Monotonic Form Research Articles

An electron microscope study of evaporating gold particles: the Kelvin equation for liquid gold and the lowering of the melting point of solid gold particles

An electron microscope has been used to observe the evaporation of small ( < 50 nm radius) gold particles resting on heated carbon substrates in a vacuum of 2.7 x 10 -4 Pa. Evaporation curves for liquid gold particles are found to agree with the predictions of Kelvin’s theory which relates vapour pressure to surface curvature and, at low evaporation rates, the evaporation curves for solid gold particles also have the expected monotonic form. However, with solid gold particles evaporating in the temperature range 1160 to 1330 K, evaporation curves have been obtained showing a characteristic ‘kinked’ nature; that is, on each curve obtained, there appears to be two sections consistent with two different rates of evaporation. These non-monotonic curves have been interpreted as being caused by the melting of the slowly evaporating solid gold particles into liquid—the difference in evaporation coefficient between solid and liquid gold giving the observed sudden change in evaporation rate. From these results a plot of particle radius against melting temperature has been obtained.