Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 54 Number 1 February 2013 35

Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B, February 2013, 54 (1), 35–41

1. IntroductionBorosilicate glasses have wide uses in technology. Due to their glass formability and low expansion coefficients they are widely used in laboratory applications. They also exhibit high solubility for other elements and are thus are the chosen glasses for sequestering radioactive wastes. This versatility derives from the flexibility of their atomic structure to accommodate other glass formers, in addition to glass modifiers. The adaptability of the network structures of borate glasses stems from the ambivalence of B atoms which, unlike Si in silicate glasses, can assume both three fold and four fold coordination, forming BO3 triangles and BO4 tetrahedra.(1)

Boroaluminosilicate glasses are technologically important due to their high mechanical strength and chemical durability and are widely used in various applications, which include optical communications, glass to metal seals, ion exchange materials, nuclear waste immobilization, etc.(2,3) It is known that the physico-chemical properties of borosilicate/boro-aluminosilicate glasses such as thermal expansion coefficients glass transition temperatures, chemical durabilities/thermal stabilities and ion exchange capacities are strong functions of composition as well as the type of additives incorporated into the glass.(4)

Seal et al(5) studied the effect of phase separation on the fracture toughness of SiO2–B2O3–Na2O glass and found that (I) with increasing silica content in sodium borosilicate glasses both the fracture toughness and hardness increase and (II) that phase separation giv-ing higher interconnectivity enhances the fracture

toughness of glasses.Ji et al(6) studied a thermodynamic approach to

assess the glass forming ability of glasses and found that according to the Gibbs free energy difference between liquid and crystal, a thermodynamic glass forming ability (GFA) parameter related to charac-teristic temperatures namely the crystallization onset temperature (Tx) and liquidus temperature (Tl). The new parameter defined as ω=Tl (Tl+Tx)/(Tx(Tl−Tx)) cor-relates well with the critical section thickness (Zc) as verified by data for glasses, including oxide glasses, which have been used to validate previous GFA pa-rameters. It is one of the most reliable and applicable GFA parameters among Trg=(Tg/Tl), γ=(Tx/(Tl+Tg)), α=(Tx/Tl), δ=Tx/(Tl−Tg), etc.

Lu Chunhua et al(7) studied the structural charac-teristics of boroaluminosilicate glasses. They found that, in B2O3–Al2O3–SiO2 glass, the boron coordination sites are mainly trigonal (B(3)) and tetrahedral (B(4)), and the aluminium coordination sites are A1(4), Al(5) with some Al(6) . With increasing BaO content in the B2O3–Al2O3–SiO2 glass, B(3) gradually changes to B (4), and Al( 5), Al( 6) changes to Al(4).

Infrared spectra of Na2O–B2O3–SiO2 and Al2O3–Na2O–B2O3–SiO2 glasses (with intermediate SiO2 con-tents) have been analyzed to calculate the N4 fraction of four coordinated borons. A reasonable agreement between the N4 values calculated from IR spectra and those determined from NMR spectroscopy (or using literature models) could be attained under certain conditions. It has been proposed that the absorption bands in the region 1000–1120 cm−1 arise from contri-butions of SiO2 and B2O3 vibrations. The contribution of an oxide is proportional to its concentration. Heat

Influence of SiO2 substitution for B2O3 on the properties of borosilicate glassesI. Kashif,*,1 E. M. Sakr, A. A. Soliman & A. Ratep*Physics Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, EgyptPhysics Department, Faculty of Girls, Ain Shams University, Heliopolis, Cairo, Egypt

Manuscript received 28 January 2012Revised version received 13 May 2012Accepted 3 July 2012

Glass samples with the molecular formula (mol%): (30+x) SiO2–(10−x)B2O3–5Al2O3–54Na2O–1MnO2 where x=0, 5, 7·5 and 10 mol% were prepared. The density, glass forming ability, fragility, Vickers hardness and magnetic susceptibility were measured and the results were used to investigate the changes caused by the replacement of boron oxide by silicon oxide. It was found that the density, glass forming ability, fragility and Vickers hardness at first increase at 5 mol% B2O3 (35 mol% SiO2) and then decrease at 2·5 mol% B2O3 (37·5 mol% SiO2) and increase again with increasing SiO2. The replacement of B2O3 by SiO2 increases the concentration of nonbridging oxygens and changes the ratio between BO4 and BO3. The higher density, glass forming ability and fragility of the sample containing 5 mol% B2O3 is due to the ratio between B2O3 and Al2O3 which equals 1, causing BO3 to change to BO4. The magnetic susceptibility increases as the silicon oxide content increases up to 37·5 mol% and then decreases.

1 Corresponding author. Email Isnham@alo.com

36 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 54 Number 1 February 2013

treatment of Na2O–B2O3–SiO2 glasses does not change the value of N4 and this indicates that the structure of the alkali borate phase is the same as that in the glass obtained from the melt and in the heat treated one.(8)

Du et al(9) studied the addition of Al2O3, to sodium borosilicate glass, they showed that the fraction of the fourfold-coordinated boron decreased in Na2O–B2O3–SiO2 glass. This is caused by the formation of a singly charged tetrahedral AlO4/2-units at the expense of BO4 group. Also they showed that the Si network in Na2O–B2O3–SiO2 glass without the addition of Al2O3 is composed of Q4 units (all bridging oxygens) and Q3 units (three bridging oxygens and one nonbridging oxygen). However, after introduction of 3% Al2O3, the Si network is mainly composed of Q4 units.

The effect of MnO2 content on Tg, density, hard-ness, glass forming ability, stability, fragility, mag-netic susceptibility and the activation energy of the glass transition of 40SiO2.5Al2O3.55Na2O glasses has been investigated. The indicative parameters of GFA, ΔTx, Trg, KH, gm and the magnetic susceptibility tend to increase as MnO2 increases up to 0·4 mol%, and decrease beyond 0·4 mol% MnO2. The glass transi-tion kinetic studies show that the glass activation energies have lower values for MnO2 contents up to 0·4 mol%. The fragility parameter Fi values of the investigated glass samples have been evaluated using two different methods indicating that the present glass system is a strong glass former according to Angell’s classification scheme. Then the experimental results show that the addition of MnO2 up to 0·4 mol% MnO2 in a 40SiO2.5Al2O3.55Na2O glass may improve the glass forming ability, thermal stability and the hardness. The glass sample with composition 40SiO2.5Al2O3.54·6Na2O.0·4 MnO2 has the best GFA and stability.(10)

Sodium borosilicate glasses are used for many ap-plications such as optical glasses, ovenware, nuclear waste immobilisation and in the electronic industry. Sodium borosilicate glasses containing manganese are used in purification water as this glass attracts heavy metals in water.

In this work,the effects of substituting SiO2 for B2O3 on density, glass forming ability, fragility, hardness and magnetic susceptibility of sodium aluminium borosilicate glass containing manganese have been studied.

2. Experimental work

A series of glass samples with compositions (mol%): (30+x)SiO2–(10−x)B2O3–5Al2O3–54Na2O–MnO2, [where x=0, 5, 7·5 and 10 mol%] were prepared using chemically pure raw materials, which were finely pul-verized. The homogeneous mixtures were melted in porcelain crucibles in an UAF 15/10 Lenton Thermal Designs programmable electrically heated furnace equipped with an automatic temperature controller.

The samples were heated to 1100±20°C and held at this temperature for 2 h; the molten material was quenched between two steel plates at room tempera-ture. The quenched samples were annealed at 300°C for 20 min, cooled in air to room temperature, and placed immediately into vacuum desiccators until used for measurement. The samples were examined using a Philips PW 3710 analytical x-ray diffraction system with a Cu anode, which confirmed their amorphous nature.

Glass density measurements were made at room temperature using the standard Archimedes principle with toluene as an immersion liquid (density=0·866 g/cm3). The experimental error was about ±0·003 g/cm3. The molar volume [Vm] was calculated from molecular weight and density.

Differential thermal analysis measurements (DTA) were carried out using a Shimadzu DTA-50 Analyser. The measurements were carried out between 25 to 1000°C under N2 gas with Al2O3 powder as a reference material, at different rates from 5 to 40°C/min, at the reported measurements were made at 25°C/min.

Vickers hardness was measured using a Zwick-3270 microhardness tester. The surfaces of the glass sam-ples were cleaned in 10% HF aqueous solution for 30 s. The applied load and the loading time were 4·9 N and 30 s, respectively. The indentations were observed using a microscope at room temperature.

The magnetic susceptibility was measured using Oxford Faraday Magnetometer (Faraday Balance).

3. Results and discussion

3.1 X-ray resultsThe x-ray diffraction patterns of the {(30+x)SiO2–(10−x)B2O3–5Al2O3–54Na2O–1MnO2} (mol%) glass samples (x=0, 5, 7·5 and 10) are shown in Figure 1. From this figure, it can be observed that all the studied glass samples exhibit a broad halo, peaking at around a diffraction angle 2θ≅32°, characteristic of an amorphous structure and confirming the absence of crystals in the samples.

3.2 Density

The density of a material is used for quality control in commercial glass manufacture. It is a measure of the mass of a material per unit volume. Figure 2 (the line is a guide for the eyes) shows the relation between the density and silicon oxide content. From Figure 2, it can be observed that the variation of density when boron oxide is substituted by silicon oxide gradually increases for SiO2 additions up to 35 mol% and then decreases up to 37·5 mol% SiO2 then increases again by more adding SiO2. Density data were used to calculate the volumes of structural units present in various types of glasses.

In the sodium aluminium silicate glass system, for

I. KashIf et al: Influence of sio2 substItutIon for b2o3 on the propertIes of borosIlIcate glasses

Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 54 Number 1 February 2013 37

Al2O3/Na2O<1 all Al ions are found in the form of AlO4 groups. The Al2O3 changed all the SiO4 nonbridging oxygens to SiO4 bridging oxygens but in sodium borosilicate glass the addition of Al2O3 changes BO4 to BO3 vibrational groups. Hence it is clear that the density in Na2O–B2O3–SiO2–Al2O3–MnO2 depends mainly on the change in the BO4 to BO3 ratio.

In contrast, in alkali borate glasses the volume of BO3 units and also the number of BO4 units changes markedly with the glass composition. With increas-ing alkali content up to 33 mol%, the fraction of BO4 units increases at the expense of the BO3 units. The lower packing density of BO3 units causes a decrease in density. Also, Figure 2 shows a signifi cant change for the replacement of 5 mol% B2O3 by SiO2. Both

[BO3] triangles and [BO4] tetrahedra are the main structural units of B2O3–Al2O3–SiO2 glasses that eff ect its properties. The increase of B2O3/Al2O3 ratios helps the progressive conversion of [BO4] to [BO3] units. However, when the B2O3/Al2O3 ratio reaches 1·3, excessive BO3 unit transforms to BO4 units.(11) With increasing B2O3/Al2O3 ratio, the density increases at 5 mol% B2O3, until the B2O3/Al2O3 ratio reaches 1 and then decreases for 1·3<B2O3/Al2O3<2.

3.3 Differential thermal analysis

DTA investigations of the glass transition tempera-ture, Tg, are useful in identifying structural changes associated with compositional changes. The DTA curves of the diff erent glasses have been measured at a heating rate of 25 K/min. They exhibited an en-dothermic peak, which is characteristic of the glass transition region (Tg), followed by an exothermic peak at higher temperatures characteristic of the crystal-lization region (crystallization onset temperature, Tx and peak crystallization temperature Tc). The exothermic peak is followed by a sharp peak just before the melting onset temperature, Tm, which may be the eutectic point of the glass samples and Tm is followed by the liquidus temperature, Tl. The eff ect of the SiO2 content on the thermal data determined at a heating rate of 25 K/min for the investigated glass samples is shown in Figure 3. All of the values of the transition temperatures are summarized in Table 1.

Also it could be seen that the glass samples have two Tg, Tc and Tm, (clear in sample containing 30 mol% SiO2) which are att ributed to the glass transi-

Figure 1. X-ray diff raction patt erns of glass samples

Figure 2. Dependence of density on the glass former con-tent (The line is a guide for the eye)

Table 1. Ratio of oxygen to former ions as a function of silica contentSiO2 Ratio Tg Tx Tl

(mol%) [O/(B+Si)] (°C) (°C) (°C)40 2 442 492 100337·5 1·94 432 469 99035 1·89 410 469 95930 1·8 404 438 926 164 247

Figure 3. DTA curves for the glasses studied

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38 Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 54 Number 1 February 2013

tion temperatures Tg1 and Tg2. This means that these investigated glass sample have two glassy phases(12) or it may be due to nano-glassy phase separation in the glassy state. One glassy phase is related to SiO2 and other to B2O3. It is clear that the sample heat treated at the fi rst Tc formed one phase and when heat treated at second Tc formed a second phase. Also two phases were formed when the sample was heat treated at a second Tc.

It is known that DTA investigations of the glass transition temperature, Tg, are useful in indicating structural changes that occur through compositional changes. This is because Tg is very sensitive to any change of the coordination number of the network forming atoms and to the formation of nonbridging oxygens.(13–16) The eff ect of silicon content on Tg for the investigated samples is shown in Figure 4 (the line is a guide for the eye). Figure 4 shows that the values of Tg increase gradually as the SiO2 content increased. The correlation between the glass transi-tion temperature and the glass structure has been studied extensively. The eff ect of any cation on Tg is related to the following factors:(17,18) (1) the density of covalent crosslinking, (2) the oxygen density of the network, and (3) the number and strength of the crosslinks between oxygen and the cation.

In sodium aluminoborosilicate glass the ratio between oxygen and the glass former oxide (borate and silicate) increases with increasing SiO2 content which causes the increase in the value of Tg. The values of Tg are quantitatively described in terms of two eff ects:(19) (1) increasing the values of Tg, creation of four-coordinated boron, aluminium and silicon (BO4, AlO4 and SiO4 units), (2) decreased values of Tg are due to the creation of nonbridging oxygens (BO3 units). The results indicate that there no anomalous structural changes (i.e. no nonlinear behaviour) has occurred.

3.4 Glass forming ability

The term glass forming ability (GFA) should not only mean the ease with which a glass can be formed from the liquid, but also that the glass has a high resist-

ance to crystallization. To assess the GFA of the glass samples, a fi rst parameter, defi ned as Trg=Tg/Tl (the reduced glass forming temperature) is considered; a low Tl and a high Tg indicates a higher tendency for glass formation and a hence a larger value of Trg indicates easier glass formation. However, Trg does not incorporate the other aspect of GFA,(1) which is the thermal stability of glass. The second parameter used to calculate the glass forming ability was ΔTx=Tx−Tg which expresses the width of the supercooled liquid region SCL, the temperature diff erence observed during heating between the onset crystallization tem-perature, Tx and the glass transition temperature, Tg. Usually, unstable glasses show a crystallization peak close to the glass transition temperature. Therefore, the temperature diff erence Tx−Tg between the two temperatures is a good indication of thermal stability because the higher the value of this diff erence, the more the delay in the nucleation process. Hence Trg and ΔTx each include only one aspect of GFA (either the ease of glass formation or the thermal stability of glass) and hence cannot measure GFA eff ectively.(20) Other parameters used to calculate the glass forming ability such γ, β, α and δ were therefore calculated and are tabulated in Table 2. These parameters are given by

Trg =(Tg/Tl) (1)

γ=TX/(Tl+Tg) (2)

α=Tx/Tl (3)

δ=Tx/(Tl−Tg) (4)

In addition a new parameter defi ned by

ω=Tl(Tl+Tx)/(Tx(Tl−Tx)) (5)

Table 2. Glass forming ability parameters for the glasses studiedSiO2 (mol%) γ Trg α δ 40 0·34 0·441 0·491 0·87737·5 0·33 0·436 0·374 0·84135 0·343 0·428 0·489 0·85430 0·329 0·436 0·473 0·839

Figure 4. Glass transition temperature as a function of silica content (The line is a guide for the eye)

Figure 5. Relationship between the new GFA parameter (ω) and silica content (The line is a guide for the eye)

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Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B Volume 54 Number 1 February 2013 39

was considered. Figure 5 (the line is a guide to the eye) shows the relation between this new GFA parameter and the silica content of the glass samples.

From Figure 5 it can be seen that the sample con-taining 5 mol% B2O3 and 35 mol% SiO2 has a higher glass forming ability. This may be due to the ratio between B and Al being equal to 1 which means that this sample has a higher number of BO4 groups (higher coordination number of boron). This indicates that the glass samples containing 35 mol% have high resistance against nucleation and crystal growth and greater thermal stability than other glass samples.

3.5 Vickers hardness

The methods for calculating the hardness of crystal-line materials do not apply to glasses because the glasses have a high degree of disorder (e.g. a broad distribution of bond angles and lengths).

In spite of this, some att empts have been made by scientists to predict the hardness of glasses. It has been suggested that structural parameters such as the number of nonbridging oxygens per tetrahedron (NBO/T) and structural density could be used for predicting hardness of glasses.(21)

Smedskjaer et al(22) describe and discuss several important factors that should be used for predicting the hardness of glasses. To do so, they studied the influences of thermal history and chemical composi-tion on the hardness of silicate glasses. By subjecting aluminosilicate glasses of diff erent compositions to various degrees of annealing, it was found that the hardness decreased with the fictive temperature. Ad-dition of Na2O to a SiO2–Al2O3 glass system caused a decrease in hardness. However, the number of nonbridging oxygens per tetrahedron (NBO/T) is not the only parameter determining hardness. In addi-tion, it was found that the eff ect of ionic radius on hardness is opposite for alkali and alkaline earth ions. Hence, changes of the structural network occurring at the atomic scale must be taken into account when predicting the eff ect of composition on hardness.

Figure 6 (the line is a guide for the eye) shows the Vickers hardness as a function of the SiO2 content. The hardnesses of the glasses studied here have various values with a minimum of 225 HV at B2O3=10 mol%. In the region of low B2O3 content (<10 mol%), the Vickers hardness increases with increasing fraction of tetrahedral SiO4 and BO4 units. The Vickers hardness increased with increasing fraction of tetrahedral SiO4 units when the B2O3 content equals 5 mol% owing to the formation of BO4 and AlO4 structural units acting as network formers. Above 5 mol% B2O3, the Vickers hardness decreased with the decreasing fraction of tetrahedral BO4 units dropping to about 250 HV at 2·5 mol% B2O3, which indicates that three-coordinated boron atoms are predominant in these glass samples (relative to four-coordinated ones).

From Figure 6 it can be seen that the hardness increases with increasing silica content. From Table 1, it can be observed that the ratio between oxygen and former increases as the silica content increases in the glass samples. The correlation between NBO and hardness is not valid for all silicate glass systems. It has been shown for basaltic glasses that an increase in NBOs can cause both an increase and a decrease of hardness.(22,23) It is well known that the structure of silica glass which has no nonbridging oxygens is not compact, but very open, and that the strength of Si–O bonds is high(24) and greater than that of B–O. This shows that adding silica causes an increase in bond strength and thus causes an increase of hardness.

From Figure 6 it can be observed that the hardness increases with increasing silica up to 35 mol% SiO2 and then decreases beyond this value. The ratio of oxygen ions to network formers increases with in-creasing silica content in the glass network as shown in Table 1. Thus the increasing the hardness up to 35 mol% SiO2 may be due to the substitution of B2O3 by SiO2 increasing the number of SiO4 groups as the Si–O bond strength is greater than the B–O bond strength. Meanwhile the decrease in the hardness beyond 35 mol% SiO2 may be due to the aluminium ions are entering the glass structure as AlO4 leading to the conversion of BO4 to BO3 units.

3.6 Fragility

The fragility of a glass former can be understood as the ease with which the respective supercooled liquid adjusts to a diff erent structure at or just above Tg under a small temperature increment.(25–27) Figure 7 (the line is a guide for the eye) shows the relation between fragility and boron content. The fragility index of the glass, in the temperature range of glass transition, can be approximately evaluated(28) using:

Fi≡2·1Tg/(Tg end−Tg) (6)

The dependence of this quantity on the glass composition is shown in Figure 5. It is found that the fragility slightly increases with increasing silicon con-tent. The fragility of the glass sample free from boron is higher than that of the samples that contain boron

Figure 6. Relationship between hardness and silica content (The line is a guide for the eye)

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oxide. To understand this composition dependence, the theoretical fragility model proposed by Vilgis is useful.(29,30) This model states that the degree of fragil-ity relates to the degree of fluctuation of the coordi-nation number of a key element, i.e. that the origin of the fragility relates to the degree of fluctuation of the coordination number. Hence the formation of the four-coordinated boron plays a dominant role in the fragility of the borate glassy systems.(31)

Both [BO3] triangles and [BO4] tetrahedra are the main structural units of B2O3–Al2O3–SiO2 glasses. Increasing the B2O3/Al2O3 ratio leads to conversion of [BO4] to [BO3] units. However, when the B2O3/Al2O3 ratio reaches 1·3, excessive [BO3] units transform to [BO4] unit.(9) With the increase in B2O3/Al2O3 ratio, the fragility decreases first at 2·5 mol% B2O3 (B2O3/Al2O3 ratio equal to 0·5) and then increases (1<B2O3/Al2O3 ratio<2).

But the fragility depends on NBO/T.(32) From Table 1 it can be seen that the number of oxygens decreases with increasing B2O3 content.

In the borosilicate glass samples, the soda content is equal to 54 mol% and the ratio between Na2O/SiO2≥1·3. This indicates that the conversion proceeds from Q2 (SiO4 tetrahedron with two nonbridging oxygens) to Q1 (SiO4 tetrahedron with three nonbridging oxy-gens); the eff ect of the ratio between B2O3/Al2O3 and Na2O/SiO2 causes the change in fragility due to the change of NBO/T for boron and silicon.

Lee et al investigated the fragility of sodium silicate glasses, Na2O–SiO2, on the basis of their viscosity and thermal properties, and found that the fragil-ity of Na2O–SiO2 glasses monotonically increases as the Na2O content increases.(33) In this case, the coordination number of silicon atoms is constant at four but the nonbridging oxygen atoms increases with increasing alkali oxide content. Aniya has also proposed an extended fragility model fragility, in which not only the coordination number but also the bond strength is considered, and has shown that the degrees of both fluctuations relate to the fragility.(34) This model may be applicable to explain the fragility of both alkali silicate and borate glasses. In the alkali silicate glasses, the atomic units, SiO4 tetrahedra,

change from Q4 to Q0 as alkali oxide content increases. It is known that Qn (n=0, 1, 2, 3, 4) systematically varies with the increase of alkali oxide such that Q4ÆQ3ÆQ2ÆQ1ÆQ0 (Q4=all bridging oxygens, Q0=all nonbridging oxygens). In contrast to silicate glasses, the situation in borate glasses is rather complicated since several structural units exist. Thus, in this paper, we have considered fi rst the coordination change of boron atoms, and found that the fluctuation in the coordination number of boron atoms correlates to the fragility of the system. This result indicates that in the case of sodium aluminium borosilicate glasses, the coordination change of the boron atoms is a more important factor in the origin of the fragility than that of the oxygen atoms.

Ikeda & Aniya have provided a new method to analyze the fragility.(35) In this model, if the fluctuation of the coordination number is known, the parameters relating to the fluctuation of the bond strength can be determined. This model can give new additional insights in to the origin of the fragility. Such analysis by this model is based on the coordination change of both boron and oxygen atoms. Also according to Vilgis(30) glass forming liquids that exhibit an ap-proximate Arrhenius temperature dependence of their relaxation times with a low value of Fi (≈16) are defi ned as strong glass formers, while the limit for fragile glass forming liquids is characterized by a high value of Fi (≈200).(25) As the values of fragility, Fi for the studied samples are within the above mentioned limit, it is reasonable to state that the glass samples are obtained from strong glass forming liquids.

3.7 Magnetic susceptibility

Figure 8 shows the relation between the magnetic susceptibility and the silica content. It can be seen that the magnetic susceptibility increases with increasing silica concentration and then decreases.

Manganese exists in diff erent valance states with diff erent concentrations in glass matrices, for example as Mn3+ in borate glasses with octahedral coordination whereas in silicate and germinate glasses it is present

Figure 8. Magnetic susceptibility as a function of silica content

Figure 7. Relationship between fragility and silica content (The line is a guide for the eye)

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as Mn2+ with both tetrahedral and octahedral envi-ronments.(36–38) Thus in the present borosilicate glass system, manganese exits as Mn3+ with octahedral coordination and Mn2+ with both tetrahedral and octahedral coordination.(10,39–43)

The initial increase in the magnetic susceptibility of the glass samples may be due to the manganese ions mostly existing as Mn2+ with a magnetic moment equal to 5·92 μB. The decrease of magnetic suscepti-bility of glass samples may be due to the change of Mn2+ to Mn3+ which has magnetic moment equal to 4·90 μB.(36)

4. Conclusions

The effect of SiO2 substitutions for B2O3 on the glass transition temperature, density (d), hardness (HV), glass forming ability (GFA), fragility (Fi) and magnetic susceptibility (χ) of aluminoborosilicate glasses has been studied. Density, GFA, fragility and hardness tend to increase for SiO2 contents up to 35 mol%, and then decrease at 37·5 mol% and increase with increasing SiO2. The glass sample containing 35 mol% SiO2 has higher density, fragility and the best GFA.

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I. KashIf et al: Influence of sio2 substItutIon for b2o3 on the propertIes of borosIlIcate glasses