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The influence of pyroxene on the rheology of peridotite is critical for understanding the dynamics of the upper mantle. Intragranular deformation of each phase is often assumed to be independent and the rheology of aggregates is calculated by mixing models. However, intergranular deformation on grain boundaries (e.g., olivine-olivine) and phase boundaries (e.g., olivine-pyroxene) could be intrinsically different and needs to be constrained experimentally. Experiments on fine-grained olivine and clinopyroxene demonstrate that 50-50 mixtures of these phases (a wehrlite) deform up to ∼30 times faster than either of the end-members when scaled to the same experimental conditions. Strain localization within layers with a high-density of phase boundaries demonstrates that phase boundaries enhance the rate of deformation relative to that of either olivine or pyroxene aggregates. Olivine AG-type crystallographic textures (i.e., axial girdles of [100] and [001] axes with [010] maxima sub-perpendicular to the shear plane) observed in deformed specimens are indistinguishable from those observed in many naturally deformed mantle peridotites. We propose that the presence of abundant olivine-pyroxene phase boundaries can decrease the viscosity of mantle shear zones, enhancing strain localization.

The chemical widths of grain and phase boundaries in deformed wehrlite (olivine–clinopyroxene; Ol–Cpx) aggregates are characterized by atom probe tomography (APT), a (laser-assisted) field evaporation technique employing time-of-flight mass spectrometry. The wehrlite was deformed to high finite strain in diffusion creep: The effective viscosity measured for the wehrlite is approximately an order of magnitude lower than that of either end-member aggregate; further, phase ordering, in which the spatial density of Ol–Cpx phase boundaries increases with accumulated strain, characterizes the deformation (Zhao et al. in Earth Planet Sci Lett 517:83–94, 2019). The mechanical results imply that the transport properties of the phase boundaries—dictated by their structure and composition—differ from those of grain boundaries. Our APT data show that, indeed, the chemical widths of crystalline Ol–Cpx phase boundaries—3.1–6.6 nm, depending on the element used for their characterization—are up to a factor of two greater than the chemical widths of crystalline Ol–Ol and Cpx–Cpx grain boundaries. Careful statistical analyses of the APT data reveal that the near-boundary compositional profiles of the presented Ol–Cpx phase boundary are consistent with—indeed, evidence for—the rheological model in which diffusion creep is rate-limited by the (mechanism-required) interfacial reactions at the Ol–Cpx phase boundaries. Such an analysis is unavailable by current electron beam/X-ray spectrometry approaches, which have not the requisite spatial precision. APT application to nanometer-scale problems in silicate petrology is challenging, particularly because signal overlap is caused by the evaporation of polyion species. We carefully outline the procedures used to acquire and discriminate the data in order to address the challenges of signal overlap.

Here we report Atom Probe Tomography (APT) analyses of grain and phase boundaries of laboratory-deformed, fine-grained mixtures of clinopyroxene and olivine (Zhao, et al., 2019). The experiments show that the mixtures deform much more rapidly than either mineral endmember. This enhanced deformation in the two-phase material is due to stress-driven reactions at the phase boundaries. Lower effective viscosities of phase mixtures may be critical to the initiation of plate tectonics and the formation of mantle shear zones.The hypothesis presented here is that the ‘bulk rock’ – a wehrlite – deforms rapidly because conversion of one phase to the other occurs at phase boundaries (e.g., Sundberg & Cooper, 2008). In this model, grain-scale transport of the shared (slowly-diffusing) mineralogical component Si4+ is not required. The near-boundary gradients of olivine-insoluble ions are presented as evidence of the phase transformation which either dissolves olivine into clinopyroxene or vice versa. The resolving power of the APT makes it a promising tool for investigating the microphysics of rock deformation, bridging the atomic scale all the way to the plate-tectonic scale.References:Sundberg M, Cooper RF (2008) Crystallographic preferred orientation produced by diffusional creep of harzburgite: effects of chemical interactions among phases during plastic flow. J Geophys Res Solid Earth 113(12):B12208.Zhao N, Hirth G, Cooper RF, Kruckenberg SC, Cukjati J (2019) Low viscosity of mantle rocks linked to phase boundary sliding. Earth Planet Sci Lett 517:83–94.

Thirty silicate glasses were synthesized as realistic analogs to those expected to exist on Mars, the Moon, and Mercury. Samples were measured using visible/near-infrared and Mossbauer spectroscopy to determine the effects of varying bulk chemistry, oxygen fugacity, and temperature on spectral properties. For Martian glasses, the fO2 during fusion strongly affects absorption band intensities in the spectra, while bulk chemistry has noticeable secondary effects on absorption band positions. Titanium and iron content drive spectral changes in lunar glasses, where Fe3+ is effectively absent. Iron-free Mercury analog glasses have much higher albedos than all other samples, and their spectral shape is a close match to some pyroclastic deposits on Mercury. Synthetic glass spectra were used as inputs into a spectral unmixing model applied to remote orbital datasets to test for the presence of glass. The model is validated against physical laboratory mixture spectra, as well as previous detections of glass-rich pyroclastic deposits on the Moon. Remote data were then used from suspected impact deposits and possible pyroclastic deposits on Mars as a new application of the model: the results reveal spatially coherent glass-rich material, and the strong spectral match of the synthetic glasses to these remotely sensed data gives new insights into the presence and character of glasses on the Martian surface. The large library of glass spectra generated here, acquired from consistently synthesized and measured samples, can serve as a resource for further studies of volcanic and impact processes on planetary bodies.

Single crystals of ferromagnesian olivine (San Carlos, AZ, peridot; ~Fo88-90) have been deformed in both uniaxial creep and load relaxation under conditions of ambient pressure, T = 1500 °C and pO2= 10–10 atm; creep stresses were in the range 40 ≤ σ1 (MPa) ≤ 220. The crystals were oriented such that the applied stress was parallel to [011]c, which promotes single slip on the slowest slip system in olivine, (010)[001]. The creep rates at steady state match well the results of earlier investigators, as does the stress sensitivity (a power-law exponent of n = 3.6). Dislocation microstructures, including spatial distribution of low-angle (subgrain) boundaries additionally confirm previous investigations. Inverted primary creep (an accelerating strain rate with an increase in stress) was observed. Load-relaxation, however, produced a singular response—a single hardness curve—regardless of the magnitude of creep stress or total accumulated strain preceding relaxation. The log-stress v. log-strain rate data from load-relaxation and creep experiments overlap to within experimental error. The load-relaxation behavior is distinctly different than that described for other crystalline solids, where the flow stress is affected strongly by work hardening such that a family of distinct hardness curves is generated, which are related by a scaling function. The response of olivine for the conditions studied, we argue, indicates flow that is rate-limited by dislocation glide, reflecting specifically a high intrinsic lattice resistance (Peierls stress).

We have performed creep and stress-reduction experiments on polycrystalline ice (grain sizes, d ≈ 30 and ≈ 245 µm) in the grain boundary sliding and dislocation creep regimes (stresses σ = 0.5–5 MPa, temperature T = 233 K) to determine the constant-hardness creep compliance under these conditions. Our results are consistent with a microstructural state-variable description of dislocation-effected deformation whose rate is accelerated by grain boundary sliding. The fine-grained specimens reveal no subgrain boundaries, indicating that the stress-sensitive microstructural feature upon which the state-variable behavior is founded may be the dislocation structure of the grain boundaries. Deviations of our constant-hardness data from the behavior predicted by the state-variable formulation allow estimation of the viscosity of the grain boundaries, which is ~4.8 × 106 Pa s at this temperature.

Light noble gas (He–Ne–Ar) solubility has been experimentally determined in a range of materials with six-member, tetrahedral ring structures: beryl, cordierite, tourmaline, antigorite, muscovite, F-phlogopite, actinolite, and pargasite. Helium solubility in these materials is relatively high, 4 × 10 −10 to 3 × 10 −7 mol g −1 bar −1 , which is ∼100 to 100,000× greater than He solubility in olivine, pyroxene, or spinel. Helium solubility broadly correlates with the topology of ring structures within different minerals. Distinctive He–Ne–Ar solubility patterns are associated with the different ring structure topologies. Combined, these observations suggest ring structures have a strong influence on noble gas solubility in materials and could facilitate the recycling of noble gases, along with other volatiles (i.e., water, chlorine, and fluorine), into the mantle. Measurements of Ne and Ar solubility in antigorite, however, are highly variable and correlated with each other, suggesting multiple factors contribute the solubility of noble gases in serpentine-rich materials.

Here we apply the relatively new analytical technique of atom probe tomography (APT) to a naturally occurring isoferroplatinum grain (Pt3Fe) from northern California to constrain its origin and the nanoscale distribution of trace elements within the grain. Each analysis detected the positions of 10 million atoms in three dimensions with sub-nanometer spatial resolution. The (111) atomic planes are clearly resolved and their orientation was confirmed by electron backscatter diffraction (EBSD). The elemental concentrations of all elements (Pt, Fe, Ir, Ni, Rh, Ru, and Cu) determined from the APT mass/charge spectra are within 2 standard deviations of electron microprobe analysis (EMPA) of the grain. The isotopic abundances determined by APT matches NIST standard compositions over a wide range of concentrations, down to 100 ppmw. Nanoscale areas free of minor and trace elements are present throughout the sample. These could be due to the random distribution of atoms. Alternatively, the Pt-Fe phase diagram indicates that order-disorder precipitates of an L12 structure could have formed as the isoferroplatinum cooled from magmatic temperatures (Nose et al. 2003). The trace element free areas could be such precipitates, which would support a high-temperature igneous origin for the isoferroplatinum, rather than formation during low-temperature serpentinization. The results highlight the unique capabilities of APT and the potential utility of knowing the location and identity of atoms in nanometric volumes.

Fatigue can cause materials that undergo cyclic loading to experience brittle failure at much lower stresses than under monotonic loading. We propose that the lithospheres of icy satellites could become fatigued and thus weakened by cyclical tidal stresses. To test this hypothesis, we performed a series of laboratory experiments to measure the fatigue of water ice at temperatures of $198$ K and $233$ K and at a loading frequency of $1$ Hz. We find that ice is \textit{not} susceptible to fatigue at our experimental conditions and that the brittle failure stress does not decrease with increasing number of loading cycles. Even though fatigue was not observed at our experimental conditions, colder temperatures, lower loading frequencies, and impurities in the ice shells of icy satellites may increase the likelihood of fatigue crack growth. We also explore other mechanisms that may explain the weak behavior of the lithospheres of some icy satellites., 33 pages, 7 figures, accepted for publication in JGR Planets

Remote sensing observations have identified aluminate spinel, in the absence of measureable olivine and pyroxene, as a globally distributed component of the lunar crust. Earlier remote sensing observations and returned samples did not indicate the presence of this component, leaving its geologic significance unclear. Here, we report visible to mid-infrared (V-IR) reflectance (300–25 000 nm) and Mossbauer spectra of aluminate spinels, synthesized at lunar-like oxygen fugacity (ƒO2), that vary systematically in Fe abundance. Reflectance spectra of particulate ( 6 Fe#. Although the 2000 and 2800 nm bands are assigned to Fe 2+ IV electronic transitions, spectra of aluminate spinels with excess Al2O3 demonstrate that the strengths of the 1000 nm bands are related to the abundance of Fe 2+ VI. The abundance of Fe 2+ VI depends on bulk Fe content as well as factors that control the degree of structural order-disorder, such as cooling rate. Consequently the strength of the 1000 nm bands are useful for constraining the Fe content and cooling rate of remotely sensed spinel. Controlling for cooling rate, particle size, and ƒO2, we conclude that spinels with >12 Fe# (

Helium partitioning between olivine, orthopyroxene, clinopyroxene, and spinel and basaltic melt has been experimentally determined under upper mantle melting conditions (up to 20 kbar and 1450 °C). Under the conditions explored, helium partition coefficients are similar in all minerals investigated ( K d He ∼ 10 − 4 ), suggesting He is evenly distributed between the minerals of spinel peridotite. This is in contrast to most incompatible elements, which are concentrated in clinopyroxene in spinel peridotite. The studied minerals have different concentrations of point defects, but similar He solubility, providing no evidence for He partitioning onto specific defects sites (e.g. cation vacancies). Upper limits on the partition coefficients for Ne and Ar have also been determined, constraining these elements to be moderately to highly incompatible in olivine at the conditions of spinel peridotite melting ( 10 − 2 and 10 − 3 , respectively). Helium partitioning in peridotite minerals varies little within the range of temperatures, pressures, and mineral compositions explored in this study. Reported partition coefficients, in combination with previous work, suggest that moderate to high degree mantle melting is not an efficient mechanism for increasing (U+Th)/He, (U+Th)/Ne, or K/Ar of the depleted mantle (DMM) through time, and consequently, supports the argument that recycling of oceanic crust is largely responsible for the relatively strong radiogenic noble gas signatures in the depleted mantle. Mantle residues with lowered (U+Th)/He, (U+Th)/Ne, and K/Ar may be produced through large extents of melting, but concentrations of noble gases will be low, unless noble gas solubility in solids deviate from Henryʼs Law at high fugacity.

Alteration of fresh rock surfaces proceeds very rapidly in most terrestrial environments so that initial stages of modification of newly exposed surfaces are quickly masked by subsequent aqueous weathering processes. The hyper-arid and hypo-thermal environment of Beacon Valley, Antarctica, is limited in terms of available liquid water and energy available for alteration, which severely slows weathering processes so that the initial stages of alteration can be studied in detail. We report on the nature of initial chemical alteration of the Ferrar Dolerite in Beacon Valley, Antarctica, using a multiplicity of approaches to characterize the process. We suggest that initial chemical alteration is primarily driven by cation migration in response to the oxidizing environment. Morphological studies of altered rock surfaces reveal evidence of small-scale leaching and dissolution patterns as well as physical erosion due to surface weakening. Within the alteration front, mineral structures are largely preserved and alteration is only indicated by discrete zones of discoloration. Mineralogical investigations expose the complexity of the alteration process; visible/near-infrared reflectance and mid-infrared emission spectroscopy reveal significant variations in mineralogical contributions that are consistent with the introduction of oxide and amorphous phases at the surfaces of the rocks, while X-ray diffraction analyses reveal no definitive changes in mineralogy or material properties. Chemical analyses reveal large-scale trends that are consistent with cation migration and leaching, while small-scale electron microprobe analyses indicate that chemical variations associated with magmatic processes are still largely preserved within the alteration rind. This work confirms the incomplete and immature chemical alteration processes at work in the McMurdo Dry Valleys. Liquid water is not a significant contributor to the alteration process at this early stage of rind development, but assists in the removal of alteration products and their local accumulation in the surrounding sediments. These results also suggest that the McMurdo Dry Valleys (and Beacon Valley, in particular) are relevant terrestrial analogs to hyper-arid and hypo-thermal alteration processes that may be dominant on the martian surface.

The formation of extended planar defects in minerals such as olivine is related to high point defect concentration and can be driven by large gradients in chemical potential, where the energy of the system is lowered by the ordering of defects along specific planes in the crystal. The presence of extended defects has the potential to create the (apparently) anomalous ionic diffusion in olivine as reported recently (Spandler and O’Neill in Contrib Mineral Petrol 159(6):791–818, 2010). High-resolution transmission electron microscopy and energy-filtered imaging were done using experimental samples designed to examine the impact of a TiO2 and f O2 on the potential to form such defects in ferromagnesian olivine. Doped basalt (5 wt% TiO2)–olivine reaction couple experiments were run at 1 atm and 1,310 and 1,410 °C for 50 h at various f O2, ranging from 102 below to 102 above the quartz–fayalite–magnetite buffer. Our results show that extended planar defects in olivine, parallel to {101}ol and occurring in ordered “clusters” with a prolate spheroid geometry ~5–25 nm across and extending up to 150 nm into the olivine, are present near the olivine–glass interfaces in all of our experimental high-TiO2 basalt–olivine samples. Increased Ti content in the olivine is associated with the defects; ordering of Ti4+ and octahedral site vacancies leads to a two- or three-layer superstructure in the olivine. Defect nucleation and growth is driven by the large TiO2 chemical potential gradient across the phase boundary at the start of the experiments, which provides access to microstructures not otherwise present.

How noble gases are recycled from the atmosphere back into the mantle has been unclear. High-pressure experiments demonstrate that noble gases are highly soluble in an important hydrous mineral in altered oceanic crust, suggesting that subduction of this type of crust may be a significant pathway for noble gas flux back into the mantle.

Large strain deformation experiments in torsion were conducted on a coarse-grained natural dunite with a pre-existing lattice preferred orientation (LPO). Experiments were conducted at conditions where deformation by diffusion creep is initially negligible. Microstructural evolution was studied as a function of strain. We observe that the pre-existing LPO persists to a shear strain of at least 0.5. At larger strains, this LPO is transformed. Relict deformed grains exhibit LPO with [100] crystallographic axes at high angles to the shear plane. Unlike previous experimental studies, these axes do not readily rotate into the shear plane with increasing strain. Partial dynamic recrystallization occurs in samples deformed to moderate strains (g . 0.5). Recrystallized material forms bands that mostly transect grain interiors. The negligible rate of diffusion creep along relict grain boundaries, as well as the limited nature of dynamic recrystallization, may account for the relatively large strains required to observe evolution of microstructures. Our data support hypotheses based on natural samples that microstructures may preserve evidence of complex deformation histories. Relationships between LPO, seismic anisotropy and deformation kinematics may not always be straightforward. As the most abundant mineral in the upper mantle, olivine plays a key role its rheological behaviour (Karato & Wu 1993; Hirth & Kohlstedt 2003). Moreover, olivine is essential to our interpretation of mantle kinematics because of its strong influence on seismic anisotropy (Nicolas & Christensen 1987; Karato et al. 2008). The generation and evolution of olivine lattice preferred orientation (LPO) has been well documented in laboratory studies (Carter & Ave Lallemant 1970; Nicolas et al. 1973; Zhang & Karato 1995; Bystricky et al. 2000; Zhang et al. 2000; Jung & Karato 2001), naturally deformed peridotites (Nicolas et al. 1971; Mercier 1985; Ben Ismail & Mainprice 1998; Warren et al. 2008; Skemer et al. 2010) and numerical simulations (Wenk et al. 1991; Wenk & Tome 1999; Tommasi et al. 2000; Kaminski & Ribe 2001; Blackman & Kendall 2002; Castelnau et al. 2009). However, there are still substantial differences between the microstructures generated or observed in these studies. Notably, studies of peridotite mylonites suggest that somewhat larger strains may be required to rotate LPO into concordance with deformation kinematics, in comparison to experimental studies and numerical simulations that have an initially random texture (Warren et al. 2008). This difference in the influence of strain on LPO may be a consequence of factors such as the initial microstructure and the conditions of deformation. Furthermore, different numerical models show distinct patterns and rates of LPO development which may differ from experimental and geological observations (Castelnau et al. 2009). Variations between models arise from differences in numerical techniques, but also the inclusion or exclusion of specific dynamic recrystallization mechanisms (Wenk et al. 1991; Wenk & Tome 1999). To expand on existing experimental studies and provide further bases for numerical investigations, we conducted a series of deformation experiments on coarse-grained natural dunite. The objective of these experiments is to improve our understanding of microstructural processes in realistic mantle materials and to help bridge the gap between the laboratory and nature. For technical reasons, many of the experimental studies of olivine deformation have been conducted on relatively fine-grained synthetic materials (e.g. Zhang & Karato 1995; Bystricky et al. 2000). As a consequence, deformation in these experiments typically includes contributions from several parallel mechanisms including dislocation creep, diffusion creep and grain-boundary sliding (Fig. 1). In contrast, many geologically observable mantle rocks are interpreted to have deformed at conditions where dislocation creep is strongly dominant. It is therefore important to understand from an From: Prior, D. J., Rutter, E. H. & Tatham, D. J. (eds) Deformation Mechanisms, Rheology and Tectonics: Microstructures, Mechanics and Anisotropy. Geological Society, London, Special Publications, 360, 211–223. DOI: 10.1144/SP360.12 # The Geological Society of London 2011. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics experimental perspective how mantle materials deform when grain-size-sensitive creep is more completely suppressed. To this end, we conducted laboratory deformation experiments on samples cored from a block of Balsam Gap Dunite, which was chosen for two reasons. First, it has a moderately large grain size which allows us to suppress activity of the diffusion creep mechanism. Secondly, it has a moderately strong pre-existing LPO. This allows us to consider how LPO might evolve in a situation where kinematics are changing, for example in a corner flow regime such as a mantle wedge or mid-ocean ridge.

A new viscoelastic creep function that incorporates both the effects of elastically-accommodated grain boundary sliding (GBS) and transient diffusion creep is proposed. It is demonstrated that this model can simultaneously describe both the transient microcreep curves and the shear attenuation/modulus dispersion in a fine-grained (d ∼ 5 µm) peridotite (olivine + 39 vol. % orthopyroxene) specimen. Low-frequency shear attenuation, , and modulus dispersion, G(ω), spectra were measured in a one-atmosphere reciprocating torsion apparatus at temperatures of 1200 ≤ T ≤ 1300°C and frequencies of 10−2.25 ≤ f ≤ 100 Hz. Reciprocating tests were complemented by a series of small stress (τ ∼ 90 kPa) microcreep experiments at the same temperatures. In contrast to previous models where the parameters of viscoelastic models are derived by fitting the Laplace transform of the creep function to measured attenuation spectra, the parameters are derived solely from the fit of the creep function to the experimental microcreep ...

Droplets of an iron-bearing calcia magnesia aluminosilicate (Fe-CMAS) melt were reacted under distinctly reducing conditions ( f O2 = 2.4 × 10−13 and 6.4 × 10−15 atm) at high temperature (~1400 °C) and ambient pressure. The low f O2 environment was maintained by a flowing gas mixture of CO and CO2, with a high content of CO. Molten metallic iron alloyed with silicon and carbon formed on the surface of the melt; no metal was observed in sample interiors. A color change from brown to pale blue confined to the outer layer of the melt indicated that essentially complete reduction of Fe3+ to Fe2+ had occurred in this region. Analysis of the reaction kinetics, particularly in comparison to melts of similar polymerization but free of CaO, reveals that the concentration of electron holes has decreased to such an extent that ionic transport in the melt is significantly slowed and the diffusion of CO as a neutral species becomes dominant and rate-limiting: molecular CO, initially incorporated into the melt as a physically dissolved species, subsequently reacts to form chemically dissolved (bonded into the melt structure) CO32− anions, consuming electron holes in the process. The chemical diffusion coefficient for CO in the reduced melt at 1400 °C is estimated as D CO ≈ 4 × 10−4 cm2/s, consistent with that of other, similarly sized molecular species (e.g., H2 and H2O) for similarly polymerized melts, as reported by other investigators. Upon quenching, the droplet acts as a closed system. Internal redox couples see the reduction of the carbonate so as to form bubbles of CO, the composition of which are confirmed with Raman spectroscopy. The open-system reduction and closed-system quenching dynamics are analyzed following an Ellingham-diagram approach.

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO(2), H(2)O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.

Wavelength-dispersive X-ray spectroscopy (WDS) was used to characterize the morphology of the reactions between a liquid bronze alloy (Cu–36 at.%Sn) and two magnesium aluminosilicate, glass–ceramic-forming glassmelts, one of which was doped significantly with ZnO. Two suites of experiments were pursued for each glassmelt, an isochronal series (30-min reactions with temperatures ranging from ∼1300 to 1400 °C) and an isothermal series (1350 °C reactions with durations ranging from 5 to 60 min). The reactions are decidedly complex. Transient behavior sees initially rapid incorporation of Cu +,2+ into the glassmelts, effected primarily by a redox couple involving the SiO 2 component of the aluminosilicate. This behavior gives way to a more dominant kinetic response in which Sn 2+,4+ is incorporated into the glassmelt in a reaction and chemical diffusion process that, in part, pulls the early-incorporated ionic copper back out of the aluminosilicate. In the case of the ZnO-doped glassmelt, coupled redox and interdiffusion of ionic Sn and ionic Zn is important in the longer-time response, giving rise to a Liesegang-band morphology. The extent of metal–silicate reaction diminishes as the temperature is increased, a thermodynamic effect related to the solution thermodynamics of the liquid bronze alloy. The reaction kinetics are interpreted following the Wagner–Schmalzried formalism for diffusion-effected redox reactions.

The thermal and mechanical evolution of Europa and comparable icy satellites—the physics behind creating and sustaining a subsurface water ocean—depends almost entirely on the mechanical dissipation of tidal energy in ice to produce heat, the mechanism(s) of which remain poorly understood. In deformation experiments, we combine steady–state creep and low-frequency, small-strain periodic loading, similar conditions in which tectonics and tidal flexing are occurring simultaneously. The data reveal that the relevant, power-law attenuation in ice (i) is non-linear, depending on strain amplitude, (ii) is independent of grain size, and (iii) exceeds in absorption the prediction of the Maxwell solid model by an order of magnitude. The Maxwell solid model is widely used to model the dynamics of planetary ice shells, so this discrepancy is important. The prevalent understanding of damping in the geophysical context is that it is controlled by chemical diffusion on grain boundaries, which renders attenuation strongly dependent on grain size. In sharp contrast, our results indicate instead the importance of intracrystalline dislocations and their spatial interactions as the critical structural variable affecting dissipation. These dislocation structures are controlled by stress and realized by accumulated plastic strain. Thus, tectonics and attenuation are coupled, which, beyond the icy satellite/subsurface ocean problem, has implications also for understanding the attenuation of seismic waves in deforming regions of the Earth's upper mantle.

The microstructures of two-phase binary aggregates of ice-I + salt-hydrate, prepared by eutectic solidification, have been characterized by cryogenic scanning electron microscopy (CSEM). The specific binary systems studied were H2O-Na2SO4, H2O-MgSO4, H2O-NaCl, and H2O-H2SO4; these were selected based on their potential application to the study of tectonics on the Jovian moon Europa. Homogeneous liquid solutions of eutectic compositions were undercooled modestly (Δ T ~ 1–5 °C); similarly cooled crystalline seeds of the same composition were added to circumvent the thermodynamic barrier to nucleation and to control eutectic growth under (approximately) isothermal conditions. CSEM revealed classic eutectic solidification microstructures with the hydrate phase forming continuous lamellae, discontinuous lamellae, or forming the matrix around rods of ice-I, depending on the volume fractions of the phases and their entropy of dissolving and forming a homogeneous aqueous solution. We quantify aspects of the solidification behavior and microstructures for each system and, with these data, articulate anticipated effects of the microstructure on the mechanical responses of the materials.

Ceramic matrix composites have to overcome two major barriers for applications in aircraft and stationary turbines/engines. One is the unacceptably high processing cost to obtain sufficient mechanical properties and the other is their poor corrosion resistance under hydrothermal oxidizing conditions typical of engines and turbines. Functional geopolymer composites provide possible solutions to the above two problems since they can be formed by technologically simple processing routes and the hydrothermal corrosion resistance can be improved by tailoring material compositions. In this paper geopolymer matrix materials with superior hydrothermal corrosion resistance were processed from the selected geomimetic compositions. The effects of processing parameters, such as particle size, extent of dissolution, and firing temperature on the compressive strength and microstructures were examined and specimens with a compression strength as high as ∼99 MPa were fabricated. Hydrothermal test experiments on KOH-derived geopolymer specimens showed that all of potassium-bonded geopolymer specimens have minimal mass change or dimensional change during hydrothermal exposure in spite of the phase change.

Wavelength-dispersive X-ray spectroscopy (WDS) has been utilized in a study of redox chemical diffusion dynamics in reaction couples of two sodium aluminoborosilicate glassmelts – both lightly doped with Fe 3+ – with two copper-base liquid metal alloys: Cu–37 at.%Ge and Cu–36 at.%Sn. The glassmelts were floated on the alloys for 30 min in a controlled, reducing atmosphere at temperatures in the range 1200–1350 °C. Despite there being a significant driving potential to incorporate Cu +,2+ into the glassmelt, such does not occur: ionic Cu concentration in the glassmelts was below the detection limits of WDS (i.e., X Cu 2+,4+ and Sn 2+,4+ diffused into the glassmelt to a depth of ∼50 μm, a distance similar to that seen for ionic Sn in the soda-lime silicate on pure Sn float process that is operated at notably lower temperatures and for dramatically shorter reaction times. For reactions with Cu–37 at.%Ge, doping the glass with ferric iron had a profound effect on the depth of penetration of ionic Ge, decreasing it significantly: Fe 3+ acts as an electron acceptor from Ge 2+ ; the resultant tetravalent cation (Ge 4+ , a network former) is all-but immobile. A similar result was not seen for reactions with the Cu–Sn alloy, a consequence of considerable reduction of the glassmelt by its pre-reaction with molybdenum components of a delivery system. The reaction morphologies seen are consistent with the Wagner/Schmalzried formulation for dynamic redox reactions.

A petromimetic (geological–analog) approach is applied to the design of alumina-fiber-reinforced glass-ceramic-matrix composites that use a refractory, trioctahedral fluoromica fiber–matrix interphase and feldspar matrixes. Studies of the solid-state reaction couples between these silicate phases are pursued to address the chemical tailorability of the interphase/matrix interface from an engineering perspective. The minimization of alumina and silica activities within polyphase, feldspar-based matrixes via MgO buffering is shown to be an effective route toward a stable fluoromica interphase. An anorthite–2-vol%-alumina (CaAl2Si2O8+α-Al2O3) substrate, chemically buffered with MgO, is shown to exhibit thermodynamic stability against fluorokinoshitalite (BaMg3[Al2Si2]O10F2), up to temperatures potentially as high as 1460°C. The key to the approach is the reduction of alumina activity via the formation of MgAl2O4 spinel. Similarly, the formation of forsterite (Mg2SiO4) stabilizes the mica in contact with matrix compositions otherwise containing excess silica. The cationic interdiffusion between solid-solution feldspars and fluoromicas also is characterized. Coupled interdiffusion of K+ and Si4+ in exchange for Ba2+ and Al3+ was observed between K-Ba solid-solution celsian and the barium-rich solid-solution end-member fluorokinoshitalite at 1300°C. A similar cationic exchange also is observed against the potassium-rich end-member fluorophlogopite (KMg3[AlSi3]O10F2), although in a reverse direction, at temperatures of

Optical and electron microscopies are used to analyze the mechanism and kinetics of internal reduction of an Fe 2+ -doped magnesium aluminosilicate melt. Melt samples are heated to temperatures in the range of 1300°-1400°C under a flowing gas mixture of CO/CO 2 , which corresponds to a p O2 range of 1 x 10 -13 -4 x 10 -13 atm. The melt experiences an internal reaction in which a dispersion of nanometer-scale iron-metal precipitates forms at an internal interface. The metal precipitates show no signs of coarsening within the samples; however, the crystals at the surface (which formed in the initial part of the reaction) do grow via vapor phase transport. The overall reaction is characterized by parabolic kinetics, which is indicative of chemical diffusion being the rate-limiting step. The diffusion of network-modifier divalent cations-particularly Mg 2+ cations-is demonstrated to be the rate-limiting factor, and its diffusion coefficient is calculated to be ∼1 x 10 -6 cm 2 /s within the temperature range of the experiments.

The damping behavior in torsion of single phase β-In3Sn has been evaluated at room temperature over a broad range of frequencies (10−4 to 103 Hz) and as a function of various mechanical and thermal treatments. The results are consistent with a model for the power-law (tan δ ∝ f −n), high-temperature-background absorption being effected by diffusional processes on grain and, particularly, on subgrain boundaries. The results are compared/contrasted with those for damping in single-phase γ-InSn4 and in the two-phase β-γ eutectic. Failure of the eutectic material to follow a composite model for damping, combined with a thermal aging effect that lowers damping only for certain frequencies, shows that the boundary-based absorption model for the high-temperature background applies, too, to phase boundaries.

A novel heating technique, electromagnetic induction heating (EMIH), uses electromagnetic radiation, ranging in frequency from a few megahertz to tens of gigahertz, to volumetrically heat silicon above 1000/spl deg/C in only a few seconds. Typical power requirements fall between 900 to 1300 W for silicon wafers 75 to 100 mm in diameter. This technique has successfully produced direct silicon wafer-to-wafer bonds without the use of an intermediate glue layer. Infrared images indicate void free bonds that could not be delaminated with knife-edge tests. In addition, four pairs of stacked wafers were bonded simultaneously in 5 min, demonstrating the potential for multiwafer bonds and high-throughput batch processing.

The volatilization kinetics of single crystal α-SiC, polycrystalline β-SiC, and SiO_2 (cristobalite or glass) were determined in H_2-CO_2, CO-CO_2, and H_2-CO-CO_2 gas mixtures at oxygen fugacities between 1 log unit above and 10 log units below the iron-wustite (IW) buffer and temperatures in the range 1151 to 1501°C. Detailed sets of experiments on SiC were conducted at 2.8 and 6.0 log units below IW (IW-2.8 and IW-6.0) at a variety of temperatures, and at 1300°C at a variety of oxygen fugacities. Transmission electron microscopic and Rutherford backscattering spectroscopic characterization of run products shows that the surface of SiC exposed to IW-2.8 is characterized by a thin ( 1100°C for P^(tot)= 10^(−6) atm and sticking coefficient = 0.01, then the calculated lifetimes would be about 10 yr for 10-μm-diameter grains, essentially independent of temperature. The results thus imply that presolar SiC grains would survive short heating events associated with formation of chondrules (minutes) and calcium-, aluminum-rich inclusions (days), but would have been destroyed by exposure to hot (≥900°C) nebular gases in less than several thousand years unless they were coated with minerals inert to reaction with a nebular gas.

Seismic imaging as a tool to understand the structure and, perhaps, the dynamics of the planet at depth involves the spatially resolved, combined study of wave velocities and velocity dispersion, wave birefringence and wave attenuation (mechanical absorption). When integrated with insights from petrology, plus-or-minus input from magneto-tellurics, specific hypotheses concerning structure at depth can be formulated (cf. Karato 1993; Karato and Karki 2001). The recent advances in tomography have allowed significant improvements in spatial resolution which also have allowed ever more exacting hypotheses of structure to be articulated. Nevertheless, as is nicely illustrated by the relatively recent seismic analyses of an accreting plate margin (the East Pacific Rise —e.g., Toomey et al. 1998; Webb and Forsyth 1998), the structures inferred at depth can vary significantly: clearly, the interpretation of seismic data is limited specifically by a lack of understanding of the physical processes by which low-frequency wave absorption occurs, particularly in the cases (a) where melt is present and/or (b) where the material is being actively plastically deformed. This chapter emphasizes the mineral physics/materials science of mechanical absorption, specifically in dense materials at elevated temperature; the interest, then, is in employing the physical study of mechanical absorption to understand natural phenomena such as those mentioned above. Beyond questions of the basic mineral physics of absorption, the guiding interest the community of scholars pursuing the ideas is in isolating actual absorption mechanisms operative in the geological setting thereby allowing for a greater discrimination in the interpretation of seismic data. At the outset, I note that this contribution does not seek to be a comprehensive review of the ideas and work involved in the physics of mechanical absorption. There are marvelous reviews in the literature that are both authoritative and current; I refer to these frequently in contemplating my own …

The high-temperature creep behavior of two fine-grained (∼3 μm) anorthite-rich glass-ceramics was characterized at ambient pressure and under a confining pressure of ∼300 MPa. Experiments were done at differential stresses of 15–200 MPa and temperatures of 1200°–1320°C. Of the two materials, one had a tabular (lathlike) grain structure with finely dispersed second phase of mullite, mostly in the form of ∼3–5 μm grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (∼400 nm) mullite precipitates concentrated at the anorthite grain boundaries. The results of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitude faster than the equiaxed material. Creep in the tabular-grained material at ambient pressure was accompanied by a significant extent of intergranular cavitation: pore-volume analysis before and after creep in this material suggested that >75% of the bulk strain was due to growth of these voids. The equiaxed material, in contrast, showed a smooth transition from Newtonian (n= 1) creep at low stresses to non-Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both materials underwent significant changes. Grain-boundary mullite precipitates in the undeformed, equiaxed-grain material were replaced by fine (∼100 nm), intragranular precipitates of silliminate and corundum because of a pressure-induced chemical reaction. This was accompanied by a significant reduction in grain size in both materials. The substantial microstructural changes at high confining pressure resulted in substantially lower viscosities for both materials. The absence of mullite precipitates at the grain boundaries changed the behavior of the equiaxed material to non-Newtonian (n= 2) at a pressure of ∼300 MPa, possibly because of a grain-boundary sliding mechanism; the tabular-grained material showed Newtonian diffusional creep under similar conditions.

Rutherford backscattering (RBS) and wavelength-dispersive X-ray (WDS) spectroscopies were employed to study the redox dynamics in reaction couples of liquid Au–28at.%Ge alloy and lightly iron-doped sodium aluminoborosilicate glasses. Glassmelts were floated in a reducing atmosphere for 30 min at temperatures from 1300°C to 1450°C. The depth of diffusive penetration of Ge 2+,4+ into the glassmelt was generally ∼20 μm, similar to Sn ions in conventionally processed float glass. Doping of the glassmelt with Fe 3+ affected significantly the penetration: with additional Fe 3+ , the internal redox reaction that forces germanium into the distinctly immobile Ge 4+ (network-former) state occurs closer to the metal–alloy/glassmelt interface. The observed chemical profiles in the glassmelt are consistent fully with a description of the float reaction as a cation-diffusion-controlled reduction of the glassmelt that involves multiple diffusion steps and internal structural reactions acting in series.

The ‘off-axis’, high-temperature compression creep behavior of bidirectionally (2D, 0/90°) reinforced CAS–II/SiC (Nicalon® fiber) composites was studied experimentally in the stress–temperature regime of 1275–1325°C and 15–50 MPa. The results indicated that the overall, high-T rheologic response of the 2D composites was intermediate to the properties of 1D composites with fiber orientations corresponding to the constituent plies in the 2D material. This behavior strongly suggested that the 2D material behaved as an isostrain laminate during creep. A simple analysis, treating the 2D material as a three-phase laminate, where the constituent plies were assigned the viscoelastic properties of the corresponding 1D materials and separated by thin layers of unreinforced matrix, fit the experimental data. In the case of 2D composites with the plies misoriented at 20 and 70° to the applied stress (20/–70° composites), however, microstructural study suggested that growth of cracks in directions perpendicular to the applied stress due to the Poisson effect would have made a significant contribution to the bulk strain. Hence, such crack growth acts as a limitation to the universal applicability of the laminate model.

The compression creep of a unidirectionally reinforced, SiC continuous fiber/calcium aluminosilicate (anorthite) glass-ceramic matrix composite was evaluated experimentally. Experiments covered the stress (−σ1) and temperature (T) ranges of 20–40 MPa and 1300–1320°C, respectively. The experiments also emphasized characterization of the rheology as a function of the angle of misorientation (ϕ) between the applied compressive load and the direction of reinforcement. For any given σ, T condition, the highest steady-state strain rate occurred for ϕ ∼ 50° (up to an order of magnitude faster than in the transverse, ϕ = 90°, case); overall composite strain in this case included a substantial contribution from displacement across the fiber-matrix interface. The data reveal that the interfacial rheology responsible for the displacement is distinctly temperature sensitive. Evaluation of the composite flow through its comparison to numerical/rheological models that scrutinize the interfacial effect implies that the interface is characterized by a non-Newtonian viscous rheology; this suggests that the interface response involves specifically the flow of the thin amorphous silica interphase that comprises a portion of the fiber-matrix interface in this material. The overall plastic response of the unidirectionally reinforced material is nevertheless rate-limited by plastic flow of the matrix and can be described by the superposition of three modes of strain, the magnitude of each being dependent specifically on ϕ.

Rutherford backscattering spectroscopy (RBS) and optical microscopy (OM) and transmission electron microscopy (TEM) have been used to characterize the oxidation process in a homogeneous, well-annealed, ferrous iron-bearing calcium–magnesium–aluminosilicate (Fe–CMAS) glass. Suites of specimens were exposed to oxidizing environments of air ( p O 2 =0.21 atm) or of argon ( p O 2 ∼10 −6 atm) within a time range 1–200 h and a temperature of ∼750°C (near the glass transition). Oxidation causes: (1) formation of crystalline Mg/Fe oxides on the free surface of the glass, and (2) an internal region that is depleted of divalent cations. In general, this morphology is unequivocal evidence of the oxidation being rate-limited and dominated by the chemical diffusion of divalent, network-modifying cations out of the glass; the cation flux is charge-compensated by an inward flux of electron holes (polarons). Specifics of the reaction morphology vary for the two oxidation atmospheres. In the case of the Ar environment, outfluxing Mg 2+ and Fe 2+ form discrete particles of crystalline (Mg,Fe) 3 O 4 on the free surface of the glass; in air, a continuous surface film forms that contains cubic γ-Fe 2 O 3 . In both cases, nm-scale ferrites precipitate at an internal reaction front; the air-oxidized case sees the presence of a second front that changes the ferrite precipitates to γ-Fe 2 O 3 . The air-oxidized case also sees substantial outfluxing of Ca 2+ . Consistent with rate-limitation by chemical diffusion, parabolic reaction kinetics characterize the oxidation reaction. The different reaction morphologies seen for the different oxidizing environments demonstrate directly the applicability of the Modified Random Network (MRN) model to the structure of the oxidized (residual) glass. Removal of Mg 2+ and Fe 2+ in the case of Ar oxidation creates internal ferrite precipitates and a residual glass that retains interconnected channels for the network-modifying cations, hence the formation of discrete crystalline oxide precipitates on the surface. In air, the internal transformation of the ferrite to γ-Fe 2 O 3 , by requiring the outward flux of Ca 2+ , sees the collapse of the interconnected channels in the remnant glass, and so a continuous oxide film forms on the free surface. The threshold network-modifier-oxide content for the existence of the interconnected channels of modifiers in the CMAS residual glass is thus estimated as ∼10 vol.%.

The impact of a chemically and texturally equilibrated melt phase on the shear creep and attenuation behaviors of polycrystalline olivine has been measured experimentally at seismic-to-subseismic frequencies. The experiments were performed on aggregates that had a particularly uniform and fine grain size (∼3 µm). The effect of the melt phase (∼5 vol%) is to decrease the (Newtonian) viscosity by a factor of ∼6; there is no dramatic disaggregation effect or melt-induced plummeting of the shear modulus. Both the melt-free and melt-bearing aggregates display an attenuation “band” of the form QG−1 ∝ f−1 4. This response cannot be attributed to a variation in microstructure; it is intrinsic to the diffusional creep behavior. There is no unique effect of the melt phase on the attenuation response: the slight increase in absorption of partial melt specimens is explained fully by the effect of the texturally equilibrated melt on aggregate viscosity.

Rutherford backscattering spectroscopy was used to investigate the persistence of cation-diffusion-limited oxidation in three, low-Fe{sup 2+}-bearing MgO-Al{sub 2}O{sub 3}-SiO{sub 2} glasses (base glass compositions along the enstatite-cordierite-liquid cotectic; total Fe levels of 0.04, 0.19, and 0.54 at%). The glasses were reacted in air at temperatures of 700--850 C ({approximately}T{sub g}), and changes in the composition of the near-surface region ({le}2.5 {micro}m) of the glass resulting from oxidation were characterized. The reaction morphology produced by oxidation at temperatures above 800 C, for all of the glasses studied regardless of Fe concentration, was consistent uniquely with an oxidation process dominated by diffusion of Fe{sup 2+} cations to the free surface that was charge compensated by a (counter) flux of electron holes into the material. In the high-Fe material (0.54 at{degree}), the activation energy for the cation-diffusion-limited reaction was estimated at {approximately}475 kJ/mol. Below 800 C, the two glasses with lowest Fe concentration displayed a reaction morphology consistent with oxidation occurring by the motion of an oxygen species. High levels of transition metal cations are not required to ensure the dominance of cation-diffusion-limited oxidation reaction in silicate glasses and melts; thus monitoring internal Fe{sup 3+}:Fe{sup 2+} equilibrium, even at trace amounts, seems untenablemore » as an indicator of the diffusion behavior of molecular or ionic oxygen.« less

A recoil implantation technique is investigated for ultrashallow p+/n junction formation. In this method, a 3–35 nm thick B layer is deposited on the wafer by magnetron sputtering. Then a medium energy (10–40 keV) Ge implant drives the boron atoms into Si by means of ion beam mixing. The remainder of the boron film is chemically etched away prior to the annealing step. Sub-60 nm deep p+/n junctions with sheet resistance less than 1000 Ω/sq and test diodes with leakage current density below 2 nA/cm2 have been formed using this method.

The high-temperature (1200–1285°C) torsional dynamic attenuation (10−3–100 Hz) and unidirectional creep behavior of a fine, uniform grain sized (d ≈ 3 μm) olivine (∼Fo92) aggregate have been measured. In all cases, the material is found to be mechanically linear (i.e., γ(t), γ ∝ σxy1), indicating that diffusional processes dominate the deformation kinetics in these experiments. The creep response displays a large decelerating transient in the strain rate leading to a nominally constant “steady state.” The attenuation behavior displays a band in QG−1 that is moderately dependent on frequency (QG−1 ≈ f−0.35) and temperature with −1.5

Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating of a variety of ceramic, glass, and polymer materials. An explanation for these controversial observations has eluded researchers for over a decade. This paper describes a series of recent experimental and theoretical investigations that provide an explanation for these intriguing observations in terms of ponderomotive forces acting on mobile ionic species. The ponderomotive phenomenon, like its conventional-plasma analog, can be described in the continuum model limit by combining the continuity, Poisson’s, and transport equations. However, the solid-state plasma version typically manifests as a result of gradients in mobile charge mobility (e.g., near physical surfaces or interfaces), whereas the conventional plasma ponderomotive transport is typically a consequence of gradients in the radiation field intensity. Both cases can be captured in a single, general, mathematical articulation developed in terms of the mobile particle fluxes.

Transient creep and attenuation (internal friction) of a β-spodumene glass-ceramic has been examined. Four-point flexural creep tests have been performed at high tempera-ture(T = 925°-1000°C) and modest stress levels (σl,max= 7-25 MPa). The flexural creep response of the glass-ceramic is characterized by a large decelerating transient that precedes the establishment of a steady state. The creep response is transformed via a numerical algorithm to give the attenuation spectrum. In addition, low-stress (2.5-30 MPa), subresonant-frequency (10 −5 Hz ≤ f ≤ 1 Hz), com-pression-compression attenuation experiments have been performed. A first-order thermodynamic analysis of the effect of effective pressure on the volume fraction of re-sidual glass suggests that the dilatational process that is associated with the flexural deformation mode should be characterized by a single loss mechanism. The predictions that are available from the flexural creep tests, as well as those that have been directly measured in experiments, in-dicate that attenuation behavior is insensitive to both fre-quency and temperature and exhibits a characteristic band of absorption. The weak frequency dependence of this an-elastic response is well-characterized by a power law of the form Q−1χfα, where 0.15 ≥ a ≥ 0.3, which suggests a physical mechanism that possesses a distribution of relax-ation times. A plausible explanation exists in the fluid-mechanics description of deformation-induced melt migra-tion, in which the crystalline β-spodumene “matrix” undergoes compaction or dilatation while the residual glass phase, which forms an interpenetrative network along grain triple junctions of the matrix, flows: the evolution with time of the compacted (and/or dilated) layer thickness of the deforming matrix produces a distribution of compli-ances.

&p.1: Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating or processing of a variety of ceramic, glass, and polymer materials. These empirical observations of microwave enhancements have been broadly called the “microwave effect”. In the past, these claims have been the source of significant controversy, due in part to the lack of a credible and verifiable theoretical explanation. Moreover, certain notable microwave heating experiments have failed to observe any resolvable reaction or transport rate enhancements. This paper describes a series of recent experimental and numerical investigations that have established the fact that strong microwave electric fields induce a (previously unknown) nonlinear driving force for (ionic) mass transport near surfaces and structural interfaces (e.g., grain boundaries) in ceramic materials. This driving force can influence reaction kinetics by enhancing mass transport rates in heterogeneous solid-state reactions. Most of the previously reported observations regarding “microwave effects” (both for and against) are consistent with the characteristics of this newly identified microwave-induced driving force. &kwd: Microwave processing · Microwave effect · Ponderomotive force · Ionic transport · Nonthermal · Sintering&bdy

The recent theory of averaged ponderomotive action of microwave fields in solids is expanded to describe quasistationary ionic currents driven by that action. Several limiting cases are explored in detail, and in all cases the effect is shown to depend upon the interfacial properties of the ionic crystal. Experimental results on the dynamics of the microwave-induced currents in AgCl and NaCl are presented. Agreement between experiment and theory provides further and stronger evidence for the general validity of the theoretical model.

Rutherford backscattering spectroscopy, in conjunction with optical and scanning electron microscopy, has been used to characterize the oxidation process in a homogeneous, well-annealed glass prepared from a nepheline-normative olivine basalt. Initially melted and annealed at an oxygen fugacity substantially below the quartz-fayalite-magnetite (QFM) buffer, the glasses were oxidized in air under the time and temperature ranges 1–100 h and 550–600°C, respectively. Oxidation causes (1) formation of crystalline CaO and MgO that partially covers the free surface of the glass and (2) an internal reaction zone that is depleted of Ca2+ and Mg2+ but enriched in Na+ The reaction morphology is uniquely consistent with a model in which oxidation occurs by the outward diffusion (to the free surface) of Ca2+ and Mg2+ that is charge compensated by an inward flux of electron holes (polarons): oxidation of the glass occurs as the oxygen/cation ratio increases, not by addition of oxygen, but rather by removal of cations. The flux of Na+ from depth in the glass to the oxidizing region, which is also charge compensated by a counterflux of electron holes, is a response to the thermodynamic driving force seeking to stabilize Fe3+ as a network former, consistent with equilibrium thermodynamic and spectroscopic studies. Growth of the oxidized/transformed glass follows parabolic (chemical-diffusion-limited) kinetics. Using a first-order, Wagnerian approach, the diffusion coefficient and driving force terms of the parabolic reaction-rate constant are separated, giving an average divalent cation diffusion coefficient of DA2+(cm2·s−1) = 9.9 × 10−2exp−210kJ·mol−1RT . The oxidation mechanism seen for the glass, that is, one dominated by diffusion of network modifying cations and not an oxygen species, is anticipated to also occur in iron-bearing aluminosilicate melts: the discrepancy between the kinetics of redox reactions and of oxygen tracer diffusion noted in the literature for melts is most likely explained in this way.

A theoretical model of microwave absorption in linear dielectric ~nonferroelectric! ionic crystals that takes into account the presence of point defects has been synthesized and specifically applied to NaCl single crystals by considering all relevant interaction mechanisms between a harmonic electric field and single-crystal ionic crystalline solids, including ionic conduction, dielectric relaxation, and multiphonon processes. The loss factor e9 has been measured by a cavity resonator insertion technique for nearly pure and Ca 21 doped NaCl single crystals at frequencies from 2 to 16 GHz and at the temperatures from 300 to 700 K. The experimental results are in good agreement with the theoretical model. The theoretical model predicts a transition between low- and high-temperature absorption processes that may partly account for the phenomenon of thermal runaway observed during microwave processing of ceramics.

A system has been developed for measuring the complex permittivities of low loss ceramic materials at frequencies from 2 to 20 GHz and over a temperature range 20–1000 °C. The measurement technique involves a modified version of the conventional cavity perturbation method. Details of the design and fabrication of the circular cylindrical cavity and the input and output coupling transmission lines are discussed. Particular features related to high‐temperature operation and temperature cycling are described. Data are presented for an illustrative measurement of the complex microwave dielectric properties of NaCl single crystals between 20 and 400 °C. The experimental results are in excellent agreement with theoretical models.