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Research Papers

Synthesis and Characterization of Solid-State Phase Change Material Microcapsules for Thermal Management Applications OPEN ACCESS

[+] Author and Article Information
Fangyu Cao

Mem. ASME
Department of Mechanical Engineering,
University of Maryland,
College Park, MD 20740
e-mail: fycao@umd.edu

Jing Ye

Department of Mechanical Engineering,
University of Maryland,
College Park, MD 20740
e-mail: ppsi523@gmail.com

Bao Yang

Department of Mechanical Engineering,
University of Maryland,
College Park, MD 20740
e-mail: baoyang@umd.edu

1Corresponding author.

Manuscript received October 2, 2013; final manuscript received February 25, 2014; published online March 12, 2014. Assoc. Editor: Calvin Li.

J. Nanotechnol. Eng. Med 4(4), 040901 (Mar 12, 2014) (5 pages) Paper No: NANO-13-1070; doi: 10.1115/1.4026970 History: Received October 02, 2013; Revised February 25, 2014

Polyalcohols such as neopentyl glycol (NPG) undergo solid-state crystal transformations that absorb/release significant latent heat. These solid–solid phase change materials (PCM) can be used in practical thermal management applications without concerns about liquid leakage and thermal expansion during phase transitions. In this paper, microcapsules of NPG encapsulated in silica shells were successfully synthesized with the use of emulsion techniques. The size of the microcapsules range from 0.2 to 4 μm, and the thickness of the silica shell is about 30 nm. It was found that the endothermic phase transition of these NPG-silica microcapsules was initiated at around 39 °C and the latent heat was about 96.0 J/g. A large supercooling of about 43.3 °C was observed in the pure NPG particles without shells, while the supercooling of the NPG microcapsules was reduced to about 14 °C due to the heterogeneous nucleation sites provided by the silica shell. These NPG microcapsules were added to the heat transfer fluid PAO to enhance its heat capacity and the effective heat capacity of the fluid was increased by 56% with the addition of 20 wt. % NPG-silica microcapsules.

FIGURES IN THIS ARTICLE
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PCMs have been receiving considerable attention in the application of thermal energy storage and transfer [1-6]. PCMs are latent heat storage materials, which provide much higher thermal energy storage density than sensible thermal storage materials [4,7-9]. Solid–liquid PCMs, such as paraffin wax and salt hydrates, are PCMs traditionally used in the field of thermal energy storage and transfer applications. However, they suffer from potential issues in the liquid phases, such as liquid leakage.

Certain molecular crystals such as polyalcohols undergo solid-state crystal transformations, during which they absorb or release a significant amount of latent heat, due mainly to the formation of hydrogen bonds among the molecules [8,10]. Polyalcohols, including NPG, pentaerythritol, etc., transform from heterogeneous crystals to homogeneously face-centered cubic crystals with high symmetry at a certain temperature. Compared with conventional solid–liquid PCMs, the solid–solid PCMs do not involve liquid phase during phase transitions, thus freeing them from concerns about liquid leakage and thermal expansion. Additionally, the phase transition temperature of polyalcohol PCMs can be tuned continuously by mixing two or more polyalcohols, making them suitable for many applications with different operation temperatures [10-12].

In this paper, the microcapsules comprised of solid–solid PCM NPG encapsulated in silica shells were successfully synthesized with the use of emulsion techniques. The chemical composition of the NPG-silica microcapsules was characterized using Fourier transform infrared spectrometry (FT-IR) and energy dispersive spectroscopy (EDS) and the phase change behavior was investigated using the differential scanning calorimetry (DSC) techniques. Suspensions of microcapsules in the heat transfer fluid polyalphaolefin (PAO) were also prepared and their effective heat capacity was characterized. To protect the PCM particles in the base fluids from damage inside heat exchangers and pumps, the PCM particles need to be encapsulated. However, the shell not only enhances the mechanical stability and reduces PCMs reactivity toward the base fluids but also effectively reduces the supercooling of the NPG particles by providing heterogeneous nucleation sites to the encapsulated PCMs. To the best of our knowledge, no research on the microencapsulation of solid–solid PCM polyalcohols has been reported in literature.

Materials.

Neopentyl glycol (NPG, 99%), cyclohexane (99%), ethanol (99.5%), hydrochloric acid (37% aqueous solution), tetraethyl orthosilicate (TEOS, 99%), and polysorbate 80 (Tween 80) were purchased from Sigma-Aldrich. Synfluid PAO was purchased from Chevron Philips Chemical Company LLC and distilled water was supplied by the University of Maryland, College Park. All chemicals were used as received.

Synthesis of Solid-State PCM Microcapsules.

The method of preparing hollow silica microspheres by hydrolysis of TEOS in W/O emulsion had been reported by O'Sullivan and Vincent [13] and Wang et al. [14]. This method was modified to synthesize the microcapsule comprising NPG encapsulated in silica shell in this study which is illustrated in Fig. 1. Highly concentrated (≥75 wt. %) NPG aqueous solution with HCl was used as the water phase of the W/O emulsion. The silicon oxide shell is formed by the hydrolysis of TEOS, as given by the equation below:

Due to the hydrophobic precursor TEOS and the hydrophilic product silica, the hydrolysis reaction of TEOS occurs at the interface between aqueous droplets and the bulk cyclohexane. During the reaction, water in the aqueous solution is consumed by the hydrolysis of TEOS and the core of the microcapsule is composed of a mixture of NPG and residual HCl, water, and ethanol, which are later removed.

The detailed synthesis procedure is described below. An aqueous solution of NPG with hydrochloric acid is used as water phase, while cyclohexane is the oil phase. In a typical preparation procedure, 1.6 g NPG and 0.01 g Tween 80 are dissolved into 0.4 g 20% HCl aqueous solution, after which the aqueous solution is poured into 250 ml cyclohexane. Vigorous agitation for 10 min is used to create a water-in-oil emulsion, and 1 ml TEOS is then added into the emulsion, which is sealed and stirred for 24 h at room temperature. The as-produced microcapsules are collected by centrifuge separation, then washed twice with cyclohexane and dried in a vacuum oven to remove any residue. The as-prepared microcapsules are dispersed into PAO later to prepare the phase changeable thermal fluid.

Characterization of Solid-State PCM Microcapsules.

The morphology and size of the NPG microcapsules were measured using scanning electronic microscopy (SEM, Hitachi SU-70) and transmission electronic microscopy (TEM, JEM-2010). Fourier transform infrared spectrometer (FT-IR Thermo Scientific Nicolet IR200) and energy dispersive X-ray spectroscopy (EDS, JEOL JEM 2100 F) were used to analyze the chemical and elemental composition of the NPG microcapsule.

The phase transition behavior of the microcapsules was examined using differential scanning calorimetry (DSC, TA-Q100), with the heating and cooling rate set to 5 °C/min in a nitrogen atmosphere. All the results were found to be repeatable within an uncertainty of 2%.

Morphologies of NPG PCM Microcapsules.

The electron microscope images of the NPG-silica microcapsules are shown in Figs. 2(a) and 2(c) and it can be seen that these microcapsules are spherical in shape with a smooth surface. The diameter of these microcapsules is in the range of 0.2–4 μm, with an average diameter of 1.0 μm. The NPG microcapsules have a relatively larger size distribution, due to the increased viscosity of the aqueous phase, NPG solution [14]. After removing NPG from the microcapsule, the silica shell collapses, but rupture of the shell was not observed, as shown in Figs. 2(b) and 2(d). The shrinkage of the silica shell is probably due to its thinness of about 30 nm, as shown in Fig. 2(d).

Chemical Composition of NPG PCM Microcapsules.

FT-IR spectra of the NPG-Silica microcapsules were measured to analyze their chemical composition. Figure 3 shows the FT-IR spectra of pure silica synthesized by the emulsion method, pure NPG, and the microcapsules comprising solid–solid PCM NPG encapsulated in a silica shell. The FT-IR spectrum of pure silica is characterized by the Si-O-Si linkage at 1070, 940, and 800 cm−1 and the broad band around 2850–3000 cm−1 due to the O-H stretch of silanol [15]. The peaks in the spectrum of pure NPG correspond to the stretch of C-C, C-H, C-O, and O-H bonds. The spectrum of the microcapsules (i.e., curve c), has all the essential spectral characteristics of both NPG and silica, shown in curves a and b, respectively, providing evidence in support of NPG encapsulated by hydrated silica shells. The elemental composition of the as-synthesized microcapsules is further confirmed using the EDS images (see Fig. 4).

Phase Change Behavior of NPG PCM Microcapsules.

Knowledge of the phase change behavior of these solid-state PCM microcapsules is critical for their application in thermal management. As shown in Fig. 5, the phase change behavior of the fluids containing pure NPG particles and NPG-silica microcapsules was studied using DSC. The latent heat of bulk NPG, Hbulk_NPG, is 124.0 J/g and the phase transition initiates at 39.1 °C, in agreement with literature values [12,16]. Corresponding to the solid–solid phase transition from simple monoclinic phase to face-centered cubic phase, the broad endothermic peak is due to the slow heat transfer rate between the bulk NPG sample and the sensor. During cooling, the exothermic peak of bulk NPG initiates at Tc−m,bulk = 27.7 °C, which corresponds to the cubic-to-monoclinic transition. The supercooling observed in bulk NPG, i.e., the difference between the transformation temperatures during heating and cooling, is about 11.4 °C.

On the other hand, the NPG microparticles release the stored thermal energy with the cubic-to-monoclinic transformation at much lower temperature Tc−m,particle = p °C during cooling, compared to the bulk NPG, Tc−m,bulk = 27.7 °C. The supercooling difference observed between bulk and microsized NPG can be explained by classical nucleation theory [17,18]. The probability of homogeneous nucleation to initialize the phase transition, J(T), is proportional to the material volume vDisplay Formula

(2)J(T)={vk(T)exp(-BT(T-Tm-c)2)forT<Tm-c0forTTm-c

where B is a coefficient associated with the supercooled substance. The pre-exponential term k(T) varies more slowly with the temperature than the exponential term, and thus can be considered to be constant [17,19]. For samples with the same chemical composition and crystalline structure, the reduction in sample volume leads to smaller probability of homogeneous nucleation and thus the larger supercooling. It should be noted that the interface between the NPG and PAO provides no heterogeneous nucleation sites for the PCM. The supercooling observed in the dispersion of pure NPG micro particles in PAO, about 43.3 °C, is too large relative to the expected temperature swing in typical thermal storage and transfer systems.

The supercooling of NPG particles was suppressed significantly in the NPG microcapsules, in which the silica shell provides effective heterogeneous nucleate sites for the crystal transformation. It is evident in Fig. 5 that the supercooling in NPG-silica microcapsules has been reduced to 14.0 °C from 43.3 °C, which occurs in pure NPG particles. The silica shell provides heterogeneous nucleating sites, which can reduce the nucleation activation energy inducing cubic-to-monoclinic transition of the NPG at the inner surface of the silica shell, resulting in decreased supercooling.

Heat Capacity of PAO Fluids Containing NPG PCM Microcapsules.

By dispersing NPG microcapsules in traditional heat transfer fluids such as PAO, the fluid heat capacity can be significantly increased. Given an operation temperature range of ΔTT = Tf − Ti), the apparent specific heat of the thermal fluid CTF can be estimated by [5,6,20]Display Formula

(3)CTF=(1-xPCM)Cbf+qΔTxPCM

in which Cbf is the specific heat of the base fluid, q is the heat capacity per unit mass of the PCM particles, and xPCM is the weight ratio of the corresponding PCM particles in the thermal fluid using DSC. The heat capacity of the NPG-silica microcapsule was found to be Hmicrocapsule = 96.0 J/g. Assuming the minimum effective ΔT = τmicrocapsule = 14.0 °C, the effective specific heat of the microcapsules, q/ΔT=8.8J/gK, is considerably larger than the specific heat of PAO (CPAO = 2.3 J/gK). Thus, a remarkable elevation of heat capacity can be expected by adding the PCM microcapsules in the base fluid PAO. As shown in Fig. 5(c), with 20 wt. % of the microcapsules in PAO, the overall heat capacity elevation is 19.2 J/g, and effective mass specific heat increases by up to 56% from 2.3 J/gK to 3.6 J/gK when the operation temperature is 14 °C, shown in Fig. 6Fig. 6

Calculated effective heat capacity of dispersion of NPG particles and dispersions of NPG-silica microcapsules

Grahic Jump LocationCalculated effective heat capacity of dispersion of NPG particles and dispersions of NPG-silica microcapsules

. It should be mentioned that a larger ΔT decreases the elevation of q/ΔT, and the operation temperature range cannot be decreased without limit. To utilize the latent heat of the PCM sufficiently, the lower boundary of the operation temperature range should be lower than the freezing point of the encapsulated PCM, and the upper boundary should be higher than the melting temperature.

Viscosity of PAO Fluids Containing NPG PCM Microcapsules.

In addition to heat capacity, other thermophysical properties of the dispersions of the microcapsules in PAO were also investigated. Viscosities of pure PAO and the suspension of 20 wt. % microcapsules in PAO were measured in the temperature range of 10–55 °C and the experimental data are shown in Fig. 7. The viscosity of pure PAO was found to decrease with increasing temperature, in good agreement with literature values [21]. Additionally, the viscosity of the pure PAO and the phase change fluid were measured at various spindle rotational speeds and exhibited a shear-independent characteristic of Newtonian fluids. It should be noted that in the application of heat transfer systems, a trade-off of increased pumping power is likely, due to the expected viscosity increase with the addition of microparticles in the thermal fluids.

Microcapsules comprised of solid–solid PCM NPG encapsulated in a silica shell were successfully synthesized using an emulsion technique. The size of the microcapsules is in the range of 0.2–4 μm, and the chemical composition of the microcapsules was characterized using the FT-IR and EDS techniques. Phase change behavior of the NPG-silica microcapsules as well as the pure NPG particles was investigated using the DSC techniques. A large supercooling, about 43.3 °C, was observed in the pure NPG particles without shell, due to the small volume of the microsized particles and the associated small probability of homogeneous nucleation. It was found that the silica encapsulation shell can reduce the supercooling to about 14 °C by providing heterogeneous nucleation sites. The NPG microcapsules were added to the heat transfer fluid PAO to enhance its heat capacity and it was determined that the effective heat capacity of the fluid could be increased by 56% by adding 20 wt. % NPG-silica microcapsules. Compared to conventional solid–liquid PCMs, the solid–solid PCMs involve no liquid phase, thus alleviating concerns about liquid leakage and thermal expansion during phase transition, which are critical to applications in heat transfer fluids.

This research was financially supported by National Science Foundation (NSF) under Grant No. 1336778.

Prakash, J., Garg, H. P., and Datta, G., 1985, “A Solar Water Heater With a Built-In Latent-Heat Storage,” Energy Convers. Manage., 25(1), pp. 51–56. [CrossRef]
Buddhi, D., Sawhney, R. L., Sehgal, P. N., and Bansal, N. K., 1987, “A Simplification of the Differential Thermal-Analysis Method to Determine the Latent-Heat of Fusion of Phase-Change Materials,” J. Phys. D: Appl. Phys., 20(12), pp. 1601–1605. [CrossRef]
Shaikh, S., and Lafdi, K., 2010, “C/C Composite, Carbon Nanotube and Paraffin Wax Hybrid Systems for the Thermal Control of Pulsed Power in Electronics,” Carbon, 48(3), pp. 813–824. [CrossRef]
Mondal, S., 2008, “Phase Change Materials for Smart Textiles—An Overview,” Appl. Therm. Eng., 28(11-12), pp. 1536–1550. [CrossRef]
Han, Z. H., Yang, B., Qi, Y., and Cumings, J., 2011, “Synthesis of Low-Melting-Point Metallic Nanoparticles With an Ultrasonic Nanoemulsion Method,” Ultrasonics, 51(4), pp. 485–488. [CrossRef] [PubMed]
Han, Z. H., Cao, F. Y., and Yang, B., 2008, “Synthesis and Thermal Characterization of Phase-Changeable Indium/Polyalphaolefin Nanofluids,” Appl. Phys. Lett., 92(24), p. 243104. [CrossRef]
Baetens, R., Jelle, B. P., and Gustavsen, A., 2010, “Phase Change Materials for Building Applications: A State-of-the-Art Review,” Energy Build., 42(9), pp. 1361–1368. [CrossRef]
Farid, M. M., Khudhair, A. M., Razack, S. A. K., and Al-Hallaj, S., 2004, “A Review on Phase Change Energy Storage: Materials and Applications,” Energy Convers. Manage., 45(9-10), pp. 1597–1615. [CrossRef]
Zhao, C. Y., and Zhang, G. H., 2011, “Review on Microencapsulated Phase Change Materials (MEPCMs): Fabrication, Characterization and Applications,” Renewable Sustainable Energy Rev., 15(8), pp. 3813–3832. [CrossRef]
Wang, X. W., Lu, E. R., Lin, W. X., Liu, T., Shi, Z. S., Tang, R. S., and Wang, C. Z., 2000, “Heat Storage Performance of the Binary Systems Neopentyl Glycol/Pentaerythritol and Neopentyl Glycol/Trihydroxy Methyl-Aminomethane as Solid-Solid Phase Change Materials,” Energy Convers. Manage., 41(2), pp. 129–134. [CrossRef]
Yan, Q., and Liang, C., 2008, “The Thermal Storage Performance of Monobasic, Binary and Triatomic Polyalcohols Systems,” Sol. Energy, 82(7), pp. 656–662. [CrossRef]
Chandra, D., Chellappa, R., and Chien, W. M., 2005, “Thermodynamic Assessment of Binary Solid-State Thermal Storage Materials,” J. Phys. Chem. Solids, 66(2-4), pp. 235–240. [CrossRef]
O'Sullivan, M., and Vincent, B., 2010, “Aqueous Dispersions of Silica Shell/Water-Core Microcapsules,” J. Colloid Interface Sci., 343(1), pp. 31–35. [CrossRef] [PubMed]
Wang, J.-X., Wang, Z.-H., Chen, J.-F., and Yun, J., 2008, “Direct Encapsulation of Water-Soluble Drug Into Silica Microcapsules for Sustained Release Applications,” Mater. Res. Bull., 43(12), pp. 3374–3381. [CrossRef]
Almeida, R. M., and Pantano, C. G., 1990, “Structural Investigation of Silica-Gel Films by Infrared-Spectroscopy,” J. Appl. Phys., 68(8), pp. 4225–4232. [CrossRef]
Divi, S., Chellappa, R., and Chandra, D., 2006, “Heat Capacity Measurement of Organic Thermal Energy Storage Materials,” J. Chem. Thermodyn., 38(11), pp. 1312–1326. [CrossRef]
Ruckenstein, E., and Djikaev, Y. S., 2005, “Recent Developments in the Kinetic Theory of Nucleation,” Adv. Colloid Interface Sci., 118(1-3), pp. 51–72. [CrossRef] [PubMed]
Santiso, E., and Firoozabadi, A., 2006, “Curvature Dependency of Surface Tension in Multicomponent Systems,” AIChE J., 52(1), pp. 311–322. [CrossRef]
Gibout, S., Jamil, A., Kousksou, T., Zeraouli, Y., and Castaing-Lasvignottes, J., 2007, “Experimental Determination of the Nucleation Probability in Emulsions,” Thermochim. Acta, 454(1), pp. 57–63. [CrossRef]
Cao, F., and Yang, B., 2014, “Supercooling Suppression of Microencapsulated Phase Change Materials by Optimizing Shell Composition and Structure,” Appl. Energy, 113, pp. 1512–1518. [CrossRef]
Synfluid PAO Databook, Chevron Phillips Chemical Company LP, Chevron Phillips Chemical LP, 2002, Synfluid PAO Databook, The Woodlands, TX.
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References

Prakash, J., Garg, H. P., and Datta, G., 1985, “A Solar Water Heater With a Built-In Latent-Heat Storage,” Energy Convers. Manage., 25(1), pp. 51–56. [CrossRef]
Buddhi, D., Sawhney, R. L., Sehgal, P. N., and Bansal, N. K., 1987, “A Simplification of the Differential Thermal-Analysis Method to Determine the Latent-Heat of Fusion of Phase-Change Materials,” J. Phys. D: Appl. Phys., 20(12), pp. 1601–1605. [CrossRef]
Shaikh, S., and Lafdi, K., 2010, “C/C Composite, Carbon Nanotube and Paraffin Wax Hybrid Systems for the Thermal Control of Pulsed Power in Electronics,” Carbon, 48(3), pp. 813–824. [CrossRef]
Mondal, S., 2008, “Phase Change Materials for Smart Textiles—An Overview,” Appl. Therm. Eng., 28(11-12), pp. 1536–1550. [CrossRef]
Han, Z. H., Yang, B., Qi, Y., and Cumings, J., 2011, “Synthesis of Low-Melting-Point Metallic Nanoparticles With an Ultrasonic Nanoemulsion Method,” Ultrasonics, 51(4), pp. 485–488. [CrossRef] [PubMed]
Han, Z. H., Cao, F. Y., and Yang, B., 2008, “Synthesis and Thermal Characterization of Phase-Changeable Indium/Polyalphaolefin Nanofluids,” Appl. Phys. Lett., 92(24), p. 243104. [CrossRef]
Baetens, R., Jelle, B. P., and Gustavsen, A., 2010, “Phase Change Materials for Building Applications: A State-of-the-Art Review,” Energy Build., 42(9), pp. 1361–1368. [CrossRef]
Farid, M. M., Khudhair, A. M., Razack, S. A. K., and Al-Hallaj, S., 2004, “A Review on Phase Change Energy Storage: Materials and Applications,” Energy Convers. Manage., 45(9-10), pp. 1597–1615. [CrossRef]
Zhao, C. Y., and Zhang, G. H., 2011, “Review on Microencapsulated Phase Change Materials (MEPCMs): Fabrication, Characterization and Applications,” Renewable Sustainable Energy Rev., 15(8), pp. 3813–3832. [CrossRef]
Wang, X. W., Lu, E. R., Lin, W. X., Liu, T., Shi, Z. S., Tang, R. S., and Wang, C. Z., 2000, “Heat Storage Performance of the Binary Systems Neopentyl Glycol/Pentaerythritol and Neopentyl Glycol/Trihydroxy Methyl-Aminomethane as Solid-Solid Phase Change Materials,” Energy Convers. Manage., 41(2), pp. 129–134. [CrossRef]
Yan, Q., and Liang, C., 2008, “The Thermal Storage Performance of Monobasic, Binary and Triatomic Polyalcohols Systems,” Sol. Energy, 82(7), pp. 656–662. [CrossRef]
Chandra, D., Chellappa, R., and Chien, W. M., 2005, “Thermodynamic Assessment of Binary Solid-State Thermal Storage Materials,” J. Phys. Chem. Solids, 66(2-4), pp. 235–240. [CrossRef]
O'Sullivan, M., and Vincent, B., 2010, “Aqueous Dispersions of Silica Shell/Water-Core Microcapsules,” J. Colloid Interface Sci., 343(1), pp. 31–35. [CrossRef] [PubMed]
Wang, J.-X., Wang, Z.-H., Chen, J.-F., and Yun, J., 2008, “Direct Encapsulation of Water-Soluble Drug Into Silica Microcapsules for Sustained Release Applications,” Mater. Res. Bull., 43(12), pp. 3374–3381. [CrossRef]
Almeida, R. M., and Pantano, C. G., 1990, “Structural Investigation of Silica-Gel Films by Infrared-Spectroscopy,” J. Appl. Phys., 68(8), pp. 4225–4232. [CrossRef]
Divi, S., Chellappa, R., and Chandra, D., 2006, “Heat Capacity Measurement of Organic Thermal Energy Storage Materials,” J. Chem. Thermodyn., 38(11), pp. 1312–1326. [CrossRef]
Ruckenstein, E., and Djikaev, Y. S., 2005, “Recent Developments in the Kinetic Theory of Nucleation,” Adv. Colloid Interface Sci., 118(1-3), pp. 51–72. [CrossRef] [PubMed]
Santiso, E., and Firoozabadi, A., 2006, “Curvature Dependency of Surface Tension in Multicomponent Systems,” AIChE J., 52(1), pp. 311–322. [CrossRef]
Gibout, S., Jamil, A., Kousksou, T., Zeraouli, Y., and Castaing-Lasvignottes, J., 2007, “Experimental Determination of the Nucleation Probability in Emulsions,” Thermochim. Acta, 454(1), pp. 57–63. [CrossRef]
Cao, F., and Yang, B., 2014, “Supercooling Suppression of Microencapsulated Phase Change Materials by Optimizing Shell Composition and Structure,” Appl. Energy, 113, pp. 1512–1518. [CrossRef]
Synfluid PAO Databook, Chevron Phillips Chemical Company LP, Chevron Phillips Chemical LP, 2002, Synfluid PAO Databook, The Woodlands, TX.

Figures

Grahic Jump Location
Fig. 1

Process of synthesizing NPG in silica microcapsules. (a) Mixing water phase into cyclohexane with surfactant; (b) adding TEOS to the mixture; (c) hydrolysis of TEOS to form silica shell; and (d) collection of microcapsules.

Grahic Jump Location
Fig. 2

(a) SEM and (c) TEM images of as-synthesized microcapsules of NPG in silica shell, and (b) SEM and (d) TEM images of wrinkled silica shell after NPG is removed. Inserted in (a) is a histogram of the particle size distribution of the NPG-silica microcapsules.

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Fig. 3

FT-IR spectra of sample (a) silica, (b) NPG, and (c) NPG-silica microcapsules

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Fig. 4

Atomic distribution of the NPG-silica microcapsules measured using the EDS technique

Grahic Jump Location
Fig. 5

DSC heating and cooling curves of samples, (a) pure, bulk NPG, (b) dispersions of pure NPG micro-particles in PAO, and (c) dispersions of 20 wt. % NPG-silica microcapsules in PAO

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Fig. 6

Calculated effective heat capacity of dispersion of NPG particles and dispersions of NPG-silica microcapsules

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Fig. 7

Dynamic viscosities of pure PAO and PAO containing 20 wt. % microcapsules in versus temperature

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