Scientists in the USA are exploring pore-free and fully-dense neodymium-doped yttrium aluminium garnet (Nd:YAG) advanced ceramics. They believe these optically transparent materials could eventually replace YAG crystals in lasers for improved quality.

‘Eliminating porosity has been one of the primary driving forces in ceramics processing science,’ explains Professor Gary Messing, at Pennsylvania State University. ‘Pores typically degrade the mechanical, electrical and optical properties of ceramic materials.’

His team is focusing on improving the optical quality of fully dense (greater than 99.999% density) Nd:YAG. Pores in most ceramics traditionally scatter light and make the material optically opaque. Eliminating porosity would allow the advanced ceramic to replace single crystals of YAG doped with neodymium or erbium, or other rare earth elements, in lasers. These crystals are made via the melt growth Czochralski process and therefore ‘have large defects that limit the size and optical quality’, says Messing.

The researchers claim to have removed the pores by carefully controlling the processing of green bodies made from powders about 200nm in particle size.

The part is either sintered in a vacuum furnace between 1,700-1,800ºC or undergoes a sinter/hot isostatic pressing (HIP) procedure. Here the material is fired at 1,600ºC, followed by HIP at 1,600-1,700ºC and around 200MPa.

Graduate Research Assistant Adam Stevenson explains, ‘During sintering, the traditionally held theory is that there is a critical pore-to-grain size ratio that must be achieved before pores will shrink and disappear. That is typically when the pores are just smaller than the average grain size.

‘This comes from analysing the forces that lead to pore closure during final-stage sintering. By HIP, we add a driving force orders of magnitude higher. We are able to get around the pore-to-grain size ratio of pressureless sintering.’

Using HIP allows the pores to close at lower temperatures than sintering alone and results in a smaller grain size for improved strength and thermal shock resistance. Tests suggest the resulting ceramic could outperform crystals as laser gain media.

Taped up

The material properties are further enhanced, says the team, via a novel tape casting method to form a green body from the ‘slurry’ of ceramic powder, organic solvent and polymer binder. The team can now cast a single tape where the composition of the material is varied (differing Nd3+ concentrations for example) perpendicular to the casting direction for ‘functionally graded’ components.

‘These architectures can aid thermal management and increase efficiency and output power of laser systems,’ notes Messing.

Jon Binner, Professor of Ceramic Materials, at Loughborough University, UK, comments that although the pore elimination process is not ‘groundbreaking’, the research is ‘another incremental step in the right direction’ for pore-free ceramics.

He adds, ‘Quite a few people are working on this. The particular bit that seems innovative is the modified casting process. This is unusual’.