Superinsulating materials

A certain superinsulation material having a thermal conductivity of 2 x 0 4 W/m - °C is used to insulate a tank of liquid nitrogen that is maintained at - 320°F 85.8 Btu is required to vaporize each pound mass of nitrogen at this temperature. Assuming that the tank is a sphere having an inner diameter (ID) of 2 ft, estimate the amount of nitrogen vaporized per day for an insulation thickness of 1.0 in and an ambient temperature of 70°F. Assume that the outer temperature of the insulation is 70°F. 

A superinsulating material is to be constructed of polished aluminum sheets separated by a distance of 0.8 mm. The space between the sheets is sealed and evacuated to a pressure of 10"5 atm. Four sheets are used. The two outer sheets are maintained at 35 and 90°C and have a thickness of 0.75 mm, whereas the inner sheets have a thickness of 0.18 mm. Calculate the conduction and radiation transfer across the layered section per unit area. For this calculation, allow the inner sheets to float in the determination of the radiation heat transfer. Evaluate properties at 65°C. 

In a similar spirit, but without any consideration about covalent cross-linking, dry nanostructured silicas have been also used for the synthesis of some sifica/polyurethane composites [48]. Silica nanopowders (e.g., finely divided silicas with characteristic dimensions between 10 nm and 100 pm) have been dispersed in various polyurethane sols. After gelation in the presence of these inorganic fillers, the polyurethane-based composites can be considered as superinsulating materials with thermal conductivities lower than 0.020 W/m K. 

Despite those differences, it appears clear that those two aerogel families present a common huge potential for hybridization with their nanostructured silica homologues, which could lead to, among others, brand new reinforced superinsulating materials. 

Table 26.1. Overview of insulation materials sorted by their ambient condition thermal conductivities conventional (red) and superinsulation materials and components (turquoise) can be classified by their respective thermal conductivity values. Phenolic and polyurethane foams mark a transition area between those two families (pink)...

High-performance insulation A common synonym for superinsulation. Materials and/ or systems with superior thermal insulation performance when compared with conventional... 

Superinsulation Insulation systems based on the use of superinsulating products and/or components and/or materials. A superinsulating materials is commonly defined by a thermal conductivity lower than the one of air (e.g., 0.025 W/m K in room conditions) and more recently lower than 0.020 W/m K... 

Aerogels are probably the best known superinsulation materials with thermal conductivity values as low as half of the value of standing air (0.025 W m ... 

The measurement of very low thermal conductivities is done directly by equilibrium methods, where typically a constant heat flux is measured to maintain a given temperature difference between a hot and a cold side. Dynamic methods rely on a transient heat pulse or wave that is sent from a material interface and travels over a known distance to reach a detector. Indirect methods then rely on physical models to calculate the thermal conductivity based on heat diffusion equations. A detailed review on the physics of heat transport in aerogels was given by Ebert [203] in the aerogels handbook. Various theoretical models exist, which allow one to determine the effective thermal conductivity of superinsulation materials based on dynamic measurement methods. 

Thermal Insulation. In addition to their low thermal conductivity, as discussed in the section above, siUca aerogels can be prepared to be highly transparent in the visible spectmm region. Thus, they are promising materials as superinsulating window-spacer. To take further advantage of its... 

More advanced insulations are also under development. These insulations, sometimes called superinsulations, have R that exceed 20 fthh-°F/Btu-m. This can be accomplished with encapsulated fine powders in an evacuated space. Superinsulations have been used commercially in the walls of refrigerators and freezers. The encapsulating film, which is usually plastic film, metallized film, or a combination, provides a barrier to the inward diffusion of air and water that would result in loss of the vacuum. The effective life of such insulations depends on the effectiveness of the encapsulating material. A number of powders, including silica, milled perlite, and calcium silicate powder, have been used as filler in evacuated superinsulations. In general, the smaller the particle size, the more effective and durable the insulation packet. Evacuated multilayer reflective insulations have been used in space applications in past years. 

Silica aerogels, a newly developing type of material, also have been produced as thermal insulations with superinsulation characteristics. The nanometer-size cells limit the gas phase conduction that can take place. The aerogels are transparent to visible light, so they have potential as window insulation. The use of superinsulations at present is limited by cost and the need to have a design that protects the evacuated packets or aerogels from mechanical damage. 

To reduce the radiation power input, several solutions are possible inserting n thermally floating shields between the cold and hot surfaces, the transmitted power is reduced by a factor (n + 1). The practical realization is the so-called superinsulation used in the dewar of Fig. 5.1 a few layers of a thin metallized insulating foil about 4 xm thick is used. To prevent thermal contact between adjacent layers, the material is often corrugated or a thin layer of fibreglass cloth is inserted between layers. 

Physical situations that involve radiation with conduction are fairly common indeed. Examples include heat transfer through superinsulation made up of separated layers of very reflective material, heat transfer and temperature distributions in satellite and spacecraft structures, and heat transfer through the walls of a vacuum flask. 

The choice of materials can be broken down into those that are used to make mechanical and electrical connections to the STM from ambient temperature and those used at low temperature for the STM itself and connections to it. Since the sample and tip need to be cooled, then stabily held at that fixed depressed temperature, the STM has to be placed in a controlled cryogenic environment. A superinsulated Dewar is used to house the STM to eliminate vibrations due to boiling liquid nitrogen used in the heat shield of glass Dewars. The STM can be immersed directly in the cryo-fluid, enclosed in a separate vacuum can suspended in the fluid, or transferred from a EIHV environment... 

There are several features of these superinsulations that are attractive for modern applications. One of these features is, obviously, reduced thermal conductivity. This permits storing or transporting larger quantities of product within the same size envelope, which could be very important in the barge shipment of liquid hydrogen. Another feature is that these insulations are easily evacuated and do not release foreign material or contaminants into the vacuum space. This property is important to space chambers, where it is desirable to insulate the cryopanels to reduce the refrigeration load. 

Since it is necessary to construct tanks of this type by field-erected techniques rather than to shop-fabricate and ship to the field, it was obvious that standard methods of applying multilayer insulation could not be used. The inner vessel could not be rotated around its axis to spirally wind these materials. In addition, the construction schedule required installation of the superinsulation within a seven-day time period. To meet these stringent requirements, quilted superinsulation, a new technique of applying multilayer insulations, covered by U.S. patents 3,007,596 3,009,600 3,009,601 and patents pending was developed. 

There have also been efforts to develop products with adaptive properties— namely, materials that change their porosity or thermal insulation in dependence of external stimuli like temperature or humidity (Crespy and Rossi, 2007 Hu et al., 2012). Materials with heat absorbing capacities like phase change materials can be found for niche applications (Xu et al., 2013 Yoo et al., 2013), but their effect seems to be too limited to get large acceptance by the consumers. Materials with very low thermal conductivity ( superinsulators ) have been used for several years in the construction... 

The strut is covered by layers of superinsulation to minimize the radiation heat leak and is cryogenically shielded by two dis -crete shields along its length. For practical purposes, the strut can be considered adiabatic between the heat shields and between the heat shields and the ends. We chose two equally spaced intermediate cooling stations at 11 K and 70 K based on refrigeration power optimization studies carried out for similar composite materials. ... 

Duer K, Svendsen S (1998) Monolithic siUca aerogel in superinsulating glazings Sol Energy 63 259-267 Husing N, Schubert U (1998) Aerogels - Airy materials Chemistry, stracture, and properties. Angew Chem Int Ed 37 23 5... 

Figure 10.17. Remarkably, low values of thermal conductivity have been obtained with these materials (e.g., as low as 0.014 W/m K as calculated from the product of density, thermal diffiisivity and specific heat), suggesting them as very promising candidates for themial superinsulation at atmospheric pressure.

Abstract The present chapter is focused on describing the intimate link which exists between aerogels and thermal superinsulation. For long, this applied field has been considered as the most pronusing potential market for these nanostructured materials. Most likely this old vision will become reality in the near future. 

Following a short presentation of the global need for superinsulation together with a closer look at the specific situation in the building sector, we propose within this synopsis a brief analysis of (1) the world s insulation markets, (2) superinsulating aerogel materials and their alternatives, (3) commercial aerogel insulation products available today, and (4) our estimation of their most likely applications worldwide in the future. We conclude this chapter with some first considerations on health, toxicity, and environmental aspects. 

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