Reacting flow coupling with thermal impacts in a single solid oxide fuel cell

Thermal impacts are the major concern for the designs of electrolyte of Solid Oxide fuel cells (SOFCs) due to the high temperature operating conditions. In this study, the coupling dynamics of electrochemical reacting flows with heat transfer and generations of thermal strains and stresses (thermal impact) of solid electrolyte and porous electrodes are investigated in a single SOFC by numerical simulations. Modeling results from a test case show that the coupling is necessary as the electrochemical and thermal properties of the cell strongly depends on temperature, meanwhile, the thermal strains and stresses on temperature gradients. The differences in current density and thermal strain gradients predicted by coupling and decoupling simulations are as larger as 20% because of the strong dependents of ionic conductivity of the electrolyte material on temperature, the maximum thermal strain, thermal stresses, and temperature are all about 5%. It is identified that the high operation voltage benefits to the thermal strain, which decreases 20% when the cell operating from 0.5 V–0.7 V.     

In situ assessment of carbon nanotube flow and filtration monitoring through glass fabric using electrical resistance measurement

Filtration of nanofillers into porous fabric media is still an issue during the preparation of advanced fiber-reinforced composites. The assessment of resin/multiwall carbon nanotube (MWCNT) flow, MWCNT filtration, and the cure monitoring of glass fiber/carbon nanotube-polyester composites by means of the measurement of the electrical resistance was introduced. The vacuum-assisted resin transfer molding technique was used. The electrical resistances measured over the span of a composite were qualitatively correlated with MWCNT flow and the degree of MWCNT filtration. It was found that while the complexity of the fabrics could likely introduce preferential deposition of MWCNTs, their filtration is mainly affected by their dispersion state in the resin suspension. Relationships among critical parameters such as the lengths and diameters of MWCNTs, the inter- and intra-tow dimensions of glass fabrics, the dispersion level of MWCNTs, and the viscosity of nanocomposite samples are discussed and correlated to the filtration, cure, and flow phenomena. We showed that our method can also serve as an early warning to obviate defects in the resulting composite.     

Optical temperature sensing based on the luminescence from YAG:Pr transparent ceramics

The YAG:Pr transparent ceramic was fabricated using a conventional solid-state reactive method to explore its possible application in optical thermometry. Photoluminescence and temperature-dependent luminescence were elaborately investigated under 452 nm excitation. The ceramic showed two intrinsic emission bands at 488 and 594 nm, which were attributed to characteristic Pr³⁺: ³P₀ → ³H₄ and ³P₁ → ³H₆ transitions, respectively. Down-conversion emissions from the two thermally coupled excited states of Pr³⁺ were recorded in the temperature range of 293–593 K. The Boltzmann distribution theory was adopted to interpret the temperature-dependent luminescence of Pr³⁺. The temperature sensitivity exhibited an increasing trend with the increase of temperature, typically, 0.0025 K⁻¹ at 593 K. The results indicated that the present ceramic was a promising candidate for optical temperature sensor.     

A subgap density of states modeling for the transient characteristics in oxide-based thin-film transistors

The modeling of the transient subgap density of states (DOS) for the investigation of trap densities in the oxide-based thin-film transistors is proposed. The study is based on both transient measurements and physical modeling. In history, the subgap DOS modeling of trap densities have been studied according to the static-state current–voltage characteristics or the capacitance–voltage curves. However, the subgap DOS modeling for the transient curves is seldom proposed. In this study, the transient model of subgap DOS is discussed for amorphous In–Ga–Zn–O (a-IGZO) thin films. This model suggests the subgap DOS exhibits a transient behavior with an exponential distribution on the band edge and a Gaussian distribution in the deep gap level. This study could be helpful to understand and optimize the transient electrical properties of a-IGZO TFTs.     

Theoretical and experimental analyses of Young's modulus and thermal expansion coefficient of the alumina-mullite system

Young's moduli (E) and thermal expansion coefficients (TECs) of the alumina–mullite–pore system (96.4–99.5% relative density) were measured for a wide mullite fraction range from 0 to 100 vol%. Both E and TEC values decreased at high mullite fractions. These properties were theoretically analyzed with four proposed model structures that were constructed by three-phase systems of mullite (or alumina) continuous phase 2–pore dispersed phase 1–alumina (or mullite) dispersed phase 3. The ratios of E(theoretical)/E(experimental) and TEC(theoretical)/TEC(experimental) were very close to unity, depending on the mullite fraction. That is, the measured E and TEC values are closely related to the change in the composite microstructure as a function of mullite fraction.     

Corrosion behaviour of porous Ni-free Ti-based bulk metallic glass produced by spark plasma sintering in Hanks' solution

Porous bulk metallic glasses (BMGs) are promising biomedical materials to be used for surgical implants. Here we report on successful formation of porous Ni-free Ti-based BMGs with a diameter exceeding 15 mm by spark plasma sintering the mixture of the gas-atomized Ti-based (Ti₄₅Zr₁₀Cu₃₁Pd₁₀Sn₄) glassy alloy powder and solid salt powder, followed by leaching treatment into water to eliminate the salt phase. Corrosion behaviour of the produced porous Ti-based BMGs was investigated in Hanks' solution. The potentiodynamic polarization curves showed that the anodic current density in the porous Ti-based BMGs slowly increased during anodic polarization, suggesting the crevice corrosion.     

Polymer spherulites: A critical review

Polymeric and non-polymeric materials often crystallize as spherulites when crystallized from viscous melts or solutions at large undercooling. The essential component of a spherulite is fibrillar crystals that grow in predominantly radial directions and branch irregularly. We review the growth, branching and twisting of crystals in the light of theoretical and experimental advances of the last decade, while maintaining an appreciation for historical context.The crucial role of self-generated fields ahead of the crystal–melt interface is developed. Pressure gradients from volume contraction have been treated, as well as impurity gradients ahead of a growing crystal; fibril width W is predicted and found to be proportional to δ^1/2, where the diffusion length δ = D/G, the quotient of diffusivity and growth rate, conveys the extent of the field gradient. Ribbon-like spherulite radii grow at a constant rate under diffusion-coupled interface control.Non-crystallographic branching is required to maintain the volume occupied by fibrillar crystals as the spherulite radius increases. Topological giant screw dislocations and induced nucleation at cilia tethered to crystals are observed mechanisms leading to branching normal to the wide dimension of lamellar crystals; but the relative importance of each of these is not yet established. Repetitive tip splitting by kinetic interface instability has been suggested as a branching mechanism in the wide dimension of lamellar crystals.Larger molecular mass reduces the spherulite growth rate, more so at low undercoolings, for reasons that remain unresolved. Miscible diluents often profoundly reduce G by lowering both thermodynamic driving force and local transport dynamics that govern the secondary nucleation rate. Spherulite blend morphology is linked to the competition between radial growth rate G and diffusivity D of the diluent, expressed as the diffusion length δ.Polymer crystals in which chain helices all have the same sense show banded spherulites, as do crystals in which the chain axes are not perpendicular to the basal surfaces. Recent analyses with optical birefringence and X-ray micro-diffraction support the presence of helicoidally twisted ribbons, although other structural arrangements have sometimes been revealed by microscopy. Assessments of twist directions in spherulites of chiral polymers point to unbalanced basal surface stress as the source of twisting, although a general mechanical analysis is lacking. Another twisting model employs regular arrays of isochiral giant screw dislocations; results are mixed for this model.     

Origin of abnormal glass transition behavior in metallic glasses

In this paper, the phenomenon of two glass-transition-like appearance in the supercooled liquid region of metallic glasses was investigated. It is confirmed that this abnormal behavior is attributed to the transition process of an amorphous state from higher energy to lower energy. The amorphous state with higher energy comes from the uneven distribution of compositions in glasses, which is mainly caused by the component with significant differences in atomic size and nonnegative values of enthalpy of mixing. The results were verified by high resolution transmission electron microscopy and energy-dispersive spectrometry.     

Improved performance of inverted polymer solar cells by utilizing alcohol-soluble oligofluorenes as efficient cathode interlayers

Two star-shaped oligofluorenes with hexakis(fluoren-2-yl)benzene as core are designed and synthesized, namely Tn0 and Tn1. Diethylamino groups are attached to the side chain of fluorene units of Tn0 and Tn1 and enable them alcohol solubility, additional hydrophobic nhexyl chains are grafted on the π-extended fluorene arms of Tn1. Power conversion efficiency (PCE) as high as 8.62% and 8.80% are achieved when utilizing Tn0 and Tn1 as cathode interlayers in inverted polymer solar cells, respectively. The work function of ITO effectively decreased by introducing interlayer, resulting in high Voc of the device, besides, the wetting properties of the interlayers can be tuned by modifying the oligofluorenes with π-extended structure, and the more hydrophobic interlayer will benefit the device performance with enhanced Jsc and FF.     

A systematic investigation into a novel method for preparing carbon fibre–carbon nanotube hybrid structures

By electrospraying solvent dispersed carbon nanotubes (CNTs) with a binder onto carbon fibre (CF), hybrid structures, with an end aim to improve interfacial bonding in composites, were formed. The electrospray parameters controlling the modification of the CNT morphologies were studied. High-speed camera observations found applied voltage was critical for determining spray mode development. Electric field simulations revealed a concentrated electric field region around each fibre. Both voltage and distance played an important role in determining the CNT morphology by mediating anchoring strength and electric field force. The forming mechanism investigation of different surface morphologies suggested that binder with appropriate wetness gives freedom to the CNTs, allowing them to orientate radially from the CF surface. Linear density (LD) measurements and thermogravimetric analysis revealed that a 10 min coating increased the LD of a single CF filament by up to 31.7% while a 1 h treatment increased fibre bundle mass by 1%.