Differential scanning calorimetry studies on V2O5–CaO–P2O5 glasses

Materials Chemistry and Physics 82 (2003) 887–891

Differential scanning calorimetry studies on V2 O5–CaO–P2 O5 glasses B. Indrajit Sharma, A. Srinivasan∗ Physics Department, Indian Institute of Technology Guwahati, Guwahati-781039, India Received 10 December 2002; received in revised form 14 June 2003; accepted 3 August 2003

Abstract The non-isothermal behavior of xV2 O5 ·40CaO·(60−x)P2 O5 (10 ≤ x ≤ 30) glasses has been studied using a differential scanning calorimeter. The variation in the glass transition temperature for different compositions has been interpreted in terms of the variation in the molar volume of oxygen ion in the glasses. The Kissinger method was used for the calculation of the activation energy for the glass transition. The activation energy decreases with an increase in V2 O5 content. The variation of the excess heat capacity at glass transition with V2 O5 content has been interpreted in terms of the variation in the fragility of the glasses. © 2003 Elsevier B.V. All rights reserved. Keywords: Differential scanning calorimeter; Glasses; Specific heat capacity; V2 O5 –CaO–P2 O5 glasses

1. Introduction Although the main interest in vanadate glasses stems from their novel electrical properties [1,2], recent reports on the possibility of their application as oxygen gas sensors [3] and optical devices [4] have generated a keen interest in the thermal stability of these glasses. Non-isothermal properties, such as those obtained from a differential scanning calorimeter study, provide a lot of insight on the thermal stability of the glass, the chemical bonding in the glass and the nature of the glassy network structure [5]. The molar volume of oxygen ion, Vo∗ , provides a simple means [6,7] to correlate the changes occurring in the structure of the glass with its macroscopic properties. Vo∗ has been used [8–10] to interpret the variation of the glass transition temperature, crystallization temperature and the thermal stability of oxide glasses as a function of composition. According to Angell [11], the excess heat capacity at glass transition could be interpreted in terms of fragility of the glassy network. This scheme has been effectively applied to many glassy systems [12]. Several methods have been proposed to obtain the activation energy of glass transition and crystallization from differential scanning calorimetry (DSC) experiments [13]. These methods are based on the assumption that the reaction temperature of a kinetically driven transformation shifts when the sample is heated at different ∗ Corresponding author. Tel.: +91-361-269-0968; fax: +91-361-269-0762. E-mail address: [email protected] (A. Srinivasan).

0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2003.08.002

constant heating rates. xV2 O5 ·40CaO·(60−x)P2 O5 glasses exhibit a change in majority charge carrier type from p-type to n-type at 20 mol% of V2 O5 [1]. There is no report of a systematic study of the non-isothermal behavior of these glasses in the literature. In this paper, a detailed study of the non-isothermal behavior of xV2 O5 ·40CaO·(60−x)P2 O5 glasses has been performed using a differential scanning calorimeter.

2. Experimental Glass samples with composition xV2 O5 ·40CaO·(60−x) P2 O5 (10 ≤ x ≤ 30) were prepared by the melt quenching technique. Appropriate amounts of high purity V2 O5 , CaCO3 and P2 O5 were melted in an alumina crucible. The melted charge was subsequently press quenched between two copper plates. The preparative conditions such as the weight of the charge, the quenching temperature (1473 K) and the temperature of the copper plates were uniformly maintained in order to ensure that all the glass samples were prepared under the same conditions. The amorphous nature of the as-quenched samples was verified using the X-ray diffraction technique. The concentration of V4+ and V5+ ions in the as-quenched glass samples was determined by iodometric titration. The ratio of high valence to low valence vanadium ion (V5+ /V4+ ) and the density of the glass samples are listed in Table 1. The glass density was measured using Archimedes’ principle with xylene as the immersion fluid. A DSC based on

888

B. Indrajit Sharma, A. Srinivasan / Materials Chemistry and Physics 82 (2003) 887–891

Table 1 Reduced vanadium ion ratio, cV = V4+ /(V4+ + V5+ ), glass density molar volume of oxygen ion Vo∗ , glass transition temperature Tg at a constant heating rate of 10 K min−1 of V2 O5 –CaO–P2 O5 glassesa Composition (V2 O5 :CaO:P2 O5 , mol%)

cV

10:40:50 15:40:45 20:40:40 25:40:35 30:40:30

0.74 0.58 0.49 0.28 0.26

a

(0.77) (–) (0.5) (–) (0.25)

Density (g cm−3 )

Vo∗ (cm3 mol−1 )

Tg (◦ C)

2.76 2.84 2.91 2.98 3.05

12.02 11.92 11.86 11.74 11.68

194 178 174 164 156

(2.799) (–) (2.963) (–) (3.052)

Values reported by Kennedy and Mackenzie [1] are given in brackets. Error in the measurement of Tg is ±2 ◦ C.

the power compensation technique (Perkin-Elmer DSC 7) was employed for recording the non-isothermal behavior of the glass samples. The instrument was calibrated using the melting point of high purity indium and zinc. All the DSC experiments were performed with ≈20 mg as-quenched glass samples taken in crimped aluminum pans under continuous nitrogen purging. The DSC curves were recorded in the temperature range of 50–500 ◦ C for different heating rates ranging from 10 to 30 K min−1 . The glass transition temperatures (Tg ) reported in this paper correspond to the on-set of the glass transition and is the averaged value of two or more independent DSC runs performed at the same heating rate. The specific heat capacity at constant pressure (Cp ) of the glass samples was determined by the method of ratios proposed by O’Neill [14]. In this method, three separate DSC runs were performed over the desired temperature range at a constant heating rate (say, 10 K min−1 ). In the first DSC run, the base line corresponding to temperature region of interest was obtained. After this, two independent DSC runs were performed under identical conditions, one with the weighed quantity of the standard reference (␣-Al2 O3 ) and the other with weighed quantity of the glass sample. The specific heat at constant pressure Cp of the sample was then determined using the relation [14]: Cp =

Cp1 m1 y my1

the temperature range of 150–200 ◦ C. The Tg values of these glasses decreased with an increase in V2 O5 content (Table 1). No sharp crystallization peaks were observed in this temperature regime. The empirical relation for the molar volume of oxygen ions in the V2 O5 –CaO–P2 O5 glass system could be expressed as Vo∗ =

[M(V2 O5 ) − 16cV ]x + M(P2 O5 )y + M(CaO)z ρ[(5 − cV )x + 5y + z]

(2)

where ρ is the glass density, cV = V4+ /Vtotal (where Vtotal = V4+ + V5+ ) is the ratio of the amount of reduced transition metal ion to that of total transition metal ion. x, y, z and M(V2 O5 ), M(P2 O5 ), M(CaO) are the molar fraction and molecular weight of V2 O5 , P2 O5 and CaO, respectively. The Vo∗ values calculated from relation (2) are listed in Table 1. Vo∗ decreased from 12.02 to 11.68 cm3 mol−1 as the V2 O5 content was increased from 10 to 30 mol%. Tg of the glasses increased with an increase in Vo∗ (and P2 O5 content). The increase in Tg with an increase in the value of Vo∗ as shown in Fig. 2 indicates that the glass structure turns more loosely packed with an increase in P2 O5 content. A similar trend in Tg has been observed in other loosely packed

(1)

where Cp1 , m1 , y1 are the specific heat at constant pressure, the weight and the ordinate displacement due to the reference material, respectively, and y the ordinate displacement due to the sample. Specific heat capacity data of ␣-Al2 O3 in the temperature range of interest was taken from the literature [15]. Although the uncertainty in the Cp data obtained by this method is rather high (typically about 4%), this method is still popular due to its relative simplicity and ease of experimentation.

3. Results and discussion Fig. 1 shows the DSC curves of xV2 O5 ·40CaO·(60−x) P2 O5 glasses recorded under a constant heating rate of 10 K min−1 . All glass compositions exhibited a glass transition marked by a distinct endothermic baseline shift within

Fig. 1. DSC curves for xV2 O5 ·40CaO·(60−x)P2 O5 (10 ≤ x ≤ 30) glasses recorded at a constant heating rate of 10 K min−1 .

B. Indrajit Sharma, A. Srinivasan / Materials Chemistry and Physics 82 (2003) 887–891

-8.8

12.0

470

11.8

x=30

11.6

T g(K)

-9.2 2

450 440

11.4 10

15

20

25

-9.4 -9.6

430

-9.8

420

-10.0

30 2.10

V2O5 (mol %)

EK R

2.20

2.25

2.30

2.35

1000/T g(K )

(3)

where ϕ is the heating rate, EK the activation energy obtained from the Kissinger plot and R the universal gas constant. The Kissinger plots obtained for various glass compositions are shown in Fig. 3. The activation energies EK obtained from least squares fits to Eq. (2) for the xV2 O5 · 40CaO·(60−x)P2 O5 glasses are shown in Fig. 4. This figure shows that the activation energies EK decrease linearly with an increase in V2 O5 content, indicating that the glassto-supercooled liquid transition becomes less hindered as the V2 O5 content is progressively increased (Table 2). In the present case, the EK values decreased from 145 to 97 kJ mol−1 as a function of V2 O5 content. The activation

Fig. 3. Kissinger’s plots for xV2 O5 ·40CaO·(60−x)P2 O5 (10 ≤ x ≤ 30) glasses (dotted lines shown in the figure are least squares fit to the data).

150 140 -1

E K(kJ mol )

glass systems [16,17]. Tg of xV2 O5 ·4Sb2 O3 ·(96−x)TeO2 glasses [8] increased with an increase in TeO2 content mimicking the behavior reported in the present studies. Glasses associated with a smaller Vo∗ value are thermally more stable than the ones with a larger Vo∗ value [10,18,19]. A comparison of Vo∗ of xV2 O5 ·40CaO·(60−x)P2 O5 glasses (11.68–12.03 cm3 mol−1 ) with that of xV2 O5 ·6Bi2 O3 · (94−x)TeO2 glasses (13.0–13.7 cm3 mol−1 [9]), xV2 O5 · 10ZnO·(90−x)TeO2 glasses (13.3–14.5 cm3 mol−1 [10]) and xV2 O5 ·4Sb2 O3 ·(96−x)TeO2 glasses (13.1–14.5 cm3 mol−1 [8]) shows that the V2 O5 –CaO–P2 O5 glasses with comparatively smaller Vo∗ values are thermally more stable than the rest. A kinetically controlled reaction temperature such as Tg shifts with heating rate. The activation energy of the reaction (which is a measure of the inhibition to the process) could be estimated if the reaction temperature TR under different heating rates is available. The Kissinger method [13,20] provides the following Arrhenius relations form from which the activation energy could be determined from DSC data: =−

2.15

-1

Fig. 2. Variation of the molar volume of oxygen ion and glass transition temperature of xV2 O5 ·40CaO·(60−x)P2 O5 (10 ≤ x ≤ 30) glasses as a function of V2 O5 content.

d[1/Tg ]

x=25

x=20

x=10 x=15

460

ln(j/T g )

*

-1

V o (cm mol )

-9.0

d[ln(ϕ/Tg2 )]

889

130 120 110 100 90 10

15

20

25

30

V2O5 (mol %) Fig. 4. Activation energies of glass transition based on Kissinger’s method for various compositions of xV2 O5 ·40CaO·(60−x)P2 O5 (10 ≤ x ≤ 30) glasses (dotted lines merely connect adjacent data points).

energies EK obtained in the present studies may be compared with the ones reported in the literature for other glass systems. EK values obtained for Se80−x Sbx Te20 glasses varied between 119.5 and 147.2 kJ mol−1 [21,22]. EK decreased with increase in V2 O5 content in xV2 O5 · 20SnO·(80−x)TeO2 (18 ≤ x ≤ 50) glasses [23]. Table 2 Activation energy for glass transition and excess heat capacity at Tg of V2 O5 –CaO–P2 O5 glasses (error in the calculation of EK is ±3 kJ mol−1 ) Composition (V2 O5 :CaO:P2 O5 , mol%)

EK (kJ mol−1 )

Cp (J g−1 K−1 )

10:40:50 15:40:45 20:40:40 25:40:35 30:40:30

145 126 121 104 97

1.9 0.94 0.33 0.52 0.66

890

B. Indrajit Sharma, A. Srinivasan / Materials Chemistry and Physics 82 (2003) 887–891

3.5

10V2O5. 40CaO. 50P2O5

-1

-1

∆Cp(Jg K )

3.0 2.5 ∆Cp

2.0 1.5 1.0 430 440 450 460 470 480 490 500

Temperature (K) Fig. 5. Variation in specific heat capacity of a typical glass sample in the vicinity of the glass transition.

Fig. 5 shows the variation of Cp data of 10V2 O5 ·40CaO· 50P2 O5 glass composition in the vicinity of the glass transition. From the Cp versus temperature curves, the specific heat capacity jump at Tg (Cp ) was calculated according to the procedure shown in Fig. 5. The excess heat capacity at the glass transition (Cp ) is the difference between the specific heat values of the glass and the supercooled liquid. Angell [11] proposed that glasses exhibiting a small Cp value show a strong resistance to structural degradation in the liquid state. Consequently, a small Cp value has been correlated with a minimum fragility [24] of the glass. The composition dependence of Cp of V2 O5 –CaO–P2 O5 glasses is shown in Fig. 6. All glasses showed a small heat capacity jump at Tg , typical of a “strong” glass [11]. Tatsumisago et al. [24] pointed out that such a “smearing out of heat capacity jump” (which gives an impression that crystallization has occurred) was observed in the case of a specific composition of Ge–As–Se glasses. V2 O5 –CaO–P2 O5

xV2O5. 40CaO. (60-x)P2O5

1.5

-1

-1

∆ Cp(Jg K )

2.0

1.0

0.5

10

15

20

25

30

V2O5 (mol %) Fig. 6. Excess heat capacity at glass transition of xV2 O5 ·40CaO· (60−x)P2 O5 (10 ≤ x ≤ 30) glasses (dotted line merely connects adjacent data points).

glasses have a general inhibition to crystallization and hence this small Cp value should be inferred as a signature of a strong glass. One can see that the glass with x = 20 mol% V2 O5 shows the minimum Cp value indicating that fragility is minimum for the glass with 20 mol% V2 O5 . xV2 O5 ·40CaO·(60−x)P2 O5 glasses exhibit a majority charge carrier reversal (MCCR) at the composition with x = 20 mol% V2 O5 [1]. It has been shown [1] that this MCCR occurs when the ratio of the concentration of the vanadium ions, V5+ /V4+ = 1. Glasses with V5+ /V4+ < 1 exhibited p-type conduction, whereas glasses with V5+ /V4+ > 1 exhibited n-type conduction. The carrier reversal and its relation to the ratio of the vanadium ion concentration in the xV2 O5 ·40CaO·(60−x)P2 O5 glasses has been verified using thermoelectric power measurements and iodometric titration [25]. The minimum Cp (and fragility) shown by the 20V2 O5 ·40CaO·40P2 O5 glass indicates that this composition undergoes a minimum structural degradation at the glass transition in comparison to the others in this series. Although the MCCR phenomenon is associated with the electronic band structure of the glass, it has been found that many physical properties of Bi and Pb modified Ge–Se glasses [26–28] exhibit anomalous behavior near the MCCR composition. Cp plots of Bi–Ge–Se [27] and Pb–Ge–Se [28] glasses exhibit a maximum and minimum, respectively, at the compositions at which the MCCR occurs indicating that the MCCR composition is marked by a drastic change in the fragility of the glasses. This observation provides support to the argument that the carrier type reversal is accompanied by perceptible changes in the glassy network structure. It is interesting to note that in spite of the large differences between the chalcogenide and oxide glasses, Cp in both these cases shows an anomaly close to the MCCR composition.

4. Conclusions A systematic study of the non-isothermal behavior of xV2 O5 ·40CaO·(60−x)P2 O5 glasses has been carried out. The Tg and Vo∗ values of these glasses increased with an increase in P2 O5 content (and decreased with an increase in V2 O5 content), indicating the characteristics of a loosely packed structure. The composition dependence of the activation energy of glass transition showed that the glass-to-supercooled liquid transition is less hindered for glasses with higher V2 O5 content. The variation of the excess heat capacity at Tg of these glasses with composition showed that the fragility of the glass is minimum at the composition at which the MCCR occurred.

Acknowledgements Financial support from Council of Scientific and Industrial Research, Government of India through project no. 03(0987)/03/EMR-II is gratefully acknowledged.

B. Indrajit Sharma, A. Srinivasan / Materials Chemistry and Physics 82 (2003) 887–891

References [1] T.N. Kennedy, J.D. Mackenzie, Phys. Chem. Glasses 8 (1967) 169. [2] L. Murawski, C.H. Chung, J.D. Mackenzie, J. Non-Cryst. Solids 32 (1979) 91. [3] H. Mori, H. Sakata, IUMRS-ICEM Symposium Proceedings, vol. 3, Materials Research Society, Taiwan, 1994, p. 101. [4] H. Hirashima, M. Ide, T. Yoshida, J. Non-Cryst. Solids 86 (1986) 327. [5] J.E. Shelby, Introduction to Glass Science and Technology, Royal Soc. Chem., Cambridge, 1997, p. 20. [6] T. Alersma, J.D. Mackenzie, J. Chem. Phys. 47 (1967) 1406. [7] C. Drake, J.A. Stephan, B. Vates, J. Non-Cryst. Solids 28 (1978) 61. [8] H. Mori, T. Kitami, H. Sakata, J. Non-Cryst. Solids 168 (1994) 157. [9] H. Mori, T. Kitami, H. Sakata, J. Ceram. Soc. Jpn. 101 (1993) 347. [10] H. Mori, J. Igarashi, H. Sakata, J. Ceram. Soc. Jpn. 101 (1993) 1351. [11] C.A. Angell, J. Non-Cryst. Solids 73 (1985) 1. [12] S.K. Lee, M. Tatsumisago, T. Minami, Phys. Chem. Glasses 36 (1995) 225. [13] J. Wolny, J. Soltys, R. Kokoszka, J. Non-Cryst. Solids 91 (1987) 209. [14] M.I. O’Neill, Anal. Chem. 38 (1966) 1331. [15] D.C. Ginnings, G.T. Furukawa, J. Am. Chem. Soc. 75 (1953) 522. [16] N. Mochida, K. Takahashi, K. Nakada, S. Shibusawa, YogyoKyokai-Shi 86 (1978) 317.

891

[17] N. Mochida, K. Takahashi, K. Nakada, S. Shibusawa, Yogyo-KyokaiShi 88 (1980) 583. [18] H. Hirashima, F. Tanaka, Seramikkusu Ronbunshi 97 (1990) 1150. [19] T. Naito, T. Namekawa, A. Kato, K. Maeda, J. Ceram. Soc. Jpn. 100 (1992) 685. [20] H.E. Kissinger, J. Res. Nat. Bur. Std. 57 (1956) 217. [21] L. Kashif, A.A. Soliman, A.M. Sanad, Phys. Chem. Glasses 39 (1998) 195. [22] R.M. Mehra, G. Kaur, P.C. Mathur, J. Mater. Sci. 26 (1991) 3433. [23] B. Indrajit Sharma, A.K. Pattanaik, A. Srinivasan, Phys. Chem. Glasses 43 (2002) 12. [24] M. Tatsumisago, B.L. Halfpap, J.L. Green, S.M. Lindsay, C.A. Angell, Phys. Rev. Lett. 64 (1990) 1549. [25] P.C. Paul, Ph.D. Thesis, North Eastern Hill University, Shillong, India, 1992. [26] B. Vaidhyanathan, S. Murugavel, S. Asokan, K.J. Rao, J. Phys. Chem. B 101 (1997) 9717. [27] M.K. Rabinal, K.S. Sangunni, E.S.R. Gopal, S.V. Subramanyam, Solid State Commun. 88 (1993) 251. [28] M.K. Rabinal, N. Ramesh Rao, K.S. Sangunni, E.S.R. Gopal, Phil. Mag. B 70 (1994) 89.