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Dielectric properties of La1.75Ba0.25NiO4 ceramics prepared by spark plasma sintering
Journal of Alloys and Compounds 490 (2010) 605–608
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Dielectric properties of La1.75 Ba0.25 NiO4 ceramics prepared by spark plasma sintering C.L. Song, Y.J. Wu ∗ , X.Q. Liu, X.M. Chen Department of Materials Science and Engineering, Zhejiang University, 38, Zheda Road, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 2 July 2009 Received in revised form 14 October 2009 Accepted 15 October 2009 Available online 24 October 2009 Keywords: Ceramics Dielectric response La1.75 Ba0.25 NiO4 Spark plasma sintering
a b s t r a c t The microstructure and dielectric properties of La1.75 Ba0.25 NiO4 ceramics prepared by spark plasma sintering (SPS) process were investigated. The interstitial oxygen could be removed by the SPS process, and this would induce the significant decrease of electrical conductivity of the sample. The grain growth could be suppressed while the pore removal could be enhanced by the SPS process. The dielectric constant and loss were both much lower than those of Ln2−x Srx NiO4 ceramics, while the activation energy of low temperature dielectric relaxation was larger. There were two dielectric relaxations in the present ceramics, the low temperature one was closely related to the charge order–disorder transition, while the high temperature one was originated from the extrinsic effects. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The giant dielectric constant materials have attracted many attentions since the discovery of CaCu3 Ti4 O12 (CCTO) because of their greatly potential applications in the microelectronics [1–8]. Recently, the giant dielectric responses up to high frequency are found in charge-ordered Ln2−x Srx NiO4 (Ln = La, Nd and Sm) ceramics [9,10], especially, the temperature-stable colossal dielectric constant (∼105 ) in a broad temperature range of 150–500 K up to 5 MHz is observed in Sm1.5 Sr0.5 NiO4 ceramics [10], this stimulates the research interests in the charge-ordered nickelates [11]. However, the rather high dielectric losses limit the practical application of these materials, and the high losses should be contributed from the high electrical conductivities of the materials. There are two methods to reduce the electrical conductivities in these materials, one is decrease of the doping content, i.e. the value of x, and the other one is replacement of the doping element strontium with barium [12,13]. Here we combine these two methods and select La1.75 Ba0.25 NiO4 as the object. The charge ordering temperature of the present material is about 140 K [13,14]. Actually it is difficult to prepare the stoichiometric ceramics by the conventional solidstate sintering process in air due to the capability of K2 NiF4 -type arrangements for accommodating extra oxygen in the interstitials of the structure [15,16]. The extra interstitial oxygen will increase the conductivity of the sample [17,18], and this will deteriorate the
∗ Corresponding author. Tel.: +86 571 87951410; fax: +86 571 87951410. E-mail address: [email protected] (Y.J. Wu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.114
dielectric loss in turn. So, the stoichiometric ceramics are expected to meet the requirement of low dielectric loss in the practical application. On the other hand, the spark plasma sintering (SPS) method has been used widely as a low temperature and rapid sintering method in the past decade [19–25]. The dense ceramics with smaller grain size, for example, nano-scale, can be easily prepared by SPS method because of its much lower sintering temperature and time [25]. The furnace chamber is under vacuum and the graphite die is used during the sintering process, these factors favor the extraction of interstitial oxygen from the sample [16], so the SPS may be a good way to prepare the stoichiometric La1.75 Ba0.25 NiO4 ceramics. In present work, La1.75 Ba0.25 NiO4 ceramics are prepared by SPS and solid-state sintering process, respectively, and the microstructures together with dielectric properties of the present ceramics are characterized. 2. Experimental details La1.75 Ba0.25 NiO4 powders were synthesized by a solid-state reaction process using La2 O3 (99.99%), BaCO3 (99.93%) and NiO (99%) as the starting materials, which were weighted and mixed by ball milling with ZrO2 balls in ethanol for 24 h, then dried and calcined at 1200 ◦ C in air for 3 h to yield the desired materials. The calcined powders were ball milled for 24 h and then dried. The obtained powders were added into a graphite die and sintered at 1025 ◦ C for 5 min under a vacuum of 6 Pa with an SPS apparatus (SPS-1050). During the period of heating and soaking, a pressure of 30 MPa was applied to the sample. The heating rate was 100 ◦ C/min from room temperature to 900 ◦ C and 62.5 ◦ C/min from 900 ◦ C to 1025 ◦ C. Then the as-sintered samples were annealed in air at 600 ◦ C for 2 h to remove the residual carbon on the surface of the samples, and dense ceramics (relative density was larger than 95% of the theoretical density) were obtained. For the samples prepared by the conventional solid-state sintering method, the calcined powders with 8 wt% polyvinyl alcohols (PVA) were pressed into pellets, and then sintered in air at 1450 ◦ C for 3 h.
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C.L. Song et al. / Journal of Alloys and Compounds 490 (2010) 605–608
The crystalline phase was identified by powder X-ray diffraction using Cu K␣ radiation (Rigaku 2550/PC). The microstructures were observed on the fracture surfaces with a field emission scanning electron microscopy (Hitachi S-4800). The dielectric characteristics and ac conductivities of these ceramics were evaluated with a broadband dielectric spectrometer (Novocontrol Turkey Concept 50) in a broad range of temperature (128–573 K) and frequency (1 Hz–10 MHz) with a heating rate of 2 K/min, and the silver paste was adopted as electrodes.
3. Results and discussion The sintering behavior of La1.75 Ba0.25 NiO4 ceramics during spark plasmas sintering process is shown in Fig. 1. A rather large thermal expansion is observed when the temperature increases from room temperature to about 980 ◦ C. The shrinkage initiates at about 980 ◦ C, and it persists in the front of soaking period at 1025 ◦ C, then it almost disappears when the soaking time goes beyond 4 min, suggesting that the densification process is almost complete. The densification temperature of SPS process is about 425 ◦ C lower than that of solid-state sintering method. The relatively low sintering temperature and short sintering time can be attributed to the merits of the SPS method. The inset of Fig. 1 shows the sintering time dependence of vacuum in the furnace chamber, there is a sharp peak in the curve and the corresponding temperature is about 1000 ◦ C. The sharp decrease of the vacuum in the chamber indicates that some gases emit from the sample. Combined with the result of electrical conductivity (see the discussion in the later chapter), we conclude that this peak should be originated from the emission of interstitial oxygen, which has come from the air during the calcining process. The X-ray diffraction (XRD) patterns of La1.75 Ba0.25 NiO4 calcined powder and ceramics prepared by solid-state sintering and SPS methods are shown in Fig. 2. It shows that the single La1.75 Ba0.25 NiO4 phase is obtained in the calcined powder and ceramics prepared by solid-state sintering method. While minor secondary phase, i.e. La2 O3 phase (PDF# 74-1144, the content is about 2.4 wt%), is found in the ceramics prepared by SPS process, it should be decomposed from the parent compound through the local discharge between the particles during the sintering process. The space group of main phase is I 4/mmm, which can be indexed by the La2 NiO4 (PDF# 72-1241) as the model, and the cell parameters are a = 3.8611(4) Å, and c = 12.753(2) Å, respectively. The cell parameters are consistent with the results of the previous work [13]. Fig. 3 shows the SEM micrographs of La1.75 Ba0.25 NiO4 ceramics prepared by solid-state sintering and SPS methods. The spark plasma-sintered sample has uniform grain size distribution and the size is about 1 m, while the inhomogeneous microstructure is observed on the solid-state sintered sample and the average
Fig. 1. Shrinkage curve and sample’s temperature as a function of sintering time during spark plasma sintering process of La1.75 Ba0.25 NiO4 ceramics. The inset is the furnace chamber’s vacuum during the SPS process.
Fig. 2. X-ray diffraction patterns of La1.75 Ba0.25 NiO4 : (a) calcined powder and ceramics prepared by (b) solid-state sintering and (c) SPS process.
size is larger than 5 m. The La2 O3 secondary phase may inhabit the grain growth of the ceramics, but it will concurrently degenerate the densification process of the ceramics, i.e., the optimal sintering temperature should be increased [26]. However, the much smaller grain size should be resulted from the much shorter sintering time during the spark plasma sintering process (16 min versus 7 h) for the sintering temperature is too low to promote the densification process in the present work. Also, more pores are observed on the solid-state sintered sample than those of the spark plasma-sintered sample as shown in the SEM micrograph. These phenomena also indicate the merits of the SPS process.
Fig. 3. SEM micrographs of the fracture surfaces of La1.75 Ba0.25 NiO4 ceramics prepared by (a) solid-state sintering and (b) SPS process.
C.L. Song et al. / Journal of Alloys and Compounds 490 (2010) 605–608
Fig. 4 shows the comparison of electrical conductivities among the samples prepared by solid-state sintering method, SPS process and the spark plasma-sintered sample annealed at 800 ◦ C in air for 2 h. The electrical conductivity of the sample prepared by SPS is much lower than that of the other two samples, while the values of the other two samples are almost same. The spark plasmasintered atmosphere is in vacuum, and the reducing atmosphere may exist for the graphite die is used during the sintering process. There are some gases emitted from the sample during the spark plasma sintering process as shown in the inset of Fig. 1, and they should be come from the oxygen element in the sample. There are two types of non-stoichiometric oxygen defects that can be existed in the present ceramics, one is the interstitial oxygen and the other is lattice oxygen vacancy. The removal of the interstitial oxygen will decrease the electrical conductivity of the sample, while the creation of lattice oxygen vacancy will increase the electrical conductivity of the sample [17,18]. So, we conclude that the low electrical conductivity of spark plasma-sintered sample should be resulted from the extraction the interstitial oxygen from the sample during the sintering process. After annealing the spark plasma-sintered sample at 800 ◦ C in air, the oxygen in air will infiltrate into the lattice to form interstitials and the interstitial oxygen will increase the electrical conductivity of the present sample. Fig. 5 shows the temperature dependence of dielectric properties for La1.75 Ba0.25 NiO4 ceramics prepared by SPS process. While no stable and credible data can be obtained from the solid-state sintered and annealed samples, and this may be deduced from the high electrical conductivities in these two samples. This emphasizes the advantage of the spark plasma sintering method over the conventional solid-state process in the present ceramics. The dielectric constant is about 500, which is much lower than that of Ln2−x Srx NiO4 (Ln = La, Nd, and Sm) ceramics [9–11]. And the dielectric loss is also much lower than that of Ln2−x Srx NiO4 ceramics. There are two relaxations in the curve of the temperature dependence of dielectric constant, one is around 100–200 K, and the other is around the room temperature. The low temperature relaxation is the onset of dielectric constant step, and a corresponding peak is found in the curve of temperature dependence of dielectric loss. This dielectric relaxation is around the charge-ordered temperature of the present ceramics [13–14], and the relaxation should be closely related to the charge order–disorder transition as shown in the previous work [9,10]. After plotting the frequency dependence of peak temperatures in the dielectric loss curve (as shown in the inset of Fig. 5b), the Arrhenius law is used to fit the linear relationship. The fitting results in the activation energy of 0.218 ± 0.003 eV,
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Fig. 5. Temperature dependence of (a) dielectric constant and (b) dielectric loss for La1.75 Ba0.25 NiO4 ceramics prepared by SPS. The inset is frequency dependence of the peak temperature of dielectric loss in the ceramics.
which is larger than that of Sm1.5 Sr0.5 NiO4 ceramics [10]. This may deduce from the different charge ordering statue in the present ceramics which will investigate in the further work. Above the room temperature, another dielectric relaxation is obvious at the low frequency, while it almost disappears at high frequency. So, this phenomenon indicates the relaxation should be originated from the extrinsic effects, such as Maxwell–Wagner effect [27]. 4. Conclusions Dense La1.75 Ba0.25 NiO4 ceramics with minor impurity have been prepared by SPS process. The interstitial oxygen can be removed by the SPS process, and this may result in the decrease of electrical conductivity of the sample. The dielectric constant and loss of the present ceramics are both much lower than those of La2−x Srx NiO4 ceramics, while the activation energy is larger. There are two dielectric relaxations in the present ceramics; the lower temperature relaxation should be closely related to the charge ordering, while the higher temperature one is originated from the extrinsic effects, such as Maxwell–Wagner effect. Although the dielectric loss is lower than that of the other charge ordering nickelate ceramics, the present ceramics is not suitable for the practical application because of its some high dielectric loss. Acknowledgement This work was supported by National Science Foundation of China under Grant Nos. 50702049 and 50832005. References
Fig. 4. Comparison of electrical conductivities of La1.75 Ba0.25 NiO4 ceramics prepared by SPS, annealed-SPS and solid-state sintering process at room temperature.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Dielectric properties of La1.75 Ba0.25 NiO4 ceramics prepared by spark plasma sintering C.L. Song, Y.J. Wu ∗ , X.Q. Liu, X.M. Chen Department of Materials Science and Engineering, Zhejiang University, 38, Zheda Road, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 2 July 2009 Received in revised form 14 October 2009 Accepted 15 October 2009 Available online 24 October 2009 Keywords: Ceramics Dielectric response La1.75 Ba0.25 NiO4 Spark plasma sintering
a b s t r a c t The microstructure and dielectric properties of La1.75 Ba0.25 NiO4 ceramics prepared by spark plasma sintering (SPS) process were investigated. The interstitial oxygen could be removed by the SPS process, and this would induce the significant decrease of electrical conductivity of the sample. The grain growth could be suppressed while the pore removal could be enhanced by the SPS process. The dielectric constant and loss were both much lower than those of Ln2−x Srx NiO4 ceramics, while the activation energy of low temperature dielectric relaxation was larger. There were two dielectric relaxations in the present ceramics, the low temperature one was closely related to the charge order–disorder transition, while the high temperature one was originated from the extrinsic effects. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The giant dielectric constant materials have attracted many attentions since the discovery of CaCu3 Ti4 O12 (CCTO) because of their greatly potential applications in the microelectronics [1–8]. Recently, the giant dielectric responses up to high frequency are found in charge-ordered Ln2−x Srx NiO4 (Ln = La, Nd and Sm) ceramics [9,10], especially, the temperature-stable colossal dielectric constant (∼105 ) in a broad temperature range of 150–500 K up to 5 MHz is observed in Sm1.5 Sr0.5 NiO4 ceramics [10], this stimulates the research interests in the charge-ordered nickelates [11]. However, the rather high dielectric losses limit the practical application of these materials, and the high losses should be contributed from the high electrical conductivities of the materials. There are two methods to reduce the electrical conductivities in these materials, one is decrease of the doping content, i.e. the value of x, and the other one is replacement of the doping element strontium with barium [12,13]. Here we combine these two methods and select La1.75 Ba0.25 NiO4 as the object. The charge ordering temperature of the present material is about 140 K [13,14]. Actually it is difficult to prepare the stoichiometric ceramics by the conventional solidstate sintering process in air due to the capability of K2 NiF4 -type arrangements for accommodating extra oxygen in the interstitials of the structure [15,16]. The extra interstitial oxygen will increase the conductivity of the sample [17,18], and this will deteriorate the
∗ Corresponding author. Tel.: +86 571 87951410; fax: +86 571 87951410. E-mail address: [email protected] (Y.J. Wu). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.10.114
dielectric loss in turn. So, the stoichiometric ceramics are expected to meet the requirement of low dielectric loss in the practical application. On the other hand, the spark plasma sintering (SPS) method has been used widely as a low temperature and rapid sintering method in the past decade [19–25]. The dense ceramics with smaller grain size, for example, nano-scale, can be easily prepared by SPS method because of its much lower sintering temperature and time [25]. The furnace chamber is under vacuum and the graphite die is used during the sintering process, these factors favor the extraction of interstitial oxygen from the sample [16], so the SPS may be a good way to prepare the stoichiometric La1.75 Ba0.25 NiO4 ceramics. In present work, La1.75 Ba0.25 NiO4 ceramics are prepared by SPS and solid-state sintering process, respectively, and the microstructures together with dielectric properties of the present ceramics are characterized. 2. Experimental details La1.75 Ba0.25 NiO4 powders were synthesized by a solid-state reaction process using La2 O3 (99.99%), BaCO3 (99.93%) and NiO (99%) as the starting materials, which were weighted and mixed by ball milling with ZrO2 balls in ethanol for 24 h, then dried and calcined at 1200 ◦ C in air for 3 h to yield the desired materials. The calcined powders were ball milled for 24 h and then dried. The obtained powders were added into a graphite die and sintered at 1025 ◦ C for 5 min under a vacuum of 6 Pa with an SPS apparatus (SPS-1050). During the period of heating and soaking, a pressure of 30 MPa was applied to the sample. The heating rate was 100 ◦ C/min from room temperature to 900 ◦ C and 62.5 ◦ C/min from 900 ◦ C to 1025 ◦ C. Then the as-sintered samples were annealed in air at 600 ◦ C for 2 h to remove the residual carbon on the surface of the samples, and dense ceramics (relative density was larger than 95% of the theoretical density) were obtained. For the samples prepared by the conventional solid-state sintering method, the calcined powders with 8 wt% polyvinyl alcohols (PVA) were pressed into pellets, and then sintered in air at 1450 ◦ C for 3 h.
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C.L. Song et al. / Journal of Alloys and Compounds 490 (2010) 605–608
The crystalline phase was identified by powder X-ray diffraction using Cu K␣ radiation (Rigaku 2550/PC). The microstructures were observed on the fracture surfaces with a field emission scanning electron microscopy (Hitachi S-4800). The dielectric characteristics and ac conductivities of these ceramics were evaluated with a broadband dielectric spectrometer (Novocontrol Turkey Concept 50) in a broad range of temperature (128–573 K) and frequency (1 Hz–10 MHz) with a heating rate of 2 K/min, and the silver paste was adopted as electrodes.
3. Results and discussion The sintering behavior of La1.75 Ba0.25 NiO4 ceramics during spark plasmas sintering process is shown in Fig. 1. A rather large thermal expansion is observed when the temperature increases from room temperature to about 980 ◦ C. The shrinkage initiates at about 980 ◦ C, and it persists in the front of soaking period at 1025 ◦ C, then it almost disappears when the soaking time goes beyond 4 min, suggesting that the densification process is almost complete. The densification temperature of SPS process is about 425 ◦ C lower than that of solid-state sintering method. The relatively low sintering temperature and short sintering time can be attributed to the merits of the SPS method. The inset of Fig. 1 shows the sintering time dependence of vacuum in the furnace chamber, there is a sharp peak in the curve and the corresponding temperature is about 1000 ◦ C. The sharp decrease of the vacuum in the chamber indicates that some gases emit from the sample. Combined with the result of electrical conductivity (see the discussion in the later chapter), we conclude that this peak should be originated from the emission of interstitial oxygen, which has come from the air during the calcining process. The X-ray diffraction (XRD) patterns of La1.75 Ba0.25 NiO4 calcined powder and ceramics prepared by solid-state sintering and SPS methods are shown in Fig. 2. It shows that the single La1.75 Ba0.25 NiO4 phase is obtained in the calcined powder and ceramics prepared by solid-state sintering method. While minor secondary phase, i.e. La2 O3 phase (PDF# 74-1144, the content is about 2.4 wt%), is found in the ceramics prepared by SPS process, it should be decomposed from the parent compound through the local discharge between the particles during the sintering process. The space group of main phase is I 4/mmm, which can be indexed by the La2 NiO4 (PDF# 72-1241) as the model, and the cell parameters are a = 3.8611(4) Å, and c = 12.753(2) Å, respectively. The cell parameters are consistent with the results of the previous work [13]. Fig. 3 shows the SEM micrographs of La1.75 Ba0.25 NiO4 ceramics prepared by solid-state sintering and SPS methods. The spark plasma-sintered sample has uniform grain size distribution and the size is about 1 m, while the inhomogeneous microstructure is observed on the solid-state sintered sample and the average
Fig. 1. Shrinkage curve and sample’s temperature as a function of sintering time during spark plasma sintering process of La1.75 Ba0.25 NiO4 ceramics. The inset is the furnace chamber’s vacuum during the SPS process.
Fig. 2. X-ray diffraction patterns of La1.75 Ba0.25 NiO4 : (a) calcined powder and ceramics prepared by (b) solid-state sintering and (c) SPS process.
size is larger than 5 m. The La2 O3 secondary phase may inhabit the grain growth of the ceramics, but it will concurrently degenerate the densification process of the ceramics, i.e., the optimal sintering temperature should be increased [26]. However, the much smaller grain size should be resulted from the much shorter sintering time during the spark plasma sintering process (16 min versus 7 h) for the sintering temperature is too low to promote the densification process in the present work. Also, more pores are observed on the solid-state sintered sample than those of the spark plasma-sintered sample as shown in the SEM micrograph. These phenomena also indicate the merits of the SPS process.
Fig. 3. SEM micrographs of the fracture surfaces of La1.75 Ba0.25 NiO4 ceramics prepared by (a) solid-state sintering and (b) SPS process.
C.L. Song et al. / Journal of Alloys and Compounds 490 (2010) 605–608
Fig. 4 shows the comparison of electrical conductivities among the samples prepared by solid-state sintering method, SPS process and the spark plasma-sintered sample annealed at 800 ◦ C in air for 2 h. The electrical conductivity of the sample prepared by SPS is much lower than that of the other two samples, while the values of the other two samples are almost same. The spark plasmasintered atmosphere is in vacuum, and the reducing atmosphere may exist for the graphite die is used during the sintering process. There are some gases emitted from the sample during the spark plasma sintering process as shown in the inset of Fig. 1, and they should be come from the oxygen element in the sample. There are two types of non-stoichiometric oxygen defects that can be existed in the present ceramics, one is the interstitial oxygen and the other is lattice oxygen vacancy. The removal of the interstitial oxygen will decrease the electrical conductivity of the sample, while the creation of lattice oxygen vacancy will increase the electrical conductivity of the sample [17,18]. So, we conclude that the low electrical conductivity of spark plasma-sintered sample should be resulted from the extraction the interstitial oxygen from the sample during the sintering process. After annealing the spark plasma-sintered sample at 800 ◦ C in air, the oxygen in air will infiltrate into the lattice to form interstitials and the interstitial oxygen will increase the electrical conductivity of the present sample. Fig. 5 shows the temperature dependence of dielectric properties for La1.75 Ba0.25 NiO4 ceramics prepared by SPS process. While no stable and credible data can be obtained from the solid-state sintered and annealed samples, and this may be deduced from the high electrical conductivities in these two samples. This emphasizes the advantage of the spark plasma sintering method over the conventional solid-state process in the present ceramics. The dielectric constant is about 500, which is much lower than that of Ln2−x Srx NiO4 (Ln = La, Nd, and Sm) ceramics [9–11]. And the dielectric loss is also much lower than that of Ln2−x Srx NiO4 ceramics. There are two relaxations in the curve of the temperature dependence of dielectric constant, one is around 100–200 K, and the other is around the room temperature. The low temperature relaxation is the onset of dielectric constant step, and a corresponding peak is found in the curve of temperature dependence of dielectric loss. This dielectric relaxation is around the charge-ordered temperature of the present ceramics [13–14], and the relaxation should be closely related to the charge order–disorder transition as shown in the previous work [9,10]. After plotting the frequency dependence of peak temperatures in the dielectric loss curve (as shown in the inset of Fig. 5b), the Arrhenius law is used to fit the linear relationship. The fitting results in the activation energy of 0.218 ± 0.003 eV,
607
Fig. 5. Temperature dependence of (a) dielectric constant and (b) dielectric loss for La1.75 Ba0.25 NiO4 ceramics prepared by SPS. The inset is frequency dependence of the peak temperature of dielectric loss in the ceramics.
which is larger than that of Sm1.5 Sr0.5 NiO4 ceramics [10]. This may deduce from the different charge ordering statue in the present ceramics which will investigate in the further work. Above the room temperature, another dielectric relaxation is obvious at the low frequency, while it almost disappears at high frequency. So, this phenomenon indicates the relaxation should be originated from the extrinsic effects, such as Maxwell–Wagner effect [27]. 4. Conclusions Dense La1.75 Ba0.25 NiO4 ceramics with minor impurity have been prepared by SPS process. The interstitial oxygen can be removed by the SPS process, and this may result in the decrease of electrical conductivity of the sample. The dielectric constant and loss of the present ceramics are both much lower than those of La2−x Srx NiO4 ceramics, while the activation energy is larger. There are two dielectric relaxations in the present ceramics; the lower temperature relaxation should be closely related to the charge ordering, while the higher temperature one is originated from the extrinsic effects, such as Maxwell–Wagner effect. Although the dielectric loss is lower than that of the other charge ordering nickelate ceramics, the present ceramics is not suitable for the practical application because of its some high dielectric loss. Acknowledgement This work was supported by National Science Foundation of China under Grant Nos. 50702049 and 50832005. References
Fig. 4. Comparison of electrical conductivities of La1.75 Ba0.25 NiO4 ceramics prepared by SPS, annealed-SPS and solid-state sintering process at room temperature.
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