Triple Point Of Argon Research Articles

Density-dependent solvation dynamics in a simple Lennard-Jones fluid

The density dependence of time correlation functions for the solvation energy in a simple Lennard-Jones liquid has been investigated by molecular dynamics computer simulations. Considering argon dissolved in liquid argon, mechanical solvation dynamics has been studied treating interactions between excited solute states and the solvent by changes in the Lennard-Jones well-depth parameter, the Lennard-Jones size parameter, and a combination of the two. Densities have been varied from supercritical to triple point densities at a constant temperature of 151 K. In addition, a thermodynamic state close to the argon triple point has been considered. All the solvation energy time correlation functions have been broken down into their partial two- and three-body contributions giving an insight into the cancellation effect of solvation dynamics. It is found that the well-depth solvation process produces slowly decaying time correlation functions for the solvation dynamics at lower densities. In this case, the solvation dynamics becomes faster with increasing density due to long time cancellations between two-body time correlations with positive amplitudes and negative three-body contributions. In contrast, the size parameter solvation process is much faster. The corresponding solvation dynamics time correlations decay rapidly already at low liquid densities with two- and three-body contributions significantly stronger correlated than the total solvation energy time correlation function. Describing the excited solute by changes in the well-depth and the size parameter, the dynamical features resemble much of the solvation dynamics obtained from changes only in the size parameter.

GM型冷凍機を用いたアルゴンの三重点の実現

The triple point of argon is realized calorimetrically using a closed cycle refrigerator (a Gifford-McMahon type refrigerator). The usage of the closed cycle refrigerator with a closed fixed-point cell of argon is found to have a high potential for an easy-handling and the most accurate realizing method, as the temperature distribution inside the cryostat is stable enough for a long period, compared with the conventional cryostat. The standard uncertainty of this system in calibrating standard platinum resistance thermometers at the triple point of natural argon is estimated to be 0.059mK including isotopic uncertainties.

Simulation studies of shear viscosity time-correlation-functions

Computer simulations of the Green–Kubo time-correlation-functions (tcfs) for the shear viscosity are reported for argon (Lennard-Jonesium) at several state points. The principal function of interest is the contribution that involves the intermolecular forces. This particular tcf is the sum of a two-body, a three-body, and a four-body term. The time-dependencies and magnitudes of these individual tcfs have been evaluated over a wide range of density at 151 K and for a single state point near the argon triple point. It is shown that all of these component tcfs are significant; that their time-dependencies at moderately short times are quite different from one another; and that the individual functions display pronounced long-time-tails at high density. However, a tail does not appear in the total tcf because of cancellation in the summation. That is, the long-time-tails appear in the partial tcfs in the ratio 2-body:3-body:4-body=1:−2:1.

Inconsistency in the ITS-90 and the triple point of mercury

The triple point of mercury appears to be assigned an incorrect temperature on the ITS-90. This conclusion is based on fixed-point correlations and the disagreement of the a-coefficients for the deviation functions associated with the calibration of platinum resistance thermometers above and below 273,16 K. The disagreement of the a-coefficients is particularly important as it implies a slope discontinuity in the ITS-90 at 273,16 K. The proposed remedy involves modification of the ITS-90 platinum resistance thermometer low-temperature reference function. With the revised reference function, it is also necessary to lower the argon triple point temperature by 0,7 mK to 83,8051 K to preserve acceptable sub-range consistency. The mercury point retains its ITS-90 assigned value of 234,3156 K. With this modification, the a-coefficient obtained from any sub-range calibration above 273,16 K may be used in the low-temperature deviation functions in place of a measurement at the mercury triple point.

An International Intercomparison of Argon Triple Point Calibration Facilities, Accommodating Long-stem Thermometers

Argon triple point calibration facilities have been compared among eight laboratories with one transfer system, employing local long-stem standard platinum resistance thermometers. The apparatus intercompared, included a sealed cell and its associated cryostat. As is evidenced by the results of long-term investigations, previously performed at the INM, cells of the type employed may show a triple-point temperature, which is stable within the reproducibility of the measurements (≃0,1 mK) over a period of about 10 years. At each laboratory the mean difference between the Argon triple-point temperature of the transfer cell (t) and the local cell (i) has been determined, using a standard resistance thermometer previously calibrated at the fixed points, according to the IPTS-68; associated repeatabilities are typically of the order of 0,1 mK. The reproducibility attained by measuring the mean difference in different laboratories, using cells of the same type and origin (INM), amounts to 0,4 mK.

Performance of the Double Yukawa Potential in the Variational Theory of Simple Liquids

Abstract A variational theory for classical fluids based on a hard-sphere reference system is used to examine the differences between performing the calculations with a Lennard-Jones pair potential or a double Yukawa one, in the range of densities up to four times that of the Argon triple point. Likewise we verify that the results achieved using the Verlet-Weis g vw(r) are quite similar to those obtained with the Percus-Yevick g PY(r).

The temperatures of the triple points of methane and argon on IPTS-68

The triple points of argon and methane were realised in a triple-point cell, adiabatically built in a flow cryostat of new design. The dependence of the triple-point temperature on the amount of the liquid phase was determined as a criterion of the purity of the gas used. For very pure argon (mole fraction of impurities less than 10−6), no measurable change of temperature could be detected in the range 25% to 90% of liquid phase. For methane (mole fraction of impurities 5×10−6), the temperature was constant between 15% and 90% of liquid phase. We found T68=83.7972 K for the temperature of the argon triple point and T68=90.6846 K for that of the methane triple point, on the International Practical Temperature Scale of 1968 (realisation of PTB), based on the oxygen condensation point. If the triple point of argon is used as a reference, with its assigned temperature T68=90.798 K, we find T68=90.6854 K.

Cellule à Point Triple de l'Argon: Instrument de Transfert de Pression

We show that the argon triple-point (68 890.0 Pa) is suitable as a fixed point of pressure. The equipment required for its realization is easily transportable. Thus, we have a convenient device for comparing fixed mercury-manobarometers in different laboratories.

The Use of Triple Point of Gases in Sealed Cells as Pressure Transfer Standards: Oxygen (146.25 Pa), Methane (11,696 Pa) and Argon (68,890 Pa)

The use of the triple point of gases as a method for obtaining highly reproducible pressure fixed points is described. In order to obtain pressure transfer standards, the method is implemented with transportable, self-contained sealed cells, containing the sample of gas, which can be easily shipped around for intercomparisons in the pressure range 0.1 to 100 kPa. Experimental results on these devices are reported for three gases, including also their absolute pressure value. Oxygen: p = 146.25 ± 0.3 Pa, T = 54.3610 K, reproducibility of the transfer standard: ±0.1 Pa; methane: p = 11 696 ± 2 Pa, T = 90.6856 K, reproducibility: ±0.7 Pa; argon: p = 68 890 ± 1.5 Pa, T = 83.7980 K, reproducibility: ±1 Pa. The temperature of the normal boiling point of argon was also measured and found to be 87.2952 ± 0.0005 K. All temperatures are given in IPTS-68, realized with the triple point of argon.

Shear viscosity of simple liquids measured at low shear rates near the triple point

Shear viscosity measurements performed at low shear rates near the triple point of argon, krypton and xenon give results which agree with those obtained at higher shear rates. The non-newtonian behavior found in non-equilibrium molecular dynamics models seems to have no counterpart in the real fluid.

A high-density fluid-perturbation theory based on an inverse 12th-power hard-sphere reference system

A variational theory is developed that is accurate at normal liquid densities and densities up to 4 times that of the argon triple point. This theory uses the inverse 12th-power potential as a reference system. The properties of this reference system are expressed in terms of hard-sphere packing fractions by using a modified form of hard-space variational theory. As a result of this ’’bootstrapping,’’ a variational procedure may be followed that employs the inverse 12th-power system as a reference but uses the hard-sphere packing fraction as the scaling parameter with which to minimize the Helmholtz free energy.

熱分析によるアルゴン三重点の精密な決定

熱分析によるアルゴン三重点の精密な決定

Temperature of the Triple Point of Methane

The temperature of the triple point of methane was determined as 90.6861 K ± 0.2 mK using the triple point of argon (83.798 K) as the reference. The reproducibility of this fixed point is at least as good as that of argon.

The Triple Points of Krypton, Argon, Nitrogen and Neon

The triple points of krypton, argon, nitrogen and neon have been realized with reproducibilities of ±0.2 mK, ±0.1 mK, ±0.1 mK, and ±0.2 mK respectively. The results suggest that the triple points of argon and nitrogen make excellent reference temperatures while those of krypton and neon appear to be less suitable, possibly due to problems associated with isotope composition.

Three-phase equation of state by the lattice gas model

A rigid lattice gas model of solid-liquid, liquid-gas, and solid-gas transformations is proposed. The total free energy is separated into a lattice part and a vibrational correction part. It is reported that the vibrational part plays an important role in obtaining the gas, liquid and solid phases and the correct phase transitions. A method of combining the two lowest order approximations (single site, two-site) in the cluster variation method is suggested in describing the lattice free energy. The critical point and the triple point of argon are reproduced fairly well. Results are compared with the expandable lattice theory demonstrating the appropriateness of the present model in the vicinity of the triple point.

Argon triple point realization cryostat for platinum resistance long stem thermometers

A high-precision realization cryostat was developed for long stem type platinum resistance thermometers at the argon triple point. Six liters of argon gas (at room temperature) are sufficient for calibration and if ultrahigh purity grade is used one may expect the results to yield better than ±1 mK accuracy.

A molecular dynamics study of the intensity and band shape of depolarized light scattered from rare-gas crystals

The induced dipole-induced dipole autocorrelation functions for systems of atoms interacting with the Lennard-Jones, the purely repulsive Lennard-Jones, and the hard sphere potentials in the solid phase are calculated by molecular dynamics. The calculated band shape for the Lennard-Jones system agrees with the experimental measurements of rare gas solids at their triple points as well as at lower temperatures where more phonon structure is evident. The band shape calculated for the hard sphere system shows no discrete phonon structure even at very high density and disagrees at the triple point of argon. A system of particles interacting with the purely repulsive Lennard-Jones potential shows an intermediate level of phonon contributions. The hard sphere and the Lennard-Jones potentials, however, each lead to a calculated intensity which is an order of magnitude too large, but give about the correct ratio of the liquid to the solid intensity at the triple point. This shows that the polarizability distortion is of similar magnitude in both the phases of argon.

Theory of melting

An expandable-lattice theory of liquid-solid transitions is proposed. The configurational free energy is divided into an intracellular part and a lattice part. It is shown that the correlation between the intracellular configurations of molecules plays an important role. By taking into account this correlation approximately, the hard-sphere transition and the triple point of argon are reproduced fairly well.

The Percus–Yevick approximation near the triple point of argon

The radial distribution function and thermodynamic properties of argon are calculated using the (12, 6) potential and the Percus–Yevick approximation. The results are compared with experimental values for argon, the machine calculations of Verlet and of McDonald and Singer, and with the perturbation theory of Barker and Henderson.