SOLID STATE SYNTHESIS OF MULTICOMPONENT RARE-EARTH OXIDE CERAMICS

Phase formation in multicomponent rare-earth oxides is determined by a combination of composition, sintering atmosphere, and cooling rate. Polycrystalline ceramics comprising various combinations of Ce, Gd, La, Nd, Pr, Sm, and Y oxides in equiatomic proportions were synthesized by solid-state sintering. The effects of composition (type and number of cations), sintering atmosphere (oxidizing, inert, and reducing), and cooling rate on phase formation were investigated. Single cubic or monoclinic phase compositions were obtained with a slow cooling of 3.3 ºC/min, indicating that rare-earth oxides follow a different phase stabilization process than that of transition metal high-entropy oxides. In an oxidizing atmosphere, both Ce and Pr induce the formation of a cubic phase, while only Ce plays that role in an inert or reducing atmosphere. Samples without Ce or Pr develop a single monoclinic phase. The phases formed at initial synthesis may be converted to a different one, when the ceramics are annealed in an atmosphere different than the original sintering atmosphere. Additionally, phase evolution of a five-cation composition was studied as a function of sintering temperature. The binary oxides used as raw materials completely dissolve into a single cubic structure at 1450ºC in air

Production of TiO2 Coated Multiwall Carbon Nanotubes by the Sol-Gel Technique

<div><p>In recent years, efforts in developing high strength-low density materials are increasing significantly. One of the promising materials to attend this demand is the carbon nanotube (CNT), to be used mainly as a reinforcing phase in lightweight metal matrix composites (MMC). In the present work, the sol-gel technique has been employed to obtain TiO2 coating on the surface of commercial multiwall carbon nanotubes (MWCNT). The aim of such coating is to improve the thermal stability of MWCNT in oxidize environment, which is necessary in most of MMC processing routes. Calcination in inert atmosphere was performed in order to crystallize a stable coating phase. The hybrid CNT/TiO2 nanocomposite was characterized by X-Ray Diffractometry (XRD), Raman spectroscopy, Thermogravimetry (TGA) and Field Emission Gun - Scanning Electron Microscopy (FEG-SEM). The coating structure was observed to change from anatase to rutile, as the calcination temperature increases from 500 to 1000°C. Results from thermogravimetric analysis showed that the samples calcined at 1000 ºC were more resistant to oxidation at high temperatures.</p></div

Crystal Growth and Phase Formation of High-Entropy Rare-Earth Aluminum Perovskites

We demonstrate for the first time the crystal growth of high-entropy rare-earth (RE) aluminum perovskites (REAlO3) using the micro-pulling-down method to inform future exploration of functional crystals. To determine how composition affects phase formation, we formulate equiatomic compositions containing five REs from the following list: Lu, Yb, Tm, Er, Y, Ho, Dy, Tb, Gd, Eu, Sm, Nd, Pr, Ce, La. To test whether combinations of REs with similar ionic radii may favor a single phase, compositions containing REs with consecutive or nonconsecutive ionic radius values were formulated. Powder and single-crystal X-ray diffraction indicate that crystals containing only REs with similar ionic radii that form orthorhombic single-RE REAlO3 are a single phase. Crystals containing REs with dissimilar ionic radii or mixtures of REs that form orthorhombic, rhombohedral, and tetragonal single-RE REAlO3 are a mixture of phases. The elemental distribution in single-phase crystals analyzed via electron probe microanalysis confirms no evidence of preferential incorporation of any of the constituent REs. The distribution and composition of secondary phases were analyzed via scanning electron microscopy and energy dispersive spectroscopy; secondary phases were seen as a small region in the center of the crystals with branching features closer to the outer surface