Lead zirconate titanate (PZT) compounds usually have a perovskite type structure, and commonly have the general formula PbxZryTizDaMb03 where D, M are dopants and at least one of a, b are none zero. Common dopants, used for PZT compounds, include donor additives such as Nb5+, acceptor additives such as Fe3+, and isovalent additives such as Sn4+. PZT compounds are ceramic materials that, when poled, exhibit valuable piezoelectric and electrooptic properties; they are commonly used for capacitor and piezoelectric transducer applications.

A number of processes have been developed for the synthesis of lead zirconate titanate compounds. The choice of a process depends upon a number of factors, including the purity, homogeneity, and yield of the product required. The process chosen may also depend upon scale of the process required; safety and waste disposal being particularly important factors for large scale syntheses. Traditionally lead zirconate titanate compounds have been synthesised by mixing metal oxide powders, followed by calcination and sintering of the mixture. Such processes usually call for high sintering temperatures and can adversely affect the properties of the product. More recently lead zirconate titanate compounds have been synthesised from a mixture of appropriate metal alkoxides, as described in Japanese Laid-Open patent application No. 86022/85. Unfortunately the use of metal alkoxides can result in traces of carbon in the product, having an adverse effect upon the product quality. Further, metal alkoxides are inflammable and hydrophilic, and the use of these precursors can result in processing and safety problems for large scale syntheses of lead zirconate titanate compounds.

The presence of carbon impurities can result in reduction of the lead component of lead zirconate titanate compounds to free lead. It is therefore desirable to reduce residual carbon content in the product by minimising the use of organic precursors, and by minimising the use of organic solvents such as alcohols. Co-precipitation processes have been developed that minimise the use of organic precursors. These co- precipitation processes involve the addition of oxalic acid to an aqueous solution of metal ions to produce a oxalate precipitate. Ammonia, usually in the form of an aqueous solution, is often used to reduce the solubility of the oxalate precipitate in the mother liquor. The oxalate precipitate is then filtered, washed and calcined to generate the lead zirconate titanate compound. TiC14 is one source of titanium that can be used in such co-precipitation processes, though this source can lead to undesirable chloride impurities in the product. A further problem with the co-precipitation processes is the solubility of the oxalate precipitate in the mother liquor, which can reduce yields. To reduce this solubility ethanol and other alcohols are often added prior to the addition of the oxalic acid. Unfortunately the use of alcohols can be unsuitable for large scale production of lead zirconate titanate compounds since these solvents are inflammable and can result in carbon impurities in the PZT product.

EP 280033 Bl describes a co-precipitation process that involves the use of ethanol or other alcohols to reduce the solubility of the mixed oxalate. The source of the titanium ions for the process disclosed in EP 280033 Bl, is titanium isopropoxide. The titanium isopropoxide is reacted with water to yield a titanium hydroxide precipitate.

The titanium hydroxide precipitate is then filtered off, washed, and then dissolved in concentrated nitric acid. Unfortunately the EP 280033 Bl method requires a relatively large amount of concentrated nitric acid and this has an adverse effect upon the product quality and yield.

According to the present invention, there is provided a process for producing a lead zirconate titanate compound, which comprises the following steps: (a) combining an aqueous solution of a lead salt with a titanium alkoxide; (b) combining the products from step (a) with concentrated nitric acid; (c) combining the products from step (b) with an aqueous solution of a zirconium salt; (d) combining the products from step (c) with an excess of oxalic acid to produce a mixture that includes an oxalate precipitate, and adjusting the pH of the solution containing the oxalate precipitate with ammonia or an amine until the pH is between 8.5 and 9.5; and (e) calcining the oxalate precipitate.

An advantage of this process is that a relatively small amount of concentrated nitric acid is required to dissolve the titanium hydroxide, which is produced when the alkoxide is combined with the water. The small amount of concentrated nitric acid results in improved product quality and yield. Only a relatively small amount of concentrated nitric acid is required because the titanium alkoxide is combined with water in the presence of the lead salt. A further advantage of this process is that the titanium hydroxide does not need to be filtered off before the addition of the concentrated nitric acid. A yet further advantage of this process is that no ethanol or other alcohol is required to reduce the solubility of the mixed oxalate, though such use is not precluded. Yet another advantage of this process is that virtually all the components of the reaction are inert, and hence the process is suitable for the large scale production of lead zirconate titanate compounds. The process is particularly useful for the production of lead zirconate titanate compounds having a perovskite type structure.

Preferably this process is used as a means for producing a lead zirconate titanate compound, characterised in that step (c) comprises the following steps: (i) combining the products from step (b) with an aqueous solution of a zirconium salt; and (ii) combining the products from step (i) with an aqueous solution of a niobium salt.

The combination of the aqueous solutions of the zirconium and niobium salts in this order is particularly advantageous, since it facilitates the solvation of the metal ions prior to the addition of the oxalic acid at step (d).

Advantageously this process is used as a means for producing a lead zirconate titanate compound characterised in that the aqueous solution of the niobium salt is an aqueous solution of hydrated niobium oxide. More advantageously this process is characterised in that the aqueous solution of the hydrated niobium oxide is prepared by a process that comprises the steps: (i) combining an aqueous solution of ammonia with niobium pentachloride to produce a precipitate; and (ii) combining the precipitate produced in step (i) with an aqueous solution of oxalic acid.

Preferably the titanium alkoxide is a titanium propoxide. More preferably the titanium alkoxide is titanium n-propoxide. The optimum choice of titanium alkoxide depends upon a number of factors. The alkoxide should have a relatively small carbon content to reduce carbon impurities in the lead zirconate titanate compond. However, the carbon content should not be so small as to make the alkoxide too reactive.

The present invention will now be described by way of example only.

Example 1 An aqueous solution of zirconium nitrate hydrate is prepared by dissolving commercially available concentrated nitric acid (10 ml) in deionised water (2 litre); commercially available white grade zirconyl nitrate hydrate (1100 g) is then dissolved in this nitric acid solution. The resulting solution is then heated to aid dissolution and further deionised water is added until the total volume is 3 litres. Once any particulate has been minimised, by heating and stirring, the particulate matter is allowed to settle and the aqueous solution of zirconium nitrate hydrate is siphoned off. This method can also be used to prepare other aqueous solutions of zirconium salts.

An aqueous solution of ammonia is prepared by combining analar grade 25% aqueous ammonia solution (200 ml) and deionised water (200 ml). The aqueous solution of ammonia is combined with niobium pentachloride (99% purity), resulting in the formation of a hydrated niobium oxide precipitate. The supernatant liquid is siphoned off once the hydrated niobium oxide precipitate has settled out. The precipitate is then vacuum filter washed with deionised water using a Watman No. 541 filter paper, until the conductivity of the filtrate is less than 0.2 mS and is then filter dried. An aqueous solution of oxalic acid is prepared by dissolving analar grade hydrated oxalic acid (336 g) in deionised water (3 litres). The dried precipitate is then added to this aqueous solution of oxalic acid. The resulting aqueous solution of hydrated niobium oxide is then vacuum filtered using Whatman No. 542 filter paper. This method can also be used to prepare other aqueous solutions of niobium salts.

Commercially available analar grade lead nitrate (717.00g) is dissolved in 3.55 litres of deionised water. This aqueous solution of lead nitrate is then added to commercially available titanium n-propoxide (28.94g) to produce a suspension of hydrated titanium oxide. 30 minutes after the addition of the lead nitrate solution, concentrated Analar grade nitric acid (10 ml) is added to dissolve the hydrated titanium oxide. This addition of concentrated nitric acid (1 Oml) is repeated a further five times, with stirring, each addition being separated by a ten minute interval. While the hydrated titanium oxide is dissolving to produce a solution containing lead and

titanium ions, a second solution of oxalic acid is prepared by dissolving analar grade oxalic acid dihydrate (914.93g) in deionised water (9.76 litre). A portion (2294g) of the aqueous solution of zirconium nitrate hydrate prepared above, is then added to the solution containing lead and titanium ions. Once a homogeneous solution containing lead, titanium, and zirconium ions is obtained, a portion (470.45g) of the aqueous solution of hydrated niobium oxide prepared above, is added and the resulting solution is stirred for 1 hour. The solution of lead, zirconium, titanium and niobium ions is then added to an excess of oxalic acid and the oxalate precipitate, thus produced, is stirred for 1 hour. The excess of oxalic acid is in the form of the second solution of oxalic acid prepared above and is approximately 1.1 times that amount required to convert all the metal ions to the corresponding oxalates. 25% aqueous ammonia (approximately 2.5 litres) is then added to the solution containing the oxalate precipitate, with continuous stirring over a period of 2.5 hours. The temperature is controlled by controlling the rate of addition of the aqueous ammonia; and should not be allowed to exceed 40°C. The addition of the ammonia is stopped once the pH of the solution reaches 9, but stirring continues for 2 hours after the ammonia addition has ceased.

Once the oxalate precipitate has settled out, the supernate is siphoned off and the residual slurry is filtered through a buchner funnel fitted with a Whatman No. 3 filter paper. The oxalate precipitate is then vacuum filter washed with deionised water until the conductivity of the filtrate is less than 3 mS. Once the precipitate has been washed it is air dried, however, it should not be allowed to dry out or crack during the washing process. The air dried precipitate is mixed with 4.5 litres of deionised water and the resulting slurry is spay dried. The dried oxalate precipitate is then calcined using the following shedule: (i) 30°C/hour to 300°C for a dwell of 1 hour, (ii) 10°C/hour to 400°C for a dwell of 1 hour, (iii) 60°C/hour to 600°C for a dwell of 2 hours, (iv) 60°C/hour to 775°C for a dwell of 16 hours. This is followed by natural cooling down to ambient temperature, to yield a lead zirconate titnate compound having niobia as a dopant. The compound has the formula: Pbo. 99n (Zro. 95Tio. o5) o. 9822Nbo. o) 7803 and has a perovskite type structure. This method is also suitable for the preparation of other lead

zirconate titanate compounds, for example compounds having the formula PbxZryTizDaMb03 where D, M are dopants and at least one of a, b are none zero.

Example 2 To a first solution, prepared by combining analar grade 25% aqueous ammonia solution (250ml) and deionised water (800 ml), is added to a second solution, prepared by adding GPR grade tin (IV) chloride (200 g) to deionised water (600 ml).

The resulting mixture, which includes a precipitate, is stirred for thirty minutes and is left to stand. The precipitate is vacuum filter washed with deionised water using a buchner funnel fitted Whatman No. 541 filter paper. The precipitate is then vacuum dried until it is dry enough to remove from the filter paper. This semi-dry precipitate is dissolved in an aqueous solution of oxalic acid, prepared by dissolving analar grade hydrated oxalic acid (200 g) in deionised water (1.944 litres). Once the precipitate has completely dissolved, a process that may take several days, the solution is vacuum filtered using a Whatman no. 542 filter paper/0.2 micron cellulose nitrate membrane filter. The filtered solution is a tin stock solution used in the subsequent steps of the process.

Commercially available analar grade lead nitrate (710. 00g) is dissolved in 3.52 litres of deionised water. This aqueous solution of lead nitrate is then added to commercially available titanium n-propoxide (28.9228g) to produce a suspension of hydrated titanium oxide. 30 minutes after the addition of the lead nitrate solution, concentrated Analar grade nitric acid (10 ml) is added, to dissolve the hydrated titanium oxide precipitate. This addition of concentrated nitric acid (1 Oml) is repeated a further five times, with stirring, each addition being separated by a ten minute interval. While the hydrated titanium oxide is dissolving, a first solution of oxalic acid is prepared by dissolving analar grade oxalic acid dihydrate (892g) in deionised water (9.84 litre). A portion (1951 g) of the aqueous solution of zirconium nitrate hydrate, prepared in example 1, is then added to the solution containing lead and titanium ions.

Once a homogeneous solution containing lead, titanium, and zirconium ions is obtained, a portion (317.9643g) of the aqueous solution of hydrated niobium oxide,

prepared in example 1, is added and the resulting solution is stirred for 1 hour. A portion (433g) of the tin stock solution is added to the first solution of oxalic acid (9.84 litre), which was prepared while the hydrated tianium oxide was dissolving, to produce a second solution of oxalic acid. The solution of lead, zirconium, titanium and niobium ions is then added to the second solution of oxalic acid, and the oxalate precipitate thus produced is stirred for 1 hour. An excess of oxalic acid is used to produce the oxalate and the excess is in the form of the second solution of oxalic acid.

The excess is approximately 1.1 times that amount required to convert the metal ions to the corresponding oxalates. 25% aqueous ammonia (approximately 2.5 litres) is then added to the solution containing the oxalate precipitate, with continuous stirring over a period of 2.5 hours. The temperature is controlled by controlling the rate of addition of the aqueous ammonia; the temperature should not be allowed to exceed 40°C. The addition of the ammonia is stopped once the pH of the solution reaches 9, but the mixture continues to be stirred for 2 hours after the ammonia addition has ceased.

Once the oxalate precipitate has settled out, the supernate is siphoned off and the residual slurry is filtered through a buchner funnel fitted with a Whatman No. 3 filter paper. The precipitate is then vacuum filter washed with deionised water until the conductivity of the filtrate is less than 3 mS. Once the precipitate has been washed it is air dried, however, it should not be allowed to dry out or crack during the washing process. The air dried precipitate is mixed with 4.5 litres of deionised water and the resulting slurry is spray dried. The dried oxalate is then calcined using the same schedule as described in example 1 followed by natural cooling down to ambient temperature, to yield lead zirconate titanate compound having niobia as a first dopant and stannic oxide as a second dopant. The compound has the formula: Pbo. 99225 (Zro. 8t5Tio. o5Sno. i35) o. 984sNbo. otss03 and has a perovskite type structure. This method is also suitable for the preparation of other lead zirconate titanate compounds, for example compounds having the formula PbxZryTizDaMb03 where D, M are dopants and at least one of a, b are none zero.