Nanofluids: Key Parameters to Enhance Thermal Conductivity and Its Applications

Utilizing phase change materials (PCMs), which can be tailored to melt or solidify at specific temperatures, as an energy storage medium provides an effective way to store latent thermal heat for diverse applications. However, conventional PCMs exhibit significant disadvantages, such as low thermal conductivity and leakage. Therefore, highly-thermally-conductive composite phase change materials (CPCMs) have received increasing research interest by virtue of integrating nanofillers with high thermal conductivity and thermal stability, such as hexagonal boron nitride (hBN), into the organic PCM matrices. While several nanomaterials-reinforced composites, such as graphene-based CPCMs, have been reviewed extensively, however, there are no critical reviews on the thermal performance of BN-enhanced organic CPCMs in energy storage applications. Therefore, this literature review highlights and comprehensively summarizes the latest research on the thermal improvements of BN-filled organic CPCMs with a focus on different types of BN allotropes, including hBN, BN nanoplatelets, BN nanosheets and BN nanotubes. Moreover, this review provides an extensive discussion on the fabrication processes, effects of functionalization and porous scaffolds, as well as the applications of BN fillers as shell materials. Specifically, the novel theoretical and experimental methodologies for achieving high thermal conductivities are reviewed and compared. Additionally, some other fundamental parameters associated with thermal properties, such as latent heat of fusion, thermal cycle stability, and thermal-to-solar efficiencies are also discussed. Finally, current challenges in the field of BN-incorporated CPCMs are listed/discussed, and recommendations for the future development of BN-enhanced organic CPCMs with better thermal properties to cater the future needs in energy storage are provided. 

Effects of boron-nitride-based nanomaterials on the thermal properties of composite organic phase change materials: A state-of-the-art review

Carbon nanotubes and nanofibers (CNFs) are well-known nano additives to produce coating materials with high electrical and thermal conductivity and corrosion resistance. In this paper, coating materials incorporating hydrogen bonding offered significantly lower electrical resistance. The hydrogen bonding formed between functionalized carbon nanotubes and ethanol helped create a well-dispersed carbon nanotube network as the electron pathways. Electrical resistivity as low as 6.8 Ω cm has been achieved by adding 4.5 wt% functionalized multiwalled carbon nanotubes (MWNT-OH) to 75%polyurethane/25%ethanol. Moreover, the thermal conductivity of polyurethane was improved by 332% with 10 wt% addition of CNF. Electrochemical methods were used to evaluate the anti-corrosion properties of the fabricated coating materials. 75%polyurethane/25%ethanol with the addition of 3.0 wt% of MWNT-OH showed an excellent corrosion rate of 5.105 × 10-3mm year-1, with a protection efficiency of 99.5% against corrosive environments. The adhesion properties of the coating materials were measured following ASTM standard test methods. 75%polyurethane/25%ethanol with 3.0 wt% of MWNT-OH belonged to class 5 (ASTM D3359), indicating the outstanding adhesion of the coating to the substrate. These nanocoatings with enhanced electrical, thermal, and anti-corrosion properties consist of a choice of traditional coating materials, such as polyurethane, yielding coating durability with the ability to tailor the electrical and thermal properties to fit the desired application. 

Enhanced electrical conductivity of anticorrosive coatings by functionalized carbon nanotubes: effect of hydrogen bonding

Phase change materials can store or release large amounts of energy during phase change. An increasing number of authors are studying the influence of the dispersion of nanometric particles on these materials. This article presents the design and experimental characterization of temporal stability, thermal conductivity, isobaric heat capacity, phase change transitions, rheological behaviour, and dielectric properties of nano-enhanced phase change materials based on carbon black (CB) dispersions in polyethylene glycol (PEG200) by using polyvinylpyrrolidone (PVP) as surfactant. We studied the temporal stability of carbon black nanoparticles dispersed in PEG200 using dynamic light scattering and spectrophotometry techniques. All the samples showed good temporal stability, since the temporal measurement of the hydrodynamic size of the nanoparticles are practically constant and the wavelength observed by UV-vis shows a small variation of around 4% for static conditions. We observed small changes in thermal conductivity and isobaric heat capacity. Nevertheless, the thermograms evidence how the latent heat clearly increases with the load of carbon black nanoparticles up to four times that of the PEG200. The viscosity studies do not show variation with shear rate, indicating a Newtonian behaviour excluding 2.0 wt% CB/PVP+PEG200 nanofluid. Additionally, we noticed frequency dependent and independent regions for permittivity. 

Thermophysical, rheological and dielectric behaviour of stable carbon black dispersions in PEG200

The demands for effective thermally conductive dielectric composite materials continue to increase due to high speed and low-loss signal propagation required in microwave frequency applications. Composites of polytetrafluoroethylene (PTFE) and boron nitride are promising for these applications. Here, for the first time, recent generation surface-engineered boron nitride nanobarbs (BNNB) material was evaluated at a low filler content in PTFE to achieve the desirable high thermal conductivity and low dielectric loss requirements which are typically difficult without a strong filler–matrix interaction and high filler loading. Composites of modified BNNB and modified PTFE were also fabricated and evaluated. These composites exhibited higher thermal conductivity (1.2–1.3 W/mK) with a lower concentration of boron nitride nanoparticles than those reported in the literature. The data were analyzed by theoretical models and interfacial thermal resistance was the lowest for the composite made from the modified PTFE and modified BNNB. Additionally, the composites displayed excellent dielectric properties including a dielectric constant ≈2.3 and loss tangent of less than 0.005 at frequencies of 1–3 GHz. These results indicate that such thermally conductive and low-loss dielectric composites have significant potential as materials for thermal management and dielectric components in microelectronics, 5G, and microwave devices. 

Polytetrafluoroethylene Nanocomposites with Engineered Boron Nitride Nanobarbs for Thermally Conductive and Electrically Insulating Microelectronics and Microwave Devices

An investigation was conducted to examine the extent to which the thermal conductivity of materials in polyethylene glycol (PEG) fluids could be improved using the concept of hydrogen bonding and long chains. Boron nitride (BN), hydroxyl functionalized multiwall nanotube (MWNT-OH), and carbon nanofibers (CNFs) were mixed and exfoliated to form hydrogen bonds between the particles and the PEG fluids. Following this bonding process, clear enhancements in the thermal conductivity were observed. In addition, the long chain boron nitride nanotubes (BNNTs) and carbon nanofibers were shown to significantly enhance the thermal conductivity, due to the resulting increase in the contact and interfacial surface areas. The results of this experimental investigation indicated that PEG400 with a loading of 7.5 wt% MWNT-OH yielded a thermal conductivity increase as high as 89.7% over the base fluid. PEG400 loaded with a combination of 2 wt% CNF and 2 wt% BNNT resulted in a thermal conductivity increase of 73.1%. These results are significantly higher than the best results previously reported in the literature and may in part be attributed to the use of a three roll milling technique to enhance contact. The results presented here will open the way for further exploration of the use of novel phase change materials for thermal energy storage applications.

Improved thermal conductivity of PEG-based fluids using hydrogen bonding and long chain of nanoparticle

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Improved thermal conductivity of PEG-based fluids using hydrogen bonding and long chain of nanoparticle