Optical and thermal properties of binary calcium phosphate and barium phosphate glasses

Optical Materials 28 (2006) 200–206 www.elsevier.com/locate/optmat

Optical and thermal properties of binary calcium phosphate and barium phosphate glasses E.T.Y. Lee *, E.R.M. Taylor Optoelectronics Research Centre, University of Southampton, Highfield, Southampton SO17 1BJ, UK Received 17 March 2004; accepted 20 December 2004 Available online 8 February 2005

Abstract Binary calcium phosphate and barium phosphate glasses corresponding to xCaO–(100
x)P2O5 and xBaO–(100
x)P2O5, respectively, have been prepared in the range of 20 6 x 6 50. Assessment of the optical and thermal properties reveal that refractive index (n) and glass transition temperature (Tg) show a minima while thermal expansion coefficient (a) and thermo-optic coefficient (dn/dT) change monotonically as the amount of modifying oxides, CaO and BaO, increase. a > 9 · 10
6/C is required for the calcium phosphate and barium phosphate glasses to exhibit negative dn/dT.  2005 Elsevier B.V. All rights reserved. PACS: 78.20.Nv; 65.60.+a; 61.43.Fs; 42.70.Ce Keywords: Thermal expansion coefficient; Thermo-optic coefficient; Calcium phosphate; Barium phosphate

1. Introduction Over the years, different phosphate glass systems have been developed for numerous applications, by specially tailoring the properties of the glasses such as the glass transition temperature and the thermal expansion coefficient. In addition, the thermo-optic coefficient of phosphates has also been utilised for a number of specialty applications. Certain phosphate compositions containing rare earth oxides are used as host materials for high-power laser applications because they possess high-stimulated emission cross-sections and low thermooptic coefficients [1,2]. Another application is the codoping of germanosilicate core fibre with P2O5 for the athermalisation of long period fibre gratings [3]. This works by making use of the negative thermo-optic coef*

Corresponding author. Tel.: +44 023 8059 4530; fax: +44 023 8059 3149. E-mail address: [email protected] (E.T.Y. Lee). 0925-3467/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2004.12.010

ficient of P2O5 [4] to minimise the temperature dependence of the resonance wavelength. The present paper seeks to assess the thermo-optic coefficient of binary calcium phosphate (designated as CaP) and barium phosphate (designated as BaP) glasses to determine their potential for athermalisation work. The paper also reports other optical and thermal properties, such as the refractive index and the thermal expansion coefficient, of the glasses and examines the compositional effects on these properties. The glasses reported here are part of a series of glasses developed for the characterisation of their dn/dT to determine their potential application in athermal planar devices such as Bragg gratings. We have chosen to investigate the suitability of calcium phosphate and barium phosphate glasses due to the fact that these phosphate glasses exhibit negative dn/dT. Table 1 compares the dn/dT data of some metaphosphate glasses [4]. Binary alkali phosphate glasses were not considered because their poor chemical

E.T.Y. Lee, E.R.M. Taylor / Optical Materials 28 (2006) 200–206 Table 1 Thermo-optic coefficient (dn/dT) of some metaphosphate glasses Glass

dn/dT (· 10
6/C)

Al(PO3)3 Mg(PO3)2 Ca(PO3)2 Ba(PO3)2 NaPO3 KPO3 P2O5

+5.0 +2.9
2.5
10.6
15.0
23.3
92.2

durability limit their practical uses, although they also exhibit negative dn/dT. To our knowledge, the results for the calcium phosphate glasses are the first reported data. For the barium phosphate glasses, we have extended the reported data down to 20BaO–80P2O5 composition.

2. Experimental 2.1. Glass preparation Different compositions of binary calcium phosphate (CaP) and barium phosphate (BaP) glasses corresponding to xCaO–(100
x)P2O5 and xBaO–(100
x)P2O5, respectively, were synthesised in the range 20 6 x 6 50. Batches of different compositions were prepared from high-purity powder of CaCO3 (99.95%), BaCO3 (99. 95%) and P2O5 (99.99%), supplied by Alfa Aesar. The reaction mixtures containing the raw materials were melted in silica crucibles, covered with a platinum lid to minimise material loss during the melting process. Melting was carried out in oxygen atmosphere for 60 min at temperatures between 1200 C and 1400 C, depending on the glass compositions. The glass melts were then poured into a graphite mould and annealed for 60 min at the respective glass transition temperature, then slowly cooled at a rate of 0.5 C/min to room temperature. Energy dispersive analysis of X-ray (EDAX) was carried out to determine the compositions of a selection of glasses. Analysis was carried out on three separate spots on each sample.

201

gen atmosphere at a rate of 10 C/min using 20–30 mg glass samples. Experimental error is estimated to be within ±5 C. Thermal expansion coefficient, a, was measured using a thermomechanical analyser (Perkin Elmer TMA7) at a heating rate of 5 C/min in the range of 50–120 C with an uncertainty of ±0.2 · 10
6/C. Samples of approximately 10 mm thick were polished using three different solutions containing 9 lm alumina powder, 3 lm alumina powder and 0.125 lm silica particles respectively, for the determination of the thermooptic coefficient, dn/dT. dn/dT was determined using an interferometer, as shown schematically in Fig. 1. The sample was held between two aluminium plates within the furnace. The back plate was fixed in place while the front plate was spring-loaded to prevent strain due to sample expansion. Measurements were carried out in the temperature range of 20–100 C at a rate of 40 C/h. An interference fringe pattern due to the reflected light from the front and back faces of the sample was obtained and used to calculate dn/dT using the following equation [5,6]:   dn k ¼
na ð1Þ dT 2lDT where k is the laser wavelength (k = 632.8 nm), l is the sample thickness, n is the refractive index, a is the thermal expansion coefficient and DT is the temperature difference between successive maxima or minima of the interference pattern. An average dn/dT value was taken and reported, as the DT parameter did not vary significantly over the temperature range of 20–100 C. The contributions to the errors in dn/dT come from a combination of uncertainties in the measurement process, refractive index and thermal expansion coefficient. Rectangular polished glass samples 2 mm thick were used for infrared measurement of relative OH concentrations using an FTIR spectrophotometer (Perkin Elmer System 2000). The spectra were recorded in the transmission mode over a range of 2000–6000 cm
1. Fig. 2 shows the typical FTIR transmission spectra for Beamsplitter

Furnace

He-Ne Laser

2.2. Property measurements Lens Detector

Sample holder

Computer T

ADC

Sample thermocouple

Fig. 1. Interferometer set-up to measure dn/dT.

Sample

The refractive index (n) of the glasses was measured at room temperature using an Abbe
60 refractometer, at the sodium D-line (k = 589.3 nm). Measurements are accurate to within ±0.0005, due to equipment limitation. The glass transition, Tg, the onset of crystallisation, Tx, and the peak crystallisation, Tp, temperatures were determined using a differential thermal analyser (Perkin Elmer DTA7). Measurements were carried out in nitro-

202

E.T.Y. Lee, E.R.M. Taylor / Optical Materials 28 (2006) 200–206 1.59

100

40

Ba

Transmission (%)

60

Ca

Refractive index,n

80

1.57 1.56

*

1.55

* Expected MTO > CNMe region for CaP

1.54 1.53

20

Expected MTO < CNMe region for BaP

Expected MTO > CNMe region for BaP

1.58

1.52 5500

5000

4500

4000

3500

3000

10

0 2500

20

30

CaP

(cm-1)

Fig. 2. Infrared transmission spectra of 20MeO–80P2O5 glasses for OH concentration measurement. The spectra are normalised for a 1 mm thick sample and corrected for Fresnel loss.

the CaP and BaP glasses. The relative OH concentration can be expressed as the absorption coefficient, rOH [7]   
ln TT 3000 5000 ð2Þ rOH ¼ l where T3000 and T5000 are the % transmission at 3000 cm
1 and 5000 cm
1 wave numbers respectively, and l is the sample thickness. 3. Results The compositions and respective optical and thermal properties of the CaP and BaP glasses studied are shown in Table 2. The samples were visually checked and found to be homogeneous and transparent. EDAX analysis carried out on a selection of glasses showed that the analysed compositions were within ±3 mol% of the nominal compositions and the level of silica contamination from the crucible was approximately 1 mol%. Fig. 3 shows the refractive index of the CaP and BaP glasses as a function of the mol% of the modifying cations (MeO where Me = Ca, Ba). The results show that the refractive index of the CaP glasses goes through a

40

50

†

BaP

‡

Ref [8] BaP data

Fig. 3. Refractive index (n) of CaP and BaP glasses. *Contains relatively higher OH content, , measured at k = 589.3 nm.

minimum point before increasing with increasing CaO mol% while that of the BaP glasses increases monotonically with BaO mol%. Similar trend is observed for the glass transition temperature of the glasses, as shown in Fig. 4. However, the minimum point observed for the CaP glasses is more pronounced and at a slightly higher CaO mol% while a minimum point can be observed at around 25 mol% BaO for the BaP glasses. The MTO > CNMe and MTO < CNMe regions in Figs. 3 and 4 will be explained later in Section 4 (Discussion). The compositional effects on the thermal expansion coefficient and the thermo-optic coefficient with modifying cation mol% of the glasses are depicted in Fig. 5 (CaP) and Figs. 6 and 7 (BaP). The results show that as the modifying cation amount increases, a increases while dn/dT progresses towards negative values. In Figs. 3–7, the lines are drawn as a guide to the eye. Available n, a and dn/dT data from Ref. [8] for barium phosphate glasses have been included for comparison with our work. The conditions of their measurements are different from ours and are listed in the respective figures and there was no mention of the OH content in their glasses. A literature search reveals no available data for calcium phosphate glasses.

Table 2 Properties of CaP and BaP glasses Composition (mol%) Sample

CaO

P2O5

CaP-1a CaP-1b CaP-1c CaP-1d

50 40 30 20

50 60 70 80

Sample

BaO

P2O5

BaP-1a BaP-1b BaP-1c BaP-1d BaP-1e

50 45 40 30 20

50 55 60 70 80

60

MeO (mol%) †

Wavenumber

Expected MTO < CNMe region for CaP

a (· 10
6/C)

dn/dT (· 10
6/C)

rOH (cm
1)

n

Tg (C)

Tx (C)

1.551 1.535 1.526 1.527

547 510 504 533

719 734 781 811

799 788 841 875

10.22 9.39 9.10 8.60


2.98
0.92
0.39 +0.36

8.08 12.86 15.27 14.28

1.585 1.575 1.569 1.556 1.547

478 458 452 421 419

651 637 645 674 704

699 673 694 766 >780

13.67 13.35 13.07 12.38 11.62


10.24
9.14
8.19
7.06
5.76

14.04 13.25 14.73 21.92 22.26

Tp (C)

550 525 500

Expected Expected MTO > CNMe MTO < CNMe region for BaP region for BaP

475 450 425

*

*

13

20

30

40

50

* 12

*

400 10

203

14

-6

Expected Expected MTO > CNMe region MTO < CNMe region for CaP for CaP

(×10 /˚C)

575

Thermal expansion coefficient,α ,

Glass transition temperature,Tg, (˚C)

E.T.Y. Lee, E.R.M. Taylor / Optical Materials 28 (2006) 200–206

11

60

10

20

30

MeO (mol%) CaP

40

50

60

BaO (mol%)

BaP

†

Measured data

Fig. 4. Glass transition temperature (Tg) of CaP and BaP glasses. *Contains relatively higher OH content.

‡

Ref [8] data

Fig. 6. Thermal expansion coefficient (a) of BaP glasses. *Contains relatively higher OH content, a measured over 50–120 C, a measured over 20–120 C.

12 -5

4

0

10

20

30

40

50

60

dn/dT -4

CaO (mol%)

*

-6

(×10-6/˚C)

8

Thermo-optic coefficient,dn/dT,

α or dn/dT (×10-6/˚C)

α

-7

*

-8 -9 -10 -11 10

20

30

40

†

Fig. 5. Thermal expansion coefficient (a) and thermo-optic coefficient (dn/dT) of CaP glasses.

The relative OH content of the glasses are given in Table 2. Generally, all the glasses have similar OH content relatively, despite different amount of modifying cations, except for samples BaP-1d and BaP-1e, which have slightly higher OH content. P2O5 is known to be hygroscopic [9] and the higher P2O5 content in these two glasses compared to the other samples contribute to the higher relative OH content.

4. Discussion Ultraphosphate glasses generally contain a considerable amount of water due to the hygroscopicity and volatility of P2O5 [9]. The presence of water results in the conversion of bridging oxygens to non-bridging oxygens through the formation of P–OH bonds, thus depolymerising the phosphate network. This changes the ultraphosphate network from that expected by the nominal alkali-to-phosphorus ratio [10]. Hence the presence of P–OH bonds must be considered when describing the

50

60

BaO (mol%) Measured data

Ref [8] data

‡

Fig. 7. Thermo-optic coefficient (dn/dT) of BaP glasses. *Contains relatively higher OH content, dn/dT measured over 20–100 C at k = 632.8 nm, dn/dT measured over 20–120 C at k = 508 nm.

structures and properties, such as the density and the glass transition temperature, of such glasses. The CaP and BaP glasses prepared in this work generally have similar OH content, even as the amount of modifying cations changes. This is possibly due to the long melting time and sealed crucible used within this work. An increased melting time reduces the OH content due to evaporation until equilibrium is achieved [11]. Thus our results show that the OH content does not affect the trend in the measured values of n, Tg, a and dn/dT. However samples BaP-1d and BaP-1e, corresponding to the compositions 30BaO–70P2O5 and 20BaO–80P2O5, respectively, contain slightly higher OH content and this will have an effect on the properties of these two glasses compared to the others. CaP glasses are generally more resistant to hydrolysis than BaP glasses because of the higher field strength, defined as the ratio of the ion valence to the square of the bond distance between the ion and oxygen, z/a2 [12], of the Ca2+

204

E.T.Y. Lee, E.R.M. Taylor / Optical Materials 28 (2006) 200–206

cation. Fig. 2 shows the IR transmission spectra through 20MeO–80P2O5 (Me = Ca, Ba) glass samples, clearly showing the different OH content in these glasses. The IR edge shown in an overtone of the asymmetric (PO2) vibration mode [13]. The refractive indices of the CaP and BaP glasses are shown in Fig. 3. For the CaP glasses, the refractive index decreases upon the initial addition of CaO and then increases with further CaO additions. For the BaP glasses, the refractive index increases monotonically with the addition of BaO. Comparison of our results for BaP glasses with Ref. [8] data shows good agreement in the range of 45–50 mol% BaO. At this point, it is difficult to explain the deviation between the two sets of data at lower BaO content because of the lack of other information from Ref. [8], such as the melting and annealing conditions, the OH content and the temperature at which the measurements were taken. The results for the glass transition temperatures of these glasses (Fig. 4) also follow similar fashions as that observed for n. There is no Tg data available from Ref. [8]. The minimum points observed in the n and Tg results for the CaP glasses have also been reported in the literature for Li and Na ultraphosphate density [14,15], Ca and Ba ultraphosphate packing density [16] and Na and Cs ultraphosphate Tg [17]. Such behaviour can be explained in terms of the structural model proposed by Hoppe [18], which describes the roles of the modifier coordination (with coordination number, CNMe) and the number of terminal oxygens (MTO) available to coordinate the modifier ions in determining the ultraphosphate glass structure. By assuming that all terminal oxygens participate in coordinating the modifier cations, the number of available terminal oxygens per modifying cation, MTO, for alkaline earth ultraphosphates having the stoichiometry xMeO–(100
x)P2O5 is [18]   200 M TO ¼ ð3Þ x Hoppes model suggests that there will be two structurally unique ultraphosphate networks, depending on the glass composition, as shown schematically in Fig. 8. In the range when MTO > CNMe, the number of available terminal oxygens exceed the coordination requirement of each Me2+ cation and so these cations can exist at isolated sites within the ultraphosphate network (Fig. 8(a)). However, when MTO < CNMe, the number of terminal oxygens are not adequate to coordinate with every single Me2+ cation and hence these cations must begin sharing the available terminal oxygens, and in the process, creating clusters of Me2+ cations and bridging neighbouring phosphate polyhedra (Fig. 8(b)). In the region of MTO > CNMe, density decreases as the phosphate network expands to accommodate the additional Me2+ cations, subsequently leading to a de-

O

O O

O

P

P

P O O-

O

Me O P O

O

O

P O

O

O P O

(a)

O

O-

O-

O O

O

P

P

O

OMe

O-

O

OMe

Me

P O-

O

O

P

P

O -

O-

O P

O

O-

O-

O-

O O

O-

O

P OO

(b)

Fig. 8. Ultraphosphate structural network when (a) MTO > CNMe and (b) MTO < CNMe.

crease in n (as per Fig. 3 for CaO < 25 mol%). The addition of the cations also depolymerises the phosphate network and reduces the structural cross-link density, causing a decrease in Tg (as per Fig. 4 for CaO < 33 mol%). In the region of MTO < CNMe, the formation of modifier sub-structure by sharing the terminal oxygens creates a more compact network and therefore increases the density and refractive index of the glass (as per Fig. 3 for CaO > 25 mol%). By sharing the terminal oxygens, the modifying cations also cross-link neighbouring phosphate polyhedra, thereby strengthening the glass network and increasing the glass Tg (as per Fig. 4 for CaO > 33 mol%). The structural transition from a network with isolated modifier sites to a network with modifier substructure can be predicted based on the coordination number of the cation. Setting MTO = CNMe (of the cation) and based on Eq. (3), the transition for Ca (CNCa  6 [18]) occurs at x = 33 mol% and the transition for Ba (CNBa  8 [18]) occurs at x = 25 mol%. These numbers coincide with the position of the minima observed for the Tg of the CaP and BaP glasses within this work. However, there is no apparent minimum point observed for the BaP glasses in the n results, although such effect has been reported in the literature [16,18]. This can be attributed to the relatively higher water content in the glasses with low modifier content, which will affect the properties of the glasses. The presence of water reduces the number of available terminal oxygens through the formation of P–OH bonds and this effectively extends the MTO < CNMe region to lower modifying cation mol% than expected theoretically or even eliminates the existence of the MTO > CNMe region. Hence the cations begin to share the available terminal oxygens the moment they are added, increasing the density and refractive index of the glasses. It has been reported that anhydrous sodium ultraphosphates exhibits a minimum in density at around 20 mol% Na2O [15], which is absent in the sodium ultraphosphate glasses prepared in open crucibles [19,20]. This confirms the water effect on the refractive index of the BaP glasses in this work. The absence of a minimum point in the n

E.T.Y. Lee, E.R.M. Taylor / Optical Materials 28 (2006) 200–206

where A = [(n2
1)(n2 + 2)/6n], Pe is the electronic polarisability and a is the thermal expansion coefficient. For a glass to exhibit negative dn/dT, it must have an a that is large enough so that 3a can overcome the effect of d(ln Pe)/dT. As shown in Figs. 5–7, when the modifying cation mol% increases, a increases, and correspondingly dn/dT decreases. There is a slight difference in our results for the BaP glasses compared to the data from Ref. [8], due to the different temperature ranges and wavelengths at which the measurements were taken and possibly the thermal history of the glasses. Fig. 9 shows the linear dependence of dn/dT on a for the CaP and BaP glasses. This linear dependence is not obvious from Eq. (4) as we do not have the measurement for d(ln Pe)/dT. However, the results show that we can deduce the dn/dT, from the semi-empirical equations defined, of any calcium phosphate glasses within the compositional range of 1.526 6 n 6 1.551 and 8.60 · 10
6/C 6 a 6 10.22 · 10
6/C and any barium phosphate glasses within the compositional range of 1.547 6 n 6 1.585 and 11.62 · 10
6/C 6 a 6 13.67 · 10
6/C. Fig. 9 also shows that in order to obtain negative dn/dT, a > 9 · 10
6/C is required for the CaP and BaP (from extrapolation) glasses. By extrapolating the results for the BaP glasses in Fig. 9, we can see that both sets of data fall on the same line. This observation implies that d(ln Pe)/dT is dominated by the phosphate matrix, irrespectively of the modifiers. As polarisability is increased by an increase in the inter-ionic distance due to a change in tempera-

2

(CaP) dn/dT = -2.088α + 18.50

0

-6

dn/dT (×10 /˚C)

data for BaP glasses and the shift of the minimum point observed at 33 mol% CaO (Tg) to 25 mol% CaO (n) suggest that the glass Tg is less sensitive to changes in the glass water content. Fig. 5 shows the a and the dn/dT of the CaP glasses while Figs. 6 and 7 show the a and dn/dT, respectively, of the BaP glasses. There is no evidence of a maximum or minimum point for a and dn/dT throughout the whole range of glasses examined, in contrast to the minimum point observed for Tg when the glass undergoes a structural network change. The results show that a of the CaP and BaP glasses increases monotonically with increasing modifier content. As discussed earlier, the addition of modifying cations reduces the cross-linking of the phosphate network, leading to an increase in the a of the glasses. The results also show that dn/dT of the glasses progresses to negative values as the modifier content increases. Such behaviour can be explained in terms of the glass thermal expansion. The link between a and dn/dT can be established by the equation for dn/dT as derived from the Lorentz–Lorenz equation [21], using the approximation of (d(lnV)/dT  3a), as shown in Eq. (4).   dn dðln P e Þ ¼A
3a ð4Þ dT dT

205

-2 -4 -6 -8

dn/dT = -2.104α + 18.90 (BaP)

-10 -12 8

10

12

α

14

(×10-6/˚C) CaP

BaP

Fig. 9. dn/dT of CaP and BaP glasses as a function of a. The lines are drawn as a guide to the eye.

ture [22], d(ln Pe)/dT can be further expressed as [o(ln Pe)/o(ln r)]T Æ [d(ln r)/dT] or ca where c is the change in the electronic polarising power of the cation due to a change in the inter-ionic distance, r is the cation–oxygen distance and a is the thermal expansion coefficient. Izumitani et al. [22] found that c increases as the ionic field strength, z/a2 increases. This implies that a cation with high-polarising power strongly attracts and expands the electron cloud of the oxygen ion, even with a small change in the inter-ionic distance, compared to a cation with low field strength. As the field strength decreases in the order of P5+ (2.1) > Ca2+ (0.35) > Ba2+ (0.26) [4], we would expect c, and subsequently d(ln Pe)/dT, to be dominated by the phosphate matrix.

5. Conclusions The optical and thermal properties have been measured for a range of binary calcium phosphate and barium phosphate glasses. The glass transition temperatures of the glasses exhibit a minimum point and such behaviour can be explained by the transition of the ultraphosphate network from a network containing modifiers at isolated sites to a network with modifier sub-structure sharing terminal oxygens. The difference in the behaviour of the refractive indices of the glasses can be attributed to the depolymerising effect of water on the phosphate network. The structural network transition, however, does not seem to affect the thermal expansion coefficients and thermo-optic coefficients of the glasses, where the systematic addition of modifiers leads to the increase of thermal expansion and the subsequent progress towards negative thermo-optic coefficient values. dn/dT is strongly dependent on a and a value of a > 9 · 10
6/C is required to obtain negative dn/dT for the calcium phosphate and barium phosphate glasses.

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Acknowledgement This research was funded by EPSRC under the EPSRC/PHOTON/GR/M98876 project
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