Exothermic Phase Research Articles

Phase-transformation-induced lubrication of earthquake sliding.

Frictional failure is not possible at depth in Earth, hence earthquakes deeper than 30-50 km cannot initiate by overcoming dry friction. Moreover, the frequency distribution of earthquakes with depth is bimodal, suggesting another change of mechanism at about 350 km. Here I suggest that the change at 30-50 km is from overcoming dry friction to reduction of effective stress by dehydration embrittlement and that the change at 350 km is due to desiccation of slabs and initiation by phase-transformation-induced faulting. High-speed friction experiments at low pressure indicate that exceeding dry friction provokes shear heating that leads to endothermic reactions and pronounced weakening. Higher-pressure studies show nanocrystalline gouge accompanying dehydration and the highest pressure experiments initiate by exothermic polymorphic phase transformation. Here I discuss the characteristic nanostructures of experiments on high-speed friction and high-pressure faulting and show that all simulated earthquake systems yield very weak transformation-induced lubrication, most commonly nanometric gouge or melt. I also show that phase-transformation-induced faulting of olivine to spinel can propagate into material previously transformed to spinel, apparently by triggering melting analogous to high-speed friction studies at low pressure. These experiments taken as a whole suggest that earthquakes at all depths slide at low frictional resistance by a self-healing pulse mechanism with rapid strength recovery.This article is part of the themed issue 'Faulting, friction and weakening: from slow to fast motion'.

TiS3 Magnesium Battery Material: Atomic-Scale Study of Maximum Capacity and Structural Behavior

Good cyclability is essential for the potential application of cathode materials. We investigated electrochemical properties of Mg in layered intercalation compound from first-principles using TiS3 as a model system. The calculations showed exothermic phase transformation upon intercalation of Mg from the electrolyte: the geometry optimization of the structure containing 0.5 Mg showed the shift of layers accompanied by change of Mg coordination from square pyramidal to trigonal prismatic. Further increase of the Mg content leads to break of the S–S bonds in the disulfide ion and conversion of the TiS3 layers into ribbons. The obtained phase is metastable and can easily and irreversibly decompose to MgS and TiS2. This means that in order to achieve full theoretical capacity of TiS3 this decomposition has to be suppressed. A very low migration barrier of 0.292–0.698 eV (depending on the Mg content) was found in the [010] direction, which is much lower than the value of analogues, such as layered and spinel ...

Characterization of Phase Separation Propensity for Amorphous Spray Dried Dispersions.

A generalized screening approach, applying isothermal calorimetry at 37 °C 100% RH, to formulations of spray dried dispersions (SDDs) for two active pharmaceutical ingredients (APIs) (BMS-903452 and BMS-986034) is demonstrated. APIs 452 and 034, with similar chemotypes, were synthesized and promoted during development for oral dosing. Both APIs were formulated as SDDs for animal exposure studies using the polymer hydroxypropylmethlycellulose acetyl succinate M grade (HPMCAS-M). 452 formulated at 30% (wt/wt %) was an extremely robust SDD that was able to withstand 40 °C 75% RH open storage conditions for 6 months with no physical evidence of crystallization or loss of dissolution performance. Though 034 was a chemical analogue with similar physical chemical properties to 452, a physically stable SDD of 034 could not be formulated in HPMCAS-M at any of the drug loads attempted. This study was used to develop experience with specific physical characterization laboratory techniques to evaluate the physical stability of SDDs and to characterize the propensity of SDDs to phase separate and possibly crystallize. The screening strategy adopted was to stress the formulated SDDs with a temperature humidity screen, within the calorimeter, and to apply orthogonal analytical techniques to gain a more informed understanding of why these SDDs formulated with HPMCAS-M demonstrated such different physical stability. Isothermal calorimetry (thermal activity monitor, TAM) was employed as a primary stress screen wherein the SDD formulations were monitored for 3 days at 37 °C 100% RH for signs of phase separation and possible crystallization of API. Powder X-ray diffraction (pXRD), modulated differential scanning calorimetry (mDSC), Fourier transform infrared spectroscopy (FTIR), and solid state nuclear magnetic resonance (ssNMR) were all used to examine formulated SDDs and neat amorphous drug. 452 SDDs formulated at 30% (wt/wt %) or less did not show phase separation behavior upon exposure to 37 °C 100% RH for 3 days. 034 SDD formulations from 10 through 50% (wt/wt %) all demonstrated thermal traces consistent with exothermic phase separation events over 3 days at 37 °C 100% RH in the TAM. However, only the 15, 30, and 50% containing 034 samples showed pXRD patterns consistent with crystalline material in post-TAM samples. Isothermal calorimetry is a useful screening tool to probe robust SDD physical performance and help investigate the level of drug polymer miscibility under a humid stress. Orthogonal analytical techniques such as pXRD, ssNMR, and FTIR were key in this SDD formulation screening to gain physical understanding and confirm or refute whether physical changes occur during the observed thermal events characterized by the calorimetric screening experiments.

Synthesis and characterization of metastable, 20 nm-sized Pna21-LiCoPO4 nanospheres

The majority of research activities on LiCoPO4 are focused on the phospho-olivine (space group Pnma), which is a promising high-voltage cathode material for Li-ion batteries. In contrast, comparably little is known about its metastable Pna21 modification. Herein, we present a comprehensive study on the structure–property relationships of 15–20nm Pna21-LiCoPO4 nanospheres prepared by a simple microwave-assisted solvothermal process. Unlike previous reports, the results indicate that the compound is non-stoichiometric and shows cation-mixing with Co ions on the Li sites, which provides an explanation for the poor electrochemical performance. Co L2,3-edge X-ray absorption spectroscopic data confirm the local tetrahedral symmetry of Co2+. Comprehensive studies on the thermal stability using thermogravimetric analysis, differential scanning calorimetry, and in situ powder X-ray diffraction show an exothermic phase transition to olivine Pnma-LiCoPO4 at 527°C. The influence of the atmosphere and the particle size on the thermal stability is also investigated.

Моделирование процесса формирования алмазоподобных фаз из структурных разновидностей тетрагонального графита

Theoretical investigations of the phase transformations of structural modifications of tetragonal L4-8 graphite in diamond-like phases was performed using the density functional theory method in local density approximation (LDA) and generalized gradient approximation (GGA). The varieties of L4-8 graphite were examined with the following packing of graphene layers: AA (P4/mmm), AB (I4/mmm), ABCD (I41/amd). The LA6 (Imma), LA7 (Cmcm), and LA10 (I41/amd) diamond-like phases can be obtained from these graphites under compression. The LA6 diamond-like phase can be obtained from L4-8 AA graphite at pressure of 44 GPa. Another phase, LA7, can be formed from tetragonal L4-8 AB graphite in the pressure range from 43 to 46 GPa. The LA10 structure can be obtained only from L4-8 graphite with the ABCD layer packing of at pressures from 32 to 40 GPa. It was established that «L4-8 AA graphite → LA6 phase», «L4-8 AB graphite → LA7 phase», and « L4-8 ABCD graphite → LA10 phase» structural transitions are exothermic phase transitions of the first kind, as a result of which an energy of 0.5, 0.5, and 0.3 eV/atom is released, respectively. The energy barriers that need to be overcome to observe reverse «LA6 → L4-8 AA graphite», «LA7 → L4-8 AB graphite» и «LA10 → L4-8 ABCD graphite» phase transitions are 0.38, 0.34, and 0.18 eV/atom for the LDA-calculations or 0.31, 0.28, and 0.13 eV/atom for the GGA-calculations, respectively. Possible ways of synthesizing LA6, LA7, and LA10 phases are a strong compression of graphite along the [001]-axis at low temperatures.

Interaction of chitosan and self‐assembled distearoylphosphatidic acid molecules at liquid/liquid and air/water interfaces. Effect of temperature

The effect of the polyelectrolyte, chitosan (CHI), on distearoylphosphatidic acid (DSPA) films was analyzed by cyclic voltammetry, surface pressure‐area isotherm, and Brewster angle microscopy. Distearoylphosphatidic acid films formed at liquid/liquid interface produced a blocking effect to the electrochemical transfer of tetraethylammonium (TEA+) cation from the aqueous to the organic phase, as can be deduced from the cyclic voltammetry experiments. Monolayers of DSPA were performed at different experimental conditions, temperature, subphase solutions (LiCl or CaCl2), and CHI concentration. Both aqueous electrolytes studied exert strong interaction with the negative‐charged polar head groups of phospholipids, which enhance the lateral interactions of the hydrocarbon chains, forming a very structured film. Under these conditions the presence of CHI produces a shift of TEA+ transfer potential toward more positive values because of the less availability of negative sites of the phospholipids for TEA+ adsorption, because they are occupied by the positive‐charged amine groups in CHI chain.Surface pressure‐molecular area isotherms for DSPA monolayers change in the presence of CHI, demonstrating that this polymer produces an expansion of the DSPA film and modifies the surface elasticity of the film. The effect of the temperature was also studied; an exothermic phase transition was observed in the surface pressure‐molecular area isotherms for DSPA‐CHI monolayers, which involves the rupture of interactions between them. Images of Brewster angle microscopy evidence an increase in the optical thickness of the DSPA films in presence of CHI that indicates that the polymer interacts with DSPA molecules at all molecular areas. Copyright © 2016 John Wiley & Sons, Ltd.

Thermal analysis of in-situ Al2O3/SiO2(p)/Al composites fabricated by stir casting process

The in-situ fabricated Al2O3/SiO2(p)/Al composites were produced by adding different quantities of SiO2 particles (viz. 0, 3, 6 and 9wt.%) into molten commercial purity Al at 750°C. Newtonian thermal analysis was conducted to investigate the solidification behaviors of these molten composites prepared by the stir casting process. The microstructure and phase identification were achieved via optical microscope, SEM (equipped with EDS) and X-ray diffraction. Results showed the dendrite arm spacing of α-Al grain decreased with the increase of SiO2 addition. The SEM image showed the special duplex phase with Al2O3 layer formed around SiO2 particle in Al matrix. The Si concentration in Al matrix increased with increasing in the SiO2 addition, resulting in the decrease of liquidus temperature of the melts. Compared with the purity Al melt, one more stage related to the exothermic Al-Si phase transformation was revealed in the cooling curves of these molten composites. Cooling rate of the purity Al melt was lower than that of the melts with SiO2 particles addition. The difference in solidification behaviors of these melts was ascribed to the effect of SiO2/Al reaction on the effective heat capacity and different amounts of latent heat released from phase transformation process.

The calorimetric analysis as a tool for studying the aging hardening mechanism of a Cu-10wt%Ni-5.5wt%Sn alloy

The transformations of a Cu-10wt%Ni-5.5wt%Sn alloy as a function of the aging time in the range from room temperature up to 600 °C have been followed by Differential Scanning Calorimetry (DSC). The results obtained have shown that this alloy undergone two overlapping exothermic phase transitions with DSC peaks at 208 °C and 305 °C, respectively, followed by an endothermic phase transformation with a DSC peak at 526 °C. The structural analysis by TEM, ED, EDX and XRD of the intermediates phases previously discriminated by DSC suggests that the first exothermic peak is associated to the spinodal decomposition of the sample, while the second one is associated to the segregation of a DO22 (CuxNi1−x)3Sn tetragonal phase coherent with the α-Cu structure of the starting alloy. The endothermic peak has been associated to the precipitation of cubic DO3 nanocrystals from the DO22 phase previously formed. The microhardness measurements carried out in combination with the structural characterization demonstrate that the aging hardening of the alloy under study is exclusively due to the formation of the coherent DO22 phase. The DO22 → DO3 transition leads to a dramatic drop of the hardness of the alloy.

Study on Application Properties of Modified Montmorillonite as Phase Change Material for Energy Storage

AbstractNew types of clay materials, I‐PCM and O‐PCM, with good heat storage and release capacity were prepared by the inorganic and organic montmorillonite (MMT) intercalated with phase change material (PCM). The PCM used was the mixture of liquid and solid paraffin. To get an optimal combination regimen, orthogonal experiments of four factors and three levels were used. All samples were examined by differential scanning calorimetry (DSC). Their cyclic endothermic and exothermic phase change latent heat were observed and calculated. Then, the most reasonable and effective experimental conditions were chosen to prepare the I‐PCM and O‐PCM with the best heat storage and release capacity. These clays were then characterized by diffusion‐oozing circle method, thermogravimetric (TG) analysis, scanning electron microscopy (SEM), X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and other testing methods. The diffusion‐oozing circle results showed that O‐PCM was almost extremely stable compared with that of I‐PCM according to the leakage circle. In XRD analysis, expansions of the spacings in the plane were observed for all O‐PCMs. This indicated that the intercalation of surfactants and paraffin molecules in the galleries of MMT was successfully achieved. TG‐DTG analysis of these clays illustrated that all O‐PCMs could be suitable for industrial applications due to their high thermal stability. DSC analysis illustrated that O‐PCM samples possessed a more stable structure together with better heat storage and release capacity compared with that of I‐PCM samples.

The mantle convection model with non-Newtonian rheology and phase transitions: The flow structure and stress fields

The mantle convection model with phase transitions, non-Newtonian viscosity, and internal heat sources is calculated for two-dimensional (2D) Cartesian geometry. The temperature dependence of viscosity is described by the Arrhenius law with a viscosity step of 50 at the boundary between the upper and lower mantle. The viscosity in the model ranges within 4.5 orders of magnitude. The use of the non-Newtonian rheology enabled us to model the processes of softening in the zone of bending and subduction of the oceanic plates. The yield stress in the model is assumed to be 50 MPa. Based on the obtained model, the structure of the mantle flows and the spatial fields of the stresses σ xz and σ xx in the Earth’s mantle are studied. The model demonstrates a stepwise migration of the subduction zones and reveals the sharp changes in the stress fields depending on the stage of the slab detachment. In contrast to the previous model (Bobrov and Baranov, 2014), the self-consistent appearance of the rigid moving lithospheric plates on the surface is observed. Here, the intense flows in the upper mantle cause the drift and bending of the top segments of the slabs and the displacement of the plumes. It is established that when the upwelling plume intersects the boundary between the lower and upper mantle, it assumes a characteristic two-level structure: in the upper mantle, the ascending jet of the mantle material gets thinner, whereas its velocity increases. This effect is caused by the jump in the viscosity at the boundary and is enhanced by the effect of the endothermic phase boundary which impedes the penetration of the plume material from the lower mantle to the upper mantle. The values and distribution of the shear stresses σ xz and superlithostatic horizontal stresses σ xx are calculated. In the model area of the subducting slabs the stresses are 60–80 MPa, which is by about an order of magnitude higher than in the other mantle regions. The character of the stress fields in the transition region of the phase boundaries and viscosity step by the plumes and slabs is analyzed. It is established that the viscosity step and endothermic phase boundary at a depth of 660 km induce heterogeneities in the stress fields at the upper/lower mantle boundary. With the assumed model parameters, the exothermic phase transition at 410 km barely affects the stress fields. The slab regions manifest themselves in the stress fields much stronger than the plume regions. This numerically demonstrates that it is the slabs, not the plumes that are the main drivers of the convection. The plumes partly drive the convection and are partly passively involved into the convection stirred by the sinking slabs.

Polymer amide in the Allende and Murchison meteorites

AbstractIt has been proposed that exothermic gas phase polymerization of amino acids can occur in the conditions of a warm dense molecular cloud to form hydrophobic polymer amide (HPA) (McGeoch and McGeoch 2014). In a search for evidence of this presolar chemistry Allende and Murchison meteorites and a volcano control were diamond burr‐etched and Folch extracted for potential HPA yielding 85 unique peaks in the meteorite samples via matrix‐assisted laser desorption time‐of‐flight mass spectrometry (MALDI TOF/MS). The amino acids after acid hydrolysis in Allende were below the level of detection but many of the Allende peaks via the more sensitive MALDI/TOF analysis could be fitted to a polymer combination of glycine, alanine, and alpha‐hydroxyglycine with high statistical significance. A similar significant fit using these three amino acids could not be applied to the Murchison data indicating more complex polymer chemistry.

The mechanism of different thermoregulation types of composite shape-stabilized phase change materials used in asphalt pavement

Considered the temperature distress forms and the impacts of temperature on asphalt pavement, three temperature thermoregulations were proposed to solve the asphalt pavement temperature problems initiatively. Based on the properties of phase change thermoregulation agent, thermoregulation types of composite phase change materials (CPCMS) were studied and the values of appropriate phase change temperature were determined by differential scanning calorimetric (DSC) test. Results show that for high temperature thermoregulation, the phase change temperature range is 35–50°C, in which with the 58# paraffin proportion increasing, the phase change enthalpy increases linearly and the phase change temperature grows exponentially. As for the low temperature thermoregulation, the phase change temperature range is −5°C to 5°C, in which with the Tetradecane proportion increasing, the phase change enthalpy grows linearly, while phase change temperature increases exponentially. In terms of the mixing temperature thermoregulation, the phase change temperature range is 5–35°C, in which when Tetradecane mass ratio are fixed as 30%, 40%, and 50%, with the 58# paraffin proportion increasing, high temperature enthalpy grows and phase change temperature increases linearly; when 58# paraffin mass ratio are fixed as 30%, 40%, and 50%, with the Tetradecane proportion increasing, low temperature enthalpy grows approximate linearly, exothermic (endothermic) high phase change temperature decreases (grows) slight linearly. Finally, this paper recommends composites mass proportion to different thermoregulations types and the corresponding enthalpy values, which intends to solve the worldwide problem of temperature distresses in asphalt pavement in the future.

Surface evolution of manganese chloride aqueous droplets resulting in self-suppressed evaporation.

The exchange kinetics of liquid water, which are of fundamental interest and have potential applications, remain unclear. A fantastic and extraordinary phenomenon was observed during the evaporation of a water droplet doped with manganese chloride. As observed from the evolution of this type of droplet, a thin film was formed on the surface with an exothermic phase transition, resulting in self-suppressed evaporation. The MnCl2-doped water droplets were maintained in a relative humidity (RH) of 50% at 40 °C for more than a week and for longer than two months at a temperature of 25 °C. In contrast, a pure water droplet can only be sustained for a few minutes. The self-suppressed evaporation of doped water may be due to the special hydration of the accumulated manganese and chloride ions at the surface, decreasing the surface tension.

Transparent and flexible films of new segmented polyurethane nanocomposites incorporated by NH2-functionalized TiO2 nanoparticles

Abstract In this work, TiO 2 nanoparticles are surface modified by NH 2 -terminated organic moieties arised from 4,4′-methylene diphenyl diisocyanate (MDI). These nanoparticles are incorporated into ether-based segmented polyurethane (SPU) matrix. MDI is utilized as monomer together with poly(tetramethylene oxide) (PTMO) comonomer for preparing the final polymer as well. The NH 2 -functionalized TiO 2 nanoparticles are covalently linked to the NCO terminals of the resulting SPU macromolecules during film preparation stage. Therefore, in addition to butylene glycol, these surface modified nanoparticles with enhanced organophilicity could play the role of the second chain extender of NCO-capped SPU macromolecules through formation of urea linkages. Optical and thermal behaviors of the transparent and flexible film (SPU/TiO 2 –MDI) is compared with those of unmodified TiO 2 (SPU/TiO 2 ) and TiO 2 -unloaded SPU films. Though the particle loading is only 5 wt.%, incorporation of TiO 2 and TiO 2 –MDI nanoparticles into the SPU polymer enhances significantly the light absorption in UV region at 300–400 nm. SEM images of the prepared films clearly show a considerable decrease in particle aggregation for TiO 2 –MDI into SPU matrix compared to that of unmodified TiO 2 . TG analyses indicate a one-step decomposition pattern with onset temperatures of about 360 and 380 °C for neat SPU and SPU/TiO 2 –MDI, respectively. Moreover, DTA thermograms of both nanocomposites show obviously two exothermic phase transitions in the thermal range of 330–440 °C.

Numerical studies of the effects of phase transitions on Venusian mantle convection

This paper presents a study on the effects of phase transitions on the mantle convection of Venus in a three-dimensional (3D) spherical shell domain. Our model includes strong depth- and temperature-dependent viscosity and exothermic phase change from olivine to spinel as well as endothermic phase change from spinel to perovskite. From extensive numerical simulations of the effects of Rayleigh number (Ra), and the Clapeyron slopes and depths of phase changes, we found the following: (1) The endothermic phase change prevents mass flow through the interface. Increasing the absolute value of the Clapeyron slopes decreases radial mass flux and normalized radial mass flux at the endothermic phase boundary, and decreases the number of mantle plumes. In other words, mass flow through the phase boundary decreases. The inhibition influence of phase changes increases, as do convective wavelengths. (2) Increasing Ra also increases the convective wavelength and decreases the number of mantle plumes, but it has less influence on the mass exchange. As Ra increases, the convective vigor increases along with the radial mass flux and the mass flow through the phase boundary; however, the normalized mass flux through the phase boundary varies little with Ra, which is different from the conclusion that increasing Ra will greatly increase the inhibition of mass flow through the phase boundary based on two-dimensional (2D) modeling. (3) Increasing the depth of endothermic phase change will slightly decrease the number of mantle plumes, but has little effect on the mass flow through the phase boundary. Consistent with previous studies, our results show that the phase change from spinel to perovskite could inhibit the mass flow through the phase boundary, but they also show that the buildup of hot materials under the endothermic phase boundary in the 3D model could not be so large as to cause strong episodic overturns of mantle materials, which is quite different from previous 2D studies. Our results suggest that it is difficult for phase changes to cause significant magmatism on Venus; in other words, phase changes may not be the primary cause of catastrophic resurfacing on Venus.

Copper binding affinity of the C2B domain of synaptotagmin-1 and its potential role in the nonclassical secretion of acidic fibroblast growth factor

Fibroblast growth factor 1 (FGF1) is a heparin-binding proangiogenic protein. FGF1 lacks the conventional N-terminal signal peptide required for secretion through the endoplasmic reticulum (ER)–Golgi secretory pathway. FGF1 is released through a Cu2+-mediated nonclassical secretion pathway. The secretion of FGF1 involves the formation of a Cu2+-mediated multiprotein release complex (MRC) including FGF1, S100A13 (a calcium-binding protein) and p40 synaptotagmin (Syt1). It is believed that the binding of Cu2+ to the C2B domain is important for the release of FGF1 into the extracellular medium. In this study, using a variety of biophysical studies, Cu2+ and lipid interactions of the C2B domain of Syt1 were characterized. Isothermal titration calorimetry (ITC) experiments reveal that the C2B domain binds to Cu2+ in a biphasic manner involving an initial endothermic and a subsequent exothermic phase. Fluorescence energy transfer experiments using Tb3+ show that there are two Cu2+-binding pockets on the C2B domain, and one of these is also a Ca2+-binding site. Lipid-binding studies using ITC demonstrate that the C2B domain preferentially binds to small unilamellar vesicles of phosphatidyl serine (PS). Results of the differential scanning calorimetry and limited trypsin digestion experiments suggest that the C2B domain is marginally destabilized upon binding to PS vesicles. These results, for the first time, suggest that the main role of the C2B domain of Syt1 is to serve as an anchor for the FGF1 MRC on the membrane bilayer. In addition, the binding of the C2B domain to the lipid bilayer is shown to significantly decrease the binding affinity of the protein to Cu2+. The study provides valuable insights on the sequence of structural events that occur in the nonclassical secretion of FGF1.

The nature of the structural phase transition from the hexagonal (4H) phase to the cubic (3C) phase of silver

The phase transition from the hexagonal 4H polytype of silver to the commonly known 3C (fcc) phase was studied in detail using x-ray diffraction, electron microscopy, differential scanning calorimetry and Raman spectroscopy. The phase transition is irreversible and accompanied by extensive microstructural changes and grain growth. Detailed scanning and isothermal calorimetric analysis suggests that it is an autocatalytic transformation. Though the calorimetric data suggest an exothermic first-order phase transition with an onset at 155.6 °C (for a heating rate of 2 K min−1) and a latent heat of 312.9 J g−1, the microstructure and the electrical resistance appear to change gradually from much lower temperatures. The 4H phase shows a Raman active mode at 64.3 cm−1 (at 4 K) that undergoes mode softening as the 4H → 3C transformation temperature is approached. A first-principles density functional theory calculation shows that the stacking fault energy of 4H-Ag increases monotonically with temperature. That 4H-Ag has a higher density of stacking faults than 3C-Ag, implies the metastability of the former at higher temperatures. Energetically, the 4H phase is intermediate between the hexagonal 2H phase and the 3C ground state, as indicated by the spontaneous transformation of the 2H to the 4H phase at −4 °C. Our data appear to indicate that the 4H-Ag phase is stabilized at reduced dimensions and thermally induced grain growth is probably responsible for triggering the irreversible transformation to cubic Ag.

Effects of mantle and subduction-interface rheologies on slab stagnation and trench rollback

Trench rollback has been a widely discussed phenomenon in recent years, and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the (upper) mantle. Here we concentrate on the effects of rheological description (yield stress, lower-mantle viscosity, viscosity of crust) in controlling the rollback and associated stagnation of slabs in the transition zone (410–660 km depth). We perform numerical simulations of slab evolution in a 2D Cartesian model with strongly nonlinear rheology combining diffusion creep, dislocation creep and a power-law stress limiter. We demonstrate that trench retreat develops in most models considered, regardless of the subducting plate age or prescribed strength. Rollback then mostly produces slabs that are horizontally deflected at the 660-km phase boundary and remain subhorizontal at the bottom of the transition zone. Slab morphologies are in agreement with stagnant, horizontally deflected structures reported in the transition zone by seismic tomography. Furthermore, if the strength of the slab is limited to less than 0.5 GPa, the slab experiences a significant amount of horizontal buckling. The amplitude of the rollback velocity is sensitive to several model parameters. As one might expect, it increases with the age of the subducting plate, thus reflecting its increasingly negative buoyancy. On the other hand, rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates. High friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation. Also, reducing the additional negative buoyancy imparted by the 410-km exothermic phase transition suppresses trench rollback. Interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models, where, for example, buoyancy effects may be counteracted by associated yield-stress weakening.

26. Optimizing tissue cryopreservation techniques for cryopreservation solution modifications

The purpose of this study was to observe how minor changes to cryopreservation solutions affect the success of cryopreservation controlled-rate freeze cycles. Cryopreservation is an important technique utilized in multiple industries. In the medical industry, specifically when used in tissue preservation, there are several components critical to the success of the cryopreservation process including maintaining a constant freeze rate. A typical cooling rate appropriate for vascular tissue, as evaluated in this study, is 1 °C/min. Due to exothermic reactions occurring during phase change, a constant freeze rate is not able to be achieved unless accounted for in a controlled-rate cryopreservation program. Different additives included in cryopreservation media may drastically alter the time and temperature at which these exothermic reactions occur. Unless modifications are made to the cryopreservation freeze program, a constant freeze rate is not achievable. This study evaluated controlled-rate cryopreservation responses to cryoprotectant modifications. Minor changes to a vascular tissue control cryoprotectant containing DMSO were made to determine the effects on the controlled freezing rate. A total of six solutions were evaluated using a controlled-rate freeze program developed to maintain a 1 °C/min freeze rate for vascular tissues cryopreserved in the control cryoprotectant. Solution compositions tested included: (1) control cryoprotectant (CCP); (2) CCP+ 15 g Dextran 40; (3) CCP+15 g Dextran 40 and 1.0 g Mannitol; (4) CCP+ 9.5 g Trehalose Dihydrate; (5) CCP+ 7.3 g trehalose Duhydrate; (6) DMEM with DMSO. The freeze program utilized accommodates for the exothermic phase change occurring when the control cryoprotectant gives off heat as it changes from a liquid to a solid. Cryopreservation reference pouches were filled with each cryoprotectant. Thermocouples were inserted in each of the pouches to monitor the temperature of the solution throughout the controlled-rate freeze. During this process, graphical data reported the temperature inside the cryoprotectant pouches and cryopreservation chamber. Observations were recorded for all solutions to determine if the freeze program was able to accommodate the exothermic phase change. The two parameters assessed were linearity over the entire controlled-rate freeze duration and cryoprotectant temperature increases during the exothermic phase change. All additives resulted in a deviation from the programmed freeze rate of 1 °C/min. The control solution, Solution 1, behaved as expected having a linear, constant freeze rate of 1 °C/min with no temperature increases during the phase change. The results for Solutions 2 and 3 demonstrated that they were unable to maintain a 1 °C/min controlled-rate freeze and had visual temperature increases during the exothermic phase change. Solutions 4, 5 and 6 did not have visual temperature increases during the phase change period, but were not able to maintain a constant controlled-rate freeze of 1 °C/min. For all test solutions evaluated, minor changes to the cryoprotectant resulted in cryopreservation runs unable to maintain a constant 1 °C/min freeze. In conclusion, controlled-rate freezing programs are sensitive to changes in the cryopreservation solution. It is important that new cryopreservation freeze programs be developed to accommodate solution changes thereby ensuring optimal cryopreservation technique.

Computational Investigation of Heat and Mass Transfer Processes in a Gel-Like Fuel Ignited by a Limited-Capacity Source

We have carried out a computational investigation of the interrelated processes of heat and mass transfer, exothermal (crystallization) and endothermal (melting and evaporation) phase transitions, and chemical reactions resulting from ignition of a gel-like condensed substance. The dependence of the main integral characteristic of the process — the ignition delay time — on the temperature of the local heating source has been established. Limiting ignition conditions have been revealed. The position of the zone of the leading oxidation reaction relative to the interface between the heated particle and the fuel has been determined. A comparison has been made between the ignition conditions of condensed substances in different aggregate states under local heating.