2.1. Material Preparation
2.1.1. Preparation of the Eutectic
CaCl2 (purity: 96%, Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China) was dissolved in distilled water (molar ratio: 1:6) to prepare CaCl2·6H2O. Then the mixture was melted in a thermal bath at a temperature of 50 °C for 15 min. Finally, the CaCl2·6H2O was cooled down to solid form.
Different amounts of MgCl2·6H2O (purity: 98%, Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China) were mixed with the CaCl2∙6H2O to prepare the CaCl2∙6H2O-MgCl2∙6H2O eutectics. The mass fractions of MgCl2·6H2O varied from 10% to 25% with an interval of 5%. Each sample was heated to 50 °C and kept for 1 h until they were fully melted.
2.1.2. Preparation of the Composite PCM
The CaCl2∙6H2O-MgCl2∙6H2O mixture with a suitable phase change temperature and relatively high latent heat was selected as the PCM to be composited with 2 wt.% SrCl2∙6H2O (purity: 99.5%, Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China) and EP (average diameter: 2–5 mm, Henan Huitong Co., Ltd., Zhengzhou, China) with mass fractions of 50 wt.%, 55 wt.%, 60 wt.%, and 65 wt.%. The CaCl2∙6H2O-MgCl2∙6H2O was impregnated into the EP at 50 °C and at a pressure of 88.1 kPa in vacuum.
A differential scanning calorimeter (DSC, Q20, TA Instruments, New Castle, DE, USA) was used to analyze the phase change characteristics of the pristine eutectic, and the composite PCMs. The DSC tests were carried out under the atmosphere of nitrogen at a flow rate of 50 mL per minute. The testing temperature rose up from −5 to 50 °C at a rate of 3 °C/min.
Under the nitrogen atmosphere (flowrate: 100 mL/min), a thermoanalyzer (STA409PC, Netzsch, Waldkraiburg, Germany) tested the mass change of each sample during the heating process from 25 °C to 600 °C at a rate of 10 °C per minute. Thermal gravimetric analysis (TGA) characterizes the thermal stability of those materials.
The thermal conductivity of the materials in solid state were measured with a thermal constant analyzer (TPS 2500, Hot Disk, Gothenburg, Sweden). The composite PCM was compressed into two cylinders 40 mm in diameter and 10 mm in height. The sensor (type: 7577, 2.01 mm in diameter, Hot Disk, Gothenburg, Sweden) was sandwiched between the two cylinders. The thermal conductivity of the EP and eutectic were also measured, respectively.
The supercooling degree of the PCMs was characterized by the cooling curves. The schematic diagram is shown in Figure 1
. Two tubes that contained the eutectic and the composite PCM composites were put into a thermal bath in which the temperature was controlled by a thermal oil (Phenyl silicone oil, flash point: 315 °C, Clearco Products Co., Inc., Willow Grove, PA, USA). Two K-type thermocouples (NR-81530, accuracy: ±0.5 °C, HILA, Taipei, China) were placed at 20 mm from the bottom of each tube. First, the sample was heated to 50 °C and kept at this temperature for more than 10 min. Then, the thermal bath temperature was cooled down to −5 °C at a rate of 1 °C/min. After 10 min at −5 °C, the oil was heated to 50 °C again. The two thermocouples were connected to a data logger (34970A, Agilent, Santa Clara, CA, USA), which recorded the data of the temperature variations for each sample.
To determine whether the composite PCM was thermally reliable, the composite PCMs underwent 500 heating and cooling thermal cycles. In each cycle, the composite PCM was cycled between 50 °C and −5 °C via a temperature-controlled instrument (BPHJS-060A, Bluepard, Shanghai, China). In each stage of the thermal cycling, the composite PCM sample was maintained at the specified temperature for 30 min to ensure that the phase change completed. The results from SEM, Fourier transform infrared spectra, and DSC were compared before and after the thermal cycling.
The X-ray diffractometer (XRD, D8-ADVANCE, Bruker, Billerica, MA, USA, Cu Kα radiation λ = 1.5406 Å) collected the diffraction patterns of CaCl2∙6H2O, MgCl2∙6H2O, and the eutectics in the 2θ ranging from 5° to 80°. The FT-IR of the pristine salts and the composite PCMs whose wavenumber ranged from 4000–400 cm−1 were obtained with the spectrophotometer (Tensor 27, Bruker, Billerica, MA, USA) at room temperature. The microstructures of EP and the CaCl2∙6H2O-MgCl2∙6H2O/EP composite PCM were observed using a scanning electron microscope (SEM, Quanta, FEI, Hillsboro, OR, USA).