Microwave Assist Sintering of Porcelain Insulators with Large Cross-section

Shawn M. Allan*Morgana Fall, Dr. Holly Shulman, Ceralink Inc

Gary Carnahan, Lapp Insulator Company

Ceralink Inc. Rensselaer Technology Park

Troy, New York

33rd International Conference and Exposition onAdvanced Ceramics and Composites

Daytona Beach, FloridaJanuary 19, 2009

OutlineCeramic Processing

Microwave Assist Technology

Sintering of Porcelain Insulators

Summary

Ceralink 1700 °C MAT Lab Kiln 2.45 GHz

Ceramic ProcessingParts are formed from powders

Casting, Pressing, Extrusion, Machining

Nearly all ceramic processing uses heatDrying 50-200 °CThermolysis of Organics 200-1000 °CCalcining/Reactions 300-1200 °CSintering 600-2400 °C

Reactions, phase changes, mass loss, densification, property shifts

Temperature, time, dimensions, heating rate dependent

Ceramic Processing ReactionsIn firing Whitewares several types of reaction occur

Reactions can be endothermic or exothermic

Degassing: remove physically adsorbed gas (water, CO2)

Dehydration: kaolin metakaolin & waterAl2O3•2SiO2•2H2O Al2O3•2SiO2 + 2H2O (g)

Dissociation: CaCO3 CaO + CO2

Ceramic SinteringAs FormedHigh temperature diffusion process

Discrete particles join, become grains in microstructureDensification, shrinkageGrains, grain boundaries, liquid phases, porosityMicrowave affects diffusion

As SinteredMicrowave sintered alumina products

Robocasting, LLCAlumina filter

High AluminaSparkplugs

Conventional SinteringPoor heat transfer

Conduction in porous structureTypically low thermal conductivity

Slow heating to prevent defects

Long sintering times grain growth limited properties

Tem

p

Temperature profile

Microwave Assist TechnologyMAT Sintering

Combines radiant heat (gas or electric) with microwave energy

Uniformity allows faster firing energy savings

Balance microwave heat with radiant heat with independent controls

Schematic of inside of

MAT electric kiln

Tem

p

Conventional Microwave MAT

Temperature profile across part thickness

MAT is a patented technology. Ceralink has exclusive license in North America,

Sublicensed exclusively to Harrop Industries for industrial MAT kilns.

MAT Firing DataRefractories

Tem

pera

ture

(ºC

)En

ergy

use

d (th

erm

s)

MAT Gas Only

Time (hrs)

Gas input MW input

MAT Conventional

Total Firing Time

MAT: 22 hrs total

Conventional: 45 hrs total

Energy consumption

E = P*tMAT: 60 mil BTU

Conventional: 110 mil BTU

Porcelain InsulatorsAlumina Replaced Porcelain

Goal: Speed up firing of electrical porcelain for time & energy savings

Challenge: Large parts with variable cross-section

Result: 78 hrs conventional firing 16 hrs MAT firing

MAT kiln

CerMAT- 17a

Feasibility

11 kg, 12” x 12”

MAT for Electrical Porcelain

Use microwaves to put heat directly into the productMAT kiln – scalableUnderstand sintering process of electrical porcelainUnderstand effects of microwave on the porcelain & process

1. Dilatometer Linear shrinkage2. TGA Mass change3. DSC Enthalpy of reactions4. Dielectric measurements Microwave heating response5. Density & porosity6. Physical observations

Microwaves - Dielectric HeatingTan δ = ε”/ε’

Conductivity increasing*

Transparent to microwavesTan δ < 0.01

Absorb microwave (heats)Tan δ ~ 0.01 - 2

Reflects microwavesTan δ > 10

20°C2.45 GHz

Alumina Zirconia Silicon Carbide Aluminum(bulk**)

Tan δ 0.001 0.015 0.08-1.05 ∞

Penetration Depth (m)

12.8 1.0 0.004 - 0.05 0.000001

* Magnetic field may also induce currents leading to heating** Powder metals have measureable dielectric properties

Microwave Heating Mechanisms

radconvcondElectricMWincs WWWWW −−−= + )(

o

o

c

E

tT

ρ

εωε

2

2"r

=∂∂

Electric heating radiation and convectionconduction through ceramic

Microwave heating internal loss mechanisms resistive heating lossesdielectric polarization lossesε” represents combination of loss mechanismstemperature gain due to microwave energy absorption is:

Dielectric Property TestingZirconia and Alumina Insulation

Measured at 2.45 GHz

Relates to conductivity changes Predicts microwave heating behavior

Higher Tan δ Better absorptionWant product to preferentially absorb

Need radiant heat at low tempAvoid thermal runaway at high temp

M TLoss of adsorbedwater

Ceramic Processing: Dielectric Properties Change

Kaolin Clay Permittivity *

** Kaolin Metakaolin

† Metakaolin Mullite

* Drying

***

†

* R. Hutcheon, Microwave Properties North

Ceramic Processing:Complex Dimensional Changes

Kaolin Clay, Al4Si4O10(OH)8

** Kaolin Metakaolin

† Metakaolin Mullite

‡ Isothermal Densification

* Drying

Cooling

* **

‡

†

Physical-Dielectric Correlation

** Kaolin Metakaolin

† Metakaolin Mullite

* Drying

* **

†

ε’

Electrical Porcelain Dielectrics

Half-power depthDepth where 50% MW absorbed

Loss Tangent

Heating

Heating

Cooling

Cooling

*

†

*

†

* Drying† Feldspar melts & Metakaolin Mullite

Temperature and Microwave Absorption

Half-power depth

12 cm

1 cm

Heating

λ λ1200 °C

Initial MAT Profile

MAT: Electric and microwave

Scaled up insulator size

Added hold to thoroughly dry

8 °C/min heating rate with MW

TGA & DSC of PorcelainLarge endotherm from loss of crystalline water

Endotherm restricts heat transfer

Requires very slow conventional heating

Microwave allowed full speed heating through transition!

A six fininsulator

loses 1 pint of water

at 550 °C!!

*

†

**

* Drying

** Kaolin Metakaolin

† Feldspar melting &Metakaolin decomposition

DSC and Loss TangentNo significant change of dielectric properties at kaolin metakaolin transition

Half power depth at 550 °C, 3x the core diameter Uniform field

Feldspar glass melting at ~975 °C, tan δ increases steadily

† Feldspar Melting

** Kaolin Metakao

†

**

†

**

Cooling DefectsFast furnace cool produced stresses

Wavy cracks on fracture surface

Added slow cooling, 5°C/min

Blue CoringFeldspar glass melts at surface ~975 °C closes pores

Residual carbon from clay causes color

Trapped gases lead to internal open porosity

Result dense surface, porous core

BlueCore

~ 1.5 cm

Open PorosityBlue core was solved

Dwell for residual carbon pyrolysis

Open porosity remained in core

Open porosity 10-20 year failure

Insulator must last 40-50 years

1 hr dwell

Density,g/cc

Open / Total Porosity, %

Surface 2.71 0 / 6.2

Core 2.51 10 / 13.1

Removing Open PorosityLonger sinter dwell better results

Open porosity was persistent

Glass melting at 975 °C still problem for core porosity removal

3 hr dwell

Density,g/cc

Open / Total Porosity, %

Surface 2.77 0 / 3.9

Core 2.61 3 / 9.7

The Final ProfileSlow heating through feldspar glass transition

No change to profile duration

Blue core and open porosity eliminated

3 hr dwell

Density,g/cc

Open / Total Porosity, %

Surface 2.76 0 / 4.5

Core 2.69 0 / 6.8

Proof of ConceptMAT SinteringLapp Insulator – Alumina replaced porcelain

MAT ~5x faster firing

Saved 62 hrs in cycle time

16 hrs vs. 78 hrs

Part quality meets spec

Firing Method Densityg/cc

Open porosity %

MAT

Conventional

2.69 0

2.76 0

MAT Profile – Time is Energy

Scale-up and Production MAT Equipment

C-Tech MAT Gas Shuttle Kiln

Harrop MAT Electric Elevator Kiln

Build teams

Get funding support

Demonstration

Cost Benefit & Manufacturability Analysis

SummaryMicrowave heating (MAT) allowed fast firing of insulators

Large cross-section benefitted despite low microwave penetration

Dielectric properties, DSC and TGA all critical to process

Lapp’s expertise with process facilitated rapid development

MAT resulted in nearly 5x shorter firing cycle

Acknowledgments

Dr. Ron HutcheonMicrowave Properties North