Behaviour of power MOSFETs at cryogenic temperatures

In this paper an investigation is reported on Siemens-power-metal-oxide-semiconductor (SIPMOS) transistors of both p and n channel types, for their suitability for cryogenic applications. The drain characteristics, temperature dependence of Rds(on) and switching behaviour have been studied in the temperature range 4.2 – 300 K in BSS91 and BSS92 MOSFETs. The experiments reveal that these types of power transistors are well suited for operations down to ≈ 30 K. However, below 30 K the operating characteristics make them unsuitable for application. This arises because of carrier freeze-out in the n− region on the substrate, which forms a drain.

Hybridization of carbon-glass epoxy composites: An approach to achieve low coefficient of thermal expansion at cryogenic temperatures

The paper is based on a study to develop carbon-glass epoxy hybrid composites with desirable thermal properties for applications at cryogenic temperatures. It analyzes the coefficient of thermal expansion of carbon-epoxy and glass-epoxy composite materials and compares it with the properties of carbon-glass epoxy hybrid composites in the temperature range 300 K to 125K. Urethane modified epoxy matrix system is used to make the composite specimens suitable for use even for temperatures as low as 20K. It is noted that the lay-up with 80% of carbon fibers in the total volume fraction of fibers oriented at 30 degrees and 20% of glass fibers oriented at 0 degrees yields near to zero coefficient of thermal expansion as the temperature is lowered from ambient to 125 K. (c) 2010 Elsevier Ltd. All rights reserved.

Deformation and strength of Ti-6Al-4V alloyed with B at cryogenic temperatures

Plastic deformation and strength of Ti-6Al-4V (Ti64) alloyed with minor additions of B at cryogenic temperatures were investigated through unnotched and notched tensile tests at 20 and 77 K Marked microstructural refinement that occurs with the trace addition of B to Ti64 was exploited for examining the role of microstructural length scales on the cryogenic plastic deformation. The tensile tests were complemented with detailed microstructural characterisation using transmission electron microscopy and electron back scattered diffraction imaging of the deformed specimens. Experimental results show that the addition of 0.30 wt% and above of B to Ti64 reduces ductility, and in turn enhances the notch sensitivity to the extent that those alloys become unsuitable for low temperature applications. However, the addition of similar to 0.10 wt% B is beneficial in enhancing the low temperature strength. An examination of the yield strength variation at various temperatures reveals that at 77 K, the colony size determines the yield strength of the alloy, just as it does at room temperature; implying dislocation-mediated plasticity continues to dominate up to 77 K At 20 K however, twinning dominates the flow response, with the activation of {11 (2) over bar1} and {5 (6) over bar1 (3) over bar} twinning in addition to {10 (1) over bar2} in the base alloy resulting in enhanced ductility of it as compared to either B-modified alloys at 20 K or the base alloy itself at 77 K The observation of a reasonable correlation between the lath aspect ratio, given by the colony-to-lath thickness ratios, and yield strength variation at 20 K suggests that coarse colony size in the base alloy allows for the activation of additional twinning mechanisms. (C) 2014 Elsevier B.V. All rights reserved.

Dielectric properties of Pb Nb2 O6 ferroelectric ceramics at cryogenic temperatures

Complex dielectric permittivity measurements in Pb Nb2 O6 ceramics were performed in a frequency and temperature range of 1 kHz-1 MHz and from 15 to 900 K, respectively. The results revealed two dielectric anomalies showing typical characteristics of relaxor ferroelectric materials at cryogenic temperatures. Comparison with other tetragonal tungsten bronze (TTB) structure-type materials suggests the existence of successive phase transitions, which until now were not reported. The observed low temperature dielectric behaviors seem to be due to intrinsic physical characteristics related to the TTB structure. © 2007 American Institute of Physics.

Particle tracking at cryogenic temperatures: the Fast Annihilation Cryogenic Tracking (FACT) detector for the AEgIS antimatter gravity experiment

The AEgIS experiment is an interdisciplinary collaboration between atomic, plasma and particle physicists, with the scientific goal of performing the first precision measurement of the Earth's gravitational acceleration on antimatter. The principle of the experiment is as follows: cold antihydrogen atoms are synthesized in a Penning-Malmberg trap and are Stark accelerated towards a moiré deflectometer, the classical counterpart of an atom interferometer, and annihilate on a position sensitive detector. Crucial to the success of the experiment is an antihydrogen detector that will be used to demonstrate the production of antihydrogen and also to measure the temperature of the anti-atoms and the creation of a beam. The operating requirements for the detector are very challenging: it must operate at close to 4 K inside a 1 T solenoid magnetic field and identify the annihilation of the antihydrogen atoms that are produced during the 1 μs period of antihydrogen production. Our solution—called the FACT detector—is based on a novel multi-layer scintillating fiber tracker with SiPM readout and off the shelf FPGA based readout system. This talk will present the design of the FACT detector and detail the operation of the detector in the context of the AEgIS experiment.

Vertical conduction properties of few-layer epitaxial graphene / n-type 4H-SiC heterojunctions at cryogenic temperatures

Vertical diodes of epitaxial graphene on n 4H-SiC were investigated. The graphene Raman spectraexhibited a higher intensity in the G-line than the 2D-line, indicative of a few-layer graphene film.Rectifying properties improved at low temperatures as the reverse leakage decreased over six ordersof magnitude without freeze-out in either material. Carrier concentration of 10 16 cm 3in the SiCremained stable down to 15 K, while accumulation charge decreased and depletion width increasedin forward bias. The low barrier height of 0.08 eV and absence of recombination-induced emissionindicated majority carrier field emission as the dominant conduction mechanism.

The structural permeability of concrete at cryogenic temperatures

The thesis describes a programme of research designed to identify concretes for application at cryogenic temperature, in particular for storage of Liquefield Natural Gas which is maintained at a temperature of -165oC. The programme was undertaken in two stages. Stage 1 involved screening tests on seventeen concrete mixes to investigate the effects of strength grade (and water/cement ratio), air entrainment, aggregate type and cement type. Four mixes were selected on the basis of low temperature strength, residual strength after thermal cycling and permeability at ambient temperature. In Stage 2 the selected mixes were subjected to a comprehensive range of tests to measure those properties which determine the leak tightness of a concrete tank at temperatures down to -165oC. These included gas permeability; tensile strength, strain capacity, thermal expansion coefficient and elastic modulus, which in combination provide a measure of resistance to cracking; and bond to reinforcement, which is one of the determining factors regarding crack size and spacing. The results demonstrated that the properties of concrete were generally enhanced at cryogenic temperature, with reduced permeability, reduced crack proneness and, by virtue of increased bond to reinforcement, better control of cracking should it occur. Of the concretes tested, a lightweight mix containing sintered PFA aggregate exhibited the best performance at ambient and cryogenic temperature, having appreciably lower permeability and higher crack resistance than normal weight concretes of the same strength grade. The lightweight mix was most sensitive to thermal cycling, but there was limited evidence that this behaviour would not be significant if the concrete was prestressed. Relationships between various properties have been identified, the most significant being the reduction in gas permeability with increasing strain capacity. The structural implications of the changing properties of the concrete have also been considered.

Cryogenic Silicon Optical Reference Cavities

Thermodynamical fluctuations in temperature and position exist in every physical system, and show up as a fundamental noise limit whenever we choose to measure some quantity in a laboratory environment. Thermodynamical fluctuations in the position of the atoms in the dielectric coatings on the mirrors for optical cavities at the forefront of precision metrology (e.g., LIGO, the cavities which probe atomic transitions to define the second) are a current limiting noise source for these experiments, and anything which involves locking a laser to an optical cavity. These thermodynamic noise sources scale physical geometry of experiment, material properties (such as mechanical loss in our dielectric coatings), and temperature. The temperature scaling provides a natural motivation to move to lower temperatures, with a potential huge benefit for redesigning a room temperature experiment which is limited by thermal noise for cryogenic operation.

We design, build, and characterize a pair of linear Fabry-Perot cavities to explore limitations to ultra low noise laser stabilization experiments at cryogenic temperatures. We use silicon as the primary material for the cavity and mirrors, due to a zero crossing in its linear coefficient of thermal expansion (CTE) at 123 K, and other desirable material properties. We use silica tantala coatings, which are currently the best for making high finesse low noise cavities at room temperature. The material properties of these coating materials (which set the thermal noise levels) are relatively unknown at cryogenic temperatures, which motivates us to study them at these temperatures. We were not able to measure any thermal noise source with our experiment due to excess noise. In this work we analyze the design and performance of the cavities, and recommend a design shift from mid length cavities to short cavities in order to facilitate a direct measurement of cryogenic coating noise.

In addition, we measure the cavities (frequency dependent) photo-thermal response. This can help characterize thermooptic noise in the coatings, which is poorly understood at cryogenic temperatures. We also explore the feasibility of using the cavity to do macroscopic quantum optomechanics such as ground state cooling.

Improved neutron radiation hardness for Si detectors: Application of low resistivity starting material and or manipulation of N-eff by selective filling of radiation-induced traps at low temperatures

Radiation-induced electrical changes in both space charge region (SCR) of Si detectors and bulk material (BM) have been studied for samples of diodes and resistors made on Si materials with different initial resistivities. The space charge sign inversion fluence (Phi(inv)) has been found to increase linearly with the initial doping concentration (the reciprocal of the resistivity), which gives improved radiation hardness to Si detectors fabricated from low resistivity material. The resistivity of the BM, on the other hand, has been observed to increase with the neutron fluence and approach a saturation value in the order of hundreds k Omega cm at high fluences, independent of the initial resistivity and material type. However, the fluence (Phi(s)), at which the resistivity saturation starts, increases with the initial doping concentrations and the value of Phi(s) is in the same order of that of Phi(inv) for all resistivity samples. Improved radiation hardness can also be achieved by the manipulation of the space charge concentration (N-eff) in SCR, by selective filling and/or freezing at cryogenic temperatures the charge state of radiation-induced traps, to values that will give a much smaller full depletion voltage. Models have been proposed to explain the experimental data.

Improved neutron radiation hardness for Si detectors: Application of low resistivity starting material and or manipulation of N-eff by selective filling of radiation-induced traps at low temperatures

Radiation-induced electrical changes in both space charge region (SCR) of Si detectors and bulk material (BM) have been studied for samples of diodes and resistors made on Si materials with different initial resistivities. The space charge sign inversion fluence (Phi(inv)) has been found to increase linearly with the initial doping concentration (the reciprocal of the resistivity), which gives improved radiation hardness to Si detectors fabricated from low resistivity material. The resistivity of the BM, on the other hand, has been observed to increase with the neutron fluence and approach a saturation value in the order of hundreds k Omega cm at high fluences, independent of the initial resistivity and material type. However, the fluence (Phi(s)), at which the resistivity saturation starts, increases with the initial doping concentrations and the value of Phi(s) is in the same order of that of Phi(inv) for all resistivity samples. Improved radiation hardness can also be achieved by the manipulation of the space charge concentration (N-eff) in SCR, by selective filling and/or freezing at cryogenic temperatures the charge state of radiation-induced traps, to values that will give a much smaller full depletion voltage. Models have been proposed to explain the experimental data.