High Temperature Thermal Properties
For temperatures other than ambient, the thermal performance and the
relative variation with pressure changes in a complex but predictable fashion. Heat transfer through these porous materials
occurs via the sum of three different mechanisms; 1) solid phase conduction, 2) gas phase conduction, and 3) radiation. As
the operating temperature increases, solid phase conduction increases with a fairly weak temperature dependence. In the gas
phase conduction, two changes occur: First, the thermal conductivity of the bulk gases increases in almost direct proportion
to absolute temperature. Secondly, increasing temperature results in an increase in the mean free path of the gas. Therefore,
higher temperatures yield higher bulk gas conductivity but this is partly offset by a reduction in the effective gas phase
conduction at a particular pressure level due to a greater percentage of molecules striking the pore walls rather than other
gas molecules. The third component, radiation, can be highly temperature dependent depending upon whether the insulation is
sufficiently opacified with infrared absorbing and/or scattering materials. A graph of both thermal conductivity and R-value
for NanoPore™ based insulation products and it's relation to temperature is shown in the figure below (note temperatures are in Kelvin).
For the lowest temperature, performance improves at all pressure values because of reduced solid phase conduction, gas phase
conduction, and radiation. However, the characteristic decrease with pressure from the Knudsen effect actually shifts to lower
pressures because of the shorter mean free path (~20 nm at 100 K versus ~60 nm at 300 K). At higher temperatures and moderate
vacuum (>100 mbar), the thermal performance is virtually independent of temperature since the higher bulk gas conduction, solid
phase conduction, and radiation terms are balanced by a decrease in the effective gas phase conductivity arising from the enhanced
Knudsen effect caused by the larger mean free path.
NanoPore™ Thermal Insulation for High Temperature
NanoPore™ HP-150 can be used at temperatures up to 300°C, but for higher temperature use NanoPore produces a special high temperature
insulation, NanoPore™ HT-170 which can be used up to 800°C and above in some cases. For applications with highly specific performance
requirements, custom grades of NanoPore™ Thermal Insulation can provided to meet a project's special needs.
High Temperature Implications for VIPs
NanoPore™ VIPs can be used at temperatures below 120-150°C, depending on the required lifetime of the product. Special high
temperature barrier materials can be used for longer lifetimes at high temperature but generally speaking higher temperature
applications will shorten VIP lifetime.
A key property of a VIP insert material, other than its’ thermal performance, is the filler's mechanical stability under a one
bar load at the operating temperature. Changes in physical dimensions during use at temperature results in loss of thermal
performance, higher probability of premature barrier failure, and loss of mechanical integrity for the entire insulation system.
NanoPore based insulation products exhibit excellent mechanical stability at temperature with almost no thermal expansion problems
or shrinkage and collapse that can occur polyurethane or open-cell polystyrene based VIPs which can occur at temperatures as low as 80°C.
This stability is not surprising when one considers that slightly modified versions of NanoPore™ are used as insulation at very high