GB2240335A

GB2240335A - Ferroelectric materials

Info

Publication number: GB2240335A
Application number: GB8926186A
Authority: GB
Inventors: Nicholas Martyn Shorrocks; Roger William Whatmore
Original assignee: GEC Marconi Ltd; Plessey Co Ltd; Marconi Co Ltd
Current assignee: Plessey Co Ltd; BAE Systems Electronics Ltd
Priority date: 1988-11-30
Filing date: 1989-11-20
Publication date: 1991-07-31
Anticipated expiration: 2009-11-20
Also published as: FR2666083A1; IT9021542A0; IT9021542A1; IT1243030B; GB2240335B; DE3943507A1; GB8926186D0

Abstract

Compositional modifications are provided for the ferroelectric material lead scandium tantalate (PST), which improve its performance as an electronic material. PST is of particular use in infrared detection applications, in the form of the sensitive material of a dielectric bolometer, but its optimum performance occurs at about 50 DEG C. The compositional modifications described enable this optimum temperature to be adjusted, to match a particular application, without loss of peak sensitivity. One modification provides the substitution of a bismuth/potassium mixture for a proportion of the lead to lower this optimum temperature. for example, to achieve the best performance at typical ambient conditions, near 20 DEG C. A second modification increases this temperature by substituting niobium for a proportion of the tantalum. This allows detector operation over a wide range of external temperatures with stabilisation by heating, but without any requirement for cooling. Both compositional modifications provide for a valence balance between substituting and substituted elements. This ensures that the electrical resistivity of the material is not lowered significantly by either modification. This is of particular importance for low noise, dielectric bolometer detectors. The material may be prepared by hot pressing/sintering methods or by thin layer deposition methods.

Description

1 IMPROVEMENT TO DIELEMIC BQLLOMETERS The present invention relates to

ferroelectric materials for use in electronic sensors and devices, especially dielectric bolometer infra-red detector applications and more particularly, to compositional modifications to the ferroelectric: lead scandium tantalate (Pb2ScTaO6) or PST.

A dielectric bolometer is a detector of incident electromagnetic radiation by virtue of its sensing temperature changes, and is a particular form of pyroelectric detector. The pyroelectric effect occurs in all materials which are crystallographic ally polar and is observed as the release of charge at the surface of such a material when it experiences a change in temperature. Pyroelectric radiation detectors exploit this effect by allowing the incident radiation to generate a temperature change in a pyroelectric material and detecting the changes released using an electronic circuit.

Materials which are polar require no electric field to generate the effect. Dielectric bolometers are similar in operation, but require an electric bias field to generate or induce the pyroelectric effect; otherwise the response is similar. Thus pyroelectric detectors operate by integrating incident radiation to produce a temperature change which in turn releases electric charge. This may be contrasted with nonintegrating detectors such as photodetectors which generate charge directly in response to incident energy of appropriate wavelength.

The bolometer element is electrically equivalent to a capacitor biased by a constant voltage. As the temperature (T) of the element 2 varies, an electrical charge is generated because of the temperature dependence of the dielectric properties of the detecting material. The charge responsivity is proportional to:

(aDlaT)v which can be termed the induced pyroelectric coefficient (p) with the electric displacement (D) induced by the bias voltage (V).

This response is similar to that of pyroelectric detectors. Consequently, the well known descriptions of the performance and applications for pyroelectric devices are substantially applicable to dielectric bolometer devices. In particular, the well known figure of merit:

FD = p/[c( P-P-Otan3)l is relevant to both types of devices. FD is derived from the ratio of the detector responsivity to its Johnson noise and is used to compare different materials. Favourable materials characteristics are dependent upon a high induced pyroelectric coefficient, a low volume heat capacity (c) and a low dielectric constant and loss (c and tan3 respectively). A bias voltage used in the operation of a dielectric bolometer leads to the additional requirement for a favourable material of high electric resistivity (p) to minimise any dc leakage current.

Ferroelectric materials have been used for both pyroelectric and dielectric bolometer detectors. For the former, materials are 3 typically selected such that the operating temperature is well below their ferroclectric-to-paraelectric transition temperature (Tc) because operation close to Tc results in a gradual loss of response over time due to de-poling of the material. With dielectric bolometers, the operating temperature range can include Tc, as the bias voltage stabilises the material's dielectric polarisation and prevents any behaviour analogous to de-poling. Close to Tc the dielectric properties vary most rapidly with temperature resulting in the strongest induced pyroelectric responses. A particular advantage of these high values of p is in large arrays of detectors where the individual elements are comparatively small, possibly with areas (A) of less than 10-8M2. As the charge signal is proportional to (pA) the high value for p compensates for a small value for A.

A large number of ferroelectric materials have been fabricated and many of these, especially those with Tc near to room temperature, could be used for dielectric bolometer applications. In particular, the material lead scandium tantalate (Pb2ScTaO6, termed PST hereafter) has many of the desirable properties required. It can be fabricated readily as a high quality ceramic of almost full theoretical density. Its mechanical properties are good, being compatible with the cutting and polishing processes typical in the fabrication of both pyroelectric and dielectric bolometer detectors. The electrical properties of PST are suitable for high performance detectors.

The electrical properties of PST, in particular p, P- and tanS, have been measured over a range of temperatures and biasing electric fields. A field of about 5 x 106VM-1 has been found to be strong

4 enough to give good performance. Figure (1) shows the variation of FD with temperature at this field, calculated from measurements on a 10Ogm thick slice of PST. The peak in FD is about a factor of two higher than that observed in common pyroelectric materials, such as triglycine sulphate, strontium barium niobate and ceramics of the lead zirconate titanate family. However, the peak performance occurs at about SO'C, well above typical operating temperatures of 15-25'C. It would be desirable to alter this temperature of peak performance, to adjust it to match a particular operating temperature without degrading the desirable properties of PST. As the bolometer response is related to the transition, this is equivalent to lowering Tc by up to WC without altering any other electrical properties. Under certain operational conditions, it may- be desirable to increase Tc. For example, for use in devices which are required to operate in hot conditions or for use in an electronic system where it is required only to heat the device, rather than temperature stabilise it by heating and cooling.

The present invention is based on the realisation that it is possible to modify the structure of PST in order to adjust the temperature at which peak operation occurs whilst maintaining the electrical resistivity sufficiently high that bias voltages required for correct operation will not create excessive leakage current such as to affect operation of the device.

Thus whilst it has been found in accordance with the invention that substitution of a certain amount of bismuth (or an equivalent material) for a proportion of the lead will lower the transition temperature (Tc), nevertheless, bismuth having a valency of three, will tend to increase the number of free electrons in the structure since lead has a valency of two only. The solution is to add a certain amount of potassium (or equivalent element) having a valency of one in order to reduce the number of free electrons.

Again it has been found possible in accordance with the invention to raise the transition temperature (Tc) by substituting a certain proportion of the tantalum with niobium, both having valences of five. In this case the electrical resistivity remains unchanged.

Thus the present invention provides a ferroelectric material comprising lead scandium tantalate where a predetermined proportion of the lead, scandium or tantalum is substituted by one or more other elements such as to change the transition temperature (Tc) by a desired amount while retaining the electrical resistivity of the material at the same level or not reducing the electrical resistivity by more than a factor of about 10.

PST is an oxide ferroelectric with a perovskite-type lattice structure. It is well known that such materials can readily be doped with a wide range of metal oxides in order to adjust their electrical properties, including Tc. Substitutions of small proportions of uranium or bismuth for lead were studied. Tc was lowered as desired, but the electrical resistivity was also reduced from 1012 1013 Qm for PST to below 109 Qm for both dopants. The addition of Bi was formulated as:

Pb2-3y+x Bi2y (Sc Ta) 06+x 6 The addition of Bi altered Tc and p as follows (for x = 0. l):

y 0 0.02 0.05 TC (OC) p (OM) 1012 - 1013 28 12 6 9 X 108 The addition of U was formulated as:

Pb2+x (SC Ta)l-y Uy 06+x The addition of U altered Tc and p as follows, (for x = 0.1):

Y TC(I1c) P(QM) 0.02 21 5 X 108 These Tc reductions are useful in certain devices, particularly detectors with larger element areas. However, the resistance reduction is a problem for small area detectors, for which it is desirable that the resistivity is high enough, such that the shot noise on the leakage current, iL, is lower than the Johnson noise associated with the dielectric loss, that is eiL < 4kBTcoCtan8 for all relevant angular frequencies, co. Here C is the detector capacitance, T the absolute temperature, kB is Boltzmann's constant 7 and e is the charge of one electron. This limit is device dependent but, for detector applications with PST, is roughly equivalent to:

p > 10110M Since the dopant (3 in the case of bismuth, 6 for uranium) differs from the substituted material (lead with valency 2 or Se/Ta with a mean valency of 4 respectively), these dopants tend to donate electrons to the lattice and enhance the conductivity.

The present invention provides for a valence balanced substitution for lead in PST, which lowers Tc in a controllable manner, without reducing its electrical resistivity and in particular, the substitution of equal molar amounts of bismuth and potassium for some of the lead. The balance is between bismuth (valency 3) and potassium (valency 1).

According to this invention, a further composition of a ceramic for dielectric bolometer detectors is:

Pb2-2y+xKyMyScTa06+.

where y is a number in the range 0 to 0.20 and x is a number in the range 0 to 0.20. A value of x greater than zero provides an excess of lead oxide in this material. This is known to be advantageous, both in compensating for lead loss during ceramic fabrication and in favouring the formation of the desired perovskite phase. The value of y determines the value of Tc; it has been observed that Tc can be estimated using the expression:

8 (25 - 650y) OC for 0 < y < 0.06 Such a ceramic, with x = 0.14 and y = 0.04, has been fabricated and assessed electrically. The measured resistivity was in the range 1012 to 2 x 1012 Qm, which is acceptably high, and Tc was about WC. Figure (2) shows the variation of FD with temperature with a bias field of 5 x 106 Vm-1 applied. If a comparison in made with Figure (1), it can be observed that the shapes of the two curves are similar, as are the peak values of the figures of merit, while the optimum operating temperature. as given by the peak of FD, has been reduced from about 50C to 25'C. This reduction corresponds to the change in Tc from 25'C to O'C, so for other values of y, the optimum operating temperature at this bias field can be predicted approximately from the expression:

(51 - 650y) C For example, measurements on a ceramic with a composition corresponding to x = 0. 14, y = 0.06 gave a peak of FD at 14'C, close to the predicted value of 12'C.

Other dopants are possible while keeping the valency balance, such as replacing potassium with lithium, sodium, rubidium or caesium (all with valency 1). It will be appreciated that as the bismuth substitution for Pb2+ reduces p because it acts as an electron donor, the addition of K+ as an electron acceptor increases the resistivity to an acceptable level. As an extension of this concept, the 1 9 compensation of the electron donor substitution of Pb2+ by Bi3+ could be achieved by an electron acceptor substitution for the SC3+1TO+ (with an average valency of 4+). This may be achieved by using for example Mn or Fe3+ substitutions. A formulation for thi's would then be:

Pb2-2y+x Bi2y(Sc Ta)1-z Mnz 06+x where z = 2y For the best performance over a wide range of operating temperatures, for example -40C to 7WC, both PST and the KBimodified material benefit from some external temperature stabilisation, to provide some heating at the lowest temperatures and some cooling at the highest. Cooling is generally more awkward to achieve than heating, involving the use, for example, of peltier effect devices.

There is some benefit, therefore, in modifying PST to increase its transition temperature, so that no cooling would be required. Again, valence balancing is required to ensure high electric resistivity. An appropriate modification is the substitution of niobium for a proportion of the tantalum, both having valencies of 5.

Lead scandium niobate, termed HN hereafter, is known to have a Tc of about 120T, so a mixture of PST and HN will have a Tc intermediate in the range 25-120'C.

The present invention provides a valence balanced substitution for tantalum in PST, which raises Tc in a controllable manner without degrading its electrical properties and in particular, the substitution of niobium for an equal molar amount of tantalum.

According to this invention, the composition of a ceramic for dielectric bolometer detectors is:

Pb2+xSeTal-wNb06+x where w is a number in the range 0 to 1.0 and x is a number in the range 0 to 0.20. The value of w determines the value of Tc; it has been observed that Tc can be estimated using the expression:

(25 + 16w + 80w2) Such a ceramic with w = 0.40 and x = 0.10 has been fabricated and assessed electrically. The measured resistivity was about 1.2 x 1012 f2m and Tc was 43'C. Figure (3) shows the variation of FD with temperature with a bias field of 5 x 106 Vm- I applied.

Comparing with unmodified PST, this modification has both increased the optimum operating temperature, as expected, and marginally improved the peak FD value. The peak FD temperature was 67'C, corresponding to the highest measurement temperature.

An alternative substitution which may also increase Tc is to substitute Sc/Ta by Ti in the following way:

Pb2+x (SC Ta)l-z Tiz 06+x PST with K and Bi can readily be fabricated in the form of a ceramic block. By way of an example, the following process has been 1 11 used to make almost fully dense ceramic with a fine grain size, typically 1-2pLm.

Weigh out equal.molar quantities of high purity SC203 and Ta205 powders, form a slurry in acetone, mill for two hours and dry.

2. React this at 1400-14STC for two hours in air to form ScTa04.

3. Mill in acetone as above for four hours, dry and repeat stage 2.

4. Weigh out and add appropriate molar quantities of high purity PbO, Bi203 and K2C204. H20 powders to a known weight of ScTa04, mill for six hours in acetone and dry.

5. Calcine this mixture by heating to 92TC for 3 hours in air.

6. Mill for 4 hours in acetone, dry and sieve the resultant powder.

Add an organic binder, for example CRANCO (trade mark) and form a slug by cold pressing.

8. Fleat slug to SOTC to burn out the binder and then to 1200'T to hot press for 6 hours in oxygen under a pressure of 2.5 tsi.

This schedule has been found appropriate for block diameters of 2-5cm and thickness of about 1cm. The block composition can be controlled accurately by the weighing stages 1 and 4. The preliminary formation of the ScTa04 phase (stages 1 and 2) has been found to be beneficial in producing the desired perovskite structure in the final ceramic.

It has been observed that the ceramic produced can be improved by subsequent annealing. A typical schedule is:

Heat to 11STC at a rate of 30WC per hour Hold at 11STC for 100 hours Cool to room temperature at a rate of 30WC per hour To avoid loss of lead, which is comparatively volatile, this process can be carried out in a lead-rich environment, provided by a spacer powder such as PST or lead zirconate. This annealing process does not produce significant grain growth, but does increase the sharpness of the ferroelectric transition and lead to an improvement in the FD figure of merit. This is associated with enhanced order of the crystal lattice, with annealing favouring the formation of a structure with alternate (111) crystal planes of tantalum and scandium.

The ceramic fabrication route used for Nb-modified PST is similar. In step 1, a mixture of Nb205 and Ta205 in appropriate molar quantities replaces the Ta205- In step 4 no Bi203 or K2C204H20 powders are required. The post-fabrication annealing described is also especially beneficial.

An alternative approach to fabrication of this material is the production of thin layers onto a suitable substrate, with the layer thickness appropriate for the application. For infra-red detectors, l 13 this could involve thickness in the range of 3-50jim. A range of techniques for the deposition of layers of mixed metal oxides are well known and can be adapted for this material. Examples are deposition from solution, chemical vapour deposition and sputtering. Attention is drawn to our copending application 8809955.1 (our reference F20477) which discloses a method of manufacturing PST.

In deposition from solution, soluble compounds of the metal cations required are synthesised and mixed in the proportions required for the final composition. Successive layers are deposited on the substrate, by evaporation of the solvent and thermal decomposition of the above compounds, until the required thickness is reached.

In chemical vapour deposition volatile compounds of the metal cations required are synthesised and are evaporated and mixed in a flowing gas stream, which passes over a heated substrate. Growth occurs by thermal decomposition of the above compounds at the substrate surface. The deposited film composition can be controlled by adjusting the evaporation rates of the metal compounds.

In deposition by sputtering, the material is sputtered off an appropriate target and deposited onto a substrate. The target could be a ceramic block and its composition controls the composition of the deposited film.

For all these techniques, a subsequent thermal annealing stage in an oxidising atmosphere may be beneficial for improving the crystallinity of the film. A typical temperature for such annealing would be in the range of 700-10OWC.

14 The technique of forming a scandium tantalate compound, as in intermediate step in the ceramic fabrication, may be beneficial in any of these thin film deposition techniques. This could be achieved by depositing as alternate layers scandium-plus-tantalum, or scandiumplus tantalum-plus-niobium and lead-plus-bismuth-plus-potassium, or lead oxides, followed by thermally-enhanced diffusion to produce a homogeneous film.

It is important that any substrate used does not react excessively with the materials in the thin film, thereby degrading its desired properties, either during deposition or in any subsequent annealing stages. Growth has been achieved successfully on alumina, sapphire, magnesium oxide and aluminium nitride. It would be possible to use more reactive substrates, protected by an inert covering layer, for example, deposition onto silicon protected by a thin alumina film.

A possible advantage of both silicon and magnesium oxide is that they can be removed readily by chemical etching, in for example, potassium hydroxide and phosphoric acid respectively. By selective removal of areas of the substrate, areas of the modified PST thin film can be isolated for subsequent processing. This is beneficial in infra-red detector applications as the substrate would act as a heat sink: removing the substrate improves the thermal insulation of the detector material and increases the detector responsivity, especially at low frequencies.

A specific example is given below of fabrication in the form of a ceramic block.

1 is Example of Preparation of Lead Scandium Tantalate Doped with Bismuth and Potassium Pre-Reaction of ScTaO- Powder Weight out 7.149g Of SC203 and 22.851g of Ta205. Ball mill together for 2 hours in acetone with Zr02 milling media in polythene pots. Dry and sieve and pre-react in A1203 crucible for 2 hours at 14501C in static air using heating and cooling rates of 300'C hrl. Crush and sieve through 25Ogm mesh and ball mill again for 4 hours. Dry and sieve and pre-react again for 2 hours at 1450'C in static air using the same heat and cool rates. Crush and sieve through a 25OpLm mesh.

Ceramic Preparatio-n To prepare a ceramic with the composition:

Pb(1.05 x 0.98)Bio.02K0.02(ScTa)0.503 Powder Fabrication Weigh out 28.710g of ScTa04 powder prepared as described above and add 45. 487g of PbO, 0.923g of Bi203 and 0.364g of K2C204.H20. All 4 compounds are then ball milled in acetone with Zr02 milling media in polyethylene for 6 hours.

The slurry is dried and sieved through 250gm mesh. The powder is calcined in an A1203 crucible at 920'C for 3 hours in static air using heating and cooling rates of 300"C hr-1, then crushed and sieved through a 250gm mesh and ball milled again for 4 hours in 16 acetone. Finally, it is dried and sieved through 100gm mesh. The resulting powder has 5wt% of a solution of a commercial acrylic ceramic binding agent (i.e., ICI Cranco) thinned with acetone added to it. This is dried with occasional stirring during drying and when almost completely dry, it is sieved through a 250gm mesh and dried thoroughly.

Ceramic Fabrication The ceramic powder and binder is first cold pressed in a hardened tool steel punch and die set of the required size. This block is placed in a Si3N4 punch and die set with about 3mm clearance all around, packed with 60# (60 mesh) fused A1203 grit, with coarser 10# fused A1203 top and bottom to facilitate removal. The die is then placed in a furnace constructed in 2 halves with SiC heating elements. The die is supported on an A1203 tube with a side tube so that 02 can be passed up through the die during hot pressing. One half of the furnace is situated in a hydraulic press which is initially operated manually during the hotpressing stage. Once the silicon nitride die has been loaded with the cold-pressed powder, the other half of the furnace is positioned in the press and the 2 furnace halves are clamped together. The furnace is heated using a suitable temperature controller to 500'C at 300'C/hr. When at SOWC, the temperature is held for 2 hours to burn out the organic binder. The 02 flux is started at room temperature and continued until the temperature reaches 1200C After 2 hours at SOTC, the temperature is raised to 1200'C at 600'C/hr. When the temperature reaches 80WC, the full pressing pressure of 21/2tsi is applied. The S Z 17 sintering temperature of 1200'C is maintained for 6 hours and the pressure of 21/2tsi is also maintained during this period.

The pressure is then released and the furnace cooled at 1OWC/hr to room temperature. When cool, the die is removed and the sample extracted. The excess A1203 is removed by rubbing the block with carborundum. The ceramic block is then waxed to a steel plate and both faces are diamond ground until all the A1203 has been removed. Finally, the sample is waxed to a jig and the diameter ground to the required size.

18

Claims

1. A ferroelectric material comprising lead scandium tantalate wherein a predetermined proportion of the lead, scandium or tantalum is substituted by one or more other elements such as to change the transition temperature (Tc) by a desired amount while retaining the electrical resistivity of the material at the same level or not reducing the electrical resistivity by a factor of more than about 10.

2. A ferroelectric material as claimed in claim 1 wherein the transition temperature is lowered by the substitution of controlled amounts of bismuth and an element selected from potassium, lithium, sodium or caesium.

3. A ferroelectric material as claimed in claim 2 wherein the material has the compostion:

Pb2-2y+x Qy Biy SC Ta 06+x where 0<y <0.20 and 0<x <0.20 and Q is an element selected from potassium, lithium, sodium or caesium.

4. A ferroelectric material as claimed in claim 1 wherein the transition temperature of the material is lowered by the substitution of controlled amounts of bismuth and manganese or iron.

19

5. A ferroelectric material as claimed in claim 4 wherein the material has the composition, Pb2-2y+x -Bi2y (SC Ta)1-z Qz 06+x where z = 2y, 0<y <0.20 and 0<x<0.20 and Q is a material selected from manganese or iron.

6. A ferroelectric material as claimed in claim 1 wherein the transistion temperature of the material is raised by the substitution of controlled amounts of niobium or titanium.

7. A ferroelectric material as claimed in claim 6, wherein the composition of the material is as follows:

Pb2+x SC Tal-w Qw 06+x where 0<w <1.0 and 0<x<0.2 or Pb2+x (ScTa)1-z Tiz 06+x where 0<z <0.25 and 0<x <0.2

8. Ferroelectric materials substantially as hereinbefore described.

9. An electric sensor incorporating a ferroelectric material as claimed in any preceding claim.

10. An electric sensor as claimed in claim 9 comprising a dielectric bolometer.

11. A dielectric bolometer incorporating a ferroelectric material comprising lead scandium tantalate wherein a predetermined proportion of the lead, scandium or tantalum is substituted by a substituent element such as to change the transition temperature of the material while retaining the electrical resistivity of the material at the same level or not reducing the resistivity by such an amount that the bias voltages required for operation of the bolometer do not create excessive leakage current such as to affect operation of the device.

12. A method of making lead scandium tantalate or a material as claimed in any of claims 1 to 10 substantially as hereinbefore described.

13. A method for forming the ferroelectric material lead scandium tantalate, the method including the steps of:(a) providing predetermined quantities of scandium oxide and tantalum oxide, and reacting the quantities together for a predetermined time period at a temperature above 1400T to form ScTa04; (b) adding a predetermined quantity of lead oxide to a predetermined quantity of ScTa04, mixing the quantities together, and calcining the mixtures by heating at a temperature between about 8000C and 10000C to form Pb2SeTaO6; (c) forming a slug of Pb2SeTaO6 and hot pressing the slug for a predetermined time to increase the density of the slug material.

-1 q 21

14. A method according to claim 13 wherein in step (a) equal molar quantities Of SC203 and Ta205 are formed as a slurry in acetone, the slurry being milled and dried.

15. A method according to claim 13 or 14 wherein in step (a) controlled amounts of niobium oxide or titanium oxide are added.

16. A method as claimed in claim 13 wherein in step (a) the ScTa04 formed is milled and dried and heated again for a predetermined time period at a temperature above 140WC.

17. A method according to claim 13 wherein in step (a) the heating is carried out at a temperature between 14000C and 14500C.

18. A method according to claim 13 wherein in step (b) the quantities are mixed by milling for a predetermined time period in acetone and then dried.

19. A method according to claim 13 wherein in step (b) the quantities are heated at a temperature of about 90WC.

20. A method according to claim 13 wherein in step (b) predetermined amounts of Bi203 and K2C204H20 are added.

2 1. A method according to claim 13 wherein in step (b) predetermined amounts of the oxide of bismuth and selected oxides of manganese, iron, potassium, lithium sodium or caesium are added.

22 22. A method according to claim 13 wherein in step (c) a slug is formed by adding a binder material and cold pressing to form a slug, and wherein the slug is heated at a relatively low temperature to burn out the binder material and then heated at a relatively high temperature whilst pressing to obtain a slug of increased density.

23. A method according to claim 22 wherein the slug is hot pressed at a temperature of about 1200'T or a plurality of hours in oxygen at a pressure of more than one top per square inch.

24. A method as claimed in Claim 13 wherein the ferroelectric material is annealed by gradually heating the material to a temperature about 10000C, holding the temperature for a predetermined time period, and then gradually cooling the material.

25. A method as claimed in Claim 24 wherein the annealing is carried out in a lead-rich environment.

published 1991 atIbePatcntCHTlcc. State House. 66/71 High Holborn. LDndonWCIR47?-Furthercoples maybe obtained from Sales Branch. Unit 6. Nine Mile Point Cwmfelinfacb. Cross Keys. Newport, NPI 7HZ. Printed by Multiplex techniques lid. St Mary Cray, Kent.