A laboratory scale route to produce defect-free monolithic ceramic specimens with tailored pore/grain microstructure has been developed. The target configuration consisted of isolated individual pores 0.5-1 µm size, immersed in a matrix of 200-300 nm sized grains with total porosity levels of 10 to 20 %. This type of microstructure is of special interest in the nuclear energy field because of its appearance at the periphery of Light Water Reactor UO2 fuel at high burn-up.
A commercial 4 mol% yttria stabilised ZrO2 (Y-PSZ) nanopowder with modified surface was used to replicate the structure observed in nuclear fuel. The target microstructure was achieved via a novel colloidal consolidation technique based on the gel-casting process. Polymethylmethacrylate (PMMA) microspheres, introduced as sacrificial templates for the pore formation, were co-dispersed and stabilised with the powder before consolidation.
Monolithic samples with tailored macropores could be successfully fabricated at laboratory scale. However, scale-up of this method for the production of samples with larger dimensions is limited first by the length of the drying process and secondly by the extremely delicate debinding step. Nevertheless, despite the high amount of organic mass to be removed during thermal decomposition of additives, an improved burn-out process was established for the macroporous/nanocrystalline bodies. The presence of PMMA-templates supports this process by affecting the matrix shrinkage, thus, greatly reducing the internal pressure and the propensity for cracking. A closed-cell porosity type was established after sintering with a volume fraction very close to the total porosity measured.
Mechanical properties, i.e. hardness, E-Modulus and fracture toughness of dense specimens were in good correspondence with literature data of Y-PSZ ceramics with similar characteristics. Thermal conductivity of dense materials has shown excellent accordance with literature data as well. In general, significant grain size effects were absent. Porosity effects on the properties were also observed, as expected.
The response of the materials on high temperature exposure was investigated in terms of indentation tests up to 900°C, superplastic deformation up to 1300°C and pore stability during ageing up to 1500°C. Microindentation from room temperature up to 900°C confirmed the high diffusional activity of the nanomaterial at relative low temperatures, also observed by sintering. Porous specimens provided under compression clear evidence for superplastic deformation, illustrated by the distortion and following complete elimination of the macropores with absolute absence of cracks. At the same time, strong grain growth suppression was manifested defining the improved superplasticity of the material.
The existence of a critical grain/pore ratio for pore size stabilization was observed. 3 µm pores showed extreme stability that could be explained by the thermodynamically derived critical ratio.
The strongly inhibited grain growth observed for the present material is in contrast with data of analogous Y-PSZ ceramics, possibly caused by a small amount of residual pores.