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Author: Subject: Aluminum hydroxide to Aluminum Oxide
D4RR3N
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[*] posted on 27-8-2013 at 08:16
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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[*] posted on 27-8-2013 at 09:38


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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[*] posted on 27-8-2013 at 12:13


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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[*] posted on 27-8-2013 at 12:20


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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[*] posted on 27-8-2013 at 12:30


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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[*] posted on 27-8-2013 at 14:00


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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[*] posted on 27-8-2013 at 22:55


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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[*] posted on 31-8-2013 at 08:38


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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[*] posted on 7-9-2013 at 06:19


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.

Quote: Originally posted by blogfast25  

To obtain annealed alumina you would need to heat well above 2072 C, then allow to cool.


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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[*] posted on 7-9-2013 at 09:00


...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).

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[*] posted on 9-9-2013 at 03:41


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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