• The relationship between technology and society
• properties of materials depend upon 3 atomic level features
• 3 types of strength
• composite materials
• what is a superconductor, and how does it work?
• semiconductors and photovoltaics, how do they work?

• stone age: Society used material as it was found, except to form it. Eg. axe head
• copper age: Society still used material as it was found, but now it could also heat and hammer it to its form
• bronze age: This society could alters more than the form of a material, is also was capable of making alloys, using temperature and hammer
• iron age: Society was able to purify compounds, for instance, getting metals from oxides. Our society has just finished this age, we can now obtain any element we want.
• Technological age: What do we do with these elements, imagination.

• all properties of materials depend upon 3 atomic level features
1.  The kinds of atoms that make it up
2.  The way in which the atoms are arranged
3.  The nature of its bonding
• e.g. of 1: lead is heavier than carbon
• e.g. of 2: graphite and diamond are both carbon compounds, but their structures are different
• e.g. of 3: materials with metallic bonding are all good conductors of electricity.

• Historically this was the most important property
• 3 types of strength:
1.  compressive: able to withstand applied pressure, e.g. bricks, diamonds, osmium

2.  tensile: able to withstand pulling apart, e.g. rope
3.  shear: able to withstand twisting, e.g. a wooden stick versus a wire.
• In the last few decades man has discovered composite materials (mixtures). E.g. concrete and rebar. Plywood, graphite composite golf clubs.

• These are materials that conduct electricity without any loss of energy, such as heat loss.
• They can levitate magnets when turned on, just by cooling.
• They only work at low temperatures, below -100° C.
• They work because of a unique crystal structure
• The crystal structure has a layer of Cu and O atoms that are strongly bonded to each other, but the layer itself is weakly bonded to the rest of the crystal.

• Under thermal motion this layer waves in the crystal like a sheet in the wind, the electrons surf these waves, moving without being the cause of any atomic motion. This is why there is no heat associated with superconductors.
• As the temperature increases, the other atoms in the crystal start to vibrate and they complicate the surfing waves. This is why superconductors only work at low temperatures.
• The layered structure means that it is difficult to make circuits. We need new superconductors that are made of polymers.
• We need new superconductors that work at room temperatures.
• Future directions: flying trains and rocket launchers, Interstellar space ships, etc.

• Let’s start with the crystal structure of diamond, as illustrated above. Note that the valence of carbon is 4, and there are 4 covalent bonds. So we imagine that there is 1 electron in each bond.
• Make it out of Si instead of C (though C is better!!!) cause it is easier and cheaper and still has 4 valence electrons.
• Substitute Al for some of the Si, and P for some others. Note that Al has 3 valence electrons and P has 5.

• We observe that P gives up one of its valence electrons to Al. That gives the Al site the right number of electrons in its bonds (4), and a charge of -1. Also P will have the right number of electrons (4), and a charge of +1. Such an arrangement of atoms is called a semiconductor.
• Put a thin sheet of P-doped Si against a thin sheet of Al-doped Si and the Al-doped sheet ends up with a negative charge while the P-doped sheet is positive.
• This makes a door in which conduction of electrons only goes in the direction from the P-side to the Al-side. Called a diode. The flow does not easily go in the opposite direction. When it does, then you have blown your diode.

• Photovoltaics get electricity from sunlight.
• They are made of a sheet of P-doped Si on top of a sheet of Al-doped Si. The sun hits the P-doped sheet, exciting its loosely bounded electrons, which then travel to the Al-doped side, because that is the only direction they can go. The P-doped side is now short of electrons. If the Al-doped side is connected to the P-side via a circuit (say through a lightbulb) then there is a flow of electrons back to the P-doped side.

• Future directions: Thin films and paint.