Mechanochemical fabrication of single phase PMN of perovskite structure

Abstract

Single phase lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), of high sintered density has been successfully prepared via a novel mechanochemical fabrication route. Nanocrystallites of perovskite PMN are triggered to form in a highly activated oxide composition consisting of PbO, MgO and Nb₂O₅. Unlike in the solid state reaction of constituent oxides at high temperatures, intermediate pyrochlore phases were not observed in the activated oxide matrix prior to the formation of perovskite PMN phase. A single phase PMN powder of perovskite structure, which consists of more or less spherical particles of 20–30 nm in size together with a minimized degree of particle agglomeration, was obtained when the constituent oxides were mechanically activated for 20 h. It was sintered to a density of ∼99% theoretical density at 1050°C for 1 h. Sintered PMN exhibits a peak dielectric constant of 18 083 at the Curie temperature at −11°C when measured at a frequency of 100 Hz.

Introduction

Relaxor type ferroelectric lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), exhibits excellent dielectric and electrostrictive properties, thus making it promising for several technologically demanding applications in electronics and microelectronics [1]. One of the major problems facing the applications of PMN and PMN-based electroceramics is the difficulty in producing a single-phase material of perovskite structure. Traditional solid state reaction using mixed oxides as the starting materials always results in the formation of one or more lead niobate-based pyrochlore phases [2], which exhibit a low dielectric constant (∼130) [3]. It is commonly believed that the presence of such a pyrochlore phase significantly degrades the dielectric properties of PMN-based ceramics.

There are at least three other types of very different fabrication routes that can be utilized to fabricate PMN and PMN-based relaxors. (i) Columbite method [4] which is a modification of the conventional mixed oxide method, whereby the constituent MgO and Nb₂O₅ are first mixed and reacted together to form the columbite phase (MgNb₂O₆), prior to mixing and reacting with PbO in the second step of calcination at an elevated temperature. It has been dominating the fabrication of PMN and PMN-based relaxor ferroelectrics for the past 15 years since its discovery in 1982, and it is the most reliable processing route for delivering a PMN of predominant perovskite phase. (ii) Wet chemistry routes, where a PMN precursor is first synthesized via one of the many wet chemistry-based routes, such as co-precipitation [5], sol–gel routes [6], citrate [7], hydrothermal [8] and partial oxalate methods [9]. Perovskite PMN phase is then formed by calcining the resulting chemical precursor at an intermediate temperature, prior to shape forming and sintering at a high temperature. Most of these wet chemistry-based processing routes use high purity inorganic or organometallic chemicals, such as nitrates, chlorides and alkoxides, as the starting materials. They are many times more expensive than the widely available ceramic oxide powders and many of these chemicals are also highly sensitive to moisture and therefore are difficult to handle in a large production scale for industrial applications. Furthermore, almost all these wet chemistry-based processing routes are very low in production yield and many of them have yet demonstrated any significant advantages over the conventional ceramic route. (iii) Molten salt method [10], [11], in which the required perovskite PMN phase is synthesized by reacting the constituent oxides in a molten medium of low melting point, such as KCl, NaCl and PbO. It is however not suitable for producing a high-purity PMN, as a result of the contamination by either the flux or excess PbO required for forming the perovskite phase [12].

Mechanical alloying was devised for synthesizing novel metals and alloy compounds [13]. Mechanochemical synthesis and mechanical activation have been utilized to prepare nanocrystalline powders, nanostructured materials and ferrite-based magnetic materials [14], [15], [16], [17]. The solid state reactions involved in these novel processes are triggered by mechanical activation, instead of calcination at high temperatures. Mechanically activated processes have also been applied to prepare several functional ceramic powders including oxides and non-oxides, such as ZrO₂ [18], PbTiO₃ [19], [20] and BaTiO₃ [21], although in many cases, they are not very successful. For most of the ceramic materials investigated, the reactivity of starting materials can be improved significantly by a mechanochemical treatment, and therefore the necessary calcination for forming the designed ceramic phase is completed at a lowered temperature. For example, when the mixture of PbO, TiO₂, Nb₂O₅ and Mg(OH)₂ was subjected to 60 min of mechanochemical treatment in a multi-ring mill, only a minor amount of perovskite 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) phase was observed [22]. The formation of perovskite phase took place when the milled powder was calcined and pyrochlore phase was observed prior to the formation of 0.9PMN–0.1PT phase. A single phase 0.9PMN–0.1PT powder required a calcination temperature of 850°C. It is apparent that the mechanochemical treatment was able to enhance the reactivity of constituent oxides and hydroxide. However, it did not result in the completion of formation of the desired ceramic phase, and further heat treatment could not be avoided. So far, nobody has demonstrated that the multicomponent compound PMN can be synthesized directly by a mechanical reaction among mixed oxides without further thermal treatment at an elevated temperature. It was even suggested that this was impossible as a result of the high positive enthalpy of formation involved [23].

In this work, a novel mechanochemical technique for synthesizing fine PMN powders is described. It starts with the low-cost, widely available oxide powders as the starting materials, and the perovskite phase of PMN is triggered to form in a mechanical activation chamber, instead of by calcination at an elevated temperature. The resulting PMN powders are then studied for powder characteristics, sintering behavior and dielectric properties.