Cryogenic Media Research Articles

Spectroscopic and Photophysical Properties of the Soluble Cn Molecules, n = 60, 70, 76/78, 84

Abstract:A report is given on molecular carbon research in Los Angeles, starting with review of the molecular precursor approach to carbon materials.1 Regarding larger molecules, mixtures of carbon molecules, Cn, are extracted2 in 25–40 percent yield from graphite evaporated under helium. Gram quantities of the mixture are then separated into pure fractions – C60, C70, C76/C78, C84, and higher (n > 88),3,4 and characterized by mass spectrometry, liquid chromatography, and 13CNMR spectroscopy. Examination of the spectroscopic and photophysical5 properties of the molecules, with particular focus on C60 in solution, reveals: (i) the first transitions appear at high energy, are electronically forbidden (in C60, the homo-lumo hu - t1u transitions), and exhibit resolved vibronic structure; (ii) the spectra sharpen in cryogenic media, allowing vibronic assignment; and (iii) relaxation occurs with near-unity quantum yield to a long-lived triplet, which also transfers energy efficiently to acceptors including O2.

Use of super-high-frequency (SHF) resonators for measuring the parameters of vapor-liquid cryogenic media

Use of super-high-frequency (SHF) resonators for measuring the parameters of vapor-liquid cryogenic media

Thermally activated release of stored chemical energy in cryogenic media

We have observed very rapid spontaneous heating of the cold solid methane moderator when irradiated by fast neutrons at low temperatures in the Intense Pulsed Neutron Source. In one instance enough heat was released to vaporize a fraction of the methane. We have also induced smaller rapid heat releases —both by slowly warming irradiated methane and by diminishing the effectiveness of cooling. An explanation for these observations is that energy stored in the form of reactive species (H, CH3,...) accumulates in solid methane irradiated at low temperatures, and is released by thermally activated diffusion and subsequent reaction. During irradiation, the density of reactive species can build up sufficiently to become unstable as the rate of heat release due to recombination exceeds the rate of heat loss to the cooled surroundings. Similar effects may occur in other cryogenic moderator materials, and may be responsible for the jets observed from cometary nuclei.

Standard system for calibrating and checking level gauges for cryogenic media under working conditions

Standard system for calibrating and checking level gauges for cryogenic media under working conditions

Calculation of conditions of transition to characteristic flow regimes of two-phase cryogenic medium in tubes and annular channels

Calculation of conditions of transition to characteristic flow regimes of two-phase cryogenic medium in tubes and annular channels

Metrological support to means of measuring flow rate for liquid cryogenic media

Metrological support to means of measuring flow rate for liquid cryogenic media

Tracking secondary transducer for a cryogenic liquid level gauge

The authors have developed a tracking level gauge that allows one to lock onto a particular resonance, giving remote sensing as the first advantage and high conversion accuracy as the second, in which the frequency of the FM oscillator is adjusted by a peak-holding regulation system to be equal to the sensor frequency. This paper shows the structural diagram and explains how the guage works. This structure was used in the basic module for a multirange level gauge for cryogenic media type URK-1, which is now being regularly produced at the Ryazan Teplopribor plant. The overall error in converting the resonant frequency to digital form in the URK-1 is not more than 2 x 10/sup -4/, while the frequency shift in the sensor in measuring a liquid nitrogen level is about 16%.

Electric Strength of Polymers Under Vacuum at Cryogenic Temperatures

Abstract The electric strength of solid polymers under high vacuum is investigated at a temperature ranging from 293°K down to that of liquid helium (5°K) under power frequency alternating- and direct-voltages. The electric strength of polymers was found to be very dependent on the pre-breakdown partial discharges when measured in cryogenic medium whether gas or liquid. When helium was used as coolant, discharges may extend very far from the electrodes and thus exerted high electric stresses on large areas of the sample under test. These discharges may produce flash-over and serious troubles. It may also lead to localised breakdown of polymers which may give apparent reduction in dielectric strength. A large increase in the electric strength of polymers was observed when used at cryogenic temperatures under vacuum; the increase was very significant for polyethylene and polyimide specially under direct voltages. The results showed promising properties and it is concluded that the high vacuum will offer the...

Cryogenic level-measuring calibration system with thermal stabilization of the transducer mounting

Existing calibration systems (float type, photoelectric with direct readout, etc.) for certifying gauges for warm liquids cannot be used in certifying cryogenic gauges on account of the specific features of cryogenic media: low boiling points (from 4.2 to 90~ and instability in the pressure, temperature, dielectric constant, etc. Therefore, the best method is to use semiconductor transducers in the calibration system. Semiconductor resistance thermometers are suitable for the purpose [I, 2]. In particular, one can use any semiconductor component with a nominal crystal diameter of 1.0-2.0 mm and a protective coating thickness not more than 0.05-0.1 mm in liquid nitrogen. These transducers have direct heating, and the output signal is displayed via a threshold relay. The direct heating of the sensitive element and the choice of relay threshold determine the mode of operation, which is based on the difference in heat transfer in liquid and vapor.

The antifriction properties of friction couples of metal and materials based on Teflon in cryogenic media

The antifriction properties of friction couples of metal and materials based on Teflon in cryogenic media

Dielectric strength of polymers at cryogenic temperatures under vacuum

Dielectric strength of solid insulation (especially polymers) when measured in a cryogenic medium (gas or liquid) is found to be highly dependent upon pre-breakdown discharges. These discharges may act in two different ways. On the one hand, especially when a fluid like helium. (whose dielectric strength is poor) is concerned, discharges may extend very far from the electrodes and thus involve high electric stresses upon large areas of the samples under test. In extreme cases, these discharges can even produce flash-over. On the other hand, discharges may lead to important deterioration and consequently localised reductions in dielectric strength. In order to reduce the effect of these discharges on the apparent dielectric strength observed in cryogenic media, the behaviour of solid insulation under high vacuum was studied. Special care was taken to minimize the effects of pre-breakdown discharges. A large increase of dielectric strength was observed in relation to certain polymers, especially polyethylene-terephthalate. The results are in good agreement with values published by different authors as intrinsic strength values. It is concluded that high vacuum may afford the solution to many insulation problems occuring at cryogenic temperatures.

A cryostat for Mössbauer experiments in the temperature range from 4 to 300° K

A DESCRIPTION is given of a gas-cooled, temperature controlled cryostat for use in Mossbauer experiments. Cold gas is supplied to the cryostat by evaporating cryogenic liquids (helium, hydrogen, or nitrogen). When cooling of source and absorber is required in a Mossbauer experiment, the velocity drive can easily be mounted within the cryostat. Theoretical and experimental values of heat influx and consumption of liquid helium are reported. The cryostat needs about 100 cm^/hr of liquid helium at 10° K and about 35 cmVhr at 75° K. In many cases, Mossbauer experiments must be carried out at low temperatures. Usually, extremely stable temperatures are not required and the heat input caused by the y-ray source is nearly zero. No serious problems concerning heat transfer from the cryogenic medium to the samples arise. Temperature flexibility is the most important consideration. Temperatures from the liquid helium range up to room temperature and above should be available continuously, and it should be possible to keep the source and absorber at the same temperature or at different ones. These considerations led us to the construction of the cryostat described below. The first such cryostat has been in use for more than two years and has operated quite well. A communication on this cryostat seems timely in that most physicists working with the Mossbauer effect are more or less unfamiliar with cryogenic equipment.

Compact impulse generator

An unconventional Stirling engine is proposed and its theoretical analysis is performed. The engine belongs to a “cryogenic heat engine” that is fueled by cryogenic medium. Conventional “cryogenic heat engine” employs liquid air as pressure source, but disregards its heat-absorbing ability. Therefore, its efficiency can only be improved by increasing vapor pressure, accordingly increasing the demand on pressure resistance and sealing. In the proposed engine, the added Stirling mechanism helps achieve its high efficiency and simplicity by utilizing the heat-absorbing ability of liquid air. On one hand, based on Stirling mechanism, gas in the hot space absorbs heat from atmosphere when expanding; gas in the cold space is cooled down by liquid air when compressed. Taking atmosphere as heat source and liquid air as heat sink, a closed Stirling cycle is formed. On the other hand, an exhaust port is set in the hot space. When expanding in the hot space, the vaporized gas is discharged through the exhaust port. Thus, an open cycle is established.To model and analyze the system, the Schmidt theory is modified to describe temperature variation in the cold space, and irreversible characteristic of regenerator is incorporated in the thermodynamic model. The results obtained from the model show that under the same working pressure, the efficiency of the proposed engine is potentially higher than that of conventional ones and to achieve the same efficiency, the working pressure could be lower with the new mechanism. Its efficiency could be improved by reducing temperature difference between the regenerator and the cold/hot space, increasing the swept volume ratio, decreasing the liquid–gas ratio. To keep outputting work, the leading angle should be set in the region [0, π].