Cryogenic Regime Research Articles

Cryostructuring of polymer systems. XXIX. Preparation and characterization of supermacroporous (spongy) agarose‐based cryogels used as three‐dimensional scaffolds for culturing insulin‐producing cell aggregates

AbstractSupermacroporous (spongy) agarose‐based cryogels were prepared by a two‐step freezing procedure (freezing at −30°C followed by incubation at a warmer subzero temperature) and subsequent thawing. The cryogels were formed as cylinders in plastic syringes and as platelike samples in flat metal molds. The characteristic feature of the gel matrices thus obtained was their heterogeneous spongelike morphology with a system of interconnected gross (50–250‐μm and larger) pores. The influence of the cryogenic processing regimes on the properties and porous morphology of such agarose cryogels was explored by flow‐through analysis, optical microscopy, thermometry, and high‐sensitivity differential scanning calorimetry. These biocompatible, spongelike matrices were used as three‐dimensional scaffolds for culturing insulin‐producing rat insulinoma cells self‐assembled in multicellular spherical aggregates (pseudoislets). The cell morphology and functional activity of such pseudoislets indicate that supermacroporous agarose‐based cryogels can be useful as a tool for engineering biohybrid insulin‐producing tissue. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Solid-state NMR of endohedral hydrogen–fullerene complexes

We present an overview of solid-state NMR studies of endohedral H(2)-fullerene complexes, including (1)H and (13)C NMR spectra, (1)H and (13)C spin relaxation studies, and the results of (1)H dipole-dipole recoupling experiments. The available data involves three different endohedral H(2)-fullerene complexes, studied over a wide range of temperatures and applied magnetic fields. The symmetry of the cage influences strongly the motionally-averaged nuclear spin interactions of the endohedral H(2) species, as well as its spin relaxation behaviour. In addition, the non-bonding interactions between fullerene cages are influenced by the presence of endohedral hydrogen molecules. The review also presents several pieces of experimental data which are not yet understood, one example being the structured (1)H NMR lineshapes of endohedral H(2) molecules trapped in highly symmetric cages at cryogenic temperatures. This review demonstrates the richness of NMR phenomena displayed by H(2)-fullerene complexes, especially in the cryogenic regime.

Transverse and Longitudinal CTE Measurements of Carbon Fibers and their Impact on Interfacial Residual Stresses in Composites

In situ transmission electron microscopy (TEM) is utilized to evaluate the coefficients of thermal expansion (CTE) of two polyacrylonitrile (PAN) based (T1000 and IM7) and one pitch-based (P55) carbon fiber in the temperature range 20–1100 C. The transverse morphology of the fibers is studied using field emission scanning electron microscope and selected area diffraction (SAD) patterns in a TEM and is co-related to the fiber transverse CTE. The PAN-based IM7 and T1000 fibers revealed a transversely isotropic structure, which was consistent with their transverse CTE measurements. Even though the pitch-based P55 fiber revealed cylindrical orthotropy, it did not translate to orthotropy in the transverse CTE. Finite element models of unidirectional IM7/977 composite, utilizing the present IM7 CTE values, are compared to experimental lamina scale CTE results where good agreement was obtained for cool-down regime. The models also indicate that the Tresca stresses in the epoxy matrix at the fiber–matrix interface exceed the allowable strength at cryogenic regimes suggesting possible fiber–matrix debonding and transverse matrix cracking.

Synthesis and thermal stability of ball-milled and melt-quenched amorphous and nanostructured Al-Ni-Nd-Co alloys

In this work the Al85Ni9Nd4Co2 alloy was used as a starting point for examining the possibility of forming bulk glassy Al-based materials by combining rapid quenching and ball milling techniques. Fine glassy powders were obtained by ball milling melt-spun amorphous ribbons using a severe cryogenic processing regime. The thermal stability data of the powders as obtained by constant-rate heating and isothermal DSC experiments together with viscosity measurements are discussed with respect to feasible consolidation conditions. The powder compaction was done by two methods (uni-axial hot pressing and extrusion) at 513 K for up to 15 min. Only the uni-axial hot pressing led to bulk Al85Ni9Nd4Co2 samples with similar glassy structure and Vickers microhardness values comparable to those of the initial melt-spun ribbons.

Guidelines for etching silicon MEMS structures using fluorine high-density plasmas at cryogenic temperatures

This paper presents guidelines for the deep reactive ion etching (DRIE) of silicon MEMS structures, employing SF/sub 6//O/sub 2/-based high-density plasmas at cryogenic temperatures. Procedures of how to tune the equipment for optimal results with respect to etch rate and profile control are described. Profile control is a delicate balance between the respective etching and deposition rates of a SiO/sub x/F/sub y/ passivation layer on the sidewalls and bottom of an etched structure in relation to the silicon removal rate from unpassivated areas. Any parameter that affects the relative rates of these processes has an effect on profile control. The deposition of the SiO/sub x/F/sub y/ layer is mainly determined by the oxygen content in the SF/sub 6/ gas flow and the electrode temperature. Removal of the SiO/sub x/F/sub y/ layer is mainly determined by the kinetic energy (self-bias) of ions in the SF/sub 6//O/sub 2/ plasma. Diagrams for profile control are given as a function of parameter settings, employing the previously published black silicon method. Parameter settings for high rate silicon bulk etching, and the etching of micro needles and micro moulds are discussed, which demonstrate the usefulness of the diagrams for optimal design of etched features. Furthermore, it is demonstrated that in order to use the oxygen flow as a control parameter for cryogenic DRIE, it is necessary to avoid or at least restrict the presence of fused silica as a dome material, because this material may release oxygen due to corrosion during operation of the plasma source. When inert dome materials like alumina are used, etching recipes can be defined for a broad variety of microstructures in the cryogenic temperature regime. Recipes with relatively low oxygen content (1-10% of the total gas volume) and ions with low kinetic energy can now be applied to observe a low lateral etch rate beneath the mask, and a high selectivity (more than 500) of silicon etching with respect to polymers and oxide mask materials is obtained. Crystallographic preference etching of silicon is observed at low wafer temperature (-120/spl deg/C). This effect is enhanced by increasing the process pressure above 10 mtorr or for low ion energies (below 20 eV).

Heat Transfer Enhancement by Fins in the Microscale Regime

The current literature contains many studies of microchannel and micro-pin-fin heat exchangers, but none of them consider the size effect on the thermal conductivity of channel and fin walls. The present study analyzes the effect of size (i.e., the microscale effect) on the microfin performance, particularly in the cryogenic regime where the microscale effect is often appreciable. The size effect reduces the thermal conductivity of microchannel and microfin walls and thus reduces the heat transfer rate. For this reason, heat transfer enhancement by microfins becomes even more important than for macroscale fins. The need for better understanding of heat transfer enhancement by microfins motivates the current study, which resolves three basic issues. First, it is found that the heat, flow choking can occur even in the case of simple plate fins or pin fins in the microscale regime, although choking is usually caused by the accommodation of a cluster of fins at the fin tip. Second, this paper shows that the use of micro-plate-fin arrays yields a higher heat transfer enhancement ratio than the use of the micro-pin-fin arrays due to the stronger reduction of thermal conductivity in micro-pin-fins. The third issue is how the size effect influences the fin thickness optimization. For convenience in design applications, an equation for the optimum fin thickness is established which generalizes the case without the size effect as first reported by Tuckerman and Pease.

Second law analysis of a plate heat exchanger with an axial dispersive wave

A second law analysis is presented for thermally dispersive flow through a plate heat exchanger. It is well known that in plate or plate fin type heat exchangers the backmixing and other deviations from plug flow contribute significantly to the inefficiency of the heat exchanger, which is of importance to heat exchangers working in the cryogenic regime. The conventional axial heat dispersion model which is used so far is found to be better than `plug flow' model but still unsatisfactory where the timescale related to heat transfer is comparable with the thermal relaxation time for the propagation of dispersion. The present work therefore considers dispersion as a wave phenomenon propagating with a finite velocity. The study discusses the nature of variation of different contributions to total exergy loss in the heat exchanger with respect to dispersion parameters of the Peclet number and propagation velocity of the dispersive wave. The practical example of the single-pass plate heat exchanger demonstrates how a second law optimization can be carried out for heat transfer equipment under such conditions.

Exergetic analysis of plate heat exchanger in presence of axial dispersion in fluid

This paper presents an exergetic analysis of a single pass counterflow plate type heat exchanger for cryogenic applications. The special focus of the analysis is the study of the effect of axial dispersion which takes the flow maldistribution into consideration. It is found that with unutilized exergy fraction as the figure of demerit the exergetic performance of the heat exchanger can be consistently presented without the ‘paradox’ which has been observed by many investigators. The axial dispersion is found to reduce the exergetic performance and so does the frictional pressure drop of the fluids. This makes it possible to identify a minimum irreversibility corresponding to a given level of dispersion. It has been observed that over a relatively broad zone near the optimum NTU the change in exergy loss is small, providing more flexibility for the designer of the heat exchanger to alleviate the penalty for exergy loss, which is particularly severe in the cryogenic regime.

Charpy Impact Tests Near Absolute Zero

Abstract We review Charpy impact testing at extreme cryogenic temperatures, especially at liquid helium temperature (4 K), considering methods of testing and calibration, thermal behavior during the various stages of testing, and correlations between Charpy absorbed energy and quantitative toughness parameters. Because of the very low specific heats of metals near absolute zero, any surface condensation of gases, convective or conductive heat transfer, or plastic deformation during a test will cause the specimen temperature to rise rapidly. Consequently, valid impact tests of alloys at 4 K can not be performed according to the procedure outlined in ASTM Methods E 23-88. During Charpy tests, the temperature of austenitic steel specimens, initially at or near 4 K, may in fact rise outside the cryogenic regime. Fracture does not occur at the intended temperature, but at an uncontrolled temperature, since materials with different work hardening rates heat differently. In view of the temperature rise variability and scatter in measurements and property correlations, we conclude that it is not possible to accurately estimate the 4 K fracture toughness of ductile steels, or rank them properly, using Charpy tests.

Friction and wear characteristics of cermet-type metal films at cryogenic temperatures

Many aerospace applications require wear surfaces to operate in cryogenic environments. Specific applications in the cryogenic regime require bearings and seals to operate at high speeds with heavy loads for relatively short periods, as in turbopumps, or at moderate speeds and loads, as in cooling system pumps. The purpose of this paper is to determine the friction coefficients and wear characteristics of sputter deposited WCCo and TiCNi alloys on Inconel 718 substrates cooled to 20 K. Composition of the deposited films was verified by energy dispersive analysis of X-rays. Wear characteristics were determined using a friction and wear test fixture which utilized a two-stage cryogenic refrigerator to cool the stationary member of the test couple. Wear results as determined by scanning electron microscopy showed the wear of TiCNi coatings to be abrasive and a function of surface finish. WCCo was shown to deform plastically as asperity peaks in the surface appeared to be burnished with minimal debris generation. Coating adhesion was verified by diamond scratch testing.

The calorimetric detection of rare events in the cryogenic regime

The use of thermal calorimetry in the cryogenic regime for the energy detection of subatomic events is discussed in relation to fundamental principles and practical feasibility. It is shown that a candidate material as cryogenic detector is single crystal silicon, whose technology allows the preparation of devices with energy resolution better than 1 eV and dead time shorter than 0.1 s. Design criteria, open problems, and possible applications are discussed.

Testing of the Euratom LCT-coil, a forced-flow cooled NbTi coil, in the international fusion superconducting magnet test facility (IFSMTF)

The Euratom-LCT-coil was tested as a single coil in the TOSKA facility at KfK Karlsruhe and in IFSMTF at ORNL Oak Ridge. Different mechanical boundary conditions and other operation parameters (mass flow, current) in the IFSMTF led to new set of results which broaden the knowledge about the coil. During cooldown the coil showed excellent heat transfer properties. The design current 11.4 kA with a max. field of 6.63 T could be reached with a ramp rate of 4 A/S without visible instabilities and thermal losses. The coil had at rated current a Stekly parameter α = 2.3 and could safely operated outside the cryogenic stability regime. The coil was 7 times dumped from currents higher than 4 kA with a peak voltage of 2.5 kV. Pressure increase was investigated under different boundary conditions and remained moderate. Dump losses measured were about 2 % of the stored energy. The coil was two times quenched during investigation of the operation limits. The quench detection system specially developed for this coil could be successfully tested with a level setting of 50 mV. For the first time the operation limits of such a coil were determined by current sharing measurement with heated helium slugs. The results are in fair agreement with those extrapolated from single strand measurements.

The technology of solid-fuel-layer targets for laser-fusion experiments

An apparatus which produces uniform solid-fuel layers in glass-shell targets for laser irradiation is described. A low-power cw laser pulse is used to vaporize the fuel within a previously frozen target which is maintained in a cold-helium environment by a cryogenic shroud. The rapid refreezing that follows the pulse forms a uniform fuel layer on the inner surface of the glass shell. This apparatus and technique meet the restrictions imposed by the experimental target chamber. The method does not perturb the target position; nor does it preclude the usual diagnostic experimets since the shroud is retracted before the main laser pulse arrives. Successful laser irradiation and implosion of solid-fuel-layer targets at KMSF have confirmed the effectiveness and reliability of this system and extended the range of laser-target-interaction studies in the cryogenic regime.

The heat pipe

The heat pipe is a device having a high thermal conductance which utilizes the transport of a vapour and rejection of latent heat to achieve efficient thermal energy transport. The theory of heat pipes is well developed. Their use in applications involving temperatures in the cryogenic regime, and with development units running as high as 2000 degrees C, shows that they can function over a large part of the temperature spectrum. Applications in spacecraft, electronics and die casting are but few of the uses for these devices