Formation of Transition Alumina Dust around Asymptotic Giant Branch Stars: Condensation Experiments using Induction Thermal Plasma Systems

Mid-infrared spectroscopic observations of oxygen-rich asymptotic giant branch (AGB) stars show the common presence of dust species that have a broad feature at ∼11–12 μm. Chemically synthesized amorphous alumina (Al2O3) is widely accepted as the source of this emission, although it is not obvious that amorphous alumina can condense in circumstellar conditions. We performed condensation experiments of Al–Si–Mg–O and Mg–Al–O gases using induction thermal plasma systems, in which small particles condense from vapors with a steep temperature gradient. The condensates were analyzed using X-ray diffraction and Fourier transform infrared spectroscopy, and observed with a transmission electron microscope. The condensed nanoparticles from the Al and O gases were transition aluminas based on face-centered cubic (fcc) packed oxygen (δ- and λ-alumina, and an unknown phase). The fcc oxygen frameworks were maintained in the condensed alumina containing small amounts of Mg and Si. Condensates from the gases of Al:Mg = 99:1 and 95:5 had δ- and γ-alumina structures. Particles with λ- and γ-alumina structures formed from starting materials of Al:Si = 9:1 and Al:Si:Mg = 8:1:1, respectively. Amorphous silica-rich particles condensed from gases of Al/(Si+Al) < 0.75. The condensed transition alumina containing ∼10% Si showed similar spectral shapes to the observed dust emission from the alumina-rich AGB star T Cep. Based on the present results, it is reasonable that the source of 11–12 μm broad emission of alumina-rich stars is not amorphous alumina, but is transition alumina containing ∼10% Si.


Introduction
Mid-infrared (MIR) dust emission from oxygen-rich asymptotic giant branch (AGB) stars are classified into several types based on fluxes at 10, 11, and 12 μm (Sloan et al. 2003). About half of the known O-rich AGB stars show clear amorphous silicate emissions at 10 and 18 μm (i.e., they are silicate-rich stars). Dust emission with a broad peak at 11-12 μm is observed for about 40% of O-rich AGB stars. This dust emission cannot be explained by amorphous silicates, but is reproduced by amorphous Al 2 O 3 (i.e., they are alumina-rich or aluminum oxide-rich stars). The distribution of gaseous SiO and AlO molecules around the alumina-rich AGB star W Hya shows that >70% of SiO molecules remain gaseous, and the AlO gas distribution matches the dust distribution observed by optical polarimetric imaging (Ohnaka et al. 2016(Ohnaka et al. , 2017Takigawa et al. 2017). These observations indicate the efficient formation of alumina dust around alumina-rich stars. Presolar grains identified in primitive meteorites and cometary dust are survivals of the circumstellar dust of evolved stars that formed prior to the birth of the solar system. Although hundreds of presolar alumina grains have been reported, analysis of the crystal structures were performed for only several of them; this showed that most of the presolar alumina grains analyzed were corundum except for one amorphous alumina grain (Choi 1998;Stroud et al. 2004;Takigawa et al. 2014Takigawa et al. , 2018. Amorphous alumina synthesized by sol-gel processing is able to reproduce the broad peak at 11-12 μm, and thus is widely used to fit the dust emission of alumina-rich stars (Begemann et al. 1997). However, such a wet chemical technique (hydrolyzation and polymerization of metal alkoxides) is different to the possible dust-formation processes occurring around stars. The structure of amorphous alumina is not unique because it depends on its density (Gutiérrez & Johansson 2002). In laboratories, amorphous alumina has been prepared not only by sol-gel processing but by the sputter deposition technique, chemical vapor deposition, anodization, and via the oxidation of Al (e.g., Chou & Nieh 1991;Maruyama & Arai 1992;Yakovleva et al. 2000;Trunov et al. 2005).
It is, however, not obvious that amorphous alumina condenses in circumstellar environments. Crystalline aluminas are divided into two types: Al 2 O 3 based on hexagonal close packed (hcp) and face-centered cubic (fcc) packed oxygen. There are more than eight transition aluminas, which are metastable alumina phases with different crystal structures, other than the stable phase of Al 2 O 3 , corundum (α-alumina). Corundum, κ, and χ alumina have hcp oxygen frameworks, whereas γ, η, θ, θ′, θ″, δ, and λ aluminas have fcc-packed oxygen. Even from gases undergoing very high cooling, transition (rather than amorphous) alumina might condense, as seen in previous condensation experiments (Tavakoli et al. 2013;Ishizuka et al. 2016). Moreover, amorphous or metastable alumina that forms under strong disequilibrium might contain other elements in their structures, such as Mg and Si, because of the low elemental abundance of Al (Al/Si ∼0.08; Lodders 2003).
An induction thermal plasma (ITP) system provides an ultrahigh temperature plasma to vaporize refractory materials and a high cooling rate for gases with a sharp temperature gradient where nanoparticles condense. Nanoparticles of silicates with various compositions and textures have been synthesized using ITP systems to study the formation of cometary silicates (Matsuno et al. 2014;Kim et al. 2017). In this study, we perform condensation experiments of gases in the Al-Si-Mg-O system using ITP furnaces. The crystal structures of the condensed particles are examined with X-ray powder diffraction (XRD) and a field-emission transmission electron microscope (FE-TEM). Their MIR spectra are then measured and compared with the dust emission of the alumina-rich star T Cephei.

Condensation Experiments
An ITP system consists of a plasma torch, a chamber, and a radio-frequency power supply (Cheng et al. 2012;Kim et al. 2017). Experiments were performed in lower (6 kW) and higher (30 kW) power ITP systems. The experimental conditions are summarized in Table 1.
The condensation experiments that used Al-Si-O gases were performed in the lower-power ITP system (ITP-L; JEOL TP-40020NPS, TP-99010FDR) at Kyoto University, Japan. A mixture of Ar and O 2 was used as the central plasma-forming gas. A plasma jet was generated at a plate power of 6 kW and an induction frequency of 13. were used as starting materials. Ar gas at 5 L minute −1 was used as a central gas and a mixture of Ar and O 2 gases at 55 and 10 L minute −1 , respectively, was used as a sheath gas. We note that the experiments of only Al and O ( f Al =1) were performed in both the low-power (ITP-L) and high-power (ITP-H) ITP systems (Al 100 and Al 100 -H) to assess differences between them.
Starting materials are injected into a discharged plasma flame with a carrier gas of Ar and are vaporized at high temperatures (∼10,000 K). They react with a discharged oxygen gas to form Al and Si oxides. Steep temperature gradients in the plasma flames provide high cooling rates for the gases (10 4 -10 5 K s −1 ) where nanoparticles subsequently condense. Condensed fine powders were collected at the upper and bottom walls of the reaction chamber in the ITP-L system, and on the wall and filter of the reaction chamber and sample collection filter in the ITP-H system.

Sample Analysis
The condensed particles were analyzed with XRD (Rigaku SmartLab) using Ni-filtered CuKα (λ=1.5418 Å) radiation at an accelerating voltage of 40 kV and a tube current of 40 mA. Silicon zero diffraction plates were used as sample holders. The scan range was 4°-95°2θ and the scan step interval was 0.02 deg/step. For Fourier transform infrared (FT-IR) spectroscopy, particles were dispersed in a KBr matrix with a mass ratio of 1:200. KBr disks of 5 mm diameter were prepared for all samples. A large pellet of 10 mm diameter with a column density of particles of 0.95 mg cm −2 was prepared to determine the reflective index of the condensed particles in Run Al 90 Si 10 . Transmission spectra of the pellets in the wavenumber range 400-7500 cm −1 (1.3-25.0 μm) were measured with JASCO MFT-680 and Thermo Nicolet iS5. Because the XRD and FT-IR spectra showed no differences between samples collected at the upper and bottom walls of the ITP-L system and between samples from the chamber wall and the filter of the ITP-H system, transmission electron microscopy (TEM) observations were performed for samples from the upper wall of the ITP-L system and from the filter in the ITP-H system. The powders randomly collected were placed on carboncoated Cu TEM grids without dispersant to avoid any reaction between the ultra-fine run products and the dispersant, and then observed with an FE-TEM (JEOL JEM-2100F) equipped with energy dispersive X-ray spectroscopy (EDS). The condensed crystalline grains were not indexed as alumina with the hcp packing of oxygen, such as corundum, but instead as aluminas with the fcc oxygen frameworks. The most common alumina based on the fcc packing of oxygen is γ-alumina (Fd3m ) γ-alumina has a defect spinel structure, where Al cations partially occupy four-or six-coordinated interstitial sites of the fcc oxygen sub-lattice. The structural Starting  (Lee et al. 1997), where Al atoms and vacancies (V) in parentheses and brackets represent those in tetrahedral and octahedral sites, respectively. Al atoms and vacancies in both sites are disordered in γ-alumina. The structural differences between γ-, θ-, λ-, and δ-aluminas are based on the differences in the degree of chemical and displacive ordering of Al cations at the interstitial sites of the fcc oxygen subcell ).

XRD and TEM Observations
The XRD patterns of γand δ-aluminas synthesized by solgel processing (Takigawa et al. 2014) are also shown in The TEM observations show that the condensed grains in all run products have spherical shapes with the size of <150 nm. The spherical shape of condensates indicates that these grains formed via a transitional liquid state (Ishizuka et al. 2016). Electron diffraction (ED) patterns were taken from >50 nm sized grains. An amorphous halo was only observed in the grains in Run Si 100 (Figure 2(a)). All grains in Run Al 7 Si 93 , and most grains in Runs Al 75 Si 25 and Al 50 Si 50 , are amorphous as well. Some mullite (Al 4+2x Si 2-2x O 10-x , x=0.17-0.59) grains were also observed for the products in Runs Al 75 Si 25 and Al 50 Si 50 , which is consistent with the XRD patterns. The ED patterns confirm that the crystalline grains in Run Al 80 Si 10 Mg 10 have a γ-alumina structure, which has an fcc oxygen framework (Figure 2(b)). Most grains in Run Al 95 Mg 5 -H show diffraction patterns of γ-alumina, and some show weak reflections due to superlattices indexed to δ-alumina (Figure 2(c)). Clear ED patterns of δ-alumina were observed for the products in Runs Al 99 Mg 1 -H and Al 100 (Figure 2(d)). The diffraction patterns of the grains in Run Al 100 -H indicate that they have an fcc oxygen framework with superlattices, but they are different to known transition aluminas based on the fcc packing of oxygen (γ-, θ-, θ′-, θ″-, λ-, δ-, and η-alumina; see Figure 2(e)). The condensed grains in Run Al 90 Si 10 contain amorphous and λ-alumina (P2 1 /c) (Figure 2(f); . No pure γ-alumina grains were observed for the product in Run Al 90 Si 10 , but the intensities of the diffraction spots due to the superlattices are weak compared with those in Runs Al 99 Mg 1 -H, Al 100 -H, and Al 100 (Figure 2(d)), which is consistent with the XRD pattern  Table 1. EDS mapping was performed for the products in Runs Al 90 Si 10 , Al 75 Si 25 , and Al 50 Si 50 . In the products of Run Al 90 Si 10 , most of the grains showed almost the same Al contents as the starting material, where the averaged Al content was f Al =0.91±0.03, and a few small grains (<10 nm in diameter) attached to larger grains (a few tens nm in diameter) are richer in Si than the starting material. The products in Run Al 75 Si 25 showed homogeneous chemical composition with the Al content similar to the starting material, but those in Run Al 50 Si 50 contains two typed grains: Al-rich and Si-rich. The Al-rich grains ( f Al ∼0.7) were relatively larger than the Si-rich grains ( f Al ∼0.3). These results indicate that Al and Si were basically incorporated into a single phase with the same ratio as the starting materials if the composition of the starting materials is rich in Al ( f Al >0.75).

FT-IR Spectra
FT-IR spectra of the experimental products are shown in Figure 3. The product in Run Si 100 shows peaks at 9.1, 12.2, and 21.2 μm, which is consistent with those of amorphous silica. With an increase in Al content, the intensities of the peaks at 9.1 and 21.2 μm decrease and the broad peaks at 11-14 and 15-18 μm become stronger. The peaks at 9.1 and 21.2 μm are attributed to vibrations of the SiO 4 tetrahedra, and the peaks at 11-14 and 15-18 μm are attributed to vibrations of the condensed AlO 4 tetrahedra and AlO 6 octahedra, respectively (Tarte 1967). The FT-IR spectrum of the condensed grains in Run Al 80 Si 10 Mg 10 shows broad double peaks at 12.3 and 13.6 μm. That in Run Al 90 Si 10 shows a broad feature with a peak at 12.2 μm and a shoulder at 13.2 μm. Many sharp peaks are observed in addition to the broad feature at ∼12.5-16.0 μm for the spectra of the products in Runs Al 95 Mg 5 -H, Al 99 Mg 1 -H, Al 100 -H, and Al 100 . These sharp peaks reflect the superlattices observed in the ED patterns. The absence of sharp peaks in the FT-IR spectrum of the product in Run Al 90 Si 10 indicates a low degree of ordering at the crystallographic sites that form superlattices. Figure 3 also compares the spectra of the γand δ-aluminas synthesized by sol-gel processing (Takigawa et al. 2014), which are the same as those used in the XRD analysis. The spectral profiles of the products in Runs Al 80 Si 10 Mg 10 and Al 90 Si 10 are similar to those of the γ-alumina, and the peak profiles of the products in Runs Al 100 , Al 100 -H, and Al 99 Mg 1 -H, and Run Al 95 Mg 5 -H are similar to that of δ-alumina. However, the peaks of these ITP products at 14-20 μm are relatively weaker than the spectra of the γand δ-aluminas. This difference comes from the contrasting shapes of the grains. It is well known that spectra are strongly dependent on grain shape (Bohren & Huffman 1983). The condensates in the ITP systems are spheres, whereas the γand δ-alumina grains formed by sol-gel processing are rhombic plates (Takigawa et al. 2014). The FT-IR spectral profile of γ-alumina with spherical shapes (Koike et al. 1995) is very similar to that of condensates in Run Al 90 Si 10 except for the peak width, as shown below.

Comparison of the Grain-formation Conditions in ITP Systems and around AGB Stars
Based on nucleation theory, Yamamoto & Hasegawa (1977) introduced dimensionless parameter of Λ to describe grain formation under various conditions. Λ defines the physical conditions of grain formation (Equation (1)).
Here, a is radius of vapor molecules, α is sticking probability, n 0 is number density of vapor molecules, k B is the Boltzmann constant, N A is Avogadro constant, T e is equilibrium temperature, h is latent heat of evaporation, and τ is a cooling timescale of the vapor. We estimated Λ of the ITP-H and ITP-L systems to compare the experimental conditions with circumstellar environments. The cooling rate of the ITP-L system was assumed to be similar to that of ITP-H system, and a value of 10 4 K s −1 was adopted (Shigeta & Watanabe 2005), which corresponds to a cooling timescale (τ T ) of ∼0.1 s. For a rough estimation, we assumed a pure Al-O system and the feeding rates of 200 and 100 mg minute −1 as typical values for ITP-H and ITP-L, respectively. The number densities of AlO molecules in the plasmas in the ITP-H and ITP-L systems were estimated from the feeding rates of the starting materials, the flow rates of the Ar and O 2 gases, and the total pressure in the chambers ( Table 1). The sticking coefficients (α c ) of the amorphous and transition alumina are not known, but are probably higher than that of corundum (0.05-0.10; Takigawa et al. 2015). Here we assumed that the α c values of the amorphous and transition aluminas were between 0.1 and 1.0. The estimated Λ values of the ITP-H and ITP-L systems are thus (0.8-8)×10 4 and (2-20)×10 4 , respectively. The gas density vaporized from the starting material is one of the important factors to determine the diameter of particles. Plasma has temperature and velocity distributions in radial and axial direction (Shigeta & Watanabe 2005), which makes a distribution of gas density vaporized from the starting material. Nucleation and growth of grains at different vapor density and temperature gradient and mixing of those grains result in a broad size distribution of produced particles ( Figure 2). Assuming that the τ T of the stellar outflow from an AGB star is 4×10 7 s, the total gas pressure is 0.01 Pa, and alumina forms from a gas of solar abundance, Λ is estimated as (0.1-1)×10 4 . The obtained Λ values of the ITP systems and the AGB wind are roughly comparable, indicating that small alumina grains as observed in the experiments may form in circumstellar environments as well. It should be noted that the cooling timescale of dust-formation regions around AGB stars needs to be more carefully discussed because the τ T of the atmosphere near the star is much slower than the outflow, and complex gas motion due to stellar pulsations should also be considered. Also, this model cannot predict formation of metastable or amorphous grains because α c only represents kinetic barriers for grain growth because diffusion distance of atoms depends on the cooling timescale, and kinetic processes controlling the crystal structures of condensed grains are not taken into account. The transformation of different metastable alumina or corundum via the annealing of amorphous or condensed metastable alumina is another issue that could possibly occur in circumstellar envelopes at low cooling rates.

Circumstellar Alumina
The FT-IR spectrum of the condensed particles in Run Al 90 Si 10 shows a broad feature at ∼12 μm without sharp peaks (Figure 3). This should not be directly compared with the circumstellar dust emission because the FT-IR spectra of the samples embedded in KBr pellets are affected by the KBr medium (Tamanai et al. 2009). To compare the observed dust emission with the transmission spectra of the experimental products, we calculated the extinction coefficient (Q ext ) of the products in Run Al 90 Si 10 in a vacuum as a function of wavelength λ (Equation (2) where a is the average radius of the particles and m is the complex refractive index (m=n + ik). From the TEM observation, grains are small spherical particles that satisfy 2πa/λ=1. The complex refractive index values of particles embedded in a KBr pellet were deduced by dispersion analysis of the measured processing of meteorites. Even if presolar metastable alumina were present in primitive meteorites, they had been lost during acid processing. This also explains the rarity of amorphous presolar alumina. Careful studies both on the chemical composition and crystal structure of individual presolar alumina grains without chemical treatment and prior to the destructive isotopic measurements are needed to conclude the presence of presolar Si-containing transition alumina grains in chondrites.