Effect of Zr 4+ Dopant on Microstructure, Densification and Thermomechanical Behaviour of ZnAl 2 O 4 Spinel
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Effect of Zr4+ Dopant on Microstructure, Densification and Thermomechanical Behaviour of ZnAl2O4 Spinel P. K. Haldar1, T.K. Parya2, S. Mukhopadhyay1 Abstract: ZnAl2O 4 spinel precursor powders (mole ratio ZnO : Al2O3 = 1 : 1) doped with 0.5–2.0 mass-% Zr4+ ions were synthesized by the coprecipitation route. The derived powders were uniaxially pressed into cylindrical pellets and sintered at 1350 °C for 1 h. The sintering behaviour, phase assemblage and microstructure of fired ZnAl2O 4 ceramics doped with different concentrations of Zr4+ were investigated. Sintered density above 97 % of the theoretical density of pure ZnAl2O 4 ceramics was achieved with the addition of 2.0 % Zr4+ . The residual flexural strength of the sintered body was as high as 120 kg.cm -2 for the highest addition of Zr4+, and it was higher than that of the Ti4+ doped ZnAl2O 4. The powder XRD patterns of sintered compacts revealed that besides ZnAl2O 4 spinel as the major crystalline phase, a minor amount of tetragonal ZrO2 phase is also present when the dopant quantity is higher. The SEM micrographs of the fractured surfaces of the sintered body demonstrated a coherent and uniform microstructure with a homogeneous distribution of microfine grains with Zr4+-doped ZnAl2O 4 compacts. Keywords: ZnAl2O 4 spinel, densification, residual flexural strength, XRD, microstructure
1. Introduction ZnAl2O4 belongs to the normal spinel family in which Zn2+ ions occupy the divalent tetrahedral interstices and trivalent Al3+ ions occupy the octahedral interstices in the FCC lattice of oxygen ions. Nowadays, ZnAl 2O4 ceramic is attracting the attention of scientists because of its extensive multifunctional applications, such as a catalyst carrier for metals, as a catalyst in dehydration of saturated alcohols to olefins, in alkylation, double-bond isomerization, hydrogenation and hydroxylation, as a potential reservoir and carrier for drug delivery, coating materials, sintering aids, sensors in electronic devices, etc. [1–10]. This is primarily due to its high mechanical resistance, high chemical durability and good thermal stability. Moreover, ZnAl 2O4 functions as a potential semiconductor suitable for ultraviolet (UV) and optoelectronic applications due to its wide optical bandgap property [11]. The traditional method of production of zinc aluminate spinel involves the solid-state
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reaction between pure zinc oxide and pure alumina as starting materials sintered at high temperatures around 1650 °C [12,13]. The main drawback of this route is the lack of homogeneity, non-stoichiometry in composition, variation of crystallographic nature and the non-uniform microstructural features of the product. There are other potential methods for the preparation of ZnAl2O4 precursors such as the co-precipitation method, modified citrate method, microwave-assisted hydrothermal method, metal-chitosan complexation method, hydrothermal method, sol–gel method, polymeric precursor method and thermolysis of organic precursors [14–21]. Recently, zinc aluminate spinel ceramics do
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