CERAMIC MATERIAL

Ceramic materials are inorganic, non-metallic materials made from compounds of a metal and a non metal. Ceramic materials may be crystalline or partly crystalline. They are formed by the action of heat and subsequent cooling. clay was one of the earliest materials used to produce ceramics, but many different ceramic materials are now used in domestic, industrial and building products. Ceramic materials tend to be strong, stiff, brittle, chemically inert, and non-conductors of heat and electricity, but their properties vary widely. For example, porcelain is widely used to make electrical insulators, but some ceramic compounds are superconductors.

CERAMIC MATERIALS

MAIN TYPES OF CERAMIC MATERIALS

Crystalline ceramicsNon-crystalline ceramics

Crystalline ceramic materials are not amenable to a great range of processing. Methods for dealing with them tend to fall into one of two categories - either make the ceramic in the desired shape, by reaction in situ, or by "forming" powders into the desired shape, and then sintering to form a solid body. Ceramic forming techniques include shaping by hand (sometimes including a rotation process called "throwing"), slip casting, tape casting (used for making very thin ceramic capacitors, etc.), injection moulding, dry pressing, and other variations. (See also Ceramic forming techniques. Details of these processes are described in the two books listed below.) A few methods use a hybrid between the two approaches.

Non-crystalline ceramic

Non-crystalline ceramics, being glasses, tend to be formed from melts. The glass is shaped when either fullymolten, by casting, or when in a state of toffee-like viscosity, by methods such as blowing to a mold. If later heat-treatments cause this glass to become partly crystalline, the resulting material is known as a glass-ceramic.

PROPERTIES OF CERAMIC MATERIALS

Mechanical properties The important mechanical properties affecting the selection of a material are:

(i) Tensile Strength: This enables the material to resist the application of a tensile force. To withstand the tensile force, the internal structure of the material provides the internal resistance.

(ii) Hardness: It is the degree of resistance to indentation or scratching, abrasion and wear. Alloying techniques and heat treatment help to achieve the same.

(iii) Ductility: This is the property of a metal by virtue of which it can be drawn into wires or elongated before rupture takes place. It depends upon the grain size of the metal crystals.

(iv) Impact Strength: It is the energy required per unit cross-sectional area to fracture a specimen, i.e., it is a measure of the response of a material to shock loading.

(v) Wear Resistance: The ability of a material to resist friction wear under particular conditions, i.e. To maintain its physical dimensions when in sliding or rolling contact with a second member.

(vi) Corrosion Resistance: Those metals and alloys which can withstand the corrosive action of a medium, i.e. corrosion processes proceed in them at a relatively low rate are termed corrosion-resistant.

(vii) Density: This is an important factor of a material where weight and thus the mass is critical, i.e. aircraft components.

Electrical properties

Semiconductors Some ceramics are semiconductors. Most of these are transition metal oxides that are II-VI semiconductors, such as zinc oxide.

While there are prospects of mass producting blue LEDs from zinc oxide, ceramicists are most interested in the electrical properties that show grain boundary effects.

Conductivity, resistivity, dielectric strength are few important electrical properties of a material. A material which offers little resistance to the passage of an electric current is said to be a good conductor of electricity.

INSULATOR Insulators have very high resistivity. Ceramic insulators are most common examples and are used on automobile spark plugs, Bakelite handles for electriciron, plastic coverings on cables in domestic wiring

CONDUCTOR In general metals are good conductors.

Magnetic Properties

Materials in which a state of magnetism can be induced are termed magnetic materials. There are five classes into which magnetic materials may be grouped: (i) diamagnetic (ii) paramagnetic (iii) ferromagnetic (iv) antiferromagnetic and (v) ferrimagnetic.

•DIAMAGNETIC

•PARAMAGNETIC• FERROMAGNETIC

•ANTIFERROMAGNETIC

• FERRIMAGNETIC

Types of Magnetic Properties

DIAMAGNETIC

Diamagnetic materials have a weak, negative susceptibility to magnetic fields. Diamagnetic materials are slightly repelled by a magnetic field and the material does not retain the magnetic properties when the external field is removed. In diamagnetic materials all the electron are paired so there is no permanent net magnetic moment per atom. Diamagnetic properties arise from the realignment of the electron paths under the influence of an external magnetic field. Most elements in the periodic table, including copper, silver, and gold, are diamagnetic.

Diamagnetism is a fundamental property of all matter, although it is usually very weak. It is due to the non-cooperative behavior of orbiting electrons when exposed to an applied magnetic field. Diamagnetic substances are composed of atoms which have no net magnetic moments (ie., all the orbital shells are filled and there are no unpaired electrons). However, when exposed to a field, a negative magnetization is produced and thus the susceptibility is negative. If we plot M vs H, we see:Note that when the

field is zero the magnetization is zero. The other characteristic behavior of diamagnetic materials is that the susceptibility is temperature independent. Some well known diamagnetic substances, in units of 10-8 m3/kg, include:quartz (SiO2)   -0.62

Calcite (CaCO3)  -0.48 water    -0.90

PARAMAGNETIC

Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic properties are due to the presence of some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum.

This class of materials, some of the atoms or ions in the material have a net magnetic moment due to unpaired electrons in partially filled orbitals. One of the most important atoms with unpaired electrons is iron. However, the individual magnetic moments do not interact magnetically, and like diamagnetism, the magnetization is zero when the field is removed. In the presence of a field, there is now a partial alignment of the atomic magnetic moments in the direction of the field, resulting in a net positive magnetization and positive susceptibility.

 Montmorillonite (clay)   13  Nontronite (Fe-rich clay)   65

 Biotite (silicate)    79

 Siderite(carbonate) 100  Pyrite (sulfide)    30

FERROMAGNETIC

Ferromagnetic materials have a large, positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and are able to retain their magnetic properties after the external field has been removed. Ferromagnetic materials have some unpaired electrons so their atoms have a net magnetic moment. They get their strong magnetic properties due to the presence of magnetic domains. In these domains, large numbers of atom's moments (1012 to 1015) are aligned parallel so that the magnetic force within the domain is strong. When a ferromagnetic material is in the unmagnitized state, the domains are nearly randomly organized and the net magnetic field for the part as a whole is zero. When a magnetizing force is applied, the domains

When you think of magnetic materials, you probably think of iron, nickel or magnetite. Unlike paramagnetic materials, the atomic moments in these materials exhibit very strong interactions. These interactions are produced by electronic exchange forces and result in a parallel or antiparallel alignment of atomic moments. Exchange forces are very large, equivalent to a field on the order of 1000 Tesla, or approximately a 100 million times the strength of the earth's field. The exchange force is a

quantum mechanical phenomenon due to the relative orientation of the spins of two electron.Two distinct characteristics of ferromagnetic materials are their:

 (1) spontaneous magnetization and the existence of

 (2) magnetic ordering temperature

In ionic compounds, such as oxides, more complex forms of magnetic ordering can occur as a result of the crystal structure. One type of magnetic ordering is call ferrimagnetism. A simple representation of the magnetic spins in a ferrimagnetic oxide is shown here.

The magnetic structure is composed of two magnetic sublattices (called A and B) separated by oxygens. The exchange interactions are mediated by the oxygen anions. When this happens, the interactions are called indirect or superexchange interactions. The strongest superexchange interactions result in an antiparallel alignment of spins between the A and B sublattice.

FERRIMAGNETIC

In ferrimagnets, the magnetic moments of the A and B sublattices are not equal and result in a net magnetic moment. Ferrimagnetism is therefore similar to ferromagnetism. It exhibits all the hallmarks of ferromagnetic behavior- spontaneous magnetization, Curie temperatures, hysteresis, and remanence. However, ferro- and ferrimagnets have very different magnetic ordering.

ANTIFERROMAGNETIC If the A and B sublattice moments are exactly equal

but opposite, the net moment is zero. This type of magnetic ordering is called antiferromagnetism.

The clue to antiferromagnetism is the behavior of susceptibility above a critical temperature, called the Néel temperature (TN). Above TN, the susceptibility obeys the Curie-Weiss law for paramagnets but with a negative intercept indicating negative exchange interactions.

CHEMICAL PROPERTIES These properties includes atomic weight, molecular weight, atomic number, valency, chemical composition, acidity, alkalinity, etc. These properties govern the selection of materials particularly in Chemical plant.

Optical properties Optically transparent materials focus on the response of a material to incoming lightwaves of a range of wavelengths. Frequency selective optical filters can be utilized to alter or enhance the brightness and contrast of a digital image. The optical properties of materials, e.g. refractive index, reflectivity and absorption coefficient etc. affect the light reflection and transmission,

Ceramic Processing Ceramic processing is used to produce commercial products that are very diverse in size, shape, detail, complexity, and material composition, structure, and cost.The purpose of ceramics processing to an applied science is the natural result of an increasing ability to refine, develop, and characterize ceramic materials. 

Ceramics are typically produced by the application of heat upon processed clays and other natural raw materials to form a rigid product. Ceramic products that use naturally occurring rocks and minerals as a starting material must undergo special processing in order to control purity, particle size, particle size distribution, and heterogeneity. These attributes play a big role in the final properties of the finished ceramic.

The next step is to form the ceramic particles into a desired shape. This is accomplished by the addition of water and/or additives such as binders, followed by a shape forming process. Some of the most common forming methods for ceramics include extrusion, slip casting, pressing, tape casting and injection molding. After the particles are formed, these "green" ceramics undergo a heat-treatment (called firing or sintering) to produce a rigid, finished product. Some ceramic products such as electrical insulators, dinnerware and tile may then undergo a glazing process. Some ceramics for advanced applications may undergo a machining and/or polishing step in order meet specific engineering design criteria.

Processing of ceramic materials

MILLING Milling is the process by which materials are reduced from a large size to a smaller size. Milling may involve breaking up cemented material (in which case individual particles retain their shape) or pulverization (which involves grinding the particles themselves to a smaller size). Milling is generally done by mechanical means, including attrition (which is particle-to-particle collision that results in agglomerate break up or particle shearing), compression (which applies a forces that results in fracturing), and impact (which employs a milling medium or the particles themselves to cause fracturing). Attrition milling equipment includes the wet scrubber (also called the planetary mill or wet attrition mill), which has paddles in water creating vortexes in which the material collides and break up.

MILLING

BATCHING Batching is the process of weighing the oxides according to recipes, and preparing them for mixing and drying.

MIXING Mixing occurs after batching and is performed with various machines, such as dry mixing ribbon mixers (a type of cement mixer), Mueller mixers, and pug mills. Wet mixing generally involves the same equipment.

FORMING Forming is making the mixed material into shapes, ranging from toilet bowls to spark plug insulators. Forming can involve: (1) Extrusion, such as extruding "slugs" to make bricks, (2) Pressing to make shaped parts, (3) Slip casting, as in making toilet bowls, wash basins and ornamentals like ceramic statues. Forming produces a "green" part, ready for drying. Green parts are soft, pliable, and over time will lose shape. Handling the green product will change its shape. For example, a green brick can be "squeezed", and after squeezing it will stay that way.

FORMING

DRYING Drying is removing the water or binder from the formed material. Spray drying is widely used to prepare powder for pressing operations. Other dryers are tunnel dryers and periodic dryers. Controlled heat is applied in this two-stage process. First, heat removes water. This step needs careful control, as rapid heating causes cracks and surface defects.

FIRING Firing is where the dried parts pass through a controlled heating process, and the oxides are chemically changed to cause sintering and bonding. The fired part will be smaller than the dried part.

CERAMIC STRUCTURES In recent years, the number and variety of materials, which are of particular interest to an engineer have increased tremendously. Each type of material has a specific composition possessing specific properties for a specific use. It is not possible for one to explain the properties of all types of these materials. A knowledge of the structure of the material helps students and engineers to study the properties of the material. Material structure can be classified as: macrostructure, microstructure, substructure, crystal structure, electronic structure and nuclear structure

MACROSTRUCTURE Macrostructure of a material is examined

by low-power magnification or naked eye. It deals with the shape, size and atomic arrangement in a crystalline material. In case of some crystals, e.g., quartz, external form of the crystal may reflect the internal symmetry of atoms. Macrostructure may be observed directly on a fracture surface or on a forging specimen. The individual crystals of a crystalline material can be visible, e.g. in a brass doorknob by the constant polishing and etching action of a human hand and sweat. Macrostructure can reveal flaws, segregations, cracks etc. by using proper techniques and one can save much expenses by rejecting defective materials at an early stage.

MACROSTRUCTURE

MICRO STRUCTURE This generally refers to the structure of the material observed under optical microscope. Optical microscopes can magnify a structure about 1500 to 3000 times linear, without loss of resolution of details of the material structure. We may note that optical microscopes can resolve two lines separately when their difference of separation is 10-7 m (= 0.1 m). Cracks, porosity, non-metallic insclutions within materials can be revealed by examining them under powerful optical microscope.

MICRO STRUCTURE

ELECTRONIC STRUCTURE This refers to the electrons in the outermost shells of individual atoms that form the solid. Spectroscopic techniques are commonly used for determining the electronic structure.

SUB STRUCTURE

When crystal imperfections such as dislocation in a structure are to be examined, a special microscope having higher magnification and resolution than the optical microscope is used. Electron microscope with magnifications 10 are used for this purpose. Another important modern microscope is field ion microscope, which can produce images of individual atoms as well as defects in atomic arrangements.

SUB STRUCTURE

NUCLEAR STRUCTURE This is studied by nuclear spectroscopic techniques, e.g., nuclear magnetic resonance (NMR) and Mössbauer spectroscopy.

CRYSTAL STRUCTURE This reveals the atomic arrangement within a crystal. X-ray diffraction techniques and electron diffraction method are commonly used for studying crystal structure. It is usually sufficient study the arrangement of atoms within a unit cell. The crystal is formed by a very large number of unit cells forming regularly repeating patterns in space.

CRYSTAL STRUCTURE

ENGINEERING APPLICATION Compressive strength makes ceramics good structural materials (e.g., bricks in houses, stone blocks in the pyramids)

• High voltage insulators and spark plugs are made from ceramics due to its electrical conductivity properties Good thermal insulation has ceramic tiles used in ovens and as exterior tiles on the Shuttle orbiter Some ceramics are transparent to radar and other electromagnetic waves and are used in radomes and transmitters

• Hardness, abrasion resistance, imperviousness to high temperatures and extremely caustic conditions allow ceramics to be used in special applications where no other material can be used.

• Chemical inertness makes ceramics ideal for biomedical applications like orthopaedic prostheses and dental implants

• Glass-ceramics, due to their high temperature capabilities, leads to uses in optical equipment and fiber insulation

INDUSTRIAL CERAMICS Thousands of engineering gears have used from advanced ceramics solutions for wear resistance, corrosion resistance & thermal resistance, providing significant lifetime added to over conventional metal gears. It is not always the best possible design solution, commonly advanced ceramics can be benefited as direct substitutes for available designs.

Typical gears include wear plates & thermal barriers, bearings for high speed and high stiffness spindles, bushes, gears and many others. Dynamic-Ceramic can provide now hundreds of case histories on the successful and cost effective application of advanced ceramics solutions in mechanical engineering applications.

Although ceramics have been used by man for many centuries, until recently their applications have been limited by their mechanical properties. Unlike metals, most ceramics materials do not exhibit a non-linear plastic region before failure. Instead, ceramics are known to be brittle and fail catastrophically. Their application in engineering applications has certainly been limited by their lack of toughness.

INDUSTRIAL CERAMICS