Development of thermal enhanced n-octadecane/porous nano carbon-based materials using 3-step filtered vacuum impregnation method

Highlights

• 3-step filtered vacuum impregnation method made for phase change material composites.

• n-Octadecane/porous nano carbon-based materials(OPNCs) was made for validate method.

• Thermal conductivity of OPNCs were increased up to 580% compared with n-octadecane.

• Through thermal analysis, OPNCs showed Latent heat capacity (J/g) 103.90–196.80.

• 3-step filtered method was determined that stable impregnation method.

Abstract

In this study, n-octadecane/porous nano carbon-based materials (OPNCs) were thermally enhanced using a 3-step filtered vacuum impregnation method. n-octadecane as phase change materials (PCMs) and supporting materials of C-300, C-500, Activated carbon (AC), Expanded graphite (EG) and Exfoliated graphite nanoplatelets (xGnP) made of the same raw material. Through scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR) analysis, n-octadecane was well impregnated in carbon-based materials not a chemical bonding. Thermal conductivities of OPNCs were increased up to 580% compared with n-octadecane by TCi. Dfferential scanning calorimetry (DSC) analysis was used to verify thermal performance of OPNCs, the latent heat capacities of OPNCs were measured from 220J/g to 393J/g. Analysis of thermal stability by thermogravimetric analysis (TGA) showed that the impregnation ratio of OPNCs was about 56% and that of EG was 88.53%. 3-step filtered vacuum impregnation method manufactured a stable and thermally enhanced OPNCs.

Introduction

The building sector is a major energy consumers with a total contribution toward global energy consumption of about 40% [1], [2]. Furthermore, building energy consumption has been rapidly increasing due to population growth and pursuit of enhanced comfort in the buildings [3], [4].

Consequently, various techniques have been presented for energy reduction in buildings. The one of techniques, Thermal Energy Storage (TES) is a useful technology for improving energy efficiency and energy saving. When TES apply to buildings, the building could be more stored energy for building energy savings than only use sensible heat [5], [6], [7], [8]. TES is applied in buildings by using Phase change materials (PCMs). PCMs provide thermal energy that is stored when it heated by melting and released when it cooled off by solidification. TES thus works in a passive way through these phase changes from solid to liquid or liquid to solid. More energy can be stored using this process compared to the use of sensible heat or latent heat [9], [10], [11], [12]. PCMs are divided into three types: organic PCMs, inorganic PCMs, and eutectic PCMs. In this study, n-octadecane is selected, which has a melting temperature of 28 °C and is classified as an organic PCM. Organic PCMs have been widely used in thermal energy storage applications due to their large latent heat and good thermal characteristics, such as little or no super cooling, low vapor pressure, good thermal and chemical stability [13], [14], [15], [16]. Despite these advantages, PCMs have weaknesses that limit their application to building materials. Firstly, PCMs incur a leakage problem when in an endothermic or exothermic state. In other words, leakage occurs with the phase change from solid to liquid or liquid to solid. Secondly, PCMs have low thermal conductivity. In order to apply PCMs to building materials for a TES system, high thermal conductivity is essential. To achieve optimal thermal performance of PCMs, they must be stored heat sufficiently, above the sensible heat capacity, which means PCMs are within the state of fusion [17].

Recently, a new type of PCM has been developed, called shape-stabilized PCMs (SSPCMs), composed of PCMs and supporting materials. PCMs need to have congruent supporting materials. If the PCMs are based on paraffin, the supporting material should have a similar skeleton to that of paraffin, such as high-density polyethylene (HDPE), polypropylene (PP), and styrene–butadiene–styrene (SBS). Paraffin can be easily dispersed into the network formed by the supporting material. Providing that the operating temperature is below the melting point of the supporting material, the SSPCMs can maintain their shape even when the paraffin changes from solid to liquid or liquid to solid [18].

Many attempts have been made to enhance the thermal conductivity and mitigate the leakage problem of PCMs. One of the best ways to apply PCMs in a real condition is to impregnate PCMs into high thermal conductivity materials such as carbon materials with porous structures [19], [20], [21].

Research by the Drzal group has shown that exfoliated graphite nanoplatelets (xGnP™), which combine the layered structure and low price of nanoclays with the superior mechanical, electrical and thermal properties of carbon nanotubes, are very cost effective and can simultaneously provide a multitude of physical and chemical property enhancements [22], [23], [24]. With similar characteristics to those mentioned above, C-300 and C-500 can be made with Exfoliated graphite nanoplatelets (xGnP) through a secondary processing. XG Sciences offers its “C-grade” materials in a variety of different surface areas. These materials have a “small flake” morphology, with particle sizes that are larger in the lower surface area materials and smaller in the higher surface area materials [25]. Activated carbon (AC) is another porous nano-sized carbon. While AC is similar to another carbon-based materials, it has a stability of structures and high thermal conductivity. In [26], it was demonstrated that AC is a noteworthy material because it has a large surface area and low density [27]. Lastly, expanded graphite (EG) has been made through the Tryba’s works [28], [29]. Using the H₂SO₄–graphite intercalation compounds (GICs) with thermal treatment, natural graphite can be transformed into graphite, then it has a considerably larger surface area than before treatment one. From this work, EG has an accordion shape as shown Fig. 1 [30], [31].

In this study, an attempt is made to address the weaknesses of PCMs such as the leakage problem that causes decay or strength decrease in application and the low thermal conductivity for the application of the TES system in buildings. Porous nano carbon-based materials were therefore used as supporting materials for the purpose of manufacturing SSPCMs. Also, the method of the 3-step filtered vacuum impregnation was conducted as it more precise and has increased productivity in comparison to the methods outlined in [3], [9], [13], [16], [18], [19], [20], [21], [26], [27], [32], [33], [34], [35], [36], [37]. The verification experiments included scanning electron microscopy (SEM) analyze the porous nano-carbon based material characteristics of the micro structure, Fourier transform infrared spectroscopy (FT-IR) for chemical stability, TCi for thermal conductivity, differential scanning calorimetry (DSC) for thermal storage performance and thermogravimetric analysis (TGA) for thermal stability.