<p>Hypersonic
flight causes ultra-high surface temperatures which are most intense on sharp
leading edges. One way of reducing the surface temperature is to apply a high emittance
ceramic (HEC) on the leading edge, increasing the radiation component of heat
transfer. An ideal HEC must have a high emittance, while also possessing a
strong ablation resistance. From a scientific standpoint, it would be helpful
if emittance could be tailored at different wavelengths. For example, materials
with tailorable emittance could be used to improve the efficiency of engines,
thermo-photo voltaic cells, and other applications. The approach used to create
a ceramic with tailorable emittance was to use two different rare-earth
elements, adding them to an ultra-high temperature ceramic (UHTC) in small
quantities. The samarium element was added to increase the emittance of the
UHTC over a large wavelength range (visible to near infrared wavelengths,
consistent with the temperature range expected for hypersonic flight), and the erbium
element was added to decrease the emittance at specific wavelength ranges. The goal of this study was to create an UHTC
with tailorable emittance while maintaining the required ablation
resistance. Therefore, ZBS billets with five different Sm to Er ratios and with
a nominal total amount of 3 mol.% dopant incorporated were prepared by sintering
in vacuum to 2000 °C. The ablation resistance was evaluated by using an oxyacetylene torch and observing at exposure
times of 60 s and 300 s, whereas the emittance was evaluated at the Air Force
Research Lab facilities via a laser heating testing. The results for the
ablation testing showed that ZrB<sub>2</sub>-SiC (ZBS) billets co-doped
with Sm and Er formed a beneficial <i>c<sub>1</sub></i>-(Sm/Er)<sub>0.2</sub>Zr<sub>0.8</sub>O<sub>1.9</sub>
oxide scale as the
majority phase, which is more thermally stable than the <i>m</i>-ZrO<sub>2</sub> oxide scale typically formed in oxidized ZBS
systems, resulting in a more adherent oxide scale to the unreacted material. The crystalline oxide scale and the amorphous
phase were formed by a convection cell mechanism where the <i>c<sub>1</sub></i>-(Sm/Er)<sub>0.2</sub>Zr<sub>0.8</sub>O<sub>1.9</sub>
crystalline islands precipitate, grow, and coalesce. Moreover, differences in surface
temperatures between ZBS samples with different dopant ratios suggest
differences in spectral absorptance/emittance between each of the five
compositions evaluated.
Despite that the emittance profiles with varying Sm:Er molar ratios were
similar because <i>m</i>-ZrO<sub>2</sub> was
formed as the major oxide phase, the emittance study showed that the erbium
oxide influences the emittance profile, as can be noted by the maximum and
minimum emittance peaks. Furthermore, results showed that the emittance varies
as a function of dopant(s) molar ratios and temperature at shorter wavelength
ranges. These changes in the emittance are caused by the different Sm and Er
concentration on the surface. Future work should be focused on producing the beneficial
<i>c<sub>1</sub></i>-(Sm/Er)<sub>0.2</sub>Zr<sub>0.9</sub>O<sub>1.8
</sub>phase directly from the manufacturing process, and therefore, maximize the
effect of varying the Sm:Er molar ratios to tailor the emittance. Nonetheless,
this study represents the first generation and reported emittance data of UHTC
doping ZBS systems with both Sm and Er elements. </p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8030867 |
| Date | 14 May 2019 |
| Creators | Angel A Pena (6624245) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/Evaluation_of_Rare-Earth_Element_Dopants_Sm_and_Er_Effect_on_the_Ablation_Resistance_and_Emittance_Tailoring_of_ZrB2_SiC_Sintered_Billets/8030867 |
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