Article Dependence of Liquid Supercooling on Liquid Overheating Levels of Al Small Particles Qingsong Mei 1,* and Juying Li 2 Received: 5 November 2015; Accepted: 17 December 2015; Published: 24 December 2015 Academic Editor: Andreas Taubert 1 Department of Materials Engineering, School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China 2 School of Mechanical Engineering, Wuhan Polytechnic University, Wuhan 430023, China; [email protected] * Correspondence: [email protected]; Tel.: +86-27-6877-2252 Abstract: The liquid thermal history effect on liquid supercooling behavior has been found in various metals and alloys; typically the degree of liquid supercooling (DT´) increases with the increase of liquid overheating (DT`) up to several to tens of degrees above the equilibrium melting point ´ ` (T0). Here we report quantitative experimental measurements on the DT -DT relationship of Al small particles encapsulated in Al2O3 shells by using a differential scanning calorimeter. We find a remarkable dependence of DT´ on DT` of Al small particles, extending to at least 340 ˝C above T0 of Al (~1.36T0), which indicates the existence of temperature-dependent crystallization centers in liquid Al up to very high liquid overheating levels. Our results demonstrate quantitatively the significant effect of liquid thermal history on the supercooling behavior of Al and its alloys, and raise new considerations about the dependence of DT´ on DT` at very high DT` levels. Keywords: liquid overheating; liquid supercooling; solidification; thermal history; Al particles 1. Introduction Solidification and melting are transformations basic to technological applications such as casting, crystal growth, and glass formation. Historically, the liquid thermal history was discovered to have a significant effect on the liquid supercooling behavior and thus, the nucleation and growth of crystals and their qualities [1–13]. The thermal history effect can be quantified by the relationship between liquid overheating (DT`), which is measured by the difference between liquid temperature and the ´ equilibrium melting point (T0), and liquid supercooling (DT ), which is measured by the difference ´ ` between T0 and the temperature of solidification. Thus far, the dependence of DT on DT has been investigated in metals/semimetals (Bi, Sn, Ga) [2–5,11,14] and alloys [6–11]; typically DT´ increases ` with an increase of DT up to several to tens of degrees above T0. Despite the fundamental and technical importance, the underlying mechanism of this phenomenon in relation to the liquid structure and the kinetics of heterogeneous nucleation of solidification is not well understood. Turnbull [14] suggested that crystals in microcavities, which are on the container or surface of impurity particles inside a melt, can be retained above T0, i.e., they have elevated melting points. These retained crystals can serve as nuclei for solidification at a certain supercooling level, which results in an increase of DT´ with the increase of DT` [14]. The cavity theory has been quantitatively validated by various experimental results [2–5], but it was considered to be not general, e.g., the dependence of DT´ on DT` in Bi, Sn, SnSb and SnPb was found to be either continuous or discontinuous [10,11], which points to the evolution of transient short range order structures in the liquid state. Obviously, more investigations of the DT`-DT´ relationship (thermal history effect) in metals and alloys are needed. In the casting of Al alloys, liquid Materials 2016, 9, 7; doi:10.3390/ma9010007 www.mdpi.com/journal/materials Materials 2016, 9, 7 2 of 8 thermal history effects on the microstructure of solidification have been reported [15,16]. Hereby, a quantitative study of the dependence of DT´ on DT` in the simple metallic system Al is considered to be interesting. For experimental measurements of the DT`-DT´ relationship, a precise control of temperature and heating/cooling rate is needed, and extraneous effects (e.g., oxidation) should be excluded [3,4]. Meanwhile, in bulk continuous systems, the factors affecting the solidification process are rather accidental, and the crystallization centers in one region may have an effect on the freezing behavior of the whole. In this study, we prepared samples of Al particles encapsulated in Al2O3 shells to eliminate possible effects of oxidation and coalescence at high temperatures. By using differential scanning calorimeter (DSC), we quantitatively measured the DT`-DT´ relationship of encapsulated Al particles and found a remarkable dependence of DT´ on DT` that extends to ultrahigh DT` levels. 2. Materials andMaterials Methods 2016, 9, 0007 quantitative study of the dependence of ΔT− on ΔT+ in the simple metallic system Al is considered to Al small particlesbe interesting. were For preparedexperimental bymeasurements means of of active the ΔT+‐Δ HT2 plasmarelationship, evaporation a precise control and of condensation, using bulk Al (withtemperature a purity and heating/cooling of 99.8%) rate as theis needed, starting and extraneous material. effects The (e.g., particles oxidation) were shouldin be situ passivated excluded [3,4]. Meanwhile, in bulk continuous systems, the factors affecting the solidification process at room temperatureare rather before accidental, exposure and the crystallization to air. The centers passivated in one region Al may particles have an effect were on the further freezing oxidized in air to produce a thickbehavior surface of the whole. oxide In this shell. study, Samples we prepared with samples different of Al particles oxidation encapsulated extents in Al2O3 wereshells produced by changing the temperature/durationto eliminate possible effects of ofoxidation oxidation. and coalescence at high temperatures. By using differential scanning calorimeter (DSC), we quantitatively measured the ΔT+‐ΔT relationship of encapsulated Al The morphologyparticles and and found size a remarkable of Al particles dependence was of Δ investigatedT− on ΔT+ that extends by a to transmission ultrahigh ΔT+ levels. electron microscope (TEM, Philips, Amsterdam, The Netherlands) and a high-resolution transmission electron microscope 2. Materials and Methods (HRTEM, JEOL, Tokyo, Japan), which was conducted on a Philips EM 420 microscope (Philips) Al small particles were prepared by means of active H2 plasma evaporation and condensation, with an acceleratingusing bulk voltage Al (with a of purity 100 of kV 99.8%) and as athe JEM starting 2010 material. high-resolution The particles were microscope in situ passivated (JEOL) with an accelerating voltageat room of temperature 200 kV, before respectively. exposure to Samples air. The passivated for TEM Al particles and HRTEM were further observations oxidized in air were prepared to produce a thick surface oxide shell. Samples with different oxidation extents were produced by by dispersing thechanging particles the temperature/duration in ethanol by of sonicationoxidation. and then, dropping on a carbon-coated copper grid. Formation ofThe an morphology oxide shell and size on of Al the particles Al particle was investigated surface by a transmission was confirmed electron microscope by TEM and HRTEM observations. The(TEM, surface Philips, Amsterdam, oxide shell The was Netherlands) found and to bea high able‐resolution to retain transmission the shape electron of microscope Al particles effectively (HRTEM, JEOL, Tokyo, Japan), which was conducted on a Philips EM 420 microscope (Philips) with upon heating andan accelerating cooling sovoltage that of the 100 particlekV and a canJEM melt2010 high and‐resolution freeze independently.microscope (JEOL) with an Thermal analysisaccelerating was voltage performed of 200 kV, on respectively. a Netzsch Samples high-temperature for TEM and HRTEM differential observations scanning were calorimeter prepared by dispersing the particles in ethanol by sonication and then, dropping on a carbon‐coated (DSC 404C, Netzsch,copper grid. Selb, Formation Germany). of an oxide The shell sample on the Al inparticle Al2 Osurface3 crucible was confirmed was measuredby TEM and in a dynamic ˝ Ar atmosphereHRTEM with observations. a gas flow The surface rate oxide of 50 shell mL/min was found to and be able a to heating/cooling retain the shape of Al particles rate of 20 C/min. The temperatureeffectively and enthalpy upon heating were and cooling calibrated so that the by particle the melting can melt and endotherms freeze independently. of pure In, Al and Ag. Thermal analysis was performed on a Netzsch high‐temperature differential scanning calorimeter (DSC 404C, Netzsch, Selb, Germany). The sample in Al2O3 crucible was measured in a dynamic Ar 3. Results andatmosphere Discussion with a gas flow rate of 50 mL/min and a heating/cooling rate of 20 °C/min. The temperature and enthalpy were calibrated by the melting endotherms of pure In, Al and Ag. As shown in Figure1a, the original Al particles are nearly spherical in shape with sizes mostly 3. Results and Discussion ranged from ~30 to ~500 nm. Al particles were encapsulated in Al2O3 shells after thermal oxidation at 500 ˝C for 90 minAs in shown air (Figure in Figure1 b,c).1a, the X-rayoriginal diffractionAl particles are (XRD)nearly spherical analysis in shape also with confirmed sizes mostly the formation ranged from ~30 to ~500 nm. Al particles were encapsulated in Al2O3 shells after thermal oxidation at of oxide shells500 (Figure °C for 901d). min Thein air Al(Figure2O 1b,c).3 shells X‐ray are diffraction able to(XRD) prevent