Variable Cryogenic Temperatures Research Articles

Development of modular cryogenic indentation apparatus for investigating micro-region mechanical properties of materials

High-precision cryogenic micromechanical testing apparatuses are of significant importance in investigating the micro-region mechanical properties and deformation behaviors of materials at cryogenic temperatures. In this study, a modular cryogenic indentation apparatus was designed and developed utilizing a contact-atmosphere hybrid refrigeration technique with a minimum temperature of −150 °C. The temperature of both the indenter tip and the specimen was independently controlled through a dual-channel proportional–integral–derivative (PID) feedback loop to eliminate contact thermal drift to within 0.1 nm/s. The application of cryogenic nitrogen (N2) atmosphere refrigeration eliminated additional stiffness at the indenter tip completely. The feasibility of the apparatus was demonstrated through calibrations and tests conducted on standard fused silica and single crystal nickel. Highly accurate indentation curves and micro-region mechanical properties of single crystal nickel were obtained at variable cryogenic temperatures. With decreasing temperature, both slip bands and pile-up around residual indentation imprints were weakened. Notably, at −150 °C, the slip bands disappeared, and the phenomenon of pile-up was replaced by sink-in. This developed cryogenic indentation apparatus will be a powerful tool to reveal the effect of cryogenic temperatures on the evolution mechanisms of materials’ micro-region mechanical properties.

DEVELOPMENT OF A CRYOGENIC FATIGUE TEST FACILITY FOR HIGH TEMPERATURE SUPERCONDUCTING TAPES

Because of the high critical current density in the intense magnetic field, wider temperature margin, resistance irradiation and good mechanical properties, high temperature superconducting tapes (BSCCO and ReBCO taps) have a huge potential application in accelerator magnets, high field magnets and superconducting electric power systems. Some expectable opportunities will be created for modern high-tech electromagnetic equipment. High temperature superconducting tapes with substrate layers usually have high tensile strength. Thus, they can be applied under an intense electromagnetic field. However, in its application process and under the operation conditions, many kinds of inevitable fatigue loads will occur, which have an impact on the current-carrying capacity of the high temperature superconducting tapes and can destroy the high temperature superconducting magnets. A cryogenic fatigue test facility for high temperature superconducting tapes is introduced, the system can be used to measure the mechanical, thermal and strain-field-dependent behaviors of fatigued superconducting wires and tapes under variable cryogenic temperatures. Based on the cryogenic fatigue test facility, the mechanical behaviors and current-carrying characteristics of tension-compression fatigued YBCO tapes were experimentally studied. The preliminary test results show that their mechanical behaviors and current-carrying characteristics showed a nonlinear dependence on the stress ratio during the fatigue test. The successful development of the cryogenic fatigue test facility will provide a basic fatigue test platform for the design and development of accelerator high temperature superconducting magnets.

Interplay between structural and magnetic-electronic responses of FeAl2O4 to a megabar: Site inversion and spin crossover

X-ray diffraction pressure studies at room temperature demonstrate that the spinel FeAl$_{2}$O$_{4}$ transforms to a tetragonal phase at ~18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ~45 GPa, after which a transformation to a Cmcm post-spinel phase is onset. This is attributable to pressure driven Fe-Al site inversion at room temperature, corroborated by signatures in the 57Fe M\"ossbauer spectroscopy pressure data. At the tetragonal to post-spinel transition, onset in the range 45-50 GPa, there is a concurrent emergence of a non-magnetic spectral component in the M\"ossbauer data at variable cryogenic temperatures. This is interpreted as spin crossover at sixfold coordinated Fe locations emanated from site inversion. Spin crossover commences at the end of the pressure range of the tetragonal phase and progresses in the post-spinel structure. There is also a much steeper volume change dV/V ~ 10% in the range 45-50 GPa compared to the preceding pressure regime, from the combined effects of the structural transition and spin crossover electronic change. At the highest pressure attained, ~106 GPa, the M\"ossbauer data evidences a diamagnetic Fe low-spin abundance of ~50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of ~33 T. This is indicative of a magnetic ground state associated with strong covalency, as well as substantive disorder from site inversion and the mixed spin-state configuration. Intriguingly magnetism survives in such a spin-diluted post-spinel lattice at high densities. The R(300 K) data decreases by only two orders of magnitude from ambient pressure to the vicinity of ~100 GPa. Despite a ~26% unit-cell volume densification from the lattice compressibility, structural transitions and spin crossover.

Cryogenic Adsorption of Nitrogen and Carbon Dioxide in Activated Carbon

Activated carbon have been used for a long time at low temperature for cryogenic applications. The knowledge of adsorption characteristics of activated carbon at cryogenic temperature is essential for some specific applications. However, such experimental data are very scare in the literature. In order to measure the adsorption characteristics of activated carbon under variable cryogenic temperatures, an adsorption measurement device was presented. The experiment system is based on the commercially available PCT-pro adsorption analyzer coupled to a two-stage Gifford McMahon refrigerator, which allows the sample to be cooled to 4.2K. Cryogenic environment can be maintained steadily without the cryogenic liquid through the cryocooler and temperature can be controlled precisely between 5K and 300K by the temperature controller. Adsorption measurements were performed in activated carbon for carbon dioxide and nitrogen and the adsorption isotherm were obtained.

Site-specific spin crossover in Fe2TiO4 post-spinel under high pressure up to nearly a megabar

X-ray diffraction studies to ~90 GPa at room temperature show that Fe$_{2}$TiO$_{4}$ ferrous inverse spinel undergoes the following sequence of structural transitions : cubic (Fd3m) to tetragonal (I41/amd)to orthorhombic(Cmcm) to orthorhombic(Pmma),at the indicated onset transition pressures. Within the Cmcm phase, site-specific spin crossover is initiated and involves only highly distorted octahedral sites constituting ~25% of all Fe locations. This is manifest as a steeper volume decrease of dV/V0 ~ 3.5% beyond ~40 GPa and an emergent diamagnetic component discerned in 57Fe M\"ossbauer spectroscopy at variable cryogenic temperatures. A subsequent Cmcm to Pmma Fe/Ti disorder-order reconfiguration is facilitated at 6-fold coordinated (octahedral) sites. The rest of the high-spin Fe in 6-fold and 8-fold coordinated sites (~75% abundance) in the Pmma phase exhibit average saturation internal magnetic fields of H$_{hf}$ ~ 42 T to ~90 GPa, typical of spin-only (orbitally quenched) Fermi-contact values. By contrast average H$_{hf}$ ~ 20 T values, signifying unquenched orbital moments, occur below the 40-45 GPa spin-crossover initiation regime in the Cmcm phase. Therefore site-specific spin crossover invokes a cooperative lattice response and polyhedral distortions at the rest of the high-spin Fe sites, translating to 3d level (sub-band) changes and consequential orbital moment quenching. Near ~90 GPa Fe$_{2}$TiO$_{4}$ is a partially spin-converted chemically ordered Pmma post-spinel structure having a persistent charge gap of ~100 meV. Despite structural symmetry changes, partial spin crossover and lattice compressibility resulting in a ~33% total reduction in unit-cell volume and corresponding 3d bandwidth broadening, strong electron correlations persist at high densification.

The Influence of Strain Rate on the Tensile Properties of a Nb–Ti/Cu Superconducting Composite Wire Under Variable Cryogenic Temperature

The tensile properties of superconducting wires usually play an essential role in practical applications. The effect of strain rate on the tension responses of commercial superconducting composite wires consisting of niobium-47wt.% Ti filaments in a copper matrix (Nb-Ti/Cu) with a Cu:superconductor ratio of ~4.3, under variable cryogenic temperature, is experimentally investigated in this paper. A variable temperature cryostat system is employed, which provides the cooling environment from room temperature to liquid nitrogen temperature, and a compact-scale cryogenic-type extensometer is utilized to measure the tensile strains on the superconducting wires. Under a range of cryogenic temperatures, the corresponding stress and strains are recorded during the elongation of the wire with a broad range of strain rates (10 -1 to 10 -4 s -1 ). The cryogenic mechanical behaviors of the Nb-Ti/Cu composite wires, including the tensile strength and elongation at fracture, as well as the yield stress, are captured experimentally. It is shown that the ultimate tensile strength and the yield strength of the Nb-Ti/Cu superconducting wires increased linearly with cryogenic temperature for the lower strain rates, while notable nonlinear features appear at a higher strain rate. The elongations always increased nonlinearly with decreasing temperature for each tested strain rate, but the elongations decreased with higher strain rate. The Young's modulus approximately increases linearly with decreasing temperature for each strain rate. Additionally, the effect of strain hardening rate derived from the stress and strain relations for the superconducting wires at different cryogenic temperatures and strain rates is discussed.

Measurement of Thermal Expansion of Superconducting Wires at Cryogenic Temperature Based on Tensile Method

The effects of variable cryogenic temperature on the thermal expansion are measured by tensile method for several commercial superconducting composite wires of NbTi/Cu, Nb3Sn/Cu, and high-temperature superconducting (HTS) Bi2212/Ag tapes. A variable temperature cryostat system is constructed to provide successive cooling environment from 300 to 77 K, and a cryogenic-type extensometer is utilized to measure the deformation of the superconducting wires arising from tensile and thermal loadings. In comparison with the mechanical deformation at room temperature, the corresponding thermal deformation at the variable cryogenic temperature is recorded and evaluated during stretching of the superconducting wires. The thermal expansion behavior of the composite wires and superconductor filaments is further captured. It shows that the thermal expansion of NbTi/Cu composite wire, NbTi filaments, Nb3Sn/Cu composite wires, and Nb3Sn filaments is almost linearly dependent upon the temperature, while those of HTS Bi2212/Ag tapes exhibit notable nonlinear features during cooling of the superconducting wires. In addition, based on the thermal expansion measured from room temperature to liquid nitrogen temperature, the thermal expansions at 4.2 K are extrapolated for low-temperature composite wires and their filaments, which are compared with the experiments using thermal methods in the existing literature. The results indicate that our variable temperature cryostat system and corresponding tensile method will be an easy way for measuring the thermal parameter of superconducting composite wires.

Cryogenic Temperature Dependence of Tensile Response of NbTi/Cu Superconducting Composite Wires

The effects of variable cryogenic temperature on the tensile response are reported for the commercial superconducting composite wires consisting of niobium-titanium filaments in a copper matrix (NbTi/Cu). A variable temperature cryostat system is constructed to provide the successive cooling environment from room temperature (RT) to the liquid nitrogen temperature (LNT), and a cryogenic-type extensometer is used to measure tensile strains of the superconducting wires. With the variable cryogenic temperature, the corresponding stress and strain relations are recorded during the wire being stretched. The mechanical behavior of the NbTi/Cu composite wire and pure NbTi filaments, including the tensile strength and elongation at fracture, yield strength, and Young's modulus, are further captured. It is shown that ultimate tensile strengths and Young's moduli of the NbTi/Cu composite wire and pure NbTi filaments are almost linearly dependent on the temperature, whereas the elongation at fracture and the yield strength exhibit notable nonlinear features with the superconducting wires' cooling. In addition, based on the mechanical properties measured from RT to LNT, the properties at the lower temperature of 4.2 K are extrapolated for the NbTi/Cu composite wire and NbTi filaments, which are compared with the experimental data in the literature.

A multiple pin, flip-chip system for microwave and gigabit per second cryogenic device testing at variable temperatures

We have developed a cryogenic probe and cryostat system to test both active and passive superconducting devices and circuits up to microwave frequencies at variable temperatures. Our system consists of two basic parts: the cryostat and the matching probe. The cryostat is a unique, very efficient, variable temperature, flow-type cryostat, where we control temperature without electrical heaters. It is magnetically shielded and allows rapid testing between 4.2 K and room temperature. Probes developed for this cryostat have a larger number of shorter cables than standard dip-type probes. They are designed to test chips or chip assemblies without additional packaging. Chips or chip assemblies can be quickly mounted and dismounted from a fixed spring-contact assembly. Each probe has 56 wide-bandwidth signal cables. We have repeatedly used these probes for testing both active and passive superconducting integrated circuits up to 20 GHz. The probe and cryostat combination provides a testing capability that is simultaneously high frequency, dc magnetic shielded, has variable cryogenic temperature, and quick turnaround.

A variable cryogenic temperature near-field scanning optical microscope

We describe a novel variable (cryogenic) temperature near-field scanning optical microscope (VT-NSOM) designed specifically for submicron imaging of materials and devices over a temperature range of 12–300 K. In high vacuum, we cool only the sample stage of the compact NSOM, thereby maintaining a large scan area (35 μm×35 μm) at low temperatures and enabling rapid (∼30 min) temperature changes. With incorporation into an external conventional optical microscope, the VT-NSOM is capable of imaging a single submicron feature over the entire temperature range. We demonstrate the performance of the instrument by examining the photoresponse of threading dislocation defects in relaxed GeSi films.

Vacuum near-field scanning optical microscope for variable cryogenic temperatures

We describe the design of a novel near-field scanning optical microscope for cryogenic temperatures and operation in vacuum. A helium flow cryostat is used for active temperature control of the sample in the range between 8 and 330 K, while all components of the near-field microscope are kept at room temperature. This design greatly simplifies near-field microscopy at variable sample temperatures and permits large piezoelectric scan ranges of up to 100×100×10 μm 3, the implementation of hardware-linearized piezoelectric scan stages, as well as flexible coarse positioning. First experiments with single GaAs quantum wires demonstrate the excellent performance of this new instrument.

57Fe Mössbauer studies at high pressures and low temperatures

A brief description is given of a new facility that permits57Fe Mossbauer spectroscopy (MS) studies in transmission geometry to be made in the range 10–300 K on absorbers pressurized in a diamond-anvil cell. Examples are given of good quality spectra which have each been obtained in 20–30 hours by using a “point” source of ∼5mCi in conjunction with absorbers that have a comparatively low-enrichment (10–20%) of57Fe. The successful execution of57Fe MS at high pressures and variable cryogenic temperatures has been achieved by making simple refinements to existing elements of high-pressure MS methodology. The facility described herein finds important application in fields of geophysics and condensed matter physics.

57Fe Mössbauer spectroscopy in a diamond-anvil cell at variable high pressures and cryogenic temperatures

A detailed description is given of a new facility for transmission 57Fe Mössbauer spectroscopy (MS) studies on absorbers in a miniature diamond-anvil cell in the temperature range 10–300 K and pressure range 0–45 GPa. Spectra with an adequate signal-to-noise ratio for a reliable analysis to be made have each been measured in 20–30 h of data accumulation time. This has been accomplished in spite of the severe constraints imposed by both the minute sample area and the high attenuation of the 14.4 keV probing radiation by the diamond anvils compared with the experimental conditions of conventional MS experiments. A custom-made Rh(57Co) ‘‘point’’ source with a high specific activity has been used in conjunction with absorbers that have an optimal enrichment (10%–20%) of 57Fe. The successful implementation of 57Fe Mössbauer spectroscopy at very high pressures and variable cryogenic temperatures has been accomplished by making relatively simple refinements to existing elements of high-pressure MS methodology.