Engineering topics

Ohsaka, K.

Publications and source records attributed to Ohsaka, K..

25 records · Page 2

NASA NTRS1992

On the melting temperatures of low-temperature phases of polymorphic metals

An improved analytical formula for determining the melting temperatures of the low-temperature phases of polymorphic metals is proposed which uses the specific heat differences at the equilibrium transition temperatures. The formula is solved by an iterative method, with no more than one iteration necessary to converge. The results obtained using the formula proposed here are generally in good agreement with the analytical solution.

Ohsaka, K.↗

NASA NTRS1991

An apparatus for the specific heat measurement of undercooled liquids

This paper describes a drop calorimeter with an electromagnetic levitator that was specifically built for enthalpy measurements of undercooled liquids, including high-melting-point metals. Design diagrams of this device and of a furnace for making a suspended drop are presented together with results of measurements on an aluminum sample.

Ohsaka, K.↗

NASA NTRS1991

Free energy change of off-eutectic binary alloys on solidification

A formula for the free energy difference between the undercooled liquid phase and the stable solid phase is derived for off-eutectic binary alloys in which the equilibrium solid/liquid transition takes place over a certain temperature range. The free energy change is then evaluated numerically for a Bi-25 at. pct Cd alloy modeled as a sub-subregular solution.

Ohsaka, K.↗

NASA NTRS1990

Undercooling of acoustically levitated molten drops

It was observed that the acoustically levitated molten SCN (succinonitrile) drops can generally be undercooled to a degree where the impurities in the drop are responsible for the nucleation of the solid phase. However, it was also observed that ultrasound occasionally terminates undercooling of the levitated drops by initiating the nucleation of the solid at an undercooling level which is lower than that found for the nucleation catalyzed by the impurities in the drop. This premature nucleation can be explained by thermodynamic considerations which predict an increase in effective undercooling of the liquid upon the collapse of cavities. Pre-existing gas microbubbles which grow under the influence of ultrasound are suggested as the source of cavitation. The highly undercooled SCN drops can be utilized to measure the growth velocity of the solid in the deeply undercooled region including the hypercooled region.

Ohsaka, K.↗

NASA NTRS1989

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.

Ohsaka, K.↗

NASA NTRS1988

On dendritic growth in undercooled melts

The role of gravity-dependent convection in the steady-state growth of dendrites in undercooled melts is investigated theoretically. The model described by Huang and Glicksman (1981) is extended and refined, using the concept of a thermal diffusion boundary layer (Burton et al., 1987) to characterize the dendritic interface. Theoretical predictions are presented in graphs and shown to be in good general agreement with published experimental data on succinonitrile. The need for careful space experiments to clarify the role of nongravity-dependent convection is indicated.

Ohsaka, K.↗

NASA NTRS1988

The glass transition, crystallization and melting in Au-Pb-Sb alloys

The glass transition, crystallization and melting of Au(55)Pb(22.5)Sb(22.5) alloys have been studied by differential scanning calorimetry DSC. Crystallization on heating above the glass transition temperature Tg (45 C) begins at 64 C. Further crystallization events are observed at 172 C and 205 C. These events were found to correspond to the formation of the intermetallic compounds AuSb2, Au2Pb, and possibly AuPb2, respectively. Isothermal DSC scans of the glassy alloy above Tg were used to monitor the kinetics of crystallization. The solidification behavior and heat capacity in the glass-forming composition range were determined with droplet samples. An undercooling level of 0.3T(L) below the liquidus temperature T(L) was achieved, resulting in crystallization of different stable and metastable phases. The heat capacity C(P) of the undercooled liquid was measured over an undercooling range of 145 C.

Lee, M. C.↗