Scholarly article on topic 'Preparation and performances of bulk porous Al foams impregnated with phase-change-materials for thermal storage'

Preparation and performances of bulk porous Al foams impregnated with phase-change-materials for thermal storage Academic research paper on "Materials engineering"

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{"Phase change materials" / "Porous Al foams" / Composites / "Thermal storage" / "Dynamic mechanical properties"}

Abstract of research paper on Materials engineering, author of scientific article — Jinghua Jiang, Yingying Zhu, Aibin Ma, Donghui Yang, Fumin Lu, et al.

Abstract Shape-stabilized phase change materials (PCMs) exhibit desirable thermal storage properties and are used for energy conservation systems. This study is focused on the preparation and thermal-/dynamic-mechanical properties of the bulk porous Al foams impregnated with organic PCMs such as paraffins and stearic acid. The results indicated that the new shape-stabilized PCMs composites could be produced by a simple and eco-friendly constant-pressure impregnation method. The filling fraction of PCMs was approximately more than 80% for the impregnated samples with vacuuming or without vacuuming. The shape-stabilized PCMs/Al-foam composites exhibited considerable latent thermal storage potential for good interface, desirable phase-change temperature range and latent heat values (72.9kJ/kg for the paraffin/Al-foam composite and 66.7kJ/kg for the stearic acid/Al-foam composite). Moreover, the impregnation of PCMs improved the dynamic-compression yield strength, durability and resistance to oxidation of Al foams and made its ideal energy-saving materials.

Academic research paper on topic "Preparation and performances of bulk porous Al foams impregnated with phase-change-materials for thermal storage"

Progress in Natural Science: Materials International 2012;22(5):440-444

Chinese Materials Research Society

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Progress in Natural Science

Progress in Natural Science: Materials International

www.elsevier.com/locate/pnsmi www.sciencedirect.com

ORIGINAL RESEARCH

Preparation and performances of bulk porous Al foams impregnated with phase-change-materials for thermal storage

Jinghua Jianga,b,n, Yingying Zhub, Aibin Maa,b,n, Donghui Yangb,c, Fumin Lub, Jianqing Chenb, Jun Shib, Dan Songa,b

aNational Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, Hohai University, Nanjing 210098, China

bCollege of Mechanics and Materials, Hohai University, Nanjing 210098, China

cDepartment of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC 27695-7919, USA

Received 21 April 2012; accepted 27 May 2012 Available online 31 October 2012

KEYWORDS

Phase change materials; Porous Al foams; Composites; Thermal storage; Dynamic mechanical properties

Abstract Shape-stabilized phase change materials (PCMs) exhibit desirable thermal storage properties and are used for energy conservation systems. This study is focused on the preparation and thermal-/dynamic-mechanical properties of the bulk porous Al foams impregnated with organic PCMs such as paraffins and stearic acid. The results indicated that the new shape-stabilized PCMs composites could be produced by a simple and eco-friendly constant-pressure impregnation method. The filling fraction of PCMs was approximately more than 80% for the impregnated samples with vacuuming or without vacuuming. The shape-stabilized PCMs/Al-foam composites exhibited considerable latent thermal storage potential for good interface, desirable phase-change temperature range and latent heat values (72.9 kJ/kg for the paraffin/Al-foam composite and 66.7 kJ/kg for the stearic acid/Al-foam composite). Moreover, the impregnation of PCMs improved

"Corresponding authors at: National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, Hohai University, Nanjing 210098, China. Tel.: +86 25 8378 7239; fax: +86 25 8378 6046.

E-mail addresses: jhjianghhu@gmail.com (J. Jiang), aibin-ma@hhu.edu.cn (A. Ma). Peer review under responsibility of Chinese Materials Research Society.

1002-0071 © 2012 Chinese Materials Research Society. Production and hosting by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pnsc.2012.05.004

the dynamic-compression yield strength, durability and resistance to oxidation of Al foams and made its ideal energy-saving materials.

© 2012 Chinese Materials Research Society. Production and hosting by Elsevier Ltd. All rights reserved.

1. Introduction

Due to the extensive demands in renewable energy applications (such as solar energy), thermal energy storage techniques have been paid great attention [1-3]. Latent heat storage using phase-change-materials (PCMs) is particularly attractive, since it provides benefits including reduction in temperature variability (thermal inertia) and high thermal energy storage density [4,5]. Various PCMs are generally divided into two main groups from their compositions (i.e. organic and inorganic PCMs) or two categories from their melting points (i.e. high-temperature PCMs above 200 °C and low-temperature PCMs below 200 °C). The high-temperature PCMs can be used in solar power plants, while the low-temperature PCMs are mainly used in waste heat recovery systems and buildings. Although organic substances exhibit desirable properties at low temperature applications (such as limited supercooling, no phase segregation and non-corrosion), they present a low thermal conductivity (~0.24 W/(m K) for paraffin waxes) [6]. In order to offset the heat storage/extraction rate during melting/solidification cycles, extensive investigations have been carried out to improve the thermal response of PCMs through adding various high thermal conductivity materials [7,8]. The methods include dispersing high conductivity particles (or fibers) into PCMs, impregnating a porous metallic (or graphitic) matrix with PCMs. Due to the high surface-area density and strong mixing capability, high porosity open-cell light-metallic foams are now regarded as one of the most promising materials for the impregnation of organic liquidsolid PCMs. However, almost all the previous investigations were focused on numerical analyses of the solid/liquid phase change heat transfer in metal foams [5,9]. There are relatively few experimental reports for fabricating shape-stabilized PCMs/porous-metallic-matrix composites as high power thermal storage materials and investigating their dynamic responses at high strain rates (>103/s). Undoubtedly, simple procedure and good dynamic mechanical behaviors are very important for its possible engineering application. This study was focused on preparation and characterization of thermal-/ dynamic-mechanical properties of bulk porous Al foams structure impregnated with PCMs (e.g. paraffins and stearic

acid). The aims of this work are to investigate the latent heat values and dynamic responses of the organic PCMs/Al-foam composites, to present an operable method of fabricating shape-stabilized PCMs with desirable thermal-mechanical properties, and to propose high power PCMs supported in a high thermal conductivity matrix.

2. Experimental details

Commercial stearic acids and sliced paraffins were used in the preparation of the shape-stabilized PCMs. The melt-infiltrating method was applied to fabricate the porous Al foam, and the matrix Al alloy. The Porous Al foam (Al-7Si-Mg) was selected as the supporter of the PCMs. The porous Al foam (see Fig. 1a) were fabricated by melt-infiltrating methods [10] and obtained with porosities of ~65-70%, pore size lower than 1 mm and apparent density of 0.80-0.94 x 103 kg/m3. Two kinds of matrix samples (10 mm x 10 mm x 24 mm and F 24 mm x 10 mm in dimension ) were machined from the as-received Al foams, polished and then used to conduct constant-pressure impregnation or vacuum impregnation (< 2 Pa, Tube type vacuum furnace LZJ-6-10) at room temperature, respectively.

The PCMs/Al-foam samples impregnated after 0.3-1.5 h were removed from the PCMs bath and the thin layer was scraped on their surfaces, then the amount of PCMs retained by porous Al-foam matrixes were evaluated by the weighing method. The latent heat values of fusion with various PCMs concentrations were found using differential-scanning-calorimetry (DSC) analyses of small PCMs/Al-foam samples, which were cut from three various positions for each PCMs/Al-foam samples. The DSC tests were performed by STA409PC/4/H differential thermal analyzer (NETZSCH corp., Germany) at a heating and cooling rate of 5 °C/min from 25 °C to 100 °C. Dynamic compression tests were carried out at the strain rates of 800 s_1 and 1000 s_1 using a split-Hopkinson pressure bar (SHPB). The detailed description of SHPB can be found in Ref. [11]. The dynamic yield strength values of PCMs/Al-foam were obtained from the average values plotted in the stress-strain curves. Metallographic observation of the PCMs/Al-foam samples was performed by using an optical microscopy (OM) and a

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Fig. 1 Macroscopic appearance of cellular Al foams (a), optical micrographs of PCMs/ Al foams composites embedded with stearic acid (b) and paraffins (c) at constant pressure impregnation.

scanning electron microscopy (SEM), to discover PCMs impregnation situation and failure mechanism of the broken samples after dynamic compression.

3. Results and discussion

Fig. 1 presents the optical micrographs of the PCMs/Al-foam composites impregnated with stearic acid (b) and paraffins (c), in which no crack exists on the boundaries of the dark-colored PCMs areas and the light-colored Al-foam areas. The good impregnation effect was obtained due to the high permeability of organic low-temperature PCMs and the strong cohesion of PCMs with the Al-foam matrix. SEM micrographs of PCMs/Al-foam composites (shown in Fig. 2) also indicate the good impregnation effect of PCMs in the Al matrix, where the filling fraction of PCMs in existing pores reaches above 80% with the small amount of residual voids (see the square area). The lower residual porosities (see Fig. 2b) demonstrate that the impregnation effect of the paraffin is better than that of the stearic acid.

To optimize the fabrication method of the shape-stabilized PCMs composite, the amount of PCMs retained by Al-foam matrixes after various processes were compared. Table 1 lists apparent density (p) and filling fraction (F) of PCMs/Al-foam samples after constant-pressure impregnation(CI), vacuum impregnation (VI) and constant-pressure plus dynamic compression (CD). Table 1 shows that the p values of these samples increase with improving filling fraction of PCMs, while the filling fraction of the paraffin is higher than that of the stearic acid. Furthermore, the variation values of the filling fraction of paraffin/Al-foam composites are lower. It indicates that the dynamic impact resistance of the paraffin/Al-foam composites is higher than that of the stearic acid/Al-foam composites. It is noted that the impregnation effect of the PCMs/Al-foam composites under constant-pressure and vacuum condition is only slightly different, thus constant-pressure impregnation is recommended due to its simple operation and low costs.

Fig. 3 presents the DSC results of the paraffin and the stearic acid for investigating the latent heat values of the shape-stabilized PCMs/Al-foam composites. The commercial paraffin has the starting phase-change temperature of 33.0 °C, the peak temperature of 64.1 °C, the finial temperature of 70.0 °C and the corresponding latent heat value of 140 kJ/kg (Fig. 3a). The stearic acid has the starting temperature of 54.7 °C, the peak temperature of 64.0 °C, the final phase-change temperature of 70.6 °C and the corresponding latent heat value of 141 kJ/kg (Fig. 3b). The results demonstrate that the latent heat values and phase-change temperature ranges of the two PCMs are appropriate for solar energy storage and energy efficient building application. The latent heat value of the shape-stabilized PCMs/Al-foam composites can be calculated from the equations as follows:

miQo + m2cDT m1 + m2

where Q is the latent heat of the composites (kJ/kg), Q0 is the latent heat of the impregnated PCMs (kJ/kg), c is the specific heat capacity of the Al foams (kJ/(kg K)), AT is the phase-change temperature range of the PCMs, then m1 and m2 are the mass fraction of the PCMs and the Al foams, respectively. Therefore, the latent heat values of the paraffin/Al-foam and the stearic acid/ Al-foam were calculated to be approximately 72.9 kJ/kg and 66.7 kJ/kg, respectively. Actually, metallic foams have so complicated interior microstructures that it is hard for us to accurately calculate the latent heat values of PCMs/Al-foam composites. While the shape-stabilized PCMs/Al-foam composites can present higher latent heat values with increasing the porosity of the Al foams. The paraffin/Al-foam composite, compared with the stearic acid/Al-foam composite, possesses the wider phase-change temperature range and the higher latent heat value. Thus it is better to adjust the ambient temperature through latent heat storage/release of the paraffin/Al-foam composite.

Fig. 4 presents the dynamic compression curves of Al foams (Fig. 4a), shape-stabilized PCMs/Al-foam composites impreg-

Fig. 2 SEM micrographs of PCMs/Al-foam composites impregnated with stearic acid (a) and paraffins (b) at constant pressure impregnation.

Table 1 Impregnation effect of the PCMs/Al-foam composites under constant-pressure or vacuum condition.

PCMs Pci (kg/m3) Fci (%) Pvi (kg m3) Fvi (%) Pcd (kg m3) Fcd (%)

Stearic acid 1.452 x 103 87.6 1.392 x 103 85.5 1.464 x 103 88.3

Paraffins 1.406 x 103 88.1 1.334 x 103 88.5 1.431 x 103 89.7

50 60 70 Temparature / °C Fig. 3 DSC measurement of PCMs: (a) paraffins and (b) stearic acid.

0.00 0.02 0,04 0.06 0.08 0.10 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 o.os o.oo 0.02 0.04 o.os 0.0s 0.10

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Fig. 4 Dynamic compressing curves: (a) Al foams, (b) paraffins/Al-foam and (c) stearic acid/Al-foam.

nated with paraffin (Fig. 4b) and stearic acid (Fig. 4c). Their dynamic yield strength values were obtained from the average values of two or three samples. It is seen in Fig. 4a that the dynamic compressing yield strength of the Al foam is 12.0 MPa. The fluctuating stress data in the stress-strain curves result from uneven distribution of pore diameters in the as-received Al foams. PCMs impregnation improves the dynamic mechanical properties of the Al foams matrix. The dynamic yield strength values of Al foams impregnated with stearic acid and paraffins are 28.0 MPa and 22.0 MPa, respectively. It is

obvious that the shape-stabilized PCMs/Al-foam composites have good dynamic impact resistance. The macrographs of the PCMs/Al-foam after dynamic compression tests (in Fig. 5) also indicate that the bonding effect in paraffins/Al-foam is better than that in the stearic acid/Al-foam composites.

Fig. 6 demonstrates SEM micrographs of the Al foam and stearic acid/Al-foam composites after dynamic compression tests. Fig. 6a clearly shows that the porous structure of the Al foam is collapsed under dynamic pressing and some pore connecting parts are transversely broken. While the white

Fig. 5 Macrographs of three dynamic-compressed samples: (a) Al foams, (b) paraffins/Al-foams and (c) stearic acid/Al-foams.

Fig. 6 SEM micrographs of two dynamic-compressed samples: (a) Al foam and (b) stearic acid/Al-foam.

particles on the Al foam section are the oxidation particles produced due to its long-term exposure in air. For the dynamic-compressed stearic acid/Al-foam samples (in Fig. 6b), the bond between the PCMs and the Al foam matrix keeps good without visible separation. Also it is noted from Fig. 6b that the impregnation of PCMs improves the durability and resistance to oxidation of Al foams, which is beneficial to its application in energy saving building.

4. Conclusions

Two kinds of shape-stabilized PCMs/Al-foam composites for thermal storage were prepared by impregnating paraffin or stearic acid into bulk open-pore Al foams under constant-pressure/vacuum condition. The effect of impregnating pressure on the filling fraction of PCMs and the apparent density of PCMs/Al-foam composites is unconspicuous. The impregnation effect of the paraffin is better than that of the stearic acid, and thus improves the filling fraction of PCMs (more than 80%) and the latent heat values of PCMs/Al-foam composites. These shape-stabilized PCMs/Al-foam composites exhibit considerable

latent thermal storage potential because of good interface, desirable phase-change temperature range (33.0-70.0 °C for the paraffins/Al-foam composite and 54.7-70.6 °C for the stearic acid/Al-foam composite), latent heat values (72.9 kJ/kg for the paraffins/Al-foam composite and 66.7 kJ/kg for the stearic acid/ Al-foam composite) and enhanced dynamic-compressing yield strength. The impregnation of PCMs also improves durability and resistance to oxidation of Al foams, which is beneficial to its application in energy saving building.

Acknowledgements

Project (51141002) supported by the National Natural Science Foundation of China, Project (200802941010) supported by the Doctoral Program of Higher Education of China, Project (AMM201007) supported by the Opening Project of Jiangsu Key Laboratory of Advanced Metallic Materials.

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