Abstract

In the pursuit of higher operating temperatures and better durability of thermal barrier coating (TBC) materials, the currently used air-plasma-sprayed (APS) 8 wt.% yttria-stabilized zirconia (8YSZ) ceramic is faced with a phase degradation problem regarding its initial metastable T prime (T′) phase, which results in residual stress and cracks during long-term operations. In this study, we focus on the relationship between the long-term phase transformation and mechanical properties, which facilitates the simulation and prediction of properties under operating conditions. Field-assisted sintering specimens of the pure T′ phase were annealed at 1300 °C for different periods of time to obtain various phase compositions. A nano/micro hybrid structure was observed as the phase transformation developed. The Vickers hardness of the materials increased by 8% due to the precipitation of fine T phase grains. On the other hand, the bending strength decreased by more than 60% when the T′ phase decomposed from 70 to 20 vol.%. A noteworthy reduction in the fracture toughness, along with a decrease in the volume percentage of the T′ phase, was also observed during long-term degradation. Effects of the T′ phase decomposition on mechanical properties are discussed in this paper. This systematic research on the mechanical properties and phase degradation of YSZ is of significant importance in predicting and evaluating the performance evolvement of TBCs.

Abstract

Abstract

The evolution of lattice structure and thermal conductivity has been studied systematically for a range of Ti-doped, Y₂O₃-stabilized ZrO₂ (YSZ) solid solutions. The mechanism of reducing the thermal conductivity by Ti doping has been determined. Ti⁴⁺ mainly substitutes for Zr⁴⁺ below a critical composition factor (x ⩽ 0.08), above which the interstitial Ti⁴⁺ need to be considered separately. The effect of lattice defects caused by mass and radius differences between Ti⁴⁺ and Zr⁴⁺ ions on the phonon scattering coefficient was discussed quantitatively. And the reduction of oxygen vacancy by interstitial Ti⁴⁺ ions which increases the thermal conductivity at high Ti doping content was also determined. Concerning the integrated phase stability and thermo-mechanical properties, Ti-doped YSZ is believed to be a promising candidate for thermal barrier coatings at higher temperature.

Abstract

Starting from theoretical calculations based on LSDA, the authors compute the lattice parameters, cohesive energies and formation enthalpies of monazite-type REPO₄ compounds. The calculated values are satisfactory compared with the experimental results from the elastic constants obtained, the mechanical moduli are evaluated using the strain–stress method. The predicted bulk, Young’s and shear moduli are in good agreement with the experiments. It is shown that the mechanical moduli are low (<200 GPa) and also increase from LaPO₄ to GdPO₄. The three-dimensional contours and their planar projections of Young’s modulus are plotted to illustrate the anisotropy in elasticity. It is found that Young’s moduli of all monazite-type REPO₄ show strong dependence on direction. The linear thermal expansion coefficients are calculated using the empirical method, and the values are in the range 9 × 10⁻⁶–12 × 10⁻⁶ K⁻¹. Using Clarke’s and Slack’s models, the thermal conductivities of REPO₄ compounds obtained are close to the experimental profiles. The observed anomalies of experimental thermal properties of monazite-type GdPO₄ are also explained based on the observed monazite to zircon-type transformation in experiment. Solving the Christoffel equation for monoclinic symmetry, the anisotropy in thermal conductivity is investigated. The results indicate that the total lattice thermal conductivities of monazite-type REPO₄ show weak dependence on direction. Meanwhile, their sound velocities exhibit strong anisotropic properties.

Abstract

Complex rare-earth silicate oxyapatite RE_9.33(SiO₄)₆O₂ (RE = La, Nd, Sm, Gd, Dy) ceramics have been synthesized and their thermal conduction characteristics investigated. When evaluated using a steady-state laser heat-flux technique under conditions ranging from room temperature to 1000 °C the materials demonstrated very low thermal conductivities (0.96–1.49 W m^–1 K^–1), especially Gd_9.33(SiO₄)₆O₂, which shows a value of 1.10–1.14 W m^–1 K^–1 in the measured temperature range. Phonon mean free path and Raman spectra were used to investigate the thermal transfer mechanism. The source of low thermal conductivity was determined to be the strong intrinsic scattering in the crystal cell, which is due to the phonon mean free path being on the inter-atomic level. Furthermore, a connection between the full width at half maximum Raman spectra and the thermal conductivity of RE_9.33(SiO₄)₆O₂ ceramics at room temperature was established. The insensitivity of the thermal conduction properties to temperature for RE_9.33(SiO₄)₆O₂ ceramics have allowed it to show great potential in high temperature thermal insulation applications.

Abstract

Compounds with an A₂B₂O₇-type pyrochlore structure have been identified as a class of materials with low thermal conductivity. To ascertain the effect of the pyrochlore structure, the thermophysical properties of rare-earth stannates which constitute the longest isostructural series with this unique crystal structure were measured and the thermal conduction behavior was investigated by analyzing the variation in the inverse phonon mean free path with temperature in terms of the phonon-scattering theory. The results show that the contraction of rare-earth ions and the accompanying deviation from the ideal pyrochlore structure result in a simultaneous increase in the Young’s modulus and the thermal expansion coefficient. Scattering due to the strain field fluctuation caused by the systemic displacement of 48f oxygen ions rather than scattering from the so-called 8a site “oxygen vacancies” and thermal defects predominates in reducing the phonon mean free path and consequently the thermal conductivity at relative low temperature. At a certain critical temperature, the thermal conduction behavior of the stannates undergoes a transition to a minimum phonon mean free path mechanism, which is responsible for the remarkable variation in temperature dependence of thermal conductivity across the compositions.

Abstract

The anisotropic elastic and thermal properties of layered compounds in the series Ln₂SrAl₂O₇ (Ln = La, Nd, Sm, Eu, Gd or Dy) are calculated from first principles using density functional theory combined with the Debye quasi-harmonic approximation. The polycrystalline values of the elastic constants and bulk, shear and Young’s moduli are consistent with those determined experimentally. All compounds in the compositional series have weakly anisotropic elastic and thermal properties. For instance, thermal expansion in the [0 0 1] direction of the tetragonal unit cell is slightly larger than along the [1 0 0] or [0 1 0] directions for most Ln₂SrAl₂O₇ compounds and the calculated in-plane thermal conductivity is always larger than that along the c-axis, parallel to the layer stacking direction.

Abstract

A superhydrophobic TiO₂ film with water contact angle greater than 170° on Hastelloy substrate was fabricated through simply dip-coating method from TiO₂ precursor solution containing TiO₂ nanoparticles with the average diameter 25 nm, followed by heat-treatment and modification with fluoroalkylsilane (FAS) molecules. The as-obtained sample was characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), and water contact angle measurement respectively. Moreover, the dynamic light scattering (DLS) size distribution of TiO₂ aggregated particles in the TiO₂ precursor solution containing P25 particles was evaluated by Laser Particle Sizer. It is found that the TiO₂ nanoparticles in TiO₂ precursor solution play a crucial role to form high superhydrophobicity. Simultaneously, the superhydrophobic TiO₂ film still showed great superhydrophobicity after corroded with strong acid or alkali solutions and protected the substrate from corrosion which should be critical to the potential application in industry.

Abstract

The electronic structure, chemical bonding and mechanical properties of Ln₂Zr₂O₇ (Ln = La, Pr, Nd, Sm, Eu and Gd) pyrochlore are investigated by local-density approximation of spin polarized scheme + U calculations (U is the Hubbard energy) and further verified by the experimental results. Ln₂Zr₂O₇ compounds are wide band gap insulators, and this is consistent with the experiment results. The calculated spin polarized density of states of them indicates that they are in a ferromagnetic state and the magnetic moment is mainly attributed to the 4f shell of the Ln atoms. For the chemical bonds in the Ln₂Zr₂O₇ crystals, the O–Zr bond is stronger than the O–Ln bond. The hardness, elastic constants, bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio of Ln₂Zr₂O₇ compounds are investigated and the theoretical values are in good agreement with the experiments. The mechanical anisotropic properties are discussed using Zener’s indexes and universal elastic anisotropic index (A^U). The sound velocities along [1 0 0], [1 1 0] and [1 1 1] directions are calculated for each Ln₂Zr₂O₇ crystal. The thermal conductivities of Ln₂Zr₂O₇ compounds are evaluated and the obtained thermal conductivity of Ln₂Zr₂O₇ is lower than that of yttria-stabilized zirconia, indicating that they could be good low thermally conductive materials at high temperature.

Abstract

Glass-like thermal conductivity is observed in (La_1−xYb_x)₂Zr₂O₇ (1/6 ⩽ x ⩽ 1/3), which exhibits great potential as a high-temperature thermal barrier coating material. In the pyrochlore-type La₂Zr₂O₇, the large 16c-site La³⁺ is weakly bonded by its surrounding 48f-site oxygen ions, and substitution of La³⁺ with smaller and heavier Yb³⁺ gives rise to a large atomic displacement parameter (ADP) of Yb³⁺ which behaves as a “rattler”, as evidenced by the X-ray diffraction refinement. The localized “rattling” of Yb³⁺ in the cation sublattice significantly scatters the heat-carrying phonons and lowers the thermal conductivity close to the amorphous limit in combination with the intrinsic oxygen vacancies in the anion sublattice. In contrast, substituting Yb³⁺ with the larger La³⁺ in Yb₂Zr₂O₇ does not result in as remarkable a decrease in the thermal conductivity as in Yb-doped La₂Zr₂O₇ due to the absence of rattling atoms. In this study, a resonant phonon scattering is proposed as a new approach to reduce the thermal conductivity of oxides which are important in various thermal engineering applications.

Abstract

Since the structural integrity of A₂B₂O₇-type pyrochlores relies mostly on the interconnecting BO₆ octahedra, Ti⁴⁺ was selected to partially substitute Zr⁴⁺ in Gd₂Zr₂O₇ in order to distort the pyrochlore structure in order to improve the material’s thermophysical properties for potential use as high-temperature thermal insulation. As evidenced by X-ray diffraction and Raman spectroscopy studies, incorporation of Ti⁴⁺ simultaneously leads to long-range ordering of the pyrochlore structure as well as local lattice distortion. These two effects have been shown to be competitive in determining the crystal energy of the Gd₂(Zr_1−xTi_x)₂O₇ series and result in a minimum value of the Young’s modulus at x = 0.3 and a maximum value of the coefficient of thermal expansion at x = 0.2. At lower temperatures, the thermal conductivity of Gd₂Zr₂O₇ was significantly reduced by Ti⁴⁺ doping, and its composition dependence was accurately modeled by taking into account the phonon scattering by mass and strain fluctuations at the B site.