**2.1 Mixing impregnation**

Mixing impregnation is based on the capillary force of the porous ceramic, which confines the solid-liquid PCM in the pores to prevent leakage and can effectively improve the overall thermal conductivity. The simple preparation process of mixing-impregnation is shown in **Figure 1**. It could be divided into two main steps: (1) preparation of ceramic porous skeleton as a carrier for the inorganic salt, and (2) infiltration of the PCM to provide enough heat capacity. Typically, a ceramic skeleton is prepared by rational pore-making techniques and high-temperature sintering, and the molten PCM penetrates the porous carrier by capillary forces.

#### **Figure 1.**

*Process flow for mixed impregnation [19] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*

**Figure 2.**

*Process flow of ternary chloride/SiC CPCM [23] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*

The basic equipment includes a high-speed mill, a high-temperature tube furnace, and a vacuum sintering furnace. Mixing impregnation consists mainly of melt impregnation and vacuum impregnation. Most of the PCMs could be adsorbed into the ceramic skeleton by melt impregnation. However, some pores may still exist inside the carrier, leading to a reduction in the total CPCMs' thermal storage density. Therefore, vacuum impregnation under high-temperature and vacuum environments can introduce more PCM to support the matrix. In general, CPCM units are subjected to continuous thermal cycling charging/discharging tests to ensure their mechanical strength and thermal stability.

Jiang et al. [20] prepared shaped-stabilized CPCMs based on modified diatomite by loading NaNO3 into the porous ceramic skeleton using melt-impregnation. The infiltration process of the sample was carried out in a muffle furnace under atmospheric pressure, followed by sandpapering of the excess cured salt. The ss-PCM has excellent cyclic thermal stability, thermal storage density, and efficiency. Wang et al. [21] used the same method to prepare high-temperature ss-CPCMs with ternary chloride salts as the PCM, porous Si3N4 as the skeleton, and heat transfer enhancers. The pore permeability of PCM in the skeleton was 88.14%, and the thermal conductivity of ss-CPCMs was significantly improved. To develop high-performance CPCMs that can be mass-produced at low cost, our group [21, 22] used vacuumimpregnation to combine SiC ceramics with high-enthalpy ternary chloride salts as shown in **Figure 2**. The controlled porosity of the skeleton results in excellent thermal properties (thermal conductivity-heat capacity). PCM relies on capillary force to enter the carrier in a vacuum environment and has a high loading capacity. Other current research about mixing-impregnation is summarized in **Table 3**, including compositions and results.

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


#### **Table 3.**

*Research related to the melt-impregnation method.*

It needs to be clarified that the mixing impregnation requires proper regulation of the pore structure of the porous skeleton. First, the porous ceramics' pore size should be within a certain range. Too small pore size will limit the microflow of the PCM, and too large pores will not provide sufficient capillary. In addition, the carrier's porosity needs to be regulated. Too low porosity reduces the total heat storage capacity, and too high porosity affects the thermal cycling stability of the CPCPMs [29]. Nomura et al. [30] investigated the effect of the ceramic materials' pore size (diatomite) in CPCMs on the melting point. Due to the nano-size effect, the smaller the pore size of diatomite, the lower the melting point of the CPCMs. Liu et al. [31] also demonstrated that increasing the porosity or decreasing the pore size could effectively increase the infiltration ratio of inorganic salt. Both the sintering temperature and impregnation temperature during the preparation process affect the CPCMs' thermal properties [32]. The former is a key factor in regulating the pore structure of the ceramic carrier, while the latter ensures the loading.

#### **2.2 Cold press-sintering**

Cold press-sintering can usually be summarized in three steps: mixing grinding, hydrostatic forming, and high-temperature sintering, as shown in **Figure 3**. First, the inorganic salts were mixed and ground with ceramic materials, then the mixture was poured into molds and pressed into desired shapes, and finally, the CPCMs were prepared by high-temperature sintering. The basic equipment includes highspeed mills, tablet presses, and high-temperature sintering furnaces. It needs to be clarified that the stability of CPCMs prepared by the cold pressing-sintering method stems from mechanical locking during pressing and mounding under the external force, and bonding after the melting and solidification of PCM/ceramic particles during the high-temperature sintering. In addition, the structure of CPMs is related to the mounding process. Conventional isostatic pressing results in a more homogeneous and dense material; uniaxial pressing produces a hierarchically arranged structure.

Porous ceramics are inherently capable of adsorbing molten salts to prevent leakage, making them the preferred target for the preparation of ceramic-based CPCMs by cold pressing-sintering. Ye et al. [33] used MgO to encapsulate PCMs.

#### **Figure 3.**

*Preparation process of cold pressed-sintered [19] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*

#### **Figure 4.**

*Process flow of TC/MgO/EG CPCMs [35] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*

They successfully obtained Na2CO3/MgO composites by using cold pressing-sintering and adding carbon nanotubes (MWCNTs) as additives. The CPCM's thermal conductivity increases with the increase in the weight fraction of MWCNTs. Deng et al. [34] prepared KNO3/diatomite-shaped-stabilized CPCMs by mixing-sintering. Diatomite has high porosity, which acts as a high-strength carrier and effectively limits the leakage of PCMs. The 65% loaded CPCMs have good physical/chemical properties with latent heat of 60.52 J/g and an effective melting point of 330°C. To further enhance the thermal performance of ss-PCM, our group [35] added EG to ternary chloride (TC)/MgO ceramics-shaped CPCMs. The specific preparation process of CPCMs is shown in **Figure 4**. MgO carrier as the supporting skeleton could improve the mechanical strength of CPCMs, and EG as the additive could increase the thermal conductivity and improve the heat transfer process. CPCMs have a promising future in high-temperature solar applications. Some other current research about cold press-sintering is summarized in **Table 4**, including compositions and results.

For the cold pressing-sintering process, the ratio between the materials, the amount of molding pressure, and the sintering temperature all have an important effect on the CPCMs' mechanical and thermophysical properties [40]. Qin et al. [32, 41] investigated the Na2SO4/diatomite CPCMs ratio. It was finally determined that CPCMs containing 45% diatomite were optimal in terms of energy density, leakage prevention, and mechanical strength. The effect of sintering temperature (300–500°C) on ceramic-based CPCM was analyzed by Ji et al. [42]. The maximum mass fraction of PCM and compressive lightness decreased with increasing sintering temperature, and the thermal conductivity remained almost constant.

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


#### **Table 4.**

*Research related to the cold press-sintering method.*
