Time-temperature Space Research Articles

An oxidation mechanism map for tungsten

A tungsten oxidation mechanism map is developed to clarify literature confusion about the dominant oxidation kinetic regime and to enable improved predictions in extreme environments. Thermogravimetry data is systematically extracted from 14 papers in the range 600–1600 °C and treated as three distinct kinetic regimes: parabolic oxidation, linear oxidation, and sublimation. A mechanism map is constructed that shows metal recession thickness contours for each regime in time-temperature space. The map enables consideration of a fusion reactor accident combining loss of coolant and vacuum, in which a tungsten first wall could reach ∼1200 °C for several weeks. Complete oxidation of the first wall and 2 mm of sublimation is predicted. Similar maps must be developed for accident tolerant tungsten alloys if reliable predictions are to be made about their performance.

A model to predict the kinetics of mass loss in wood during thermo-vacuum modification

Abstract Mass loss (ML) of wood caused by thermal degradation is one of the most important features of the thermal treatments and referred to as an indicator of intensity and quality of the process. The ML is proportional to the quantity of the effective heat power exchanged during the treatment process, represented by the area of the temperature profile versus time during the process. In this paper a model for the ML prediction based on the relative area was discussed. The model proposed an analytical solution to take into account the non-linear trend of ML when plotted versus temperature and time as observed in isothermal experiments. The model was validated comparing calculated and measured final ML of samples treated during thermal modification tests with different temperature profiles. The results showed that the relative area calculated in a transformed time-temperature space improves the correlation with the measured ML.

Cure‐induced phase separation of epoxy/DDS/PEK‐C composites and its temperature dependency

AbstractThe cure‐induced phase separation approach for thermoplastic toughened thermosets of epoxy/DDS (4,4′‐diaminodiphenyl sulfone)/PEK‐C (modified polyetherketone) is studied by in situ rheology, real‐time TOM (transmission optical microscope) in wide time–temperature space. Rheology studies show that there are two critical gel transformations corresponding to the loose entangled thermoplastic network at the threshold of phase separation and the percolated denser thermosetting network at chemical gelation point, respectively, within broad cure temperature range. The phase separation time may vary by using rheological or morphological methods, but its temperature dependencies can be well described by Arrhenius equation with similar phase separation activation energy Ea(ps). It is found in the present systems that Ea(ps) keeps unchanged with varying PEK‐C content and different epoxy monomers, while changes along the stoichiometric ratio of hardener, which presumably affects the chemical environment of the blends. The quantitative description of time/temperature dependence of phase separation is of practical importance for the design of cure paths in processing and optimizing the properties of TP/TS composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Exploiting 3D Spatial Sampling in Inverse Modeling of Thermochronological Data

The development of quantitative models for fission track annealing (Laslett et al. 1987; Carlson 1990; Laslett and Galbraith 1996; Ketcham et al. 1999) and more recently, helium diffusion in apatite (Wolf et al. 1996; Farley 2000), has allowed direct inference of the temperature history of the host rocks, and a more indirect inference of denudation chronologies (see Kohn et al. this volume, and references therein). An example of a model prediction of AFT parameter and (U-Th)/He age for a specified thermal history is given in Figure 1⇓. Various approaches exist to extract a thermal history model directly from the data, and these focus around inverse modeling (Corrigan 1991; Gallagher 1995; Issler 1996; Willett 1997; Ketcham et. al. 2000). The user specifies some constraints on the thermal history (e.g., upper and lower bounds on the temperature time, and heating/cooling rate), and then typically some form of stochastic sampling is adopted to infer either the most likely thermal history (ideally with some measure of the uncertainty of the solution), and/or a family of acceptable thermal histories. In both the forward and inverse approaches, the thermal history is typically parameterized as nodes in time-temperature space, with some form of interpolation between the nodes. Figure 1. A typical forward model—the thermal history is specified, and having chosen and annealing/diffusion model, we can predict the apatite fission track parameters (age, length distribution), and (U-Th)/He data. PRZ and PAZ are the partial retention zone, and partial annealing zones, over which the He and AFT systems are most sensitive on geological timescales. Over recent years, one of the major applications of low temperature thermochronology has been the study of long term denudation as recorded in the cooling history of surface samples. More recently, some studies have specifically tried to …

Mineral assemblages formed in Oxford Clay fired under different time–temperature conditions with reference to brick manufacture

SUMMARY Time–temperature–transformation (TTT) diagrams have been determined for high-lime (10.2%) and moderate-lime (5.2%) bulk raw materials from the Peterborough Member of the Oxford Clay Formation of King’s Dyke [TL 525 297] and Orton [TL 517 293] brickworks. Study of the chemistry and mineralogy of the brickclays provides the starting point for 79 firing experiments on each raw material, at temperatures between 800 and 1100°C, and for firing times at maximum temperature from 15 minutes to 336 hours. Quantified X-ray diffraction of each fired briquette enables individual TTT diagrams to be constructed for each phase. Finally, a summary diagram shows the distribution of mineral phase assemblages in time–temperature space. The original mineralogy (quartz, illite, kaolinite, chlorite, calcite, aragonite, K-feldspar, albite, pyrite, gypsum, anatase and apatite) changes on heating to a mixture of calc-silicates (anorthite, melilite and pyroxene) with quartz, hematite, anhydrite and glass in commercial bricks and also wollastonite and cordierite when longer firing times and temperatures are used. The mineralogical changes can be followed in quantified equations using the actual compositions and quantities of the phases. The calc-silicates are produced by lime (from calcite) reacting with the clay minerals or their breakdown products such as metakaolinite. The glass has granitic affinities. The data provide a case for considering shorter commercial firing times.

Modélisation inverse des histoires thermiques par traces de fission dans l'apatite

The problem of modelling thermal histories lies in the exploration of a time-temperature space, usually so broad, in order to identify the optimal paths. For overcoming this difficulty, many approaches were proposed, using linear and non-linear optimisation algorithms. Generally, these approaches do not take into account the experimental data (fission track age [FTA] and fission track length distribution (FTLD]) to better aim the search strategy. The present work shows that experimental data hold some precious information, for which it should be known how to extract it. In fact, it allows us to tighten the time-temperature space of search, supposed to contain the optimal solutions. A genetic algorithm is also used in this work to perform the search for these optimal solutions.

The mechanism of nanocrystallization driven by the Fe/Si ratio in Fe73.5Cu1Nb3Si22.5−xBx alloys

The kinetics of nanocrystallization were studied in two amorphous Fe–Cu–Nb–Si–B based alloys with different Fe/Si ratios. Differential scanning calorimetry and transmission Mössbauer spectroscopy data were collected in a range of time-temperature space. Nanocrystallization is preceded by a relaxation of the amorphous phase that appears to involve changes in its short range order (SRO) parameters [T. Pradell, N. Clavaguera, J. Zhu, and M. T. Clavaguera-Mora, J. Phys.: Condens. Matter 7, 4129 (1995)] which are consistent with Cu-clustering. This process is followed by formation of Fe(Si) embryos with ∼20 at. % Si, regardless of alloy composition. Nanocrystals with DO3 structure grow, and change composition via a diffusion controlled process that is stopped by soft impingement [D. Crespo, T. Pradell, M. T. Clavaguera-Mora, and N. Clavaguera, Phys. Rev. B 55, 3435 (1997)]. The rate of this process depends on both temperature and alloy composition. Nanocrystallization is enhanced by prior annealing at conditions which induce SRO relaxation, however, prior annealing at an intermediate temperature (where crystal nucleation occurs) will retard nanocrystallization.

Evolving temperature histories from apatite fission-track data

A procedure for quantifying thermal histories from apatite fission-track data is presented. Given an appropriate annealing algorithm, or forward model, the problem is to determine a best-fit thermal history and quantify the resolution of this solution. Also, a priori geological information should be readily incorporated. Due to the non-linear nature of the problem, conventional inversion methods such as iterative least squares are inappropriate. The procedure described here relies on an initial stochastic search of a broad range of potential thermal histories using a genetic algorithm (GA). These techniques are extremely efficient in defining the regions of time-temperature space where good data-fitting solutions occur. However, the best model found with a genetic algorithm is generally non-optimal. Therefore, a multi-dimensional direct search method is used to update the best GA solution. A variety of standard statistical methods are considered to define confidence regions around this refined best thermal history.

An observation concerning the contribution of temperature on the behaviour of grain boundary segregants for varied microstructures in a NiCrMoV steel

In this paper, the authors examine thermodynamic interactions amongst trace and alloying elements at the grain boundaries of low alloy steels through measurement of grain boundary segregation isotherms and determination of time-temperature space plots (interaction maps). In a more recent work, a rational of the effects of microstructure on grain boundary segregation behaviour in a low alloy, NiCrMoV steel, was attempted. In this paper the effects of microstructure (martensitic, bainitic, ferrite-pearlite) on grain boundary segregation processes were elucidated by defining a parameter, referred as interaction severity, Q, which gives an idea of the magnitude of interaction between the two elements. The interaction severity, Q, for a cooperative interaction process was defined as the ratio of the grain boundary coverage of the interacting elements (e.g. for Cr-N, cooperative interaction process, Q = {theta}{sub N}/{theta}{sub Ce}, where {theta} is the grain boundary coverage of the interacting element) and for site-competitive process, it was defined as the sum of the grain boundary coverage of the interacting elements (e.g. for S-Sn site competitive interaction process, Q = {theta}{sub S} + {theta}{sub SN}).

Effects of structural relaxation on cationic tracer diffusion in silicate melts

The glass transition in silicate melts is a curve in time-temperature space marking the transition of the melt structure from an unrelaxed, disequilibrium glass to a relaxed, equilibrium liquid. Tracer diffusivity data obtained in glasses vs. liquids cannot be compared without consideration of the effects of this transition. For tracer diffusivity experiments, two time scales are important, the time duration of the experiment ( τ d) and the inverse of the jump frequency ( τ p) of the tracer. When the time duration of the experiments reaches the relaxation time-scale ( τ d = τ s) of the melt a transition occurs from diffusion in an unrelaxed matrix (undergoing vibrational thermal expansion) to diffusion in a relaxed matrix (undergoing equilibrium, configurational and elastic, thermal expansion). At this transition, an inflection is observed in the temperature dependence of cationic tracer diffusivity. At temperatures below the inflection, the diffusivity is Arrhenian whereas at temperatures above the diffusivity is non-Arrhenian. At high temperatures the tracer diffusivities of the cations approach the value of diffusivity obtained from the Eyring relation ( τ p = τ s). The contrasting, high-temperature, composition dependence of Na and Li vs. Co, Cs, Sr, Ba, Eu, Fe and C diffusivities can be explained in terms of the Eyring (network O and Si) diffusivity influencing the latter group. The contrasting high- vs. low-temperature, composition dependence of Ba and Sr diffusivities can be similarly explained. These latter observations indicate that all cationic diffusivities will be within a log 10 unit of the Eyring oxygen diffusivity in melts with viscosities below 10 P.

Diagenesis and Preservation of Porosity in Norphlet Formation (Upper Jurassic), Southern Alabama

Sandstones of the Norphlet Formation (Upper Jurassic) in the vicinity of Lower Mobile Bay, Alabama, have porosities greater than 20% and permeabilities up to about 1 darcy at depths of more than 20,000 ft (6,096 m). Typically, pre-Tertiary sandstones buried to such depths have approximately 6% (±2%) porosity; thus, deep Norphlet sandstones contain up to 14% excess porosity. This porosity is largely intergranular and appears to be preserved primary porosity. When the diagenetic scenario for the Norphlet is reconstructed in a time-temperature framework, it is apparent that porosity preservation is not the result of any single diagenetic event, but is due to a continuous series of diagenetic conditions that overlapped in time. The most important of these circumstances were the following. (1) The lack of pervasive early calcite or anhydrite cements found in the large eolian dunes (10-300 ft or 3-91 m high) that comprise the best Norphlet reservoirs. This lack of cement may have set the stage for subsequent events by keeping open pathways through which fluids were later able to efficiently contact the sandstone. (2) Early grain-coating clay/iron oxide rims reacted to form chlorite in the 176°-248°F (80°-120°C) thermal wind w. The grain-rimming chlorite may have inhibited subsequent quartz cementation. Up to 10.5% porosity may be due to the porosity preserving effect of chlorite. (3) Migration of hydrocarbons and development of geopressures, beginning at temperatures as low as 230°F (110°C), retarded further cementation. Up to 4.5% porosity may be due to the combined effects of hydrocarbons and geopressures. (4) A series of CO2-generating reactions occurred, including decarboxylation of organic acids, thermal cracking of liquid hydrocarbons, and thermochemical sulfate reduction. As a result of these reactions, late carbonate cements were not abundant. Thermochemical sulfate reduction (275°-356°+F or 135°-180°+C) has been of particular importance in the deep Norphlet. Reaction of hydrocarbons with anhydrite has resulted in the local removal of nodular anhydrite and has also affected gas quality (i.e., H2S content). The preserved porosity in Norphlet sandstones is the result of a combination of circumstances. The fact that these conditions overlapped in time-temperature space may be as important as any single factor.

Time-Temperature Reconstructions of Diagenetic Systems: ABSTRACT

Predicting the distribution of porosity and permeability enhancement in hydrocarbon reservoirs can be achieved by integrating the generation of carboxylic acids, phenols, mineral oxidants, and liquid hydrocarbons in time-temperature space. Such predictive models can be constructed by linking data from oil-field water chemistry, source rock geochemistry, clay mineralogy, clastic diagenesis, thermal modeling and basin analysis. The detailed organic and inorganic geochemistry and the thermal scenarios used in the time-temperature analysis must be basin specific. Predictive time-temperature models using kerogen-specific kinetic parameters have been developed for two tectonic settings: rift or pull-apart basins, and intermontane or Laramide basins. From these integrated reconstructions, the optimum conditions and capacity for porosity and permeability enhancement can be predicted. The optimum conditions for porosity and/or permeability enhancement are: (1) short migration distances, (2) rapid evolution from organic solvent generation to the liquid hydrocarbon window (thermal environments associated with crustal attenuation or overpressuring could cause such perturbations), (3) adequate fluid flux (organic acids are highly water soluble), and (4) available conduits in potential reservoir rocks (fractures, unconformities, or preserved original porosity). End_of_Article - Last_Page 868------------

Supercooling and the crystallization of plagioclase, olivine, and clinopyroxene from basaltic magmas

AbstractEffects of supercooling have been studied in a range of basaltic melts by isothermal and constant cooling rate experiments at one-atmosphere, using the wire-loop container method. The nucleation of plagioclase in these melts is systematically controlled by supercooling (-ΔT), time below the liquidus temperature, and initial superheating (+ΔT). The temperature at which the melt is initially superheated prior to supercooling controls the temperature at which crystal nucleation first takes place in a supercooled melt undergoing cooling; the greater the + ΔT the larger the degree of supercooling required prior to nucleation of the liquidus phase during cooling. In all six compositions investigated there are at least two fields in time-temperature space, one in which the liquidus phase always fails to nucleate on supercooling (favoured by small -ΔT) and one (favoured by large -ΔT) in which it always nucleates. These two fields may be separated by another in which the liquidus phase may or may not crystallize. Supercooling phenomena are not restricted to the liquidus phase but can also occur when the melt becomes saturated with subsequent phases. It is shown that the composition of plagioclase varies systematically with -ΔT and it is demonstrated that isothermal supercooling ‘lines’ can be used with a relatively high degree of accuracy to predict when nucleation of the liquidus phase will take place during constant cooling rate experiments.

Tensile and indentation behavior of rigid PVC

Abstract The purpose of this study was the evaluation by mechanical testing of the durability of PVC. To that end, the mechanical behavior of PVC was subjected to systematic testing by traction and indentation methods, in order to define the brittle transition in time-temperature space. In a second portion we began a study of the change in the brittle transition by indentation methods after artificial weathering of the PVC.