Structure model and properties of alkali borate melts

Transcription

1 ZHANG, Z. nd REDDY, R.G. Structure model nd properties of lkli borte melts. VII Interntionl Conference on Molten Slgs Fluxes nd Slts, The South Africn Institute of Mining nd Metllurgy, 00. Structure model nd properties of lkli borte melts Z. ZHANG nd R.G. REDDY Deprtment of Metllurgicl nd Mterils Engineering, The University of Albm, Tuscloos, USA Experimentl nd thermodynmic modelling studies on the structure of lkli borte melts nd properties re discussed. A thermodynmic structure model for lkli borte melts is developed to describe the distribution of structurl groups in the lkli borte melts s function of composition. A qulittive interprettion for the boron nomly in viscosity hs been proposed, which regrds the boron nomly s mnifesttion of the vrious ntitheticl effects cused by the introduction of lkli oxide into boric oxide. It hs been found tht the viscosity of lkli borte melts increses or decreses ccording to the chnges in rigidity nd sptil connectivity of the glss network structure, which is the function of temperture nd composition. These concepts with some key exmples re discussed in reltion to their usefulness nd prcticlity in industril slgs. Keywords: lkli borte melts; structurl model; ctivities; structurl groups; viscosity; Introduction The clssic structurl work on borte glsses dtes bck to Zchrisen nd Wrren et l.,. The existence of the boron nomly in the therml expnsion coefficient vs. glss composition curves for the sodium borte glsses served s the bsis in the erly models for the structure of the lkli borte glsses. Biscoe nd Wrren suggested tht the decrese in the therml expnsion coefficient between 0 nd 6 mol% N O ws the result of the progressive chnge of boron from three- to four-fold coordintion s sodium oxide replced boric oxide in the glss composition. The minimum of the therml expnsion coefficient t 6 mol% N O ws believed to be the result of n increse in the concentrtion of non-bridging oxygen toms nd decrese in the concentrtion of tetrhedrlly coordinted boron. The NMR observtions of Bry nd coworkers,5, which indicted tht the concentrtion of fourfold boron continued to increse up to t lest 0 mol% N O, rised considerble doubt bout the vlidity of this interprettion of the erlier mesurements. Bry et l. developed n elborte model for the lkli borte glsses, which considers the principl types of structurl sites. From the melting-point depressions 6 nd infrred spectroscopy 7, Krogh-Moe developed n idelized model for the structure of lkli borte glsses nd concluded tht lkli borte melts consist of four structurl groups, depending on their compositions: boroxol (B O 9/ ), pentborte (B 5 O 8 ) -, triborte (B O 5 ) - nd diborte (B O 7 ) -. The proportions of the threefold coordinted boron toms to the fourfold coordinted boron toms in these structurl units re shown in Tble I. Arujo proposed sttisticl thermodynmic model to clculte the frction of boron toms tht re in tetrhedrl coordintion in lkli borte glsses s function of composition nd temperture 8,9. In the present study, bsed on rections between intermedite compounds, which re confirmed by the spectroscopic studies in the literture, thermodynmic model is developed to describe the structure of lkli borte glsses nd simulte the distributions of structurl species in the sodium borte melt. In this model, complete types of structurl species, including (O - BO / ) - nd (O -B O / ) - s bsic units in lkli borte melts, contining 50% nd 80% nonbridging oxygen (NBO) respectively, nd free oxygen O - contining 00% NBO (the / subscript is used to identify the connection to two ctions) re tken into considertion in the entire composition rnge. Structure model of binry borte melts X-ry diffrction studies,0 on vitreous boric oxide (pure) hve shown tht the bsic building units re (BO / ) tringles, which re linked with neighbouring ones by three Tble I The structurl units in the lkli borte melts Structurl units in lkli Rtio of threefold boron toms to borte melts fourfold boron toms boroxol (B O.5 ) :0 pentborte (B 5 O 8 ) - : tetrborte (B 8 O ) - 6: triborte (B O 5 ) - : diborte (B O 7 ) - : (O - BO / ) - :0 STRUCTURE MODEL AND PROPERTIES OF ALKALI BORATE MELTS 87

2 bridging oxygen toms, thereby forming n lmost plnr structure of continuous B-O-B bonds. The B-O bond distnce is 0.7 nm. The bonding distnce between the lyer is ~0. nm, which is much greter distnce; thus the bonds re weker. The strong B-O bonds within the plnr structure retin two-dimensionl configurtion, while the weker inter-sheet bonds permit reltively esy sliding. The Rmn spectrum of B O glss is perhps the strongest support for the concept of boroxol rings,. The pek t 808 cm - is scribed to the symmetricl stretching of the boroxol rings. As temperture increses, the intersheet distnce increses, resulting in decrese in viscosity. Some reserch hs shown tht the equilibrium between boroxol BO / tringle shifts to the right with incresing temperture. Whtever the nture of the ring configurtion, the tringulr connections in the B O glss do not mke very rigid structure. Generlly, sodium borte glss is treted s binry system. B O is considered s single component. The ctivity dt of boric oxide nd sodium oxide for the binry system re vilble in literture. In the cse of structurl study, the binry system B O -N O cn lso be considered s multicomponent system composed of (B O 9/ ), (BO / ), (B 5 O 8 ) -, (B 8 O ) -, (B O 5 ) -, (B O 7 ) -, (O - BO / ) -, (O - B O / ) -, nd O -. To clculte ctivities of these structurl species, reltionships between these ctivities nd the ctivities of boric oxide nd sodium oxide hve to be estblished. NMR spectr reported by Youngmn nd Zwnziger nd Hssn et l. indicte structure for vitreous B O with : rtio of boroxol rings to boron trioxide unites. Thus, the chemicl component B O is best represented by the boroxol group nd the boron trioxide group in terms of structurl species. Bsed on mss blnce, the reltionship between B O nd (B O 9/ ) s well s (BO / ) is given by nbo n + n ( [] BO 9/ ) ( BO/ ) An equl mount of the sme substnce is represented on ech side of the bove expression using two different composition coordinte systems. The totl Gibbs energy of the system will not chnge with vrious composition coordinte systems. From this, Equtions [b] nd [c] cn be obtined. o G( left) Σµ ( left) ( µ BO + RTnBO ) O G( right) Σµ ( right) ( µ BO + RTn 9/ BO 9 ) [b] / + O µ + RTn [c] Since ech (B O 9/ ) unit is in equilibrium with three (BO / ) units t ny given temperture, nd the equilibrium my shift with chnging temperture, hence, it follows tht ( BO 9/ ) ( BO/ ) nd BO BO 9/ BO/ K BO/ o BO9/ ( BO BO ) / / [] [b] K O is the equilibrium constnt for rection []. Substituting eqution [b] into [c], Eqution [] cn be obtined. K BO [] 9 o BO By the progressive introduction of lkli oxide into the borte melts, boron toms re prtilly converted from threefold coordintion stte (B ) to fourfold coordintion stte (B ). The distribution nd concentrtion of complex structurl groups in the melt chnge grdully such tht the proportion of B in the melt increses progressively to mximum nd then decreses due to the increse of nonbridging oxygen (NBO) in the melt. Initil ddition of O - to the boroxol rings minly leds to the formtion of the pentborte group (B 5 O 8 ) - by prtilly converting B to B. Every O - converts two boron toms from B to B. The formtion rection of the structurl units cn be expressed s rection []: 0 ( BO 9/ )+ O ( BO 5 8) [] nd K 5 K 5 * [b] BO 5 8 BO 9/ O BO O where K * is the equilibrium constnt for rection [], nd K (K * 5/6 K ō ) is combined equilibrium constnt. K * will be equl to K when K 0 is equl to one. For simplicity, the chrge numbers re not included in writing the ctivity Eqution [b] nd other equtions given below. With further ddition of O - to the borte glss melt, the tetrborte group content increses t the expense of the pentborte group. Every O - converts two boron toms from B to B, nd the B to B rtio in the melt chnges from : to :: 8( BO 5 8) + O 5( BO 8 ) [5] nd BO 8 BO 5 8 O BO O K 5 K K [5b] With the further ddition of O - to the borte glss melt, the triborte group content increses t the expense of the tetrborte group. Every O - converts two boron toms from B to B. The B to B rtio in the melt chnges from : to :: ( ) + ( ) BO O 8 BO nd K K 8 k 5 K 5 BO 5 BO 8 O BO O [6] [6b] As more O - is dded to the borte glss melt, the diborte group content increses t the expense of the triborte group. Every O - converts two boron toms from B to B. The B to B rtio in the melt chnges from : to :: BO O BO nd / 8 5 ( ) ( ) + ( ) 5 7 K K K K 5 K 5 ( ) BO 7 BO 5 O BO O [7] [7b] The diborte group is the structurl group tht cn ccommodte the mximum number of B. All the possible positions for n oxygen tom to go into tetrhedrl coordintion re filled. The borte glss melt cnnot 88 MOLTEN SLAGS FLUXES AND SALTS

3 ccommodte more O - by the B to B conversion. Further ddition of O - leds to the formtion of the non-bridging ionic species (O - BO / )-, designted s (BO ) - for convenience, nd the brekdown of the borte glss network, which lowers the viscosity. Every O - converts two boron toms from B bck to B. Chin structures re formed when tringle unit is connected with its neighbours only by two bridging oxygen toms: BO O BO [8] ( ) + ( ) nd [8b] Thus, by the progressive introduction of the lkli oxide into the borte melt, the formtion of the non-bridging ionic species (O -B O / ) -, designted s (B O 5 ) - for convenience, occurs, the ioniztion of the boron tringle increses, nd the number of bridging bonds per tringle decreses: BO O B O [9] nd 7 K K K K K K ( ) ( ) + ( ) 5 K K K K 6 K K 5 K 5 ( ) BO 5 6 BO O 6 5 BO O [9b] By knowing K i in Equtions [b] to [9b], BO, nd O, the ctivities of ll the structurl groups cn be clculted from Equtions [b] to [9b] respectively. By E.M.F. mesurements of binry lkli borte melts, Itoh et l. 5, 6 evluted the prtil molr Gibbs energies for N O-B O, Li O-B O, nd K O-B O melts in the composition rnges of B O to M O B O with high ccurcy, compred to ll vilble dt 7. Therefore, MO, the ctivities of lkli oxides in the binry borte melts, referred to the pure solid M O, cn be clculted from the vlues of G M O. The reltive molr Gibbs energies nd the ctivities of B O in the binry borte melts, referred to pure B O, re derived with the help of the Gibbs-Duhem reltion: X dg + X dg [0] MO MO BO BO 0 BO 5 BO7 O 5 BO O The ctivities of B O in lkli borte melts re given in Figure. The ctivity of B O in sodium borte melt t K cn be expressed by three-prmeter Sigmoidl eqution, with R : [] Figure shows tht when the mole frction of B O in the sodium borte melt t K reduces from.0 to 0.667, the ctivity of B O decreses from to 0.0. From to 0.0 of X BO, the ctivity of B O will tend to zero. Although the experimentl dt 5,6 re vilble only in the composition rnges 0 to 0. of X NO, the regressed eqution extended the ctivity vlues of B O to the entire composition rnge. Agin, with the help of the Gibbs- Duhem reltion, smoothed curve of NO vs X NO ws obtined, which mde it possible to do further model clcultions nd evlute the distribution of structurl groups in the melt for the entire composition rnge. Figure shows the ctivities of N O nd B O in the binry sodium borte melt t K s function of in the entire composition rnge. Likewise, the ctivity of B O in lithium borte melt t 98 K cn be expressed s Eqution [], with R : BO [] nd the ctivity of B O in potssium borte melt t 75 K cn be expressed s eqution [], with R : BO BO. 06 ( XBO ) + exp XBO ( XBO ) + exp ( XBO ) + exp ( 0 XBO ) ( ) ( 0 XBO ) [] Determintion of equilibrium constnts To clculte the ctivities nd the distribution of structurl groups in the sodium borte melt, the equilibrium constnts K i for the bove formtion rections given by Equtions [] to [9] hve to be determined. No proper K i vlues cn Figure. Activities of B O in lkli borte melts (dt fter Itoh et l. 5, 6 ) Figure. Activities of sodium oxide nd boric oxide s function of X NO t K STRUCTURE MODEL AND PROPERTIES OF ALKALI BORATE MELTS 89

4 Tble II The determined equilibrium constnts for the sodium borte melt t K Equilibrium constnt Vlue Eqution K [b] K [5b] K [6b] K [7b] K [8b] K [9b] be obtined, however, from the literture or thermodynmic dtbses becuse the thermodynmic dt required re either not vilble or re incomplete. Severl uthors hve ttempted to clculte the distribution of structurl groups in lkli borte melts with method similr to tht of the present study by considering the equilibrium between the structurl units 5,8. They, however, hve filed to present correct formtion rections nd to determine the proper K i vlues. Therefore, the ctivities of ll the structurl units in the melts for the entire composition rnge hve not been presented correctly. As result, these uthors hve ssumed tht the lkli borte melts re idel solutions. From the ctivities of B O in the binry melts, shown in Figures nd, it cn be found esily, however, tht the lkli borte melts re fr from idel solutions. In Eqution [b], B5O8 nd K re unknown. To solve the two unknowns, n dditionl condition is needed. Fortuntely, there is specil condition vilble for solving this problem. When we sy tht lkli borte melts consist of boroxol (B O 9/ ), pentborte (B 5 O 8 ) -, tetrborte (B 8 O ), triborte (B O 5 ) -, nd diborte [B O 7 ] - groups, we hve recognized tht the lkli borte melts include ll these stoichiometric compounds. Tht is, the sodium borte melt is solution mixture of liquid boroxol, liquid pentborte, liquid tetrborte, liquid triborte, nd liquid diborte. According to the definition of ctivity, the ctivity of pure condensed phse, which is liquid or solid phse, is unity t one br. For pure liquid pentborte, the glss formul N O 5B O corresponds to the stoichiometric composition of 6.67 mol% N O nd 8. mol% B O. This mens tht t this composition, the ctivity of the pentborte group referred to its pure liquid pentborte is unity nd hs mximum vlue. Therefore, by substituting BO nd O t X NO nd K into Eqution [b], K cn be clculted. K * cn lso be determined if the reltion of K 0 s function of temperture is known. For pure liquid tetrborte, the glss formul N O B O corresponds to the stoichiometric composition of 0 mol% N O nd 80 mol% B O. This mens tht t this composition the ctivity of the tetrborte group referred to its pure liquid tetrborte is unity nd hs mximum vlue. By substituting the determined K, BO, nd O t 0.0 X NO nd K into Eqution [5b], K cn be determined. Consequently, t the stoichiometric compositions tht correspond to the glss formule N O B O, N O B O, N O B O nd N O B O, the rest equilibrium constnts K, K, K 5, nd K 6 in Equtions [6b] to [9b] respectively cn lso be determined. Therefore, ll the equilibrium constnts re determined without ny further ssumption or djustble prmeters. Ech equilibrium constnt hs fixed vlue determined Figure. Activities of structurl groups in the sodium borte melt s function of composition t K only by corresponding temperture, pressure, BO, nd O. All the determined equilibrium constnts t K re tbulted in Tble II. Results nd discussion Using the bove K i, BO, O nd vlues t K nd given composition, the ctivities of the structurl groups in the sodium borte melt referring to their pure liquid sttes cn be clculted respectively. Figure shows the ctivities of ll the structurl groups in the sodium borte melt s function of the mole frction of N O t K in the entire composition rnge. All of the compositions, t which the ctivities of the structurl groups rech mximum, exctly mtch the stoichiometric compositions of the corresponding glss formul. The ctivities decrese when the compositions re wy from the stoichiometric compositions of the corresponding glss formul. This mens tht Eqution [] represents correct reltionship between the two different composition coordinte systems. Otherwise, the pek positions of the ctivities would shift to the right or left of the stoichiometric compositions. At ny composition between pure B O nd pure N O, the sodium borte melt consists of more thn one structurl group. For exmple, s it cn be seen from Figure, t 0. of X NO, the melt is minly composed of diborte, triborte, nd [O - BO / ] -. Ech structurl group is only frction of the sodium borte melt. At given composition, Figure. Effective concentrtions of structurl groups in the sodium borte melt s function of composition t K 80 MOLTEN SLAGS FLUXES AND SALTS

5 the higher the ctivity of structurl group reltive to the totl ctivities of the structurl groups, the higher the effective concentrtion of the structurl group in the sodium borte melt. Therefore, the effective concentrtion of ech structurl group in the sodium borte melt cn be represented by the ctivity frction of the structurl group. The ctivity frction of the structurl group i, t given composition, cn be expressed s the following eqution: f i i i n i i [] where f i is the ctivity frction of component i. The effective concentrtions of ll the structurl groups in the sodium borte melt s function of the composition re shown in Figure. Vrious structurl groups re distributed in different composition rnges. Between the compositions of pure N O nd pure B O, ll of the ctivity frctions of the structurl groups re less thn one. In the cse of equl ctivity coefficients for ll components, the ctivity frction becomes the mole frction. Tht is, the idel solution is only one specil cse of the non-idel solution. The initil introduction of the sodium oxide into the boric oxide glss results in significnt decrese in the effective concentrtion of the boroxol group. Simultneously, the effective concentrtions of pentborte, tetrborte, nd triborte groups strt to increse t different rtes. The threefold coordinted boron toms re prtilly converted to the fourfold coordinted boron toms. At bout 0. X NO, the effective concentrtion of the boroxol group is close to minimum. At 0., 0.0, nd 0.7 of X NO, the effective concentrtions of pentborte, tetrborte, nd triborte groups rech their mxim, 0.0, 0., nd 0. respectively. The ctivity frction of the diborte group strts to increse significntly from 0.0 X NO, reches mximum vlue of 0.9 t 0.5 X NO, nd then grdully decreses to zero t bout 0.60 X NO due to the B to B conversion by NBO. The effective concentrtion of the [O - BO / ] - group tht contins 50% NBO strts to increse t 0.0 X NO nd reches mximum vlue of 0.76 t 0.9 X NO. The ctivity frction of the [O - B O / ] - group tht contins 80% NBO first ppers t bout 0.0 X NO nd reches mximum vlue of 0.58 t 0.68 X NO. At over 0.5 X NO, due to the excess of sodium oxide content, free oxygen O - ppers, nd the effective concentrtions of other intermedite compounds in the melt grdully decrese to zero with the increse in X NO. By summrizing the frctions of B, B, nd NBO in ech type of the structurl units, the totl frctions of B, B, nd NBO in the sodium borte melt cn be clculted by the following expressions: Figure 5: Frctions of B, B, nd NBO in the sodium borte melt s function of composition t K Figure 5 shows frctions of B, B, nd NBO in the sodium borte melt s function of the mole frction X NO. The frction of boron toms tht re in tetrhedrl coordintion in totl boron toms cn be clculted s the following: N B f fb + f B B [8] Compred with NMR results reported by Bry nd O Keefe nd Svnson et l. 9, Arujo 9 clculted nd plotted the frction of boron toms in tetrhedrl coordintion s function of the composition of lkli borte glsses. Their results compred with the clculted vlues from the present model re replotted in Figure 6. The present modelling result is in good greement with their results in simulting the increse of the fourfold coordinted boron toms s function of the composition. Figure 6 lso reveled some uncertinty of quntifiction between NMR mesurements on the mximum nd the decrese of the fourfold coordinted boron toms s function of the composition. The higher frctions of fourfold boron toms resulted from previous studies re f B Σ fi % B 00 ( ) [5] Σ fi % B fb 00 nd f NBO ( ) Σ fi (% NBO) 00 where f i is the ctivity frction of the structurl group. [6] [7] Figure 6. The frction of boron toms in tetrhedrl coordintion in lkli borte glsses s function of composition STRUCTURE MODEL AND PROPERTIES OF ALKALI BORATE MELTS 8

6 probbly due to tht low temperture ws pplied for the mesurements nd clcultion. The present thermodynmic structure model is lso in greement with Bry et l. nd Krogh-Moe s models, which derived from NMR observtions nd from melting point depression mesurements respectively 0,. The compositions, t which ccording to the present model the boroxol group disppers, the mximum effective concentrtions of the tetrborte group nd the diborte group pper, nd NBO first occurs, re in close greement with their results, lthough, it is impossible to compre the concentrtions of structurl groups clculted in this study with those from Bry s work. This thermodynmic structure model includes complete intermedite compounds for sodium borte melt, covers the entire composition rnge, nd quntittively explictes the equilibrium reltions between ll the intermedite compounds. It should be noted tht the present structurl model does not ssume the temperture independence of the structurl species of the melt. Arujo 8,9 nd Vedishchev et l. studied the temperture dependence of structurl species in the borte melts. Some recent NMR studies lso documented the structure chnges with temperture. In this study, the ctivities nd distribution of the structurl groups in the sodium borte melt s function of composition re clculted using EMF experimentl dt t K. As we know, MO is function of composition nd temperture. It follows tht the clculted distribution of the structurl groups in the lkli borte melts my chnge when MO chnges with temperture. The method used in this study is very vluble. It my be used for other binry systems with intermedite compounds. The thermodynmic properties of these intermedite compounds cn be quntittively estimted from the corresponding properties of pure oxides. Therefore, by using this method, the thermodynmic dtbses cn be enriched significntly with relible experimentl dt. Structurl interprettion of the boron nomly Silicte melts exhibit smooth vrition in physicl properties on chnging the silic content t constnt temperture -6. On the other hnd, lkli borte melts Figure 7. Isotherml viscosities of lkli borte melts (dt fter Shrtsis et l. 7 ) exhibit mximum nd minimum, or one of them, in physicl properties, such s viscosity, therml expnsivity, molr volume, nd other properties, with continuous vrition in B O content. This phenomenon hs been discussed in detil 7-9. Referring to the experimentl dt of Shrtisis nd coworkers 7, the isotherml viscosities of lkli borte melts re plotted s function of lkli oxide content in Figure 7. Between 800ºC nd 900ºC, the viscosities of lkli borte melts show minim t 0 mol% M O nd mxim t 0 mol% M O. Since it seems tht the boron nomly definitely does exist in the temperture rnge tht the lkli borte glsses re of Newtonin type, it would be informtive to study the reltionship of the boron nomly nd the structure of borte glsses. As Shelby 0 hs pointed out the disppernce of the boroxol group, mximum in the concentrtion of the tetrborte group, the first ppernce of the diborte group, nd the first ppernce of the (O - BO / ) - group ll occur t 0 mol% N O. Exctly t 0 mol% sodium oxide, the viscosity mximum of the sodium borte melts is found to occur. This strongly suggests tht there is direct structurl connection between wht hppens t this composition nd the presence of the boron nomly. Shrtsis nd coworkers 7 erlier postulted tht t ny given temperture, there were two opposing tendencies cused by the introduction of lkli oxide into boric oxide, nmely () the wekening effect of forming singly bonded oxygen toms nd () the strengthening effect of forming fourfold coordinted boron groups. They suggested tht the dominnt ction of the oxygen toms introduced in the low lkli oxide rnge ws to brek the B-O-B bonds nd form B-O bonds, which lowered the viscosity. Some of the oxygen toms introduced, however, might lso hve formed fourfold coordinted boron groups tht would hve hd n opposing effect on viscosity. As more lkli oxide ws dded, the equilibrium ws shifted in the direction of incresing the concentrtion of fourfold boron groups so tht the downwrd trend of viscosity ws first hlted nd then reversed. At high lkli oxide concentrtions, where ll of the possible positions for n oxygen tom to go into tetrhedrl coordintion re filled, only the formtion of singly bonded oxygen is possible, nd further introduction of lkli oxide cn only reduce the viscosity. It cn be seen from Figure 5 tht, by the progressive introduction of N O, boron toms were grdully converted from threefold coordintion stte (B ) to fourfold coordintion stte (B ). The incresed connectivity in the network structure cused the therml expnsion to decrese nd flow-relted properties to decrese, which ment the viscosity incresed. The concentrtion of the fourfold boron continued to increse up to 0. X NO nd then ws converted bck to threefold coordinted boron. The non-bridging oxygen in the melt only ppered t bove 0. X NO. Therefore, the monotonic increse of f B nd no ppernce of f NBO in the composition rnge of 0 to 0. X NO, shown in the present structure model, do not support Shrtsis nd coworkers 7 postultion. There must be some other reson tht cused the viscosity minimum to occur. An lterntive explntion is needed. Glss-forming B O is regrded s reltively strong liquid. For oxide glsses in generl, strong covlent bondings prevent the structure from ny drstic chnges when temperture is rised bove glss trnsition 8 MOLTEN SLAGS FLUXES AND SALTS

7 temperture T g. Above T g the high degree of connectivity results in high viscosity. The initil introduction of the lkli oxide into the boric oxide melts, s Equtions [] to [7] hve suggested, does not led to the ppernce of non-bridging oxygen in the borte melt. However, it does result in the increse of bond ioniztion nd the disorder of the rndom network of the glss melt. The bonds in the glss network re converted from strong covlent bonds to weker ionic bonds. This decreses the bonding energy, in other words, the rigidity of the glss network nd the viscosity of the melt. After the conversion of boron from threefold coordintion stte to fourfold coordintion stte, ll the oxygen remins bridging; the extr negtive chrges on the fourfold boron groups re stisfied by lkli ions M + in the vicinity. The electron trnsfer from the ions M + occurs s distributed chrge density over lrge effective dimeter of the structurl group, nd is not loclized between the M tom nd ny specific oxygen. Bsed on the bove discussion, qulittive interprettion for the boron nomly is proposed. Of course, the vlidity of these interprettions is mtter of opinion. The explntion is stted s follows: The boron nomly tht ppered in the temperture rnge in which the lkli borte glsses re of Newtonin type, is mnifesttion of the vrious ntitheticl effects in equilibrium. At ny given temperture nd composition, the structure of lkli borte glsses tends to chnge in two opposing directions cused by the introduction of lkli oxide into boric oxide nd reches n equilibrium between the two opposing tendencies. The dominnt effect on the structure by the initil introduction of lkli oxide into boric oxide is the ioniztion of the covlent bonded network. Associted with lkli ions in their vicinity, nionic structurl groups, such s pentborte, tetrborte, nd triborte, re formed. Increse of the ioniztion of the covlent bonded network nd the disorder of the rndom network of the glss melt reduces bond energy in the melt nd cuses the viscosity to decrese. In the mentime, the initil introduction of the lkli oxide leds to the conversion of the boron tom from threefold coordintion stte to fourfold coordintion stte. The incresed rigidity nd sptil connectivity in the network structure tends to cuse the therml expnsion to decrese nd the viscosity to increse. A lower temperture or higher lkli content is in fvour of the ltter effect. As more lkli oxide is dded, the structure-strengthening effect of converting B to B outweighs the structurewekening effect of ionizing the bonds in the melt. Therefore, the equilibrium is shifted in the direction of strengthening the network structure so tht the downwrd trend of viscosity is first hlted t 0. X NO, nd then reversed. Although the concentrtion of the fourfold boron toms continues to increse up to 0. X NO, the upwrd trend of the viscosity is hlted immeditely, once the non-bridging oxygen in the melt occurs t 0. X NO. The structurewekening effect of the non-bridging oxygen outweighs the structure-strengthening effect of converting B to B. Visser nd Stevels 0 suggested tht the effect of non-bridging oxygen on decresing the viscosity ws lrger thn tht of converting B to B on incresing the viscosity. Therefore, there is no justifiction for ssuming tht the effect of f B nd f NBO on the viscosity of lkli borte glsses is numericlly equl but opposite in sign. As more lkli oxide is dded, the incresed non-bridging oxygen breks the connection nd rigidity of the network; therefore, the viscosity of glss decreses. Summry Bsed on the thermodynmic considertions, quntittive structure model for lkli borte melts is developed to describe the distribution of structurl groups in the lkli borte melts s function of the composition. The equilibrium reltions between the ctivities of the structurl groups re quntittively explicted over the entire composition rnge. Frctions of threefold coordinted boron toms nd fourfold coordinted boron toms in the sodium borte glss re clculted for the entire composition rnge nd compred with results reported in the literture. A qulittive interprettion for the boron nomly in viscosity hs been proposed, which regrds the boron nomly s mnifesttion of the vrious ntitheticl effects cused by the introduction of lkli oxide into boric oxide. At ny given temperture nd composition, the structure-wekening effects of bond ioniztion in the glss network nd the bond brek by non-bridging oxygen re in equilibrium with the structure-strengthening effect of converting boron toms from the threefold coordintion stte to the fourfold coordintion stte. The viscosity increses or decreses ccording to the chnges in the sptil connectivity of the glss network structure, governed by dominnt effect. Acknowledgements The uthors re plesed to cknowledge the finncil support for this reserch by the Deprtment of Defence ARO nd U.S. DOT (CAVT) Grnt No. DTMFH6, Prometllurgy funds nd ACIPCO. References. ZACHARIASEN, W.H. The Atomic Arrngement in Glss. J. Am. Chem. Soc., vol. 5, no pp WARREN, B.E., KRUTTER, H., nd MORNINGSTAR, O., Fourier Anlysis of X-ry Ptterns of Vitreous SiO nd B O. J. Am. Cerm. Soc., vol. 9, no pp BISCOE, J. nd WARREN, B.E., X-Ry Diffrction Study of Sod-Boric Oxide Glss. J. Am. Cerm. Soc., vol., no pp BRAY, P.J., nd O KEEFE, J.G., Nucler Mgnetic Resonnce Investigtions of the Structure of Alkli Borte Glsses, Phys. Chem. Glsses, vol., no pp BRAY, P.J., NMR Studies of Bortes. Borte Glsses: Structure nd Properties, nd Applictions (Mterils Science Reserch, vol. 7), Pye, L.D., Frechette, V.D. nd Kreidi, N.J. (eds.). Plenum, New York, 978. pp KROGH-MOE, J. Structurl Interprettion of Melting Point Depression in the Sodium Borte System. Phys. Chem. Glsses, vol., no pp KROGH-MOE, J. Interprettion of the Infrred Spectr of Boron Oxide nd Alkli Borte Glsses. Phys. Chem. Glsses, vol. 6, no pp STRUCTURE MODEL AND PROPERTIES OF ALKALI BORATE MELTS 8

8 8. ARAUJO, R.J. Sttisticl Mechnics of Chemicl Disorder: Appliction to Alkli Borte Glsses. J. Non-Cryst. Solids, vol. 58, 98. pp ARAUJO, R.J. Temperture Dependence of Boron Coordintion in Alkli Borte Glsses s n Exmple of Second Order Trnsition. Phys. Chem. Glsses, vol., no pp GREEN, R.L. X-Ry Diffrction nd Physicl Properties of Potssium Borte Glsses. J. Am. Cerm. Soc., vol. 5, no., 9, pp MIYAKA, M., SUZUKI, T., MORIKAWA, H., TAKAGI, Y., nd MARUMO, F. Structurl Anlysis of Molten B O. J. Chem. Soc., Frdy Trns., Prt I, vol. 80, 98, pp GASKELL, P.H., PARKER, J.M., nd DAVIS, E.A. The Structure of Non-crystlline Mterils. New York, Tylor & Frncis, Inc., 98.. pp.. HASSAN, A.K., TORELL, L.M., nd BÖRJESSON, L. Structurl Chnges of B O through the Liquidglss Trnsition Rnge: A Rmn-scttering Study. Phys. Rev. B, vol. 5, no., 99. pp YOUNGMAN, R.E. nd ZWANZIGER, J.W. Multiple Boron Sites in Borte Glss Detected with Dynmic Angle Spinning Nucler Mgnetic Resonnce, J. Non-Cryst. Solids, vol. 68, 99. pp ITOH, H., SASAHIRA, A., MAEKAWA, T., nd YOKOKAWA, T. Electromotive-force Mesurements of Molten Oxide Mixtures. J. Chem. Soc., Frdy Trns., vol. 80, no., 98. pp ITOH, M., SATO, S., nd YOKOKAWA, T. E.M.F. Mesurements of Molten Mixtures of Lithium Oxide +, Sodium Oxide +, nd Potssium Oxide + Boron Oxide. J. Chem. Thermodynmics, vol. 8, 976. pp SHAKHMATKIN, B.A. nd VEDISHCHEVA, N.M. Thermodynmic Studies of Oxide Glss-forming Liquids by the Electromotive Force Method. J. Non- Cryst Solids, vol. 7, 99. pp REDDY, R. G., YEN, J. Y., nd ZHANG, Z. Chemicl Properties of Sodium Borte Cotings. Elevted Temperture Cotings: Science nd Technology II, Proceedings of symposium held during the TMS Annul Meeting 96 (Anheim, CA, Februry 8, 996). Dhotre, N.B. nd Hmpikin, J.M. (eds.). TMS, 996. pp SVANSON, S.E., FORSLIND, E. nd O KEEFE, J.G. Nucler Mgnetic Resonnce Study of B Coordintion in Potssium Borte Glsses. J. Phys. Chem., vol. 66, 96. pp SHELBY, J. E. Therml Expension of Alkli Borte Glsses. J. Am. Cerm. Soc., vol. 66, no., 98, pp GRISCOM, D.L. Borte Glss Structure. Borte Glsses: Structure, Properties, nd Applictions (Mterils Science Reserch, vol. ). Pye, L.D., Frechette, V.D., nd Kreidel, N.J. (eds.). New York, Plenum, 978. pp. 9.. VEDISHCHEVA, N.M., SHAKHMATKIN, B.A., SHULTZ, M.M., nd WRIGHT, A.C. Simultion of the Structure of Borte Glsses nd Melts on the Bsis of Thermodynmic. Proc. Second Intl. Conf. On Borte Glsses, Crystls, nd Melts, Wright, A.C. (ed.). The Society of Glss Technology, Sheffield, 997. pp. 5.. TURKDOGAN, E.T. Pheologicl nd Trnsport Properties. Physico-Chemicl Properties of Molten Slgs nd Glsses, London, The Metls Society, 98. pp. 6.. ZHANG, Z. nd REDDY, R. G. Viscosity of Led Silicte Slgs, Minerls & Metllurgicl Processing, vol. 9, no., 00, pp REDDY, R. G., nd HEBBAR, K., Viscosity of FeO- SiO Slgs, Minerls & Metllurgicl Processing, vol. 8, no., 00, pp ZHANG, Z., nd REDDY, R. G., Distribution of Vrious Structurl Species in Alkli Borte Melts, EPD Congress, P. R. Tylor, (ed.), TMS, Wrrendle, USA, 00, pp SHARTSIS, L., CAPPS, W., nd SPINNER, S. Viscosity nd Electricl Resistivity of Molten Alkli Bortes. J. Am. Cerm. Soc., vol. 6, no. 0, 95. pp VARSHNEYA, A.K. The Boron Anomly. Fundmentls of Inorgnic Glsses, New York, Acdemic Press, Inc., 99. pp LI, P., GHOSE, A.C., nd SU, G. Viscosity Determintion of Boron Oxide nd Binry Bortes. J. Am. Cerm. Soc., vol. 5, no pp VISSER, T.J.M. nd STEVELS, J.M. Rheologicl Properties of Boric Oxide nd Alkli Borte Glsses. J. Non-Cryst. Solids, vol. 7, 97. pp MOLTEN SLAGS FLUXES AND SALTS