Space Shuttle Heat Shield - What is its characteristic?

Dear Folks,
I am wondering what are characteristic of heat shield tiles of the shuttle would be. First it needs to have a high heat insulation as well
as high heat dissipation. I guess it must be quite porous and fully crystalline.
Am I having any misunderstanding? Please correct me.
Thanks SC
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The tiles on the bottom of the shuttle are quite simple. They are comprised of high purity silica with a porosity around 90%. Because of the low solids content, heat transfer is dominated by thermal conductivity of the gas within the pores. This material works well for the belly of the spacecraft as structural characteristics are not of key importance. The carbon-carbon composites are used for the leading edges of the vessel.
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Some tiles (the black ones) also have a high emissivity coating to dissipate heat.
Gregg snipped-for-privacy@yahoo.com wrote:

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