THEREOF

The present invention relates to a method of preparing a thin film element which is a dielectric exhibiting piezoelectric properties comprising a bismuth-based solid solution ceramic material as well as uses thereof. In particular, the bismuth-based solid solution ceramic material of use in the present invention has a general chemical formula (I): xA- yB-ZiCi-z ₂ C ₂ ; wherein A is a first bismuth based perovskite component; B is a second bismuth based perovskite component; Ci and C ₂ are dopant perovskite components; and wherein: x+y+zi+z ₂ = 1 ; x, y ¹ 0; and (zi + z ₂ ) > 0.

Actuator materials are needed to generate electric-field induced strains for a wealth of devices including, for instance, mechanical relays, digital cameras, and ink-jet printers. The composition and crystal structure of the actuator material are critical to determining the actuator characteristics. Common actuator materials include piezoelectric materials which undergo physical changes in shape when exposed to an external electric field. However, dielectric materials that do not exhibit the piezoelectric effect may also potentially find application as actuators.

In principle, all dielectric materials exhibit electrostriction, which is characterised by a change in shape under the application of an electric field. Electrostriction is caused by displacement of ions in the crystal lattice upon exposure to an external electric field; positive ions being displaced in the direction of the field and negative ions displaced in the opposite direction. This displacement accumulates throughout the bulk material and results in an overall macroscopic strain (elongation) in the direction of the field. Thus, upon application of an external electric field, the thickness of a dielectric material will be reduced in the orthogonal directions characterized by Poisson's ratio. Electrostriction is known to be a quadratic effect, in contrast to the related effect of piezoelectricity, which is primarily a linear effect observed only in a certain class of dielectrics (those lacking a crystallographic centre of symmetry).

The critical performance characteristics for an actuator material include the effective piezoelectric coefficient, d _33* , the temperature dependence of d _33* and the long-term stability of d _33* in device operation. Lead zirconate titanate (PZT), Pb(Zr _x Tii _-x )0 ₃ , and its related solid solutions, are a well-known class of ceramic perovskite piezoelectric materials that have found use in a wide variety of applications utilising piezoelectric actuation. However, as a result of emerging environmental regulations, there has been a drive to develop new lead-free actuator materials.

Significant attention has been given to electric field induced strain behaviour of alternative lead-free dielectric materials for potential actuator applications, examples of which include (K,Na)Nb0 ₃ -based materials, (Ba,Ca)(Zr,Ti)0 ₃ -based materials and (Bi,Na,K)Ti0 ₃ -based materials. Ceramics with the perovskite structure have been of particular interest in this regard. The constituent ions allow the unit cell to deform easily, giving rise to various ferroelectrically-active non-cubic perovskite phases such as those with tetragonal, rhombohedral, orthorhombic or monoclinic symmetry. The relatively large structural tolerance for ionic substitution is beneficial for chemical modifications, enabling functional properties to be tailored. When an external electric field is applied, these perovskite-structured ceramics are deformed along with the changes in their macroscopic polarisation state.

The perovskite compound bismuth sodium titanate (Bi _0.5 Na _0.5 )TiO ₃ (“BNT”) has, in particular, been studied extensively in the pursuit of lead-free actuator materials, including solid solutions comprising BNT with other components intended to enhance BNT’s dielectric and piezoelectric properties. WO 2012/044313 and WO 2012/044309 describe a series of lead-free materials based on ternary compositions of BNT and (Bio _. sKo _. s) Ti0 ₃ (“BKT”) in combination with (Bio _.5 Zn _0.5 )Ti0 ₃ (“BZT”), (Bi _0.5 Nio _.5 )Ti0 ₃ (“BNiT”), or (Bi _0.5 Mg _0.5 )TiO ₃ (“BMgT”) which exhibit piezoelectric properties. WO 2014/116244 also describes ternary compositions of BiCo0 ₃ together with perovskites such as BaTi0 ₃ (“BT”), (Na,K)Nb0 ₃ (“KNN”), BNT and BKT.

When the intended use of a solid solution ceramic material is in actuator applications, the ceramic material may be employed in the form of a thin film product. Preparation of products for such thin film microelectromechanical systems (MEMS) applications typically involves chemical solution deposition using chemical precursors, or sputtering (e.g. RF magnetron sputtering) using solid state sintered or hot-pressed ceramic targets. Where chemical solution deposition is used, it has been surprisingly found by the inventors that certain conditions relating to crystallisation of the deposited solution can have a significant impact on the ferroelectric and piezoelectric properties, as well as morphology, of the resulting thin film. WO 2017/158344 describes a piezoelectric thin film element comprising PZT suitable for use in actuators, particularly an actuator for a droplet deposition head in a droplet deposition apparatus. That document describes a chemical solution deposition process where a bulk PZT thin film layer is formed from three solutions, each solution having a Zr/Ti content which is different from that of any other and at least one solution having an excess lead content greater than that of any other, such that the bulk PZT thin film layer has a substantially uniform lead content and Zr/(Zr+Ti) ratio in its thickness direction. In the examples of WO 2017/158344, precursor layers are formed by spin coating each of the sol-gel solutions on to the electrode, drying and pyrolysing the spin-coated layer before annealing at 700 °C with a 10 °C/second temperature ramp, 60 seconds holding time and 2 SLPM 0 ₂ flow.

US 6,337,032 B1 describes a sol-gel precursor solution for forming a lead-based perovskite ferroelectric material for integrated circuits, preferably selected from PZT, lead lanthanum zirconium titanate (PLZT), lead magnesium niobate (PMN) and lead iron niobate (PFN). US 6,337,032 B1 describes a rapid annealing step performed as part of preparing the thin film for such integrated circuit applications. A rapid thermal annealing process (RTP), preferably in an oxygen containing atmosphere, is said to follow a 100 °C/s ramp rate to an annealing temperature of, most preferably, 600 °C to 800 °C, where the temperature is held for a time sufficient to induce crystallisation to form the desired crystallographic phase. It is also said that crystallisation temperature and processing times may be reduced by annealing in the presence of oxygen, ozone and water vapour, as compared to annealing in dry oxygen. The principal focus of US 6,337,032 B1 is, however, maximising remanent polarization, which is important for integrated circuit applications, as opposed to any potential displacement properties of the thin film material.

Jeon ef a/., J. Am. Ceram. Soc., 96 (2013), pp 2172, describes the synthesis of lead-free 72.5BNT-22.5BKT-5BMgT thin films via chemical solution deposition where formation of phase-pure perovskite films was shown to be possible at temperatures from 600 °C to 700 °C. The solutions were spin cast on a 100 nm Pt/33 nm TiOx/500 nm Si0 ₂ /Si substrate at 3000 rpm for 30 s. After each spin, the wet film was pyrolysed at 300 °C for 5 min and then annealed for 10 min at varying temperatures ranging from 600 °C to 700 °C in a preheated box furnace in air. Ferroelectric and piezoelectric properties of the resulting films were found to be improved upon increasing annealing temperature from 600 °C to 700 °C. No information is provided regarding the use of RTP or its effects on resulting piezoelectric properties or film morphology of the bismuth-based films.

It has now been surprisingly found that improvements in the ferroelectric and piezoelectric properties, as well as morphology, of bismuth-based thin films obtained from chemical solution deposition may be achieved where the annealing temperature of the ceramic material is reached by following a certain heating regime. In particular, improvements are obtained when utilising rapid thermal processing (RTP) in order to heat the deposited solution at a specific ramp rate of from 70 to 150 °C/s. By using a ramp rate in this specific range for reaching the annealing temperature, it has been found that the nature of the resulting crystallisation affords a bismuth-based thin film having particularly advantageous properties as compared to thin films obtained using alternative ramp rates achievable with RTP or much slower heating rates, for instance achievable simply using a preheated furnace. The benefits of the invention are also enhanced by utilising a particular flow rate of oxygen-containing gas during the RTP.

The present invention therefore offers a means for enhancing the ferroelectric and piezoelectric properties of bismuth-based films such that they may become more viable as alternatives to conventional lead-based piezoelectric materials (e.g. PZT), which have been used traditionally, particularly in MEMS applications.

SUMMARY

Thus, in a first aspect, the present invention provides a method for fabricating a lead- free thin film element comprising a substrate and a thin film formed thereon, wherein said thin film is a solid solution ceramic material having a major proportion of a perovskite phase and having the formula (I) below:

(I): xA-yB-ZiCi-z ₂ C ₂ wherein A is a first bismuth based perovskite component; B is a second bismuth based perovskite component; Ci and C ₂ are dopant perovskite components; and wherein: x+y+zi+z ₂ = 1 ; x, y ¹ 0; (zi + z ₂ ) > 0; said method comprising:

performing steps i) and ii) below one or more times:

i) depositing a precursor solution for the ceramic material of formula (I) on to the substrate by chemical solution deposition to form a deposited precursor solution on the substrate;

ii) drying and pyrolysing the deposited precursor solution to form a coating;

followed by step iii) below:

iii) crystallising the coating by rapid thermal processing to form a film of the solid solution ceramic material of formula (I); wherein crystallising in step iii) involves heating the coating to a crystallisation temperature of from 600 °C to 800 °C; and wherein the temperature is increased at a ramp rate of from 70 to 150 °C/s up to the crystallisation temperature.

The invention also provides an actuator component comprising a thin film element obtainable, preferably obtained, from the methods described herein, as well as a droplet deposition apparatus comprising the actuator component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURES 1a-c: show XRD diffractograms for three layer bismuth-based films prepared in accordance with the Examples using RTP at different heating ramp rates and with crystallisation after each coating layer“each” or after coating of the final layer (“end”) (Figures 1a and 1 b) and employing different oxygen volumetric flow rates (Figure 1c);

FIGURES 2a-h: show atomic force microscopy images (1 pm and 5 pm) for three layer bismuth-based films prepared using RTP at different heating ramp rates;

FIGURES 3a-f: show graphs of room temperature electromechanical strain and polarisation versus electric field strength measured at 1 kHz for three layer bismuth- based films prepared using RTP at different heating ramp rates; FIGURES 4a-f: show graphs of dielectric constant and dielectric loss determined for three layer bismuth-based films prepared using RTP at different heating ramp rates. These were collected with a 50 mV oscillation voltage; and

FIGURE 5: shows a graph of d33* values for the different ramp rates for certain thin films prepared in the Examples.

DETAILED DESCRIPTION

Chemical solution deposition (CSD) is commonly employed in the fabrication of thin films and involves the use of appropriate precursor compounds provided together with one or more solvents in the form of a precursor solution which is deposited on a substrate and further treated. The precursor solution which is used for deposition on the substrate may be prepared by combining individual solutions of the cation constituents of the desired solid solution ceramic material. As will be appreciated, individual precursor solutions for components A, B or C ₁ /C ₂ in formula (I) above may also be separately prepared before being combined into a single precursor solution to be used for deposition on to the substrate.

The crystalline solid solution ceramic materials of formula (I) comprise a major proportion of a perovskite crystallographic phase (i.e. above 50 vol.%). Additional crystalline phases that may be present collectively represent a minor proportion of the ceramic material’s microstructure. The lattice dimensions of the dominant crystalline phase, and the physical and chemical properties of the solid solution, are continuous functions of composition. The lattice symmetry may change within said composition range by uniform distortion of the structure as the composition changes. In preferred embodiments the ceramic material of formula (I) comprises at least 70 vol.%, more preferably at least 80 vol.%, even more preferably at least 90 vol.%, yet more preferably at least 95 vol.% of a perovskite crystallographic phase. Most preferably, the solid solution ceramic material of formula (I) is substantially homogeneous (i.e. phase pure), having only a perovskite crystalline phase.

The term“solid solution” used herein refers to a mixture of two or more crystalline solids that combine to form a new crystalline solid, or crystal lattice, that is composed of a combination of the elements of the constituent compounds. As will be appreciated, the solid solution ceramic materials according to formula (I) referred to herein may consist essentially of its constituent crystalline compounds as well as dopants and inevitable impurities. The solid solution exists over a partial or complete range of proportions or mole ratios of the constituent compounds, where at least one of the constituent compounds may notionally be considered to be the "solvent" phase (i.e. the constituent compound(s) which are correspond to a major mole fraction of the solid solution).

The term“dopant” (as opposed to“perovskite dopants Ci and C ₂ ”) used herein refers to a metallic component which may be dissolved in the solid solution of the ceramic materials of the invention in order to modify performance or engineering characteristics of the ceramic material, without having any material impact on the overall phase and symmetry characteristics of the solid solution. For instance, dopants may be used to modify grain size and domain mobility, or to improve resistivity (e.g. by compensating for excess charge carriers), temperature dependence and fatigue properties.

Examples of suitable dopants include materials comprising a metallic cation, preferably selected from Mn, Mg, Nb and Ca, for example Mn0 ₂ , MgO, Nb ₂ 0 ₅ and CaO. Preferably, the solid solution ceramic materials of the invention contain less than 5 at.%, preferably less than 3 at.%, more preferably less than 2.5 at.% of dopant. In other preferred embodiments, the solid solution ceramic materials of the invention contain no dopant.

The bismuth-based film of formula (I) is a solid solution formed of components that, individually, exhibit a perovskite structure, “ABX ₃ ”, where 'A' and 'B' are cations of different sizes, and X is an anion that bonds to both. As will be appreciated by the skilled person, it is understood that multiple cations can occupy the A- or B- sites of the perovskite structure.

In formula (I), xA-yB-ZiC _r z ₂ C ₂ , A corresponds to a first bismuth based perovskite component; B is a second bismuth based perovskite component; Ci and C ₂ are optional dopant perovskite components, and wherein: x+y+zi+z ₂ = 1 ; x, y ¹ 0; and (zi + z ₂ ) > 0.

Perovskite components A and B of formula (i) correspond to a major molar proportion of the solid solution ceramic material (i.e. greater than 50 mol.%). In other words, (x + y) > 0.5. In preferred embodiments, (x + y) > 0.6; more preferably (x + y) > 0.7, still more preferably (x + y) > 0.75; and even more preferably (x + y) > 0.8.

The optional and different dopant perovskite components Ci and C ₂ are collectively present as a minor molar fraction (i.e. less than 50 mol.%) of the solid solution ceramic material and are generally added to modify the microstructure of the solid solution ceramic material, for instance to optimise ferroelectric and piezoelectric properties. In other words, (Ci + C ₂ ) < 0.5. In preferred embodiments, (Ci + C ₂ ) < 0.2; more preferably (Ci + C ₂ ) < 0.15, still more preferably (Ci + C ₂ ) < 0.10.

In some embodiments, A of formula (I) is (Bio _.5 Nao _.5 )Ti0 ₃ and B of formula (I) is (Bi _0.5 K _0.5 )TiO _3. In related embodiments, (Ci + C ₂ ) = 0, such that the solid solution ceramic material of formula (I) is (Bi0.5Na0.5)Ti03-(Bi0.5K0.5)Ti03 (“BNT-BKT”).

In some embodiments, Ci and/or C ₂ of formula (I) are independently selected from SrHf0 ₃ , SrZr0 ₃ , Bi(Mgo _.5 Tio _.5 )0 ₃ , Bi(Zno _.5 Tio _.5 )0 ₃ , Bi(Nio _.5 Tio _.5 )0 ₃ , KNb0 ₃ , NaNb0 ₃ , (Ko _.5 Na _0.5 )Nb0 ₃ , (Bio _.5 Lio _.5 )Ti0 ₃ , (Bio _.5 Nao _. s)Hf0 ₃ and (Bio _.5 Ko _.5 )Hf0 _3. In related embodiments, the solid solution ceramic material of formula (I) is (Bi _0.5 Na _0.5 )TiO ₃ - (Bio _.5 Ko _.5 )Ti0 ₃ -Bi(Mgo _.5 Tio _.5 )0 ₃ (“BNT-BKT-BMT”).

In other embodiments, the solid solution ceramic material has a composition according to the formula (la):

(la): x(Bio.5Nao.5)Ti0 ₃ -y(Bio.5Ko.5)Ti0 ₃ -ZiSrHf0 ₃ -z ₂ SrZr0 ₃ x+y+z ₁₊ z ₂ = 1 ; x, y ¹ 0; {z _{\ +} z ₂ ) > 0.

In preferred embodiments, 0.25 < x < 0.65, more preferably 0.35 < x < 0.55, most preferably 0.40 < x < 0.50 in formula (la).

In preferred embodiments, 0.25 < y < 0.75, more preferably 0.35 < y < 0.55, most preferably 0.40 < y < 0.50 in formula (la).

In preferred embodiments, 0.01 < (åi+z ₂ ) < 0.15, more preferably 0.02 < (zi+z ₂ ) < 0.10, most preferably 0.04 < (zi+z ₂ ) < 0.08 in formula (la). In preferred embodiments, 0.40 < x < 0.50; 0.40 < y < 0.50; and 0.02 < (åi+z ₂ ) < 0.10 in formula (la).

In other embodiments, one of z1 and z2 = 0, such that only one of SrHf0 ₃ and SrZr0 ₃ is present in the solid solution.

The precursor solution which is used for depositing on a substrate in accordance with the present invention may comprise the dissolved precursors for the ceramic material of formula (I) in any suitable concentration which is adequate for providing a film of sufficient thickness following evaporation of the solvent following drying after deposition. For example, the concentration may range from 0.2 mol/L to 0.8 mol/L, preferably from 0.25 mol/L to 0.75 mol/L. In some embodiments, the precursor solution may have a total weight concentration of soluble precursor compounds for the solid solution ceramic material of formula (I) of at least 10 wt.%. For example, the precursor solution may have a total weight concentration of soluble precursor compounds for the solid solution ceramic material of formula (I) of from 10 to 40 wt.%, from 10 to 30 wt.%, from 12 to 28 wt.%, or from 15 to 25 wt.%.

The consistency of the liquid phase precursor solution is not particularly limited and may, for instance, be provided in the form of a sol-gel solution. Such sol-gel solutions typically comprise organic compounds, such as surfactants, to provide a colloidal suspension of precursor compounds.

Examples of suitable precursors useful in preparation with the bismuth-based thin films include titanium (IV) isopropoxide, titanium butoxide, bismuth acetate, bismuth nitrate, bismuth 2-ethylhexanoate, barium acetate, barium nitrate, barium 2-ethylhexanoate, sodium acetate trihydrate, sodium nitrate, potassium acetate, potassium nitrate, magnesium acetate tetrahydrate, magnesium nitrate, zinc acetate and zinc nitrate. Suitable solvents that may be employed in these methods where appropriate include alcohols (for example, methanol, ethanol, 2-methoxyethanol and 1 -butanol) and organic acids (for example, acetic acid and propionic acid). Suitable stabilisers that may be employed in these methods where appropriate include acetylacetone and diethanolamine. It is well-known that bismuth, sodium, and potassium are all volatile, particularly at process temperatures typical of perovskite crystallization. The vapour pressures of K ₂ 0 and Na ₂ 0 are especially high, being comparable with that of PbO. To compensate for the high volatility of certain cations, precursor solutions may be prepared with amounts of excess cations added thereto (overdoping). Such overdoping is common in CSD- prepared PZT thin films (for example, up to 20 mol%-40 mol% excess Pb ²⁺ can be added, depending on solution chemistry). In a similar manner, bismuth cation precursor solutions used in connection with the present invention, as well as precursor solutions of other cations, particularly those comprising sodium and potassium, may be overdoped and the skilled person is able to determine an appropriate level of overdoping.

Without being bound by any particular theory, it is believed that overdoping in order to ensure that the desired stoichiometry is achieved is beneficial to the durability/resistance to conduction of the resulting films. If there is a stoichiometric imbalance, point defects, which contribute to the space charge limited current, can occur. As will be appreciated, improving resistance to leakage/breakdown may then necessitate applying higher fields to achieve the desired degree of displacement.

Generally, a molar excess of each cation is provided in the precursor solution to account for volatilization during RTP. Typically, each cation is overdoped in the precursor solution to no more than 30 mol.% or no more than 20 mol.%. In preferred embodiments, each cation is overdoped in the precursor solution from 2 to 18 mol.%, more preferably from 4 to 14 mol.%.

A suitable range of overdoping for bismuth cation precursor solutions used in connection with the present invention is from 2 to 10 mol.%, preferably from 4 to 10 mol.%, more preferably from 5 to 9 mol.%, most preferably from 6 to 8 mol.%.

A suitable range of overdoping for sodium cation precursor solutions used in connection with the present invention is from 5 to 20 mol.%, preferably from 6 to 18 mol.%, more preferably from 6 to 14 mol.%, most preferably from 8 to 12 mol.%.

A suitable range of overdoping for potassium cation precursor solutions used in connection with the present invention is from 5 to 20 mol.%, preferably from 6 to 18 mol.%, more preferably from 6 to 14 mol.%, most preferably from 8 to 12 mol.%. A suitable range of overdoping for magnesium cation precursor solutions used in connection with the present invention is from 1 to 10 mol.%, preferably from 2 to 8 mol.%, more preferably from 3 to 7 mol.%, most preferably from 4 to 6 mol.%.

In accordance with step i), the precursor solution is deposited on a substrate by any suitable means of which the skilled person is aware. Such means include dip- or spin- coating, preferably spin coating, which are commonly utilised in chemical solution deposition processes.

Dip-coating involves immersing a substrate in a container of precursor solution and depositing the precursor solution on to the substrate as the substrate is withdrawn, typically at constant speed. The speed at which the substrate is withdrawn from the container of precursor solution effectively determines the thickness of the coating, faster withdrawal giving rise to a thicker coating.

In contrast, spin-coating typically involves the use of a spin-coat apparatus in which a substrate may be secured and spun in the proximity of a dispenser from which the precursor solution may be dispensed on to the substrate whilst being spun or shortly before spinning commences. Rotation of the substrate typically occurs at a rate of from 2000 rotations per minute (rpm) to 4000 rpm, preferably from 2500 rpm to 3500 rpm, more preferably from 2750 rpm to 3250 rpm, for example 3000 rpm.

The duration over which rotation is typically conducted relates to the desired thickness of the coating layer; longer rotation times resulting in thinner coating layers. Suitable time periods over which rotation is conducted during spin-coating is from 20 seconds to 200 seconds, preferably from 30 seconds to 120 seconds, more preferably from 30 seconds to 60 seconds.

Typically, deposition and rotation where spin-coating is employed, or immersion and withdrawal where dip-coating is used, is conducted so as to provide a coating layer having a thickness of from 10 nm to 500 nm, preferably from 15 to 200 nm, more preferably from 20 to 100 nm. As discussed in more detail below, multiple layers may be deposited so as to form a film having a multi-layer laminate structure. For example, the laminate may suitably include at least 2 layers and preferably less than 50 layers. In other examples, the laminate has from 2 layers to 20 layers, from 2 layers to 10 layers, or from 2 layers to 5 layers. It will be appreciated that the thickness of the coating layer depends on the deposition process, such as spin speed for spin-coating or withdrawal speed for dip-coating, as well as the viscosity and solid content of the precursor solution.

Each additional layer is formed directly on the previously deposited layer with no intervening layers there between, provided that the previously deposited layer has undergone at least drying and pyrolysis, and in some cases also crystallization, as discussed in more detail below. Alternatively, the thin film may be formed from only a single deposited layer having a sufficient thickness. The final thickness of the thin film may suitably be in the range of from 0.3 pm to 5 pm, preferably in the range of from 0.5 pm to 3 pm.

Deposition of the precursor solution is typically conducted at from room temperature (for example, 20 °C) up to a temperature of 100 °C. Higher deposition temperatures may lead to near simultaneous evaporation of the solvent of the precursor solution upon deposition.

As the skilled person is aware, it is possible to heat the substrate upon which the precursor solution is to be deposited in order to facilitate drying and pyrolysis. For example, spray pyrolysis may be used which typically relies on the use of a heated substrate.

Prior to deposition on the bare substrate, it is usual for the substrate to be cleaned to remove residue and dirt. Removing residual organics on the substrate surface typically involves washing with low molecular weight volatile compounds, such us methanol, ethanol, isopropanol and/or acetone followed by evaporation of the volatile compounds by heating the substrate. The substrate may, for instance, be heated to a temperature of up to 350 °C, suitably for a duration of from 30 seconds to 3 minutes, to ensure complete evaporation of the volatile compounds. Other techniques including ultrasonic bath and plasma etching may also be relied upon as part of the substrate cleaning process.

The nature of the substrate which is employed in the present invention is not particularly limited. Suitable substrate materials include metals selected from platinum, iridium or ruthenium, as well as materials coated with such metals, for example, metallized silicon wafers (preferably M/Ti/Si0 ₂ /Si, where M is a metal selected from platinum, iridium or ruthenium, preferably platinum). Platinum is particularly preferred as a substrate material as a result of its high conductivity and high chemical stability, as well as its resistance to oxidation in oxygen environments at high temperatures.

In some embodiments, the substrate is a diaphragm which is: i) a metal or metal oxide layer that functions as a lower electrode; or ii) a non-metal layer coated with a metal layer that provides an electrically conductive lower electrode. In preferred embodiments, the diaphragm is a layer of platinum, iridium, iridium oxide or ruthenium, more preferably platinum.

Following deposition of the precursor solution, forming of the thin film typically involves drying to remove the solvent of the precursor solution and pyrolysing to cause reaction of the precursor compounds so as to form an amorphous layer comprising the ceramic material according to formula (I). The crystallization step subsequently transforms the amorphous layer into a thin film comprising a major proportion of the perovskite phase, preferably at least 90 vol.% of the perovskite phase, more preferably at least 95 vol.% of the perovskite phase, even more preferably at least 98 vol.% of the perovskite phase, and most preferably 100 vol.% of the perovskite phase. As will be appreciated, drying and pyrolysing may be carried out in a single step, or as distinct sub-steps in the method of the invention. Thereafter, crystallization of the precursor layer is effected by heating to a crystallisation temperature / annealing temperature (i.e. to a temperature where changes in the microstructure of the initially amorphous coating layer are possible).

The conditions for drying and pyrolysing are not particularly limited so far as there is no negative impact on the morphology and/or porosity of the resulting amorphous coating layer or any decomposition of the materials at high temperature. Generally, the minimum temperature for drying is dictated by the lowest-boiling point solvent that is present in the precursor solution. Similarly, the minimum temperature for the pyrolysis step is dependent upon the precursor having the lowest pyrolysis temperature. Suitable temperatures over which drying may be conducted are from 60 °C to 250 °C, preferably from 100 °C to 200 °C, and suitably over a time period of from 1 to 5 minutes, preferably from 2 to 3 minutes. Suitable temperatures over which pyrolysis may be conducted are from 150 °C and 500 °C, preferably from 250 °C to 450 °C, more preferably from 300 °C to 375 °C, and suitably over a time period of from 1 to 10 minutes, preferably 2 minutes to 8 minutes, more preferably from 3 to 5 minutes.

The crystallisation temperature used in accordance with the method of the present invention is from 600 °C to 800 °C. This range of crystallisation temperatures has been found to be important, in combination with the specific ramp rate employed, for achieving the benefits of the invention.

“Crystallisation temperature” or“annealing temperature” used herein is intended to mean the temperature to which the amorphous coating layer or layers is heated and at which temperature a transition from an amorphous to a crystalline phase is possible (i.e. an annealing temperature for the material).

In particularly preferred embodiments, the crystallisation temperature is within a range from 650 °C to 750 °C, preferably from 675 °C to 725 °C, for example 700 °C.

Rapid Thermal Processing (RTP) is a versatile optical heating method which can be used for semiconductor processing as well as for controlled heating of objects which are in the form of sheets or discs, including thin and thick films. Heating is generally conducted in a chamber having one or more walls including a transparent portion, typically comprising quartz, so as to allow transmittance of radiation from powerful heating lamps. Radiation from a halogen lamp, an infrared lamp, or an ultraviolet lamp may suitably provide the source of heat in the RTP. Preferably, tungsten-halogen lamps are used

The radiation from the lamps is directed through the transparent portion(s) of the chamber wall(s) on to the surface of the object to be heated. Provided the object to be heated absorbs light in the spectral region transmitted by the heating lamps, RTP accommodates for rapid changes in temperature and process gas for the different material processes and conditions. Examples of suitable RTP apparatuses include AG Associates Model 410 Heat Pulser, which utilises a halogen lamp heating source, and Accutherm AW610 from Allwin21 Corp.

By utilising RTP, it is possible to closely control the temperature to which the coated substrate is heated to reach the crystallisation temperature (i.e. an annealing temperature), as well as the ramp rate at which the temperature of the coated substrate is increased from a lower temperature which prevails prior to heating during RTP to the higher crystallisation temperature achieved during RTP. Reference herein to“ramp rate” is therefore intended to mean the rate of increase in temperature upon heating by RTP up to the crystallisation temperature. Moreover, reference to ramp rate herein is intended to mean the average (mean) ramp rate over the period of time over which temperature is increasing (bT/btime) during RTP and up to reaching the crystallisation temperature. The ramp rate may be accurately controlled using the RTP and the average ramp rate may be readily ascertained based on the data outputted by the RTP apparatus.

As will be appreciated, it is possible for the RTP phase to include one or more holding steps, where temperature is increased before being held at a constant temperature, which is lower than the crystallisation temperature, before the temperature is further increased to a higher holding temperature or the crystallisation temperature. Heating in this manner can help stabilise the system before heating to the crystallisation temperature and can help avoid overshooting the desired crystallisation temperature as result of the heating ramp. For the avoidance of doubt, the ramp rate according to the present invention refers to the rate of temperature increase which results in achieving the crystallisation temperature. Preferably, where there are any holding steps at elevated temperature which are incorporated into the RTP phase, such holding steps are at temperatures of less than 375 °C, more preferably less than 350 °C, and most preferably less than 325 °C.

In accordance with the present invention, the ramp rate at which the temperature of the coated substrate is increased to the crystallisation temperature is from 70 to 150 °C/s. This range has been found by the inventors to confer surprising benefits in terms of the combination of piezoelectric properties of the resulting bismuth-based films, as well as their morphology. Without being bound by any particular theory, this particular range of ramp rate is thought to be particular beneficial in promoting nucleation and crystal growth in the bismuth-based amorphous coating layer(s) in a manner which gives rise to advantageous grain size/morphology and other crystallographic properties, such as crystal orientation, of the solid solution. In particularly preferred embodiments, the ramp rate is from 75 to 125 °C/s, more preferably from 85 to 115 °C/s, even more preferably from 90 to 110 °C/s, most preferably from 95 to 105 °C/s.

As the skilled person will appreciate, a suitable starting temperature of the coated substrate from which heating to the crystallisation temperature in RTP may be conducted may range from room temperature up to the temperature at which pyrolysis in the preceding step is conducted. The starting temperature of the coated substrate in the RTP will be from room temperature (for example, 20 °C) up to the temperature of the preceding pyrolysis step, and typically a temperature lying between those temperatures.

As mentioned above, the temperature of the coated substrate may be elevated and held at a hold temperature (typically below 375 °C) before heating to the crystallisation temperature in RTP is undertaken. For example, in some embodiments, the coated substrate is heated to an initial hold temperature of from 150 and 250 °C, preferably from 175 and 225 °C, most preferably from 190 to 210 °C, where the elevated temperature is held, for instance, from 30 seconds to 5 minutes, before further heating is commenced to ramp the temperature up to the crystallisation temperature at the desired ramp rate.

Heating of the coated substrate by RTP may be at least partially conducted in the presence of oxygen over the course of heating. Provision of a source of oxygen may, however, not be over the entire duration of RTP. Providing a source of oxygen is believed to: i) control volatilization; ii) control the oxidation states of the cations; and iii) minimise the concentration of oxygen vacancies, the extent of which has been found to be advantageous, particularly when using the preferred volumetric flow rates described below. The enhanced control over volatilization using the preferred volumetric flow rates has also been found by the inventors to be particularly advantageous in combination with overdoping as discussed hereinbefore for ensuring a desired stoichiometry is achieved so as to maximise phase purity.

In particular, the beneficial properties associated with the bismuth-based thin films of the invention have been found to be enhanced by conducting the heating as part of the crystallisation step iii) whilst being contacted by an oxygen-containing gas which is provided at a particular volumetric flow rate. Generally, an oxygen-containing gas may suitably be provided during the crystallisation step at a volumetric flow rate of from 0.1 to 12 standard litres per minute (SLPM). However, in particularly preferred embodiments, the volumetric flow rate of oxygen-containing gas may be from 1 to 4 SLPM, more preferably from 1.5 to 3 SLPM, most preferably from 1.75 to 2.25 SLPM, for example 2.0 SLPM. At these preferred volumetric flow rates, the phase purity of the resulting crystalline films has been found by the inventors to be maximised and the beneficial ferroelectric and piezoelectric properties of the films have been found to be enhanced.

The oxygen-containing gas may consist essentially, or preferably consist, of oxygen or may contain oxygen together with only inert diluents, such as nitrogen or argon gases. In some embodiments, the oxygen-containing gas may comprise oxygen in an amount of at least 50 vol.%, at least 60 vol.%, at least 70 vol.%, at least 80 vol.% or at least 90 vol.%. In particularly preferred embodiments, the oxygen-containing gas may comprise oxygen in an amount of at least 98 vol.%, more preferably at least 99 vol.%, most preferably at least 99.9 vol.%. In other embodiments, the oxygen-containing gas may be air. In still further embodiments, the oxygen-containing gas may comprise water vapour and ozone.

Once the crystallisation temperature has been reached by using RTP, this temperature may be maintained for a period of time (i.e. a“hold time”), which may suitably range from 1 to 15 minutes. In preferred embodiments, the crystallisation temperature is held for 2 to 10 minutes, more preferably from 4 to 6 minutes. The RTP apparatus may be programmed to implement heating so as to achieve and maintain a specific crystallisation temperature.

As part of the process of the present invention, the following steps i) and ii) are performed one or more times: i) depositing a precursor solution for the ceramic material of formula (I) on to the substrate by chemical solution deposition to form a deposited precursor solution on the substrate; and ii) drying and pyrolysing the deposited precursor solution to form a coating. Following performance of steps i) and ii) once, or for a desired number of cycles, step iii) is subsequently performed: iii) crystallising the coating by rapid thermal processing to form a film of the solid solution ceramic material of formula (I). As will be appreciated,“one or more times” used herein in reference to performing steps i) and ii) means that these steps may be performed a single time, thereby forming a single layer of coating on the substrate before crystallisation is performed in step iii), or a plurality of times involving repeated cycles of precursor solution deposition and drying/pyrolysing, thereby providing a plurality of coating layers on the substrate before crystallisation is performed in step iii). It will therefore be understood that, where steps i) and ii) are performed a plurality of times, deposition of further precursor solution as part of a repeat cycle of steps i) and ii) will be on to a previously deposited, dried and pyrolysed coating layer, and therefore not directly on to the substrate. Drying and pyrolysing of the further deposited precursor solution(s) may be conducted under substantially the same or different drying and pyrolysing conditions as the preceding coating layer. It will also be understood that where steps i) and ii) are conducted a plurality of times before crystallisation in step iii), the resulting laminate of coating layers will undergo crystallisation collectively at the same time when crystallisation is conducted as part of step iii).

As mentioned hereinbefore, the method of the invention is preferably utilised in order to prepare a multi-layer thin film laminate on the substrate. As will be appreciated, such a laminate structure may be achieved in a variety of ways. For example, a multi-layer thin film laminate on the substrate may be formed by depositing further precursor solution on to the first coating formed directly on the substrate following a first round of steps i) and ii) and prior to crystallisation in step iii), as described above. Thus, in some embodiments, steps i) and ii) of the method are performed a plurality of times prior to crystallisation in step iii); preferably wherein steps i) and ii) are performed from 2 to 5, more preferably 3 or 4 times, so as to form a multi-layer thin film laminate on the substrate.

Additionally or alternatively, following formation of the thin film after the crystallisation step iii) of the method, the method may further comprise depositing further precursor solution on to the film of solid solution ceramic material formed on the substrate and repeating the cycle of drying and pyrolysing according to step ii) and crystallising according to step iii) to form a further solid solution ceramic film layer, thereby providing a multi-layer thin film laminate on the substrate. Drying and pyrolysing of the further deposited precursor solution may be conducted under substantially the same or different drying and pyrolysing conditions as the preceding coating layer. The formation of additional thin film layers may be repeated until a desired thickness of the multi-layer thin film laminate is reached. As will be appreciated, crystallisation may in some embodiments be conducted after each round of deposition of additional precursor solution and formation of a coating layer following drying and pyrolysing. This approach is referred to as“each layer” crystallisation herein, for instance, as indicated in the Examples, and is generally preferred over applying multiple coatings before a single crystallisation step at the end. In some embodiments, the number of layers of the multi- layer thin film laminate that are formed by including further round(s) of deposition of precursor solution, drying/pyrolysing and crystallisation may range from 2 to 5, preferably from 2 to 4 layers, for example 3 layers.

Thus, in some embodiments, following crystallisation step iii) of the method of the invention, the method further comprises: a) depositing further precursor solution on to the film of solid solution ceramic material formed on the substrate; b) drying and pyrolysing according to step ii); c) optionally repeating steps a) and b) to provide further coating layers; and d) crystallising according to step iii) to form one or more further solid solution ceramic film layers so as to provide a multi-layer thin film laminate on the substrate.

Where a multi-layer thin film laminate is formed on the substrate, the nature of the precursor solution used may be different or substantially the same at each occurrence it is used for forming a further layer of the multi-layer laminate. In preferred embodiments, the composition of the precursor solution(s) deposited for forming a laminate of thin film layers is different to that of the precursor solution which is used to deposit directly onto the substrate initially. This allows the composition of the precursor solution which is to be deposited directly on to the substrate to be optimized for that purpose and for forming a base coating upon which other coating layers may be advantageously grown. In other preferred embodiments, when a multi-layer thin film laminate having 3 or more layers is formed, the precursor solutions deposited after a first coating layer is formed on the substrate are substantially the same.

Any combination of additional steps for increasing the number of layers of the thin-film laminate may be adopted, provided there is a final step of crystallisation to ensure all layers of the laminate have undergone a transition from an amorphous to a crystalline state.

In a further aspect, the present invention also provides an actuator component comprising a thin film element obtainable, preferably obtained, by the methods described herein. Such actuator components may find use in a droplet deposition apparatus and therefore the present invention also provides a droplet deposition apparatus comprising such an actuator component. Droplet deposition apparatuses have widespread usage in both traditional printing applications, such as inkjet printing, as well as in 3D printing and other materials deposition or rapid prototyping techniques.

Accordingly, in a related aspect, there is also provided a method of actuating the actuator component, said method comprising the step of applying an electric field to the actuator component.

An actuator component suitable for use in a droplet deposition apparatus may, for instance, comprise a plurality of fluid chambers, which may be arranged in one or more rows, each chamber being provided with a respective actuator element and a nozzle. The actuating element is actuatable to cause the ejection of fluid from a chamber of the plurality through a corresponding one of the nozzles. The actuating element is typically provided with at least first and second actuation electrodes configured to apply an electric field to the actuating element, which is thereby deformed, thus causing droplet ejection. Additional layers may also be present, including insulating, semi-conducting, conducting, and/or passivation layers. Such layers may be provided using any suitable fabrication technique such as, for example, a deposition/machining technique, e.g. sputtering, CVD, PECVD, MOCVD, ALD, laser ablation etc. Furthermore, any suitable patterning technique may be used as required, such as photolithographic techniques (e.g. providing a mask during sputtering and/or etching).

The actuating element may, for example, function by deforming a wall bounding one of the fluid chambers of the actuator component. Such deformation may in turn increase the pressure of the fluid within the chamber and thereby cause the ejection of droplets of fluid from the nozzle. Such a wall may be in the form of a membrane layer which may comprise any suitable material, such as, for example, a metal, an alloy, a dielectric material and/or a semiconductor material. Examples of suitable materials include silicon nitride (Si ₃ N ₄ ), silicon oxide (Si0 ₂ ), aluminium oxide (Al ₂ 0 ₃ ), titanium oxide (Ti0 ₂ ), silicon (Si) or silicon carbide (SiC).

The droplet deposition apparatus typically comprises a droplet deposition head comprising the actuator component and one or more manifold components that are attached to the actuator component. Such droplet deposition heads may, in addition, or instead, include drive circuitry that is electrically connected to the actuating elements, for example by means of electrical traces provided by the actuator component. Such drive circuitry may supply drive voltage signals to the actuating elements that cause the ejection of droplets from a selected group of fluid chambers, with the selected group changing with changes in input data received by the head.

To meet the material needs of diverse applications, a wide variety of alternative fluids may be deposited by droplet deposition heads as described herein. For instance, a droplet deposition head may eject droplets of ink that may travel to a sheet of paper or card, or to other receiving media, such as textile or foil or shaped articles (e.g. cans, bottles etc.), to form an image, as is the case in inkjet printing applications, where the droplet deposition head may be an inkjet printhead or, more particularly, a drop-on- demand inkjet printhead.

Alternatively, droplets of fluid may be used to build structures, for example electrically active fluids may be deposited onto receiving media such as a circuit board so as to enable prototyping of electrical devices. In another example, polymer containing fluids or molten polymer may be deposited in successive layers so as to produce a prototype model of an object (as in 3D printing). In still other applications, droplet deposition heads might be adapted to deposit droplets of solution containing biological or chemical material onto a receiving medium such as a microarray.

Droplet deposition heads suitable for such alternative fluids may be generally similar in construction to printheads, with some adaptations made to handle the specific fluid in question. Droplet deposition heads which may be employed include drop-on-demand droplet deposition heads. In such heads, the pattern of droplets ejected varies in dependence upon the input data provided to the head. The tensile stresses associated with the thin film element prepared in accordance with the present invention can affect field-induced strains and the magnitude of the effective piezoelectric coefficient d ₃₃ ^* . The skilled person is able to determine the extent of residual tensile stresses associated with a fabricated thin film and take steps to control such stresses (for example, utilising thermal anneals to relieve stress, by designing the device architecture to achieve a desired stress state, and by adjusting film thickness) in order to gain the maximum benefit of the field-induced strains associated with the solid solution ceramic materials utilised.

This approach can also, for instance, be utilised when the solid solution ceramic material is fabricated as a thin film forming part of an actuator component of a droplet deposition apparatus, described in further detail below. The skilled person is able to accommodate for, or mitigate, intrinsic stresses resulting from the configuration of the actuator component so as to optimise the performance of the actuator component. Thus, as applied to the droplet deposition apparatus, the skilled person is able to ensure that the gain or loss of electric-field induced strain resulting from the application of an ejection waveform to an actuator element formed of the bismuth-based ceramic material is sufficient to cause ejection of a droplet. In one example, this might be accomplished by appropriate design of the ejection waveform. This may, for instance, include identifying a suitable amplitude for the ejection waveform (e.g. suitable peak-to-peak amplitude) and/or identifying suitable maximum and minimum voltage values (with the characteristic phase transition occurring upon change between maximum and minimum voltage values). The thus-designed ejection waveform may accommodate for, or mitigate, the effect that intrinsic stresses have on the conditions necessary to achieve satisfactory electric-field induced strain to cause ejection of a droplet.

The present invention will now be described by reference to the following Examples which are intended to be illustrative of the invention and in no way limiting.

Examples

Precursor Solution Preparation for BNT-BKT Thin Films

Bismuth acetate Bi(OOC ₂ H ₃ ) ₃ , sodium acetate trihydrate Na(00C ₂ H ₃ )*3H ₂ 0, potassium acetate K(OOC ₂ H ₃ ), and titanium isopropoxide Ti[OCH(CH ₃ ) ₂ ] ₄ were used as solution precursors. First, titanium isopropoxide was stabilized with acetic acid in a 1 :4 of molar ratio in a dry atmosphere to prevent the reaction of the titanium precursor with atmospheric H ₂ 0. Subsequently, bismuth acetate was dissolved in propionic acid at room temperature. Sodium and potassium acetates were dissolved separately in methanol. 2.5x as much propionic acid by volume was used to dissolve the bismuth acetate compared to the amount of methanol used to dissolve the other separate cation acetates. Next, an appropriate weight of the Bi, Na, and K precursor solutions was carefully dropped into the Ti solution using a syringe to achieve a 0.5M final solution molarity. To compensate for the high volatility of the cations, varying amounts of excess cations were added when making the solutions (overdoping). In particular, excesses were as follows: 8% Bi, 16% Na, 16% K.

General Method for Thin Film Preparation

The Pt substrates were first ultrasonically cleaned in Acetone, Methanol, and Isopropanol successively for 5 minutes each. To ensure the substrates were dry of all solvents, they were then heated on a hotplate to 300 °C for 1 minute. The substrate was then mounted onto a spin coater (using a vacuum mount) and precursor solution was spread drop-wise across the entire substrate surface. The substrate was then spun at 3000 RPM for 30 seconds. The coated substrate was then placed on a hotplate at 100 °C for one minute as a drying step, then moved to a second hotplate where it was pyrolysed at 350°C for 4 minutes (all steps within a fume hood). Optionally, deposition, drying/pyrolysing steps could be repeated to provide further coating layers. After this, the substrate was placed in an RTP apparatus and heated to an initial hold temperature of 200 °C before being heated to a crystallization temperature of 700 °C using a controlled ramp rate under controlled flow of ultra-high purity (UHP) oxygen (greater than 99.9 vol.% oxygen). The crystallisation temperature of 700 °C was held for 5 minutes before cooling. Optionally, this process could be repeated again on top of the most recent crystallized layer until the desired thickness is achieved.

Particular conditions of the RTP in terms of heating rate and oxygen-containing gas flow rate are addressed in the specific examples below.

Examples 1 to 16

80(Bio _.5 4,Na _0.66 )Ti0 ₃ -20(Bio _.54 ,Ko _.66 )Ti0 ₃ thin films comprising three layers were prepared in accordance with the above general method using a precursor solution containing 8 mol.% excess bismuth cation, and 16 mol.% excess of each of sodium and potassium cations. The concentration of the precursor solutions before spinning in each case is 0.5 molar and is comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume.

In some examples (Examples 1 , 3, 5, 7, 9, 11 and 13 to 16), crystallisation was undertaken after each round of coating of precursor solution (i.e. each round of deposition, drying/pyrolysing) to form a three layer film. In other examples (Examples 2, 4, 6, 8, 10 and 12), two further rounds of spin coating, drying and pyrolising were undertaken before crystallisation of the three layer coating was completed. Various heating ramp rates were adopted for RTP, which was conducted under the flow of oxygen at various volumetric flow rates, as indicated in Table 1 below.

Table 1

X-ray diffraction analysis was completed for a number of the resulting three-layer films of the above Examples. Results of the analysis of the films of Examples 1 , 2, 5, 6, 11 and 12 are provided in Figure 1a; those results for Examples 1 , 3, 5, 7, 9 and 11 (prepared with crystallisation after each coating) are provided in Figure 1 b; and those results for Examples 13 to 16 are provided in Figure 1c. Atomic force microscopy images (1 pm and 5 pm) for the films of Examples 3, 5, 7, and 9 (prepared with crystallisation after each coating) were obtained, and are shown in Figures 2a-h. In addition, Double Beam Laser Interferometry (DBLI) analysis using an AixACCT Piezoelectric Characterization System was conducted for the films of Examples 1 , 3, 5, 7, 9, and 11 and results in the form of graphs of electromechanical strain and polarisation measured at 1 kHz and at fixed temperature versus electric field strength are presented in Figures 3a-f. Determinations of dielectric constant (e _G ) and dielectric loss (tan d) for the films of Examples 1 , 3, 5, 7, 9, and 11 were also were measured over a range of frequencies using an HP 4192A LCR Meter at an oscillation level of 50 mV and the results are provided in Figures 4a-f. A summary of these results is also provided in Table 2 below (dielectric constant (e _G ) and dielectric loss (tan d) values at 1 kHz provided therein).

Table 2

As can be seen from the X-ray diffraction patterns of Figures 1a and 1b, crystallisation after each coating or once after three coatings had no impact on the perovskite phase purity exhibited by the films. The X-ray diffraction patterns of Figure 1c also show that a phase pure sample was prepared using a 2 SLPM oxygen flow rate in the RTP. Other volumetric flow rates were able to provide thin films having a substantial majority of the desired perovskite phase but without 100% phase purity.

The 1 pm square atomic force microscopy images of three layer films crystallised after each coating layer shown in Figures 2a-h show that optimal film morphology is seen for the 100 °C/s ramp rate. This is judged by lace of surface features and the clearest morphology with the largest grain size.

Figure 5 shows a plot of d33* values for the different ramp rates used in Examples 1 , 3, 5, 7, 9, and 11 , as shown in T able 2 above. Figure 5 further demonstrates the surprising improvement in terms of piezoelectric properties in the films prepared using a ramp rate in accordance with the present invention, and particularly at a ramp rate of 100 °C/s where the d33* value is maximised.

Examples 17 and 18

80(Bio _.5 4,Na _0.66 )Ti0 ₃ -20(Bio _.54 ,Ko _.66 )Ti0 ₃ thin films comprising three layers were prepared in accordance with the above general method except that various hold times for crystallisation were tested as indicated in Table 3 below. The precursor solution used contained 8 mol.% excess bismuth cation, and 16 mol.% excess of each of sodium and potassium cations. The concentration of the precursor solutions before spinning in each case was 0.5 molar and was comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume. A heating ramp rate of 100 °C/s was used for the RTP and without a flow of oxygen.

Table 3

Examples 19 to 22

80(Bio _.5 4,Na _0.66 )Ti0 ₃ -20(Bio _.54 ,Ko _.66 )Ti0 ₃ thin films comprising three layers were prepared in accordance with the above general method. The precursor solution used contained 8 mol.% excess bismuth cation, and 16 mol.% excess of each of sodium and potassium cations. The concentration of the precursor solutions before spinning in each case was 0.5 molar and was comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume. A heating ramp rate of 100 °C/s was used for the RTP and various oxygen volumetric flow rates adopted, as indicated in Table 4 below. Table 4

Examples 23 to 25 80(Bio _.5 4,Na _0.66 )Ti0 ₃ -20(Bio _.54 ,Ko _.66 )Ti0 ₃₃ thin films comprising three layers were prepared in accordance with the above general method. Different precursor solutions were investigated with different cationic excesses, as indicated in Table 5 below. The concentration of the precursor solutions before spinning in each case was 0.5 molar and was comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume. A heating ramp rate of 100 °C/s was used for the RTP and an oxygen volumetric flow rate of 2 SLPM adopted.

Table 5

Examples 26 to 29

80(Bio _.5 4,Na _0.66 )Ti0 ₃ -20(Bio _.54 ,Ko _.66 )Ti0 ₃ thin films comprising three layers were prepared in accordance with the above general method except that various crystallisation temperatures were investigated as indicated in Table 6 below. The precursor solution used contained 6 mol.% excess bismuth cation, and 12 mol.% excess of each of sodium and potassium cations. The concentration of the precursor solutions before spinning in each case was 0.5 molar and was comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume. A heating ramp rate of 100 °C/s was used for the RTP and an oxygen volumetric flow rate of 2 SLPM adopted.

Table 6

Examples 30 and 31

80(Bio _.5 4,Na _0.66 )Ti0 ₃ -20(Bio _.54 ,Ko _.66 )Ti0 ₃ thin films comprising three layers were prepared in accordance with the above general method except that various pyrolysis temperatures were investigated as indicated in Table 7 below. The precursor solution used contained 6 mol.% excess bismuth cation, and 12 mol.% excess of each of sodium and potassium cations. The concentration of the precursor solutions before spinning in each case is 0.5 molar and is comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume. A heating ramp rate of 100 °C/s was used for the RTP and an oxygen volumetric flow rate of 2 SLPM adopted.

Table 7

Examples 32 to 59

72.5BNT-22.5BKT-5BMgT thin films comprising three layers were prepared in accordance with the above general method except that various pyrolysis temperatures and different cationic excesses were investigated, as indicated in Tables 8a-d below. The concentration of the precursor solutions before spinning in each case was 0.5 molar and was comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume. A heating ramp rate of 100 °C/s was used for the RTP and an oxygen volumetric flow rate of 2 SLPM adopted.

Table 8a

Table 8b

Table 8c

Table 8d

Examples 60 to 63

72.5BNT-22.5BKT-5BMgT thin films comprising three layers were prepared in accordance with the above general method except that various pyrolysis durations were investigated as indicated in Table 9 below. The precursor solution used contained 4 mol.% excess bismuth cation, and 8 mol.% excess of each of sodium and potassium cations. The concentration of the precursor solutions before spinning in each case was 0.5 molar and was comprised of a solvent mixture of approximately 50% propionic acid, 40% methanol and 10% acetic acid by volume. A heating ramp rate of 100 °C/s was used for the RTP and an oxygen volumetric flow rate of 2 SLPM adopted. Table 9