Vacuum insulation panel core materials and modelling the thermal conductivity of granular materials
Two potential alternative core materials, diatomaceous earth (DE) and glass bubbles (GB), were considered for cost effective and long life-time vacuum insulation panels (VIP). The pore size distribution of the DE was determined quantitatively by mercury porosimetry and nitrogen sorption. The majority of pores in DE centered around 1 μm. The average pore size of GB was estimated to be 50 μm via scanning electron microscopy. Subsequently, the relationship between pore size and gaseous thermal conductivity was established using the Kaganer relation. Composites of DE and fumed silica (FS) showed remarkable insulating performance whereby thermal conductivity only rose by 11 mW/m.K as pressure increased to 10,000 Pa. A similar analysis on composites of GB and fumed silica showed an increment of 16 mW/m.K. It was estimated that the duration for a typical VIP to reach 10,000 Pa is approximately 50 100 years depending on the type of barrier used. [30] Hence, both composites are capable of providing long vacuum insulating service life. Furthermore, DE/FS composites would greatly reduce the cost of VIPs because of the inexpensive nature of DE. Both composites show superior life-time when compared to glass fibers, which are already commercially used in VIPs.
Thermal Conductivity of Eight Halide Crystals in the Temperature Range 220°K to 390°K
The thermal conductivity of eight halide crystals has been measured over the temperature range 220°K to 390°K. The thermal conductivities of potassium bromide, potassium chloride, and sodium chloride vary inversely with temperature for the lower part of this temperature range, and are nearly independent of temperature for the higher temperatures of the range measured. The thermal conductivities of cesium bromide, cesium iodide, and silver chloride are nearly independent of temperature throughout the entire temperature range. The thermal conductivities of barium fluoride and calcium fluoride vary more rapidly with temperature than anticipated from a T−1 relationship; however, the accuracy of the data is not sufficient to establish the exact form of the variation of the thermal conductivity with temperature. The comparative method used for these measurements is described.
Thermal properties of silica aerogels between 1.4 and 330 K
Thermal properties of monolithic silica aerogels with densities 62–262 kg m −3 in the temperature range T ≈ 1.4−330 K have been examined. The solid thermal conductivity, λ, of aerogels shows the same variation with T as vitreous silica between 100 and 330 K and as Vycor between 20 and 330 K. However, the absolute value for λ is reduced by a factor g ≈ 100−400. Between 5 and 50 K, it is found that λ ∝ T δ , with δ ≈ 1.7. Around 1.4 K, α ≈ 0.8 holds. The data collected for thermal conductivity are carefully corrected for radiative contributions. The temperature dependence of the specific heat, c aerogel ( T ), is comparable to c vitr.silica ( T ) between 5 and 330 K; however, its absolute value is higher by about 30%. Below 5 K, c aerogel / T 3 shows a characteristic increase with decrasing temperature. These results can be correlated with the vibrational density of states derived from inelastic neutron scattering. The measured macroscopic sound velocity, υ, of a 71 kg m −3 aerogel in the cryogenic region is independent of temperature within 5%. Vibrating-reed measurements of Δυ/υ below 1 K show no important contributions from two-level systems. All data can be explained by a model, which takes into account the structural hierarchy of the aerogel.