Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.

https://link.springer.com/content/pdf/10.1007%2Fs10853-010-5113-0.pdf

Review of the anatase to rutile phase transformation

This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current-voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain-boundary defect model which explains both the instability and the stability under use conditions.

Application of Zinc Oxide Varistors

Mechanisms of cement hydration

Perovskite lead-free dielectrics for energy storage applications

Combustion synthesis has emerged as a facile and economically viable technique for the preparation of advanced ceramics, catalysts and nanomaterials. Recent innovations in the combustion and processing parameters have resulted in a better understanding of combustion phenomena and control of microstructure and property of the products.

/pdf/combustion-synthesis-an-update-3l5d2c0u1f.pdf

Combustion synthesis: an update

Titania powders were synthesized by the thermal hydrolysis of titanium tetrachloride with the yield of above 85% in a mixed solvent of n-propanol and water. The morphology of the precipitates was controlled by adjusting the volume ratio of n-propanol to water (RH ratio) of the mixed solvent. Precipitates obtained with an RH ratio of 0 were fine, and highly agglomerated. In contrast, an RH ratio of 3 resulted in precipitates consisting of uniform and discrete particles. According to observations of the zeta potentials of precipitates and the dielectric constants of solvents, the discrete particles obtained with an RH ratio of 3 resulted from the low zeta potential and dielectric constant. The result of Fourier transform infrared (FTIR) spectroscopy showed the chemical interaction of particle surface with the solvent alcohol, which decreased the zeta potential of precipitates with an increase of RH ratio. Adding hydroxypropyl cellulose (HPC) as a steric dispersant made it possible to reduce the size of particles to the submicrometer range. Effects of the solvent on the formation and morphology of the resulting particles were investigated.

Effect of Solvent on Titania Particle Formation and Morphology in Thermal Hydrolysis of TiCl4

Dense SiC ceramics were obtained by pressureless sintering of [beta]-SiC and [alpha]-SiC powders as starting materials using Al[sub 2]O[sub 3]-Y[sub 2]O[sub 3] additives. The resulting microstructure depended highly on the polytypes of the starting SiC powders. The microstructure of SiC obtained from [alpha]-SiC powder was composed of equiaxed grains, whereas SiC obtained from [beta]-SiC powder was composed of a platelike grain structure resulting from the grain growth associated with the [beta][r arrow][alpha] phase transformation of SiC during sintering. The fracture toughness for the sintered SiC using [alpha]-SiC powder increased slightly from 4.4 to 5.7 MPa [center dot] m[sup 1/2] with holding time, that is, increased grain size. In the case of the sintered SiC using [beta]-SiC powder, fracture toughness increased significantly from 4.5 to 8.3 MPa [center dot] m[sup 1/2] with holding time. This improved fracture toughness was attributed to crack bridging and crack deflection by the platelike grains.

Effects of α-SiC versus β-SiC starting powders on microstructure and fracture toughness of SiC sintered with Al2O3-Y2O3 additives

A novel method is demonstrated which yields a spherical ZrO2 powder of narrow size distribution through heating of a zirconyl chloride solution with an alcohol–water mixture as the solvent. The kind and composition of the solvent mixture greatly influenced the behavior of the precipitation and the morphology of the resulting particles. When 1-propanol or 2-propanol was employed as the alcohol of the solvent mixture, the resulting particles had a spherical shape and a narrow size distribution. The particle size and the particle agglomeration level could be controlled by the amount of hydroxypropyl cellulose (HPC) in the solution. As-prepared amorphous powder was crystallized to a mixture of metastable tetragonal phase and monoclinic phase at about 460°C. The metastable tetragonal phase was converted to the monoclinic phase as the calcination temperature was increased. After calcination, the spherical shape of the zirconia powder was retained, while its particle size was decreased slightly.

Preparation of Monodisperse and Spherical Zirconia Powders by Heating of Alcohol–Aqueous Salt Solutions

The fracture-toughness-determining mechanism of silicon carbide with additions of yttria and alumina was studied. Observations of indentation crack profiles revealed that significant crack deflection had occurred. Median deflection angles increased with increased volume fractions of the second phases, which was accompanied by increased fracture toughness.

Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using Β-SiC powder with the addition of 6 wt% Al2O3 and 4 wt% Y2O3. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the Β-SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the Β→α phase transformation. With increasing holding time, the fraction of Β→α phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m1/2 to 8.3 MPa m1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.

Chong Hee Kim

Papers

Effect of Solvent on Titania Particle Formation and Morphology in Thermal Hydrolysis of TiCl4

Effects of α-SiC versus β-SiC starting powders on microstructure and fracture toughness of SiC sintered with Al2O3-Y2O3 additives

Preparation of Monodisperse and Spherical Zirconia Powders by Heating of Alcohol–Aqueous Salt Solutions

Toughening Behavior of Silicon Carbide with Additions of Yttria and Alumina

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives