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An Electrostatic Levitator for High-Temperature Containerless Materials Processing in L-G Won-Kyu Rhim, Sang K

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An Electrostatic Levitator for High-Temperature Containerless Materials Processing in L-G Won-Kyu Rhim, Sang K An electrostatic levitator for high-temperature containerless materials processing in l-g Won-Kyu Rhim, Sang K. Chung, Daniel Barber, Kin F. Man, Gary Gutt, Aaron Rulison,a) and R. Erik Spjutb) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109 (Received 15 March 1993; accepted for publication 19 July 1993) This article discusses recent developments in high-temperature electrostatic levitation technology for containerless processing of metals and alloys. Presented is the first demonstration of an electrostatic levitation technology which can levitate metals and alloys (2-4 mm diam spheres) in vacuum and of superheating-undercooling-recalescence cycles which can be repeated while maintaining good positioning stability. The electrostatic levitator (ESL) has several important advantages over the electromagnetic levitator. Most important is the wide range of sample temperature which can be achieved without affecting levitation. This article also describes the general architecture of the levitator, electrode design, position control hardware and software, sample heating, charging, and preparation methods, and operational procedures. Particular emphasis is given to sample charging by photoelectric and thermionic emission. While this ESL is more oriented toward ground-based operation, an extension to microgravity applications is also addressed briefly. The system performance was demonstrated by showing multiple superheating-undercooling-recalescence cycles in a zirconium sample ( T, = 2 128 K) . This levitator, when fully matured, will be a valuable tool both in Earth-based and space-based laboratories for the study of thermophysical properties of undercooled liquids, nucleation kinetics, the creation of metastable phases, and access to a wide range of materials with novel properties. I. INTRODUCTION limited primarily to electrical conductors. (ii) In an ESL, sample heating and levitation do not interfere with each A capability for containerless materials processing is other so that the sample temperature can be varied over a important for the study of thermophysical properties of wide range, whereas the electromagnetic field in an EML is undercooled liquid states, nucleation kinetics, and for the intrinsically coupled to sample heating, which limits the production of various metastable phases with novel prop- lowest temperature it can attain for a given sample density. erties in different materials. The electrostatic levitator de- (iii) The ESL, through employment of feedback control, scribed in this article provides the capability of superheat- provides quiescent positioning during sample processing, ing, undercooling, and solidifying metals and alloys in a whereas a molten sample levitated by an EML is subjected clean (contactless and high vacuum) and quiescent envi- to strong internal flow which may cause severe shape dis- ronment. This levitator is an extension of the one previ- tortion, prevents accurate temperature measurements, and ously described’ in which sample levitation was limited to perhaps causes premature nucleation, therefore preventing low density materials at ambient conditions. deeper undercooling. (iv) The ESL provides a more open Suspension of charged particles by electrostatic forces view, whereas the levitation and heating coils in an EML has a rather long history.24 To our knowledge, however, are closely wound around the levitated sample, severely none of these past applications have addressed high- restricting access to the diagnostic instruments. However, temperature materials processing of large samples. The ad- one major drawback of the ESL is that it requires either a vantages of the present high-temperature high-vacuum high-vacuum or a high-pressure environment in order for a electrostatic levitator (ESL) when compared with the elec- high electric field to be applied without causing gas break- tromagnetic levitator (EML) 5 are (i) The ESL can accom- down. modate a broad range of materials, including metals, semi- We describe in this article an electrostatic levitator conductors, and insulators, since maintaining a sufficient which has demonstrated for the first time the capability to surface charge on the sample is the only requirement to process materials at high temperature in a clean environ- generate a levitation force. In contrast, an EML relies on ment. A brief review of the basic principles of an electro- eddy currents induced in a conducting sample by an ap- static levitator is given, followed by a description of the plied rf magnetic field. Mutual interaction between the general architecture of the levitator, electrode design, po- eddy currents and the applied field results in a levitation sition control hardware and software, sample heating, force. Materials that the EML can levitate are, therefore, charging, and the operational procedures. The system per- formance is demonstrated using zirconium samples which “NASA-NRC Resident Research Associate. melt at 2128 K. We conclude with a discussion on some “Department of Engineering, Harvey Mudd College, Claremont, CA. intrinsic problems and aspects that need to be improved in 2961 Rev. Sci. Instrum. 64 (lo), Ott 1993 0034-6748/93/64(10)/2961/10/$6.00 @ 1993 American Institute of Physics 2961 Downloaded 13 Jan 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp Top Electrode sample 0 5D -I-- Electrode To Vacl: PWYtp FIG. I. Schematic diagram of an electrostatic levitator in which the sample position is actively controlled only along the vertical direction. It relies on a two-dimensional potential well for centering in the lateral direction. FIG. 2. Schematic diagram of the high-temperature high-vacuum elec- trostatic levitator designed for ground-based applications. I is the sample, the future, and also point out how some of the difficulties 2 is the electrode assembfy, 3 is the focusing lens, 4 is the spherical we face in ground-based laboratories can be overcome in reflector, 5D and 60 are the position detectors, 5L and 6L are the He-Ne lasers, 7 is the I kW xenon lamp, 8 is the video camera with a telephoto the microgravity environment of space. lens, and 9 is the pyrometer. Ii. THE PRINCIPLES OF ELECTROSTATIC Ill. THE EXPERIMENTAL HARDWARE LEVITATION A schematic diagram of the present high-temperature/ In electrostatic levitation the positioning of a charged high-vacuum electrostatic levitator is shown in Fig. 2, The sample is achieved through the application of feedback- electrode assembly is housed in a cylindrical vacuum controlled electrostatic fields that are generated by a set of chamber and all the necessary equiement for levitation, appropriately positioned electrodes around the sample. heating, and diagnostics are located around the chamber, Since a three-dimensional electrostatic potential minimum The 30 &stainless steel chamber can be evacuated to an does not exist ( Earnshaw’s theorem6), electrostatic sample ultimate vacuum of 5 X lo-* Torr by a vibration-free mag- positioning is only possible with an actively controhed ap- netically suspended turbomolecular pump (Osaka model plied electric field. Our system uses a feedback control to TH250M ) backed by a roughing pump (Danielson model correct any deviation in sample position from a preset po- TDIOO). sition. The electrode assembly shown schematically in Fig, 3 Figure 1 is a schematic diagram of an electrostatic is located at the center of the chamber. The basic difFerence levitator with one-dimensional control capability. This sys- between this and the assembly shown in Fig. 1 is the ad- tem employs single-axis position control along the vertical dition of two pairs of side electrodes surrounding the bot- direction to overcome the gravitational force, mg. The ver- tom electrode. Damping voltages applied on these side tical position information from a position detector is com- electrodes prevent sample oscillation in the lateral direc- pared to a preset z coordinate to generate an error signal which is processed by a computer to generate a control signal according to a feedback algorithm. The control sig- nal is amplified and applied to the top electrode to main- tain the sample at the preset position. The force-balance equation for levitation when the sample is positioned at the center of a pair of infinite parallel electrodes is given ap- proximately by ,: ,: : : mg= f&V/L, ,,. where M is the mass of the sample carrying a charge Q, ,:,: and V is the voltage difference between the two electrodes separated by a distance L. For Q,=O.69 x 10m9 C, m = 140 4 mg, and L= 10 mm, V is approximately - 10 kV. As explained elsewhere,7 the electrode arrangement 0 shown in Fig. 1 provides a two-dimensional potential well in the horizontal direction when it is operated in the pres- ence of a gravitational force on Earth. In the microgravity FIG. 3. Schematic diagram of the electrode assembly (side and top views), where 1 is the sample, 2 is the top electrode, 3 is the bottom environment of space a different electrode arrangement electrode, 4’s are the side electrodes, and 5 is the hole which allows access and control method would be required. to the sample storage system. 2962 Rev. Scl. Instrtim., Vol. 64, No. 10, October 1993 Electrostatic levitator 2962 Downloaded 13 Jan 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp tions. Without the side electrodes the lateral position in- stability would introduce uncertainties into the diagnostic measurements. RulW an Two orthogonal HeNe lasers (30 mW each) together ml inpa with two position detectors provide the three-dimensional Qhlhb position information which is used by the microcomputer smvo mnpm to generate a feedback signal. A Macintosh IIfx computer and 12 bit Analog to Digital (A/D) and Digital to Analog Ti%T (D/A) converter cards were used for data acquisition and .t.ul(n position control. The computer is equipped with a Motor- m”*mnau ola 50 MHz 68030 microprocessor and 68882 floating WA WA otiputs j y$&W“*l!dS‘” ”lih point math coprocessor.
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