Temperature-independent Parameters Research Articles

Stability and thermoelectric performance of doped higher manganese silicide materials solidified by RGS (ribbon growth on substrate) synthesis

Abstract Large scale deployment of thermoelectric devices requires that the thermoelectric materials have stable electrical, thermal and mechanical properties under the conditions of operation. In this study we examine the high temperature stability of higher manganese silicide (HMS) materials prepared by the RGS (ribbon growth on substrate) technique. In particular we characterize the effect of element substitution on the structural and electrical changes occurring at the hot side of temperatures of thermoelectric devices relevant to this material (600 °C). Only by using suitable substitution (4% vanadium at the Mn site) can we obtain temperature-independent structural parameters in the range 20 °C–600 °C, a condition that results in stable electrical properties. Additionally, we show that 4% vanadium substitution at the Mn site offers the best thermoelectric figure of merit among the different compositions reported here with ZTmax = 0.52, a value comparable to the state of the art for HMS materials. Our analysis suggests that ionized impurity scattering is responsible for the better performance of this material.

BayMBT: A Bayesian calibration model for branched glycerol dialkyl glycerol tetraethers in soils and peats

Accurate temperature records for the deep geological past are a vital component of paleoclimate research. Distributional changes of branched glycerol dialkyl glycerol tetraether (brGDGT) lipids in geological archives including paleosoils are a promising indicators to infer past continental air temperatures. However, the ‘orphan’ status of the brGDGTs, the potential effect of temperature-independent parameters on their relative distribution, and the uneven geographical distribution of the soils used for calibration contribute to the high uncertainty of brGDGT-based transfer functions (root mean squared error, RMSE: ±5 °C). Here, we expand the soil dataset from the previous calibration(s) with new and published soil data. We use Bayesian statistics to calibrate the relationship of the 5-methyl brGDGTs (MBT′5Me) and mean annual air temperature (MAAT). The addition of soils from warm (>28 °C) environments from India substantially increases the upper limit of the Bayesian calibration (BayMBT) from 25 to 29 °C, aiding in the generation of temperature records for past greenhouse climates, such as the Eocene. The BayMBT model also effectively minimizes the structured MAAT residuals prevalent in previous calibrations, therefore giving the opportunity to explore confounding factors on the calibration. We formulate a set of alternative calibration models to test the effect of specific environmental parameters and show that soils at mid-latitudes with temperature seasonalities >20 °C are not well described by the BayMBT model. We find that the MBT′5Me index is best correlated to the average temperature of all months >0 °C, called the BayMBT0 model. This finding supports the hypothesis that brGDGT production ceases or slows down in the winter months. However, a persistent feature of the BayMBT model and previous calibrations is the significant scatter at mid-latitudes, which is speculatively linked with a possible increase in diversity of microbial brGDGT-producing communities in these locations.

Optical Characterization of III-V Multijunction Solar Cells for Temperature-Independent Band Gap Features

A recently developed method to characterize the band gap energies of III-V optosemiconductors was utilized to determine temperature-invariant band gap features of multijunction solar cells. The method is based on measuring electroluminescent spectra of the solar cells at different temperatures. The normalized spectra reveal temperature-invariant energy values of the different junctions which are further converted to band gap energies. The method utilization requires a calibrated spectroradiometer and a temperature controlled mounting base for the solar cell under test, however, no knowledge about the absolute temperature of the cell under measurement. The method was tested on GaAs and GaInP solar cells that consist of single and dual junctions. The band gap energies were also derived from spectral response measurements. The differences of the determined band gap energies from the literature values were smaller than 1.1%. Compared with other band gap determination methods, the developed method yields temperature-invariant band gap characteristics; with a known uncertainty, that separated the different junctions in a multijunction device without individual biasing for the different junctions. In addition, a temperature-independent characterization parameter ensures that the operating conditions of the device under test do not affect the results.

The Vapor-Liquid Phase Diagram of Pure Methane Using Temperature-Dependent Interaction Parameters: A Monte Carlo Simulation

Adopting temperature-dependent interaction parameters in the Lennard-Jones potential, the vapor-liquid phase diagram of methane was produced using NVT Gibbs Ensemble Monte Carlo technique. Published second virial coefficient data were used to fit a simple two-parameter temperature-dependent model for the interaction parameters. The simulations were carried out in the temperature range 120-190 K. The critical density and temperature were evaluated using Ising-scaling model. Using the temperature-dependent interaction parameters in the simulation has reduced the root mean square deviation by 94.7% compared to the temperature-independent interaction parameters. The evaluated critical temperature was enhanced using temperature-dependent interaction parameters, whereas the simulations using temperature-independent interaction parameters predict a better critical density value

TEMPERATURE-DEPENDENT THERMOMECHANICAL MODELING OF SOFT TISSUE DEFORMATION

Modeling of thermomechanical behavior of soft tissues is vitally important for the development of surgical simulation of hyperthermia procedures. Currently, most literature considers only temperature-independent thermal parameters, such as the temperature-independent tissue specific heat capacity, thermal conductivity and stress–strain relationships for soft tissue thermomechanical modeling; however, these thermal parameters vary with temperatures as shown in the literature. This paper investigates the effect of temperature-dependent thermal parameters for soft tissue thermomechanical modeling. It establishes formulations for specific heat capacity, thermal conductivity and stress–strain relationships of soft tissues, all of which are temperature-dependent parameters. Simulations and comparison analyses are conducted, showing a different thermal-induced stress distribution of lower magnitudes when considering temperature-dependent thermal parameters of soft tissues.

Fluid phase equilibrium prediction of acid gas solubility in imidazolium-based ionic liquids with the Peng-Robinson and the PC-SAFT models

In this research, the solubility of two single gases, hydrogen sulfide and carbon dioxide, in the sixteen ionic liquids ([emim][eFAP], [emim][EtSO4], [emim][OTf], [emim][Tf2N], [bmim][BF4], [bmim][OTf], [bmim][PF6], [bmim][Tf2N], [hmim][PF6], [hmim][Tf2N], [omim][PF6], [omim][Tf2N], [HOemim][BF4], [HOemim][OTf], [HOemim][PF6] and [HOemim][Tf2N]) are modeled using the Peng-Robinson and the PC-SAFT equations of state. For the ionic liquids, the pure component parameters of the PC-SAFT EoS have been obtained by the predictions of the model to both of experimental H2S and CO2 solubility data in ionic liquids as well as experimental liquid density data. Also, the critical properties and acentric factor of the ionic liquids were estimated using the modified Lydersen-Joback-Reid method. In the next step, the temperature-independent interaction parameters between the ionic liquids and the gases for both equations of state were calculated by fitting to the experimental VLE data and the obtained results were analyzed. In most of the studied binary systems, the percent average absolute deviation was lower than 10% that shows good agreement between the experimental and predicted values.

Mechanistic Understanding of Improved Performance of Graphene Cathode Inverted Organic Light-Emitting Diodes by Photoemission and Impedance Spectroscopy.

Modification of multilayer graphene films was investigated for a cathode of organic light-emitting diodes (OLEDs). By doping the graphene/electron transport layer (ETL) interface with Li, the driving voltage of the OLED was reduced dramatically from 24.5 to 3.2 V at a luminance of 1000 cd/m2. The external quantum efficiency was also enhanced from 3.4 to 12.9%. Surface analyses showed that the Li doping significantly lowers the lowest unoccupied molecular orbital level of the ETL, thereby reducing the electron injection barrier and facilitating electron injection from the cathode. Impedance spectroscopy analyses performed on electron-only devices (EODs) revealed the existence of distributed trap states with a well-defined activation energy, which is successfully described by the Havriliak-Negami capacitance functions and the temperature-independent frequency dispersion parameters. In particular, the graphene EOD showed a unique high-frequency feature as compared to the indium tin oxide one, which could be explained by an additional parallel capacitance element.

In-situ high-energy X-ray characterization of neutron irradiated HT-UPS stainless steel under tensile deformation

The tensile deformation behavior of a high-temperature, ultrafine-precipitate strengthened (HT-UPS) stainless steel was characterized in-situ with high-energy X-ray diffraction at 20 and 400 °C. The HT-UPS samples were neutron irradiated to 3 dpa at 400 °C. Significant irradiation hardening and ductility loss were observed at both temperatures. Lattice strain evolutions of the irradiated samples showed a strong linear response up to near the onset of the macroscopic yield, in contrast to the unirradiated HT-UPS which showed a pronounced non-linear behavior well below the macroscopic yield. While the room-temperature diffraction elastic moduli in the longitudinal direction increased after irradiation, the 400 °C moduli were similar before and after irradiation. The evolution of the {200} lattice strain parallel to the loading axis (ε{200}L) showed unique characteristics: in the plastic regime, the evolution of ε{200}L after yield is temperature-dependent in the unirradiated specimens but temperature-independent in the irradiated specimens; and the value of ε{200}L at the yield is an irradiation-sensitive, temperature-independent parameter. The evolution of ε{200}L corresponds well with the dislocation density evolution, and is an effective probe of the deformation-induced long-range internal stresses in the HT-UPS steel.

A multi-state model updating method for structures in high-temperature environments

A multi-state model updating method is studied to solve the problem of updating finite element model of structures in high-temperature environments. The key to the problem is to update temperature dependent parameters like elastic modulus and temperature independent parameters like density properties simultaneously in different states. Traditional multi-model updating method only deals with problems with constant parameters in different boundary conditions. To develop the method, specific constraint matrix was designed and an improved Sequential Quadratic Programming method was used to solve the problem. The vibration test of a carbon fibre reinforced bismaleimide composite laminate plate in the high-temperature environments was used to verify the method. The finite element model of the plate in different temperature boundary conditions was updated simultaneously utilizing the presented method. Compared with the traditional method, the multi-state model updating method got a similar updating result. In the meanwhile, it remains the physical meaning of parameter-temperature relations and was proved to be computational efficient with the introduction of additional constraints.

On the temperature independence of statistical model parameters for cleavage fracture in ferritic steels

The work relates to the effect of temperature on the model parameters in local approaches (LAs) to cleavage fracture. According to a recently developed LA model, the physical consensus of plastic deformation being a prerequisite to cleavage fracture enforces any LA model of cleavage fracture to observe initial yielding of a volume element as its threshold stress state to incur cleavage fracture in addition to the conventional practice of confining the fracture process zone within the plastic deformation zone. The physical consistency of the new LA model to the basic LA methodology and the differences between the new LA model and other existing models are interpreted. Then this new LA model is adopted to investigate the temperature dependence of LA model parameters using circumferentially notched round tensile specimens. With the published strength data as input, finite element (FE) calculation is conducted for elastic-perfectly plastic deformation and the realistic elastic-plastic hardening, respectively, to provide stress distributions for model calibration. The calibration results in temperature independent model parameters. This leads to the establishment of a ‘master curve’ characteristic to synchronise the correlation between the nominal strength and the corresponding cleavage fracture probability at different temperatures. This ‘master curve’ behaviour is verified by strength data from three different steels, providing a new path to calculate cleavage fracture probability with significantly reduced FE efforts.

Extension of 2C Association Scheme to Polyols Phase Equilibria

The application of the 2C association scheme proposed by De Villiers et al. (Ind. Eng. Chem. Res. 2011 50, 8711–8725) in the association term of cubic plus association equation of state (CPA EoS) is extended to polyol containing systems to correlate the binary and predict the ternary mixtures. First, the CPA parameters of pure components were successfully adjusted by vapor pressure and saturated liquid density experimental data. Then, the performance of the aforementioned scheme is compared to the other classical schemes by modeling the vapor–liquid equilibria (VLE) and liquid–liquid equilibria (LLE) of binary {polyol/small alcohol}, {polyol/water}, {fatty acid ester/small alcohol}, and {polyol/alkane} and ternary {small alcohol/poyol/fatty acid ester} and {small alcohol/water/poyol} mixtures. Applying a simple and robust phase equilibria algorithm proposed by Privat et al. (Comput.Chem. Eng. 2013, 50, 139–151) to binary and ternary systems, the VLE and LLE correlative performance of CPA EoS coupled with various combined schemes are very satisfactory over extensive temperature ranges and shows that each model is within or close to experimental uncertainty. Then, using two temperature-independent binary interaction parameters (BIPs) fitted with binary VLE and LLE data, the predictive performance of different models is further evaluated in ternary systems phase equilibria (LLE and VLE), and acceptable results are achieved at different temperatures. Eventually, it has been demonstrated that 2C scheme exhibits remarkable precision in predicting the VLE and LLE of polyol containing systems.

Modeling binary vapor–liquid equilibrium data containing perfluorocarbons using the Peng–Robinson and the PC-SAFT equations of state

In this communication, thermodynamic modeling of twenty four binary systems containing perfluorocarbon compounds is studied using the Peng–Robinson and the PC-SAFT equations of state. Interaction parameters of binary systems were estimated through fitting to experimental equilibrium pressures and vapor phase compositions. Very good results were obtained through using temperature-independent interaction parameter for both equations of state. According to the results, percent deviations in pressure were varying between 0.5 and 7.7 for the PR EoS and 0.2 and 6.5 for the PC-SAFT EoS. Also, percent deviations in vapor phase composition were varying between 0.1 and 6.6 for the PR EoS and 0.1 and 7.4 for the PC-SAFT EoS. Furthermore, when binary system has critical point, phase equilibrium calculations using the PR EoS do not converge through common methods. For solving this problem, a solution is suggested that was absolutely effective on all the studied systems.

On-the-fly Doppler broadening of unresolved resonance region cross sections

Two methods for computing temperature-dependent unresolved resonance region cross sections on-the-fly within continuous-energy Monte Carlo neutron transport simulations are presented. The first method calculates Doppler broadened cross sections directly from zero-temperature average resonance parameters. In a simulation, at each event that requires cross section values, a realization of unresolved resonance parameters is generated about the desired energy and temperature-dependent single-level Breit-Wigner resonance cross sections are computed directly via the analytical ψ−χ Doppler integrals. The second method relies on the generation of equiprobable cross section magnitude bands on an energy-temperature mesh. Within a simulation, the bands are sampled and interpolated in energy and temperature to obtain cross section values on-the-fly. Both of the methods, as well as their underlying calculation procedures, are verified numerically in extensive code-to-code comparisons. Energy-dependent pointwise cross sections calculated with the newly-implemented procedures are shown to be in excellent agreement with those calculated by a widely-used nuclear data processing code. Relative differences at or below 0.1% are observed. Integral criticality benchmark results computed with the proposed methods are shown to reproduce those computed with a state-of-the-art processed nuclear data library very well. In simulations of fast spectrum systems which are highly-sensitive to the representation of cross section data in the unresolved region, k-eigenvalue and neutron flux spectra differences of <10 pcm and <1.0% are observed, respectively. The direct method is demonstrated to be well-suited to the calculation of reference solutions — against which results obtained with a discretized representation may be assessed — as a result of its treatment of the energy, temperature, and cross section magnitude variables as continuous. Also, because there is no pre-processed data to store (only temperature-independent average resonance parameters) the direct method is very memory-efficient. Typically, only a few kB of memory are needed to store all required unresolved region data for a single nuclide. However, depending on the details of a particular simulation, performing URR cross section calculations on-the-fly can significantly increase simulation times. Alternatively, the method of interpolating equiprobable probability bands is demonstrated to produce results that are as accurate as the direct reference solutions, to within arbitrary precision, with high computational efficiency in terms of memory requirements and simulation time. Analyses of a fast spectrum system show that interpolation on a coarse energy-temperature mesh can be used to reproduce reference k-eigenvalue results obtained with cross sections calculated continuously in energy and directly at an exact temperature to within <10 pcm. Probability band data on a mesh encompassing the range of temperatures relevant to reactor analysis usually require around 100 kB of memory per nuclide. Relative to the case in which probability table data generated at a single, desired temperature are used, minor increases in simulation times are observed when probability band interpolation is employed.

Application of the modified group-contribution PC-SAFT to nitrile and their mixtures

The modified group-contribution PC-SAFT (NguyenHuynh, Fluid Phase Equilibria2016, 430, 33–46) is applied to simultaneously predict vapor-liquid and liquid-liquid equilibria of nitrile + alkane mixtures (from n-butane to n-hexadecane) using a single temperature-independent binary interaction parameters. Nitriles were considered specific polar and associative compounds. The EoS parameters were regressed on pure compound data and the binary interaction parameter was fitted on liquid-liquid phase equilibrium data of nitrile + n-hexane mixture. The association and dipolar interactions of nitrile in PC-SAFT were investigated with an emphasis on the difficulties in modeling LLE of nitrile + alkanes, water and aromatics binary mixtures.The modeling results were however disappointing when nitrile was studied as unique dipolar or associative compound. It was found in fact that a cross-association interaction between nitrile and aromatic was necessary to accurately calculate the mixtures phase equilibria. The results were finally compared with the induced Polar PC-SAFT EoS.

High pressure solubility of methane in the ionic liquid 1-hexyl-3-methylimidazolium tricyanomethanide

In this paper, new experimental measurements of methane (CH4) solubility in the ionic liquid (IL) 1-hexyl-3-methylimidazolium tricyanomethanide ([hmim][TCM]) are presented. The solubility data were obtained using a synthetic method based on bubble-point pressure measurements for concentrations ranging from (2–10) CH4 mole% and within a temperature range of (293–363K) and pressures up to 10MPa. The results obtained in this work were compared with those obtained for other ILs. The IL [hmim][TCM] was found to show the lowest CH4 solubility, which is desired for application of this IL as gas sweetening solvent (low co-absorption of hydrocarbons). Moreover, the CH4 solubility was compared to the CO2 solubility in the same IL, showing that [hmim][TCM] can selectively absorb CO2 from natural gas. The experimental solubility data of CH4 in [hmim][TCM] were accurately modeled using the PR-EoS with only one temperature-independent binary interaction parameter.

Phase equilibrium of CCS mixtures: Equation of state modeling and Monte Carlo simulation

To understand the role played by the impurities (such as N2, Ar, H2, CO, SO2, O2 and NO) during the processes of Carbon Dioxide Capture and Storage (CCS), it is essential to know the thermodynamic properties of the CO2-impurities mixtures under the conditions of CO2 capture, transport and storage. Considering the variety of composition of these gas mixtures, it is necessary to have at one’s disposal suitable models to predict their thermodynamic properties. In this work, two thermodynamic models: the E-PPR78 (Enhanced Predictive Peng-Robinson, 1978) and the PC-SAFT (Perturbed-Chain Statistical Associating Fluid Theory) models, are applied for describing the phase equilibria properties of 77 binary CCS mixtures containing CO2, gas impurities (SO2, O2 and NO), water and hydrocarbons. Our research results indicate that both models are able to accurately predict the phase behavior of binary CCS mixtures. It was however necessary to adjust the binary interaction parameters (kij,PC-SAFT) within the PC-SAFT model to improve the prediction accuracy. Compared to the PC-SAFT model with one temperature-independent binary interaction parameter, the E-PPR78 model normally shows better prediction accuracy for the investigated systems. In addition, to extend the experimental database which was built for the evaluations of Equation-of-State (EoS) modeling, the Monte Carlo (MC) simulation method is employed in this work to generate phase-equilibrium data of a few CCS mixtures deemed as insufficiently described by experimental measurements reported in the open literature.

Thermodynamic representation of ionic liquids phase equilibrium with the Wilson-NRF model

Ionic liquids (ILs) with their limitless combinations of cations and anions can offer an optimal solvent for a specific purpose especially separation processes such as liquid-liquid extraction (LLE). The present work inquired phase behavior of 32-types ILs with various cations (including imidazolium, pyridinium, piperidinium, morpholinium, pyrrolidinium and phosphonium-based) and various anions (including Cl, FeCl4, BF4, B(CN)4, PF6, NTf2, FAP and alkylsulfates) in LLE systems. Calculation of LLE for 38 binary (220 Tie-Line) and 52 ternary (545 Tie-Line) systems were carried out using Wilson-Non-Random-Factor (Wilson-NRF) model. All the data were in the temperature range of 298.15–408.15K. Therefore, in order to obtain an temperature-independent interaction parameters, the adjustable parameter assumed to be as a function of T as: λij=aij+bij/T. All the interaction parameters either for binary or ternary systems were directly regressed from the experimental data using an optimization program based on two objective functions. All the results were expressed by root mean square deviation (rmsd) between the calculated and experimental compositions (the solubility data from the literature). The applied model had very good satisfactory results for ternary and binary systems with rmsd values of 8.78 and 4.08 respectively. Besides, the effect of cation-type, anion-type and the other organic compound on the model behavior was illustrated. Finally, the results represented that Wilson-NRF model could be satisfying model for evaluation of thermodynamic behavior of various LLE containing ILs.

Incorporation of a volume translation in an equation of state for fluid mixtures: which combining rule? which effect on properties of mixing?

In a previous work, we showed that the incorporation of a temperature-dependent volume-translation parameter in an equation of state for pure components alters necessarily the prediction of 1-phase and 2-phase properties. On the contrary, if a temperature-independent volume translation parameter is used, most of the 1-phase and 2-phase properties are preserved.In the present study, the influence of the incorporation of a volume translation in an equation of state, on mixture properties (essentially phase equilibria and properties of mixing) is addressed. It is first assumed that the volume-translation parameter of a mixture (c) is related to mole fractions through a quadratic mixing rule involving mutual volume translation parameters (cij). It is found that only a simple arithmetic-mean combining rule for cij makes it possible to preserve phase equilibria. Using such a combining rule entails that the quadratic mixing rule for the c parameter degenerates to a linear mixing rule.Secondly, it is proved that properties of mixing (gM, hM, sM, cPM …) are preserved if a linear mixing rule is used for the c parameter and altered otherwise.

A simple model for temperature-independent kij of the PR-vdW model for mixtures containing HCs, HFCs, PFCs, HFOs, CO2, RE170 and R13I1

The purpose of this paper is to establish a simple model for temperature-independent binary interaction parameter kij of the PR-vdW model (Peng-Robinson equation of state with van der Waals one-parameter mixing rules). The model was proposed as kij = wjki + wikj (i ≠ j). In the present model, every pure component was considered to have a constant contribution ki to kij, constant wi is the contribution coefficient of component j mixing with component i. kij can be determined by summing the contribution coefficient of each pure component times its contribution. By regressing the vapor-liquid equilibrium (VLE) data of binary mixtures, 119 kij were obtained and used to optimize the constant contribution ki and contribution coefficient wi. kij of 351 binary systems were predicted and used to describe the VLE data of binary and ternary mixtures. Better performs could be found in the present model than the available models on VLE description of the mixtures.

Correlation and prediction of liquid–liquid equilibria for alcohol/hydrocarbon mixtures using PC-SAFT equation of state at high pressure up to 150 MPa

In this paper, we present a systematic study of the ability of the PC-SAFT equation of state to predict of liquid–liquid equilibria (LLE) for alcohol/hydrocarbon mixtures at high pressure on the basis of LLE data at 0.1 MPa. The adjustable temperature-independent binary interaction parameters, kij, in the PC-SAFT model were determined from LLE data at 0.1 MPa. The new proposed parameters set for methanol and ethanol with the two-site association scheme reproduce vapor pressures and liquid density with the same accuracy of PC-SAFT original parameters but show a better representation of the LLE data. In addition to using pure component PC-SAFT parameters derived from the standard method of fitting to liquid density and saturated vapor pressure, the new PC-SAFT parameters set for methanol and ethanol are determined from experimental association energy. The new association energy parameter of methanol and ethanol shows better agreement with the experimental results at approximately 298 K compared to all previously published PC-SAFT parameters.Different binary methanol + hydrocarbon and ethanol + hydrocarbon systems at high pressure up to 150 Mpa were tested using the PC-SAFT. LLE calculations and prediction show good agreement with experimental data for almost considered systems.