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Chemical durability, electrical and dielectric properties of the ternary system (50-x)K2O–xMnO–50P2O5 phosphate glasses
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 13 (2019) 466–473
www.materialstoday.com/proceedings
ICMES 2018
Chemical durability, electrical and dielectric properties of the ternary system (50-x)K2O–xMnO–50P2O5 phosphate glasses W. Ahminaa, M. El Moudaneb*, A. Shaimc, M. Zriouila, M. Taibid a
LCAM, Faculty of Sciences, University of Mohammed V, Rabat, Morocco. LMNE, Faculty of Sciences, University of Mohammed V, Rabat, Morocco. d LME, Faculty of Sciences, University of IbnZohr V, Agadir, Morocco. c LPCM, Normal Superior School (ENS), University of Mohammed V, Rabat, Morocco. b
Abstract K2O–MnO–50P2O5 glasses containing different concentrations of MnO ranging from 20 to 35 mol % have been prepared using conventional melt-quenching technique. The amorphous nature of the samples was asserted by X–ray diffraction. The chemical durability of these glasses increases with rising MnO content. Glasses containing more than 30 mol%MnO had an excellent chemical durability. The electrical and dielectric parameters were measured in the frequency range from 10 KHz to 1 MHz in the temperature range from room temperature to 550°C. It was observed that the values of ac conductivity augment on increasing frequency. The conductivity of all the glasses increases with temperature following the Arrhenius law. Activation energy was found to increase with raise in concentration of manganese oxide and decrease with the increase in the frequency. It was also observed that the values of dielectric constant and loss factor enhance with the increase in temperature and decrease with increase in frequency in all the glasses studied. These results agree with a closer structure and act in a manner that Mn2+ enters the glassy matrix as a network former character. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018. Keywords: Phosphate; Oxide glasses; Chemical Durability; Electrical conductivity; Dielectric parameters.
* Corresponding author. Tel.: +212 (0) 6 62 37 98 85 ; fax: + 212 (0) 5 37 77 42 61. E-mail address:[email protected] 2214-7853 Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018.
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1. Introduction In the last decade, the study of the phosphate glasses represents an exceptional part in the large field of material science for the reason that of their applications in the different domains. Phosphate glassy materials are of interest for a wide variety of applications, including laser glasses [1,2], radioactive waste entrapment [3,4], release of therapeutic ions [5–7], and, owing to their solubility in water, as fertilizers in agriculture [8]. In the recent years, the application of phosphate glasses has been greatly expanded in the electronic field since of their high thermal, chemical stability, high mechanical strength and optical transparency as compared to silicate or borate materials. A great deal of interest has been paid to phosphate glassy materials for use as solid electrolytes in the production of solid state batteries [9-11].In addition to high ionic conductivities, these glasses have several advantages for instance absence of grain boundaries, isotropic property, ease of formation and more stability. Alkali phosphate glasses have attracted more consideration as of their high ionic conductivity, low melting point and strong glass-forming character [12].The impedance spectroscopy [13] of alkali phosphate glasses reveals that the conductivity in these glasses is ionic in nature and increase with growing alkali ions. A model described in literature [14] shows that the direct current (DC) electrical conductivity of glasses increases strongly with increasing modifier content. Oxide glasses containing large amounts of transition metal ions show interesting electrical properties [15]. This behavior is strongly influenced by the simultaneous existence in the glass network of the transition metal ion in two different valence states, due to redox processes occurring in the melt at high temperatures in the course of preparation. The aim of the present study is to investigate the effect of MnO on chemical durability, electrical and dielectric properties of phosphate glasses (50-x)K2O-xMnO-50P2O5 with 20 ≤ x ≤ 35, mol %. The alternating current (AC) electrical conductivity, activation energy, dielectric constant and loss factor are measured over a wide range of frequency and temperature in order to expose comprehensive information concerning the conduction mechanism and to study the phenomenon ofthe dispersion of electrical parameters with frequencies. 2. Material and Methods 2.1. Glass preparation The starting materials for obtaining (50-x)K2O–xMnO–50P2O5 glass system with 20≤x≤30 mol% were (NH4)2HPO4, MnCO3 and K2CO3 of reagent grade purity. The batches were introduced in an alumina crucible and melted at 1100°C during 30 min. Details of the procedure adopted in the preparation of these glasses, have been described in our earlier work [16]. Table 1 gives the chemical compositions of the studied samples. Table 1.Nominal and analyzed glass compositions of(50-x)K2O-xMnO-50P2O5 glasses [16]. Glass
K2O nominal/analyzed
MnO nominal/analyzed
P2O5 nominal/analyzed
G1
30/29.84
20/18.95
50/49.97
G2
25/24.33
25/24.08
50/50.20
G3
20/19.97
30/28.20
50/50.85
G4
15/14.95
35/33.85
50/50.81
The amorphous state of all as-quenched samples was confirmed by powder X-ray diffractometry (XRD), using a Phillips D5000 apparatus equipped with a CuKα X-ray source and a Ni filter (λ = 1.54 Å). No Bragg peaks were detected in a wide range of 2θ angles between 10°and 80°. 2.2. Dissolution rate Dissolution rate (DR) was measured from calculated weight loss of the samples immersed in distilled water at 25°C. Circular samples were polished with different grades of SiC papers, cleaned with distilled water. The weight of the samples was measured by suspending them in a beaker filled with 150 ml distilled water. The beaker is further
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placed in an oven at 25 °C and the weight of glass samples has been taken at the end of 6, 12, 24 hours, 14, 21 and 14 days. The dissolution rate has been determined by the formula:
DR =
∆W A. t
Where ΔW is the weight loss (g), A is the surface area (cm2), and t is the immersion time (min). For chemical durability, all tests were made in duplicate and the average value of DR is reported herein. The evolution on the initial pH of solution wasmeasured during dissolution time. 2.3. Electrical measurements Four samples were cut and finely polished and shaped as disks with two parallel faces. A thin coating of silver paste was applied to the two opposite larger-area faces to serve as electrodes. Each disk has been placed between silver electrodes; slight pressure was applied to make good electrical contact. The dielectric constant (ε′ ) and ionic conductivity () were deduced from the measurements of capacitance C and dissipation factor tgδ at 10KHz, 100 kHz and 1 MHz in the room temperature to 550°C using an automatic controlled LCR meter type HP 4284A. The dielectric constant (ε′) is expressed as the following: ε =
Cd ε .A
Where ε the permittivity of free space, (C) iscapacitance, d is thickness of the glass sample, and A is cross sectional area of the sample. 3. Results and Discussion
3.1. Dissolution rate Figure 1 shows the dissolution rates DR,which was defined as the weight loss of the glass expressed as g.cm-2.min-1, of the glasses with various contents of manganese oxide in 25 °C distilled water for 6, 12, 24hours , 7, 14, 21 and 28 days.It indicates that the dissolution rate decreases slightly between 20 and 30 mol% MnO. For MnO additions>30 mol%, the dissolution rate decreases rapidly as MnO amount increases. In our case we assume that the higher dissolution rate for less than 25% MnO corresponds to the release of metaphosphate chains into the solution, the addition of less than 25 mol%MnO, shows that MnO has a smaller effect on the glass structure with infinite long chains of PO43- tetrahedral. The same effect was observed for small ZnO additions in NaPO3 glass [17]. When MnO concentration augment, the metaphosphate chains are broken into smaller groups of short chain phosphates such as P4O136-, P3O105- and P2O74-, which are linked to manganese through P–O–Mn bonds. These results could be explained by the increase in the cross links between the phosphate units when the manganese oxide is added and thus confirming its reticulation effect. The phosphate glasses dissolution begins by the hydration process [18]. This reaction consists in the diffusion of water into the glass surface and the migration of entire phosphate chains into the solution. It seems that the large field strength of Mn2+ increases gradually the reticulation of the phosphate network and creates a less sensitive P-O-Mn bonds toward hydration than the P-O-K and P-O-P bonds and therefore enhances the chemical durability of the glasses [19]. The improvement in chemical durability and increase in density, and Tg values with increasing MnO content [16]are all consistent with stronger bonding in the glass.The improved durability of phosphate glasses is attributed to the replacement of the easily hydrated P-O-P bond by corrosion resistant Mn-O-P bond. As the MnO content increases, the number of Mn-O-P bonds also increases. It was found that the G4 glasshas the highest chemical durability. Figure 2 shows the evolution of the pH as a function of the MnO content. It is noticed that the pH of the solution decreases continuously during a times being less than 12 hours, and beyond this value it slows down progressively. These results show that from an initial value of 6 for distilled water, increases continuously with the immersion time to reach a maximum value of 10.56.
W. Ahmina et al / Materials Today: Proceedings 13 (2019) 466–473
469
-5.5
6 hours
-2
-1
logDR (gcm min )
-5.0
12 hours 24 hours 7 days
-6.0
14 days 21 days -6.5
28 days 20
25
30
35
% MnO
Fig.1. Composition dependence of dissolution rates DR for (50-x)K2O–xMnO–50P2O5 glasses after immersion in distilled water at 25 °C for 6 h, 12 h, 24h , 7 days, 14 days, 21 days and 28 days.
10
pH
9
28 days 21 days 14 days 7 days 24 hours 12 hours 6 hours
8
7 20
25
30
35
% MnO Fig.2. Composition dependence of pH for (50-x)K2O–xMnO–50P2O5 glasses after immersion in distilled water at 25°C for 6 h, 12 h, 24h , 7 days, 14 days, 21 days and 28 days.
Because of the higher durability of the glasses containing more than 25 mol%MnO, the pH decreased only slightly from its initial value. However, there was a much larger decrease in the pH for the solutions in which glasses containing less than 25 mol%MnO were immersed. This decrease in pH is consistent with the larger DR values. The basification of the pH = 6 solution can be attributed to the alkali extraction by ion exchange mechanisms between the protons of the solution and the network modifying elements. This exchange process is responsible for the increase of pH due to the increase of residual OH- ions in the solution.
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3.2. Electric-dielectric studies As an example, Figs. 3 and 4 represent the variation of dielectric constant ε′ and loss tgδ with temperature at different frequencies of G3 glass. The value of dielectric constant is found to increase considerably at higher temperatures, particularly at lower frequencies. The same behaviour has observed for all the samples studied. The rate of the increase of ε′ with temperature is found to be the highest for G1 sample and minimum for G4 [20].It can be noted that, the values of dielectric losses show little variation with frequency up to about 300 °C. Beyond this temperature, dielectric loss shows a stronger dependence of temperature and frequencies. It should be noted that the same trend was obtainedfor all the compositions studied 10, 20 and 40 mol% of MnO. The dielectric parameters are found to rise at the higher temperatures, especially at lower frequencies. The large increase in the values of ε′ and tgδ with temperature can be attributed to space charge polarization due to the bonding defects produced in the glass network [21-25] that is contributed to space charge. The augmenting in temperature leads to two modifications in dipolar polarization; 1) the decrease in intermolecular force increases the orientation vibration; 2) the increase in thermal agitation disorders the orientation vibration. The dielectric constant becomes bigger at lower frequencies and at higher temperatures, which is a usual observation in oxide glasses and is not a manifestation for spontaneous polarization [26-27]. As the frequency increases the ionic and dipolar polarization decrease and finally disappear due to the inertia of the ions. As the temperature augments these two processes starts increasing [26]. 160
100 KHz 10 KHz
150
1 MHz
'
140
130
120
110
100
200
300
400
500
600
T[K]
Fig.3. Variation of dielectric constant with temperature at different frequencies of G3 glass.
The following relation is used in estimating the a.c. conductivity of all the glasses (ac) at different temperatures. =
′
against 1/T for G3 Where ω is the frequency and ε is the vacuum dielectric constant. Fig. 5 shows the plots of glass at 10 KHz, 100 KHz and 1 MHz. The linear relationship reveals that the temperature dependence of conductivity follows Arrhenius form, suggesting that the conductivity is thermally activated. From these plots, it is evident that the conductivity of the glasses increases with increasing frequency. The similar trend has been observed in G1, G2 and G4 glasses, and in other glassy materials, for example Li2O-PbO-Bi2O3-P2O5phosphate glasses [28].
W. Ahmina et al / Materials Today: Proceedings 13 (2019) 466–473
8
100 KHz
471
1 MHz
10 KHz
tg
6
4
2
0
500
600
700
] K [ T
400
800
Fig.4. The variation of tg with temperature at different frequencies of G3 glass.
logm)
-1
-2
-4
-6
1 MHz 100 KHz
-7
10 KHz -8 1,2
1,4
1,6
1,8
2,0
2,2
-1
1000/T [K ] Fig.5. The variation of a.c. conductivity ac with 103/T at different frequencies of G3 glass.
The values of activation energies for conduction for different glass samples are summarized and represented in Table 2 and Figure 6. Activation energy increase with MnO content and decreases with the raise in the frequency. Table 2.Activation energy Ea (eV) for (50-x)K2O–xMnO–50P2O5 glasses at different frequencies. Glass
10 KHz
100 KHz
1 MHz
G1
1.26
1.01
0.97
G2
1.38
1.17
1.08
G3
1.54
1.38
1.29
G4
1.73
1.58
1.42
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W. Ahmina et al / Materials Today: Proceedings 13 (2019) 466–473
18
10 KHz 100 KHz
Ea (ev)
16
1 MHz 14
12
10
20
25
% MnO
30
35
Fig. 6. Variation of activation energy with MnO contents at different frequencies
The increase of conductivity with temperature can be explained as follow: as temperature increases, the amplitude of lattice vibration increases and the ions can be close to each other, at the same time the charge carriers gain addition thermal energy, when gives high probability for the hopped charge carrier, between ions on the octahedral sites, this leads to an increase in the charge carriers mobility and consequently the conductivity. Moreover, the values of Ea show a decrease with the increase in frequency. On the other hand, increase in the applied field enhances the charge carrier’s jumps between the localized states. Consequently, the activation energy decreases with the rising of frequency of measurements [29-31]. The dispersion of conductivity with frequencies may be attributed to exchange interaction between electric dipole moments on octahedral sites, where the electric dipoles are parallel and the stronger exchange interaction between them is found. 4. Conclusion This work highlights the effect of MnO adding on some properties ofa series of phosphate glasses with the formula (50-x)K2O-xMnO-50P2O5. The higher field strength of Mn2+ results in the shrinking of the phosphatenetwork. The substitution of the P-O-K and P-O-P bonds by theP-O-Mn bonds increases the reticulation and enhances the chemicaldurability of the glasses. The a.c. conductivity acis found to increase and activation energy is observed to decrease with the increasing in the frequency.The reduce in the dielectric parameters vis, ε' and tanδ, with rising frequency is due to the decrease of the dipolar polarization, indeed, the usual behaviour of dielectric constant and loss tangent growing with temperature can be explained by the mounting of the orientation polarization and the electrical conductionlosses values. References [1] [2] [3] [4] [5] [6] [7]
V. Petrov, U. Griebner, D. Ehrt, W. Seeber, Opt. Lett., 22 (1997) 408-410. J.F. Philipps, T. Topfer, H. Ebendorff-Heidepriem, D. Ehrt, R. Sauerbrey, N.F. Borrelli, Appl. Phys. B: Lasers Opt., 72 (2001) 175-178. P.A. Bingham, R.J. Hand, O.M. Hannant, S.D. Forder, S.H. Kilcoyne, J. Non-Cryst. Solids, 355 (2009) 1526-1538. D.E. Day, Z. Wu, C.S. Ray, P. Hrma, J. Non-Cryst. Solids, 241 (1998) 1-12. J.P. Pessan, N.S. Al-Ibrahim, M.A.R. Buzalaf, K.J. Toumba, J. Appl. Oral Sci., 16 (2008) 238-244. W.M. Allen, B.F. Sansom, P.T. Gleed, C.B. Mallinson, C.F. Drake, Vet. Rec., 115 (1984) 55-57. W.M. Allen, B.F. Sansom, C.B. Mallinson, R.J. Stebbings, C.F. Drake, Vet. Rec., 116 (1985) 175-177.
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P. Knott, Glastech. Ber., 62 (1989) 29-4. C.J. Quinn, P.D. Frayer, G.H. Beall, Polym. Mater. Encycl., 4 (1996) 2766-2777. B.H. Jung, D.N. Kim, H.S. Kim, J. Non-Cryst. Solids, 351 (2005) 3356-3360. R.K. Brow, J. Non-Cryst. Solids, 263&264 (2000) 1-28. S.W. Martin, J. Am. Ceram. Soc., 74 (1991) 1767-1784. S. Sanghi, S. Rania, A. Agarwal, V.P. Seth, Physica B, 404 (2009) 1969-1973. B. Roling, M. Meyerb, A. Bundeb, K. Funkea, J. Non-Cryst. Solids, 226 (1998) 138-144. L. Murawski, C.H. Chung, J.D. Mackenzie, J. Non-Cryst. Solids, 32 (1979) 91-104. W. Ahmina, M. El Moudane, M. Zriouil, M. Taibi, Phase transitions, 89 (2016) 1051-1061. L. Montagne, G. Palavit, R. Delaval, J. Non-Cryst. Solids, 223 (1998) 43-47. B.C. Bunker, G.W. Arnold, J.A. Wilder, J. Non-Cryst. Solids, 64 (1984) 291-316. J.R. Jones, A.G. Clare, John Wiley & Sons, Ltd., Chichester, UK. 2012. W. Ahmina, M. El Moudane, M. Zriouil, M. Taibi, J. Mater. Environ. Sci., 8 (2017) 4193-4198. V. Ravi Kumar, N. Veeraiah, S. Buddhudu, V. Jaya TyagaRaju, J. Phys. III France, 7 (1997) 951-961. S. BalaMurali Krishna, P.SrinivasaRao, B. Sreedhar, D. Krishna Rao, Phys. B. Condens. Matter, 404 (2009) 3898-3905. G. SahayaBhaskaran, G. Little Flower, D. Krishna Rao, N. Veeraiah, J. Alloys Compd., 431 (2007) 303-312. A. Mogus-Mlilankovic, B. Santic, C.S. Ray, D.E. Day, J. Non-Cryst. Solids, 263 (2000) 299-304. P. Srinivasa Rao, C.H. Rajyasree, A. Ramesh Babu, P.M. VinayaTeja, D. Krishna Rao, J. Non-Cryst. Solids, 357 (2011) 3585-3591. P. Ramesh babu, R. Vijay, P. Nageswara Rao, N. Veeraiah, D. Krishna Rao, Mat. Res. Bull., 48 (2013) 4618-4627. S. Yusub, P. Srinivasa Rao, D. Krishna Rao, J. Alloys Compd., 663 (2016) 708-717. M. El Moudane, M. El Maniani, A. Sabbar, A. Ghanimi, M. Tabyaoui, A. Bellaouchou, A. Guenbour, Mat. Res. Bull., 72 (2015) 241-244. M.S. Hossain, R. Islam, K.A. Khan, J. Optoelectron. Adv. Mater., 9 (2007) 2192-2199. R. Ertugrul, A. Tataroglu, Chin. Phys. Lett., 29 (2012) 077304-077309. H.A.M. Ali, H.S. Soliman, M. Saadedin, K. Sawaby, Mater. Sci. Semicond. Process., 18 (2014) 141-145.
ScienceDirect Materials Today: Proceedings 13 (2019) 466–473
www.materialstoday.com/proceedings
ICMES 2018
Chemical durability, electrical and dielectric properties of the ternary system (50-x)K2O–xMnO–50P2O5 phosphate glasses W. Ahminaa, M. El Moudaneb*, A. Shaimc, M. Zriouila, M. Taibid a
LCAM, Faculty of Sciences, University of Mohammed V, Rabat, Morocco. LMNE, Faculty of Sciences, University of Mohammed V, Rabat, Morocco. d LME, Faculty of Sciences, University of IbnZohr V, Agadir, Morocco. c LPCM, Normal Superior School (ENS), University of Mohammed V, Rabat, Morocco. b
Abstract K2O–MnO–50P2O5 glasses containing different concentrations of MnO ranging from 20 to 35 mol % have been prepared using conventional melt-quenching technique. The amorphous nature of the samples was asserted by X–ray diffraction. The chemical durability of these glasses increases with rising MnO content. Glasses containing more than 30 mol%MnO had an excellent chemical durability. The electrical and dielectric parameters were measured in the frequency range from 10 KHz to 1 MHz in the temperature range from room temperature to 550°C. It was observed that the values of ac conductivity augment on increasing frequency. The conductivity of all the glasses increases with temperature following the Arrhenius law. Activation energy was found to increase with raise in concentration of manganese oxide and decrease with the increase in the frequency. It was also observed that the values of dielectric constant and loss factor enhance with the increase in temperature and decrease with increase in frequency in all the glasses studied. These results agree with a closer structure and act in a manner that Mn2+ enters the glassy matrix as a network former character. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018. Keywords: Phosphate; Oxide glasses; Chemical Durability; Electrical conductivity; Dielectric parameters.
* Corresponding author. Tel.: +212 (0) 6 62 37 98 85 ; fax: + 212 (0) 5 37 77 42 61. E-mail address:[email protected] 2214-7853 Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Materials and Environmental Science, ICMES 2018.
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467
1. Introduction In the last decade, the study of the phosphate glasses represents an exceptional part in the large field of material science for the reason that of their applications in the different domains. Phosphate glassy materials are of interest for a wide variety of applications, including laser glasses [1,2], radioactive waste entrapment [3,4], release of therapeutic ions [5–7], and, owing to their solubility in water, as fertilizers in agriculture [8]. In the recent years, the application of phosphate glasses has been greatly expanded in the electronic field since of their high thermal, chemical stability, high mechanical strength and optical transparency as compared to silicate or borate materials. A great deal of interest has been paid to phosphate glassy materials for use as solid electrolytes in the production of solid state batteries [9-11].In addition to high ionic conductivities, these glasses have several advantages for instance absence of grain boundaries, isotropic property, ease of formation and more stability. Alkali phosphate glasses have attracted more consideration as of their high ionic conductivity, low melting point and strong glass-forming character [12].The impedance spectroscopy [13] of alkali phosphate glasses reveals that the conductivity in these glasses is ionic in nature and increase with growing alkali ions. A model described in literature [14] shows that the direct current (DC) electrical conductivity of glasses increases strongly with increasing modifier content. Oxide glasses containing large amounts of transition metal ions show interesting electrical properties [15]. This behavior is strongly influenced by the simultaneous existence in the glass network of the transition metal ion in two different valence states, due to redox processes occurring in the melt at high temperatures in the course of preparation. The aim of the present study is to investigate the effect of MnO on chemical durability, electrical and dielectric properties of phosphate glasses (50-x)K2O-xMnO-50P2O5 with 20 ≤ x ≤ 35, mol %. The alternating current (AC) electrical conductivity, activation energy, dielectric constant and loss factor are measured over a wide range of frequency and temperature in order to expose comprehensive information concerning the conduction mechanism and to study the phenomenon ofthe dispersion of electrical parameters with frequencies. 2. Material and Methods 2.1. Glass preparation The starting materials for obtaining (50-x)K2O–xMnO–50P2O5 glass system with 20≤x≤30 mol% were (NH4)2HPO4, MnCO3 and K2CO3 of reagent grade purity. The batches were introduced in an alumina crucible and melted at 1100°C during 30 min. Details of the procedure adopted in the preparation of these glasses, have been described in our earlier work [16]. Table 1 gives the chemical compositions of the studied samples. Table 1.Nominal and analyzed glass compositions of(50-x)K2O-xMnO-50P2O5 glasses [16]. Glass
K2O nominal/analyzed
MnO nominal/analyzed
P2O5 nominal/analyzed
G1
30/29.84
20/18.95
50/49.97
G2
25/24.33
25/24.08
50/50.20
G3
20/19.97
30/28.20
50/50.85
G4
15/14.95
35/33.85
50/50.81
The amorphous state of all as-quenched samples was confirmed by powder X-ray diffractometry (XRD), using a Phillips D5000 apparatus equipped with a CuKα X-ray source and a Ni filter (λ = 1.54 Å). No Bragg peaks were detected in a wide range of 2θ angles between 10°and 80°. 2.2. Dissolution rate Dissolution rate (DR) was measured from calculated weight loss of the samples immersed in distilled water at 25°C. Circular samples were polished with different grades of SiC papers, cleaned with distilled water. The weight of the samples was measured by suspending them in a beaker filled with 150 ml distilled water. The beaker is further
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W. Ahmina et al / Materials Today: Proceedings 13 (2019) 466–473
placed in an oven at 25 °C and the weight of glass samples has been taken at the end of 6, 12, 24 hours, 14, 21 and 14 days. The dissolution rate has been determined by the formula:
DR =
∆W A. t
Where ΔW is the weight loss (g), A is the surface area (cm2), and t is the immersion time (min). For chemical durability, all tests were made in duplicate and the average value of DR is reported herein. The evolution on the initial pH of solution wasmeasured during dissolution time. 2.3. Electrical measurements Four samples were cut and finely polished and shaped as disks with two parallel faces. A thin coating of silver paste was applied to the two opposite larger-area faces to serve as electrodes. Each disk has been placed between silver electrodes; slight pressure was applied to make good electrical contact. The dielectric constant (ε′ ) and ionic conductivity () were deduced from the measurements of capacitance C and dissipation factor tgδ at 10KHz, 100 kHz and 1 MHz in the room temperature to 550°C using an automatic controlled LCR meter type HP 4284A. The dielectric constant (ε′) is expressed as the following: ε =
Cd ε .A
Where ε the permittivity of free space, (C) iscapacitance, d is thickness of the glass sample, and A is cross sectional area of the sample. 3. Results and Discussion
3.1. Dissolution rate Figure 1 shows the dissolution rates DR,which was defined as the weight loss of the glass expressed as g.cm-2.min-1, of the glasses with various contents of manganese oxide in 25 °C distilled water for 6, 12, 24hours , 7, 14, 21 and 28 days.It indicates that the dissolution rate decreases slightly between 20 and 30 mol% MnO. For MnO additions>30 mol%, the dissolution rate decreases rapidly as MnO amount increases. In our case we assume that the higher dissolution rate for less than 25% MnO corresponds to the release of metaphosphate chains into the solution, the addition of less than 25 mol%MnO, shows that MnO has a smaller effect on the glass structure with infinite long chains of PO43- tetrahedral. The same effect was observed for small ZnO additions in NaPO3 glass [17]. When MnO concentration augment, the metaphosphate chains are broken into smaller groups of short chain phosphates such as P4O136-, P3O105- and P2O74-, which are linked to manganese through P–O–Mn bonds. These results could be explained by the increase in the cross links between the phosphate units when the manganese oxide is added and thus confirming its reticulation effect. The phosphate glasses dissolution begins by the hydration process [18]. This reaction consists in the diffusion of water into the glass surface and the migration of entire phosphate chains into the solution. It seems that the large field strength of Mn2+ increases gradually the reticulation of the phosphate network and creates a less sensitive P-O-Mn bonds toward hydration than the P-O-K and P-O-P bonds and therefore enhances the chemical durability of the glasses [19]. The improvement in chemical durability and increase in density, and Tg values with increasing MnO content [16]are all consistent with stronger bonding in the glass.The improved durability of phosphate glasses is attributed to the replacement of the easily hydrated P-O-P bond by corrosion resistant Mn-O-P bond. As the MnO content increases, the number of Mn-O-P bonds also increases. It was found that the G4 glasshas the highest chemical durability. Figure 2 shows the evolution of the pH as a function of the MnO content. It is noticed that the pH of the solution decreases continuously during a times being less than 12 hours, and beyond this value it slows down progressively. These results show that from an initial value of 6 for distilled water, increases continuously with the immersion time to reach a maximum value of 10.56.
W. Ahmina et al / Materials Today: Proceedings 13 (2019) 466–473
469
-5.5
6 hours
-2
-1
logDR (gcm min )
-5.0
12 hours 24 hours 7 days
-6.0
14 days 21 days -6.5
28 days 20
25
30
35
% MnO
Fig.1. Composition dependence of dissolution rates DR for (50-x)K2O–xMnO–50P2O5 glasses after immersion in distilled water at 25 °C for 6 h, 12 h, 24h , 7 days, 14 days, 21 days and 28 days.
10
pH
9
28 days 21 days 14 days 7 days 24 hours 12 hours 6 hours
8
7 20
25
30
35
% MnO Fig.2. Composition dependence of pH for (50-x)K2O–xMnO–50P2O5 glasses after immersion in distilled water at 25°C for 6 h, 12 h, 24h , 7 days, 14 days, 21 days and 28 days.
Because of the higher durability of the glasses containing more than 25 mol%MnO, the pH decreased only slightly from its initial value. However, there was a much larger decrease in the pH for the solutions in which glasses containing less than 25 mol%MnO were immersed. This decrease in pH is consistent with the larger DR values. The basification of the pH = 6 solution can be attributed to the alkali extraction by ion exchange mechanisms between the protons of the solution and the network modifying elements. This exchange process is responsible for the increase of pH due to the increase of residual OH- ions in the solution.
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3.2. Electric-dielectric studies As an example, Figs. 3 and 4 represent the variation of dielectric constant ε′ and loss tgδ with temperature at different frequencies of G3 glass. The value of dielectric constant is found to increase considerably at higher temperatures, particularly at lower frequencies. The same behaviour has observed for all the samples studied. The rate of the increase of ε′ with temperature is found to be the highest for G1 sample and minimum for G4 [20].It can be noted that, the values of dielectric losses show little variation with frequency up to about 300 °C. Beyond this temperature, dielectric loss shows a stronger dependence of temperature and frequencies. It should be noted that the same trend was obtainedfor all the compositions studied 10, 20 and 40 mol% of MnO. The dielectric parameters are found to rise at the higher temperatures, especially at lower frequencies. The large increase in the values of ε′ and tgδ with temperature can be attributed to space charge polarization due to the bonding defects produced in the glass network [21-25] that is contributed to space charge. The augmenting in temperature leads to two modifications in dipolar polarization; 1) the decrease in intermolecular force increases the orientation vibration; 2) the increase in thermal agitation disorders the orientation vibration. The dielectric constant becomes bigger at lower frequencies and at higher temperatures, which is a usual observation in oxide glasses and is not a manifestation for spontaneous polarization [26-27]. As the frequency increases the ionic and dipolar polarization decrease and finally disappear due to the inertia of the ions. As the temperature augments these two processes starts increasing [26]. 160
100 KHz 10 KHz
150
1 MHz
'
140
130
120
110
100
200
300
400
500
600
T[K]
Fig.3. Variation of dielectric constant with temperature at different frequencies of G3 glass.
The following relation is used in estimating the a.c. conductivity of all the glasses (ac) at different temperatures. =
′
against 1/T for G3 Where ω is the frequency and ε is the vacuum dielectric constant. Fig. 5 shows the plots of glass at 10 KHz, 100 KHz and 1 MHz. The linear relationship reveals that the temperature dependence of conductivity follows Arrhenius form, suggesting that the conductivity is thermally activated. From these plots, it is evident that the conductivity of the glasses increases with increasing frequency. The similar trend has been observed in G1, G2 and G4 glasses, and in other glassy materials, for example Li2O-PbO-Bi2O3-P2O5phosphate glasses [28].
W. Ahmina et al / Materials Today: Proceedings 13 (2019) 466–473
8
100 KHz
471
1 MHz
10 KHz
tg
6
4
2
0
500
600
700
] K [ T
400
800
Fig.4. The variation of tg with temperature at different frequencies of G3 glass.
logm)
-1
-2
-4
-6
1 MHz 100 KHz
-7
10 KHz -8 1,2
1,4
1,6
1,8
2,0
2,2
-1
1000/T [K ] Fig.5. The variation of a.c. conductivity ac with 103/T at different frequencies of G3 glass.
The values of activation energies for conduction for different glass samples are summarized and represented in Table 2 and Figure 6. Activation energy increase with MnO content and decreases with the raise in the frequency. Table 2.Activation energy Ea (eV) for (50-x)K2O–xMnO–50P2O5 glasses at different frequencies. Glass
10 KHz
100 KHz
1 MHz
G1
1.26
1.01
0.97
G2
1.38
1.17
1.08
G3
1.54
1.38
1.29
G4
1.73
1.58
1.42
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18
10 KHz 100 KHz
Ea (ev)
16
1 MHz 14
12
10
20
25
% MnO
30
35
Fig. 6. Variation of activation energy with MnO contents at different frequencies
The increase of conductivity with temperature can be explained as follow: as temperature increases, the amplitude of lattice vibration increases and the ions can be close to each other, at the same time the charge carriers gain addition thermal energy, when gives high probability for the hopped charge carrier, between ions on the octahedral sites, this leads to an increase in the charge carriers mobility and consequently the conductivity. Moreover, the values of Ea show a decrease with the increase in frequency. On the other hand, increase in the applied field enhances the charge carrier’s jumps between the localized states. Consequently, the activation energy decreases with the rising of frequency of measurements [29-31]. The dispersion of conductivity with frequencies may be attributed to exchange interaction between electric dipole moments on octahedral sites, where the electric dipoles are parallel and the stronger exchange interaction between them is found. 4. Conclusion This work highlights the effect of MnO adding on some properties ofa series of phosphate glasses with the formula (50-x)K2O-xMnO-50P2O5. The higher field strength of Mn2+ results in the shrinking of the phosphatenetwork. The substitution of the P-O-K and P-O-P bonds by theP-O-Mn bonds increases the reticulation and enhances the chemicaldurability of the glasses. The a.c. conductivity acis found to increase and activation energy is observed to decrease with the increasing in the frequency.The reduce in the dielectric parameters vis, ε' and tanδ, with rising frequency is due to the decrease of the dipolar polarization, indeed, the usual behaviour of dielectric constant and loss tangent growing with temperature can be explained by the mounting of the orientation polarization and the electrical conductionlosses values. References [1] [2] [3] [4] [5] [6] [7]
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