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  1. Abstract

    The solid‐state synthesis of perovskite BiFeO3has been a topic of interest for decades. Many studies have reported challenges in the synthesis of BiFeO3from starting oxides of Bi2O3and Fe2O3, mainly associated with the development of persistent secondary phases such as Bi25FeO39(sillenite) and Bi2Fe4O9(mullite). These secondary phases are thought to be a consequence of unreacted Fe‐rich and Bi‐rich regions, that is, incomplete interdiffusion. In the present work, in situ high‐temperature X‐ray diffraction is used to demonstrate that Bi2O3first reacts with Fe2O3to form sillenite Bi25FeO39, which then reacts with the remaining Fe2O3to form BiFeO3. Therefore, the synthesis of perovskite BiFeO3is shown to occur via a two‐step reaction sequence with Bi25FeO39as an intermediate compound. Because Bi25FeO39and the γ‐Bi2O3phase are isostructural, it is difficult to discriminate them solely from X‐ray diffraction. Evidence is presented for the existence of the intermediate sillenite Bi25FeO39using quenching experiments, comparisons between Bi2O3behavior by itself and in the presence of Fe2O3, and crystal structure examination. With this new information, a proposed reaction pathway from the starting oxides to the product is presented.

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    Free, publicly-accessible full text available February 1, 2025
  2. Abstract

    Thin films based on PbZr1−xTixO3and K1−xNaxNbO3are increasingly being commercialized in piezoelectric MEMS due to the comparatively low drive voltages required relative to bulk actuators, as well as the facile approach to making sensor or actuator arrays. As these materials are incorporated into devices, it is critically important that they operate reliably over the lifetime of the system. This paper discusses some of the factors controlling the electrical and electromechanical reliability of lead zirconate titanate (PZT)-based piezoMEMS films. In particular, it will be shown the gradients in the Zr/Ti ratio through the depth of the films are useful in increasing the lifetime of the films under DC electrical stresses.

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    Free, publicly-accessible full text available September 21, 2024
  3. Abstract

    To fulfill the demands of more bandwidth in 5G and 6G communication technology, new dielectric substrates that can be co‐fired into packages and devices that have low dielectric loss and improved thermal conductivity are desired. The motivation for this study is to design composites with low dielectric loss (tan δ) and high thermal conductivity (κ), while still limiting the electrical conductivity, for microwave applications involving high power and high frequency. This work describes the fabrication of high‐density electroceramic composites with a model dielectric material for cold sintering, namely sodium molybdate (Na2Mo2O7), and fillers with higher thermal conductivity such as hexagonal boron nitride. The physical properties of the composites were characterized as a function of filler vol.%, temperature, and frequency. Understanding the variation in measured properties is achieved through analyzing the respective transport mechanisms.

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    Free, publicly-accessible full text available October 1, 2024
  4. Abstract

    The cold sintering process (CSP) is a low temperature processing technique that utilizes a transient phase to synthesize dense ceramics. However, some CSP parts contain microflaws that arise due to inhomogeneities in pressure, temperature, and transient phase. This work uses 20 MHz ultrasound to verify the presence of defects in CSP ZnO samples of varying densities (84%–97%). Acoustic metrics used in this work include wave speed, which is affected by differences in the effective elastic properties of the medium, and attenuation, which quantifies wave energy loss due to scattering from defects. Wave speed maps were inhomogeneous, suggesting density gradients which were verified with scanning electron microscopy. In addition, it was demonstrated that the pores produced by cold sintering are anisometric, which increases the anisotropy in the elastic properties. High attenuation regions (>300 Np/m) are present in all samples independent of relative density and correspond to defects identified in X‐ray computed tomography (XCT) which were as small as 38 µm in effective diameter. However, some high attenuation spots do not correspond to visible defects in XCT, which suggests the presence of features undetectable with XCT such as residual secondary phases at the grain boundaries.

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  5. Abstract

    This review highlights the unique techniques for patterning liquid metals containing gallium (e.g., eutectic gallium indium, EGaIn). These techniques are enabled by two unique attributes of these liquids relative to solid metals: 1) The fluidity of the metal allows it to be injected, sprayed, and generally dispensed. 2) The solid native oxide shell allows the metal to adhere to surfaces and be shaped in ways that would normally be prohibited due to surface tension. The ability to shape liquid metals into non‐spherical structures such as wires, antennas, and electrodes can enable fluidic metallic conductors for stretchable electronics, soft robotics, e‐skins, and wearables. The key properties of these metals with a focus on methods to pattern liquid metals into soft or stretchable devices are summari.

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  6. Abstract

    Knowledge about phase transitions in doped HfO2and ZrO2‐based films is crucial for developing future ferroelectric devices. These devices should perform in ambient temperature ranges with no degradation of device performance. Here, the phase transition from the polar orthorhombic to the nonpolar tetragonal phase in thin films is of significant interest. Detailed electrical and structural characterization is performed on 10 nm mixed HfxZr1‐xO2binary oxides with different ZrO2in HfO2and small changes in oxygen content. Both dopant and oxygen content directly impact the phase transition temperature between the polar and nonpolar phase. A first‐order phase transition with thermal hysteresis is observed from the nonpolar to the polar phase with a maximum in the dielectric constant. The observed phase transition temperatures confirm trends as obtained by DFT calculations. Based on the outcome of the measurements, the classification of the ferroelectric material is discussed.

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  7. Abstract

    Antiferroelectric (AFE) materials are of great interest owing to their scientific richness and their utility in high‐energy density capacitors. Here, the history of AFEs is reviewed, and the characteristics of antiferroelectricity and the phase transition of an AFE material are described. AFEs are energetically close to ferroelectric (FE) phases, and thus both the electric field strength and applied stress (pressure) influence the nature of the transition. With the comparable energetics between the AFE and FE phases, there can be a competition and frustration of these phases, and either incommensurate and/or a glassy (relaxor) structures may be observed. The phase transition in AFEs can also be influenced by the crystal/grain size, particularly at nanometric dimensions, and may be tuned through the formation of solid solutions. There have been extensive studies on the perovskite family of AFE materials, but many other crystal structures host AFE behavior, such as CuBiP2Se6. AFE applications include DC‐link capacitors for power electronics, defibrillator capacitors, pulse power devices, and electromechanical actuators. The paper concludes with a perspective on the future needs and opportunities with respect to discovery, science, and applications of AFE.

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  8. Abstract

    Ceramics such as lead zirconate titanate (PZT) tend to dissolve incongruently, and thus pose a challenge in the cold sintering process. Moist lead nitrate has previously been shown to enable a cold sinter‐assisted densification of PZT by a viscous phase sintering mechanism. In this paper, lead acetate trihydrate is demonstrated to lower the required temperature of the cold sintering step to 200°C. This densification process was described as a two‐step process: cold sintering of PZT with lead acetate trihydrate and post‐annealing the as‐cold sintered PZT ceramics. Unlike in the case of lead nitrate, PZT densification with lead acetate trihydrate occurs by a liquid phase assisted sintering mechanism, leading to an as‐cold sintered relative density of 84% at 200°C. After performing a post‐anneal step at 900°C, >97% relative densities were achieved in samples that were cold sintered with lead acetate trihydrate. This step not only densified PZT but also refined the grain boundaries. In the post‐annealed samples, the room‐temperature relative permittivity at 100 Hz was ~1600, slightly higher than that reported in samples that used lead nitrate as a sintering aid; the loss tangent was about 3.8%. For measurements at 10 Hz, the remanent polarization in both cases was ~28 µC/cm2. Both Rayleigh analysis and aging studies showed that a higher irreversible contribution to the permittivity exists in samples that used lead nitrate as a cold sintering aid.

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  9. Free, publicly-accessible full text available December 1, 2024
  10. In many commercially utilized ferroelectric materials, the motion of domain walls is an important contributor to the functional dielectric and piezoelectric responses. This paper compares the temperature dependence of domain wall motion for BaTiO3 ceramics with different grain sizes, point defect concentrations, and formulations. The grain boundaries act as significant pinning points for domain wall motion such that fine-grained materials show smaller extrinsic contributions to the properties below the Curie temperature and lower residual ferroelectric contributions immediately above the Curie temperature. Oxygen vacancy point defects make a modest change in the extrinsic contributions of undoped BaTiO3 ceramics. In formulated BaTiO3, extrinsic contributions to the dielectric response were suppressed over a wide temperature range. It is believed this is due to a combination of reduced grain size, the existence of a core-shell microstructure, and a reduction in domain wall continuity over the grain boundaries.

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    Free, publicly-accessible full text available November 15, 2024