A-level Physics (Advancing Physics)/Metals
There are several physical properties of metals you need to know about:
Metals consist of positive metal ions in a 'soup' or 'sea' of free (delocalized) electrons. This means that the electrons are free to move through the metal, conducting an electric current.
The electrostatic forces of attraction between the negatively charged electrons and the positively charged ions holds the ions together, making metals stiff. Stiffness is indicated by the Young modulus of the material, so a stiff material deforms little for a given tensile stress.
Since there are no permanent bonds between the ions, they can move about and slide past each other. This makes metals ductile.
Metals are tough for the same reason as they are ductile: the positive ions can slide past each other while still remaining together. So, instead of breaking apart, they change shape, resulting in increased toughness. This effect is called plasticity. When a tough material breaks the ratio of 'energy used / new surface area created' is very large.
When a metal is stretched, it can return to its original shape because the sea of electrons which bonds the ions together can be stretched as well.
The opposite of tough: a material is likely to crack or shatter upon impact or force. It will snap cleanly due to defects and cracks.
Metals are malleable because their atoms are arranged in flat planes that can slide past each other. Their bonds are non-directional. Metals also contain dislocations which mean that ions in the structure can be moved unilaterally rather than as a whole layer, which takes less energy to do.
Diffusive transformation: occur when the planes of atoms in the material move past each other due to the stresses on the object. This transformation is permanent and cannot be recovered from due to energy being absorbed by the structure
Diffusionless transformation: occurs where the bonds between the atoms stretch, allowing the material to deform elastically. An example would be rubber or a shape memory metal/alloy (often referred to as SMA) such as a nickel-titanium alloy. In the shape memory alloy the transformation occurs via the change of phase of the internal structure from martensitic to deformed martensitic, which allows the SMA to have a high percentage strain (up to 8% for some SMA's in comparison to approximately 0.5% for steel). If the material is then heated above a certain temperature the deformed martensite will form austenite, which returns to twinned martensite after cooling.
1. Would you expect a metal to have more or less conductivity than a semiconductor? Why?
2. How can the stress-strain graph for a metal be explained in terms of ions in a sea of electrons?
3. As a metal heats up, what happens to its conductivity? Why?