K. Ohsaka

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.

Specific volumes of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy in the liquid, glass, and crystalline states

The specific volumes of the Zr41.2Ti13.8CU12.5Ni10.0Be2.25 alloy as a function of temperature, T, are determined by employing an image digitizing technique and numerical calculation methods applied to the electrostatically levitated spherical alloy. The linear fitting of the volumes of the alloy in the liquid, V-l, glass, V-g, and crystalline V-c, states in the temperature ranges shown in parentheses are V-l(T) = 0.1583 + 8.877 x 10(-6)T(cm^(3)/g) (700-1300 K); V-g(T) = 0.1603 + 5.528 x 10^(-6)T (400-550 K); V-c(T) = 0.1583 + 6.211 x 10(-6)T(400-850 K). The average volume thermal expansion coefficients within the temperature ranges are determined to be 5.32, 3.39, and 3.83 x 10^(-5) (1/K) for the liquid, glass, and crystalline states, respectively.

DYNAMIC NUCLEATION OF ICE INDUCED BY A SINGLE STABLE CAVITATION BUBBLE

Evidence is presented of the dynamic nucleation of ice induced by an isolated stable cavitation bubble in undercooled water. The evidence supports the notion of dynamic nucleation as a result of the freezing point shift due to high pressure pulses associated with a collapsing bubble. It is found that the cavitation bubble requires minimum undercooling, 5 K, to initiate nucleation on ice. This undercooling may be attributed to the temperature rise of the water due to the heat generated by compression of the bubble or the undercooling required for normal ice to nucleate on a high pressure phase.

Densities of Si determined by an image digitizing technique in combination with an electrostatic levitator

We have determined the densities of Si in the liquid, rhol(T), and solid, rhos(T), states as a function of temperature, T, by employing an image digitizing technique and numerical calculation methods in combination with an electrostatic levitator. The obtained density data can be fitted with the following equations: rhol(T) = rhol(Tm) – 1.71 × 10^–4(T – Tm) – 1.61 × 10^–7(T – Tm)2(g/cm^3); rhos(T) = rhos(Tm) – 2.63 × 10^–5(T – Tm)(g/cm^3),where Tm is the melting point, 1687 K, and rhol(Tm) and rhos(Tm) are 2.580 and 2.311 (g/cm^3), respectively. The error involved in the determination is estimated to be ±0.006 (g/cm3). The rhol(T) value smoothly varies through Tm and does not indicate a reported anomalous density variation. The rhol(Tm) value is 2% larger than the literature value and the coefficient of the linear temperature dependence is approximately half of a reported value. The rhos(Tm) value closely agrees with the literature value.

Apparatus for measuring the growth velocity of dendritic ice in undercooled water

A unique apparatus for measuring the dendritic growth velocity of ice crystals in undercooled water is described and a preliminary result at undercooling levels between 5 and 8.5 K is presented. The result appears to show a significant deviation from the theoretical prediction based on the thermal diffusion model possibly due to sluggish interface kinetics which start playing a role in dendritic growth at these undercooling levels. Discussion includes necessary modifications of the diagnostic technique in order to eliminate uncertainties in the preliminary data and improve the accuracy in future measurements.

Undercooling of acoustically levitated molten drops

Abstract The effect of ultrasound on the undercooling of an acoustically levitated molten drop is investigated by measuring the onset temperature of solidification. The measurement indicates that ultrasound occasionally terminates undercooling by initiating the nucleation of a solid at an undercooling level which is lower than that determined for nucleation catalyzed by the impurities in the drop. The results are interpreted by thermodynamic considerations which indicate a significant increase in effective undercooling of the liquid, beyond the level set by the impurities upon the collapse of acoustically driven pre-existing gas microbubbles.

Gibbs free energy difference between the undercooled liquid and the beta phase of a Ti-Cr alloy

The heat of fusion and the specific heats of the solid and liquid have been experimentally determined for a Ti60Cr40 alloy. The data is used to evaluate the Gibbs free energy difference, ΔG, between the liquid and the β phase as a function of temperature to verify a reported spontaneous vitrification (SV) of the β phase in Ti‐Cr alloys. The results show that SV of an undistorted β phase in the Ti60Cr40 alloy at 873 K is not feasible because ΔG is positive at the temperature. However, ΔG may become negative with additional excess free energy to the β phase in the form of defects.

Specific volumes and viscosities of the Ni-Zr alloys and their correlation with the glass formability of the alloys

Abstract The specific volumes and viscosities of the Ni–Zr liquid alloys as a function of temperature are determined by employing a digitizing technique and numerical analysis methods applied to the optical images of the electrostatically levitated liquid alloys. The results show that the specific volume of the NiZr 2 alloy is significantly smaller than that of an ideal mixture of the constituent metals, thereby indicating the presence of the associated species in the form of NiZr 2 . The presence of NiZr 2 species can account for the observed properties; the specific volume anomaly above the melting point under a moderate cooling rate and a relatively small undercooling level prior to crystallization. The results also show that the viscosities of the compound forming alloys are larger than those of the eutectic alloys, thereby indicating the presence of the NiZr and NiZr 2 species. The associated species are stable and exist at temperatures well above their melting points. It is concluded that a small specific volume or a large viscosity resulting from the presence of the associated species primarily facilitates the glass formation of the Ni–Zr alloys by slowing down the growth of the crystalline phases.

Gibbs free-energy difference between the glass and crystalline phases of a Ni-Zr alloy

The heats of eutectic melting and devitrification, and the specific heats of the crystalline, glass, and liquid phases have been measured for a Ni24Zr76 alloy. The data are used to calculate the Gibbs free-energy difference, DeltaGAC, between the real glass and the crystal on an assumption that the liquid-glass transition is second order. The result shows that DeltaGAC continuously increases as the temperature decreases in contrast to the ideal glass case where DeltaGAC is assumed to be independent of temperature.

Melting and solidification of acoustically levitated drops

A single axis acoustic levitator has been used to suspend a drop of commercial grade succinonitrile in air. The drop is subjected to thermal cycling in order to observe melting and solidification of the drop in the acoustically levitated state. Melting takes place over a temperature rise of a few degrees, which is typical for an impure material. Solidification initially progresses along the drop surface because of the temperature gradient developed in the drop. The resultant solid drop shows a dendritic surface feature with a cavity in some circumstances. An attempt is made to calculate the growth velocity of the dendrites.