Nanoparticles of alumina and oxides of elements of main groups I and II of the periodic table, and their preparation

1. A process for the preparation of nanoparticles wherein the nanoparticles consist of 50-99.99% by weight of alumina in the rhombohedral α-modification and 0.01-50% by weight of a metal oxide of elements of main group I or II of the Periodic Table comprising the steps of adding a crystallization nuclei, oxide formers comprising elements of main groups I and II of the Periodic Table to aluminum chlorohydrate, thermally treating the combined crystallization nuclei and oxide formers, and aluminum chlorohydrate for between 0.5 and 30 minutes to form agglomerates and comminuting the agglomerates.

2. The process as claimed in claim 1 , wherein the aluminum chlorohydrat is a compound of the chemical formula Al 2 (OH) x Cl y , wherein x is a number from 2.5 to 5.5, y is a number from 3.5 to 0.5, and the sum of x+y is always 6.

3. The process as claimed in claim 1 , wherein the crystallization nuclei is very finely dispersed α-Al 2 O 3 , hematite or diaspore.

4. The process as claimed in claim 3 , wherein the very finely dispersed α-Al 2 O 3 crystallization nuclei have a mean particle size of less than 0.1 μm.

5. The process as claimed in claim 1 , wherein the combination of aluminum chlorohydrate, crystallization nuclei and oxide formers is an aqueous suspension.

6. The process as claimed in claim 1 , wherein the thermally treating step is carried out in a sliding-bat kiln, chamber furnace, tube furnace, rotary kiln, microwave oven, or in a fluidized-bed reactor.

7. The process as claimed in claim 1 , wherein the thermally treating step is carried out at a temperature below 100° C.

8. The process as claimed in claim 1 , wherein the thermally treating step is carried out at from 700 to 1100° C.

9. The process as claimed in claim 5 , wherein the aqueous suspension comprising aluminum chlorohydrate and nuclei is injected directly into the thermally treating step apparatus without prior removal of the any water.

10. The process as claimed in claim 1 , wherein the agglomerates are comminuted in a subsequent step by wet or dry milling.

11. The process as claimed in claim 1 , wherein the agglomerates are comminuted by ultrasound.

12. The process as claimed in claim 1 , wherein the agglomerates are comminuted at from 20 to 90° C.

13. The process as claimed in claim 1 , wherein the suspensions obtained in the comminuting step are converted into a powder with soft agglomerates by spray-drying, freeze-drying or other drying methods.

14. The process as claimed in claim 1 , wherein the suspensions obtained in the comminuting step are centrifuged and the clear to slightly opalescent supernatants comprising nanoparticles <20 nm are separated off.

15. The process as claimed in claim 14 , wherein the suspensions obtained after the centrifuging are dried by spray-drying, freeze-drying or other drying methods and a readily redispersible powder is formed thereby.

16. The process as claimed in claim 5 , wherein the aqueous suspension is first dried and the dried product is then thermally treated.

17. The process as claimed in claim 1 , wherein the thermally treating step is carried out in from 0.5 to 10 minutes.

18. The process as claimed in claim 1 , wherein the thermally treating step is carried out in from 2 to 5 minutes.