In power electronic applications, what is called "heat sink paste" is used to fill irregularities between mating surfaces, and reduce the thermal resistance at a mechanical interface. A traditional composition for heat sink paste is zinc oxide (ZnO) mixed with silicone grease. More modern formulations, such as used between a microproccessor IC package and a heat sink are composed of silver (Ag) powder mixed with silicon grease. Wafers of beryllium oxide (BeO) have also been used. ZnO is cheaper than silver, and both of these substances are safer to use than BeO, which becomes hazardous if powderized.