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Definitions

• the present invention relates to a new process for the manufacture of bismuth sodium titanate (BNT) based materials.
• BNT products of the process are also encompassed by the invention, as is their use in the manufacture of electronic and optic products, for application in devices such as transducers, ultrasound diagnostic equipment, and piezoelectric transformers, actuators and sensors.
• Ferroelectric materials have many important applications as functional materials in electronics and optics.
• a large number of ferroelectric ceramics exploit properties that are an indirect consequence of ferroelectricity, such as dielectric, piezoelectric, pyroelectric and electro-optic properties.
• dielectric, piezoelectric, pyroelectric and electro-optic properties With the development of ceramic processing and thin film technology, many new applications have emerged.
• the biggest uses of ferroelectric ceramics have been in areas such as dielectric ceramics for capacitor applications, ferroelectric thin films for non-volatile memories, piezoelectric materials for medical ultrasound imaging and actuators, and electro-optic materials for data storage and displays.
• PZT lead zirconate titanate
• PbZr x Tii- x 0 3 is a binary solid solution of PbZrCb (orthorhombic) and PbTiCb (tetragonal perovskite).
• PZT has a perovskite type structure with the Ti 4+ and Zr 4+ ions occupying the B site at random.
• lead-based ferroelectric compounds include lead niobate, PbNbiCL, which has a tungsten bronze type structure.
• the tungsten bronze family has a more open structure compared with the perovskites and a wide range of cation and anion substitutions are possible without loss of ferroelectricity.
• Lead bismuth niobate (PbBLNbiOs) is another example. This material has a bismuth oxide layered type structure consisting of comer-linked perovskite-like sheets separated by (Bi Cf) 2- layers. The plate like crystal structure of these compounds leads to highly anisotropic ferroelectric properties. However, the piezoelectric properties are not good because of a very low poling efficiency.
• Poling is a process used to induce piezoelectric behaviour in a ferroelectric ceramic by applying a direct current electric field at a high temperature that is below the Curie point of the material.
• the spontaneous polarisation within each domain of the ceramic is orientated in the direction of the applied field, leading to a net polarisation in the poling direction, but the domains in a ceramic cannot fully align along the poling axis because the orientation of the polarisation is restricted by the symmetry of the crystal structure.
• BNT has the general formula (Bio .5 Nao .5 )Ti0 3 and is a perovskite-type ferroelectric material. It has a high Curie temperature (T c ) of 320 °C and a large remanent polarisation (P r ) of 38 mC/cm 2 at room temperature. BNT -based materials are particularly attractive candidates for use in lead-free piezoelectric/ferroelectric materials.
• BNT is typically synthesized by low-cost and industrial scale solid state reaction of bismuth oxide, sodium carbonate and titanium oxide.
• the main challenge is the provision of single-phase, fine-grained, reproducible and dense ceramics of given
• solution based precursor employed during spray pyrolysis enables atomic scale homogeneity, rendering low temperature post-treatment sufficient for homogeneous cation distribution in the final ceramic.
• Lower temperature and shorter holding times during heat treatments renders the stoichiometric and homogeneous control obtained by spray pyrolysis unmatched by competing BNT production technologies.
• the synthesis route of the present invention starts with a stable solution of cations to build up the material.
• spray pyrolysis is used to directly prepare the oxide powder.
• spray pyrolysis a large scale preparation of high quality powder is possible.
• the process of the present invention can be applied to form a vast range of BNT materials.
• the invention provides a process for the preparation of a bismuth sodium titanate (BNT) compound of formula (I);
• B is one or more of Ti, Nb, Ta, Zr, Fe, Nd, Eu and Co;
• said process comprising spray pyrolysis of a solution comprising Bi ions, Na ions, Ti ions and, if present, metal (A) and/or metal (B) ions.
• the invention provides a BNT compound of formula (I) prepared by a process as hereinbefore defined.
• the invention provides a ferroelectric composition comprising a BNT compound as hereinbefore defined.
• the invention provides the use of a ferroelectric composition as hereinbefore defined in the manufacture of electronic and/or optic devices.
• the invention provides a piezoelectric transformer comprising a BNT compound as hereinbefore defined.
• the invention provides a process comprising sintering a BNT compound of formula (I) as hereinbefore defined in air.
• the invention relates to a process for the preparation of a bismuth sodium titanate (BNT) and BNT materials produced by that process.
• the BNT materials of the invention are represented by the general formula
• A is one or more of Bi, Na, Li, K, Mg, Ca, Sr, Ba, La, Al, Cu, Eu, Ag and Zn;
• B is one or more of Ti, Nb, Ta, Zr, Fe, Nd, Eu and Co;
• General formula (I) covers both stoichiometric and non stoichiometric compounds.
• the cations and anions are generally present in such ratios that a stable perovskite structure is formed.
• the compounds of the invention are essentially stoichiometric.
• the invention will therefore be further defined with reference to stoichiometric compounds.
• a and B may be present as a single metal or a mixture of two or more metals.
• a and B are each a single metal.
• the compound of formula (I) can be doped such that the value of x is between 0 and a maximum of 0.5, more preferably between 0 and 0.3. It is another aspect of the invention that the compound of formula (I) can be doped such that the value of y is between 0 and a maximum of 0.5, more preferably between 0 and 0.3.
• Preferred doping metal ions (A) are Bi, Na, Li, K, Ca, Sr or Ba, especially Bi, Na, Li, K or Ba, such as Ba or K.
• Preferred doping metal ions (B) are Ti, Nb, Ta or Zr, especially Nb.
• metal ion A can be a metal or a mixture of two or more metals selected from Ba, Li, Na and K.
• the metal is Ba, Na or K, or a mixture of Ba, Na and K, and most preferably a mixture of Ba, Na and K.
• alkali metals e.g. Na and K
• the preferred molar ratio of each metal is in the range 0.4:0.6 to 0.6:0.4, more preferably 0.45:0.55 to 0.55:0.45, most preferably 1 : 1.
• Particularly preferable sub-groups of formula (I) are: (Bio .5-z Nao .5 )i- 3 ⁇ 4z 4 x Tii-yS 0 3 , where A is one or more of Bi, Na, Li, K, Ca, Sr, Ba; B is one or more of Ti, Nb, Ta, Zr; 0 ⁇ x ⁇ 0.5; 0 ⁇ y ⁇ 0.5; and -0.1 ⁇ z ⁇ 0.1; or
• A is one or more of Bi, Na, Li, K, Ba
• B is one or more of Ti, Nb, Ta, Zr; 0 ⁇ x ⁇ 0.3; 0 ⁇ y ⁇ 0.3; and -0.1 ⁇ z ⁇ 0.1.
• the process of the invention involves the spray pyrolysis of a solution containing all the metal ions required to form the desired compound of formula (I).
• the invention relies therefore on the formation of a solution of the metal ions necessary to form the BNT material of formula (I). It is therefore necessary to provide soluble metal salts of each reactant metal ion.
• any convenient metal salt can be used if it is soluble.
• Preferred metal salts are nitrates, alkoxides, carboxylates, ketonates, oxalates and/or dioxalates, and salts of organic acids, especially citrates and acetates. Particularly preferred are nitrates.
• the solution required can be prepared by dissolving salts in the solvent simultaneously or sequentially or could involve the preparation of one or more precursor solutions containing one or more of the reactant metal ions.
• the skilled man can devise ways of making the solution required.
• the invention utilises precursor solutions of each metal ion required which are subsequently mixed before being subjected to spray pyrolysis.
• Soluble metal salts can be prepared using any convenient counter ion (e.g. a nitrate, chloride, sulphate and so on) or using one or more complexing agents with appropriate solvent.
• Suitable complexing agents could be hexadentate, pentadentate, tetradentate, tridentate or bidentate ligands, in particular amino polycarboxylic acid chelating agents.
• Suitable ligands include EDTA, cyclohexanediamine tetraacetic acid (CDTA), citric acid, malic acid, maleic acid, DOTA, DTPA and ethylene diamine.
• the solution is an aqueous solution, i.e. comprising water (e.g. distilled water).
• aqueous solution i.e. comprising water (e.g. distilled water).
• water e.g. distilled water.
• metal salts are generally readily soluble so all manner of salts can be used here although it is essential that the counter ion/complexing agent used with the metal ion A, B, Bi and/or Ti does not form insoluble salts with any other metal ion present.
• the ions of Bi, Na and Ti and optionally A and B ions can then be mixed in an appropriate ratio (e.g. depending on the molarity of each solution used and the desired stoichiometry of the final product) and the solution which forms can be spray pyrolysed.
• Stirring, sonication or other mixing techniques can again be used to ensure homogeneity of the solution prior to or during spray pyrolysis.
• mixing takes place over a period of at least 6 hours.
• the solvent used throughout the manufacturing process comprises water (e.g. comprises at least 80wt%, such as at least 90 wt% water). Most preferably the solvent consists of water.
• the pressure at the nozzle in the spray pyrolysis process may be in the range 1 to 3 bars. This may be achieved using a nozzle 0.5 to 5 mm, preferably 1 to 2 mm in diameter.
• the atomising gas can be any gas which is inert to the reactant materials, including air.
• Atomisation occurs into a furnace at temperatures of at least 500 °C. Ideally, the temperature is in the range 700 to 1200°C, preferably 750 to 950°C.
• the processes of the invention do not comprise the use of ultrasound at the nozzle.
• the powder which exits the furnace (e.g. at a temperature of approximately 400 to 550°C) can be collected by a cyclone or a filter.
• the particles of this invention can be formed without any deposition surface. We do not need to spray pyrolyse onto a surface to induce particle formation rather, our particles form due to evaporation of the solvents from the atomized droplets, preferably formed by the dual-phase nozzle described below. Deposition is in fact a potential problem as it can lead to scraping of the formed particles from the deposition surface and hence contamination. Deposition onto a surface also leads to the formation of a layer of material on that surface, i.e. to the formation of a film. The present invention primarily concerns the formation of distinguishable particles and not a thin film on a substrate. It is prefered to collect particles in a cyclone thus creating a free flowing powder and not a film.
• the spray pyrolysis method of the invention preferably employs a dual phase nozzle arrangement to ensure atomisation, narrow particle size distribution and high yields.
• the particle precursor materials are provided in the form of an emulsion or solution which is atomised in a two phase nozzle and flows into a hot zone such as a furnace. In the hot zone each droplet is converted to a particle and any solvent/emulsion components etc are decomposed into gas.
• Particle size can be controlled by e.g. varying droplet size. Droplet size is in turn controlled by manipulation of the nozzle outlet.
• mixing means for producing a flow mixture of solution, e.g. aqueous solution, and air, is combined with an atomizing nozzle to provide a nozzle assembly which produces a jet of very fine droplets.
• the nozzle assembly is used in conjunction with a hot zone such as a furnace.
• the mixing means comprises a pipe, external of the hot zone having separate, spaced inlets for the solution and air.
• a reducing diameter nozzle is positioned in the pipe bore between the inlets, for accelerating the air.
• the air contacts the solution and turbulently moves down the pipe bore to produce what is known as a "bubbly flow" mixture.
• the mixture is fed to the nozzle, which is inside of the hot zone.
• the nozzle has an inlet; a first contraction section of reducing diameter for accelerating the flow, preferably to supersonic velocity, whereby the droplets are reduced in size; a diffuser section of expanding diameter wherein the mixture decelerates and a shock wave may be induced; a second contraction section operative to accelerate the mixture more than the first contraction section; and an orifice outlet for producing a jet or spray.
• the nozzle assembly is designed to give droplets in the region of 1 to 10 pm.
• the product of the spray pyrolysis reaction normally contains minor amounts of non-combusted organic material and traces of anions from the salt that is not decomposed which can be removed by calcination (e.g. at 400 to 1200°C, preferably 550 to 1000°C, e.g. about 600 to 800°C).
• calcination e.g. at 400 to 1200°C, preferably 550 to 1000°C, e.g. about 600 to 800°C.
• the powder is usually exposed to high temperature for a period of 1 to 24 hours, preferably 4 to 12 hours. It has been found that the final crystallite size can be controlled by the temperature at which calcination takes place, lower
• the primary particles or crystallites can aggregate to form soft agglomerates. Prior to calcination these agglomerates are often of the order of less than 20 microns and hence still represent a free flowing powder. Post calcination the agglomerates have broken down into smaller aggregates of the primary particles, preferably in the range 10 to 700 nm, especially 20 to 500 nm in diameter.
• the calcination step also improves the crystallinity of the formed powder. It is a preferable feature of the invention therefore that the BNT compound is a crystalline solid.
• the product formed after calcination and optional milling can be used directly.
• the invention provides a BNT obtained by the spray pyrolysis process as hereinbefore defined and after calcination.
• the powder formed after calcination is milled, typically wet milled to reduce its volume. This is not essential; however, milling ensures a more consistent particle size distribution by breaking down any remaining agglomerates into the primary particles or crystallites.
• Any suitable type of milling method can be used, such as jet milling or ball milling. Milling can also be effected in the presence of water, ethanol, isopropanol, acetone or other solvent. Yttria-stabilized zirconia is a typical grinding medium. Milling can also be effected by impacting particles by a jet of compressed air or any inert gas.
• Powders formed by this process are single phase and are typically submicron in size and non agglomerated.
• Doping of the BNT materials formed by this spray pyrolysis process can be achieved readily be employing a soluble doping metal ion salt or complex in an appropriate amount.
• the soluble doping metal ion compound can be added into to a precursor solution of metal ions or to the mixed solution prior to spray pyrolysis in an appropriate amount depending on the amount of doping meal ion which is desired to be present. It may obviously be necessary to reduce the amount of one or more of the other precursor solution or other metal salts to reflect the amount of doping metal ion being added.
• a convenient way of introducing the doping metal ions into solution is through their nitrates as these are often soluble in water.
• a solution of barium nitrate or barium nitrate solid can be added to the mixed solution.
• the compounds formed by the process of the invention are thus preferably ferroelectric, preferably piezoelectric.
• Ferroelectricity is a spontaneous electric polarization of a material that can be reversed by the application of an external electric field.
• Piezoelectricity is the ability to generate an electric potential in response to applied mechanical stress.
• the compounds of the invention are lead free and represent environmentally acceptable alternatives to the materials used today as piezoceramics.
• the invention provides a ferroelectric, preferably piezoelectric, BNT compound of formula (I) formed by the process as hereinbefore defined.
• the invention provides a ferroelectric, preferably piezoelectric, composition comprising the BNT compound of formula (I) formed by the process as hereinbefore defined. It is preferable that the ferroelectric composition according to the invention will comprise 10-50 wt % of compound (I), more preferably 15-30 wt. %, most preferably 20-25 wt.%, relative to the total weight of the composition as a whole.
• sintering of the compounds may be required. Pressing and sintering can be carried out using known conditions. For example, pressing is typically carried out at ambient temperature in any standard press. The sintering temperature is highly dependent on the
• the sintering has to be done close to the solidus temperature. Sintering can therefore occur at temperatures up to the solidus temperature of the compound, for example up to 1400°C, preferably 800 to 1250°C, especially less than 1225°C. Sintering can last 0 to 20 hrs, e.g. 2 to 12 hrs.
• the particles of the intention are preferably converted into a green body and then sintered.
• the green body can be formed by any convenient techniques such as by pressing or by casting or the like.
• the sintering process of the invention is carried out in air.
• the invention provides a compound of formula (I) as hereinbefore defined obtainable by a process in which the compound is sintered in air, e.g. to have a density relative to its theoretical density of at least 92%.
• the inventors have surprisingly found that it is possible to produce a solid solution of bismuth sodium titanates of Formula (I) with typical particle sizes in the nanometer range. These particles are of interest as potential replacements for PZT as they are easier to sinter.
• the expected perovskite crystal structure should give rise to a good poling efficiency and thereby good piezoelectric properties.
• the present invention also comprises therefore the use of the compounds of the invention for manufacturing electronic and optic devices, including medical diagnostic devices such as ultrasound devices, and/or piezoelectric transformers.
• Such devices comprising the afore-mentioned ferroelectric composition are also encompassed by the invention.
• Preferred devices are in particular transducers, ultrasound diagnostic equipment, gas ignitors, accelerometers, displacement transducers, actuators, sensors and piezoelectric transformers.
• said ferroelectric composition can further comprise a polymer.
• the polymer should have a density lower than the ferroelectric compound. Examples are polyvinylidene fluoride, poly(vinylidene fluoride) hexafluoroproylene or epoxy polymers.
• Figure 1 X-ray diffractogram ofBNT-BT material
• Powder X-ray diffractograms were obtained from 20 to 70° with a Bruker D2 Phaser, equipped with a Lynxeye detector and Ni- and Cu-filters using CuKcr radiation accelerated at 40kV and 40 mA.
• Scanning electron micrographs were captured on a field emission gun SEM (Zeiss Ultra 55, Limited Edition) using the in-lens secondary electron detector.
• the Bi-cation precursor solution was left stirring at room temperature overnight (12 hours).
• ME ammonia
• the Ba-cation precursor solution was left stirring at room temperature overnight (12 hours).
• concentrations of the Ti-solution and a commercial sodium hydroxide, NaOH (Alfa Aesar) were accurately measured by thermogravimetric analysis.
• appropriate amount of each solutions Ti, Na, Bi and Ba
• pH adjustment with 25% ammonia is to -7 at each step.
• the solution was left stirring overnight (12 hours) at room temperature and resulted in a homogeneous aqueous
• Bio .5076 Nao .47 Bao . o 6 Ti0 3 solution which was spray pyrolyzed using a pilot scale equipment.
• the solution was fed into a water-cooled lance at a rate of 200 ml/min and atomised in a nozzle with aid of pressurised air.
• the droplets were transported through the hot zone in a furnace tube, at a set temperature of 900 °C, by an underpressurised air stream where the time of flight inside the furnace tube was less than one second.
• the product, called green powder was collected in a cyclone and phase purity is documented in the x-ray diffractograms given in Figure 1.
• the green powder was calcined at 600 °C for 6 h to remove residues and increase crystallinity.
• the calcined powder was further dry milled for 30 minutes to reduce the volume and further ball milled for 24 h in 2-propanol with yttria-stabilized zirconia as grinding media.
• the resulting single phase powder was a submicron non-agglomerated powder as shown in the SEM micrograph of Figure 2.
• aqueous precursor solutions were prepared and mixed in stoichiometric proportions in a similar manner as for Example 1.
• the precursor solution was spray pyrolyzed as described under Example 1 and the resulting green powder was treated in a similar manner.
• Dry powder uniaxial pressing of green bodies is the most relevant industrial preparation method for bulk ceramic materials.
• BNT-BT powders were pressed into cylindrical bodies of various dimensions (5 to 15 mm in diameter and 1 to 20 mm in height) using a compacting force of 75 Mpa.
• double acting stainless-steel dies were employed.
• the die walls were coated with an ethanol-stearic acid solution and allowed to dry, leaving stearic acid as a lubricant.
• the press was left for 2 minutes before releasing the force.
• the green bodies were finally removed from the die by employing the press to push the bottom punch steadily.
• the samples were sintered in air with heating rate of 200 to 400 °C/h up to temperatures in the range 950-1200 °C for 2 to 6h.
• Figure 3 shows the surface microstructure of a BNT-BT pellet of 5 mm in diameter sintered at 1200 °C for 2h.
• the aqueous precursor solutions for Ba, Na, Ti, and Bi cations were prepared and mixed stoichiometrically in a similar manner as for Example 1.
• the aqueous precursor solution of Nb was prepared by dissolving C 4 H 4 NNb0 9 *8H 2 0 in purified water and stirred overnight.
• the exact Nb concentration in the resulting solution was determined thermogravimetrically, and the potassium solution was prepared by dissolving stoichiometric amounts of KNO 3 in purified water.
• appropriate amounts of each solution Ti, Na, K, Nb, Bi and Ba
• the solution was left stirring overnight (12 hours) at room temperature, resulting in a homogeneous aqueous
• aqueous precursor solutions for Na, Ti, and Bi cations were prepared in a similar manner as for Example 1.
• K and Li precursor solutions were prepared by dissolving stoichiometric amounts of KNO3 and LiNCb in purified water. In accordance with the concentrations, appropriate amounts of each solution (Ti, Na,
• the aqueous precursor solutions for Bi, Na and K cations were prepared in a similar manner as for Example 5.
• the Sr solution precursor was made by dissolving a stoichiometric amount of Sr(NC>3)2 anhydrate in purified water. In accordance with the concentrations, stoichiometric amounts of each solution were added in the following sequence: Ti, Na, Bi, K and Sr, and neutralized after NaOH and Bi additions. The solution was left stirring overnight (12 hours) at room temperature, resulting in a homogeneous aqueous Bio .48 Nao .4032 Ko . o768Sro . o4Ti03 solution. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder was treated in a similar manner.

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