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Title:
ACTUATOR MATERIAL AND METHOD THEREOF
Document Type and Number:
WIPO Patent Application WO/2019/175721
Kind Code:
A1
Abstract:
The present invention relates to the preparation of non-textured polycrystalline piezoceramic exhibiting ultrahigh electrostrain by solid-state sintering. The pseudo-ternary ferroelectric alloy system BiFeO3-PbTiO3-LaFeO3 exhibiting electrostrain of about 1.3 % is prepared as per the chemical formula (l-x)Bi1-yLayFeO3-xPbTiO3. The compositions with x > 0.40 exhibit high electrostrain when chemically tuned by Lanthanum (La).

Inventors:
RANJAN, Rajeev (Indian Institute of Science, C V Raman RoadKarnataka, Bangalore 2, 560012, IN)
NARAYAN, Bastola (Indian Institute of Science, C V Raman RoadKarnataka, Bangalore 2, 560012, IN)
Application Number:
IB2019/051875
Publication Date:
September 19, 2019
Filing Date:
March 08, 2019
Export Citation:
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Assignee:
INDIAN INSTITUTE OF SCIENCE (C V Raman Road, Bangalore 2, 560012, IN)
International Classes:
C04B35/462; H01L41/00
Domestic Patent References:
WO2009119322A12009-10-01
Foreign References:
CN100337980C2007-09-19
US6783588B22004-08-31
Attorney, Agent or Firm:
KRISHNAMURTHY, Rama et al. (IBHA IP Solutions, #303 1st Main, 1st Block,Bel Layout, Vidyaranyapur, Karnataka Bangalore 7, 560097, IN)
Download PDF:
Claims:
WE CLAIM:

1. A pseudo ternary alloy, polycrystalline ceramic composition of formula I:

(l-x) Bii_yLayFe03-(x)PbTi03

Formula I

2. The pseudo ternary alloy, polycrystalline ceramic composition as claimed in claim 1, wherein x>0.45 and 20<y<0.35.

3. The pseudo ternary alloy, polycrystalline ceramic composition as claimed in claim 1, wherein the composition exhibits coexistence of cube like (CL) and tetragonal (T) phases.

4. The pseudo ternary alloy, polycrystalline ceramic composition of claim 1, wherein the composition is of grain size ranging from about 6 to 8 //m.

5. A process of preparation of pseudo ternary alloy, polycrystalline ceramic composition as claimed in claim 1, the process comprising steps of:

a) mixing oxides or carbonate powders of Bismith, Lead, Iron, Titanium and Lanthanum using solvent to obtain a mixture;

b) calcinating the mixture at temperature between 700°C to l000°C for 2 h in alumina crucibles;

c) mixing the calcined powder with a binder;

d) pelletizing the powder for uniaxial loading; and

e) sintering the pelletized powder at a temperature ranging from 950 - 1100 °C to obtain pseudo ternary alloy, polycrystalline ceramic composition (l-x) Bii_y LayFeCL-xPbTiCL.

6. The process of preparation of pseudo ternary alloy composition as claimed in claim 5, wherein the binder is selected from a group comprising natural polymer, synthetic polymers, synthetic binders and combination thereof.

7. The process of preparation of pseudo ternary alloy composition as claimed in claim 5, wherein the powder is pelletized by using a low melting additive selected from a group comprising Mn02, CuO and Si02.

8. An actuator material comprising a pseudo ternary alloy, polycrystalline ceramic composition of formula I:

(l-x) Bii_yLayFe03-xPbTi03

Formula I.

wherein electrostrain of the composition ranges from 1% to 1.3%.

Description:
Actuator material and method thereof

TECHNICAL FIELD

The present invention relates to piezoelectric ceramic compositions. More particularly, it relates to non-textured polycrystalline piezoceramic exhibiting ultrahigh electrostrain. Specifically, the invention is in relation to pseudo-ternary ferroelectricalloy system(l-x) Bi y La y Fe0 3 - (x)PbTi0 3 exhibiting high field electrostrain of about 1.3% at 80kV/cm in non- texturedpolycrystalline ceramic specimens. The invention is also in relation to the method of preparation of the ferroelectricalloy system BiFe0 3 -PbTi0 3 -LaFe0 3 .

BACKGROUND

Piezoelectrics find large application in ultrasonic transducers for medical imaging, high precision control of mirrors in space telescopes, micro robotics, guidance systems in military, automotive industry, for structural health monitoring and the like. It is ubiquitously known that Piezoelectric actuators transform input electrical energy to mechanical energy, and the wide-ranging applicationsare attributed to their compactness, quick response time, large impact force, and accurate displacement.

The most important parameter of a piezoelectric material with regard to its use as actuators is the electric field induced strain known as electro strain. In patent document of CN105154976, PSN-PMN-PT-PZ ferroelectric single-crystal material is disclosed to induce large electrostrain but growth of high-quality single crystals requires a delicate control of many different experimental parameters over a long period of time. A slight unintentional variation in any one of the experimental conditions, even for a short duration, can ruin the quality of the crystal and its intended property. In some cases, it is not possible to grow single crystals of the most desired compositions of piezoelectric alloys and also its not economical on a large-scale preparation. EP1628351 discloses a single crystal piezoelectric material and a piezoelectric element which has to be suitably cut along specific plane for obtaining large electrostrain in excess of 1 %.

Patent document CN 201510377643 discloses a method for preparing Nal/2Bil/2Tinon- texturedpolycrystalline piezoceramic with maximum electrostrain of 0.7 %, which is associated with a cubic -like to rhombohedral transformation in Nal/2Bil/2Ti0 3 -based piezoceramic.

Owing to the large applications and the ever-demanding need of piezoelectric systems with large electrostrain for suitable adaptability, it is necessary to develop systems that produce high electrostrain having simple fabrication leading to economical industrial production.

SUMMARY OF INVENTION

Accordingly the present invention helps to mitigate limitations within the prior art relating to piezoelectric systems with large electrostrain. In accordance with an embodiment of the invention there is provided a synergistic composition of a pseudo-ternary ferroelectric alloy polycrystalline ceramic of formula I.

(l-x) Bii- y La y Fe0 3 - (x)PbTi0 3

Formula I.

In accordance with an embodiment of the invention there is provideda pseudo ternary alloy polycrystalline ceramic composition of formula (l-x) Bii -y La y Fe0 3 -xPbTi0 3 whereinfor each value of x<0.45, there is a critical value of y which is likely to show large electrostrain is provided. In yet another embodiment of present invention, a process of preparation of said pseudo ternary alloy, polycrystalline ceramic composition is provided, the process comprising steps of:a)mixing oxides or carbonate powders of Bismith, Lead, Iron, Titanium and Lanthanum using solvent to obtain a mixture; b)calcinating the mixture at temperature between 700°C to l000°C for 2 h in alumina crucibles; c)mixing the calcined powder with a binder;d)pelletizing the powder for uniaxial loading; ande) sintering the pelletized powder at a temperature ranging from 950 - 1100 °C to obtain pseudo ternary alloy, polycrystalline ceramic composition (l-x) Bi l-y LayFeCL-xPbTiCL.

BRIEF DESCRIPTION OF FIGURES

The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.

Figure 1: (a)The evolution of the x-ray powder diffraction pattern of sintered 0.55Bii_ y La y FeO 3 -0.45PbTiO 3 (BF-PT45:La (30) as a function of La concentration (y). (b) the variation of the tetragonal and cubic-like lattice parameters (a T , cp, ape ) with La concentration.

Figure 2: (a) Electric -field dependent unipolar strain of 0.55Bii -y La y Fe0 3 -0.45PbTi0 3 for different La concentration (y). (b) shows < ¾* (the large-signal piezoelectric coefficient) for La concentration (y) calculated from unipolar electrostrain curves (c) depicts the evolution of the pseudocubic {200} pc XRD Bragg profile of BF-PT45:La as function of La concentration (d) shows the composition-temperature phase diagram of BF-PT45:La. Figure 3: Evolution of the {200} pc XRD Bragg profile of BF-PT:La (y=0.30) as a function of

(a) increasing field and (b) decreasing field, (c) shows the domain switching fraction (h 0 02)·

Figure 4: Graphical estimation of the electro strain.

Figure 5: Scanning electron micrograph of sintered specimens of (a) BF-PT45:La (y=0.30) and (b) 0.44BNZ-0.56 PT.

Figure 6: Dielectric measurements as a function of temperature are carried out for 0.55Bii_ y La y FeO 3 -0.45PbTiO 3 at different La concentration (y).

Figure 7: XRD reflection pseudocubic {200} pc of (a) unpoled and (b) poled PT-BNZ powder, (c) shows Pseudocubic { 111 } pc and { 200 } pc neutron powder diffraction (NPD) Bragg profiles of (a), PT-BNZ and (d) shows the strain in the corresponding sample (e) shows the domain switching in PT-BNZ from poled powder to poled pellet and then under 35kv/cm field and after removal of field.

Figure 8: (a) X-ray powder diffraction patterns of (x)BNZ - (l-x)PbTiO 3 0.38 < x<0.41. (b) X-ray diffraction patterns of unpoled and poled 0.44 BNZ - 0.56 PT.

Figure 9: (a) shows Temperature dependent relative permittivity of BF-PT45:La (y=0.30) at different frequencies, (b) shows the Vogel-Fulcher fitting (c) the temperature evolution of the pseudo-cubic { 200} pc Bragg profile while heating poled BF-PT:45La30, (d) the temperature dependence of the lattice parameters of the tetragonal and the cubic -like phase.

Figure 10: Unipolar strain vs electric field of BF-PT45:La30 repeated cycle at (a) 60kV/cm,

(b) 70kV/cm, (c) 75kV/cm and (d) 80kV/cm. (e) bipolar strain vs electric field study of the

BF-PT:La30 at 80kV/cm. DESCRIPTION OF INVENTION

The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the figures, description and claims. It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.

The present invention discloses a new class of piezoelectric polycrystalline ceramic exhibiting ultrahigh electrostrain ranging from 1% to 1.3%. It is observed that the strain is associated with large spontaneous lattice strain of the ferroelectric phase and the extensive reverse switching of the non- 180° ferroelectric-ferroelastic domains. And therefore, any chemical modification which can bring about the following features:(i) make the system behave like a relaxor ferroelectric ;(ii) induce long-range ferroelectric distortion upon application of strong electric field with a reasonably large spontaneous lattice strain ~ 2 %; (iii) induce low symmetry (monoclinic) of the ferroelectric phase, and (iv) very large reverse switching of ferroelastic domains upon switching off the field are potentially interesting piezoelectrics which can give ultrahigh electro strain > 1 %.

The present invention claims to design non-textured poly crystalline piezoelectric ceramic which can show electrostrain of more than 1 % at electric field of ~ 80 kV/cm. This is achieved by suitable compositional tailoring of the alloy system ( 1 -x)BiFc0 3-(x) PbTi0 3 exhibiting very large tetragonality, and by use of appropriate additives to help grain growth and reduce the leakage current. The chemical modification is done such that it drives the system towards a relaxor ferroelectric state and show very large reversible switching of ferroelectric -ferroelastic domains after switching of the electric field. Such a state and very large electrostrain (~ 1 %) is likely to be achieved by stoichiometric and off- stoichiometric chemical modifications of the parent solid solution series ( 1 -x)BiFc0 3 _ (x) PbTi0 3 . The system can be driven towards the relax or ferroelectric state by partial isovalent substitutions of rare-earth ions (such as La, Dy, Ce, and the like) at the Bi and/or Pb site; and/or partial replacement of Pb by cations such as Ca, Sr, Ba, Li, Na, K, and/or partial replacement of Ti by Zr, Hf, Sn.

Among the rare earths Lanthanum is cheapest. For the specific case of Lanthanum modification ultrahigh electrostrain is achieved for x >0.45; and 0.20<y<0.35. A large electrostrain of 1.3 % is achieved in the case of Lanthanum modification. The important features which is associated with such a large electrostrain is that even while becoming relax or ferroelectric, the system retains sufficiently large tetragonality ~ 2 % and it shows large reverse switching of the non-l80-degreeferroelectric domains.

And therefore, in yet another form, the inventionprovides a novel polycrystalline ceramic composition comprising Lanthanum with formula (l-x) Bii -y La v Fc 0 3 - x P h T i 0 3 whereinfor each value of x>0.45, there is a critical value of y which is likely to show large electro strain. The novel composition comprising Lanthanum shows electrostrain of 1.3 % at 80 kV/cm when prepared using Mn02 as minor additive. The high electrostrain is associated with large spontaneous lattice strain of the piezoelectric phase, domain miniaturization, and very large recoverable domain switching due to presence of competing mechanisms associated with domains of smaller and longer coherence lengths. The realization of ultrahigh electrostrain in polycrystalline piezoceramics provides a cost-effective alternative to the costly single crystals for ultrahigh actuation. Another aspect of invention is a method of preparation of the ferroelectric alloy system (l-x) Bii. y La y FcCT - x PbT i O 3 , more specifically BiFe0 3 -PbTi0 3 -LaFe0 3 . The polycrystalline ceramic specimens are prepared by conventional solid-state sintering technique using analytical grade high purity powders of B1 2 O 3 , PbO, Fe 2 0 3 , Ti0 2 and La 2 0 3 . For better homogeneity, it is advisable to prepare the specimens by sol -gel synthesis, or by using stable carbonates (instead of oxides) of some of the cations. Said powders are densified with a suitable binder.

The binder is selected from a group comprising natural polymer, synthetic polymers, synthetic binders and combination thereof.

In the present invention powders are densified 2% polyvinyl alcohol (PVA) as a binder is used, before sintering at temperature 950°C to about 1150 °C temperature.

Alternatively, the ceramic is also made by cold compacting under isostatic pressure of about 500 MPa.0.2 - 0.5 weight percent of a low melting additive is added to the calcined powder to reduce the leakage current in the samples.

The low melting additive is selected from a group comprising Mn0 2 , CuO and Si0 2.

Typicallysaid powders are pelletized under uniaxial loading. The green pellet obtained after uniaxial pressing is sintered in the range (depending on the composition) at temperature ranging from about 950°C to about 1150 °C for about 2 h. The sintering temperature needs to be optimized in each caseby ensuring that the grain sizes are > 5 microns for the realization of the large electro strain.

The method for fabrication of polycrystalline non-textured piezoceramics is relatively easy, less energy consuming and can be prepared in large quantity with reproducible properties. The polycrystalline ceramic can be used as inexpensive actuator material and can provide large displacement on application of electric field. EXPERIMENTAL

A. Preparation of Poly crystalline ceramic specimencontaining Lanthanum

i. 0.55Bii -y La y FeO 3 -0.45PbTiO 3 is prepared by conventional solid-state sintering technique in the composition range 0<y<0.40. Analytical grade high purity powders of Bΐ 2 0 3 , PbO, Fe 2 0 3 , Ti0 2 and La 2( ¾ (Alfa Aesar, purity > 99.5 %) are mixed thoroughly for about 8 h using Fritsch Ball mill in a zirconia bowl with zirconia balls and acetone as the mixing media. The mixed powders are calcined at 800°C for 2 h in alumina crucibles. The calcined powders are mixed with 2% polyvinyl alcohol (PVA) as a binder. 0.2% of Mn0 2 is added in each composition to reduce the leakage current in the samples. The powders are pelletized under uniaxial loading. The green pellet obtained after uniaxial pressing is then sintered in the range (depending on the composition) 950 - 1100 °C for 2 h on alumina plate surrounded by the powders of the same composition and covered with an alumina crucible to minimize the volatilization of PbO and Bi 2 0 3 . ii. 0.60Bii -y La y Fe0 3 -0.40PbTi0 3 (BF-PT40:Lal00y) is prepared by conventional solid- state sintering technique in the composition range 0<y<0.40. Analytical grade high purity powders of Bi 2 0 3 , PbO, Fe 2 0 3 , Ti0 2 and La 2 0 3 (Alfa Aesar, purity > 99.5 %) are mixed thoroughly for about 8 h using Fritsch Ball mill in a zirconia bowl with zirconia balls and acetone as the mixing media. The mixed powders are calcined at 800°C for 2 h in alumina crucibles. The calcined powders are mixed with 2% polyvinyl alcohol (PVA) as a binder. 0.2% of Mn0 2 is added in each composition to reduce the leakage current in the samples. The powders are pelletized under uniaxial loading. The green pellet obtained after uniaxial pressing is then sintered in the range (depending on the composition) 950 - 1100 °C for 2 h on alumina plate surrounded by the powders of the same composition and covered with an alumina crucible to minimize the volatilization of PbO and B1 2 O 3 .

B. Comparative study of electro strain property of Polycrystalline ceramic specimen

The piezoelectric system of present invention B i Fc O 3 - P b T i O 3 - La Fc O 3 is compared with analogous alloy system (l-x)PbTi0 3 -(x)Bi(Nii /2 Zri /2 )0 3 (PT-BNZ).

A series of (x)BNZ-(l-x)PT (abbreviated as PT-BNZ) is prepared in the composition range 0.38< x < 0.44 by conventional solid state synthesis technique. Dried reagent grade oxides of B1 2 O 3 , PbO, NiO, Z1 O 2 and T1O 2 (Alfa Aesar, purity > 99.5 %) are mixed according to stoichiometric formulas of each composition. The mixtures are ball milled in agate vials in acetone medium for 8 h using Fritsch Ball Mill. Milled powders are dried and calcined at 850°C for 6 h in alumina crucibles. The calcined powders are then ball milled in ethanol medium to break the agglomeration. The calcined powders are mixed with binder and pressed in to disc under uniaxial pressure. The pellets are sintered in the range 1100-1150 °C for 3 h.

Similar to BF-PT45:La(y), PT-BNZ also exhibits a composition driven tetragonal to cubic- like transformation at x ~ 0.59 (Figure 8), and poling induced cubic -like to a tetragonal transformation, Figure 7a, b. The critical composition of PT-BNZ however shows electrostrain about 0.3 % at 80 kV/cm. The value is merely 1/4* the colossal electrostrain obtained in specimen BF-PT45:La30 of present invention. While the low electrostrain in PT- BNZ can partly be rationalized by arguing that the tetragonality of PT-BNZ (c/a -1 ~ 0.011) is nearly half the tetragonality in BF-PT45:La30 (c/a-l ~ 0.023), what is more important to note is the lack of reverse domain switching in the former (Figure 7d). After poling the irreversibly switched fraction of the tetragonal domains is 0.50 (Figure 7c), which is significantly larger than in BF-PT45:La30 (h = 0.10). This considerably lessens the scope for additional domain switching on application of field in the same direction, as evident from Figure 7c. It is to be noted that the large difference in the electrostrain of BF-PT:La and PT- BNZ cannot be attributed to the difference in the grain size of the specimens as both specimens exhibit grain size in the range 6-8 mhi ( Figure 5). The low symmetry (Cc) of the ferroelectric phase may aid to the efficient domain switching in BF-PT45:La(y=0.30) as it allows more domain variants to appear, which in turn increases the probability of finding compatible variants across neighboring grains for efficient collective switching to take place across the piezoceramic body. In contrast, the ferroelectric phase of PT-BNZ has higher symmetry (P4mm), as there is no evidence of superlattice reflections corresponding to octahedral tilt in the NPD pattern (Figure 7c).

C. Structural and functional characterization

X-ray powder diffraction patterns are collected using Rigaku (SmartLab) rotating anode- based diffractometer with CuK a radiation working in Bragg Brentano reflection geometry. The diffraction patterns of“unpoled specimens” are obtained by grinding the sintered pellets to powder and subsequent annealing at about 700°C for 1 h to get rid of stress induced ferroelastic changes in the specimens, if any, incurred during the grinding process. The XRD patterns of the poled specimens are collected after grinding the poled pellets to powder. Neutron powder-diffraction (NPD) experiment is carried out at the SPODI diffractometer at FRM II, Germany using a wavelength of 1.548 A. Rietveld refinement is carried out using the program FULLPROF. Microstructural features of the pellets are examined by Scanning Electron Microscopy (FEI, Quanta 200). Ferroelectric and electrostrain measurements are carried out with Radiant Precision Premier II loop tracer and a MTI photonic sensor, respectively. Dielectric measurements are carried out on fired-on silver electroded circular discs of ~ lmm thickness and -10 mm diameter. The local structure and chemistry in both the unpoled and poled samples is investigated using high resolution transmission electron microscopy (TEM) on a C s corrected Titan G2 microscope operated at 200 kV. Electron transparent samples are prepared both in unpoled and poled states via conventional mechanical polishing followed by ion milling. While reciprocal space analysis is performed by obtaining selected area electron diffraction (SAED) patterns of regions (-100 nm diameter) along various zones, real space analysis is performed on atomic resolution images obtained via high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Chemical maps are obtained using energy dispersive spectroscopy (EDS) in a ChemiSTEM set-up (4 solid-state detectors placed symmetrically about the optical axis) with simultaneous HAADF-STEM imaging.

Figure 1 and Figure 2cshows the evolution of the pseudocubic { 200 } pc x-ray powder diffraction Bragg profile of BF-PT45:La(y) as a function of La concentration. The two tetragonal peaks (002) T and (200) T come close with increasing La concentration suggesting decrease in the tetragonal spontaneous lattice strain (c/a-l). The tetragonality decreases from 0.14 at y = 0.0 (La free) to 0.023 at y = 0.30 (Figure lb). For x > 0.25, a third peak appears in between the two (002) T and (200) T tetragonal peaks. This peak corresponds to a cubic -like (CL) phase. The significantly large increase in electrostrain for y = 0.25 -0.32, therefore has strong correlation with the onset of the CL phase. The large-signal piezoelectric coefficient ( < ¾3*) for La concentration (y) is calculated from unipolar electrostrain curves (Figure 2(b)). The composition-temperature phase diagram of BF-PT45:La is shown in Figure 2(d). The transition temperatures in Figure 2(d)are obtained from the temperature dependent dielectric measurements (Figure 6) and high temperature XRD measurements.

The domain switching behaviour of BF-PT45:La30 is investigated using XRD measurements in-situ with electric field. For the sake of establishing a correlation with measured unipolar electrostrain shown in Figure 2a, diffraction measurements are carried out under unipolar electric -field on a poled pellet. Figure 3 shows the in-situ electric field dependent evolution of the {200} pc profile. Although it is proved that the most plausible structure of BF- PT45:La30 is monoclinic Cc, for interpretation of the XRD results, it is assumed that the structure are tetragonal-like (as the XRD data originally suggests). The diffraction pattern of the poled pellet shows the pseudocubic {200} c profile to split into two tetragonal peaks (002) T and (200)t· The small peak in between the (002) T and (200) T corresponds to the remnant cubic-like phase that survives after poling. In a normal tetragonal ferroelectric perovskite poling often increases the intensity of the (002) t above (200) T due to irreversible reorientation of the tetragonal domains. Interestingly, this is not the case for BF-PT45:La30 as the intensity ratio of (002) T and (200) T is very close to the value in preferred orientation free powder pattern. The extent of domain switching fraction ( h ) is estimated using the equation below:

I 002 I 200

where, Ioo2 and I 2 oo are the integrated intensities of the (002) T and{200} T reflectionsrespectively in thepresence of field. / 002 and / 2oo are the integrated intensities of the (002) T and { 200 } T reflections respectively before application of the field. After poling the switched fraction is merely 0.10. With increasing field, 77002 increases again and reaches a value ~ 0.4 at 60 kV/cm, and returns to 0.10 at E = 0 kV/cm, Figure 3c. It is found that the peak position of the residual cubic -like phase shifts synchronously to lower 20 value as the switching fraction of the tetragonal domain increases. This suggests strong coupling between the two phenomena. On decreasing the field, lattice strain of the cubic -like phase relaxes and, by its coupling, forces the tetragonal domains to reverse switch. The minor cubic -like phase (~ 30 %), which is most likely to be representative of regions near the strong random field centers (wherein long-range coherence could not be achieved by field), therefore provides the necessary restoring force for reversing the domain wall motion of the tetragonal domains when the field is reduced. By taking into consideration the contribution of tetragonal domain switching and the lattice strain of the cubic -like phase, the estimated electrostrain is very close to the observed value [Figure 4].

Dielectric measurements as a function of temperature are carried out for 0.55Bii -y La y Fe0 3 - 0.45PbTiO 3 at different La concentration (y) (Figure 6). The dielectric anomaly temperatures correspond to the Curie points of the different compositions. This is also verified with temperature dependent XRD measurements which shows disappearance of the tetragonal phase above the dielectric maximum temperature.

Temperature dependent relative permittivity of BF-PT45:La (y=0.30) at different frequencies is shown in Figure 9 (a). The shifting of the permittivity peak towards higher temperature with increasing frequency confirms the relaxor like behaviour of this composition. Figure 9(b) shows the Vogel-Fulcher fitting of the co vs T m extracted from the plots in Figure 9(a).

Where coo is preexponential factor and T f is usually referred to as the freezing temperature of the dynamic polar regions. The best fit is obtained for T f =173 0C and pre-exponential factor wo=6.15c10 h Hz. Figure 9 (c) shows the temperature evolution of the pseudo-cubic {200}pc Bragg profile while heating poled BF-PT:45La30. It is evident from this figure that the cubic- like phase, starts appearing above l500°C. This is consistent with the freezing temperature of ~ 170° C, predicted from the Vogel-Fulcher analysis. Figure 9(d) shows the temperature dependence of the lattice parameters of the tetragonal and the cubic -like phase. The tetragonal peaks survive till 2500°C, which is close to the dielectric maximum temperature.

The electrostrain plots are found to be reproducible in different unipolar cycling of field on the same specimen (Figure 10). For example, the colossal strain of 1.3 % is repeatedly obtained in each cycle until the pellet fractures (in the 10* cycle due to buildup of stress in the ceramic). The number of cycles before fracture increases considerably for lower field amplitudes (Figure 10). Electrostrain of 1.3 % obtained by present invention at 80 kV/cm is close to the record strain of 1.6 % at the same field in [001] oriented single crystal of PZN- PT. The non-saturating nature of electrostrain at 80 kV/cm suggests that it might still be possible to achieve electrostrain well beyond 1.3 % on further increase of the field.

The present invention thus provides a non-textured polycrystalline piezoceramic with ultrahigh electrostrain more than 1% which can be of high utility in various fields where piezoceramics is of high demand. The ease of fabrication of the system provides added advantage as it can be produced on an industrial scale for facile adoption in variety of applications. The invention offers a solution to replace expensive single crystals for adoption as actuator material.