We've discovered a metal BETTER than firebrick! (Just one caveat)

So as some of you might recall my father and I have been making the Liberator Rocket Heaters now for some time with good success. However many of you expressed legitimate concerns that metal will eventually corrode and burn out. At the same time, insulation was requested. We all know that insulation is beneficial, but even aerogel or other similar products need a stable material, like firebrick or metal, to keep it held in place. Well, after months and months of research we have discovered a metal that is non-reactive with oxygen even at high temperatures and can even withstand temperatures FAR exceeding that of even the best firebricks and refractory's. This metal has a melting point of 3017 degree's. Celsius! Thats 5462 F. melting point. This metal is used in vacuum furnaces, nuclear reactors, and aircraft turbines. Whats the downside? Well, for a 12" X 24" section of Tantalum, which is only enough to line the horizontal burn tube, not even the heat riser, would run $6,800.00 dollars. This metal is Tantalum.

I know that high temperature refractory metals exist. If there is anybody with any knowledge or sources on an alloy that would work in a RMH I would like to ask of you to please forward me this information.

I'm also thinking about starting a kickstarter to make a gravity fed woodchip burning Rocket Heater that can also burn cordwood, sticks, staves, pellets, and agricultural waste lined with firebricks that will be EPA approved and building code compliant. I'll need 80 thousand dollars to do that. Let me know what you guys think.

http://www.eaglealloys.com/tantalum/

I used to work with a company that flowformed tantalum. It is a very expensive material, but you can use it nearly paper thin, so a little goes a long way. They used to make things that looked like the big metal tub in a washing machine out of a piece of tantalum the size of a dinner plate.

Flowforming is an interesting process that seems like it would be perfect for making rocket mass heater parts:
Flowforming Demo

Well, that got me off on an interesting tangent: https://en.wikipedia.org/wiki/Refractory_metals

My suggestion would be to design and sell (if it can be made cheap enough) a pellet burn basket for people making pellet RMH systems.

There are ethical issues associated with using Tantalum. First, it is considered a conflict resource, and is considered to be driving warfare in some of the poorest parts of the world.

Second - it is rare and valuable. It has more valuable uses in society than solving an engineering problem which can be avoided by (a) using cheap and abundant fire bricks or (b) designing a component out of cheap steel that can be easily replaced.

https://en.wikipedia.org/wiki/Tantalum

Back when I was doing engineering work, an older engineer advised me that "if you have to use Unobtanium in your design, you need to redesign it."  There are times when exotic materials are the right solution, but usually it seems to be better to rethink the design.

330 Stainless is about as good as it reasonably gets. Rated to 2100F. It's what InStove uses for their grates. I bought 26 3x8 sheets for $75 each off Craigslist. Retail is over $300 a sheet. I'm using it for rocket stove liners, inside insulating firebrick, for rocket stoves for schools in Haiti.

in my 6" RMH Hybrid with cook top and dutch oven thread, we discuss non corrosive and hi-temp metals.  

  i figured inconel and/or similar nickel alloys would be great, about the same price as stainless or slightly higher.

check out my 6" RMH Hybrid thread or research NICKLE alloys, such as inconel.

  good luck man.   ive since/recently moved on to using firebricks, since building the other 2 stoves, mostly for peace of mind because all the anti-metal advice.  im sure there is a metal that would work out there, but ive moved onto a different construction.  i havent had any metal meltdowns yet, but when i see the burn chamber turn red hot, its kind of cause for concern.  i have more peace of mind, and less criticism from the rocket gurus with my latest brick cored build.  lol .    i know exhaust manifolds and hedders last for decades and they are metal, so you should be able to figure something out, but definately check out inconel/nickle alloys and dont give up on your ideas.

Very interesting discussion indeed.  I tend to agree with those sticking with fire brick.  I use fire brick for the entire J tube.  The design is pretty much a standard RMH except the interior of the 55 gallon drum is also lined with veneer fire brick.  I currently have the stove disassembled because it is a "design in progress."  i have been using a rocket stove outdoors for the last 4 years that is made entirely of fire brick.  It is a J tube that is an excellent cooking device and has not demonstrated any heat stress deterioration..... Yet.  Since it is just stacked brick repair is a "Piece of cake" or in Paul's case a Piece of Pie.  I have been working on a heater that uses 2 55 gallon drums stacked that looks like one of those conventional stacked drum woodstoves.  My design if successful will use the second drum as the heat sink.  I want the RMH to be effective in heating situations that won't allow a large heat sink.  It will be moveable since both drums have removeable lids. It is quite heavy when assembled but can be disassembled down to bare 55 gallon drums.  

Robert Fairchild wrote:330 Stainless is about as good as it reasonably gets. Rated to 2100F. It's what InStove uses for their grates. I bought 26 3x8 sheets for $75 each off Craigslist. Retail is over $300 a sheet. I'm using it for rocket stove liners, inside insulating firebrick, for rocket stoves for schools in Haiti.

Thank you VERY much! This is exactly what we've been looking for. I am 100% confident that in our super small 4.5" combustion chamber that this stainless steel will never burn out. Ever. Now its just a matter of finding a supplier.

Research basalt, it is an abundant natural materials from volcanos. It is used today for many heat and strength purposes.  I believe it maybe relatively inexpensive compared to your other metallic material listed.

Ronald Cline wrote:Research basalt, it is an abundant natural materials from volcanos. It is used today for many heat and strength purposes.  I believe it maybe relatively inexpensive compared to your other metallic material listed.

I just looked it up, nowhere near the temperature resistance and rating necessary.
http://smarter-building-systems.com/smarter-building-basalt-faqs/

Do you know how much surface area of your design will exceed this temperature?

Ronald Cline wrote:Do you know how much surface area of your design will exceed this temperature?

Certainly not a lot. Its just the horizontal burn tube and internal riser. I'm more concerned about the burn tube, that gets by far the hottest. The riser can get away with regular Stainless steel.

I don't know, but I wonder if you use compressed dry clay powder instead of pre-fired brick, would it work.  Mixing perlite with it may even boost its performance.

Ronald Cline wrote:I don't know, but I wonder if you use compressed dry clay powder instead of pre-fired brick, would it work.  Mixing perlite with it may even boost its performance.

Castable refractory's already exist for 3000 F. and up.
http://www.hightemptools.com/castablerefractory.html

I'm trying to get away from refractory's because they will fall apart during shipping and transit. I want something thats permanent, will allow me to use aerogel or other super-insulating material between the metal/refractory, and will never burn out, and also be affordable. That search has been incredibly difficult.

Molded graphite. I have no idea of the expense.

Hello Sky,

I haven't seen GeoPolymers mentioned. If for whatever reason you would have interest in something else than metal that can be casted and have very high temp rating (like Granite, but better), you should maybe look into GeoPolymers. I am planning on casting a core for mine when I get to make my Rocket Mass Heather Core and also planning on using it for construction (like making a floor that can withstand the heat of welding slag or plasma cutting slag or even molten metal directly). There's a lot of people referring to AAM (Alkaline Activated Materials) as GeoPolymers and they are 2 completely different beasts. AAM are normally concrete variants that uses alkalies (sodium hydroxide/potassium hydroxide) to increase strength. We all know that calcium based materials will pop like pop corn at those temps. This is where GeoPolymers are very interesting and may be a cheap alternative with much more heat tolerance and durability. A Geopolymer is a rock Polymer with different ratios of Aluminates and silicates (no calcium or very little) in a perfect ratio to make them bind in a complex polymer. With GeoPolymers, they also use alkalies to soften up the raw material but what makes it different from AAM is to ensure proper aluminates and silicates ratios for the intended purposes. Depending on the ratios, you could get something like a rock epoxy that can be used with fiberglass and non-toxic as well, on the other side of the range, you also can get concrete like materials that could be as hard as 100Mpa and that can be casted to any form you wish. The tricky part is to find raw materials that have very little other elements than aluminates and silicates or they can contaminate, so to speak, the mix and make it less stable. It's very common to get a Meta-Kaolin material as the raw material, but that can become expensive. The cheapest material I came across is Fly Ash (You need type F, not C as the C type contains around 10% Calcium Oxide and will lessen the quality of the polymer). FlyAsh is a very cheap material that companies like Boral are trying to get rid of for very cheap. There's already a recipe that was made available by the GeoPolymer Institute in France for free for humanity to use. The pdf that covers ratios and what you need to look at in the raw materials is located at:

GEOASH.pdf
https://www.geopolymer.org/fichiers_pdf/GEOASH.pdf

Just don't confuse geopolymers for concrete, they are really different as it gets.

I hope this helps.

Cheers

Jan

Why not use cast iron or even boiler plate - good enough for wood stoves and cooking ranges for centuries past.  I used recycled heavy steel tube for my rocket stove and I am confident it will last many years

Chris Southall wrote:Why not use cast iron or even boiler plate - good enough for wood stoves and cooking ranges for centuries past.  I used recycled heavy steel tube for my rocket stove and I am confident it will last many years

A properly functioning rocket stove will reach temperatures that will burn out cast iron and steel.  This has been demonstrated over and over ad nauseam

Permies.com does not recommend the use of metal in the burn tunnel or heat riser of a rocket stove or mass heater.  Please check out this thread - using metal in the burn tunnel and heat riser

Chris Southall wrote:Why not use cast iron or even boiler plate - good enough for wood stoves and cooking ranges for centuries past.  I used recycled heavy steel tube for my rocket stove and I am confident it will last many years

Cast iron would be fine for the top plate. But not the best for a burn tube.

I was going to shout "asbestos!!", but then I looked it up what these geopolymers are.
They are not asbestos

But it appears to me that you are seriously upcycling some waste materials if this works.

Highly interested also in the 'burn everything' RMH!
Our petrol fired floor heating went 3 years ago, the wood stove is still keeping us warmisch, but some replacement would be great. Cord wood in abundance, but as my parents are agging and I'm considering leaving the house, they started considering the day they'll no longer be able to easily navigate stairs with wood in their arms and the floors are not strong enough to just build a RMH on top except for the cellar floor (where the broken petrol stove still sits).

Tyrr Vangeel wrote:I was going to shout "asbestos!!", but then I looked it up what these geopolymers are.
They are not asbestos

But it appears to me that you are seriously upcycling some waste materials if this works.

Highly interested also in the 'burn everything' RMH!
Our petrol fired floor heating went 3 years ago, the wood stove is still keeping us warmisch, but some replacement would be great. Cord wood in abundance, but as my parents are agging and I'm considering leaving the house, they started considering the day they'll no longer be able to easily navigate stairs with wood in their arms and the floors are not strong enough to just build a RMH on top except for the cellar floor (where the broken petrol stove still sits).

I'll be honest, I looked into asbestos (chrysotile) because it is heat resistant up to 4500 F. and (was) cheap and abundant. It makes me curse the EPA and OSHA for their actions. I'm sure when used responsibly and in certain situation asbestos is effective and safe and I believe a RMH would be one of those uses. I digress.

Sky,  I believe your current design incorporates cooling fins to help deal with the heat issue in a steel core. Why not pursue that concept and go for an air-cooled core like the Steel RMH I am currently using which is up and running for this cooling season. See pics on my recent posts. Simplicity rather than complexity is often the solution!

Bruce Woodford wrote:Sky,  I believe your current design incorporates cooling fins to help deal with the heat issue in a steel core. Why not pursue that concept and go for an air-cooled core like the Steel RMH I am currently using which is up and running for this cooling season. See pics on my recent posts. Simplicity rather than complexity is often the solution!

It currently is air cooled. However, we want a material that will unquestionably never burn out. Everybody here thinks the steel will burn out despite the steel never exceeding 850 F (which means it wont burn out). Besides, insulation will make it burn better, cleaner, and more efficiently. That stainless 330 seems like the way to go.

It's not the high Temps per se.
It's that at higher Temps oxygen is pulled out of the  liner.
That's why metal liners burn out.

A liner needs to be non reactive to oxygen stripping.

Tom Robertson wrote:It's not the high Temps per se.
It's that at higher Temps oxygen is pulled out of the  liner.
That's why metal liners burn out.

A liner needs to be non reactive to oxygen stripping.

I think thats a little off. Steel including stainless steel and other metals do not contain oxygen in them. They oxidize (rust) when oxygen BONDS with the metal, and "burn out" is a type of accelerated oxidation (rust, oxygen bonding to the metal) metals when they reach their point of incandescence will become much more susceptible to bonding with oxygen, so you got it a little backwards there.

And BTW, on the periodic table almost every element is a metal. Metal is not "doomed" as even firebrick use metal, particularly alumina, to resist heat. Its just a matter of finding the RIGHT metal. As I said originally tantalum would definitely work, and so would tungsten. But they just cost thousands of dollars per sheet.
https://en.wikipedia.org/wiki/Refractory_metals

So yes, the high temps are whats causing this because elements become more reactive with eachother at higher temperatures. Its why an incredibly hot fire, or a deflagration wave thats being ionized by an electrical source, will actually be able to burn nitrogen. Thats normally not possible, but at temps above 4000 F. and with the right conditions the nitrogen will actually bond with oxygen and form NOX. At high temperatures steel, then stainless steel, and so on, will bond with oxygen. Of course each element has its own properties and some metals have high temperature resistance because they will not bond with oxygen. Tantalum really can't bond with oxygen for the most part, it requires exposure to hydrofluoric acid, and other similar oxidizers, to corrode. Not all oxidizers are oxygen. For example, CLF3 is 8 times better at oxidizing than pure Oxygen itself.

What it really comes down to is, a liner needs to be non reactive and resistant to bonding with oxygen at higher temperatures.

I'm going off what I've read.
What's your take on metal disintegration in the riiser?

Tom Robertson wrote:I'm going off what I've read.
What's your take on metal disintegration in the riiser?

The science says that so long as the metal does not reach or exceed the point of incandescence the metal will last just as long in the form of a rocket heater as it would if it were made into a chair or table or whatever. BTW disintegrate isn't quite accurate, the metal if its exceeding its temperature limit is corroding/oxidizing. If it were disintegrating it would be destroyed at the subatomic level into subatomic particles. Actually, if you can get a rocket heater to disintegrate at the subatomic level please let me know. Because thats probably producing mondo amounts of energy (and a lot of radiation).

Sky Huddleston wrote:The science says that so long as the metal does not reach or exceed the point of incandescence the metal will last just as long in the form of a rocket heater as it would if it were made into a chair or table or whatever.

Well made chairs don't generally glow when you use them though, unlike the metal in business end of a rocket heater.

Ok take a chair made of copper for example and leave it out side ; It will turn greenish  but apart from that nothing much
Take a piece of copper place it in a flame about 400c like a bunsen burner  well below any incandescent stuff and watch it turn black oxidise and eventually turn to powder . Reaction rates are dependant on temperature . lots of stuff can happen before things go incandescent. I think it a mistake to be concerned only with that part of the situation

Sky Huddleston wrote:

What it really comes down to is, a liner needs to be non reactive and resistant to bonding with oxygen at higher temperatures.

Sky, i'll let the idea out.

You know in France, we have those aluminized steel pipes for small stoves. And since the surface of the aluminium coating turns to pure alumina in maters of seconds, they are protected. I haven't done that extensive a testing on these. But they stand the heat for a while.

I think the trick, for a lasting metal heat riser, would be to use a tungsten molybdenum stainless steel pipe, with a proper alumina coating inside.

Another lead could be

http://www.google.fr/url?url=http://web2.eng.nu.ac.th/nuej/file/journal/NUEJ_Vol6_1_2011_paper08.pdf&rct=j&frm=1&q=&esrc=s&sa=U&ved=0ahUKEwixvPXS_ejPAhWHWBoKHYPLC_oQFgglMAI&usg=AFQjCNHvZYkewwtmDGveteFkmULPwXYDOQ


again alumina coated.

I cannot get your link to work santamax
Alumina is tricky stuff sometimes it works other times it does not. and when it fails it can fail big time.
A classic simple experiment is to wipe a bit of aluminium with mercury and watch it dissolve . I would be worried about using it .

Why not try tungsten ? Its not that expensive as far as i could find, it melts at 3422c..

Just no clue where you can buy it.

Price - "A broad range of prices for finished tungsten products would be from $25 to $2500 per kilo, with the majority of products in the $100 to $350 per kilo range."  see here http://www.tungsten.com/tips/tungsten-and-costs/
Bricks not so much but then they are more difficult to sell and make money on

David

Burra Maluca wrote:

Sky Huddleston wrote:The science says that so long as the metal does not reach or exceed the point of incandescence the metal will last just as long in the form of a rocket heater as it would if it were made into a chair or table or whatever.

Well made chairs don't generally glow when you use them though, unlike the metal in business end of a rocket heater.

^If the metal is kept cool via fins or has a high enough point of incandescence then even in a Rocket Heater it wont be an issue.

David Livingston wrote:Ok take a chair made of copper for example and leave it out side ; It will turn greenish  but apart from that nothing much
Take a piece of copper place it in a flame about 400c like a bunsen burner  well below any incandescent stuff and watch it turn black oxidise and eventually turn to powder . Reaction rates are dependant on temperature . lots of stuff can happen before things go incandescent. I think it a mistake to be concerned only with that part of the situation

^I'm mainly talking about steel, nobody uses copper to make a Rocket Heater, much too low of a temperature resistance. Reaction rates for steel almost identically correlate to incandescence. Again other elements have other properties.

I would be interested to find out where this idea about pulling oxygen out of the metal comes from.  To me it is vaguely like the idea that "when steel crystallizes, it breaks".

Incandescence should only be a function of temperature.  Many metals melt long before incandescence comes into play.

Lots of knowledge of high temperature metal usage comes from gas turbine knowledge.  Cooling of hot parts is something that is done, as is the coating of parts with glass and/or ceramic.  It is common to see yttrium oxide in these coatings.  You would be unlikely to see any alkali metal oxide in such a coating (too high a vapour pressure).  Some other metals that could be considered based on the temperature are rhenium, tungsten and alloys of tungsten with rhenium.

Some high temperature coatings do not revolve around glass/ceramic.  One that comes to mind, is iridium (a platinum group metal).

I haven't done it myself, but seen in written a few times.  It is possible to get aluminum to the point where the oxide skin is holding the object together.  All the aluminum metal inside the skin has become liquid.  Pierce the oxide skin, and the metal comes out and the object fails.

In terms of intermetallic compounds, it may be that nickel, iron or titanium aluminide could be considered.  They all probably need an oxide (glass, ceramic) coating.

On the basis of temperature alone, carbon/carbon would work.  There is far too much oxygen in your environment for carbon/carbon to last.  It would need a coating.  At some point, you will have built up full density through the fibre region, and then you would more or less switch to producing a layer for surface properties.  You may be able to get a carbon based coating that is similar to an aerogel, or you might go for a carbon filled with particulate oxide.  With particulate oxide, you may then be able to coat the material with an oxide (glass, ceramic) similar to a metal.

Thermal fatigue can be an issue with all these coatings, as you are going through a rather large change in temperature between stove not running, and stove running.

If you go to oxide coatings, you may also have to worry about sulfur.  Sulfur may affect your refractories already.

Onions, garlic and related plants can concentrate sulfur.  Do any trees concentrate sulfur?

It isn't just sulfur, it is all the heavier than oxygen chalcogenides.  There are some plants that concentrate
selenium  Possibly the same ones concentrate tellurium.

The chalcogenide problem largely doesn't exist with gas turbines, as the fuel is usually specified to be low sulfur.  Natural gas might be an exception in this regard.

Gordon Haverland wrote:I would be interested to find out where this idea about pulling oxygen out of the metal comes from.  To me it is vaguely like the idea that "when steel crystallizes, it breaks".

Incandescence should only be a function of temperature.  Many metals melt long before incandescence comes into play.

Lots of knowledge of high temperature metal usage comes from gas turbine knowledge.  Cooling of hot parts is something that is done, as is the coating of parts with glass and/or ceramic.  It is common to see yttrium oxide in these coatings.  You would be unlikely to see any alkali metal oxide in such a coating (too high a vapour pressure).  Some other metals that could be considered based on the temperature are rhenium, tungsten and alloys of tungsten with rhenium.

Some high temperature coatings do not revolve around glass/ceramic.  One that comes to mind, is iridium (a platinum group metal).

I haven't done it myself, but seen in written a few times.  It is possible to get aluminum to the point where the oxide skin is holding the object together.  All the aluminum metal inside the skin has become liquid.  Pierce the oxide skin, and the metal comes out and the object fails.

In terms of intermetallic compounds, it may be that nickel, iron or titanium aluminide could be considered.  They all probably need an oxide (glass, ceramic) coating.

On the basis of temperature alone, carbon/carbon would work.  There is far too much oxygen in your environment for carbon/carbon to last.  It would need a coating.  At some point, you will have built up full density through the fibre region, and then you would more or less switch to producing a layer for surface properties.  You may be able to get a carbon based coating that is similar to an aerogel, or you might go for a carbon filled with particulate oxide.  With particulate oxide, you may then be able to coat the material with an oxide (glass, ceramic) similar to a metal.

Thermal fatigue can be an issue with all these coatings, as you are going through a rather large change in temperature between stove not running, and stove running.

I have nothing really to add to this, good stuff. All I have to say is that the incandescence often times correlates with accelerated oxidation, particularly in steel, stainless steel, nickel, copper, and other common metals. Much like how CO output correlates with grams per hour of fine particles even though they are different. Oxide coatings are usually expensive.

I have nothing really to add to this, good stuff. All I have to say is that the incandescence often times correlates with accelerated oxidation, particularly in steel, stainless steel, nickel, copper, and other common metals. Much like how CO output correlates with grams per hour of fine particles even though they are different. Oxide coatings are usually expensive.

Materials science is my field.  

If Fe/Ni/Cu surfaces give rise to light emission at elevated temperature in an oxygen environment, you might think of it as combustion.  Maybe.  Different metals will have different emission curves as a function of temperature, but nominally yellow heat for steel should be the same for cast iron, nickel, stainless, copper, tungsten, tantalum, rhenium, iridium, ....

Some kind of surface chemistry on the fine particles?  There is a chemical equilibrium between CO, CO2 and C.

The coatings are cheaper than making the bulk material out of a resistant material.

But that is sort of why I brought up the aluminides.  Unlike aluminum metal, the part stays solid.  Like aluminum, the surface is protected in large part by alumina.  It can help to add other things to this alumina layer.  Ductility has been a historical problem, but if you are comparing them to refractories they are probably okay.  I suspect your cheapest one is going to be iron aluminide, and it probably has the worse properties of the 3.  But, it has been a while since I read much about it.

Cold spraying might form a useful coating, but I think the various hot spraying methods might be better for you.

I do think your limiting factors are thermal fatigue of the coating (leading to spalling) and sulfur (selenium, tellurium).

I didn't know much about your stoves, and just read a bit at Wikipedia (before my first reply).