Patent Publication Number: US-5294576-A

Title: Mullite ceramic compound

Description:
This application is a continuation-in-part of application Ser. No. 07/728,017, filed Jul. 8, 1991, now abandoned, which is a continuation of application Ser. No. 07/299,547, filed Jan. 11, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a mullite ceramic composition, and more particularly, to a mullite ceramic composition suitable for use as a material for electronic parts, such as a circuit board or a ceramic package for a semiconductor device. 
     2. Description of the Related Art 
     Current requirements for a high signal propagation speed in electronic parts, high-density packaging, and low cost, mean that a package and a substrate material which form the electronic parts must have a low dielectric constant, a thermal expansion coefficient close to that of a semiconductor element, a high mechanical strength, a high thermal conductivity, and a low production cost. 
     To satisfy these requirements ceramics such as an aluminum nitride ceramic, a silicon carbide ceramic, a low-temperature-fireable ceramic and a mullite ceramic have been developed. 
     The mullite ceramic is fired (or sintered) by adding in solid solution an alkaline earth element compound such as magnesium oxide to mullite powder as a sintering aid, but in a flattering process using a reburning step to straighten a bent fired ceramic, a problems arises in that flecks or spots are generated on the surface of or within the ceramic. 
     Particularly, in a white mullite ceramic, the generation of flecks greatly deteriorates the value thereof as a commercial product, and even if a coloring agent is added to the mullite ceramic, inhomogeneity of color occurs due to the generated flecks. 
     The present inventors have developed a mullite ceramic composition using a rare earth element compound such as yttrium oxide as a substitute for the alkaline earth element compound, as a sintering aid, to improve the mechanical strength of the above-mentioned conventional mullite ceramic, as disclosed in JP-A-62-140444. 
     Nevertheless, although this mullite ceramic has a high mechanical strength (flexural strength) flecks are still generated on the surface of or within the ceramic. 
     Further, to obtain a strong brazing adhesion to a metallized layer formed on a mullite ceramic surface, a large amount of the rare earth element compound is required, but this inclusion of a large amount of the rare earth in the compound causes flecks to be generated in a firing step before the flattering process. Further, even if the amount of rare earth contained is reduced, although flecks are not generated in the first firing step, such flecks are generated in the flattering step using a second firing process. 
     Further, flecks are easily generated when the firing temperature is relatively low, and therefore, it is difficult to set the most suitable firing conditions. 
     Furthermore, even if a coloring agent is added to the ceramic, the same problem arises, i.e., inhomogeneity of color caused by the flecks. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a mullite ceramic composition wherein flecks causing inhomogeneity are not generated in a sintered body, and having other improved properties. 
     According to the present invention there is provided a mullite ceramic composition which comprises mullite powder as a raw material; at least one of rare earth element and alkaline earth element compounds; and at least one of the Periodic Table Group VB compounds of a vanadium compound, a niobium compound and a tantalum compound, said mixture being fired. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will now be explained. 
     The mullite ceramic composition of the present invention comprises a mullite powder, at least one of rare earth element and alkaline earth element compounds added as a sintering aid, which are conventional additives, and further comprises at least one of a the Group VB selected from a vanadium compound, a niobium compound, and a tantalum compound. 
     According to the present invention, V 2  O 5 , VF 5 , and VCl 3  vanadium compounds are preferably used. Also Nb 2  O 5 , NbF 5 , and NbCl 5  niobium compounds, and Ta 2  O 5 , TaF 5 , and TaCl 5  tantalum compounds are preferably used. &#34;In the mullite&#34; ceramic according to the present invention, no conventional flecks causing the inhomogeneity were generated, even though a flattering process was carried out, the amount of yttrium oxide was increased, and the firing temperature was varied. Further, even when a coloring agent such as MoO 3  was applied to the mullite ceramic of the present invention, no generation of flecks or spots occurred. 
     Although the reason for the appearance of the fleck or the spot within or on the surface of the conventional fired mullite is not clear, it is thought that the sintering aid, e.g., rare earth element compound, reacts with other elements to form a covalent composition, at a temperature of 1250° C. or less, according to a phase diagram whereby the formation temperature of liquid phase is lowered, and at the same time, the speed of the reaction between the mullite particles is varied, and the liquid phase composition is partly differentiated. 
     The present inventors have found that the generation of flecks or spots can be prevented by raising the formation temperature of the above-mentioned liquid phase by an addition of a vanadium compound, a niobium compound, or a tantalum compound. 
     The total amount of the vanadium, niobium and tantalum compounds is not particularly limited, but is advantageously from 0.1 to 10% by weight. When the total amount of such compounds is more than 10% by weight the resultant fired body is unpreferably colored yellow. When the amount of such compounds is less than 0.1% by weight, the effect of the prevention of the generation of flecks or spots is reduced. 
     The optimum total amount of vanadium compound, niobium compound or tantalum compound depends on the particle size or the particle purity of mullite powder or the kind or amount of the sintering aid. For example, when 0.5 to 3% by weight of yttrium oxide is added to mullite powder having a particle size of 1 μm as a sintering aid, about 0.1 to 0.2% by weight of niobium oxide is preferably added. 
     The total amount of the rare earth element compound and the alkaline earth element compound used is preferably from 0.1 to 20% by weight. The rare earth element compound and the alkaline earth element compound can be used separately or as a mixture thereof. 
     As the rare earth element, Y 2  O 3 , YF 3 , YCl 3 , La(CO 3 ) 3 , LaF 3 , La(CO 3 ) 2 , Ce 2  (CO 3 ) 3 , CeO 2  and CeF 3 , are preferably used, and as the alkali earth element compound, MgO, CaO, MgF, MgCl, CaCO 3 , CaF 2 , SrO, SrCO 3 , BaO, BaCO 3  are preferably used. The optimum total amount of the sintering aid depends on the particle size or the particle purity of the mullite powder. For example, when the particle size of the mullite powder is about 1 μm, an amount of 0.5 to 3% by weight of yttrium oxide is advantageously used. Generally, the larger the particle size of the mullite powder, the greater the amount of the yttrium oxide used. 
     The mullite powder, is usually produced by mixing alumina and silicon dioxide and calcining the mixture to form mullite and pulverizing. 
     High purity mullite powder is recently obtained by, for example an electro-fusion method, and when a high purity mullite powder is used, a dense mullite ceramic composition having a high mechanical strength is obtained. If cost is not considered, a high purity and finely granulated mullite powder can be used in a method of thermal decomposition while admixing an intermediate product consisting of alkoxide or salts of Al or Si. 
     The firing of the present invention can be carried out at a temperature of 1300° to 1850° C. In this case, metal conductor metallized by using a conventional tungsten or molybdenum-manganese can be co-fired under the same conditions as conventionally required, i.e., at a temperature of 1500° to 1600° C. 
     The dielectric constant of the obtained mullite ceramic is low, the flexural strength is greater than that of the conventional product, and the same brazing properties were obtained. Note, the thermal expansion coefficient was slightly increased but this did not cause a problem. 
     The use of the rare earth element compound or the mixture of the rare earth element compound and alkaline earth element compound as a sintering aid, provided a remarkably improved flexural strength, metallized strength, and brazing strength than when only the alkali earth element compound was used. For example, when yttrium oxide is used as the sintering aid, the brazing strength is about twice that obtained when magnesium oxide is used. 
     Further, when metallized strength after firing is measured, the peel strength of an outer portion lead of the electric parts was 400 g/lead, in a case of addition of magnesium oxide. However, with the addition of 2% by weight of yttrium the peel strength, was enhanced to 700 g/lead. Further in addition of 5% by weight of yttrium the peel strength is remarkably increased. 
     The mullite ceramic composition is generally white, but when a powder having a particle size of 2 μm or less is used as the mullite powder, the fired mullite ceramic body has an improved translucency. Therefore, when a fired mullite ceramic substrate having a thickness of about 0.5 mm is placed over printed matter, large letters about 2 mm square in size can be read. 
     Accordingly, the mullite ceramic composition can be preferably used not only as an electronic part substrate and a ceramic package material but also as a material requiring a translucency, such as an arc tube surrounding body. On the other hand, when the mullite ceramic composition is used as a material which requires a shading property, it can be colored by applying a coloring agent thereto. A coloring agent which does not affect the inherent properties of the mullite product should be selected. 
     Coloring agents such as TiO 2 , Fe 2  O 3  and Cr 3  O 3  are conventionally used for an alumina ceramic, but when 1% by weight of Cr 2  O 3  is applied to the mullite ceramic, the coloring is unsatisfactory. Further, Fe 2  O 3  and TiO 2  deteriorate the dielectric property of the fired ceramic body, in particular, TiO 2  remarkably and unpreferably increases the dielectric constant. 
     When MoO 3  is used, if even 1% by weight of MoO 3  is added a satisfactory coloring can be obtained. Further, when a small amount of Cr 2  O 3  is added to the mullite ceramic of the present invention, together with the MoO 3 , the coloring is improved and the mechanical strength is also improved, without lowering the inherent dielectric property of the product. 
     When a large amount of coloring agent, e.g., a total amount of 4% by weight, is added to a conventional alumina ceramic, the shading property is unsatisfactory, and the dielectric is increased. 
     However, as explained above, a colored mullite ceramic composition to which MoO 3  was added can be preferably used as a ceramic package requiring a property of shielding a semiconductor element from light. 
     To explain the present invention in more detail, examples of the present invention will now be described. 
    
    
     EXAMPLE 1 
     Two parts by weight of yttrium oxide and 1 part by weight of niobium oxide were added to 100 parts by weight of electro-fused mullite powder having an average particle diameter of about 2 μm, and an n-butanol-toluene mixed solution was mixed therewith by ball milling for 24 hours, as a dispersion medium. 
     Then 4 parts by weight of di-n-butyl phthalate and 8 parts by weight of polyvinyl butylar were added to the resultant mixture, and mixed by ball milling for 24 hours. The resultant mixture was vacuum-defoamed and cast into a sheet by a doctor blade process. 
     The density of the obtained green sheet was 1.91 g/cm 3 . This green sheet was then fired for 4 hours at a temperature of 1600° C. under atmospheric pressure in a weak reduction atmosphere, to obtain a fired body. 
     The density of the fired body was 3.16 g/cm 3 . 
     EXAMPLE 2 
     One part by weight of MoO 3  was added to the raw material composition of Example 1, and a green sheet was formed by the same processes as in Example 1, and fired. The density of the obtained green sheet was 1.93 g/cm 3  and the density of the fired body was 3.20 g/cm 3 . 
     EXAMPLE 3 
     Two parts by weight of MgO and 1 part by weight of niobium oxide were added to the mullite powder of Example 1, and a green sheet was formed by the same processes as in Example 1, then fired. The density of the obtained green sheet was 1.92 g/cm 3 , and the density of the fired body was 3.20 g/cm 3 . 
     EXAMPLE 4 
     One and a half parts by weight of tantalum oxide were added instead of the 1 part by weight of niobium oxide of Example 1, and a green sheet was formed by the same processes as in Example 1, then fired. The density of the obtained green sheet was 1.94 g/cm 3 , and the density of the fired body was 3.13 g/cm 3 . 
     EXAMPLE 5 
     One part by weight of vanadium oxide was added instead of the 1 part by weight of niobium oxide of Example 1, and a green sheet was formed by the same processes as in Example 1, then fired. The density of the obtained green sheet was 1.87 g/cm 3 , and the density of the fired body was 3.05 g/cm 3 . 
     COMPARATIVE EXAMPLE 1 
     Two parts by weight of Y 2  O 3  was added to 100 parts by weight of electro-fused mullite powder having an average particle diameter of about 2 μm and a green sheet was formed by the same processes as in Example 1, and fired. The density of the green sheet was 1.92 g/cm 3 , and the density of the fired body was 3.14 g/cm 3 . 
     COMPARATIVE EXAMPLE 2 
     One part by weight of the molybdenum oxide was added to the same raw material composition as in comparative example 1, and a green sheet was formed by the same processes as in Example 1, then fired. The density of the green sheet was 1.94 g/cm 3 , and the obtained density of the fired body was 3.16 g/cm 3 . 
     COMPARATIVE EXAMPLE 3 
     Two parts by weight of MgO were added to the same mullite powder as in Example 1, and a green sheet was formed by the same processes as in Example 1, and fired. The density of the obtained green sheet was 1.92 g/cm 3 , and the density of the fired body was 3.11 g/cm 3 . 
     The results of measurements of the properties of the obtained mullite ceramics are shown in Table 1. When the sintering aid was added as a haloid, it was changed to oxide during firing. Thus the effect of the haloid was the same as that of the oxide. 
     
                                           TABLE 1                                 
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                                           THERMAL                        
                           DIELECTRIC                                     
                                   FLEXURAL                               
                                           EXPANSION     FLECK            
                           CONSTANT                                       
                                   STRENGTH                               
                                           COEFFICIENT   GENER-           
       ADDITIVES           (1 MHz 20° C.)                          
                                   (kg/mm.sup.2)                          
                                           (×10.sup.-6 /°C.) 
                                                    COLOR                 
                                                         ATION            
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Example 1                                                                 
       Y.sub.2 O.sub.3 (2 wt %) Nb.sub.2 O.sub.5 (1 wt %)                 
                           6.5     22.7    4.5      White                 
                                                         --               
Comparative                                                               
       Y.sub.2 O.sub.3 (2 wt %)                                           
                           6.4     21.2    4.2      White                 
                                                         ∘    
Example 1                                                                 
Example 2                                                                 
       Y.sub.2 O.sub.3 (2 wt %) MoO.sub.3 (1 wt %) Nb.sub.2 O.sub.5       
                           7.0     26.0    4.9      Black                 
                                                         --               
       (1 wt %)                                                           
Comparative                                                               
       Y.sub.2 O.sub.3 (2 wt %) MoO.sub.3 (1 wt %)                        
                           6.9     24.5    4.5      Black                 
                                                         ∘    
Example 2                                                                 
Example 3                                                                 
       MgO (2 wt %) Nb.sub.2 O.sub.5 (1 wt %)                             
                           6.6     17.6    4.3      White                 
                                                         --               
Comparative                                                               
       MgO (2 wt %)        6.4     16.1    4.1      White                 
                                                         ∘    
Example 3                                                                 
Example 4                                                                 
       Y.sub.2 O.sub.3 (2 wt %) Ta.sub.2 O.sub.5 (1.5 wt                  
                           7.0     20.6    4.3      White                 
                                                         --               
Example 5                                                                 
       Y.sub.2 O.sub.3 (2 wt %) V.sub.2 O.sub.5 (1 wt %)                  
                           6.5     21.9    4.5      Black                 
                                                         --               
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