Vaporization of solid lithium nitride

Journal of Nuclear Materials 91 (1980) 200-204 0 North-Holland Publishing Company

VAPORIZATION Hitoshi

OF SOLID LITHIUM NITRIDE

KIMURA, Mitsuru ASANO and Kenji KUBO

Institute of Atomic Energy, Kyoto University, Uji, Kyoto, Japan

Received 30 January 1980

The vaporization of solid lithium nitride has been studied by a mass spectrometric Knudsen effusion method. The solid was found to vaporize congruently to Li(g), Liz(g) and N*(g), and partial pressures of them may be represented by the equations: log pLf (Pa) = (11.487 ? 0.102) - (9.670 f 0.081) 103/T, log pLfs (Pa) = (14.306 + 0.209) - (14.090 ?r 0.166) 103/T and log pNZ (Pa) = (10.959 r 0.157) - (9.638 f 0.124) 103/r in the temperature range 739-859 K. No identification of LiN(g) was made. From the combination of the determined enthalpy of the reaction LisN(c) = 3Li(g) + O.SNa(g) with appropriate literature data, the enthalpy AH&rg, the free energy A&o8 and the entropy A&r8 of formation for solid lithium nitride have been obtained to be (-171.3 * 7.7) kJ/mol, (-135.4 + 7.7) kJ/mol and (-120.4 * 36.5) J/mol . K, respectively.

tained by calorimetry [4-61. The free energy of formation AC; for Li,N(c) has also been given by the emf method [7], by heat capacity measurements of Li3N(c) [8] and a method based on the solubility of Li3N in lithium and the decomposition pressure of nitrogen over Li3N(c) [2]. The purposes of this investigation are first to determine partial pressures of Li(g), Liz(g) and N,(g) over Li,N(c) by a mass spectrometric Knudsen effusion method and then to obtain values of w, AC; and the entropy of formation A$ for Li3N(c) by the combination of the determined enthalpy of the reaction

1. Introduction Lithium metal is regarded as a potential material for the blanket of fusion reactors because of the favorable tritium breeding gain, excellent heat transfer characteristics and low melting point [ 11. On the contrary of these merits, lithium metal will react easily with nitrogen, carbon, oxygen and hydrogen presented in the surroundings. Thes reaction products not only contaminate liquid lithium metal but also give rise to the embrittlement and the corrosion of structural materials of interest to fusion reactors. Lithium nitride Li3N is known to be one of these reaction products. Accordingly, it is very important to determine decomposition pressures over Li,N(c) and thermodynamic quantities for the formation of Li3N(c) for the evaluation of the chemical behavior of Li3N in the liquid lithium blanket. Hitherto, decomposition pressures over Li3N(c) have been measured by Yonco et al. [2]. They have identified only nitrogen as decomposition gaseous species. However, when the decomposition vaporization occurs to Li3N(c), the detection of lithium gaseous species should be made from the consideration of vapor pressures over lithium metal [3]. The enthalpy of formation A@ for Li3N(c) has been ob-

Li3N(c) = 3 Li(g) + 0.5 N,(g) , with appropriate

literature

(1)

data [9].

2. Experimental

Experiments were carried out with a 0.2 m radius, 90’ sector single focusing Hitachi RMdK mass spectrometer equipped with a molybdenum Knudsen cell. The cell has an inside diameter of 7 mm and a height of 9 mm. The diameter of the cutter shaped effusion orifice is 0.5 mm. Before use, the cell was degassed at 200

201

H. Kimura et aLlVaporization of solid lithium nitride

1700 K for a long time. The powder sample of Li,N(c) supplied by Mitsuwa Chemical Co. was loaded into the cell in an argon atmosphere and preheated in the cell at 860 K under a background pressure of 2 X lo-’ Pa. The temperature of the sample was measured with a well calibrated thermocouple inserted into the bottom of the cell. Ion intensities used for deter~nations of partial pressures were measured with ionizing electrons, 3 eV above the ionization potential of the respective ion species; IP(Li’) = 5.392 eV [lo], IP(Li’,) = 4.86 eV [3] and IP(N’,) = 15.58 eV [ 1I]. In order to clarify the portion of the gaseous species originating from the cell, shutterable ion intensities were measured. The resolution of the mass spectrometer was suffcient to measure the intensity of the 14Nl ion without interference of the background ‘2C160* ion at the same nominal mass number. Usually, the resolution was held at 3800 in peak width at half maximum. The measured ion intensity 1i of the gaseous species i was converted into the corresponding partial pressure pi at temperature T by the relation pi = kZiT/ Unini, where k is the pressure calibration constant, ui is the relative ionization cross-section, Ti is the gain of the electron multiplier and nf is the isotopic abundance ratio. The value of k was obtained from equilibrium constants at various temperatures calculated for the reaction [9]

Liatg) = 2 LW ,

Identified ion species are Li’, Li;, Na’ and Nl. The relative ion intensities of 7Li+, ‘Lii, 23Na*, 14N; were 1.00, 7.3 X 10S3, 1.8 X 10m3, 3.4 X lo-‘, respectively. The intensity of the LiN+ ion can be estimated from the intensities of the Lit and N: ions and the calculated equilibrium constant for the reaction z91 LiN(g) = Li(g) t 0.5 N,(g) .

The estimation shows that the intensity of the LiN* ion is much below the detection limit of the mass spectrometer, and actually no identification of the LiN’ ion was made in the present search. The origin of the Na’ ion is probably an impurity in the Li3N(c) sample. However, since the intensity of the 23Na+ion is much less than that of the ‘Li+ ion, the amount of the sodium impurity in the sample is considered to be very small. 3.2. Partial pressures of h(g), L&(g) and N,(g) The partial pressures of Li(g), l,(g) and N,(g) over L&N(c) are shown in fig. 1. The lines drawn by the least-squares treatment of data are given by the

-f

(IO

13~

C-Q

and measured ratios of i(Lii) to p(Li’) to be (2.40 + 0.53) X 1013 Pa - at%/A . K. Atomic ionization crosssections were taken from Mann [ 121 and molecular ionization cross-sections were calculated by taking 0.75 of the sum of Mann’s atomic cross-sections [ 131. Multip~er gains were measured to be 1.59 X 106, 1.95 X lo6 and 2.42 X lo6 for ‘Li+, ‘Li; and r4Nz, respectively. The isotopic abundance ratio of ‘Li in the sample was measured to be (92.6 + 0.4) at% and that of 14N was taken from literature [ 141.

(3)

1

O-

-lQ 4

-2 -

0. g -3 -

-4 -

-5 -

3. Results and discussion 3.1. Identification of gaseous species

A search for gaseous species effusing from the Knudsen cell was done at a temperature of 859 K,

-611

I ^_

1 ,

1.2

103/T

1.3

1.4

( K-')

Fig. 1. Partial pressures of Li(g), L&(g) and N2&) over LisN(c) as a function of the reciprocal temperature.

202

H. Kimura et al./Vaporization of solid lithium nirn’de

following equations: logpI,i (Pa)=(11.487

Table 1 Third-law enthalpies of the reaction LisN(c) = 3 Li(g) + 0.5

f 0.102)

- (9.670 f 0.081) 103/T)

Nz(s)

(4)

1ogpLiz (Pa)= (14.306 * 0.209) - (14.090 f 0.166) 103/r ) logpN,

(5)

(Pa) = (10.959 + 0.157)

- (9.638 + 0.124) 103/T,

(6)

in the temperature range 739-859 K. No visible reaction occurs between the Li3N(c) sample and the molybdenum Knudsen ceil. The ratio PLi/PN? is 3.09 f 0.29 at experimental temperatures. From eqs. (4) and (6), partial molar enthalpies of vaporization for Li(g) and N,(g) from the Li,N(c) sample are determined to be m&(Li, g) = (185.0 + 1 S) kJ/mol and @&(NZ, g) = (184.6 + 2.4) kJ/mol, respectively. The agreement of both values is good. Chemical analysis showed that the ratios Li/N were 3.01 _+0.03 for the lithium nitride sample before use and 2.99 + 0.01 for the lithium nitride residue after experiments, respectively. Furthermore, X-ray diffraction patterns of the sample and the residue showed only lines of Li,N(c) [ 151. From these results, solid lithium nitride was found to vaporize congruently and the main vaporization reaction may be given by eq. (1). Since the ratio PLiZ/pLi is very small, the contribution of Liz(g) to the whole lithium amount of gaseous lithium species is negligible. The theoretical ratio PLi/PN2 in eq. (1) is 2.99, which is in good agreement with the experimental one. Yonco et al. [2] have measured decomposition pressures over Li,N(c) between 933.6 and 1050.8 K. They have reported that the gaseous samples taken after pressure measurements consist of >99 at% nitrogen and that there is no evidence of nonstoichiometry in the Li3N(c) sample. In this work, the gaseous composition over Li,N(c) is as shown in fig. 1, and analytical results of solid phases showed almost stoichiometric composition of Li3N(c). 3.3. Thermodynamic

quantities for the formation

of

LW(cl

From the measured partial pressures of Li(g) and N,(g) over LijN(c), one can calculate the enthalpy of

(TK)

ba3.5)

739 153

4.99 x 10-19 2.13 X lo--l8

759 765 114 781 786 816 822 842 853 859

6.68 1.66 6.23 1.36 2.30 8.66 1.68 1.96 6.11 1.08

X X X X X X X X X X

lo-‘* 10-l’ 10-l’ lo-l6 lo-l6 1O-‘5 lo-l4 lo-l3 lo-l3 lo-‘*

-A[(G’+ $98)/T] (J/m01 . K)

AH0298 (kJ/mol)

431.5 431.3 437.2 437.1 437.0 436.9 436.8 436.2 436.1 435.7 435.5 435.4

653.1 654.7 654.2 653.5 652.5 653.2 653.9 653.8 654.0 652.4 652.6 653.1 av. 653.4 * 0.7

reaction (aga) for eq. (1). Third-law values are shown in table 1 together with equilibrium constants and changes in free energy functions for eq. (1). In the calculations, free energy functions for Li(g) and N,(g) were taken from JANAF tables [9] and those for Li,N(c) from Osborne and Flotow [8]. The thirdlaw average value &,, = (653.4 + 0.7) kJ/mol is in agreement with the second-law value @2g, = (659.5 + 5.5) kJ/mol within combined errors. The errors quoted here refer to standard deviations. The combination of the third-law average value with the of vaporization for lithium metal enthalpy AH’&s(Li, c) = (160.7 + 1.7) kJ/mol [9] gives the enthalpy of formation for Li3N(c): 3 Li(c) t 0.5 N,(g) = Li3N(c) ,

(7)

tobeAH&(Li3N,c)=-(171.3*5.l)kJ/mol. value of error in the The additional AH&s(L&N, c) arises from uncertainties in measurements of partial pressures and also from estimated probable errors in the free energy functions for LieN(c). Errors in isotopic abundance ratios &i/Hi are negligible and those in temperature measurements are small. When the experimental error in the pressure calibration constant Ak/k = +0.22 and estimated errors in ion intensities, ionization cross-sections and gains of the electron multiplier are Ali/Zi = +0.2,

203

H. Kimura et al./Vaporization of solid lithium nitride Table 2 Thermodynamic

quantities for the formation of LisN(c)

Investigators

JANAF tables [ 91 Bonomi et al. [ 71 O’Hare et al. [ 61 Yonco et al. [ 21 Osborne et al. [ 8) This work

Method

Calorimetry EMF Calorimetry Solubility Heat capacity Vaporization

--AH”&9 8 (kJ/mol)

-A&9a

-As”fz9a

(kJ/mol)

(J/mol . K)

197.5 f 4.2 170.1 a) 164.93 f 1.09 163.6 a) 164.56 f 1.09 b, 171.3 * 7.7

126.2 128.91 f 1.13 122.2 128.64 * 1.10 135.4 * 7.7

147.3 a) 120.83 * 0.75 138.9 a) 120.46 f 0.54 120.4 * 36.5

a) Values at 0 K. b, Revised from the value [6].

Aoi/ci = +0.2 and Ayi/yi = kO.1, respectively, one obtains errors in the measured partial pressures to be &i/pi = kO.4. Taking into account errors in partial pressures and estimated probable errors in the free energy functions for LiaN(c) being kO.05 J/mol * K [S], the value of the enthalpy of formation for LisN(c) is finally calculated to be Af&s(LiaN, c) = --(171.3 f 7.7) kJ/mol. The free energy of formation for LiaN(c) is obtained by combining the above value with the free energy functions for LiaN(c) [S], Li(c) [9] and N,(g) [9] to be AG’&s(LisN, c) = --(135.4 rt 7.7) kJ/mol. Furthermore, the entropy of formation for LiaN(c) is calculated from the relation AS = (AI! - AG)/T to be AS&.&LisN, c) = -(120.4 f 36.5) J/mol * K. Bonomi et al. [7] have obtained values of AGxLisN, c) by galvanic-cell emf measurements in the temperature range 823-973 K to be AGxLisN, c) = (0.1473T - 170.1) kJ/mol. O’Hare and Johnson [6] have obtained A&&s(LiaN, c) by solution calorimetry in both acidic and alkaline media. From experiments in aqueous hydrochloric acid solution and in aqueous lithium hydroxide solution, a weighted mean value of A$&(LisN, c) = 4164.93 + 1.09) kJ/mol has been obtained. They have also calculated values of AG&s(Li3N, c) and A?&,s(LiaN, c) from entropies for Li(c), N,(g) and LisN(c). Yonco et al. [2] have obtained the relation AGgLisN,c) = (0.1390T - 163.6) kJ/mol from a thermodynamic analysis of the solubility of nitrogen as a form of LiaN in the liquid lithium metal and decomposition pressure of nitrogen over LisN(c). Osborne and Flotow [S] have reported the heat capa-

city of Li,N(c) from 5 to 350 K and calculated AG$LisN, c) from the entropy of LisN(c) obtained by integration of the heat capacity. They used the same LisN(c) sample as used by O’Hare and Johnson [6], while they have revised the weighted mean value of AII&(LisN, c) [6] to -(164.56 + 1.09) kJ/mol based upon accurate mass analysis of the ratio 6Li/ ‘Li. The values of M&,s, AC&s and AS&as for LisN(c) obtained in this work are listed in table 2 in comparison with those given by previous workers [2,6-91. The present values of A$&(LisN, c) and AG&(Li3N,c) are in agreement with those by O’Hare and Johnson [6] and also by Osborne and Flotow [S] within combined errors, respectively. Yonco et al. [2] have given no estimated errors in their values of AGxLisN, c). The accuracy of the relation between AGxLisN, c) and temperature given by Bonomi et al. [7] is somewhat doubtful as they have described. However, values of AG&&,isN, c) and A&,(LisN, c) by Yonco et al. [2] and by Bonomi et al. [7] are in agreement with the corresponding present values within probable errors, respectively. Two early values, A@‘a9s(Li3N, c) = -(197.5 f 4.2) kJ/mol [5] and AE&s(LisN, c) = -(196.7 + 8.4) kJ/mol [4], obtained by calorimetry have been compiled in JANAF tables [9]. The agreement of those values is apparently good, as can be seen in table 2, however, they are more negative than the recent values. The value of AS&s(Li3N, c) obtained in this work agrees quite well with those found by O’Hare and Johnson [6] and by Osborne and Flotow [S] . From above comparisons, it is concluded

204

H. Kimura et al/Vaporization

that partial pressures of Li(g), Liz(g) and N*(g) over LisN(c) determined in this work are reliable.

Acknowledgements The authors are grateful to Prof. Dr. S. Magari of the Institute of Atomic Energy, Kyoto University for his continuous encouragement. This investigation was supported by a special grant for Installations of the Ministry of Education of Japan.

References [l] E. Veleckis, R.M. Yonco and V.A. Maroni, Intern. Symp. on Thermodynamics of Nuclear Materials, Jiihch, 1979, IAEASM-236156. [2] R.M. Yonco, E. Veleckis and V.A. Maroni, J. Nucl. Mater. 57 (1975) 317. [3] C.H. Wu, J. Chem. Phys. 65 (1976) 3181.

of solid lithium nitride

[4] A. Guntz, Compt. Rend. (Paris) 123 (1896) 995. [S] B. Neumann, C. Kroger and H. Haebler, Z. Anorg. AIlgem. Chem. 204 (1932) 81. [6] P.A.G. O’Hare and G.K. Johnson, J. Chem. Thermodyn. 7 (1975) 13. ‘I A. Bonomi, M. Hadate and C. Gentaz, in: Proc. Intern. Symp. on Molten Salts, Ed. J.P. Pemsler (Electochemical Society, Princeton, 1961) p. 78. ‘I D.W. Osborne and H.E. Flotow, J. Chem. Thermodyn. 10 (1978) 675. 1 D.R. StuII and H. Prophet, JANAF Thermochemical Tables, 2nd. ed., NSRDS-NSB 37 (Dow Chemical, Midland, 1971). [lo] American Institute of Physics Handbook (McGraw-Hill, 1972) 7-10. [ll] R.E. Honig, J. Chem. Phys. 16 (1948) 105. [ 121 J.B. Mann, Recent Developments in Mass Spectroscopy, Eds. K. Ogata and T. Hayakawa (University of Tokyo Press, 1970) p. 814. [13] J. Drowart and P. Goldfinger, Angew. Chem. 79 (1967) 589. [14] Ref. [lo] 7-6. [lS].JCPDS Powder Diffraction card (Joint Committee on Powder Diffraction Standards) File No. 2-0301.