PROCESS FOR THE PREPARATION OF BISMUTH SODIUM TITANATE

Abstract

The invention provides a process for the preparation of a bismuth sodium titanate (BNT) compound of formula (I) wherein 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; 0<x<0.8; 0<y<0.8; and −0.1<z<0.1; 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.

Claims

1. A process for the preparation of a bismuth sodium titanate (BNT) compound of formula (I);
(Bi.sub.0.5-zNa.sub.0.5).sub.1-xA.sub.xTi.sub.1-yB.sub.yO.sub.3  (I) wherein 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; 0≤x≤0.8; 0≤y ≤0.8; and −0.1≤z≤0.1; wherein the process comprises spray pyrolysis of a solution comprising Bi ions, Na ions, Ti ions and, when present, metal (A) and/or metal (B) ions.

2. The process of claim 1, wherein: A is one or more of Bi, Na, Li, K, Ca, Sr, and Ba; B is one or more of Ti, Nb, Ta, and Zr; 0≤x≤0.5; 0≤y≤0.5; and −0.1≤z≤0.1.

3. The process of claim 1, wherein: A is one or more of Bi, Na, Li, K, and Ba; B is one or more of Ti, Nb, Ta, and Zr; 0≤x≤0.3; 0≤y≤0.3; and −0.1≤z≤0.1.

4. The process of claim 1, wherein A and B are each a single metal.

5. The process of claim 1, wherein only one of A or B is present.

6. The process of claim 1, wherein the spray pyrolysis is performed by atomizing the solution into a furnace at a temperature of at least 500° C.

7. The process according to any preceding claim of claim 1, wherein said-the solution is an aqueous solution.

8. The process of claim 7, wherein the aqueous solution is prepared from water soluble precursors comprising at least one metal citrate or metal nitrate.

9. The process of claim 1, wherein the BNT compound forms a free flowing powder directly after spray pyrolysis.

10. The process of claim 1, wherein the BNT compound formed directly after spray pyrolysis does not adhere to any surface.

11. The process of claim 1, wherein the spray pyrolysis is carried out using a dual phase nozzle arrangement.

12. The process of claim 1, wherein the particles obtained directly after spray pyrolysis have a perovskite structure.

13. The process of claim 1, wherein the spray pyrolysis produces a fine powder product and the process further comprises: collecting the fine powder product by cyclone; and calcining the fine powder product at a temperature in the range of 400-1200° C.

14. The process of claim 13 wherein the calcination is carried out at a temperature of 550 to 1000° C.

15. The process of claim 13, further comprising: milling the fine powder product after calcination.

16. A BNT compound of formula made by the process of claim 1.

17. A ferroelectric composition comprising the BNT compound of claim 16.

18. A method of use of the ferroelectric composition of claim 17, the method comprising using the ferroelectric composition in the manufacture of electronic and/or optic devices.

19. A piezoelectric transformer comprising the BNT compound of claim 16.

20. The process of claim 1, further comprising: sintering the BNT compound of formula (I) in air.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0072] FIG. 1: X-ray diffractogram of BNT-BT material

[0073] FIG. 2: SEM image of BNT-BT powder

[0074] FIG. 3: SEM of sintered BNT-BT pellet

[0075] FIG. 4: Strain and polarisation curves of BNT-BT material

EXAMPLES

Test methods

X-Ray Diffraction

[0076] 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 CuKa radiation accelerated at 40 kV and 40 mA.

SEM

[0077] Scanning electron micrographs were captured on a field emission gun SEM (Zeiss Ultra 55, Limited Edition) using the in-lens secondary electron detector.

Strain & Polarization

[0078] A piezoelectric testing system (TF Analyzer 2000, aix ACCT Systems GmbH) was used to investigate strain and polarization behaviour of the pellet samples in relation to an applied electric field. Ferroelectric hysteresis (P-E) loops were measured by linear bipolar 0.1 Hz frequency cycling.

Example 1 Bi.SUB.0.5076.Na.SUB.0.47.Ba.SUB.0.06.TiO.SUB.3 .(BNT-BT) Powders Prepared by Spray Pyrolysis

[0079] A ratio of 1:2 titanium isopropoxide, C.sub.12H.sub.28O.sub.4Ti, and citric acid, C.sub.6H.sub.8O.sub.7, was dissolved in distilled water in molar concentrations ˜2 M to prepare the Ti-cation precursor solution. Bismuth citrate, C.sub.6H.sub.5BiO.sub.7, and ethanolamine, C.sub.2H.sub.7NO, were mixed in a ratio 1:1.5 in distilled water in molar concentrations ˜1 M. pH was increased to 7-8.5 with 25% ammonia (NH.sub.3) until suspensions begins to clear. The Bi-cation precursor solution was left stirring at room temperature overnight (12 hours). A ratio of 1:1:2 barium nitrate, Ba(NO.sub.3).sub.2, EDTA, C.sub.10H.sub.16N.sub.2O.sub.8, and citric acid, C.sub.6H.sub.8O.sub.7, was dissolved in distilled water in molar concentrations ˜0.2 M. pH was increased to 6-7 with 25% ammonia (NH.sub.3) until suspensions begins to clear. The Ba-cation precursor solution was left stirring at room temperature overnight (12 hours). The concentrations of the Ti-solution and a commercial sodium hydroxide, NaOH (Alfa Aesar) were accurately measured by thermogravimetric analysis. In accordance with the concentration of the Ti and Na-solutions, appropriate amount of each solutions (Ti, Na, Bi and Ba) were sequentially added with pH adjustment with 25% ammonia (NH.sub.3) to ˜7 at each step. The solution was left stirring overnight (12 hours) at room temperature and resulted in a homogeneous aqueous Bi.sub.0.5076Na.sub.0.47Ba.sub.0.06TiO.sub.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 FIG. 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 FIG. 2.

Example 2 Bi.SUB.0.5.Na.SUB.0.5.TiO.SUB.3 .(BNT) Powders Prepared by Spray Pyrolysis

[0080] The 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.

Example 3 The Fabrication of BNT-BT Pellets and Sintering

[0081] 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. For all pressing experiments, 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 6 h. FIG. 3 shows the surface microstructure of a BNT-BT pellet of 5 mm in diameter sintered at 1200° C. for 2 h.

Example 4 Bi.SUB.0.46.Na.SUB.0.47.K.SUB.0.01.Ba.SUB.0.06.Ti.SUB.0.98.Nb.SUB.0.02.O.SUB.3 .or 0.92Bi.SUB.0.5.Na.SUB.0.5.TiO.SUB.3.-0.06BaTiO.SUB.3.-0.02K.SUB.0.5.Na.SUB.0.5.NbO.SUB.3 .(BNT-BT-KNN) Powders Prepared by Spray Pyrolysis

[0082] 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.sub.4H.sub.4NNbO.sub.9*8H.sub.2O 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.sub.3 in purified water. In accordance with the concentrations, appropriate amounts of each solution (Ti, Na, K, Nb, Bi and Ba) were sequentially added and neutralized. The solution was left stirring overnight (12 hours) at room temperature, resulting in a homogeneous aqueous Bi.sub.0.46Na.sub.0.5K.sub.0.01Ba.sub.0.06Ti.sub.0.98Nb.sub.0.02O.sub.3 solution. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder was treated in a similar manner.

Example 5 Bi.SUB.0.5.Na.SUB.0.385.K.SUB.0.1.Li.SUB.0.015.TiO.SUB.3 .or 0.77Bi.SUB.0.5.Na.SUB.0.5.TiO.SUB.3.-0.2Bi.SUB.0.5.K.SUB.0.5.TiO.SUB.3.-0.03Bi.SUB.0.5.Li.SUB.0.5.TiO.SUB.3 .(BNT-BKT-BLT) Powders Prepared by Spray Pyrolysis

[0083] The 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 KNO.sub.3 and LiNO.sub.3 in purified water. In accordance with the concentrations, appropriate amounts of each solution (Ti, Na, K, Li and Bi) were sequentially added and neutralized. The solution was left stirring overnight (12 hours) at room temperature, resulting in a homogeneous aqueous Bi.sub.0.5Na.sub.0.385K.sub.0.1Li.sub.0.015TiO.sub.3 solution. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder was treated in a similar manner.

Example 6 Bi.SUB.0.48.Na.SUB.0.4032.K.SUB.0.0768.Sr.SUB.0.04.TiO.SUB.3 .or 0.96Bi.SUB.0.5.(Na.SUB.0.84.K.SUB.0.16.).SUB.0.5.TiO.SUB.3.-0.04SrTiO.SUB.3 .(BNKT-ST) Powders Prepared by Spray Pyrolysis

[0084] 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(NO.sub.3).sub.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 Bi.sub.0.48Na.sub.0.4032K.sub.0.0768Sr.sub.0.04TiO.sub.3 solution. The precursor solution was spray pyrolyzed as described in Example 1 and the resulting green powder was treated in a similar manner.