Maxim Sokol
Published: 2017
Total Pages:
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"Transparent polycrystalline ceramics consist of small crystalline grains densified into bulk materials. Properly sintered ceramics display unique combinations of properties, such as hardness, fracture toughness, elastic modulus, transparency, absorption coefficient, thermal conductivity, dopant absorption, emission characteristics and optical isotropy, that are very close or even better than those of single crystals. Moreover, polycrystalline ceramics offer various advantages over single crystals, such as cost-effectiveness, the possibility for large-scale production, shape control and improved mechanical properties. As such, polycrystalline ceramics have become increasingly attractive for a wide range of applications, both military and civil. The development of transparent polycrystalline ceramics for laser oscillation and armor materials is an important goal of materials technology. In 1995, Ikesue presented efficient laser oscillation the first time, using polycrystalline Nd:YAG. Polycrystalline magnesium aluminate spinel (PMAS) is another attractive transparent ceramic that can be utilized as armor material due to its simple synthesis, excellent mechanical properties and high-level of transparency over a wide range of wavelengths. Extensive investigations have been conducted by various groups around the world, given the economic implications of the possible application of polycrystalline ceramics. Conventional technological approaches for the fabrication of the polycrystalline ceramics are based on prolonged pressureless sintering (PLS) at relatively high temperatures, hot pressing (HP) and hot isostatic pressing (HIP). While these approaches allow for the fabrication of transparent ceramics with adequate functional properties, they are time-consuming and very expensive. In recent decades, Spark Plasma Sintering (SPS) technology has been employed for the fabrication of polycrystalline ceramics. The SPS process involves simultaneous application of an electric field, temperature and pressure and permits rapid powder consolidation at temperatures significantly lower than those used with conventional sintering processes. The aims of this dissertation were to understand and investigate the effect of SPS parameters (i.e., dwell time, temperature, pressure, heating rate) and sintering additives (mainly LiF) on functional properties of polycrystalline transparent ceramics for a wide range of laser and armor applications. The present study addresses both fundamental aspects of functional properties and the fabrication of transparent polycrystalline ceramics with controlled microstructure by high-pressure SPS process. The fundamental aspects of transparent ceramic sintering and the effects of SPS under high pressure were investigated in depth and are discussed. This doctoral research can be divided into two main parts. The first part is related to the fabrication of PMAS by high-pressure SPS (HPSPS) techniques (up to 1.0 GPa) and characterization of the optical and mechanical properties of the material generated. Codoped PMAS suitable for Q-switching applications with an adequate saturable absorption was successfully fabricated using a combined SPS approach followed by HIP treatment. The second part of the work describes the SPS-based fabrication of Nd:YAG specimens and characterization of their mechanical and optical properties, including lasing efficiency and power threshold. HPSPS-processed Nd:YAG displayed laser generation characteristics comparable with commercially available, conventionally sintered specimens. The obtained results point to the ability to fabricate cost-effective nanostructured polycrystalline ceramics with controlled microstructure and unique combinations of optical, thermal and mechanical properties. It was established that the hardness values of these materials followed the well-known Hall-Petch relation down to grain sizes of 30 nm. For grain sizes lower than 30 nm, an inverse Hall-Petch relation was clearly observed. The results of this investigations were published in seven international peer-reviewed journals with high ranking and were presented at more than twenty international conferences."--abstract.