Whisker reinforcement of glass-ceramics

Silicon carbide whisker reinforcement of anorthite and cordierite glass ceramics has been studied. At 25 vol% whisker loading the flexural strengths increased from 65–103 MPa to 380–410 MPa, the fracture toughnesses increased from 1.0–1.5 MPa m^1/2 to 5.2–5.5 MPa m^1/2. The strengths decline to 240–276 MPa at 1200 °C. The reasons for the decrease in strength with temperature are discussed. Whiskers from two different sources with differences in diameters and aspect ratios were evaluated and the effect of the whisker morphology on the composite properties was studied. It was found that larger diameter, higher aspect ratio whiskers result in improved composite performance. The composites were also characterized in terms of their thermal properties, i.e. thermal expansions and thermal conductivities. The thermal expansion coefficient from 25–1000 °C for anorthite-based composite was 4.6×10^–6 °C^–1 and that for the cordierite-based composite was 3.62×10^–6 °C^–1. The thermal conductivities at 1000 °C were 3.75 and 4.1 Wm^–1 K^–1 for cordierite and anorthite composites, respectively.     

Thermal Diffusivity/Conductivity and Specific Heat of Mullite-Zirconia-Silicon Carbide Whisker Composites

The thermal diffusivity of mullite-ZrO₂-SiC_w materials was determined from 25° to 1000°C for composites in which the ZrO₂ had different amounts of monoclinic and tetragonal phase, and varying SiC _w content. At 25°C the thermal diffusivity of the matrix materials increased with increasing amounts of monoclinic phase, and little or no anisotropy was seen. Composites with SiC _w additions showed significant increases in thermal diffusivity and anisotropy as compared with the matrix materials. With increasing temperature, thermal diffusivities of the matrices and their respective composites decreased, and at 1000°C differences were small. Specific heat was determined from 25° to 700°C and thermal conductivity values calculated. Specific heat increased with temperature and was not composition dependent, except for one sample. Thermal conductivity values as a function of temperature followed the same trends as the thermal diffusivity values.     

Structural drivers controlling sulfur solubility in alkali aluminoborosilicate glasses

If the direct feed approach to vitrify the Hanford's tank waste is implemented, the low activity waste (LAW) will comprise higher concentrations of alkali/alkaline-earth sulfates than expected under the previously proposed vitrification scheme. To ensure a minimal impact of higher sulfate concentrations on the downstream operations and overall cost of vitrification, advanced glass formulations with enhanced sulfate loadings (solubility) are needed. While, the current sulfate solubility predictive models have been successful in designing LAW glasses with sulfate loadings <2 wt.%, it will be difficult for them to design glass compositions with enhanced loadings due to our limited understanding of the fundamental science governing these processes. In this pursuit, this article unearths the underlying compositional and structural drivers controlling the sulfate solubility in model LAW glasses. It has been shown that the preferentially removes non-framework cations from the modifier sites in the silicate network, thus, leading to the polymerization in the glass network via the formation of ring-structured borosilicate units. Furthermore, though the sulfate solubility slightly decreases with increasing Li⁺/Na⁺ in the glasses, the prefers to be charge compensated by Na⁺, as it is easier for to break Na–O bonds instead of Li–O bonds.     

A study of 3D cover glass design that improves handheld device drop reliability

Cover glass in commercial handheld devices is now evolving from flat (2D) to curved (3D) shapes. For example, some commercial devices have utilized sled‐shape cover glass, 1 which partially covers long edges of the device. According to the patents published by key handheld manufacturers, 2 , 3 we can expect more variety of 3D shaped cover glass for handheld devices in the market. In this study, we have focused on the reliability of 3D cover glass when it is dropped to a rigid surface. The key parameters under study are the corner/edge bend radius and angle of the cover glass, which determines the 3D shape of the cover glass. To achieve this goal, we developed a finite element model to simulate the drop 4 - 13 of a handheld device with 3D‐shaped glass. The model uses explicit algorithm to simulate the high speed impact on the device during the drop test. The glass performance was evaluated based on contact force between the glass and the ground and maximum principal stress in the glass. We showed that to avoid severe damage because of first impact between the glass and the ground, the bend angle of 3D glass has to be in the range between 0 and 45°. For drop angles of 45° and higher, with the proposed glass bend angle, the impact can be taken over by the edge of the back cover of the device. In addition, we showed that optimum glass bend radius is in the range of 3.8 mm and larger. This is required to reduce stress in glass because of impact. The approach and conclusions from the current study can serve as a general guideline to improve the 3D cover glass reliability of a handheld device.     

Nanoscale Lacing by Electrons

The ability to harness the optical or electrical properties of nanoscale particles depends on their assembly in terms of size and spatial characteristics which remains challenging due to lack of size focusing. Electrons provide a clean and focusing agent to initiate the assembly of nanoclusters or nanoparticles. Here an intriguing route is demonstrated to lace gold nanoclusters and nanoparticles in string assembly through electron‐initiated nucleation and aggregative growth of Au(I)‐thiolate motifs on a thin film substrate. This size‐focused assembly is demonstrated by controlling the electron dose under transmission electron microscopic imaging conditions. The Au(I)‐thiolate motifs, in combination with the molecularly mediated alignment, facilitate the interstring electrostatic and intrastring aurophilic interactions, which functions as a molecular template to aid electron‐initiated 1D lacing. The findings demonstrate a hierarchical route for the 1D assemblies with size and spatial tunable catalytic, optical, sensing, and diagnostic properties.     

High-gain observers in the presence of measurement noise: A switched-gain approach

This paper considers output feedback control using high-gain observers in the presence of measurement noise for a class of nonlinear systems. We study stability in the presence of measurement noise and illustrate the tradeoff when selecting the observer gain between state reconstruction speed and robustness to model uncertainty on the one hand versus amplification of noise on the other. Based on this tradeoff we propose a high-gain observer that switches between two gain values. This scheme is able to quickly recover the system states during large estimation error and reduce the effect of measurement noise in a neighborhood of the origin of the estimation error. We argue boundedness and ultimate boundedness of the closed-loop system under switched-gain output feedback.     

Developing a Simplified Analytical Thermal Model of Multi-chip Power Module

The study of the thermal behavior of power modules has become a necessity regarding the known rapid development in modern power electronics, and the prediction of temperature variation has generally been performed using transient thermal equivalent circuits. In this paper we have developed a simplified analytical thermal model of a power hybrid module. This analytical method is used to evaluate the thermal parameters of a device. The model takes into account the thermal mutual influence between the different module chips based on the analytical method. The thermal interaction between components is dependent on the boundary condition, the dissipated power value in the different components and the number of operating chips constituting the module. This effect is modelled as a source energy and a thermal resistance simply computed tanks to reasonably low measurement applied on the module. The derived thermal models offer an excellent trade-off between accuracy, efficiency and CPU-cost.     

Density and Young׳s Modulus of ternary glasses close to the eutectic composition in the CaO–Al2O3–SiO2-system

The elastic properties and the density of ternary glass forming systems within the CaO–SiO₂–Al₂O₃-system (CAS) were evaluated. Different glass compositions near the lowest eutectic (1170 °C) composition within the CaO–Al₂O₃–SiO₂-system have been melted from pure raw materials. Their target compositions differed not more than 4 wt% for each component. Exact chemical compositions were measured by x-ray fluorescence. The density, and acoustic properties were determined and the Young׳s Moduli were derived herefrom. It was of special interest to obtain information on these properties and their dependencies upon small variations in the composition. The density values were between 2.600 and 2.667 g cm⁻³ and the packing density factors V_p of the oxides glasses using the ionic radii of Pauling were in the range from 0.559 to 0.571. The determined data were compared to different model calculations. Density model calculations show relative deviations between 2 and 6%. The values calculated from the model for Young׳s Modulus by Makishima and Mackenzie (1973) [1] were somewhat smaller than the measured ones. The correction by Rocherulle et al. (1989) [3] of the Makishima model showed better agreement with the measured values.     

Crystallization of a highly viscous multicomponent silicate glass: Rigidity percolation and evidence of structural heterogeneity

A magnesium bearing multicomponent silicate glass was annealed at the temperature slightly above glass transition temperature (T_g) for various periods of time. The crystalline phase of fluorophlogopite with the chemical formula of NaMg₃AlSi₃O₁₀F₂ is precipitated in the floppy regions, which leads to the formation of a core-shell structure. Considering that there exists only four- or six-fold coordinated Mg²⁺ in the studied glass, the fraction of four-fold coordinated Mg²⁺ is determined by comparing the maximum size of the crystallite and the floppy region according to the percolation theory. In addition, a striking phenomenon of endotherm above glass transition in heat capacity (C_p) curve is observed for the first time when prolonging the annealing time, which is ascribed to the amorphous heterogeneous structure surrounding the crystallites.     

Toward revealing atomic deformation mechanics in lithium disilicate and β-quartz containing glass-ceramics

The mechanics of glass-ceramics subjected to sharp contact or other loading conditions remain elusive, even after being commercialized in many industrial applications. We present work herein to reveal atomic details of such deformations that are otherwise extremely difficult to probe experimentally for a lithium disilicate (LS2) and β-quartz containing glass-ceramics via molecular dynamics simulations. Specifically, the materials are comprised of LS2 and β-quartz nanocrystals in a residual glass matrix. Regardless of the deformation mechanism, whether it be nanoindentation or crack propagation for samples with pre-existing flaws, we observe that the LS2 nanocrystal itself undergoes substantial deformation, either by activating dislocations, forming an amorphization zone, or by initiating microcracks at glass-crystal interfaces or weak crystallographic planes. In contrast, the β-quartz nanocrystal is not easily deformed and remains almost intact with minimal plastic deformation, thereby forcing shear flow and crack propagation pathways to predominately occur in the residual glass and/or at interfaces. The dramatic difference between the crystalline phases also manifests itself in the deformation mode of interfaces under pure shear loading, in which shear bands preferably occur at the LS2-glass interfaces, while cavities form at the β-quartz-glass interfaces. These observations significantly advance our understanding of glass-ceramics and pave ways to exploit the understanding for more applications.