Dr. Thad L. Druffel Profile

Dr. Thad L. Druffel

at Univ of Louisville

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Publications (3)

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Proceedings Article | 7 May 2009 Paper

Demonstration of flexible nanocomposite NIR mirror

Thad Druffel, Eric Grulke

Proceedings Volume 7298, 729812 (2009) https://doi.org/10.1117/12.820234

KEYWORDS: Thin film coatings, Polymers, Refractive index, Thin films, Nanocomposites, Reflectivity, Nanoparticles, Optical coatings, Mirrors, Metals

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Thin film metal oxide coatings have been used for commercial electromagnetic filters from the UV to infra red regions for over half a century. Deposition onto a substrate has typically been accomplished using vapor deposition techniques and more recently sol-gel methods. These coatings provide very good optical performance under abrasion, thermal cycles and variable humidity when applied on substrates with similar thermal and mechanical properties. When conventional metal oxide coatings are applied to flexible, relatively soft substrates such as polymers, mismatches in mechanical properties can reduce interfacial adhesion or accelerate mechanical failures. The authors recently showed that a thin film polymer nanocomposite can be applied on a polymer substrate and maintain adhesion even under high strains. This paper describes the demonstration of an IR mirror using fifteen discrete layers with an IR-reflectance that exceeds 90 percent at 1064 nm and transparent in the visible spectrum. We will present the results with thin film stacks containing over 15 discrete layers for IR mirror applications, and our recent work shows that the technology can produce thin film stacks containing 30 layers or more. Furthermore these coatings have high flexibility and can be applied to curved polymer substrates. These IR mirrors can withstand thermal cycling and large strains much better than those made using the state of the art techniques.

Proceedings Article | 4 September 2008 Paper

The role of nanoparticles in visible transparent nanocomposites

Thad Druffel, Omar Buazza, Matt Lattis, Scott Farmer, Megan Spencer, Natalia Mandzy, E. Grulke

Proceedings Volume 7030, 70300F (2008) https://doi.org/10.1117/12.795273

KEYWORDS: Nanoparticles, Polymers, Refractive index, Particles, Air contamination, Nanocomposites, Thin films, Photorefractive polymers, Silica, Titanium dioxide

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Metal oxide nanoparticles can be used in thin film polymer systems to engineer specific material properties while maintaining visible transparency. High loadings of nanoparticles in a polymer can manipulate refractive index, modulus, and UV absorption over a wide range. Because the polymer binders can be allowed to dominate the physical properties, these systems are ideal where materials undergo large strains. While stable dispersions of sub-100nm diameter CeO2, ZnO, and SiO2 are well understood and commercially available, our group also developed a stable dispersion of TiO2 nanoparticles. These metal oxides are significantly harder than the host polymer, have high UV absorption, and cover a large refractive index range. Our group has successfully incorporated these materials into PMMA thin films with loadings up to 60% by volume (approaching the theoretical close packing of spheres). These thin film nanocomposites have been successfully incorporated into 30 layer, sharp cut optical filters that easily withstand large strains induced by mechanical loading and thermal cycling. In these films we have adhered to the rule that nanoparticle diameter should be one-tenth the wavelength of visible light. As the thickness of the overall filter stack increases, light scattering is intensified, so the dimensions and refractive indices of the nanoparticles become critical for highly transparent systems. We study here the interactions of particle dimensions, refractive index, loading, thickness, and transparency in nanocomposites.

Proceedings Article | 29 August 2008 Paper

Nanocomposite thin films exhibiting high mechanical and optical flexibility

Thad Druffel, Omar Buazza, Matt Lattis, Scott Farmer

Proceedings Volume 7067, 70670A (2008) https://doi.org/10.1117/12.795294

KEYWORDS: Refractive index, Nanoparticles, Nanocomposites, Reflectivity, Thin films, Polymers, Optical filters, Ultraviolet radiation, Particles, Thin film coatings

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Nanocomposites are created by doping host polymers with nanoparticles that typically have higher or lower refractive indices. The ability to tailor the mechanical and optical performance of these composites has led to their increased use in transparent materials. Nanocomposites maintain the elastic properties of the binding polymers and exhibit infinite refractive index tunability between the limits of the system. These unique properties provide distinct benefits for multilayer, thin-film optical filters. Because the nanoparticles are dispersed in a fluid or bound in a polymer matrix in use, toxicity risks that may be associated with raw particles are reduced. Using a stable dispersion of titanium dioxide nanoparticles and a UV curable monomer, we were able to design and produce several quarter-wave filters that demonstrate control of the height and width of the passband through adjustment of the organic/inorganic ratio and layer count. The volume loading of the metal oxides can be adjusted from zero to near the theoretical packing density of spheres, allowing refractive index to be controlled over a large range. Because metal oxide particles exhibit high UV absorption, these additives provide UV protection to the host polymer and the filter's substrate. Additionally, significant improvements in abrasion resistance are often observed in films loaded with nanoparticles at the concentrations of interest.

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