Self-propagating high-temperature synthesis in Y-N-H systems

Pergamon PII:

SELF-PROPAGATING

Im. J. Hydrogen Energy, Vol. 21, No. 11112,pp. Y55 959, 1996 Copyright (Cl 1996International Association f’or Hydrogen Energy Elsevier ScienceLtd Printed in Great Britain. All rights reserved sO360-31!J9(%073-0 0360-3190~9651’.00i0O~~

HIGH-TEMPERATURE SYSTEMS

SYNTHESIS IN Y--N-H

A. G. ALEKSANYAN and S. K. DOLUKHANYAN Institute of Chemical Physics, Armenian National Academy of Science.Sevak str. 5!2. Yervan, 375044Armenia (Receivedforpublication

12 April 1996)

Abstract-The combustion processesin Y-H, Y-N and Y-N-H systems are studied by the self-propagatinghightemperaturesynthesis(SHS)method.The main regularitiesof the combustionprocesses and of the formationof end productsare determined.Di- and trihydrides, nitride and hydro-nitride of yttrium are created.The combustionof yttrium in a gasmixture is dependenton the partial pressureratio of the reactinggases.The reactionscan proceedin three different directions,leading to the formation of yttrium hydrides,nitrides and hydro-nitrides.Copyright i’ 1996 International Associationfor HydrogenEnergy

ticularly applies to making YH,, the conditions for which are difficult to generate. For example, the reaction The goal of the present investigation is to study the com- between YH, and hydrogen starts at 350°C. continues at bustion processesin the Y-N-H systemand the synthesis 200°C and finishes after 48 hours [3]. of yttrium hydro-nitride. All the combustion processesin Interaction of yttrium with hydrogen in the eomthe Y-N and Y-H systemswere studied simultaneously. bustion regime was studied within the range of pressures All three systemshave not been studied before using the up to 30 atm. The yttrium shavings readily ignite in SHS method. The investigation of the ternary Y-N-H hydrogen at PH2 = 1 atm. The combustion product is systemis impossible without knowledge of the main regu- yttrium hydride. The characteristics of yttrium hydride larities of the combustion in the binary systems. are indicated in Table 1. As can be seen fro& Tabie 1 These investigations were carried out by SHS [l, 21. the influence of hydrogen pressure here is cansiderabte, SHS is a heterogeneous process occurring in the regime particularly on the metals of group IV. Hydrogen content of a directed combustion without any external heat sour- in the hydride grows with increasing ms pressure. On ces by virtue of the internal resources of the system. a level with dihydride phase the trihydride phase with hexagonal close-packed lattice appears. From 20 atm and higher the hydrogen pressure does not intiemze the EXPERIMENTAL chemical and phase compositions. With the increase of Yttrium of grade ITM-1 was ground by crushing a pressure, the temperature and the rate of combustion = I%@-15oO”C, and compact ingot in an argon atmosphere. Hydrogen and grow in the ranges of TEomb nitrogen with a “pure” classification were used. The u comb = 8.2-9.4 sm/s, respectively. The temwature of obtained shavings were pressed into cylindrical tablet combustion approaches the yttrium me& point ‘& Ieads forms, or were added into quartz glass. The combustion to the yttrium melting. However the me&ing of the initial reaction was initiated by an incandescent tungsten spiral. metal does not create any diffusion ditiulties for hydrogen. Figure 1 depicts the dependencesof hydragen content in the combustion products and temperature on COMBUSTION OF THE Y-H SYSTEM hydrogen pressure. INTRODUCTION

It is known that yttrium with hydrogen forms hydrides with MeH, and MeH, compositions. Yttrium dihydride has a fee structure. With increased hydrogen content the trihydride phase (MeH,) appears [3]. The traditional mode of the yttrium hydride synthesis is based on the thermal processing of the metals in a hydrogen atmosphere. It is a difficult process. This par955

COMBUSTION OF Y--N SYSTEM The phase diagram of the Y-N system has not been sufficiently studied [4]. In this system, only one nitride of the NaCl type with a fee structure has been found (lattice

A. G. ALEKSANYAN

956

and S. K. DOLUKHANYAN

Table 1. Characteristics of yttrium hydrides

PH* atm. 2 5

tHz content wt. %

Lattice parameters A Crystal structure fee fee fee

20

2.0 2.03 2.20 3.15

hcp

30

3.28

hcp

15

experiment

literature

a = 5.228

a = 5.209 a = 5.209

a= a= a= c= a= c=

5.224 5.226 3.661 6.630 3.659 6.632

Actual formula YH, 8, YH, 84 YHz YH,,,

a = 3.672 c = 6.625

YH, 96

yttrium melting. At low pressures(up to 2-3 atm), Tcomb is slightly higher than the Tmeltof yttrium, therefore the samplesmelt without the form changing. At PH, > 3 atm the cylindrical sample melts and flattens. As in-the combustion process, the sample melting takes place at a temperature below Tmeltof yttrium nitride (T,,,, = 2600°C) [4, 51.One can conclude that the nitride formation occurs after combustion but not in the combustion front. The annealing of intermediate combustion products by gas throw out of the reaction spaceshowed that the hcp phase of nitrogen solid solution in yttrium is not observed. This is related to the narrow zone of combustion and the high nitrogenation rate of this combustion regime. I I II I Yttrium melts completely at nitrogen pressureshigher 10 2 5 Pressure (a!@Hz than 10 atm. It apparently creates some difficulties for Fig. 1. The influence of H, pressure on the combustion tem- nitrogen diffusion, causing low nitrogen content in the perature of yttrium (1) and hydrogen concentration in the end combustion products. Analogous phenomena have been products (2). observed in Refs. 16, 71where the SHS-processesin the Ti-N and Zr-N systemshave been studied. In these systems as a result of an increase in Tcomb, melting of metals parameter u = 4.887 A). In Ref. [5] two methods of takes place, which leads to the halting of after-nitroyttrium nitride synthesis have been described. genation. This sharply reduces the nitrogen content and One of them consists of nitrogenation of yttrium shavleads to the formation of nitrogen solid solution in ings in the presence of ammonium at 700-l 100°C. The titanium and zirconium. second method of YN formation consists of the interIn the Y-N system, upon the increase of the nitrogen action of yttrium hydride with gaseous nitrogen or pressure, despite the decreasein nitrogen content in the ammonium at T = 900°C. combustion products, the nonstoichiometric nitride fee We have studied yttrium combustion within the range phase is always formed throughout the whole invesof nitrogen pressure l-90 atm. In this range the comtigated pressure range. It is a distinct peculiarity of bustion product is yttrium nitride. The results of the chemical and X-ray phase analysis yttrium combustion in nitrogen. The nitrogen solid soluof combustion products of the Y-N system are given in tion phase in yttrium is not found. Table 2. From Table 2 it can be seen that Tcomb and Ucomb Unfortunately, in the framework of this study we cangrow with the increasing nitrogen pressure, leading to the not determine the range of homogeneity of nonTable 2. Characteristics of yttrium nitrides PN2 atm. 1.5 3 5

Phasecomposition and formula YNo.,, YNo 7 ~cl.75

10

YN,.,,

90

YN,.,

fee fee fee fee fee

N* content wt.% 10.3

10.0 10.46 8 7.05

Crystal lattice, A

“C

sm/s

4.868 4.861 4.865 4.855 4.846

1650 1750 1780 1990 2500

6.5 6.8 7.0 7.2 9.5

Tcomb

u comb

057

SHSPROCESSESIN Y-N-H SYSTEMS stoichiometric yttrium nitrides. In scientific publications, information about these compounds was not available. The chemical and X-ray phase analysis data show that upon decreasing nitrogen content in yttrium nitride, the lattice parameters in the fee phase decline. It again proves the formation of nonstoichiometric yttrium nitrides at relatively high nitrogen pressure. Our experiments show that SHS yttrium nitride, similar to YN obtained by the traditional method is unstable in air. We also obtained yttrium nitride by yttrium hydride combustion in nitrogen at PN, = 10 atm. To realize the combustion reaction in the YH2-N, system the metallic yttrium up to lo-20 wt% was first added to YH2:

to move on to analysis of the combustion process in the ternary system. The Y-N-H system’s combustion was studied over a wide range of pressures and partial pressure ratios 2 < PH2/PN2 < 0.5 (total pressure up to 15 atm). The results of the chemical and X-ray-phase analyses of combustion products of the Y-N-H systems, obtained at different partial ratios PH,/PNI, are given in Table 3. It was found that yttrium combustion in the mixture of two reacting gasesproceeds in three different ways by analogy with the combustion of titanium, zirconium and hafnium in the nitrogen and hydrogen mixture [Xl. YH. /” Y+N:+H2-+YN,H,.+YN.,+YHT \ YN

YH,+lO%Y+NZ+YN+Hzf. Thus, analysis of experimental data indicates that the combustion of yttrium in nitrogen occurs like the previously studied combustion of group IV metals in nitrogen. The main regularities, typical for combustion of these metals in nitrogen, are indentical to those for combustion of yttrium. However, in this case, an increase of nitrogen pressure causes the formation of fee phase nonstoichiometric yttrium nitrides, but not the solid solution in metal as in the case of group IV metals. COMBUSTION

OF THE Y-N-H

SYSTEM

The determination of the main characteristics and of the mechanism of the end products’ formation of the combustion in Y-H and Y-N systemsmakes it possible

1 11 III

When the hydrogen pressure is higher than that of nitrogen (PH2/Py2 > 2) the formation of hydrides is favourable (reaction I). In this casethe nitrogen does not take part in the reaction. The rate and temperature of the process are typical for the reaction of hydride formation (Tcm = 12OG1300°C). Another reaction, when the nitrogen pressure is higher than that of hydrogen has a tendency towards the nitride formation (PHI/P,, < 0.5). Here hydrogen does not take part in the reaction (reaction III). The temperature of combustion is typieal for nitride formation ( Tcomb= 16O&lSOO”C). Finally, when PHI/PN2 < 1.0, the products of reaction II represent

Table 3. The combustion parameters and characteristics of products Y--N-H system

H*

PH.

content wt.%

p,

PH, atm

PN* atm

P 11. P >2

10

u.

6

5 2

1.8 1.85

1

2

0.83

P 0.5$

u.

< 1

PHI --.. < 0.5 PN.

W

content wt.%

Phase composition Y H, (fee) YH, (fee)

2.49

YH,H,+YN hcp fee YN,H, +YN hcp see YN,H,.+YN hcp fee YN,H,.+YN fee hcp YN,H, + YN hcp fee

5

10

1.2

3.9

3

5

1.29

2.8

4

5

1.1

2.5

2

2

0.9

3.75

3

3

0.89

2.51

YN,rH,+ YN

1.2

5.14

hcp fee YN,H, + YN fee hcp

-

8.45 9.13

YN (kc) YN (fee)

5

5

3 5

10 20

Lattice parameters A

~,““,I. fc 1150 1250

a=3.649 c = 3.713 a = 3.65 c = 5.788 a = 3.658 c = 5.791 a = 3.65 c = 5.79 a = 3.658 c = 3.827 a= 3.65 c = 5.79 a = 3.657 c = 5.781

1700 1750

1550 !570 1600 2000 2200

A. G. ALEKSANYAN and S. K. DOLUKHANYAN

958

?H, %N2(wt) 3.14 %Ha(wt) 0.89 hexagonal

a = 3.658/3 c= 5.827A

20

30

40

50

al

70

80

YH, cubic a = 52077A

+ YN cubic a = 4.877A

all”

Y yttrium a_ a = 3.6474,

z! -1: $ 20

30

40

50

60

(degrees) Fig. 2.

3 70

8

i

c=5.7306A hexagonal

80

20

yttrium hydro-nitride with hexagonal close-packed lattice and two fee phases-YH, and YN (< 5-10%). The monophase yttrium hydro-nitride was not obtained. Reaction II was started by nitrogen, therefore the temperature of combustion approaches the temperature of the nitrogen reaction (Tc,,m,,= 1600-1700°C). Figure 2 illustrates X-ray diagrams of yttrium and its compounds synthesized during the experiment. Comparing the results obtained during the combustion of Y-N-H with those of analogous systems such as Ti-N-H [6], Zr-N-H [7] and Hf-N-H [8], one can conclude that the main tendency of interaction of the transition metals of group IV and of the group III metal (yttrium) with the mixture of gases is identical. For the combustion of metals in a mixture of gases,the most interesting reaction is the formation of metal hydro-nitride. It is a two-stage reaction: the formation of nitrogen solid solution in metal (first stage) and the interaction of this solution with hydrogen (second stage). The analysis of experimental data indicates that the formation of yttrium hydro-nitride is also two-stage.

Apparently in the first stage, the combustion reaction is carried out by nitrogen forming the solid solution of nitrogen in yttrium. In the second stage, while cooling down, hydrogen is introduced into the crystal lattice, stabilizing it and arresting the afternitriding process. However, the annealing of the intermediate products does not permit the clear separation of the combustion stage from the after-combustion stage as in the case of TiN,H, and ZrN,H,. The chemical and X-ray-phase analyses show that the intermediate products are identical with the end products of combustion. The annealing of the binary Y-N system products does not lead to the discovery of the nitrogen solid solution in yttrium, however the fact that the yttrium hydro-nitride is formed with an hcp structure in the Y-N-H system proves the existence of the hcp phase of nitrogen solid solution in yttrium fixed by hydrogen. REFERENCES 1. A. G. Merzhanovand I. P. Borovinskaya,Dokl. AN SSSR 204,336(1972).

SHS PROCESSES IN Y--N-H SYSTEMS

959

2. S. K. Dolukhanyan, H. G. Hakobian and A. G. Aleksanian,

melnie metalli i ikh soedinenia, pp. 128-I 34. Naukova Dumka.

Int. J. of Se&Propagating High-Temperature Synthesis, l(4), 530(1992). 3. Metal Hydrides. (ed. W. M. Muller) Moskva, Atomizdat

Kiev (1970). 6. S. K. Dolukhanyan, A. G. Aleksanian, G. B. Seranian. N. N. Aghadjanian and A. B. Nalbandian, Dokl. AN SSSR 276(l), 136-140 (1984). 7. A. G. Aleksanian, S. K. Dolukhanian, A. B. Nalbandian and A. G. Merzhanov, Fizikagorenia i vzriva, (3), 73-77 (1985). 8. S. K. Dolukhanyan, A. G. Aleksanian and H. G. Hakobian. Int. J. Hydrogen Energy 20(5), 391-395 (1995).

(1973). 4. 0. N. Carbon, R. R. Lichtenberg and .I. C. Warner, In Proceedings Redkozemelnie metalli, splavi i soedinenia, pp. 15&l%. Nauka, Moscow (1973). 5. M. D. Ljutaja and A. B. Goncharuk, In Proceedings Redkoze-