Adsorption of3He and4He on copper and on argon-coated copper below 20 K

Measurements have been made of adsorption isotherms of ³ He and of ⁴ He on copper and on a monolayer of argon deposited on copper in the temperature range 6.18–18.55 K and in the pressure range 0.25 to 75 Torr. From these many isotherms, calculations have been made of the isosteric heat of adsorptionQ _st/R. In the limit of zero coverage on the argon monolayerQ _st/R=76±2 K for ³ He and 76±2 K for ⁴ He. For adsorption on the bare copper,Q _st/R is difficult to extrapolate to zero coverage, but it probably lies (for both ³ He and ⁴ He) between 135 and 165 K. At theoretical monolayer helium coverage,Q _st/R=44±2 K for ³ He on the argon monolayer and 47±2 K for ⁴ He. At theoretical monolayer helium coverage on the bare copper,Q _st/R=61±4 K for ³ He and 77±5 K for ⁴ He. The results are compared with theoretical evaluations for helium adsorbed on an argon monolayer and with some previous experimental data, and the agreement is found to be fair. All the data are summarized in tables. Finally, a review is given of evaluations, including those from this work, of the monolayer capacity of ³ He and ⁴ He on the substrates studied.Work supported by a contract with the Department of Defense (Themis Program) and with the Office of Naval Research and by a Grant from the National Science Foundation.     

Helium adsorption on exfoliated graphite

High precision adsorption isotherms of³He and⁴He on bare Grafoil and on Grafoil coated with a monolayer of argon have been measured in the temperature range 4–20 K, using a high resolution, pressure-sensitive capacitance gauge located at 4.2 K close to the sample chamber. This gauge obviated thermomolecular pressure effects and corrections. The isotherms yielded the following resultant data: The monolayer coverage V_m for⁴He on bare Grafoil was 0.42 cm³ (STP)/m² and for³He on bare Grafoil was 0.395 cm³ (STP)/m²: the isosteric heat of adsorptionQ _st/R of the second layer of³He on bare Grafoil was 23.5 K andQ _st/R for³He on argon-coated Grafoil for the first layers was 47.5 K. Also, the data for³He and⁴He on bare Grafoil at and just aboveV _m have been used by Novaco (see subsequent paper) for determination of the virial coefficients of the gas phase occuring in the early formation of the second layer.     

Adsorption isotherms and heats of adsorption of neon and hydrogen on zeolite and charcoal between 20 and 90 K

Measurements have been made of the adsorption isotherms of neon and hydrogen on Linde synthetic zeolite type 13X and on Fisher activated coconut charcoal in the temperature ranges 20–37 and 77–90 K. In the lower temperature range, below the critical points of each adsorbate, it was found that the adsorption isotherms, when plotted giving the volume V, of gas adsorbed in cm³ (STP) per gram of adsorbent as a function of the parameter (p/p ₀)^T/100, yielded a unique isotherm curve independent of temperature for each adsorbate-adsorbent system (p ₀ is the saturated vapor pressure at each temperature of measurement). This result is what would be expected from potential theory, as emphasized by Chesteret al. in 1974. It was found, moreover, for neon on both charcoal and zeolite 13X that the unique temperature-independent curve for each system was the same for adsorption data taken below the triple point T_tr of bulk neon as for data taken above T_tr. From the data, the isosteric heats of adsorption Q_st were calculated at various temperatures and coverages and these showed a marked dependence on coverage, Q_st decreasing rapidly with increasing coverage.     

Anomalous 4He Adsorption to in situ Baked C60

We present data for ⁴ He adsorption to thermally evaporated C ₆₀ films for 1.30 K < T < 2.17 K. This work minimizes possible effects due to the presence of adsorbed species other than ⁴ He. We observe an anomalous mass adsorption to the C ₆₀ films close to ⁴ He saturation which amounts to 3 times the expected mass loading due to the saturated ⁴ He film on the C ₆₀ surface at T = 1.30K.     

Low-temperature adsorption isotherms of 3He and para-H2 on grafoil and their normalization

Measurements have been made of the adsorption isotherms of ³He and of para-hydrogen (p-H₂), on bare Grafoil, the former in the temperature range 0.88–1.23 K, and the latter in the temperature range 10.0–20.0 K. For p-H₂ the quantity of material adsorbed plotted against (p/p ₀) ^T yields a single temperature-independent curve, as would follow from the potential theory for multilayer adsorption, and as emphasized by Chester et al. in 1974. The same behavior was found for ³He on Grafoil, except that deviations from a single temperature-independent curve occurred at the higher temperatures. For p-H₂ on Grafoil it is to be noted that the isotherms taken below the triple point of free, bulk p-H₂ fit the same single temperature-independent curve as do the isotherms measured at temperatures between the triple point and the boiling point. From the data, the isosteric heats of adsorption were calculated over a wide range of coverage and this provided further experimental evidence for the similarity between the uppermost layers of thick adsorbed films and the free liquid of the same substance.Supported by the National Science Foundation and the Brazilian National Council of Scientific and Technological Development (CNPq).     

Thin Film Adsorption of 4He To C60

Low temperature adsorption isotherms of ⁴ He on C ₆₀ films are presented. Data is obtained by monitoring the frequency of C ₆₀ -coated quartz crystal microbalances versus pressure as ⁴ He gas is added to an experimental cell at constant temperature. Results which indicate excess low coverage ⁴ He adsorption to C ₆₀ are presented for T=1.5 K and T=1.68 K.     

Adsorption of 4He on neon-coated exfoliated graphite

Adsorption isotherms of ⁴He on Grafoil coated with a monolayer of neon were made for the temperature range of 2–10 K and in the pressure range of 0.10–15.00 Torr, using a standard volumetric method. From these isotherms the isosteric heats of adsorption were calculated as a function of coverage. The binding energy of ⁴He on Ne was obtained from the isosteric heat for V 0, yielding a value of 39.4 K. The differential molar entropy and the internal molar energy were calculated as a function of coverage. A comparison of monolayer coverage obtained by analyzing different procedures is presented. Whenever possible the results are compared with theoretical evaluations and previous experimental data.Work supported by FINEP, CNPq, and CEPG. Part of the Cooperative Program UFRJ/Stevens Institute of Technology, sponsored by CNPq/NSF International Program.     

Sub-boundaries in3He and4He single crystal and their migration below 1K observed by X-ray topography

The observation of alterations in crystal structure at ultra-low temperatures by X-ray diffraction, made possible by the realization of synchrotron radiation use after a quarter of this century, plays an important role in obtaining an understanding of the base structure of solids. For that, two types of³He-⁴He dilution refrigerators with a modified version of the top-loading facilities were installed at the BL-3C₂ and 6C₁ stations of the Photon Factory. In BL-3C₂, the behaviors of lattice defects in solid helium have been studied by X-ray topography. In this note, the migrations of sub-boundaries in³He and⁴He single-crystals are reported as being the result of an annealing effect. After annealing hcp⁴He single crystals for 80min at 0.5K, no change in crystallographic orientations could be easily observed from white SR X-ray topographs. In the same type topographs of bcc³He single crystals after annealing for several hours at 03K, migration of sub-boundaries were conspicuously discerned.     

Multilayer adsorption of3He and4He on zeolite from 4 K to 80 K

The adsorption isotherms of⁴He, N₂, and argon have been measured on synthetic zeolite (Linde Molecular Sieve 13X) at 78 K, and of³He and⁴He, also on zeolite 13X, in the temperature range 4 K to 20 K. The results are presented in tabular and graphical form. The N₂ isotherms, which showed characteristic step-like behavior, served to assess the specific surface area, which was 527 m² g^–1 based on a standard N₂ molecular area of 16.2 Ų. It also provided a value ofE ₁ equal to 2530 cal mole^–1. The argon isotherm at 78 K yielded a specific surface area for the zeolite 13X in fair agreement with that from the N₂ data. Nine isotherms were taken for⁴He between 4 K and 20 K and four for³He in the same temperature range. These isotherms permitted good evaluations of the isosteric heats of adsorption to be made and plotted as a function of coverage, yielding, for⁴He,Q _st =1580 j mole^–1 at zero coverage,Q _st =1030 j mole^–1 at monolayer coverage andE ₂=480 j mole^–1 at two-layer coverage. For³He, which showed everywhere smaller Q _st values. Q _st =1420 at zero coverage. By use of the Steele equation applied to⁴He, we found that the monolayer coverageV _m1 0.29 cm³ (STP) m^–2, and the second-layer coverage,V _m2 0.10 cm³ (STP) m^–2.Supported in part by a grant from the National Science Foundation and by contracts with ONR and the Department of Defense (Themis).     

4He Adsorption and Superfluid Transition on C60

We present adsorption isotherm data of⁴He on C₆₀ for 1.50 K<T<1.68 K determined by measurements of the frequency of quartz crystal microbalances (QCM’s) coated with C₆₀ films. We find a Kosterlitz-Thouless transition with a jump in the areal superfluid density close to predictions. By comparing the adsorbed⁴He on two QCM’s we derive an upper limit for the amount of⁴He intercalated into C₆₀ at low temperature of 0.05⁴He atoms per C₆₀ molecule. The low coverage portion of the adsorption data shows an apparent excess adsorption of⁴He onto the C₆₀.     

Dielectric-Constant Gas-Thermometry Scale from 2.5 K to 36 K Applying 3He, 4He, and Neon in Different Temperature Ranges

New dielectric-constant gas-thermometry (DCGT) measurements were performed at PTB in the temperature range from 23 K to 36 K. For the first time, besides helium, neon was also used as a measuring gas. The measurements with helium and neon yielded thermodynamic temperature data in the range between 27 K and 36 K. A combination of these data, with former measurements in the range between 2.5 K and 26 K made using ³He and ⁴He, lead to a temperature scale which coincides with the ITS-90 within a few tenths of a millikelvin even at highest temperatures. The discussion of the uncertainty budget for neon follows the procedures already proved for helium. Furthermore, a comparison between new highly accurate ab initio calculations for the second and third density virial coefficients of helium, and the extended DCGT results are given. For the third virial coefficient of helium, the presented data allow the first experimental check of the new ab initio calculation for the range above 24 K.     

Motional states of3He and4He in the one-dimensional channels of K-L zeolite

The behavior of³He and⁴He in a one-dimensional channel (10 Å in diameter) of K-L zeolite has been studied by measuring isosteric heat of sorption and heat capacities in the temperature range 0.1–1.5 K as a function of the amount of helium adsorbed. At smaller amounts, below 40% of full channel, the He adatoms seem to be bound on the potential minima of the wall. However, at larger amounts adsorbed, between 40 and 70%, a one-dimensional translation of the excess adatoms over 40% is suggested by the results that the molar heat capacities atT1 K are about the magnitudeR/2 of a one-dimensional gas and that the heat of sorption atT=0 K is almost constant.Research Institute for Catalysis, Hokkaido University, Sapporo, Hokkaido, Japan.     

Adsorption of4He,Ar, and N2 on exfoliated graphite and of4He on Ar-coated exfoliated graphite at low temperatures

The results of measurements of adsorption isotherms are given for ⁴ He, Ar, and N ₂ on bare exfoliated graphite (Grafoil) and for ⁴ He on Grafoil covered with a monolayer of argon. The temperatures employed were 77.3 K for the argon and N ₂ , and the range 4.2–18.55 K for the⁴He. The pressure range was from 0.25 to 150 Torr. The Grafoil surface area was calculated from the argon isotherm at 77.3 K, yielding a value of 19.9 m ² /g or a total surface area of 672 m ² for the 33.81 g of adsorbent used in these experiments. From the ⁴ He isotherms the isosteric heats of adsorptionQ _st /R were calculated. In the limit of zero coverage,Q _st /R on the argon monolayer was 82 K, and on the bare GrafoilQ _st /R lies between 130 and 170 K. At theoretical monolayer coverage on the argon-coated GrafoilQ _st /T=68 K. These results are compared with theoretical evaluations and with previous calorimetric data on other graphites.Work supported in part by a grant from the National Science Foundation and by contracts with DOD (Themis Program) and ONR.     

Formation of layers and gas and liquid states of helium in 8–13 Å diameter pores of Y zeolites

Evolution of layers and their motional states for³He and⁴He adsorbed in Y zeolites have been studied by means of isosteric heat of sorption and heat capacity measurements. The two kinds of Y zeolites used in this study have pores interconnected in a ordered fashion, as well as the same diameter 8–13 Å. However, they are different potential profiles on the pore wall. The heat of sorption clearly shows localization potentials at Na⁺ ion sites on the pore wall for Na-Y zeolites. In contrast, Silica-Y zeolite has few localization sites because it contains very few cations. We find that a solid layer forms on the pore wall at 38% of full pore adsorption amount for both Y zeolites. Upon increasing the amount adsorbed, the adatoms on the solid layer behave as a gas whose molar heat capacity is a half of the gas constant. From 50 to 65 %, the second layer on the pore wall is in a semi-quantum liquid state since the heat capacity becomes proportional to temperature. Finally, the possibility of Fermi degeneracy of adsorbed³He in zeolitic micropores is discussed.     

Multilayer isotherms of neon adsorbed on exfoliated graphite

Isotherms of neon adsorbed on compressed exfoliated graphite were measured, using a standard volumetric method, in the temperature interval of 12–24 K for the four first layers. The critical temperatures for the first three layers were determined to be 16.0±1.0, 19.0±1.0 K, and 18.0±1.0 K, respectively. From the isotherms, the isosteric heats of adsorption were calculated as a function of coverage, indicating some interesting features, which may be attributed to possible phase transitions. The isosteric heat of adsorption for the fourth layer on,Q _st/R=.275 K, is comparable to the three-dimensional latent heat of sublimation 256 K at the triple point. The binding energies for the first, second, and third layers were obtained from the isosteric heats as V → 0, yielding 302.5, 237.5, and 227.5 K, respectively. The differential molar entropies and internal molar energies were also calculated as a function of coverage. The possibility of a coexistence region with two solids having different structures in the phase diagram for the first layer is discussed. Whenever possible, the results are compared with theoretical evaluations and previous experimental data.     

Adsorption of3He on cesium

Adsorption isotherms of³He on cesium substrates have been measured in the temperature range from 0.2 K to 1.5 K. At liquid-vapor coexistence³He wets cesium at all temperatures studied. Step-like features are found in the isotherms which are similar to the prewetting transitions of⁴He on Cs substrates, but the width of these steps is ∼20 times wider for³He than for⁴He. In the case of³He on Cs, the steps are located at a chemical potential about 0.6 K below liquid-vapor coexistence. If the low temperature behavior is interpreted to be first order prewetting, the prewetting critical point temperature is 0.6±0.1 K.     

Extraordinary behavior of4He on hydrogen and deuterium surfaces

Using third sound resonance below 1 K, for⁴He on solid H₂ and D₂ surfaces, we have found new phenomena. On H₂ we find surface superfluidity for⁴He coverages less than one layer, with the zero temperature Kosterlitz-Thouless (KT) transition extrapolated to a coverage of 0.35±0.05 layers. On both H₂ and D₂ we find a new line of transitions below the KT transitions. On H₂, at zero temperature, the new line of transitions starts at zero temperature at a coverage of 0.6±0.1 layers of⁴He and has a slope of 0.58 (K/layer) of the universal KT slope. We also find, on H₂, two modes of surface wave propagation between the new transition and the KT transition. Below the new transition we also find anomalous third sound absorption which increases exponentially with temperature by five orders of magnitude.     

Theoretical Study of Quantum Gel Formation in?Superfluid 4He

Bosonic density functional theory calculations were carried out for neon, argon, and fluorine based systems in superfluid ⁴He with an emphasis on the formation of dimeric species in the liquid. These atomic species display relatively strong binding with helium and hence their solvation structures in the liquid exhibit highly localized liquid helium layers around them. These solvent layers modify the gas phase dimer potentials by inclusion of a recombination barrier, which provides stabilization for the solvated atoms. Of closed shell species neon is shown to exhibit a recombination barrier of 3 K for the dimer and up to 5.8 K for specific cluster geometries. For argon, the liquid induced potential barrier is only 0.7 K and it has a rather large amount of excess energy available along the recombination coordinate indicating that it is not possible to stabilize argon atoms in superfluid helium. Atomic fluorine shows the most pronounced effect with the recombination barrier of 26.8 K for producing ground state F₂. It is concluded that neon and fluorine atoms are good candidates to form impurity based quantum gels in bulk superfluid helium.     

The thermal conductivity of and convective onset in liquid3He at saturated vapor pressure above 1.3 K

This paper presents thermal conductivity data for liquid³He taken at saturated vapor pressure (s.v.p.), and also the critical Rayleigh number at which convection begins. The temperature in this experiment ranged from 1.3 K to 3.2 K. We compare the thermal conductivity results with those of earlier experimenters, and find they agree well. The convective onset measurements are the first reported for³He, and we find the critical Rayleigh number Ra_c = 1500 ± 105, which is similar to the experimental values reported for⁴He. References are provided for the thermodynamic properties necessary for calculating the Rayleigh number Ra at s.v.p.     

Structural and Superfluid Properties of the 4He Monolayer on a C28 Molecule

We have performed path-integral Monte Carlo calculations to study the adsorption of N ⁴He atoms on a single C₂₈ fullerene molecule. Radial density distributions show a layer-by-layer growth of ⁴He with the first adlayer being located at a distance of 5.3 Å from the center of C₂₈. The monolayer is found to show a commensurate structure at N=16 with each of the 16 adsorption sites on the molecular surface being occupied by one ⁴He atom. As more helium atoms are adsorbed beyond N=16, the adlayer is in a mixed state consisting of 4 localized atoms at the hexagonal faces and the other atoms delocalized over the pentagonal faces. Another structurally-ordered state is observed at N=32, where the helium layer shows the same crystalline structure with an icosahedral symmetry as observed for 32 ⁴He atoms on a C₆₀ molecule. It is found that more ⁴He atoms can be squeezed into the first layer to disrupt this icosahedral structure when enough ⁴He atoms are added in the second layer. Finally we observe the reentrant superfluid response of the monolayer with superfluidity being quenched completely at the ordered states of N=16 and 32.