Doping effects on protonic conduction in SrCeO3- and BaCeO3-based perovskites

Electrical conductivity of SrCe_1-x M _x O₃ and BaCe_1-x M _x O₃ (M = rare earth) protonic conductors with the perovskite structure was examined as a function of doping level. The anomalous properties of BaCe_1-x Fr _x O₃ ceramics in the two-phase region (0.2 <x < 0.4) were interpreted in terms of percolating clusters.     

Grain boundary analysis of MgO-doped Al2O3

Grain boundaries of MgO-doped alumina also containing CaO have been investigated by various authors. The general conclusion of this work is that Ca is enormously enriched at the grain boundaries compared with Mg, which was found either to be slightly or not at all enriched at the grain boundaries. Electron scanning chemical analyses of predominant intergranular fracture surfaces of alumina samples containing different amounts of MgO (100, 250, 1000 and 2000 ppm), as described in this paper, show that Mg is enriched at the grain boundaries below the solubility limit. Above the solubility limit it is probable that MgAl₂O₄ precipitates contribute considerably to the Mg content measured at the boundaries. The amount of Mg segregated at the grain boundaries was found to be roughly independent of the amount of MgO added as long as the firing conditions were kept constant. The enrichment found, together with the effect of small amounts of MgO on the development of the microstructure, indicate the possibility that segregated Mg may still be active in the grain growth process.     

Grain boundary migration in Fe-3mass%Si alloy bicrystals with twist boundary

Abstract

The characteristics of grain boundary migration in Fe-3mass%Si alloy bicrystals with ∑3(011), ∑5(001) and ∑9(011) coincidence twist boundaries and random twist boundaries were examined to obtain an information on the development of {110}(001) (Goss) texture. The bicrystals were annealed at 1223 K for an appropriate time and the grain boundary migration speed was evaluated.

The ∑5 001l and ∑9 011l twist boundaries showed higher migration speed than ∑3(011) twist boundaries, and the random twist boundaries migrated faster than other boundaries. The migration speed decreased with increasing annealing time due to an increase in the edge components of the lattice misfits in the migrated boundaries. The grain boundary migration was also sensitive to the deviation angle (?θ) from the ideal orientation relationship for a coincidence boundary. The increase of ?θ accelerated the boundary migration. The motion of the grain boundary was influenced by plastic strain. Migration of the ∑9 twist boundary was more suppressed by plastic strain than that of the random boundary. On the basis of characteristics of the grain boundary migration, the effect of inhibitor on the Goss texture was discussed. © 2000 Elsevier Science Ltd. All rights reserved.     

Grain boundary migration in Fe-3mass%Si alloy bicrystals with twist boundary

The characteristics of grain boundary migration in Fe-3mass%Si alloy bicrystals with Σ3〈011〉, Σ5〈001〉 and Σ9〈011〉 coincidence twist boundaries and random twist boundaries were examined to obtain an information on the development of {110}〈001〉 (Goss) texture. The bicrystals were annealed at 1223 K for an appropriate time and the grain boundary migration speed was evaluated.The Σ5〈001〉 and Σ9〈011〉 twist boundaries showed higher migration speed than Σ3〈011〉 twist boundaries, and the random twist boundaries migrated faster than other boundaries. The migration speed decreased with increasing annealing time due to an increase in the edge components of the lattice misfits in the migrated boundaries. The grain boundary migration was also sensitive to the deviation angle (Δθ) from the ideal orientation relationship for a coincidence boundary. The increase of Δθ accelerated the boundary migration. The motion of the grain boundary was influenced by plastic strain. Migration of the Σ9 twist boundary was more suppressed by plastic strain than that of the random boundary. On the basis of characteristics of the grain boundary migration, the effect of inhibitor on the Goss texture was discussed.     

Grain boundary diffusion and wetting in the analysis of intergranular penetration

Intergranular penetration of liquid bismuth has been analysed in two pure metals, Cu at 500 °C and Ni at 700 °C, used either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves have developed in Cu–Bi, while micrometer-thick films were observed in Ni–Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicate a zone of monolayer Bi segregation followed by a diffusion-type profile over a distance of the order of 100 μm for Cu–Bi and a nanometer-thick film followed by similar diffusion-type profile for Ni–Bi. In both cases the kinetics of intergranular penetration and embrittlement has been shown to be parabolic. It is concluded that no wetting occurs in Cu–Bi system at 500 °C while Bi wets Ni at 700 °C. It is postulated that the mechanism of intergranular penetration operates at a very tip of the penetration front, as opposed to the tip of liquid Bi film observed by scanning electron microscopy, and must be based on diffusion rather than wetting phenomena. Some suggestions are formulated for the future research in the area of intergranular penetration that can be split in two phenomena: grain boundary wetting above the wetting transition temperature and grain boundary diffusion below.     

Formation and microwave dielectric properties of Sr(Ga0.5Ta0.5)O3-based complex perovskites

The Sr(Ga0.5Ta0.5)O3-based perovskites with O2- and/or Sr2+ vacancies were formed by changing the A-site and/or B-site cation ratios. The Sr-deficient perovskites with a limited composition of Sr0.86(Ga0.36Ta0.64)O3 could be obtained, whereas oxygen vacancies were hardly created. The B′-site Ga3+ cation could be replaced with large Sc3+, In3+, Y3+, Nd3+ and La3+. The crystal symmetry of the complex perovskites changed from cubic (B′=Ga3+, Sc3+, In3+) to tetragonal (B′=La3+) through rhombohedral (B′=Y3+, Nd3+). The dielectric permittivities of these ceramics indicated no distinct dependence on the B′-site cation species. The temperature coefficient of permittivity might be associated with the symmetry change of the perovskite phases. Higher microwave Qf values, >Qf=38 000 GHz, were obtained for our complex perovskite ceramics, except with B′=La3+. This revised version was published online in July 2006 with corrections to the Cover Date.     

Grain boundary reorientation in copper

The present route to grain boundary engineering (GBE) is usually based on multiple annealing twinning which can only be applied to a certain subset of materials, namely those that twin prolifically. A more general approach has been highlighted recently, following experimental evidence that certain boundary planes in iron bicrystals are ‘special’, and that this classification is not based on misorientation. It was suggested that, under suitable conditions, individual interfaces could reorient the most energetically advantageous orientations. This approach concurs with a similar concept of ‘grain boundary plane engineering’, proposed previously. In the present article we explore this concept and report the effect of long duration, low temperature annealing on the distribution of boundary misorientation and planes in copper. The new findings give support to the possibility of grain boundary structure optimisation via controlled annealing. To have established that grain boundary plane reorientation is feasible opens up new avenues and challenges in the field of grain boundary research. This could have significant impact both scientifically in terms of understanding grain boundary structure and technologically in the field of GBE.     

Formation of secondary phases and microdiscs on flux-grown HoFeO3 crystals

Results of optical and scanning electron microscopic and EDAX studies, carried out on structures exhibited by HoFeO₃ crystals grown from the PbO-PbF₂-B₂O₃ flux system, are reported. Aluminium and silicon are present as impurities in the crystals studied. EDAX of certain structures indicate formation of magnetoplumbite (PbO · 6Fe₂O₃) during the flux growth of HoFeO₃. Crystallization of HoOF on the HoFeO₃ crystal surfaces is also indicated; the process taking place almost at the end of HoFeO₃ crystal growth. A variety of microdisc patterns on HoFeO₃ crystal surfaces are illustrated. Their formation is attributed to the covering process of impurity phases by the rapidly advancing growth fronts on the HoFeO₃ crystal surfaces. Experimental evidence in support of this is offered by exposing the impurity phase buried under the microdisc. Precipitation of the secondary phases during the flux growth of HoFeO₃ crystals and their influence on the latter is discussed.     

CaCu3Ti4O12中的晶界弛豫

研究了CaCu3Ti4O12在低温低频下的内耗曲线特征,发现金属中来源于晶界的内耗规律也适合CaCu3Ti4O12材料.测量了晶界弛豫激活能的大小和不同温度下该材料的特征弛豫时间.与常温的情况相比,低温下CaCu3Ti4O12的特征弛豫时间明显增加.分析表明:这种弛豫时间的增加来源于在畴区上极化弛豫的动态慢化效应.     

Grain boundary wetting in ceramic cuprates

Grain boundary (GB) wetting by a chemically compatible liquid in ceramic cuprates and its influence on transport properties of ceramics are considered, using model Bi₂CuO₄-Bi₂O₃ system as an example. It is shown that the GB wetting transition enhances significantly ionic conductivity and diffusional permeability of the ceramics. The experimental data on grain boundary wetting transition, electrical conductivity, diffusivity, and high temperature corrosion of copper are discussed.     

Grain boundary grooving in molybdenum bicrystals

The morphologies of grain boundary grooves formed after annealing of three Mo bicrystals at the temperature close to the melting point were studied with the aid of scanning force microscopy. The ratio of grain boundary to surface energy was calculated from the surface slopes measured at the root of the groove. This ratio was about 0.3 for the grain boundary grooves with the sharp roots, and it was significantly lower than 0.3 for the grooves with the blunted roots. It was shown that the blunting of the root of the grain boundary grooves is associated with the grain boundary migration during annealing. A model of grain boundary grooving accompanied by the instantaneous boundary shift was formulated. The surface topographies predicted by the model were in a good agreement with the experimentally measured ones. It was shown that grain boundary grooving process can be completely suppressed at the singular surfaces. This, together with the grain boundary migration during annealing results in characteristic groove morphology with the blunted root and leads to a significant underestimation of the grain boundary energy calculated from surface slopes.     

Grain growth and secondary recrystallization in iron

The effect of impurities and of slight deformations as factors modifying the kinetics of normal grain growth after primary recrystallization in pure iron is investigated. Samples of pure iron and Armco iron, each with or without a content of precipitated oxides, were examined.Inclusions inhibit grain growth up to high temperatures, whilst the influence of strain depends on its amount: very low deformations (2%) generally block grain growth; for deformations around 5%, secondary recrystallization takes place; higher deformations (10%) accelerate the early stages of growth, leading to not very large final grain dimensions. 2.0% elongation has been found to be, in our conditions, the critical strain limit for secondary recrystallization.     

Grain boundary complexions and pseudopartial wetting

The important class of grain boundary (GB) complexions includes the few nanometer thick layers having composition which strongly differs from that of the abutting grains. Such GB complexions are frequently called intergranular films (IGFs) and can be observed close to the lines of wetting, prewetting and premelting complexion transitions in the bulk phase diagrams. In the majority of systems, the direct transition between complete and partial GB wetting takes place (by changing temperature, pressure, etc.). However, in certain conditions the so-called pseudopartial (or pseudoincomplete, or frustrated complete) GB wetting appears in a phase diagram between complete and partial wetting. In case of pseudopartial GB wetting, the thin GB layer of a complexion (IGF or 2-D interfacial phase) can coexist with large droplets (or particles) of the wetting phase with a non-zero dihedral (contact) angle. Thus, such IGFs can be observed in the two-phase (or multiphase) fields of bulk phase diagrams, in the broad intervals of concentrations, temperature and/or pressure. The IGFs driven by the pseudopartial GB wetting can drastically modify the properties of polycrystals. In this review, we discuss this phenomenon for the technologically important Fe–Nd–B-based hard magnetic alloys, WC–Co cemented carbides and Al-based light alloys.     

Surface energy and textural boundary conditions between A and B phases of 3He

The behavior of the order parameter A _ai (x) is investigated for a planar interface between ³He-A and ³He-B. Boundary conditions are derived and the surface energy is calculated in excellent agreement with experiment. Our energy is 30% lower than a previous estimate which assumed the presence of a planar phase in the transition region.Alexander von Humboldt Fellow.     

Non-stoichiometry, grain boundary transport and chemical stability of proton conducting perovskites

The interrelationship between defect chemistry, non-stoichiometry, grain boundary transport and chemical stability of proton conducting perovskites (doped alkaline earth cerates and zirconates) has been investigated. Non-stoichiometry, defined as the deviation of the A : M molar ratio in AMO₃ from 1 : 1, dramatically impacts conductivity, sinterability and chemical stability with respect to reaction with CO₂. In particular, alkaline earth deficiency encourages dopant incorporation onto the A-atom site, rather than the intended M-atom site, reducing the concentration of oxygen vacancies. Transport along grain boundaries is, in general, less favorable than transport through the bulk, and thus only in fine-grained materials does microstructure impact the overall electrical properties. The chemical stability of high conductivity cerates is enhanced by the introduction of Zr. The conductivity of BaCe_0.9–x Zr _x M_0.1O₃ perovskites monotonically decreases with increasing x (increasing Zr content), with the impact of Zr substitution increasing in the order M = Yb Gd Nd. Furthermore, the magnitude of the conductivity follows the same sequence for a given zirconium content. This result is interpreted in terms of dopant ion incorporation onto the divalent ion site.     

Grain boundary plane distributions and single-step versus multiple-step grain boundary engineering

The ‘five-parameter’ (i.e. both misorientation and grain boundary plane) distribution in type 304 austenitic stainless steel has been measured and evaluated for an ‘as-received’ (AR) specimen and specimens undergoing both single-step grain boundary engineering processing (SSGBE) and multiple-step grain boundary engineering processing (MSGBE) comprising three iterations. The results showed that the fundamental requirement for twinning-related GBE is to maximise concomitantly the proportion of both Σ3 and Σ9 boundaries, which in turn supports the development of special planes in the grain boundary network. 1 1 0 and 1 1 1 tilt and twist boundaries play a key role in the formation of ‘special’ grain boundary planes. MSGBE added increased proportions of Σ3 boundaries and resulted in development of different characteristics in the planes distribution compared to SSGBE. These modifications are likely to result in improved grain boundary properties after MSGBE compared to SSGBE.     

Grain boundary crystallography in two Fe-C-P alloys

Abstract

Misorientation, grain growth and brittle fracture were investigated in two iron - carbon alloys containing 0.06 wt-% phosphorus (0.06P) and 0.12 wt-% phosphorus (0.12P) after selected heat treatment schedules. A 'fracture surface serial sectioning' technique was devised and combined with misorientation measurements to reconstruct specimens after fracture. Anomalous grain growth occurred in the 0.06P specimen only, after 1000°C annealing. This was attributed to the inhomogeneous distribution of phosphorus at the interfaces. No evidence was found for the direct influence of misorientation angle distributions or coincidence site lattice distributions on anomalous grain growth. The proportion of Σ3s increased greatly after annealing at 1000°C, attributed to the twinning that developed in the austenite range. There was strong evidence that Σ3s were in general more resistant to brittle fracture than were random boundaries. It is suggested that alloys of this type could be 'grain boundary engineered' to improve fracture resistance.