Search

The low-frequency dielectric properties of cotton fabrics in the fabric thickness direction are examined in relation to the fabric construction, thread count and solid volume fraction (SVF) at five different relative humidity levels. It is found that the real part of the relative permittivity increases with increase in thread count, and this is due to the associated increase in SVF. On the other hand, the imaginary part of the relative permittivity and loss tangent (dielectric loss) show no clear monotonic tendency to the SVF at highly moist environments. Permittivity is observed to change depending on fabric constructions under the controlled SVF. These observations indicate that, in addition to the SVF, the fabric geometrical parameters, which are influenced by construction methods, also need to be considered in low-frequency dielectric analysis. [ABSTRACT FROM AUTHOR]

Indentation-induced surface and subsurface deformation mechanisms in directionally solidified WC-WC eutectoids are investigated. The microstructures of the WC-WC composites consist of primary WC regions, WC-rich regions, and lamellar-type eutectoid regions, which form during the sequential phase transformations during laser processing. Indentation-induced surface crack profiles indicate little propensity for debonding at the WC-WC eutectoid interfaces, although some interactions with larger-scale microstructural features are observed. In the subsurface indentation regions, dislocation networks predominantly composed of partial dislocation pairs having Burgers vectors $$ b = \frac{1}{6}\left\langle {2\bar{1}\bar{1}3} \right\rangle $$ are observed in the primary WC regions, whereas shear banding and grain refinement are commonly observed in the micrometer-scale WC-rich regions. Deformation in the sub-micrometer eutectoid regions is distinct, with the dislocation activity primarily limited to the WC phase, with Burgers vectors of $$ b = \frac{1}{3}\left\langle {1\bar{2}13} \right\rangle $$ that terminate at the interfaces. [ABSTRACT FROM AUTHOR]

Crystallographic orientation relationships and interfaces in directionally solidified WC-WC eutectoids are investigated by electron microscopy and electron back-scattered diffraction. The WC-WC eutectoids are prepared by a laser surface processing method, which unidirectionally melts and resolidifies ceramic powder substrates. Lamellar-type microstructures are observed in all samples with preferred nominal growth directions of [ $$ \overline{1} 2\overline{1} 0 $$ ]WC//[ $$ \overline{1} 2\overline{1} 0 $$ ]WC along the solidification direction. The majority of interface habit planes are found to be (0001)WC//(0001)WC. The interfaces are found to be semicoherent, with a misfit Burger's vector of 1/3[ $$ 2{\bar{1}} {\bar{1}} 0 $$ ]. An intermediate layer is identified at the interface and is associated with a change in the stacking sequence of the close-packed (0001) tungsten planes. [ABSTRACT FROM AUTHOR]

The spatial redistribution of non-stoichiometric point defects in rutile TiO 2 is studied as a function of voltage and time. Single crystals are equilibrated initially to a well-defined stoichiometry with n-type conductivity and a carrier concentration on the order of 10 18 cm −3 . The crystals are subsequently electroded with Pt contacts that exhibit Schottky behavior. When subjected to an applied voltage of 15 V, a time-dependent increase and saturation in the leakage current is observed, which is associated with an accumulation of point defects and an attendant decrease in stoichiometry at the cathode electrode. This local change in stoichiometry degrades the Schottky barrier, leading to asymmetric electrodes and thus macroscopic rectifying behavior. Cathodoluminescence spectroscopy shows that Ti interstitials dominate the point defect redistribution process. Under larger applied voltages, of around 30 V, qualitatively different behavior is observed in which the resistivity increases as a function of time. This behavior is associated with condensation of point defects into a region of extended defects and Magnéli phases near the cathode, sufficient to increase the bulk stoichiometry and resistivity. These experiments demonstrate that a one-dimensional drift-diffusion process, as opposed to filamentary growth, dominates in these experimental conditions and that the Pt–TiO 2 –Pt system remains closed, with no significant oxygen transport across the Pt–TiO 2 interfaces. We believe this is the first observation of a second higher-voltage regime in which the bulk stoichiometry and thus resistivity is increased as large concentrations of defects condense into metallic Magnéli phases in the near-electrode regions. [ABSTRACT FROM AUTHOR]

Abstract: Samples of B4C–TiB2 eutectic are laser processed to produce composites with varying microstructural scales. The eutectic materials exhibit both load dependent and load independent hardness regimes with a transition occurring between 4 and 5N indentation load. The load-independent hardness of eutectics with a microstructural scale smaller than 1μm is about 31GPa, and the indentation fracture toughness (5–10N indenter load) of the eutectics is 2.47–4.76MPam1/2. Indentation-induced cracks are deflected by TiB2 lamellae, and indentation-induced spallation is reduced in the B4C–TiB2 eutectic compared to monolithic B4C. Indentation-induced amorphization in monolithic B4C and the B4C phase of the eutectic is detected using Raman spectroscopy. Sub-surface damage is observed using TEM, including microcracking and amorphization damage in B4C and B4C–TiB2 eutectics. Dislocations are observed in the TiB2 phase of eutectics with an interlamellar spacing of 1.9μm. [Copyright &y& Elsevier]

The boron carbide ( B4C) titanium diboride ( TiB2) ceramic eutectic is being investigated for armor and tribological applications. Electron diffraction shows [11 [ABSTRACT FROM AUTHOR]

Ceramic/metal interfaces have vast technological significance and are used in a variety of applications. This has prompted a concerted effort to develop fundamental structure/property relationships for these structures. This article critically reviews recent advances in complementary experimental and theoretical studies of ceramic/metal interfaces to provide an overview of progress made towards developing these relationships. In particular, the focus is on the structure, characterization, and modeling of non-reactive, bulk ceramic/metal interfaces. This includes determining interface atomic structure and chemistry and investigating impurity segregation to the interfaces. Additionally, the most important advances in the experimental and theoretical modeling of interfacial structure and chemistry are reviewed and comments are provided on the future outlook of this field. [Copyright &y& Elsevier]

Studies the atomic structures of two symmetric tilt grain boundaries in yttria-stabilized cubic-zirconia. Use of Z-contrast scanning transmission electron microscopy; Boundaries composed of periodic arrays of highly symmetric structural units; Oxygen coordination that is similar between the bulk and grain boundary; Proposed atomic models of the grain boundaries.

Visible photoluminescence at 515 nm of ruthenium-doped multiwall carbon nanotubes, fabricated on quartz substrates using a chemical vapor deposition technique, is reported. The well-aligned nanotubes serve as templates for the luminescent, residual ruthenium-iron catalyst particles contained within the nanotubes, restricting the particle size to about 10 nm. The synthesis technique can be readily extended to other luminescent dopants; moreover, since nanotube arrays can be readily grown from patterned substrates, nanotube-based optoelectronic devices may be achieved. [ABSTRACT FROM AUTHOR]

Scientists use imaging to identify objects of interest and infer properties of these objects. The locations of these objects are often measured with error, which when ignored leads to biased parameter estimates and inflated variance. Current measurement error methods require an estimate or knowledge of the measurement error variance to correct these estimates, which may not be available. Instead, we create a spatial Bayesian hierarchical model that treats the locations as parameters, using the image itself to incorporate positional uncertainty. We lower the computational burden by approximating the likelihood using a noncontiguous block design around the object locations. We use this model to quantify the relationship between the intensity and displacement of hundreds of atom columns in crystal structures directly imaged via scanning transmission electron microscopy (STEM). Atomic displacements are related to important phenomena such as piezoelectricity, a property useful for engineering applications like ultrasound. Quantifying the sign and magnitude of this relationship will help materials scientists more precisely design materials with improved piezoelectricity. A simulation study confirms our method corrects bias in the estimate of the parameter of interest and drastically improves coverage in high noise scenarios compared to non-measurement error models. [ABSTRACT FROM AUTHOR]

The solid‐state synthesis of perovskite BiFeO3 has been a topic of interest for decades. Many studies have reported challenges in the synthesis of BiFeO3 from starting oxides of Bi2O3 and 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 Bi2O3 first reacts with Fe2O3 to form sillenite Bi25FeO39, which then reacts with the remaining Fe2O3 to form BiFeO3. Therefore, the synthesis of perovskite BiFeO3 is shown to occur via a two‐step reaction sequence with Bi25FeO39 as an intermediate compound. Because Bi25FeO39 and the γ‐Bi2O3 phase are isostructural, it is difficult to discriminate them solely from X‐ray diffraction. Evidence is presented for the existence of the intermediate sillenite Bi25FeO39 using quenching experiments, comparisons between Bi2O3 behavior 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. [ABSTRACT FROM AUTHOR]

Urgent care visits account for the highest volume of patients in many primary health care settings and most of these visits are triggered by illness, injury or the onset of new and worrisome symptoms. Many of these visits are one time encounters, episodic in nature, requiring no followup or ongoing therapy, however, a number of patients seek more complex care in the urgent care environment. Patients, on the other hand, rarely complain about the lack of comprehensive care, but they are quick to grumble about long lead times for appointments.

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of ∼135 μC/cm2 charge in Al1−xBxN via electron microscopy, Al1−xBxN ferroelectric domain patterns poled down to 400 nm in width via scanning probe microscopy, and full polarization retention after over 1000 h of 200 °C baking and a 2× enhancement relative to ZnO in the nonlinear optical response of Zn1−xMgxO. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed. [ABSTRACT FROM AUTHOR]

The onset of flash sintering is generally considered to be predominantly a function of the conductivity of a given ceramic, the furnace temperature, and the electric field. However, the evolution of the point defect profiles in ionic conductors such as yttria-stabilized zirconia (YSZ) can complicate the picture of homogeneous Joule heating and thermal runaway during DC flash sintering. Here, 8 mol% YSZ pellets were partially flash sintered under a DC bias using various current densities and hold times. The electrode geometry was varied to modulate the oxygen ion flux available to the cathode to compare the effects on the resulting oxygen vacancy inhomogeneity. The contribution from fundamental and experimental factors to the cathodic reduction reaction is also discussed, such as interface reaction kinetics and sample geometry. Local reduction of the ceramic was inevitably observed under all current densities and the resulting microstructural inhomogeneity was explained as the result of a transient conductivity asymmetry. This asymmetry was the result of the enhanced electronic conductivity in the cathode region to a value significantly greater than the ionic conductivity of near-stoichiometric YSZ. The link between the local conductivity, voltage and Joule heating is mathematically demonstrated to result in an asymmetric heating profile. Image 1 [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Schottky barrier of BaTiO 3 /Ni and BaTiO 3 /NiO interfaces are 0.68 and 1.49 eV. • Schottky barrier rapidly altered by deposition of NiO. • No significant influence of polarization on Schottky barrier height. Abstract Barium Titanate and Ni-based multilayer ceramic capacitors have wide commercial applicability, and interfaces are critical to the overall device behavior as they can help control unwanted leakage currents. Here we make use of photoemission methods to investigate the electrostatic barriers formed at BaTiO 3 /Ni(O) interfaces to understand the implications for electron injection. We find the interface Fermi level in BaTiO 3 to evolve smoothly during Ni deposition with a Schottky barrier height for electrons of 0.68 eV, whereas with NiO the Fermi level evolves rapidly with an electron injection barrier of 1.49 eV. In-situ poling shows the Schottky barrier at the BaTiO 3 /Ni interface is not significantly altered by ferroelectric polarization, consistent with the good screening of the Ni electrode. This study presents a direct quantitative measurement of the interface barrier heights and highlights the significance of the oxidation state of the electrode. [ABSTRACT FROM AUTHOR]

Ribbonlike magnetoelastic sensors can be considered the magnetic analog of an acoustic bell; in response to an externally applied magnetic field impulse the sensors emit magnetic flux with a characteristic resonant frequency. The magnetic flux can be detected external to the test area using a pick-up coil, enabling query remote monitoring of the sensor. The characteristic resonant frequency of a magnetoelastic sensor changes in response to mass loads [L. D. Landau and E. M. Lifshitz, Theory of Elasticity, 3rd ed. (Pergamon, New York, 1986), p. 100]. Therefore, remote query chemical sensors can be fabricated by combining the magnetoelastic sensors with a mass changing, chemically responsive layer. In this work magnetoelastic sensors are coated with humidity-sensitive thin films of ceramic, nanodimensionally porous TiO[sub 2] to make remote query humidity sensors. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ferroelectric wurtzites have the potential to revolutionize modern microelectronics because they are easily integrated with multiple mainstream semiconductor platforms. However, the electric fields required to reverse their polarization direction and unlock electronic and optical functions need substantial reduction for operational compatibility with complementary metal-oxide semiconductor (CMOS) electronics. To understand this process, we observed and quantified real-time polarization switching of a representative ferroelectric wurtzite (Al0.94B0.06N) at the atomic scale with scanning transmission electron microscopy. The analysis revealed a polarization reversal model in which puckered aluminum/boron nitride rings in the wurtzite basal planes gradually flatten and adopt a transient nonpolar geometry. Independent first-principles simulations reveal the details and energetics of the reversal process through an antipolar phase. This model and local mechanistic understanding are a critical initial step for property engineering efforts in this emerging material class. [ABSTRACT FROM AUTHOR]

Time-dependent increases in electrical conductivity in dielectrics exposed to a direct-current voltage bias can be detrimental to device reliability and lifetime, while the same phenomenon may be exploited for resistive switching functionality. The applied electric field induces the electromigration of charged point defects, resulting in spatially inhomogeneous stoichiometry and modifications in the electrode electrostatics. Herein, we monitor the time-dependent changes in leakage current in rutile TiO 2 with platinum electrodes. Current–voltage measurements before and after the extended electrical biasing demonstrate that changes in the electrostatics of the cathode Schottky barrier are responsible for the time-dependent increases in leakage current. We also find that electromigration is accelerated along the ⟨ 001 ⟩ relative to the ⟨ 010 ⟩ crystallographic direction in contrast to what would be predicted by high-temperature diffusion kinetics. We find the observed anisotropic behavior to be consistent with a change in the defect charge state of the titanium interstitials and dominant transport path at lower temperatures. A comparison of optical color front migration kinetics to the temporal evolution of leakage current further supports the interpretation that ionic electromigration near the cathode interface is responsible for the increased leakage current in the electrically degraded state. [ABSTRACT FROM AUTHOR]

Plan view scanning transmission electron microscopy was used to investigate the microstructural connections to device resistance in inline phase change switch devices. It was revealed that massive structural changes occur in GeTe during switching, most notably the formation of an assembly of voids along the device centerline and large GeTe grains on either side of an "active region." Restructuring of this variety was tied to changes in ON-state resistance with increasing pulse number, where initially porous and fine-grained (10-20 nm) GeTe was converted to large crystalline domains comprising the majority of the RF gap (400-700 nm). A phenomenological model for this microstructure is presented in which the OFF pulse melts a given width of GeTe, and upon cooling crystalline material outside the melt region acts as a template for an inward-propagating crystalline growth front. The voids observed along the device centerline were correlated to increasing OFF state resistance and a relatively stable ON state with increasing pulse number via a series resistance model. As a result of this analysis, OFF state resistance was suggested as an early indicator of device reliability. An improved GeTe deposition process was implemented to limit void formation, which is shown to have a more stable OFF-state resistance with increasing pulse number. [ABSTRACT FROM AUTHOR]

Directionally solidified WC-W2C ceramics containing 40 at% carbon, corresponding to the WC-W2C eutectoid composition, were produced by laser surface melt processing. The resulting microstructures showed a lamellar-type eutectic/eutectoid microstructure with the WC minor phase embedded in the W2C matrix phase. The interlamellar spacing (λ) in the eutectoid regions followed the relationship Vλ3.8 = constant, with the smallest spacing of 331 ± 36 nm achieved in the 3.24 mm/s processed sample. The indentation hardness increased with decreasing interlamellar spacing, and a Vickers indentation hardness of 28.5 GPa was achieved in the sample with the smallest interlamellar spacing. The directionally solidified WC-W2C materials show enhanced indentation mechanical properties in comparison to previously reported WC-Co composites and WC-based materials. [ABSTRACT FROM AUTHOR]

To characterize the inhomogeneous field distributions and their effect on ferroelectric/ferroelastic domain switching in potassium sodium niobate-based piezoceramics, a partially electroded orthorhombic phase K 0.5 Na 0.5 NbO 3 ceramic sample containing a distinct edge was investigated using micro-beam, high-energy XRD with applied fields. Under the field application, the partial-electrode sample exhibits orientation-dependent and position-sensitive domain switching. The experimental measurements of micro-scale domain texture correlate reasonably with simulated electric fields calculated from a finite element analysis. Both experimental and simulated results reveal that the domain switching is induced inhomogeneously up to the millimeter-scale across the sample and that the electric field is significantly concentrated near the electrode edge. Integrating the experimental results with the model demonstrates that the local electric field directions and amplitudes drive the local orientation dependence of domain switching and, conversely, the direction of the local electric field can be inferred from the direction in which the maximum domain switching is measured. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, we measure DC and AC conductivity and Hall voltage to determine the origin of electrical insulating properties of Fe-doped β-Ga2O3 single crystals, which are measured perpendicular to the 2 ¯ 01 crystallographic plane. We find that electrical conduction is predominantly controlled by free electrons in the temperature range 230–800 °C with the mutual compensation of the impurity donor (Si) and acceptor dopant (Fe), explaining the low concentration of free electrons and Fermi level pinning over a wide range of temperatures. Furthermore, the negative temperature-dependence of the carrier mobility indicates that it is limited by optical phonon scattering. Importantly, we find electrical conductivity to be largely independent of oxygen partial pressure (pO2) from air to 10−4 atm at 600 °C, but it becomes slightly dependent on pO2 at 800 °C, as intrinsic non-stoichiometric point defects begin to influence the charge balance. [ABSTRACT FROM AUTHOR]

The effect of gnomonic distortion on orientation indexing of electron backscatter diffraction patterns is explored through simulation of electron diffraction patterns for sample‐to‐detector geometries associated with transmission Kikuchi diffraction (TKD) and electron backscatter diffraction (EBSD). Simulated data were analysed by computing a similarity index for both Hough transformed data and simulated patterns to determine the sensitivity of each method for detecting subtle differences in the effect of gnomonic distortions on electron diffraction patterns. These results indicate that the increased gnomonic distortions in electron diffraction patterns for a TKD geometry enhance the sensitivity for detecting subtle differences in interband angles. Additionally, the utilisation of a Hough transform‐based indexing approach further enhances the sensitivity. Electron backscatter diffraction (EBSD) and transmission Kikuchi diffraction (TKD) have become vital tools for studying materials. These probes provide information about how locally arranged. EBSD and TKD utilise different experimental configurations that have impacts the measured data. The projection of the electron diffraction Ewald's sphere onto the face of the detectors results in Gnomonic distortions, and the selection of experimental configuration affects the magnitude of these distortions. In this work we used simulated dynamical electron diffraction patterns (EDPs) to understand the how differences in the EBSD and TKD experimental configurations impacts Gnomonic distortions and how these distortions effect the sensitivity of each method for detecting subtle differences in interband angles. These results indicate that the increased gnomonic distortions in electron diffraction patterns for a TKD geometry enhance the sensitivity for detecting subtle differences in interband angles. These results also suggest that a traditional Hough transform approach for indexing EDP data is less sensitive to detecting slight deviations than direct analysis of EDPs. [ABSTRACT FROM AUTHOR]

Multilayer capacitors consist of multiple, often hundreds of capacitors connected in parallel to maximize volumetric efficiency. As the dielectric and electrode layer thicknesses are scaled down, microstructural imperfections become increasingly influential on the device electrical performance. Specifically, the presence of nonplanar and discontinuous electrodes can lead to local field enhancements while the relative morphologies of two adjacent electrodes determine variations in the local dielectric thickness. To study the effects of electrode morphologies, an analytical approach is taken to calculate the field enhancement and leakage current with respect to an ideal parallel-plate capacitor. It is shown that the electrode roughness causes the leakage current to increase with respect to that of the ideal flat parallel-plate capacitor. To further include the effects of local curvature on electric field enhancements, finite element methods are used to calculate field distributions within capacitor structures containing rough interfaces and porosity. In these simulations, the effects of electrode pore diameters, dielectric layer thickness, and the amplitude and wavelength of the electrode roughness are studied. [ABSTRACT FROM AUTHOR]

Multilayer ceramic capacitors ( MLCCs), owing to their processing conditions, can exhibit microstructure defects, such as electrode porosity and roughness. The effect of such extrinsic defects on the electrical performance of these devices needs to be understood to achieve successful miniaturization. To understand the influence of microstructural defects on field distributions and leakage current, the three-dimensional (3-D) microstructure of a local region in MLCCs is reconstructed using a serial-sectioning technique in the focused ion beam. This microstructure is then converted into a finite element model to simulate the perturbations in electric field due to the presence of electrode defects. The electric field is significantly enhanced in the vicinity of such defects, and this is expected to have a bearing on the leakage current density of these devices. To simulate the scaling effects, the dielectric layer thickness is reduced in the 3-D microstructure keeping the same electrode morphology. It is seen that the effect of microstructure defects is more pronounced as one approaches thinner layers, leading to higher local electric field concentrations and a concomitant drop in insulation resistance. [ABSTRACT FROM AUTHOR]

Abstract: The optical properties and structure of anodic oxides are dependent on the anodization conditions. For tantalum oxide formed in dilute phosphoric acid, the anodic oxide forms as two chemically distinct layers, referred to as a bi-layer, where the inner layer is pure tantalum oxide and the outer layer contains incorporated phosphate. The complex dielectric functions of the individual inner and outer layers are determined using spectroscopic ellipsometry. The dielectric functions of the bi-layers are compared to mono-layer oxides formed in sodium sulfate, and the effects of hydrothermal sealing are explored. For bi-layer oxides formed to 70V in phosphoric acid, the inner layer band gap is 4.37±0.02eV and the outer layer band gap is 3.86±0.06eV. Thin anodic oxides (∼6–15nm) are best described by a mono-layer oxide model and exhibit higher optical absorption with a band gap of 3.98±0.08eV. This study shows that spectroscopic ellipsometry is a valuable tool in assessing processing–property relationships of multi-layer anodic films. [Copyright &y& Elsevier]

Abstract: Both pulsed-DC biased and commercial ion-beam sputtered VO x thin films maintain a face-centered-cubic nanocrystalline phase, even for stoichiometries of x >1.5, which is well outside the bulk equilibrium solubility range for cubic VO x . Many of these highly nonstoichiometric films exhibit a high density of microtwins, which give rise to unusual fine structure in the selected-area electron diffraction patterns, namely: an additional defect ring; a significant broadening of the {200} ring; pairs of parallel rod features which are tangent to the additional defect ring; and additional fine-structure features between the {200} and {220} rings. The formation of the microtwins is correlated with the coalescence of vanadium vacancies along the {111} twin planes in the crystalline lattice. [Copyright &y& Elsevier]

Abstract: Au-assisted metalorganic chemical vapor deposition was employed for the synthesis of GaSb nanowires. X-ray energy dispersive spectrometry indicates that the composition of catalyst particles depends on the reactor pressure at constant growth temperature and V/III ratio. The catalyst particles were ternary Au–Ga–Sb alloys for nanowires grown at 100 and 200Torr, suggesting a vapor–liquid–solid (VLS) nanowire growth mechanism. At 300Torr, the catalyst particles were pure Ga and the self-catalytic VLS mechanism was responsible for the growth of the GaSb nanowires. The diameter and growth rate of the nanowires, as well as the contact angle between the catalyst particle and the nanowire, were found to be dependent on the composition of the catalyst particles. [Copyright &y& Elsevier]

Nickel manganite spinel thin films prepared by chemical solution deposition exhibit negative temperature coefficient of resistance (TCR) values between −3.3 and −4.5%/K. In contrast to bulk thermistors, dense films could be prepared completely within the spinel phase field. Thus, decomposition into the NiO phase and a Mn-rich spinel, which is problematic in bulk ceramics, is minimized in thin films. For films prepared outside of the single-phase field, phase separation cannot always be detected using X-ray diffraction. In such cases, transmission electron microscopy is useful in identifying decomposition. It is found that the lattice parameters for films with compositions ranging between Mn/(Mn+Ni)=0.14 and 0.77 are smaller than the values reported for nickel manganite spinels, suggesting cation deficiency. Single-phase spinel films are compared with single-phase bixbyite films synthesized between 630° and 930°C. The bixbyite phase exhibits lower TCR and lower resistivity (TCR=−3.1 to −3.3%/K and resistivity values=400–1600 Ω·cm) compared with spinel (TCR=−3.6 to −4.1%/K and resistivity values=3500–21 000 Ω·cm). Composite films (achieved by controlling the pyrolysis to create a low local pO2 during annealing) show intermediate values (TCR=−3.0 to −3.8%/K and resistivity values=470–6600 Ω·cm). [ABSTRACT FROM AUTHOR]

The formation of a directionally solidified non-oxide eutectic surface layer based on the 75 mol% B4C and 25 mol% TiB2 eutectic composition has been developed using a continuous-wave, high-power, CO2 laser. To prevent the oxidation of nonoxide compounds and to reduce thermal stress formation, an atmospherically controlled furnace capable of back-heating samples up to 1100°C during the laser processing has been employed. The effect of the laser scan rate on the eutectic microstructure formation is investigated. It is found that at relatively slow laser scan rates, ∼2–4 mm/s, the formation of a colony-type eutectic microstructure with submicrometer scale features is observed. At higher heating rates up to 42 mm/s, disordered eutectic grains with nanometer-scale TiB2 lamellae form. The dependence of the TiB2 interlamellar spacing on the crystallization rate in the laser solidified B4C–TiB2 eutectic has the same general trend as in the eutectic produced by a floating zone method. However, this method allows the production of much smaller microstructural length scales, on the order of 100 nm, as a consequence of the high crystallization velocity. [ABSTRACT FROM AUTHOR]

Abstract: In situ substrate curvature measurements were used to monitor stress evolution during metal organic chemical vapor deposition (MOCVD) of Al x Ga1− x N epilayers (x=0, 0.5, 0.7) on 6H-SiC (0001) substrates that were grown using compositionally graded AlN to Al x Ga1− x N buffer layers. The results were compared to identical growths carried out using thin (∼80nm) AlN buffer layers in order to assess the impact of the graded layer on the mean film stress and structural properties of Al x Ga1− x N. Compositionally graded (AlN–GaN) layers effectively increased the mean compressive stress in GaN epilayers grown on SiC compared to samples grown on AlN buffer layers, with the mean stress in the GaN layer increasing with buffer layer thickness. Increasing buffer thickness correlated with a decrease in the threading dislocation density measured by plan-view-TEM. In contrast, the mean stress in high Al-fraction Al x Ga1− x N epilayers (x=0.5 and 0.7) was not significantly altered by the use of AlN to Al x Ga1− x N graded buffer layers and in some cases, the graded layer leads to a decrease in mean compressive stress in the films. The differences in stress evolution are explained in terms of the available compressive strain energy, which is suggested to be responsible for dislocation bending observed in the GaN graded layer samples. [Copyright &y& Elsevier]

Microstructural control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the present-day challenges for maintaining an increase in capacitive volumetric efficiency. The present paper continues a series of investigations aimed at understanding and controlling the microstructural stability of ultrathin Ni electrodes in MLCCs. Here, a kinetic approach based on the control of sintering profiles is used. Ni–BaTiO3 MLCC chips (0805-type with 300 active layers) are nonisothermally sintered up to 900°–1300°C with different heating rates in the range from 200° to 3000°C/h. In general, the continuity of the Ni electrodes increases with heating rate. However, a strong nonlinear dependence of Ni electrode continuity on sintering temperature is observed. It is concluded that a low-melting interfacial liquid (Ni,Ba,Ti) alloy layer initiates at temperatures between 1000° and 1100°C when the Ni electrodes are under tension. This interfacial liquid phase accelerates a stress-induced diffusion and is the key cause of the severe electrode discontinuities during heating. At higher temperatures (above 1100°C), where compressive stresses are active, the interfacial liquid alloy layer facilitates some recovery of the Ni electrode microstructure. The formation of the interfacial liquid alloy layer can be kinetically controlled using fast-heating rates, which improves the Ni electrode continuity. [ABSTRACT FROM AUTHOR]

A novel approach to pressureless sintering based on the combination of rapid-rate sintering, rate-controlled sintering, and two-step sintering under a controlled atmosphere is proposed. This combined sintering method facilitates control of grain and pore morphology. The application of this sintering approach for pure nanocrystalline barium titanate powder enables the suppression of grain growth during the intermediate and final stages of sintering and the production of fully dense ceramics with 108 nm grain size. The grain growth factor is 3.5, which is three and 17 times smaller than rate-controlled and conventional sintering, respectively. [ABSTRACT FROM AUTHOR]

We explore the synergistic effects of co‐doping BaTiO3 with a judicious combination of acceptors and donors to control the point defect chemistry and electrical properties, with the goal of simultaneously limiting the electronic and ionic conductivities over broad temperature and oxygen partial pressure (pO2) ranges. Specifically, we compare the temperature‐ and pO2‐dependent electrical properties of BaTiO3 ceramics acceptor‐doped with either Mn or Mg and co‐doped with a Y donor. This study, which is the first of a two‐part series, presents the electrical properties as a function of pO2, temperature, and time, focusing on the grain‐interior electrical response. The DC and AC electrical conductivity measurements reveal that co‐doping with Mn and Y can result in (1) increased electrical resistivity over a broad temperature range, (2) pO2‐independent electrical conductivity in oxidizing conditions, and (3) improved time‐dependent dielectric degradation resistance. These behaviors are attributed to a Fermi level pinning effect, as is explained in the companion paper, which presents complementary density functional theory (DFT)‐based grand‐canonical defect chemistry models. The collective experimental and computational studies demonstrate that the pO2‐independent electrical conductivity in the Mn and Y co‐doped BaTiO3 is attributed to a Fermi level pinning mechanism arising from the multivalent Mn dopant, and the background reservoir of positive charge provided by the predominant substitution of Y on the Ba sites. The enhanced degradation resistance is attributed to a reduced oxygen vacancy concentration relative to the other doping chemistries. [ABSTRACT FROM AUTHOR]

Grain boundary segregation of yttrium in titanium dioxide is studied systematically as a function of dopant concentration. Local grain boundary defect chemistry is quantitatively determined by analytical transmission electron microscopy and compared to equilibrium space-charge segregation models that incorporate both the electrostatic and elastic strain-energy driving forces for solute segregation. The thermodynamic models show that the elastic strain-energy driving force dominates in this system and that the electrostatic driving force contributes a 10–20% correction to the total solute segregation. In comparison to the experimental results, the theoretical models provide good predictions for several grain boundary parameters including local stoichiometry, solute interfacial excess and space-charge layer thickness. [Copyright &y& Elsevier]

A first‐principles informed grand canonical defect chemistry model capable of accounting for non‐stoichiometry and partial equilibration of different sub‐lattices is developed and used to study Mg and Mn doped, and (Mg+Y) and (Mn+Y) co‐doped BaTiO3 to elucidate the role of Mn and Y in improving the resistivity and resistance degradation of BaTiO3 as observed by Ryu et al. in Part I of this series of papers. The model qualitatively captures the behavior of the samples in all conditions, reproducing the observed carrier plateau and increased resistivity of (Mn+Y) co‐doped BaTiO3, and expected trends in the concentrations of free oxygen vacancies with doping. These trends reflect the observed differences in degradation characteristics, and help explain the substantially improved degradation resistance of the (Mn+Y) co‐doped samples. Our model adds to the mechanism proposed by Yeoh et al. that the Fermi level is pinned by the multivalent character of MnTi in (Mn+Y) co‐doped BaTiO3 by giving insight into the role of barium vacancies, the site preferences of the dopants, and defect complexes in this mechanism. These insights provide a set of criteria in the search for sets of co‐dopants with similar behaviors. [ABSTRACT FROM AUTHOR]

Thin film metallic coatings applied to alumina and silicon substrates are investigated for their use as a catalyst to help grow high-quality multiwall carbon nanotubes (MWNTs). Substrate coatings examined include Fe, Tb:Fe, Ni, Cu, and Ni:Fe, with xylene used as the hydrocarbon source. Coating the substrate with Tb90Fe10 and Ni80Fe20 facilitated dense and uniform growth of MWNTs without graphitic particles; Ni and Fe substrate coatings produced graphitic particles in addition to the MWNTs, while Tb and Cu were found to be completely inactive with no MWNT growth. Many of the MWNTs grown over Ni:Fe have a helical appearance, while the MWNTs grown over Tb90Fe10 did not contain catalyst particles [ABSTRACT FROM AUTHOR]

Examines the structure of the cubic-zirconia tilt grain boundary on density functional theory. Computation of grain boundary; Stabilization mechanism of zirconia; Comparison between z-scanning and pristine stoichiometric grain boundary.

In situ high-energy X-ray diffraction (XRD) was performed on lead-zirconate-titanate-based ferroelectric materials with composition near the morphotropic phase boundary (MPB). The utilization of the two-dimensional area detector in in situ field-dependent experiments enables the complete analysis of the material response with respect to all azimuthal angles at each field amplitude. The studies reveal that the field-induced phase transition from tetragonal to rhombohedral is dependent on crystal orientation in Nb-doped PbZr 0.53 Ti 0.47 O 3 that is in close compositional proximity to the MPB. However, only domain wall motion is activated in Nb-doped PbZr 0.50 Ti 0.50 O 3 , which is further in composition from the MPB. This synchrotron-based XRD characterization approach illustrates the importance in evaluating the orientation-dependence of phase transitions in piezoelectric and ferroelectric polycrystalline materials. [ABSTRACT FROM AUTHOR]

• Novel controls system bridges gap between conventional and flash sintering. • FEA and in-situ flash sintering IR measurements correlated with microstructure. • Densification and grain growth trends follow classic dependency on thermal profile. • Bulk conductivity depends on flash sintering thermal profile, including quench. • Flash sintered samples consistently exhibit higher grain boundary conductivity. In flash sintering experiments, the thermal history of the sample is key to understanding the mechanisms underlying densification rate and final properties. By combining robust temperature measurements with current-ramp-rate control, this study examined the effects of the thermal profile on the flash sintering of yttria-stabilized zirconia, with experiments ranging from a few seconds to several hours. The final density was maximized at slower heating rates, although processes slower than a certain threshold led to grain growth. The amount of grain growth observed was comparable to a similar conventional thermal process. The bulk electrical conductivity correlated with the maximum temperature and cooling rate. The only property that exhibited behavior that could not be attributed to solely the thermal profile was the grain boundary conductivity, which was consistently higher than conventional in flash sintered samples. These results suggest that, during flash sintering, athermal electric field effects are relegated to the grain boundary. [ABSTRACT FROM AUTHOR]

Compositionally graded AlGaN buffer layers enable the growth of thicker crack free layers of GaN on (111) Si than is possible with an AlN buffer layer. Using cross sectional transmission electron microscopy and in situ stress measurements, it is shown that a compressive growth stress is incorporated in the GaN layer when the graded AlGaN buffer layer is thick enough to accommodate all microstructural evolution, which is primarily a reduction in threading dislocation density with thickness during growth. Most of the dislocation density reduction is observed to occur when the film is growing under a compressive stress. This compressive stress arises from the changing lattice parameter due to grading and helps to offset the tensile stress generated by microstructural evolution. It also helps to decrease the tensile thermal expansion mismatch stress during cooling and thus reduces film cracking. [ABSTRACT FROM AUTHOR]

Thin (∼10 nm) AlN interlayers have previously been used to mitigate stress and cracking in GaN epitaxial layers grown on Si substrates. However, multiple AlN interlayers are typically required for the growth of thick (>1 μm) GaN as the initial compressive mismatch stress introduced by the AlN interlayer transitions to a tensile stress within 0.5 μm. To better understand the reasons for the transition, in situ monitoring and transmission electron microscopy have been used to study stress and structural evolution in undoped GaN layers deposited on high temperature (1050–1100 °C) AlN interlayers by metal-organic chemical-vapor deposition. The results show that transition of the initial compressive stress to a final tensile stress is associated with a reduction in the density of dislocations introduced either by the pseudosubstrate or the interlayer itself. [ABSTRACT FROM AUTHOR]

The crystallization behavior of amorphous barium titanate (BaTiO3) thin films was studied as a function of annealing temperature from 500 to 1000 °C. Quantitative phase analysis by grazing incidence X-ray diffractometry revealed that the metastable hexagonal phase preferentially nucleates at lower temperatures with a transition to stable cubic nuclei at higher temperatures. The predominance of the metastable hexagonal-phase nucleation at lower temperatures suggests that it has the lowest nucleation barrier, in accordance with the Ostwald's step rule. To help induce the nucleation of the cubic phase at lower temperatures, we explored the effects of electric fields on the crystallization behavior and found that direct-current fields moderately enhanced the cubic phase fraction in the 500–700 °C temperature range. Although the nucleation barrier of the cubic phase in the presence of an electric should be lowered more significantly with respect to that of the hexagonal phase because of the former's higher relative permittivity, the effect arising from the change in volume free energy should only become significant at electric field strengths in the range of MV cm−1, an order of magnitude higher than the experimentally accessible fields in the present study. This suggests alternative, perhaps interface-mediated, mechanisms by which the electric field modifies the nucleation behavior of BaTiO3. [ABSTRACT FROM AUTHOR]

Data‐driven science and technology have helped achieve meaningful technological advancements in areas such as materials/drug discovery and health care, but efforts to apply high‐end data science algorithms to the areas of glass and ceramics are still limited. Many glass and ceramic researchers are interested in enhancing their work by using more data and data analytics to develop better functional materials more efficiently. Simultaneously, the data science community is looking for a way to access materials data resources to test and validate their advanced computational learning algorithms. To address this issue, The American Ceramic Society (ACerS) convened a Glass and Ceramic Data Science Workshop in February 2018, sponsored by the National Institute for Standards and Technology (NIST) Advanced Manufacturing Technologies (AMTech) program. The workshop brought together a select group of leaders in the data science, informatics, and glass and ceramics communities, ACerS, and Nexight Group to identify the greatest opportunities and mechanisms for facilitating increased collaboration and coordination between these communities. This article summarizes workshop discussions about the current challenges that limit interactions and collaboration between the glass and ceramic and data science communities, opportunities for a coordinated approach that leverages existing knowledge in both communities, and a clear path toward the enhanced use of data science technologies for functional glass and ceramic research and development. [ABSTRACT FROM AUTHOR]