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Dept. of Materials Science & Engineering , National Taiwan University
Chapter 3 Furnaces
Lecturer: Professor Wei (韋文誠教授)
3/5/2013
助教: 許哲誠 –舊物二號館220室
0971 097 628, [email protected]
Dept. of Materials Science & Engineering , National Taiwan University
High temperature Processes
of Ceramics
1. Expensive step;
2. Necessary for ceramics:
sintering, reactions, phase transformation,
glass melting, crystal growth;
3. Tend to get desirable properties;
4. Contamination
5. Safety
Types of Furnaces
• Based on the heat source:
(1) Combustion (燃燒式)
(2) Electrically Resistive (電阻式)
(3) Focused Infrared (紅外線)或 visible light (可見光)
(4) Direct heating, microwave (微波) or
radiofrequency (RF)
Type of furnace
(1) periodic or batch type (批次式)
(2) continuous (連續式 )
Terminology
• Furnace vs. kiln (interchangeable)
Furnace vs. ovens (for drying)
* Combustion (oxidative process) vs. burning
Combustion Furnaces
Combustion Heat Generation
• Need to know the heat
capacity of each species
• Cp is the molar heat
capacity
• Diabatic flame
temperature
Electrically Heated Furnaces
Powder from electricity
P=VI=I2R
so, heat Q =RI2t (Joule or kWh)
• Several advantages
(1) easy to measure power input
(2) easy to control heating rate and temperature
(3) the furnace can be operated in an atmosphere
independent of the heating source
Batch (or Continuous) Operation
Continuous Operation
•Tube burner
•Thermal shock
•Design time-temperature profile
Indirect Heating: 1. Induction
• Induction furnace
-external Cu coil
-applied power + impedance
of materials
-good susceptor: graphite
-60-1000 Hz
-up to 3000oC
-up to kA
• Microwave heating
• Arc-image furnaces
Indirect Heating: comparison
• Induction furnace:
good in clean, fast, process reproducible, be automated,
local heating is possible
• Microwave heating:
high frequency 0.915 or 2.45 GHz
home: 700-1200 W, industry: up to 5 kW
• Arc-image furnaces
clean, fast and used for crystal growth.
• Laser or electron beam heating
temperature control is poor
Heating Elements
• Cost and lift time are also the concern of using the
elements
SiC Heating Elements
• Most widely used non-oxide
• < 1500oC
• >1500oC oxidation is apparent
Manufacturing of SiC
(1) In-situ reaction (or called carbon-reduction) method
mixture of SiO2 + C is heated to 1900oC
(2) Reaction bonding method
green (SiC+C powder) bars is immersed in melted Si.
(3) Sintering
SiC powder with sintering aids (for instance, B+C
powder, Al2O3+Y2O3 sintered up to 2300oC.
MoSi2 Heating Elements
•
•
•
•
Brittle
Up to 1800oC short period
Normal <1700oC
Room temperature resistivity is 2.5x10-7ohm-m, but
increases to 4x10-6 ohm-m.
• Manufacturer: Kanthal Super, made the element in
composite with Al-Silicate glass (20 vol%)
ZrO2 and SnO2 Heating Elements
• Cubic zirconia (c-ZrO2)
ex. Ca or Mg doped
• For >2000oC heating;
• In air or oxidation cond.
• Need preheating to
1000oC, then becomes
conductive.
• SnO2 as electrode in glass
melting furnace;
• E-conductivity is high at
glass melting temp.;
• Resist to corrosion
• Not discolor the glass.
• 60 cm long 15 cm dia.,
1400oC sintered electrode;
• Free surface of glass
batch is low prevent
evaporation of low melted
species, ex Pb, B.
Graphite, Mo and W Heating Element
• Graphite is good
conductor and less vapor
in reducing atmosphere
upto 3000oC;
• Reactive with oxide
ceramics;
• Susceptive to oxidation;
• All metal oxides are
eventually reduced if
contacted with graphite.
• Mo oxidizes to MoO3
>700oC;
• Mo is good conductor
and less vapor up to
1500oC;
• W is good in inert
atmosph. Up to 3000oC;
• Susceptive to oxidation;
Contamination by W Heating Element
 a-SiC sample sintered in
W-furnace for 12 hr at
1300oC.
• Fine W particles are
coming from the
evaporation of W-element
during the heat treatment.
Refractory in an Industrial Glass Melting
Furnaces
• 70% are
preformed
bricks (定型磚)
• Hearth (爐床)
Refractory (耐火材)
• Iron and steel
making
• Cu & Al melting
• Cement & ore
processing
• Petro-industry
Dolomite 白雲石
Zircon 鋯英石 ZrSiO4
Refractory for Blast Furnace
• Slag: protective layer on
melted iron, consisted of
limestone, SiO2 (from
ore), ash and various
oxides;
Thermal Conductivity of Porous Insulators
• Or a simple form, when kc > kp
Furniture, Tubes and Crucibles
• Important crucible
materials;
• Boat (船型坩堝)
setter plate (棚板)
Furniture on Push Car
Firing Process (Schedule)
• Heat-up stage, soaking stage, and cooling stage
• Quartz transformation a to b at 537oC  induce ….
• Soaking period (持溫時間)
Heat Transfer in Furnace
• By Conduction/convection/radiation
• Conduction: from heating element to air and
•
hot air to ceramics
• Convection: hot air flowing in furnace;
is important at lower temperature;
• Radiation: hot wall or furniture to others by visible light,
and is the most important at high temperature
Temperature measurement by
Thermo-couple
• What do the alloys mean? Constantan, chromel, alumel?
Pyrometers (光譜測溫儀)
• Radiation effect of object;
• Disappearing filament optical pyrometers
compare with standard incandescent source!
• Advantages
Dis-advantages
Temperature measurement: Pyrometric
Cones
• Sensitive to time
and temperature
• Useful for
ceramic sintering
Safety
• High temperature hazards
protective goggle, gloves, hood, and safety glass
• Electric hazards
>100 mA is lethal, Good insulation from high voltage,
power
• Chemical hazards in reducing cond.
1. the formation of CO
2. The formation of CN and HCN,
Conclusions
• Very important tools;
• Safety issues;
• A tour to lab.
參考資料

Chapter 7 in Ceramic Materials, Science and
Engineering, Springer, 2007