Process for the preparation of semiconducting ceramics composed of metal oxides, in particular of tin oxide, especially for varistors

Abstract

Process for the preparation of a semiconducting ceramic based on doped tin oxide SnO 2 by a process of “PADO” (Precursor Alloy Direct Oxidation) type applied to an alloy of tin and of doping metals or by a process of “PADO” type applied to tin, the doping metals being added in the form of oxides to the powder subjected to sintering.

1 . A process for preparing a substantially homogenous semiconducting ceramic composition comprising base metal oxide and at least one doping metal oxide, the process comprising: 
 a) melting and mixing under a neutral or reducing atmosphere said base metal oxide and doping metals and/or salts of doping metals, in order to obtain a homogeneous liquid mixture;    b) preparing a powder from said homogenous liquid mixture with a predetermined particle size, directly or after an optional operation of particle size classification, each of the particles of said powder being homogeneous; and    c) completely or partially oxidizing said powder to obtain a homogeneous powder which is completely or partially oxidized, thereby preparing said substantially homogenous semiconducting ceramic composition comprising a base metal oxide and at least one doping metal oxide.    
   
   
       2 . The process according to  claim 1 , wherein the oxide or oxides of doping metals are selected from the group consisting of oxides of cobalt, chromium, manganese, niobium, tantalum, and transition metals.  
   
   
       3 . A process according to  claim 1 , wherein the oxide or oxides of doping metals are selected from the group consisting of cobalt, manganese, niobium and tantalum oxides.  
   
   
       4 . A process according to  claim 1 , wherein the doping metal oxides are selected from the group consisting of cobalt oxide, manganese oxide, niobium oxide and tantalum oxide.  
   
   
       5 . A process according to  claim 1 , wherein the proportion by weight of the doping metal or metals and/or salt or salts of doping metals in step a) is selected from the group consisting of 90% by weight, 95% by weight, 99% by weight, and 99.995% by weight.  
   
   
       6 . A process according to  claim 1 , wherein the doping metal or metals and/or salt or salts of doping metals in step a) comprises one or more among the following oxides in the following proportions by weight: 
 0.1 to 3% of cobalt oxide,    0.01 to 3% of chromium oxide,    0.01 to 3% of manganese oxide,    0.01 to 0.5% of niobium oxide,    0.01% to 0.5% of tantalum oxide, or    0.01 to 0.5% of one or more transition metal oxides,    0.01 to 0.5% of one or more lanthanide metal oxides, such as lanthanum oxide.    
   
   
       7 . A process according to  claim 1 , wherein the said doping metal or metals and/or salt or salts of doping metals in step a) comprises the following oxides in the following proportions by weight: 
 SnO 2 : 98.24%;    Co 3 O 4 : 0.05%;    MnO 2 : 1.69%;    Nb 2 O 5 : 0.01%; or    Ta 2 O 5 : 0.01%.    
   
   
       8 . A process according to  claim 1 , wherein the powder of step b) is prepared by cooling, optionally rapidly, the liquid mixture of step a).  
   
   
       9 . A process according to  claim 8 , wherein step a) further comprises casting the liquid mixture in an ingot mould under a neutral or reducing atmosphere prior to the cooling of step b).  
   
   
       10 . A process according to  claim 1 , wherein step b) comprises spraying or atomizing by a stream of gas or of liquid, and quenching quenched.  
   
   
       11 . A process according to  claim 9 , wherein step b) comprises spraying or atomizing with a stream of gas or of liquid, and quenching.  
   
   
       12 . A process according to  claim 10  or  11 , wherein in the homogeneous liquid is sprayed or atomized with a stream of neutral or reducing gas, such as hydrogen, nitrogen, argon or their mixtures.  
   
   
       13 . A process according to  claim 10  or  11 , wherein the homogeneous liquid is sprayed or atomized with a stream of oxidizing gas, such as air, air enriched in oxygen, or oxygen optionally enriched in water vapour.  
   
   
       14 . A process according to either one of claims  8  and  9 , wherein the powder of step b) is prepared by abrasion or milling.  
   
   
       15 . A process according to  claim 1 , wherein the powder of step b) is separated into several particle size fractions.  
   
   
       16 . A process according to  claim 1 , wherein the powder in step c) is partially oxidized to a percentage selected from the group consisting of 50 to 99.9% by weight, 55 to 80 or 85% by weight, and 60 to 70% by weight.  
   
   
       17 . A process according to  claim 1 , wherein the powder in step c) is oxidized to a percentage of 64% by weight.  
   
   
       18 . A process according to  claim 1 , wherein the partial or complete oxidation of step c is carried out by bringing the said particles into contact with an oxidizing gas from a temperature and/or for a period of time which is (are) sufficient to produce a desired percentage of oxides of metals in the powder, for example for the oxidation to be complete.  
   
   
       19 . A process according to  claim 18 , wherein the oxidizing gas comprising oxygen is optionally enriched in water vapour and/or in carbon dioxide.  
   
   
       20 . A process according to claims  18  or  19 , wherein the oxidation of step c) is carried out at a temperature of greater than or equal to 400° C., for a period of time of 1 second to 6 hours.  
   
   
       21 . A process according to  claim 1  further comprising shaping the product of step c) by compacting it in the form of pellets.  
   
   
       22 . A process according to  claim 21 , wherein the shaping of step d) is carried out at a temperature of 1100 to 1350° C. for a period of time of greater than or equal to 30 minutes.  
   
   
       23 . A process for the preparation of a semiconducting ceramic comprising a base metal oxide and at least one doping metal oxide the following successive steps: 
 tin is placed in a crucible;    the tin is melted, so as to obtain liquid tin;    a tin powder with a predetermined particle size is prepared from the liquid tin, directly or after an optional operation of particle size classification;    all the tin powder with a predetermined particle size or a predetermined particle size fraction of the powder is completely or partially oxidized;    one or more doping metal oxide(s) is (are) added to the partially or completely oxidized tin powder;    the partially or completely oxidized tin powder is mixed with the doping metal oxide(s);    the mixture of partially or completely oxidized tin powder and of the doping metal oxide(s) is shaped;    the shaped powder is sintered.    
   
   
       24 . A process according to  claim 23 , wherein the doping metal oxide or oxides added to the completely or partially oxidized tin powder are from the group consisting of cobalt, chromium, manganese, niobium and tantalum oxides and oxides of transition metals.  
   
   
       25 . A process according to  claim 23 , wherein the oxide or oxides of doping metals are selected from the group consisting of cobalt, manganese, niobium and tantalum oxides.  
   
   
       26 . A process according to  claim 25 , wherein the oxide or oxides of doping metals are selected from the group consisting of doping metal oxides, cobalt oxide, manganese oxide, niobium oxide and tantalum oxide.  
   
   
       27 . A process according to  claim 23 , wherein the tin oxide is added in a proportion of greater than or equal to a percentage selected from the group consisting of 90% by weight, 95% by weight, 99% by weight, and 99.995% by weight.  
   
   
       28 . A process according to  claim 27 , wherein the doping metal oxide(s) are added to the partially or completely oxidized tin powder in a percentage selected from the group consisting of: 
 0.1 to 3% of cobalt oxide,    0.01 to 3% of chromium oxide,    0.01 to 3% of manganese oxide,    0.01 to 0.5% of niobium oxide,    0.01% to 0.5% of tantalum oxide,    0.01 to 0.5% of one or more transition metal oxides, and    0.01 to 0.5% of one or more lanthanide metal oxides.    
   
   
       29 . A process according to  claim 28 , wherein the doping metal oxides are added to the partially or completely oxidized tin powder in a percentage by weight such that it makes it possible to obtain a ceramic comprising, as complement to 100% by weight of tin oxide, the following oxides in the following proportions by weight: 
 SnO 2 : 98.24%;    Co 3 O 4 : 0.05%;    MnO 2 : 1.69%;    Nb 2 O 5 : 0.01%; or    Ta 2 O 5 : 0.01%.    
   
   
       30 . A process according to  claim 23 , wherein the tin powder with a predetermined particle size is prepared in the liquid phase by melting bulk tin to give a liquid metal which is sprayed or atomized by a stream of gas or of liquid and cooled, quenched.  
   
   
       31 . A process according to  claim 23 , wherein the liquid metal is sprayed or atomized by a stream of neutral or reducing gas.  
   
   
       32 . A process according to  claim 23 , wherein the tin powder with a predetermined particle size is prepared in the solid phase by abrasion or milling.  
   
   
       33 . A process according to  claim 23 , wherein the tin powder is partially oxidized to a percentage selected from the group consisting of 50 to 99.9% by weight, 55 to 80 or 85% by weight, and 60 to 70% by weight.  
   
   
       34 . A process according to  claim 23 , wherein the tin powder is oxidized to a percentage of 64% by weight.  
   
   
       35 . A process according to  claim 23 , wherein the tin powder is separated into several particle size fractions.  
   
   
       36 . A process according to  claim 23 , wherein the tin powder with a predetermined particle size is partially or completely oxidized by bringing the said powder into contact with an oxidizing gas from a temperature and/or for a period of time which is (are) sufficient to produce a desired percentage of tin oxide in the powder.  
   
   
       37 . A process according to  claim 36 , wherein the operation in which the particles are brought into contact with the oxidizing gas is carried out at a temperature of greater than or equal to 400° C., for a period of time of 1 second to 6 hours.  
   
   
       38 . A process according to  claim 36  or  37 , wherein the oxidizing gas is chosen from gases comprising oxygen optionally enriched in water vapour and/or in carbon dioxide.  
   
   
       39 . A process according to  claim 23 , wherein in which the mixture of the partially or totally oxidized tin powder and of the doping metal oxide(s) is shaped by compacting it in the form of pellets.  
   
   
       40 . A process according to  claim 23 , wherein the sintering is carried out at a temperature of 1100° C. to 1350° C. for a period of time of greater than or equal to 30 minutes.  
   
   
       41 . The composition produced according to the process of  claim 1 .  
   
   
       42 . The composition produced according to the process of  claim 23.