CERAMIC

Abstract

There is disclosed a piezoelectric ceramic having the composition: a[PbTiO.sub.3]-b[SrTiO.sub.3]-c[BiFeO.sub.3]-d[(K.sub.xBi.sub.1-x)TiO.sub.3]; wherein 0.4<x<0.6; 0.1<a<0.4; 0.01<b0.2; c0.05; d0.01; and a+b+c+d=1 optionally comprising an A- or B-site metal dopant in an amount of up to 2 at. %.

Claims

1.-18. (canceled)

19. A piezoelectric ceramic comprising: a Berlincourt d.sub.33 value 250 pm/V; a Curie temperature, T.sub.C, of 300 C.; and a fracture toughness, K.sub.1C(10.sup.6), of 3 MPam.sup.1/2.

20. A piezoelectric ceramic according to claim 19 that further comprises at least one of: a coercive field, Ec, of |1.75| kV/mm; a Vickers hardness, H.sub.v, (10.sup.9)3.5 kgf/mm.sup.2; or a flexural strength, , 100 MPa.

21. A piezoelectric ceramic according to claim 19, wherein: the piezo ceramic material has a composition of:
a[PbTiO.sub.3]-b[SrTiO.sub.3]-c[BiFeO.sub.3]-d[(K.sub.xBi.sub.1-x)TiO.sub.3]; wherein 0.4x0.6, 0.1a0.4, 0.01b0.2, c0.05, d0.01, and a+b+c+d=1; and the piezo electric material further comprises at least one of an A-metal site dopant or a B-metal site dopant in an amount up to 2 at. % per respective metal site.

22. A piezoelectric ceramic according to claim 21, wherein: 0.3c0.7; and 0.05d0.25.

23. A piezoelectric ceramic according to claim 21, wherein 0.2a0.3.

24. A piezoelectric ceramic according to claim 21, wherein 0.02b0.15.

25. A piezoelectric ceramic according to claim 21, wherein: 0.23a0.27; and 0.025b0.10.

26. A piezoelectric ceramic according to claim 21, wherein: 0.4c0.6; and 0.1d0.2.

27. A piezoelectric ceramic according to claim 21, wherein x is 0.5.

28. A piezoelectric ceramic according to claim 21, wherein the at least one of an A-metal site dopant or a B-metal site dopant comprises at least one of Ti, Zr, W, Nb, V, Ta, Mo or Mn.

29. A piezoelectric ceramic according to claim 28, wherein the at least one of an A-metal site dopant or a B-metal site dopant comprises at least one of Mn or Nb and is present in an amount of from 0.25 at. % to 2 at. % per respective metal site.

30. A piezoelectric ceramic according to claim 19, wherein the piezo electric ceramic is a solid solution substantially free of non-perovskite phases comprising PbTiO.sub.3, SrTiO.sub.3, BiFeO.sub.3 and (K.sub.xBi.sub.1-x)TiO.sub.3 wherein 0.4x0.6.

31. A process for preparing a piezoelectric ceramic comprising: (A) preparing an intimate mixture of a substantially stoichiometric amount of a compound of each of Bi, K, Ti, Fe, Pb, and Sr; (B) converting the intimate mixture into an intimate powder; (C) inducing a reaction in the intimate powder to produce a mixed metal oxide; (D) manipulating the mixed metal oxide into a sinterable form; and (E) sintering the sinterable form of the mixed oxide to produce the ceramic, wherein the ceramic has a composition of
a[PbTiO.sub.3]-b[SrTiO.sub.3]-c[BiFeO.sub.3]-d[(K.sub.xBi.sub.1-x)TiO.sub.3]; wherein 0.4x0.6, 0.1a0.4, 0.01b0.2, c0.05, d0.01, and a+b+c+d=1; and the intimate mixture comprises at least one of an A-metal site dopant or a B-metal site dopant in an amount up to 2 at. % per respective metal site.

32. A process according to claim 31, wherein step (E) further comprises a hot pressing step.

33. A process according to claim 31, wherein the at least one of an A-metal site dopant or a B-metal site dopant comprises at least one of Ti, Zr, W, Nb, V, Ta, Mo or Mn.

34. A process according to claim 33, wherein the at least one of an A-metal site dopant or a B-metal site dopant comprises at least one of Mn or Nb and is present in an amount of from 0.25 at. % to 2 at. % per respective metal site.

35. A device comprising a piezoelectric element comprising a ceramic that has a composition of:
a[PbTiO.sub.3]-b[SrTiO.sub.3]-c[BiFeO.sub.3]-d[(K.sub.xBi.sub.1-x)TiO.sub.3]; wherein: 0.4x0.6, 0.1a0.4, 0.01b0.2, c0.05, d0.01, and a+b+c+d=1; and the ceramic comprises at least one of an A-metal site dopant or a B-metal site dopant in an amount up to 2 at. % per respective metal site.

36. An actuator for a droplet deposition head comprising the device according to claim 35.

37. An actuator according to claim 36, wherein the actuator includes alternate firing and non-firing chambers.

Description

[0102] In order that the invention may be fully understood, it is now described having regard to the following Examples and the accompanying drawings. It will be appreciated, however, that the Examples and drawings as well as the description do not necessarily limit the scope of the invention.

[0103] FIG. 1 shows a cross section of a side shooter droplet deposition head as described in EP 0 364 136 B1.

[0104] FIG. 2 shows a cross section of an end shooter droplet deposition head as described in EP 1 885 561 B1.

[0105] FIG. 3 shows a cross section of a side shooter droplet deposition head with firing and non-firing fluidic chambers.

[0106] FIG. 4 shows a cross section of a roof mode droplet deposition head.

[0107] FIG. 5 is a flow chart of a ceramic production method according to an embodiment of the present invention.

[0108] FIG. 6 is a polarisation-field loop for a ceramic comprising a quaternary composition C (in Tables 2 and 3 below) of 0.25625[PbTiO.sub.3]-0.075[SrTiO.sub.3]-0.52125[BiFeO.sub.3]-0.1475[(K.sub.0.5Bi.sub.0.5)TiO.sub.3] which is collected at 10 Hz and room temperature.

[0109] FIG. 7 is a bipolar strain-field loop for the ceramic comprising the quaternary composition C collected at 10 Hz and room temperature.

[0110] FIG. 8 is a unipolar strain-field loop for the ceramic comprising the quaternary composition C collected at 1 Hz and room temperature.

[0111] FIG. 9 is an X-ray diffraction pattern of the ceramic comprising the quaternary composition C. The pattern is used to calculate the difference in volumes of the tetrahedral and rhombohedral unit cells.

EXAMPLES

[0112] The Examples relate to ceramics produced by the addition of amounts of SrTiO.sub.3 to a ternary composition of 0.256[PbTiO.sub.3], 0.521[BiFeO.sub.3] and 0.188(K.sub.0.5Bi.sub.0.5)TiO.sub.3hereinafter composition X.

[0113] Composition X is an example of a ternary composition having a set amount of PbTiO.sub.3 and amounts of BiFeO.sub.3 and (K.sub.0.5Bi.sub.0.5)TiO.sub.3 which are optimised to lie in the morphotropic phase boundary regionviz., the region that displays the highest piezoelectric activity for the compositional range.

[0114] The Examples show that substitution in this ternary composition with SrTiO.sub.3 results in quaternary compositions that form a ceramic solid solution having very desirable properties such as exceptional toughness.

Preparation of Ceramics According to the Invention

[0115] The moisture content and loss on ignition of the precursor materials was performed and adjustments to obtain the desired compositions were made according to the moisture and loss on ignition results.

[0116] Bi.sub.2O.sub.3, Fe.sub.2O.sub.3, K.sub.2CO.sub.3, Pb.sub.3O.sub.4, SrCO.sub.3 and TiO.sub.2 were weighed out according to the stoichiometry required for the specific composition and combined. The resultant powder was then blended prior to particle size reduction to a mean particle diameter d.sub.50<1.5 m using a pin mill. The resultant powder was calcined between 700 C. and 900 C. for between 2 and 16 hours. A second particle size reduction to a d.sub.50 of <1.5 m was then performed.

[0117] Further homogenization and particle size reduction, to 0.2 m<d.sub.50<0.8 m, was then carried out using an attrition mill. Addition of a binder-softener system (Zusoplast G63 and Optapix AC112 both from Zschimmer & Schwarze GmbH & Co KG) was required prior to spray drying of the resultant slurry.

[0118] The ceramic materials were produced by uni-axially pressing of the powder, then subsequent isostatic pressing between 25 and 100 MPa and subsequent iso-pressing of the samples between 50 MPa and 200 MPa. Burn-out of the binder from the samples was carried out up to 600 C prior to sintering. Sintering of the samples between 980 C. and 1080 C. for between 2 and 128 hours was performed to deliver a ceramic material with a final grain size between 1.5 m and 6 m.

[0119] The ceramic materials were prepared for testing of piezoelectric and other properties by grinding and cutting to form pellets (diameter 10 mm and thickness 1 mm) or bars (of dimensions 25 mm3 mm0.5 mm). Electrodes were formed where necessary by applying silver termination ink (Gwent Electronic Materials silver termination paste) to opposite faces of the pellets or bars and then firing according to the manufacturer's recommendations. Where necessary, poling was carried out at 100 C. at 5 kV/mm.

[0120] The steps of the process are shown schematically in FIG. 5 and can be considered to comprise a series of initial powder processing steps and a final series of sample processing steps.

Properties of a Ceramic According to the Present Invention

[0121] An exemplary material according to the present invention was developed from and compared to X.

[0122] Ceramic materials of quaternary composition are obtained as described above by substituting amounts of SrTiO.sub.3 in place of the corresponding amount of BiFeO.sub.3.

[0123] Samples were prepared for testing of the piezoelectric properties of the ceramic as follows: after sintering, the ceramic parts were ground and formed into pellets of diameter 10+/0.5 mm and thickness 1+/0.1 mm. Electrodes were prepared by screen printing a silver electrode frit (Gwent Electronic Materials silver termination paste) and firing on according to the manufacturer's recommendations.

[0124] The piezoelectric activity (d.sub.33), Curie temperature (T.sub.c) and relative electric permittivity .sub.r were determined as described in detail below for four samples comprising quaternary compositions containing different amounts of SrTiO.sub.3. A powder X-ray diffraction pattern of the crystalline structure of each of the quaternary compositions was also obtained.

[0125] The Berlincourt d.sub.33 value and the relative electrical permittivity at room temperature (RT) and at the Curie temperature (T.sub.c) are shown for each sample in Table 1.

[0126] As can be seen, increasing the amount of SrTiO.sub.3 in the quaternary compositions is seen to deliver an increase in piezoelectric activity of the samples. The Curie temperature of the samples decreases with increasing amounts of SrTiO.sub.3 in the compositions, but remains greater than or equal to 375 C.

TABLE-US-00001 TABLE 1 Properties of quaternary ceramic compositions based on 0.256[PbTiO.sub.3], 0.521[BiFeO.sub.3] and 0.188(K.sub.0.5Bi.sub.0.5)TiO.sub.3 with the specified amounts of SrTiO.sub.3 added in place of BiFeO.sub.3. Amount of SrTiO.sub.3 added % in place of BiFeO.sub.3 T.sub.c d.sub.33 Rhombohedral (molar fraction).sub. .sub.r @ RT ( C.) .sub.r @ T.sub.c (pC/N) phase 0 714 488 13200 175 52.51 0.03 782 452 12100 239 16.81 0.05 805 445 10500 221 12.76 0.075 826 398 12000 236 17.94 0.10 850 375 11500 233 15.54

[0127] Note also that the percentage of rhombohedral phases is greatly reducedindicating a shift of the resultant compositions from the morphotropic phase boundary region.

[0128] Further ceramic materials of quaternary composition are obtained as described above by reintroducing an amount of BiFeO.sub.3 in place of (K.sub.0.5Bi.sub.0.5)TiO.sub.3 in the composition of Table 1 containing 0.075 (molar fraction) SrTiO.sub.3. These compositions are optimised for piezoelectric activity through adjustment of the position in the phase diagram towards the morphotropic phase boundary.

[0129] The piezoelectric activity d.sub.33, Curie temperature T.sub.C, relative electric permittivity .sub.r at the Curie temperature and the coercive field E.sub.c were determined as described in detail below for four samples A to D comprising quaternary compositions containing different amounts of SrTiO.sub.3.

[0130] Table 2 shows the Berlincourt d.sub.33 value, the relative electric permittivity .sub.r at the Curie temperature T.sub.c and the coercive field of each sample E.sub.c. As may be seen, the compositions provide good piezoelectric activity and high Curie temperature in the samples together with a high coercive field.

TABLE-US-00002 TABLE 2 Curie temperature (T.sub.c), piezoelectric coefficient (d.sub.33) and coercive field (E.sub.c) of quaternary ceramic compositions containing 0.075 (molar fraction) strontium titanate from Table 1 in which an amount of BiFeO.sub.3 is reintroduced in place of (K.sub.0.5Bi.sub.0.5)TiO.sub.3. Amount of BiFeO.sub.3 reintroduced T.sub.c d.sub.33 E.sub.c(kV/mm) Composition (molar fraction) ( C.) (pC/N) negative positive A 0.02 367 269 1.93 1.87 B 0.03 355 273 1.89 1.85 C 0.04 389 280 2.26 2.08 D 0.05 387 245 2.44 2.49 X n/a 488 175 2.7 2.72

[0131] The d.sub.33 values presented in Tables 1 and 2 were obtained from a Wide Range d.sub.33 Meter (APC International, USA). The meter applies a predetermined stress and calculates the charge generated and from the stress, the direct piezoelectric coefficient d.sub.33. It enables a very rapid determination of d.sub.33 in the samples. Instructions for use of the meter can be found at the following web address: https://www.americanpiezo.com/images/stories/d_tester/d33-Meter_InstructionManual-updated.pdf.

[0132] The meter is turned on for a period of at least 30 minutes to ensure stability. After this, the meter is zeroed and its accuracy checked against the piezoelectric validation component supplied by the manufacturer. The sample is placed between two metal contacts and brought into contact by turning a screw until vibrations are no longer felt. After the establishment of an electrical connection, the screw is turned a further turn. The vibrational force was 250 mN.

[0133] Note that there are no standards for the calibration and validation of such meters, as the results are dependent on applied stress, geometry, frequency and stress (see, for example, National Physical Laboratory, UK, Good Practice Guide 44, ISSN 1368-6550).

[0134] The d.sub.33 values were verified by comparison of values, taken on bars (of dimensions 1 mm by 1 mm by 3.5 mm) formed from the pellets, with PZT bar control samples measured by resonance according to CENELEC EN 50324-2:2002 (Piezoelectric Properties of Ceramic Materials and ComponentsPart 2: Methods of Measurement and PropertiesLow Power. CENELEC (2002)). These bars were poled along the long dimension.

[0135] As mentioned above, the d.sub.33 measured using the Berlincourt method is on average 11% higher than the values measured using the resonance method. The difference is a composite of random and systematic error, and difference in the measurements, frequency, stress and voltage.

[0136] The Curie temperatures T.sub.C and relative permittivities .sub.r at T.sub.C and room temperature presented in Tables 1 and 2 were determined in the normal way. The capacitance of the (unpoled) pellets was measured as a function of temperaturethe Curie temperature being identified as that for maximum capacitance.

[0137] A Keysight 4299A Precision Impedance Analyser was used to measure capacitance at 1 MHz during cooling of the samples from a temperature above the Curie temperature (for example 550 C.) at a rate of 2 C. per second.

[0138] Two metre cables terminating in silver wires were used to connect to the samples. The silver wires were tensioned slightly to provide a spring contact to the sample, establishing a sound electrical connection.

[0139] A small tube furnance (Lenton LTF 12/38/250) was used to control the temperature of the samples and the temperatures were measured using a K-type thermocouple in close proximity to the sample and independently checked against a calibrated thermocouple and thermocouple monitor.

[0140] The capacitance at room temperature was measured at 1 kHz in accordance with CENELEC (2002).

[0141] The relative permittivity of the (poled) pellets at the Curie temperature and at room temperature were calculated from the measured capacitance and the geometry at the respective frequencies of 1 MHz and 1 kHz.

[0142] The coercive field was determined at 20 to 25 C. by recording polarisation-electric field loops (PE) using an AixAcct TF 2000 ferroelectric tester at a frequency of 10 Hz and an electric field between 5 and 6 kV/mm (at least double the coercive field). The coercive field is the field at which the polarisation shown in the PE loop is zero. The value Ec is taken as the average coercive field during positive and negative electric field excursions. The pellets were immersed in silicone oil to prevent electric discharge during the measurement.

[0143] The Vickers hardness H.sub.v and the fracture toughness K.sub.1C of each quaternary composition were determined by Vickers indentation on pellets polished using 1 m diamond. A load of 0.3 kg was applied using a Wilson Hardness Tukon 1202 machinecalibrated and validated to DIN EN ISO 6507-2 by the manufacturer within a year of the testing. Hardness values were calculated (using a modulus of 70 GPa assumed from the d.sub.33 resonance data) from the size of the indent and fracture toughness from the length of the propagating cracks. Ten (10) measurements per sample were taken in each case and the mean value was calculated.

[0144] The flexural strength was calculated from measurements made on polished (using 1 m diamond) bars (25 mm3 mm0.5 mm) formed from the quaternary compositions after grinding. An Instron ElectroPuls E3000 system was used together with a 250 N load cellthe system being calibrated to ISO 7500-1:2004 by UKAS accredited Instron Calibration Laboratory (High Wycombe, UK).

[0145] A 4-point flexural testing was carried out with a lower head spacing of 20 mm, an upper head spacing of 6.66 mm and a head speed of 2 mm/min. The flexural strength, , was calculated from the load at failure, F, using the length of the lower head, L, and the width, b, and thickness, d, of the sample, according to the equation =FL/(bd.sup.2). Six results were collected per bar and the mean value calculated.

[0146] The Berlincourt d.sub.33 values, the Vickers hardness, the fracture toughness and the flexural strength of each of the compositions A to D are shown in Table 3.

TABLE-US-00003 TABLE 3 Fracture toughness (K.sub.1c), Vickers hardness (H.sub.v) and/or flexural strength () values of composition X, quaternary compositions according to the invention (A, B, C & D) and PZT materials (PZT-4, PZT PIC 151) of the prior art. Composition/ Berlincourt K.sub.1c (10.sup.6) Stan. H.sub.v Stan. Stan. standard d.sub.33 (pC/N) [MPa.m.sup.1/2) Dev. (10.sup.9) [kgf/mm.sup.2] Dev. (MPa) Dev. X 175 A 269 5.2 0.9 4.1 0.3 149.3 2.9 B 273 3.9 0.7 4.5 0.2 144.4 5.2 C 280 7.6 1.5 4.1 0.3 152.5 4.8 D 254 7.2 1.1 4.2 0.2 149.8 1.3 PZT-4 250* 0.7-1.8 [1] 2.5 [1] PZT PIC 151 555 45 [2] [1] C. T. Sun and S. B. Park, Measuring fracture toughness of piezoceramics by Vickers indentation under the influence of electric fields, Ferroelectrics, vol. 248, no. 1, pp. 79-95, 2000.; * not disclosed here, but calculated from the value (225 pC/N) obtained by resonance and given at https://ntrs.nasa.gov/archive/nasa/casi.ntris.gov/19980236888.pdf. [2] T. Fett, D. Munz, and G. Thun, Tensile and bending strength of piezoelectric ceramics, J. Mater. Sci. Lett., vol. 18, no. 23, pp. 1899-1902, 1999; not disclosed here, but calculated from from the value 500 pC/N obtained by resonance and given at the following web address: https://www.piceramic.com/en/service/downloads/catalogs-brochures-certificates/.

[0147] Table 3 also shows the piezoelectric activity of Composition X and two ferroelectrically hard PZT materials known to the art. The Vickers hardness, the fracture toughness or the flexural strength of these materials are reported in the indicated references ([1] and [2]). The piezoelectric activity of these materials is found in data otherwise available to the skilled person.

[0148] Note that the fracture toughness, the Vickers hardness and the flexural strength can be compared with the indicated references notwithstanding that slightly different methods and some variation in the equipment and load are used, because the reported fracture toughness and flexural strength values for PZT-4 and PZT PIC 151 are obtained on multiple samples across a wide range of loads.

[0149] As can be seen in Table 3, the quarternary compositions according to the invention show significantly higher fracture toughness K.sub.1C as compared to PZT-4 and PZT PIC 151 (3 MNm.sup.3/2 or more as compared to less than 1 MNm.sup.3/2). Furthermore, the quaternary compositions exhibit higher Vickers hardness and higher flexural strength than is observed for PZT (other literature references suggest PZTs flexural strength varies between 50 and 86 MPa).

[0150] FIG. 6 is a polarisation-field loop for a ceramic comprising the quaternary composition C (of Tables 2 and 3) 0.25625[PbTiO.sub.3]-0.075[SrTiO.sub.3]-0.52125[BiFeO.sub.3]-0.1475[(K.sub.0.5Bi.sub.0.5)TiO.sub.3] collected at 10 Hz and room temperature. As may be seen, the remanent polarisation is similar to that observed in PZT.

[0151] FIG. 7 is a bipolar strain-field loop for the ceramic comprising the quaternary composition C collected at 10 Hz and room temperature. As may be seen, the total peak to peak strain is 0.49%.

[0152] FIG. 8 is a unipolar strain-field loop for the ceramic comprising the quaternary composition C collected at 1 Hz and room temperature. As may be seen, the total strain exceeds 0.18% at 4 kV/mm and the high field d.sub.33 (max strain/max field) is 431 pm/V.

[0153] FIG. 9 shows a powder X-ray crystallography analysis of the ceramic comprising the quaternary composition C, viz., of a ceramic with a good combination piezoelectric activity, high Curie temperature, fracture toughness, high flexural strength and Vickers hardness.

[0154] The analysis was to define (a) the relative proportions of each of the tetrahedral and rhombohedral phases and (b) the size/volume of each unit cell type. The difference in primitive unit cell size between tetragonal and rhombohedral phase is known as a pre-requisite for an enhanced transformation toughening mechanism (as described in T. P. Comyn, T. Stevenson, S. A. Qaisar, A. J. Bell, High performance piezoelectric materials Actuator 2012) which underlies the highly improved fracture toughness of the ceramics of the present invention.

[0155] In FIG. 9 the peaks at ca. 44.6 and 46.4 relate to the tetragonal phase, and the single peak at ca. 46 relates to the rhombohedral phase. From these positions the difference in the volumes of the rhombohedral and tetragonal phases was calculated at 1.24%.

[0156] Note that the techniques described in detail here can be applied broadly and do not necessarily limit the scope of the invention. A further refinement of the process by which the material of composition C is performed may, for example, lead to further improvements in the material properties of the ceramic.