Low-temperature Boundary Research Articles

Protein refolding Versus aggregation: computer simulations on an intermediate-resolution protein model

Computer simulations are performed on a system of eight model peptide chains to study how the competition between protein refolding and aggregation affects the optimal conditions for refolding of four-helix bundles. The discontinuous molecular dynamics algorithm is utilized along with an intermediate-resolution protein model that we developed for this work. Physically, the model is much more detailed than any model used to date for simulations of protein aggregation. Each model residue consists of a detailed, three-bead backbone and a simplified, single-bead side-chain. Excluded volume, hydrogen bond, and hydrophobic interactions are modeled with discontinuous (i.e. hard-sphere and square-well) potentials. Simulations efficiently sample conformational space, and complete folding trajectories from random initial configurations to two four-helix bundles are possible within two days on a single processor workstation. Folding of the bundles follows two main pathways, one through a trimeric intermediate and the other through an intermediate with two dimers. The proportion of trajectories that follow each route is significantly different for the eight-peptide system in this work than in a previously studied four-peptide system, which yields one four-helix bundle, suggesting, as our previous simulations have, that protein folding properties are strongly influenced by the presence of other proteins. Folding of the bundles is optimal within a fixed temperature range, with the high-temperature boundary a function of the complexity of the protein (or oligomer) to be folded and the low-temperature boundary a function of the complexity of the protein’s environment. Above the optimal temperature range for folding, the model chains tend to unfold; below the optimal range, the model chains tend to aggregate. As has been seen previously, aggregates have substantial levels of native secondary structure, suggesting that aggregates are composed largely of partially folded intermediates, not denatured chains.

High pressure synthesis of cubic boron nitride from Si-hBN system

Cubic boron nitride (cBN) was synthesized from hexagonal boron nitride (hBN) in the presence of silicon. The cBN forming pressure-temperature region was determined at pressures up to 7.7 GPa. Near perfect octahedral cBN crystals could be synthesized under the P-T conditions near the low temperature boundary of the cBN-forming region. When the temperature was above 1700°C at 6.5 GPa, the transformation rate of cBN from hBN was very high and the cBN crystals had an oriented columnar morphology. This suggests that silicon has strong catalytic ability for cBN formation from hBN. The energy-disperse X-ray analysis (EDXA) identified that silicon was homogeneously distributed in the cBN crystals.

Structural style, deformation mechanisms and paleodifferential stress along an exposed crustal section: constraints on the rheology of quartzofeldspathic rocks at supra- and infrastructural levels (Bohemian Massif)

Investigations of Variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the Teplá Barrandian unit (Bohemian Massif). The study focuses on determining differential stress, deformation mechanisms and geometry of Cadomian metagreywackes and Cambrian granitoids which show different grade of Variscan Barrovian-type metamorphism ranging from the greenschist-/subgreenschist-facies boundary (semibrittle deformation regime) to the high-grade amphibolite facies.The semibrittle deformation regime (T=ca. 300°C) is characterized by large-scale thrusting within a folding environment and by similar intensities of cataclasis, diffusive mass transfer, and crystal plasticity of quartz, the latter including low-temperature migration recrystallization. Within Cadomian quartz veins of the greenschist-facies part, dislocation creep of quartz operated at differential stresses ranging from ca. 70MPa in the lowermost greenschist-facies to ca. 20MPa in the uppermost greenschist-facies. These values have been derived by paleopiezometry using the grain size of recrystallized quartz. They are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes. A marking increase in recrystallized grain size of quartz occurs within the lower greenschist-facies, where low-temperature grain boundary migration is replaced by subgrain rotation. Thus, causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level. Pressure solution of quartz and development of discrete crenulation cleavage was common in fine-grained metagreywackes of the lower greenschist-facies, whereas the upper greenschist-facies (T=ca. 500–570°C) is characterized by increasing intensity of metamorphic reactions, grain coarsening and development of a schistosity. The entire greenschist-facies level forms a suprastructure where viscosity contrasts between mica- and quartz-rich layers were sufficiently high for buckle folding. Strain-protected intercalated metagranitoids, on the other hand, do not show pervasive deformation fabrics.The amphibolite-facies part forms an infrastructure characterized by pervasive mylonitic shearing of both Cambrian granitoids and metagreywackes. It is suggested that the combination of the following processes has contributed to weaken the amphibolite-facies level making nonlocalized flow possible: (1) increased recrystallization of feldspar; (2) crystal-plastic softening of quartz due to high-temperature grain boundary migration and activity of prism slip; and (3) enhanced diffusive mass transfer in the form of metamorphic reactions.The structural breaks at the semibrittle crustal level and at the greenschist-/amphibolite-facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust.

On By-Product Flames with Implications Towards Emission of Organics During Waste Incineration

A three-step reaction mechanism is employed to study the production and release of by-products in flames. The emphasis is towards understanding the production of organic by-products that are formed by vapor-phase reactions during, for example, waste incineration. The reaction mechanism includes (1) reaction of the fuel with an oxidizer to produce products plus a radical that is needed to form the by-product; e.g., H or Cl, (2) reaction of the radical with the fuel to generate a by-product, and (3) consumption of the by-product with an oxidizer to form products. The resulting problem is solved by high-activation-energy asymptotic analysis and the solution identifies the flame temperature and flame location as well as the amount of by-product at the low-temperature boundaries of the by-product generation and consumption zones. These data indicate conditions where the by-product will be released into the products of combustion, either by leakage through the oxidizer side or by transport into the cooler, nonreactive region of the fuel side. Results show that the amount of by-product increases as the Damkohler number for the by-product consumption reaction, Da 3 , decreases, and there exists an extinction Damkohler number, Da 3,E , below which a high amount of by-product exists in the combustion products. The effects of reactivity of the generation and consumption reactions on by-product production are also discussed. The net by-product production is found to be, to a large extent, independent of the Lewis number of the radical, Le R , for Le R < 1. On the other hand, the effect of the Lewis number of the by-product, Le S , is more complex; for fuel-air flames the by-product leakage, flame temperature and Da 3,E display a non-monotonic dependence on Le S . This result is explained by considerations of enthalpy loss via by-product transport away from the oxidizer and its subsequent effect on flame temperature and reaction intensity for the by-product consumption reaction. In addition to studying fuel-air flames, diluted fuel-oxygen flames are studied wherein the air side nitrogen is substituted into the fuel such that the flame temperature is the same as for fuel-air flames. The relative flame locations are different, however, and this difference is found to dramatically reduce the production of the by-product. This result is a consequence of a changing flame structure. For the fuel-air flame, the temperature gradient in the oxidizer side of the reaction region is much steeper than that in the fuel side. The generation region, which is located at the fuel side of the oxidation region, is then much broader than the consumption region located on the oxidizer side of the oxidation region. Therefore, the residence time for the generation reaction is much larger than that of the consumption reaction, leading to a higher generation rate and a lower consumption rate of the by-product. When the inert gas is shifted to the fuel stream, the flame is located on the fuel side of the stagnation plane so that all conditions are reversed This then implies conditions that make it more difficult to generate, and relatively easy to oxidize by-product. Consequently the by-product concentration is much lower for the second case.

A two-stage process with temperature-shift for enhanced anthocyanin production in strawberry cell suspension cultures

A two-stage process with temperature-shift has been developed to enhance the anthocyanin yield in suspension cultures of strawberry cells. The effect of the temperature-shift interval and the shift-time point was investigated for the optimization of this strategy. In this process, strawberry cells were grown at 30 degrees C (the optimum temperature for cell growth) for a certain period as the first stage, with the temperature shifted to a lower temperature for the second stage. In response to the temperature shift-down, anthocyanin synthesis was stimulated and a higher content could be achieved than that at both boundary temperatures but cell growth was suppressed. When the lower boundary temperature was decreased, cell growth was lowered and a delayed, sustained maximum anthocyanin content was achieved. Anthocyanin synthesis was strongly influenced by the shift-time point but cell growth was not. Consequently, the maximum anthocyanin content of 2.7 mg.g-fresh cell(-1) was obtained on day 9 by a temperature-shift from 30 degrees C, after 3-d culture, to 15 degrees C. The highest anthocyanin yield of 318 mg.L-1 on day 12 was achieved when the temperature was shifted from 30 degrees C, after 5-d culture, to 20 degrees C. For a global optimization of both the yield and productivity, the optimum anthocyanin yield and productivity of 272 mg.L-1 and 30.2 mg.L-1.d(-1) on day 9 were achieved by a two-stage culture with a temperature-shift from 30 degrees C after 3 d to 20 degrees C.

Thermal stresses from large volumetric expansion during freezing of biomaterials.

Thermal stresses were studied in freezing of biomaterials containing significant amounts of water. An apparent specific heat formulation of the energy equation and a viscoelastic model for the mechanics problem were used to analyze the transient axi-symmetric freezing of a long cylinder. Viscoelastic properties were measured in an Instron machine. Results show that, before phase change occurs at any location, both radial and circumferential stresses are tensile and keep increasing until phase change begins. The maximum principal tensile stress during phase change increases with a decrease in boundary temperature (faster cooling). This is consistent with experimentally observed fractures at a lower boundary temperature. Large volumetric expansion during water to ice transformation was shown to be the primary contributor to large stress development. For very rapid freezing, relaxation may not be significant, and an elastic model may be sufficient.

Soot formation in the pyrolysis of halogenated hydrocarbons. Part I, binary mixtures of carbon tetrachloride with hydrogen and iron pentacarbonyl

AbstractMeasurements on carbon particle growth during CCl4 thermal decomposition were carried out behind both incident and reflected shock waves. Three kind of mixture compositions have been investigated: 4000 ppm CCl4 in argon; 200 to 4000 ppm CCl4 and 200 to 8000 ppm Fe(CO)5 in argon; 4000 ppm CCl4 and 4000 ppm hydrogen in argon. The temperature range 1200&lt;T/K&lt;3200, pressure around 25 bar. Kinetics of the particle growth was detected via the attenuation of the laser beam (He‐Ne laser, 632.8 nm).The low temperature boundary for soot particles detection and magnitude of soot yield in CCl4 pyrolysis is the same as that for most of the hydrocarbons studied. The temperature change during CCl4 pyrolysis is an important factor for its kinetic interpretation. The first order rate constants of soot growth kt/[C]are higher than those for all hydrocarbons, including benzene under comparable conditions. At T&lt;2200 K the induction times tindare close to those for hydrocarbons with respect to both, absolute values and activation energy. In the middle temperature range studied, 2200&lt;T/K&lt;2500, an “induction‐less behaviour” of soot growth was observed; tind at T2500 K show a negative activation energy. Hydrogen additives significantly act on soot formation characteristics depending on H/Cl‐ratio.Chemical reaction in mixtures CCl4/Fe(CO)5 was observed even at relatively low temperatures behind incident shock waves. However, iron additives only slightly change the soot yield as compared to CCl4 pyrolysis. A qualitative interpretation of experimental findings observed is suggested based on the consideration of the hierarchy of characteristic times in reacting systems followed by the growth of soot particles.

UNUSUAL RETROGRADE CONDENSATION AND ASPHALTENE PRECIPITATION IN A MODEL HEAVY OIL SYSTEM

ABSTRACT A partial phase diagram for the system athabasca bitumen vacuum bottoms (ABVB) (24.6 wt% / 2 mole % ) + dodecane (73.8 wt. % / 47 mole % ) + hydrogen (1.6 wt % / 51 mole % ) was constructed in the temperature range 425 K to 725 K and the pressure range 2 MPa to 7 MPa using an X-ray view cell apparatus. This fluid system is shown to exhibit two phase L1V and three phase L1 L2 V phase behaviour over parts of this P-T region. The shape of the low temperature boundary between the L1V and L1 L2 V zones is characteristic of dilute asymmetric mixtures where a heavy liquid phase, L2 appears then disappears within the light liquid phase L1 on isothermal compression. Such phase behaviour is referred to as unusual retrograde condensation and is of both practical and theoretical interest. Transitions between the multiphase regions were found to be reversible at temperatures less than 660 K., in all cases. At higher temperatures irreversible “ asphaltene precipitation” arose within the L2 phase “ Asphaltene precipitation” did not arise in the absence of the L2 phase, i.e.: within the L1 V region, even at temperatures in excess of 700 K. These data provide a strong link between “ asphaltene precipitation” and multiphase behaviour, and demonstrate a physical rather than kinetic basis for asphaltene precipitation at elevated temperatures

Low-temperature phonon thermal conductivity of the quasi-onedimensional single crystals (Ta1-xNbxSe4)2I

We report on thermal conductivity measurements of the quasi-onedimensional single crystals (Ta1−xNbxSe4)2I (x = 0, 0.008, 0.01) between 50 mK and 6 K. The thermal conductivity of a (TaSe4)2I sample obeys a cubic variation below 0.8 K, in good agreement with the boundary scattering regime. For two Nb doped samples, the thermal conductivity between 0.1 K and 1 K obeys a quasiquadratic regime, which is ascribed to extended “linear or planar defects,” specific to this one-dimensional structure, and introduced either by doping or by successive thermal cycling, while it obeys a cubic variation below 0.1 K, as in the case of pure (TaSe4)2I. In the vicinity of 1 K and 2 K for pure and 1 at. % Nb doped samples, thermal conductivity shows a very sharp peak, which exceeds the low-temperature cubic boundary Casimir regime, or the quasi quadratic regime, respectively. The presence of the peak in these systems is explained by a phonon Poiseuille flow, originating from their strong anharmonicity, amplified by the lattice anisotropy.

Sputtering erosion of beryllium coated plasma facing components—general considerations and analysis for ITER detached plasma regime

Sputtering erosion is a key performance issue for beryllium coatings in future fusion reactors. Past analysis of this subject for various edge plasma regimes—as briefly reviewed here—shows acceptable erosion only for low duty factor devices. Erosion analysis is now focusing on a very low boundary temperature (≤2 eV) `detached plasma' regime for the International Thermonuclear Experimental Reactor (ITER) Vertical Director design. Erosion and tritium codeposition calculations are made for this regime using a coupled WBC/REDEP/DEGAS + code analysis. The analysis includes calculations of low energy, oblique incidence, sputtering yields using the VFTRIM-3D code calibrated to fit recent data on D–Be sputtering. We find that beryllium performs better than carbon for detached conditions due to the absence of chemical sputtering. Beryllium performs worse than vanadium or tungsten. Net erosion rates of beryllium coated first wall components, due mainly to physical sputtering by charge exchange D–T neutrals, vary from ∼0 to 2 cm per burn-year along the wall. Tritium co-deposition in redeposited beryllium surface layers may be high.

Atmospheric and ionospheric response to trace gas perturbations through the ice age to the next century in the middle atmosphere. Part I—chemical composition and thermal structure

Abstract The aim of this first part of a series of two articles is to examine the potentially important role of the changing atmosphere on middle atmosphere ionization. The period covered is the last glacial maximum to the middle of the next century. Important parameters of interest to the ionization are changes in temperature and the concentration of several neutrals, especially H 2 O, NO x , H 2 SO 4 , CH 3 CN, aerosols, etc. This part deals with the changes in all these neutrals and the temperature affecting the ionization using an interactive two dimensional model of chemistry, radiation and dynamics. This article constructs an input field for article II which is devoted to changes in the middle atmosphere ionization. For the past, values of the forcing parameters are known whereas, for the future, we assumed scenarios for the forcing parameters and made simulations. Special attention has been given to highlight changes at 30 °N. The two dimensional model used here considers ‘frozen’ circulation in the troposphere. Although increased thermal emission by perturbing the lower boundary temperature has been considered while simulating for future, it is still a constraint in the present model.

Lawsonite-type phase transitions in hennomartinite, SrMn2[Si2O7](OH)2.H2O

Phase transitions similar to those observed at low temperatures in lawsonite,CaA12[Si207](OHh-H20, were recorded at -95 and -150°C in hennomartinite, SrMn2[Si207](OHhH20. The room temperature structure of the natural phase was refined from X-ray singlecrystal data in space group P21enand that of the > 150°C phase at 245 °C in space group Cmem. The structures are isotypic to the low-temperature and room temperature structures of lawsonite. The P2,en structure of hennomartinite exhibits twinning parallel to (100) with additional disorder mainly involving the H positions. As indicated by a split o position (05), disorder and twinning must also be considered in the Cmem structure. Similar to lawsonite, the driving force of the phase transitions is apparently the development of cooperative hydrogen bonds. At low temperatures the H atoms shift toward neighboring 0 atoms forming strong hydrogen bonds. The transition temperatures were determined by monitoring the intensities of critical reflections over the temperature range from -163 to 245°C. The low-temperature boundary of the Cmem phase is characterized by the appearance of the 017 reflection (among other C-centering forbidden reflections) below 150(5)°C. The high-temperature boundary of the twinned room temperature phase is characterized by a dramatic enhancement of the intensity of the 406 reflection below 95(5) 0C.Between 95 and 150°C (in analogy to lawsonite) the average space group Pmen is proposed, which is the mediating sub-super group between P21enand Cmem. After one heating and cooling cycle, splitting of hkO reflections (observed in w scans) below -100°C indicates that the true, relaxed room temperature structure represents a monoclinic derivative of the natural, initially determined P21enaverage structure. Moreover, a decrease of the a unit-cell parameter and the unit-cell volume during the 245°C data collection suggests a temperature-controlled structural evolution of hennomartinite from disorder and small twin domains in the natural phase to large twin domains caused by annealing at elevated temperatures.

The RR Lyrae Variable Stars in the Globular Cluster M15

view Abstract Citations (71) References (63) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The RR Lyrae Variable Stars in the Globular Cluster M15 Silbermann, N. A. ; Smith, Horace A. Abstract New CCD photometry is presented for RR Lyrae variable stars in the globular cluster M15. This photometry, mainly in V and R, with some additional B and I, is used to construct lightcurves of 44 RR Lyraes and one Cepheid in M15. One new low-amplitude RR Lyrae star was discovered. The observable parameters of the light curves are used, in conjunction with theoretical photometric indices, to obtain effective temperatures and bolometric magnitudes for the RR Lyrae stars. We confirm that the regions occupied by RRab and RRcd variables in M15 have little or no overlap in effective temperature. Photometry for variable and nonvariable horizontal branch stars in Ml5 is used to establish the high and low temperature boundaries of the instability strip. There is evidence that the blue edge of the M15 RR Lyrae instability strip becomes cooler at higher luminosities. Employing Cox's value of the mass of RRd stars, 0.75 Msun, and the pulsation equation, the absolute magnitude of the RR Lyrae stars in M15 is estimated to be <MV>= +0.36±0.12 mag. However, this result depends upon the adopted mass, bolometric correction, and color-temperature relation, which remain significantly uncertain. The implications of the absolute magnitude determination for the ages of the globular clusters are discussed. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117555 Bibcode: 1995AJ....110..704S Keywords: STARS: VARIABLES: RR LYRAE; GLOBULAR CLUSTERS: INDIVIDUAL: M15 full text sources ADS | data products SIMBAD (82) CDS (1) Related Materials (1) Erratum: 1996AJ....111..567S

Finite amplitude thermal convection with spatially modulated boundary temperatures

Finite amplitude thermal convection in a fluid layer between two horizontal walls with different fixed mean temperatures is considered when spatially modulated temperatures with amplitudes L 1 * and L u * are prescribed at the lower and upper walls, respectively. The nonlinear steady problem is solved by a perturbation technique, and the preferred mode of convection is determined by a stability analysis. In the case of a resonant wavelength excitation, regular or non-regular multi-modal pattern convection can be preferred for some ranges of L 1 * and L u *, provided the wave vectors for such patterns are contained in a certain subset of the wave vectors representing a linear combination of modulated upper and lower boundary temperatures. In the case of non-resonant wavelength excitation, a three (two) dimensional solution in the form of multi-modal (rolls) pattern convection can be preferred, even if the boundary modulations are one (two) or two (one) dimensional, provided the wavelengths of the modulations are not too small. Heat transported by convection can be enhanced by boundary modulations in some ranges of L 1 * and L u *.

An approximate closed-form for the entropy of dimer monomer mixture

Abstract A new method to construct a formula for the entropy of mixture systems is suggested. Using the exact values of the entropy at high and low temperature boundaries, an approximate formula of the entropy in the entire range can be obtained. This method is applied to the A, B mixture on square lattices as a first example and then extended to the A-A, B (dimer monomer) mixture system. This method is a “macroscopic” way of derivation. Therefore it is free from the difficulties of counting the number of configurations in which all dimers must avoid each other. With non-zero coupling energies between monomers and dimers in a A-A, B mixture, our result indicates a unique phenomenon that in some narrow range of chemical potential, decreasing temperature will make more dimers stay in lattice but further decreasing will reverse the tendency and more monomers will occupy the lattice. This phenomenon is confirmed by our Monte Carlo simulations and can be easily explained by the closed formula.

Preparation and properties of block copolymers with two stat-copoly(oxyethylene/oxypropylene) blocks

Diblock copolymers, each with two stat-copoly(oxyethylene/oxypropylene) blocks, were prepared by sequential anionic copolymerization in the presence of a crown ether, and characterized by gel permeation chromatography and NMR spectroscopy. Aqueous solutions of the copolymers were investigated by static and dynamic light scattering and differential scanning calorimetry. Standard thermodynamic micellization functions were derived from critical micelle temperatures determined by static light scattering for solutions covering a range of concentrations. The lower and upper temperature boundaries of the gel regions were determined and the standard thermodynamic gelation functions derived. Typical values obtained for the standard enthalpies were +360kJ (mol of chains) −1 for micellization, +20 kJ (mol of micelles) −1 for gelation at the lower temperature boundary, and −15 kJ (mol of micelles) −1 for gelation at the upper temperature boundary: the latter quantities corresponding roughly to + 1 or −1 kJ (mol of chains) −1 respectively. Gelation in aqueous ethanol solution was also investigated.

Time-resolved fluorescence probing of oil-in-water microemulsions stabilised by non-ionic surfactants

Time-resolved fluorescence (TRF) of fluorescent and quencher probe molecules has been used to investigate the dynamics, mean size and polydispersity of oil-in-water (o/w) droplet microemulsions stabilised by a range of alkyloply(oxyethylene glycol ether) non-ionic surfactants. Close to the low-temperature (solubilisation) phase boundary of the microemulsions, there is no significant rate of exchange of probe molecules between the droplets. Exchange occurs on the experimental timescale of a few µs as the temperature is increased away from the phase boundary. An analysis of the measured mean aggregation numbers of the o/w microemulsion droplets indicates that the surfactant tailgroups in the curved monolayers coating the droplets are rather densely packed with areas per surfactant tail in the range 0.2–0.3 nm2. The surfactant tall areas found in this work for relatively small microemulsion droplets are singificantly lower than observed previously by dynamic light scattering (DLS). We show here that areas derived using DLS data are mainly sensitive to the tailgroup areas for large droplets whereas the TRF data give accurate values for small droplets. Combination of the TRF and DLS data shows that the tailgroup areas of the surfactants studied here increase with increasing droplet size. The TRF method is more sensitive to the aggregate polydispersity than radiation scattering methods. The relative radius polydispersities for the microemulsion systems tested here are all in the range 0.1–0.2 and decrease with increasing overall surfactant chain length.

The influence of radiation-induced segregation on precipitate stability in austenitic steels

The influence of radiation-induced segregation of major components and silicon in austenitic steels on the stability of M 23C 6, γ' and G phases under irradiation is theoretically studied. The low temperature boundary T L of M 23C 6 carbide stability and the high temperature boundary T H of γ' and G silcides stability are estimated by comparing the steady-state chromium and silicon concentrations at point defect sinks with corresponding solubility limits for these precipitates. It is shown that both T L and T H are decreased with increasing sink density and decreasing point defect generation rate. The value of T L drops with increasing carbon content while the value of T H does with decreasing silicon concentration.

The numerical simulation of diamond synthesis from acetylene flames

The mechanism of diamond growth from an oxy-acetylene flame is discussed using a numerical simulation which includes both gaseous reactions in the low-temperature boundary layer near the substrate and detailed surface reactions on the growing film. It is found from these simulations that the CH 3-precursor model is sufficient to explain the growth rate for wide ranges in the substrate temperature T s and the gas mixture ratio, and that the fractional density of the surface adsorbing bond (the overall sticking probability) is about 10 −3 at T s = 1250 K. It is also shown that the growth rates for other deposition methods are satisfactorily explained by the present model.

Diffusion along [001] tilt boundaries in the Au/Ag system—II. atomistic modeling and interpretation

The low temperature boundary diffusion data obtained in Part I were modeledusing elementary vacancy, interstitialcy and interstitial defect mechanisms. First, boundary structures were calculated using the embedded atom method. Point defect formation energies at different boundary sites and migration energies betweenthem were then calculated, and values of δ b D b were determined using recently developed expressions for this quantity. The results indicate that the diffusion is dominated by a small number of jumps having relatively small activation energies and large partial correlation factors. Also, interstitial-related mechanisms make important (and perhaps dominant) contributions to the diffusion. Both the data and modelling indicate Arrhenius pre-exponential factors considerably smaller than those reported at high temperatures. It is suggested that additional jumps (possessing higher effective activation energies) exist, and that thei contributions become dominant at high temperatures. Finally, the observed δ b D b vs misorientation behavior is found to be quatitatively consistent with the structural unit model.