Paul-François Paradis

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.

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 ρ_1(T) = 6.24 × 10^3 – 0.29(T – T_m) kg/m^3 with T_m = 2128 K, and the corresponding volume expansion coefficient as α_1 = 4.6 × 10^(−5)/K. Similarly, over the 1250–2100 K range, the measured density of the solid can be expressed as ρ_s(T) = 6.34 × 10^3 – 0.15(T – T_m), giving a volume expansion coefficient α_s = 2.35 × 10^(−5)/K. The constant pressure heat capacity of the liquid phase could be estimated as C_(pl)(T) = 39.72 – 7.42 × 10^(−3)(T – T_m) J/(mol/K) if the hemispherical total emissivity of the liquid phase e_(T1) 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 e_(Ts)(T) = 0.29 – 9.91 × 10^3 (T – T_m). 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 × 10^3 – 0.244 (T – T_m) mN/m and as η(T) = 4.83 – 5.31 × 10^(−3)(T – T_m) mPa s, respectively.

Maxwell–Wagner effect in hexagonal BaTiO3 single crystals grown by containerless processing

Oxygen-deficient hexagonal BaTiO3 single crystals, with dielectric constant e′∼105 and loss component tan δ∼0.13 at room temperature and a linear temperature dependence of e′ in the range 70–100K, was analyzed by impedance spectroscopy analysis. Two capacitors, bulk and interfacial boundary layer, were observed, and the colossal dielectric constant was mainly dominated by the interfacial boundary layers due to Maxwell–Wagner effect. After annealing the oxygen-deficient hexagonal BaTiO3 at 663K, the e′ and tanδ became, respectively, 2×104 and 0.07 at room temperature. This work showed an important technological implication as annealing at lower temperatures would help to obtain materials with tailored dielectric properties.

Study of the aerodynamic trap for containerless laser materials processing in microgravity

In the context of containerless laser processing of glasses in microgravity, a systematic study of the aerodynamic trap (ADT) has been done on the ground at both ambient and very high temperatures (≳2000 K). This work yielded a better understanding of the ADT and helped in improving its design. Experiments indicate that restoring force and sample stability depend upon the diffuser’s interior angle, flow rate, and ratio of sample to diffuser’s throat diameters. It was found that the trap’s potential energy curve versus position had a barrier height that increased with flow rate but decreased with increasing angle of the diffuser. Small angle diffusers show a greater spatial extent of the potential well, higher sphere‐to‐wall distances, and greater sample stability than larger angle diffusers. Low flow rates give quieter environments (smaller oscillations and perturbations due to the gas flow) than higher flow rates even though they are sufficient to trap the sample and damp external perturbations. Heat los...

Surface Tension and Viscosity Measurements of Liquid and Undercooled Alumina by Containerless Techniques

Electrostatic levitation and multi-beam radiative heating overcame contamination and sample position instability problems associated with handling of liquid alumina. This allowed the measurements of the surface tension and viscosity in the superheated and undercooled states using the oscillation drop method. Over the 2190–2500 K interval, the surface tension of alumina was measured as σ(T)=0.64–8.2×10-5 (T-Tm) (N/m), where Tm, the melting temperature, is 2327 K. Similarly, on the same temperature range, the viscosity was determined as η(T)=3.2exp [43.2×103/(RT)] (mPas). Both sets of data agree well with the literature values.

Persistence of Covalent Bonding in Liquid Silicon Probed by Inelastic X-Ray Scattering

Metallic liquid silicon at 1787 K is investigated using x-ray Compton scattering. An excellent agreement is found between the measurements and the corresponding Car-Parrinello molecular dynamics simulations. Our results show persistence of covalent bonding in liquid silicon and provide support for the occurrence of theoretically predicted liquid-liquid phase transition in supercooled liquid states. The population of covalent bond pairs in liquid silicon is estimated to be 17% via a maximally localized Wannier function analysis. Compton scattering is shown to be a sensitive probe of bonding effects in the liquid state.

Non-contact Thermophysical Property Measurements of Liquid and Supercooled Platinum

The density and the isobaric heat capacity of alumina in its liquid and undercooled states were measured using an electrostatic levitation furnace. Over the 2175 to 2435 K temperature interval, the density can be expressed as ρ(T)=2.93×103-0.12(T-Tm) (kgm-3) with Tm=2327 K, yielding a volume expansion coefficient α(T)=4.1×10-5 (K-1). In addition, the isobaric heat capacity can be estimated as CP(T)=153.5+3.1×10-3(T-Tm) (Jmol-1K-1) if the hemispherical total emissivity of the liquid remains constant at 0.8 over the 2120 K to 2450 K interval. The enthalpy and entropy of fusion have also been calculated respectively as 109.0 kJmol-1 and 46.8 Jmol-1K-1.

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.

Physical properties of liquid and undercooled tungsten by levitation techniques

Maintaining deep undercooling melts represents a formidable challenge when dealing with tungsten due to its high vapor pressure, its melting temperature (Tm=3695K), and the risk of contamination. Using electrostatic levitation, properties of liquid tungsten were measured above the melting temperature as well as in the undercooled phase. Over the 3125–3710K interval, the density was measured as ρ(T)=1.67×104−1.08(T−Tm)kgm−3. Similarly, the surface tension was measured as σ(T)=2.478×103−0.31(T−Tm) over the 3360–3700K range. At Tm, the data agree well with the literature values. The excellent processing conditions also offer opportunities to achieve reproducible and controlled formation of metastable phases.

Viscosity measurements of molten refractory metals using an electrostatic levitator

Viscosities of several refractory metals (titanium, nickel, zirconium, niobium, ruthenium, rhodium, hafnium, iridium and platinum) and terbium have been measured by the oscillation drop method with an improved procedure. The measured data were less scattered than our previous measurements. Viscosities at their melting temperatures showed good agreement with literature values and some predicted values.