Scientists at Tomsk Polytechnic University (TPU) have developed technological approaches to the post-processing of fluorescent ceramics, which make it possible to enhance the luminescence of the material without using expensive equipment and additional chemical reagents.
One of the key problems of modern optoelectronics and LED technologies is to increase the efficiency of radiation conversion of luminescent materials. Traditional phosphors in the form of powders, despite their high radiation conversion efficiency, have low thermal conductivity and are prone to degradation. Ceramic materials are practically free of these disadvantages, however, in a dense polycrystalline medium, a significant part of the generated light is lost due to total internal reflection on a smooth surface.
To solve these problems, TPU materials scientists have proposed a technology for post-processing luminous ceramics based on mechanical modification of the surface of the material.
"Existing approaches, such as chemical etching and plasma treatment, either require aggressive reagents and special equipment, or change the surface composition. We have proposed an alternative — a mechanical modification of the surface, which does not change the composition of the material, but allows you to control its optical response. At the same time, the effect of mechanical roughness on luminescence was previously studied mainly for single-layer ceramics. Our work is the first systematic study of the properties of a functionally gradient ceramic structure," said Daria Deulina, one of the authors of the study, a research engineer at the Scientific and Educational Innovation Center "Nanomaterials and Nanotechnology" at TPU.
The scientists obtained samples of functionally gradient ceramics based on yttrium-stabilized zirconium dioxide and yttrium-aluminum garnet modified with cerium and europium ions.
Ceramics were treated with abrasive silicon carbide paper of various grain sizes. Laboratory experiments have shown that this approach has made it possible to precisely adjust the arithmetic mean surface roughness. Thus, for a composite activated by cerium ions, an increase in surface roughness to 0.83 microns led to an increase in photoluminescence efficiency from 20 to 24%. According to scientists, this improvement is directly related to the coarser-grained microstructure of the material.
"It was found that coarse-grained abrasives effectively removed the non-luminescent, mechanically damaged surface layer formed during the manufacturing process without damaging the large, deeply embedded luminescent cerium centers. As a result, access to undamaged active areas was restored, which increased the overall light output. Thus, abrasive surface treatment serves as a highly efficient post—treatment method to optimize the optical characteristics of this ceramic system," explained Daria Deulina.
At the same time, it was possible to identify a fundamentally different reaction to mechanical modification, depending on the elemental composition of the material and its microstructure. In fine-grained materials with europium, it was not possible to improve the characteristics by changing the surface.
"This highlights the critical role of microstructure, especially grain size, in determining the reaction of luminescent ceramics to surface machining. The results presented here provide important recommendations for the rational design of technological protocols in the development of highly efficient fluorescent ceramics and functional gradient components for advanced applications in optoelectronics," said Damir Valiev, project leader, Associate Professor of Materials Science at the Engineering School of New Production Technologies of TPU.
The research was carried out with the support of the Russian Science Foundation.
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