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Advancements in magnetoencephalography (MEG) present new opportunities for clinical pediatric centers faced with the challenge of offering a sensitive diagnostic evaluation for epilepsy in infants and young children. A noninvasive brain imaging method, which measures the magnetic fields generated by the electrical activity of the human brain, MEG provides significant advantages that make it particularly suitable for pediatric use. First, patient preparation is quick and easy and, unlike functional magnetic resonance imaging/magnetic resonance imaging, operates silently. As such, brain activity can be measured in a much more child-friendly environment. Second, although similar to electroencephalography in providing millisecond temporal resolution of neural activity, MEG offers advantages over electroencephalography in superior spatial resolution given the sensitivity of MEG to intracellular neural currents and the relative “transparency” of magnetic fields to biological tissues. Finally, when combined with magnetic resonance imaging, MEG data contribute unique information to the evaluation and localization of foci beyond that obtained with other neuroimaging technologies. As an example, the utility of MEG has been shown in determining irritative onset zones associated with epileptiform activity and for the functional mapping of nearby eloquent areas. This article reviews MEG theory along with new methods and technological advances, which result in improvements in pediatric epilepsy care. After a brief introduction to MEG technology, recent hardware developments (optimized for pediatrics), improved signal processing methods, and technological developments are reviewed. Next, two clinical case studies are presented to provide examples of MEG source localization methods applied to identify epileptogenic foci in pediatrics and compare established and recently developed analysis methods. We conclude that MEG is a unique and powerful imaging method for epilepsy treatment and patient care, increasingly a focal point in medical settings aiming to excel in the treatment of epilepsy both for diagnostics and surgical outcomes with near-future pediatric MEG system also providing optimal clinical assessments in infants and young children.

The evolution of sodium, oxygen and hydrogen ion conducting ceramic electrolytes will be traced with particular attention given to oxygen ion conductors. These materials such as the stabilized zirconias are critical to the commercial success of oxygen sensors, the solid oxide fuel cell (SOFC) and devices separating oxygen from air. They also can play significant roles as protective coatings for other oxygen ion conductors and in the synthesis of sodium-ion conducting ceramic composites. This overview will also include an assessment of mixed ionic/electronic conducting oxides employed in oxygen separation devices, the partial oxidation of methane, and in the development of composite anode and cathode structures in the SOFC. The development of rechargeable beta batteries for electric vehicles and load leveling energy storage systems employing sodium-ion conducting β-Al 2 O 3 electrolytes will be placed into perspective with the evolution of SOFC power generation systems.

We report the case of a 61-year-old man with a history of a Bilroth II procedure who presented to the emergency department after 12 hours of severe epigastric pain and one episode of coffee-ground emesis. Computed tomography and esophagogastroduodenoscopy revealed intussuscepted jejunum through a gastrojejunostomy that required emergency operative reduction. This case illustrates the rare complication of acute jejunogastric intussusception more than 20 years after a Bilroth II procedure.

We discuss the case of a 59-year-old woman who presented to the emergency department with complaints of diplopia for five days. Physical examination disclosed an isolated complete right abducens nerve palsy. Angiography revealed a giant aneurysm from the postcavernous portion of the right internal carotid artery. This case illustrates an unusual etiology of a unilateral abducens nerve palsy.

A 24-year-old woman with a recent diagnosis of infectious mononucleosis presented with fever and left upper quadrant pain. Abdominal computed tomography revealed a splenic abscess that at laparotomy was found to have ruptured. This case illustrates a rare cause of splenic abscess and a rare complication of infectious mononucleosis.

A 77-year-old woman with an indwelling gastrostomy tube presented with a clinical picture of pneumonia and small bowel obstruction. During gastrostomy closure and performance of a feeding jejunostomy, inadvertent transmigration of the gastrostomy tube was found to be the etiology of the small intestinal obstruction. This case illustrates a rare complication of enteric feeding tubes.

Study objectives: This study was designed to establish the frequency, magnitude, and possible etiologies of the dysfunction of activity-sensing internal pacemakers during helicopter aeromedical transport. Design: Two models of Medtronic Activitrax ® pacemakers were attached externally to healthy adult volunteers. Each volunteer then was loaded into the helicopter and subjected to a flight sequence. Pacemaker firing rates throughout this sequence were recorded. Setting: On separate days, Aerospatiale Dauphin and Twinstar helicopters completed a total of 23 flights. Type of participants: Four healthy adult volunteers, two men and two women, participated. Interventions: These included intra- and inter-flight threshold reprogramming and external magnet application. Results: The average preflight pacemaker rate of 65 beats per minute increased to an average in-flight rate of 105 beats per minute, which resolved to preflight rates on shutdown. This pattern was consistently extinguished with external magnet application. Conclusion: The effect of rotor motion and flight vibration on the rate-response of the Activitrax ® pacemaker is both predictable and easily preventable. Possible guidelines for the safe transport of these patients, using pacemaker reprogramming or external magnet application, are examined.

Summary Helicopter and fixed-wing air medical transportation provides an important role in the management of critically-ill patients. As the use of cardiac pacemakers continues to grow, knowledge of their expanding capabilities and sophistication is important. The environments of our “airborne intensive care units” are subject to many sources of electromagnetic and vibrational interference. Although pacemaker shielding mechanisms have become quite eleborate, further studies are needed to define their reliability in modern aircraft. Further, the possible effects of electromagnetic and vibrational interference upon inflight reprogramming require further study.

The advantages of the stress-relaxation technique can be effectively realized by applying the procedure to conventional four-point bend tests usually employed in the deformation studies of ceramic materials. A test system and procedure for determining plastic strain rate-stress relationships at elevated temperatures (up to 1600° C) by the stress-relaxation method is described. An analysis to calculate true plastic strains and stresses from measured deflections and loads is presented and it is shown that such an analysis requires minimum assumptions regarding the materials behaviour. Preliminary results obtained on an iron-doped MgO specimen are discussed and compared with the constant load test results obtained on identical specimens. Sources of error in the four-point bend stressrelaxation tests and the methods to minimize them are also discussed.

A critical analysis was performed on galvanic cell techniques which employ calcia‐stabilized zirconia electrolytes. The presence of low‐level electron or hole conduction in the ionic conducting range of the electrolyte was found to be responsible for oxygen permeation through the electrolyte which can either oxidize or reduce a single‐phase oxide and, hence, lead to drift in the open‐circuit emf with time. Taking oxygen permeation into account quantitatively, a series of isothermal titration experiments in wüstite was performed in which the cell emf was found to be linear with composition (i.e., oxygen‐iron molal ratio). These results are in excellent agreement with a recent composite analysis of all thermodynamic data in the Fe ‐ O system. Previous cell studies in which deviant behavior has been reported are probably due to improper galvanic cell techniques. No evidence was found to support the existence of more than one‐phase or second‐order transitions within the wüstite phase.

The load relaxation technique was applied to double- cantilever-beam specimens for the determination of diagrams of the stress intensity factor (K1) vs crack velocity (V) for soda-lime glass. The effect of machine compliance was explicitly incorporated into the velocity equation. The data were in good agreement with published data from different techniques. It was shown that a hard machine (small compliance) is best suited for the load-relaxation technique. The use of a soft machine (high compliance) will lead to catastrophic failure during load relaxation. A single-edge notched-beam specimen in bending is unsuitable for the relaxation technique since catastrophic failure will occur during relaxation regardless of machine compliance.

High-temperature (1160 to 1450‡ C) deformation of dense polycrystalline (10 to 90 Μm) Al2O3 and MgO doped with Fe (up to 2.65 cation %) was studied by stress relaxation, dead-load creep and creep recovery. In some cases, all three deformation tests were conducted on a single specimen. A comparison of strain rate-stress data calculated from both stress relaxation and dead-load creep experiments revealed discrepancies in both the magnitude of the strain rates and the dependence between the strain rate and stress. These differences were attributed to the existence of anelastic deformation effects. The correction of stress relaxation data in the low stress regime for linear anelasticity led to strain rate-stress data in reasonably close agreement with results obtained from dead-load creep tests conducted in the viscous creep regime. Creep recovery experiments indicated that anelastic deformation in these ceramic materials was relatively insensitive to changes in temperature and grain size over the range of variables studied.

The critical stress-intensity factor, K1C, and the fracture strength, σf, have been investigated on both hot-pressed and sintered lithia-stabilized β”-alumina. The hot-pressed material possessed a strong preferred orientation with many of the basal planes aligned perpendicular to the direction of hot-pressing. Both K1C and σf were found to be orientation-dependent. Two regimes of fracture were identified. In fine-grained material (

An analysis of mass transport in the diffusional creep of iron-doped, polycrystalline MgO was conducted. Creep regimes in which magnesium grain boundary, oxygen grain boundary, and magnesium-lattice diffusion were rate-controlling were identified. An analytical procedure was developed for the estimation of the diffusion constants for these three processes.

The steady state creep of polycrystalline (5–60 μm) rutile, which is doped with 1 cation % tantalum, is controlled by a Nabarro-Herring lattice diffusion process at 1100 to 1200° C. Doping with tantalum significantly depresses the steady state creep rate by lowering the concentrations of titanium interstitials and oxygen vacancies. The concentrations of these defects, and hence the steady state creep rate of doped rutile, can be increased by decreasing the oxygen partial pressure below 10−7 to 10−8 atm at 1200° C. Tentative evidence is presented in support of the hypothesis that the steady state creep of polycrystalline, undoped rutile at 950 to 1100° C is controlled by interfacial defect creation and/or annihilation at grain boundaries. Interfacial controlled deformation rates are probably due to the large concentrations (and perhaps high mobilities) of cation and anion lattice defects which are present in pure rutile equilibrated in both oxidizing and reducing atmospheres. The steady state creep rate was a very weak inverse function of the grain size and essentially independent of the oxygen partial pressure.

The effect of doping simultaneously with iron and titanium was studied in dense, polycrystalline alumina over a range of grain sizes (10 to 100 μm) and temperatures (1250° to 1550°C). In the double-doped system, the titanium concentration was varied between 0.05 and 0.15 cation %, whereas the iron-dopant level was varied between 0.05 and 6 cation %. For iron concentrations below about 2 to 3%, the aluminum vacancy concentration was dominated by the presence of quadrivalent titanium in substitutional solid solution and Nabarro-Herring diffusional creep at 1450°C was rate-limited by aluminum lattice diffusion. As the iron-dopant level was increased, the concentration of divalent iron became comparable to that of quadrivalent titanium, leading to a suppression in the cation lattice diffusivity at an iron-to-titanium ratio of ∼60. These results suggested that, at the dopant levels and temperatures studied, more than 98% of the iron was in the trivalent state. The diffusional creep of polycrystalline alumina doped with a single iron impurity (0.2 to 2%) was reinterpreted in terms of simultaneous contributions of aluminum lattice and grain-boundary diffusion, consistent with a grain-size dependence corresponding to a mixture of Nabarro-Herring and Coble creep. Aluminum grain-boundary diffusion was found to be significantly enhanced by the presence of iron in solid solution. Evidence is presented to suggest that the diffusional creep of polycrystalline Al2O3 doped with a single titanium dopant is interface-controlled. Interfacial kinetics can be promoted by several factors, including (1) a small grain size, (2) a high cation lattice diffusivity, (3) slow cation grain-boundary diffusion, and (4) the presence of a grain-boundary second phase.

Nasicon ceramic electrolytes were processed and characterized using a number of different techniques. The materials were sintered at T = 1100/sup 0/C to 1375/sup 0/C. Results of this work demonstrate that Nasicon ceramics with compositions around x = 2 (i.e. Na/sub 3/Zr/sub 2/Si/sub 2/PO/sub 12/), are not monophase, possess poor mechanical strength and an anomalous thermal expansion behavior, and are subject to chemical and phase instability during densification at high temperatures. Wet-chemical techniques, X-ray diffraction, and SEM analyses were used to observe the phase and chemical decomposition of these materials. The products of this decomposition process are correlated with the poor physical properties.

The construction of deformation mechanism maps for a polycrystalline ionic solid in which anion and cation transport are coupled has been demonstrated. Because of anioncation ambipolar coupling, two regimes of Coble creep are possible in systems where anion grain boundary transport is rapid: (1) rate-controlled at low temperatures and small grain sizes by cation grain-boundary diffusion, and (2) rate-limited at high temperatures and large grain sizes by anion grain-boundary diffusion. A new type of deformation mechanism map was introduced in which the temperature and grain size were primary variables. This map was shown to be particularly useful for materials which deform primarily by diffusional creep mechanisms. Ambipolar diffusional creep theory was used to construct several deformation mechanism maps for polycrystalline MgO and magnesiowustite over wide ranges of stress, grain size, temperature and composition.

Steady-state creep experiments were performed on hot-pressed polycrystalline MgO doped with Fe. Dead-load 4-point bend creep tests were conducted at stresses of 26 to 270 kg/cm2, at temperatures of 1250° to 1450°C, in O2 partial pressures of 1 to 10−9 atm, on specimens with grain sizes of 10 to 65 μm. Viscous steady-state creep was always observed when the grain size was stable. Experiments at variable PO2's and temperatures were used to identify regimes of high (117 ± 10 kcal/mol) and low (81 ± 5 kcal/mol) activation energy. In the latter, creep rates were nearly independent of Fe dopant concentration and PO2, whereas in the former creep rates were enhanced by increasing PO2's and Fe dopant levels. The high- and low-activation-energy regimes were interpreted as diffusional creep controlled primarily by Mg lattice diffusion and O grain-boundary diffusion, respectively.

Grain growth was studied at 1300°, 1400°, and 1500°C in nearly theoretically dense (0.1 to 0.8% porosity) hot-pressed magnesium oxide (99.99+%) and magnesiowustite (0.10 and 0.48 wt% Fe2O3). Small amounts of porosity had large effects on the kinetics of grain growth. Grain growth is probably porosity-controlled in MgO at all temperatures and in magnesiowustite at 1400° and 1500°C if pores remain on grain boundaries or at grain intersections. The tendency for the entrapment of porosity is enhanced as the temperature is increased and as the dopant (Fe2O8) concentration is decreased. Small amounts of porosity (

The transient creep rate in polycrystalline Fe-doped MgO (grain size 4 to 19 μm) follows the exponential time relation The initial strain rate, , depends linearly on stress, and the relaxation time, τ, is thermally activated. Two mechanisms for the transient were considered, i.e. (1) viscous grain-boundary sliding with elastic accommodation and (2) transient diffusional creep. Although the results are not conclusive, transient diffusional creep appears to be the most promising mechanism.

The Cahn-Fullman method was used to determine three-dimensional grain size distributions from experimentally measured two-dimensional chord distributions in high-purity MgO and magnesiowustite (0.10 and 0.48 wt% Fe2O3). Using the standard deviation of the three-dimensional distributions, the constant which relates average three- and two-dimensional grain sizes was evaluated by the procedure of Mendelson. The characteristics of the grain size distributions were correlated with steady state, abnormal, and terminal grain growth in MgO and magnesiowustite from 1300° to 1500°C. The distribution parameters were particularly useful in characterizing anomalous (i.e. abnormal and terminal) grain growth behavior.

The decomposition of boehmite, or alpha alumina monohydrate, was studied in water vapor and nitrogen atmospheres between 430° and 500°C. Powder samples used consisted of agglomerates of very small boehmite crystallites. The rate of decomposition was determined with a transducer which measured the loss of sample weight due to the liberation of gaseous water. A linear rate law is obeyed by the decomposition of boehmite and, because of the characteristics of the fractional weight loss-time curve, an interface model was suggested as the mechanism of decomposition. The activation energy was 70 kcal/mole. The effect of water vapor on the rate of decomposition was investigated. A simple forward-back reaction mechanism did not satisfactorily explain the data. Two models, either of which could be valid, were proposed to describe the relation between decomposition rate and water vapor pressure: (1) An adsorption-desorption process of water on the boehmite was assumed, and (2) an intermediate species of Al2O3 entering into the decomposition reaction was assumed.

Polycrystalline MgO and MgO-Fe2O3 solid solutions (0.10 to 8.08 wt% Fe2O3) were fabricated to almost theoretical density by vacuum hot-pressing. Specimens were creep-tested in air under four-point dead-load conditions between 1000° and 1400°C at stresses between 50 and 550 kg/cm2. Steady-state creep was never achieved in the experiments, which sometimes lasted more than 50 h. The strain rate vs time (t) data were described by an equation of the form =c1/(t+C2)p, which is consistent with the assumptions that creep occurs at least in part by a “viscous” mechanism and that grain growth occurs simultaneously. Doping MgO with Fe2O3 enhanced the viscous contributions to creep and inhibited the nonviscous ones. Creep rates in these specimens increased with increasing Fe2O3 additions. The occurrence of simultaneous grain growth during the high-temperature creep of magnesiowustite (i.e. MgO-Fe2O3 solid solutions) was used in establishing the strain rate vs grain size dependence. The results of this study are consistent with a transition between grain boundary and lattice diffusion mechanisms as the grain size increases (4 to 44 μan). The creep of polycrystalline MgO is a mixed process in that viscous and nonviscous (dislocation) contributions are present.

Polycrystalline samples, thin single crystals, and—325 mesh powders of brucite decomposed according to a first-order rate equation consistent with a random nucleation process in beds of micron-sized particles formed by disruption during the very early stages of the process and consistent with electron and optical microscope observations. Kinetic data for the decomposition of thick single crystals were more complex but could be fitted to an interface-model equation which predicts that decomposition will be controlled by the movement of an interface reaction along the basal plane. Physical disruption of these crystals was confined initially to the outer edges of the crystals and proceeded along the basal plane in the form of a contracting disk. Since the decomposition occurs in cracked material and not at a strict interface, the fit of the data to the interface model may be fortuitous.

The decomposition of brucite was studied directly in electron and optical microscopes. A small amount of decomposition caused marked changes in the physical state of the brucite crystal. The decomposition was interpreted as a normal nucleation and growth process in which magnesium oxide nuclei formed coherently with the brucite matrix. This process accounts for: (a) the structural relations between parent and product phases, (b) the small crystallite size of the product, (c) the cracking process occurring in the initial stages of decomposition, and (d) the range of d spacings observed for the magnesium oxide formed during the decomposition.

A mass transport equation which takes into account parallel diffusion paths for both anions and cations was derived and applied to the diffusional creep of polycrystalline ionic solids. From the results of the analysis, several limiting conditions were found for grain-boundary- and lattice-diffusion-controlled kinetics. These conditions depend on temperature, grain size, and type and concentration of cation impurities. Examples of these limiting situations are given for the creep of polycrystalline Fe-doped MgO and transition-metal-doped Al2O3. Summary A mass transport equation which takes into account parallel diffusion paths for anions and cations was derived and applied to the diffusional creep of polycrystalline ionic solids. The effect of grain size and cation impurities of variable valence in solid solution on the relative contributions of lattice and grain-boundary diffusion of different ionic species in polycrystalline MgO and Al2O3 was examined. Depending on the temperature, grain size, impurity level, and O2 partial pressure, several limiting conditions were found: Limit I: At very small grain sizes and reasonably small cation lattice diffusivities the creep rate will be controlled by the slower-moving ion in the grain-boundary regions (i.e. Coble creep). Limit II: For intermediate grain sizes and cation lattice diffusivities the creep rate will be controlled by cation lattice diffusion when anion transport is significantly faster near grain boundaries than in the lattice (i.e. Nabarro-Herring creep). Limit III: For an appropriate combination of large grain size and high cation lattice diffusivity the creep rate will be controlled by anion boundary diffusion (i.e. Coble creep). Well-defined examples of limits I and II have been observed in the creep of Fe-doped polycrystalline MgO, and tentative evidence exists for limit III. Most results of studies of creep in polycrystalline Al2O3 (doped and undoped) fall within limit II, with some overlap with limit III. The model developed in the present work explains much of the data in the literature in which creep rates correspond to cation lattice mobilities. It is concluded that in the creep of polycrystalline ionic solids anion transport near grain boundaries is rapid and can, in some circumstances, be rate-controlling. It should also be possible to apply this model to sintering and thermal-grooving data for such systems, particularly for Al2O3, in which cation lattice diffusion is frequently observed to be rate-controlling.32

Steady-state creep was studied in hot-forged polycrystalline Al2O3 (3 to 42 μm) of nearly theoretical density doped with≤1 cation % of Fe, Ti, or Cr. Tests were conducted at stresses between 10 and 550 kg/cm2 at 1375° to 1525°C under O2 partial pressures of 0.88 to 10−10 atm. Except in the 10-μm Fe-doped material tested at very small stresses, slightly nonviscous creep behavior was generally observed. The effects of Po2 on the creep rate indicated that increased concentration of a divalent (Fe2+) or quadrivalent (Ti4+) impurity in solid solution enhances the creep rate of polycrystalline Al2O3. The activation energies for the creep of Fe- and Ti-doped Al2O3 samples (148 and 145 kcal/mol, respectively) were significantly higher than that for Cr-doped material (114 kcal/mol). Taking into account the effects of Po2, temperature, and grain size, it was concluded that the steady-state creep of transition-metal-doped Al2O3 is controlled by cation lattice diffusion.

A quantitative rheological model was developed for predicting the non-Newtonian flow behavior of aqueous halloysite suspensions. Halloysite clay from the large Utah deposits was used for this investigation because of its unique tubular morphology. The flow behavior of the suspensions was characterized by the semi empirical Ree-Eyring generalized viscosity equation. The β parameter in the Ree-Eyring equation, a measure of the relaxation time for the flow process, was very sensitive to variations in the pH and volume fraction solids. When the logarithm of 0 was plotted versus pH (4 to 11) for halloysite suspensions containing 15, 20, and 25 wt% solids, linear plots with increasing slopes were obtained. Attempts were made to show the physical significance of the parameters introduced in the model.

An analysis of strains and stresses in four-point bending creep tests in the limit of small beam deflections resulted in a general equation which relates the load-point deflection, the applied load, the creep exponent (N), and the geometrical parameters of the loading system. Measurements of load-point deflection rates, which are experimentally easy to accomplish in ceramic systems, vs the applied load lead to the direct determination of the creep exponent and the creep compliance in a steady-state creep test. The creep compliance is a function of the temperature, grain size, and all other factors except stress. The elastic equation relating the load-point deflection and the outer fiber strain is strictly valid for viscous creep and approximately valid for nonviscous creep (i.e. N>1) if the ratio of the distance between the support points to the distance between the load points is not very large.

All data available in the literature regarding oxygen activities and composition limits for the wustite phase were analyzed critically. The most reliable activity data were collected into a composite in which (1) the partial molal free energy of oxygen varied linearly with composition and reciprocal temperature and (2) the partial molal enthalpy and entropy of oxygen varied linearly with composition and were independent of temperature. These linear variations of the thermodynamic properties are inconsistent with the presence of additional phases which have been reported within the wustite phase field. The composition limits of the wustite phase, which were computed from the composite, were in excellent agreement with Darken and Gurry's original phase diagram.

A mixed diffusional creep mechanism in a polycrystalline ceramic can cause the apparent activation energy to vary with temperature and grain size. For mechanisms involving parallel transport paths for the same ion (e.g., cation lattice and grain-boundary diffusion), the process with the lowest activation energy will be dominant (i.e., rate-limiting) at low temperatures and the process with the highest activation energy will dominate at high temperatures. However, for mechanisms involving coupled and parallel diffusional steps (e.g., cation lattice and anion grain-boundary diffusion), the process with the lowest activation energy will be dominant at high temperatures whereas the high-activation-energy process will dominate at low temperatures. Examples of these effects are presented for the diffusional creep of polycrystalline MgO and Al/sub 2/O/sub 3/ doped with Fe. Variations in creep activation energy with grain size and temperature are only significant when the difference in activation energies for the competing processes is significant and the temperature range investigated is large.

The sodium ion resistivity of lithia-stabilized polycrystalline β”-alumina was measured as a function of temperature for fine-grained and coarse-grained specimens with a chemical composition of 8.80 Na20-0.75 Li2O-90.45 A12O3 (wt%). A model is presented which explains the dependence of sodium ion resistivity on grain size. Using the model the activation energy was determined for the transport of sodium ions across a grain boundary in this form of sodium β”-alumina.

It is well known that under conditions of small stress, fine grain size and enhanced lattice and/or grain boundary diffusion polycrystalline ceramics will deform at elevated temperatures by one or more diffusional creep mechanisms.[1–9] In this paper creep deformation maps will be utilized to illustrate the role of soluble (aliovalent) dopants, grain size, and oxygen fugacity on the relative contributions of (1) lattice diffusion, (2) grain boundary diffusion, (3) interfacial defect reactions at grain boundaries, and (4) power law or dislocation deformation mechanisms on the creep deformation of polycrystalline magnesium and aluminum oxides.

The critical stress-intensity factor, K1C, and the fracture strength, σf, have been investigated on both hot-pressed and sintered lithia-stabilized β”-alumina. The hot-pressed material possessed a strong preferred orientation with many of the basal planes aligned perpendicular to the direction of hot-pressing. Both K1C and σf were found to be orientation-dependent. Two regimes of fracture were identified. In fine-grained material (

Steady-state creep experiments were performed on hot-pressed polycrystalline MgO doped with Fe. Dead-load 4-point bend creep tests were conducted at stresses of 26 to 270 kg/cm2, at temperatures of 1250° to 1450°C, in O2 partial pressures of 1 to 10−9 atm, on specimens with grain sizes of 10 to 65 μm. Viscous steady-state creep was always observed when the grain size was stable. Experiments at variable PO2's and temperatures were used to identify regimes of high (117 ± 10 kcal/mol) and low (81 ± 5 kcal/mol) activation energy. In the latter, creep rates were nearly independent of Fe dopant concentration and PO2, whereas in the former creep rates were enhanced by increasing PO2's and Fe dopant levels. The high- and low-activation-energy regimes were interpreted as diffusional creep controlled primarily by Mg lattice diffusion and O grain-boundary diffusion, respectively.

The exaggerated grain growth during high temperature (1585° – 1600°C) sintering of Li2O-stabilized β″-alumina compositions is essentially a process or very rapid growth of stabilized 3″ nuclei to large dimensions (>100 µm) while the matrix remains extremely fine-grained (< 3 µm). The final grain size is therefore inversely proportional to the cube root of the volume fraction originally occupied by these nuclei~1 Such a relationship was reported by Cutler in the case of seeded alpha alumina powders and more recently by Lacour and Paulus3 in the case of barrium-hexaferrite. The occurrence of an exaggerated grain growth during sintering of ceramics has been shown to depend on the flux-growth mechanism wherein a film or channel of liquid phase surrounds the growing nuclei. The growth of nuclei to large dimensions takes place by the solution of surrounding fine grains in the liquid and their precipitation on the growing crystal surface. While Cooy4 and Stuijts5 reported accidental impurities as the cause of liquid phase formation, Lacour and Paulus3 showed that local inhomogenities can also give rise to a liquid phase responsible for the flux growth.

In this paper recent developments in the processing and characterization of polycrystalline β″-alumina ceramic electrolytes will be discussed, β″-alumina ceramics have potential uses in diverse devices such as the electrolyte or sodium-ion separator in (1) the sodium-sulfur energy storage battery 1 designed for either electric utility load leveling or automotive propulsion, (2) a high temperature sodium heat engine or thermoelectric device.2 (3) a high electrowinning of sodium metal from molten sodium salts, (4) the purification of sodium metal and (5) various electrochemical devices for measuring sodium and perhaps even oxygen activities.

The sodium ion resistivity of lithia-stabilized polycrystalline β”-alumina was measured as a function of temperature for fine-grained and coarse-grained specimens with a chemical composition of 8.80 Na20-0.75 Li2O-90.45 A12O3 (wt%). A model is presented which explains the dependence of sodium ion resistivity on grain size. Using the model the activation energy was determined for the transport of sodium ions across a grain boundary in this form of sodium β”-alumina.

Results are presented for the subcritical crack growth behavior of polycrystalline β”-alumina in water as a function of microstructure. A load relaxation technique using double cantilever-beam-type specimens and dynamic fatigue experiments using rectangular bar- and ring-type specimens were utilized to evaluate subcritical crack growth parameter N. The value of N ranged from 64–96 in the load relaxation experiments and from 26–76 in the dynamic fatigue experiments. The N values obtained from the two techniques for identical microstructures indicated no general agreement. This discrepancy is discussed on the basis of the spatial variation in material properties and dynamic effects of crack propagation.

Steady state, constant load, creep experiments have been conducted on pure and doped, polycrystalline magnesium and aluminum oxides to determine: 1) the effect of solid solution impurities (e.g. Fe, Cr, Ti) on deformation mechanisms, particularly as they relate to the concentration of lattice defects 2) the effects of impurities, atmosphere, and grain size on the relative contribution to viscous deformation of lattice and grain boundary diffusion of both cationic and anionic species 3) the relative roles of viscous and non-viscous deformation mechanisms.

Nach Brook entspricht in Oxiden ( ;z.B. Al2O3), die mit Verunreinigungen von variabler Valenz ( ;z.B. Ti) dotiert sind, die empirische Aktivierungsenergie fur das Sintern nicht nur der Defektbewegung, sondern auch der Defekterzeugung.

Das stationare Kriechverhalten von fast zu theoretischer Dichte heisgeprestem Al2O3 mit Fe, Ti oder Cr-Dotierungen 1% wird untersucht.