Technical Articles

Materials in design
21/10/05

Mike Thomas, Chief Technology Officer, Morgan Technical Ceramics, gives designers and manufacturers an insight into advanced ceramic materials and their properties 

Today’s advanced ceramics offer powerful physical, thermal and electrical properties that make them highly resistant to melting, bending, stretching, corrosion, wear, high voltages and currents. This has opened up development opportunities for manufacturers in a wide range of industries such as aerospace, defence, automotive, medical, electronics, telecommunications, scientific equipment and semiconductor processing.

Advanced ceramics such as Alumina, Zirconia, Silicon Carbide, Silicon Nitride and Titania based materials, each with their own specific characteristics, provide a cost-effective, high performance alternative to traditional materials such as metals, plastics and glass. The demand posed by new and changing applications is to improve operation at a reduced cost. New materials are constantly being developed to address this need for individual applications.

There are two factors that designers should consider when choosing a material – the desired properties for the application and the available materials. By fully understanding the materials, designers can then make well-informed choices.

Desired properties for the application

Dielectric properties
Ceramic components are increasingly being used by electrical systems designers. The properties of the component material which significantly affect the performance of the system are: dielectric constant (Er); electrical loss; and temperature stability. Dielectric ceramic materials are formulated to optimise these three parameters for a range of DC to microwave frequencies and low to high power applications. These include AM/FM commercial radio transmitters, frequency filters used in wireless cellular/radio systems, antenna products, dielectric resonator oscillators and microwave radar systems.

The dielectric constant is an inherent property of the material. The higher the dielectric constant of the material, the smaller the size of the component to achieve the same frequency or capacitance. This means a smaller, lighter, cost effective product.

Better system performance, in terms of improved frequency selectivity or improved power handling capability, can be achieved through very low electrical loss or through a high ratio of ‘energy stored’ to ‘energy dissipated’ per cycle (Q). Electrical loss is generally proportional to the dielectric constant. In other words, a higher Er means a greater electrical loss. It is therefore necessary to strike a balance between all aspects of the material’s performance.

Performance is also improved in a system that has zero frequency drift with temperature. This means that the temperature of the surroundings does not affect the signal and the final product can operate just as effectively from -30°C to 50°C. By making subtle compositional modifications, a high permittivity can be adjusted accurately to how it responds to variations in temperature fluctuations. However, not all ceramic materials can be modified in this manner to provide the ideal combination of desired Er and loss tangent coupled with a zero or near zero temperature coefficient.

An example of material designed in response to demand from the telecommunications market for smaller sized, high performance antennae is Morgan Technical Ceramics’ D43. It is an attractive, low-cost alternative to metallic resonant cavities and is manufactured to provide smaller sized filter systems that retain a high performance. It has a dielectric constant of 43 and is based on a barium/zinc/cobalt/niobate composition. The material offers a Q of 22,000 at 2GHz and good temperature stability to make it an excellent dielectric material for microwave and RF applications. 

In response to an increasing need for higher performance base station cavity filters Morgan Technical Ceramics has developed the ceramic D34. This cost-effective solution offers a dielectric constant close to 34, and depending on the application, a Q of greater than 40,000 (at 2GHz). This allows the cavity filter size to be reduced while still achieving a very good filter profile. The material’s frequency drift is inherently stable with temperature offering a near linear response; it can be supplied to compensate for frequency drift within the system by specifying an alternative option for temperature coefficient. 

Physical properties
Physical properties such as strength, hardness, wear resistance, corrosion resistance and thermal stability are considered when choosing a material. Each of these characteristics is determined by specific materials; for a break down of materials and their characteristics see fig 1. Designers look for those materials which give the best combination of characteristics for an excellent performance. 

Materials engineered for their physical properties include laser reflectors. Demands for higher overall efficiency of the laser system and efficiency in the transfer of radiation from the source to the laser rod mean a high reflectance material is needed to form the pumping chamber cavity surrounding the laser rod and lamp. Traditionally the reflectors have been metal coated with materials such as gold. Now, a special Alumina material, Sintox, has been developed as a highly cost effective alternative to metal reflectors providing a higher level of reflectance in some cases. In a similar manner, this material has also been used extensively for reflectors in housings for high intensity lamps.

The Alumina material is chosen for its particle size which gives the material excellent microstructural control of defined porosity and provides a good scatter of the laser light. The Alumina can be glazed to further increase reflectivity and to seal the porosity, making the ceramic laser cavity impervious to the cooling fluid. It has a high strength to cope with stresses experienced in regular servicing of the laser and is chemically resistant to the cooling solution. It has a good thermal conductivity and excellent dimensional and electrical stability at all operating temperatures. 

A demand for longer lasting bearings in pumps prompted Morgan Technical Ceramics to research the possibilities of manufacturing bearings from Alumina materials. Hilox is a fully dense 96% Alumina - its exceptionally high hardness offers a higher resistance to wear from abrasive particles such as black iron oxide present in heating plants containing mild steel parts, than carbon-steel. The reduction in wear leads to lower noise levels and in many cases longer life and lower overall system maintenance requirements.

Materials

Having identified the material properties needed for individual applications, designers should then consider the various ceramic materials available to them. Alumina is one of many ceramic materials optimized for mechanical, electrical and chemical properties, but there are other materials that designers may consider for their specific characteristics:

Alumina 
Alumina is a versatile material that offers a combination of good mechanical and electrical properties. It is suitable for a wide range of applications including seal rings, laser tubes, ballistic armour, electrical insulators, threadguides, medical prostheses, electronic substrates, thermocouple tubes, grinding media and wear components. It has a good strength and stiffness, good hardness and resistance to wear. Alumina is available in many grades ranging from 60% to >99.9% with additives designed to enhance properties such as strength. It can be formed using a variety of ceramic processing methods and can be processed net-shaped or machined to produce a variety of sizes and shapes. In addition it can be readily joined to metals or other ceramics using specially developed metallising and brazing techniques.

Zirconia
Zirconia offers chemical and corrosion resistance at high temperatures up to 2400°C – well above the melting point of Alumina. In its pure form, crystal structure changes limit use in mechanical/temperature applications, but stabilised Zirconias with Calcium, Magnesium or Ytrium Oxide additives can produce materials with very high strength, hardness and in particular, toughness. The material has low thermal conductivity (20% that of Alumina) and is an ionic conductor above 600°C which benefits applications such as fuel cells where ionic movement within a solid material is required. This has lead to applications in oxygen sensors and high temperature fuel cells. Typical applications include: precision ball valves (balls and seats), high density grinding media, threadguides, cutting blades, medical prostheses, pump seals, valves and impellors, radio frequency heating susceptors and metrology components.

Titania
While Titania is used in large quantities in powder form, bulk sintered material has limited mechanical applications because of its relatively low fracture toughness compared to Alumina. In pure form the material is electrically insulative and its main application is used in thread guides in high speed processing of synthetic fibres. This is because it is relatively soft and can be more easily processed than Alumina to give a kinder surface to sensitive chemical fibres. For threadguide applications where static electricity is a problem the material can be produced in a reduced form that is able to dissipate electrical charge. Titania is moderate in strength, has a high thermal conductivity, has low friction, is corrosion resistant, and has high dielectric properties in pure form.

Silicon Nitride
Silicon Nitride has good high temperature strength, creep resistance and oxidation resistance. In addition, its low thermal expansion coefficient gives good thermal shock resistance compared to most ceramic materials. It has a high fracture toughness, high hardness, chemical and wear resistance. Silicon Nitride is produced in three main types; Reaction Bonded Silicon Nitride (RBSN), Hot Pressed Silicon Nitride (HPSN) and Sintered Silicon Nitride (SSN). RBSN gives a relatively low-density product compared with hot pressed and sintered Silicon Nitride. HPSN and SSN materials offer better physical properties suitable for more demanding applications. Typical applications include: bearing balls and rollers, cutting tools, valves, turbocharger rotors for engines, turbine blades, glow plugs, molten metal handling, thermocouple sheaths, welding jigs and fixtures and welding nozzles. 

Silicon Carbide
Silicon Carbide is a highly wear resistant material with good mechanical properties including high temperature strength and thermal resistance of up to 1650 C. It has a low density, high hardness and wear resistance and excellent chemical resistance. Its characteristics mean the material is ideal for applications such as fixed and moving turbine components, seals, bearings, ball valve parts and semiconductor wafer processing equipment.

Conclusion

Many factors determine the material from which components are manufactured. It is most important to consider the application and the performance requirements based on thermal, mechanical, electrical and chemical properties. Ceramic materials’ hardness, physical stability, extreme heat resistance, chemical inertness, biocompatibility, superior electrical properties and, not least, their suitability for use in mass produced products, make them one of the most versatile groups of materials in the world. As applications make greater demands on any one or combination of these properties, ceramics not only become the materials of choice, but in many cases, the only viable option in terms of materials that can survive in the extreme conditions of the application.


Morgan Technical Ceramics is a specialist in manufacturing products from a range of materials including ceramic, glass and precious metal. It also produces piezoelectric and dielectric materials for the use in electronic applications. The company has established an enviable reputation for providing value-added solutions through world-class research and development, innovative design and application engineering. Morgan Technical Ceramics supplies a range these products and materials expertise to manufacturers worldwide in industries such as, automotive, aerospace, medical, telecommunications, laser and semiconductor.