Ceramics

Chemical Structure of Ceramics

Crystalline

Spinel

Spinel (MgAl₂O₄) is a transparent ceramic composed of magnesium oxide alumina with the beneficial properties of high hardness, strength and abrasion resistance. The crystal lattice structure of spinel is a face centered crystal (FCC) structure. Optical quality transparent spinel can be produced by hot press or sintering techniques. Other grades of spinel can be produced by electric furnace co-fusion of high purity magnesia with alumina or alumina containing compounds followed by solidification and cooling. Applications of spinel ceramic material include armor, watch crystals, optics, windows, defense technologies, cutting tools and lasers.

Garnet

Transparent yttrium aluminum garnet (Y₃Al₅O₁₂ or YAG) is a synthetic crystalline material. YAG is commonly used as a material used in solid state lasers. YAG lasers incorporate YAG laser rods of varying diameters to produce infrared light. The use of dopants at different concentrations can effectively change the laser properties.

Perovskite

A perovskite is any material with the same crystal structure of perovskite mineral comprised of calcium titanate (CaTiO₃). The crystal lattice structure of petrovskite is a simple cubic (SC) structure. Perovskite materials exhibit properties such as superconductivity, high thermal power, ferroelectricity, and magnetoresistance making these compounds useful for applications in sensors, memory devices and fuel cell electrodes. Additionally, recent research has shown that trihalide perovskites can be valuable solar cell materials.

Hexagonal

Hexagonal ceramic crystal lattice systems are frequently associated with graphite and corundrum (Al2O3) or other ceramic compounds having similar chemical structure.

Cubic

Cubic crystal systems include simple cubic (SC), face centered cubic (FCC) and body centered cubic (BCC) lattice structures.

Non-crystalline

Non-crystalline ceramics are amorphous (glass-like) materials formed from melts. Additional heat processing can render the glass partly crystalline; such materials may be referred to as glass-ceramic.

Types of Ceramics by Use

White Ware

White ware ceramics are often referred to colloquially as pottery. These include earthenware, often composed of clay, quartz, and feldspar, and stoneware, which is comprised primarily of clay. Porcelain, a well-known variety of white ware ceramic, is composed of a specific type of clay called kaolin.

Structural

:Structural ceramic products include bricks, ceramic pipes, and floor and roof tiles.

Refractory

Refractory ceramic materials are capable of retaining their strength and structure at high temperature. Refractory materials are often used as lining for furnaces, kilns, incinerators, and high temperature reactors.

Technical Ceramics

Technical ceramics are engineered for specific functions, and typically exhibit both the heat resistance typical of refractory materials, as well as resilience to mechanical or chemical stresses found in a given application. These are found as mechanical components such as bearings in engines, coatings on jet engine turbine blades, and as biomedical implants, among other applications.

Functional Ceramic Types

Piezoelectric

Piezoelectric ceramics have the property of developing an electric charge upon exposure to mechanical stress. Piezoelectric ceramics are broadly classified as either hard or soft doped in reference to the variable ferroelectric properties such as the mobility of dipoles and polarization properties. The electric response to the mechanical stimulation is known as the direct piezoelectric effect and the mechanical response to electric stimulation is known as the converse piezoelectric effect. Hard doped piezoelectric materials can be exposed to high electrical and mechanical stresses making them useful for high power applications. Soft doped piezoelectric ceramics exhibit easy polarization even and are thus suitable for sensing applications, receivers, actuators and low power transducers.

Ferroelectric / Ferromagnetic

Ferroelectric ceramics have spontaneous electric polarization that can be reversed in the presence of an electric field. These properties make ferroelectric compounds one of the most prominent and useful ceramics for electronics applications. Ferromagnetic ceramics include ferrites and magnetic garnets. These materials are used in electronic components such as electromagnets, transformers and inductors due to their high electrical resistances. Spherical ferromagnetic iron powder is composed of high-purity iron microsphere particles with unique electromagnetic properties for use in electronics, power injection molding, sensors, and radiation shielding.

Semiconductor

Semiconductor ceramics are used for their conductive properties with application in electronics, computing and photovoltaics.

Methods of Synthesis

There are a wide variety of processes used to synthesize ceramic materials. Ceramic fabrication processes can be divided into four generic categories: powder, vapor, chemical, and melt processes.

Powder Processing

The heating and cooling method most commonly used to synthesize advanced ceramics is also known as powder processing and involves four steps including powder preparation, shape forming, high temperature sintering and component finishing. Each process step can be achieved using one or more methods as shown below:

Vapor Processes

Vapor processes are typically used to produce ceramic coatings on a the surface of another material. Chemical vapor deposition involves bringing gas-phase precursors in contact with a heated surface where a chemical reaction then takes place, forming a coating. Application of ceramic coatings to numerous products such as electronics, corrosion resistant surfaces and cutting tools is commonly achieved by chemical vapor deposition. Alternatively, several approaches to sputtering, in which a plasma is used to physically knock individual molecules of an intended coating material off of a target, allowing them to be re-deposited on the material to be coated, may be used to produce some types of ceramic coatings.

Chemical Processes

Sol-gel techniques may also be used in advanced ceramics fabrication. Sol-gel technology involves the formation of a suspension of sol, or colloidal particles, which is converted to a gel by chemical processing. The gel is dried and sintered to form the ceramic product.

Pyrolysis is a technique in which precursor compounds are heated to high temperatures in an oxygen-free atmosphere, leading to their decomposition and the subsequent formation of a chemically distinct product. Pyrolysis of silane or silazane-based polymers or of various organometallic compounds may be used to produce a variety of non-oxide ceramics.

Melt Processes

Melt processes are typically used to manufacture glasses. The starting materials are melted together and then cooled and formed under controlled conditions. Thermal spraying, also considered a melt process, can be used to apply glassy ceramic coatings onto a substrate.

Select Applications for Ceramic Materials

Coatings: Ceramic coatings are applied to numerous products, ranging from electronic components to cast iron cookware, to increase hardness and wear resistance.

Defense: Transparent crystalline ceramics such as spinels and garnets are used for transparent armor and glassware. In addition, ceramics are also found on the ends of heat seeking missiles and heat resistant ballistic materials.

Tools and Abrasives: Numerous tools such as drills, cutting equipment, and grinding tools often incorporate ceramics. For example, many cutting tools and drills are coated with ceramic materials due to their high resistance to heat.

Biomedical Devices: Dental implants and orthopedic devices often incorporate ceramics due to their wear resistance and biocompatibility.

Optical and Scintillation Crystals: Both crystalline and glass ceramics have found applications in laser technologies, optics, sensors, and scintillation (radiation detection).

Porous Ceramics: Porous ceramics such as ceramic foams have applications as membranes, filters, or molecular capture devices.

Thermal Insulation: Heat insulation properties of ceramics are used widely in electronics, coatings, and aerospace applications.

Semiconductor Devices: High purity ceramics function as dielectric materials in a majority of semiconductor devices and electronics, in addition to playing a role in process applications such as CVD, wafer fabrication and handling equipment, melting crucibles, sub-assembly components, and wafer calibration. Typical materials include alumina, alumina nitride, silicon carbide, pyrolytic boron nitride, silicon carbide, and zirconia.