Articles written in Bulletin of Materials Science

• Sintering behaviour of spinel–alumina composites

Study of alumina–magnesia binary phase diagram reveals that around 40–50 wt% alumina dissolves in spinel (MgAl2O4) at 1600°C. Solid solubility of alumina in spinel decreases rapidly with decreasing temperature, which causes exsolution of alumina from spinel phase. Previous work of one of the authors revealed that the exsolution of alumina makes some interlocking structures in between alumina and spinel phases. In the present investigation, refractory grade calcined alumina and spinel powder were used to make different batch compositions. Green pellets, formed at a pressure of 1550 kg cm-2 were fired at different temperatures of 1500°, 1550° and 1600°C for 2 h soaking time. Bulk density, percent apparent porosity, firing shrinkage etc were measured at each temperature. Sintering results were analysed to understand the mechanism of spinel–alumina interactions. SEM study of present samples does not reveal the distinct precipitation of needle shaped 𝛼-alumina from spinel, but has some effect on densification process of spinel–alumina composites. Microstructural differences between present and previous work suggest an ample scope of further work in spinel–alumina composites.

• Sintering behaviour of spinel–alumina composites

• Function of magnesium aluminate hydrate and magnesium nitrate as MgO addition in crystal structure and grain size control of 𝛼-Al2O3 during sintering

Chemically pure reactive alumina (𝛼-Al2O3) which is commercially available was used for densification study in presence of widely accepted dopant MgO as additive. MgO was added both as spinel (MgAl2O4) forming precursor i.e. magnesium aluminate hydrate, and magnesium nitrate. Sintering investigations were conducted in the temperature range 1500–1600°C with 2 h soaking. Structural study of sintered pellets was carried out by extensive XRD analysis. Scanning electron mode SEM images of the specimens were considered to understand the effect of both types of additions. Addition of MgO within and beyond optimum amount had some effect on development of microstructure of sintered bodies. Densification, around 99% ρth, with fine grain microstructure was achieved. These different types of additions caused two distinct changes in crystal structure of alumina-one small contraction and the other expansion of unit cell parameters.

• Effect of agglomeration during coprecipitation: Delayed spinellization of magnesium aluminate hydrate

Precipitation of magnesium aluminate hydrate with faster addition of ammonia at desired pH causes agglomeration. Agglomerated powder, without any further treatment, on calcination forms intermediate compounds at low temperatures (≤ 900°C). The intermediate compounds on further heat treatment (≥ 1000°C) decompose into MgO, MgAl2O4 and 𝛼-Al2O3. Effect of agglomeration and absorption of foreign ions such as Cl, SO$^{2-}_{4}$, and NH$^{+}_{4}$ in complex compounds probably cause loss of Al3+ and Mg2+ ions during heat treatment, and stoichiometry changes. Powders prepared by continuous method with better control of process parameters than batch process yields better spinellization.

• MgAl2O4–𝛾-Al2O3 solid solution interaction: mathematical framework and phase separation of 𝛼-Al2O3 at high temperature

Although existence of MgAl2O4–𝛾-Al2O3 solid solution has been reported in the past, the detailed interactions have not been explored completely. For the first time, we report here a mathematical framework for the detailed solid solution interactions of 𝛾-Al2O3 in MgAl2O4 (spinel). To investigate the solid solubility of 𝛾-Al2O3 in MgAl2O4, Mg–Al spinel (MgO–𝑛Al2O3; 𝑛 = 1, 1.5, 3, 4.5 and an arbitrary high value 30) precursors have been heat treated at 1000°C. Presence of only non-stoichiometric MgAl2O4 phase up to 𝑛 = 4.5 at 1000°C indicates that alumina (as 𝛾-Al2O3) present beyond stoichiometry gets completely accommodated in MgAl2O4 in the form of solid solution. 𝛾 → 𝛼 alumina phase transformation and its subsequent separation from MgAl2O4 has been observed in the Mg–Al spinel powders (𝑛 &gt; 1) when the 1000°C heat treated materials are calcined at 1200°C. In the mathematical framework, unit cell of MgAl2O4 (Mg8Al16O32) has been considered for the solid solution interactions (substitution of Mg2+ ions by Al3+ ions) with 𝛾-Al2O3. It is suggested that combination of unit cells of MgAl2O4 takes part in the interactions when 𝑛 &gt; 5 (MgO–𝑛Al2O3).

• Enhancement of MgAl2O4 spinel formation from coprecipitated precursor by powder processing

Although low temperature fast coprecipitation technique has been used to synthesize stoichiometric (MgO–nAl2O3, 𝑛 = 1) MgAl2O4 spinel forming precursor, delayed spinellization has always been the concern in this process. In this article, the precursor of this ‘fast technique’ has been used for bulk production by further processing by high speed mixing with solvents and mechanical activation by attrition milling in terms of superior spinellization. At 1000°C, MgAl2O4 – 𝛾-Al2O3 solid solution and MgO phases are formed (spinel formed by 1000°C is regarded as primary spinel). At higher temperatures, due to large agglomerate size, MgO can not properly interact with the exsolved 𝛼-Al2O3 from spinel solid solution to form secondary spinel; and consequently spinellization gets affected. Solvent treatment and attrition milling of the coprecipitated precursor disintegrate the larger agglomerates into smaller size (effect is more in attrition). Then MgO comes in proper contact with exsolved alumina, and therefore total spinel formation (primary + secondary) is enhanced. Extent of spinellization, for processed calcined samples where some alumina exists as solid solution with spinel, can be determined from the percentage conversion of MgO. Analysis of the processed powders suggests that the 4 h attrited precursor is most effective in terms of nano size (&lt; 25 nm) stoichiometric spinel crystallite formation at ≤ 1100°C.

• Bulletin of Materials Science

Volume 43, 2020
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• Editorial Note on Continuous Article Publication

Posted on July 25, 2019