PRODUCTION OF LEAD-FREE PIEZOCERAMICS IN AQUEOUS SURROUNDINGS

Abstract

The invention relates to a method for producing ceramics having piezoelectric properties in predominantly aqueous suspending agents.

Claims

1. A method for producing a ceramic having piezoelectric properties, wherein predominantly aqueous suspending agents are used.

2. The method for producing a ceramic having piezoelectric properties according to claim 1, wherein, in a first method step, the raw materials are mixed in predominantly aqueous suspending agents and are milled, wherein a suspension having isotropic distribution is created.

3. The method for producing a ceramic having piezoelectric properties according to claim 2, wherein the isotropic distribution of the suspension is fixed in the subsequent method step.

4. The method for producing a ceramic having piezoelectric properties according to claim 3, wherein the fixing of the isotropic distribution takes place by freezing of the suspension.

5. The method for producing a ceramic having piezoelectric properties according to claim 4, wherein the freezing takes place in a liquid, solid or gaseous freezing medium.

6. The method for producing a ceramic having piezoelectric properties according to claim 5, wherein the temperature of the freezing medium is below the melting temperature of the suspension, and preferably far below the melting temperature.

7. The method for producing a ceramic having piezoelectric properties according to claim 6, wherein the suspension is flash-frozen.

8. The method for producing a ceramic having piezoelectric properties according to claim 5, wherein the freezing medium is selected from the group consisting of liquid or gaseous nitrogen, liquid or gaseous air, liquid or gaseous oxygen, or other liquid or gaseous organic or inorganic media.

9. The method for producing a ceramic having piezoelectric properties according to claim 4, wherein the freezing takes place by way of injection into a freezing medium, whereby protogranules are created.

10. The method for producing a ceramic having piezoelectric properties according to claim 3, wherein the fixing of the isotropic distribution takes place by way of physical methods, such as pressure changes, solid media or the like, by way of gelling processes, by way of flocculation of the dispersion, or by the addition of additives.

11. The method for producing a ceramic having piezoelectric properties according to claim 1, wherein the suspending agent is removed under exclusion of capillary forces and without any noteworthy liquid phase being created.

12. The method for producing a ceramic having piezoelectric properties according to claim 11, wherein the removal of the suspending agent takes place by way of sublimation.

13. The method for producing a ceramic having piezoelectric properties according to claim 3, wherein the fixed isotropic distribution is transferred into a desired piezoelectric ceramic material by way of a reaction process.

14. The method for producing a ceramic having piezoelectric properties according to claim 13, wherein the reaction process is a temperature treatment (calcination), whereby a calcinate is obtained.

15. The method for producing a ceramic having piezoelectric properties according to claim 14, wherein a compressible powder is produced from the calcinate by way of a granulation process.

16. The method for producing a ceramic having piezoelectric properties according to claim 15, wherein the granulation process is the spray drying or the spray freeze granulation.

17. A suspension for producing a ceramic having piezoelectric properties, wherein the suspension comprises 40 to 60% ceramics, preferably selected from bismuth sodium titanate (BNT), bismuth sodium titanate-barium titanate (BNT-BT), potassium sodium niobate (KNN) or bismuth sodium titanate-potassium sodium niobate (BNT-KNN) ceramic, or mixtures thereof, 40 to 60% water, and <20%, preferably <5% (organic) additives.

Description

EXAMPLE 1

[0029] The method is carried out based on the following method steps: 1) Production of the Bi.sub.2O.sub.3Na.sub.2CO.sub.3TiO.sub.2 Suspension

[0030] Bi.sub.2O.sub.3, Na.sub.2CO.sub.3, TiO.sub.2, water and grinding balls were weighed in a container. This suspension was subsequently homogenized or deagglomerated.

[0031] 2) Spray Freeze Granulation of the Bi.sub.2O.sub.3Na.sub.2CO.sub.3TiO.sub.2 Suspension

[0032] The ceramic suspension was sprayed in trays filled with liquid nitrogen and thereby flash-frozen. The slip was supplied by way of a hose connection via the reservoir. The product trays were filled with liquid nitrogen at the start of every spraying process. Due to evaporation of the liquid nitrogen, the fill level was checked regularly between the spray units, and nitrogen was replenished if needed. This ensured that the frozen droplets were continuously covered with liquid nitrogen, so as to prevent the slip from melting again. After the spraying process, the product trays containing the frozen slip droplets and the liquid nitrogen were placed in a freeze dryer. As soon as the liquid nitrogen had fully evaporated, except for minor residue, the freeze-drying process was started, which is to say the pressure in the freeze dryer was lowered. A time of approximately 24 h was established for the main drying process of the granules. A subsequent drying step was dispensed with.

[0033] 3) Production of the Bi.sub.2O3-Na.sub.2CO.sub.3TiO.sub.2BaTiO.sub.3 Powder Mixtures

[0034] For the Bi.sub.2O3-Na.sub.2CO.sub.3TiO.sub.2BaTiO.sub.3 powder mixtures, the spray freeze granules and the BaTiO.sub.3 powder were weighed in different proportions (see Table 1) in containers. After the addition of several grinding balls, the containers were placed on a cylinder mill and mixed until dry so as to generate a homogeneous powder mixture.

TABLE-US-00001 TABLE 1 Molar ratios of the raw materials Molar ratio BT BNT 0.055 0.945 0.06 0.94 0.065 0.935 0.07 0.93 0.075 0.925 0.08 0.92 0 1

[0035] 4) Calcination

[0036] The Bi.sub.2O.sub.3Na.sub.2CO.sub.3TiO.sub.2BaTiO.sub.3 powder mixtures were distributed among trays. The calcination of the powder mixtures to yield BNT-BT was carried out at 900? C.

[0037] 5) BNT-BT Slip Production

[0038] Following the calcination, the BNT-BT powders were milled using a planetary ball mill. After the grinding ball had been separated, organic additives were added to the slips. The slips were homogenized again on the cylinder mill.

[0039] 6) Spray Freeze Granulation of the BNT-BT Slip

[0040] Analogous to step 2) 7) Pressing of the BNT-BT Components

[0041] The compacts were produced with the aid of a uniaxial press using a pressing pressure of approximately 200 MPa.

[0042] 8) Debinding and Sintering

[0043] The green compacts were debound at 800? C. Thereafter, the pressed bodies were placed in capsules and sintered.

[0044] 9) Metallizing and Polarizing

[0045] The thickness of the sintered tablets was set to 1 mm by grinding of the surfaces. The metallization on both sides took place using a silver paste, which was applied by way of screen printing and baked at 700 to 900? C. For polarization, a trapezoidal voltage profile was applied, wherein the maximum voltage was varied between 2 and 5 kV.

[0046] To provide a better understanding, the procedure of Example 1 is shown as a flow chart in FIG. 1.

[0047] In this way, different piezoceramics were produced, the characteristic parameters of which were determined by way of the methods defined in DIN 50324 1-3 (status date: December 2002).

TABLE-US-00002 TABLE 2 Test results for piezoceramics according to the invention BNT-BT6 BNT d.sub.33 (pC/N) 200 87 k.sub.p (%) 30 16 k.sub.t (%) 50 44 ?.sup.T.sub.33/?.sub.0 1000 460 ? (g/cm.sup.3) 5.8 5.8