16.9 Device Design

Advanced ceramics is an interdisciplinary subject, spanning physical chemistry, metallurgy and chemical engineering (refractories in furnaces); solid-state chemistry, physics and advanced ceramics fabrication techniques (batteries and fuel cells), and electrical and electronic engineering (electronic components and insulators). So electroceramicists must understand the conductive, dielectric, optical, piezoelectric and magnetic as well as the physical properties of materials. Ferroelectrics also have non-linear electrical, electromechanical and electro-optic behaviour, making design even more difficult. The lack of a comprehensive theory explaining some of the effects increases the difficulty of design further.

To achieve the desired effects the electroceramacist needs to carry out considerable tailoring of the many parameters of the target material, optimising the initial raw materials and the process route. As some important ingredients are only required in small amounts to achieve the desired result, considerable care needs to be exercised to achieve homogeneity in the final ceramic.

Even when the required effect has been achieved, there are problems because some materials exhibit more than one of these effects, and some all of them. For example, if a piezoelectric material is used to measure vibration it may also suffer interference from temperature effects if the material is also pyroelectric (and vice-versa). These unwanted effects have to be minimised or ideally designed out of products, particularly sensors, often using very inventive engineering.

Specific types of advanced ceramics and their applications are described in the following sections:

16.10 Magnetic Ceramics

The earliest use of a magnetic “ceramic” was of the naturally occurring lodestone (the black ore, magnetite, Fe3O4) reportedly by the Chinese around 2,600 BC, possibly as a crude magnet for direction finding. It was reportedly linked to the military success of Emperor Huang-Ti. The Greeks used it for navigation before 800 BC, and it is they who coined the word magnet possibly after Magnesia in Turkey where it was mined, and the Vikings also used it before 900 AD. Iron (ferric) oxide or haematite was the major component of the original ceramic magnets, hence the name “ferrites”.

Illustration of domains and their tendency
to align to the magnetic field

Ferrites are made up of a large number of minute domains each with spontaneous magnetisation. Normally these domains are randomly oriented so the ferrite is non-magnetic. However an applied magnetic field makes some of the domains align with the field or grow at the expense of others thereby resulting in a nett magnetisation. When the field is removed many domains remain in place producing a permanent magnet. This magnetisation can be removed with a high reverse field, the magnitude of which depends on the ferrite’s “hardness” or “coercive force”. Each grain in a polycrystalline magnetic ceramic may have a number of domains with different magnetisation directions, or the whole grain or several grains may form a domain. As mentioned earlier, the spontaneous magnetisation in ferrite material reduces with increased temperature becoming zero at the Curie temperature.

To manufacture polycrystalline ferrite ceramics, the component oxides can be milled in water or alcohol for better mixing, calcined in air in a kiln at around 1000 ºC, milled again to a particle size of about 1 micron, mixed with a binder to a slurry, dried to granules around 50-300 microns and dry pressed to the required shape. However to obtain the best results the oxides have to have an optimum size spread for controlled grain growth during sintering and this is better achieved by chemical synthesis. This latter technique can produce a narrow range of high purity sub-micron particles, helping subsequent compaction and minimising pores. The methods used include precipitation from a solution, sol-gel, spray drying or freeze-drying. As an example, in the case of nickel zinc ferrite the original powder obtained by precipitation has particles of some 0.03 microns diameter.