Bicrystal Grain Boundary Research Articles

Vortex dynamics and resistivity in HTS films with low-angle tilt grain boundaries

Abstract The model of thermally assisted vortex transfer along a row of equidistant parallel linear pins under the Lorentz force influence is elaborated and applied for the resistive state in HTS bicrystalline film in dc magnetic field parallel to the c -axis. It is assumed that the low-angle grain boundary of bicrystal is formed by the row of c -oriented edge dislocations, which are strong pins for vortices. It is shown that strong (nearly exponential) dependence of the critical current density across the grain boundary on the misorientation angle of grains in bicrystal j c ( θ ), as well as its nonmonotonous dependence on applied magnetic field j c ( B ) observed in some experiments, can be explained in the framework of suggested model.

Nonlinear Dielectric Properties at Oxide Grain Boundaries

Abstract The electric activity of grain boundaries in donor-doped SrTiO3 bicrystals is studied using a combination of variable temperature impedance spectroscopy, current–voltage measurements and scanning probe microscopy. The dominant transport mechanism changes from thermionic emission to diffusive at about 220 K. The temperature dependence of interface capacitance is interpreted in terms of field-dependent dielectric constant in the vicinity of the interfaces. Simplified expressions for the temperature dependences of interface potential, charge and capacitance for a weakly nonlinear dielectric are derived to describe the experiment. The complex nature of these materials requires incorporation of such effects as nonlinear polarization and multiple simultaneous transport mechanisms.

Controlling factor for nucleation of martensite at grain boundary in Fe-Ni bicrystals

A favourable nucleation of martensites at a grain boundary was investigated using Fe-Ni bicrystals containing a <211> symmetric tilt boundary with various tilt angles, focusing on the martensite-start temperature ( Ms), the morphology of martensites and the variant selection. Near the grain boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected and their variants changed depending on the tilt angle. In addition, the equivalent variants were symmetrically adjoined at the boundary to maintain the compatibility of shape strain of martensites across the boundary. Such characteristic nucleation can be regarded as a self-accommodation across the boundary, which is called cooperative nucleation (CN). The CN effectively reduces the strain energy due to the formation of martensites compared with the independent nucleation within a grain resulting in an increase in Ms. The characteristic variant selection of martensites can also be explained from the minimisation of strain energy.

Grain Boundary Structures and High Temperature Deformations in Alumina Bicrystals

Alumina bicrystals were successfully fabricated by hot joining technique at 1500°C in air to obtain two kinds of [0001] symmetrical tilt grain boundaries, i.e., Σ7 and Σ31. Their atomic structures and high-temperature deformation behaviors were investigated by high resolution transmission electron microscopy (HRTEM) and compressive creep tests, respectively. HRTEM observation revealed that grain boundaries in the alumina bicrystals were directly bonded without any amorphous phases, and that the atomic structure of Σ7 was different from that of Σ31 due to their geometrical misorientation and grain boundary planes. It was also found that grain boundary sliding rate in Σ7 was much smaller than that in Σ31. These results indicate that grain boundary sliding strongly depends on geometrical coherency and atomic structure of grain boundaries.

Chemical preferential doping in grain boundaries of melt textured YBa2Cu3Oy superconductors

Diffusion method has been developed to preferentially dope the grain boundaries of YBa2Cu3Oy bicrystals and polycrystals. Ca and Fe have been used as dopants in this study. The distribution of the dopants is highly localized in the grain boundaries. The critical current density of a YBa2Cu3Oy textured polycrystalline sample is significantly enhanced by doping Ca in grain boundaries; however, it is significantly decreased by doping Fe in grain boundaries. The Ca-doping effect has been explained by charge-carrier compensation in the grain boundaries, which reduces the grain boundary resistance and thus increases the grain boundary critical current. The Fe-doping effect has been explained by worsening the hole loss in grain boundary region, which increases the grain resistance and thus decreases the grain boundary critical current.

Migration and reorientation of grain boundaries in Zn bicrystals during annealing in a high magnetic field

Zinc bicrystals with 89° 〈1 0 1 ̄ 0〉 symmetric tilt boundary inclined at different angles to the free surfaces are annealed in the field of 17 T. Boundaries migrate reorienting almost perpendicular to the surfaces. The observed effect is due to combination of magnetically induced and capillarity driven boundary migration. The absolute boundary mobility was measured to be about 2.5×10 −8 m 4/J s.

Cleavage cracking resistance of high angle grain boundaries in Fe–3%Si alloy

High angle grain boundaries in steel offer an important resistance to the propagation of cleavage cracks that affects the fracture toughness and can modulate the ductile-to-brittle transition temperature of fracture downward. This behavior has been studied now in bicrystals of Fe–3%Si alloy in detail. It was noted that the twist misorientation across a high angle boundary has a more profound effect on cleavage fracture resistance than the tilt misorientation. Specific measurements of such resistance over a random selection of high angle grain boundaries in bicrystals and associated fractographic studies have led to quantitative models of the resistance that high angle grain boundaries offer to cleavage cracks. The study has also revealed a transition from pure cleavage to mixed cleavage around 0 °C for this alloy above which the observed definite increment of fracture work could be associated with the sigmoidal plastic bending and rupture of ligaments left between separate cleavage strips in the adjoining grain.

Grain boundary liquid metal wetting: A synchrotron micro-radiographic investigation

The penetration of liquid metals into the grain boundaries (GB) of solid metals can lead to severe embrittlement of the solid. Grain boundary wetting, which is the precursor process of liquid metal embrittlement, is still far from being well understood. We report in situ observations of the propagation kinetics of liquid metal into the GB of bicrystals and polycrystals. The experiments are performed using synchrotron radiation micro-radiography. This imaging technique is well adapted for the in situ non-destructive bulk characterization of such phenomenon due to suitable spatial and temporal resolutions. Experiments have shown that the Ga penetration front in aluminum bicrystals propagates roughly with linear time dependence, whereas a discontinuous behaviour occurs in polycrystals.

Metallic and non-metallic longitudinal conductance of grain boundaries inbicrystalline and polycrystalline germanium doped with mercury andantimony

The influence of temperature and magnetic and electric fieldson the grain boundary (GB) longitudinal conductance σat low temperatures in mercury-doped polycrystals and bicrystalsof germanium was studied. It was shown that the σ(T), σ(E) andσ(H)dependences for GBs in bicrystals are determined by metallicand hopping mechanisms, whereas we could not find a hoppingchannel in bicrystalline samples. The behaviour of the σ(H) dependencesfor low magnetic fields (super-linear) and the linearity of the current–voltage characteristicsat T < 100K indicate merely the presence of a metallic mechanism in bicrystalline samples.All of the results obtained indicate that during the zone-melting process, thesegregation of impurities at grain boundaries develops differently in bicrystallineand polycrystalline ingots.

Cleavage cracking resistance of large-angle grain boundaries in Fe-3 wt% Si alloy

Abstract Large-angle grain boundaries in steel offer an important resistance to the propagation of cleavage cracks that affects the fracture toughness and can modulate the ductile-to-brittle transition temperature of fracture downwards. This behaviour has been studied in bicrystals of Fe-3 wt% Si alloy in detail. It was noted that the twist misorientation across a large-angle boundary has a more profound effect on cleavage fracture resistance than the tilt misorientation does. Specific measurements of such resistance at −20°C over a random selection of large-angle grain boundaries in bicrystals and associated fractographic studies have led to quantitative models of this resistance. The study has also revealed a transition from pure cleavage to mixed cleavage at around 0°C for this alloy, above which the observed increment of fracture work could be associated with the sigmoidal plastic bending and rupture of ligaments left between separate primary cleavage strips in the adjoining grain.

Temperature dependence of magnetoresistance and nonlinear conductance of the bicrystal grain boundary in epitaxial La0.67Ba0.33MnO3 thin films

We investigated conduction through an artificial grain-boundary junction made in La0.67Ba0.33MnO3 thin films, deposited on a 36.7° SrTiO3 bicrystal substrate using a laser ablation technique. The grain boundary exhibits substantial magnetoresistance at low temperatures and also shows nonlinear I–V characteristics. Analysis of temperature dependence of the dynamic conductance allows us to identify three carrier transport mechanisms across the grain boundary. These mechanisms exist in parallel, and at a given temperature one mechanism may dominate. Particularly, at higher temperatures (T&amp;gt;175 K) the transport across the grain boundary involves spin–flip scattering, which we establish leads to decrease of the bicrystal grain-boundary contribution in magnetoresistance. At lower temperature (4.2–45 K), tunneling through a disordered oxide at the grain boundary dominates, whereas in the temperature range from 100 to 175 K, carrier transport is dominated by inelastic tunneling via pairs of manganese atoms.

Electrical transport properties of Bi2223 [ [formula omitted]] tilt grain boundary junctions

This work deals with experiments on the electrical transport of epitaxial Bi 2Sr 2Ca 2Cu 3O x (Bi2223) thin films fabricated by pulsed laser deposition on (1 0 0) oriented SrTiO 3 single crystal and bicrystalline substrates with [0 0 1] tilt grain boundaries. The current–voltage characteristics and the temperature dependence of the critical current density J c( T) across grain boundaries of thin film Bi2223 bicrystals have been measured as a function of tilt angle ( Θ). For Θ=0–36.8°, the critical current density decreases exponentially with increasing tilt angle. This orientation dependence is very similar to that observed for YBa 2Cu 3O 7 bicrystals.

Stacking-fault formation in [001] small-angle symmetric tilt grain boundaries in cubic zirconia bicrystals

Small-angle symmetric [001] tilt grain boundaries in cubic zirconia bicrystals with misorientation angles 2θ =1.0° and 2θ =5.0° have been fabricated by diffusion bonding. High-resolution electron microscopy observations revealed that the 1.0° boundary consists of a periodic array of mixed dislocations with Burgers vector b =( a /2)[101] or b = ( a /2)[101], while the 5.0° boundary consists of a periodic array of edge dislocations with Burgers vector b = ( a /2)[100], associated with stacking faults at alternate intervals. This suggests that there is a critical angle for structural transitions in the series of the [001] small-angle tilt grain boundaries.

Grain boundary segregation in Ni-Sb alloy polycrystals

In order to compare the grain boundary segregation of antimony and sulphur in bi- and polycrystals, the chemical composition of grain boundaries was studied by AES on fracture surfaces of polycrystalline Ni-Sb alloy (0.485 at.% Sb, 0.003 at.% S) samples annealed in the temperature range 773-1173 K. In contrast to the previously studied {133} bicrystals, dominant antimony segregation was observed at the grain boundaries of samples annealed in the above temperature range. By quantification of the AES results, the temperature dependence of grain boundary composition was determined and correlated with the Guttmann model of interfacial solute segregation in multicomponent systems. Consideration of various levels of grain boundary saturation and mutual interactions enabled the determination of the values of segregation enthalpy (ΔH 0 Sb = -63 kJ mol -1 , ΔH 0 S = -70 kJ mol -1 ) and entropy (ΔA 0 Sb = -20 J (mol -1 K -1 ), ΔS 0 S = +12 J (mol -1 K -1 )) of both segregants. Extraordinarily high values of antimony concentration were detected at grain boundaries of bicrystals as well as polycrystals after annealing at high temperatures. It is highly probable that an extended two-dimensional precipitation of nickel antimonides at grain boundaries occurs during the corresponding heat treatment.

Electrical activity of grain boundaries in silicon bicrystals and its modification by hydrogen plasma treatment

Electrical activity of grain boundaries (GBs) and its transformation under the influence of low-energy hydrogen plasma treatment in p-type silicon bicrystalline samples cut from EFG silicon polycrystals were investigated. Comprehensive studies have enabled one to investigate the electrical activity of GBs relative to the minority (MIC) and majority (MAC) carriers and to demonstrate the possibility of controlling this activity by different processing methods. These studies allowed for establishing the correlation between the type, structure and individual electrical activity of GBs and also thermal pre-history of samples. Among the tested modes, hydrogenation was found to be the most radical method of electrical activity modification for all types of GBs. In the process, results on hydrogenation of GBs in EFG silicon crystals depend essentially on three factors: type of GBs, state of ribbons (as-grown or annealed) and concurrence of grain boundary dangling bonds and boron passivation effects.

Structure of [110] tilt grain boundaries in zirconia bicrystals.

Cubic stabilized zirconia bicrystals with [110] symmetric tilt grain boundaries were fabricated by diffusion bonding of two single crystals with the composition of ZrO2-9.6mol%Y2O3. The structures of symmetric tilt small angle grain boundary and two types of symmetric tilt sigma3 grain boundaries with different grain boundary planes were observed by transmission electron microscopy (TEM). High-resolution transmission electron microscopy (HREM) observations clarified that the [110] small angle tilt grain boundary consists of periodic array of b = a/2[110] type edge dislocations. This result is consistent with Frank's dislocation model for small angle grain boundary. HREM observation also revealed that the 70.5 degrees sigma3 grain boundary shows atomically coherent grain boundary structure with the boundary plane of [111], while the 109.5 degrees sigma3 grain boundary accompanies grain boundary facets taking [111]/[115] asymmetric grain boundary plane. Because of the very low surface energy of [111] plane and/or high lattice matching of [111] and [115] type planes, the grain boundary faceting may be preferred in spite of increasing grain boundary area to about 6%. TEM-energy-dispersive X-ray spectroscopy (EDS) analyses were performed on both sigma3 grain boundaries, and the segregation of yttrium ions to the boundaries was detected in both cases. The amount of segregation is about the same in both sigma3 boundaries. It can be concluded that the segregation of yttrium ions to sigma3 grain boundary exists in cubic zirconia.

Grain boundary transport properties in YBa/sub 2/Cu/sub 3/O/sub x/ coated conductors

Critical current data obtained as a function of magnetic field on an isolated grain boundary (GB) of a coated conductor and two other types of bicrystal GBs of YBa/sub 2/Cu/sub 3/O/sub x/ show a peak in the critical current and an unusual hysteresis. These results provide support for a new mechanism for enhanced GB critical currents, arising from interactions of GB vortices with pinned Abrikosov vortices in the banks of a GB, as suggested by Gurevich and Cooley (1994). A substantial fraction of this enhancement, which can exceed a factor of ten, also occurs upon surpassing the critical current of the grains after zero field cooling. A bulk GB and thin film GBs show qualitatively identical results.

Transport properties of bicrystal Josephson junctions in Y-Ba-Cu-O films grown by liquid phase epitaxy

We studied charge transport across 24/spl deg/, 36/spl deg/, 45/spl deg/ symmetrical and 45/spl deg/ asymmetrical grain boundaries (GB's) in YBCO bicrystals grown by liquid phase epitaxy (LPE). We show that transport properties of symmetrical GB's are similar to those in bicrystals formed by various physical vapour deposition (PVD) methods while for 45/spl deg/ asymmetrical GB's, a difference between transport properties of LPE and PVD bicrystals has been found. In particular, in contrast to PVD bicrystals, selected LPE samples demonstrate regular magnetic interference patterns. In an agreement with transmission electron microscopy data, this result suggests better quality of 45/spl deg/ asymmetrical GB's in LPE films.

HRTEM Study on Σ7 Grain Boundary in Aluminium Bicrystals with and without Ga Doping

A HRTEM investigation was carried out on the Σ7{213} tilt grain boundary in bicrystals of both high purity aluminium and same aluminium doped with 10 ppm gallium. It was determined by image simulation that the bright contrast in the TEM images show the positions of atom columns. By measuring contrast profiles it was found that in the specimen of aluminium-10 ppm gallium, the atoms which are closest to the grain boundary, are anomalously shifted along the ‹121› direction compared to equivalent atom positions of the pure aluminium sample. This anomalous shift makes the disparity in the grain boundary structure of both samples. It is assumed that this anomalous shift is most likely the reason for the higher mobility of the grain boundary of the aluminium-10 ppm gallium sample.

Static and Dynamic Electron Holography of Electrically Active Grain Boundaries in SrTiO3

The dynamics of electrostatic potential barriers at grain boundaries (GBs) in Nb-doped SrTiO3 bicrystals is investigated using a unique combination of bulk and in-situ TEM electrical measurements across isolated GBs, coupled with electron holography under in-situ applied bias. The Nb bulk-doped bicrystals exhibit a positive GB potential that suppresses reversibly under applied bias greater than the nonlinearity threshold in the current-voltage curve. This suppression is interpreted as break-down of the potential barrier to current transport. The results on Nb bulk-doped bicrystals have been compared to those in which Mn has been added as a grain boundary specific dopant. This acceptor doping of the grain boundary causes an appreciable increase in the grain boundary resistance and extension of the nonlinear regime. A preliminary account of static electron holography shows a relatively flat potential profile across the GB, indicating probable compensation of donor states at the GB core with Mn-acceptors. Interestingly, the phase profile under applied bias in this case exhibits a reversible “dip” at the GB which is interpreted as an activation of GB trap states due to Mn-acceptor dopants trapping extra electrons (the majority charge carriers) at the GB core, inducing a negative GB potential, and diminishing current transport until the threshold bias is exceeded. The synergistic combination of nanoscale TEM measurements coupled with traditional macroscopic electrical measurements is emphasized.