Preparation and characterization of phase transition/graphite foam composite materials

Preparation of graphite foam

The materials used in the template method included carbon source (AR mesophase pitch), template (polyurethane foam) and thickener (poval). Pores per linear inch (PPI) of the polyurethane foam was 40. Its volume density was 0.028 g • cm^-3 and the average pore size was 0.48 mm.

Experimental procedure: first, grind mesophase pitch into powder with burnisher and screen the powder with the 400 mesh sieve to ensure its granularity less than 37 μm. Then cut polyurethane foam into cubes and put them into sodium hydroxide aqueous solution for removing the non-link films in the polyurethane foam, followed by washing with distilled water until neutral and drying for 6 h at 100°C. Next prepare pitch liquid (35%) with the ground pitch powder and add poval (2%) into it to enhance the adhesive ability of pitch. After that infiltrate the dried polyurethane foam with mesophase pitch liquid, then drying for 12 h at 100°C to evaporate water. After drying, put the foam in an argon atmosphere, raise the temperature to 280°C at the rate of 1°C/min and keep the foam at 280°C for 1 h so as to make the pitch soften and adhere to the foam skeleton, followed by putting the foam in air, raise the temperature to 300°C at the rate of 10°C/h and keep the foam at 300°C for 1 h, then raise the temperature to 350°C at the same rate so that the template and organic additive were burned off. After oxidation, put the foam in argon, raise the temperature to 1000°C at the rate of 50°C /h and keep the foam at 1000°C for 1 h, followed by raising the temperature to 1400°C at the same rate and keep the foam at 1400°C for 1 h for carbonization, at last, raise the temperature to 2400°C at the rate of 600°C/h and keep the foam at 2400°C for 1 h for graphitization.

Infiltrating graphite foam with phase change materials

Ideal infiltration generally infiltrates melting phase change materials through porous media by capillary force without external force, which requires high wettability between solid and liquid materials. When the pore size of porous media reaches 200 µm, capillary force and barometric pressure are at the same level, which is not favorable for infiltration. To achieve better infiltration, in this paper, infiltration of phase change materials was conducted under vacuum environment, which could avoid the obstruction of air and achieve more sufficient infiltration. Under a vacuum degree of -0.85 bar, the infiltration rate was 20% higher than that in air.

Taking eicosane as an example, the specific experimental procedure can be summarized as follows: first, cut graphite foam into appropriate size and clean it with ultrasonic wave. Then, place it into drying oven for drying, and measure its dried weight and volume. Next put the dried graphite foam into a beaker and cover it with enough eicosane to overwhelm the foam during melting. After that, put the beaker into drying oven and vacuumize the oven and raise the temperature to 50°C keeping steady for 1 h. At last, take the beaker out and cool it down until complete crystallization of eicosane and remove residual eicosane on the surface.

Graphite foam

The volume density of graphite foam at different stages can be calculated through measuring weight and volume, as shown in Figure 1.

It can be seen that the weight of graphite foam reduces quite a lot at the early stage of heating, which may be caused by decomposition of urethane foam in this temperature range. From 350°C to 1000°C, the weight loss of graphite foam is little. It is mainly caused by the loss of high-molecular weight elements in the oxidized pitch. After carbonization and graphitization, the weight loss goes on because of volatilization.

The volume loss presents a similar variation trend to the weight loss. After heat treatment in nitrogen at 280°C, the volume of graphite foam reduces by 28% because of the shrink of template. After heat treatment in air at 350°C, the volume reduces further by 7% because of the decomposition of template. During heat treatment, the volume of graphite foam continuously reduces, which decreases by nearly 50% after graphitization.

Overall, the density of graphite foam presents a continuously increasing trend. It increases sharply when the temperature is lower than 280°C. Then from 350°C, the increasing trend slows down, and the density increases at a slow rate.

Figure 2 shows the scanning electron microscopy (SEM) image of graphite foam. As can be seen, the pores of graphite foam distribute evenly and show a dodecahedron structure. Meanwhile, the pore walls are smooth, indicating the template method can successfully replicate the pore structure of polyurethane foam. During preparation, porous structure of graphite foam is relatively intact, but the volume reduces drastically, thus leading to the smaller pore size of graphite foam relative to that of template. In addition, due to by-hand infiltration, the sample is not completely even and some pore walls are thicker.

The porosity of graphite foam was measured by the mercury intrusion method. The porosity is determined as 84.9% and the pore size mainly concentrates in the size range of 176-251 µm. As the distribution of pore size shows, the structure of graphite foam is even, but the pore size is significantly lower relative to that of template due to the volume shrink during the heat treatment.

The samples were cut into cubic specimens with a side length of 20 ± 0.5 mm and then the specimens were subjected to a compressiv e load by 0.2 mm/min until it was destroyed. Figure 3 shows the load-displacement curve of graphite foam. Graphite foam belongs to fragile foam materials. It shows transitory linear elasticity under low load and begins to break when the stress reaches its compressive strength. This kind of break belongs to brittle failure and the pore walls will crack one layer by one layer. Once one layer cracks, the stress will be reassigned to the remaining walls. As a result, the curve shows a repeating zigzag. Besides, from the linear elasticity stage, the compressive strength of graphite foam can be easily calculated. The compressive strength of graphite foam is 0.32 MPa, which is not very high. Combined with the SEM image, it can be deduced that the low compressive strength is caused by the high porosity and the microstructure flaw of pore walls.

X-ray diffractometer was used to test the microcrystalline structure and graphitization degree of graphite foam. Graphitization degree can be calculated through the following formula proposed by Mering and Marie:

g = 0.3440 − d 002 0.3440 − 0.3354

where d₀₀₂ = λ ⁄(2sinθ) is the interlayer spacing of (002) surface, λ is the wave length of x-ray, θ is the half diffraction angle, 0.3440 (nm) is the interlayer spacing of non-carbonization foam and 0.3354 (nm) is the interlayer spacing of ideal graphite crystalline.

Figure 4 shows the X-ray diffraction analysis (XRD) pattern of graphite foam. The temperature of graphitization cannot be over 2400°C because of equipment limitation, thus graphitization of the prepared graphite foam is not complete. After graphitization at 2400°C, the degree of graphitization reaches 89.53% and the interlayer spacing is 0.3363 nm, which is close to that of ideal interlayer spacing (0.3354 nm). That is to say, graphite foam can be prepared successfully.

The graphite foam sample was cut into disks with a diameter of 12.7 mm and a thickness of 4 mm and then covered with graphite. Afterwards, the heat conductivity of graphite foam at room temperature was determined through laser heat conductivity, and the value was 5.44 W/(m∙K), which was lower than the numerical result of Sedeh and Khodadadi (15) but higher than 4.68 W/(m∙K) of Karthik et al (16).

PCM/GF

Heat storage capacity is the most important index of phase change heat storage materials. The higher the latent heat, the more heat can be absorbed during phase transition. However, in practical applications, latent heat is not the only thing that needs to be considered. For instance, cabin interspacing of spacecraft is limited, so that the size of heat sink for electronic devices must be small enough. From this aspect, the heat storage capacity per unit volume seems more important. Table I shows the theoretical heat storage capacity of four kinds of PCM/GF. As can be seen, the theoretical heat storage capacity of acetamide, xylitol, and eicosane PCM/GF is below 200 J/g, while the heat storage per unit volume of erythritol, xylitol, and acetamide PCM/GF is relatively high, which is above 200 MJ/m³.

Differential scanning calorimetry (DSC) can get the endothermic or exothermic heat during physics or chemical changes, based on the thermophysical characteristics of the material that can be computed by heat analysis software. Figure 5 shows the DSC curves of four kinds of PCM/GF and the corresponding computation results are given in Table II. The results show that the infiltration has no effect on melting point and the latent heat storage capacity is lower relative to the theoretical value. Among the four kinds of materials, erythritol composite material has the highest melting point and the highest enthalpy of fusion.

Under ideal conditions, phase change materials are generally absorbed on the intercommunicating pore structure with the help of capillarity of graphite foam. In practical applications, phase change materials may leak after melting, so that it is necessary to investigate the stability of graphite foam. The specific steps are as follows: firstl, weigh the sample and package it with filter paper, then put the sample into the drying oven and heat it to the melting point of phase change materials. After heat preservation of two hours, take out the sample and cool it down. Finally, open the package and weigh the sample. Repeat above steps for nine more times and calculate the weight loss ratio to compare the stability of graphite foam.

After ten cycles, the weight loss ratios of eicosane, acetamide, xylitol, and erythritol PCM/GF are 19.8%, 46.2%, 2.1% and 6.9%, respectively. Based on the prepared graphite foam skeleton, xylitol PCM/GF has the best stability, and acetamide PCM/GF has the worst, due to the high volatility of acetamide. Further, it can be inferred that the stability of PCM/GF depends on the heat stability of PCM and the matching degree between PCM and graphite foam. The higher the viscosity of PCM and the smaller the pore size of graphite foam, the better the stability of PCM/GF.

The compressive strengths of eicosane, acetamide, xylitol, and erythritol PCM/GF are 2.27 MPa, 3.32 MPa, 9.08 MPa and 4.53 MPa, respectively. Among the four composite materials, the compressive strength of xylitol PCM/GF is the highest (9.08 MPa) and that of eicosane PCM/GF is the lowest (3.32 MPa). The compressive strengths of xylitol and eicosane are 28 times and 10 times higher than that of graphite foam. Due to the vacuum environment during infiltration, infiltration rate of the composite materials is very high and almost all pores are filled with PCM. Thus, PCM/GF can be regarded as compact materials, so that the compressive strength can be improved significantly.

The heat conductivities of eicosane, acetamide, xylitol, and erythritol PCM/GF at room temperature are 4.32 W/(m∙K), 4.58 W/(m∙K), 5.26 W/(m∙K) and 5.14 W/(m∙K), respectively. The heat conductivity of pure PCM is generally not good. For example, the heat conductivity of erythritol is 0.733 W/(m∙K), but the heat conductivity of erythritol PCM/GF is 7 times higher than that of pure erythritol. In contrast, in the research of Karthik et al, the heat conductivity of erythritol PCM/GF is 3.77 W/(m∙K) [lower than 5.14 W/(m∙K)] and its fusion enthalpy is 251.2 J/g (lower than 266.3 J/g).