Abstract:
Nanoscalar, pyrogenically produced yttrium-zirconium mixed oxide which has a BET surface area of between 1 and 600 m 2 /g, a total chloride content of less than 0.05 wt. % and does not exhibit a conversion into the monoclinic phase during storage is produced by atomizing yttrium compounds and zirconium compounds and reacting them in a flame. The mixed oxide can be used as ceramic raw material.

Description:
REFERENCE TO A RELATED APPLICATION 
     Provisional application 60/310,836 filed Aug. 9, 2001 is relied on and incorporated herein by reference. 
    
    
     INTRODUCTION AND BACKGROUND 
     This invention relates to nanoscalar, pyrogenically produced yttrium-zirconium mixed oxide, a process for its production, as well as its use. 
     The production of pyrogenic oxides and mixed oxides by flame hydrolysis of vaporisable metal chlorides or metalloid chlorides is known. (Ullmanns EnzykIopadie der technischen Chemie, 4th Edition, Volume 21, page 44 (1982)). 
     Also known is a process for producing nanoscalar, pyrogenically produced oxides and/or mixed oxides of metals and/or metalloids, which is characterised in that organometallic and/or organometalloid substances, optionally dissolved in a solvent, optionally in a flame, are converted into the oxides at temperatures of above 200° C. The educts may be organometalloid and/or organometallic pure substances or any mixtures of these, or be used as solutions in organic solvents (EP 001 07 237.0-2111). 
     Zirconium oxide produced by this process has the disadvantage that the original tetragonal phase rearranges to form the monoclinic phase after a month, even during normal storage. This conversion runs parallel with a volume expansion. Because mouldings are destroyed during this process, the use of these products in ceramic applications is ruled out. 
     An object, therefore, of the present invention is to produce nanoscalar, pyrogenically produced zirconium oxide which does not have these disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides nanoscalar, pyrogenically produced yttrium-zirconium mixed oxide which has a BET surface area of between 1 and 600 m 2 /g and a total chloride content of less than 0.05 wt. %, preferably of less than 0.02 wt. %, and does not exhibit a conversion into the monoclinic phase during storage at room temperature, nor even during calcination (ca. 1000° C.). The yttrium-zirconium mixed oxide according to the invention exhibits a stable tetragonal phase. By nanoscalar yttrium-zirconium mixed oxides are meant those having a particle size of equal to or less than 100 nanometers. 
     The invention also provides a process for producing the nanoscalar, pyrogenically produced yttrium-zirconium mixed oxide, which is characterised in that yttrium compounds and zirconium compounds, optionally dissolved or dispersed in a solvent, are atomized and then converted into the yttrium-zirconium mixed oxide in a flame, preferably an oxyhydrogen flame, at temperatures of above 200° C. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention will be further understood with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic flow diagram illustrating the process of the invention, and 
         FIG. 2  is a schematic representation of an atomizer nozzle suitable for use in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The present invention will now be described in further detail. 
     Suitable compounds of yttrium and of zirconium can be introduced, in the form of a very finely divided liquid spray, into a high-temperature reaction chamber. In the high-temperature reaction chamber, which is designed preferably in the form of a closed flow tube, particle formation can take place at temperatures of above 200° C., possibly with inert or reactive gases being additionally introduced as carrier gas into the high-temperature reaction chamber, and the powder can be recovered by known methods of gas-solid separation, by means of filters, cyclones, washers or other suitable separators. 
     For this, solutions of organometallic and/or organometalloid substances (precursors) in organic solvents, or even the pure substances (precursors), can be converted into the oxides, optionally in a flame, at elevated temperatures, optionally above 200° C. 
     The precursors used may be compounds of the type MeR, wherein R represents an organic group such as, for example, methyl, ethyl, propyl, butyl, or the corresponding alkoxy variants. 
     The solvents used may be organic solvents, such as alcohols, particularly lower alcohols for example, propanol, n-butanol, isopropanol, and/or water. 
     The zirconium can also be introduced into the flame in the form of an aqueous dispersion of zirconium dioxide. 
     The precursor can be introduced at a pressure of 1 to 10000 bar, preferably of 2 to 100 bar. Amorphous particles and compact spheres can be produced at a temperature of at least 200° C. Fine particles can be obtained at a temperature of 1800° C. to 2400° C. 
     An advantage of the process according to the invention is that the precursor can be introduced into the combustion chamber not in gaseous form, but as a liquid. Here, at pressures of up to 10000 bar it is possible, using at least one single-fluid nozzle, to produce a spray of very fine droplets (average size of droplets, depending on the pressure in the nozzle, is from &lt;1-500 μm), which then undergoes combustion and yields the yttrium-zirconium mixed oxide in solid form. At pressures of up to 100 bar, at least one two-fluid nozzle can be used. 
     The droplets can be produced by using one or more two-fluid nozzles and the gas used in the two-fluid atomization process may be reactive or inert. 
     The advantage ensuing from the use of a two-fluid nozzle is that the droplets are produced by means of a gas jet. This gas jet may contain oxygen or nitrogen. A very intensive mixing of the oxidizing agent with the precursor can thereby be achieved. An additional supply of fuel can also be introduced into the immediate environment of the droplets, should the precursor be unreactive or the vapour pressure of the precursor not be high enough to ensure a rapid reaction. 
     Through the use of organometallic precursors in solvents, homogeneous solvent mixtures of various compounds corresponding to the formula MeR (precursor) can be easily prepared in any ratios of concentrations and are passed preferably in liquid form to a flame, in order to obtain the corresponding pyrogenic mixed oxides which are low in chloride. Yttrium-zirconium mixed oxides which, owing to the sharply differing evaporation behavior of the raw materials, previously were difficult to synthesize or could not be synthesized at all, are easily accessible by means of the process according to the invention. 
     Another advantage of the process according to the invention is that not only can the liquid precursor be mixed with other liquid precursors, but optionally fine particles, for example, pyrogenic oxides such as zirconium oxide, can also be dispersed in the precursor and thereby a coating of the particles dispersed in the precursor can be obtained during the reaction. 
     The conversion of the precursors into the oxides is carried out preferably in an oxyhydrogen flame. Besides hydrogen, other combustible gases such as, for example, methane, propane, ethane, may be used. 
     As the organometallic precursors are themselves a good fuel, a further advantage of the process according to the invention is that one can dispense completely with the supporting flame, with a consequent saving of expensive raw materials, for example, hydrogen. 
     Moreover, the properties of the oxides, for example, the BET surface area, can be influenced by varying the quantity of air (for the combustion) and/or by altering the nozzle variables. 
     The pyrogenically produced yttrium-zirconium mixed oxide according to the invention can be used as filler, as supporting material, as a catalytically active substance, as the starting material for preparing dispersions, as polishing material for polishing metal discs or silicon discs in the electronics industry (CMP application), as ceramic raw material for gas sensors or in the fuel cell or as dental material or for preparing membranes, in the cosmetics industry, as an additive in the silicone and rubber industry, for adjusting the rheology of liquid systems, for heat stabilization, in the coatings industry, as colored pigment, as heat-insulating material and as an antiblocking agent. 
     EXAMPLES 
     The zirconium precursors used, Zr(NO 3 ) 4 , Zr(O-n-C 3 H 7 ) 4  or zirconium oxide sol, were reacted in a hydrogen flame according to the process described in US 2001/0036437 and US 2003/0206854 which is are relied on and incorporated herein by reference. 
     The arrangement of burners which can be used according to the invention is shown diagrammatically in  FIG. 2 . 
     The solutions containing the Zr-precursors and Y-precursors as given in Table 1 were atomized into the reaction tube by means of a nozzle, under nitrogen pressure. The reaction tube contained a burning oxyhydrogen flame, consisting of hydrogen and air. The temperature 0.5 m below the flame was 800 to 1000° C. The yttrium-zirconium mixed oxide obtained was removed in filters. The properties of the products obtained are listed in Table 2. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Zr-precursor 
                 Solvent for Zr- 
                 Solution of Y- 
                 Solvent for 
                   
               
               
                 Example 
                 wt. % 
                 precursor 
                 precursor wt. % 
                 Y-precursor 
                 Nozzle 
               
               
                   
               
             
             
               
                 1 
                 Zr(NO 3 ) 4  15% 
                 H 2 O 
                 Y(NO 3 ) 3  10% 
                 H 2 O 
                 2-fluid 
               
               
                 2 
                 ZrO 2  30% 
                 H 2 O 
                 Y(NO 3 ) 3  10% 
                 H 2 O/n- 
                 2-fluid 
               
               
                   
                   
                   
                   
                 propanol 
               
               
                 3 
                 Zr(O-n-C 3 H 7 ) 4   
                 n-Propanol 
                 Y (NO 3 ) 3  10% 
                 n-propanol 
                 2-fluid 
               
               
                   
                 74% 
               
               
                 4 
                 Zr(O-n-C 3 H 7 ) 4   
                 n-propanol 
                 Y(NO 3 ) 3  10% 
                 H 2 O 
                 3-fluid 
               
               
                   
                 74% 
               
               
                 5 
                 Zr(O-n-C 3 H 7 ) 4   
                 n-propanol 
                 Y(NO 3 ) 3  10% 
                 methanol 
                 3-fluid 
               
               
                   
                 74% 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Y 2 O 3  wt. %, 
                   
                   
               
               
                   
                 BET m 2 /g 
                 Phase a) RT b) 
                 based on the 
                   
                 Cl 
               
               
                   
                 a) RT b) 
                 900° C. for two 
                 total mixed 
                 Particle size nm 
                 content 
               
               
                 Example 
                 800° C. 
                 hours 
                 oxide 
                 (TEM) 
                 % 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 a) 11 b)8.4 
                 a) tetragonal b) 
                 3.5 
                 50 
                 0.02 
               
               
                   
                   
                 tetragonal 
               
               
                 2 
                 a) 2.4 
                 a) tetragonal 
                 5.9 
                   
                 0.01 
               
               
                 3 
                 a) 60 b) 29 
                 a) tetragonal b) 
                 3.0 
                 12 
                 0.02 
               
               
                   
                   
                 tetragonal 
               
               
                 4 
                 a) 43 b) 31 
                 a) tetragonal b) 
                 5.7 
                 20 
                 0.01 
               
               
                   
                   
                 tetragonal 
               
               
                 5 
                 a) 68 b) 42 
                 a) tetragonal b) 
                 5.3 
                 14 
                 0.01 
               
               
                   
                   
                 tetragonal 
               
               
                   
               
             
          
         
       
     
     Further variations and modifications of the foregoing will be apparent to those skilled in the art and are intended to be encompassed by the claims appended hereto. 
     German priority application 101 38 573.0 is relied on and incorporated herein by reference.