S. Lampenscherf et al., Stress development due to capillary condensation in powder compacts: A two-dimensional model study, J AM CERAM, 83(6), 2000, pp. 1333-1340
A model experiment is presented to investigate the relationship between the
humidity-dependent liquid distribution and the macroscopic stress in a par
tially wet powder compact. Therefore, films of monosized spherical particle
s were cast on silicon substrates. Using environmental SEM the geometry of
the liquid necks trapped between particles was imaged as a function of rela
tive humidity. Simultaneously the macroscopic stress in the substrate adher
ed particle film was measured by capacitive deflection measurement. The exp
erimentally found humidity dependence of the liquid neck size and the macro
scopic film stress are compared with model predictions. The circle-circle a
pproximation is used to predict the size of the liquid necks between touchi
ng particles as a function of the capillary pressure. Using the modified Ke
lvin relation between capillary pressure and relative humidity, we consider
the effect of an additional solute which may be present in the capillary l
iquid. The results of the stress measurement are compared with the model pr
edictions for a film of touching particles in hexagonal symmetry. The contr
ibution of the capillary interaction to the adhesion force between neighbor
ing particles is calculated using the integrated Laplace equation. The resu
lting film stress can be approximated relating this capillary force to an e
ffective cross section per particle. The experimentally found humidity depe
ndence of the liquid neck size is in good agreement with the model predicti
ons for finite solute concentration. The film stress corresponds to the mod
el predictions only for large relative humidities and shows an unexpected i
ncrease at small values. As is shown with an atomic force microscope, the r
eal structure of the particle-particle contact area changes during the wet/
dry cycle. A solution/reprecipitation process causes surface heterogeneitie
s and solid bridging between the particles. It is claimed that the existenc
e of a finite contact zone between the particles gives rise to the unexpect
ed increase of the stress at small relative humidities.