INEXPENSIVE, OFF THE SHELF HYBRID MICROWAVE SYSTEM Page: 3 of 5
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The microwave susceptor is 200 grams of
granular SiC (silicon carbide) encased in a 6" tall
annular quartz encasement around a 1" center hole
where the charge tube slides in and out. The 2" OD
quartz encasement is set in a 2" ID x 4" OD x 8" tall
alumina insulation tube that has a 1" thick bottom to
Fig.2 Charge tube with alumina foam filter
Once the quartz encasement is inserted into the
insulation tube, a 1" thick alumina insulation ring is
placed on top of the quartz encasement and some
alumina wool is stuffed above that to ensure no heat
shine reaches the metal above. The 3" tall stainless
steel plasma shield goes around the outside of the
insulation and is positioned against the top of the
microwave oven cavity. The bottom of the shield is
about 3/8" below the top of the SiC in the susceptor
unit. These pieces are shown in Fig.3.
contents. Without the plasma shield, a plasma can form
at pressures from 3 Torr up to 80 Torr. With the
plasma shield, a plasma can only form at pressures less
than 9 Torr. As a result, the heating process can be
started with about 15 Torr of an inert gas.
III. SYSTEM CONTROL
III.A. Microwave Heating Control
The magnetron produces the microwaves in the
oven. Control of the magnetron was accomplished by
inserting an ON/OFF relay in the line going to the step
up transformer prior to the magnetron. Automatic
computer control of the relay was based on either the
pyrometer temperature reading or the charge tube
thermocouple temperature reading versus an operator
set point. Manual computer control of the relay was
based on a percent output inputted by the operator. The
cycle time is 32 seconds, so a 50% output would
produce 16 seconds ON, then 16 seconds OFF and so
on. To operate the oven, the operator would still have
to enter a time on the oven front panel and press the
start button. All original safety interlocks on the oven
remained in use, which includes the door interlock and
the cavity over-temperature interlock. Both interlocks
shut off the oven.
III.B. Temperature Response
Using the magnetron control scheme described
above, the temperature of the sample in the charge tube
rapidly heats to process temperature and remains at
temperature for the duration of the measurement. Fig. 4
shows the pyrometer (IR2) reading for a process run,
and the pressure increase in the charge tube for an
initial pressure of 400 Torr.
Fig.3 Plasma Shield and Quartz Encasement
The plasma shield extends down from the top of
the microwave cavity to about 1/4" below the top of
the susceptor. The purpose of this shield is to reduce
the direct impingement of microwaves into the
charge tube to prevent a plasma from forming inside
the charge tube at low pressures. A plasma causes
uncontrolled rapid heating of the tube and its
Fig. 4 Pressure rise due to gas heating
The sample temperature rose to 1100 C, remained for
about 30 minutes, and was increased to 1150 C for
PTime vs R2 Temp
-Time vs Pressure
- Pressure decrease
-e s r --se
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Walters, T; Paul Burket, P & John Scogin, J. INEXPENSIVE, OFF THE SHELF HYBRID MICROWAVE SYSTEM, article, June 21, 2007; [Aiken, South Carolina]. (https://digital.library.unt.edu/ark:/67531/metadc881416/m1/3/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.