Dual-phase Material Research Articles

Optimization of strontium molybdate based composite anode for solid oxide fuel cells

Perovskite-type SrMoO3 (SMO) is composited with commonly used electrolytes Y0.08Zr0.92O2 (YSZ), La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) and Gd0.1Ce0.9O2 (GDC) to form dual-phase materials, which are characterized as potential anode materials for solid oxide fuel cells. Introduction of any of the electrolytes YSZ, LSGM and GDC can remarkably enhance the electrochemical performance of SrMoO3. The polarization resistance of the composite anode in a symmetrical cell decreases in the sequence of SMO–LSGM, SMO–YSZ and SMO–GDC. The content of GDC in SMO–xGDC composite anode has a considerable effect on the catalytic activity of the electrode. The SMO–60GDC composite shows the lowest polarization resistance of 0.107 Ω cm2 at 850 °C. With SMO–60GDC composite as anode, the maximum power densities of 584, 452 and 257 mW cm−2 are achieved at 900, 850 and 800 °C, respectively, for LSGM electrolyte supported cell with La0.58Sr0.4Co0.2Fe0.8O3 as cathode. The cell exhibits considerably stable performance under a constant voltage of 0.71 V at 800 °C in a short-term cycle test.

Evaluation of La0.7Ca0.3Cr0.95Zn0.05O3–δ–Gd0.1Ce0.9O2–δ Dual-phase Material and its Potential Application in Oxygen Transport Membrane

In this work, a dual-phase material consisting of La0.7Ca0.3Cr0.95Zn0.05O3–δ (LCCZ, 40 wt%) and Gd0.1Ce0.9O2–δ (GDC, 60 wt%) was synthesized. Properties including phase structure, sintering behavior, electrical conductivity and oxygen permeability for LCCZ–GDC were evaluated. The results show that dense LCCZ–GDC dual-phase disks were obtained at the sintering temperature of 1250, 1300, 1350 and 1400 °C by tape casting and high temperature sintering method. The grain sizes of both GDC and LCCZ grew up with the increasing of sintering temperature. The average grain size of GDC was about 0.5, 0.8, 1.4, 1.8 μm while the average grain size of LCCZ was about 0.8, 1.5, 1.8 and 2 μm after sintering at 1250, 1300, 1350 and 1400 °C, respectively. Oxygen flux of LCCZ–GDC decreased with the increase of sintering temperature from 1250 to 1400 °C. The oxygen flux of LCCZ–GDC sintered at 1250 °C reached 0.079 mL/min/cm2 at 975 °C with a membrane thickness of 800 μm. Dual-phase material of LCCZ–GDC will be a promising oxygen transport membrane material for its low sintering temperature and good microstructure.

Characterization of the Orthotropic Elastic Constants of a Micronic Woven Wire Mesh via Digital Image Correlation

Woven structures are steadily emerging as excellent reinforcing components in dual-phase composite materials subjected to multiaxial loads, thermal shock, and aggressive reactants in the environment. Metallic woven wire mesh materials in particular display good ductility and relatively high specific strength and specific resilience. While use of this class of materials is rapidly expanding, a significant gap in property characterization remains. This research classifies the homogenized, orthotropic material properties of a representative twill dutch woven wire mesh through the use of in-plane uniaxial tensile experiments incorporating a Digital Image Correlation (DIC) strain measurement technique. Values for elastic modulus and Poisson’s ratio are calculated from the experimental data, and shear modulus values are identified by means of constitutive modeling. This approach establishes a reproducible method for characterizing the in-plane elastic response of micronic metallic woven materials via macro-scale uniaxial tensile tests, and shows that a homogenous orthotropic constitutive model may be employed to describe the macro-scale elasticity of this class of materials with reasonable accuracy.

Review of SBSE Technique for the Analysis of Pesticide Residues in Fruits and Vegetables

Stir bar sorptive extraction (SBSE) is a microextraction technique, introduced to overcome the problem of limited extraction capacity and fragile fiber coatings inherent in the solid phase microextraction technique. The major limitations of the SBSE technique are that only polydimethylsiloxane has been commercially available, this reduces its use to non-polar analytes, and its tedious reconstitution step which can lead to loss of analytes and introduction of impurities. The current trend has been aimed at the use of other materials, some of which are commercially available, such as restricted access materials, carbon adsorbents, molecularly imprinted polymers, ionic liquids, microporous monoliths, sol–gel prepared coatings and dual phase material. This has greatly helped in widening the applications of SBSE for pesticide analysis in fruits and vegetables and other matrices. The introduction of a thermal desorption unit which eliminates the reconstitution step of the stir bar in organic solvents before instrumental analysis has helped to automate the extraction method online with gas chromatography. This paper reviews the use of SBSE in pesticide residues analysis in fruits and vegetables, with a view on sample preparation steps, method optimization and validation of analytical figures of merit.

Ce0.85Sm0.15O1.925–Sm0.6Sr0.4Al0.3Fe0.7O3 dual-phase membranes: One-pot synthesis and stability in a CO2 atmosphere

As alternatives of single-phase mixed conducting materials, dual-phase materials have been suggested as candidates for application as oxygen separation membranes, since it is difficult to meet all the requirements in a single-phase membrane material. The influence of synthetic methods on the performance of the 75 wt.%Ce0.85Sm0.15O1.925-25 wt.%Sm0.6Sr0.4Al0.3Fe0.7O3 (SDC-SSAF) dual-phase membranes has been investigated. Three one-pot methods, i.e. the solid state reaction (SSR), EDTA-citrate complex (EC) and co-precipitation (CP) methods, were used to prepare the SDC-SSAF powder. The structure, surface morphologies, electrical conductivity, oxygen permeation, and stability in a CO2 atmosphere were investigated. It was found that the membrane derived from the SSR method shows the highest oxygen permeation flux and total conductivity. The significant differences between the performances of the dual-phase membrane derived from the different methods relates to the different microstructures developed during membrane preparation, which further influences the formation of a continuous electronic Conduction network across the membranes. The stability of the dual-phase membrane was studied by treating the membrane materials under a CO2 atmosphere and by sweeping the membrane with pure CO2. The results show that the membrane is CO2-stable and is potentially integrated with the oxyfuel process for CO2 capture. (C) 2013 Elsevier B.V. All rights reserved.

Ductile fracture initiated by interface nucleation in two-phase elastoplastic systems

Dual-phase materials often show damage nucleation along the interface between the two phases followed by void growth and coalescence. This failure mechanism is investigated using a microstructure model with both phases deforming plastically according to a physics-based hardening law. The interface is modeled as a cohesive zone whose constitutive behavior is described by a bi-linear traction–separation law. The interface is specified with a weak site where damage first initiates representing the presence of a small particle sitting along the interface. A parametric study is conducted through finite element unit cell calculations to examine the effect of the morphological and rheological factors on the complete ductile damage process. The mismatch of phase properties influences the local stress triaxiality evolution, which, in turn, significantly affects damage nucleation and interface decohesion involving possible crack arrest. The strength contrast appears to be more critical than the phase morphology in defining the ductility of typical two-phase metallic systems. The nucleation strain decreases by a factor of 10 when the imposed overall stress triaxiality increases from 0.67 to 2.0.

Three-dimensional simulations of microstructural evolution in polycrystalline dual-phase materials with constant volume fractions

The microstructural evolution of a polycrystalline dual-phase material with a constant volume fraction of the phases was investigated using large-scale three-dimensional phase-field simulations. All materials parameters are taken to be isotropic, and microstructures with volume fractions of 50/50 and 40/60 were examined. After an initial transient, the number of grains decrease from ∼2600 to ∼500. It was found that the mean grain size of grains of both phases obeyed a power law with an exponent of 3, and the microstructural evolution was found to be controlled by diffusion. Steady-state distributions of grain sizes and topology were determined. It was found that the grain size distributions were in good agreement with experimentally characterized size distributions for solid particles coarsening in a liquid matrix, and that the distributions of the number of faces were in good agreement with the topology of single-phase grain structures as determined by experiment and simulation. The evolution of size and number of faces for the minority and majority phase grains in the 40/60 volume fraction simulation is presented and discussed. Non-constant curvature across some interphase boundaries was observed, even though the interfacial energies are isotropic.

Nitrogen-sensitized dual phase titanate/titania for visible-light driven phenol degradation

A dual-phase material (DP-160) comprising hydrated titanate (H2Ti3O7·xH2O) and anatase (TiO2) was synthesized in a low-temperature one-pot process in the presence of triethylamine (TEA) as the N-source. The unique structure exhibits strong visible light absorption. The chromophore is linked to Ti–N bonds derived from both surface sensitization and sub-surface (bulk) doping. From transmission electron microscope (TEM) and textural studies by N2 physisorption, the composite exists as mesoporous particles with a grain size of ∼20nm and mean pore diameter of 3.5nm, responsible for the high surface area (∼180m2/g). DP-160 demonstrated photocatalytic activity in the degradation of phenol under visible light (λ>420nm). The activity of the composite was further enhanced by a small addition (0.001M) of H2O2, which also gave rise to some visible light activity in the control samples. This effect is believed to be associated with the surface peroxo-titanate complex. GC–MS analyses showed that the intermediate products of phenol degradation induced by visible light irradiation of DP-160 did not differ from those obtained by UV (band-gap) irradiation of TiO2. The overall performance of the composite is attributed to efficient excitation via inter-band states (due to N-doping), surface sensitization, improved adsorptive properties of aromatic compounds due to the N-carbonaceous overlayer, and the presence of heterojunctions that are known to promote directional charge transfer in other mixed-phase titanias like Degussa P25.

Modeling the Material Microstructure Effects on the Surface Generation Process in Microendmilling of Dual-Phase Materials

The anisotropic behavior of the material microstructure when processing multiphase materials at microscale becomes an important factor that has to be considered throughout the machining process. This is especially the case when chip-loads and machined features are comparable in size to the cutting edge radius of the tool, and also similar in scale to the grain sizes of the phases present within the material microstructure. Therefore, there is a real need for reliable models, which can be used to simulate the surface generation process during microendmilling of multiphase materials.This paper presents a model to simulate the surface generation process during microendmilling of multiphase materials. The proposed model considers the effects of the following factors: the geometry of the cutting tool, the feed rate, and the workpiece material microstructure. Especially, variations of the minimum chip thickness at phase boundaries are considered by feeding maps of the material microstructure into the model. Thus, the model takes into account these variations that alter the machining mechanism from a proper cutting to ploughing and vice versa, and are the main cause of microburr formation. By applying the proposed model, it is possible to estimate more accurately the resulting roughness because the microburr formation dominates the surface generation process during microendmilling of multiphase materials. The proposed model was experimentally validated by machining two different samples of dual-phase steel under a range of chip-loads. The roughness of the resulting surfaces was measured and compared to the predictions of the proposed model under the same cutting conditions. The results show that the proposed model accurately predicts the roughness of the machined surfaces by taking into account the effects of material multiphase microstructure. Also, the developed model successfully elucidates the mechanism of microburr formation at the phase boundaries, and quantitatively describes its contributions to the resulting surface roughness after microendmilling.

Towards a Phase Field Model of the Microstructural Evolution of Duplex Steel with Experimental Verification

A phase field model to study the microstructural evolution of a polycrystalline dual-phase material with conserved phase fraction has been implemented, and 2D simulations have been performed. For 2D simulations, the model predicts the cubic growth well-known for diffusion-controlled systems. Some interphase boundaries are found to show a persistent non-constant curvature, which seems to be a feature of multi-phase materials. Finally, it is briefly outlined how this model is to be applied to investigate microstructural evolution in duplex steel.

A clinically focused discussion of luting materials

A luting agent's primary function is to fill the minute void between an indirect restoration (definitive or provisional) and tooth (or implant abutment) and mechanically lock the restoration in place to prevent dislodgement during function. The purpose of this paper is to provide a clinically focused discussion on the broad spectrum of luting materials currently available to help the general practitioner make appropriate choices. Resins are typically formulated for a specific function or restoration and offer strength, aesthetics, flexible working times, and very low solubility yet are technique sensitive, expensive and often hard to clean-up. Glass-ionomers offer good strength and optical properties plus the potential for fluoride release/recharge but may have short working times, are sensitive to moisture or dehydration early on, and take time to fully set. Resin-modified glass-ionomers are hybrid, dual-phase materials which are manipulated like glass-ionomer but set quicker and are stronger. Zinc phosphate cement, used successfully for over a century to lute well-fitting metal and metal-ceramic definitive restorations, is a very inexpensive, rigid material which displays very high early compressive strength yet acidity and solubility can be problems. Polycarboxylate cement (a hybrid of zinc phosphate) has lower compressive strength but high tensile strength and may be less injurious to the pulp. Zinc oxide eugenol and zinc oxide non-eugenol cements typically have good sealing abilities but their relatively low compressive and tensile strengths, inherent brittleness, and high solubility limit usage to provisional restorations or implant supported crowns. Claims for multi-purpose or universal use by manufacturers can be somewhat confusing and overwhelming. Even so, the busy general practitioner must have sufficient knowledge to help choose an appropriate luting agent for each unique clinical situation.

Investigation of the Machining Behavior of Metal Matrix Composites (MMC) Using Chip Formation Simulation

The machining of metal matrix composites (MMC) induces cyclic loadings on tools, which creates new challenges for machining. In particular the distributed reinforcement, consisting of silicon carbide (SiC) or aluminum oxide (Al2O3), evokes especially high mechanical loads. The development of metal matrix composites is pointing towards higher fractions of reinforcements, which affects the resulting forces and temperatures. In this regard the influence of varying particle filling degrees, particle diameters, cutting velocities and tool geometries in terms of rake angle and cutting edge radius have been investigated by means of cutting simulation. For the process a self-designed continuous remeshing routine was used for which a dual phase material behavior has been implemented. The developed simulation model enables investigations of the machining behavior of metal matrix composites to the extent that ideal process strategies and tool geometries can be identified by multiple simulations.

Assessment of CO 2 stability and oxygen flux of oxygen permeable membranes

Oxygen permeation measurements on Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3− δ (BSCF), SrCo 1− x Nb x O 3− δ (SCN; x = 0.1, 0.2), Sr(Co 0.8Fe 0.2) 1− y Ti y O 3− δ (SCFT; y = 0.1, 0.2, 0.4), La 2NiO 4 (L2N), Ca 0.5Sr 0.5Fe 0.2Mn 0.8O 3− δ (CSFM) and the dual phase material Ce 0.8Sm 0.2O 1.9 + Ag (CS + Ag) were carried out using CO 2/N 2 mixtures as sweep gas. A breakdown of the O 2 permeation was observed for BSCF and SCN even at low CO 2 concentrations. A protective layer of Ce 0.8Gd 0.2O 1.9 on BSCF shifts the permeation breakdown to higher CO 2 concentrations but was insufficient for pure CO 2 sweep. SCFT samples show a stable O 2 permeation with CO 2 sweep, but only at temperatures of at least 900 °C. The O 2 permeation for CO 2 sweep drops down with time, especially at lower temperatures. The time dependency was fitted by a logarithmic regression to predict the total loss of the O 2 permeation. An excellent stability against CO 2 was found only for L2N, CSFM and Cs + Ag. However, the O 2 permeation values amounted to less than one order of magnitude below BSCF.

Mechanisms of void formation during tensile testing in a commercial, dual-phase steel

A detailed analysis of the microstructure and failure mechanism of a dual-phase steel material as a function of strain was conducted. Accordingly, three tensile tests were performed and interrupted at different strain levels in order to investigate void nucleation, void growth and void coalescence. Scanning electron microscopy analysis revealed that void nucleation occurs by ferrite grain-boundary decohesion in the neighborhood of martensite grains. Further, void initiation could be observed between closely situated martensite grains. Martensite morphology and distribution has a significant impact on the accumulation of damage. The mechanism of failure was found to be influenced by deformation localization due to microstructural inhomogeneity. Based on the experimental observations and simulation results, a model describing the failure mechanism is proposed for dual-phase steel material.

In-situ Neutron Diffraction Study on Work-hardening Behavior in a Ferrite-Martensite Dual Phase Steel

Strength and work-hardening in steels are discussed from the viewpoint of heterogeneous deformation. In-situ neutron diffraction studies made it clear that misfit strains between grains accompanied grain-scaled internal stresses (intergranular stress). In a dual phase steel, the intergranular stress was superposed on the phase stress. These results show good agreement with the predictions of a simple dual-phase material model: the strong martensite phase yields higher stress than the macro-yield stress, resulting in high strengthening of (ferrite + martensite) dual phase steels. Both long-range internal stress and short-range ones such as forest dislocation hardening may cause resistance to dislocation motion in the steels. Therefore, work-hardening takes place more effectively with higher internal stress and larger volume fraction.

Dual-Phase Materials Extrusion Process for Rapid Fabrication of Ceramic Dental Crown

A novel ceramic forming process, dual-phase materials extrusion process based on solid freeform fabrication (also known as rapid prototyping), has been developed. It is a kind of selective slurry extrusion (SSE) technique previously proposed in our group. The dual-phase materials extrusion machine was constructed with micro-extrusion system and 3-axial positioning system. To demonstrate the usability of this machine, a functionalized dental crown was directly fabricated with support material and dental material from 3D model (STL format) of a human tooth. The fabrication time is usually in 20-50 min.

Micro-Macro Description for Elastoplasticity of Materials with Lamellar Microstructure Using Anisotropic Eshelby Tensor

The elastoplastic behavior of dual-phase materials with lamellar microstructure is analyzed using a micro-macro approach. It involves the Hill's self-consistent scheme incorporating general anisotropic Eshelby tensor, and it can take into account both inherent and plastic deformation induced anisotropies. A high-precision numerical algorithm related to rotational ellipsoidal inclusion and anisotropic Eshelby tensors is developed. The elastoplastic behavior of both macroscopically anisotropic and isotropic materials is analyzed and compared with the result using isotropic Eshelby tensor. It shows that the difference between the result with an isotropic Eshelby tensor and that with an anisotropic Eshelby tensor is negligible if plastic deformation is not severe. The effect of the shape of the ellipsoidal lamellar inclusion on the material responses, the computation time and the convergence of numerical processes are also investigated. It shows that the convergence of the numerical process with a general anisotropic Eshelby tensor is as good as that with an isotropic Eshelby tensor, but for the former the computation time is much more. The shape of the ellipsoidal inclusion also strongly affects the computation time.

Porous pSiCOH Ultralow-k Dielectrics for Chip Interconnects Prepared by PECVD

Porous pSiCOH materials with ultralow dielectric constants (ulk) have been developed with dielectric constants reaching values below 2.0. The pSiCOH films, composed of Si, C, O, and H atoms, are prepared by plasma-assisted chemical vapor deposition (PECVD), showing that PECVD can be used to prepare porous materials. The pSiCOH films are deposited as dual-phase materials, and the porosity is formed in them by curing the as-deposited films by thermal anneals, electron beams, or UV irradiation. The porous pSiCOH dielectrics are suitable for integration in the interconnects of VLSI chips of the 45-nm-and-beyond technology nodes, and films with dielectric constants of 2.4 have already been implemented in upcoming 45-nm products. This review discusses the fabrication of pSiCOH films and the characterization of their structure, porosity, and electrical and mechanical properties. The limitations of some of the characterization techniques of these amorphous thin films are also addressed.

On the control of the liquid-phase distribution in multi-material assemblies processed by liquid-phase sintering

Proper control of composition gradients in components processed by liquid-phase sintering requires the control of the migration of the liquid phase during sintering. A model considering isotropic interface energies has been developed for expressing the variation of the solid–liquid interface area with liquid volume fraction during the final stage of liquid-phase sintering ( i.e. when residual porosity has disappeared). The model allows the computation of the driving forces for shape accommodation as a function of the dihedral angle ψ, liquid volume fraction, u, particle volume, V p, and average particle coordination, n c. It particularly enlightens the coupling between the effects of ψ and of n c. The model allows a new insight into the control of microstructural evolution during liquid-phase sintering of assemblies of dual-phase materials with different average particle size. Taking ψ = 0° for the WC–Co system, the model was found to predict with a good accuracy the distribution of Co at equilibrium in assemblies made by heat-treating at 1400 °C under protective atmosphere stacks of cylinders of WC–Co cermets differing in mean WC particle size and cobalt content.

A microstructure-based analysis of cyclic plasticity of pearlitic steels with Hill’s self-consistent scheme incorporating general anisotropic Eshelby tensor

A pearlitic steel is composed of numerous pearlitic colonies with random orientations, and each colony consists of many parallel lamellas of ferrite and cementite. The constitutive behavior of this kind of materials may involve both inherent anisotropy and plastic deformation induced anisotropy. A description of the cyclic plasticity for this kind of dual-phase materials is proposed by use of a microstructure-based constitutive model for a pearlitic colony, and the Hill’s self-consistent scheme incorporating anisotropic Eshelby tensor for ellipsoidal inclusions. The corresponding numerical algorithm is developed. The responses of pearlitic steel BS11 and single-phase hard-drawn copper subjected to asymmetrically cyclic loading are analyzed. The analytical results agree very well with experimental ones. Compared with the results using isotropic Eshelby tensor, it is shown that the isotropic approximation can provide acceptable overall responses in a much simpler way.