Here is the most concise information on alumina ceramics on the internet.
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Alumina ceramic is a highly durable industrial oxide ceramic known for its remarkable hardness. Produced from bauxite, it can be shaped through various methods including injection molding, die pressing, isostatic pressing, slip casting, diamond machining, or extrusion. Its sintered crystalline structure is similar to that of sapphire and ruby.
Alumina ceramic is primarily used as an insulation material for electrical components. Additionally, it is valued for its outstanding strength, resistance to corrosion, and wear properties.
Products crafted from alumina ceramic—some of which are depicted in the image below—are resistant to wear, chemicals, erosion, corrosion, and high temperatures. They are also bioinert, making them ideal for use in medical implants.
Alumina ceramics are considered technical ceramics due to their excellent properties and favorable price-to-performance ratio. They are classified based on their alumina content, which ranges from 70% to 99.9%. Higher purity alumina provides superior wear and corrosion resistance.
Alumina ceramics are composed of a white granular material resembling table salt or a fine, silky, dense white powder. There are three primary types of alumina: hydrated, calcined, and tabular. Each type is available in various grades.
The different types of alumina vary based on their content of soda (Na2O), iron (Fe2O3), and silica (SiO2), as well as their chemical purity and the specific properties of the powder used in their production.
Calcined alumina is produced by heating aluminum oxide to 1050°C (1900°F). This process removes all impurities and water, resulting in a very pure form of alumina with a purity of 99.99%. It achieves a hardness rating of 9 on the Mohs scale, just below the 10 rating of diamonds.
Hydrated alumina, also known as alumina hydroxide, is valued for its use in glazes due to its ability to remain suspended in glaze slurries and its adhesive properties.
Tabular alumina is created by heating aluminum oxide to 1650°C (3000°F). It boasts high thermal capacity, exceptional temperature stability, and excellent strength. This type of alumina is produced by sintering calcined alumina balls, which are then crushed into a powder. Tabular alumina is known for its high refractoriness, mechanical strength, wear resistance, chemical purity, dielectric properties, and resistance to corrosion in both acids and alkalis.
Alumina maintains its properties in both oxidizing and reducing atmospheres up to 1650°C (2900°F) and in vacuum environments up to 2000°C (3600°F). At 1000°C, it retains 50% of its tensile strength compared to room temperature. Unlike metals, which weaken at high temperatures, alumina ceramics preserve their strength and remain unchanged when temperatures return to normal.
Abrasion occurs when a material is worn down through friction and rubbing. The ability of a material to resist abrasion means it maintains its original structure even after enduring mechanical wear. Alumina ceramics exhibit high abrasion resistance due to their inherent hardness.
Alumina demonstrates excellent resistance to both acids and alkalis at high temperatures because it is chemically inert. This lack of chemical reactivity makes it resistant to the effects of various chemicals, including solvents and salt solutions.
Density refers to a material's mass divided by its volume, typically expressed in grams per cubic centimeter (g/cm³). In this context, mass is measured in grams and volume in cubic centimeters. The density of a material is inversely related to its volume.
Alumina ceramics are produced from fine particles that minimize the presence of voids in the material. Fewer voids contribute to higher density and volume. For instance, at 77°F (25°C), alumina ceramics have a density of 3.965 g/cm³ at standard atmospheric pressure.
To achieve an optimal wear surface, manufacturers grind the alumina powder to a sub-micron (nanometer) level, resulting in a grain size of less than five microns after firing. Ceramic with 95% alumina content will have a grain size of 30 to 40 microns after firing, leading to larger voids and a higher wear rate. Conversely, ceramic with 87% alumina content will exhibit a grain size of three to five microns after firing, resulting in fewer voids and improved wear resistance.
The mechanical characteristics of a material are defined by its ability to withstand stress and strain. Alumina is noted for its exceptional strength and hardness, which increase with the purity of the various grades.
Thermal conductivity refers to a material's capacity to transfer heat. For instance, cooking pans need high thermal conductivity to cook food effectively. When a material is heated, its atoms vibrate more vigorously, causing their bonds to stretch. This phenomenon is described by the coefficient of thermal expansion. Due to their robust atomic bonds, alumina ceramics exhibit a low coefficient of thermal expansion, enhancing their stability under high temperatures.
Alumina ceramics possess high resistivity, which minimizes thermal shock. Furthermore, higher purity forms of alumina have even greater resistivity.
Alumina ceramics have exceptional dielectric properties compared to metals or plastic. Dielectric loss is a measurement of the amount of lost energy when a ceramic is subjected to electromagnetic voltage. With high conductivity, electrons move freely when exposed to electromagnetic voltage. The purpose of an insulator is to inhibit the flow of electrons. Alumina ceramics are able to withstand extreme exposure to electrical charges without any dielectric loss. This makes it a perfect insulation material because of its dielectric equality. Electric current is unable to pass through it.
Hardness evaluates a material's capacity to resist mechanical wear and abrasion. Alumina ceramics surpass steel and tungsten carbide tools in hardness. On the Rockwell hardness scale, alumina ceramics range from HRA80-90, placing them just below diamonds and above stainless steel.
The raw material for alumina ceramics is derived from bauxite, an aluminum-rich, clay-like substance found just a few meters below the earth's surface. The process for producing alumina, known as the Bayer process, was developed in 1887.
The extracted bauxite is transported to a processing facility, where it is ground to obtain aluminum oxide. The resulting aluminum oxide appears as a white powder.
The ground bauxite is washed, dried, and mixed with caustic soda and lime to create a slurry. This slurry is then heated in a digester to 300°F (145°C) and subjected to 50 pounds of pressure for several hours to dissolve the aluminum compounds.
The slurry is transferred through a series of flash tanks to lower the pressure and temperature. These flash tanks cool the slurry at atmospheric pressure and release steam.
Impurities in the slurry, such as sand, iron, and other insoluble elements, settle at the bottom of the tank. The liquid at the top is filtered and washed multiple times to recover the alumina before proceeding to the next stage.
The filtered clear sodium aluminate is transferred into a series of precipitators, where alumina particles are introduced to initiate the process. The alumina forms around these seeds, causing them to settle at the bottom of the tank. The settled alumina is then removed and sent to thickening tanks for further filtration.
The final step in extracting alumina involves heating to remove water from the alumina hydrate. It undergoes additional filtration and washing to eliminate impurities and moisture. A conveyor, as shown in the diagram, transports the hydrate to calcination, a gas-fired, inclined kiln that rotates to ensure uniform heating.
Modern processing techniques employ fluid bed calcining, where alumina particles are suspended above a hot air screen and heated to the desired temperature.
The refined powder is blended with other materials before undergoing the forming process. The combination of alumina with these additional substances determines the final grade of the ceramics. Common mixing methods include spray dried powder, aqueous slip, and ceramic dough feedstock.
Spray drying creates a granulated powder suitable for uniaxial and isostatic pressing. The powder is mixed with a solvent, like water, and a binder is added before spraying to provide the necessary strength for pressing.
In aqueous slip processing, the raw materials are milled in water with an organic dispersant using a ball mill, resulting in a concentrated slurry. This slip is then subjected to high pressure to produce green density components.
The ceramic dough feedstock process involves blending water, alumina, a binder, and a plasticizer to create a clay-like substance. High-quality feedstock is essential to achieve the desired green density for consolidation, reducing shrinkage during sintering.
Consolidation involves shaping the ceramic part for handling and further processing. Various consolidation methods include extrusion, uniaxial and isostatic pressing, electrophoretic deposition, and slip casting.
Injection molding involves mixing fine powder with a binder to create a feedstock, which is then heated to form a viscous slurry. This slurry is injected into molds to create the green part. After molding, the binder is removed, and the green part is densified through sintering to form the final product.
Extrusion involves forcing dough feedstock through a die under pressure, shaping it as it emerges. The binders in the mixture help maintain the shape of the form as it dries.
Isostatic pressing can be either wet or dry bag pressing. In this process, sprayed powder is enclosed in a rubber or polyurethane bag and subjected to isostatic pressure. Wet bag pressing is typically used for creating simple shapes, whereas dry bag pressing can be used to produce more complex shapes.
Slip casting, also known as drain casting, involves pouring a liquid aqueous slip into a plastic mold. Water is then drained from the mixture, leaving a consolidated layer of powder on the mold’s interior surfaces. As the cast’s thickness increases, excess slip is removed. Pressure casting applies additional pressure to the slip, resulting in thicker and more solid parts. A simplified version of the slip casting process is illustrated in the diagram below.
Uniaxial pressing involves compacting alumina powder by applying pressure in one direction using a piston, punch, or plunger. This process can be performed with hydraulic or mechanical presses that feature a top punch, bottom punch, and die.
Electrophoretic deposition (EPD) is used in various industrial coating and painting processes. In ceramic production, an electrostatic charge causes ceramic particles suspended in a liquid to deposit onto a mold's surface. After achieving the desired thickness, the mold is either removed from the slip container or the slip is poured off.
During sintering, the green ceramic part is heated to increase its density. At these high temperatures, processes such as particle rearrangement, grain growth, and pore elimination occur. For alumina ceramics, this process is referred to as solid-state sintering. Sintering often results in shrinkage of up to 20% to 25%. The uniformity of this shrinkage and the quality of the final product largely depend on the density of the green material.
Alumina can be machined in either its green or fully dense state. In its green form, alumina can be easily shaped into intricate designs. However, it is important to note that it will shrink by about 20% during sintering, which alters the dimensions of the piece. As a result, alumina machined in its green state will not maintain precise tolerances through sintering. To ensure dimensional accuracy, alumina should be machined after the sintering process using a diamond-coated wheel.
Diamond grinding is performed after sintering to achieve tight tolerances, enhance surface finish, and eliminate any imperfections. The diamond processes used include grinding, cutting, honing, lapping, and polishing. Due to the hardness of alumina ceramics, diamond tooling is essential for these finishing tasks.
The aim of alumina ceramics is to deliver components that exhibit high-temperature resistance, exceptional hardness, electrical insulation, and chemical inertness for various applications. Fortunately, several manufacturers have perfected their equipment for producing these ceramics. Below are five notable brands of machines used in the United States and Canada to produce alumina ceramics, including specific models and their unique features:
Features: CeraNova Corporation's High-Speed Ceramics Milling Machine is engineered for precise milling of alumina ceramics. It employs cutting-edge technology to achieve high material removal rates and superior surface finish. The machine includes options for high-speed spindles, multi-axis control, and specialized tooling for efficient ceramic machining.
Features: CoorsTek provides a variety of ceramic grinding media, including the alumina-based CeraSurf™. These media are suited for applications like ball and attritor milling. CeraSurf™ offers exceptional wear resistance, chemical inertness, and consistent particle size distribution, enhancing grinding efficiency and product quality.
Features: Sacmi USA Group's PH Series Presses are hydraulic machines tailored for alumina ceramic production. They offer precise control over pressure, speed, and stroke, enabling the creation of complex shapes with high density. The presses come with an intuitive interface, advanced automation features, and compatibility with downstream processes.
Features: Harper International's pusher furnaces are designed for the sintering of alumina ceramics. These furnaces provide controlled high-temperature atmospheres essential for sintering. They feature precise temperature control, uniform heating profiles, and adjustable residence times, supporting high throughput and various ceramic shapes and sizes.
Features: LECO Corporation's AMH43 Moisture Determination Analyzer is used for measuring moisture in alumina ceramics and other materials. It features a precision balance, high-temperature drying, and advanced software algorithms for accurate moisture content analysis. The analyzer offers rapid testing, ease of use, and customizable parameters.
Keep in mind that the availability and specifications of these models may change, so it is advisable to consult with the manufacturers for the most current information.
Alumina is a key engineering material known for its outstanding mechanical and electrical properties, making it suitable for a diverse array of applications. Some of its common uses are outlined below.
Alumina ceramics are widely used as insulation materials due to their excellent dielectric and thermal shock resistance. In high-frequency applications, extremely pure and dense alumina, fired at temperatures up to 1600°C, is utilized. This makes alumina ceramics essential for insulating components in television and satellite transmitter tubes, microwave generators, and lasers, where strength and durability are crucial.
Alumina ceramic labware is ideal for laboratory environments where high temperatures and contamination-free conditions are required. Its chemical and corrosion resistance, combined with high hardness, strength, durability, and wear resistance, makes alumina ceramics an excellent choice for various lab applications.
In the electronics industry, alumina ceramics are utilized for interconnectors, resistors, and capacitors. They serve as an economical and durable material for substrates in hybrid integrated circuits, surface mount devices, and sensors.
Alumina ceramic cutting tools are known for their strength and thermal conductivity. Previously costly, alumina cutting tools are now made more affordable through the use of composites and are produced by sintering and die pressing. These composites are commonly used as alternatives for abrasive and cutting tools, as well as in extrusions, nozzles, friction components in piston engines, and machinery where wear resistance is critical. Additionally, alumina ceramics are uniquely employed in the blades of Olympic ice skates.
Alumina ceramic is utilized in ballistics for its exceptional shock resistance, making it suitable for body armor in tanks, helicopters, and bulletproof vests. Alumina ceramics serve as the first line of defense against projectiles, designed to absorb the initial impact. After the alumina ceramic has absorbed this impact, a subsequent layer of material is used to dissipate the remaining kinetic energy, rendering the projectile inert.
Alumina ceramics are chemically inert, resistant to wear, and capable of achieving a highly polished finish, making them valuable as biomaterials. As a result, alumina ceramics are used in various medical applications, including artificial joints, bone spacers, cochlear implants, and dental implants. They are also employed in manufacturing tubes and scientific equipment. These advantageous properties make alumina ceramics an excellent choice for replacing body parts.
Alumina ceramic sheets offer versatile applications due to their resistance to extreme temperatures. Sheets with 99% alumina content are ideal for use in vacuum environments. Additionally, their machinability enables them to be shaped for use in harsh conditions, including high humidity environments
Since alumina ceramic sheets can withstand temperatures up to 2600 °F (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 (0.0254 cm) up to 0.5 in (1.27 cm) and are available in various 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 are defined by their alumina content, which ranges from 70% to 99.9%. The specific percentage of alumina depends on the proportions of additional elements included in the mixture.
Alumina ceramics are characterized by their electrical properties, mechanical strength, wear resistance, chemical and corrosion resistance, and thermal stability, and they are dense and non-porous. These features make them suitable for applications such as electrical packaging, bushings, grinding media, wear-resistant components, and various industrial uses. For example, the tube lining shown below is composed of 92% alumina ceramics.
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.
This grade offers similar characteristics to 94% alumina, with enhanced compressive and flexural strength, and excellent hermeticity. It is ideal for applications such as ceramic-to-metal feedthroughs, X-ray component feedthroughs, high-voltage bushings, and medical implants. Additionally, 95% alumina can be molded into body armor for military use.
Primarily used in medical applications, 96% alumina combines mechanical, electrical, thermal, and chemical properties. These attributes make it suitable for wear-resistant nozzles, guides, blood valves, electrical connector housings, and other industrial uses.
This grade can be metal-coated for high-temperature brazing assemblies and serves effectively as an electrical insulator. It is suitable for use in high-vacuum systems, laser equipment, X-ray tubes, electron microscopes, microwave windows, and insulation for medical devices.
97.5% alumina is a high-quality, fine-grain material used for electrical and mechanical components requiring thick metal coatings and high resistance to abrasion and chemicals. It is employed in metal-coated components such as conductor and resistor networks and dielectric layers, and is utilized in the defense industry, medical field, and scientific research.
This grade provides operational stability and reliability, maintaining electrical and dimensional stability across a wide temperature range. It is used in laser components, electro-optical devices, flow measurement instruments, sensors, and X-ray equipment.
98.6% alumina is a lightweight material used as armor for military vehicles and structures. It is capable of stopping small arms fire and medium-caliber projectiles.
99% alumina is extremely hard and is utilized in components for rotary and reciprocating pumps handling chemicals. It is used in shafts, bearings, thrust washers, plungers, and counterface seals in chemical pumps.
99.5% alumina is a highly pure form of alumina used in semiconductor applications, including chambers and fixtures. It features low particle generation and is vacuum-tight.
This grade has a 98% reflectance efficiency at 1064 nanometers (nm) and a 96% reflectance at wavelengths from 500 to 2000 nm, making it ideal for laser reflectors. It is suitable for use in pumping chambers for flash lamps and continuous wave lasers.
99.8% alumina is extremely pure and developed for semiconductor applications. It offers excellent chemical and plasma resistance with high dielectric properties for high-voltage applications. It is used in various semiconductor processes, including PVD, CVD, CMP oxide etching, ion implantation, and photolithography, among others.
99.9% alumina is an ultra-pure form suitable for severe applications such as plasma etching components and nuclear-grade insulators. The properties of 99.9% alumina are detailed 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 |
Zirconia toughened alumina is produced by combining aluminum oxide with 10% to 20% zirconium oxide. The resulting material has exceptional strength, toughness, hardness, and outstanding wear resistance. It provides a design solution that is less expensive than using zirconia alone, which can cost anywhere from $75 to $400 per carat. The cost of zirconia varies in accordance with its purity and quality.
The process of combining alumina oxide with zirconia oxide enhances the fracture toughness of Zirconia Toughened Alumina (ZTA). When zirconia is subjected to stress, its crystal structure transforms, leading to a volume expansion that compresses any cracks within the alumina matrix. This transformation significantly increases the structural strength of alumina, making it suitable for a wide range of demanding applications.
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