GB2338478A

GB2338478A - Ceramic composites; resonators

Info

Publication number: GB2338478A
Application number: GB9913923A
Authority: GB
Inventors: Neil Mcneil Alford; Stuart James Penn
Original assignee: South Bank University Enterprises Ltd
Current assignee: South Bank University Enterprises Ltd
Priority date: 1998-06-15
Filing date: 1999-06-15
Publication date: 1999-12-22
Anticipated expiration: 2019-06-15
Also published as: GB9913923D0; GB9812833D0; GB2338478B

Abstract

A ceramic composite material for use in dielectric resonators comprises titanium oxide as the major constituent with a minor constituent of a metal compound which has an ionic radius of between 0.5 Angstroms and 0.9 Angstroms and has a formal charge of below 4+; preferably the composites have Q value of greater than 5000 and a dielectric constant of greater than 90. The metal compound may be an oxide of zinc, copper, magnesium, manganese, yttrium, iron, aluminium, a rare earth metal or transition metal. The material is used as a resonant element located in a resonator housing.

Description

173 Second Filing.doc 13-Jun-99 18:44 2338478 Ceramic Composites 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.

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'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.

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.

However in some areas the performance of devices based on room temperature DRs is inadequate and three Examples are (1) output filter banks for satellites where a reduction in insertion loss results in reduced satellite power consumption (2) in cellular communications where extremely narrow band filters are required in the ground stations to reduce interference, increase 173 Second.Filing.doc 13-Jun-99 - 18:44 sensitivity and improve channel capacity and (3) there is an increasing demand from radar technology, sensitive microwave measurement and communications for very low noise microwave oscillators and microwave secondary frequency standards.

The dielectric constant or permittivity is an important property of the dielectric material. The higher the value of the dielectric constant the greater the miniaturisation potential for a dielectric resonator, however in general as the dielectric constant increase there is an increase in the dielectric loss i.e. the loss increases.

The ideal DR material has a high dielectric constant (F,, a low loss tangent (tan 3) and a very small temperature coefficient of resonant frequency ('C,). However materials which have all these properties have been difficult to devise.

is One of the earliest resonator materials was Barium Titanate (BaTiO 3 or BaTi 4 0 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 20 band width MB 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 massively 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.

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 particulate ceramic material can be shaped in a 173 Second Filing.doc 13-Jun-99 - 18:44 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. 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 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. The sintering process involves less expensive capital equipment and is less energy intensive than for forming single crystals.

The major problem with dielectric ceramics is that their dielectric loss is much higher than single crystals.

Single crystals usually display Q factors very much higher than their sintered is polyerystalline counterparts and single crystal materials can exhibit very low loss.

This is usually attributed to the granular nature of the polycrystalline materials and the random orientation of the crystal axes and the absence of grain boundaries and the greater perfection in the structure of single crystals and single crystals are usually of far higher purity.

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 devised an improved dielectric material.

The invention provides a composite ceramic material which comprises a major constituent of titanium oxide and a minor constituent of a metal compound which has an ionic radius of between 0.5 Angstroms and 0.9 Angstroms and has a formal charge of below 4+.

The ionic potential is defined as the charge/ionic radius and the ionic radius can be determined by the method described in R.D. Shannon in Acta Crystallographica A vol 32 p 751 (1976).

173 Second riling.doc 13-jun-99 - 18:44 The preferred materials are metal oxides such as zinc, copper, magnesium, manganese, yttrium, iron aluminium, rare earth and transition metal oxides of the above properties. The amount of the metal oxide in the composite is preferably less than 20% by weight and more preferably less than 15% by weight. The materials preferably have less than 10 mol % of the metal oxide and more preferably less than 5 mol % Preferably the composites of the invention have Q value greater than 5, 000 at 3 GHz at a temperature of 25 C and more preferably greater than 10,000 and preferably the composites have a dielectric constant greater than 90.

Zirconium tin titanate ceramics are well known as dielectric resonator materials but their composition is fixed at such a composition to include far more zirconium than in the present invention where the amount of zirconium is too low to form any zirconium titanate.

The composites of the present invention can be made by any conventional methods e.g. by sintering at elevated temperatures e.g. above 1000C and as described above.

The composites of the invention can be used in dielectric resonators and the invention also comprises a composite of the invention in combination with a resonator housing and means for coupling electromagnetic energy into the housing; wherein the dielectric material is a resonant element located in the resonator housing.

The invention is illustrated in the following Examples.

Examples

In the Examples the ceramic powders were pressed in a 13mm diameter stainless steel die press at a pressure of I OOMPa. The pressed samples were sintered in air or 0 0 oxygen at temperatures ranging between 1000 C -1600 C. The sample density was then measured by noting the sample mass and dimensions. The dielectric constant and dielectric loss experiments were carried out using a parallel plate resonator and employing a modified Haki-Coleman technique (B.W. Haki and P.D. Coleman " A 173 Second Filing.doc 13-Jun-99 - 18:44 dielectric resonator method of measuring inductive capacities in the millimetre range" MEE Trans. Microwave Theory Tech., Vol. 8, p402-410, (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 4mm thick and 1 Omm in diameter. The sample dimensions were approximately 1 Omm diameter, 4mm thick discs. The measurements were made using a Hewlett Packard HP8719C vector network analyser with 1Hz resolution and the TEOII mode was examined. All dielectric measurements were carried out at room temperature in air at a relative humidity of approximately 50%. No special precautions were taken to prevent the adsorption of water to the sample surface which might have been expected to have an adverse effect on the dielectric loss. The loss factors are presented in terms of the Q factor i.e. tan deltd'.

TiO 2 sintered in air to near full density displays a very poor Q which was measured at is 1,500. This is due to the fact that TiO 2 is easily reduced. Small deviations ftom stoichiometric TiO 2 causes a random distribution of point defects. As the defect concentration increases their interaction increases and ordering can occur. Long range ordering produces shear structures, at composition around TiO 1.9 Magnelli phases exist but this degree of reduction is severe and is associated with a darkening of the material to blue-black. A slight reduction to TiO 1.998 causes defects to order on the (132) planes in a series of shear structures. In the 100% TiO 2 samples studied here, sintering in air has caused a slight reduction resulting in a dense ceramic with a brown-tan colour.

In the following Examples, the frequency of measurement was approximately 3 GHZ and the measurements were made at room temperature. Examples 1 to 5 are examples of various titanium dioxide composites for comparison.

Example 1

TiO 2 was taken and sintered in air at 1400 C Material Q Dielectric constant Porosity % Ti02 1997 100 1.71 173 Second Tiling.doc 13-Jun-99 18:44 1 Example 2

0 TiO 2 was taken and sintered in air at 1300 C Material Q Dielectric constant Porosity % Ti02 533 98 2.77 Note that the Q value has dropped this is to be expected as the sample is more. porous see for Example an explanation of such phenomena by N. McN. Alford and S. J. Perin Journal of Applied Physics volume 80 1996).

Example 3

0 TiO 2 was taken and sintered in air at 1200 C Material Q Dielectric constant Porosity % Ti02 6287 88 7.14 Note that the Q value has increased despite the fact that the sample is more porous.

This is attributed to the presence of oxygen. At a sample porosity of 7. 14% the pores are interconnected and allow the oxygen present in the air to reach the ceramic through the interpenetrating pore network. The grain size of the sample is of the order of a few micrometers and hence the diffusion length is far smaller in comparison with the denser ceramic, where there is no interpenetrating pore network.

In a denser ceramic the diffusion length is the size of the ceramic sample which, in this case, is a cylinder approximately 1Omm diameter and 5mm thick. The diffusion coefficient for oxygen in TiO 2 is 10-" cm'/sec at approximately 1000C with an activation energy of around 50k-eal/mole[see VY'D Kingery, H. Bowen and DR Ulilmann 'Introduction to ceramics' John Wiley and sons, 2nd edition 9761 which is very low indeed.

Example 4

TiO 2 was taken and sintered at 11 OOC 173 Second'Filing.doc 13-Jun-99 18:44 is Material Q Dielectric constant Porosity % Ti02 1859 61 22.63 It was observed that at high density (low porosity as in Examples 1 and 2) the Q value for the TiO 2 was low in comparison with samples where there was open and interpenetrating porosity (Example 3). This is attributed to a reduction in the oxygen content. For Example F.A. Grant (Reviews of Modem Physics vol 31 no 3 pp646 674 1959) in which the electrical conductivity of TiO 2 is seen to increase as the number of oxygen vacancies increases i.e. as oxygen is removed from the sample. At high temperatures of sintering the porosity becomes closed and oxygen cannot penetrate the sample as the ceramic body cools in the furnace. The low Q for Example 4 is due to the high degree of porosity in the sample Example 5 In order to determine. if oxygen could be diffused into the ceramic at high densities TiO 2 was taken and sintered at 1400C in flowing oxygen.

Material Q Dielectric constant Porosity % Ti02 4437 94.3 1.23 As can be seen the effect of sintering in an oxygen atmosphere has increased the Q from the values 500-2000 as in Example 1 and 2 to a value of 4437. This indicates the beneficial nature of the oxygen in increasing the Q but the value is A series of TiO 2 metal oxide composites were made as described above and their properties measured, the results are shown in the Table in which it can be seen that the compounds of the invention have the requisite Q values.

Table

Dopant ion ionic radlus. r (A) ionic potential, 0 =7W Q.

K 1.38 0.73 2691 Ne 1.02 0.98 2810 Ba2+ 1.35 1.48 c4000 Sil+ 1.18 1.70 <4000 Ca+ 1.00 2.00 <4000 Zn24- 0.74 2.70 16274 0.73 2.74 15951 me 0.72 2.78 15335 La3+ 1.032 2.91 <4000 Mn', 0.67 2.99 15994 Mn14- 0.58 5.17 NP 0.983 3.05 i <4000 3.13 <4000 Y3+ 0.900 3.33 10232 Fe3+ 0.55 5.46 16713 Zr" 0.72- 5.56 <4000 A13_+ 0.535 5.61 14538 Sn4+ 0.69 5.80 <4000 TP 0.64 7.81 <4000 0.64 7.81 <4000 V. 0.400 10.00 - <4000 From R.D. Shannon, Acta Crysudlographica A, 32,1976) p 751, Revised effective ionic radii aDd systematic studies of interatomic distances in halides and chalcogenides; ra& taken for V1 coordination cations 173 Second tiling.doc 13-Jun-99 18:44 11

Claims

1. A composite ceramic material which comprises a major constituent of titanium oxide and a minor constituent of a metal compound which has an ionic radius of between 0.5 Angstroms and 0.9 Angstroms and has a formal charge of below 4+.

2. A composite material as claimed in claim 1 in which the metal compound is an oxide of zinc, copper, magnesium, manganese, yttrium, iron aluminium, a rare earth or transition metal oxide which has an ionic radius of between 0.5 Angstroms and 0.9 Angstroms and has a formal charge of below 4+.

3. A composite material as claimed in claimed 2 in which the metal oxide is present in a mount of less than 20% by weight of the composite.

3. A composite material as claimed in claimed 2 in which the metal oxide is present in of less than 15% by weight of the composite.

4. A composite material as claimed in claimed 2 which has less than 10 mol % of the metal oxide.

5. A composite material as claimed in claimed 2 which has less than 5 mol%.

6. A composite material as claimed in any one of claims 1 to 5 which has a Q value 0 greater than 5,000 at 3- GHz at a temperature of 25 C

7. A composite material as claimed in any one of claims 1 to 5 which has a Q value greater than 10,000 at 3 GHz at a temperature of 25 C

8. A composite material as claimed in any one of claims 1 to 7 which has a dielectric constant greater than 90.

9. A dieletric resonator which comprises a composite as claimed in any one of claims 1 to 8 as the dielectric material in combination with a resonator housing and means for coupling electromagnetic energy into the housing; wherein the dielectric material is a resonant element located in the resonator housing.