Journal of Non-Crystalline Solids 371–372 (2013) 41–46

Contents lists available at SciVerse ScienceDirect

Journal of Non-Crystalline Solids

j ourna l homepage: www.e lsev ie r .com/ locate / jnoncryso l

Preparation of homogeneous barium borosilicate glass usingmicrowave energy

Ashis Kumar Mandal a,⁎, Dinesh Agrawal b, Ranjan Sen a

a Glass Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja S. C. Mullick Road, Kolkata – 700 032, Indiab Materials Research Institute, The Pennsylvania State University, University Park, PA, 16802, United States

⁎ Corresponding author. Tel.: +91 33 2473 5829; faxE-mail address: ashis@cgcri.res.in (A.K. Mandal).

0022-3093/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jnoncrysol.2013.04.044

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 December 2012Received in revised form 8 April 2013Available online xxxx

Keywords:Microwave processing;Barium borosilicate glass;Energy efficient process;Glass melting process, self stirring

Bariumborosilicate glass having composition inwt.% (46.5) SiO2, (26.5) B2O3, (16.5)Na2O (10.5) BaOwasmeltedat 1473 K in a microwave furnace and also in a conventional resistance heating furnace. A comparative study ofthe properties of both the glasses was conducted by adopting standard methods of characterisation. Glass tran-sition temperature was recorded as 825 K and 829 K for glass samples prepared inmicrowave and conventionalheating respectively. Low evaporation of volatile component was observed in microwave prepared glass whichleads to less density of glass in comparison with glass prepared in conventional method. ‘Self stirring’ effect inmicrowave prepared glass sample was observed in annealed glass obtained without pouring it into preheatedmold. Identical structure and optical transmission were found in both the glasses. OH content was estimatedlow in glassmelted inmicrowave heating over conventional heating.Microwave heating can be a potential alter-native method of preparing glass with substantial reduction of energy and processing time.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Microwave processing of materials draws serious attention of scien-tists and engineers over conventional processing methods mainly dueto energy saving and shorter processing time, improved product unifor-mity and yield, improvedmicrostructures and properties, and synthesisof new materials [1]. Fundamentally, energy is delivered directly tomaterials through molecular interaction with the electromagnetic fieldin microwave heating system. In contrast, energy is transferred to thematerial through convection, conduction, and radiation of heat fromthe surfaces of the material in conventional thermal processing via ther-mal conductivity mechanism, making it a slow process. In microwaveheating, electromagnetic energy is converted to thermal energy via di-electric or magnetic losses [2]. Microwave generates heat within entirevolume of material at the same time resulting volumetric heating of ma-terial. Temperature gradient in microwave processing exists in the re-verse direction in compare to conventional heating methods.

There are many diverse areas where microwave energy is beingapplied such as sintering [3], thermal toughening [4], joining, drying[5,6], glass sealing and others. Major constituents, being transparentto microwave at room temperature, for glass making with this routewas not studied much. However, the constituents of glasses start cou-pling with microwave energy and thereby generating heat above cer-tain critical temperature. Hence, hybrid furnace is useful, where themixture of rawmaterials is preheated up to 773–873 K by conventionalheating and further heating till meltingwas carried outwithmicrowave

: +91 33 2473 0957.

rights reserved.

radiation [3]. The use of microwave heating for processing of glass hasalso been reported such asmelting of ion conducting glass [7], synthesisof tin containing molybdo-phosphate and tungsto-phosphate glass [8],preparation of inorganic glasses [9], niobium phosphate glasses [10],and iron phosphate glasses [11]. In the reported works, glass composi-tions were selected with considerable amount of microwave absorber(50% or more) in it. Also, studies were undertaken in domestic micro-wave ovenwhere understanding of process parameter for glassmeltingwas difficult.

Our objective in this study is to develop a glassmelting process usingmicrowave heating with different glass compositions. Present studyreports preparation of barium borosilicate glass around 1473 K in a mi-crowave furnace. Barium borosilicate has been reported to have utiliza-tion in immobilization of sulfate bearing high-level radioactive liquidwaste [12].

2. Experimental

The glass having a composition ofwt.% (46.5) SiO2, (26.5) B2O3, (16.5)Na2O (10.5) BaO was selected for this study. The source of SiO2, primaryglass-forming oxide in the glass composition, is quartz (from BremthalerQuarzitwerk GmbH (BQW), Germany) and for B2O3, Na2B4O7 · 10H2O(from Merck 99.5% pure) was used. The alkaline oxide, Na2O wasobtained from carbonate source (Na2CO3 from Loba Chemie 99.9%pure). BaCO3 (from BDH lab 99.5% pure) is introduced to obtain BaO inthe glass. Approximately, 60 g of glass batch with the composition wasmixed thoroughly in an agate mortar and formed pellet at 8 t pressurein a hydraulic pellet press. The pellet was placed in an alumina cruciblewith microwave transparent insulation cage in air atmosphere in a

Fig. 1. A. Crucible with molten glass prepared in microwave furnace. B. Annealed glass melted in microwave radiation (BBSMW). C. Annealed glass melted in resistance heating furnace(BBSRH). D. Polished glass (15×20 mm; 2 mm thickness) sample both BBSRH and BBSMW.

42 A.K. Mandal et al. / Journal of Non-Crystalline Solids 371–372 (2013) 41–46

2.45 GHz,multimodemicrowave furnace cavity (fromM/S EnerziMicro-wave Systems Pvt. Ltd., India) using water cooled magnetron (model:2M265-M12WJ-Panasonic) with maximum output power: 3.0 kW andheated at a rate of 15 K/min to 25 K/min up to 1473 K and held for1 h. A small amount microwave susceptor (SiC) was placed around thecrucible to preheat the sample inmicrowave furnace. Temperaturemea-surement of sample was carried out by IR pyrometer. Temperature andmicrowave power was recorded by DAQSOFT software. The moltenglass was cast into a preheated steel mold and annealed at 823 K for2 h followedby controlled cooling of the glass to eliminate thermal stress.The annealed glass sample, prepared in microwave furnace, is termed asBBSMW (barium borosilicate glass prepared by microwave furnace).

Same composition of glass wasmelted in electrical resistance heatingfurnace at 1473 K with a maximum heating rate 5 K/min. The moltenglass was manually stirred intermittently to obtain homogeneous glassand at 1473 K the molten mass was kept for 2 h. The molten glass waspoured into preheated steel mold and annealing of cast glass was doneas discussed earlier. The glass is termed as BBSRH (barium borosilicateglass prepared by electrical resistance heating furnace).

The densities of glass samples were measured by followingArchimedes' principle using water as buoyancy liquid on Mettler Tolladobalance fitted with density measurement kit with an accuracy ± 0.001.

Fig. 2. Microwave induced ‘Self stirring’ effect as observed in glass prepared withoutpouring.

XRD analysis was performed on 94 X'Pert, PANalytical using Ni-filteredCuKα radiationwithwavelength of 1.5406 Å to confirmamorphous char-acteristic of sample. The scanning was carried out in the range of 5–90°with a step size of 0.05° per minute. Chemical analysis of the glasseswas performed using inductively coupledplasma atomic emission spec-troscopy (ICP-AES) for BaO and Na2O in a spectro analytical instrument,Germany and volumetric titrimetry for SiO2 and B2O3. Glass transitiontemperature was measured by DTA analysis (NETZSCH STA 409 C/CD)at a heating rate of 10 K/min in flowing air with an accuracy ± 0.5 K.Refractive indices (RI) of glass samples prepared by both methods weremeasured at five wavelengths (473, 532, 633, 1064, and 1552 nm) onPrism Coupler (Metricon Model-2010, NJ, USA) fitted with five differentlasers as illuminating sources with an accuracy ± 0.0004 (maximum).The optical transmittance spectra were recorded on a Perkin ElmerUV–Vis spectrophotometer (Model Lambda 20, Perkin Elmer,Waltham,MA, USA) in the wavelength range 200–1100 nm. FTIR measurementswere performed on FTIR spectrophotometer (Model: 1615, Perkin

Fig. 3. XRD profile of A) BBSRH and B) BBSMW.

Table 1Refractive Index (at five different wave lengths) and density of BBSRH and BBSMW samples.

473 nm 532 nm 633 nm 1064 nm 1552 nm Density, g/cm3

(±0.0004) (±0.0004) (±0.0004) (±0.0004) (±0.0004) (±0.001)BBSMW 1.5409 1.5374 1.5333 1.5247 1.5180 2.66BBSRH 1.5485 1.5448 1.5403 1.5306 1.5243 2.68

43A.K. Mandal et al. / Journal of Non-Crystalline Solids 371–372 (2013) 41–46

Elmer corporation, USA) with a spectral resolution of 4 cm−1 over aspectral range of 400–1500 cm−1. Also, FTIR transmission spectrawere studied over spectral region 400 to 7000 cm−1 for OH contentanalysis.

The hardness of the glasses was measured by micro-indentationon the polished surface of the glasses using a micro-indentation hard-ness testing system (Clemex CMT, Longueuil, Canada, accuracy of stan-dard sample ± 3%) equipped with a conical Vicker's indenter at anindent load of 200 g. The diagonals of the Vicker's indents were carefullymeasured using optical microscope and subsequently, hardness wascalculated by taking average of ten indentation impressions obtainedby identical loading condition using standard equation.

3. Results and discussion

Fig. 1 displays photographs of crucible with molten glass preparedin microwave furnace (A), annealed glass melted in microwave radi-ation (B), annealed glass prepared in conventional resistance heatingfurnace (C) and two polished glass samples (15 mm × 20 mm) of2 mm (±0.01) thick from both BBSRH and BBSMW (D). Fig. 1 (A) de-picts clear transparent molten glass resulting from uniform heating inmicrowave furnace. As BBSMW was prepared without employing anyexternal stirring, homogeneity was achieved due to microwave induced‘self stirring’ effect during melting [4]. SiC susceptor used in the processacted to provide preheating of the glass mixture since at room tempera-ture the glass batch does not couple with microwave radiation, once thebatchmixture is heated to ~ 773 K–873 K, it starts coupling withmicro-wave and volumetric heating is achieved. This was confirmed by homo-geneous and uniform microstructure observed in the glass sampleprepared by microwave method. But this was not the case in electricalresistance heating where homogeneity is achieved by adopting externalstirring of molten glass.

To understand the effect of ‘self stirring’ in microwave heating, an-other batch of same composition was melted in microwave furnace at1473 K and crucible with molten glass was directly transferred toannealing furnace without pouring into preheatedmold. After annealinga sample was cut and polished from the glass as shown in Fig. 2. Thesample shows the presence of definite pattern of striations inside itconfirming the presence of ‘microwave stirring’. In microwave heatingdielectrically lossymaterials directly couplewithmicrowaves generatingheat within the material without heating the surrounding atmospherearound the sample. This often causes heat loss from the surface creatinghigher temperatures in the interior than on the surface. This thermalgradient generates convection current in the molten glass resultingself-stirring effect. This ‘self stirring effect’ is the evidence of microwavecoupling with glass at higher temperature and this appears to contribute

Table 2Chemical Analysis Result of BBSRH and BBSMW samples.

Oxide phase Batch, wt.% BBSRH BBSMW

SiO2 46.5 59.97 (±0.24) 55.02 (±0.24)Na2O 16.5 14.36(±0.06) 14.30 (±0.06)BaO 10.5 10.39 (±0.04) 10.19 (±0.04)B2O3 26.5 14.52 (±0.16) 19.72 (±0.16)

in improving homogeneity in glass. Thus glassmelting inmicrowave fur-nace can be advantageouswith the view point of elimination or substan-tial reduction of stirring arrangement, which is, however, essential forconventional glass melting methods.

The X-ray diffraction pattern for the BBSRH and BBSMW samplesare shown in Fig. 3. A similar broad hump and a small hump in boththe XRD curves of BBSRH and BBSMW confirm amorphous nature.Absence of sharp peak in XRD profile also confirms no crystallinephase left in the sample prepared using microwave.

Refractive Indices at five different wave lengths viz. 473 nm,532 nm, 633 nm, 1064 nm and 1552 nm and densities of BBSRHand BBSMW are presented in Table 1. Density of glass BBSMW isfound to be slightly less than that of BBSRH. This is mainly due tohigher loss of volatile material in the glass melted in conventionalfurnace. Chemical analysis results, presented in Table 2, confirms higheramount of B2O3 (wt. % 19.72) in BBSMW than in BBSRH(wt. % 14.52).This signifies more evaporation loss of volatile constituent (i.e. B2O3)in conventional heating. The data in Table 2 further indicates that com-position of glass for BBSMW is closer to the batch composition.

Optical properties for BBSRH and BBSMW were calculated fromtheir measured refractive indices using relevant expressions and werepresented in Table 3. The refractive indices at standard wavelengths nd(at 587.6 nm), nF′ (at 486.1 nm) and nC′ (at 656.3 nm) was estimatedfrom the dispersion curves presented in Fig. 4, obtained by fitting themeasured refractive indices with the Cauchy Eq. (1).

n λð Þ ¼ Aþ Bλ2 þ

Cλ4 þ ð1Þ

where, n is refractive index at wavelength λ.The Abbe number V of a material is defined as Eq. (2)

V ¼ nd–1ð Þ= nF–ncð Þ: ð2Þ

where nd, nF and nC are the refractive indices correspond to the wave-lengths 587.6 nm, 486.1 nm and 656.3 nm respectively. Abbe numberwas calculated 72.78 for BBSMW and 66.34 for BBSRH. This signifiesthe low optical dispersion for BBSMW glass sample and this is usefulfor reduction of chromatic aberration in lenses.

Fig. 5 illustrates the DTA thermogram for both the glasses BBSRH(A) and BBSMW (B). Glass transition temperature (Tg) 825 K forBBSMW and 829 K for BBSRH samples can be identified from the DTAthermograph. This difference of Tg values is attributed to slight differ-ence in the composition of the glasses resulting from loss of volatile

Table 3Abbe no, mean diffraction and reflection loss for BBSRH and BBSMW samples were cal-culated from their measured refractive indices.

Sample ID Nd Nf Nc Nd − 1 Meandiffraction

Abbe no Reflectionloss

Nf–Nc

BBSMW 1.535 1.54 1.533 0.535 0.007 72.78 4.31%BBSRH 1.542 1.548 1.539 0.543 0.008 66.34 4.39%

Fig. 4. RI vswavelength fitted by Cauchy curvefitting equation (A— BBSRH; B— BBSMW). Fig. 6. UV–VIS optical transmission spectra of BBSRH (A) and BBSMW (B).

44 A.K. Mandal et al. / Journal of Non-Crystalline Solids 371–372 (2013) 41–46

materials and also different process of cooling. In microwave heating,time taken to pour the molten glass into preheated mold is higherthan that of conventional heating due to stepwise shut down of micro-wave furnace and removal of crucible from the insulation box. Thismakes the cooling rate lower than that of conventionally preparedglass and results into slight difference in glass transition temperatureprepared by two methods.

Fig. 6 illustrates UV–VIS optical transmission spectra in the wave-length range 200–1100 nm. Percentage optical transmission can beidentified from the spectra 90.4% for BBSRH and 91.5% at (~710 nm)for BBSMW indicating slightly improved homogeneity of glass meltedusing microwave radiation.

The FTIR reflectance spectra of samples BBSRH and BBSMW areshown in Fig. 7. Both the spectra of these glasses consist of similarbroad and sharp bands in different regions (400–1500 cm−1) signify-ing identical structure. The peak ~ 446 cm−1 is assigned to Si–O–Sibend vibration [13]. The bands at around 1020 cm−1 peak are attributedto a stretching vibration of B–O–Si linkage [14]. The peak ~ 700 cm−1 isdue to the bending of B–O–B linkages in the borate glassy network [15].Both the FTIR spectra for BBSRH and BBSMW shows that borate networkis seen in three infrared spectral regions as reported on alkali borate glass

Fig. 5. DTA curves A for BBSRH and B for BBSMW.

[16–18]. These regions are: (1) 1200–1500 cm−1 is due to the asym-metric stretching of the B–O band of triangle [BO3] units, (2) 800–1200 cm−1 is due to the B–O stretching vibration of tetrahedral [BO4]units and (3) 600–800 cm−1 due to bond bending vibration of B–O–Bgroups [19,20].

Fig. 8 depicts FTIR transmission spectra in the range 400 to7000 cm−1 for both BBSRH and BBSMW samples and the observedband at ~ 2820 nm (~3545 cm−1) is considered due to the stretchingvibration of free hydroxyl group [21]. The absorption coefficient, αOH,at 3545 cm−1 can be used as a measure of the OH- concentration, assuggested by Ebendorff-Heidepriem [22], can be expressed as:

αOH ¼ logT0

TD

� �� 1

dð3Þ

where, To be the highest transmission at ~ 6000 cm−1, TD is the trans-mission at ~ 3545 cm−1 and d is the thickness of the sample. UsingEq. (3), the αOH is calculated 4.88 cm−1 for BBSRH and 4.23 cm−1 forBBSMW. OH content is determined (from 30.αOH) 146.6 for BBSRHand 126.9 ppm for BBSMW [23]. In microwave heating, volumetricheating drives the OH away from the glass melt as water strongly

Fig. 7. The FTIR reflectance spectra of BBSRH and BBSMW samples in the wave numberrange 400–2000 cm−1.

Fig. 8. FTIR transmission spectra in the range 400 to 7000 cm−1 in both BBSRH andBBSMW.

Fig. 10. Temperature (a) and variation of microwave power (b) with time for the studiedglass prepared in microwave furnace.

45A.K. Mandal et al. / Journal of Non-Crystalline Solids 371–372 (2013) 41–46

absorbs microwave at 2.45 GHz frequency [1]. Thus, low OH− concen-trations glass can be prepared in microwave heating over conventionalheating, which often found beneficial for special application.

Micro indentation impressions A for BBSRH and B for BBSMW arepresented in Fig. 9. The average hardness was found to be 6.301 GPafor BBSMW and 6.049 GPa for BBSRH samples. Slightly higher value ofhardness in the microwave prepared glass again confirms that micro-wave produces more uniform and homogeneous microstructuresproviding improved mechanical properties.

Temperature and variation of microwave power with time for thestudied glass produced in microwave furnace are presented in Fig. 10.This illustrates that the maximum microwave power consumptionis ~ 0.8 kWwith average consumption ~0.6 kWduring the glassmeltingprocess. Microwave power is ~ 65–70% of input electrical power takingmagnetron operating efficiency into account and actual electrical powerconsumption to operatemicrowave furnace including utility and accesso-ries was monitored by Electrical Power Meters. Total electrical powerconsumption in microwave furnace for the melting of the studied glasswas recorded 4.5 kWH including the additional power load for utilitieslike water cooling station to flow cooling water inside chamber, magne-tronhousing, diaphragmpumpetc. Electrical power requirement in resis-tance heating furnace would be 3–4 time more. Also time requirementfor total melting process in microwave furnace is ~2 h. Heating rate inmicrowave furnace was maintained at about 15 to 25 K/min. In case ofconventional furnace, the same is maintained 5 K/min to protect life ofheating element and also refractory lining inside the furnace. Thus total

Fig. 9. Micro-indentation images o

time required for full cycle of melting is around 7–8 h. Moreover micro-wave furnacewas switched off before taking outmolten glass for casting,further reducing power consumption and also time whereas in conven-tional furnace fast heating or cooling is restricted.

4. Conclusions

Barium borosilicate glass has been successfully melted in micro-wave furnace at 1473 K. XRD analysis confirms the amorphous natureof sample prepared in microwave heating. A notable observation ofmicrowave induced ‘Self stirring’ effect which offer an added advan-tage and may be effective in eliminating or substantial reduction ofstirring arrangement during glass melting using microwave radiation.Density of glass prepared in microwave processing is slightly lessthan that of glass prepared in conventional processing due to lowloss of volatile constituent and this has been confirmed by chemicalanalysis. Glass transition temperature was recorded less in micro-wave prepared glass. UV–VIS spectra reveal that percentage transmis-sion remains identical or slightly improved in certain wavelengthrange in microwave prepared glass. OH concentration was found tobe less in the glass melted by microwave heating over resistanceheating. Total electrical power consumption in microwave furnacewas recorded 4.5 kWH including the additional power load for utili-ties. Also total time needed to prepare the glass is around 2 h.Hence, the microwave heating could be a potential method of prepar-ing glass with substantial reduction of energy and processing time.

f (A) BBSRH and (B) BBSMW.

46 A.K. Mandal et al. / Journal of Non-Crystalline Solids 371–372 (2013) 41–46

Acknowledgements

This work was funded by Other Laboratory Project OLP0369 of theinstitute. Authors are thankful to Director, CSIR-CGCRI for his kindsupport and encouragement to pursue this work. Authors sincerelyacknowledge the contribution of staff in GSTS, CSIR-CGCRI for theirhelp as and when required.

References

[1] W.H. Sutton, Cer. Bull. 68 (1989) 376–386.[2] E.T. Thostenson, T.-W. Chou, Composites: Part A 30 (1999) 1055–1071.[3] J. Majling, P. Znasik, D. Agrawal, J. Cheng, R. Roy, J. Mater. Res. 10 (1995)

2411–2414.[4] M.P. Knox, G.J. Copley, Glas. Technol. 38 (1997) 91–96.[5] Medeni Maskan, J. Food Eng. 44 (2000) 71–78.[6] H. Segerer, Am. Ceram. Sot. Bull. 77 (1998) 64–66.[7] D.J. Duval, B.L. Phillips, M.J.E. Terjak, S.H. Risbud, J. Solid State Chem. 131 (1997)

173–176.[8] S. Chenu, J. Rocherullè, R. Lebullenger, O. Merdrignac, F. Chevire, F. Tessier, H.

Oudadesse, J. Non-Cryst. Solids 356 (2010) 87–92.[9] B. Vaidhyanathan, M. Ganguli, K.J. Rao, J. Solid State Chem. 113 (1994) 448–450.

[10] L. Ghussn, J.R. Martinelli, J. Mater. Sci. 39 (2004) 1371–1376.[11] F.J.M. Almeida, J.R. Martinelli, C.S.M. Partiti, J. Non-Cryst. Solids 353 (2007)

4783–4791.[12] C.P. Kaushik, R.K. Mishra, P. Sengupta, Amar Kumar, D. Das, G.B. Kale, Kanwar Raj,

J. Nucl. Mater. 358 (2006) 129–138.[13] S. Abiraman, H.K. Varma, T.V. Kumari, P.R. Umashankar, A. John, Bull. Mater. Sci.

25 (2002) 419–429.[14] A.S. Tenny, J. Wong, J. Chem. Phys. 56 (1972) 5516–5523.[15] H. Dwoeidari, Zeid Abou, E.I. Damraway, J. Phys. D: Appl. Phys. 24 (1991)

2222–2228.[16] J. Krogh Moe, Phys. Chem. Glasses 6 (1965) 46–54.[17] E.I. Kamitsos, M.A. Karakassides, G.D. Chyrssikos, J. Phys. Chem. 91 (1987)

1073–1079.[18] E.I. Kamitsos, A.P. Patsis, M.A. Karakassides, G.D. Chyrssikos, J. Non-Cryst. Solids

126 (1990) 52–67.[19] P. Perince, S. Esposits, A. Arone, Phys. Chem. Glasses 39 (1998) 222–227.[20] P. Perince, A. Arone, A. Marrotta, Mater. Chem. Phys. 30 (1992) 195–198.[21] A.M. Efimov, V.G. Pogareva, Chem. Geol. 229 (2006) 198–217.[22] H. Ebendorff-Heidepriem, W. Seeber, D. Ehrt, J. Non-Cryst. Solids 163 (1993)

74–80.[23] H. Toratani, H. Meissner, T. Izumitani, S. Stokowski, J. Non-Cryst. Solids 95-96

(1987) 701–708.