Abstract:
Low dielectric loss ceramic materials useful in dielectric resonators comprise alumina doped with a minor amount, preferably below 2% by weight of a metal or semi metal oxide, preferably an oxide of titanium, niobium, yttrium or zirconium. The compositions can be made by sintering the powders together below their melting point and materials with a with a Q value of greater than 35,000 at 9-10 GHz and at 25° C., can be obtained.

Description:
RELATED APPLICATIONS 
     This Application is a Continuation-In-Part of application Ser. No. 09/101,986 filed Jul. 13, 1998, now abandoned, which is a 371 of PCT/GB96/03150 based upon G.B. Application 95/26339.8 having a Priority Date of Dec. 22, 1995. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the production of low dielectric loss ceramic materials, particularly ceramic materials useful in dielectric resonators. In current microwave communication, technology dielectric resonators (DRs) are key elements for filters, low phase noise oscillators and frequency standards. DRs possess resonator quality factors (Q) comparable to cavity resonators, strong linearity at high power levels, weak temperature coefficients, high mechanical stability and small size. 
     BACKGROUND OF THE INVENTION 
     Ceramic dielectric materials are used to form thermally stable DRs as key components in a number of microwave subsystems which are used in a range of consumer and commercial market products. These products range from Satellite TV receiver modules (frequency converter for Low Noise Broadcast (LNB)), Cellular Telephones, PCN&#39;s. (Personal Communication Networks Systems) and VSAT (Very Small Aperture Satellite) systems for commercial application to emerging uses in transportation and automobile projects, such as sensors in traffic management schemes and vehicle anti-collision devices. Dielectric Resonators may be used to determine and stabilise the frequency of a microwave oscillator or as a resonant element in a microwave filter. New systems of satellite TV transmission, based on digital encoding and compression of the video signals, determine the need for improved DR components. The availability of advanced materials will also enable necessary advances in the performance of DRs used for other purposes as referred to above. 
     In this specification, the term ‘ceramic’ means any solid inorganic particulate material, the particles of which can be caused to sinter together by the application of heat. The term ceramic has been used also to describe single crystals of inorganic materials such as alumina, titania, etc. 
     Low dielectric loss materials are highly desirable in the area of communications over a wide frequency range. As an example, resonators using dielectric sintered ceramics may be used in base stations required for mobile communications. The materials used are often complex mixtures of elements. 
     One of the earliest resonator materials was Barium Titanate (BaTiO 3  or BaTi 4 O 9  see, for example, T Negas et al American Ceramic Society Bulletin, vol. 72, pp 80-89 1993). The dielectric loss of a material is referred to as the tan delta and the inverse of this quantity is called the Q (Quality Factor). The Q factor of a resonator is determined by choosing a resonance and then dividing the resonant frequency by the band width 3 dB below the peak. 
     The losses in ceramic materials may be associated with molecules or defects which can be spatially oriented (Debye loss), due to the inertia of free charges, e.g. electrons in a metal or resonant absorption at certain frequencies. It is considered that extrinsic factors such as impurities and e.g. oxygen vacancy concentration as well as microstructure are of overriding importance. Single crystals or ‘perfect’ crystals have a lower loss than corresponding poly-crystalline materials. The difference between a ‘perfect’ single crystal and a polycrystalline ceramic are thought to be due to the huge differences in microstructure and perfection between the two and are clear indicators why it is considered impossible to achieve a dielectric loss approaching that of single crystal counterparts in sintered materials. 
     A single crystal is made from a melt. The melting temperatures of these crystals is extremely high. For example, the melting point of alumina is 2072C, of magnesia 2852C, of zirconia 2700C, of yttria 2410C and titania 1850C. 
     The particulate ceramic material can be shaped in a variety of ways, for example, by uniaxial powder pressing, by isostatic pressing, by slip-casting or by polymer processing and extrusion. The resultant shape is then sintered at high temperature and this is associated with a shrinkage and a decrease in the volume of the body. The sintering step can take place in air or in special atmospheres be they oxidising, reducing or inert. 
     Sintering a ceramic involves taking a fine powder of the material, pressing it into the desired shape and then heating it to temperatures less than their melting point (usually about 75% of the melting point). The powders sinter together in an effort to reduce surface energy and this is accomplished by the reduction in surface area until the porosity is reduced substantially or entirely. The sintering process involves less expensive capital equipment and is less energy intensive. 
     The major problem with dielectric ceramics is that their dielectric loss is much higher than single crystals. Single crystal materials can exhibit very low loss and this is usually attributed to the absence of grain boundaries and the greater perfection in their structure. The problem with single crystals is that they are time consuming to manufacture and they are extremely expensive. For example, a single crystal of alumina in cylindrical form is around 10,000 times more expensive than an identically shaped sintered alumina. 
     We have now discovered sintered ceramic materials with a low dielectric loss and a method for making them. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a doped alumina ceramic which has a high Q value. The object is achieved by forming the ceramic from a homogenous mixture of alumina containing less than 165 ppm impurities, less than 1% weight of a metal oxide of a metal of cation ionic radius of 0.55 to 1, in which the alumina has a particle size of 0.01 to 2 microns and a density of at least 98% theoretical. 
     The metal or semi-metal oxide, which forms the minor part of the materials of the invention, are preferably oxides of elements of Group III and IV of the periodic table such as Ti, Nb, Y and Zr. The minor part is preferably present in an amount of less than 2% by weight of the total weight of the composition and, more preferably, less than 1% by weight. 
     The materials of the present invention can be made by homogenously mixing the alumina powder and the dopant powder. In order to achieve homogenous mixing the particle size of the dopant should be the same or less than that of the alumina powder. Alternatively homogenous mixing can achieved by precipitating from solution. The mixed powders can be formed into the ceramic by mixing the powders of alumina and the minor component, for example, of particle size of 0.01 to 2 microns and pressing the mixture into a shape and then heating to a temperature below its melting point, typically 75% of the melting point. The powders sinter together until the porosity is substantially or entirely reduced. Preferably the temperature of sintering for powders such as alumina is less than 1600° C. and more preferably between 1500° C. and 1600° C. In place of the metal oxide used as the dopant a salt of the metal which forms the oxide on the conditions of forming the ceramic can be used e.g. the metal carbonate. 
     It is very surprising that the addition of dopant oxides with a low Q factor to alumina can increase the Q factor of the resulting material very considerably and can approach the value of single crystals. It was considered that any material with grain boundaries must inevitably show high dielectric loss. 
     The dielectric loss of polycrystalline sintered alumina has been measured by several workers and the results vary widely. For example, Ceramic Source, vol. 6 1990, American Ceramic Society (publ) reports the loss of alumina as Q=1000 at 10 GHz and 25° C. 
     The highest Q previously measured in alumina at room temperature (i.e. approximately 25° C.) is by Woode et al (R A Woode, E N Ivanov, M E Tobar and D G Blair ‘Measurement of dielectric loss tangent of alumina at microwave frequencies and room temperature’ Electronics Letters, vol. 30 no. 25, Dec. 8, 1994) who measured a Q of 23,256. This Article noted that purity alone was a poor indicator of the dielectric loss tangent. We have found that although an impure alumina will give a poor Q, a very pure alumina is not a guarantee of a high Q. Pure alumina can have compounds added to it in order to assist the sintering process. These additions should not adversely influence the Q. So, for example, magnesia may be added to a very pure alumina and this will assist the sintering but will not adversely affect the Q if added in small quantities. However, adverse effects are observed with impurities such as alkali salts (sodium and potassium) and metallic elemental impurities such as iron. 
     The invention also comprises a sintered ceramic material comprising an alumina and minor amount of a metal or semi-metal oxide which has a Q value greater than 25,000, more preferably greater than 30,000 and even more preferably greater than 45,000 at 9 to 10 GHz and at 25° C. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described in the following Examples. 
     The aluminas used were commercially available aluminas and the analyses of the powders used in the examples are given in Table 1 with the impurities given in parts per million based on the total weight of the sample. 
     The aluminas used had a particle size of 0.01 to 2 microns. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Chemical analyses of powders used (ppm) 
               
             
          
           
               
                   
                 “A” 
                 “B” 
                 “C” 
               
               
                   
                   
               
             
          
           
               
                   
                 Na 2 O 
                 800 
                 300 
                 20 
               
               
                   
                 MgO 
                 400 
                 1000 
                 16 
               
               
                   
                 SiO 2   
                 400 
                 400 
                 85 
               
               
                   
                 Fe 2 O 3   
                 300 
                 100 
                 29 
               
               
                   
                 CaO 
                 200 
                 40 
               
               
                   
                 SO 3   
               
               
                   
                 CuO 
                   
                   
                 13 
               
               
                   
                 surface area m 2 g 1   
                 7 
                 5 
                 5-10 
               
               
                   
                 Sample % Density 
                 97.8 
                 98.2 
                 98 
               
               
                   
                   
               
             
          
         
       
     
    
    
     EXAMPLES 
     Examples 1 to 3 are comparative examples. 
     Powders A-C inclusive were pressed in a 13.5 mm diameter stainless steel die press at a pressure of 100 MPa. The pressed samples were sintered in air at a temperature of 1550° C. for 300 mins. The sample density was then measured and the dielectric constant and dielectric loss experiments were carried out using a parallel plate resonator and employing a modified Haaki-Coleman technique described in B. W Haaki and P. D Coleman ‘A dielectric resonator method of measuring inductive capacities in the millimeter range’, IFEE Trans. Microwave Theory Tech. Vol. 8, p.402-410 (1960). Here the dielectric puck is placed not directly onto the lower copper plate but onto a low loss material with a much lower dielectric constant. We have used a quartz crystal 4 mm thick and 10 mm in diameter. The sample dimensions were approximately 10 mm diameter, 4 mm thick discs. The measurements were made using a Hewlett Packard HP8719C vector network analyser with 1 Hz resolution and the TE011 mode was examined. All dielectric measurements were carried out at room temperature in air at a relative humidity of approximately 30%. No special precautions were taken to prevent the adsorption of water to the sample surface. The loss measurements are presented in terms of the Q factor, i.e. tan delta −1 , the measuring frequency was 9-10 GHz at a temperature of 25° C. The density of the sintered products was in the range 98% to 99.5% theoretical i.e. they had a porosity of 2% to 0.5%. 
     Example 1 
     Sample A showed a Q of 3,500 while B had a Q factor less than 1000 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 A 
                 B 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Q 
                 3,500 
                 &lt;1000 
               
               
                   
               
             
          
         
       
     
     Example 2 
     The chemically purer powder C was made into dense discs. Sample C2, was subjected to a sintering temperature of 1600° C. for 300 mins. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 C2 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 12,000 
               
               
                   
                   
               
             
          
         
       
     
     Example 3 
     The same alumina powder as used in C, was sintered at 1500° C. for 300 minutes. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 C 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 16,000 
               
               
                   
                   
               
             
          
         
       
     
     Examples 4 and 5 and 10 to 12 are examples of the invention and Examples 8 and 9 are comparative examples. 
     Examples 4 to 6 
     The dopant powder of particle size smaller than that of the alumina powder was homogenously mixed with the alumina powder and formed into a ceramic as above. 
     In the second series of experiments, Alumina C was doped with TiO 2 . The addition of TiO 2  influences the sintering temperature at which the material achieves high density and for this reason the aluminas doped with TiO 2  were sintered at 1500° C. To check the effect of a reduction in the sintering temperature on the pure alumina, alumina C was also sintered at 1500° C. TiO 2  sintered in air to full density displays a very poor Q which was measured at 1,500. This is due to the fact TiO 2  is easily reduced. Small deviations from stoichiometric TiO 2  causes a random distribution of point defects. As the defect concentration increases, their interaction increases and ordering can occur. The dielectric loss has suffered in comparison with the single crystal Q value which was measured at 7,000. 
     Example 4 
     A pure alumina (alumina C) was doped by 0.5% weight of Titanium dioxide powder. (Sample name CT0.5). The mixture was sintered at 1500° C. as in Example 3 and the Q measured as in Example 1, and was found to be 47,000. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 CTO.5 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 47,000 
               
               
                   
                   
               
             
          
         
       
     
     Example 5 
     A pure alumina (alumina C) was doped with 1% by weight of TiO 2  (sample name CT1.0) and sintered at 1500° C., i.e. at the same temperature as that used for samples C in Example 3. The Q was found to be 35,000. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 CT1.0 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 35,000 
               
               
                   
                   
               
             
          
         
       
     
     Example 6 
     A pure alumina (alumina C) was doped with 5% by weight of TiO 2  (sample name CT5.0) and sintered at 1500° C., i.e. at the same temperature as that used for samples C in Example 3. The Q was found to be 12,650. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 CT5.0 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 12,650 
               
               
                   
                   
               
             
          
         
       
     
     Example 7 
     A pure alumina (alumina C) was doped with 10% by weight of TiO 2  (sample name CT10.0) and sintered at 1500° C., i.e. at the same temperature as that used for samples C in Example 3. The Q was found to be 7,300. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 CT10.0 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 7,300 
               
               
                   
                   
               
             
          
         
       
     
     Example 8 
     A pure TiO2 was sintered to full density in air at 1400° C. The purpose of this example is to demonstrate that the Q of TiO 2  is much lower than the Q of Al 2 O 3  and hence demonstrate that it might be expected that any addition of TiO 2  to Al 2 O 3  would have the effect of decreasing the Q rather than enhancing the Q. Using a mixtures rule, common in the description of dielectrics, it might be expected that at 0.5% TiO 2  in Al 2 O 3  the Q should be 33,500 when in fact the measured Q is 47,500. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 TiO2 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 1,500 
               
               
                   
                   
               
             
          
         
       
     
     Example 9 
     By was of comparison, three single crystals (SC1-3) of sapphire (Al 2 O 3 ) were tested under identical conditions. As expected, the Q of the single crystals was higher than the Q of the sintered aluminas but, surprisingly, the Q factors for the sintered aluminas were only a factor of approximately two lower. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 SC1 
                 SC2 
                 SC3 
               
               
                   
                   
               
             
             
               
                   
                 Q 
                 100,000 
                 100,000 
                 100,000 
               
               
                   
                   
               
             
          
         
       
     
     Examples 10, 11 and 12 
     Dopants other than TiO 2  close to Ti in the periodic table, i.e. Nb, Zr and Y, were also added to alumina C. The dopants were added at the 0.25, 0.5, 1 and 5 wt % level a total of 12 different compositions. All samples were sintered at 1500° C. for 300 mins in air. In none of the measurements did the Q factor exceed the highest value for the TiO 2  addition. However, there are indications of increases in the Q which is surprising as all three dopants have a higher dielectric loss in comparison with Al 2 O 3 . 
     Examples 
     
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                 Example 10 
                 Example 11 
                 Example 12 
               
               
                 Dopant 
                 Nb 2 O 5   
                 Y 2 O 3   
                 ZrO 2   
               
               
                   
               
             
             
               
                 Q (0.25% dopant) 
                   
                   
                 26,000 
               
               
                 Q (0.5% dopant) 
                 30,000 
                 25,400 
                 28,400 
               
               
                 Q (1.0% dopant) 
                   
                 36,000 
               
               
                 Q (5.0% dopant) 
                 30,000 
                 30,000 
                 17,000 
               
               
                   
               
             
          
         
       
     
     Example 13 
     The effect of cation ionic radius was determined by forming a series of ceramics with a range of metal oxides. In some case the carbonates were used for convenience, these form the oxides under the conditions of forming the ceramics. The ceramics were made as above and their Q values measured and the results shown, in Table 2 
     The mol wt % is shown to show the comparative effectiveness of the dopants, the weight per cent in all cases was less than 2 wt %. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Cation ionic 
                   
               
               
                   
                 Dopant 
                 Radius (A°) 
                 Q 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 K 2 CO 3   
                 1.38 
                  1060 
               
               
                   
                 Na 2 CO 3   
                 1.02 
                 too low to measure 
               
               
                   
                 BaCO 3   
                 1.35 
                 12032 
               
               
                   
                 SrCO 3   
                 1.18 
                 15790 
               
               
                   
                 CaCO 3   
                 1 
                 24045 
               
               
                   
                 ZnO 
                 0.74 
                 36475 
               
               
                   
                 CuO 
                 0.73 
                 41290 
               
               
                   
                 MgO 
                 0.72 
                 37845 
               
               
                   
                 MnCO 3   
                 0.67 
                 37891 
               
               
                   
                 La 2 O 3   
                 1.032 
                  5972 
               
               
                   
                 Nd 2 O 3   
                 0.983 
                 40384 
               
               
                   
                 Sm 2 O 3   
                 0.958 
                 38901 
               
               
                   
                 Y 2 O 3   
                 0.9 
                 43726 
               
               
                   
                 Fe 2 O 3   
                 0.55 
                  1311 
               
               
                   
                 ZrO 2   
                 0.72 
                 32607 
               
               
                   
                 TiO 2   
                 0.605 
                 41614 
               
               
                   
                 Nb 2 O 5   
                 0.64 
                 41210