Impact of Slurry Composition on Properties of Cellular Alumina: A Computed Tomographic Study

Abstract

Fine-pored, 45 ppi (pores per linear inch) alumina foams are prepared from ceramic slurries with varying contents of additives (deflocculant, binder) and solid loading following a standardized procedure. Rheological key parameters (yield stress, high-shear viscosity) of the respective slurries are determined by approximation of the experimental flow curves with appropriate rheological models. The resulting ceramic foams are characterized by computed tomography (CT) followed by a morphometric analysis of the reconstruction volume data. The main scope of the work involves the development of a procedure to reliably define the binarization threshold during these morphometric calculations, which is based on the analysis of the differential course of the total porosity results from calculations performed at varying binarization threshold values ("differential thresholding"). A very good match of the CT porosity results with experimental data is achieved, despite the unfavorable CT voxel resolution to foam structure fineness relation. The CT evaluation results are finally correlated to the rheological properties of the respective slurries used in foam manufacturing. The dominant slurry composition parameters are the weight fraction of the ceramic powder and the binder concentration. Increasing binder and solid content result in an increased yield stress and viscosity of the respective dispersion and consequently in a decreased porosity and cell size of the finally manufactured cellular ceramic.

Hierarchical‐Porous Ceramic Foams by a Combination of Replica and Freeze Technique

“…For preparing the slurries solid loadings of 20 − 40 vol% were chosen, which is relative low compared to standard slurries, used for the replica technique (e. g., 47 vol% alumina). In this case, high solid loadings are preferred, as a higher mass is to be applied on the template foams, and a more uniform coating and therefore higher mechanical strengths are the result due to the characteristic shear‐thinning behavior and the presence of a distinct shear stress . However, low solid loadings are necessary to achieve a higher porosity and different pore sizes inside of the foam struts as a result of the freeze‐drying technique.…”

“…The pore size and structure thickness distributions were calculated using the software package CTAnalyzer 1.17.7.2 (Skyscan/ Bruker microCT, Kontich, Belgium). The CTAnalyzer data import and binarization steps preceding the actual morphological calculations were performed using the differential thresholding technique . After the binarization threshold was determined, the 3D analysis was conducted with the CTAnalyzer to calculate the different geometric parameters relating to the foam's microstructure.…”

Reticulated Replica Ceramic Foams: Processing, Functionalization, and Characterization

“…These should be omitted if a parallel plate measuring system with a smooth plate surface has been used. [21] Another model, combining both, the yield point and the shear thinning behavior, is the BinghamPapanastasiou model, which has been essentially developed for the numerical simulation of flow properties of fluids and may be also appropriate for the description of the rheological behavior of a ceramic slip. [22] Essential for a ceramic slurry characterization are its yield point and its viscosity at higher shear rates starting at about 100 s À1 .…”

“…[24] The latter mentioned directly influences the total porosity and the strut thickness of the resulting cellular structures. [21,25] Nevertheless, a variety of papers dealing with foam preparation do not report the rheological parameters (completely) or the viscosity behavior is just plotted without an extraction of the parameters. For example, in ref.…”

“…Taking the wall slip effect into account, viscosity measurements can be described by the Herschel-Bulkley model, as has been recently demonstrated for a alumina slurry system with different grain size distribution. [21] By varying the concentration of the binder (PVA), the yield stress can be adjusted and the influence on foam's relative density, the cell size distribution, and the specific strut surface is obvious especially for a high solid loading with alumina.…”