Metallic foams: Radiative properties/comparison between different models

The aim of this study is to determine the radiative properties, which are the extinction coefficient, the scattering albedo and the scattering phase function, of highly porous open-cell aluminium foam, using more-or-less simple predictive models, and to compare all these models. The radiative properties are predicted using geometric optics laws to model the interaction of radiation with the particles forming the foam. Moreover, the particles forming the foam are large compared with the considered wavelength and are supposed to be sufficiently distant from each other to scatter radiation independently. Thus, the radiative characteristics of the foam can be determined by adding the contributions of each particle. A particular attention is paid on microstructure analysis and modelling. We considered different kinds of cell shapes and struts cross-section, using microscopic and tomographic analysis. Furthermore, a new phase function modelling is presented. Finally, we compare the results of each method with the radiative properties obtained from experimental measurements of directional and hemispherical transmittances and hemispherical reflectance.     

Thermal radiative properties of phenolic foam insulation

Thermal insulation has long been an important subject in engineering. Foam insulations have become the most widely used insulations due to their low cost and ease of procesing. In recent years, phenolic foams find increasing applications because of their fire retardation. This paper presents experimental results of thermal radiative properties of phenolic foams, with or without activated carbon. Transmittance spectra were first taken using FTIR for samples of various densities. Extinction coefficient spectra were then obtained by applying Beer's law. Finally, by using the diffusion approximation, the Rosseland mean extinction coefficients and radiative thermal conductivities were obtained for various temperatures. Results show that the extinction coefficient increases with sample density. The addition of activated carbon increases the extinction coefficient slightly.     

单层元胞结构金属泡沫的透射特性

本文构建了开孔泡沫铝的简化几何模型,并利用有限积分法模拟了具有单层元胞结构的金属泡沫材料在线性极化平面波垂直入射情况下的透射率。基于菲涅耳-基尔霍夫衍射理论分析了孔隙率、孔径尺寸、材料厚度和骨架结构对金属泡沫材料辐射特性的影响。当尺度参数较大时,金属泡沫材料的固相支架结构满足良导体条件,宏观电磁屏蔽效应显著,金属泡沫材料呈现"非透明"性质。随着入射电磁波波长逐渐接近于孔径尺寸,散射效应越来越显著,金属泡沫材料的"半透明"性质开始显现,不同孔隙率的金属泡沫材料的透射率以相近的规律随波长变化。随着波长的进一步减小,衍射效应对于金属泡沫材料透射特性的影响逐渐占据主导地位,采用菲涅耳-基尔霍夫衍射理论可以较好地描述透射能流在孔隙结构内的分布。当衍射效应占据主导地位时,对于相同孔隙率金属泡沫材料,孔径尺寸对衍射光学行为影响不大,而材料厚度、孔隙率和骨架结构会显著影响金属泡沫材料的透射率。     

Sensitivity of trace gas abundances retrievals from infrared limb emission spectra to simplifying approximations in radiative transfer modelling

The accuracy of trace gas abundances retrieved from spectrally resolved infrared limb emission measurements relies, among other things, on the appropriate modelling of radiative transfer through the actual atmosphere. We quantify the mapping of several commonly applied simplifications in radiative transfer modelling on the trace gas abundances retrieval error at the example of the Michelson interferometer for passive atmospheric sounding (MIPAS)/environmental satellite (ENVISAT) space borne Fourier transform infra-red limb emission experiment. The Karlsruhe optimized and precise radiative transfer algorithm (KOPRA) which was used as a tool for this study will be introduced. KOPRA supports accurate modelling of the particular instrument requirements of MIPAS and the observation scenarios during the ENVISAT mission, in particular with respect to its viewing direction and its altitude coverage of the atmosphere. We show that disregarding of horizontal temperature inhomogeneities and non-local thermodynamic equilibrium effects, insufficient accuracy in modelling of field-of-view and apodisation effects, and disregarding individual profiles of isotopic species play the key roles in radiative transfer modelling and lead to systematic retrieval errors which can by far exceed the expected random retrieval error caused by measurement noise.     

In situ observations of compressive behaviour of aluminium foams by local tomography using high-resolution X-rays

We have investigated the compressive behaviour of closed-cell aluminium foams using a high-resolution X-ray CT. The microstructures of cell walls or Plateau borders in the foams were visualized in 3D using the local tomography technique which is a high-resolution CT method to reconstruct a region of interest within a large sample. The shapes and the 3D distribution of micropores, particles, and regions of solute segregation in the foams are evaluated, comparing the cell walls with the Plateau borders. Under compressive loads, the damage behaviour of such microstructures has been observed using an in situ test rig. It is found that the microcracks were mainly initiated from the cell walls and the micropores with large diameters were also damaged. The crack initiation sites are classified from the results. In addition, a method for non-destructive characterization of elastic and plastic deformation in the foams, which is called a 3D microstructure gauge (MG) method, is presented. Thousands of micropores as markers on each load were automatically matched by the information of those volumes and surface areas. The local strain mapping by the MG indicates that the edges of the micropores with large diameters have large strain under compression and this is consistent with the crack analyses.     

MORPHOLOGY OF SEMICRYSTALLINE FOAMS BASED ON POLYETHYLENE

The matrix polymer morphology of a collection of low-density polyethylene (LDPE) foams manufactured from a high-pressure nitrogen solution process is analyzed at different length scales. This morphology is compared with that of the extruded LDPE, which was used to produce the foams. The main sources for the differences between the morphologies of both kinds of material seem to be related to the geometrical arrangement of the polymer in thin cell walls and to the complicated mechanical and thermal history of the polymer that comprises the cell walls of the foams.     

Viscosity effects in foam drainage: Newtonian and non-newtonian foaming fluids

We have studied the drainage of foams made from Newtonian and non-Newtonian solutions of different viscosities. Forced-drainage experiments first show that the behavior of Newtonian solutions and of shear-thinning ones (foaming solutions containing either Carbopol or Xanthan) are identical, provided one considers the actual viscosity corresponding to the shear rate found inside the foam. Second, for these fluids, a drainage regime transition occurs as the bulk viscosity is increased, illustrating a coupling between surface and bulk flow in the channels between bubbles. The properties of this transition appear different from the ones observed in previous works in which the interfacial viscoelasticity was varied. Finally, we show that foams made of solutions containing long flexible PolyEthylene Oxide (PEO) molecules counter-intuitively drain faster than foams made with Newtonian solutions of the same viscosity. Complementary experiments made with fluids having all the same viscosity but different responses to elongational stresses (PEO-based Boger fluids) suggest an important role of the elastic properties of the PEO solutions on the faster drainage.     

Functionalized alginate/chitosan biocomposites consisted of cylindrical struts and biologically designed for chitosan release

A novel alginate/chitosan composite scaffold was developed. The composite scaffolds were fabricated at low temperature using a three-axis robot system connected to a micro-dispenser and a core/shell nozzle. The structure of the composite scaffolds included hollow struts; deposited chitosan on the inner walls (core region) of the struts reacted electrostatically with the alginate layer (shell region). The fabricated, highly porous composite scaffolds exhibited excellent mechanical properties and controllable chitosan release, which was closely dependent on the weight fraction of the alginate in the shell region. The tensile strength in the dry state was ∼1.8-fold greater than that of pure alginate scaffold due to the ionic interaction between alginate and chitosan. To determine the feasibility of using the developed scaffold in tissue regeneration applications, in vitro cellular responses were evaluated using osteoblast-like-cells (MG63). The cell proliferation on the composite scaffold was ∼3.4-fold greater than that on the pure alginate scaffold. Alkaline phosphate activity and calcium deposition of the composite scaffold after 14 and 21 days of cell culture were significantly enhanced (1.6- and 1.8-fold greater, respectively) compared with those of the pure alginate scaffold. These results suggested that the alginate/chitosan composite scaffolds with a controlled chitosan release have great potential for use in regenerating various tissues.     

Properties of acrylonitrile butadiene rubber (NBR)/poly(lactic acid) (PLA) blends and their foams

Abstract

Thermoplastic elastomers and their foams were prepared by blending elastomeric acrylonitrile butadiene rubber (NBR) and rigid poly(lactic acid) (PLA) with various PLA compositions ranging between 0 and 40%. The thermal and mechanical properties and the morphologies of the blends with various PLA contents were investigated through universal testing machine, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscope analysis. The rheological properties during gel formation were in situ monitored through the evolution of torque with curing time. Furthermore, the microcellular structures and physical properties of the NBR/PLA foams prepared using organic blowing agents were studied. The NBR/PLA blends showed a two-phase morphology made of a continuous NBR matrix and micron or submicron nodules and the tensile strength and modulus; also, hardness of the NBR/PLA blends increased with the increase of the added PLA content. While the foamed samples exhibited a similar cell structure and foaming ratio to that of the pure NBR, the cell formation was considerably reduced as the added PLA content exceeded 30%. We conclude that the mechanical properties of NBR thermoplastic elastomer as well as its foams can be controlled by a judicious introduction of rigid and biodegradable PLA.     

Numerical modelling of foam Couette flows

A numerical computation based on a tensorial visco-elasto-plastic model based on continuous mechanics is compared to experimental measurements on liquid foams for a bidimensional Couette flow between two glass plates, both in stationary and transient cases. The main features of the model are elasticity up to a plastic yield stress, and viscoelasticity above it. The effect of the friction of the plates is taken into account. The numerical modelling is based on a small set of standard material parameters that are fully characterised. Shear localisation as well as acute transient observations are reproduced and agree with experimental measurements. The plasticity appears to be the fundamental mechanism of the localisation of the flow. Finally, the present approach could be extended from liquid foams to similar materials such as emulsions, colloids or wet granular materials, that exhibit localisation.     

Magnetic and transport properties of YBa2Cu3Oy superconductor foams

The recently reported superconducting YBa₂Cu₃O_y (Y123) foams are highly interesting and promising for variety of applications. In this report we present first magneto-transport measurements of the superconducting properties of these foams. The investigations reveal the superconducting properties being similar to those of bulk melt processed materials. The 123 foams reveal a T_c of 92 K and have a magnetization J_c of 40,000 A/cm² at 77 K and 0 T. The measurements of magnetic hysteresis versus field show a high anisotropy of the critical current density up to J_c^ab/J_c^c7.     

Numerical investigation of the effect of soot aggregation on the radiative properties in the infrared region and radiative heat transfer

The effect of aggregation on soot radiative properties in the infrared region of the spectrum is numerically investigated using Rayleigh-Debye-Gans theory for fractal aggregates (RDG-FA). In order to use the RDG-FA theory for a wide range of aggregate sizes and wavelengths, the predicted phase functions, scattering and absorption coefficients are compared with a more accurate theory, the integral equation formulation for scattering—IEFS. The importance of scattering when compared with absorption is investigated, as well as the effect of aggregation on the phase function shape and on the scattering cross section. It is concluded that in the case of small aggregates formed with small primary particles the scattering coefficient is negligible compared with the absorption coefficient, and scattering and aggregation of primary particles can be ignored. Thus, the Rayleigh approximation can be used leading to isotropic scattering. In the case of large aggregates constituted by large primary particles, aggregation becomes important and the scattering cross section is of the same order of magnitude of the absorption cross section. Moreover, the phase function becomes highly peaked in the forward direction. Therefore, the Rayleigh and the equivalent volume Mie sphere approximations are not valid, and the RDG-FA method emerges as a good compromise between accuracy and simplicity of application. However, radiative transfer calculations between two infinite, parallel, black walls show that scattering may always be neglected in the calculation of total radiative heat source and heat fluxes to the walls. The minor influence of scattering on the accuracy of the predictions is explained by the shift between the spectral region where scattering is important and the region where the spectral radiative heat source is large.     

The method of lines solution of discrete ordinates method for radiative heat transfer in cylindrical enclosures

A radiation code based on method of lines solution of discrete ordinates method for radiative heat transfer in axisymmetric cylindrical enclosures containing absorbing-emitting medium was developed and tested for predictive accuracy by applying it to (i) test problems with black and grey walls (ii) a gas turbine combustor simulator enclosing a non-homogeneous absorbing-emitting medium and benchmarking its steady-state predictions against exact solutions and measurements. Comparisons show that it provides accurate solutions for radiative heat fluxes and can be used with confidence in conjunction with CFD codes based on the same approach.     

A preliminary study of pure metal surfaces using Auger Electron Spectroscopy (AES), X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectroscopy (SIMS)

A system is described which incorporates Auger spectroscopy (AES), x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) in one UHV chamber. The system has been used to determine sensitivity factors for XPS for a number of pure metals. These factors are then compared with ion yields from surfaces using SIMS. By performing similar XPS measurements in another, more conventional, system, comparison has been made of the electron transmissions of the double-pass cylindrical analyser in the XPS (retarding) mode and of the hemispherical analyser.     

A three-dimensional radiative transfer model to interpret ranging scatterometer measurements from a pine forest

This paper addresses the propagation of microwaves in forested media. It interprets the vertical distribution of backscatter acquired by a ranging scatterometer in a pine forest at X band (9.5 GHz) and vertical incidence by means of modelling. A plane parallel radiative transfer model for the forest canopy exhibits a large discrepancy between the measured and estimated distribution. This is attributed to failure to represent the horizontal heterogeneity in the forest. Taking into account the height variability of the trees significantly improves the estimated distribution at the top of the canopy but the backscatter from the lower layers and the ground is still underestimated. To deal with this, a fully three-dimensional (3D) radiative transfer model is developed. By modelling precisely the propagation of the wave through the medium, the 3D approach allows accurate estimation of the ground backscatter and slightly improves the estimated backscatter from the lower layers. It is demonstrated that 3D modelling is required to estimate accurately the vertical distribution of backscatter, total backscatter and the height of the centre of backscatter in the experimental data. These results have implications for the interpretation of more general measurements scenarios, especially as regards forest height measurement by radar interferometry.     

A novel sample preparation method to avoid influence of embedding medium during nano-indentation

The effect of the embedding medium on the nano-indentation measurements of lignocellulosic materials was investigated experimentally using nano-indentation. Both the reduced elastic modulus and the hardness of non-embedded cell walls were found to be lower than those of the embedded samples, proving that the embedding medium used for specimen preparation on cellulosic material during nano-indentation can modify cell-wall properties. This leads to structural and chemical changes in the cell-wall constituents, changes that may significantly alter the material properties. Further investigation was carried out to detect the influence of different vacuum times on the cell-wall mechanical properties during the embedding procedure. Interpretation of the statistical analysis revealed no linear relationships between vacuum time and the mechanical properties of cell walls. The quantitative measurements confirm that low-viscosity resin has a rapid penetration rate early in the curing process. Finally, a novel sample preparation method aimed at preventing resin diffusion into lignocellulosic cell walls was developed using a plastic film to wrap the sample before embedding. This method proved to be accessible and straightforward for many kinds of lignocellulosic material, but is especially suitable for small, soft samples.     

Study of the 49Ti energy levels using the (γ, γ′) and the (γ, n) reactions

An unbound level in ⁴⁹Ti at 8884 keV was photoexcited using a γ-beam obtained from the Cr(n, γ) reaction. The γ-decay and n-decay properties of this level were studied using angular distributions and polarization measurements. Thus spin and parity assignments of some levels in ⁴⁹Ti were made. The neutron width and the total radiative widths of the 8884 keV level were determined to be: Γ_n = 0.25 ± 0.05 eV, Γ_γ = 2.55 ± 0.80 eV.     

Rheology of the Cu-H2O nanofluid in porous channel with heat transfer: Multiple solutions

Dynamics of nanofluid comprising a base fluid (water) with copper (Cu) nanoparticles have been considered in channel with porous walls under magnetic field influence. The channel walls are considered to be permeable in order to analyze the wall mass transfer phenomenon. Relevant mathematical modelling has been performed and the derived PDEs are converted into coupled nonlinear ODEs by using suitable transformations. Computations have been made numerically by employing the shooting technique. It is noted that multiple solutions occur for the variation of suction Reynolds number, solid volume fraction and magnetic parameters which are interpreted in detail.