Thermal Conductivity and Stability of Hydrocarbon-Based Nanofluids with Palladium Nanoparticles Dispersed by Modified Hyperbranched Polyglycerol.

Palladium nanoparticles, which were prepared by modified hyperbranched polyglycerol (mHPG) as stabilizers, can be dispersed well in nonpolar organic solvents and form highly stable nanofluids. The influences of three mHPG products modified with cyclohexanethiol (CSHPG), dodecanethiol (DSHPG), and octadecanethiol (OSHPG) on the preparation and stability of the palladium nanoparticles were investigated. The stability and thermal conductivity enhancement of the hydrocarbon-based nanofluids with Pd@mHPG (Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG) compared to the corresponding base fluid were investigated at different temperatures. The average diameters of nanoparticles stabilized by CSHPG, DSHPG, and OSHPG are within 2.7-3.6 nm. The palladium nanoparticles could be dispersed well in the nonpolar base fluid such as decalin. The nanofluids with Pd@DSHPG and Pd@OSHPG could remain stable for up to 330 days at room temperature. The nanofluid with Pd@DSHPG or Pd@OSHPG could be stable for more than 24 h at 110 °C. The thermal conductivity of the nanofluids improved with increasing temperature and the mass fraction of nanoparticles compared to the corresponding base fluid. The long alkyl chain-modified HPG can give better protection for nanoparticles from agglomeration and assist metal nanoparticles in enhancing the thermal conductivity of nanofluids.

Introduction

As an innovative kind of thermal fluid based on nanotechnology, nanofluids can significantly improve the efficiency of the energy conversion process.[1−3] A nanofluid refers to a stable colloidal mixture of nanoparticles in a base fluid.[4] Since the suspended nanoparticles in nanofluids could improve the thermal conductivity of a common fluid, they have tremendous potential for different industrial cooling applications such as heat exchangers, cooling devices, solar collectors, and so on.[5,6] Moreover, the nanofluid could be regarded as promising coolants and fuels to be utilized in an aircraft power system.[7] A growing number of studies concentrate on nanofluids suspending metal nanoparticles used in various thermal fields.[8−10] However, the preparation and use of nanofluids with metal nanoparticles still face some limitations, especially the acquisition of a solid–liquid suspension with good stability, which is still a major challenge.[11,12]

The small size, high surface activity, and van der Waals interactions among metal nanoparticles make them difficult to be dispersed well in the base fluid.[13,14] A nanofluid with long-term stability is what we expected. The agglomeration and sedimentation of nanoparticles are hindrances to acquire nanofluids with high stability, which may even cause pipe blockage and finally influence the performance of fluids in usage.[15] Preparation of metal nanoparticles with good dispersing ability is the key to obtain an oil-based nanofluid with good stability and high thermal conductivity enhancement. The conventional method is adding surfactants or modification agents, while excess surfactants may lead to uneven phonon scattering and affect important properties, including viscosity and heat transfer ability of nanofluids.[16−18]

The metal nanoparticles could be dispersed by the hyperbranched polymers. As a class of polymer with highly branched three-dimensional spherical structures, the hyperbranched molecules have unique physical and chemical properties, such as good solubility, low viscosity, and so on.[19−21] Furthermore, the hyperbranched polymer, which has a large number of functional groups, can be used as a kind of effective surface modifier to stabilize metal nanoparticles.[22] The nanoparticles could be suspended in fluids without increasing the pressure drop and pumping power caused by increasing the density and viscosity of nanofluids.[23,24] Liu et al.[25] synthesized the hyperbranched polymer-decorated Au nanoparticles in aqueous media with a remarkable stability at 200 °C. Skaria et al.[26] proposed a convenient approach to prepare hyperbranched polymers with a thioether shell for stabilization of coinage nanoparticles in organic media. Hyperbranched polyglycerol (HPG) has high-flexible aliphatic polyether backbones and possesses the ability to stabilize metal nanoparticles close to that of dendrimers. The internal spatial structure could limit the growth of particles and could endow nanoparticles with small particle sizes and a large specific surface area. Through modification of HPG with nonpolar alkyl groups, the highly organosoluble amphiphilic structures with the hydrophobic shells could be obtained.[19] However, only a few applications of hyperbranched polymers in oil-based or hydrocarbon-based nanofluids have been reported to date.[27,28]

In this work, the hydrophobic palladium nanoparticles are prepared with long-chain alkyl- or cycloalkyl group-modified hyperbranched polyglycerol (mHPG) as dispersants and stabilizers. mHPG could provide metal nanoparticles salient dispersibility and stability in oil base fluids, and the process allowed us to prepare palladium nanofluids with good stability. The influences of three mHPG products modified with cyclohexanethiol (CSHPG), dodecanethiol (DSHPG), and octadecanethiol (OSHPG) on the preparation and stability of the palladium nanoparticles were investigated. The thermal stability of nanofluids in the temperature range of 110–140 °C was investigated. The effects of the palladium nanoparticles (Pd@OSHPG, Pd@DSHPG, and Pd@CSHPG) that enhance the thermal conductivity of nanofluids were studied, and the viscosity of nanofluids was measured.

Results and Discussion

Characterization of HPG and mHPG

The 1H NMR spectra of HPG, HPG-MA, and mHPG (CSHPG, DSHPG, and OSHPG) are shown in Figure a. The significant signals for ether bonds of HPG appear at 3.2–4.0 ppm. After the successful transesterification reaction, vinyl group signals can be seen around 5.6 and 6.1 ppm, and methyl signals of the methacryloyl group appear at 1.9 ppm. After the thiol-ene click reaction of HPG-MA with alkylthiol, methyl and methylene signals for alkyl groups of CSHPG, DSHPG, or OSHPG can be observed between 0.8 and 2.1 ppm. The disappearance of signals for C=C bonds at 5.6 and 6.1 ppm also indicate that the three mHPG products have been successfully prepared.

Figure 1

(a) 1H NMR spectra and (b) FTIR spectra of HPG, HPG-MA, and mHPG (CSHPG, DSHPG, and OSHPG).

The FTIR results of HPG, HPG-MA, and mHPG are shown in Figure b. The broad absorption bands of the O–H and C–O vibration of HPG appear around 3400 and 1120 cm–1, respectively. After transesterification, the absorption peak of C=C (around 1570 cm–1) is observed. The absorption bands around 3003 and 1720 cm–1 belong to the stretching vibrations of =C—H and C=O, respectively. In the FTIR spectra of CSHPG, DSHPG, and OSHPG, the absorption band of C=C decreases and the typical band of S–C appears in a range of 600–700 cm–1. The signals of the 1H NMR and FTIR spectra are consistent with previous reports.[29,30]

Characterization of Pd@mHPG

The morphology and size distribution of Pd@mHPG (Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG) are observed by transmission electron microscopy (TEM) (Figure ). Three kinds of Pd nanoparticles were well dispersed. The average particle sizes of Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG are about 3.4, 2.7, and 3.6 nm, respectively. It can be seen that the prepared Pd nanoparticles have small particle sizes (2.7–3.6 nm) and narrow distribution due to the encapsulation capabilities of mHPG. The outer shell of the hyperbranched polymer can protect the nanoparticles from aggregation between multiple molecules and provide good dispersiblity of the Pd nanoparticles in nonpolar base fluids. The particle size contributes in dispersion stability and predominantly influences the thermal conductivity of the nanofluids. The small nanoparticle size could enhance the surface-to-volume ratio and improve interfacial heat exchange resulting from the improved particle specific surface area and mobility of the small nanoparticles.[31,32] In other words, the small particle size means that the metal nanoparticle has a large specific surface area and good thermal conductivity. As a contrast, the Pd nanoparticles stabilized by dodecanethiol (Pd@S) were prepared, and the TEM image of Pd@S is shown in Figure S1 of the Supporting Information. The average particle size of Pd@S is about 3.0 nm with unobvious aggregation.

Figure 2

TEM images and size distributions of (a) Pd@CSHPG, (b) Pd@DSHPG, and (c) Pd@OSHPG.

Figure shows the X-ray photoelectron spectroscopy (XPS) spectrum of Pd@DSHPG. It is clear that the Pd nanoparticles contain S, C, Pd, and O elements. This suggests the formation of Pd@DSHPG. The Pd 3d spectrum shows two peaks (336.8 and 335.8 eV) for 3d5/2 and 342.05 eV for Pd 3d3/2. The binding energy of 335.8 eV (Pd 3d5/2) corresponds to the metallic Pd0. The peak around 336.8 eV corresponding to Pd2+ is attributed to anchoring with the thioether structures.

Figure 3

(a) Survey XPS spectrum of Pd@DSHPG. (b) High-resolution XPS spectrum of Pd in Pd@DSHPG.

Stability of Nanofluids

Considering that nanofluids need to be stored for a long time, the stability of nanofluids of Pd@mHPG under different conditions was investigated. The schematic diagram of the stability of nanofluids with 0.1 wt % nanoparticles at room temperature is shown in Figure . It can be seen that the nanoparticles stabilized by different mHPG products are well dispersed in the base liquid to form a uniform and quasi-homogeneous system (photograph shown in Figure a). The stability of nanofluids with Pd@CSHPG, Pd@DSHPG, or Pd@OSHPG was tested against time at room temperature by a UV–vis spectrograph. The relative content of metal nanoparticles in the nanofluid is calculated by the ratio of absorbance at different storage times to the initial absorbance. The relative content of Pd@DSHPG or Pd@OSHPG in the nanofluid remains almost unchanged, and the nanofluids with Pd@DSHPG and Pd@OSHPG remain stable for 330 days (Figure b). The relative content of Pd@CSHPG in the nanofluid decreased significantly in the first few days, and little precipitation is observed at the bottom of the liquid. The rate of deposition becomes relatively slow in the following days. The CSHPG encapsulated nanoparticles are less stable than the other two. It is probably because the peripheral ligand of CSHPG is the cyclohexyl groups, and the interaction between metal nanoparticles and CSHPG is limited due to the steric hindrance of CSHPG. The metal nanoparticles gradually agglomerate with increasing storage time, and more than 50% of the Pd@CSHPG nanoparticles are stably dispersed for 330 days. The TEM images of Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG in nanofluids after a 330 day storage period are shown in Figure S2. It can be seen that the Pd@CSHPG nanoparticles are partly agglomerated, and Pd@DSHPG and Pd@OSHPG in nanofluids remain highly dispersed.

Figure 4

(a) Photographs of nanofluids with Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG with a mass fraction of 0.1%. (b) Relative content of the suspended particle in nanofluids with storage time at room temperature.

The nanoparticles could agglomerate at high temperatures. The stability of three nanofluids under different temperatures was evaluated by a UV–vis spectrograph. The nanofluids were considered to be unstable when there was precipitation, and the absorbance decreased markedly. The heating process stopped when the absorbance dropped to zero. The heating stability times for nanofluids with 0.1 wt % Pd@CSHPG, Pd@DSHPG, or Pd@OSHPG at different temperatures (110–140 °C) are shown in Figure . Every experimental result for thermal stability is the average of three measurements. It was shown that the thermal stability times for three nanofluids decreased with increasing temperature. The nanofluid with Pd@DSHPG and Pd@OSHPG could be stable for more than 24 h at 110 °C. The stability time of a nanofluid with Pd@CSHPG is less than that of Pd@DSHPG or Pd@OSHPG at the same heating temperature. The heating stability for the nanofluids with 0.1 wt % Pd@S is shown in Figure S3. The thermal stability times for the nanofluids with Pd@S decreased with increasing temperature, and the thermal stability time is less than 24 h at 110 °C. DSHPG or OSHPG could utilize its backbone to encapsulate the metal nanoparticles, while the steric hindrance of CSHPG limits the interaction with metal nanoparticles as the peripheral ligand of CSHPG is the cyclohexyl groups. The progressive breaking of the skeleton of polymer molecules with increasing temperature causes the coated metal nanoparticles to be exposed, and the thermal stability of the nanoparticles reduces significantly. The hyperbranched molecules modified with alkyl chains could effectively protect the metal nanoparticles in a certain temperature range, which expands the applicable conditions for the storage and application of nanofluids.

Figure 5

Thermal stability of nanofluids with Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG at a mass fraction of 0.1%.

Thermal Conductivity of Nanofluids

Compared with traditional heat transfer fluids, nanofluids have opened up new directions in the field of thermal conductivity enhancement.[6,33] The thermal conductivity of nanofluids with Pd@CSHPG, Pd@DSHPG, or Pd@OSHPG was measured for different mass fractions at different temperatures. In this work, the improvement effect of nanoparticles on the thermal conductivity for nanofluids is expressed as the ratio of thermal conductivity (k/k0). k is the thermal conductivity of the nanofluid, and k0 is that of the nonpolar base fluid (decalin). As shown in Figure , the thermal conductivity of nanofluids with Pd@CSHPG, Pd@DSHPG, or Pd@OSHPG increased with increasing temperature and/or the mass fraction of nanoparticles. The experimental results showed obvious improvements in thermal conductivity for three nanofluids compared to the thermal conductivity for the base fluid. Figure a shows the variation of thermal conductivity for nanofluids with Pd@CSHPG at different mass fractions with temperatures. At 25 °C, the enhancement of thermal conductivity is about 1, 3, or 5% for 0.05, 0.1, or 0.3 wt % of Pd@CSHPG, respectively. As the temperature increased to 50 °C, the improvements in thermal conductivity of the nanofluids were observed to be approximately 16, 20, and 35%. For 0.1 wt % Pd@DSHPG, the enhancement of thermal conductivity of the nanofluid is 4% at 20 °C and 30% at 50 °C. For the nanofluids with Pd@OSHPG, the thermal conductivity increases significantly. When the added nanoparticles of Pd@OSHPG increases from 0.05 to 5 wt %, the enhancement of thermal conductivity for nanofluids is from 15 to 74% at 50 °C. The increase in thermal conductivity is nonlinear with increasing temperature and the mass fraction, which matches with other reports.[25] This nonlinear behavior depends on the nature of the nanoparticles and base fluids. The distance between particles decreases with increasing particle concentration. More particles cause the collision between particles in the fluid to be more severe, and the energy transfer is more efficient. Moreover, the palladium nanoparticles stabilized by long alkyl chain-modified HPG have small particle sizes and uniform distribution and show better stability and thermal conductivity.

Figure 6

Ratio of thermal conductivity (k/k0) of nanofluids with (a) Pd@CSHPG, (b) Pd@DSHPG, and (c) Pd@OSHPG. (d) Comparison of k/k0 for nanofluids with different Pd nanoparticles at a mass fraction of 0.1%.

For better understanding, Figure d compares the effect of thermal conductivity enhancement for nanofluids with different Pd nanoparticles at the same mass fraction (0.1 wt %). In the temperature range of 25–50 °C, the nanofluid with Pd@OSHPG has higher thermal conductivity at the same mass fraction. The order of nanoparticles in nanofluids for thermal conductivity enhancement is Pd@OSHPG > Pd@DSHPG > Pd@CSHPG. The thermal conductivity enhancement of Pd@mHPG is larger than that of Pd@S for the nanofluids with the same nanoparticle concentration. The longer alkyl chain-modified HPG can give better protection for nanoparticles and assist metal nanoparticles in enhancing the thermal conductivity of nanofluids.

Figure represents the density and viscosity measurements of the base fluid and nanofluids with 0.1 wt % nanoparticles at 25–50 °C. It can be seen that the density and viscosity decrease with increasing temperature, which is due to a weakening of intermolecular forces. At 50 °C, the increases of density and viscosity for nanofluids compared to those for the base fluid are less than 0.12 and 2%, respectively. In the literature,[33] the viscosity and density of distilled water and GNP-Ag were reported. The results shows that the viscosity increases by about 30% for nanofluids with 0.1 wt % nanoparticles compared to the viscosity of the base fluid (40 °C). Meanwhile, the density increases by about 0.09% for 0.1 wt % nanoparticles compared to the density of the base fluid. In general, the change of viscosity for the Pd nanofluids prepared in this paper is very small and negligible compared to the literature. The results could be useful for engineering application.

Figure 7

(a) Densities and (b) viscosities of the base fluid and nanofluids at different temperatures.

Conclusions

Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG were prepared with modified HPG. The prepared Pd nanoparticles were quasi-spherical in shape and small in particle size (3.4, 2.7, and 3.6 nm). Stability studies indicated that the modified hyperbranched molecules enhanced the dispersion and stability of nanoparticles in the nonpolar base fluid without any additives. The nanofluids with Pd@DSHPG and Pd@OSHPG remain stable for up to 330 days, and the CSHPG encapsulated nanoparticles are less stable than the other two. The nanofluid with Pd@DSHPG or Pd@OSHPG could be stable for 24 h at 110 °C. With increasing temperature, the thermal stability of nanofluids decreased. The thermal conductivity study showed that the thermal conductivity enhanced with increasing temperature and the mass fraction of nanoparticles. The experimental results showed significant improvements in thermal conductivity for hydrocarbon-based nanofluids compared to the corresponding base fluid. In general, the palladium nanoparticles stabilized by long-chain alkyl-modified HPG show better stability and thermal conductivity. The results could supply useful information for designing smart coolants for a variety of thermal management systems.

Experimental Section

Materials and Reagents

Potassium tetrachloropalladate (98%), cyclohexanethiol (98%), 1-dodecanethiol (98%), 1-octadecanethiol (97%), glycidyl methacrylate (97%), decalin (98%), toluene (99%), and 4-dimethylaminopyridin (99%) were all purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Benzophenone (99%), tetrabutylammonium bromide (TOAB; 98%), and dimethyl sulfoxide (DMSO) were obtained from Macklin Biochemical Co., Ltd. (Shanghai, China). HPG (Mn = 6659, Mw/Mn = 1.3) was prepared in our laboratory, according to the literature.[34,35] The hyperbranched polyglycerols were synthesized by anionic ring-opening polymerization, and the monomers in the reactions should be added very slowly to ensure low polydispersity.

Preparation of Palladium Nanoparticles and Nanofluids

mHPG products (CSHPG, DSHPG, and OSHPG) were prepared according to the following procedure, as shown in Scheme . mHPG with thioether structures was synthesized by click chemistry of HPG-MA (transesterification product of HPG with GMA[29]) and cyclohexanethiol, 1-dodecanethiol, or 1-octadecanethiol. The modification to terminal alkyl groups imparted hydrophobicity to HPG, giving the modified product mHPG (CSHPG, DSHPG, and OSHPG) good solubility in the oil phase.

Scheme 1

Preparation Process of mHPG (CSHPG, DSHPG, and OSHPG)

The oil-soluble palladium nanoparticles were prepared by the phase-transfer method. Typically, 20 mL of K2PdCl4 (0.61 mmol) aqueous solution and 1.40 mmol of tetrabutylammonium bromide (TOAB) were dissolved in 100 mL of toluene. The mixture was stirred for 1 h. After standing stratification, the organic phase was collected. Then, 0.2 g of CSHPG, DSHPG, or OSHPG was added. After that, 20 mL of NaBH4 (4.00 mmol) aqueous solution was added dropwise, and the reaction was stirred vigorously overnight. The organic phase was removed by rotary evaporation at 45 °C. Finally, the product was obtained by vacuum drying at 25 °C. A series of oil-based nanofluids were prepared by dispersing a certain amount of Pd@CSHPG, Pd@DSHPG, and Pd@OSHPG in decalin with ultrasonication.

To better understand the role of Pd@mHPG, the dodecanethiol-coated Pd nanoparticles (Pd@S) were synthesized by the phase-transfer method; the details are shown in the Supporting Information.[36,37]

Characterization

Nuclear magnetic resonance spectra (1H NMR) were carried out by a Bruker Advance III 600 MHz spectrometer. HPG and mHPG were tested with CH3OD and CDCl3 as the solvents, respectively. Fourier transform infrared spectra (FTIR) were acquired by a Nicolet 5700 spectrometer (Thermo Scientific). The morphology of Pd nanoparticles was performed on a JEM-2100 transmission electron microscope (JEOL). The elemental composition and chemical state of nanoparticles were recorded by an EscaLab 250Xi X-ray photoelectron spectroscope (Thermo Scientific). The stability of nanofluids was monitored by UV–vis spectroscopy on an Evolution 201 spectrophotometer (Thermo Scientific). The thermal conductivity of nanofluids was measured by using a KD2 Pro thermometer with a KS-1 probe sensor (Decagon Devices). The result was reported as average after at least six measurements for each temperature and concentration after equilibrium of nanofluids. The viscosities and densities of the base fluid and nanofluids were measured by a Lovis 2000 M automatic microviscometer and DMA 5000 M density meter (Anton Paar), respectively.