How do you make a ceramic membrane?

[f95toli]

Take a look at this link, on the page you will see a ceramic membrane which is used to make membranes for speaker cones.

I work with ceramic thin films and am familiar with different kinds of PVD, but that is on substrates; how do you make a membrane?
Noe also that there are two layers growing together.

My guess is that they do not grow the membrane, they simply anneal a thin aluminiym film in a oxygen rich reactive atmosphere.

Does anyone know for sure?

Edit: Sorry, forgot the link. Look at the SEM picture
Accuton

 

[Gibsons]

link?

 

[f95toli]

Fixed

 

[CycloWizard]

Making membranes is sort of a black art more than a science. I talked to a membrane specialist at Pall-Gelman Corp a couple years ago when dealing with their membrane filters and he explained the basic process. The material is cast with a known thickness on a large slab. The slab can be held at a certain temperature, ramped, or what have you to achieve the desired effects. Likewise, pressure can be varied in the room (like an autoclave). The casting slab can even emit gases and so on. I got the feeling they just played with the conditions of the casting process until it gave them the pore distribution they wanted, and this agrees with what I know of polymeric membrane production as well. Lots of hand-waving. 😛

 

[Howard]

The simplest method (IMO) would be to deposit a coating on a substrate and then etch the substrate, leaving the coating (membrane, now) by itself.

EDIT: The pic looks like they started anodizing an extremely thin layer of aluminum? It is alumina, after all.