Lead-free Ferroelectric Materials Research Articles

Switchable Photoelectric Response in High-Temperature Leadless Molecular Ferroelectric [C4N2H14][BiI5

Lead-free molecular ferroelectrics have garnered considerable attention for their promising potential, but such species with narrow band gap and sensitive photoelectric response are yet inadequate. Herein, we demonstrated the bulk ferroelectric photovoltaic effect in a novel lead-free molecular ferroelectric [C4N2H14][BiI5] with a Curie temperature (Tc) of 366 K and a narrow band gap (Eg) of 1.92 eV. The transformation of the crystal structure from the polar space group P21 to the nonpolar space group P21/m was elucidated using single-crystal X-ray diffraction. Room-temperature (RT) hysteresis loop reveals the intrinsic ferroelectricity of [C4N2H14][BiI5] with a relative small coercive field (Ec ∼ 0.27 kV/cm), saturation polarization (Ps ∼ 1.87 μC/cm2), and remanent polarization (Pr ∼ 1.61 μC/cm2). [C4N2H14][BiI5]-based solar device exhibits significant PV effects with a steady-state photocurrent (Jsc) of 3.54 μA/cm2 and a photovoltage (Voc) of 0.34 V under AM 1.5 G illumination, which can be significantly improved by adjusting the ferroelectric polarization, reaching a maximum Jsc of 140 μA/cm2 and Voc of 0.51 V. This work offers a promising avenue for lead-free molecular ferroelectric materials in the field of optoelectronic devices.

Low field assisted electrocaloric effect and energy storage responses through optimization of morphotropic phase boundary by sintering temperature for lead-free Ba0.95Ca0.05Sn0.09Ti0.91O3 ceramics

The development of lead-free ferroelectric materials is highly desired for next-generation energy storage and superior electrocaloric effect (ECE) applications. However, one of the significant challenges in lead-free dielectric materials is to obtain enhanced ECE and energy storage density at lower applied electric fields, needed for realization of sustainable and efficient energy solutions. Herein, the sol-gel elaborated lead-free Ba0.95Ca0.05Sn0.09Ti0.91O3 ceramics have been synthesized and sintered at temperatures 1100 and 1200 ˚C. The sample sintered at 1200 ˚C exhibits low field assisted improved ∆T ∼ 0.42 K and ∆S ∼ 0.52 J/kg/K values and are closely associated with presence of MPB and relaxor characteristics. The enhanced recoverable energy density (Wrec) and total energy density (Wtot) are also obtained as 79 mJ/cm3 and 87 mJ/cm3 for the same sample sintered at 1200 ˚C with excellent conversion efficiency of 91 % under a very low applied electric field of 30 kV/cm. The results demonstrate that the optimization of sintering temperature significantly affects the MPB evolution, microstructure and emergence of relaxor characteristics owing to vibrant polar-nano regions that result in enhanced ECE and energy storage characteristics. The present work would be beneficial for advancement of lead-free ferroelectric materials as an efficient electrocaloric cooling and sustainable green energy solutions.

FTIR characterization analysis of BaTiO3 nanoparticles synthesized using Terminalia catappa leaf extract

Ferroelectric materials are materials that are in great demand in high-density memory applications, such as ferroelectric random access memory, ferroelectric field effect transistors, negative capacitance field effect transistors, and ferroelectric tunnel junctions. Barium titanate (BaTiO3) is a lead-free ferroelectric material with a high dielectric constant and low dielectric loss. The method used is sol gel for the synthesis of BaTiO3 solution, extract for ketapang (Terminalia catappa) leaves, and characterized by FTIR. The FTIR spectrum synthesized with ketapang leaf extract displays several characteristic peaks in the mid-infrared region (4000 – 400 cm-1). These peaks can be determined by the specific vibration mode of the structure and organic compounds contained in the ketapang leaf extract. BaTiO3 has a cubic perovskite structure with a peak wave number of 600 cm-1. Ketapang leaf extract is incorporated into the BaTiO3 structure. BaTiO3 has a Ti–O functional group. Ketapang leaf extract has C–O, C=O, C–H, and O–H functional groups.

Comment on “Ferroelectric composite-based piezoelectric energy harvester for self-powered detection of obstructive sleep”

SrTi2O5 particles were claimed by Panda et al. in J. Materiomics 2023; 9:609 as a new lead-free ferroelectric material with orthorhombic symmetry and space group of Cmm2, being, therefore, employed as a base of piezoelectric energy harvesters. However, in this comment we express concerns regarding the presence of the piezoelectricity in the studied material and the interpretation of the structural, microstructural, and ferroelectric results in that publication as those associated with SrTi2O5. We also note that the presented dielectric results are contradictory and that many important details are missing.

Double hysteresis (P-E) loops in the transparent composite containing BaTiO3 at room temperature

A transparent composite sample containing BaTiO3 was synthesized by using the melting quenching method. The presence of a glassy phase in this composite sample was detected through XRD analysis, and this was further confirmed by DSC study. TEM and SAED analyses provided evidence that the BaTiO3 nanoparticles/clusters are embedded within the borate glass matrix, establishing the composite nature of this sample. FTIR spectrum of the present sample revealed that the glass matrix is composed of two structural groups (BO3 with NBO’s, and BO4), along with the distinct groups for BaTiO3. XPS spectra of the present composite sample indicated the presence of more than one type of boron, barium, titanium and oxygen. DSC and dielectric studies of the present composite sample revealed the presence of the phase transition temperature (Tc). Dielectric constant (ɛr) and dielectric loss (tan δ) curves of the present composite sample displayed an anomaly peak in the vicinity of TC. The optical transmission spectrum of the present composite sample exhibit two transmission bands of Ti3+ (3d1) ions in tetragonal distorted sites. At room temperature, the present transparent composite sample exhibited double hysteresis loops for BaTiO3 at low electric fields. The results obtained can be used for the development of lead-free ferroelectric material.

Surface ferromagnetism of lead-free ferroelectric bismuth sodium titanate materials

The role of complex surface defect on the magnetic at the (110) surface of bismuth sodium titanate (Bi0.5Na0.5TiO3) was discussed based on the first-principles calculation. The first-principle calculations for various types of surface defects exhibited the existence of magnetic moments for selected chemical and position defects. Specifically, Na and Bi vacancies induced large magnetic moments of 0.52 µB/f.u and 0.50 µB/f.u, respectively, which were larger than that of Ti vacancies of 0.01 µB/f.u. Interestingly, oxygen vacancies did not induce local magnetic moments. Furthermore, significant magnetic moments of 0.50 µB/f.u and 0.49 µB/f.u were obtained for Na and Bi interstitial defects, while the local magnetic moments were slightly achieved around 0.03 µB/f.u and 0.04 µB/f.u for Ti and O interstitial defects, respectively. Anti-site defects between Bi and Na at A-site in perovskite ABO3 structure exhibited magnetic moments of 0.55 µB/f.u for Na anti-site at Bi-site and 0.39 µB/f.u for Bi anti-site at Na-site. Interestingly, anti-site defects between the A-site and B-site in perovskite ABO3 structure resulted in larger magnetic moments, with values of 0.57 µB/f.u and 0.53 µB/f.u obtained for Ti anti-site defects at the Bi-site and Na-site, respectively. Additionally, magnetic moments of 0.50 µB/f.u and 0.54 µB/f.u were achieved for Bi and Na anti-site defects at the Ti-site, respectively. We expected that our work further contributed to the understanding of the role of surface defects in the magnetism of Bi0.5Na0.5TiO3 materials in integrating ferromagnetic properties into lead-free ferroelectric materials for smart electronic device applications.

Characterization of dielectric and piezoelectric responses in 0.76(Na0.5Bi0.5)TiO3-0.06BaTiO3-0.18(K0.5Bi0.5)TiO3 ceramics synthesized by the solid-state method

In this research, the authors explore the structural, dielectric, electrical insulating, electromechanical, and piezoelectric characteristics of ceramics that have been modified using BaTiO3 and (K0.5Bi0.5)TiO3.as key components. Specifically, they explore the composition (1-x-y)(Na0.5Bi0.5)TiO3-xBaTiO3-y(K0.5 Bi0.5)TiO3, where x (mol.%) varies between 0, 6, and NBT, and y (mol.%) varies between 0, 18, and KBT. These ceramics were fabricated using a solid-state synthesis technique. The optimized calcination temperature for achieving a pure perovskite phase was determined to be 1000 °C for a duration of 4 h. The research involves a comprehensive analysis of the composition, microstructure, electrical insulating, and electromechanical characteristics of the ceramics. Rietveld analysis of X-ray diffraction (XRD) data was employed to accurately ascertain the morphotropic phase boundary (MPB) within this material. In this paper, we discuss the impact of crystallite size on the shifts in Raman band frequencies. Sintering at 1100 °C for 4 h was found to enhance densification, with scanning electron microscopy (SEM) images illustrating a nearly uniform distribution of compact grains. Electrical insulating assessments demonstrate that each of the ceramics displays a scattered phase transition around the temperature Tm, with a diffusivity ranging from 1.4 to 1.8, and a shift of Tm towards elevated temperatures. Exceptional piezoelectric properties were observed at the morphotropic phase boundary (MPB), including a d33 value of 149 pC/N and electromechanical coupling factors kp of 0.297. These findings are attributed to the coexistence of rhombohedral and tetragonal phases, as well as precise grain size control. This research presents a novel approach to enhancing lead-free ferroelectric materials based on the composition 0.76(Na0.5Bi0.5)TiO3-0.06BaTiO3-0.18(K0.5Bi0.5)TiO3.

Electrocaloric effect with enhanced temperature stability in an interfacial polarization controlled ferroelectric laminated composite

Research on zero-emission lead-free ferroelectric materials is fascinating in the field of solid-state refrigeration due to the observed large electrocaloric phenomenon. However, one of the challenges involved is realizing a large adiabatic temperature change ( ΔT ) over a wide temperature span. Here, a laminated ferroelectric structure with layers, having successive phase transitions covering room temperature, is proposed to overcome this problem. BaTi1−x Sn x O3 with x = 0.08, 0.12, and 0.15 compositions exhibiting the required ferroelectric Curie temperatures was chosen for the component layers. The designed interfacial polarization engineered laminated structure with 0.15/0.08/0.12 compositional layers yielded a value of adiabatic temperature change ΔT max ⩾ 0.6 °C by an indirect method using Maxwell’s thermodynamic equations over a wide temperature span (30 °C–85 °C). The observed results are correlated to the interrelated cumulative effect of reduced interfacial polarization, homogeneous electric field distribution, field-induced phase transitions, and enhanced domain switching, which are the manifestation of the designed laminated geometry.

Quantification of Crystalline Phases in Hf0.5Zr0.5O2 Thin Films through Complementary Infrared Spectroscopy and Ab Initio Supercell Simulations.

In the quest for thinner and more efficient ferroelectric devices, Hf0.5Zr0.5O2 (HZO) has emerged as a potential ultrathin and lead-free ferroelectric material. Indeed, when deposited on a TiN electrode, 1-25 nm thick HZO exhibits excellent ferroelectricity capability, allowing the prospective miniaturization of capacitors and transistor devices. To investigate the origin of ferroelectricity in HZO thin films, we conducted a far-infrared (FIR) spectroscopic study on 5 HZO films with thicknesses ranging from 10 to 52 nm, both within and out of the ferroelectric thickness range where ferroelectric properties are observed. Based on X-ray diffraction, these HZO films are estimated to contain various proportions of monoclinic (m-), tetragonal (t-), and polar orthorhombic (polar o-) phases, while only the 11, 17, and 21 nm thick are expected to include a higher amount of polar o-phase. We coupled the HZO infrared measurements with DFT simulations for these m-, t-, and polar o-crystallographic structures. The approach used was based on the supercell method, which combines all possible Hf/Zr mixed atomic sites in the solid solution. The excellent agreement between measured and simulated spectra allows assigning most bands and provides infrared signatures for the various HZO structures, including the polar orthorhombic form. Beyond pure assignment of bands, the DFT IR spectra averaging using a mix of different compositions (e.g., 70% polar o-phase +30% m-phase) of HZO DFT crystal phases allows quantification of the percentage of different structures inside the different HZO film thicknesses. Regarding the experimental data analysis, we used the spectroscopic data to perform a Kramers-Kronig constrained variational fit to extract the optical functions of the films using a Drude-Lorentz-based model. We found that the ferroelectric films could be described using a set of about 7 oscillators, which results in static dielectric constants in good agreement with theoretical values and previously reported ones for HfO2-doped ferroelectric films.

Intermediate bands and p-orbital tuning of the band structure of BiFeO3 for photovoltaic application.

BiFeO3 is a lead-free inorganic ferroelectric material that holds promise for photovoltaic applications. However, its larger band gap poses a challenge for efficient sunlight absorption, limiting its efficiency in such applications. In this work, nitrogen, which has lower electronegativity compared to oxygen, was selected to tune the band gap of BiFeO3. It was discovered that the band gap of BiFeO3-xNx (x = 0.125, 0.25, 0.375, and 0.5) was reduced from the original value of 3.1 eV for the parent phase to 1.45-1.77 eV, which originated from the intermediate bands caused by the stronger interaction between Fe 3d and N 2p and the valence band maximum shifted towards higher energy levels caused by the introduction of N 2p. In particular, the band gaps of 1.45 and 1.49 eV for BiFeO2.75N0.25 and BiFeO2.625N0.375, respectively, were closer to the optimal band gap of approximately 1.4 eV for photovoltaic materials. Accompanied with the reduction in the band gap, the optical absorption coefficient of N-substituted BiFeO3 in the visible range was evidently increased. Noticeably, N-substituted BiFeO3 showed good ferroelectricity with a decreased band gap and increased optical absorption. The presented results thus demonstrate that N substitution is a promising way to improve the ferroelectric photovoltaic properties of BiFeO3.

ELECTRICAL PROPERTIES OF MULTI-VALENT ALTERNATE ABO3 PEROVSKITE (KNN) MULTI-FUNCTIONALFERROELECTRIC CERAMICS

Ecological-friendly and unhazardous materials are being prepared by the scientific community in search of novel lead-free ferroelectric materials. K0.5Na0.5NbO3belonging to the ABO3 perovskite was found to be promising ferroelectric lead-free material. A new set of compound, namely, double dopant LKNZN compound was prepared by solid state route. Here, Lithium is partially replaced with Sodium, at A-site, and Zirconium substituted with Niobium, at B-site. The compositional formula of the present investigations is: [((K0.5Na0.5)(1-x)Lix) (Zr0.025Nb0.98)O3], where x=0, and 0.025 9 (labelled as KNN1 and KNN2). In addition we have prepared (K0.5(Na0.475Li0.025)) (Zr0.025Nb0.98)O3 (labelled as KNN3). XRD patterns obtained at room temperaturehave shown maximum intensity peak of (111), confirms the formation of a single-phase. SEM pictures of KNN1 and KNN2 have shown cubical grain with bimodal grain distribution, whereas KNN3 consist of circular bimodal grain distribution. Delectric measurements were studied on the said above samples. It is observed a hump around 200°Cand dielectric transition at 4520°C for all the samples. From this it is evident that: (i) the samples are transforming from orthorhombic phase to tetragonal phase near 200°C (ii) around 420°C, samples transform from tetragonal phase to cubic phase. Theresults were corroborated to the tolerance factor and lattice distortions.

Atomic‐scale insights into electronic, structural, dielectric, and ferroelectric properties of Ba(Zr, Ti)O3 perovskites

Ba(Zr, Ti)O3 perovskites are promising lead-free piezoelectric and relaxor ferroelectric materials for energy storage and harvest devices, of which the ferroelectric mechanism has long been ambiguous. We theoretically investigated the ferroelectric mechanism from the electronic and atomic scale using first-principles calculation based on density functional theory and density functional perturbation theory. With increasing zirconium content, it is obtained a lattice expansion and a decrease in the ferroelectric polarization in agreement with experiment. An unstable zone-center phonon mode is observed in the polar ferroelectric phase, which tends to stabilizein the nonpolar paraelectric phase, which is associated with the engineering of the relative displacement of the B-site ions that alters the short-range force. The newly formed Ti/Zr (dzx,dyz)-O (2px,2py) π-type bonds are discovered to be the origin of the Ba(Zr, Ti)O3ferroelectric instability and polarization. Local relaxation strains caused by lattice misalignment of the ionic displacements of Ti ions and Zr ions suppress the polarization of Ba(Zr, Ti)O3 by counteracting the off-centering of Ti ions and adjacent Zr ions in certain directions.

Superior energy storage performance of BNT-based ferroelectric ceramics based on maintaining high polarization and breakdown strength

Bi[Formula: see text]Na[Formula: see text]TiO3 (BNT)-based lead-free ceramics with superior ferroelectric properties are considered to be extremely advantageous in energy storage capacitors for future green technologies. Here, we demonstrate an approach to achieve both ultrahigh energy density [Formula: see text] and efficiency [Formula: see text] by regulating the multiscale electropolar structures and microstructure. A satisfactory energy storage performance of a high [Formula: see text] of [Formula: see text], and a decent [Formula: see text] of 80% under [Formula: see text] are attained in the 0.5(BNT-CS)-0.5SB[Formula: see text]T ceramic (abbreviated as BNT-0.2SBT). Moreover, BNT-0.2SBT exhibits superior power density ([Formula: see text]), ultrafast discharge time ([Formula: see text][Formula: see text]ns) at [Formula: see text], and good temperature stability. The findings in this work not only demonstrate that a valid candidate, but also provide a new idea of how to achieve both high-energy storage density and efficiency in lead-free ferroelectric materials.

A brownmillerite electronic material LiBiFe2O5: structural, dielectric, electrical, and ferroelectric properties for device application

ABSTRACT A lead-free brownmillerite ferroelectric material LiBiFe2O5 (LBFO) has been synthesized via a solid-state reaction technique. Using the procedure of N-TREOR method of X-ray diffraction, the crystal structure of the prepared sample is found to be monoclinic with lattice parameters: a = 13.6726 Å, b = 4.1560 Å, c = 8.5317 Å, and β = 102.89˚. Based on analysis of electrical data, the existence of Koop's and Maxwell–Wagner relaxation phenomena and dielectric relaxation in the material has been confirmed. This analysis has also shown a high impact of the dielectric permittivity (ϵr) and low tangent loss (tanδ) on using the material in some electronic devices. A detailed study of the frequency-temperature-dependent ac conductivity has suggested the presence of Jonscher's universal power law in the material. As the material has thermistor constant (β) values of 4019.26, 11535.75, and 484.36 in different temperature ranges indicating high-temperature NTC thermistor application.

Defect engineering in barium titanate ferroelectric ceramic showing simultaneous enhancement of piezoelectric coefficient and mechanical quality factor

For high-power on-resonant piezoelectric applications, piezoelectric materials with high piezoelectric coefficient (d33) and mechanical quality factor (Qm) are in high demand. However, previous work has shown that the d33 and Qm are generally inversely proportional, implying that it is difficult to improve these properties simultaneously. Here we report the simultaneous enhancement of d33 and Qm in (Mn, Zr) co-doped BaTiO3 (BT-xMn-yZr) ceramics. Compared with pure BaTiO3 ceramics, the piezoelectric coefficient (d33) of BT-0.5%Mn-5%Zr ceramic is increased from 190 pC/N to 321 pC/N, and the Qm is increased from 290 to 513. Furthermore, the Qm of BT-1%Mn-5%Zr ceramic reaches 1470, and the d33 is 248 pC/N. When both d33 and Qm are considered, the obtained properties outperform commercial hard Pb(Zr,Ti)O3 and other typical lead-free ferroelectric ceramics. The experimental results show that the enhanced d33 and Qm of these ceramics originate from the synergistic effect of defect engineering-induced domain configuration effect and pinning effect. This work provides insights into the design of high-performance high-power lead-free ferroelectric materials.

Insight of the Influence of Cobalt Substitution on the Structural, Microstructural, Optical and Electrical Properties of Tetragonal Barium Titanate Ceramics

The development of lead-free ferroelectric materials is essential for upcoming applications in energy and information storage. We comprehensively report the structural, optical, dielectric and electrical characteristics of BaTi1-x Co x O3 (0.00 ≤ x ≤ 0.10) ceramics prepared using the conventional solid-state route process. The synthesized sample features a single-phase tetragonal structure with P4mm symmetry, as confirmed by the X-ray diffraction investigation. SEM imaging clearly shows that the material was polydispersive. UV–vis diffuse reflectance spectra of the BaTi1-xCoxO3 ceramics has been carried out in order to calculate the optical band gap of each composition. For the prepared samples, Cole-Cole plot of the complex impedance was examined over a wide temperature and frequency range. The compounds displayed non-Debye type dielectric relaxation. The Ferroelectric-Paraelectric phase transition is clearly visible in the T-dependent dielectric spectra for all the samples. After E-poling the ceramics, a well-defined genuine saturated hysteresis loop was established.

Superior energy storage performance and excellent multiferroic properties of BaTi1-xGdxO3 (0 ≤ x ≤ 0.06) ceramics

Ceramic based capacitors with superior energy storage performance and high storage energy density play an important role in modern electronic devices. Several ferroelectric materials with high dielectric constant and relaxor behavior become the most promising systems for realizing higher energy storage performance. In the present work, environmentally lead-free BaTi1-xGdxO3 (0 ≤ x ≤ 0.06) nanostructures were synthesized using the sol-gel auto-combustion process. The doping of Gd at the Ti site causes the transformation of the ferroelectric behavior to the relaxor ceramics, which is important for finding the large energy storage efficiency of the material. The surface morphology of undoped and doped samples was examined through the SEM studies. Ferroelectric study evidence that the system is going to a more symmetric phase due to a reduction in the polarization parameters and enhancement in the leakage current density. Moreover, the energy discharge density and energy storage efficiency were boosted in x = 0.02 and x = 0.04 samples with energy storage efficiencies reaching ∼81% and 83%, respectively. The linear enhancement in the magnetic moment and the dielectric constant in the doped sample make the system favorable for multifunctional applications. The samples' piezoelectric constant (d33) remains preserved under Gd doping. Hence, the present study opens a propagable and feasible route to synthesize novel lead-free ferroelectric material for energy storage applications.

Structural Refinement, Dielectric and Electromechanical Properties of (1-x)NBT-xKBT Piezoceramics in the Morphotropic Phase Boundary Region

Lead-free ferroelectric materials of sodium-potassium bismuth titanate, (1-x)NBT-xKBT systems were synthesized by a hydrothermal process. In this way, the appropriate conditions for the hydrothermal synthesis of NBT and KBT (i.e., concentrations of synthetic precursors, solution pH and temperature) are given graphically. Ceramics of (1-x)NBT-xKBT with (x(mol.%) = 0; 12; 16; 20; 30 and 100) were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The rhombohedral-tetragonal morphotropic phase boundary (MPB) was confirmed to be in the region of 0.12 ≤ x ≤ 0.20 for (1-x)NBT-xKBT at ambient temperature. Scherrer's formula and the Williamson-Hall (W-H) analysis were used to examine the average crystallite size and lattice strain. Raman spectroscopy was effectively applied to study the structural evolution of the (1-x)NBT-xKBT phase. The ceramics exhibited a high temperature of maximum dielectric permittivity at (Tmax = 343 °C at 100 kHz) along with electromechanical coupling factors (kp = 0.34, d33 = 147 pC/N). Based on the composition of all specimens, the results indicate a diffuse phase transition, probably of second order, between ferroelectric and paraelectric phases.

Improved dielectric energy storage performance of Na0.5Bi0.5TiO3-based lead-free relaxation ferroelectric ceramics achieved by domain structural regulation and enhanced densification

There is still a problem of low energy storage density in dielectric capacitors which is a core component of power systems. For the improvement of the energy storage density, the linear dielectric material CaTiO3 (CT) was introduced in Na0.5Bi0.5TiO3 (NBT) ceramics in this paper. By modifying the A site, a new relaxor ferroelectric ceramic was successfully synthesized and attained a recoverable density (Wrec) of 2.34 J/cm3 at x = 0.18. Moreover, the preparation process was optimized in this paper. Through the viscous polymer process (VPP) route, the energy density (WA) of 82NBT-18CTVPP ceramic further reaches 6.45 J/cm3 at 340 kV/cm, with efficiency (η) up to 75% and a Wrec of 4.82 J/cm3. At the same time, the change of Wrec is small at temperature (30–150 °C) and frequency (1 Hz–300 Hz), which demonstrates its excellent stability. The discharge power density reaches about 180 MW/cm3 and the discharge time is 0.117 μs, which indicates its excellent pulse discharge performance. The results show that 82NBT-18CT lead-free relaxation ferroelectric material is expected to become ideal for high-energy storage applications.

Enhanced functional reversibility in lead-free ferroelectric material over long cycle pyroelectric energy conversion

Enhanced functional reversibility in lead-free ferroelectric material over long cycle pyroelectric energy conversion