## 2 Materials and Methods
### Characterization of the DP Composite  
An FESEM (field emission scanning electron microscope, SU-70, Japan) was used to observe the microstructure of diatomite and the DP composite. The magnification was up to 5700 times, and the testing voltage was 15 kV. An EDS (energy-dispersive spectrometer) was also employed to characterize the change in the chemical elements of diatomite before and after absorbing the paraffin.  
A BET (Brunauer Emmett Teller, NOVA1200e, America) was used to identify the pore size distribution and the total pore volume of diatomite.  
DSC (differential scanning calorimetry, DSC-200F3, America) was used to determine the melting and solidification temperatures of paraffin and the DP composite, as well as their corresponding latent heat values. The samples were heated/cooled at 2 \({}^{\circ}\)C/min over the temperature range of 0-60 \({}^{\circ}\)C. Besides this, the specific heat capacities of the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
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Table 1: Characteristics of the main materials used in this study.  
diatomite/paraffin/gypsum composites were measured by DSC with the operating temperatures ranging from 0 to 60 \({}^{\circ}\)C. Meanwhile, the impregnation ratio of paraffin in the DP composite was calculated using the following equation:  
\[\gamma 1=\frac{\Delta H_{DP}}{\Delta H_{PCM}} \tag{1}\]  
where \(\gamma 1\) is the impregnation ratio of paraffin in the composite. \(\Delta H_{DP}\) and \(\Delta H_{PCM}\) represent the measured heat enthalpy of DP and pure paraffin through DSC, respectively.  
TGA (thermogravimetric analysis, TGA Q50, America) was used to evaluate the thermal stability of DP. The sample was heated at 10 \({}^{\circ}\)C/min over the temperature range of 0-600 \({}^{\circ}\)C. The mass percentage of paraffin in diatomic was computed using the formula below:  
\[\gamma 2=\frac{\Delta m_{DP}}{\Delta m_{PCM}} \tag{2}\]  
where \(\gamma 2\) is the mass percentage of paraffin. \(\Delta m_{DP}\) refers to the DP's weight expressed with unit mass (100%); \(\Delta m_{PCM}\) represents the mass of paraffin.  
An FT-IR (Fourier transform infrared spectrometer, Perkin Elmer Spectrum 100, America) was used to verify the chemical structures of paraffin, diatomite and the DP composite. The wavelengths used ranged from 500 cm\({}^{-1}\) to 4000 cm\({}^{-1}\).  
A thermal cycle test (Temp & Humi Programmable Chamber, BE-TH-150, China) was performed to assess the thermal reliability of DP. The DP was placed in a thermal cycling chamber and underwent up to 600 thermal cycles of melting and solidification. In each thermal cycle, the temperature was increased from 10 \({}^{\circ}\)C to 40 \({}^{\circ}\)C at a rate of 2 \({}^{\circ}\)C/min. Then the temperature was kept at 40 \({}^{\circ}\)C for 40 min. After that, the temperature was decreased from 40 to 10 \({}^{\circ}\)C at the same rate, and finally the temperature was kept at 10 \({}^{\circ}\)C for 30 min. Each thermal cycle took 190 min to complete. Any changes in phase change temperatures and latent heats before and after the thermal cycling tests (300 and 600 cycles) were revealed by the DSC analyses.