1 9 A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 2 2 at THz frequencies through THz-TDS. The refractive index at 0.8 THz of pristine glasses was found to increase relative to increasing (Ca+Al)/Si, consistent with density trends. All laser-modified glasses measure positive refractive index changes at 0.8 THz, however silicate-rich glasses, or reduced (Ca+Al)/ Si composition, exhibit larger refractive index changes. THz-TDS is an effective nondestructive technique to measure densification or rarefaction of the glass structure and corresponding larger glass network. CAS glasses along the tectosilicate join show densification from laser irradiation, due to higher measured THz refractive index values following irradiation. It is suspected the densification is due to the ability of aluminum to convert from four to five-fold coordination due to laser irradiation. In an earlier study, the authors examined the CAS glass system and laser-induced structural modification using molecular dynamic simulations[26]. CAS compositions along the tectosilicate join were selected and modeled using the Large-Scale Atomic/ Molecular Massively Parallel Simulator (LAMMPS) with simulated laser exposure applied through a heat flux cross defined region or hotspot/focal region. Localized density changes, coordination number distribution, and short and intermediate range order bond angles and distances between Si and Al species were analyzed and discussed. It was proposed that densification of CAS glass due to femtosecond laser irradiation occurs through conversion of AlIV to AlV and distortions of silica inter-tetrahedral bonding environments including Si-O bond distances and SiO-Si bond angles. Compositions with a lower Al2O3/SiO2 ratio, reflecting a silica-rich glass, densify more due to increased concentration of AlV, which was found to increase with pulse energy. Addition of Al and Ca, at the expense of Si, increases the packing density of the glass structure and network, reducing laser exposure modification to the overall network. EXAMINING HA-GLASS COMPOSITES The authors have characterized HA-glass composites[27], specifically HA and calcium zinc silicate glass composites. THz-TDS is used to determine the refractive index and dielectric constant values, which are correlated to the glass content within defined composites. THz-TDS serves as a nondestructive tool for examining HA-glass composites. Hydroxyapatite (Ca10(PO4)6(OH)2; HA) is the main mineral component of hard tissues, allowing for use in bone repair and replacement. The refractive index and dielectric constant were proven to be viable in determination of glass content found in HA-glass composites from 0.2-1.5 THz, above which cannot be measured due to surface roughness. Dielectric constant and refractive index Fig. 4 — Refractive index of SLS and ABS glasses over 0.3-0.8 THz.
RkJQdWJsaXNoZXIy MTYyMzk3NQ==