Emeritus Professor Graeme Murch

This comprehensive resource for fellows/trainees and candidates for recertification in gastroenterology summarizes the field in a modern, fresh format. Prominent experts from around the globe write on their areas of expertise, and each chapter follows a uniform structure. The focus is on key knowledge, with the most important clinical facts highlighted in boxes. Color illustrations reinforce the text. © 2010 Blackwell Publishing Ltd.

In this contribution, a guide is given to the history of various applications of the Monte Carlo method to solid-state diffusion using atomistic (Kinetic Monte Carlo) and phenomenological (Lattice Monte Carlo). This is followed by a detailed exposition of how a basic Kinetic Monte Carlo calculation is performed to determine tracer and collective diffusion quantities as well as how the atomic jump frequency is included. This is followed by an exposition of how a basic phenomenological diffusion problem can be addressed in which the effective diffusion coefficients and concentration profiles are determined by the Lattice Monte Carlo method.

In this paper, the diffusion isotope effect and the Manning factor are investigated by means of Molecular Dynamics simulations in liquid Cu-Ag alloys. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms that are moving cooperatively in a basic diffusion event as ¿seen¿ by a given (tracer) atom. On average, in the considered alloys and considered temperatures, this is limited to between 5 and 15. This is consistent with results of Molecular Dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, a majority of the neighbouring atoms are involved in a basic diffusion event. Results for the Manning factor (MD simulation) for Cu-Ag liquid alloys are seemingly in agreement with the direct exchange mechanism where only two atoms are involved in the elementary diffusion event. This is not in apparent agreement with the isotope effect results. It was shown, however, that any ring mechanism, or, more complex, cage mechanism are, in fact, a combination of several simultaneously happening direct exchanges. Any other possible mechanisms for diffusion in liquids is most likely a combination of direct exchanges as well. It can be seen then that the collective nature of all considered mechanisms is very similar and follows the direct exchange signature properties.

In the present study, 10 mol% concentrations of calcium and gadolinium oxides were chosen as dopants to stabilise zirconium oxide (ZrO2). The main goals of this research were to study the lattice thermal conductivity, oxygen diffusion coefficient, oxygen ionic conductivity and thermotransport of these materials. Therefore, molecular dynamics (MD) simulations based on the Green-Kubo formalism were applied over a wide temperature range (from 800 K to 1800 K) to calculate the thermal and ionic conductivities and thermotransport. These calculations employed a reliable Buckingham type interatomic potential. The integration of an autocorrelation function was applied to calculate the ionic conductivity and, (approximately) the oxygen tracer diffusion coefficient. It was found that the results were in reasonable agreement with available experimental data. The Onsager cross-coefficients (LOq = LqO) were estimated by using the Green-Kubo formalism as well. The results were also compared with the findings of our previous study on yttria-stabilised zirconia (YSZ).

The present study addresses the impact loading of functionally graded metal syntactic foams (FG-MSF). For comparison, samples of the same material were also compression loaded at quasi-static velocities. Samples of A356 aluminium FG-MSF were produced using counter-gravity infiltration casting with combination of equal-sized layers of expanded perlite (EP) and activated carbon (AC) particles. A modified Split Hopkinson Pressure Bar test set-up was used to impact the FG-MSFs from their EP or AC layers at 55 m/s or 175 m/s impact velocities. A high-speed camera captured the deformation of the samples during testing. It was shown that increasing the loading velocity enhanced both the compressive proof strength and energy absorption of the impacted FG-MSF from both layers, confirming a dynamic strengthening effect of the foam. The samples impacted from both layers at 55 and 175 m/s showed a transition and a shock mode of deformation, respectively. The impacted samples at 55 m/s experienced lower final average strain values compared to 175 m/s.

In this overview, we summarize the phenomenon of thermotransport (the close coupling of mass transport and heat transport) in two fast ion conductors: yttria-doped zirconia and gadolinia-doped ceria. We focus on two recent molecular dynamics calculations using the Green-Kubo formalism. We show that the Onsager thermotransport cross coefficient (mass-heat coupling) is negative, meaning that oxygen ions would drift, in principle, to the hot side in a temperature gradient. Simulation results presented in this overview show reasonable agreement with available experimental data for thermal conductivity. Results of this study suggest that the coupling between mass and heat transport in oxygen ion electrolytes could have significant effect for practical applications.

This research study introduces a novel functionally graded metal syntactic foam. Counter-gravity infiltration casting is employed to manufacture MSF layers with dissimilar metallic matrices. Each layer combines expanded perlite particles with a different matrix alloy, i.e., ZA27 or pure aluminum. Dissimilar MSF layers are combined either during casting or using different joining techniques following manufacturing. The dissimilar matrices introduce a controlled gradient of the physical and mechanical properties of the resulting functionally graded foam material. Quasi-static compression testing shows that the initial deformation is controlled by the weaker aluminum syntactic foam layer. Following partial densification of this layer, deformation transitions toward the layer containing the higher strength ZA27 matrix alloy.

This study addresses the dynamic compression of functionally-graded (FG) metal syntactic foams (MSF). Cylindrical MSFs are manufactured by combining a ZA27 alloy with equal sized layers of expanded perlite (EP) and activated carbon (AC) particles. For comparison, uniform MSFs containing either particle type are manufactured with different aspect ratios. Samples are tested at the loading velocities 0.2 mm·s-1 (quasi-static) or 284 mm·s-1 (dynamic) to probe for changes of the deformation mechanism and effective mechanical properties. It is shown that uniform MSFs with a lower aspect ratio exhibit an increased overall strength. The underlying mechanism is a change of the shear failure mode, which has been closely studied by combining infrared (IR) imaging with dynamic compression. EP-MSFs exhibit a strength reduction at the higher loading velocity whereas AC-MSFs show no significant change. The dynamic deformation of FG-MSFs originates in the weaker EP layer and thus closely resembles the deformation behavior of the EP-MSFs. At higher strains, the deformation transitions to the AC layer and the stress-strain response changes accordingly.

This study focuses on a number of transport phenomena in yttria-stabilized zirconia (YSZ). A molecular dynamics simulation based on the Green-Kubo formalism is applied to calculate the lattice thermal conductivity, oxygen diffusion coefficient, ionic conductivity and thermotransport at different concentrations (i.e., 4, 8, 10, 12, 16 and 20 mol% of Y2O3) over a temperature range from 700 K to 1500 K. The results show that the YSZ has a low thermal conductivity in comparison with pure zirconia. The oxygen tracer diffusion coefficient, as calculated from the mean square displacements, and also the ionic conductivity show an activation energy of 0.85eV. The Onsager cross coefficient associated with thermotransport is negative, meaning that the drift of anions in a temperature gradient would be from a cold region to a hot region. All of the simulation results presented show reasonable agreement with available experimental data.

One of the most technologically beneficial engineering ceramics is yttria stabilized zirconia (YSZ). As a result, research interest about YSZ has been intensive for many years. In this study, the lattice thermal conductivity and oxygen diffusion coefficient of YSZ are investigated at different temperatures (from 700 K to 1300 K) and zero pressure with the Green-Kubo formalism. We find that the lattice thermal conductivity decreases as the temperature increases, particularly at low temperatures and it shows a slightly temperature independence at high temperatures. The results demonstrate that the YSZ has quite a low thermal conductivity compared with pure zirconia. We also show that the oxygen tracer diffusion coefficient, as calculated from the mean square displacements, has an activation energy of 0.85eV.

The problem of estimation of the tracer diffusion coefficients is solved by utlizing a novel radiotracer-diffusion couple technique in the absence of a suitable radioisotope of one of the components and reliable thermodynamic parameters. This is demonstrated by generating reliable and reproducible mobility data in the alloys of the Fe¿Ga system with a strong composition dependence of the diffusion coefficients. Tracer- (59Fe) and inter-diffusion are simultaneously measured in three couples Fe/Fe-16 Ga, Fe/Fe-24 Ga and Fe-16 Ga/Fe-24 Ga at 1143 K in at.%). The results obtained for the couples with different end-members are in an excellent agreement with each other for the overlapping composition intervals. The influence of the molar volume on the measured tracer- (59Fe) and inter-diffusion coefficients is evaluated. Using thermodynamic calculations, the Ga tracer diffusion coefficient and the vacancy wind factor are determined via the Darken-Manning relation for the composition range of 0¿24 at.% Ga. The results in this study confirm the reliability of the radiotracer-diffusion couple technique for producing highly accurate diffusion data. As a result, this helped for optimizing the mobility description of the bcc phase of the Fe¿Ga system. The Ga tracer diffusion coefficients are further estimated via experimental determination of the ratio of the Fe and Ga tracer diffusivities at the Kirkendall marker planes and utilizing the Fe tracer diffusion coefficients measured directly by the radiotracer method.

We simulated thermodiffusion and ionic conductivity in doped ceria by molecular dynamics. Four approaches to estimate the ionic conductivity were applied using the auto-correlation function and collective displacement of oxygen ions, the tracer diffusion coefficient from the mean squared displacement, and the displacement in an electric field. The Green-Kubo method was applied to determine the mass-heat coupling cross-coefficients LOq and LqO of the Onsager transport matrix. This is the first calculation of these coefficients in an ionic compound. The simulated parameters are negative throughout the applied temperature range suggesting net transport of anions from a cold to a hot region in a temperature gradient. The results emphasize that the coupling between mass and heat transport in an oxygen ion electrolyte could have an impact for practical applications.

In this paper, the diffusion isotope effect and diffusion mechanism are investigated by means of molecular dynamics simulations in two liquid alloys, Ni-Ag and Ni-Cu. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms ND which are moving cooperatively in a basic diffusion event as experienced by a given atomic species. It is shown that the composition dependence of NNDD is typically very small. However, the temperature dependence of this parameter is much more pronounced. In addition, it is shown that, on average, in these alloys and temperatures considered, NNDD is limited to the range: 5 < ND < 17. This is consistent with results of molecular dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, depending on temperature, the neighbouring atoms are all involved in the basic diffusion event.

Novel foam filled tubes were manufactured via a highly reproducible and cost effective in situ process. Stainless steel tubes were filled with ultralight porous expanded perlite particles and molten aluminium infiltrated the gaps between these particles. During casting, a ternary intermetallic phase was formed between the liquid aluminium and steel tube as a result of a chemical reaction. Quasi-static uni-axial compression testing was applied on the foam, empty tube, and foam-filled tube samples. Additional samples were subjected to quasi-static and dynamic three-point bending tests. The results of the quasi-static testing indicate that the foam filling improves the energy absorption capacity of tubes by 2.23 and 3.9 times for compressive and bending loading conditions, respectively. The dynamic bending tests indicate that both empty tubes and foam filled tubes exhibit a positive strain rate sensitivity. It is further shown that a larger tube wall thickness increases the energy absorption of both empty and foam-filled tubes. Foam-filling further increases the energy absorption capacity and, more importantly, the energy absorption efficiency. The impact of foam filling is more substantial in the case of tubes with lower thickness.

In this paper, a new, lighter, version of the highly accurate Moleko, Allnatt and Allnatt formalism for describing both tracer (self) and collective diffusion kinetics in multicomponent random alloys is presented. Verification of the resulting expressions is performed by means of kinetic Monte Carlo simulation. The accuracy of the new formalism is much higher than that of the combined Manning and Holdsworth and Elliott formalism discussed recently. Using this formalism the possible range of the tracer diffusion ratio of the highest to the lowest atomic component is examined for equiatomic (or near equiatomic) binary, ternary, quaternary and quinary alloys. It is shown that in the random alloy model, the correlation effect is the highest with a reduction of the fastest tracer diffusion by 40¿55%, when moving from two pure metals to their equiatomic binary alloy. By adding the third component (with an intermediate mobility) this effect can be further increased with a possible total reduction of the fastest tracer diffusion by up to 70% (depending on the combinations of mobilities), while adding the fourth component brings this reduction up to 80% and with a possible maximum of up to 85% reduction for the 5-component alloy (again depending on the combinations of mobilities). But the slowest diffusing components are not affected by this. This suggests that kinetics arguments alone are not enough for explaining the sluggish diffusion observed of all atomic components in (equiatomic) high-entropy alloys.

In this paper, extensive Monte Carlo simulation results are reported on tracer and collective diffusion correlation effects in the random ternary alloy, as an example of a multicomponent alloy system. The problem of analytically describing both collective and tracer diffusion kinetics is also addressed for the random multicomponent alloy by application of a combination of the Manning theory and Holdsworth and Elliott theory. It is found that the overall results from the combined theory agree reasonably well with Monte Carlo results. This combined approach is much more accurate than Mannings approach itself and much more manageable than the almost exact, but unfortunately difficult to use, self-consistent theory of Moleko, Allnatt and Allnatt. Some relations between the Onsager phenomenological coefficients and tracer diffusion coefficients are derived and are tested with our Monte Carlo data. Good agreement is found.

Equilibrium molecular dynamics simulation in conjunction with the Green-Kubo formalism is employed to study the transport properties of a model Ni50Al50 melt with the embedded-atom method potential developed in [G.P. Purja Pun, Y. Mishin, Phil. Mag., 2009, 89, 3245]. The principal objective of the work is to quantitatively characterise and analyse thermotransport in the system, i.e. diffusion driven by a temperature gradient. In addition, direct phenomenological coefficients for mass and thermal transport are also evaluated and analysed in the process. Furthermore, the results obtained are compared with previously published data for a different model of Ni50Al50 melt with an alternative embedded-atom method potential for the alloy as well as with experiment where possible. It is found that both potentials are able to consistently predict both direct transport coefficients over a wide temperature range. However, these two potentials are found to be inconsistent in characterising the cross-coupled heat and mass transport, predicting even different directions (sign) of the heat of thermotransport. The origin of this difference is discussed in the paper in detail.

Abstract A novel metallic syntactic foam is produced using a counter-gravity infiltration casting method. To this end, expanded perlite particles are combined with an aluminium alloy matrix. This enables close control of geometry at a relatively low production cost. The mechanical properties of the material are studied using finite element analysis. Numerical calculation models are generated directly from micro-computed tomography in order to capture their complex internal geometry. For verification purposes, numerical results are compared with experimental measurements of similar samples where available. But in contrast to experimental testing the numerical analysis is non-destructive and hence allows the repeated testing of samples in multiple loading directions. Thus, material anisotropy can be investigated for the first time. To this end, the quasi-elastic gradient, the 1% offset yield stress and the plateau stresses are obtained from virtual compression tests in three perpendicular directions (one coincides with the casting direction). Results indicate a weak anisotropy of the mechanical properties.

In the present paper (1) the Hall method (HM) (specifically designed for determining the interdiffusion coefficient at the low and high composition limits of the corresponding interdiffusion composition profile) is further developed in order to be applied to the whole composition profile resulting in the Extended Hall method (EHM); (2) A comparative study of the HM, EHM, Boltzmann-Matano (BM) and Sauer and Freise (SF) methods is performed using composition profiles generated by computer simulation. The results clearly indicate that the HM/EHM technique is only applicable when the interdiffusion coefficient is constant (i.e. independent of composition) or almost constant at the low composition regions. In all other cases, the BM and SF methods give the best agreement with the input interdiffusion function even at the ends of the composition profiles.

This paper addresses the computation of the effective diffusivity in new bioactive glass (BG) based tissue engineering scaffolds. High diffusivities facilitate the supply of oxygen and nutrients to grown tissue as well as the rapid disposal of toxic waste products. The present study addresses required novel types of bone tissue engineering BG scaffolds that are derived from natural marine sponges. Using the foam replication method, the scaffold geometry is defined by the porous structure of Spongia Agaricina and Spongia Lamella. These sponges present the advantage of attaining scaffolds with higher mechanical properties (2¿4¿MPa) due to a decrease in porosity (68¿76¿%). The effective diffusivities of these structures are compared with that of conventional scaffolds based on polyurethane (PU) foam templates, characterised by high porosity (>90¿%) and lower mechanical properties (>0.05¿MPa). Both the spatial and directional variations of diffusivity are investigated. Furthermore, the effect of scaffold decomposition due to immersion in simulated body fluid (SBF) on the diffusivity is addressed. Scaffolds based on natural marine sponges are characterised by lower oxygen diffusivity due to their lower porosity compared with the PU replica foams, which should enable the best oxygen supply to newly formed bone according the numerical results. The oxygen diffusivity of these new BG scaffolds increases over time as a consequence of the degradation in SBF.

Free-surface-induced L10 chemical long-range ordering phenomena in a nanolayer, a nanowire and a cubic nanoparticle of FePt were studied by means of Monte Carlo simulations. The system was modeled with nearest-neighbor and next-nearest-neighbor interatomic pair interactions deduced from ab initio calculations. The generated samples, the dimensionality of which was determined by appropriate periodic boundary conditions imposed upon the generated supercells, were initially either perfectly ordered in the c-variant L10 superstructure ((001)-oriented monatomic planes), or completely disordered in the fcc crystalline structure. Vacancy-mediated creation of equilibrium atomic configurations was modelled by relaxing the systems at temperatures below the 'order-disorder' transition point using the Glauber algorithm implemented with the vacancy mechanism of atomic migration. The (100)-type-surface-induced heterogeneous nucleation of L10-order domains was observed and quantified by means of an original parameterization enabling selective determination of volume fractions of particular L10-variants. Due to the specific competition between the three kinds of (100)-type free surfaces, the initial c-L10 variant long-range order appeared to be the most stable in the cubic nanoparticle. The initially disordered samples were transformed by the creation of a specific L10 domain structure with a mosaic of particular L10-variant domains at the surfaces and almost homogeneous long-range order in the inner volume. The analysis of correlation effects revealed that chemical ordering was initiated at the free surfaces.

A novel filler material is introduced to produce high strength metal-matrix syntactic foam. Packed beds of pumice particles, a low-cost natural porous volcanic glass, with the size range of 2.8-4mm were infiltrated with molten aluminium alloy. The resulting syntactic foams were subjected to microstructural observations and chemical analysis. Furthermore, quasi-static compression testing was applied on heat treated samples. The material strength and the deformation mechanism under compressive loading of the pumice particles and foams were investigated. The results indicate that there is a limited chemical reaction between the particles and matrix. The pumice particles considerably enhance the strength of the foam and result in an average plateau stress of 68.25MPa and specific energy absorption of 24.8MJ/m3. Pumice particles show higher strength in the direction of tubular pores. The mechanical anisotropy of pumice particles is likely to cause a slight variation in the directional properties of the foams.

Abstract This paper addresses the thermal properties of syntactic metal foam made by embedding expanded perlite particles in A356 aluminium matrix. Lattice Monte Carlo (LMC) analyses are conducted to determine the thermal characterisation of the foam. For increased accuracy, the complex geometry of the metallic foam is captured by micro-computed tomography imaging. Using the resulting detailed geometric models, the effective thermal conductivity tensor is computed with possible thermal anisotropy taken into consideration. The numerical results are verified by comparison with experimental measurements. To this end, an improved steady-state method is used to correct for thermal contact resistance. Furthermore, the effective heat capacity, average density and thermal diffusivity of perlite - metal syntactic foam are determined.

A recently developed novel approach of simultaneous analysis of isotope and interdiffusion profiles in binary alloy systems is significantly advanced in order to apply to the case of multicomponent alloy systems. The resulting relations for the tracer or self-diffusion coefficients allow for the avoiding of the explicit solution of the interdiffusion equations. This remarkable result means that in the experimental implementation of this new technique there is no need for multiple diffusion couples. Only two profiles of the same component are necessary for the complete analysis. These can be two different isotopes or just two spatially different parts of the same atomic species. Descriptions of three possible experimental implementations of the novel technique combined with the Sauer-Freise method are discussed. Therefore, the new development is ready to be applied experimentally and can provide valuable insight into otherwise very difficult diffusion investigations into multicomponent alloys including high-entropy alloys.

The recently introduced analytical model for the heat current autocorrelation function of a crystal with a monatomic lattice [Evteev et al., Phil. Mag. 94 (2014) p. 731 and 94 (2014) p. 3992] is employed in conjunction with the Green¿Kubo formalism to investigate in detail the results of an equilibrium molecular dynamics calculations of the temperature dependence of the lattice thermal conductivity and phonon dynamics in f.c.c. Ni. Only the contribution to the lattice thermal conductivity determined by the phonon¿phonon scattering processes is considered, while the contribution due to phonon¿electron scattering processes is intentionally ignored. Nonetheless, during comparison of our data with experiment an estimation of the second contribution is made. Furthermore, by comparing the results obtained for f.c.c. Ni model to those for other models of elemental crystals with the f.c.c. lattice, we give an estimation of the scaling relations of the lattice thermal conductivity with other lattice properties such as the coefficient of thermal expansion and the bulk modulus. Moreover, within the framework of linear response theory and the fluctuation-dissipation theorem, we extend our analysis in this paper into the frequency domain to predict the power spectra of equilibrium fluctuations associated with the phonon-mediated heat dissipation in a monatomic lattice. The practical importance of the analytical treatment lies in the fact that it has the potential to be used in the future to efficiently decode the generic information on the lattice thermal conductivity and phonon dynamics from a power spectrum of the acoustic excitations in a monatomic crystal measured by a spectroscopic technique in the frequency range of about 1¿20¿THz.

Packed beds of expanded perlite (EP) particles with three different size ranges (1-1.4, 2-2.8, and 4-5.6. mm) have been infiltrated with molten Al to produce EP/A356 Al syntactic foam. A T6 heat treatment was applied to the foams. The effects of EP particle size on microstructural, geometrical, and mechanical properties of the foams were investigated. The EP particle size determines the number of cells across the sample diameter (7-25). It also influences the microstructural characteristics of the cell-wall alloy and the homogeneity of the cell-wall geometry. Enhanced microstructural characteristics and a greater geometrical homogeneity of the cell-wall in the case of smaller EP particles result in superior mechanical properties. The compressive deformation becomes more uniform by decreasing the EP particle size resulting in smoother and steeper stress-strain curves. As a result, these foams exhibit higher plateau stresses and improved energy absorption. The number of cells across the sample diameter does not have a significant effect on the mechanical properties of the samples considered.

The vibrational contribution to the thermal transport properties of liquid Cu is investigated in detail in the temperature range 1300-1800 K. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials for Cu. It is found that the temporal decay of the heat current autocorrelation function of the liquid Cu model can be described by a single exponential function, which is characterized in the studied temperature range by a constant value of the heat flux relaxation time of about 0.059 ps. The vibrational thermal conductivity of the liquid Cu model slightly decreases with temperature from about 1.1 W/(mK) at 1300 K to about 1 W/(mK) at 1800 K. Near the melting temperature it is about 30% lower than the vibrational thermal conductivity of the f.c.c Cu model. The calculated thermal diffusivity of the liquid Cu model is demonstrated to retain a constant value of about 2.7 × 10-7 m2/s in the studied temperature range, which is about two orders of magnitude higher than the atomic diffusivity in the model at the melting temperature. The vibrational contribution to the total thermal conductivity of liquid Cu is found to slightly decrease with temperature, being estimated as about 0.7-0.5% in the temperature range of 1400-1800 K. Furthermore, the applicability of some simple theoretical treatments of vibrational thermal transport in liquid Cu is discussed.

This paper addresses the mechanical behavior of robocast PCL-Bioglass^® scaffolds. These structures can be used as 3rd generation implants in tissue engineering to support the regrowth of damaged tissue, in particular bone. After successful tissue regeneration the scaffolds slowly dissolve leaving no foreign material permanently inside the body. However, to avoid mechanical separation from surrounding tissue they must exhibit similar mechanical properties. The present study introduces a detailed numerical study focusing on the determination of effective mechanical material properties, their anisotropy, and mechanical degradation due to scaffold resorption. In order to accurately capture the complex scaffold geometry, micro-computed tomography scans of actual samples are conducted. The resulting three-dimensional data are directly converted into finite element calculation models. Numerical compressive tests of these unmodified models are repeated for three perpendicular directions to investigate mechanical anisotropy, after which the effect of scaffold degradation due to exposure to body fluid is simulated. To this end, two different resorption models, namely surface erosion and bulk degradation, are applied to the micro-computed tomography data. The modified geometry data are then converted into calculation models and numerical compression tests then allow the prediction of the mechanical properties of partially resorbed scaffolds.

Abstract This paper addresses the mechanical characterization of polycaprolactone (PCL)-bioglass (FastOs®BG) composites and scaffolds intended for use in tissue engineering. Tissue engineering scaffolds support the self-healing mechanism of the human body and promote the regrowth of damaged tissue. These implants can dissolve after successful tissue regeneration minimising the immune reaction and the need for revision surgery. However, their mechanical properties should match surrounding tissue in order to avoid strain concentration and possible separation at the interface. Therefore, an extensive experimental testing programme of this advanced material using uni-axial compressive testing was conducted. Tests were performed at low strain rates corresponding to quasi-static loading conditions. The initial elastic gradient, plateau stress and densification strain were obtained. Tested specimens varied according to their average density and material composition. In total, four groups of solid and robocast porous PCL samples containing 0, 20, 30, and 35% bioglass, respectively were tested. The addition of bioglass was found to slightly decrease the initial elastic gradient and the plateau stress of the biomaterial scaffolds.

The thermal resistance of a crystal lattice with a monatomic unit cell due to three-phonon scattering processes is investigated in detail theoretically. A general expression for the lattice thermal conductivity is derived from a combined analysis based on: (i) the Boltzmann equation and (ii) data on the heat current autocorrelation function obtained via molecular dynamics simulations in conjunction with the Green-Kubo formalism. It is argued that the phonon gas in a monatomic lattice conducts heat as if it consisted of two distinct parts (two 'thermal fluids'), so that the lattice thermal conductivity can be decomposed into contributions from these two parts. The origin of the behaviour of the phonon gas, which is explored in the present work, is due to an intrinsic interplay between Umklapp and normal three-phonon scattering processes. New insight into the nature of the lattice thermal conductivity is demonstrated and the results of the present work are in agreement with previous studies in this area.

Tracer diffusivities provide the most fundamental information on diffusion in materials, and are the foundation of robust diffusion databases that enable the use of the Onsager phenomenological formalism with no major assumptions. Compared to traditional radiotracer techniques that utilize radioactive isotopes, the secondary ion mass spectrometry (SIMS)-based thin-film technique for tracer diffusion is based on the use of enriched stable isotopes that can be accurately profiled using SIMS. An overview of the thin-film method for tracer diffusion studies using stable isotopes is provided. Experimental procedures and techniques for the measurement of tracer diffusion coefficients are presented for pure magnesium, which presents some unique challenges due to the ease of oxidation. The development of a modified Shewmon-Rhines diffusion capsule for annealing Mg and an ultra-high vacuum system for sputter deposition of Mg isotopes are discussed. Optimized conditions for accurate SIMS depth profiling in polycrystalline Mg are provided. An automated procedure for correction of heat-up and cool-down times during tracer diffusion annealing is discussed. The non-linear fitting of a SIMS depth profile data using the thin-film Gaussian solution to obtain the tracer diffusivity along with the background tracer concentration and tracer film thickness is demonstrated. An Arrhenius fit of the Mg self-diffusion data obtained using the low-temperature SIMS measurements from this study and the high-temperature radiotracer measurements of Shewmon and Rhines (Trans. AIME 250:1021-1025, 1954) was found to be a good representation of both types of diffusion data over a broad range of temperatures between 250 and 627¿°C (523 and 900¿K).

Hydrogen is expected to become a commonly used energy carrier on the global scale in the near future. However, hydrogen as a fuel is environmentally friendly only when generated from water using renewable energy, such as solar energy. Therefore, intensive research aims to develop a new generation of solar materials, which may be used for the production of hydrogen fuel from water using solar energy. The highly promising candidates for solar energy conversion are photosensitive oxide semiconductors (POSs), particularly the TiO2-based semiconductors, which may be used for converting solar energy into the chemical energy required for hydrogen generation from water, as well as water purification (removal of microbial agents and toxic contaminants from water). The present work considers an R&D strategy for developing TiO2- based systems capable of converting solar energy into the chemical energy via water oxidation. The effect of surface versus bulk semiconducting properties on the performance of POSs is considered in terms of partial and total water oxidation. The progress requires modification of the key performance-related properties (KPPs) in order to enhance the light-induced reactivity of the POSs with water. The most recent approach in the development of POSs with enhanced performance is deposition of metallic islets of different size and shape in order to induce a plasmonic effect. The development of high-performance POSs can be achieved through a multidisciplinary approach. It is shown that defect disorder has a critical effect on the light-induced reactivity of POSs and the solar energy conversion. Therefore, defect engineering may be applied in the development of high-performance POSs. This work considers the hurdles in the development of high-performance POSs for specific applications and formulates the key questions that must be addressed to overcome these hurdles. The concepts developed for TiO2may be expanded for other metal oxides.

In this paper, an extended version of the matrix method is derived in order to address diffusion kinetics for the full anisotropic three-dimensional h.c.p. structure. It is shown that the diffusion anisotropy can be properly addressed with a model of 13 atom-vacancy frequencies which is an extended version of the well-known 5-frequency model for the f.c.c. lattice. Both tracer and phenomenological diffusion coefficients are calculated using this new approach. Extended Monte Carlo simulations are performed in order to cross-check some of the results of the matrix method. Applications of the proposed model to experimental diffusion data are discussed. © 2014 Taylor & Francis.

This paper addresses the numerical analysis of the mechanical properties of titania foam intended for use in tissue engineering scaffolds. Special focus is given to a PDLLA coating that has been shown to distinctly increase the mechanical strength of the scaffold. Micro-computed tomography (µCT) data of a real scaffold are obtained and converted into numerical calculation models. Finite element simulations are performed alternately with and without the PDLLA coating. In addition, a strut model containing a single micro-crack is analysed. Numerical results indicate that filling the crack with PDLLA significantly increases the strength of the strut by attenuating stress concentrations. © 2013 Elsevier Ltd. All rights reserved.

Owing to the increasingly apparent climate change, it becomes imperative to use renewable energy in the production of fuel that is environmentally friendly. At the same time, there is a need to introduce the related education programs to develop the skills of the technical staff working at the front line of rapidly developing renewable energy technologies. Hydrogen is expected to be the fuel in the near future. At present hydrogen fuel is mainly produced using steam reforming of methane (SRM). However, hydrogen generation using the SRM results in emission of greenhouse gases and climate change. Therefore, there is a common consensus that the SRM technology will soon be challenged by the technologies of solar hydrogen generation using photoelectrochemical cells (PEC). However, the PEC technology will be the ultimate winner only if the effects related to climate change and pollution are fully monetised. While such radical development is difficult for implementation due to economic reasons, the increasingly urgent need to reduce climate change dictates the need to increase competitiveness of the PEC method. This imposes the need to increase the efficiency of the solar energy conversion and reduce the costs of the related raw materials and devices. The development of renewable energy-related technologies, such as those related to solar hydrogen, imposes the need to introduce education programs in order to train technical and research staff working at the front line of rapidly developing sustainable energy systems. The present work considers such programs addressing a range of energy-related topics, such as hydrogen energy, electrochemical energy, photoelectrochemical energy and alternative renewable energy as well as industrial ecology and energy policy. It is concluded that implementation of these programs is urgently needed in order to protect the environment through sustainable development. © 2014 Hydrogen Energy Publications, LLC.

This paper addresses an innovative syntactic foam (SF) formed by counter-gravity infiltration of a packed bed of low-cost expanded perlite (EP) particles with molten A356 aluminium. The uniform distribution of EP particles in foams causes an even density throughout the height. Due to the low density (~0.18g/cm3) of EP, the average density of these foams is only 1.05g/cm3 which is considerably lower than most studied SFs. Owing to the high porosity of the filler material (~94%), the total porosity of the new foam reaches 61%. Microstructural observations reveal no sign of damage or unintended EP particle infiltration. EP shows a good wettability whilst essentially no reaction occurs at the EP-metal interface. Under compression, EP/A356 syntactic foam shows stress-strain curves consisting of elastic, plateau and densification regions. On account of its consistent plateau stress (average value 30.8MPa), large densification strain (almost 60%), and high energy absorption efficiency (88%) EP/A356 syntactic foam is an effective energy absorber. © 2014 Elsevier B.V.

This paper addresses the use of the Lattice Monte Carlo method for the thermal characterization of composite materials. An optimized approach that minimizes computational time is presented. The key aspect of the approach is the avoidance of the need to model the local thermal inertia. A combined finite element and Lattice Monte Carlo analysis is conducted on a model composite for a formal verification of the effective thermal diffusivity and conductivity calculated by the optimized Lattice Monte Carlo method. The effective thermal inertia is calculated separately by making use of the energy conservation law. © 2014 Elsevier Ltd. All rights reserved.

Tissue engineering scaffolds are designed to support tissue self-healing within physiological environments by promoting the attachment, growth and differentiation of relevant cells. Newly formed tissue must be supplied with sufficient levels of oxygen to prevent necrosis. Oxygen diffusion is the major transport mechanism before vascularization is completed and oxygen is predominantly supplied via blood vessels. The present study compares different designs for scaffolds in the context of their oxygen diffusion ability. In all cases, oxygen diffusion is confined to the scaffold pores that are assumed to be completely occupied by newly formed tissue. The solid phase of the scaffolds acts as diffusion barrier that locally inhibits oxygen diffusion, i.e. no oxygen passes through the scaffold material. As a result, the oxygen diffusivity is determined by the scaffold porosity and pore architecture. Lattice Monte Carlo simulations are performed to compare the normalized oxygen diffusivities in scaffolds obtained by the foam replication (FR) method, robocasting and sol¿gel foaming. Scaffolds made by the FR method were found to have the highest oxygen diffusivity due to their high porosity and interconnected pores. These structures enable the best oxygen supply for newly formed tissue among the scaffold types considered according to the present numerical predictions.

Equilibrium atomic configurations and the kinetics of "order- order" and surface segregation processes in B2-ordering stoichiometric A-50 at.%B binary thin films are investigated by means of Semigrand Canonical Monte Carlo (SGCMC) and Kinetic Monte Carlo (KMC) simulations. The (100)-oriented films are modeled with an Ising-type Hamiltonian with previously evaluated pair-interaction energy parameters yielding the effect of "triple-defect disordering". The SGCMC simulations provide equilibrium vacancy concentration and atomic configuration in the films with B-atom termination of both free surfaces achieved at high temperatures by the generation of an antiphase boundary. Despite strong vacancy surface segregation, the thermodynamic activation energy for their formation inside the films is the same as in the bulk material. KMC simulations implemented with the SGCMC-determined equilibrium vacancy concentration reveal very slow relaxation of the films towards equilibrium configuration. The B-termination of the (100) free surfaces is produced by A-atom diffusion inwards into the films mediated by vacancies segregating on surfaces. © 2014 Elsevier Ltd. All rights reserved.

In this paper, a new formalism of a combined tracer and interdiffusion experiment for a binary interdiffusion couple is developed. The analysis requires an interdiffusion couple that initially contains a thin layer of tracers of one or both of the constituent elements at the original interface of the couple (sandwich interdiffusion experiment). This type of interdiffusion experiment was first performed in 1958 by J.R. Manning. The theoretical analysis presented in this paper is based on a newly developed phenomenological theory of isotopic interdiffusion combined with the Boltzmann-Matano formalism. This new analysis now provides the means to obtain the composition dependent interdiffusion coefficient and tracer diffusion coefficients simultaneously from analysis of the interdiffusion and tracer profiles in a single sandwich interdiffusion experiment. The new analysis is successfully applied to the results of Mannings original sandwich interdiffusion experiment in the Ag-Cd system (six couples in total) and is validated with an independent determination of the Ag and Cd tracer diffusion coefficients by Schoen using the conventional thin film technique. Suggestions for further development of the sandwich interdiffusion experiment and analysis to the case of multicomponent alloys are provided.

This paper addresses the mechanical properties of Corevo® aluminium foam. The effective Young's modulus, Poisson's ratio, and material yield stress are determined. To this end, samples are tested using uni-axial compressive testing. In addition, micro-computed tomography data of the complex material geometry are obtained and converted into finite element calculation models. The numerical analysis further enables the testing of mechanical material anisotropy and plastic deformation within the material's meso-structure. © 2013 Elsevier B.V. All rights reserved.

The phonon-mediated thermal conductivity of f.c.c. Cu is investigated in detail in the temperature range 40-1300 K. The calculations are performed in the framework of equilibrium molecular dynamics making use of the Green-Kubo formalism and one of the most reliable embedded-atom method potentials for Cu. It is found that the temporal decay of the heat current autocorrelation function (HCACF) of the Cu model at low and intermediate temperatures demonstrate a more complex behaviour than the two-stage decay observed previously for the f.c.c. Ar model. After the first stage of decay, it demonstrates a peak in the temperature range 40-800 K. A decomposition model of the HCACF is introduced. In the framework of that model we demonstrate that a classical description of the phonon thermal transport in the Cu model can be used down to around one quarter of the Debye temperature (about 90 K). Also, we find that above 300 K the thermal conductivity of the Cu model varies with temperature more rapidly than, following an exponent close to -1.4 in agreement with previous calculations on the Ar model. Phonon thermal conductivity of Cu is found to be about one order of magnitude higher than Ar. The phonon contribution to the total thermal conductivity of Cu can be estimated to be about 0.5% at 1300 K and about 10% at 90 K. © 2013 © 2013 Taylor & Francis.

An analytical treatment of decomposition of the phonon thermal conductivity of a crystal with a monatomic unit cell is developed on the basis of a two-stage decay of the heat current autocorrelation function observed in molecular dynamics simulations. It is demonstrated that the contributions from the acoustic short-and long-range phonon modes to the total phonon thermal conductivity can be presented in the form of simple kinetic formulas, consisting of products of the heat capacity and the average relaxation time of the considered phonon modes as well as the square of the average phonon velocity. On the basis of molecular dynamics calculations of the heat current autocorrelation function, this treatment allows for a self-consistent numerical evaluation of the aforementioned variables. In addition, the presented analysis allows, within the Debye approximation, for the identification of the temperature range where classical molecular dynamics simulations can be employed for the prediction of phonon thermal transport properties. As a case example, Cu is considered.

This paper reports on an investigation of the compressive properties of Corevo® foam. Corevo® foam is a cellular metal manufactured by the infiltration casting of salt dough with aluminium. Corevo® foam samples with different porosities are tested by using quasi-static compression loading. Their mechanical properties (i. e.: effective Young's modulus, Poisson's ratio, initial yield stress and material yield stress) are then compared to reveal the importance of the density difference. In addition, three-dimensional finite element analysis is performed on models generated from micro-computed tomography (µCT). The results of two different pore sizes are obtained and compared in the scope of this work. These numerical results are verified by comparison with the experimental analysis. Sound agreement is found. Numerical analysis in this work also includes the investigation of the mechanical material anisotropy and plastic deformation. © (2014) Trans Tech Publications, Switzerland.

Self-diffusion of component atoms and order-order relaxations in a B2-ordering binary system AB showing a tendency for triple-defect formation were consistently simulated by means of two Monte Carlo techniques. In view of a strict correlation between antisite-defect and vacancy concentrations the Kinetic Monte Carlo (KMC) simulations were implemented with a temperature-dependent vacancy concentration determined by means of Semi-Grand Canonical Monte Carlo (SGCMC) simulations. The Ising model of the system was completed with local-configuration-dependent saddle-point energy parameters related to vacancy mediated atomic jumps. The simulations elucidated the atomistic origin of the experimentally observed low rate of order-order relaxations in NiAl, as well as reproduced the experimental relation between the activation energies for order-order kinetics and Ni self-diffusion in NiAl. Higher value of the deduced activation energy for atomic migration with respect to the effective energy barriers related to individual atomic jumps indicated their high correlation. © 2013 Taylor & Francis Group, LLC.

In this paper, carbon diffusion in cementite is studied by molecular dynamics simulation. An assumption that carbon-carbon interaction occurs only indirectly via neighbouring iron atoms is used. An interstitial mechanism of carbon diffusion in cementite is revealed. The principal tracer diffusion coefficients and activation parameters of carbon diffusion in cementite are calculated for the temperature range 1223-1373 K and compared with the available published experimental data.

The presence of atomic oxygen at internal metal-ceramic oxide interfaces significantly affects the physical properties of the interfaces which in turn affects the bulk properties of the material. We address this problem for the case of a constant source of oxygen at the surface and periodic arrangements of ceramic oxide (MgO) inclusions embedded in a metal (Ag) matrix. We simulate the time-dependence of the oxygen concentration into the material using a newly developed lattice Monte Carlo method that takes into account a constant source of diffusant.

In this paper, we show how lattice¿based random walks of virtual particles directed by Monte Carlo methods (Lattice Monte Carlo) can be used to address a variety of complexphenomenologically mass diffusion problems. Emphasis is put on the practical details of doing thecalculations. It is shown how concentration depth profiles can be determined: this is exemplifiedwith diffusion in the presence of isolated dislocation pipes, grain boundary slabs, and oxygensegregation at interfaces in metal-ceramic oxide composites. It is also shown how effectivediffusivities can be determined in materials. We also show how temperature profiles and the effective thermal conductivity can be determined for the thermal diffusion analogue of massdiffusion. A detailed comparison is made in one case with the results of the Finite Element method.