Thick sputtered tantalum (Ta) coatings on polished Inconel were investigated as a potential replacement for bulk refractory metal substrates used for high-temperature emitters and absorbers in thermophotovoltaic energy conversion applications. In these applications, high-temperature stability and high reflectance of the surface in the infrared wavelength range are critical in order to sustain operational temperatures and reduce losses due to waste heat. The reflectance of the coatings (8 and 30 μm) was characterized with a conformal protective hafnia layer as-deposited and after one hour anneals at 700, 900, and 1100 °C. To further understand the high-temperature performance of the coatings, the microstructural evolution was investigated as a function of annealing temperature. X-ray diffraction was used to analyze the texture and residual stress in the coatings at four reflections (220, 310, 222, and 321), as-deposited and after anneal. No significant changes in roughness, reflectance, or stress were observed. No delamination or cracking occurred, even after annealing the coatings at 1100 °C. Overall, the results of this study suggest that the thick Ta coatings are a promising alternative to bulk substrates and pave the way for a relatively low-cost and easily integrated platform for nanostructured devices in high-temperature energy conversion applications.

Thermophotovoltaics (TPV) is a high-temperature energy conversion scheme with no moving parts and low-maintenance requirements that allows for scalable electricity production with high specific energy from a variety of heat sources. In TPV systems, thermal radiation from a heat source at high temperature drives a suitable small bandgap photovoltaic cell. The heat can be produced by hydrocarbon combustion, ideal for portable power sources;1–4 by radioisotopes, ideal for space missions and remote missions requiring power sources with long lifetimes and low maintenance;5 or by solar radiation absorbed by a suitable absorber and converted to heat.6,7

The selective emitter tailors the photonic density of states to produce spectrally confined selective emission of light matching the bandgap of the photovoltaic cell, enabling high heat-to-electricity conversion efficiency.8–11 The selective emission can either originate from the natural material properties, such as in ytterbia or erbia emitters,12 or can be engineered through microstructuring.13–19 Microstructured thermal emitters and absorbers have been studied and fabricated on single crystal tungsten (W) bulk substrates8,9,13,15–17 as well as polycrystalline tantalum (Ta) and Ta–W alloy bulk substrates11,20 by etching a periodic pattern into the metallic substrate. Refractory metals, such as Ta and W, are used in high-temperature energy conversion applications due to their high melting temperature, low vapor pressure, and low infrared emissivity.21 

Bulk Ta substrates, in particular, are used for selective emitters in TPV systems and for selective absorbers in solar energy systems, as well as for low-emissivity insulating coatings in thermoelectric and solid oxide fuel cell systems. Bulk emitters must be mechanically integrated to the heat source, which is made from a different material than the emitter. In the case of combustion TPV, the heat source can be an Inconel microburner, and in the case of radioisotope TPV, the heat source can be a platinum–iridium enclosure. These materials are not suitable for photonic crystal fabrication as they are difficult to etch and do not have the required optical properties. A TPV emitter directly integrated into the system in the form of a coating would require less material, potentially decreasing the fabrication and postfabrication complexity and integration cost, and would reduce parasitic edge radiation from the thickness of the emitter. Using a Ta coating as a functional layer on different substrates, selected and matched to the system's needs, would decouple the requirements of the functional layer and the substrate.

Previously, the optical and thermomechanical properties of sputtered W coatings and their evolution at high temperatures were studied as a potential fabrication route for high-temperature nanostructured surfaces.22 This study revealed that sputtered W layers were a promising approach to a high-temperature, high-reflectance coating for a photonic crystal substrate, if the challenge of delamination at high temperatures could be overcome. In another study, evaporated thin Ta coatings were found to have low film density and high surface roughness, resulting in low reflectance in the near infrared wavelength range, which made them unsuitable as a photonic crystal substrate.23 Thus, in this study thicker, denser, highly reflective Ta coatings were deposited via ion-assisted DC magnetron sputtering and investigated as a potential substrate for photonic crystal fabrication.

Thick Ta sputtered coatings have been extensively studied over the past few decades. The coatings have been found to contain one or both of two distinct phases: a stable body-centered cubic (BCC) α-Ta phase that has desirable physical properties, and a metastable tetragonal β-Ta phase that transforms to the BCC phase at around 750 °C is brittle, and has a much higher resistivity and lower reflectance than the α-Ta phase.24,25 Three aspects—the factors that cause the β phase instead of the stable α phase,24–33 the sputtering parameters on the morphology and residual stresses of the coatings,29,32–37 and the effects of annealing on the properties of Ta coatings38—have all been previously investigated. Generally, these studies have focused on the stability and the properties of the β phase, on relatively moderate temperatures (≤750 °C), and on low vacuum conditions.39–45 

However, if α-Ta coatings are ultimately to be used as high-temperature (≥900 °C) thermal emitters, their ability to withstand high-vacuum high-temperature anneals must be assessed. Moreover, their optical properties (which are known to be significantly affected by sputtering34–36,46,47 and annealing23) must also be known. Therefore, the present study evaluates the thermal stability and high-temperature properties of thick sputtered Ta coatings in order to gauge their potential for high-temperature energy conversion applications. The coatings were sputtered on Inconel, a readily available low-cost nickel-chromium-based superalloy used in combustion TPV applications.

Overall, the results of this study suggest that the thick Ta coatings are a promising alternative to bulk substrates and will pave the way for a relatively low-cost and easily integrated platform for nanostructured devices in high-temperature energy conversion applications.

Tantalum coatings were fabricated via ion-assisted DC magnetron sputtering on polished Inconel 625 substrates at a deposition temperature of 300 °C at 2 kW. Prior to deposition, the substrates were cleaned with solvents (acetone, methanol, and isopropanol) and by oxygen plasma to promote adhesion of the coatings. Argon was used as the sputtering gas and the vacuum chamber pressure was 2 mTorr with a base pressure of 5 × 10−6 Torr. A 40 V bias was applied on the samples and the discharge filament was run at 40 A for the secondary plasma to increase ion bombardment. The thicknesses of the samples were determined by contact profilometry after deposition and found to be 8 and 30 μm via witness coupons. A 20 nm conformal layer of hafnia (HfO2) was deposited via atomic layer deposition (ALD) at 250 °C, using tetrakis dimethylamino hafnium and water as precursors, to prevent degradation of the coatings at high temperatures.23 

In order to characterize the optical properties as a function of temperature, the samples were annealed at 700, 900, and 1100 °C for one hour in a quartz-lined Inconel tube furnace in vacuum (5 × 10−6 Torr base pressure), at a slow heating and cooling rate of 2 °C/min. The reflectance of the Ta samples was obtained experimentally at room temperature and after each annealing run using an automated spectroradiometric measurement system (Gooch & Housego OL750), scanning the wavelength from 1 to 3 μm.

The prepared samples were imaged by atomic force microscopy (AFM), using the Veeco Nanoscope V Dimension 3100, to characterize the surface and roughness of the coatings as-deposited and after each anneal. In order to quantify and compare the roughness of the coating, the average one-dimensional surface roughness Ra and root-mean-square (RMS) roughness Rq were calculated from the AFM images. In addition, the autocorrelation R(r), which measures the correlation of surface heights separated laterally by the distance r, and height-to-height correlation H(r) functions were calculated. The autocorrelation was fitted using the R(r)=exp((r/ξ)2α) approximation model where ξ is the lateral correlation length, defined as the value of r at which R(r) decreases to 1/e of its original value, and α is the roughness exponent, extracted from the slope of the corresponding height-to-height correlation function analysis.48 

The residual stress of the coatings was determined via X-ray diffraction (XRD) analysis (Rigaku Smartlab) using a sin2ψ methodology,49,50 as-deposited and after each anneal. After measuring the d-spacing at several sample rotations ψ, the residual stress σ was calculated (under the assumption of a bi-axial stress state in the film) as

σ=1d01S2dsin2ψ,
(1)

in which d0 is the stress-free lattice parameter (taken as the lattice parameter at ψ = 0), and the partial derivative d/sin2ψ is calculated from a fit of the results. The x-ray elastic constant of Ta depends on the crystallographic plane of interest, and is given by S2 = (1 + ν)/E, where ν is Poisson's ratio and E is the elastic modulus. In this work, four Ta diffraction peaks were analyzed ([220], [310], [222], and [321]), and their respective residual stresses were averaged. The values of S2 for each peak, calculated from single crystal elastic constants51–55 as the Neerfeld limit (i.e., the average of the Voigt and Reuss limits), are listed in Table I.

Table I.

X-ray elastic constants of Ta.

hklS2 (10−6 MPa−1)
220 7.022 
310 7.852 
222 6.591 
321 7.022 
hklS2 (10−6 MPa−1)
220 7.022 
310 7.852 
222 6.591 
321 7.022 

The phase and texture of the sputtered Ta coatings were characterized via XRD analysis and compared after different anneal temperatures. The texture of the films was evaluated using pole figure analysis and compared after different anneal temperatures.

In this study, the topographical results, including the roughness Rq, the lateral correlation length ξ, and the roughness exponent α, of the 8 and 30 μm coatings were characterized, as shown in Fig. 1. In all cases, no significant change was observed, thereby revealing that neither the height variation nor the arrangement of this variation was affected by annealing.

Fig. 1.

(Color online) Comparison of the 8 μm and 30 μm coatings in terms of (a) reflectance of the coatings measured at a wavelength of 2 μm, (b) RMS roughness Rq, (c) lateral correlation length ξ, and (d) roughness exponent α, as-deposited, as well as after the deposition of the protective HfO2 conformal layer at 250 °C, and after anneal at 700, 900, and 1100 °C.

Fig. 1.

(Color online) Comparison of the 8 μm and 30 μm coatings in terms of (a) reflectance of the coatings measured at a wavelength of 2 μm, (b) RMS roughness Rq, (c) lateral correlation length ξ, and (d) roughness exponent α, as-deposited, as well as after the deposition of the protective HfO2 conformal layer at 250 °C, and after anneal at 700, 900, and 1100 °C.

Close modal

The reflectance of the coatings, especially in the IR wavelength range, is of utmost importance for the intended use in high-temperature energy conversion applications. The optical properties of the coatings as-deposited and after annealing in vacuum at different elevated temperatures were characterized. As shown in Fig. 2, the reflectance of the Ta sputtered coatings was exceptionally high and comparable to that of polished bulk samples. Dense metallic coatings have been shown to have similar optical properties as their bulk counter parts.24–26,36,37 On average, the reflectance of the 8 μm coating was higher in the measured range than that of the 30 μm coating. Surface roughness causes a decrease in reflectance and can introduce a wavelength dependence through surface plasmon effects.56 The initial increase in reflectance after annealing can be attributed to coating densification, decreased porosity of the layer, and grain growth.57 The subsequent decrease can be attributed to the observed increase in surface roughness. Overall, a minimal net change in reflectance was observed, thus meeting the roughness and reflectance requirements for thermal emitter fabrication.

Fig. 2.

(Color online) Reflectance of the 8 μm (a) and 30 μm (b) Ta coating with the protective HfO2 conformal layer after one hour anneals at 700, 900, and 1100 °C compared with flat polished Ta. All reflectance measurements were made at room temperature.

Fig. 2.

(Color online) Reflectance of the 8 μm (a) and 30 μm (b) Ta coating with the protective HfO2 conformal layer after one hour anneals at 700, 900, and 1100 °C compared with flat polished Ta. All reflectance measurements were made at room temperature.

Close modal

The phase and texture of the sputtered Ta coatings were characterized via XRD analysis. Previous studies have shown that elevated deposition temperatures promote the formation of BCC α-Ta coatings, whereas lower deposition temperatures lead to β-Ta (Refs. 24, 25, and 31). Additionally, it has been shown that α-Ta is the predominant phase in thick sputtered Ta coatings, with increasing β-Ta content with decreasing thickness.32,33,37 As the thickness of the coatings studied here is in the micron range and the deposition was carried out at a substrate temperature of 300 °C, the existence of the beta phase is unlikely. This is confirmed in Fig. 3, which shows an excellent agreement between the experimental XRD peaks and the reference α-Ta peaks.

Fig. 3.

(Color online) XRD diffraction peaks all originating from α-phase; no β-phase is present. (The small peak to the left of the (110) peak is the Cu β-peak from the instrument.)

Fig. 3.

(Color online) XRD diffraction peaks all originating from α-phase; no β-phase is present. (The small peak to the left of the (110) peak is the Cu β-peak from the instrument.)

Close modal

From pole figure analysis, a fiber texture was observed in both films, as revealed in Fig. 4 by the lack of x and y preference. For the 8 μm coating, the surface was aligned with the (110) crystallographic planes, as demonstrated by the surface normals in the center of the (220) pole figure. This is a common growth direction for BCC films due to its high density, and has been observed in many other studies of sputtered Ta films.24,25,30–33 Interestingly, the thicker coating exhibited a (111) fiber texture. This transition in texture with thickness has been previously detected,33 and has been attributed to stress build-up in the growing film which favors a texture that reduces the stress. The pole figures were obtained both before annealing and after one hour anneals at 700 and 900 °C, and no change in texture was observed.

Fig. 4.

(Color online) In both films, a strong (110) fiber texture was observed. The (220) surface normals can be seen in the center of the pole figure. The other projections (211) and (200) show circles, signifying that there is no x and y preference.

Fig. 4.

(Color online) In both films, a strong (110) fiber texture was observed. The (220) surface normals can be seen in the center of the pole figure. The other projections (211) and (200) show circles, signifying that there is no x and y preference.

Close modal

The calculated residual stress, as defined in Eq. (1) is plotted as a function of the annealing temperature and coating thickness in Fig. 5. These results indicate that the as-deposited coatings were highly stressed (values ranging from 1250 to 1750 MPa) and in compression. Values of this magnitude are common for thin films and coatings and have been observed previously in sputtered Ta samples.30,32–36,39,40,42–44

Fig. 5.

(Color online) Residual stress of the 8 and 30 μm coatings calculated as-deposited and after anneal at 700, 900, and 1100 °C. The line represents the average of the four diffraction peaks considered at each temperature.

Fig. 5.

(Color online) Residual stress of the 8 and 30 μm coatings calculated as-deposited and after anneal at 700, 900, and 1100 °C. The line represents the average of the four diffraction peaks considered at each temperature.

Close modal

The stress caused by differences in thermal expansion (thermal stress) was calculated, using the thermal expansion data in Ref. 58 (assuming that the thickness of the coating is negligible in comparison to that of the substrate) and taking the deposition temperature to be 673 K, and found to be about −800 MPa. The stress caused by the deposition (intrinsic stress) was found to be approximately −450 to −950 MPa. Compressive intrinsic stresses are expected in films sputtered with low sputtering pressures, high substrate biases, normal incidence, high target mass, low sputtering gas mass, and a low substrate temperature, as these will produce large particle momenta while maintaining low atomic mobility.46,47 As such, Ta coatings deposited with an argon gas (high target to gas mass ratio), at 300 °C (low homologous temperature of 0.2TM) are expected to be in compression. The occurrence of this intrinsic compressive stress may be beneficial, as it is indicative of dense coatings with optical properties that approach those of bulk materials (i.e., high reflectivity).34–37,42,46,47 On the other hand, it is also worth noting that high compressive stresses can lead to coating buckling and poor adhesion.

The change in residual stress after annealing is shown in Fig. 5. For the 30 μm coating, a slight increase in compressive stress with temperature was observed, but all results were well within error of one another. For the 8 μm coating, the residual stress was found to be relatively stable. The only exception was the 1100 °C anneal, which caused a sudden increase in compressive stress. However, due to the large scatter in the results from all four diffraction peaks, this increase was not statistically significant. This thermally stable residual stress is in contrast to the previous studies on Ta coatings in which an increase in the compressive stress, increase in the tensile stress, or a reversal from compressive to tensile stress was observed with annealing.39,40,43

The changes in residual stresses with annealing can be attributed to a number of processes: plastic deformation, phase transformation, grain growth, and impurities or oxidation. Indeed, at high temperatures, the thermal stress may cause the coating to yield, thereby altering the residual stress. However, as the substrate's thermal expansion coefficient is larger than that of the coating, the thermal stresses will tend to decrease the compressive stress, and ultimately produce a net tensile stress on the order of 1500 MPa at 1373 K. This signifies that plastic deformation may be unlikely at the temperatures investigated and with this combination of substrate and coating. Previous work (on β to α transformations39,43) has shown that phase transitions can result in a relaxation of the residual stress. However, as the coatings studied are already in the stable α phase, transformations will not occur. Additionally, at elevated temperatures, the reduction of the grain boundary area of the coating would reduce its excess volume and produce a net tensile stress.46,47 However, Ta is unlikely to experience significant grain growth at the temperatures of interest,59–64 and this effect may be negligible. Finally, an increase in the impurity content of the coatings, as well as the formation of tantalum pentoxide (Ta2O5), has been associated with an increase in the compressive stress,40–42,44,45 which is caused by an associated increase in the volume of the unit cells. However, the protective HfO2 coating, as well as the high-vacuum conditions, may prevent any significant oxidation, and would therefore prevent volumetric distortion.

In TPV applications, bulk photonic crystal substrates add weight to the system and must be mechanically integrated with the heat source which is often made from a different material. An integrated photonic crystal TPV system would require less material, potentially decreasing the fabrication and postfabrication complexity and integration cost. Using a Ta coating as a functional layer on different substrates, selected and matched to the system's needs, would decouple the requirements of the functional layer and the substrate. In this study, we found that the sputtered Ta layers withstand temperatures up to 1100 °C without delamination and can be used as a viable photonic crystal substrate.

The reflectance of the Ta sputtered coatings was found to be exceptionally high as-deposited and comparable to that of polished bulk samples. The reflectance of the 8 μm coating was found to be higher than that of the 30 μm coating due to lower surface roughness. The reflectance of the coatings did not increase significantly after one hour anneals at 700–1100 °C in vacuum. No significant change in roughness was observed, as expected from the reflectance measurements, thus meeting the roughness and reflectance requirements for thermal emitters. The pole figures, obtained by XRD diffraction before and after the anneals, revealed a fiber texture for all peaks for both coatings at all temperatures. The 8 μm coating exhibited a (110) texture while the 30 μm coating exhibited a (111) texture, attributed to stress build-up. The Ta sputtered coatings were found to be in a state of compressive stress, thereby producing dense coatings with high reflectivity. For the 8 μm coating, the residual stress was found to be relatively stable. For the 30 μm coating, a slight increase in compressive stress with temperature was observed, but all results were well within error of one another. Annealing the coatings was not found to produce any significant changes in the residual stress, demonstrating that phenomena such as plastic deformation, phase transformations, grain growth, and oxidation were not significant under these conditions. Both 8 and 30 μm coatings were found to be suitable substrates for photonic crystal fabrication; however, the 8 μm coating exhibited all in all slightly better optical and thermomechanical properties.

Overall, the results of this study suggest that these thick Ta coatings are a promising alternative to bulk substrates as a relatively low-cost and easily integrated platform for nanostructured devices for high-temperature energy conversion applications.

The authors would like to thank: L. Gibbons at H.C. Starck for waterjetting of the Inconel substrates; J. Guske for assistance with XRD measurements at CMSE; and V. Rinnerbauer for insightful discussions. XRD analysis was done at the MIT Center for Materials Science and Engineering XRD SEF. ALD was done at CNS at Harvard University, a member of the National Nanotechnology Infrastructure Network (NNIN), supported by the National Science Foundation under NSF Award No. ECS-0335765. Fabrication was supported by the Solid-State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. DE-SC0001299/DE-FG02-09ER46577. Research was also supported by the U.S. Army Research Laboratory and the U.S. Army Research Office through the Institute for Soldier Nanotechnologies, under Contract No. W911NF-13-D-0001.

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