Water Quenched and Acceptor-Doped Textured Piezoelectric Ceramics for Off-Resonance and On-Resonance Devices.

Piezoelectric materials should simultaneously possess the soft properties (high piezoelectric coefficient, d33 ; high voltage coefficient, g33 ; high electromechanical coupling factor, k) and hard properties (high mechanical quality factor, Qm ; low dielectric loss, tan δ) along with wide operation temperature (e.g., high rhombohedral-tetragonal phase transition temperature Tr-t ) for covering off-resonance (figure of merit (FOM), d33  × g33 ) and on-resonance (FOM, Qm  × k2 ) applications. However, achieving hard and soft piezoelectric properties simultaneously along with high transition temperature is quite challenging since these properties are inversely related to each other. Here, through a synergistic design strategy of combining composition/phase selection, crystallographic texturing, defect engineering, and water quenching technique, <001> textured 2 mol% MnO2 doped 0.19PIN-0.445PSN-0.365PT ceramics exhibiting giant FOM values of Qm  × k 31 2 $k_{31}^2$ (227-261) along with high d33  × g33 (28-35 × 10-12 m2 N-1 ), low tan δ (0.3-0.39%) and high Tr-t of 140-190 °C, which is far beyond the performance of the state-of-the-art piezoelectric materials, are fabricated. Further, a novel water quenching (WQ) room temperature poling technique, which results in enhanced piezoelectricity of textured MnO2 doped PIN-PSN-PT ceramics, is reported. Based upon the experiments and phase-field modeling, the enhanced piezoelectricity is explained in terms of the quenching-induced rhombohedral phase formation. These findings will have tremendous impact on development of high performance off-resonance and on-resonance piezoelectric devices with high stability.

Ultrahigh Durability Perovskite Solar Cells.

Unprecedented conversion efficiency has been demonstrated for perovskite solar cells (PSCs), however, their stability and reliability continue to be challenge. Here, an effective and practical method is demonstrated to overcome the device stability issues in PSCs. A CF4 plasma treatment method is developed that results in the formation of a robust C-F x layer covering the PSC device, thereby, imparting protection during the operation of solar cell. PSCs exposed to fluorination process showed excellent stability against water, light, and oxygen, displaying relatively no noticeable degradation after being dipped into water for considerable time period. The fluorination process did not have any impact on the morphology and electrical property of the top Spiro-OMeTAD layer, resulting in a conversion efficiency of 18.7%, which is identical to that of the pristine PSC. Under the continuous Xe lamp (AM 1.5G, 1 sun) illumination in ambient air for 100 h, the fluorinated PSCs demonstrated 70% of initial conversion efficiency, which is 4000% higher than that of the pristine PSC devices. We believe this breakthrough will have significant impact on the transition of PSCs into real world applications.

Simultaneously Achieving Large Piezoelectricity and Low Dielectric Loss in High-Temperature BiScO3-PbTiO3-Based Ceramics.

High-temperature piezoelectric materials are pivotal to technology applications fields including defense, aerospace, nuclear energy, and oil well logging. However, the acquisition of excellent piezoelectric properties is usually at the cost of temperature stability (reduced Curie temperature and increased high-temperature dielectric loss), which hinders the application of piezoelectric ceramics in harsh environments. In this study, we investigated the effect of Nb5+ donor and Mn2+/3+ acceptor doping on the dielectric and piezoelectric properties of BiScO3-PbTiO3 (BS-PT)-based ceramics. In contrast to the acceptor doping, it was found that the donor doping not only enhances the piezoelectric properties but also effectively suppresses the dielectric loss at a high temperature by reducing the oxygen vacancy concentration. Eventually, we simultaneously attained an excellent piezoelectric performance (d33 is 553 pC/N at room temperature and 1528 pC/N at 400 °C, respectively) and a low dielectric loss (less than 2% in the temperature range of 150-300 °C) but still with a high Curie temperature (TC ∼ 445 °C) in Nb5+-doped BS-PT ceramics. Furthermore, different in situ measurements were used to demonstrate the remarkable temperature stability up to a high depolarization temperature of ∼400 °C. This work represents significant progress in high-temperature piezoelectric materials and provides a guideline for future efforts on enhancing the piezoelectricity and suppressing the dielectric loss at high temperature.

Heterovalent-doping-enabled atom-displacement fluctuation leads to ultrahigh energy-storage density in AgNbO3-based multilayer capacitors.

Dielectric capacitors with high energy storage performance are highly desired for next-generation advanced high/pulsed power capacitors that demand miniaturization and integration. However, the poor energy-storage density that results from the low breakdown strength, has been the major challenge for practical applications of dielectric capacitors. Herein, we propose a heterovalent-doping-enabled atom-displacement fluctuation strategy for the design of low-atom-displacements regions in the antiferroelectric matrix to achieve the increase in breakdown strength and enhancement of the energy-storage density for AgNbO3-based multilayer capacitors. An ultrahigh breakdown strength ~1450 kV·cm-1 is realized in the Sm0.05Ag0.85Nb0.7Ta0.3O3 multilayer capacitors, especially with an ultrahigh Urec ~14 J·cm-3, excellent η ~ 85% and PD,max ~ 102.84 MW·cm-3, manifesting a breakthrough in the comprehensive energy storage performance for lead-free antiferroelectric capacitors. This work offers a good paradigm for improving the energy storage properties of antiferroelectric multilayer capacitors to meet the demanding requirements of advanced energy storage applications.

Giant Energy Density via Mechanically Tailored Relaxor Ferroelectric Behavior of PZT Thick Film.

Relaxor ferroelectrics (RFEs) are being actively investigated for energy-storage applications due to their large electric-field-induced polarization with slim hysteresis and fast energy charging-discharging capability. Here, a novel nanograin engineering approach based upon high kinetic energy deposition is reported, for mechanically inducing the RFE behavior in a normal ferroelectric Pb(Zr0.52 Ti0.48 )O3 (PZT), which results in simultaneous enhancement in the dielectric breakdown strength (EDBS ) and polarization. Mechanically transformed relaxor thick films with 4 µm thickness exhibit an exceptional EDBS of 540 MV m-1 and reduced hysteresis with large unsaturated polarization (103.6 µC cm-2 ), resulting in a record high energy-storage density of 124.1 J cm-3 and a power density of 64.5 MW cm-3 . This fundamental advancement is correlated with the generalized nanostructure design that comprises nanocrystalline phases embedded within the amorphous matrix. Microstructure-tailored ferroelectric behavior overcomes the limitations imposed by traditional compositional design methods and provides a feasible pathway for realization of high-performance energy-storage materials.

Near-ideal electromechanical coupling in textured piezoelectric ceramics.

Electromechanical coupling factor, k, of piezoelectric materials determines the conversion efficiency of mechanical to electrical energy or electrical to mechanical energy. Here, we provide an fundamental approach to design piezoelectric materials that provide near-ideal magnitude of k, via exploiting the electrocrystalline anisotropy through fabrication of grain-oriented or textured ceramics. Coupled phase field simulation and experimental investigation on <001> textured Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 ceramics illustrate that k can reach same magnitude as that for a single crystal, far beyond the average value of traditional ceramics. To provide atomistic-scale understanding of our approach, we employ a theoretical model to determine the physical origin of k in perovskite ferroelectrics and find that strong covalent bonding between B-site cation and oxygen via d-p hybridization contributes most towards the magnitude of k. This demonstration of near-ideal k value in textured ceramics will have tremendous impact on design of ultra-wide bandwidth, high efficiency, high power density, and high stability piezoelectric devices.

Ultrahigh Piezoelectric Performance through Synergistic Compositional and Microstructural Engineering.

Piezoelectric materials enable the conversion of mechanical energy into electrical energy and vice-versa. Ultrahigh piezoelectricity has been only observed in single crystals. Realization of piezoelectric ceramics with longitudinal piezoelectric constant (d33 ) close to 2000 pC N-1 , which combines single crystal-like high properties and ceramic-like cost effectiveness, large-scale manufacturing, and machinability will be a milestone in advancement of piezoelectric ceramic materials. Here, guided by phenomenological models and phase-field simulations that provide conditions for flattening the energy landscape of polarization, a synergistic design strategy is demonstrated that exploits compositionally driven local structural heterogeneity and microstructural grain orientation/texturing to provide record piezoelectricity in ceramics. This strategy is demonstrated on [001]PC -textured and Eu3+ -doped Pb(Mg1/3 Nb2/3 )O3 -PbTiO3 (PMN-PT) ceramics that exhibit the highest piezoelectric coefficient (small-signal d33 of up to 1950 pC N-1 and large-signal d33 * of ≈2100 pm V-1 ) among all the reported piezoelectric ceramics. Extensive characterization conducted using high-resolution microscopy and diffraction techniques in conjunction with the computational models reveals the underlying mechanisms governing the piezoelectric performance. Further, the impact of losses on the electromechanical coupling is identified, which plays major role in suppressing the percentage of piezoelectricity enhancement, and the fundamental understanding of loss in this study sheds light on further enhancement of piezoelectricity. These results on cost-effective and record performance piezoelectric ceramics will launch a new generation of piezoelectric applications.

Design and Fabrication of 15-MHz Ultrasonic Transducers Based on a Textured Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3 Ceramic.

Ultrasound medical imaging is an entrenched and powerful tool for medical diagnosis. Image quality in ultrasound is mainly dependent on performance of piezoelectric transducer elements, which is further related to the electromechanical performance of the constituent piezoelectric materials. With rising need for piezoelectric materials with better performance and low cost, a highly 〈001〉 textured piezo ceramic, Pb(Mg1/3Nb2/3)O3-Pb(Zr, Ti)O3, has been developed. Recently, textured ceramic materials can be produced at low cost and exhibit high piezoelectric strain constants and large electromechanical coupling coefficients. In this work, 15-MHz ultrasonic transducers with an effective aperture of 2.5 mm in diameter based on these highly 〈001〉 textured ceramics have been successfully fabricated. The fabricated transducers achieved a central frequency of 15 MHz, a fractional bandwidth of 67% (at -6 dB), a high effective electromechanical coupling coefficient [Formula: see text] of 0.55, and a low insertion loss (IL) of 21 dB. Ex vivo ultrasonic imaging of a porcine eyeball was used to assess the tomography quality of the transducer. The results show that utilized textured ceramic has a great potential in developing ultrasonic devices for biomedical imaging purposes.

A New Method for Evaluation of the Complex Material Coefficients of Piezoelectric Ceramics in the Radial Vibration Modes.

The determination of complex elastic, piezoelectric, and dielectric coefficients of piezoelectric ceramics is important for precision engineering devices. Here, a novel method for determining the optimal material coefficients is presented. This method minimizes the average relative error in the values of conductance, susceptance, resistance, and reactance obtained from the 1-D model in the IEEE Standard (ANSI/IEEE Std 176-1987) and the experimental measurements of the first and second radial modes. Poisson's ratio is assumed to be a complex number in addition to the elastic, piezoelectric, and dielectric coefficients in the present method. The global minimum of the average relative error is found by searching the minimum among all local minima of the average relative error, which are obtained with the Levenberg-Marquardt modification of Newton's method from randomly chosen initial conditions. The optimal material coefficients of an APC 850 disk and an APC 855 disk are calculated with this method. The uncertainties in the optimal material coefficients are evaluated by calculating the minimum average relative error when the real or imaginary part of each coefficient is prescribed.

Electrocaloric Performance of Multilayer Ceramic Chips: Effect of Geometric Structure Induced Internal Stress.

Driven by an ever-growing demand for environmentally benign cooling systems, the past decade has witnessed the booming development in the field of electrocaloric (EC) cooling technology, which is considered as a promising solid-state cooling approach. Multilayer ceramic chip capacitors (MLCCs) represent the optimum structure for EC cooling elements because of large breakdown strengths, low driving voltages, and high macroscopic volumes of active EC materials. However, fundamental relationships between the geometric parameters of MLCCs and the EC coefficient are less understood. In this study, 0.92Pb(Mg1/3Nb2/3)O3-0.08PbTiO3 (PMN-PT) MLCCs with controlled configurations, such as active/inactive layer thickness, number of layers, and active volume ratio, were fabricated, and their EC performance was evaluated. The electric properties of the MLCCs are confirmed to be closely related to the geometric structure, which influences not only the heat flow but also the internal stress, resulting in the variability of EC performance and reliability/breakdown strength. The internal stress arises due to the residual thermal stress originating from the densification-related shrinkage, thermal expansion mismatch during the sintering, and clamping stress arising from the inactive area due to the large strain from the active area under a high electric field. The geometric structure-based stress distribution and the magnitude of stress on the active layers in MLCCs were determined by finite element modeling (FEM) and correlated with the experimental EC coefficients. The results reveal that a low inactive volume percentage is beneficial toward increasing the breakdown field and enhancement of EC performance because of reduced clamping stress on active EC material.

Electric Field Control of Magnetic Permeability in Co-Fired Laminate Magnetoelectric Composites: A Phase-Field Study for Voltage Tunable Inductor Applications.

Control of magnetic permeability through electric field in magnetoelectric materials promises to create novel voltage tunable inductors (VTIs). VTIs synthesized using co-fired ceramic processing exhibit many advantages over traditional epoxy bonding method, but the internal residual stress in co-fired VTIs resulting from thermal expansion mismatch hinders a full exploitation of the tunability of permeability. To find the optimal condition for high tunability of co-fired VTIs, domain-level phase field modeling and computer simulation are employed to study co-fired magnetoelectric composites comprising NiZn ferrite and PZT. Two key factors important toward increasing the inductor tunability are systematically investigated: intrinsic magnetocrystalline anisotropy of the ferrite material and internal residual stress caused by the co-firing process. The simulations indicate that in order to achieve a large tunability, the tuned permeability should be confined within the linear region of the reciprocal of susceptibility and stress. Additionally, both magnetocrystalline anisotropy and residual stress should be as small as possible. These results provide a design strategy for realizing high-tunability co-fired VTIs.

3D printed graphene-based self-powered strain sensors for smart tires in autonomous vehicles.

The transition of autonomous vehicles into fleets requires an advanced control system design that relies on continuous feedback from the tires. Smart tires enable continuous monitoring of dynamic parameters by combining strain sensing with traditional tire functions. Here, we provide breakthrough in this direction by demonstrating tire-integrated system that combines direct mask-less 3D printed strain gauges, flexible piezoelectric energy harvester for powering the sensors and secure wireless data transfer electronics, and machine learning for predictive data analysis. Ink of graphene based material was designed to directly print strain sensor for measuring tire-road interactions under varying driving speeds, normal load, and tire pressure. A secure wireless data transfer hardware powered by a piezoelectric patch is implemented to demonstrate self-powered sensing and wireless communication capability. Combined, this study significantly advances the design and fabrication of cost-effective smart tires by demonstrating practical self-powered wireless strain sensing capability.

Low-Temperature Co-Fired Unipoled Multilayer Piezoelectric Transformers.

The reliability of piezoelectric transformers (PTs) is dependent upon the quality of fabrication technique as any heterogeneity, prestress, or misalignment can lead to spurious response. In this paper, unipoled multilayer PTs were investigated focusing on high-power composition and co-firing profile in order to provide low-temperature synthesized high-quality device measured in terms of efficiency and power density. The addition of 0.2 wt% CuO into Pb0.98Sr0.02(Mg1/3Nb2/3)0.06(Mn1/3Nb2/3)0.06(Zr0.48Ti0.52)0.88O3 (PMMnN-PZT) reduces the co-firing temperature from 1240 °C to 930 °C, which allows the use of Ag/Pd inner electrode instead of noble Pt inner electrode. Low-temperature synthesized material was found to exhibit excellent piezoelectric properties ( , , %, pC/N, and °C). The performance of the PT co-fired with Ag/Pd electrode at 930 °C was similar to that co-fired at 1240 °C with Pt electrode (25% reduction in sintering temperature). Both high- and low-temperature synthesized PTs demonstrated 5-W output power with >90% efficiency and 11.5 W/cm3 power density.

Colossal tunability in high frequency magnetoelectric voltage tunable inductors.

The electrical modulation of magnetization through the magnetoelectric effect provides a great opportunity for developing a new generation of tunable electrical components. Magnetoelectric voltage tunable inductors (VTIs) are designed to maximize the electric field control of permeability. In order to meet the need for power electronics, VTIs operating at high frequency with large tunability and low loss are required. Here we demonstrate magnetoelectric VTIs that exhibit remarkable high inductance tunability of over 750% up to 10 MHz, completely covering the frequency range of state-of-the-art power electronics. This breakthrough is achieved based on a concept of magnetocrystalline anisotropy (MCA) cancellation, predicted in a solid solution of nickel ferrite and cobalt ferrite through first-principles calculations. Phase field model simulations are employed to observe the domain-level strain-mediated coupling between magnetization and polarization. The model reveals small MCA facilitates the magnetic domain rotation, resulting in larger permeability sensitivity and inductance tunability.

Fabrication of Lead-Free (CH3 NH3 )3 Bi2 I9 Perovskite Photovoltaics in Ethanol Solvent.

The toxicity of lead present in organohalide perovskites and the hazardous solvent systems used for their synthesis hinder the deployment of perovskite solar cells (PSCs). Herein, an environmentally friendly route toward bismuth-based, lead-free (CH3 NH3 )3 Bi2 I9 perovskites that utilize ethanol as the solvent is described. Using this method, dense and homogeneous microstructures were obtained, compared to the porous, rough microstructures obtained using dimethylformamide. Photovoltaic performances were enhanced, with an open-circuit voltage of 0.84 V measured.

Compositionally Graded Multilayer Ceramic Capacitors.

Multilayer ceramic capacitors (MLCC) are widely used in consumer electronics. Here, we provide a transformative method for achieving high dielectric response and tunability over a wide temperature range through design of compositionally graded multilayer (CGML) architecture. Compositionally graded MLCCs were found to exhibit enhanced dielectric tunability (70%) along with small dielectric losses (<2.5%) over the required temperature ranges specified in the standard industrial classifications. The compositional grading resulted in generation of internal bias field which enhanced the tunability due to increased nonlinearity. The electric field tunability of MLCCs provides an important avenue for design of miniature filters and power converters.

Correlation between tunability and anisotropy in magnetoelectric voltage tunable inductor (VTI).

Electric field modulation of magnetic properties via magnetoelectric coupling in composite materials is of fundamental and technological importance for realizing tunable energy efficient electronics. Here we provide foundational analysis on magnetoelectric voltage tunable inductor (VTI) that exhibits extremely large inductance tunability of up to 1150% under moderate electric fields. This field dependence of inductance arises from the change of permeability, which correlates with the stress dependence of magnetic anisotropy. Through combination of analytical models that were validated by experimental results, comprehensive understanding of various anisotropies on the tunability of VTI is provided. Results indicate that inclusion of magnetic materials with low magnetocrystalline anisotropy is one of the most effective ways to achieve high VTI tunability. This study opens pathway towards design of tunable circuit components that exhibit field-dependent electronic behavior.

Giant piezoelectric voltage coefficient in grain-oriented modified PbTiO3 material.

A rapid surge in the research on piezoelectric sensors is occurring with the arrival of the Internet of Things. Single-phase oxide piezoelectric materials with giant piezoelectric voltage coefficient (g, induced voltage under applied stress) and high Curie temperature (Tc) are crucial towards providing desired performance for sensing, especially under harsh environmental conditions. Here, we report a grain-oriented (with 95% <001> texture) modified PbTiO3 ceramic that has a high Tc (364 °C) and an extremely large g33 (115 × 10-3 Vm N-1) in comparison with other known single-phase oxide materials. Our results reveal that self-polarization due to grain orientation along the spontaneous polarization direction plays an important role in achieving large piezoelectric response in a domain motion-confined material. The phase field simulations confirm that the large piezoelectric voltage coefficient g33 originates from maximized piezoelectric strain coefficient d33 and minimized dielectric permittivity ɛ33 in [001]-textured PbTiO3 ceramics where domain wall motions are absent.

Giant strain with ultra-low hysteresis and high temperature stability in grain oriented lead-free K0.5Bi0.5TiO3-BaTiO3-Na0.5Bi0.5TiO3 piezoelectric materials.

We synthesized grain-oriented lead-free piezoelectric materials in (K0.5Bi0.5TiO3-BaTiO3-xNa0.5Bi0.5TiO3 (KBT-BT-NBT) system with high degree of texturing along the [001]c (c-cubic) crystallographic orientation. We demonstrate giant field induced strain (~0.48%) with an ultra-low hysteresis along with enhanced piezoelectric response (d33 ~ 190pC/N) and high temperature stability (~160°C). Transmission electron microscopy (TEM) and piezoresponse force microscopy (PFM) results demonstrate smaller size highly ordered domain structure in grain-oriented specimen relative to the conventional polycrystalline ceramics. The grain oriented specimens exhibited a high degree of non-180° domain switching, in comparison to the randomly axed ones. These results indicate the effective solution to the lead-free piezoelectric materials.

Impact of Capacitive Effect and Ion Migration on the Hysteretic Behavior of Perovskite Solar Cells.

In the past five years, perovskite solar cells (PSCs) based on organometal halide perovskite have exhibited extraordinary photovoltaic (PV) performance. However, the PV measurements of PSCs have been widely recognized to depend on voltage scanning condition (hysteretic current density-voltage [J-V] behavior), as well as on voltage treatment history. In this study, we find that varied PSC responses are attributable to two causes. First, capacitive effect associated with electrode polarization provides a slow transient non-steady-state photocurrent that modifies the J-V response. Second, modification of interfacial barriers induced by ion migration can modulate charge-collection efficiency so that it causes a pseudo-steady-state photocurrent, which changes according to previous voltage conditioning. Both phenomena are strongly influenced by ions accumulating at outer interfaces, but their electrical and PV effects are different. The time scale for decay of capacitive current is on the order of seconds, whereas the slow redistribution of mobile ions requires several minutes.