Mark Lysek

Phase diagrams of multilayer adsorbed methane

We propose multilayer phase diagrams based on recent observations of methane adsorbed on graphite and gold (111). Methane wets graphite at all temperatures observed, but it fully wets gold only above the bulk triple point. Possible reasons for this difference in behavior are discussed. The methane-graphite phase diagram also includes layer by layer condensation with critical points leading to a predicted bulk roughening temperature, and an extension of the bulk melting curve into the multilayer film region where it closes at a triple point with a compressed incommensurate first layer solid phase. Experimental evidence for these features is presented and discussed.

A fully automated scanning ratio calorimeter for use in adsorption studies

This article describes a scanning ratio calorimeter that has been developed for studying multilayer adsorbed films. The scanning technique presented is able to accumulate highly precise data much more quickly than other calorimetric techniques. The rms noise in the total heat capacity of the sample cell is 0.02% at 70 K and 0.04% at 120 K for scan rates of 2 K/h. Sources of error in the heat capacity measurement are analyzed in detail. The calorimeter is completely automated and can be operated continuously using a personal computer.

The multilayer melting transition in methane adsorbed on graphite

High resolution heat capacity measurements of multilayer methane adsorbed on graphite are presented and analyzed. The evidence indicates the presence of two wetting transitions: a first-order dewetting transition at T_w = 90.48 K, and a continuous wetting transition at the triple point, T_t = 90.66 K. This behavior is to be expected in connection with the melting transition in any system where both solid and liquid wet the surface. Heat capacity measurements can provide a valuable diagnostic tool for the wetting behavior of films too thick to be investigated by other means. In the thin film limit, we find that the latent heat of melting vanishes at about 4 layers.

Design and Testing of an Improved Cryopump for the Confined Helium Experiment

A cryopump flown on the space shuttle for the Lambda Point Experiment1 (LPE) was improved and redesigned for use in the Confined Helium Experiment (CHeX). The successful Lambda Point Experiment (LPE) flew on the space shuttle in October 1992. The Confined Helium Experiment (CHeX) will launch in October 1997. CHeX uses high resolution thermometers2 to measure the heat capacity of liquid helium when confined within finely spaced parallel disks. To eliminate heat flow between the calorimeter and the cryogenic bath, a cryopump is used to reduce the pressure in the experimental probe. The LPE cryopump trapped activated charcoal behind a nuclepore filter. The new cryopump uses activated charcoal glued to copper plates to eliminate the nuclepore filter. Vibration tests verified that the cryopump does not generate charcoal dust contamination. Early performance tests indicated that the probe vacuum is significantly lower with the new pump, and that the pressure falls more rapidly than with the previous design. Recent tests with the CHeX experimental probe show a significant reduction in the heat leak between the calorimeter and the cryogenic bath.

Observation of a first-order triple-point dewetting transition in methane adsorbed on graphite

We report the results of high-resolution heat capacity measurements of multilayer methane adsorbed on graphite near the bulk triple point. These measurements indicate that the films studied, from 5 to 18 layers thick, undergo two wetting transitions in the process of melting. The first is a first-order dewetting transition at T_w ∼ 90.5 K, and the second is a continuous wetting transition at the triple point, T_t = 90.66 K. The melting transition in the film begins at T_w, and moves to lower temperatu for thinner films.

The Methane/Graphite Phase Diagram

Methane adsorbed on graphite has been one of the most intensely studied of all multilayer film systems. A rather detailed phase diagram for the system was proposed by Goodstein et al. in 1984, [1] elaborated and embellished by Wortis in 1984,[2] and modified by Pettersen et al. in 1986.[3] The purpose of proposing the phase diagram was to stimulate further research by presenting hypotheses to be tested and drawing attention to open questions. The authors of the phase diagram had to confront some of the most interesting and vexing issues in the field of adsorbed films today: Does the adsorbed solid wet the substrate, and if so, why? Is there evidence of roughening, capillary condensation, or surface melting? Most intriguing of all, can one observe a dimensional crossover, from 2D to 3D behavior in the melting transition? The authors proposed answers to all of those questions. The purpose of this paper is to reexamine those proposed answers in the light of subsequent experimental and theoretical work on methane/graphite and other related systems. As we shall see, although much has been learned, many of the questions remain open. This is very much a work in progress.

Cryogenic And Thermal Aspects of The SIRTF Warm Launch Concept

A fundamentally new concept is being explored for cryogenic operation of the Space Infrared Telescope Facility (SIRTF). In this concept, the helium dewar and instruments would be launched at liquid helium temperature while the telescope and external radiation shields are at room temperature. Once on-orbit, the telescope and the radiation shields would be cooled radiatively to about 70 K. Helium vapor is then used to bring the telescope down to its final operating temperature of 5.5 K. We present a design which meets SIRTF requirements and discuss its merit.

Cryogenic telescope test facility

An optical test Dewar has been constructed with the unique capability to test mirrors of diameter ≤ 1 m, f≤6, at temperatures from 300 to 5 K with a ZYGO Mark IV/V interferometer. The facility possesses extensive thermometry throughout for characterization of the test chamber thermal environment and Dewar performance. Optical access is controlled with cryogenically cooled shutters. The entire Dewar is vibration isolated by 40 dB where the fundamental resonances of the Dewar structure are highest. The facility has been brought on line for its first user, the Infrared Telescope Technology Testbed (ITTT) for the Space Infrared Telescope Facility (SIRTF) at JPL. The design requirements for this facility and the resultant design and implementation experiences and challenges will be presented.

A Test of Commercial Squid Systems for Response to EMI and Temperature Changes

Tests of interface electronics for thermal response and electromagnetic interference (EMI) susceptibility were conducted on two unmodified commercial SQUID systems to determine their suitability for use on NASA space missions. The zero offset and open loop gain were measured as a function of preamplifier temperature for one RF and one DC SQUID system while the electronics were in vacuum. Suitable performance can be obtained from these systems operating from 0 to 40°C if the preamplifier temperature is controlled. Measurements of EMI susceptibility were performed on one RF and several DC SQUID systems by exposing the preamplifiers to fields up to 2 V/m at frequencies from 10 KHz to 250 MHz. The RF SQUID system was sensitive at narrow bands near the 200 Mhz modulation frequency. The DC SQUID systems were very sensitive to EMI in broad bands over the frequency range studied, at field intensities as low as 0.5 V/m. These preliminary tests show that the DC SQUID systems tested will need extensive modifications in order to reduce their EMI susceptibility to acceptable levels.

Phases and Phase Transitions in the First Few Layers of Methane, Argon and Krypton Adsorbed on Graphite

The equilibrium phase diagrams of monolayers of many substances adsorbed on graphite have long been studied as examples of 2-dimensional (2D) matter. One typically observes 2D gas and liquid phases, and solid phases that may be commensurate or incommensurate with the substrate lattice. Many experimental techniques have been used, but thermodynamic measurements are generally the most useful for tracing out phase boundaries. Recent advances in technique have made it possible to use thermodynamic measurements to study the phase diagrams of the second and higher layers, up to the fifth or sixth. These advances include ultra high resolution scanning calorimetry, and a detailed understanding of the role of capillary condensation in corners and pores of the graphite foam substrate. We find a rich array of phases and phase transitions in multilayer methane, argon and krypton. The second and third layers typically have distinct 2D gas, liquid and solid phases evidenced by 2D triple points and critical points. We observe phase transitions between solid phases that are commensurate and incommensurate with the layer below. We also observe melting of the first layer at higher temperatures, even when one to five additional layers are adsorbed on top of it. In argon and krypton, but not in methane, a strange new phenomenon is observed at temperatures above the gas-liquid critical point of the nth layer for n> 3. Below that temperature, nthlayer gas coexists with a condensed nth layer. At some temperature above it, a new coexistence region is observed in which a partial nth layer coexists with a partial n + 1st layer. This behavior is thought to be evidence for a theoretically predicted phase transition of the bulk interface, called the preroughening transition.