Structural and acoustic investigations of calcium borate glasses

Article abstract of DOI:10.1002/pssa.200622140

Acoustic and structural properties of xCaO ·(1-x) B ₂O₃ (0.2 ≤ x ≥ 0.7) glass system have been studied at ambient temperature. The ultrasonic velocity and attenuation for different compositions of calcium borate glasses were measured

Full text of DOI:10.1002/pssa.200622140

phys. stat. sol. (a) 203, No. 10, 2356–2364 (2006) / DOI 10.1002/pssa.200622140
Structural and acoustic investigations of calcium borate glasses
Manupriya1, K. S. Thind*, 2, G. Sharma2, V. Rajendran3, K. Singh4, A. V. Gayathri Devi3
and S. Aravindan3
1
Department of Applied Sciences, Amritsar College of Engineering & Technology, Amritsar, India
Department of Physics, Guru Nanak Dev University, Amritsar, India
Department of Physics, Nano Science and Technology Research Centre, Mepco Schlenk Engineering
2
3
College, Sivakasi, Virudhunagar (Dt), Tamilnadu, India
School of Physics & Material Science, Thapar Institute of Engineering & Technology, Patiala, India
4
Received 15 March 2006, revised 14 June 2006, accepted 18 June 2006
Published online 8 August 2006
PACS 61.43.Fs, 62.20.Dc, 62.65.+k, 78.30.Ly
Acoustic and structural properties of xCaO · (1–x) B2O3 (0.2 ≤ x ≥ 0.7) glass system have been studied at
ambient temperature. The ultrasonic velocity and attenuation for different compositions of calcium borate
glasses were measured at 5 MHz at room temperature (303 K) using a pulse echo overlap method. The
elastic moduli, Debye temperature, Poisson’s ratio and other acoustic parameters have been obtained from
experimental data. Fourier transform infrared spectroscopic analysis has been used to further support the
results of acoustic studies. The results obtained indicate that the calcium oxide enters the glass network as
a modifier by occupying the interstitial spaces in the network and by increasing the nonbridging oxygen
atoms (NBOs).
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
Recently, calcium-containing borate glasses have received much attention for their potential as bioactive
materials [1–4]. The level of bioactivity of bioactive glasses is composition dependent [5], by altering
the bioactive glass composition, the rate of resorption and hence its bioactivity can be controlled. The
measurement of ultrasonic parameters such as velocities, acoustic impedence, microhardness and Pois-
son’s ratio as a function of composition is of great interest for complete characterization of glasses as
they provide useful information about co-ordination, cross-link density and structural changes [6–8].
Density and molar volume are sensitive and informative about the changes that occur in the structure of
glass networks [9, 10].
Fourier transform infrared spectroscopy (FTIR) plays a significant role in understanding the structural
aspects of glass networks due to modifier doping [11–13]. According to Stoch and Sroda [14], glass is a
solid body with a flexible random structure. The construction and uniformity of the structure depends on
the chemical composition, the origin and the history of glass. Randomness of the arrangement of the
elements of the structure of glass is stabilized by a strong, polymeric network with covalent-ionic chemi-
cal bonds, as in silicate, phosphate and borate glasses or by inhomogeneities as clusters of a stabilizing
admixture. Thus, the paper aims to throw light on the structure of calcium borate glasses as a function of
composition through the investigation of the density, molar volume, elastic moduli, and Fourier trans-
form infrared spectra.
* Corresponding author: e-mail: kulwantsthind@yahoo.com, Phone: 91-183-2258802-09 (3346 Ext.); Fax: 91-183-2258819,
2258820
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phys. stat. sol. (a) 203, No. 10 (2006)
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Table 1 Sample name, nominal composition, Poisson’s ratio in xCaO · (1 – x) B2O3 (0.2 ≤ x ≥ 0.7) glass
system at 300 K.
sample name nominal composition
(mole fraction)
Poisson’s ratio
CaO (x)
B2O3 (1–x)
Ca1
Ca2
Ca3
Ca4
Ca5
Ca6
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.7
0.6
0.5
0.4
0.3
0.280
0.285
0.279
0.280
0.280
0.297
2 Experimental techniques
2.1 Sample preparation
Glass samples of the type xCaO · (1–x) B2O3 (where x is mole fraction) were prepared by employing the
melt-quenching technique. The chemical compositions of all these glasses are shown in Table 1. Com-
mercial grade chemicals of CaCO3 (Fluka Chemical Company) and H3BO3 having 99.99% purity level
were used as starting materials. Appropriate amounts of oxides were weighed by using an electronic
balance having an accuracy of 0.001 g. The chemicals were then mixed in a pestle and mortar for 30 min.
The mixture of the glass compositions was melted in an alumina crucible at 1523 K for about 20 min.
The melt was stirred many times to ensure homogeneity of the glass mixture using a quartz tube. The
glass samples were then annealed in a separate annealing furnace for 12 h. The samples were ground
mechanically using different grades of SiC abrasives and aluminum oxide. The polishing was done with
cerium oxide to obtain flatness.
2.2 Density and molar volume measurements
The density was obtained from Archimedes’ principle using benzene as the liquid. The density was de-
termined employing the following relation:
Wa
ρ =
¥ ρb ,
(1)
Wa - Wb
where Wa is the weight in air, Wb the weight in liquid and ρb the density of the liquid. All the weight
measurements were made using a digital balance (M/s. Sartorius, Model: BP221S, USA). The accuracy
in the measurement of weight is 0.1 mg. The experiment was repeated five times to get an accurate
value of density. The overall accuracy in the density measurement is 0.5 kg m–3 and hence, the percent-
age error in the measurement of density is 0.05%. The molar volume, Vm, is given by using the molec-
ular weight of glass and density as follows:
Vm = M/ρ .
(2)
2.3 Ultrasonic velocity and attenuation measurements
The ultrasonic velocity (both longitudinal and shear) measurements were carried out using a high-power
ultrasonic pulser receiver system (Fallon Ultrasonic Inc. Ltd., FUI1050, Canada) with a 100 MHz digital
storage oscilloscope (HP-54600B) using the cross-correlation technique employing the pulse echo me-
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Manupriya et al.: Structural and acoustic investigations of calcium borate glasses
thod described in our earlier studies [15]. The longitudinal and shear ultrasonic waves are generated,
respectively, by X and Y-cut transducers. Precise transit time measurements were carried out by the
cross-correlation technique described elsewhere [16, 17]. The precise ultrasonic velocities (UL and US) in
each glass were obtained by measuring the precise transit time (t) using the relation
2d
U =
,
(3)
t
where d is the thickness of the sample. The percentage of error in the measurement of velocity is 1%.
The attenuation coefficient of the ultrasonic waves (both longitudinal and shear) was measured, oper-
ated at a frequency of 5 MHz using a contact-type transducer. A uniform pressure was maintained be-
tween the transducer and the glass during the contact measurements. Among the various available
couplants for room temperature measurements, machine oil and honey were found to be more suitable to
get a steady back-wall echo train in the oscilloscope screen, respectively, for longitudinal and shear
waves. The peak amplitude of successive back-wall echoes from the glasses was used to determine the
attenuation coefficient using the relation,
-20
2(m - n) d
I
Ê
ˆ
Ê
m ˆ
˜
α =
log
,
(4)
Á
Ë
˜
¯
Á
Ë
¯
In
where Im and In are, respectively, the maximum amplitude (voltage) of the m-th and n-th pulse echoes.
The percentage error in the attenuation measurement is 2%.
2.4 Elastic constants
Elastic moduli such as longitudinal (L), shear (G), bulk (K), Young’s (Y) and Poisson’s ratio (σ), Debye
temperature (θD), acoustic temperature (Z), microhardness (H) have been obtained by employing the
measured density (ρ), longitudinal velocity (UL) and shear velocity (US) values using the standard rela-
tions as discussed elsewhere [18, 19].
2.5 Fourier transform infrared spectroscopy
Infrared absorption spectra of powdered glass samples were analyzed by FTIR studies. FTIR absorption
spectra were recorded at room temperature in the 400–4000 cm–1 range using a Shimadzu FTIR-8700
spectrometer. 4.0 mg of each tested sample was mixed with 200 mg of KBr in an agate mortar and then
pressed into pellets of 13 mm diameter. For each sample the spectrum represents an average of 20 scans,
which were normalized to the spectrum of the blank KBr pellet.
3 Results and discussions
The variation of density (ρ) and molar volume (Vm) as a function CaO content is shown in Fig. 1. Pure
borate glass (density = 1837 kg m–3, molar volume = 37.90 × 10–6 m3 mol–1 [20]) is mostly made up of a
random network of boroxyl units with boron in three-fold co-ordination (BO3). CaO enters the glass
network structure in the form of a network modifier [21] by producing fourfold co-ordinated boron (BO4)
by crosslinking the planar triangles and tightens and strengthens the network [22]. Density is a tool to
reveal the degree of change in structure with composition of glasses [9, 10]. The increase in density with
increase in CaO content can be attributed to an increase in BO4 units at the expense of the BO3 units that
have lower density. Furthermore, replacement of a low-density oxide, B2O3 (2.46 × 103 kg m–3) by a
high-density oxide, CaO (3.3 × 103 kg m–3) contributes to the increase in density. The decrease in molar
volume (Fig. 1) is due to the high field strength of the Ca2+ (0.33 × 1020 m–2 [23]). The change in slope in
the plots of density and molar volume at x = 0.5 can be explained as for x ≤ 0.5, the modifier (CaO) con-
verts BO3 units in to BO4 units, i.e. the fraction of N4 increases with the increase in CaO content and
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phys. stat. sol. (a) 203, No. 10 (2006)
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2900
2800
2700
2600
2500
2400
28
26
24
22
20
Density
Molar Volume
0.2
0.3
0.4
0.5
0.6
0.7
CaO Content (mole fraction)
Fig. 1 Variation of density and molar volume with calcium oxide content. The drawn lines are guides to
the eye.
reaches its maximum for Ca4 (x = 0.5). For x > 0.5, the added modifier causes the formation of nonbridg-
ing oxygen ions (NBOs) in borate matrix. This type of ion represents broken bonds in the network. Units
containing NBOs have greater volume than those without NBOs and the formation of such units appears
as a change in slope for density and molar volume plots. The compositional dependence of the measured
ultrasonic velocities and attenuation (both longitudinal and shear) are shown in Figs. 2 and 3, respec-
tively. The ultrasonic velocity increases at first with increase in added CaO content and attains its maxi-
mum value for Ca4 (x = 0.5) and with further increase in modifier content it decreases. This increase is
attributed to the contraction of network structure because of the conversion of BO3 units to BO4 units
with modifier content [24]. Furthermore, an increase in modifier content leads to an increase in ortho-
3800
UL
6800
US
3700
6600
3600
6400
3500
6200
3400
0.2
0.3
0.4
0.5
0.6
0.7
CaO Content (mole fraction)
Fig. 2 Variation of ultrasonic longitudinal velocity (UL) and shear velocity (US) with calcium oxide con-
tent. The drawn lines are guides to the eye.
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Manupriya et al.: Structural and acoustic investigations of calcium borate glasses
0.5
0.4
0.3
Longitudinal Atten.
Shear Atten.
0.44
0.40
0.36
0.32
0.2
0.3
0.4
0.5
0.6
0.7
CaO Content (mole fraction)
Fig. 3 Variation of attenuation (longitudinal and shear) with calcium oxide content. The drawn lines are
guides to the eye.
and metaborate units (NBOs), this in turn results in loose packing of atoms that causes a weakening of
the glass structure and hence, a decrease in velocity beyond x = 0.5. Attenuation of an ultrasonic wave
(Fig. 3), which is a measure of loss of ultrasonic energy, shows its minima for glass Ca4 (x = 0.5) con-
firming maximum N4 units for this structure and hence the most rigid structure. The variations of longi-
tudinal (L) and shear modulus (G) are shown in Fig. 4, both register increase initially with increased CaO
content, but for x ≥ 0.5 the longitudinal modulus attains a degree of constancy, whereas the shear
modulus shows a similar trend to that of the velocity. Young’s modulus sheds light on the physicochemi-
cal and mechanical properties of glasses [25]. The change in chemical bond and bond strength in glass
structures are normally incorporated in Young’s modulus. Both the Young’s and bulk moduli are shown
135
40
L
G
120
36
105
32
28
90
0.2
0.3
0.4
0.5
0.6
0.7
CaO Content (Mole fraction)
Fig. 4 Variation of longitudinal (L) and shear modulus (G) with calcium oxide content. The drawn lines
are guides to the eye.
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phys. stat. sol. (a) 203, No. 10 (2006)
2361
104
96
88
80
72
K
Y
80
72
64
56
0.2
0.3
0.4
0.5
0.6
0.7
CaO Content (Mole fraction)
Fig. 5 Variation of bulk (K) and Young’s (Y) modulus with calcium oxide content. The drawn lines are
guides to the eye.
in Fig. 5. The trend of Young’s modulus is similar to the trend of velocity, showing Ca4 as the most rigid
structure. The bulk modulus (Fig. 5) increases with increase in modifier content (CaO) and shows its
maxima for Ca6, which implies the increase in number of bonds per unit volume with increased modifier
content because of the high field strength of the cation that maintains connectivity to a large extent even
with presence of NBOs [26]. The Poisson’s ratio of a glass system depends on the crosslink density, a
network with higher crosslink density will have a lower Poisson ratio and vice versa [25]. In the present
study, the Poisson’s ratio (Table 1) remains almost constant (varies from 0.280 to 0.297) with modifier
12000000
Z
H
9.0
11000000
7.5
10000000
9000000
6.0
0.2
0.3
0.4
0.5
0.6
0.7
CaO Content (mole fraction)
Fig. 6 Variation of acoustic impedance (Z) and microhardness (H) with calcium oxide content. The
drawn lines are guides to the eye.
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Manupriya et al.: Structural and acoustic investigations of calcium borate glasses
580
560
540
520
0.2
0.3
0.4
0.5
0.6
0.7
CaO Content (mole fraction)
Fig. 7 Variation of Debye temperature (θD) with calcium oxide content. The drawn lines are guides to
the eye.
content changing from 0.2 to 0.7 mole fraction. For small values of the modifier the connectivity is at-
tributed to the N4 units present in the structure, with the increase in the modifier NBOs dominate but
connectivity is not compromised because of the nature of the cation (Ca2+) [26]. Hardness explores the
information about the elastic strength and surface behaviour of glass. Figure 6 shows that the trend of
microhardness, which is closely related to Young’s modulus, has similar dependences confirming that
the densification of structure is maximum for Ca5. The variation of acoustic impedance is also in accor-
dance with the variation of Young’s modulus, which supports the presence of NBOs after x > 0.5. The
Debye temperature (θD) represents the temperature where nearly all the modes of vibrations are excited
in the solid. The maximum value of Debye temperature (Fig. 7) for Ca4 and Ca5 suggests the most rigid
structure. The results of acoustic studies are further supported by FTIR analysis of the glass system.
Figure 8 shows the FTIR spectra for different xCaO · (1–x) B2O3 glasses (0.2 ≤ x ≥ 0.7). Borate glasses
show three active infrared spectral regions: 600–850 cm–1 is attributed to the bending vibrations of bo-
rate segments, 850–1200 cm–1 is attributed to the stretching vibration of structural groups containing
BO4 tetrahedra such as diborate, triborate, tetraborate and pentaborate groups [27–29]. Finally, the re-
gion between 1200 and 1500 cm–1 arises from the B–O stretching vibration of BO3 units such as metabo-
rate chains and rings, pyroborate and orthoborate groups [28–32]. The presence of the band centered at
~1037 cm–1 in the present studied system confirms the presence of diborate (BO4) groups [33, 34]. The
intensity of this band is maximum for Ca4 and this peak shifts to lower wave number, i.e. to ~990 cm–1
and ~925 cm–1 for Ca6. The peak at ~690 cm–1 is attributed to the vibrations of oxygen bridging the two
trigonal boron atoms. The sharpness of this peak also decreases with the increased content of the modi-
fier (CaO) and no such peak is observed for Ca6. The high intensity of high-frequency bands around
1353 cm–1 and 1396 cm–1 for high modifier content (x = 0.7) indicates the presence of the NBOs due to
the metaborate and pyroborate chains. The presence of the peak at ~925 cm–1 for Ca6 shows the appear-
ance of the orthoborate group in the structure of the borate glasses with higher CaO modifier (x = 0.7)
[35]. These results confirm that the calcium enters into the borate glass network as a modifier and occu-
pies open spaces to form BO4 units and for high calcium oxide content (x > 0.5) the structure is domi-
nated by orthoborate, metaborate and pyroborate units with an abundance of NBOs. In the present sys-
tem maximum N4 units are present for Ca4. This result is in good agreement with the studies of alkaline
borate glasses by Kamitsos [23].
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phys. stat. sol. (a) 203, No. 10 (2006)
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Ca6
Ca5
Ca4
Ca3
Ca2
Ca1
4000 3500 3000 2500 2000 1750 1500 1250 1000 750 500
1/cm
Fig. 8 FTIR spectra of xCaO · (1 – x) B2O3 (0.2 ≤ x ≥ 0.7) glass system.
4 Conclusions
The acoustic properties of calcium borate glasses in the xCaO · (1 – x) B2O3 glass system have been
studied with different CaO contents. The results of acoustic and density measurements are confirmed by
using FTIR spectroscopy. In view of current results we can conclude that the addition of CaO modifier in
the pure borate glass leads to the conversion of BO3 units to BO4 up to 0.5 mole fraction of CaO content
making Ca4 the most rigid structure. Further increase in modifier leads to the formation of NBOs domi-
nant units in the glass network. These NBOs dominant units lead to the softening/weakening of the struc-
ture as evident from the acoustic results, hence the acoustic measurements have potential applications in
studying the structure of glasses.
Acknowledgements The authors are grateful to Thiru A. Vairaprakasam, Correspondent and Dr. S. Balakrishnan,
Principal, Mepco Schlenk Engineering College for their constant encouragement in this collaborative work and also
to INMAS, New Delhi (VR, AVG & SA) for the financial support.
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