Here is the most concise information on alumina ceramics on the internet.
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Alumina ceramic is an industrial oxide ceramic that has high hardness, is long wearing. It is manufactured from bauxite and can be shaped using injection molding, die pressing, isostatic pressing, slip casting, diamond machining and extrusion.
Products made from alumina, some of which are shown in the image below, are wear, chemical, erosion, corrosion, and high temperature resistant and bioinert, making them perfect for medical implants.
Alumina ceramics are a technical ceramic due to their properties and price to performance ratio. The classification of alumina ceramics is based on their alumina content, which can vary from 70% to 99.9%. The higher the purity of alumina, the stronger is its wear and corrosion resistance.
Alumina ceramics are made from a white granular material that is similar to table salt or a very fine silky dense powder. The three general types of alumina are hydrated, calcined, and tabular. Each type has a variety of grades.
The types of alumina vary according to the amount of soda (Na2O), iron (Fe2O3) and silica (SiO2) they contain as well as their chemical purity and the properties of the powder used in the production process.
To produce calcined alumina, aluminum oxide is heated to 1050° C or 1900° F. The super heating removes all chemicals and water creating a very pure, 99.99% pure, with a 9 on the Mohs hardness scale, which is just below a diamond‘s Mohs rating of 10.
Alumina hydrate, or alumina hydroxide, is used as a glaze because of its ability to stay in suspension in glaze slurries and adhesive qualities.
Tabular alumina is produced by heating aluminum oxide to 1650° C or 3000° F. It has a high heat capacity, excellent thermal temperature, strength, and volume stability. It is formed from sintering balls of calcined alumina, which are crushed to form a powder. Tabular alumina has high refractory properties, mechanical strength, wear resistance, chemical purity, dielectric properties, and corrosion resistance in acids and alkaline.
Alumina is used in oxidizing and reducing atmospheres up to 1650°C or 2900°F as well as vacuum environments of 2000°C or 3600°F. At 1000° C, it keeps 50% of the tensile strength it has at room temperature. While metals are weakened by high temperatures, alumina ceramics retain their strength when they return to normal temperatures and are unchanged.
Abrasion wears down a material by rubbing it away by friction. The resistance to abrasion means a material will maintain its original structure even after mechanical wear. Alumina ceramics are high in abrasion resistance due its hardness.
Alumina is resistant to acids and alkalis at high temperatures because it is inert, not chemically reactive, which makes it resistant to the effects of chemicals such as solvents and salt solutions.
The density of a material is its mass divided by its volume, which is read as grams per cubic centimeter (g/cm3) where grams is mass and cubic centimeters is volume. The colume and density of the material are inversely proportional. Alumina ceramics are made from fine particles that do not allow for voids in the material. The fewer voids means the material has high volume and density. The density of alumina ceramics varies according to the temperature. At 25° C, its density is 3.965 g/m3 at standard atmospheric pressure.
Manufacturers need to grind the powder to a sub micron, nanometers level, to achieve an after firing grain size of less than five microns, which produces the fewest voids and makes the best wear surface. Ceramic with a 95% alumina content, after firing, will have a grain size of 30 to 40 microns, large voids, and a high wear rate. Ceramic with an 87% alumina content, after firing, will have a grain size of three to five microns.
The mechanical properties of a material is determined by its strength, which is the amount of stress and strain it can endure. Alumina has superior strength and hardness that improves with the purity of the different grades.
Alumina has high resistivity and reduces thermal shock. The increased purity of alumina increases its resistivity.
Alumina ceramics make a perfect insulation material because of its dielectric equality, the inability of an electric current to pass through them.
Hardness tells the ability of a material to be able to endure mechanical wear and abrasion. Alumina ceramics are harder than steel and tungsten carbide tools. They are harder than sapphire and are excellent for mill linings and bearings. According to Rockwell hardness, alumina ceramics are at HRA80-90, second only to diamonds and above stainless steel.
The powder material for alumina ceramics comes from the processing of bauxite, which is an aluminum rich clay like material located a few meters below the earth‘s crust. The method for manufacturing alumina was discovered in 1887 and is called the Bayer process. Below is a description of the process.
The mined bauxite is taken to a processing plant where it is ground.
The ground bauxite is washed, dried, and dissolved in a mixture of caustic soda and lime to form a slurry that is heated in a digester to 300° F or 145° C and placed under 50 lbs. of pressure for several hours to dissolve the aluminum compounds.
The slurry is pumped through a series of flash tanks to reduce the pressure and heat on the material. The flash tanks cool the material at atmospheric pressure and flash off steam.
The impurities in the slurry, like sand, iron, and other elements that do not dissolve, settle to the bottom of the tank. The liquor at the top of the tank moves through a set of filters and is washed to recover the alumina. It is filtered several times before moving on.
The filtered clear sodium aluminate is pumped into a series of precipitators. Particles of alumina are added to begin the process. The alumina grows around the seeds causing them to fall to the bottom of the tank to be removed and sent to thickening tanks. The material is filtered again.
The final step in the extraction of alumina involves a heating process to remove water from the alumina hydrate. It is filtered and washed again to remove impurities and moisture. A conveyor, seen in the diagram, moves the hydrate to calcination, a gas fired kiln on an incline that rotates to ensure even heating.
Newer methods of processing use a fluid bed calcining that suspends the alumina particles above a screen of hot air and calcined.
The powder from refining is mixed with other materials before being put through the forming process. The blend of alumina and other substances determines the grade of the ceramics. Three of the mixing processes are spray dried powder, aqueous slip, and ceramic dough feedstock.
Spray drying produces a granulated powder for uniaxial and isostatic pressing. The raw powder is milled in a solvent, such as water. A binder is added before the spraying process, which gives the material strength for pressing.
The aqueous process produces a slurry for casting.
In the ceramic dough feedstock process, water, alumina, a binder, and plasticizer are mixed to produce a clay like material.
Consolidation is the forming of the ceramic part for handling and forming. Four of the many methods for consolidation are extrusion, uniaxial and isostatic pressing, electrophoretic deposition, and slip casting.
In the injection molding process, fine powder is mixed with a binder to form the feedstock, which is heated to form a viscous slurry and injected molded to form the green part. The binder is removed to form the brown part, which is densified through sintering.
Extrusion uses dough feedstock that is deformed under pressure. The binders in the mix help the form keep its shape as it dries.
Isostatic can be wet or dry bag pressing. Sprayed powder is placed in a bag of rubber or polyurethane and placed under isostatic pressure. Wet bag pressing produces simple shapes, while dry bag pressing can produce very complex shapes.
Slip casting, or drain casting, uses a fluid aqueous slip that is poured into a mold made of plastic. Water is drained from the mixture leaving a consolidated powder mix on the sides of the mold. As the thickness of the cast increases, the remaining slip is drained. With pressure casting, pressure is applied to the slip creating thicker walled and more solid parts. A simplified version of slip casting can be seen in the diagram below.
Uniaxial pressing is a compacting of alumina powder by applying pressure in a single axial direction using a piston, punch, or plunger. Presses for uniaxial pressing can be hydraulic or mechanical that have a top punch, bottom punch, and die.
EPD is used for several industrial coating and painting processes. For ceramic production, an electrostatic charge consolidates ceramic particles from suspension and deposits them on the surface of a mold. Once the necessary thickness is achieved, the mold is removed from the slip container or the slip is poured off.
During sintering, the consolidated green ceramic part is fired to give it density. At the high temperatures of sintering, particle rearrangement, grain growth, and pore elimination take place. For alumina ceramics, this is known as solid state sintering.
Diamond grinding is done after sintering to match tight tolerances, improve the surface finish, and remove any flaws. Diamond processes include grinding, cutting, honing, lapping, and polishing. The diamond tooling is necessary due to the hardness of alumina ceramics.
Alumina is a major engineering material that has excellent mechanical and electrical properties, which makes it applicable for a wide range of uses. A few of its uses are listed below.
Alumina‘s dielectric and thermal shock properties make it an excellent insulating material for electrical components.
Alumina ceramic labware is used for laboratory applications with high-temperature conditions that need to be contamination free. The chemical and corrosion resistance, high hardness, strength, durability, and wear resistance of alumina ceramics makes it a perfect solution for lab applications.
Alumina ceramics are used in the electronics industry for interconnectors, resistors, and capacitors. It is an economical and durable material for substrates for hybrid integrated circuits, surface mount devices, and sensors.
Alumina ceramic cutting tools have strength and thermal conductivity. Though alumina cutting tools were very expensive, at one time, the material has been engineered with the use of composites to be cost effective and are manufactured by sintering and die pressing.
The high shock resistance of alumina ceramics makes it ideal for body armor for protection of tanks, helicopters, and bullet proof jackets.
Since alumina ceramics are inert, they are insoluble in chemical reagents, have wear resistance, and can have a high polished finish, which makes alumina ceramics useful as biomaterial. Alumina ceramics are used for artificial joints, bone spacers, cochlear implants, and teeth implants. Tubes and scientific products are also made from alumina ceramics.
There are a wide variety of ways to use alumina ceramic sheets due to their ability to withstand extreme temperatures. The 99% alumina content makes the sheets suitable for use in vacuum applications. Their machinability allows them to be shaped and placed in harsh environments, including ones with high humidity, since the sheets are unaffected by water.
Since alumina ceramic sheets can withstand temperatures up to 2600° F or 1426° C, the sheets are often used to make gaskets for furnaces and boilers and can be easily shaped using any form of cutting tool. The consistency and strength of alumina ceramic sheets varies depending on the percentage of alumina content. Thicknesses range from 0.01 in or 0.0254 cm up to 0.5 in or 1.27 cm in and are available in a wide variety of lengths and widths. These lengths and widths are limited by the size of the ovens available to finish them.
The grades and types of alumina ceramics is determined by the percentage of alumina the material contains, which can vary from 70% to 99.9%. The percentage of alumina fluctuates according to the amount of other elements that are added to the mixture.
92% Alumina features electrical properties, mechanical strength, wear resistance, chemical and corrosion resistance, thermal stability, and is dense non-porous. It is used for electrical packaging, bushings, grinding media, wear resistant components, and industrial applications. The tube lining below is made from 92% alumina ceramics.
94% Alumina has low thermal expansion, high volume resistivity, abrasion and chemical resistance, dielectric constant, and accepts manganese metal coating for high temperature brazing. It can be used as a pressure sensor, bearing coatings, electron tube, and laser components.
95% Alumina has similar properties to 94% alumina with added qualities of compressive and flexural strength and excellent hermeticity. It is good for ceramic to metal feed throughs, X-ray component feed throughs, high voltage bushings, and medical implants. This percentage of alumina can be molded into body armor for military applications.
96% Alumina is used for medical applications. It has a combination of mechanical, electrical, thermal, and chemical properties. The combination of these properties makes them suitable for wear nozzles, wear guides, blood valves, electrical connector housings, and other industrial applications.
97% Alumina can be metal coated for high temperature brazing assemblies and serve as an electrical insulator. It can be used for high vacuum systems, laser equipment, X-ray tubes, electron microscopes, microwave windows, and insulation for medical equipment.
97.5% Alumina is a high quality, fine grain alumina used for electrical and mechanical components that need a thick metal coating and where abrasive and chemical resistance is required. It can be used with metal coated components like conductor and resistor networks, and dielectric layers. This percentage of alumina can be used in the defense industry, medicine, and scientific research to name a few.
97.6% Alumina is an essential part of components that require operational stability and reliability. It is electrically and dimensionally stable in a wide range of temperatures. It is used for laser components, electro optical devices, flow measurement, sensors, and X ray equipment.
98.6% Alumina is a light weight alumina for use as armor for military vehicles and structures. It can stop small arms fire and medium caliber cannon.
99% Alumina is extremely hard and is used as a component on rotary and reciprocating pumps that handle chemicals. Shafts, bearings, thrust washers, plungers, and counter face seals on pumps that handle chemicals use 99% alumina material.
99.5% Alumina is a very pure form of alumina that is used for semiconductors, semiconductor chambers and fixtures. It has low particle generation and is vacuum tight.
99.7% Alumina has a 98% reflectance efficiency at 1064 nanometers (nm) and a 96% reflectance at wavelengths of 500 to 2000 nm making it perfect for laser reflectors. It can be used for pumping chambers for flash lamps and continuous wave lasers.
99.8% Alumina is very high purity and has been developed for semiconductors. It has excellent chemical and plasma resistance with dielectric properties for high voltage applications. It can be used for semiconductor applications that include PVD, CVD, and CMP oxide etching, ion implants, and photolithography to name a few. This percentage of alumina has wide use in the semiconductor industry.
99.9% Alumina is the ultra-pure alumina, which can be applied in severe applications, such as plasma etching components and nuclear grade insulator components. The properties of 99.9% alumina can be seen in the chart below from CoorsTek.
| Property | Units | Value |
| Flexural Strength, MOR (20 °C) | MPa | 350 - 600 |
| Fracture Toughness, KIc | MPa m1/2 | 4.0 - 5.0 |
| Thermal Conductivity (20 °C) | W/m K | 28 - 35 |
| Coefficient of Thermal Expansion | 1 x 10-6/°C | 8.0 - 8.5 |
| Maximum Use Temperature | °C | 1750 - 1800 |
| Dielectric Strength (6.35mm) | ac-kV/mm | 8.7 |
| Dielectric Loss (tan δ) | 1MHz, 25 °C | 10-4 to 10-3 |
| Volume Resistivity (25°C) | Ω-cm | 1014 to > 1015 |
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