Thermodynamic Stability Limit Research Articles

Stabilizing Protic and Aprotic Liquid Electrolytes at High-Bandgap Oxide Interphases

Approaches for regulating electrochemical stability of liquid electrolytes in contact with solid-state electrodes are a requirement for efficient and reversible electrical energy storage in batteries. Such methods are particularly needed in electrochemical cells in which the working potentials of the electrodes lie outside the thermodynamic stability limits of the liquid electrolyte. Here we study electrochemical stability of liquids at electrolyte/electrode interfaces protected by nanometer thick, high electrical bandgap ceramic phases. We report that well-designed ceramic interphases extend the oxidative stability limits for both protic and aprotic liquid electrolytes, in some cases by as much as 1.5 V. It is shown further that such interphases facilitate stable electrodeposition of reactive metals such as lithium at high Coulombic efficiency and in electrochemical cells subject to extended galvanostatic cycling at a current density of 3 mA cm–2 and at capacities as high as 3 mAh cm–2. High-resolution c...

(Battery Division Research Award) Forming Interphases

Solid electrolyte interphases (SEI) enable the battery chemistries to maximize energy density by operating beyond the thermodynamic stability limits of non-aqueous electrolytes 1 ,2. The chemical building blocks of SEI come from the decomposition of either solvents or salt anion, depending on a host of interwoven parameters such as the HOMO/LUMO energy levels of these components (reduction/oxidation stability) as well as their interactions (solvation sheath structures). Designing electrolytes for new battery chemistries requires understanding of interphasial chemistries and the ability to manipulate them. This talk summarizes our efforts in interfacing various electrode chemistries to electrolytes of non-aqueous, aqueous and hybrid natures. Reference s : [1] K. Xu, Chem. Rev., 2004, 104, 4303 [2] K. Xu, Chem. Rev., 20 1 4, 1 1 4, 11503

A High‐Voltage Electrochemical Cell Operating with Two Aqueous Electrolytes of Different pH Values

AbstractA concept of a carbon/carbon electrochemical cell using aqueous electrolytic solutions of potassium hydroxide and sodium sulfate as catholyte and anolyte, respectively, and a cation exchange membrane as separator is presented. Owing to the pH difference between the two electrolytes, the theoretical potential difference between water oxidation and reduction is enlarged. Experimentally, an optimized cell could be charged up to 1.5 V, while the operating potential range of the electrodes fitted in the thermodynamic stability limits of the electrolytes. The cell demonstrated a good life span, with constant capacitance and resistance values, when it was kept at imposed voltage of 1.5 V for a total number of 120 hours.

Water-in-Salt Electrolytes for High-Voltage Supercapacitors

The rising ecological problems, fossil fuel curtail and global concerns dramatically demand the development of sustainable energy storage systems. Though lithium-ion batteries are capable of providing adequate energy density, their deficient performance upon fast charging limits its applications in many high-power storage systems. On the other hand, supercapacitors (SCs) are a special class of energy storage devices that fill the gap between traditional dielectric capacitors (1-100 kW kg−1) and batteries (~100 Wh kg−1). However, SCs are power devices that can be fully charged or discharged in seconds; consequently, their energy density (~ 5 Wh kg−1) is lower than batteries. To mitigate this issue, several types of device configurations were constructed with various micro/nanoelectrodes with larger surface area which cause high power and energy density supercapacitors1,2. Among several other strategies, enlarging the voltage window for a device is given utmost priority, since the energy density is proportional to the voltage squared. Today, the state of the art technique is the usage of organic electrolyte solutions such as, acetonitrile (ACN) or propylene carbonate (PC), with an optimized salt content. Despite these solvents offering excellent ionic conductivity and wide voltage window; low boiling point and high viscosity poses safety concerns and critically slows down the device performance upon extended cycling. Considerable research has been carried out with ionic liquids which are well known for wide voltage window operations, but their high viscosity falloff the ionic conductivity at room temperature. The balance of low viscosity, extended electrochemical stability window, and high dielectric constants can encounter all sorts of problems with existing electrolytes while maintaining high energy output3. In this context, aqueous based electrolytes hold significant advantages of low cost, high safety, and high power; it is green and a key component for sustainable electrochemical energy storage system. However, early decomposition of water sets operation of these devices in a narrow voltage window (<1.5 V) which is a substantial drawback to obtain high energy density output for its application in supercapacitor’s. Recent studies on developing high voltage aqueous electrolytes for Li-ion battery application4,5 though provides safety solution, has shown reasonable energy density in the context of rapidly evolving battery power technologies. Herein, we report a supercapacitor application of “Water-in-Salt” electrolytes in comparison with conventional aqueous (low voltage < 1.5 V) and no-aqueous electrolytes systems. Extensive electrochemical studies including cyclic and linear voltammetry demonstrate that these “Water-in-Salt” electrolytes are stable up to 3 V without gas evolution reactions. The stability mechanism can be ascribed to the high salt concentration which strongly perturb the ionic clouds at the interface and forms a solid electrolyte interphase layer that postpones the evolution reactions. The extended electrochemical stability window allows the operation of SCs at voltages significantly above the thermodynamic stability limit of water. To realize efficacies of electrolyte, a symmetrical SC was fabricated with nano-porous carbon and activated carbon as anode and cathodes respectively with a glass fiber separator. The symmetric supercapacitor with ‘Water-in-Salt’ electrolytes allowed a maximum operating voltage of 2.9 V and delivered 120 F g-1 of specific capacitance. Benefitting from the large potential window, the SCs showed a high energy density of 12.5 Wh kg-1. In addition, its excellent performance at different current densities (0.1 A g-1 to 10 A g-1) and prolonged cycle life paves a way for the development of high-voltage aqueous supercapacitors. Hence, this class of electrolytes can remarkably enhance the energy density and safety performance of next-generation high-energy supercapacitors.

Synthesis of Urea Derivatives using Carbon Dioxide as Carbonylation Reagent in Ionic Liquids

Urea and its derivatives, which are usually generated through the carbonylation reaction between amines and carbonylation reagent, have been found widespread applications in agriculture and pharmaceuticals. Among the carbonylation reaction, it is the most appealing and promising strategy that employs CO2 as a green carbonylation reagent. However, CO2 inherent thermodynamic stability and kinetic inertness limit its application. Apart from being regarded as one of the green solvents, ionic liquid is also an efficient organocatalyst for CO2 capture or activation due to the interaction between CO2 with cation or anion of ionic liquid. In this mini-review, we have summarized representative synthetic methodologies of urea derivatives using carbon dioxide as a green carbonylation reagent and using ionic liquids as solvents and/or organocatalysts. Keywords: Activation, carbon dioxide, carbonylation, ionic liquid, organocatalyst, urea derivatives.

How Solid-Electrolyte Interphase Forms in Aqueous Electrolytes.

Solid-electrolyte interphase (SEI) is the key component that enables all advanced electrochemical devices, the best representative of which is Li-ion battery (LIB). It kinetically stabilizes electrolytes at potentials far beyond their thermodynamic stability limits, so that cell reactions could proceed reversibly. Its ad hoc chemistry and formation mechanism has been a topic under intensive investigation since the first commercialization of LIB 25 years ago. Traditionally SEI can only be formed in nonaqueous electrolytes. However, recent efforts successfully transplanted this concept into aqueous media, leading to significant expansion in the electrochemical stability window of aqueous electrolytes from 1.23 V to beyond 4.0 V. This not only made it possible to construct a series of high voltage/energy density aqueous LIBs with unprecedented safety, but also brought high flexibility and even "open configurations" that have been hitherto unavailable for any LIB chemistries. While this new class of aqueous electrolytes has been successfully demonstrated to support diversified battery chemistries, the chemistry and formation mechanism of the key component, an aqueous SEI, has remained virtually unknown. In this work, combining various spectroscopic, electrochemical and computational techniques, we rigorously examined this new interphase, and comprehensively characterized its chemical composition, microstructure and stability in battery environment. A dynamic picture obtained reveals how a dense and protective interphase forms on anode surface under competitive decompositions of salt anion, dissolved ambient gases and water molecule. By establishing basic laws governing the successful formation of an aqueous SEI, the in-depth understanding presented in this work will assist the efforts in tailor-designing better interphases that enable more energetic chemistries operating farther away from equilibria in aqueous media.

(Invited) Vertical GaN Devices Enabled By Selective Area P-Type Doping

A key element for enabling high performance vertical GaN power electronic devices is the capability to realize selective area p-type doping. The ability to arbitrarily define p-type regions through ion implantation eliminates the need for epitaxial regrowth on etched surfaces, which is currently a well-known technological roadblock. However, activation of implanted Mg is also a challenge, since activation of the Mg acceptors requires annealing at temperatures above the thermodynamic stability limits of GaN in atmosphere. Implementing the symmetric multi cycle rapid thermal annealing (SMTRA) technique has been shown to activate up to ~10% of the implanted Mg dopant atoms. This technique includes a temporary thermally stable capping layer, annealing in a nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. Vertical GaN junction barrier Schottky (JBS) diodes and Schottky barrier diodes (SBDs) with implanted junction termination extension (JTE) are demonstrated using the SMRTA process, as are process module development toward trench MOSFET devices.

The spinodal of single- and multi-component fluids and its role in the development of modern equations of state

The spinodal represents the limit of thermodynamic stability of a homogeneous fluid. In this work, we present a robust methodology to obtain the spinodal of multicomponent fluids described even with the most sophisticated equations of state (EoS) available. We elaborate how information about the spinodal and its uncertainty can contribute both in the development of modern EoS and to estimate their uncertainty in the metastable regions. Inequality constraints are presented that can be exploited in the fitting of modern EoS of single-component fluids to avoid inadmissible pseudo-stable states between the vapor and liquid spinodals. We find that even cubic EoS violate some of these constraints.With the use of a selection of EoS representative of modern applications, we compare vapor and liquid spinodal curves, superheat and supersaturation limits from classic nucleation theory (CNT), and available experimental data for the superheat limit. Computations are performed with pure species found in natural gas, binary mixtures, as well as a multi-component natural gas mixture in order to demonstrate the scalability of the approach. We demonstrate that there are large inconsistencies in predicted spinodals from a wide range of EoS such as cubic EoS, extended corresponding state EoS, SAFT and multiparameter EoS. The overall standard deviation in the prediction of the spinodal temperatures were 1.4 K and 2.7 K for single- and multi-component liquid-spinodals and 6.3 K and 26.9 K for single- and multi-component vapor spinodals.The relationship between the measurable limit of superheat, or supersaturation, and the theoretical concept of the spinodal is discussed. While nucleation rates from CNT can deviate orders of magnitude from experiments, we find that the limit of superheat from experiments agree within 1.0 K and 2.4 K with predictions from CNT for single- and multi-component fluids respectively. We demonstrate that a large part of the metastable domain of the phase diagram is currently unavailable to experiments, in particular for metastable vapor. Novel techniques, experimental or with computational simulations, should be developed to characterize the thermodynamic properties in these regions, and to identify the thermodynamic states that define the spinodal.

Near-Field IR Imaging and Spectroscopy of Interfaces and Interphases in Li-Ion Electrodes

Most Li-ion battery chemistries involve the creation or transformation of materials under non-equilibrium environments. Because these chemical energy storage systems operate beyond the thermodynamic stability limits of the electrolytes, reactions between electrodes and electrolyte result in the formation of new phases and interphases, which hinder further electrolyte decomposition and assure battery operation over the lifespan of the application. The basic physico-chemical properties and long-term stability of these surface layers known as the solid electrolyte interphase (SEI) determine the performance and durability of the entire system [1,2]. However, our understanding of how and why some materials function, and, equally importantly, why some materials with many ideal characteristics actually fail, is mainly rudimentary and empirical. The requirements for long-term stability of Li-ion batteries are extremely stringent and necessitate control of the chemistry at a wide variety of temporal and structural length scales. However, the spatial resolution of standard optical spectroscopic and imaging techniques is limited by diffraction to light beam sizes on the order of the wavelength of the source i.e., ∼10 μm for IR and∼500 nm in the visible range. The advent of near-field optical methods during the past decades has led to the development of new advanced techniques for chemical analysis. This presentation provides an overview of novel imaging and spectroscopic optical methodologies, which exploit micro and nano-manipulation techniques and single crystal model electrodes to provide sufficient sample definition suitable for advanced multifunctional far- and near-field optical scanning probes and electrochemical characterization techniques. The SEI chemistry is known to be highly heterogeneous at the nanoscale, and although many compounds have been suggested as contributing, their distribution, relative concentration and functionality remain poorly understood [2]. To address this challenge we used the apertureless near-field scanning optical microscopy (aNSOM) (3-9), which combines the high resolution of AFM with the chemical specificity of IR spectroscopy, to resolve the SEI chemistry at high (∼20 nm) spatial resolution. Examples of detailed in situ molecular characterization of graphite Sn and Si electrodes surface and bulk processes at the nano-level scale exceeding the diffraction limit will be discussed [7-9]. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, under contract no. DE-AC02-05CH11231.

Manipulating interphases in batteries

Charge-transfer across electrolyte/ electrode interfaces dictates kinetics and reversibility of cell reactions within electrochemical devices. When the device operates beyond the thermodynamic stability limits of electrolytes, ‘interphases’ form at the interfaces [1,2]. In the pursuit of new battery chemistries that promise ambitious energy densities, electrolytes often take the blame for failing to support the cell reactions, while in most cases the real culprit is the interphase instead of the bulk electrolyte components themselves. Being able to manipulate the chemistry and morphology of such an important sub-component holds the key to the success of these new battery chemistries. Electrolytes and interphases are independent but intimately related entities, with the former serving as the chemical parent to the latter.The importance of interphase is best highlighted by lithiumion batteries (LIB), whose intercalationchemistry is made possible only with the formation of interphases during the cell activation. Although not fully recognized in pre-LIB era, interphases are prevalent in nearly all batteries operating above 3.0 V, with the venerable Li/thionyl chloride primary battery as an example [3]. It was the success of LIB that brought the science of interphases under the spotlight.The fundamental understanding achieved in the past two decades has allowed us to effectively manipulate the chemistry and properties of interphases, hence decoupling them further from bulk properties. The most extreme example is perhaps represented by the ‘Water-in-Salt’ electrolyte, where the electrochemical stability window of water was expanded to >3.0 V thanks to a designed interphase based on LiF, far beyond what Pourbaix limits (1.23 V) define for hydrogen and oxygen evolutions [4,5].This successful separation of interphases from their aqueous parental electrolyte opens an avenue to the ‘uncharted water’ of high-voltage aqueous electrochemistries [6], inspiring more aggressive manipulation of interphases for new battery chemistries. It is generally accepted now that an interphase must insulate electrons while conducting ions that are of significance to the cell reactions (such as Li+ for LIB). It was this electrolyte nature that earned it the popular name ‘SEI’ (solid electrolyte interphase) [7]. Due to significant departure of their potentials from the thermodynamic limits of electrolytes, anode surfaces in LIB induce most of the interphasial chemistry [2]. Chemical analyses on these interphase have identified the electrolyte solvents as the major chemical contributors to interphases, while the salt anion participation remains elusive. This remains true in part because of the ‘diluted’ electrolyte solutions (∼1.0 M) used in the state-of-the-art LIB, in which the Li+-solvation-sheath is sufficiently populated with solvent molecules. According to the formationmechanismproposed by Xu et al. [8–10], such a primary solvation-sheath serves as the chemical precursor of SEI via a transient ternary intercalation compound stage, so reduction of solvent molecules (mostly carbonate esters) dominates the surface chemistry (Fig. 1, left). This ‘solvent-signature’ on SEI will be reduced or even eliminated when the salt is used at unusually

Recent Achievements on High Voltage Olivine-Type Cathode Materials

The development of high power and high energy density cathode materials for rechargeable Li-ion batteries suitable with high operation voltage is a challenge. In this regard, the fundamental understanding of the thermodynamic stability and performance limits of high voltage cathode materials is essential, since one allows us to predict/determine the intrinsic voltage limit of the cathode material [1, 2] and tailor the components of the battery cell to provide a high Li+ diffusion rate in the bulk material and across the interface. The main reasons which limit to use high voltage LiMPO4 (LMPO, M=Co, Ni) cathode materials for electric vehicle applications are low electronic conductivity, which results from the olivine structure where the valence electrons of the transition metal are isolated from each other by the polyanion (PO3- 4), and an instability of the electronic structure of LMPO at the electrode-electrolyte interface caused by the decomposition of liquid electrolytes at high voltage potential. Low electronic conductivity of the Co- and Ni based olivine-type cathode materials makes it difficult to use electronic spectroscopy techniques which give unambiguous information on the density of the occupied and unoccupied states near the Fermi level (E F), on the oxidation and spin states of the transition metals, as well as on the chemical composition at the electrode-electrolyte interface [3-5]. In this presentation, we demonstrate our latest achievements on the study of the electronic structure of the layered- and olivine- type film cathode materials and its correlation with the electrochemical performance, Fig. 1. We demonstrate that the electronic structure of the bulk cathode material and the cathode-electrolyte interface is a key parameter which governs the thermodynamic stability of the whole battery cell. By plotting the experimentally obtained energy diagrams of the LiCoO2 (LCO) layered structure and LCPO-olivine structure cathode materials, we demonstrate the role of the inductive effect [7] which leads to the increase of the voltage window by the substitution of oxygen (O2-) by the polyanion. We discuss the role of morphology of the LiMPO4 film cathode materials on the Li-ion rate and demonstrate that high electronic conductivity can be achieved without using the carbon additives to the active material, Figs. 1(b). On the example of a LCPO film with different lithium content, x, we demonstrates the gradual shift of E F to the valence band maximum due to the increase of the work function of the cathode material in accordance with the rigid band model. By the analysis of the evolution of the core-levels and valence band structure as a function of lithium content in LCPO, we demonstrate that the charge compensation caused by Li+ removal takes place at the Co site resulting in the change of Co2+ ions (x=1.0) with the high spin (HS) state to the Co3+state in the accordance with DFT calculations [8]. The possible physicochemical mechanisms contributed to the degradation of the LMPO batteries are also discussed. [1] J. B. Goodenough, Y. Kim, Challenges for Rechargeable Li Batteries. Chem. Mat. (2010) 22, 587. [2] D. Ensling, G. Cherkashinin, S. Schmid, S. Bhuvaneswari, A. Thissen, W. Jaegermann, Nonrigid Band Behavior of the Electronic Structure of LiCoO2 Thin Film during Electrochemical Li Deintercalation. Chem. Mater. (2014) 26, 3948. [3] G. Cherkashinin, D. Ensling, W. Jaegermann, LiMO2 (M=Ni, Co) Thin Film Cathode Materials: a Correlation Between the Valence State of Transition Metals and the Electrochemical Properties. J. Mater. Chem. A (2014) 2, 3571. [5] D. Becker, G. Cherkashinin, R. Hausbrand, W. Jaegermann, Adsorption of Diethyl Carbonate on LiCoO2 Thin Films: Formation of the Electrochemical Interface. J. Phys. Chem. C (2014) 118,962. [6] G. Cherkashinin, M. Motzko, N. Schulz, T. Späth, W. Jaegermann, Electron Spectroscopy Study of Li[Ni,Co,Mn]O2/Electrolyte Interface: Electronic Structure, Interface Composition, and Device Implications. Chem. Mater. (2015) 27, 2875. [7] A. K. Padhi, K. S. Nanjundaswami, J. B. Goodenough, Phospho- Olivines as Positive- Electrode Materials for Rechargeable Lithium Batteries, J. Electrochem. Soc. (1997) 144, 1188. [8] F. Zhou, M. Cococcioni, C. A. Marianetti, D. Morgan, G. Ceder, First- Principles Prediction of Redox Potentials in Transition-Metal Compounds with LDA+U. Phys. Rev. B (2004) 70, 235121. Figure 1

Bulk Properties of the Oxygen Reduction Catalyst SrCo0.9Nb0.1O3−δ

The perovskite SrCo0.9Nb0.1O3−δ (SCN) has excellent electrochemical activity toward oxygen reduction, and it is also valuable as a possible model material for other state-of-the-art perovskite catalysts based on strontium and cobalt, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF). Here we report thermogravimetric, conductivity, and diffraction measurements from SCN. We find that the thermodynamic stability limits of SCN are slightly more favorable than those reported for BSCF, although both materials exhibit a slow oxidative partial decomposition under likely operating conditions. In SCN, this decomposition is thermodynamically preferred when the average formal oxidation state of cobalt is greater than ∼3.0+, but due to sluggish kinetics, metastable SCN with higher cobalt valence can be observed. The oxygen stoichiometry 3−δ varies from 2.45 to 2.70 under the conditions studied, 500–1000 °C and 10–4–1 bar O2, which encompass both stable and metastable behavior. The electronic conductivity is p-type and thermally...

A diffuse interface Lox/hydrogen transcritical flame model

We present a diffuse-interface all-pressure flame model that transitions smoothly between subcritical and supercritical conditions. The model involves a non-equilibrium liquid/gas diffuse interface of van der Waals/Korteweg type embedded into a non-ideal multicomponent reactive fluid. The multicomponent transport fluxes are evaluated in their thermodynamic form in order to avoid singularities at thermodynamic mechanical stability limits. The model also takes into account condensing liquid water in order to avoid thermodynamic chemical instabilities. The resulting equations are used to investigate the interface between cold dense and hot light oxygen as well as the structure of diffusion flames between cold dense oxygen and gaseous-like hydrogen at all pressures, either subcritical or supercritical.

Titania-carbon nanocomposite anodes for lithium ion batteries--effects of confined growth and phase synergism.

As lithium-ion batteries (LIB) see increasing use in areas beyond consumer electronics, such as the transportation sector, research has been directed at improving LIBs to better suit these applications. Of particular interest are materials and methods to increase Li(+) capacity at various charge/discharge rates, to improve retention of Li(+) capacity from cycle-to-cycle, and to enhance various safety aspects of electrode synthesis, cell construction, and end use. This work focuses on the synthesis and testing of three-dimensionally ordered macroporous (3DOM) TiO2/C LIB anode materials prepared using low toxicity precursors, including ammonium citratoperoxotitanate(IV) and sucrose, which provide high capacities for reversible Li(+) insertion/extraction. When the composites are pyrolyzed at 700 °C, the carbon phase restricts sintering of TiO2 crystallites and keeps the size of these crystallites below 5 nm. Slightly larger crystallites are produced at higher temperatures, alongside a titanium oxycarbide phase. The composites exhibit excellent capacities as LIB anodes at low to moderate charge/discharge rates (in the window from 1 to 3 V vs Li/Li(+)). Composites pyrolyzed at 700 °C retain over 200 mAh/g TiO2 of capacity after 100 cycles at a C/2 rate (C = 335 mA/g), and do not suffer from extensive cycle-to-cycle capacity fading. A substantial improvement of overall capacities, especially at high rates, is attained by cycling the composite anodes in a wider voltage window (0.05 to 3 V vs Li/Li(+)), which allows for Li(+) intercalation into carbon. At currents of 1500 mA/g of active material, over 200 mAh/g of capacity is retained. Other structural aspects of the composites are discussed, including how rutile TiO2 is found in these composites at sizes below the thermodynamic stability limit in the pure phase.

Estimation of the Thermodynamic Limit of Overheating for Bulk Water from Interfacial Properties

The limit of overheating or expanding is an important property of liquids, which is relevant for the design and safety assessment of processes involving pressurized liquids. In this work, the thermodynamic stability limit—the so-called spinodal—of water is calculated by molecular dynamics computer simulation, using the molecular potential model of Baranyai and Kiss. The spinodal pressure is obtained from the maximal tangential pressure within a liquid–vapor interface layer. The results are compared to predictions of various equations of state. Based on these comparisons, a set of equations of state is identified which gives reliable results in the metastable (overheated or expanded) liquid region of water down to $$-55$$ MPa.

Optical emission and nanoparticle generation in Al plasmas using ultrashort laser pulses temporally optimized by real-time spectroscopic feedback

With an interest in pulsed laser deposition and remote spectroscopy techniques, we explore here the potential of laser pulses temporally tailored on ultrafast time scales to control the expansion and the excitation degree of various ablation products including atomic species and nanoparticulates. Taking advantage of automated pulse-shaping techniques, an adaptive procedure based on spectroscopic feedback is applied to regulate the irradiance and enhance the optical emission of monocharged aluminum ions with respect to the neutral signal. This leads to optimized pulses usually consisting in a series of femtosecond peaks distributed on a longer picosecond sequence. The ablation features induced by the optimized pulse are compared with those determined by picosecond pulses generated by imposed second-order dispersion or by double pulse sequences with adjustable picosecond separation. This allows to analyze the influence of fast- and slow-varying envelope features on the material heating and the resulting plasma excitation degree. Using various optimal pulse forms including designed asymmetric shapes, we analyze the establishment of surface pre-excitation that enables conditions of enhanced radiation coupling. Thin films elaborated by unshaped femtosecond laser pulses and by optimized, stretched, or double pulse sequences are compared, indicating that the nanoparticles generation efficiency is strongly influenced by the temporal shaping of the laser irradiation. A thermodynamic scenario involving supercritical heating is proposed to explain enhanced ionization rates and lower particulates density for optimal pulses. Numerical one-dimensional hydrodynamic simulations for the excited matter support the interpretation of the experimental results in terms of relative efficiency of various relaxation paths for excited matter above or below the thermodynamic stability limits. The calculation results underline the role of the temperature and density gradients along the ablated plasma plume which lead to the spatial distinct locations of excited species. Moreover, the nanoparticles sizes are computed based on liquid layer ejection followed by a Rayleigh and Taylor instability decomposition, in good agreement with the experimental findings.

Thermodynamic stability limits of simple monoatomic materials

This computational study addresses the thermodynamical stability of superheated crystals. Molecular dynamics simulations are employed to derive the caloric curves of the solid and liquid phases of a material. Caloric curves are used to derive thermodynamic state functions, the parameters of the equilibrium melting phase transition, and the regions of thermodynamical stability of the liquid and solid phases. Molecular dynamics trajectories are also analyzed to gain insight on the mechanisms leading to the instability of the homogeneous superheated solid phase. This study shows that in simple and homogeneous solids the configurational entropy is not zero and that its excitations can occur without disrupting the crystallinity of the lattice. The superheating and supercooling limits of the solid and liquid phases are found to correspond to states of equal entropy and enthalpy.

Liquid–vapour spinodal of pure helium 4

Unlike gases, liquids can be overheated or stretched only up to a limit. The determination of the mean-field thermodynamic stability limit—the so-called spinodal—is a very difficult theoretical and a more-or-less impossible experimental task. Based on a recent semi-empirical method, the spinodal pressure of helium-4 at given temperature is determined, using liquid–vapour surface tension, interface thickness and vapour pressure data.

Estimation of the stability limit of metallic crystals under conditions of isothermal stretching

Based on the results of analysis of behavior of the thermodynamic properties of metals, a semiempirical method is suggested for calculating the limit of thermodynamic stability of crystalline state under conditions of isothermal stretching. The pressure and volume on the spinodal at temperatures of 0 and 300 K are estimated for a group of metals, and the stability limit of lead is calculated in the temperature range from 0 K to the melting point. The correlation between limiting pressures on the melting line and on the spinodal of crystal at T → 0 is revealed.

The modified Simon equation and some properties of substances on the melting line

Two versions of modified Simon equation, which contain a correction factor with additional parameters, are suggested for describing the melting lines of elementary substances. It is demonstrated that the generalizations found for the Simon equation (the universality of component Δ\(\tilde s\)1 of entropy of melting, the convergence of the extreme position of melting line at T → 0 with the limits of thermodynamic stability of coexisting crystal and liquid, and the correlation of the pole of melting line p* with internal pressure pi in the condensed phase in the vicinity of the triple point) retain stability to the variation of the form of dependence pmelt = f(T).