The interior of a bulk superconductor cannot be penetrated by a weak magnetic field, a phenomenon known as the Meissner effect. When the applied magnetic field becomes too large, superconductivity breaks down. Superconductors can be divided into two types according to how this breakdown occurs. In type-I superconductors, superconductivity is abruptly destroyed via a first order phase transition when the strength of the applied field rises above a critical value Hc. This type of superconductivity is normally exhibited by pure metals, e.g. aluminium, lead, and mercury. Examples of intermetallics exhibiting type-I superconductivity include tantalum silicide (TaSi2), BeAu, and β-IrSn4. The covalent superconductor SiC:B, silicon carbide heavily doped with boron, is also type-I.