ABOUT CERAMICS PRODUCTS

ABOUT CERAMICS The word ceramics is derived from Keramos the Greek word for pottery or earthen ware materials formed by the action of heat or earthy raw materials. The evolution of pottery to present ceramic components has broadened the term ceramic so that today the term is used to define a class of materials ,which are inorganic nonmetallic. They are compounds of metallic and non metallic elements in which the atoms are held together by strong covalent or ionic bonds. The art and science of making these materials is called Ceramic technology. This group includes traditional ceramics such as pottery, tiles, porcelain, refractories, structural clay products, abrasives, porcelain enamels, cement and glass and technical ceramics such as electronic and magnetic materials, ferroelectrics, manufactured single crystals, glass ceramics and a variety of other product. Because of the unique chemical, electrical, magnetic, thermal and structural properties of ceramic materials, they are a basic component of the metallurgical industry. Abrasives are essential to the machine tool and automobile industries. Glass products are essential to the automobile Industry as well as architectural, electronic, and electrical industry. UO2 fuels are essential to the nuclear power industry. Cements are essential to the architectural and building industry. Various special electrical and magnetic ceramics are essential to the developments of computers and many other electronic devices. As a matter of fact, almost every industrial production line, office and home is dependent on ceramic materials. CERAMIC PROCESSES A major characteristic of ceramics familiar to everyone is that they are brittle and fractured with little or no deformation. This behavior stands in contrast to metals, which yield and deform. As a result, Ceramics cannot be formed into shape by normal deformation processes used for metals. Two basic processes have been developed for shaping ceramics. One is to use fine ceramic particles mixed with liquid or binder or lubricants or pore spaces. The combination that has rheological properties which permits shaping. Then by heat treatment the fine particles are agglomerated into a cohesive, useful product. The essential; of this procedure are first to find or prepare fine particles, shape them and then stick them back together by heating. The second basic process is to melt the material to form a liquid and then shape it during cooling and solidification; this is most widely practiced in forming glasses. RAW MATERIALS The mineral raw materials used in the ceramic industry are mainly inorganic nonmetallic crystalline solids formed by complex geological processes. Their ceramic properties are largely determined by the crystal structure and chemical composition of their essential constituents and the nature and amount of accessory minerals present. The mineralogical characteristics of such material and therefore their ceramic properties are subjected to wide variations among different occurrences or even within the same occurrences, depending on the geological environment in which it was formed as well as the physical and chemical modifications that have taken place during subsequent geological history. Since silicate and aluminum silicate are widely distributed, they are also inexpensive and thus provide the backbone of high tonnage products of the ceramic industries and determine to a considerable extent its form. Low grade clays are advisable almost everywhere as a result the manufacture of building bricks and tile not requiring exceptional properties is a localized industry for which extensive beneficiation of raw materials is not appropriate. In contrast for fine ceramic requiring the use of better controlled raw materials, the raw materials are normally beneficiated by mechanical concentrations, froth flotation and other relatively inexpensive processes. For materials in which the value added during manufacture is high, such as magnetic ceramics, nuclear fuel materials, electronic ceramics, and specialized refractories, chemical purification and even chemical preparation of raw materials may be necessary and appropriate. FABRICATION METHODS The critical affecting forming and firing processes are the raw materials and their preparation. The most important parameters of concern are raw material purity, particle size and the particle size distribution of the raw materials. In addition to a desired particle size and particle size distribution, intimate mixing of materials is necessary for uniformity of properties with the body and for the reaction of individual constituents during the firing processes. For preparing slurries or a fine grain plastic mass, it is the usual practice to use wet mixing with raw materials placed together in ball mills or a blunger. Shear stresses developed in the mixing process improve the properties of the plastic mix and ensure the uniform distribution of the fine grain constituents. For dewatering the well milled mix, either a filter press may be used ,or more commonly spray drying in which the droplets of the slurry is dried with counter current of warm air to maintain their uniform composition during their drying. The resulting aggregates normally one millimeter or so in size, flow and deform readily in subsequent forming. The simplest method of compacting a ceramic shape consists of a forming a dry or slightly damp powder, usually with an organic binder, in metal die at sufficiently high pressures to form a dense, strong piece .It is relatively cheap and can form shapes to close tolerances .Pressures in the range of 3000 to 30000 psi are commonly used. Automatic dry pressing at high rates of speed has been developed to a high rate of effectiveness. One modification of the dry pressing method which leads to a more uniform density is to enclose the sample in a rubber mold inserted in hydrostatic chamber to make pieces by hydrostatic molding, in which the pressure is more uniformly applied. This method is widely used for the manufacture of sparkplug insulator and special electric component in which a high degree of uniformity and high level of product quality are required. Other fabrication techniques include Extrusion in which a stiff plastic mix extrude through die orifice; Jiggering and Jolleying which consist of placing a lump of soft clay on the surface of plaster of Paris mould and rotating it about 400 rpm while pulling a profile to down on the surface to spread the clay and form the upper surface; slip casting, which consist of pouring ceramic suspension into porous plaster of Paris mould, where the mould sucks the liquid form the contact area ,and hard layer is built on the surface. DRYING AND FIRING The dryer step, in which the liquid is removed, must be carefully controlled for satisfactory result i.e. to obtain a defect free product. During drying, the initial drying rate is independent of the water content. As the liquid evaporates, the particle becomes pressed more closely together and shrinkage occurs until they are in a contact in a solid structure free from water film. During shrinkage period, stress and possibly cracks may develop because of local variation in the liquid content; during this period rate must be carefully controlled. Once the particles are in contact, drying can be continued at a more rapid rate without difficulty. After drying ceramic ware is normally fired to temperatures ranging from 700C to 800C depending on the composition and properties desired. During firing process either a viscous liquid or sufficient atomic mobility in the solid is developed to permit chemical reactions, grain growth and sintering; the last consist of allowing the forces of surface tension to consolidate the ware and reduce the porosity. Several different types of kilns are used for firing ware. Chamber kilns of either the up-draft or the down-draft are widely used for batch firing. The general availability of more précised temperature controls for gas, oil and electric heating and demand for ware uniformity have lead to the increased use of tunnel kilns in which a temperature profile is maintained. Constant and the ware is pushed through through the kiln to provide a précised firing schedule under conditions such that effective control can be obtained. The rate of temperature raised and the temperature uniformity must be controlled to avoid variation in porosity and shrinkage. CERAMIC PRODUCTS The diversity of ceramic products range from conventional aluminum-silicate products such as pottery, porcelain, refectories structural clay products abrasive porcelain enamels, cements and glass to new or advanced non-silicate formulations. From the point of view of historical development, raw materials used and tonnage produced, ceramic products are classified as, traditional ceramics and advance ceramic. TRADITIONAL CERAMICS These are defined as those comprising the silicate industries primarily clay products, cement and silicate glasses. These groups include pottery, porcelain, conventional aluminum-silicate refectories, structural clay products, abrasive and porcelain enamels. ADVANCED CERAMICS This category includes those products which are developed in order to fulfill a particular need in greater temperature resistance, superior mechanical properties, special electric properties, and greater chemical receptivity. These products include –Pure oxide ceramics, nuclear fuel, Electro-optic ceramics, Magnet ceramics, Single crystals, Nitrides, Carbides Borides, Ferro electric ceramics Molecular sieves Non-silicate glasses Glass ceramics and composites. The demand of new and better properties has led to the development of new materials; the availability of new materials has led to new uses based on their unique properties. This cycle of new ceramics- new uses –new ceramics has accelerated with the attainment of better understanding or ceramics and their properties.