High Temperature and Very High Temperature Materials

[Astronuc]

A family member shared an article about thermovoltaics being developed at MIT with support from National Renewable Energy Lab (NREL).

A new heat engine with no moving parts is as efficient as a steam turbine​

https://news.mit.edu/2022/thermal-heat-engine-0413

The article mentions that the "design can generate electricity from a heat source of between 1,900 to 2,400 degrees Celsius, or up to about 4,300 degrees Fahrenheit."

Not too many materials are solid at those temperatures. Graphite (carbon) is mentioned as the material in the hottest part. "The system would absorb excess energy from renewable sources such as the sun and store that energy in heavily insulated banks of hot graphite."

Forms of carbon are relatively inexpensive compare to exotic refractory metals and ceramics.

Otherwise, only tungsten (mp 3410°C), rhenium (3180°C), osmium (3045°C), tantalum (2996°C), or some compound, or carbide (e.g., a carbide of Ta or Hf, or both) would be capable of handling temperatures up around 2400°C. While Mo has a melting point of 2617°C, it would creep/flow at 2400°C if there were any appreciable load/stress.

Melting points of carbides are approximately: 3887°C (HfC), 3875°C (TaC), 3532-3550°C (ZrC), 3480-3500°C (NbC), 3140°C (TiC), and 2830° C (VC). Of course, one has to look at the thermochemical/thermodynamic stability of the metals or compounds at such temperatures.

 

[Vanadium 50]

Inconel? (Or a member of the Inconel family) It's intended to be strong even near its melting point.

 

[Baluncore]

Forms of carbon are relatively inexpensive compare to exotic refractory metals and ceramics.

Otherwise, only tungsten (mp 3410°C), rhenium (3180°C), osmium (3045°C), tantalum (2996°C), or some compound, or carbide (e.g., a carbide of Ta or Hf, or both) would be capable of handling temperatures up around 2400°C.

The problem to avoid comes at the contact between different elemental metals. An alloy of the two metals forms at the contact, with a lower melting point for the eutectic alloy. That alloying mechanism also reduces durability and limits the upper temperature of thermocouples.

 

[anorlunda]

"The system would absorb excess energy from renewable sources such as the sun and store that energy in heavily insulated banks of hot graphite."

So they want to do this?

 

[Astronuc]

So they want to do this?

I suppose that is possible, but I don't know if that facility is considered.

The problem to avoid comes at the contact between different elemental metals. An alloy of the two metals forms at the contact, with a lower melting point for the eutectic alloy. That alloying mechanism also reduces durability and limits the upper temperature of thermocouples.

Yes, that is another key concern.

Inconel? (Or a member of the Inconel family) It's intended to be strong even near its melting point.

Ni melts around 1430°C, so it would not be suitable for temperatures proposed for the device mentioned in the OP. Typically, for a mechanical system like a jet aircraft turbine, one design for a maximum service temperature of something like 1000 to 1100°C, or a homologous temperature of 0.75 to 0.80 (based on m.p. of Ni). ASM International's Specialty Handbook, Heat Resistant Materials puts the melting points of 718 and X750 at 1260°C and 1290°C, respectively, so the homologous temperatures under the aforementioned service temperatures would be ~0.83 - 0.90 for 718 and ~0.81 - 0.88 of X750. The ASM HRM handbook gives the working range for 718 as 915 to 995°C, or homologous temperatures of about 0.775 to 0.827, based on m.p. of 1260°C. I'll have to look into this further, since in the same article, the incipient melting temperature of X750 is given as 1393°C, which is ~100°C high given in another table.

Since the introduction of Ni-based superalloys, Co-based alloys have been introduced. Cannon-Muskegon makes a variety of Ni and Co alloys. One of their main families is the CSMX alloys.

https://www.tms.org/superalloys/10.7449/1996/Superalloys_1996_35_44.pdf

https://www.tms.org/superalloys/10.7449/2004/Superalloys_2004_45_52.pdf

https://www.tms.org/superalloys/10.7449/2012/Superalloys_2012_179_188.pdf

https://cannonmuskegon.com/cmsx-10/ - elements like Re and W are added for creep strength at high temperature.

Jet engines are usually overhauled on a schedule of something like 10 K hours. The higher the temperature, the shorter time between overhauls. Jet aircraft turbine blades are internally cooled with air bleed from the compressor, so the turbine blade root and disc would see lower temperatures than the mid radius and surface of the blade. At least that would be the objective.

If one considers spacecraft propulsion, it might require continuous service of several years, so something like 60 000 hours or 100 000 hours, which means higher strength materials or lower temperatures.

 

[anorlunda]

I suppose that is possible, but I don't know if that facility is considered.

But how else could they use solar energy to get temperatures that high?

 

[Baluncore]

I would be concerned that the advantage over steam turbines would be lost due to the requirement for higher temperature insulation.

How do you get the available energy into an incandescent storage chamber, without it radiating outwards at the same rate?

 

[Baluncore]

How do you charge, insulate and regulate an incandescent storage? I believe that the energy storage must involve a pumped thermal transfer fluid. That provides a number of ways to regulate the energy transfer and storage.

Having only one component of a complex system makes it difficult to evaluate the prospects. Fair comparisons should be made with existing systems.

As an example, the Andasol solar power project in Spain uses 390°C molten salt for energy storage and conversion. It is a stable technology that works and has been replicated.
https://en.wikipedia.org/wiki/Andasol_Solar_Power_Station
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