GB2196621A - Particulate oxide materials - Google Patents

Abstract

A method of preparing an oxide material in a particulate form comprises the steps of (a) providing a metal organic precursor of the required oxide material, (6) dissolving said precursor in a suitable organic solvent, (c) hydrolysing the solution to form a gel, (d) dispersing the gel into a suitable dispersion medium and (e) spray drying the resulting suspension to produce a powder form of the material. The spray dried powder may be calcined to convert the material to a particulate oxide. The resulting powder is suitable for sintering to form a dense ceramic body of e.g. lead zirconate-titanate.

Description

SPECIFICATION Particulate oxide materials This invention relates to particulate oxide materials. In particular it relates to a method of preparation of such materials which may be suitable for eventually forming a dense, hot pressed and sintered body of the material.

An example of a use for such a body is that of a lead-containing ferroelectric oxide ceramic body which is used in transducer applications.

The traditional methods for making metal oxide ceramics of this type involve reacting the component metal oxides at elevated temperatures. Consolidation of the reacted powder is achieved by sintering or hot-pressing at a higher temperature, whilst mixing and subdivision of the powders utilises mechanical milling. Reproducible manufacture of a high quality, contaminant-free end product can often be difficult to achieve as a result of this complex and lengthy series of operations.

There is therefore a need for improved, more reproducible processing which has motivated research into methods starting from solutions of metal organic precursors. Several undesirable features of powder based processing can be avoided in this way. The present invention was devised to provide a complete process which would be capable of producing a complex, optimised electronic ceramic formulation in a reproducible manner.

According to the invention, there is provided a method of preparing an oxide material in a particulate form, the method comprising the steps of forming a metal organic precursor of the required oxide material, dissolving said precursor in a suitable organic solvent, making a gel of the solution by a controlled hydrolyis operation, dispersing the gel into a suitable dispersion medium and spray drying the resulting suspension to produce a powder form of the material. Some examples of suitable dispersion media include water and methoxyethanol.

The resulting material powder may then be calcined in air to restore the amorphous gel form of the material to a particulate oxide form. The calcination may be effected at a relatively low temperature such as 600"C.

Where a particulate form composed of mixed oxides of different metals is required, the required mixture of the various metallic components may be made at the stage of dissolving the precursors in the organic solvent.

If it is required for the powdered oxide material to be used eventually to form a dense ceramic body, then the prepared powder may be hot pressed and sintered to achieve the necessary shape needed in the finished body.

Use of the powder form of the material manufactured by the method already described has been found to allow hot-pressing to take place at a lower temperature than with powder produced by a conventional mechanical milling method. The use of a lower hot-pressing temperature can allow the production of a fine grained ceramic body which still has the requisite high density.

One type of ceramic body which has been produced by the method of the invention is that for a pyroelectric detector array device.

By way of example, a particular embodiment of the invention will now be described with reference to the accompanying drawing, in which: Figure 1 shows a block diagram of the different stages in the method of the invention, and, Figure 2 is a perspective view of a fine grained sintered ceramic body made by the method.

As an initial example of the process of the invention, the preparation of a lead zirconate titanate (PZT) ceramic composition will now be described. The preparation required as starting materials lead acetate, Pb(OAc)2,3H2O, zirconium n-butoxide butanol complex, Zr(O Bu)4.BuOH and titanium n-butoxide, Ti(O'Bu)4, or alternatively titanium isopropoxide..

The detailed steps in the process were as follows: 1. Dissolve lead acetate, Pb(OAc)2.3H2O in methoxyethanol at 70"C, to give a solution about 0.3M in Pb.

2. Distil to 2/3 of original volume, to remove the water of crystallisation introduced, at 125"C.

3. Cool to 90"C.

4. Add zirconium complex Zr(OBu)4.BuOH and titanium complex Ti(OBu)4 in the desired ratios. These viscous liquid alkoxides are weighed into dropping funnels, and residues adhering to the funnel sides are washed over with methoxyethanol, to ensure good compositional control.

5. Concentrate at 125"C to approximately 1M in Pb.

6. Cool to room temperature.

7. Acidify by introducing a small amount of 70% nitric acid, to give a nitric acid concentration of between 10-3 and 10-4M.

8. Hydrolyse by adding water, with stirring, to obtain a homogeneous gel. The significance of the amount of water used will be further discussed below.

At the time the method was first proposed the necessity for the concentration step was not fully appreciated. It has also been established that the system is quite immune to the precipitation of solid material. There is no need to dilute the water used for the hydrolysis with methoxyethanol, or to add it particularly slowly, as had been previously supposed. The result of the processing stages described above is the production of a transparent, homogeneous gel. The colour of the PZT precursor solutions and gels made was yellow to orange, the colours being caused by the presence of impurities in the alkoxides used.

The soft nature of the undried gel suggested that it might be easy to disperse into a fine suspension in a suitable medium. For the development of such a process a Silverson Machines laboratory mixer emulsifier (standard model) was used. The working head of this machine comprises a vaned rotor surrounded by a perforated cylindrical screen.

Subdivision of suspended particulates is mainly achieved by the application of a large shearing force, as they are forced through the screen by centrifugal action. For the experiments described, the machine was always used at maximum speed. A variety of screen types are available, and the one used was recommended for emulsification of gelatinous material.

Initial experiments were made using lead zirconium titanate PbZrOgTiO,03 precursor gel.

The first experiment performed used about 29 of gel in 1.5 litres of water, subsequently solid material loadings of about 20 times greater were routinely used. The action of the emulsifier rapidly produced an opaque white suspension. Particle sizes were estimated by squeezing droplets of the mixture between glass slides, and examining with a microscope. Thirty seconds of blending produced a typical particle size of about 50 micrometres, which was reduced to 5 micrometres after five minutes. The particle size was similar after a total dispersion time of twenty minutes, suggesting that no further breakdown occurred. Experience with bulk gel samples indicated that these particles would shrink to less than half their original diameter on drying.

Some settling of the suspended particles began almost immediately, and was severe after standing for several hours. It was estimated that suspension made by this method should be passed on to the next stage in any process within one hour.

Means were then investigated for removing the dispersion medium, without reagglomeration of the suspended particles. When suspensions were filtered, the cake of gel particles obtained repidly clogged the filter paper used.

Oven drying of this material at 150"C, and also of the emulsion itself, produced large flakes or granules of material rather than a powder. It was concluded that more elaborate methods of drying had to be employed to avoid reagglomeration of the particles. The spray drying technique was identified as appropriate and this was investigated as described below.

A potential problem with the dispersion technique outlined would be that metallic components not fully incorporated into the gel network could be extracted into solution. In the case of the simple system used, only lead would be prone to this effect, having been initially introduced as a water soluble precursor. Other components in more complex materials could be similarly affected, however. A crude attempt was made to estimate the amount of lead extracted in this way, by passing the suspension through a 0.5 micrometre membrane filter, then testing the filtrate with an equal volume of 10% potassium iodide solution. It was found that, about thirty hours after preparation of a suspension, up to ten percent of the lead present may have been extracted into solution, judging from the intensity of the precipitate formed.Immediately after preparing a suspension, only small traces of lead in solution, certainly less than one percent of that present, were detected.

The fate of dissolved material during the spray drying process will be discussed in the next section.

One way in which the extraction of components into solution could be avoided would be by employing a non-polar dispersion medium.

Dispersions could be produced in methoxyethanol in a very similar fashion to water, but this liquid is still quite polar. Toluene, selected as a typical non-polar liquid, proved ineffective as a dispersion medium. Large particles of material persisted in the suspension, and sedimented very rapidly.

The dispersions of the lead zirconium titanate PbZrOgTiO 1 3 precursor gel in water were yellow, and those in methoxyethanol lilac in colour.

A final refinement of the dispersion process was to control the temperature rise associated with the emulsification operation. This was achieved by placing an iced water jacket around the beaker used to prepare the suspension. Temperature rises were limited to about 7"C during a thirty minute run.

For an initial assessment of the applicability of spray drying to the production of oxide powder from gel, a solution or suspension to be dried is fed into a Buchi 190 mini spray dryer by a peristaltic pump. The spray is injected into a heated airstream, and the whole apparatus is maintained under reduced pressure. Powder is collected in a cyclone, and the residual airstream exhausted. For the work described, the heater controls were set to give an inlet temperature of about 200"C, and an outlet temperature of about 100"C. Other parameters (pumping rate and gas flow rates) were set to produce powder at the maximum rate possible, consistent with the above requirements.

The suspensions were prepared by emulsifying 279 of gel into 600ml of dispersion medium for ten minutes. These were then immediately spray dried, which took about fortyfive minutes. Any sedimented material at the bottom of the flask containing the suspension was not fed into the spray dryer. Using suspensions based on water and methoxyethanol, about 1 .8g of free-flowing white powder was collected in each case. Using toluene as the dispersion medium, only a very small amount of powder was collected, as the coarse particles persisting in this suspension rapidly clogged the feed tube.

The powder collection efficiency of the cyclone was estimated at only twenty to thirty percent. Some material collected on the walls of the apparatus. This was in the form of a fairly coherent film, rather than a powder which could be scraped off. The major reason for loss of powder was thought to be finer particles being carried out of the exhaust. The size of the particles collected was much smaller than those found in many typical spray drying applications.

The two relatively large powder samples obtained from the suspensions in water and methoxyethanol were characterised by the range of techniques outlined below. The smaller sample from the toluene suspension was examined by SEM only.

The nature of any retained organic components in the powders was investigated first, by means of TGA and DTA. Experimental conditions were as described for bulk gel samples. This work showed that the spray dried powders had an organic content very similar to corresponding bulk gel samples dried at 150"C.

For a further example, the preparation of a fully dense ceramic material of the composition Pb(zroô4Feo.14Nbo ,4Tioo8) o995U0005 03, a member of the so-called PZFNTU family of materials which was developed for use in pyroelectric detectors will be described.

As depicted in Fig. 1, the preparation for production of PZFNTU begins with making 1 the appropriate metal organic precursor compounds (lead acetate, uranyl acetate, iron acetylacetonate, niobium ethoxide, titanium butoxide and zirconium butoxide complex) and dissolving these 2 in a suitable solvent, in this instance 2-methoxyethanol, to form the solution.

Suitable treatment of the solution, followed by a controlled hydrolysis operation 3 leads to the formation of a homogeneous gel.

The low mechanical strength of the gel state is then exploited by forming a dispersion 4 of the gel material in a suitable liquid medium using a standard mixer emulsifier. A number of dispersion media have been evaluated and their influence on particle morphology assessed. In particular, water and methoxyethanol were both found to produce suspensions stable enough to permit easy spray drying. A spheroidal particle morphology was preferred and this was found to be obtainable by using water as the dispersion medium.

Processing conditions similar to those previously employed were used, though the dispersion time was increased to thirty minutes.

Since water was identified as the most promising dispersion medium for the PZT precursor gel, this was used exclusively. Less settling of the suspensions was observed than with the PZT precursor gel, and yields of powder up to about fifty percent were obtained. The powder collected was yellow in colour, and apparently very fine and free-flowing.

An attempt was made to increase the rate at which powder could be produced by increasing the solid loading in the suspension fivefold (that is, about 1359 of gel was dispersed into 600ml portions of water). This produced no problems in processing, but significantly larger particle sizes resulted.

The dispersion is then treated in a spray drying stage 6 which enables the dispersion medium to be removed without permitting reagglomeration of the gel particles. Spray drying conditions have been identified which produce free-flowing powders with particle sizes typically in the range of from 3.0 to 0.05 micrometres diameter.

One benefit of the combined dispersion and spray drying process is the relatively high degree of control over particle size and morphology that is possible by manipulation of the processing conditions. Spheroidal, non-agglomerated particles that are very desirable for ceramic body fabrication can readily be produced. The gel particles also appear to retain the atomic scale mixing proportions of the various metallic components that are present in the starting solution.

The spray dried powder initially contains residual organic species. These are readily removed by calcination 7 in air at relatively low temperatures such as 600"C. Higher calcination temperatures, required in traditional processing operations to react the oxide starting materials, are not necessary. Calcination at these moderate temperatures does not cause any undesirable changes in the powder, and in some cases this temperature may be sufficient to convert the initially amorphous gel material to the required crystalline mixed metal oxide ceramic composition.

The aforementioned method of producing metal oxide powder 8 is suitable for a number of different applications in which particulate oxide material are used. Some examples of these applications are pigments, phosphors, catalysts and component materials in composites. The process is thus suitable for almost any application where fine complex oxide powders of controlled particle size and morphology are required. The possibility of making a powder with a molecular level of homogeneity will also be of interest for many applications.

However, the oxide powder may additionally be used to form a sintered ceramic body as depicted In Fig. 2. To do this, the standard ceramic hot-pressing and sintering techniques are employed. These techniques are for hot processing a temperature of between 1120 and 1200"C in oxygen for six hours with a pressure of 22 tons per square inch being applied when the sample reached 800 C. Sintering was then carried out at 1 2000C for six hours.

Initial results in the case of PZFNTU indicate that fully densified material may be obtained more readily than with a conventionally produced powder. For example, the hot-pressing temperatures may be reduced significantly below 1 1000C to give a fine grained ceramic material, whilst the requisite high density is maintained. A hot-pressing temperature of between 900 and 1000 C has been proposed.

The resulting ceramic body 9 can be conveniently processed in an identical way to that employed with conventional material for the preparation of electroded devices etc. such as for manufacture of advanced pyroelectric detector arrays.

The foregoing description of embodiments of the invention has been given by way of example only and a number of modifications may be made without departing from the scope of the invention as defined in the appended claims. For instance, although the invention has been described for making a single member of the PZFNTU family, it can clearly be modified to make other members of this family or even other mixed metal oxide ceramic materials. Further examples of ceramic materials produced by this route include yttria stabilised zirconia (Y203, 8-12% wt.% in Zero2) for infra red windows. It should also be possible to utilise the process with other gels such as the citrate gels used to produce calcium lanthanum sulphide (CaLa2S4).

Claims (7)

1. A method of preparing an oxide material in a particulate form, the method comprising the steps of providing a metal organic precursor of the required oxide material, dissolving said precursor in a suitable organic solvent, hydrolysing the solution to form a gel, dispersing the gel into a suitable dispersion medium and spray drying the resulting suspension to produce a powder form of the material.

2. A method as claimed in Claim 1, including the further step of calcining the spray dried powder to convert the material to a particulate oxide form.

3. A method as claimed in Claim 2, in which the said calcination step is effected at a temperature of 600 C.

4. A method as claimed in any one of Claims 1 to 3, in which the step of dissolving a precursor in the solvent is effected by adding precursors of two or more different metals to said solvent.

5. A method of preparing an oxide material substantially as hereinbefore described with reference to the accompanying drawing.

6. A particulate oxide material when prepared by a method as claimed in any one of Claims 1 to 5.

7. A hot pressed and sintered dense ceramic body when prepared from particulate oxide material as claimed in any one of Claims 1 to 6.