Showing papers on "Grain boundary published in 2009"

Unique nanostructures and enhanced thermoelectric performance of melt-spun BiSbTe alloys

Abstract: We report a melt spinning technique followed by a quick spark plasma sintering procedure to fabricate high-performance p-type Bi0.52Sb1.48Te3 bulk material with unique microstructures. The microstructures consist of nanocrystalline domains embedded in amorphous matrix and 5–15 nm nanocrystals with coherent grain boundary. The significantly reduced thermal conductivity leads to a state-of-the-art dimensionless figure of merit ZT∼1.56 at 300 K, more than 50% improvement of that of the commercial Bi2Te3 ingot materials.

Experimental observations of stress-driven grain boundary migration.

Abstract: In crystalline materials, plastic deformation occurs by the motion of dislocations, and the regions between individual crystallites, called grain boundaries, act as obstacles to dislocation motion. Grain boundaries are widely envisaged to be mechanically static structures, but this report outlines an experimental investigation of stress-driven grain boundary migration manifested as grain growth in nanocrystalline aluminum thin films. Specimens fabricated with specially designed stress and strain concentrators are used to uncover the relative importance of these parameters on grain growth. In contrast to traditional descriptions of grain boundaries as stationary obstacles to dislocation-based plasticity, the results of this study indicate that shear stresses drive grain boundaries to move in a manner consistent with recent molecular dynamics simulations and theoretical predictions of coupled grain boundary migration.

Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects

Abstract: Ferromagnetism usually only occurs in materials containing elements that form covalent 3d and 4f bonds. Its occurrence in pure carbon is therefore surprising, even controversial. A systematic magnetic force microscope study indicates that ferromagnetism in graphite is the result of localized spins that arise at grain boundaries.

Large modulation of carrier transport by grain-boundary molecular packing and microstructure in organic thin films

Abstract: Solution-processable organic semiconductors are central to developing viable printed electronics, and performance comparable to that of amorphous silicon has been reported for films grown from soluble semiconductors. However, the seemingly desirable formation of large crystalline domains introduces grain boundaries, resulting in substantial device-to-device performance variations. Indeed, for films where the grain-boundary structure is random, a few unfavourable grain boundaries may dominate device performance. Here we isolate the effects of molecular-level structure at grain boundaries by engineering the microstructure of the high-performance n-type perylenediimide semiconductor PDI8-CN2 and analyse their consequences for charge transport. A combination of advanced X-ray scattering, first-principles computation and transistor characterization applied to PDI8-CN2 films reveals that grain-boundary orientation modulates carrier mobility by approximately two orders of magnitude. For PDI8-CN2 we show that the molecular packing motif (that is, herringbone versus slip-stacked) plays a decisive part in grain-boundary-induced transport anisotropy. The results of this study provide important guidelines for designing device-optimized molecular semiconductors.

High Total Proton Conductivity in Large-Grained Yttrium-Doped Barium Zirconate

Abstract: Barium zirconate has attracted particular attention among candidate proton conducting electrolyte materials for fuel cells and other electrochemical applications because of its chemical stability, mechanical robustness, and high bulk proton conductivity. Development of electrochemical devices based on this material, however, has been hampered by the high resistance of grain boundaries, and, due to limited grain growth during sintering, the high number density of such boundaries. Here, we demonstrate a fabrication protocol based on the sol−gel synthesis of nanocrystalline precursor materials and reactive sintering that results in large-grained, polycrystalline BaZr_(0.8)Y_(0.2O3−δ) of total high conductivity, 1 × 10^(−2) Scm^(−1) at 450 °C. The detrimental role of grain boundaries in these materials is confirmed via a comparison of the conductivities of polycrystalline samples with different grain sizes. Specifically, two samples with grain sizes differing by a factor of 2.3 display essentially identical grain interior conductivities, whereas the total grain boundary conductivities differ by a factor of 2.5−3.2, depending on the temperature (with the larger-grained material displaying higher conductivity).

Magnetization study of nanograined pure and Mn-doped ZnO films: Formation of a ferromagnetic grain-boundary foam

Abstract: In order to elucidate room-temperature (RT) ferromagnetism (FM) in ZnO, we have analyzed a multitude of experimental publications with respect to the ratio of grain-boundary (GB) area to grain volume. FM only appears if this ratio exceeds a certain threshold value ${s}_{\text{th}}$. Based on these important results nanograined pure and Mn-doped ZnO films have been prepared, which reveal reproducible RT FM and magnetization proportional to the film thickness, even for pure ZnO films. Our findings strongly suggest that grain boundaries and related vacancies are the intrinsic origin for RT ferromagnetism.

Giant magnetic-field-induced strains in polycrystalline Ni–Mn–Ga foams

Abstract: The magnetic shape-memory alloy Ni-Mn-Ga shows, in monocrystalline form, a reversible magnetic-field-induced strain (MFIS) up to 10%. This strain, which is produced by twin boundaries moving solely by internal stresses generated by magnetic anisotropy energy, can be used in actuators, sensors and energy-harvesting devices. Compared with monocrystalline Ni-Mn-Ga, fine-grained Ni-Mn-Ga is much easier to process but shows near-zero MFIS because twin boundary motion is inhibited by constraints imposed by grain boundaries. Recently, we showed that partial removal of these constraints, by introducing pores with sizes similar to grains, resulted in MFIS values of 0.12% in polycrystalline Ni-Mn-Ga foams, close to those of the best commercial magnetostrictive materials. Here, we demonstrate that introducing pores smaller than the grain size further reduces constraints and markedly increases MFIS to 2.0-8.7%. These strains, which remain stable over >200,000 cycles, are much larger than those of any polycrystalline, active material.

Charge-Transport Anisotropy Due to Grain Boundaries in Directionally Crystallized Thin Films of Regioregular Poly(3-hexylthiophene)

Abstract: Figure 1: Phase mode AFM image of directionally crystallized P3HT, revealing the unique grain structure. The white arrow indicates the direction of the fiber axis and polymer chain axis. The scan is 4 by 4 μm. (Left) The cartoon shows a possible orientation of crystallites within the directionally crystallized film, as derived from XRD. Red crystallites are “edge-on” with their pi-pi stacking repeat in the plane of the substrate. Grey crystallites are termed “face-on,” with their pi-pi stacking repeat out of the plane of the substrate. In this cartoon of the grains, the longest axis is long the polymer chain backbone (001), the next longest along the alkyl-stacking direction (100) and the shortest along the pi-pi stacking direction (010), see Figure 2b for reference. (Right) Charge Transport Anisotropy Due to Grain Boundaries in Directionally Crystallized Thin Films of Regio-Regular Poly(3-hexylthiophene)

Grain Boundary Defect Elimination in a Zeolite Membrane by Rapid Thermal Processing

Abstract: Microporous molecular sieve catalysts and adsorbents discriminate molecules on the basis of size and shape. Interest in molecular sieve films stems from their potential for energy-efficient membrane separations. However, grain boundary defects, formed in response to stresses induced by heat treatment, compromise their selectivity by creating nonselective transport pathways for permeating molecules. We show that rapid thermal processing can improve the separation performance of thick columnar films of a certain zeolite (silicalite-1) by eliminating grain boundary defects, possibly by strengthening grain bonding at the grain boundaries. This methodology enables the preparation of silicalite-1 membranes with high separation performance for aromatic and linear versus branched hydrocarbon isomers and holds promise for realizing high-throughput and scalable production of these zeolite membranes with improved energy efficiency.

Effect of Mn substitution on electrical and magnetic properties of Bi0.9La0.1FeO3

Abstract: Polycrystalline samples of Bi0.90La0.10(Fe1−xMnx)O3 (x=0, 0.05, 0.10, 0.15, and 0.20) were prepared using a novel mechanical activation followed by a conventional solid-state reaction technique. The formation of the desired materials was confirmed using x-ray diffraction. The electrical and magnetic properties of the materials were investigated at different Mn concentrations as a function of temperature. Both dielectric constant and loss tangent increased with the increase in Mn content in the system. The grain and grain boundary contributions have been estimated using impedance spectroscopy analysis. Both grain and grain boundary conductivity increased with a rise in temperature for all Mn concentrations. The value of activation energy for both grain and grain boundary is nearly the same, and decreased with an increase in Mn concentration. There is a systematic increase in the value of magnetization on increasing Mn concentration.

Three-dimensional grain mapping by x-ray diffraction contrast tomography and the use of Friedel pairs in diffraction data analysis.

Abstract: X-ray diffraction contrast tomography (DCT) is a technique for mapping grain shape and orientation in plastically undeformed polycrystals. In this paper, we describe a modified DCT data acquisition strategy which permits the incorporation of an innovative Friedel pair method for analyzing diffraction data. Diffraction spots are acquired during a 360 degrees rotation of the sample and are analyzed in terms of the Friedel pairs ((hkl) and (hkl) reflections, observed 180 degrees apart in rotation). The resulting increase in the accuracy with which the diffraction vectors are determined allows the use of improved algorithms for grain indexing (assigning diffraction spots to the grains from which they arise) and reconstruction. The accuracy of the resulting grain maps is quantified with reference to synchrotron microtomography data for a specimen made from a beta titanium system in which a second phase can be precipitated at grain boundaries, thereby revealing the grain shapes. The simple changes introduced to the DCT methodology are equally applicable to other variants of grain mapping.

Grain boundary segregation and thermodynamically stable binary nanocrystalline alloys

Abstract: A free-energy function for binary polycrystalline solid solutions is developed based on pairwise nearest-neighbor interactions. The model permits intergranular regions to exhibit unique energetics and compositions from grain interiors, under the assumption of random site occupation in each region. For a given composition, there is an equilibrium grain size, and the alloy configuration in equilibrium generally involves solute segregation. The present approach reduces to a standard model of grain boundary segregation in the limit of infinite grain size, but substantially generalizes prior thermodynamic models for nanoscale alloy systems. In particular, the present model allows consideration of weakly segregating systems, systems away from the dilute limit, and is derived for structures of arbitrary dimensionality. A series of solutions for the equilibrium alloy configuration and grain size are also presented as a function of simple input parameters, including temperature, alloy interaction energies, and component grain boundary energies.

Aging effects on the microstructure and creep behavior of Inconel 718 superalloy

Abstract: Standard heat treatment (HT1) for Inconel 718 superalloy is solutionizing at 1095 °C, 1 h/AC, then aging at 955 °C, 1 h/AC + 720 °C, 8 h/FC 57 K/h to 620 °C, 8 h/AC. In order to study the aging effects of the δ phase, two more conditions HT2 (no aging condition 955 °C) and HT3 (955 °C, 3.5 h/AC) were studied in this research. Lever arm creep tests were performed at 650 °C under constant stress 625 MPa. Since HT2 produces no δ phase, the stress rupture life, creep elongation to failure and steady state creep rate of HT2 are largest among these three aging conditions. However, increasing the 955 °C aging time, the stress rupture life, creep elongation and steady state creep rate raise slightly as compared to HT1, because platelet δ phase is more uniformly nucleated and more direction oriented at grain boundaries. Fractographs show ductile fracture patterns mostly and, small portion of inter-granular fracture in the HT2 specimens. Generally only inter-granular fracture is observed in the other two cases of HT1 and HT3. Besides twinning and dislocation mechanisms, grain boundary sliding is also activated, so that creep elongation to failure of HT2 specimens could reach 5.6%, whereas 1% for the other two schemes.

Electronic and optical properties of boron-doped nanocrystalline diamond films

Abstract: We report on the electronic and optical properties of boron-doped nanocrystalline diamond (NCD) thin films grown on quartz substrates by CH4/H2 plasma chemical vapor deposition. Diamond thin films with a thickness below 350 nm and with boron concentration ranging from 1017 to 1021 cm−3 have been investigated. UV Raman spectroscopy and atomic force microscopy have been used to assess the quality and morphology of the diamond films. Hall-effect measurements confirmed the expected p-type conductivity. At room temperature, the conductivity varies from 1.5×10−8 Ω−1 cm−1 for a nonintentionally doped film up to 76 Ω−1 cm−1 for a heavily B-doped film. Increasing the doping level results in a higher carrier concentration while the mobility decreases from 1.8 down to 0.2 cm2 V−1 s−1. For NCD films with low boron concentration, the conductivity strongly depends on temperature. However, the conductivity and the carrier concentration are no longer temperature dependent for films with the highest boron doping and the NCD films exhibit metallic properties. Highly doped films show superconducting properties with critical temperatures up to 2 K. The critical boron concentration for the metal-insulator transition is in the range from 2×1020 up to 3×1020 cm−3. We discuss different transport mechanisms to explain the influence of the grain boundaries and boron doping on the electronic properties of NCD films. Valence-band transport dominates at low boron concentration and high temperatures, whereas hopping between boron acceptors is the dominant transport mechanism for boron-doping concentration close to the Mott transition. Grain boundaries strongly reduce the mobility for low and very high doping levels. However, at intermediate doping levels where hopping transport is important, grain boundaries have a less pronounced effect on the mobility. The influence of boron and the effect of grain boundaries on the optoelectronic properties of the NCD films are examined using spectrally resolved photocurrent measurements and photothermal deflection spectroscopy. Major differences occur in the low energy range, between 0.5 and 1.0 eV, where both boron impurities and the sp2 carbon phase in the grain boundaries govern the optical absorption.

Elastic strain at interfaces and its influence on ionic conductivity in nanoscaled solid electrolyte thin films--theoretical considerations and experimental studies.

Abstract: Ionic transport in solids parallel to grain or phase boundaries is usually strongly enhanced compared to the bulk. Transport perpendicular to an interface (across an interface) is often much slower. Therefore in modern micro- and nanoscaled devices, a severe influence on the ionic/atomic transport properties can be expected due to the high density of interfaces. Transport processes in boundaries of ionic materials are still not understood on an atomic scale. In most of the studies on ionic materials the interfacial transport properties are explained by the influence of space charge regions. Here we discuss the influence of interfacial strain at semicoherent or coherent heterophase boundaries on ionic transport along these interfaces in ionic materials. A qualitative model is introduced for (untilted and untwisted) hetero phase boundaries. For experimental verification, the interfacial oxygen ionic conductivity of different multilayer systems consisting of cubic ZrO2 stabilised by aliovalent dopands (YSZ, CSZ) and an insulating oxide is investigated as a function of structural mismatch. Recent results on extremely fast ionic conduction in YSZ/SrTiO3 thin film systems (“colossal ionic concuctivity at interfaces”) is discussed from the viewpoint of strain effects.

Weak-link behavior of grain boundaries in superconducting Ba(Fe1−xCox)2As2 bicrystals

Abstract: We show that despite the low anisotropy, strong vortex pinning, and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1−xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe1−xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry.

Simple and rapid synthesis of α-Fe2O3 nanowires under ambient conditions

Abstract: We propose a simple method for the efficient and rapid synthesis of one-dimensional hematite (α-Fe2O3) nanostructures based on electrical resistive heating of iron wire under ambient conditions. Typically, 1–5 μm long α-Fe2O3 nanowires were synthesized on a time scale of seconds at temperatures of around 700 ° ⊂. The morphology, structure, and mechanism of formation of the nanowires were studied by scanning and transmission electron microscopies, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman techniques. A nanowire growth mechanism based on diffusion of iron ions to the surface through grain boundaries and to the growing wire tip through stacking fault defects and due to surface diffusion is proposed.

Grain boundary segregation and thermodynamically stable binary nanocrystalline alloys

Abstract: A free-energy function for binary polycrystalline solid solutions is developed based on pairwise nearestneighbor interactions. The model permits intergranular regions to exhibit unique energetics and compositions from grain interiors, under the assumption of random site occupation in each region. For a given composition, there is an equilibrium grain size, and the alloy configuration in equilibrium generally involves solute segregation. The present approach reduces to a standard model of grain boundary segregation in the limit of infinite grain size, but substantially generalizes prior thermodynamic models for nanoscale alloy systems. In particular, the present model allows consideration of weakly segregating systems, systems away from the dilute limit, and is derived for structures of arbitrary dimensionality. A series of solutions for the equilibrium alloy configuration and grain size are also presented as a function of simple input parameters, including temperature, alloy interaction energies, and component grain boundary energies.

Recrystallization mechanism of as-cast AZ91 magnesium alloy during hot compressive deformation

Abstract: Dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy during hot compression at 300 °C and the strain rate of 0.2 s−1 was systematically investigated by electron backscattering diffraction (EBSD) analysis. Twin DRX and continuous DRX (CDRX) are observed in grains and near grain boundaries, respectively. Original coarse grains are firstly divided by primary { 1 0 1 ¯ 2 } tensile twins and { 1 0 1 ¯ 1 } compression twins, and then { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins are rapidly propagated within these primary compression twins with increasing compressive strain. Some twin-walled grains are formed by the mutual crossing of twins or by the formation of the { 1 0 1 ¯ 1 }–{ 1 0 1 ¯ 2 } double twins and furthermore, subgrains divided by low-grain boundaries in the double twins are also formed. Finally, DRXed grains are formed by the in situ evolution of the subgrains with the growth of low-angle boundaries to high-angle grain boundaries in twins. CDRX around the eutectic Mg17Al12 phases at grain boundaries occurs together with the precipitation of discontinuous Mg17Al12 phase and the fragmentation of the precipitates during compression. The discontinuous fragmented precipitates distribute at the newly formed CDRXed grain boundaries and have remarkable pinning effect on the CDRXed grain growth, resulting in the average grain size of about 1.5 μm.

Modeling study of thermoelectric SiGe nanocomposites

Abstract: Nanocomposite thermoelectric materials have attracted much attention recently due to experimental demonstrations of improved thermoelectric properties over those of the corresponding bulk material. In order to better understand the reported data and to gain insight into transport in nanocomposites, we use the Boltzmann transport equation under the relaxation-time approximation to calculate the thermoelectric properties of $n$-type and $p$-type SiGe nanocomposites. We account for the strong grain-boundary scattering mechanism in nanocomposites using phonon and electron grain-boundary scattering models. The results from this analysis are in excellent agreement with recently reported measurements for the $n$-type nanocomposite but the experimental Seebeck coefficient for the $p$-type nanocomposite is approximately 25% higher than the model's prediction. The reason for this discrepancy is not clear at the present time and warrants further investigation. Using new mobility measurements and the model, we find that dopant precipitation is an important process in both $n$-type and $p$-type nanocomposites, in contrast to bulk SiGe, where dopant precipitation is most significant only in $n$-type materials. The model also shows that the potential barrier at the grain boundary required to explain the data is several times larger than the value estimated using the Poisson equation, indicating the presence of crystal defects in the material. This suggests that an improvement in mobility is possible by reducing the number of defects or reducing the number of trapping states at the grain boundaries.