Freeze casting for near‐net‐shaping of dense zirconium diboride ceramics

Abstract

Zirconium diboride (ZrB2) was formed into dense complex shapes using freeze casting as a near‐net‐shaping technique. Aqueous‐based formulations were compared with nonaqueous (cyclohexane) based formulations in terms of rheological behavior, particle packing in the green body, sintered density, macroscale porosity, and cracking. The influence of particle solids concentration and freezing rate was investigated. The aqueous formulations were found to be deficient in that they produced macroscale porosity that could not be eliminated during sintering resulting in low density and large pores in the final shaped objects. The nonaqueous‐based system was able to produce complex shaped objects with significantly reduced macroscale porosity. The higher concentration of solids in the nonaqueous‐based formulations was primarily responsible for the reduced macroscale porosity and enabled higher sintered densities (up to 90%‐91.5% of theoretical density for fast freezing). The microstructure of the ZrB2 formed at fast freezing rates and high solids content typically had isolated pores in the order of 5‐10 μm in size, mainly found along grain boundaries (grain sizes between 20 and 50 μm). Although this rapid freezing produced denser components, it tended to produce objects with internal cracks. When slower freezing rates were used, intricate complex shaped objects could be produced without cracks but their density was only between 65% and 80% of theoretical density.

Direct ink writing of hierarchical porous alumina‐stabilized emulsions: Rheology and printability

“…Other pore formation processes include induced phase separation; for instance, ice templating, in which the freezing process creates directional dendritic pore networks after sublimation 9 . The prevalence of crack‐like defects, 10 however, remains a challenge.…”

Particle Packing Theory Guided Thermal Conductive Polymer Preparation and Related Properties

“…Rheological analysis is taken as an effective method to evaluate the processing properties of polymer. 50 Generally, when the particle size of the fillers in the system is properly proportioned, the spheres can cooperate with each other to roll. 51 The better fluidity of the system, the smaller viscosity.…”

Cracking during freeze‐drying in dense freeze cast ceramics: Role of drying rate

“…However, the examination of some objects produced by this dense freeze-casting process has revealed the presence of significant defects, specifically, numerous large-scale internal cracks. 18 Critically, the causes behind these cracks have not been reported to date. In this study, the influence of freezing and freeze-drying rates on the internal cracking is investigated.…”

“…Even though freeze casting inherently results in pore formation, by using a modified freeze‐casting process with appropriate conditions, it has been possible to produce near‐dense components 15–22 . First, by maximizing the solids loading of ceramic in suspension, the resulting total porosity can be minimized 1,17 .…”

“…1,14 Even though freeze casting inherently results in pore formation, by using a modified freeze-casting process with appropriate conditions, it has been possible to produce near-dense components. [15][16][17][18][19][20][21][22] First, by maximizing the solids loading of ceramic in suspension, the resulting total porosity can be minimized. 1,17 Second, by applying high solidification rate via flash freezing, the individual pore size can also be minimized.…”