Abstract: In order to improve the low thermal conductivity of phase change materials, this experiment prepared paraffin/gypsum composite materials using desulfurization gypsum and paraffin as the main materials, and then modified them with micrometer sized iron powder and nano graphite sheets as thermal conductive fillers. The experimental results indicate that (1) in paraffin/gypsum composite materials, the appropriate paraffin content is 18%; (2) The addition of thermal conductive fillers has little effect on the apparent density and porosity of the material, but it will reduce the mechanical properties of the material and significantly increase the thermal conductivity of the material; (3) The paraffin/gypsum composite material with 18% paraffin and 5% nano graphite flakes has better performance. At this time, the compressive and flexural strengths of the material after 7 days are 4.91MPa and 2.66MPa, respectively, and the thermal performance is good. The thermal conductivity reaches 1.6678W · (m · K) -1, the phase transition temperature is 67.89 ℃, and the latent heat of phase transition Δ H is 74.65J · g-1. In summary, this composite material has a high thermal conductivity and good phase change energy storage effect, and can be further researched and promoted as an energy-saving material for buildings.
Keywords: phase change materials; thermal conductivity; Desulfurization gypsum; Nano graphite sheet; latent heat
To solve the problem of energy shortage, China advocates opening up sources and reducing consumption, in order to improve energy utilization efficiency and reduce energy consumption. Building energy consumption has always accounted for a large proportion of total energy consumption, mainly including air conditioning, cooling, heating, etc. The application of phase change materials in the construction industry can effectively reduce building energy consumption, as phase change materials can store thermal energy, thereby reducing thermal penetration [1,2]. Therefore, the development of phase change materials is of great significance for building energy efficiency. Based on this, Jiang Dahua et al. [3] combined the phase change material nonadecanoic acid with 1-octadecanol by melt adsorption with diatomaceous earth powder to obtain a phase change material. The results showed that this phase change material not only had good heat storage performance with gypsum made gypsum board, but also improved the leakage problem of phase change material; Shu Zhao et al. [4] made composite phase materials using fatty acid phase change materials, expanded graphite, and other materials. The results showed that when 2%~10% expanded graphite was added, the thermal conductivity of the material increased by 29.7%~708.2%; Yang Kun et al. [5] used expanded graphite as a carrier and composite phase change paraffin as the main phase change material to produce a composite shaped phase change material. The results showed that under the optimal ratio, the latent heat of phase change of the material was 83.56 J · g-1, the thermal conductivity was 0.5316 W · (m · K) -1, and it had seasonal adaptability. Considering the low thermal conductivity of these phase change materials, further improvement is needed to enhance their thermal storage performance. Regarding this, based on paraffin/gypsum composite materials, two types of thermal conductive fillers, micro sized iron powder and nano graphite sheets, were used for modification, and the material properties were studied.
1 Experimental section
1.1 Materials and Equipment
Desulfurized gypsum (Industrial Pure Shijiazhuang Zhenxing Building Materials Co., Ltd.); Expandable Graphite (Industrial Pure Qingdao Yucheng Graphite Co., Ltd.); Paraffin (Industrial Pure Henan Yuyang Wax Industry Co., Ltd.); Micro scale iron powder (industrial pure Gongyi Baidehui Metallurgical Materials Factory).
ST-S5000 electronic universal testing machine (Dongguan Sitai Instrument); FRQ-1008HT ultrasonic cleaner (Hangzhou Fantawild Ultrasonic Technology); DRS-D type thermal conductivity meter (Shanghai Etison Instrument Technology); DHG-9075AE type blast drying oven (Wuxi Maret Technology Co., Ltd.); HCR-2 Differential Calorimetry Scanner (Beijing Hengjiu Experimental Equipment Co., Ltd.).
1.2 Experimental Methods
1.2.1 Preparation of nano graphite sheets: The nitric acid stripping method is used to prepare nano graphite sheets from expanded graphite. The steps are as follows [6,7]:
(1) Take 1.0g of expanded graphite and place it in a crucible. Keep it in a muffle furnace at a constant temperature of 950 ℃ for 10 seconds.
(2) Add 0.5g of the reaction product to 50mL of concentrated HNO3 and stir thoroughly for 30 minutes in a water bath at a constant temperature of 70 ℃.
(3) Process the mixture using a vacuum filtration machine, wash thoroughly with deionized water, and then dry for 24 hours to obtain nano graphite sheets.
1.2.2 Preparation of desulfurization gypsum based composite materials
Referring to GB/T17669.3-1999, different gypsum and paraffin ratios were used to investigate the optimal paraffin content in the material, and the effects of single addition of micrometer sized iron powder or single addition of nano graphite flakes on the material properties were studied. Please refer to Tables 1 and 2 [8, 9] for details. The preparation steps of desulfurization gypsum based composite material samples are as follows:
(1) Add desulfurization gypsum to the muffle furnace, calcine at a constant temperature of 350 ℃ for 3 hours, naturally cool down and pass through a 0.63mm sieve to obtain desulfurization gypsum powder.
(2) Mix an appropriate amount of desulfurized gypsum powder and thermal conductive reinforcement material together, then add water to stir and vibrate.
(3) Melt an appropriate amount of paraffin wax in a constant temperature fruit and add it to the steps
(2) Stir and mix in the mixture.
(4) By using a drying oven, the material was kept at a vacuum degree of -0.9 MPa and a temperature of 80 ℃ for 6 hours, and then cooled and solidified to obtain a sample of desulfurization gypsum based composite material.
1.3 Performance Testing
The apparent density and porosity of the material are analyzed by measuring the sample mass, length, etc. using electronic balance and other equipment [10, 11].
The mechanical properties of material samples are analyzed using a universal testing machine based on two indicators: flexural strength and compressive strength.
Refer to GB/T 10294-2008 standard for thermal conductivity and use a thermal conductivity meter to test the material sample and analyze its thermal conductivity [12].
The thermal performance is measured using a differential scanning calorimetry (DSC) to measure the phase transition temperature and latent heat of the material.
2 Results and Discussion
2.1 Effects of Paraffin Content
2.1.1 Apparent density and porosity (Figure 1)
As shown in Figure 1, with the increase of paraffin content, the overall apparent density of the material shows a decreasing trend, but the change is relatively small. The apparent density of the material with a paraffin content of 6% is 1.195g · cm-3. When the paraffin content is 24%, the apparent density of the material decreases to 1.145g · cm-3, a decrease of 4.18%.
Meanwhile, with the increase of paraffin content, the porosity of the material decreases significantly. The maximum porosity of the material is 51.23% when the paraffin content is 6%. When the paraffin content is 12% and 18%, the porosity of the material is 46.59% and 41.72%, respectively. When the paraffin content reaches 24%, the porosity of the material increases slightly to 42.35%. Overall, as the amount of paraffin increases, the apparent density and porosity of paraffin/gypsum composite materials show a decreasing trend.
2.1.2 Mechanical Properties (Figure 2)
As shown in Figure 2, with the increase of paraffin content, the 7-day flexural strength of the material decreases, while the 7-day compressive strength shows fluctuating changes. When the paraffin content is 6%, the mechanical properties of the material are optimal, with a 7-day compressive and flexural strength of 11.25MPa and 4.19MPa, respectively. When the paraffin content is 12%, the mechanical properties of the material decrease, and the 7-day compressive and flexural strengths are 8.92MPa and 3.84MPa, respectively. The 7-day compressive strength of the material with a paraffin content of 18% increased slightly to 9.67 MPa, while the 7-day flexural strength decreased to 3.43 MPa. The material mixed with 24% paraffin has the worst mechanical properties, with the lowest 7-day compressive and flexural strength of 7.94MPa and 3.18MPa, respectively.
From this, it can be concluded that an increase in paraffin content will have a negative effect on the mechanical properties of paraffin/gypsum composites. In practical applications, paraffin/gypsum composite materials are essentially phase change energy storage materials, and the paraffin in them is beneficial for improving the latent heat of phase change of the material. Therefore, in order to increase the latent heat of phase transition of the material, the amount of paraffin should not be too low. Overall, the recommended paraffin content is 18%.
2.2 Effects of Thermal Conductive Fillers
Keeping the paraffin content at 18% constant, investigate the effects of two thermal conductive fillers, micrometer sized iron powder and nano graphite flakes, on material properties.
2.2.1 Apparent density and porosity (Figure 3)
As shown in Figure 3 (a), with the increase of the content of micrometer sized iron powder thermal conductive filler, the apparent density of the material first increases and then decreases, and then continues to increase, while the porosity first increases and then decreases. The apparent density and porosity of the blank material sample are 1.161g · cm-3 and 41.72%, respectively. When 5% micron sized iron powder is added, the apparent density of the material is 1.202g · cm-3, and the porosity is 43.21%. In addition, as shown in Figure 3 (b), with the increase of the amount of nano graphite sheet thermal conductive filler, the material pores first increase and then decrease. When 5% nano graphite flakes are added, the apparent density of the material is 1.170g · cm-3 and the porosity is 43.92%. However, compared with the blank sample, the material doped with thermal conductive filler has smaller changes in apparent density and porosity. This indicates that the addition of thermal conductive fillers has a relatively small effect on the apparent density and porosity of paraffin/gypsum composite materials.
2.2.2 Mechanical Properties (Figure 4)
As shown in Figure 4 (a), with the increase of micrometer sized iron powder content, the compressive and flexural strength of the material sample showed fluctuating changes after 7 days, but overall showed a downward trend. The mechanical properties of the blank material sample are the best, with a 7-day compressive strength of 9.67 MPa and a 7-day flexural strength of 3.43 MPa. When 2.5% micron sized iron powder is added, the mechanical properties of the material decrease. The 7-day compressive strength decreases to 5.38MPa, a decrease of 44.36%, while the 7-day flexural strength decreases to 2.12MPa, a decrease of 38.19%. When 10% micron sized iron powder is added, the 7-day compressive strength of the material is 3.77 MPa, and the 7-day flexural strength is 2.10 MPa. It can be seen that the addition of micrometer sized iron powder can significantly reduce the mechanical properties of the material. As shown in Figure 4 (b), with the increase of the content of nano graphite sheets, the compressive and flexural strength of the material decrease after 7 days. When 2.5% nano graphite flakes are added, the 7-day compressive strength of the material decreases to 8.05MPa, and the 7-day flexural strength decreases to 3.25MPa. The material sample mixed with 10% nano graphite flakes has the worst mechanical properties, with the 7-day compressive and flexural strengths at their minimum values of 3.40MPa and 1.93MPa, respectively. It can be seen that the addition of nano graphite sheets will also reduce the mechanical properties of the material. However, compared to micrometer sized iron powder, the negative impact of nano graphite sheets on the mechanical properties of the material is relatively small.
After analysis, when no thermal conductive filler is added, the paraffin particles at the interface between the paraffin and gypsum in the material system are tightly bound to the gypsum crystals, resulting in better mechanical properties and higher strength of the material. However, when micrometer sized iron powder is added, the metal powder will agglomerate in the material system, causing flocculation and destroying the original rod-shaped structure of gypsum crystals, resulting in a decrease in the mechanical properties of the material. In addition, nano graphite sheets have a porous skeleton structure that can encapsulate phase change materials in the material system. Compared with micrometer sized iron powder, the bonding between nano graphite sheets and the material system is tighter, and the mechanical properties are better. In summary, when adding micrometer sized iron powder, the recommended dosage is 7.5%; When incorporating nano graphite flakes, the recommended dosage is 5%.
2.2.3 Thermal conductivity: Thermal conductivity tests were conducted on S3 and S6 samples, and the results are shown in Figure 5.
As shown in Figure 5, the thermal conductivity of the blank sample is the smallest, only 0.1652W · (m · K) -1. This indicates that when used as a building phase change energy storage material, the heat transfer efficiency of the blank sample is low, therefore, the energy storage efficiency is low. To improve the energy storage efficiency of phase change materials, it is necessary to increase the thermal conductivity. From the figure, it can be seen that the thermal conductivity coefficients of samples S3 and S6 are both relatively high, with values of 1.6074 and 1.6678 W · (m · K) -1, respectively. This indicates that the addition of both micron sized iron powder and nano graphite sheets as thermal conductive fillers can significantly enhance the material's heat transfer ability, increase the material's thermal conductivity, and the enhancement effect of adding nano graphite sheets on the material's heat transfer ability is better than that of micron sized iron powder. After analysis, the thermal conductivity of micrometer sized iron powder itself is as high as 80W · (m · K) -1, while the nano graphite sheet is obtained from expandable graphite, which also has a high thermal conductivity of 300W · (m · K) -1 [13, 14]. Therefore, both types of thermal conductive fillers can enhance the thermal conductivity of the material. At the same time, nano graphite sheets have a unique layered structure with protrusions at the edges, which increases the contact surface between graphite sheets and various substrates, forming a three-dimensional network structure and increasing the number of thermal conductivity channels. Therefore, the thermal conductivity of the material is higher [15]. Overall, nano graphite sheets have a better thermal conductivity enhancement effect on materials.
2.2.4 Thermal Performance: Further thermal performance tests were conducted on S3 and S6 specimens, and the results are shown in Figure 6.
As shown in Figure 6, the DSC curves (changes) of each material are basically consistent. The phase transition temperature of the blank sample is 67.78 ℃, and the latent heat of phase transition Δ H is 81.28J · g-1. The phase transition temperature of the S3 sample is 67.83 ℃, and the latent heat of phase transition Δ H is 85.84J · g-1. The phase transition temperature of the S6 sample is 67.89 ℃, and the latent heat of phase transition Δ H is 74.65J · g-1. From this, it can be seen that the difference in phase transition temperature between sample S3 and sample S6 is relatively small, but the latent heat of phase transition in sample S3 is slightly higher than that in sample S6. Overall analysis shows that composite materials with added thermal conductive fillers have better thermodynamic properties and are more effective in energy storage than blank samples. Therefore, both S3 and S6 samples have good thermal properties.
3 Conclusion
Prepare paraffin/gypsum composite materials using desulfurization gypsum and paraffin as the main materials, and optimize the paraffin content. And micrometer sized iron powder and self-made nano graphite sheets were used as thermal conductive fillers to investigate the performance improvement effect of thermal conductive fillers on paraffin/gypsum composite materials. The main conclusions are as follows:
(1) When micron sized iron powder is used as the thermal conductive filler, the S3 sample with 7.5% micron sized iron powder has good mechanical properties, with a 7-day compressive strength of 4.34 MPa and a 7-day flexural strength of 1.84 MPa. At the same time, the thermal conductivity of the material is 1.6074W · (m · K) -1, the phase transition temperature is 67.83 ℃, and the latent heat of phase transition Δ H is 85.84J · g-1.
(2) When using nano graphite sheets as thermal conductive fillers, the S6 sample with 5% nano graphite sheets added has good mechanical properties, with a 7-day compressive strength of 4.91MPa and a 7-day flexural strength of 2.66MPa. At the same time, the thermal conductivity of the material is 1.6678W · (m · K) -1, the phase transition temperature is 67.89 ℃, and the latent heat of phase transition Δ H is 74.65J · g-1.
(3) This experiment suggests that the optimal paraffin content should be 18%, and the preferred thermal conductive filler should be nano graphite sheets. The paraffin/gypsum composite material mixed with 18% paraffin and 5% nano graphite flakes has better comprehensive performance and can be used as a composite phase change material to improve the thermal storage performance of buildings.
Reprinted: China Construction Research Institute Gypsum Industry Branch
