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Abstract Temperature limitations in nickel‐base superalloys have resulted in the emergence of SiC‐based ceramic matrix composites as a viable replacement for gas turbine components in aviation applications. Higher operating temperatures allow for reduced fuel consumption but present a materials design challenge related to environmental degradation. Rare‐earth disilicates (RE 2 Si 2 O 7 ) have been identified as coatings that can function as environmental barriers and minimize hot component degradation. In this work, single‐ and multiple‐component rare‐earth disilicate powders were synthesized via a sol‐gel method with compositions selected to exist in the monoclinic C 2/ m phase ( β phase). Phase stability in multiple cation compositions was shown to follow a rule of mixtures and the C 2/ m phase could be realized for compositions that contained up to 25% dysprosium, which typically only exists in a triclinic, P , phase. All compositions exhibited phase stability from room temperature to 1200°C as assessed by X‐ray diffraction. The thermal expansion tensors for each composition were determined from high‐temperature synchrotron X‐ray diffraction and accompanying Rietveld refinements. It was observed that ytterbium‐containing compositions had larger changes in the α 31 shear component with increasing temperature that led to a rotation of the principal axes. Principal axes rotation of up to 47° were observed for ytterbium disilicate. The results suggest that microstructure design and crystallographic texture may be essential future avenues of investigation to ensure thermo‐mechanical robustness of rare‐earth disilicate environmental barrier coatings. 

We report on the synthesis of self-intercalated Nb1+xSe2 thin films by molecular beam epitaxy. Nb1+xSe2 is a metal-rich phase of NbSe2 where additional Nb atoms populate the van der Waals gap. The grown thin films are studied as a function of the Se to Nb beam equivalence pressure ratio (BEPR). X-ray photoelectron spectroscopy and x-ray diffraction indicate that BEPRs of 5:1 and greater result in the growth of the Nb1+xSe2 phase and that the amount of intercalation is inversely proportional to the Se to Nb BEPR. Electrical resistivity measurements also show an inverse relationship between BEPR and resistivity in the grown Nb1+xSe2 thin films. A second Nb-Se compound with a stoichiometry of ∼1:1 was synthesized using a Se to Nb BEPR of 2:1; in contrast to the Nb1+xSe2 thin films, this compound did not show evidence of a layered structure.

 

Ferroelectric hafnium oxides are poised to impact a wide range of microelectronic applications owing to their superior thickness scaling of ferroelectric stability and compatibility with mainstream semiconductors and fabrication processes. For broad-scale impact, long-term performance and reliability of devices using hafnia will require knowledge of the phases present and how they vary with time and use. In this Perspective article, the importance of phases present on device performance is discussed, including the extent to which specific classes of devices can tolerate phase impurities. Following, the factors and mechanisms that are known to influence phase stability, including substituents, crystallite size, oxygen point defects, electrode chemistry, biaxial stress, and electrode capping layers, are highlighted. Discussions will focus on the importance of considering both neutral and charged oxygen vacancies as stabilizing agents, the limited biaxial strain imparted to a hafnia layer by adjacent electrodes, and the strong correlation of biaxial stress with resulting polarization response. Areas needing additional research, such as the necessity for a more quantitative means to distinguish the metastable tetragonal and orthorhombic phases, quantification of oxygen vacancies, and calculation of band structures, including defect energy levels for pure hafnia and stabilized with substituents, are emphasized.

 

While ferroelectric HfO₂shows promise for use in memory technologies, limited endurance is one factor that challenges its widespread application. Herein, endurance is investigated through field cycling W/Hf_0.5Zr_0.5O₂/W capacitors above the coercive field while manipulating the time under field using bipolar pulses of varying pulse duration or duty cycle. Both remanent polarization and leakage current increase with increasing pulse duration. Additionally, an order of magnitude decrease in the pulse duration from 20 to 2 μs results in an increase in endurance lifetime of nearly two orders of magnitude from 3 × 10⁶to 2 × 10⁸cycles. These behaviors are attributed to increasing time under field allowing for charged oxygen vacancy migration, initially unpinning domains, or driving phase transformations before segregating to grain boundaries and electrode interfaces. This oxygen vacancy migration causes increasing polarization before creating conducting percolation paths that result in degradation and premature device failure. This process is suppressed for 2 μs pulse duration field cycling where minimal wake‐up and lower leakage before device failure are observed, suggesting that very short pulses can be used to significantly increase device endurance. These results provide insight into the impact of pulse duration on device performance and highlight consideration of use of conditions when endurance testing.

 

Abstract Materials with tunable thermal properties enable on-demand control of temperature and heat flow, which is an integral component in the development of solid-state refrigeration, energy scavenging, and thermal circuits. Although gap-based and liquid-based thermal switches that work on the basis of mechanical movements have been an effective approach to control the flow of heat in the devices, their complex mechanisms impose considerable costs in latency, expense, and power consumption. As a consequence, materials that have multiple solid-state phases with distinct thermal properties are appealing for thermal management due to their simplicity, fast switching, and compactness. Thus, an ideal thermal switch should operate near or above room temperature, have a simple trigger mechanism, and offer a quick and large on/off switching ratio. In this study, we experimentally demonstrate that manipulating phonon scattering rates can switch the thermal conductivity of antiferroelectric PbZrO 3 bidirectionally by −10% and +25% upon applying electrical and thermal excitation, respectively. Our approach takes advantage of two separate phase transformations in PbZrO 3 that alter the phonon scattering rate in different manners. In this study, we demonstrate that PbZrO 3 can serve as a fast (<1 second), repeatable, simple trigger, and reliable thermal switch with a net switching ratio of nearly 38% from ~1.20 to ~1.65 W m −1 K −1 . 

Abstract As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices 1–9 . However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity 10,11 , are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries 12,13 , the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses. 

The presence of the top electrode on hafnium oxide‐based thin films during processing has been shown to drive an increase in the amount of metastable ferroelectric orthorhombic phase and polarization performance. This “Clamping Effect,” also referred to as the Capping or Confinement Effect, is attributed to the mechanical stress and confinement from the top electrode layer. However, other contributions to orthorhombic phase stabilization have been experimentally reported, which may also be affected by the presence of a top electrode. In this study, it is shown that the presence of the top electrode during thermal processing results in larger tensile biaxial stress magnitudes and concomitant increases in ferroelectric phase fraction and polarization response, whereas film chemistry, microstructure, and crystallization temperature are not affected. Through etching experiments and measurement of stress evolution for each processing step, it is shown that the top electrode locally inhibits out‐of‐plane expansion in the HZO during crystallization, which prevents equilibrium monoclinic phase formation and stabilizes the orthorhombic phase. This study provides a mechanistic understanding of the clamping effect and orthorhombic phase formation in ferroelectric hafnium oxide‐based thin films, which informs the future design of these materials to maximize ferroelectric phase purity and corresponding polarization behavior.