Ceramics can be diffusion bonded, but conventional diffusion bonding using radiant heating can demand bonding times as long as 8 hours. This is particularly so for materials such as alumina, which are bonded at temperatures approaching 1600°C.
Using a microwave heat source, bonding times can be reduced by an order of magnitude (although pressure is still required). Direct coupling of the microwave with the ceramic results in volumetric heating. Microwaves are being considered as an energy source for joining ceramics such as alumina, zirconia, mullite, silicon nitride and silicon carbide.
Very high purity alumina is difficult to heat with microwaves, because of its low inherent dielectric properties: this makes joining difficult. Use of interlayers, including sealing glasses, and alumina gels has been investigated for producing joints with high purity alumina. Alumina gels offer the advantage that, at the joining temperature, the gel transforms into colloidal ?-alumina which subsequently sinters to provide a homogeneous interface.
In contrast to pure alumina, impure, 85% alumina is joined easily using microwave heating, showing bond strengths equivalent to that of the parent material.