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The environmental barrier coating (EBC) has been studied to protect the silicon carbide blades of the next-generation gas turbine engine of aircraft from superheated water vapor. The upper ceramic layer of EBC is brittle, therefore, cracks can occur under the impact of foreign debris and spread toward the lower layers, which leads to the destruction of this structure. Therefore, self-healing structural ceramics is a promising solution to this problem, but they still have some critical drawbacks. First, a healing agent such as SiC is highly volatile in water vapor. Second, the uniform distribution of the healing agent in the matrix can cause internal stress, which can break the composite. Finally, in most cases, the self-healing function can only be used once. Here, functionally graded Y2Ti2O7-Y2TiO5-TiN composites are produced by hot pressing and nitriding and then their self-healing function is demonstrated. The oxidation of TiN to TiO2, associated with volumetric expansion, can heal surface cracks. In addition, the bending strength of the composite can be restored. The self-healing ability is renewable at least until the second heating cycle. These optimistic results can pave the way for the development of a new class of materials with permanent self-healing ability. [ABSTRACT FROM AUTHOR]

Yttrium titanate (Y2Ti2O7) thin films, having pyrochlore-type structure, were synthesized in single phase by sol-gel synthesis technique. Single-phase formation was confirmed by X-ray diffraction (XRD) study. Surface morphology of thin films was studied by using FESEM and average grain size was found to be in nanometer range. Retention of stoichiometry in thin films was confirmed by EDX analysis. Optical property of Y2Ti2O7 thin films was studied with the help of UV-Vis spectrometer and the energy band gap was found to be ∼3.5 eV. Dielectric properties of thin films are also studied and discussed in details. [ABSTRACT FROM AUTHOR]

Highly transparent yttrium titanate (Y 2 Ti 2 O 7) ceramics were fabricated by vacuum sintering using co-precipitated powders for the first time. The effects of the powder calcination temperature on the phase composition, morphology of the calcined powders, and on the microstructure and transmittance of the Y 2 Ti 2 O 7 ceramics were investigated. When the calcination temperature was above 850 °C, pure phase Y 2 Ti 2 O 7 nanopowders with high sintering activity were obtained. Transparent Y 2 Ti 2 O 7 ceramics were obtained after vacuum sintered at 1600 °C for 6 h and annealed at 1100 °C for 5 h in air. The highest transmittance reached 73% at 1000 nm when the calcination temperature was 1150 °C. The measured refractive index of Y 2 Ti 2 O 7 ceramics was higher than 2.24 at the wavelength range of 350–1000 nm, making it a promising candidate for optical devices. [ABSTRACT FROM AUTHOR]

The nanocrystalline yttrium titanate (YTO) ceramic was prepared by polymerized sol-gel method. Structural analysis performed with x-ray diffraction and Raman spectroscopy showed the cubic-pyrochlore structure of YTO. Impedance and dielectric spectroscopy have been performed in a high temperature region varying from 550 to 700 °C. The YTO grains were found to participate dominantly in the process of electrical conduction. The YTO showed the negative thermal coefficient of resistance type material for electrical conduction. The dc conductivity of YTO followed the Arrhenius behavior and the activation energy was calculated to be 1.15 eV. The dielectric constant has increased with increasing the measuring temperature. The ac conductivity had shown the Universal behavior described by Jonscher's power law and the corresponding value of the frequency exponent ( n ) was found to be less than unity. The CBH model was successfully employed and the conduction was proved to occur through hopping mechanism and the value of maximum barrier height ( W M ) was obtained as 0.17 eV. Electrical modulus spectra were fitted with Bergman proposed KWW function and the dielectric relaxation occurred in YTO was of non-Debye type because the magnitude of stretching factor (β) was found to be less than unity (ranging from 0.77 to 0.89). The electrical modulus and impedance characteristics of YTO were properly scaled so that they fall in a single Universal plot, that implied that the respective mechanisms in the nanocrystalline YTO were reported to be temperature independent. [ABSTRACT FROM AUTHOR]

Transparent Y 2 Ti 2 O 7 pyrochlore was fabricated via the direct solid-state reaction of Y 2 O 3 and TiO 2 by hot pressing at 1400 °C under 20 MPa in vacuum, followed by annealing at 700–1000 °C for 4–10 h in air. A small amount of excess Y 2 O 3 was added to the stoichiometric composition to promote densification via the formation of more oxygen vacancies. The black-colored samples obtained after hot pressing exhibited transparency upon annealing owing to the restoration of oxygen ions in the oxygen vacancies, where the transmittance depended significantly on the annealing conditions. The highest transmittances of 55.1 and 74% were obtained at 800 and 2400 nm, respectively, for the Y 2 Ti 2 O 7 annealed at 800 °C for 8 h. The bandgap energy was 3.75 eV for the transparent Y 2 Ti 2 O 7 , while that of the black Y 2 Ti 2 O 7 without annealing was 0.29 eV due to the defect level formed by the oxygen vacancies. A model was proposed to explain the transparency behavior of Y (2+2X) Ti 2 O (7-X) owing to the variations in the atmosphere and excess Y 2 O 3 in terms of oxygen vacancies and ions present in a unit cell. Transparent Y 2 Ti 2 O 7 pyrochlore: Annealing and constant cationic B lattice model. [Display omitted] • Fabricating a transparent Y 2 Ti 2 O 7 pyrochlore via direct solid-state reaction of Y 2 O 3 and TiO 2 using hot pressing. • Examining the effects of annealing on physical and optical properties of Y 2 Ti 2 O 7 associated with defect level formation. • Proposing the 'constant cationic B lattice model' to explain the number of oxygen vacancies and ions in Y 2 Ti 2 O 7 unit cell. [ABSTRACT FROM AUTHOR]

Nanosized perovskite YTiO3 with the mean crystallite size of 18 nm was synthesized for the first time by mechanochemical treatment. The mechanochemical solid state reaction between commercial Y2O3 powder and mechanochemically synthesized TiO powder in molar ratio 0.5:1 was completed for 3 h in a high-energy planetary ball mill in argon atmosphere. The heating in vacuum at 1150 °C for 12 h transforms nanosized YTiO3 to a well-crystallized single-phase perovskite YTiO3. Both samples were characterized by X-ray diffraction (XRD) and thermogravimetric (TGA/DTA) analyses, as well as superconducting quantum interference device magnetometer (SQUID) measurements.

1-D nanostructures are deemed to be important building blocks for future optical and optoelectronic nanodevices. We have used electrospinning methods to synthesize nanofibers and freestanding, non-woven nanofibers membranes of single phase yttrium titanate and ErYTiO (x = 0-12 %) with diameters less than 150 nm and have characterized the physical, thermal and optical properties. We have demonstrated that such fibers can be synthesized at a relatively low cost and high production volume with a narrow diameter range. No clear phases are observed up to a calcination temperature of 700 °C and pyrochlore phase appear at 725 °C which are also confirmed by the TGA. Single pyrochlore phase is observed at temperatures greater than 900 °C. No other oxide phases are present. [ABSTRACT FROM AUTHOR]

Highlights: [•] Co-doped (Y0.99 − x Ho0.01Yb x )2Ti2O7 nanophosphors were fabricated by gel-combustion method. [•] The effect of calcination and Yb3+ doping on upconverting spectra of nanophosphors was studied. [•] The dependence of upconverting intensity on the excitation power was examined. [Copyright &y& Elsevier]

Abstract: The tribological behavior of different low infrared (IR) emission ceramic materials from the titanate group has been investigated at room temperature under gross slip fretting conditions against 100Cr6 ball bearing steel and α-alumina as the counterface materials. For all material pairs, friction and wear depended largely on the relative humidity of the environment. When paired with steel the low IR-emission ceramic disks showed higher wear resistance. Under dry conditions, almost no wear was found on the low IR-emission ceramic specimen. The high wear on the steel counter body is caused not only by tribo-oxidation, which is the main wear mechanism, but also by abrasion. In dry conditions a COF of 0.6 and in humid conditions a COF of 0.2 have been measured. When paired with α-alumina the results on wear are just opposite to those with steel counterbody and the low IR-emission ceramic materials show much lower wear resistance under all conditions. This is explained by the predominance of abrasion and the relatively large difference in hardness between the low IR-emission ceramic and α-alumina. In high humidity environments the results point to the formation of stable reaction layers causing lower friction and wear. [Copyright &y& Elsevier]

Abstract: Pure-phase pyrochlore oxide Y2Ti2O7 nanocrystals with good dispersity were successfully synthesized via a glycine-nitrate gel-combustion approach. The preparation process and products were monitored and characterized by Fourier transform-infrared spectra, thermogravimetry-differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The detailed results suggested that the phase and nature of as-obtained products are highly dependent of the pH value of the precursor solution and the amount of glycine. Under the appropriate conditions that the pH value of the precursor is 2.0, the fuel-to-oxidant ratio is 2.0, and the calcination temperature is 800°C, the as-prepared products are almost sphere-like or ellipsoid, exhibits perfect crystalline phase, good dispersity and a narrow particle distribution with an average size diameter of 20–30nm. This soft-chemistry route can be also extended to prepare Y2Ti2O7-based functional nanomaterials and other pyrochlore-oxide nanocrystals. [Copyright &y& Elsevier]

Pyrochlore-type yttrium titanate (YTiO) nanoparticles were successfully synthesized by a simple soft-chemistry technique viz. citric acid sol-gel method (CAM). The preparation process was monitored by X-ray diffraction, thermogravimetric-differential thermal analysis and Fourier transform-infrared experiments and the microstructures and average size of as-prepared products were characterized by transmission electron microscopy and high resolution transmission electron microscopy images. It was found that compared with traditional solid state reaction (SSR), YTiO nanopowders were synthesized at a relatively low temperature (750°C) for shortened reaction time. Detailed analysis showed that the as-prepared YTiO with good dispersibility and narrow size distribution were quasi-spherical; the average size was about 20-30 nm, also, the obtained products had higher BET surface area (50 m/g). These properties are very helpful for a photocatalyst to achieve excellent activity and may result in better behaviour in hydrogen storage. [ABSTRACT FROM AUTHOR]

Abstract: The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y2Ti2O7, have been determined over the temperature range 2K⩽T⩽300K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ D, of ca. 970K) and an assignment of the remaining 63 optic modes, as well as a correction for Cp −Cv . From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H(T=298.15K)−H 0=34.69kJmol−1 and S(T=298.15K)−S 0=211.2JK−1 mol−1. The thermal conductivity shows a peak at ca. θ D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y2Ti2O7 is 2.8Wm−1 K−1, close to the calculated theoretical thermal conductivity, κ min, for fully coupled phonons at high temperatures. [Copyright &y& Elsevier]

Abstract: 17O MAS NMR and XRD studies of precursor-derived Y1.6Zr0.4Ti2O7.2 and Y1.2Zr0.8Ti2O7.4 have been performed to investigate the development of local and long-range order in these materials as they evolve from a metastable amorphous state upon heating. Zirconium titanate (ZrTiO4) was also investigated to help interpret the 17O NMR spectra of the ternary compositions. Consistent with earlier studies, crystallization was observed at 800°C to form a fluorite structure and a small amount of rutile; weak broad reflections were also observed which were ascribed to the presence of small pyrochlore-like ordered domains or particles within the fluorite phase. As the temperature was increased further, the sizes of these domains grew along with the concentration of rutile. At the highest temperature studied (1300°C), the reflections of the thermodynamic phases, pyrochlore and zirconium titanate (ZrTiO4), dominated the XRD pattern. The 17O NMR spectra revealed a series of different peaks that were assigned to different 3- and 4-coordinate O local environments. The data were consistent with the formation of a metastable phase Y2− x Zr x Ti2− y Zr y O7+ x with pyrochlore-like ordering but with Zr substitution on both cation sites of the pyrochlore structure. At low temperatures, doping on the A (Y3+) sites predominates (i.e., x>y), consistent with the fact that the pyrochlore develops out of a more disordered fluorite-like, phase. As the temperature is raised, the Zr doping on the A site decreases and the metastable phase at this temperature can now be written as Y2− x ′Zr x ′Ti2− y ′Zr y ′O7+ x ′ (i.e., x′

Abstract: Er3+/Yb3+ codoped pyrochlore-type (Y0.99-xEr0.01Ybx)2Ti2O7 (x=0, 2.5, 5.0, 7.5, 10.0 and 12.5mol%) nanocrystals have been successfully prepared by the sol–gel process with citric acid as chelator. The samples annealed at 800°C for 1h exhibit good crystallinity and narrow size distribution with average size of 20–30nm. Upon 980nm excitation at room temperature, green (at~525 and 545nm) and red (at~660nm) upconversion emissions were observed, which are ascribed to the (2H11/2, 4S3/2)→ 4I15/2 and 4 F9/2 → 4I15/2 transitions, respectively. And also, it has been found that the annealing temperatures and the Yb3+ ion doping concentrations have greatly influenced on the upconversion luminescence intensity. The results show that Y2Ti2O7 is an excellent host material for efficient upconversion nanophosphors. [Copyright &y& Elsevier]