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Effective thermal conductivity of MgH2 compacts containing expanded natural graphite under a hydrogen atmosphere
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 3 4 9 e3 5 5
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Effective thermal conductivity of MgH2 compacts containing expanded natural graphite under a hydrogen atmosphere Jae-Hyeok Shim a,*, Mirae Park a, Young Hyo Lee b, Sanggon Kim c, Yeon Ho Im b,c, Jin-Yoo Suh a, Young Whan Cho a a
High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea b Department of Energy Storage and Conversion Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea c School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea
article info
abstract
Article history:
Expanded natural graphite (ENG) was added to enhance the effective thermal conductivity
Received 5 July 2013
of MgH2, which is one of the important parameters in the design of MgH2-based hydrogen
Received in revised form
storage tanks. Cylindrical MgH2 compacts containing up to 20 wt% ENG flakes with various
3 September 2013
average sizes (20, 50, 200, 350 and 1200 mm) were fabricated to measure the effective
Accepted 16 September 2013
thermal conductivity of MgH2eENG mixtures. The radial direction effective thermal con-
Available online 7 November 2013
ductivity of the compacts was measured under a hydrogen atmosphere up to 70 bar. The conductivity was significantly enhanced by the addition of ENG flakes, reaching
Keywords:
9.3 W m1 K1 at 20 wt% ENG at 1 bar of hydrogen. It was observed that hydrogen pressure
Hydrogen storage
and the size of ENG flakes influenced the conductivity together with the amount of ENG. As
Magnesium hydride
hydrogen pressure increased up to 20 bar, the conductivity continued to increase. On the
Expanded natural graphite
other hand, the conductivity very slowly increased above 20 bar, exhibiting a saturation
Thermal conductivity
tendency. It relatively rapidly increased with increasing average flake size up to 200 mm and
Hydrogen pressure
then gradually decreased with further increasing size up to 1200 mm, exhibiting the maximum value at an average flake size of 200 mm. This trend might be determined by the competition between the thermal resistance at ENG/MgH2 interfaces and the formation of conductive networks of ENG flakes. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Magnesium hydride (MgH2) is a promising solid-state hydrogen storage material due to its high gravimetric (7.6 wt % H2) and volumetric (110 kg H2/m3) hydrogen densities.
Moreover, its good reversibility and cycle performance provide strong advantages over other alkali metal (or alkalineearth metal) binary and complex metal hydrides. Nevertheless, its high equilibrium temperature (285 C at 1 bar hydrogen) and slow sorption kinetics are serious drawbacks
* Corresponding author. Tel.: þ82 2 958 6760; fax: þ82 2 958 5449. E-mail address: [email protected] (J.-H. Shim). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.092
350
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 3 4 9 e3 5 5
that need to be overcome. The addition of transition metal catalytic additives such as V [1], Cr2O3 [2], TiO2 [3], Nb2O5 [4] and NbF5 [5], using ball milling has been shown to be effective in improving the sorption kinetics of MgH2. Recently, there have been attempts to use MgH2 as a solution to store large quantities of hydrogen from renewable energy, such as solar and wind power (e.g. Ref. [6]), as Mg is relatively cheap and abundant and has a high hydrogen capacity. During hydrogenation, a large amount of heat is released due to the highly exothermic hydrogenation reaction with an enthalpy change of 75 kJ/mol H2, which can significantly elevate the temperature of hydrogen storage tanks containing MgH2. If the temperature reaches the equilibrium temperature corresponding to applied hydrogen pressure, the hydrogenation reaction will be stopped or significantly retarded. Therefore, the reaction heat needs to be dissipated as quickly as possible to prevent a serious increase in the temperature of storage tanks during hydrogenation. On the other hand, the heat should be supplied into MgH2 against the endothermic reaction during dehydrogenation. To facilitate the thermal management of storage tanks containing MgH2, the effective thermal conductivity of the powder beds of MgH2, whose thermal conductivity is inherently low (w1 W m1 K1) [7], needs to be significantly improved, in both radial and axial directions of hydrogen storage tanks. Ishido et al. [8] measured the effective thermal conductivity of packed MgH2 powder with a porosity of 0.57e0.65 in the temperature range between room temperature and 200 C under various hydrogen pressures from vacuum to 40 bar. The measured conductivity clearly increases with increasing hydrogen pressure, although it exhibits a saturation tendency above 20 bar. It also tends to increase with decreasing porosity. The measured values are in the range of 0.9e1.6 W m1 K1, except those at vacuum, which are very low (w0.04 W m1 K1). Kapischke and Hapke [9] measured the effective thermal conductivity of hydrogenated Mg powder containing 2 wt% Ni with a porosity of approximately 0.4 in a temperature range of 100e400 C at hydrogen pressure up to 50 bar. The measured values are dependent mainly on the extent of hydrogenation. The effective thermal conductivity significantly decreases as the hydrogenation fraction increases. At the fully hydrogenated state (i.e. MgH2), the measured conductivity reaches approximately 4 W m1 K1. They also used an oscillating heating method to measure the effective thermal conductivity of the same materials, which produces a value of 1.8e2.4 W m1 K1 at the fully hydrogenated state, which is similar to the value of a MgH2 nanostructure containing a small amount of vanadium recently measured by Chen et al. [10]. To enhance the effective thermal conductivity of MgH2 powder beds, there have been attempts to incorporate expanded natural graphite (ENG) flakes, which are known to have very high in-plane thermal conductivity and have been applied to LaNi5 for the same purpose [11e13], into MgH2. Chaise et al. [14] fabricated disc-shaped MgH2 compacts (porosity of approximately 0.4) containing up to 10 wt% ENG by compressing a mixture of MgH2 powder ball milled with 4 at% TieVeCr alloy and ENG flakes (0.1 to several millimeters in size) under a uniaxial stress of 100 MPa. Then, they measured the effective thermal conductivity of the compacts
in both radial and axial directions at room temperature. The addition of the ENG flakes remarkably raised the radial direction effective thermal conductivity of the compacts, which reached approximately 4 W m1 K1 at 5 wt% ENG and approximately 7 W m1 K1 at 10 wt% ENG. On the other hand, the axial direction conductivity tended to increase very slightly. This anisotropy was attributed to the alignment of the ENG flakes perpendicular to the compression direction during uniaxial compression, which was also observed in the measurement of Mg alloyeENG composites by Pohlmann et al. [15]. Later, Delhomme et al. [16] measured the effective thermal conductivity of the same type of compacts after dehydrogenationerehydrogenation cycles. Interestingly, the effective thermal conductivity tended to increase as the number of cycles increased. The purpose of this study was to investigate the behavior of the effective thermal conductivity of MgH2 compacts containing ENG flakes under a hydrogen atmosphere. A miniaturized measurement system of effective thermal conductivity was used for this work, which requires a small sample volume of 15 ml. Based on this system, we measured the radial direction effective thermal conductivity of MgH2 compacts containing ENG flakes under hydrogen pressure up to 70 bar. Also, we investigated the effect of ENG flake size on the effective thermal conductivity by incorporating ENG flakes with various average sizes from 20 to 1200 mm.
2.
Experimental procedures
MgH2 (98% purity, Alpha Aesar, w50 mm size) and NbF5 (98% purity, SigmaeAldrich, w2 mm size) were purchased as raw materials. Also, various ENG powders of more than 95% purity were purchased from the SGL group (average particle sizes of approximately 50, 200, 350, 500 and 1200 mm) and Graphit Kropfmu¨hl (average particle size of approximately 20 mm). Fig. 1 shows the morphology of ENG powders with average particle sizes of 20, 200, and 1200 mm observed by scanning electron microscopy (SEM) (FEI Inspect F50). The particles exhibited flake morphologies with an average thickness of approximately 1, 3, and 10 mm for average particle sizes of 20, 200 and 1200 mm, respectively. A 10 g sample of MgH2 with 5 wt% NbF5 was charged into a 300 ml hardened steel vial together with eleven 12.7 and seventeen 9.5 mm diameter Cr-steel balls and milled at a speed of 350 rpm for 30 min under Ar atmosphere using a Fritsch P5 planetary mill. The ball-to-powder weight ratio was approximately 15:1. NbF5 was added as a catalyst to enhance the reaction kinetics of MgH2. Also, NbF5 is believed to play a role in suppressing the reaction between MgH2 and oxygen or water vapor by forming protective layers, when MgH2 is handled in air [17]. The milled samples were mixed with up to 20 wt% ENG at 100 rpm for an hour under Ar atmosphere in a tumbling mill. In this process, balls were not used to maintain the original size of the ENG flakes. The mixture was so homogeneous that the segregation of ENG flakes was not observed. Next, 8 g samples of the MgH2eENG mixtures were fabricated into compacts by compressing the mixtures in a mold with a uniaxial stress of 100 MPa in air using a hand press
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Fig. 1 e SEM micrographs of ENG powders with average flake sizes of (a) 20, (b) 200 and (c) 1200 mm.
(Carver 3851). Each compact was a cylinder with a diameter of approximately 22 mm and a height of approximately 24 mm, and its density reached approximately 60% of the theoretical density, which is in agreement with the results of Chaise et al. [14]. As shown in Fig. 2, there were five holes at the bottom of
Fig. 2 e MgH2 compact containing ENG flakes fabricated by compressing with a uniaxial stress of 100 MPa.
the compacts, which were made from five vertical rods at the bottom of the mold, for the insertion of a micro sheath heater and thermocouples during the effective thermal conductivity measurement. The effective thermal conductivity of the compacts was measured at room temperature under hydrogen pressure up to 70 bar using a miniaturized measurement system developed through numerical simulation-aided design, as shown in Fig. 3(a). This system was based on an absolute steady-state method using a simple configuration similar to a transient hot-wire system. The total volume of this cylindrical measurement cell was 15 ml, so that a small amount of sample is required to measure the effective thermal conductivity. As described in Fig. 3(b), a micro sheath heater (30 U) with a diameter of 0.5 mm and a length of 15 mm was inserted at the center of the bottom of the measurement cell. Four micro Ktype thermocouples with the same size as the micro sheath heater were positioned along the radial direction. As shown in Fig. 3(b), the minimum intervals in radial direction (D) were 1.5 mm, which was good enough to measure the effective thermal conductivity of the MgH2eENG mixtures in this work. There was a clearance of about 20 mm between a compact and the cell to facilitate the compact load. Furthermore, the microgaps between thermocouples (or heater) and the measurement cell were filled with indium to minimize the contact
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Fig. 3 e (a) Cylindrical cell for the measurement of effective thermal conductivity, (b) bottom view of the cell and (c) schematic illustration of the effective thermal conductivity measurement setup.
resistances. Finally, this measurement cell was assembled to a flange type container, as shown in Fig. 3(c). The effective thermal conductivity in this system was determined by the absolute steady-state method using the measured temperature differences with the applied power of the micro sheath heater. The Fourier’s law of the steady-state heat transfer was utilized for calculation of the effective thermal conductivity using the steady-state temperature difference in the radial direction of the measurement cell. The radial heat flow required in this method was obtained directly from the applied power of the micro sheath heater. After the MgH2 compacts were loaded into the cell, the entire setup was initially evacuated up to 103 mbar. Then, a constant power (10 W) was provided in the microheater using a DC power supply, while the cell was maintained at a constant ambient temperature in a thermostatic oven. After hydrogen was fed into the cell, the predetermined hydrogen pressure was maintained during the measurement period. All temperatures of the thermocouples were monitored at an interval of 1 min until a steady state was reached. The accuracy of the measurement cell was evaluated using LaNi5 as a reference material. The standard measurement errors of the effective thermal conductivity for the LaNi5 samples were within 0.05 W m1 K1 under hydrogen pressure ranging from 10 to
50 bar, indicating that the accuracy of this system is good enough for this work. A detailed analysis of the reliability of this measurement system according to miniaturization will be published elsewhere [18].
3.
Results and discussion
A SEM micrograph of the cross-section of the MgH2 compact containing 5 wt% ENG with an average flake size of 200 mm is shown in Fig. 4. The dispersed ENG flakes clearly tended to be aligned perpendicularly to the compression direction, which is expected to improve the radial direction effective thermal conductivity, as reported by a number of studies [11e15]. Fig. 5 shows the radial direction effective thermal conductivity of the MgH2 compacts containing the ENG flakes with an average size of 200 mm as a function of the ENG content measured under hydrogen pressure of 1 bar. Without ENG, the measured value was approximately 1.9 W m1 K1, which agreed with that measured by Ishido et al. [8] under similar conditions. The effective thermal conductivity clearly increased with increasing ENG content. While it increased slowly with increasing ENG content up to 10 wt%, it exhibited a quite high value at 20 wt% ENG (approximately 9 W m1 K1)
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Fig. 4 e SEM micrograph of the cross-section of the MgH2 compact containing 5 wt% ENG with an average flake size of 200 mm.
out of the trend at low ENG contents. This relatively high conductivity value seems to be related to the formation of a continuous pathway for thermal conduction through the connection of ENG flakes at high ENG contents. This behavior is quite commonly observed in polymer composites containing inorganic particles that are incorporated for the enhancement of effective thermal conductivity [19e21]. The measured conductivity values at 5 and 10 wt% ENG were slightly lower than those observed by Chaise et al. [14]. This difference might be attributed to the presence of a small
Fig. 5 e Radial direction effective thermal conductivity of the MgH2 compacts containing the ENG flakes with an average size of 200 mm as a function of the ENG content measured under hydrogen pressure of 1 bar.
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amount of the metallic additive (TieVeCr alloy) in their samples, which could additionally contribute to the enhancement of effective thermal conductivity. The contribution of NbF5 in our compacts is not clear, as the thermal conductivity of NbF5 is not known. However, the contribution of NbF5 might be lower than that of the metallic additive, because the thermal conductivity of metal halide is generally lower than that of metal [22]. Fig. 6 plots the effective thermal conductivity of the MgH2 compacts with and without 5 wt% ENG (average flake sizes of 20, 200, and 1200 mm) against hydrogen pressure. The plots indicate that the effective thermal conductivity of all the compacts depended on hydrogen pressure, although the values of the compact with ENG were higher than those of the compact without ENG. It increased relatively rapidly with increasing hydrogen pressure up to 20 bar and then exhibited a saturation trend above 20 bar. Very similar behavior was observed in the work of Ishido et al. [8] who measured the effective thermal conductivity of MgH2 compacts under a hydrogen atmosphere. In their work, the saturation trend above 20 bar was distinct after the rapid increase in effective thermal conductivity up to 20 bar, although their values were slightly lower than the measurements in this study. This type behavior is also effective for other metal hydrides such as LaNi5 and TiCrMn hydrides [23e26], which exhibit a saturation trend above 10e20 bar. It is known that filling gas can greatly influence the effective thermal conductivity of porous media. The effect of filling gas will be greater if hydrogen, which has a relatively high thermal conductivity, is filled. The dependence of the effective thermal conductivity of porous media on gas pressure is usually explained by the Knudsen number (Kn), which is defined by the mean free path of gas molecules divided by the pore size in this case [24]. At Kn > 0.01, the mean free path approaches the order of magnitude of the pore size; the collision between gas molecules increases with increasing gas pressure, and the exchange of energy between gas molecules is directly proportional to the number of gas molecules. Therefore, the effective thermal conductivity
Fig. 6 e Radial direction effective thermal conductivity of the MgH2 compacts with and without 5 wt% ENG flakes with an average size of 20, 200 and 1200 mm as a function of hydrogen pressure.
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increases with increasing gas pressure in this region. At Kn < 0.01, the mean free path is much lower than the pore size; the stagnant gas at high pressure can be considered as a continuum with a thermal conductivity independent of gas pressure, which seems to explain the saturation trend of the effective thermal conductivity above 20 bar. The effective thermal conductivity of the MgH2 compact containing a higher ENG content with an average flake size of 200 mm (10 and 20 wt%) is plotted as a function of hydrogen pressure in Fig. 7. On the whole, the effect of hydrogen pressure on the thermal conductivity was similar to the result of the compact with 5 wt% ENG. However, the conductivity values at 10 and 20 wt% ENG were higher than those at 5 wt%. The conductivity reached 5.7 and 13.4 W m1 K1 at 10 and 20 wt% ENG, respectively, under hydrogen pressure of 70 bar. The effective thermal conductivity of the MgH2 compact containing 5 wt% ENG under hydrogen pressure of 1 and 20 bar is presented as a function of the average flake size of ENG (Fig. 8). It increased relatively rapidly with increasing flake size up to 200 mm, irrespective of hydrogen pressure. Then, it tended to gradually decreases as the size further increased. Therefore, it exhibited the maximum around the size of 200 mm (3.4 and 4.5 W m1 K1 at 1 and 20 bar, respectively). Although there have been many reports on the enhancement of the effective thermal conductivity of metal hydrides and polymers using ENG [11e16,27e30], the influence of the size of ENG on the effective thermal conductivity has not been previously investigated. It is known that the thermal conduction in non-metals is accomplished by the flow of phonons or lattice vibration [31]. The rapid decrease in effective thermal conductivity with decreasing flake size below 200 mm might be explained by the effect of the interfacial resistance of thermal conduction. As the flake size decreases, the interfacial area between MgH2 and ENG increases; hence, the thermal resistance increases due to phonon scattering at the interfaces [32]. It is not easily understood why the effective thermal conductivity gradually decreased with increasing flake size above 200 mm. This phenomenon seems to be related to the connectivity of the conductive pathways of ENG flakes. As indicated by Zheng et al. [27], the probability of the
Fig. 7 e Radial direction effective thermal conductivity of the MgH2 compacts with and without 10 and 20 wt% ENG flakes with an average size of 200 mm as a function of hydrogen pressure.
Fig. 8 e Radial direction effective thermal conductivity of the MgH2 compact containing 5 wt% ENG flakes as a function of average flake size of ENG measured under hydrogen pressure of 1 and 20 bar. formation of a network of the conductive pathways decreases with increasing ENG flake size, which leads to decreased effective thermal conductivity. Therefore, it is thought that the influence of ENG flake size on the effective thermal conductivity is determined by the competition between the interfacial resistance of thermal conduction and the connectivity of thermal conduction pathways. It is believed that the 5 wt% addition of ENG to MgH2 in this study led to the formation of percolated networks of ENG flakes to some extent, as the percolation threshold of ENG in polymer is known to be quite low (1e3 wt%) [33]. The form factor of each ENG flake might not have a significant influence on the conductivity behavior because no distinct difference in the morphology of ENG flakes with respect to the average size was found (Fig. 1).
4.
Conclusions
Cylindrical MgH2 compacts containing up to 20 wt% ENG flakes with an average size of 20e1200 mm were fabricated by the uniaxial compression at 100 MPa. A small amount of NbF5 was also included to enhance the kinetics of the dehydrogenation and hydrogenation reactions of MgH2 and to protect MgH2 from oxygen and water vapor in air. The density of the compacts reached approximately 60% of the theoretical density. It was observed that the dispersed ENG flakes were aligned perpendicularly to the compression direction. The radial direction effective thermal conductivity of the compacts, which is one of the key parameters in the design of hydrogen storage tanks, was measured under a hydrogen atmosphere up to 70 bar using a miniaturized system for measuring the effective thermal conductivity. The addition of ENG flakes clearly contributes to the enhancement of the radial direction effective thermal conductivity. The measured conductivity almost linearly increased with increasing ENG content up to 10 wt% (4.4 W m1 K1 at 10 wt% ENG under hydrogen pressure of 1 bar) and it tended to relatively rapidly increase above 10 wt% (9.3 W m1 K1 at 20 wt% ENG under hydrogen
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 3 4 9 e3 5 5
pressure of 1 bar). It was found that hydrogen pressure influenced the conductivity. It increased with increasing hydrogen pressure up to 20 bar, whereas it exhibited a saturation trend above 20 bar. This saturation trend is explained by stagnant gas molecules with a low mean free path at high pressure, whose effective thermal conductivity is independent of gas pressure. Also, the measured conductivity was found to be a function of the average ENG flake size. It relatively rapidly increased with increasing flake size up to 200 mm and then gradually decreased with further increasing size, exhibiting the maximum value around 200 mm. This behavior seems to be determined by the competition between the thermal resistance at ENG/MgH2 interfaces and the formation of the conductive networks of ENG flakes.
Acknowledgment This work has been supported by the KIST Institutional Programs (Project Nos. 2E24022 and 2E24070).
references
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Effective thermal conductivity of MgH2 compacts containing expanded natural graphite under a hydrogen atmosphere Jae-Hyeok Shim a,*, Mirae Park a, Young Hyo Lee b, Sanggon Kim c, Yeon Ho Im b,c, Jin-Yoo Suh a, Young Whan Cho a a
High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea b Department of Energy Storage and Conversion Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea c School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea
article info
abstract
Article history:
Expanded natural graphite (ENG) was added to enhance the effective thermal conductivity
Received 5 July 2013
of MgH2, which is one of the important parameters in the design of MgH2-based hydrogen
Received in revised form
storage tanks. Cylindrical MgH2 compacts containing up to 20 wt% ENG flakes with various
3 September 2013
average sizes (20, 50, 200, 350 and 1200 mm) were fabricated to measure the effective
Accepted 16 September 2013
thermal conductivity of MgH2eENG mixtures. The radial direction effective thermal con-
Available online 7 November 2013
ductivity of the compacts was measured under a hydrogen atmosphere up to 70 bar. The conductivity was significantly enhanced by the addition of ENG flakes, reaching
Keywords:
9.3 W m1 K1 at 20 wt% ENG at 1 bar of hydrogen. It was observed that hydrogen pressure
Hydrogen storage
and the size of ENG flakes influenced the conductivity together with the amount of ENG. As
Magnesium hydride
hydrogen pressure increased up to 20 bar, the conductivity continued to increase. On the
Expanded natural graphite
other hand, the conductivity very slowly increased above 20 bar, exhibiting a saturation
Thermal conductivity
tendency. It relatively rapidly increased with increasing average flake size up to 200 mm and
Hydrogen pressure
then gradually decreased with further increasing size up to 1200 mm, exhibiting the maximum value at an average flake size of 200 mm. This trend might be determined by the competition between the thermal resistance at ENG/MgH2 interfaces and the formation of conductive networks of ENG flakes. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Magnesium hydride (MgH2) is a promising solid-state hydrogen storage material due to its high gravimetric (7.6 wt % H2) and volumetric (110 kg H2/m3) hydrogen densities.
Moreover, its good reversibility and cycle performance provide strong advantages over other alkali metal (or alkalineearth metal) binary and complex metal hydrides. Nevertheless, its high equilibrium temperature (285 C at 1 bar hydrogen) and slow sorption kinetics are serious drawbacks
* Corresponding author. Tel.: þ82 2 958 6760; fax: þ82 2 958 5449. E-mail address: [email protected] (J.-H. Shim). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.092
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that need to be overcome. The addition of transition metal catalytic additives such as V [1], Cr2O3 [2], TiO2 [3], Nb2O5 [4] and NbF5 [5], using ball milling has been shown to be effective in improving the sorption kinetics of MgH2. Recently, there have been attempts to use MgH2 as a solution to store large quantities of hydrogen from renewable energy, such as solar and wind power (e.g. Ref. [6]), as Mg is relatively cheap and abundant and has a high hydrogen capacity. During hydrogenation, a large amount of heat is released due to the highly exothermic hydrogenation reaction with an enthalpy change of 75 kJ/mol H2, which can significantly elevate the temperature of hydrogen storage tanks containing MgH2. If the temperature reaches the equilibrium temperature corresponding to applied hydrogen pressure, the hydrogenation reaction will be stopped or significantly retarded. Therefore, the reaction heat needs to be dissipated as quickly as possible to prevent a serious increase in the temperature of storage tanks during hydrogenation. On the other hand, the heat should be supplied into MgH2 against the endothermic reaction during dehydrogenation. To facilitate the thermal management of storage tanks containing MgH2, the effective thermal conductivity of the powder beds of MgH2, whose thermal conductivity is inherently low (w1 W m1 K1) [7], needs to be significantly improved, in both radial and axial directions of hydrogen storage tanks. Ishido et al. [8] measured the effective thermal conductivity of packed MgH2 powder with a porosity of 0.57e0.65 in the temperature range between room temperature and 200 C under various hydrogen pressures from vacuum to 40 bar. The measured conductivity clearly increases with increasing hydrogen pressure, although it exhibits a saturation tendency above 20 bar. It also tends to increase with decreasing porosity. The measured values are in the range of 0.9e1.6 W m1 K1, except those at vacuum, which are very low (w0.04 W m1 K1). Kapischke and Hapke [9] measured the effective thermal conductivity of hydrogenated Mg powder containing 2 wt% Ni with a porosity of approximately 0.4 in a temperature range of 100e400 C at hydrogen pressure up to 50 bar. The measured values are dependent mainly on the extent of hydrogenation. The effective thermal conductivity significantly decreases as the hydrogenation fraction increases. At the fully hydrogenated state (i.e. MgH2), the measured conductivity reaches approximately 4 W m1 K1. They also used an oscillating heating method to measure the effective thermal conductivity of the same materials, which produces a value of 1.8e2.4 W m1 K1 at the fully hydrogenated state, which is similar to the value of a MgH2 nanostructure containing a small amount of vanadium recently measured by Chen et al. [10]. To enhance the effective thermal conductivity of MgH2 powder beds, there have been attempts to incorporate expanded natural graphite (ENG) flakes, which are known to have very high in-plane thermal conductivity and have been applied to LaNi5 for the same purpose [11e13], into MgH2. Chaise et al. [14] fabricated disc-shaped MgH2 compacts (porosity of approximately 0.4) containing up to 10 wt% ENG by compressing a mixture of MgH2 powder ball milled with 4 at% TieVeCr alloy and ENG flakes (0.1 to several millimeters in size) under a uniaxial stress of 100 MPa. Then, they measured the effective thermal conductivity of the compacts
in both radial and axial directions at room temperature. The addition of the ENG flakes remarkably raised the radial direction effective thermal conductivity of the compacts, which reached approximately 4 W m1 K1 at 5 wt% ENG and approximately 7 W m1 K1 at 10 wt% ENG. On the other hand, the axial direction conductivity tended to increase very slightly. This anisotropy was attributed to the alignment of the ENG flakes perpendicular to the compression direction during uniaxial compression, which was also observed in the measurement of Mg alloyeENG composites by Pohlmann et al. [15]. Later, Delhomme et al. [16] measured the effective thermal conductivity of the same type of compacts after dehydrogenationerehydrogenation cycles. Interestingly, the effective thermal conductivity tended to increase as the number of cycles increased. The purpose of this study was to investigate the behavior of the effective thermal conductivity of MgH2 compacts containing ENG flakes under a hydrogen atmosphere. A miniaturized measurement system of effective thermal conductivity was used for this work, which requires a small sample volume of 15 ml. Based on this system, we measured the radial direction effective thermal conductivity of MgH2 compacts containing ENG flakes under hydrogen pressure up to 70 bar. Also, we investigated the effect of ENG flake size on the effective thermal conductivity by incorporating ENG flakes with various average sizes from 20 to 1200 mm.
2.
Experimental procedures
MgH2 (98% purity, Alpha Aesar, w50 mm size) and NbF5 (98% purity, SigmaeAldrich, w2 mm size) were purchased as raw materials. Also, various ENG powders of more than 95% purity were purchased from the SGL group (average particle sizes of approximately 50, 200, 350, 500 and 1200 mm) and Graphit Kropfmu¨hl (average particle size of approximately 20 mm). Fig. 1 shows the morphology of ENG powders with average particle sizes of 20, 200, and 1200 mm observed by scanning electron microscopy (SEM) (FEI Inspect F50). The particles exhibited flake morphologies with an average thickness of approximately 1, 3, and 10 mm for average particle sizes of 20, 200 and 1200 mm, respectively. A 10 g sample of MgH2 with 5 wt% NbF5 was charged into a 300 ml hardened steel vial together with eleven 12.7 and seventeen 9.5 mm diameter Cr-steel balls and milled at a speed of 350 rpm for 30 min under Ar atmosphere using a Fritsch P5 planetary mill. The ball-to-powder weight ratio was approximately 15:1. NbF5 was added as a catalyst to enhance the reaction kinetics of MgH2. Also, NbF5 is believed to play a role in suppressing the reaction between MgH2 and oxygen or water vapor by forming protective layers, when MgH2 is handled in air [17]. The milled samples were mixed with up to 20 wt% ENG at 100 rpm for an hour under Ar atmosphere in a tumbling mill. In this process, balls were not used to maintain the original size of the ENG flakes. The mixture was so homogeneous that the segregation of ENG flakes was not observed. Next, 8 g samples of the MgH2eENG mixtures were fabricated into compacts by compressing the mixtures in a mold with a uniaxial stress of 100 MPa in air using a hand press
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Fig. 1 e SEM micrographs of ENG powders with average flake sizes of (a) 20, (b) 200 and (c) 1200 mm.
(Carver 3851). Each compact was a cylinder with a diameter of approximately 22 mm and a height of approximately 24 mm, and its density reached approximately 60% of the theoretical density, which is in agreement with the results of Chaise et al. [14]. As shown in Fig. 2, there were five holes at the bottom of
Fig. 2 e MgH2 compact containing ENG flakes fabricated by compressing with a uniaxial stress of 100 MPa.
the compacts, which were made from five vertical rods at the bottom of the mold, for the insertion of a micro sheath heater and thermocouples during the effective thermal conductivity measurement. The effective thermal conductivity of the compacts was measured at room temperature under hydrogen pressure up to 70 bar using a miniaturized measurement system developed through numerical simulation-aided design, as shown in Fig. 3(a). This system was based on an absolute steady-state method using a simple configuration similar to a transient hot-wire system. The total volume of this cylindrical measurement cell was 15 ml, so that a small amount of sample is required to measure the effective thermal conductivity. As described in Fig. 3(b), a micro sheath heater (30 U) with a diameter of 0.5 mm and a length of 15 mm was inserted at the center of the bottom of the measurement cell. Four micro Ktype thermocouples with the same size as the micro sheath heater were positioned along the radial direction. As shown in Fig. 3(b), the minimum intervals in radial direction (D) were 1.5 mm, which was good enough to measure the effective thermal conductivity of the MgH2eENG mixtures in this work. There was a clearance of about 20 mm between a compact and the cell to facilitate the compact load. Furthermore, the microgaps between thermocouples (or heater) and the measurement cell were filled with indium to minimize the contact
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Fig. 3 e (a) Cylindrical cell for the measurement of effective thermal conductivity, (b) bottom view of the cell and (c) schematic illustration of the effective thermal conductivity measurement setup.
resistances. Finally, this measurement cell was assembled to a flange type container, as shown in Fig. 3(c). The effective thermal conductivity in this system was determined by the absolute steady-state method using the measured temperature differences with the applied power of the micro sheath heater. The Fourier’s law of the steady-state heat transfer was utilized for calculation of the effective thermal conductivity using the steady-state temperature difference in the radial direction of the measurement cell. The radial heat flow required in this method was obtained directly from the applied power of the micro sheath heater. After the MgH2 compacts were loaded into the cell, the entire setup was initially evacuated up to 103 mbar. Then, a constant power (10 W) was provided in the microheater using a DC power supply, while the cell was maintained at a constant ambient temperature in a thermostatic oven. After hydrogen was fed into the cell, the predetermined hydrogen pressure was maintained during the measurement period. All temperatures of the thermocouples were monitored at an interval of 1 min until a steady state was reached. The accuracy of the measurement cell was evaluated using LaNi5 as a reference material. The standard measurement errors of the effective thermal conductivity for the LaNi5 samples were within 0.05 W m1 K1 under hydrogen pressure ranging from 10 to
50 bar, indicating that the accuracy of this system is good enough for this work. A detailed analysis of the reliability of this measurement system according to miniaturization will be published elsewhere [18].
3.
Results and discussion
A SEM micrograph of the cross-section of the MgH2 compact containing 5 wt% ENG with an average flake size of 200 mm is shown in Fig. 4. The dispersed ENG flakes clearly tended to be aligned perpendicularly to the compression direction, which is expected to improve the radial direction effective thermal conductivity, as reported by a number of studies [11e15]. Fig. 5 shows the radial direction effective thermal conductivity of the MgH2 compacts containing the ENG flakes with an average size of 200 mm as a function of the ENG content measured under hydrogen pressure of 1 bar. Without ENG, the measured value was approximately 1.9 W m1 K1, which agreed with that measured by Ishido et al. [8] under similar conditions. The effective thermal conductivity clearly increased with increasing ENG content. While it increased slowly with increasing ENG content up to 10 wt%, it exhibited a quite high value at 20 wt% ENG (approximately 9 W m1 K1)
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Fig. 4 e SEM micrograph of the cross-section of the MgH2 compact containing 5 wt% ENG with an average flake size of 200 mm.
out of the trend at low ENG contents. This relatively high conductivity value seems to be related to the formation of a continuous pathway for thermal conduction through the connection of ENG flakes at high ENG contents. This behavior is quite commonly observed in polymer composites containing inorganic particles that are incorporated for the enhancement of effective thermal conductivity [19e21]. The measured conductivity values at 5 and 10 wt% ENG were slightly lower than those observed by Chaise et al. [14]. This difference might be attributed to the presence of a small
Fig. 5 e Radial direction effective thermal conductivity of the MgH2 compacts containing the ENG flakes with an average size of 200 mm as a function of the ENG content measured under hydrogen pressure of 1 bar.
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amount of the metallic additive (TieVeCr alloy) in their samples, which could additionally contribute to the enhancement of effective thermal conductivity. The contribution of NbF5 in our compacts is not clear, as the thermal conductivity of NbF5 is not known. However, the contribution of NbF5 might be lower than that of the metallic additive, because the thermal conductivity of metal halide is generally lower than that of metal [22]. Fig. 6 plots the effective thermal conductivity of the MgH2 compacts with and without 5 wt% ENG (average flake sizes of 20, 200, and 1200 mm) against hydrogen pressure. The plots indicate that the effective thermal conductivity of all the compacts depended on hydrogen pressure, although the values of the compact with ENG were higher than those of the compact without ENG. It increased relatively rapidly with increasing hydrogen pressure up to 20 bar and then exhibited a saturation trend above 20 bar. Very similar behavior was observed in the work of Ishido et al. [8] who measured the effective thermal conductivity of MgH2 compacts under a hydrogen atmosphere. In their work, the saturation trend above 20 bar was distinct after the rapid increase in effective thermal conductivity up to 20 bar, although their values were slightly lower than the measurements in this study. This type behavior is also effective for other metal hydrides such as LaNi5 and TiCrMn hydrides [23e26], which exhibit a saturation trend above 10e20 bar. It is known that filling gas can greatly influence the effective thermal conductivity of porous media. The effect of filling gas will be greater if hydrogen, which has a relatively high thermal conductivity, is filled. The dependence of the effective thermal conductivity of porous media on gas pressure is usually explained by the Knudsen number (Kn), which is defined by the mean free path of gas molecules divided by the pore size in this case [24]. At Kn > 0.01, the mean free path approaches the order of magnitude of the pore size; the collision between gas molecules increases with increasing gas pressure, and the exchange of energy between gas molecules is directly proportional to the number of gas molecules. Therefore, the effective thermal conductivity
Fig. 6 e Radial direction effective thermal conductivity of the MgH2 compacts with and without 5 wt% ENG flakes with an average size of 20, 200 and 1200 mm as a function of hydrogen pressure.
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increases with increasing gas pressure in this region. At Kn < 0.01, the mean free path is much lower than the pore size; the stagnant gas at high pressure can be considered as a continuum with a thermal conductivity independent of gas pressure, which seems to explain the saturation trend of the effective thermal conductivity above 20 bar. The effective thermal conductivity of the MgH2 compact containing a higher ENG content with an average flake size of 200 mm (10 and 20 wt%) is plotted as a function of hydrogen pressure in Fig. 7. On the whole, the effect of hydrogen pressure on the thermal conductivity was similar to the result of the compact with 5 wt% ENG. However, the conductivity values at 10 and 20 wt% ENG were higher than those at 5 wt%. The conductivity reached 5.7 and 13.4 W m1 K1 at 10 and 20 wt% ENG, respectively, under hydrogen pressure of 70 bar. The effective thermal conductivity of the MgH2 compact containing 5 wt% ENG under hydrogen pressure of 1 and 20 bar is presented as a function of the average flake size of ENG (Fig. 8). It increased relatively rapidly with increasing flake size up to 200 mm, irrespective of hydrogen pressure. Then, it tended to gradually decreases as the size further increased. Therefore, it exhibited the maximum around the size of 200 mm (3.4 and 4.5 W m1 K1 at 1 and 20 bar, respectively). Although there have been many reports on the enhancement of the effective thermal conductivity of metal hydrides and polymers using ENG [11e16,27e30], the influence of the size of ENG on the effective thermal conductivity has not been previously investigated. It is known that the thermal conduction in non-metals is accomplished by the flow of phonons or lattice vibration [31]. The rapid decrease in effective thermal conductivity with decreasing flake size below 200 mm might be explained by the effect of the interfacial resistance of thermal conduction. As the flake size decreases, the interfacial area between MgH2 and ENG increases; hence, the thermal resistance increases due to phonon scattering at the interfaces [32]. It is not easily understood why the effective thermal conductivity gradually decreased with increasing flake size above 200 mm. This phenomenon seems to be related to the connectivity of the conductive pathways of ENG flakes. As indicated by Zheng et al. [27], the probability of the
Fig. 7 e Radial direction effective thermal conductivity of the MgH2 compacts with and without 10 and 20 wt% ENG flakes with an average size of 200 mm as a function of hydrogen pressure.
Fig. 8 e Radial direction effective thermal conductivity of the MgH2 compact containing 5 wt% ENG flakes as a function of average flake size of ENG measured under hydrogen pressure of 1 and 20 bar. formation of a network of the conductive pathways decreases with increasing ENG flake size, which leads to decreased effective thermal conductivity. Therefore, it is thought that the influence of ENG flake size on the effective thermal conductivity is determined by the competition between the interfacial resistance of thermal conduction and the connectivity of thermal conduction pathways. It is believed that the 5 wt% addition of ENG to MgH2 in this study led to the formation of percolated networks of ENG flakes to some extent, as the percolation threshold of ENG in polymer is known to be quite low (1e3 wt%) [33]. The form factor of each ENG flake might not have a significant influence on the conductivity behavior because no distinct difference in the morphology of ENG flakes with respect to the average size was found (Fig. 1).
4.
Conclusions
Cylindrical MgH2 compacts containing up to 20 wt% ENG flakes with an average size of 20e1200 mm were fabricated by the uniaxial compression at 100 MPa. A small amount of NbF5 was also included to enhance the kinetics of the dehydrogenation and hydrogenation reactions of MgH2 and to protect MgH2 from oxygen and water vapor in air. The density of the compacts reached approximately 60% of the theoretical density. It was observed that the dispersed ENG flakes were aligned perpendicularly to the compression direction. The radial direction effective thermal conductivity of the compacts, which is one of the key parameters in the design of hydrogen storage tanks, was measured under a hydrogen atmosphere up to 70 bar using a miniaturized system for measuring the effective thermal conductivity. The addition of ENG flakes clearly contributes to the enhancement of the radial direction effective thermal conductivity. The measured conductivity almost linearly increased with increasing ENG content up to 10 wt% (4.4 W m1 K1 at 10 wt% ENG under hydrogen pressure of 1 bar) and it tended to relatively rapidly increase above 10 wt% (9.3 W m1 K1 at 20 wt% ENG under hydrogen
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pressure of 1 bar). It was found that hydrogen pressure influenced the conductivity. It increased with increasing hydrogen pressure up to 20 bar, whereas it exhibited a saturation trend above 20 bar. This saturation trend is explained by stagnant gas molecules with a low mean free path at high pressure, whose effective thermal conductivity is independent of gas pressure. Also, the measured conductivity was found to be a function of the average ENG flake size. It relatively rapidly increased with increasing flake size up to 200 mm and then gradually decreased with further increasing size, exhibiting the maximum value around 200 mm. This behavior seems to be determined by the competition between the thermal resistance at ENG/MgH2 interfaces and the formation of the conductive networks of ENG flakes.
Acknowledgment This work has been supported by the KIST Institutional Programs (Project Nos. 2E24022 and 2E24070).
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