Eric D. Marquardt

Cryogenic Material Properties Database

NIST has published at least two references compiling cryogenic material properties. These include the Handbook on Materials for Superconducting Machinery and the LNG Materials & Fluids. Neither has been updated since 1977 and are currently out of print. While there is a great deal of published data on cryogenic material properties, it is often difficult to find and not in a form that is convenient to use. We have begun a new program to collect, compile, and correlate property information for materials used in cryogenics. The initial phase of this program has focused on picking simple models to use for thermal conductivity, thermal expansion, and specific heat. We have broken down the temperature scale into four ranges: a) less than 4 K, b) 4 K to 77 K, c) 77 K to 300 K, and d) 300 K to the melting point. Initial materials that we have compiled include oxygen free copper, 6061-T6 aluminum, G-10 fiberglass epoxy, 718 Inconel, Kevlar, niobium titanium (NbTi), beryllium copper, polyamide (nylon), polyimide, 304 stainless steel, Teflon, and Ti-6Al-4V titanium alloy. Correlations are given for each material and property over some of the temperature range. We will continue to add new materials and increase the temperature range. We hope to offer these material properties as subroutines that can be called from your own code or from within commercial software packages. We will also identify where new measurements need to be made to give complete property prediction from 50 mK to the melting point.

A Cryogenic Catheter for Treating Heart Arrhythmia

Progress in the development of a cryogenic catheter to treat heart arrhythmia is discussed. This system uses a mixed-gas Joule-Thomson (J-T) refrigerator to cool the tip of a catheter that can be inserted into the body through the large veins leading into the heart. The cryogenic catheter is intended to treat heart arrhythmia characterized by an abnormally rapid heart rate, although the system has a wide variety of other medical applications. Approximately 2 million people in the U.S. suffer from rapid-rate heart arrhythmia. Catheter therapy has proven to be a more effective and less expensive method of treatment than alternatives such as drugs or surgery. A cryogenic catheter has significant advantages over existing catheters used in this form of therapy. The catheter has coaxial tubes for the high and low pressure streams with a miniature heat exchanger and J-T orifice at the catheter tip. The high pressure is maintained at 2.5 MPa. The largest diameter is 3 mm, the length is 90 cm, and all but the last 10–20 mm is flexible. The gas mixture has been optimized for the required operating conditions using nonflammable and low ozone depletion gases. Low cost techniques have been incorporated into the fabrication of the cold tip so that each catheter can be disposable. Several prototype catheters have been built. No-load temperatures down to 85 K were achieved with the cold tip exposed to ambient air. Using room temperature gelatin to simulate tissue heat loads, catheter tip temperatures of 160 to 175 K have been achieved, and ice balls about 26 mm in diameter weighing 11 g were created. We estimate that ice balls about 10 mm in diameter weighing 1.5 to 2.0 g are required to treat ventricular arrhythmia. Although the heat load in our experiments was less than the in vivo load, we think the current refrigeration power is sufficient to meet the clinical requirement.

Regenerator Behavior with Heat Input or Removal at Intermediate Temperatures

Regenerators with finite losses are capable of absorbing a limited amount of heat at intermediate temperatures along their length. This paper discusses a simple analytical model and a rigorous numerical model of regenerator behavior under the influence of heat input or heat removal at intermediate temperatures as well as the influence of a steady mass flow superimposed on the oscillating mass flow within the regenerator. The finite time-averaged enthalpy transport through the regenerator undergoes a discontinuity at the location of the heat input to satisfy the First Law of Thermodynamics. The discontinuous enthalpy flow leads to a discontinuous temperature gradient in the axial direction and to an increase in the regenerator loss that must be absorbed at the cold end. However, the increased loss is less than the heat input at the intermediate temperature, which allows the regenerator to provide a certain amount of cooling without the need for a separate expansion stage. This phenomenon is particularly useful for shield cooling and for precooling a gas continuously or at discrete regenerator locations prior to liquefaction at the cold end. For continuous precooling the total heat load can be reduced by as much as 23%.

Design Optimization of Linear-Arm Flexure Bearings

Flexure bearings have been used in linear-resonant compressors to maintain a non-contacting clearance seal between the piston and cylinder. This paper examines the linear-arm flexure bearing which was recently introduced. Linear-arm flexures have a higher radial stiffness and a higher radial-to-axial stiffness ratio than the spiral-arm flexures which are commonly used. Finite element analysis was used to study a wide range of linear-arm flexure sizes and materials. From this modeling, design graphs have been developed to assist in early compressor design stages and to provide the initial geometry for a more detailed finite element analysis. For a given stroke, diameter, stress, and Young’s modulus, the optimized material thickness and flexure arm geometry can be determined to maximize the radial stiffness. The radial stiffness, radial-to-axial stiffness ratio, and axial rotation can then be closely approximated from the design graphs presented here. A range of stroke-to-diameter ratios up to 0.2 was analyzed. A linear-arm flexure bearing was fabricated, and the measured stiffness agreed with that calculated by the finite element model to within 12%.

Vapor Precooling in a Pulse Tube Liquefier

Experiments were performed to study the effects of introducing vapor into a dewar where a coaxial pulse tube refrigerator was used as a liquefier in the neck of the dewar. We were concerned about how the introduction of vapor might impact the refrigeration load as the vapor barrier in the neck of the dewar is disturbed. Three experiments were performed where the input power to the cooler was held constant and the nitrogen liquefaction rate was measured. The first test introduced the vapor at the top of the dewar neck. Another introduced the vapor directly to the cold head through a small tube, leaving the neck vapor barrier undisturbed. The third test placed a heat exchanger partway down the regenerator where the vapor was pre-cooled before being liquefied at the cold head. This experiment also left the neck vapor barrier undisturbed. Compared to the test where the vapor was introduced directly to the cold head, the heat exchanger test increased the liquefaction rate by 12.0%. The experiment where vapor was introduced at the top of the dewar increased the liquefaction rate by 17.2%. A computational fluid dynamics model was constructed of the dewar neck and liquefier to show how the regenerator outer wall acted as a pre-cooler to the incoming vapor steam, eliminating the need for the heat exchanger.

Cryogenic Technology Information Database Program

The Chemical Propulsion Information Agency (CPIA) and Technology Applications, Inc. (TAI), in collaboration with the National Institute of Standards and Technology (NIST), has reconstructed and updated the database of the former National Bureau of Standards (NBS) Cryogenic Data Center. NBS maintained a database of cryogenic technical documents that served the national need until the early 1980s. The electronic database, maintained on a mainframe computer, was a highly specific bibliography of cryogenic literature and thermophysical-property data that covered 100 years of data. However, since then, the database has not been maintained for use. We have undertaken a project to convert the NBS database to a personal-computer (PC) platform and to backfill the database with citations of cryogenic literature dating from 1980 up to the present.