The procedure of this method was described by Chung (1974).
The error
margin of this method is approximately 10%.
Morphological and microchemical analyses were
carried out using a LEO VP-1431 scanning electron
microscope with an energy dispersive X-ray spectrometer
(SEM-EDX). Before the SEM analysis, samples were coated
with a thin film (thickness: 25 nm) of gold using a sputter
coater to make the sample conductive.