Parasitic Light Absorption Processes in Transparent Polycrystalline MgAl₂O₄ and YAG

Abstract

Parasitic radiation absorption by impurities located in transparent polycrystalline ceramic hosts may significantly impair their functioning. At least two classes of impurity which can act as parasitic absorbers were encountered: transition or rareearth metal cations and neutral species (molecules or atoms) trapped in lattice void spaces. The most ubiquitous parasitic absorbers, able to affect transmission in the visible domain, have been identified for the case of MgAl 2 O 4 -spinel and Y 3 Al 5 O 12 -garnet hosts; their spectral effect was described and explained. In the case of spinel, S 8 type molecules and carbon atom clusters are the most damaging impurities, with Ce 3+ and Y 2+ also of significance for YAG host. Oxygen vacancy formation assists some of the redox processes which generate uncontrolled absorption. For instance, the Ti 4+ reduction to Ti 3+ is facilitated by such a process.

Spectroscopic study of the discoloration of transparent MgAl2O4 spinel fabricated by spark-plasma-sintering (SPS) processing

“…In the case of carbon, since the single atoms or atom clusters absorb the light at all the visible range without showing any local absorption bands, the T in spectra exhibit featureless curves; the T in value monotonously increases with increasing k. The T in spectra of the SPSed spinels [14][15][16] also showed the featureless T in spectrum curves. If this is also the case for the present material, DT in,(PH-AR) % 10% at k > 400 nm can be mainly ascribed to the parasitic light absorption caused by the glassy carbon, such as non-crystalline carbon atoms and/or atom clusters [78].…”

“…Goldstein et al [78] suggested that both single carbon atoms and/or carbon atom clusters become a light absorber for transparent spinel. The absorption caused by impurities has generally been defined as the parasitic light absorption in order to distinguish from those of point defects (vacancies).…”

Assessment of carbon contamination in MgAl₂O₄ spinel during spark-plasma-sintering (SPS) processing

“…However, if individual carbon atoms exist within the grains, they create absorption bands in the transparent spectrum due to the formation of point defects. 31) This is not the case in the present study because the spectrum monotonously changed with the wavelength of the incident light.…”

Transparent Ceramics at 50: Progress Made and Further Prospects

“…Certain intrinsic and extrinsic point defects [like host lattice ion vacancies or interstitials, transition‐metal or rare‐earth cations (TM + or RE + ), OH − anions, individual (or small clusters) of absorber, neutral atoms, or molecules (e.g., C 0 , S 8 )] are able to selectively absorb and/or emit various (usually narrow) portions of the spectrum domain for which the TCs discussed here are supposed to be transparent (0.1–15 μm) . Some of these absorber/emitter units are undesired impurities, while others are dopants added on purpose; the latter confer new functionalities to passive hosts, leading to applications for TCs in areas like lasers, phosphors, scintillators, filters, etc.…”

“…Part of the EMR which is expected to transit through the TC is absorbed by dopants, e.g., in fluorescent parts, or by various impurities, impairing (sometimes dramatically) the functionality of the host. As a first step in reducing such adverse effects, it is important to identify all potentially negative agents as, for example, the detection of carbon inside hot‐pressed or HIPed parts . During HIP at 200 MPa, carbon diffused into the parts even without any open porosity and at starting (before HIP) bulk densities of 97%–99% TD; this phenomenon occurs frequently at over 1500°C and is quasi‐absent below 1400°C.…”