WO2018091359A1

WO2018091359A1 - Light color conversion device

Info

Publication number: WO2018091359A1
Application number: PCT/EP2017/078831
Authority: WO
Inventors: Mark Thomas Johnson; Franciscus Johannes Gerardus HAKKENS; Roland Alexander VAN DE MOLENGRAAF
Original assignee: Koninklijke Philips N.V.
Priority date: 2016-11-15
Filing date: 2017-11-10
Publication date: 2018-05-24

Abstract

A color conversion device comprises an electroactive material layer. Passive light conversion particles or droplets are provided within the electroactive material layer. The light conversion function of the overall device is dependent on the deformation of the electroactive material layer by one or more transparent or translucent actuation electrode layer in response to an electrical signal applied to actuation electrodes. As a result, the device can be used in a system for generating a desired light output color or in a system for analyzing deformation based on detected optical properties of the electroactive polymer layer.

Description

Light color conversion device

FIELD OF THE INVENTION

This invention relates to devices for performing light color conversion.

BACKGROUND OF THE INVENTION

There are many situations in which light output from a system is desired to have a particular color, and there are also many systems in which analysis of an incident color is performed as part of a sensing function.

The generation of a desired color can be achieved using active electrochromic materials, which change color in response to an applied voltage or charge. However, this requires active control circuitry.

Many light sources can only efficiently generate light with a particular wavelength or band of wavelengths. It is well known to use passive color conversion layers or films such as filled polymers and ceramics to convert from one wavelength to another. Phosphors are widely used for converting light to a longer (lower energy) wavelength, for example for converting a blue LED output to yellow.

Such color conversion films are widely used in lighting, optical analysis, spectroscopy, and displays.

In a passive optical conversion layer, the ratio of converted and not converted light is fixed. Thus, a color conversion film generally does not provide a controllable color output.

SUMMARY OF THE INVENTION

There is a need for a light conversion arrangement which provides controllable light color in a simple and energy efficient manner.

The invention is defined by the claims.

According to the invention, there is provided a color conversion device comprising:

an electroactive material layer comprising an electroactive material; at least one transparent or translucent actuation electrode layer for inducing deformation of the electroactive material layer in response to an electrical signal applied to actuation electrodes in the layer or layers; and passive light conversion particles or droplets within the electroactive material layer, wherein the light conversion function of the overall device is dependent on the deformation of the electroactive material layer.

In this way, an electroactive material is used as the matrix of a composite with light conversion particles or droplets. The color conversion ratio can thus be tuned or can be influenced by an external force, which provides mechanical deformation of the electroactive material layer. The particles or droplets are passive in the sense that they need no external electrical or mechanical actuation. Instead, they are photoresponsive materials which require no non-optical stimulus. This avoids the need to make electrical or physical contact with the particles or droplets. Instead, the deformation of the electroactive material layer alters the number of particles or droplets in a region through which light travels, for example by altering the thickness of the layer presented to a light path. By changing the particle or droplet density along the light path, the light conversion function is changed.

The device may be actively driven to provide a controllable light conversion function, or it may be subjected to an external force which induces deformation which is then detected based on the prevailing light conversion function. Thus, the device may be used as part of a control system or as part of a sensing/analysis system.

The light conversion particles or droplets for example comprise ceramic phosphor particles. They may for example comprise YAG:Ce ceramic particles. However other phosphor particles may be used. The particles may be organic or inorganic, and they may be based on polymers, ceramics or quantum dots.

The electroactive material layer may comprise a plurality of different types of light conversion particles or droplets at different locations within the electroactive material layer. In this way, different light conversion functions may be performed depending on which part of the electroactive material layer the light passes through. This enables control of a wide range of output colors or enables more easy detection.

The electroactive material layer may further comprise scattering particles.

These can be used to ensure than the light path is exposed to a larger volume of the electroactive material layer.

In one set of examples, the device is for controlling a light output color, wherein the actuation electrodes are for inducing deformation of the electroactive material in response to an electrical signal applied to the actuation electrodes. In the context of the device, translucent or transparent means at least partially translucent for light processed by the device.

The color conversion function can thus be tuned by operation of an

electroactive material actuator. Compared to applying external forces by using an electric or piezoelectric motor, the actuation using an electroactive material provides better isotropic deformation and hence more accurate homogenous color conversion. It also enables reduced cost and smaller form factors.

In another set of examples, the device is for implementing a color conversion function in response to external deformation of the electroactive material layer. The device is then suitable for use in an optical analysis system, for example for detecting strain in the electroactive material layer. The total deformation of the electroactive material layer is measured. This is based on the sum of all effects which include deformation as a result of an applied voltage, effects of counterforces (external forces), effects of environmental conditions, temperature, etc.

The invention according to the first set of examples thus also provides a light generation system for generating light with a controllable color, comprising:

a light source with an output with a fixed output color; and

a color conversion device as defined above.

The light source for example comprises a blue LED, and the color conversion particles or droplets then provide a wavelength shift along the wavelength spectrum towards (or even to) red.

The color conversion device may have a first actuation state in which the electroactive material layer is in the path of the output of the light source and a second actuation state in which the electroactive material layer is outside the path of the output of the light source. This enables a large difference in the light conversion function between extreme actuation levels. There may be a range of intermediate actuation levels to enable a fine adjustment.

The system may further comprise an optical fiber into which the light with a controllable color is provided, wherein the optical fiber has light outcoupling structures for outcoupling light of different colors at different points along the optical fiber.

This arrangement enables not only an output color to be controlled, but also a location at which the light output is generated. This may have application in a catheter with different color light output at different locations, for example for local heating. The invention according to the second set of examples thus also provides an optical analysis system, comprising:

a light source with a first output color;

a color conversion device as defined above; and

an analysis system for analyzing the light after at least one passage through the electroactive material layer, thereby to determine the deformation of the electroactive material layer.

The system may comprise a scanner for scanning the light source output over the area of the color conversion device. For example a reflector may be provided on the opposite side of the color conversion device to the light source. This means the light source and analysis system are located in the same area.

The features of the first and second sets of examples can be combined, in a device that has two modes of operation: optically sensing external forces; and actively varying a light output using the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Figure 1 shows a known electroactive polymer device which is not clamped;

Figure 2 shows a known electroactive polymer device which is constrained by a backing layer;

Figure 3 shows the general operation of color conversion device; Figure 4 shows a light generating device using the color conversion device; Figure 5 shows an optical analysis system using the color conversion device;

Figure 6 shows a first alternative way of operating the color conversion device;

Figure 7 shows how a catheter can be used to provide the light output to a desired location;

Figure 8 shows a second alternative way of operating the color conversion device; and

Figure 9 shows the effect of adding a filler to an EAP material, and shows the strain versus the applied electric field for different particle concentrations. DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a color conversion device comprising an electroactive material layer. Passive light conversion particles or droplets are provided within the electroactive material layer. The light conversion function of the overall device is dependent on the deformation of the electroactive material layer. As a result, the device can be used in a system for generating a desired light output color or in a system for analyzing deformation based on detected optical properties of the electroactive polymer layer.

The invention makes use of an electroactive material (EAM), which is a class of materials within the field of electrically responsive materials. When implemented in an actuation device, subjecting an EAM to an electrical drive signal can make them change in size and/or shape. This effect can be used for actuation and sensing purposes.

There exist inorganic and organic EAMs.

A special kind of organic EAMs are Electroactive polymers (EAPs).

Electroactive polymers (EAP) are an emerging class of electrically responsive materials. EAPs, like EAMs can work as sensors or actuators, but can be more easily manufactured into various shapes allowing easy integration into a large variety of systems. Other advantages of EAPs include low power, small form factor, flexibility, noiseless operation, and accuracy, the possibility of high resolution, fast response times, and cyclic actuation. An EAP device can be used in any application in which a small amount of movement of a component or feature is desired, based on electric actuation. Similarly, the technology can be used for sensing small movements. The use of EAPs enables functions which were not possible before, or offers a big advantage over common sensor / actuator solutions, due to the combination of a relatively large deformation and force in a small volume or thin form factor, compared to common actuators. EAPs also give noiseless operation, accurate electronic control, fast response, and a large range of possible actuation frequencies, such as 0 - 20 kHz.

As an example of how an EAM device can be constructed and can operate, Figures 1 and 2 show two possible operating modes for an EAP device that comprises an electroactive polymer layer 14 sandwiched between electrodes 10, 12 on opposite sides of the electroactive polymer layer 14.

Figure 1 shows a device which is not clamped to a carrier layer. A voltage is used to cause the electroactive polymer layer to expand in all directions as shown.

Figure 2 shows a device which is designed so that the expansion arises only in one direction. To this end the structure of Figure 1 is clamped or attached to a carrier layer 16. A voltage is used to cause the electroactive polymer layer to curve or bow. The nature of this movement arises from the interaction between the active layer which expands when actuated, and the passive carrier layer which does not.

Figure 3 shows a color conversion device. An electroactive material layer 30 comprises an electroactive material. Light conversion particles or droplets 32 are embedded within the electroactive material layer 30.

There may be solid particles or liquid or gel droplets within a binding matrix, or encapsulated droplets effectively forming particles. For the description that follows, solid particles are assumed.

The electroactive material is used as the matrix of a composite with light conversion particles 32. The light conversion particles are for example phosphors which may be ceramic or polymer or based on quantum dots. They may be organic or inorganic. Light passing through the electroactive material layer is partially converted into light with a longer wavelength.

The device is deformable between different states of deformation, two of which are shown in Figure 3. In the left part of the image, the electroactive material layer is in a relaxed state. In the right part of the image, the electroactive material layer is in a deformed state. In the deformed state, the layer is thinner as it has expanded in plane, for example as shown in Figure 1.

The light conversion function of the device is dependent on the deformation of the electroactive material layer 30. In particular, there is a light path through the layer, so that the thickness of the layer influences the path distance over which the light interacts with the light conversion particles 32. This then affects the color conversion ratio.

The color conversion ratio can thus be tuned or can be influenced by an external force.

For color tuning, the electroactive material layer forms part of an actuator. For this purpose, actuation electrodes are provided in at least one layer, in accordance with the invention. This layer or each of the layers is at least partially transparent or translucent to the light being processed. This would be light with an initial color before conversion and light with a resulting color after conversion by the device. The transparency of electrodes may be evaluated using regular spectroscopy techniques in absorbance or reflectance modes for the light at hand.

The electrodes may be made of an intrinsically transparent material such as ITO, Indium Tin Oxide, or they may be made translucent by being very thin, and could be metallic as is used in OLED screens, their thickness being for example 5 - 50 nm. The electrodes may be partially open discontinuous electrodes having light openings so that opaque electrode material may be used. For example, the layers could be wire grids of preferably metallic electrodes; or they could be interdigitated comb-like electrodes, again preferably metallic.

As described below, some arrangements may have an electrode layer on each side of the electroactive material layer. It is also possible to provide an electrode layer on one side only for example using interdigitated comb electrodes.

The level of actuation determines the ratio of converted to not-converted light, causing a color shift. In this way, in-situ color conversion becomes possible, for wavelength tuning, sweeping, control or correction.

The prevailing color conversion ratio can also be used to measure the strain in the electroactive material layer by analyzing the wavelength distribution of the interacted light, after transmission or reflected. The average strain as well as local strain distribution can be measured.

By way of example, in Figure 3, the electroactive material layer 30 includes a yellow phosphor 32, for instance YAG:Ce ceramic particles.

The incident light 34 may then be blue. In the left image, half of the blue light 34 is converted into yellow 36 and half is transmitted as blue 34'. The transmitted blue light mixes with the converted yellow light forming for instance warm white. As a result of actuation as shown in the right image, the amount of converted light 36 is reduced, changing the color to cool white. Any color between these two actuation extremes can be selected in- situ. Also continuous wavelength sweeps are possible.

Figure 4 shows a light generation system 40 embodying the invention for generating light with a controllable color. It comprises a light source 41 with an output with a fixed output color and a color conversion device 42 as explained above. The color conversion device includes control electrodes 44 for actuating the electroactive polymer material layer.

The light source can for example be a blue LED with high efficiency. Of course, a wide range of incident wavelengths and wavelength distributions is possible and a wide range of converted wavelengths are also possible based on the phosphor types used.

Light can be transmitted through the electroactive material layer in any desired directions so that the actuation can be arranged to shift the resulting recombined wavelength distribution towards higher or lower wavelengths. The wavelength is increased by the light conversion particles, but the actuation can be designed to place more or fewer particles in the optical path.

Scattering particles may also be provided in the electroactive material layer in order to increase the effective path length and hence level of color conversion.

Figure 5 shows an optical analysis system 50, comprising a light source 51 with a first output color. A color conversion device 52 as explained above is provided. In this case, the color conversion device is subjected to an external load which induces deformation as well as being subject to other effects causing strain within the layer. The light can be routed to the color conversion device 52 through a light guide 53. Although not shown, the colour conversion device can have the actuation electrodes 44 as described above, in accordance with the invention.

The color conversion device 52 has a top reflector 54 so that incident light from the light source passes through the electroactive material layer, is reflected and passes back through the electroactive material layer. An analysis system 56 comprises a light sensor and processor for analyzing the spectrum of the reflected light.

The left image shows the relaxed state, which gives rise to a first yellow content 58 (assuming a blue light source and yellow phosphor). A light spectrum is also shown with the blue and yellow peaks (B, Y). The right image shows a deformed state, which give rises to a second, lower, yellow content 58 and the corresponding light spectrum is also shown.

The color conversion effect can thus be used for measuring the strain in the electroactive material layer. Depending on the strain situation, the spectrum shifts towards a lower or higher frequency. The directions of light incidence and actuation can be parallel to each other or perpendicular (as in the example shown) or anything between.

It is also possible to scan the color conversion device 52 with a narrow light beam so that the strain distribution in the electroactive material layer can be determined. Figure 5 schematically shows a scanning system 62 for this purpose. A narrow beam of light is scanned over the electroactive material layer by the scanning system, and the transmitted (single pass) or reflected (double pass) light colour is then analyzed.

In this example the light is reflected by the reflector 54 on top of the device, so that there are two passes through the phosphor containing layer. Of course it is also an option to analyze transmitted light as mentioned above. Similarly, reflection may be used in the light generating system of Figure 4. The examples above show that deformation of the electroactive material layer results in changed path length for the light passing through the layer. Another option is to arrange that the layer is either within the optical path or outside it, depending on the level of deformation (whether actuated or externally controlled).

Figure 6 shows a first deformation state on the left, where the layer 30 is to the side of the optical path resulting in no light conversion. The second deformation state on the right has the layer 30 in the path thus providing some light wavelength conversion. There may be a range of intermediate actuation levels to enable a fine adjustment.

Figure 7 shows an implementation of the system which further comprises an optical fiber 70 into which the light with a controllable color is provided using a light generator 40 as described above. The optical fiber 70 has light outcoupling structures 72a, 72b such as Bragg gratings for outcoupling light of different colors at different points along the optical fiber.

This arrangement enables not only an output color to be controlled, but also a location at which the light output is generated. This may have application in a catheter with different color light output at different locations, for example for local heating.

This local heating at positions along a catheter (or guide wire) is for example of interest for controlling stiffness as part of a control of the steering of a catheter.

Catheterization has become one of the most widely used procedures in cardiovascular analysis and treatment. For example in abdominal aneurism repair a stent is placed in an aneurism, which is a weakened part of the abdominal aorta, to prevent further widening and ultimately rupture of the aneurism. In the case of a so-called Fenestrated Endo Vascular Abdominal aneurism Repair (FEVAR) procedure, the renal arteries need to be stented as well. Here, a combination of catheters and guide wires is used to bring the stent into position: first a soft guide wire with a pre-formed tip and a catheter are used to navigate to the renal arteries. In this step often several guide wires, each with a different stiffness, are tried before the surgeon succeeds in positioning the tip of the guide wire and catheter in the renal artery. After this step, while keeping the catheter in position, the soft guide wire is removed and a stiff guide wire is introduced.

When the stiff guide wire is in place, the catheter is removed and a catheter with the stent is railed over the stiff guide wire in order to position the stent in the renal artery. Hereby it is essential that the guide wire is sufficiently stiff in order to be able to guide the catheter with the stent. The applicant has proposed the use of a guide wire with a controllable stiffness in order to reduce the amount of steps needed to bring the stent in place as it enables the use of only one guide wire, thereby reducing the exposure to harmful X-rays and contrast agents.

The use of phase change materials (PCMs) to control the stiffness of a guide wire has been proposed. The system as described above with reference to Figure 7 can be used for optically heating a PCM for stiffness control.

Independent control of multiple segments of the catheter tip becomes possible with the use of a single optical fiber. Tilted or blazed Bragg gratings enable control of the location along the length of the fiber at which the light is coupled out, depending on the color control provided at the input to the catheter, as shown in Figure 7.

To heat up the PCM, a cladding layer may be provided with a high absorption coefficient such that it heats up by the light that is coupled into the cladding. Heat could be transferred by thermal conduction from the cladding to the PCM. Another approach is to remove the cladding from the optical fiber such that the light is not coupled into cladding modes but into radiation modes. For this, the PCM surrounding the optical fiber core has a high absorption coefficient such that the light is converted into heat. This could for example be achieved by adding a black absorber (such as black carbon particles) to the PCM.

The examples above are based on an electroactive material layer with one type of light converting particle. The layer may comprise a plurality of different types of light conversion particles at different locations within the electroactive material layer as shown in Figure 8.

In this example there are two types of light conversion particle, on in region 80 and one in region 82.

The top image shows a first deformation state in which the optical path is laterally offset from the layer.

The middle image shows a second deformation state in which the optical path is aligned with region 80 providing a first light conversion function.

The bottom image shows a third deformation state in which the optical path is aligned with region 82 providing a second light conversion function.

In this way, different light conversion functions may be performed depending on which part of the electroactive material layer the light passes through. This enables control of a wide range of output colors or enables more easy detection.

Different patterns of light conversion particles may for example be formed by 3D printing or dispensing. The phosphor particles need to interact with the light. Therefore the diameter should not be much shorter than the wavelength of light. Very small but many particles have the same color conversion as a few larger ones. However, many small particles increase the stiffness more, thereby limiting actuation. In most cases EAPs have a multilayer structure to reduce driving voltage, and the sub layers are a few microns thick. The stacked multilayer is for example ΙΟΟμιη thick. The particles will be significantly thinner than the layer or sub layer thickness. The particles for example have around Ιμιη diameter, and generally between 30 nm and 3 μιη.

Some examples of suitable filler have been given above. They may be phosphors, ceramics, polymers, quantum dots, and they may be organic or inorganic.

Ceramic particles are the most easy to implement and will be very stable in a polymer matrix:

Some examples of ceramic phosphors are

YAG:Ce (yellow)

GdYAG:Ce (yellow)

CaAlSiN3 :Eu (red) Ca can be replaced by Sr or Ba.

(Zn, Cd)S:Ag (yellow)

ZnS:Cu (green)

Some examples of quantum dots (wherein the size determines wavelength of emission) are:

CdSe/GdS (core/shell)

InP/ZnS

CuInS/ZnS

Organic phosphors and organometallic phosphors may also be used.

EAPs can be used as actuators in a wide range of applications. Examples are catheter or guide wire steering, shaver heads (closeness control), ultrasound tissue contact control, variable focus lenses, ultrasound mirror actuation, optical mirror actuation. In many cases, a controlled shape of the actuator (or position control of the component the EAP is actuating) is desired. However, the local position as a function of applied voltage depends also on the load, temperature, aging etc. By measuring the strain, a shape or position feedback control can be realized making shape controlled actuation very accurate.

Although in the detailed description herein above the construction and operation of devices and systems according to the invention have been described for EAPs, the invention can in fact be used for devices based on other kinds of EAM material. Hence, unless indicated otherwise, the EAP materials hereinabove can be replaced with other EAM materials. Such other EAM materials are known in the art and the person skilled in the art will know where to find them and how to apply them. A number of options will be described herein below.

A common sub-division of EAM devices is into field-driven and current or charge (ion) driven EAMs. Field-driven EAMs are actuated by an electric field through direct electromechanical coupling, while the actuation mechanism for current or charge driven EAMs involves the diffusion of ions. The latter mechanism is more often found in the corresponding organic EAMs such as EAPs. While field driven EAMs generally are driven with voltage signals and require corresponding voltage drivers/controllers, current driven EAMs generally are driven with current or charge signals sometimes requiring current drivers. Both classes of materials have multiple family members, each having their own advantages and disadvantages.

Field driven EAMs can be organic or inorganic materials and if organic can be single molecule, oligomeric or polymeric. For the current invention they are preferably organic and then also oligomeric or even polymeric. The organic materials and especially polymers are an emerging class of materials of growing interest as they combine the actuation properties with material properties such as light weight, cheap manufacture and easy processing.

The field driven EAMs and thus also EAPs are generally piezoelectric and possibly ferroelectric and thus comprise a spontaneous permanent polarization (dipole moment). Alternatively, they are electro strictive and thus comprise only a polarization (dipole moment) when driven, but not when not driven. Alternatively they are dielectric relaxor materials. Such polymers include, but are not limited to, the sub-classes: piezoelectric polymers, ferroelectric polymers, electrostrictive polymers, relaxor ferroelectric polymers (such as PVDF based relaxor polymers or polyurethanes), dielectric elastomers, liquid crystal elastomers. Other examples include electrostrictive graft polymers, electrostrictive paper, electrets, electroviscoelastic elastomers and liquid crystal elastomers.

The lack of a spontaneous polarization means that electrostrictive polymers display little or no hysteretic loss even at very high frequencies of operation. The advantages are however gained at the expense of temperature stability. Relaxors operate best in situations where the temperature can be stabilized to within approximately 10 °C. This may seem extremely limiting at first glance, but given that electrostrictors excel at high frequencies and very low driving fields, then the applications tend to be in specialized micro actuators. Temperature stabilization of such small devices is relatively simple and often presents only a minor problem in the overall design and development process.

Relaxor ferroelectric materials can have an electrostrictive constant that is high enough for good practical use, i.e. advantageous for simultaneous sensing and actuation functions. Relaxor ferroelectric materials are non-ferroelectric when zero driving field (i.e. voltage) is applied to them, but become ferroelectric during driving. Hence there is no electromechanical coupling present in the material at non-driving. The electromechanical coupling becomes non-zero when a drive signal is applied and can be measured through applying the small amplitude high frequency signal on top of the drive signal, in accordance with the procedures described above. Relaxor ferroelectric materials, moreover, benefit from a unique combination of high electromechanical coupling at non-zero drive signal and good actuation characteristics.

The most commonly used examples of inorganic relaxor ferroelectric materials are: lead magnesium niobate (PMN), lead magnesium niobate-lead titanate (PMN-PT) and lead lanthanum zirconate titanate (PLZT). But others are known in the art.

PVDF based relaxor ferroelectric based polymers show spontaneous electric polarization and they can be pre-strained for improved performance in the strained direction. They can be any one chosen from the group of materials herein below.

Polyvinylidene fluoride (PVDF), Polyvinylidene fluoride - trifluoroethylene (PVDF-TrFE), Polyvinylidene fluoride - trifluoroethylene - chlorofluoroethylene (PVDF- TrFE-CFE), Polyvinylidene fluoride - trifluoroethylene - chlorotrifluoroethylene) (PVDF- TrFE-CTFE), Polyvinylidene fluoride- hexafluoropropylene (PVDF - HFP) , polyurethanes or blends thereof.

The current driven EAMs and EAPs comprise conjugated polymers, Ionic Polymer Metal Composites, ionic gels and polymer gels.

Examples of ionic-driven EAPs are conjugated polymers, carbon nanotube (CNT) polymer composites and Ionic Polymer Metal Composites (IPMC).

The sub-class dielectric elastomers includes, but is not limited to: acrylates, polyurethanes, silicones.

The sub-class conjugated polymers includes, but is not limited to: polypyrrole, poly-3,4-ethylenedioxythiophene, poly(p-phenylene sulfide), polyanilines.

The materials above can be implanted as pure materials or as materials suspended in matrix materials. Matrix materials can comprise polymers. To any actuation structure comprising EAM material, additional passive layers may be provided for influencing the behavior of the EAM layer in response to an applied drive signal.

The actuation arrangement or structure of an EAP device can have one or more electrodes for providing the control signal or drive signal to at least a part of the electroactive material. Preferably the arrangement comprises two electrodes. The EAP may be sandwiched between two or more electrodes. This sandwiching is needed for an actuator arrangement that comprises an elastomeric dielectric material, as its actuation is among others due to compressive force exerted by the electrodes attracting each other due to a drive signal. The two or more electrodes can be also be embedded in the elastomeric dielectric material. Electrodes can be patterned or not.

A substrate can be part of the actuation arrangement. It can be attached to the ensemble of EAP and electrodes between the electrodes or to one of the electrodes on the outside.

The electrodes may be stretchable so that they follow the deformation of the

EAM material layer. This is especially advantageous for EAP materials. Materials suitable for the electrodes are also known, and may for example be selected from the group consisting of thin metal films, such as gold, copper, or aluminum or organic conductors such as carbon black, carbon nanotubes, graphene, poly-aniline (PANI), poly(3,4-ethylenedioxythiophene) (PEDOT), e.g. poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). Metalized polyester films may also be used, such as metalized polyethylene terephthalate (PET), for example using an aluminum coating.

Some arrangements may have electrode layers on each side of the electroactive material layer. It is also possible to provide an electrode layer on one side only for example using interdigitated comb electrodes. If electrodes are on one side only of a single layer actuator, that side of the device may implement a reflection, so that non- transparent electrodes may then be used. Similarly, for a stack structure, if the optical path is in the plane of the layers, then non-transparent electrodes may be used.

The materials for the different layers will be selected for example taking account of the elastic moduli (Young's moduli) of the different layers.

Additional layers to those discussed above may be used to adapt the electrical or mechanical behavior of the device, such as additional polymer layers. The invention makes use of composite materials which combine an

electroactive material (in particular a polymer) and other particles (which will be termed generally as a "filler") for changing the optical properties.

The way such composite materials can be manufactured will now be discussed as well as the effects on the physical and electrical properties of the electroactive material.

The example of dielectric elastomer electroactive materials will first be presented. These are sandwiched between two electrodes to create dielectric electroactive polymer actuators. Silicone rubbers are the main applied elastomer group. The deformation is the result of attractive forces between the positively and negatively charged electrodes.

Compounding of particles in silicones is widely used on an industrial scale. As an example ultrasound transducer lenses are made of silicone (PDMS, Polydimethylsiloxane) filled with iron and silicon oxide particles to increase acoustic impedance and wear resistance. PDMS (silicone) compounds containing rutile (Ti02) are widely used to increase the refractive index or to create white reflecting materials.

With respect to the performance of a dielectric electroactive polymer, compounding with non-conducting hard particles such as ceramics has two main significant effects. First, the stiffness of the material increases requiring larger forces to obtain the same strain levels. Another effect is that the dielectric constant of the composite changes (in general that of the filler will be higher than that of silicones, which is close to 3). Whether the strain effect depending on voltage is positive or negative depends on the dielectric constant of the particles and on particle size as more small particles have a larger effect on stiffness.

This is discussed in S. Somiya, "Handbook of Advanced Ceramics: Materials, Applications, Processing, and Properties," in Nonlinear Dielectricity of MLCCs, Waltham, Academic Press, 2013, p. 415. By way of example, adding particles increases the dielectric constant but also increases the stiffness.

Thus, compounding fillers into elastomers to influence the properties of a dielectric electroactive polymer is known. Adding high dielectric constant particles to increase the dielectric constant of the elastomer and therefore potentially the effectivity, has been widely investigated.

Figure 9 shows the effect of adding BaTi03 to a silicone elastomer in an EAP.

It shows the strain versus the applied electric field for different particle concentrations, wherein the particles are Ιμιη particles of density 6g/cm3. At around 20 wt% the EAP strain as a function of field strength is enlarged, as the positive effect of the increased dielectric constant outweighs the negative effect of the increased stiffness. Increasing the dielectric constant increases the strain depending on voltage whereas the increase in stiffness decreases strain.

Silicone elastomers are in general prepared by mixing two components. One of them contains a Pt or peroxide curing catalyst. The different components can be mixed in a high speed mixer. In the same process, the filler can be added or the filler may already be premixed in one or both components. The filler material is in general applied in a solvent which evaporates during processing. After or during mixing in a high speed mixer in general vacuum is applied to remove air (and or solvents) inclusions. After this the mixture can be casted and cured. Curing temperature and time depends on the polymer grade but is typically around 80 oC for 10 minutes. Most particles are compatible with silicones as long as they do not inactivate the catalyst (for instance sulphur containing materials). Peroxide curing silicones are less sensitive.

Silicones can be injection molded (liquid silicone rubbers, LSR). The two components are injected on a screw, after passing a (static) mixer, of the LSR injection molding machine. The filler particles may be pre-mixed in one or both components. The material is transported by a cold screw and injected into a hot mold where it cures fast depending on temperature. As the LSR has very low viscosity very thin sections can be realized. Typical curing temperatures are close to 180 oC and times around 30 seconds to one minute.

Besides casting and injection molding a number of other shaping technologies are available to produce silicon rubber compound components also in the form of thin films. Examples are extrusion (foils and profiles), rolling of foils, lamination and rolling of multilayers, doctor blade film casting, spin coating and screen printing.

The filling can be performed locally at the point of manufacture, for example by using multi shot injection molding (2 shot or overmolding), silicone dispensing and over casting or silicone additive manufacturing (i.e. 3D printing)

The example of piezoelectric polymer composites will next be presented. Piezoelectric polymer composites containing a compound of PVDF (a matrix polymer) and ceramic particles such as PZT have been investigated. Manufacturing technologies like solvent casting and spin coating are suitable. Also, cold and hot pressing techniques are suitable. After dissolving the PVDF, evaporation of solvent until a viscous mix is obtained and mixing in the filler particles may then be performed. PVDF polymer based composites with a well dispersed grain size distribution and intact polymer matrix may be realized. The example of relaxor electrostrictive polymer actuators will next be presented.

These are a class of semi crystalline terpolymers that can deliver a relatively high force with medium strain. Therefore these actuators have a wide range of potential applications. Relaxor electrostrictive polymers have been developed from "normal" PVDF polymers by employing proper defect modifications. They contain: vinylidene fluoride (VDF), trifluoroethylene (TrFE), and 1, 1-chlorofluoroethylene (CFE) or Chlorotrifluoro ethylene (CTFE).

Addition of defects in the form of chemical monomers, like 1, 1- chlorofluoroethylene (CFE) which are copolymerized with the VDF-TrFE, eliminate the normal ferroelectric phase, leading to a relaxor ferroelectric with electromechanical strain greater than 7% and an elastic energy density of 0.7 J/cm3 at 150 MV/m. Furthermore is has been described that by introducing defects via high electron irradiation of the P(VDF-TrFE) copolymers, the copolymer can also be converted from a "normal" ferroelectric P(VDFTrFE) into a ferroelectric relaxor.

The materials may be formed by polymer synthesis as described in F. Carpi and et. al, "Dielectric Elastomers as Electromechanical Transducers: Fundamentals,

Materials, Devices, Models and Applications of an Emerging Electroactive Polymer

Technology," Oxford, Elsevier, 2011, p. 53. This discloses a combination of a suspension polymerization process and an oxygen-activated initiator. This films can be formed by pouring the solution on a glass substrate and then evaporating the solvent.

The desired filler can be added to the solvent before film casting. After casting, the composite can then be annealed to remove the solvent and increase crystallinity. The crystallization rate can reduce depending on filler concentration and particle size distribution. Stretching will align molecule chains and will become more difficult as particles can pin molecular chains. The dielectric constant will increase for most additives which reduces the required actuation voltage to reach a certain strain. The material stiffness will increase reducing strain.

The manufacturing process thus involves forming a polymer solution, adding particles, mixing, followed by casting (e.g. tape casting) potentially combined with lamination. Alternatives are spin coating, pressing etc.

Local variations in concentration can be realized using dispensing and or 3D solvent printing. Layer thicknesses between 10 to 20μιη are for example possible with 3D printing processes. In all examples, the addition of the filler generally has an effect on the breakdown voltage. The maximum strain that can be reached with an electroactive polymer is determined by the maximum voltage that can be applied, which is the breakdown voltage (or dielectric strength).

The breakdown voltage of polymers is related to the dissociation of polymer molecules under an applied external field. The addition of filler particles in a polymer matrix can have a significant influence on the breakdown voltage. Especially larger particles can locally increase fields. Therefore compounding polymers with particles in the sub-micron range has a lower negative effect on voltage breakdown. Furthermore the polymer - filler interface structure can strongly influence voltage breakdown.

Agglomeration of particles is another effect that reduces breakdown voltage. However, by modifying particle surfaces, preventing agglomeration and improving the interface structure, the negative effect of voltage breakdown levels can be reduced. However, the filled polymers will obtain a lower breakdown strength than unfilled polymers, leading to lower actuation strain.

In conclusion, for dielectric electroactive polymers, compounding with particles can be achieved using a wide range of industrial compounding and shaping technologies. In order to keep the effect on stiffness and therefore stroke reduction for an actuator limited, smaller concentrations are preferred. For a given volume concentration, not too small particles are also preferred to keep the effect on stiffness limited. A soft base polymer can be selected to compensate for the rise in stiffness. Increased dielectric constant can enable actuation at reduced voltages. In order to maintain the dielectric strength, particle size and concentration should be limited and measures can be taken to improve the polymer - filler interface as well as particle dispersion. Local concentration variations can be printed.

For relaxor type electro active polymers compounding with particles is also possible. Similar trends with respect to the influence of particle concentration and size, on stiffness and dielectric strength are comparable to the effects described above. Particles can be added after polymerization. Dissolved polymers can be shaped using various technologies such as tape casting and spin coating. Also local concentration variations are possible.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A light color conversion device (42, 52) comprising:

an electroactive material layer (30) comprising an electroactive material;

at least one transparent or translucent actuation electrode layer (44) for inducing deformation of the electroactive material layer in response to an electrical signal applied to actuation electrodes in the layer or layers; and

passive light conversion particles or droplets (32) within the electroactive material layer, wherein the light color conversion function of the device is dependent on the deformation of the electroactive material layer.

2. A device as claimed in claim 1, wherein the light conversion particles or droplets comprise ceramic phosphor particles.

3. A device as claimed in claim 2, wherein the light conversion particles comprise YAG:Ce ceramic particles.

4. A device as claimed in any preceding claim, wherein the electroactive material layer (30) comprises a plurality of different types of light conversion particles or droplets at different locations within the electroactive material layer.

5. A device as claimed in any preceding claim, wherein the electroactive material layer further comprises scattering particles.

6. A device as claimed in any preceding claim, for implementing a color conversion function in response to external deformation of the electroactive material layer.

7. A light generation system for generating light with a controllable color, comprising:

a light source (41) with an output with a fixed output color; and a color conversion device as claimed in any preceding claim.

8. A system as claimed in claim 7, wherein the light source comprises a blue

9. A system as claimed in claim 7 or claim 8, wherein the color conversion device has a first actuation state in which the electroactive material layer (30) is in the path of the output (34) of the light source and a second actuation state in which the electroactive material layer is outside the path of the output of the light source.

10. A system as claimed in claim 8, 9 or 10, further comprising an optical fiber (70) into which the light with a controllable color is provided, wherein the optical fiber has light outcoupling structures (72a, 72b) for outcoupling light of different colors at different points along the optical fiber.

11. A system as claimed in claim 10, wherein the outcoupling structures (72a, 72b) comprise Bragg gratings.

12. An optical analysis system (50) comprising:

a light generation system (40) as claimed in claim 7 or claim 8, wherein the colour conversion device (52) is for implementing a color conversion function in response to external deformation of the electroactive material layer; and

an analysis system (56) for analyzing the light after at least one passage through the electroactive material layer, thereby to determine the deformation of the electroactive material layer.

13. A system as claimed in claim 12, comprising a scanner (62) for scanning the light source output over the area of the color conversion device (52).

14. A system as claimed in claim 12 or claim 13, comprising a reflector (54) on the opposite side of the color conversion device to the light source.