Synthesis of Mg±Al spinel powder via precipitation using ammonium bicarbonate as the precipitant Ji-Guang Li *, Takayasu Ikegami, Jong-Heun Lee, Toshiyuki Mori, Yoshiyuki Yajima National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan Received 27 April 2000; received in revised form 23 June 2000; accepted 1 July 2000 Abstract A precursor for Mg±Al spinel has been synthesized via the precipitation method, using ammonium bicarbonate as the precipitant. The precursor was composed of crystalline ammonium dawsonite hydrate [NH 4 Al(OH) 2 CO 3 . H 2 O] and hydrotalcite [Mg 6 Al 2 (CO 3 )(OH) 16 . 4H 2 O] phases. The precursor converted to pure spinel phase at 900  C via two steps upon calcination: (i) decomposi- tion of hydrotalcite at lower temperatures (400±800  C) and (ii) solid-state reaction between MgO (decomposed from hydrotalcite) and g-Al 2 O 3 (derived from NH 4 Al(OH) 2 CO 3 . H 2 O) at higher temperatures (>800  C). The eect of calcination temperature on par- ticle morphology and sinterability of the resultant spinel powders were investigated. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Calcination; MgAl 2 O 4 ; Powders±chemical preparation; Sintering; Spinels 1. Introduction Magnesium aluminate spinel (MgAl 2 O 4 ) is an impor- tant ceramic material considering its high melting point (2135  C), high resistance against chemical attack, 1 good mechanical strength both at room temperature and ele- vated temperatures, 2,3 low dielectric constant, 4 and excellent optical properties. 5 Dense spinel ceramics could potentially ®nd frequent applications in diverse engineering ®elds. However, since the spinellization reaction is accompanied by 5% volume expansion, 6 dense spinel ceramics of stoichiometric composition with high-purity characteristics are dicult to fabricate directly from mixtures of individual Al 2 O 3 and MgO powders via solid-state reaction using the conventional pressureless-sintering technique. In practice, dense materi- als are produced by a two-stage ®ring process: calcining the powder mixture at 1600  C to complete the spinelli- zation reaction, followed by ball milling and re®ring at even higher temperatures. Synthesis of highly reactive, phase-pure spinel powder seems to be a crucial step to the successful fabrication of dense ceramic materials at reasonable sintering temperatures. It is well accepted that the wet-chemical processing of multi-cation oxides provides considerable advantages of good mixing of the starting materials and excellent chemical homogeneity of the ®nal product. In recent years, several types of wet-chemical techniques or wet-chemical related techniques have been developed and successfully used for the production of pure spinel powders at relatively low temperatures. These methods include hydroxide coprecipitation, 7 9 sol±gel of metal alkoxides or inorganic salts, 10 14 spray-drying, 15,16 freeze-drying, 17,18 modi®ed Pechini process, 19 ¯ame spray pyrolysis, 20 and combustion synthesis. 21 The above methods have their own respective advantages and disadvantages. For spray-drying (pyrolysis), freeze- drying, modi®ed Pechini process and combustion synth- esis, chemical composition (Mg/Al molar ratio) of the starting salt solution was easily kept to the ®nal pro- duct. Unfortunately, the resultant spinel powders gen- erally do not show desirable sinterability. Suyama and Kato 15 reported that the spinel powder prepared by the spray pyrolysis technique only densi®ed to 93% of the theoretical density after pressureless sintering at 1590  C for 2 h. Spinel powders synthesized by sol±gel of metal alkoxides are usually characterized by high reactivity and less agglomeration. 10 12 However, metal alkoxides are expensive and most of the solvents for them are toxic. Besides, special measures are required during manip- ulating alkoxide materials due to their high sensitivity to 0955-2219/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00188-6 Journal of the European Ceramic Society 21 (2001) 139±148 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +81-0298-51-3354; ext.2247; fax: +81- 0298-52-7449. E-mail address: jgli@nirim.go.jp (J.-G. Li,).