Daniel M. Pasquevich

A thermogravimetric analyser for corrosive atmospheres and its application to the chlorination of ZrO2-C mixtures

Abstract We report on a thermogravimetric analyser, built on the basis of a Cahn 2000 electrobalance, suitable for using with corrosive atmospheres. We discuss the corrections for buoyancy and gas flow effects, which strongly modify thermogravimetric curves. p]As an application, we studied the kinetics of a reaction between Cl 2 and a mixture of ZrO 2 and C. We were able to measure the uptake of Cl 2 by C and the reaction rate within the first 50 s. We present evidence of a transition to quite a different reaction rate at longer times.

Formation of pseudobrookite through gaseous chlorides and by solid-state reaction

Pseudobrookite (Fe2TiO5) was prepared from Fe2O3–TiO2 mixtures at 850 and 950 °C in air, argon and chlorine atmospheres. In the latter case, an experimental arrangement in which Fe2O3 and TiO2 were placed in separated compartments under the same chlorine atmosphere was also used. Pseudobrookite was identified by X-ray powder diffraction and microstructurally characterized by scanning electron microscopy. The very different pseudobrookite morphologies in each atmosphere allow us to propose that the formation mechanism involves Ti4+ diffusion in the Fe2O3 surface in air and argon atmospheres and vapour transport through FeCl3(g) and TiCI4(g) in the chlorine atmosphere.

Zirconium recovery from zircaloy shavings

A chlorination process for recovering Zr from zircaloy scrap has been studied. Zircaloy chlorination was possible at temperatures as low as 220 °C. The scale microstructure and its effect on the zircaloy reactivity was analysed using Thermogravimetric analysis (TGA), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDXS) and scanning electron microscopy (SEM) techniques. A solid-solid phase transformation took place into the oxide scale during the zircaloy chlorination. Zirconium, as ZrCl4(g), was separated from the oxide scale and chlorides of Cr and Fe. The effect of the reaction temperature was also analysed.

Enhancing effect of Cl2 atmosphere on transition aluminas transformation

In the present study we examine the changes in phase composition and microstructure undergone by solids after thermal treatments performed on transition aluminas in chlorine and air atmospheres. The thermal transformation to the stable α-Al2O3 was faster for the chlorine-heated samples than for the air-heated ones, and for chlorinated samples single crystals of α-Al2O3 were observed. We propose a mechanism by which AlCl3(g) formation allowed vapour mass transport that contributed to the growth of crystals of α-Al2O3. Hydroxyls contained in transition aluminas could enhance gaseous transport and could also react with chlorine molecules. The subsequent α-Al2O3 crystallization would be assisted by water vapour.

Mass effects on the surface reaction of chlorine with carbon

The reaction of chlorine with carbon surfaces has been studied by thermogravimetric techniques. The carbon samples increase in mass at a rate which increases with sample mass. This effect is interpreted as being due to the adsorption of the HCl formed by the reaction of chlorine with chemically bound hydrogen.

Effect of reaction temperature on the chlorination of a Sm2O3–CeO2–C mixture

Abstract The intrinsic kinetics of the chlorination of an hematite-titania-carbon mixture was studied by means of scanning electron microscopy and isothermal and non-isothermal thermogravimetry between 580 and 1123 K. It was observed that various reactions take place simultaneously and that a carbon surface deactivation occurs between 733 and 820 K. The latter was attributed to chemisorption of iron chlorides on the carbon surface.

Enhancement of the martensitic transformation of tetragonal zirconia powder in the presence of iron oxide

Abstract The martensitic t → m phase transformation in air is enhanced by the presence of iron oxide during heating and follows the same mechanism as in the absence of iron. X-ray diffraction and scanning electron microscopy show the phase transformation and the morphology changes occurring during heating.

Cu-Zn-Al alloys characterization: phases thermal stability and dezincation

As a first stage in the study of chlorination of ternary alloys, characterization studies involving thermodynamics and temperature effects are necessary to know the changes in the structure, which might arise from the heating that precedes the chlorination reactions. The initial beta-alloy was quenched from 900oC. After different non-isothermal treatments mass loss was observed due to a dezincation process at temperatures above 70oC0. It was found that the heating rate affects the Zn loss starting temperature. Occurrence of low temperature phases due to thermal treatments was observed. The peaks observed in the sample XRD profiles allow to identify the presence of alpha and gamma phases for temperatures below 525oC, and the beta phase for higher temperatures. At 240oC the beta phase disappears after 9 hours, while at 315oC the beta peak was absent after 2 hours. XRD and SEM techniques were used to analyse non-isothermal measurements and thermal treatments.

Kinetic study of the + transformation reaction in a CuZnAl alloy

Abstract In the present work we study the phase transformation reaction of a Cu-20Zn-7Al (wt.%) alloy by isothermal and non-isothermal in situ X-ray diffraction. An effective activation energy for each phase is obtained for the range between 275 and 395°C. The experimental data are also adjusted with the JMA equations. Finally, we present the TTT (time–temperature–transformation) diagram corresponding to the precipitation of alpha and gamma for the lower temperatures. Below 340°C, we found for this alloy that the gamma phase precedes the formation of alpha.

CHARACTERIZATION OF ULTRAFINE ZIRCONIA PARTICLES PRODUCED BY A GAS-SOLID REACTION

Abstract The microstructure of a zirconia powder prepared from a gas-solid reaction was determined by SEM, EDXS, DRX and TEM analysis. At low temperatures (773 K) and up to 0.5 of reaction degree two types of particles were formed: a melaslable tetragonal ZrO2 phase fixed on the Fe2O3 grain surfaces and isolated grains of monoclinic ZrO2 phase.