Stycast 2850FT Research Articles

In situ detection of delamination and critical current degradation caused by the thermal stress in epoxy impregnated REBa2Cu3O7−δ coils

Epoxy impregnated superconducting coils have better structural integrity and thermal stability. However, for REBa2Cu3O7−δ (REBCO, RE=Rare earth) coils, the mismatch of thermal expansion coefficients between epoxy and REBCO tapes is a serious problem. In this work, the temperature distribution, stress evolution during the cooling process, critical current distribution, and delamination sites inside REBCO coils impregnated using Stycast 2850FT have been studied. We measured the temperature distribution and the hoop strain in the penultimate turn of impregnated coils during the cooling process and analyzed the thermal stress evolution. No damage was observed for coils with the ratio between outer and inner diameter Ro/Ri<1.93. The delamination behavior occurred in coils with Ro/Ri>2.37, where the coils even exhibited a two-stage delamination. The delamination mechanism of REBCO coils was proposed from three aspects: the mechanical analysis, the critical current degradation, and the microscopic analysis. In this work, we found that the actual delamination behavior may appear earlier than the steady-state temperature, and temperature distribution will push the radial stress peak toward the inner radius of the coil, making the inner turn more susceptible to delamination. Multiple delamination locations were accurately predicted and confirmed. The measured compressive hoop strain first increased from −4212 με to −4684 με with the increase in Ro/Ri and then decreased to −3835 με obviously due to delamination. This work reveals in detail the delamination mechanism in impregnated REBCO coils, which is of great significance for the development of damage-free coils.

Critical current degradation and delamination crack observation of epoxy-coated REBCO superconducting tapes after thermal cycles in liquid nitrogen

The delamination and critical current (Ic) degradation caused by thermal stress after epoxy impregnation are threats for the application of REBa2Cu3O7-x(REBCO, RE = Rare earth) superconducting magnets. In this work, two types of REBCO tapes were coated by Stycast 2850FT with controlled coating geometries. Critical currents of coated samples after thermal cycles in liquid nitrogen were measured. Ic degradation was found in coated samples with a free no-coating edge, when the surface coating layer was thicker than 1000 μm. It was also found that additional edge coating can help to suppress the Ic degradation. Samples with degraded Ic after thermal cycles showed an obvious delamination phenomenon. The morphology and location of delamination cracks were carefully observed by using focused Ion beam, scanning electron microscope, and transmission electron microscope. Delamination cracks propagated within the REBCO layer and stopped at reaching the Silver/REBCO interface. Simulations by finite element method suggest that delamination cracks are generated by the stress accumulation within the REBCO layer, which could be reduced by a full epoxy coating on both tape surface and edges.

Specific heat of epoxies and mixtures containing silica, carbon lamp black, and graphite

We present specific heat measurements in the 2 – 20 K range of several commercially available epoxies (Stycast 1266, Stycast 2850FT, EPO-TEK 377, and EPO-TEK 377H) and mixtures of one of these epoxy binders and fillers such as silica, carbon lamp black, and graphite. The results are briefly described in the context of the Debye model.

Improved Copper-Epoxy Adhesion by Laser Micro- and Nano-Structuring of Copper Surface for Thermal Applications.

The objective of this work is the enhancement of metal-to-metal bonding to provide high thermal conductivity together with electrical insulation, to be used as heat sinks at room and cryogenic temperatures. High thermal conductive metal (copper) and epoxy resin (Stycast 2850FT) were used in this study, with the latter also providing the required electrical insulation. The copper surface was irradiated with laser to induce micro- and nano-patterned structures that result in an improvement of the adhesion between the epoxy and the copper. Thus, copper-to-copper bonding strength was characterized by means of mechanical tensile shear tests. The effect of the laser processing on the thermal conductivity properties of the Cu/epoxy/Cu joint at different temperatures, from 10 to 300 K, is also reported. Using adequate laser parameters, it is possible to obtain high bonding strength values limited by cohesive epoxy fracture, together with good thermal conductivity at ambient and cryogenic temperatures.

Epoxy-based thermally conductive adhesives with effective alumina and boron nitride for superconducting magnet

Heat management is one of the key technologies for reliable and high-performance electric devices. We have developed thermally conductive adhesives based on a commercial thermally conductive epoxy, Stycast 2850FT (S2850), with an additional unfilled resin and boron nitride (BN). The optimal mixing ratio of BN to epoxy resin was 10 wt%, and the thermal conductivity (κ) of this mixture was 1.65 W/m·K at 25 °C, 1.6 times higher than that of S2850. Such enhancement of κ may be attributed to the formation of more thermal conductive paths generated by the effective additives inside the composite adhesive layers, which was confirmed with elemental mapping analysis. Our epoxy-based thermally conductive adhesives with improved κ was then implemented for a superconducting magnet. Because of the enhanced heat dissipation of our composite adhesive, the superconducting magnet showed enhancement of 42.7%, 14.8%, and 76.6% in the cooling rate, the minimum quench energy, and the normal zone propagation velocity in the longitudinal direction at 70% of the current-carrying capacity, respectively, which are important parameters in the operational performance of a superconducting magnet, compared to a coil with S2850.

Permittivity and permeability of epoxy–magnetite powder composites at microwave frequencies

Radio, millimeter, and sub-millimeter astronomy experiments as well as remote sensing applications often require castable absorbers with well known electromagnetic properties to design and realize calibration targets. In this context, we fabricated and characterized two samples using different ratios of two easily commercially available materials: epoxy (Stycast 2850FT) and magnetite (Fe3O4) powder. We performed transmission and reflection measurements from 7 GHz up to 170 GHz with a vector network analyzer equipped with a series of standard horn antennas. Using an empirical model, we analyzed the data to extract complex permittivity and permeability from transmission data; then, we used reflection data to validate the results. In this paper, we present the sample fabrication procedure, analysis method, parameter extraction pipeline, and results for two samples with different epoxy-powder mass ratios.

Analysis of thermo-physical properties of materials suitable for thermal stabilization of superconducting tapes for high-voltage superconducting fault current limiters

High-temperature superconducting coated conductors (CC) are attractive materials for high-voltage resistive fault current limiters. Commercially produced CC tapes cannot withstand electric fields over 100 V m−1, and therefore, they have to be modified by addition of thermal stabilization layer. We investigated several composite systems suitable for such thermal stabilization. As a matrix of composites, Stycast resins were used, which contain various contents of powder particles from SiC, SiO2, PbTiO3, ZrW2O8, or synthetic diamond. The thermo-physical properties such as thermal expansion, heat capacity, thermal diffusivity and direct-current electrical conductivity were measured because they are influenced by filler content in the composite. The possible highest amount of the powder can slightly increase the thermal conductivity and decrease the thermal expansion of the system to an acceptable value needed for the coating of CCs. A side effect is simultaneous drop of heat capacity; however, its value stays still high enough for the purpose of heat sinking. Overall, the best results were achieved in system with 20 vol% of SiC filler in Stycast 2850FT + Catalyst 23LV resin matrix. There the contribution of the semiconducting SiC addition to the electrical conductivity of composite was negligible.

Investigation of the Dependences of the Attenuation Properties of Cryogenic Metal-Powder Filters on the Preparation Method

We fabricated low-pass metal powder filters for use in low-noise measurements at cryogenic temperatures and investigated their attenuation characteristics for different wire-turn densities, metalpowder shapes, and preparation methods at frequencies up to 20 GHz. We used nominally 30-μmsized stainless-steel 304L powder and mixed it with low-temperature binders. The low-temperature binders used were Stycast 2850FT (Emerson and Cumming) with catalyst 23LV and GE-7031 varnish. A 0.1-mm insulated copper wire was wound on preformed powder-mixture bobbins in the shape of a circular rod and was encapsulated in metal tubes with the powder mixture. All the fabricated powder filters showed a large attenuation at high frequencies with a cut-off frequency near 1 GHz. For filters of the same wire length, a lower wiring density showed a larger attenuation, which implies that the amount of powder in close contact with the wire determines the attenuation. Filters made of a powder/varnish mixture showed significantly larger attenuations than those of a powder/stycast mixture, and the attenuation improved with increasing powder ratio in the mixture. The low-temperature thermal conductivities of a 2 : 1 powder/Stycast mixture and a 5 : 1 powder/varnish mixture showed similar values at temperatures up to 4.2 K.

Design, Manufacturing, and Characterization of Conical Blackbody Targets With Optimized Profile

This paper describes the design, manufacturing, and characterization of conical blackbody targets to be integrated in millimeter and submillimeter wave radiometers. In particular, we present the electromagnetic properties of the binding material Stycast 2850FT, which is suitable at cryogenic temperatures, with microwave absorbing fillers between 100 and 420 GHz. On the basis of these extensive measurements we have determined the parameters of a mixing formula, which can be applied to calculate the composite properties with an arbitrary concentration of the filling material. Furthermore, we demonstrate that significant improvements of the electromagnetic properties of a calibration target can be achieved with an optimization of the cone profile. We compare the coherent backscattering S11 of conical targets with a common linear and an exponential-shaped cone. Numerical calculations have been carried out showing a reduced reflectivity of targets with an exponential cone profile. The coherent backscattering S11 of two target prototypes has been measured in different frequency bands between 105 and 410 GHz experimentally validating the simulations.

Study on the fabrication of low-pass metal powder filters for use at cryogenic temperatures

We fabricated compact low-pass stainless-steel powder filters for use in low-noise measurements at cryogenic temperatures and investigated their attenuation characteristics for different wire lengths, shapes, and preparation methods up to 20 GHz. We used nominally 30-micrometer-sized SUS 304L powder and mixed with Stycast 2850FT by Emerson and Cumming with catalyst 23LV. A 0.1 mm insulated copper wire was wound on preformed powder-mixture spools in the shape of a right-circular cylinder, a flattened elliptic cylinder and a toroid, and the coils were encapsulated in metal tubes or boxes filled with the powder mixture. All the fabricated powder filters showed a large attenuation at high frequencies with a cut-off frequency near 1 GHz. However, the toroidal filter showed prominent ripples corresponding to resonance modes in the 0.5-m-long coil wire. A filter with a 2:1 powder/epoxy mixture mass rate and a wire length of 1.53 m showed an attenuation of -93 dB at 4 GHz and the attenuation was linearly proportional to the wire length. As the powder-to-epoxy ratio increased, the high-frequency attenuation increased. An equally-spaced single-layer coil structure was found to be more efficient in attenuation than a double-layer coil. Geometry of the metal filter-case affected noise ripples with the least noise in a circular-tube case.

Ramping Operation of the Conduction-Cooled High-Temperature Superconducting Magnet for an Active Magnetic Regenerator System

An active magnetic regenerator (AMR) system requires fast ramp rate of magnetic field variation. The magnetic field change enables a magnetic refrigerant to create magnetocaloric effect for magnetic refrigeration. In this paper, a conduction-cooled high-temperature superconducting (HTS) magnet has been thermally analyzed and tested for an AMR system. A GdBCO conductor insulated with polyimide tape was wound by dry winding and standard double-pancake coil method. Stycast 2850FT was applied to the edge of the tape conductor so that the cooling plates, which were located at the top and bottom of the coil, should effectively cool the magnet. The whole thermal bridging between the magnet and a two-stage GM cryocooler is analyzed by the numerical simulation and a Joule heating experiment of the HTS magnet at normal state. The inductance of the HTS magnet was measured as 350 mH. The HTS magnet is intended to be operated with maximum 1 T/s at 20 K, cooled by the cryocooler. A solenoid switch and an external dump resistor were employed in order to discharge the HTS magnet. A continuous ramping operation test was conducted with a 60-A peak current and a maximum 20-A/s ramp rate by 3-kW dc power supply. This paper describes the experimental results of the ramping operation of the HTS magnet for an AMR system, and the technical issues on the results are discussed.

The Broadband Anti-reflection Coated Extended Hemispherical Silicon Lenses for Polarbear-2 Experiment

Polarbear-2 (PB-2) is a next-generation receiver that is part of the Simons Array cosmic microwave background (CMB) polarization experiment which is located in the Atacama desert in Northern Chile. The primary scientific goals of the Simons Array are a deep search for the CMB B-mode signature of gravitational waves from inflation and the characterization of large-scale structure using its effect on CMB polarization. The PB-2 receiver will deploy with 1897 dual-polarization sinuous antenna-coupled pixels, each with a directly contacting extended hemispherical silicon lens. Every pixel has dual polarization sensitivity in two spectral bands centered at 95 and 150 GHz, for a total of 7588 transition edge sensor bolometers operating at 270 mK. To achieve the PB-2 detector requirements, we developed a broadband anti-reflection (AR) coating for the extended hemispherical lenses that uses two molds to apply two layers of epoxy, Stycast 1090 and Stycast 2850FT. Our measurements of the absorption loss from the AR coating on a flat surface at cryogenic temperatures show less than 1 % absorption, and the coating has survived multiple thermal cycles. We can control the diameter of the coating within 25 \({\upmu }\)m and translation errors are within 25 \({\upmu }\)m in all directions, which results in less than 1 % decrease in transmittance. We also find the performance of the AR-coated lens matches very well with simulations.

Cryogenic thermometry for refrigerant distribution system of JT-60SA

JT-60SA is a fully superconducting fusion experimental device involving Japan and Europe. The cryogenic system supplies supercritical or gaseous helium to superconducting coils through valve boxes or coil terminal boxes and in-cryostat pipes. There are 86 temperature measurement points at 4 K along the distribution line. Resistance temperature sensors will be installed on cooling pipes in vacuum. In this work, two sensor attachment methods, two types of sensor, two thermal anchoring methods, and two sensor fixation materials have been experimentally evaluated in terms of accuracy and mass productivity. Finally, the verification test of thermometry has been conducted using the sample pipe fabricated in the same way to the production version, which has been decided by the comparison experiments. The TVO sensor is attached by the saddle method with Apiezon N grease and the measurement wires made of phosphor bronze are wound on the pipe with Stycast 2850FT as the thermal anchoring. A Cernox sensor is directly immersed in liquid helium as a reference thermometer during the experiment. The measured temperature difference between the attached one and reference one has been within ±15 mK in the range of 3.40-4.73 K. It has satisfies the accuracy requirement of 0.1 K.

Temperature dependence of helium diffusion through common epoxies

Helium gas at room temperature is known to diffuse through the epoxies commonly used in various low temperature applications, which can complicate leak detection. The helium flux typically decreases with decreasing temperature. We have measured the flux of helium that passes though thin sections of as-cast clear Stycast 1266, Stycast 2850FT (black) and TRA-BOND 2151 (blue) epoxies as a function of temperature in the range 130K < T < 300K. We analyze the data to create normalized (to constant sample thickness and pressure differential) data for comparison. We report the preliminary temperature-dependent fluxes we have measured, which show significant differences among the epoxies studied.

Development of hard X-ray and gamma-ray spectrometer using superconducting transition edge sensor

Abstract Superconducting transition edge sensors (TES) are used for high-resolution X-ray spectroscopy. In our group, we developed an Ir-TES and reported an energy resolution of 6.9 eV FWHM at 5.9 keV ( Kunieda et al., 2004 ). In this study, we have designed a new TES detector using a superconducting tin (Sn) absorber to detect high energy photons over 100 keV. The Sn absorber is coupled to an Ir/Au superconducting film which is deposited on an ultra-thin SiN membrane (500 nm thick) with a small amount of epoxy post (Stycast 2850FT) by handling with a flip-chip bonding machine. The measured energy resolution is 485 eV FWHM at 60 keV and is better than that of HPGe detector.

Development of a Gamma-Ray Detector With Iridium Transition Edge Sensor Coupled to a Pb Absorber

We have recently started to develop a high-resolution gamma-ray spectrometer for material defect analysis. Our gamma-ray detector is a microcalorimeter consisting of an iridium/gold bilayer transition edge sensor (TES) thermometer and a bulk Pb absorber, which is directly coupled to a TES with Stycast 2850FT epoxy. This paper describes our TES based gamma-ray detector and the first experimental results for 662 keV and 1173 keV incident gamma-rays. We report an energy resolution of 4.7 keV at 662 keV and 2.9 keV at 1173 keV.

Cryogenic thermometry with a common diode: Type BAS16

Cryogenic test experiments often require a large number of temperatures to be monitored. In order to reduce cost, we investigated the feasibility of low-cost common diodes. We chose the Philips BAS16 diode in a type SOT23 package. By means of Stycast 2850FT, these diodes were glued into alumina holders. In total, 20 sensors were assembled and tested. With an excitation current of 100 μA and considering a temperature range from 50 K to room temperature, a quadratic least-squares-fit was obtained: T = 486.1 − 363.7 V − 38.3 V 2. All sensors agreed with this fit within an error of 1.3 K.

Linear thermal expansivity (1.5–300 K) and heat capacity (1.2–90 K) of Stycast 2850FT

Linear thermal expansivities, α, have been obtained from 1 to 300 K for a sample of Stycast 2850FT epoxy that initially was cured at room temperature (X1), and then at 95 °C (X2). Heat capacities, Cp, were measured from 1 to 95 K for X2. The α’s for X1 and X2 are roughly equal to and twice that of copper near room temperature, but differ by as much as 10% at low temperature. The α’s approach those for copper near 100 K, but have a much smaller temperature dependence at lower temperatures. The different shapes of the α(T) and Cp(T) relations give a Grüneisen parameter, Γ, which decreases from 1.2 at 10 K to 0.5 at 95 K. Both α and Cp show apparent changes in lattice properties near 4.5 K, and a low temperature anomaly (Γ=3.5) that is characteristic of amorphous solids. Sample relaxation (shortening) effects from 300 K to 110 K were more serious for X2 than for X1. Because of differences related to the curing procedure, these data should be used only semiquantitatively in the design of experiments in which Stycast 2850FT will be used as a construction material. Possible Stycast-related problems with a capacitance dilatometer are discussed.

Elastic properties of several amorphous solids and disordered crystals below 100 K

We have measured the internal friction and speed of sound variation at temperatures between 60 mK and room temperature for amorphous CdGeAs2, Polystyrene, and Stycast 2850FT epoxy, and the disordered crystals (ZrO2)0.89(CaO)0.11 and (CaF2)0.74(LaF3)0.26. A comparison of our results with an extensive review of previously published data shows a remarkable similarity in the internal friction of disordered solids below ∼5 K. The low temperature elastic behavior of these solids is adequately described by the standard tunneling model, from which one finds a nearly universal density of tunneling states for glasses. Internal friction above ∼10 K for different materials, however, displays a wide range of magnitudes and temperature dependence that is far from universal. Attempts to directly link the tunneling states observed by internal friction at low temperatures to configurational states of localized oscillators existing at high temperatures must take into account this striking variation among disordered solids above 10 K.