Nanomaterials Resarch Laboratory
Background
“Nano-eutectic” breakthrough for thermal storage (2011~2013): My innovative contribution is the usage of nano-eutectics as an efficient thermal storage medium, which is a drastic departure from conventional approaches and thinking. Premature results in some cases were dismissive of their unique benefit due to the lack of fundamental understanding. In particular, the heat capacity of the hybrid media was commonly understood to decrease along with the concentration of nanoparticles. My passion and mission are to unequivocally correct this misconception through the use of theoretical and experimental evidences. My research first focused on laying rigorous physical grounds relating the concentration of nanoparticles and resulting heat capacity. I discovered, in the process, that nano-eutectics’ heat capacity could be significantly enhanced when suitable materials are appropriately synthesized. This is a game-changing technology for solar thermal and other energy industries as it can lead to millions of dollars in savings when appropriately designed. To reverse the course and correct a misconception that many researchers in the field believed for over two decades is my point of pride as a researcher in the academia.
Impact of nano-eutectic breakthrough: (2014~present): Following the publication of my breakthrough findings in 2011 (fundamental)[1,2] and 2013~14 (verification) [3,4], several research groups, listed in Table 1, followed my approach by citing my researches and corroborating my results. Subsequently, I have been extensively asked to review the manuscripts of other researchers, who also cited my previous researches. However, they are not included in Table 1 as their manuscripts are still under review. My influential work is reflected by my rapidly increasing citation index, which has been recognized as the fourth most cited article in the International Journal of Heat and Mass Transfer (IJHMT) since 2011 among 5,800 articles, excluding review papers.
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[1] Shin, D., & Banerjee, D. (2011). International journal of heat and mass transfer, 54(5),1064
[2] Shin, D., & Banerjee, D. (2011). Journal of heat transfer, 133(2), 024501
[3] Shin D, Tiznobaik H, Banerjee D (2014) Applied Physics Letters 104: 121914
[4] Tiznobaik H, Shin D (2013) Applied Physics Letters 102: 173906;
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For over two decades, it was commonly understood that the addition of nanoparticles into liquids increases only the effective thermal conductivity and decreases the effective heat capacity. [5,6] Hence, I aimed to understand the underlying mechanism behind the enhanced heat capacity phenomena. About two years later, I found that a distinct heat storage mechanism exists for molten salt nano-eutectic, and I also experimentally verified this mechanism. As nanoparticles in water, oil, or ethylene glycol are the primary reason for thermal conductivity enhancement of liquid media, nanoparticles in molten salts induce salts themselves to form a fractal-like nanostructure that is primarily responsible for enhancing heat capacity of molten salts (Figure 1).
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[5] Zhou S, Ni R (2008) Applied Physics Letters 92: 093123
[6] Namburu, P. K., et al. (2007). Micro & Nano Letters, 2(3), 67-71
Topic 1: Discovering the underlying mechanism of molten salt nano-eutectic
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Figure 1 (a) homogeneously dispersed salt compound A (bright) and salt compound B (dark). (b) When a nanoparticle is loaded, A is more attracted than B due to the difference in electrostatic interactions. A moves closer to and B moves away from the nanoparticle. (c) A portion of ionic compound A near the nanoparticle starts to forms a fractal-like nanostructure.
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The nanostructural changes and their effects on heat capacity are not yet thoroughly understood and thus need further investigation. However, due to the extremely small size of these nanostructures, it is challenging to investigate these phenomena using the current experimental techniques. Thus, a computational investigation should be proposed to fill the gaps in understanding molten salt nano-eutectics. Molecular dynamics (MD) simulation can be useful in this case. MD simulation is a computational technique used to model the behavior of atoms or molecules. Furthermore, it is used for simulating physical, chemical, and thermodynamic properties of various fluids or gases under high pressure and temperature. In a general MD simulation, the equation of motion is solved for each atom at each time step by employing a particular model for the force fields based on several fundamental material interactions such as van der Waals force, Coulomb forces, etc. Therefore, the positions and velocities of each atom are updated at each time step by computing interactions with the force fields. These position and velocity information of each atom are used to simulate thermophysical properties of a system using the space–time correlations from statistical mechanics. MD simulations have been well established for the calculations of thermodynamic properties using equilibrium models, Green-Kubo method, and non-equilibrium direct methods. Figure 2 shows an example of the inorganic nano-eutectic MD domain that my group constructed.
Topic 2: A computational approach to mimic the salts’ nanostructural changes
Figure 2 A MD domain of molten salt nano-eutectic.
Previous studies focused on molten salt nano-eutectics by relying on salts’ unique feature to form a fractal-like nanostructure. Hence, the technology was only viable for molten salt media. For this reason, I developed a new idea to synthesize nanostructures that are chemically bonded to a nanoparticle, as shown in Figure 3; and this approach does not require a base medium, such as salts, to form a fractal-like nanostructure. Moreover, the technology can be used for any fluids. Initially, the approach was applied to polyalphaolefin, which is a nonpolar liquid used as airforce radar coolant – thus making my work a potential Air Force Office of Scientific Research (AFOSR) sponsored research. The preliminary results have shown that its properties have been enhanced by 44.5% (heat capacity), 27% (thermal conductivity), and 25% (viscosity). The increase in viscosity may have a negative effect on the pumping power; thus, it is necessary to perform a figure of merit (FOM) analysis for heat storage fluid [7] and heat transfer fluid [8]. FOM results show that this approach can enhance fluid’s performance for heat storage & transfer by 168% & 103%.
7 Bonilla CF (1957) Nuclear engineering
8 Lenert A, Nam Y, Wang EN (2012) Annual Review of Heat Transfer 15
Topic 3: Fabricating ionically bonded sub-nanostructure to be used for non-molten salt media
Figure 3 A schematic of chemically bonded nanostructure, a potential universal nano-additive.
Topic 4: Fabricating ionically bonded sub-nanostructure to be used for non-molten salt media
I developed a new in-situ synthesis protocol for molten salt nano-eutectics. Without the addition of nanoparticles, through the use of the thermal decomposition of carbon residue or inorganic compound eutectic, which may be unstable at high temperatures, I found that these materials are decomposed into nanoparticles under molten salt conditions (Figure 4). Once the nanoparticles are formed, they induce surrounding salts to form an interconnected nanonetwork that can significantly improve thermal properties. The results of the protocol was submitted for a patent (WO2015095068 A1).
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Figure 4 In-situ formed nano-particles in a molten salt
Topic 5: Nanostructure-embedded phase change energy storage
I developed a new fabrication method to mimic the nanostructural change in a phase change material (PCM). With this method, one can enhance the thermal properties of the material near its phase transition: hence, the overall effective enthalpy can be increased with only a marginal decrease in its heat of fusion. This idea can be used to develop novel PCM systems and initially attracted an industry partner (Mitsubishi, Japan) and three year, proof-of-concept project was supported to develop a hydrated salt-based PCM and expected to continue for a long term collaboration. The idea can be extended for various engineering applications such as engine warm-up, building insulation, portable energy storage, etc – thus potentially can be several industry sponsored research projects.
Figure 5 Encapsulated water in the initial results.
Previous studies only focused on the effect of salts’ structural changes outside of the nanoparticles. Hence, I focused on developing a new idea encapsulating supercritical water inside the nanoparticles. The preliminary theoretical estimate showed that heat capacity could be enhanced up to 160% with only a nanoparticle concentration of 1%. Through my collaboration with a well-known researcher in nano-encapsulation, we successfully synthesized water-encapsulated silica nanoparticles, as shown in Figure 5.
Topic 6: Supercritical water-encapsulated molten salt nano-eutectics
Figure 6 Super-non-wetting surface by molten salt.
I developed a cost-effective anticorrosion coatings for structure materials that are compatible with the developed storage fluids. With this approach, we can provide a cost-effective anticorrosion coatings on existing structure material without a significant change in the concentrating solar power (CSP) system structure. If successful, it will displace the incumbent CSP storage systems and will eventually pave the way for the next generation CSP market. The preliminary test has confirmed a high contact angle (~133°C) of molten salt on the nano-engineered surface.
Topic 7: Corrosion resistance for molten salt nano-eutectics by super-non-wetting surface
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Research Direction 1: Molten salt based solar thermal fluid system
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Developing novel synthesis methods to prepare molten salt-based thermal fluid and energy storage for solar thermal power systems. Several synthesis methods were investigated.
One of them resulted in a patent application (Publication #US20160318067 A1)
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Developing a novel energy storage fluid for solar thermal power systems. Several nitrate salt mixtures are mixed with various nanoparticles that can enhance the storage performance up to 100%.
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Research Direction 2: Nanoparticle embedded energy storage system
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Developing a nanoparticle embedded phase change heat storage system that can minimize heat loss when charging and discharging energy + improving thermophysical properties.
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Research Direction 3: Nano-engineered dielectric fluid system
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Developing a “universal” nano-additive that can induce nano-structural change in common engineering fluids for enhancing their heat transfer performance.
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(1st demonstration in literature)
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Research Direction 4: High-temperature anti-corrosion
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Developing a simple nano-coating on a structure material that can provide molten salt-phobic surface to minimize corrosion.
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