Materials, science and technology
A functional ceramic is a ceramic chosen for what it does (its function), not just for its look. To get the function we control four things: the chemical recipe, the purity of the powder, the size and shape of the grains, and the firing. Change one of them and the property changes.
Common fine ceramics: alumina (aluminium oxide), zirconia (zirconium oxide), silicon carbide, silicon nitride, barium titanate and lead zirconate titanate (PZT).
Mechanical functions
The atoms in these ceramics are held by very strong bonds. So they are very hard, do not wear, do not rust and stay strong at high heat.
- Alumina and zirconia: knife blades, pump parts, hip joints, dental crowns.
- Silicon carbide and silicon nitride: bearings, engine parts, cutting tool tips.
The weak point is that they are brittle. Makers use tiny grains and zirconia to stop cracks growing.
Electrical functions
- Insulators: they do not let current pass. Alumina in spark plugs and porcelain on power-line poles.
- Dielectrics: they store charge. Barium titanate is used in tiny capacitors inside phones.
- Piezoelectric ceramics (PZT): squeeze them and they make voltage; give them voltage and they change shape. Used in lighters, sensors, buzzers and ultrasound scanners.
- Others: thermistors (resistance changes with heat), zirconia oxygen sensors in cars, ferrite magnets, and some superconductors.
Optical functions
Light passes through a material only if there is nothing to scatter it. Tiny pores and grain edges scatter light, so ordinary ceramic looks white and cloudy. If we fire the powder very carefully until almost no pores are left, the ceramic becomes translucent or clear.
- Clear alumina tube: inside street lamps.
- Laser crystals such as YAG: make a strong narrow light beam.
- Glass-ceramic: cooker tops that take heat without cracking.
- Phosphor ceramics: glow when hit by light or electrons (white LEDs).
Try it
Try it at home. Find a gas lighter that clicks (not the wheel type). The click hits a small ceramic crystal; the squeeze makes a spark. In the 3D, pick Electrical and drag the slider to see volts rise with force. Then pick Optical and take the pores down to zero.
Key formulas and definitions
- Key terms: functional ceramic, insulator, dielectric, piezoelectric, translucent, pore
- Piezo output (simple model): V = k x F, so double the force gives double the voltage
- Light after n layers: I = I0 x T^n (T = fraction passed by one layer)
- Few pores -> more light through; many pores -> more scattering
Worked examples
1. A piezo sensor gives 2 V for every 1 kN of force. What voltage does it give at 3.5 kN?
V = k x F = 2 x 3.5 = 7 V.
2. One thin ceramic layer lets 80% of light pass. How much passes through three such layers?
Each layer multiplies the light by 0.8. 0.8 x 0.8 x 0.8 = 0.512, so 51.2%.
3. Why is a ceramic knife better than a steel knife for slicing a long time without sharpening, but worse if you drop it?
The ceramic is much harder, so its edge wears slowly. But it is brittle, so a drop on a hard floor can chip or break it. Steel is softer but tough, so it bends rather than breaks.
Common mistakes
- Thinking all ceramics are insulators. Some are semiconductors, ionic conductors or even superconductors.
- Saying the piezo effect needs a battery. Squeezing alone makes the voltage.
- Believing ceramics cannot be see-through. With almost no pores they are.
- Forgetting that hard does not mean tough. Ceramics are hard but brittle.