Chemical Synthesis of Pure and Gd-Doped CaZrO3 Powders

SYNTHESIS of PURE and Gd-DOPED CaZrO₃ POWDERS
by
SELF-PROPAGATING COMBUSTION SYNTHESIS
and
ACID-BASE TITRATION VIA EDTA

I. Erkin Gonenli and A. Cuneyt TAS
Dept. of Metallurgical and Materials Engineering
Middle East Technical University
Ankara, Turkey

* I. E. Gonenli and A. C. TAS, "Chemical Synthesis of Pure and Gd-doped CaZrO₃ Powders,"
Journal of The European Ceramic Society, Vol. 19 (13-14), 2563-2567 (1999). (----> download cazro3.pdf)

* I. E. Gonenli and A. C. TAS, "Chemical Synthesis of Pure and Gd-doped CaZrO₃ Powders,” "Processing and Characterization of Electrochemical Materials and Devices," Ceramic Transactions, Vol. 109, pp. 153-162, (Eds.) P.N. Kumta, A. Manthiram, S.K. Sundaram, and Y-M. Chiang, 2000, The American Ceramic Society, USA, ISBN 1-57498-096-3.

* IV. Ceramics Congress, September 22-25, 1998, Proceedings Book, pp. 525-530, Eskisehir, Turkey .

Aqueous solutions of calcium chloride (CaCl2.2H2O) and zirconium oxychloride (ZrOCl2.8H2O), in appropriate volumetric amounts, were used as the starting chemicals in the synthesis of phase-pure CaZrO3 powders. Rare earth element doping (upto 25 at%) was performed by using the aqueous chloride solutions of either gadolinium (Gd). Formation of the CaZrO3 powders were achieved by two chemical synthesis techniques: (a) self-propagating combustion synthesis, and (b) precipitation in the presence of EDTA by the technique of acid-base titration. Phase characterization was performed by powder XRD (X-ray diffraction).

INTRODUCTION

There is a growing interest in calcium zirconate-based oxides for potential sensor/device applications at elevated temperatures. In particular, several studies have been reported on the use of calcium zirconate-based systems for monitoring oxygen (1-3), humidity and hydrogen (4-8). In these studies, sintered polycrystalline samples were used to characterize carrier types and the concentrations of ionic (proton or oxygen) and electronic charge carriers as a function of temperature, impurity distribution, and oxygen and/or water vapor partial pressures. Pretis et al. (9) reported that undoped calcium zirconate (CaZrO3) is a p-type semiconductor in air. When doped with oxides such as Al2O3, Y2O3 and MgO or with a small excess of ZrO2 or CaO, it becomes predominantly an oxygen-ion conductor (1, 2, 7). For a sample doped with trivalent cations such as indium, scandium and gallium, it may become predominantly a proton conductor when exposed to a hydrogen-containing atmosphere (steam) at temperatures ranging from 600-1000°C (4, 5). The protonic conduction, however, tends to diminish at higher temperatures and can be replaced by electronic (hole) conduction, especially in a dry air atmosphere (4).

CaZrO3 has also been studied for its potential use as high-temperature thermistor material (8). The electrical response of calcium zirconate (prepared by the solid state reactive firing of CaCO3 and ZrO2 powders at 1400°C) was found to be sensitive to methane, but was practically unaffected by humidity and and carbon monoxide. The use of a calcium zirconate-based thermistor is, therefore, limited to atmospheres without methane and/or possibly other hydrocarbon gases. The dramatic response to methane, however, makes CaZrO3 a potential candidate material for hydrocarbon sensing.

In this paper, we report results of two different chemical powder preparation techniques for synthesis of fine powders of pure and Gd-doped (5-15 at%) CaZrO3. This study, to our knowledge, has been the first attempt in the relevant literature for the synthesis of calcium zirconate powders by chemical means, rather than conventional solid state firing and calcination practices.

EXPERIMENTAL PROCEDURE

Starting chemicals used in this study were reagent-grade CaCl2.2H2O (Riedel de Haen), ZrOCl2.8H2O (Merck), Gd2O3 (+99.9%, Ames Laboratory, IA, USA), EDTA (C10H14N2Na2O8.2H2O, Merck), NaOH (Merck), and Urea (CH4N2O, Riedel de Haen). GdCl3 stock solutions were prepared by reacting the oxide powders in an HCl solution of correct stoichiometry. Two different synthesis techniques were employed to produce pure and Gd-doped calcium zirconate powders.

In the first technique, i.e., “self-propagating combustion synthesis (SPCS) (10, 11),” appropriate amounts of Ca- and Zr-chloride salts (in the case of Gd-doping, proper aliquots of Gd-chloride solutions were added) were first dissolved in distilled water. Urea of proper amount (which serves as the fuel / oxidant in the combustion reaction) was then added to this solution. The solution, following its transfer into a Pyrex beaker, was placed into an electric furnace pre-heated to 500 ± 10°C. The combustion reaction was completed in less than 15 minutes yielding an amorphous and foam-like powder body. This powder body was then ground, and calcined for crystallization in a stagnant air atmosphere in 17 h at 1200°C.

In the second technique, again appropriate amounts of Ca-, Zr- and/or Gd-chloride salts/solution were dissolved in distilled water. EDTA was used as a chelating agent. The cation solution at room temperature was added in a drop-wise manner into a concentrated NaOH solution. The formed precipitates were filtered, washed with distilled water, dried at 90°C, and later calcined at 1200°C for 17 h.

Phase characterization of the calcium zirconate powders were achieved by powder XRD (X-ray diffraction). The diffractometer (Rigaku Corp., Model: D/Max-B, Japan) was used with a Cu radiation tube operated at 40 kV and 20 mA. The XRD data were collected at a step size of 0.05° and count time of 1 s. Powder morphology was investigated by SEM (scanning electron microscopy; JEOL Corp., JSM-6400, Japan).

Download XRD chart

Download another XRD chart here

Synthesis of Pure and Gd-doped CaZrO3 Powders (in the presence of EDTA)

Another XRD plot here

SEM micrograph of phase-pure CaZrO₃ powders produced in the presence of EDTA in the precipitation solutions
(powders calcined at 1200°C in loose form for 17 h)

Download an SEM pic

Phase-pure CaZrO₃powders produced by the EDTA-precipitation technique were sub-micron,
and had the lattice parameters of a = 8.0159, b = 5.7516, c = 5.5954 Å,
with an orthorhombic (space group: Pnma (62)) unit cell of the volume of 257.97 Å³.

Download pH chart

REFERENCES

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