Compositions – Electrically conductive or emissive compositions – Metal compound containing
Patent
1991-10-08
1994-06-07
Group, Karl
Compositions
Electrically conductive or emissive compositions
Metal compound containing
252519, 252520, 252521, 501134, 501135, H01B 108
Patent
active
053187237
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to ceramic materials having low resistivities and processes for their production.
DESCRIPTION OF RELATED ART
Electrically conductive ceramic materials are used in many fields where advantage is taken of their characteristic electrical properties. Thus, for examples, conductive ceramics are used in the manufacture of electrical components such as resistance elements, capacitors and semi-conductor devices. Generally, however, ceramic materials have relatively high resistivities, typically greater than 10.sup.-1 .OMEGA..cm. Thus, for example, evaporated films of stannic oxide (SnO.sub.2) are widely used as electrical resistance elements, but resistivities less than 10.sup.-1 .OMEGA..cm are difficult to obtain. Resistances of 1 to 10 .OMEGA..cm can be observed in freshly prepared samples of cadmium oxide (CdO) but this material is relatively unstable. Resistivities in the range 10.sup.-1 to 10.sup.-3 .OMEGA..cm have been observed in sintered samples of cadmium stannate (see T. Hashimi et al., J. Electrochem. Soc. 134 (1987) pp. 1591-1594) where the use of such relatively low resistivity ceramic materials in the manufacture of electrodes for secondary electrochemical cells is described. However hitherto, the production of ceramic materials having resistivities substantially less than 10.sup. -3 .OMEGA..cm has never been described.
The production of such low resistivity ceramic materials would be highly desirable, as it would enable such materials to be used in place of metals, which typically have resistivities in the range 10.sup.-3 to 10.sup.-7 .OMEGA..cm. Thus, for example, low resistivity ceramics could be used in corrosive environments, where hitherto expensive noble metals such as platinum and gold have hitherto been used.
SUMMARY OF THE INVENTION
We have now developed a highly conductive ceramic material which has a resistivity at ambient temperature of less than 10.sup.-3 .OMEGA..cm.
The ceramic materials according to the invention would normally not have resistivities lower than 10.sup.-7 .OMEGA..cm. Samples have, however, been produced with resistivities less than 10.sup.-4 .OMEGA..cm.
The conductive ceramic materials according to the invention generally comprise an ionic lattice of metal ions and oxygen ions wherein at least a portion of the metal ions are ions of a metal capable of existing in a plurality of valence states, and wherein ions of at least two different metals are present, one of which is capable of existing in a +2 oxidation state and one of which is capable of existing in a +4 oxidation state and wherein at least one of said two different metals is present in the form of ions having different oxidation states.
The presence of ions of different oxidation states is achieved according to one embodiment of the invention by exposing a ceramic starting material to reducing conditions, for example by exposing it to a reducing atmosphere. Alternatively ions of one or more of the metals may be maintained in different oxidation states by incorporating one or more dopants in the ceramic material. Examples include electron-donating dopants, such as trivalent metal oxides, e.g., Sb.sub.2 O.sub.3 or Bi.sub.2 O.sub.3.
The invention also provides a method of producing a ceramic material having a resistivity at ambient temperature of less than 10.sup.-3 .OMEGA..cm, which comprises forming a ceramic starting material comprising at least two different metals in the form of oxides, one of which is capable of existing in a +2 oxidation state and one of which is capable of existing in a +4 oxidation state, said ceramic starting material having a resistivity greater than 10.sup.-3 .OMEGA..cm and exposing the ceramic starting material to reducing conditions, generally at an elevated temperature. Preferably the ceramic starting material is exposed to a reducing atmosphere, for example gaseous hydrogen although other methods may be employed, for example electrochemical polarization.
The selection of the metal oxide components of the ceramic start
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patent: 4284521 (1981-08-01), Payne et al.
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patent: 5008163 (1991-04-01), Smith et al.
"Ceramics as Electrical Materials" vol. 5, pp. 292, 294 & 299 Sprechsaal 107. Jahrgang-pp. 1057-1060, 1974.
Journal of the Electrochemical Society, Jul. 1987, pp. 1591-1594.
Elmwood Sensors, Ltd.
Group Karl
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