Showing papers on "Grain boundary published in 1982"

A quantitative demonstration of the grain boundary diffusion mechanism for the oxidation of metals

Abstract: Below 1000°C the oxidation of nickel cannot be controlled by the diffusion of ions through the bulk crystal lattice of the pure oxide, because the measured oxidation rates are several orders of magnitude faster than would be predicted on this basis. Short-circuit diffusion through oxide grain boundaries or dislocations has usually been held responsible, but there has hitherto been no proper quantitative confirmation of this mechanism. We report measurements of the oxide scale thickness and oxide grain size as a function of time during the oxidation of high-purity nickel in the temperature range 500–800°C. All the oxidation experiments were carried out in pure oxygen at a pressure of one atmosphere. The measured parabolic oxidation rate constants have been compared with those calculated from grain boundary diffusion data obtained in our previous work, using a grain boundary diffusion model for the oxidation process. The quantitative agreement between measured and calculated oxidation rates shows c...

Conductivity behavior in polycrystalline semiconductor thin film transistors

Abstract: CdSe thin film transistor (TFT) structures which have been ion implanted with 50 keV 52Cr, 50 keV 27Al, or 15 keV 11B have a very steeply rising conductivity above some threshold dose and exhibit modulated transistor characteristics over certain ranges of implant dose, even though there is no thermal annealing during or after ion implantation. These results are interpreted using a model based on grain boundary trapping theory. The dependence of leakage current on implant dose, and of drain current (at a fixed dose) on gate voltage are described very well by this model, when the drain voltage is very small. Using this simple model, the important parameters of the polycrystalline CdSe film, namely the trap density per unit area in the grain boundary, the donor density, grain size, and electron mobility can be deduced. The effect of thermal annealing on implanted and unimplanted CdSe TFT’s has also been studied and the model appears to give a general description of the conductivity behavior in polycrystallin...

Thermal analysis of trapped hydrogen in pure iron

Abstract: The relative amount of trapped hydrogen and the activation energy for its evolution from various lattice defects in iron were calculated by monitoring the pressure change caused by release of hydrogen from charged specimens heated at uniform heating rates. Hydrogen release peaks were observed at 385 K, 488 K, and 578 K, respectively, when the hydrogen charged specimen were heated at 2.6 K per minute. Analysis suggests that the peak at 385 K corresponds to hydrogen release from grain boundaries, and the peak at 488 K corresponds to release from dislocations, while the peak at 578 K results from release from micro voids. The activation energies for evolution of trapped hydrogen were determined experimentally from measured peak temperatures at different heating rates and were found to be 17.2 KJ/mol, 26.8 KJ/mol, and 35.2 KJ/mol, respectively, in grain boundaries, dislocations, and microvoids. It was also observed that most of hydrogen is trapped on dislocations if the density of specimen is greater than 98.95 pct, and in microvoids if less than 98.95 pct.

A model for the effect of grain size on the yield stress of metals

Abstract: It is proposed that the grain-size dependence of the yield stress of metals is due to the elastic incompatibility stresses at the grain boundaries. For clarity, the process of yielding is divided into three stages. In the first stage (prior to microyielding), the differences in elastic properties arising from the elastic anisotropy of adjacent grains establish localized stress concentrations at the grain boundaries. In the second stage, the stress concentrations at the grain boundaries result in localized plastic flow; this is the microyield region. The generation and motion of these 'geometrically necessary' dislocations attenuates the stress concentrations; a work-hardened grain-boundary layer is formed. As the stress is increased, the polycrystalline metal acts as a composite material consisting of a continuous network of a work-hardened grain-boundary material and discontinuous bulk material. The bulk is prevented from flowing plastically because the continuous network of work-hardened grain-...

Effect of impurities on sintering and conductivity of yttria-stabilized zirconia

Abstract: The effect of low concentrations of Fe2O3, Al2O3 and Bi2O3 on the sintering behaviour of (ZrO2)083 (YO15)017, made by alkoxide synthesis, has been investigated The best results are achieved with Bi2O3 as a sinter agent and a relative density of 95% is obtained at 1200 K The effects of these impurities on the electrical conductivity of the bulk and the grain boundaries has been investigated using frequency dispersion analysis (101-106 Hz) All investigated impurities have a negative influence on both the bulk and grain-boundary conductivity For Fe2O3 and Al2O3 grain-boundary segregation factors of about two are calculated

Deuterium passivation of grain-boundary dangling bonds in silicon thin films

Abstract: Hydrogen passivation of silicon grain boundaries has been investigated by using deuterium as a readily identifiable isotope which duplicates hydrogen chemistry. Deuterium detection with high sensitivity was achieved with secondary‐ion mass spectrometry. Diffusion of deuterium in single‐ crystal silicon and polycrystalline silicon thin films at low temperatures (e.g., 350 °C) clearly demonstrates enhanced diffusion along grain boundaries. Defects at grain boundaries were detected by electron‐spin resonance and identified as silicon‐dangling bonds. Deuterium passivation of grain boundaries is revealed by correlated deuterium diffusion and dangling‐bond annihilation in polycrystalline silicon films.

Critical Microstructures for Microcracking in Al2O3‐ZrO2 Composites

Abstract: The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al2O3/ZrO2 composites. The degree of transformation was found to be dependent on the volume fraction of ZrO2 and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr02, the ZrO2 particles were found to pin the Al2O3 grain boundaries, thus limiting the Al2O3 grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO2. Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO2) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO2 at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO2 grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO2 size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO2. Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO3 decoration technique, near the indentations.

Grain boundary diffusion mechanisms in metals

Abstract: In recent years it has become well established that fast diffusion along grain boundaries plays a key role in many important metallurgical processes including cases where net mass is transported along boundaries which act as sources and/or sinks for the fluxes of atoms. In addition, considerable advances have been made in understanding grain boundary structure, and new techniques have become available for studying kinetic phenomena in grain boundaries. This lecture will attempt to review our current knowledge of the atomistic mechanisms responsible for these grain boundary diffusion phenomena. Relevant aspects of the structure of grain boundaries and the point and line defects which may exist in grain boundaries are described first. The important experimental observations are then discussed. Diffusion models are then taken up, and it is concluded that the atomic migration occurs by a point defect exchange mechanism which, in at least the vast majority of boundaries in simple metals, most likely involves grain boundary vacancies. The grain boundary sources and/or sinks required to support divergences in the atomic (vacancy) fluxes are grain boundary dislocations. Phenomena therefore occur which resemblethe Kirkendall Effect in the bulk lattice in certain respects. Additional topics are discussed which include effects of boundary structure on boundary diffusion and the question of whether or not boundary diffusion is faster along migrating than stationary boundaries.

Preparation and properties of TiO2 varistors

Abstract: TiO2 varistors with low resistivity were developed to operate at low voltages and have a reasonably high nonlinearity index. The varistor properties of TiO2 ceramics are due to two different groups of dopants. Pentavalent cation dopants are used to decrease the TiO2 lattice resistivity to ≃10–100 Ω cm at 25°C. Divalent cation dopants with large ionic radii are introduced to the grain boundary region by segregation process during cooling to provide a localized transport barrier. Thus, the conductive grain lattice and the resistive grain boundaries result in a low voltage varistor.

Growth of single‐crystal CoSi2 on Si(111)

Abstract: Single‐crystal CoSi2 films have been grown under ultrahigh vacuum conditions on Si (111) by both standard deposition and molecular beam epitaxy techniques. Films were analyzed by Rutherford backscattering spectroscopy and channeling, transmission electron microscopy, and low energy electron diffraction. The films are free of grain boundaries but are rotated 180° about the normal to the Si surface. The crystalline perfection, as measured by channeling, is the best yet reported for an epitaxial silicide system. The expected hexagonal misfit dislocation arrays, along with a coarser triangular defect structure, are confined to the plane of the interface.

Microstructural Analysis of Sintered High-Conductivity Zirconia with Al203 Additions

Abstract: Transmission electron microscopy (at 100 and 1000 kV potential) and analytical scanning transmission electron microscopy were used to study α-Al203 second-phase particles and their interactions with grain boundaries in two high-conductivity Y203/Yb203 stabilized zirconia ceramics containing deliberate additions of the alumina as a sintering aid. Most of the Al203 particles were intragranular and microanalysis showed that they contained inclusions rich in Zr or Si plus Zr. Al2O3 particles at grain boundaries were frequently associated with amorphous cusp areas rich in Si and Al. The results suggest that the Al203 acts as a scavenger for SiO2, removing it from grain-boundary localities. A model is proposed whereby this process occurs as the boundaries meet the second-phase particles, assisted by rapid grain-boundary diffusion. Such an ZrO2-Al2O3-SiO2 interaction and partitioning is predicted thermodynamically and offers a possible explanation for the improvements in ionic conductivity brought about by Al2O3 additions, as reported in the literature.

The thermodynamics of interactive co-segregation of phosphorus and alloying elements in iron and temper-brittle steels

Abstract: The thermodynamics of co-segregation and precipitation of P and alloying elements (transition metals M and carbon) involved in temper embrittlement of steels are studied quantitatively on the basis of the regular solution model for co-segregation. The equations of this model are fitted to the available Auger data for grain boundary segregation in high purity iron-base alloys and commercial steels, allowing the determination of the intrinsic segregation energies ΔGi o and of the binary βP gb, βc gb and ternary βPC gb, sMP gb interaction coefficients in the grain boundaries. This analysis shows that Ni, Cr, and Mo do not segregateper se in iron whereas Mn does weakly, and that the segregation of these elements is essentially driven by that of P through the strongβMP gb attractive interaction energyat the boundaries. This energy, which increases in the order Ni, Mn, Cr, Mo, is remarkably close to the bulk values βMP B in the corresponding phosphides as calculated on the basis of solubility data. The scavenging of P by M elements with largebulk M-P interactions is shown to play a determining role in low Mo and high (12 pct) Cr steels. The beneficial role of carbon is complex since it drives Mo to the grain boundaries due to the large Mo-C attraction, but it also strongly opposes P segregation due to the large repulsive P-C interaction.

Role of interfaces in kinetics of internal shape changes

Abstract: The author discusses the role of interfacial properties, notably interfacial energy σ and mobility μ, in a series of solid-state reactions that involve the change of size and shape of second-phase precipitates. The following topics are discussed: (i) precipitate growth from a supersaturated solid solution; (ii) competitive growth of precipitates to reduce their interfacial energy, Ostwald ripening; and (iii) migration of a high-angle matrix grain boundary through a dispersion of coherent precipitates.

Grain boundary strengthening in strongly textured magnesium produced by hot rolling

Abstract: The grain size dependence of the yield stress in hot rolled 99.87 pct magnesium sheets and rods was measured in the temperature range 77 K to 420 K. Hot rolling produced strong basal textures and, for a given grain size, the hot rolled material has a higher strength than extruded material. The yield strength-grain size relation in the above temperature range follows the Hall-Petch equation, and the temperature dependencies of the Hall-Petch constants σ0 and k are in support of the theory of Armstrong for hcp metals that the intercept σ0 is related to the critical resolved shear stress (CRSS) for basal slip (easy slip) and the slope k is related to the CRSS for prismatic slip (difficult slip) occurring near the grain boundaries. In the hot rolled magnesium, σ0 is larger and k is smaller than in extruded material, observations which are shown to result from strong unfavorable basal and favorable {1010} textures, respectively. Texture affects the Hall-Petch constants through its effect on the orientation factors relating them to the CRSS for the individual slip systems controlling them.

Compressive creep of Si3N4/MgO alloys

Abstract: Highly localized strain fields are observed at grain boundaries in crept specimens of Si3N4/MgO alloys which were frozen under stress. These fields disappear upon annealing. Unresolved asperities between the grain pairs appear to give rise to the strain field during deformation. Viscoelastic effects responsible for primary creep and strain recovery are explained in terms of grain-boundary sliding on the glassy interphase which is accommodated by the elastic strain arising at the asperities. Each boundary containing an asperity can be modelled as a simple Kelvin element. The spectrum of these boundaries within the bulk gives rise to a spectrum of relaxation times that is observed for the strain recovery effect. The highly stressed region at the asperity also gives rise to the higher chemical potential required to drive diffusional creep. Although the source of the asperities was not observed, the possibility of opposing ledges of either single or multiple interplanar height is discussed.

Grain boundary states and the characteristics of lateral polysilicon diodes

Abstract: In lateral n+p−p+ diodes made in LPCVD polycrystalline silicon films, the energy distribution of the traps at the grain boundaries is found to be U shaped. They have a density of about 1012 cm−2 and a capture cross section of about 10−16 cm2. The forward current of the diodes is ascribed to recombination, the reverse current to field-enhanced generation via these traps.

The Effect of Hot Rolling Condition and Chemical Composition on the Onset Temperature of γ-α Transformation after Hot Rolling

Abstract: The effect of hot rolling condition and chemistry of steels on the onset temperature of γ-α transformation after hot rolling was investigated using a thermal analyzer developed for the measurement of Ar3 temperature after hot rolling. The changes of Ar3 temperature and austenitic microstructure in connection with reheating temperature, rolling temperature and reduction rate were investigated in a Si-Mn and a Nb-bearing steels. The results were analyzed based on the changes of the effective interfacial area per unit volume (Sv) that included both of the recrystallized and unrecrystallized grain boundaries and deformation band as nucleation sites for ferrite. Although the Sv value increased with the refinement of recrystallized γ grain or the increase of rolling reduction below recrystallization temperature of austenite, the latter resulted in a much greater change of Ar3 temperature in the Nb steel. This was considered to be due to a reduced amount of dissolved Nb atoms around the grain boundary or deformation band through the strain induced precipitation of Nb(CN).The effect of chemistry and plate thickness on the Ar3 temperature after controlled rolling was studied in a large number of steels with different chemistry, and the relation between the Ar3 and chemistry was quantitatively established based on the multiple regression analysis.