Aluminium oxide powder, dispersion and coating composition

1. An aluminium oxide powder in the form of aggregates of primary particles, wherein each particle has a BET surface area of from 10 to 90 m 2 /g and comprises as crystalline phases, in addition to gamma-aluminium oxide and/or theta-aluminium oxide, at least 30% by weight of delta-aluminium oxide, and wherein the aluminium oxide powder has a tamped density of 10 to 200 g/l.

2. The aluminium oxide powder according to claim 1 , wherein the BET surface area is 30 to 70 m 2 /g.

3. The aluminium oxide powder according to claim 1 , wherein the content of delta-aluminium oxide is 70% to 95% by weight.

4. The aluminium oxide powder according to claim 2 , wherein the content of delta-aluminium oxide is 70% by weight to 95% by weight.

5. The aluminium oxide powder according to claim 1 , comprising amorphous aluminium oxide.

6. The aluminium oxide powder according to claim 1 , wherein the BET surface area is 40 to 60 m 2 /g, the content of delta-aluminium oxide is 80% by weight to 90% by weight and the content of theta-aluminium oxide and/or gamma-aluminium oxide is 10 to 20% by weight.

7. The aluminium oxide powder according to claim 1 , wherein the BET surface area is 70 to 90 m 2 /g, the content of delta-aluminium oxide is 30% by weight to 40% by weight, the content of theta-aluminium oxide is 0 to 5% by weight and the content of gamma-aluminium oxide is 60 to 70 wt. % by weight.

8. A process for the preparation of the aluminium oxide powder according to claim 1 , the process comprising:

vaporizing aluminium chloride and transferring the vapour into a mixing chamber with a carrier gas and

separately from the vaporizing of aluminium chloride, transferring hydrogen and primary air, which can optionally be enriched with oxygen and/or preheated, into the mixing chamber to yield a reaction mixture of aluminium chloride vapour, hydrogen and primary air,

subsequently igniting the mixture of aluminium chloride vapour, hydrogen and air with a flame burner so that the flame burns in a reaction chamber which is separated from ambient air,

introducing additional secondary air into the reaction chamber, and

separating a solid obtained in the reaction chamber from gases and subsequently treating the solid with steam and optionally air,

wherein

the primary air/secondary air ratio is 0.01 to 2,

the exit speed v B of the reaction mixture from the burner is at least 10 m/s,

a lambda value is 1 to 4,

a gamma value is 1 to 3 and

the value of gamma*v B /lambda is greater than or equal to 55.

9. A dispersion comprising 10 to 60 wt. % of the aluminium oxide powder according to claim 1 .

10. The dispersion according to claim 9 , wherein the average aggregate diameter of the aggregates of primary particles in the dispersion is less than 200 nm.

11. A method of chemical-mechanical polishing, the method comprising applying the dispersion according to claim 9 to a surface to be polished.

12. A coating composition comprising the dispersion according to claim 9 and at least one binder.

13. A coating composition comprising the dispersion according to claim 10 and at least one binder.

14. A coating composition according to claim 12 , wherein the content of binder, based on the aluminium oxide powder, is between 3 and 150 wt. %.

15. A coating composition according to claim 13 , wherein the content of binder, based on the aluminium oxide powder, is between 3 and 150 wt. %.