semangat

Muhamad Amirul Ariffin

Dr. Dmitri Kopeliovich

Alumina (aluminum oxide) is the most important, widely used and cost effective oxide ceramic material.

The technical alumina ceramics contain at least 80% of aluminum oxide (AL₂O₃).

Small amounts of silica (SiO₂), magnesia (MgO) and zirconia (ZrO₂) may be added to alumina ceramics.

Addition of zirconia to alumina ceramic results in considerable increase of the material fracture toughness.

Alumina possesses strong ionic bonding, which determines the material properties:

High mechanical strength (flexural strength) and hardness;

High wear resistance;

High resistance to chemical attacks of strong acids and alkali even at high temperatures;

High stiffness;

Excellent insulating properties;

Low coefficient of thermal expansion;

Good fracture toughness;

Good thermal conductivity;

Good biocompatibility.

Aluminum ceramics parts are manufactured by the following technologies: uniaxial (die) pressing, isostatic pressing, injection molding, extrusion and slip casting. The parts may be machined in "green" condition before sintering (firing).

Aluminum ceramics are widely used in electronics and electrical engineering, metallurgical processes, chemical technologies, medical technologies, mechanical engineering, military equipment.

Aluminum ceramics are used for manufacturing insulators, capacitors, resistors, furnace tubes, sealing refractory parts, foundry shapes, wear pads, thermocouple protection tubes, cutting tools and polishing/grinding powders, ballistic armor, laboratory equipment, bio-ceramic parts for orthopedic and dental surgery, bearings.

Properties of some alumina ceramics

(Materials Data)

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Magnesia ceramics

Dr. Dmitri Kopeliovich

Magnesia Ceramic is a ceramic material consisting of at least 90% of Magnesium Oxide (MgO).

Magnesium Oxide is produced from natural minerals such as magnesite (magnesium carbonate), magnesium chloride rich brine, and seawater.

Magnesia is characterized by high melting temperature 5070 ºF (2800 ºC) and high thermal and chemical stability, therefore it is widely used in refractory applications.

Magnesia ceramics are stable up to 4170 ºF (2300 ºC) in oxidizing atmosphere and up to 3090 ºF (1700 ºC) in reducing atmosphere.

Magnesia ceramic materials are produced in tight high density form (porosity less than 1%) and in porous form (porosity up to 30%).

Magnesia may be doped by small amount of Yttrium Oxide (Y₂O₃) as sintering aid.

Addition of carbon to magnesia ceramic allows to increase its chemical resistance to Steel making basic slags.

Magnesia ceramics possess the following properties:

High thermal stability;

High resistance to molten metals (iron, steel, aluminum), slags and semiconductor compounds;

Good corrosion resistance even at high temperatures;

Good insulating properties;

Good thermal conductivity;

Infrared transparency.

The main disadvantage of fine grain dense magnesia ceramic is low thermal shock resistance.

Magnesia ceramics are used for manufacturing high temperature crucibles, thermocouple tubes, heating elements, foam ceramic filters for molten metal, linsulators, steel making refractories, kiln furniture.

Properties of some magnesia ceramics

(Materials Data)

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Zirconia ceramics

Dr. Dmitri Kopeliovich

Zirconia Ceramic is a ceramic material consisting of at least 90% of Zirconium Dioxide (ZrO₂).

Zirconium Oxide is produced from natural minerals such as Baddeleyite (zirconium oxide) or zirconium silicate sand.

Pure zirconia changes its crystal structure depending on the temperature:

At temperatures below 2138 ºF (1170 ºC) zirconia exists in monoclinic form.

At the temperature 2138 ºF (1170 ºC) monoclinic structure transforms to tetragonal form which is stable up to 4300 ºF (2370 ºC).

Tetragonal crystal structure transforms to cubic structure at 4300 ºF (2370 ºC).

Structure transformations are accompanied by volume changes which may cause cracking if cooling/heating is rapid and non-uniform.

Additions of some oxides (MgO, CaO,Y₂O₃) to pure zirconia depress allotropic transformations (crystal structure changes) and allow to stabilize either cubic or tetragonal structure of the material at any temperature.

The most popular stabilizing addition to zirconia is yttria (Y₂O₃), which is added and uniformly distributed in proportion of 5.15%.

Depending on sintering temperature and other processing parameters, the following forms of stabilized zirconia may be prepared:

Fully stabilized zirconia (FSZ) with cubic crystal structure;
Partially stabilized zirconia (PSZ) with mixed structure (cubic+tetragonal);
Polycrystalline tetragonal zirconia (TZP) with metastable tetragonal structure of very fine zirconia grains sintered at low temperature.

The following characteristics are typical for Zirconia Ceramics:

High density – up to 380 lb/ft³ (6.1*10³ kg/m³);

Low thermal conductivity – 10% of that of alumina ceramics;

High fracture toughness;

Very high flexural strength and hardness;

High maximum service temperature – up to 4350 ºF (2400 ºC).

coefficient of thermal expansion similar to that of cast iron;

Modulus of elasticity similar to steel;

High chemical resistance;

High resistance to molten metals;

Good wear resistance;

Low coefficient of friction;

Oxygen ion conductivity (used for oxygen sensors and high temperature fuel cells).

Zirconia ceramics are used for manufacturing Extrusion dies, powder compacting dies, cutting tools, balls and seats for ball valves, thread and wire guides, pump seals, impellers and shaft guides, engine parts, oxygen sensors, fuel cells membranes,high temperature heaters for electric furnaces, bearings (e.g., bearings for submersible pumps).

Properties of some zirconia ceramics

(Materials Data)

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Aluminum titanate ceramics

Aluminum titanate is a ceramic material consisting of a mixture of alumina (Al₂O₃) and titania (TiO₂) forming solid solution with stoichiometric proportion of the components: Al₂O₃*TiO₂ or Al₂TiO₅.

Aluminium titanate is prepared by heating of a mixture of alumina and titania at temperature above 2460°F (1350°C).

The powder is then sintered at a temperature in the range 2550 - 2910°F (1400 - 1600°C) in air atmosphere.

Pure Aluminum Titanate is unstable at the temperatures above 1380°F (750°C) when the solid solution decomposes into two separate phases Al₂O₃ and TiO₂. Aluminum Titanate ceramics are doped with MgO, SiO₂ and ZrO₂in order to stabilize the solid solution structure.

The distinctive property of Aluminum Titanate ceramics is their high thermal shock resistance which is a result of very low coefficient of thermal expansion.

The following characteristics are typical for Aluminum Titanate Ceramics:

Low coefficient of thermal expansion;

Low Modulus of Elasticity;

High Thermal Shock Resistance;

Low Thermal Conductivity;

Low wettability in molten non-ferrous metals;

Good chemical resistance;

Good wear resistance.

Disadvantage of Aluminum Titanate ceramics is relatively low mechanical strength caused by micro-cracks formed as a result of anisotropy of thermal expansion along the three primary axes of the crystal lattice (a single crystal of Aluminum Titanate expands along two axes and contract along the third axis when heated).

Aluminum Titanate ceramic materials ceramics are used for manufacturing crucibles, launders, nozzles, riser tubes, pouring spouts and thermocouples for non-ferrous metallurgy, portliner and cylinder linerrs in automotive engines, master moulds in the glass industry, spacing rings of catalytic converters.