Abstract
Phosphor converters for solid state lighting applications experience a strong thermal stress under high-excitation power densities. The recent interest in laser diode based lighting has made this issue even more severe. This research presents an effective approach to reduce the thermal quenching effect and damage of laser-excited phosphor-silicone converters using thermally conductive hexagonal boron nitride (hBN) particles. Herein, the samples are analyzed by employing phosphor thermometry based on the photoluminescence decay time, and thermo-imaging techniques. The study shows that hBN particle incorporation increases the thermal conductivity of a phosphor-silicone mixture up to 5 times. It turns out, that the addition of hBN to the Eu^{2+} doped chalcogenide-silicone converters can increase the top-limit excitation power density from 60 to 180 W cm^{-2}, thus reaching a 2.5 times higher output. Moreover, it is shown that the presence of hBN in Ce^{3+} activated garnet phosphor converters, may increase the output power by up to 1.8 times and that such converters can withstand 218 W cm^{-2} excitation. Besides, hBN particles are also found to enhance the stability of the converters chromaticity and luminous efficacy of radiation. This means that the addition of hBN particles into silicone-based phosphor converter media is applicable in a wide range of different areas, in particular, the ones requiring a high optical power output density.
Highlights
Phosphor converters for solid state lighting applications experience a strong thermal stress under high-excitation power densities
It is known that hexagonal boron nitride crystals are transparent for visible light, non-toxic, inert, low-cost, and have a high thermal conductivity (TC) value which is in the range between 180 and 400 W m− 1 K− 137,38. hBN powder is characterized by the reflection of light and a significantly wider TC range depending on the crystal plane and varying between 2.5 and 600 W m− 1 K− 139,40
It was shown that by using a simple and relatively low-cost material like hBN powder, the TC of silicone matrix used for phosphor converters, can be increased five times
Summary
Phosphor converters for solid state lighting applications experience a strong thermal stress under high-excitation power densities. HBN particles are found to enhance the stability of the converters chromaticity and luminous efficacy of radiation This means that the addition of hBN particles into silicone-based phosphor converter media is applicable in a wide range of different areas, in particular, the ones requiring a high optical power output density. The addition of high TC particles to increase the TC value of polymer-based composites was mostly applied for power electronics in order to enhance the thermal dissipation of the devices For this purpose, polymers are mixed with carbon composites, various metals, nitrides or oxides[36]. In this paper the effect of hBN particles on the thermal, PL and spectral properties of silicone-based Eu2+ activated chalcogenide and YAG:Ce phosphor converters for high power density lighting applications is presented. A more comprehensive technique, is based on the PL decay time measurements in frequency domain, and allows to accurately measure the temperature of optically active phosphor particles within the c onverter[47,48]
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