Technology Brief 3: Microwave Ovens Microwave Absorption

Technology Brief 3: Microwave Ovens Microwave Absorption
TECHNOLOGY BRIEF 3: MICROWAVE OVENS
1
Technology Brief 3: Microwave Ovens
Percy Spencer, while working for Raytheon in the 1940s on the design and construction of magnetrons for radar, observed
that a chocolate bar that had unintentionally been exposed to microwaves had melted in his pocket. The process of
cooking by microwave was patented in 1946 and by the 1970s, microwave ovens had become standard household items.
Microwave Absorption
A microwave is an electromagnetic wave whose frequency lies in the 300 MHz–300 GHz range (see Fig. 1-16.) When a
material containing water is exposed to microwaves, the water molecule reacts by rotating itself so as to align its own
electric dipole along the direction of the oscillating electric field of the microwave. The rapid vibration motion creates
heat in the material, resulting in the conversion of microwave energy into thermal energy. The absorption coefficient
of water, α(f ), exhibits a microwave spectrum that depends on the temperature of the water and the concentration
of dissolved salts and sugars present in it. If the frequency f is chosen such that α(f ) is high, the water-containing
material will absorb much of the microwave energy passing through it and convert it to heat. However, it also means
that most of the energy will be absorbed by a thin surface layer of the material, with not much energy remaining to heat
deeper layers. The penetration depth δp of a material, defined as δp = 1/2α, is a measure of how deep the power carried
by an EM wave can penetrate into the material. Approximately 95% of the microwave energy incident upon a material
is absorbed by the surface layer of thickness 3δp . Figure T3-1 displays calculated spectra of δp for pure water and two
materials with different water contents. The frequency most commonly used in microwave ovens is 2.54 GHz. The
magnitude of δp at 2.54 GHz varies between ∼ 2 cm for pure water and 8 cm for a material with a water content of only
20%. This is a practical range for cooking food in a microwave oven; at much lower frequencies, the food is not a good
absorber of energy (in addition to the fact that the design of the magnetron and the oven cavity become problematic),
and at much higher frequencies, the microwave energy will cook the food very unevenly (mostly the surface layer).
50
T = 20◦C
3δp
30
Penetration Depth δp (cm)
40
95% of energy
absorbed in
this layer
Chocolate bar
20
Food with 20% water
Microwave oven frequency (2.54 GHz)
10
Food
with 5
0% w
ater
Pure water
0
1
2
3
Frequency (GHz)
4
5
Figure TF3-1: Penetration depth as a function of frequency (1–5 GHz) for pure water and two foods with different
water contents.
2
TECHNOLOGY BRIEF 3: MICROWAVE OVENS
Whereas microwaves are readily absorbed by water, fats, and sugars, they can penetrate through most ceramics,
glass, or plastics without loss of energy, thereby imparting little or no heat to those materials.
Oven Operation
To generate high-power microwaves (∼ 700 watts) the microwave oven uses a magnetron tube, which requires the
application of a voltage on the order of 4000 volts. The typical household voltage of 115 volts is increased to the required
voltage level through a high-voltage transformer. The microwave energy generated by the magnetron is transferred
into a cooking chamber designed to contain the microwaves within it through the use of metal surfaces and safety
Interlock switches. Microwaves are reflected by metal surfaces, so they can bounce around the interior of the chamber
or be absorbed by the food, but not escape to the outside. If the oven door is made of a glass panel, a metal screen
or a layer of conductive mesh is attached to it to ensure the necessary shielding; microwaves cannot pass through
the metal screen if the mesh width is much smaller than the wavelength of the microwave (λ ≈ 12 cm at 2.5 GHz). In
the chamber, the microwave energy establishes a standing-wave pattern, which leads to an uneven distribution. This is
mitigated by using a rotating metal stirrer that disperses the microwave energy to different parts of the chamber.
Metal
screen
Stirrer
Interlock switch
Magnetron
4,000 V
High-voltage
tranformer
115 V
(a) Cavity
Magnetron
Over lamp
Cooling vents
Magnetron cooling fan
Magnetron filament
terminals
Controller
(hidden
behind
bracket)
High voltage
capacitor
High voltage
diode
Interlock switches
High voltage transformer
(b) Typical microwave oven electronics bay. [Photo courtesy of John Gallawa (microtech@gallawa.comn)]
Figure TF3-2: (a) Microwave oven cavity and (b) electronics bay.
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