- Email: [email protected]
Enhancement of electrical conductivity in lithium vanadate glasses by nanocrystallization
Solid State Ionics 175 (2004) 691 – 694 www.elsevier.com/locate/ssi
Enhancement of electrical conductivity in lithium vanadate glasses by nanocrystallization J.E. Garbarczyk*, P. Jozwiak, M. Wasiucionek, J.L. Nowinski Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland Received 20 August 2004; accepted 24 August 2004
Abstract Electronically conducting nanomaterials were prepared by annealing glasses of the Li2O–V2O5–P2O5 system at crystallization temperature Tc. The electrical conductivity was measured for initial glassy samples and during the thermal treatment up to 400 8C. The presence of crystalline V2O5 grains formed after annealing at Tc was confirmed by X-ray diffractometry (XRD). The average size of these grains was estimated to about 30 nm. The nanomaterials obtained by annealing at Tc exhibit much higher conductivity (up to 101 S/cm at 360 8C) and much lower activation energy (E=0.27 eV) than the initial glasses. Moreover, such nanomaterials are thermally stable up to Tc=360 8C whereas the initial glasses of that composition are stable only to T=260 8C. This considerable enhancement of electrical conductivity after nanocrystallization is ascribed to formation of extensive and dense network of electronic conduction paths which are situated between V2O5 nanocrystals and on their surfaces. Further annealing at higher temperature T=398 8C leads to considerable growth of V2O5 crystallites (up to 1 Am) and formation of other crystalline phase. These phenomena lead to disappearance of aforementioned bconduction tissueQ for electrons and substantial reduction of electronic conductivity. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystallization; Electronic–ionic conduction; Lithium vanadate–phosphate glasses; Impedance spectroscopy
1. Introduction Silver and lithium glasses based on glass-forming oxides such as V2O5 and P2O5 exhibit high ionic conductivity (up to 102 S/cm) due to transport of Ag+ or Li+ ions [1]. In addition, glasses with high content of V 2O5 exhibit considerable electronic conduction [2]. Purely ionic conducting glasses can be used as solid electrolytes and those exhibiting mixed electronic–ionic conduction can be employed as cathode materials in novel advanced electrochemical cells. Electrical conductivity of glasses of the above systems can be enhanced by nanocrystallization [3]. The idea of nanocrystallization as a method of additional increase of electrical conductivity of glasses utilizes a fact that during formation of crystalline grains of nano- or
* Corresponding author. Tel.: +48 22 6607267; fax: +48 22 6282171. E-mail address: [email protected] (J.E. Garbarczyk). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.08.025
submicrometer size at grain surfaces, there are formed defective regions of increased electrical conductivity. The main goal of this work was synthesis and characterization of materials of the Li2O–V2O5–P2O5 system exhibiting improved electrical conductivity and thermal stability compared to those of the as-received glasses. Nanocrystallization was induced by annealing glasses under study at temperatures close to crystallization temperature.
2. Experimental Glasses of the Li2O–V2O5–P2O5 system were prepared by a standard melt quenching technique. Chemical compositions were described by a formula xLi2OS(1002x)V2O5S xP2O5 for x=15, 25, 35, 40, 45. The resulting material was fully amorphous, as confirmed by X-ray diffractometry patterns. In order to find characteristic temperatures such as glass transition temperature T g and crystallization temper-
692
J.E. Garbarczyk et al. / Solid State Ionics 175 (2004) 691–694
ature Tc, the glasses were studied by differential scanning calorimetry (DSC) using a Perkin-Elmer Pyris 1 DSC calorimeter [2]. The temperature range was 20–500 8C and the heating rate was set to 20 K/min. Structural properties of the materials under study, i.e. degree of amorphousness, presence of crystalline phases formed during annealing, their identification and average grain size were determined by X-ray diffractometry (XRD). A Philips X’Pert PRO setup was used for these studies. XRD patterns were collected at room temperature. The glasses were nanocrystallized by annealing at temperature close to crystallization temperature Tc determined from DSC thermograms. Electrical parameters of the samples, such as total electrical conductivity, were studied by impedance spectroscopy (IS) in the wide (20–4008C) temperature range. In order to measure electrical conductivity, gold electrodes were sputtered onto opposite flat surfaces of the samples. These electrodes were blocking for ions and reversible for electrons. The IS experiments were carried out using a setup consisting of an Impedance Analyzer Solartron 1260 and a temperature regulation and stabilization system. Frequency range was 10 mHz to 10 MHz, and amplitude of ac signal was set to 30 mV rms. All IS measurements were fully automated and run under computer control, temperature programming included. The acquired spectra were numerically analyzed on the basis of the equivalent circuit approach [4] using a computer package described in Ref. [5].
3. Results and discussion From among a series of samples of compositions, we focused our studies on the samples of the composition 15Li2OS70V2O5S15P2O5 corresponding to x=15. High content of vanadium and relatively low content of Li2O cause these glasses to be predominantly electronic conductors (via hopping of electrons between V4+ and V5+ centers). DSC traces of glasses of this composition show that these glasses easily crystallize at about 310 8C [6]. For some other compositions (x=25, 35) crystallization occurred at higher temperatures, and for glasses with high Li2O content (x=40, 45), it was not observed in the temperature range under study (i.e. up to 500 8C). Fig. 1 shows temperature dependence of conductivity of a glass of composition 15Li2Od 70V2O5d 15P2O5 during a cycle consisting of heating to 311 8C, annealing at that temperature for 7 h and cooling down to room temperature. Up to temperature ca. 260 8C, the temperature dependence of conductivity r(T) followed the Arrhenius formula: E rT ¼ r0 exp ð1Þ kT where r 0-preexponential factor, E-activation energy, and k-Boltzmann constant.
Fig. 1. Temperature dependence of electrical conductivity of a sample 15Li2Od 70V2O5d 15P2O5 during its heating and cooling together with a DSC thermogram taken at first heating run.
Up to 130 8C activation energy was equal to 0.41 eV, which is close to an often cited estimate of activation energy for electronically conductive V2O5-rich glasses [7]. In the range 130–260 8C, the activation energy is slightly higher (0.50 eV), but the Arrhenius-like dependence is preserved. The change of slope of the temperature dependence of conductivity at ca. 130 8C is related to the influence of ionic component of the total electrical conductivity which becomes non-negligible at that temperature. The detailed analysis of that phenomenon is going to be published in a separate article. At ca. 260 8C, which is very close to the glass transition temperature T g determined from DSC scan (inserted in Fig. 1), the conductivity sharply increases. Up to 311 8C, it changes in the obviously non-Arrhenian way. Remarkably good fit to the temperature dependence of conductivity in this range is given by the Vogel–Tammann– Fulcher (VTF) formula, widely used to fit temperature dependencies of conductivity of many polymers and glassforming liquids: rT ¼ Aexp
B ; T T0
ð2Þ
where A, B-constants and T 0-so-called bideal glass transition temperatureQ.
J.E. Garbarczyk et al. / Solid State Ionics 175 (2004) 691–694
It should be noted that in this case the best-fit parameter T 0=236F27 8C is about 25 8C lower than T g determined from DSC thermograms (261 8C), which is a common empirical relation between T g and T 0 temperatures for many glass-forming systems. The heating at this stage stopped at the crystallization temperature Tc (marked at the DSC curve in Fig. 1). Annealing at that temperature lasted for about 7 h and did not lead to any change of conductivity. On cooling, the temperature dependence of conductivity followed the Arrhenius equation (Eq. (1)) but with much lower average activation energy (0.27 eV) than during heating. The absolute values of conductivity were at the cooling stage more than 10 times higher than those for initial glass on heating. For example, at 261 8C, conductivity value on cooling was 5.1102 S/cm while on heating it was only 4.7103 S/cm. In order to check if this enhanced conductivity is stable and reproducible, we performed a series of conductivity measurements during the second heating–cooling cycle (denoted by labels 3 and 4 in Fig. 2). On heating, the conductivity closely followed its dependence from the cooling stage in the first cycle, up to temperature exceeding Tc=311 8C. This means that the material formed during the first run exhibits the Arrhenius-like increase of conductivity to 360 8C, with no time-dependent effects up to that temperature. In the first run, major changes of conductivity occurred at 260 8C (Fig. 1). This means that after the annealing, the upper temperature limit of stability was raised from 260 to about 360 8C. Only at T cVc360 8C and, to a greater extent, at 398 8C, the conductivity started to deviate
Fig. 2. Thermal history of a glass of composition 15Li2Od 70V2Od 15P2O5 during nanocrystallization by annealing. Numbers in boxes denote consecutive heating and cooling stages. For clarity, the experimental points are omitted. Further details in the text.
693
Fig. 3. XRD patterns of the glass 15Li2Od 70V2O5d 15P2O5: (a) before annealing; (b) after 7 h of annealing at Tc=311 8C; (c) after additional annealing for 5 h at 398 8C. Filled circles mark peaks corresponding to crystalline V2O5. Open circles denote unidentified crystalline phase.
from Arrhenius-like behavior. It even started to decrease at the highest temperature of ca 400 8C. After a drop of conductivity by more than one order of magnitude observed at 400 8C, the sample was cooled down to room temperature (stage 4 in Fig. 2). The conductivity changed with temperature according to Arrhenius formula (Eq. (1)). Activation energy was 0.50 eV in the whole 20–400 8C temperature range. All above listed effects were ascribed to structural changes occurring in initially heated glasses, and further changes in annealed glass-crystalline materials. In order to investigate these changes of structure, a series of XRD experiments were done on samples at different stages of nanocrystallization process. The results for samples of composition 15Li2Od 70V2O5d 15P2O5 are shown in Fig. 3. In the case of as-received materials (Fig. 3a), there is only a wide halo observed with no indication of diffraction peaks. This confirms the amorphous state of initial glasses. A pattern shown in Fig. 3b was collected for a sample after its annealing at crystallization temperature Tc=311 8C, determined from DSC studies (Fig. 1). It contains a number of peaks corresponding to a crystalline phase superimposed on a wide halo indicating that there is still considerable amount of amorphous phase. The XRD pattern in Fig. 3b is a clear indication that annealing at temperature Tc only starts to produce small crystallites in the glass matrix. This is a valuable hint, since it means that by adjusting the temperature of annealing, one can control the amount of crystalline grains formed in the material and thus improve its electrical conductivity. The crystalline phase was identified as V2O5. Average size of crystallites was estimated from the widths of diffraction peaks, using the Scherrer formula, to ca. 30 nm. The second annealing, this time at temperature 398 8C, considerably exceeding temperature Tc and slightly exceeding temperature T cV (Fig. 2), leads to massive
694
J.E. Garbarczyk et al. / Solid State Ionics 175 (2004) 691–694
crystallization (Fig. 3c). After this stage, the XRD pattern does not contain a halo, peaks corresponding to V2O5 are much stronger and, additionally, there are some peaks corresponding to the phase which we have not identified yet. The estimate of average grain size at this stage is close to 100 nm; however, microscopic observations revealed also the presence of larger grains (up to 1 Am). As mentioned above, such material exhibited much lower conductivity than the nanomaterial obtained by annealing at temperature Tc=311 8C. The observed changes of conductivity and structure of the material under study can be explained in the following way. The most important for electronic conduction in the glasses of the Li2O–V2O5–P2O5 system with high amount of V2O5 is the spatial distribution of V4+ and V5+ ions which are centers of hopping for electrons [7]. In the initial glass, there is a random distribution of such centers. The annealing at Tc=311 8C leads to formation of nanocrystallites of V2O5 embedded in the glass matrix. Since the average size of these grains is small (ca. 30 nm), the interface between crystalline and amorphous phases is very extensively ramified and strongly influences overall electrical properties of the nanomaterial. In particular, it may contain the enhanced concentration of V4+ and V5+ centers distributed on the surface of V2O5 crystallites. Material annealed at much higher temperature (398 8C) is polycrystalline with large grains of V2O5 (up to 1 Am). It does not contain any amorphous phase, but there are traces of other, not identified yet, crystalline phases (probably from Li2O–P2O5 system). In this case, the interfacial bconducting tissueQ is reduced and the conductivity of the material considerably decreases. As a result, the effective electronic resistivity is a sum of interand intragrain resistances connected in series, which causes a decrease of electronic conduction and an increase of the effective activation energy of this conduction. It seems that the V2O5 grains in final material are too large and too close-packed to make possible a considerable long-range ionic transport. Similar increase of electrical conductivity after nanocrystallization, as discussed for a glass of composition 15Li2Od 70V2O5d 15P2O5 (x=15), was also observed for glasses with x=25 and x=35. Those glasses crystallized at higher temperatures (Tc=385 8C and 435 8C, respectively). In the case of glasses with high content of Li2O (x=40, 45) exhibiting predominantly ionic conduction, no crystallization was observed up to 500 8C.
4. Summary Selected glasses of the Li2O–V2O5–P2O5 system have been transformed into nanomaterials via annealing at crystallization temperature Tc determined from DSC thermograms. After annealing they consist of small crystallites of V2O5 (average size ca. 30 nm) embedded in glassy matrix. The resulting materials exhibit much higher electrical conductivity and are thermally stable to higher temperature than the initial glasses. Further annealing of these nanomaterials at higher temperature (ca. 400 8C) causes their massive crystallization. Average grain size of V2O5 crystallites increases to ca 1 Am. Moreover, other crystalline phase appears, which has not been identified so far. Electrical conductivity of this polycrystalline material is much lower than before that stage of annealing. These changes of electrical conductivity have been ascribed to changes in microstructure of the material. When crystalline grains are small and their concentration is high, the interface between glass matrix and these grains forms an extensively ramified system of conducting paths for electrons—a bconducting tissueQ. When grains are large and amorphous phase disappears, then these interfacial regions also disappear and conductivity of the material considerably decreases.
Acknowledgements Authors gratefully acknowledge financial support from State Committee for Scientific Research under Grant PBZ/ KBN-013/T08.
References [1] T. Minami, Y. Takuma, M. Tanaka, J. Electrochem. Soc. 124 (1977) 1659. [2] J.E. Garbarczyk, M. Wasiucionek, P. Jo´z´wiak, L. Tykarski, J.L. Nowin´ski, Solid State Ionics 154–155 (2002) 367. [3] S. Adams, K. Hariharan, J. Maier, Solid State Ionics 86–88 (1996) 503. [4] J.R. Macdonald, Impedance Spectroscopy: Emphasizing Solid State Materials and Sytems, Wiley-Interscience, New York, 1987. [5] J. Dygas, PhD Thesis, Northwestern University, Evanston, 1986. [6] P.J. Jo´z´wiak, PhD Thesis, Warsaw University of Technology, Warsaw, 2003. [7] J.E. Garbarczyk, M. Wasiucionek, P. Machowski, W. Jakubowski, Solid State Ionics 119 (1999) 9.
Enhancement of electrical conductivity in lithium vanadate glasses by nanocrystallization J.E. Garbarczyk*, P. Jozwiak, M. Wasiucionek, J.L. Nowinski Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warszawa, Poland Received 20 August 2004; accepted 24 August 2004
Abstract Electronically conducting nanomaterials were prepared by annealing glasses of the Li2O–V2O5–P2O5 system at crystallization temperature Tc. The electrical conductivity was measured for initial glassy samples and during the thermal treatment up to 400 8C. The presence of crystalline V2O5 grains formed after annealing at Tc was confirmed by X-ray diffractometry (XRD). The average size of these grains was estimated to about 30 nm. The nanomaterials obtained by annealing at Tc exhibit much higher conductivity (up to 101 S/cm at 360 8C) and much lower activation energy (E=0.27 eV) than the initial glasses. Moreover, such nanomaterials are thermally stable up to Tc=360 8C whereas the initial glasses of that composition are stable only to T=260 8C. This considerable enhancement of electrical conductivity after nanocrystallization is ascribed to formation of extensive and dense network of electronic conduction paths which are situated between V2O5 nanocrystals and on their surfaces. Further annealing at higher temperature T=398 8C leads to considerable growth of V2O5 crystallites (up to 1 Am) and formation of other crystalline phase. These phenomena lead to disappearance of aforementioned bconduction tissueQ for electrons and substantial reduction of electronic conductivity. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystallization; Electronic–ionic conduction; Lithium vanadate–phosphate glasses; Impedance spectroscopy
1. Introduction Silver and lithium glasses based on glass-forming oxides such as V2O5 and P2O5 exhibit high ionic conductivity (up to 102 S/cm) due to transport of Ag+ or Li+ ions [1]. In addition, glasses with high content of V 2O5 exhibit considerable electronic conduction [2]. Purely ionic conducting glasses can be used as solid electrolytes and those exhibiting mixed electronic–ionic conduction can be employed as cathode materials in novel advanced electrochemical cells. Electrical conductivity of glasses of the above systems can be enhanced by nanocrystallization [3]. The idea of nanocrystallization as a method of additional increase of electrical conductivity of glasses utilizes a fact that during formation of crystalline grains of nano- or
* Corresponding author. Tel.: +48 22 6607267; fax: +48 22 6282171. E-mail address: [email protected] (J.E. Garbarczyk). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.08.025
submicrometer size at grain surfaces, there are formed defective regions of increased electrical conductivity. The main goal of this work was synthesis and characterization of materials of the Li2O–V2O5–P2O5 system exhibiting improved electrical conductivity and thermal stability compared to those of the as-received glasses. Nanocrystallization was induced by annealing glasses under study at temperatures close to crystallization temperature.
2. Experimental Glasses of the Li2O–V2O5–P2O5 system were prepared by a standard melt quenching technique. Chemical compositions were described by a formula xLi2OS(1002x)V2O5S xP2O5 for x=15, 25, 35, 40, 45. The resulting material was fully amorphous, as confirmed by X-ray diffractometry patterns. In order to find characteristic temperatures such as glass transition temperature T g and crystallization temper-
692
J.E. Garbarczyk et al. / Solid State Ionics 175 (2004) 691–694
ature Tc, the glasses were studied by differential scanning calorimetry (DSC) using a Perkin-Elmer Pyris 1 DSC calorimeter [2]. The temperature range was 20–500 8C and the heating rate was set to 20 K/min. Structural properties of the materials under study, i.e. degree of amorphousness, presence of crystalline phases formed during annealing, their identification and average grain size were determined by X-ray diffractometry (XRD). A Philips X’Pert PRO setup was used for these studies. XRD patterns were collected at room temperature. The glasses were nanocrystallized by annealing at temperature close to crystallization temperature Tc determined from DSC thermograms. Electrical parameters of the samples, such as total electrical conductivity, were studied by impedance spectroscopy (IS) in the wide (20–4008C) temperature range. In order to measure electrical conductivity, gold electrodes were sputtered onto opposite flat surfaces of the samples. These electrodes were blocking for ions and reversible for electrons. The IS experiments were carried out using a setup consisting of an Impedance Analyzer Solartron 1260 and a temperature regulation and stabilization system. Frequency range was 10 mHz to 10 MHz, and amplitude of ac signal was set to 30 mV rms. All IS measurements were fully automated and run under computer control, temperature programming included. The acquired spectra were numerically analyzed on the basis of the equivalent circuit approach [4] using a computer package described in Ref. [5].
3. Results and discussion From among a series of samples of compositions, we focused our studies on the samples of the composition 15Li2OS70V2O5S15P2O5 corresponding to x=15. High content of vanadium and relatively low content of Li2O cause these glasses to be predominantly electronic conductors (via hopping of electrons between V4+ and V5+ centers). DSC traces of glasses of this composition show that these glasses easily crystallize at about 310 8C [6]. For some other compositions (x=25, 35) crystallization occurred at higher temperatures, and for glasses with high Li2O content (x=40, 45), it was not observed in the temperature range under study (i.e. up to 500 8C). Fig. 1 shows temperature dependence of conductivity of a glass of composition 15Li2Od 70V2O5d 15P2O5 during a cycle consisting of heating to 311 8C, annealing at that temperature for 7 h and cooling down to room temperature. Up to temperature ca. 260 8C, the temperature dependence of conductivity r(T) followed the Arrhenius formula: E rT ¼ r0 exp ð1Þ kT where r 0-preexponential factor, E-activation energy, and k-Boltzmann constant.
Fig. 1. Temperature dependence of electrical conductivity of a sample 15Li2Od 70V2O5d 15P2O5 during its heating and cooling together with a DSC thermogram taken at first heating run.
Up to 130 8C activation energy was equal to 0.41 eV, which is close to an often cited estimate of activation energy for electronically conductive V2O5-rich glasses [7]. In the range 130–260 8C, the activation energy is slightly higher (0.50 eV), but the Arrhenius-like dependence is preserved. The change of slope of the temperature dependence of conductivity at ca. 130 8C is related to the influence of ionic component of the total electrical conductivity which becomes non-negligible at that temperature. The detailed analysis of that phenomenon is going to be published in a separate article. At ca. 260 8C, which is very close to the glass transition temperature T g determined from DSC scan (inserted in Fig. 1), the conductivity sharply increases. Up to 311 8C, it changes in the obviously non-Arrhenian way. Remarkably good fit to the temperature dependence of conductivity in this range is given by the Vogel–Tammann– Fulcher (VTF) formula, widely used to fit temperature dependencies of conductivity of many polymers and glassforming liquids: rT ¼ Aexp
B ; T T0
ð2Þ
where A, B-constants and T 0-so-called bideal glass transition temperatureQ.
J.E. Garbarczyk et al. / Solid State Ionics 175 (2004) 691–694
It should be noted that in this case the best-fit parameter T 0=236F27 8C is about 25 8C lower than T g determined from DSC thermograms (261 8C), which is a common empirical relation between T g and T 0 temperatures for many glass-forming systems. The heating at this stage stopped at the crystallization temperature Tc (marked at the DSC curve in Fig. 1). Annealing at that temperature lasted for about 7 h and did not lead to any change of conductivity. On cooling, the temperature dependence of conductivity followed the Arrhenius equation (Eq. (1)) but with much lower average activation energy (0.27 eV) than during heating. The absolute values of conductivity were at the cooling stage more than 10 times higher than those for initial glass on heating. For example, at 261 8C, conductivity value on cooling was 5.1102 S/cm while on heating it was only 4.7103 S/cm. In order to check if this enhanced conductivity is stable and reproducible, we performed a series of conductivity measurements during the second heating–cooling cycle (denoted by labels 3 and 4 in Fig. 2). On heating, the conductivity closely followed its dependence from the cooling stage in the first cycle, up to temperature exceeding Tc=311 8C. This means that the material formed during the first run exhibits the Arrhenius-like increase of conductivity to 360 8C, with no time-dependent effects up to that temperature. In the first run, major changes of conductivity occurred at 260 8C (Fig. 1). This means that after the annealing, the upper temperature limit of stability was raised from 260 to about 360 8C. Only at T cVc360 8C and, to a greater extent, at 398 8C, the conductivity started to deviate
Fig. 2. Thermal history of a glass of composition 15Li2Od 70V2Od 15P2O5 during nanocrystallization by annealing. Numbers in boxes denote consecutive heating and cooling stages. For clarity, the experimental points are omitted. Further details in the text.
693
Fig. 3. XRD patterns of the glass 15Li2Od 70V2O5d 15P2O5: (a) before annealing; (b) after 7 h of annealing at Tc=311 8C; (c) after additional annealing for 5 h at 398 8C. Filled circles mark peaks corresponding to crystalline V2O5. Open circles denote unidentified crystalline phase.
from Arrhenius-like behavior. It even started to decrease at the highest temperature of ca 400 8C. After a drop of conductivity by more than one order of magnitude observed at 400 8C, the sample was cooled down to room temperature (stage 4 in Fig. 2). The conductivity changed with temperature according to Arrhenius formula (Eq. (1)). Activation energy was 0.50 eV in the whole 20–400 8C temperature range. All above listed effects were ascribed to structural changes occurring in initially heated glasses, and further changes in annealed glass-crystalline materials. In order to investigate these changes of structure, a series of XRD experiments were done on samples at different stages of nanocrystallization process. The results for samples of composition 15Li2Od 70V2O5d 15P2O5 are shown in Fig. 3. In the case of as-received materials (Fig. 3a), there is only a wide halo observed with no indication of diffraction peaks. This confirms the amorphous state of initial glasses. A pattern shown in Fig. 3b was collected for a sample after its annealing at crystallization temperature Tc=311 8C, determined from DSC studies (Fig. 1). It contains a number of peaks corresponding to a crystalline phase superimposed on a wide halo indicating that there is still considerable amount of amorphous phase. The XRD pattern in Fig. 3b is a clear indication that annealing at temperature Tc only starts to produce small crystallites in the glass matrix. This is a valuable hint, since it means that by adjusting the temperature of annealing, one can control the amount of crystalline grains formed in the material and thus improve its electrical conductivity. The crystalline phase was identified as V2O5. Average size of crystallites was estimated from the widths of diffraction peaks, using the Scherrer formula, to ca. 30 nm. The second annealing, this time at temperature 398 8C, considerably exceeding temperature Tc and slightly exceeding temperature T cV (Fig. 2), leads to massive
694
J.E. Garbarczyk et al. / Solid State Ionics 175 (2004) 691–694
crystallization (Fig. 3c). After this stage, the XRD pattern does not contain a halo, peaks corresponding to V2O5 are much stronger and, additionally, there are some peaks corresponding to the phase which we have not identified yet. The estimate of average grain size at this stage is close to 100 nm; however, microscopic observations revealed also the presence of larger grains (up to 1 Am). As mentioned above, such material exhibited much lower conductivity than the nanomaterial obtained by annealing at temperature Tc=311 8C. The observed changes of conductivity and structure of the material under study can be explained in the following way. The most important for electronic conduction in the glasses of the Li2O–V2O5–P2O5 system with high amount of V2O5 is the spatial distribution of V4+ and V5+ ions which are centers of hopping for electrons [7]. In the initial glass, there is a random distribution of such centers. The annealing at Tc=311 8C leads to formation of nanocrystallites of V2O5 embedded in the glass matrix. Since the average size of these grains is small (ca. 30 nm), the interface between crystalline and amorphous phases is very extensively ramified and strongly influences overall electrical properties of the nanomaterial. In particular, it may contain the enhanced concentration of V4+ and V5+ centers distributed on the surface of V2O5 crystallites. Material annealed at much higher temperature (398 8C) is polycrystalline with large grains of V2O5 (up to 1 Am). It does not contain any amorphous phase, but there are traces of other, not identified yet, crystalline phases (probably from Li2O–P2O5 system). In this case, the interfacial bconducting tissueQ is reduced and the conductivity of the material considerably decreases. As a result, the effective electronic resistivity is a sum of interand intragrain resistances connected in series, which causes a decrease of electronic conduction and an increase of the effective activation energy of this conduction. It seems that the V2O5 grains in final material are too large and too close-packed to make possible a considerable long-range ionic transport. Similar increase of electrical conductivity after nanocrystallization, as discussed for a glass of composition 15Li2Od 70V2O5d 15P2O5 (x=15), was also observed for glasses with x=25 and x=35. Those glasses crystallized at higher temperatures (Tc=385 8C and 435 8C, respectively). In the case of glasses with high content of Li2O (x=40, 45) exhibiting predominantly ionic conduction, no crystallization was observed up to 500 8C.
4. Summary Selected glasses of the Li2O–V2O5–P2O5 system have been transformed into nanomaterials via annealing at crystallization temperature Tc determined from DSC thermograms. After annealing they consist of small crystallites of V2O5 (average size ca. 30 nm) embedded in glassy matrix. The resulting materials exhibit much higher electrical conductivity and are thermally stable to higher temperature than the initial glasses. Further annealing of these nanomaterials at higher temperature (ca. 400 8C) causes their massive crystallization. Average grain size of V2O5 crystallites increases to ca 1 Am. Moreover, other crystalline phase appears, which has not been identified so far. Electrical conductivity of this polycrystalline material is much lower than before that stage of annealing. These changes of electrical conductivity have been ascribed to changes in microstructure of the material. When crystalline grains are small and their concentration is high, the interface between glass matrix and these grains forms an extensively ramified system of conducting paths for electrons—a bconducting tissueQ. When grains are large and amorphous phase disappears, then these interfacial regions also disappear and conductivity of the material considerably decreases.
Acknowledgements Authors gratefully acknowledge financial support from State Committee for Scientific Research under Grant PBZ/ KBN-013/T08.
References [1] T. Minami, Y. Takuma, M. Tanaka, J. Electrochem. Soc. 124 (1977) 1659. [2] J.E. Garbarczyk, M. Wasiucionek, P. Jo´z´wiak, L. Tykarski, J.L. Nowin´ski, Solid State Ionics 154–155 (2002) 367. [3] S. Adams, K. Hariharan, J. Maier, Solid State Ionics 86–88 (1996) 503. [4] J.R. Macdonald, Impedance Spectroscopy: Emphasizing Solid State Materials and Sytems, Wiley-Interscience, New York, 1987. [5] J. Dygas, PhD Thesis, Northwestern University, Evanston, 1986. [6] P.J. Jo´z´wiak, PhD Thesis, Warsaw University of Technology, Warsaw, 2003. [7] J.E. Garbarczyk, M. Wasiucionek, P. Machowski, W. Jakubowski, Solid State Ionics 119 (1999) 9.