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John Binner - Week 9 - Coggle Diagram
John Binner - Week 9
Transparent armour
We need materials that are transparent but also resistant to impact, scratches, abrasion etc.
Although ceramics appear opaque, actually they are intrinsically transparent. It is the presence of grain boundaries that make them appear opaque.
Photons can interact with the electronic structure of the atoms in a material by exciting electrons up into the conduction band from the valence band. If this happens the radiation is absorbed and the materials is opaque to the incident radiation wavelength. If not, the energy passes through the material.
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Transparency
The issue of photon energy vs band gap energy depends on the wavelength of the light. E = hv, = h*(c/lambda)
Ceramics that are used are transparent to visible light as well as both ultraviolet and infrared for different applications.
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Toughening glass
Surface heat treatment
Flame toughening, known as tempering, glass sheets are heated to a plastic state at around 650 C, and then moved into a quench area where they are cooled by a series of high pressure air nozzles.
This rapid cooling (quenching) induces a high compressive stress in the glass surface whilst the centre remains in an expanded state. Although the physical characteristics remain unchanged, the additional stresses created within the glass increase its toughness and strength because cracks need to overcome the surface compression before they can form and grow.
Higher max peak stress, but more concentrated in the centre.
Ion exchange
Chemical tempering - relies on replacing some of the sodium ions within the glass with potassium ions, which are larger than sodium ions, so strain the surrounding lattice and make dislocation movement more difficult, strengthening the material. Results in more even distribution of compressive stress field, but lower max peak stress value.
If the compressive layer is penetrated by a scratch or an impact, the stresses will be released and the glass will break into a number of small particles, rather than the long shards associated with broken glass.
Aircraft sensors
External lights and sensors on aircraft are continually exposed to factors such as particulate abrasion, thermal and chemical stresses as they fly through the atmosphere.
To protect them they need transparent covers. Those based on polymers such as polycarbonates or acrylics do not have required properties, whilst ceramics do. Borosilicate glasses offer excellent performance for civilian aircraft whilst materials such as sapphire can be used by the military.
Although denser and heavier and more brittle, they are significantly more abrasion resistant whilst having a much lower CTE and much wider and much wider operating temperature range than polymeric alternatives.
Space vehicle windows
The space shuttles often suffered damage when they flew and satellites and the international space station also get hit from time to time.
Orbital craft travel at extremely high speeds (thousands of miles per hour) so any impact can cause extreme damage. Less than a centimetre is not a problem, up to a centimetre can cause critical damage and up to 10 cm can shatter a spacecraft or satellite into pieces.
Bullet resistant glass
Contains multiple layers of regular glass and polycarbonate. Much thicker and heavier than regular glass, so much better energy absorption characteristics
The more layers, the greater the level of protection.