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Heterogeneous stacked electronics are one of the major topics in flexible electronics applications, which require high performances. At the same time, P(VDF-TrFE)-based polymers with unique characteristics, e.g., ferroelectric and flexible properties, also bring attention to heterogeneous stacked device applications. Since the interfacial characteristic has strong effects with performance and reliability, therefore, it becomes one of the key issues in P(VDF-TrFE)-based heterogeneous stacked devices. In this article, a mechanism of an interface trap charge-induced low-k interfacial layer was proposed and examined by experimental methods. To analyze the low-k interfacial layer effects, both general-doping (few 1015 cm−3) p-type and n-type silicon substrate were used. These experimental results showed that different types of silicon substrates have distinct interface properties. Then, different interface trap density ( $1.75\times 10^{12}$ – $4.1\times 10^{11}$ eV−1 cm−2) silicon substrates were fabricated to further analyze the low-k interfacial layer. These showed the substrate with the lowest interface trap density has less low-k interfacial effect than other substrates do. Based on these experimental verifications, the mechanism of interface trap charge-induced low-k interface characteristic was proposed. As a consequence, this interface trap–charge-induced behavior is a necessary consideration for applications of heterogeneous stacked devices.

Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) has been used for gaining insights into perovskite solar cells (PSCs). However, the importance of selecting ion beam parameters to eliminate artifacts in the resulting depth profile is often overlooked. In this work, significant artifacts were identified with commonly applied sputter sources, i.e., an O

The origin of ferroelectricity and antiferroelectricity in zirconia (ZrO2) ultrathin films has become a topic of great interest in recent years. The normal ferroelectricity in ZrO2 is widely considered to originate from the high-pressure noncentrosymmetric Pca21 orthorhombic (o) phase. While the antiferroelectric-like behavior is regarded as the result of a phase transition from the centrosymmetric and paraelectric P42/nmc tetragonal (t) phase to the ferroelectric o-phase under electrical loading. This study reports an effective technique to selectively produce a ferroelectric or antiferroelectric ZrO2 ultrathin film (∼15 nm) without compositional manipulation. The technique is based on tailoring the crystal orientation of the Pt bottom electrode on which the ZrO2 ultrathin film is deposited. By correlating the results of the XRD, HRTEM, and electric field-polarization (P-E) analyses, it is found that a cubic phase (111)-oriented Pt electrode promotes the textured growth of the ferroelectric o-phase (111) planes in the ZrO2 ultrathin film; while a random-oriented Pt electrode leads to a t-phase dominated system, resulting in an antiferroelectric-like pinched P-E hysteresis. The modulation of ZrO2 ferroelectricity via the crystal orientation of the Pt bottom electrode can be achieved with a low thermal budget (< 400 °C), making it highly favorable for process integration and device scaling in a variety of nanoelectronics and nanoelectromechanical applications.

In this study, tailoring the microstructures and ferroelectric(FE)/antiferroelectric(AFE) properties of nanoscale ZrO2 thin films is demonstrated with an intentional introduction of sub-nanometre interfacial layers. The ferroelectricity of ZrO2 thin films is significantly enhanced by the HfO2 interfacial layers, while the TiO2 interfacial layers lead to a dramatic transformation of ZrO2 from ferroelectricity into antiferroelectricity. The HfO2 and TiO2 interfacial layers boost the formation of the polar orthorhombic phase with (111)-texture and the non-polar tetragonal phase with (110)-texture in the FE/AFE ZrO2 thin films, respectively, as evidenced by grazing incidence, out-of-plane, and in-plane X-ray diffraction measurements. Furthermore, the modulation of ferroelectricity and antiferroelectricity of nanoscale ZrO2 thin films by the HfO2/TiO2 interfacial layers can be achieved without high-temperature annealing, which is highly advantageous to process integration. The findings demonstrate the important role of the interfaces in the effective tuning of FE/AFE properties of nanoscale thin films.

Large stable ferroelectricity in nanoscale undoped zirconia (ZrO 2 ) thin films prepared without post-annealing has been demonstrated for the first time. Remanent polarizations up to 12 μC cm −2 were obtained in the as-deposited ZrO 2 thin films prepared by remote plasma atomic layer deposition at 300 °C substrate temperature on the Pt electrode. Ferroelectric crystallization of the films was achieved without post-annealing, which is highly beneficial to the application of the films in non-volatile memories and ultralow-power nanoelectronics. The existence of the ferroelectric orthorhombic phase with noncentrosymmetric space group Pbc 2 1 in the as-deposited ZrO 2 thin films was confirmed by high-resolution transmission electron microscopy.

Wurtzite aluminum nitride (AlN) of space group P63mc has long been recognized as a non-ferroelectric material, lacking the polarization switching ability. This paper reports the induction of ferroelectricity in a single crystalline epitaxial AlN ultrathin film with a thickness of 8−10 nm. The ferroelectric AlN epilayer was grown on a single crystalline GaN layer, forming a [0001]-oriented AlN/GaN epitaxial heterostructure with two reversible polar variants: [000-1] and [0001]. The AlN epilayer exhibited soft ferroelectricity with large switching currents and a polarization value of ~3.0 μCcm-2 during a 180° polarization switch. The AlN epilayer was prepared by the atomic layer deposition technique at 300°C in conjunction with in-situ atomic layer annealing. The two-dimensional electron gas (2DEG) at the AlN/GaN interface could be manipulated by the ferroelectric switching in the AlN epilayer. Strain engineering via lattice mismatch at the AlN/GaN interface was the key to creating a ferroelectric AlN/GaN heterojunction. Based on the reciprocal space mapping analysis, the AlN ferroelectricity is believed to be stemming from the out-of-plane compressive strain and inplane tensile strain present in the [0001]-oriented AlN epilayer. The discovery of low-temperature prepared, CMOScompatible AlN ultrathin films with soft ferroelectric characteristics will undoubtedly spur new fundamental and applied research in low-dimensional ferroelectric systems based on the AlN/GaN heterojunction.

Negative capacitance (NC) has been proposed to realize sub-Boltzmann steep-slope transistors in recent years. We provide experimental evidences and theoretical view for ferroelectric NC and inductance induced by polarization switching, based on an as-deposited nanoscale ferroelectric zirconium oxide (ZrO2) layer (nano-f-ZrO2). The experimental results are demonstrated in nano-f-ZrO2, including resistor–inductor–capacitor oscillations, positive reactance in Nyquist impedance plot, enhancement of capacitance, and sub-60 mV/dec subthreshold swing of nanoscale transistors. The theoretical analysis shows that ferroelectric polarization switching yields an effective electromotive force which is similar in behavior to Lenz’s law, leading to inductive and NC responses. Nano-beam electron diffraction reveals ferroelectric multi-domains in nano-f-ZrO2. Under small-signal operation, the switching of net polarization variation in ferroelectric multi-domains contributes to the ferroelectric inductance and NC. Nano-f-ZrO2 provides a pronounced inductance compared to conventional inductors, which would have impacts in a variety of applications including transistors, filters, oscillators, and radio-frequency-integrated circuits. Ferroelectric negative capacitance could be used to overcome the Boltzmann limit for next-generation energy-efficient transistors. This study demonstrates the performance capability of nanothick ZrO2 observing negative capacity and ferroelectric inductance and characterizes the role of multidomains in the observed behaviors.

An ~6.5 nm pure ZrO2 thin film with a giant ferroelectric remanent polarization (Pr) of ~50 μ Ccm − 2 and an effective piezoelectric coefficient (d33) of 7−9 pm/V is reported. The film was prepared on a (111)-oriented Pt electrode using plasma-enhanced atomic layer deposition at 300 °C, followed by annealing at 400 °C, and exhibited a preferred orientation of the orthorhombic (111) planes in the in-plane direction. The Pr value of ~50 μ Ccm − 2 is the largest reported to date for both perovskite and fluorite nanoscale ferroelectric thin films ( μ Ccm − 2 in nanoscale ferroelectrics on a Pt electrode. The giant Pr, ascribed to the preferred crystal orientation, was confirmed by the positive-up negative-down (PUND) polarization measurement with a long delay time to allow the relaxation of polarization. The effective d33 was obtained using piezoresponse force microscopy with an interferometric displacement sensor to minimize the frequency-dependent artifacts and the effects of cantilever dynamics. The low-temperature preparation of the textured ZrO2 ultrathin film with a giant Pr is extremely advantageous for device scaling and process integration in advanced nanoelectronics.

The effects of an implant on temperature distribution in a tissue-mimicking hydrogel phantom during the application of therapeutic ultrasound were investigated. In vitro experiments were conducted to compare the influences of plastic and metal implants on ultrasound diathermy and to calibrate parameters in finite element simulation models. The temperature histories and characteristics of the opaque (denatured) areas in the hydrogel phantoms predicted by the numerical simulations show good correlation with those observed in the in vitro experiments. This study provides an insight into the temperature profile in the vicinity of an implant by therapeutic ultrasound heating typically used for physiotherapy. A parametric study was conducted through numerical simulations to investigate the effects of several factors, such as implant material type, ultrasound operation frequency, implant thickness and tissue thickness on the temperature distribution in the hydrogel phantom. The results indicate that the implant material type and implant thickness are the main parameters influencing the temperature distribution. In addition, once the implant material and ultrasound operation frequency are chosen, an optimal implant thickness can be obtained so as to avoid overheating injuries in tissue.

Large ferroelectricity has been demonstrated in nanoscale bilayer thin films consisting of undoped ZrO2 and HfO2 layers on conventional Pt/Ti/SiO2/Si substrates for the first time. The bilayer thin films had thicknesses between 7.5 and 18 nm and their crystalline phases, symmetry groups, polarization hystereses and composition depth profiles were characterized to understand the influence of the stacking order and thickness of the ZrO2 and HfO2 layers on ferroelectricity. Two factors: (1) effective confinement of the HfO2 layer by the ZrO2 layer and Si substrate to promote the ferroelectric orthorhombic phase and (2) reduction of the bulk characteristics of the ZrO2 and HfO2 layers to minimize the paraelectric monoclinic phase are the key to stable ferroelectricity with a large remanent polarization. These factors can be manipulated by the stacking order and thickness of the layers, and consequently, producing ferroelectric behaviors ranging from minor to fully developed hystereses. The undoped ZrO2/HfO2 bilayer thin film is a simpler design and can be fabricated more easily compared to the doped HfO2 or solid solution HfxZr1-xO2 thin films reported in the literature, which require not only precise composition control but also the less available TiN electrodes for stable ferroelectricity. The chosen Pt/Ti/SiO2/Si substrate for the ZrO2/HfO2 bilayer thin films is readily available and has been widely used in MEMS applications. These advantages make the ZrO2/HfO2 bilayer thin films favorable for silicon-based device integration.

Large stable ferroelectricity in hafnium zirconium oxide (HZO) solid solution ultrathin films (including pure zirconia (ZrO2) and hafnia (HfO2)) and ZrO2/HfO2 bilayer ultrathin films of thickness ranging from 5–12 nm, prepared by thermal atomic layer deposition or remote plasma atomic layer deposition (RP-ALD) has been demonstrated. Ferroelectric crystallization of the ZrO2 ultrathin film with high-pressure orthorhombic (o) space group Pbc21 could be achieved without post-annealing due to the plasma-induced thermal stresses experienced by the film during the RP-ALD process. In contrast, for the ZrO2/HfO2 bilayer ultrathin film, due to the high crystallization temperature of HfO2, post-annealing was needed to achieve sufficient confinement of the sandwiched HfO2 layer by the ZrO2 top layer and Si bottom substrate to promote the high-pressure ferroelectric o-phase in HfO2. The ferroelectric properties of the HZO ultrathin films prepared by RP-ALD were highly dependent on the Hf-to-Zr ratio — an increasing amount of HfO2 has been found to be detrimental to the ferroelectricity, mainly due to the high crystallization temperature of HfO2. Without post-annealing, the ferroelectricity of the HZO ultrathin films was governed by the relative amounts of the amorphous phase and the ferroelectric o-phase induced by the plasma treatment. While with post-annealing, the ferroelectricity was governed by the relative amounts of the ferroelectric o-phase and the non-ferroelectric monoclinic (m) phase.

In this paper, a high performance micro piezoelectric energy harvester (PEH) fabricated on stainless substrates is presented. A PZT piezoelectric active layer with a thickness of about 10 μm was deposited on a stainless steel substrate by the aerosol deposition method. The cantilever beam-shaped PEH was then fabricated by metal-MEMS processing of the PZT/stainless steel composite structure. The size of the cantilever PEH transducer developed was about 1 cm2 and a proof mass was attached to tune its resonant frequency to around 120 Hz for harvesting mechanical vibrations from direct drive AC motors. The PEH transducer showed an output voltage and an output power of 8.9 Vp-p and 107.8 μW, respectively, when connected with optimal load and excited under 0.5 g acceleration level. In order to realize the fatigue behavior and reliability of the PEH in field applications, the PEH transducer was driven at its own resonant frequency and tested under 1.0 g acceleration level for millions of cycles and the vibration modes were measured with a laser scanning vibrometer. The PEH transducer had an operating lifetime of about 1.8 million cycles at 1.0 g cyclic loading based on the shift of its resonant frequencies and the decrease in electrical output. The experimental results show the resonant frequencies of the first, second and third modes were all shifted to lower frequencies with increasing operation cycle number due to the development of microcracks in the ceramic PZT active layer. However, the same PEH transducer could survive millions of cycles (in the high millions) at 0.5 g cyclic loading without any significant changes in the resonant frequencies and electrical output. The results confirm the operating limits of the PEH transducer and suggest further protection and reinforcement are required for the transducer to operate at high acceleration loadings.

An integrated computational framework was developed in this study for modeling high-intensity focused ultrasound (HIFU) thermal ablation. The temperature field was obtained by solving the bioheat transfer equation (BHTE) through the finite element method; while, the thermal lesion was considered as a denatured material experiencing phase transformation and modeled with the latent heat. An equivalent attenuation coefficient, which considers the temperature-dependent properties of the target material and the ultrasound diffraction due to bubbles, was proposed in the nonlinear thermal transient analysis. Finally, a modified thermal dose formulation was proposed to predict the lesion size, shape and location. In-vitro thermal ablation experiments on transparent tissue phantoms at different energy levels were carried out to validate this computational framework. The temperature histories and lesion areas from the proposed model show good correlation with those from the in-vitro experiments.

The ability of N-isopropylacrylamide (NIPAM)-based hydrogel phantoms to mimic tissues with different acoustic and thermal properties under high-intensity focused ultrasound (HIFU) ablation was investigated. These phantoms were designed to model the formation of thermal lesions in tissues above the threshold temperature of protein denaturation. By adjusting the concentration of acrylic acid (AAc) in the NIPAM-based hydrogel phantoms, the cloud point (i.e., lower critical solution temperature) of the phantoms could be tailored to produce HIFU thermal lesions similar to those formed in different swine tissues in terms of size and shape. Additionally, energy thresholds for inducing transient or permanent bubbles in the phantoms during HIFU ablation were also identified to shed light on the onset of cavitation or material damage. The NIPAM-based hydrogel phantoms developed in this study possess major advantages such as transparent, reusable and tailorable properties, and are practical tools for characterizing an ablative device (or treatment) to determine its efficacy and safety.

Photo-driven materials are an emerging research field due to its real-time definable capabilities and wide ranging potential applications. Previous research in the field has been limited to opto-electrical interactions. In this study, we investigated an opto-mechanical composite material of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] and titanium oxide phthalocyanine (TiOPc). The characteristics of this developed composite material were examined using SEM, FTIR, and XRD analytical methods. Our results showed that a remnant polarization and coercive field can be increased with a proportional increase of TiOPc in the composite. Experimental results show that the optimal composition of the TiOPc to P(VDF-TrFE) was 10 wt% (d33 = 18 pC/N, 30% of impedance variation at 100 Hz). Our developed composite material has good potential as an opto-configurable mechanism for various applications.

Titania–ceria (TiO 2 –CeO 2 ) nanostructured composites based on the design of coating the surfaces of anodized TiO 2 nanotube arrays with small band gap CeO 2 nanoparticles have been constructed and characterized to demonstrate the effectiveness of the TiO 2 –CeO 2 semiconductor heterojunction in enhancing the photocurrent response of TiO 2 -based photoelectrodes. The TiO 2 –CeO 2 heterojunction was confirmed to possess conduction and valence band offsets (0.81 and 1.59 eV, respectively) which promote the separation of photoinduced electron–hole pairs. The photocurrent densities of the TiO 2 –CeO 2 composites prepared with low annealing temperatures were about 25–40% larger than that of the anatase TiO 2 nanotube arrays. When the nanoparticle-on-nanotube architecture of the TiO 2 –CeO 2 heterostructure was maintained under specific processing conditions, the benefits of having a high specific surface area, a small band gap component capable of absorbing visible light, and a favorable heterojunction were achieved together for photocurrent enhancement.

The concept of thermal dose is widely adopted in the simulation of thermal therapies to estimate the formation of ablation lesions. However, such approaches typically use empirical assumptions, and treat the formation of a lesion as an end result from heating and not as a factor that could influence the temperature profile. In this study, the forming of lesions during high-intensity focused ultrasound (HIFU) therapy is interpreted as a denaturation process involving an irreversible phase transformation, and is a critical factor influencing the evolution of the temperature profile. Based on this notion and HIFU experiments performed on the pork tenderloin and egg white-based gel phantom, an important but often neglected phenomenon during the formation of the HIFU ablation lesion – referred to as the thermal confinement – was revealed. The thermal confinement, resulted from latent heat effects, exhibits features such as a substantial time delay in reaching maximum temperature around the ultrasound focal spot after turning off HIFU, and a lower-than-expected peak temperature attained during the ablation. A theoretical model capable of predicting the thermal confinement phenomenon and the changes in the temperature profile associated with it was successfully constructed. In addition, a modified bioheat transfer equation with the effect of latent heat from denaturation was proposed. The simulated temperature profiles further revealed the extent of the influence of the thermal confinement is strongly dependent on the position relative to the ultrasound focal spot.

Plasmonic enhancement of photoluminescence from bulk silicon was achieved by spectrally tailoring coupled localized surface plasmon resonance (LSPR) in the Al2O3 cover/nanostructured platinum (nano-Pt)/Al2O3 spacer/silicon multilayer structures prepared by atomic layer deposition (ALD). Agreement between the simulation and experimental data indicates that the plasmonic activity originates from absorption enhancement due to coupled LSPR. Because of the optimized dielectric environment deposited by ALD around the nano-Pt layer, absorption of the multilayer structure was enhanced by the precise tuning of coupled LSPR to coincide with the excitation wavelength. This accurate plasmonic multilayer structure grown by ALD with high precision, tunability, uniformity, and reproducibility can be further applied in efficient light-emitting devices.

Polymerization of N-isopropylacrylamide (NIPAM) with acrylic acid (AAc) has been adopted to fabricate reusable tissue-mimicking hydrogel phantoms designed for the real-time visualization and examination of thermal lesion formation in ablation and hyperthermia therapies. It is shown that the cloud point temperature of the NIPAM-based hydrogel phantoms can be adjusted by the concentration of AAc to represent the threshold temperature of pain (42 °C) or tissue damage (52 °C). The mechanical, thermal and acoustic properties of the developed phantoms are similar to those of human soft tissues. The ability of the phantoms to provide visualization of thermal lesions produced by either microwave or high-intensity focused ultrasound (HIFU) ablation was examined. Evolution of the optical transparency of the phantoms with temperature was found to be a stable hysteretic behavior and reproducible in consecutive heating–cooling cycles, demonstrating the reusability of the phantoms. By processing the optical images of the phantoms at different stages of the heating process, a thermal lesion can be considered formed (i.e., threshold temperature reached) when the grayscale value reaches the half-saturation point. The image processing method proposed for the NIPAM-based hydrogel phantoms is shown to be independent on the type of heating device used.

Episodic release of bioactive compounds plays an important role in biological systems. "On-demand" release systems which based on polymeric materials and activated by external stimuli may provide the necessary functionality. Here we describe an ultrasound-responsive hydrogel based on N-isopropylacrylamide (NIPAM) and N,N'-methylenebisacrylamide (MBAm), which is suitable for triggered release of two large molecules: bovine serum albumin (BSA, 66kDa) and dextran (3-5kDa). It is shown that the release amount of these two large molecules increased with increasing hydrogel temperature, and the application of ultrasound further increased the release. By simply adjusting the contents of NIPAM and MBAm, the difference of BSA release between the presence and absence of ultrasound could be adjusted from 2.7 to 84 folds. There was also a positive correlation between the ultrasound intensity and release amount. These properties made the NIPAM-based hydrogel a tunable platform for focal drug delivery.

The crystalline phases and ferroelectric properties of Zr x Hf1-x O2 (ZHO) ultrathin films (5.6–5.8 nm in thickness) prepared by remote plasma atomic layer deposition (RP-ALD) before and after post-annealing and wake-up cycling have been investigated in this study. For the films with high ZrO2 percentages, the plasma bombardment from RP-ALD was able to induce partial ferroelectric orthorhombic (o) crystallization during the film deposition stage. The as-deposited pure ZrO2 ultrathin film exhibited a fully developed ferroelectric polarization hysteresis after wake-up cycling. For the films containing the ferroelectric o-phase in the as-deposited state, post-annealing the films followed by a wake-up cycling procedure could greatly promote ferroelectric switching while maintaining low leakage. The ferroelectric properties of the ZHO ultrathin films were highly dependent on the ZrO2-to-HfO2 ratio and could be tailored by various combinations of post-annealing and wake-up cycling. An increase in the amount of HfO2 has been found to be detrimental to the ferroelectricity, mainly due to the high crystallization temperature of HfO2. For the post-annealed ZHO ultrathin films, the ferroelectricity was governed by the relative amounts of the ferroelectric o-phase and the non-ferroelectric monoclinic (m) phase. The Zr0.5H0.5O2 ultrathin film was the only composition which exhibited a large, stable polarization hysteresis after post-annealing but before wake-up cycling. The annealed Zr0.5H0.5O2 ultrathin film is believed to contain a suitable mix of o and m phases to produce loose grain boundaries, allowing the ferroelectric o-phase crystallites to switch freely under electrical loading.

High-performance plasmonic ZnO/Al2O3 spacer/nano-Pt/Al2O3/substrate multilayer structures were prepared by atomic layer deposition, in which the localized surface plasmons (LSP) were supported by the uniformly dispersed nanostructured Pt (nano-Pt) layer and a precise Al2O3 spacer was introduced to suppress luminescence quenching. Over 100-fold enhancement of photoluminescence from the ZnO emitter, which facilitates the observation of significant quantum confinement effect in the nanoscale ZnO layer as thin as ∼2 nm, was achieved. The giant plasmonic enhancement can be deduced from the optimized Al2O3 spacer thickness, which leads to the reduced coupling with nonradiative high-order LSP modes as well as the overlap between the LSP resonance wavelength and the luminescence/absorbance spectra of the ZnO emitter. This plasmonic multilayer structure with high enhancement, accuracy, tunability, uniformity, and reproducibility can be further applied in sensitive bioassays/biosensing and efficient solid-state lig...

This study investigates the microstructure, elemental distribution, diffuse phase transition (DPT) and crystalline phases of BNBK 85.4/2.6/12 lead-free ferroelectric ceramics, a high-strain bismuth sodium titanate-based relaxor system with complex A-site compositions. TEM electron diffraction studies indicate that the ceramics are close to a compositionally driven morphotropic phase boundary between a conventional rhombohedral ( R ) structure and a nanoscale tetragonal ( T ) structure. The ceramics exhibit a core–shell grain microstructure – the shell region shows a cubic average symmetry with the presence of tetragonal distortions; while the core region is of a mixture of T and R phases. Analysis of the corresponding superlattice reflections and dark-field images reveals an R phase-dominant core with the presence of R -twin domains. The high resolution TEM image of the shell region reveals the existence of T -phase nano-domains. The DPT behavior is mainly attributed to the heterogeneous distribution of cations of high valences and concentrations across the core–shell grains.

A hybrid method for estimating temperature with spatial mapping using diagnostic ultrasound, based on detection of echo shifts from tissue undergoing thermal treatment, is proposed. Cross-correlation and zero-crossing tracking are two conventional algorithms used for detecting echo shifts, but their practical applications are limited. The proposed hybrid method combines the advantages of both algorithms with improved accuracy in temperature estimation. In vitro experiments were performed on porcine muscle for preliminary validation and temperature calibration. In addition, thermal mapping of rabbit thigh muscle in vivo during high-intensity focused ultrasound heating was conducted. Results from the in vitro experiments indicated that the difference between the estimated temperature change by the proposed hybrid method and the actual temperature change measured by the thermocouple was generally less than 1 °C when the increase in temperature due to heating was less than 10 °C. For the in vivo study, the area predicted to experience the highest temperature coincided well with the focal point of the high-intensity focused ultrasound transducer. The computational efficiency of the hybrid algorithm was similar to that of the fast cross-correlation algorithm, but with an improved accuracy. The proposed hybrid method could provide an alternative means for non-invasive monitoring of limited temperature changes during hyperthermia therapy.

The multi-axial responses of barium titanate (BaTiO3) and hard lead zirconate titanate (PZT-4D) are measured for stress and electric field loadings, and are compared to the response of soft lead zirconate titanate (PZT-5H) taken from a previous study. First, poled ferroelectric specimens are subjected to an electric field at an angle to the original poling direction. Second, unpoled ferroelectric specimens are loaded by a uniaxial compressive stress and a parallel, proportional electric field. The switching surfaces of BaTiO3 and PZT-4D are constructed from the experimental measurements, and compared with existing data for PZT-5H. The measured responses are then used to evaluate the accuracy of existing micromechanical and phenomenological models of ferroelectric switching.

The optical absorption and photoelectric properties of oxygen-deficient titania (TiO 2 ) nanostructures consisting of anatase nanotubes and rutile film layer were investigated. The nanostructures were prepared by electrochemical anodization followed by long-time annealing at four temperatures – 450, 550, 650 and 750 °C. Various characterization techniques, including X-ray photoelectron spectroscopy depth profiling, revealed that elemental stable zones (structural regions in which the concentrations of O and Ti are stable) formed within the TiO 2 nanostructures at high annealing temperatures (650 and 750 °C) have O/Ti atomic ratios significantly less than 2. A direct relationship between oxygen vacancy concentration and annealing temperature was established on the basis of this finding. Measurement of the optical absorption spectra of the TiO 2 nanostructures revealed a blue-shift in the absorption edge along with a notable increase in the long-wavelength absorption due to the presence of oxygen vacancies. This observation is in agreement with the first-principles calculations of the absorption coefficients of anatase Ti n O 2 n −1 and Ti n O 2 n −2 structures, in which the oxygen vacancy concentration can be adjusted by varying the supercell size. The contrary photocurrent responses of the TiO 2 nanostructures under ultraviolet and visible light were measured. A strong photocurrent response under filtered visible light (λ > 500 nm) was found for the TiO 2 nanostructures annealed at 650 and 750 °C, which suggests that the dominant positive effect of oxygen vacancies exceeds the adverse impact of other features associated with thickening of the rutile film layer at high annealing temperatures, such as a reduction in the specific surface area and an increased charge recombination rate.

Lead-free ferroelectric ceramics with a morphotropic phase boundary (MPB) composition 85.4% (Bi0.5Na0.5)TiO3–2.6%BaTiO3–12.0% (Bi0.5K0.5)TiO3 (BNT-BT-BKT at a molar ratio of 85.4: 2.6: 12.0) doped with 0.8 mol% Nb2O5 were studied for their crystalline phases and microstructure. The crystalline phases were identified using X-ray diffractometry (XRD) with the contents determined using the Rietveld refinement technique. The phase-transformation-induced microstructure was analyzed using transmission electron microscopy (TEM) and the crystal symmetries were determined using the convergent-beam electron diffraction (CBED) technique. Samples sintered at 1200°C contain a mixture of cubic (C-), tetragonal (T-) and rhombohedral (R-) phases at a ratio of C/T/R = 56.6: 28.4: 15.0 wt%. Two types of grains are produced: one characterized by a featureless contrast consisting of nano-scale T-domains dispersed in a C-phase matrix; the other a core-shell structure with a shell containing twin and anti-phase-boundary (APB) ...

Bulk barium titanate (BaTiO3) ceramic specimens with various amounts of abnormal grains are prepared. A direct current (DC) electric field of 6 MV m−1 is applied to the specimens and their lifetimes are evaluated. Comparing to the specimens with only small normal grains, the time to failure of the specimens with coarse abnormal grains is significantly shorter. It is found that the BaTiO3 specimens would fail within 200 h when abnormal grains are present in the microstructure. However, the lifetimes of the specimens containing abnormal grains vary significantly from one to another. The Weibull statistics is adopted to estimate the extent of data scatter. The statistical analysis indicates that the amount of abnormal grains has little influence on the lifetime performance of bulk BaTiO3 ceramics under large DC electric fields. In most of the failed BaTiO3 specimens, regardless of their lifetimes, large through-thickness round holes with recrystallization features are present. A mixed failure mode consisting of avalanche and thermal breakdowns is observed for the failed specimens.

We investigated the design parameters of a compact pot-like ultrasonic sensor which possesses a highly anisotropic beam pattern. As the sensor size is small due to its application constraint, the parameters are thus highly coupled to one another. We analyzed the respective effects of the parameters in the case where there is a vertical beam width reduction. The parameters investigated include resonant frequency, vibrating plate width-expanded angle, and ratio of thickness discontinuity of the vibrating plate. Numerical models developed by combining finite-element analysis and spatial Fourier transforms were adopted to predict the far-field radiating beam pattern of the various design configurations. The displacement distribution of the vibrating plate was measured using a microscopic laser Doppler vibrometer and the far-field pressure beam patterns were measured using a standard microphone in a semianechoic environment. The three configurations we used to validate the simulation model resulted in an H-V ratio of 2.67, 2.68 and 3.13, respectively which all agreed well with the numerical calculations. We found that by increasing the operating resonant frequency from 40kHz to 58kHz, the vertical far-field beam width of an ultrasonic sensor can be reduced by 31.62%. We found that the vertical beam width can be significantly reduced when the ratio of the thickness discontinuity of the vibrating plate decreases from 1 to 0.4 and is incorporated with its optimal width-expanded angle of the vibrating plate. It appears that an ultrasonic sensor with this type of anisotropic beam pattern can be ideally adopted for today's automotive applications.

In the present study, bulk barium titanate ceramic specimens with bimodal microstructures are prepared and their dielectric and fatigue strengths under an alternating electric field are investigated. It is found that both the dielectric and fatigue strengths decrease with increasing amount of coarse grains. The scatter of the fatigue strength is characterized with the Weibull statistics. The extent of scatter of the fatigue strength data correlates strongly with the size distribution of the coarse grains. Such correlation is resulted from the presence of intrinsic defects within the microstructure. Direct microstructure evidences are provided.

Large strain actuation in barium titanate (BaTiO3) single crystals subjected to combined uniaxial stress and electric field is examined. A maximum strain of about 0.45% is measured under a combined loading of 2.7 MPa compressive stress and ±1.25 MV m −1 cyclic electric field. Above 2.7 MPa, the crystal does not cycle fully between the in-plane and out-of-plane polarized states due to large compressive stress, and consequently, a considerable reduction in actuation strain is apparent. The hysteresis evolution of the crystal under combined electromechanical loading reveals incomplete switching characteristics and a considerable disproportion of slope gradients at zero electric field for the measured polarization and strain hysteresis curves. A likely cause for the disproportion of slope gradients is the cooperative operation of multiple 90 ◦ switching systems by which “polarization-free” strain changes are induced. An in situ domain observation study reveals the formation of bubble-like micro-domains prior to the macroscopic 90◦ switching of the crystal bulk. The presence of these bubble-like “switching weak points” indicates that regions within the BaTiO3 single crystal do not necessarily switch 90

This article documents both modeling and experimental studies developed to investigate the switching behavior of ferroelectric single crystals. The theoretical model makes a priori ansatz that switching follows the evolution of a particular domain pattern. The choice of this configuration is dictated by the requirement that domains remain compatible during evolution, giving rise to a low-energy path for the overall switching. The construction of this pattern is achieved using multirank laminates. It offers an advantage of specifying different types of domain wall movements, leading to a distinction for the switching types. A loading experiment is performed on a barium titanate (BaTiO3) single crystal with a constant compressive stress and a cyclic electric field. Both 180 � and 90 � coercive fields are measured as input parameters required for the theoretical framework. The simulation results show good agreement with the observed strains measured by the present and other available experiments. It is found that depolarization has a non-trivial influence on attainable actuation strains. r 2008 Elsevier Ltd. All rights reserved.

The polarization switching characteristics of lead-free a (Bi 0.5 Na 0.5 )TiO 3 – b BaTiO 3 – c (Bi 0.5 K 0.5 )TiO 3 (abbreviated as BNBK 100 a /100 b /100 c ) ferroelectric ceramics are investigated. For the first time, the strain hystereses of BNBK compositions inside and outside the morphotropic phase boundary (MPB) are presented. The total induced electrostrain ( e 33,total ) and apparent piezoelectric coefficient ( d 33 ) first increase dramatically and then decrease gradually as the BNBK composition moves from the tetragonal phase to the MPB and then to the rhombohedral phase. The measured polarization hystereses indicate that the BNBK compositions situated near the rhombohedral side of the MPB typically possess higher coercive field ( E c ) and remanent polarization ( P r ), while the compositions situated near the tetragonal side of the MPB possess higher apparent permittivity. BNBK 85.4/2.6/12 a composition well within the MPB, exhibits an e 33,total of ∼0.14%, an apparent d 33 of 295 pCN −1 and a P r of 22.5 μC cm −2 . These property values suggest a candidate material for lead-free actuator applications.

Various amounts of silver particles, 0.08–7.7 mol%, are mixed with zinc oxide powder and subsequently co-fired at 800–1200 ◦ C. The effects of Ag addition on the microstructural evolution and electrical properties of ZnO are investigated. A small Ag doping amount ( 3.8 mol%). It is evident that a tiny amount of Ag (∼0.08 mol%) may dissolve into the ZnO lattice. High-resolution TEM observations give direct evidences on the segregation of Ag solutes at the ZnO grain boundaries. The grain boundary resistance of ZnO increases 35-fold with the presence of Ag solute segregates. The Ag-doped ZnO system exhibits a nonlinear electric current–voltage characteristic, confirming the presence of an electrostatic barrier at the grain boundaries. The barrier is approximately 2 V for a single grain boundary. © 2007 Elsevier Ltd. All rights reserved.

Plasmonic silver nanostructures and a precise ZnO cover layer prepared by capacitively coupled plasma atomic layer deposition (ALD) were exploited to enhance the Raman scattering from nanoscale ultrathin films on a Si substrate. The plasmonic activity was supported by a nanostructured Ag (nano-Ag) layer, and a ZnO cover layer was introduced upon the nano-Ag layer to spectrally tailor the localized surface plasmon resonance to coincide with the laser excitation wavelength. Because of the optimized dielectric environment provided by the precise growth of ZnO cover layer using ALD, the intensity of Raman scattering from nanoscale ultrathin films was significantly enhanced by an additional order of magnitude, leading to the observation of the monoclinic and tetragonal phases in the nanoscale ZrO2 high-K gate dielectric as thin as ∼6 nm on Si substrate. The excellent agreement between the finite-difference time-domain simulation and experimental measurement further confirms the so-called [absolute value]E(->)[absolute value](4) dependence of the surface-enhanced Raman scattering. This technique of plasmonic enhancement of Raman spectroscopy, assisted by the nano-Ag layer and optimized dielectric environment prepared by ALD, can be applied to characterize the structures of ultrathin films in a variety of nanoscale materials and devices, even on a Si substrate with overwhelming Raman background.

Lead zirconate titanate (PZT) thick films have been successfully grown on Pt/Ti-coated (1 0 0) Si substrates by a novel aerosol plasma deposition (APD) method at room temperature. The dielectric constant (K) and loss tangent (tan δ) of the as-deposited film measured at 100 kHz are 223 and 0.034, respectively. The dielectric properties of the film are improved considerably by subsequent microwave annealing: K = 745 and tan δ = 0.024 are achieved for films which are microwave-annealed at 600 W for 3 min, and K = 1049, tan δ = 0.027, and remanent polarization (Pr) = 32 μC cm−2 for films annealed at 800 W for 3 min. These values are comparable to those of PZT films grown by conventional deposition methods with high substrate and/or post-annealing temperatures.

The actuation stiffness of a set of steel Kagome Double-Layer Grid (KDLG) structures with brazed joints is measured experimentally and compared with predictions by the finite element method. The predicted actuation stiffnesses for the perfect KDLGs much exceed the measured values, and it is argued that the low values of observed actuation stiffness are due to the presence of geometric imperfections introduced during manufacture. In order to assess the significance of geometric defects upon actuation stiffness, finite element calculations are performed on structures with a stochastic dispersion in nodal position from the perfectly periodic arrangement, and on structures with wavy bars. It is found that bar waviness has the dominant effect upon the actuation stiffness. The predicted actuation stiffness for the imperfect structures are in satisfactory agreement with the measured values assuming the same level of imperfection between theory and experiment.

Encapsulation of taxol (paclitaxel) by liposomes provides an environment that enhances the solubility of taxol and avoids side effects caused by Cremophor EL, the emulsifier currently used. In this study, a suitable formula and production method for the liposomal delivery system were investigated. A combination of bath-type sonication and passage through ultrafiltration membranes was found to be the best way to produce liposomes particles of uniform size in the range of 100–200 nm. Phosphatidylcholine and phosphatidylglycerol in a 7 : 3 lipid molar ratio together with 40%, cholesterol, 25% α-tocopherol and 3% taxol (in mol/mol lipids), further resulted in liposomes with homogeneous and stable particle sizes. This taxol delivery method was found to have superior therapeutic effects in tests conducted on mice with transplanted liver tumors in comparison with taxol delivered in ethanol/Cremophor EL formula.

The goal of this study is to investigate photocatalytic semiconductor systems which are layered thin film composites built from perovskite oxide materials with characteristics such as small and large band gaps and/or ferroelectricity. In order to improve the efficiency of photocatalysis, semiconductor heterojunctions within the developed composites have been designed to possess electronic band offsets favoring the separation of photo-induced electron and hole (e−/h+) pairs. Furthermore, the remanent polarization of the ferroelectric component within the composites has been utilized to induce favorable band bending at the material interface, lowering the potential barrier for electron transfer. The band offsets and ferroelectric polarization could be considered as built-in electric fields; how they interact with photo-induced e−/h+ would greatly affect the photocatalytic properties of the composites. In this study, various perovskite oxide thin film materials — large band gap strontium titanate (SrTiO3), small band gap silver niobate (AgNbO3) and ferroelectric lead lanthanum titanate (PLT) — were combined to form layered thin film composites. The composites were then adopted as photoanodes in a photoelectrochemical cell and detailed characterization of their photocurrent response was carried out under different light irradiation and ferroelectric polarization conditions. Electronic band offsets at the material interface (i.e., heterojunction) were determined by ultraviolet-visible spectrophotometry and ultraviolet photoelectron spectroscopy. Electric field poling of the ferroelectric component was achieved by non-contact corona charging. Our results have shown that the band offsets at the SrTiO3-AgNbO3 heterojunction were about 1.0 eV in conduction band edge and 0.4 eV in valence band edge, promoting the rapid separation of photo-induced charge carriers; i.e., the flow of e− from SrTiO3 to AgNbO3 and the flow of h+ from AgNbO3 to SrTiO3. It was found that ferroelectric PLT could be used as a seeding layer for the low-temperature (500 °C) growth of SrTiO3/AgNbO3 thin film composites on ITO/glass substrates, forming a layered structure of SrTiO3/AgNbO3/PLT/ITO. In addition, the photocurrent density of the composites could be increased by depositing gold nanoparticles at the PLT-ITO interface. When the polarization of the PLT layer was poled toward the AgNbO3 layer, the potential barrier associated with the flow of e− to the ITO electrode was reduced by favorable band bending created at the AgNbO3-PLT interface. This resulted in a significant increase in photocurrent density.

This study investigates the microstructure, optical absorption and photoelectric properties of nanostructured composites of TiO 2 nanotube arrays and SrTiO 3 or CeO 2 nanoparticles. The composites were fabricated by anodization and hydrothermal methods and their UV-visible and ultraviolet photoelectron spectra (UV-Vis and UPS) were measured to determine the band structures of the TiO 2 -SrTiO 3 and TiO 2 -CeO 2 heterojunctions. The heterojunctions are designed to promote the separation of photo-induced electron and hole (e-/h+) pairs when the nanostructured composites are adopted in photocatalytic or photoelectrode applications. Approximately 1.0 and 0.8 eV shifts in conduction band position were determined for the TiO 2 -SrTiO 3 and TiO 2 -CeO 2 heterojunctions, respectively. The photocurrent densities of the TiO 2 - SrTiO 3 and TiO 2 -CeO 2 composites were about 20 to 40% larger than that of the TiO 2 nanotube arrays under identical irradiation conditions. The size of the SrTiO 3 and CeO 2 nanoparticles, which could be controlled by the hydrothermal temperature and time, and the concentration of oxygen vacancies within the TiO 2 nanotubes were identified to be the key factors governing the photocurrent densities of the nanostructured composites.

Non-invasive temperature measurement of tissues deep inside the body has great potential for clinical applications, such as temperature monitoring during thermal therapy and early diagnosis of diseases. We developed a novel method for both temperature estimation and thermal mapping that uses ultrasound B-mode radiofrequency data. The proposed method is a hybrid that combines elements of physical and statistical models to achieve higher precision and resolution of temperature variations and distribution. We propose a dimensionless combined index (CI) that combines the echo shift differential and signal intensity difference with a weighting factor relative to the distance from the heat source. In vitro experiments verified that the combined index has a strong linear relationship with temperature variation and can be used to effectively estimate temperature with an average relative error

Strain hysteresis evolution of a (001)-oriented 5 × 5 × 2 mm3 cuboidal barium titanate (BaTiO3) single crystal during a combined electromechanical loading sequence in the non-variant [110] direction is investigated. The goal is to compare the strain behaviors of the BaTiO3 single crystal subjected to loading in the variant [001] and non-variant [110] directions. The simultaneous application of compressive stress and electric field in the [110] direction was achieved by machining the square cuboid crystal into a hexagonal prism and applying the loads parallel to the hexagonal side faces of the prism (i.e., perpendicular to the [001] and 45° to the [100] and [010] directions). The room temperature strain hystereses show that the maximum total electro strains produced from loading and measuring in the [110] and [001] directions (denoted as e[110],max,RT and e[001],max,RT, respectively, where the last term of the subscript describes the testing temperature) are 0.20% at 3.0 MPa and 0.45% at 2.7 MPa, respectively. The ratio between e[110],max,RT and e[001],max,RT is 0.44, which is in good agreement with the ratio, predicated by the analytical calculations. Factors which may influence the strain behavior, such as the bias stress level, depolarization field and switching coercivities, are examined by repeating the loading experiment at 55 °C. The strain hystereses measured at 55 °C show that e[110],max,55 is 0.19% at 11.9 MPa — this maximum [110] strain is similar to the one obtained at room temperature, but is only achieved with a much larger bias stress. When the out-of-plane depolarization field and the in-plane switching coercivities are reduced at 55 °C, more domains are randomized in the in-plane variant directions during electric field unloading by the depolarization fields. Therefore, a much larger bias stress is required at 55 °C to switch a sufficient number of domains to the out-of-plane variant directions at small electric fields, which can then be switched back to the in-plane variant directions at high electric fields, producing strain in the [110] direction. The strain hysteresis study has revealed that the combined effect of the depolarization field and switching coercivity is a critical factor governing the strain behavior of the BaTiO3 single crystal.Copyright © 2012 by ASME

The photoelectric properties of oxygen-deficient titanium dioxide (TiO2) nanotube arrays are investigated in this study. The TiO2 nanotube arrays are prepared by anodization, followed by annealing at 450 to 750 °C for 3 h in air to form different crystalline phase mixtures. When the annealing temperature is increased, several phenomena are observed: (1) the ratio of anatase to rutile decreases, (2) the anatase nanotubes are shortened and (3) the thickness of the dense rutile film layer underneath the anatase nanotubes increases. The efficiency of visible light absorption of the nanotube arrays is enhanced with increasing annealing temperature. This is believed to be caused by the ionic defects, especially the oxygen vacancies, generated during the annealing procedure, enabling the absorption of low-energy radiations. The X-ray photoelectron spectroscopy (XPS) depth profile analysis provides the supporting evidence on the chemical nonstoichiometry (i.e., oxygen-deficiency) of the TiO2 nanotube arrays annealed at high temperature. With increasing annealing temperature, a decrease and an increase in the photocurrent density of the nanotube arrays under UV and visible light (wavelength > 500 nm) irradiations, respectively, are detected. The decrease of the photocurrent density under UV irradiation is caused by the reduction in the specific surface area (i.e., anatase nanotubes transform into rutile film with vigorous annealing). In contrast, the increase of the photocurrent density under visible light irradiation is contributed to the oxygen vacancies in the nanostructure, providing extra electron energy levels (locating below the conduction band of TiO2) within the band structure.

To improve the efficiency of water splitting by photocatalysis, a semiconductor heterojunction made of titanium dioxide (TiO2 ) and strontium titanate (SrTiO3 ) is constructed to promote the separation of photo-induced electron and hole (e− /h+ ) pairs. The photocatalytic behaviors of the TiO2 -SrTiO3 composite film and nanostructure fabricated by the sol-gel and hydrothermal methods are studied. UV-visible and ultraviolet photoelectron spectroscopies (UV-Vis & UPS) are adopted to identify the band structure of the TiO2 -SrTiO3 heterojunction. For the composite film system, an approximately 0.5 eV band shift at the heterojunction improves the separation of photoinduced e− /h+ pairs. The photocurrent density of the composite film is about 2–3 times larger than that of the TiO2 or SrTiO3 film alone. For the composite nanostructure system, it is composed of an array of TiO2 nanotubes coated with SrTiO3 nanoparticles. An approximately 0.2 eV band shift at the heterojunction is determined for the composite nanostructure. It is found that the size of the SrTiO3 nanoparticles, which can be controlled by the hydrothermal temperature and time, is a key factor in influencing the photocurrent density of the TiO2 -SrTiO3 composite nanostructure.Copyright © 2011 by ASME