Zirconium Dioxide (ZrO2).
Melting Point: 2715 °C
Zirconia, is a white crystalline oxide of zirconium. Its commonest naturally occurring form, with a monoclinic crystalline structure, is called baddeleyite. Its cubic crystalline form, called ‘cubic zirconia’, is rarely, if ever found in nature, but is made in various colours for sale as a gemstone.
It is used as a refractory, in insulation, abrasives, enamels and glazes, in high-temperature fuel cells and oxygen sensors.
Zirconium dioxide is one of the most studied ceramic materials. Pure ZrO2 has a monoclinic crystal structure at room temperature and transitions to tetragonal and cubic at increasing temperatures.
The volume expansion caused by the cubic to tetragonal to monoclinic transformation induces very large stresses, and will cause pure ZrO2 to crack upon cooling from high temperatures. Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic phases: magnesium oxide (MgO), yttrium oxide, (Y2O3), calcium oxide (CaO), and cerium oxide (CeO), amongst others.
Zirconia is very useful in its ‘stabilized’ state. In some cases, the tetragonal phase can be metastable. If sufficient quantities of the metastable tetragonal phase is present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion.
This phase transformation can then put the crack into compression, retarding its growth, and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia. A special case of zirconia is that of tetragonal zirconia polycrystaline or TZP, which is indicative of polycrystalline zirconia composed of only the metastable tetragonal phase.
The cubic phase of zirconia also has a very low thermal conductivity, which has led to its use as a thermal barrier coating or TBC in jet turbine and diesel engines to allow operation at higher temperatures. The laws of thermodynamics tell us that the higher temperature you operate an engine at, the greater the possible efficiency (see Carnot heat engine). As of 2004, a great deal of research is ongoing to improve the quality and durability of these coatings.
Stabilized zirconia is used in oxygen sensors and fuel cell membranes because it has a unique ability to allow oxygen ions to move freely through the crystal structure at high temperatures. This high ionic conductivity (and a low electronic conductivity) makes it one of the most useful electroceramics.
Single crystals of the cubic phase of zirconia are commonly used as a substitute for diamond in jewellery. Like diamond, cubic zirconia has a cubic crystal structure and a high index of refraction.
Discerning a good quality cubic zirconia gem from a diamond is difficult, and most jewellers will have a thermal conductivity tester to identify cubic zircona by its low thermal conductivity (diamond is a very good thermal conductor). This state of zirconia is commonly called “cubic zirconium” or “zircon” by jewellers, but these names are not chemically accurate.
Zirconium dioxide also occurs as a white powder and possesses both acidic and basic properties. Zirconium silicate (ZrSiO4), a natural mixture of zirconia and silica, is called zircon. Its transparent form is used as a gemstone, and its opaque form as a refractory. There is a red variety called hyacinth and a yellow or brown variety from Sri Lanka called jargon.
Olivine.
The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two end members of the series: forsterite (Mg-rich) and fayalite (Fe-rich). It gives its name to the group of minerals with a related structure (the olivine group) which includes monticellite and kirschsteinite. Olivine occurs in both mafic and ultramafic igneous rocks, and as a primary mineral in certain metamorphic rocks.
It is one of the most common minerals on Earth.
Olivine is usually coloured olive-green (hence the name), though it may be reddish from the oxidation of iron. It has a conchoidal fracture and is rather brittle. The hardness of olivine is 6.5-7, its relative density is 3.27-3.37 and it has a vitreous lustre. It is transparent to translucent.
Olivine crystallizes from magma that is rich in magnesium and low in silica, which forms mafic to ultramafic rocks such as gabbro, basalt, peridotite, and dunite. The metamorphism of impure dolomite or other sedimentary rocks with high magnesium and low silica content also seems to produce Mg-rich olivine, or forsterite.
Olivine or high pressure structural variants also constitute over 50% of the Earth’s upper mantle making it one of the Earth’s most common minerals by volume. Olivine has also been discovered in meteorites, on Mars, and on Earth’s moon.
Bentonite.
Bentonite is an absorbent aluminium phyllosilicate generally impure clay consisting mostly of montmorillonite, Chemical Formula: (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·nH2O.
Two types exist: swelling bentonite which is also called sodium bentonite and non-swelling bentonite or calcium bentonite. It forms from weathering of volcanic ash, most often in the presence of water. Bentonite expands when wet – sodium bentonite can absorb several hundred percent of its dry weight in water.
It is commonly used in drilling fluids, used to make slurry walls, and used to form impermeable barriers (ie plug old wells, as a liner in the base of landfills to prevent migration of leachate into the soil).
Much of bentonite’s usefulness in the drilling and geotechnical engineering industry comes from its unique rheological properties. Relatively small quantities of bentonite suspended in water form a viscous, shear thinning material. Most often, bentonite suspensions are also thixotropic, although rare cases of rheopectic behaviour have also been reported. At high enough concentrations (60 grams of bentonite per litre of suspension), bentonite suspensions begin to take on the characteristics of a gel (material with finite yield strength).
Bentonite is named after Benton Formation (a geological stratum, at one time Fort Benton Formation) in eastern Wyoming’s Rock Creek area. Most high grade commercial sodium bentonite mined in the US comes from the area between the Black Hills of South Dakota and the Big Horn Basin of Montana.
Sodium bentonite is also mined in the south-western US, in Greece, and in other regions of the world. Calcium bentonite is mined in the south eastern US. Bentonite can be used in cement, adhesives, ceramic fillers, cosmetics, and cat litter. Bentonite clay is also used in pyrotechnics to make end plugs and rocket nozzles. Most is used as drilling mud in the oil and gas well drilling industries.
It is also sold in health food stores as a drink. Bentonite clay is also extensively used in the foundry industry as a binder for green sand
The colour of bentonite ranges from white to light olive green, cream, yellow, earthy red, brown and sometimes sky blue when fresh but yellowing rapidly with exposure to air. When wet it is highly plastic and slippery. Bentonite feels and appears greasy or waxy, there are two basic types of bentonite depending on whether they contain sodium or calcium in the crystal lattice.
1. Swelling Bentonites – sodium bentonite contains sodium in the platy molecular structure; has strong swelling properties (ability to swell to many] times its dry volume when wet) and possesses a high dry-bonding strength.
2. Non-swelling Bentonites – calcium bentonite contains calcium in its structure; has a far lower capacity to swell when wet and usually exhibits greater adsorptive characteristics.
Uses of Bentonite.
Swelling bentonite has many uses, due to its high plasticity and gelling property, its ability to take on large quantities of water and its capacity to swell to many times its dry volume.
The applications for swelling bentonite include:
Well-drilling mud, where several pounds of bentonite are added per cubic foot of water to form a gel which lubricates the drill bit, helps to keep the walls of the borehole intact, reduces loss of drilling fluid and, when circulated during drilling, helps to bring the cuttings to the surface.
- Pelletizing and binding agent for iron ore prior to smelting.
- Binder and digestive aid in animal feed.
- Sealing agent for reservoirs, lagoons and other engineering projects.
- Plasticizer in ceramic clay bodies.
- Binder of foundry sand.
- Carrier of insecticide and pesticide.
- Agent in water and effluent purification.
- Absorbent material for cat litter.
The use of bentonite as a binder in foundry green sand moulds is still considered a major application for which the swelling variety is preferred, although non-swelling bentonite is used for some foundry purposes.