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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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A.V.Gayathri Devi et al. / International Journal of Engineering Science and Technology Vol. 2(6), 2010, 2483-2490 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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A.V.Gayathri Devi et al. / International Journal of Engineering Science and Technology 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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A.V.Gayathri Devi et al. / International Journal of Engineering Science and Technology 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.  

   

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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.  

   

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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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