**3.1 Porous oxide and non-oxidized ceramic materials**

Some porous oxide (MgO, Al2O3) and non-oxide materials (SiC, AlN) could be prepared into porous ceramic by suitable processes to provide encapsulation space for inorganic salts. The pore structure of CPCMs prepared from them tends to be uniform, and the leakage of CPCMs could be limited by capillary force and surface tension. In addition, these materials possess excellent high-temperature resistance

and physical/chemical stability, making them a reliable choice for the application of medium-high-temperature CPCMs.

MgO has become a widely used carrier material in ceramic-based CPCMs due to its advantages of high thermal conductivity, high specific heat, good wettability with inorganic salts, and wide application temperature range. Li et al. [43] conducted an in-depth study on MgO-based CPCMs and analyzed the effects of particle size and density on CPCMs' microstructure and thermal properties. The results demonstrate that light and small MgO could realize denser structures and better encapsulation. They also found that CPCMs prepared from similar-sized compositions present higher thermal conductivity and mechanical strength. Sang et al. [44] prepared shaped CPCMs using ternary chloride as PCMs and MgO as the carrier (the optimal ratio 5:5). The CPCMs were uniformly embedded in the pore structure of MgO ceramics by cold compression and mixed sintering. Similarly, Ye et al. [33] added MWCNTs as thermal conductivity enhancers to the Na2CO3/MgO CPCMs. The results showed that MgO as a porous support matrix ensured shape stabilization. Our group encapsulated ternary chlorides with MgO as a carrier and EG as a thermally conductive additive. The CPCM has good mechanical strength and thermal stability. The microstructure and the thermal properties of the composite are shown in **Figure 5**.

Porous Al2O3 is another low-cost ceramic material with excellent thermal stability and corrosion resistance and has also been applied for medium-high temperature CPCMs. Jiang et al. [45] prepared Na2SO4-NaCl high-temperature CPCMs by cold press sintering. Alumina as the encapsulation skeleton has a microporous coral mesh structure, which can effectively prevent the leakage of PCM. After 100 thermal cycles, the CPCM's latent heat and solidification temperature only decreased by 0.4% and 0.3% without phase separation and chemical reaction. The molten PCM possessed good chemical compatibility with the alumina skeleton. Ji et al. [42] prepared NaNO3-KNO3/EG/Al2O3 shape-stable CPCMs at different sintering temperatures, with Al2O3 and EG as skeletal support materials. The results showed that the PCM loading mass fraction and compressive strength decreased with increasing sintering temperature. The thermal conductivity of CPCM is independent of the sintering temperature but increases with EG mass fraction.

Porous SiC has the advantages of high porosity, good permeability, and high thermal conductivity. The porous SiC ceramics obtained through an effective preparation process have high strength, low density, interconnected pore structure, and stable thermal properties. The porous SiC ceramics encapsulated with inorganic salt can solve the low thermal conductivity and leakage of the traditional PCM, and it is

#### **Figure 5.**

*(a) SEM image after sintering, (b) thermal conductivity with different proportions, and (c) variations of mass loss after 100 cycles [35] (Reprinted with permission from Elsevier [OR APPLICABLE SOCIETY COPYRIGHT OWNER]).*
