Thermal contraction, cryogenics

Different structural materials have different thermal contraction coefficients, meaning that accommodations should be made for their different dimensions at cryogenic temperatures. If not, problems associated with safety (e.g., leaks) may arise. Generally, the contraction of most metals from room temperature (300 K) to a temperature close to the liquefaction temperature of hydrogen (20 K) is <1%, whereas the contraction for most common structural plastics is from 1% to 2.5% [23]. 

The low-temperature thermal conductivity of different materials may differ by many orders of magnitude (see Fig. 3.16). Moreover, the thermal conductivity of a single material, as we have seen, may heavily change because of impurities or defects (see Section 11.4). In cryogenic applications, the choice of a material obviously depends not only on its thermal conductivity but also on other characteristics of the material, such as the specific heat, the thermal contraction and the electrical and mechanical properties [1], For a good thermal conductivity, Cu, Ag and A1 (above IK) are the best metals. Anyway, they all are quite soft especially if annealed. In case of high-purity aluminium [2] and copper (see Section 11.4.3), the thermal conductivities are k 10 T [W/cm K] and k T [W/cm K], respectively. 

S. Pattanayak, S. Kanagaraj Thermal contraction of FRPs and its measurements at cryogenic temperature. Proceedings of Beijing International Cryogenic Conference, p. 185 (2000)... 

The final electrical connections to the STM can be done with copper wires. A small amount of helium is used as an exchange gas to anchor the temperature of the whole assembly to the cryogenic fluid. The body of the STM can be made out of copper, which will respond quickly to temperature changes for variable temperature measurements and provide a uniform temperature environment for the tunnel junction. One has to estimate the differential thermal contraction of the component parts to make sure that a tunnel junction separation set at room temperature is sufficiently large to prevent tip crash on cooling. Other materials like Macor or Invar , which closely match the thermal expansion properties of the piezoelectric transducers, are used as well but take more time to thermally stabilize. Some references are given in [6.30-6.43]... 

Low temperatures can also affect materials by thermal contraction. The thermal expansion coefficient is a function of temperature. For many materials, which are cooled down from room to cryogenic temperature, more than 90 % of the total contraction experienced will have already taken place at 77 K. Rule-of-thumb figures of thermal contraction are 0.3 % in iron-based alloys, 0.4 % in aluminum, or over 1 % in many plastics [43]. Cryogenic vessels or piping systems must account for this contraction to avoid large thermal stresses. 

Any calorimeter designed for cryogenic work is subject to thermal contraction of one or both of the insulation boundary surfaces. For the calorimeter at hand, the geometric modification due to thermal expansion and contraction was calculated using data of Corruc-cini and Gniewek [ ]. Calculations show that an error as high as 4.6% can be introduced into the results unless the expansion and contraction phenomena are taken into account in evaluation of the data [ ]. 

While there are apparent temperature dependent efiects on the LAD screen itself (pores may shrink at reduced temperatures) due to apparent higher FTS pressure drop with reduced temperatures, recent comprehensive analysis conducted by Darr et al. (2015) show that the screen pores do not appreciably shrink at cryogenic temperatures, and that screen properties vary hy less than 1% at LH2 temperatures. For example, the coefficient of thermal contraction can be used to show physical screen property variation with temperature ... 

All of these systems share to some degree several typical design problems associated with cryogenic liquid transfer. One class of difficulties results from cooling the system down from ambient to cryogenic temperature. Evidence of cooldown is in the form of two-phase flow, thermal contraction, and line bowing. Thermal contraction of a transfer line must not result in contact between the inner and outer lines, a condition most frequently encountered at changes in direction of the transfer lines. Expansion joints, bellows, and U-bends have been employed to solve the problem of thermal contraction. 

Another concern of a cryogenic transfer line designer is thermal contraction because the inner line is cooled from ambient to cryogenic temperatures while the outer line remains at room temperature. The degree of thermal contraction from room temperature to liquid hydrogen temperature for a copper pipe is about 0.033 m for each 10 m of pipe length. 

The estimated uncertainty in cryogenic flow measurement using head-type meters ranges from 1 to 3%. This is composed of the uncertainty in bias shift caused by thermal contraction of the material, uncertainty in the effect of increased Reynolds number, and a large imprecision traceable to the methods of pressure measurement and pressure tap design. 

At cryogenic temperatures, mechanical properties are influenced by stresses arising from differential thermal contractions, and thermal transport phenomena are influenced by phonon mismatch at filler-matrix boundaries. 

Thermal Contraction. Thermal contraction at low temperatures is determined by the push-rod method measuring the elongation outside a cryostat, direct measurement on the sample within a cryostat by capacitive strain gauges, and reflective or interferometric methods. The last method involves cryogenic problems (see Fig. 12). A laser beam is directed onto a tubular specimen fixed within a cryostat. A mirror and a transparent mirror are placed on the bottom and top of the specimen, respectively. Both reflected beams are focused on an... 

The three rear lens elements are to be used at cryogenic temperature, and the manufacturing specifications must account for both the change in index of refraction and the geometric contraction with temperature. This is straightforward for the spherical surfaces but not for the asphere. Li practice, the problem was solved by manual iteration. Trial lens constants were supplied, and the sag after thermal contraction was calculated and compared to the sag desired. Adjusting the constants eventually led to a lens that will contract to the desired cold shape. 

A second important factor is the difficulty of the experiment. Some measurements, such as liquid densities at ambient temperatures, provide accurate data easily, but in other cases experiments may be difficult or infeasible. Complications that would argue against experiments include high temperatures and/or pressures (or very low temperatures or pressures requiring cryogenic or vacuum equipment) chemicals that are unstable, corrosive, or toxic and chemicals that are not available in sufficient purity or quantity. Some properties, such as high-pressure phase equilibria and the thermal conductivity of polar fluids, are more difficult to measure accurately. A few laboratories do have capabilities for more difficult measurements, so the option of contracting with such a lab for measurements may be considered. 

Thermistors are usually made from ceramic metal oxide semiconductors, which have a large negative temperature coefficient of electrical resistance. Thermistor is a contraction of thermal-sensitive-resistor. The recommended temperature range of operation is from -55 to 300°C. The popularity of this device has grown rapidly in recent years. Special thermistors for cryogenic applications are also available [12]. 

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