Noncontact determination of the thermal conductivity of fibres

Because of the small size and the fragility of ceramic fibres, noncontact methods are highly advantageous for their thermal characterisation. The development of such a noncontact method for the direct measurement of the thermal conductivity of fibres and wires is described. The presented measurement method uses a high-power laser diode emitting in the visible spectrum as a line-shaped heat source and a high resolution infrared camera as a spatial temperature sensor. The latter detects the stationary temperature profile adjacent to the locally heated fibres as a result of heat losses by radiation and convection. An evaluation of this temperature allows the derivation of the thermal conductivity in the direction of the fibre axis. Variation of the heating power and of the sample geometry by investigating single fibres or bundles of fibres can be used to check the consistency of the results and to improve accuracy. The noncontact method was applied to fibre samples with diameters in the range of tens of micrometres at ambient temperatures. Tests were performed with glass fibres of known thermal conductivity. Hereby a light-absorbing layer was deposited on the samples by spraying. The method was also applied to ceramic fibres of the Si/B/N/C system. In this case there was no need for any coatings because of the small reflectivity in the infrared spectrum. A thermal conductivity of (1.1 +/- 0.3) W m(-1) K-1 was obtained along the axis of the Si/B/N/C fibres. The presented measurement method, which is also applicable to foils and plates, stresses simplicity of preparation and wide applicability rather than accuracy. However, it offers the possibility of being easily extended to dynamic measurement procedures and different heater and sample geometries, which leads to a reduction of the uncertainties in the determined thermal conductivity.