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Author: Subject: Carbon Foam Refractory Made From Bread
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[*] posted on 26-7-2020 at 09:56

I did a few exploratory experiments with these carbon foams and a sodium silicate solution (waterglass). I tried three basic treatments:

* repeatedly soaking in waterglass, then dilute sulfuric acid, then rinsing

(Na2O)x·(SiO2)y + xH2SO4 -> xH2O + ySiO2 + xNa2SO4

the idea was to coat or permeate the foam with silica. The foam was baked to dry after each dunking in the waterglass, which led to the formation of many tiny sodium silicate bubbles. These structures survive the acid treatment as this photomicrograph show. At high magnification, there appear to be small silicate needles on the carbon.

roastedSilicateWide.jpg - 1.3MB roastedSilicateBub.jpg - 959kB

It seemed to stand up to a MAPP gas torch pretty well; afterwards, the pores of the carbon foam seemed to have been filled with a bubbly/crystalline silicate

blazedSilicate.jpg - 1MB

* soaking in boric acid, baking to dehydrate to boric trioxide, then soaking in waterglass

(Na2O)x·(SiO2)y + zB2O3 -> (Na2O)x·(SiO2)y·zB2O3

Boric acid was dissolved in absolute EtOH, and the carbon foam soaked in the solution. It was dried, then baked to dehydrate, then soaked again. After a couple of repetitions, this was dunked in waterglass and baked dry again. The result was a fluffy, flakey white crust on the foam. When hit with the MAPP gas, the whole structure crumpled, suggesting that the boric/silicate soak had permeated and disrupted the carbon network, such that it lost all integrity when the intersitial material melted. When examined, the silicate/boric material had melted into droplets clinging to the foam surface, which cooled into tiny beads containing air bubbles. I wonder what role the foam geometry (voids, ridges) have in forming the beads?

glassDroplets.jpg - 1MB

* soaking in copper acetate, then soaking in waterglass, then dilute sulfuric

The idea here was to coat the foam with a mixed copper/silicate layer, like you'd see in a chemical garden. After soaking in an ethanol/copper acetate solution and drying, a chunk of foam was cut open. The salt appeared to be mostly but not entirely superficial, with a small amount appearing to have made its way into the interior. When hit with a torch, a thin layer of copper was left firmly adhering to the surface.

After soaking in waterglass, drying, and then sealing with dilute acid, the surface had a light green, foamy appearance. When this was torched, it seems to withstand a lot of heat; afterwards there is a glassy red-brown residue on the surface. Under the microscope it looks to be a mix of copper metal and silicate. It also looks like the silicate is wetting the carbon surface much better in this case than the others.

I wonder to what extent the cupric ion is being reduced by the acetate ion vs. by the carbon substrate...

redFoam.jpg - 981kB wetRedGlass.jpg - 1.3MB

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