Chemical control of thermal expansion in zeolites with the LTA topology

Carey, Thomas (2013). Chemical control of thermal expansion in zeolites with the LTA topology. University of Birmingham. Ph.D.

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Abstract

Zeolites are microporous materials that have been commonly shown to exhibit remarkable negative thermal expansion (NTE) behaviour in their purely siliceous forms, with reported thermal expansion coefficients ranging from −3 x 10–\(_6\) K–\(_1\) to −26.1 x 10–\(_6\) K–\(_1\). In contrast, very little research has been reported on the aluminium-containing structures which are widely used for various commercial applications. Compounds exhibiting this property, which has only been observed in a small number of solids, are of considerable technological interest as their inclusion in devices or composite materials can counterbalance the more usual expansion on heating and contraction on cooling, thereby reducing the incidence of thermally induced failures. Here, we report an investigation into the effect that changing the chemical composition of the zeolite framework and intrapore species has on thermal expansion properties of zeolites with the LTA topology. Variable-temperature powder X−ray diffraction studies were used to determine the thermal expansion coefficients of the chemically modified zeolites over a sub-ambient temperature range and investigate the structural basis behind their thermal behaviour. Dramatic changes in the thermal expansion behaviour (from strong negative to weak positive) of the zeolites were observed as the structures were modified through ion-exchange, dehydration, varying the Al content in the framework and loading the pores with silver nitrate. The zeolitic pores contents have been shown to play a key role in the manner in which LTA-zeolites react to temperature variation, especially in the case of intra-porous water molecules. Detailed atomistic structural mechanisms behind the observed NTE behaviour have been produced for the more simplistic systems. Several key breakthroughs have also been achieved in understanding the formation of the superlattice when silver nitrate is incorporated into the zeolite pores and with regards to solving the unit cell structure.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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