PS24
ガスジェット浮遊による過冷却液滴からの準安定
γ-Al2O3 成長とその融解熱測定
Growth of metastable γ-Al2O3 from a supercooled
droplet and its heat of fusion measurement using anaerodynamic levitation
兵頭慧 1, 小畠秀和 1, 後藤琢也 1,
Kei HYODO1, Hidekazu KOBATAKE1, Takuya GOTO1
1

同志社大学，Doshisha University#1,

1. Introduction
The heat of fusions of oxides is one of the fundamental thermodynamic properties that govern the heattransport process during the solidification of functional materials1) or the crystallization of minerals frommagma2). Conventionally, the heat of fusion has been measured using drop calorimeters3) or differential
scanning calorimeters. However, for the oxide with high melting temperature, it becomes difficult task tomeasure the heat of fusion due to chemical interactions with the contact materials. Additionally, heat loss
through radiative heat transfer causes experimental uncertainty in the measurement using the drop
calorimetry at elevated temperature4). Furthermore, conventional measurements are limited to stable phases,
despite the growing interest in the thermodynamic properties of metastable oxides for functional materialprocessing5). To overcome these difficulties, the heat of fusion of oxide materials has been measured based on
hypercooling (ΔThyp) limit determination using an aerodynamic levitator. Al2O3 was used for the evaluation ofthe validity of this technique.
2. Experiment
A spherical α-Al2O3 sample (diameter: 2 mm) was levitated in an Ar gas flow using the aerodynamic levitator
and then melted by CO2 laser irradiation. The temperature of the samples was measured with a two-color
radiation thermometer, which was calibrated based on Wien's law using the liquidus temperature. From thetemperature profile during the solidification process, the hypercooling limit was determined. Using thehypercooling limit, the heat of fusion of the Al2O3 was determined. The phase of the obtained sample wasevaluated by X-ray diffraction (XRD).
3. Measurement Principle

1 of 3Our previous investigation6,7) have shown that there is a linear relation between the degree of supercooling(ΔT) and the plateau duration (∆t).
(1)

𝛥𝐻f = 𝐶p 𝛥𝑇 + 𝛼𝛥𝑡Here, ΔHf denotes the heat of fusion, Cp is the heat capacity of liquid phase at constant pressure, and α
represents the term accounting for heat loss caused by forced convection by gas flow and by radiation. The xintercept of this line corresponds to the hypercooling limit. Using the hypercooling limit, the heat of fusion
simply can be expressed as(2)𝛥𝐻f = 𝐶p 𝛥𝑇.
4. Results and discussion

Figure 1 shows the temperature profile of the sample. From the recalescence process, the both plateau
temperature duration (Δt) and degree of undercooling (ΔT) can be obtained. Figure 2 shows the correlation
between thermal plateau time (Δt) and degree of undercooling (ΔT). From the fitting line, ΔThyp wasdetermined to be 393 K.
Using the heat capacity of liquid Al2O3 (192.464 J mol-1 K-1) in literature8), the heat of fusion of Al2O3 wasdetermined as 75.7 kJ mol-1. This value is 23% smaller than the heat of fusion of α-Al2O3 in literature8).
Meanwhile XRD analysis showed that the metastable phase γ-Al2O3 was formed during the solidification
process. Further, it was found that γ-Al2O3 appeared at larger undercooling region; ΔT > 101 K. Moreover, theintensity of γ-Al2O3 peak in XRD increased as increase of the degree of undercooling.

These results inferredthat metastable γ-Al2O3 can be formed at larger undercooling. The determined of heat of fusion of Al2O3 inthis study was close to the that of γ-Al2O3 in the NIST-JANAF database (78.5 kJ mol-1) indicating that the heatof fusion of metastable γ-Al2O3, was directly determined.

Figure 1. A typical temperature profile of the recalescence of Al2O3.
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Figure 2. The correlation between thermal plateau time (Δt) and degree of undercooling (ΔT)

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1)© 2024 by the authors. Submitted for possible open access publication under theterms and conditions of the Creative Commons Attribution (CC BY) license(http://creativecommons.org/li censes/by/4.0/).

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