CERAMIC

Abstract

The present invention relates to a ceramic comprising (or consisting essentially of) a solid solution containing Bi, K, Ti and Fe (and optionally Pb) which exhibits piezoelectric behaviour.

Claims

1. A ceramic comprising a solid solution of formula:
x(Bi.sub.aK.sub.1-a)TiO.sub.3yBi[Fe,B]O.sub.3zPbTiO.sub.3 wherein 0.4a0.6; 0<x<1; 0<y<1; 0<z0.5; x+y+z=1; and B is a B-site metal dopant which has a higher valency than the valency of the metal which it substitutes or has a mixed valency, wherein B is present on the B-site in an amount greater than 0 and less than or equal to 50 at %, wherein the ceramic is substantially free of non-perovskite phases, and wherein the average valence of the A-site plus the average valence of the B-site is 6.

2. A ceramic comprising a solid solution of formula:
x(Bi.sub.aK.sub.1-a)TiO.sub.3yBi[Fe,B]O.sub.3zPbTiO.sub.3 wherein 0.4a0.6; 0<x<1; 0<y<1; 0<z0.5; x+y+z=1; and B is a B-site metal dopant selected from the group consisting of Ti, Zr, W, Nb, V, Ta, Mo and Mn, wherein B is present on the B-site in an amount greater than 0 and less than or equal to 50 at %, wherein the ceramic is substantially free of non-perovskite phases.

3. A ceramic comprising a solid solution of formula:
x(Bi.sub.ak.sub.1-a)TiO.sub.3yBi[Fe,B]O.sub.3zPbTiO.sub.3 wherein 0.45a0.6; 0<x<1; 0<y<1; 0<z0.5; x+y+z=1; and B is a B-site metal dopant selected from the group consisting of Ti, Co and Ni, wherein B is present on the B-site in an amount greater than 0 and less than or equal to 50 at %, wherein the ceramic is substantially free of non-perovskite phases.

Description

[0095] The present invention will now be described in a non-limitative sense with reference to Examples and accompanying Figures in which:

[0096] FIG. 1: A plot of theoretical density vs measured density of solid solutions xKBTxBF where x=0.1 to 0.9;

[0097] FIG. 2: X-ray diffraction patterns of solid solutions xKBT1xBF where x=0.1 to 0.6;

[0098] FIG. 3: Permittivity vs temperature plots for solid solutions xKBT1xBF where x=0.4, 0.5 and 0.6;

[0099] FIG. 4: Curie point of solid solutions xKBT1xBF as a function of composition;

[0100] FIG. 5: Strain-field response for various solid solutions xKBT1xBF;

[0101] FIG. 6: Room temperature dielectric constant of solid solutions xKBT1xBF as a function of composition;

[0102] FIG. 7: X-ray diffraction patterns for 0.6BiFe.sub.0.9Co.sub.0.1O.sub.3 0.4Bi.sub.1/2K.sub.1/2TiO.sub.3 and 0.6BiFe.sub.0.8Co.sub.0.2O.sub.3 0.4Bi.sub.1/2K.sub.1/2TiO.sub.3;

[0103] FIG. 8: Strain-field plot for 0.6BiFe.sub.0.8Co.sub.0.2O.sub.3 0.4Bi.sub.1/2K.sub.1/2TiO.sub.3;

[0104] FIG. 9: X-ray diffraction patterns for (a) 0.475BF0.45KBT0.075PT, (b) 0.55 BF0.3 KBT0.15 PT and (c) 0.625 BF0.15 KBT0.225 PT;

[0105] FIG. 10: Polarisation-field loop for 0.55 BF0.3 KBT0.15 PT collected at 0.1 Hz and room temperature;

[0106] FIG. 11: Bipolar strain-field loop for 0.55 BF0.3 KBT0.15 PT collected at 0.1 Hz and room temperature;

[0107] FIG. 12: Unipolar strain-field loop for 0.55 BF0.3 KBT0.15 PT collected at 0.1 Hz and room, temperature; and

[0108] FIG. 13: Permittivity vs temperature plots for 0.55 BF0.3 KBT0.15 PT (labelled 0.15PT) and 0.625 BF0.15 KBT0.225 (labelled 0.225PT).

EXAMPLE 1

Experimental Procedure

[0109] A sample of the solid solution x(Bi.sub.aK.sub.1-a)TiO.sub.3(1x)BiFeO.sub.3 was synthesised using a mixed oxide process at each of nine compositions where x=0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 respectively The end point compositions where x=0 and x=1 were prepared for comparative purposes. The formal compositions were: [0110] x=0.1 0.9BiFeO.sub.30.1(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0111] x=0.2 0.8BiFeO.sub.30.2(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0112] x=0.3 0.7BiFeO.sub.30.3(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0113] x=0.4 0.6BiFeO.sub.30.4(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0114] x=0.5 0.5BiFeO.sub.30.5(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0115] x=0.6 0.4BiFeO.sub.30.6(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0116] x=0.7 0.3BiFeO.sub.30.7(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0117] x=0.8 0.2BiFeO.sub.30.8(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0118] x=0.9 0.1BiFeO.sub.30.9(Bi.sub.0.5K.sub.0.5)TiO.sub.3

[0119] The precursor powders (Bi.sub.2O.sub.3, Fe.sub.2O.sub.3, TiO.sub.2 and K.sub.2CO.sub.3 99.9% purity, Sigma-Aldrich) were dried at 130 C. for 24 hours in order to remove any moisture and to permit accurate weighing. The powders were weighed in the correct proportions to fabricate the target oxides listed above (see Table 1) and ball milled with yttria stabilised zirconia beads in 2-propanol for 17 hours. The resulting slurry was dried under heat lamps whilst stirring and sieved through 300 micron mesh nylon gauze.

TABLE-US-00001 TABLE 1 Weight of Weight of Weight of Weight of Total Total Weight - Composition Bi.sub.2O.sub.3 Fe.sub.2O.sub.3 K.sub.2CO.sub.3 TiO.sub.2 Weight CO.sub.2 (x) (g) (g) (g) (g) (g) (g) 0 148.593 51.047 0 0 200 200 0.1 145.837 47.349 2.277 5.263 200.725 200 0.2 142.524 43.417 4.697 10.858 201.496 200 0.3 138.995 39.229 7.275 16.818 202.317 200 0.4 135.228 34.758 10.0273 23.179 203.193 200 0.5 131.199 29.976 12.971 29.985 204.130 200 0.6 126.878 24.847 16.128 37.282 205.136 200 0.7 122.235 19.334 19.522 45.126 206.217 200 0.8 117.228 13.392 23.180 53.581 207.381 200 0.9 111.816 6.967 27.135 62.723 208.641 200 1.0 105.946 0 31.424 72.637 210.007 200

[0120] The mixture of dried, milled powders was calcined in covered alumina crucibles to induce a chemical reaction to produce the desired perovskite phase. The temperature programme for this step was: heat at 150 C./hour to 800 C., dwell at 800 C. for 4 hours and cool at 300 C./hour to room temperature.

[0121] The powder was sieved through a 300 micron mesh and milled as described above. The powder was then made into pellets by loading 0.6 g into a 10 mm die set to be pressed at 50 MPa.

[0122] Sintering temperatures of 850 C., 950 C., 975 C., 1000 C., 1040 C. and 1050 C. were tried in order to achieve high density ceramics. Heating was carried out at 50 C./hour to 600 C. and then at 300 C./hour to the desired sintering temperature. Cooling from the sintering temperature was carried out at 150 C./hour to 600 and then at 300 C./hour to room temperature. A lower cooling rate was used in order to minimize thermal shock.

[0123] Prior to density, X-ray diffraction, electrical and electromechanical analyses, the sintered pellets were ground flat and parallel to nominally 1 mm thickness.

Results

[0124] The optimum sintering temperatures and resultant densities are shown in Table 2.

TABLE-US-00002 TABLE 2 Optimum sintering temperature/ Composition C. Density/g cm.sup.3 x = 0.1 975 7.5 x = 0.2 975 7.2 x = 0.3 1000 7.1 x = 0.4 1000 6.4 x = 0.5 1040 6.8 x = 0.6 1025 6.4

[0125] The density is also shown in FIG. 1 plotted against the theoretical density calculated from x-ray diffraction analysis.

[0126] X-ray diffraction analysis was carried out on the sintered pellets in order to confirm the crystal structure. The outcome of this analysis is shown in FIG. 2. An interpretation of FIG. 2 shows all the compositions to be a single phase of rhombohedral symmetry. There are no secondary, deleterious non-perovskite phases.

[0127] The Curie point is the temperature at which a ferroelectric material transforms to paraelectric. In the present xKBT1xBF system, this occurs with the transition from a polar rhombohedral structure to a non-polar cubic structure and was measured by plotting the relative permittivity vs temperature (see FIG. 3). A maximum in the relative permittivity vs temperature curve denotes the Curie point. The Curie point of the compositions x=0.1 to 0.4 is such that no peak could be found using this technique (ie the Curie point is >ca. 600 C.).

[0128] In order to determine the Curie point of the compositions x=0.3 and 0.4, the crystal structure was studied as a function of temperature. This showed that the composition x=0.3 has a Curie point of 720 C. and the composition x=0.4 has a Curie point of 700 C.

[0129] From the known Curie point of BiFeO.sub.3, it is assumed that the Curie point for the compositions x=0.1 and 0.2 is between 720 and 820 C. However at these temperatures, K.sub.2O and Bi.sub.2O.sub.3 become volatile and the composition modifies. The data is shown in FIG. 4.

[0130] The piezoelectric activity of the various compositions is shown in FIG. 5. The composition x=0.6 exhibits the optimum piezoelectric activity.

[0131] When driven at the same drive field (7.5 kVmm.sup.1), the piezoelectric activity (defined as the maximum strain/maximum electric field) for each composition is shown in Table 3. No strain was generated for the composition x=0.1 at this electric field.

TABLE-US-00003 TABLE 3 Piezoelectric activity/pm Composition V.sup.1 x = 0.6 330 x = 0.5 130 x = 0.4 90 x = 0.3 45 x = 0.2 25 x = 0.1

[0132] FIG. 6 shows the room temperature dielectric constant as a function of composition.

EXAMPLE 2

[0133] x=0.6 y=0.05 (1x) Bi(Fe.sub.(1-y)Co.sub.y)O.sub.3x(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0134] x=0.6 y=0.1 (1x) Bi(Fe.sub.(1-y)Co.sub.y)O.sub.3x(Bi.sub.0.5K.sub.0.5)TiO.sub.3 [0135] x=0.6 y=0.2 (1x) Bi(Fe.sub.(1-y)Co.sub.y)O.sub.3x(Bi.sub.0.5K.sub.0.5)TiO.sub.3

[0136] The precursor powders (Bi.sub.2O.sub.3, Fe.sub.2O.sub.3, TiO.sub.2, K.sub.2CO.sub.3 and CoO 99.9% purity, Sigma-Aldrich) were dried at 130 C. for 24 hours in order to remove any moisture and to permit accurate weighing. The powders were weighed in the correct proportions to fabricate the target oxides listed below (see Table 4) and ball milled with yttria stabilised zirconia beads in 2-propanol for 17 hours. The resulting slurry was dried under heat lamps whilst stirring and sieved through 300 micron mesh nylon gauze.

TABLE-US-00004 TABLE 4 Weight of Weight of Weight of Weight of Weight of Total Total Weight- Composition Bi.sub.2O.sub.3 Fe.sub.2O.sub.3 K.sub.2CO.sub.3 TiO.sub.2 CoO Weight CO.sub.2 (y) (g) (g) (g) (g) (g) (g) (g) 0.05 126.928 23.614 16.135 37.295 1.167 205.1378 200 (5% Cobalt) 0.10 126.976 22.380 16.140 37.309 2.334 205.140 200 (10% Cobalt) 0.2 127.074 19.908 16.153 37.338 4.671 205.1437 200 (20% Cobalt)

[0137] The mixtures of dried, milled powders were calcined in covered alumina crucibles to induce a chemical reaction to produce the desired perovskite phase. The temperature programme for this step was: heat at 150 C./hour to 800 C. for 4 hours and to cool at 300 C./hour to room temperature.

[0138] The powder was again sieved through a 300 m mesh and milled as described above with 1 wt % Glascol HA40 binder and sieved a final time. The powder was then fabricated into pellets by loading 0.6 g into a 10 mm diameter die and pressed at 30 MPa for 5 minutes. The pellets were then cold isostatic pressed for five minutes at 350 MPa.

[0139] Sintering temperatures of 1000 C. and 1025 C. were attempted in order to obtain high density ceramics. The sintering regime was as follows: 50 C./hour to 600 C. and then at 300 C./hour to the sintering temperatures outlined above. Cooling from the sintering temperature was carried out at 150 C./hour to 600 C. and then at 300 C./hour to room temperature. Cooling rates were lower to minimize thermal shock.

[0140] Prior to density and X-ray diffraction analysis, the pellets were ground flat and parallel to 1 mm in thickness.

[0141] Prior to electrical testing such as strain field loops and permittivity v temperature analysis, the pellets were ground flat and parallel to 0.3 mm.

Results

[0142] FIG. 7 shows X-ray diffraction patterns for 0.6BiFe.sub.0.9Co.sub.0.1O.sub.30.4Bi.sub.1/2K.sub.1/2TiO.sub.3 and 0.6BiFe.sub.0.8Co.sub.0.2O.sub.30.4Bi.sub.1/2K.sub.1/2TiO.sub.3. The patterns are single phase perovskite and showed no secondary non-perovskite phases.

[0143] The strain/electric field response for composition x=0.6, y=0.2 is shown in FIG. 8. Using an applied electric field of 4 kV mm.sup.1, a strain of 0.44% was generated with a high field d.sub.33=1100 pm V.sup.1 which is far higher than is observed in PZT.

EXAMPLE 3

[0144] A sample of the solid solution x(Bi.sub.aK.sub.1-a)TiO.sub.3y(BiFeO.sub.3)(1xy)PbTiO.sub.3 was synthesised using the mixed oxide process described in the previous Examples at each of the following compositions [0145] 0.475BF0.45KBT0.075PT [0146] 0.55BF0.3KBT0.15PT [0147] 0.625BF0.15KBT0.225PT

[0148] The powders were weighed in the correct proportions to fabricate these target oxides (see Table 5).

TABLE-US-00005 TABLE 5 Weight of Weight of Weight of Weight of Weight of Total Total Weight - Bi.sub.2O.sub.3 Fe.sub.2O.sub.3 K.sub.2CO.sub.3 TiO.sub.2 PbO Weight CO.sub.2 Composition (g) (g) (g) (g) (g) (g) (g) 0.475BF- 120.680 28.064 11.505 31.027 12.387 203.663 200 0.45KBT- 0.075PT 0.55BF- 115.058 30.982 7.313 25.356 23.620 202.329 200 0.3KBT- 0.15PT 0.625 BF- 109.937 33.640 3.494 20.189 33.853 201.113 200 0.15KBT- 0.225PT

[0149] X-ray diffraction data are given in FIG. 9.

Results

[0150] Table 6 shows density and sintering temperature for the three compositions.

TABLE-US-00006 TABLE 6 Sintering Composition Temp ( C.) Density (kg/m3) Tc ( C.) 0.475BF-0.45KBT0.075PT 1045 7000 0.55 BF-0.3 KBT-0.15 PT 1035 7250 525 0.625 BF-0.15 KBT-0.225 1025 7300 590

[0151] FIG. 10 is a polarisation-field loop for 0.55 BF0.3 KBT0.15 PT collected at 0.1 Hz and room temperature. The remnant polarisation and coercive field are similar to that observed in PZT.

[0152] FIG. 11 is a bipolar strain-field loop for 0.55 BF0.3 KBT0.15 PT collected at 0.1 Hz and room temperature. The total peak to peak strain is >0.7%.

[0153] FIG. 12 is a unipolar strain-field loop for 0.55 BF0.3 KBT0.15 PT collected at 0.1 Hz and room temperature. The total strain exceeds 0.43% at 7.5 kV/mm. The high field d33 (max strain/max field) is 575 pmV.sup.1.

[0154] FIG. 13 illustrates permittivity vs temperature plots for 0.55 BF0.3 KBT0.15 PT (labelled 0.15PT) and 0.625 BF0.15 KBT0.225 (labelled 0.225PT). From the maximum in dielectric constant, the inferred ferroelectric paraelectric transition temperatures are 520 C. and 590 C. respectively. The data was collected on cooling from high temperature at a frequency of 100 kHz.

SUMMARY

[0155] The composition 0.55 BF0.3 KBT0.15 PT exhibits a high field d33 of 575 pm V.sup.1 which is higher than most hard commercial PZT materials and a T.sub.c of 525 (PZT max=350). In addition, the density is lower than that of PZT (typically 7700 to 7900) which may be useful for some applications. The total strain (both bipolar and Unipolar) exceeds that exhibited by conventional PZT based materials.