Low-to-negative thermal expansion is an important property for materials in a wide range of applications including space applications, microwave components, construction, mirrors, dental fillings and high precision ion dilatometers. Powder X-ray diffraction (PXRD) has been used to investigate the thermoresponsive behavior of Nb6.7W10.3O47, Ta14.8W1.2O46, Ta16W18O94 and BaPbO3.
Calibration and benchmarking of the diffractometer was performed to ensure accurate results are obtained. Continuous Online X-ray Analysis (COXA) enabled the close monitoring of phase transitions, changes in any lattice parameters in response to thermal stimuli. When COXA is coupled with Rietveld refinement, coefficients of thermal expansion (CTE) could be determined accurately.
The tungstates were synthesized using a precipitation reaction and heat treatment from chlorides of the metals (NbCl5; TaCl5) and tungstic acid (H2WO4). The products, Nb6.7W10.3O47, Ta14.8W1.2O46 and Ta16W18O94, underwent a COXA analysis within the temperature range 30 ˚C to 900 ˚C.
Nb6.7W10.3O47 has a mean volumetric CTE of 6.704 x 10-6 ˚C-1, and therefore has low positive thermal expansion. The tantalum tungstates were found to have low positive thermal expansion as well. Ta14.8W1.2O46 and Ta16W18O94 have mean volumetric CTEs of 2.101 x 10-5 ˚C-1 and 9.851 x 10-6 ˚C-1.
BaPbO3 underwent two phase transitions between 30 ˚C and 550 ˚C from orthorhombic Imma to I4/mcm to cubic Pm-3m. The transition from Imma to I4/mcm took place at 270 ˚C. The phase transition from I4/mcm to Pm-3m took place at 390 ˚C.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/13946 |
| Date | 25 February 2014 |
| Creators | Nyoni, Michelle Sibonokuhle |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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