Investigating the Trade‐off between Optical and Mechanical Properties during Constrained Sintering of Mesomorphic Ceramic Films

Abstract

Robust, transparent, and birefringent inorganic films are demanded for polarization control of high‐power lasers. While single crystals or films obtained via glancing angle deposition exhibit desirable optical properties and laser damage resistance, these methods are limited by cost and scalability. Mesomorphic ceramics as inorganic solids with liquid crystalline superstructure offer appealing transparency and birefringence but lack mechanical robustness due to their high porosity. Here, the effect of sintering on optical and mechanical properties of mesomorphic ceramics is evaluated. Films prepared by blade coating are sintered under varying conditions. Constrained sintering accomplished crystallite growth, densification, and morphological changes including necking as well as cracking while preserving the crystallographic orientation. The extent of sintering as a function of thermal treatment is quantified by morphology, surface area loss, and crystallite growth. Moreover, activation energies for surface diffusion and grain growth are estimated by surface area analysis and X‐ray diffraction peak narrowing, respectively. After sintering, birefringence decreases while Young's modulus and hardness improve as the film densifies. Upon partial sintering, mesomorphic ceramics retain transparency, high birefringence, and enhanced modulus. Laser‐induced damage threshold is measured as well. The reported results represent an important step toward the assembly and sintering of robust waveplates with high laser damage resistance.

Shear-Assembly of Crack-Free, Mesomorphic Ceramics from Zinc Oxide Nanorods for Waveplates

“…Zinc oxide (ZnO) nanorods were synthesized and dispersed in ethanol after surface functionalization following our previous report. 9 Zinc acetate dihydrate (13.18 g, Honeywell, 99.0+%) and potassium hydroxide (5.40 g, Fisher Chemical, 86.4%) were dissolved separately in 120 mL of methanol (99.8+%) and mixed by stirring under reflux conditions at 60 °C for a 2-h reaction. After concentrating the resultant suspension into a volume of 12 mL, the suspension was further refluxed for 5 days.…”

“…57 According to our recent report, E̅ f were determined as 35.3 GPa for mesomorphic ceramics after calcination and a higher E̅ f would be expected due to sintering. 9 Therefore, it is suggested that thermal stress becomes more concentrated at the free surface upon cooling the sintered 2-μm-thick film, leading to more severe cracking.…”

“…7 Furthermore, sintering for mechanical enhancement causes severe cracks, leading to optical transmission loss, for films thicker than 700 nm. 9 Examples of cracks observed in mesomorphic ceramic films after calcination are shown in Figure 1.…”

“…In our recent work, transparent mesomorphic ceramic thin films with large, uniform birefringence over centimeter dimensions were prepared by blade-coating of ZnO nanorods suspended in a lyotropic nematic sol, followed by calcination . The resulting thin films were sintered to enhance mechanical properties without sacrificing optical transparency and birefringence . In addition to uniform birefringence, waveplate coatings must be sufficiently thick and crack free and have smooth surfaces.…”

“…However, cracks and grooves inevitably form when fabricating mesomorphic ceramic films beyond a micrometer in thickness . Furthermore, sintering for mechanical enhancement causes severe cracks, leading to optical transmission loss, for films thicker than 700 nm . Examples of cracks observed in mesomorphic ceramic films after calcination are shown in Figure .…”