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ALTHOUGH many studies have dealt with the alkali-containing aluminosilicate glasses, little work has been reported on the physical parameters of glasses at the SiO,-rich end of the pseudobinary SiOz-A1203 system.Glasses were made using an Oz-fed radio-frequency plasma torch, as described elsewhere.' Separate Ar streams were saturated with SiCl, and Al(CH3), at room temperature. The gas streams were then mixed and injected into the exit gas from the plasma torch. Buildup of the oxidized SiOz-A1203 glass occurred on a fused silica disk with a surface temperature (uncorrected optical pyrometer) of = 1800°C. The glass was usually deposited for 3 h at a rate of = 4 mm/h; the boule diameter was 5 cm. Bubbles occurred,' particularly at the highest A1 concentrations, but bubble-free 1-cm c u k s and rods up to 4 cm long were obtained from all but one specimen.Compositions of the glasses formed were determined by emission spectroscopic analysis. The Si-A1 ratios corresponded closely to those in the vapor phase, as calculated from the vapor pressures, indicating no preferential volatilization in the oxidation and deposition processes. Density was determined by the Archimedes technique using CCll at 22°C. Thermal expansion was measured with an automatic recording dilatometer.2 The OH content of the samples, determined from the intensity of the 3700 cm-' absorption band as described by Dodd and Fraser? ranged from 29 to 86 ppm, depending on the humidity.' Acoustic velocity was measured at 20MHz by the pulse superposition technique in a one-ended delay line configuration; the criterion of McSkimmin and Andreatch4 for correct "in-phase" determination was used.Equations for the least-squares linear regressions are given in Figs. 1-3 in terms of M , the mol% Alz03 content. Both the longitudinal and shear acoustic velocities at 20 MHz increased linearly with A1 content (Fig. l), although most impurities reduce the acoustic velocities in fused silica. This behavior opens the possibility of constructing a low-loss acoustic waveguide using the combination fused silicalfused silica plus alumina, which would have the advantage that the acoustic wave would be guided in the lower-velocity medium, i.e. the undoped fused silica, which would /" V'3.762+0.00943 M (f0.005) km/r 3.76 0 1 2 3 4 5 6 MOLE X At203 Fig. 1. Acoustic velocities of fused silica containing A1203.be expected to have a much lower acoustic loss than doped fused silica? Density and refractive index results are given in Figs. 2 and 3. The dashed lines, from Refs. 7 and 8, are interpolations from data at higher concentrations based on rapidly quenched samples. Lines calculated by the method of Huggins and Sun* are also shown. The effect of annealing on the density and refractive index was determined by heating specimens for various times at 980°C. Only in the case of specimens with the highest Al concentrations was any significant change observed; the effect of a total of 22 days of annealing is shown by the arrows in Figs. 2 and 3. Light scattering was seen with a laser be...