Thermal management using electrospun material

Electrospun material has been investigated for use in thermal management. Thermal management may involve having the electrospun material as a passive thermal insulator/conductor, or an active temperature regulating device. The later involves absorption of thermal energy when the temperature is high and releasing energy when the temperature is low. Selection of additives to be used in the electrospun fibers will determine its properties for thermal management. In other cases, electrospun mat has been used as a supporting role in a thermal management system.

A highly thermal conductive polymeric material may be used for heat dissipation in electrical and electronic devices while keeping weight to a minimum. Datsyuk et al (2013) fabricated a thermal conductive carbon nanotube/polybenzimidazole polymer nanofiber using core-shell electrospinning. The carbon nanotube was found to be highly aligned along the length of the fiber core for 1.94% wt carbon nanotubes loading. The in-plane thermal conductivity of mat was 18 W/mK. In the direction normal to the flat surface of the mat, it is thermally insulating with through-plane conductivities of 0.010 - 0.014 W/mK.

In some applications, the thermal conductive material needs to be flexible. Chen et al (2021) constructed a flexible fiber membrane that uses carbon nanotube and electrospun polyurethane to achieve high thermal conductivity. Acidified multiwalled carbon nanotubes (a-MWCNTs) were first mixed into thermoplastic polyurethane (TPU) solution followed by electrospinning to form a nanofibrous membrane. The electrospun a-MWCNTs/TPU nanofiber membrane was then submerged into a suspension containing a-MWCNTs and ultra-sonicated to get the a-MWCNTs to adhere onto the surface of the fibers. The resultant composite fibrous membrane with a-MWCNTs both inside and on the surface gave a horizontal direction and vertical direction thermal conductivity of 3.50 W/mK and 1.79 W/mK respectively, which corresponds to 18 times and 10 times higher compared to pure TPU fiber membranes. Although there is a reduction in the elongation at break compared to pure TPU electrospun membrane, the composite membrane still showed a high elongation at break of 350%.

Miniaturization of electronic components and circuits creates a challenging environment which requires thermal management and thermomechanical reliability from its die attach and thermal interface material. Electrospun membrane has been tested as a joint interface material between a solder matrix and the electronic component. Zanden et al (2014) electrospun polyimide fiber mesh and surface functionalized it to form a surface layer of silver. The final composite was formed by infiltration of Sn-Ag-Cu matrix into the fibrous network. This solder matrix nano-polymer composite (SMNPC) exhibited a high heat transfer capability with a through-plane and in-plane thermal conductivity of about 22 W/mK and 42 W/mK respectively which is close to that of direct soldered interface. The elastic modulus is reduced by 65% compared to pure Sn-Ag-Cu and thermal cycling tests showed reliable thermomechanical performance.

High porosity of electrospun fibrous mesh is able to trap air which potentially gives it a good thermal insulation property. This is confirmed using thermal conductivity tests which show that decreasing fiber diameter leads to an increase in thermal resistance [Gibson et al 2007]. However, at high compression level, the amount of air trapped within the inter-fiber pockets are reduced resulting in a decrease in its insulating properties. Using electrospun polyacrylonitrile membrane, Gibson et al (2007) showed that as its thickness increases, the increment in thermal resistance is much less than other insulating materials such as waterfowl down clusters, melt-blown pitch, polyester filament wool etc. This is due to the physical arrangement of the nanofibers which does not encourage loft. It has been suggested that integrating electrospun nanofibers with other materials may take advantage of the better insulation property of nanofibers while providing greater resilience to the composite material.

In some applications, the insulation property provided by the relatively thin nanofiber membrane is useful in small devices. Jiang et al (2012) used electrospun polyethylene oxide (PEO) as thermal insulating membrane to regulate seed growth in microfluidic chips. In agreement with the result from Gibson (2007),increment of the thermal insulating property of the membrane reduces as the electrospinning deposition time increases. With increasing electrospun fiber thickness, the pore size reduces [Jiang et al 2012]. This will contribute to smaller air pockets and reducing insulation performance. In the setup by Jiang et al (2012), thermal radiation is the main heat transfer mechanism. They hypothesized that the insulation property of electrospun membrane comes from the scattering of the electromagnetic waves.

Electrospun fibers have been investigated for use as a reinforcement material in composite. Since electrospun mat is also demonstrated to be a good thermal insulator, it has been tested for use as a reinforcement material for brittle thermal insulating material. Silica aerogel is a highly thermal insulating material with a thermal conductivity as low as 0.013 W/mK. However, its poor mechanical stability limits its use in many applications. Wu et al (2013) showed that with the addition of electrospun polyvinylidene fluoride (PVDF) web, the mechanical stability can be significantly improved while maintaining a low thermal conductivity. When tested with electrospun PVDF microparticles, beaded nanofibers and smooth nanofibers, the smooth nanofibers give the highest compressive strength, bending strength and lowest thermal conductivity.

Microstructure of electrospun PVDF	Thermal conductivity of aerogel composites (W/mK)	Compressive strength (MPa)	Bending strength (MPa)
Microparticles	0.039	2.74	0.12
Beaded nanofibers	0.032	4.56	0.79
Smooth nanofibers	0.027	5.23	1.1
Pure aerogel	0.024	0.75	-
Comparison of thermal conductivity and mechanical properties of aerogel with various electrospun PVDF support [Wu et al. Journal of Nanomaterials, vol. 2013, Article ID 375093, 8 pages, 2013. doi: 10.1155/2013/375093. This work is licensed under a Creative Commons Attribution 3.0 Unported License].

Morphology and microstructure of three electrospun PVDF webs (a, d, and g) microparticles electrospun from 18 wt.% PVDF; (b, e, and h) combined microparticles and nanofibers electrospun from 23 wt.% PVDF; (c, f, and i) nanofibers electrospun from 28wt.% PVDF. [Wu et al. Journal of Nanomaterials, vol. 2013, Article ID 375093, 8 pages, 2013. doi: 10.1155/2013/375093. This work is licensed under a Creative Commons Attribution 3.0 Unported License].

A simple form of active temperature regulation device is to use a phase change materials (PCM). These materials store thermal energy when it changes from solid to liquid state (endothermic process) and give out the energy when it reverts back (exothermic phase). Potential applications of PCMs include thermal energy storage, thermal protection and maintaining the temperature of an enclosed environment. Electrospun fibers have been explored for this application by encapsulating PCMs. Its high surface area and aspect ratio potentially gives it rapid heat transfer properties and flexibility. Hun and Yu (2014) used core-shell electrospinning to encapsulate natural soy wax within polyurethane. They were able to load up to 60% of the fibers with wax without showing leakage during heating or cooling cycles. The composite showed endothermic and exothermic behaviors with two well defined peaks within the temperature range of 0 to 80° while pure polyurethane does not exhibit any peaks. Lin et al (2023) incorporated boron nitride nanoparticles into phase change materials (PCM) for thermal management applications. Boron nitride exhibits high thermal conductivity which facilitates the transfer of heat into the phase change material. Using coaxial electrospinning, the PCM material, polyethylene glycol (PEG) forms the core and the sheath consists of boron nitride nanosheet (BNNS) and thermoplastic polyurethane (TPU). The low Young's modulus of TPU gave the resultant nanocomposite high flexibility. The composite fibers were aligned on a rotating drum during electrospinning and an additional layer of boron nitride was electrosprayed on the nanofiber layer. A final step of mold-pressing was used to bind the electrosprayed boron nitride nanosheet (BNNS-es) to the nanofiber layer. The resultant phase change nanocomposite (PCN) film showed a high in-plane thermal conductivity of 28.3 Wm^-1K^-1 with 32 wt% BNNS loading. In contrast, the through-plane thermal conductivity was only about 0.9 Wm^-1K^-1. Yi et al (2019) constructed core-shell electrospun fibers for application in thermo-regulated textile. The sheath material is made of polyvinyl butyral (PVB) while the core is pure octadecane. To maintain a molten octadecane, the electrospinning ambient temperature was kept at 50°C. The resultant composite was able to achieve a high latent heat up to 118 J g^-1. To facilitate conversion of solar to thermal energy, hexagonal cesium tungsten bronze (Cs_xWO₃, a near infrared absorber) was introduced to PVB. Cs_xWO₃ is also able to absorb infrared light to enhance user comfort. The multi-components electrospun fibers retained a high latent heat up to 96.9 J g^-1. A 100 thermal cycle between 25 and 50 °C showed a slight reduction in latent heat capacity. This shows that electrospinning can be used to fabricate composite containing PCM for the purpose of temperature regulation.

Scheme of the preparation process of the PEG@TPU/BNNS-es nanocomposite films [Lin et al 2023]

[+]

[-] comments powered by Disqus

Content [Materials]