Agar-Based Aqueous Gel Casting of Barium Titanate Ceramics

Abstract

Agar was used for gel casting barium titanate (BaTiO3) ceramics. Aqueous suspensions of bimodal BaTiO3 powders, with 500 and 100 nm nominal particle sizes, were prepared with a 85:15 coarse:fine ratio. BaTiO3 suspensions were mixed with agar solutions from 2 to 8 wt%, with 0.5 or 1.0 wt% agar (as a function of BaTiO3 solids content) in the final gel‐casting suspension. BaTiO3 green densities up to ∼53.2% of theoretical were obtained. Gel casting of high aspect ratio features (up to 7:1) could be routinely performed. Cast bodies were also amenable to computer‐numerical‐controlled based green machining.

Biopolymer‐Based Gel Casting of Ferroelectric Ceramics

“…This process is invariably thermo‐reversible. Figure a demonstrates a typical heating/cooling response with respect to the viscosity of agar/H 2 O mixtures . On heating, there is an initial viscosity increase as the agar powder starts to dissolve.…”

“…There is a further increase in viscosity upon cooling and, finally, a steeper increase upon gelation. The oscillatory response for a 4 wt% agar solution, during cooling, is shown in Figure b . Here, the viscoelastic characteristics are highlighted by the relative values of G ′ and G ″.…”

“…Gel casting investigations have looked at factors such as the agar concentration and the ceramic powder solids content. Agar and agarose have also been used for gel casting porcelains, porous cordierite, silicon nitride, and BaTiO 3 ceramics . Millan et al have compared a range of polysaccharide systems, notably agar, agarose and carageenan.…”

“…With that in mind, Santacruz et al utilized an overpressure technique to generate high‐concentration agar gels, such that the final polymer/water content of the cast suspensions could be reduced while maintaining a suitable agar concentration. This approach was subsequently combined with the use of multi‐modal ceramic powders to obtain high‐suspension solids loadings while maintaining relatively low viscosity . Using this later approach, gel cast components could be readily green machined following casting and drying.…”

“…Polysaccharide and protein gelling systems are outlined, paying attention to the ceramics that have also been processed using such approaches. Our recent investigations of the colloidal characterization and forming of barium titanate (BaTiO 3 ) ferroelectric ceramics are also reviewed in this context, as a case study of colloidal processing and gel casting . The use of blended, nominally bimodal particle sizes was investigated to lower suspension viscosities while maintaining a high‐solids content.…”