Thermal superinsulation 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. 

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. 

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. 

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... 

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. 

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. 

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... 

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.

Based on recent developments in the field, it can be stated that aerogels still offer the greatest potential for nonevacuated superinsulation systems and consequently must be considered as an amazing opportunity for sustainable development. This chapter of the handbook bridges the gap between those dealing with thermal insulation properties of aerogel materials in general (Chap. 21) and the various commercial products described in Part XV. 

VIP and VG offer outstanding thermal resistance because evacuation of the porous core material or the glazing cavity, respectively, results in a drastic reduction of heat transport by gas molecules. Aerogels on the contrary are nonevacuated superinsulators. Their low thermal conductivity is correlated with the pore structure of these materials. Before we briefly touch on the effect of the unique structural features of aerogels on heat transport, let us recapitulate the basics. Generally, heat is transferred by conduction, convection, and radiation. In porous materials there are five possible contributions to the total heat transfer, namely ... 

Most commercially relevant superinsulating Si02 aerogels have densities between 80 and 200 kg m. Their thermal conductivity values are dominated by conduction through the solid silica particle network at high densities and a combination of radiation and gaseous conduction through the air inside the pores at low densities. To produce the lowest conductivity materials, it is necessary to find an optimum between those two. It seems apparent that... 

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