Refractory Ablative Heat Shields for Spacecraft: A Path to Addressing Climate Change?

[NNadir]

The paper I will discuss in this post is this one: Zirconium-Doped Hybrid Composite Systems for Ultrahigh-Temperature Oxidation Applications: A Review (Giridhar Gudivada and Balasubramanian Kandasubramanian, Ind. Eng. Chem. Res., 2019, 58 (12), pp 4711–4731)

This paper itself is not about climate change, and the reason I am posting it here in the E&E section, rather than the Science group, where it may be equally appropriate if not more appropriate, is solely based on my own speculations, speculations connected with some insight into how superalloy turbines, wherein the surfaces are protected by thermal barrier coatings, work. These types of turbines are generally utilized in dangerous fossil fuel combustion systems such as combined cycle gas - and far more rarely in combined cycle integrated gasification coal plants - and in dangerous petroleum fueled jet aircraft, but they it is clear that they might well be adapted for use in cleaner and safer nuclear systems. One feasible avenue - not the only avenue, but certainly one likely to be important - is the high temperature, high pressure reformation of biomass. Some, but not all, of the energy invested in reforming the biomass can be recovered by allowing the resultant gases, likely to be a mixture of steam, hydrogen, and carbon dioxide (or, if the water has been consumed, carbon monoxide) to expand against a turbine. The hydrogen/carbon oxide mixtures (syn gas) will then be available to displace all of the current uses for dangerous fossil fuels, including those that represent sequestration in products. In this case, particularly in the case of extreme temperatures that are ideal for many reasons of efficiency, the velocity and temperature of the gases, particularly at critical points like nozzles, are likely to approximate those found on the surfaces of vehicles experiencing re-entry or launch at supersonic speeds.

Thus, the relevance of this materials science paper to climate change can be established.

The introduction to this review indicates what the subject is really about, which is not turbines, but high speed aircraft and space craft:

Ablative materials are degenerative composite systems which, by design, are processed to degrade at projected rates when exposed to high aerodynamic heat rates (?10^5 BTU/ft^2) at high temperatures (?8000 °C). Ablative materials have diverse applications1?5 in the fields of aerospace as a protective layer for leading edges6 of the control surface, in medicine for curing various diseases in form of ablating lasers and in space technology as thermal protecting systems at hyperthermal7?9 environments. In the field of medicine, ablation2 phenomenon is used to cure tumors and treat irregularities in heart pulse rates; by focusing high dosages of energy over a small volume, as in the case of ablative radiography or catheter ablation for atrial fibrillation; however, in the case of aerospace technology, heat energy is insolated upon a larger surface that is to be considered. The term “ablation” in medical terminology implies the complete removal of material from the host system, as in the case of tumors and in the case of atrial fibrillation, the paths of unnecessary impulses are cut down, whereas, for the field aerospace technology, only a part of the system is necessarily required to ablate at known uniform rates under stable operating conditions. A technical understanding of the phenomenon ablation, as early as 1983, states that,

“Ablation is a complex energy dissipative process whereby a material undergoes combined thermal, chemical, and mechanical degradation accompanied by a physical change or removal of surface material”.10

The degradation process has been a keen interest among the scientific community for many years and has evolved many techniques to converge upon a common idea, i.e., how an ablative material functions under severe aerodynamic conditions. Recently, the multiphase modified matrix technology, unlike simple single-phase matrix systems of two classes mentioned in the next section has offered a platform for yielding knowledge investment from a multidisciplinary background of science and engineering for design the ablative materials. Therefore, the performance of modern ablatives is tending toward euclidative application of ultrahightemperature ceramics, potentially with zirconium diboride, because of its quick and timely response to the cataclysmic reentry environments as witnessed by the thermokinetic approach and experimental procedures that are discussed in this article.

1.1. Re-entry Vehicle Structures. Re-entry11?14 vehicle structures are marvels of modern engineering that have made human space travel conceivable by guaranteed safe landings, surviving the extreme re-entry conditions that are discussed in the next section...

The main idea of these kinds of systems is two fold: They are designed to dissipate some heat by exploiting the very high heat of vaporization of very high melting (and vaporizing) systems while protecting inner layers from heating beyond their melting points by containing materials that have extremely low thermal conductivity.

One of the best descriptions of this phenomenon is the paper published by the great Princeton University Scientist Emily Carter on the occasion of her induction into the National Academy of Scientists: Atomic-scale insight and design principles for turbine engine thermal barrier coatings from theory (Kristen A. Marino, Berit Hinnemann, and Emily A. Carter, April 5, 2011 108 (14) 5480-5487) The full paper is available open sourced on line, but for convenience I reproduce an excerpt of the introduction here:

Aircraft and power plants share a common source of usable energy: Both employ turbine engines that combust fuel to either propel airplanes or produce electricity. At a time in which efficient use of energy is paramount, improving the efficiency of turbine engines is one means to contribute to this global challenge. Turbine engines operate via the Brayton cycle, which offers lower carbon dioxide emissions and lower cost for power generation than other possible alternatives. Their efficiency can be increased by increasing the inlet temperature...

...However, high-temperature operation, under oxidizing conditions, poses serious demands on the materials...

...Materials must be found that are robust under such harsh operating conditions. Engineers over the past few decades have improved greatly the thermomechanical properties of the metal alloy comprising, e.g., the turbine blades, and have created a multilayer coating for the blades that protects against both heat and corrosion, referred to as a thermal barrier coating (TBC). These materials advances, along with internal component cooling, have been astonishingly successful, allowing the gas temperature to exceed the melting point of the metal alloy from which the engine components are constructed!

The class of multilayered materials largely discussed in the review introduced in this post are ablative, designed to erode (slightly) in use, whereas the layered materials for turbines to which Dr. Carter eludes in her paper, are not. Both papers however discuss the chemistry and material properties of zirconium: Dr. Carter's refers to "YSZ" or yttrium stabilized zirconia, and predicts that a Hf analogue - hafnium is a (relatively rare) cogener of zirconium and titanium - may be superior based on in silico calculations. In the current paper under discussion however, the layered material discusses a material doped with zirconium boride, an extreme refractory.

The paper has a nice graphic showing the classes of refractory layered materials:



The caption:

Figure 1. Classification of materials for the thermal protection system.

Of particular interest are the ultra-high temperature ceramics which are described in the text as follows:

1.3. Ultrahigh-Temperature Ceramics (UHTC) for Ablative Applications. The UHTCs are the ceramics with melting points greater than 2700 °C.1 These materials possess properties like good oxidation resistance, ablation resistance, thermal expansion, and damage tolerance among other characteristic features which are discussed in later sections. The best contender among UHTCs for ablative is ZrB2; nevertheless, there are other ceramics like tantalum carbide (TaC), hafnium-diboride (HfB2), and hafnium carbide (HfC) with melting point temperatures higher than that of ZrB2 but there are other aspects, such as cost, ease of processing, availability, and temperature range of chemical activity (1500? 1800 °C). It is necessary for the ceramic to be used as a matrix modifier while ablation during the re-entry phase that they have to respond to the changes in the environment in the vicinity of the boundary layer.49?53

Technologies based on tantalum, are best avoided, since tantalum is a fairly rare and easily depleted element, and - although it is widely used in cell phones - is a conflict metal. Small amounts of it are synthesized in certain types of nuclear reactors, generally ship borne reactors, in control rods by neutron capture in hafnium, but hafnium, utilized in this fashion because of its high neutron capture cross section is relatively rare, and is found as an impurity in all zirconium ores, from which it must be removed for nuclear applications.

Zirconium ceramics, of which zirconium boride is one example, have extremely high melting points, according to the review, better than 3000°C, however they are said to exhibit poor thermal shock resistance and low fracture toughness and, as is true of many ceramic materials, they are brittle. Thus they, as suggested by Dr. Carter's paper, are utilized as composite materials.

This schematic cartoon, showing how ablative thermal shields work, gives a feel for how the layering works:



The caption:

Figure 3. Representation of the ongoing ablation process.

An issue with layered systems however, is that the properties of the materials must be closely matched, specifically thermal expansion and factors like Young's modulus, or "stiffness."

Some mathematics connected with these considerations are described:

…mathematical evaluation of mechanical properties have been undertaken for fracture at high time rate of thermal loads based on certain assumptions which state that the model (Figure 4) is a two well-bonded plate, which does not consider the interface damage, there is no heat exchange between the UHTC plate and base plate, both the layers geometrically confirm with each other which make calculation easier, finally plate is continuous, isotropic, elastic, and is restricted in the domain of small deformation hypothesis.

The equations for the effective linear expansion in the ceramic layer then are given by eq 1:

(1)

where ?x1 is the elongation in the ceramic plate without external restrictions and ?x? is the value of restricted elongation due to complementary compressive stress at the interface. From the above equation, it is evident that the net elongation of the system when assumed the changes in Young’s modulus (Yc) and Poisson’s ratio (?c) for ceramic material are devoid of temperature changes, could generate an internal stress ? in the ceramic plate plausibly at the interface, which was derived by Li et al.,87 as shown in eq 2:


(2)

Considering the above equation with the effects of temperature would be modified to eq 3.

Here, the pressure stress or internal compressive stress has been taken into account by considering Young’s modulus (Y) and the coefficient of thermal expansion (? :

(3)

The most effective way to mitigate failure by thermal shock is to increase the critical temperature difference of rupture (CTDR). According to Wang et al., the CTDR increases as the temperature of surroundings increase, up to a certain extent, and then decreases. The governing equation for CTDR by Wang et al.,82 is as shown in eq 4

(4)

where h is the heat-transfer coefficient, tS is the thickness of the ceramic plate, and R? is a constant parameter called as second thermal shock resistance parameter and is given by eq 5:

(5)

Note that the physical properties of mechanical interest, such as the Young’s modulus and fracture stress, vary along with temperature as described by eq 6.



where B1, B2, B3, and Bo are material constants and Yo is Young’s modulus at ambient conditions. The fracture stress as a function of temperature is given according to Li et al.,87 as in eq 7. In order to understand the dependence of the fracture stress with temperature, it has to be inferred from the function of Young’s modulus that it is dependent on temperature in a transcendental fashion and so does the fracture strength.



It is mentioned that the term



belongs to a temperature-dependent fracture surface energy term82 and indicates that, as the operating temperature approaches the melting point, the ratio ? has a tendency to unity; as a result, the temperature-dependent fracture stress tends toward theoretical stress, leading to failure. The thermal shock resistance can be increased by incorporating microflaws into the ceramics, like crack, pores, grains, residual stress due to thermal expansion anisotropy, and, as such, eliminating the initial rupture temperature reaching the danger zone of temperature for thermal shock resistance as reported by Kou et al.,88 and Wang et al., for materials such as hafnium diboride and zirconium diboride, respectively, along with mathematical reasoning.82 These microflaws also cause deterioration in mechanical performance, as reported by Wang et al., which is testified by the following equations of fracture mechanics...

There is a lot of similar information in this wonderful review and it will not be possible to cover all of the things covered therein. Regrettably the paper is not open sourced, and must be accessed in a library.

It may be useful though, to look at the pictures.

The fine details of how these materials, for which the above text gives some feel, results in changes to the material as it performs, and in some cases, these changes improve the materials performance.

This cartoon evokes as much:





The caption:

Figure 5. Schematic of ablation cycle of a material

These changes can actually enhance the properties of the material. For example, a zirconium boride carbon composite will become coated with ZrO2 in an oxidizing environment, and enhance temperature resistance, since ZrO2 has well known thermal barrier properties, and, as discussed above, can be modified with yttrium to give the widely used "YSZ" material.

Silicon carbide is a well known and widely used refractory ceramic. When doped with zirconium boride, graphene or graphene oxide can form. The following graphic relates to a consequence of this structural rearrangement, which that the material can be utilized as an oxygen reduction electrode in fuel cells in the presence of platinum dopants, thus showing the further the utility and versatility of these materials. Note that if the carbon involved in the graphene and silicon carbide is obtained by air capture (by any means) the carbon is effectively sequestered.



The caption:

Figure 6. Combined effect of graphene and ZrB2 under the influence of an ionized platinum on oxidation properties at low temperatures.

While not explicitly described as such in this review, the paper from which the graphic immediately above comes can be found in this open sourced paper, which, if interested, the reader can easily access and read merely by clicking on the link below.

Nano Conductive Ceramic Wedged Graphene Composites as Highly Efficient Metal Supports for Oxygen Reduction (Mu et al, Scientific Reports volume 4, Article number: 3968 (2014)

In the case of a re-entry vehicle the temperature driven evolution of the material is evoked by the following cartoon:



The caption:

Figure 7. Illustration of ZrB2–SiC response in a typical re-entry environment.

A more detailed representation:



The caption:

Figure 8. Detailed illustration of highlighted area for the typical response of ZrB2–SiC to re-entry environment.

The addition of additional elements are being evaluated to improve the performance of these materials:

Many scientists have investigated the aforementioned studies and concluded that mechanical alloying with rare-earth elements forms a multilayer protective glass coating, yet each layer may still be multiphase. Tan et al.104 modified ZrB2 with samarium and thulium through two processes: first, chemical doping by CVI technique and second, by dry mixing in a ball mill, followed by compaction in a press. Furthermore, they reported that chemically doped ZrB2 best performs by enhanced surface emissivity, which is an ingenious technique to deal with ablating environment as radiation can transfer 90% of heat. It is required to recollect that the addition of one atom to another effects cation field strength and the addition of transitional metals to ZrB2 due to optimal cation field strength (eq 8), there would be immiscibility, which increases viscosity, as explained by the Einstein?Stokes equation (eq 9) of the melt at oxidation temperatures. As a result, oxygen transport into the material reduces in proportion to the increasing viscosity of the melt. In addition, mechanical mixing has not given many admirable results, when compared to chemically modified ZrB2, as reported by Monteverde et al.105

(8)

(9)

where C denotes cation field strength, Z denotes valency, r denotes ionic radius, D denotes diffusion rate, K denotes Boltzmann constant, ? denotes viscosity, p denotes particle dimension, and T denotes absolute temperature…

…Another innovative idea to form multilayer was reported by Zhang et al.,107 by doping zirconium diboride with tungsten carbide (WC), which lead to the formation of dual glass layer (Figure 10), the top layer was porous and depleted of tungsten oxide and appeared light in complexion, while the bottom layer was rich in WC and appeared dark and dense...

Like zirconium, samarium is a fission product, and thus given the high energy to mass density of nuclear energy even when compared with dangerous fossil fuels, appreciable quantities may be available in the reprocessing of used nuclear fuels, especially when the timing of the reprocessing is utilized to minimize or maximize residual radioactivity for some isotopes in some of these elements. The higher lanthanides beyond europium are not appreciably represented as fission products, for example thulium, although small amounts may be formed in a kind of earthbound aufbau process - the manner in which heavy elements are formed in stars in the s-process - in "breed and burn" nuclear reactors, the kind I personally favor. This would result in the use of fission products with high neutron capture cross sections, as represented by the heaviest lanthanide fission products as neutron shields (and in some cases heat sources to maintain metal coolants in liquid states during shut down) In any case, not all of these strategies result in positive outcomes, and the matter should remain an area of materials science research.

Figure 10:




The caption:

Figure 10. Effect of modifying ZrB2 with WC on the formation of barrier coat.

Overall, these effects are summarized in the following graphic:



The temperature gradient that these materials generally experience is shown in this cartoon:



The caption:

Figure 12. Temperature profile of the ablative material.

There is a nice evocation of the thermodynamics of these systems - thermodynamics being the science most routinely ignored by those "efficiency will save us" and "batteries will save us" types that have lead us to the horror of the dangerous fossil fuel waste carbon dioxide's concentrations being permanently well above 410 ppm (and rapidly rising). The discussion includes some very beautiful and fun differential equations as well as an evocation of the Arrhenius equation, Arrhenius being the guy who told us in the late 19th century that what is happening would happen with respect to climate change:

It has also been mentioned that there is a continuous thermal gradient that exists through the char region, reaction/ pyrolysis zone, and unaffected virgin material (Figure 12). An Arrhenius-type temperature-dependent reaction rate (eq 12) has been mentioned and explained as follows:

A significant work presented by Norman et al., where the temperature distribution was presented as a function of char depth and energy balance (eq 13).

The first term apparently is the rate of heat flow through the nonporous part of the material, calculated from Fourier’s principles for one-dimensional (1-D) heat flow, the second term excludes the conductive heat flow into the trapped gases inside the voids, which could otherwise flow away to the surface with the velocity ?s, the fourth term is probably the heat rate exchanged between the hot entrapped gases and elemental material of depth dx, while the gases are expelled out of reaction zone of the material with no effectiveness of heat exchange taken into consideration; finally, the last term has been mentioned by Norman et al.,57 as a result of heat of decomposition. With the above analysis, where, for eq 13, ϵ is fractional void in the solid (for a unit length volume fraction and area fraction do not vary significantly), ?s is a relative velocity between the material surface and incoming mass of air, ?s is the density of the material, cps stands for the specific heat of material, cpg represents the specific heat of gases, ṁg is the mass rate of gases, ks is the conductivity of the solid, ?E is the activation energy of phenolic matrix (11 kcal mol?1), MW is a constant with the value of 10, and, finally, kg denotes the conductivity of gas...

There is then a discussion of the preparation methods of these materials, a subject I personally find interesting because of my interest in printable nuclear reactor cores composed of ultrahigh temperature ceramics represented by actinide nitrides. I just have time for the cartoons.



The caption:

Figure 13. Depiction of the sol–gel process.

The caption:

Figure 15. Schematic of a typical CVI process.

There's a depiction of the test equipment:



The caption:

Figure 16. Schematic of the ablation test.

And a graphic illustrating the over all concepts:



The caption:

Figure 17. Design parameters of an ablative material.

Some phenolic resin chemistry of carbon relative to the building of these materials:



The caption:

Figure 18. Mechanism of coalescence of phenol rings during pyrolysis.

This post is, I'm sure, highly esoteric, even for my posts, many of which fit into the category of "esoteric."

I write them to fix concepts in my mind, and post them on the off chance that there are people interested in the practical scientific and engineering issues of addressing climate change which, trust me, are way beyond anything being discussed politically and popularly. There are scientists working long and hard hours to build the intellectual infrastructure by which we may save what remains to be saved, and any attention they get, improves whatever small chances remain for our planet.

Irrespective of your interest in the practical approaches to addressing and even reversing climate change, and the Herculean engineering tasks they represent, I trust you're having a nice weekend.

[safeinOhio]

1. Details are way over my head, but

how cool is that.
Thanks.

[customerserviceguy]

2. I'm sure you lost most people here

when you mentioned the word "nuclear", many here think that nuclear power is part of the problem rather than being part of the solution.

Me, you lost me when it started looking like Sheldon Cooper's whiteboard.

[NNadir]

4. Anyone who is familiar with anything I write here...

...will recognize that for purely environmental reasons, I am a pro-nuclear as anyone could possibly be.

In fact, I frequently state my opinion, built over 30 years of intense review of the science, that nuclear energy is the only practical technology that can do anything reasonable to save what is left and not yet destroyed.

I recognize that the refusal to be educated exists, as much as I hate to say it, on the left as much as on the right.

As a life long Democrat, I feel about the rote anti-nuke members of our party, much as a pre-Trump Republican scientist whose politics focused on a belief in libertarian small government might have felt about creationists, as something as an embarrassment to be tolerated.

I'm actually not all that tolerant, but, over the years here, I've learned to avail myself of the "ignore" button for the dumbest and most egregious anti-nukes.

As I frequently point out, one of America's most prominent climate scientists has pointed out that opposition to nuclear power is - although he certainly didn't use the word - obscene.

Prevented Mortality and Greenhouse Gas Emissions from Historical and Projected Nuclear Power (Pushker A. Kharecha* and James E. Hansen Environ. Sci. Technol., 2013, 47 (9), pp 4889–4895)

The point is that opposition to nuclear power kills people.

Who is Sheldon Cooper?

Thanks for your comment.

[customerserviceguy]

5. I'm one of those

who agree with you. Nuclear energy, properly managed, is a viable way of getting to a zero emission economy.

I was referring to "The Big Bang Theory" TV show.

[littlemissmartypants]

3. A good lesson in rare earth metals. Thanks for the post, NNadir. nt