319
is 237 W/(mK). Overall thermal conductivity of porous materials
depends on convection in the pores, conduction in the pores, conduction in the solid, and radiation. Heat transfer in porous materials is very complex, and individual mechanisms are complex and
cannot be easily summarized. Briefly, conduction in a porous material increases with the increase of pressure around it and inside the
pores; convective heat transfer increases with the increase of the
motion of air inside the pores; and radiation increases significantly
with an increase in temperature and with an increase in pore size.
Hence overall thermal conductivity decreases with a decrease of the
pressure inside the pores, a decrease in temperature, and a decrease
of pore diameter. Conduction in the solid part of the porous matrix
is defined by the type and amount of material used. Direct material
conduction effects are reduced due to the very large percentage of
porosity normally present.
Designs intended to improve insulation qualities depend on
varying these several parameters. In buildings, temperature levels
are given design parameters that cannot be varied much. Pore
diameter can be varied with material interventions. In some device
designs using porous materials, the pressure inside the pores can
be varied. There are many advantages to introducing nanoscale
pores. In nanoporous materials, convection inside the pores is
largely negligible due to the small space available for the motion
of molecules. The “mean-free path” (see Section 4.4) of air molecules is larger than the size of pores. So, though in conventional
porous materials molecules of gas mainly collide with each other
inside the pores, inside nanopores they also constantly collide
with the pore wall, which leads to suppressed gas conduction.
Conduction in the gas will also decrease with any decreases of
pressure that can be obtained. Ideally, a vacuum inside the pores
produces the best insulating properties. Practically, the smaller
the pores become, the smaller is the vacuum that needs to be
achieved for the same insulating properties. Radiation is lower for
small pores and for low temperatures. From all these considerations, the value of using nanostructures is significant. The goal is
to create incredibly small pores and evacuate them, which would
significantly reduce conduction in gases, decrease the radiation
(because of the small pores), and reduce the effect of conduction
in solids due to large porosity.
There are a great many ways that nanotechnologies can be used to
create one or another type of nanofoam. The intent is invariably to
create porous structures that entrap air. A well-known example of
a highly porous nanoscale material with high insulating properties
The Thermal Environment
is 237 W/(mK). Overall thermal conductivity of porous materials
depends on convection in the pores, conduction in the pores, conduction in the solid, and radiation. Heat transfer in porous materials is very complex, and individual mechanisms are complex and
cannot be easily summarized. Briefly, conduction in a porous material increases with the increase of pressure around it and inside the
pores; convective heat transfer increases with the increase of the
motion of air inside the pores; and radiation increases significantly
with an increase in temperature and with an increase in pore size.
Hence overall thermal conductivity decreases with a decrease of the
pressure inside the pores, a decrease in temperature, and a decrease
of pore diameter. Conduction in the solid part of the porous matrix
is defined by the type and amount of material used. Direct material
conduction effects are reduced due to the very large percentage of
porosity normally present.
Designs intended to improve insulation qualities depend on
varying these several parameters. In buildings, temperature levels
are given design parameters that cannot be varied much. Pore
diameter can be varied with material interventions. In some device
designs using porous materials, the pressure inside the pores can
be varied. There are many advantages to introducing nanoscale
pores. In nanoporous materials, convection inside the pores is
largely negligible due to the small space available for the motion
of molecules. The “mean-free path” (see Section 4.4) of air molecules is larger than the size of pores. So, though in conventional
porous materials molecules of gas mainly collide with each other
inside the pores, inside nanopores they also constantly collide
with the pore wall, which leads to suppressed gas conduction.
Conduction in the gas will also decrease with any decreases of
pressure that can be obtained. Ideally, a vacuum inside the pores
produces the best insulating properties. Practically, the smaller
the pores become, the smaller is the vacuum that needs to be
achieved for the same insulating properties. Radiation is lower for
small pores and for low temperatures. From all these considerations, the value of using nanostructures is significant. The goal is
to create incredibly small pores and evacuate them, which would
significantly reduce conduction in gases, decrease the radiation
(because of the small pores), and reduce the effect of conduction
in solids due to large porosity.
There are a great many ways that nanotechnologies can be used to
create one or another type of nanofoam. The intent is invariably to
create porous structures that entrap air. A well-known example of
a highly porous nanoscale material with high insulating properties
The Thermal Environment