Evidence for the Microwave Effect During Hybrid Sintering

Abstract

A microwave/conventional hybrid furnace has been used to sinter three ceramics with different microwave absorption characteristics under pure conventional and a range of microwave/conventional hybrid heating regimes. The precursor powder particle size was also varied for each material. In each case it was ensured that every sample within a series had an identical thermal history in terms of its temperature/time profile. An increase in both the onset of densification and the final density achieved was observed with an increasing fraction of microwave energy used during sintering, the effect being greatest for the materials that absorbed microwaves most readily. Twenty‐three percent greater densification was observed for submicron zinc oxide powder, the material with the largest microwave absorption capability, when sintered using hybrid heating involving 1 kW of microwave power compared with pure conventional power under otherwise identical conditions. For the ceramic with the lowest microwave absorption characteristic, alumina, the increase in densification was extremely small; partially stabilized zirconia, a moderate microwave absorber, was intermediate between the two. Temperature gradients within the samples, a potential cause of the effect, were assessed using two different approaches and found to be too small to explain the results. Hence, it is believed that clear evidence has been found to support the existence of a genuine “microwave effect.”

Improvement of microstructural properties of 3Y-TZP materials by conventional and non-conventional sintering techniques

“…Microwave radiation for sintering of ceramic components has recently appeared as a newly focused scientific approach [9][10][11][12][13][14][15][16]. Microwave sintering has several advantages such as rapid end volumetric heating, improved production rate, enhancement in densification and grain growth prohibition of ceramics [17][18][19].…”

Microwave sintering of mullite-Al2O3 from kaolin precursor

“…However, kaolinite, mullite and alumina present low dielectric losses, and it is known that the development of liquid phase increases the body's dielectric loss, thus considerably increasing the microwave absorption characteristics of compacts. Other authors 27,28 have shown that materials with very low dielectric loss display an extremely small, almost negligible difference in densification when processed by microwave energy compared to conventional heating.…”

Dense nanostructured zirconia by two stage conventional/hybrid microwave sintering

“…Although recent results from Loughborough have now confirmed the existence of the socalled 'microwave effect' during ceramic sintering [9], it is believed that the major effect influencing these results is that a much faster heating rate was used. Due to the volumetric nature of the hybrid heating, 20 o C min -1 could be used without risking any damage to the samples from thermal stresses, whilst the radiant heating was limited to only 7 o C min -1 .…”