Glasses in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2 were studied with respeet to their crystallization and viscosity. Glasses in the metaphosphate range are composed of phosphate chains, and possess a comparably small tendeney to crystallization. Here, AlPO4 in a cristobalite-like modification and Ca2P2O7 are formed. Glasses in the intermediate range between metaphosphate and invert glass structures are composed of smaller phosphate units and show higher erystal growth velocities as well as higher viscosities. Additions of MgO, AI2O3 and TiO2 result in a strengthening of the glass network, and hence, inereasing viscosities and a notably smaller tendeney to crystallization

Knoche, Silke

Rüssel, Christian

Vogel, Jürgen

Wange, Peter

English

Repositorium für Naturwissenschaften und Technik

Original PaperC r y s t a l l i z a t i o n a n d v i s c o s i t y o f p h o s p h a t e m e l t s i n t h e S y s t e mN a 2 0 - C a O - M g O - P 2 0 5 - A l 2 0 3 - T i 0 2Peter Wange, Jürgen VogeP, Silke Knoche^ and Christian RüsselOtto-Schott-Institut für Glaschemie, Friedrieh-Schiller-Universität Jena, Jena (Germany)Glasses in the System Na20-CaO - M g O -P205 -A l203 -T i02 w e r e studied with respeet to their crystallization and viscosity.Glasses in the metaphosphate ränge are composed of phosphate chains, and possess a comparably small tendeney to erystallization.Here, AIPO4 in a cristobalite-like modification and Ca2P207 are formed. Glasses in the intermediate ränge between metaphosphateand invert glass structures are composed of smaller phosphate units and show higher erystal growth velocities as well as higherviscosities. Additions of MgO, AI2O3 and Ti02 result in a strengthening of the glass network, and hence, inereasing viscosities anda notably smaller tendeney to erystallization.1 . IntroductionPhosphate glasses show  a viscosity-temperature behaviourdifferent from that of Silicate glasses. At comparable net-work former/network modifier ratios, phosphate glasses ex-hibit notably lower viscosities [1]. By analogy, also the crys-tallization behaviour is different. In Silicate glasses, controlled volume crystallization might be enabled by micro-phase Separation and subsequent nucleation and crystalgrowth. This is scarcely possible in phosphate glasses be-cause they usually do not exhibit phase Separation. Phos-phate glasses mostly crystallize from the surface [2], how-ever, it is also possible to initiate volume crystallization byadding nucleating agents such as noble metals or zirconiato the glass batch. For this purpose, phosphate glasses inthe invert glass ränge are most advantageous, however, alsometaphosphate glasses are suitable. Using this strategy, thepreparation of bioactive phosphate glass-ceramics has beenenabled, as previously reported in [3 and 4]. Their chemicalsolubility and also their bioresorptivity can be adjusted bythe chemical composition. The glasses which these glass-ceramics have been prepared from also show some interest-ing properties, especially with respect to their corrosion be-haviour in slightly acidie and neutral Solut ions [5]. As recently reported in [5], also small changes in the chemicalcomposition notably affect the corrosion behaviour. Herethree composition ranges can be distinguished: the invertglass ränge ([P2O5] < 41 mol%), the metaphosphate ränge([P2O5]  > 47mol%) and the ränge in between. The pro-nounced tendency to crystallization, however, is a strongReceived 17 February, revised manuscript  2 June 2004.Now with: Institut für Physikalisehe Hochtechnologie, Fried-rieh-Schiller-Universität Jena, Jena (Germany).Now with: SCHOTT AG, Mainz (Germany).limitation with respect to other applications. This paperprovides  a study on the viscosity-temperature dependencyand the crystallization of phosphate glasses in the SystemN a 2 0  CaO  M g O  P2O5  AI2O3  T i02 with compo-sitions in the metaphosphate glass ränge and in the compo-sition ränge between the invert glass and the metaphosphateglass ränge.2. ExperimentalGlasses in the System N a 2 0 - C a O - M g O - P 2 0 5 - A l 2 0 3 -Ti02 were melted from the raw materials Ca(P03)2,Mg(P03)2, Α1(Ρθ3)3, Na2C03, Al(OH)3 and Ti02 (anatase).Here, the metaphosphate ränge ([P2O5] > 47 mol%) and anintermediate ränge (41 mol% < [P2O5] < 47 mol%) betweeninvert and metaphosphate glass ränge were considered. Thebatch compositions are summarized in table 1. Some of thecompositions were analytically controlled by inductivelycoupled plasma emission optical spectroscopy ( ICP-FOS).The analytical compositions deviate from the batch compositions especially with respect to the P2O5 concentration.Due to P2O5 evaporation during melting, the P2O5 con-centration in the glass was approximately 1 0 % (relative)lower than in the batch. In order to measure viscosities athigh temperatures,  a rotation viscometer (Bahr 403, Bahr,(Germany)) was used. The viscosities were continuouslymeasured as a function of the temperature. In the tempera-ture ränge studied, crystallization did not occur. Glasseswith strong tendency to crystallization could only be studiedin a very limited temperature ränge. To study the crystalliza-tion behaviour, DTA profiles (Shimadzu DTA 5, Shimadzu(Japan)) of powdered glass samples (grain sizes 250 to315 pm) were recorded. Here, Jg as well as the crystalliza- - - - - ­---Crystallization and viscosity of phosphate melts in the System Na20-CaO -MgO -P205-Al203-Ti02Table 1. Glass eompositions studied (in mol%)sample [Na.O] [Κ^Ο] [CaO] [MgO] [Ρ2Ο5] [ΑΙ2Ο3] [TiOJ [SiOJ [B2O3]Α 16 22 8 47 5 2Β 8 8 22 6 45 6 5 - -C 14.9 1.9 7.3 8.6 0.5 58.6 8.2D 15.6 - 10.9 - - 1.2 - 72.3 -Ε 19.6 - 24.1 - 52.1 4.2 - - -F 19 - 23.4 52.3 4.0 1.3 - -G 19 - 11.4 12 52.3 4 1.3 - -Η 18.4 - 10.6 12 52.5 3.9 2.6 - -I 20.3 - 25 - 47.8 4.4 2.5 - -Κ 19.5 - 23.7 4.3 52.5 - - - -L 18.2 - 22.1 4.1 53 - 2.6 - -Μ 20.6 - 30.6 - 40.9 5.5 2.4 - -Ν 16 - 26 4 47 5 2 - -Ο 12 4 18 8 47 6 5 - -Ρ 15.7 25.5 3.9 46.1 4.9 3.9 - -R 10 6 20 6 45 6 5 2 ZnO 2 BaOS 16 - 26 4 47 5 2 -Table 2. Temperatures attributed to viseosities at 10 and103 Pa s sample Γ, in C Γ3 in C Γι Γ3 in ΚΑ 798 625 173Β 812 655 157Ε 720 560 160F 761 582 179G 781 598 183Η 802 612 190I 736 580 156Κ 680 510 170L 710 530 180Μ 942 705 2370 830 640 184tion temperature (exothermic peak in the DTA profile) wereobserved. To determine crystal growth velocities, small glasssamples were thermally treated in a gradient furnace andsubsequently the crystal sizes were determined in an opticalmicroscope. In all samples, mainly surface crystallizationwas observed. The experimental parameters (time and tem-perature ränge) had to be adjusted to the respective glasscompositions. The experimental error in the crystal growthvelocity was ± 10 %.3. Results and discussion3.1 Viscos i t iesFigure  1 presents viscosities in the ränge of 10 to 10^·^ Pa s of two phosphate glasses (samples Α and B) and two Silicateglasses (samples  C and D). The composition of sample Α isin the metaphosphate ränge and that of sample Β betweenthe metaphosphate and the invert glass ränge. The highestviscosities at all temperatures studied are those of sampleD,  a Silicate glass with  a composition comparable to a floatglass. Sample  C possesses somewhat lower viscosities thansample D. It has a composition typical of a bioresorbablefibre glass for thermal insulating composites. The two phos-600 700 800 900temperature in °C1000Figure 1. Viscosi ty- temperature d e p e n d e n c y o f t w o phosphatea n d t w o Si l icate glasses; - : s a m p l e A, — : sample B, : s a m p l e C, a n d : sample D.phate glasses have much lower viscosities than the Silicateglasses. Here, sample Α possessing  a P2O5 concentration of47 mol% has lower viscosities than sample Β (45 mol%P2O5). Glass Α lies in the metaphosphate ränge, while glassΒ is attributed to the intermediate composit ion ränge. Bothglasses also contain Ti02 and AI2O3, which are incorpor-ated as AIO4 and TiOe units and interconnect the phosphatechains to a certain extent. In the metaphosphate glass(sample A) the viscosity is lower, al though the phosphatechains should be longer than in sample B. The effect ofTi02 on the viscosity is illustrated in figure 2. Sample Εdoes not contain T i02 while sample  F contains 1.3mol%Ti02. The molar ratios of the other melt components arekept constant. The comparably small T i02 concentrationleads to viscosities around four times higher than those ofthe glass without titania. The viscosity-temperature curve isshifted around 40 Κ to higher temperatures in the titaniacontaining glass. Samples  G and Η possess titania concen-trations of 1.3 and 2.6 mol%. Also here, the molar ratios ofthe other melt components were kept constant. The samplewith the larger T i02 concentration possesses the higher vis-_ _ — ---°  ° ­550 600 650 700temperature in °C -750 800Figure 2. Viseosity-temperature dependeney of some phosphateglasses; : sample E, : sample F, : sample G, and : sample H.cosities. The effect is not as pronounced as in sample Ε andF The viscosity-temperature curve is shifted by around20 Κ to higher temperatures. Sample G and F possess thesame molar Ti02 concentration, however, sample F con-tains 12mol% MgO and 11.4mol% CaO, while sample G contains 23.4 mol% C a O and does not contain any MgO.The sum of the molar alkaline earth concentrations as wellas the molar concentrations of all other components are thesame. It is seen that a partial Substitution of M g O for C a Oleads to an increase in viscosity. The effect is clearly signifi-cant, the viscosity-temperature curve is shifted around 20 Κto higher temperatures. It should be noted that in all com-positions shown in figure 2, an increase in viscosity alsoresults in an increase in the workability ränge. This can beillustrated by the difference of the temperatures attributedto viscosities of 10 and 10·̂  Pa s. These differences are 160,179, 183 and 190 Κ for the samples E, F, G and H, respec-tively. Table 2 shows viscosities at 10 and 10^ Pa s of all glasscompositions studied. The results can be summarized as follows: decreasing P2O5 concentrations led to an increase inthe viscosities, this is especially illustrated by the sample Μwhich has the lowest P2O5 concentration of all samplesstudied (40.9 mol%). The incorporation of AI2O3 or T i02leads to an increase in viscosity as well as the replacementof C a O by MgO. The largest workability ränge { Τ \ { η10 P a s ) Τ ^ ( η  10̂ ^ P a s )  237 K) is observed at thecomposition M.It has already been reported [6 and 7] that phosphateunits, especially and units, are joined by AIO4 andTiOl~ polyhedra. This leads to a stabilization of the glassstructure, which results in increasing viscosities. The replacement of CaO by M g O has the same effect (as shownabove): it joins the phosphate units, while Ca^^ acts as net-work modifier which increases the number of nonbridgingoxygens.3.2 CrystallizationIn figures 3 and 4, DTA profiles of the samples Μ and Αare respectively shown. In sample Μ which has an invertglass structure, two strong exothermic peaks at tempera-400 500 600 700 800- temperature in °C -900 1000Figure 3. DTA profile of sample Μ (heating rate 10 K/min).400 500 600 700 800temperature in °C900 1000Figure 4. DTA profile of sample Α (heating rate 10 K/min).tures of around 550 and 680 C are seen. This is a hint ofthe formation of two different crystalline phases. As provedby X R D , in this case, AIPO4 (in cristobalite-like structure) isthe phase formed first. A t higher temperatures, additionallyCa2P207 is formed. In the DTA profile of sample Α (seefigure 4), two exothermic peaks at 560 and 660 °C are observed. Sample Α is a glass in the metaphosphate ränge anddoes not show as intense exothermic peaks as sample M.The observed peaks are attributed to the formation of thesame crystalline phases (AIPO4 and Ca2P207). In the caseof invert glass structures, the crystal growth velocities weretoo large to be studied in the entire temperature ränge usingconventional tempering techniques. This was also due to thelarge crystal density, caused by a large nucleation rate. Inthe metaphosphate ränge, the determination of crystalgrowth velocities using this technique, however, was pos-sible. In figure 5, crystal growth velocities of the sample R are shown as a function of temperature for the two crystal-line phases formed. On all samples surface crystallizationwas observed. The phases formed can clearly be distinguished in optical micrographs as shown in figures  6 and7: AIPO4 in cristobalite-like modification forms cubic crys-tals (see figure 6), while Ca2P207 occurs as dentrites. Themaximum in the crystal growth velocity of AIPO4 is ob - — — -° = -- = =---Crystallization and viscosity of phosphate melts in the System NagO-CaQ-MgO-PsOs-AlgGs-TiOsΕcöοω>25020015010050Ονν ν ν ν 400 500 600 700 800 900 1000temperature in C ^Figure 5. Crystal growth veloeity of sample  R as a funetionof temperature; · : AIPO4 (eristobahte-like modifieation), andV: Ca2P207.400 500 600 700 800 900 1000 temperature in C ^ Figure 8. Crystal growth veloeity of sample  S as a functionof temperature; · : AIPO4 (cristobalite-like modification), andV: Ca2P207.^100pmFigure 6. Optieal mierograph of sample Ρ after tempering at800 C for 0.5 h. The cubic crystals are AIPO4.served at 650 °C. At higher temperatures, the crystal growthvelocity strongly decreases and becomes negligible at850 C. The crystallization of Ca2P207 occurs at higher tem-peratures and is maximum at 910 C. The maximum is fairlybroad, the crystal growth velocities at 800 and 950 C areonly around 12 % lower than at 910 C. At the maxima bothcrystal growth velocities are 210 pm/h. Figure  8 presentscrystal growth velocities of the sample S. Also here, AIPO4rX ^ ^ ^ ^ ^I 1100 pm600 800 1000 temperature in °C1200Figure 7. Optieal mierograph of sample  R after tempering at950 C for  1 h. The crystalline phase is Ca2P207.Figure 9. Crystal growth velocities as a function of the tempera-ture; O: sample A, • : sample B, T: sample O, and • : sampleP, erystal phase: AIPO4 (cristobalite-like modifieation).as well as Ca2P207 crystallizes. The maximum crystalgrowth velocity of AIPO4 is observed at 550 °C. Ca2P207crystallizes at higher temperatures, the attributed maximumis observed at 900 °C. The maximum values of the crystalgrowth velocities are 80 and 85 pm/h and hence notablysmaller than those of sample R.In figure 9, crystal growth velocities (crystal phase:AIPO4 (cristobalite-like modification) of samples A, B, Οand Ρ are shown. The samples Α and Ο have the same P2O5concentration (47 mol%), while that of sample Ρ is slightlysmaller (46.1 mol%). These samples show maxima in thecrystal growth velocity of 700, 800 and 950 C, respectively.They all lie in the ränge of 40 to 75 pm/h. Sample Α pos-sesses the smallest Ti02 concentration and shows the maxi-mum at the lowest temperature. Sample Ρ has a larger Ti02concentration (4 mol%) and 4 % C a O are replaced by MgO.Here, the maximum is shifted to considerably larger temperatures (950 °C). In comparison to sample A, sample Οhas  a larger AI2O3 concentration (6 mol%) and 4 % Na20are replaced by K2O, which also leads to a shift in the maxi-mum to higher temperatures. Sample Β has the smallest° — ° ° -° ° ° ° ° -° Ρ. WANGE ET AL.: CRYSTALLIZATION AND VISCOSITY OF PHOSPHATE nnelts IN THE System N A 2 0 - C a O - M g O - P 2 0 5 - A L 2 0 3 - T I 0 2Α 9005025 30 35 40 45([P205]-[MgO]-[TI02]-2[AL203]) IN MOL%-Figure 10. Temperatures attributed to the viseosities OF 10 ( O )and 103 Pa s ( ) as a funetion of [P205]-[MgO]-2[AL203]-[TI02](in MOL%).P2O5 concentration and shows the highest crystal growthvelocities.From figures 5, 8 and 9, it can be concluded that themaximum crystal growth velocity strongly depends on theP2O5 concentration. For 45 mol% P2O5, it is around200 pm/h and drops to 40 to 85 pm/h for 47 mol% P2O5. Itshould be noted that the increase in the P2O5 concentrationleads to a decrease in viscosity. That means, the higher crys-tal growth velocities are observed in the glass with thehigher viscosity. The concentrations of MgO, AI2O3 andTi02 are of minor importance for the maximum crystalgrowth velocity, however, they determine at which tempera-ture the maximum occurs. With increasing AI2O3 and Ti02concentrations the maximum is shifted to higher tempera-ture, while MgO seems to have the opposite effect. Thedominating effect of units in the invert glasses favoursthe crystallization, also because of the occurrence ofA l - O - P bonds. Here, the crystallization of AIPO4 occurs.3.3 General remarksThe viscosity-temperature dependence of glasses in the Sys-tem studied is strongly affected by the P2O5 concentrationand components such as AI2O3, T i02 and MgO. While in-creasing P2O5 concentrations lead to decreasing viscosities,the addition of AI2O3, T i02 or M g O has the opposite effect.In figure 10, the temperatures attributed to viscosities of 10and l O ^ P a s are plotted against [P205 ] - [Ti02 ] - [MG0]-2[Αΐ2θ3]. For both viscosities, linear correlations are obtained. Within the limits of error, the correlation lines runparallel. Metaphosphate structures generally show lowerviscosities than invert glass structures.As recently reported [5], also the chemical durability isstrongly affected by the P2O5 concentration and the respect-ive additions. Here an increase in the T I 0 2 , MgO and AI2O3concentration leads to a higher resistance to chemical at-tack. However, an increase in the P2O5 concentration de-creases the corrosion rate. With respect to the corrosion be-haviour, an increase in the P2O5 concentration results in a strengthening of the glass network. However, it also resultsin lower viscosities and hence  a more mobile glass structureat high temperatures.The maximum crystal growth velocity decreases with in-creasing P2O5 concentration. This is a surprising effect,since increasing P2O5 concentrations lead to a lower vis-cosity, and hence also should lead to higher crystal growthvelocities. While the crystal growth velocity is scarcely af-fected by the T I 0 2 , MgO and AI2O3 concentrations, theP2O5 concentration strongly affects the crystallization be-haviour. If it is decreased by only  2 mol%, the crystal growthvelocity drops by around the factor 3. This is of specialimportance for the preparation of glass-ceramics. Especiallywith respect to the evaporation of P2O5 during melting, spe-cial attention has to be paid to the homogeneity of the melt.4. References[1] Seholze, H.: Glass. Nature, structure and properties. Berlinet al.: Springer, 1991. Ρ 164 ff[2] Vogel, W.: Glaschemie. 3rd ed. Berlin et al.: Springer, 1992.Ρ 319 ff.[3] Vogel, W.; Höland, W.: The development of bioglass ce-ramics for medieal applications. Angew. Chem. Int. Ed.Engl. 26 (1987) pp. 527-544.[4] Höland, W.; Wange, P ; Naumann, K. et al.: Control ofphase formation processes in glass-eeramies for medieineand technology J. Non-Cryst. Solids 129 (1998) pp.152-162.[5] Vogel, J., Wange, P , Knoche, S. et al.: Chemical solubihtyof phosphate glasses in the System NA20-CaO-MgO-P205-Αΐ2θ3-Τ1Θ2 in aqueous S o l u t i o n s of different pH-values.Glass Sei. Technol. 77 (2004) pp. 82 -87 .[6] Schtin, S.; Slemichin, S.: The structural part of Ti in KAl-phosphate glasses. (Orig. russ.) Fiz. Khim. Stekla 5 (1979) pp. 171-177.[7] Perry, C ; Knizer, D.: Magnetic exchange between Ti^^ions in titanium phosphate glass. J. Appl. Phys. 50 (1979)pp. 1601-1603.Contact:Prof Dr. C. RüsselOtto-Schott-Institut für GlaschemieFriedrieh-Schiller-Universität JenaFraunhoferstraße 6 D-07743 JenaE-mail: ccr@rz.uni-jena.de-· -

Crystallization and vicosity of phosphate melts in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2

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Crystallization and vicosity of phosphate melts in the system Na2O-CaO-MgO-P2O5-Al2O3-TiO2

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