Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Autogenously or by activation of dry coated particles
Patent
1996-03-13
1998-03-31
Derrington, James
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Autogenously or by activation of dry coated particles
264604, 264614, 264669, 264681, 264109, 264332, C04B 3546
Patent
active
057334897
DESCRIPTION:
BRIEF SUMMARY
The invention relates to the production of articles comprising suboxides of titanium, in particular Magneli phase oxides of the formula TiO.sub.x where x is 1.55 to 1.95.
EP-A-047595 discloses a method of making an article comprising a suboxide of titanium, the method comprising, preparing the suboxide from ititanium dioxide by reduction in hydrogen and carbon; binding the suboxide into the shape of an article; and subjecting the bonded shape to a heat treatment.
Articles of these suboxides can be used as electrodes in electro chemical processes and for a variety of industrial purposes. The articles can be used for example in electrodialysis, as metal recovery devices and as self-cleaning electrochlorinators. Such articles tend to be porous and therefore wicking of electrolyte on to a ceramic-metal junction at a connection point can be a problem. We have now discovered how to make articles of the suboxides which are characterised by the unique combination of being resistant to chemical corrosion and non-porous or pore-free to the extent that the problem of wicking is avoided. This invention is based on the discovery that this combination of properties may be achieved by controlling the parameters of a heat treatment.
According to the invention in one aspect there is provided a method for the purpose specified comprising binding Magneli phase powder into the shape of an article and subjecting the bonded shape to heat treatment characterised in that the heat treatment is carded out at from about 1100.degree. C. to about 1200.degree. C. and a pressure of from about 3000 (20685 Pa) to about 5000 psi (34475 Pa) and over from about 3 hours to about 6 hours whereby the heat treated article is substantially corrosion resistant and non-wicking in use.
According to the invention in a more specific aspect there is provided a method of making a substantially corrosion resistant and non-wicking article comprising a titanium suboxide, the method comprising the steps of: mixing a Magneli phase powder and a binder therefor to form a preform, removing the binder and subjecting the preform to heat treatment characterised by controlling the temperature and pressure and time as defined above whereby the heat treated article is substantially corrosion resistant and non-wicking in use.
Our investigations suggest that the pressure and temperature are inversely related. It is preferred to operate at the maximum pressures possible with available equipment, such as from about 3000 (20685 Pa) to about 3500 psi (34475 Pa), depending on the size of the workpiece and the mechanical capabilities of the press. This permits the pressing times to be minimised. At these pressures, temperatures in the range of from about 1100.degree. to about 1150.degree. C. can advantageously be used and the pressing can be completed within a working day.
Temperatures for the heat treatment should not exceed about 1200.degree. C. While higher temperatures were expected to reduce necessary pressing times without undue adverse affects, we have found that higher temperatures make the product susceptible to corrosion.
In one overall preferred method according to the invention, relatively porous powdered Magneli phase titanium suboxide is first prepared from titania, using hydrogen or carbon reduction, for example as disclosed in U.S. Pat. Nos. 4,422,917 and 5,173,215 the contents of each of which are hereby incorporated by reference. The powder is preferably between about 1 and 80 microns average diameter, more preferably between 1 and 15 microns. The powder is combined with a low residue binder, e.g. a polymeric binder such as methyl cellulose, or polyester based materials in a solvent therefor, and shaped, e.g. cast or laid up into a preform such as a flat sheet or tape strand. Thinner preforms, for example 1 to 2 mm, are preferred to ensure that the material is completely densified during processing. The preform is then held at about 300.degree. C. at slightly elevated pressure for sufficient time to remove the binder. The binder-free precursor is then subje
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