Improvement in the Hydrogen Storage Properties of MgH2 by Adding NaAlH4

Young Jun Kwak; Myoung Youp Song; Ki-Tae Lee

doi:10.20944/preprints202401.0201.v1

Submitted:

01 January 2024

Posted:

03 January 2024

You are already at the latest version

Abstract

Milled MgH2, NaAlH4, MgH2–10NaAlH4, MgH2–30NaAlH4, MgH2–50NaAlH4, and MgH2–2Ni-10NaAlH4 samples were prepared by milling in hydrogen atmosphere (reactive mechanical milling, RMM). Decomposition temperatures of milled MgH2, NaAlH4, MgH2–10NaAlH4, and MgH2–30NaAlH4 were examined in a Sieverts-type hydrogen-absorption and release apparatus, in which the hydrogen pressures were kept constant during hydrogen absorption or release. As the content of NaAlH4 in the sample increased, the temperature at the highest peak in the ratio of increase in Hr to increase in T, dHr/dT, versus T curve decreased. The higher content of NaAlH4 is believed to have made effects of reactive mechanical milling stronger. Effects of the NaAlH4 content on the temperatures of intermediate reactions were studied for MgH2 – NaAlH4 composites. Phase formation in the cycled MgH2–50NaAlH4 were investigated. The hydrogen-storage properties of MgH2–30NaAlH4 were compared with those of Mg-based alloys in which oxide, halides, or fluoride was added.

Keywords:

hydrogen storage

;

MgH2

;

NaAlH4

;

decomposition temperature

;

hydrogen absorption

;

release rates

Subject:

Chemistry and Materials Science - Materials Science and Technology

1. Introduction

Clean alternative energies have drawn interest to prevent air pollution and climate change. One of the clean alternative energies is hydrogen energy, which is the energy produced by the reaction of hydrogen with oxygen, producing water as a by-product. The problems to be solved for applying the hydrogen energy to practical use are hydrogen production and storage. One of the methods to store hydrogen is using metal hydrides. We are interested in synthesizing Mg-based hydrides with high reaction rates and high hydrogen storage capacities.

Many works were performed to improve the hydriding and dehydriding kinetics of Mg [1,2,3,4,5,6]. Researchers were interested in improving the hydrogen storage properties of MgH₂ by adding NaAlH₄ with a high hydrogen-storage capacity [7,8,9,10,11]. Ali and Ismail [7] reviewed the hydrogen storage properties of the Mg–Na–Al system. The complex hydride NaAlH₄ releases hydrogen via three step reactions:

At 458 – 503 K, 3NaAlH₄ → Na₃AlH₆ + 2Al + 3H₂

(1)

At 533 K, Na₃AlH₆ → 3NaH + Al + ½H₂

(2)

At 708 K, 3NaH → 3Na + (3/2)H₂.

(3)

Ali and Ismail [7] reported that addition of NaAlH4 could destabilize the MgH2 effectively and the hydrogen storage properties of NaAlH4 has could also be improved by adding MgH₂. The MgH₂–NaAlH₄ system exhibited much better dehydriding properties than unary MgH₂ and NaAlH₄.

Ismail et al. [10] reported that the following reaction takes place within the temperature range from 443 to 485 K:

NaAlH₄ + MgH₂ → NaMgH₃ + Al + 1.5H₂

(4)

A mixing decomposition of (4) the reaction of MgH₂ with Al and (5) the decomposition of the excessive MgH₂ occurs reversibly between 553 K and 603 K [10].

17MgH₂ + 12Al ⇔ Mg₁₇Al₁₂ + 17H₂

(5)

MgH₂ ⇔ Mg + H₂

(6)

They also reported that NaMgH₃ decomposes between 603 K and 633 K by the following reversible reaction:

NaMgH₃ ⇔ NaH + Mg + H₂

(7)

NaH decomposes between 633 K and 648 K by the following reversible reaction [10]:

NaH ⇔ Na + (1/2)H₂.

(8)

In this work, milled MgH₂, NaAlH₄, MgH₂–10NaAlH₄ (with a composition of 90 wt% MgH₂ + 10 wt% NaAlH₄), MgH₂–30NaAlH₄ (70 wt% MgH₂ + 30 wt% NaAlH₄), MgH₂–50NaAlH₄ (50 wt% MgH₂ + 50 wt% NaAlH₄), and MgH₂–2Ni-10NaAlH₄ (88 wt% MgH₂ + 2 wt% Ni + 10wt% NaAlH₄) samples were prepared by milling in hydrogen atmosphere (reactive mechanical milling, RMM). Decomposition temperatures of milled MgH₂, NaAlH₄, MgH₂–10NaAlH₄, and MgH₂–30NaAlH₄ were examined heating at a rate of 5 ∼ 6 K in a Sieverts-type hydrogen-absorption and release apparatus and phase formation in cycled MgH₂–50NaAlH₄ were investigated.

2. Experimental Procedures

As starting materials, were used MgH₂ (magnesium hydride, purity 98%, Alfa Aesar), NaAlH₄ (Hydrogen-storage grade, Aldrich), and Ni (average particle size 2.2–3.0 μm, purity 99.9 % metal basis, C typically < 0.1%, Alfa Aesar).

Reactive mechanical milling (RMM) was carried out in a planetary ball mill (Planetary Mono Mill; Pulverisette 6, Fritsch). A mixture with the planned composition (total weight = 8 g) was mixed in a stainless-steel container (with 105 hardened steel balls, total weight = 360 g) sealed hermetically, the sample to ball weight ratio being 1/45. All samples were handled in a glove box in argon atmosphere. The disc revolution speed was 400 rpm. The mill container (volume of 250 ml) was then filled with high purity hydrogen gas of about 12 bar. The RMM was performed for 6 h (by repeating milling for 15 min and pause for 5 min 24 times). Hydrogen was refilled every 2 h milling (every eight milling times).

The absorbed or released hydrogen quantity was measured as a function of time by a volumetric method, using Siverts-type hydrogen-absorption and release apparatus previously described [21]. The hydrogen pressure in the reactor was kept as 12 bar during hydriding by dosing the quantity of hydrogen absorbed from a reservoir of known volume. The hydrogen pressure in the reactor was kept as 1.0 bar during dehydriding by removing the quantity of hydrogen released from the reactor to a reservoir of known volume. 0.5 g of the samples was used for these measurements. Samples after hydrogen absorption-release cycling were characterized by X-ray diffraction (XRD) with Cu Ka radiation, using a Bruker D8 Advance (Karlsruhe, Germany) powder diffractometer.

3. Results

The quantity of released hydrogen, H_r (wt% H), was calculated using the sample weight as a standard.

Figure 1 shows quantity of released hydrogen (H_r) versus temperature T curves and the ratio of increase in H_r to increase in T, dH_r/dT, versus T curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄ samples. The samples were heated at a heating rate of 5 ∼ 6 K in 1.0 bar hydrogen. Table 1 presents the temperatures (K) at peaks in the dH_r/dT versus T curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH4 samples. The highest peaks appear at 638, 600, 592, and 455 K, respectively, for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄.

Figure 2 shows the quantity of released hydrogen (H_r) versus temperature curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄ samples. The samples were heated at a heating rate of 5 ∼ 6 K in 1.0 bar hydrogen. The points were marked so that they correspond to the beginning and ending points of the peaks in Figure 1. Table 2 presents the temperatures (K) at the marked points in the H_r versus T curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄ samples.

Hydrogen release begins at 627 K for the as-milled MgH₂. Hydrogen release from the NaAlH₄ begins at 438 K and the slope of the H_r versus temperature curve then changes at 488, 495, and 625 K. MgH₂-10NaAlH₄ begins to release hydrogen at 508 K and slopes of the H_r versus temperature curves change at 525 K and 550 K. MgH₂-30NaAlH₄ begins to release hydrogen at 480 K and slopes of the H_r versus temperature curves change at 541, 562, and 633 K.

As the content of NaAlH₄ in the sample increases, the temperature at the highest peak decreases. The higher content of NaAlH₄ is believed to have made effects of reactive mechanical grinding stronger, lowering the temperatures for the reaction. The effects of reactive mechanical grinding are thought to be creation of defects, making cracks and clean surfaces, and decreasing particle sizes.

The XRD pattern of MgH₂-50NaAlH₄ dehydrided after number of cycles, n, of 4 at 593 K is shown in Figure 3. The sample contains Al, MgO, Al₃Mg₂, MgH₂, and Mg₁₇Al₁₂. Even though the sample was dehydrided, a small amount of MgH₂ remains. Al, formed from the reactions (4), is believed to react with Mg (formed by decomposition of MgH₂) and form Al₃Mg₂ and Mg₁₇Al₁₂ by the following reactions:

3Al + 2Mg → Al₃Mg₂

(9)

12Al + 17Mg → Mg₁₇Al₁₂.

(10)

Liu et al. [12] reported that when the temperature is increased to 633 K, a large amount of hydrogen has been released and two new phases, Mg₁₇Al₁₂ and Mg, are formed while the preformed Al and Al₃Mg₂ disappear. Samples were easily ignited and combusted during treatment, making difficult the obtention of XRD patterns and leading to the formation of a strong peak of MgO and relatively weak peaks of other phases.

Figure 3. XRD pattern of MgH₂-50NaAlH₄ dehydrided after number of cycles, n, of 4 at 593 K.

Figure 4 shows the change of H_a versus time t curve at 593 K in 12 bar hydrogen with cycle number, n, for MgH₂-30NaAlH₄. The effective hydrogen-storage capacity is defined as the quantity of hydrogen absorbed for 60 min (wt% H). As n increases from one to two, the hydrogen-absorption rate for 1 min increases, and from n = 2 to n =4 the initial hydriding rate decreases. In a similar way, the effective hydrogen-storage capacity increases as n increases from one to two, and from n = 2 to n = 4 the effective hydrogen-storage capacity decreases. The activation is considered to have been completed after n = 2. At n = 2, MgH₂-30NaAlH₄ absorbs 4.09 wt% H for 1 min, 7.17 wt% H for 10 min, and 7.42 wt% H for 60 min.

The change of H_r versus t curve at 593 K in 1.0 bar hydrogen with n for MgH₂-30NaAlH₄ is shown in Figure 5. As the number of cycles (n) increases from one to four, the hydrogen-release rate for 1 min decreases. The hydrogen-release rate for 1 min at n = 1 and n = 2 are very similar. The quantity of hydrogen released for 60 min increases as n increases from one to two, and from n = 2 to n =4 the quantity of hydrogen released for 60 min decreases. At n = 2, MgH₂-30NaAlH₄ releases 1.31 wt% H for 1 min, 4.95 wt% H for 10 min, and 7.34 wt% H for 60 min.

During hydrogen absorption (Figure 3) and release (Figure 4), the reactions (5) and (6) are believed to occur.

Figure 6 shows changes in H_a versus time t curve at 593 K in 12 bar hydrogen and H_r versus t curve at 593 K in 1.0 bar hydrogen with cycle number, n, for MgH₂-30NaAlH₄. The curves show that activation is completed after n = 2, showing the highest hydrogen-absorption rate, the highest hydrogen-release rate, the largest effective hydrogen-storage capacity, and the largest quantity of hydrogen released for 60 min.

The H_a versus t curves in 12 bar hydrogen at 593 K or 623 K at n = 1 and n = 2 for MgH₂-50NaAlH₄ are shown in Figure 7. Because the hydrogen-absorption rate was low, experiments were performed several times. Even though the samples were handled in an Ar atmosphere, the samples were ignited and combusted partly, leading to low hydrogen-absorption rates and low effective hydrogen storage capacities. For n = 1 at 593 K, the MgH₂-50NaAlH₄ sample does not absorb hydrogen. For n = 2 at 623 K, the MgH₂-50NaAlH₄ sample does not absorb hydrogen either, probably because the difference between the applied hydrogen pressure (12 bar) and the equilibrium plateau pressure at 623 K (6.38 bar [13]) of the Mg-H system is small. At n = 2, the MgH₂-50NaAlH₄ sample absorbs 0.99 wt% H for 1 min, 1.36 wt% H for 10 min, and 3.19 wt% H for 60 min at 593 K.

Figure 8 shows the H_r versus t curves in 1.0 bar hydrogen at 593 K or 623 K at n = 1 ∼ 4 for MgH₂-50NaAlH₄. Hydrogen-release rates are low and the quantities of hydrogen released for 60 min are small. As n increases, the initial hydrogen-release rate and the quantity of hydrogen released for 60 min increase slightly. At n = 2, the MgH₂-50NaAlH₄ sample releases 1.03 wt% H for 5 min and 1.29 wt% H for 60 min at 593 K. When the temperature increases from 593 K to 623 K, the initial hydrogen-release rate and the quantity of hydrogen released for 60 min increase slightly. Partial ignition and combustion in the samples led to low initial hydrogen-release rates and small quantities of hydrogen released for 60 min.

H_a versus t curves in 12 bar hydrogen at 593 K for activated MgH₂-30NaAlH₄, MgH₂-2Ni-10NaAlH₄ after RMM, activated Mg-10Fe₂O₃ [14,15], activated Mg-10TaF₅ [16,17], and activated Mg-10VCl₃ [18] are shown in Figure 9. The Mg-10Fe₂O₃ [14,15], Mg-10TaF₅ [16,17], and Mg-10VCl₃ [18] samples were also prepared by RMM under the conditions similar to those for preparing MgH₂-30NaAlH₄, MgH₂-2Ni-10NaAlH₄. MgH₂-2Ni-10NaAlH₄ did not require activation after reactive mechanical milling (RMM). The H_a versus t curve of MgH₂-2Ni-10NaAlH₄ after RMM is used for comparison with the H_a versus t curves of other activated samples. Activated MgH₂-30NaAlH₄ has the highest hydrogen-absorption rate for 2.5 min, followed in order by MgH₂-2Ni-10NaAlH₄ after RMM, activated Mg-10VCl₃, activated Mg-10TaF₅, and activated Mg-10Fe₂O₃. Activated MgH₂-30NaAlH₄ has the highest effective hydrogen-storage capacity, followed in order by activated Mg-10VCl₃, MgH₂-2Ni-10NaAlH₄ after RMM, activated Mg-10Fe₂O₃, and activated Mg-10TaF₅. MgH₂-30NaAlH₄ has much higher hydrogen-absorption rate for 2.5 min (2.20 wt% H/min) and much larger effective hydrogen-storage capacity (7.42 wt% H) than the other samples.

Figure 10 shows H_r versus t curves in 1.0 bar hydrogen at 593 K for activated MgH₂-30NaAlH₄, MgH₂-2Ni-10NaAlH₄ after RMM, activated Mg-10Fe₂O₃, activated Mg-10TaF₅, and activated Mg-10VCl₃. Activated MgH₂-30NaAlH₄ has the highest hydrogen-release rate for 2.5 min, followed in order by activated Mg-10VCl₃, activated Mg-10TaF₅, MgH₂-2Ni-10NaAlH₄ after RMM, and activated Mg-10Fe₂O₃. Activated MgH₂-30NaAlH₄ has the largest quantity of hydrogen released for 60 min, followed in order by activated Mg-10VCl₃, MgH₂-2Ni-10NaAlH₄ after RMM, activated Mg-10TaF₅, and activated Mg-10Fe₂O₃, and activated. MgH₂-30NaAlH₄ has much higher hydrogen-release rate for 2.5 min (0.86 wt%/min) and much larger quantity of hydrogen released for 60 min (7.34 wt% H) than the other samples.

4. Discussion

From the results of Fig. 2, it is believed that for the NaAlH₄, the reaction (1) [decomposition of NaAlH₄] begins to occur at 438 K and the reaction (2) [decomposition of Na₃AlH₆] begins to occur at 495 K. For the MgH₂-10NaAlH₄ sample, it is believed that the reaction (4) occurs between the point 1 (508 K) and the point 2 (525 K), the reactions (5) and (6) begins to occur after the point 3 (550 K), and then the reaction (7) and the reaction (8) occur consecutively. For the MgH₂-30NaAlH₄ sample, it is believed that the reaction (4) occurs between the point 1 (480 K) and the point 2 (541 K), the reactions (5) and (6) begins to occur after the point 3 (562 K), and then the reaction (7) and the reaction (8) occur consecutively. The reaction (4) for the MgH₂-30NaAlH₄ begins to occur at 28 K lower temperature than that for the MgH₂-10NaAlH₄.

Samples were easily ignited and combusted during treatment, making difficult the obtention of XRD patterns and leading to the formation of a strong peak of MgO and relatively weak peaks of other phases. Liu et al. [12] reported that when the temperature is increased to 633 K, the preformed Al and Al₃Mg₂ disappear. In Fig. 3 (the XRD pattern of MgH₂-50NaAlH₄ dehydrided after number of cycles, n, of 4 at 593 K), the Al₃Mg₂ phase is observed. The dehydriding temperature (593 K) lower than that in the work of Liu et al. [12] is believed to have led to this result.

As the content of NaAlH₄ in the sample increases, the temperature at the highest peak in the ratio of increase in H_r to increase in T, dH_r/dT, versus T curve decreases. The higher content of NaAlH₄ is believed to have made effects of reactive mechanical milling stronger. However, too much content of NaAlH₄ (as in MgH₂-50NaAlH₄) leads to worse hydrogen-storage properties (hydrogen-absorption rate, hydrogen-release rate, and hydrogen-storage capacity).

RMM creates defects and cracks. The propagation of cracks makes the particles finer. Defects can be used as the sites active for nucleation. Decrease in particle size reduces the diffusion distance of hydrogen atoms. The added materials and formed phases are believed to make the effects of RMM stronger. The expansion of lattice due to hydrogen absorption and the contraction of lattice due to hydrogen release causes the effects similar to those of RMM. However, the effects of lattice expansion and contraction will be weaker than those of RMM. The MgH₂-30NaAlH₄ sample has a higher hydrogen-absorption rate for 2.5 min, a larger effective hydrogen-storage capacity, a higher hydrogen-release rate for 2.5 min, and a larger quantity of hydrogen released for 60 min than the other samples, showing that the effects of RMM and hydrogen absorption-release cycling are stronger in the MgH₂-30NaAlH₄ sample, compared with those for the other samples. Reportedly, nucleation can be facilitated by creating active nucleation sites by mechanical treatment and/or alloying with additives [19]; the diffusion distance of hydrogen can also be decreased by the mechanical treatment and/or alloying of Mg with additives, thereby reducing the magnesium particle size [20]; in addition, the hydrogen mobility can be improved by additives that create microscopic paths of hydrogen [20]; a rough surface of magnesium particles having many cracks and defects is thus considered more advantageous for hydrogen absorption [21].

In our future work, milled NaAlH₄ will be prepared by reactive mechanical milling. The quantity of released hydrogen (H_r) versus temperature T curve and the ratio of increase in H_r to increase in T, dH_r/dT, versus T curve for the milled NaAlH₄ will be obtained and studied in detail. Behaviors of the milled NaAlH₄, which are different from those of NaAlH_4, are expected.

As shown in Figure 9 and Figure 10, addition effects of NaAlH₄, oxide, halides, or fluoride to MgH₂ or Mg are different. Which kinds of properties such as physical properties (hardness, toughness, surface area, and microstructure, etc.) and chemical properties affect the hydrogen-storage properties will be investigated.

5. Conclusions

In this work, milled MgH₂, NaAlH₄, MgH₂–10NaAlH₄ (with a composition of 90 wt% MgH₂ + 10 wt% NaAlH₄), MgH₂–30NaAlH₄ (70 wt% MgH₂ + 30 wt% NaAlH₄), MgH₂–50NaAlH₄ (50 wt% MgH₂ + 50 wt% NaAlH₄), and MgH₂–2Ni-10NaAlH₄ (88 wt% MgH₂ + 2 wt% Ni + 10wt% NaAlH₄) samples were prepared by reactive mechanical milling (RMM). As the content of NaAlH₄ in the sample increased, the temperature at the highest peak in the ratio of increase in H_r to increase in T, dH_r/dT, versus T curve decreased. The higher content of NaAlH₄ is believed to have made effects of reactive mechanical milling stronger. MgH₂-50NaAlH₄ dehydrided after four cycles contained Al, MgO, Al₃Mg₂, MgH₂, and Mg₁₇Al₁₂. Hydriding in 12 bar hydrogen and dehydriding in 1.0 bar hydrogen at 593 K of MgH₂-30NaAlH₄ are performed by the reversible reactions MgH₂ ⇔ Mg + H₂ and 17MgH₂ + 12Al ⇔ Mg₁₇Al₁₂ + 17H₂. Activation of MgH₂–30NaAlH₄ was completed after two hydrogen absorption-release cycles. MgH₂–30NaAlH₄ was the best Mg-based composite among Mg-based alloys in which an oxide, a halide, a fluoride, or a complex hydride was added, with a high hydrogen-absorption rate for 2.5 min (2.20 wt% H/min) and a large effective hydrogen-storage capacity (7.42 wt% H).

Author Contributions

Conceptualization, Kwak, Y.J.; Song, M.Y.; Lee, K.-T.; methodology, Kwak, Y.J.; Song, M.Y.; Lee, K.-T.; formal analysis, Kwak, Y.J.; Song, M.Y.; Lee, K.-T.; investigation, Kwak, Y.J.; Song, M.Y.; Lee, K.-T.; data curation, Kwak, Y.J.; writing—original draft preparation, Kwak, Y.J.; Song, M.Y.; writing—review and editing—Kwak, Y.J.; Song, M.Y.; Lee, K.-T.; project administration, Kwak, Y.J.; Lee, K.-T.; funding acquisition, Kwak, Y.J.; All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2021R1C1C2009103). This work was supported by Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (20213030040110).

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author, [M Y S].

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Quantity of released hydrogen (H_r) versus temperature T curves and dH_r/dT versus T curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄ samples. The samples were heated at a heating rate of 5 ∼ 6 K in 1.0 bar hydrogen.

Figure 2. Quantity of released hydrogen (H_r) versus temperature T curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄ samples. The samples were heated at a heating rate of 5 ∼ 6 K in 1.0 bar hydrogen.

Figure 4. Change in quantity of absorbed hydrogen (H_a) versus time t curve at 593 K in 12 bar hydrogen with cycle number, n, for MgH₂-30NaAlH₄.

Figure 5. Change in quantity of released hydrogen (H_r) versus t curve at 593 K in 1.0 bar hydrogen with cycle number, n, for MgH₂-30NaAlH₄.

Figure 6. Changes in H_a versus t curve at 593 K in 12 bar hydrogen and H_r versus t curve at 593 K in 1.0 bar hydrogen with cycle number, n, for MgH₂-30NaAlH₄.

Figure 7. H_a versus t curves in 12 bar hydrogen at 593 K or 623 K at n = 1 and n = 2 for MgH₂-50NaAlH₄.

Figure 8. H_r versus t curves in 1.0 bar hydrogen at 593 K or 623 K at n = 1 ∼ 4 for MgH₂-50NaAlH₄.

Figure 9. H_a versus t curves in 12 bar hydrogen at 593 K for activated MgH₂-30NaAlH₄, MgH₂-2Ni-10NaAlH₄ after RMM, activated Mg-10Fe₂O₃, activated Mg-10TaF₅, and activated Mg-10VCl₃.

Figure 10. H_r versus t curves in 1.0 bar hydrogen at 593 K for activated MgH₂-30NaAlH₄, MgH₂-2Ni-10NaAlH₄ after RMM, activated Mg-10Fe₂O₃, activated Mg-10TaF₅, and activated Mg-10VCl₃.

Table 1. Temperatures (K) at peaks in the dH_r/dT versus T curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄ samples.

	Peak	Highest Peak	Peak	Peak
milled MgH₂		638
MgH₂-10NaAlH₄	513	600
MgH₂-30NaAlH₄	512	592
NaAlH₄		455	552	631

Table 2. Temperatures (K) at the marked points in the H_r versus T curves for milled MgH₂, MgH₂-10NaAlH₄, MgH₂-30NaAlH₄, and NaAlH₄ samples.

Marked Points	1	2	3	4
milled MgH₂	627	673
MgH₂-10NaAlH₄	508	525	550
MgH₂-30NaAlH₄	480	541	562	633
NaAlH₄	438	488	495	625

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