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Author: Subject: Graphite or silicon carbide?
Morgan
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[*] posted on 30-8-2018 at 12:54
Graphite or silicon carbide?


At the end there's a quick voltmeter shot and I was wondering if silicon carbide could be forming or not?

Coating Quartz Glass Tube In Graphite
https://youtube.com/watch?v=oReCziJFHVM

I was looking to see if SiC is conductive and saw this.
https://www.sciencedirect.com/science/article/pii/S002231150...
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Metallus
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[*] posted on 12-9-2018 at 05:17


I work with SiC, silica and carbon in my institute, and I've seen SiC forming on the surface of carbon only at T>1900°C and under applied pressure.

I don't think that SiC would realistically form in such "bland" conditions where quartz is not even close to its melting point.
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[*] posted on 12-9-2018 at 07:32


Around the 25 second mark the entire tube is fully lit up, white hot on the end. He said he was using a welding mask over the camera to film it. Last week he offered to send me a piece if I wanted to test it. But what's an easy test?
https://youtube.com/watch?v=oReCziJFHVM

Some Wiki tidbits on silicon carbide

"The simplest process to manufacture silicon carbide is to combine silica sand and carbon in an Acheson graphite electric resistance furnace at a high temperature, between 1,600 °C (2,910 °F) and 2,500 °C (4,530 °F). Fine SiO2 particles in plant material (e.g. rice husks) can be converted to SiC by heating in the excess carbon from the organic material.[15] The silica fume, which is a byproduct of producing silicon metal and ferrosilicon alloys, can also be converted to SiC by heating with graphite at 1,500 °C (2,730 °F)."

"Alpha silicon carbide (α-SiC) is the most commonly encountered polymorph. It's formed at temperatures greater than 1700 °C and has a hexagonal crystal structure (similar to Wurtzite). The beta modification (β-SiC), with a zinc blende crystal structure (similar to diamond), is formed at temperatures below 1700 °C.[29] Until recently, the beta form has had relatively few commercial uses, although there is now increasing interest in its use as a support for heterogeneous catalysts, owing to its higher surface area compared to the alpha form."
I saw they sell beta SiC on Alibaba for sandpaper.

"Pure silicon carbide can also be prepared by the thermal decomposition of a polymer, poly(methylsilyne), under an inert atmosphere at low temperatures."

But anyway, it was just a possibility I was entertaining that a carbide was forming. He was leaning toward the graphite and maybe he and you are right. The gas he was using was propane and O2. He said he might try introducing methane or acetylene (I can't remember which) from inside the tube to exclude air. I like the mirror-like appearance of the deposit in the clip and wanted to know just how conductive it was. He was uncertain in his test.
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[*] posted on 13-9-2018 at 00:09


Keep in mind that you would need homogeneously mixed powders and inert atmosphere to get appreciable amounts of SiC. In that video only the contact surface of the quartz is exposed and in a oxidising environment. Now:

1) If the layer of deposited graphite was thick, then you could hope to form some layer of SiC at the interface between quartz and graphite, with the external coating still being graphite.
2) If the layer of graphite was thin, then you could hope to form some layer of SiC which would the the most external layer. However, SiC starts oxidising at temperatures as low as 600°C and its oxidation is quite fast at T>1200°C. If the flame is as hot, then any external SiC layer would just be oxidised back to silica.

Even sintered SiC (full density SiC, hot pressed at 2100°C and >40 MPa) is oxidised in air at T>1200°C. We use it for high temperature testing. When we get a new SiC support, after the first test it forms a thin blue oxide layer of SiO2 (the color changes with the thickness. Further uses increase the oxide thickness until it turns grey/transparent).

That's why, realistically speaking, what is being formed is most likely just graphite.
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[*] posted on 13-9-2018 at 04:08


Here's some tidbit I suggested to him, just some avenue to try. In addition to the inert atmosphere, some hydrogen is employed. I guess it would be an ambitious project unless you had the means. I wonder if there's some practical application for carbide coated quartz tubing?

Reduction of SiO2 to SiC Using Natural Gas
Abstract
This paper presents a preliminary study of SiC production by use of natural gas for reduction of silica. Direct reduction of SiO2 by gas mixtures containing CH4, H2, and Ar was studied at temperatures between 1273 K and 1773 K (1000 °C and 1500 °C). Silica in form of particles between 1 and 3 mm and pellets with mean grain size 50 µm were exposed to the gas mixture for 6 hours. Influence of temperature and CH4\H2 ratio was investigated. Higher temperature and CH4 concentration resulted in greater SiC production. Two kinds of SiC were found: one was deposited between SiO2 particles, the other one was deposited inside the SiO2 particles. Although the exact reaction mechanisms have not been determined, it is clear that gas-phase reactions play an important role in both cases. The reaction products were analyzed by Electron Probe Micro Analyzer.
https://link.springer.com/article/10.1007/s40553-014-0027-4

Effects of gas atmosphere on reduction of quartz and its reaction with silicon carbide for silicon production - Research Online - UOW
PDFUniversity of Wollongong › au › ro › cig9
"The quartz was crushed to < 70 µm, uniformly mixed with graphite and pressed into
pellets. Reduction was studied in isothermal and temperature programmed reduction
experiments in a fix bed reactor in argon, hydrogen and argon–hydrogen gas mixtures.
The carbothermal reduction of quartz in hydrogen was faster than in argon. Formation
of silicon carbide started at 1300 °C in argon, and 1200 °C in hydrogen. Synthesis of
silicon carbide in hydrogen was close to completion in 270 minutes at 1400 °C, 140
minutes at 1500 °C, and 70 minutes at 1600 °C. Faster carbothermal reduction rate in
hydrogen was attributed to the involvement of hydrogen in the reduction reactions by
directly reducing silica and/or indirectly, by reacting with graphite to form methane as
an intermediate reductant."
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