High-temperature Electrostatic Levitator Research Articles

Minimizing convection effects to measure diffusion in liquid droplets during high-temperature electrostatic levitation

We present an approach to reduce the convective flow in an electrostatically levitated liquid droplet to such an extent that diffusion is the dominant mechanism for mass transport, thus enabling direct measurements of atomic diffusion in reactive liquids at elevated temperatures. Convection is minimized by containerless processing, and reducing temperature variations in the sample. The diffusion tracer is deposited in situ in the electrostatic levitation device used for containerless processing. Uniform noncontact heating of the sample is achieved by laser heating with multiple beams arranged symmetrically, e.g., in a tetrahedral geometry. The expected temperature variations and the resulting convection flows are estimated for a Zr-based glass-forming alloy. The analysis suggests that diffusion experiments are possible throughout the entire undercooled liquid temperature range of this alloy and, in microgravity, up to 50K above the liquidus temperature.

Thermophysical Properties of Molten Germanium Measured by the High Temperature Electrostatic Levitator

Thermophysical properties of molten germanium have been measured using the high-temperature electrostatic levitator at the Jet Propulsion Laboratory. Measured properties include the density, the thermal expansivity, the hemispherical total emissivity, the constant-pressure specific heat capacity, the surface tension, and the electrical resistivity. The measured density can be expressed by ρ liq=5.67×103−0.542 (T−T m ) kg·m−3 from 1150 to 1400 K with T m=1211.3 K, the volume expansion coefficient by α=0.9656×10−4 K−1, and the hemispherical total emissivity at the melting temperature by e T, liq(T m)=0.17. Assuming constant e T, liq(T)=0.17 in the liquid range that has been investigated, the constant-pressure specific heat was evaluated as a function of temperature. The surface tension over the same temperature range can be expressed by σ(T)=583−0.08(T−T m) mN·m−1 and the temperature dependence of the electrical resistivity, when r liq(T m)=60μμΩ·cm is used as a reference point, can be expressed by r e, liq(T)=60+1.18×10−2(T−1211.3)μΩ·cm. The thermal conductivity, which was determined from the resistivity data using the Wiedemann–Franz–Lorenz law, is given by κ liq(T )=49.43+2.90×10−2(T−T m) W·m−1·K−1.

Non-contact measurements of thermophysical properties of titanium at high temperature

Four thermophysical properties of both solid and liquid titanium measured using the high-temperature electrostatic levitator at JPL are presented. These properties are density, thermal expansion coefficient, constant pressure heat capacity, and hemispherical total emissivity. For the first time, we report the density, the thermal expansion coefficient, and the ratio of the constant pressure heat capacity to the hemispherical total emissivity of undercooled titanium over a wide range of temperatures. Over the 1650 K to 2000 K temperature span, the liquid density can be expressed asρ (T)/(kg · m−3)= 4.208 · 103− 0.508 · (T−Tm)/K withTm= 1943 K, and the corresponding volume expansion coefficient asα= 1.169 · 10−4K−1. Similarly, over the 1540 K to 1940 K temperature range, the measured density of the solid can be expressed asρ (T)/(kg · m−3)= 4.321 · 103− 0.212 · (T−Tm)/K, giving a volume expansion coefficient α= 4.76 · 10−5K−1. The constant pressure heat capacity of the liquid phase could be estimated as Cp, m(T)/(J · K−1· mol−1)= 45.5 − 3.21 · 10−3· (T−Tm)/K if the hemispherical total emissivity of the liquid phase ϵTremains constant at 0.34 over the 1650 K to 2000 K temperature range. Over the 1540 K to 1940 K temperature span, the hemispherical total emissivity of the solid phase could be rendered asϵT (T) = 0.297 + 5.952 · 10−5· (T−Tm)/K. The enthalpy of fusion has also been calculated as 14.3 kJ · mol−1.

Thermophysical properties measurement of molten silicon by high-temperature electrostatic levitator: density, volume expansion, specific heat capacity, emissivity, surface tension and viscosity

Thermophysical properties of molten silicon measured by the high-temperature electrostatic levitator at JPL are presented. The properties include the density, the volume expansion, the constant pressure specific heat capacity, the hemispherical total emissivity, the surface tension and the viscosity. Over the temperature range investigated (1350–1850 K), the measured liquid density showed a quadratic nature expressed by ρ( T)=2.58−1.59×10 −4 ( T− T m)−1.15×10 −7 ( T− T m) 2 g/cm 3 with T m=1687 K. The volume expansion with respect to the melting point could be expressed by V( T)/ V m=1+6.18×10 −5 ( T− T m)+4.72×10 −8 ( T− T m) 2. The hemispherical total emissivity of molten silicon at the melting temperature was 0.18, and the constant pressure specific heat was determined over a 500 K around the melting point. The measured surface tension and the viscosity could be expressed respectively by σ( T)=765−0.016 ( T− T m) mN/m, and η( T)=0.75−1.22×10 −3 ( T− T m) mPa s.

Laser-induced rotation of a levitated sample in vacuum

A method of systematically controlling the rotational state of a sample levitated in a high vacuum using the photon pressure is described. A zirconium sphere was levitated in the high-temperature electrostatic levitator and it was rotated by irradiating it with a narrow beam of a high-power laser on a spot off the center of mass. While the laser beam heated the sample, it also rotated the sample with a torque that was proportional both to the laser power and the length of the torque arm. A simple theoretical basis was given and its validity was demonstrated using a solid zirconium sphere at ∼2000 K. This method will be useful to systematically control the rotational state of a levitated sample for the containerless materials processing at high temperature.

Thermophysical properties of zirconium at high temperature

Six thermophysical properties of both solid and liquid zirconium measured using the high-temperature electrostatic levitator at the Jet Propulsion Laboratory are presented. These properties are density, thermal expansion coefficient, constant pressure heat capacity, hemispherical total emissivity, surface tension, and viscosity. For the first time, we report the densities and the thermal expansion coefficients of both the solid as well as liquid Zr over wide ranges of temperatures. Over the 1700–2300 K temperature span, the liquid density can be expressed as ρ1(T) = 6.24 × 103 – 0.29(T – Tm) kg/m3 with Tm = 2128 K, and the corresponding volume expansion coefficient as α1 = 4.6 × 10−5/K. Similarly, over the 1250–2100 K range, the measured density of the solid can be expressed as ρs(T) = 6.34 × 103 – 0.15(T – Tm), giving a volume expansion coefficient αs = 2.35 × 10−5/K. The constant pressure heat capacity of the liquid phase could be estimated as Cpl(T) = 39.72 – 7.42 × 10−3(T – Tm) J/(mol/K) if the hemispherical total emissivity of the liquid phase εT1 remains constant at 0.3 over the 1825–2200 K range. Over the 1400–2100 K temperature span, the hemispherical total emissivity of the solid phase could be rendered as εTs(T) = 0.29 – 9.91 × 103 (T – Tm). The measured surface tension and the viscosity of the molten zirconium over the 1850–2200 K range can be expressed as ς(T) = 1.459 × 103 – 0.244 (T – Tm) mN/m and as η(T) = 4.83 – 5.31 × 10−3(T – Tm) mPa s, respectively.

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.

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) μΩ cm over Tm∼1160 K, rsolid=10.77+1.421×10−2(T−Tm) μΩ cm over 700 K∼Tm, and the thermal conductivity as determined by the Wiedemann–Franz–Lorenz law from the resistivity data was given by κliq(T)=94.61+4.41×10−2(T−Tm) W m−1 K−1, κsolid(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.

Measurements of thermophysical properties of molten silicon by a high-temperature electrostatic levitator

Several thermophysical properties of molten silicon measured by the high-temperature electrostatic levitator at JPL are presented. They are density, constant-pressure specific heat capacity, hemispherical total emissivity, and surface tension. Over the temperature range investigated (1350<T m<1825 K), the measured liquid density (in g·cm−3) can be expressed by a quadratic function,p(T)=p m−1.69×10−4(T−T m)−1.75×10−7(T−T m)2 withT m andp m being 1687 K and 2.56 g·cm−3, respectively. The hemispherical total emissivity of molten silicon at the melting temperature was determined to be 0.18, and the constant-pressure specific heat was evaluated as a function of temperature. The surface tension (in 10−3 N·m−1) of molten silicon over a similar temperature range can be expressed by σ(T)=875–0.22(T−T m).

Containerless liquid to solid nucleation pathways in two representative grades of commercially available zirconium

Experimental measurements were conducted to determine the solid metal nucleation pathways of radiatively cooling, molten zirconium spheres of two different commercially available purity grades in a high-vacuum, high-temperature electrostatic levitator. The ensemble distribution of maximum undercooling temperatures was interpreted using Poisson statistics to determine the temperature dependence of the solid metal nucleation rate. For a sample of nominally 99.95% pure zirconium, the results are consistent with heterogeneous solid metal nucleation either on static catalyst particles at least ∼30 nm diameter or on a surface coating. For a sample of nominally 99% pure zirconium, however, it appears that heterogeneous solid metal nucleation occurred either on a polydispersion of ∼10 nm (mean diameter) static catalyst particles or on dynamic catalyst particles that precipitated from a solution that became supersaturated as the melt cooled.

A noncontact measurement technique for the density and thermal expansion coefficient of solid and liquid materials

A noncontact measurement technique for the density and the thermal expansion coefficient of refractory materials in their molten as well as solid phases is presented. This technique is based on the video image processing of a levitated sample. Experiments were performed using the high-temperature electrostatic levitator (HTESL) at the Jet Propulsion Laboratory in which 2–3 mm diam samples can be levitated, melted, and radiatively cooled in vacuum. Due to the axisymmetric nature of the molten samples when levitated in the HTESL, a rather simple digital image analysis can be employed to accurately measure the volumetric change as a function of temperature. Density and the thermal expansion coefficient measurements were made on a pure nickel sample to test the accuracy of the technique in the temperature range of 1045–1565 °C. The result for the liquid phase density can be expressed by ρ=8.848+(6.730×10−4)×T (°C) g/cm3 within 0.8% accuracy, and the corresponding thermal expansion coefficient can be expressed by β=(9.419×10−5) −(7.165×10−9)×T (°C) K−1 within 0.2% accuracy.

A Method Based on Radiative Cooling for Detecting Structural Changes in Undercooled Metallic Liquids

Abstract We introduce a structure-sensitive parameter for undercooled melts which can be measured in containerless processing experiments. We have established that the ratio, R(T), of hemispherical total emissivity eT(T) to constant-pressure specific heat cp(T) can serve as an indicator which is sensitive to any changes in short range atomic order in undercooled metallic melts. R(T) ≡ eT(T)/cp(T) values for nickel, zirconium, and silicon have been obtained using the high temperature electrostatic levitator while the levitated melts were undergoing purely radiative cooling into the deeply undercooled region. R(T) plots for undercooled liquid nickel and zirconium indicate no significant change in short-range structure from their melting temperatures to 15% undercooling. In contrast, liquid silicon shows marked shortrange structural changes beginning above its melting temperature and extending throughout the undercooled region. The short-range structure of liquid silicon is related to the highly-directional covalent bonding which characterizes its solid form. The nickel and zirconium data show that eT varies linearly with T, in support of metal emissivity theories.