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High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application
Journal Pre-proof High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application Liang Yan, Qiuyun Fu, Muhammad Humayun, Dongxiang Zhou, Mei Wang, Geng Wang, Xiaoyun Gao, Zhiping Zheng, Wei Luo PII:
S0272-8842(19)33339-5
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.149
Reference:
CERI 23518
To appear in:
Ceramics International
Received Date: 19 September 2019 Revised Date:
13 November 2019
Accepted Date: 17 November 2019
Please cite this article as: L. Yan, Q. Fu, M. Humayun, D. Zhou, M. Wang, G. Wang, X. Gao, Z. Zheng, W. Luo, High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.149. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application Liang Yan, Qiuyun Fu*, Muhammad Humayun, Dongxiang Zhou, Mei Wang, Geng Wang, Xiaoyun Gao, Zhiping Zheng, Wei Luo, Engineering Research Center for Functional Ceramics of Ministry of Education, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China *Author to whom correspondence should be addressed. E-mail address: [email protected].
Abstract: It is a great challenge to prepare chip thermistors with high positive temperature coefficient of resistivity (PTCR) jump and simultaneously low room temperature resistivity by reduction-reoxidation
method.
In
this
work,
the
influence
of
Mn
doping
on
Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3 based chip thermistors were carefully investigated and Mn doping was proved to be an effective way in preparation of high-performance PTCR ceramics with low resistivity. The results showed that Mn can’t create a high enough potential barrier in un-oxidized samples but can obviously increase the value of potential barrier height after reoxidation. The trap activation of Mn d states near Curie point helps to reduce room temperature resistivity and increase PTCR jump in reoxidized samples. As a result, samples with extremely low resistivity and high PTCR jump of 103.4 were reported for the first time.
Keywords: PTCR; Reduction-reoxidation; Mn; Sodium bismuth titanate;
1. Introduction It is well understood that the resistivity of donor-doped barium titanate increases
significantly during phase change (ferroelectric–paraelectric transition) [1-4]. This phenomenon is known as the Positive temperature coefficient resistivity (PTCR) effect. In past few decades, thermistors applications based on the donor doped barium titanate (BT) have been widely used as functional or protective components in electronics [5, 6]. To meet the requirements of miniaturization and low resistance in electronic industry, chip PTC thermistors were invented
[7]
. Chip thermistors which used as current limiting
protection applications are often connected in series with the working components. Low resistive means low energy consumption. Chip thermistors often have parallel metal inner electrodes, so they must be sintered in a protective atmosphere to prevent oxidation of the metal electrodes. In order to produce PTCR effect, they are reoxidized at a low temperature (≤ 800 °C) in the air atmosphere, which is called reduction-reoxidation method
[8, 9]
. In this
process, the grain boundary is formed during sintering, and the reoxidation temperature is lower than the sintering temperature. Thus, the grain boundaries have very limited oxidation efficiency during reoxidation. Low oxidation efficiency produces low surface acceptors, including VTi′′′′ and Oadd′′, which leads to a low PTCR jump of chip thermistors. One way of improving the PTCR jump is to increase the reoxidation temperature or extending reoxidation time. Through this approach, the PTCR jump was increased to ~ 103.3 and the value of room temperature resistivity was recorded as 727 Ω·cm [10]. Controlling the microscopic morphology of ceramics has also been proven to be an effective means in improving the PTCR jump. By using the double stage thermal processing method, the PTCR jump can be further improved to ~104 and the resistivity can be decreased to ~ 136 Ω·cm [11]. Unfortunately, it requires a very long sintering and soaking time and the resistivity is not low enough. According to our recent report [12], doping of (Bi0.5Na0.5)TiO3 can greatly improve the density of Oadd′′, thus PTCR jump reached a high value of 103.6 and the resistivity was decreased to 18 Ω cm. Until now, it’s difficult to improve the oxidation degree of the grain boundary. Further, the way to enhance VTi′′′′ and Oadd′′ has been nearly exhausted. However, the performance is still not satisfactory and new surface acceptors need to be developed. Mn, as a 3d transition element, is widely used in preparing traditional BT based
thermistors. Many researchers have reported the significantly enhanced PTCR jumps of the traditional thermistors which were sintered in the air atmosphere. Generally, it is accepted that Mn enters the Ti4+ site and acts as acceptors. Heywang [13] reported that 3d transition elements favorably segregate at the grain boundaries and improves the density of surface acceptors and PTCR jump. Later findings suggested that Mn doesn't just act as an acceptor. For example, Miki et al.
[13]
carefully calculated the energy levels of several Mn-related
centers and draw a conclusion that trap activation of Mn d states near Curie point significantly increases the PTCR effect. It has been reported that Mn can also increase the thickness of insulating grain boundary layer because acceptors MnTi′′ have energy levels equal to or higher than VBa′′ [14]. Recent report shows that Mn acceptor can partially compensate Bi donor in Mn doped BNT-BT ceramics, thereby decreasing the resistivity and synchronously enhancing the PTCR jump[15]. According to the above studies, doping Mn into traditional BT based thermistors is very effective. However, very few papers reported the influence of Mn on chip thermistors prepared by the reduction-reoxidation method and with Ba/Ti ≥1. Some researchers even believe that Mn doping has a limited effect on chip thermistors when sintered in a protective atmosphere. The main surface acceptors of chip thermistors are VTi′′′′ and Oadd′′, which are produced during reoxidation process [10, 16]. But there is little evidence to support this view. In order to increase the PTCR jump and synchronously decreasing ρ25°C, herein, Mn is introduced and the effect of Mn doping on BNT-BT based chip thermistors is well investigated. 2. Experimental procedure (Bi0.5Na0.5)TiO3 were prepared by using commercially available Bi2O3, NaCO3, and TiO2. Detailed synthetic pathway had been reported in published papers
[17]
.
(Bi0.5Na0.5)TiO3 powder was then mixed with BaCO3, TiO2, La2O3 according to the formula of Ba0.993(Bi0.5Na0.5)0.003La0.004TiO3. Mn(NO3)2 was used for Mn doping. Mn was doped in addition rather than instead of Ti. The weighed mixture was calcined at ~1150 °C, and then
0.3 mol% BaCO3 was added to it. After ball-milling, the ceramic powders were dried in an oven overnight and then mixed with dispersant, defoamer, solvent and binder through ball milling for more than 19 hours. The slurry was then employed to prepare green sheets (~55 µm) by tape casting method
[17]
. The sheets were then stacked, pressed, and cut into
rectangular-blocks to form green pellets. Since we were mainly explore basic materials in this work, we did not prepare internal electrodes. The binder was removed by heating at 280 °C for more than 48 hours. The green pellets were then fired at 1050-1150 °C in the presence of high-purity N2 for 2 h. The reoxidation process was carried out at a moderately temperature of 600–850 °C for 2 h in air atmosphere. The In-Ga counter electrode was used in measurement of electrical performance. AWXB R-T Test System (Wuhan, China) was used in measurement of Resistance-temperature (R-T) characteristics, from room temperature to 250 °C. The value of PTCR jump was defined as the ratio of maximal resistance and minimum value. Gemini SEM 300-71-12 was used in observing the microstructure, all samples has been polished and thermal etched. The average grain size was counted and estimated with software (Nano Measurer ver. 1.2) on more than 100 grains. WK6550B impedance analyzer was used for obtaining Impedance spectroscopy with the measurement range of 30 Hz to 30 MHz. After sintering, ceramics were characterized by X-ray powder diffraction (XRD-7000s, Shimadzu, Japan). The EPR spectra were obtained by a JEOL JES-FA200 (ESR/EPR) spectrometer operating at 9.42 GHz.
3. Results and discussion 3.1 Microstructures and densification As shown in Fig. 1, the Mn-doped sample exhibits a pure tetragonal phase perovskite
structure without any noteworthy secondary phase. Fig. 2 shows SEM images of the cross-section of the samples with different Mn content. A typical polygonal morphology of ceramics can be seen. Based on the cross-section image, average grain sizes of the samples were estimated to be 1.91 µm, 1.57 µm, 1.50 µm, and 1.37 µm, respectively. The average grain sizes of samples sintered at different temperature were also obtained and calculated. As shown in Fig. 3 (a), the average grain size of samples decreases with the increase in Mn content. The relative density of ceramics was measured by Archimedes method (Fig. 3(b)). According to the changes in density and SEM images, Mn doping inhibits grain growth and the number of pores of ceramics increases with increase in Mn content. This is due to the low mobility of oxygen atoms [18], which play an important role in the grain growth upon addition of Mn. 3.2 Electrical properties Fig. 4 (a) shows the resistivity variation with temperature for different samples as a function of Mn content. All samples exhibited a remarkable PTCR effect. Detailed electrical parameters of samples sintered at different temperature as a function of Mn content are shown in Fig. 4 (b) and (c). As the figure shows, room temperature resistivity and PTCR jump increase with the increase in Mn content. The relatively low value of PTCR jump for samples sintered at 1150 °C is related to relatively large relative density, which leads to the insufficient oxidized grain boundaries. The rise in PTCR jump where x ≥ 0.05 is attributed to the decreased grain size and ceramics density, as shown in Fig. 3 (b). For comprehensive consideration of low room-temperature resistivity and high PTCR jump, samples with 0.02 mol% of Mn was selected to be the representative one, which holds a low room temperature
resistivity of 6.3 Ω·cm and high PTCR jump of 103.5. Compared with samples in published papers which listed in Table 1, our representative samples have low resistivity and simultaneously high PTCR jump. 3.2.1 Room temperature resistivity As a useful tool in separating resistance of grain (Rg) and resistance of grain boundary (Rgb), impedance spectroscopy was used. A typical cole-cole plot is shown in Fig 5 (a), which means that there no any grain shell in the samples. With the help of software (Zview), the value of Rg and Rgb were obtained by using an equivalent circuit model of Rg (Cgb//Rgb) [19]. As shown in Fig. 5 (b), the value of Rg was small and changed little when Mn content increased. And the value of Rgb increases in an accelerated manner. It means that the Mn, which acts as an acceptor, should be segregated at the grain boundaries thus increases Rgb and have little influence on Rg. On the other hand, Mn doping inhibits the grain growth and increase in the number of grain boundaries accelerates the increase of Rgb. 3.2.2 PTCR jump As shown in Fig. 6 (a) and (b), the unoxidized samples had very low ρ25 °C and PTCR jump. Heywang–Jonker model has been accepted by most researchers and successfully used in explaining a variety of phenomena that are related to the PTCR effect. Heywang–Jonker model describes the relationship among potential barrier height (Φ0), the density of effective acceptors (Ns), the density of effective donors (Nd) and relative dielectric constant (εr ) as shown in equation 1 and 2: ⁄8
= b=
(1) (2)
Where e is the elementary charge, ε0 represents the vacuum permittivity, Nd=1/eµρg, b represents the thickness of depletion layer. According to equation (1), low value of Φ0 in un-oxidized samples is related to low Ns and high Nd. ESR was used to examine the valence state of Mn ions in the samples. As shown in Fig. 7 (a), there are no ESR signals arising from Mn at 25 °C. Thus, Mn ions should act as acceptors, and the valence state of Mn ions should be +3, [20, 21]. The low value of the PTCR jump in un-oxidized samples should be attributed to the large number of oxygen vacancies in the un-oxidized grain boundary and their ionization. The oxygen vacancies have very low ionization energies and generate electrons at room temperature, thus act as donors [22]: ×
•
→
+
(3)
And: •
→
••
+
(4)
Electrons generated by ionization of oxygen vacancies compensate acceptors. Therefore, the potential barrier height is very low and the PTCR effect is weak. Nevertheless, the PTCR jump increased with an increase in Mn content. As can be seen from the above analysis, Mn act as acceptors and its doping can increase surface acceptors. However, in the presence of oxygen vacancies ionization, PTCR effect of unoxidized samples doped with Mn is still weak. After reoxidation at 800 °C, both PTCR jump and RT resistivity of reoxidized were significantly improved. As shown in Fig.4 (c), the PTCR jump increases from 102.4 to 103.8, as the content of Mn boost from 0 to 0.08 mol%. Smaller changed PTCR jump and RT resistivity of samples sintered at 1150 °C may relate to higher relative density [23-25]. Herein, the potential barrier height was calculated and compared with Arrhenius-plots (Fig. 8 (a)) and drew in Fig.
8 (b). [26]. As shown in Fig. 8 (b), the potential barrier increases from 0.61 to 0.99 eV when x boost from 0 to 0.08%. It can be seen from Fig. 5 (b), that Rg changes a little when Mn content increase from 0 to 0.08 mol%. According to equation (1), increased Φ0 should be attributed to the increased Ns.
As shown in Fig. 7 (b) and (c), an ESR signal of six hyperfine peaks arose after phase transition. The value of g was calculated to be 2.002, arises from +1/2,mI↔-1/2,mI, which indicates that the valence state of Mn is changed from +3
[20, 21, 27]
to +2
[28]
. after the
paraelectric-ferroelectric structural transition. This phenomenon is the same as that of Mn in traditional thermistor sintered in air atmosphere. Mn doping produces MnTi′ before phase transition which has little influence on the room temperature resistivity and produces MnTi′′ after the phase transition which further improves Ns. Another factor that affects Ns is the density of ceramics [23-25]. Analysis of unoxidized samples has shown that the densities of Oadd′′ and VTi′′′′ are very low before reoxidation and are generated during reoxidation. As the content of Mn increases, the density of ceramics decreases, which leads to an increase in the Oadd′′ and VTi′′′′ and Ns. The thickness of the depletion layer was also estimated by equation (2) [26]
. As the resistivity of grain changes little with the increase of Mn content, and Ns increases
with the increase in Mn content, the thickness of depletion layer that mainly created by VTi′′′′, MnTi′′, and Oadd′′ is increased from 0.76 nm to 1.16 nm. It can be seen from the above analysis that Mn doping can effectively increase the density of surface acceptors and the PTCR jump after reoxidation. 3.2.3 Temperature coefficient Temperature coefficients (αT) are often used to measure the response speed of PTCR
ceramics, as expressed in the form of equation 5: (
=
)
(5)
It can be used to measure the slope and steepness of the R-T curves. As shown in Fig. 4 (a), with the increase in Mn content, the R-T curves become much steeper. Here, temperature coefficients (α0/ Δ T) on measurement point were used for representing the αT and were calculated by: /
=
!"
∆
#
(6)
R0 is the resistance on the measurement point (T0) and R1 is the resistance on the measurement point (T1) next to T0. ΔT=T1-T0. The maximum temperature coefficient (αTmax) and its corresponding temperature (Tαmax) are plotted in Fig. 9. Tαmax is hardly varied when Mn content changes and αTmax increased with the increase in Mn content. According to equations (1) and (5): (
α =
&'
≈
&*+
,-.
= /01 × #
2& *4
13 )
(7)
Where ρT = ρg + ρgb is the total resistivity of ceramics, for a much smaller value of ρg compared with ρgb, ρg was ignored. According to equation (5), Ns also affect the
temperature coefficient. Thus, the temperature coefficient increased with the increase in Mn content. 3.3 Effect of reoxidation temperature According to the previous literature
[25, 29]
, reoxidation temperature affects Ns and PTCR
jump. Herein, the influence of reoxidation temperature on the electrical performance of BNT-BT based chip ceramics was evaluated. As shown in Fig. 10 (a), with an increase in reoxidation temperature, PTCR jump slightly increases, but the resistivity first increases and
then decreases. Impedance spectra were used to analyze the unusual changes in room temperature resistivity. Fitting by software (Zview), the resistance of grain and grain boundaries were obtained and listed in table 2. According to Table 2, the value of Rg is small and changes little when reoxidized at different temperatures. Rgb increased to 20 Ω when samples were reoxidized at 700 °C and then dropped to 11 Ω, when samples were reoxidized at 800 °C. This anomaly phenomenon is due to the second phase at grain boundaries. According to Zu et. al [9], the Ba2TiO4 exists at the grain boundary when sintering aid BaCO3 is used. Ba2TiO4 can be consumed by reacting (when reoxidation temperature ≥ 800 °C) with Ba6Ti17O44, which is generated by the oxidization of grain boundaries. The reaction can establish new low-resistance pathways at grain boundaries and reduce the room temperature resistance. Thus, Rgb firstly increases and then decreases. Based on the above analysis, 800 °C is the best reoxidation temperature.
4. Conclusions In this work, Mn doped BNT-BT based chip PTCR materials were prepared and Mn doping was proved to be an effective way in preparing low-resistivity high-performance chip thermistors. The analysis shows that Mn doping cannot create a high enough potential barrier in un-oxidized samples but can obviously increase the value of potential barrier height after reoxidation. The valence change of Mn during phase change also contributes to the preparation of low-resistivity chip thermistors. Additionally, Mn doping effectively increases the thickness of depletion layer which helps improve the continuity of the depletion layer in low-resistivity thermistors. As a result, the representative samples exhibit extremely low resistivity of 6.3 Ω·cm and high PTCR jump of 103.4. Herein, we report for the first time the
preparation of thermistors with ρ25°C lower than 10 Ω·cm and PTCR jump higher than 103.
Acknowledgments This work is supported by National Key Research and Development Program of China (Grant 2017YFB0406405), National Natural Science Foundation of China (Grant No.61571203) and Innovation Team Program of Hubei Province (Grant No.2019CFA004). The authors acknowledge the assistance by the Analytical and Testing Center of Huazhong University of Science and Technology.
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Fig. 1 XRD patterns of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+0.06 mol% Mn at room temperature. Fig. 2 SEM images of the cross-section for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn which were sintered at 1125 °C: (a) x= 0, (b) x= 0.02, (c) x= 0.04, and (d) x= 0.06. Fig. 3 (a) average grain size and (b) relative density of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples as a function of x. Fig. 4 (a) R-T curves, (b) Room temperature resistivity and (c) PTCR jump of Ba0.996 La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples sintered at different temperatures and reoxidized at 800 °C. Fig. 5 (a) Impedance spectra and (b) separated Rg and Rgb of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples sintered at 1125 °C and reoxidized at 800 °C. Fig. 6 (a) Room-temperature resistivity and (b) PTCR jump of unoxidized Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples sintered at different temperatures. Fig. 7 ESR spectra of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+0.06 mol% Mn samples (a) before reoxidation and measured at room temperature, (b) after reoxidation and measured at room temperature, (c) after reoxidation and measured at 140 °C. Fig. 8 (a) The Arrhenius-plots (lnρ against 103/T) and (b) estimated potential barrier heights for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn sintered at 1125 °C and reoxidized at 800 °C Fig. 9
The maximum temperature coefficient (αTmax)and its corresponding temperature
(Tαmax) for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn sintered at 1125 °C and reoxidized at 800 °C Fig. 10 (a) Room-temperature resistivity and PTCR jump, (b) Impedance spectra for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+0.02 mol% Mn sintered at 1125 °C and reoxidized at different temperature
Table. 1 Some of representative samples prepared by the reduction-reoxidation method in published papers. Main components Ba0.997-x(Bi0.5Na0.5)xLa0.003TiO 3 +0.3 mol% BaCO3 Ba0.997La0.003TiO3+ x mol% BaCO3 BaLa0.004TiMn0.0005O3+2 mol% BN Ba0.984Y0.016TiO3+0.025 BaCO3+0.05 BN (Bam-0.202Ca0.2La0.002)TiO3+0.01 SiO2 (Ba0.882-xSr0.12Smx)TiO3 Ba0.85Ca0.15Y0.002Ti1.01O3+0.25 mol%SiO2+0.06 atm% Mn
PTCR jump
RT resistivity
Year
The author
103.5
18.4 Ω cm
2019
Yan et .al[30]
103.7
28 Ω cm
2018
Zu et .al[24]
104
136 Ω cm
2017
Gao et .al[11]
103.3
727 Ω cm
2012
Liu et .al [10]
~ 90
5 Ω cm
2007
Niimi et .al [31]
~ 1000
23 Ω cm
2007
Niimi et .al [32]
~22
5 Ω cm
1994
Kanda et .al[33]
Table. 2 the resistance of grain and grain boundaries of samples Ba0.996La0.003(Bi0.5Na0.5)0.004TiO3+0.02 mol% Mn sintered at 1125 °C and reoxidized at different temperature obtained from impedance analysis. Reoxidation temperature
--
700°C
750°C
800°C
850°C
Rg (Ω)
2.1
4.1
3.6
3.0
3.3
Rgb (Ω)
3.8
20.3
15.6
11.1
12.7
This work is supported by National Key Research and Development Program of China (Grant 2017YFB0406405), National Natural Science Foundation of China (Grant No.61571203) and Innovation Team Program of Hubei Province (New microwave devices for next generation wireless communication systems). There are no interests to declare.
S0272-8842(19)33339-5
DOI:
https://doi.org/10.1016/j.ceramint.2019.11.149
Reference:
CERI 23518
To appear in:
Ceramics International
Received Date: 19 September 2019 Revised Date:
13 November 2019
Accepted Date: 17 November 2019
Please cite this article as: L. Yan, Q. Fu, M. Humayun, D. Zhou, M. Wang, G. Wang, X. Gao, Z. Zheng, W. Luo, High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.149. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
High-performance PTCR ceramics with extremely low resistivity for multilayer chip thermistor application Liang Yan, Qiuyun Fu*, Muhammad Humayun, Dongxiang Zhou, Mei Wang, Geng Wang, Xiaoyun Gao, Zhiping Zheng, Wei Luo, Engineering Research Center for Functional Ceramics of Ministry of Education, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, 430074, China *Author to whom correspondence should be addressed. E-mail address: [email protected].
Abstract: It is a great challenge to prepare chip thermistors with high positive temperature coefficient of resistivity (PTCR) jump and simultaneously low room temperature resistivity by reduction-reoxidation
method.
In
this
work,
the
influence
of
Mn
doping
on
Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3 based chip thermistors were carefully investigated and Mn doping was proved to be an effective way in preparation of high-performance PTCR ceramics with low resistivity. The results showed that Mn can’t create a high enough potential barrier in un-oxidized samples but can obviously increase the value of potential barrier height after reoxidation. The trap activation of Mn d states near Curie point helps to reduce room temperature resistivity and increase PTCR jump in reoxidized samples. As a result, samples with extremely low resistivity and high PTCR jump of 103.4 were reported for the first time.
Keywords: PTCR; Reduction-reoxidation; Mn; Sodium bismuth titanate;
1. Introduction It is well understood that the resistivity of donor-doped barium titanate increases
significantly during phase change (ferroelectric–paraelectric transition) [1-4]. This phenomenon is known as the Positive temperature coefficient resistivity (PTCR) effect. In past few decades, thermistors applications based on the donor doped barium titanate (BT) have been widely used as functional or protective components in electronics [5, 6]. To meet the requirements of miniaturization and low resistance in electronic industry, chip PTC thermistors were invented
[7]
. Chip thermistors which used as current limiting
protection applications are often connected in series with the working components. Low resistive means low energy consumption. Chip thermistors often have parallel metal inner electrodes, so they must be sintered in a protective atmosphere to prevent oxidation of the metal electrodes. In order to produce PTCR effect, they are reoxidized at a low temperature (≤ 800 °C) in the air atmosphere, which is called reduction-reoxidation method
[8, 9]
. In this
process, the grain boundary is formed during sintering, and the reoxidation temperature is lower than the sintering temperature. Thus, the grain boundaries have very limited oxidation efficiency during reoxidation. Low oxidation efficiency produces low surface acceptors, including VTi′′′′ and Oadd′′, which leads to a low PTCR jump of chip thermistors. One way of improving the PTCR jump is to increase the reoxidation temperature or extending reoxidation time. Through this approach, the PTCR jump was increased to ~ 103.3 and the value of room temperature resistivity was recorded as 727 Ω·cm [10]. Controlling the microscopic morphology of ceramics has also been proven to be an effective means in improving the PTCR jump. By using the double stage thermal processing method, the PTCR jump can be further improved to ~104 and the resistivity can be decreased to ~ 136 Ω·cm [11]. Unfortunately, it requires a very long sintering and soaking time and the resistivity is not low enough. According to our recent report [12], doping of (Bi0.5Na0.5)TiO3 can greatly improve the density of Oadd′′, thus PTCR jump reached a high value of 103.6 and the resistivity was decreased to 18 Ω cm. Until now, it’s difficult to improve the oxidation degree of the grain boundary. Further, the way to enhance VTi′′′′ and Oadd′′ has been nearly exhausted. However, the performance is still not satisfactory and new surface acceptors need to be developed. Mn, as a 3d transition element, is widely used in preparing traditional BT based
thermistors. Many researchers have reported the significantly enhanced PTCR jumps of the traditional thermistors which were sintered in the air atmosphere. Generally, it is accepted that Mn enters the Ti4+ site and acts as acceptors. Heywang [13] reported that 3d transition elements favorably segregate at the grain boundaries and improves the density of surface acceptors and PTCR jump. Later findings suggested that Mn doesn't just act as an acceptor. For example, Miki et al.
[13]
carefully calculated the energy levels of several Mn-related
centers and draw a conclusion that trap activation of Mn d states near Curie point significantly increases the PTCR effect. It has been reported that Mn can also increase the thickness of insulating grain boundary layer because acceptors MnTi′′ have energy levels equal to or higher than VBa′′ [14]. Recent report shows that Mn acceptor can partially compensate Bi donor in Mn doped BNT-BT ceramics, thereby decreasing the resistivity and synchronously enhancing the PTCR jump[15]. According to the above studies, doping Mn into traditional BT based thermistors is very effective. However, very few papers reported the influence of Mn on chip thermistors prepared by the reduction-reoxidation method and with Ba/Ti ≥1. Some researchers even believe that Mn doping has a limited effect on chip thermistors when sintered in a protective atmosphere. The main surface acceptors of chip thermistors are VTi′′′′ and Oadd′′, which are produced during reoxidation process [10, 16]. But there is little evidence to support this view. In order to increase the PTCR jump and synchronously decreasing ρ25°C, herein, Mn is introduced and the effect of Mn doping on BNT-BT based chip thermistors is well investigated. 2. Experimental procedure (Bi0.5Na0.5)TiO3 were prepared by using commercially available Bi2O3, NaCO3, and TiO2. Detailed synthetic pathway had been reported in published papers
[17]
.
(Bi0.5Na0.5)TiO3 powder was then mixed with BaCO3, TiO2, La2O3 according to the formula of Ba0.993(Bi0.5Na0.5)0.003La0.004TiO3. Mn(NO3)2 was used for Mn doping. Mn was doped in addition rather than instead of Ti. The weighed mixture was calcined at ~1150 °C, and then
0.3 mol% BaCO3 was added to it. After ball-milling, the ceramic powders were dried in an oven overnight and then mixed with dispersant, defoamer, solvent and binder through ball milling for more than 19 hours. The slurry was then employed to prepare green sheets (~55 µm) by tape casting method
[17]
. The sheets were then stacked, pressed, and cut into
rectangular-blocks to form green pellets. Since we were mainly explore basic materials in this work, we did not prepare internal electrodes. The binder was removed by heating at 280 °C for more than 48 hours. The green pellets were then fired at 1050-1150 °C in the presence of high-purity N2 for 2 h. The reoxidation process was carried out at a moderately temperature of 600–850 °C for 2 h in air atmosphere. The In-Ga counter electrode was used in measurement of electrical performance. AWXB R-T Test System (Wuhan, China) was used in measurement of Resistance-temperature (R-T) characteristics, from room temperature to 250 °C. The value of PTCR jump was defined as the ratio of maximal resistance and minimum value. Gemini SEM 300-71-12 was used in observing the microstructure, all samples has been polished and thermal etched. The average grain size was counted and estimated with software (Nano Measurer ver. 1.2) on more than 100 grains. WK6550B impedance analyzer was used for obtaining Impedance spectroscopy with the measurement range of 30 Hz to 30 MHz. After sintering, ceramics were characterized by X-ray powder diffraction (XRD-7000s, Shimadzu, Japan). The EPR spectra were obtained by a JEOL JES-FA200 (ESR/EPR) spectrometer operating at 9.42 GHz.
3. Results and discussion 3.1 Microstructures and densification As shown in Fig. 1, the Mn-doped sample exhibits a pure tetragonal phase perovskite
structure without any noteworthy secondary phase. Fig. 2 shows SEM images of the cross-section of the samples with different Mn content. A typical polygonal morphology of ceramics can be seen. Based on the cross-section image, average grain sizes of the samples were estimated to be 1.91 µm, 1.57 µm, 1.50 µm, and 1.37 µm, respectively. The average grain sizes of samples sintered at different temperature were also obtained and calculated. As shown in Fig. 3 (a), the average grain size of samples decreases with the increase in Mn content. The relative density of ceramics was measured by Archimedes method (Fig. 3(b)). According to the changes in density and SEM images, Mn doping inhibits grain growth and the number of pores of ceramics increases with increase in Mn content. This is due to the low mobility of oxygen atoms [18], which play an important role in the grain growth upon addition of Mn. 3.2 Electrical properties Fig. 4 (a) shows the resistivity variation with temperature for different samples as a function of Mn content. All samples exhibited a remarkable PTCR effect. Detailed electrical parameters of samples sintered at different temperature as a function of Mn content are shown in Fig. 4 (b) and (c). As the figure shows, room temperature resistivity and PTCR jump increase with the increase in Mn content. The relatively low value of PTCR jump for samples sintered at 1150 °C is related to relatively large relative density, which leads to the insufficient oxidized grain boundaries. The rise in PTCR jump where x ≥ 0.05 is attributed to the decreased grain size and ceramics density, as shown in Fig. 3 (b). For comprehensive consideration of low room-temperature resistivity and high PTCR jump, samples with 0.02 mol% of Mn was selected to be the representative one, which holds a low room temperature
resistivity of 6.3 Ω·cm and high PTCR jump of 103.5. Compared with samples in published papers which listed in Table 1, our representative samples have low resistivity and simultaneously high PTCR jump. 3.2.1 Room temperature resistivity As a useful tool in separating resistance of grain (Rg) and resistance of grain boundary (Rgb), impedance spectroscopy was used. A typical cole-cole plot is shown in Fig 5 (a), which means that there no any grain shell in the samples. With the help of software (Zview), the value of Rg and Rgb were obtained by using an equivalent circuit model of Rg (Cgb//Rgb) [19]. As shown in Fig. 5 (b), the value of Rg was small and changed little when Mn content increased. And the value of Rgb increases in an accelerated manner. It means that the Mn, which acts as an acceptor, should be segregated at the grain boundaries thus increases Rgb and have little influence on Rg. On the other hand, Mn doping inhibits the grain growth and increase in the number of grain boundaries accelerates the increase of Rgb. 3.2.2 PTCR jump As shown in Fig. 6 (a) and (b), the unoxidized samples had very low ρ25 °C and PTCR jump. Heywang–Jonker model has been accepted by most researchers and successfully used in explaining a variety of phenomena that are related to the PTCR effect. Heywang–Jonker model describes the relationship among potential barrier height (Φ0), the density of effective acceptors (Ns), the density of effective donors (Nd) and relative dielectric constant (εr ) as shown in equation 1 and 2: ⁄8
= b=
(1) (2)
Where e is the elementary charge, ε0 represents the vacuum permittivity, Nd=1/eµρg, b represents the thickness of depletion layer. According to equation (1), low value of Φ0 in un-oxidized samples is related to low Ns and high Nd. ESR was used to examine the valence state of Mn ions in the samples. As shown in Fig. 7 (a), there are no ESR signals arising from Mn at 25 °C. Thus, Mn ions should act as acceptors, and the valence state of Mn ions should be +3, [20, 21]. The low value of the PTCR jump in un-oxidized samples should be attributed to the large number of oxygen vacancies in the un-oxidized grain boundary and their ionization. The oxygen vacancies have very low ionization energies and generate electrons at room temperature, thus act as donors [22]: ×
•
→
+
(3)
And: •
→
••
+
(4)
Electrons generated by ionization of oxygen vacancies compensate acceptors. Therefore, the potential barrier height is very low and the PTCR effect is weak. Nevertheless, the PTCR jump increased with an increase in Mn content. As can be seen from the above analysis, Mn act as acceptors and its doping can increase surface acceptors. However, in the presence of oxygen vacancies ionization, PTCR effect of unoxidized samples doped with Mn is still weak. After reoxidation at 800 °C, both PTCR jump and RT resistivity of reoxidized were significantly improved. As shown in Fig.4 (c), the PTCR jump increases from 102.4 to 103.8, as the content of Mn boost from 0 to 0.08 mol%. Smaller changed PTCR jump and RT resistivity of samples sintered at 1150 °C may relate to higher relative density [23-25]. Herein, the potential barrier height was calculated and compared with Arrhenius-plots (Fig. 8 (a)) and drew in Fig.
8 (b). [26]. As shown in Fig. 8 (b), the potential barrier increases from 0.61 to 0.99 eV when x boost from 0 to 0.08%. It can be seen from Fig. 5 (b), that Rg changes a little when Mn content increase from 0 to 0.08 mol%. According to equation (1), increased Φ0 should be attributed to the increased Ns.
As shown in Fig. 7 (b) and (c), an ESR signal of six hyperfine peaks arose after phase transition. The value of g was calculated to be 2.002, arises from +1/2,mI↔-1/2,mI, which indicates that the valence state of Mn is changed from +3
[20, 21, 27]
to +2
[28]
. after the
paraelectric-ferroelectric structural transition. This phenomenon is the same as that of Mn in traditional thermistor sintered in air atmosphere. Mn doping produces MnTi′ before phase transition which has little influence on the room temperature resistivity and produces MnTi′′ after the phase transition which further improves Ns. Another factor that affects Ns is the density of ceramics [23-25]. Analysis of unoxidized samples has shown that the densities of Oadd′′ and VTi′′′′ are very low before reoxidation and are generated during reoxidation. As the content of Mn increases, the density of ceramics decreases, which leads to an increase in the Oadd′′ and VTi′′′′ and Ns. The thickness of the depletion layer was also estimated by equation (2) [26]
. As the resistivity of grain changes little with the increase of Mn content, and Ns increases
with the increase in Mn content, the thickness of depletion layer that mainly created by VTi′′′′, MnTi′′, and Oadd′′ is increased from 0.76 nm to 1.16 nm. It can be seen from the above analysis that Mn doping can effectively increase the density of surface acceptors and the PTCR jump after reoxidation. 3.2.3 Temperature coefficient Temperature coefficients (αT) are often used to measure the response speed of PTCR
ceramics, as expressed in the form of equation 5: (
=
)
(5)
It can be used to measure the slope and steepness of the R-T curves. As shown in Fig. 4 (a), with the increase in Mn content, the R-T curves become much steeper. Here, temperature coefficients (α0/ Δ T) on measurement point were used for representing the αT and were calculated by: /
=
!"
∆
#
(6)
R0 is the resistance on the measurement point (T0) and R1 is the resistance on the measurement point (T1) next to T0. ΔT=T1-T0. The maximum temperature coefficient (αTmax) and its corresponding temperature (Tαmax) are plotted in Fig. 9. Tαmax is hardly varied when Mn content changes and αTmax increased with the increase in Mn content. According to equations (1) and (5): (
α =
&'
≈
&*+
,-.
= /01 × #
2& *4
13 )
(7)
Where ρT = ρg + ρgb is the total resistivity of ceramics, for a much smaller value of ρg compared with ρgb, ρg was ignored. According to equation (5), Ns also affect the
temperature coefficient. Thus, the temperature coefficient increased with the increase in Mn content. 3.3 Effect of reoxidation temperature According to the previous literature
[25, 29]
, reoxidation temperature affects Ns and PTCR
jump. Herein, the influence of reoxidation temperature on the electrical performance of BNT-BT based chip ceramics was evaluated. As shown in Fig. 10 (a), with an increase in reoxidation temperature, PTCR jump slightly increases, but the resistivity first increases and
then decreases. Impedance spectra were used to analyze the unusual changes in room temperature resistivity. Fitting by software (Zview), the resistance of grain and grain boundaries were obtained and listed in table 2. According to Table 2, the value of Rg is small and changes little when reoxidized at different temperatures. Rgb increased to 20 Ω when samples were reoxidized at 700 °C and then dropped to 11 Ω, when samples were reoxidized at 800 °C. This anomaly phenomenon is due to the second phase at grain boundaries. According to Zu et. al [9], the Ba2TiO4 exists at the grain boundary when sintering aid BaCO3 is used. Ba2TiO4 can be consumed by reacting (when reoxidation temperature ≥ 800 °C) with Ba6Ti17O44, which is generated by the oxidization of grain boundaries. The reaction can establish new low-resistance pathways at grain boundaries and reduce the room temperature resistance. Thus, Rgb firstly increases and then decreases. Based on the above analysis, 800 °C is the best reoxidation temperature.
4. Conclusions In this work, Mn doped BNT-BT based chip PTCR materials were prepared and Mn doping was proved to be an effective way in preparing low-resistivity high-performance chip thermistors. The analysis shows that Mn doping cannot create a high enough potential barrier in un-oxidized samples but can obviously increase the value of potential barrier height after reoxidation. The valence change of Mn during phase change also contributes to the preparation of low-resistivity chip thermistors. Additionally, Mn doping effectively increases the thickness of depletion layer which helps improve the continuity of the depletion layer in low-resistivity thermistors. As a result, the representative samples exhibit extremely low resistivity of 6.3 Ω·cm and high PTCR jump of 103.4. Herein, we report for the first time the
preparation of thermistors with ρ25°C lower than 10 Ω·cm and PTCR jump higher than 103.
Acknowledgments This work is supported by National Key Research and Development Program of China (Grant 2017YFB0406405), National Natural Science Foundation of China (Grant No.61571203) and Innovation Team Program of Hubei Province (Grant No.2019CFA004). The authors acknowledge the assistance by the Analytical and Testing Center of Huazhong University of Science and Technology.
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Fig. 1 XRD patterns of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+0.06 mol% Mn at room temperature. Fig. 2 SEM images of the cross-section for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn which were sintered at 1125 °C: (a) x= 0, (b) x= 0.02, (c) x= 0.04, and (d) x= 0.06. Fig. 3 (a) average grain size and (b) relative density of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples as a function of x. Fig. 4 (a) R-T curves, (b) Room temperature resistivity and (c) PTCR jump of Ba0.996 La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples sintered at different temperatures and reoxidized at 800 °C. Fig. 5 (a) Impedance spectra and (b) separated Rg and Rgb of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples sintered at 1125 °C and reoxidized at 800 °C. Fig. 6 (a) Room-temperature resistivity and (b) PTCR jump of unoxidized Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn samples sintered at different temperatures. Fig. 7 ESR spectra of Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+0.06 mol% Mn samples (a) before reoxidation and measured at room temperature, (b) after reoxidation and measured at room temperature, (c) after reoxidation and measured at 140 °C. Fig. 8 (a) The Arrhenius-plots (lnρ against 103/T) and (b) estimated potential barrier heights for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn sintered at 1125 °C and reoxidized at 800 °C Fig. 9
The maximum temperature coefficient (αTmax)and its corresponding temperature
(Tαmax) for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+x mol% Mn sintered at 1125 °C and reoxidized at 800 °C Fig. 10 (a) Room-temperature resistivity and PTCR jump, (b) Impedance spectra for ceramics Ba0.996La0.004(Bi0.5Na0.5)0.003TiO3+0.02 mol% Mn sintered at 1125 °C and reoxidized at different temperature
Table. 1 Some of representative samples prepared by the reduction-reoxidation method in published papers. Main components Ba0.997-x(Bi0.5Na0.5)xLa0.003TiO 3 +0.3 mol% BaCO3 Ba0.997La0.003TiO3+ x mol% BaCO3 BaLa0.004TiMn0.0005O3+2 mol% BN Ba0.984Y0.016TiO3+0.025 BaCO3+0.05 BN (Bam-0.202Ca0.2La0.002)TiO3+0.01 SiO2 (Ba0.882-xSr0.12Smx)TiO3 Ba0.85Ca0.15Y0.002Ti1.01O3+0.25 mol%SiO2+0.06 atm% Mn
PTCR jump
RT resistivity
Year
The author
103.5
18.4 Ω cm
2019
Yan et .al[30]
103.7
28 Ω cm
2018
Zu et .al[24]
104
136 Ω cm
2017
Gao et .al[11]
103.3
727 Ω cm
2012
Liu et .al [10]
~ 90
5 Ω cm
2007
Niimi et .al [31]
~ 1000
23 Ω cm
2007
Niimi et .al [32]
~22
5 Ω cm
1994
Kanda et .al[33]
Table. 2 the resistance of grain and grain boundaries of samples Ba0.996La0.003(Bi0.5Na0.5)0.004TiO3+0.02 mol% Mn sintered at 1125 °C and reoxidized at different temperature obtained from impedance analysis. Reoxidation temperature
--
700°C
750°C
800°C
850°C
Rg (Ω)
2.1
4.1
3.6
3.0
3.3
Rgb (Ω)
3.8
20.3
15.6
11.1
12.7
This work is supported by National Key Research and Development Program of China (Grant 2017YFB0406405), National Natural Science Foundation of China (Grant No.61571203) and Innovation Team Program of Hubei Province (New microwave devices for next generation wireless communication systems). There are no interests to declare.