Noncontact electrical resistivity measurement technique for molten metals

A noncontact technique of measuring the changes in electrical conductivity (or resistivity) of conducting liquids is reported. The technique is based on a conducting drop that is levitated by the high-temperature electrostatic levitator in a high vacuum. This technique, which utilizes the principle of the asynchronous induction motor, measures the relative changes in torque as a function of temperature by applying a rotating magnetic field to the sample. Changes in electrical resistivity are related to the changes in measured torque using the formula developed for the induction motor. Validity of this technique was demonstrated using a pure aluminum sample around its melting temperature. When the measurement results were calibrated by a literature value of resistivity at the melting point, our resistivity data around the melting point could be expressed by rliq = 24.19 + 1.306 × 10–2(T – Tm) µOmega cm over Tm ~ 1160 K, rsolid = 10.77 + 1.421 × 10–2(T – Tm) µOmega cm over 700 K ~ Tm, and the thermal conductivity as determined by the Wiedemann–Franz–Lorenz law from the resistivity data was given by kappaliq(T) = 94.61 + 4.41 × 10–2(T – Tm) W m – 1 K – 1, kappasolid(T) = 211.13 – 7.57 × 10–2(T – Tm) W m – 1 K – 1. Both electrical resistivity and thermal conductivity are in close agreement with the literature, confirming the validity of the present technique

Thermophysical Property Measurements of Molten Semiconductors in 1-g and Reduced-g Condition

Understanding and controlling the formation kinetics of varieties of crystal imperfections such as point defects, non uniform distribution of doping atoms, and impurity atoms in growing crystals are very important. Theoretical (numerical) modeling of the crystal growth process is an essential step to achieving these objectives. In order to obtain reliable modeling results, input parameters, i.e. various thermophysical parameters, must be accurate. The importance of accurate thermophysical properties of semiconductors in crystal growth cannot be overly emphasized. The total hemispherical emissivity, for instance, has a dramatic impact on the thermal environment. It determines the radiative emission from the surface of the melt which determines to a large extent the profile of the solidified crystal. In order to understand the convection and the turbulence in a melt, viscosity becomes an important parameter. The liquid surface tension determines the shape of the liquid-atmosphere interface near the solid-liquid-atmosphere triple point. Currently used values for these parameters are rather inaccurate, and this program intends to provide more reliable measurements of these thermophysical properties. Thus, the objective of this program is in the accurate measurements of various thermophysical properties which can be reliably used in the modeling of various crystal growth processes. In this program, thermophysical properties of molten semiconductors, such as Si, Ge, Si-Ge, and InSb will be measured as a function of temperature using the High Temperature Electrostatic Levitator at JPL. Each material will be doped by different kinds of impurities at various doping levels. Thermophysical properties which will be measured include: density, thermal expansion coefficient, surface tension, viscosity, specific heat, hemispherical total emissivity, and perhaps electrical and thermal conductivities. Many molten semiconductors are chemically reactive with crucibles. As a result, these dispersed impurities in the melts tend to substantially modify the properties of pure semiconductors. Sample levitation done in a vacuum clearly helps maintain the sample purity. However, in the 1-g environment, all gravity caused effects such as convection, sedimentation and buoyancy are still present in the sample. In addition, large forces needed to levitate a sample in the presence of the gravity can cause additional flows in the melt. The use of the High Temperature Electrostatic Levitator (HTESL) for the present research is a recent development and little is known about the flows induced by the electrostatic forces. In this ground base program, we will define the limits of HTESL technology as various thermophysical properties of molten semiconductors are measured

Electrostatic levitation technology for thermophysical properties of molten materials

Measurements of thermophysical properties of undercooled liquids often require some kind of levitator which isolates samples from container walls. We introduce in this presentation a high temperature/high vacuum electrostatic levitator (HTHVESL) which promises some unique capabilities for the studies of thermophysical properties of molten materials. Although substantial progress has been made in the past several months, this technology is still in the development stage, therefore, in this presentation we only focus on the present state of the HTHVESL(1) and point out other capabilities which might be realized in the near future

Transverse-magnetization recovery in the rotating frame

The transverse relaxation of F19 nuclei in Teflon in the rotating frame at exact resonance has been studied by using rf fields large compared to the local field in this solid. Various pulse sequences are explored which serve to trace out the decay of the magnetization in the rotating frame and, further, to recover the magnetization lost under the action of secular dipolar terms and of the inhomogeneity of the rf magnetic field. It is found theoretically, and partially confirmed experimentally, that the rotary free-induction decay can be refocussed even after the spin system has presumably attained a steady state in the rotating frame, contrary to the assumption of the spin-temperature approximation

NMR line narrowing by means of rotary spin echoes

The dipolar broadened F19 resonance line in Teflon has been narrowed by irradiating the solid with strong continuous rf magnetic fields under magic-angle conditions. The transverse magnetization in the rotating frame lost under the action of rf and dc magnetic field inhomogeneities has been recovered by means of rotary spin echoes produced by dc magnetic field pulses. The variation of the longitudinal relaxation with direction and strength of the effective magnetic field in this compound has also been measured

Thermal expansion of liquid Ti–6Al–4V measured by electrostatic levitation

The liquid density of Ti–6Al–4V was measured over a temperature range from 1661 to 1997 K that included undercooling by as much as 280 K. The sample was levitated in an electrostatic levitator and video imaging technique was used to capture the volume changes as a function of temperature. Over the temperature range the liquid density can be expressed by rholiq(T)=4123–0.254 (T–Tm) kg/m^3, where the melting temperature Tm is 1943 K. The corresponding volume expansion coefficient is alphaliq=6.05×10^–5 K^–1 near Tm

Feedback controlled electrostatic and electromagnetic sample positioners

Four different sample positioners are discussed. The four systems share a common operating principle in that the sample positioning is achieved by feedback controlled forces which can be electrostatic, dielectrophoretic, or electromagnetic. The first system is the electrostatic liquid drop positioner which operates at the near ambient position. The second system is the tetrahedral electrostatic positioner which is being developed for the high temperature materials processing in vacuum. The third system is essentially the the same tetrahedral system above except that the position control is achieved by dielectrophoretic forces in the pressurized gas environment. Finally, the feasibility of a feedback controlled electromagnetic positioner is discussed

Noncontact technique for measuring surface tension and viscosity of molten materials using high temperature electrostatic levitation

A new, noncontact technique is described which entails simultaneous measurements of the surface tension and the dynamic viscosity of molten materials. In this technique, four steps were performed to achieve the results: (1) a small sample of material was levitated and melted in a high vacuum using a high temperature electrostatic levitator, (2) the resonant oscillation of the drop was induced by applying a low level ac electric field pulse at the drop of resonance frequency, (3) the transient signals which followed the pulses were recorded, and (4) both the surface tension and the viscosity were extracted from the signal. The validity of this technique was demonstrated using a molten tin and a zirconium sample. In zirconium, the measurements could be extended to undercooled states by as much as 300 K. This technique may be used for both molten metallic alloys and semiconductors