Taiwan and Vietnam have taken successful measures to combat the spread of COVID-19 at the early s... more Taiwan and Vietnam have taken successful measures to combat the spread of COVID-19 at the early stages. Many authors attributed the successful policies to the lessons learned by these countries during the severe acute respiratory syndrome (SARS) pandemic in 2002.(Ohara, 2004) This manuscript provides a summary of recent early-stage policies that were successful in mitigating the spread and creating resilience against the negative consequences of COVID-19 in Taiwan and Vietnam. Crucially, these policies go beyond and complement social isolation. As social isolation is expected to have a negative socio-economic impact on the population and adherence is likely to decrease with time(Armitage and Nellums, 2020; Weems et al., 2020), it is important to consider a broad range of policies to promote a steady control of the COVID-19 spread. Initially, we provide a brief introduction to some general concepts related to COVID-19. Thereafter, we introduce a concise review of policies and their d...
Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a larg... more Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr 0.2 Ti 0.8 )O 3 -0.35(Ba 0.7 Ca 0.3 )TiO 3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of DT EC ¼ 0.33 K at $65 C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about $50%. The discrepancy is attributed to the relaxor character of this material. V C 2015 AIP Publishing LLC. [http://dx.
In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1 – x)Ba(Zr 0... more In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1 – x)Ba(Zr 0.2 Ti 0.8)O 3 –x(Ba 0.7 Ca 0.3)TiO 3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, DT direct = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase.
Coupling of order parameters provides a means to tune functionality in advanced materials includi... more Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed " rotopolarization ". These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi 1/2 Na 1/2 TiO 3-0.25SrTiO 3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2015
The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at r... more The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at room temperature. One layer consisted of a nonergodic relaxor, (Bi Na )TiO 1/2 1 /2 3 -7 , BaTiO 3 with an electric-field-induced longrange ferroelectric order, whereas the other is understood to be an ergodic relaxor [(Bi Na )TiO 1/2 1 /2 3 -25 ] SrTiO 3 that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of a bilayer with low diffusion between the two components is also demonstrated. By taking advantage of the different macroscopic strain-and polarization-electric-field responses of the two constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of the volume fractions and macroscopic properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics.
Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a larg... more Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr 0.2 Ti 0.8 )O 3 -0.35(Ba 0.7 Ca 0.3 )TiO 3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of DT EC ¼ 0.33 K at $65 C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about $50%. The discrepancy is attributed to the relaxor character of this material. V C 2015 AIP Publishing LLC. [http://dx.
Coupling of order parameters provides a means to tune functionality in advanced materials includi... more Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed "rotopolarization". These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi1/2Na1/2TiO3-0.25SrTiO3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.
Contributions to the piezoelectric response of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO... more Contributions to the piezoelectric response of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 ceramics are quantified by small signal measurements made as functions of bias field and temperature. The highest fraction of intrinsic contributions is observed far from phase boundaries, of extrinsic contributions around phase boundaries, and of irreversible switching in the orthorhombic phase. The largest piezoelectric response, d 33 ¼ 475 6 85 pC/N, is found near the orthorhombic to tetragonal phase boundary due to both reversible and irreversible switching. A peak in reversible switching above the Curie temperature for all compositions suggests a line of critical points associated with first order phase transitions, indicating that concurrence of triple and tricritical points in the zero-field phase diagram is not the responsible mechanism of enhanced piezoelectricity. V C 2015 AIP Publishing LLC.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2015
The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at r... more The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at room temperature. One layer consisted of a nonergodic relaxor, (Bi Na )TiO 1/2 1 /2 3 -7 , BaTiO 3 with an electric-field-induced longrange ferroelectric order, whereas the other is understood to be an ergodic relaxor [(Bi Na )TiO 1/2 1 /2 3 -25 ] SrTiO 3 that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of a bilayer with low diffusion between the two components is also demonstrated. By taking advantage of the different macroscopic strain-and polarization-electric-field responses of the two constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of the volume fractions and macroscopic properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics.
The design of core-shell materials affords additional degrees of freedom to tailor functional pro... more The design of core-shell materials affords additional degrees of freedom to tailor functional properties as compared to solid solution counterparts. Although to date most of the work in core-shell materials has focused on dielectrics, piezoelectric coreshell ceramics may gain similar interest. Generalities of core-shell functional ceramics features are addressed in this work. A model system, Bi 1/2 Na 1/2 TiO 3 -SrTiO 3 , is introduced to discuss structure-property relationships. We demonstrate that this system features a core-shell microstructure for the composition corresponding to 25 at.% Sr. The material is studied by means of macroscopic functional properties and in situ structural characterization techniques at different length scales, such as X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The evolution of the core-shell with field and temperature determines its functional properties. The high strain of the system, 0.3% at 4 kV/mm, is due to an electric-field-induced phase transition of the core and shell. Upon field removal the core remains in a poled state, whereas the shell is characterized by a reversible transformation. The reversibility of the phase transition of shells and associated switching are key features in the observed giant strain. Dielectric anomalies are found to be related to changes in oxygen octahedral tilting angles within the core and shell. Recent work, however, has shown the potential of multiphase structures such as ceramic/ceramic composites 23-28 and core-shell materials. 29 This contribution focuses on the D. Johnson-contributing editor Manuscript No. 35120.
The morphotropic phase boundary composition Bi1/2Na1/2TiO3-20 mol. % Bi1/2K1/2TiO3 was chosen as ... more The morphotropic phase boundary composition Bi1/2Na1/2TiO3-20 mol. % Bi1/2K1/2TiO3 was chosen as initial material to do selective A-site aliovalent doping replacing Na and K by 1 at. % La, respectively. The materials were studied macroscopically by measuring dielectric and electromechanical properties. The Na-replaced material has a lower freezing temperature Tfr, lower remanent polarization and remanent strain, and thus a higher degree of ergodicity than the K-replaced material. These results are contrasted with local poling experiments and hysteresis loops obtained from piezoresponse force microscopy. The faster relaxation of the tip-induced local polarization and the lower remanent state in bias-on and -off loops confirm the higher degree of ergodicity of the Na-replaced material. The difference in functional properties is attributed to small variations in chemical pressure achieved through selective doping. Raman results support this working hypothesis.
Optical temperature sensor based on upconversion emission in Er-doped ferroelectric 0.5Ba(Zr0.2Ti... more Optical temperature sensor based on upconversion emission in Er-doped ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ceramic Appl. Phys. Lett.
The evolution of ferroelectric domains in the lead-free BaðZr 0.2 Ti 0.8 ÞO 3 -xðBa 0.7 Ca 0.3 ÞT... more The evolution of ferroelectric domains in the lead-free BaðZr 0.2 Ti 0.8 ÞO 3 -xðBa 0.7 Ca 0.3 ÞTiO 3 (abbreviated as BZT-xBCT) piezoelectric ceramic is investigated in situ under an applied electric field using transmission electron microscopy (TEM). Poling-induced, reversible, transformation from a multidomain to a single-domain state is monitored for a large variety of compositions. For all studied materials, this transformation occurs with the appearance of an intermediate nanodomain state at moderate poling fields. According to our results, under high poling fields, a single-domain state vanishes and multiple domains reappear within the grains. Upon further cycling, switching between two different multidomain states occurs. For all BZT-xBCT compositions that we investigate, no sign of the electric-field-induced structural changes is detected using the selected area electron-diffraction (SAED) patterns, which are devoid of the reflection splitting or any detectable changes during electrical poling. The extrinsic contribution to the piezoelectric properties is found to dominate in the BZT-xBCT piezoceramic.
The temperature-dependent crack growth resistance behavior of the lead-free (1 À x)Ba(Zr 0.2 Ti 0... more The temperature-dependent crack growth resistance behavior of the lead-free (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 (x = 0.385) ceramic was characterized using compact-tension specimens from 25°C to 60°C. The observed plateau fracture toughness at 25°C was found to be approximately 37% lower than commercial Pb(Zr,Ti)O 3 . At elevated temperature, the maximum fracture toughness displayed a decrease, which was found to be related to the temperature-dependent elastic and ferroelastic properties. Mechanical measurements are presented that demonstrate decreasing effective switching strain, coercive stress and Young's modulus with increasing temperature.
In response to the current environmental regulations against the use of lead in daily electronic ... more In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the marketdominating lead-based ones, representatively Pb(Zr x Ti 1-x )O 3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO 3 and (Bi 1/2 Na 1/2 )TiO 3 -based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi 1/2 Na 1/2 )TiO 3 -based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.
Materials in two composition regimes-K 0.5 Na 0.5 NbO 3 -CaZrO 3 (BNT-BT-KNN-CZ), were synthesize... more Materials in two composition regimes-K 0.5 Na 0.5 NbO 3 -CaZrO 3 (BNT-BT-KNN-CZ), were synthesized via the mixed oxide route and their structural, dielectric and electrical properties were investigated. CZ was identified to render the two local maxima in permittivity more diffused. This resulted in temperature-insensitive relative permittivity spectra with average values from ∼470 up to ∼2300 and operational windows of at least ∼400 • C with less than 15% of variation in the temperature range from −100 up to above 500 • C. Moreover, loss factors are below ∼10% and RC constants range from ∼0.03 s up to ∼4 s at 300 • C. The materials of current investigation are highly attractive for developing capacitors of wide temperature usage.
Taiwan and Vietnam have taken successful measures to combat the spread of COVID-19 at the early s... more Taiwan and Vietnam have taken successful measures to combat the spread of COVID-19 at the early stages. Many authors attributed the successful policies to the lessons learned by these countries during the severe acute respiratory syndrome (SARS) pandemic in 2002.(Ohara, 2004) This manuscript provides a summary of recent early-stage policies that were successful in mitigating the spread and creating resilience against the negative consequences of COVID-19 in Taiwan and Vietnam. Crucially, these policies go beyond and complement social isolation. As social isolation is expected to have a negative socio-economic impact on the population and adherence is likely to decrease with time(Armitage and Nellums, 2020; Weems et al., 2020), it is important to consider a broad range of policies to promote a steady control of the COVID-19 spread. Initially, we provide a brief introduction to some general concepts related to COVID-19. Thereafter, we introduce a concise review of policies and their d...
Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a larg... more Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr 0.2 Ti 0.8 )O 3 -0.35(Ba 0.7 Ca 0.3 )TiO 3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of DT EC ¼ 0.33 K at $65 C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about $50%. The discrepancy is attributed to the relaxor character of this material. V C 2015 AIP Publishing LLC. [http://dx.
In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1 – x)Ba(Zr 0... more In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1 – x)Ba(Zr 0.2 Ti 0.8)O 3 –x(Ba 0.7 Ca 0.3)TiO 3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, DT direct = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase.
Coupling of order parameters provides a means to tune functionality in advanced materials includi... more Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed " rotopolarization ". These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi 1/2 Na 1/2 TiO 3-0.25SrTiO 3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2015
The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at r... more The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at room temperature. One layer consisted of a nonergodic relaxor, (Bi Na )TiO 1/2 1 /2 3 -7 , BaTiO 3 with an electric-field-induced longrange ferroelectric order, whereas the other is understood to be an ergodic relaxor [(Bi Na )TiO 1/2 1 /2 3 -25 ] SrTiO 3 that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of a bilayer with low diffusion between the two components is also demonstrated. By taking advantage of the different macroscopic strain-and polarization-electric-field responses of the two constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of the volume fractions and macroscopic properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics.
Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a larg... more Solid solutions of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr 0.2 Ti 0.8 )O 3 -0.35(Ba 0.7 Ca 0.3 )TiO 3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of DT EC ¼ 0.33 K at $65 C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about $50%. The discrepancy is attributed to the relaxor character of this material. V C 2015 AIP Publishing LLC. [http://dx.
Coupling of order parameters provides a means to tune functionality in advanced materials includi... more Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed "rotopolarization". These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi1/2Na1/2TiO3-0.25SrTiO3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.
Contributions to the piezoelectric response of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO... more Contributions to the piezoelectric response of (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 ceramics are quantified by small signal measurements made as functions of bias field and temperature. The highest fraction of intrinsic contributions is observed far from phase boundaries, of extrinsic contributions around phase boundaries, and of irreversible switching in the orthorhombic phase. The largest piezoelectric response, d 33 ¼ 475 6 85 pC/N, is found near the orthorhombic to tetragonal phase boundary due to both reversible and irreversible switching. A peak in reversible switching above the Curie temperature for all compositions suggests a line of critical points associated with first order phase transitions, indicating that concurrence of triple and tricritical points in the zero-field phase diagram is not the responsible mechanism of enhanced piezoelectricity. V C 2015 AIP Publishing LLC.
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2015
The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at r... more The macroscopic electromechanical behavior of lead-free bilayer composites was characterized at room temperature. One layer consisted of a nonergodic relaxor, (Bi Na )TiO 1/2 1 /2 3 -7 , BaTiO 3 with an electric-field-induced longrange ferroelectric order, whereas the other is understood to be an ergodic relaxor [(Bi Na )TiO 1/2 1 /2 3 -25 ] SrTiO 3 that undergoes a reversible electric-field-induced macroscopic nonpolar-to-polar transition. Microstructural evidence of a bilayer with low diffusion between the two components is also demonstrated. By taking advantage of the different macroscopic strain-and polarization-electric-field responses of the two constituents, internal mechanical and electrical fields can be developed that enhance the unipolar strain over that expected by a rule of mixtures approximation, thereby improving the properties needed for application of such materials to actuator systems. It is possible through further tailoring of the volume fractions and macroscopic properties of the constituents to optimize the electromechanical properties of multilayer lead-free ferroelectrics.
The design of core-shell materials affords additional degrees of freedom to tailor functional pro... more The design of core-shell materials affords additional degrees of freedom to tailor functional properties as compared to solid solution counterparts. Although to date most of the work in core-shell materials has focused on dielectrics, piezoelectric coreshell ceramics may gain similar interest. Generalities of core-shell functional ceramics features are addressed in this work. A model system, Bi 1/2 Na 1/2 TiO 3 -SrTiO 3 , is introduced to discuss structure-property relationships. We demonstrate that this system features a core-shell microstructure for the composition corresponding to 25 at.% Sr. The material is studied by means of macroscopic functional properties and in situ structural characterization techniques at different length scales, such as X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The evolution of the core-shell with field and temperature determines its functional properties. The high strain of the system, 0.3% at 4 kV/mm, is due to an electric-field-induced phase transition of the core and shell. Upon field removal the core remains in a poled state, whereas the shell is characterized by a reversible transformation. The reversibility of the phase transition of shells and associated switching are key features in the observed giant strain. Dielectric anomalies are found to be related to changes in oxygen octahedral tilting angles within the core and shell. Recent work, however, has shown the potential of multiphase structures such as ceramic/ceramic composites 23-28 and core-shell materials. 29 This contribution focuses on the D. Johnson-contributing editor Manuscript No. 35120.
The morphotropic phase boundary composition Bi1/2Na1/2TiO3-20 mol. % Bi1/2K1/2TiO3 was chosen as ... more The morphotropic phase boundary composition Bi1/2Na1/2TiO3-20 mol. % Bi1/2K1/2TiO3 was chosen as initial material to do selective A-site aliovalent doping replacing Na and K by 1 at. % La, respectively. The materials were studied macroscopically by measuring dielectric and electromechanical properties. The Na-replaced material has a lower freezing temperature Tfr, lower remanent polarization and remanent strain, and thus a higher degree of ergodicity than the K-replaced material. These results are contrasted with local poling experiments and hysteresis loops obtained from piezoresponse force microscopy. The faster relaxation of the tip-induced local polarization and the lower remanent state in bias-on and -off loops confirm the higher degree of ergodicity of the Na-replaced material. The difference in functional properties is attributed to small variations in chemical pressure achieved through selective doping. Raman results support this working hypothesis.
Optical temperature sensor based on upconversion emission in Er-doped ferroelectric 0.5Ba(Zr0.2Ti... more Optical temperature sensor based on upconversion emission in Er-doped ferroelectric 0.5Ba(Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 ceramic Appl. Phys. Lett.
The evolution of ferroelectric domains in the lead-free BaðZr 0.2 Ti 0.8 ÞO 3 -xðBa 0.7 Ca 0.3 ÞT... more The evolution of ferroelectric domains in the lead-free BaðZr 0.2 Ti 0.8 ÞO 3 -xðBa 0.7 Ca 0.3 ÞTiO 3 (abbreviated as BZT-xBCT) piezoelectric ceramic is investigated in situ under an applied electric field using transmission electron microscopy (TEM). Poling-induced, reversible, transformation from a multidomain to a single-domain state is monitored for a large variety of compositions. For all studied materials, this transformation occurs with the appearance of an intermediate nanodomain state at moderate poling fields. According to our results, under high poling fields, a single-domain state vanishes and multiple domains reappear within the grains. Upon further cycling, switching between two different multidomain states occurs. For all BZT-xBCT compositions that we investigate, no sign of the electric-field-induced structural changes is detected using the selected area electron-diffraction (SAED) patterns, which are devoid of the reflection splitting or any detectable changes during electrical poling. The extrinsic contribution to the piezoelectric properties is found to dominate in the BZT-xBCT piezoceramic.
The temperature-dependent crack growth resistance behavior of the lead-free (1 À x)Ba(Zr 0.2 Ti 0... more The temperature-dependent crack growth resistance behavior of the lead-free (1 À x)Ba(Zr 0.2 Ti 0.8 )O 3 -x(Ba 0.7 Ca 0.3 )TiO 3 (x = 0.385) ceramic was characterized using compact-tension specimens from 25°C to 60°C. The observed plateau fracture toughness at 25°C was found to be approximately 37% lower than commercial Pb(Zr,Ti)O 3 . At elevated temperature, the maximum fracture toughness displayed a decrease, which was found to be related to the temperature-dependent elastic and ferroelastic properties. Mechanical measurements are presented that demonstrate decreasing effective switching strain, coercive stress and Young's modulus with increasing temperature.
In response to the current environmental regulations against the use of lead in daily electronic ... more In response to the current environmental regulations against the use of lead in daily electronic devices, a number of investigations have been performed worldwide in search for alternative piezoelectric ceramics that can replace the marketdominating lead-based ones, representatively Pb(Zr x Ti 1-x )O 3 (PZT)-based solid solutions. Selected systems of potential importance such as chemically modified and/or crystallographically textured (K, Na)NbO 3 and (Bi 1/2 Na 1/2 )TiO 3 -based solid solutions have been developed. Nevertheless, only few achievements have so far been introduced to the marketplace. A recent discovery has greatly extended our tool box for material design by furnishing (Bi 1/2 Na 1/2 )TiO 3 -based ceramics with a reversible phase transition between an ergodic relaxor state and a ferroelectric with the application of electric field. This paired the piezoelectric effect with a strain-generating phase transition and extended opportunities for actuator applications in a completely new manner. In this contribution, we will present the status and perspectives of this new class of actuator ceramics, aiming at covering a wide spectrum of topics, i.e., from fundamentals to practice.
Materials in two composition regimes-K 0.5 Na 0.5 NbO 3 -CaZrO 3 (BNT-BT-KNN-CZ), were synthesize... more Materials in two composition regimes-K 0.5 Na 0.5 NbO 3 -CaZrO 3 (BNT-BT-KNN-CZ), were synthesized via the mixed oxide route and their structural, dielectric and electrical properties were investigated. CZ was identified to render the two local maxima in permittivity more diffused. This resulted in temperature-insensitive relative permittivity spectra with average values from ∼470 up to ∼2300 and operational windows of at least ∼400 • C with less than 15% of variation in the temperature range from −100 up to above 500 • C. Moreover, loss factors are below ∼10% and RC constants range from ∼0.03 s up to ∼4 s at 300 • C. The materials of current investigation are highly attractive for developing capacitors of wide temperature usage.
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Papers by Matias Acosta