Ceramic Science and Technology

Prof. Antonio Licciulli
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Academic year  2002-2003

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Introduction to ceramics

 

Definition

The word ceramic can be traced back to the Greek term keramos, meaning "a potter" or "pottery." Keramos in turn is related to an older Sanskrit root meaning "to burn." Thus the early Greeks used the term to mean "burned stuff" or "burned earth" when referring to products obtained through the action of fire upon earthy materials1. Ceramics can be defined as inorganic, nonmetallic materials. They are typically crystalline in nature and are compounds formed between metallic and nonmetallic elements such as aluminum and oxygen (alumina-Al2O3), calcium and oxygen (calcia - CaO), and silicon and nitrogen (silicon nitride-Si3N4).
Ceramic Segments
The broad categories or segments that make up the ceramic industry can be classified as follows:
  • structural clay products
  • whitewares
  • refractories
  • glasses
  • abrasives
  • cements
  • advanced ceramics
Most of the above segments can be further broken down into more specific product classifications as seen on the table below. In 1974, the U.S. market for the ceramic industry was estimated at $20 million. Today, the U.S. market is estimated to be over $35 billion.
Industry of Ceramic Segments
Industry Segment Common Examples
Structural clay products Brick, sewer pipe, roofing tile, clay floor and wall tile (i.e., quarry tile), flue linings
Whitewares Dinnerware, floor and wall tile, sanitaryware, electrical porcelain, decorative ceramics
Refractories Brick and monolithic products are used in iron and steel, non-ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemicals industries
Glasses Flat glass (windows), container glass (bottles), pressed and blown glass (dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical fibers)
Abrasives Natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused alumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure blasting of materials
Cements Used to produce concrete roads, bridges, buildings, dams, and the like
Advanced ceramics  
Structural
Wear parts, bioceramics, cutting tools, and engine components
Electrical
Capacitors, insulators, substrates, integrated circuit packages, piezoelectrics, magnets and superconductors
Coatings
Engine components, cutting tools, and industrial wear parts
Chemical and environmental
Filters, membranes, catalysts, and catalyst supports
Overview of Ceramic and Glass Manufacturing
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. Chemically prepared powders also are used as starting materials for some ceramic products. These synthetic materials can be controlled to produce powders with precise chemical compositions and particle size.

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.

The processing of glass products is different than for ceramics. In glass production, raw materials such as silica, lime, and soda ash are melted in a furnace, then formed into the desired shape (i.e.; pressed plate, fibers, molded bottle, plate glass, etc.) while still molten. After the molten glass is formed, it is quickly cooled, "freezing" the glass into place to form the finished product. The glass typically undergoes additional processing steps such as cutting, etching, coating, grinding, decorating, or heat treating (tempering).

Some of the challenges that face ceramic and glass manufacturers, and suppliers include availability and cost of raw materials, quality and cost of labor, changing markets, quality control, capital for expansion, import pressure, and environmental, health and safety standards.

 

Structure and Properties
The properties of ceramic materials, like all materials, are dictated by the types of atoms present, the types of bonding between the atoms, and the way the atoms are packed together. This is known as the atomic scale structure. Most ceramics are made up of two or more elements. This is called a compound. For example, alumina (Al2O3), is a compound made up of aluminum atoms and oxygen atoms.

The atoms in ceramic materials are held together by a chemical bond. The two most common chemical bonds for ceramic materials are covalent and ionic. For metals, the chemical bond is called the metallic bond. The bonding of atoms together is much stronger in covalent and ionic bonding than in metallic. That is why, generally speaking, metals are ductile and ceramics are brittle.

Another structure that plays an important factor in the final property of a material is called microstructure. The microstructure of a material is the structure that can be seen using a microscope, but seldom with the naked eye. For ceramics, the microstructure can be entirely glassy (glasses only); entirely crystalline; or a combination of crystalline and glassy. In the last case, the glassy phase usually surrounds small crystals, bonding them together.

The atomic structure primarily effects the chemical, physical, thermal, electrical, magnetic, and optical properties. The microstructure also can effect these properties but has its major effect on mechanical properties and on the rate of chemical reaction. Due to ceramic materials wide range of properties, they are used for a multitude of applications. In general, most ceramics are:
  • hard,
  • wear-resistant,
  • brittle,
  • refractory,
  • thermal insulators,
  • electrical insulators,
  • nonmagnetic,
  • oxidation resistant,
  • prone to thermal shock, and
  • chemically stable.
Of course there are many exceptions to these generalizations. For example, borosilicate glasses (glasses that contain silica and boron as major ingredients) and certain glass ceramics (glasses that contain a crystalline phase) and NZP ceramics are very resistant to thermal shock and are used in applications such as ovenware, stove tops and kiln furniture respectively. Also, some ceramics are excellent electrical conductors and an entire commercial market is based on the fact that certain ceramics (ferrites) are magnetic.
History
Archeologists have uncovered human-made ceramics that date back to at least 24,000 BC. These ceramics were found in Czechoslovakia and were in the form of animal and human figurines, slabs, and balls. These ceramics were made of animal fat and bone mixed with bone ash and a fine claylike material. After forming, the ceramics were fired at temperatures between 500-800C in domed and horseshoe shaped kilns partially dug into the ground with loess walls. While it is not clear what these ceramics were used for, it is not thought to have been a utilitarian one. The first use of functional pottery vessels is thought to be in 9,000 BC. These vessels were most likely used to hold and store grain and other foods.

It is thought that ancient glass manufacture is closely related to pottery making, which flourished in Upper Egypt about 8,000 BC. While firing pottery, the presence of calcium oxide (CaO) containing sand combined with soda and the overheating of the pottery kiln may have resulted in a colored glaze on the ceramic pot. Experts believe that it was not until 1,500 BC that glass was produced independently of ceramics and fashioned into separate items.

Since these ancient times, the technology and applications of ceramics (including glass) has steadily increased. We often take for granted the major role that ceramics have played in the progress of humankind. Below are just a few examples of how important ceramics are to society.
Impact on Society
Refractories

We all know that metals and their alloys are used to make automobiles, machinery, planes, buildings, and a myriad of other useful things possible. But metal production would not be possible without the use of a ceramic material called refractories. Refractories can withstand volatile and high-temperature conditions encountered in the processing of metals. Refractory ceramics are enabling materials for other industries as well. The chemical, petroleum, energy conversion, glass and other ceramic industries all rely upon refractory materials.

Uses in the Construction Industry

The multi-billion dollar construction industry encompasses areas such as commercial buildings, residential homes, highways, bridges, and water and sewer systems. These areas, which we often take for granted, would not be possible without ceramic materials. Products such as floor, wall and roofing tile, cement, brick, gypsum, sewer pipe, and glass are the building blocks in the world of construction.

What would our homes and businesses be if we did not have windows? The applications of glass in the construction industry include various types of windows to let in natural light. Approximately three billion square feet of glass is produced each year to make various types of windows. To put things in perspective, imagine a 200-foot wide glass highway stretching from New York to Los Angeles. The different types of glass for windows include safety, stained, tinted, laminated and non-reflective. Additionally, glass fibers are used for insulation, ceiling panels and roofing shingles, helping us stay warm and dry.

Clay brick is used to build homes and commercial buildings because of its strength, durability, and beauty. Brick is the only building product that will not burn, melt, dent, peel, warp, rot, rust or be eaten by termites. Brick comes in approximately 10,000 different colors, textures, sizes, and shapes. Ceramic tile is used in applications such as flooring, walls, countertops, and fireplaces. Tile also is a very durable and hygienic construction product that adds beauty to any application.

Lighting Electrical

An important invention that changed the lives of millions of people was the incandescent light bulb. This important invention by Thomas Edison in 1879 would not be possible without the use of glass. Glass properties of hardness, transparency, and its ability to withstand high temperatures and hold a vacuum at the same time made the light bulb a reality.

The evolution of lighting technology since this time has been characterized by the invention of increasingly brighter and more efficient light sources. By the middle of twentieth century, methods of lighting seemed well established - with filament and fluorescent lamps for interiors, neon lamps for exterior advertising and signs, and sodium discharge lamps for streets. Since this time, light-emitting diode (LED) technology has been developed with applications in watches, instrument panel indicators, telecommunications (optical fiber networks), data storage (CD technology), and document production (laser printers).

Electrical Applications

The vast electronic industry would not exist without ceramics. Ceramics have a wide range of electrical properties including insulating, semi-conducting, superconducting, piezoelectric and magnetic. Ceramics are critical to products such as cell phones, computers, television, and other consumer electronic products.

Magnetic storage of data has developed in parallel with semiconductor computer chips and has been equally vital to computing and information handling. Without magnetic storage there would be no Internet, no personal computers, no large databases and no gigabytes, terabytes and exabytes of data which computers now manipulate. Today more than 150 million hard disc drives and 50 million videocassette recorders are made annually worldwide.

Ceramics are even being used to enhance our sporting activities. Piezoelectric ceramics (piezoceramics) are being used to make "smart" sporting goods equipment. That is, sporting goods that can respond to its surrounding environment in order to increase its effectiveness. Uses include snow skis, baseball/softball bats, and shock absorbers in mountain bicycles.

For example, K2 Corporation uses a control module produced by Active Control eXperts inside a line of its skis. The control module contains a piezoceramic material that dampens vibrations from the ice and snow, helping keep the skis on the snow and thus enhancing stability, control, and ultimately speed. Worth, Inc. now has a line of bats that incorporates a similar module. The piezoceramic control module helps to increase the bats sweet spot and reduces unwanted sting on off-center hits.

Ceramic spark plugs, which are electrical insulators, have had a large impact on society. They were first invented in 1860 to ignite fuel for internal combustion engines and are still being used for this purpose today. Applications include automobiles, boat engines, lawnmowers, and the like. High voltage insulators make it possible to safely carry electricity to houses and businesses.

Communication

Glass optic fibers have provided a technological breakthrough in the area of telecommunications. Information that was once carried electrically through hundreds of copper wires can now be carried through one high-quality, transparent, silica (glass) fiber. Using this technology has increased the speed and volume of information that can be carried by orders of magnitude over that which is possible using copper cable.

Optical fibers are a reliable conduit for delivering an array of interactive services, using combinations of voice, data and video. Whether it's multimedia and video applications, high-speed data transmissions and Internet access, telecommuting or sophisticated, on-demand services, optical fibers make it easier to communicate.

Medical

Ceramics are becoming increasingly useful to the medical world. Surgeons are using bioceramic materials for repair and replacement of human hips, knees, and other body parts. Ceramics also are being used to replace diseased heart valves. When used in the human body as implants or even as coatings to metal replacements, ceramic materials can stimulate bone growth, promote tissue formation and provide protection from the immune system.

Dentists are using ceramics for tooth replacement implants and braces. Glass microspheres smaller than a human hair are being used to deliver large, localized amounts of radiation to diseased organs in the body. Ceramics are one of the few materials that are durable and stable enough to withstand the corrosive effect of bodily fluids.

Imaging systems are critical for medical diagnostics. Modern ceramic materials play an important role in both ultrasonic and X-ray computed tomography (CT) systems. Transducers utilizing lead zirconate titanate (PZT) based piezoelectric ceramics are the heart of ultrasonic systems. Transducers generate the ultrasonic acoustic waves and detect the reflected signals to form the image.

Ultrasound can be used to examine many parts of the body including the abdomen, breasts, female pelvis, prostate, thyroid and parathyroid, and the vascular system. Most commonly, ultrasound is used during the first, second, or third trimester of pregnancy. New developments in ultrasound now enable doctors to see reliable images of ulcerated plaque or irregularities in the blood vessels - something doctors have never been able to see before. This may allow doctors to better identify those at risk for stroke because of its ability to more clearly illustrate the flow of blood, the walls of the carotid artery, and the movement of plaque.

X-ray CT scans are now a common diagnostic procedure in hospitals and clinical sites to image selected regions inside the human body for detection of cancer and other diseases. For CT scans, an x-ray detector is a crucial component that must have high efficiency to obtain high quality images. In 1988 GE Medical Systems unveiled an ultrahigh-performance detector with a breakthrough ceramic scintillator which gives better images using lower x-ray doses to the patient.

Environmental and Space Applications

Ceramics play an important role in addressing various environmental needs. Ceramics help decrease pollution, capture toxic materials and encapsulate nuclear waste. Today's catalytic converters in trucks and cars are made of cellular ceramics and help convert noxious hydrocarbons and carbon monoxide gases into harmless carbon dioxide and water. Advanced ceramic components are starting to be used in diesel and automotive engines. Ceramics light-weight, high-temperature and wear resistant properties results in more efficient combustion and significant fuel savings. Ceramics also are used in oil spill containment booms that corral oil so it can be towed away from ships, harbors, or offshore oil drilling rigs before being burned off safely.

Reusable, lightweight, ceramic tile make NASA's space shuttle program possible. The 34,000 thermal barrier tiles protect the astronauts and the shuttles aluminum frame from the extreme temperatures (up to approximately 1600C) encountered upon re-entry into the earth's atmosphere.

While the list could go on, one can begin to grasp the critical impact that ceramics have played in our past and present and will continue to play in the future.

source: American Ceramic Society

References:

1 "Report of the Committee on Definition of the Term Ceramics;" Journal of the American Ceramic Society, Vol. 3, No. 7, July 1920, pp. 526-542.