5:15 PM - 6:45 PM
[MIS18-P02] Growth of metastable γ-Al2O3 from a supercooled droplet and its enthalpy of fusion measurement using an aerodynamic levitation
Keywords:aerodynamic levitation, heat of fusion, metastable
The enthalpy of fusion of an oxide is one of the fundamental thermodynamic properties, which govern the heat transport process during the solidification of functional materials or mineral crystallization from magma. In addition to the thermodynamically stable materials, metastable phase of the oxide is also attractive for functional material processing.
Conventionally, the enthalpy of fusion has been measured using drop calorimeters or differential scanning calorimeters. However, due to the high melting points of oxides, measuring the enthalpy of fusion becomes complex due to chemical interactions with the container material. Heat loss through radiative heat transfer during the measurement poses a challenge. To overcome these difficulties, he enthalpy of fusion measurement for the oxide material based on the hypercooling limit determination using an aerodynamic levitator has been conducted. Al2O3 was used for the measurement for the measurement to evaluate its adequacy.
Our previous investigation have shown that there is a linear relation between the degree of supercooling (T) and the plateau duration (t) , expressed as Hf = CpT + alphat. Here, Hf denotes the enthalpy of fusion, Cp is the liquid's heat capacity at constant pressure, and alpha represents the term accounting for heat loss caused by forced convection by gas flow and radiation. From this linear relation, the hypercooling limit Thyp, can be determined when the plateau time is zero. Using the hypercooling limit, the enthalpy of fusion can be determined as a product of Thyp and Cp.
A spherical alpha-Al2O3 sample was levitated in an Ar gas flow in the aerodynamic levitator and then melted by a CO2 laser irradiation.
Using the liquid Al2O3 (192.464 J mol-1 K-1) in literature, the enthalpy of fusion of Al2O3 was calculated to be 85.3 kJ mol-1 based on these values. The experimentally determined heat of fusion was 23% smaller than the enthalpy of fusion of alpha-Al2O3 in literature. In-situ observation of the solidification process using the high-speed camera revealed the solidified crystalline re-melted and eventually the entire sample completely solidified. XRD analysis also showed that the metastable phase gamma-Al2O3 was formed during the solidification process.
From these experimental results and heat of fusion measurement, the following scenario of the solidification under supercooled conditions can be proposed,
1)Metastable gamma-Al2O3 nuclei formed as primary crystals and then grew.
2)Under small undercooled condition, stable alpha-Al2O3 nucleated form during the metastable gamma-Al2O3 growth.
3)Due to the release of heat of fusion of alpha-Al2O3, metastable gamma-Al2O3 was partially re-melted, and then both alpha-Al2O3 and gamma-Al2O3 solidified.
The enthalpy of fusion of Al2O3 measured in this study showed a value close to the enthalpy of fusion of gamma-Al2O3 in the NIST-JANAF database (78.5 kJ mol-1) indicating that the enthalpy of fusion of metastable gamma-Al2O3, solidified from supercooling, was directly determined.
Conventionally, the enthalpy of fusion has been measured using drop calorimeters or differential scanning calorimeters. However, due to the high melting points of oxides, measuring the enthalpy of fusion becomes complex due to chemical interactions with the container material. Heat loss through radiative heat transfer during the measurement poses a challenge. To overcome these difficulties, he enthalpy of fusion measurement for the oxide material based on the hypercooling limit determination using an aerodynamic levitator has been conducted. Al2O3 was used for the measurement for the measurement to evaluate its adequacy.
Our previous investigation have shown that there is a linear relation between the degree of supercooling (T) and the plateau duration (t) , expressed as Hf = CpT + alphat. Here, Hf denotes the enthalpy of fusion, Cp is the liquid's heat capacity at constant pressure, and alpha represents the term accounting for heat loss caused by forced convection by gas flow and radiation. From this linear relation, the hypercooling limit Thyp, can be determined when the plateau time is zero. Using the hypercooling limit, the enthalpy of fusion can be determined as a product of Thyp and Cp.
A spherical alpha-Al2O3 sample was levitated in an Ar gas flow in the aerodynamic levitator and then melted by a CO2 laser irradiation.
Using the liquid Al2O3 (192.464 J mol-1 K-1) in literature, the enthalpy of fusion of Al2O3 was calculated to be 85.3 kJ mol-1 based on these values. The experimentally determined heat of fusion was 23% smaller than the enthalpy of fusion of alpha-Al2O3 in literature. In-situ observation of the solidification process using the high-speed camera revealed the solidified crystalline re-melted and eventually the entire sample completely solidified. XRD analysis also showed that the metastable phase gamma-Al2O3 was formed during the solidification process.
From these experimental results and heat of fusion measurement, the following scenario of the solidification under supercooled conditions can be proposed,
1)Metastable gamma-Al2O3 nuclei formed as primary crystals and then grew.
2)Under small undercooled condition, stable alpha-Al2O3 nucleated form during the metastable gamma-Al2O3 growth.
3)Due to the release of heat of fusion of alpha-Al2O3, metastable gamma-Al2O3 was partially re-melted, and then both alpha-Al2O3 and gamma-Al2O3 solidified.
The enthalpy of fusion of Al2O3 measured in this study showed a value close to the enthalpy of fusion of gamma-Al2O3 in the NIST-JANAF database (78.5 kJ mol-1) indicating that the enthalpy of fusion of metastable gamma-Al2O3, solidified from supercooling, was directly determined.