Sciencemadness Discussion Board

magnesium aluminate refractory up to 1800C.

rockyit98 - 16-1-2021 at 02:18

I need refractory that withstand at least 1800C to make microwave kiln but the fire clay that i have only can handle up to 1400C. i can increase its up t 1700C by adding Al2O3 but not good enough. MgO is good up to 2500C but need precalcining MgCO3 at 750C to make MgO and another at 1500C to make dead burnt MgO so there i a chicken and the egg problem. then i learned about Spinel type of gemstone. It has the formula MgAl2O4 and melt about 2135°C.wiki
Attachment: MgAl2O4.pdf (847kB)
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to make the material at convenience i decided to use one pot precipitation of amorphous stuff form MgSO4 and NaAlO2 solution mixed till pH of 7.0 if possible. NaAlO2 is from reacting Al dross form casting with diluted NaOH so slower the reaction and the excess water will boil away. after solid stuff is settled decanting the resulting Na2SO4 and keeping it so by adding hydrated lime i can recycle the NaOH. the powder paste at bottom is washed thoroughly with warm water and dried. resulting powder is heated to 750C or more to get the final product.

i will upload the pics within few weeks because my kiln needs repairing and my order still in progress.

unionised - 16-1-2021 at 08:27

If you use MgCl2 and AlCl3 solutions with excess ammonia to produce the mixed oxide/ hydroxide then you won't have to remove sodium and sulphate before calcining it.
The NH4Cl will evaporate.
You will get a higher purity product and that should mean a higher melting point.

You can get the chlorides by dissolving the metals (or hydroxides/ carbonates whatever) in aqueous HCl

As an interesting variant you could see if you can use solutions of the citrates in citric acid/ water.
You can coat a substrate with that and then burn off the organics to leave the mixed oxides.

wg48temp9 - 16-1-2021 at 09:36

If you are going to coat the refractory with a microwave absorber to form the heating element, what do you intend to use as the absorber ?

rockyit98 - 16-1-2021 at 10:28

Quote: Originally posted by wg48temp9  
If you are going to coat the refractory with a microwave absorber to form the heating element, what do you intend to use as the absorber ?

SiC can up to 1600C.
Quote: Originally posted by unionised  
If you use MgCl2 and AlCl3 solutions with excess ammonia to produce the mixed oxide/ hydroxide then you won't have to remove sodium and sulphate before calcining it.
The NH4Cl will evaporate.
You will get a higher purity product and that should mean a higher melting point.

You can get the chlorides by dissolving the metals (or hydroxides/ carbonates whatever) in aqueous HCl

As an interesting variant you could see if you can use solutions of the citrates in citric acid/ water.
You can coat a substrate with that and then burn off the organics to leave the mixed oxides.

my point was to do it with thing that are easy to find like MgSO4 from bath salt or fertilizer and NaOH from hardware store and Al from scrap. plus NH4OH is toxic and smelly also residual ammonium chloride is bad for the heating element as it corrosive. and i have to go to a water treatment supply to get poly aluminum chloride and chemical shop to get MgCl2 in this current time.

yobbo II - 16-1-2021 at 18:49


You can purchase alumina bricks for this. They are very expensive. I believe that the alumina must be very pure and that makes them expensive.

Yob

Twospoons - 16-1-2021 at 21:30

You will want to remove as much sodium as possible - any residue may act as a flux.

unionised - 17-1-2021 at 04:08

Quote: Originally posted by Twospoons  
You will want to remove as much sodium as possible - any residue may act as a flux.

... or not add it in the first place....

rockyit98 - 17-1-2021 at 17:47

Quote: Originally posted by unionised  
Quote: Originally posted by Twospoons  
You will want to remove as much sodium as possible - any residue may act as a flux.

... or not add it in the first place....

even so called high alumina bricks have Na impurity because of Bayer process.I think 50C water will remove most of Na+.

wg48temp9 - 22-1-2021 at 05:16

Quote: Originally posted by rockyit98  
Quote: Originally posted by wg48temp9  
If you are going to coat the refractory with a microwave absorber to form the heating element, what do you intend to use as the absorber ?

SiC can up to 1600C.



My understanding is the absorption of microwaves by silicon carbide is dependent on its purity and the type of impurity. I tried about 0.1g of my powdered silicon carbide spread out on the base of a ceramic mug in an empty 800W microwave oven. It did not get noticeably hot to the touch after 40s.

I suggest you try your silicon carbide first.

rockyit98 - 22-1-2021 at 06:16

Quote: Originally posted by wg48temp9  


My understanding is the absorption of microwaves by silicon carbide is dependent on its purity and the type of impurity. I tried about 0.1g of my powdered silicon carbide spread out on the base of a ceramic mug in an empty 800W microwave oven. It did not get noticeably hot to the touch after 40s.

I suggest you try your silicon carbide first.


@wg48temp9 no wonder. at high-temperature dielectric loss increases,
which derives from the combined effect of the polarization
relaxation and the electric conductance of SiC. The microwave
absorption demonstrates that SiC powders enhanced microwave
absorption coupled with improved broadband absorption, specifically increased microwave absorption properties at higher temperature.

unionised - 23-1-2021 at 05:30

Quote: Originally posted by rockyit98  
....I think 50C water will remove most of Na+.

Unless it's present as this
https://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/...

rockyit98 - 23-1-2021 at 10:02

Quote: Originally posted by unionised  
Quote: Originally posted by rockyit98  
....I think 50C water will remove most of Na+.

Unless it's present as this
https://authors.library.caltech.edu/5456/1/hrst.mit.edu/hrs/...

https://en.wikipedia.org/wiki/Beta-alumina_solid_electrolyte

wg48temp9 - 28-1-2021 at 05:14

So I decided to try a small pile of my 400 mesh silicon carbide between two wires connected to about 2,000Vrms (HT of a MOT with the HT winding of an other MOT as a ballast. No noticeable conduction uless the wires wire touched and then separated by about 5mm which was probably just an arc drawn out.
So my silicon carbide is high resistance.

However I did notice a curious effect. A cloud of silicon carbide dust appeared above the high voltage wire (the other wire was at earth voltage). It looked like it was suspended by the high AC voltage. If the wires touched, shorting out the voltage, the dust dropped down.

macckone - 28-1-2021 at 17:24

If you want to use clay, you can use the clay you have then add powdered mullite.
You are going to want to add aluminum hydroxide and mullite and then work the clay until it is fully consistent.

Other methods, get a fire brick of the appropriate temperature and cut a hole in it and line it with graphite powder.

Buy one, $42 on amazon: https://www.amazon.com/Large-Microwave-Kiln-Glass-Fusing/dp/...

Use sorel cement and put it in a charcoal furnace to fuse it and drive off the HCl.

A charcoal furnace will also work to calcine magnesium oxide.

Final method, phosphate bonded refractory, you can use magnesia. It doesn't need to be fully calcined. You basically add phosphoric acid to a wet paste, there needs to be some magnesium hydroxide present to complete the reaction.
Aluminum oxide also works but the maximum recommended working temperature is below 1800C.

One of the questions is does the whole contraption reach 1800C, because if it is just the area in contact with the graphite, then you can probably use a lower melting point for the majority, paint on milk of magnesia and then a mix of graphite and magnesia. It will calcine in situ, I haven't tried it using this method because I am a fan of sorel and I can put it on the BBQ pit. Magnesium Oxychloride will calcine completely at 500C with the evolution of a LOT of HCl.
Do that part outside in a BBQ pit on a windy day.

RogueRose - 29-1-2021 at 01:12

I'm thinking you could use plasma arc to heat the MgO. Get a few carbon rods and some 6-12v batteries or some good power supplies and then pile up some MgO around the arc and heat it up. I would think this might be the easiest way to get the temp you are looking for. Another idea is to maybe use a clay/terrocatta pot and put the MgO inside that.

densest - 31-3-2021 at 11:57

You can buy refractory coatings & sprays. Highest I've used is ZrO for 1800C. Worked pretty well to protect graphite from air at red heat. There are lots of formulations for different atmospheres and substrates. The manufacturers' data sheets brag about particle size control and proprietary binders.

Out of curiosity what are you melting at 2500C?

rockyit98 - 1-4-2021 at 00:55

Quote: Originally posted by densest  
You can buy refractory coatings & sprays. Highest I've used is ZrO for 1800C. Worked pretty well to protect graphite from air at red heat. There are lots of formulations for different atmospheres and substrates. The manufacturers' data sheets brag about particle size control and proprietary binders.

Out of curiosity what are you melting at 2500C?


only up to 1800c or so for a induction heater .I'm gonna give it a go with making iron alloys, but I still need to build one first. getting component is hard and also not enough spare time.

WGTR - 1-4-2021 at 15:29

I will reiterate the point that macckone made.

High temperature refractory materials are generally not insulative the way that the lower temperature ones are. However, there’s only 400 degrees difference between 1800C and 1400C. The refractory material only needs to be thick and insulative enough that the “cold” side of if is no higher than 1400C when it is surrounded by the 1400C firebrick.

rockyit98 - 2-4-2021 at 06:34

Quote: Originally posted by WGTR  
I will reiterate the point that macckone made.

High temperature refractory materials are generally not insulative the way that the lower temperature ones are. However, there’s only 400 degrees difference between 1800C and 1400C. The refractory material only needs to be thick and insulative enough that the “cold” side of if is no higher than 1400C when it is surrounded by the 1400C firebrick.

the walls must be thin for high temperature crucibles and it's insulation, otherwise they tend to crack because of different thermal expansion at different temperatures .so it's better to use insulating wool for 1400C and insulating powder in between the crucible insulation (crucible sleeve) and that.

Mateo_swe - 22-4-2021 at 10:00

I saw a video of a guy making a microwave kiln using silicon carbide grit size F80.
He used waterglass as a binder for the silicon carbide grit and made a 4mm thick ring used as the walls for the microwave kiln.
He then added ceramic wool around the outside of the silicon carbide ring and sprayed it with waterglass.
Maybe waterglass cant be used as a binder for the silicon carbide in your application as you want so high temperatures but maybe something else can be used as the binder, something that can handle the temps.
The silicon carbide seems to absorb the heat making it a good material for microwave kilns and it melts at 2830°C.
Here is a link to the video i stubled upon a while ago with the guy making a microwave kiln.
https://www.youtube.com/watch?v=rAIyLWEKt8U

I dont know if it can be of any help but the creating of the silicon carbide ring was interesting.