3. Preparation: characterization and properties of paraffin/porous silica ceramic composites

A form-stable composite of paraffin/porous silica composite was prepared by sol-gel method and melt infiltration. The properties of the composite will be characterized by means of DSC, SEM, and FTIR. Furthermore, the mass fractions of the paraffin in silica matrices prepared by different molar ratios of EtOH/TEOS were recorded to determine the relation between the mass fraction of paraffin and the silica matrices.

#### 3.1. Materials and methods

#### 3.1.1. PCM

The technical grade paraffin (n-eicosane: the chemical formula C20H42) used in this study was supplied by Nanyang Chemicals Company in China. Thermophysical data of the PCM are given in Table 1. The melting temperature ranges, and fusion heats of the PCMs were measured by a DSC instrument (NETZSCH STA449C) with heating rate at 10C/min in the temperature range of 20–80C.

#### 3.1.2. Preparation of n-eicosane/silica nanocomposite

The preparation process of n-eicosane/silica nanoporous composite is shown in Figure 4. Firstly, the support materials—porous silica for the PCM—were synthesized by a two-step sol-gel process. Secondly, reinforced fibers (main compositions: SiO2 > 99.95%) were exploited as structural reinforcement matrix.

The fibrous materials were uniformly pre-shaped in a mold and sol-gel mixtures and closely infiltrated the fibers by pouring them into the mold. Then, the mold includes silica sol-gel mixtures with fiber material uniformly distributed within which were supercritically dried in an autoclave above the critical temperature and critical pressure after gelation and aging in ethanol. The illustration of supercritical fluid drying equipment is shown in Figure 4. The final bulk silica nanoporous materials are obtained.

Figure 4. Experimental preparation process of n-eicosane/silica nano-composite.

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2.6. Enlightening

160 Advances in Some Hypersonic Vehicles Technologies

ceramic composites

3.1. Materials and methods

perature range of 20–80C.

as structural reinforcement matrix.

3.1.1. PCM

mass fraction of paraffin and the silica matrices.

3.1.2. Preparation of n-eicosane/silica nanocomposite

bulk silica nanoporous materials are obtained.

We performed combined thermodynamic/heat transfer analysis to obtain the total mass, thickness, and temperature excursion as functions of percentage area of PCM under given maximum energy and thermal flux based on the composite structural model and the measured thermophysical data. The relationship between the area percentage of PCM and the excursion temperature is the hyperbolic function, showing the nanocomposite better temperature control management. The relationship between the area percentage of PCM and total mass and thickness shows the nanocomposite better temperature control without much mass or volume quantities. These results are attributed to the strong interaction between the n-eicosane and the

silica skeleton, which exhibits novel temperature management and energy utilization.

3. Preparation: characterization and properties of paraffin/porous silica

A form-stable composite of paraffin/porous silica composite was prepared by sol-gel method and melt infiltration. The properties of the composite will be characterized by means of DSC, SEM, and FTIR. Furthermore, the mass fractions of the paraffin in silica matrices prepared by different molar ratios of EtOH/TEOS were recorded to determine the relation between the

The technical grade paraffin (n-eicosane: the chemical formula C20H42) used in this study was supplied by Nanyang Chemicals Company in China. Thermophysical data of the PCM are given in Table 1. The melting temperature ranges, and fusion heats of the PCMs were measured by a DSC instrument (NETZSCH STA449C) with heating rate at 10C/min in the tem-

The preparation process of n-eicosane/silica nanoporous composite is shown in Figure 4. Firstly, the support materials—porous silica for the PCM—were synthesized by a two-step sol-gel process. Secondly, reinforced fibers (main compositions: SiO2 > 99.95%) were exploited

The fibrous materials were uniformly pre-shaped in a mold and sol-gel mixtures and closely infiltrated the fibers by pouring them into the mold. Then, the mold includes silica sol-gel mixtures with fiber material uniformly distributed within which were supercritically dried in an autoclave above the critical temperature and critical pressure after gelation and aging in ethanol. The illustration of supercritical fluid drying equipment is shown in Figure 4. The final

Figure 4. Experimental preparation process of n-eicosane/silica nano-composite.

#### 3.1.3. Pore structure modification of silica matrix infiltrated with paraffin

The molar ratios of EtOH and TEOS were designed as the reactants to prepare the porous silica. The final silica showed a different pore structure with the change of EtOH/TEOS. The mass fraction of the melted paraffin infiltrated into the porous silica was closely related to the pore structure of silica. The mass fractions of the paraffin in silica matrices prepared by different molar ratios of EtOH/TEOS were recorded to determine the relation between the mass fraction of paraffin and the silica matrices.

the E = 10 and E = 20 silica. Pore structural characteristics of the samples obtained from the

Porous Ceramic Matrix Phase Change Composites for Thermal Control Purposes of Hypersonic Vehicle

Figure 6 shows the pore size distributions of samples with different molar ratios of EtOH/ TEOS measured by nitrogen desorption method (BJH method). The network formations of silica are known to be affected by water/Si, catalyst, pH, temperature, and ethanol/Si [19]. Results show that the EtOH/TEOS molar ratio causes the difference of silica network structures because the greater ethanol content retards the gelation time and results in the network

Microphotographs are taken from the fracture surface of the silica samples using JSM-6360LV scanning electron microscope. SEM of the microstructures of samples with the three kinds of EtOH/TEOS molar ratios of 2, 10, and 20 is given in Figure 7. The pore size increases when the molar ratios of the EtOH/TEOS increase by comparing with the samples with the three kinds of molar ratios of the EtOH/TEOS; the pore size of the sample with the EtOH/TEOS molar ratio of 20 is bigger than the other two types of the EtOH/TEOS molar ratios of 2 and 10. Several

) C V (cm3

<sup>g</sup> <sup>1</sup>

) Davera (nm) DBJH (nm)

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structures with larger pores when the other basic compositions hold constant.

<sup>g</sup> <sup>1</sup>

 618.3 583 44.31 3.594 23.3 23.4 624.8 577.7 88.42 4.042 25.9 32.4 565.4 518.9 83.86 4.534 32.1 38.5 606.5 552.8 94.22 8.051 53.1 59.5 596.7 573 91.55 5.219 56 60.7

Note: Smulti, multiple point BET values; Ssingl, mono-point BET values; V, pore volume; Davera, mean pore diameter;

Table 2. Pore characteristics of the samples with various molar ratios of EtOH/TEOS (E) obtained from the analysis of N2

Figure 6. The pore size distributions of samples with different molar ratios of EtOH/TEOS measured by nitrogen

analysis of N2 sorption data were shown in Table 2.

<sup>g</sup> <sup>1</sup>

) Ssingl (m2

EtOH/TEOS Smulti (m<sup>2</sup>

DBJH, pore diameter by BJH method.

sorption data.

desorption method (BET).

#### 3.2. Characterization and properties

#### 3.2.1. Pore structure modification of porous silica matrices

The adsorption-desorption measurements were performed by ASAP 2000 micromeritic apparatus. Analysis of the surface areas of the porous silica samples was conducted by Brunauer-Emmett-Teller (BET) method [16]. The method also determines the external/mesoporous surface area and extends the analysis of adsorbed gas from the lower P/P0 range of Langmuir monolayer and BET multilayer adsorption to the higher P/P0 range. The pore size is also determined using the Barrett-Joyner-Halenda (BJH) method [17, 18].

The N2 adsorption-desorption isotherms of the silica samples are given in Figure 5. It shows that the sorption isotherms of the samples with the moral ratios of the EtOH/TEOS (E) = 2, E = 3, and E = 5 belong to Type IV mesoporous structures according to IUPC classification, nitrogen absorption of which was linearly increased with the relative pressure until 0.92 and then increased abruptly and saturated at near saturated pressure during absorption process. During desorption, nitrogen was dissociated, and the cumulative absorbed volume decreased greatly in the region of 0.90–0.95. For E = 10 porous silica, the region of great desorption change shifted to 0.97–1. It was shifted to higher region for E = 20 porous silica, and the steep slopes may be associated with capillary condensation in larger pores (>50 nm) and classified as Type II according to the Kelvin equation and IUPC classification. N2 sorption data indicates that the E = 2, E = 3, and E = 5 silica are comprised of small-sized pores compared to those of

Figure 5. The N2 adsorption-desorption isotherms of the silica samples.

the E = 10 and E = 20 silica. Pore structural characteristics of the samples obtained from the analysis of N2 sorption data were shown in Table 2.

3.1.3. Pore structure modification of silica matrix infiltrated with paraffin

mass fraction of paraffin and the silica matrices.

3.2.1. Pore structure modification of porous silica matrices

using the Barrett-Joyner-Halenda (BJH) method [17, 18].

Figure 5. The N2 adsorption-desorption isotherms of the silica samples.

3.2. Characterization and properties

162 Advances in Some Hypersonic Vehicles Technologies

The molar ratios of EtOH and TEOS were designed as the reactants to prepare the porous silica. The final silica showed a different pore structure with the change of EtOH/TEOS. The mass fraction of the melted paraffin infiltrated into the porous silica was closely related to the pore structure of silica. The mass fractions of the paraffin in silica matrices prepared by different molar ratios of EtOH/TEOS were recorded to determine the relation between the

The adsorption-desorption measurements were performed by ASAP 2000 micromeritic apparatus. Analysis of the surface areas of the porous silica samples was conducted by Brunauer-Emmett-Teller (BET) method [16]. The method also determines the external/mesoporous surface area and extends the analysis of adsorbed gas from the lower P/P0 range of Langmuir monolayer and BET multilayer adsorption to the higher P/P0 range. The pore size is also determined

The N2 adsorption-desorption isotherms of the silica samples are given in Figure 5. It shows that the sorption isotherms of the samples with the moral ratios of the EtOH/TEOS (E) = 2, E = 3, and E = 5 belong to Type IV mesoporous structures according to IUPC classification, nitrogen absorption of which was linearly increased with the relative pressure until 0.92 and then increased abruptly and saturated at near saturated pressure during absorption process. During desorption, nitrogen was dissociated, and the cumulative absorbed volume decreased greatly in the region of 0.90–0.95. For E = 10 porous silica, the region of great desorption change shifted to 0.97–1. It was shifted to higher region for E = 20 porous silica, and the steep slopes may be associated with capillary condensation in larger pores (>50 nm) and classified as Type II according to the Kelvin equation and IUPC classification. N2 sorption data indicates that the E = 2, E = 3, and E = 5 silica are comprised of small-sized pores compared to those of Figure 6 shows the pore size distributions of samples with different molar ratios of EtOH/ TEOS measured by nitrogen desorption method (BJH method). The network formations of silica are known to be affected by water/Si, catalyst, pH, temperature, and ethanol/Si [19]. Results show that the EtOH/TEOS molar ratio causes the difference of silica network structures because the greater ethanol content retards the gelation time and results in the network structures with larger pores when the other basic compositions hold constant.

Microphotographs are taken from the fracture surface of the silica samples using JSM-6360LV scanning electron microscope. SEM of the microstructures of samples with the three kinds of EtOH/TEOS molar ratios of 2, 10, and 20 is given in Figure 7. The pore size increases when the molar ratios of the EtOH/TEOS increase by comparing with the samples with the three kinds of molar ratios of the EtOH/TEOS; the pore size of the sample with the EtOH/TEOS molar ratio of 20 is bigger than the other two types of the EtOH/TEOS molar ratios of 2 and 10. Several


Note: Smulti, multiple point BET values; Ssingl, mono-point BET values; V, pore volume; Davera, mean pore diameter; DBJH, pore diameter by BJH method.

Table 2. Pore characteristics of the samples with various molar ratios of EtOH/TEOS (E) obtained from the analysis of N2 sorption data.

Figure 6. The pore size distributions of samples with different molar ratios of EtOH/TEOS measured by nitrogen desorption method (BET).

kinds of form-stable paraffin/porous silica composites were successfully prepared in our recent study. Using the silica matrices synthesized from the EtOH/TEOS ratio of 10, the PCM showed better thermal absorption characteristics, which is suitable for the fields of requiring cooling at high temperatures such as aircraft electronics and spacecraft devices.

paraffin meant considerable latent heat energy storage potential. The pore size of porous silica becomes larger with the increase of the EtOH/TEOS molar ratios; when the molar ratios of EtOH/TEOS are 10 and 20, the average pore size of the synthesized silica are 53.1 and 56.0 nm, respectively. The maximum mass percentage of paraffin as PCM in the E = 10 and E = 20 silica matrices reached to 75 wt% and over than those of the E = 2 silica matrix. When the molar ratios of EtOH/TEOS are 10 and 20, the synthesized silica matrices are suitable to serve as

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The morphologies of silica nanoporous materials and PCM/silica composite were observed with a field emission scanning electron microscope (FE-SEM: JSM-6700F, JEOL, Japan, with a thin Pt-Pd coating). Figure 9 compares the SEM of the microstructures of porous silica and the composite. The microstructure of the porous silica (Figure 9a) is porous and connected each other, while the microstructure of the composite (Figure 9b) shows that the paraffin was dispersed uniformly into the porous network of silica ceramics, which used as supporting material and provided a mechanical strength to the PCM. The maximum mass fraction of paraffin dispersed into the composites was measured as 75%, and there was no leakage of the paraffin from the surface of the composite up to this mass fraction even over its melting

FTIR spectra of the paraffin and the silica and the paraffin/porous silica composite are shown in Figure 10. In FTIR spectra of the paraffin, the peaks at the wave numbers of 2935 and 2860 cm<sup>1</sup> are caused by stretching vibration of C–H, peaks at around 1500 cm<sup>1</sup> belong to the deformation vibration of –CH2 and –CH3, and the peak at 750 cm<sup>1</sup> represents the rocking vibration of –CH2. In FTIR spectra of the silica, the peaks at 1012 and 810 cm<sup>1</sup> are caused by bending vibration of Si–O, and an intense band at the wave number of 1090 cm<sup>1</sup> is typical of

Figure 9. The SEM of the microstructures of porous silica (a) and the PCM/nano-composite (b).

supporting materials of the PCM.

3.2.3. Structure analysis

temperature.

3.2.4. Chemical properties

#### 3.2.2. The mass fractions of the paraffin

The relation between the mass fraction of the paraffin infiltrated the three kinds of molar ratios of EtOH/TEOS (E = 2, E = 10, E = 20) silica matrices and the infiltration time was plotted in Figure 8. The mass fraction of the paraffin in porous silica increases with the infiltration time, and the E = 10 and E = 20 silica matrix composites reached to 75% at 180 s, while E = 2 silica matrix composite reached to 68%, and then they all increase a little with further time. It indicates that the maximum mass fractions of the paraffin as PCM in E = 10 and E = 20 silica matrices are over than one of the E = 2 silica matrices and the bigger mass fraction of the

Figure 7. SEM of the microstructures of samples with the three kinds of EtOH/TEOS molar ratios of 2, 10, and 20.

Figure 8. The relation between the mass fractions of the paraffin infiltrated the three kinds of molar ratios of EtOH/TEOS (E = 2, E = 10, E = 20) silica matrices and the infiltration time.

paraffin meant considerable latent heat energy storage potential. The pore size of porous silica becomes larger with the increase of the EtOH/TEOS molar ratios; when the molar ratios of EtOH/TEOS are 10 and 20, the average pore size of the synthesized silica are 53.1 and 56.0 nm, respectively. The maximum mass percentage of paraffin as PCM in the E = 10 and E = 20 silica matrices reached to 75 wt% and over than those of the E = 2 silica matrix. When the molar ratios of EtOH/TEOS are 10 and 20, the synthesized silica matrices are suitable to serve as supporting materials of the PCM.

## 3.2.3. Structure analysis

kinds of form-stable paraffin/porous silica composites were successfully prepared in our recent study. Using the silica matrices synthesized from the EtOH/TEOS ratio of 10, the PCM showed better thermal absorption characteristics, which is suitable for the fields of requiring cooling at

The relation between the mass fraction of the paraffin infiltrated the three kinds of molar ratios of EtOH/TEOS (E = 2, E = 10, E = 20) silica matrices and the infiltration time was plotted in Figure 8. The mass fraction of the paraffin in porous silica increases with the infiltration time, and the E = 10 and E = 20 silica matrix composites reached to 75% at 180 s, while E = 2 silica matrix composite reached to 68%, and then they all increase a little with further time. It indicates that the maximum mass fractions of the paraffin as PCM in E = 10 and E = 20 silica matrices are over than one of the E = 2 silica matrices and the bigger mass fraction of the

Figure 7. SEM of the microstructures of samples with the three kinds of EtOH/TEOS molar ratios of 2, 10, and 20.

Figure 8. The relation between the mass fractions of the paraffin infiltrated the three kinds of molar ratios of EtOH/TEOS

(E = 2, E = 10, E = 20) silica matrices and the infiltration time.

high temperatures such as aircraft electronics and spacecraft devices.

3.2.2. The mass fractions of the paraffin

164 Advances in Some Hypersonic Vehicles Technologies

The morphologies of silica nanoporous materials and PCM/silica composite were observed with a field emission scanning electron microscope (FE-SEM: JSM-6700F, JEOL, Japan, with a thin Pt-Pd coating). Figure 9 compares the SEM of the microstructures of porous silica and the composite. The microstructure of the porous silica (Figure 9a) is porous and connected each other, while the microstructure of the composite (Figure 9b) shows that the paraffin was dispersed uniformly into the porous network of silica ceramics, which used as supporting material and provided a mechanical strength to the PCM. The maximum mass fraction of paraffin dispersed into the composites was measured as 75%, and there was no leakage of the paraffin from the surface of the composite up to this mass fraction even over its melting temperature.

## 3.2.4. Chemical properties

FTIR spectra of the paraffin and the silica and the paraffin/porous silica composite are shown in Figure 10. In FTIR spectra of the paraffin, the peaks at the wave numbers of 2935 and 2860 cm<sup>1</sup> are caused by stretching vibration of C–H, peaks at around 1500 cm<sup>1</sup> belong to the deformation vibration of –CH2 and –CH3, and the peak at 750 cm<sup>1</sup> represents the rocking vibration of –CH2. In FTIR spectra of the silica, the peaks at 1012 and 810 cm<sup>1</sup> are caused by bending vibration of Si–O, and an intense band at the wave number of 1090 cm<sup>1</sup> is typical of

Figure 9. The SEM of the microstructures of porous silica (a) and the PCM/nano-composite (b).

the bending vibration of Si–O. And, the peak at 3449 cm<sup>1</sup> represents the stretching vibration of functional group of Si–OH. For the composite, the peaks at the wave numbers of 3500, 2935, 2860, 1500, 1090, and 750 cm<sup>1</sup> have corresponding vibration, and no significant new peaks were observed. The FTIR spectra illustrate that the composite is just a physical combination of silica ceramics and paraffin.

56.3C is 165.16 kJ/kg. TG curve indicates that weight of the composite changes very a few;

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Numerical and experimental studies are proposed to predict and investigate the thermal absorption characteristics of porous silica infiltrated with PCM for thermal protection applications. The numerical simulation was performed using a volume-averaging technique, and a finite volume modeling (FVM) was used to discretize the heat diffusion equation. The phase

Three porous silica cylindrical disks (100 mm in diameter and 10 mm thick) with the three different solid-liquid PCMs (two kinds of paraffin and one kind of xylitol) were fabricated as the solid matrices of the silica-PCM composites according to our recent study. The

The composites (100 mm in diameter and 10 mm thick) were then introduced into the experimental setup (see Figure 12 left). The temperature was recorded as it varied with time. The top and bottom walls of the container were insulated by adiabatic materials. For the sake of validation of the numerical model and assumptions, the temperature of the cold face of

A schematic diagram of the composite and two-dimensional grids is given in Figure 12. (Sample dimensions: 180 mm in diameter and 120 mm thick). Due to the symmetry and regularity, the samples were formulated with two-dimensional axissymmetric coordinates

Mass before infiltration (kg) 7.20E1 7.20E1 7.20E1 Mass after infiltration (kg) 2.79 2.83 4.16 PCM (paraffin or xylitol) mass rate (%) 74.19 74.56 82.69 Phase change point (K) 324 329 363

) 3.05E3 3.05E3 3.05E3

) 172 165 198

) 0.38 0.38 0.46

) 914.75 927.87 1363.93

) 2.4 2.4 3.8

C58# C64# X98#

composite X98# as a function of time was deduced by the numerical simulation.

weight loss of the composite is less than 0.2%.

4.1. Experimental procedure

4.2. Numerical model

Sample volume (m3

Density (kgm<sup>3</sup>

Heat storage capacities (kJ(kg)<sup>1</sup>

Thermal conductivity (Wm<sup>1</sup> K<sup>1</sup>

Table 3. Properties of the porous silica matrix composites.

Specific heat (kJ(kg)<sup>1</sup> K<sup>1</sup>

4. Applications: thermal protection purposes

change process was modeled using the enthalpy-porosity method.

thermophysical data for these samples are given in Table 3.

#### 3.2.5. Thermal properties

The DSC curves of the paraffin and the composite are shown in Figure 11. From Figure 1a, the latent heat of the paraffin is 182.22 kJ/kg (T<sup>m</sup> = 56.8C). TG curve shows that weight of paraffin hardly changes, which indicates the paraffin used as PCM has good thermal chemical stability. Figure 11b indicates that heat storage capacity of the composite happened at melting point

Figure 10. FTIR spectra of the paraffin and the silica and the paraffin/porous silica composite.

Figure 11. The DSC-TG curves of the paraffin (a) and the PCM/silica composite (b).

56.3C is 165.16 kJ/kg. TG curve indicates that weight of the composite changes very a few; weight loss of the composite is less than 0.2%.
