Supercritical Argon Research Articles

Thermodynamic crossovers in supercritical fluids

Can liquid-like and gas-like states be distinguished beyond the critical point, where the liquid-gas phase transition no longer exists and conventionally only a single supercritical fluid phase is defined? Recent experiments and simulations report strong evidence of dynamical crossovers above the critical temperature and pressure. Despite using different criteria, many existing theoretical explanations consider a single crossover line separating liquid-like and gas-like states in the supercritical fluid phase. We argue that such a single-line scenario is inconsistent with the supercritical behavior of the Ising model, which has two crossover lines due to its symmetry, violating the universality principle of critical phenomena. To reconcile the inconsistency, we define two thermodynamic crossover lines in supercritical fluids as boundaries of liquid-like, indistinguishable, and gas-like states. Near the critical point, the two crossover lines follow critical scalings with exponents of the Ising universality class, supported by calculations of theoretical models and analyses of experimental data from the standard database. The upper line agrees with crossovers independently estimated from the inelastic X-ray scattering data of supercritical argon, and from the small-angle neutron scattering data of supercritical carbon dioxide. The lower line is verified by the equation of states for the compressibility factor. This work provides a fundamental framework for understanding supercritical physics in general phase transitions.

The Minima of Viscosities.

The Trachenko-Brazhkin equation of the minimal possible viscosity is analysed, emphasising its validity by the account of multibody interactions between flowing species through some effective masses replacing their true (bare) masses. Pressure affects the effective masses, decreasing them and shifting the minimal viscosity and the temperature at which it is attained to higher values. The analysis shows that effective masses in the Trachenko-Brazhkin equation are typically lighter compared bare masses; e.g., for tin (Sn) the effective mass is m = 0.21mSn, whereas for supercritical argon (Ar), it changes from m = 0.165mAr to m = 0.129mAr at the pressures of 20 and 100 MPa, respectively.

Molecular hydrodynamic theory of the velocity autocorrelation function.

The velocity autocorrelation function (VACF) encapsulates extensive information about a fluid's molecular-structural and hydrodynamic properties. We address the following fundamental question: How well can a purely hydrodynamic description recover the molecular features of a fluid as exhibited by the VACF? To this end, we formulate a bona fide hydrodynamic theory of the tagged-particle VACF for simple fluids. Our approach is distinguished from previous efforts in two key ways: collective hydrodynamic modes and tagged-particle self-motion are modeled by linear hydrodynamic equations; the fluid's spatial velocity power spectrum is identified as a necessary initial condition for the momentum current correlation. This formulation leads to a natural physical interpretation of the VACF as a superposition of products of quasinormal hydrodynamic modes weighted commensurately with the spatial velocity power spectrum, the latter of which appears to physically bridge continuum hydrodynamical behavior and discrete-particle kinetics. The methodology yields VACF calculations quantitatively on par with existing approaches for liquid noble gases and alkali metals. Furthermore, we obtain a new, hydrodynamic form of the self-intermediate scattering function whose description has been extended to low densities where the Schmidt number is of order unity; various calculations are performed for gaseous and supercritical argon to support the general validity of the theory. Excellent quantitative agreement is obtained with recent MD calculations for a dense supercritical Lennard-Jones fluid.

Investigation of flow and heat transfer characteristics of supercritical argon in conical Hampson heat exchangers for a J-T cryocooler

The Hampson-type heat exchanger is critical for the rapid cooling of a conical J-T cryocooler, and its internal flow and heat transfer characteristics directly affect the performance of the conical cryocooler. The flow and heat transfer characteristics of supercritical argon are studied using a numerical model of a conical helical tube in this work. The simulated results show that supercritical argon flow in the conical helical tube is mainly influenced by the combined effect of variable thermophysical properties and the secondary flow produced by centrifugal force. The buoyancy has little impact on heat transfer. The heat transfer coefficient reaches its maximum at the pseudo-critical region. A more significant heat transfer coefficient and faster heat exchanger cooling rate can be achieved by increasing the mass flow rate and maintaining the fluid in the tube close to the pseudo-critical region. At a cone angle of 50°, the maximum total heat exchange quantity is observed. With a greater temperature drop per unit length, a stronger heat transfer effect can be achieved close to the outlet of the tube. Based on the numerical analyses, a new correlation for heat transfer is proposed.

Supercritical argon adsorption in connected pores composed of cavity and neck: effect of cavity size and temperature

Supercritical argon adsorption in connected pores composed of cavity and neck: effect of cavity size and temperature

Phase distribution in nanochannels of supercritical fluid with different fluid–wall interactions

Supercritical fluids (SFs) are classically regarded as single-phase fluids without bubbles or interfaces, but a recent study shows nanobubbles in SFs under unconfined conditions. The objective of this paper is to explore the phase distribution under confined conditions. Molecular dynamics simulations are performed for supercritical argon. Two walls containing the SF have equal fluid–wall interactions with equal and unequal wall temperatures. An external force is applied on the top wall to control the pressure at 1.5Pc, in which Pc is the critical pressure. Periodic boundary conditions are applied on the four side surfaces of the simulation box. The study indicates that the bulk fluid density is not only dependent on pressure and temperature, but also on fluid–wall interactions, this result deviates from the classical theory, where density depends on only pressure and temperature. For strong fluid–wall interactions, three- or five-layer structures are found, including liquid-like (LL) layers on the walls and two-phase-like (TPL) and gas-like (GL) layers (depending on bulk density) in the channel core. For weak fluid–wall interactions, the phase distribution becomes GL on the wall, and TPL and LL (depending on bulk density) in the channel core, which is inverse to those of strong fluid–wall interactions. Correspondingly, the phase distributions for strong and weak fluid–wall interactions can be analogous to annular or Leidenfrost patterns at subcritical pressures, respectively. The density profile is symmetric against the channel centerline at equal wall temperatures, but symmetry-breaking may exist when applying different wall temperatures. This work provides a phase-distribution link between subcritical and supercritical pressures, which is useful for the design and analysis of SF systems.

The Phonon Theory of Liquids and Biological Fluids: Developments and Applications.

Among the three basic states of matter (solid, liquid, and gas), the liquid state has always eluded general theoretical approaches for describing liquid energy and heat capacity. In this Viewpoint, we derive the phonon theory of liquids and biological fluids stemming from Frenkel's microscopic picture of the liquid state. Specifically, the theory predicts the existence of phonon gaps in vibrational spectra of liquids and a thermodynamic boundary in the supercritical state. Direct experimental evidence reaffirming these theoretical predictions was achieved through a combination of techniques using static compression X-ray diffraction and inelastic X-ray scattering on deeply supercritical argon in a diamond anvil cell. Furthermore, these findings inspired and then led to the discovery of phonon gaps in liquid crystals (mesogens), block copolymers, and biological membranes. Importantly, phonon gaps define viscoelastic crossovers in cellular membranes responsible for lipid self-diffusion, lateral molecular-level stress propagation, and passive transmembrane transport of small molecules and solutes. Finally, molecular interactions mediated by external stimuli result in synaptic activity controlling biological membranes' plasticity resulting in learning and memory. Therefore, we also discuss learning and memory effects─equally important for neuroscience as well as for the development of neuromorphic devices─facilitated in biological membranes by external stimuli.

Comparison of the corrosion of materials in supercritical carbon dioxide, air, and argon environments

The corrosion resistance of structural alloys was investigated in conditions that simulate supercritical carbon dioxide sCO2 power fluid environments at 20 MPa and 550°C. Complementary air and Ar experiments were conducted at 1 atm. The alloys investigated in this study were as follows: Gr. 91, ferritic martensitic steel; 316L, austinetic stainless steel; HR 120, Ni-Fe based superalloy; and IN625, Ni based superalloy. The results for the air and sCO2 exposed samples indicated parabolic mass-gain with respect to time and sCO2 was more aggressive. The austinetic alloys exposed to sCO2 developed recrystallized zones under the oxide. The same alloys exposed to air showed signs of recrystallization that was detectable with TEM.

Parametric Characterization of Copper Metal Coatings Produced by Supercritical Argon Electroplating

An electroplating process under supercritical argon applied to a copper sulfate bath with no additives is presented in this work. Adjusting electroplating parameters such as temperature, pressure and current density changed the deposition behavior, thus changing the coating properties. The finest grains (~ 20.91 nm) were obtained at 2500 psi and 35°C, which were relatively higher pressure and lower temperature than those found for SC-CO2 processes, respectively, 2000 psi and 50°C. Mechanical, electrical and corrosion analyses revealed the coating properties were enhanced compared with those produced by the conventional process. Finally, the results point to optimal fabrication parameters of 35°C, 5 A/dm2 and 2500 psi, achieving the relatively best coating properties and highest current efficiency (~ 93%).

Fabrication of Cu coatings by ultrasound-assisted supercritical argon electroplating

This work reports a novel ultrasound-assisted supercritical argon electroplating process (US-SC-Ar) with no additives to produce copper metal coatings. Introduction of ultrasound accelerates metal ion mobility, yielding densely-packed structures with finer grains and better aesthetic finish. Temperature of 35 °C, pressure of 2500 psi, current density of 5 A/dm2, and acoustic power density of 0.094 W/cm3, produced coatings with enhanced mechanical and corrosion behavior compared to those produced by conventional or silent SC-Ar processes.

Molecular Dynamics Simulation of a Jet in a Binary System at Supercritical Environment.

With the development of large-thrust liquid rocket engines, the behavior of liquid in supercritical conditions arouses increasing public interest. Due to the high pressure and temperature of the combustion chamber, fuel reaches its critical point much more easily, and enters supercritical conditions. Due to the drastic changes in the physical properties of the fluid near the critical point, it is usually difficult to simulate the fluid motion using traditional computational fluid dynamic methods; but molecular dynamics (MD) can simulate fluid motion at the molecular level. In view of the engineering application, the physical properties of a binary system consisting of argon and nitrogen, and the stability of subcritical jets sprayed into supercritical environment, has been studied here using the MD method. First, the molecular dynamic simulation of the equation of state (EOS) of the mixture was put forward. Four conditions, with different mixing ratios of nitrogen, were designed. The results showed that the mixing ratio of nitrogen noticeably affected the results; these results were compared with the Soave-Redich-Kwong (SRK) EOS. Second, a simulation was conducted of subcritical nitrogen jet sprayed into a supercritical argon environment. After analyzing the results, the jet density and temperature distributions were obtained and the disturbance growth rate of the shear layer was analyzed.

Feasibility study on sc-Ar electroplating for metal coating fabrication

ABSTRACTFor this preliminary work, Cu metal coatings were produced by electroplating under supercritical argon (sc-Ar) on copper sulphate bath with no additives. Coatings were also produced by supercritical carbon dioxide (sc-CO2) and traditional electroplating for comparison. Mechanical and electrical properties were analysed for all coatings. It was found that sc-CO2 electroplating had the highest grade of grain refinement; however, this meant relatively higher degree of internal stresses, while the sc-Ar electroplating displayed more moderate grain refinement, hence relatively lower internal stress. Advantages of the proposed the sc-Ar method are faster fabrication time compared to the traditional method and desirably lower internal stresses compared to the sc-CO2 method.

Effects of Corrosion in Supercritical CO2 on the Microstructural Evolution in 800H Alloy

This study investigates corrosion of Fe–Ni-based alloy 800H that were exposed to supercritical CO2 (sCO2), ambient air and argon gas at pressures up to 20 MPa, at 650 and 750 °C for up to 1000 h. This alloy and other comparable metal alloys are expected to be used in sCO2 heat exchanger cycles as proposed in the DOE Advanced Ultra-Supercritical program. Alloy 800H is considered for this application, because it meets the high-temperature strength and creep rupture requirements and is a lower cost alternative to other Ni-based alloys. The oxidation performance and microstructural changes due to exposure in sCO2 have been evaluated and compared with exposures in air and Ar. The 800H alloy showed similar oxide scale thicknesses in sCO2 as in air. A recrystallized zone was observed beneath the oxide formed in air and sCO2. No such zone was observed after exposure to Ar, suggesting this recrystallization was associated with the oxidation process and not simply an effect of surface finishing. A wider recrystallized zone was observed underneath the oxide formed in sCO2 than in air. The effect of air and sCO2 on internal oxidation and carburization was investigated as well, showing that air led to more internal oxidation but less internal carburization than sCO2. It was concluded that the carbon species provided by the sCO2 atmosphere in conjunction with the increased grain boundary density in the recrystallized zone allowed for more ingress of carbon into the base metal, which resulted in a higher densities of carbides beneath the oxide scale.

Rates of Molecular Vibrational Energy Transfer in Organic Solutions.

For condensed-phase reactions, commonly used kinetic models assume that energy exchange from and to solvent molecules is much faster than any reactive steps. However, it is becoming increasingly evident that this does not always hold true. In this work, we use molecular dynamics simulations to explore the time scale for energy transfer between solvent and solute in some typical organic solvents. As a reference, energy transfer between solvent molecules is also considered. The time scale is found to depend most strongly on the identity of the solvent. Energy transfer occurs fastest, with a time scale of roughly 10 ps, for ethanol, DMSO or THF, while it is noticeably slower in dichloromethane and especially supercritical argon, where a time scale well in excess of a hundred picoseconds is found. This suggests that the experimental search for nonthermal effects on selectivity and reactivity in organic chemistry should pay special attention to the choice of solvent, as the effects may occur more frequently in some solvents than in others.

Statistical correlations for thermophysical properties of Supercritical Argon (SCAR) used in cooling of futuristic High Temperature Superconducting (HTS) cables

High Temperature Superconducting (HTS) cables are emerging as an alternative to conventional cables in efficient power transmission. However, these HTS cables require cooling below the critical temperature of superconductors used to transmit larger currents. With the invention of high temperature superconductors whose critical temperatures are up to 134K (Hg based), it is a great challenge to identify a suitable coolant which can carry away the heating load on the superconductors. In order to accomplish such challenge, an attempt has been made in the present work to propose supercritical Argon (SCAR) as the alternative to cool the HTS cables. Further, a statistical correlation has been developed for the thermophysical properties such as density, viscosity, specific heat and thermal conductivity of SCAR. In addition, the accuracy of developed correlations is established with the help of few statistical parameters and validated with standard database available in the literature. These temperature dependent accurate correlations are useful in predicting the pressure drop and heat transfer behaviour in HTS cables using numerical or computational techniques. In recent times, with the sophistication of computer technology, solving of various complex transport equations along with the turbulence models became popular and hence the developed correlations would benefit the technological community. It is observed that, a decrease in pressure, density and viscosity are found to be decreasing whereas the thermal conductivity and specific heat increase significantly. It can be concluded that higher heat transfer rate and lower pumping power can be achieved with SCAR as coolant in the HTS cables.

Interfacial tension and gas solubility of molten polymer polyethylene glycol in contact with supercritical carbon dioxide and argon

Interfacial tension plays a key role in understanding many polymer engineering processes and applications at high pressure. Supercritical fluids are promising media for reducing interfacial tension and increasing the solubility of gas into polymers melts.An experimental technique modified for studies of the interfacial interactions of liquids in equilibrium with gas in a glass-windowed equilibrium cell based on the means of the capillary rise method was applied for the binary systems of CO2+PEG (polyethylene glycol) and argon+PEG. The molecular masses of PEGs differed from Mw=400gmol−1 up to Mw=10000gmol−1 up to a pressure of 30.0MPa at a constant temperature of 343K. This technique was validated by conducting measurements of the interfacial tension between CO2 and pure water at a temperature of 318K. The accuracy of the obtained results compared to the ones found in the literature was better than 4%. Additionally, the effect of temperature was also investigated by measuring the solubilities and the interfacial tensions in the systems CO2+PEG 1500 and argon+PEG 1500 within the range of temperatures from 328K up to 363K.

Burst design and signal processing for the speed of sound measurement of fluids with the pulse-echo technique.

The pulse-echo technique determines the propagation time of acoustic wave bursts in a fluid over a known propagation distance. It is limited by the signal quality of the received echoes of the acoustic wave bursts, which degrades with decreasing density of the fluid due to acoustic impedance and attenuation effects. Signal sampling is significantly improved in this work by burst design and signal processing such that a wider range of thermodynamic states can be investigated. Applying a Fourier transformation based digital filter on acoustic wave signals increases their signal-to-noise ratio and enhances their time and amplitude resolutions, improving the overall measurement accuracy. In addition, burst design leads to technical advantages for determining the propagation time due to the associated conditioning of the echo. It is shown that the according operation procedure enlarges the measuring range of the pulse-echo technique for supercritical argon and nitrogen at 300 K down to 5 MPa, where it was limited to around 20 MPa before.

Dynamics and drift motion of O2− in supercritical argon

A method is presented for the study of the mean motion and the dynamics of ions under the action of an electrostatic field in supercritical fluids through a nonequilibrium molecular dynamics simulation technique. In the case of O2− in Ar, the ion–neutral interaction potential is determined through the reproduction of the experimental mobility data. This enabled the prediction of the radial distribution around the ion and the dynamics of the ion motion. In addition, the diffusion coefficient components, parallel and perpendicular to the electric field, have been calculated and found to be almost equal to one another and to depend weakly to the Ar density. The appearance of a shell around an ion is confirmed and found to be strengthened with density increase. The method has been applied at moderate Ar densities and low field strengths but can be extended to denser systems and stronger field strengths at the expense of high amounts of computer time.

Collection efficiency of photoelectrons injected into near- and supercritical argon gas

Injection of photoelectrons into gaseous or liquid dielectrics is a widely used technique to produce cold plasmas in weakly ionized systems for investigating the transport properties of electrons. We report measurements of the collection efficiency of photoelectrons injected into dense argon gas for T = 152.7 K, close to the critical temperature T(c) ≈ 150.9 K, and for T = 200.0 K. The high-field data agree with the Young-Bradbury model and with previous measurements below T(c) and at an intermediate temperature above T(c). The effective, density-dependent electron-atom momentum transfer scattering cross section can be deduced. However, the weak-field data near T(c) show large deviations from the theoretical model. We show that the electron behavior at weak field is influenced by electrostriction effects that are only important near the critical point.

Observations of a thermodynamic liquid–gas critical coexistence line and supercritical fluid phase bounds from percolation transition loci

We extend previous investigations into the thermodynamics of liquid state boundaries by focusing on the origins of liquid–gas criticality. The singular point hypothesis of van der Waals is re-examined in the light of recent knowledge of the hard-sphere percolation transitions and further analysis of simulation results for the supercritical properties of the square-well fluids. We find a thermodynamic description of gas–liquid criticality that is quite different from both van der Waals hypothesis and modern mean-field theory. At the critical temperature (Tc) and critical pressure (pc), in the density surface ρ(p,T), there is no critical point. Using tabulations of experimental ρ(p,T) data, for supercritical argon, and also water, as examples, at Tc a liquid phase coexists with a vapor phase determined by percolation transition densities. In the ρ(p,T) surface, there is a line of critical coexistence states of constant chemical potential at the intersection of two percolation loci in the p–T plane. For temperatures above this line, there exists a supercritical mesophase bounded by percolation transition loci. Below the line of critical states there is the familiar subcritical liquid–vapor two-phase coexistence region. Unlike the hypothetical van der Waals critical point, all thermodynamic state points on the line of critical states are consistent with Gibbs phase rule.