Shear Strength and Interfacial Toughness Characterization of Sapphire–Epoxy Interfaces for Nacre-Inspired Composites

The common tensile lap-shear test for adhesive joints is inappropriate for brittle substrates such as glasses or ceramics where stress intensifications due to clamping and additional bending moments invalidate results. Nevertheless, bonding of glasses and ceramics is still important in display applications for electronics, in safety glass and ballistic armor, for dental braces and restoratives, or in recently developed bioinspired composites. To mechanically characterize adhesive bondings in these fields nonetheless, a novel approach based on the so-called Schwickerath test for dental sintered joints is used. This new method not only matches data from conventional analysis but also uniquely combines the accurate determination of interfacial shear strength and toughness in one simple test. The approach is verified for sapphire–epoxy joints that are of interest for bioinspired composites. For these, the procedure not only provides quantitative interfacial properties for the first time, it also exemplarily suggests annealing of sapphire at 1000 °C for 10 h for mechanically and economically effective improvements of the interfacial bond strength and toughness. With increases of strength and toughness from approximately 8 to 29 MPa and from 2.6 to 35 J/m², respectively, this thermal modification drastically enhances the properties of unmodified sapphire–epoxy interfaces. At the same time, it is much more convenient than wet-chemical approaches such as silanization. Hence, besides the introduction of a new testing procedure for adhesive joints of brittle or expensive substrates, a new and facile annealing process for improvements of the adhesive properties of sapphire is suggested and quantitative data for the mechanical properties of sapphire–epoxy interfaces that are common in synthetic nacre-inspired composites are provided for the first time

Facile Deposition of YSZ-Inverse Photonic Glass Films

An alternative all-colloidal and single-step deposition method of yttrium-stabilized zirconia (YSZ)-infiltrated polymeric photonic glass films is presented. Heterocoagulation of oppositely charged polystyrene (PS) microspheres and YSZ nanocrystals in aqueous dispersions created PS/YSZ core–shell spheres. These composite particles were deposited on glass substrates by a simple drop-coating process. Heterocoagulation impaired self-assembly of the particles, resulting in a disordered structure. Burn-out of the polymer yielded a random array of YSZ shells. The effect of the filling fraction of YSZ between these shells was explored. YSZ-inverse photonic glass films with a thickness below 40 μm achieved 70% reflectance of the incident radiation over a broad wavelength range between 0.4 and 2.2 μm. The YSZ structures demonstrated structural stability up to 1000 °C and maintained high reflectance up to 1200 °C for several hours, thus enabling applications as broadband reflectors at elevated temperatures

Bottom-up Fabrication of Multilayer Stacks of 3D Photonic Crystals from Titanium Dioxide

A strategy for stacking multiple ceramic 3D photonic crystals is developed. Periodically structured porous films are produced by vertical convective self-assembly of polystyrene (PS) microspheres. After infiltration of the opaline templates by atomic layer deposition (ALD) of titania and thermal decomposition of the polystyrene matrix, a ceramic 3D photonic crystal is formed. Further layers with different sizes of pores are deposited subsequently by repetition of the process. The influence of process parameters on morphology and photonic properties of double and triple stacks is systematically studied. Prolonged contact of amorphous titania films with warm water during self-assembly of the successive templates is found to result in exaggerated roughness of the surfaces re-exposed to ALD. Random scattering on rough internal surfaces disrupts ballistic transport of incident photons into deeper layers of the multistacks. Substantially smoother interfaces are obtained by calcination of the structure after each infiltration, which converts amorphous titania into the crystalline anatase before resuming the ALD infiltration. High quality triple stacks consisting of anatase inverse opals with different pore sizes are demonstrated for the first time. The elaborated fabrication method shows promise for various applications demanding broadband dielectric reflectors or titania photonic crystals with a long mean free path of photons