Preparation of Fully Cubic Calcium-Stabilized Zirconia With 10 mol% Calcium Oxide Dopant Concentration by Microwave Processing Anirudh P. Singh, w Navdeep Kaur, and Ajay Kumar Department of Applied Sciences and Humanities, Shaheed Bhagat Singh College of Engineering and Technology, Ferozepur 152 004, India Kanchan L. Singh Lala Lajpat Rai Institute of Engineering and Technology, Moga 153001, India A novel, fast, and cheaper method has been developed for the synthesis of fully cubic calcium-stabilized zirconia (ZrO 2 ) of composition Ca 0.1 Zr 0.9 O 1.9 by dissolution of calcium oxide in monoclinic ZrO 2 for the first time using microwave energy. In this process, the precursors have been prepared by the mixed- oxide method taking the constituents in their stoichiometric ratio. The samples have been allowed to absorb microwave ra- diation in the presence of a polymeric susceptor. The susceptor absorbs the microwave radiation at room temperature and in- creases the temperature of the sample, where it starts interacting with microwave radiation. The susceptor burns off at a higher temperature without reacting with the sample. The cubic Ca 0.1 Zr 0.9 O 1.9 has been prepared at a temperature of 11001C within 5 min. I. Introduction Z IRCONIA (ZrO 2 ) technology for sensors has been very suc- cessful in the market place, and it has pushed forward the development of solid oxide fuel cell materials. One of the major commercial success of ZrO 2 as electrolyte/solid-state ionics in the last century and till today has been its ubiquitous use as ex- haust sensors in almost every automobile worldwide, besides its applications in other devices. 1–3 The main difference is that the power output of the sensor is low so partially stabilized ZrO 2 can be used as an electrolyte. At higher power, fully stabilized ZrO 2 must be used if the electrolyte is to remain stable for long periods. Homogeneous doping with appropriate elements is one approach to obtain materials with high electrical conductivity. The basic idea in the case of ZrO 2 is to prepare a solid solution with a compound containing an ion with different valence. The cubic phase of ZrO 2 can be stabilized by addition of an aliova- lent cation having cubic symmetry and size comparable with that of a zirconium ion and can have better conductivity. Owing to the lower cost of calcium-stabilized ZrO 2 , it is still used com- mercially. 4 Cubic ZrO 2 stabilized with calcium oxide (CaO) is a nonstoi- chiometric substance with a defect fluorite structure, the defects being anion vacancies. The general formula of cubic calcium- stabilized ZrO 2 is Ca x Zr 1Àx O 2Àx . The cubic phase extends over a wide range of dopant concentrations with corresponding large anion vacancy concentrations. 5 The variation of isothermal elec- trical conductivity (with dopant content) of a binary oxide sys- tem based on ZrO 2 exhibits a maximum at or near the lowest dopant concentration required to stabilize the cubic phase and decreases with increasing dopant concentration. This trend is accompanied by an increase in the activation energy for con- duction. The decrease in conductivity with increasing dopant concentration is contrary to the dilute solution model and has not been fully accounted for in a quantitative way, although defect ordering, 6 clustering, 7,8 electrostatic interaction, 9,10 pre- cipitation of a second phase, 7 etc. have been invoked. A large number of works have been carried out to stabilize the cubic phase of ZrO 2 to prepare a solid solution of the ZrO 2 and CaO system. However, there is considerable disagreement about the existence of a cubic phase region in the ZrO 2 –CaO system, which depends upon the methods of preparation. 11 Considerable efforts have been made in the past and are still continuing to reduce the concentration of CaO for the forma- tion of fully cubic-stabilized ZrO 2 in order to improve the elec- trolytic property of ZrO 2 . Recently, it has been found that a wide variety of chemical reactions are accelerated by microwave irradiation of react- ants. 12–16 Excellent reviews have been written to overview the developments in this rapidly growing field. 17–19 The major ad- vantages of microwave processing are lower reaction tempera- tures and shorter processing times compared with conventional synthetic procedures. However, the great majority of the reac- tions are in solution phases. High dielectric loss tangents of the polar solvents are responsible for the effective coupling to the microwave field. Relatively few reports have appeared on micro- wave solid-state synthesis of complex oxides. In the synthesis of BaTiO 3 , it has been observed that nonstoichiometry of TiO 2 is responsible for the enhanced coupling of microwave. 20 Also, in the synthesis of PZT it has been reported that use of stabilized ZrO 2 and reduced TiO 2 not only enhances the kinetics of for- mation of PZT but changes the mechanism also. 15 The reduc- tion in processing time and temperature is believed to be due to higher diffusion rates induced by a microwave field. In the present, work fully cubic calcium-stabilized ZrO 2 of composition Ca 0.1 Zr 0.9 O 1.9 has been synthesized for the first time using microwave energy at a temperature of 11001C from a precursor prepared by the mixed oxide method taking calcium carbonate (CaCO 3 ) and monoclinic ZrO 2 in their stoichiometric ratio. The microwave radiation was allowed to interact with a polymeric susceptor, which helped in heating the sample to a temperature, where it started absorbing the microwave radi- ation. In order to optimize the minimum temperature of for- mation of cubic ZrO 2 , reaction kinetics were studied. Also, the effects of temperature on density were studied. II. Experimental Procedure The multimode microwave system used in this study was a modified domestic microwave oven of 2.45 GHz and maximum 1.2 kW power output. The microwave furnace was fabricated in the lab [Lab report]. The precursors of calcium-stabilized ZrO 2 (Ca x Zr 1Àx O 2Àx ) of compositions x 5 0.10 and x 5 0.08 were D. Agarwal—contributing editor w Author to whom correspondence should be addressed. e-mail: anips123@rediffmail. com Manuscript No. 21923. Received June 21, 2006; approved September 22, 2006. J ournal J. Am. Ceram. Soc., 90 [3] 789–796 (2007) DOI: 10.1111/j.1551-2916.2006.01379.x r 2007 The American Ceramic Society 789