Mesomorphic Ceramic Film Fabricated via Blade Coating of a Lyotropic Nematic Liquid Crystal for High-Power Lasers

Abstract

Mesomorphic ceramic films are fabricated over large areas by blade coating of a lyotropic nematic sol of zinc oxide nanorods. Upon calcination to remove organics, continuous monodomain films with a uniform thickness result as transparent ceramics, exhibiting uniform birefringence over centimeter dimensions. At low blade-coating velocities, the dry film thickness varies inversely with the coating velocity, whereas at high coating velocities, it increases with increasing velocity as prescribed by the Landau–Levich theory. The thickness–velocity scaling in the Landau–Levich regime depends on the shear-thinning behavior of the lyotropic nematic sol. The optimization of the coating process for the minimization of optical defects leads to transparent monodomain films that exhibit a uniform in-plane birefringence of 0.032 ± 0.002 from 580 to 1690 nm. After calcination, the resulting mesomorphic ceramic films span large areas (3.5 cm × 2.5 cm), are optically transparent with uniform thickness (0.60 ± 0.03 μm), and exhibit smooth surface finish with an average surface roughness of 20 nm. The alignment of zinc oxide crystallites along the coating direction within mesomorphic ceramics is confirmed by X-ray diffraction texture analysis and scanning electron microscopy imaging. As explained by the effective medium theory, the measured in-plane birefringence of 0.118 ± 0.003 is primarily contributed by form birefringence over intrinsic birefringence of zinc oxide in mesomorphic ceramics. The reported approach to highly ordered mesomorphic ceramics directly from lyotropic nematic sols, avoiding the gel state, could be generally applicable to other mineral liquid crystals, thereby benefiting the manufacture of optics for high-power lasers.

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

“…Zinc oxide nanorods prepared from zinc acetate solutions in methanol [ 13 ] exhibit lengths of 302 ± 68 nm and diameters of 12 ± 1 nm, respectively. Transparent green bodies for were fabricated via blade coating of the lyotropic nematic sol of ZnO nanorods in ethanol at 2.32 cm s −1 from a gap of 8 µm.…”

“…Transparent green bodies for were fabricated via blade coating of the lyotropic nematic sol of ZnO nanorods in ethanol at 2.32 cm s −1 from a gap of 8 µm. [ 13 ] The as‐coated green body films covered an area of 3.5 cm × 2.5 cm, exhibiting uniform birefringence of 0.034 ± 0.004 and a transmittance ≥ 0.87 from 600 to 1690 nm. After calcination, thermogravimetric analysis confirms the complete removal of organic ligands while the morphology and bulk uniaxial orientation of nanorods are preserved as indicated by scanning electron microscope (SEM) imaging and texture analysis, separately (see Figures S1 and S2, Supporting Information).…”

“…Local differences in grain growth are attributed to free rotation of rods about their long axes during assembly, as previously demonstrated. [ 13 ] At the early‐stage sintering, neck formation is facilitated by the coherence of crystal planes in <100> directions where the rods’ lateral surfaces are in contact. [ 20 ]…”

“…Continuous monodomain green body films are obtained by blade‐coating of lyotropic nematic sols followed by calcination to form transparent mesomorphic ceramic thin films. [ 13 ] We systematically sinter such uniaxially oriented films to study how the nanorods’ anisotropy affects surface energy reduction, and we examine how the extent of sintering is accompanied by crystallite growth, densification, and morphological changes including neck growth and crack formation. Furthermore, we correlate the sintered films’ morphology and porosity with optical and mechanical properties to enable sintering of large aperture, ceramic‐based waveplates with improved properties, and a laser damage resistance test is conducted on an optimized film.…”

“…Due to their optical anisotropy, mesomorphic ceramics are under consideration as alternative waveplate materials. [13][14][15][16][17] Unlike inorganic single crystals processed into conventional waveplates, our group has recently developed a mesomorphic ceramic film that can be fabricated over large areas by blade coating a lyotropic nematic sol followed by calcination of the resulting oriented nanoparticle assembly. [13] Birefringence of the calcined films is attributed mainly to form birefringence originating in the anisotropic packing of rods with interstitial pores rather than intrinsic birefringence of the nanocrystal.…”