HBA Microwave by Dr Sir Rabnawaz of DMME department of PIEAS university

HBA Microwave by Dr Sir Rabnawaz of DMME department of PIEAS university. This presentation include the detailed operational and functional working of microwave oven.

1. DR. Kaleem Ahmad
R-Block

2. -A form of electromagnetic energy
-An electric component as well as a magnetic
-wavelengths from1 m to 1 mm with corresponding frequenciesfrom
300 MHz to 300 GHz

3. Transparent material

Absorbing material Reflecting material a)Metals
b)Conductors
c)Nopenetration
d)Reflected
a)(Metal)oxides
b)Lossyinsulator
c)Penetratepartially
d)Absorber
a)Teflon,Quartzglass
b)Lowlossinsulator
c)Penetratetotally
d)Transparent

4. Microwave
Absorb

6. Energy transfer Energy conversion
External heating source Internal heating
Heat Flow: outside to inside Inside to outside
Material independent Material dependent
Energy losses Highly efficient
Sample
FURNACEINSULATION
HEATING ELEMENT
MICROWAVE PORT
Sample
INSULATION
MICROWAVE CAVITY
CONVENTIONAL
MICROWAVE

7. •Significantreductioninmanufacturingcostdueto
energysavingsandshorterprocessingtime
•Volumetricandrapidinternalheating
•Enhanceddensificationandsuppressedgrain
growthduetofastheatingrate
•Greater microstructure controlleadsto
developmentofnovelmicrostructure

8. •Specificenergyconsumption inprocessof
sinteringalumina-basedceramicsattemperature
of1600˚Cisabout4kWh/kgforMWheating
versus59kWh/kgforfastconventionalheatingin
aresistiveoven
•Low-temperature firing
•Good on-off control
•Uniform heating by special design of insulation
and susceptors
•Moreuniformandfinermicrostructureand
improvedmechanicalpropertiesofmaterial

9. Ability to achieve very high temperatures
Provides Fine Microstructure
Better Mechanical Properties
Superior Quality Sintered Products
Nano powders processing
Heating is materials dependent: Selective heating
Eco -Friendly
BETTER –FASTER –CHEAPER-GREENER

10. •Microwave process is 6 to 12 times faster
and required only 5% to 50% of the energy
used by the conventional sintering process
Apparentactivationenergyfor
aluminasinteredbythemicrowave
andconventionalprocess
•Synthesis of new materials

12. •Inability to heat low dielectric loss materials
•Thermal runaway
•Sample cracking
•Arcing and plasma formation
•Reaction with thermal insulation
•Poor control over sintering process
•Inability to measure temperature accurately

13. Microwave Materials
Interaction

14. The dielectric loss tangent defines the ability of a material to convert
electromagnetic energy into heat energy, at certain frequency and
temperature
ε´-dielectric constant, describes the ability of a molecule to be polarized
by electric field
ε´´-dielectric loss, describes the efficiency with which the energy of the
electromagnetic irradiation can be converted into heat

15. Dielectric Properties
•The study of dielectric properties is
concerned with the storage and dissipation
of electric and magnetic energy in
materials. It is important to explain
various phenomena in electronics, optics,
and solid-state physics.
•The term "dielectric" was coined by
William Whewell (from "dia-electric") in
response to a request from Michael
Faraday.

16. Permittivity
•In electromagnetism, permittivityis the
measure of how much resistance is encountered
when forming an electric field in a medium.
•In other words, permittivityis a measure of
how an electric field affects, and is affected by, a
dielectric medium. Permittivity is determined by
the ability of a material to polarize in response to
the field, and thereby reduce the total electric
field inside the material.
•Thus, permittivityrelates to a material's ability
to transmit (or "permit") an electric field.

17. Electric Susceptibility
•Permittivityis directly related to
electric susceptibility, which is a
measure of how easily a dielectric
polarizes in response to an electric
field.

18. Units of Permittivity
•In SI units, permittivity ε is
measured in farads per meter (F/m);
electric susceptibility χ is
dimensionless. They are related to
each other through
•where ε
ris the relative permittivity of
the material, and ε
0= 8.85… ×10
−12
F/m is the vacuum permittivity.

19. Permittivity and Organization
of Electric Charges
•Inelectromagnetism,theelectricdisplacementfieldDrepresents
howanelectricfieldEinfluencestheorganizationofelectrical
chargesinagivenmedium,includingchargemigrationand
electricdipolereorientation.Itsrelationtopermittivityinthevery
simplecaseoflinear,homogeneous, isotropicmaterialswith
"instantaneous"responsetochangesinelectricfieldis
•wherethepermittivityεisascalar.Ifthemediumisanisotropic,
thepermittivityisasecondranktensor.
•Ingeneral,permittivityisnotaconstant,asitcanvarywiththe
positioninthemedium,thefrequencyofthefieldapplied,
humidity,temperature,andotherparameters.Inanonlinear
medium,thepermittivitycandependonthestrengthofthe
electricfield.Permittivityasafunctionoffrequencycantakeon
realorcomplexvalues.

21. Dielectric
•A dielectricis an electrical insulator that may be polarized
by an applied electric field. When a dielectric is placed in an
electric field, electric charges do not flow through the
material, as in a conductor, but only slightly shift from their
average equilibrium positions causing dielectric
polarization. Because of dielectric polarization, positive
charges are displaced toward the field and negative charges
shift in the opposite direction. This creates an internal
electric field that partly compensates the external field
inside the dielectric.
•If a dielectric is composed of weakly bonded molecules,
those molecules not only become polarized, but also
reorient so that their symmetry axis aligns to the field.

22. The Use of Term INSULATOR
in Dielectric
•Although the term "insulator" refers to a
low degree of electrical conduction, the
term "dielectric" is typically used to
describe materials with a high
polarizability. The latter is expressed by a
number called the dielectric constant. A
common, yet notable example of a
dielectric is the electrically insulating
material between the metallic plates of a
capacitor. The polarization of the dielectric
by the applied electric field increases the
capacitor's surface charge.

23. Electric susceptibility
•Theelectricsusceptibilityχeofa
dielectricmaterialisameasureof
howeasilyitpolarizesinresponseto
anelectricfield.This,inturn,
determinestheelectricpermittivity
ofthematerialandthusinfluences
manyotherphenomena inthat
medium,fromthecapacitanceof
capacitorstothespeedoflight.

24. WAVE PROPAGATION
•Microwave propagation in air or in materials depends
on the dielectricand the magneticproperties of the
medium.
•The electromagnetic properties of a medium are
characterized by complex permittivity ()and complex
permeability (μ), where:
έdielectric constant & ’’ dielectric loss factor
•έis not constant but can vary significantly with
frequency and temperature
•Similarly;
μ' permeability &μ'' magnetic loss factor
Microwave Fundamentals

25. The dielectric loss tangent defines the ability of a material to convert
electromagnetic energy into heat energy, at certain frequency and
temperature
ε´-dielectric constant, describes the ability of a molecule to be polarized
by electric field
ε´´-dielectric loss, describes the efficiency with which the energy of the
electromagnetic irradiation can be converted into heat

26. ״
ε ο
σ
ωε ״ ״
ο
εεσ
tanδ
έέωέε
   ״ 2
ο
ο
σ
=ωε(ε )׀E׀
ωε
 Dipolar Loses
Conductive Losses
P״ ״
ο
σ
εε
ωε
r


27. Perspective
•Engineers were working on Short-wave
Transmission
–Contracted Artificial Feverand Observed;
•Heatgenerated directly in the objectitself;
•(No transferof heat is involved)
•Associated apparatusneed not be heated.
•The surfacesof the materialneed not be affected.
•The people who work with the equipment have cooler
working conditions.
•No gasesare involved and thus corrosion of surfacesis
eliminated.
•The material can be heated from the inside-out.
•Finally, objects of unusual size or shapecan be heated.
•First Control on MW (During 2
nd
World War).
•Used in Radar System
•1952 1
st
MW Oven
•Working to Overcome Problems
Introduction

28. Materials Interaction
•Electric-Field Interaction with;
•Conductor Electric Current flows due to free electrons
–Resistive Heating
–Reflection of MW from conductors not effective heating
•Insulators Electrons do not flow freely Heating due to;
–Electronic cloud reorientation
–Re-orietation of induced or permanent dipoles.
•Food heating is based;
–Primarily on the dipole behavior of the water molecule in the food
and the dipole's interaction with microwaves.
–Because microwaves generate rapidly changing electric fields,
these dipoles rapidly change their orientations in response to the
changing fields.
•Coupling;
•If the field change is occurring near the natural frequency at
which reorientation can occur, a maximum in energy consumed
is realized, and optimum heating occurs. In this Scenario the
material is said well "coupled”.
Introduction

29. •Properties of greatest importance in MW-
processing of a dielectric are;
•The permittivity(often called the dielectric constant)
•The loss tangent, tanδ.
–The complex permittivity( ) is a measure of the ability
of a dielectric to absorband to storeelectrical potential
energy.
–Real permittivity( ) penetration of MWinto the
material.
–Loss factor( ) material's ability to storethe energy.
–tanδ[Loss Tangent] ability of the material to convert
absorbed energy into heat. (most importantproperty in
MW processing)
–For optimum coupling, a balanced combination of;
•Moderate ( ), to permit adequate penetration.
•High loss, maximum ( ) and tanδ.
Introduction
Materials Interaction Cont…

30. •The trick in MW processing is to find;
–Polarizable material
–Whose dipoles can reorient rapidlyin response to changing
electric field strength.
•Fortunately, many materials satisfy these requirements.
•However, if these materials are poor thermal conductors;
–Heat does not rapidly dissipate from hot regionto the
surrounding regions.
–This difficulty is compounded, because the dielectric loss
dramatically increaseswith temperature.
–Thus, the hot region becomes even hotter, sometimes
resulting in local melting. These regions are known as "hot
spots"
–Hybrid systems(microwave heating with other heat sources )
are used to reduce uneven heating.
Materials Interaction Cont…
Introduction

31. Cost Consideration
•Microwaves generation needs electrical energy.
•Electricalenergy is generated primarily from fossil fuels.
–The conversion of the energy in the fuel to electrical energy is
< 40% efficient.
–In addition, microwave generators(magnetrons, etc.) are not
generally better than 85% efficientin converting electric
power to microwaves.
–Microwaves are not perfectly coupled to the material (90%
couplingwould be very good),
•So the total energy generatedis probably < 30%of the
energy content of the fossil fuelused in generating the MW
heat energy.
•This means there are real limitations to the economics of
bulk heating.
–Direct heating with fossilfuels makes much more efficient use
of energy
–Microwaves can only be economicallycompetitive when electric
heating is mandatedor the selective heatingcapability of
microwaves, or some other factor.
Introduction

32. MWinteractwithceramicmaterialsleadingtovolumetric
heatingbydielectriclossmechanism.
1.Flowofconductingcurrent(IonicConduction)
2.Dipolarreorientation

37. Relationship between the dielectric loss factor and ability to absorb
microwave power for some common materials

38. •Direct and hybrid MW heating
•Full power heating using high heating rates
•Multi step heating using low heating rates
•MicrowavesinteringwithPre/Posthotisostaticpressing
•Microwavesinteringfollowedbyannealing
•Variabledwelltimeatmaximumsinteringtemperature

39. •Susceptors
•Are they necessary
•Effect of placement of susceptors
•Relation between total load and container
volume (Filling Factor)
•Casket design
•Do not absorb all the
microwave

40. •Astemperature increasesabove critical
temperaturelosstanδbeginstoincreaserapidly
causesaconditionofthermalrunawayinMW
heatedmaterial.
•Importantaspect
•Causeundesirablehotspots/cracking
Temperature

42. Cr
2O
3
•Behave like material A
•At low power the build up of heat require more
time to
•Trigger thermal runaway
•Significant heat dissipated to environment

43. W.H. Sutton, "Microwave processing of ceramic materials," American
Ceramic Society Bulletin, 68 [2] 376-386 (1989).

44. Loss Tangent as a Function of
Temperature for different
materials at 8-10 GHz
•Compositional additive/impurities
•Softening of intergranular amprphous phase
•Leads to increase in local conductiivty
W.H. Sutton, "Microwave processing of ceramic materials," American
Ceramic Society Bulletin, 68 [2] 376-386 (1989).

45. 0.40
0.50
0.60
0.70
0.80
0.90
1.00
900 1000 1100 1200 1300 1400 1500 1600
Temperature (ºC)
Relative Density
Conventional
Sintering
Microwave Sintering 200°C
K.H. Brosnan, G.L. Messing and D.K. Agrawal, “Microwave Sintering of Alumina at 2.45 GHz,”
J. Am. Ceram. Soc. 86[8] 1307-12, 2003.

46. •Indirect MW hybrid Heating
–Use of Susceptors
•“Picket fence” arrangement
•Direct hybrid heating
–MW and conventional heating
simultaneously

47. Dielectric Loss
Factor
Temperature
Silicon
Carbide
Zirconia
Change in dielectric loss factor with temperature

49. •Samples held between zirconia setters
•Heating assisted by silicon carbide susceptors

50. Modified Kitchen Microwave 1.1 KW
Power ControllerThermal Insulation
Sic Susceptors
Absorb MW
Radiant Heat

51. A 1 kg, 20 cm high pure alumina ceramic specimen sintered in a 30 GHz millimetre-wave gyrotron system. The sintering
temperature was 1600 ◦C with a hold time at this temperature of 20 min; the maximum millimeter-wave power was 4 kW.
For comparison, an unsintered specimen is also shown on the left.
High temperature microwave processing of materials. J. Physics D 34(2001)

53. •Microwave irradiation is becoming increasingly
interesting method in heating materials offers
clean, cheap, convenient, and instantaneous
method of heating.
•Microwave heating can improve yield and increase
productionrate
•Selective Heating
•Knowledgeofthedielectricpropertiesofmaterials,
aswellastheirdependence onfrequency,
temperature, chemical composition, and
microstructureisofparamountimportanceforthe
intelligentuseofmicrowaveenergyforhigh-
temperatureprocessing

54. •By utilizing the enhanced densification and
suppress grain growth property of MW sintering
at lower temperature as compared to
conventional sintering we may be able to
significantly improve mechanical properties of
ceramics on reproducible basis.
•Therearealsosomechallengessuchaslackof
experiencedmanpower,lackoffundamentaldata
ondielectricpropertiesatelevatedtemperature
etc.

55. 1.K. Ahmad, W. Pan, and W.J. Si, "Microwave sintering of alumina-
silicon carbide nanocomposites," Key Engineering Materials, 336-338 II
1072-1075 (2007).
2.D.E. Clark, "Microwave processing: present status and future promise,
" pp. 3-21, Vol. 14, Ceramic Engineering and Science Proceedings.Edited.
Pub by American Ceramic Soc, Westerville, OH, USA, Cocoa Beach, FL, USA,
1993.
3.E. Kubel, "Advancements in Microwave Heating Technology,
" Industrial Heating, 72 [1] 43-53 (2005).
4.W.H. Sutton, "Microwave processing of ceramic materials," American
Ceramic Society Bulletin, 68 [2] 376-386 (1989).
5.C. Zhao, J. Vleugels, C. Groffils, P.J. Luypaert, and O. Van der Biest,
"Hybrid sintering with a tubular susceptor in a cylindrical single-mode
microwave furnace," Acta Materialia, 48 [14] 3795-3801 (2000).

56. THANKS