Siu Wing Or

Structural transformation and ferroelectromagnetic behavior in single-phase Bi1−xNdxFeO3 multiferroic ceramics

Single-phase Bi1−xNdxFeO3 (BNFOx) (x=0–0.2) multiferroic ceramics were prepared to study the effects of Nd substitution on their crystal structure and ferroelectromagnetic behavior. Rietveld refinement of x-ray diffraction data showed a continual transformation of crystal structure from the rhombohedral structure of BNFOx=0 (BiFeO3) to a triclinic structure in BNFOx=0.05–0.15 and a pseudotetragonal structure in BNFOx=0.175–0.2. Ferroelectromagnetic measurements revealed the existence of ferroelectricity with remnant polarization of ∼9μC∕cm2 in BNFOx=0–0.175, paraelectricity in BNFOx=0.2, and weak ferromagnetism with remnant magnetizations of 0.07–0.227emu∕g in BNFOx=0.15–0.2. Magnetoelectric coupling was obvious in BNFOx=0.15–0.175 near the Neel temperature of ∼380°C.

Multiferroicity in polarized single-phase Bi0.875Sm0.125FeO3 ceramics

Single-phase Bi0.875Sm0.125FeO3 ceramics were prepared, and their crystal structure and multiferroic properties were studied. The triclinic structure with P1 space group was confirmed in the ceramics by refining the x-ray diffraction data. Large piezoelectric d33 coefficient of 29pC∕N, together with high remnant polarization of 15.09μC∕cm2, was measured at room temperature, suggesting the existence of long-range ferroelectric order on a macroscopic scale. The observed small remnant magnetization of 0.071emu∕g at room temperature as a result of the collapse of the space-modulated spin structure indicated the presence of long-range canted antiferromagnetic order on a macroscopic scale. The coexistence of the long-range ferroelectric and canted antiferromagnetic orders allowed the magnetoelectric effect below the antiferromagnetic Neel temperature of 265°C near which magnetoelectric coupling was obvious.

Dynamic magnetomechanical properties of Terfenol-D/epoxy pseudo 1-3 composites

Terfenol-D/epoxy pseudo 1-3 composites were fabricated by embedding and aligning Terfenol-D particles with a size distribution of 10–300μm in a passive epoxy matrix using six Terfenol-D volume fractions (υf) ranging from 0.22 to 0.72. The dependence of the dynamic relative permeability (μr33T), elastic modulus (E3H), and dynamic strain coefficient (d33) on υf was investigated as a function of magnetic bias field (HBias). The HBias response data showed that the built-in non-180° domain states related to residual compressive stresses in the composites result in a significant decrease in μr33T for HBias 0.5. The present study provides a useful guide to optimize the composite properties for transducer design.

Enhanced piezoelectric and pyroelectric effects in single-phase multiferroic Bi1−xNdxFeO3(x=0–0.15) ceramics

Single-phase multiferroic Bi1−xNdxFeO3 (x=0–0.15) ceramics are polarized and their piezoelectric and pyroelectric effects are reported. The success in polarizing the ceramics originates from low electrical conductivities controlled predominantly by the Poole-Frenkel conduction mechanism. Limited internal traps due to reduced oxygen vacancies in the ceramics result in sufficiently low leakage current densities of <30mA∕m2 even at a high electric field of 145kV∕cm. The piezoelectric d33 coefficients of the ceramics before and after annealing at 550°C for 4h are measured to be ∼28 and ∼24pC∕N, respectively. These values are higher than those of most major lead-free high-temperature piezoelectric ceramics.

Converse magnetoelectric effect in three-phase composites of piezoceramic, metal cap, and magnet

We report experimentally and theoretically the converse magnetoelectric (CME) effect in a three-phase piezoceramic-metal-cap-magnet composite made by sandwiching a thickness-polarized Pb(ZrxTi1−x)O3 (PZT) disk between two truncated conical brass caps and two thickness-magnetized SmCo disks. The reported CME effect originates from the product of the converse piezoelectric effect in the PZT disk, the mechanical transformation/amplification effect in the brass caps, and the magnetic induction effect in the SmCo disks. The composite exhibits a large CME coefficient (αB) in excess of 2mG∕V with a flat response in the broad frequency range of 0.1–100kHz. The measured magnetic flux density shows an extremely linear relationship to the applied voltage with amplitude varying from 10to100V over a wide range of detection distance of 0.7–6.5mm. This electromechanomagnetically coupled effect enables applications of the composite in coil-free magnetic flux control devices, featuring smaller Joule heating loss, wider operational bandwidth, and greater property-tailorable flexibility compared to conventional electromagnet-based devices.We report experimentally and theoretically the converse magnetoelectric (CME) effect in a three-phase piezoceramic-metal-cap-magnet composite made by sandwiching a thickness-polarized Pb(ZrxTi1−x)O3 (PZT) disk between two truncated conical brass caps and two thickness-magnetized SmCo disks. The reported CME effect originates from the product of the converse piezoelectric effect in the PZT disk, the mechanical transformation/amplification effect in the brass caps, and the magnetic induction effect in the SmCo disks. The composite exhibits a large CME coefficient (αB) in excess of 2mG∕V with a flat response in the broad frequency range of 0.1–100kHz. The measured magnetic flux density shows an extremely linear relationship to the applied voltage with amplitude varying from 10to100V over a wide range of detection distance of 0.7–6.5mm. This electromechanomagnetically coupled effect enables applications of the composite in coil-free magnetic flux control devices, featuring smaller Joule heating loss, wider op...

Raman scattering spectra and ferroelectric properties of Bi1−xNdxFeO3(x=0–0.2) multiferroic ceramics

Single-phase Bi1−xNdxFeO3 (BNFOx) (x=0–0.2) multiferroic ceramics were prepared to study the effects of Nd substitution on their crystal structures, Raman scattering spectra, and ferroelectric properties. Rietveld refinement of x-ray diffraction data showed a gradual change in crystal structure from rhombohedral to pseudotetragonal via triclinic with increasing x. The evolvement of Raman normal modes with increasing x suggested that such structural change is accompanied by the weakening of long-range ferroelectric order. The simultaneous occurrence of abrupt mode evolvement and ferroelectric-paraelectric transition in BNFOx=0.175 and BNFOx=0.2 was explained according to the change of Bi–O covalent bonds as a result of the decline in the stereochemical activity of the Bi lone electron pair at 0.175⩽x⩽0.2.

Structural transformation and ferroelectric?paraelectric phase transition in Bi1?x Lax FeO3 (x = 0?0.25) multiferroic ceramics

Bi1−xLaxFeO3 (x = 0–0.25) multiferroic ceramics were prepared to study the compositional driven structural transformation and ferroelectric– paraelectric phase transition. Rietveld-refined crystal structure parameters revealed the existence of a rhombohedral R3c symmetry in BiFeO3, a triclinic P1 symmetry in Bi0.95La0.05FeO3, Bi0.9La0.1FeO3 and Bi0.85La0.15FeO3, a pseudotetragonal P4mm symmetry in Bi0.8La0.2FeO3 and a pseudotetragonal P4/mmm symmetry in Bi0.75La0.25FeO3. La substitution led to the ferroelectric–paraelectric phase transition accompanying abrupt changes of Raman scattering spectra in Bi0.8La0.2FeO3 and Bi0.75La0.25FeO3, besides the release of weak ferromagnetism in Bi0.85La0.15FeO3, Bi0.8La0.2FeO3 and Bi0.75La0.25FeO3. Ferroelectricity with remnant polarization of >9.8 µC cm−2 and piezoelectricity with d33 coefficient of >25 pC/N were confirmed in all compositions except for Bi0.75La0.25FeO3.

Converse magnetoelectric effect in laminated composites of PMN-PT single crystal and Terfenol-D alloy

We have found experimentally and theoretically that laminated composites comprising one layer of length-magnetized Tb0.3Dy0.7Fe1.92 (Terfenol-D) magnetostrictive alloy sandwiched between two layers of thickness-polarized, electro-parallel-connected 0.7Pb(Mg1∕3Nb2∕3)O3–0.3PbTiO3 (PMN–PT) piezoelectric single crystal have a large converse magnetoelectric effect characterized by a large magnetic induction in response to an applied ac voltage. The reported converse magnetoelectric effect originates from the product of the converse piezoelectric effect in the PMN–PT layers and the converse magnetostrictive effect in the Terfenol-D layer. Large converse magnetoelectric coefficient in excess of 105mG∕V is obtained in the composites at a low magnetic bias field of 170Oe. The measured magnetic induction has an excellent linear relationship to the applied ac voltage with amplitude varying from 50to160V. These made the composites to be a promising material for direct realization of core-free magnetic flux control de...

Reduced ferroelectric coercivity in multiferroic Bi0.825Nd0.175FeO3 thin film

Bi0.825Nd0.175FeO3 (BNFO) thin film is grown on Pt∕TiO2∕SiO2∕Si substrate by pulsed laser deposition, and its multiferroic properties are compared with those of BiFeO3 (BFO) thin film. With limited Fe2+ ions and its twinborn oxygen vacancies, both samples show low dielectric losses of <0.026 at 100Hz and high maximum ferroelectric polarizations of ∼34μC∕cm2. The ferroelectric coercive field of BNFO is reduced by ∼40%, reaching a low value of 235kV∕cm, compared to that of BFO due to the increased ratio of 180° and curved ferroelectric domains to total ferroelectric domains and better nucleation of the ferroelectric domains at the BNFO∕Pt interface. The Raman scattering spectra confirm that the ferroelectric polarizations of both samples originate in the stereochemical activity of the Bi lone electron pair. Weak ferromagnetism is observed in both samples as a result of the limited amount of Fe2+ ions and γ‐Fe2O3 impurity.

Enhanced magnetoelectric effect in longitudinal-transverse mode Terfenol-D/Pb(Mg1/3Nb2/3)O3-PbTiO3 laminate composites with optimal crystal cut

Magnetoelectric (ME) laminate composite consisting of optimal crystal cut thickness-polarized piezoelectric 0.7Pb(Mg1∕3Nb2∕3)O3–0.3PbTiO3 (PMN-PT) single crystal and the length-magnetized magnetostrictive Tb0.3Dy0.7Fe1.92 (Terfenol-D) alloy has been fabricated. The cut optimization of PMN-PT crystal greatly enhances the longitudinally magnetized-transversely polarized (L‐T) mode ME effect, which has a superior ME voltage coefficient αE of ∼3.02V∕cmOe in low frequency band. Near the resonance frequency of 95kHz, the coefficient dramatically increases and reaches the maximized value of 33.2V∕cmOe, which is almost two times larger than the previously reported ⟨001⟩-oriented PMN-PT crystal based L‐T mode laminate composite.