Progress update on failure mechanisms of advanced thermal barrier coatings: A review

A novel photocatalyst, titanium dioxide (TiO2) doped with neodymium (Nd), was prepared by the sol–gel method and used for the photocatalytic reduction of Cr(VI) under UV illumination, in order to determine its photocatalytic properties. A series of Nd3+–TiO2 catalysts prepared with different Nd3+ dosages were characterized by XRD, Raman spectroscopy, SEM, EDX, TEM, EDS and XPS spectroscopy. In the experiments, formic acid was used as a hole scavenger to enhance the photocatalytic reduction reaction. The experiments demonstrated that Cr(VI) was effectively reduced in aqueous Nd3+–TiO2 suspension by more than 95% within 60 min, while the pH of the solution increased from 3.1 to 3.35 due to the consumption of formic acid. The experimental results indicate that the presence of Nd3+ in TiO2 catalysts substantially enhances the photocatalytic reaction of chromium(VI) reduction. It was found that the optimal dosage of 1–3 wt% Nd3+ in TiO2 achieved the fastest reaction of Cr(VI) reduction under the experimental conditions. The neodymium ions deposited on the TiO2 surface behave as sites at which electrons accumulate. The improved separation of electrons and holes on the modified TiO2 surface allows more efficient channeling of the charge carriers into useful reduction and oxidation reactions rather than recombination reactions. The presence of sacrificial electron donors such as formic acid enhances the photocatalytic reduction of Cr(VI). The Cr(VI) adsorbed on the surface of the TiO2 particles was observed to be almost completely photoreduced to Cr(III).

Preparation, characterization and application of Nd–TiO2 photocatalyst for the reduction of Cr(VI) under UV light illumination

Nanocrystalline titanium dioxide (TiO2) particles were prepared by a modified alkoxide method under acidic conditions at temperatures ranging from 60°C to 90°C. The reaction temperature was used to control the crystalline phase of the TiO2 particles. At 60°C and 75°C rutile was formed whilst at 90°C anatase and brookite were formed.

Preparation of nanosized crystalline TiO 2 particles at low temperature for photocatalysis

Investigation of the effect of sol–gel synthesis variables on structural and photocatalytic properties of TiO2 nanoparticles

Doped and undoped nanostructured titanium dioxide was synthesized via a sol–gel method under varying conditions to determine the effects of neodymium ion doping on the titania lattice. Specifically, the effects of doping on the anatase to rutile phase transformation were studied. Samples were analyzed by a variety of techniques, including X-ray diffraction, Rutherford back-scattering spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectrometry to investigate the thermodynamic and structural effects of dopant addition. Analysis of the anatase to rutile phase transformation in TiO 2 showed a maximum increase in activation energy for 0.1 mol% Nd 3+ doped TiO 2 with no further response at higher dopant concentrations. This was supported by structural analyses, which showed distortion only along the c -axis of anatase TiO 2 with a maximal distortion at 0.1 mol% Nd 3+ and no further response at higher concentration. This is due to a combination of interstitial and substitutional accommodation of the dopant ions.

http://www.physics.udel.edu/%7Eismat/2003p3.pdf

Neodymium ion dopant effects on the phase transformation in sol-gel derived titania nanostructures

The effect of microstructural features on the electrical properties of TiO2 (rutile) was studied using impedance spectroscopy in conjunction with X-ray diffraction and scanning electron microscopy. The electrical properties of TiO2 were measured at 350 °C after sintering in air at different temperatures. It was found that the electrical resistivity increased significantly after the interconnected pores closed in the TiO2 due to sintering. However, further decrease of porosity in the TiO2 had little effect on its resistivity prior to the closure of pores. On the other hand, after the sintered specimens were annealed in vacuum, the resistivity of TiO2 decreased considerably without apparent change of the microstructure in TiO2. Moreover, the resistivity, measured at room temperature, decreased after the interconnected pores closed due to sintering. These results suggest that the electrical conduction in TiO2 after sintering in air is mainly due to ionic conduction, while the conduction in the sintered TiO2 after annealing in vacuum was mainly due to electronic conduction. The closure of the open pores in the TiO2 reduced the surface ionic conduction but increased the electronic conduction route. Overall, the microstructural features of TiO2 have characteristic effects on its electrical properties.

Effect of microstructural features on the electrical properties of TiO2

In this work, impedance spectroscopy (IS) has been used to evaluate the degradation of thermal barrier coatings (TBCs) due to oxidation at 1150 o C. The spallation of TBCs was found to be induced by the development of thermally grown oxides (TGO) produced from the oxidation of TBCs. The change in the electrical properties of the TGOs was found to be related to the change in the microstructure and microchemistry of the TGO. The resistivity of the TGO due to oxidation from 10 to 1000 h decreased rapidly, which corresponded to the increase in porosity in the TGO and the compositional change of the TGO from α-Al 2 O 3 to a mixture of α-Cr 2 O 3 and (Ni,Co)(Cr,Al) 2 O 4 spinel. The slow decrease in the resistivity of the TGO from oxidation for 1000–2000 h indicated that there was little change in the composition of the mixed oxides, although the growth rate of the TGOs was relatively fast during this oxidation period. The disappearance of α-Al 2 O 3 in the TGO caused a rapid oxide growth during oxidation and led to the spallation of TBCs.

Evaluation of degradation of thermal barrier coatings using impedance spectroscopy

Oxidation of air-plasma-sprayed (APS) thermal barrier coatings (TBCs) was carried out in air at 950 °C and was investigated using impedance spectroscopy coupled with scanning electron microscopy and X-ray diffraction. After oxidation for between 500 and 3000 h, a continuous alumina scale was formed at the bond coat/top coat interface in the TBCs, and was evaluated using impedance spectroscopy. The impedance spectra of the oxidised TBCs showed that there were four relaxation processes, which were attributed to the yttria-stabilised zirconia (YSZ) bulk of the TBCs, the thermally grown alumina scale, the YSZ grain boundary, and the metal electrode effect. Impedance analysis showed that the resistivity of the alumina scale increased with increasing oxidation time, demonstrating the thermally grown oxide (TGO) growth. The resistance of the YSZ grain boundaries also increased after oxidation for 2000 h, suggesting the composition change at grain boundaries after a long-term oxidation.

Shenhua Song

Papers

Effect of microstructural features on the electrical properties of TiO2

Evaluation of degradation of thermal barrier coatings using impedance spectroscopy

An impedance spectroscopy study of high-temperature oxidation of thermal barrier coatings