Separation of Cavitation-Strain and Creep-Strain During Deformation

A significant fraction of the strain in high-temperature deformation may arise from cavitation. A methodology is described for separating the creep strain from the cavitation strain. Such a distinction is necessary because cavitation and creep are mechanistically different processes.     

Passive Oxidation of an Effluent System: The Case of Polymer-Derived SiCO

The oxidation kinetics of non-oxides depend on the inward diffusion of oxygen from the environment through the passivating silica overgrowth, the outward diffusion of the effluent species, e.g., CO, produced by the oxidation reaction at the interface, and the chemical driving forces for diffusion. An analysis that combines these factors into a unified theory is presented. The analysis is applied to experiments on the oxidation of polymer-derived amorphous silicon oxycarbide (SiCO) ceramics containing different amounts of carbon. The comparison between theory and experiment suggests that the activity of the so-called "free carbon" in SiCO is likely to be less than unity, which explains why the oxidation of SiCO is passive in nature. Further, the analysis provides quantitative answers to the following questions: (a) How is the effective diffusivity for the parabolic rate constant related to the composition of the substrate, the inward diffusivity of oxygen, and the outward diffusivity of CO? (b) How does the rate constant depend on the activity of carbon in the substrate and on the activity of carbon in the environment? (c) How is the pressure of CO generated at the interface related to the carbon activity and the diffusion coefficients? The analysis points towards the need for systematic experiments in controlled O₂/CO₂ environments for a more complete understanding of the oxidation kinetics of carbon-based ceramics.     

Crystallization Maps for SiCO Amorphous Ceramics

Amorphous silicon–oxycarbide (SiCO) can retain large mole fractions of carbon when it is made from controlled pyrolysis of silicon-based polymers. The crystallization resistance of these ceramics, which is quite remarkable, varies with the carbon content. In high-carbon materials, crystallization is usually accompanied by weight loss (resulting from the carbothermal reduction of silica), whereas phase separation can lead to crystallization without significant weight loss in the low-carbon materials. A simple graphical method is developed to estimate the composition of the crystallized material, and the concomitant weight loss. The results are presented as maps for a quick estimate of crystallization and weight loss for any composition. Experiments with a medium–high carbon SiCO are used to quantify the degree of crystallization and the associated weight loss at 1300°C and at 1350°C; these results show that, in the case of medium high carbon content, crystallization begins with phase separation but becomes quickly dominated by weight loss.     

Structure-Optical Property Correlation of Epitaxial Potassium Niobate Thin Films Deposited on Magnesium Oxide (100) Substrates Using a Strontium Titanate Transition Layer

Epitaxial thin films of orthorhombic KNbO₃ (100)/(010) were deposited on MgO (100) single-crystal substrates by pulsed laser ablation using an ε120 Å SrTiO₃ (110) transition layer in between. From X-ray diffraction, the orientation relationships were determined as (010) [100] or (100) [010] KNb0₃∥ (110) [110] SrTiO₃∥ (100) [011] MgO and (010) [001] or (100) [001] KNbO₃∥ (110) [001] SrTiO₃∥ (100) [011] MgO. The measured film refractive indices at 632.8 nm were 2.213 ± 0.003, 2.278 ± 0.003, and 2.285 ± 0.003, respectively, along MgO [100], [011], and [011] directions. A model was developed to correlate the measured effective refractive indices of the film to area fractions of domain variants in the film. Using the model, the area fraction of domains with their polarization axis normal to the substrate was estimated to be 60.0 ± 2.7%.     

Optimization of design and characterization of a novel micro-pumping system with peristaltic motion

In this work, we present a novel design of peristalsis based micro pump with optimized fluid chambers possessing improved discharge efficiency per unit volume of the pumping architecture and reduced reverse flow. Such designs are very often important from the standpoint of blood cell sorting assays where a full delivery of fluid containment within the pumping chamber is critical. The paper uses FLUENT and COMSOL simulations to look at the fluid flow within the pumping chamber due to the deflecting actuator membrane during pumping cycle. The resulting effect of fluid-membrane interaction has been evaluated on different chamber designs for observing the lateral velocity distribution profile of fluid in the connecting channels. It has been observed through particle image velocimetry (PIV) that the optimized design has minimized chamber retainability with maximum deflection of the actuator membrane and minimum reverse flow component. Optimized geometrical profile formulated above was seen to allow the maximum contact area between actuating membrane and fluid containment thus reducing the problem of fluid retainability. Other experimental studies show that the new design has much lower percentage retainability of biological and other fluids contained within the chambers which makes it a comparatively high efficiency micropumping system with respect to the conventional design with circular membrane and chambers. The experimental evaluation of the new micro pump design has shown its least count to be 0.1 μl/min which is very well comparing with some of the other micropumping mechanisms like electro-osmotic, magneto-hydrodynamic mechanisms (Laser and Santiago in J Micromech Microeng 14:35, 2004; Iverson et al. 2008) and additionally provides better discharge efficiency per unit volume of the pumping architecture, lower retainability, minimized reverse flow and precise pumping of fluids.     

Correction to: Efficient room temperature methanol sensors based on polyaniline/graphene micro/nanocomposites

Iranian Polymer Journal - The article listed above was initially published with typo error in Eq. (4).     

Performance Evaluation of QAM for Improvement of BER

Wireless Personal Communications - This paper presents performance evaluation of the mary quadrature amplitude modulation (M-QAM). BER performance is evaluated on operation of The Binary coding,...     

A coupled discrete element-finite difference model of selective laser sintering

Selective laser sintering (SLS) is an additive manufacturing technology whereby one can 3D print parts out of a powdered material. However, in order to produce defect free parts of sufficient strength, the process parameters (laser power, scan speed, powder layer thickness, etc.) must be carefully optimized depending on material, part geometry, and desired final part characteristics. Computational methods are very useful in the quick optimization of such parameters without the need to run numerous costly experiments. Most published models of this process involve continuum-based techniques, which require the homogenization of the powder bed and thus do not capture the stochastic nature of this process. Thus, the aim of this research is to produce a reduced order computational model of the SLS process which combines the essential physics with fast computation times. In this work the authors propose a coupled discrete element-finite difference model of this process. The powder particles are modeled as discrete, thermally and mechanically interacting spheres. The solid, underneath substrate is modeled via the finite difference method. The model is validated against experimental results in the literature and three-dimensional simulations are presented.     

Mechanical Design for Accommodating Thermal Expansion Mismatch in Multilayer Coatings for Environmental Protection at Ultrahigh Temperatures

The design of coatings is like designing a system. Every coating has one or more specific functions that determine the choice of materials, and its architecture. In the case of environmental barrier coatings the topcoat must be chemically inert to the atmosphere. In high-temperature applications the stresses arising from thermal expansion mismatch between the topcoat and the substrate must be ameliorated. In this article we consider the design of an intermediate layer of a multilayer coating system with the explicit objective of managing thermal expansion difference between the topcoat and the substrate. The design is based upon a columnar architecture where the columns serve as flexible beams to accommodate relative displacement without fracture. The value of the maximum stresses in the beam and in the topcoat are calculated and used to develop a map with fail and safe regimes. The safe region is defined by the prevention of fracture in the beams, since their fracture would precipitate delamination of the topcoat. As a rule of thumb the topcoat thickness should be less than the width of the columns for safe operation (this condition changes somewhat with the aspect ratio of the columns). A larger aspect ratio of the columns also promotes safe design. We further consider how the tractions induced by the thermal stresses on the surface of the substrate may influence the intrinsic fracture strength of the substrate. The stresses in the coating are predicted to have an insignificant effect on the intrinsic fracture strength of the substrate.     

Phase evolution during reactive flash sintering of Li6.25Al0.25La3Zr2O12 starting from a chemically prepared powder

Reactive flash sintering (RFS) of a chemically prepared multiphase precursor powder was performed to fabricate Li_6.25Al_0.25La₃Zr₂O₁₂ (Al-LLZO) ceramics. This approach allowed for obtaining single-phase dense samples in a remarkably short processing time of 30 s, at a furnace temperature of 600 °C, with an electric field of 50 V cm^?1 and a current limit of 150 mA mm^-2. The ceramics display high bulk conductivity of 0.18 mS cm^?1 at room temperature. Furthermore, phase evolution is studied by in-situ X-ray diffraction during: i) conventional heating and ii) RFS under current rate mode. As expected, the intermediate phases progressively dissolved into the Al-LLZO matrix by conventional heating. On the other hand, RFS promoted the growth of the intermediate La₂Zr₂O₇, an effect that was overcome by the thermally driven formation of Al-LLZO at higher temperatures. The observed different reaction pathway suggests that RFS can be used for stabilizing phases that are not thermodynamically favored upon conventional heating.