…And Men With Funny Hats. …Superconductors… …Semiconductors… A Journey to the Land of Lattices… A Journey Through Space… A Journey Through Time…

Geometry of Crystalline Structures Adopted by Ionic Compounds Face centred cubic lattice e.g. NaCl Single unit cell Sodium chloride has the coordination number 6:6, as the sodium ion has six neighbouring chloride ions, and visa versa. Body centred cubic lattice e.g. CsCl Single unit cell Caesium chloride has the coordination number 8:8, as the caesium ion has 8 neighbouring chloride ions, and visa versa.

= Superconductors What are they? Ceramic materials, such as magnesium diboride. Superconductors are materials which conduct electricity with no loss of energy. They have zero resistance. Metal alloys such as niobium-titanium. Superconductors can be metal elements like mercury or silver. Superconductors were, in the past, impractical because they had to be super cooled by liquid helium which scientists were complaining was “Really deer to buy.” = Ceramic superconductor’s critical temperature for superconductivity isn’t as close to zero degrees Kelvin, which means liquid nitrogen can be used in place of helium. This is less expensive.

The Meissner Effect In a weak applied field, a superconductor "expels" all magnetic flux. This allows a magnetic object to “float” above a superconductor. Scientists have come to the conclusion that this effect is “pretty class”. Material after cooling, in superconductive state Material before cooling Arrows represent magnetic field lines

Applications for Superconductors Power Cables: Right now, up to 10% of electrical energy carried by power cables is lost as heat Using a superconductor could eliminate this energy loss. It would also allow power lines to be hidden underground.

Applications for Superconductors Other uses include: Magnetic Resonance Imaging Electronics Superconductors create high magnetic fields, this allows doctors to see inside parts of the body that hitherto had not been visible outside of investigative surgery. Using superconductors to craft electronic switches could advance microprocessor technology making them extremely fast.

Applications for Superconductors Other uses include: Transport Oh, and that car from I, Robot Trains may be able to hover over tracks which means frictionless, high speed travel like the Maglev Train in Shanghai, China.

Their conductivity increases with temperature and exposure to light. Semiconductors Semiconductors are materials that have a conductivity lying somewhere between non-metals which have no free charged particles, and metals, which have free moving electrons. Their conductivity increases with temperature and exposure to light. Examples are silicon and germanium, metalloids that have four valence electrons.

Doping P-type N-type Ga Ga Ga Ga Ga Ga Si Si Ga Ga Electron Positive hole Free electron In a p-type semiconductor, and impurity from group three such as gallium or indium is added, this creates three bonds with the group four semiconductor leaving a gap where an electron should be. This is known as a “positive hole” as it contains no negative electron. In an n-type conductor, an impurity from group five such as antimony or arsenic is added to a group four semiconductor, four bonds are created leaving one free one to act as a charge carrier.

The Photovoltaic Effect When light energy falls on a semiconductor, the conductivity increases. P-N Junction The energy a photon delivers to an electron releases it from n-type side of the junction and the depletion layer causes a potential difference. N-type P-type + - - + - + - + + - - + + - Depletion layer

Questions Magnesium chloride has the same coordination number as sodium chloride. What shape is the magnesium chloride crystal? Which of the following statements referring to the structures of sodium chloride and caesium chloride is correct? A: There are eight chloride ions surrounding each sodium ion. B: There are eight chloride ions surrounding each caesium ion. C: The chloride ions are arranged tetrahedrally round the sodium ions. D: The chloride ions are arranged tetrahedrally round the caesium ions. Answer B: caesium chloride is a body centred lattice with a coordination of 8:8. Answer: Sodium chloride has a coordination number of 6:6, like sodium chloride, magnesium chloride is a face centred lattice.

Explain the Meissner effect. Questions Explain the Meissner effect. A superconductor is a material A : Whose electrical conductivity decreases with decreasing temperature B: That does not conduct electricity unless doped with another material C: That can conduct electricity with zero resistance D: Whose electrical conductivity increases with increasing temperature. Answer C: Superconductors that are cooled to extremely low temperatures past a critical point can conduct electricity with no energy loss. Answer: When cooled below a critical point, a superconductor repels all magnetic flux, this allows a magnetic object to “float” above it.

How does a photovoltaic cell convert light energy into electricity? Questions How does a photovoltaic cell convert light energy into electricity? Silicon can be converted into an n-type semiconductor by adding A: Boron B: Carbon C: Arsenic D: Aluminium. Answer: Photons release electrons, they gather at the p-n junction and form a depletion layer creating a potential difference. Answer C: Using a group five element like arsenic to dope silicon will leave one free electron turning it into an n-type semiconductor.

Also, Bowl Cuts Are Cool.