The complex relative permittivity ε'_r - jε"_(req) of water in equilibrium with its vapour is measured from room temperature to 280°C at a frequency of 2.426 GHz. The water sample, sealed in a thick-walled borosilicate capillary tube, is heated to a preset temperature and is rapidly introduced in a rectangular TE_(103) resonant cavity. The change in resonant frequency and Q-factor are measured and are used to compute the real and imaginary parts of the complex permittivity of water. Results show a monotonic decrease of ε'_r with temperature. More interestingly, ε"_(req) initially decreases with rising temperature, reaching a minimum at 220°C before increasing gradually with temperature. We attribute these effects to a combination of a weakening of the hydrogen bonding network and to enhanced ionic dissociation of water molecules at high temperatures.
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