Boundary Composition Research Articles

The structure and composition of olivine grain boundaries: 40 years of studies, status and current developments

Interfaces in rocks, especially grain boundaries in olivine dominated rocks, have been subject to about 40 years of studies. The grain boundary structure to property relation is fundamental to understand the diverging properties of polycrystalline samples compared to those of single crystals. The number of direct structural observations is small, i.e. in range of 100 micrographs, and the number of measurements of properties directly linked to structural observations is even smaller. Bulk aggregate properties, such as seismic attenuation, rheology and electrical conductivity, are sensitive to grain size, and seem to show influences by grain boundary character distributions. In this context we review previous studies on grain boundary structure and composition and plausible relations to bulk properties. The grain boundary geometry is described using five independent parameters; generally, their structural width ranges between 0.4–1.2 nm and the commonly used 1 nm seems a good approximation. This region of enhanced disorder is often enriched in elements that are incompatible in the perfect crystal lattice. The chemical composition of grain boundaries depends on the bulk rock composition. We determined the 5 parameter grain boundary character distribution (GBCD) for polycrystaline Fo $$_{90}$$ and studied structure and chemistry at the nm-scale to extend previous measurements. We find that grain boundary planes close to perpendicular to the crystallographic c-direction dominate the grain boundary network. We conclude that linking grain boundary structure in its full geometric parameter space to variations of bulk rock properties is now possible by GBCD determination using EBSD mapping and statistical analyses.

Impact of Ni content on the thermoelectric properties of half-Heusler TiNiSn

Thermoelectric properties vary dramatically between two boundary compositions of Ni-rich and Ni-poor TiNiSn, up to 25% in zT.

Probing local structure of the morphotropic phase boundary composition of Na0.5Bi0.5TiO3–BaTiO3 using rare-earth photoluminescence as a technique

(1-y)Na0.5Bi0.5TiO3-(y)BaTiO3 (NBT-BT) is one of the most investigated lead-free piezoelectric system in the recent past. Unlike the conventional piezoelectric alloys wherein the morphotropic phase boundary (MPB) compositions exhibiting maximum piezoelectric response, exhibit coexistence of ferroelectric phases with different symmetries on the global scale, the MPB composition (y = 0.06) of NBT-BT shows a cubic-like phase in the unpoled state. On poling it transforms to a non-cubic ferroelectric phase. Here we exploit the sensitivity of the photoluminescence (PL) property of doped rare-earth ions with regard to the local symmetry of the host crystal to investigate the local structure of the cubic-like phase of the MPB composition of NBT-BT. We performed a comparative study of the PL response by doping separately Er and Eu in very dilute concentration in the (i) MPB composition (y = 0.06) exhibiting a cubic-like phase, (ii) a sub-MPB composition (y = 0.03) exhibiting rhombohedral phase, and (iii) above the MPB composition (y = 0.10) exhibiting tetragonal phase. We found that both the Er and the Eu PL spectra of the MPB composition (y = 0.06) exhibits more number of Stark lines in its unpoled cubic-like phase as compared to that in the field-stabilized rhombohedral phase. The additional Stark lines in the cubic-like global phase are identified to be that of the tetragonal structure. Our study therefore confirms that what appears as a cubic-like phase on the global scale has intimately connected tetragonal and rhombohedral local structures. The success of our experiments suggests that rare-earth PL can be used as a simple yet powerful tool to investigate local structures in scenarios wherein the global structure need not comply with the local structure.

Effect of local chemical composition of grain boundaries on corrosive resistance and mechanical properties of ultrafine-grained titanium alloys

Effect of local chemical composition of grain boundaries on corrosive resistance and mechanical properties of ultrafine-grained titanium alloys

Influence of grain boundaries on the distribution of components in binary alloys

Based on the free-energy density functional method (the Cahn–Hilliard equation), a phenomenological model that describes the influence of grain boundaries on the distribution of components in binary alloys has been developed. The model is built on the assumption of the difference between the interaction parameters of solid solution components in the bulk and at the grain boundary. The difference scheme based on the spectral method is proposed to solve the Cahn-Hilliard equation with interaction parameters depending on coordinates. Depending on the ratio between the interaction parameters in the bulk and at the grain boundary, temperature, and alloy composition, the model can give rise to different types of distribution of a dissolved component, namely, either depletion or enrichment of the grain-boundary area, preferential grainboundary precipitation, competitive precipitation in the bulk and at the grain boundary, etc.

Characteristics of martensitic and strain-glass transitions of the Fe-substituted TiNi shape memory alloys probed by transport and thermal measurements

The electrical resistivity, Seebeck coefficient, thermal conductivity, and specific heat of Ti50Ni50-xFex (x = 2.0–10.0 at.%) shape memory alloys (SMAs) were measured to investigate the influence of point defects (Fe) on the martensitic transformation characteristics. Our results show that the Ti50Ni48Fe2 and Ti50Ni47Fe3 SMAs have a two-step martensitic transformation (B2 → R and R → B19′), while the Ti50Ni46Fe4, Ti50Ni44.5Fe5.5, and Ti50Ni44Fe6 SMAs display a one-step martensitic transition (B2 → R). However, the compounds Ti50Ni42Fe8 and Ti50Ni40Fe10 show strain glass features (frozen strain-ordered state). Importantly, the induced point defects significantly alter the martensitic transformation characteristics, namely transition temperature and width of thermal hysteresis during the transition. This can be explained by the stabilization of austenite B2 phase upon Fe substitution, which ultimately leads to the decrease in enthalpy that associated to the martensitic transition. To determine the boundary composition that separates the R-phase and strain glass systems in this series of SMAs, a Ni-rich specimen Ti49Ni45Fe6 was fabricated. Remarkably, a slight change in Ti/Ni ratio converts Ti49Ni45Fe6 SMA into a strain glass system. Overall, the evolution of phase transformation in the Fe-substituted TiNi SMAs is presumably caused by the changes in local lattice structure via the induced local strain fields by Fe point defects.

Influence of microstructural characteristics on ionic conductivity of ceria based ceramic solid electrolytes

Solid electrolyte powders based on gadolium-doped and samaria-codoped ceria of compositions Ce0.8Gd0.2−xSmxO1.9 (x = 0.00; 0.01; 0.03; 0.05) were sintered using the polymeric precursor method (Pechini). The X-ray diffractometry results confirmed the formation of a single crystalline phase, that is, a solid solution. Test specimens were compacted using uniaxial cold pressing followed by two-stage sintering. Relative densities were higher than 96% of theoretical density in all the samples. Ionic conductivity was determined using impedance spectroscopy, obtaining values of 10−2Scm−1 at 700°C, with the best results for codoped electrolytes, primarily the Ce0.8Gd0.15Sm0.05O1.9. system. This study aimed at correlating ionic conductivity variations of codoped electrolytes with microstructural alterations (on a nanometric scale) resulting from the addition of codopants. Energy dispersive spectroscopy (EDS) was used to analyze the grain boundary compositions of the electrolytes produced and assess possible dopant segregation in these regions. Interplanar spacings corresponding to the crystallographic planes of the cubic (fluorite-type) phase were identified using high-resolution transmission electron microscopy (HRTEM) images. Extra diffraction spots and diffuse stains were observed in the doped and codoped electrolytes, using selected area electron diffraction (SAED), confirming the presence of amorphous nanodomains at the atomic level. However, the codoped system exhibited diffraction patterns with no diffuse stains, indicating that codoping favored obtaining nanodomain-free regions. These regions displayed better structural homogeneity, which may explain the enhanced ionic conductivity in codoped systems.

Investigation of thermally sensitised stainless steels as analogues for spent AGR fuel cladding to test a corrosion inhibitor for intergranular stress corrosion cracking

A small proportion of irradiated Advanced Gas-cooled Reactor (AGR) fuel cladding can be susceptible to intergranular stress corrosion cracking (IGSCC) when stored in pond water containing low chloride concentrations, but corrosion is known to be prevented by an inhibitor at the storage temperatures that have applied so far. It may be necessary in the future to increase the storage temperature by up to ∼20 °C and to demonstrate the impact of higher temperatures for safety case purposes. Accordingly, corrosion testing is needed to establish the effect of temperature increases on the efficacy of the inhibitor. This paper presents the results of studies carried out on thermally sensitised 304 and 20Cr-25Ni-Nb stainless steels, investigating their grain boundary compositions and their IGSCC behaviour over a range of test temperatures (30–60 °C) and chloride concentrations (0.3–10 mg/L). Monitoring of crack initiation and propagation is presented along with preliminary results as to the effect of the corrosion inhibitor. 304 stainless steel aged for 72 h at 600 °C provided a close match to the known pond storage corrosion behaviour of spent AGR fuel cladding.

Segregation-induced ordered superstructures at general grain boundaries in a nickel-bismuth alloy.

The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains. This discovery shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.

Giving grain boundaries more structure

Metallurgy The properties of metals change depending on the composition and structure of grain boundaries in polycrystalline materials. Yu et al. discovered a surprising grain boundary superstructure in a nickel-bismuth alloy. Previously, the structure was only known to exist in a specific type of uncommon grain boundary, and experiments had focused on bicrystals. Unexpectedly, this alloy has grain boundary superstructures across a wide range of boundaries in polycrystalline samples. This likely also occurs in other alloys, which opens an avenue for grain boundary engineering to tune the physical properties of metals and ceramics. Science , this issue p. [97][1] [1]: /lookup/doi/10.1126/science.aam8256

Phase transitional behavior in lead-free (1-x)(Li0.12Na0.88)NbO3-xBaTiO3 (0 ≤ x ≤ 0.40) ferroelectric ceramics

ABSTRACT(1-x) Li0.12Na0.88NbO3-xBaTiO3 (0.0 ≤ x ≤ 0.40) ferroelectric ceramics were prepared using conventional ceramics route and their phase transitional behavior is investigated by using dielectric spectroscopy. The phase and microstructure analysis confirm a homogeneous solid solution and an existence of morphotropic phase boundary (MPB) composition near x = 10–12.5. A partial phase diagram has been proposed based on temperature-dependent dielectric permittivity ϵ′(T) studies. The observed diffused phase transition in ϵ′(T) was characterized by an acceptable and competent characterizing parameter (D), defined by Uchino et al. [J Am Ceram Soc 2010;93:4011], was measured and validated. Interestingly, a crossover from ferroelectric (FE) diffused phase transition (DPT) to relaxor ferroelectric (RFE) transition is found for the composition x ≥ 0.225. The ferroelectric-diffused phase transition (FE-DPT) is characterized by a frequency-independent temperature of dielectric maxima (Tm), while an RFE is found to have a frequency-dependent Tm.

Ferroelectric, upconversion emission and optical thermometric properties of color-controllable Er3+-doped Pb(Mg1/3Nb2/3)O3-PbTiO3-Pb(Yb1/2Nb1/2)O3 ferroelectrics

The (0.98-x)(0.6Pb(Mg1/3Nb1/3)O3-0.4PbTiO3)-xPb(Yb1/3Nb1/3)O3-0.02Pb(Er1/2Nb1/2)O3 ((0.98-x)(PMN-PT)-xPYN:Er3+) ceramics were prepared through a solid-state reaction method. The phase structure, piezoelectric response, ferroelectric performance and upconversion emission of the ceramics were systematically investigated. The phase structure, the electrical and optical properties are strongly related to the content of PYN. The optimized piezoelectric response and upconversion emissions of the ceramics were achieved near x=0.12, which locates in the morphotropic phase boundary (MPB) composition. Furthermore, the temperature sensing behaviors of the resultant compounds based on the thermally coupled levels of 2H11/2 and 4S3/2 of Er3+ ions in the temperature range of 133–573K were studied by utilizing the fluorescence intensity ratio technique. Additionally, the thermal effect, which is induced by the laser pump power, of the studied ceramics is also investigated and the produced temperature is enhanced from 268 to 348K with the pump power rising from 109 to 607mW.

In situ 2D diffraction as a tool to characterize ferroelectric and piezoelectric thin films

In this paper the application of 2D x-ray diffraction (XRD2) as a technique to characterize in situ during electrical cycling the properties of a ferroelectric and piezoelectric thin film is discussed. XRD2 is one type of XRD on which a 2D detector is used instead of a point detector. This technique enables simultaneous recording of many sample information in a much shorter time compared to conventional XRD. The discussion is focused especially on the data processing technique of the huge data acquired. The methodology to calculate an effective piezoelectric coefficient, analyze the phase and texture, and estimate the domain size and shape is described in this paper. This methodology is then applied to a lead zirconate titanate (PZT) thin film at the morphotropic phase boundary (MPB) composition (i.e. Pb[Zr0.52Ti0.48]O3) with a preferred orientation of (1 0 0). The in situ XRD2 characterization was conducted in the European synchrotron radiation facility (ESRF) in Grenoble, France. Since a high-energy beam with vertical resolution as small as 100 nm was used, a cross-sectional scan of the sample was performed over the entire thickness of the film. From these experimental results, a better understanding on the piezoelectricity phenomena in PZT thin film at MPB composition were achieved, providing original feedback between the elaboration processes and functional properties of the film.

Electric field dependent local structure of (KxNa1−x)0.5Bi0.5TiO3

The in situ X-ray Pair Distribution Function (PDF) characterisation technique has been used to study the behaviour of (Kx Na1-x)0.5 Bi0.5 TiO3, as a function of electric field. As opposed to conventional X-ray Bragg diffraction techniques, PDF is sensitive to local atomic displacements, detecting local structural changes at the Angstrom to nanometre scale. Several field-dependent ordering mechanisms can be observed in x = 0.15, 0.18 and at the morphotropic phase boundary (MPB) composition x = 0.20. X-ray total scattering shows suppression of diffuse scattering with increasing electric field amplitude, indicative of an increase in structural ordering. Analysis of PDF peaks in the 3-4 A range shows ordering of Bi-Ti distances parallel to the applied electric field, illustrated by peak amplitude redistribution parallel and perpendicular to the electric field vector. A transition from ‹110› to ‹112› type off-centre displacements of Bi relative to the neighbouring Ti atoms are observable with increasing x. Analysis of PDF peak shift with electric field show the effects of Bi-Ti redistribution and onset of piezoelectric lattice strain. The combination of these field-induced ordering mechanisms is consistent with local redistribution of Bi-Ti distances associated with domain reorientation and an overall increase in order of atomic displacements.

Crystal structure, impedance, and multiferroic property of SrZrO3 and MnO2 modified 0.725BiFeO3−0.275BaTiO3 ceramics

Abstract SrZrO 3 modified and SrZrO 3 +MnO 2 co-modified 0.725BiFeO 3 −0.275BaTiO 3 : x SrZrO 3 + y MnO 2 (BF-BT: x SZ+ y MnO 2 , x , y = 0–0.075) multiferroic ceramics were prepared and the structure, resistance and multiferroic properties were comparatively investigated. SZ plays a more dominant role than MnO 2 in making a phase transition from rhombohedral to pseudocubic around x = 0.02, y = 0, which implies that SZ has formed solid solution with BF-BT whereas most Mn cations may have not entered into lattice of host but exist as second phase isolated particles. The modifications have complex effects on impedance spectra but the SZ-induced MPB has significantly increased resistance. As a result, such modifications show great effects on multiferroics. Briefly, the rhombohedral-pseudocubic morphotropic phase boundary composition shows peak ferroelectricity with remanent polarization of 8.2 μC/cm 2 and switchable remanent magnetization of 0.22 emu/g. Then the ferroelectricity and magnetism weaken slightly with increasing SZ contents. However, the introduction of MnO 2 results in monotonously suppressed ferroelectricity but enhanced magnetization. These results not only provide promising multiferroic material with switchable polarization and magnetization at room temperature, but also may stimulate further work on co-modified BF-BT solid solutions with optimized multiferroic properties.

The role of grain boundary microchemistry in irradiation-assisted stress corrosion cracking of a Fe-13Cr-15Ni alloy

A novel explanation based on the variation of grain boundary composition is proposed to elucidate the localized nature of irradiation-assisted stress corrosion cracking, i.e., intergranular cracking is observed only at specific sites of random high-angle grain boundaries, in a Fe-13Cr-15Ni austenitic model alloy subjected to proton irradiation and straining in a high-temperature water environment. Specifically, this work presents electron microscopy characterization of multiple cracked and un-cracked grain boundary sites and the neighboring oxides. The depletion of Cr and enrichment of Ni, as expected due to radiation-induced segregation, is observed at grain boundaries far from the crack tip (for cracked sites) or the metal surface (for un-cracked sites), and such modification of grain boundary composition is enhanced in the vicinity of the corrosion reaction front. Unexpectedly, grain boundary sites beyond the crack tip always present a lower Ni content and higher Cr content than the un-cracked sites on the same grain boundary. Overall, it is proposed that the site-specific susceptibility to stress corrosion cracking is governed by the grain boundary microchemistry, which determines not only the quantity of Cr, but also the efficiency of Cr transport to the reaction front and thus the protectiveness of the inner oxide. These effects of local composition may be further coupled with the local structure of and the local stresses interacting with the random high-angle grain boundaries.

Extrinsic contributions to piezoelectric Rayleigh behavior in morphotropic PbTiO3 - BiScO3

In situ, high-energy, time-resolved X-ray diffraction experiments are utilized to quantify contributions from non-180° domain wall motion to the macroscopic electromechanical coupling effect in the morphotropic phase boundary composition 0.64PbTiO3-0.36BiScO3 during the application of weak electric field amplitudes. Macroscopic piezoelectric coefficient measurements are compared with diffraction data. The results demonstrate a linear contribution of electric-field-amplitude-dependent non-180° domain wall motion at small field amplitudes, and therefore domain wall motion contributes directly to the Rayleigh behavior of piezoelectric coefficients.

Grain size effect on structure and electrical properties of lead-free Na0.4K0.1Bi0.5TiO3 ceramics

Abstract Sodium potassium bismuth titanate ceramics (Na 0.4 K 0.1 Bi 0.5 )TiO 3 with grain sizes of 0.58, 0.93, 1.19 and 1.83 µm were prepared by varying sintering time. Grain size dependent structural and electrical properties of NKBT ceramics at morphotropic phase boundary composition have been investigated for the first time. Large internal compressive stress preclude the formation of tetragonal phase in sub-micron sized samples. Phase co-existence in 1.83 µm sample has been well modelled with rhombohedral ( R 3 m ) and tetragonal ( P 4 mm ) using Rietveld refinement. Relative permittivity increases with grain size and all the samples exhibits relaxor behavior. Strong coupling between grain boundary and domain walls hinder domain switching and polarization reversal. Hence reduction in the remnant polarization was observed in sub-micron sized samples. Excellent piezoelectric constant of 215 pC/N for sample with grain size of 1.83 µm is due to large amount of non-180° domain variants than that of samples with lowest grain size.

Polarization and space charge performance in PVDF with MPB composition BCZT doped composite films

ABSTRACTThe morphotropic phase boundary composition 47(Ba0.7Ca0.3)TiO3–0.53Ba(Zr0.2Ti0.8)O3 (calcium barium zirconate titanate, BCZT) were doped in poly(vinylidene fluoride) matrix by solution casting method. The effect of BCZT particles on polarization characteristics of composite films were investigated through dielectric spectra, hysteresis, and thermally stimulated discharge current measurements. It reveals that relative dielectric constant, remnant polarization, and energy storage density gradually increase and reach the maximum value with 39 at 100 Hz, 1.17 μC/cm2, and 3.69 J/cm3 at the concentration of 20 vol % BCZT. The Gaussian method was employed to decompose thermally stimulated discharge current composite peaks and an approximate model was used to analyze the activation energy levels of these films. The results indicated that the introduction of BCZT is effective for the charge carrier traps and activation energy distribution in the relaxation process of composite films. The decreased molecular movements, and increased interfaces formed between the BCZT grains and the poly(vinylidene fluoride) matrix may be responsible for these performances. And a new relaxation peak appeared at about 370 K in composites when BCZT concentration exceeded 20 vol %, which was considered to be caused by the ferroelectric‐paraelectric phase transition of BCZT. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45362.

Characterization methodology for lead zirconate titanate thin films with interdigitated electrode structures

The accurate evaluation of ferroelectric thin films operated with interdigitated electrodes is quite a complex task. In this article, we show how to correct the electric field and the capacitance in order to obtain identical polarization and CV loops for all geometrical variants. The simplest model is compared with corrections derived from Schwartz-Christoffel transformations, and with finite element simulations. The correction procedure is experimentally verified, giving almost identical curves for a variety of gaps and electrode widths. It is shown that the measured polarization change corresponds to the average polarization change in the center plane between the electrode fingers, thus at the position where the electric field is most homogeneous with respect to the direction and size. The question of maximal achievable polarization in the various possible textures, and compositional types of polycrystalline lead zirconate titanate thin films is revisited. In the best case, a soft (110) textured thin film with the morphotropic phase boundary composition should yield a value of 0.95Ps, and in the worst case, a rhombohedral (100) textured thin film should deliver a polarization of 0.74Ps.