Method For Preparing A Sol-Gel Solution Which Can Be Used For Preparing A Barium Titanate Ceramic Doped With Hafnium And/or With At Least One Lanthanide Element

Abstract

The invention relates to a method for preparing a sol-gel solution which can be used to prepare a barium titanate ceramic doped with hafnium and/or with at least one lanthanide element, comprising the following steps: a) a step to place a first mixture comprising a barium carboxylate and a diol solvent in contact with a second mixture comprising a titanium alkoxide and a hafnium alkoxide and/or an alkoxide of a lanthanide element in a monoalcohol solvent; b) a step to distil the mixture resulting from step a) to remove at least part of the monoalcohol solvent; c) a step to add acetic acid, under heat, to the distilled mixture of step b).

Claims

1. A method for preparing a sol-gel solution which can be used to prepare a barium titanate ceramic doped with hafnium and/or with at least one lanthanide element, comprising the following steps: a) a step to place a first mixture comprising a barium carboxylate and a diol solvent in contact with a second mixture comprising a titanium alkoxide and a hafnium alkoxide and/or an alkoxide of a lanthanide element in a monoalcohol solvent; b) a step to distil the mixture resulting from step a) to remove at least part of the monoalcohol solvent; and c) a step to add acetic acid, under heat, to the distilled mixture of step b).

2. The method according to claim 1, wherein the barium titanate ceramic is doped solely with hafnium, wherein the second mixture comprises a titanium alkoxide and a hafnium alkoxide.

3. The method according to claim 1, wherein the barium carboxylate is represented by following formula (II)
(RCOO).sub.2Ba(II) where R is an alkyl group having 1 to 3 carbon atoms.

4. The method according to claim 1, wherein the barium carboxylate is barium acetate of formula (CH.sub.3COO).sub.2Ba.

5. The method according to claim 1, wherein the diol solvent is an alkylene glycol having a number of carbon atoms ranging from 2 to 5.

6. The method according to claim 1, wherein the hafnium alkoxide and/or lanthanide element alkoxide and titanium alkoxide contained in the second mixture respectively meet following formulas (II) and (III):
X(OR).sub.4(II)
Ti(OR).sub.4(III) where: X is the hafnium element or lanthanide element; and R is a linear or branched alkyl group, wherein the branched alkyl group has at least 3 carbon atoms.

7. The method according to claim 1, wherein the hafnium alkoxide and titanium alkoxide are hafnium isopropoxide and titanium isopropoxide respectively.

8. The method according to claim 1, wherein the monoalcohol solvent is an aliphatic monoalcohol solvent having 1 to 6 carbon atoms.

9. The method according to claim 1, wherein the monoalcohol solvent has the same number of carbon atoms as each alkoxy ligand (RO) of the alkoxides of formulas (II) and (III) defined in claim 6.

10. The method according to claim 1, wherein the barium titanate doped with hafnium meets following formula (IV):
BaHf.sub.xTi.sub.1-xO.sub.3(IV) where 0.05<x?0.2.

11. The method according to claim 1 which, after step c), further comprises the following steps: d) a step to agitate the sol-gel solution obtained at step c); and e) a step to dilute the sol-gel solution resulting from step d) in a predetermined ratio with a diol solvent the same as the one used at step a).

12. A method for producing a material in barium titanate doped with hafnium and/or with at least one lanthanide element, said method comprising: f) a step to implement the method such as defined in claim 1; g) at least one step to deposit on a substrate a layer of sol-gel solution obtained at step f); and h) a heat treatment step of said layer for conversion thereof to barium titanate doped with hafnium and/or with at least one lanthanide element.

13. A method for producing a powder of barium titanate doped with hafnium and/or with at least one lanthanide element, successively comprising the following steps: a step to implement the method such as defined in claim 1; a gelling step of a sol-gel solution obtained at the preceding step; and a heat treatment step of the resulting gel to obtain said powder.

14. A method for producing a compacted object in barium titanate doped with hafnium, successively comprising the following steps: a step to implement the method for preparing a powder such as defined in claim 13; a step to compact the powder in a mould having a shape corresponding to that of the compacted object it is desired to obtain; and a step to sinter the object obtained above for consolidation thereof.

15. A method for preparing a composite sol-gel solution, comprising the following steps: a step to implement the method for preparing a sol-gel solution such as defined in claim 1; a step to add to said sol-gel solution a powder of barium titanate doped with hafnium and/or with at least one lanthanide element, after which the composite sol-gel solution is obtained consisting of a dispersion in which the powder forms the dispersion phase whilst the sol-gel solution forms the continuous dispersion medium.

16. A method for preparing a material in barium titanate doped with hafnium and/or with at least one lanthanide element, successively comprising the following steps: a) preparing a composite sol-gel solution with a method such as defined in claim 15; b) depositing a layer of a composite sol-gel solution such as defined above via liquid process on a substrate; c) repeating b) one or more times to obtain a stack of at least two layers; d) heat treating said layers for conversion thereof to the corresponding ceramic(s); e) impregnating the stack obtained at c) with a sol-gel solution the same as or differing from that used at a) but is a precursor of a barium titanate ceramic doped with hafnium and/or with at least one lanthanide element; f) optionally repeating step e) one or more times; g) heat treating said stack to convert the sol-gel solution impregnating the stack to the corresponding ceramic.

Description

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

Example 1

[0170] This example illustrates the preparation of pellets of barium titanate doped with hafnium, and more specifically meeting the formula BaH.sub.f0.05Ti.sub.0.95O.sub.3, this preparation comprising the following steps: [0171] a step to prepare a precursor sol-gel solution of barium titanate having the above-mentioned formula (step a); [0172] a step to prepare a powder from the sol-gel solution resulting from step a) (step b); [0173] a step to prepare pellets from the powder obtained at step b) (step c).

a) Preparation of a Precursor Sol-Gel Solution of Barium Titanate of Formula BaH.SUB.f0.05.Ti.SUB.0.95.O.SUB.3

[0174] This step illustrates the preparation of a precursor sol-gel solution of barium titanate of formula BaH.sub.f0.05Ti.sub.0.95O.sub.3 from the following alkoxide precursors: titanium isopropoxide symbolised Ti(OiPr).sub.4, hafnium isopropoxide symbolised Hf(OiPr).sub.4 and barium acetate symbolised Ba(OAc).sub.2.

[0175] In a first round-bottom flask, titanium isopropoxide (6.67 g; 0.0234 mole) and hafnium isopropoxide (0.9 g; 0.00217 mole) are respectively added to isopropanol (5 g; 0.083 mole). The whole is left under agitation for 1 hour for proper dispersion of the hafnium isopropoxide in solution.

[0176] In parallel, a second round-bottom flask surmounted by a distillation assembly is charged with barium acetate (6.48 g; 0.0255 mole)) in ethylene glycol (14.68 g; 0.236 mole) under agitation, to prevent the formation of an insoluble block. To the resulting mixture, the content of the first flask is added after the above-mentioned agitation operation. The whole is then distilled until the temperature of the reaction mixtures reaches 160? C. After distillation, the solution is opaque. It is found under infrared analysis that 90% by weight of the isopropanol has been removed and that esters have been formed.

[0177] Once the temperature has returned to 100? C., acetic acid (14 g; 0.23 mole) is slowly added to the distillate until total dissolution of the non-dissolved precursors and a clear yellow solution is obtained.

[0178] A substantial amount of water is added to the sol-gel solution under agitation to form a gel. The gel is then dried in an oven.

[0179] The powder collected from the oven is ground and calcined at 400? C. to remove residual organic components, sources of carbonation.

[0180] The powder obtained is again ground and annealed this time at 900? C., to obtain a powder of barium titanate doped with crystallized hafnium having a perovskite crystal structure.

[0181] The powder is then subjected to an attrition operation in an attrition drum containing 75 g of powder, 150 g of ethanol and 1600 g of zirconium beads, the resulting mixture being placed under agitation for 20 minutes at 1500 rpm. After this operation, a finer powder is recovered that can be used to form pellets the density of which will be greater after sintering, compared with pellets produced with non-attrited powders.

[0182] The pellets are produced using a uniaxial press. A binder (more specifically polyvinyl alcohol) is added to the powder and the resulting mixture is placed in a 16 mm mould and pressed at 20 MPa. The formed pellets are then sintered at 1500? C. for 5 hours.

[0183] Characterization of these pellets is performed by impedance measurement after polarising the pellets in an oil bath subjected to an electric field (150? C.; 2 kV/mm).

[0184] The piezoelectric properties obtained are of particular interest notably having a k.sub.t value (corresponding to the coupling constant in thickness mode, which approximately corresponds to the ratio between supplied mechanical energy and received mechanical energy) of 0.47 and a dielectric constant of 840 (at 1.8 MHz).

Example 2

[0185] This example illustrates the preparation of pellets in barium titanate doped with hafnium, and more specifically meeting formula BaH.sub.f0.1Ti.sub.0.9O.sub.3, the preparation following similar modalities to those in Example 1 except in respect of the quantities of reagents which are the following: [0186] Barium acetate: 6.48 g; [0187] Titanium isopropoxide: 6.49 g; [0188] Hafnium isopropoxide: 1.2 g; [0189] Ethylene glycol: 14.68 g; [0190] Isopropanol: 5 g; and [0191] Acetic acid: 14 g.

[0192] In addition, the pellets are sintered at a temperature of 1350? C. from powders that have not been subjected to attrition treatment.

[0193] These pellets have a dielectric constant of 1230.

Example 3

[0194] This example illustrates the preparation of a precursor sol-gel solution of barium titanate doped with hafnium of formula BaH.sub.f0.075Ti.sub.0.925O.sub.3, the preparation being carried out following similar modalities to those in Example 1 except in respect of the quantities of reagents which are the following: [0195] Barium acetate: 6.48 g; [0196] Titanium isopropoxide: 6.67 g; [0197] Hafnium isopropoxide: 0.90 g; [0198] Ethylene glycol: 14.68 g; [0199] Isopropanol: 5 g; and [0200] Acetic acid: 14 g.

Example 4

[0201] This example illustrates the preparation of a material in the form of a thin layer from sol-gel solutions prepared as in Examples 1, 2 and 3, and all aged for 45 days and each having a concentration of 0.9 M.

[0202] Before deposition, the sol-gel solution is diluted with ethylene glycol (40 mL) in particular to adapt the wettability and viscosity of the solution to the substrate and to deposition conditions, the substrate in this example being a portion of a platinum-silicon wafer (Si 500 ?m/SiO.sub.2 2.5 ?m/Ti 15 nm/Pt 150 nm) and the deposition technique used in this example being spin coating.

[0203] The sol-gel solution thus diluted is deposited via spin coating for 3 minutes at 3000 rpm and then dried over a hot plate first for 5 minutes at 50? C. and then 10 minutes at 360? C. Several layers (total of 3) are stacked following the same treatment and the resulting stack is heat treated at 600? C. over a hot plate followed by oven annealing (Rapid Thermal AnnealingRTA) for 1 minute at 750? C. The coating obtained does not exhibit any carbonate groups under infrared analysis, indicating the strong density of the coating obtained.

[0204] The coating obtained also displays strong permittivity at 1 kHz, of respectively 200 for a film derived from the sol-gel solution in Example 1 (e=180 nm), 231 for a film derived from the sol-gel solution in Example 3 (e=500 nm), 215 for a film derived from the sol-gel solution in Example 2 (e=400 nm), this being of possible particular interest in the field of microelectronics.

Example 5

[0205] The solution in Example 1 is aged for 45 days and then diluted with ethylene glycol to obtain a solution of 0.4 M concentration. This is obtained by adding 12 mL of ethylene glycol to 10 ml of the aged solution. 50 grams of this solution are mixed with 5.5 grams of barium titanate powder doped with hafnium prepared following the protocol in Example 1. The composite solution is left under agitation for 40 days. Stack deposition is then performed by dip coating at a rate of 5 cm/min (5 layers). Heat treatment at 600? C. (5 minutes) over a hot plate is conducted for each layer. Final annealing is carried out at 900? C. (2 minutes), in an RTA oven on the entire stack.

[0206] The films are then impregnated with a non-loaded sol of 0.5 M concentration. Heat treatment is applied at 600? C., over a hot plate for 1 hour for each impregnation layer. Final annealing in an RTA oven at 900? C. is then carried out for 2 minutes.

[0207] Aluminium electrodes are placed on the composite layer of 45 ?m thickness for electromechanical characterization. The measured coupling coefficient is 0.45 i.e. close to that of the bulk material.