Pore Migration Research Articles

Phase-field study of pore-grain boundary interaction

During final stage sintering, a complex interplay of densification and grain growth dominates microstructural evolution. Grain growth starts, when pore drag effects become less important due to pore shrinkage. This grain growth then decreases the driving force available for sintering. Accordingly, the interplay of pores and grain boundaries needs to be considered in detail. A phasefield model was extended to treat pore dynamics under consideration of pressure stability. To study pore attachment and detachment at moving interfaces, an idealized hexagonal microstructure with a constant driving force relationship for pore migration is constructed. Additionally, realistic polycrystalline microstructures were used. The model is in good agreement with experiments and analytic equations. Three different cases were observed in the realistic microstructure: pore attachment at the moving interface, partial and total pore detachment. However, in the partial case, the initial location of pores was found to be important: pores tend to migrate from quadruple junctions over triple junctions to grain boundary planes, where they eventually detach. This results in a variation of pore detachment, which is not captured in analytic equations. Therefore large simulation setups are required to reflect the impact of initial pore location on pore drag effects.

In situ X-ray tomographic microscopy of Kirkendall pore formation and evolution during homogenization of pack-aluminized Ni–Cr wires

The Kirkendall effect, usually associated with the formation of undesirable fields of micron-size pores, can be used to fabricate hollow nano- and microstructures, where all pores have coalesced into a single cavity. Here, this concept is used with the goal of converting 50 μm diameter Ni-20wt.%Cr wires into Ni–Cr–Al tubes by diffusion of a surface-deposited aluminide coating. The formation and evolution of Kirkendall pores, created by imbalanced diffusion fluxes of Al, Ni and/or Cr, are investigated using both in situ X-ray tomographic microscopy and ex situ annealing studies. The tomography results indicate that the pores nucleate near the interface between the Ni–Cr core and β-NiAl(Cr) reaction layer that develops upon homogenization. They then grow and coalesce near the center of the radial cross-section of the wire. This is followed by a stagnation period until the pores migrate longitudinally due to the influence of a temperature gradient imposed by the laser heating used for annealing. The ex situ experiments are in qualitative agreement with the tomography study with the exception of the secondary pore migration along the wire length, as expected because the ex situ study is conducted under isothermal conditions. Detailed features of the microstructure were discernable only upon metallographic preparation and include Cr-rich precipitates and an α-Cr(Al,Ni) rejection region that formed during coating and homogenization, respectively. In situ X-ray tomographic microscopy is thus demonstrated to be a viable technique to use in conjunction with more conventional ex situ metallographic techniques to conduct interdiffusion and Kirkendall pore studies.

Abstract B35: Vinyl sulfone analogues of lysophosphatidylcholine irreversibly inhibit autotaxin and prevent angiogenesis in melanoma

Abstract Autotaxin is an enzyme discovered in the conditioned medium of cultured melanoma cells and identified as a protein that strongly stimulates motility. Autotaxin is a unique ectonucleotide pyrophosphatase and phosphodiesterase that facilitates the removal of a choline headgroup from lysophosphatidylcholine to yield lysophosphatidic acid, which is a potent lipid stimulator of tumorigenesis. Thus, autotaxin has received renewed attention because it has a prominent role in malignant progression and represents a promising area of research with significant translational potential. Specifically, we sought to develop active site-targeted irreversible inhibitors as anti-cancer agents. Our goal was to discover agents with therapeutic efficacy in melanoma. For this purpose, we synthesized a set of lysophosphatidylcholine analogs that function as irreversible inhibitors of autotaxin and inactivate the enzyme. The analogs were tested in cell viability assays using multiple cancer cell lines. The IC50 values ranged from 6.74 – 0.39 μM, consistent with a Ki of 3.50 μM for inhibition of autotaxin by the analog CVS-16 (10b). A control compound, GWJ-A-10, which is missing the critical moiety, had little effect on cell viability and did not inhibit autotaxin. In addition, CVS-16 (10b) significantly inhibited melanoma cell wound closure and pore migration (data not presented herein). Most importantly, CVS-16 (10b) significantly inhibited melanoma progression in an in vivo tumor model by preventing angiogenesis. Taken together, this suggests that CVS-16 (10b) is a potent autotaxin inhibitor with significant biological activity, both in vitro and in vivo. Citation Format: Mandi Murph, Guowei Jiang, Molly Altman, Jia Wei, Sterling Tran, Duy Nguyen, Ali Alshamrani, William Hardman, Jianxing Zhang, Glenn Prestwich. Vinyl sulfone analogues of lysophosphatidylcholine irreversibly inhibit autotaxin and prevent angiogenesis in melanoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Melanoma: From Biology to Therapy; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(14 Suppl):Abstract nr B35.

Abstract 2672: Vinyl sulfone analogues of lysophosphatidylcholine irreversibly inhibit autotaxin and prevent angiogenesis in melanoma

Abstract Autotaxin (ATX) is an enzyme discovered in the conditioned medium of cultured melanoma cells and identified as a protein that strongly stimulates motility. This unique ectonucleotide pyrophosphatase and phosphodiesterase facilitates the removal of a choline headgroup from lysophosphatidylcholine (LPC) to yield lysophosphatidic acid (LPA), which is a potent lipid stimulator of tumorigenesis. Thus, ATX has received renewed attention because it has a prominent role in malignant progression and represents a promising area of research with significant translational potential. Since ATX inhibitors may have broad implications for therapy, we sought to create a novel series of inhibitors for use against cancer, especially melanoma. Herein we report on the synthesis and biological activity of a targeted affinity label for the active site of ATX, which has a critical threonine residue that acts as a nucleophile in the lysophospholipase D reaction to liberate choline. We synthesized a set of vinyl sulfone analogs of ATX that function as irreversible inhibitors of ATX and inactivate the enzyme. We commenced biological testing of this compound with viability assays against multiple cancer cell lines whereby the IC50 values ranged from 6.74 - 0.39 μM, which was consistent with the Ki of the vinyl sulfone (3.50 μM). In addition, this compound was also able to significantly inhibit melanoma cell wound closure and pore migration. Most importantly, the vinyl sulfone significantly inhibited melanoma progression using an in vivo tumor model by preventing angiogenesis. Taken together, this suggests that the vinyl sulfone is a potent and irreversible ATX inhibitor with significant biological activity both in vitro and in vivo. Note: This abstract was not presented at the meeting. Citation Format: Mandi M. Murph, Molly Altman, Wei Jia, Duy Nguyen, Jada Fambrough, William J. Hardman, Guowei Jiang, Damian Madan, Jianxing Zhang, Glenn D. Prestwich. Vinyl sulfone analogues of lysophosphatidylcholine irreversibly inhibit autotaxin and prevent angiogenesis in melanoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2672. doi:10.1158/1538-7445.AM2014-2672

Phase field simulation of grain growth in porous uranium dioxide

A novel phase field model has been developed to investigate grain growth in porous polycrystalline UO2. Based on a system of Cahn–Hilliard and Allen–Cahn equations, the model takes into consideration both the curvature driven grain boundary motion and pore migration by surface diffusion. As such, the model accounts for the interaction between pore and grain boundary kinetics, which tends to retard the growth process. The phase field model parameters are found in terms of measurable material properties. Hence, quantitative results that can be compared with experiments were obtained. The model has been used to investigate the effect of porosity on the kinetics of grain growth in UO2. It is found that, as the amount of porosity increases, grain growth in UO2 gradually changes from boundary controlled growth to pore controlled growth. For high porosity levels, the grain growth completely stops after a short evolution time. It is also found that the inhomogeneous distribution of pores leads to abnormal grain growth even without taking into account the anisotropy in grain boundary energy and mobility. The effects of porosity, temperature and initial microstructure on grain growth were thoroughly investigated. The model predictions are in good agreement with published experimental results of grain growth in UO2.

Modelling actinide redistribution in mixed oxide fuel for sodium fast reactors

In this paper, a model for the actinide redistribution mechanism in mixed oxide fuel for a sodium fast reactor is presented and compared with measured data collected from post irradiation examination on fuel from the JOYO experimental reactor. The model considers that solid-state thermal diffusion and vapour transport can simultaneously contribute to the plutonium and americium radial profiles. The effect of fuel non-stoichiometry on actinide transport as well as on pore velocity is taken into account. The model is embedded into the TRANSURANUS fuel rod performance code and calculates the evolution of the Am and Pu concentrations as a function of the radial temperature profile. The calculated actinide distribution is in good agreement with the experimental data. The results confirm that under normal operation conditions with a decreasing fuel power during its life-time, redistribution via pore migration is extremely important only during the central hole formation at the beginning of life, whereas thermal diffusion represents the dominant effect for long-term irradiation.

Experimental investigations on welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steel

Tungsten Inert Gas (TIG) welding is considered as one of the cleanest welding methods. It is generally adopted for thinner materials with moderate weld joint strengths. Welding of sintered porous materials continues to be a challenge due to the inherent porosity of the parent metals. The present research work attempts to address some of the issues relating to the welding behaviour of sintered and forged Fe–0.3%C–3%Mo low alloy steels under TIG welding. Rectangular strips of size 70mm×15mm×5mm, obtained by blending, compacting and sintering of elemental powders of iron, graphite and molybdenum, were upset forged – both hot and cold in order to obtain alloy steel strips of various porosities. Two identical alloy steel strips of equal density were then welded both along longitudinal and transverse directions, by TIG welding, employing filler metal of suitable composition. The welded strips were then subjected to tensile test, hardness test, microstructural and Scanning Electron Microscope (SEM) fractography studies. Cold/hot upsetting of the sintered alloy preforms has led to enhanced density. As a result of improved density, their tensile strength and hardness values were also found to be enhanced. The welded alloy exhibited higher tensile strength compared to the un-welded base metal, due to strengthening by residual stress. Similarly, the strength and hardness of the welded alloy strips were found to be enhanced with increase in density. The tensile strength of welded joint is found to be higher compared to that of the base metal due to alloy metals segregation, rapid cooling and formation of acicular ferrite at the weldment of welded joint. No porosity was observed in the weld metal or Heat Affected Zone (HAZ) of the weld joint. However, the base metal had numerous micro pores, though pore migration towards weldment has not been observed.

Three Dimensional Monte Carlo Simulation of Microstructure Evolution in Presence of Pores and Impurities for Three-Phase Nanocomposite Ceramic Tool Materials

A modified three-dimensional Monte Carlo (MC) model in presence of pores and impurities for three-phase nanocomposite ceramic tool materials is successfully established in this paper. Pore migration by surface diffusion is incorporated into the MC model and it is applied to observe and scientific quantitative characterization of three dimensional microstructure evolution and densification process. Some modifications are applied to the simulation algorithm to improve the computing efficiency. The influence of pores on the particle and impurity loaded grain boundaries is simulated and investigated for the study of grain growth kinetics. The relationship between porosity and density is also analyzed. The results indicate that the higher the porosity is, the lower the density will be.

Phase-field modeling of temperature gradient driven pore migration coupling with thermal conduction

Pore migration in a temperature gradient (Soret effect) is investigated by a phase-field model coupled with a heat transfer calculation. Pore migration is observed towards the high temperature domain with velocities that agree with analytical solution. Due to the low thermal conductivity of the pores, the temperature gradient across individual pores is increased, which in turn, accelerates the pore migration. In particular, for pores filled with xenon and helium, the pore velocities are increased by a factor of 2.2 and 2.1, respectively. A quantitative equation is then derived to predict the influence of the low thermal conductivity of pores.

OS1-2-3 固体酸化物形燃料電池燃料極の構造変化に関するナノ・メゾスケール解析(OS1 マルチスケール現象のシミュレーション技術(2))

The electrochemical performance of a solid oxide fuel cell (SOFC) is strongly linked with the evolution of an anode microstructure that is composed of a porous Ni-YSZ cermet. Ni coarsening is widely recognized phenomena which may lead to the loss of SOFC performance during long-term operation at high temperature. In this study, we have developed three-dimensional Monte Carlo simulations based on a two phase Q-state Potts model in order to numerically predict the microstructural evolution of the complicated cermet anode during operation. Ni coarsening was captured by incorporating the three distinct kinetic algorithms: grain growth, pore migration and pore annihilation. The starting configuration of each simulation was taken from the experimentally reconstructed Ni-YSZ cermet structure using a dual beam FIB-SEM measurement. Our calculations reveal that the triple-phase boundary length because of the constraint effect of the YSZ phase. This trend quite agrees with recent experimental measurements, which support the validity of our numerical modeling.

J056051 SOFC燃料極の微視構造発展予測に向けた3次元モンテカルロシミュレーション

The electrochemical performance of a solid oxide fuel cell (SOFC) is strongly linked with the evolution of an anode microstructure that is composed of a porous Ni-YSZ cermet. In particular, Ni coarsening is widely recognized phenomena which may lead to the loss of SOFC performance during long-term operation at high temperature. In this study, we have performed three-dimensional kinetic Monte Carlo (kMC) simulations based on a two phase Q-state Potts model in order to numerically predict the microstructural evolution of the complicated cermet anode during operation. Ni coarsening was captured by incorporating the three distinct kinetic algorithms: grain growth, pore migration and pore annihilation. The starting configuration of each simulation was taken from the experimentally reconstructed Ni-YSZ cermet structure using a dual beam FIB-SEM measurement. Our calculations reveal that the triple-phase boundary (TPB) length because of the constraint effect of the YSZ phase. This trend quite agrees with recent experimental measurements, which support the validity of our numerical modeling.

Preparation and characterization of micro components fabricated by micro powder injection molding

Abstract Given the dimension of the micro component to be replicated, it is crucial to study sintering for dimensional and property control in micro powder injection molding. In the paper, 316 L stainless steel micro components with two different microsize structure dimensions, ϕ100 μm × height 200 μm and ϕ60 μm × height 191 μm, are fabricated from in-house feedstock. Shrinkage, microstructure, microindentation hardness and sintering trajectory are investigated for the micro components. The linear shrinkage rate of the micro component reaches the maximum at 1180 °C, after which it begins to decline. Porosity (volume fraction of pores) decrease, pore migration and breakaway, grain growth and the formation of a dense layer are observed for the microsize structures. The microindentation hardness of the dense layer in the rim of the microsize structures is lower than the microindentation hardness of the substructure. It is found that if the relative density is below 0.98, densification is promoted when the structure size reduces from the substructure in the millimeter regime to the microsize structures of ϕ60 μm. This is attributed to the oxides on the debound micro components and internal gas pressure produced by hydrogen sintering.

Parallel simulation of 3D sintering

A three-dimensional parallel implementation of a Monte Carlo model for the microstructure evolution during sintering is presented. The model accounts for the main phenomena occurring during sintering, including grain growth, pore migration and vacancy annihilation. The parallel implementation is based on the SPPARKS code and enables the simulation of systems with large number of particles. Several examples are shown and results are compared with a serial implementation as well as experimental data available.

Characteristics of pore migration controlled by diffusion through the pore-filling fluid

We analyze drag and drop of pores filled with a fluid phase, e.g., water or melt, in which the constituting elements of the solid matrix are dissolved. Assuming that the diffusion through the fluid-phase dominates bulk transport kinetics, we address the problem of pore motion and calculate the pore mobility and the critical velocity of elongated and lenticular pores on a grain boundary for arbitrary dihedral angle. The found variations in critical velocity and mobility with dihedral angle are modest for given volume of pores with the two considered geometries. For given pore size, however, the dependence on dihedral angle accounts for several orders of magnitude in pore mobility and critical velocity.

Mesoscale Simulations of Microstructure Evolution in a Temperature Gradient

AbstractThe evolution of pore and grain structure in a nuclear fuel environment is strongly influenced by the local temperature, and the temperature gradient. The evolution of pore and grain structure in an externally imposed temperature gradient is simulated for a hypothetical material using a Potts model approach that allows for porosity migration by mechanisms similar to surface, grain boundary and volume diffusion, as well as the interaction of migrating pores with stationary grain boundaries. First, the migration of a single pore in a single crystal in the presence of the temperature gradient is simulated. Next, the interaction of a pore moving in a temperature gradient with a grain boundary that is perpendicular to the pore migration direction is simulated in order to capture the force exerted by the pore on the grain boundary. The simulations reproduce the expected variation of pore velocity with pore size as well as the variation of the grain boundary force with pore size.

Modelling of Grain Growth Kinetics in Porous Ceramic Materials under Normal and Irradiation Conditions

Effect of porosity on grain growth is both the most frequent and technologically important situation encountered in ceramic materials. Generally this effect occurs during sintering, however, for nuclear fuels it also becomes very important under reactor irradiation conditions. In these cases pores and gas bubbles attached to the grain boundaries migrate along with the boundaries, in some circumstances giving a boundary migration controlled by the movement, coalescence and/or sintering of these particles. New mechanisms of intergranular bubble and pore migration which control the mobility of the grain boundary under normal and irradiation conditions are reviewed in this paper.

Radial redistribution of actinides in irradiated FR-MOX fuels

The redistributions of neptunium, plutonium and americium during two kinds of short-term irradiation tests for 10 min and 24 h at high linear heating rate around 430 W cm −1 were studied in the uranium and plutonium mixed oxide fuel containing Am and/or Np. It was found in the irradiation test for 24 h that the concentrations of Pu and Am increased toward the central void, but there was no change in the concentration of Np. The obtained experimental redistributions of Am and Pu were analyzed, based on both pore migration and thermal diffusion models. As a result, the calculated redistributions of Pu and Am showed good agreements with the experimentally obtained ones.

Pore migration under high temperature and stress gradients

An analytical model is proposed for the evolution of ellipsoidal pores by surface diffusion under the influence of large temperature gradients and the associated thermoelastic stress field. It is found that both of these influences affect the migration velocity of a pore as well as its shape. The shape of a pore is determined by competition between the thermoelastic stress field, the interfacial energies of the pore and grain boundaries, and the kinetics of the mass transport process. The dramatic case of void migration in a uranium dioxide nuclear fuel rod is considered, in which very large temperature gradients of the order of 4 × 10 5 K/m are predicted. It is found that crack-like pores are expected to form on the radial grain boundaries prevalent in the fuel rod microstructure, and that these crack-like pores lead to the eventual structural failure of the component. This agrees with experimental observations. The temperature gradient is the dominant driving force for the migration of near-spheroidal and prolate pores, whereas the stress field is the dominant driving force for oblate crack-like pores.

Some generic capillary-driven flows

This paper deals with numerical simulations of some capillary-driven flows. The focus is on the wetting phenomenon in sintering-like flows and in the imbibition of liquids into a porous medium. The wetting phenomenon is modeled using the coupled Cahn–Hilliard/Navier–Stokes system. The Cahn–Hilliard equation is treated as a system where the chemical potential is solved first followed by the composition. The equations are discretised in space using piecewise linear functions. Adaptive finite element method is implemented with an ad hoc error criterion that ensures mesh resolution along the vicinity of the interface. In the 3D case we use parallel adaptive finite element method. First, a basic wetting of a liquid drop on a solid surface is shown and is established the independence of the dynamic contact angle on the interface width. In addition, the dependence of the dynamic contact angle on the Capillary number is matched with experimental data. Next, some generic sintering-like flows with a fixed matrix is presented. Different geometries in 2D and 3D are considered. We observed rapid wetting, precursor films, coalescence, breakup of melt drops as well as pore migration and elimination that are all microstructural characteristics of a liquid phase sintering. Finally, the effect of equilibrium contact angles on imbibition of liquid into a porous medium is studied.

Multi‐Scale Study of Sintering: A Review

An integrated approach, combining the continuum theory of sintering with a kinetic Monte‐Carlo (KMC) model‐based mesostructure evolution simulation is reviewed. The effective sintering stress and the normalized bulk viscosity are derived from mesoscale simulations. A KMC model is presented to simulate microstructural evolution during sintering of complex microstructures taking into consideration grain growth, pore migration, and densification. The results of these simulations are used to generate sintering stress and normalized bulk viscosity for use in continuum level simulation of sintering. The advantage of these simulations is that they can be employed to generate more accurate constitutive parameters based on most general assumptions regarding mesostructure geometry and transport mechanisms of sintering. These constitutive parameters are used as input data for the continuum simulation of the sintering of powder bilayers. Two types of bilayered structures are considered: layers of the same particle material but with different initial porosity, and layers of two different materials. The simulation results are verified by comparing them with shrinkage and warping during the sintering of bilayer ZnO powder compacts.