Inorganic phosphor materials play a crucial role in the creation of white light from blue and near-UV solid-state sources, and understanding the intricacies of phosphor structure is key to set the stage for improved, more efficient functionality. The following dissertation begins with an introduction to the fundamentals of solid state lighting, phosphors, and a summary of some recent advances in the field. This is followed by a study to understand structural ordering in the framework of the commercial green-emitting phosphor material, β-Si6−zAlzOzN8−z, as a function of Al content. SiAlON ceramics, solid solutions based on the Si3N4 structure, are important, lightweight structural materials with intrinsically high strength, high hardness, and high thermal and chemical stability. Described by the chemical formula β-Si6−zAlzOzN8−z, from a compositional viewpoint, these materials can be regarded as solid solutions between Si3N4 and Al3O3N. A key aspect of the structural evolution with increasing Al and O (z in the formula) is to understand how these elements are distributed on the β-Si3N4 framework. The average and local structure evolution of highly phase-pure samples of β-Si6−zAlzOzN8−z with z = 0.050, 0.075, and 0.125 are studied using a combination of X-ray diffraction, nuclear magnetic resonance studies, and density functional theory calculations. Synchrotron X-ray diffraction establish sample purity and indicate subtle changes in average structure with increasing Al content in these compounds. 27Al solid-state magic angle spinning nuclear magnetic resonance (NMR) experiments, coupled with detailed ab initio calculations of NMR spectra of Al in different AlOqN4−q tetrahedra (0 ≤ q ≤ 4), reveal a tendency of Al and O to cluster in these materials, more specifically, a high propensity for AlON3 tetrahedral motifs to be distributed on the SiAlON framework. Independently, the calculations suggest an energetic preference for Al–O bond formation, instead of a random distribution, in the β-SiAlON system.
Next, an average structure and coordination environment analysis of the robust and efficient green-emitting phosphor, β-SiAlON:Eu2+ (β-Si6–zAlzOzN8−zEu0.009), is combined with a range of property measurements to elucidate the role of Al content (z) on luminescence properties, including the red shift of emission and the thermal quenching of luminescence as a function of increasing Al content z. Average structure techniques reveal changes in polyhedral distortion with increasing z for the 9-coordinate Eu site in β-SiAlON:Eu2+. X-ray absorption near edge structure (XANES) is used to confirm that the majority of the activator Eu is in the Eu2+ state, exhibiting the symmetry-allowed and efficient 4f75d0→4f65d1 transitions. Room temperature and temperature-dependent luminescence indicate an curious increase in thermal stability with increasing z over a small range due to an increasing barrier for thermal ionization, which is correlated to an increase in the quantum yield of the phosphor. The works shows that specific emission properties can be targeted via compositional tuning, such as narrower emission β-SiAlON:Eu2+ (low z) for lower temperature operation, or maximum quantum yield and improved thermal stability (higher z up to 0.125) for high flux and/or high temperature operation.
With some fundamentals of phosphor structure-property relationships elucidated, we can now take steps to engineer these materials into useful morphologies. High power light emitting diodes (LEDs) and laser diodes (LDs) are being explored for white light generation and visible light communication, and thermally stable encapsulation schemes for color-converting inorganic phosphors are essential to their success. In the first example of thermally robust phosphors for laser-based lighting in the current thesis, the canonical blue-emitting phosphor, high purity Eu-doped BaMgAl10O17 (BAM:Eu2+), has been prepared using microwave-assisted heating (25 min) and densified into a ceramic phosphor using spark plasma sintering (30 min), resulting in translucent samples that emit blue light when incident with a UV laser diode. Results of the refinement on synchrotron X-ray diffraction data demonstrate the viability of microwave assisted heating for faster preparation of phase pure BaMgAl10O17:Eu2+ . The emission properties of the initial powder and the translucent sample have been studied using both a xenon lamp and a violet LD, and reveal the quantum yield of the starting powder does not change from densification into a translucent sample, and could likely be improved using optimized starting powders. Results indicate promise for uses of this blue phosphor in laser-based applications, as well as demonstrating a viable and fast way to prepare dense monolithic phosphors for laser light conversion.
As eluded to, a higher flux from high power LEDs and laser sources results in more conversion and therefore more conversion losses in the phosphor. This generates self-heating, surpassing the stability of current encapsulation strategies used for light-emitting diodes, usually based on silicones. Therefore, encapsulation-free phosphor ceramics are one solution to replace temperature limitations of resins and glasses and support the next generation of laser-based white lighting. In the next section of the dissertation, we again utilize spark plasma sintering (SPS), this time for preparing ceramic phosphor composites of the canonical yellow-emitting phosphor Ce-doped yttrium aluminum garnet (Ce:YAG) combined with a chemically compatible and thermally stable oxide, α-Al2O3. SPS allows for compositional modulation and control of density. Phase fraction, microstructure, and luminescent properties of ceramic composites with varying compositions are studied here in detail. The relationship between density, thermal conductivity, and temperature rise during laser-driven phosphor conversion is elucidated, showing that only modest densities are required to mitigate thermal quenching in phosphor composites. Additionally, the scattering nature of the ceramic composites makes them ideal candidates for laser-driven white lighting in a reflection geometry, where Lambertian scattering of blue light offers great color uniformity. Furthermore, a luminous flux >1000 lumens is generated using a single commercial LD coupled to a single phosphor element. With increased light output, drastically lower operating temperatures, and white color points, these composites offer an advantage over the exemplary phosphor material alone. To ensure the widespread commercial adoption of these devices, warm white emitters must be developed by including some red or amber emission in a device, for example. This will be achieved through advances in lasers and phosphors, and some of the challenges and preliminary work conducted at UCSB will be discussed.
Finally, a brief comparison of single crystal emitters and phosphor powders is conducted to clarify the origin of thermal quenching in phosphors and to elucidate the difference between the intrinsic role of composition versus the ability to dissipate heat. The canonical yellow phosphor, Ce:YAG, has high thermal stability due to a high quenching temperature (>700 K) for low (<1% Ce) doping. In the present work, temperature dependent emission and photoluminescent lifetime measurements show nearly identical lifetime behavior from 77K to 503K for a single crystal and a powder derived from grinding the single crystal, reminding us that the thermal quenching temperature is primarily dictated by composition and not form. The results demonstrate the discrepancy between temperature dependent emission and lifetime measurements. In the present study, the quenching temperature of the Ce:YAG is observed to be a result of low activator doping regardless of whether it is in single crystal or powder in silicone form. The present results suggest that doping, which is an intrinsic materials property that dictates thermal quenching onset temperature and the intensity of emission for a given temperature, and thermal conductivity, which will dictate the operating temperature of the phosphor and is therefore an engineering consideration, must be understood for a given system. As seen presently, the phosphor in silicone and single crystal differ in thermal conductivity by two orders of magnitude, but display nearly identical quenching behavior. With Ce:YAG single crystals being explored for transmission LD-based white lighting, the complex interplay between absorption, thermally conductivity, and the thermally quenching temperature, must be optimized.