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Sun Oct 1, 2017, 03:05 AM

Working with one of the most refractory and hardest materials known Tantalum Hafnium Pentacarbide.

Generally, most people are aware that spacecraft and supersonic aircraft require refractory (high melting) materials to avoid being burned up by air friction.

Since the end of the "space race" and the "cold war" there has been less interest in refractory materials than there might have been some years back, something I know from having attended a bunch of presentations of materials science departments while my son was selecting a school.

Be that as it may, whether it is generally known or not, is that future generations, owing to our inattention, fixation on dumb ideas that don't work, and general irresponsibility, will require refractory materials to reverse whatever can be reversed from our willingness to screw them over by dumping trillion ton quantities of dangerous fossil fuel waste (at a rate of over 30 billion tons per year) while we all wait, insipidly, like Godot, for the grand solar and wind Nirvana that never comes (and never will come.)

I could discuss that for hours, but rather than do so, I'd rather simply focus on a paper I collected some time ago on a rather remarkable material that fits the bill, Ta4HfC5, tetratantalum hafnium pentacarbide.

The paper I'll discuss is this one: Reduced-temperature processing and consolidation of ultra-refractory Ta4HfC5 (Int. Journal of Refractory Metals and Hard Materials 41 (2013) 293299)

The introduction gives a nice description of the remarkable properties of this material:

Carbides, nitrides, and borides are of interest for many applications because of their high melting temperatures, high elastic moduli, and high hardness. Among all refractory compounds, 4TaC-HfC ranks among the highest, with an estimated melting temperature of 3942 C and hardness of approximately 20 GPa at 100 g-f [13]. TaC is the most metallic of the IV and V transition metal monocarbides. It has the NaCl-type structure (B1, space group Fm3−m) and an exceptionally high melting point of 3880 C [47]. TaC's relatively good oxidation resistance and resistance to chemical attack have been attributed to strong covalent-metallic bonding [8]. Other relevant properties of TaC include high strength, high hardness (11 to 26 GPa), wear resistance, fracture toughness (KIC ≈ 12.7 MPa-m1/2), low electrical resistivity (42.1 μΩ-cm at 25 C), and high elastic modulus (up to 550 GPa). TaC is also reported to exhibit a ductile-to-brittle transition in the temperature range 17502000 C that allows it to be shaped above the DBTT, and it also exhibits ductility of 33% at 2160 C [912]. Similarly, HfC also crystallizes in the NaCl-type structure (B1, space group Fm3−m, close packed), and exhibits a high melting point (3890 C, the highest among the binary metallic compounds) [1315]. HfC also has good chemical stability, high oxidation resistance, high hardness (up to 33 GPa [16]), high electrical and thermal conductivity, and a high Young's modulus (up to 434 GPa) [1724]. HfC has found applications in coatings for ultrahigh-temperature environments due to its high hardness, excellent wear resistance, good resistance to corrosion, and low thermal conductivity. HfC is also found in high-temperature shielding, field emitter tips, and arrays (HfC has the lowest work function of all transition metal carbides). In addition, HfC can be used as a reinforcing phase in tool steels [2527].

However, if you reflect on it for even a moment, you realize the difficulty of working with such a material. It cannot be worked easily, and as it's melting point is higher than almost any container in which it can be processed, it certainly can't be cast. It's melting point is even higher than remarkable materials like uranium nitride, thorium nitride and thorium carbide. (Thorium oxide, which is mildly radioactive, has been widely used for ceramic refractory crucibles for handling molten metals.)

Such materials can only be handled by sintering, which involves heating them to roughly two thirds of their melting temperature (pretty extreme in any case) and applying extreme pressure, conditions under which the elements can diffuse to a smaller to larger extent.

In this case, the authors milled hafnium carbide and tantalum carbide powders for a long period of time (18 hours) and placed them in a graphite press under a pressure roughly 1000 times atmospheric pressure, and heated them at 1500 C (much lower than the melting point) and got pretty decent tetratantalum hafnium tetracarbide. (Machining this stuff is yet another problem, not addressed here.)

This material is not ready for prime time, nor will it ever be.

Tantalum is mostly utilized in cell phones, where it is a constituent of the supercapacitors on which those devices depend. The mining of tantalum is a great human tragedy, the "coltan" issue. (Tantalum is always found in ores that also contain niobium which was formerly known as columbium, hence the name "coltan" for the ore.) Tantalum is one of the "conflict" elements, and mining it is simply a horror.

This disturbing documentary, "Blood Coltan" is available on line:

Hafnium is a side product of the nuclear industry. It is always found in ores of its cogener zirconium, which is widely used in nuclear reactors. Typically the amount of hafnium in zirconium ores is on the order of 1-3% However, since hafnium has a very large neutron capture cross section (and is sometimes used in control rods, particularly in small reactors like those on nuclear powered ships) it must be removed from zirconium before the zirconium can be used in nuclear reactors.

It is possible however, to obtain pure hafnium free zirconium from used nuclear fuel, where it is a major fission product. The chemical separation of hafnium and zirconium is nontrivial, as is the chemical separation of niobium and tantalum, owing to the "lanthanide contraction." It is possible to obtain monoisotopic zirconium, zirconium-90, (which is lighter than "natural zirconium) from the decay of the fission product Sr-90, itself a useful heat source. Thus at some point it may be cheaper to utilize fission product zirconium instead of natural zirconium, at least it would be so in a sensible world run by intelligent and responsible people, a nuclear powered world.

But we don't live in such a world. (One may hope that future generations will be smarter than ours.)

This said, this information might be useful under many imaginable exotic conditions, and I found it interesting.

Have a nice Sunday.

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Reply Working with one of the most refractory and hardest materials known Tantalum Hafnium Pentacarbide. (Original post)
NNadir Oct 2017 OP
Canoe52 Oct 2017 #1
NNadir Oct 2017 #2

Response to NNadir (Original post)

Mon Oct 2, 2017, 10:32 AM

1. Very interesting, thanks for sharing. Video won't play btw.

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Response to Canoe52 (Reply #1)

Mon Oct 2, 2017, 06:38 PM

2. Try this.


Not a direct link to the video, but you can apparently watch it there.

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