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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.
This issue contains 9 papers from The American Ceramic Society’s 40th International Conference on Advanced Ceramics and Composites, held in Daytona Beach, Florida, January 24-29, 2016. This issue includes papers presented in the 10th International Symposium on Advanced Processing and Manufacturing Technologies for Structural and Multifunctional Materials and Systems (Symposium 8), Additive Manufacturing and 3D Printing Technologies (Focused Session 4), and Field Assisted Sintering (Focused Session 5).
Until recently, ceramic materials were considered unsuitable for optics due to the numerous scattering sources, such as grain boundaries and residual pores. However, in the 1990s the technology to generate a coherent beam from ceramic materials was developed, and a highly efficient laser oscillation was realized. In the future, the technology derived from the development of the ceramic laser could be used to develop new functional passive and active optics. Co-authored by one of the pioneers of this field, the book describes the fabrication technology and theoretical characterization of ceramic material properties. It describes novel types of solid lasers and other optics using ceramic materials to demonstrate the application of ceramic gain media in the generation of coherent beams and light amplification. This is an invaluable guide for physicists, materials scientists and engineers working on laser ceramics.
This book describes spark plasma sintering (SPS) in depth. It addresses fundamentals and material-specific considerations, techniques, and applications across a broad spectrum of materials. The book highlights methods used to consolidate metallic or ceramic particles in very short times. It highlights the production of complex alloys and metal matrix composites with enhanced mechanical and wear properties. Emphasis is placed on the speed of the sintering processes, uniformity in product microstructure and properties, reduced grain growth, the compaction and sintering of materials in one processing step, various materials processing, and high energy efficiency. Current and potential applications in space science and aeronautics, automation, mechanical engineering, and biomedicine are addressed throughout the book.
In this work we present results of fabrication (powder synthesis and densification) and in-depth characterization of doped YAG ceramics by spark plasma sintering (SPS). SPS is an advanced one-stage, rapid, near-net shape densification technique combining uniaxial pressure with resistive heating. Various Ce, Nd and multi-doped samples with adequate optical properties have been successfully fabricated. The effect of residual porosity characteristics in both phosphor and laser materials was discussed and important conclusions implicating all pressure assisted sintering techniques were presented. -- From the abstract.
Sintering is one of the final stages of ceramics fabrication and is used to increase the strength of the compacted material. In the Sintering of Ceramics section, the fabrication of electronic ceramics and glass-ceramics were presented. Especially dielectric properties were focused on. In other chapters, sintering behaviour of ceramic tiles and nano-alumina were investigated. Apart from oxides, the sintering of non-oxide ceramics was examined. Sintering the metals in a controlled atmosphere furnace aims to bond the particles together metallurgically. In the Sintering of Metals section, two sections dealt with copper containing structures. The sintering of titanium alloys is another topic focused in this section. The chapter on lead and zinc covers the sintering in the field of extractive metallurgy. Finally two more chapter focus on the basics of sintering,i.e viscous flow and spark plasma sintering.
This book represents the first ever scientific monograph including an in-depth analysis of all major field-assisted sintering techniques. Until now, the electromagnetic field-assisted technologies of materials processing were lacking a systematic and generalized description in one fundamental publication; this work promotes the development of generalized concepts and of comparative analyses in this emerging area of materials fabrication. This book describes modern technologies for the powder processing-based fabrication of advanced materials. New approaches for the development of well-tailored and stable structures are thoroughly discussed. Since the potential of traditional thermo-mechanical methods of material treatment is limited due to inadequate control during processing, the book addresses ways to more accurately control the resultant material's structure and properties by an assisting application of electro-magnetic fields. The book describes resistance sintering, high-voltage consolidation, sintering by low-voltage electric pulses (including spark plasma sintering), flash sintering, microwave sintering, induction heating sintering, magnetic pulse compaction and other field-assisted sintering techniques. Includes an in-depth analysis of all major field-assisted sintering techniques; Explains new techniques and approaches for material treatment; Provides detailed descriptions of spark plasma sintering, microwave sintering, high-voltage consolidation, magnetic pulse compaction, and various other approaches when field-assisted treatment is applied.
The majority of functional materials today are based on ceramic materials which find use in a wide range of applications that include magnetic, electronic, optical, thermoelectric (TE) and piezoelectric energy. The properties and reliability of functional ceramic materials are highly depended on the density, grain size and existence of heterogeneities in the microstructure. It is a well-known fact that there is property enhancement at finer grain sizes for most functional materials through a multitude of mechanisms depending on the application. However, what remains a challenge is the success in maintaining fine-grained microstructures using conventional sintering methods. The use of such methods results in uncontrollable grain growth and coarse microstructures which negate the benefits of fine-grained related properties. The use of spark plasma sintering (SPS) technique offers an opportunity to produce fine-grained microstructures with minimum grain growth. However, grain refinement is not always guaranteed during SPS sintering especially under high-temperature sintering conditions. Therefore, sintering conditions that allow densification with minimal grain growth are well suited for microstructural refinement. A modified two-step sintering (TSS) methodology in SPS has proven to yield promising results and has potential use in the production of functional ceramic materials with controlled microstructures.