Crystallization of pyroxene phases and physico-chemical properties of glass-ceramics based on Li2O–Cr2O3–SiO2 eutectic glass system

Materials Chemistry and Physics xxx (2014) 1e8

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Crystallization of pyroxene phases and physico-chemical properties of glass-ceramics based on Li2OeCr2O3eSiO2 eutectic glass system S.M. Salman, S.N. Salama, H.A. Abo-Mosallam* Glass Research Department, National Research Centre, Dokki, Cairo, Egypt

h i g h l i g h t s
Glass ceramics based on Li2OeCr2O3eSiO2 eutectic (1028 ± 3  C) glass were prepared.
LiCrSi2O6 and LiFeSi2O6 phases form monomineralic pyroxene solid solution phase.
Solid solution between LiAlSi2O6 or LiInSi2O6 phases and LiCrSi2O6 phase doesn't form.
Microhardness values of the crystalline materials ranged between 5282 and 6419 MPa.
Chemical stables of the glass-ceramics were better in alkaline than in acidic media.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 May 2014 Received in revised form 12 October 2014 Accepted 19 October 2014 Available online xxx

The crystallization characteristics, crystalline phase assemblages and solid solution phases developed due to thermally crystallized glasses based on the Li2SiO3eLi2Si2O5eLiCrSi2O6 (1028 ± 3  C) eutectic glass system by replacing some trivalent oxides instead of Cr2O3 were investigated. The microhardness and chemical durability of the glass-ceramics were also determined. Lithium meta and disilicate (Li2SiO3 and Li2Si2O5), lithium gallium silicate (LiGaSiO4), and varieties of pyroxene phases, including Cr-pyroxene phase, i.e. lithium-kosmochlor (LiCrSi2O6), lithium aluminum silicate (LiAlSi2O6), lithium indium silicate (LiInSi2O6) and pyroxene solid solution of Li-aegerine type [Li (Fe0.5, Cr0.5) Si2O6] were the main crystalline phases formed in the crystallized glasses. There is no evidence for the formation of solid solution or liquid immiscibility gaps between LiAlSi2O6 or LiInSi2O6 phases and LiCrSi2O6 phase. However, LiCrSi2O6 and LiFeSi2O6 components were accommodated in the pyroxene structure under favorable conditions of crystallization to form monomineralic pyroxene solid solution phase of the probably formula [Li (Fe0.5, Cr0.5) Si2O6]. The type and compatibility of the crystallized phases are discussed in relation to the compositional variation of the glasses and heat-treatment applied. The microhardness values of the crystalline materials ranged between 5282 and 6419 MPa while, the results showed that the chemical stability of the glass-ceramics was better in alkaline than in acidic media. © 2014 Published by Elsevier B.V.

Keywords: Glasses Crystallization Corrosion Hardness

1. Introduction The glass-ceramics are produced by the controlled nucleation and crystallization of a base glass [1]. Glass-ceramics offer numerous advantages compared with parent glasses such as improved thermo-physical properties, higher strength, wear resistance, often making them better suited than glasses for various applications [2]. The binary lithiaesilica glass system has gained great interest in the preparation of glasses and glasseceramic materials [3]. The known lithium silicate glass-ceramics usually

* Corresponding author. E-mail address: [email protected] (H.A. Abo-Mosallam).

contain as main components SiO2, Li2O, A12O3, alkali metals such as Na2O or K2O and nucleating agents such as P2O5. In addition, they can contain as further components for example alkali metal oxides and/or alkaline earth metal oxides and/or ZnO. Glass-ceramics are also known which contain small quantities of further metal oxides and in particular coloring and fluorescent metal oxides [4]. Lithium meta and di-silicate phases might be formed depending on the SiO2/Li2O ratio, the presence of nucleating agents, thermal history of parent glasses [2]. The lithium silicate based glasseceramics has crystal phases transfer according to the selected heat treatment schedules, the Li2SiO3 crystal phases had changed into Li2Si2O5 in glasseceramics under relatively lower crystallization temperature (about 650  C), and the Li2Si2O5 crystal phases can be resolved into Li2SiO3 in glasseceramics at higher temperature (about 850  C) on

http://dx.doi.org/10.1016/j.matchemphys.2014.10.033 0254-0584/© 2014 Published by Elsevier B.V.

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S.M. Salman et al. / Materials Chemistry and Physics xxx (2014) 1e8

the contrary [5].The systematic study of the ternary system Li2OeCr2O3eSiO2 appears to be that of Hummel et al. [6], who prepared chromium analog of spodumene (a-LiCrSi2O6) and studied the partial solid solution series formed by Cr3þ4Al3þand Liþ4Naþ substitutions. They found that chromium analog of spodumene undergoes some kind of changes between 1250 and 1300  C although the products were not identified. The phase equilibrium in the system Li2OeCr2O3eSiO2 was studied by Izquierdo and West [7]. They reported that six subsolidus compatibility triangles and six ternary invariant points were located. The highest solidus, temperature is 1283  C, but liquidus temperatures are much higher for many compositions. The system has two eutectic points SiO2eLi2Si2O5eLiCrSi2O6 (1032 ± 2  C) and Li2SiO3eLi2Si2O5eLiCrSi2O6 (1028 ± 3  C). Glass-ceramics based on pyroxene group have attracted interest due to the excellent controllability of their properties. Pyroxenes are capable of a wide range of isomorphous substitution in their crystal structure and having the necessary physical and chemical characteristics, and may form the basis for production of many crystalline and glass-ceramic materials [8]. Pyroxenes belong to the silicates of the general composition AMSi2O6 where A stands for mono- or divalent elements while M represents di- or trivalent elements. Their crystal structures possess orthorhombic or monoclinic symmetries which can accept a wide variety of M and A elements, especially the 3d-transition metals. In most cases Si may be also substituted by Ge. Thus, pyroxenes offer a quite broad and flexible class of materials for physical investigations [9]. A comparative study of the structural variation and crystal chemistry of LiMe3þSi2O6 has been provided by Redhammer and Roth [10], where synthetic end members of the LiMe3þSi2O6 solid solution series (Me3þ ¼ Al, Ga, In, V, Fe, Sc, and Cr) were investigated by single-crystal X-ray diffraction. The purpose of the present work is to study the crystallization characteristics of glasses based on the eutectic (1028 ± 3  C) glass composition in the lithium chromium silicate system. The role of partial replacement of chromium by different trivalent oxides in determining the crystallization behavior, the phase relation, the extent of solid solution phases formed and the physico-chemical properties of the obtained glassceramics are also considered. 2. Experimental techniques 2.1. Batch composition and glass preparation The five glass compositions presented in Table 1 were prepared by a conventional melt technique from high-purity chemicals: lithium carbonate (Li2CO3), chromium oxide (Cr2O3), Quartz (SiO2), aluminum oxide (Al2O3), indium oxide (In2O3), gallium oxide (Ga2O3) and ferric oxide (Fe2O3). Batches were melted in platinum crucibles at 1400e1450  C for 2 h with continuous stirring at intervals to achieve homogeneity. The melts were cast in warm

stainless steel molds in the required dimensions. The prepared samples were immediately transferred to an annealing muffle furnace regulated at 450  C. The furnace was switched off after 1 h and left to cool to room temperature. 2.2. Differential thermal analysis (DTA) DTA analysis was conducted for all the glass samples using a differential thermal analysis (DTA) by (SDTQ 600 V20.9 Build 20) under inert gas to determine the glass transition temperature (Tg) and crystallization temperature (Tc). About 20 mg powder sample was placed in an alumina crucible and subjected to a heating rate of 10  C/min from ambient temperature to 1100  C in a flowing high purity argon environment. The onset point of an endothermic drift on the DTA curve corresponding to the beginning of the glass transition range is reported as Tg while the peak of the exothermic is taken as Tc. In the thermo grams, the heat flow is measured and plotted against the temperature of the furnace. 2.3. Glass-ceramic formation and heat treatment regime The progress of crystallization in the glasses was followed using double stage heat-treatment regimes for each glass to obtain thermally treated glasseceramic materials of holocrystalline mass with minimum residual glassy phase without deformation. The glasses were first heat treated in a muffle furnace at endothermic temperature of each glass composition; the glass sample was soaked for 5 h which is sufficient to provide sufficient nucleation sites. The muffle was then raised to the exothermic temperature, specific for each glass for10 h. A heating rate of 10  C/min was used during the double stage heat-treatment regimes. After crystallization, the muffle furnace was switched off and the samples were allowed to cool down to room temperature. 2.4. X-ray diffraction analysis (XRD) Identification of the crystal phases formed due to the course of crystallization was conducted by X-ray diffraction (XRD) analysis of the powdered samples. XRD experiments were performed by an Xray diffractometer (PW 1830; PANalytical) using Ni filtered Cu-Ka radiation with scanning speed of 2 (2q) per minute. Diffraction pattern was recorded within Bragg angle in a 2q ¼ 5e80 range using and phases were identified by JCPDS numbers (ICDDePDF2 database). 2.5. Scanning electron microscope (SEM) Investigations the microstructure of the heat treated samples were studied by SEM (JEOL type JXA-840A Electron Probe Microanalyzer operated at 30 kV). Samples were fractured and the fracture positions were immersed in 1% (HF þ HNO3) solution for 60s

Table 1 The composition and DTA data of the glasses studied. Sample

Oxide constitutions (Mole%) Li2O

Cr2O3

SiO2

Al2O3

Ga2O3

In2O3

Fe2O3

G1a G2 G3 G4 G5

31.24 31.24 31.24 31.24 31.24

6.26 3.13 3.13 3.13 3.13

62.50 62.50 62.50 62.50 62.50

0.0 3.13 0.0 0.0 0.0

0.0 0.0 3.13 0.0 0.0

0.0 0.0 0.0 3.13 0.0

0.0 0.0 0.0 0.0 3.13

(Tg)

Tc

545 533 495 490 481

718 715 685 714 675

Tc  Tg

e 746 e e e

173 182 190 224 192

Tg ¼ the transition temperature. TC ¼ the crystallization temperature. a The base glass G1 based on eutectic Li2SiO3eLi2Si2O5eLiCrSi2O6 ¼ 1028 ± 3  C system.

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sequentially washed with distilled water in an ultrasonic cleaner and then air-dried. The fracture surfaces were coated with a thin layer of gold by sputtering method. SEM was carried out to analyze the morphology of crystals in the final glasseceramics.

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2.7. Chemical durability measurements

In the present work, microhardness was measured by using Vicker's microhardness indenter (Shimadzu, Type-HMV, Japan). Grinding and well polishing were necessary to obtain polished, smooth and flat parallel surfaces of glass-ceramic samples before indentation testing. The glass samples were cut using a low speed diamond saw after controlled heat treatment the specimens dry ground using 500, 800, and 1000 grit SiC paper and polished carefully using 3 and 1 mm diamond paste to obtain smooth and flat parallel surfaces glasseceramic samples before indentation. At least six indentation readings were made and measured for each sample. Testing was made by load 100 g and the loading time was fixed for all crystalline samples (15 Sec.). The measurements were carried out under normal atmospheric conditions. The Vicker's microhardness value was calculated from the following equation [11].

In the present work, the powder method was applied to assess the chemical durability of the glass-ceramic materials. The samples were crushed in an agate mortar and then sieved to obtain particles with diameter ranging between 0.63 and 0.32 mm [12e14]. Very fine grains can't be used because they will stick together and to the wall of the container and a large portion of the sample will therefore become shielded from the attack by aquase solutions. The grains were then washed by decantation in ethyl alcohol three times and were then dried. The dried sample was accurately weighed (1 g) in a G4-sintered glass crucible, which was then placed in a 300 ml polyethylene beaker. The samples were tested for their chemical durability in 0.1 M HCl and 0.1 M NaOH solution, 200 ml of the acidic or alkaline solutions were introduced into the polyethylene beaker. The polyethylene beaker with its contents was covered by polyethylene cover to reduce evaporation. The chemical durability was expressed as the weight loss percent. The experiments were carried out at 95  C for 1 h to the different crystalline samples. The sintered glass crucible was then transferred and kept in an oven at 120  C for 1 h, then transferred to cool in a desiccator. After cooling, the total weight loss percent of the samples were calculated.

.
.  Hv ¼ A P d2 kg mm2

3. Results

2.6. Microhardness measurements

Where A is a constant equal to 1.8545 takes into account the geometry of squared based diamond indenter with an angle 136 between the opposing faces, p is the applied load (g) and d is the average diagonal length (mm). The microhardness values are converted from kg/mm2 to MPa by multiplying with a constant value 9.8.

Fig. 1. DTA data of the investigated glasses.

3.1. Crystallization characteristics The DTA thermographs of fine glass powders are shown in Fig. 1. The shapes of all the curves are similar and confirm the glassy nature of all the glasses. All the glass compositions feature has a single endothermic dip before the onset of crystallization, which corresponds to the Tg with the inflection point situated between 481 and 545  C. The transition temperature gives information on

Fig. 2. X-ray diffraction patterns of the crystallized glasses (G1eG4).

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both the strength of interatomic bonds and the glass network connectivity, in a similar way that the melting temperature does for crystalline solids. The exothermic peaks corresponding to the crystallization temperature (Tc) are observed in the temperature range 675e746  C indicate that the crystallization reaction takes place in the glass. From the measured values of Tg and Tc, the glass forming ability parameter DT ¼ (Tc  Tg), that give the information on the stability of the glass against devitrification for all the glass ceramics are presented in Table 1. Figs. (2 and 3), presents the X-ray diffractograms of the investigated bulk glass heat treated within the crystallization temperature. It was seen from the figures, peaks corresponding to the different crystalline phases are marked with different symbols, and the crystalline phases were identified by matching the peak positions of the intense peaks with PCPDF standard cards. The X-ray diffraction analysis of the eutectic glass G1 crystallized at 545  C/ 5 he720  C/10 h (Fig. 2, the pattern I, Table 2), showed that lithium disilicate-Li2Si2O5 (JCPDS card No. 40-0376) was crystallized together with lithium metasilicate phase-Li2SiO3 (JCPDS card No. 83-1517) and lithium chromium silicate -LiCrSi2O6 phase of pyroxene family, (JCPDS card No. 24-0612). The partial replacement of Cr2O3 by Al2O3 in the eutectic glass i.e. G2 led to the crystallization of lithium aluminum silicate eLiAlSi2O6 (JCPDS card No. 76-0921) together with lithium disilicate, lithium metasilicate and lithium chromium silicate phases which developed by heating the glass at 535  C/5 h and then at 745  C/10 h [Fig. 2, pattern II, Table 2]. The addition of 3.13 mol. % of Ga2O3 instead of Cr2O3, i.e. G3, heattreated at 495 C/5 he685 C/10 h led to the development of lithium gallium silicate e LiGaSiO4 (JCPDS card No. 39-0279), lithium disilicate and lithium chromium silicate phases, however the lithium metasilicate phase could not be detected as proved by the X-ray diffraction analysis (XRD) (Fig. 2, Pattern III, Table 2). The X-ray diffraction analysis of the crystallization products of glassceramic specimen G4, thermally treated at 490 C/5 he715 C/10 h revealed that the portion exchange of Cr2O3 by In2O3 in the eutectic glass led to develop lithium indium silicate-LiInSi2O6 (JCPDS card No. 39-0279), lithium disilicate, lithium metasilicate and lithium chromium silicate phases [Fig. 2, Pattern IV, Table 2]. Mineralogically, the partially swap of Fe2O3 instead of Cr2O3 in the eutectic glass led to form pyroxene solid solution of Li-aegerine typeLi(Cr,Fe)Si2O6 (JCPDS card No. 24-0612), lithium disilicate and lithium metasilicate as indicated by the X-ray diffraction of glass G5 heat-treated at 480 C/5 he675 C/10 h. However, no lithium chromium silicate -LiCrSi2O6 phase could be detected (Fig. 3, Pattern II, Table 2).

Table 2 Crystalline phases developed and the properties of the investigated glass-ceramic samples. Sample Heattreatment ( C/h)

Crystal phases developed Average microhardness MPa

Weight loss %

G1

Li2Si2O5, Li2SiO3,LiCrSi2O6 6419

4.39 3.20

Li2Si2O5, Li2SiO3, LiAlSi2O6,LiCrSi2O6 Li2Si2O5, LiGaSiO4,LiCrSi2O6 Li2Si2O5, Li2SiO3, LiInSi2O6,LiCrSi2O6 Li2Si2O5, Li2SiO3,Li(Cr,Fe) Si2O6

5713

6.90 5.18

5282

8.65 7.42

5890

6.29 4.59

6086

4.91 3.45

G2 G3 G4 G5

545/5 e720/10 535/5 e745/10 495/5 e685/10 490/5 e715/10 480/5 e675/10

HCl NaOH

The scanning electron micrographs of a fractured surface of the crystallized glasses are present in Figs. 4 and 5. They clearly showed that, prismatic-like microstructure was developed for crystalline eutectic glass G1 (Fig. 4). The partial replacement of Cr2O3 by Al2O3 in the eutectic glass, i.e. G2 led to form fine fibrous microstructure. However, the effect of partial replacement of Cr2O3 by Ga2O3 of i.e G3 led to the formation of prismatic-like microstructure. The partially Fe2O3/Cr2O3 swap in the eutectic glass, i.e. G5, led to change the microstructure to fine aggregates of prismatic-like microstructure (Fig. 5). 3.2. The properties 3.2.1. Microhardness The Vickers microhardness property was determined for the resulting glass-ceramics. Fig. 6 exhibit a summary of the microhardness values of the investigated crystalline products. Vickers hardness of the glass-ceramic samples is ranging from 5282 to 6419 MPa G1eutectic sample exhibits the highest hardness value, while the sample G3 with Ga2O3/Cr2O3 replacement represents the lowest microhardness value. Generally, the partial replacement of Cr2O3 by different trivalent oxides in the base glass led to the decrease of the microhardness values of the investigated crystalline glasses as compared with the data of the eutectic sample. 3.2.2. The chemical durability The chemical durability characterization of the crystallized glasses was conducted in order to evaluate the chemical resistance in hot acidic and alkaline media. This property is strictly related to the crystalline phases formed, microstructure and the glassy matrix between the phases in the glass-ceramics formed. Fig. 7 shows the chemical stability of the glass-ceramics tested in 0.1 M NaOH and 0.1 M HCl solution at 95  C. The results of weight loss of glassceramic samples showed that the chemical stability of the materials was better in alkaline than in acidic media. The crystalline material G1 and G5 demonstrated chemical resistance greater than the other crystalline samples in both acid and alkaline media. The weight loss of the glass-ceramics in 0.1 M NaOH and 0.1 HCL media for an hour show the resistance of the samples against solutions attack which increased in the order: G1 > G5 > G4 > G2 > G3. 4. Discussion 4.1. Crystallization characteristics

Fig. 3. X-ray diffraction patterns of the crystallized glasses (G1 and G5).

A crystallization process in glasses is driven by the mobility of the atoms and their capacity to form a crystalline network. Once the activation energy for ordering atoms in a lattice is reached, the

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5

Fig. 4. SEM micrographs of fracture surface of glasseceramic (G1eG4).

crystallization process will take place [15]. The transition temperature of the glasses Tg shows an obvious correlation with the change in the coordination number of the network former and the construction of the non-bridging oxygen (NBO), which means destruction of the network structure [16]. Generally, Tg shows a distinct increase when the coordination number of the network former increases. On contrary to that, a construction of NBO causes a decrease into the Tg. As it is known [17], when an oxide of a trivalent element is introduced in a silicate structure, MO4 tetrahedral units can be formed substituting the SiO4 ones; alternatively, the cation introduced can be allocated in the holes of the structure as network modifying ions. Indeed, the addition of different trivalent oxide instead of chromium oxide in the Li2SiO3eLi2Si2O5eLiCrSi2O6 eutectic system remarkably changed the glass transition temperatures Tg and crystallization temperatures Tc. In all cases, the partial insertion of the different trivalent

Fig. 5. SEM micrographs of fracture surface of glasseceramic G1 and G5.

Fig. 6. Microhardness value of the crystallized glasses.

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Fig. 7. Chemical durability of the crystallized glasses in acidic and alkaline media.

oxides instead of chromium oxide in the eutectic glass decreased the glass transition temperature. This may be attributed to the ion field strength of the different trivalent element added which possess a lower field strength than that of chromium. This may account for the decrease of the coherency of glass structure [18]. Therefore, the tendency of glass-crystalline conversion becomes easier by the addition of the different trivalent ions instead of Cr2O3. The compositional dependence of thermal stability factor, DT ¼ Tc  Tg is also included in Table 1. The difference between Tc and Tg in the eutectic glass is about 173  C and it increases with the partially M2O3/Cr2O3 replacement. These results indicating that the thermal stability of the prepared glasses against devitrification increases with addition the different trivalent element at the expense of chromium. Experimentally, the eutectic Li2SiO3eLi2Si2O5eLiCrSi2O6 (1028 ± 3  C) glass had high crystallization tendency upon heattreatment and form bulky crystallization mass. This may be explained by the phase separation of the lithia-silica glass that is caused by the incompatibility of Li2O with SiO2 [19]. The highest concentration of lithium is known to promote the crystallization of silicate glasses. Liþ is a very mobile ion reducing a viscosity of the glass and, hence, simplifying the ions rearrangement, which is a necessary step in the devitrification [20]. On the other hand, the existence Cr2O3 work as an effective nucleation agent able to induce bulk nucleation in the investigated glasses. The Cr2O3 oxide is characterized by a low solubility in silicate glass melts, resulting in the direct formation of Cr-based spinels, which then appear as nuclei for pyroxene glass-ceramics [21]. The phase development during crystallization of the present eutectic glass revealed that, lithium disilicate-Li2Si2O5, lithium metasilicate-Li2SiO3 and pyroxene Li-kosmochlor LiCrSi2O6 phases were formed. The results showed that the phases formed by controlled heat treatment of the eutectic glass are in agreement with those found by Izquierdo and West [7] who mentioned that lithium disilicate, lithium metasilicate and Li-kosmochlor phases were precipitated during the crystallization of the eutectic point 1028 ± 3  C in the Li2OeCr2O3eSiO2 glass system. The effect of partial addition of different trivalent elements instead of Cr2O3 in the eutectic glass on the crystallization characteristics was investigated. The occurrence of various phases formed in the present crystallized glasses was the function of replacement element type and the used crystallization parameters. The partially swap Cr2O3 by Al2O3 in the eutectic glass, i.e. G2 led to form lithium aluminum silicate-LiAlSi2O6 solid solution phase together with lithium disilicate, lithium metasilicate and Li-

kosmochlor phases. Compositions based on Li2O.Al2O3.SiO2 (LAS) are the earliest glass-ceramics developed, and have achieved great industrial and economic importance. The family of lithium alumino-silicates comprises compounds with the general formula LiAlSixO2xþ2. The main crystalline phases formed in this system are metastable solid solutions of high quartz or keatite structure, such as b-eucryptite, b-spodumene and virgilite [22]. We found that, there is no evidence for the formation of solid solution phases or liquid immiscibility gaps between spodumene-LiAlSi2O6 ss phase and Li-kosmochlor-LiCrSi2O6 phases. This can explain the formation of spodumene ss phase content together with the pyroxene kosmochlor phase. On other hand, the formation of lithium aluminum silicate solid solution phase which is accompanied by a concomitant decrease in the lithium disilicate phase as indicated from the decrease of their main characteristics line intensities of the XRD. Riello et al., [23] reported that the formation of b-spodumene and/or b-eucryptite led to a corresponding decrease in the amount of lithium silicate phase. The partial addition of Ga2O3 instead of Cr2O3 in the eutectic glass, i.e. G3, led to the crystallization of lithium disilicate Li2Si2O5, lithium chromium silicate LiCrSi2O6 in addition to the formation of eucryptite-like structure of lithium gallium silicate LiGaSiO4 phase. The possibility to prepare eucryptite-like LiGaSiO4 glasseceramics discovered and studied by Quintana and West [24]. It was found that, heat treatment of glass with composition Li2O:Ga2O3:SiO2 ¼ 1:1:2 at temperatures of 600e650  C results in the formation of metastable LiGaSiO4 crystalline phase. This phase irreversibly transforms into stable eucryptite-like LiGaSiO4 phase during additional heat treatment at 900  C for 5 min. The formation of eucryptite-like LiGaSiO4 coinciding with the disappearance of lithium metasilicate phase in the eutectic glass may be attributed to the residual free silica which can be combined with the equivalent amount of Ga2O3 and lithium metasilicate molecule to form LiGaSiO4 molecule according to the following equation.

Li2 SiO3 þ SiO2 þ Ga2 O3 /2 LiGaSiO4 The present study revealed that when In2O3 and Cr2O3 are present together in lithium silicate glass e.g. G4, the phase distribution can be formulated assuming that Li2O can combine with an equivalent amount of In2O3 (or Cr2O3) and SiO2 to form LiInSi2O6 and LiCrSi2O6 mineral phases which related to the pyroxene family together with lithium meta and di-silicate phases in the crystallized glass. Among pyroxenes those with the M2 site occupied by Li are a peculiar group. The general formula of Li-pyroxenes is LiMe3þSi2O6; in nature only the end member with Al as Me3þ cation is found, named spodumene, but end members with Me3þ ¼ Al, Fe, Cr, Ni3þ, V, Ga, In, Sc and intermediate compositions have been synthesized and studied [10]. However, there was no evidence for the formation of isomorphous between LiInSi2O6 and LiCrSi2O6 crystals. The limits of the isomorphous substitution by different ions in the pyroxene structures depend mainly on two factors. The compositions of the initial host pyroxene crystal, i.e. the nature of those elements sharing in the building of the initial host pyroxene crystal and the conditions of crystallization, namely the degree of equilibria of the mineral-forming process [25]. For the sample G5 the partial replacement of Cr2O3 by Fe2O3 in the eutectic glass give rise to lithium disilicate and lithium metasilicate phase to be formed together with pyroxene solid solution of Li-aegerine type. Theoretically, on the basis of the petrochemical calculation of the chemical composition into normative mineral molecules lithium disilicate Li2Si2O5, lithium metasilicate Li2SiO3, Li-aegerine (LiFeSi2O6) and Li-kosmochlor (LiCrSi2O6) phases could be formed from the G5 glass composition. It seemed therefore, that the Li-aegerine phase LiFeSi2O6 goes into solid solution with the Li-

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kosmochlor LiCrSi2O6 to form pyroxene solid solution (LiFeSi2O6e LiCrSi2O6), i.e. there is a preferential tendency for Li-kosmochlor to accommodate Li-aegerine in its structure forming pyroxene solid solution of Li-aegerine type-Li(Cr,Fe) Si2O6. The displacement of the major characteristics d-spacing lines of the pyroxene variety towards higher 2q values may support the suggestion that the Fe3þ was incorporated in the pyroxene solid solution of kosmochlor type. The resulting pyroxene solid solution phase may exhibit the following formula Li(Cr 0.5, Fe 0.5)Si2O6. Furthermore the addition of Fe2O3 instead of Cr2O3 in eutectic glass led to improve the crystallization tendency of the glasses. Williamson [26] described the effect of iron on the nucleation and crystal growth rates of silicate glasses. He concluded that a spinel phase was the first detectable phase that has been crystallized, this spinel phase provides a surface upon which the main crystal phases can deposit and grow. Other factors that helped to improve the crystallization of the glass (G5), presence of Fe2O3 with Cr2O3 next to each other. Karamanov et al., [21] proved that a small percentage of Cr2O3 strongly affects the spinel formation, thereby reducing the time and temperature of the thermal treatment and enhancing the degree of crystallization of iron containing silicate glasses. 4.2. Properties 4.2.1. Microhardness Hardness is one of the most characteristic properties of materials and often plays a key role in the progression of civilization because it has enabled progressively more sophisticated devices and machines to be constructed [27]. The hardness value is usually defined as the ratio of the indentation load and either the surface or projected area of residual indents. The microhardness of crystallized glasses depends not only on the type of precipitating phases, but also on their size, shape, and natural wetting as well with the emergence or absence of internal cracks [28]. However, the degree of crystallinity should be also taken into consideration. In general, the porosity and grain size are key factors affecting the strength of ceramic materials [29]. Vicker's microhardness values of glass-ceramic samples are tabulated in Table 2. Fig. 6 shows a decrease in hardness value of the crystallized samples with partial replacement of Cr2O3 with different trivalent oxides in the glasses. The data indicated that the glass ceramic sample G1 represents high hardness values. This may be attributed to crystallization of the high hardness Cr-pyroxene member LiCrSi2O6 phase together with lithium meta and disilicate phases. Ceramics and glass-ceramics based on pyroxenes have attracted interest due to the excellent controllability of their properties. Pyroxenes are capable of a wide range of isomorphous substitutions in their crystal structure and having a distinct physical and chemical characteristic [9]. The Observed decreases in the microhardness value of crystallized sample G3 with Ga2O3/Cr2O3 replacement may be attributed to the formation of coarse-grained microstructure as compared with eutectic e.g. G1 aside from the formation of low mechanical LiGaSiO4 phase instead of high mechanical pyroxene member phases. 4.2.2. Chemical durability In the glass ceramic materials, the chemical durability is a function of many factors such as the ratios of the glass oxides added and consequently the nature and concentration of the crystalline phases, microstructure and phase composition of the residual glassy. It was stated that the initial attack will be on the residual glassy phase which is thought to play an important role in determining the resistance of glass ceramic to chemical attack. The solubility of both the formed crystals and the residual glassy phase in leaching solution has almost an equally important influence on

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the chemical stability of glass ceramics [30]. The glass ceramic, in general, possess good chemical stability and that they compare favorably in this respect with other ceramic materials [31]. The weight loss of glass ceramic samples in 0.1 M HCl and 0.1 M NaOH solution at 95  C temperature are shown in Fig. 7. The results show that the chemical resistance of the glass-ceramic specimens have the same behavior in acidic and alkaline solutions, but the chemical stability was better in alkaline than in acidic media. This may be attributed to the crystallization of a large proportion lithium meta and disilicate phases, which show weak resistance to corrosion in acidic media. With respect to the acid mineral stability, some minerals are very resistant to the action of acids (nearly insoluble), others show weak resistivity and decompose or gelatinize under treatment with acid solutions Monmaturapoj et al., [32] reported that lithium meta- and disilicates phases are weak resistivity and decomposed by acidic solution. The data also indicate that the glass ceramic samples G1 and G5 represent the highest values in the chemical resistance in acidic solutions or alkaline solution. This may be ascribed to the formation of nearly unattacked Cr-pyroxene phase, i.e., lithium-kosmochlor (LiCrSi2O6) crystals and its solid solution form Li(Cr,Fe)Si2O6 in G1 and G5 respectively. Varieties of pyroxene phases and their solid solution based glass-ceramic exhibited higher chemical resistance than its parent glass, because pyroxene is a stable crystal phase whose structure is made up of silica chains and different cations positioned between the chains [33and34]. Therefore, the possible explanation of the reduced durability of G3 glass-ceramic compared to the all crystallized specimens could lie in the fact that the major crystal phase (LiGaSiO4) may be more soluble than the pyroxene group minerals and this reduces the overall chemical resistance of the glass-ceramic. 5. Conclusion The crystallization characteristics and physico-chemical properties of glasses based on the Li2SiO3eLi2Si2O5eLiCrSi2O6 (1028 ± 3  C) eutectic glass modified by partial swap of Cr2O3 by different trivalent oxides were investigated. The presence of different trivalent oxides beside the Cr2O3 greatly plays an important role in crystallization characteristics, the type of crystalline phases developed and solid solution formed by thermal treatment of the investigated glasses. In all cases, the insertion of the different trivalent oxides instead of Cr2O3 in the eutectic glass improves thermal stability against the devitrification. No evidence for the formation of solid solution between LiAlSi2O6 or LiInSi2O6 phases and kosmochlor LiCrSi2O6 phase could be achieved from the results obtained. The Li-aegerine LiFeSi2O6 and Lithium-kosmochlor, LiCrSi2O6 phases are a member of the pyroxene group minerals. The LiCrSi2O6 and LiFeSi2O6 phases were accommodated in the pyroxene structure under favorable conditions of crystallization to form monomineralic pyroxene solid solution phase. The microhardness and the chemical stability of crystallized glasses have been investigated. The microhardness values of the studied crystalline products ranged between 5282 and 6419 MPa while, the corrosion behavior indicated that chemical durability was better in alkaline than in acidic media. The present results provided good information about the role of glass oxide constituents and heat treatment regime in determining the crystalline phase assemblages, the compatibility between the phases, the nature of the solid solution formed and the chemical and mechanical properties of the glass-ceramics obtained. References [1] W. Holand, G.H. Beall, Handbook of Advanced Ceramics Materials, Applications, Processing, and Properties, second ed., Elsevier, USA, UK, The Netherlands, 2013, pp. 371e382.

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[2] W. Holand, G.H. Beall, Glass-Ceramic Technology, The American Ceramics Society, Westerville, OH, USA, 2002. [3] M.H. Lewis, Glasses and Glasseceramics, Chapman and Hall, London, 1989. [4] C. Ritzberger, W. Holand, M. Schweiger, V. Rheinberger, Lithium Silicate Glass Ceramic and Glass with Transition Metal Oxide US 2011/0257000 A1. [5] J. Cheng, D. Xiong, H. Li, H. Wang, Crystallization behaviors of R2OeAl2O3eSiO2 glasseceramics for use as anodic bonding materials, J. Alloys Compd. 507 (2010) 531e534. [6] F.A. Hummel, K. Auh, G.H. Johnson, Synthesis of the chromium analogue of (a) spodumene and its solubility relationships with ureyite and b-spodumene, Mater. Res. Bull. 5 (1970) 5301e5306. [7] G. Izquierdo, A.R. West, Phase equilibria in the system Li2O-Cr2O3-SiO2, J. Am. Ceram. Soc. 62 (1979) 136e137. [8] R.M. Thompson, R.T. Downs, Model pyroxenes II: structural variation as a function of tetrahedral rotation, Am. Mineral. 89 (2004) 614e628. [9] R.M. Thompson, R.T. Downs, G.J. Redhammer, Model pyroxenes III: volume of C2/c pyroxenes at mantle P, T, and X, Am. Mineral. 90 (2005) 1840e1851. [10] G.J. Redhammer, G. Roth, Structural variation and crystal chemistry of LiMe3þSi2O6 clinopyroxenesdMe3þ ¼ Al, Ga, Cr, V, Fe, Sc and In, Z. Krist. 219 (2004) 278e294. [11] R.L. Smith, G.E. Sandland, An accurate method of determining the hardness of metals, with particular reference to those of a high degree of hardness, Proc. Inst. Mech. Eng. I (1922) 623e641. [12] S.N. Salama, S.M. Salman, Chemical stability of some manganese glass- ceramics, Mater. Chem. Phys. 37 (1994) 338e343. [13] A.A. Ahmed, H.A. E1-Batal, N.A. Ghoneim, F.A. Khalifa, Leaching of some lithium silicate glasses and glass-ceramics by HCl, J. Non-Cryst. Solids 41 (1980) 57e70. [14] L. Wu, Y. Li, Y. Teng, G. Meng, “Preparation and characterization of borosilicate glass-ceramics containing zirconolite and titanite crystalline phases”, J. NonCryst. Solids 380 (2013) 123e127. zaro, S.C. Santos, C.X. Resende, E.A. dos Santos, Individual and combined [15] G.S. La effects of the elements Zn, Mg and Sr on the surface reactivity of a SiO2$CaO$Na2O$P2O5 bioglass system, J. Non-Cryst. Solids 386 (2014) 19e28. [16] M.M. El-Desoky, M.S. Al-Assiri, Structural and poloronic transport properties of semiconducting CuO eV2O5eTeO2 glasses, Mater. Sci. Eng. B 137 (2007) 237e246. [17] H. Rawson, Inorganic Glass-forming Systems, Academic Press, London, 1967. [18] R.H. Doremus, Glass Science, second ed., John Wiley and Sons Inc, New York, 1994. [19] J.E. Shelby, S.R. Shelby, Phase separation and the properties of lithium calcium silicate glasses, Phys. Chem. Glas. 41 (2000) 59e64.

[20] E. Willhauk, R. Harikantha, Glass ceramics for household appliances, in: H. Bach, D. Krause (Eds.), Low Thermal Expansion Glass-ceramics, second ed., Springer -Verlag, Heidelberg, 2005, pp. 51e58. [21] A. Karamanov, P. Pisciella, M. Pelino, The effect of Cr2O3 as a nucleating agent in iron-rich glass-ceramics, J. Eur. Ceram. Soc. 19 (1999) 2641e2645. [22] G. Müller, Low Thermal Expansion Glass Ceramics, Springer-Verlag, H. Bach, Berlin, 1995, pp. 13e25. [23] P. Riello, P. Canton, N. Comelato, S. Polizzi, M. Verita, G. Fagherazzi, H. Hofenister, S. Hopfe, Nucleation and crystallization behaviour of glassceramic materials in the Li2O-SiO2-Al2O3 system of interest for their transparency properties, J. Non-Cryst. Solids 288 (2001) 127e139. [24] P. Quintana, A. West, Compound formation and phase equilibria in the system Li4SiO4eLiGaSiO4, J. Solid-State Chem. 81 (1989) 257e269. [25] W.A. Deer, R.A. Howie, J. Zussman, An introduction to the rock-forming minerals, in: Third ELBS Impression, CommonWealth, Printing Press Ltd., Hong Kong, 1992. [26] J. Williamson, The kinetics of crystal growth in an aluminosilicate glass containing small amounts of transition-metal ions, Mineral. Mag. 37 (I970) 759e770. [27] P. Zhang, S.X. Li, Z.F. Zhang, General relationship between strength and hardness, Mater. Sci. Eng. A 529 (2011) 62e73. [28] I.L. Denry, J.A. Holloway, Elastic constants, Vickers hardness, and fracture toughness of fluorrichterite-based glasseceramics, Dent. Mater. 20 (2004) 213e219. [29] K. Lin, W. Zhai, S. Ni, J. Chang, Y. Zeng, W. Qian, Study of the mechanical property and in vitro biocompatibility of CaSiO3 ceramics, Ceram. Int. 31 (2005) 323e326. [30] A. Lodding, in: D.E. Clark, B. Zoitos (Eds.), Corrosion of Glass, Ceramics and Ceramic Superconductors, Noyes Publications, Park Ridge, New Jersey, 1992. [31] P.W. McMillan, Glass-ceramics, Academic Press, London, New York, 1979. [32] N. Monmaturapoj, P. Lawita, W. Thepsuwan, Characterization and properties of lithium disilicate glass ceramics in the SiO2-Li2O-K2O-Al2O3 System for Dental Applications, Adv. Mater. Sci. Eng. 2013 (2013). Article ID 763838, 11 pages. [33] Sh A. Faiziev, M.S. Paizullakhanov, E.Z. Nodirmatov, R. Yu Akbarov, M.A. Zufarov, Synthesis of pyroxene pyroceramics in large solar furnace with ZrO2 crystallization nucleator, Appl. Sol. Energy 44 (2008) 139e141. [34] H.A. Abo-Mosallam, S.N. Salama, S.M. Salman, Contribution of gallium oxide content to the crystallization characteristics and chemical stability of sodium phlogopite glasses, Ceram. Int. 40 (2014) 8037e8044.

Please cite this article in press as: S.M. Salman, et al., Crystallization of pyroxene phases and physico-chemical properties of glass-ceramics based on Li2OeCr2O3eSiO2 eutectic glass system, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.10.033