Low Interfacial Free Energy Research Articles

Alternating one-phase and two-phase crystallization mechanisms in octahedral patchy colloids.

Colloidal systems possess unique features to investigate the governing principles behind liquid-to-solid transitions. The phase diagram and crystallization landscape of colloidal particles can be finely tuned by the range, number, and angular distribution of attractive interactions between the constituent particles. In this work, we present a computational study of colloidal patchy particles with high-symmetry bonding-six patches displaying octahedral symmetry-that can crystallize into distinct competing ordered phases: a cubic simple (CS) lattice, a body-centered cubic phase, and two face-centered cubic solids (orientationally ordered and disordered). We investigate the underlying mechanisms by which these competing crystals emerge from a disordered fluid at different pressures. Strikingly, we identify instances where the structure of the crystalline embryo corresponds to the stable solid, while in others, it corresponds to a metastable crystal whose nucleation is enabled by its lower interfacial free energy with the liquid. Moreover, we find the exceptional phenomenon that, due to a subtle balance between volumetric enthalpy and interfacial free energy, the CS phase nucleates via crystalline cubic nuclei rather than through spherical clusters, as the majority of crystal solids in nature. Finally, by examining growth beyond the nucleation stage, we uncover a series of alternating one-phase and two-phase crystallization mechanisms depending on whether or not the same phase that nucleates keeps growing. Taken together, we show that an octahedral distribution of attractive sites in colloidal particles results in an extremely rich crystallization landscape where subtle differences in pressure crucially determine the crystallizing polymorph.

Low Interfacial Free Energy Describes the Bulk Ordering Transition in Colloidal Cubes.

Many hard faceted nanoparticles are known to undergo disorder-to-order phase transitions following a classical nucleation and growth mechanism. In a previous study [J. Phys. Chem. B 2018, 122, 9264-9273], it was shown that hard cubes undergo a nonclassical phase transition with a bulk character instead of originating from consolidated nuclei. Significantly, an unusually high fraction of ordered particles was observed in the metastable basin of the disordered phase, even for very low degrees of supersaturation. This work aims to substantiate the conjecture that these unique properties originate from a comparatively low interfacial free energy between the disordered and ordered phases for hard cubes relative to other hard particle systems. Using the cleaving wall method to directly measure the interfacial free energy for cubes, it is found that its values are indeed small; e.g., at phase coexistence conditions, it is only one-fifth that for hard spheres. A theoretical nucleation model is used to explore the broader implications of low interfacial tension values and how this could result in a bulk ordering mechanism.

The effect of grain boundary structure on chromium carbide precipitation in alloy 600

The aim of the present work is to study the effect of grain boundary structure on the precipitation of chromium carbides in Alloy 600. The material was processed by thermo-mechanical treatment consisting of hot rolling (at 1000 °C) and a short-time annealing treatment (for 10 min) to generate a high fraction of twins and disrupted random boundaries network. These samples were then given sensitization heat treatment at 704 °C for various times in the range 0.5–100 h, in order to introduce varying degrees of carbide precipitation at the interface regions, depending on the structure of grain boundaries and triple junctions. The characteristics of chromium carbides (identified as Cr23C6) precipitation on different kinds of grain boundaries and triple junctions were investigated using field emission scanning electron microscopy (FESEM) and electron backscatter diffraction (EBSD) based orientation imaging microscopy (OIM) techniques. The propensity for chromium carbides precipitation increased with increasing aging time. However, it was largely dependent on the structure of grain boundaries. Low angle grain boundaries (LAGBs) and low-Σ coincidence site lattice (CSL) boundaries were observed to retard the precipitation of carbides. In contrast, on random high angle grain boundaries (HAGBs), coarse and discrete carbides were observed. Amongst the low-Σ CSL boundaries, the deviation in misorientation was observed to have a profound effect on the precipitation of chromium carbides. Low-deviation twin (Σ3) boundaries (having misorientation deviation within 1°) were found to be strongly resistant against carbide precipitation. However, on high deviation twin and Σ9 boundaries, needle-like carbides evolved. Based on the results, it can be concluded that among the special (i.e., low-Σ CSL) boundaries, only low-deviation twin boundaries have sufficiently low interfacial free energy to completely subdue the precipitation of chromium carbides.

Performance Characterization of Laser Powder Bed Fusion Fabricated Inconel 718 Treated with Experimental Hot Isostatic Processing Cycles

Inconel 718 alloy fabricated by selective laser melting (SLM) (or laser powder-bed fusion (LPBF)) has been post-process heat-treated by stress-relief anneal at 1065 °C; stress-relief anneal (1065 °C) + solution treatment (at 720 °C) + aging (at 620 °C); hot isostatic pressing (HIP) (at 1120–1200 °C); stress-relief anneal + HIP; and stress-relief anneal + HIP + solution treatment + aging. Microstructure analysis utilizing optical metallography revealed primarily equiaxed grain structures (having average diameters ranging from ~30 to 49 microns) containing annealing twins, and a high concentration of carbide precipitates in all HIP-related treatments in the grain boundaries and intragrain regions. However, no precipitates nucleated on the {111} coherent annealing twin boundaries because of their very low interfacial free energy in contrast to regular grain boundaries. The mechanical properties for the as-fabricated Inconel 718 exhibited a yield stress of 0.64 GPa, UTS of 0.98 GPa, and elongation of 26%. Following stress-relief anneal at 1065 °C, the yield stress dropped to 0.60 GPa, while the elongation increased to 43%. The associated grain structure was an irregular, somewhat elongated, recrystallized structure. This structure was preserved at a stress anneal at 1065 °C + solution treatment + aging, but grain boundary and intragrain precipitation resulted in a doubling of the yield stress to 1.3 GPa and a reduced elongation of 12.6%. The results of HIP-related post-process heat treatments involving temperatures above 1060 °C demonstrated that the yield stress and elongations could be varied from 1.07 to 1.17 GPa and 11.4% to 19%, respectively. Corresponding Rockwell C-scale hardness values also varied from 33 for the as-fabricated Inconel 718 to 53 for simple post-process HIP treatment at 1163 °C.

On the Origin of Seemingly Nonsurface-Active Particles Partitioning between Phase-Separated Solutions of Incompatible Nonadsorbing Polymers and Their Adsorption at the Phase Boundary.

We have computed the free energy per unit area (i.e., interfacial tension) between a solid surface and two coexisting polymer solutions, where there is no specific interaction between the particles and either polymer, via self-consistent field calculations. Several different systems have been studied, including those where the two polymers differ in molecular weight (Mw) by a factor of ∼2 or where the polymers have the same Mw, but one set of chains is branched with the other linear. In the absence of any enthalpic contribution resulting from adsorption on the solid particle surface, the differences in the free energy per unit area resulting from the polymer-depleted regions around the particles in the two coexisting phases are found to be ∼1 μN m-1. Although this value may seem rather small, this difference is more than capable of inducing the partitioning of particles of 100 nm in size (or larger) into the phase with the lower interfacial free energy at the solid surface. By examining the density profile variation of the polymers close to the surface, we can also infer information about the wettability and contact angle (θ) of solid particles at the interface between the two coexisting phases. This leads to the conclusion that for all systems of this type, when the incompatibility between the two polymers is sufficiently large, θ will be close to 90°.

Systematic dependence of kinetic and thermodynamic barriers to homogeneous silica nucleation on NaCl and amino acids

Abstract

(Invited) High-Sn Concentration MOCVD-Grown Strained GeSn Thin Films Evaluated Using HAXPES and XRD Base on Synchrotron Technique

1. Introduction GeSn alloy is an attractive material as a high hole mobility channel substituting Si and as a stressor for strained channels. Furthermore, modulation of band structure from indirect to direct gap by the increasing Sn composition up to 6-10 % is expected to improve the performance of optical devices [1, 2]. Then, a high-quality GeSn thin film with higher Sn concentration is a promising for higher performance and integrated devices on the Si platform. However, it has been difficult to increase Sn concentration, because the solubility limit of Sn within a GeSn film is considered to be approximately 1 atomic % [3]. Hence, a growth technique which enables the suppression of Sn segregation during GeSn growth and a characterization to analyze the surface segregation occurred with a thickness of several nm, are essential. For these purpose, we have developed a metal-organic chemical-vapor-deposition (MOCVD) method with new source gases that is safe, uniform, and industrially applicable [4]. In addition, we also investigated a hard X-ray photoelectron spectroscopy (HAXPES) and X-ray diffraction (XRD) on synchrotron technique to analyze thin GeSn films. In this presentation, we introduce high Sn concentration GeSn thin film exceeding 6% prepared by combining the new MOCVD growth and synchrotron analysis methods. 2. Experimental Thin GeSn films with the thickness of typically 30-100 nm were grown on (001) Ge substrates at low temperature (320-360 degrees) by the MOCVD method using Ge (t-C4H9GeH3) and specially prepared Sn ((C2H5)4Sn) source gases. The Sb was doped from the atmosphere with residual triisopropyl-antimony [(i-C3H7)3Sb]. The target compositions of Sn were 2%, 3%, 6%, respectively. To clarify the crystallinity of GeSn films, HAXPES and XRD were carried out at SPring-8. Since the inelastic mean free path (IMFP) is several times deeper than that for conventional X-ray photoelectron spectroscopy (XPS), HAXPES is expected to be useful to identify simultaneously the variation of chemical state at a surface part and the underlying bulk part of a GeSn film [5]. The Sn and Sb concentrations were calculated by the combination of Rutherford back-scattering spectroscopy (RBS) and secondary ion mass spectrometry (SIMS) measurements. Cross-sectional TEM was also carried out for the evaluation of the crystal quality. 3. Results and discussion Initially, we observed the splitting of the Sn3d5/2 spectrum into two peaks for 3% Sn composition GeSn film by the HAXPES measurement, whereas the spectrum of a low Sn composition (2%) GeSn film remained single (Fig.1). The binding energy of the newly split peak (M2) was lower than that of the Sn3d5/2 peak (M1) and the peak position of M1 approximately coincided with that of the 2% GeSn film. To clarify the newly split Sn3d5/2 peak (M2), total reflection mode HAXPES (TR-HAXPES) measurement was also carried out for the 3% GeSn film. As a result, only a single Sn3d5/2 spectrum was observed by the measurement. Since the position of the observed peak by the TR-HAXPES, closely coincided with the split peak (M2), the M2 was identified as a peak derived from the Sn segregation formed at the film surface. Hence, it is concluded that depth profile characterization of the Sn chemical state within a GeSn film possible by combining the HAXPES and TR-HAXPES measurements. In order to further increase the Sn concentration, we employed Sb as a surfactant [6]. As a result, neither extreme change of XRD spectrum due to strain relaxation or asymmetric structure of HAXPES by the Sn segregation was observed for the newly grown GeSn film of 6% Sn. A decrease in the broad peak intensity on the high binding energy side due to surface oxidation was also confirmed. Since the surface is more stabilized by Sb with lower interfacial free energy, it is expected that suppression of Sn segregation and uniform Sn concentration in depth direction are simultaneously achieved. The concentration distribution of Ge and Sb were confirmed by the RBS and SIMS, and it was revealed that the Sn concentration was uniform in the depth direction, and the composition of Sn and Sb was 6.6 at.% and 0.5 at.%, respectively. Hence, by combining newly developed MOCVD technique and synchrotron technique, it was confirmed that uniform GeSn thin layer with higher Sn composition was realized. Reference [1] S.Zaima, JJAP 52 (2013) 030001, [2] R.Cjhen et al., APL 99, (2011) 181125, [3] C. Thurmond et al., J.Chem.Phys. 25,(1956)799, [4] K.Suda et al., ECS J.SSST, 4 (2015) 152. [5] K.Usuda et al., MRS spring meeting, (2016), EP11.6.10. [6] X. Yang et al., IEEE Photon.Tech.Lett.12,(2000) 128. Figure 1

Composition and automated crystal orientation mapping of rapid solidification products in hypoeutectic Al-4 at.%Cu alloys

Rapid solidification can produce metastable phases and unusual microstructure modifications in multi-component alloys during additive manufacturing or laser beam welding. Composition and phase mapping by transmission electron microscopy have been used here to characterize the morphologically distinct zones developing in hypoeutectic Al-4 at.% Cu alloy after pulsed laser melting for different crystal growth rate regimes. Deviations of the compositions of the alloy phases from equilibrium predictions and unique orientation relationships between the solidification transformation products have been determined. Specifically, for the columnar growth zone at solidification rates of 0.8ms−1< v <va=1.8ms−1, two distinct orientation relationships were established between the concomitantly forming non-equilibrium phases, supersaturated α-Al solid solution and the discontinuously distributed α-Al2Cu-based θ′-phase, which can be described as {110}θ∥ {001}α, [001]θ∥ [110]α and {001}θ∥ {001}α, [100]θ∥ [100]α. These orientation relationships permit formation of coherent interphase interfaces with low interfacial free energy. This endows a kinetic advantage to the thermodynamically less stable θ′-Al2Cu phase relative to the more stable equilibrium θ-Al2Cu phase during formation of the morphologically modified eutectic of the columnar growth zone grains, since repeated nucleation is required to establish the discontinuous distribution of θ′-Al2Cu phase.

Remineralization of partially demineralized dentine substrate based on a biomimetic strategy

Dentine remineralization is clinically significant for prevention and treatment of dentine caries, root caries, and dentine hypersensitivity. However, dentine remineralization is more difficult than enamel remineralization due to the abundant presence of organic matrix in dentine. The objective of this study was to develop a biomimetic method to facilitate remineralization of demineralized dentine through phosphorylation of type I collagen in demineralized dentine using sodium trimetaphosphate. The experimental results indicated that the effect of fluoride on remineralizing dentine was limited when residual mineral crystals were lacking on the surface of demineralized dentine, whereas the phosphorylation and Ca(OH)(2) pretreatment enhanced surface remineralization of the partially demineralized dentine. This biomimetic methodology resulted in favorable surface properties (i.e. highly negative charge and low interfacial free energy between substrate and aqueous medium) for crystal nucleation, and thus could be a promising method to remineralize superficially demineralized dentine lesions.

Structure-property interplay of poly(amide-imide) and TiO2 nanoparticles impregnated poly(ether-sulfone) asymmetric nanofiltration membranes

Poly(amide-imide) (PAI) and TiO2 nanoparticle-incorporated poly(ether-sulfone) (PES) asymmetric nanofiltration membranes with an integrally dense skin layer were prepared by diffusion induced phase separation. The prepared membranes were characterized by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), atomic force microscopy (AFM), pure water flux, water content and contact angle technique to investigate the influence of PAI on the properties of the membranes. Intermolecular interactions between the components in blend membranes were established by ATR-FTIR and microcrystalline morphology was confirmed by XRD. The SEM analysis showed that blend membranes have an integrally dense skin layer adequate for nanofiltration and a spongy sub-layer along the entire membrane cross section. The contact angle measurements indicated that, hydrophilicity of the virgin PES membranes was improved by the addition of PAI and TiO2 nanoparticles due to the preferential orientation of these components towards the membrane surface during immersion precipitation. Surface free energy parameters of the membrane such as surface free energy, interfacial free energy, work of adhesion and spreading coefficient were calculated. The efficiency of these membranes in the separation of mixture solutions of divalent salt and surfactant were found to be improved significantly. Fouling stability of the membranes studied by bovine serum albumin (BSA) as the model foulant revealed improved fouling resistance. Chlorine stability of the modified membranes was also investigated. From the results, it was revealed that low interfacial free energy membranes prepared by the incorporation of PAI and TiO2 nanoparticles may be valuable in fouling stability industrial separations.

Biomimetic deposition of calcium phosphate minerals on the surface of partially demineralized dentine modified with phosphorylated chitosan

The aim of this study is to remineralize partially demineralized dentine sections using phosphorylated chitosan (P-chi) based on mimicking the nucleating role of the phosphorylated noncollagenous proteins in the biomineralization of dentine. The surface of partially demineralized dentine sections, mainly composed of type I collagen, were modified by covalent immobilization of P-chi on the collagen surface. The dentine sections were subsequently put into remineralizing solution for remineralization, and then the remineralizing effect was investigated. The remineralization effect of this methodology was compared with that of fluoride. Moreover, the influence of surface properties of samples on remineralizing effect was investigated. The experimental results indicated that the effect of fluoride on remineralization of dentine was limited when residual crystals were lacking on the surface of partially demineralized dentine, whereas the covalent immobilization of P-chi can significantly induce deposition of calcium phosphate minerals on the surface of the partially demineralized dentine. This biomimetic methodology resulted in favorable surface properties (i.e., highly negative charge and low interfacial free energy between substrate and remineralizing medium) for crystal nucleation. Thus, P-chi can facilitate surface remineralization of dentine and thereby could find application in the minimally invasive management of dentine caries and dentine hypersensitivity.

Preparation, Morphology, Performance, and Hydrophilicity Studies of Poly(amide-imide) Incorporated Cellulose Acetate Ultrafiltration Membranes

Fouling-resistant cellulose acetate (CA) membranes were prepared by the phase inversion technique using hydrophilic poly(amide-imide) (PAI) as the modification agent. The prepared membranes were characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), pure water flux, water content, porosity, and contact angle technique to investigate the influence of PAI on the final properties of the membranes. Intermolecular interactions between the components in blend membranes were established by ATR-FTIR, and semicrystalline nature was confirmed by XRD. SEM analysis showed that blend CA membranes have a thinner top layer and higher porosity in the sublayer. AFM surface roughness analysis data substantiate the enhanced surface porosity with an increase in PAI content, while the mean pore size decreases. The contact angle measurements indicated that the hydrophilicity of the CA membranes was improved by the addition of PAI due to the preferential orientation of functional groups towards the surface. Moreover, the surface free energy parameters of the membrane such as surface free energy, interfacial free energy, work of adhesion, and spreading coefficient were calculated. From the results, it was revealed that low interfacial free energy membranes prepared by the incorporation of PAI may be valuable in fouling-resistant industrial separations.

Morphology, thermal behavior and mechanical properties of binary blends of compatible biosourced polymers: Polylactide/polyamide11

Blends of entirely biosourced polymers, namely polylactide (PLA) and polyamide11 (PA11), have been melt-compounded by twin-screw extrusion without the use of any compatibilizing agent. The crystallization and melting behavior, the morphology and mechanical properties of the melt-compounded binary blends have been investigated over the whole composition range. Albeit immiscibility prevails in all blends, the micronic and sub-micronic dispersion of the minor phase reveals a self-compatibilization behavior of the PLA/PA11 system as directly evidenced via scanning electron microscopy. For PLA compositions below 50%, PLA appears to be dispersed as globules in the PA11 matrix. With increasing PLA content beyond 50%, the blends exhibit co-continuous intertwined phases, then thread-like PA11 phase dispersed in a PLA matrix. For PLA composition greater than 80%, PA11 displays globular dispersion. In the case of PLA-rich blends, the crystallization of PA11 from the melt displayed a contribution of “homogeneous nucleation” corroborating the high degree of dispersion of the minor phase. However, the earlier crystallization of PA11 upon cooling did not promote that of PLA suggesting low interfacial free energy at the boundaries of the phase-separated domains, i.e. roughly neutral interactions between the unlike species. The non-symmetric structural behavior of the blends over the whole composition range was found to influence the mechanical properties. If the elastic modulus of the blends roughly obeys an additive mixture law at room temperature (RT), this is not the case above the Tg of both PLA and PA11. More particularly in the PLA-rich range, the thread-like dispersion of the stiff PA11 component plays the role of in situ fibrillar reinforcement of the soft PLA matrix. The strain at break and the yield stress also do not obey a simple mixture law, both at RT and above Tg. Perspectives of morphological and mechanical improvements of PLA/PA11 blends are discussed.

Computational study of cation influence on anion propensity for the air–water interface

Molecular dynamics simulations with polarizable interactions were carried out to investigate how cation charge and ion pairing influence the anion propensity for the air–water interface of water. Higher cation charge resulted in greater interfacial activity for chloride, along with stronger NaCl binding. It was also found that artificially weakening NaCl interactions without changing charge has only a minor influence on chloride interfacial activity. Furthermore, we found that for a single NaCl contact ion pair in water that it had a much lower interfacial free energy than chloride alone, showing that the presence of a cation enhances anion interfacial activity.

How Substrate Properties Control Cell Adhesion. A Physical–Chemical Approach

Most living cells derived from solid tissues require an adhering surface to live in vitro conditions. A good understanding of the relationships between the behavior of cells and the physicochemical properties of substrates such as the surface free energy, the surface polarity, the presence of functional groups and surface charges is of prime importance for the optimization of adhesion, spreading and proliferation of cells. Polystyrene and treated polystyrene surfaces were characterized by determining their surface free energies using wettability measurements. The knowledge of the surface properties of the culture substrates provides a good view of the influence of the substrate properties on cell adhesion. However, this study shows that it is not directly the surface free energy of materials that controls cell adhesion but rather the interfacial free energy between the culture medium and the substrate. The interfacial free energy between the culture medium and the solid surface controls the adsorption of serum components that may inhibit or promote cell adhesion. One of the components inhibiting cell adhesion is serum albumin. The results indicate that the adsorption of serum albumin is related to the interfacial free energy between the culture medium and the substrate. Hydrophilic substrates, such as plasma treated polystyrene substrates, have a lower interfacial free energy with water than hydrophobic polystyrene leading to a lower adsorption of proteins inhibiting cell adhesion. In addition, it is observed that there is a competition between proteins inhibiting (serum albumin) and proteins promoting cell adhesion (such as fibronectin).

Bi-seeded emulsion polymerization in the presence of poly(methyl methacrylate) and polystyrene

Bi-Seeded emulsion polymerization was investigated in the presence of polymethyl methacrylate (PMMA) and polystyrene (PSt). The experimental results indicate, when using styrene (St) as the second-stage monomer, the latexes had bimodal size distribution and the two kinds of seeds had competition for St. It is found that when the second-stage polymerization was carried out with large PSt latex particles and small PMMA latex particles as seeds, the PMMA seeds increased more evidently and more St polymerized on them. The possible reason is that the specific surface area of seeds decides the polymerization locus and St is apt to polymerize on smaller seeds. Nevertheless, when using large PMMA and small PSt as bi-seeds, the PMMA seed particles grew more obviously and showed a higher chance of obtaining more St than that of the PSt seed particles. The explanation is the formation of PMMA/PSt composite particles has lower interfacial free energy change, which has more significant effect on the polymerization locus of second-stage monomer than the specific surface area. Adding methods have little effect on the distribution of second-stage monomer in bi-seeded polymerization system, but through dropping method, it is easier to form composite particles with core-shell structure. In addition, anomalous particles were produced when adding crosslinker into St.

Surface characterization and blood compatibility of poly(ethylene terephthalate) modified by plasma surface grafting

Poly(ethlene terephthalate) (PET) used in artificial heart valve sewing rings were treated by argon plasma discharge and grafted by different molecular water-soluble polymer—polyethylene glycols (PEG). The surface properties of the plasma treated and grafted films were determined by contact angle measurement, X-ray photoelectron spectroscopy (XPS), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The contact angle of water decreased from 83.5° to 38.7° and the interfacial energy diminished from 30.7 to 6.3 mN/m. The ratio of oxygen to carbon increased from 0.15 to 0.25. All the results revealed that PEG chains were successfully grafted onto the surface of the PET films. The interaction between the surface-modified PETs and blood components was investigated to evaluate the blood compatibility of the samples. Activated partial thromboplastin time (APTT) of the PET-PEG was significantly longer than that of the untreated PET. Scanning electron microscopy (SEM) and optical microscopy indicated that adhered, aggregated and morphologically changed platelets were significantly reduced by grafting PEG chains onto the PET films. The in vitro blood compatibility tests suggested that the blood compatibility of PET grafted with PEG was related to the molecular weight of PEG. The best blood compatibility was achieved when the grafted molecular weight of PEG was 6000. The hemocompatibility improvement can be explained by the low interfacial free energy, sterically hindered effects of PEG chains, and maintaining of the normal conformation.

Effects of carbon content, deformation, and interfacial energetics on carbide precipitation and corrosion sensitization in 304 stainless steel

Utilizing transmission electron microscopy analysis of (110)/(110) grain boundaries in 304 stainless steel thin films prepared from specimens having 0.011, 0.025, 0.05, and 0.07% C, the M 23C 6 precipitate density was measured for grain boundary misorientations, Θ, for aging times of 10 and 50 h at 670°C, and for unstrained and 20% strained conditions. The density of precipitates in grain boundaries increased with increasing carbon content above 0.011% C and with increasing angles of misorientation. The propensity for precipitation also increased with straining to 20% and with increasing aging time, i.e. at high misorientation angles the sizes of precipitates increased with aging time. Essentially no precipitates were observed on grain boundaries at 0.011% C and no precipitates were observed on coherent ( Σ3) twin boundaries. Average energies at 670°C were calculated for random, high-angle grain boundaries, non-coherent twin boundaries, and coherent twin boundaries from experimental data and corresponding temperature coefficients. Precipitation observations suggested a critical or threshold energy requirement for precipitation. Correspondingly, a carbon content threshold also seems to exist for precipitation. It appears that of the special ( Σ) boundaries, only coherent twin ( Σ3) boundaries possess sufficiently low interfacial free energy to deter precipitation and sensitization, leading to the conclusion that only a twin-dominant grain boundary character could assure a significantly reduced corrosion sensitivity.

Silicide formation by solid-state diffusion in MO/Si multilayer thin films

The solid-state reaction of Mo/Si multilayer thin films produced by the r.f. magnetron sputtering technique was examined using differential scanning calorimetry (DSC) and X-ray diffraction and was explained by the concepts of effective driving force and effective heat of formation. In constant scanning-rate DSC, there were two exothermic peaks representing the formation of h-MoSi2 and t-MoSi2, respectively. The activation energy for the formation of h-MoSi2 was 1.5 eV, and that of t-MoSi2 was 7.8 eV. Nucleation was the rate-controlling mechanism for each silicide formation. The amorphous phase was not formed in the Mo/Si system as predicted by the concept of effective driving force. h-MoSi2, the first crystalline phase, was considered to have lower interfacial free energy than t-MoSi2, and by increasing the temperature, it was transformed into more stable t-MoSi2.

Surface nucleation of μ-cordierite in cordierite glass: thermodynamic aspects

The heterogeneous nucleation rate of μ-cordierite at the surface of cordierite glass (2MgO·2Al 2O 3·5SiO 2) is discussed in terms of classical nucleation theory (CNT). Calculations are based on experimental viscosity data and estimates of the melting temperature and melting enthalpy of μ-cordierite from calorimetric measurements. A virtual melting temperature of the metastable μ-cordierite, T μ M, was estimated, by means of X-ray measurements, from the primary occurrence of μ-cordierite after brief thermal treatment. T μ M was found to lie in the range 1300–1467°C. The molar enthalpy of melting, ΔH μ M, was calculated from calorimetric measurements and literature data. Referring to the molar weight of the formula unit, M = 585 g mol −1, ΔH μ M could be narrowed to the range 170–190 kJ mol −1. The heterogeneous volume nucleation rate, I∗(T), was estimated from the apparent nucleation rate of μ-cordierite which was previously studied at fractured and polished surfaces of cordierite glass by means of transmitting light microscopy. I∗(T) steadily increases up to 235 K above the glass transition temperature at 815°C without reaching a maximum. In the present paper, this exceptional temperature-dependence is discussed in terms of CNT. The best fit of experimental data was obtained for T μ M ≈ 1467°C, a low liquid-crystal interfacial free energy, σ, and a low activity of nucleating substrates, Ф, if σФ 1 3 ≈ 100 mJ m −2 is valid.