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Romain Gaume Romain Gaume

The Optical Ceramics group conducts research on transparent polycrystalline materials for high-power lasers, nuclear and radiological scintillator detectors, and applications in nonlinear optics.

Hybrid optical nanocomposites: a low-cost solution to radiation sensors

Nanocomposites, in particular inorganic-organic hybrids, allow for a wide range of combinations of phases with broad applications including solar energy harvesting, lighting, imaging, radiation sensing, lasers, magneto-optics and plasmonics. Intense research eff orts on optical nanocomposites are being motivated by the fascinating prospect that these materials can assume the optical properties of the inorganic phase and yet be processed with the shape versatility, low-cost and ease of polymeric materials.

In the area of radiation sensing for example, the United States Departments of Homeland Security (DHS) and Customs and Border Protection (CBP) have been tasked to screen every cargo container crossing domestic borders for illicit radioactive material. This is accomplished by using gamma-ray spectrometers capable of discriminating regulated special nuclear materials from non-threatening radioisotopes. To this end, scintillation detector systems, specifically thallium-doped sodium iodide (Tl:NaI) single crystals, are by far the most popular due to their reasonable performance and cost. In recent years, however, the demand for scintillator materials with improved lightyield, timing and energy resolution resulted in a wealth of new materials with att ractive properties. Remarkable single-crystals, such as Ce:LaBr3 and Eu:SrI2, with light outputs and energy resolution surpassing those of Tl:NaI, have been discovered but, the difficulty of their growth and size-scalability are currently limiting their commercialization.

Transparent ceramic scintillators, in which CREOL’s Optical Ceramics Laboratory led by Dr. Romain Gaume is actively engaged in, off er valuable alternatives to this scalability issue. Yet, in a serendipitous research twist, Gaume’s group developed an innovative method to fabricate monolithic transparent hybrid nanocomposites with very high particle loading and high refractive index mismatch tolerance between the inorganic and organic constituents. By providing adequate radiation stopping power, such high-loading fraction composites would yet provide another means to scale-up the size of scintillation detectors.

Hybrid scintillator composites consist of nano- to micro-scale luminescent phosphors dispersed in liquid monomers that are cured into near net-shape bulk size scintillators. This fabrication strategy uses inexpensive organic matrices and does not require the growth of large crystals. The dispersion of particles in an organic host however is of paramount importance to obtain composites of high optical quality. Nano-scale powders tend to agglomerate into clusters, and the refractive index mismatch between the powders and the organic phase leads to light scatt ering even when the refractive indices of the inorganic and organic phases are closely matched. The new concept proposed by the CREOL team takes advantage of two complementary techniques: (i) the production of highquality ceramic powder-compacts (so called “green-bodies”) used and being developed for the fabrication of transparent ceramics and (ii) a polymer impregnation technique. Ceramic green-bodies, made by consolidation of unagglomerated particles, form an open network of interconnected porosity suitable for liquid monomer impregnation.

The characteristic of the so-called ceramic open-porosity determines the inorganic loading of the composite once infiltrated by the monomer and the size of the polymer inclusions which form aft er in-situ polymerization. Thus, the inorganic volume fraction of the nanocomposites can be varied through the use of diff erent green-body forming techniques.

This new approach was demonstrated on acrylate-based nanocomposites containing over 60 vol% (i.e. 84 wt%) CaF2 nanoparticles. Because of its versatility, the technique could be advantageously used om other applications including optical sensing, low-power lasers, Christiansen filters, information storage and even beyond the realm of optical materials.

This summary was adapted from S. Chen and R. Gaume’s recent paper published in Applied Physics Letters, December 2015.

Hybrid Optical Nanocomposites The UCF logo displayed through a transparent CaF2-acrylate nanocomposites containing over 84 wt% CaF2.
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