The Sintering of Transparent Polycrystalline MgAl2O4 Spinel
Scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron diffraction, and energy dispersive spectroscopy (EDS) show compacts hot pressed with sulfate-derived powders (~ 800 ppm S) contain areas of sub-micron grains with boundaries decorated with an amorphous impurity-rich phase (Figure 1). Calculation of grain boundary area and spectrophotometry indicate the impurity phase scatters electromagnetic radiation from the visible to the mid-infrared (IR), rendering compacts opaque. Higher purity (< 100 ppm) flame-spray pyrolysis powder compacts do not contain these features and display much higher in-line transmission. However, MgO nodules and associated stress fields (due to slightly MgO-rich stoichiometry not detectable with X-ray diffraction) scatter UV (ultra-violet) and shorter visible wavelengths.
Figure 1. SEM, TEM, and spectrophotometry results for hot pressed spinel compacts.
LiF sintering additive dramatically reduces scatter and absorption in hot pressed compacts. However, it results in grain growth and associated impurity concentration into second phases that scatter shorter visible wavelengths. However, the extent of impurity phases is reduced, resulting in near-theoretical in-line transmittance from the visible to the mid-IR (infra-red). Hot press experiments and chemical analysis using laser ablation inductively coupled plasma mass spectroscopy and optical emission spectroscopy (LA ICP-MS/OES) determined that impurity content was lower when LiF was used. Simultaneous thermal analysis coupled with mass spectroscopy (STA-MS) and simulation with HSC thermodynamic software determined that LiF reacts in the vapor phase with impurities to produce volatile species that can be removed by applying pressure and forcing densification at a temperature that is higher than the volatilization range (Figure 2).
Figure 2. STA-MS results for hot pressed spinel compacts.
Single crystal annealing experiments, hot press and experiments with varying stoichiometry and varying shielding from carbon, simulation with HSC thermodynamic software, spectrophotometry, and electronic impedance spectroscopy (EIS), indicate that the formation of aluminum oxycarbides may be responsible for absorption and that lithium may be responsible for eliminating it. Although LiF addition imparts transparency, it also causes grain boundary embrittlement. Measurement of lattice parameter, stoichiometry, and impurity variation at grain boundaries using TEM convergent beam electron diffraction (CBED), TEM electron energy loss spectroscopy (EELS), S/TEM-EDS, secondary ion mass spectroscopy (SIMS), atomic force microscopy (AFM), and electronic force microscopy (EFM), suggests a combination of additive segregation, point defects, and tensile strain may be the cause.
Novel ways of sintering that rely on defects have also been investigated, including flash sintering and neutron irradiation. Sintering studies using variable atmosphere dilatometry with irradiated and non-irradiated powders, electronic impedance spectroscopy (EIS), IR-ellipsometry, and Raman spectroscopy have been used to quantify defects and understand how they affect sintering behavior.