GB2510211A

GB2510211A - Composition containing oxide nanoparticles

Info

Publication number: GB2510211A
Application number: GB1313332.7A
Authority: GB
Inventors: Gareth Wakefield; Jonathan Moghal; Martin Ryan Gardener
Original assignee: Oxford Energy Technologies Ltd
Current assignee: Oxford Energy Technologies Ltd
Priority date: 2013-07-26
Filing date: 2013-07-26
Publication date: 2014-07-30
Also published as: GB201313332D0

Abstract

A composition comprises oxide nanoparticles and a binder, which when formed as a coating, has a pencil hardness of at least 4H according to ASTM D3363 - 05(2001)e2. Preferably, the nanoparticles comprise tin-doped indium oxide, aluminium-doped zinc oxide, fluorine-doped zinc oxide, antimony-doped tine oxide, or silica. The binder may be (i) a thermally-curable binder, such as tetraethylorthosilicate, methyltrimethoxysilane, or a polyurethane or (ii) a UV-curable binder, such as pentaerythritol triacrylate, 1,6-hexanediol diacrylate, or trimethylolpropane triacrylate. The nanoparticles may be porous or non-porous and may have a random close-packed structure. Coatings, optical elements, ophthalmic elements, solar cells, windows, glass panels, low-K dielectrics, and battery electrodes comprising the composition are also disclosed. A method of preparing a coating is also claimed, which comprises (a) applying a dispersion or solution of oxide nanoparticles in one or more solvents to a substrate to form a layer, (b) applying a binder solution to the layer, and (c) curing the binder.

Description

NANOPARTICLE THIN FILMS FOR OPTICAL AND ELECTRONIC APPLICATIONS

[001] The invention relates to a process of producing a highly loaded nanoparticle thin film where the particles are loaded close to the random dose packing limit and consequently the film has resistance to abrasive wear and other environmental stresses.

[002] BACKGROUND INFORMATION

[003] In polymer or sol-gel films, inorganic fillers or particles play a significant role in controlling the physical and mechanical properties of those films. In particular, fillers such as silica particles, clays or glass beads are used to enhance the mechanical properties of the composite film. Modification of particle size, particle dispersion and particle aspect ratio are also used to add further degrees of control to the properties of the composite material. Using conventional micrometre sized fillers requires a relatively high loading (usually approximately 20% volume) to achieve enhanced stiffness, albeit at a cost of reduced toughness and ductility.

Moving towards nanoparticle fillers enables the advantageous properties of inorganic additives to be retained whilst reducing any negative effects. However, this is only true for an optimum loading of the nanoparticle filler. In general, as long as any agglomeration effects in the nanoparticles are minimised, it is possible to add up to approximately 30 vol% filler to a polymer or sol-gel and enhance its mechanical properties over the neat materials. Above such loadings mechanical properties are diminished over the neat materials. In practice, due to particle agglomeration, optimum additive levels are often much lower than 30% vol. [004] For example, Xiao-Lin Xie et al (Polymer, 45 (2004) 6665-6673) describe the loading of CaCO3 nanoparticle fillers in PVC and show that the Youngs modulus and impact resistance of the nanocomposite PVC is enhanced as a function of nanoparticle loading up to 5 wt% (equivalent to approximately 2 vol%) before decreasing as loading increases. In a further example, Yuchun Ou et al (Journal of Polymer Science B: Polymer Physics, 36 (1998] 789-795) describe the addition of silica nanoparticles to Nylon-6 and the consequently reinforcing qualities which again show an optimum at 5 wt% [equivalent to approximately 2 vol%) loading. For a full review see SJ Tjong (Materials Science and Engineering R, 53 (2006) 73-197).

[0051 There are a number of other properties of nanoparticles which would be useful if incorporated into host sol-gel or polymer binders to make novel thin film structures; for example the refractive index, dielectric constant and electrical conductivity properties of the film could be modified. According to the effective medium approximation the macroscopic properties of a material can be approximated as the weighted sum of the properties of the individual components.

Consequently the volume fraction of the nanoparticles in the film needs to be significant in order to make a substantial change to the macroscopic properties of the film, and this must be attained without compromising the mechanical properties of the film such that it becomes useless in practical application. For example, if one wishes to manufacture an anti-reflection coating by incorporating low refractive index nanoparticles into a typical sol-gel or polymer matrix one requires the reflection to be reduced to <1.5% for the film to be considered suitably anti-reflective. Incorporating low index particles of, for example, hollow silica nanoparticles of refractive index 1.30 requires a nanoparticle loading of 60% volume in order to reduce the films effective refractive index to less than 1.40 and to consequently reduce the reflectivity to 1.5%. Such a high volume loading would seriously compromise the mechanical properties of a thin film according to conventional nanoparticle loading techniques into typical sol-gel or polymer host lattices.

[0061 It is possible to obtain thin films of nanoparticles that have both high volume content of nanoparticles and film mechanical toughness if the particles are jammed' within the film. Jamming is a phenomenon observed in various granular macroscopic media but is associated with an inability of particles to move with respect to each other -either the entire system moves or the external force is resisted. As a volume system fills up with randomly distributed particles the jamming point is reached when all particles touch but are at zero pressure. Once the pressure between the particles becomes non-zero the system is jammed. If a system is comprised of infinitely hard, frictionless spheres then it will remain jammed at all positive pressures and the jamming point (i.e. volume fraction of spheres) will tend towards the random close packing limit as the system dimensions tends towards infinite. It is difficult to define random exactly but it is generally accepted that the jamming point for a random close packed (RCP) system is 64%. This can be compared with the maximum ordered packing (face centred cubic) volume of 74%.

Note that the jamming threshold is dimensionless and does not depend on sphere size. In a real system of particles the jammed volume has a finite elastic modulus (reversible deformation] and yield strength (irreversible deformation).

[0071 The critical aspect of a jammed particle system is that the system will only undergo yield as a function of the entire system since individual particles cannot move with respect to each other. Practically, this acts to distribute any external force within the entire system rather than concentrating it on a single particle or small cluster. This dissipation of force results in a system which can withstand far higher external impact or pressure than an unjammed system and this effect is increased as intrinsic particle hardness is increased; i.e. inorganic particles are preferred to get maximum mechanical strength. The shear modulus increases abruptly at the jamming threshold as a result of this and the smoothing of the force network topology has been demonstrated by computer simulations. See, for example, Alexander OM Siemens and Martin van Hecke, Physica A 389 (2010) 4255- 4264 and L Kondic et al, EPL 97 (2012) 54001.

[0081 In practice an array of nanoparticles is not infinite or at the RCP limit and in a thin film a significant proportion may be close to the surface. Surface particles are not jammed since they can move in a direction normal to the substrate and there will be small regions within the film where particles can move with respect to each other. Consequently a binder is required to maintain film integrity; however the RCP nature of the film results in the intrinsic strength of the binder needing to be much less than required for a non-jammed system. The binder system is designed such that adhesion to both the nanoparticles and the substrate is obtained. The result is a jammed thin film nanocomposite.

[009] The two key mechanical components of thin film robustness are film hardness (resistance to deformation) and the coefficient of friction between the film and any external force load. In practice the coefficient of friction is a dimensionkss system measurement that relates a force normal on a substrate producing a force of friction parallel to the substrate such that: Ff«=uF [0010] The coefficient of friction can be either static) relating to the ability to initiate motion between two surfaces or dynamic, relating to the ability to maintain motion between two surfaces. Static coefficients of friction tend to be higher than dynamic ones. Although the coefficient of friction is system dependent it can, in practice, generally be lowered by a reduction in surface roughness and surface energy of a nanoparticle film. Typically a reduction in surface energy can be obtained by a fluorine based coating on the nanocomposite thin film.

[0011] The hardness and coefficient of friction of thin films can be measured using a nanoindentation system in scratch and wear mode. Thin films also need to pass various ASTM standards such as pencil hardness and steel wool abrasive wear. In pencil hardness testing a series of pencils of increasing hardness are drawn across a surface under a standard load and the film is inspected for scratches. The hardest pencil which does not produce a scratch defines the pencil hardness of the coating.

The results can be ambiguous for ultrathin films since not only is the film required to resist the applied pencil the effects of any graphite deposition on a film will significantly change the optical properties and become very visible. Consequenfly, a nanocomposite film requires both hardness and low coefficient of friction to withstand pencil impact and minimise graphite deposition.

S

[00121 A steel wool test is an abrasive wear test in which surface degradation is caused by relative motion causes wear debris and material transfer from one surface to another. Surface roughness or surface features may be easily removed and increase the wear rate, consequently it is important to minimise surface features and reduce the coefficient of friction which can transfer energy into surface features and increase the probability of their removal from the surface.

[0013] STATEMENT OF INVENTION

[0014] This invention relates to a composition comprising oxide nanoparticles and a binder) wherein the composition, when formed as a coating, when tested for pencil hardness according to ASTM D3363 -0S(2001)e2 has a hardness of at least 4th preferably at least ÔH, more preferably at least BH. An alternative equivalent hardness testing standard is 15015184:2012(E).

[00151 This invention also relates to a composition comprising oxide nanoparticles and a binder, wherein the composition, when formed as a coating, has a hardness of at least 3GPa, preferably at least 4GPa, more preferably at least 4.SGPa, as measured by nanoindentation.

[0016] In some embodiments, the composition is a coating.

[0017] The term "nanoparticles" is used in relation to this invention to refer to particles having a mean diameter in the range 1-lOOnm. Preferably the nanoparticles have a mean diameter in the range 1-SOnm, more preferably in the range 10-4Onm, even more preferably in the range 20-3Onm. The particles are preferably in the size range 20-3Onm in order to reduce any surface roughness of the film to less than 3Onm.

[0018] It is preferred that the nanoparticles have a random close packed structure.

Preferably, in the random close packed structure the nanoparticles have a density of at least 50%, preferably at least SS%, more preferably at least 60%. The maximum density for a random close packed structure is preferably about 64%. In this context, the term "density" is used to refer to the volume fraction of the nanoparticles in the composition.

[0019] It is preferred that the oxide nanoparticles comprise indium oxide, aluminium oxide, zinc oxide, tin oxide or silica. More preferably, the oxide nanoparticles comprise tin-doped indium oxide, aluminium-doped zinc oxide, fluorine-doped zinc oxide, antimony-doped tin oxide or silica. In some embodiments, the nanoparticles comprise silica.

[0020] The oxide nanoparticles may either be porous or non-porous. In a particularly preferred embodiment, the nanoparticles comprise porous silica nanoparticles. It is preferred that the pores of the porous silica nanoparticles are randomly oriented. The term "randomly oriented" is used in relation to this invention to refer to pores which do not form a repeating [or partly or entirely symmetrical) structure. Examples of this are pores which have a tortuous path, and/or are disordered, and/or are non-uniform, and/or are non-periodic, and/or are irregular and/or are asymmetric.

[0021] Preferably, the porous particles have a mean pore diameter in the range 1-SOnm. It is preferred that the nanoparticles are mesoporous, ie they have a mean pore diameter in the range 2-SOnm. Preferably, substantially all of the pores (more preferably all of the pores) have a mean pore diameter in the range 1-lOnm, preferably in the range 1-Snm, more preferably in the range 1-3nm.

[0022] It is preferred that the porous nanoparticles have a continuous porous structure, ie they are not hollow. Preferably, none of the pores has a pore diameter greater than 50% of the diameter of the nanoparticle. More preferably, none of the pores has a pore diameter greater than 25%, even more preferably 15%, of the diameter of the nanoparticle.

[00231 In a preferred embodiment, when the oxide nanoparticles comprise indium oxide, aluminium oxide, zinc oxide or tin oxide (more preferably tin-doped indium oxide, aluminium-doped zinc oxide, fluorine-doped zinc oxide, or antimony-doped tin oxide), they are non-porous.

[0024] Preferably, the reflectance for incident light on a substrate having one surface coated with the composition at at least one wavelength in the range from 400nm to 700nm is less than 3%, more preferably less than 2.5%.

[0025] This invention also relates to a coating) optical element, ophthalmic element) solar cell, window, glass panel, low-K dielectric, or battery electrode comprising the composition as defined above. This invention also relates to transparent conducting oxides comprising the composition as defined above. Preferably) the transparent conducting oxides comprise the non-porous particles mentioned above) most preferably non-porous oxide nanoparticles comprising tin-doped indium oxide, aluminium-doped zinc oxide, fluorine-doped zinc oxide, or antimony-doped tin oxide.

[0026] This invention also relates to a method of preparing a coating comprising oxide nanoparticles and a binder, the method comprising the steps of: (a) applying to a substrate a solution or dispersion of oxide nanoparticles in one or more solvents in order to form a nanoparticle-containing layer, (b) applying to the nanoparticle-containing layer a binder solution comprising a binder and a solvent, (c) curing the binder.

[0027] It is preferred that the method comprises, between steps (b) and (c), the step of allowing the binder solution to diffuse into the nanoparticle containing layer.

Preferably, the one or more solvents in step (a) is selected such that the nanoparticles are entirely within 80% of the Hansen solubility sphere radius for that particle as defined by Hansen solubility parameters.

B

[00281 Preferably, the oxide nanoparticles used in the method are those defined in paragraphs [0017] to [00231 above.

[0029] Preferably, the one or more solvents in step (a] comprises an alcohol) preferably isopropanol, and/or a ketone, preferably methyl ethyl ketone and/or methyl isobutyl ketone.

[0030] It is preferred that, when the oxide nanoparticles comprise silica) the one or more solvents in step (a] comprises isopropanol. In particular, when the oxide nanoparticles comprise uncoated silica (eg without additional surface functionalisation), it is preferred that the one or more solvents in step (a) consists essentially of isopropanoL Uncoated porous silica particles preferably use isopropanol (IPA] as a solvent which has a relative energy difference (RED) of 0.5.

The RED is the radius of the solubility sphere so 0.5 is halfway out Preferably) when the oxide nanoparticles comprise silica and have acrylate-containing groups on their surface, the one or more solvents in step (a) comprises isopropanol and methyl isobutyl ketone. A preferred solvent composition for these particles is methyl isobutyl ketone:isopropanol 70%:30%v/v (their RED is 0.74).

[0031] When more than one solvent is used in step (a), the solvents should preferably be chosen such they all have a vapour pressure and surface tension within 10% of one another or that the solvents with a lower vapour pressure have a lower surface tension and ones with a higher vapour pressure have a higher surface tension. As solvent evaporates the higher vapour pressure solvents evaporate first) leading to a depletion layer of these at the surface. This in turn leads to the surface having a higher surface tension then the bulk of the material if this solvent possesses a lower surface tension than the other solvents in the blend. This then exacerbates randomly forming surface structures and leads to striations and defects in the resulting film.

[00321 It is preferred that the binder is a (i] thermally-curable binder, preferably tetraethylorthosilicate and/or methyltrimethoxysilane and/or a polyurethane, or (H] a UV-curable binder, preferably pentaerythritol triacrylate and/or 1, 6 -hexanediol diacrylate and/or trimethylolpropane triacrylate.

[0033] It is preferred that in steps (a] and (b] the applying is by spin coating. In some embodiments, step [a) comprises two or more separate and sequential applications of the nanoparticle solution or dispersion. It is preferred that, before step (c], the method comprises the step of removing excess binder solution, preferably by spinning the substrate, more preferably at by spinning the substrate at at least 3000rpm. When the method includes the step of allowing the binder solution to diffuse into the nanoparticle containing layer, it is preferred that the binder is allowed to diffuse into the nanoparticle-containing layer for at least 2 seconds before either the step of removing excess binder solution and/or step [c].

[0034] This invention also relates to coatings obtainable by the method described above.

[0035] A process for producing a randomly close packed film of nanoparticles by depositing from a solvent or solvent blend in which the attractive forces between the nanoparticles are minimised to reduce agglomeration and maximise the ability of capillary action to produce a randomly close packed layer. A process whereby an inorganic or organic binder system is then deposited around the nanoparticles in the layer to provide mechanical robustness and a flat surface. A process whereby a low surface energy surface is obtained by incorporation of a suitable fluorine component either within the binder or as a separate surface layer.

[0036] A randomly close packed film of nanoparticles in which the volume of nanoparticles is >50% of the film volume and the pores between the nanoparticles are filled with an inorganic or organic binder system. The film roughness is c2Onm to reduce friction forces.

[00371 DETAILED DESCRIPTION

[00381 There are three practical aspects to producing a randomly close packed film of nanoparticles from dispersion in a solvent Firstly, inter particle attractive forces must be minimised. Secondly, the carrier solvent must evaporate slowly enough to allow the particles to settle into their lowest energy state and, thirdly) capillary forces act to pull the particle film together [capillary forces between particles partially submerged in solvent on a substrate are attractive since the wetting of the particles creates a deformation of the solvent meniscus and consequently an attractive force).

[00391 Hansen Solubillty Parameters (HSP] predict if a given nanoparticle will disperse in a solvent or solvent blend. Forces acting on the nanoparticle surface are split into three components -dispersive or Van der Waals, intermolecular dipole forces and hydrogen bonding. All solvents also have their own set of HSPs and the difference between the measured nanoparticle forces and the solvent space defines a sphere within which the nanoparticle will disperse. Blends of solvents can also be used to minimise these forces. After the deposition of a layer of solvent containing dispersed nanoparticles on a surface the particle-particle distance decreases as the solvent evaporates. The Van der Waals interaction between the nanoparticles increases rapidly as particle separation reduces and this tends to drive particle agglomeration during the final stages of particle film drying. This tendency towards agglomeration is balanced against by the capillary forces acting to produce a close packed film but should be reduced as much as possible to allow capillary forces to dominate.

[00401 Therefore, given any nanoparticle system, the Hansen Solubility Parameters should be measured in accordance with techniques described in the literature (see) for example) Hansen C.M. (2007). Hansen Solubility Parameters: A userTs handbook) Second Edition, CRC Press). A solvent or solvent blend should be chosen such that the particle sits in the middle of the HSP sphere, i.e. the relative energy difference is as low as possible, so that attractive forces acting upon the nanoparticle are minimised. The solvent blend may be further modified by the incorporation of low, typically <20%, levels of high boiling point solvents which act to slow solvent evaporation in the final stages of film formation and allow increased time for the capillary forces to drive close packing of the nanoparticle film. This effect must be attained without the high boiling point solvent increasing the viscosity of the solution as the principal solvent evaporates as this will tend to act as a retarding force on the capillary forces driving close packed film formation. Typical examples of high boiling point solvents are di-basic esters (DBE), 1-butanol and diglycol ethers such as the DowinalTM series from Dow Chemical.

[00411 A dispersion of nanoparticles in a solvent or solvent blend designed such that the van der Waals interactions are minimised is then prepared, as in Example 1, at low solids content in order produce a thin film of close packed particles. The dispersion is then coated on to any suitable substrate by standard wet chemical coating techniques. Examples of coating techniques are a) spin coating, in which a volume of the dispersion is dropped on to a spinning substrate, b) dip coating in which a substrate is dipped in to a bath of the nanoparticle dispersion and c) roll-to-roll coating, in which a polymer film is passed through a bath of the particle dispersion in a continuous process.

[00421 Examples of suitable substrates are glass, quartz, transparent conducting oxides and polymers including polyethylene terephthalate [PET), poly methylmethacrylate [PMMA), triacetyl cellulose [TAC), poly carbonate [PC), Polyethylene naphthalate [PEN), CR 39 and hardcoated variations of these polymers.

[00431 The nanoparticles may also be coated with various chemical functionalities, such as thiols, acrylates, hydroxyls, epoxies and others, in order to provide a chemical mechanism whereby the particles can strongly bind to the binder system.

[00441 Once the close packed particle thin film is deposited to a suitable thickness the next stage is to introduce a binder between the particles. The binder should be chosen such that it may easily ingress into the pores between the particles as capillary forces pull the binder containing solvent into the porous layer. It should consist of single molecule or monomer materials which may be consequently cured into a three dimensionally cross-linked films or low molecular weight resins. High molecular weight materials should be avoided as they will tend to separate out and sit on top of the close packed particle layer as the solvent penetrates into the pores.

The binder solids loading of the binder carrier solvent) which can be completely different to the particle carrier solvent, should be chosen such that enough binder is deposited to penetrate completely through the nanoparticle layer and promote adhesion to the film substrate but does not result in a thick, typically >2Onm, excess binder layer on top of the particle layer.

[00451 The binder may be deposited by either dynamic (substrate spinning), static (substrate stationary and central liquid drop deposited prior to spinning) or flooding (substrate stationary and completely flooded) spin coating. In the static and flooding cases the substrate is immediately accelerated to minimise evaporation of the carrier solvent prior to spinning.

[00461 Following the incorporation of the binder in to the pores or voids between the nanoparticle the thin film must be cured to ensure mechanical robustness. This is typically undertaken by thermal baking or ultraviolet light exposure but in some cases may be by infrared heating or electron beam exposure.

[0047] Typical types of binder for thermal curing are silicate systems such as tetra ethyl ortho silicate (TEOS), methyl trimethoxy siliane (M-TMOS) or other alkoxysilanes.

[0048] Typical]JV curable binders include pentaerythritol triacrylate (PETA), 1, 6 -hexanediol diacrylate (HDDA], and trimethylolpropane triacrylate (TMPTA). These are all IJV curable monomers which require the presence of a photoinitiator such as benzophenone to initiate curing.

[00491 A binder material based on polyurethanes may also be used in cases where thermal curing on to a polymer substrate is required. Polyurethane materials are highly diverse with a wide range of possible hardness, density, tensile strength, elongation, abrasion and chemical resistance depending on the preparation method and selection of precursor materials. The binder formulation typically the reaction involves a polyol, polyisocyanate and, in some cases, a catalyst Mixtures of multiple polyois and polyisocyanates are also well known. Typical examples of materials that may be used to achieve such coatings are a] Polyols -2,3-Butanediol, 1,2- Hexanediol, 2,2-Di-n-butyl-1,3-propanediol, 2-Methyl-1,3-propanediol, 3,3-Dimethyl-1,2-butanediol and Multranol'TM 4025. These materials have a viscosity range between 50 and 700cP, b) Polyisocyanates -DesmodurT H, DesmodurlM I, DesmodurTM W, DesmodurTM N 75 BA, DesmodurTM XP 2580, DesmodurT 2730 and BaytecTM WP-260 and c) Catalysts -Dibutyltin dilaurate, hydrated monobutyltin oxide, butyl chlorotin dihydroxide, butyltin tris(2-ethylhexanoate), dibutyltin diacetate, dibutyltin dioxide, butyl stannoic acid, dioctyltin dilaurate, dioctyltin maleate, stannous oxide, stannous oxalate, stannous bis(2-ethylhexanoate).

Organobismuth, organozinc and tertiary amine catalysts may also be used in polyurethane binder.

[00501 It is possible to use both inorganic, for example, porous silica, silica, zinc oxide, and organic nanoparticles to form the system. In the case of organic nanoparticles, for example polystyrene, polycarbonate, poly methylmethacryalte and others, the organic nanoparticles may be removed after the formation of the film, for example by firing or chemical dissolution, resulting in a high surface area layer of the binder material. Such inverse structures may find benefit in, for example, electrodes for batteries.

[0051] A fluorocoating may be applied after the film is cured by spin, dip or roll-to-roll coating and it typical used as low solids content in a suitable solvent For a silicate type binder a fluorosilicate is used in which the silicates bond together and the fluorine component is the surface layer. It is also possible to use a polymer based fluorine containing material for binders which are based on organic systems.

In certain cases a fluorine containing compound may be added to the binder such that it migrates to the thin film surface during curing. This is possible with both inorganic and organic based binders.

[0052] Typical fluorosilicates include 1H,1H,2H,2H-Perfluorodecyltriethoxysilane(Triethoxy-1H,1H,2H, 2H-perfluorodecylsilane), (3,3,3-Trifluoropropyl]trimethoxysilane, 1H,1H,2F1,2H, perfluorooctyltriethoxysilane, nonafluorohexyltriethoxysilane and fluoroalkylacrylates. Typical fluoropolymers include Fluorolink S-b, a perfluoroether from Solvay solexis, and general perfluoroethers.

[00531 SUMMARY OF FILM MICROSTRUCTURE

[00541 The final thin film should consist of dose packed nanoparticles with a volume fraction greater than 50% and the remaining volume consisting of a cured binder system. The binder system should penetrate completely through to the substrate and provide adhesion between the substrate and the film and, in addition, should bind to the surface of the nanoparticles in any regions where the nanoparticles are not touching. The binder should not form a layer on the surface of the nanoparticles in excess of lOnm. The surface roughness of the film should be less than or equal to the diameter of the particles and not increased by the binder.

The film may also have a fluorinated coating to reduce the coefficient of friction between the film and any abrasive object The combination of low surface roughness and coefficient of friction and the force dissipation that occurs as a result of the nanoparticles being close packed and in contact with each other imparts mechanical robustness to the thin film nanocomposite. The specific properties of the nanoparticles at high loading impart the desired functionality of the thin film; for example optical, dielectric, conductive or thermal properties.

[00551 Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 shows films of porous silica particles produced from a) ethanol and b) propanol.

FIGURE 2 shows a cross-section of a close packed anti-reflection coating with a so-ge1 binder. The film thickness is llOnm (note that the image is taken at 45° viewing angle).

FIGURE 3 shows an image of an BH pencil scratch on the anti-reflection coating produced in Example 2.

FIGURE 4 shows a reflectance curve of the anti-reflection coating produced in Exampk 2.

FIGURE 5 shows a transmission curve of the anti-reflection coating produced

in Example 3.

FIGURE 6 shows a cross-section of a close packed anti-reflection coating with a WI cured PETA binder.

[0056j APPLICATIONS [0057] 1) Antireflection coatings [0058j Nanoparticles of 25nm mesoporous silica with a pore fraction of 67% are deposited in a close packed film and the voids between the particles are filled with a suitable binder and cured. The pore fraction of the nanoparticles reduces the refractive index below then optimises this for the substrate, which may be glass or polymer. Mesoporous particles have a refractive index of 1.16 at 67% pore volume.

When packed at 55 vol% into a dose packed film with a binder of refractive index 1.50 (typical of polymer and sol-gel binders) of thickness lO6nm the resulting coating has a refractive index of 1.3 at SSOnm and robust mechanical properties.

This film has a reflectance of 0.35% at SbOnm incident light. A typical ghss or polymer substrate has a reflectance of 4-5% per surface.

[00S91 Any sing'e layer anti-reflection coating has an intrinsic colour due to the fact the reflectance curve is not flat throughout the visible and has a minimum at wavelength A=4nd where n is the refractive index of the film and d is the film thickness. This colour varies with viewing angle due to variations in the path length of the incident light and this colour shift can be varied by changing the thickness of the coating and hence the minimum wavelength. As the particles are close packed a reduction in particle diameter allows a finer control over the minimum wavelength and colour shift under viewing angle change. It is preferred that the partide size of the mesoporous silica is tess than SOnm, most preferaffly less than 3Onm.

[0060] 2) Low-K dielectrics [0061] Mesoporous carbon doped silica nanoparticles have a refractive index of 1.12 and hence a low dielectric constant. A critical aspect to using particle based porous materials as low-k dielectric materials is their resistance to chemomechanica polishing. This resistance can be increased and dielectric constant minimised by using randomly close packed layers of mesoporous carbon doped silica in a sol-gel binder.

[0062] 3) Transparent conducting oxides [0063j Using nanoparticles of conducting oxides; such as ITO, ZnO:Al, ZnO:F, Sb doped Sn02, to form transparent conducting layers reduces cost since vacuum deposition techniques are not required. As the layer is close packed the layer is over the percolation threshold and there are continuous conduction pathways through the film. The close packing and binder material make the film robust enough to withstand further processing.

[00641 4] Battery electrode [0065] Nanoparticles of organics such as polystyrene are assembled in a dose packed structure prior to metallic or inorganic electrode materials being deposited around them. The polymer nanoparticles are then removed by either etching of firing leaving a type of tinverse opal' structure of the electrode. Alternatively, electrode materials such as LiMnO2 may be combined with metallic elements by preferential deposition into the void left by the organic nanoparticle.

[0066] SYNTHESIS OF POROUS SILICA NANOPARTICLES [0067] Porous silica nanoparticles are typically prepared by the hydrolysis of an alkoxysilane (such as tetramethylorthosilicate and tetraethylorthosillcate) followed by co-condensation of the hydrolysed precursor to produce an inorganic silica polymer. To produce particulate structures the reaction is catalysed by the presence of a base, which accelerates the condensation reaction. Any suitable base may be employed) for instance ammonia, NaOH or KOH. Thus the reaction is typically performed in an alkaline solution, which is typically an aqueous solution of the base.

Typically this reaction will result in large, dense spherical silica particles.

[0068] The inclusion of an organic templating agent, preferably triethanolamine (TEA], results in structural modification of the particle and the development of a randomly oriented pore structure. For example, if triethanolamine is in the same solution as the above reaction, then the space occupied by the organic templating agent cannot be occupied by the silica, and hence the silica grows around the organic templating agent, resulting in an intimately mixed organic/inorganic particle. Removal of the organic templating agent, by a solvent that dissolves the agent and not silica, results in a silica particle with pores resulting from the organic templating agent removal. Organic templating agent removal is never complete because the surface energy increase of completely removing the agent from the silica surface is too large. Hence a degree of organic templating agent is retained within the silica nanoparticles on the internal surfaces of the pores.

[0069] The overall particle size is controlled by forming an oil in water emulsion.

The emulsion droplets act to halt growth of the particle beyond the domains of the droplet The droplet size is controlled by the ratio of oil, water and emulsifying agent type and concentration. Under appropriate conditions, particle diameter and distribution of diameters can therefore be kept within a preferred range of 20-30 nm.

[0070] The porous silica particles) fabricated as described above, are such that the pore structure is randomly oriented and the internal surface of the pores is coated with a hydrophobic kyer and the external surface of the particle is hydrophilic.

[0071] Functionalisation of the particles may be carried out either during or post synthesis. Typicafly, particles are precipitated in polymer containing micelles in which the organic templating agent templates a pore structure and the particle size is controlled by the micelle. After silica precipitation around the temphte within the micelle, at which point functionahsation may be carried out, the organic templating agent is removed from the pores by subsequent washing steps which resuks in the formation of a c3Onm mesoporous silica nanopartide of >60% pore volume.

Suitable functionisations for acrylate binding are thiols, acrylates, amines and methacrylates -these are formed as silanes, for example trimethoxy silane, which attaches to the silica leaving the functionalisation to bind to the acrylate binder.

[0072] EXAMPLES

[0073] Preparation of acrvlate functionalised mes000rous silica particles lused in

Example 3 below]

[0074] A solution A was prepared comprising ig triethanolamine and 9g deionised water. A s&ution B was also prepared from SOg CTAC (25% in water) and 450g de-ionised water. 2.4g of solution A was then added to 120g 30 of solution B. The resulting mixture was heated to 70°C whilst stirring.

[0075] After stirring for 30 mins, 9.6mL of silane solution, prepared from mixing 3.84g of phenyltriethoxysilane and 33.28g of tetraethoxysilane, was added. This mixture was left stirring at 70°C for 60 mins. At this point, an amount -2wt% of the total silane [0.803g) of 3-(methacryloxy) propyl trimethoxysilane was added and the mixture stirred for a further 10 mins.

[0076] The mixture was then removed from heat and 120ml of an acidified ethanol solution, made from lOOmI of 37% HCI diluted to l000ml with EtOH, was added and stirred. 44mL of the resulting solution was decanted into centrifuge tubes and spun at 15000rpm for 10 mins. The supernatant was poured off and l8mL of Ammonium nitrate solution (lOg Ammonium nitrate in SOOmL EtOH] was added to each tube.

The solid was redispersed using vortex and ultrasonication, before each tube was filled with de-ionised water and spun at 21000rpm for 10 mins. The supernatant was again removed and lSmL of the acidified ethanol solution added to each tube.

The solid was again redispersed using vortex and ultrasonication, before each tube was filled with de-ionised water and spun at 21000rpm for 10 mins. The superantant removed again and 10 ml of final dispersion solvent (ie 70%wt methylisobutylketone/ 30%wt isopropanol] added to each tube. The solid was redispersed using vortex and ultrasonication. The resulting dispersions were combined and stored in plastic bottles.

[0077] 1] Hansen solubility parameters and their effect on close packine.

[0078] Solutions of 1.4% w/v 25nm mesoporous silica in either ethanol or isopropanol are used as a source of particles. Measurements of the relative energy difference of mesoporous silica in are 0.5 for isopropanol and 0.7 for ethanol. This indicates that films produced from isopropanol would be expected to show closer packing then their ethanol equivalents. This effect is shown in Figure 1 -Figure la shows films deposited from ethanol and Figure lb shows films deposited from isopropanol. Nanoparticle films deposited from isopropanol are clearly more closely packed.

[0079] 21 Production of a random close packed anti-reflection coating.

[00801 A solution of 1.4% w/v 2Snm mesoporous silica in isopropanol is used as a source of particles (solution A]. Two binder solutions comprising of 1.458g tetraethyl orthosilicate (TEOS], 1.458g hydrochloric acid and 17.08g ethanol (Solution B] and 0.68g methyltrimethoxysilane (MTMOS], 0.5g hydrochloric and 18.81g ethanol (Solution C) were prepared. The binder solutions (B and C) were stirred for 24h and then mixed together in an 80:20 ratio of TEDS to MTMOS (Solution D). Glass substrates were prepared by washing in isopropanol at 70C for minutes and are then dried. The dimensions of the substrates are 5cm x 5cm.

[0081] The anti-reflection coating is prepared using a spin coater. A substrate is spun at 4500rpm and 5004 of solution A is deposited on substrate which continues spinning for 40 seconds. After 20 seconds another SOOpI of solution A is deposited on substrate. Once the spin coater has stopped the substrate is flooded with 3ml of Solution D. After a dwell period of S second the spin coater is ramped up to 5000rpm for 30 seconds to remove excess binder. The sample is then cured in an oven for one hour at 180C. In Figure 2 a cross-sectional of the film is shown -the film is llOnm thick and it can be seen that the nanoparticles are close packed and the binder penetrates to the substrate. Figure 3 shows a pencil scratch from an 8H pencil on the film. There is graphite deposited from the pencil but the film remains undamaged indicating its high mechanical strength. Figure 4 shows the corresponding reflectance curve, for reference the reflectance of a blank glass substrate is 4-4.5%.

[0082] 31]JV cured antireflection coating on TAC Itriacetyl cellulose] substrate.

[0083] TAC (triacetyl cellulose) sheets were cleaned by 10 mm sonication in 1% water solution of Triton -X, followed by rinsing with water and 10 mm ultrasonic treatment in IPA (isopropyl alcohol]. After drying samples were submerged in 10% wt. NaOH for S mm, rinsed with water, sonicated in water for 10 mm, submerged in 1% Acetic acid solution for S mm, rinsed with water and sonicated in water for 10 mm.

[00841 3 layers of 0.2ml 1.4wt% acrylate functionalised mesoporous silica particles in a solvent comprising of 70%MIBK: 30%IPA were deposited by spincoating onto TAC sheets at 4000 Rpm. The sample was then "flooded" with 2.5%wt PETA (Pentaerythritcd triacrykte] solution in a 70:30 MIBK:IPA solvent which a'so contained 3wt% of total solids of benzopheone to act as a photoinitiator. After 10 seconds any excess of binder was removed by spinning at 4000 Rpm for 30 sec.

[0085] The antireflection coating was liv cured under a medium pressure mercury lamp.

[0086] Figure 5 shows the increase in transmission of a samp'e of TAC coated on a single side with the antireflection coating described in the example. It shows an increase in TAC film transmission of up to 2.5%, corresponding to a reflection of 1% (c.f 3.5% on uncoated TAG]. The micrograph shows a cross-sectiona' scanning electron microscope image of the antireflection coating produced in Figure 6. The film is consists of close packed particles and the PETA binder and the film is uniform with a thickness of lóOnm.

Claims

CLAIMS1. A composition comprising oxide nanoparticles and a binder, wherein the composition, when formed as a coating, when tested for pencil hardness according to ASTM D3363 -O5[2001)e2 has a hardness of at least 4H, preferably at least 6H, more preferably at least BH.
2. A composition as claimed claim 1, wherein the nanoparticles have a random close packed structure.
3. A composition as claimed in claim 2, wherein in the random close packed structure the nanoparticles have a density of at least 50%, preferably at least 55%, more preferably at least 60%.
4. A composition as claimed in any one of the preceding claims, wherein the oxide nanoparticles comprise tin-doped indium oxide, aluminium-doped zinc oxide, fluorine-doped zinc oxide, antimony-doped tin oxide or silica.
5. A composition as darned in claim 4 wherein the oxide nanoparticles comprise silica.
6. A composition as claimed in any of the preceding claims, wherein the oxide nanoparticles are either porous or non-porous.
7. A composition as claimed in claim 6, wherein the oxide nanoparticles are porous silica nanoparticles and the pores are randomly oriented.
8. A composition as claimed in any of the preceding claims, wherein the reflectance for incident light on a substrate having one surface coated with the composition at at least one wavelength in the range from 400nm to 700nm is less than 3%, more preferably less than 2.5%.
9. A coating, optical element, ophthalmic element, solar cell, window, glass panel, low-K dielectric, or battery electrode comprising the composition as claimed in any of the preceding claims.
10. A method of preparing a coating comprising oxide nanoparticles and a binder) the method comprising the steps of: (a) applying to a substrate a solution or dispersion of oxide nanoparticles in one or more solvents in order to form a nanoparticle-containing layer, (b) applying to the nanoparticle-containing layer a binder solution comprising a binder and a solvent, (c) curing the binder.
11. A method as claimed in claim 10 comprising, between steps (b) and (c), the step of allowing the binder solution to diffuse into the nanoparticle containing layer.
12. A method as claimed in either claim 10 or claim 11, wherein the one or more solvents in step (a) is selected such that the nanoparticles are entirely within 80% of the Hansen solubility sphere radius for that particle as defined by Hansen solubility parameters.
13. A method as claimed in any one of claims 10 to 12, wherein when more than one solvent is used in step [a)) the solvents all have a vapour pressure and surface tension within 10% of one another.
14. A method as claimed in any one of claims 10 to 13 comprising the oxide nanoparticles of any one of claims 4 to 7.
15. A method as claimed in claim 14, wherein when the oxide nanoparticles comprise silica, the one or more solvents in step (a) comprises isopropanol.
16. A method as claimed in claim 15, wherein when the oxide nanoparticles comprise silica and have acrylate-containing groups on their surface, the one or more solvents in step [a) comprises isopropanol and methyl isobutyl keone.
17. A method as daimed in any one of daims 10 to 16, wherein the binder is a (iJ thermally-curable binder, preferably tetraethylorthosilicate and/or methyltrimethoxysilane and/or a polyurethane, or [ii] a UV-curable binder) preferably pentaerythritol triacrybte and/or 1, 6 -hexanediol diacrylate and/or trimethylolpropane triacrylate.
18. A method as claimed in any one of claims 10 to 17, wherein in steps (a] and (b] the applying is by spin coating.
19. A method as claimed in any one of claims 10 to 18, wherein step (a] comprises two or more separate and sequential applications of the nanoparticle solution or dispersion.
20. A method as claimed in any one of claims 10 to 19 comprising, before step(c], the step of removing excess binder solution, preferably by spinning the substrate, more preferably at by spinning the substrate at at least 3000rpm.
21. A method as claimed in claim 11, wherein the binder is allowed to diffuse into the nanoparticle-containing layer for at least 2 seconds before either the step of removing excess binder solution and/or step (c).
22. A coating obtainable by the method of any one of claims 10 to 21.