Material candidates for thermally robust applications of selective thermophotovoltaic emitters

As the majority of the input energy in power generation or energy consumption processes goes to waste as heat, thermophotovoltaic (TPV) devices enable energy recovery from (waste) heat. In TPV devices, the power output and conversion efficiency are impacted by thermal emitters. Since TPV devices operate at higher temperatures, emitters that can withstand hot environments without significant degradation of their emission performance are required. Refractory metals are commonly used as the emitter material due to their higher melting point and optical properties. This paper reviews physical and chemical properties of 15 refractory metals that may affect the emitter's performance at high temperatures: melting point, crystal structure, lattice constant, standard reduction potential, diffusion coefficient, Young's modulus, thermal expansion coefficient, and refractive index. Biological hazards and prices of the metals are also explored. Then, selective TPV emitters fabricated with the refractory metals are compared regarding their thermal stability. Finally, material properties are discussed toward achieving thermally robust TPV emitters.

Figure 1

Thermophotovoltaic (TPV) waste heat recovery. (a) Energy flow with TPVs. (b) Key components of a TPV device. $hf$ and $E_g$ are photon energy and band gap energy, respectively.

Figure 2

Comparison of a bulk structure (a) and a subwavelength structure (b). λ is the wavelength of the incident light. The size of $p$ relative to λ distinguishes bulk and subwavelength structures. Objects in blue and gray represent different materials.

Figure 3

Grains of a single crystal (a) and a polycrystal (b). Black arrows in the grains represent the orientation of the atomic stacking within that grain. All grains are in the hexagonal close-packed (hcp) structure.

Figure 4

Conventional unit cells for the fcc (a), bcc (b), and hcp (c) lattices. The coloring of the spheres is meant only to guide the eye—it is not meant to imply different atoms. Given their symmetry, the fcc and bcc lattices are typically given a single lattice constant, marked $a$, which applies along all three Cartesian axes. The hcp is marked with both its in-plane, $a$, and out-of-plane, $c$, lattice constants. The inset in (c) is a top view of the hcp unit cell.

Figure 5

(a) An fcc lattice where the nearest neighbor distance is marked in red. (b) The diamond cubic lattice is represented by a single fcc with a two-atom basis (left) and with a single-atom basis and the accompanying interlocked fcc lattices (right).

Figure 6

(a) A schematic view of lattice-mismatch-induced strain. Top: an epilayer with a lattice constant greater than that of the underlayer undergoes compressive strain (blue arrows), shrinking the in-plane lattice constant and stretching the out-of-plane constant. Bottom: an epilayer with a lattice constant smaller than that of the underlayer undergoes tensile strain (blue arrows), stretching the in-plane and shrinking the out-of-plane lattice constants, respectively. (b) Delamination of thin strips of Ni from a Au/Si substrate (reprinted with permission from Ref. [35]). (c) Delamination of a thin Si layer from a quartz substrate.

Figure 7

Delamination due to a higher Young's modulus upon thermal expansion. Materials with a higher (a) and lower (b) Young's modulus are compared. Atoms and atomic bonding are represented by balls and lines, respectively. The upward (↑) and downward (↓) arrows represent high and low values of the property, respectively.

Figure 8

Periodic Table of elements. Metals with a melting point $\left(T_m\right)$ higher than $1700{}^{\circ }\mathrm{C}$ are boxed in red.

Figure 9

Properties of the refractory metals from Table 1. Melting point (a), lattice constant (b), standard reduction potential (c), self-diffusion coefficient (D) (d), Young's modulus (e), linear thermal expansion coefficient (CTE) (f), and typical prices (g) are shown. Lattice constants are shown along with those of common substrate or coating materials for TPV emitters: Si [55], ${\mathrm{HfO}}_2$ [112], and ${\mathrm{Al}}_2{\mathrm{O}}_3$ [113]. The Young's moduli [114, 115, 116] and CTEs [113, 117, 118] of these three materials are shown as well.

Figure 10

Refractive index of the refractory metals. Data are separated for optically isotropic (a) and anisotropic (b) metals. $n$ and $k$ are real and imaginary parts of the refractive index, respectively. In the legend, s.c. and p.c. stand for single crystal and polycrystal, respectively. The orientation of incident electric field ($E$) relative to the orientation of the longer axis of hcp unit cell, the $c$ axis, is notated. Data are from [52, 119]. For polycrystalline Ru, Hf, and Re, refractive indices are calculated from the relationships $\varepsilon =({\varepsilon }_{\left|\right|}+2{\varepsilon }_{\perp })/3$ and $\sqrt{\varepsilon }=n+ik$, assuming randomized grain orientations [120], where ε is permittivity. We note that ${\varepsilon }_{\left|\right|}$ and ${\varepsilon }_{\perp }$ are permittivities corresponding to electric fields parallel and perpendicular to the $c$ axis of the hcp unit cell, respectively [120].

Figure 11

A V-based emitter. (a) Emitter structure with three nanosized disks. White and blue layers are the metal (either all Ti or V or W) and ${\mathrm{SiO}}_2$, respectively. (b) Simulated spectral radiance curves of the emitter with V disks and a blackbody corresponding to 2000 K. (Reprinted with permission from Ref. [60].)

Figure 12

Cr-based emitters. (a) Emitter with Cr (metal) and ${\mathrm{SiO}}_2$ (dielectric) planar layers [62]. The layer structure (a1,a2) and measured absorption (Expt.-Cr) (a3) are shown. $d_d=85\mathrm{nm}$ and $d_m=8\mathrm{nm}$. (b) Emitter with a Cr ring resonator [63]. The unit cell (b1) and simulated absorption (b2) are shown. Materials in orange and gray are Cr and a dielectric, respectively. $w=4\mu \mathrm{m}$, $r=25\mu \mathrm{m}$, $p_x=p_y=70\mu \mathrm{m}$, and $h=26\mu \mathrm{m}$. ((a), (b) are reprinted with permission from Refs. [62, 63], respectively. © The Optical Society.)

Figure 13

Nb-based metamaterials with 5 nm thick ${\mathrm{Al}}_2{\mathrm{O}}_3$ coating. (a)–(c) Scanning electron microscopy (SEM) images of the emitter's top surface before or after annealing in vacuum. (d) Measured transmittance of the emitter before and after annealing. (Reprinted with permission from Ref. [68]. © (2018) American Chemical Society.)

Figure 14

A Mo-based emitter. (a) Emitter structure. Thicknesses of the top and bottom Mo layers are 100 nm and 1 $\mu \mathrm{m}$, respectively. (b) Measured emission of the emitter at several temperatures up to $1000{}^{\circ }\mathrm{C}$ in vacuum. (c) Measured reflectance of the emitter before and after annealing at $1000{}^{\circ }\mathrm{C}$ for 3 h in vacuum. (d,e) are SEM images of undeformed and deformed Mo disks after annealing, respectively. (Reprinted with permission from Ref. [69].)

Figure 15

A Ru-based photonic crystal. (a) A cross-section SEM image at an oblique angle. (b) Top-view SEM images before (left) and after (right) annealing at $1000{}^{\circ }\mathrm{C}$ for 24 h in inert atmosphere (95% Ar + 5% ${\mathrm{H}}_2$). (Reprinted with permission from Ref. [75].)

Figure 16

Ta-based photonic crystal emitters with and without 20 nm thick ${\mathrm{HfO}}_2$ coating. (a) Emissivity of the ${\mathrm{HfO}}_2$-coated structure measured at room temperature before and after annealing in vacuum. (b), (c) are SEM images of the ${\mathrm{HfO}}_2$-coated structure (oblique view) before and after annealing at $900{}^{\circ }\mathrm{C}$ for 144 h in vacuum, respectively. (d) Emissivity of the uncoated structure measured at room temperature before and after annealing in vacuum. (Reprinted with permission from Ref. [80]. © The Optical Society.)

Figure 17

W-based nanostructures for TPV emitters. (a) Results of the W-CNT structure (no surface coating of W) before and after annealing in vacuum [97]. SEM images (a1), measured emittance (a2), and XRD peaks (a3) are shown. The blue and red curves in the XRD results correspond to W. (b) Results of the 20 nm thick ${\mathrm{HfO}}_2$-coated metasurface before and after annealing in vacuum [98]. An SEM image (b1) and measured absorptance (b2) are shown. ((a), (b) are reprinted with permission from Refs. [97, 98], respectively.)

Figure 18

An Ir-based emitter. Absorbance results (for $d=225\mathrm{nm}$) measured at room temperature are shown for unpolarized light. The incidence angles are denoted in the legend. (Reprinted with permission from Ref. [103].)

Figure 19

Pt-based emitters (without surface coating). (a) Surface SEM image (a1) and measured emissivity (a2) of the two-dimensionally patterned metamaterial [106]. (b) Results of the three-dimensionally patterned metamaterial [107]. Surface SEM images before (b1) and after (b3), (b4) annealing with corresponding measured reflectance (b2) are shown. Annealing conditions are written on top of each SEM image. ((a), (b) are reprinted with permission from Refs. [106, 107], respectively.)

Figure 20

Measured reflectance ($R$) of Pt-based MM emitters with and without 150 nm thick ${\mathrm{Al}}_2{\mathrm{O}}_3$ coating after annealing at $1027{}^{\circ }\mathrm{C}$ in Ar atmosphere. The legend shows annealing time, where the font color corresponds to the plot color. ε is emissivity. (Reprinted with permission from Ref. [108].)

Figure 21

Simulated relative stress in layers at $100{}^{\circ }\mathrm{C}$. Thickness ratio of the layers used in simulation is Ir:sapphire = 1:10 (a), Ti:sapphire = 1:10 (b), and Ir:Ti:sapphire = 1:0.1:10 (c). Note that the resulting screenshots are cut at the sapphire. Layers are assumed to have been deposited at room temperature. Material properties at room temperature are used for simplified simulation.

Figure 22

Price comparison of TPV emitters from Table 3. The constituent material of the emitter is denoted by the element symbol on top of each data point.

Figure 23

Desired properties for a good material candidate for TPV emitters. Upward (↑) and downward (↓) arrows indicate higher and lower, respectively.