GB2382574A

GB2382574A - Dielectric ceramic composition

Info

Publication number: GB2382574A
Application number: GB0128909A
Authority: GB
Inventors: Robert Ian Scott; Christopher John Hampson; Christine Nora Elgy
Original assignee: MORGAN MATROC Ltd; Morgan Advanced Ceramics Ltd
Current assignee: MORGAN MATROC Ltd; Morgan Advanced Ceramics Ltd
Priority date: 2001-12-03
Filing date: 2001-12-03
Publication date: 2003-06-04
Also published as: GB0128909D0

Abstract

A dielectric ceramic composition comprises: <CHE>(Ba x Sr (1-x) )(Zn (1-y) CO y ) 1/3 Nb 2/3 O 3 </CHE> wherein 0 < x < 1, 0 < y < 1 and x * y. The composition may also include Ga and Al as part of the crystal structure in amounts up to 10 at%. The dielectric compositions may be used as resonators in the microwave frequency region.

Description

DIELECTRIC CERAMIC COMPOSITION This invention relates to a dielectric ceramic composition, and a method of controlling independently the dielectric properties of said composition.

Dielectric materials designed for use as resonators in the microwave frequency region must have three important properties:

(a) A high relative permittivity (dielectric constant)-this allows the physical size to be 1 reduced (governed by-,) since the physical size is related to the wavelength being worked upon, and the wavelength is related to Sr.

(b) The ceramic should exhibit a small dielectric loss at high frequency, to result in a large value of the (Q) quality factor. In practical terms Q should exceed 20,000 and preferably exceed 34, 000.

(c) The dielectric ceramic should exhibit a resonant frequency that changes little with temperature and can be controlled readily. The temperature coefficient of resonant

, 0. 10-6 C-1, 10. 10-6 frequency Tf may usefully be in the range 30. 10' C'', or preferably 10. 10' C"', and tunable through 0.

Dielectric materials designed for use as resonators in the microwave frequency region and with a dielectric constant (sir) of around 30 are conventionally made from tantalum-containing materials. However, the main drawback of this is the cost of tantalum. It is also possible to manufacture dielectric materials with an Sr of around 40 from niobate materials of the form :- Ba (ZnI/3Nb2/3) 03 (BZN), alone and as mixtures with other niobate materials such as :-

Sr (Znlf3Nb2l3) 03 (SZN) and :Ba (COlf3Nb2/3) 03 (BCN).

Onoda et al. (Jpn. J. Appl. Phys. 21 pp 1707-1710 (1982) ) discloses a tantalum-free dielectric material comprising a solid solution of BZN and SZN. Compositions of the form :-

(Ba (Zn,/3Nb2/3) 03) x (Sr (Zn,/3Nb2/3) 03) ()-.) where 0 < x < 1 were investigated for dielectric properties suitable for use at 10 GHz. In particular, the composition x=0.3 was found to give a dielectric constant (sir) of 40, a Qo value of 3050 and a temperature coefficient of resonant frequency (if) of-5 ppm/ C. In this case, the dielectric properties are controlled by the variation of the Ba/Sr ratio.

Endo a/. (Y. /K. C < ?/' :. & ? c. 70 C-215-C-218 (1987)) discloses a tantalum-free dielectric material comprising a solid solution of BCN and BZN. Compositions of the form :- (Ba (Coi/3Nb2/3) 03) x (Ba (Zn,/3Nb2/3) 03) (.-x) were found to give 8r in the region of 30-40 at 6.5 GHz. In particular, compositions wherein x = 0.7 gave a high Qo value of 11000 and Tf of 0 ppm/ C. In this case the value of Tf is controlled by varying the Zn/Co ratio. The authors also noted that the value of Qo is controllable by nitrogen addition to the annealing atmosphere.

Japanese Patent Application no. JP5298922 discloses a dielectric material comprising a solid solution of BCN and SZN. Compositions of the form :- (Ba (Co1/3Nb2/3)O3)x(Sr (Zn1/3Nb2/3)O3)(1-x) where x is limited to the range 0.3 < x < 0.8, give high unloaded 00 values and a if value of 0 ppm/ C. Here the dielectric properties are controlled by the ratio of Ba/Sr, which is tied to the ratio of Co/Zn.

These conventional niobate dielectric compositions are all based on binary systems. This gives only one degree of freedom when altering the composition, as variation of one component consequently produces variation in the other. In practical terms, this causes the dielectric properties of the composition (Er, Tf) to be linked, and it is not possible to separate them. In all of the above examples it is not possible to change the value of if without affecting the value of 8r. Consequently this imposes severe limitations on choice of raw materials, cost and applications to which such compositions are suited. None of the above publications have explored more complex mixtures such as BCN/BZN/SZN or BCN/SZN/SCN (where SCN is (Sr, (Co (,-,) Nb2/3) 03)).

Hughes, Iddles & Reaney (Applied Physics Letters Vol. 79, no. 18 pages 2952-2954) propose adding barium gallium titanates to mixed BZN/BCN dielectrics to achieve a zero Tf. Such materials do not solve the problem of the cost of tantalum compounds.

A problem with the use of binary materials comprising cobalt is that accurate compositional control is difficult, due to the volatilisation of Co during firing. Also, especially at high firing temperatures, contamination of refractory kiln linings and kiln furniture with Co occurs, limiting the life of the refractories and requiring dedicated kilns and kiln furniture, thus considerably adding to the cost and difficulty of manufacture of such materials.

There is also a need to form dielectric materials with a value ofsr of approximately 30 and a high Qo value. Such materials are conventionally provided by tantalum compounds. The major drawback of this is the high cost of tantalum. Therefore the formation of a high Qo tantalum-free microwave dielectric composition with a value ofsr of approximately 30 would also be advantageous.

The sintering atmosphere is also known to influence the value of Qo, as discussed in Endo et al. The authors found that sintering in a nitrogen atmosphere increased the value of Qo in comparison with materials sintered in an Os atmosphere. The values of Qo also increased with annealing time, regardless of the atmosphere used. However, long annealing times are not practical in an industrial situation.

The applicants have realised that what is needed is a method by which the dielectric properties of a tantalum-free material may be controlled independently. This would allow materials to be designed to meet a particular need or specification. In order to do this, compositions other than binary systems must be explored.

Accordingly the invention provides a dielectric ceramic composition comprising:

(Ba. Sr (i)) (Zn (i. y) Coy)]/3Nb2/303 wherein 0 < x < l, 0 < y < l and x y.

The invention further provides a dielectric ceramic composition wherein the composition further comprises Ga as part of the crystal structure.

The invention yet further provides a dielectric ceramic composition wherein the composition further comprises Al as part of the crystal structure.

Up to 10% of Ga or Al may be substituted into the crystal structure.

The invention yet further provides a resonator comprising said dielectric composition.

The invention yet still further provides a method of controlling the properties of a dielectric resonator independently by providing a composition comprising: (BaxSr . x)) (Zn . y) Coy)]/3Nb2/303 and wherein 0 < x < l, 0 < y < l and x y.

Further features of the invention will be apparent from the claims in the light of the following description.

The invention is illustrated by the following description and examples and with reference to the drawings, in which: Figure 1, is a schematic representation of binary dielectric compositions disclosed in the prior art plotted together with the more complex dielectric compositions in accordance with the present invention; Figure 2, is a similar but simplified such representation showing particular ranges of interest for a zero temperature coefficient dielectric; and Figure 3, is a similar but simplified such representation contrasting the approach of the prior art to the present invention.

Figure 1 shows a simple comparison of the inventive compositions over the prior art. Figure 1

plots on the Y axis the number of moles of Sr present in a mole of the dielectric having the formula :- (BaxSr (1~x)) (Zn (]-y) Coy))/3Nb2/303 against, on the X axis, the number of moles of Co present. The Onoda et al., compositions correspond to the formula in which y = 0 and lie along the Y axis. The compositions disclosed by Endo et al. correspond to the formula in which x = 1 and

lie along the X axis. JP5298922 corresponds to the formula in which x = y and lie along the diagonal line from Sr = 1 mole to Co = 1/3 mole.

This is an excellent illustration of the limitations of binary systems-dielectric properties may only be altered by traversing along these lines, and all properties can only be altered in the same way. It is thus very difficult to provide dielectrics of specified dielectric constant grand temperature coefficient of resonant frequency Tf.

Plotted on Figure I are the dielectric constants Sr and temperature coefficient of resonant frequency if (in units ofMK'') shown in the above mentioned literature (as circles with italic text) and like values for the compositions of the present invention (as diamonds with bold text) for which :- 0 < x < 1, 0 < y < 1 and x &num; y.

The dotted triangle shown connects points from the prior art with approximately zero temperature coefficient of resonant frequency and it seemed to the applicants that by exploring in and around this region, useful compositions of approximately zero temperature coefficient of resonant frequency could be obtained. As a preliminary investigation a number of compositions were made and tested and these are indicated by the diamonds on Figure 1. From these results it can be seen (as indicated in Figure 2) that there are in fact two regions of low temperature coefficient of resonant frequency (indicated as the two double headed arrows in Figure 2) separated by a region of higher temperature coefficient of resonant frequency.

One region seems to correspond approximately to Sr levels in the range 0.55 to 0.75 moles per formula mole (x = 0.45 to 0.25) and may lie between 0.6 to 0.7 (x = 0.4 to 0.3). The other region seems to correspond approximately to Sr less than 0. 1 moles (x = 0.9 or more).

By moving away from the previous compositions in which variation in properties was achieved by varying x, or y, (or both identically together) in the above formula, a further degree of freedom in terms of compositional variation is achieved. This is highly advantageous in that it is possible to it is possible to provide a range of materials of constant relative permittivity and specified temperature coefficient of frequency (rf) over the practical range of interest (usually : 1 : 5 ppm/C). For example, by moving along the broad double-headed arrow of Figure 3 it is possible to have relatively constant permittivity (approximately 33. 3) while varying the temperature coefficient of frequency (from-2 to 5.9).

The ability to vary the value of Sr whilst the value of 1 : f remains constant at 0 ppm/ C is also of interest.

Dielectric compositions of the form (BaxSr. xZn. yOyNbC are of particular interest for forming resonators with an Sr of approximately 30 (e. g 30-35), and replacing tantalumcontaining materials. They may also provide useful alternatives to existing commercial materials with Sr-36 and Er-39.

Illustrative examples of dielectric ceramic compositions in accordance with the invention

were manufactured using the following method. Starting materials of a high purity were z : l weighed out, together with de-ionised water. The starting materials were mixed and pulverised in a mill for approximately 15 hours, until the average particle size was below 1 m. A slip was formed by the addition of the de-ionised water.

Impurities may be introduced by the milling action, depending on the type of media and milling time, although these are not present above 0.5 wt%.

The resulting slip was dried to < 1 % moisture content, and calcined at 900-1250 C, typically for 4-12 hours.

The calcined material was re-milled after mixing with pure de-ionised water for 15-24 hours

until the average particle size was below 2, um. Binders accounting for up to 6% by weight of the overall mixture were added, and the mixture dried to < 0. 5% moisture content. The resultant powder was compacted to shape using a hydraulic press and tooling in a manner familiar to those skilled in the art to shape and sintered in air at a temperature of 1150- 1500 C for 4-24 hours until a dense ceramic material was obtained. Optionally additional heat treatment at or below the sintering temperature for periods up to 100 hours was used to improve the dielectric properties of the material.

Suitable sintering aids (which may typically be chosen from at least one of zirconia, barium I zirconate, tungsten oxide, barium tungstate, tin oxide, aluminium, or aluminium-containing compounds) were added either with the raw materials or after the calcination stage. This list should not be taken to exclude any other suitable sintering aids known within the art.

The dielectric ceramic compound may further comprise gallium and/or aluminium additions, up to 10at%. These elements are substituted for Zn and/or Co into the crystal structure of the

composition. This may be advantageous as it appears to lead to a reduction in the firing temperature required to obtain a dense ceramic. This reduces the energy required to produce the ceramic material, and consequently reduces refractory costs and production times. A further advantage is that lowering the firing temperature reduces the volatilisation of Co from the material during firing. This reduces kiln contamination and improves compositional control. Such additives may also be used with the binary niobate compositions described above.

It should further be noted that some degree of substitution by other like elements can be tolerated; for example, limited substitution of the Ba or Sr by other alkaline earth metals such as Ca is acceptable.

Results obtained are indicated in Table 1 which shows the sample number, composition, permittivity, temperature coefficient of resonant frequency 1r, and quality factor Q*f of a number of samples.

Microwave measurements were made by the cylindrical resonance method, to determine the microwave quality factor (Q) and the temperature coefficient of resonant frequency err). This was undertaken over a frequency range 6.5 to 8.5 GHz. The Q value was subsequently converted into a figure of merit, by the relationship Qu fur = constant

where Qo is tan 0 -1 and fr is the resonant frequency.

The temperature coefficient of resonant frequency (tif) was measured between-30 and +70 C ( 1 C) with the resonant frequency at 25 C as a reference ; the relative permittivity (8r) was measured using the parallel plate methodology.

Table 1

Sample Ba Sr Zn Co Nb Ga Al O Permittivity @f Q*f Number ppm/C GHz 01SMP1 0.95 0.05 0.315 0.018 0.667 0 0 3.0005 38.2 34.2 67000 01 SMP2 0. 950. 050. 060. 273 0. 667 0 3. 00053335. 9 78000 01 SMP3 0. 95 0.05 0.003 0.33 0.667 0 0 3.0005 29.9 1 62000 01SMP4 0. 15085OJ150. 018 0. 667 0 0 3.0005 38.2 -10.6 24000 01 SMP5 0. 05 0.95 0.3 0.033 0.667 0 0 3.0005 36.4 -31.4 28000 01SMP6 0.05 0.95 0.003 0.33 0.667 0 0 3.0005 31.1 -56.3 18000 01SMP7 0.35 0.65 0.2 0.133 0.667 0 0 3.0005 38 -14.1 32500 01SMP8 0.3075 0.6925 0.315 0.018 0.667 0 0 3.0005 40 -6.3 33000 01SMP9 0.325 0.675 0.26 0.073 0. 667 0 0 3. 000538. 8 10. 7 28000 01SMP10 0.64 0.36 0.166 0.167 0.667 0 0 3.0005 37.8 80.5 53000 01SMP11 0.6 0.4 0.233 0.1 0. 667 03. 0005 39. 8 95. 1 51000 E89 0. 9854 0. 0146 0. 117 0. 208 0. 6641 0.0125 0 3.0005 34.4 4.31 34,931 As is well known, Q values depend highly upon density and firing conditions (including firing atmosphere). The Q values given in Table 1 should be taken as indicative only, as the conditions used were not optimised for the test materials. The examples show that usable Q values are achieved and with optimisation higher values appear readily achievable. The results also show that permittivity and temperature coefficient of resonant frequency can also be controlled independently for this system.

It should be noted that the composition of the material can, and frequently will, deviate from the stoichiometric ratios so that it has a composition (BaxSr(1-#)(1~k)(Zn(1-y)Coy)(1/3=1)Nb(2/3~m)O3 wherein 0 < x < I

0 < y < 1 x my 0 < k < 0. 1 0 0

and the present invention encompasses such compositions. Such deviations from stoichiometry can be intentional or accidental. It is well known in the art that there can be errors in raw materials analyses, and that there are losses during firing that can cause deviations from stoichiometry. Preferably k, I and m are as small as possible.

Claims

CLAIMS 1. A dielectric ceramic composition comprising : (BaxSr(1-x))(Zn(1-y)Coy)1/3Nb2/3O3 wherein 0 < x < 1, O < y < l andx&num;y.
2. A dielectric composition, as claimed in Claim 1, in which the composition is non- stoichiometric and has the formula (BaxSr (i. x)) (I k) (Zn (1-y)Coy)(1/3=1)Nb(2/3=m)O3 wherein 0 < x < 1 0 < y < 1

x e y O < k < O. l 0 0.
3. A dielectric ceramic composition, as claimed in Claim 1 or Claim 2, wherein the composition further comprises Ga as part of the crystal structure.
4. A dielectric ceramic composition, as claimed in Claim 3, wherein the amount of Ga is less than or equal to 10 at%.
5. A dielectric ceramic composition, as claimed in any of Claims I to 4, wherein the composition further comprises Al as part of the crystal structure.
6. A dielectric ceramic composition, as claimed in Claim 5, wherein the amount of Al is less than or equal to 10 at%.

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7. A dielectric ceramic composition, as claimed in any of Claims 1 to 6, wherein the composition further comprises at least one of zirconia, barium zirconate, tungsten

oxide, barium tungstate, tin oxide, aluminium or aluminium-containing compounds as z : l sintering aids.
8. A dielectric ceramic composition, as claimed in any of Claims I to 7, in which x lies between 0.25 and 0.45.
9. A dielectric ceramic composition, as claimed in Claim 8, in which x lies between 0. 3 and 0.4.
10. A dielectric ceramic composition, as claimed in any of Claims I to 7, in which x is equal to or greater than 0.9.
11. A dielectric composition, as claimed in any of Claims 1 to 10, in which the temperature coefficient of frequency (if) lies in the range 5 ppm/C.
12. A resonator comprising a dielectric ceramic composition as claimed in any preceding Claim.
13. A method of controlling the dielectric properties of a resonator independently by providing a composition comprising: (BaxSr ()-.)) (Zn (]-y) Coy) l/32/303

and wherein 0 < x < 1, 0 < Y < 1 and x "* y.
14. A method as claimed in Claim 13, in which the composition is non-stoichiometric and has the formula (BaxSr (,-x)) (I : i-i,) (Zn (). y) Coy) (t/3 ;) Nb (2/3 m) 03 wherein

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O < x < 1 0 < y < 1

x $ y 0 < k < 0. 1 0 < 1 < 0. 03 0 < m < 0. 07and k+1+m > O.
15. A method as claimed in Claim 13 or 14, wherein the composition further comprises Ga as part of the crystal structure.
16. A method as claimed in Claim 15, wherein the amount of Ga is less than or equal to 10 at%.
17. A method as claimed in any of Claims 13 to 16 wherein the ternary composition further comprises Al as part of the crystal structure.
18. A method as claimed in Claim 17 wherein the amount of Al is less than or equal to 10 at%.
19. A method as claimed in any of Claims 12 to 18 wherein the ternary composition comprises at least one of zirconia, barium zirconate, tungsten oxide, barium tungstate, tin oxide, aluminium or aluminium-containing compounds as sintering aids.