Isothermal diffusion coefficient measurements of antimony doped molten germanium using the modified shear cell technique.

摘要

A comparative study on the effectiveness of two different methods to reduce buoyancy driven convection to obtain an intrinsic diffusion coefficient was investigated. Both methods used the shear cell technique and antimony doped molten germanium. In the first method, the effectiveness of microgravity was investigated. In the second method, a ground-based isothermal heat pipe furnace system was used to reduce the thermal gradients. The effect of capillary size was also investigated using four capillary diameters (3.0 mm, 2.0 mm, 1.5 mm and 1.0 mm).; The antimony concentration in the solidified capillaries was characterized using Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and a measured diffusion coefficient (D_measured) was determined using the long-time solution to Fick's 2nd Law. The data obtained was extrapolated to a ‘zero diameter’ capillary to yield an intrinsic diffusion coefficient (D_intrinsic).; The microgravity experiments returned a D_measured value for the two 3.0 mm capillaries of (5.0878 ± 0.4687) × 10−4 cm2/s and (5.6472 ± 0.5948) × 10 −4 cm2/s, with a value of (3.6911 ± 0.2563) × 10−4 cm2/s for the 1.5 mm capillary. Because of the capillary diameter dependence on D_measured, it was concluded that the microgravity results were contaminated with convection.; The D_intrinsic value for Sb-doped molten Ge at 1000°C was found to be (1.2230 ± 0.0624) × 10−4 cm2/s. It was found that there was no influence of capillary size. This implies that there was no contribution from buoyancy driven convection for the ground-based isothermal furnace results.; This D_intrinsic value was extrapolated to 938°C using (1) the Stokes-Einstein equation and (2) the Arrhenius equation. For the Stokes-Einstein equation: D₍₉₃₈₎ = 0.9670 × 10 −4 cm2/s. For the Arrhenius equation: D ₍₉₃₈₎ = 1.0334 × 10−4 cm2/s with a range of (1.0242 to 1.0470) × 10−4 cm 2/s.; The extrapolated diffusion coefficient values was compared to the recalculated mean value of 0.522 × 10−4 cm2/s with a range of 0.240 to 0.693 × 10−4 cm 2/s reported by Burton et al. Thus, the isothermal furnace measurements were slightly above this, however, the extrapolated D_intrinsic value at 1000°C was more precise.