what is the diffrence between dielectric &insulator? how is it possible to use Al as dielectric in capacitors?

Answer 1

Dielectric are generally insulators but they start conducting at high value of electric field applied across them. No aluminum can't be used as dielectric as it is conductor and results in short circuiting of two plates of capacitor if fully filled, though we can partially fill it without touching any of the two plates

Answer 2

A dielectric, or electrical insulator, is a substance that is highly resistant to the flow of electric current. Although a vacuum is also an excellent dielectric, the following discussion applies primarily to physical substances.

An insulator is a material or object that prevents the flow of electrical charges, thereby preventing the flow of an electric current.

The term electrical insulator has the same meaning as the term dielectric, but the two terms are often used in different contexts. Conductors and semiconductors, which permit the flow of charge are the opposite of electrical insulators. Very pure semiconductors are insulators at low temperatures unless doped with impurity atoms that release extra charges to help carry the current. A few materials (such as silicon dioxide) are almost ideal electrical insulators, a property that is invaluable in flash memory technology. Teflon is another almost ideal insulator, making it a valuable material for long term charge storage in electrets. A much larger class of materials, for example rubber and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).