Microstructure, property and processing relation in gradient porous cathode of solid oxide fuel cells using statistical continuum mechanics

This paper investigates the relation between microstructure, macroscopic transport properties, and fabrication processing for a gradient porous cathode of solid oxide fuel cells (SOFCs). Functionally graded porous cathode with smooth variations in pore size is composed of lanthanum strontium manganite (LSM) fabricated on yttria stabilized zirconia (YSZ) electrolyte substrate using a multi-step spray pyrolysis (SP) technique at various deposition conditions. Two-dimensional (2D) serial-sections of the gradient porous microstructure obtained by FIB-SEM are fully characterized using statistical correlation functions. Results of statistical analysis of the microstructures revealed that the SP processing technique is capable of generating statistically identical and homogeneous microstructures with smooth gradient in pore size resulting from changing the processing parameters. Strong contrast statistical approach is also used to predict the in-plane temperature dependent effective electrical conductivity of the gradient porous cathode and the results are compared to the experimental data.     

The calculation of thermal conductivity,viscosity and thermodynamic properties for nanofluids on the basis of statistical nanomechanics

The paper features the mathematical model of calculation of thermophysical properties for nanofluids on the basis of statistical nanomechanics. Calculation of properties for nanofluids for real substances is possible by the classical and statistical mechanics. Classical mechanics has no insight into the microstructure of the substance. Statistical mechanics, on the other hand, calculates the properties of state on the basis of molecular motions in a space, and on the basis of the intermolecular interactions. The equations obtained by means of classical thermomechanics are empirical and apply only in the region under observation. The main drawback of classical thermomechanics is that it lacks the insight into the substance of microstructure. Contrary to classical mechanics, statistical mechanics calculates the thermomechanic properties of state on the basis of intermolecular and intramolecular interactions between particles in the same system of molecules. It deals with the systems composed of a very large number of particles.The results of the analysis are compared with experimental data and show a relatively good agreement. The analytical results obtained by statistical mechanics are compared with the experimental data and show relatively good agreement.     

Micro-damage propagation in ultra-high vacuum seals

The paper addresses a fundamental problem of tightness of ultra-high vacuum systems (UHV) at cryogenic temperatures in the light of continuum damage mechanics (CDM). The problem of indentation of a rigid punch into an elastic–plastic half-space is investigated based on rate independent plasticity with mixed kinematic and isotropic hardening. The micro-damage fields are modeled by using an anisotropic approach with a kinetic law of damage evolution suitable for ductile materials and cryogenic temperatures. The model has been experimentally validated and the results are used to predict the onset of macro-cracking (loss of tightness) and the corresponding load (contact pressure). The algorithm is applied in the design of UHV systems for particle accelerators.     

Pressure-driven water flow through carbon nanotubes: Insights from molecular dynamics simulation

Pressure-driven water flow through carbon nanotubes (CNTs) is examined using molecular dynamics simulation. The results are compared to reported experimental flow rate measurements through similarly sized CNTs and larger carbon nanopipes. By using molecular dynamics simulation to predict the variation of water viscosity and slip length with CNT diameter, we find that flow through CNTs with diameters as small as 1.66 nm can be fully understood using continuum fluid mechanics. Potential mechanisms to explain the differences between the flow rates predicted from simulation and those measured in experiments are identified and discussed.     

Effect of interface on the thermal conductivity of carbon nanotube composites

This paper presents the effect of interface on the equivalent thermal conductivity of the carbon nanotube composites. The element free Galerkin method has been utilized as a numerical tool to evaluate the thermal conductivity of the composites. The numerical results have been obtained using continuum mechanics approach for a model composite problem, and it was found that the interface has a major effect on the thermal conductivity of the composites. The effect of interface on the effective conductivity of the composite is small for short nanotubes as compared to long nanotubes. Interface thickness also plays an important role on the effective thermal conductivity of the composite. Nanotube anisotropy has got a small effect on effective thermal conductivity of the composites. Transverse thermal conductivity of the composite has got nearly linear variation with nanotube length.     

The prediction of effective thermal conductivities perpendicular to the fibres of wood using a fractal model and an improved transient measurement technique

The porous microstructure of wood samples on their sections perpendicular to the fibres were analyzed using the scanning electron microscope images. The fractal dimensions of these images were calculated using the box-counting method, respectively. They are all approximately equal to 1.4, although the distribution and the scale of wood fibres are extremely different. Then, a fractal model for predicting the effective thermal conductivities of wood was established using the thermal resistance method. In addition, we measured the effective thermal conductivity of wood via an improved transient plane source measurement method. The calculated results by the proposed model are in good agreement with the experimental data as well as the literature data. The comparison shows clearly that this fractal model can be used to accurately and effectively predict the effective thermal conductivities perpendicular to the fibres of wood.     

Numerical analysis on transport properties of self-humidifying dual catalyst layer via 3-D reconstruction technique

Developing a fuel cell model with fundamental structural properties such as distribution of pore size, geometrical network of individual phase, and volume-specific interfacial area are critical in evaluating the accurate cell performance. Therefore, herein, by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) tomography, three-dimensional (3-D) microstructure of CLs is reconstructed from two real-time samples: (i) High tortuosity humidifying catalyst layer (HTH CL) and (ii) standard catalyst layer. From the reconstructed microstructure, water imbibition behavior at different levels of capillary pressure is simulated and the effective transport properties such as gas permeability, gas diffusivity, surface area and water permeability are derived as well. By coupling the effective structural and transport properties, a 2D model is developed to predict the performances of the two CLs, at relative humidity (RH) levels of 20% and 100%. Since the effective transport properties are derived from real-time samples, this 2D model is expected to have a greater accuracy in predicting the fuel cell performance. Finally, the mechanism of self-humidifying MEA at lower and higher RH conditions (20% RH and 100% RH) is demonstrated as a function of liquid water saturation in the cathode CL and water dry-out in the anode CL.     

Thermophysical Properties of Zirconia Toughened Alumina Ceramics with Boron Nitride Nanotubes Addition

Abstract

The demand for high thermal conductivity substrates with electrically insulating materials are increasing with the emerging markets in power electronics and mobile telecommunication device packages. Effective heat transfer in those packages is important to provide high performance and reliability of the product. This paper mainly presents the thermophysical properties of zirconia toughened alumina ceramics with the addition of small amount of boron nitride nanotubes (BNNTs). The effects of the boron nanotubes addition on the sintering behavior, the microstructure and the thermal properties of the yttria-stabilized zirconia toughened alumina (YZTA), nanocomposite ceramics are investigated. The addition of 0.3?wt% boron nitride nanotubes into the YZTA matrix enhanced the thermal diffusivity as well as a mechanical strength. Above all, the addition of boron nitride nanotubes greatly decreased the coefficient of thermal expansion (CTE) of the composites in which the CTE of pure alumina increases with increasing temperatures. Moreover, the BNNTs added YZTA composites revealed a drastic decrease in CTE at high temperature range, 400–800?°C. This enhanced thermal stability of YZTA–BNNT composites may have a potential application to the high temperature structural ceramics and high power semiconductor packaging substrate.     

Combustion and flame spreading phenomena in gas-permeable explosive materials

Combustion and flame spreading in gas-permeable beds of solid propellant grains are analyzed in this paper. A theoretical model based on continuum mechanics concepts is first formulated. The Lax-Wendroff finite difference technique is then used to generate some numerical solutions. The sensitivity of the model to propellant characteristics, heat transfer and drag correlations, and propellant bed packing density is established by the numerical results. It is concluded that propellant properties, particularly the energy release rate and the burning rate index, are the most critical parameters in this problem. The lack of adequate heat transfer and drag correlations for reactive flows is also noted.     

Fatigue life evaluation of high pressure hydrogen storage vessel

By combining the micromechanics and continuum damage mechanics, a theoretical model is proposed to perform the fatigue evaluation of high pressure hydrogen storage vessel under cyclic internal pressure, which concentrates on the fatigue properties of the aluminum liner. Results show that the fatigue lifetime of vessel relates to the finite element mesh size, crack density and ratio in an element, cyclic loading amplitude and stress status at the liner. Effects of the mesh size and crack density on the fatigue lifetime of vessel are discussed. In addition, numerical results are also compared with those by experiments.     

Effect of B on microstructure and properties of low thermal expansion superalloy

Abstract

The effect of B on microstructure and various properties including coefficient of thermal expansion (CTE), HV hardness, and both smooth and notch stress rupture properties of modified Thermo-Span alloy was studied. The results show that B hardly dissolves in matrix. Increasing B content constrains the formation of Laves phase and grain boundary (GB) precipitation of Laves and G phases, but promotes the formation of M(Co, Fe)NbB boride. In low B doped alloy, its intrinsic high susceptibility to intergranular cracks leads to reduced rupture life and notch sensitivity. Increasing B improves grain boundary cohesion, tying up vacancies and reducing GB diffusion, which constrains the nucleation and propagation of intergranular microcracks, prolongs the rupture life and eliminates the notch sensitivity in the new alloy. Compared with conventional Thermo-Span alloy, the B doped modified alloy shows lower CTE and improved notch sensitivity.     

On the physical interpretation of the internal heat transfer coefficient in a solid-fluid mixture

The continuum theory of mixtures, specially developed to model multiphase phenomena, in which the phases are treated as overlapping continuous constituents, requires additional source terms, absent in a continuum mechanics approach, in order to couple the transport processes among the constituents. A physical interpretation for the internal heat source, present in the energy balance equations for a solid-fluid mixture, is proposed by relating the heat transfer processes between both constituents in two distinct approaches: continuum theory of mixtures and continuum mechanics. This leads to an analogy between the local temperature difference and the usual heat transfer coefficients.     

Macroscopic thermal properties of real fibrous materials: Volume averaging method and 3D image analysis

A general method combining the volume averaging technique and image analysis is proposed to determine the effective thermal conductivity tensor of real fibrous materials featuring local anisotropic thermal properties. The application of mathematical morphology tools on 3D images of wood based fibrous insulators allows a thorough investigation of the microstructure of these materials. A representative elementary volume is determined and the geometrical structure and local anisotropy are studied and quantified. The classical closure problem coming from the one equation model is solved on the 3D thermal conductivity tensor field and the effective thermal conductivity is computed. Good agreement with available experimental data is achieved.     

Exploration of alloy 441 chemistry for solid oxide fuel cell interconnect application

Alloy 441 stainless steel (UNS S 44100) is being considered for application as an SOFC interconnect material. There are several advantages to the selection of this alloy over other iron-based or nickel-based alloys: first and foremost alloy 441ss is a production alloy which is both low in cost and readily available. Second, the coefficient of thermal expansion (CTE) more closely matches the CTE of the adjoining ceramic components of the fuel cell. Third, this alloy forms the Laves phase at typical SOFC operating temperatures of 600-800 °C. It is thought that the Laves phase preferentially consumes the Si present in the alloy microstructure. As a result it has been postulated that the long-term area specific resistance (ASR) performance degradation often seen with other ferritic stainless steels, which is associated with the formation of electrically resistive Si-rich oxide subscales, may be avoidable with alloy 441ss. In this paper we explore the physical metallurgy of alloy 441, combining computational thermodynamics with experimental verification, and discuss the results with regards to Laves phase formation under SOFC operating conditions. We show that the incorporation of the Laves phase into the microstructure cannot in itself remove sufficient Si from the ferritic matrix in order to completely avoid the formation of Si-rich oxide subscales. However, the thickness, morphology, and continuity of the Si-rich subscale that forms in this alloy is modified in comparison to non-Laves forming ferritic stainless steel alloys and therefore may not be as detrimental to long-term SOFC performance.     

Damping Characteristics of Cylindrical Laminates with Viscoelastic Layer Considering Temperature- and Frequency-Dependence

The vibration and damping characteristics of the cylindrical hybrid panels with viscoelastic layers were investigated by using a full layerwise shell finite element method which can consider temperature and frequency dependent material properties. The present layerwise shell theory can accurately represent the zig-zag in-plane and out-of-plane displacements of multilayered hybrid structures and can fully consider the transverse shear and normal strains and the perfect cylindrical geometry. The remarkable differences of the frequency response functions of cylindrical hybrid panels including constrained layer damping and co-cured sandwiched models were observed. Present results show that the damping mechanics of hybrid panels with viscoelastic layers may be greatly affected by their temperature and frequency dependent material properties and that the present full layerwise theory can be used to accurately predict the vibration and damping characteristics of hybrid shells with viscoelastic layers.     

Simulations of flow and heat transfer in a serpentine heat exchanger having dispersed resistance with porous-continuum and continuum models

Numerical simulations of flow and heat transfer in a serpentine heat exchanger configuration are presented to demonstrate application of porous media techniques in heat exchanger analyses. The simulations are conducted using two different approaches. In the first approach, a porous continuum homogeneous model (PCM), or macroscopic model, is applied. The solid and fluid phases are modeled as a single, homogeneous medium having anisotropic effective properties that are calculated separately from unit cell scale analyses and are made available to the macroscopic analysis. In the second approach, a continuum heterogeneous model (CM), or microscopic model, is employed to solve the momentum and energy equations for the fluid phase. The solid phase, a regular interruption to the flowfield, is, in this example, composed of square rods in a spatially periodic pattern. Because the microscopic model includes computation of all the flow features, computation time is considerable. A comparison shows the advantage of using the porous-continuum model, a large savings of computation time. This is particularly valuable in parametric studies. The effective properties in the macroscopic model include permeability values, Forchheimer coefficients, thermal dispersion coefficients, and heat transfer coefficients, constructed from results of periodic unit cell scale analyses, done separately. The results from the microscopic model are volume averaged over the porous medium representative elementary volume (REV) in order to generate averaged values for comparison to the results of the macroscopic model. Profiles of average velocity and temperature at various axial and longitudinal locations within the serpentine section of the example heat exchanger show agreement between the volume-average of the microscopic model results and the macroscopic model results. Further, comparisons are discussed in terms of local and global residuals from the models. It is found that local residuals of the calculations correlate well with the dimensionless product of the streamline curvature (the inverse of the curvature radius) and the scale of the unit cell. Global residuals, which are local residuals averaged over REVs, correlate with packing number (number of unit cells within the serpentine section). The packing number is used for estimating the global residual errors incurred when using the macroscopic model.     

Study of the mechanical behavior of paper-type GDL in PEMFC based on microstructure morphology

As the softest part in a proton exchange membrane fuel cell (PEMFC), the gas diffusion layer (GDL) could have a large deformation under assembly pressure imposed by bipolar plate, which would have an impact on the cell performance. So, there is an urgent need to clearly reveal the mechanical behavior of GDL under certain pressure. In this paper, the mechanical behavior of paper-type GDL of PEMFC is studied, considering the complex contact environment in the fibrous layered structure. The microstructure of GDL is reconstructed stochastically, then the stress-strain relationship of GDL is explored from the perspective of solid mechanics by using the finite element method. Based on microstructure morphology, it is found that contact pairs and pore space of microstructure are two key factors determining the nonlinearity of the compressive curve. The equivalent Young's modulus increases with the decrease of porosity and carbon fiber diameter but it is not very sensitive to the carbon paper thickness. The results indicate that with the increase in acting pressure, the average porosity of the carbon paper decreases, and the nonuniformity of porosity along the through-plane direction increases. Furthermore, a reasonable explanation for the increase of concentration loss and the decrease of ohmic loss is given from the microstructure findings of the present study.     

Nonlocal thermodynamic response of a rod

Thermally induced behavior of a rod under the influence of a moving heat source is studied in the present work. The thermal behavior is modeled using the non-Fourier heat conduction model and the elastodynamic behavior of the rod is modeled using the nonlocal continuum mechanics. Laplace transformation and Riemann-sum approximation methods are used in the mathematical formulations. Based on the temperature history, the thermally induced nonlocal deformation and nonlocal stress behavior of the rod are studied. The influences of nonlocal scale parameter and speed of the heat source are studied in detail. The results presented in this work are helpful in the design of nanoscale welding, grinding, metal cutting devices, etc., that make use of the thermodynamic behavior within the nanoscale rod.     

Use of the reference stress method in estimating the life of pipe bends under creep conditions

The effect that the change of ovality of pipe bends, due to creep under internal pressure, has on the stationary-state stress state has been investigated using a reference stress approach. Attention has been focused on practical pipe bend geometries and loading conditions in power plant applications.Prediction of failure times based on stationary-state stresses, for cases in which ovality changes are neglected and in which ovality is included, indicate that the change of ovality can be very significant in some practical situations. Failure times obtained using a creep continuum damage mechanics approach have also been compared with those obtained using the stationary-state predictions with ovality changes included. These results indicate that even sophisticated damage mechanics analyses are inadequate unless the effects of the changing ovality which occurs are taken into account.     

The indentation analysis triggering internal short circuit of lithium‐ion pouch battery based on shape function theory

A novel indentation theory with homogenized, isotropic, and continuum model based on different shape functions is investigated to predict the mechanical behavior triggering internal short circuit of lithium‐ion pouch battery (LIPB) under indentation loading. By taking energy conservation principle into account, the relationship among indentation force, indentation displacement, and indentation region of LIPB is proposed. The results conclude that the effects of deformation region and indentation displacement play an important role in mechanical behavior triggering internal short circuit. The theoretical results based on sine, cosine, and quadratic shape functions agree well with experimental results. Both of increasing compressed yield stress of jellyroll core and punch radius and decreasing flow stress of soft casing and thickness of soft casing can avoid triggering internal short circuit of LIPB. According to current research, the indentation loading is reduced to flat compression when punch radius approaches infinity, and the internal short circuit of LIPB under flat compression is very difficult to be triggered. Effectiveness and application scope of different shape functions are also discussed. The theoretical model provides guidance for improving mechanical behavior, decreasing internal short circuit, and optimizing structure of LIPB in industrial manufacture.