Hydrides ScFe(Ni)2Hx: preparation and properties

International Journal of Hydrogen Energy 26 (2001) 449–452

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Hydrides ScFe(Ni)2Hx : preparation and properties V.N. Fokin ∗ , E.E. Fokina, B.P. Tarasov, I.I. Korobov, S.P. Shilkin Institute of Problems of Chemical Physics of Russian Academy of Sciences, Chernogolovka 142432, Moscow Region, Russia

Abstract The interaction of the intermetallic compounds ScFe2 and ScNi2 with ammonia was investigated at an initial pressure of ammonia of 0.6 – 0.8 MPa in the presence of 10 mass% (from the quantity of intermetallide entered in reaction) of NH4 Cl as a process activator. The possibility of obtaining both crystalline hydrides of the intermetallic compounds and amorphous products of a reaction in a high-dispersed state was shown at various temperatures. It has been established that the decomposition ◦ of the intermetallides in ammoniacal medium occurs at ¿ 450 C. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Intermetallic compound; Ammonia; Hydride

1. Introduction At present, the problems of elaboration of new methods of synthesis, the conditions of formation and the stabilization of high-dispersed compounds — hydrides, nitrides, borides, carbides, oxides, etc. — attract the attention of a wide circle of researchers working in the >eld of preparative inorganic chemistry. A new method of the synthesis of the ultradispersed titanium hydride crystallized into a tetragonal lattice and stabilized by a small quantity of nitrogen has been o?ered [1]. The method is based on the interaction of titanium with ammonia in the presence of ammonium chloride, and depending on the conditions of interaction and the degree of dispersivity of initial titanium, titanium nitride, titanium hydridonitride or titanium hydride can be obtained in a high-dispersed state [1,2]. Since the intermetallic compounds (IMC) and their hydride phases formed by rare-earth metals, Sc, Y, Ti, Zr and 3d-transition metals are considered as perspective materials in metalhydride technologies, for the creation of permanent magnets of new generation, in electrochemical sources of energy, etc., it is of interest to investigate an

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Corresponding author.

opportunity of obtaining hydrides and other derivatives of IMC in high-dispersed state by the above-mentioned method — by the interaction of IMC with ammonia in the presence of NH4 Cl (by analogy with [1,2]). IMC ScFe2 and ScNi2 , the reactions of which with hydrogen, nitrogen–hydrogen mixture and ammonia are investigated in detail in works [3– 6], are chosen as the model subjects. According to [7], the compound ScFe2 melts congruently and has two polymorphic modi>cations: 3 -phase ◦ (MgNi2 -type structure) forms above 1200 C, 1 -phase ◦ (MgZn2 -type structure) forms below 1200 C. At a 36.5 at% content of Sc the cubic Laves phase 2 -ScFe1:74 (MgCu2 -type structure), forming by the peritectic reaction ◦ ◦ at 1525 C and breaking down at 1295 C with the formation of 3 -phase, is found. ◦ The compound ScNi2 is formed from a melt at 1310 C. It has the homogeneity region of 64 –70 at% Ni and belongs to the structural type of MgCu2 [8]. The interaction of IMC ScFe2 with hydrogen was investigated in a number of works [3,4,9,10]. The modi>cation 1 -ScFe2 absorbs hydrogen reversibly up to a composition ◦ ScFe2 H3:2 at 0.7 MPa and 20 C [9]. As shown in [10], the cubic phase 2 -ScFe1:74 is hydrogenated in the presence of Sc at room temperature up to a composition of ScFe1:74 H3:0 . At room temperature and under hydrogen pressure up to 6 MPa the hydride phase of the composition 3 -ScFe2 H3:0 was obtained. After pressure removal this phase remains till

0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 0 ) 0 0 0 7 7 - X

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2.5 –2.7 H atoms=ScFe2 and decomposes at 230 C with the formation of the monohydride ScFe2 H1:0 , which at further ◦ heating up to 450 C does not evolve hydrogen [3]. The hydride phase ScNi2 H2:0 formed evolves, at pressure removal, a part of hydrogen up to the composition ScNi2 H1:6 ◦ and is decomposed at heating up to 115 C with complete evolution of hydrogen [3]. IMC ScFe2 (a 3 -phase) and ScNi2 interact with nitrogen– hydrogen mixture under a pressure of 5 MPa at room temperature with the formation of hydride phases ScFe2 H2:7 and ScNi2 H1:4 , accordingly. At an increase of the temperature ◦ up to 450 C there is a decomposition of the initial metallic matrices on the scandium nitride and -Fe or Ni [5]. At treatment of indicated IMC by gaseous ammonia under ◦ a pressure of 1 MPa at 450 C, the decomposition of the initial intermetallides on the scandium nitride and -Fe or ◦ Ni occurs, and at 525 C for ScFe2 among the crystalline products of a reaction, only iron nitride of the composition Fe4 N is found [6]. The present work is devoted to the research of an interaction of IMC ScFe2 (a 1 -phase) and ScNi2 with ammonia in the presence of ammonium chloride with the purpose of obtaining high-dispersed products. 2. Experimental The initial samples of IMC ScR 2 were prepared by alloying scandium (purity, 99.8%) with iron (purity, 99.9%) or nickel (purity, 99.99%) under ∼ 0:2 MPa of pure argon in an electroarc furnace with non-spent tungsten electrode. The bake-out of alloys was carried out in quartz ampoules ◦ at 800 C for 250 h with subsequent quenching in cold water. ScFe2 powders with a particle size of 50 mcm were prepared by crushing the alloy bead in a metallic mortar with ◦ subsequent treatment of a material in the vibromill at 20 C for 30 min and with screening the necessary fraction. The speci>c surface of such powder is 0:13 m2 =g. ScNi2 powders with a particle size of 50 mcm (the speci>c surface is 0:24 m2 =g) were prepared by the method of hydride dispergation. For ScFe2 , the MgZn2 structure was con>rmed with lattice J and c = 8:0371 A, J for ScNi2 the constants a = 4:9650 A J was con>rmed. MgCu2 structure (a = 6:9052 A) Ammonium chloride was dried by an evacuation for 9 h ◦ at 150 C. The ammonia, desiccated by metallic sodium, had a purity of 99.99%. The X-ray graphical investigations were performed on an automatic assembly consisting of a di?ractometer ADP-1 (Cu K -emission) and a computer IBM PC=AT. The determination error of the crystal lattice constants did not exceed J 0:005 A. The speci>c surface of the samples was determined on an amount of low-temperature adsorption of krypton after the removal of volatile products from a solid phase under a

◦

vacuum of 1:3 × 10−3 Pa at a temperature of 300 C for 15 h and was calculated by the Brunauer–Emmet–Teller method. The determination error was ±10%. The thermogravimetric investigations were carried out on a derivatograph of type 3434C with microprocessor control in a stream of argon. The contents of hydrogen in the reaction products were de>ned by a standard method of burning the samples in a stream of oxygen. The contents of nitrogen were determinated by the Kjeldal method. The contents of chlorine were analyzed by the method of turbidimetry. The process of IMC nitriding was investigated at an initial pressure of ammonia of 0.6 – 0.8 MPa by using ammonium chloride, as activator, added in the quantity of 10 mass% from the quantity of IMC used in the reaction. The mixture of IMC and NH4 Cl powders was subjected to vibration milling at room temperature for 30 min. The interaction of the prepared mixture with ammonia was conducted in the laboratory plant of high pressure with a capacity of 60 cm3 . The given quantity of prepared mixture (1.0 –1.5 g), placed in a stainless-steel container, was loaded into a reactor-autoclave; traces of oxygen and moisture were removed under a vacuum of ∼1 Pa at room temperature for 30 min; ammonia was brought up to a pressure of 0.6 – 0.8 MPa and the reactor-autoclave was kept at the same temperature for 30 min. Further, the reactor was heated up to ◦ various temperatures for 3 h, was cooled up to ∼20 C and was again heated up. After the realization of a given quantity of heating–cooling cycles, ammonia was discharged into a bu?er vessel, and ammonium chloride was removed under a vacuum of ∼1 Pa, if necessary, by heating the re◦ actor up to 180–200 C for 2 h. The reaction products were unloaded in an inert atmosphere and analyzed. 3. Results and discussion The results of an interaction of ScFe2 and ScNi2 with the average size of the particles of 50 mcm with ammonia in the presence of 10 mass% NH4 Cl are given in Table 1. In the reaction considered there is an increase of pressure in the system at the expense of an evolution of hydrogen and nitrogen, the process termination is determined by a cessation of the pressure change. As follows from the data of Table 1, an interaction of ◦ ◦ ScFe2 at 150 C and ScNi2 at 200 C with ammonia (samples 1,8) gives rise to the crystalline hydride phases ScFe2 H1:5 J and c = 8:1328 A J and with lattice constants a = 4:9704 A J The particle ScNi2 H1:0 with lattice constant a = 7:0683 A. size, calculated in an approximation of their ball form, is equal to 130 and 145 nm, respectively. ◦ The rise of the temperature to 200 –350 C for ScFe2 (sam◦ ples 2– 4) and to 300–350 C for ScNi2 (samples 10, 11) is accompanied, on data of the X-ray phase analysis, by the formation of the amorphous products of a reaction. At a tem◦ perature of 400 C for ScFe2 (sample 5) and ScNi2 (sam-

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Table 1 Conditions and investigation results of the interaction of ScR 2 with ammonia Sample no.

Initial IMC

1 2 3 4 5 6 7 8 9 10 11 12 13 14

ScFe2 ScFe2 ScFe2 ScFe2 ScFe2 ScFe2 ScFe2 ScNi2 ScNi2 ScNi2 ScNi2 ScNi2 ScNi2 ScNi2

Sample no.

Phase composition of interaction products

1 2 3 4 5 6 7 8 9 10 11 12 13 14

ScFe2 H1:5 X-ray X-ray X-ray ScFe2 Hx + -Fe -Fe+ ScN0:87 -Fe+ ScN0:87 + ScH2 ScNi2 H1:0 ScNi2 Hx + Ni X-ray X-ray Ni Ni + ScNx Ni + ScNx + ScH2

Conditions of synthesis ◦

T ( C)

Time (h)

PNH (MPa)

150 200 250 350 400 450 500 200 250 300 350 400 450 500

31 33 33 31 31 33 29 33 33 33 33 33 33 31

0.74 0.65 0.70 0.72 0.76 0.76 0.68 0.68 0.67 0.72 0.64 0.68 0.66 0.72

Lattice constants of interaction products J a (A)

J c (A)

4.9704 amorphous amorphous amorphous 4.9425 2.8666 2.8681 4.4950 2.8656 4.5039 4.7641 7.0683 6.9156 3.5252 amorphous amorphous 3.5250 3.5230 4.4144 3.5204 4.3709 4.7820

8.1328

ple 12) the partial decomposition of the metallic matrices of IMC with a precipitation of -Fe or Ni takes place. The complete disintegration of the matrices of initial IMC oc◦ curs at 450 –500 C (samples 6, 7, 13 and 14) and leads to obtaining the homogeneous mixtures of the nitride and the hydride of scandium with iron or nickel. The calculated lattice constants of iron, nickel, scandium hydride and scandium nitride, as well as of the hydride phases of IMC, coincide well with the literature data [11–13].

8.1682 — — — — — — — — — — — — — — —

Speci>c surface of interaction products (m2 =g) 7.4 5.3 3.5 5.9 3.8

10.1 6.8 5.1 10.8 3.4 4.2 1.8 8.0

The thermal decomposition of the obtained hydride phases of IMC occurs with an isolation of the parent quantity of ◦ hydrogen at temperatures of 250–270 C and comes to an ◦ end at 750–800 C. According to the data of the X-ray phase and chemical analyses, the solid products of a thermolysis represent the initial IMC. Thus, the interaction of ScFe2 and ScNi2 with ammonia in the presence of NH4 Cl can be described by the reactions, the schemes (1) – (3) of which are shown below

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V.N. Fokin et al. / International Journal of Hydrogen Energy 26 (2001) 449–452

(R = Fe; Ni). Especially, it is necessary to note, that scheme (1) is suitable for obtaining the high-dispersed hydride phases of IMC. NH Cl

4 ScR 2 + NH3 −−−− −→ ScR 2 Hx + N2 ; ◦ 150–200 C

(1)

NH Cl

4 −→ the X-ray amorphous products; ScR 2 + NH3 −−−− ◦ 200–350 C

NH Cl

4 −→ R + ScNx + ScHy + H2 : ScR 2 + NH3 −−−− ◦ 450–500 C

(2) (3)

Acknowledgements This work is supported by the Russian Basic Research Foundation (grant No. 98-03-32751). References [1] Fokin VN, Fokina EE, Tarasov BP, Shilkin SP. Int J Hydrogen Energy 1999;24:111. [2] Fokin VN, Shilkin SP, Fokina EE, Mozgina NG. Zh Obshch Khim 1997;67:705 (in Russian).

[3] Burnasheva VV, Ivanov AV, Yartys’ VA, Semenenko KN. Izv Akad Nauk SSSR, Ser Neorg Mater 1981;17:980 (in Russian). [4] Semenenko KN, Sirotina RA, Savchenkova AP, Burnasheva VV, Lototskii MV, Fokina EE, Troitskaya SL, Fokin VN. J Less-Common Met 1985;106:349. [5] Semenenko KN, Shilkin SP, Burnasheva VV, Volkova LS, Govorkova LV, Mozgina NG. Zh Obshch Khim 1984;54:491 (in Russian). [6] Shilkin SP, Volkova LS. Zh Neorg Khim 1992;37:2417 (in Russian). [7] Bodak OI, Kotur BYa, Gavrilenko IS, Markiv VYa, Ivanchenko GI. Dokl Akad Nauk SSSR, Ser A 1978;4:366 (in Russian). [8] Markiv VYa, Gavrilenko IS, Pet’kov VV, Belyavina NN. Metallo>zika, Respublikanskii mezhvedomstvennyi sbornik 1978;73:39 (in Russian). [9] Niarcos D, Viccaro PJ, Dunlap BD, Aldred AT. Hyper>ne Interact 1981;9:563. [10] Burnasheva VV, Fokina EE, Fokin VN, Troitskaya SL, Semenenko KN. Zh Neorg Khim 1984;29:1379 (in Russian). [11] Protasov VI. Fiz Kristallizatsii 1967;11:591 (in Russian). [12] Aivazov MI, Rezchikova TV, Gurov SV. Izv Akad Nauk SSSR, Ser Neorg Mater 1977;13:1235 (in Russian). [13] Alefeld G, VPolkl J, editors. Hydrogen in metals. II. Application-oriented properties. Berlin: Springer, 1978.