Materials Chemistry of Ceramics

Ceramics have high hardness and mechanical strength, excellent chemical and thermal stabilities, and particular electromagnetic and optical properties. Based on these unique properties, they have been widely used in our life, industry activities, and various environmental and energy fields. The durability of ceramics is derived from strong chemical bonding, that is, covalent and ionic bonds, and tight atomic packing. The electrical properties are dominated by crystal and electronic band structures and typically classified into insulator, dielectric, semiconductor, and metallic conductor. Typical covalent crystals are diamond, graphite, and hexagonal boron nitride (h-BN). Diamond, having 3D carbon network, is extremely hard, whereas graphite and h-BN are soft due to the hexagonal layer structure. Structural principle in ionic crystal is the alternate arrangement of cation and anion different in charge and size. The coordination configuration consisting of centered cation and surrounding anions is the elemental unit: for example, silica is constructed by SiO₄ tetrahedrons and titania by TiO₆ octahedrons. Lattice defects, such as vacancy and interstitial atom, provide a large influence on semiconductor and ionic conductor. Glass has a random network structure and has been applied in optical fields. Recent trend is the dimensional control of morphology from zero- to three-order. New functions of ceramics have been created using nanoparticle, nanofiber, nanosheet, hybrid, and nanoporous materials.

Phase equilibrium and phase diagram advise us what is the stable substance under certain circumstances and what condition is needed for the stability of the substance. This chapter describes the fundamental to understand the phase equilibrium and the phase diagram: the phase rule, the phase transformation with the change in the temperature or the pressure, the relationship between the temperature and the pressure in the phase transformation (so-called the Clausius–Clapeyron equation), and the phase change in compositional variation. The ways of reading and understanding of one-, two-, and three-component phase diagrams are explained using several practical phase diagrams.

Solid state reactions are mainly used to synthesize fine raw particles and to sinter advanced ceramics. To control the powder characteristics and sintering process, several analyses have been conducted based on classical thermodynamics and kinetics. In this chapter, we describe the thermodynamics and kinetics including the temperature-dependent standard free energies of formation and diffusion coefficients. Then, practical studies of solid state reaction kinetics are introduced using classical theories, such as the Jander and Ginstling-Brounstein equations. Finally, we discuss classical sintering theory using a two-particle model. Although several novel sintering methods recently have been developed, classical sintering theory is still important.

Ceramic powders are synthesized using chemical solution methods, such as organic acid salt, precipitant generation, alkaline hydrolysis, and alkoxide hydrolysis. The powder synthesis by precipitation and hydrolysis and the synthesis of monodispersed particles are described. The formation of complex alkoxide of lithium niobate (LN) precursor is confirmed by ⁹³Nb nuclear magnetic resonance spectroscopy. Stoichiometric LN films are synthesized on various substrates via chemical solution route. The synthesis of the designed precursor in solution is a key for the low-temperature crystallization of high-quality LN films with preferred orientation. Several functional ceramic thin films of multicomponent oxides are also synthesized using chemical solution process.

Chemical functions of ceramics are strongly related to the particle size, crystalline plane, morphology, and synthesis methods. In this chapter, the physical and chemical properties of fine ceramics materials are introduced. Environmentally friendly soft chemical processes, including solvothermal/hydrothermal process and mechanochemical process, are the effective methods for the synthesis of various functional materials and mixed anion type visible light-induced photocatalysts. The mixed anion type photocatalytic compounds consisting of N/O, N/F/O, S/O, and N/C/O show excellent visible light absorption ability, indicating the potential applications in environmental purifications with high solar light utilization efficiency. Various functions such as full-spectra active long wavelength light-induced photocatalyst, full-time active photocatalyst system, UV shielding, oxygen storage capacity (OSC), and hydrophilicity of oxide ceramics based on the precise design of components, particle size, and morphology are introduced also.

Technologies applied to characterizing ceramics for biomedical applications have essential roles in the development of artificial bones and teeth. By recovering biological function hampered by aging and disease and maintaining a high quality of life for patients, healthcare is of increasing significance in an aged society such as Japan. At present, healthcare includes not only technologies that sustain life but also those that can extend healthy lives. In this chapter, ceramics useful for recovering biological function are introduced.

In this chapter, mechanical properties of ceramics are described on the basis of both standards (JIS and ISO) together, for strength, hardness, creep, and abrasion resistance of dense sintered body. As a notable point, the characteristic value of mechanical property of ceramics depends on the size and quantity of microscopic and macroscopic defects (endogenous defect and exogenous one). Furthermore, the anisotropy and crystallinity in microstructure of the sample have an influence on the actual value. Besides, the measurement value is influenced by the measurement condition (different jig and device) even if it is measured with the same instrument and equipment. Nevertheless, the characterization of ceramics requires to evaluate more accurate mechanical property by using an appropriate test method.

Electric properties of ceramics range from insulator to superconductor, which are basically based on their crystal structures. Defect structures such as vacancy and substituted atoms affect some kind of electric properties. Intentional doping is frequently conducted to improve the electric properties. Different from the single crystal, ceramics contain grain boundary giving some unique functions. The conductivity or power generation of some kinds of ceramics changes depending on factors such as temperature, ambient gas, and applied pressure. Such property changes can be used as sensor materials. In addition to the microscopic crystal structure, some electromagnetic functions can be modified by changing the macroscopic structures such as micrometer order pores and layers.

This chapter provides an overview on essential principle of optical properties of ceramics which involves the phenomena in physics of the interaction between light and materials. Firstly, the light-matter interactions and how light behaves are described by considering the five key factors: scattering, absorption, transmission, reflection, and refraction. Also, this explanation includes three main groups of optical ceramics, transparent, translucent, and opaque, as material object and how to classify them. This chapter deals not only with physical backgrounds but also with giving the descriptions and examples of optical ceramics based on four crucial categories: transparent ceramic, single crystal, polycrystalline ceramic, and phosphor. Each sort is interpreted in detail with attribute, fabrication process, examples of material, and related applications.

Organic-inorganic hybrid materials have developed rapidly to become an established part of ceramic science and technology. In this chapter, among preparation methods of organic-inorganic hybrid materials, the sol-gel process, intercalation, and surface modification, which have a close relationship with ceramic materials, are described with an emphasis on their synthetic aspects and their applications. To begin with, classification of organic-inorganic hybrid materials based on the type of their interfacial chemical bonds is introduced. Following a brief definition of the sol-gel process, the basic chemistry of the sol-gel process is explained. Various preparation methods for sol-gel-derived organic-inorganic hybrid materials, including bridged silsesquioxane and mesostructured materials, are then described, and representative applications of sol-gel-derived organic-inorganic hybrid materials are briefly discussed. For intercalation, which is characteristic of layered materials, the basic concept is introduced first, after which the applicable reaction mechanisms for intercalation are demonstrated with the grafting reaction, which is related to intercalation. Representative applications of intercalation compounds are then exhibited with applications of nanosheets obtained via the exfoliation of layered materials. The process of surface modification of metal oxide nanoparticles and the application of the resulting surface-modified metal oxide nanoparticles are then demonstrated.