Effect Of Triple Junctions Research Articles

Effect of Triple Junctions and Grain Size on Oxygen Diffusion Into the Surface Layer of Material

Effect of Triple Junctions and Grain Size on Oxygen Diffusion Into the Surface Layer of Material

Experimental Investigation of the Triple-Junction Effect on the Electric Discharge Characteristics and Modeling of the Maximum Discharge Current

The electrical discharge and breakdown characteristics of a triple junction (TJ) in high voltage (HV) electrical insulation are becoming an important issue in recent industrial applications. In this paper, a comparative study is conducted between three insulation materials used in HV: silicone, porcelain, and heat tempered glass, with electrode shape parameters such as inter-electrode distance (d), applied voltage (V), and geometry investigated for positive polarity, using an experimental approach. For optimum test organization to support our research and highlight the parameters and their interactions, we have established a mathematical regression model of the discharge current. It is polynomial, with good performance and a high level of precision. Besides, this yields one monovariable and another two-variable quadratic model. The latter can simulate the electric discharge simultaneously with two variables: V and d, and an adjusted determination coefficient () of nearly 0.99. Consequently, the two-variable quadratic model has been adapted to V and d ranges of 10 to 50 kV and 1.9 to 8 cm, respectively. Finally, this study provides us with new applicable knowledge about the TJ effect on electrical insulation with recommendations to optimize the design for higher performance.

Molecular dynamics studies of the grain-size dependent hydrogen diffusion coefficient of nanograined Fe

Nanograined materials have much denser grain boundary (GB) networks than their coarse-grained counterparts, thereby the hydrogen (H) diffusion and trapping behaviors in nanograined materials, which are strongly influenced by GBs, may differ greatly from those in coarse-grained materials. In the present research, the grain-size dependent hydrogen diffusion coefficient in nanograined Fe is studied by theoretical analysis and molecular dynamics (MD) simulations. A theoretical model based on thermodynamics is developed. The GB-related material parameters required by the model are then obtained by fitting the MD simulation results. Finally, the grain-size dependent diffusion coefficients are compared with model predictions to evaluate the validity of the model. It is found that the trapping effect of triple junctions that usually ignored in coarse-grained materials becomes increasing important as grain size and temperature decrease. Due to the strong trapping effect of GBs in nanograined Fe, H diffusion is slowed down by the GBs.

Critical assessment 30: Grain growth – Unresolved issues

ABSTRACTGrain growth is a broad field of study that covers several scientifically and technically interesting characteristics. Normal grain growth kinetics and its associated parabolic growth law and self-similar size distributions, grain boundary pinning, abnormal grain growth, the effect of triple junctions and quadruple points as well as grain growth in thin films are the most important current topics of grain growth research. In this critical assessment of grain growth in metals, we present and discuss some of the most important unresolved issues that have been a subject of research for many years.

Effect of triple junctions on deformation twinning in a nanostructured Cu-Zn alloy: A statistical study using transmission Kikuchi diffraction.

Scanning electron microscopy transmission Kikuchi diffraction is able to identify twins in nanocrystalline material, regardless of their crystallographic orientation. In this study, it was employed to characterize deformation twins in Cu/10 wt % Zn processed by high-pressure torsion. It was found that in 83% of grains containing twins, at least one twin intersects with a triple junction. This suggests that triple junctions could have promoted the nucleation of deformation twins. It should be cautioned that this technique might be unable to detect extremely small nanoscale twins thinner than its step size.

Initiation, evolution, and saturation of coupled grain boundary motion in nanocrystalline materials

Coupled grain boundary (GB) motion in sheared nanocrystalline materials composed of Ni, Al, or Cu is investigated by atomistic simulation methods, and the effects of grain size and temperature are evaluated. Due to the pinning effect of triple junctions, saturation of coupled GB motion is observed in all the nanocrystals except Cu. The two components of coupled GB motion, normal migration (NM) and tangential motion (TM), initiate and saturate at nearly equivalent nominal shear strains. Accompanied with coupled GB motion, massive dislocations and stacking faults are found to form within some grains, and the elementary structure units in the observed GBs transform from in-order to out-of-order. Compared with nanocrystalline Ni, the coupled GB motion in nanocrystalline Al has a reduced shear strain threshold and saturated NM displacement. The effects of grain size and temperature are similar in both NM and TM, so that their influence on coupled GB motion is slight.

Influence of Grain Formation on Electrical Properties of La<sub>0.67</sub>Sr<sub>0.33</sub>MnO<sub>3</sub>

In this work, polycrystalline perovskite manganites of La0.67Sr0.33MnO3 (LSMO) sample was prepared by two wet chemical methods: co-precipitation (CP) and sol-gel (SG) methods. Both samples were sintered at high temperature (1300 °C) for longer duration (24 hours) to investigate the effect of grains formation on its electrical properties. XRD results show that both samples were forming single LSMO phase with hexagonal structure (R-3c). SG sample shows smaller grain size (~220nm) however the CP sample (~4.45μm) was about 20-fold larger. Slower grain growth takes place in SG sample due to the triple junction effect where grain growth in nanocrystal is limited. Terrace patterns are noticed on the surface of CP sample which is suspected as the occurrence full crystallization or recrystallisation. TP of SG sample was shifted to lower temperature (298 K) due to the significant magnetically disordered layer across the grain boundaries which had weakened double exchange effect. SG sample displays higher extrinsic MR (-10.8%, 1 kG) and intrinsic MR (-25.1%, 10 kG) at 80 K due to the Core-Shell effect in the nanograin. However, grain boundaries (shell) effect is weakening in full crystallite CP sample. Hence, only intrinsic MR can be observed which is-15.6% at 10 kG applied field. Consequently, extrinsic MR is dominant in sol-gel sample however intrinsic MR is dominant in co-precipitation sample. Therefore, the grain size and microstructure formation affect the Tp, resistivity and magnetoresistive effect.

A phase-field model for grain growth with trijunction drag

A phase-field model has been developed to study the effect of triple junction (TJ) mobility on 2-D grain growth kinetics. The method captures the results of past work such as a linear increase in the average grain size with time, but can also follow the transition from TJ-limited to grain boundary energy-limited growth. The distribution of grain boundary curvature is examined. In the low TJ mobility simulations the distribution has a peak at zero curvature and approaches the grain boundary mobility-limited steady-state distribution at larger sizes. Even for extremely low TJ mobility, a small fraction of the grain boundary length has non-zero curvature and thus a lack of self-similarity is observed for all TJ-limited simulations, even when the average size is increasing linearly in time. We find that the topology of the grain structure is independent of the degree of TJ drag, within the range of parameters employed in the simulation. The effects of TJ mobility increase as the grain size decreases, suggesting that TJ mobility can play a significant role in nanocrystalline grain growth kinetics.

Crack initiation site at the interface between a nano-component and substrate

In order to determine the effect of triple junction on the crack initiation, we use the finite element method to investigate the effect of misorientation between Cu grains on the stress concentration near the triple junction in comparison to that near a free edge. The results reveal that the triple junction represents a greater danger for crack initiation than the free edge in nano-components over a wide range of grain combinations. The stress concentration near the triple junction is governed by the difference in stiffness between the grains. Thus, the combination of the stiffest and softest grains induces the highest stress concentration.

Effect of triple junctions of nanotubes on strengthening and fracture toughness of ceramic nanocomposites

A theoretical model has been proposed for describing the influence of triple junctions of nanotubes on the strengthening of a nanocomposite. It has been assumed that the slip of nanotubes along the boundary with the matrix takes place via the nucleation and glide of prismatic dislocation loops enveloping the nanotubes. These loops are retarded by the triple junctions of nanotubes, which leads to a strengthening and increase in the fracture toughness (crack resistance) of the nanocomposite. It has been shown that, in order for the dislocation loop to overcome the triple junction, the shear stress acting on the loop should exceed a certain critical level. This critical stress increases as the radius and wall thickness of the nanotube decrease. The inference has been made that the triple junctions of thinnest nanotubes, such as single-walled carbon nanotubes, should lead to the greatest strengthening and to an increase in the crack resistance of these nanocomposites.

Field enhancement at a triple junction in arrangements consisting of three media

The field singularity occurs at an edge point where wedge-like interfaces of dielectrics meet. This phenomenon is well known as Takagi-effect or triple-junction effect in the cases of two dielectrics. Its significance is twofold; (1) the negative phenomena of possible discharge or breakdown inception, and (2) the positive application of the field enhancement to field emission or electron beam generation in vacuum. This paper describes the analytical expression which gives the exponent of the field singularity in three dielectric media, and applications of this expression in various simple conditions. The analytical expression is also compared with the value given from the numerical field calculation.

Nanosecond pulse corona charging of polymers

A needle-plane electrode system was used to study charging properties of non woven polypropylene (PP) films exposed to nanosecond pulse discharge. A discovery of polarity reversal charging (PRC) was found that the accumulated charges on the non woven PP films sometimes show an opposite polarity against the applied pulses. Different polarity and different width of pulses produce different turning point of pulse peak (TPPP) where polarity of the net charges turns. The TPPP of negative pulse increases with the increase of pulse width, while that of positive one decreases. In order to make the charging process clear, PP films were also used and the pulsed electro-acoustic (PEA) space charge measuring method was used. The measurements showed that charges of different polarities, whose amount increases with the growth of pulse peak and charging time, have been simultaneously injected into the bulk of the PP films. Based on the mechanism of gaseous discharge, a combination effect of high filed injection (HFI) and return stroke injection (RSI) caused by a triple-junction effect is put forward here and the PRC phenomena is also discussed

Drag effect of triple junctions on grain boundary and grain growth kinetics in aluminium

The motion and geometry of connected 2D grain boundary systems with triple junctions in aluminium was investigated in situ in an SEM with a specially designed heating stage. The results show that triple junctions can have a marked influence on grain boundary motion and grain growth kinetics. The grain area change with annealing time was recorded in situ in the SEM. An analysis of the experimental data reveals that there is no unique relationship between growth rate and the number n of grain sides (von Neumann–Mullins relation). This is attributed to the effect of triple junction drag on grain growth.

Triple Junction Controlled Grain Growth

Grain growth in 2D polycrystals was simulated under the supposition that triple junctions possess a restricted mobility and so impede the migration of grain boundaries. A parameter 0 L = 0 D m taking into account the effect of triple junctions was varied in the range from 0.003 to 270 (m is the ratio of the triple junction mobility to that of grain boundary and 0 D the initial grain diameter). It was shown that at 0 L <0.4–0.5, i.e. at a small 0 D and small m, the growth kinetics becomes linear. It is supposed that the effect of triple junctions on grain growth can be observed in nanocrystalline materials.

Effect of Triple Junctions on Grain Boundary and Grain Growth Kinetics in Aluminium - 10 ppm Magnesium Sheet

The motion and geometry of connected grain boundary systems with triple junctions in aluminium -10 ppm magnesium was investigated in-situ with a special designed SEM heating stage. The results show that triple junctions can have a marked influence on grain boundary motion. The grain area change with annealing time was from a hot stage in the SEM. An analysis of the experimental data reveals that there is no unique relationship between growth rate and the number n of grain sides (Von Neumann-Mullins relation). This is attributed to the effect of triple junction drag on grain growth.

Yield stress of nanocrystalline materials: role of grain-boundary dislocations, triple junctions and Coble creep

A theoretical model is suggested which describes the strengthening of nanocrystalline materials due to the effects of triple junctions of grain boundaries as obstacles for grain-boundary sliding. In the framework of the model, a dependence of the yield stress characterizing grain-boundary sliding on grain size and triple-junction angles is revealed. With this dependence we have found that, in as-fabricated nanocrystalline materials, the yield stress depends upon a competition between conventional dislocation slip and grain-boundary sliding. On the other hand, yield stress dependence on grain size in heat-treated nanocrystalline materials is described as that caused by a competition between conventional dislocation slip and Coble creep. Grain-size and triple-junction angle distributions are incorporated into the consideration to account for distributions of grain size and triple-junction angles, occurring in real specimens. The results of the model are compared with experimental data from as-fabricated and heat-treated nanocrystalline materials and shown to be in good agreement.

Strengthening and softening mechanisms in nanocrystalline materials under superplastic deformation

A theoretical model is suggested which describes the strengthening and softening mechanisms in nanocrystalline materials under high-strain-rate superplastic deformation. In the framework of the model, the strengthening occurs due to the effects of triple junctions of grain boundaries (GBs) as obstacles for GB sliding. Deformation-induced migration of GBs and their triple junctions gives rise to local softening in nanocrystalline samples under superplastic deformation. With representations of the model, dependences of the yield stress on parameters characterizing the geometry of triple junctions and plastic strain degree are revealed. The results of the model account for experimental data for nanocrystalline materials exhibiting superplasticity, reported in the literature.

The effects of triple junctions and grain boundaries on hardness and Young’s modulus in nanostructured Ni–P

Hardness and Young’s modulus were measured on a series of nanocrystalline Ni–P samples. With decreasing grain size, a transition from regular to inverse Hall–Petch relationship and a reduction in Young’s modulus at the smallest grain sizes was observed, which can be attributed to grain boundary and triple junction effect.

Field emission property of chemical vapor deposited diamond overlayer films

The effect of diamond overlayer films on field emission properties has been studied. When conductive diamond layers are deposited on high quality diamond films, the non-diamond carbon contents in the films are increased with increasing overlayer thickness. The increase of overlayer thickness improves significantly both the electron emission currents and the number of emission sites. The threshold field of the diamond films grown at 2% CH 4/H 2 decreases from 12.0 to 7.7 V μm −1 with post-growth at 10% CH 4/H 2 for 5 min. The enhancement of field emission is explained by the improvement of electron transport through the high quality diamond layer by the field enhancement across the diamond layer. On the contrary, when the non-conductive diamond layers are grown on the conductive diamond films, both the electron emission currents and the number of emission sites degrade markedly with the increase of overlayer thickness while there is no substantial change in structure and morphology of the films. These degradations are attributed to both the electron transport through the overlayer and the reduction of the triple junction effect.

Modelling of the effect of triple junctions on grain boundary migration by a cellular automaton

Changes of the grain shapes caused by the grain boundary migration are modelled by a cellular automaton in a quadricrystal. It is shown that a simple local transition rule is sufficient to describe the grain boundary migration also in the configurations containing triple junctions. A process during which two grains shrink and finally disappear is quantitatively assessed. The method of cellular automata is an efficient tool, in particular, for the simulation of those stages of the grain growth processes that are out of equilibrium.