Bicrystal Boundary Research Articles

Characterization of grain boundary segregation in an Fe-Si alloy

In studying grain boundary segregation by Auger electron spectroscopy, several questions arise during data evaluation. The most important ones are: (1) How are segregants distributed between the two fracture surfaces which expose the boundary for measurements? (2) What is the best way to extract information about chemical composition from Auger spectra? (3) Is chemical composition the relevant characteristic of grain boundary segregation? These issues are discussed in detail, using solute segregation at numerous well-characterized grain boundaries in Fe-Si alloy bicrystals as a model system. It is shown that segregants are equally distributed between the two fracture surfaces for symmetrical grain boundaries whereas uneven distribution is observed for asymmetrical grain boundaries. Both types of segregant distribution can be described on the basis of a zig-zag intergranular fracturing. The atomic composition of the grain boundary is evaluated from measured Auger spectra using a method which accounts for the solute depth distribution. Auger electron attenuation and primary electron backscattering. Contradictory solute concentration dependences on orientation suggest that the chemical composition of grain boundaries in multicomponent systems may not be representative of interfacial segregation. It is shown that the grain boundary segregation anisotropy can be clearly and directly characterized by segregation enthalpies.

Non-linear mechanical relaxation associated with the viscous sliding of grain boundaries in aluminium bicrystals

The internal friction of high-purity aluminium bicrystals with a [110] symmetric tilt boundary having a misorientation angle of 129.5° was measured with a forced-vibration method. A pronounced internal friction peak (versus temperature) was observed around 200°C with frequency of vibration about 1 Hz. The activation energy concerned was found to be 0.88 eV. Similar experiments were also performed with bicrystals having misorientation angle 60° and the activation energy was found to be 0.92 eV. An amplitude effect was observed within the temperature range of the internal friction peak. This shows that the relaxation process associated with the grain boundary relaxation of bicrystals is non-linear. It is considered that the basic process associated with the observed internal friction peak is viscous sliding of the boundary, which drags the dislocation substructure in the vicinity of the boundary. This dragging process controls the viscous sliding along the boundary, so that an internal friction peak can appear. However, by virtue of the interaction between the dislocation substructure and the boundary of the bicrystal during the boundary sliding, the controlling action is complicated and the relaxation process exhibits a non-linear behaviour.

High‐Resolution Transmission Electron Microscopy Studies of a Near Σ11 Grain Boundary in α‐Alumina

The atomistic structure of a near Σ11 (N211) grain boundary in ultrapure α‐AI2O3 bicrystals was determined by quantitative high‐resolution transmission electron microscopy (HRTEM). High‐resolution imaging revealed an atomically sharp interface with a characteristic periodic pattern at (0111) ‖ (0111) facets. The pattern was analyzed by comparing the HRTEM micrographs with simulated images of different structures that had been relaxed by static‐lattice calculations. The best agreement with experiment was reached for the grain boundary structure with the lowest energy (1.7 J·m−2). Although this structure reproduces the relative translation state of the adjacent crystals to within 0.02 nm, the calculated structure was not in complete agreement with the periodic pattern at the grain boundary. The origin of this difference is probably due to a slight misalignment of the two crystals adjacent to the boundary.

Direct Determination of Grain Boundary Atomic Structure in SrTiO3

AbstractIn the electroceramic SrTiO3 the grain boundary atomic structure governs a variety of electrical properties such as non-linear I-V characteristics. An understanding of this atomic structure-property relationship for individual grain boundaries requires a technique which probes both composition and chemical bonding on an atomic scale. Atomic structure models for [001] tilt boundaries in SrTiO3 bicrystals have been determined directly from experimental data, by combining high-resolution Z-contrast imaging to locate the cation columns at the boundary, with simultaneous electron energy loss spectroscopy to examine light element coordination at atomic resolution. In this paper we compare and contrast the grain boundary structure models of symmetric and asymmetric boundaries in SrTiO3.

YBCO microwave grain boundary mixer using a SrTiO/sub 3/ bicrystal substrate

A microwave mixer was patterned on a microstrip transmission line of superconducting YBCO (YBa/sub 2/Cu/sub 3/O/sub 7-x/). The YBCO film was epitaxially laser deposited on a SrTiO/sub 3/ bicrystal substrate. A weak link was constructed by patterning a microbridge in the microstrip at the bicrystal boundary. Microwave signals at 9.000 GHz and 9.941 GHz were applied at the input of the microstrip line. An output intermediate frequency signal was observed at 941 MHz and was detected as the transmitted signal. The microbridge junction, which behaved as a resistively shunted Josephson junction (RSJ), was current biased slightly above the critical current I/sub c/. The mixer conversion loss was measured at the input and output ports of the device package. The mixing efficiency was determined at the device junction. This efficiency was compared with the calculated efficiency determined by numerical solution of the Josephson equation for the weak link junction. >

Thermodynamics of Faceting ξ3 Grain Boundaries in Cu

The specific energy γ b of a grain boundary generally depends not only on the misorientation of the adjacent grains but also on the grain boundary plane inclination. That is why a single grain boundary between grains of fixed misorientation is not necessarily planar or smoothly curved. It may facet to minimize its free energy. We have observed grain boundary facets forming at initially planar Σ3 grain boundaries in diffusion-bonded Cu bicrystals. The facets were characterized crystallographically by metallographic methods. The tendency to form facets and the morphology of the facets vary in a characteristic way with the initial grain boundary inclination

Deformation texture of channel-die deformed aluminum bicrystals with S orientations

The evolution of crystal lattice orientations in two aluminum bicrystals with S orientations, during channel die compression has been studied in detail. The boundary planes of the bicrystals were parallel to the plane of compression. The textures developed in each component crystal and at the boundary between the two crystals were examined as a function of deformation. In one of the bicrystal assemblies, the initial orientations of the component crystals were found to be unstable. They rotated to a common orientation. Hence, the bicrystal boundary, initially high angle in nature, became a low angle boundary. In the other bicrystal, the initial orientations of the component crystals were found to be largely stable. Except for some minor orientation adjustments, they remained essentially in their initial orientations, and the bicrystal boundary retained its high angle misorientation. Results of theoretical analyses using a numerical model appear to explain the texture development and the behavior at the boundary of the bicrystals very satisfactorily.

Weak link behavior of single grain boundary Josephson junction in BaPb1−xBixO3 bicrystals

The single grain boundaries in bulk bicrystals of the oxide superconductor BaPb 1−x Bi x O 3 show properties of Josephson junctions. Josephson supercurrent and the system of self-excited current resonances were found in I–V characteristic, measured critical current oscillating in applied magnetic field. The temperature dependence of the critical current exhibit anomalous non monotonous behavior at low temperature and a power-law behavior with the critical exponent close to 3 2 near critical temperature. Auger microscopy studies of the grain boundary were carried out to investigate a structure of the junction. Experimental findings are discussed from the point of view of the tunneling barrier with a semiconducting properties in the BaPb 1−x Bi x O 3 bicrystals.

Fatigue of aluminium-lithium alloys: Dudgeon, H.D. Diss. Abstr. Int. Oct. 1991 52, (4), 301 pp

Prior finite element method calculations have previously shown that high cycle fatigue cracks start at twin boundaries, because elastic interactions produce the highest stresses there. This investigation was undertaken to test this idea.Three diffusion-bonded bicrystals of 70-30 α-brass, having a boundary plane at 45 ° to the loading axis, were fatigued at low strain amplitudes. The component crystals were so oriented that bicrystal 1 produced low elastic interaction stresses and bicrystals 2 and 3 were of matrix-twin orientations, with the highest total stresses on slip systems nearly parallel to the twin boundary. Only for the initial few cycles did bicrystals 1 reveal no slip bands near the boundary, whereas bicrystals 2 and 3 produced a significant slip band nearly parallel to the twin boundary at the outset. With repeated cycling, the matrix of bicrystal 2 also showed a secondary slip band at the boundary where it produced large incompatibility strains and subsequent cracking. Fatigue cracks were initiated along slip band extrusions away from the bicrystal boundary in bicrystal 1 and along the twin boundary in bicrystal 3. For all three bicrystals, cracking commenced where the stresses and therefore the strains were the highest.

Atomic Relaxation Modes in Grain Boundaries

High-resolution electron microscopy (HREM) in combination with computer simulations of the fully relaxed atomic structures and energies of symmetric and asymmetric grain boundaries (GBs) in Au tilt bicrystals has been applied to a study of atomic relaxation modes. These investigations indicate that: (1) Atomic relaxations are typically dominated by short-range interactions and the tendency of the solid to assume a local atomic environment similar to the bulk. (2) Misfit localizations are likely within a structural unit whenever the GB unit cell is large. (3) When there is a length mismatch along the GB the atomic relaxations can assume quasiperiodic character, generating densely spaced regions of structural disorder, akin to misfit dislocations. (4) Atomic relaxations can take the form of stacking disorder.

Critical current and Josephson properties of single grain boundary in BaPb 1- xBi xO 3 bicrystals

The critical current and transport properties of the single grain boundary in bulk bicrystals of the oxide superconductor BaPb 1− x Bi x O 3 were measured. The I − V characteristic clearly shows a Josephson supercurrent and a system of self-excited current resonances, and the measured critical current oscillates in an applied magnetic field. Estimates of the Josephson junction size, based on the period of these oscillations, gives values close to the geometric size of a single grain boundary and we conclude that the grain boundary is single and an unbroken Josephson barrier exists, with some inhomogeneities in the spatial distribution of the critical current. The temperature dependence of the grain boundary junction critical current was investigated, including effects of thermally activated phase slippage. The temperature dependence of the critical current (without reduction due to fluctuations) in terms of the power law I c ≈ (1 − T/ T c) 1.6 near the critical temperature was evaluated by resistive transition analysis.

Carrier trapping and recombination at copper-decorated grain boundaries in silicon

Abstract The effects of carrier trapping and recombination at copper-decorated grain boundaries in silicon bicrystals have been investigated by means of deep-level transient spectroscopy and photocapacity experiments. The analysis of the experimental data makes use of a specific model for the recombination properties of the copper microprecipitates in the boundary plane, regarded as metallic inclusions in rectifying contact with the silicon matrix. The value of the recombination velocity of the boundary in particular, as deduced from these experiments, is in accordance with a direct determination by electron-beam-induced current measurements on similar samples.

Two-Dimensional Transport Properties of Germanium Bicrystal at Low Temperature

Two-dimensional properties of grain boundaries in germanium bicrystals are studied at liquid helium temperature ranging from 1.6 to 4.2 K. The experimental results indicate that the magnetoresistivity for 8° tilt angle increases exponentially with the magnetic field intensity square in weak magnetic field and with the linear magnetic field intensity in strong magnetic field. The phenomena are explainable with the percolation theory.

Quasi-particle and Josephson tunnelling characteristics of the single-grain boundary in BaPb1-xBixO3 bicrystals

Bulk bicrystals of the oxide superconductor BaPb1-xBixO3, with parameter x=0.27 and critical temperature 10.9 K, were grown by crystallization in a PbO-Bi2O3-BaO melt. Current-voltage (I-V) characteristics of the single-grain boundary, constituting a plane between superconducting blocks, were measured. The I-V characteristics had a shape typical of the Josephson junction with a critical current density of lower than 0.1 A cm2 at 4.2 K. The temperature dependences of the critical current density of the single-grain boundary were measured. Clear peaks on the dynamic conductance against bias voltage dependences of the bicrystals were observed and interpreted as energy-gap peculiarities. The temperature dependence of the energy gap of BaPb1-xBixO3 and the ratio 2 Delta (0)/kTc=3.6 were determined.

アルミニウム双結晶の疲労寿命におよぼす結晶粒界の影響

In order to examine the effect of grain boundary on fatigue life, aluminum bicrystals with a longitudinal boundary were prepared by the Bridgman method. They were tested by a bending fatigue testing machine under the constant strain amplitude of 0.13% and cyclic frequence of 30.4Hz. The results obtained were as follows:(1) The fatigue life decreased with increasing the saturation moment, Ms, at N=103.(2) The existance of grain boundaries in aluminum bicrystals possibly shortens the fatigue life.

Quantitative High-Resolution Electron Microscopy of Grain Boundaries in α-Al2O3

AbstractDetailed structural characterization of a near Σ11 grain boundary in ultra-pure α-A12O3 bi-crystals was performed by means of high-resolution transmission electron microscopy (HRTEM). High-resolution imaging revealed a characteristic periodic pattern along the grain boundary. In addition to HRTEM studies, atomistic simulations based on an ionic model were used to calculate three-dimensional structure models that were compared with the experimentally obtained images of the grain boundary. The comparison between the simulated and experimental HRTEM images showed good agreement for the theoretically proposed grain-boundary structure with the lowest grain-boundary energy of 1.8 Jm−2.

YBa2Cu3O7−x Grain Boundary Weak Links on MgO Bicrystal

ABSTRACTThe characteristics of YBa2Cu3O7−x (YBCO) thin films by laser ablation on MgO bicrystals have been investigated. The bicrystals were fabricated by hot pressing two single crystals with the configuration of [001] tilt boundaries. The YBCO films were epitaxial grown with C-axis normal to the both adjacent grains of bicrystals. The FWHM about (005) reflection was 0.4–0.5 degree, indicating the high degree of the oriention for the film with small mosaic spread. Our preliminary study showed that the typical value of Jc on either side of the bicrystal boundary was 0.4–10×106 A/cm2 at 15K, while that across the 10° tilt boundaries was 0.3–9×105 A/cm2 at 15K. These results implied that the artificial grain boundaries effectively weakened the supercurrent, and therefore, the weak-link properties of artificial boundaries were more easily controllable than those of naturally occuring grain boundaries.

Electrical transport across grain boundaries in bicrystals of YBa 2Cu 3O 7δ

The superconducting properties of about 20 bulk-scale flux-grown bicrystals of YBa 2Cu 3O 7−δ have been measured in order to deduce the nature of the coupling across their grain boundaries. Bicrystals with two types of misorientation relationship were studied: (i) θ [001] (C-type) and (ii) 90° [010] plus an additional rotation about the a I | c II axis (P-type). A general trend for a transition from flux pinning (FP) to Josephson Junction (JJ) to resistive (R) behavior was observed as the misorientation angle increased. However, FP character was observed well into the high-angle regime, in particular, for P-type bicrystals, and JJ behavior was observed at as low as 10° [001]. The J α values of two bicrystals were increased by a second oxygen anneal. In one case (28° [001] boundary), extended oxygenation produced a change form JJ to mixed JJ/FP behavior, markedly raising the transport critical current density (J α) at all fields. HRTEM has been performed on two of the same bicrystals which were electromagnetically characterized. A thin second phase layer unobservable by conventional TEM was observed in a resistive boundary, whereas no such phase was detected at a JJ boundary. We conclude that the character of the boundary cannot be predicted either from its misorientation angle or the magnitude of its J c.

Vibrating capacitor measurement of potential on crystal surface

Abstract The surface potential on pure and doped sodium chloride single crystals fits conventional defect theory, but the potential on grain boundaries in bicrystals is difficult to explain. The electrical width of the grain boundary is also an uncertain quantity that needs further investigation.

Transport processes and noise in YBa 2Cu 3O 7−δ grain boundary junctions

In the high temperature superconductors the weak link nature of extended defects as grain boundaries is responsible for the deterioration of the superconducting transport properties. From the study of individual grain boundaries in thin-film bicrystals of YBa 2Cu 3O 7−δ general relations describing the superconducting transport and noise characteristics of grain boundaries in the high temperature superconductors were found. Firstly, the critical current density J gb c decreases as the misorientation angle between the adjoining grains increases. Secondly, the J gb c ϱ N products, where ϱ N is the normal resistance times unit area of the grain boundaries, scale proportional to about (1/ϱ N) q with q ranging between 1 and 1.5. Thirdly, all grain boundaries show large amounts of low frequency 1/f noise. The transport and noise characteristics can be explained by a junction model which is based on an insulating layer at the grain boundary interface containing a large number of localized defect states.