Coarsening Effects Research Articles

Fabrication and characterization of poly(l-lactic acid) 3D nanofibrous scaffolds with controlled architecture by liquid–liquid phase separation from a ternary polymer–solvent system

Poly(l-lactic acid) (PLLA) three-dimensional (3D) scaffold with macro/micropores and nanofibrous structure was fabricated by phase separation from a ternary PLLA/dioxane/water system. The pore size was mainly determined by the coarsening effects in the phase separation process, while the nanofibrous structure was due to the formation of PLLA microcrystallite domains in the gelation process. Increasing the gelation temperature or the content of water in the mixed solvent system, the pore size definitely increased and macropores up to 300 μm were observed. However, coalescence of nanofibers occurred, even platelet-like structure appeared at gelation temperatures higher than 12 °C or the proportion of water exceeded 12%. X-ray diffraction (XRD) and Differential Scanning Calorimetry (DSC) analyses demonstrated that the crystallinity degree increased with increasing the gelation temperature or the non-solvent volume ratio in the mixed system. Moreover, the results indicated that α′ was mainly corresponding to the nanofibers structure, while α crystal was detected in the platelet-like structure. Scanning electron micrograph (SEM) and methyl thiazolyl tetrazolium (MTT) assays indicated that the nanofibrous scaffold provided a better attachment and viability of MSCs (rat derived mesenchymal stem cells) than the platelet-like scaffold.

Investigation of sintering mechanisms of alumina using kinetic field and master sintering diagrams

Strain rates during sintering of alumina at different heating rates between 0.2 and 20 K/min have been measured non-contact using a thermooptical measuring technique. The shrinkage data were represented in a kinetic field diagram. It was shown that coarsening effects can be indirectly derived from the position and slope of iso-strain lines identified in the kinetic field. In addition temperature gradients which frequently lead to an erroneous interpretation of shrinkage kinetics were identified. Uni-axial loads up to 6 MPa were applied during sintering. The strain rates were measured parallel and perpendicular to the load. They were processed in separate kinetic field diagrams. It was shown that creep leads to a parallel shift of the iso-strain lines either to lower temperatures, for the strain rates parallel to the load, or to higher temperatures, for the strain rates perpendicular to the load. The individual kinetic field diagrams for a specific load were assembled in 3d, with the load used as z-axis. They formed a master sintering diagram (MSD) where the iso-strain lines were replaced by iso-strain surfaces. The parallel shift of the iso-strain lines with load enabled a simple representation of sintering kinetics for various loads during the entire range of densification, temperatures and heating rates.

Immobilization of hydrothermally produced TiO2 with different phase composition for photocatalytic degradation of phenol

Hydrothermally produced TiO2 powders with different phase composition (anatase, rutile and mixed phase) were immobilized on glass fibers and tested in the phenol mineralization process. Both H2O2 and O2 were used as oxygen donors, and their performances were compared with those of the same TiO2 samples as slurries.The catalytic properties of the immobilized different crystalline phases, rutile and anatase, show the same trend as the slurry samples: pure rutile displays the highest catalytic efficiency in the presence of H2O2, while samples containing anatase improve the photodegradation efficacy with O2. It was suggested that the stability of the photogenerated electron–hole couple allows high activity of rutile in the presence of H2O2, while the relevant oxygen chemisorption on anatase causes high catalytic activity in the presence of O2. A four parameters kinetics model shows that both reaction steps, the phenol degradation and the mineralization of the intermediates, are photoactivated by TiO2.Photoactivity of the coated glass fibers is generally lower than that of slurries, even if their efficiencies are almost comparable when the oxidation is performed by H2O2, while much lower when the oxygen donor is O2. As a matter of fact, the morphology of immobilized catalysts shows the presence of chestnut burr aggregates of large rutile crystalline rods on the glass fiber, which are much less compact than the aggregates of small anatase particles. This preserves rutile surface area from the coarsening effects; thus, when rutile is the more active species, as in the presence of H2O2, the photocatalytic activity is less affected by immobilization.

Phase diagram of the Holstein polaron in one dimension

The behavior of the 1D Holstein polaron is described, with emphasis on lattice coarsening effects, by distinguishing between adiabatic and nonadiabatic contributions to the local correlations and dispersion properties. The original and unifying systematization of the crossovers between the different polaron behaviors, usually considered in the literature, is obtained in terms of quantum to classical, weak coupling to strong coupling, adiabatic to nonadiabatic, itinerant to self-trapped polarons and large to small polarons. It is argued that the relationship between various aspects of polaron states can be specified by five regimes: the weak-coupling regime, the regime of large adiabatic polarons, the regime of small adiabatic polarons, the regime of small nonadiabatic (Lang-Firsov) polarons, and the transitory regime of small pinned polarons for which the adiabatic and nonadiabatic contributions are inextricably mixed in the polaron dispersion properties. The crossovers between these five regimes are positioned in the parameter space of the Holstein Hamiltonian.

STRING TENSION AND REMOVAL OF LATTICE COARSENING EFFECTS IN MONTE CARLO RENORMALIZATION GROUP

We study the computation of the static quark potential under decimations in the Monte Carlo Renormalization Group (MCRG). Employing a multirepresentation plaquette action, we find that fine-tuning the decimation prescription so that the MCRG equilibrium self-consistency condition is satisfied produces dramatic improvement at large distances. In particular, lattice coarsening (change of effective lattice spacing on action-generated lattices after decimation) is nearly eliminated. Failure to correctly tune the decimation, on the other hand, produces large coarsening effects, of order 50% or more, consistent with those seen in previous studies. We also study rotational invariance restoration at short distances, where no particular improvement is seen for this action.

Densification maps for spark plasma sintering of nanocrystalline MgO ceramics: Particle coarsening and grain growth effects

Recently densification maps for spark plasma sintering (SPS) of nanocrystalline MgO were constructed using a HIP model and constant particle size during the process. Here, the effects of particle coarsening during the early stage and grain growth during the later stage of densification were integrated into the model. Densification diagrams were calculated between 500 °C and 900 °C, pressures range 30–500 MPa and initial particle size 10–40 nm. Plastic yield and diffusion process dominated during the densification in SPS. The power law creep exhibited negligible densification rates. Threshold pressure exists for full densification of nanocrystalline compact with minimal grain growth. Particle coarsening and grain growth resulted in densification patterns that follow the experimental SPS results. The calculated densities were lower than the experimental results and fall short in describing the experimental densification rates of nanocrystalline MgO. Therefore additional densification mechanisms with faster kinetics should be incorporated.

Numerical Approaches for microscopic modelling of solute redistribution during solidification of binary alloys

In the present study, two numerical approaches for single-domain modelling of microsegregation during solidification of binary alloys are presented. In the first approach, the concentration jump at the moving solid/liquid interface is formulated using a volumetric term and a Boolean function. The governing solute redistribution equation, valid for the whole domain comprising the solid and liquid regions, is derived in terms of the liquid phase composition. The effects of microstructure coarsening on microsegregation has been described and included in the model. In the second approach, the continuum mixture theory is utilized to derive a single domain solute redistribution equation in terms of the mixture composition. The solidification front motion and dendrite arm coarsening effects are accommodated by considering the representative elementary volume to consist of solid, interdendritic, and extradendritic liquid phases. Numerical solutions have been obtained using a control-volume based finite-difference method with a fixed grid. Good agreement has been observed between the predictions of the present fixed-domain models and the exact analytical and experimental results.

Aging in the ferroic random-field Ising model system strontium-barium niobate

Large aging effects, investigated via temporal dependences of the dielectric response, are found to be inherent in the uniaxial ferroelectric relaxor systems $\mathrm{SBN}:\mathrm{Cr}$ and $\mathrm{SBN}:\mathrm{Ce}$. The results are discussed on the basis of domain coarsening and domain-wall pinning effects. Owing to the different charge disorder induced by the ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ dopants, a fundamental difference between both systems is established. While $\mathrm{SBN}:\mathrm{Ce}$ shows only a weak breaking of ergodicity, the $\mathrm{SBN}:\mathrm{Cr}$ system experiences true ergodicity breaking.

Macroporous poly( l-lactide) of controlled pore size derived from the annealing of co-continuous polystyrene/poly( l-lactide) blends

A detailed study on the static annealing of co-continuous polystyrene/poly( l-lactide) (PLLA) blends is presented. The effects of temperature, time at temperature, viscosity of the phases and interfacial modification on the coarsening of the blend are discussed. In this paper, polystyrene and PLLA are blended at compositions of 50/50 and 60/40 to form co-continuous morphologies. These co-continuous morphologies are coarsened under quiescent annealing conditions, and the subsequent removal of the polystyrene phase leaves a macroporous PLLA structure. The microstructure is analyzed using three different techniques: the BET nitrogen adsorption technique, mercury intrusion porosimetry and SEM combined with image analysis. It is shown that static annealing can be used to generate a series of co-continuous networks with controlled pore sizes ranging from 1 to hundreds of microns. A non-linear pore size growth rate is observed for these systems due to the degradation of PLLA and this study indicates that controlled degradation can be used as an additional tool for morphology control. Compatibilized polystyrene/PLLA blends demonstrate significantly reduced coarsening effects due to the reduction of interfacial tension. The coarsening rate of the co-continuous structure was examined in terms of the pore size, R and this growth rate is discussed in terms of a previously proposed coarsening mechanism. This approach is a route towards the preparation of a macroporous PLLA structure with pore sizes in the range required for scaffolds for tissue regeneration.

Fracture toughness of alumina and ZTA ceramics: microstructural coarsening effects

The influence of zirconia particles addition and microstructural coarsening, as a result of different heat treatments, on the fracture toughness of a monolithic alumina and a zirconia-toughened-alumina (ZTA) composite is studied. It is observed that addition of zirconia particles results in an enhancement of fracture toughness with respect to that exhibited by the alumina matrix. Similarly, microstructural coarsening within ZTA is found to produce a fracture toughness increment, mainly associated with the effect of zirconia particle size on their phase transformability. The obtained mechanical properties and grain growth kinetics are discussed in terms of microstructural features and operative toughening mechanisms.

Effect of heat treatment on the electrical resistivity of near-eutectic Sn-Ag-Cu Pb-Free solder alloys

The electrical resistivity of solder joints prepared from Sn-Ag, Sn-Ag-Cu, and Sn-Ag-Cu-X alloys (where X = Co, Fe, or Bi) was characterized by a four-point probe technique and interpreted in terms of microstructure and composition. The resistivity is also reported of drawn solid wires of these alloys. The solder-joint samples were prepared by hand soldering to copper substrates and were electrically characterized over a temperature range from 293–423 K, covering the anticipated range of elevated-temperature operation for Pb-free solders. Selected joint specimens were measured before and after a 72-h heat treatment at 423 K. Metallographic inspection of the solder joints was performed to characterize coarsening effects and to determine the degree to which these changes affected electrical conduction.

Computational and experimental investigation of microsegregation in an Al-rich Al–Cu–Mg–Si quaternary alloy

A new micromodel was developed to predict the microstructure and microsegregation in multicomponent alloys during dendritic solidification. The micromodel was directly coupled with multicomponent phase diagram calculations using a user friendly and robust phase diagram calculation engine—PanEngine. Solid back diffusion, undercooling and coarsening effects were included in this model, and the experimentally measured cooling curves were used as the inputs to carry out the microsegregation calculations. Microsegregation in Al–4.5 wt%Cu–1 wt%Si–0.5 wt%Mg alloy was experimentally investigated from directional solidification and electron probe microanalysis. Calculated results using this model are in accord with the experimental data, while results from the Scheil model deviate significantly from the experimental data.

Acoustic Emission Analysis for Fatigue Prediction of Lap Solder Joints in Mode Two Shear

This current research method combined the disciplines of Fractography, ConFocal analysis, and Acoustic Emission to discover and identify failure modes of eutectic (62%Sn36%Pb2%Ag) solders (with and without rosin core flux) and lead free (99CSn-Cu) solder under cyclic displacement controlled Mode II shear fatigue. Results show a strong pattern of acoustic emission correlating to stages of failure. Three distinct stages were found: “Stage I” (crack initiation), “Stage II” (nucleation), and “Stage III” (failure and/or propagation). Time domain analysis shows distinct waveforms for each stage of fatigue. The frequency domain analysis shows a distinct pattern in the PSD. The combination of these two trends allows for a basis to predict solder joint fatigue in an acoustical manner utilizing acoustic emissions. Fractographic results support the trends seen in the acoustic emission analysis for each stage of fatigue. Dominant failure modes of specimens appeared to be intermetallic crack growth, grain coarsening, and dislocation energy effects. These failure modes can be explained in both the time and frequency responses of these specimens. The conclusion is that utilizing acoustic emissions with the disciplines of Acoustic Emissions, Fractography, ConFocal analysis has uncovered and identified failure modes of eutectic (62%Sn36%Pb2%Ag) solders (with and without rosin core flux) and lead free (99CSn-Cu) solder under cyclic displacement controlled Mode II shear fatigue.

Sintering Effects on the Porous Characteristics of Functionally Gradient Ceramic Membrane Structures

A method to drain cast porous ceramics has been conceived and established, where samples were shown to have a functionally gradient cross-section with a continuously increasing mean particle size between the two principal surfaces.Ceramic discs approximately 45 mm in diameter, and 3.3 mm thick were cast by sedimentation. These green bodies were dried prior to sintering. Maximum sintering temperature and the length of the sintering soak time were varied for samples made from suspensions of both 5 and 10 volume percent solids. Mercury porosimetry was used to obtain the porosity and pore size distribution in each sample. Additionally, a number of atomic force microscopy (AFM) measurements were made on some samples in order to correlate bulk porous properties with those on the outside surfaces.The results showed that as the sintering temperature increased, the densification of the bodies proceeded more rapidly. In general, the longer the sintering soak time, the denser the samples became as well. For the samples prepared at the lower temperatures however, the porosity showed less of a soak time dependence. The green body had a clustered and asymmetric microstructure, which contributed to differing degrees of localized densification and coarsening effects depending on the sintering temperature. Densification effects were more pronounced with the samples made from the more concentrated suspenisions.There was an inverse correlation between the bulk and surface pore dimensions, attributable to the different size scales of particles in the two regions. The much finer surface layer particles were able to undergo some amount of densification while surface diffusion sintering mechanisms were primarily at work elsewhere in the structure.

Precipitate size refinement by CeO2 and Y2BaCuO5 additions in directionally solidified YBa2Cu3O7

Directional solidification of YBa2Cu3O7 has been carried out through a Bridgman technique, and the influence of Y2BaCuO5 and CeO2 additives on the size of Y2BaCuO5 precipitates has been investigated. It is demonstrated in this work that the most efficient procedure to reduce the size of the Y2BaCuO5 precipitates is to increase the concentration of nucleation centers present in the peritectic decomposition of YBa2Cu3O7−x. A small concentration (0.3−1 wt. %) of CeO2 has a strong influence on the solidification process and on the size of Y2BaCuO5 precipitates. It is shown that when CeO2 is added, further refinement of the size of precipitates results from the formation of nanometric Y2O3 particles which further enhance the multinucleation effect. We have also observed that coarsening effects are avoided with CeO2 additives.

Coarsening effects on the formation of microporous membranes produced via thermally induced phase separation of polystyrene-cyclohexanol solutions

The effects of coarsening on microstructure formation in highly viscous polystyrene-cyclohexanol solutions and membranes made from them were studied by scanning electron microscopy and mercury intrusion porosimetry. Using thermally induced phase separation and a freeze-drying technique, it was demonstrated that the polymer membrane microstructure can be tailored by controlling the quench route and coarsening time. For systems undergoing phase separation by spinodal decomposition, resulting in a well-interconnected, microporous structure with nearly uniform pore sizes, it was found that extending the phase separation time prior to freezing and solvent removal can result in a significant increase in pore or cell size which is highly dependent on both quench depth and coarsening time. The coarsening rate of the cell size can be expressed as a power law in time. For relatively deep quenches, the initial growth-rate exponent has a value of 1 3 in agreement with classical theories for coarsening by Ostwald ripening or coalescence, while for shallow quenches smaller exponents were observed, in agreement with studies involving isopycnic polystyrene-diethyl malonate systems. At longer coarsening times, a crossover to a much faster growth rate was observed yielding an exponent of 1.0, consistent with the expectations for the hydrodynamic flow mechanism of coarsening. Novel, complex microporous membrane structures with pore sizes of two characteristic length scales were also produced in this system using a two-step temperature jump process.

Coarsening Effects on Microstructure Formation in Isopycnic Polymer Solutions and Membranes Produced via Thermally Induced Phase Separation

The phase separation of isopycnic polystyrene-diethyl malonate solutions has been studied by investigating the microstructure of polymer membranes. Polymer solutions underwent spinodal decomposition and coarsening via a thermally induced phase separation procedure, and supercritical CO 2 extraction was employed to remove solvent, resulting in microporous membranes. At relatively short coarsening times, the coarsening rate of the cell size can be expressed as a power law in time with the exponent increasing with increasing quench depth; for deep quenches, the growth rate has an exponent of 1/3 in agreement with the classic theories for coarsening by Ostwald ripening or coalescence

A general criterion for high-cycle multiaxial fatigue failure: McDiarmid, D.L. Fatigue Fract. Eng. Mater. Struct. 1991 14, (4), 429–453

Transient Liquid Phase Bonding (TLPB) makes use of the isothermal solidification of a liquid interlayer through the formation of intermetallic compounds (IMC). This work investigates the Ag3Sn IMC formation in a thin, electroplated Ag–Sn interlayer system appropriate for TLPB – while covering as-coated conditions, growth initiation up to a few minutes of holding time – with experimental and numerical methods. Both solid-state and solid–liquid reactions at 200, 250 and 300 °C, respectively, were covered under identical and near-isothermal conditions with a novel experimental setup. Morphological aspects like the lateral IMC grain size or the scallop roughness were proven to strongly depend on temperature. Growth kinetics were captured by power laws, growth exponents n were determined as 0.24, 0.36 and 0.29, parabolic growth constants k as 1.32 ⋅ 10−11, 4.04 ⋅ 10−10 and 7.24 ⋅ 10−10 cm2/s at 200, 250 and 300 °C, respectively, and the activation energy Q as 30.6 kJ/mol. Predominant diffusion mechanisms, i.e. volume or GB diffusion, as well as coarsening effects were identified as temperature and time dependent at early stages. A mathematical model was developed based on the Arrhenius relationship including a growth correction function zIMC,corr to calculate and predict IMC growth as a function of temperature and time. The study was completed by multiphase-field simulations which targeted to mimic kinetic and morphological features of Ag3Sn IMC under various conditions. Good agreement between experiments and simulations was found for a limited set of parameters which allowed drawing conclusions about the dominant transport mechanisms for the reacting elements.

Defect generation in LPE garnet films during annealing

AbstractThe defects induced in LPE garnet films during annealing processes near 1300°C were investigated by X‐ray topography, etching, scanning electron microscopy and Nomarski interference microscopy. The results show that besides loops and more complicated dislocation configurations point defects play an essential role for the stress relaxation mechanism in the film. The observed coarsening effects on the film surface are due to the high surface free energy of {111} faces on garnets. A consistent explanation of the various experimental observations is given by assuming the creation of oxygen vacancies during annealing.

Grain coarsening effects of chromium and iron in Au-Ag-Cu alloys

Chromium and iron in concentrations as low as 0.3–0.5 per cent have been found to cause a marked coarsening of the microstructure of 18 carat Au-Ag-Cu alloys on annealing. Where minimum grain growth during annealing is required, as in the fabrication of rings and chains, the concentrations of these metals should therefore be kept at a low level.