16.6 Single Crystal Ceramics

Although some materials such as electronic quality quartz is found naturally in Brazil, 3000 tons or 90% of crystal materials annually are now grown synthetically. The first synthetic crystal was grown by the flame-fusion method in 1891. Using this method alumina powder was melted using the newly developed oxyhydrogen torch and the crystals formed in a crucible beneath. Single crystal ceramics are now produced by a variety of different methods. The hydrothermal method starts with a solution that is subjected to high pressure and temperature in an “autoclave” (a steel pressure vessel). This method can produce single crystals of materials such as quartz 4 cms diameter and 12 cms long. Another method is called “molten salt synthesis” where the components are melted with a flux and crystallise on cooling, which is used for barium titanate and lead zinc niobium titanate. A third is to melt the pure material and to use a seed crystal to slowly draw a large crystal out of the melt (as used to make very large crystals of silicon semiconductor), which is used for barium titanate, lithium niobate and barium zinc ferrite. As an example, yttrium aluminium garnet, the single crystal ceramic used in lasers, is drawn from a melt in an iridium crucible over 4 to 8 weeks.

16.7 Solid-State Sintering for Polycrystalline Ceramics

As we know, conventional ceramics are produced at a temperature when some of the components melt or vitrify and penetrate the porous microstructure, and when cooled the whole becomes a fine-grained matrix that includes the higher melting temperature components. This firing process is also known in the scientific community as “liquid phase sintering”, as some components melt into liquids. However, this can produce the wrong sort of grain boundaries and make this process unacceptable for many electronic devices.

The firing process required for advanced ceramics is called “solid-state sintering” that produces a dense mixture of porous-free polycrystalline material without any component having to melt. To achieve the necessary quality of material it is essential to control the purity and size of the starting powders. Sintering converts the compacted powder into a denser structure of crystallites joined to one another by grain boundaries. The grain boundaries are not as dense as the crystallites so permit gas to escape in the early stages of sintering, which typically takes a few hours at 80 to 90% of the melting temperature.  It takes longer to process than with liquid sintering, as the mechanisms for consolidation of the crystals and removal of pores is slower. As an example when pure alumina powder is heated, the particles first become closer together, the whole aggregate shrinks and there is a decrease in porosity as the temperature and time increase. With continued increase in temperature and time, it changes from a multi-particle aggregate into a ceramic with some closed pores containing pressurised gases. At sufficiently high temperatures the gases diffuse from the alumina to leave a completely non-porous transparent ceramic.

As an example, one transparent electro-optic material has to be made by hot pressing, where an appropriate quantity of the prepared material is placed in a silicon carbide mould and heated to 1250 ºC for 18 hours at 2000 psi (14 MPa). This method can produce optical quality discs up to 6” diameter.

Without stringent processing controls, the physical and other unique properties of the ceramic component can be degraded, defeating the very strengths of ceramic electronic components – their stability and reliability.

16.8 Finishing

After sintering some ceramics are sliced into wafers to be ground, lapped, polished, drilled, coated, have electrodes fitted and “poled”. Poling is the application of a magnetic field to a ferrite or an electric field to a ferroelectric ceramic so that the respective magnetic or electric domains are aligned, forming “permanently” magnetised ferrites or “permanently” polarized ferroelectrics. It is referred to as poling after the magnetisation or poling (creation of North and South poles) in an iron rod by stroking with a permanent magnet. Poling is carried out as the ceramic is cooling through its “Curie” point. The Curie temperature is the transition temperature of a ferrite or ferroelectric ceramic above which it changes phase from a domain structure that is spontaneously magnetised/polarised, to a disordered state. Conversely it changes phase on cooling through the Curie temperature to reform a domain structure.