Heat-resistant materials

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Zirconia has a very high melting point, about 2700C. This is the primary reason why it is used as a refractory.

However, after some preliminary searches, I found another common ceramic material with a melting point higher than that of zirconia - magnesia (MgO), with a melting temperature of around 2800C. Why isn't there extensive research being done on magnesia for use in refractory ceramics?
 

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  • #2
Integral
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First off I am not an expert in materials, but I do know that there is more to refractory materials then just melting point. A key property is the thermal diffusivity, that is how it adsorbs and conducts heat. You must also consider things like toxicity if the material is to be handled by living creatures.

Others should be able to add more details.
 
  • #3
PerennialII
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MgO refractory bricks are used in steel furnaces for one, so at least to some extent magnesia is being used in such applications. What am left thinking is whether MgO can match the inertness of ZrO2 & what other reasons are there .... :confused:
 
  • #4
Astronuc
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The type of thermal ceramic depends upon the application (temperature and environment) and cost. The higher the temperature or more aggressive the environment, the higher the cost because the material will be special.

Here are some examples of refractory ceramics - http://www.thermalceramics.com/literature/datasheets.asp
from http://www.thermalceramics.com/home.html

High Temperature Insulating Firebrick - primarily alumina-silica
http://www.thermalceramics.com/upload/pdf/114-2.pdf

See also - http://en.wikipedia.org/wiki/Firebrick
 
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  • #5
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Great links, but I'm still puzzled as to why MgO isn't as used as often as alumina or zirconia in high-tech refractories.

Currently most engineering is focused on ceramic coatings on a metal substrate. Is this due to the high cost of the ceramic? If I remember correctly there is a lot of focus on silicon nitride.
 
  • #6
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Integral said:
I do know that there is more to refractory materials then just melting point.

Yes. This is exactly what I want to find out. Is there something wrong with the mechanical behavior of MgO at high temperatures?
 
  • #7
PerennialII
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...... looked at couple of general material property references since books had on ceramics had pretty much nothing on this, comparatively speaking. Overall comparing MgO to other similar engineering ceramics, what properties found can be summarized as follows:
  • strength at high temperatures bit better than ZrO2, not as good as Al2O3 or SiC
  • fracture toughness lower than most others, like Al2O3, Si3N4, SiC, and (naturally&especially) ZrO2
  • Average ceramic with respect to modulus, nearly up to par with Al2O3, bit better than ZrO2
  • Corrosion wise has issues in strong acids compared to other ceramics, otherwise seems ok.
  • heat capacity pretty equal to alumina, thermal conductivity was given to be higher than that of Al2O3 (ZrO2 has one order of magnitude smaller), depending on reference & alloying etc. by a factor of 1.1 to 1.5.

..... some beneficial thermal properties do appear to arise, high temperature corrosion :confused: ... should find something about those furnaces etc. appls where it is used.
 
  • #8
Astronuc
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Silicon carbide and silicon nitride are structural ceramics with relatively high thermal conductivities. Oxide cermaics have lower thermal conductivities, so are used for insulation.

I would expect that MgO is less expensive than ZrO2, and may be slightly more expensive than Al2O3, since Al is produced at higher tonnage than Mg.
 
  • #9
PerennialII
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....... on the basis of mechanical properties (took a look at some old ceramics related lecture notes), it seems that with high purity (>90%) MgO (and some mixtures for example with alumina) using criteria such as "hot strength" and "hot stiffness" operating temperatures approx. 400 C higher can be attained than with "standard firebrick" (say around and lower than 50% alumina). Said that has to do with the "kneeing" of properties (mechanical properties deteriorate rapidly at a window of few hundred degrees) at higher temperatures (and subsequently starting viscoplastic deformation), a feature to which MgO would be less succeptible (a feat of polycrystals, the temperature increases with purity).
 
  • #10
There are numerous properties that refractories are required to have based on the application in which they are used. Basically, in no order of importance, these properties include resist maximum service temperatures, resist spalling, support heavy loads, resist abrasion and erosion and the ability to resist corrosion by gases or liquids. The term used to describe or differentiate refractory materials is known as refractoriness. Strange huh.

The use of MgO over other materials may make sense in one application but make no sense in another. You also have to understand that rarely is a refractory made completely out of one mineral. They are often composed of two or three main minerals along with a few other minerals at much lower levels. It is the combination of these minerals that, when sintered for the first time, make up the overall performance characteristics of a given refractory. On top of all that, if look at a furnace as a whole system, you will never see one refractory material being used throughout the entire furnace. You may have super-duty fireclay bricks in one region, mullite in another and still castable materials in other areas.

There is never an easy answer!
 
  • #11
chemisttree
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From Merck index:

Magnesium oxide (magnesia) Takes up CO2 and H2O from the air. The light form more readily than the heavy... Combines with water to form magnesium hydroxide.... pH of saturated solution 10.3 (anything over 10 is considered hazardous).

uses: Antacid, laxative, (treatment for) hypomagnesemia, manuf refractory crucibles, fire bricks...
-------------------------------------------

This stuff is unstable in moist environments and will carbonate to produce magnesuim carbonate. It will settle your stomach while giving one the hershey squirts. Hmmmm, I'll stick with hafnium boride or silicon nitride, thank you!
 

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