Rapid reduction of sodium occupancy in type II silicon clathrate by chemical etching
The invention relates to a method to produce a type II silicon clathrate, a method to produce a type I clathrate, and a method to decrease sodium in silicon clathrates.
1. A method to produce a type II silicon clathrate, comprising:
heating NaH powder and Si powder for a heating time between about 24 hours to about 72 hours at a heating temperature between about 350° C. and about 450° C. in the absence of oxygen;
milling the NaH powder and the Si powder;
mixing the milled NaH powder and milled Si powder to produce NaSi powder;
annealing the NaSi powder for an annealing time between about 1 hour and about 24 hours at an annealing temperature of between about 200° C. and about 300° C. to produce annealed NaSi powder; and
thermally decomposing the NaSi powder in a chamber, wherein the chamber comprises a cold plate, wherein the temperature of the cold plate is less than a melting point of sodium, wherein the chamber is maintained at a decomposition temperature between about 300° C. and about 400° C., and wherein a pressure in the chamber is between about 0.5 mTorr and about 5 mTorr to produce a product, wherein a phase fraction of the product comprises greater than about 80 wt. % of the type II silicon clathrate.
2. The method of claim 1 , wherein the decomposition temperature is about 370° C.
3. The method of claim 1 , further comprising providing sodium powder to the chamber.
4. The method of claim 1 , wherein the temperature of the cold plate is about 25° C.
5. The method of claim 1 , wherein the phase fraction of the product further comprises between about 3 wt. % to about 10 wt. % of a type I silicon clathrate, between about 2 wt. % to about 10 wt. % of silicon, and between about 0 wt. % to about 10 wt. % of sodium.
6. The method of claim 1 , further comprising decreasing a weight percentage of sodium in the product, comprising:
rinsing the product with an alcohol;
etching the product in the presence of an acid solution to produce a slurry; and
filtering the slurry.
7. The method of claim 6 , wherein the alcohol is ethanol.
8. The method of claim 6 , wherein the acid solution comprises hydrofluoric acid and nitric acid.
9. The method of claim 8 , wherein the acid solution comprises between about 1 vol. % to about 5 vol. % of the hydrofluoric acid and between about 5 vol. % to about 15 wt. % of HNO 3 .