A.V.Gayathri Devi et al. / International Journal of Engineering Science and Technology
Vol. 2(6), 2010, 2483-2490
Ultrasonic characterisation of calcium
phosphate glasses and glass-ceramics with
addition of TiO2
A. V. GAYATHRI DEVI1, V. RAJENDRAN *1 AND N. RAJENDRAN2
1
Centre for Nano Science and Technology, K. S. Rangasamy College of Technology, Tiruchengode-637 215, Tamil
Nadu, India
2
Department of Chemistry, CEG Campus, Anna University Chennai, Chennai-600 025, Tamil Nadu, India
*
Corresponding author,e-mail: [email protected], fax:+91-4288-274880, Phone:+91-4288-274880,274741-4
Abstract
Calcium phosphate based glasses with different concentrations of TiO2 (0 to 2.5 mol%) were prepared and
their corresponding glass-ceramics were obtained by controlled heat treatments. The amorphous nature of glasses
and the presence of crystalline phases in glass-ceramics were studied through XRD studies. Density, molar volume,
ultrasonic velocities, attenuation, elastic constants and microhardness of glass and glass-ceramics were used to study
structural and mechanical properties. The results indicate that the added TiO2 increases the cross link density of
phosphate glasses and thus results in higher network stability. The glass-ceramics exhibits higher mechanical
strength when compared with its corresponding glasses.
Keywords: Glasses, Glass-ceramics, TiO2, Elastic properties, Mechanical properties
1. Introduction
Recently, the interest on different kinds of bioactive glasses1and glass-ceramics2-4 gained momentum due to
their inherent physical, chemical, mechanical and bioactive properties. Generally, phosphate based bioactive glasses
find wide applications in biomedical field due to their properties. The existence of higher solubility in aqueous
solution results in limitation of their long term applications. Several attempts have been made to increase the
chemical durability of glasses by changing its composition, addition of metal ions and subjecting them into different
thermal treatment conditions5,6. Addition of metal oxides plays a dominant role during the glass formation and
crystallisation process5. Generally, addition of metal ions creates ionic cross linking between non-bridging oxygens
(NBOs) of two different phosphate chains resulting in long-term stability7 and higher mechanical strength8. Among
the various modifying ions, titania is found to be more effective in improving the chemical stability and mechanical
properties of these glasses9.
Preparation of glass-ceramics by controlled heat treatment produces large amounts of calcium phosphate
(Ca-P) crystals. It is believed that precipitation of Ca-P/HAp is the best approach to obtain materials suitable for
bone replacement/regeneration applications. The objective of development of ceramics is to improve the mechanical
properties of the biomaterial with reduced Young’s modulus. The distinct advantages of glass-ceramics have high
microstructural uniformity, the absence of porosity and the minor changes in volume during the conversion of
glasses into glass-ceramics.
In order to use glass and glass-ceramics for particular clinical applications, one should explore the property
by knowing its structure. Several methods are used to explore the material properties either destructively or nondestructively. Ultrasonic non-destructive characterisation of materials is a versatile tool to investigate the change in
microstructure, deformation process and mechanical properties of materials10,11.
Phosphate based glasses especially P2O5-CaO-Na2O-TiO2, which is studied extensively in terms of its
mechanical properties12 degradation13 bioactivity and non-toxicity14. However, so far, to our knowledge, no work is
carried out to explore the structural properties of glasses along with its ceramics using Ultrasonic technique. In the
present investigation, the structural properties of phosphate glasses and glass-ceramics under the influence of TiO2
are explored through density, elastic moduli, and microhardness.
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2. Materials and methods
2.1 Preparation of bioactive glasses and glass-ceramics
The bioactive glasses of 47P2O5-30.5CaO-(22.5 - x)Na2O-xTiO2 system for different TiO2 contents namely
x = 0, 0.5, 1.0, 1.5, 2.0 and 2.5 mol% (hereafter termed as BT0, BT0.5, BT1.0, BT1.5, BT2.0 and BT2.5
respectively) were prepared employing normal melting and annealing technique15. Reagent grade (Aldrich)
ammonium dihydrogen phosphate (99.999%), Na2CO3 (99.9%), CaCO3 (99.995%) and TiO2 (99%) were used as
starting materials for preparing the glasses. The powder mixtures were heated at 573 K for 1 h in porcelain crucibles
to evaporate ammonia and water. The batches were melted in a Pt.2% Rh crucible. The melting was carried in an
electric furnace at 1473 K for 2 h. The melts were stirred for every 30 min to promote good homogeneity. The melts
were cast into the preheated stainless steel moulds and were transferred into a muffle furnace regulated at 623 K for
1 h and then left to cool to room temperature.
In order to determine the glass transition temperature (Tg) and suitable heat treatment temperatures,
differential thermal analysis (DTA) was carried out using a Thermal Analyser (Perkin Elmer Diamond, USA) under
a stream of high purity nitrogen atmosphere. The scanning temperature ranges from 301 to 1273 K. The values were
chosen from the obtained TG-DTA curve. On the basis of the DTA results, a crack-free glass-ceramics were
obtained by scheduled two–step heat treatments are given in Table 2. Each glass was heated slowly to the first
chosen temperature (T1) for the formation of nuclei sites and after holding for 5 h, it was then further heated to reach
the second chosen temperature (T2) for the further crystallisation process and hold it for 10 h. The sample was left
to cool inside the furnace to room temperature.
2.2 X-ray diffraction analysis
XRD was obtained from X-ray Diffractometer (Bruker AXS, Model: D8 Advance, UK) using CuK as a
radiation source at a scanning rate of 2per min. The XRD analysis was used to confirm the amorphous nature of
the glasses and the crystalline phases formed in the glass-ceramics.
2.3 Density and molar-volume measurements
By knowing the weight using a digital balance (Sartorius, Model: BP221S, USA) in air Wa, weight in buoyant
Wb and the density of buoyant ρb, the density of glass and glass-ceramics were measured employing the Archimedes
principle. i.e.,
ρ=
Wa
ρb ,
W aW b
(1)
In order to get more accurate value of density of glasses and glass-ceramics, the experiment was repeated at
least for five times. The accuracy of digital balance and density measurements are respectively ±0.0001 g and ±0.5
kg m-3. The percentage of error in measurement is ± 0.05. The molar volume Vm was calculated using the molecular
weight of glass and density as follows:
(2)
Vm = M /ρ,
2.4 Ultrasonics
The ultrasonic velocities and attenuation (longitudinal and shear) were measured at 5 MHz using the cross
correlation technique employing the pulse echo method. A high power ultrasonic Pulser Receiver (Olympus NDT,
5900 PR, USA) and a digital storage oscilloscope (DSO) (Lecroy, Wave Runner 104 MXi 1GHz, USA) were used
for recording ultrasonic (rf) signals. The precise transit time t was measured employing the cross-correlation
technique. The velocity and attenuation were calculated similar to our earlier studies16,17.
2.5 Elastic constants
Elastic moduli such as longitudinal (L), shear (G), bulk (K), Young’s (E) and Poisson’s ratio (),
microhardness (H) were obtained from the experimental values of density (ρ), longitudinal velocity(UL) and shear
velocity (US) as described elsewhere18.
3. Results and Discussion
The nominal glass compositions along with its sample code are given in Table 1.
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Vol. 2(6), 2010, 2483-2490
Table 1 Glass composition along with its sample code, density (), molar volume (Vm), longitudinal (UL) and shear
(US) velocity, longitudinal (L) and shear (S) attenuation, Poisson’s ratio () and microhardness (H)
Sample
code
Nominal glass composition
mol%
P2O5
CaO
Na2O
TiO2



x10-3
kg m-3
Ultrasonic
velocity
Attenuation
Vm
UL
US
L
S




H
BT0
47
30.5
22.5
0
2599
37.62
4947
2743
0.3688
0.2344
0.2780
2.8937
BT0.5
BT1.0
BT1.5
BT2.0
BT2.5
47
47
47
47
47
30.5
30.5
30.5
30.5
30.5
22.0
21.5
21.0
20.5
20.0
0.5
1.0
1.5
2.0
2.5
2596
2602
2607
2610
2618
37.69
37.63
37.59
37.58
37.49
4851
4892
4995
5042
5107
2672
2714
2799
2810
2853
0.3774
0.3302
0.3172
0.2745
0.2328
0.2458
0.2263
0.1906
0.1586
0.1125
0.2822
0.2777
0.2711
0.2747
0.2732
2.6908
2.8406
3.1163
3.0951
3.2225
The absence of any diffraction peaks in the XRD pattern in all glasses confirms its amorphous nature
(Figure not given). XRD pattern of heat treated glasses exhibits crystalline peaks along with the residual glassy
phase are given in Table 2.
Table 2 Glass-ceramics sample code along with the thermal treatment temperatures, crystal phases, density (),
molar volume (Vm), longitudinal (UL) and shear (US) velocity, longitudinal (L) and shear (S) attenuation,
Poisson’s ratio () and microhardness (H)
Sample
code
Heat
treatment
temperatures
K
T1
T2
AT0
680
953
AT0.5
690
963
AT1.0
AT1.5
AT2.0
AT2.5
695
700
705
710
968
973
978
983
Predominant crystal phases
NaPO3, -Ca2P2O7, ,TCP, Ca(PO3)2
NaPO3,  Ca2P2O7, TCP, Na5Ti(PO4)3, Ca(PO3)2
-Do-Do-Do-Do-




x10-3
kg m-3
Ultrasonic
velocity
Attenuation
UL
US
L
S


H
2536
5248
3010
0.6314
0.2372
0.2548
3.7575
2614
5966
3432
0.8494
0.3550
0.2527
5.0747
2633
2606
2623
2665
5992
4635
6012
7122
3480
2650
4026
4054
1.1804
0.6834
0.8782
0.9699
0.5605
0.3876
0.4043
0.5338
0.2456
0.2572
0.0935
0.2603
5.4086
2.9621
11.5237
6.9986
Density measurements are widely used to study the effects of composition on glass structure19,20. These
measurements are usually employed to control the homogeneity of glass, but the value of density itself is not a
useful structural parameter. On the contrary, the determination of molar volume from density data can provide
information on different aspects of the glass structure. It is found that an initial decrease in density when TiO2 is
introduced into the base glass. Further, a gradual increase in density with addition of TiO2 from 1.0 to 2.5 mol% is
obtained as given in Table 1.
As per our earlier discussion21, addition of metal oxide like TiO2 leads to breaking of the P-O-P bonds in
phosphate network results in the formation of terminal oxygens. Thus, the presence of titanium in phosphate
network acts as both a network former and modifier. As a result, an initial decrease in density with addition of TiO2
(0.5 mol%) is observed. Addition of TiO2 (> 0.5 mol%) results creation of ionic cross linking between non-bridging
oxygens of two different phosphate chains, thereby reinforcing the glass structure. A continuous increase in density
with increase in TiO2 concentration is due to the molecular weight of TiO2 being larger than Na2O22. Density of
glass-ceramics follows a different trend due to the presence of crystalline phases. Initially, it increases up to 1.0
mol% of TiO2 and then it shows a sudden decrease at 1.5 mol% and then increases gradually up to 2.5 mol% as
given in Table 2.
It is observed that Vm increases for the initial addition of TiO2 and it decreases monotonically with further
increase in TiO2 content23 as given in Table 1. The molar volume increases as a result of the creation of nonbridging oxygens (NBOs), which break the bonds of P-O-P linkages and thereby increases the spaces in the network.
When TiO2 is substituted for Na2O there is a continuous decrease in molar volume Vm and it can be explained by the
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Vol. 2(6), 2010, 2483-2490
formation of Ti-O bonds with a covalent nature more than that of Na-O bonds, which reticule the phosphate network
and leads to close structure of the glasses.
The measured values of the longitudinal and shear ultrasonic velocities and attenuation with variation of
TiO2 content are listed in Table 1. The observed sudden decrease in velocity and an increase in attenuation are due to
the weakening of glass structure. Addition of TiO2 in the phosphate glass system results in the splitting of P-O-P
bonds and it causes the conversion of bridging oxygens (BOs) into NBOs. The further addition of TiO2 leads to an
increase in longitudinal ultrasonic velocity from 4892 to 5107 m s-1 and shear velocity from 2714 to 2853 m s-1 are
noticed. At the same time, a decrease in attenuation is noticed for longitudinal (0.3302 to 0.2328) and shear waves
(0.2263 to 0.1125). The observed increase in velocities and a decrease in attenuation are attributed to the increase in
rigidity of glass network.
It is generally accepted that the controlled heat treatment of glasses above the glass transition temperature
(Tg) and crystallisation temperature (Tc) lead to the glasses into their corresponding polycrystalline glass-ceramics10.
The thermal treatment of phosphate glasses results in the release of stresses from the glass and the possible
formation of crystalline phases along with the residual glassy phases10. The observed increase in density and
velocities is mainly due to the densification during thermal treatments. Ultimately, the densification leads to more
loss of sound energy i.e., attenuation as shown in Table 2. The existence of crystalline phases in the glass-ceramics
also has an effect on attenuation measurements.
Elastic properties are very informative about the structure of solids and they are directly related to the inter
atomic potentials. Glasses being isotropic and have only two independent elastic constant: longitudinal and shear
elastic moduli. These two parameters are obtained from the longitudinal and shear velocities and density of the
glass. Elastic moduli of different silica and phosphate glasses have studied experimentally24-27. The knowledge on
the elastic property of glasses and glass-ceramics is very important to use as a potential candidate for particular bone
implant applications28. Attempt has been made to examine the relationships between the elastic and physicochemical
properties of cortical bone29. The elastic properties of materials are dependent on the orientation and location at
which they are evaluated. Young’s modulus of Cortical and Cancellous bone are reported as 3-30 GPa and 0.1-0.4
GPa30.
22.00
21.00
66.00
20.00
63.00
19.00
Shear modulus (GPa)
Longitudinal modulus (GPa)
69.00
L
G
18.00
60.00
0
0.5
1
1.5
2
2.5
Content of TiO2 (mol%)
3
Fig. 1. Variation of Longitudinal and Shear modulus of Glasses with different TiO2 content. ISSN: 0975-5462
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40
55
39
53
38
51
37
49
Young's modulus (GPa)
Bulk modulus (GPa)
A.V.Gayathri Devi et al. / International Journal of Engineering Science and Technology
Vol. 2(6), 2010, 2483-2490
K
Y
36
47
0
0.5
1
1.5
2
Content of TiO2 (mol%)
2.5
3
150
50
125
40
100
30
75
20
Shear modulus (GPa)
Longitudinal Modulus (GPa)
Fig. 2. Variation of Bulk and Young’s modulus of Glasses with different TiO2 content. The results indicate that the elastic moduli showed an anomalous with an initial addition of TiO2 and it
increases with further addition of TiO2 content as shown in Fig.1 and 2. In BT0 and BT2.5 glasses, the measured
longitudinal and shear moduli ranges respectively from 63.60 to 68.28 GPa and 19.56 to 21.31GPa. Similarly, the
Young’s and bulk moduli ranges from 49.98 to 54.26 GPa and 37.53 to 39.87 GPa for BT0 to BT2.5 glasses. The
increase in elastic moduli is due to an increase in the rigidity of glasses20. Amaral et al.24 reported that the Young’s
modulus of Bioglass 45S5 and Cerabone were 35 and 118 GPa.
L
G
50
10
0
0.5
1
1.5
2
Content of TiO2 (mol%)
2.5
3
Fig. 3. Variation of Longitudinal and Shear modulus of Glass-Ceramics with different TiO2 content. ISSN: 0975-5462
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Vol. 2(6), 2010, 2483-2490
140
70
115
60
90
50
65
40
K
Y
30
Young's Modulus (GPa)
Bulk Modulus (GPa)
80
40
0
0.5
1
1.5
2
Content of TiO2 (mol%)
2.5
3
Fig. 4. Variation of Bulk and Young’s modulus of Glass-Ceramics with different TiO2 content. The measured longitudinal, shear, Young’s and bulk modulus for glass-ceramics follow the same trend of
variation as that of density. In AT0 and AT2.5 glass-ceramics, longitudinal and shear moduli varies from 69.84 to
135.19 GPa, and 22.98 to 43.80 GPa respectively, while Young’s and bulk modulus varies from 57.68 to 110.42
GPa and 39.20 to 76.79 GPa respectively (Fig.3 and 4). The key requirement for the clinical success of bioceramics
is the matching of the mechanical behavior of the implant with the host tissue31. In most cases the goal is to increase
strain to failure and decrease the elastic modulus32. The bioactive ceramics with a too high Young’s modulus are
liable to induce bone resorption owing to the stress shielding which surrounds them33. Thus, it is expected to
fabricate bioactive ceramics with Young’s modulus analogous to that of human cortical bone.
Poisson’s ratio for glasses varied from 0.2780 to 0.2732 and it shows a highest value for BT0.5 (0.2822)
and a lowest for BT1.5 (0.2711). The decrease in Poisson’s ratio with increase in TiO2 content in glass network is
attributed to increase the crosslink density of glass network as proposed by Higazy and Bridge34. It is well known
that Poisson’s ratio is affected by the changes in the cross-link density of glass network. A high cross-link density
has Poisson’s ratio in order of 0.1 to 0.2, while a low cross-link density has Poisson’s ratio between 0.3 and 0.535,36.
The observed Poisson’s ratio for glass-ceramics ranges from 0.2548 to 0.2603. Poisson’s ratio for the cortical bone
at the initial load cycle is 0.2837. In the present investigation, the variation in Poisson’s ratio of glasses and glassceramics with change in TiO2 content is very negligible and is very close to that of cortical bone (Table 1 and 2).
Microhardness expresses the stress required to eliminate the free volume (deformation of the network) of the
glass. The increase in the microhardness indicates the increase in the rigidity of glass. Microhardness obtained from
the Poisson’s ratio and Young’s modulus show an increasing trend except for the glasses BT0.5 and BT2.0.
Microhardness for glass-ceramics shows a higher value than its glasses, it shows a minimum value for the glassceramic AT1.5 as shown in Tables 1 and 2.
4. Conclusion
Phosphate based glass and glass-ceramics with addition of TiO2 content are prepared respectively using
normal melt quenching and controlled heat treatments. The presence of TiO2 in calcium phosphate glass network
results in higher rigidity which is explored from the observed increase in glass density and decrease in molar
volume. Due to the higher bonding ability of TiO2, it results an increase in ultrasonic velocity and decrease in
attenuation. The elastic constants results support the above observation.
The thermal treatment of phosphate glasses results in the release of stresses from the glass and the possible
formation of crystalline phases along with the residual glassy phases. It is interesting to note a higher magnitude in
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Vol. 2(6), 2010, 2483-2490
the density, velocity and attenuation compared with its glasses. Elastic modulus and Microhardness show a
remarkable increase in its magnitude which is required higher for implant applications.
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