EP1373161A1 - A microwave dielectric ceramic composition and a process for the preparation thereof - Google Patents

A microwave dielectric ceramic composition and a process for the preparation thereof

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Publication number
EP1373161A1
EP1373161A1 EP01934303A EP01934303A EP1373161A1 EP 1373161 A1 EP1373161 A1 EP 1373161A1 EP 01934303 A EP01934303 A EP 01934303A EP 01934303 A EP01934303 A EP 01934303A EP 1373161 A1 EP1373161 A1 EP 1373161A1
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Prior art keywords
range
composition
formula
microwave dielectric
dielectric constant
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German (de)
French (fr)
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Naseemabeevi Jawahar Isuhak
Thomas Sebastian Mailadil
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Council of Scientific and Industrial Research CSIR
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Council of Scientific and Industrial Research CSIR
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Priority claimed from PCT/IN2001/000077 external-priority patent/WO2002079115A1/en
Publication of EP1373161A1 publication Critical patent/EP1373161A1/en
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  • Conventional ceramics for use in such applications include BaO-TiO 2 system, Ba(Mg!
  • Ba 5 Nb Oi5 type hexagonal perovskites have high dielectric constant and high Q factor.
  • the ceramics have hexagonal crystal structure.
  • C. Veneis, P. K. Davies, T. Negas and S. Bell (Mater. Res. Bull.
  • Yet another object of the present invention is to provide to tune the microwave dielectric properties of the above ceramics and hence to achieve temperature compensation by stacking dielectric resonators with positive and negative its.
  • Figure 1 shows the variation of ⁇ r and Tf with x.
  • Figure 2 shows the effective ⁇ r versus volume fraction of 5ZnO-2Nb 2 O5.
  • Figure 3 shows the effective Tf versus volume fraction of 5ZnO-2Nb 2 O5.
  • the dielectric constant is in the range
  • the ceramic composition is of the formula Ba 5 Ta 4 Oi5 and wherein the dielectric constant is 28 ⁇ 1, quality factor freq ency product greater than 32000 and temperature variation of resonant frequency 8 ⁇ 4 ppm °C.
  • the ceramic composition is of the, formula 5ZnO-2Nb 2 O 5 , and wherein the dielectric constant is 22 ⁇ 1, quality factor- frequency product greater than 88,000 and temperature variation of resonant frequency -73 ⁇ 5ppm/°C.
  • the ceramic composition is of the formula xZnO- (5-x)MgO- 2Nb 2 O 5 wherein 0 ⁇ x ⁇ 5.
  • the ceramic composition is of the formula xCaO - (5-x)ZnO - 2Nb 2 O 5 wherein 0 ⁇ x ⁇ l and wherein the dielectric constant is in the range 20+ 1 and 21 + 1, quality factor-frequency product is in the range 44,000 to 79,000 and temperature variation of resonant frequency is in the range -55 ⁇ 3 and -69 ⁇ 5 ppm °C.
  • the ceramic composition is of the formula 0.5CaO-4.5ZnO-2Nb 2 O5 wherein the dielectric constant is 21 ⁇ 1, quality factor-frequency product > 79,000 and temperature variation of resonant frequency is in the range -55 ⁇ 3 ppm °C.
  • the ceramic composition is of the formula A5B' x B" 4 .
  • the invention also relates to stacked resonators consisting of the above ceramics with opposite T f values to tune the T f to near to zero values.
  • the stacked resonators are between Ba5Nb O 15 and 5ZnO-2Nb 2 O5 ceramics wherein the volume fraction of 5ZnO-2Nb 2 O 5 is in the range 0.6 and 0J where the dielectric constant varies from 26 to 30 and Tf varies between 20 and -20 ppm/°C.
  • 0 ⁇ x ⁇ l when A' Ca and A' - Zn.
  • the microwave dielectric ceramic comprises MgsT ⁇ O ⁇ Mg 5 Ta O ⁇ 5 ; Sr J Ta O ⁇ 5 ; BajT ⁇ uOis; Mg 5 Nb 4 O ⁇ 5 ; Mg 5 Nb 4 O ⁇ 5 ; 5ZnO-2Nb 2 O 5 ; 5CaO-2Ta 2 O 5 and 5CaO-2Nb 2 O 5 .
  • temperature compensation is achieved by stacking the resonators with positive and negative temperature coefficients of resonant frequency by preparing the perovskites of the invention in the powder form, moulding of the powder in the suitable shape, drying, sintering and final treatment.
  • a 5 B Oi 5 ceramic in A 5 B Oi 5 ceramic the valency of A is two and that of B is five.
  • the solid solutions or mixture phases with the general formula xA ⁇ - (5-x)A" - 2Nb 2 O5 are prepared by mixing calcium carbonate or magnesium oxide and zinc oxide with niobium pentoxide in the x: 5-x: 2 ratio.
  • the mixture phases are prepared for using the solid state ceramic route.
  • a 5 B4O1 5 ceramic the valency of A is two and that of B is five.
  • the calcium, Magnesium and zinc are attempted to substitute at A site keeping niobium at B site to get the respective niobates.
  • barium, strontium, calcium and magnesium are attempted to substitute at A site in order to obtain the respective tantalates.
  • the atoms of a particular site may be replaced by another atom of the same valency and nearly the same ionic radius to form solid solution phases with intermediate dielectric constant and temperature coefficient of resonant frequencies.
  • the A 5 B 4 O 15 system provides ceramic materials with large range of dielectric constant, Q factor and positive and negative temperature coefficient of resonant frequencies and hence provides good scope for tuning the dielectric properties of ceramics with similar structure.
  • the present system of ceramics is useful for applications as dielectric resonators in communication systems and as substrates in microwave integrated circuits. Compared to the use of alumina substrates they decrease the size not only for strip line resonators and filters but also for all microwave circuits. It is also possible to use these dielectric materials in the fabrication of devices such as circulators, phase shifters etc. for impedance matching.
  • the dielectric resonators based on the above compounds are also useful for the fabrication of dielectric resonator antennas. Solid solution formation in the above system enables to tune the dielectric constant and temperature variation of resonant frequency in the Ba 5 Nb 4 Oi5 type hexagonal perovskites.
  • the oxides or carbonates of A are wet mixed with niobium pentoxide/tantalum pentoxide in the molar ratio.
  • the mixed powder is calcined in the range 1100°C - 1400°C and cooled to the room temperature. The calcine is ground well, PVA is added as the binder, dried and again ground.
  • the resultant fine powder is pellettized in the appropriate size for the measurement (5- 9 mm in height and 11 mm in diameter).
  • the careful design of the dimensions of the samples is a prerequisite for the accuracy of the microwave dielectric measurements.
  • the height of the sintered samples should be less than their diameter for the accuracy of the results.
  • the sintering temperatures of the samples were optimized at different temperatures in the range 1220-1625°C.
  • the sintered samples are polished well to avoid any irregularities on the flat surface and are used for measurements
  • the microwave dielectric constant is measured using Hakki- Coleman dielectric post resonator method.
  • the resonator is placed between two gold-coated copper metallic plates and microwave energy is coupled through an E- field probe to excite various resonant modes. Among the various resonant modes the TEon mode is selected for the measurement.
  • the above ceramics resonate at frequencies between 4 and 10 GHz
  • the quality factors of the samples are measured at the TE 01 ⁇ mode resonant frequency using a cavity method [Jerzy Krupka, Krzytof Derzakowsky, Bill Riddle and James Baker Jarviz, Meas. Sci. Technol. 9(1998), 1751- 1756].
  • the inner wall of the copper metallic cavity is silver coated.
  • the sample is mounted on a cylindrical quartz crystal. The measurement is done in the transmission mode
  • the temperature variation of resonant frequency (T f ) can be measured by noting the variation of TEoi ⁇ mode resonant frequency with temperature.
  • Table-I The microwave dielectric data for the system of materials is presented in Table- 1.
  • the magnesium oxide powder is heated at 1000°C for 3 hours to convert the small percentage of carbonate into oxide.
  • the preparation and characterization of the compounds follow the same procedure described in Example 1.
  • the calcination and sintering are done at temperatures in the range 1100 - 1250°C and 1250°C - 1400°C respectively.
  • the results are shown in Table-2.
  • a plot of the variation of ⁇ r and T f with x is also shown in Fig- 1.
  • Table-2 The microwave dielectric properties of xZnO-(5-x)MgO-2Nb 2 O 5
  • the preparation and characterization of the compounds follow the same procedure described in EXAMPLE- 1.
  • the calcinations and sintering are done at temperatures in the range 1050- 1075°C and 1 190-1200°C respectively. Results are shown in Table-3.
  • the magnesium oxide powder is usually heated at 1000°C for 3 hours to convert the small percentage of carbonate into oxide weighed before cooling.
  • the preparation and characterization of the compounds follow the same procedure described in Example 1.
  • the calcination are done in the temperature range 1250- 1400°C for 4 to 8 hours and sintering in the range 1435-1600°C for 2 to 4 hours.
  • Table-4 Table-4
  • the formation of solid solution between the above two ceramics for the tuning of microwave dielectric properties is not possible due to large difference in the ionic radii of Ba and Zn and also due to the difference in crystal structure. Hence a stacked resonator between, the. above ceramics is tried.
  • the Ba 5 Nb 4 Oi 5 is formed from a stoichiometric mixture of high purity BaCO 3 and Nb2 ⁇ 5 by calcining at 1200-1225°C for 5 hour and sintered at 1200°C for 2 hour where as 5ZnO-2Nb 2 O 5 is formed from a stoichiometric mixture of high purity ZnO and Nb 2 O 5 at 1050°C for 4 hour and sintered at 1380°C for 2 hour through the solid state route.
  • the preparation conditions are well controlled such that diameters of the final sintered pellets are nearly the same (The mean deviation is ⁇ 0.5 %).
  • the sintered pellets were polished well.
  • the microwave dielectric constants and Q factors of the pellets were characterized accurately using the cavity method.
  • the dimensions and the microwave parameters measured for the samples were shown in Table-5.
  • the average dielectric constant measured for Ba5Nb O 15 pellets is 39.5 and that of 5ZnO-2Nb 2 O 5 pellets is 22 with less than 0.3% deviation.
  • the average value of Qxf 'for BajNb 4 Oi5 samples is 21845 with a maximum deviation of 16% and those for 5ZnO-2Nb2 ⁇ 5 are 77,560 and 4% respectively.
  • the pellets of one type say 5ZnO-2Nb2 ⁇ 5 are placed over the other in between two gold-coated copper metallic plate (the Hakki-Coleman setup) and microwave is applied.
  • the pellets can be glued together using low-loss ceramic glues like cyanoacrylic.
  • the resonant structure act like a single dielectric resonator mounted in the set up.
  • the equivalent dielectric resonator can be assumed to have a dielectric constant ⁇ cf rwith length and diameter is respectively obtained from the sum of lengths and average of the diameters of the individual pellets.
  • the TEo ⁇ mode resonant frequency of the resonant system is noted.
  • the pellets are reversed and the resonant frequency is noted.
  • the effective dielectric constant ( ⁇ C fr) is calculated using the formulae suggested by Hakki and Coleman.
  • the experiment is repeated for different possible combinations of the pellets.
  • the results are tabulated in Table-6.
  • the variation of the dielectric constant with volume fraction is also given in Fig-2.
  • the quality factors were measured using a cavity method described in Example- 1.
  • the Q factor and resonant frequency vary with the reversal of the pellets.
  • the experiment is repeated with different possible combination of pellets.
  • the results were tabulated in Table-7
  • the inventive system of microwave ceramics has high dielectric constant, high quality factor and small temperature variation of resonant frequencies.
  • Ba 5 Ta 4 Oi5 with dielectric constant of 28-29, quality factor greater than 5500 and low T between 4 and 13 pprn °C is a potential material for practical applications.
  • the 5ZnO-2Nb 2 Os samples show very high Q factor which is greater than 1200Q, high dielectric constant of 22 and intermediate ⁇ f of -65 to -75 ppm/°C.
  • the 5AO-2B 2 O 5 ceramic compositions give the A 5 B O ⁇ 5 type ceramics only when Ba, Sr and Mg are used at the A site.
  • the mixture phases formed from 5ZnO-2Nb 2 O 5 and 5CaO-2Nb 2 ⁇ 5 have negative Tf whereas 5CaO-2Ta2 ⁇ 5 has positive ⁇ r.
  • the phases were identified to be AB 2 O6, A 2 B 2 O or A3B 2 O 8 type ceramics.
  • the ceramic system are useful for tuning the dielectric properties of the hexagonal perovskites to the extent, which is permissible by substitution, doping, solid solution or by forming mixtures without much degradation of the required properties.
  • the magnesium site is attempted to replace with zinc, which resulted in the multiphase ceramics with the compositional formula xZnO- (5-x) MgO-Nb ⁇ Os.
  • the above said ceramics show ⁇ r in the range 1 1 to 22, Qxf between 18000 and 89000 and Tf between -54 ⁇ 3 and -73 ⁇ 3 ppm °C.
  • the system gives ceramics with very high Qxf in the range 36,000 to 89,000 with ⁇ r in the range 18 ⁇ 1 to 22+ 1 for 1.5 ⁇ x ⁇ 5.
  • the zinc site in the 5ZnO-2Nb 2 O 5 mixture system is tried to replace with calcium and the resulted mixture phased ceramics may be represented by the compositional formula xCaO - (5-x)ZnO - 2Nb 2 O 5 .
  • the results are summarized in Table-3.
  • the substitution of Ca up to x 0.5 decreases the Tf from -73 ⁇ 3 to -55 ⁇ 3 ppm °C.
  • the microwave dielectric properties were given in Table-4.
  • the MgsNb . x Ta x Oi5 ceramics have ⁇ r of 11 with high quality factor.
  • the set of materials in the 5AO-2B 2 O 5 provide microwave dielectrics with a wide range of ⁇ r (1 1-42), Q factor up to 88,000 and positive and negative ⁇ »- (between -73 and 140 ppm °C) useful for applications.
  • the microwave dielectric properties can be suitably tuned by stacking cylindrical resonators with negative Tf over those with positive Xr and vice versa.
  • the microwave dielectric response of the Ba 5 Nb 4 Oi5-5ZnO-2Nb 2 O5 stacked resonator system is given in Table-6 and Table-7.
  • the Q factor of the system increases with the volume fraction of 5ZnO-2Nb 2 O 5 whereas effective dielectric constant shows a reverse trend and it decreases from 39 to 22.
  • the T f gradually decreases from high positive value to high negative value with 5ZnO-2Nb 2 O5.
  • the effective dielectric constant is between 26 and 30, Qxf between 26000 and 34000 and T f between 20 and - 20 ppm/°C.
  • Stacking provides a method suitable for tuning the dielectric properties of ceramics having high dielectric constant and Q factor even if their Tf values are very high.
  • the inventive system of materials provides a large range of dielectric constant, quality factor with small temperature variation of resonant frequencies. 2. It provides dielectric resonator materials, which are useful for tuning the hexagonal perovskite with high dielectric constant and Q factor.
  • Some of the ceramics in the system have high dielectric constant and Q factor and low Tf suitable for practical applications.

Abstract

The present invention discloses a novel microwave dielectric composition of the general formula 5AO-2B2O5 wherein A = Ba, Sr, Ca, Mg or Zn and B = Nb or Ta, and a process for the preparation thereof. In one embodiment of the invention, the dielectric constant is in the range 11+/-1 to 42+/-1, quality factor frequency product in the range 2000 and 88,000 and temperature coefficient of resonant frequency in the range +140+/-7 and -73+/-5 ppm/ C.

Description

A MICROWAVE DIELECTRIC CERAMIC COMPOSITION AND A PROCESS FOR THE PREPARATION THEREOF
Field of the invention The present invention relates to a microwave dielectric ceramic composition and to a process for the preparation thereof. More particularly, the present invention relates to a microwave dielectric ceramic composition of the formula 5AO-2B2O5 (A = Ba, Sr, Ca, Mg, Zn; B= Nb, Ta) and to a process for the preparation thereof. Background of the invention Dielectric resonators require high dielectric constant (εr = 10 - 100) for miniaturization, high quality factor (Q > 2000) for selectivity and low temperature variation of resonant frequency (if < |20| ppm/°C) for stability while used in practical circuits for microwave applications. Conventional ceramics for use in such applications include BaO-TiO2 system, Ba(Mg! 3Ta23)O3, Ba(Zn1 3Ta2/3)O3, (Zr, Sn)Tiθ3 etc. But their applications are limited by the low dielectric constants because the size of the system is inversely proportional to the εr 1 2. Materials with different dielectric constants are required for different applications.
Ba5Nb Oi5 type hexagonal perovskites have high dielectric constant and high Q factor. H. Sreemoolanadhan, M. T. Sebastian and P. Mohanan (Mater. Res. Bull 30, 1996, pp 653) have reported the dielectric properties of the solid solution BaxSr5. xNb Ou (x= 0, 1, 2, 3, 4, 5) wherein the system show dielectric constants in the range 38-50 and Qxf in the range 6,600-44,000 GHz. The ceramics have hexagonal crystal structure. C. Veneis, P. K. Davies, T. Negas and S. Bell (Mater. Res. Bull. 3 1(5) 1996 pp 431- 437) have reported that Ba5Nb4Oi5 has dielectric constant of 39, Qxf = 26,000 and Tf = + 78 ppm °C. The high tf values of the ceramics render them unsuitable for practical applications. Objects of the invention
The main object of the present invention is to provide a novel microwave dielectric composition 5AO-2B2O5 (A = Ba, Sr, Ca, Mg, Zn; B= Nb, Ta) and achieving temperature compensation by stacking the resonators with positive and negative temperature coefficients of resonant frequency which obviates the drawbacks detailed above.
Another object of the present invention is to provide novel microwave dielectric ceramic composition by tailoring the dielectric properties of the high performance ceramics in the 5AO- 2B2O5 (A = Ba, Sr, Ca, Mg, Zn; B= Nb, Ta) system either by forming solid solution phases or by forming mixtures like xZnO- (5- x)MgO-2Nb2O5( 0<x<5).
Yet another object of the present invention is to provide to tune the microwave dielectric properties of the above ceramics and hence to achieve temperature compensation by stacking dielectric resonators with positive and negative its. Brief description of the accompanying drawings
Figure 1 shows the variation of εr and Tf with x.
Figure 2 shows the effective εr versus volume fraction of 5ZnO-2Nb2O5. Figure 3 shows the effective Tf versus volume fraction of 5ZnO-2Nb2O5.
Summary of the invention
Accordingly the present invention provides a novel microwave dielectric composition of the general formula 5AO-2B2O5 wherein A = Ba, Sr, Ca, Mg or Zn and B= Nb or Ta. In one embodiment of the invention, the dielectric constant is in the range
11 ± 1 to 42 ± 1, quality factor - frequency product in the range 2000 and 88,000 and temperature coefficient of resonant freqμency in the range + 140 + 7 and -73 + 5 pρm °C.
In another embodiment of the invention, the ceramic composition is of the formula Ba5Ta4Oi5 and wherein the dielectric constant is 28 ± 1, quality factor freq ency product greater than 32000 and temperature variation of resonant frequency 8±4 ppm °C.
In another embodiment of the invention, the ceramic composition is of the, formula 5ZnO-2Nb2O5, and wherein the dielectric constant is 22 ± 1, quality factor- frequency product greater than 88,000 and temperature variation of resonant frequency -73 ± 5ppm/°C.
In another embodiment of the invention, the ceramic composition is of the formula xA' - (5-x) A" - 2[yB'-(l-y) B"]2O5 ( A', A" = Ba, Sr, Ca, Mg, Zn; B\ B"= Nb, Ta) wherein 0<x<5 and 0<y<l . In another embodiment of the invention, the ceramic composition is of the formula xZnO- (5-x)MgO- 2Nb2O5 wherein 0<x< 5.
In another embodiment of the invention, wherein 1.5<x<5 and wherein the dielectric constant is in the range 18 ± 1 and 22 + 1, quality factor-frequency product is in the range 36000 to 89000 and temperature variation of resonant frequency is in the range -56 ± 3 and -73 ± 3 ppm/°C.
In another embodiment of the invention, the ceramic composition is of the formula xCaO - (5-x)ZnO - 2Nb2O5 wherein 0<x<l and wherein the dielectric constant is in the range 20+ 1 and 21 + 1, quality factor-frequency product is in the range 44,000 to 79,000 and temperature variation of resonant frequency is in the range -55 ± 3 and -69 ± 5 ppm °C.
In another embodiment of the invention, the ceramic composition is of the formula 0.5CaO-4.5ZnO-2Nb2O5 wherein the dielectric constant is 21 ± 1, quality factor-frequency product > 79,000 and temperature variation of resonant frequency is in the range -55 ± 3 ppm °C.
In another embodiment of the invention, the ceramic composition is of the formula A5B'xB"4.xO15 (A= Ba, Sr, Mg) [0<x<4J wherein the dielectric constant in the range 11 ± 1 and 36 ± 1, quality factor-frequency product between 3,000 and 25,000 and temperature variation of resonant frequency in the range -36 ± 3 and
+35± 3 ppm/°C.
The invention also relates to stacked resonators consisting of the above ceramics with opposite Tf values to tune the Tf to near to zero values.
In another embodiment of the invention, the stacked resonators are between Ba5Nb O15 and 5ZnO-2Nb2O5 ceramics wherein the volume fraction of 5ZnO-2Nb2O5 is in the range 0.6 and 0J where the dielectric constant varies from 26 to 30 and Tf varies between 20 and -20 ppm/°C.
In another embodiment of the invention, the microwave dielectric composition is of formula 5AO-2B2O5 wherein A = Ba, Sr, Ca or Mg, Zn; B= Nb or Ta, said process comprising reacting a carbonate or oxide of A with a pentoxide of B.
In another embodiment of the invention, the solid solutions or mixture phases with the general formula xA'- (5-x)A" - 2Nb2O5 ( A', A"= Ca, Mg or Zn) is prepared by mixing calcium carbonate or magnesium oxide and zinc oxide with niobium pentoxide in the x: 5-x: 2 ratio. In another embodiment of the invention, 0<x<l when A'= Ca and A' - Zn.
In another embodiment of the invention, x = 0.5, 1, 1.5, 2.0, 2.25, 2.5, 2J5, 3.0, 3.5, 4.0 and 4.5 when A'= Zn and A"= Mg, the mixture phases being prepared using the solid state ceramic route. In another embodiment of the invention, the solid solution is prepared using BaCO3, SrCO3 or MgO with Nb2O5 and Ta2O5 in the appropriate molar ratio for 5AO - (x/2)Nb2O5 - ((4-x)/2)Ta2O5 (x= 1, 2, 3) where A= Ba, Sr and Ca.
In one embodiment of the invention, the microwave dielectric ceramic comprises MgsT^O^ Mg5Ta Oι5; SrJTa Oι5; BajT∑uOis; Mg5Nb4Oι5; Mg5Nb4Oι5; 5ZnO-2Nb2O5; 5CaO-2Ta2O5 and 5CaO-2Nb2O5.
In another embodiment of the invention temperature compensation is achieved by stacking the resonators with positive and negative temperature coefficients of resonant frequency by preparing the perovskites of the invention in the powder form, moulding of the powder in the suitable shape, drying, sintering and final treatment.
In another embodiment of the invention the solid solutions or mixture phases with the general formula xA'- (5-x)A" - 2Nb2O5 ( A', A"= Ca, Mg or Zn) are prepared by mixing calcium carbonate or magnesium oxide and zinc oxide with niobium pentoxide in the x: 5-x: 2 ratio. In another embodiment of the invention, in A5B Oi5 ceramic the valency of A is two and that of B is five. Detailed description of the invention
The solid solutions or mixture phases with the general formula xAη- (5-x)A" - 2Nb2O5 ( A', A"= Ca, Mg or Zn) are prepared by mixing calcium carbonate or magnesium oxide and zinc oxide with niobium pentoxide in the x: 5-x: 2 ratio. When A'= Ca and A"= Zn, 0<x<l . When A'= Zn and A"= Mg and x = 0.5, 1, 1.5, 2.0, 2.25, 2.5, 2.75, 3.0, 3.5, 4.0 and 4.5, the mixture phases are prepared for using the solid state ceramic route.
For 5AO - (x 2)Nb2O5 - ((4-x)/2)Ta2O5 (x= 1, 2, 3) where A- Ba, Sr and Ca the solid solutions are prepared using BaCO3, SrCO3 or MgO with Nb2O5 and Ta2θ5 in the appropriate molar ratio.
The stacking of resonators having opposite Tr values is studied to achieve temperature compensation.
In A5B4O15 ceramic the valency of A is two and that of B is five. The calcium, Magnesium and zinc are attempted to substitute at A site keeping niobium at B site to get the respective niobates. In another series keeping the tantalum at B, barium, strontium, calcium and magnesium are attempted to substitute at A site in order to obtain the respective tantalates. In the case of ceramics having the similar structure, the atoms of a particular site may be replaced by another atom of the same valency and nearly the same ionic radius to form solid solution phases with intermediate dielectric constant and temperature coefficient of resonant frequencies.
The A5B4O15 system provides ceramic materials with large range of dielectric constant, Q factor and positive and negative temperature coefficient of resonant frequencies and hence provides good scope for tuning the dielectric properties of ceramics with similar structure.
The present system of ceramics is useful for applications as dielectric resonators in communication systems and as substrates in microwave integrated circuits. Compared to the use of alumina substrates they decrease the size not only for strip line resonators and filters but also for all microwave circuits. It is also possible to use these dielectric materials in the fabrication of devices such as circulators, phase shifters etc. for impedance matching. The dielectric resonators based on the above compounds are also useful for the fabrication of dielectric resonator antennas. Solid solution formation in the above system enables to tune the dielectric constant and temperature variation of resonant frequency in the Ba5Nb4Oi5 type hexagonal perovskites.
The detailed description of this invention will now be presented with specific examples. It is to be understood that the invention is not limited to the details of the illustrated examples.
EXAMPLE-1 The A5B4O15 type compounds are prepared by allowing the respective carbonates or oxides of A (A= Ca, Mg, Zn) to react with the pentoxides of B (Nb or Ta) through the solid-state ceramic route. The oxides or carbonates of A are wet mixed with niobium pentoxide/tantalum pentoxide in the molar ratio. The mixed powder is calcined in the range 1100°C - 1400°C and cooled to the room temperature. The calcine is ground well, PVA is added as the binder, dried and again ground. The resultant fine powder is pellettized in the appropriate size for the measurement (5- 9 mm in height and 11 mm in diameter). The careful design of the dimensions of the samples is a prerequisite for the accuracy of the microwave dielectric measurements. The height of the sintered samples should be less than their diameter for the accuracy of the results. The D/L (D= Diameter; L= length) ratio of 2 - 2.3 is preferable for the Q factor measurements. The sintering temperatures of the samples were optimized at different temperatures in the range 1220-1625°C. The sintered samples are polished well to avoid any irregularities on the flat surface and are used for measurements The microwave dielectric constant is measured using Hakki- Coleman dielectric post resonator method. The resonator is placed between two gold-coated copper metallic plates and microwave energy is coupled through an E- field probe to excite various resonant modes. Among the various resonant modes the TEon mode is selected for the measurement.
The above ceramics resonate at frequencies between 4 and 10 GHz The quality factors of the samples are measured at the TE01δ mode resonant frequency using a cavity method [Jerzy Krupka, Krzytof Derzakowsky, Bill Riddle and James Baker Jarviz, Meas. Sci. Technol. 9(1998), 1751- 1756]. The inner wall of the copper metallic cavity is silver coated. The sample is mounted on a cylindrical quartz crystal. The measurement is done in the transmission mode The temperature variation of resonant frequency (Tf) can be measured by noting the variation of TEoiδ mode resonant frequency with temperature. The Tf is calculated using the formula τf = (1/f) x (ΔffΔT) where Δf is the variation of resonant frequency from the room temperature (usually 20°C) resonant frequency and ΔT is the difference in temperature from room temperature. The microwave dielectric data for the system of materials is presented in Table- 1. Table -I
The microwave dielectric properties of A5B4O15 ceramics
* Heated MgO is used for calcinations
** Measurement is not possible due to poor resonance EXAMPLE-2
The ceramics with the general formula xZnO- (5-x)MgO-2Nb2O5 are prepared by mixing magnesium oxide, zinc oxide and niobium pentoxide in the x:5-x:2 with x=0.5, 1, 1.5, 2.0, 2.25, 2.5, 2.75, 3.0, 3.5, 4.0 and 4.5. The magnesium oxide powder is heated at 1000°C for 3 hours to convert the small percentage of carbonate into oxide. The preparation and characterization of the compounds follow the same procedure described in Example 1. The calcination and sintering are done at temperatures in the range 1100 - 1250°C and 1250°C - 1400°C respectively. The results are shown in Table-2. A plot of the variation of εr and Tf with x is also shown in Fig- 1. Table-2 The microwave dielectric properties of xZnO-(5-x)MgO-2Nb2O5
Example-3
The ceramics with the general formula xCaO - (5-x)ZnO - 2Nb2O5 are prepared by mixing calcium carbonate , zinc oxide and niobium pentoxide in the x:5- x:2 with x=0.1, 0.2, 0.5, 1.0. The preparation and characterization of the compounds follow the same procedure described in EXAMPLE- 1. The calcinations and sintering are done at temperatures in the range 1050- 1075°C and 1 190-1200°C respectively. Results are shown in Table-3. The ceramics are glossy and do not resonate for x =1.0.
Table-3 Microwave dielectric properties of xCaO- (5-x)ZnO - 2Nb2O5
EXAMPLH- 4 Ba, Sr and Mg) are prepared by mixing the respective oxides or carbonates of A with niobium pentoxode or tantalum pentoxide in the appropriate ratios for x=l, 2,3. The magnesium oxide powder is usually heated at 1000°C for 3 hours to convert the small percentage of carbonate into oxide weighed before cooling. The preparation and characterization of the compounds follow the same procedure described in Example 1. The calcination are done in the temperature range 1250- 1400°C for 4 to 8 hours and sintering in the range 1435-1600°C for 2 to 4 hours. The results are shown in Table-4. Table-4
Microwave dielectric properties of AsN t-xTaxOu ceramics (A= Ba, Sr and Mg)
Example 5 Ba b4Oi5-5ZnO-2Nb2θ5 stacked resonators
The hexagonal type BasNb4Oi5 is reported to have εr = 39.0, high Qxf up to 25,000 and τf = +78 ppm/°C where as 5ZnO-2Nb O5 has εr = 22, high Qxf up to 88,000 and Tf= -73 ppm/°C. The formation of solid solution between the above two ceramics for the tuning of microwave dielectric properties is not possible due to large difference in the ionic radii of Ba and Zn and also due to the difference in crystal structure. Hence a stacked resonator between, the. above ceramics is tried. The Ba5Nb4Oi5 is formed from a stoichiometric mixture of high purity BaCO3 and Nb2θ5 by calcining at 1200-1225°C for 5 hour and sintered at 1200°C for 2 hour where as 5ZnO-2Nb2O5 is formed from a stoichiometric mixture of high purity ZnO and Nb2O5 at 1050°C for 4 hour and sintered at 1380°C for 2 hour through the solid state route. The preparation conditions are well controlled such that diameters of the final sintered pellets are nearly the same (The mean deviation is < 0.5 %). The sintered pellets were polished well. The microwave dielectric constants and Q factors of the pellets were characterized accurately using the cavity method. The dimensions and the microwave parameters measured for the samples were shown in Table-5. The average dielectric constant measured for Ba5Nb O15 pellets is 39.5 and that of 5ZnO-2Nb2O5 pellets is 22 with less than 0.3% deviation. The average value of Qxf 'for BajNb4Oi5 samples is 21845 with a maximum deviation of 16% and those for 5ZnO-2Nb2θ5 are 77,560 and 4% respectively. The pellets of one type say 5ZnO-2Nb2θ5 are placed over the other in between two gold-coated copper metallic plate (the Hakki-Coleman setup) and microwave is applied. The pellets can be glued together using low-loss ceramic glues like cyanoacrylic. The resonant structure act like a single dielectric resonator mounted in the set up. The equivalent dielectric resonator can be assumed to have a dielectric constant εcfrwith length and diameter is respectively obtained from the sum of lengths and average of the diameters of the individual pellets. The TEoπ mode resonant frequency of the resonant system is noted. The pellets are reversed and the resonant frequency is noted. The effective dielectric constant (εCfr) is calculated using the formulae suggested by Hakki and Coleman. The experiment is repeated for different possible combinations of the pellets. The results are tabulated in Table-6. The variation of the dielectric constant with volume fraction is also given in Fig-2. The quality factors were measured using a cavity method described in Example- 1. The Q factor and resonant frequency vary with the reversal of the pellets. The experiment is repeated with different possible combination of pellets. The results were tabulated in Table-7.
Table-5 The microwave characteristics Ba5Nb Oi5 and 5ZnO-2Nb2θ5 samples
Could not be measured due to the undersize of sample
L0 TabIe-6
The εr and Tf of the stacked resonators between Ba5Nb4O15 and 5ZnO-2Nb2θs
Table-7
The resonant frequency and Q factor of the stacked resonators (Measured using cavity resonator setup)
The inventive system of microwave ceramics has high dielectric constant, high quality factor and small temperature variation of resonant frequencies. Ba5Ta4Oi5 with dielectric constant of 28-29, quality factor greater than 5500 and low T between 4 and 13 pprn °C is a potential material for practical applications. The 5ZnO-2Nb2Os samples show very high Q factor which is greater than 1200Q, high dielectric constant of 22 and intermediate τf of -65 to -75 ppm/°C. The 5AO-2B2O5 ceramic compositions give the A5B Oι5 type ceramics only when Ba, Sr and Mg are used at the A site. The mixture phases formed from 5ZnO-2Nb2O5 and 5CaO-2Nb2θ5 have negative Tf whereas 5CaO-2Ta2θ5 has positive τr. The phases were identified to be AB2O6, A2B2O or A3B2O8 type ceramics. The ceramic system are useful for tuning the dielectric properties of the hexagonal perovskites to the extent, which is permissible by substitution, doping, solid solution or by forming mixtures without much degradation of the required properties.
In order to tune the microwave dielectric properties of the Mg5Nb Oi5 ceramics, the magnesium site is attempted to replace with zinc, which resulted in the multiphase ceramics with the compositional formula xZnO- (5-x) MgO-Nb∑Os. The above said ceramics show εr in the range 1 1 to 22, Qxf between 18000 and 89000 and Tf between -54 ± 3 and -73 ± 3 ppm °C. The system gives ceramics with very high Qxf in the range 36,000 to 89,000 with εr in the range 18 ± 1 to 22+ 1 for 1.5 < x < 5. In another attempt, the zinc site in the 5ZnO-2Nb2O5 mixture system is tried to replace with calcium and the resulted mixture phased ceramics may be represented by the compositional formula xCaO - (5-x)ZnO - 2Nb2O5. The results are summarized in Table-3. The substitution of Ca up to x = 0.5 decreases the Tf from -73 ± 3 to -55 ± 3 ppm °C. For x =1 the samples do not resonate. The replacement of B site niobium with tantalum in the hexagonal A5Nb Oi5 (A= Ba, Sr, Mg) ceramics gives AsNb4. xTaxOi5 (A= Ba, Sr, Mg) solid solution phases. The microwave dielectric properties were given in Table-4. The ceramics has Ba5Nb .xTaxOi5 (x= 1, 2, 3) solid solutions show high εr from 26 to 32, low Tf from +14 to + 35 ppm/°C and high Qxf from 4849 to 21683 GHz. Sr5Nb4-χTaxNb4O15 (x= 1, 2, 3) show high εr between 32 and 36 for Kx<3. The MgsNb .xTaxOi5 ceramics have εr of 11 with high quality factor. Hence the set of materials in the 5AO-2B2O5 provide microwave dielectrics with a wide range of εr (1 1-42), Q factor up to 88,000 and positive and negative τ»- (between -73 and 140 ppm °C) useful for applications.
The microwave dielectric properties can be suitably tuned by stacking cylindrical resonators with negative Tf over those with positive Xr and vice versa. The microwave dielectric response of the Ba5Nb4Oi5-5ZnO-2Nb2O5 stacked resonator system is given in Table-6 and Table-7. The Q factor of the system increases with the volume fraction of 5ZnO-2Nb2O5 whereas effective dielectric constant shows a reverse trend and it decreases from 39 to 22. The Tf gradually decreases from high positive value to high negative value with 5ZnO-2Nb2O5. When the volume fraction of 5ZnO-2Nb2θ5 0.6 to 07 the effective dielectric constant is between 26 and 30, Qxf between 26000 and 34000 and Tf between 20 and - 20 ppm/°C. Stacking provides a method suitable for tuning the dielectric properties of ceramics having high dielectric constant and Q factor even if their Tf values are very high. The main advantages of the present invention are
1. The inventive system of materials provides a large range of dielectric constant, quality factor with small temperature variation of resonant frequencies. 2. It provides dielectric resonator materials, which are useful for tuning the hexagonal perovskite with high dielectric constant and Q factor.
3. Some of the ceramics in the system have high dielectric constant and Q factor and low Tf suitable for practical applications.
4. Achieving temperature compensation by stacking the resonators with positive and negative temperature coefficient of resonant frequencies coefficient of resonant frequency to near to zero by stacking dielectric resonators with positive and negative Tf.

Claims

We claim
1. A microwave dielectric composition of the general formula 5AO-2B2O5 wherein A = Ba, Sr, Ca, Mg or Zn and B= Nb or Ta.
2. A microwave dielectric composition as claimed in claim 1 wherein the dielectric constant is in the range l l ± l to 42 ± 1, quality factor - frequency product in the range 2000 and 88,000 and temperature coefficient of resonant frequency in the range + 140 ± 7 and -73 ± 5 ppm/°C.
3. A microwave dielectric composition as claimed in claim 1 wherein the ceramic composition is of the formula BasT^Ou and wherein the dielectric constant is 28+ 1, quality factor frequency product greater than 32000 and temperature variation of resonant frequency 8 ± 4 ppm °C.
4. A microwave dielectric composition as claimed in claim 1 wherein the ceramic composition is of the formula 5ZnO-2Nb2θ5, and wherein the dielectric constant is 22 ± 1, quality factor-frequency product greater than 88,000 and temperature variation of resonant frequency -73 ± 5ppm/°C.
5. A microwave dielectric composition as claimed in claim 1 wherein the ceramic composition is of the formula xA' - (5-x) A" - 2[yB'-(l-y) B"]2O5 ( A', A" = Ba, Sr, Ca, Mg, Zn; B', B"= Nb, Ta) wherein 0<x<5 and 0<y<l.
6. A microwave dielectric composition as claimed in claim 1 wherein the ceramic composition is of the formula xZnO- (5-x)MgO- 2Nb2O5 wherein 0<x< 5.
7. A microwave dielectric composition as claimed in claim 6 wherein 1.5<x<5 and wherein the dielectric constant is in the range 18 ± 1 and 22 ± 1, quality factor- frequency product is in the range 36000 to 89000 and temperature variation of resonant frequency is in the range -56 ± 3 and -73 ± 3 ppm °C.
8. A microwave dielectric composition as claimed in claim 1 wherein the ceramic composition is of the formula xCaO - (5-x)ZnO - 2Nb2O5 wherein 0<x<l and wherein the dielectric constant is in the range 20 ± 1 and 21 ± 1, quality factor- frequency product is in the range 44,000 to 79,000 and temperature variation of resonant frequency is in the range -55 ± 3 and -69 ± 5 ppm/°C.
9. A microwave dielectric composition as claimed in claim 8 wherein the ceramic composition is of the formula 05CaO-4.5ZnO-2Nb2Oj wherein the dielectric constant is 21 ± 1, quality factor-frequency product > 79,000 and temperature variation of resonant frequency is in the range -55 ± 3 ppm °C.
10. A microwave dielectric composition as claimed in claim 1 wherein the ceramic composition is of the formula A5B'xB"4-xO15 (A= Ba, Sr, Mg) [0<x<4] wherein the dielectric constant in the range l l ± l and 36 ± 1, quality factor-frequency product between 3,000 and 25,000 and temperature variation of resonant frequency in the range -36 ± 3 and +35 ± 3 ppm/°C.
1 1. Stacked resonators consisting of the ceramics of any of the preceding claims with opposite Tf values to tune the τrto near to zero values.
12. Stacked resonators as claimed in claim 1 1 comprising between Ba5Nb Oi5 and 5ZnO-2Nb2O5 ceramics wherein the volume fraction of 5ZnO-2Nb2O5 is in the range 0.6 and 0.7 where the dielectric constant varies from 26 to 30 and Tf varies between 20 and -20 ppm °C.
13. A process for the preparation of microwave dielectric composition of formula 5AO-2B2O5 wherein A = Ba, Sr, Ca or Mg, Zn; B= Nb or Ta, said process comprising reacting a coarbonate or oxide of A with a pentoxide of B.
14. A process as claimed in claim 13 wherein the solid solutions or mixture phases with the general formula xA'- (5-x)A" - 2Nb2O5 ( A', A"= Ca, Mg or Zn) is prepared by mixing calcium carbonate or magnesium oxide and zinc oxide with niobium pentoxide in the x: 5-x: 2 ratio.
15. A process as claimed in claim 13 wherein, 0<x<l when A'= Ca and A' - Zn.
16. A process as claimed in claim 13 wherein, x = 0.5, 1, 1.5, 2.0, 2.25, 2.5, 2.75,
3.0, 3.5, 4.0 and 4.5 when A'= Zn and A"= Mg, the mixture phases being prepared using the solid state ceramic route.
17. A process as claimed in claim 13 wherein the solid solution is prepared using
BaCO3, SrCO3 or MgO with Nb2O.<i and Ta2O5 in the appropriate molar ratio for 5AO - (x/2)Nb2O5 - ((4-x)/2)Ta2O5 (x= 1 , 2, 3) where A= Ba, Sr and Ca.
EP01934303A 2001-03-30 2001-03-30 A microwave dielectric ceramic composition and a process for the preparation thereof Withdrawn EP1373161A1 (en)

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