Download INDUCTION HEATING FOR CRYSTAL GROWING

Journal of Crystal Growth
11
(1971) 50—52
North-Holland Publishing Co.
LOW-FREQUENCY INDUCTION HEATING FOR CRYSTAL GROWING
C. S. DUNCAN, R. I-f. HOPKINS and R. MAZELSKY
Westinghouse Research Laboratories, Pittsburgh, Pennsylvania /5235, (IS. A.
Received 25 March 1971; revised manuscript received II May 197l
Low frequency (lO kHz) induction, largely overlooked by crystal growers as a means of heating, offers some
unique advantages especially for growing crystals of high melting point materials. One such system, successfully used to grow silicate-oxyapatite laser hosts (MP
2170 ~C) with minimal crucible damage is described below.
1. Introduction
2. Characteristic features of crystal growing by low
frequency induction heating
current, approximately 86°~
of the total heat developed
in a material occurs within the depth D from the
surface. The heating depth at 10 kHz is about 7 times
greater than at 500 kHz and nearly 22 times greater
than at 5 MHz in non-magnetic materials. For example
at 1600 ~Cthe heating depth of platinum, a commonly
used crucible material, is 2.8 mm at 10 kHz, 0.40 mm
at 500 kHz and only 0.13 mm at 5 MHz.
The greater depth of heating possible with low
frequencies can be extremely important whenever
crucibles or susceptors must be maintained at temperatures approaching their melting points for prolonged intervals. This is true because crucible walls
often contain high resistivity regions caused by inclu-
Probably the greatest advantage of low frequency
heating is its relatively large “skin effect” or depth of
material heating, which can be calculated from the
r~lation’),
sions, poor welds, or inhomogeneous grain growth
during service. Under these conditions the variation in
resistance along the path traversed by the induced
current within the crucible wall produces hot spots.
Although induction heating is often used to power
crystal growing apparatus, crystal growers are largely
unfamiliar with the advantages that low (‘-.~ 10 kHz)
frequency systems can provide. We describe briefly
below one type of low frequency system that is especially suited for the growth of high-melting-point
compounds. For the benefit of those unfamiliar with
induction heating some basic principles are also discussed; details are available in standard texts~).
D=
3570 / p
-
/
~/
‘~
With high frequency heating the melting temperature
of the crucible material may be locally exceeded
at
these points resulting in catastrophic failure2). Lo~
frequency induction heating minimizes this problem
since the induced currents penetrate the bulk rathei
than simply the surface of the crucible wall. This pro
vides a more uniform or average resistance path, w
well as allowing the required power to be dissipatec
from a larger volume of the material. Both these effect
tend to eliminate hot spots, thus prolonging crucibi
life.
Several other features of low frequency heating ar
notable. The power source (motor-generator) can b
cm,
(fit)
where
p = resistivity in ohm-cm for the material being
heated,
p = permeability of the material being heated, = I
for non-magnetic materials,
f = induction heating frequency (Hz),
D = depth (cm) in the material being heated, at which
the current density reaches 1/c (—~37~)of the
density value of the surface.
Since power is proportional to the square of the
50
LI
LI
LOW-FREQUENCY
INDUCTION HEATING
located far from the crystal growing furnace, connected
to the work coil only by coaxial cable, thus freeing the
laboratory of unnecessary noise and providing for
efficient equipment layout. Ten kHz motor-generators
are more stable to line voltage fluctuations than high
frequency induction or resistance heating sources
because only the generator field supply, which requires
about one percent of the rated output power, is sensifive to line voltage changes. Finally, low frequency
generators feature long life and low maintenance and
a cost comparable or below that of a tube-type 450
kHz generator of equal power rating.
3. Systems for crystal growing by low-frequency
induction heating
Power source. The basic source of power for lowfrequency induction heating consists of a 10 kHz
alternating current generator driven by a synchronous
motor. This “M—G Set” combination, well known in
commercial induction heating applications, is supplied
as standard industrial equipment, and has proven to be
highly reliable when used in continuous production
service.
Control system. Control systems for 10 kHz induction heating may be similar to those commonly used
for crystal growing with tube-type radio-frequency
generators which operate in the 450 kHz to 20 MHz
range, or with resistance heating. The suggested principal components of a 10 kHz control systems for crystal
51
FOR CRYSTAL GROWING
intended temperature. The input signal to the measuring
potentiometer is provided by the sensing element described in (1) above.
(3) A control amplifier with stepless dc output, and
response features which allow it to be tuned to the
thermal and electrical time rate characteristics of the
crystal growing system. Proportional, reset, rate and
approach actions should be provided in order that the
output may be adjusted for proper response. The input
to the control amplifier consists of the error signal
voltage supplied by the control slidewire described in
(2) above.
(4) A thyristor power supply which provides a controlled current to the field winding of the 10 kHz
generator. This supply receives its input signal from
the output of the control amplifier in (3) above.
Capacitance tuned work-coil. The transmission line
from the 10 kHz generator should be terminated by a
tuned circuit consisting of the work-coil and a suitable capacitance in parallel connection.
Block diagram of typical 10 kffz crystal growing
system. Fig. 1 illustrates schematically how the components described above can be assembled to form a
crystal growing system.
Crucible Multi-Range
and
TunIng
Work Coil
Capacitor
8
8
U~~
8-
-
10 Kilohertz
Generator
Q :::~
Synchronous
Motor
To AC
(I) A sensing element which provides a voltage pro5~1)
portional to the variable quantity to be controlled.
~
rT__
Some examples of sensing elements include: thermoField
couple, resistance thermometer, thernial radiation deTransformer
T~r~tor
tector, power transducer, and an emf source. For
Supply
crystal growing purposes the variable quantity to be
controlled must be either temperature or a quantity to
~r~i~r
which it is closely related such as the work-coil voltage.
(2) A self-balancing measuring potentiometerMeasuring
recorder with adjustable zero-suppression and range
Potentiometer
(span), and a control slidewire. Adjustable zero suppression and range are desirable in order to provide an Fig. I. Suggested 10kHz system with work-coil voltage control.
e’cpanded range for greater accuracy in measuring the
controlled variable. A control slidewire is required in 4. Crystal growth with a 10 kHz system
order to provide an error signal voltage which is
We have successfully Czochralski-grown many highproportional to the difference between the measured melting-point compounds including gadolinium alutemperature (or other controlled variable) and the minate3), yttrium aluminum garnet, strontium barium
52
C. S. DUNCAN,
R. I-I. HOPKINS
AND
R. MAZELSKY
niobate, sapphire, fluorapatite4), and the silicate oxyapatites°)by means of the low frequency system discussed above. Even when open, poorly insulating
furnace arrangements were used4) melting of the platinum or iridium crucibles was rarely observed and
many growth runs were made before shape changes
gradually rendered crucibles useless. Large relatively
perfect crystals of CaLa
4(Si04)30 (MP 2170 ~C)have
been grown in routine fashion from iridium crucibles
having 1.5 mm thick walls.
Withable
450tokHz
2) were
growheating
spinel
Cockayne
and
Chesswas
(MP 2105 °C)without crucible failure only by resorting
to thick (3 mm) walled crucibles and the use of an iridium disk to reduce radial temperature gradients.
The improved durability of expensive iridium crucibles
can be an important economic advantage of 10 kHz
heating.
when materials must be grown at temperatures approaching the melting point of the crucible. Such systems
also have the advantage of high reliability, stability,
and competitive cost with presently used systems.
5. Conclusions
Growth 2 (1968) 209.
4) R. Mazeisky, R. C. Ohlmann and K. B. Steinbruegge, J.
Electrochem. Soc. 115 (1968) 68.
5) R. H. Hopkins, G. W. Roland, K. B. Steinbruegge and W. D.
Low frequency induction heating, often ignored in
crystal growing applications, can be especially useful
Acknowledgements
The authors wish to express their appreciation to
W. E. Kramer, W. B. Stickel, E. 1’. A. Metz, W. A.
Stewart, R. L. John, W. Gaida, and R. P. Storrick for
their
which many
formedvaluable
the basiscontributions
for this paper.to the programs
References
I) See, for example, E. May, Industrial High Frequency Electron
Power (Wiley, New York, 1950).
2) B. Cockayne and M. Chesswas, J. Mater. Sci. 2 (1967) 398.
3) R. Mazelsky, W. E. Kramer and R. ft Hopkins, J. Crystal
Partlow, J. Electrochem. Soc.
118 (1971) 638.