Bismuth pyrochlore microwave dielectric materials

This application claims priority to U.S. Provisional Application No. 60/214,938 filed Jun. 29, 2000.

The present invention relates to dielectric ceramic compositions for microwave applications and, more particularly, to Bi 2 O 3 —ZnO—Ta 2 O 5 dielectric ceramic compositions for microwave devices.

In recent years, communication systems have developed which use microwaves (frequency band ranging from 300 MHz to 300 GHz). These systems include wireless telephones, car phones, cellular phones, satellite broadcasting systems, and the like. As a result, there is an increasing demand for dielectric ceramics with better electrical properties for use components such as resonator devices, band pass filters, and microwave integrated circuits.

Bismuth based pyrochlores have recently become of interest for use as high frequency dielectric materials. One of the bases for this interest is that: they can be fired at low temperatures. In contrast to conventional microwave dielectric materials which require sintering temperatures of more than 1600° K, Bismuth pyrochlores can be sintered at less than about 1400° K. In addition, their dielectric properties such as a low loss of tan δ of 10 −4 and a K of up to about 150 make Bismuth pyrochlores promising dielectric material candidates.

For use in microwave communications systems which operate at high frequencies, dielectric materials should have properties such as high dielectric constant (“K”); high quality factor (“Q”); and stable temperature coefficient of capacitance (“TCC”). However, it is very difficult to develop dielectric materials which have a stable TCC as well as high K and high Q. A need therefore continues to exist for a dielectric material which has a high K, a high Q value and a stable TCC.

FIG. 1 shows a ternary phase diagram of Bi 2 O 3 —ZnO—Ta 2 O 5 and a compositional space defined by vertices A, B and C.

The present invention provides Bi 2 O 3 —ZnO—Ta 2 O 5 dielectric materials which have both high K and high Q, and which can be fired at low temperatures such as less than about 1000° C.

Compounds within the compositional space defined by vertices A, B and C of the Bi 2 O 3 —ZnO—Ta 2 O 5 system shown in FIG. 1 are produced. These compounds are illustrated by Bi 2 (ZnTa 2 ) x O 6x+3 where 0.57≦x≦1.0, by Bi 2 (ZnTa y ) 2/3 O ((5y+11)/3) where 1.0≦y≦3.0, as well as by Bi 2 (ZnTa y ) 2/3 O ((5y+11)/3) where 1.0≦y≦3.0 with the proviso that y is not=2.0. In FIG. 1, vertex A is defined by 0.125 mol % Ta 2 O 5 , 0.125 mol % ZnO, 0.75 mol % Bi 2 O 3 ; vertex B is defined by 0.125 mol % Ta 2 O 5 , 0.75 mol % ZnO, 0.125 mol % Bi 2 O 3 ; and vertex C is defined by 0.6875 mol % Ta 2 O 5 , 0.125 mol % ZnO, 0.1875 mol % Bi 2 O 3 .

Mixed phases and solid solutions on the tie line between the compounds of examples 5 and 8 within the compositional space A-B-C of FIG. 1, as defined by the formula r(Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 )-((1−r)(Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 )) where 0<r<1, also are produced.

These compounds typically have a high K, high Q, a low TCC, and low TCF over the frequency range of 1 MHz-28 GHZ, and can be sintered between about 850° C. to about 1000° C., preferably between about 850° C. to about 950° C. Borosilicate glass in an amount of up to about 5 wt. % based on the weight of compound, preferably Bi 2 (ZnTa 2 ) 2/3 O 7 , may be added to the compound.

The Bi 2 O 3 —ZnO—Ta 2 O 5 dielectric compounds of the invention have outstanding K, Q, TCC and temperature coefficient of resonant frequency (“TCF”). Typical properties include a K of 50-80, such as K>60 at 5 GHz, low dielectric loss (tan δ<0.003) such as a tan δ<0.001 at 5 GHz, a Q>300 at 5 GHz, a Q f >2000 at 5 GHZ, a TCF<40 ppm/° C. over the temperature range of −50° C. to +125° C., a TCC<50 ppm/° C. such as a TCC of <30 ppm/° C. over the temperature range of −50° C. to +125° C.

In a first embodiment, compounds of Bi 2 (ZnTa 2 ) x O 6x+3 where 0.57≦x≦1.0, of Bi 2 (ZnTa y ) 2/3 O ((5y+11)/3 ) where 1.0≦y≦3.0 and of Bi 2 (ZnTa y ) 2/3 O ((5y+11)/3) where 1.0≦y≦3.0 with the proviso that y is not=2.0 are produced. Manufacture of these compounds is illustrated in examples 1-14.

EXAMPLES 1-14

In manufacture of compounds of the formula Bi 2 (ZnTa 2 ) x O 6x+3 , ZnO and Ta 2 O 5 are reacted at 1000° C. to produce (ZnTa 2 ) x O 6x according to equation (1):

x (ZnO)+ x (Ta 2 O 5 )→(ZnTa 2 ) x O 6x   (1)

The (ZnTa 2 ) x O 6x then is reacted at 1000° C. with Bi 2 O 3 according to equation (1A) to produce a compound corresponding to Bi 2 (ZnTa 2 ) x O 6x+3 :

Bi 2 O 3 +(ZnTa 2 ) x O 6x →Bi 2 (ZnTa 2 ) x O 6x+3   (1A)

In manufacture of (ZnTa 2 ) x O 6x , reagent grade ZnO of 99.9% purity from Aldrich Chemical Co. and reagent grade Ta 2 O 5 of 99.9% purity from Aldrich Chemical Co. are milled in deionized water in a ball mill. Milling is performed for 24 hours using yttrium-stabilized zirconia balls to produce a blend that has a particle size range of 0.3 to 1.5 microns, and an average particle size of 1.0 micron. The resulting milled particle blend is dried in air at 120° C. for 16 hours. The resulting dried particles are calcined at 1000° C. in an open alumina crucible for 4 hours to produce (ZnTa 2 O 6 ) x .

Bi 2 O 3 is mixed with the (ZnTa 2 O 6 ) x powder. The resulting mixture is ball milled for 24 hours using yttrium-stabilized zirconia balls to produce a particle size range of 0.5 to 1.3 microns, and an average particle size of 0.8 microns. The milled particles are dried in air at 120° C. for 16 hours and calcined in an open alumina crucible at 800° C. for 4 hours. The milled particles are blended with 1 wt. %, based on the weight of the calcined particles, of polyvinyl alcohol. The resulting mixture is uniaxially cold pressed at 6000 PSI and sintered in an open alumina crucible at 950-1100° C. to produce a sintered disk that measures 10 mm diameter and 1 mm thick.

In manufacture of compounds of the formula Bi 2 (ZnTa y ) 2/3 O ((5y+11)/3) , ZnO and Ta 2 O 5 are reacted at 1000° C. to produce (ZnTa y ) 2 O 5y+2 according to equation (2):

2ZnO+ y Ta 2 O 5 →(ZnTa y ) 2 O 5y+2   (2)

The (ZnTa y ) 2 O 5y+2 then is reacted with 3Bi 2 O 3 according to equation (2A) at 950-1100° C. to produce a compound corresponding to Bi 2 (ZnTa y ) 2/3 O ((5y+11)/3) :

⅓(ZnTa y ) 2 O 5y+2 +Bi 2 O 3 →Bi 2 (ZnTa y ) 2/3 O ((5y+11)/3)   (2A)

In manufacture of (ZnTa y ) 2 O 5y+2 , reagent grade ZnO of 99.9% purity from Aldrich Chemical Co. and reagent grade Ta 2 O 5 of 99.9% purity from Aldrich Chemical Co. are milled in deionized water in a ball mill. Milling is performed for 24 hours using yttrium-stabilized zirconia balls to produce a blend that has a particle size range of 0.3 to 1.5 microns, and an average particle size of 1.0 micron. The resulting milled particle blend is dried in air at 120° C. for 16 hours. The resulting dried particles are calcined at 1000° C. in an open alumina crucible for 4 hours to produce (ZnTa y ) 2 O 5y+2 .

Bi 2 O 3 is mixed with the (ZnTa y ) 2 O 5y+2 powder. The resulting mixture is ball milled for 24 hours using yttrium-stabilized zirconia balls to produce a particle size range of 0.5 to 1.3 microns, and an average particle size of 0.8 microns. The milled particles are dried in air at 120° C. for 16 hours and calcined in an open alumina crucible at 800° C. for 4 hours. The milled particles are blended with 1 wt. %, based on the weight of the calcined particles, of polyvinyl alcohol. The resulting mixture is uniaxially cold pressed at 6000 PSI and sintered in an open alumina crucible at 950-1100° C. to produce a sintered disk that measures 10 mm diameter and 1 mm thick.

The amounts of reactants, sintering temperatures, and the compositions of the resulting compounds produced in examples 1-14 are shown in Table 1. Compounds 1-10 also are shown in FIG. 1 .

EXAMPLES 15-23

In a second embodiment, composites and solid solutions of the formula r(Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 )-((1−r) (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 ), 0<r<1, are produced as the reaction products of mixtures of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 .

To illustrate, a series of mixtures of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 (r=1) and (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 (r=0) powders are prepared according to the formula r(Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 )-((1−r) (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 ), 0<r<1. These mixtures are prepared for (r) values of 0, 0.2, 0.3, 0.4, 0.5, 0.6, 0.85, and 1.0, which correspond to examples 15-23, respectively. The powders are ball milled with yttrium stabilized zirconia balls to an average particle size of 1 micron. The milled powders are dried at 120° C. for 16 hours, mixed with 1 wt. % organic binder, and uniaxially compressed at 6000 PSI into 10 mm thick disks of 1 mm thickness. The disks are sintered at 1000° C. for 4 hours in air to produce the solid solution.

The Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and the (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 employed in examples 15-23 are produced as described below.

Manufacture of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7

15.55 gms. of reagent grade ZnO of 99.9% purity from Aldrich Chemical Co. and 84.45 gms. of reagent grade Ta 2 O 5 of 99.9% purity from Aldrich Chemical Co. are milled in deionized water in a ball mill for 24 hours using yttrium-stabilized zirconia balls to produce a blend that has a particle size range of 0.3 to 1.5 microns, and an average particle size of 1.0 micron. The milled particle blend is dried in air at 120° C. for 16 hours. The resulting dried particles are calcined at 1000° C. in an open alumina crucible for 4 hours to produce ZnTa 2 O 6 .

57.19 gms. Bi 2 O 3 are mixed with 42.81 gms. of the ZnTa 2 O 6 powder. The resulting mixture is ball milled for 24 hours using yttrium-stabilized zirconia balls to produce a particle size range of 0.5 to 1.3 microns, and an average particle size of 0.8 microns. The milled particles are dried in air at 120° C. for 16 hours and calcined in an open alumina crucible at 800° C. for 4 hours. The milled particles are blended with 1 wt. %, based on the weight of the calcined particles, of polyvinyl alcohol. The resulting mixture is uniaxially cold pressed at 6000 PSI and sintered in an open alumina crucible at 950° C. to produce a sintered disk of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 that measures 10 mm diameter and 1 mm thick.

Manufacture of (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7

15.55 gms. of reagent grade ZnO of 99.9% purity from Aldrich Chemical Co. and 84.45 gms. of reagent grade Ta 2 O 5 of 99.9% purity from Aldrich Chemical Co. are milled in deionized water in a ball mill. Milling is performed for 24 hours using yttrium-stabilized zirconia balls to produce a blend that has a particle size range of 0.3 to 1.5 microns, and an average particle size of 1.0 micron. The milled particle blend is dried in air at 120° C. for 16 hours. The resulting dried particles are calcined at 1000° C. in an open alumina crucible for 4 hours to produce ZnTa 2 O 6 .

45.85 gms. Bi 2 O 3 and 2.67 gms. ZnO are mixed with the 51.48 gms. ZnTa 2 O 6 . The resulting mixture is ball milled for 24 hours using yttrium-stabilized zirconia balls to produce a particle size range of 0.5 to 1.3 microns, and an average particle size of 0.8 microns. The milled particles are dried in air at 120° C. for 16 hours and calcined in an open alumina crucible at 800° C. for 4 hours. The milled particles are blended with 1 wt. %, based on the weight of the calcined particles, of polyvinyl alcohol. The resulting mixture is uniaxially cold pressed at 6000 PSI and sintered in an open alumina crucible at 950° C. to produce a sintered disk of (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 that measures 10 mm diameter and 1 mm thick.

In manufacture of the Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 compounds as described above, reagent grade oxides of Bi 2 O 3 , ZnO and Ta 2 O 5 of a purity >99.9% is used. It should be noted however, that non-reagent grade oxides of about 99% purity also can be used. In addition, binders other than polyvinyl alcohol can be used. Examples of other organic binders which may be used include but are not limited to polyethylene glycol, methylcellulose, carboxymethylcellulose, ethylcellulose, hydroxpropylcellulose, polyethylene oxide base high polymers, acrylic base high polymers, maleic anhydride base high polymers, starch, gelatine, polyoxyethylene alkyl ether, polyvinyl butyrol and waxes. In addition, it should be noted that ball milling may be done in media other than deionized water. Examples of suitable media include acetone.

EXAMPLES 24-27

In another aspect, the Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 compounds each may be mixed with glass such as a P 2 O 5 type glass, a PbO type glass, and a Bi 2 O 3 type glass, preferably a borosilicate glass, more preferably a ZnO—B 2 O 3 —SiO 2 type borosilicate glass, and then fired. The amount of glass added to these compounds may be up to about 5 wt. % based on the weight of the compound, preferably about 0.5 wt. %.

To illustrate, a borosilicate glass of the composition ZnO—B 2 O 3 —SiO 2 is added to Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 to produce a blend. The blend then is ball milled in water with yttrium stabilized zirconia balls for 24 hours to produce an average particle size of 0.5 microns. The resulting milled powder is then mixed with 1 wt. % of polyvinyl alcohol binder based on the weight of the milled power. The resulting blend is uniaxially compressed at 6000 PSI into a pellet.

The sintering temperatures of various blends of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and the ZnO—B 2 O 3 —SiO 2 borosilicate glass of composition 60 wt. % ZnO—30 wt. % B 2 O 3 —10 wt. % SiO 2 are shown in Table 2. The dielectric properties, as measured according to the procedures described below, of the blend of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and 0.5 wt. % borosilicate glass sintered at 850° C., when measured at room temperature a frequency of 100 KHZ, are K=58.9, Q=1400 and TCC=50.0.

Reacted blends of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and glass, (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 and glass, as well as mixtures thereof, also may be used to prepare solid solutions and composites as in the manner described above.

Dielectric Property Measurement

Gold electrodes then are sputtered onto each side of the sintered disk and the dielectric properties evaluated. The dielectric properties of each of sintered Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 , sintered Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 with glass, sintered (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 , as well as sintered solid solutions of Bi 2 (Zn 1/3 Ta 2/3 ) 2 O 7 and (Bi 3/2 Zn 1/2 ) (Zn 1/2 Ta 3/2 )O 7 are measured.

Measurement of dielectric properties such as (K, tan δ and TCC) at low frequencies of 1 KHz to 1 MHz is done while cooling at a rate of 2° C./min. over the temperature range of +150° C. to −170° C. in conjunction with a computer interfaced temperature chamber—chamber from Delta Design Corp., San Diego, Calif. The temperature is monitored with a K-type thermocouple or a Pt sensor. Measurements are made by using a Hewlett-Packard 4284 Inductance-Capacitance-Resistance (“LCR”) meter. An AC field of 0.1 V/mm is applied to 10 mm diameter sintered pellets.

The (TCC, ppm/° C.) is calculated from the slope of the dielectric constant (K) over the temperature range of +120° C. to −55° C. and the dielectric constant at 25° C. Measurement of dielectric properties (K, tan δ and TCC) at high frequencies of 400 MHz to 20 GHZ is done over the temperature range of +150° C. to −170° C. by using the well known Hakki-Coleman method with a Hewlett-Packard HP 8510C network-spectrum-analyzer.

The measured properties are shown in Tables 3 to 7.