1. Introduction

Phase change materials (PCMs) can store or release heat in high density during phase transition in the form of latent heat at nearly constant temperatures. Latent heat thermal energy storage systems (LTES) are thus becoming a promising platform of efficient use of solar energy, residue heat, and regulation of temperature [1-4]. Being the core of LTES, PCMs have been modified extensively to possess specific regulating temperature, minor supercooling and stable heat storage capacity [5]. A series of potential PCMs such as some inorganic salt hydrates like Glauber’s salt [6, 7], and organic compounds as paraffin [8, 9] are found to have good performance. In most cases, latent heat is absorbed or released when solid-liquid (s-l) transition or solid-solid (s-s) (e.g. C₁₄Zn in this work) happen at the transition temperature.

However, in practical use it is still a challenge to find ideal PCM candidates considering the wide temperature range of regulation with high efficiency [10]. A new way to solve this problem is to use the phase change materials, as the case of C₁₀Zn and C₁₂Zn, in controlled porous glasses (CPGs) embedded in nanopores [11, 12]. These nano-sized compounds show size-dependent phase transition temperatures and latent heat, still with good storage performance in thermal cycling just as the bulk counterpart does. That is, a series of "new" PCMs can be obtained by the nanoencapsulation method using those PCMs with good performance already found [13-15].

Basically, transition temperature of materials in nanopores can be estimated from the Gibbs-Thomson equation for s-l transition [16]. As is known, the equation predicts the depression of melting/ freezing temperature being proportional to inverse diameter of tiny particles. To our knowledge, the size-dependency of s-s transition temperature and enthalpy is much less discussed in literature. In the case of Cu₂S nanosolids [17] and Fe nanowires [18], the s-s phase transition temperature (T_c) is reported to follow Eqs. (1) and (2), respectively,

(1)

(2)

where T_c is the transition temperature. d and D are the atomic diameter and nanoparticle (NP) diameter, respectively. The parameter α is a shape factor, defined as the surface area ratio between a nonspherical NP and aspherical one with identical volume. L_bcc→fcc is the corresponding bulk latent heat per particle during bcc→fcc lattice transformation. V^at is atomic volume; γ is the specific surface free energy. Despite of the difference in other terms, not the purpose for detailed discussion in this work, both the equations predict the linear dependency of the T_c on 1/D, the NP size. Therefore, the s-s transition temperature of C₁₄Zn in nanopores may be controlled to some extent just as the Gibbs-Thomson equation predicts for s-l transition.

Generally, the properties of materials in nanopores may be influenced by not only the size of pores but also interface interactions and pore morphologies [16, 19, 20]. However, strong interface interactions and complex pore morphologies sometimes could cause deviations from predicted properties or polymorphs, which makes the regulation of properties of PCMs more complicated. That is, a repulsive wall of the PCM in the nanopores is more favorable for this purpose. Therefore, it is necessary to examine the possible influence of different porous media. Besides the CPGs in the previous work, other porous materials such as silica gel, activated carbons and SBA-15 may be used [8, 16]. Also, this can provide a choice of cheaper porous materials for practical use, if other conditions allow.

In this paper, we report the preparation, characterization of heat storage performance of C₁₄Zn embedded in silica gel with different pore sizes. The structure and phase transition of the layered compounds C_nZn have been characterized previously. The C_nZn bulk materials are suitable for thermal energy storage because they have high latent heat during the s-s transitions and stable heat storage capacities in thermal cycling [11, 12]. The morphology, phase transition and thermal cycling of C₁₄Zn in the nanopores are analyzed using SEM, DSC and XRD. A comparison of C₁₄Zn inside silica gel with C₁₀Zn and C₁₂Zn in CPGs is also discussed.

2. Experimental 2.1. Materials

N-Tetradecylamine (1-C₁₄H₂₉NH₂, C₁₄NH₂) with a purity in a mass fraction 0.98, zinc chloride and hydrochloric acid are analytical grades, were purchased from Aladdin Regent Co., Shanghai. Four types of silica gel (Tosoh Finechem Co., and SiliCycle Inc, Canada) were used as confinement matrix with pore size of 15, 30, 100, and 200 nm, and pore volume of 0.8, 0.8, 0.8, 1 mL/g, respectively. These materials were used as received.

2.2. Preparation of C₁₄Zn and the composites embedded in silica gel (C₁₄Zn/SG)

The compound C₁₄Zn was prepared using the literature method [11, 16, 21]. In this way, C₁₄NH₂, ZnCl4·2H₂O and HCl are weighed in a molar ratio of 2:1:2 and slowly dissolved in anhydrous ethyl alcohol with slight excess HCl. The solution with the several reactants is refluxed for 2 h and cooled to room temperature. The colorless tiny crystals of C₁₄Zn precipitated from the solvent in several hours. The as-prepared C₁₄Zn is filtered and recrystallized for three times from anhydrous ethanol. At last, the compound is dried in an oven to constant weight.

Loading of C₁₄Zn in silica gel takes a physical impregnation procedure as following. The C₁₄Zn, and SG powder of a mass of about 10 mg are weighted into a glass tube, followed by the addition of a certain volume of anhydrous ethanol until they are just immersed in. The mixture is slowly heated in a water bath to about 78 ℃ and stays for 3 h. Then, the white powder left in the bottom of the glass tube is evacuated at room temperature for 1 h and at about 60 ℃ for 3 h. C₁₄Zn in the sample can be determined from the silica gel powder before and after loading. In all the samples, C₁₄Zn was controlled to occupy 95%-96.5% of the total pore volume of the silica gel. Here, the pore fullness φ value is defined as φ=m_C14Zn/(D_C14Zn·m_g·V_p), where mC₁₄Zn is the mass of pure C₁₄Zn, D_C14Zn is the density of C₁₄Zn, m_g the total mass of the silica gel, V_p the specific pore volume of the silica gel. The density D_C14Zn uses the literature value from the single crystal with space group and cell parameters: monoclinic, P2₁/c, α=7.3785(9) Å, b=11.5970(11) Å, c=37.695(3) Å, β=95.549(10)°, D=1.220 g cm^-1, Z=4 [12, 22].

2.3. DSC, XRD and SEM analysis

Thermal analysis of the samples is completed on a DSC Q10 (TA Instruments) under a high purity nitrogen atmosphere at 20 mL min^-1 with a scanning rate of 5 ℃/min in the heating process and 2 ℃/min in the cooling process. The mass of the samples containing silica gel and C₁₄Zn is about 2-5 mg, sealed in aluminum pans. The temperature and enthalpy change was calibrated using high purity adamantine (T_trs=-64.43 ℃, Δ_trsH=24.05 J/g), high purity water (T_fus=0.01 ℃, Δ_fusH=335 J/g), and high purity indium standard (T_fus=156.598 ℃, Δ_fusH=28.57 J/g). In most cases the mean transition temperatures were reproducible within 0.5 ℃.

The XRD diffraction patterns were obtained on a Panalytical Xpert Pro diffractometer. The diffractometer uses Cu Ka radiation (1.54 Å) source at a power of 40 mA/40 kV. The samples were scanned in the angle range of 3°≤2θ≤45° with a step-size of 0.02° at room temperature. XRD analysis of C₁₄Zn/SG composites are performed in the diffraction experimental station (4B9A) of Beijing Synchrotron Radiation Facility (BSRF), using the six circle diffractometer Huber 5020 and the scintillation crystal detector Huber 9910, with a synchrotron radiation wavelength of 1.54 Å. Data were collected in steps of Δ(2θ)=0.02° also at the rate of 1 ℃ in the cooling process.

Surface morphologies of the porous glasses CPGs coated with gold film were characterized by field-emission high resolution scanning electron microscopy (SEM, Hitachi s-4800 and FEI F250).

3. Results and discussion 3.1. SEM and XRD characterization

As seen SEM images in Fig. 1a, the bulk C₁₄Zn shows a typical layered structure of the C_nZn compounds. The multiple layers can be seen from the nearly straight or irregular edges. This means it is well crystallized as the expected structure in the bulk material. The image Fig. 1b of the C₁₄Zn/SG (d=15 nm) composite displays a seemingly dense matrix carrying some white irregular particles. The matrix should be the porous silica gel, however its backbones and pores are not clear because of the resolution of SEM equipment. The particles on the surface are probably of the C₁₄Zn outside the pores. According to DSC and XRD analysis later, this part of C₁₄Zn, also the other SG of different sizes, cannot be detected and its influence is very small. The image of the C₁₄Zn/SG (d=30 nm) composite in Fig. 1c shows backbones and very small pores of the matrix without regular packing. Some white particles are also seen on the surface. The image of the C₁₄Zn/SG (d=100 nm) composite in Fig. 1d clearly displays the silica gel backbones and the pores spreading within them. These are threedimensional connected pores arranged irregularly, consistent with the reported structure. C₁₄Zn on the surface is not obvious. From these images, the target compound is mostly loaded in the nanopores of the silica gel.

In Fig. 2, C₁₄Zn in the bulk and embedded in the silica gel (d=15, 30, and 100 nm) are scanned in the X-ray diffraction angle range of 2θ=3-45°, covering all the characteristic peaks of the PDF card of the bulk compound. At the test temperature, C₁₄Zn in the bulk and in the nanopores is in the low temperature stable phase. The pure sample shows strong (00l) reflections, which appear at the same 2θ angles indexed on the basis of the cell parameters from single crystals within reasonable errors [22-24]. In contrast, the in-plane diffractions are weak. It indicates the bulk compound has the layered structure of the perovskite-type, which is consistent with the SEM image shown above. Similar patterns, for example the (00l) reflections of layers, are observed in the C₁₄Zn/SG (30, and 100 nm) composites. However, the intensities of the diffractions in the nanopores become weak relative to the bulk, common phenomena in the nano-sized system. The characteristic diffractions of (00l) planes can be easily observed in the C₁₄Zn/SG (15 nm) composite while with even weaker intensities and wider peaks. Comparing the results, the diffraction peaks of the nano-sized C₁₄Zn can all find the same reflections in the bulk, no new ones displayed. As the specific lamellar ordering as evidenced from the (00l) peaks kept, C₁₄Zn in the nanopores still possesses the same structure as the bulk. This can be ascribed to the highly selforganizing trend of the alkylammonium salt due to the polar ZnCl₄^- head. Considering the s-s transition without liquid phase, the interactions between C₁₄Zn and silica gel wall should be weak, the same as observed in C₁₀Zn and C₁₂Zn in nanopores of CPGs [11]. In the case, the size effect will dominate the properties of C_nZn embedded in SG or CPGs. These results can explain the nano-sized C₁₄Zn of its thermal properties in thermal cycling as will be shown later.

3.2. DSC analysis

Fig. 3 shows the DSC curves of C₁₄Zn and C₁₄Zn/SG composites in the heating process. The pure compound displays a large and a small peak centered at 97.02 ℃, and 94.46 ℃, respectively. As its fusion temperature is 180 ℃, the two peaks come from s-s transitions because of the movements of alkyl chains under elevated temperatures. However, the specific structure of the solid in higher temperature is still unclear.

C₁₄Zn in silica gel (d=30, 100, and 200 nm) shows only one peak at each size. Because silica gel does not have any thermal anomaly at the test temperatures, the endothermic peak of the composites is attributed to the s-s transitions of C₁₄Zn. These peaks become widened and weak in reference to the bulk. Considering the general weakened trend of the transition in nanoscale, it is reasonable to attribute it to the same origin as seen in the higher temperature peaks of the bulk. In this case, the small peaks in the bulk are suppressed or too weak to be detected in the nanopores.

C₁₄Zn in silica gel (d=15 nm) has a broad peak centered around 89.7 ℃. This much broadened peak probably is because of a wide distribution of the pore size, a result of poor control of the product during preparation. Again, the peak is ascribed to the higher temperature peak in the bulk.

The weakening of the transition peak in smaller peak is seen in C₁₄Zn/SG, DSC curve not shown here. In that case, a very weak peak can be detected, which is not considered as thermal energy storage purpose.

With the broadened peak, the nano-sized C₁₄Zn also shows a shift of the position to lower temperature relative to the bulk. A larger shift is obtained in smaller pores, similar to those found in other nanoscale systems. The depression of the transition temperature is reasonable according to the previous work on the basis of the weak interface interaction of C₁₄Zn to the pore walls as evidenced from the XRD analysis in Section 3.1. The detailed data will be presented in the following.

3.3. Transition temperature

From the DSC curves as partly seen in Fig. 3, the phase transition temperatures (T_d) of C₁₄Zn and the composites can be obtained from onset point on each curve. The transition temperatures are 89.7 ℃ (15 nm), 93.4 ℃ (30 nm), 96.07 ℃ (100 nm), 96.34 ℃ (200 nm), 94.46 ℃ (lst, bulk), 97.02 ℃ (2nd, bulk), respectively, depicted in Fig. 4a. The T_d data may be well fitted as a function of reciprocal of the pore diameter (1/d), taking the bulk T_bulk as a starting and reference point, expressed in Eq. (3),

(3)

where T_bulk is the transition temperature of the bulk C₁₄Zn (97.02 ℃) in the higher temperature. The fitted coefficient a_s=110.1 (R²=0.9994), with dimension ℃ nm.

Clearly, the size dependency of T_d of C₁₄Zn in silica gel has a linear relationship, following Eqs. (1) and (2). The slope as thus corresponds to that determined by atom size and shape factor (Eq. (1)), or surface energy, latent heat and atom volume (Eq. (2)). This linear relationship of T_d -1/d means the transition temperature is governed mainly by the particles size or pore diameter. The simple size dependence of T_d indicates the weak interface interactions between C₁₄Zn nanoparticles and the pore wall, which is consistent with the analysis on basis of the XRD results. The straight line of fitted T_d to 1/d of C₁₄Zn in silica gel is almost parallel to the other two for C₁₀Zn and C₁₂Zn in porous glasses CPGs (dashed lines), which is spaced by difference of their bulk transition temperatures. This means their slopes are near to each other. In addition, the nano-sized C₁₄Zn appears at higher temperature range than C₁₂Zn, and then C10Znat each size. As for the bulk, this is ascribed to the stronger lateral interactions among longer alkyl chains than the shorter ones, just as the case of normal alkanes.

The constant slopes of the three lines are understandable if they follow the theoretical relationship by Eq. (1), since the molecular chains of C_nZn (n=10, 12, 14) are similar in sizes so that the shape factor α and the atomic size d therein are close to each other and could be unchanged or barely changed under the experimental conditions. If the Eq. (2) is workable, the contribution of L/Tbulk (bulk entropy change ΔS) of the transition to the three slopes, Δ(γV)/(L/T_bulk), should be mainly responsible for the slight difference among them, considering their Δ(γV) values should be close again because of the similarity of these molecules. Then, the order of 1/ΔS determines and actually is the same as that of the absolute values of the experimental slope, C₁₀Zn > C₁₂Zn > C₁₄Zn. The slope means the sensitivity of the changing of T_d to the variation or regulation of the particle/pore size. Smaller DS or latent heat during phase transformation favors more sense or larger changing in T_d values, however would lower the heat storage density. In the case, one needs to balance the latent heat and extent of regulating the phase change temperature for practical use.

In Fig. 4b, depression of the s-s transition temperature (ΔT_s) of the C₁₄Zn/SG composites is displayed, calculated by difference between onset point of the transition peak in the nanopores to the bulk (T_bulk -T_d), together with those for C₁₀Zn and C₁₂Zn in porous glasses CPGs (dashed lines). These data can also be well fitted for a linear relationship with the reverse pore diameter (1/d), expressed in Eq. (4),

(4)

where the fitted coefficients b_s=1.13 (R²=0.9943), with dimension nm. As derived from Eq. (3), their coefficients have a relationship of b_s=a_s/T_bulk. The ΔT_s value increases with the decreasing of the pore diameter. In the smallest pores (d=15 nm), the shift of the phase change temperature reaches 7.3 ℃. In other words, the ΔT_s value can be predicted by changing the pore size of silica gel. On observation of literature data, the fitted straight lines of C₁₄Zn/SG and C₁₂Zn/CPG are almost superimposed, indicating almost a same slope that is already reflected by the parallel lines as shown in Fig. 4a of the two types of composites. The straight line for C₁₀Zn/CPG appears at a position of slight higher of the former two, exhibiting a bit larger absolute value of its slope. For thermal energy storage, the ΔT_s values correspond to a "window" of regulating temperatures but not the only the point of T_bulk. The expression by Eq. (4) has been found also in the s-l and s-s transitions of some small organic molecule liquids confined in porous glass, which is consistent with the Gibbs-Thomson equation [16, 25].

3.4. Heats of transition

From the DSC curves, the s-s latent heat or enthalpy change (ΔH_d, J/g) of the nano-sized C₁₄Zn can be determined from the heat absorbed divided by a mass of the pure compound after subtracting the part of silica gel in each sample. The normalized latent heat is displayed in Fig. 5, which may be fitted as linear relation with the reverse pore size by Eq. (5),

(5)

where coefficient c_s=7.23(R²=0.9999). ΔH_bulk is bulk latent heat at higher temperature, 104.4 J/g; the transition in lower temperature is 2.8 J/g. The ΔH_d values of C₁₄Zn in the nanopores of SG (15, 30, 100, and 200 nm) are 54.06, 79.24, 96.86, and 100.7 J/g, respectively. The linear relationship of ΔH_d -1/d by Eq. (5) is same as that for C₁₀Zn and C₁₂Zn/CPG (dashed lines), and for the Gibbs-Thomson equation of s-l transition. As observed in other s-s and s-l transitions, the latent heat of nanosized C₁₄Zn decreases as the pore size becomes smaller. In the smallest pores (15 nm), it still can retain~52% of the heat storage capacity of the bulk, which is acceptable as a PCM for thermal energy storage. Comparing the three C_nZn, the nanosized C₁₄Zn is higher at each size in the ΔH_d value that the other two. Again, this is attributed to the stronger interactions among alkyl chains and the higher lattice energy in C₁₄Zn, a same trend in the T_d values as discussed in Section 3.3.

3.5. Heat storage performance in thermal cycling

As is known, heat storage capacity recovery and supercooling can be estimated from thermal cycling tests, which is important for a PCM candidate. For this purpose, the s-s phase transition temperature and latent heat of the C₁₄Zn/SG composites and the bulk compound are measured before and after 10-time thermal cycling using DSC in both the heating and cooling processes. As seen in Fig. 6, all of the nano-sized C₁₄Zn and the bulk material keep nearly same shapes after thermal cycling. It means each nanosized sample maintains its transition temperature and latent heat just as the bulk does, which is beneficial for use of thermal energy storage. Actually, their latent heat change within±5% and transition temperature varies within 0.5 ℃.

According the DSC curves in Fig. 6, the transition peak of the C₁₄Zn/SG composite on cooling always appears at lower temperature range relative to that on heating. The span between the onset point of peaks on heating and on cooling can be used to signify the extent of the supercooling of all the samples, which are determined to be 13.4 (15 nm), 11.1 (30 nm), 8.06 (100 nm), 7.29 (200 nm), and 5.53 ℃ (bulk, higher temperature), respectively. Therefore, the extent of supercooling becomes larger as the pore size decreases. This trend is same as those observed in C₁₀Zn and C₁₂Zn embedded in CPGs [11].

To our knowledge, there are no detailed discussion and theoretical models dealing with the supercooling of s-s transition described in this work. Presumably, the supercooling of the C₁₄Zn/ SG composites may be qualitatively estimated from the kinetics of melting (at T_m) and freezing (at T_f) of metal nanoparticles, where the supercooling (ΔT_H) is expressed as ΔT_H ∝ γ^1.5T_m/(ΔT_HmT_f^0.5) [26, 27]. Taking the surface energy g as nearly constant, the combined term T_m/T_f^0.5 is relatively insensitive to d as the exponent can be reduced at a large portion. In the case, the supercooling would mainly depend on the term latent heat ΔT_Hm. As the ΔT_Hm value of the nano-sized compound decreases with the decreasing pore diameter (Eq. (5)), the supercooling tends to be larger in smaller pores, which is consistent with the experimental results.

In summary, the layered structure compound C₁₄Zn is successfully loaded in the porous silica gel. In the nanopores of d=15-200 nm, the s-s phase transition of C₁₄Zn takes place at a temperature range of the bulk to 7.3 ℃ below, and with a latent heat of~52% of the bulk. The nano-sized C₁₄Zn shows a linear dependency of the phase transition temperature of latent heat on the reverse pore diameter, which is analogous to the Gibbs-Thomson equation for s-l phase transition. The phase transition is consistent with the reported model for s-s phase transition in sizedependent property. The C₁₄Zn/SG composites display stable transition temperature and heat storage capacity in a short-term thermal cycling, just like the bulk does. The supercooling of the composites becomes larger as the pore size decreases, however still acceptable. The thermal properties may be understood from the structure, where the layered structure is kept in the nano-sized particles. Therefore, the nano-sized C₁₄Zn is also suitable candidates for thermal energy storage as the bulk material with the advantages of the tunable thermal properties. This method should be usable to other phase change materials, extending its application scope.

4. Conclusion

In this work, the phase change material with a layered structure, C₁₄Zn, has been successfully embedded into silica gel with different pore diameters to obtain a series of composites. The C₁₄Zn/silica gel composite shows size-dependent thermal properties: The s-s transition temperature, the heat and the latent heat storage capacity during the thermal cycling, and the supercooling all can be expressed as a function of the reverse pore diameter. It may mean the C₁₄Zn/silica gel composite is "a new PCM", which can be defined by its specific transition temperature or the pore. With the decreasing pore size, C₁₄Zn in silica gel (15-200 nm) shows reduced latent heat of the s-s transition temperature ranging from 89.7 ℃ to 96.34 ℃ with adsorbed heat of 54.06-100.7 J/g. Obviously, in the smallest pores materials, silica gel (15 nm), the latent heat of C₁₄Zn is fairly large and is still applicable for thermal energy storage.

Acknowledgment

We thank the financial support from National Natural Science Found of China (No. 21273138).