Thick-film gas-sensitive element

Abstract

A laminar gas-sensitive thick film consisting of ceramic semiconductor and metallic catalyzer is formed across a pair of electrodes carried by a ceramic substrate, and the nature of metallic catalyzer on an outer surface layer of the thick film is different from that of metallic catalyzer in that portion of the thick film which is in the proximity of the electrodes.

What is claimed is: 
     
       1. A thick-film gas-sensitive element of a laminate structure, comprising a ceramic substrate having a pair of electrodes disposed on an upper surface thereof; and a laminate gas-sensitive thick film deposited on said ceramic substrate upper surface so as to extend across the ends of the pair of electrodes, said .Iadd.laminate .Iaddend.gas-sensitive thick film .[.consisting of ceramic semiconductor and metallic catalyzer distributed in the ceramic semiconductor and said gas-sensitive thick film.]. having an inner .[.portion.]. .Iadd.layer .Iaddend.adjacent said substrate upper surface and at least one outer surface layer disposed on said inner .[.portion.]..Iadd.layer .Iaddend., .Iadd.said inner layer of said gas-sensitive thick film consisting of ceramic semiconductor and a first metallic catalyzer distributed in the ceramic semiconductor, said first metallic catalyzer containing 0.2 to 3 mole % rhodium (Rh) based on the amount of ceramic semiconductor in said inner layer and being a member selected from the group consisting of Rh, mixtures and alloys of Rh and platinum (Pt), and mixtures and alloys of Rh and palladium (Pd); and said outer surface layer of the gas-sensitive thick film consisting of ceramic semiconductor and a second metallic catalyzer distributed in the ceramic semiconductor, said second metallic catalyzer containing less than 0.2 mole % Rh based on the amount of ceramic semiconductor in said outer surface layer and being a member selected from the group consisting of Rh, Pt, Pd and mixtures and alloys thereof. .Iaddend..[.said outer surface layer of the gas-sensitive thick film containing a smaller amount of metallic catalyzer than said inner portion of the gas-sensitive thick film..].  .[. 
     
     
       2.  A thick-film gas-sensitive element as set forth in claim 1, wherein the metallic catalyzer in the outer surface layer of the gas-sensitive thick film contains less than 0.2 mole % of rhodium (Rh) based on amount of the ceramic semiconductor therein, while the metallic catalyzer in the inner portion of the gas-sensitive thick film contains 0.2-3 mole % of rhodium (Rh) on a similar basis..]. 
     
     
       3. A thick-film gas-sensitive element as set forth in claim 1, wherein the .Iadd.second .Iaddend.metallic catalyzer in the outer surface layer of the gas-sensitive thick film has a larger grain size than .[.that.]. .Iadd.the first metallic catalyzer .Iaddend.in the inner .[.portion.]. .Iadd.layer .Iaddend.of the gas-sensitive thick film in the proximity of the electrodes. 
     
     
       4. A thick-film gas-sensitive element as set forth in claim 1, wherein the ceramic semiconductor in the outer surface layer of the gas-sensitive thick film has a larger grain size than that in the inner .[.portion.]. .Iadd.layer .Iaddend.of the gas-sensitive thick film in the proximity of the electrodes. 
     
     
       5. A thick-film gas-sensitive element as set forth in claim 1, wherein the .Iadd.second .Iaddend.metallic catalyzer in the outer surface layer of the gas-sensitive thick film contains platinum (Pt) particles with a grain size of larger than 0.5 μm, while the .Iadd.first .Iaddend.metallic catalyzer in the inner .[.portion.]. .Iadd.layer .Iaddend.of the gas-sensitive thick film contains particles of metal .[.selected from the group consisting of rhodium (Rh) and compounds thereof at a concentration of 0.2-3 mole % based on amount of ceramic semiconductor therein,.]. .Iadd.having a .Iaddend.grain size of .[.metal particles in the inner portion being.]. less than 0.5 μm. 
     
     
       6. A thick-film gas-sensitive element as set forth in claim 1, wherein the gas-sensitive thick film has a laminate structure formed of fired paste layers of ceramic semiconductor, different layers having different dispersions of metallic catalyzer produced by impregnation of different solutions of salt of metallic catalyzer in the fired paste layers followed by thermal cracking of the salt. 
     
     
       7. A thick-film gas-sensitive element as set forth in claim 1, wherein the gas-sensitive thick film has a laminate structure formed of fired paste layers of ceramic semiconductor, different layers having different dispersions of metallic catalyzer produced by different mixing of .[.metal.]. .Iadd.metallic .Iaddend.catalyzer particles in different ceramic semiconductor pastes followed by laminate spreading and firing of the ceramic semiconductor pastes. 
     
     
       8. A thick-film gas-sensitive element as set forth in claim 1, wherein the gas-sensitive thick film has a laminate structure formed of fired paste layers of ceramic semiconductor, said fired paste layers including at least one layer having a dispersion of metallic catalyzer produced by impregnation of solution of salt of the metallic catalyzer in the fired paste layer followed by thermal cracking of the salt, said fired paste layers including at least one other layer having a different dispersion of metallic catalyzer produced by mixing of particles of the metallic catalyzer in ceramic semiconductor paste followed by laminate spreading and firing of the ceramic semiconductor paste. 
     
     
       9. A thick-film gas-sensitive element as set forth in claim 1, further comprising a layer of ceramic particles bonded to the upper surface of said ceramic substrate adjacent the ends of said electrodes, said ceramic particles being made of the same material as said ceramic substrate and said ceramic particles forming projections on said ceramic substrate and having recesses defined therebetween to provide a roughened surface portion on said ceramic substrate. .Iadd.10. A thick-film gas-sensitive element as set forth in claim 1, wherein the first metallic catalyzer is selected from the group consisting of Rh, RhPt and RhPd. .Iaddend. .Iadd.11. A thick-film gas-sensitive element as set forth in claim 1, wherein the second metallic catalyzer is selected from the group consisting of RhPt and Pt.