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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.