Patent Description:
It has been known to produce synthetic opal using a process, in which monodispersed spheres (i.e. spheres of substantially the same diameter) are arranged in an ordered structure to form a 'photonic crystal', and are stabilised by a filler. The diameter of the particles is in the range of the wavelengths of visible light, such that the photonic crystal diffracts light in the visible spectrum to create the well-known play of colors displayed by an opal. To enable this diffraction of light the refractive index of the filler must differ from the refractive index of the particles. Typically, the monodispersed spheres are silica (SiO<NUM>) and the filler is zirconium oxide (ZrO<NUM>), though other materials may be used.

<CIT> describes such a process of making synthetic opals using a sol gel process. Silica spheres are dispersed in water by stirring and are left to sediment, where they will naturally arrange in mass having an ordered structure. The mass is dried and then immersed in a zirconium-containing solution. During the immersion, zirconium oxide is precipitated out by hydrolysis to form the filler. Finally, the structure is calcined to leave a finished synthetic opal.

While such processes are effective in producing synthetic opals, they are typically slow, and require many process steps. A high degree of ordering of the photonic crystal is necessary to produce quality opals, and a long period is therefore required for sedimentation. Drying, hydrolysis and sintering steps also take a long time, meaning that manufacture typically takes many weeks from start to finish. Furthermore, each individual opal stone must be separately created, with each process step carries out on each individual stone. The process is therefore heavily reliant on manual input and as such is typically conducted as small batch processes. Attempts to speed up the process or reduce manual input tend to compromise the quality of the opal, and as such, high through-put manufacture of high quality synthetic opal has been historically difficult.

<CIT> discloses a biosensor comprising photonic crystals dispersed in a solvent and carrying a ligand molecule on their surface. The photonic crystals may be realised as multilayer spherical particles, with a silica core laminated by alternating titanium oxide and silica layers. The layers are formed by chemical reactions, catalysed by ammonia, while the particles are dispersed in a lower alcohol. The particles are washed and dried after each lamination step and then redispersed by an ultrasonic treatment for the next lamination step.

<CIT> discloses a method for making hollow carbon nanospheres using monodispersed silica spheres. The method comprises preparing silica spheres coated with a polystyrene layer, including subjecting a solution of silica spheres and polystyrene precursors to ultrasonic dispersion.

<CIT> discloses a printing formulation comprising monodisperse particles, a curing material, an initiator and a solvent. The monodisperse particles may be synthesised in a solution containing ammonia, with the solid particles separated from the liquid and then re-dispersed in ethanol by mechanical stirring and ultrasonic treatment.

<CIT> discloses a method for measuring pH by using photonic crystal wettability, which comprises preparing an ethanol dispersion of microspheres and preparing silica photonic crystal films. The method includes purifying an ethanol dispersion of microspheres using "centrifugation-ultrasound", i.e., subjecting the solution to an ultrasonic treatment followed by centrifugation in order to obtain a concentrated particle solution.

The present invention has been devised to mitigate or overcome at least some of the above-mentioned problems.

According to an aspect of the present invention there is provided a method of making a liquid dispersion for the manufacture of a photonic crystal. The method comprises dispersing monodispersed spheres in a liquid to form a liquid dispersion, subjecting the liquid dispersion to an ultrasonic treatment, adding ammonia solution to the liquid dispersion after subjecting the liquid dispersion to said ultrasonic treatment; and subjecting the liquid dispersion, including the added ammonia solution, to a further ultrasonic treatment.

Subjecting the liquid dispersion to an ultrasound treatment in this way breaks up agglomerations of the monodispersed spheres in the liquid dispersion. When the liquid dispersion is used to make a photonic crystal, the spheres are allowed to settle under gravity to form an ordered structure that defines the photonic crystal. Agglomerations break the ordering of the structure, which reduces the quality of the visual appearance of the photonic crystal. In particular, it reduces the transparency of the crystal and the 'fire' of the crystal (a high fire meaning that when the photonic crystal disperses the incoming light into different wavelengths, the beams corresponding to the different wavelengths are well-separated, such that different colours are distinctly visible in the outgoing light). Thus, the liquid dispersion produced by this method has fewer agglomerations and thus when used to make photonic crystals it produces a more highly ordered crystal structure, resulting in superior transparency and fire in the finished photonic crystal.

Ammonia is beneficial because it guards against re-agglomeration of the spheres after the ultrasound treatment has broken up agglomerations. In this way, the use of the ultrasound treatment and the use of ammonia act synergystically to remove agglomerations and avoid their reformation even after the ultrasound treatment has been carried out.

The method may comprise dispersing the monodispersed spheres in water to form the liquid dispersion.

A ratio of water to ammonia solution by weight may preferably be between approximately <NUM> : <NUM> and <NUM> to <NUM>, and is preferably approximately <NUM> : <NUM>. This amount of ammonia is high enough to guard against re-agglomeration effectively, but low enough to reduce health and safety concerns.

In particularly preferred embodiments, the dispersion may be subjected to first, second and third ultrasonic treatments. Applying three ultrasound treatments has been found to be particularly effective in removing agglomerations to provide high quality photonic crystals.

The method may comprise allowing the liquid dispersion to cool between the ultrasonic treatments.

The method may comprise allowing the liquid dispersion to cool for a cooling period, and the or each cooling period may have a duration that is between approximately <NUM> minutes and approximately <NUM> minutes, preferably approximately <NUM> minutes or approximately <NUM> minutes.

Where multiple ultrasonic treatments are used, the ammonia may be added after the first treatment or after subsequent treatments such as the second treatment. Adding the ammonia at a later stage helps to reduce evaporation of the ammonia during ultrasound treatments, which might otherwise be a health and safety concern.

The method may comprise subjecting the liquid dispersion to the or each ultrasonic treatment for an ultrasonic treatment period. The ultrasonic treatment period may have a duration that is between approximately <NUM> seconds and approximately <NUM> minutes, and that is preferably approximately <NUM> seconds or approximately <NUM> minutes.

A ratio of water to monodispersed spheres by weight may be between approximately <NUM> : <NUM> and approximately <NUM> : <NUM>, preferably approximately <NUM> : <NUM>.

The monodispersed spheres may have a mean diameter of between <NUM> and <NUM>, and may preferably have a mean diameter of <NUM>, <NUM>, <NUM> or <NUM>.

The method may comprise continuously feeding the dispersion into a first ultrasound volume to undergo a first ultrasonic treatment. The method may comprise continuously feeding the dispersion from the first ultrasound volume to a first cooling volume to undergo cooling.

The method may comprise continuously feeding the dispersion from the first cooling volume to a second ultrasound volume to undergo a second ultrasonic treatment. In this case, the method may comprise continuously feeding the dispersion from the second ultrasound volume to a second cooling volume to undergo cooling. Where two ultrasound volumes are used, the method may comprise pumping the dispersion through the first ultrasound volume at a first flow rate and pumping the dispersion through the second ultrasound volume at a second flow rate, the second flow rate being lower than the first flow rate.

In other embodiments, after cooling, the method may comprise continuously feeding the dispersion from the first cooling volume back to the first ultrasound volume to undergo a second ultrasonic treatment. In this case, the method may comprise continuously feeding the dispersion from the first ultrasound volume back to the first cooling volume to undergo cooling, or to a second cooling volume to undergo cooling.

The method may comprise pumping the dispersion through the first ultrasound volume for the first ultrasonic treatment at a first flow rate and pumping the dispersion through the first ultrasound volume for the second ultrasonic treatment at a second flow rate, the second flow rate being lower than the first flow rate.

The invention also encompasses a method of making a photonic crystal. The method comprises: making a liquid dispersion comprising monodispersed spheres according to the method described above; providing a mould having a liquid-receiving cavity; filling the cavity with the liquid dispersion; allowing the monodispersed spheres to sediment; allowing the sedimented spheres to dry; and filling spaces between the spheres with a filler material to form the photonic crystal.

The invention further encompasses a method of making a plurality of photonic crystals. The method comprises: making a liquid dispersion according to the method described above; providing a mould arrangement having a plurality of liquid-receiving cavities; simultaneously filling at least some of the plurality of cavities with the liquid dispersion; allowing the monodispersed spheres to sediment; allowing the sedimented spheres to dry; and filling the spheres with a filler material to form the photonic crystals.

In either method described above, after the drying stage, the sedimented spheres are referred to as an 'opal cake'. The method may comprise a step of calcining the or each opal cake, which may comprise heating the or each opal cake.

The or each opal cake may be transferred to a heating volume such as a furnace to undergo heating. The transfer step may include removing the or each opal cake from the mould to a support surface to transfer the or each opal cake to the heating volume. In this case, the method may comprise arranging the or each opal cake on the support surface so that a 'meniscus' of the or each opal cake (defined by the exposed liquid surface in the mould) faces upwardly, away from the support surface. In this way, the or each opal cake does not rest on the uneven 'meniscus' surface but instead rests on its flat base surface during the calcining step, which reduces the chance of breakage during calcining. To arrange the or each opal cake in this way, the or each opal cake may be inverted from the mould onto a temporary support surface, and then inverted once more from the temporary support surface to the support surface for the calcining stage.

The or each calcined opal product may be located in the mould for the filling stage. In this case, the mould may be filled with a first immersion liquid to allow the first immersion liquid to infiltrate between the ordered spheres. The mould may then be filled with a second immersion liquid that reacts with the first immersion liquid to form the filler.

After the filling stage the or each filled opal product may be sintered, which may comprise heating the or each filled opal product. The or each filled opal product may be transferred to a heating volume such as a furnace to undergo heating. This may involve transferring the or each filled opal product to a support surface in the manner already described above in relation to the calcining step.

In any of the aspects described above the monodispersed spheres may be monodispersed silica spheres.

In any of the aspects described above, the photonic crystal may be a synthetic opal.

Apparatus for making a photonic crystal, which is exemplified in the forgoing description as a synthetic opal, is illustrated in <FIG>. The apparatus comprises a mould <NUM>, upper and lower supports <NUM>, <NUM>, <NUM> and a container <NUM>.

The mould <NUM> comprises a plurality of cavities <NUM>, each cavity being shaped substantially as a cylinder having a diameter of approximately <NUM>. Each cavity <NUM> is open at the top and base of the mould <NUM> to define lower and upper openings 12a, 12b. Walls <NUM> of the cavities <NUM> are tapered slightly inwards towards the top, such that the diameter of the cavity is slightly larger at the base of the mould <NUM> than at the top of the mould <NUM> to define a taper angle of <NUM>°. This taper facilitates de-moulding of the opals that are formed in the cavities <NUM>.

A lower surface <NUM> of the mould <NUM> is provided with a rim <NUM> in the form of a downward protrusion that extends at least partly around the periphery of the lower surface <NUM>. An upper surface <NUM> of the mould <NUM> is provided with a detent <NUM> that at least partially surrounds the periphery of the upper surface <NUM>.

The mould <NUM> is a plate that is made from a plastics material, in particular polypropylene or Teflon, or from any other suitable material. In this example, the mould <NUM> has a width of <NUM>, a length of <NUM> and a height of between <NUM> and <NUM>, though any suitable dimensions may be used. The cavities <NUM> have a mean diameter of <NUM>. A large number of cavities <NUM>, for example <NUM> cavities, are provided on a single mould <NUM>.

A lower support <NUM> in the form of a lower support plate is provided to close off the lower openings 12a of the cavities <NUM> at the base of the mould <NUM>, and an upper support <NUM> in the form of an upper support plate is provided to close off the upper openings 12b of the cavities <NUM> at the top of the mould <NUM>.

A further lower support in the form of a sinter plate <NUM> is also provided, which is interchangeable with the lower support plate <NUM> as needed during the process. The sinter plate <NUM> is made of a material that can withstand high temperatures, such as a fused silica.

A container <NUM> is configured to receive a liquid, and to receive the mould <NUM> and support plate <NUM>, <NUM>. The container <NUM> comprises a base <NUM> and side walls 24a, 24b, and is open at the top to define an opening <NUM>. An inlet 26a is provided on one side wall, in this case the left side wall 24a, to allow liquid to flow into the container <NUM>, and an outlet 26b is provided on another side wall, in this case the right side wall 24b, to allow liquid to flow out of the container <NUM>.

The base <NUM> of the container <NUM> is formed with a recess <NUM> at its centre. The recess <NUM> is dimensioned to receive the lower support <NUM>. The upper support <NUM> may close off the opening <NUM> of the container <NUM> so as to form a 'lid' of the container <NUM>.

The container <NUM> is made from a plastics material, in particular polypropylene of Teflon, or from any other suitable material. The dimensions of the container <NUM> are slightly larger than the mould <NUM> (i.e. slightly larger than <NUM> in width and <NUM> in length and <NUM> to <NUM> high) so that the mould <NUM> fits snugly into the container <NUM>.

A process for making a pre-prepared liquid dispersion for use with the above apparatus in making a synthetic opal will now be described with reference to <FIG>.

As shown in <FIG>, monodispersed silica spheres <NUM> and demineralised water <NUM> are first added to a container <NUM>.

In a particular embodiment, <NUM> of monodispersed silica spheres <NUM> are added to <NUM> or <NUM> of water <NUM>. However, it will be appreciated that other volumes may be used, which may be in a silica sphere : water ratio of <NUM> : <NUM> by weight, or in any other suitable ratio. For example, the silica sphere : water ratio may be as low as <NUM>: <NUM> or as high as <NUM>:<NUM>. In the finished opal product, the concentration of silica spheres will affect the total volume of silica and hence the size of the resulting opal. Since the diameter of the opal is fixed by the mould, the only variable dimension is the height, thus in practice the concentration will affect the height of the opal. The concentration may therefore be varied according to the height that is desired in the finished opal.

"Monodispersed" means that the spheres are all of substantially the same diameter. It will be appreciated that a small degree of variation is to be expected in practice. For monodispersed spheres <NUM>, this variation is typically less than <NUM>% of the specified diameter: so for example, monodispersed spheres <NUM> having a mean diameter of <NUM> might be expected to have a diameter range of <NUM> to <NUM>.

The monodispersed silica spheres <NUM> are specifically selected such that the mean diameter is between <NUM> and <NUM>. This selected diameter is within the wavelength of visible light, which will give rise to the desired colour effects in the finished opal.

The specific particle size is selected according to the desired colour of the final opal. For example, <NUM> spheres are selected for a blue colour, <NUM> spheres or <NUM> spheres are selected for a green/blue colouring, and <NUM> spheres are selected for a red/green colouring.

The mixture is stirred and subjected to ultrasonic dispersion as illustrated in <FIG>. In this step, the container <NUM> is placed in a soundproof enclosure <NUM> and subjected to ultrasonic vibrations for a period of <NUM> minutes using, for example, a Branson S450A CE 400W, Sonotrode <NUM> with an ultrasound frequency of <NUM>, though other ultrasound devices may be used operating at other suitable frequencies. The ultrasonic vibrations act to disperse the silica spheres <NUM> particularly effectively within the liquid and to split up agglomerated groups of silica spheres <NUM>. Avoiding agglomerations results in a more ordered structure when the silica spheres <NUM> are sedimented, which improves the quality of the finished opal.

The ultrasonic vibrations cause the liquid to heat up during the ultrasound process. When the ultrasound vibrations are halted, the liquid is allowed to cool back to room temperature in a water bath at <NUM> for a period of <NUM> minutes (though other appropriate cooling times may be used). After cooling, the liquid is stirred once more, and the process of ultrasonic treatment for a five minute interval, followed by cooling for a <NUM> minute interval, is repeated to provide for particularly thorough dispersion. Breaking the ultrasound process into multiple steps in this way avoids overheating of the liquid whilst still breaking up agglomerated particles effectively.

Ammonia solution is then added to the container <NUM>. In this particular embodiment, <NUM> or <NUM> of <NUM> % ammonia solution is added, providing a silica:water:ammonia ratio of <NUM>:<NUM>:<NUM> by weight, though other suitable amounts and ratios could be used, for example between <NUM> to <NUM> of ammonia in <NUM> of water.

The inventors have found that after the ultrasound process has broken up agglomerates, the silica particles <NUM> are particularly prone to re-agglomeration, as a result of their small and uniform size. The ammonia solution acts to ionise the surfaces of the silica spheres <NUM>, which provides repellent electrostatic forces that prevent this re-agglomoration of the spheres <NUM>. This ensures that the silica spheres <NUM> remain separate as the liquid is used in later processes. The mixture is stirred, and the ultrasound and cooling processes are repeated for a final time.

After cooling, the mixture is stirred once more and is then ready for use in making synthetic opals.

In another embodiment, the liquid dispersion is prepared in a continuous in-line process using the apparatus depicted schematically in <FIG>.

According to this embodiment, a mixture comprising monodispersed silica spheres, demineralised water and ammonia solution in the appropriate ratios is pumped through a series of mixing volumes in the form of mixing tanks, ultrasonic treatment volumes in the form of ultrasonic cells, and cooling volumes in the form of cooling tanks.

To this end, the apparatus comprises the following components arranged in series: a mixing volume in the form of a mixing tank <NUM> having one or more inlets for receiving components of the liquid mixture, a first ultrasound volume <NUM>, a first cooling volume <NUM>, a second ultrasound volume <NUM>, a second cooling volume <NUM>, a third ultrasound volume <NUM> and a third cooling volume <NUM> having an outlet for dispensing the liquid mixture. Pumps (not shown) pump the liquid mixture from one tank to the next via the ultrasound cells.

The ultrasound volumes may be any appropriate apparatus capable of applying an ultrasound treatment to liquid passing through it. In this case the ultrasound volumes are exemplified as ultrasonic cells, such as for example a Branson S450A CE 400W, Sonotrode <NUM> with an ultrasound frequency of <NUM>.

The mixing and cooling volumes are exemplified as tanks. The pumps may be any suitable peristaltic pump, for example a Hei-FLOW Precision <NUM> or Hei-FLOW Precision <NUM> pump, that may pump the liquid at any suitable flow rate, for example at a flow rate that is between approximately <NUM>/minute and approximately <NUM>/min.

The mixture is pumped from the mixing tank <NUM> through the first ultrasonic cell <NUM> at a first pumping rate V1, which in this example is a rate of <NUM>/minute. The mixture undergoes a first ultrasonic treatment as it is pumped through the first ultrasonic cell <NUM>. In this example, the cell <NUM> has a volume of approximately <NUM>, such that liquid is in the cell for an average time of approximately <NUM> seconds. The liquid is then pumped through the first cooling tank <NUM>, at the same rate V1, where it is stirred and cooled. In this example, the tank <NUM> has a volume of approximately <NUM> litres, and typically contained <NUM> litres of liquid, such that liquid is in the tank <NUM> for an average time of <NUM> minutes.

The mixture is then pumped through the second ultrasonic cell <NUM> at a second pumping rate V2, where it undergoes a second ultrasonic treatment. The second pumping rate is preferably less than the first pumping rate, and in this example is <NUM>/minute. A higher pumping rate is preferable for the first ultrasonic treatment because during the first cycle, while larger agglomerations are still present in the liquid which might otherwise tend to clog the system, the higher pumping rate tends to reduce the tendency for clogging. Once the liquid has undergone its first ultrasonic treatment and agglomerations are reduced, a lower flow rate can be safely used.

In this example, the second cell <NUM> also has a volume of approximately <NUM>, such that liquid is in the cell for an average time of approximately <NUM> seconds. The mixture is then pumped through the second cooling tank <NUM> at the same rate V2, where it is stirred and cooled. In this example, the second cooling tank <NUM> has a volume of <NUM> litres and typically contains approximately <NUM> litres of liquid, such that liquid is typically in the second cooling tank <NUM> for an average time of <NUM> minutes.

Finally, the mixture is pumped through the third ultrasonic cell <NUM> to a third cooling tank <NUM> at a third pumping rate V3, where it undergoes a third ultrasonic treatment. The third pumping rate is preferably the same as the second pumping rate, which in this example is <NUM>/minute. In this example, the third cell <NUM> has a volume of approximately <NUM>, such that liquid is in the cell for an average time of <NUM> seconds. The mixture is then pumped through the third cooling tank <NUM> at the same rate V3, where it is stirred and cooled. In this example, the third cooling tank <NUM> has a volume of <NUM> litres and typically contains approximately <NUM> litres of liquid, such that liquid is typically in the third tank <NUM> for an average time of <NUM> minutes.

Any or all of the cooling tanks <NUM>, <NUM>, <NUM> may be cooled to cool the liquid mixture back to room temperature between ultrasonic treatments. Any or all of the tanks <NUM>, <NUM>, <NUM>, <NUM> may comprise stirring apparatus to stir liquid in the tanks <NUM>, <NUM>, <NUM>, <NUM>.

This continuous inline system is particularly advantageous as it allows the liquid dispersion to be made continuously to meet the high volume demands of a manufacturing process. The system may be a closed system, allowing the various treatment stages to be carried out without exposing the liquid mixture to the atmosphere. This firstly helps to preserve purity of the liquid mixture, and secondly helps to avoid loss of ammonia from the solution, which ensures sufficient ammonia is present to reduce undesirable agglomerations, and is beneficial for reasons of health and safety.

Other examples of an inline system that may be used are shown schematically in <FIG>.

In the example of <FIG>, the system comprises two tanks <NUM>, <NUM> in fluid communication with a single ultrasonic cell <NUM>, via pumps. The tanks <NUM>, <NUM>, ultrasonic cell <NUM> and pump are all substantially the same as those described above in relation to <FIG>.

The dispersion liquid is mixed in one tank <NUM> (or may be pre-mixed and poured into the tank <NUM>), and is pumped between the tanks <NUM>, <NUM> via the ultrasonic cell <NUM> multiple times to undergo multiple ultrasonic treatments. For example, once mixed, the dispersion liquid may be pumped from the first tank <NUM>, through the ultrasonic cell <NUM>, to the second tank <NUM> to undergo ultrasonic treatment. Once pumped into the second tank <NUM>, the liquid may then be pumped from the second tank <NUM>, back through the ultrasonic cell <NUM>, into the first tank <NUM>. The liquid may then be pumped a third time from the first tank <NUM>, through the ultrasonic cell <NUM>, to the second tank <NUM> to undergo a third and final ultrasonic treatment.

In the example of <FIG>, the system comprises two tanks <NUM>, <NUM> with a plurality of ultrasonic cells <NUM>, <NUM>, <NUM> between them, and a pump located between the tanks <NUM>, <NUM>. The dispersion liquid is mixed in a first tank <NUM> (or a pre-mixed liquid may be poured into the first tank), and is pumped from the first tank <NUM> to the second tank <NUM> via the plurality of ultrasonic cells <NUM>, <NUM>, <NUM> to undergo multiple ultrasonic treatments in succession before being cooled in the second tank <NUM>. In this case, the pumping rate may be identical through each of the ultrasonic cells <NUM>, <NUM>, <NUM>.

The process of making synthetic opals using the apparatus of <FIG> and the pre-prepared liquid dispersion produced by the method of <FIG> or using the apparatus of any of <FIG>, will now be described with reference to <FIG>.

As shown in <FIG>, the lower support plate <NUM> is first arranged in the recess <NUM> on the base <NUM> of the container <NUM>, so that the lower support plate <NUM> protrudes slightly above the base <NUM>. The container <NUM> is then filled with pre-prepared liquid dispersion <NUM> from the inlet 26a. The volume of liquid dispersion <NUM> that is added to the container <NUM> is slightly less than the total volume of the cavities <NUM> of the mould <NUM>. For example, each cavity may hold approximately <NUM> microlitres of liquid, so that if the mould contains <NUM> cavities, a total of <NUM> of liquid is initially introduced into the container <NUM>.

Referring to <FIG>, the mould <NUM> is lowered into the container <NUM> through the opening <NUM> at the top, and hence is lowered into the liquid <NUM>. As the mould <NUM> is lowered the cavities <NUM> fill with the liquid <NUM> through the openings 12a in the base of the mould <NUM>. Once the mould <NUM> has been fully lowered as shown in <FIG>, the mould <NUM> sits on top of the lower support plate <NUM>, with the lower support plate <NUM> surrounded by the rim <NUM> of the mould <NUM>, and the cavities <NUM> almost entirely filled with liquid <NUM>.

Because the total volume of liquid <NUM> in the container <NUM> is slightly less than the total volume of all the cavities <NUM>, the liquid does not completely fill the cavities <NUM>, leaving a small air gap at the top of each cavity <NUM> to avoid the liquid spilling over the mould <NUM>.

The upper support <NUM> is then arranged over the mould <NUM> and the container <NUM> as shown in <FIG>, so as to close the opening. In this way the upper support <NUM> acts as a lid that guards against evaporation of the liquid <NUM>.

In this configuration, the apparatus is left to sediment for a period of time, during which the silica spheres settle into an ordered structure. Because of the combination of ultrasound treatments and ammonia in the liquid dispersion, the silica spheres are particularly resistant to agglomeration. As a result, there are very few agglomerates in the resulting ordered structure, meaning that the structure is particularly highly ordered. Because of this high degree of ordering, the resulting sedimented spheres are particularly highly ordered, with very few agglomerated regions disturbing the ordered structure.

As can be seen in <FIG>, the settled silica spheres form a sedimented mass <NUM> at the base of each cavity <NUM> in the mould <NUM>.

The sedimentation period is typically a week. During this time, the apparatus is kept at room temperature (<NUM> - <NUM>) and in a vibration free area, so as to avoid disturbing the silica spheres as they settle.

After sedimentation is complete, the upper support <NUM> is removed. The remaining liquid is then allowed to evaporate in a 'drying' process. During this drying process, the roomtemperature and vibration-free conditions of the sedimentation stages are maintained. Additionally, the drying process must take place in an environment with a low and well-controlled air flow so as to allow evaporation of the liquid whilst avoiding any disturbance to the spheres as they dry.

The drying stage is complete when the remaining liquid has been completely evaporated, leaving only dried opal 'cakes' <NUM> remaining in the cavities <NUM> of the mould <NUM>, as shown in <FIG>. This process typically takes several days.

The dried opal cakes <NUM> that result after the drying process have a planar side <NUM> that lies against the lower support plate <NUM>, and a meniscus side <NUM> that faces upward into the cavity <NUM>. The planar side <NUM> is substantially flat with a planar surface defined by the surface of the lower support plate <NUM>. The meniscus side <NUM> is not flat, but is instead shaped to follow the meniscus that was present at the sediment-liquid interface after sedimentation occurred.

The dried opal cakes <NUM> are then transferred to the sinter plate <NUM> for calcining. If the dried opal cakes <NUM> were arranged with the meniscus side <NUM> against the sinter plate <NUM>, the dried opal cakes <NUM> would not be stable during the sintering process due to the non-planar shape of the meniscus side <NUM>. It is therefore preferable to arrange the dried opal cakes <NUM> with their planar sides <NUM> against the sinter plate <NUM>. However, it is also preferable to minimise disturbance of the dried opal cakes <NUM>, which are in a fragile state after drying.

To accommodate this balance between the need for a particular orientation and the need to minimise disturbance, the apparatus undergoes an inversion procedure to reorient the dried opal cakes.

As shown in <FIG>, the upper support <NUM> is arranged in place again and the apparatus is inverted. The container <NUM> and lower support <NUM> are then removed leaving the mould <NUM> and upper support <NUM> in place. The sinter plate <NUM> is then arranged in place of the lower support <NUM> as shown in <FIG>, in the recess <NUM> of the container <NUM>. Referring to <FIG>, the apparatus is inverted once more. Finally, as shown in <FIG>, the upper support <NUM> and the mould <NUM> are removed to leave and the dried opal cakes <NUM> in place on the sinter plate <NUM>.

The inversion processes above ensure that the dried opal cakes <NUM> can be arranged on the sinter plate <NUM> with minimal disturbance, and with the planar side <NUM>, rather than the meniscus side <NUM>, arranged on the sinter plate <NUM>.

As shown in <FIG>, the sinter plate <NUM> and dried opal cakes <NUM> are then placed in a furnace <NUM> and are subject to a calcination treatment. During this treatment, the dried opal cakes <NUM> are heated from room temperature to <NUM> within <NUM> hours. The dried opal cakes <NUM> are then held at <NUM> for <NUM> hours and finally cooled down to room temperature by passive cooling. After the calcination stage, the dried opal cakes <NUM> have formed calcined opal products <NUM>, which have undergone shrinkage of typically <NUM> %, and which have and increased mechanical stability compared to the dried opal cakes <NUM>.

After calcination, the sinter plate <NUM> and calcined opal products <NUM> are removed from the furnace <NUM>. The sinter plate <NUM> is replaced with the lower support <NUM> in a reversal of the inversion process described above.

In this reversed inversion process, as shown in <FIG>, the mould <NUM> is arranged back in place over the calcined opal products <NUM> and the sinter plate <NUM>, calcined opal products <NUM> and mould <NUM> are placed in the container <NUM>. The upper support <NUM> is placed over the mould <NUM> and container <NUM>. As shown in <FIG>, the apparatus is then inverted. Referring to <FIG>, the container <NUM> and sinter plate <NUM> are removed, and referring to <FIG>, the lower support <NUM> and the container <NUM> are arranged back in place. Finally, as shown in <FIG>, the apparatus is inverted once more and the upper support <NUM> is removed.

In the next stages, the voids between the ordered silica spheres are filled with a filler material, which in this case is ZrO<NUM>.

A first infiltration solution <NUM> is prepared by mixing ethanol and a zirconium solution of <NUM> % zirconium-n-propoxide in <NUM>-propanol, in a ratio of <NUM> ethanol to <NUM> zirconium solution.

Referring to <FIG>, the mould <NUM> is lifted slightly away from the calcined opal products <NUM> to provide a clearance <NUM> around each calcined opals product <NUM>. The first infiltration solution <NUM> is then introduced into the container <NUM> through the inlet 26a. Because of the clearance <NUM> around each calcined opal cake <NUM> the first infiltration solution <NUM> can enter the cavities <NUM> in the mould <NUM> to immerse the calcined opal products <NUM> in the first infiltration solution <NUM>.

The upper support <NUM> is then arranged in place as shown in <FIG>, and the apparatus is kept in this closed configuration for <NUM> hours to allow the solution to infiltrate and fill any voids in the calcined opal products <NUM> to form infiltrated opal products <NUM>. Referring the spheres to <FIG>, the upper support <NUM> is then removed and any remaining solution <NUM> in the container <NUM> is drained through the outlet 26b, leaving the infiltrated opal products <NUM> in the cavities <NUM> of the mould <NUM>.

As shown in <FIG>, a second infiltration solution <NUM> is then introduced into the container <NUM> through the inlet 26a to immerse the infiltrated opal products <NUM>. The second infiltration solution may be any liquid that is capable of causing the zirconium-n-propoxide in the first infiltration solution <NUM> to react to precipitate ZrO<NUM> that fills the voids between the silica spheres. In this example, the second infiltration solution is <NUM> mol/I solution of HCl, though other solutions may be used, such as HCl at other concentrations, or purified water.

After the second infiltration solution has been introduced, the upper support <NUM> is replaced, and the apparatus is left in this configuration for a further <NUM> hours. During this time, a reaction between the HCl in the solution <NUM> and the zirconium-n-propoxide in the first infiltration solution <NUM> contained in the infiltrated opal products <NUM> causes the formation and precipitation of ZrO<NUM>, filling the voids between the silica particles in the infiltrated opal products <NUM>. This process results in reacted opal products <NUM>.

As shown in <FIG>, the upper support <NUM> is removed and the excess solution <NUM> is drained from the container <NUM> via the outlet 26b, leaving the reacted opal products <NUM> in place.

In a final stage, the reacted opal products <NUM> are sintered to produce the final raw opal product. For this sintering process, the reacted opal products <NUM> must be arranged once more on the sinter plate <NUM> with the planar side <NUM> arranged against the sinter plate <NUM>. To arrange the reacted opal products <NUM> on the sinter plate <NUM> in this way, the inversion process already described in relation to <FIG> is repeated once more. For brevity, this process will not be described again in detail.

The sinter plate <NUM> and reacted opal products <NUM> are placed in the furnace <NUM> once more to undergo sintering. In this sintering process, the reacted opal products <NUM> are heated from room temperature to <NUM>,<NUM> over a period of <NUM> hours, and held at <NUM>,<NUM> for <NUM> hours. The furnace <NUM> is then controlled-cooled to <NUM> over a period of <NUM> hours before being passively cooling to room temperature.

After the sintering process, the sinter plate <NUM> is removed from the furnace <NUM> with raw opal products <NUM> on the sinter plate <NUM>, as shown in <FIG>. The raw opal products <NUM> can then be processed as required, for example by cutting or grinding into a required shape to give a finished opal.

Alternatively, as shown in <FIG> and <FIG>, the mould takes to form of vial arrangement.

<FIG> shows the assembled vial arrangement, which takes the form of a specially-designed microwell plate arrangement <NUM> made of untreated polypropylene.

The arrangement <NUM> consists of a support in the form of a frame <NUM>, shown in isolation in <FIG>, and a plurality of mould modules <NUM> in the form of vial arrays. Variations of different vial arrays 152a, 152b, 152c, 152d, 152e, are shown in isolation <FIG>. A plurality of such vial arrays <NUM> can be fitted into and removed from the frame <NUM> as desired to provide a modular mould arrangement, as will be explained.

Referring to <FIG>, the vial arrays 152a each comprise a plurality of vials <NUM> that are connected together. Each vial <NUM> is defined by a surrounding wall <NUM> and is open at its upper end, and closed at its base by a substantially planar base wall <NUM> (not visible in <FIG>, but visible in <FIG>). In each vial array, the vials <NUM> may have different forms: in particular, they may have different shaped cross sections and/or different diameters. Table <NUM> below indicates the shapes and dimensions of the vials <NUM> in the vial arrays 152a, 152b, 152c, 152d, 152e in each of <FIG>.

Considering in particular the vial array 152a of <FIG>, the vial array 152a comprises a mould body <NUM> having a plurality of vials <NUM> that are connected together in a single piece. In the example of <FIG>, the surrounding wall <NUM> of each vial <NUM> is shaped as a cylindrical shell, so that each vial <NUM> has a circular cross-section.

On an upper surface of the main body <NUM>, adjacent to the open ends of the vials <NUM>, the ends of the main body <NUM> terminate with two spaced-apart lugs <NUM>. The lugs <NUM> extend outwardly, away from the vials <NUM>, in a direction that is perpendicular to a longitudinal axis L of the vials <NUM>.

A catch <NUM> is provided in the space between the two lugs <NUM>. The catch <NUM> comprises a downwardly-depending tab 155a that extends in a direction parallel to the longitudinal axes of the vials. At the downward-most end of the tab 155a is a protrusion in the form of a ridge 155b that extends outwardly in the same direction as the lugs <NUM>. In use, the ridge 155b engages with a corresponding recess on the frame <NUM> to secure the vial array 154a to the frame <NUM>.

Referring to <FIG>, the frame <NUM> comprises front and rear end walls 156a, 156b and left and right side walls 156c, 156d. Together, the walls 156a, 156b, 156c, 156d define a perimeter that surrounds a central space <NUM>. At front and rear ends of the frame <NUM>, a horizontal upper wall 156e extends inwardly a short distance from the respective front or rear wall. These horizontal upper walls 156e act as reinforcements that lend structural integrity to the frame <NUM>.

The left and right side walls 156c, 156d have inner surfaces that face towards the central space <NUM>. The inner surfaces are provided with elongate recesses <NUM> that are spaced apart at regular intervals along the walls 156c, 156d. Each recess <NUM> has a width that is substantially the same as the width of the ridge 155b on the catch <NUM> of the vial array 152a. A spacing between the recesses is substantially equal to a width of a single vial array 152a.

The recesses <NUM> extend only part of the way up the left and right side walls 156c, 156d. In particular, each recess <NUM> terminates a short distance away from the uppermost surface of the left or right side wall 156c, 156d. This distance from the uppermost surface corresponds to the length of the downwardly-depending tab member 155a of the catch <NUM> of the vial array <NUM>.

The central space <NUM> defined by the frame <NUM> comprises a plurality of vial 'zones' 157a, 157b, 157c, 157d, 157e, 157f. In use, each zone 157a, 157b, 157c, 157d, 157e, 157f receives a different vial array, and each zone 157a, 157b, 157c, 157d, 157e, 157f is therefore associated with a pair of recesses <NUM>, one recess of the pair being on the left side wall 156c and one recess of the pair being on the right side wall 156d.

To fit the vial array <NUM> into the frame <NUM>, the user arranges the vial array <NUM> over the frame <NUM>, with the vial array <NUM> aligned with a corresponding vial zone, and hence with the catches <NUM> of the vial array <NUM> aligned with a corresponding pair of recess <NUM> in the frame <NUM>. The user pushes the vial array <NUM> downwardly, so that the side walls 156c, 156d of the frame push against the ridges 155b of the catches <NUM>. The vial array <NUM> has been pushed down sufficiently far when the lugs <NUM> of the vial array <NUM> are brought into contact with support surfaces <NUM> defined by the tops of the left and right side walls 156c, 156d of the frame <NUM>. At this point, the ridges 155b of the catches <NUM> have reached the recess <NUM> in the side walls 156c, 156d, and the ridges 155b snap into place in the recesses, thereby effecting a snap fit.

Further vial arrays are fitted to the other zones of the frame <NUM>. Some or all of these further vial arrays may be identical to the first vial array. Alternatively, different vial arrays 152a, 152b, 152c, 152d, 152e may be arranged in place in different 'zones' of the frame <NUM>, thereby allowing opals of different shapes and sizes to be made in the same frame and as part of the same batch. This allows great flexibility in the manufacturing process, so that supply can easily be adapted in accordance with demand.

The method of making opals using a vial arrangement, such as the vial arrangement of of <FIG>, or a conventional vial array such as a standard microwell plate, will now be described with reference to <FIG>.

In this case, the vial array is shown schematically, and the remaining apparatus is similar to the apparatus described above in relation to <FIG>, except that the container and upper and lower support plates may be omitted from the apparatus, and the vials <NUM> may be filled simultaneously using a multi-head pipette <NUM>.

In this filling stage, shown in <FIG>, the vials <NUM> are simultaneously filled with the pre-prepared liquid dispersion <NUM> described above via a multi-head pipette <NUM> having pipette head at spacings that correspond to the positions of the vials in the vial array. For example, for a <NUM>-vial array, the multi-head pipette may be a T. ® Eppendorf reusable <NUM>-tip pipette. Each vial is filled with a set volume of the liquid dispersion <NUM>, for example <NUM> microlitres of dispersion, i.e. a total of <NUM> over the entire <NUM> vial array.

In the event that the microplate arrangement comprises different vial arrangements in different zones, different multi-head pipettes may be used to fill vials in different zones as appropriate.

In one example not depicted in the figures, the vials are closed off using individual screwcap lids during the subsequent sedimentation stage to avoid disturbance during sedimentation. In this example, after sedimentation is complete, the lids are removed for the drying stage.

In an alternative example not depicted in the figures, to accelerate the sedimentation stage and reduce the cost of the process, the lids are omitted, and instead the microwell plate is placed inside a humidity-controlled atmosphere such as a closed desiccator. Humidity within the desiccator is controlled by providing a dish on the bottom of the desiccator that is filled with a dilute ammonia solution. The ammonia solution has a concentration that is equal to the concentration of the ammonia solution added to the pre-prepared liquid dispersion, which in this example is <NUM> %. In a particular example, the relative humidity is maintained at <NUM> %.

In this example, the drying process may also take place in a desiccator, though for the drying process no ammonia solution is present. Conducting the drying stage in the desiccator additionally speeds up the drying process.

After the drying process the dried opal cakes <NUM> are calcined. Since the vials <NUM> are closed at their base, the inversion process that is necessary to arrange the dried opal cakes <NUM> with their planar sides <NUM> against the sinter plate <NUM> must be carried out in a different manner to that described above.

Referring to <FIG>, with the dried opal cakes <NUM> arranged in a first vial array <NUM>, a second vial array <NUM> is placed over the first vial array <NUM>, with the openings of corresponding vials aligned. Next, as shown in <FIG>, the arrangement is inverted. Under the action of gravity, the dried opal cakes <NUM> gently slide into the second array of vials <NUM>, in an inverted position shown in <FIG>, i.e. with the meniscus sides <NUM> of the dried opal cakes <NUM> facing towards the closed bases <NUM> of the second vial array <NUM> and the planar sides <NUM> facing towards the openings <NUM> of the second vial array <NUM>.

The first vial array <NUM> is then set aside, and the sinter plate <NUM> is arranged over the openings <NUM> of the second vial array <NUM> as shown in <FIG>. As shown in <FIG>, the arrangement is then inverted once more, and as shown in <FIG>, the dried opal cakes <NUM> then slide onto the sinter plate <NUM>, with their planar sides <NUM> against the plate <NUM>. Finally, as shown in <FIG>, the second vial array <NUM> is removed leaving the dried opal cakes <NUM> in place on the sinter plate <NUM> for calcination.

The calcination process is substantially the same as the calcination process already described above. After calcination, the calcined opal products are arranged back in the first vial array <NUM> in a reverse of the inversion process described. In this case, the second vial array <NUM> is arranged over the calcined opal products and the arrangement is inverted. The sinter plate <NUM> is removed, and the first vial array <NUM> is arranged over the second vial array <NUM> with the openings aligned. The arrangement is inverted again, and the calcined opal products slide back into the first vial array with their planar sides facing the closed base of the vial.

The vials are then filled with the infiltration liquid already described above using the multihead pipette, and are subject to the same infiltration process. The infiltration liquid is removed by pipette and replaced with an HCl solution in the same manner as described above, and the same precipitation of ZiO<NUM> occurs.

The excess HCl solution is removed and the reacted opal products are then inverted onto the sinter plate <NUM> making use of the second vial array <NUM> in the manner already described, before being subjected to the sintering treatment already described to produce the raw opal product.

The above process provides an effective means of producing large quantities of high-quality synthetic opals in an efficient manner. The through-put is higher as a result of the ability to create many opals simultaneously, and the reduction of time required for individual process steps. A single mould can be used to produce opals of different shapes and sizes as required according to demand, thereby providing a flexible manufacturing process. The quality of the finished opals, in particular the 'fire' and transparency of the opals, is improved as a result of improved ordering of the silica spheres.

A liquid dispersion was made using the batch method described in relation to <FIG> above. Silica spheres of <NUM> diameter were used, with <NUM> of silica spheres in <NUM> water. <NUM> of ammonia with <NUM> % concentration was added to the dispersion. The liquid dispersions underwent no, one, two or three ultrasound treatments as indicated in Table <NUM> below.

The liquid dispersions were then used to make opal cakes using the microtiter plate method described above. Each vial of the microtiter plate was filled with <NUM> microlitres of dispersion liquid and the dispersion liquid was left to settle, then dried and calcined. In a first impregnation stage, a first impregnation liquid was pipetted into the vials and left for <NUM> hours to impregnate, which consisted of <NUM> of <NUM>% zirconium (IV) n-propoxide in <NUM>-propanol with <NUM> absolute ethanol. In a second impregnation stage, a second impregnation liquid was pipetted into the vials and left for <NUM> hours to impregnate, which consisted of <NUM> <NUM> mol/L HCl.

After the final sintering step, the resulting opal cakes were inspected visually to determine i) structural integrity and ii) aesthetic quality. Structural integrity was assessed based on whether the opal cake was a) whole and free of cracks or flaws b) whole but with cracks or flaws present or c) broken into multiple pieces. The assessment of aesthetic quality was based on the observed transparency and 'fire' of the finished opal. An opal of high quality will have a high transparency and a high fire (i.e. when the opal disperses the incoming light into different wavelengths, the beams corresponding to the different wavelengths are well-separated, such that different colours are distinctly visible in the outgoing light).

Results of the visual inspection are shown in Table <NUM> below.

The inventors found that without the ultrasound treatment the presence of agglomerates meant that the liquid dispersion could not be passed through a pipette tip, because the tip became clogged immediately. With only one or two ultrasound treatments, the yield was good and the aesthetic quality adequate. With three ultrasound treatments the yield of 'good' samples was the highest, and the aesthetic quality was superior. The method used to prepare Samples <NUM> and <NUM> fall within the scope of the claims; those for Samples <NUM> and <NUM> do not.

Liquid dispersion samples were made as per Sample <NUM> of Example <NUM> above (i.e. with three ultrasound treatments), but with a varying quantity of ammonia solution in the liquid. Opal cakes were made using the same method described above in relation to Example <NUM>, and the same visual inspections were carried out.

The inventors found that the presence of at least <NUM> of ammonia solution resulted in an improved yield, and that a presence of at least <NUM> of ammonia provides a superior aesthetic quality. For reasons of health and safety, it is desirable to keep the ammonia quantity to a minimum, and thus a quantity of <NUM> was selected as a providing a particularly desirable balance between opal quality and health and safety considerations. The method used to prepare Sample <NUM> does not fall within the scope of the claims.

Although in the examples and embodiments above the spheres are silica spheres and the filler material is zirconia, other suitable materials may be employed for the spheres and filler materials. Any suitable number of photonic crystals may be manufactured using the method above, and the photonic crystals may be of any appropriate shape or size.

Claim 1:
A method of making a liquid dispersion for the manufacture of a photonic crystal, the method comprising:
dispersing monodispersed spheres in a liquid to form a liquid dispersion;
subjecting the liquid dispersion to an ultrasonic treatment;
adding ammonia solution to the liquid dispersion after subjecting the liquid dispersion to said ultrasonic treatment; and
subjecting the liquid dispersion, including the added ammonia solution, to a further ultrasonic treatment.