Aaron J. Rulison

An electrostatic levitator for high‐temperature containerless materials processing in 1‐g

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 (Tm=2128 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.

Metallic glass formation in highly undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 during containerless electrostatic levitation processing

Various sample sizes of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 with masses up to 80 mg were undercooled below Tg (the glass transition temperature) while electrostatically levitated. The final solidification product of the sample was determined by x-ray diffraction to have an amorphous phase. Differential scanning calorimetry was used to confirm the absence of crystallinity in the processes sample. The amorphous phase could be formed only after heating the samples above the melting temperature for extended periods of time in order to break down and dissolve oxides or other contaminants which would otherwise initiate heterogeneous nucleation of crystals. Noncontact pyrometry was used to monitor the sample temperature throughout processing. The critical cooling rate required to avoid crystallization during solidification of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy fell between 0.9 and 1.2 K/s.

Experimental determination of a time–temperature-transformation diagram of the undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy using the containerless electrostatic levitation processing technique

High temperature high vacuum electrostatic levitation was used to determine the complete time–temperature–transformation (TTT) diagram of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass forming alloy in the undercooled liquid state. This is the first report of experimental data on the crystallization kinetics of a metallic system covering the entire temperature range of the undercooled melt down to the glass transition temperature. The measured TTT diagram exhibits the expected ‘‘C’’ shape. Existing models that assume polymorphic crystallization cannot satisfactorily explain the experimentally obtained TTT diagram. This originates from the complex crystallization mechanisms that occur in this bulk glass‐forming system, involving large composition fluctuations prior to crystallization as well as phase separation in the undercooled liquid state below 800 K.

Scale‐up of electrospray atomization using linear arrays of Taylor cones

Linear arrays of Taylor cones were established on capillary electrode tubes opposite a slotted flat plate counterelectrode to investigate the feasibility of increasing the liquid throughput rate in electrospray atomizers. It was found that individual Taylor cones could be established on each capillary over a wide range of the capillary radius to spacing ratio R/S. The onset potential Vs required to establish the cones varied directly with R/S, but the liquid flow rate per cone and current per cone were nearly independent of R/S for a given overpotential ratio P=V/Vs. Only six working capillaries were used, but the results per cone are applicable to larger arrays of cones since end effects were minimized.

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<Tm<1825 K), the measured liquid density (in g·cm−3) can be expressed by a quadratic function,p(T)=pm−1.69×10−4(T−Tm)−1.75×10−7(T−Tm)2 withTm andpm 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−Tm).

Electrostatic containerless processing system

We introduce a materials science tool for investigating refractory solids and melts: the electrostatic containerless processing system (ESCAPES). ESCAPES maintains refractory specimens of materials in a pristine state by levitating and heating them in a vacuum chamber, thereby avoiding the contaminating influences of container walls and ambient gases. ESCAPES is designed for the investigation of thermophysical properties, phase equilibria, metastable phase formation, undercooling and nucleation, time–temperature–transformation diagrams, and other aspects of materials processing. ESCAPES incorporates several design improvements over prior electrostatic levitation technology. It has an informative and responsive computer control system. It has separate light sources for heating and charging, which prevents runaway discharging. Both the heating and charging light sources are narrow band, which allows the use of optical pyrometry and other diagnostics at all times throughout processing. Heat is provided to th...

HEMISPHERICAL TOTAL EMISSIVITY AND SPECIFIC HEAT CAPACITY OF DEEPLY UNDERCOOLED ZR41.2TI13.8CU12.5NI10.0BE22.5 MELTS

High-temperature high-vacuum electrostatic levitation (HTHVESL) and differential scanning calorimetry (DSC) were combined to determine the hemispherical total emissivity epsilon T, and the specific heat capacity cp, of the undercooled liquid and throughout the glass transition of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 bulk metallic glass forming alloy. The ratio of cp/epsilon T as a function of undercooling was determining from radiative cooling curves measured in the HTHVESL. Using specific heat capacity data obtained by DSC investigations close to the glass transition and above the melting point, epsilon T and cp were separated and the specific heat capacity of the whole undercooled liquid region was determined. Furthermore, the hemispherical total emissivity of the liquid was found to be about 0.22 at 980 K. On undercooling the liquid, the emissivity decreases to approximately 0.18 at about 670 K, where the undercooled liquid starts to freeze to a glass. No significant changes of the emissivity are observed as the alloy undergoes the glass transition.

A noncontact measurement technique for the specific heat and total hemispherical emissivity of undercooled refractory materials

A noncontact measurement technique for the constant pressure specific heat (c(pl)) and the total hemispherical emissivity (epsilon(T1)) of undercooled refractory materials is presented. In purely radiative cooling, a simple formula which relates the post-recalescence isotherm duration and the undercooling level to c(pl) is derived. This technique also allows us to measure epsilon(Tl) once C(pl) is known. The experiments were performed using the high-temperature high-vacuum electrostatic levitator at JPL in which 2-3 mm diameter metallic samples can be levitated, melted, and radiatively cooled in vacuum. The averaged specific heats and total hemispherical emissivities of Zr and Ni over the undercooled regions agree well with the results obtained by drop calorimetry: C(pl,av(Zr)=40.8+/-0.9 J/mol K, epsilon(Tl,av) (Zr)=0.28+/-0.01, c(pl,av)(Ni)=42.6+/-0.8 J/mol K, and epsilon(Tl,av)(Ni)=0.16+/-0.01.

Constant-pressure specific heat to hemispherical total emissivity ratio for undercooled liquid nickel, Zirconium, and Silicon

Radiative cooling curves of nickel, zirconium, and silicon melts that were obtained using the high-temperature, high-vacuum electrostatic levitator (HTHVESL) have been analyzed to determine the ratio between the constant-pressure specific heat and the hemispherical total emissivity,cp(T)/∈T(T).This ratio determined over a wide liquid temperature range for each material allows us to determinecp(T) if ∈T(T) is known orviceversa.Following the recipe, the hemispherical total emissivities for each sample at its melting temperature, ∈T(T)m, have been determined usingcp(Tm) values available in the literature. They are 0.15, 0.29, and 0.17, for Ni, Zr, and Si, respectively.

Ablation of silicate particles in high-speed continuum and transition flow with application to the collection of interplanetary dust particles

A model for the ablation and deceleration of spheres in continuum and slip flow is presented. Experiments were conducted in which initially spherical 7.1 micron diameter soda-lime glass particles were launched from vacuum at ~4500 m s^(-1) through a 0.5 mil (13 micron) plastic film into a capture chamber containing xenon at 0.1 and 0.2 atm and 295 K. Samples of ablated particles were collected and inspected using scanning electron microscopy (SEM). It was found that the ratio of the ablated particle radius (R_f) to the initial radius (R_0) depends on the gas pressure such that at 0.1 atm, R_f/R_0 = 0.67 ± 0.08, and at 0.2 atm, R_f/R_0 = 0.88 ± 0.08. The model agrees with these results if the heat of ablation Q is set to 1.5 ± 0.2 MJ kg^(-1). This value of Q approximately corresponds to the energy needed to raise the particle temperature from 295 to 1300 K, the working point of soda-lime glass. This indicates that the mechanism of ablation is melting and blowing of material from the particles surface.