Grain Boundary Oxidation Research Articles

熱延鋼板の内部酸化層が冷間圧延後の再結晶焼鈍時におけるＳｉ，Ｍｎ表面濃化挙動に及ぼす影響

By the heat treatment of a ultra low carbon hot-rolled steel containing 0.31mass%Si-1.02mass%Mn, not only readily oxidizable elements such as Si and Mn but also Fe were internally oxidized at the steel surface region. The scale of the steel acted as the source of oxygen for internal oxidation. The composition of the grain-boundary oxides differed from those in the grain, presumably due to the difference in oxygen potential ; oxides at the grain-boundary consisted mainly of Fe-Si-Mn-O, while oxides in the grain consisted mainly of Si-Mn-O. The internal oxidation layer remained after scale pickling and cold rolling. Fe containing grain-boundary oxides, arranged in layers by the cold rolling process, were considered to act as the oxidant of Si and Mn, which diffuse toward the steel surface in recrystallization annealing. By not only the low Si and Mn activity due to the depletion layer formed during the heat treatment, but also the redox reaction between the grain-boundary oxides and solute Si and Mn, the amounts of Si and Mn selective surface oxidation were remarkably decreased. The resultant suppression of the selective surface oxidation brought about improvement in the wettability by molten Zn of the annealed sheet steel surface.

Ultralow loss polycrystalline alumina

Polycrystalline alumina with extremely low microwave dielectric loss is reported with properties analogous to a theoretical ensemble of randomly oriented, single crystal sapphire grains. By avoiding deleterious impurities and by careful control of microstructure, we show that grain boundaries in aluminum oxide have only a limited influence on the dielectric loss. A method of measuring the electric permittivity and loss tangent of low-loss microwave ceramic dielectrics is reported. The electrical parameters such as relative permittivity and loss tangent are extracted using the radial mode matching technique. The measured values for ultralow loss polycrystalline aluminum oxide agree well with theoretical values modelled on an ensemble of randomly oriented anisotropic single crystal sapphire grains.

Grain-size-dependent thermopower of polycrystalline cerium oxide

The effect of grain boundaries on the defect chemistry of polycrystalline cerium oxide was investigated by measuring the thermopower of cerium oxide samples with grain sizes in the range of 0.13–11.5 Am. The samples were prepared by sintering pellets from a single batch of nanocrystalline 500 ppm Gd-doped cerium oxide powder at different temperatures. A change in the sign of the Seebeck coefficient as function of the grain size was observed, indicating a transition from predominantly ionic conductivity at large grain size to electronic conductivity at small grain size. The experimental results were analyzed using the space charge model for ionic solids, which has already been successfully employed in the analysis of the grain-size-dependent electrical conductivity of cerium oxide. The agreement between the experimental data and the space charge model, regarding both electrical conductivity and thermopower, suggested that the essential effect of the grain boundaries in cerium oxide is the accumulation/depletion of charge carriers in space charge layers, whereas the effect of microstructure on charge carrier mobilities is negligible. From the analysis of experimental results, a value of DU=0.7 V was obtained for the space charge potential at the grain boundaries in cerium oxide. D 2002 Elsevier Science B.V. All rights reserved.

A Microstructural Study of Preferential Oxidation at the Grain Boundaries of Ni–Cr Alloys

The oxidation behavior of alloy grain boundaries in model Ni–Cr alloys was investigated. Two binary alloys with nominal wt.% compositions of Ni–10Cr and Ni–20Cr were used. Oxidation was performed in air for 50 hr at 1000°C. The grain boundaries intersecting the alloy surface in Ni–10Cr did not exhibit oxidation, whereas the alloy formed a thick (≈60 μm) oxide layer which formed inwardly. The grain boundaries in this alloy showed a “passivating influence” at the adjacent regions and retarded oxide formation. An examination of the Ni–20Cr cross section revealed preferential oxidation to a depth of 65 μm at the alloy grain boundaries intersecting its surface, while the oxide at the surface was a few micrometers thick. It is noted that the extent to which the grain-boundary oxidation differs from the alloy surface oxidation depends on the Cr content of the alloy. It is also considered that the grain-boundary oxidation behavior in different Ni–Cr alloys changes as a function of Cr content.

Effects of the Amorphous Oxide Intergranular Layer Structure and Bonding on the Fracture Toughness of a High Purity Silicon Nitride

ABSTRACTThe microstructural evolution and structural characteristics and transitions in the thin grain-boundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si3N4-2wt% Y2O3ceramic for ∼200 hr at 1400°C in air, the fracture toughness can be increased by ∼100%, coincident with a change in fracture mechanism from transgranular to intergranular. Structural phase transformations occur in the thin grain boundaries during oxidation that are revealed by XRD, EDX, Raman and EELS analyses. They affect the local bonding of atoms. It is concluded that only specific crystal “building blocks”, i.e., tetrahedra, are transformed along the grain boundary and the resulting difference in the atomic structure of the oxide interface is seen directly to alter the macroscopic fracture behavior.

Crystallographic control of the barrier structure in zinc oxide varistors

Scanning electron microscope (SEM) based remote electron beam induced current (REBIC) microscopy has been used to investigate the electrical characteristics of individual grain boundaries in a zinc oxide based varistor. Although some grain boundaries showed the expected ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces are electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias to the grain boundary of several ten’s of millivolts is necessary to restore a symmetrical barrier structure. The orientations of grains on either side of each grain boundary were determined using electron backscattered pattern (EBSP) analysis and the grain boundary plane orientation was calculated by observing the lateral shift of the grain boundary EBIC contrast peak with SEM beam voltage and, hence, penetration depth. It was found that the electrical asymmetry is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the barrier structure.

Atomic Scale Analysis of Oxygen Vacancy Segregation At Grain Boundaries in Ceramic Oxides

The properties of ceramic oxides being developed for such varied applications as fuel cells, ionic transporting membranes, high-Tc superconductors, ferroelectrics and varistors are dominated by the presence of grain boundaries. Key to controlling the electronic properties of the grain boundaries in these materials is a fundamental understanding of the complex relationship between structure, composition and local electronic structure. The ability to characterize and directly correlate these parameters on the atomic scale is afforded by the combination of Z-contrast imaging and electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM). Furthermore, the recent development of in-situ heating capabilities in the JEOL 201 OF STEM/TEM permits atomic resolution analysis to be performed at elevated temperatures and the interactions of grain boundaries with the oxygen vacancies determined.Figure 1 shows an example of the type of experiment that can be performed using these methods.

Crystal plane influence of the EBIC contrast in zinc oxide varistors

Scanning electron microscopy (SEM) based remote electron beam induced current (REBIC) microscopy has been used to investigate the electrical characteristics of individual grain boundaries in a zinc oxide based varistor. Although some grain boundaries showed ‘bright and dark’ contrast consistent with symmetrical, opposed, electric fields on either side of a charged grain boundary, the majority of interfaces were found to be electrically asymmetric showing only either bright or dark contrast. In these cases, the application of an external voltage bias across the grain boundary of several 10′s of mV was necessary to restore the symmetrical structure. The orientations of grains on either side of grain boundaries showing each of these contrast types were determined using electron backscattered diffraction (EBSD) analysis and the grain boundary plane orientation was established using depth resolved EBIC. It was found that the asymmetry in the electrical structure is governed by the orientations of the grain boundary planes on either side of the interface, demonstrating some crystallographic control of the electrical character of the barrier structure.

Formation of Potential Barrier Related to Grain‐Boundary Character in Semiconducting Barium Titanate

Resistance–temperature (R–T) characteristics were measured directly at single‐grain boundaries in 0.1‐mol%‐niobium‐doped barium titanate bicrystals that had been fabricated from polycrystalline sinters, to determine a geometrical grain‐boundary character dependence of the positive temperature coefficient of resistivity (PTCR) effect. Both random boundaries and low‐Σ boundaries exhibit a similar grain‐boundary character dependence of the PTCR effect through a simple geometrical analysis, using the coincidence of reciprocal lattice points. Differences of the R–T characteristics in individual boundaries have been explained in terms of the formation of a potential barrier that is associated with the oxidation of grain boundaries during cooling, after sintering or annealing. The grain‐boundary character is likely to affect the diffusivity of O2− ions and, hence, is crucial to the formation of the potential barrier.

Non-Stoichiometry at Tilt Grain Boundaries in SrTiO3

Abstract The origin of electrically active grain boundaries in perovskite oxides and related materials remains controversial. The stoichiometry of the grain boundary core structure and the role of oxygen vacancies are the key issues involved. Previous results have given no indications of any non-stoichiometry at SrTiO3 grain boundaries, despite the fact that SrTiO3 is the system where atomic column EELS resolution has been demonstrated [1], and where atomic resolution images of the core structures have been obtained by Z-contrast imaging [2,3]. Here we show EELS spectra from individual dislocation cores in an 8° [100] tilt grain boundary in SrTiO3 that show significant oxygen depletion. The low angle grain boundary with well-separated dislocation cores makes it possible to study individual cores with high energy resolution in EELS. Previous work either averaged over all structural units in the grain boundary, sacrificing spatial resolution [2,3], or, used a very low probe current to obtain the highest spatial resolution [1].

Theoretical Investigations of Interfaces in Electroceramic Materials

Some recent computer modeling studies of grain boundaries in electroceramics are reviewed. The report focuses on rutile and zinc oxide, which both have electronic device applications and describes computational methodologies employed in these investigations, both classical and quantum mechanical. Although the number of calculations is limited, there are indications that undoped symmetric grain boundaries in rutile and zinc oxide are not electrically active whereas doped boundaries do exhibit interface states in the bandgap, which will influence the electrical properties of the material.

Theoretical Investigations of Interfaces in Electroceramic Materials

Some recent computer modeling studies of grain boundaries in electroceramics are reviewed. The report focuses on rutile and zinc oxide, which both have electronic device applications and describes computational methodologies employed in these investigations, both classical and quantum mechanical. Although the number of calculations is limited, there are indications that undoped symmetric grain boundaries in rutile and zinc oxide are not electrically active whereas doped boundaries do exhibit interface states in the bandgap, which will influence the electrical properties of the material.

Simulating the oxygen K-edge spectrum from grain boundaries in ceramic oxides using the multiple scattering methodology

In this paper we demonstrate the use of the multiple scattering methodology to interpret oxygen K-edge spectra from both the bulk and grain boundaries in a variety of ceramic oxides. The experimental electron energy loss spectra (EELS) used in this study, were obtained from a dedicated scanning transmission electron microscope (STEM). Using the STEM to obtain the spectra has the advantage that each spectrum can be acquired with atomic spatial resolution. While the energy resolution is limited to ∼0.8 eV, and the angular integration in the microscope apertures precludes momentum resolved spectroscopy, this unprecedented spatial resolution allows the electronic structure at individual defect sites to be determined. Additionally, as the microscope can also provide an atomic resolution image of the defect, the relationship between the atomic structure of the defect and its local electronic structure can be determined. In practice, this is achieved by using the structure observed in the image to build the real space atomic cluster for multiple scattering simulations. Detailed interpretation of the simulations of oxygen K-edge spectra from bulk MgO, CaO, SrTiO 3, TiO 2, MnO 2, Mn 3O 4, Mn 2O 3 and MnO are presented. In addition, the simulations from grain boundaries in TiO 2 (undoped) and SrTiO 3 (undoped and Mn doped) are discussed in relation to quantifying the changes in the local electronic structure that are a direct consequence of the defect structure. The simulations are used to make interpretations of the structure–property relationships at these grain boundaries.

Grain Boundary Kinetics in Oxide Ceramics with the Cubic Fluorite Crystal Structure and its Derivatives

Kinetics of grain boundaries in oxides with the cubic fluorite structure and its derivatives has been investigated using fine grain ceramics that are fully dense. Grain growth measurements in these materials have provided information on grain boundary diffusivity over a diffusion distance of the order of the initial grain size. With the addition of solute cations, grain boundary mobility can be varied over many orders of magnitude, often with very different activation energies. This is caused by the variation of defect population and the defect-solute association. Definitive evidence for solute drag has also been observed in some cases, but solute drag can not be confirmed as a general mechanism in solid solutions. Lastly, while grain boundary at low temperature may continue to serve as a fast diffusion path, it may not be able to migrate because of additional pinning mechanisms such as those exerted by grain boundary nodal points or lines. This means that sintering without grain growth is possible, opening up an avenue for obtaining ultrafine ceramics by pressureless sintering.

Computer simulation of general grain boundaries in rocksalt oxides

Computer simulation of general grain boundaries in rocksalt oxides

The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, anα-Al2O3 layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr2O3 to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al2O3grains. The intermediate and inner layers consist of columnarAl2O3 grains and equiaxedCr2O3 grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with the“dynamic segregation model” to account for the effects of reactive elements on thegrowth of Al2O3 scales.

Correlating Atomic Scale Experimental Observations to Develop Three-Dimensional Structural Models for Grain Boundaries in Oxides

: Determining the three-dimensional atomic structure of grain boundaries is a crucial first step toward understanding how these defects control the overall bulk properties of materials. In this report we discuss the correlation of experimental atomic resolution Z-contrast images and electron energy loss spectrometry (EELS) to achieve this goal. Initial structural analysis is afforded through empirical bond-valence potentials. This structure is then refined using multiple scattering analysis of the energy loss spectra. These techniques are demonstrated in the analysis of a 27 degrees MgO [001] tilt grain boundary. Through this analysis, we were able to determine specific atomic locations of Ca dopants found present at this grain boundary.

Depth resolved conductive mode imaging of varistor grain boundaries

An SEM conductive mode method is described for estimating the angle a grain boundary makes with the specimen surface, by varying the beam energy. Application of this technique to sloping grain boundaries in zinc oxide eliminates grain boundary geometry as a cause for a certain type of conductive mode contrast that is incompatible with existing models for varistor behaviour. Factors responsible for this contrast are discussed. Estimation of the grain boundary declination is also of use in crystallographic studies of interfaces in polycrystalline zinc oxide, enabling the grain boundary plane to be estimated and hence providing the necessary information for calculating the grain boundary structure without recourse to (destructive) sectioning techniques.

Interfacial reactions between zirconia and titanium

Titanium alloys have excellent properties such as high specific strength and good corrosion resistance. However, they are extremely reactive to ceramics at high temperatures, resulting in a chemical reaction affected-surface. The interstitial elements (e.g., C,N,O,H) from the ceramic have a great tendency to enter into the titanium alloys and cause the deterioration of mechanical properties. Economos and Kingery found that titanium melt could penetrate along the grain boundaries of oxides, such as Al{sub 2}O{sub 3}, ZrO{sub 2}, and MgO, at 1,800 C, leading to the alternation of these oxides. Ruh indicated that the titanium could react with zirconia and up to 10 at% of Zr and O elements from ZrO{sub 2} were retained in the solid solution of Ti, while the zirconia was transformed into oxygen deficient zirconia. Saha also revealed that ZrO{sub 2{minus}x} and {alpha}-Ti(O) were formed after Ti reacted with ZrO{sub 2}. However, the interfacial reactions between zirconia and titanium has not been fully elucidated to date. In this study, the microstructure of the interface between zirconia and titanium has been investigated using an analytical transmission electron microscope with a dedicated energy dispersive spectrometer (TEM/EDS), and the mechanism of the interface reactions is also discussed.

A review of the oxidation of uranium dioxide at temperatures below 400°C

A critical review of the extensive literature on the air oxidation of UO 2 at temperatures below 400°C is presented. The key parameters that affect the rate of UO 2 oxidation are examined systematically and their importance to the reaction rate is evaluated. The formation of U 3O 7/U 4O 9 on unirradiated UO 2 powders follows the discrete-layer mechanism and displays diffusion-controlled kinetics. In contrast, U 3O 8 formation on unirradiated UO 2 displays sigmoidal `nucleation-and-growth' kinetics. Low-temperature oxidation of used fuel tends to proceed by rapid grain-boundary oxidation followed by simultaneous intragranular oxidation throughout the sample. The best estimates of the activation energy for the formation of U 3O 7/U 4O 9 are 95.7 kJ mol −1 for UO 2 powders, 98.6 kJ mol −1 for sintered pellets and ∼106 kJ mol −1 for used fuel. The activation energy for the formation of U 3O 8 is temperature dependent. The best estimate of the activation energy below ∼325°C is 154 kJ mol −1, but all the kinetic data incorporate substantial approximations so that further study is required to properly predict the behaviour of used fuel under low-temperature (<150°C) dry-air storage conditions, based on high-temperature (200 to 350°C) laboratory data.