D4RR3N
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Aluminum hydroxide to Aluminum Oxide
Aluminum hydroxide powder melts around 300C
I want to form a solid cake of Aluminum oxide, what temperature will I need to bake the hydroxide powder. Different sources on the net are saying
different things ie 180, 200, 300, 988c first two cannot be correct since aluminum hydroxide melts around 300C
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ElectroWin
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i would lean toward the high end of this scale.
i managed to sinter some impure alumina from the hydroxide on my kitchen stove, at home. i got a temperature of ~ 700 C for approx 1 hour. after
cooling, the result was hard surface with visible bubbling having taken place.
notes on rhs of this chart are helpful.
http://www.ceramicartdaily.net/PMI/KilnFiringChart.pdf
"Between 480-700°C chemical water (referred to as “water
smoke”) is driven off."
[Edited on 2013-8-27 by ElectroWin]
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blogfast25
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Aluminium oxide and aluminium hydroxide are two vastly different things. The melting point of aluminium oxide (aka 'alumina') is
about 2072 C.
Aluminium hydroxide doesn't really 'melt': as you heat it, it gradually loses water until you end up with anhydrous alumina. To obtain truly anhydrous
alumina may take some time, even at 900 C or above. The lower the temperature the higher the chances the material will retain small amounts of water.
To obtain annealed alumina you would need to heat well above 2072 C, then allow to cool. What's obtained that way is equivalent to Corundum, the
extremely hard (Mohs 9) mineral. Like the mineral form, annealed alumina is extremely hard and very resistant to chemical attack.
What is your purpose with the 'aluminium oxide cake'?
[Edited on 27-8-2013 by blogfast25]
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D4RR3N
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Quote: Originally posted by ElectroWin  | i would lean toward the high end of this scale.
i managed to sinter some impure alumina from the hydroxide on my kitchen stove, at home. i got a temperature of ~ 700 C for approx 1 hour. after
cooling, the result was hard surface with visible bubbling having taken place.
notes on rhs of this chart are helpful.
http://www.ceramicartdaily.net/PMI/KilnFiringChart.pdf
"Between 480-700°C chemical water (referred to as “water
smoke”) is driven off."
[Edited on 2013-8-27 by ElectroWin] |
700C on a kitchen stove! didn't think that was even possible, I put it on my kitchen gas stove and I couldn't even get it to melt (the spoon was
glowing orange).
When it converted to Aluminum oxide was it a solid lump or was it powdery?
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D4RR3N
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| Quote: Originally posted by blogfast25 | Aluminium oxide and aluminium hydroxide are two vastly different things. The melting point of aluminium oxide (aka 'alumina') is
about 2072 C.
Aluminium hydroxide doesn't really 'melt': as you heat it, it gradually loses water until you end up with anhydrous alumina. To obtain truly anhydrous
alumina may take some time, even at 900 C or above. The lower the temperature the higher the chances the material will retain small amounts of water.
To obtain annealed alumina you would need to heat well above 2072 C, then allow to cool. What's obtained that way is equivalent to Corundum, the
extremely hard (Mohs 9) mineral. Like the mineral form, annealed alumina is extremely hard and very resistant to chemical attack.
What is your purpose with the 'aluminium oxide cake'?
[Edited on 27-8-2013 by blogfast25] |
Wikipedia says it melts at 300C
http://en.wikipedia.org/wiki/Aluminium_hydroxide
I'm using Aluminum hydroxide as a starting point because I have no chance of melting Aluminum oxide but should be able to heat aluminum hydroxide
until it decomposes into a cake of solid aluminum Oxide. Aluminum oxide is a very polar molecule, I want to experiment with making an Electret with
it.
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ScienceSquirrel
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A cake of aluminum oxide will have different properties depending on how it is prepared.
Suppose you start from an aluminium sulphate solution and you add a base.
Depending on the base you use, you will get different precipitates; sodium hydroxide, sodium carbonate or ammonia solution will all make voluminous
precipitates but they will vary in composition and basicity.
Similarly starting from a solution of sodium aluminate and adding hydrochloric or acetic acid to a specified pH and then drying it will make quite
different products.
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D4RR3N
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| Quote: Originally posted by ScienceSquirrel | A cake of aluminum oxide will have different properties depending on how it is prepared.
Suppose you start from an aluminium sulphate solution and you add a base.
Depending on the base you use, you will get different precipitates; sodium hydroxide, sodium carbonate or ammonia solution will all make voluminous
precipitates but they will vary in composition and basicity.
Similarly starting from a solution of sodium aluminate and adding hydrochloric or acetic acid to a specified pH and then drying it will make quite
different products.
|
But wouldn't it be easier to start with pure aluminum hydroxide powder since it is cheap and easy to get?
http://www.ebay.co.uk/itm/320915180593?lpid=54&device=c&...
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Zephyr
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| Quote: Originally posted by D4RR3N | | Quote: Originally posted by ScienceSquirrel | A cake of aluminum oxide will have different properties depending on how it is prepared.
Suppose you start from an aluminium sulphate solution and you add a base.
Depending on the base you use, you will get different precipitates; sodium hydroxide, sodium carbonate or ammonia solution will all make voluminous
precipitates but they will vary in composition and basicity.
Similarly starting from a solution of sodium aluminate and adding hydrochloric or acetic acid to a specified pH and then drying it will make quite
different products.
|
But wouldn't it be easier to start with pure aluminum hydroxide powder since it is cheap and easy to get?
Aluminium sulfate can be made by adding aluminium hydroxide, Al(OH)3, to sulfuric acid, H2SO4:
2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3·6H2O
http://www.ebay.co.uk/itm/320915180593?lpid=54&device=c&... |
Aluminium sulfate can be made by adding aluminium hydroxide, Al(OH)3, to sulfuric acid, H2SO4:
2 Al(OH)3 + 3 H2SO4 → Al2(SO4)3·6H2O
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ElectroWin
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| Quote: Originally posted by D4RR3N | 700C on a kitchen stove! didn't think that was even possible,[....]
When it converted to Aluminum oxide was it a solid lump or was it powdery? |
i achieved this high T with lots of insulation.
the result was a solid lump; and hard.
on the contrary, annealing occurs below the glass transition temperature for the ceramic. if m.p. of Al2O3 is 2072 then you should be well below that.
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12AX7
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...Or if it's not a glassy ceramic, then the temperature where sufficient diffusion occurs to affect internal stress and crystal form. This is
usually about half the melting point (absolute temperature), or maybe 900C for Al2O3. Expect to wait a very long time for it to do anything; high
pressure helps (i.e., hot isostatic pressing).
Tim
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AJKOER
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If heating on the stove, try adding turbulence (accompanied with noise) to the flame by knocking the metal lid on the top of the stove burner slightly
to the side.
I have witnessed a hotter flame (per the time it takes for a sample to glow) and I recall one source noting that a perturbed flame is measurably
hotter (100 or more centigrade per my recollection).
Per page 14 in this reference "Turbulent Flames and the Role of Chemistry" at http://www.google.com/url?sa=t&rct=j&q=&esrc=s&a... ), for a 2/1 air methane mix in a turbulent flame a cited temperature of 1,350 K.
Per this reference, "Study of a Turbulent Non-Premixed Methane-Air Reacting Flame", per page 76, a much hotter temperature is visible on the graph
with a high temperature in excess of 1,700 K (link: http://ijamm.bc.cityu.edu.hk/ijamm/outbox/Y2012V8N12P70C6684... ).
[Edited on 9-9-2013 by AJKOER]
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