Figure 1. Filled inverted triangles indicate peaks arising from the fcc-packed oxygen. Peaks indicated by open diamonds are attributed to the superlattices of alumina based on the fcc packing of oxygen unknown phase. Periclase open rectangle is a residue of the starting material. 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.
Some additional peaks were also observed for the product in Run Al -H, as indicated in Figure 1.
These additional diffraction peaks do not match any known hcp or fcp alumina phases. The spherical shape of condensates indicates that these grains formed via a transitional liquid state Ishizuka et al. An amorphous halo was only observed in the grains in Run Si Figure 2 a. 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.
Figure 2. White circles indicate SAD areas. Superlattice reflections of transition aluminas are indicated in panels d — f. FT-IR spectra of the experimental products are shown in Figure 3. The product in Run Si shows peaks at 9. With an increase in Al content, the intensities of the peaks at 9. The peaks at 9.
That in Run Al 90 Si 10 shows a broad feature with a peak at 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. This difference comes from the contrasting shapes of the grains. 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 gas density vaporized from the starting material is one of the important factors to determine the diameter of particles. 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.
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. 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.
The complex refractive index values of particles embedded in a KBr pellet were deduced by dispersion analysis of the measured spectrum Equation 3 ; Koike et al. The result of dispersion analysis is shown in Figure 4 a. Figure 4 b compares the calculated spectra of the product in Run Al 90 Si 10 with those of a porous-amorphous alumina by Begemann et al. Here, spherical grains much smaller than the wavelength in a vacuum were assumed. If larger grain sizes are assumed, the peaks will broaden Min et al.
The spectrum of the grains with continuous distribution of ellipsoids CDE show a main peak at However, it is unlikely that the CDE-like shape of transition alumina forms via a metastable liquid state. Figure 4. Absorption coefficients of the product in Run Al 90 Si The orange line shows the calculated spectra of condensates in Run Al 90 Si 10 in a vacuum.
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The orange line is the calculated absorption coefficient of the product in Run Al 90 Si 10 in vacuum. The gray line is for spherical porous amorphous alumina Begemann et al. The dotted black line indicates the residual spectrum of T Cep subtracted by Al 90 Si A stellar blackbody of K normalized at 3. These peaks might relate to emissions from amorphous Mg—Fe silicates or Ca-included aluminosilicate glass Dorschner et al.
No presolar transition alumina grain containing Si was reported previously. This is probably because of the sample selection for isotopic measurements. Most of the presolar alumina were identified from acid residues of meteorites Choi ; Nittler et al. 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.
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