Mass spectrometric investigation of the vaporization of li2tio3(s)

Journal of Nuclear Materials 110 (1982) 158-163 North-Holland Publishing Company

158

MASS SPECTROMETRIC

Hamazo NAKAGAWA

INVESTIGATION

OF THE VAPORIZATION

OF Li,TiO,(s)

*, Mitsuru ASANO and Kenji KUBO

Institute of Atomic Energy, Kyoto University,

Vji, Kyoto 611, Japan

Received 3 May 1982, accepted 17 May 1982

The vaporization of Li,TiO,(s) has been investigated by the mass spectrometric Knudsen effusion method. Partial pressures of Li(g), LiO(g), Li,O(g), Li,O(g) and O,(g) over Li,TiO,(s) have been obtained in the temperature range 1180-1628 K. When the vaporization of Li,TiO,(s) proceeds, the content of Li,O in the Li,TiO,(s) sample decreases. The phase of the sample is a disordered Li,TiO, solid solution above 1486 K. The enthalpies of formation and the atomization energies for LiO(g) and Li,O(g) have been evaluated from the partial pressures to be AHQLiO, g)=65.4* 17.4 kJ/mol, AHto(Li,O, g)= - 207.5 * 56.6 kJ/mol, D,O(LiO)= 340.5 * 17.4 kJ/mol and D,O(Li,O) = 93 1.6* 56.6 kJ/mol, respectively.

1. Introduction Vaporization data of many lithium compounds are very important for the choice of a suitable breeding blanket of D-T fueled fusion reactors. For this reason, thermochemical studies of the vaporization of Li,O [l-4], lithium aluminates [5-81, lithium silicates [9,10], Li,S [ll], Li,N [12], Li,C, [13,14], LiAl [14], LiPb [l&16], LiBi [16] and LiSi [17] have been made. Recently, Misra et al. [18] have carried out a parametric study of the Li,TiO, candidate blanket in terms of operating temperature limit, tritium retention and release rate in situ processing of blankets for tritium recovery and blanket coolants. So, it is of practical interest to study the vaporization of Li,TiO,(s). The phase diagram in the Li,O-TiO, system has ,been studied, and four compounds, Li,TiO,, Li ,TiO,, Li,Ti,O,, and Li,Ti,O, are known [19]. Among them, the existence of two polymorphs of Li,TiO, is established. The ordered polymorph with a rock salt superstructure is the stable form. The disordered polymorph with a metastable cubic is stable above the order-disorder transition temperature 1486 K, and it melts at 1820 K. In the present work, in order to clarify the vaporization behavior of Li,TiO,(s), partial pressures of vapor species have been investigated by the mass spectrometric Knudsen effusion method in the temperature range 1180- 1628 K across the order-disorder transition tem* Present address: Ube Industries, Ltd., Ube, Yamaguchi 755, Japan

0022-3 115/82/0000-0000/$02.75

0 1982 North-Holland

perature of Li,TiO,(s). From the partial pressures, enthalpies of reaction for the gaseous equilibria have been obtained to evaluate enthalpies of formation and atomization energies for LiO(g) and Li,O(g).

2. Experimental The experiments were carried out with a 0.2 m radius, 60” sector single-focusing Hitachi RMdK mass spectrometer equipped with a platinum Knudsen cell in a tungsten holder. The cell has an inside diameter of 7 mm and a height of 9 mm. The diameter of the cutter shaped orifice is 0.5 mm. The powder sample of Li,TiO,(s), supplied by Ventron Corp., was loaded in the cell and preheated at 1630 K under a background pressure of 2 X lOA Pa. The temperature of the sample was measured with a well calibrated thermocouple inserted into the bottom of the cell. Vapor species effusing from the cell were ionized with electrons accelerated to 3 eV above the ionization potentials of the respective ions: IP(Li+) = 5.4 eV, IP(LiO+) = 9.0 eV, IP(LiZO+) = 6.4 eV and IP(Li30+) = 4.6 eV. These values were determined by the linear extrapolation method. The estimated uncertainty is 0.2 eV. The measured ion intensity 1, of the vapor species i was converted into the corresponding partial pressure pi at the temperature T by the relation pi = kIiT/uiyini, where k is the pressure calibration constant, ui is the relative ionization cross-section, y, is the gain of the electron multiplier and ni is the isotopic abundance ratio. The value of k was obtained from direct compari-

159

H. Nakagawa et al. / Vaporization of Li,TiO&)

son of 7Li+ ion intensities with vapor pressures of Li(g) over lithium metal [20] at various temperatures to be k=(1.43&0.18) X 1013 Pa.at%/A-K. Atomic ionization cross-sections were taken from Mann [21] to be uLi = 3.29 and u. = 1.27, and molecular cross-sections were estimated by the same method as described by Kordis and Gingerich [22] to be eLio = 3.42, uLizo = 4.91 and QLi,o = 6.27. Multiplier gains were obtained by the pulse counting method [23] to be yLi = 1.44 X 106, yLio = 2.31 X 106, yLizo = 2.53 X lo6 and yLi,o = 2.50 X 106. The isotopic abundance ratio of ‘Li in the sample was determined to be 92.6 * 0.4 at% and that of I60 was taken from literature [24].

T

O-1 $

-2 -

n -3 ," - -4-5 -6

3. Results and discussion

(K)

1 6

5

I\, 104/T

3.1. Identification

of gaseous species

A search for gaseous species effusing from the Knudsen cell was done at a temperature of 1618 K. Identified ion species were Li+ , LiO+, L&O+ and Li30+. Any ion species associated with titanium such as Ti+ , TiO+ , TiO: , LiTiO: and Li,TiO: were not detected. Partial pressures of Ti(g), TiO(g) and TiO,(g) over Li,TiO,(s) can be estimated from the following equilibrium reactions; Li,TiO,(s) = Li,O(g) + TiO,(g), TiO,(g) = TiG(g) + 0.50,(g) and TiO,(g) = Ti(g) + O,(g) with use of the determined partial pressure of Li *o(g) and the calculated one of O,(g) over Li *Ti03(s). The estimations at 1618 K are pTi= 1.6X lo-l9 Pa, prio = 1.2 X lo-i2 Pa and pTio2 = 6.2 X 10e9 Pa. The gaseous species of Li,(g) [ 1,2] and Li,O,(g) [ 1,2,4] over Li,G(s) have been studied mass-spectrometrically. The partial pressures of Li2(g) and Li202(g) over Li,TiO,(s) can also be estimated from the equilibria 2 Li(g) = Li,(g) and 2 LiG(g) = Li,O,(g) to be pLi2 = 7.3 X 10V7 Pa and = 2.2 X 10m6 Pa at 1618 K. These estimated presPLi,O, sures are below the detection limit of the present mass spectrometer. 3.2. Partial pressures over Li,TiO,(s) The determined partial pressures of Li(g), LiO(g), Li ,0(g) and Li,G(g) over Li,TiO,(s) are shown in fig. 1 in the temperature range 1180- 1628 K across an order-disorder transition temperature of 1486 K for Li,TiO,(s). No breaks are observed at 1486 K, although the enthalpy of the transition is compiled to be 11.5 kJ/mol[20]. Thus, without a divided temperature range at 1486 K, partial pressure data were fitted by the linear least squares method. The resulting equations are as

L 6

7

9

(It-')

Fig. 1. Experimentally determined partial pressures of Li(g), LiO(g), L&O(g) and LisO(g) and calculated ones of O,(g) over Li,TiO,(s) as a function of reciprocal temperature.

follows: logpLi = (10.14 * 0.10) - (16.66 2 0.14) 103/T (1180-1628 K), logp,,

= (11.73 * 0.31) - (23.60 * 0.48) 103/T (1478-1628 K),

logpLizo=

(15.37cO.11)

- (28.09*0.16)

103/T

(1402-1628 K), logp,lo

= (7.75 * 1.32) - (20.02 * 2.09) 103/T (1561-1628 K),

where the unit of pressure is Pascal. The errors quoted in the above equations are the standard deviations of the slopes and intercepts. Uncertainties in the partial pressures arise from errors in measurements of ion intensities and also conversions of the intensities to corresponding partial pressures. From experimental and probable errors, the uncertainties in the partial pressures are estimated to be Api/pi = 0.33. Since measurements of partial pressures of O,(g) were difficult by the interference of the background pressure of O,(g) in the ion source region, the pressures were calculated from the gaseous equilibrium Li 2O(g) = ZLi(g) + 0.50,(g) over Li,TiO,(s). The enthalpy of reaction for the above equilibrium may be obtained from the enthalpies of formation for Li,O(g), AHpo(Li,O, g) = - 148.1 * 15.8 kJ/mol [9] and for Li(g), AHpo(Li, g)

H. Nakagawa et al. / Vaporization of Li,TiO,(s)

160

= 159.1 kJ/mol [20] to be AH,?‘= 466.3 * 15.8 kJ/mol. Then, combining the value of AHP, with the free energy functions [20] for Li,O(g), Li(g) and O,(g) and with the determined partial pressures of Li,O(g) and Li(g), one can calculate the partial pressures of O,(g) as shown a solid line in fig. 1. The least squares equation is represented by

log PO, = (20.17 * 0.81) - (37.23 * 1.23) 103/T. in the temperature range 1402-1628 K. The uncertainty in the calculated pressures is estimated to be ApO,/pO, = 3.65. 3.3. Variation of the chemical constituent during vaporization

of Li,TiO,(s) 3.4. Enthalpies Li,TiO,(s)

After the vaporization experiment, the sample residue was removed from the Knudsen cell for X-ray analysis. The powder X-ray diffraction pattern of the residue at room temperature indicated lines of only ordered Li,TiO, [25]. However, the X-ray pattern of a separate sample, which had been allowed to vaporize freely for 3 h at 15 10 K under a background pressure of 1 X lO-4 Pa, showed lines of a mixture of ordered Li,TiO, [25] and Li,Ti,O,, [19]. Since the pattern of Li 4Ti s 0 i2 was very similar to that of ordered Li ,TiO,, it was somewhat difficult, although not impossible, to decide the presence of Li 4Ti s 0 ,z . According to the phase diagram in the Li,O-TiO,

Table 1 Third-law

enthalpies

of the reaction

system [ 191, disordered Li,TiO, forms an extensive range of solid solution with both excess Li,O and excess TiO, above 1486 K; Li,Ti,O,, is stable only below 1288 K. This means that the phase of the separate sample at 1510 K is the disordered Li,TiO, solid solution rich in TiO,, and the disordered LizTiO, solid solution decomposes during cooling to room temperature to ordered Thus, it can be concluded that Li,TiO, and Li,Ti,O,,. when Li(g), LiO(g), Li,O(g), Li,O(g) and O,(g) are vaporized from Li,TiO,(s), the content of Li,O in the Li,TiO,(s) sample decreases, while the phase of the sample is the disordered Li*TiO, solid solution above 1486 K.

Li,O(g)=Li(g)

T

-RlnK,

V-1

(J/mol . K)

- A[(G; (J/mol . K)

1478 1489 1496 1505 1519 1526 1534 1543 1550 1561 1569 1583 1597 1611 1628

129.1 130.1 125.2 125.8 124.2 123.6 122.2 123.8 121.8 121.2 119.5 117.6 117.8 116.2 114.6

118.9 118.9 119.0 119.0 119.0 119.0 119.0 119.0 119.0 119.0 119.0 119.0 119.0 119.0 119.0

over

In this section, the free energy functions for Li(g), LiO(g) and Li,O(g) were taken from the JANAF Tables [20] and those for Li,O(g) from Wu et al. [2]. In order to clarify the reliability of the determined partial pressures over Li,TiO,(s), third-law enthalpies of reaction for the pressure dependent equilibrium Li,O(g)

= Li(g) + LiO(g)

(1)

were calculated. The results are shown in table 1 together with equilibrium constants and changes in the free energy functions. An average value is AH:(l) = 372.6 * 4.5 kJ/mol. From the consideration of uncer-

+ LiO(g)

H,O)/Tl

of reaction for gaseous equilibria

AK0 (1) (kJ/mol) 366.6 370.8 365.2 368.5 369.4 370.1 370.0 374.7 373.3 375.0 374.2 374.6 378.1 379.0 380.3 av. 372.624.5

H. Nakagawaet al. / Vaporizationof L.i,TiO,(s) Table 2 Comparison

161

For the equation of the enthalpies

of reaction

Li,O(g)=

Li(g)+

Lie(g) Investigator

Vaporization material

AH,“, (1) (kJ/mol)

Berkowitz et al. [26] White et al. [27] JANAF Tables [20] Kudo et al. [1] Kimura et al. [4] Ikeda et al. [8] Ikeda et al. [8] Nakagawa et al. [9] Present work

Li ro(s) Li so(s) Li&s) Lisa(s) Li,O(s) LisAlO., y-LiAlO,(s) Li s SiO,(l) Li,TiO,(s)

376.6 400 408.92 23.4 372.8* 5.9 369.1* 2.3 (389.1) a) (386.6) =) 385.8* 6.4 372.6* 7.3

a) Values in parentheses from partial pressure

are calculated by the present data in ref. [8].

authors

Li,O(g) = Li(g) + Li,O(g),

(2)

third-law enthalpies of reaction are obtained as shown in table 3. Taking into account the uncertainties in partial pressures and the estimated uncertainty in the free energy functions for Li,O(g) being 0.10 [2], one determines the enthalpy of reaction for eq. (2) to be AHP, (2) = 218.5 * 54.4 kJ/mol from an average value in table3. A comparison of the present value with reported ones is shown in table4, where the present value is also in reasonable agreement with previous ones [ 1,4]. From the comparison in tables 2 and 4, it is clear that the partial pressures of Li(g), LiO(g), Li,O(g) and Li,O(g) determined in the present work are reliable. The enthalpy of reaction for the pressure independent equilibrium Li,O(g) + LiO(g) = 2 Li,O(g)

tainties in partial pressures being Api/pi = 0.33, a value of AI-Ql) = 372.6 * 7.3 kJ/mol is obtained. The value is in poor agreement with the corresponding second-law value of AH:‘(l) = 230.1 f 14.3 kJ/mol. In general, however, the third-law value is more reliable. The enthalpies of reaction for eq. (1) have already been obtained from partial pressures of Li(g), LiO(g) and Li,O(g) over Li,O(s) [1,4,20,26,27] and Li,SiO,(l) [9]. Third-law values in previous works [1,4,9,20,26,27] are listed in table2 in comparison with the present value. From the reported values of pLir pLio and pLizo over LisAlO, [8] and y-LiAlO,(s) (81, one can calculate third-law enthalpies of reaction for eq. (1) to be 389.1 kJ/mol and 386.6 kJ/mol, respectively, which are listed in parentheses in table2. As can be seen in the table, the present value is in reasonable agreement with previous ones.

Table 3 Third-law

enthalpies

of the reaction

Li,O(g)

(3) may be evaluated to be AHP, (3) = - 154.1 f 54.9 kJ/mol from A HP, (1) and AHt (2). The value of AHP, (3) is compared with reported ones [ 1,2,4] in table 5. 3.5. Enthalpies of formation and atomization energies for LiO(g) and Li,O(g) The enthalpy of formation and the atomization energy for LiO(g) may be calculated from the enthalpy of reaction for eq. (1). The combination of AHP, (1) with the enthalpy of formation for Li(g) AHG(Li, g) = 159.1 kJ/mol[20] and that for Li,O(g) AHP, (Li,O, g) = 148.1 * 15.8 kJ/mol [9] yields an enthalpy of formation for LiO(g) to be AHG(Li0, g) = 65.4 * 17.4 kJ/mol. The atomization energy for LiO(g) D,O(LiO) = 340.5 r+ 17.4 kJ/mol is obtained by combining the value of A HP, (1) with the atomization energy for Li,O(g) D,O(Li,O) = 713.1 * 15.8 kJ/mol [9]. By the same manner as the

= Li(g) + Li,O(g)

T

-RlnK,

W)

(J/mol . K)

- A](G (J/mol.K)

1561 1569 1583 1597 1611 1628

57.9 57.9 54.0 51.5 50.3 46.3

84.5 84.5 84.4 84.3 84.3 84.2

H,O)/Tl

AH; (2) (kJ/mol) 222.3 223.3 219.1 217.0 216.8 216.5 av. 218.5 k4.0

162 Table 4 Comparison

H. Nakagawa

of the enthalpies

of reaction

Li,O(g)=

et al. / Vaporization

Li(g)+

Li ,0(g) Investigator

Vaporization material

A JG (2) (kJ/mol)

Kudo et al. [l] Kimura et al. [4] Present work

Li,O(s) Li *O(s) Li zTiOs(s)

186.6* 3.8 221.3-c 4.8 218.5k54.4

Table 5 Comparison

of the enthalpies

of reaction

Li,O(g) + LiO(g) = 2

Li ,0(g) Investigator

Vaporization material

AH:, (3) (kJ/mol)

Kudo et al. [l] Wu et al. [2] Kimura et al. [4] Present work

Li &s) Li *o(s) Lizo(s) Li sTiO,(s)

-

186.22 7.1 192.9* 1.7 147.7 f 9.4 154.1 k54.9

of Li,TiO,(s)

table6 along with reported values. The values obtained in the present work are in agreement with those by vaporization studies of ‘other lithium compounds [ 1,2,4,8,9,20,26-281. The present authors [9] have pointed out that the 2.1-2.3 times of the value of D,O(LiO) is equal to that of D,O(Li,O). The 2.1 times of the present value of D,O(LiO) is equal to D,O(Li,O) [9]. Wu et al. [2] have assumed a planar rhombus for a Li,O(g) structure. Under this assumption, one can predict that the value of D,O(Li,O) is roughly equal to half the sum of D,O(Li,, sq) for planar square Li,(g) and D,O(Li,O,, without O-O bond) for planar rhombic Li,O,(g) without oxygen-oxygen bond. However, the prediction is not realizable, because the value of D,O(Li,, se) has been computed to be 267.5 kJ/mol[29] and those of D,“(Li,O,, without O-O bond) have been obtained to be 1115.O-t31.0 kJ/mol [l], 1167.3* 16.7 kJ/mol[2] and 1034.4 r+ 13.0 kJ/mol[4]. No reasonable explanation is proposed for this contradiction.

Acknowledgements

above calculation for LiO(g), the enthalpy of formation and the atomization energy for Li,O(g) are calculated from the value of AHP, (2) to be AZSHpo(Li,O,g)= - 207.5 * 56.6 kJ/mol and D,O(Li,O) = 93 1.6 * 56.6 kJ/mol, respectively. These values are summarized in

Table 6 Comparison Species

of the enthalpies

of formation

AH:, (kJ/mol)

LiOW 66.9k20.9 84.1220.9 69.0* 6.3 34.72 13.8 76.82 8.5 (72.0) a) (82.6) a) 84.5 * 12.8 65.4~ 17.4 Li so(g)

-202.5223.4 -228.0r41.8 - 195.5 k48.6 - 207.5 2 56.6

‘) Values in parentheses

are calculated

and the atomization

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.

energies

for LiO(g) and Li,O(g)

D,o (kJ/mol)

Vaporization material

Investigator

341.0 338.9 321.7 336.83- 6.3 373.2* 13.8 329.1 f 8.5 (333.9) a) (323.3) ‘) 321.4* 12.8 340.52 17.4

Li2W Li2W Li2W

Berkowitz et al. [26] White et al. [27] JANAF Tables (201 Hildenbrand [28] Kudo et al. [l] Kimura et al. [4] Ikeda et al. [8] Ikeda et al. [8] Nakagawa et al. [9] Present work

932.6-23.4 958.1 e41.8 919.6-48.6 931.6k56.6 by the present

authors

y-LiAlO,(s) Li2W Li 2%)

Li,AlO,(l) y-LiAlO,(s) Li 2Si0,(l) Li 2Ti03(s) Li 2O(s) Li,O(s) Li2W

Li,TiOs(s) from partial

pressure

Kudo et al. [l] Wu et al. [2] Kimura et al. (41 Present work data in ref. [8].

H. Nakagawa et al. / Vaporization of Li,TiO,(s)

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[16] A. Neubert, H.R. Ihle and K.A. Gingerich, J. Chem. Phys. 73 (1980) 1406. [17] H.R. Ihle, C.H. Wu, M. Miletic and K.F. Zmbov, Advances in Mass Spectrometry, Ed. N.R. Daly, Vol. 7A (Heyden, London, 1978) p. 670. [ 18) B. Misra, R.G. Clemmer and D.L. Smith, Trans. Am. Nucl. Sot. 34 (1980) 50. [19] G. Izquierdo and A.R. West, Mater. Res. Bull. 15 (1980) 1655. [20] D.R. Stull and H. Prophet, JANAF Thermochemical Tables, 2nd ed., NSRDS-NBS 37 (Dow Chemical Co., Midland, 1971). [21] J.B. Mann, Recent Developments in Mass Spectroscopy, Eds. K. Ogata and T. Hayakawa (University of Tokyo Press, Tokyo, 1970) p. 814. [22] J. Kordis and K.A. Gingerich, J. Chem. Phys. 58 (1973) 5141. (231 M. Asano, H. Kimura and K. Kubo, Mass Spectrosc. 27 (1979) 157. [24] American Institute of Physics Handbook (McGraw-Hill, New York, 1972) 7-6. 125) JCPDS Powder Diffraction Card (Joint Committee on Powder Diffraction Standards) File No. 8-249. [26] J. Berkowitz, W.A. Chupka, G.D. Blue and J.L. Margrave, J. Chem. Phys. 63 (1959) 644. [27] D. White, K.S. Seshadri, D.F. Dever, D.E. Mann and M.J. Linevsky, J. Chem. Phys. 39 (1963) 2463. [28] D.L. Hildenbrand, J. Chem. Phys. 57 (1972) 4556. [29] A.L. Companion, J. Chem. Phys. 50 (1969) 1165.