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Electrical properties and microstructure of glassy-crystalline Ag+-ion conducting composites synthesized by a high-pressure method
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Solid State Ionics 179 (2008) 1278 – 1281 www.elsevier.com/locate/ssi
Electrical properties and microstructure of glassy-crystalline Ag + -ion conducting composites synthesized by a high-pressure method M. Wasiucionek a,⁎, M. Foltyn a , J.E. Garbarczyk a , J.L. Nowinski a , S. Gierlotka b , B. Palosz b a
b
Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Received 1 August 2007; received in revised form 11 December 2007; accepted 24 December 2007
Abstract Rigid, mechanically reinforced, composite Ag+-ion conductors based on xAgI·(100-x) (0.67Ag2O·0.33B2O3), 40 ≤ x ≤ 60, glasses and hard insulating powders of α-Al2O3 (2 µm) or ZrO2 (1 µm) were prepared by a high-pressure method. Microstructure of the composites, depended on the content of AgI in the glasses and the temperature of the high-pressure stage of the synthesis. In all cases, however, it consisted of the evenly distributed phases (conductive and insulating), which did not interpenetrate at a nanometer length scale. It was found that the high electrical conductivity of the composites, though somewhat lower than that of the glasses, generally followed the temperature- and composition dependencies of the latter. An important advantage of the composites over the corresponding glasses was their higher microhardness and nonbrittleness, which, contrary to the glasses, enabled their post-synthesis machining, cutting or polishing. © 2008 Elsevier B.V. All rights reserved. Keywords: High-pressure synthesis; Solid electrolytes; Silver borate glasses; Composite electrolytes; Ionic conduction; Microhardness
1. Introduction Ag+-ion conductive glasses of the systems AgI–Ag2O– MxOy (MxOy = B2O3, P2O5, MoO3, etc.) exhibit high electrical conductivity (up to 5·10− 3 S·cm− 1 at room temperature) [1,2] of purely ionic character. Such high conductivity makes these glasses interesting as potential solid electrolytes in all-solid power sources for electronic devices operating at room temperature. However the glasses of the above mentioned systems have some important disadvantages. One of them is their brittleness, which limits the possibilities of cutting thin wafers out of glass batches and makes difficult post-synthesis shaping and machining of the samples. In last years we have started work on synthesis of mechanically reinforced composites based on AgI–Ag2O–MxOy (MxOy = B2O3, P2O5, V2O5) glasses and selected hard ceramic powders (such as ZrO2, α-Al2O3, diamond or SiC), and characterization of their main physical properties [3–5]. A concept of these composites consisted in merging high electrical conductivity (of ⁎ Corresponding author. Tel.: +48 22 234 8539; fax: +48 22 628 2171. E-mail address: [email protected] (M. Wasiucionek). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.12.073
purely cationic character) of the glasses with good mechanical properties of the ceramic powder matrix. In the syntheses we wanted to avoid a high-temperature stage, present in alternative routes of preparation of Ag + -ion conducting composites based on AgI and e.g. nanosize γ-Al2O3 [6–8]. This precaution measure was undertaken because it had been known that massive crystallization of the glassy ion- or mixed electronic-ionic conductive materials leads to deterioration of their electrical characteristics (e.g. Ref. [9]). For the synthesis we have chosen a high-pressure method consisting of a simultaneous application of high-pressure (in most cases close to 8 GPa) and moderate temperature (100–200 °C). Under these conditions the softened glassy phase is forced to fill voids between hard grains of the filler (ZrO2, α-Al2O3, diamond or SiC), non-deformable under high pressure. It has been found that some powders commonly used as fillers in composites, e.g. SiO2 or γ-Al2O3, undergo deformation at high-pressures of several GPa such as applied in this work and cannot be used in the synthesis. The aim of this study was to present selected electrical, mechanical and microstructural characteristics of the novel glassy-crystalline composite ionic conductors. These composites, based on ionically conducting glasses of the
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system AgI–Ag2O–B2O3 and hard powder fillers, were prepared by a high-pressure route. 2. Experimental Silver-borate glasses of compositions xAgI·(100-x) (0.67Ag2O· 0.33B2O3), where 40≤x ≤ 60, were prepared by a standard meltquenching method. The composites, containing equal volume fractions of the glass and the selected ceramic powder α-Al2O3 (2 µm) and ZrO2 (1 µm), were synthesized by a high-pressure method. The premixed batches placed between boron nitride plugs
Fig. 2. Conductivity isotherms of glasses and composites at 50 °C. Data for glasses are marked by solid squares, for composites with α-Al2O3 by open circles and composites with ZrO2 by full circles. Solid lines are the guide for the eye only.
were inserted into a tubular graphite heater and installed in an especially profiled calcium carbonate gasket. The role of the latter was to transmit externally applied high uniaxial pressure to the sides of the synthesized batch, and thus to create quasi-isostatic conditions. The whole load was mounted between anvils of a big hydraulic press. The batch was pressed at 7.7 GPa and simultaneously heated up to either 100, 150 or 200 °C. Final products had form of hard pellets of diameter 5 mm and thickness ca. 2 mm. Contrary to the glasses, the composites were non-brittle, and could be safely polished or cut in wafers as thin as ca 100 µm. The Scanning Electron Microscopy (SEM) observations were carried out on a LEO 1530 apparatus. Impedance measurements were done using a set-up based on a Solartron 1260 Impedance/ Gain Phase Analyzer coupled with a Keithley Current Amplifier 428 via a computer-controlled switch. The latter automatically connected the amplifier to the system when the measured impedance was high (i.e. N 10 MΩ) and did not connect it in the other case. The use of the current amplifier extended the range of reliably measured impedances from ca. 107 Ω for a standard Solartron equipment to 1012 Ω for the system with the amplifier. This arrangement was vital for reliability of the measurements carried out at the lowest temperatures — near 90 K. Numerical analysis of the impedance spectra was done using a FIRDAC software package [10]. Mechanical properties of glasses and composites were represented by their microhardness, which was measured using the Vickers method. 3. Results and discussion 3.1. Microstructure
Fig. 1. SEM micrographs of the composites: a) 50AgI·33Ag2O·17 B2O3: α-Al2O3 (2 µm) synthesized at 100 °C, b) 60AgI·27Ag2O·13 B2O3: α-Al2O3 (2 µm) synthesized at 200 °C and c) 55AgI·30Ag2O·15B2O3: ZrO2 (1 µm) synthesized at 150 °C. Pressure of the synthesis was equal to 7.7 GPa.
Microstructure of the synthesized composites depended on several factors, but mainly on the state of amorphousness of the as-quenched batches used for the synthesis and the temperature of synthesis. In the case of the composites based on glasses corresponding to x = 50 and prepared at 100 °C the microstructure consisted of a continuous glassy phase containing embedded grains of alumina (Fig. 1a). In the opposite case of composites based on samples with x = 60 and synthesized at 200 °C one could see a number of small crystallites and glassy droplets covering
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crystallization of the glass, whose high content of AgI makes it very susceptible to crystallization. This applies especially to the interfacial regions being in contact with foreign phases (grains of ZrO2) which may act as crystallization nuclei. 3.2. Temperature dependences of conductivity
Fig. 3. Temperature dependences of conductivity of glasses and composites in the low-temperature range: glasses (x = 50 — open triangles, x = 60 — open squares) and composites (x = 50; α-Al2O3 — solid triangles, x = 60; α-Al2O3 — solid squares; x = 55; ZrO2 — open circles). Solid lines represent the best-fits using the Arrhenius formula.
surfaces of the alumina grains (Fig. 1b). In the intermediate situation of composites based on glasses with x = 55 and prepared at 100 or 150 °C there is a continuous glassy phase with embedded ceramic grains, additionally decorated with ramified interfaces between both components (Fig. 1c). All these microstructural features are in agreement with the expectations. The composites presented in Fig. 1a were based on glasses with AgI content (50 mol% AgI), which is below limiting value of 55– 60 mol% AgI determining the amorphous region achievable by the standard press-quenching method. The temperature applied during the high-pressure synthesis of the composites (100 °C) did not exceed the crystallization temperature of the glassy component. Therefore it is natural that the resulting composites consist of the unmodified glassy phase containing embedded grains of the powder (as can be seen in Fig. 1a). In the case of composites based on batches corresponding to x = 60 the situation is different, because presence of 60 mol% of AgI leads to some degree of crystallization of the press-quenched samples. Moreover the high-pressure stage of the synthesis carried out at 200 °C leads to further crystallization of the glassy phase within the composite. The effect can be seen in Fig. 1b. Finally the pressquenching of the batch of composition corresponding to x = 55 may lead, but not necessarily, to some crystallization, because this content of AgI (55 mol%) lies at the border of the amorphous region achievable using quenching rates of the standard pressquenching method. Since also the temperature applied during the high-pressure stage of the synthesis (150 °C) is not high enough to promote any important crystallization of the glassy phase, the microstructure of the composite consists of the continuous glassy phase with embedded grains of zirconia (Fig. 1c). The visible distinction between the microstructures in Fig. 1a and c is the fractal-like structure of the interfaces in latter case and relatively smooth interfaces in the former one. The presence of the ramified “decorations” at the glass-ceramic grain interfaces of the composites visible in Fig. 1c is caused by limited and localized
Glasses of the ternary system AgI–Ag2O–B2O3, especially those with high contents of AgI (above 40 mol% of AgI) are very good conductors of Ag+ cations. However an increase in AgI content above ca 55 mol% does not lead to the conductivity enhancement, because of unavoidable crystallization phenomena, which may prevent the increase in conductivity due to formation of poorly conducting β−AgI crystallites. Additionally massive crystallization may lead to appearance of blocking boundaries between neighboring grains. Conductivity isotherms at 50 °C of a series of glasses whose composition was given by x from the 0.4–0.6 range are shown in Fig. 2. It can be seen that the conductivity reaches its maximum at x = 55 which is close to the border of the amorphous region of the system under study. The corresponding conductivity isotherm of the composites based on the glasses is also shown in Fig. 2. It can be seen that: the conductivity values for composites are systematically lower by a factor of 5–7 than those of the respective glasses, they depend on x in a similar way as the conductivity values of the corresponding glasses and, finally, they seem not to depend visibly on the nature of the ceramic powder used (α-Al2O3 or ZrO2). The gap between values of the conductivity of the glasses and the respective composites was confirmed by conductivity measurements in the low-temperature range down to ca 90 K (Fig. 3). The activation energies in both cases remain the same (0.30 eV for glasses vs. 0.30–0.32 eV for composites). Only for the composites based on ZrO2 the activation energy value is slightly lower (0.28 eV). These facts indicate that the effective conductivity of the composites is entirely due to the conductivity of their glassy phase, which remains basically the same as in the as-quenched form. Slightly lower activation energy of the composite based
Fig. 4. Composition dependence of the microhardness of glasses xAgI·(100 — x) (0.67Ag2O·0.33B2O3) for 40 ≤x ≤ 60 (solid squares) and composites based on these glasses and α-Al2O3 (2 µm) (open circles) or ZrO2 (1 µm) (solid circles). Solid lines are the guide for the eye.
M. Wasiucionek et al. / Solid State Ionics 179 (2008) 1278–1281
on glass x = 55 and ZrO2 may arise from the presence of the ramified interfacial regions of nanometric dimensions seen in Fig. 1c. Such regions usually contain more defects than the bulk glassy phase, which is advantageous from the point of view of the ionic transport. It is necessary to notice that the presence of the second dispersed phase in an ionic conductor (here glasses of the AgI– Ag2O–B2O3 system) does not lead to a conductivity enhancement similar to that reported for a number of Ag+-ion conducting composites based on AgI and inert fillers like γ-Al2O3 [6,11,12]. There are at least two important reasons of absence of such an enhancement effect in the case of the composites under study. Firstly, the characteristic dimensions of regions occupied by both components are above the mesoscopic scale, i.e. above ca 100 nm. Moreover they do not interpenetrate each other at distances shorter than 100 nm. This is well visible in the SEM micrographs (Fig. 1a–c). The proximity of most of the conducting phase to the interfaces with the insulating one (distances between any part of the conducting phase and the nearest interface not exceeding ca 100 µm) was an important factor for the conductivity enhancement of the AgI: γ-Al2O3 composites studied in [6,11,12]. Secondly, the room temperature conductivity of glassy phase studied (ca 10− 3 S·cm− 1) is by several orders of magnitude higher than of β-AgI (below 10− 6 S·cm− 1). Consequently its further enhancement by the introduction of a second phase is unlikely, contrary to the case of composites AgI: γ-Al2O3 [6,11,12]. It should however be noticed that the ionic conductivity of the as-synthesized composites under study can be additionally enhanced by their appropriate thermal treatment. The treatment consisted in heating samples up to temperature corresponding to the beginning of the crystallization of the glassy phase (determined by Differential Scanning Calorimetry) and cooling down to room temperature. The effect of such procedure applied to glasses of the AgI– Ag2O–B2O3 system was a partial nanocrystallization. Newly formed crystallites had grain size of order of 30 nm [2]. 3.3. Mechanical properties Contrary to the as-received glasses the composite samples were non-brittle and machinable. It was possible to cut the composites into wafers as thin as 100 µm and polish them without breaking. At the moment for a quantitative measure of the improvement of the mechanical properties of the composites we have used their microhardness measured by the Vickers method. The results of these measurements are given in Fig. 4. It can be seen that the microhardness of the glasses slightly decreases as the AgI content increases. This is understandable, taking into account that the glass network in the glasses of the AgI–Ag2O–B2O3 system built of B–O bonds is firstly weakened by the presence of a Ag2O (a glass modifier), which causes breaking of many of
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these bonds. The addition of a high amount of AgI (a dopant) leads to a further decrease in the mechanical strength of the overall structure. Microhardness of the composites is systematically higher than that of the glasses, but the dependence of microhardness on content of silver iodide is similar — it decreases with an increase in x. Large deviations in the experimental values of microhardness of most composites may be caused by their heterogeneous character. Since microhardness is only an indication of mechanical properties of the composite material, further studies on mechanical properties are planned. Nevertheless the observations confirm that the mechanical properties of the composites are dominated by those of the glassy phase and that the grains of the hard second phase reinforce the composite, in most cases substantially. Further comparative studies on mechanical properties of the glasses and composites are under way. 4. Conclusions Ionically conducting composites based on highly conductive Ag+-ion glasses and hard ceramic powders (α-Al2O3 and ZrO2) were synthesized by a high-pressure method. Their electrical conductivity, though high, was systematically lower than that of the parent glasses. From the SEM micrographs of the composites it is clear that both phases: the conducting and the insulating one, do not interpenetrate each other at nanometer length scales, which is a prerequisite of a conductivity enhancement effect, observed in composites based on AgI and insulating second phases like γ-Al2O3. The presence of the second phase led to a mechanical reinforcement of the samples, reduced their brittleness and made them machinable. References [1] T. Minami, J. Non-Cryst. Solids 56 (1983) 15. [2] M. Foltyn, M. Wasiucionek, J. Garbarczyk, J.L. Nowinski, Solid State Ionics 176 (2005) 2137. [3] M. Zgirski, J.E. Garbarczyk, S. Gierlotka, B. Palosz, M. Wasiucionek, J.L. Nowinski, Solid State Ionics 176 (2005) 2141. [4] M. Foltyn, M. Wasiucionek, J.E. Garbarczyk, J.L. Nowinski, B. Palosz, S. Gierlotka, Glass Technol.: Eur. J. Glass Sci. Technol. A 47 (2006) 138. [5] M. Foltyn, M. Wasiucionek, J.E. Garbarczyk, J.L. Nowinski, S. Gierlotka, B. Palosz, J. Power Sources 173 (2007) 795. [6] H. Yamada, A.J. Bhattacharyya, J. Maier, Adv. Funct. Mater. 16 (2006) 525. [7] K. Tadanaga, K. Imai, M. Tatsumisago, T. Minami, J. Electrochem. Soc. 147 (2000) 4061. [8] Woo Lee, Han-Ill Yoob, Jin-Kyu Lee, Chem. Commun. 24 (2001) 2530. [9] J.E. Garbarczyk, P. Jozwiak, M. Wasiucionek, J.L. Nowinski, Solid State Ionics 177 (2006) 2585. [10] J.R. Dygas, M.W. Breiter, Electrochim. Acta 44 (1999) 4163. [11] J.-S. Lee, St. Adams, J. Maier, Solid State Ionics 136–137 (2000) 1261. [12] N.F. Uvarov, B.B. Bokhonov, A.A. Politov, P. Vanek, J. Petzelt, J. Mater. Synth. Process. 8 (2000) 327.
Solid State Ionics 179 (2008) 1278 – 1281 www.elsevier.com/locate/ssi
Electrical properties and microstructure of glassy-crystalline Ag + -ion conducting composites synthesized by a high-pressure method M. Wasiucionek a,⁎, M. Foltyn a , J.E. Garbarczyk a , J.L. Nowinski a , S. Gierlotka b , B. Palosz b a
b
Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland Received 1 August 2007; received in revised form 11 December 2007; accepted 24 December 2007
Abstract Rigid, mechanically reinforced, composite Ag+-ion conductors based on xAgI·(100-x) (0.67Ag2O·0.33B2O3), 40 ≤ x ≤ 60, glasses and hard insulating powders of α-Al2O3 (2 µm) or ZrO2 (1 µm) were prepared by a high-pressure method. Microstructure of the composites, depended on the content of AgI in the glasses and the temperature of the high-pressure stage of the synthesis. In all cases, however, it consisted of the evenly distributed phases (conductive and insulating), which did not interpenetrate at a nanometer length scale. It was found that the high electrical conductivity of the composites, though somewhat lower than that of the glasses, generally followed the temperature- and composition dependencies of the latter. An important advantage of the composites over the corresponding glasses was their higher microhardness and nonbrittleness, which, contrary to the glasses, enabled their post-synthesis machining, cutting or polishing. © 2008 Elsevier B.V. All rights reserved. Keywords: High-pressure synthesis; Solid electrolytes; Silver borate glasses; Composite electrolytes; Ionic conduction; Microhardness
1. Introduction Ag+-ion conductive glasses of the systems AgI–Ag2O– MxOy (MxOy = B2O3, P2O5, MoO3, etc.) exhibit high electrical conductivity (up to 5·10− 3 S·cm− 1 at room temperature) [1,2] of purely ionic character. Such high conductivity makes these glasses interesting as potential solid electrolytes in all-solid power sources for electronic devices operating at room temperature. However the glasses of the above mentioned systems have some important disadvantages. One of them is their brittleness, which limits the possibilities of cutting thin wafers out of glass batches and makes difficult post-synthesis shaping and machining of the samples. In last years we have started work on synthesis of mechanically reinforced composites based on AgI–Ag2O–MxOy (MxOy = B2O3, P2O5, V2O5) glasses and selected hard ceramic powders (such as ZrO2, α-Al2O3, diamond or SiC), and characterization of their main physical properties [3–5]. A concept of these composites consisted in merging high electrical conductivity (of ⁎ Corresponding author. Tel.: +48 22 234 8539; fax: +48 22 628 2171. E-mail address: [email protected] (M. Wasiucionek). 0167-2738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2007.12.073
purely cationic character) of the glasses with good mechanical properties of the ceramic powder matrix. In the syntheses we wanted to avoid a high-temperature stage, present in alternative routes of preparation of Ag + -ion conducting composites based on AgI and e.g. nanosize γ-Al2O3 [6–8]. This precaution measure was undertaken because it had been known that massive crystallization of the glassy ion- or mixed electronic-ionic conductive materials leads to deterioration of their electrical characteristics (e.g. Ref. [9]). For the synthesis we have chosen a high-pressure method consisting of a simultaneous application of high-pressure (in most cases close to 8 GPa) and moderate temperature (100–200 °C). Under these conditions the softened glassy phase is forced to fill voids between hard grains of the filler (ZrO2, α-Al2O3, diamond or SiC), non-deformable under high pressure. It has been found that some powders commonly used as fillers in composites, e.g. SiO2 or γ-Al2O3, undergo deformation at high-pressures of several GPa such as applied in this work and cannot be used in the synthesis. The aim of this study was to present selected electrical, mechanical and microstructural characteristics of the novel glassy-crystalline composite ionic conductors. These composites, based on ionically conducting glasses of the
M. Wasiucionek et al. / Solid State Ionics 179 (2008) 1278–1281
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system AgI–Ag2O–B2O3 and hard powder fillers, were prepared by a high-pressure route. 2. Experimental Silver-borate glasses of compositions xAgI·(100-x) (0.67Ag2O· 0.33B2O3), where 40≤x ≤ 60, were prepared by a standard meltquenching method. The composites, containing equal volume fractions of the glass and the selected ceramic powder α-Al2O3 (2 µm) and ZrO2 (1 µm), were synthesized by a high-pressure method. The premixed batches placed between boron nitride plugs
Fig. 2. Conductivity isotherms of glasses and composites at 50 °C. Data for glasses are marked by solid squares, for composites with α-Al2O3 by open circles and composites with ZrO2 by full circles. Solid lines are the guide for the eye only.
were inserted into a tubular graphite heater and installed in an especially profiled calcium carbonate gasket. The role of the latter was to transmit externally applied high uniaxial pressure to the sides of the synthesized batch, and thus to create quasi-isostatic conditions. The whole load was mounted between anvils of a big hydraulic press. The batch was pressed at 7.7 GPa and simultaneously heated up to either 100, 150 or 200 °C. Final products had form of hard pellets of diameter 5 mm and thickness ca. 2 mm. Contrary to the glasses, the composites were non-brittle, and could be safely polished or cut in wafers as thin as ca 100 µm. The Scanning Electron Microscopy (SEM) observations were carried out on a LEO 1530 apparatus. Impedance measurements were done using a set-up based on a Solartron 1260 Impedance/ Gain Phase Analyzer coupled with a Keithley Current Amplifier 428 via a computer-controlled switch. The latter automatically connected the amplifier to the system when the measured impedance was high (i.e. N 10 MΩ) and did not connect it in the other case. The use of the current amplifier extended the range of reliably measured impedances from ca. 107 Ω for a standard Solartron equipment to 1012 Ω for the system with the amplifier. This arrangement was vital for reliability of the measurements carried out at the lowest temperatures — near 90 K. Numerical analysis of the impedance spectra was done using a FIRDAC software package [10]. Mechanical properties of glasses and composites were represented by their microhardness, which was measured using the Vickers method. 3. Results and discussion 3.1. Microstructure
Fig. 1. SEM micrographs of the composites: a) 50AgI·33Ag2O·17 B2O3: α-Al2O3 (2 µm) synthesized at 100 °C, b) 60AgI·27Ag2O·13 B2O3: α-Al2O3 (2 µm) synthesized at 200 °C and c) 55AgI·30Ag2O·15B2O3: ZrO2 (1 µm) synthesized at 150 °C. Pressure of the synthesis was equal to 7.7 GPa.
Microstructure of the synthesized composites depended on several factors, but mainly on the state of amorphousness of the as-quenched batches used for the synthesis and the temperature of synthesis. In the case of the composites based on glasses corresponding to x = 50 and prepared at 100 °C the microstructure consisted of a continuous glassy phase containing embedded grains of alumina (Fig. 1a). In the opposite case of composites based on samples with x = 60 and synthesized at 200 °C one could see a number of small crystallites and glassy droplets covering
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crystallization of the glass, whose high content of AgI makes it very susceptible to crystallization. This applies especially to the interfacial regions being in contact with foreign phases (grains of ZrO2) which may act as crystallization nuclei. 3.2. Temperature dependences of conductivity
Fig. 3. Temperature dependences of conductivity of glasses and composites in the low-temperature range: glasses (x = 50 — open triangles, x = 60 — open squares) and composites (x = 50; α-Al2O3 — solid triangles, x = 60; α-Al2O3 — solid squares; x = 55; ZrO2 — open circles). Solid lines represent the best-fits using the Arrhenius formula.
surfaces of the alumina grains (Fig. 1b). In the intermediate situation of composites based on glasses with x = 55 and prepared at 100 or 150 °C there is a continuous glassy phase with embedded ceramic grains, additionally decorated with ramified interfaces between both components (Fig. 1c). All these microstructural features are in agreement with the expectations. The composites presented in Fig. 1a were based on glasses with AgI content (50 mol% AgI), which is below limiting value of 55– 60 mol% AgI determining the amorphous region achievable by the standard press-quenching method. The temperature applied during the high-pressure synthesis of the composites (100 °C) did not exceed the crystallization temperature of the glassy component. Therefore it is natural that the resulting composites consist of the unmodified glassy phase containing embedded grains of the powder (as can be seen in Fig. 1a). In the case of composites based on batches corresponding to x = 60 the situation is different, because presence of 60 mol% of AgI leads to some degree of crystallization of the press-quenched samples. Moreover the high-pressure stage of the synthesis carried out at 200 °C leads to further crystallization of the glassy phase within the composite. The effect can be seen in Fig. 1b. Finally the pressquenching of the batch of composition corresponding to x = 55 may lead, but not necessarily, to some crystallization, because this content of AgI (55 mol%) lies at the border of the amorphous region achievable using quenching rates of the standard pressquenching method. Since also the temperature applied during the high-pressure stage of the synthesis (150 °C) is not high enough to promote any important crystallization of the glassy phase, the microstructure of the composite consists of the continuous glassy phase with embedded grains of zirconia (Fig. 1c). The visible distinction between the microstructures in Fig. 1a and c is the fractal-like structure of the interfaces in latter case and relatively smooth interfaces in the former one. The presence of the ramified “decorations” at the glass-ceramic grain interfaces of the composites visible in Fig. 1c is caused by limited and localized
Glasses of the ternary system AgI–Ag2O–B2O3, especially those with high contents of AgI (above 40 mol% of AgI) are very good conductors of Ag+ cations. However an increase in AgI content above ca 55 mol% does not lead to the conductivity enhancement, because of unavoidable crystallization phenomena, which may prevent the increase in conductivity due to formation of poorly conducting β−AgI crystallites. Additionally massive crystallization may lead to appearance of blocking boundaries between neighboring grains. Conductivity isotherms at 50 °C of a series of glasses whose composition was given by x from the 0.4–0.6 range are shown in Fig. 2. It can be seen that the conductivity reaches its maximum at x = 55 which is close to the border of the amorphous region of the system under study. The corresponding conductivity isotherm of the composites based on the glasses is also shown in Fig. 2. It can be seen that: the conductivity values for composites are systematically lower by a factor of 5–7 than those of the respective glasses, they depend on x in a similar way as the conductivity values of the corresponding glasses and, finally, they seem not to depend visibly on the nature of the ceramic powder used (α-Al2O3 or ZrO2). The gap between values of the conductivity of the glasses and the respective composites was confirmed by conductivity measurements in the low-temperature range down to ca 90 K (Fig. 3). The activation energies in both cases remain the same (0.30 eV for glasses vs. 0.30–0.32 eV for composites). Only for the composites based on ZrO2 the activation energy value is slightly lower (0.28 eV). These facts indicate that the effective conductivity of the composites is entirely due to the conductivity of their glassy phase, which remains basically the same as in the as-quenched form. Slightly lower activation energy of the composite based
Fig. 4. Composition dependence of the microhardness of glasses xAgI·(100 — x) (0.67Ag2O·0.33B2O3) for 40 ≤x ≤ 60 (solid squares) and composites based on these glasses and α-Al2O3 (2 µm) (open circles) or ZrO2 (1 µm) (solid circles). Solid lines are the guide for the eye.
M. Wasiucionek et al. / Solid State Ionics 179 (2008) 1278–1281
on glass x = 55 and ZrO2 may arise from the presence of the ramified interfacial regions of nanometric dimensions seen in Fig. 1c. Such regions usually contain more defects than the bulk glassy phase, which is advantageous from the point of view of the ionic transport. It is necessary to notice that the presence of the second dispersed phase in an ionic conductor (here glasses of the AgI– Ag2O–B2O3 system) does not lead to a conductivity enhancement similar to that reported for a number of Ag+-ion conducting composites based on AgI and inert fillers like γ-Al2O3 [6,11,12]. There are at least two important reasons of absence of such an enhancement effect in the case of the composites under study. Firstly, the characteristic dimensions of regions occupied by both components are above the mesoscopic scale, i.e. above ca 100 nm. Moreover they do not interpenetrate each other at distances shorter than 100 nm. This is well visible in the SEM micrographs (Fig. 1a–c). The proximity of most of the conducting phase to the interfaces with the insulating one (distances between any part of the conducting phase and the nearest interface not exceeding ca 100 µm) was an important factor for the conductivity enhancement of the AgI: γ-Al2O3 composites studied in [6,11,12]. Secondly, the room temperature conductivity of glassy phase studied (ca 10− 3 S·cm− 1) is by several orders of magnitude higher than of β-AgI (below 10− 6 S·cm− 1). Consequently its further enhancement by the introduction of a second phase is unlikely, contrary to the case of composites AgI: γ-Al2O3 [6,11,12]. It should however be noticed that the ionic conductivity of the as-synthesized composites under study can be additionally enhanced by their appropriate thermal treatment. The treatment consisted in heating samples up to temperature corresponding to the beginning of the crystallization of the glassy phase (determined by Differential Scanning Calorimetry) and cooling down to room temperature. The effect of such procedure applied to glasses of the AgI– Ag2O–B2O3 system was a partial nanocrystallization. Newly formed crystallites had grain size of order of 30 nm [2]. 3.3. Mechanical properties Contrary to the as-received glasses the composite samples were non-brittle and machinable. It was possible to cut the composites into wafers as thin as 100 µm and polish them without breaking. At the moment for a quantitative measure of the improvement of the mechanical properties of the composites we have used their microhardness measured by the Vickers method. The results of these measurements are given in Fig. 4. It can be seen that the microhardness of the glasses slightly decreases as the AgI content increases. This is understandable, taking into account that the glass network in the glasses of the AgI–Ag2O–B2O3 system built of B–O bonds is firstly weakened by the presence of a Ag2O (a glass modifier), which causes breaking of many of
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these bonds. The addition of a high amount of AgI (a dopant) leads to a further decrease in the mechanical strength of the overall structure. Microhardness of the composites is systematically higher than that of the glasses, but the dependence of microhardness on content of silver iodide is similar — it decreases with an increase in x. Large deviations in the experimental values of microhardness of most composites may be caused by their heterogeneous character. Since microhardness is only an indication of mechanical properties of the composite material, further studies on mechanical properties are planned. Nevertheless the observations confirm that the mechanical properties of the composites are dominated by those of the glassy phase and that the grains of the hard second phase reinforce the composite, in most cases substantially. Further comparative studies on mechanical properties of the glasses and composites are under way. 4. Conclusions Ionically conducting composites based on highly conductive Ag+-ion glasses and hard ceramic powders (α-Al2O3 and ZrO2) were synthesized by a high-pressure method. Their electrical conductivity, though high, was systematically lower than that of the parent glasses. From the SEM micrographs of the composites it is clear that both phases: the conducting and the insulating one, do not interpenetrate each other at nanometer length scales, which is a prerequisite of a conductivity enhancement effect, observed in composites based on AgI and insulating second phases like γ-Al2O3. The presence of the second phase led to a mechanical reinforcement of the samples, reduced their brittleness and made them machinable. References [1] T. Minami, J. Non-Cryst. Solids 56 (1983) 15. [2] M. Foltyn, M. Wasiucionek, J. Garbarczyk, J.L. Nowinski, Solid State Ionics 176 (2005) 2137. [3] M. Zgirski, J.E. Garbarczyk, S. Gierlotka, B. Palosz, M. Wasiucionek, J.L. Nowinski, Solid State Ionics 176 (2005) 2141. [4] M. Foltyn, M. Wasiucionek, J.E. Garbarczyk, J.L. Nowinski, B. Palosz, S. Gierlotka, Glass Technol.: Eur. J. Glass Sci. Technol. A 47 (2006) 138. [5] M. Foltyn, M. Wasiucionek, J.E. Garbarczyk, J.L. Nowinski, S. Gierlotka, B. Palosz, J. Power Sources 173 (2007) 795. [6] H. Yamada, A.J. Bhattacharyya, J. Maier, Adv. Funct. Mater. 16 (2006) 525. [7] K. Tadanaga, K. Imai, M. Tatsumisago, T. Minami, J. Electrochem. Soc. 147 (2000) 4061. [8] Woo Lee, Han-Ill Yoob, Jin-Kyu Lee, Chem. Commun. 24 (2001) 2530. [9] J.E. Garbarczyk, P. Jozwiak, M. Wasiucionek, J.L. Nowinski, Solid State Ionics 177 (2006) 2585. [10] J.R. Dygas, M.W. Breiter, Electrochim. Acta 44 (1999) 4163. [11] J.-S. Lee, St. Adams, J. Maier, Solid State Ionics 136–137 (2000) 1261. [12] N.F. Uvarov, B.B. Bokhonov, A.A. Politov, P. Vanek, J. Petzelt, J. Mater. Synth. Process. 8 (2000) 327.