*Medium-High Temperature Composite Phase Change Materials Based on Porous Ceramics DOI: http://dx.doi.org/10.5772/intechopen.114185*

an excellent skeleton material for composite phase change materials. Luo et al. [46] encapsulated PCM through gradient SiC foam as the backbone for fast and stable thermal storage. The fabrication strategy of gradient SiC foams is shown as (a). The strong capillary force generated by the gradient pore structure effectively limits the leakage of PCM. In addition, the interconnected SiC backbone enables the thermal conductivity of CPCMs to be increased to 1.9 W/(m∙K) and realizes an efficient solar thermal storage process. Our group regulates the porosity of SiC ceramics based on starch pore-formation, which in turn optimizes the thermal conductivity and mechanical strength of CPCMs [23]. A continuous and stable fast heat transfer channel was constructed by high-temperature sintering and loaded with ternary chloride salt to ensure effective heat storage density. The cell shape, microscopic pore structure, and temperature evolution with time during charging/discharging of CPCM are shown in **Figure 6**. The CPCMs have a thermal conductivity of 19.72 W/(m∙K) and an effective heat storage density of 513.46 kJ/kg.

In addition to the ceramic materials mentioned above, other materials could be used as skeleton options for medium-high-temperature ceramic-based CPCM. They are inherently porous or capable of generating porous skeletons to load inorganic salts. They also improve the thermal conductivity of the CPCM, limit PCM leakage, and have good thermal cycle stability. The studies of CPCMs based on other porous oxide and non-oxidized ceramics are summarized in **Table 5**, including compositions and results.

## **3.2 Porous clay-like mineral materials**

Clay minerals such as diatomite, and expanded perlite (EP), inherently have high-quality porous structures and large specific surface area, which can better realize the embedding of PCMs as the core material and effectively reduce the leakage. They serve as a ceramic skeleton matrix for CPCMs and are capable of long cycles and stable operation at medium-high temperatures, maintaining good chemical compatibility and stability, making them an ideal choice for commercial applications. The wide range of raw materials, low cost, and high performance make porous clay mineral skeleton materials an ideal choice for commercial applications.

Diatomite is a natural mineral material whose main component is SiO2. It has a unique porous structure with a specific surface area of 40–65 m<sup>2</sup> /g and a porosity of 80–90%. This gives diatomite excellent adsorption properties, which in turn

#### **Figure 6.**

*(a) the cell shape, (b) the microscopic pore structure, and (c) the temperature evolution during charging/ discharging of CPCM [23] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*


#### **Table 5.**

*Research related to porous oxide and non-oxidized ceramic materials.*

encapsulate PCM with good thermal stability. In addition, diatomite is abundant, readily available, low-cost, and characterized by high temperature and corrosion resistance. Therefore, it has been widely used as a skeleton support material for CPCMs over the years.

Qin et al. [41] prepared high-temperature Na2SO4/diatomite-shaped CPCMs by mixing-sintering. The diatomite acts as a shape-stabilized skeleton and possesses excellent chemical-thermal stability with the PCM. They experimentally determined that 45% diatomite was the optimal formulation for CPCMs with an energy density of more than 360 KJ/kg in the range of 700–900°C. Qian et al. [52] used diatomite as matrix and loaded three different PCMs by vacuum impregnation to prepare low, medium, and high CPCMs for solar thermal utilization. They also proved that the melting point, latent heat, and subcooling of CPCMs are constant through their self-designed experimental device. However, since the main component of diatomite is the natural SiO2, it has the disadvantage of low thermal conductivity (0.2 ~ 0.4 W/ (m∙K)) compared with metal encapsulation materials. Li et al. [53, 54] synthesized MnO2-modified diatomite by hydrothermal reaction and then prepared CPCMs by vacuum-impregnation. The preparation process of MnO2-modified diatomite is shown in **Figure 7**. After characterization, it was proved that the ceramic-based CPCMs possessed a faster heat transfer rate and better photothermal conversion capability. In addition, they also treated the diatomite with microwave acid to obtain higher porosity and added EG to improve thermal conductivity. Jiang et al. [19] used CaCO3-modified diatomite as a porous ceramic skeleton for CPCMs. Due to the densecontinuous skeleton, the CPCMs' thermal conductivity was improved by 129% and exhibited excellent thermal stability in 500 thermal cycles.

Expanded perlite (EP) is a traditional thermal insulation material. Its volume expands rapidly after high-temperature calcination, allowing it to acquire a foamy honeycomb structure with strong adsorption properties, which could be used to prepare high-performance CPCMs. Based on the excellent adsorption capacity of EP, Li et al. [55] prepared NaNO3/EP shaped CPCMs. The results showed that skeletons at different treatment temperatures (300–900°C) exhibited different adsorption strengths, with the highest adsorption after heat treatment at 500°C for 2 h. The CPCMs have good thermal conductivity and thermal cycling stability.

*Medium-High Temperature Composite Phase Change Materials Based on Porous Ceramics DOI: http://dx.doi.org/10.5772/intechopen.114185*

#### **Figure 7.**

*Preparation process of MnO2 modified diatomite [53] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*

#### **Figure 8.**

*Preparation of composite FSPCMs [57] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*

Zhao et al. [56] regulate the porous structure of EP by polyvinyl alcohol (PVA) to increase the nanopore size and specific surface area. Then they proposed novel CPCMs with high excellent loading (73.1%) and heat storage capacity (174.6 J/g). After 500 thermal cycling experiments, the samples possessed good thermal stability and leakage resistance. Zuo et al. [57] optimized the EP-based CPM by adding graphite, and the basic preparation process is shown in **Figure 8**. Characterization results demonstrate that the addition of graphite improves the thermal stability and thermal conductivity of CPCMs. EP/P50/GP80 exhibit the highest heat transfer rates and have good chemical stability.
