O₃ thin film resistor with a protective multilayer consisting of Al₂O₃ and ZrO₂ remained stable at 1360 °C, verifying the protection method effectively increased the thermal stability of In₂O₃ thin films.     

Characterization of single crystalline a-TiO2 films on MgAl2O4(100) substrates by MOCVD

Anatase phase TiO₂ (a-TiO₂) films have been deposited on MgAl₂O₄(100) substrates at the substrate temperatures of 500–650 °C by the metal organic chemical vapor deposition (MOCVD) method using tetrakis-dimethylamino titanium (TDMAT) as the organometallic (OM) source. The structural analyses indicated that the TiO₂ film prepared at 600 °C had the best single crystalline quality with no twins. The out-of-plane and in-plane epitaxial relationships of the film were a-TiO₂(001)||MgAl₂O₄(100) and TiO₂[100]||MgAl₂O₄[100], respectively. A uniform and compact surface with stoichiometric composition was also obtained for the 600 °C-deposited sample. The average transmittance of all the TiO₂ films in the visible range exceeded 91% and the optical band gap of the films varied from 3.31 to 3.41 eV.