Bulk Thermodynamics Research Articles

Al-Air Batteries: Fundamental Thermodynamic Limitations from First-Principles Theory.

The Al-air battery possesses high theoretical specific energy (4140 W h/kg) and is therefore an attractive candidate for vehicle propulsion. However, the experimentally observed open-circuit potential is much lower than what bulk thermodynamics predicts, and this potential loss is typically attributed to corrosion. Similarly, large Tafel slopes associated with the battery are assumed to be due to film formation. We present a detailed thermodynamic study of the Al-air battery using density functional theory. The results suggest that the maximum open-circuit potential of the Al anode is only -1.87 V versus the standard hydrogen electrode at pH 14.6 instead of the traditionally assumed -2.34 V and that large Tafel slopes are inherent in the electrochemistry. These deviations from the bulk thermodynamics are intrinsic to the electrochemical surface processes that define Al anodic dissolution. This has contributions from both asymmetry in multielectron transfers and, more importantly, a large chemical stabilization inherent to the formation of bulk Al(OH)3 from surface intermediates. These are fundamental limitations that cannot be improved even if corrosion and film effects are completely suppressed.

Evidence for experimentally unobserved strange hadrons from QCD thermodynamics and their impact on freeze-out parameter

We analyze the influence of experimentally not yet observed strange hadrons – predicted by quark model calculations as well as lattice QCD – on QCD bulk thermodynamics. By comparing lattice QCD results of various combinations of net strangeness fluctuations to hadron resonance gas (HRG) model calculations with different strange hadron content, we find evidence for such additional hadronic states from QCD thermodynamics. We further investigate the impact of such states on the freeze-out parameter in heavy ion collisions. We show that the freeze-out temperature as determined from yields of strange baryons over anti-baryons is, in agreement with lattice QCD, lowered by 5–8 MeV in comparison with the conventional HRG model calculations.

New approach to lattice QCD thermodynamics from Yang–Mills gradient flow

Abstract The bulk thermodynamics in lattice gauge theory is studied on the basis of the Yang–Mills gradient flow with a fictitious time t. The energy density and the pressure of SU ( 3 ) gauge theory at fixed temperature are obtained from the thermal average of the well-defined energy–momentum tensor. The continuum limit is taken in a controlled manner from the t-dependence of the flowed data.

Thermodynamics ofSU(3)gauge theory from gradient flow on the lattice

A novel method to study the bulk thermodynamics in lattice gauge theory is proposed on the basis of the Yang-Mills gradient flow with a fictitious time t. The energy density (epsilon) and the pressure (P) of SU(3) gauge theory at fixed temperature are calculated directly on 32^3 x (6,8,10) lattices from the thermal average of the well-defined energy-momentum tensor (T_{mu nu}^R(x)) obtained by the gradient flow. It is demonstrated that the continuum limit can be taken in a controlled manner from the t-dependence of the flowed data.

Trapping of Mobile Pt Species by PdO Nanoparticles under Oxidizing Conditions.

Pt is an active catalyst for diesel exhaust catalysis but is known to sinter and form large particles under oxidizing conditions. Pd is added to improve the performance of the Pt catalysts. To investigate the role of Pd, we introduced metallic Pt nanoparticles via physical vapor deposition to a sample containing PdO nanoparticles. When the catalyst was aged in air, the Pt particles disappeared, and the Pt was captured by the PdO, forming bimetallic Pt-Pd nanoparticles. The formation of metallic Pt-Pd alloys under oxidizing conditions is indeed remarkable but is consistent with bulk thermodynamics. The results show that mobile Pt species are effectively trapped by PdO, representing a novel mechanism by which Ostwald ripening is slowed down. The results have implications for the development of sinter-resistant catalysts and help explain the improved performance and durability of Pt-Pd in automotive exhaust catalytic converters.

Temperature and pressure based NMR studies of detergent micelle phase equilibria.

Bulk thermodynamic and volumetric parameters (ΔGmic°, ΔHmic°, ΔSmic°, ΔCp,mic°, ΔVmic°, and Δκmic°) associated with the monomer–micelle equilibrium, were directly determined for a variety of common detergents [sodium n-dodecyl sulfate (SDS), n-dodecyl phosphocholine (DPC), n-dodecyl-β-d-maltoside (DDM), and 7-cyclohexyl-1-heptyl phosphocholine (CyF)] via 1H NMR spectroscopy. For each temperature and pressure point, the critical micelle concentration (cmc) was obtained from a single 1H NMR spectrum at a single intermediate concentration by referencing the observed chemical shift to those of pure monomer and pure micellar phases. This permitted rapid measurements of the cmc over a range of temperatures and pressures. In all cases, micelle formation was strongly entropically favored, while enthalpy changes were all positive, with the exception of SDS, which exhibited a modestly negative enthalpy of micellization. Heat capacity changes were also characteristically negative, while partial molar volume changes were uniformly positive, as expected for an aggregation process dictated by hydrophobic effects. Isothermal compressibility changes were found to be consistent with previous measurements using other techniques. Thermodynamic measurements were also related to spectroscopic studies of topology and micelle structure. For example, paramagnetic effects resulting from the addition of dioxygen provided microscopic topological details concerning the hydrophobicity gradient along the detergent chains within their respective micelles as detected by 1H NMR. In a second example, combined 13C and 1H NMR chemical shift changes arising from application of high pressure, or upon micellization, of CyF provided site-specific details regarding micelle topology. In this fashion, bulk thermodynamics could be related to microscopic topological details within the detergent micelle.

Nucleation-Suppressed Phase Stabilization in Fe–Au Nanoparticles

Four nanoparticle compositions, Fe–21, 35, 47, and 67 at. % Au, have been prepared to study the phase stability and solid-state transformation in confined Fe–Au nanoalloys. The formation of two phases, predicted from bulk thermodynamics, has been suppressed in all compositions. Instead, a single phase solid solution forms after heat treatment at 600 °C and slow cooling. However, bulk phase relationships, signified by the precipitation of α-Fe upon cooling, was observed in larger particles (>20 nm) with composition Fe–35 at. % Au. The suppression of the phase transformation/precipitation in small particles is explained thermodynamically, as the free energy decrease associated with the phase transformation does not exceed the increase in energy due to the introduction of an interphase interface (grain boundary) within the cluster. A general equation has been derived to predict the critical cluster size below which transformations are inhibited, which agrees well with the observed experimental results.

Thermodynamics of the QCD transition from lattice

We discuss recent developments in lattice QCD for the bulk thermodynamics of the transition. We review the current status of the equation of state, the possible implications of a magnetic field and the fluctuations of conserved charges, like the net electric charge or baryon number. We also show predictions for higher cumulants, which will be experimentally available in near future.

QCD bulk thermodynamics and conserved charge fluctuations with HISQ fermions

After briefly reviewing recent progress by the HotQCD collaboration in studying the 2+1 flavor QCD equation of state, we will focus on results on fluctuations of conserved charges by the BNL-Bielefeld and HotQCD collaborations. Higher order cumulants of the net-charge distributions are increasingly dominated by a universal scaling behavior, which arises due to a critical point of QCD in the chiral limit. Considering cumulants up to the 6th order, we observe that they generically behave as expected from universal scaling laws, which is quite different from cumulants calculated within the hadron resonance gas model. Taking ratios of these cumulants, we obtain volume independent results that can be compared to the experimental measurements. We will argue that the freeze-out chemical potentials and the freeze-out temperature, usually obtained by a HRG model fit to the measured hadronic yields, can also be obtained in a model independent way from ab-initio lattice QCD calculations by utilizing observables related to conserved charge fluctuations. Further, we will show that the freeze-out strangeness and electric charge chemical potentials can be fixed by imposing strangeness neutrality and isospin asymmetry constraints in the lattice QCD calculations, in order to accommodate conditions met in heavy ion collisions. All results have been obtained with the highly improved staggered quark action (HISQ) and almost physical quark masses on lattices with temporal extent of Nτ = 6, 8, 10, 12.

A Unified Theoretical Framework to Model Bulk, Surface and Interfacial Thermodynamic Properties of Immiscible Liquid Alloys

Bulk, surface and interface thermodynamics of immiscible liquid alloys are considered within a unified theoretical framework. For bulk thermodynamic functions the exponential and the combined linear-exponential equations are discussed, obeying the 4th law of thermodynamics. Surface phase transition is discussed in details. For surface and interface thermodynamics the monolayer Butler equation is compared to the multilayer model. During further assessment of bulk thermodynamic data of immiscible liquid alloys their experimentally measured surface tension and interfacial energy should be also taken into account, coupled with the models presented here.

Using First Principles Thermodynamics to Predict the Shape Surface Structure and Reactivity of Solvated Nanoparticles at High Temperatures and Pressures.

ABSTRACTHydrothermal nanoparticle synthesis uses high temperature and pressure water to control the chemical processes that lead to specific compositions and structures. Analyses of the chemistry associated with this process have been mainly restricted to bulk thermodynamics in the form of quantities such as solubilities and empirical models based on experimental observations. In this paper we demonstrate for NiO and NiFe2O4 particles how effective reference chemical potentials derived from first principles calculations can be used to predict cluster shapes, nucleation barriers and surface reactivity. Implications for controlling the nanoparticle size and shape by adjusting pH and temperature will be discussed, as well as implications of these results in forming nanostructured materials by cluster condensation.

Computational Fluid Dynamics of a Cylindrical Nucleation Flow Reactor with Detailed Cluster Thermodynamics

Particle formation and growth with H(2)SO(4) molecules in an axially symmetric flow reactor was simulated with computational fluid dynamics. A warm (~310 K) gas containing H(2)SO(4) flows into a cooled section (296 K) that induces particle formation. The fluid dynamics gives flow fields, temperatures, and reactant and cluster distributions. Particle formation and growth are simulated with detailed H(2)SO(4) cluster kinetics with chemistry based on measured small cluster thermodynamics and on bulk thermodynamics for large clusters. Results show that particle number densities have power law dependencies on sulfuric acid of ~7, in accord with the thermodynamics of the cluster chemistry. The region where particle formation rates are largest has a temperature that is within 3 K of the wall. Additional simulations show that the H(2)SO(4) concentration in this region is 5 to 10 times greater than the measured H(2)SO(4): this information allows for direct comparisons of experiment and theory. Experiments where ammonia was added as a third nucleating species were simulated with a three-dimensional model. Ammonia was dispersed quickly and particle formation during this mixing was seen to be low. Downstream of the initial mixing region, however, ammonia greatly affected particle formation.

Higher spin black holes from CFT

Higher spin gravity in three dimensions has explicit black holes solutions, carrying higher spin charge. We compute the free energy of a charged black hole from the holographic dual, a 2d CFT with extended conformal symmetry, and find exact agreement with the bulk thermodynamics. In the CFT, higher spin corrections to the free energy can be calculated at high temperature from correlation functions of W-algebra currents.

PHASE FIELD STUDY OF POLARIZATION VORTEX IN FERROELECTRIC NANOSTRUCTURES

Ferroelectric nanostructures are attracting considerable attention due to their unusual physical properties and potential applications in memory devices and nanoelectromechanical systems. It has been found that low-dimensional ferroelectrics, such as ferroelectric nanodots, ferroelectric nanotubes and ferroelectric thin films, exhibit polarization vortices or vortex-like domain structures due to the strong depolarization field and the size effect. The polarization vortex is regarded as a new toroidal order in ferroelectrics which is different from the rectilinear order of polarization. The vortex states of polarization are bistable and can be switched from one state to the other, which holds the potential application in next generation ferroelectric memories. This paper briefly reviews the recent work on the phase field studies of polarization vortex in ferroelectric nanostructures. The homogeneous bulk thermodynamics of ferroelectrics is first introduced based on the Landau–Devonshire theory. To describe the inhomogeneous polarization distribution in ferroelectrics, the phase field model including interface thermodynamics is then presented in the form of time-dependent Ginzburg–Landau equations.

Effects of ion solvation on phase equilibrium and interfacial tension of liquid mixtures

We study the bulk thermodynamics and interfacial properties of electrolyte solution mixtures by accounting for electrostatic interaction, ion solvation, and inhomogeneity in the dielectric medium in the mean-field framework. Difference in the solvation energy between the cations and anions is shown to give rise to local charge separation near the interface, and a finite Galvani potential between two coexisting solutions. The ion solvation affects the phase equilibrium of the solvent mixture, depending on the dielectric constants of the solvents, reflecting the competition between the solvation energy and translation entropy of the ions. Miscibility is decreased if both solvents have low dielectric constants and is enhanced if both solvents have high dielectric constant. At the mean-field level, the ion distribution near the interface is determined by two competing effects: accumulation in the electrostatic double layer and depletion in a diffuse interface. The interfacial tension shows a nonmonotonic dependence on the salt concentration: it increases linearly with the salt concentration at higher concentrations and decreases approximately as the square root of the salt concentration for dilute solutions, reaching a minimum near 1 mM. We also find that, for a fixed cation type, the interfacial tension decreases as the size of anion increases. These results offer qualitative explanations within one unified framework for the long-known concentration and ion size effects on the interfacial tension of electrolyte solutions.

First-principles study of the nucleation and stability of ordered precipitates in ternary Al–Sc–Li alloys

First-principles density functional calculations are used to study the nucleation and stability of L1 2-ordered precipitates in Al–Sc–Li alloys. For dilute Al alloys, there are three possible ordered L1 2 precipitates: Al 3Sc, Al 3Li and an Al 3Sc/Al 3Li core/shell structure. To calculate the nucleation behavior, information about bulk thermodynamics (both static total energies and vibrational free energies), interfacial energetics and coherency strain is required. The study finds the following: (1) the coherency strain energies for forming coherent interfaces between Al/Al 3Sc, Al/Al 3Li and Al 3Sc/Al 3Li are relatively small, owing to the small atomic size mismatches in these systems; (2) the sublattice site preferences of Sc and Li are calculated, and it is demonstrated that Sc and Li share the same sublattice sites in both Al 3Sc(L1 2) and Al 3Li(L1 2), in agreement with recent experimental results; (3) the calculated solubilities of Sc and Li in α-Al alloys are in good agreement with experimental values and, for Sc, agree well with prior first-principles results; (4) the interfacial energies for Al/Al 3Sc, Al/Al 3Li and Al 3Sc/Al 3Li for (1 0 0), (1 1 0) and (1 1 1) interfaces are calculated: the values of the Al/Al 3Sc interfacial energies are significantly larger than those of the Al/Al 3Li and Al 3Sc/Al 3Li interfaces; (5) combining the bulk and interfacial energies yields the nucleation barriers and critical radii for Al 3Sc and Al 3Li precipitates; and (6) the energetic stability of the Al 3Sc/Al 3Li core/shell structure is compared with individual Al 3Sc and Al 3Li nuclei, and the range of precipitate sizes for which the core/shell structure is energetically favored is determined quantitatively.

Driving force for phase segregation in polymer mixtures

Using the general bulk thermodynamics and a molecular equation-of-state model, we provide a heuristic argument on driving forces for phase segregation in polymer mixtures. Particular attention is given to the immiscibility loop emerged under pressure for a binary mixture of polymers of relatively low sizes, which differ in their compressibility. As the disparity in the equation-of-state properties governs the entire phase diagram for a given system, such compressibility effect alters its character as being entropically or enthalpically contributing to phase segregation according to the lower or upper critical solution (order-disorder) transition.

Intrinsic metastability of orthorhombic HfTiO4 in thin film hafnia-titania

Orthorhombic (o) HfTiO4 is crystallized when sputter deposited hafnia-titania nanolaminates with ultrathin layers and bilayer (HfO2)0.5(TiO2)0.5 composition are annealed between 573 and 1173 K. However, o-HfTiO4 demixes after annealing at 1273 K, a result not predicted from bulk thermodynamics. X-ray diffraction and Raman microscopy are used here to study structural changes as o-HfTiO4 demixes upon long-term annealing at 1273 K into Ti-doped monoclinic HfO2 and Hf-doped rutile TiO2. We conclude that o-HfTiO4 crystallized at low temperature is intrinsically metastable. A space group symmetry analysis shows that demixing can be accomplished by a continuous phase transition chain.

Continuum thermodynamics of theSU(Nc)gluon plasma

We study the thermodynamics of $\mathrm{SU}({N}_{c})$ pure gauge theories for ${N}_{c}=3$, 4 and 6. The continuum and thermodynamic limits of bulk quantities such as the pressure ($p$), energy density ($ϵ$) and the entropy density ($s$) are taken by using several different lattice spacings and volumes. There is no window of temperature in which a nontrivial conformal theory describes bulk thermodynamics. We extract the latent heat of the first-order deconfinement phase transitions and observe good scaling with ${N}_{c}$. For all quantities that we measure, strong ${N}_{c}$ scaling holds, except, possibly, very close to the transition temperature, ${T}_{c}$; however we are unable to find strong evidence for scaling with the 't Hooft coupling in thermal quantities at the small values of ${N}_{c}$ which we study.

Adsorbate-Induced Changes in the Surface Composition of Bimetallic Clusters: Pt−Au on TiO2(110)

The growth, surface composition, and chemical activity of bimetallic Pt−Au clusters on TiO2(110) have been investigated. Scanning tunneling microscopy (STM) experiments demonstrate that the deposition of Au on Pt clusters results in the formation of bimetallic Pt−Au clusters due to the seeding of the mobile Au atoms at existing Pt nuclei. The composition of the top surface layer of the clusters was studied by low energy ion scattering (LEIS) for bulk compositions ranging from 25%−87.5% Pt with total metal coverages of 0.25 and 0.50 ML. For both coverages, the cluster surfaces consisted of nearly pure Au at Pt compositions of 50% and lower; however, a mix of Au and Pt atoms were found at the cluster surfaces at higher fractions of deposited Pt. These results are consistent with bulk thermodynamics, which predicts a Pt core−Au shell structure based on the lower surface free energy of Au compared to Pt and the large bulk miscibility gap for the two metals. The adsorption of CO on the Pt−Au clusters at room temperature promotes the diffusion of Pt to the surface of the clusters, and this phenomena is most pronounced for the clusters that are initially pure Au at the surface. Density functional theory calculations demonstrate that it is thermodynamically favorable for Pt to diffuse to the cluster surface in order to bind to CO. In contrast, the extent of CO2 production via sequential adsorption of O2 and CO on the Pt−Au clusters reflects the surface Pt content before adsorption. For CO oxidation, the first step in the reaction is the dissociation of O2 at Pt sites. Since this process requires more than one contiguous Pt site, it is not surprising that O2 dissociation cannot occur on the Pt−Au clusters that are ∼100% Au at the surface before CO exposure, given the low probability for ensembles of Pt sites to form at the surface.