Synthesis of low expansion ceramics in lithia–alumina–silica system with zirconia additive using the powder precursor in the form of hydroxyhydrogel

Abstract

Powder precursors lithium aluminosilicate (LAS) of low expansion characteristics were synthesized by following the wet interaction technique in the hydroxyhydrogel form using lithium carbonate (Li₂CO₃), hydrated aluminium nitrate [Al(NO₃)₃·9H₂O], fume silica (SiO₂) and zirconium oxychloride (ZrOCl₂·8H₂O). Dilatometry, X-ray diffraction (XRD), IR analysis, Scanning Electron Microscopy (SEM) and Image analysis were done to study sintering, characteristics phase transformation and microstructure on the sintered specimens. ZrO₂ was found to promote the formation of β-eucryptite, and grain size of the sintered specimens could be related to the amount of ZrO₂ present in the starting powder precursor. The thermal expansion characteristics were also found to be strongly influenced by ZrO₂ content in the specimens.

Introduction

Low expansion ceramics are used in electronic devices, cookware, in precision parts of heat engines components and spark plugs [1], [2] and also have been widely used in the glass and ceramic industry as lithia-bearing fluxes and low-expansion fillers in white-ware bodies [3], [4]. It has been known that thermal shock resistance of ceramics is influenced by the thermal expansion coefficient especially when the rate of heat transfer of the ceramics in question is large. The interesting properties of such materials are mainly attributed to their crystal structures, which consists of flexible and strongly bonded three-dimensional networks developing a rigid stable hexagonal lattice containing structural holes or interstices which can be partially or fully occupied by several ions such as Na⁺, Li⁺, Ca⁺⁺, Sr⁺⁺, and Ba⁺⁺ [5].

The important crystalline phases present in Li₂O–Al₂O₃–SiO₂ system include β-eucryptite (Li₂O·Al₂O₃·2SiO₂), β-spodumene (Li₂O·Al₂O₃·4SiO₂), virgilite (Li₂O·Al₂O₃·6SiO₂), petalite (Li₂O·Al₂O₃·8SiO₂) as well as metastable solid solutions [6] that are derived from the hexagonal high quartz structures by the substitution of Al³⁺ and Li⁺ for Si⁴⁺. These solid solutions are denoted as β-quartz (ss) and have the general compositions Li₂O·Al₂O₃·nSiO₂, where n varies from 2 to 10 [7], [8]. Table 1 [9], [10], [11], [12], [13] shows the list of materials based on lithium aluminosilicate (LAS) family known to have negative to zero to positive coefficient of thermal expansion (α) over a temperature range.

Yang et al. [6] reported that on heating the lithium aluminosilicate gel monoliths derived by sol–gel technique, β-eucryptite crystals were first precipitated at around 750 °C, followed by precipitation of β-spodumene crystals at 830 °C. He observed that at higher temperatures the latter grew at the expense of the former phase. The crystallized specimens exhibited very low thermal expansion coefficient ranging from −13 to +12 × 10⁻⁷/°C at temperatures from room temperature to 700 °C depending on the heat treatment temperature of the gels. Large glass ceramic monoliths in the system Li₂O–Al₂O₃–SiO₂ was prepared by Orcel and Hench [14] with the help of drying control chemical additives (DCCA). Wang et al. [15] reported that the solid LAS₄ precursor powders in this system wet each other due to a sufficient amount of the liquid phase and hence it was possible to obtain a densified sample with the addition of LiF as sintering aids [15]. He reported that precursor powder exhibited appreciable grain growth and the grain size varied from 1.0 to 25 μm with a non-uniform distribution resulting in discontinuous grain growth. In another study [16], it was reported that the grain size grew from 8 to 25 μm with a remarkable discontinuous grain growth of β-spodumene when sintered at 1050 °C for 5 h. The coefficient of thermal expansion of the sintered bodies decreased from 8.3 to 5.2 × 10⁻⁷/°C (25–900 °C) as the LiF addition increased from 0 to 5 wt.%. The crystalline phases identified were a combination of β-eucryptite and eucryptite of different crystalline geometry. Beall [17] reported that extremely efficient nucleation was achieved in lithium aluminosilicate glasses by additions of TiO₂ and ZrO₂ in roughly equivalent molar concentrations of about 4 mol%. With this addition, it was possible to obtain a highly crystalline microstructure after heat treatment at 850 °C and the resulting glass ceramic yielded a single-phase material and had a coefficient of thermal expansion of up to 7 × 10⁻⁷/°C (0–500 °C) and thus it produced outstanding thermal shock resistance. Abdel-Fattah and Abdellah reported [18] the values of negative and positive coefficient of linear thermal expansion of three different samples, sintered at different temperatures, as (−) 14.7 × 10⁻⁷/°C (75–150 °C), (−) 8 × 10⁻⁷/°C (75–150 °C) and (−) 1.3 × 10⁻⁷/°C (75–150 °C) and (−) 23.7 × 10⁻⁷/°C (500–575 °C), (+) 24 × 10⁻⁷/°C (500–575 °C), (+) 32 × 10⁻⁷/°C (500–575 °C), respectively, prepared by solid–solid interactive method in the system LiO₂–Al₂O₃–SiO₂. Wang [19] mentioned that at 800 °C the crystalline phase comprised the major phase of β-spodumene (Li₂O·Al₂O₃·4SiO₂) and the minor phase of β-eucryptite (Li₂O·Al₂O₃·2SiO₂). After calcinations at 1000 and 1200 °C, the XRD patterns showed pronounced growth of β-spodumene. The diffraction lines of β-eucryptite disappeared at 1200 °C.

It appears from the above discussion that tailoring of composition may produce sintered materials of desired thermal expansion characteristics. This tailoring is possible either by adjusting the composition to synthesize biphasic material or the composition may be selected in such a way that a phase transition is effected during the densification producing desired phases to manifest desired thermal expansion. In all the previous works of other authors, the initial composition comprised of lithium oxide, alumina and silica that produce mostly β-eucryptite, β-spodumene and phases consisting of lithiumaluminate spinel, lithiumaluminate, corundum and combination thereof. Formation and separation of such phases requires considerable higher processing temperature and as a consequence controlling the processing parameters become more complex. In addition, the above formation and segregation of undesirable phases exerts undesirable influence on the desired properties. These difficulties or drawbacks may be overcome by taking broadly two measures namely (i) appropriate processing of right type of raw materials to make powder precursor in the form of hydroxyhydrogel [20], [21], [22], [23] which on further processing gives the desired product. This may be carried out by forming a hydroxyhydrogel type of intermediate powder precursor [24] in the system Li₂O–Al₂O₃–SiO₂; and (ii) using a nucleating material which catalyses the formation of desirable phase in the sintered composites exhibiting low thermal expansion in the desired temperature range.

The present investigation was undertaken to synthesize powder precursor of lithium aluminosilicate by following wet interaction technique [24]. The sintering characteristics of the powder precursors were characterized by the thermal analysis, IR spectroscopy and microstructural analysis. The effect of ZrO₂ on the phase formation was analysed with respect to various physicochemical parameters.