Dr Victor Kitso Manisa

This paper explores the processing of an alumina matrix composite with a percolating network of graphene oxide (GPO), which exhibits a moderate electric resistivity and a near zero temperature coefficient of resistance. Different formulations of GPO–alumina composites were processed using a water–base blending, and, the pellets were densified by pressureless sintering under Argon flow. Electrical conduction at room temperature was achieved in the 2 wt % GPO–alumina composite sintered at 1400 °C, and, the 3 wt % GPO–alumina composites sintered at 1400, 1550 and 1700 °C. An investigation of the degradation of electrical conductivity was used to identify potential stable operating regimes in which these materials could be used as heaters. Thermogravimetric analysis using the Ozawa–Flynn–Wall method, was used to determine the kinetic parameters of a 3 wt % GPO composite sintered at 1400 °C which, had an activation energy for GPO degradation of 195 ± 68 kJ/mol and, an estimated thermal lifetime of 8.7 ± 0.8 years for a conversion of 0.5 wt % (failure criterion) at an application temperature of 340 °C.

The current material used as Dyson space heater is BaTiO3 (BT) (operating temperature ~ 200 °C). BT has a temperature dependent electric resistance behaviour, which presents challenges for temperature control in a heater. The engineering challenge in this work was therefore to design a material with temperature independent electrical conduction: near zero temperature coefficient of resistivity (TCR). Different formulations of multiwall carbon nanotube–alumina (MWCNT–Al2O3) and graphene oxide–alumina (GPO–Al2O3) composites were prepared. Electrical conduction at room temperature was achieved in some of the composites due to percolation of the conducting phase. An investigation of the degradation of electrical conductivity was used to identify potential stable operating regimes in which these materials could be used as heaters. Thermogravimetric analysis (TGA) using the Ozawa–Flynn–Wall method and thermal ageing studies, were used to determine the kinetic parameters of a pressureless sintered 2wt% MWCNT–1550 °C composite, and a 3wt% GPO–1400 °C composite. TGA on the 2wt% MWCNT–1550 °C composite resulted in an activation energy for the decomposition of the MWCNT as 182 ± 19 kJ/mol, with an estimated lifetime of ~3.0 ± 0.1 years when cycling at 340 °C with the failure of the heater defined as a 0.5 % loss in MWCNT mass in the composite. The 3wt% GPO–1400 °C composite had an activation energy for GPO degradation of 195 ± 68 kJ/mol and, an estimated thermal lifetime of 8.7 ± 0.8 years for similar parameters. The testing of samples as heaters (power output), and comparison of thermal conductivities of the samples showed that the 2wt% MWCNT–1550 °C and the 2wt% GPO–1400°C–ox composites performed better than the BT samples. The near zero TCR, thermal stability and power output results achieved, are key towards use of these composite materials as space heaters.