Experimental Study of an Enhanced Phase Change Material of Paraffin/Expanded Graphite/Nano-Metal Particles for a Personal Cooling System

A composite phase change material (PCM) was prepared by incorporating paraffin (PA) with expanded graphite (EG) and nano-metal particles to improve the thermal conductivity and reduce the leakage performance of PA once it melts and, consequently, develop a more efficient PCM for a personal phase change cooling system. A series of experiments was carried out by a scanning electron microscope, a differential scanning calorimeter, a hot-disk thermal analyzer, and leakage tests on the composite PCM with various mass fractions of EG and metals (i.e., Cu, Al, Ni, and Fe). Through comprehensive consideration of the thermal conductivity, leakage, and homogeneity, a composite PCM with the optimal proportion (PA-EG11%-Cu1.9%) was screened out. Its thermal conductivity was significantly improved nine times, while the phase change enthalpy showed a minimal decrease. In addition, the relationships of the composite PCM with its temperature and density were systematically investigated. The experimental results are important for determining the proper package density of PCM for application into a personal cooling system because its weight is crucial for the system design and benefits the performance comparison of various PCMs prepared under various conditions. Lastly, the heat storage efficiency of the PA–EG–Cu material was investigated using heat storage tests. Cooling performance clearly improved compared to the PCM without nano-particles added.


Introduction
Heat stress caused by operations in high-temperature environments (including military, automotive, iron, steel, metallurgical plants, glass manufacturing, and mining [1]) has become a major threat to human health [2]. Thermal fainting can endanger human life when severe. With the thermal protection of workers in high-temperature environments, a personal cooling system is a good countermeasure for heat stress [3].
Cooling clothing includes liquid cooling clothing [4], gas cooling clothing [5], and phase change material (PCM) cooling [6]. The cooling medium of gas cooling clothes is air, which is composed of basic clothing and air compressors. Cold air is piped to various parts of the human body for cooling. Liquid cooling clothes are composed of a pre-cooling device and basic clothing. The cooling medium of this type of cooling clothing are an ice water mixture, water, water and vinyl glycol below 0 • C, etc. [7]. Cooling takes the form of frozen liquid flowing to the entire human body through a pipeline composite PCM in the cooling suit was determined. Thus, a high-efficiency cooling PCM that could be applied to cooling clothes was prepared.

Experimental Materials and Instruments
The base organic PCM is PA (paraffin)(Guangzhou Wengjiang Chemical Reagent Co., Ltd, Guangzhou, China, purity ≥ 98.0%), and its parameters are shown in Table 1. The number of EG mesh is 50, the purity is more than 99%, the rate of expansion is 600 mL/g, nano-Cu, nano-Al, nano-Fe, and nano-Ni (Changsha Tianjiu Metal Materials Co., Ltd, Changsha, China).

Preparation of PA/EG/Nano-Metal Composite PCMs
The density of nano-metals is relatively high, and the direct use of melt blending will cause the phenomena of sinking and agglomeration of nano-metal particles. This study uses the two-step method to prepare a PA/EG/nano-metal composite PCM. Figure 1 shows the experimental process. First, the PA is melted in a water bath at 70 • C. Next, the nano-metal is added into an ultrasonic shaker at 40 • C and then oscillated for 2 h. Lastly, EG is added with mechanical stirring for 2 h to obtain a PA/EG/nano-metal composite PCM. The prepared samples are shown in Table 2.

Adsorption Experiments
EG with a loose porous mesh structure has good adsorption, radiation resistance, and flame retardancy. A PA-EG composite PCM was produced by absorbing the liquid PA into the EG, which was added at ratios of 5%, 8%, 11%, and 15%. Figure 2 is a cylindrical model with a bottom surface diameter of 10 mm and a height of 10 mm after mold treatment. The PA-EG model was placed in a PA-EG11-Al1.9 PCM22 PA-EG11-Ni3

Adsorption Experiments
EG with a loose porous mesh structure has good adsorption, radiation resistance, and flame retardancy. A PA-EG composite PCM was produced by absorbing the liquid PA into the EG, which was added at ratios of 5%, 8%, 11%, and 15%. Figure 2 is a cylindrical model with a bottom surface diameter of 10 mm and a height of 10 mm after mold treatment. The PA-EG model was placed in a 45 • C incubator for 3 h, and the leakage of PA on the filter paper was observed.

Adsorption Experiments
EG with a loose porous mesh structure has good adsorption, radiation resistance, and flame retardancy. A PA-EG composite PCM was produced by absorbing the liquid PA into the EG, which was added at ratios of 5%, 8%, 11%, and 15%. Figure 2 is a cylindrical model with a bottom surface diameter of 10 mm and a height of 10 mm after mold treatment. The PA-EG model was placed in a 45 °C incubator for 3 h, and the leakage of PA on the filter paper was observed.

Performance Characterization of Composite PCM
SEM: Morphology analysis of composite PCMs was performed by SEM. The experimental instrument used in the SEM was a Hitachi SU8010 with a resolution of 1.0 nm (15 kV), 1.4 nm (1 kV, WD = 1.5 nm, deceleration mode), and 2.0 nm (1 kV, WD = 1.5 nm, normal mode). The electron microscope has a magnification of 30×-8,000,000×. The experiment can be used to characterize the morphology, particle size, and dispersion of the sample. The homogeneity of the composite material was investigated after the addition of different nano-metals at different proportions to determine whether the nano-metal particles have serious agglomeration.
DSC experiment: Take a mg sample in the N2 atmosphere. Set the initial temperature to −10 °C, increase to 100 °C at a heating rate of 5 °C/min, and then drop it back to −10 °C at a cooling rate of 5 °C/min. The phase change temperature and enthalpy of the composite PCMs with different metals and different metal proportions were studied.
The thermal conductivity of different composites was measured using a hot-disk thermal property tester. The model of probe polyimide film was 7577, the size was r = 2.001 mm, repeatability was <1%, accuracy was <3%, and temperature was 20 °C. The thermal conductivity of the composite

Performance Characterization of Composite PCM
SEM: Morphology analysis of composite PCMs was performed by SEM. The experimental instrument used in the SEM was a Hitachi SU8010 with a resolution of 1.0 nm (15 kV), 1.4 nm (1 kV, WD = 1.5 nm, deceleration mode), and 2.0 nm (1 kV, WD = 1.5 nm, normal mode). The electron microscope has a magnification of 30×-8,000,000×. The experiment can be used to characterize the morphology, particle size, and dispersion of the sample. The homogeneity of the composite material was investigated after the addition of different nano-metals at different proportions to determine whether the nano-metal particles have serious agglomeration.
DSC experiment: Take a mg sample in the N 2 atmosphere. Set the initial temperature to −10 • C, increase to 100 • C at a heating rate of 5 • C/min, and then drop it back to −10 • C at a cooling rate of 5 • C/min. The phase change temperature and enthalpy of the composite PCMs with different metals and different metal proportions were studied.
The thermal conductivity of different composites was measured using a hot-disk thermal property tester. The model of probe polyimide film was 7577, the size was r = 2.001 mm, repeatability was <1%, accuracy was <3%, and temperature was 20 • C. The thermal conductivity of the composite PCM was tested at different temperatures and densities, and the effects of temperature and density on the thermal conductivity of the composite PCM were investigated.

Thermal Storage Experiment of Composite PCM
The heat storage experiment verified the heat storage efficiency after the addition of nano-metal particles. Figure 3 shows the experimental process. The prepared composite PCM is placed in a refrigerator for freezing treatment and then placed in a 50 • C thermostatic water bath. The data acquisition system measures the temperature rise of the sample through a thermocouple.
PCM was tested at different temperatures and densities, and the effects of temperature and density on the thermal conductivity of the composite PCM were investigated.

Thermal Storage Experiment of Composite PCM
The heat storage experiment verified the heat storage efficiency after the addition of nano-metal particles. Figure 3 shows the experimental process. The prepared composite PCM is placed in a refrigerator for freezing treatment and then placed in a 50 °C thermostatic water bath. The data acquisition system measures the temperature rise of the sample through a thermocouple.

Thermal Cycle Experiment of Composite PCM
Place the prepared composite PCM into a refrigerator at −10 °C to freeze. Next, put the composite PCM in a water bath at 50 °C and leave it for 0.5 h. After the phase change is complete, place the composite PCM in a refrigerator at −10 °C to freeze, repeat the process, and weigh the composite PCM five times per cycle at 100 cycles.

Volume Expansion Test of Composite PCM
The composite PCM was pressed into a cylinder with a bottom area of 10 mm and a height of 10 mm. The composite PCM cylinders with different densities were pressed separately, placed into the refrigerator for freezing treatment, and then placed in a 40 °C incubator. After 2 h of constant temperature treatment, the bottom surfaces of the composite PCM with different densities were measured to determine the micrometer changes in diameter and height. Figure 4 shows the leakage of PA. The leakage of 5% and 8% EG was serious, which far exceeds the diameter of the material itself and covers almost the entire filter paper. However, with the increase of EG content, the leakage situation was effectively improved. Figure 4c,d reveal that, when the EG content reaches 11% and 15%, their leakage situations are not very different, and the leakage was almost within the size range of the material. It can completely absorb the PA, and the situations for the 11% and 15% EG were the same. This outcome means that, when the EG content reaches 11%, PA can be completely adsorbed. The addition of too much EG has a greater impact on the overall latent heat performance of the material. Hence, the optimal addition amount is 11%.

Thermal Cycle Experiment of Composite PCM
Place the prepared composite PCM into a refrigerator at −10 • C to freeze. Next, put the composite PCM in a water bath at 50 • C and leave it for 0.5 h. After the phase change is complete, place the composite PCM in a refrigerator at −10 • C to freeze, repeat the process, and weigh the composite PCM five times per cycle at 100 cycles.

Volume Expansion Test of Composite PCM
The composite PCM was pressed into a cylinder with a bottom area of 10 mm and a height of 10 mm. The composite PCM cylinders with different densities were pressed separately, placed into the refrigerator for freezing treatment, and then placed in a 40 • C incubator. After 2 h of constant temperature treatment, the bottom surfaces of the composite PCM with different densities were measured to determine the micrometer changes in diameter and height. Figure 4 shows the leakage of PA. The leakage of 5% and 8% EG was serious, which far exceeds the diameter of the material itself and covers almost the entire filter paper. However, with the increase of EG content, the leakage situation was effectively improved. Figure 4c,d reveal that, when the EG content reaches 11% and 15%, their leakage situations are not very different, and the leakage was almost within the size range of the material. It can completely absorb the PA, and the situations for the 11% and 15% EG were the same. This outcome means that, when the EG content reaches 11%, PA can be completely adsorbed. The addition of too much EG has a greater impact on the overall latent heat performance of the material. Hence, the optimal addition amount is 11%.     The worm-like microstructure of EG is filled with PA and nano-metal particles, which have a good supporting effect on PA and the nano-metal particles. From observing the microstructure of composites with different nano-metal particles, it can be concluded that the addition of nano-Cu particles and the composite material of nano-Al particles has a relatively uniform distribution in the composite material system. The content in a unit volume is also larger than the other two kinds of nano-metal particles where no agglomeration of nano-Cu and nano-Al are observed after composition with PA-EG. The result indicates that the nano-Cu and nano-Al particles have good compatibility with PA-EG.   The worm-like microstructure of EG is filled with PA and nano-metal particles, which have a good supporting effect on PA and the nano-metal particles. From observing the microstructure of composites with different nano-metal particles, it can be concluded that the addition of nano-Cu particles and the composite material of nano-Al particles has a relatively uniform distribution in the composite material system. The content in a unit volume is also larger than the other two kinds of nano-metal particles where no agglomeration of nano-Cu and nano-Al are observed after composition with PA-EG. The result indicates that the nano-Cu and nano-Al particles have good compatibility with PA-EG.                  Figure 14 reveals the comparison of the phase transition temperature and phase transition enthalpy of the composite PCM with different metals. Figure 15 reveals the comparison of the phase transition temperature and phase transition enthalpy of the composite PCM with different proportions. Compared with PA-EG and PA, the phase transition temperature of the composite PCM with nano-metal particles increased by 1.69 • C and 2.03 • C, respectively. The ideal temperature of the human body is at approximately 30 • C and increasing amplitude has no effect on the application of composite PCM. Compared with PA-EG and PA, the minimum value of the phase change enthalpy of PA-EG-Cu decreased by 29.24 and 46.57 J/g, respectively. The enthalpy of phase change and the temperature of the phase change of the composite PCM have little effect on different metal particles. Different addition ratios have almost no effect on the phase transition temperature of the composite PCM, but have a greater effect on the phase change enthalpy, with a difference of 13.81 J/g.

DSC (differential scanning calorimeter) Analysis
Figures 10-13 show the DSC curves of PA, PA-EG, and composite PCM with different metals and different addition ratios. Figure 14 reveals the comparison of the phase transition temperature and phase transition enthalpy of the composite PCM with different metals. Figure 15 reveals the comparison of the phase transition temperature and phase transition enthalpy of the composite PCM with different proportions. Compared with PA-EG and PA, the phase transition temperature of the composite PCM with nano-metal particles increased by 1.69 °C and 2.03 °C, respectively. The ideal temperature of the human body is at approximately 30 °C and increasing amplitude has no effect on the application of composite PCM. Compared with PA-EG and PA, the minimum value of the phase change enthalpy of PA-EG-Cu decreased by 29.24 and 46.57 J/g, respectively. The enthalpy of phase change and the temperature of the phase change of the composite PCM have little effect on different metal particles. Different addition ratios have almost no effect on the phase transition temperature of the composite PCM, but have a greater effect on the phase change enthalpy, with a difference of 13.81 J/g.        Figure 11. DSC analysis of PA-EG.       Figure 12. DSC analysis of PA-EG-nano-metal particles.
Materials 2020, 13, x FOR PEER REVIEW 9 of 16      Figure 13. DSC with different mass fractions analysis of PA-EG-Cu.

Thermal Conductivity
Thermal conductivity is a crucial parameter for PCM because it reflects the material's heat transfer rate. The higher the thermal conductivity is, the faster the heat absorption will be. Heat absorption, in turn, can increase the efficiency of PCM in practical applications. In this study, different metal particles were added as thermal conductivity enhancers to improve the thermal conductivity of PA-EG-based composite PCM. The test results are shown in Figure 16. The thermal conductivity of PA is very low at 0.216 W/(m·K), while the thermal conductivity of PA-EG is 1.073 W/(m·K), which is an increase of about five times. The growth range is approximately 7.7-9 times when nano-metal particles are added. Four nano-metal particles have different effects on the thermal conductivity of composite PCM. Adding nano-Cu particles works best. EG and nano-metal particles have an enhanced effect on the thermal conductivity of composite PCM while nano-metal particles have a much smaller proportion than EG. When the thermal conductivity is increased, the effect on the phase change enthalpy of the composite PCM is reduced.

Thermal Conductivity
Thermal conductivity is a crucial parameter for PCM because it reflects the material's heat transfer rate. The higher the thermal conductivity is, the faster the heat absorption will be. Heat absorption, in turn, can increase the efficiency of PCM in practical applications. In this study, different metal particles were added as thermal conductivity enhancers to improve the thermal conductivity of PA-EG-based composite PCM. The test results are shown in Figure 16. The thermal conductivity of PA is very low at 0.216 W/(m·K), while the thermal conductivity of PA-EG is 1.073 W/(m·K), which is an increase of about five times. The growth range is approximately 7.7-9 times when nano-metal particles are added. Four nano-metal particles have different effects on the thermal conductivity of composite PCM. Adding nano-Cu particles works best. EG and nano-metal particles have an enhanced effect on the thermal conductivity of composite PCM while nano-metal particles have a much smaller proportion than EG. When the thermal conductivity is increased, the effect on the phase change enthalpy of the composite PCM is reduced.

Thermal Conductivity
Thermal conductivity is a crucial parameter for PCM because it reflects the material's heat transfer rate. The higher the thermal conductivity is, the faster the heat absorption will be. Heat absorption, in turn, can increase the efficiency of PCM in practical applications. In this study, different metal particles were added as thermal conductivity enhancers to improve the thermal conductivity of PA-EG-based composite PCM. The test results are shown in Figure 16. The thermal conductivity of PA is very low at 0.216 W/(m·K), while the thermal conductivity of PA-EG is 1.073 W/(m·K), which is an increase of about five times. The growth range is approximately 7.7-9 times when nano-metal particles are added. Four nano-metal particles have different effects on the thermal conductivity of composite PCM. Adding nano-Cu particles works best. EG and nano-metal particles have an enhanced effect on the thermal conductivity of composite PCM while nano-metal particles have a much smaller proportion than EG. When the thermal conductivity is increased, the effect on the phase change enthalpy of the composite PCM is reduced. In general, the higher the temperature is, the higher the thermal conductivity of the material is. Figure 17 shows the thermal conductivity of PA-EG-Cu at different temperatures. The measured densities are 0.15 and 0.3 cm 3 , and the thermal conductivity is linearly related to temperature.   In general, the higher the temperature is, the higher the thermal conductivity of the material is. Figure 17 shows the thermal conductivity of PA-EG-Cu at different temperatures. The measured densities are 0.15 and 0.3 cm 3 , and the thermal conductivity is linearly related to temperature. In general, the higher the temperature is, the higher the thermal conductivity of the material is. Figure 17 shows the thermal conductivity of PA-EG-Cu at different temperatures. The measured densities are 0.15 and 0.3 cm 3 , and the thermal conductivity is linearly related to temperature.       Figure 20 shows the thermal conductivity of PA-EG-Cu at different densities. Thermal conductivity has a linear relationship with density. As the density of the composite material increases, the thermal conductivity of the composite material increases. This outcome has great significance for the production and application of composite PCM.    Figure 20 shows the thermal conductivity of PA-EG-Cu at different densities. Thermal conductivity has a linear relationship with density. As the density of the composite material increases, the thermal conductivity of the composite material increases. This outcome has great significance for the production and application of composite PCM.  Figure 20 shows the thermal conductivity of PA-EG-Cu at different densities. Thermal conductivity has a linear relationship with density. As the density of the composite material increases, the thermal conductivity of the composite material increases. This outcome has great significance for the production and application of composite PCM. Table 3 shows the effect of density on the volume expansion of composite PCM after heating. When the compressed density is below 0.6 cm 3 , the phase change of the composite PCM can be well supported in the EG, and the volume will not change. When the compressed density is greater than 0.6 cm 3 , the worm structure in the EG is compressed severely due to the compression force being too large. Therefore, a volume change occurs during the phase transition. At a density of 0.9 cm 3 , the volume expands by 66%. Given the thermal conductivity of the composite PCM and the volume expansion after heating, the package density should be 0.6 cm 3 for application in the cooling suit.  Table 3 shows the effect of density on the volume expansion of composite PCM after heating. When the compressed density is below 0.6 cm 3 , the phase change of the composite PCM can be well supported in the EG, and the volume will not change. When the compressed density is greater than 0.6 cm 3 , the worm structure in the EG is compressed severely due to the compression force being too large. Therefore, a volume change occurs during the phase transition. At a density of 0.9 cm 3 , the volume expands by 66%. Given the thermal conductivity of the composite PCM and the volume expansion after heating, the package density should be 0.6 cm 3 for application in the cooling suit.  Figure 21 shows the mass loss of the composite PCM after 100 cycles. The figure reveals that the magnitude of the mass loss is only 0.66%. In addition, the decrease becomes increasingly smaller in the future and is basically a stable state. Thus, the thermal stability of this composite PCM is good.   Figure 21 shows the mass loss of the composite PCM after 100 cycles. The figure reveals that the magnitude of the mass loss is only 0.66%. In addition, the decrease becomes increasingly smaller in the future and is basically a stable state. Thus, the thermal stability of this composite PCM is good.

1
When the proportion of EG is 11%, almost no leakage of molten PA occurs. The reticular structure of EG can effectively suppress the agglomeration of nano-metal Cu and Al, which makes it evenly distributed and well compounded in the PA-EG-based composite PCM. 2 A screening of the different nano-metal particles and different mass fractions added reveals that PA-EG (11%)-Cu (1.9%) is the best-performing cooling PCM. As this composite PCM substantially improves the thermal conductivity, its phase change enthalpy decreases very little, and the phase change temperature satisfies the most suitable temperature range of the human

1
When the proportion of EG is 11%, almost no leakage of molten PA occurs. The reticular structure of EG can effectively suppress the agglomeration of nano-metal Cu and Al, which makes it evenly distributed and well compounded in the PA-EG-based composite PCM. 2 A screening of the different nano-metal particles and different mass fractions added reveals that PA-EG (11%)-Cu (1.9%) is the best-performing cooling PCM. As this composite PCM substantially improves the thermal conductivity, its phase change enthalpy decreases very little, and the phase change temperature satisfies the most suitable temperature range of the human body. Thus, it can be well applied in cooling clothes. 3 The comparison of heat storage analysis between screened PA-EG-Cu and PA-EG indicates that the heat storage speed of the composite PCM with the addition of nano-metal Cu particles is accelerated and the heat storage capacity is strengthened. The thermal stability of PA-EG-Cu is analyzed, and its mass loss tends toward a stable state in the later stage. Thus, it has a high life in the application process.