The synthesis of single-phase materials exhibiting a controlled, isotropic positive or negative thermal expansion

Abstract

Thermal expansion is a fundamental physical property of a material and it is considered crucial for some high precision applications. The aim of this project is to synthesise and characterise new single-phase materials which exhibit controlled thermal expansion or zero thermal expansion (ZTE). This idea originated from the fact that in the last decade several materials with negative thermal expansion (NTE) were synthesised. Recent publications showed that this property can be altered through chemical and physical modifications, in such a way that the coefficient of thermal expansion (CTE) is tuned from negative to zero to positive. The projects discussed in this work involved both organic and inorganic frameworks. The metal organic frameworks HKUST-1 and UiO-66 were used, as these are known to contract upon heating. These frameworks were subjected to methods that hinder contracting mechanisms, such as the introduction of guest molecules and functionalized chains and also the introduction of defects. In the case of inorganic frameworks, the family of Prussian blue analogues was chosen. Here a methodology was constructed which involved doping of hexacyanoferrates(III) which exhibit negative thermal expansion with first row transition metals, in an attempt to tune the coefficient of thermal expansion. Powder X-ray diffraction data conducted at room temperature showed structural changes within both metal organic frameworks. This was followed by variable temperature powder X-ray diffraction experiments which concluded that the coefficient of thermal expansion did indeed change. This was observed when toluene and n-pentane were added as guest molecules, when 1,2,4,5-benzene tetracarboxylic was added as a functionalised linker and via introduction of defects using acetic acid, formic acid, oxalic acid and isophthalic acid. In some samples the magnitude of negative thermal expansion was enhanced, in others near-zero thermal expansion was reached, whilst in some samples even positive thermal expansion (PTE) occurred. Similar tuning in the coefficient of thermal expansion was observed in one of the series of doped hexacyanoferrates(III) which involved the Cu/Co metal cations.