Arrhenius Temperature Dependence Research Articles

Epoxy-based/BaTiO3 nanodielectrics: Relaxation dynamics, charge transport and energy storage

Epoxy/BaTiO3 nanocomposites developed at low nanofiller concentrations with their dielectric relaxations, charge transport dynamics and capacitive energy storage were investigated via dielectric spectroscopy and dc charge-discharge experiments. Two relaxation processes were observed, namely the α- and β-relaxations, attributed to the dynamic Tg process and local motions, respectively. Another process located at low frequencies and high temperatures was identified as interfacial polarization. The charge transport properties were examined via the ac conductivity and impedance formalisms, indicating a beneficial effect with the increase of the nanofiller content. The dc conductivity was calculated from the ac spectra and exhibited Arrhenius temperature dependence. The theoretical energy density was found to increase up to 3.5 times comparing to the neat epoxy resin, highlighting the reinforcing dielectric/capacitive effect of the BaTiO3 nanoparticles on the epoxy resin. Comparisons between the theoretical energy densities and experimentally determined values were also discussed in detail.

Determination of the activation energy of tritium diffusion in ceramic breeders by reactor power variation

In this work, the analysis of experimental results on the irradiation of lithium ceramics Li2TiO3 is presented. Lithium ceramics samples were irradiated at the WWR-K research reactor (Almaty, Kazakhstan) under continuous pumping conditions at a temperature of ~610°C for ~21 days with a thermal neutron flux of ~5•1013 n/(cm2•s). During the experiment, the release of tritium-containing molecules (HT, T2, HTO and T2O) was recorded by a mass-spectrometric method. During the reactor experiment with a frequency of about one to two times a day, a short-term (less than 10 minutes) reduction of reactor power by 25-50 % was conducted, with subsequent regain. In this case, as expected, samples temperature and the released flows of tritium-containing molecules decreased. This mode of reactor operation and obtained data allowed us to propose a method for assessing the temperature dependence of the diffusion coefficient of tritium in ceramics under reactor irradiation. This method assumes that with a rapid change in the reactor power and the sample temperature, the tritium concentration in the surface region does not change much. In contrast, the diffusion coefficient (which has an Arrhenius temperature dependence) varies greatly. It determines the decrease in tritium flows from the sample. In the paper, several sections of reactor power decrease/increase are analyzed, which are used to evaluate the diffusion coefficients of tritium in ceramics under reactor irradiation.

Structure, density and viscosity of water

We analyzed the possibilities of the use of the cluster model of water to assess its viscosity. The Nemethy-Scheraga model was used in our study. In a simplified version, this model implies the presence of water cluster that are linked by hydrogen bonds as well as individual molecules (monomolecules) interacting only by van der Waals forces. The paper gives an estimation of average cluster size. Based on the experimental temperature dependences of viscosity and density, the content of monomolecules in water was approximately determined. In the first case, the ratio of the viscosity of water to monomolecules was estimated from the inverse Arrhenius temperature dependence of viscosity by considering experimental activation energy ~18.6 kJ mol–1 (0÷300C) and energy of dispersion interactions ~7.4 kJ mol–1. Then, the volumetric content of monomolecules was estimated by using the inverse Betchelor's formula, which relates the viscosity of the suspension (clusters) and dispersion medium (monomolecules) to their ratio. On the other hand, a similar estimation was performed based on the density of water, clusters that were considered similar to ice floes, and the estimated density of monomolecules. Both estimates showed that the volumetric content of water not bound into clusters does not exceed 9%. It was concluded that the structure of water most likely corresponds to the clathrate model, according to which some of the H2O molecules move into the middle of ice-like clusters, and vacancies are stabilized by H3O+–OH– pairs.

Dynamics in the plastic crystalline phase of cyanocyclohexane and isocyanocyclohexane probed by 1H field cycling NMR relaxometry.

Proton Field-Cycling (FC) nuclear magnetic resonance (NMR) relaxometry is applied over a wide frequency and temperature range to get insight into the dynamic processes occurring in the plastically crystalline phase of the two isomers cyanocyclohexane (CNCH) and isocyanocyclohexane. The spin-lattice relaxation rate, R1(ω), is measured in the 0.01-30MHz frequency range and transformed into the susceptibility representation χNMR ″ω=ωR1ω. Three relaxation processes are identified, namely, a main (α-) relaxation, a fast secondary (β-) relaxation, and a slow relaxation; they are very similar for the two isomers. Exploiting frequency-temperature superposition, master curves of χNMR ″ωτ are constructed and analyzed for different processes. The α-relaxation displays a pronounced non-Lorentzian susceptibility with a temperature independent width parameter, and the correlation times display a non-Arrhenius temperature dependence-features indicating cooperative dynamics of the overall reorientation of the molecules. The β-relaxation shows high similarity with secondary relaxations in structural glasses. The extracted correlation times well agree with those reported by other techniques. A direct comparison of FC NMR and dielectric master curves for CNCH yields pronounced difference regarding the non-Lorentzian spectral shape as well as the relative relaxation strength of α- and β-relaxation. The correlation times of the slow relaxation follow an Arrhenius temperature dependence with a comparatively high activation energy. As the α-process involves liquid-like isotropic molecular reorientation, the slow process has to be attributed to vacancy diffusion, which modulates intermolecular dipole-dipole interactions, possibly accompanied by chair-chair interconversion of the cyclohexane ring. However, the low frequency relaxation features characteristic of vacancy diffusion cannot be detected due to experimental limitations.

Studies on kinetics of isomerization of gamma oryzanol at air-water interface

We report the studies on Langmuir monolayer of gamma oryzanol (γ-Or) in trans and cis states and kinetics of photo-isomerization and thermal isomerization of γ-Or at air-water interface. The surface manometry studies show that the monolayer of trans−γ-Or on compression exhibits gas and liquid expanded phases with a small region of gas-liquid expanded coexistence region. The monolayer collapses at a surface pressure of 34 mNm−1. In contrast to this, the cis−γ-Or monolayer on compression exhibits an extended gas-liquid expanded coexistence region between gas and liquid expanded phases before collapsing at a surface pressure of 38 mNm−1. The studies on isomerization of γ-Or in the Langmuir monolayer show that the photo-isomerization process follows first order kinetics and thermal isomerization follows second order kinetics. The rate constants of photo-isomerization and thermal isomerization of γ-Or at air-water interface obtained at different temperatures show Arrhenius temperature dependence. The activation energy for the photo-isomerization was found to be 85.34 kJ/mol and for thermal isomerization it was found to be 51.33 kJ/mol.

On the coupling between ionic conduction and dipolar relaxation in deep eutectic solvents: Influence of hydration and glassy dynamics.

We have studied the ionic conductivity and the dipolar reorientational dynamics of aqueous solutions of a prototypical deep eutectic solvent (DES), ethaline, by dielectric spectroscopy in a broad range of frequencies (MHz-Hz) and for temperatures ranging from 128 to 283 K. The fraction of water in the DES was varied systematically to cover different regimes, starting from the pure DES and its water-in-DES mixtures to the diluted electrolyte solutions. Depending on these parameters, different physical states were examined, including low viscosity liquid, supercooled viscous liquid, amorphous solid, and freeze-concentrated solution. Both the ionic conductivity and the reorientational relaxation exhibited characteristic features of glassy dynamics that could be quantified from the deviation from the Arrhenius temperature dependence and non-exponential decay of the relaxation function. A transition occurred between the water-in-DES regime (<40 wt. %), where the dipolar relaxation and ionic conductivity remained inversely proportional to each other, and the DES-in-water regime (>40 wt. %), where a clear rotation-translation decoupling was observed. This suggests that for a low water content, on the timescale covered by this study (∼10-6 to 1 s), the rotational and transport properties of ethaline aqueous solutions obey classical hydrodynamic scaling despite these systems being presumably spatially microheterogeneous. A fractional scaling is observed in the DES-in-water regime due to the formation of a maximally freeze-concentrated DES aqueous solution coexisting with frozen water domains at sub-ambient temperature.

Effects of yttrium addition on thermoplastic formability of Zr-Cu-Ni-Al amorphous alloy under non-isothermal condition

A low-cost and Be-free Zr-based amorphous system with excellent thermoplastic formability has been developed by doping a small amount of Y element. Based on the thermodynamic test data, Y addition can increase the supercooled liquid region of Zr-based amorphous alloy when it is less than 2 at%, leading to a relatively wider temperature range of thermoplastic forming. The compression experimental results reveal that (Zr0.507Cu0.28Ni0.9Al0.123)98.5Y1.5 (ZrY1.5) alloy possesses the largest Dmax value (the maximum diameter of the compressed cylindrical specimen) than other Zr-based amorphous alloys prepared in this study, thereby confirming its superior thermoplastic formability. A lower viscosity can be observed on the TMA curves of ZrY1.5 alloy, suggesting that appropriate Y addition decreases the viscous resistance of amorphous supercooled liquid. However, the more obvious deviation from Vogel–Fulcher–Tamann theoretical model indicates the weak Arrhenius temperature dependence of viscous flow behavior. The parameter ΔTTPF/D*, where ΔTTPF represents the temperature range of thermoplastic forming and D* denotes the fragility index, is found to be an effective indicator for evaluating the thermoplastic formability of the investigated Y addition Zr-based amorphous system. Finally, Zr-based amorphous nanopillar arrays have been successfully fabricated under non-isothermal condition, which further proves that Y addition Zr-based amorphous alloy is a promising candidate for nanomolding material.

Observation of the relaxation process in fluoroelastomers by dielectric relaxation spectroscopy

The complex dielectric permittivity of fluoroelastomers in response to both frequency and temperature variations was thoroughly investigated by dielectric relaxation spectroscopy. To characterize the various types of relaxation processes, the dispersion spectra of permittivity in the frequency domain were successively deconvoluted using a newly developed simulation program that uses the empirical Havriliak-Negami model based on the Debye dielectric model and conductivity contribution. At 233 K ≤ T ≤ 290 K, the α and β relaxation processes were independently observed; they merged at T ≥ 290 K. At T > 303 K, and both Maxwell-Wagner-Sillars (MWS) relaxation, known as interfacial polarization, and conductivity relaxation processes were observed. From the α relaxation process, which is related to the glass transition phenomenon, the glass transition temperature Tg was determined using the Vogel-Fulther-Tamman-Hesse (VFTH) temperature dependence law; the result was similar to the estimate of Tg obtained using differential scanning calorimetry. The temperature dependence of conductivity complies well with VFTH behaviors, whereas the β and MWS processes can be described using the Arrhenius temperature dependence law. The corresponding activation energies of the rotational side groups and DC conductivity were obtained.

Modeling creep behavior in ceramic matrix composites

In this work, a three-dimensional viscoplasticity formulation with progressive damage is developed and used to investigate the complex time-dependent constituent load transfer and progressive damage behavior in ceramic matrix composites (CMCs) subjected to creep. The viscoplasticity formulation is based on Hill's orthotropic plastic potential, an associative flow rule, and the Norton-Bailey creep power law with Arrhenius temperature dependence. A fracture mechanics-informed isotropic matrix damage model is used to account for CMC brittle matrix damage initiation and propagation, in which two scalar damage variables capture the effects of matrix porosity as well as matrix property degradation due to matrix crack initiation and propagation. The Curtin progressive fiber damage model is utilized to simulate progressive fiber failure. The creep-damage formulation is subsequently implemented as a constitutive model in the generalized method of cells (GMC) micromechanics formulation to simulate time-dependent deformation and material damage under creep loading conditions. The developed framework is used to simulate creep of single fiber SiC/SiC microcomposites. Simulation results are in excellent agreement with experimental and numerical data available in the literature.

Dipole relaxation process and giant dielectric permittivity in Eu3+-doped CdMoO4 single crystal

Single crystal of Eu3+-doped cadmium molybdate (Cd0.9268▯0.0244Eu0.0488MoO4, where ▯ denotes cationic vacancies) has been successfully grown by the Czochralski method in air and under 1 MPa. X-ray diffraction analysis indicates that as-grown single crystal exhibits tetragonal scheelite-type structure (a = b = 5.16188(14) Å; c = 11.2080(5) Å; space group I41/a). Eu3+ ions do not show long-range order and they are randomly distributed in CdMoO4 framework substituting Cd2+ ones. UV–vis diffuse reflectance measurements revealed very close optical band gap (Eg) values, i.e. ∼1.74 eV along [100] and [001] crystallographic directions that are twice smaller than Eg of microcrystalline pure CdMoO4 as well as powder Eu3+-doped single crystal. Magnetic and electrical studies of Eu3+-doped cadmium molybdate single crystal showed a paramagnetic and n-type semiconducting behaviour with the metal-insulator transition above 350 K along both crystallographic directions. Dielectric results analysis using the Cole-Cole fit function revealed that the dipole relaxation process has different time scale depending on the crystallographic direction and exhibits Arrhenius temperature dependence for both studied directions. This fact is accompanied by the colossal dielectric permittivity with εr > 8⋅103. The above results are considered in the framework of narrow europium multiplets of energy comparable to thermal energy.

The Dynamics of Hydrated Proteins Are the Same as Those of Highly Asymmetric Mixtures of Two Glass-Formers.

Customarily, the studies of dynamics of hydrated proteins are focused on the fast hydration water ν-relaxation, the slow structural α-relaxation responsible for a single glass transition, and the protein dynamic transition (PDT). Guided by the analogy with the dynamics of highly asymmetric mixtures of molecular glass-formers, we explore the possibility that the dynamics of hydrated proteins are richer than presently known. By providing neutron scattering, dielectric relaxation, calorimetry, and deuteron NMR data in two hydrated globular proteins, myoglobin and BSA, and the fibrous elastin, we show the presence of two structural α-relaxations, α1 and α2, and the hydration water ν-relaxation, all coupled together with interconnecting properties. There are two glass transition temperatures Tgα1and Tgα2 corresponding to vitrification of the α1 and α2 processes. Relaxation time τα2(T) of the α2-relaxation changes its Arrhenius temperature dependence to super-Arrhenius on crossing Tgα1 from below. The ν-relaxation responds to the two vitrifications by changing the T-dependence of its relaxation time τν(T) on crossing consecutively Tgα2 and Tgα1. It generates the PDT at Td where τν(Td) matches about five times the experimental instrument timescale τexp, provided that Td > Tgα1. This condition is satisfied by the hydrated globular proteins considered in this paper, and the ν-relaxation is in the liquid state with τν(T) having the super-Arrhenius temperature dependence. However, if Td < Tgα1, the ν-relaxation fails to generate the PDT because it is in the glassy state and τν(T) has Arrhenius T-dependence, as in the case of hydrated elastin. Overall, the dynamics of hydrated proteins are the same as the dynamics of highly asymmetric mixtures of glass-formers. The results from this study have expanded the knowledge of the dynamic processes and their properties in hydrated proteins, and impact on research in this area is expected.

Using temperature dependent fluorescence to evaluate singlet fission pathways in tetracene single crystals.

The temperature-dependent fluorescence spectrum, decay rate, and spin quantum beats are examined in single tetracene crystals to gain insight into the mechanism of singlet fission. Over the temperature range of 250 K-500 K, the vibronic lineshape of the emission indicates that the singlet exciton becomes localized at 400 K. The fission process is insensitive to this localization and exhibits Arrhenius behavior with an activation energy of 550 ± 50 cm-1. The damping rate of the triplet pair spin quantum beats in the delayed fluorescence also exhibits an Arrhenius temperature dependence with an activation energy of 165 ± 70 cm-1. All the data for T > 250 K are consistent with direct production of a spatially separated 1(T⋯T) state via a thermally activated process, analogous to spontaneous parametric downconversion of photons. For temperatures in the range of 20 K-250 K, the singlet exciton continues to undergo a rapid decay on the order of 200 ps, leaving a red-shifted emission that decays on the order of 100 ns. At very long times (≈1 µs), a delayed fluorescence component corresponding to the original S1 state can still be resolved, unlike in polycrystalline films. A kinetic analysis shows that the redshifted emission seen at lower temperatures cannot be an intermediate in the triplet production. When considered in the context of other results, our data suggest that the production of triplets in tetracene for temperatures below 250 K is a complex process that is sensitive to the presence of structural defects.

Material characterization of a pultrusion specific and highly reactive polyurethane resin system: Elastic modulus, rheology, and reaction kinetics

This study presents a detailed material characterization study of a pultrusion specific polyurethane resin system. Firstly, the chemical behaviour was characterized by utilizing differential scanning calorimetry. The cure kinetics was fitted well to an autocatalytic cure kinetics model with Arrhenius temperature dependency. The resin system did not show any inhibition time. Then, the viscosity evolution was observed using a rheometer. From these measurements, the gelation point was estimated from the criterion of cross-over point of storage and loss modulus curves. The corresponding cure degree of gelation point was found to be 0.79 using the cure kinetics model. The complex viscosity evolution through dynamic scans was predicted well with a cure degree and temperature dependent viscosity model. The elastic modulus and glass transition temperature of fully and partially cured samples were measured by means of DMA in tension mode. A five step cure hardening instantaneous linear elastic model was fitted well to a wide range of cure degree values after gelation. Lastly, the fitted material models were employed in a case study of a typical pultrusion process to visualize the effects of pulling speed on the material property evolution.

Electronic voltage control of magnetic anisotropy at room temperature in high- κ SrTiO3/Co/Pt trilayer

To improve power efficiency and endurance of magnetic memory technologies, a voltage-controlled mechanism is desirable. The voltage control of magnetic anisotropy (VCMA) effect in MgO stacks is a promising option, however, its strength is too low for memory applications. Replacing the standard MgO layer by an oxide with a higher permittivity $\ensuremath{\kappa}$ may help improve the VCMA strength. We demonstrate a VCMA effect up to $\ensuremath{\xi}=75$ fJ/Vm at room temperature in a Co\ensuremath{\setminus}Pt bilayer grown on atomic layer deposited (ALD) high-$\ensuremath{\kappa}\phantom{\rule{4pt}{0ex}}{\mathrm{SrTiO}}_{3}$ (STO). After treating the STO surface with isopropanol, a thin ${\mathrm{CoO}}_{x}$ interfacial layer is observed, enabling VCMA. Upon cooling down from room temperature to 200 K, the VCMA effect strength increases by a factor of two. This increase is incompatible with the expected Arrhenius temperature dependence for an ionic effect and thus we argue that the observed VCMA effect is electronic. Electronic VCMA is desirable for adequate memory endurance, and hence the approach proposed here has great potential for applications.

Multidimensional Hydrogen Tunneling in Supported Molecular Switches: The Role of Surface Interactions.

The nuclear tunneling crossover temperature (T_{c}) of hydrogen transfer reactions in supported molecular-switch architectures can lie close to room temperature. This calls for the inclusion of nuclear quantum effects (NQEs) in the calculation of reaction rates even at high temperatures. However, computations of NQEs relying on standard parametrized dimensionality-reduced models quickly become inadequate in these environments. In this Letter, we study the paradigmatic molecular switch based on porphycene molecules adsorbed on metallic surfaces with full-dimensional calculations that combine density-functional theory for the electrons with the semiclassical ring-polymer instanton approximation for the nuclei. We show that the double intramolecular hydrogen transfer (DHT) rate can be enhanced by orders of magnitude due to surface fluctuations in the deep-tunneling regime. We also explain the origin of an Arrhenius temperature dependence of the rate below T_{c} and why this dependence differs at different surfaces. We propose a simple model to rationalize the temperature dependence of DHT rates spanning diverse fcc [110] surfaces.

Intrinsic electrical properties of cable bacteria reveal an Arrhenius temperature dependence

Filamentous cable bacteria exhibit long-range electron transport over centimetre-scale distances, which takes place in a parallel fibre structure with high electrical conductivity. Still, the underlying electron transport mechanism remains undisclosed. Here we determine the intrinsic electrical properties of the conductive fibres in cable bacteria from a material science perspective. Impedance spectroscopy provides an equivalent electrical circuit model, which demonstrates that dry cable bacteria filaments function as resistive biological wires. Temperature-dependent electrical characterization reveals that the conductivity can be described with an Arrhenius-type relation over a broad temperature range (− 195 °C to + 50 °C), demonstrating that charge transport is thermally activated with a low activation energy of 40–50 meV. Furthermore, when cable bacterium filaments are utilized as the channel in a field-effect transistor, they show n-type transport suggesting that electrons are the charge carriers. Electron mobility values are ~ 0.1 cm2/Vs at room temperature and display a similar Arrhenius temperature dependence as conductivity. Overall, our results demonstrate that the intrinsic electrical properties of the conductive fibres in cable bacteria are comparable to synthetic organic semiconductor materials, and so they offer promising perspectives for both fundamental studies of biological electron transport as well as applications in microbial electrochemical technologies and bioelectronics.

Photovoltaic Effect in Phthalocyanine-Based Organic Solar Cells: 1. Thermal Ionization of Molecular Excitons

Based on the previously obtained data [1, 2], according to which the photoelectric quantum yield (Y) in crystals of the β-form of metal-free phthalocyanine and copper phthalocyanine is the same for electrons and holes and has an Arrhenius temperature dependence with an activation energy EY equal to the difference between the band gap and the molecular exciton (ME) energy, a three-dimensional stochastic model of thermal ionization of MEs is proposed, which is considered as a sequence of reversible transitions from the ME to the lowest charge-transfer (CT) state and then to a CT state with an increasing charge-transfer distance. A relationship has been found between EY and the radial density of CT states at which the probability of charge separation is greater than that of the decay of these states.

Delay of re-entanglement kinetics by shear-induced nucleation precursors in isotactic polypropylene melt

Rheological measurements were carried out to in-situ monitor the re-entanglement process of large amplitude oscillatory shear flow (LAOS) modified partially disentangled isotactic polypropylene (i-PP) melt. The characteristic entanglement recovery time (τent) was essentially longer than the average reptation time (τD) of fully entangled melt. At temperatures above 200 °C, the Arrhenius temperature dependence of τent and τD are the same, displaying a normal chain diffusion-controlled mechanism with flow activation energy of 41.4 kJ/mol. However, when the temperature is lower than 200 °C, τent is unexpectedly longer than expectation extrapolated from high temperatures, and a much higher activation energy of 98.6 kJ/mol is observed for the re-entanglement process. Based on our understanding, the re-entanglement process of shear-induced disentanglements in i-PP melt was postponed by the existence of quasi-ordered clusters created by shear at relatively low temperatures and their dissipation turns to be the rate-determining step for the whole recovery. Moreover, morphological study under polarized optical microscope showing much higher nucleation density which originates from the flow-induced precursors corroborates our hypothesis. Meanwhile, the surviving time of LAOS-generated nucleation precursors is comparable to the recovery time of disentangled chains, suggesting that the delay effect of nucleation precursors on chain diffusion may last over the entire re-entanglement process. Notwithstanding the obstruction of “extra” physical entanglements for chain diffusion, the molecular weight (Mw) dependence of entanglement recovery still follows the reptation model-based power law: τent=(a+b)Mw3.4, where a and b is the contribution of chain diffusion and dissipation of precursors, respectively.

Shock tube ignition delay time measurements for methyl propanoate and methyl acrylate: Influence of saturation on small methyl ester high‐temperature reactivity

AbstractWe report ignition delay time measurements for methyl propanoate (MP) and methyl acrylate (MA), carried out in a high‐pressure shock tube. Experiments were performed behind reflected shock waves across a temperature range of 989‐1 367 K, for fuel‐air mixtures at equivalence ratios of ϕ = 0.5, 1.0, and 2.0, and nominal pressures of 1 and 4 MPa. Ignition delay times were found to decrease with increasing temperature, equivalence ratio, and pressure, and are well described with correlations involving Arrhenius temperature dependence and power‐law dependence on equivalence ratio and pressure. Ignition delay times are compared with model predictions from literature kinetic models, with the models of Zhang et al (Energy &amp; Fuels 2014; 28(11): 7194‐7202) and Bennadji et al (International Journal of Chemical Kinetics 2011; 43(4): 204‐218.) in good agreement with measured ignition delay times for MP and MA, respectively. Kinetic sensitivity analysis shows that the reactions most important for modeling ignition fall into two categories: initiation reactions (ie, decomposition and H‐atom abstraction) and C0‐C1 chemistry controlling the pool of small radicals. The unsaturated MA was observed to have lower reactivity than MP, due to its greater bond strengths for C─C and C─H bonds, resulting in slower rates for initiation reactions.

Quasi-equilibrium predictions of water desorption kinetics from rapidly-heated metal oxide surfaces

Controlling sub-microsecond desorption of water and other impurities from electrode surfaces at high heating rates is crucial for pulsed power applications. Despite the short time scales involved, quasi-equilibrium ideas based on transition state theory (TST) and Arrhenius temperature dependence have been widely applied to fit desorption activation free energies. In this work, we apply molecular dynamics (MD) simulations in conjunction with equilibrium potential-of-mean-force (PMF) techniques to directly compute the activation free energies (ΔG*) associated with desorption of intact water molecules from Fe2O3 and Cr2O3 (0001) surfaces. The desorption free energy profiles are diffuse, without maxima, and have substantial dependences on temperature and surface water coverage. Incorporating the predicted ΔG* into an analytical form gives rate equations that are in reasonable agreement with non-equilibrium molecular dynamics desorption simulations. We also show that different ΔG* analytical functional forms which give similar predictions at a particular heating rate can yield desorption times that differ by up to a factor of four or more when the ramp rate is extrapolated by 8 orders of magnitude. This highlights the importance of constructing a physically-motivated ΔG* functional form to predict fast desorption kinetics.