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One of the attempts to improve γ-alumina catalytic performance lies in the use of additives that prone to segregate on its interfaces. The catalytic activity depends on the acidity or basicity of the doping oxides. Li2O is one of these oxides that when is loaded on γ-alumina surface decreases the number of Lewis acid sites. However, little is known about the Li2O distribution in the γ-alumina structure and interfaces. Thus, a detailed study of the lithium oxide distribution in the nanostructure of γ-alumina is the aim of this study. Nanometric powders were synthesized by the polymeric precursor method. The nanopowders were doped with Li+ ions (0.1, 0.5, 1, 3, 5, and 10 mol%) followed by calcination at 650 °C and 750 °C for 15 h. The powders were characterized by XRD, XRF, FTIR-DRIFT, BET specific surface area (SSA) and helium pycnometry. The surface excess due to the lithium oxide segregation was determined by the selective lixiviation method which has proved to be effective in quantifying surface excess of segregated ions at oxide interfaces. The crystallite sizes of the nanopowders varied from 3.3 nm to 9.2 nm, while their SSA were between 82.0 m2/g to 119.9 m2/g. The chemical analyses suggest that the lithium segregates in the γ-alumina interfaces at 650 °C, but it volatilizes from the surface after calcination at 750 °C.

The primary characteristic of nanopowders is the high surface area and consequently high fraction of atoms on the interfaces, which changes the energy of the system. The additive distribution in the nanopowder interfaces is a fundamental aspect to control the energy, particle size, and final properties of nanopowders. In this work, the surface excess was determined using a selective lixiviation method, where a low-water-soluble oxide, SnO2, was used as the matrix, and a high-water-soluble oxide, ZnO, was used as the additive. The XPS analysis confirmed that ZnO segregated on SnO2 surfaces. However, after acid lixiviation the same analysis showed an undetectable surface concentration of ZnO. The evaluation of the nanostructure change and surface composition enables us to calculate the heat of segregation for the grain boundary(∆Heg^gb = -47.2 kJ mol-1)and surface(∆Hseg^s=-36.4 kJ.mol-1) and the interface energy reduction because of segregation. At low ZnO concentrations, the additive solubilizes in the bulk and promotes particle growth. However, the segregation to the grain boundary and surface determines the relative stability of each interface, which promotes hard agglomeration and particle size stabilization at intermediated ZnO amounts. At high ZnO concentrations, the surface segregation stabilizes the solid-gas interface and decreases the agglomeration and final particle size. This article is protected by copyright. All rights reserved.

Ostrich eggshells are a potentially abundant and a high purity and low cost source of calcium to produce β-Tricalcium Phosphate (β-TCP) and Hydroxyapatite (HA), important calcium phosphates used as biomaterials. Here, we use a wet precipitation procedure to synthesize these phosphates using ostrich eggshells as a source of calcium ions. The biphasic precipitated powder, calcined at 800ºC, is a mixture of both Hydroxyapatite and β-Tricalcium Phosphate, also known as the biomaterial Biphasic Calcium Phosphate (BCP). Physico-chemical properties of the final powder product show water and CO32- groups absorbed in the particles surface, 0.1 to 100 µm particles size distribution and 11.70 m2/g of specific surface area.

Flash sintering (FS) is a current-assisted sintering technique able to densify ceramics in short periods of time (just a few seconds) at temperatures significantly lower than in conventional sintering processes. FS technique was firstly reported for yttrium-stabilized zirconia and later it had been proved successful for a large range of oxide materials that present ionic conduction by oxygen vacancies. This paper describes the use of FS on a sodium ion conductor based on a model compound, the beta-alumina. Different electrode materials have been tested, i.e., silver and platinum. The impact of the electrode reaction on the current flow, and thus, on the sintering efficiency is shown for the first time. It appears that the densification by FS can only be possible if the current collectors, i.e., the electrodes, are specifically designed to enable reversible electrochemical reactions at the interfaces between the electrodes and the ionic compound, insuring the current flow through the powder compact.

The intense violet color and the high catalyst activity of Cr-doped SnO2nanoparticles have motivated several authors to understand the solid solution formation and the oxidation state of chromium ions after synthesis. Recent work has demonstrated the ability of surface segregation in chromium-doped tin oxide system but the oxidation state is still misunderstood. Calcium oxide addition changes the color of (Cr,Ca) codoped nanocrystalline tin dioxide pigments from violet to yellow simultaneously to a high particle size stabilization demonstrating that co-segregation could be associated to color change due to chemical environment change of chromium ions and specific surface area increase. High solubility of Cr+6and Ca+2allow us to determine the surface excess of both cations by ionic chromatography and the color change after surface solubilization.