Abstract:
A ceramic composite contains one spinel phase and one or more than one oxides phases. The composite is derived from the mixtures of two oxides, A 2  O 3  and BO, with different molar ratio, in which A and B are metallic ions of three and two valences respectively. The oxides are heated at a high temperature to form a composite containing a spinel and residual oxides. The formed spinel and residual oxides also undergo densification under high temperature. After the treatment at high temperature, not only the spinel is formed but also the composite is densified. The performance of the composite is superior in its strength, toughness and high-temperature stability.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a ceramic composite, particularly a ceramic composite containing a spinel phase and one or more oxide phases. 
     BACKGROUND OF THE INVENTION 
     Ceramics have great potential in industrial applications because they are superior in their hardness, chemical stability and refractory characteristics. Currently, most ceramics used in the industry are single phase ceramics. However, when using these single phase ceramics, they have the following drawbacks: 
     1. It is difficult to produce dense single phase ceramics with a small grain size. The strength of ceramics is proportional inversely to the square root of the grain size. That is, the strength becomes greater as the grain becomes smaller. The strength of ceramics, on the other hand, will decrease drastically as the amount of porosity increases. Therefore, the ceramics must be completely densified. During the densification process, however, the grainls often grow rapidly and the strength is affected as a result. 
     2. The high temperature stability is not satisfactory. When the single phase ceramics are used at high temperatures, their grains will grow rapidly. Therefore, the strength of ceramics will be decreased the longer they are in use. 
     3. Other than some partially stabilized zirconium oxides, the toughness of single phase ceramics usually is not high. Low toughness causes the ceramics to become brittle. This situation results in increased difficulties when using these single phase ceramics. 
     In order to overcome the above drawbacks, multi-phase ceramics, i.e. ceramic composites, have been developed. A ceramic composite consists of adding a ceramic additive into another ceramic base. The blended additive, correctly composed, can hinder the growth of grains and create a final product with a smaller grain size and better high temperature stability. Furthermore, because the additive has a different Young modulus than that of the ceramic base, the cracks will propagate in a twisted way. This will increase the toughness of the product. Currently, on the market, the ceramic composites include alumina/SiC whiskers and alumina/zirconia composites. The two phases in these systems do not react at high temperature or have only a limited reaction. These composites often have superior properties than the single phase ceramics. The drawback, however, is that they are more expensive. 
     Because of the single phase ceramics&#39; drawbacks and the expense for the composites, the inventor, after long research, has invented an improved ceramic composite. 
     The main purpose of the invention is to provide a ceramic composite with excellent properties to be used in industry and to provide a preparation method to produce the ceramic composite. The advantages of this method include the use of cheap raw materials and simple, easily controlled production steps. 
     SUMMARY OF THE INVENTION 
     In order to achieve the above objectives, the ceramic composite according to the invention comprises: 
     1-99 weight % of a spinel phase; and 
     99-1 weight % of an oxide phase, 
     in which the oxide phase comprises an A 2  O 3  oxide, a BO oxide, or both, and the spinel phase comprises a BA 2  O 4  spinel; and wherein A is a three-valence metal ion and B is a two-valence metal ion. 
     Selectively, the oxide phase of the ceramic composite of the present invention further contains a M 2  O 3  oxide; and the spinel phase further contains a BM 2  O 4  spinel and B(A y  M 2-y )O 4  spinel, in which M is a three-valence metal ion and different from A and 0&lt;y&lt;2. 
     Selectively, the oxide phase of the ceramic composite of the present invention further contains a DO oxide and the spinel phase further contains a DA 2  O 4  spinel and (B x  D 1-x )A 2  O 4  spinel, in which D is a two-valence metal ion and different from B and 0&lt;x&lt;1. 
     The metal ion A of the ceramic composite of the present invention is selected from the three-valence metal ion group consisting of aluminum, chromium, and iron. 
     The metal ion M of the ceramic composite of the present invention is selected from the three-valence metal ion group consisting of aluminum, chromium, and iron. 
     The metal ion B of the ceramic composite of the present invention is selected from the two-valence metal ion group consisting of nickel, zinc, magnesium, iron, cobalt, manganese, and copper. 
     The metal ion D of the ceramic composite of the invention is selected from the two-valence metal ion group consisting of nickel, zinc, magnesium, iron, cobalt, manganese, and copper. 
     The BA 2  O 4  of the ceramic composite of the invention, for example, is a nickel aluminate spinel, zinc aluminate spinel, magnesium aluminate spinel, cobalt aluminate spinel, zinc chromate spinel, magnesium chromate spinel, iron chromate spinel, zinc ferrite spinel, manganese ferrite spinel or copper ferrite spinel. 
     A suitable method of preparing the ceramic composite of the present invention comprises the following steps: 
     (a) mixing one or more oxide powders from each type of A 2  O 3  and BO oxide powder to give a mixed powder; 
     (b) slip casting, die pressing, or cold-isostatic pressing the mixed powder to form a green body; and 
     (c) calcining the green body at a temperature from 1000° C. to 2000° C. to form a ceramic composite containing a BA 2  O 4  spinel phase and an oxide phase; 
     in which A and B are defined as above. 
     In step (a) of the above preparation method, the molar mixing ratio of A 2  O 3  and BO oxide powders is not equal to 1.0. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     This invention uses two or more oxides from two different types of oxides A 2  O 3  and BO as the starting materials (A and B are defined as above) and these oxides will react at high temperature to produce a ceramic composite with excellent properties. The high temperature reaction of this invention is the spinel reaction. For example, 
     
         A.sub.2 O.sub.3 +BO→BA.sub.2 O.sub.4, 
    
      in which A 2  O 3  and BO are oxides and BA 2  O 4  is a spinel. The reaction of synthetic spinel can occur among oxides in the table below. 
     
                       TABLE 1______________________________________A.sub.2 O.sub.3    +         BO     →  BA.sub.2 O.sub.4______________________________________Al.sub.2 O.sub.3          NiO            NiAl.sub.2 O.sub.4Al.sub.2 O.sub.3          ZnO            ZnAl.sub.2 O.sub.4Al.sub.2 O.sub.3          MgO            MgAl.sub.2 O.sub.4Al.sub.2 O.sub.3          FeO            FeAl.sub.2 O.sub.4Al.sub.2 O.sub.3          CoO            CoAl.sub.2 O.sub.4Cr.sub.2 O.sub.3          ZnO            ZnCr.sub.2 O.sub.4Cr.sub.2 O.sub.3          MgO            MgCr.sub.2 O.sub.4Cr.sub.2 O.sub.3          FeO            FeCr.sub.2 O.sub.4Fe.sub.2 O.sub.3          ZnO            ZnFe.sub.2 O.sub.4Fe.sub.2 O.sub.3          MnO            MnFe.sub.2 O.sub.4Fe.sub.2 O.sub.3          CuO            CuFe.sub.2 O.sub.4______________________________________ 
    
     Listings on Table 1 are only examples. When the molar ratio of A 2  O 3  and BO is not 1:1, besides the formation of spinel, residual oxides with a higher mole number will be present. Three types of oxides can be used to form spinel. For example, magnesium oxide, aluminum oxide, and chromium(III) oxide can form Mg(Al y  Cr 2-y )O 4  spinel, in which 0&lt;y&lt;1. 
     The spinel-containing ceramic composite prepared by this invention can have a spinel content of 1% to 99% (weight) and the residual oxides are 1% to 99% (weight). 
     A method suitable to prepare the spinel-containing ceramic composite of this invention is, for example, selecting one or more oxides of the A 2  O 3  and BO oxides, in which the A 2  O 3  oxide can be a three-valence oxide such as aluminum oxide, chromium(III) oxide, iron(III) oxide, etc.; the BO oxide can be a mono-oxide such as nickel oxide, zinc oxide, magnesium oxide, iron(II) oxide, cobalt(II) oxide, copper(II) oxide, etc. These oxides have a different mole number. The powders are ground with a ball mill and mixed. The material can be formed into a particular shape in a plaster mold by slip casting. Or after drying, the powder mixture can be poured into a steel mold to be pressed into a particular shape. Or the powder mixture is poured into a plastic mold and pressed into a particular shape by cold-isostatic pressing. The formed green body is then put into a high temperature furnace and heated. The spinel reaction occurs at this stage. Furthermore, because the oxides have different mole numbers, there will be one or more oxides that have not reacted completely and which remain in the composite. These residual phases and the newly formed spinel are maintained at the high temperature for a period of time. This increases the density. In the end, a dense spinel-containing ceramic composite is obtained. 
     EXAMPLES 1˜5 
     A nickel oxide powder (NiO, Johnson Matthey Co., USA) with an average diameter of 16 μm and an aluminum oxide powder (TM-DR, Ta mei Chem. Co. Ltd., Tokyo, Japan) with an average diameter of 0.2 μm, under the premise that the two were at different mole numbers and a different ratio, were added into a ball mill. Partially stabilized zirconium oxide balls and alcohol were also added into the ball mill. The powder mixture was ground for 4 hours. After being dried, the powder was poured into a steel mold and a uniaxial pressure of 20 MPa was applied to form a rectangular specimen. The specimen was put in a plastic mold and pressurized to 250 MPa by cold-isostatic pressing to strengthen the green compact. The Green compact was put into a sintering furnace and sintered at 1700° C. for one hour. A dense specimen was prepared. The specimen density was measured by the Archimedes&#39; method. The specimen was polished and ground with a diamond paste. The specimen was thermally etched by heat treatment at 1450° C. for half an hour so as to reveal the grain boundaries. The grain size was measured by using the linear intercept technique. The results are shown in Table 2. 
     
                                           TABLE 2__________________________________________________________________________                                   Grain Size in Each Phase of theWeight Percent of Starting              In the Ceramic Composite                                   Ceramic Composite, μmMaterials                 Nickel                           Density of the  NickelAluminum      Aluminum                     Aluminate                           Ceramic Aluminum                                           AluminateExampleOxide %      Nickel Oxide %              Oxide %                     Spinel %                           Composite                                   Oxide   Spinel__________________________________________________________________________1    100   0       100    0     98.7    15.1    --2    95.6  4.4     90.8   9.2   98.8    10.7    7.13    87.1  12.9    72     28    98.2    10.0    9.44    79.7  20.3    56     44    96.6    12.5    9.45    56.6  43.4    0      100   97.8    --      16.8__________________________________________________________________________ 
    
     Comparing the ceramic composite in Table 2 with the single phase aluminum oxide (Example 1) and the single phase nickel aluminate spinel (Example 5), the rate of grain growth in the ceramic composite is slower. Therefore, the ceramic composite has better high temperature stability than the single phase material. 
     EXAMPLES 6˜8 
     The specimens used in Examples 6 to 8 were prepared by the same procedures as the specimens used in Examples 1 to 5. Strength values of Example 6 to 8 specimens are listed on Table 3. These values are measured by the four-point bending method (JIS R1601). 
     
                       TABLE 3______________________________________Weight Percent of Starting                   Strength at RoomMaterials, %            Temperature,Example  Aluminum Oxide                Nickel Oxide                           MPa______________________________________6      100           0          2867      95.6          4.4        3098      79.7          20.3       309______________________________________ 
    
     Table 3 shows that strength of the ceramic composites of Examples 7 and 8 is greater than that of the single phase ceramic. 
     EXAMPLES 9˜10 
     76 mole % of aluminum oxide powder (AES11, Sumitomo Co., Japan) and 24 mole % of zinc oxide powder (ZnO, CML Co., Australia) were added into a ball mill. Partially stabilized zirconium oxide balls and alcohol were also added into the ball mill. The powder mixtures were ground for 2 hours. After being dried, the powder mixtures were die pressed at 90 MPa followed by sintering to obtain a dense specimen. The sintering temperature was 1650° C. and was maintained at the temperature for 5 hours. The specimen density was measured by the Archimedes&#39; method and its strength was measured by the four-point bending method according to JIS R1601. 
     
                                           TABLE 4__________________________________________________________________________Before Sintering        After Sintering(1650° C./5 hours)Aluminum Oxide     Aluminum Oxide                            Zinc Aluminate                                    Density of theExampleMole %   Zinc Oxide Mole %                   Mole %   Spinel, Mole %                                    Composite, %                                            Strength,__________________________________________________________________________                                            MPa 9   100       0        100       0      98.6    24110    76      24         68      32      98.9    377__________________________________________________________________________ 
    
     Example 10 on Table 4 shows that the spinel ceramic composite has better strength than that of the single phase ceramic of Example 9. 
     EXAMPLE 11˜13 
     The specimens used in Examples 11 to 13 were prepared by the same procedures as the specimens used in Examples 1 to 5. After specimens of Example 11 to Example 13 were polished, a Vickers&#39; diamond indenter was nailed onto the polished surface of the specimens. The formula derived by B. R. Lawn et. al. (B. R. Lawn, A. G. Evans, D. B. Marshall, J. Amer. Ceram. Soc., 63, 574, 1980) was used to calculate the fracture toughness values (K IC ) of the specimens. The Young Modulus required by the calculation was obtained by the Ultrasonic Resonance Method. 
     
                                           TABLE 5__________________________________________________________________________Volume % In Each Phaseof the Ceramic Composite  Young Modulus,                              K.sub.IC Value,ExampleAluminum Oxide         Nickel Aluminate Spinel                     GPa      MPa__________________________________________________________________________11   100       0          392      3.012   72       28          323      3.613   56       44          295      3.7__________________________________________________________________________ 
    
     From Table 5, it can be seen that the spinel ceramic composite of Examples 12 and 13 have higher toughness than that of the single phase ceramic of Example 10. 
     To sum up, the spinel-containing ceramic composite of this invention has the advantages of high strength and toughness. Also because the grain growth rate of the composite in a high temperature environment is slower than that of the single phase ceramic, the composite has better high temperature stability. Therefore, the composite has great potential in high temperature applications. The spinel-containing ceramic composite can be made by using cheap oxides as the starting material. The oxides are mixed at different mole ratios and processed at high temperature to obtain the composite. Therefore, the ceramic composite of this invention has high commercial value and meets the elements of an invention application. Any variation or modification according to the characteristics and spirit of the claims of this invention belongs to the scope of this invention.