Nucleation and crystallisation kinetics of a Na-fluorrichterite based glass by differential scanning calorimetry (DSC)

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1 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite Nucleation and cystallisation kinetics of a Na-fluoichteite based glass by diffeential scanning caloimety (DSC) J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo * Goup of Glassy and Ceamic Mateials, Instituto de Ciencias de la Constucción Eduado Tooja, CSIC, C/ Seano Galvache, Madid, Spain * Coesponding autho; addess: momeo@ietcc.csic.es Abstact The pesent pape shows the esults of a nucleation and cystallisation study of a Nafluoichteite glass caied out by dynamic scanning caloimety (DSC). The kinetic study was pefomed using diffeent pocedues (Kissinge, Matusita-Sakka and Kissinge-Akahia- Sunose (KAS) methods), and the Avami paamete was detemined fom the Ozawa and Malek appoximations and the Malek equation. The esults have indicated the coexistence of suface and bulk cystallisation in the devitification pocess of the studied glass. The kinetic study has shown that the activation enegy of the cystallisation pocess is ove 400 kj/mol and that the mechanism poposed is a Johnson-Mehl-Avami mechanism with n equal to 3, which implies that the cystallisation develops though the thee-dimensional gowth of cystals. The study of the vaiation of the activation enegy with cystallisation using the KAS method has shown that the cystallisation pocess undegoes a multiple step mechanism, whee the main pat of the whole pocess coesponds to the thee-dimensional gowth of cystals. The mechanism poposed was confimed by applying the Péez-Maqueda et al. citeion. Keywods: fluoichteite; kinetic; isoconvesional method; glass-ceamic 1. Intoduction Glass-ceamics ae mateials composed fom at least one cystalline and glassy phase, and these mateials ae poduced by a contolled cystallisation pocess fom a base glass. Glass-ceamics show excellent technological popeties, which ae usually bette than othe mateials such as glasses, metals o oganic polymes [1] because of the wide vaiety of compositions and micostuctues that can be developed. The most citical featues fo the design of glass-

2 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite ceamics ae the composition and the micostuctue. The cystallisation mechanism of the base glass, as well as the chemical and physical popeties of the final glass-ceamic, ae contolled by the composition, wheeas the micostuctue is esponsible fo the mechanical and optical popeties. Duing the last yeas of the twentieth centuy, many studies focused on impoving the mechanical stength of glasses and glass-ceamics wee conducted. Geneally, glasses ae tough unde compession but weak unde tension. In addition, the mechanical stength values that ae obtained expeimentally fo glasses ae usually two o thee odes of magnitude lowe than thei theoetical stength. The mechanical weakening of the glass aises fom the existence of mico-cacks on the suface. Theefoe, the mechanical stength can be inceased by geneating compessive stesses in the glass suface that neutalise these mico-cacks. Glass stengthening is typically pefomed via two mechanisms: themal teatments, such as tempeing o quenching, o using a chemical bath, which modifies the chemical composition of the glass suface though ion exchange [2]. Stengthening in glass-ceamics can be achieved fom the same suface einfocement mechanisms that occu in glasses as well as fom intenal stengthening that occus fom the development of cystals with acicula o od-like mophologies. Most glass-ceamics contain silicates as the cystalline phases. In 1991, Beall [3] indicated that inosilicates o chain silicates, which contain single chains based upon SiO - 3 o double chains based on Si 4 O - 11, wee of geat elevance among the cystalline silicates of inteest in glassceamics because they confe high stength and factue toughness. Beall evaluated the chain silicate compositions descibed in the minealogical liteatue fo thei glass-foming natue and ability to devitify at low pessues. Glass-ceamics, with flexual stengths exceeding 200 MPa and factue toughness geate than 3 MPa.m 1/2, wee developed using thee families of chain silicates: enstatite, potassium fluoichteite and canasite. Enstatite (MgSiO 3 ) is a single chain silicate that is epesentative of the pyoxene mineal goup. Potassium fluoichteite (KNaCaMg 5 Si 8 O 22 F 2 ) is a double chain silicate that is a membe of the amphibole mineal goup. Finally, canasite (Ca 5 Na 4 K 2 Si 12 O 30 F 4 ) is an unusual quaduple chain silicate. Following the initial eseach that was conducted by Beall, the fluoichteite glass-ceamic has become the subject of geat inteest and diffeent studies have established the evolution of phases duing cystallisation though an ealy phase sepaation in the base glass, which fist stimulates the cystallisation of tetasilisic fluomica ((K,Na)Mg 2.5 Si 4 O 10 F 2 ) at 650ºC and then

3 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite diopside (CaMgSi 2 O 6 ) at 700 C. The eaction of diopside with mica and the esidual glass leads to the development of K-fluoichteite at 750 C [4,5]. Diffeent studies have exploed the effects of vaying magnesium, sodium, silicon o fluoine content on the micostuctue and popeties of these glass-ceamics [6-10] as well as the effects of substituting Na + fo othe alkaline cations (Li +, K + ) [11,12] o substituting Mg 2+ with Ca 2+ [13]. Futhemoe, the effects of P 2 O 5 on the cystallisation of fluoichteite have been studied duing the last decade because it has been demonstated that K-fluoichteite fluoapatite glass-ceamics that ae fomed with additions of P 2 O 5 exhibit excellent in vito biocompatibility and may be good candidates fo bone substitution mateials [14-16]. Richteite glass-ceamics have also been studied to examine the possibility of poducing oiented glass-ceamics with diffeent methods. Theefoe, Ashbee [17] has extuded geen glasses though opposed dies at tempeatues nea thei espective cystallisation tempeatues, and Keding et al. [18] have epoted on the use of electochemical nucleation to induce oientation of the cystals. In geneal, the studies mentioned above pimaily focused on studying the minealogical and micostuctual featues of fluoichteite glass-ceamics using chaacteisation techniques such as X-ay diffaction (XRD) o scanning electon micoscopy (SEM). Howeve, knowledge of the cystallisation kinetic paametes and the mechanism that govens the devitification pocess of fluoichteite-based glasses is still limited and must be studied futhe. Non-isothemal methods ae commonly used fo the kinetic analysis of solid-state eactions. One of the most employed methods fo detemining the kinetic paametes of the cystallisation pocess is the Kissinge method, which was developed to detemine the activation enegy without having any pevious knowledge of the eaction ode. Fo cystal gowth studies, the asknown Avami, o Johnson-Mehl-Avami model is the most utilised fo detemining the eaction ode of the cystallisation pocess. Futhemoe, isoconvesional methods have been used fo detemining the activation enegy as a function of the eacted faction, and without using any pevious assumptions in the kinetic model fitted by the eaction to discen whethe a pocess is multi- o single-step. The aim of the pesent wok is to obtain the optimum nucleation paametes (tempeatue and time) and the kinetic paametes (activation enegy and eaction mechanism) that ae involved in the cystallisation of a Na-fluoichteite base glass. This study is pat of a wide eseach examining the use of these mateials as components in glaze compositions fo ceamic tiles. The kinetic study was pefomed using multi-heating ate pocedues (Kissinge and Matusita-Sakka

4 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite methods) and the isoconvesional method of Kissinge-Akahia-Sunose. The n-avami paamete was detemined fom the Ozawa and Malek appoximations and Malek equation. The poposed mechanism was confimed by applying the Péez-Maqueda et al. citeion. 2. Expeimental 2.1. Mateials and Methods A paent glass, heeafte designated R glass, fom the SiO 2 -CaO-MgO-Na 2 O-F system was pepaed fom melting a mixtue of pue aw mateials (53.14 SiO 2, 7.13 CaCO 3, MgCO 3, 7.62 Na 2 CO and 4.20 MgF 2 (wt %)) in an electic oven. The batch was mixed and placed into an alumina-silica cucible, which was subsequently heated at a ate of 10ºC/min up to 1450ºC whee the tempeatue was held fo 2 hous. The melt was poued into a bass mould to fom a glass ba, which was then annealed fo 2 hous at 550ºC to avoid intenal stesses. The annealing tempeatue was chosen to be 100ºC lowe than glass tansition tempeatue (T g ) detemined fom the esults of diffeential scanning caloimety (DSC) conducted peviously on a sample of glass with the same composition. The chemical analysis of the esulting glass was detemined by X-ay fluoescence (XRF), and to veify the glass was in an amophous state, the as-annealed glass was analysed using X-ay diffaction (XRD). The cystallisation behaviou was detemined by DSC on two samples of diffeent paticle sizes: a fine sample of glass powde that was gound and sieved to a paticle size < 63 m and a coase sample obtained fom cutting the glass ba into monolithic samples (2 x 2 x 3 mm). DSC tests wee pefomed using a SETARAM Labsys Themal Analyze. The samples wee placed into platinum cucibles and calcined Al 2 O 3 was used as efeence mateial. The tempeatue pecision given by the themal analyze is ± 0.1 ºC. To evaluate the cystalline phases that developed duing cystallisation, both samples of diffeent paticle sizes wee subjected to a themal teatment fo 1 hou at a tempeatue that was slightly geate than the Tp, which is the tempeatue at the maximum of the cystallisation peak. XRD pattens wee collected fom powdeed samples using Ni-filteed CuKα adiation on a Philips X-ay diffactomete opeating at 30 ma and 50 kv. The scanning speed was set at 2 /min with time pe step of 0.02s. Phase identification was pefomed using the Intenational Cente fo Diffaction Data (PDF) database [19]. To bette undestand the cystallisation sequence that takes place duing the devitification pocess of R glass, bulk samples wee cystallised fo 60 min ove ºC, and thei micostuctue was obseved using field emission scanning electon micoscopy (FESEM) in a

5 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite HITACHI S-4800P micoscope using an acceleation voltage of 20 kv. FESEM specimens wee polished to 1 µm finish using diamond pastes following initial ginding with SiC powde. The samples wee subsequently etched fo 10 s in a solution of 5% HF, ultasonically washed with distilled wate and ethylic alcohol, died and then coated with Au-Pd in a Balzes SCD 050 sputte. The optimum nucleation paametes (time and tempeatue) wee detemined fo the monolithic glass samples using a two-step expeiment consisting of a fist isothemal step at diffeent nucleation tempeatues followed by a dynamic (non-isothemal) step until cystallisation is completed, which is detected by the completion of the exothemic cystallisation peak in the DSC cuve. The heating ate, β, must fulfil the condition that no new nuclei may be fomed duing cystallisation. This condition is accomplished using high heating ates (unless 10 ºC/min) fom the nucleation step to the final cystallisation pocess. It is impotant to emembe that it is pefeable to obtain the nucleation paametes fom coase samples, because nucleation must be avoided to be a function of the suface of paticles. Duing the isothemal step, the samples wee heated in the DSC at a ate of 50ºC/min fom oom tempeatue up to a tempeatue ange aound the T g ( ºC) and then kept at the chosen tempeatue fo 15 min; subsequently, the samples wee heated using the same heating ate up to 1400ºC. The plot of the invese of the tempeatue at the maximum of the cystallisation peak (T p ) vesus the nucleation tempeatue allows the detemination of the optimum nucleation tempeatue (T N ) as the maximum of the cuve. Once the optimum tempeatue was fixed, the optimum nucleation time (t N ) was detemined using a simila test sequence whee the glass samples wee subjected to an isothemal step at T N fo diffeent times ( min). Similaly, the plot of the invese of T p vesus the nucleation time gives the optimum nucleation time as the onset of a plateau in the cuve. The method descibed, including non-isothemal steps, is based on the easonable assumption that the invese of the cystallisation peak (1/T p ) is popotional to the density of nuclei that ae fomed duing the nucleation stage. The cystallisation kinetic study examined monolithic glass samples using the conditions that wee detemined fom the nucleation study, but by vaying the heating ate (10, 20, 30, 40 and 50 K min -1 ) fom the nucleation tempeatue to 1200 ºC. Each measuement was epoduced thee times in ode to estimate expeimental eos.

6 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite 2.2. Theoy To study the cystallisation kinetics, values of the activation enegy (E a ), pe-exponential facto (k 0 ) and mechanism (f (α)) of the pocess wee calculated by applying non-isothemal methods. Fo non-isothemal data, the eaction ate equation is expessed as: (1) whee α is the extent of the eaction, β is the heating ate (K/min), T is the tempeatue and R is the gas constant. The Johnson-Mehl-Avami (JMA) mechanism is typically used fo cystallisation [20-22], which is expessed in the diffeential fom as: (2) whee n is the Avami exponent, which is associated to the mophology of cystal gowth. By developing Eq. (1) and taking the logaithms, the activation enegy can be obtained by applying the Kissinge method [23]: (3) whee T p is the tempeatue at the maximum of the exothemic peak and cte is a constant. Matusita & Sakka [24] have poposed anothe equation when the numbe of nuclei is influenced by the heating ate. (4)

7 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite If the value of m is equal to n, it is indicated that the sample is well nucleated and that thee ae no new nuclei fomed duing the cystallisation pocess; wheeas if n m, it is indicated that new nuclei ae developed duing cystallisation. The Avami paamete, n, can be detemined fom the equation poposed by Ozawa [25]: (5) whee α is estimated by evaluating the patial aea of the cystallisation peak fom exothems that ae obtained at diffeent heating ates at a fixed tempeatue, T. The value of n can also be detemined fom the elation poposed by Malek [26] (heeafte designed as Malek appoximation) in the following fom: (6) whee E aα is the activation enegy that is obtained fom the isoconvesional method, as explained below. Futhe evaluation of n can be achieved fom the plot of f (α) vesus α. The value of n is detemined fom the maximum of cuve (α M ) using the equation poposed by Malek [26] (heeafte designed as Malek equation): (7) The isoconvesional method that is employed in this wok is based on dynamic DSC analyses. The equation fo the eaction ate, which is employed to study the degee of cystallisation, can be expessed by Eq. (1). Consideing that the ate equation (Eq. (1)) is valid fo dynamic pocess, the ate equation can be expessed in its integal fom as: (8)

8 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite Using the Muay and White appoximation [27] of the tempeatue integal, the following equation is obtained: (9) whee T α is the tempeatue of a fixed degee of cystallisation. Equation (9) is known as the Kissinge-Akahia-Sunose (KAS) [28] model-fee method that can be applied to diffeent degees of convesion. Accoding to Eq. (9), the activation enegy and the pe-exponential facto can be calculated using the slope and the odinate values expessed by the linea elationship between Ln β/t 2 α and 1/T α. This method does not equie knowledge of the convesion-dependent function (f(α) o g(α)), and the only assumption is that the pocess follows the same eaction mechanism fo a given degee of convesion, egadless of the cystallisation tempeatue. The appopiate convesion function can be disciminated by applying the Péez-Maqueda et al. citeion [29], who have dawn the lines Ln[β(dα/dT)/f(α)] vesus 1/T fo a set of convesion functions and fo all heating ates. A given convesion function coesponds to one of the following family of cuves: (a) non-linea cuves; (b) linea cuves (paallels) but with the intecept dependent upon the heating ate, and (c) a single line fo all heating ates. The eal convesion function coesponds to case (c), a single line fo all heating ates. In the pesent pape, and accoding to Mocioiu et al. [30] and Cadenato et al. [31], the convesion function is detemined fom the plots of Ln[β(g(α)/T 2 ] vesus (1/T) fo the diffeent heating ates. The kinetic schedule that was followed was fist the application of the non-isothemal Kissinge method fo the detemination of the activation enegy, and the Avami paamete was subsequently detemined fom the Ozawa and Malek appoximations and fom the Malek equation. The Matusita-Sakka method was applied using these paametes to obtain the m value. Then, the isoconvesional Kissinge-Akahia-Sunose method was used to undestand the multiple mechanisms and thei meaning. Finally, the esults wee contasted using the Péez- Maqueda citeion to test the feasibility of the poposed Avami mechanism.

9 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite 3. Results Table 1 shows the esults fom the chemical analysis of the investigated R glass, which is located in the SiO 2 -ich pat of the tenay SiO 2 -MgO-CaO glass fomation system (Figue 1). Figue 2 shows the XRD patten fom a powdeed sample of the as-annealed R glass. The absence of elevant diffaction peaks, along with the pesence of the typical wide band that coesponds to the glassy matix (amophous halo), indicates the complete amophous state of the sample. Table 1. Chemical analysis of R glass detemined by XRF Component % SiO Al 2 O MgO CaO 7.81 Na 2 O 6.13 K 2 O 0.32 F SiO 2 Cistobalite Tydimite Two Liquids Rich Wollastonite Pseudowollastonite 1600 Diopside R glass 50 CaO Akemanite 50 MgO Figue 1. Phase diagam of the SiO 2 -MgO-CaO system

10 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite Figue 2. X-ay diffaction patten of the annealed R glass sample Figue 3 pesents the DSC themogams fom fine (< 63 µm) and coase samples obtained fom the R glass. The fist endothemic jump in the baseline of the themogams coesponds to the glass tansition tempeatue (T g ), which appeas at 647ºC in both cuves. Just afte the endothemic effect associated with the T g, the DSC cuves show a slight endothemic dop (a detailed view is also shown in Fig. 3). The detailed box also depicts the typical shape of a DSC cuve aound the T g of a quenched glass without futhe annealing (dotted line). The R glass cuves show an endothemic descent below the dotted line that indicates an enthalpic elaxation pocess. Both cuves show a well-defined exothemic peak that coesponds to a cystallisation pocess. Howeve, the tempeatues at the onset and at the maximum of the cystallisation peak shift with the size of the glass paticle. The fine sample shows the cystallisation pocess occuing at lowe tempeatues and with highe intensity than the coase sample. Once the cystallisation peak is completed, the tempeatue incease induces an exothemic pocess with a minimum (T m ) at 1171 and 1192 ºC fo the fine and coase samples, espectively, which is indicative of the melting of the main cystalline phases that fomed duing heating.

11 mw / mg J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite 2 Coase Fine T p T p 0 T g -2-4 Typical T m -6 T m Tempeatue (ºC) Figue 3. DSC themogams fo fine (<63 µm) and coase (monolithic) samples ecoded at 50 ºC/min To evaluate the cystalline phases that developed duing the cystallisation pocess, both samples wee subjected to a themal teatment fo 1 hou at a tempeatue that was slightly above the tempeatue at the maximum of thei cystallisation peaks. The tempeatues chosen wee 950 C and 1100 C fo the fine and coase samples, espectively. Figue 4 shows the XRD pattens of the cystallised glass samples. The same cystalline phases ae identified in both samples, specifically, fluoichteite (Na 2 CaMg 5 Si 8 O 22 F 2 ), diopside (CaMgSi 2 O 6 ) and a magnesium silicate. Howeve, the elative amounts of the diffeent phases vay depending on the paticle size of the glass. Fluoichteite is the main cystalline phase that developed in the coase sample, wheeas diopside is the main cystalline phase in the fine sample. As fo magnesium silicate, fosteite (Mg 2 SiO 4 ) and enstatite (MgSiO 3 ) appea as mino phases in the fine and coase samples, espectively.

12 Counts (cps) Counts (cps) J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite 250 Coase e d d e e e d d Theta Fine f d d d d f d f f d d f d d f d Theta Figue 4. X-ay diffaction pattens fo a coase glass sample heat teated at 1100 ºC fo 1 hou, and a fine glass sample heat teated at 950 ºC fo 1 hou ( = fluoichteite, d = diopside, e = enstatite and f = fosteite) As shown in Figue 5, the optimum nucleation tempeatue is 675ºC as obtained fom the plot of the invese of the tempeatue at the maximum of the cystallisation peak vesus nucleation tempeatue in the coase (monolithic) glass sample. Figue 6 shows the vaiation of (1/T p ) with nucleation time in monolithic samples that wee nucleated at 675ºC. The onset of the plateau (80 min) in the plot indicates the optimum nucleation time. To bette undestand the cystallisation sequence that occus duing the devitification pocess of the R glass, bulk samples wee cystallised fo 60 min at tempeatues of 860, 880, 900 and 920 ºC. Figue 7 shows the micostuctue obseved by FESEM in the heat-teated samples.

13 1/Tp (ºC -1 ) J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite 1.0x x x x x T N (ºC) Figue 5. Plot of the invese of the tempeatue at the maximum of the cystallisation peak (T p ) vs. the nucleation tempeatue fo glass samples nucleated fo 15 min. Lines ae dawn to guide eyes /T p (ºC -1 )x t N (min) Figue 6. Plot of the invese of the tempeatue at the maximum of the cystallisation peak (T p ) vs. nucleation time fo glass samples nucleated at 675 ºC. Lines ae dawn to guide eyes The activation enegy fo the cystallisation was detemined using the Kissinge method (Eq. (3)) on both fine (< 63 µm) and nucleated monolithic samples (Figue 8). The value of the activation enegy (E a ), as detemined fom the slope of the plot of Ln ( /T 2 p ) vesus (1/T p ), was 440 ± 18 and 401 ± 22 fo the fine and coase samples, espectively.

14 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite Figue 7. FESEM micogaphs of R glass samples heat teated fo 60 min at a) 860 ºC; b) and c) 880 ºC; d) and e) 900 ºC; f) 920 ºC Table 2 pesents the Avami exponents detemined by applying the Ozawa (Eq. 5) and Malek (Eq. 6) appoximations to the nucleated monolithic samples. Tempeatues of 1165, 1190 and 1200 K wee chosen fo the Ozawa appoximation. These values ae vey close to each othe because the condition 0 < α < 1 fo all heating ates is only satisfied ove the K inteval. The values of n, as detemined fom the slope of the plot of dln[-ln(1- )] vesus (1/T) at diffeent heating ates (Malek appoximation, Eq. (6)), ae in the inteval Figue 9 depicts the Malek cuves that wee obtained fo diffeent heating ates. All cuves show a simila shape, which indicates that the cystallisation pocess follows a single eaction mechanism, egadless of the heating ate [32]. The values of n as detemined by applying M in the Malek equation (Eq. 7) ae also shown in Table 2.

15 f ( ) nomalized Ln( /Tp 2 ) J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite o.o = slope = R 2 = E-4 8.2E-4 8.3E-4 8.4E-4 8.5E-4 1/T p (K -1 ) Figue 8. Kissinge plot fo R glass nucleated at 675 ºC fo 80 min. Line epesents the least squae fitting though the data points K/min 20 K/min 30 K/min 40 K/min 50 K/min Figue 9. Malek cuves obtained fo diffeent heating ates Table 2. Values of n Avami paamete detemined fom Ozawa (Eq. 5) and Malek (Eq. 6) appoximations and Malek equation (Eq. 7) Ozawa appoximation β Malek method Malek plot T (K) n (K/min) n α max n ± ± ± aveage 3.19 ± ± 0.36

16 E a (kj/mol) J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite Fig. 10 shows the vaiation of E aα with the degee of convesion, α, as detemined with the KAS isoconvesional method (Eq. (9)). The plot depicts thee eaction steps duing the cystallisation pocess, an incease in E aα at lowe convesion degees, constant E aα ( 389 ± 7 kj/mol) in the inteval = , and finally, a decease in the activation enegy fom = 0.8 to the end of the eaction Figue 10. Plot of the isoconvesional activation enegy vs. the convesion degee (α) detemined by the Kissinge-Akahia-Sunose method. Lines ae dawn to guide eyes To evaluate the accuacy of the poposed cystallisation mechanism (n = 3), the Péez-Maqueda et al. citeion was applied ove the inteval of convesion = , whee the activation enegy that was obtained using the KAS isoconvesional method is constant. Fig. 11 shows the plots of Ln[β(dα/dT)/f(α)] vesus 1/T fo diffeent A n mechanisms (n = 2, 2/3, 3 and 4). Accoding to Péez-Maqueda et al., the coect kinetic model is that which shows kinetic paametes that ae independent of the heating ate. The plots fo the A 2, A 2/3 and A 4 mechanisms lead to a family of lines with simila slopes, but diffeent y-intecepts. Nevetheless, the lines coesponding to the plot of the A 3 model fit a staight line, indicating that the cystallisation pocess poceeds though a thee dimensional gowth of cystals.

17 Ln( g( )/T) Ln( g( )/T) Ln( g( )/T) Ln( g( )/T) J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite -10 A 2-10 A 2/ Kmin -1 20Kmin -1 30Kmin -1 40Kmin -1 50Kmin E-4 8.2E-4 8.4E-4 8.6E-4 1/T (K -1 ) Kmin -1 20Kmin -1 30Kmin -1 40Kmin -1 50Kmin E-4 8.2E-4 8.4E-4 8.6E-4 1/T (K -1 ) A A Kmin -1 20Kmin -1 30Kmin -1 40Kmin -1 50Kmin E-4 8.2E-4 8.4E-4 8.6E-4 1/T (K -1 ) Kmin -1 20Kmin -1 30Kmin -1 40Kmin -1 50Kmin E-4 8.2E-4 8.4E-4 8.6E-4 1/T (K -1 ) Figue 11. Peez-Maqueda plot of Ln(βg(α) T -2 ) vs. T -1, fo diffeent A n mechanisms: A 2, A 2/3, A 3 and A Discussion The chemical composition of the R glass is located in the SiO 2 -ich pat of the tenay SiO 2 - MgO-CaO glass fomation system (Figue 1). Howeve, Beall [3] epoted that the incopoation of fluoine and alkali ions into the glass composition consideably modifies the tenay system and that cystalline phases can be developed that ae not expected in a simple SiO 2 -MgO-CaO system. The analysed R glass composition is vey close to the composition of SiO 2, MgO, 2-6 CaO, 2-6 Na 2 O, 2-7 K 2 O, and 2-5 F (labelled as Rich in Fig.1), which leads to the devitification of the ichteite chain silicate [1]. Indeed, the T g value of the R glass (647ºC) is simila to the value of T g = 635ºC that was epoted by Hamzawy and Abdel- Hameed [11] fo a glass with a stoechiometic fluoichteite composition. Moeove, the tempeatue at the maximum of the cystallisation peak is 891 and 1080ºC fo fine and coase samples, espectively, which ae in ageement with the values epoted in the liteatue fo glasses with the stoechiometic composition of fluoichteite [11, 18]. The cystallisation of

18 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite fluoichteite fom the R paent glass has been also veified in Fig. 4, which shows that fluoichteite is the main cystalline phase developed in the coase glass sample. The enthalpic elaxation pocess obseved in Fig. 3 is chaacteistic of both annealed glasses and melts that ae cooled using slow cooling ates [33, 34]. When a glass spends enough time at tempeatues that ae elatively close to the T g (by eithe annealing o slow cooling ates), thei molecules may have enough enegy to cause some eaangement in the amophous stuctue. Theefoe, this endothemic pocess is elated to the enegy equied by the annealed glass to ecove its pevious intenal stuctue befoe continuing with the devitification pocess that occus duing heating. Both fine (< 63 µm) and coase DSC cuves show a well-defined exothemic peak that coesponds to a cystallisation pocess. Howeve, the fine sample shows the cystallisation pocess occuing at lowe tempeatues and with highe intensity than the coase sample, which indicates that both the suface and bulk cystallisation mechanisms coexist, although the suface cystallisation is pedominant ove the bulk cystallisation mechanism. The optimum nucleation paametes (tempeatue an time) fo R glass ae 675ºC and 80 min, espectively, as detemined fom Fig. 5 and Fig.6. The fomation of the maximum numbe of nuclei fom the paent glass will occu unde the optimum nucleation teatment and additional new nuclei will not be fomed duing heating. The calculated optimum nucleation tempeatue (675 ºC) is highe than the T g (647 ºC), which, accoding to Zanotto [35] and Hu and Tsai [36], indicates that the pocess mainly occus though volume cystallisation whee the nuclei ae homogeneously distibuted thoughout the bulk glass. Nevetheless, Fig. 3 clealy shows a cystallisation peak in the DSC cuve ecoded fom the fine sample (< 63 µm), which indicates that suface cystallisation is also possible in this glass. Anothe way to pedict the cystallisation mechanism is though the educed glass tansition tempeatue (T g = T g /T m, whee T g is the glass tansition tempeatue and T m is the melting tempeatue, both values in Kelvin). Accoding to James [37] and Zanotto [35, 38], glasses that show a T g > only exhibit suface cystallisation that is mostly heteogeneous, wheeas a T g < indicates homogeneous volume nucleation. The calculated values of T g fom Fig. 3 ae 0.64 and 0.63 fo the fine and coase samples, espectively, which indicates that this glass pesents heteogeneous nucleation (suface cystallisation). Consideing both esults, they indicate that both cystallisation mechanisms, volume and suface, simultaneously occu in the R glass. This occuence is confimed when the cystallisation sequence that occus duing the devitification pocess of the R glass is obseved (Fig. 7). Duing ealy stages of the devitification pocess

19 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite (Fig. 7a), the suface cystallisation mechanism is pedominant. The cystal gowth stats at the glass suface, esulting in the fomation of a cystalline shell ( 110 µm) that entiely coves the glass sample, while cystallisation is not obseved in the inteio. As the tempeatue inceases (Fig. 7b), the suface gowth continues to be pevalent and the depth of the cystallisation shell inceases ( 225 µm). Howeve, at a cetain moment, the enegy of the system is enough to stat bulk cystallisation and spheulitic cystals ( 300 µm) begin to gow and ae distibuted unifomly thoughout the entie volume of the glass sample (Fig. 7c). At this point, the cystallisation mechanism changes and volume cystallisation becomes pedominant ove suface cystallisation (Fig. 7d,e) even though the cystallisation shell continues to gow ( 550 µm). Inceasing the heating tempeatue esults in a futhe development of the numbe and size ( 1000 µm) of spheulitic cystals. Finally, the cystallisation pogess leads to an enlagement of both the cystalline shell ( 920 µm) and spheulites ( 1200 µm), which come into contact with each othe and with the cystallisation shell (Fig. 7f). Theefoe, the esidual glass phase is constained to the spheulite-spheulite and spheulite-shell intephases and the cystal gowth is steically hindeed. The activation enegy fo the cystallisation detemined using the Kissinge method (Fig. 8) was 440 and 401 kj/mol fo the fine and coase samples, espectively. The E a fo the cystallisation of the fine glass sample is appoximately 10% highe than the coesponding value fo the monolithic glass. The diffeence in the values of the activation enegy could explain the diffeence that was peviously obseved in the degee of cystallisation of these glass samples following themal teatment at tempeatues that ae slightly above the tempeatue at the maximum of thei espective cystallisation peaks (Fig. 3). The highe the activation enegy, the geate the enegy equied to stat the cystallisation pocess. As a esult, afte 60 min of themal teatment, the fine glass sample is less cystallised and shows a highe amount of esidual glass than the monolithic glass sample that has lowe activation enegy. The aveage of the n paamete that was obtained by applying the Ozawa (Eq. 5) and Malek (Eq. 6) appoximations and the Malek equation (Eq. 7) is appoximately 3 (Table 2), indicating a thee-dimensional gowth of cystals duing the glass cystallisation pocess. In the pesent study, it was assumed that no new nuclei ae developed fa away fom the optimum nucleation point (T N, t N ). To contast this assumption, the Matusita & Sakka method has been applied to the kinetic esults. Fom the values of n and E a obtained fom the Kissinge method, the m value detemined by applying Eq. (4) was 3.10 ± The similaity of the n and m values confims

20 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite that no new nuclei ae fomed duing cystallisation. The mechanism that is fixed fo n = m = 3 is a thee-dimensional gowth of cystals. The vaiation of E aα with the degee of convesion, α, as detemined with the KAS isoconvesional method (Fig. 10) depicts thee eaction steps duing the cystallisation pocess. The initial incease of E aα indicates the existence of paallel eactions, wheeas the constant value of E aα duing the majoity of the cystallisation pocess indicates that one mechanism pimaily govens the eaction ove this inteval. Finally, the deceasing of E aα at highe degees of convesion likely coesponds to a change in the cystallisation mechanism fom a kinetic to a diffusion egime [39]. The thee steps evealed fom the isoconvesional method can be coelated with the cystallisation sequence shown in Fig. 7. Duing the fist stage ( < 0.2), both the suface and bulk cystallisation mechanism occu simultaneously (Fig.7a-c), wheeas in the inteval = , bulk cystallisation pevails ove suface cystallisation and govens the devitification pocess (Fig. 7d-e). Finally, at highe degees of convesion ( > 0.8), the cystal gowth is steically hindeed (Fig. 7f) and E aα deceases. The accuacy of the poposed cystallisation mechanism (n = 3) is cooboated by applying the Péez-Maqueda et al. The lines coesponding to the plot of the A 3 model (Fig. 11) fit a staight line, indicating that the cystallisation pocess poceeds though a thee dimensional gowth of cystals. The activation enegy calculated fom the slope of the lines is 394 ± 2 kj mol -1, which is in ageement with that obtained fom the Kissinge and KAS methods. 5. Conclusions The cystallisation pocess of a Na-fluoichteite glass (designed R glass) was investigated by means of DSC, FESEM and XRD methods. The optimum nucleation paametes (tempeatue and time) wee also detemined using DSC. The cystallisation mechanism was studied by diffeent methods, namely, non-isothemal (Kissinge and Matusita&Sakka methods) and isoconvesional methods (Kissinge-Akahia-Sunose). The Avami paamete, n, was detemined fom thee diffeent appoaches: Ozawa appoximation, Malek appoximation and

21 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite Malek equation. The Péez-Maqueda et al. citeion was applied to check the accuacy of the calculated n paamete. Fom the pesented esults, the following conclusions can be dawn: - The optimum nucleation tempeatue and time wee detemined to be 675ºC and 80 min, espectively. - The optimum nucleation tempeatue (675ºC) is highe than the T g (647ºC), indicating that the cystallisation pocess takes place mainly though volume cystallisation. Howeve, the educed glass tansition tempeatues (T g ) ae 0.64 and 0.63 fo the fine and coase samples, espectively, which indicates that the R glass pesents heteogeneous nucleation (suface cystallisation). Consideing both esults, they indicate that both suface and bulk cystallisation mechanisms coexist duing the cystallisation pocess. - The activation enegy of the cystallisation pocess as detemined using the Kissinge method on monolithic glass samples is 401 kj/mol, which is in good ageement with the values of 398 and 394 kj/mol detemined fom the KAS method and Péez-Maqueda et al. citeion, espectively. - The aveage of the n paamete obtained though thee diffeent appoaches is appoximately 3, indicating thee-dimensional gowth of cystals duing the cystallisation of the R glass. - The KAS isoconvesional method showed thee steps duing the cystallisation pocess: an incease in E aα at lowe degees of convesion that coesponds to a paallel cystallisation mechanism (suface and bulk); constant E aα in the inteval = , which is indicative of the pevalence of bulk cystallisation ove suface cystallisation; and finally, a decease in the activation enegy at highe degees of convesion indicates that the cystal gowth is steically hindeed. - The cystallisation of the R glass leads to a multiphase glass-ceamic mateial consisting of diopside (CaMgSi 2 O 6 ), fluoichteite (Na 2 CaMg 5 Si 8 O 22 F 2 ) and a magnesium silicate as fosteite (Mg 2 SiO 4 ) o enstatite (MgSiO 3 ) as cystalline phases togethe with a esidual glassy phase - The glass paticle size influences the development of cystalline phases. Fluoichteite is the main cystalline phase developed in a coase monolithic sample, while diopside mainly esults duing the cystallisation of a fine (<100 µm) sample.

22 J.M. Péez, R. Casasola, J.Ma. Rincón, M. Romeo. Nucleation and cystallisation kinetics of a Nafluoichteite Acknowledgements The authos would like to acknowledge Ms. P. Díaz fo the technical suppot of the expeimental wok. R. Casasola and J.M. Péez expess thei gatitude to the Spanish National Reseach Council (CSIC) fo thei contact though the JAE Pogam (JAEPe and JAEDoc , espectively), co-financed by the Euopean Social Fund. The financial suppot though the poject MAT is also ecognised. Refeences [1] W. Hölland, G. Beall, Glass-ceamic technology, second ed., The Ameican Ceamic Society, Ohio, [2] J.M.F. Navao, El vidio, thid ed., Consejo Supeio de Investigaciones Científicas, Madid, [3] G.H. Beall, J. Non-Cyst. Solids 129 (1991) [4] A.A. Oma, A.W.A. El-Shennawi, E.M.Hamzawi, Key Eng. Mate (1997) [5] M. Misaneh, I.M. Reaney, P.F. James, Phys. Chem. Glasses 43C (2002) [6] A.W.A. El-Shennawi, A.A. Oma, E.M.A. Hamzawy, in: Choudhay MK, Huff NT, Dummond CH (Eds.), Poceedings of the 18th Intenational Congess on Glass, Ameican Ceamic Society, Westeville, OH, [7] I.L. Deny, J.A. Holloway, J. Biomed. Mate. Res. 53 (2000) [8] I.L. Deny, J.A. Holloway, J. Biomed. Mate. Res. 63 (2002) [9] E.M.A. Hamzawy, C. Leonelly, G.C. Pellacani, Glass. Technol. 44 (2003) [10] E.M.A. Hamzawy, C. Leonelli, Glass. Technol. 46 (2005) [11] E.M.A. Hamzawy, S.A.M.Abdel-Hameed Ceam. Int. 35 (2009) [12] M. Misaneh, I.M. Reaney, P.F. James, P.V. Hatton, J. Am. Ceam. Soc. 89 (2006) [13] M. Misaneh, I.M. Reaney, P.V. Hatton, P.F. James, J. Am. Ceam. Soc. 87 (2004) [14] M. Misaneh, I.M. Reaney, P.V. Hatton, S. Bhakta, P.F. James, J. Non-Cyst. Solids 354 (2008) [15] S. Bhakta, K. Pattanayak, H. Takadama, T. Kokubo, C.A. Mille, M. Misaneh, I.M. Reaney, I. Book, R. van Noot, PV. Hatton, J. Mate. Sci.: Mate. Med. 21 (2010) [16] S. Bhakta, K.H. Gillingham, M. Misaneh, C.A. Mille, I.M. Reaney, I. Book, R. van Noot, P.V. Hatton, J. Mate. Sci.: Mate. Med. 22 (2011) [17] K.H.G. Ashbee, J. Mate. Sci. 10 (1975) [18] R. Keding, D. Stachel, C. Rüssel, J. Non-Cyst. Solids 283 (2001) [19] Powe Diffaction File Release 2000, Data Sets 1-50 plus PDF *, JCPDS- Intenational Cente fo Diffaction Data,

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