For the specific purpose of melting aluminum in a microwave, you would need a material that is suitable for absorbing microwaves effectively and is also resistant to the extreme heat generated during the melting process. Graphite, as you mentioned, is not ideal for this purpose, as it reflects microwaves rather than absorbing them.
One material that is commonly used for this application is silicon carbide (SiC). Silicon carbide crucibles have high electrical conductivity and excellent microwave absorption properties. They can handle the intense heat produced during aluminum melting and efficiently convert microwave energy into heat, which promotes uniform and rapid heating of the aluminum.
When using a silicon carbide crucible for aluminum melting in a microwave, it's important to ensure that the crucible is of high quality and designed for this purpose. Using a regular ceramic or metal crucible in a microwave can lead to reflection of microwaves and potential damage to the microwave oven.
As with any specialized application, it's recommended to consult with experts in materials and microwave engineering to ensure proper safety and effectiveness when designing a crucible for melting aluminum in a microwave.