- - I Thermodynamic I. Evaluation of Surface Tension of Molten Salt Mixtures based on Thermodynamic Databases. } BulkPha~

Transcription

1 valuation of Surfae Tension of Molten Salt Mixtures based on Thermodynami Databases Toshihiro Tanaka, Mahoba Ogawa, Tamotsu Ueda and Shigeta Hara Department of Materials Siene and Proessing, Faulty of ngineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565, Japan. Tel ABSTRACT The authors disussed the appliation of thermodynami solution databases, whih have been onstruted so far to alulate thermodynami properties and phase diagrams, to the evaluation of surfae tension of molten salt mixtures. 1. INTRODUCTION During the last 3 years, various thermodynami databases have been onstruted to be mainly applied to the alulation of phase diagrams of alloys, salts and oxides 1. The aumulation and assessment of thermodynami data and phase equilibrium information to establish those databases is sometimes alled CALPHAD (Computer Calulation of Phase Diagrams) approah(2), whih has been reognied to be useful in various aspets of materials siene and engineering 1, 2. If it would be possible to use the thermodynami databases to evaluate physial properties of liquid solutions as well as phase equilibria as shown in Fig.l, we ould enlarge not only the utility of those thermodynami databases but also the understanding of the physial properties of the liquid solutions. The authors have applied the thermodynami databases to the evaluation of the surfae tension of liquid alloys and molten ioni mixtures We still have some disrepanies of the alulated results with experimental values in some molten salt mixtures, whih show large downward urvatures of the omposition dependene of the surfae tension of those mixtures. In the present work, we disuss some thermodynami onsiderations to the evaluation of surfae tension of molten salt mixtures in some ommon ion systems to fit the alulated results with the experimental values of the surfae tension of those solutions. 2. THRMODYNAMIC QUATIONS FOR SURFAC TNSION OF SOLUTIONS We have applied the ombination of the thermodynami treatments by Butler 6, Speiser et al. 7, 8 and Monma & Sudo 9 to evaluate surfae tension a of A B binary liquid solutions 4,s. Butler derived the following equation [1] assuming an equilibrium between a bulk phase and a surfae phase 6. The surfae phase is regarded as a hypothetial independent phase and onsists of the outermost monolayer of the surfae as shown in Fig.2. I Thermodynami I Databases _ I l...,,..,, I CALPHAD I Calulation of Phase Diagrams, Phase quilibria, Chemial Reations, et. valuation of Physial Properties of Multi-omponent Molten Alloys and Salt Mixtures (Surfae Tension, Visosity, Molar Volume et.) where R is the gas onstant, T temperature, ax surfae tension of pure liquid X, Ax molar surfae area in a monolayer of pure liquid X (X=A or B)..._ Surfae Phase } BulkPha Figure 1 CALPHAD Approah and its xtension to valuate Physial Properties of Melts. Figure 2 quilibrium between "Surfae Phase" and "Bulk Phase". MOLTN SLAGS, FLUXS AND SALTS '97 CONFRNC - 263

2 Ax in q.[l] an be obtained from the following equation. N Avogadro number, Vx molar volume of pure liquid X. L in q.[2] is set to be unity for molten salts 4,5. Nxs and NxB in q.[1] are mole frations of a omponent X in the surfae phase and the bulk phase, respetively; Gx,S(T,Ni) in q.[l] is the partial exess Gibbs energy of X in the surfae phase as a funtion of T and Ni ; Gx,B(T,NBB) partial exess Gibbs energy of X in the bulk phase as a funtion of T and NBB (X=A or B). Sine Gx,B(T,NBB) in the bulk phase in q.[1] an be obtained diretly from thermodynami databases, we only need the additional information on Gx,Sr,Ni) in the surfae phase. Speiser et al. 7 8 proposed their own models for Gx 5(T,NB 5 ) as follows : Speiser et al. 7 8 proposed /3 =3/4 for molten alloys, but we have determined the values of f3 for molten alloys and molten salt mixtures on the basis of the trial by Monma & Sudo 9 as follows 4,5: The surfae tension ax of pure molten salt X at its melting point is assumed to be determined by the following relation. Ax. ax=lnol!3vi/3. ax =(-UxB) - (-Uxs) = ( - UxB) - /3 *. (- UxB) = (1 - f3 * ) ' 6.Hva,X [4] where UxB and ux5 are binding energies of X in the bulk phase and the surfae phase, and /3 * =Ux 5 /UxB In the above equation, the surfae entropy terms are negleted, and ( - U xb ) is assumed to be approximately equal to evaporation energy 6.Hva,X of X at the melting point. Furthermore, we assume the following relation; /3 * (=Ux 5 JU{) : for pure substanes = f3{=gx 5(T,NB 5 ) / Cix,B(T,NBS)}: for solutions [5] From the linear relation between ax and /::,.Hva!(LN 113 Vx 213 ) in q.[4] and the above assumption in q.[5], we have determined /3 as follows 4, 5 : /3 =.83 for liquid alloys [ 6] /3 =.94 for molten salt mixtures and oxide mixtures [7] 3. PROCDUR OF CALCULATION OF SURFAC TNSION OF SOLUTIONS The surfae tension [2] a of a solution an be alulated as follows: <l> Setting T and NBBof a solution. <2> Determining the values for ax, Vx and exess Gibbs energies in the bulk phase at the above T and NBB, and substituting them in q.[l]. <3> One pair between the two equations on the right-hand side of q.[1] beomes the equation with unknown N 5. This equation is solved for NB 5, and the value of NB is substituted again into e.g. the first equation of the right-hand side of q.[1] to alulate the surfae tension a on the left-hand side ofq.[1]. 4. APPLICATION OF THRMODYNAMIC DATABASS TO VALUATION OF SURFAC TNSION OF MOLTN SALT MIXTURS We have used Pelton's thermodynami databasel, 11, whih has been assessed to provide alulation of thermodynami properties and phase diagrams of salt mixtures aording to the CALPHAD approah, to alulate the surfae tension of some molten salt mixtures. Common ion systems onsisting of the following ions have been treated here: Cations : Li+, Na+, K+, Rb+, es+ Anions : F-, Cl-, Br-, r- Surfae tension data of pure omponent salts, a x in q.[l], were taken from the NIST databasel2. We obtained Vx in q.[2] from the seleted density of pure omponent salts in the NIST database 12 with the molar weights of ations and anions. Figures 3 and 4 shows the omparison of the alulated results obtained from qs.[1 ], [2], [3] and [7] for the surfae tension of various molten salt mixtures with the experimental values stored in NIST databasel2. As an be seen from these figures, the alulated results in ommon ation systems in Fig.3 agree with the experimental data, but in ommon anion systems in Fig.4, there exist some disrepanies in some molten salt mixtures, whih have large downward urvatures of the omposition dependene of the surfae tension, although the disrepanies are almost within the unertainties of the experimental values 4,5. 5. ADDITIONAL CONDITIONS FOR CALCULATION OF SURFAC TNSION OF MOLTN SALT MIXTURS 5.1 Consideration of Relaxation Layer In ioni solutions, the interation between different ions extends over the outermost monolayer of the surfae beause of their Coulomb's fore, whih is one of the longdistane fores. In the previous work 5, as a possible way to derive an alternative equation for q.[3] onsidering those interations among ions, we introdued the "relaxation layer" between the surfae phase and the bulk phase. The relaxation layer is onsidered to be within the bulk phase region, but to be influened by the surfae phase. An equation has been proposed as shown in q.[8] MOLTN SLAGS, FLUXS AND SALTS '97 CONFRNC

3 Z9 8 7 U) 25 CsCI-Csl (1K) Q2 Q4 Q6 QB Mole fration of Csl LiCI-LiF (173K) go '-'-m'-co'-"le...,f'-'ra...tio'-'-n-'-oi-'-"l"'if_---. O RbBr RbCI (111K)?85 i;;;so u -@ 75!,---,,L,,...--.,,'-,-----,,-'-:---_,,,.,,,...-- n2 n4 os ns ,--M_o_le,f_ra tio_n_o.f_r_b_c_i.--_, Iao 65 t:: CsBr-CsCI (17K) UJ 5r---o.2,,...--o.4----,o.6,,...---o.a;;---, Mole ration of CsCI & Csl () -@11 LiBr-LiCI (17K) U) o!---.,.,-k----,,,...,..-...,..-1,---,rt,-- 25?2 $15 too UJ LiF-Lil (11K) 5 o!,--o..i.,.2,-1.4., ,,..i., i Mole fration of Li I ,----, KB r-kci (173K) j t:: UJ BOO!,--O..i.,.2-o,-1.4.,..--.1e-6----,,..i.,.8- Mole fration of KCI LiBr-Lil (11K) _--.-_M_ol,e_fr_a_t_io_no_f_Li_l--.---, NaBr-NaCI (113K) _ _ @ UJ 9,-,..,,_--,,,..,..--.,,,-,, l12.ig (/) Mole fration of LiCI LiBr-LiF (113K) _.,,,..-/ Mole fration of LiF. '/ '..... : xpe. 12 qs.[1,2,3&7] - -- : Cale. qs.[1,2,7-1] ,, ,, NaCI-NaF (1273K) 516 : Mole fration of NaF..... : xpe. 12 qs.[1,2,3&7] - -- : Cale. qs.[1,2,7-1] Figure 3 Calulated Results for the Surfae Tension of Molten Sal Mixtures in Common Cation Systems with the xperimental Values 12. Figure 3 (Continued) Calulated Results for the Surfae Tension of Molten Sal Mixtures in Common Cation Systems with the xperimental Values 12. In q.[8], the first term in the right-hand side shows the ontribution of the interation among ions in the outermost monolayer, and the seond term shows the ontribution of the relaxation layer, i.e., the effet of the interation among ions in the surfae phase to the bulk phase region, whih is MOLTN SLAGS, FLUXS AND SALTS '97 CONFRNC - 265

4 Mole fration of NaBr U) RbBr-LIBr (173K) Mole fration of LiBr 1 9 2l 8 NaBr-CsBr (173K) :fo g!1 1iJ 8 't: U) 1 " U) Ql 175 ""' Mole fration of KBr Mole fration of RbCI &CsCI NaCJ-LICJ (173K) '"i,..._, NaCl-KC! (173K) -...;:.,,..._._ Mole fration of LiCI & KCI LIF-KF (173K) 514 o!---o.2--o.4---o.l6--o.a-- Mole fration of KF ';;;16 " KF-NaF (173K) 14, 7'=---,,..,._--,-l,:----,--I------I Mole fration of NaF : xpe. 12 qs.[1,2,3&7] qs.[1,2,7-1] Figure 4 Calulated Results for the Surfae Tension of Molten Sal Mixtures in Common Anion Systems with the xperimental Values UJ _ M_o_le_,.fra_t_i_o_n_of,..C_s_B_r--- 2?22 2 LIF-NaF (12K) 18!-----=':,----,,..,..--,,-':---,,-I _ 12---Mole=-.:;.fr=at::::.:::ionofN=aF'---- 2?11 1 LICJ-KCI (173K) " 9 %, -o.2--o o o molefration.:...-mkci- CsBr-LIBr (173K) 9 ';;; _/ U) 71L------'---'-----' Mole fration of LiBr ' " 8 (/) CsCI-LICI ,/ : xpe. 1 2 qs.(1,2,3&7] qs.[l,2,7-1] Figure 4 (Continued) Calulated Results for the Surfae Tension of Molten Sal Mixtures in Common Anion Systems with the xperimental Values 12. a funtion of temperature T and the mole fration of B in the bulk phase, NBB. The value of S shows the fration of the effet of the relaxation layer to Gx,S(T,Ni) 5.2 Consideration of Volume Frations In the present work, we have also introdued the idea proposed by Grjotheim et al. 13 and the reent trial by MOLTN SLAGS, FLUXS AND SALTS '97 CONFRNC

5 Ye & Sahai14 to onsider the following volume frations Nv,BB and Nv,Bs instead of NBB and Ni in qs.[1] and [8] as follows : NvBB=VBNBB/{VA(l-NBB)+VBNBB} [9] Nv:i=VBNil{VA(l-NB 5 )+V#Bs} [1] The above two groups desribed that they obtained the downward urvatures of the omposition dependenies of the surfae tension of molten salt mixtures when they adopted the volume frations in Guggenheim's equation15 for the surfae tension of ideal binary solutions. The hain urves in Figs. 3 and 4 show the alulated results from the ombination of qs. [l ], [2], [7] [1] when we set the value of Sin q.[8] to be.1. As shown in these figures, the alulated results have been revised to agree with the experimental values in the ommon anion systems in Fig.4. On the other hand, the alulated results obtained from qs.[1], [2], [7][1] generate some disrepanies with the experimental values in the ommon ation systems as shown in Fig.3 for, e.g. LiCl-LiF, KCl KF et. When we ompare the omposition dependenies of molten salt mixtures in the ommon ation systems in Fig.3 with those in the ommon anion systems in Fig.4, we have found that the latter systems intend to have larger downward urvatures than the former systems. We have arried out only the phenomenologial disussion on the omposition dependenies of molten salt mixtures in the present work. Furthermore, the value of S=O.l was determined by a parameter-fitting proedure, and its physial meaning is still obsure. Therefore, in order to understand the surfae tension of molten salt mixtures in more details from the standpoint of thermodynami and mirosopi view, we need the information of atomi strutures near the surfae and the further aumulation of the experimental data on the surfae tension as well as thermodynami properties of molten salt mixtures. 6. CONCLUDING RMARKS The surfae tensions of some molten salt mixtures in the ommon ion alkali-halide systems have been alulated using the thermodynami data, whih have been taken from the database assessed aording to the CALPHAD approah. We have adopted some thermodynami onsiderations to fit the alulated results with the experimental values. Although we still have some problems to revise the proedures to alulate the surfae tension of molten salt mixtures, the present approah will enable us to develop a multi-funtional thermodynami data bank system, whih will be of wide appliability in the evaluation of physio-hemial properties of molten salt mixtures with the simultaneous alulation of the phase equilibria in those mixtures. RFRNCS 1. C.W. Bale and G. riksson, "Metallurgial Thermodynamial Databases - A Review", Can. Metal!. Quar., 29, (199), pp T. Nishiawa, "Progress of CALPHAD", Mat. Trans. JIM., 33, (1992), pp T. Tanaka and T. Iida, "Appliation of A Thermodynami Database to the Calulation of Surfae Tension for Iron-base Liquid Alloys", Steel Researh, 65, (1994), pp T. Tanaka, K. Hak, T. Iida and S. Hara, "Appliation of Thermodynami Databases to the valuation of Surfae Tensions of Molten Alloys, Salt Mixtures and Oxide Mixtures", Z. Metallkd., 87, (1996),pp T. Tanaka and S. Hara, "Appliation of Thermodynami Databases to Calulation of Surfae Tension of Molten Salt Mixtures", Pro. Molten Salts K, The letrohemial So., Los Angeles, (1996). 6. J.A.V. Butler, "The Thermodynamis of the Surfaes of Solutions", Pro. Roy. So. A. 135, (1935), pp R. Speiser, D.R. Poirier and K. Yeum, "Surfae Tension of Binary Liquid Alloys", Sripta Metal!., 21, (1987), pp K.S.Yeum, R. Speiser and D.R. Poirier, "stimation of the Surfae Tensions of Binary Liquid Alloys", Metall. Trans., B, 28, (1989), pp K. Monma and H. Sudo, "Thermodynamis of Surfae Tension", J. Jpn. Inst. Metals, 25, (1961), pp A.D. Pelton, "A Database and Sublattie Model for Molten Salts", CALPHAD, 12, (1988), pp J.M. Sangster and A.D. Pelton, in "Phase Diagrams for Ceramists", Vol.VII, L.P. Cook, ditor, Amer. Ceram. So., Westerville, (1983). 12. "NIST molten salt database", National Institute of Standards and Tehnology, (1987). 13. K. Grjotheim, J.L. Holm, B. Lillebuen and H. A. Oye, "Surfae Tension of Liquid Binary and Ternary Chloride Mixtures", Ata Chem. San., 26 (1972), pp J. Ye and Y. Sahai, "Surfae Tension and Density of Molten Salts Based on quimolar NaCl-KCl with Addition of Fluorides", Mat. Trans. JIM, 37 (1996), pp A. Guggenheim, "Mixtures", Clarendon Press, London, (1952). MOLTN SLAGS, FLUXS AND SALTS '97 CONFRNC - 267