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ChemEngineering 2018, 2(2), 15; doi:10.3390/chemengineering2020015

Article
Study of Hydrogen Interactions with Nd2Fe17 and Nd2Fe14B by Means of Calorimetric Method
1
Faculty of Chemistry, Lomonosov Moscow State University, Vorobyevy Gory, 119991 Moscow, Russia
2
MISiS, Leninskiy Prospekt 4, 119049 Moscow, Russia
*
Author to whom correspondence should be addressed.
Received: 30 January 2018 / Accepted: 27 March 2018 / Published: 9 April 2018

Abstract

:
Hydrogen interactions with Nd2Fe17 and Nd2Fe14B was investigated by means of the calorimetric method with application of differential heat-conducting calorimeters that were of the Tean-Calvet type. The reaction of hydrogen absorption and desorption was carried out at 250 and 300 °C for Nd2Fe17, while the pressure-composition-isotherms (P-C-T) and enthalpy change with hydrogen concentration in the intermetallic compound (IMC) were obtained. The Nd2Fe14B-H2 system was studied at 50 °C and the dependence of the enthalpy change with hydrogen concentration in the intermetallic compound was also obtained. Based on the measured data, the assumption about the order of filling the interstitial sites by hydrogen atoms was made.
Keywords:
Nd2Fe17-H2; Nd2Fe14B-H2; calorimetry; thermodynamics; enthalpy

1. Introduction

It is well known that in the production of the magnetic materials of the Nd-Fe-B and Re2Fe17-type (Re = rare earth metals), the process of hydrogenation–dehydrogenation (HD) was used. However, this process was not studied completely until now.
Generally, scientists studied magnetic properties [1,2,3] and the influence of hydrogen on the structure of alloys. To better understand how hydrogen insertion influences physical and magnetic properties of alloys, it is necessary to know the thermodynamic parameters of the process of hydrogen interaction with magnet materials. Unfortunately, the studies in which the thermodynamic properties of HD processes are studied directly by means of calorimetric method are practically negligible. The authors in a previous study [4] studied hydrogen desorption from R2Fe17HX (R = Nd and Dy) compounds with x ≤ 5 using differential scanning calorimetry (DSC). In previous works [5,6], the thermodynamic parameters of hydrogen interactions with Sm2Fe17 (the partial molar enthalpy and the partial molar entropy) were determined from pressure-composition isotherms. Ram et al. [7] investigated the hydrogen desorption from Nd2Fe14BHX (X~5) by means of differential scanning calorimetry.
In the present work, we continue our research of hydrogen interaction with the systems of R2Fe17 and Nd2Fe14B [8,9,10] and summarize the obtained results.

2. Materials and Methods

The initial sample of Nd2Fe17 was prepared by the arc melting of a stoichiometric mixture of constituent metals of 2–17 (23.3 mas. % Nd and 76.7 mas. % Fe) in a furnace with a non-consumable tungsten electrode on a water-cooled boat in a purified argon atmosphere under a pressure of 2 atm. The purity grade of starting metals was 99.999% for Fe and 99.98% for Nd. Following this, the sample was annealed in a quartz tube at 1100 °C for 40 h to ensure homogeneity.
The details of the synthesis of the materials and results of X-ray Diffraction analysis (XRD) were described elsewhere [8,9]. According to XRD data [8], the synthesized Nd2Fe17 sample has the Th2Zn17 type structure. The X-ray analysis of Nd2Fe17H4.6 showed that the hydrogenation of the crystal structure of the initial compound was retained, while the anisotropic distortion of the unit cell in the base plane occurs. That agrees with literary data [11] Lattice parameters of Nd2Fe14B and its hydride coincided with reference data too [12,13,14,15].The chemical composition of the initial samples of Nd2Fe17 and Nd2Fe14B was checked by the roentgen-fluorescent analysis on the spectrometer Rigaku Primus II. According to the obtained data, the initial sample of Nd2Fe17 contained 76.69 mas. % Fe and 22.92 mas. % Nd, while the initial sample of Nd2Fe14B contained 28.7 mas. % Nd, 70.1 mas. % Fe and 1 mas. % B [9].
The investigation of hydrogen interactions with Nd2Fe17 and Nd2Fe14B was carried out by means of the calorimetric method with application of the differential heat-conducting calorimeter DAK-12, which was connected with Sievert’s-type volumetric installation for gas-dose feeding to measure the quantity of absorbed or evolved hydrogen by means of the volumetric method. The apparatus scheme, experimental procedure and analysis of collected data were described in a previous study [9,16]. The use of such complex apparatus permits us to measure P-C-T and ∆H-C-T (where P is the equilibrium hydrogen pressure, C is the hydrogen concentration in the intermetallic compound (IMC), C = H/IMC, T is the experimental temperature and ∆H is the reaction enthalpy), simultaneously. The purity of hydrogen was 99.9999%.
The partial molar enthalpy ∆Habs. (des) was determined from the heat effect of the reaction for Nd2Fe17:
2 Nd2Fe17 + y/2 H2 ↔ 2 Nd2Fe17Hy,
or from the heat effect of the reaction for Nd2Fe14B:
Nd2Fe14B + y/2 H2 ↔ Nd2Fe14BHy.
The heat effect of the reaction was calculated according to the following equation [17]:
Q = S•A/Δn
where S is the area of voltage–time plot; A is the receptiveness, determined from an electrical calibration; and Δn is the number of absorbed/desorbed H2 molecules.
In a previous study [18], it was shown that the measured heats corresponded to the enthalpies of reaction when H2 or ½ H2 is expressed per mole.
The measurement error in the present work is reflected in Table 1 and Table 2. It was determined in accordance with recommendation of International Union of Pure and Applied Chemistry (IUPAC) [19] as a standard deviation of the mean value:
δ = √ΣΔ2[m(m − 1)]−1
where Δ is the deviation from the mean value and m is the number of data points.

3. Results and Discussion

3.1. P-C Measurements

The reaction of hydrogen interactions with the Nd2Fe17 sample was studied in the present work at 250 and 300 °С. Absorption and desorption processes were carried out. In the Figure 1, P-C (P- equilibrium hydrogen pressure, C = H/IMC) isotherms are shown for these processes.
As one can see on the obtained plots of the P = f(C) dependence, there is no plateau region, which is the characteristic feature of the formation of metal and IMC hydrides. We obtained the same result as a previous study [8] for the hydrogen reaction with Nd2Fe17 at 200 °C, which are consistent with results shown in references [20,21].
However, Figure 2a,b shows the absorption and desorption isotherms collected at 200 and 250 °C, which do not coincide with the region of 1.8 < C < 4.3, while the absorption and desorption isotherms measured at 300 °C coincide completely (see Figure 2c).
In other words, in the Nd2Fe17-H2 system, there is a small pressure hysteresis. Previously, we observed such hysteresis in the Sm2Fe17-H2 system at 250 °C [10] and Nd2Fe17-H2 system at 200 °C [8]. We obtained the following values of hysteresis for the Nd2Fe17-H2 system at 200 and 250 °С at a hydrogen concentration С~3.6 ln(10/4.9) = 0.84 and ln(13/10) = 0.60, respectively.
Similar phenomena were observed previously in references [22,23,24,25] for AB2-H2 systems, including Zr(Fe0.75Cr0.25)2-H2, ZrCrV-H2, ZrCrFe1.2-H2 and Ti0.9Zr0.1Mn1.3V0.7-H2 systems. The authors explained this phenomenon by the formation of hydride phases.
Thus, the presented data permit us to suggest that in the Nd2Fe17-H2 system at 200 and 250 °C, the existence of hydride phases is possible.

3.2. Calorimetric Results

The calorimetric investigation of the hydrogen interactions with Nd2Fe17 in the present work was carried out at 250 and 300 °С. As a result, we obtained the dependences of the change in the partial molar enthalpy of absorption and desorption with hydrogen concentration C (C = H/ Nd2Fe17) in the intermetallic compound ΔHabs. (des.) = f(C) (see Figure 3a–c and Table 1).
The data which we obtained earlier for the Nd2Fe17-H2 system at 200 °С [10] are presented in Table 1.
Analyzing the data presented in the Table 1 and Figure 3a–c, it should be noted that in the Nd2Fe17-H2 system, there are two regions with constant values of ∆Habs. at 200 and 250 °С. The increase in reaction temperature results in shrinkage of a length of the part with the constant values of ∆Habs. At 300 °С, there is only one region with constant values of ∆Habs. (des.). Furthermore, it should be noted that an increase in the reaction temperature of hydrogen interaction with Nd2Fe17 results in the decrease in values of partial molar enthalpy in terms of absolute magnitude.
Previously, Isnard et al. [26,27] investigated the hydride phases of R2Fe17, where R is the light rare-earth element with rhombohedral structure R-3m, by means of neutron diffraction analysis and determined that hydrogen atoms in these intermetallic compounds occupied octahedral 9e and tetrahedral 18g sites. It was determined that 9e sites were filled completely and 18g sites were occupied partially.
In a previous study [4], authors studied the hydrogen desorption from Nd2Fe17Hx by means of differential scanning calorimetry (DSC) and they determined that for the Nd2Fe17Hx sample, the DSC curve at a high hydrogen concentration (X = 5) had two peaks, which were namely high-temperature (HT) and low-temperature (LT). At the smaller hydrogen concentrations in metallic matrix (X = 1, 2, 3), there was one high-temperature peak in the DSC curve. The authors theorized that hydrogen atoms occupied one type of interstitial site in this case.
At X = 3 on the DSC curve, the clear shoulder appears. Based on these data, the authors concluded that hydrogen H1 occupied the pseudo-octahedral interstitial 9e site and hydrogen H2 occupies the tetrahedral 18g site. In addition, the authors noted that 9e sites that occupied hydrogen H1 possibly consisted of two energy non-equivalent interstitial sites, which experimentally verified the existence of the clear shoulder in the plots (see Figure 2 (top) and refer to a previous study [4]).
The calorimetric results that we obtained in the present work and in previous studies [8,10] are consistent with the data obtained in reference [4]. We may assume that hydrogen atoms fill 9e sites during absorption at 200 and 250 °C at the range of hydrogen concentration of 0 < C < 2.0 (ΔHabs. = −85.05 ± 0.65 kJ/mol H2 at 200 °C). Furthermore, at C > 2.0, the occupancy of 9e sites by hydrogen atoms results in less heat evolution (ΔHabs. = −80.64 ± 1.00 kJ/mol H2 at 200 °C). In other words, we may conclude that the 9e position consists of two non-equivalent energy sites. The increase in experimental temperature up to 300 °C results in the appearance of one region with the constant enthalpy values. In this case, we may assume that smoothing of two energy levels in the 9e site takes place at 300 °C.
At the hydrogen concentration C > 2.7 in the intermetallic compound, the values of partial molar enthalpy of hydrogen absorption decrease sharply in absolute magnitude when hydrogen atoms start to occupy 18g site (see Figure 3 and Table 1). In the plots ΔHabs. = f(C), there are no regions with constant enthalpy values. It is difficult to assume that we deal with the formation of stable hydride in this case as suggested by a previous study [4]. However, the P-C isotherms in Figure 2a,b show some hysteresis in the range of 1.8 < C < 4.3.
Comparing results of the calorimetric study for the Nd2Fe17-H2 and for Sm2Fe17-H2 systems (presented in Table 2 made on the basis of the results in reference [10]), one can notice that the values of ∆Habs. (des.) for the Nd2Fe17-H2 system are higher in absolute value compared to the Sm2Fe17-H2 system.
This phenomenon may be explained by the fact that the samarium radius is less than the neodymium radius due to lanthanum contraction, which leads to a decrease in the volume of the hole available for filling by hydrogen. In other words, the stability of ternary hydrides in the range of 0 < C < 3.0 depends on the unit cell volume of the alloy. This suggestion was obtained from reference [21].
Cuevas et al. [4] investigated hydrogen desorption from Nd2Fe17HX and Dy2Fe17HX hydrides by the DSC method. It was determined that the heat of reaction of hydrogen desorption was equal to 29.2 ± 0.8 kJ/mol H2 for both R2Fe17HX compounds. However, our results show that values of enthalpy absorption and desorption differ for the Nd2Fe17-H2 and Sm2Fe17-H2 systems and the enthalpy values change depending on reaction temperature and hydrogen concentration in the metallic matrix.
In a previous study [26], the authors noted that both sites exhibit different behavior depending on the temperature. The structure analysis shows that D1 is the most thermally stable and the most occupied site at higher hydrogen concentrations.
The study of hydrogen with Nd2Fe14B was carried out in the present work at 50 °С. The results of calorimetric study of hydrogen interaction with Nd2Fe14B are presented in Figure 4 and in Table 3.
As seen in Figure 4 in the plot of ΔHabs./des. = f(C) dependence, it is possible to mark three parts with constant values of enthalpy absorption and two parts with constant values of enthalpy desorption. The values of absorption and desorption enthalpy in these regions are presented in Table 3.
As one can see from these data, the enthalpy values of absorption and desorption coincide at absolute values on the regions of 2.0 < C < 3.0 and 3.2 < C < 3.7.

4. Conclusions

The reaction of hydrogen interactions with Nd2Fe17 and Nd2Fe14B was investigated by means of the calorimetric method at different temperatures. It was shown that the values of enthalpy absorption and desorption changed with reaction temperature and hydrogen concentration in the intermetallic compounds.
It was demonstrated that the character of the change of the enthalpy values correlated with the extent of filling of crystallographic holes.
The observed pressure hysteresis might be the evidence of the beginning of hydride phase formation, which requires further study.
The role of enthalpy in the occupation of crystallographic holes and hydride phase formation needs further research to be completely characterized.

Author Contributions

Elena Anikina and Victor Verbetsky conceived and designed the experiments; Elena Anikina performed the experiments; Elena Anikina and Victor Verbetsky analyzed the data; Alexander Savchenko contributed reagents, materials and analysis tools; Elena Anikina and Victor Verbetsky wrote the paper.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Absorption and desorption isotherms for Nd2Fe17-H2 at 250 and 300 °C.
Figure 1. Absorption and desorption isotherms for Nd2Fe17-H2 at 250 and 300 °C.
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Figure 2. (a) Absorption and desorption isotherms for the Nd2Fe17-H2 system at 200 °C according to reference [8]; (b) absorption and desorption isotherms for the Nd2Fe17-H2 system at 250 °C; and (c) absorption and desorption isotherms for the Nd2Fe17-H2 system at 300 °C.
Figure 2. (a) Absorption and desorption isotherms for the Nd2Fe17-H2 system at 200 °C according to reference [8]; (b) absorption and desorption isotherms for the Nd2Fe17-H2 system at 250 °C; and (c) absorption and desorption isotherms for the Nd2Fe17-H2 system at 300 °C.
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Figure 3. (a) Absorption enthalpy vs. composition at 200 °C for the Nd2Fe17-H2 system. The different symbols refer to different runs of determination [8]; and (b) Absorption enthalpy vs. composition at 250 °C for the Nd2Fe17-H2 system. The different symbols refer to different runs of determination. (c) Absorption enthalpy vs. composition at 300 °C for the Nd2Fe17-H2 system. The different symbols refer to different runs of determination.
Figure 3. (a) Absorption enthalpy vs. composition at 200 °C for the Nd2Fe17-H2 system. The different symbols refer to different runs of determination [8]; and (b) Absorption enthalpy vs. composition at 250 °C for the Nd2Fe17-H2 system. The different symbols refer to different runs of determination. (c) Absorption enthalpy vs. composition at 300 °C for the Nd2Fe17-H2 system. The different symbols refer to different runs of determination.
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Figure 4. Absorption and desorption of enthalpy vs. composition at 50 °C for the Nd2Fe14B-H2 system. The different symbols refer to different runs of determination. Filled symbols refer to absorption of H, while the open symbols refer to the desorption.
Figure 4. Absorption and desorption of enthalpy vs. composition at 50 °C for the Nd2Fe14B-H2 system. The different symbols refer to different runs of determination. Filled symbols refer to absorption of H, while the open symbols refer to the desorption.
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Table 1. The values of hydrogen absorption and desorption enthalpy in the Nd2Fe17-H2 system.
Table 1. The values of hydrogen absorption and desorption enthalpy in the Nd2Fe17-H2 system.
T, °CH/IMC∆Habs. ±δ, kJ/mol H2H/IMC∆Hdes. ±δ, kJ/mol H2Ref.
3000–2.0−80.5 ± 0.90.7–1.981.8 ± 1.3This work
2500–1.3−84.2 ± 0.30.8–2.080.0 ± 0.2This work
2501.4–1.8−79.8 ± 1.3 This work
2000–2.0−85.95 ± 0.65 [8]
2002.0–2.7−80.64 ± 1.01.9–2.776.48 ± 0.85[8]
Table 2. The values of hydrogen absorption and desorption enthalpy in the Sm2Fe17-H2 system.
Table 2. The values of hydrogen absorption and desorption enthalpy in the Sm2Fe17-H2 system.
T, °CH/IMC∆Habs. ±δ, kJ/mol H2H/IMC∆Hdes. ±δ kJ/mol H2
2500.3–2.0−81.7 ± 0.50.5–0.781.6 ± 0.5
2502.1–3.0−75.6 ± 1.01.0–2.376.8 ± 0.8
2000.8–1.6−80.2 ± 0.81.2–1.6~81
2001.8–2.6−74.9± 2.21.6–2.673.0 ± 1.1
Table 3. The values of hydrogen absorption and desorption enthalpy in the Nd2Fe14B-H2 system.
Table 3. The values of hydrogen absorption and desorption enthalpy in the Nd2Fe14B-H2 system.
T, °CH/IMC∆Habs. ±δ, kJ/mol H2H/IMC∆Hdes. ±δ, kJ/mol H2
500.3–0.9−69.3 ± 1.3
502.0–3.0−36.6 ± 1.62.0–3.038.7 ± 0.8
503.2–3.7−34.0 ± 1.33.2–3.733.5 ± 1.8

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