CAREER: Enhanced Power Generation in a Nanoscale-Gap Thermophotovoltaic Device due to Radiative Heat Transfer Exceeding the Blackbody Limit

Transcription

1 CAREER: Enhanced Power Generation in a Nanoscale-Gap Thermophotovoltaic Device due to Radiative Heat Transfer Exceeding the Blackbody Limit Mathieu Francoeur Assistant Professor, Mechanical Engineering, University of Utah, Salt Lake City, UT INTRODUCTION 1.1 Objective, Intellectual Merit and Broader Impacts The objective of this research is to test the hypothesis that power generation in a nanoscale-gap thermophotovoltaic (TPV) device can be enhanced by a factor of 20 to 30, compared to conventional TPV systems, due to radiation heat transfer exceeding the blackbody limit. This hypothesis will be tested by measuring radiative heat fluxes and TPV performances in an experimental apparatus involving planar surfaces separated by a nanosize gap maintained via spring-like spacers. The current world energy consumption is about 14 TW, among which less than 1% is coming from clean and renewable sources [1]. By 2050, this global demand is expected to reach 25 to 30 TW. In order to minimize the environmental impacts of this increasing energy consumption, experts estimate that about 20 TW should come from carbon-free renewable energy resources. Baxter et al. [1] pointed out the importance of nanoengineering to develop low-cost and highefficiency renewable energy technologies, and emphasized solar TPV power generators that could greatly benefit from nanoscale design. In such devices, solar irradiation is absorbed by a radiator, which, in turn, re-emits thermal radiation in a spectrally selective fashion toward cells generating electricity. TPV power generators are not restricted to solar applications, as any kind of heat source can be used. For instance, direct thermal-to-electrical energy conversion via TPV devices could diminish waste heat (58% of the 110 EJ consumed annually in the US is lost to heat [2]) in various systems such as combustion chambers, photovoltaic (PV) cells, and personal computers. Beyond their versatility, TPV power generators are expected to be quiet, lowmaintenance, modular, safe, and pollution-free [3-6]. The power output of a TPV system is about 10 4 Wm -2 for a thermal source in the range of 1300 K to 2000 K [3,5]. TPV power generation is limited by Planck s blackbody distribution. In order to improve TPV performances, Whale and Cravalho [7,8] suggested spacing the radiator and the cells by a subwavelength vacuum gap. At sub-wavelength distances, radiation heat transfer is in the near-field regime, such that energy exchange can exceed by a few orders of magnitude the blackbody predictions due to tunneling of waves evanescently confined normal to the surface of a thermal source [9-48]. For typical thermal radiation temperatures, near-field effects are dominant when bodies are separated by a few tens to a few hundreds of nanometers. For this reason, a TPV system capitalizing on the near-field effects of thermal radiation is referred to as a nanoscale-gap TPV device, or more simply, a nano-tpv device. Theoretical studies have demonstrated that a power output enhancement by a factor of 20 to 30 is achievable at nanosize gaps [49,50]. Experimentally, DiMatteo et al. [51-53] reported shortcircuit current enhancement by a factor of 5 [51,52], and a power output enhancement of uncertain magnitude [53]. Hanamura et al. [54-56] measured a short-circuit current increase by a factor of 3 [55] and 3.7 [56]. While these efforts demonstrated qualitatively near-field enhancement in nano-tpv power generators, no clear and systematic quantification of these effects has been reported in the literature. 1

2 The research proposed here is radically different from the state-of-the-art. (1) Radiative heat flux between planar surfaces separated by a gap as small as 20 nm, maintained via spring-like spacers, will be measured. Fluxes at nanosize gaps have been measured between a microsphere and a surface [39-42,45,46] and between a sharp tip and a plate [36,37], while fluxes between planar surfaces has only been reported for microsize gaps [38,40,44]. This research will experimentally verify the predicted d -2 power law (d is the gap thickness) of the radiative heat flux in the extreme near-field (~10 nm to 200 nm) [12]. Additionally, measurements will be, for the first time, performed with optically thin films in order to verify the newly discovered d -3 power law of the flux [35]. Experiments will be repeated at various gaps, temperatures and using different materials such as silicon carbide (SiC), tungsten and indium tin oxide (ITO). (2) Nano- TPV performances (power output, fill factor, conversion efficiency) will be measured for a gap as small as 20 nm in order to demonstrate a power generation enhancement by a factor of 20 to 30. This research will provide quantitative nano-tpv performance analysis as a function of the gap, temperatures, and materials. Experimental data will be compared for the first time against a model coupling near-field thermal radiation, charge and heat transport [57]. This project will have major impacts in heat transfer, thermal radiation at the nanoscale, clean and renewable power generation as well as waste heat recovery. This research will spark the development of nano-tpv power generators outperforming their macroscale counterpart by providing enhanced power generation by a factor of 20 to 30. These systems might find applications in direct thermal-to-electrical energy conversion in electronic devices, such as personal computers and solar PV cells [58], or in harvesting heat from the human body. The PI s career goals are to advance discovery and to promote near-field thermal radiation and its application to clean energy production. This will be accomplished via an integrated research and educational plan. A new course on near-field thermal radiation will be offered to undergraduate and graduate students. The lectures and class notes will be made available to the public through thermalhub and YouTube. Feedback will lead to a manuscript on near-field thermal radiation that will be made freely available. Additionally, the proposed research will enhance educational infrastructures by integrating the experimental bench in the undergraduate and graduate curriculum of the Department of Mechanical Engineering. Finally, K-12 outreach will be accomplished via the Utah Science Olympiad [59] and the conception of a demo-kit initiating high school students to thermal-to-electrical energy conversion. Participants at the Science Olympiad excelling in Mechanical Engineering disciplines will be offered departmental scholarships and sponsored summer research internships. 1.2 Previous Research, Educational and Outreach Activities by the PI The PI has been contributing to the emerging area of near-field thermal radiation over the past years, and, as a result, he co-wrote a chapter for the fifth edition of Thermal Radiation Heat Transfer [48]. He demonstrated that near-field thermal spectra can be tuned via coupling of surface phononpolaritons in nanostructures [30,60-62] and Mie resonance-based metamaterials [63]. The PI also successfully implemented a coupled near-field thermal radiation, charge and heat transport model for predicting nano-tpv performances [57]. Other research focuses on radiative polaritons in thin films [64,65], optical tomography [66,67] and characterization of nanoparticles [68-72]. For this last project, the PI built an experimental device, performed measurements in the microwave band in collaboration with the Fresnel Institute (Marseille, France), and is involved in the inversion procedure. The PI has a pending patent regarding a cascaded PV / nano-tpv system for optimizing solar energy conversion [58]. 2

3 The PI enhanced the undergraduate and graduate curriculum of the Department of Mechanical Engineering by developing a new course called Nanoscale Heat Transfer, offered for the first in the 2011 fall semester. The PI is also pursuing international collaborations with the CETHIL (Lyon, France) and the Fresnel Institute. A MS student working in PI s lab spent three months during the 2012 summer at the Fresnel Institute performing measurements of scattered evanescent waves in the microwave band. Finally, the PI is involved since 2012 in the Utah Science Olympiad as the representative of the Department of Mechanical Engineering and as an event coordinator for the competitions Keep the Heat and Thermodynamics. 2. BACKGROUND AND LITERATURE SURVEY 2.1 Description of a Nano-TPV Power Generator The working principle of nano-tpv power generators (Fig. 2.1) can be described as follows. By decreasing the gap separating the radiator and the cells to a sub-wavelength distance (i.e., below the dominant wavelength emitted as predicted by Wien s law [48]: λ w T = 2898 µm K), radiation transfer exceeds the blackbody predictions due to tunneling of evanescent modes. Higher radiation absorption by the cells thus leads to enhanced photocurrent generation, and potentially higher power heat input output. More specifically, the radiator is maintained at a temperature T rad via an external heat source. Radiation is converted into electricity via TPV cells that are separated by a vacuum gap of thickness d from the radiator. For the temperatures involved in nano-tpv devices (300 K 2000 K), d must be of the order of a few tens to a few radiator T rad evanescent mode vacuum d < λ w hundreds of nanometers in order to generate significant power from evanescent modes. A major technological propagating modes p-doped region depletion region challenge in nano-tpv power generation is to maintain a nanosize gap. TPV cell T cell n-doped region Cells with absorption bandgaps of about 1.1 ev (visible band) are usually employed in solar PV applications. Since TPV radiators operate at lower temperatures (400 K 2000 K), thermal emission occurs mostly in the near infrared (NIR) and infrared such that cells with bandgaps lower than 1.1 ev are required (~0.5 ev to 0.75 ev). thermal management system Figure 2.1. Schematic of a nano-tpv power generator. Typical TPV cells are made of III-V binary compounds, such as gallium antimonide (GaSb) and gallium arsenide (GaAs), as well as their ternary and quaternary III-V alloys [3,5,73]. Finally, a thermal management system might be needed to keep the cell around 300 K. When the cells are illuminated, the absorption of a wave with an energy E ( = ω e, where is the reduced Planck s constant, ω is the angular frequency and e is the electron charge) equal or larger than the bandgap E g of the cells generates electron-hole pairs (EHPs). EHPs produce a photocurrent if they reach the depletion region formed at the junction of positively (p) and negatively (n) doped semiconductors (Fig. 2.1 shows the specific case of a p on n junction) [74,75]. Radiation with E < E g does not contribute to photocurrent generation, and is dissipated in the form of heat via absorption by the lattice and the free carriers. Additionally, radiation with E > E g releases its excess of energy into heat, a phenomenon called thermalization [57,76]. Non-radiative recombination of EHPs also contributes in raising the cell temperature [57,76]. While the photocurrent generated by 3

4 the cells is essentially insensitive to the temperature, the power generated is greatly affected by the thermal effects due to an increase of the dark current with increasing temperature [57]. Heat sources within the cells can be minimized by matching near-field emission and absorption spectra of the radiator and the cells. 2.2 Literature Survey of Nano-TPV Modeling In nano-tpv devices, radiation heat transfer cannot be handled via the classical tools based on Planck s blackbody distribution [48,77]. Near-field effects of thermal radiation are accounted for by solving the Maxwell equations combined with fluctuational electrodynamics, where thermal emission is modeled as fluctuating currents [9,78,79]. Details regarding near-field thermal radiation modeling can be found in textbooks [47,48] and various papers [9-35,80,81]. Figure 2.2 presents the monochromatic radiative flux between two bulks of SiC. Results show a significant enhancement of the flux as the gap d decreases. The resonant peak at a frequency of rad/s (corresponds to an energy of 0.12 ev and a vacuum wavelength of µm) is due to surface phonon-polaritons [12,30,35,60-62,82,83]. For a 10-nm-thick gap, the far-field blackbodies ω [rad/s] Figure 2.2. Monochromatic radiative heat flux between two SiC bulks separated by a vacuum gap of thickness d. total heat flux (i.e., integrated over the entire spectrum) is about 1300 times larger than the blackbody predictions. Theoretical investigations of nano-tpv power generation are scarce [7,8,49,50,57,84-91], and are summarized in Table 2.1. The literature cited in Table 2.1 shows that power generation can be enhanced by a factor of 20 to 30 when the gap between the radiator and the cells is a few tens of nanometers [49,50,57]. The gain in conversion efficiency is marginal, however, due to the mismatch between the emission and absorption spectra of the radiator and the cells [49,50,57]. Laroche et al. [49] showed that enhanced power generation can be improved by an additional factor of 15 when using a plasmonic source as opposed to a broadband emitter (tungsten). None of the works in Table 2.1, on the other hand, accounted for the thermal impacts on nano-tpv performances. This was done by the PI via a coupled near-field thermal radiation, charge and heat transport model [57,68]. Figure 2.3(a) shows the maximum power output (P m ), the radiation absorbed by the cell ( q ), and the conversion efficiency (η c ) of a nano-tpv device made of a tungsten radiator and indium gallium antimonide (In 0.18 Ga 0.82 Sb) cells (bandgap of 0.56 ev at 300 K). The radiator and the cells are maintained at 2000 K and 300 K, respectively. Results show that the radiation absorbed by the cells and the power output increase as the gap d decreases. For 100-nm-, 20-nm- and 10- nm-thick gaps, power generation is enhanced by a factor of 6, 24 and 36, respectively, compared to large gaps (d > 5 µm). Figure 2.3(b) reveals that the power generated is quite sensitive to the temperature of the cell, due to an increasing dark current as T cell increases. Solution of the coupled near-field thermal radiation, charge and heat transport model suggests that it is challenging to maintain the cells at room temperature for gaps below 100 nm with a tungsten q ω [Wm -2 (rad/s) -1 ] T emitter = 300 K T receiver = 0 K d = 1 µm d = 10 nm d = 100 nm abs cell 4

5 radiator due to high heat dissipation within the cells. Modeling the thermal management device as a convective boundary, heat transfer coefficients as high as 10 5 Wm -2 K -1 are needed to maintain the cells around 300 K [57]. Table 2.1. Summary of theoretical and numerical modeling of nano-tpv power generation devices. Authors and Reference Whale and Cravalho [7,8] Whale [84] Pan et al. [85] Narayanaswamy and Chen [86] Laroche et al. [49] Park et al. [50] Geometry (1D for all cases) Two bulks separated by a vacuum gap Two bulks separated by a vacuum gap Two bulks separated by a vacuum gap Bulk radiator and thin film TPV cell separated by a vacuum gap Two bulks separated by a vacuum gap Two bulks separated by a vacuum gap (cell discretized into layers) Near-field radiative transfer model Fluctuational electrodynamics Fluctuational electrodynamics Analogy with total internal reflection Fluctuational electrodynamics Fluctuational electrodynamics Fluctuational electrodynamics Material for the radiator Drude model Drude model Lossless dielectric Cubic boron nitride Tungsten and Drude model Tungsten Material for the TPV cell In 1-x Ga x As (bandgaps from 0.36 to 1.4 ev). In 1-x Ga x As (single and multijunctions). Lossless dielectric Fictitious direct bandgap semiconductor (bandgap of 0.13 ev). GaSb (bandgap of 0.7 ev) In 0.18 Ga 0.82 Sb (bandgap of 0.56 ev). Main assumptions Original features Main results - 100% quantum efficiency - No thermal effects in TPV cells - 100% quantum efficiency - No thermal effects in TPV cells - radiation propagation in TPV cells modeled via Beer s law - No performance analysis of nano- TPV device - Thermal emission is not modeled directly - 100% quantum efficiency - No thermal effects in TPV cells - No performance analysis - 100% quantum efficiency - No thermal effects in TPV cells - No thermal effects in TPV cells First modeling work on nano-tpv devices Consideration of multi-junction for TPV cells N/A Proposed to use surface polaritons for spectral control Studied the possibilities that the near-field might affect the lifetime of EHPs - The TPV cell is discretized into control volumes - Calculation of radiation penetration depth within the TPV cell - Solution of the coupled near-field thermal radiation and charge transport problem. Near-field effects increase significantly the electrical power output, with marginal gain of conversion efficiency. Increase of conversion efficiency of about 5% when using TPV cells with two junctions. Maximum radiative flux between objects with refractive indices n is n 2 times the free space blackbody distribution. Surface phonon-polaritons increase photon overexcitation efficiency by a factor 2 compared to a blackbody source. - The near-field does not affect the lifetime of EHPs - Conversion efficiencies between 10% and 35% with Drude radiator. - Conversion efficiency from 17% to 23% for vacuum gaps between 2 nm and 10 µm. - Generation of 1MWm -2 of electrical power for a 10 nm thick vacuum gap η c T rad = 2000 K T cell = 300 K q abs and P cell m [Wm-2 ] P m abs qcell d [nm] η c [%] q abs and P cell m [Wm-2 ] q abs, d = 20 nm cell q abs, d = 50 nm cell q abs, d = 100 nm cell P m, d = 20 nm P m, d = 50 nm P m, d = 100 nm T rad = 2000 K T cell [K] (a) Figure 2.3. (a) Radiation absorbed by the cell, electrical power output, and conversion efficiency as a function of d. (b) Radiation absorbed by the cell and electrical power output as a function of the temperature of the cell and d. (b) 5

6 Power output and heat dissipation within the cells can be maximized and minimized, respectively, by matching the near-field thermal emission and absorption spectra of the radiator and the cells. As suggested in Refs. [49,86], radiators with surface polariton resonance matching the absorption bandgap of the cells constitute good candidates. Minimization of the flux at frequencies other than resonance can be achieved by using thin films, as suggested by Francoeur et al. [60]. Metals such as gold or silver support high-frequency surface plasmon-polaritons that cannot be excited thermally [82,88,92]. Polar crystals such as SiC or cubic boron nitride support surface phonon-polaritons at thermally accessible frequencies. However, the resonance is around 10 µm, thus corresponding to an energy of about 0.12 ev. Currently, no TPV cells have such a low absorption bandgap. The same argument is applicable to doped silicon supporting surface plasmon-polaritons around 10 µm [93]. Due to current technological limitations related to the cells, radiators with surface polariton resonance in the NIR (~0.50 ev to 0.75 ev) are needed. Artificial structures such as electromagnetic metamaterials [63,80,81,94-100] and photonic crystals [ ] could be employed to create NIR resonance in close proximity of a thermal source. Semiconductors such as ITO, aluminum zinc oxide (AZO) and gallium zinc oxide (GZO) might constitute simpler choices for nano-tpv radiators, as they support surface plasmonpolaritons in the NIR [104]. 2.3 Literature Survey of Near-Field Radiative Flux and Nano-TPV Measurements Measurements of radiative heat flux at nanosize gaps have been achieved between a large sphere and a surface [39-42,45,46] as well as between a sharp tip and a surface [36,37]. These results showed good agreement with predictions based on fluctuational electrodynamics. On the other hand, experimental validation of near-field radiative heat flux between planar surfaces at nanosize gap is still an open research area [38,40,44, ]. A number of experimental investigations involving planar surfaces were performed from 1968 to 1994 [ ]. Whale [7] reported in a single figure these data, and concluded that they were inconsistent, suspect to invalidity, divergent from theoretical predictions, and insufficient to infer a general trend and length scale for near-field thermal radiation between planar surfaces. More recent experiments involving glass plates were conducted by Hu et al. [38], where the gap was maintained via polystyrene particles. The gap was assumed to be equivalent to the maximum particle size of 1.6 µm. Experimental results showed a flux exceeding by 35% the blackbody predictions. While the method used in this work is relatively simple, the gap between the surfaces is large, thus significantly limiting heat transfer enhancement. Ottens et al. [44] measured near-field radiative heat flux between sapphire plates. The set-up employed three stepper motors allowing variations of the gap from 2 µm to 100 µm. The parallelism between the surfaces was monitored via capacitor plates and stepper motors. Results demonstrated an enhancement of the flux by 27% over the blackbody predictions. While the set-up developed by the authors is very precise, it is difficult to apply the same concept to nano-tpv power generation, as the long-term objective of this research is to produce a real engineering device. Experiments on TPV systems at sub-wavelength gaps have been reported by two groups [51-56]. DiMatteo et al. [51,52] experimented on a system comprised of a silicon radiator and indium arsenide (InAs) cells cooled down by a thermoelectric module. The gap was maintained via 1- µm-tall silicon dioxide posts. The experimental set-up allowed for a slight variation of the gap using a piezoactuator flexing the heater chip by a fraction of microns. However, the distance the heater chip flexed was not documented. By decreasing the gap for initial radiator temperatures of 6

7 348 K, 378 K and 408 K, it was observed that the short-circuit current increased by a factor of 5. A dynamic test was also performed, where the piezoactuator was oscillating between frequencies of 200 Hz and 1000 Hz, thus causing the vacuum gap also to oscillate. Results showed that variations of the short-circuit current somehow followed in-phase the gap oscillation frequency, thus leading the authors to conclude that the increase of the current was due to tunneling of evanescent waves. As pointed out by the authors, while this work demonstrated near-field enhancement, no quantitative data were reported. The experimental device was later refined by DiMatteo et al. [53], where indium gallium arsenide (InGaAs) cells were used. The updated setup employed tubular spacers minimizing heat conduction between the radiator and the cells, and allowed measurements of the full I-V curve (i.e., current versus voltage). Although an enhancement of the power generated was observed, its actual magnitude was uncertain. Note that DiMatteo is the founder, CEO and Chairman of MTPV (Micron-gap Thermal PhotoVoltaics) commercializing micron-gap TPV devices [111]. However, no data are available regarding these systems beyond the documentation in Refs. [51-53]. Nano-TPV experiments were conducted by Hanamura et al. [54], where the radiator and cells were made of tungsten and GaSb, respectively. The temperature gradient was maintained via a CO 2 laser and a water-cooled copper block. In order to avoid heat transfer by conduction, the radiator and the cell were mounted separately. The gap and the parallelism between the surfaces were controlled via micro-stages. I-V characteristics were reported for large gaps (few tens to few hundreds of micrometers). The authors pointed out the difficulty of measuring gaps below 10 µm. The temperature of the emitter and the power generated were reported in the near-field for gap ranges of 1 µm (i.e., uncertainty of 1 µm in the measured gap). Despite these uncertainties, an enhancement of the power generated was qualitatively observed. Recent results [55] using the same apparatus showed an enhancement by a factor of 3 of the short-circuit current for gaps below 1.5 µm. An enhancement by a factor of 3.7 of the short-circuit current was later reported [56]. To summarize, experimental data of near-field radiative heat flux between planar surfaces separated by a nanosize gap have never been reported in the literature. Near-field enhancement of nano-tpv power generation has been qualitatively observed by two groups, but no quantitative data have been reported. Additionally, the maximum enhancement documented thus far is by a factor of 5. The research proposed in the next section will break these boundaries and is likely to expedite the development of TPV devices operating at nanosize gaps. 3. RESEARCH PLAN 3.1 Task 1: Design and Fabrication of the Experimental Bench The objective of this task is to design and fabricate the experimental bench allowing measurements of nearfield heat flux and nano-tpv performances. More specifically, the idea is to produce a relatively simple apparatus that does not require sophisticated gap control such as stepper motors. In that way, the outcome of the research activities will serve as a firm foundation to the development of nano-tpv prototypes. A schematic of the proposed experimental device is shown in Fig L e d heat pad metal films surfaces A emitter (radiator) Q rad Q par L r receiver (TPV cells) copper spreader thermoelectric module ~ mm spacers Q cool Vacuum metal films Figure 3.1. Schematic of the proposed experimental apparatus. T hot T hot,s T cold,s T cold 7

8 The temperature gradient between the emitter and the receiver is maintained via a heat pad and a thermoelectric (TEC) module. The emitter and the receiver are separated by a gap of thickness d via spring-like spacers. The receiver is mounted on a copper heat spreader, which is in turn located on the TEC. The temperatures T hot and T cold, respectively located at the heat pad-emitter and receiver-copper interfaces, will be measured via thermocouples. The entire device is located in a vacuum chamber to minimize heat transfer by gas conduction through the gap. Heat transfer by conduction between the emitter and the receiver occurs mostly through the spacers. The sides of the device shown in Fig. 3.1 will be covered with aluminum foil to avoid radiative losses to the surroundings, as suggested in [38]. Note that the system will be designed in such a way that no more than 2% of the heat rate is due to the aforementioned parasitic heat transfer. Employing spring spacers for maintaining the gap emitter between the emitter and the receiver has the following advantages: (1) it increases the thermal silicon nitride receiver resistance by conduction when compared to posts or ~ µm tubes; (2) it can be easily integrated into nano-tpv prototypes; (3) it allows the ability to conveniently Figure 3.2. Schematic of a spring-like spacer. control and measure the gap via the spring constant; and (4) it allows small, but uniform, variations of the gap, which is not possible with rigid posts or nanoparticles. A possible spring design is shown in Fig The springs will be fabricated using the Nanofab facilities available at the University of Utah [112]. The silicon nitride spring shown in Fig. 3.2 can be fabricated normal to the substrate via the following procedure. Thin layers of silicon nitride will be deposited using chemical vapor deposition with the Oxford Plasmalab 80 Plus. Photolithography and reactive ion etching in the LAM 590 will be used to pattern nitride layers into the desired structures. Sacrificial layers of germanium will be deposited using the Denton SJ20C e-beam evaporator. The germanium layers will be used as structural supports during the spring fabrication process. Xenon difluoride etching [113,114] of the sacrificial layers will be conducted with the XACTIX XETCH system. Xenon difluoride etching is employed since it does not require ion bombardment and therefore reduces the risk of damaging the fragile nitride layers. Note that the resolution of the fabrication process, normal to the substrate, is on the scale of a few nanometers, while the minimum horizontal feature size is approximately 3 µm. Preliminary studies revealed that a separation gap as small as 20 nm can be maintained via these spacers. ~ nm to µm Characterization of the fabricated spring will be done via a Zygo NewView optical profiler. The spring constant will be predicted via COMSOL simulations, and will be measured experimentally by fabricating samples coated with metallic films at each end. An electrostatic force will be induced by applying a voltage to the films. The resulting deflection will be measured via the Zygo NewView optical profiler, and the spring constant will be experimentally determined using force and deflection data. Springs will be used as spacers between the emitter and the receiver. The gap will be conveniently controlled and measured by depositing thin metallic films at the corners of both the emitter and the receiver (see Fig. 3.1). The application of a voltage to the films will induce an electrostatic force, and will thus create a slight deflection of the springs. Using the spring constant in combination with the applied force, the deflection of the spacers will be determined and thus the gap will be known. It will be possible to vary slightly the gap by changing the voltage applied to the metallic films. Note that variations of the gap as a function of the applied 8

9 voltage are quite limited. For instance, if the vertical dimension of the spring is around 20 nm, the gap will be allowed to vary by a few nanometers. It will thus be necessary to fabricate various samples for gaps in the tens of nanometers up to tens of micrometers. It is important to note that Fig. 3.2 shows a possible spring design that will be fabricated and tested. Other designs will be proposed and tested until an optimal solution is reached. The final design will also account for thermo-mechanical constraints in the device. This analysis will be performed by employing AutoCAD in tandem with COMSOL. In addition to the spring fabrication, calibration and testing, the roughness and possible bow of the surfaces constitute another bottleneck. Chemical mechanical planarization of the surfaces will be performed with the Strasbaugh 6EC. Roughness as low as a few nanometers can be achieved via this instrument. The roughness and flatness of the surfaces will be characterized using the Zygo NewView optical profiler to ensure that the emitter and receiver are not in contact. Note that the Zygo NewView optical profiler has a vertical resolution on the order of an angstrom. 3.2 Task 2: Near-Field Radiative Heat Flux Measurements The objective of this second task is to measure radiative heat fluxes between planar surfaces separated by nanoscale gaps. This will be done using the device shown in Fig Steady-state measurements will be performed for pre-defined sets of temperatures T hot and T cold by adjusting the power supplied to the heat pad and the TEC. The radiative heat flux exchanged between the emitter and the receiver will be retrieved by dividing the difference between the cooling heat rate Q cool (measured) and the parasitic heat rate Q par (estimated) by the area A of the surfaces. The parasitic heat rate Q par includes conduction through the spacers, possible conduction through rarefied gas within the gap, and radiative losses to the surroundings. The design will ensure that parasitic heat flow does not exceed 2% of the cooling rate. Experimental results will be compared against predictions based on fluctuational electrodynamics; a number of codes for performing calculations in 1D layered geometry are available in PI s lab [21]. For this purpose, the temperatures T hot,s and T cold,s (see Fig. 3.1) are required and will be obtained as follows: Thot, s = Thot Qcool ( Le / kea) and Tcold, s = Tcold + Qcool ( Lr / kr A) where k e and k r are the thermal conductivities of the emitter and the receiver, respectively. Measurements will be performed at large gaps in the far-field regime down to the extreme near-field at 20 nm using various spring-like spacers. Experiments will be repeated for different materials, such as tungsten, SiC and ITO, and for various temperatures T hot and T cold. This will provide for the first time flux measurements at nanoscale gaps between planar surfaces. It is well-known that the flux between two bulks supporting surface polaritons in the infrared follows a d -2 power law in the extreme near-field (~10 nm to 200 nm) [12]. As depicted in Fig. 3.3, Francoeur et al. [35] recently showed that the flux (or equivalently, the radiative heat transfer coefficient h r ) may follow h r [Wm -2 K -1 ] t = 1 nm, bulk t = 5 nm, bulk t = 10 nm, bulk t = 50 nm, bulk bulk, bulk d [nm] Figure 3.3. Radiative heat transfer coefficient h r between a SiC bulk and a SiC film of thickness t as a function of the gap thickness d. d -3 T = 300 K d -2 9

10 a d-3 power law if one of the layers is an optically thin film. This research will enable experimental demonstration of the coexistence of d-2 and d-3 power laws by using a thin film as the emitter or receiver, which has never been done in the past. The thin film will be coated on a low-emitting substrate such as glass. Near-field heat flux measurements at nanosize gaps between planar surfaces will significantly impact the heat transfer community and the development of energy conversion devices. It will allow for the first time verification of fluctuational electrodynamics-based simulations involving bulks and films in 1D layered geometry. Such a verification is also crucial for nano-tpv performance predictions and design based on a coupled near-field thermal radiation, charge and heat transport model. 3.3 Task 3: Nano-TPV Performance Measurements The objective of this task is to measure nano-tpv J-V characteristics (i.e., photocurrent versus applied voltage), derive the performance indicators, and compare the data against numerical predictions. This will be done using the device shown in Fig. 3.1, where the receiver will be replaced by TPV cells. Initial testing will be performed with a tungsten radiator and GaSb cells. This choice is motivated by the fact that the various temperature- and doping-dependent properties needed for nano-tpv simulations for both materials are already available in PI s lab [57]. J-V characteristics for the nano-tpv system described in section 2.2 are shown in Fig. 3.4 as a function of the cell temperature for a 20-nm-thick gap. J-V characteristics allow the determination 2.0x10 of various performance indicators: Tcell = 300 K 1.8x10 maximum power output, fill factor and Jsc Tcell = 350 K 1.6x10 conversion efficiency. The maximum power Tcell = 400 K 1.4x10 Tcell = 450 K generated Pm is the product of the Tcell = 500 K 1.2x10 Trad = 2000 K photocurrent Jm and voltage Vm maximizing 1.0x10 d = 20 nm the area under the J-V characteristics. The 8.0x10 fill factor can be calculated by taking the 6.0x10 ratio of Pm over the product of Jsc and Voc, Voc 4.0x10 defined respectively as the short-circuit current and the open-circuit voltage. The 2.0x conversion efficiency is the ratio of Pm over abs V [V] the radiation absorbed by the cell qcell. The Figure 3.4. J-V characteristics of nano-tpv power procedure described in Task 2 allows generation for a 20-nm-thick gap and various measuring the net radiative heat flux temperatures of the cells. between the emitter and the receiver. The abs same technique can be applied for evaluating qcell, provided that the measured net heat flux is corrected by the radiation emitted by the cell at Tcold,s. Note that flux measurements will be performed at open-circuit to ensure that the power is not partially dissipated as electrical power [115,116]. Nano-TPV performances will then be compared, for the first time, against a model coupling near-field thermal radiation, charge and heat transport [57] J [Am-2] As pointed out by Francoeur et al. [57], nano-tpv performances are sensitive to the temperature of the cell Tcell. For instance, Fig. 3.4 shows that the power output significantly decreases as Tcell increases. Therefore, for a fixed radiator temperature, nano-tpv performances will be monitored 10

11 as a function of T cell and compared against numerical predictions. This will allow quantification of the efficiency loss per degree Kelvin, as done for solar PV cells [117] The difficulty of maintaining the temperature gradient between the radiator and the cells when using a broadband thermal source, such as tungsten, was discussed in section 2.2. To better match the absorption bandgap of the cells, a radiator with surface polariton resonance in the NIR is needed. ITO is a good candidate, as this material supports surface plasmon-polaritons in the NIR. Indeed, the plasma frequency of ITO can be engineered between 0.44 ev and 6.99 ev by varying the tin doping level [104]. Figure 3.5 shows the near-field heat flux between two bulks of ITO when the plasma frequency is fixed at 1.55 ev [118]. Results show a sharp resonance in the NIR at a frequency of rad/s corresponding to an energy of 0.70 ev and a vacuum wavelength of 1.76 µm. The flux for a 20- nm-thick gap exceeds the blackbody predictions in the entire spectrum. The flux can be minimized at all frequencies, except at resonance, by decreasing the volume of the thermal source (e.g., by using thin films) [60]. Therefore, materials such as ITO will be employed for the radiator. For a fixed radiator temperature and power supplied at the TEC, nano-tpv performances and the cell temperature will be recorded as a function of the radiator material. This will allow experimental determination of optimal nano-tpv radiators. Measurements will be repeated at various separation gaps. The overall objective is to demonstrate power generation enhancement by a factor of 20 to 30, which can be achieved with a 20-nm-thick gap (see Fig. 2(a)). Finally, it will be necessary to construct a database of temperature- and doping-dependent properties for nano-tpv simulations. 3.4 Research Timeline and Resources The research activities will be Table 3.2. Research timetable. conducted over a 60-month period. Project Milestones and Tasks vs. Months The total cost of the project, including Task 1: Design and fabrication of the experimental bench direct and indirect costs, is $496,411. Data acquisition system design and testing This budget is based on 1.0 personmonth per year for the first four years analysis, and testing Design of spacers, and testing on various prototypes Integration of spacers in samples, thermo-mechanical constraint Task 2: Near-field radiative heat flux measurements and 1.5 person-month for the fifth Fabrication of samples with various gaps and materials year for the PI, support for one PhD Calibration of the system in the far-field regime Measurements in the near-field regime student for four years, one MS student Construction of a database of near-field heat flux simulation results for two years, and undergraduate Task 3: Nano-TPV performance measurements summer internships for four years. Fabrication of samples with various radiators, cells, and gaps Measurement of nano-tpv performance indicators The tasks to be accomplished by the Construction of a database of doping- and temperaturedependent properties for nano-tpv simulations PI and the students are described in Construction of a database of nano-tpv simulation results the Budget Justification portion of this proposal. Note that all the aforementioned personnel will be involved in educational and q ω [Wm -2 (rad/s) -1 ] T emitter = 1200 K T receiver = 0 K blackbodies (1-6) d = 20 nm ω [rad/s] Figure 3.5. Monochromatic radiative heat flux between two ITO bulks separated by a 20-nmthick vacuum gap. (7-12) (13-18) (19-24) (25-30) (31-36) (37-42) (43-48) (49-54) (55-60) 11

12 outreach activities, as explained in section 4. Additionally, the budget accounts for equipment purchasing (vacuum chamber, data acquisition system, etc.), cost for materials consumed in the laboratory, Nanofab facility access fees and participation in conferences. The major milestones of the research project and associated timetable are summarized in Table INTEGRATION OF RESEARCH IN EDUCATION AND OUTREACH The PI s research and educational career objectives are to advance discovery and to promote near-field thermal radiation and its application to clean energy generation. The educational and outreach plan proposed below will serve as a firm foundation for fulfilling these goals. 4.1 Development of Educational Materials Near-field thermal radiation is a relatively unknown, emerging area of heat transfer engineering. Most undergraduate and graduate engineering students are unaware of the fact that the blackbody concept is valid when all parts of space are larger than the radiation wavelength [119]. With the progresses in nanotechnology and nanopatterning procedures, near-field thermal radiation should not be treated as a pure conceptual phenomenon nor a stand-alone research area. As near-field radiative heat transfer problems are becoming increasingly important in thermal management of MEMS/NEMS, thermal imaging [120], thermal rectification [121,122] and energy conversion devices, this area should be integrated into the undergraduate and graduate engineering curriculum. The integration of near-field thermal radiation in the undergraduate and graduate curriculum will be accomplished by developing a new course called Energy Transfer by Electromagnetic Waves and Near-Field Thermal Radiation. This elective class will be available to undergraduate and graduate students of the College of Engineering. The class will also initiate students to areas currently not covered in the mechanical engineering program such as imaging, characterization and plasmonics. The course will be heavily based on class notes developed by the PI. Additionally, lectures will be recorded and will be made available to the general public, along with the class notes, through PI s website, thermalhub and YouTube. The course will be offered for the first time during the 2013 fall semester, and will be repeated every two years. Feedback will contribute to strengthening and improving the lectures and class notes. This class format will also contribute to the dissemination of new discoveries and advances in near-field thermal radiation to the scientific community in a comprehensive manner. Secondly, the PI is proposing to morph the class notes into a monograph at the end of the fiveyear period of this proposal. Currently, near-field radiation heat transfer is briefly discussed in Chen s textbook [123], in Chapters 8 to 10 of Zhang s textbook [47], and in Chapter 16 (coauthored by the PI) of Howell, Siegel and Mengüç s book [48]. While these references provide relevant and useful information, a self-contained, engineering-oriented monograph intended for undergraduate and graduate students, as well as researchers wanting to begin in the area of nearfield thermal radiation, is still needed. The monograph will be made freely available on PI s website, similar to the Heat Transfer textbook by Lienhard IV and Lienhard V [124]. This electronic format will allow frequent updates of the manuscript (~2 years) based on readers feedback, and will include recent advances and discoveries. These new educational materials will promote near-field thermal radiation, educate new researchers in the field, and hopefully spark exciting new discoveries. 12

13 4.2 Enhancement of Infrastructures for Undergraduate and Graduate Education The proposed research will have direct impacts on undergraduate and graduate educational infrastructures in the College of Engineering. More specifically, the experimental apparatus capable of measuring radiative flux and nano-tpv performances will be integrated as a mandatory laboratory in the undergraduate Heat Transfer class (MEEN 3650), and the newly developed near-field thermal radiation course described in section 4.1. The PI is responsible for the theoretical and experimental portions of MEEN This class is offered once a year to undergraduate mechanical and chemical engineering students; the typical enrollment is around 150 students. The practical portion of MEEN 3650 consists of six experiments. Undergraduate students have the responsibility of performing the experiments, under the supervision of a graduate teaching assistant, collecting, processing and interpreting data, as well as producing technical reports. Current experiments are outdated, and fail to spark student excitement and interest for heat transfer. In their strategic plan, the thermal science group of the Department of Mechanical Engineering (Ameel, Chen, Francoeur, Udell) proposed updating the practical portion of MEEN 3650 by having modern, up-to-date experiments matching students interests in renewable energy. The experimental device proposed here will replace the outdated blackbody cavity laboratory. This cutting-edge experiment will demonstrate radiation transfer between planar surfaces, the view factors, the spectral nature of thermal radiation, the limit of Planck s blackbody distribution, and will illustrate a practical application of thermal radiation to power generation. The integration of the experimental bench in the undergraduate curriculum will also promote training of graduate students acting as teaching assistants. Reports submitted by the students and feedback by the teaching assistants will contribute to improving the system. The experimental apparatus will also be integrated into the near-field thermal radiation class. The expected enrollment for the first edition of this course is about 15 students. Students in the class will produce their own samples (fabricated at the Nanofab), measure near-field radiative heat flux, and evaluate nano-tpv performances. Students will have to develop their own fluctuational electrodynamics-based code for predicting near-field heat flux, while nano-tpv performances will be predicted via a model provided by the instructor. The students will gain experience in programming, nanofabrication and design, experimental measurements and data processing. The feedback provided by the students will assist with improving the experimental bench. The samples designed and fabricated by the students might result in breakthrough discoveries and new ways of generating power with nano-tpv devices. The experimental apparatus will be made available to other researchers and courses, such as MEEN 5800/6800 Sustainable Energy Engineering (~60 students) and MEEN 7670 Advanced Radiation Heat Transfer (~15 students). 4.3 Teaching and Training of Undergraduate / Graduate Students and K-12 Outreach Teaching and training of highly qualified personnel will be promoted via the involvement of two graduate (one PhD, one MS) and four undergraduate students, through summer internships, in the research and mentoring activities. Key research personnel (PI, graduate and undergraduate students) will be involved in K-12 outreach via the Utah Science Olympiad [59]. The Utah Science Olympiad was hosted for the first time by the University of Utah in This annual state competition promotes science, 13

14 technology and engineering to about 1000 junior high and high school students (7 th to 12 th graders) via hands-on learning and coaching interactions with parents, teachers, college students, and professors. The Department of Mechanical Engineering has been actively involved in the Utah Science Olympiad by awarding 16 scholarships of $1,000. The PI participated in the 2012 edition of the Utah Science Olympiad as the representative of the Department of Mechanical Engineering and as an event organizer for the competitions Keep the Heat and Thermodynamics. The PI will pursue his involvement with the Science Olympiad for years to come. In the framework of this project, the PI is proposing to promote direct thermal-to-electrical energy conversion via an interactive, portable demo-kit. The demo-kit will be designed and built by undergraduate students, supervised by graduate students, and will be presented at regional (four per year) and state (one per year) competitions. These activities will develop links and contacts with high school teachers interested in employing the demo-kit in a classroom environment. Undergraduate and graduate students will be responsible for developing a one-hour teaching session to be presented in high schools along with the interactive demo-kit. Feedback from students and teachers will help improve the demo-kit that will be in constant mutation and evolution. The Department of Mechanical Engineering is committed to providing scholarships to high school students participating in the Utah Science Olympiad for years to come. Additionally, the PI is proposing to give the opportunity to two high school students (per year) excelling in heat transfer to perform a one-month summer internship in PI s lab. This internship will be accompanied by a $1,000 fellowship. High school students will be mentored by undergraduate and graduate students, and will participate actively in enhancing the demo-kit and will be involved in the research activities in PI s lab. Continuous involvement of high school students will help strengthen the outreach activities and will significantly contribute to PI s research by assisting undergraduate and graduate students. The success of these outreach efforts will be measured via student and teacher feedback, Utah student performances at national competitions, and via the evaluation of the number of students enrolling in science and engineering programs at the University of Utah. The organization committee of the Utah Science Olympiad, with which the PI works in close collaboration, is keeping track of students participating in the event. These statistics will allow measurement of the impacts of the Olympiad to the state of Utah in terms of college enrollment, and will allow performing adjustments to the strategic plan in the case results do not meet the expectations. 4.4 Education/Outreach Timetable and Resources The educational and outreach activities will be conducted over a 60- month period. Funding is 1: Development of educational materials Preparation of near-field thermal radiation course requested for the design, Near-field thermal radiation teaching fabrication and maintenance of the demokit, as well as for the Integration in MEEN 3650 Heat Transfer Integration in near-field thermal radiation course 3: K-12 Outreach fellowships that will be awarded to high school Table 4.1. Education and outreach timetable. Educational/Outreach Milestones and Tasks vs. Months Preparation of near-field thermal radiation monograph 2: Enhancement of infrastructures for undergraduate and graduate education Design, farication, and maintenance of the demo-kit Presentation of demo-kit at Science Olympiad and high schools Summer internships for high school students Summer 2013 Fall 2013 Spring 2014 Summer 2014 Fall 2014 Spring 2015 Summer 2015 Fall 2015 Spring 2016 Summer 2016 Fall 2016 Spring 2017 Summer 2017 Fall 2017 Spring