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Sat Dec 9, 2017, 02:04 AM

The Remarkable Thermal Stability of the MAX Phase Zr2Al4C5

Last edited Sat Dec 9, 2017, 08:26 AM - Edit history (1)

There are many thermochemical cycles known for the decomposition of water into hydrogen and oxygen (in separate compartments) as well as thermochemical cycles for the decomposition of carbon dioxide into carbon monoxide and oxygen, again in separate compartments.

Carbon monoxide can also be disproportionated to give elemental carbon and carbon dioxide; this is known as the Bouardard reaction. Thus it is theoretically possible given a source of high temperatures to reverse coal combustion.

Carbon monoxide can - and industrially is - used to make hydrogen: This is known as the "water gas reaction:" CO + H2O <-> H2 + CO2. Industrially this important reaction, which is used to make 99% of the hydrogen on earth, is driven by the partial combustion of dangerous natural gas, a fuel that like oil and coal is destroying the planetary atmosphere.

However, if carbon monoxide were made instead from carbon dioxide of course, this would have possibly the effect of reversing the effects of combustion dangerous fossil fuels and directly dumping dangerous fossil fuel waste, chiefly (but not limited to) carbon dioxide.

These cycles have been broadly studied and are fairly well known.

Examples of thermochemical water splitting cycles are the "sulfur iodine" cycle, the UT-3 (CaBr2) cycle, the copper chloride cycle, and others.

Examples of thermochemical carbon dioxide splitting cycles are the tin oxide carbon dioxide and water splitting cycle, the cerium dioxide carbon dioxide splitting cycle, and one of my absolute favorites, the zinc oxide cycle, among others.

In order to get grants one must appeal to the useless fantasy about solar energy, in particular thermal solar energy plants, which have not worked, are not working and will not work, which is why in some cases they are described as "solar thermochemical cycles" but there is no practical reason that they would not work with cleaner, safer, and more practical and sustainable energy, nuclear energy.

The basic problem with many of these cycles - most of these cycles - is that they require fairly high temperatures in corrosive environments. The most famous of these cycles, the sulfur iodine cycle involves the thermal decomposition of two strong acids, sulfuric acid into sulfur dioxide, oxygen and water and the thermal splitting of hydroiodic acid, HI, into hydrogen and elemental iodine.

This is a serious materials science problem.

The extremely important reason that it would be worth solving this materials science problem is that the use of such cycles, coupled with heat transfer to thermoelectric devices or brayton/rankine combined cycle devices, would be extremely efficient overall. In general the greater the heat difference involved in a thermal process, the more work or exergy can be derived from it.

It is regrettable that research into high temperature refractory materials in many materials science departments that I toured with my son while we were researching universities for him to attend seems to have been deprioritized with the major aerospace problems having been more or less solved, but that said, there is yet still some that is of interest.

Egyptian-American scientist, Michel Barsoum at Drexel University has been a world leader in the development of the MAX phases (My son actually met him during one of the tours; he was admitted there but chose to go elsewhere.)

There are many different MAX phases, and I came across an interesting one that I encountered in a paper I came across tonight in my unexplored files is the one described in the title of this post, Zr2Al4C5 a ternary compound of the elements zirconium, aluminum, and carbon, all earth abundant elements. (Zirconium is also a prominent fission product.) The paper is this one:

Thermal stability of bulk Zr2Al4C5 ceramic at elevated temperatures (Zhang et al, Int. Journal of Refractory Metals and Hard Materials 30 (2012) 102106)

The authors succinctly and accurately describe what the MAX phases are and why they are interesting:

MAX phases are nano layered ceramics with the general formula MAX, where M is an early transition metal, A is a Group A element, and X is either carbon or nitrogen. These materials exhibit a unique combination of the characteristics of both ceramics and metals [14]. The domain of layered ternary transition-metal carbide extends beyond the MAX phases to a new family...


They go on to describe a relatively new class of these compounds which are, again, ternary composites of zirconium (or its cogener hafnium) aluminum and carbon.

Here is what they say about the compound described in the title:

Among these compounds, Zr2Al4C5 ceramics exhibit perfect high-temperature mechanical properties. Young's modulus decreases slowly with increasing temperature. At 1580 C, Young's modulus is 293 GPa, which is approximately 81% of that at room temperature. Simultaneously, the strength at 1400 C is 371 MPa, which is approximately 10% higher than that at room temperature [9,10]. Zr/HfAlC compounds demonstrate excellent elastic stiffness and strengths of up to the temperature range for ultrahigh-temperature applications.


Wow.

The research in this paper involves finding out how high temperatures can go before the MAX phase decomposes. (The authors note that these temperatures, the decomposition temperature of the various MAX phases - there are a lot of them - vary with the conditions to which they are exposed; they differ in the presence of vacuums, under various gases, inert and otherwise, and other chemical environments.)

Here are what they find out and conclude.

The high-temperature thermal stability of Zr2Al4C5 under Ar atmosphere has been studied by thermal expansion analysis. The presented thermal expansion analysis result is in good agreement with the XRD and SEM results. Zr2Al4C5 was susceptible to decomposition at temperatures above 1900 C through sublimation of high vapor pressure of Al, which resulted in the formation of a little amount of Al and Zr2Al3C5 on the surface layer. Ternary-phase Zr2Al4C5 and/or Zr3Al4C6 decomposed to ZrC and Al4C3 above 1900 C due to weaker covalent bonds between ZrC slabs and Al4C3-type layers. Zr2Al3C5 further decomposed to ZrC1−x and Al4C3 at 2000 C, and the amount of decomposing phase was found to slowly increase. The dissociation of Zr2Al4C5 was not complete at the end of the experiment, implying that the process never reached completion because of very slow kinetics. The present study clearly indicates that thermal expansion analysis, when combined with XRD and SEM, can provide a practical way for studying the thermal stability of ultra-high temperature materials.


High temperature refractory materials can often be protected beyond their melting (or in this case decomposition) point by coating with thermal barrier coatings, the most widely used one being zirconium oxide. To the extent that this type of coating would be useful in extreme environments is questionable. It would certainly not be stable in the presence of decomposing sulfuric acid or hydroiodic acid, but it would be interesting to understand the stability of the MAX phase or modified version in question under these conditions.

It is interesting to note that zirconium, aluminum, and carbon are all fairly transparent to neutrons, and the stability of MAX phases in neutron fluxes is an active area of research. I personally believe that these phases might do remarkable things, should the world survive puerile orange fools and his traitorous apologists and fellow nut cases.

This is esoteric, I know, but interesting.

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