- Email: [email protected]
Investigation of the hydrogen interaction with Ti0.9Zr0.1Mn1.3V0.7 by means of the calorimetric method
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 4 1 ( 2 0 1 6 ) 1 1 5 2 0 e1 1 5 2 5
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation of the hydrogen interaction with Ti0.9Zr0.1Mn1.3V0.7 by means of the calorimetric method E.Yu. Anikina*, V.N. Verbetsky Lomonosov Moscow State University, Chemical Department Moscow, 119991, Russia
article info
abstract
Article history:
In the present work we studied the hydrogen interaction with the stoichiometric
Received 30 September 2015
Ti0.9Zr0.1Mn1.3V0.7 Laves phase compound C14 at pressure up to 50 atm and the tempera-
Received in revised form
ture range from 52 to 130 C by means of calorimetric method and plotting of pressure-
7 December 2015
composition isotherms. The processes of hydrogen absorption and desorption were car-
Accepted 18 December 2015
ried out. The findings enabled us to draw the following conclusions: 1) the experimental
Available online 11 January 2016
temperature influences the thermodynamic parameters of studied processes and 2) depending on reaction conditions the formation of one or two hydride phases takes place.
Keywords:
© 2015 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Intermetallic compound Hydride Ti0.9Zr0.1Mn1.3V0.7-H2 system Calorimetry
Introduction Multicomponent intermetallic compounds (IMC) AB2 with hexagonal structure C14 Laves phase are very important and used for storage, transportation and purification of hydrogen. It is well known that such IMCs absorb significant amount of hydrogen, they possess high rate of hydrogen reaction with IMCs and they are resistant to degradation during cycling. The main thermodynamic parameter characterizing the process of hydrogenation is the change of enthalpy of hydrogen reaction with metallic matrix at absorption/desorption processes (DHabs.(des.)). The value of DHabs.(des.), character of its change depending on hydrogenation rate, the temperature of the experiment permit us to understand the mechanism of the hydrogen reaction with the intermetallic alloy better. A lot
of works are devoted to the investigation of the hydrogen interaction with IMCs. Generally the experimental method which is applied for determination of enthalpy change DH and entropy change DS is a calculation of the thermodynamic parameters on the basis of the Van't Hoff equation though the values of enthalpy and entropy calculated in such a way may significantly deviate from real characteristics as this method supposes that the enthalpy and the entropy are independent from the temperature which is not always the case, therefore, this assumption may leads to the errors. There are few works, in which thermodynamic properties of intermetallic compound e hydrogen system (IMC-H2 system) were investigated by the calorimetric method [1e6]. The application of the twincell heat conducting calorimeter permits us to determine directly the heat reaction of hydrogen interaction with IMC. As it was shown in Ref. [7] in this case the measured heats
* Corresponding author. Tel.: þ7 495 939 96 77; fax: þ7 495 932 88 46. E-mail address: [email protected] (E.Yu. Anikina). http://dx.doi.org/10.1016/j.ijhydene.2015.12.126 0360-3199/© 2015 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Materials and methods The initial intermetallic compound Ti0.9Zr0.1Mn1.3V0.7 was prepared by arc melting of a stoichiometric mixture of pure metals in a furnace with a non-consumable tungsten electrode on a water-cooled boat in a purified argon atmosphere under pressure 2 atm. The purity grade of starting metals was better than 99.9%. As a high volatility of manganese in comparison with another metals used for preparing initial compound 4 mass% extra Mn was added to compensate a weight loss during melting. Titanium sponge was used as a getter for purification of argon from admixtures. For that reason it was melted in the furnace before alloying of the starting metals. The button of sample was tuned over and re-melted four times and then annealed in a sealed quartz vessel at 1100 C for 240 h to ensure homogeneity. The phase composition and unit-cell parameters of initial intermetallic compound Ti0.9Zr0.1Mn1.3V0.7 and its hydrides were determined by X-ray diffraction on a DRON-2 powder diffractometer (Cu Ka radiation) and XRD data showed that the parent sample was single-phase compound with the hexagonal Laves phase structure C 14 (MgZn2) with refined units lattice parameters a ¼ 4.922 Å, c ¼ 8.040 Å, c/a ¼ 1.633, V ¼ 168.82 Å3. The refinement of diffraction profiles was performed using the RIETAN-2000 software [8]. To prevent of a hydrogen losses from the hydride phases during their X-ray study under ambient conditions we should passivate synthesized hydrides. Therefore at first we cooled autoclave with hydrogenated sample to the liquid nitrogen temperature, further we depressed of hydrogen pressure to atmospheric pressure, opened autoclave and exposed it to air at 77 K for 2 h [9]. The study of hydrogen interaction with Ti0.9Zr0.1Mn1.3V0.7 was carried out by a calorimetric method. The twin-cell heat conducting calorimeter Tian-Calvet type connected to a conventional Sieverts'-type volumetric apparatus for measuring by a volumetric method of an amount of absorbed or evolved hydrogen was applied. The apparatus scheme, the experimental procedure and the analysis of the collected data were described elsewhere [3]. As a source of pure hydrogen for hydrogenation of Ti0.9Zr0.1Mn1.3V0.7 in this study LaNi5Hx hydride was used. A purity of hydrogen was 99.9999%. Absorption (desorption) relative molar enthalpy DHabs.(des.) was determined from the heat effect of the reaction: Ti0.9Zr0.1Mn1.3V0.7 HX þ y/2H2 4 Ti0.9Zr0.1Mn1.3V0.7 HXþY
(1)
Since Ti0.9Zr0.1Mn1.3V0.7 reversibly reacts with hydrogen the same sample (10,831.7 $ 106 mol Ti0.9Zr0.1Mn1.3V0.7) was used in all experiments. Before each run the residual hydrogen was desorbed at 350e450 C under high vacuum outside the calorimeter. The hydrogen concentration in the sample was calculated from the volumetric measurements using the ideal gas low for pressure < 1 atm, the Van e der e Waals equation of state for pressure below 20 atm and the modified Van e der e Waals equation for pressure above 20 atm [10]. The experimental error in this work was expressed in accordance with recommendations of IUPAC [11] as a standard deviation of the mean value d ¼ √SD2[n(n1)]1, where D is the deviation from the mean value and n is the number of data points.
Results and discussion P-C-T measurements The Ti0.9Zr0.1Mn1.3V0.7-H2 system was studied in the temperature range 52e130 C and hydrogen pressure up to 50 atm. The PeC (P-equilibrium hydrogen pressure, C]H/IMC) desorption isotherms were measured for 52, 72, 100 and 130 C and for 52 and 72 C the PeC absorption isotherms were measured too. Since the overall duration of experiment was about six months it was necessary to check the sample characteristics. For this purpose the runs of hydrogen desorption at 52 C were repeated from time to time. Despite of the multicomponent composition of the sample under investigation the PeC and DHdes. ¼ f(C) dependences measured during these runs provide reproducibility within the present experimental error. In Fig. 1 from the represented data one can notice that the compound under investigation has a good hydrogen capacity (2.9H/IMC at 52 C and hydrogen pressure P ¼ 36 atm). However the reversible hydrogen capacity at this temperature is smaller as the equilibrium hydrogen pressure at the low hydrogen concentration in the IMC is very low. Earlier the authors marked such phenomenon in the work [12] when they studied the effect of different substitutional metals in the TiMn2-based alloys. It has been determined that the increase of the vanadium content in the alloy increased the length of a-region and reduced the reversible hydrogen capacity. V has a strong affinity to hydrogen. The
P, atm
correspond to enthalpy of reaction when expressed per mole H2 or ½ H2. In the present work we continue our study of the (Ti,Zr)(MnV)2±X-H2 system where (Ti,Zr)(MnV)2±X are intermetallic compounds (IMC) with Laves phase structure C 14 by means of the calorimetric method. Earlier we studied the IMCH2 systems in which IMCs had nonstoichiometric compositions [3e6]. Now we have chosen the Ti0.9Zr0.1Mn1.3V0.7 compound with stoichiometric composition as the subject under the investigation.
100
52C 72C
10
100C 130C
1 0 0.1
0.5
1
1.5
2
2.5
3
H/IMC
0.01
Fig. 1 e Desorption isotherms for the Ti0.9Zr0.1Mn1.3V0.7-H2 system.
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P, atm
occupation of V some B sites in the AB2 Laves phase alloys leads to formation of some interstitial sites with a strong affinity for hydrogen acting as trap. In our compound under study V atoms occupy 35% B site in the AB2. The rise of the experimental temperature leads to the increase of equilibrium pressure in the Ti0.9Zr0.1Mn1.3V0.7-H2 system and as a result to larger amount of evolved hydrogen. As one can see from Fig. 1 the plateau slope does not change in practice with rising of experimental temperature when pressures are expressed as logPH2. In Fig. 2 the PeC isotherms of the hydrogen absorption and desorption, measured at 72 C, are plotted. As one can see from the obtained data in the region 0 < C < 2 these isotherms practically coincide. However in the region 2.0 < C < 2.8 the small pressure hysteresis is observed (ln(Pabs./Pdes.) ¼ 0.29 at C ¼ 2.5), though the sharp change of slope of the PeC curves is observed at C~2.0. For C > 2.8 this system exhibits the complete reversible behaviour in the PeC characteristics, the simple process of the hydrogen solution takes place. It may be an experimental verification of an existence of the two-phase equilibrium in this concentration region. Similar phenomena were observed previously in the works [13e15] for AB2-H2 systems: Zr(Fe0.75Cr0.25)2-H2, ZrCrFeH2, and ZrCrFe1.2-H2, respectively. The partial molar enthalpy values of hydride decomposition for the Ti0.9Zr0.1Mn1.3V0.7-H2 system were obtained by applying the Van't Hoff equation to the PeC isotherms. The plot of the calculated DHdes. ¼ f(C) dependence is presented in Fig. 3. As one can see from this plot there is the initial region (0.3 < C < 0.4) where the enthalpy values decrease after that some increase took place up to ~30 kJ/molH2 and thereupon in the region 0.5 < C < 1.0 the enthalpy values remained steady (~27.34 ± 0.05 kJ/molH2). At the hydrogen concentration in the IMC more 1 (C > 1) the gradual increase of enthalpy values is observed up to 35 kJ/molH2 and in the region 2.15 < C < 2.50 enthalpy values are equal 34.57 ± 0.78 kJ/molH2. Such an increase of the partial molar enthalpy values of the hydrogen reaction with IMC along with the C axis we observed previously in the works [4e6] for the Ti0.9Zr0.1Mn1.3V0.5-H2 and Ti0.9Zr0.1Mn1.5V0.8-H2 systems, as well as for the Ti0.1Zr0.9MnCr-H2 and ZrMnCr-H2 systems [16].
60 50 40 30
72C-abs. 72C-des.
20 10 0 0
0.5
1
1.5
2
2.5
3 H/IMC
Fig. 2 e Absorption and desorption isotherms for the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 72 C. Filled symbols refer to absorption of H2 and open symbols to desorption.
Fig. 3 e Partial molar desorption enthalpy calculated from van't Hoff plot for the Ti0.9Zr0.1Mn1.3V0.7-H2 system (a temperature range 52e130 C).
Calorimetric results The partial molar enthalpy values of the hydrogen absorption and desorption obtained by the calorimetric method are presented in Table 1. As one can see from the presented data in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 52 and 72 C there are three regions of hydrogen concentration in the IMC under investigation where the enthalpy values are constant for the process of hydrogen absorption or desorption. It should be noticed that the regions of the hydrogen concentration in the metallic matrix with the constant enthalpy values coincide for the absorption and desorption processes. As well it should be noticed that for every hydrogen concentration region these enthalpy values agree in absolute magnitude within the limits of experimental error (for absorption and desorption processes). In Fig. 4 the plot of the DHdes. ¼ f(C) dependence at 52 C is presented. One can see that the enthalpy values increase with the increase of hydrogen concentration in the metallic matrix. At 72 C in the Ti0.9Zr0.1Mn1.3V0.7-H2 system as well as at 52 C there are three regions with constant enthalpy values (see Table 1). For 72 C the processes of absorption and desorption were studied and obtained data show that the enthalpy values in each of these regions coincide in absolute magnitude. In addition it should be noticed that the enthalpy values do not change at transition from 52 to 72 C. For 100 and 130 C only the hydrogen desorption from hydride IMC was studied and the enthalpy values were measured for these process. The data presented in Table 1 and Figs. 5 and 6 show that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 100 and 130 C there are two regions where the enthalpy values are constant. The enthalpy values increase with the increase of hydrogen concentration in the sample under consideration for each temperature but comparing the enthalpy values obtained at different temperatures it can be noticed that the increase of experimental temperature leads to insignificant reduction of enthalpy values.
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Table 1 e Temperature dependence of reaction enthalpy for the Ti0.9Zr0.1Mn1.3V0.7-H2 system. Temperature, C 52
72
Range (absorption)
DH, kJ/mol H2 30.5 33.8 38.4 31.0 36.0 38.3
0.2e0.8 1.0e1.8 1.8e2.8 0.5e1.5 1.6e2.1 2.1e2.6
100 130
It was necessary to clarify the situation occurring in the Ti0.9Zr0.1Mn1.3V0.7-H2 system and to understand whether we deal with the formation of three hydride phases or the formation of ordered hydrogen solid solution which takes place at hydrogen concentration in the IMC C < 1 and then in the system under the investigation the some lattice transformation occurs and the hydride phase starts to form. We hydrogenated the sample under the investigation up to composition of Ti0.9Zr0.1Mn1.3V0.7H~1 at room temperature and carried out X-ray powder diffraction analysis. X-ray analysis data showed that Ti0.9Zr0.1Mn1.3V0.7H~1 at room temperature there was the single-phase compound with the hexagonal Laves phase structure C14, its crystal lattice remained unchanged relative to the crystal lattice of the initial compound but unit cell parameters slightly increased (a ¼ 5.0168 Å, c ¼ 8.23 Å, V ¼ 179.38 Å3, DV/V ¼ 2.7%) in comparison with the initial compound. Therefore it can be suggested that at C < 1 we deal with the formation of ordered hydrogen solid solution in Ti0.9Zr0.1Mn1.3V0.7. Moreover we hydrogenated the parent sample up to composition Ti0.9Zr0.1Mn1.3V0.7H3 at room temperature and X-ray analysis data revealed that Ti0.9Zr0.1Mn1.3V0.7H3 was the single-phase compound with hexagonal Laves phase C14 structure, the volume of the initial unit cell increased on ~23%. Thus obtained experimental data of the DHabs.(des.) ¼ f(C) dependence permit us to assume that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 52 and 72 C two hydride phases can form depending on the hydrogen concentration in the metallic matrix: di-hydride (or Ti0.9Zr0.1Mn1.3V0.7H2d) and
Fig. 4 e Desorption enthalpy vs. composition at 52 C.
± 0.9 ± 2.0 ± 2.3 ± 0.6 ± 1.1 ± 1.2
Range (desorption)
DH, kJ/mol H2
0.2e0.8 1.0e1.8 1.8e2.8 0.2e1.3 1.4e1.8 1.9e2.7 0.1e1.2 1.5e2.5 0.1e1.2 1.3e2.0
31.1 ± 0.6 33.9 ± 0.4 38.9 ± 0.6 29.5 ± 0.3 34.5 ± 1.0 39.2 ± 1.1 29.1 ± 0.4 36.0 ± 0.6 29.9 ± 0.5 34.3 ± 0.6
tri-hydride (or Ti0.9Zr0.1Mn1.3V0.7H3d). Observed regions of the constant enthalpy values correspond to two stages of the formation of hydride phases in the regions of hydrogen concentrations 1.0 < C < 1.8 and 1.8 < C < 2.8. At the hydrogen concentration C > 2.8 further solution of hydrogen in Ti0.9Zr0.1Mn1.3V0.7H3d occurs. As the enthalpy values of formation or decomposition in IMC at 52 and 72 C increase in absolute magnitude with increasing of the hydrogen concentration in the IMC though we suppose that this phenomenon is connected to some minor structural distortions of the metallic matrix. To confirm this assumption it is necessary to carry out in situ neutron powder diffraction study in the temperature range 50e70 C. We calculated the partial molar entropy values of hydrogen desorption from hydride Ti0.9Zr0.1Mn1.3V0.7 for each studied temperature of the experiment on the basis of the obtained data of the enthalpy and the equilibrium hydrogen pressures and plotted the DSdes. ¼ f(C) dependences (see Figs. 7e9). As one can see from presented plots the partial molar entropy values decrease on the region of ordered hydrogen solid solution (0 < C < 1). Such reduction of the entropy may be due to the fact that in this hydrogen concentration in the IMC the configuration entropy is very large as the intermetallic compound under the study has a complex structure. Such phenomenon was observed in the works [17e19]. With increasing of hydrogen concentration in the metallic matrix the reduction of configuration entropy takes place and at 52 C we can observe the region of constant entropy values DSdes.
Fig. 5 e Desorption enthalpy vs. composition at 100 C. Different symbols refer to different series of determination.
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Fig. 6 e Desorption enthalpy vs. composition at 130 C. Different symbols refer to different series of determination.
Fig. 8 e Desorption entropy vs. composition at 100 C. Different symbols refer to different series of determination.
(1 < C < 1.8, DSdes.~100 J/mol H2K). Further the DSdes. values rather increase passing through broad maximum and decrease. Increasing of the experimental temperature leads to the changes in the shapes of DSdes.-C plots. At transition to higher temperatures of experiment one can see that extent of the region of ordered hydrogen solid solution in the IMC increases (at 100 C up to ~1.5H/IMC). And for 130 C this tendency remains. It is necessary to pay attention to the fact that the entropy values for the Ti0.9Zr0.1Mn1.3V0.7-H2 system are very low at 100 and 130 C, significantly lower than for other IMC-H2 systems. This also may be connected with the existence of high configurational entropy.
Conclusions Thus the study of the Ti0.9Zr0.1Mn1.3V0.7-H2 system by means of calorimetric method in the temperature range from 52 to 130 C and hydrogen pressure up to 50 atm showed that in the system under the investigation the partial molar enthalpy values of hydrogen interaction with IMC change at transition from one temperature to another. It was shown that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system there is large region of ordered solid solution of hydrogen in IMC and as well the formation of
Fig. 9 e Desorption entropy vs. composition at 130 C. Different symbols refer to different series of determination.
two hydride phases takes place Ti0.9Zr0.1Mn1.3V0.7H~2 and Ti0.9Zr0.1Mn1.3V0.7H~3 at 52 and 72 C furthermore the values of the partial molar enthalpy of formation of tri-hydride is higher than di-hydride in absolute magnitude. On the base of these data we can assume that at 52 and 72 C in the system under study the increase of the hydrogen concentration in the metallic matrix leads to some minor structural distortions of the metallic matrix of the initial IMC. Subsequent increasing of the experimental temperature leads to that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at hydrogen pressure up to 50 atm one hydride Ti0.9Zr0.1Mn1.3V0.7H~2 is formed. Comparing the results obtained by direct calorimetric measurements and calculated on the base of the Van't Hoff equation one can note that the calorimetric method gives more complete and exact picture of processes taking place in the IMC.
references
Fig. 7 e Desorption entropy vs. composition at 52 C.
[1] Sirotina RA, Mitrokhin SV, Zakharova MA, Verbetsky VN. Calorimetric investigation of multicomponent Laves phase
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 4 1 ( 2 0 1 6 ) 1 1 5 2 0 e1 1 5 2 5
[2]
[3]
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[8]
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with hydrogen and deuterium. J Alloys Compd 1993;202:41e5. Flanagan TB, Luo W, Clewley JD. The characterization of multi-component metal hydrides using reaction calorimetry. Z Phys Chem 1993;179:35e44. Anikina EYu, Verbetsky VN. Calorimetric investigation of the hydrogen interaction with Ti0.9Zr0.1Mn1.1V0.1. J Alloys Compd 2002;330e332:45e7. Anikina EYu, Verbetsky VN. Calorimetric investigation of the hydrogen interaction with Ti0.9Zr0.1Mn1.3V0.5. NATO Sci Ser II Math Phys Chem 2004;172:539e46. Anikina EYu, Verbetsky VN. Investigation of the hydrogen interaction with Ti0.9Zr0.1Mn1.3V0.5 by calorimetric method. In: Veziroglu TN, et al., editors. NATO Science Series A: chemistry and biology. Hydrogen materials science and chemistry of carbon nanomaterials. Springer Science; 2006. p. 427e34. Anikina EYu, Verbetsky VN. Calorimetric investigation of the hydrogen interaction with Ti0.9Zr0.1Mn1.2V0.1. Int J Hydrogen Energy 2011;36:1344e8. http://dx.doi.org/10.1016/ j.ijhydene.2010.06.085. Boureau G, Kleppa OJ. Significance of thermal effects associated with solid-gas reaction in Tian-Calvet calorimeter. J Chem Thermodyn 1977;9:543e8. Izumi F. Rietveld analysis and MEM-based whole-pattern fitting under partial profile relaxation. J Rigaku 2000;17e1:34e45. Zotov TA, Sivov RB, Movlaev EA, Mitrokhin SV, Verbetsky VN. IMC hydrides with high hydrogen dissociation pressure. J Alloys Compd 2011;509S:S839e43.
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[10] Hemmes H, Drissen A, Griessen R. Thermodynamic properties of hydrogen at pressure up to 1 Mbar and temperature between 100 and 1000K. J Phys C Solid State Phys 1986;19:3571e85. [11] Currie LA, Svehla G. Nomenclature for the presentation of results of chemical analysis. (IUPAC Recommendations 1994). J Pure Appl Chem 1994;66:595e608. [12] Liu Bin-Hong, Kim Dong-Myung, Lee Ki-Young, Lee JaiYoung. Hydrogen storage properties of TiMn2-based alloys. J Alloys Compd 1996;240:214e8. [13] Grant DM, Murray JJ, Post ML. The thermodynamics of the system for Zr(Fe0.75Cr0.25)2 eH2 using of heat conduction calorimetry. J Solid State Chem. 1990;87(2):415e22. [14] Sirotina RA, Umerenko EA, Verbetsky VN. Calorimetric investigation of hydrogen interaction with intermetallic compound ZrCrFeeH2. Russ Neorg Mater 1996;32(6):710e4. [15] Ivanova TV, Verbetsky VN. Calorimetric investigation of the hydrogen interaction with ZrCrFe1.2. J Alloys Compd 1999;293e295:34e7. [16] Guo Xiumei, Wu Erdong. Thermodynamics of hydrogenation for Ti1-xZrxMnCr Laves phase alloys. J Alloys Compd 2008;455:191e6. [17] Oesterreicher H, Bittner H. Studies of hydride formation in Ti1-xZrxMn2. Matter Res Bull 1978;13:83e8. [18] Sinha VK, Pourarian WE. Hydrogenation characteristics of Zr1-xTixMnFe. J Less-Common Met 1982;87:283e96. [19] Li G, Nishimiya N, Saton H, Kamegashira N. Crystal structure and hydrogen absorption of TixZr1-xMn2. J Alloys Compd 2005;393:231e8. http://dx.doi.org/10.1016/ j.jallcom.2004.08.097.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation of the hydrogen interaction with Ti0.9Zr0.1Mn1.3V0.7 by means of the calorimetric method E.Yu. Anikina*, V.N. Verbetsky Lomonosov Moscow State University, Chemical Department Moscow, 119991, Russia
article info
abstract
Article history:
In the present work we studied the hydrogen interaction with the stoichiometric
Received 30 September 2015
Ti0.9Zr0.1Mn1.3V0.7 Laves phase compound C14 at pressure up to 50 atm and the tempera-
Received in revised form
ture range from 52 to 130 C by means of calorimetric method and plotting of pressure-
7 December 2015
composition isotherms. The processes of hydrogen absorption and desorption were car-
Accepted 18 December 2015
ried out. The findings enabled us to draw the following conclusions: 1) the experimental
Available online 11 January 2016
temperature influences the thermodynamic parameters of studied processes and 2) depending on reaction conditions the formation of one or two hydride phases takes place.
Keywords:
© 2015 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Intermetallic compound Hydride Ti0.9Zr0.1Mn1.3V0.7-H2 system Calorimetry
Introduction Multicomponent intermetallic compounds (IMC) AB2 with hexagonal structure C14 Laves phase are very important and used for storage, transportation and purification of hydrogen. It is well known that such IMCs absorb significant amount of hydrogen, they possess high rate of hydrogen reaction with IMCs and they are resistant to degradation during cycling. The main thermodynamic parameter characterizing the process of hydrogenation is the change of enthalpy of hydrogen reaction with metallic matrix at absorption/desorption processes (DHabs.(des.)). The value of DHabs.(des.), character of its change depending on hydrogenation rate, the temperature of the experiment permit us to understand the mechanism of the hydrogen reaction with the intermetallic alloy better. A lot
of works are devoted to the investigation of the hydrogen interaction with IMCs. Generally the experimental method which is applied for determination of enthalpy change DH and entropy change DS is a calculation of the thermodynamic parameters on the basis of the Van't Hoff equation though the values of enthalpy and entropy calculated in such a way may significantly deviate from real characteristics as this method supposes that the enthalpy and the entropy are independent from the temperature which is not always the case, therefore, this assumption may leads to the errors. There are few works, in which thermodynamic properties of intermetallic compound e hydrogen system (IMC-H2 system) were investigated by the calorimetric method [1e6]. The application of the twincell heat conducting calorimeter permits us to determine directly the heat reaction of hydrogen interaction with IMC. As it was shown in Ref. [7] in this case the measured heats
* Corresponding author. Tel.: þ7 495 939 96 77; fax: þ7 495 932 88 46. E-mail address: [email protected] (E.Yu. Anikina). http://dx.doi.org/10.1016/j.ijhydene.2015.12.126 0360-3199/© 2015 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Materials and methods The initial intermetallic compound Ti0.9Zr0.1Mn1.3V0.7 was prepared by arc melting of a stoichiometric mixture of pure metals in a furnace with a non-consumable tungsten electrode on a water-cooled boat in a purified argon atmosphere under pressure 2 atm. The purity grade of starting metals was better than 99.9%. As a high volatility of manganese in comparison with another metals used for preparing initial compound 4 mass% extra Mn was added to compensate a weight loss during melting. Titanium sponge was used as a getter for purification of argon from admixtures. For that reason it was melted in the furnace before alloying of the starting metals. The button of sample was tuned over and re-melted four times and then annealed in a sealed quartz vessel at 1100 C for 240 h to ensure homogeneity. The phase composition and unit-cell parameters of initial intermetallic compound Ti0.9Zr0.1Mn1.3V0.7 and its hydrides were determined by X-ray diffraction on a DRON-2 powder diffractometer (Cu Ka radiation) and XRD data showed that the parent sample was single-phase compound with the hexagonal Laves phase structure C 14 (MgZn2) with refined units lattice parameters a ¼ 4.922 Å, c ¼ 8.040 Å, c/a ¼ 1.633, V ¼ 168.82 Å3. The refinement of diffraction profiles was performed using the RIETAN-2000 software [8]. To prevent of a hydrogen losses from the hydride phases during their X-ray study under ambient conditions we should passivate synthesized hydrides. Therefore at first we cooled autoclave with hydrogenated sample to the liquid nitrogen temperature, further we depressed of hydrogen pressure to atmospheric pressure, opened autoclave and exposed it to air at 77 K for 2 h [9]. The study of hydrogen interaction with Ti0.9Zr0.1Mn1.3V0.7 was carried out by a calorimetric method. The twin-cell heat conducting calorimeter Tian-Calvet type connected to a conventional Sieverts'-type volumetric apparatus for measuring by a volumetric method of an amount of absorbed or evolved hydrogen was applied. The apparatus scheme, the experimental procedure and the analysis of the collected data were described elsewhere [3]. As a source of pure hydrogen for hydrogenation of Ti0.9Zr0.1Mn1.3V0.7 in this study LaNi5Hx hydride was used. A purity of hydrogen was 99.9999%. Absorption (desorption) relative molar enthalpy DHabs.(des.) was determined from the heat effect of the reaction: Ti0.9Zr0.1Mn1.3V0.7 HX þ y/2H2 4 Ti0.9Zr0.1Mn1.3V0.7 HXþY
(1)
Since Ti0.9Zr0.1Mn1.3V0.7 reversibly reacts with hydrogen the same sample (10,831.7 $ 106 mol Ti0.9Zr0.1Mn1.3V0.7) was used in all experiments. Before each run the residual hydrogen was desorbed at 350e450 C under high vacuum outside the calorimeter. The hydrogen concentration in the sample was calculated from the volumetric measurements using the ideal gas low for pressure < 1 atm, the Van e der e Waals equation of state for pressure below 20 atm and the modified Van e der e Waals equation for pressure above 20 atm [10]. The experimental error in this work was expressed in accordance with recommendations of IUPAC [11] as a standard deviation of the mean value d ¼ √SD2[n(n1)]1, where D is the deviation from the mean value and n is the number of data points.
Results and discussion P-C-T measurements The Ti0.9Zr0.1Mn1.3V0.7-H2 system was studied in the temperature range 52e130 C and hydrogen pressure up to 50 atm. The PeC (P-equilibrium hydrogen pressure, C]H/IMC) desorption isotherms were measured for 52, 72, 100 and 130 C and for 52 and 72 C the PeC absorption isotherms were measured too. Since the overall duration of experiment was about six months it was necessary to check the sample characteristics. For this purpose the runs of hydrogen desorption at 52 C were repeated from time to time. Despite of the multicomponent composition of the sample under investigation the PeC and DHdes. ¼ f(C) dependences measured during these runs provide reproducibility within the present experimental error. In Fig. 1 from the represented data one can notice that the compound under investigation has a good hydrogen capacity (2.9H/IMC at 52 C and hydrogen pressure P ¼ 36 atm). However the reversible hydrogen capacity at this temperature is smaller as the equilibrium hydrogen pressure at the low hydrogen concentration in the IMC is very low. Earlier the authors marked such phenomenon in the work [12] when they studied the effect of different substitutional metals in the TiMn2-based alloys. It has been determined that the increase of the vanadium content in the alloy increased the length of a-region and reduced the reversible hydrogen capacity. V has a strong affinity to hydrogen. The
P, atm
correspond to enthalpy of reaction when expressed per mole H2 or ½ H2. In the present work we continue our study of the (Ti,Zr)(MnV)2±X-H2 system where (Ti,Zr)(MnV)2±X are intermetallic compounds (IMC) with Laves phase structure C 14 by means of the calorimetric method. Earlier we studied the IMCH2 systems in which IMCs had nonstoichiometric compositions [3e6]. Now we have chosen the Ti0.9Zr0.1Mn1.3V0.7 compound with stoichiometric composition as the subject under the investigation.
100
52C 72C
10
100C 130C
1 0 0.1
0.5
1
1.5
2
2.5
3
H/IMC
0.01
Fig. 1 e Desorption isotherms for the Ti0.9Zr0.1Mn1.3V0.7-H2 system.
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P, atm
occupation of V some B sites in the AB2 Laves phase alloys leads to formation of some interstitial sites with a strong affinity for hydrogen acting as trap. In our compound under study V atoms occupy 35% B site in the AB2. The rise of the experimental temperature leads to the increase of equilibrium pressure in the Ti0.9Zr0.1Mn1.3V0.7-H2 system and as a result to larger amount of evolved hydrogen. As one can see from Fig. 1 the plateau slope does not change in practice with rising of experimental temperature when pressures are expressed as logPH2. In Fig. 2 the PeC isotherms of the hydrogen absorption and desorption, measured at 72 C, are plotted. As one can see from the obtained data in the region 0 < C < 2 these isotherms practically coincide. However in the region 2.0 < C < 2.8 the small pressure hysteresis is observed (ln(Pabs./Pdes.) ¼ 0.29 at C ¼ 2.5), though the sharp change of slope of the PeC curves is observed at C~2.0. For C > 2.8 this system exhibits the complete reversible behaviour in the PeC characteristics, the simple process of the hydrogen solution takes place. It may be an experimental verification of an existence of the two-phase equilibrium in this concentration region. Similar phenomena were observed previously in the works [13e15] for AB2-H2 systems: Zr(Fe0.75Cr0.25)2-H2, ZrCrFeH2, and ZrCrFe1.2-H2, respectively. The partial molar enthalpy values of hydride decomposition for the Ti0.9Zr0.1Mn1.3V0.7-H2 system were obtained by applying the Van't Hoff equation to the PeC isotherms. The plot of the calculated DHdes. ¼ f(C) dependence is presented in Fig. 3. As one can see from this plot there is the initial region (0.3 < C < 0.4) where the enthalpy values decrease after that some increase took place up to ~30 kJ/molH2 and thereupon in the region 0.5 < C < 1.0 the enthalpy values remained steady (~27.34 ± 0.05 kJ/molH2). At the hydrogen concentration in the IMC more 1 (C > 1) the gradual increase of enthalpy values is observed up to 35 kJ/molH2 and in the region 2.15 < C < 2.50 enthalpy values are equal 34.57 ± 0.78 kJ/molH2. Such an increase of the partial molar enthalpy values of the hydrogen reaction with IMC along with the C axis we observed previously in the works [4e6] for the Ti0.9Zr0.1Mn1.3V0.5-H2 and Ti0.9Zr0.1Mn1.5V0.8-H2 systems, as well as for the Ti0.1Zr0.9MnCr-H2 and ZrMnCr-H2 systems [16].
60 50 40 30
72C-abs. 72C-des.
20 10 0 0
0.5
1
1.5
2
2.5
3 H/IMC
Fig. 2 e Absorption and desorption isotherms for the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 72 C. Filled symbols refer to absorption of H2 and open symbols to desorption.
Fig. 3 e Partial molar desorption enthalpy calculated from van't Hoff plot for the Ti0.9Zr0.1Mn1.3V0.7-H2 system (a temperature range 52e130 C).
Calorimetric results The partial molar enthalpy values of the hydrogen absorption and desorption obtained by the calorimetric method are presented in Table 1. As one can see from the presented data in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 52 and 72 C there are three regions of hydrogen concentration in the IMC under investigation where the enthalpy values are constant for the process of hydrogen absorption or desorption. It should be noticed that the regions of the hydrogen concentration in the metallic matrix with the constant enthalpy values coincide for the absorption and desorption processes. As well it should be noticed that for every hydrogen concentration region these enthalpy values agree in absolute magnitude within the limits of experimental error (for absorption and desorption processes). In Fig. 4 the plot of the DHdes. ¼ f(C) dependence at 52 C is presented. One can see that the enthalpy values increase with the increase of hydrogen concentration in the metallic matrix. At 72 C in the Ti0.9Zr0.1Mn1.3V0.7-H2 system as well as at 52 C there are three regions with constant enthalpy values (see Table 1). For 72 C the processes of absorption and desorption were studied and obtained data show that the enthalpy values in each of these regions coincide in absolute magnitude. In addition it should be noticed that the enthalpy values do not change at transition from 52 to 72 C. For 100 and 130 C only the hydrogen desorption from hydride IMC was studied and the enthalpy values were measured for these process. The data presented in Table 1 and Figs. 5 and 6 show that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 100 and 130 C there are two regions where the enthalpy values are constant. The enthalpy values increase with the increase of hydrogen concentration in the sample under consideration for each temperature but comparing the enthalpy values obtained at different temperatures it can be noticed that the increase of experimental temperature leads to insignificant reduction of enthalpy values.
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Table 1 e Temperature dependence of reaction enthalpy for the Ti0.9Zr0.1Mn1.3V0.7-H2 system. Temperature, C 52
72
Range (absorption)
DH, kJ/mol H2 30.5 33.8 38.4 31.0 36.0 38.3
0.2e0.8 1.0e1.8 1.8e2.8 0.5e1.5 1.6e2.1 2.1e2.6
100 130
It was necessary to clarify the situation occurring in the Ti0.9Zr0.1Mn1.3V0.7-H2 system and to understand whether we deal with the formation of three hydride phases or the formation of ordered hydrogen solid solution which takes place at hydrogen concentration in the IMC C < 1 and then in the system under the investigation the some lattice transformation occurs and the hydride phase starts to form. We hydrogenated the sample under the investigation up to composition of Ti0.9Zr0.1Mn1.3V0.7H~1 at room temperature and carried out X-ray powder diffraction analysis. X-ray analysis data showed that Ti0.9Zr0.1Mn1.3V0.7H~1 at room temperature there was the single-phase compound with the hexagonal Laves phase structure C14, its crystal lattice remained unchanged relative to the crystal lattice of the initial compound but unit cell parameters slightly increased (a ¼ 5.0168 Å, c ¼ 8.23 Å, V ¼ 179.38 Å3, DV/V ¼ 2.7%) in comparison with the initial compound. Therefore it can be suggested that at C < 1 we deal with the formation of ordered hydrogen solid solution in Ti0.9Zr0.1Mn1.3V0.7. Moreover we hydrogenated the parent sample up to composition Ti0.9Zr0.1Mn1.3V0.7H3 at room temperature and X-ray analysis data revealed that Ti0.9Zr0.1Mn1.3V0.7H3 was the single-phase compound with hexagonal Laves phase C14 structure, the volume of the initial unit cell increased on ~23%. Thus obtained experimental data of the DHabs.(des.) ¼ f(C) dependence permit us to assume that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at 52 and 72 C two hydride phases can form depending on the hydrogen concentration in the metallic matrix: di-hydride (or Ti0.9Zr0.1Mn1.3V0.7H2d) and
Fig. 4 e Desorption enthalpy vs. composition at 52 C.
± 0.9 ± 2.0 ± 2.3 ± 0.6 ± 1.1 ± 1.2
Range (desorption)
DH, kJ/mol H2
0.2e0.8 1.0e1.8 1.8e2.8 0.2e1.3 1.4e1.8 1.9e2.7 0.1e1.2 1.5e2.5 0.1e1.2 1.3e2.0
31.1 ± 0.6 33.9 ± 0.4 38.9 ± 0.6 29.5 ± 0.3 34.5 ± 1.0 39.2 ± 1.1 29.1 ± 0.4 36.0 ± 0.6 29.9 ± 0.5 34.3 ± 0.6
tri-hydride (or Ti0.9Zr0.1Mn1.3V0.7H3d). Observed regions of the constant enthalpy values correspond to two stages of the formation of hydride phases in the regions of hydrogen concentrations 1.0 < C < 1.8 and 1.8 < C < 2.8. At the hydrogen concentration C > 2.8 further solution of hydrogen in Ti0.9Zr0.1Mn1.3V0.7H3d occurs. As the enthalpy values of formation or decomposition in IMC at 52 and 72 C increase in absolute magnitude with increasing of the hydrogen concentration in the IMC though we suppose that this phenomenon is connected to some minor structural distortions of the metallic matrix. To confirm this assumption it is necessary to carry out in situ neutron powder diffraction study in the temperature range 50e70 C. We calculated the partial molar entropy values of hydrogen desorption from hydride Ti0.9Zr0.1Mn1.3V0.7 for each studied temperature of the experiment on the basis of the obtained data of the enthalpy and the equilibrium hydrogen pressures and plotted the DSdes. ¼ f(C) dependences (see Figs. 7e9). As one can see from presented plots the partial molar entropy values decrease on the region of ordered hydrogen solid solution (0 < C < 1). Such reduction of the entropy may be due to the fact that in this hydrogen concentration in the IMC the configuration entropy is very large as the intermetallic compound under the study has a complex structure. Such phenomenon was observed in the works [17e19]. With increasing of hydrogen concentration in the metallic matrix the reduction of configuration entropy takes place and at 52 C we can observe the region of constant entropy values DSdes.
Fig. 5 e Desorption enthalpy vs. composition at 100 C. Different symbols refer to different series of determination.
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Fig. 6 e Desorption enthalpy vs. composition at 130 C. Different symbols refer to different series of determination.
Fig. 8 e Desorption entropy vs. composition at 100 C. Different symbols refer to different series of determination.
(1 < C < 1.8, DSdes.~100 J/mol H2K). Further the DSdes. values rather increase passing through broad maximum and decrease. Increasing of the experimental temperature leads to the changes in the shapes of DSdes.-C plots. At transition to higher temperatures of experiment one can see that extent of the region of ordered hydrogen solid solution in the IMC increases (at 100 C up to ~1.5H/IMC). And for 130 C this tendency remains. It is necessary to pay attention to the fact that the entropy values for the Ti0.9Zr0.1Mn1.3V0.7-H2 system are very low at 100 and 130 C, significantly lower than for other IMC-H2 systems. This also may be connected with the existence of high configurational entropy.
Conclusions Thus the study of the Ti0.9Zr0.1Mn1.3V0.7-H2 system by means of calorimetric method in the temperature range from 52 to 130 C and hydrogen pressure up to 50 atm showed that in the system under the investigation the partial molar enthalpy values of hydrogen interaction with IMC change at transition from one temperature to another. It was shown that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system there is large region of ordered solid solution of hydrogen in IMC and as well the formation of
Fig. 9 e Desorption entropy vs. composition at 130 C. Different symbols refer to different series of determination.
two hydride phases takes place Ti0.9Zr0.1Mn1.3V0.7H~2 and Ti0.9Zr0.1Mn1.3V0.7H~3 at 52 and 72 C furthermore the values of the partial molar enthalpy of formation of tri-hydride is higher than di-hydride in absolute magnitude. On the base of these data we can assume that at 52 and 72 C in the system under study the increase of the hydrogen concentration in the metallic matrix leads to some minor structural distortions of the metallic matrix of the initial IMC. Subsequent increasing of the experimental temperature leads to that in the Ti0.9Zr0.1Mn1.3V0.7-H2 system at hydrogen pressure up to 50 atm one hydride Ti0.9Zr0.1Mn1.3V0.7H~2 is formed. Comparing the results obtained by direct calorimetric measurements and calculated on the base of the Van't Hoff equation one can note that the calorimetric method gives more complete and exact picture of processes taking place in the IMC.
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Fig. 7 e Desorption entropy vs. composition at 52 C.
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