You can do carbon activation with KOH in a nickel crucible introduced in a stainless steel tube as the reactor. Moreover, activation of carbon with KOH produces carbonates, and hence a caustic foam with comes out of the crucible and touches the reactor. So the materials should resist these conditions. Nickel is acceptable, but zirconium would be even better. Zr crucibles are available for sale, but expensive. Hot KOH will always destroy your quartz tube, so don't use it.

I agree with the answer of Dr. Neeraj Kumar ........

Neeraj Kumar Thank you for your nice answer, also Luma M. Ahmed thank you for your comment. May I ask one more? Why Zr crucible is better then Nickel? Can a Nickel crucible be damaged?

As zirconium has better mechanical strength at high temperature than most other metals, zirconium crucibles are ideal for melting other metals. Compared with nickel crucibles, Zirconium crucibles cost more but have better cost effectiveness as the number of fusions can be made in a zirconium crucible is much higher than other metals. Zirconium crucible requires minimum care and won't act as a catalyst for oxidation and reaction unlike platinum. The corrosion resistance of Zirconium comes from a tightly adhered oxide, zirconia, which forms almost instantaneously. It is resistant to melts of alkali (Na, K, Li) carbonates, hydroxides, peroxides, borates, nitrates, chlorides and some fluorides, or combinations of above. It is also completely resistant to most solvents of all concentrations except hydrofluoric acid. In case of Ni crucible, it might damage in harsh chemical conditions.

Dr. Neeraj Kumar what about alumina crucibles ?