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Microwave Sintering of Ceramics
Transcript of Microwave Sintering of Ceramics
Most notably used in households to heat food
First used to sinter ceramics in the 1960's
Proven in all sorts of material systems: Oxides
Many combinations Mechanisms of Microwave Heating Three Different Interactions: Reflection:
The microwave energy reflects off the material.
No heat created
Generally occurs with most metals Transmission:
The microwave energy passes through the material.
No interaction with molecules, no heat created.
Fused Silica is an example of a material which is transparent. Absorption
Microwave energy interacts with molecules or atoms
Interaction causes vibration about lattice sites
Energy from vibration creates heat Absorption is the desired reaction, but most materials interact
in some combination of all three. Mechanisms of Microwave Heating Reflection, Transmission, or Absorption is
Frequency of Microwaves
Geometry of the Material
Dielectric loss characteristics of material Hybrid Microwave Heating For materials that do no absorb well: A material with high microwave absorption is used.
The material is called a susceptor.
An example of a susceptor material is Silicon Carbide.
The material is positioned around the green body, and
heat is conducted into the green body by the susceptor. Dielectric properties of materials change significantly
with temperature. As the green body heats up, it
also begins absorbing microwave energy. Advantages over
Conventional Heating Coventional ovens heat from the outside in:
Produces large temperature gradients
Requires long ramp up Microwave Heating is Volumetric:
Temperature gradients, if any, are smaller
Heating occurs instantaneously, shorter ramp up Microwave Enhanced Diffusion... Microwave Enhanced Diffusion An effect microwave energy has while sintering materials
Fully Dense Samples at decreased sintering temperature and time. Yttrium stabilzed zirconia pellets A Few Examples Yttrium stabilized zirconia sintered using coventional heating, hybrid microwave heating, and direct microwave heating. Direct Microwave Hybrid Microwave Conventional SEM Images of same yttrium stabilized zirconia
Each sample was fully dense.
Grain size is about 0.7 microns for all
But, direct microwave sample is more uniform
Also, hardness values show a small difference. Theory of Microwave Enhanced Diffusion First Theories suggested that there were temperature measurement inaccuracies.
Microwave EM field may interact with thermocouples
However, this was disproved
Other Theories focused on the factors that effect diffusion flux: Variable terms in the overall flux term are:
Q = activation energy
Temperature Microwave Effects on Flux Activation Energy
The microwave energy could have an additive effect on energy of the system, making it appear as though the activation energy is lower.
In ionic materials, the EM field could increase the movement of ions, therefore increasing the overall driving force for material transport.
However, these two theories have been shown to be improbable. Temperature Microwave energy interacts most efficiently with defects in a material
One large defect in a polycrystalline material is grain boundaries
Grain boundaries make up about 50% of the total volume
Microwave energy may be preferentially heating the grain boundaries.
Grain boundaries reach higher temperatures than grains.
Overall temperature is an average of the two different temperatures. In Summary The mechanics of microwave sintering are still in the process of being understood.
However, microwave energy can be a very useful and tool for powder processing
Lower sintering times
Lower sintering temperatures
Possibility to improve final properties
Works in many material systems
Has been shown to require 1/10 the energy compared with conventional sintering! Thank you! Preferential Heating We have higher temperatures at the grain boundaries than inside the grain.
At lower temperatures lattice diffusion from inside the grain is the main method of densification
Higher temperature at grain boundaries means that grain boundary diffusion takes over.
Grain boundary diffusion provides quicker densification than lattice diffusion. Note: This is a new theory, and requires further study.