Abstract:
In the production of low-thermal-expansive zirconyl phosphate ceramic, whose chemical formula is (ZrO) 2 .P 2  O 7  and whose chemical composition is 2ZrO 2 .P 2  O 5  by sintering of the fine-powder compact, three kinds of reaction aids are employed. They are a densifying aid, a grain-growth-suppressing aid and a solid-solution-forming aid. The densifying aid is always employed, the grain-growth-suppressing aid is employed for producing a more thermal-resistant ceramic for high temperature use and the solid-solution-forming aid is employed for producing a more low-thermal-expansive ceramic.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for the production of high-strength and thermal-shock-resistant ceramic of phosphate, [ZrO] 2 .P 2  O 7  or 2ZrO 2 .P 2  O 5 . 
     In the ZrO 2  --P 2  O 5  -system many compounds are known, i.e. zirconium phosphate (ZrO 2 ).P 2  O 7  or 2ZrO 2 .P 2  O 5 ), NaZr 2  P 3  O 12  --type compounds, hydrated-compounds of this system and amorphous materials of this system. They have different chemical composition and different crystal structure. They are classified into two groups. The first two are used as low-thermal-expension ceramics, though the refractory temperature of (ZrO) 2 .P 2  O 7  is higher (1700° C.) than that of NaZr 2  P 3  O 12  (1300° C.). The last two are used as catalyzer or special-function-ceramics. 
     It has hitherto been known that a ceramic of phosphate polycrystalline ceramic was low-thermal-expansive. However, its powder compact alone has not densified and it has not been known whether the low thermal expansion of the sintered ceramic was due to the crystal itself or due to microcracks in the ceramic like aluminum titanate. 
     The inventor has found that the powder compact of zirconyl phosphate was able to be densified when a metal oxide was used as densifying aid, the crystal has a low thermal-expansion-coefficient, 1.7×10 -6  /°C., and the transition grain size for microcracking was 7 μm (J. Amer. Ceram. Soc., Vol. 68, No. 5, May 1985, pp. 273-278). 
     Moreover, the inventor has found that a high-strength and thermal-resistant ceramic of zirconyl phosphate could be produced when he used a grain-growth-suppressing aid, and that the solid solution of zirconyl phosphate with niobium oxide had a lower thermal-expansion-coefficient, 0.3×10 -6  /°C. at 5 wt% niobium oxide, than the pure phase. The present invention is based on these findings. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to produce a dense polycrystalline zircony phosphate ceramic. 
     It is another object of the invention to produce a more thermal-resistant zirconyl phosphate ceramic without microcracks. 
     It is a further object of the invention to produce a more low-thermal-expansive ceramic of the solid solution crystals. 
     These objects can be performed by a high temperature process always by using at least one densifying aid, moreover by using a grain-growth-suppressing aid and/or a solid-solution-forming aid according to the purpose of the use. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The feature of the present invention, therefore, consists of the production of a low-thermal-expansive zirconyl phosphate ceramic by sintering the powder compact with at least one densifying aid, moreover together with each one or both of a grain-suppressing aid and a solid-solution-forming aid. 
     In the process of the invention, the powder as a raw material is α-phase of (ZrO) 2 .P 2  O 7 , which irreversibly transforms from β-phase at above 1100° C. and which is the only thermodynamically stable phase up to 1700° C. above which decomposition occurs, so that the densifying temperature is preferably above 1200° C. and below 1700° C. Also, it is preferable that the powder is prepared by precipitation from an aqueous solution or have finer grain size than 1 μm. 
     In the process of the present invention, the densification of the powder compact is performed under the presence of a liquid phase, which is melted metal-orthophosphate formed by the reaction of zirconyl phosphate with a densifying aid, which is a metal oxide. So that the effect of the metal oxide on densification appears near the melting temperature of the metal-phosphate. The effective oxide on the densification is ZnO, MgO, Bi 2  O 3 , MnO 2 , Co 2  O 3 , NiO, TiO 2 , CeO 2 , Nb 2  O 5  ; these oxides are effective above 0.2 wt%, preferably effective between 1.0-3.0 wt% except up to 10% of Ta 2  O 5  and the property of the ceramic becomes bad over 10wt%. 
     The maximum thermal-expansion anisotropy of α-zirconyl phosphate crystal is 11.4×10 -6  /°C. of the expansion difference between the b-axis and the c-axis in the crystal. According to the anisotropy, large stress generates in the large-grain ceramic in the cooling period after firing the ceramic, then microcracks form at the grain boundary and/or in the grain, and the microcracks deteriorate the strength of the ceramic. The microcracks do not form below a certain grain size, so that a transition grain size for microcracking exists, that is 7 μm for the zirconyl phosphate. 
     In the process of the present invention, therefore, a microcrack-free ceramic, which is high-strength and thermal-resistant, is produced by suppressing the grain growth below the 7 μm with a metal oxide, among which SiO 2  was the most effective. The grain-growth-suppressing aid or its reaction product with zirconyl phosphate, e.g. ZrSiO 4 , deposits in grain boundary and then the grains are not able to contact each other and to grow. 
     In the process of the present invention, a more low-thermal-expansive ceramic is produced by sintering the powder compact of the solid solution of [ZrO] 2 .P 2  O 7  crystal with niobium oxide. The solid solution is prepared by firing a mixture of the both powders above 1200° C. and below 1400° C. above which the crystals grow abnormally. 
    
    
     The present invention will be further illustrated in detail in following Examples. It will be evident to those skilled in the art that the ratios, ingredients in the following formulations and the order of operations can be modified within the scope of the present invention. Therefore, the present invention is not to be interpreted as being limited by the following Examples. 
     EXAMPLE 1 
     The α-[ZrO] 2 .P 2  O 7  fine powder, perferably below 3 μm, was mixed with 1 wt% of fine MgO powder and 2 wt% of fine SiO 2  powder. The resultant powder was in the shape of bar, 1.5×5×0.15 cm, then the bar was sintered at 1400° C. for 24 hr. The resultant polycrystalline ceramic had 1650 kg/cm 2  of bending strength and 2.0×10 -6  /°C. of thermal-expansion-coefficient (0° C.-1000° C.), and 3.0 μm of the average grain size. 
     EXAMPLE 2-25 
     The procedure of Example 1 was repeated but various compositions of raw materials, reaction conditions as shown in Table 1 and 2 and there were thus obtained various properties of α-[Zr.O] 2 .P 2  O 7  ceramics. 
     
                       TABLE 1______________________________________Composition of Raw Materials (wt %)Example  zirconyl  densifying grain-growth-No.    phosphate aid        suppressing aid______________________________________ 2     98.0      ZnO      2.0            0 3     98.0      MgO      2.0            0 4     98.0      MgO      2.0            0 5     99.6      ZnO      0.2 SiO.sub.2  0.2 6     99.0      MgO      0.5 SiO.sub.2  0.5 7     97.0      MgO      1.0 SiO.sub.2  2.0 8     90.0      MgO      7.0 SiO.sub.2  3.0 9     94.0      MgO      2.0 ZrSiO.sub.4                                    4.010     95.0      MgO      1.0 3MgO.4SiO.sub.2.H.sub.2 O                         (Talc)     4.011     97.0      ZnO      1.0 SiO.sub.2  2.012     95.0      Bi.sub.2 O.sub.3                     2.0 SiO.sub.2  3.013     93.0      MnO.sub.2                     5.0 SiO.sub.2  2.014     92.0      Fe.sub.2 O.sub.3                     3.0 CaMgSiO.sub.4                                    5.015     93.0      Co.sub.2 O.sub.3                     3.0 Fe.sub.2 SiO.sub.4                                    4.016     96.0      NiO      2.0 SiO.sub.2  2.017     96.0      TiO.sub.2                     2.0 SiO.sub.2  2.018     93.0      CeO.sub.2                     3.0 ZnSiO.sub.4                                    4.019     94.0      MgO      1.0            CeO.sub.2                     2.0 SiO.sub.2  3.020     93.0      Ta.sub.2 O.sub. 5                     5.0 SiO.sub.2  2.021     90.0      Ta.sub.2 O.sub.5                     7.0 SiO.sub.2  3.022     95.0      Nb.sub.2 O.sub.5 *                     3.0 SiO.sub.2  2.023     93.0      Nb.sub.2 O.sub.5 *                     6.0 SiO.sub.2  2.024     90.0      Nb.sub.2 O.sub.5 *                     7.0 SiO.sub.2  3.025     92.0      Nb.sub.2 O.sub.5 *                     5.0 ZrSiO.sub.4                                    3.0______________________________________ *densifying aid and/or solidsolution-forming aid 
    
     
                       TABLE 2______________________________________Reaction Condition and Properties of Products               Average         ThermalEx-   Firing &amp; Annealing               Grain    Bending                               Expansionample Conditions    Size     Strength                               Coefficient*No.   (°C.)-(hr)               (μm)  (kg/cm.sup.2)                               (× 10.sup.-6 /°C.)______________________________________ 2    1400°-4               130       190   -0.5 3    1300°-5               3.0      1850   1.9 4    1300°-5 &amp; 1300°-6               15        510   1.0 5    1400°-2               6.0      1200   1.9 6    1400°-10               6.0      1150   2.0 7    1600°-2               6.0      1100   2.0 8    1500°-3               5.5      1450   2.4 9    1400°-5 &amp; 1400°-16               3.5      1650   2.110    1400°-24               3.5      1750   2.111    1500°-4               4.5      1550   2.312    1400°-4               4.0      1300   2.213    1400°-1 &amp; 1300°-16               3.0      1650   2.414    1300°-4               3.5      1450   2.415    1300°-4               3.0      1850   2.316    1400°-4               3.5      1350   2.217    1600°-4               5.0      1150   2.318    1600°-4               4.5      1250   2.319    1500°-6               4.0      1750   2.220    1700°-2               6.0       970   2.321    1600°-5 &amp; 1600°-4               4.0      1600   2.522    1400°-5               4.0      1650   1.723    1400°-5 &amp; 1400°-10               3.5      1750   0.624    1500°-4               4.0      1550   0.925    1600°-2               5.5      1350   0.8______________________________________ *0°-1000° C.