Nb-Doped 0.8BaTiO₃-0.2Bi(Mg_0.5Ti_0.5)O₃ Ceramics with Stable Dielectric Properties at High Temperature

Abstract

Nb-doped 0.8BaTiO₃-0.2Bi(Mg_0.5Ti_0.5)O₃ ceramics were prepared by conventional solid-state method. The dielectric properties and the structural properties were investigated. When Nb₂O₅ is doped into 0.8BT-0.2BMT system, a small amount of Ba₄Ti₁₂O₂₇ secondary phase is formed. The lattice parameters gradually increase with the Nb₂O₅ doping. It is found that the temperature-capacitance characteristics greatly depend on Nb₂O₅ content. With the addition of 3.0 mol% Nb₂O₅, a 0.8BT-0.2BMT ceramic sample could satisfy the EIA X9R specification. This material is promising for high-temperature MLCC application.

1. Introduction

Multilayer ceramic capacitors (MLCC) have been extensively used in many kinds of electronic products in the last few decades. Some harsh conditions—such as downhole drilling, aerospace, and automotive environment—require the capability of MLCCs to withstand temperatures over 200 °C or more [1,2]. By contrast, the X7R (−55 to 125 °C, ΔC/C_{25 °C} ≤ 15%) and the X8R (−55 to 150 °C, ΔC/C_{25 °C} ≤ 15%)-type MLCCs defined by the Electronic Industries Association (EIA) standards are not competent because their ceiling temperatures are 125 °C and 150 °C, respectively [3,4]. Therefore, it is of great significance to exploit a material used for X9R (−55 to 200 °C, ΔC/C_{25 °C} ≤ 15%)-type MLCCs, and develop a new temperature-stable dielectric material with high-temperature resistance, high dielectric constant, and low dielectric loss.

As well known, high permittivity MLCCs are generally derived from perovskite structures, such as barium titanate (BT) dielectric ceramics. As the Curie temperature (T_c) of BT is approximately 130 °C, however, it is difficult to prepare pure BaTiO₃ (BT) ceramics with weak temperature dependence [5]. Recently, Bi(Me)O₃ based perovskite (Me = Sc, Al, Zn_1/2Ti_1/2, etc.) were of interest for the study and development of new dielectric ceramics because of their relatively high dielectric constant, broad diffused dielectric behavior, and high Curie temperature [6,7,8,9]. Therefore, perovskite solid solutions based on BT-Bi(Me)O₃ composition with a high permittivity are developed and explored for high temperature capacitor applications [10]. The 0.8BT-0.2BMT ceramic with moderate dielectric constant and dielectric loss is reported to possess lower temperature variation of capacitance in high temperature regions [11]. The Nb₂O₅ doped BT systems are reported for potential application in advanced X8R capacitors, but the Nb₂O₅ dopant depressed dielectric constant in exchange for temperature stability [12]. The Nb⁵⁺ ion was found to diffuse into the crystal lattice and form the chemically inhomogenous regions, called ‘core-shell’ structure, showing good dielectric stability [13,14,15]. In this study, the 0.8BT-0.2BMT dielectric system is modified only by Nb₂O₅ to improve the dielectric temperature stability.

2. Experimental Procedure

Nb-doped 0.8BaTiO₃-0.2Bi(Mg_0.5Ti_0.5)O₃(0.8BT-0.2BMT) ceramics were prepared using solid-state reaction method. BaTiO₃ (99.9%), Bi₂O₃ (99.0%), MgO (99%), and TiO₂ (99.0%) powders and Nb₂O₅ (99.5%) with different doped levels (0 ≤ x ≤ 3 mol%) were batched stoichiometrically and ball milled in a polyethylene jar with yttria-stabilized zirconia balls and de-ionized water for 4 h at a rate of 280 rpm. Then the mixtures were dried and calcined in alumina crucibles at 900 °C for 2 h. Subsequently, the calcined powders were then re-milled in de-ionized water using zirconia balls for 4 h. After drying, the resulting powders were granulated with a few drops of PVA binder and pressed into disk with 12 mm in diameter and 1 mm in thickness. Samples were sintered at 1120 °C for 2 h in air with a heating rate of 3 °C/min and then cooling with furnace to ambient temperature.

Crystal structure of the samples was determined at room temperature by X-ray powder diffraction (XRD) (Cu Kα radiation, PANalytical X’Pert Pro, Almelo, The Netherlands). Rietveld refinement of the crystal structures was performed using the GSAS-EXPGUI program [16]. Microstructure and composition analysis of the ceramic pellets were performed by scanning electron microscopy (SEM, Inspect F, FEI Company, Eindhoven, The Netherlands), which was operated at 20 kV. After samples were fired at 800 °C with silver paste on both sides, the room temperature dielectric constant and dielectric loss were measured by use of a precision LCR meter (4284A, Aglient) at 1 kHz and 1 Vrms. The temperature-capacitance characteristics of the samples were measured from –55 °C to 200 °C with LCR meter (4284A, Aglient) and an automatic temperature controller at 1 kHz and 1 Vrms with a rate of 2 °C/min. The insulation resistance of the specimens was recorded by a Digital Super Megohm Meters (DSM-8104, HIOKI, Nagano, Japan) at 25 °C. The bulk density was measured using the Archimedes method.

3. Results and Discussion

Figure 1 shows the XRD patterns of well-sintered Nb₂O₅ doped 0.8BT-0.2BMTceramic disks. All the samples presented a desired perovskite structure and no secondary phase was discovered for sample without Nb₂O₅ dopants. It indicated that homogeneous solid solution of 0.8BT-0.2BMT was obtained. However, a small amount of secondary phase Ba₄Ti₁₂O₂₇ is found in all Nb-doped 0.8BT-0.2BMT samples. The phase structures are analyzed by General Structure Analysis System (GSAS) Rietveld refinements for 0.8BT-0.2BMT with various amount of Nb₂O₅, as shown in Figure 2. The main phase is detected as BaTiO₃ (P4mm, ICSD No. 086286). The lattice parameters and reliability factors are listed in

Table 1. Lattice parameters from Rietveld refinement for the samples.

Samples	Lattice Parameters			Reliability Factors
	a (Å)	c (Å)	V (Å3)	Rwp	Rp	χ2
+0.0 mol% Nb2O5	4.0096	4.0131	64.5182	0.0786	0.0598	1.678
+1.0 mol% Nb2O5	4.0112	4.0140	64.5842	0.0873	0.0651	2.187
+2.0 mol% Nb2O5	4.0131	4.0165	64.6856	0.0780	0.0579	1.555
+3.0 mol% Nb2O5	4.0150	4.0179	64.7695	0.0847	0.0621	1.943

. All the lattice parameters are obtained with high reliability as R_wp < 10%, R_p < 8%. The lattice parameters calculated from the Rietveld refinements increase continuously with the increase of Nb₂O₅ content. This indicated the larger ionic radius substitution that occurred according to Bragg’s law. In other words, Nb⁵⁺ mainly enters the Ti-sites as a donor because the Nb⁵⁺ (0.64 Å) is a considerably larger radius than that of Ti⁴⁺ (0.605 Å) in six-fold coordination [17]. Meanwhile, Nb substitution on Ti site in the BaTiO₃ lattice will leave the excess Ti out of the grains, which is responsible for secondary phase formation at the grain boundary.

Figure 3 shows the microstructure of various amount of Nb₂O₅ doped 0.8BT-0.2BMT ceramics. It is observed that all the Nb-doped 0.8-0.2BZT ceramics are in high densification with homogeneous grain size being on the order of several micrometers. There is a small change of the grain size as the Nb₂O₅ content increases, where the average grain size is about 1.5 μm. Some stick grains, pointed by arrows in the Nb₂O₅ doped samples, are considered as secondary phase (Ba₄Ti₁₂O₂₇). This result is highly consistent with XRD analysis.

Main dielectric and electrical properties at room temperature of the samples are listed in

Table 2. Dielectric and electrical properties of the samples at room temperature.

Sample	εr	tan δ	ΔC/C25 °C < ±15%	Resistivity (Ω·cm)	Density (g/cm3)
+0.0 mol% Nb2O5	1683	5.0%	-	1.0 × 1013	6.214
+1.0 mol% Nb2O5	1247	1.5%	−26~197 °C	6.4 × 1011	6.219
+2.0 mol% Nb2O5	925	0.7%	−40~200 °C	5.9 × 1012	6.148
+3.0 mol% Nb2O5	764	0.5%	−55~200 °C	1.3 × 1013	6.159

. As we can see, dielectric constant at 25 °C decreased with increasing Nb₂O₅ content. In the Nb₂O₅ modified BT-BMT system, the Nb⁵⁺ will diffuse into the crystal lattice to form the chemically inhomogeneous structure (‘core-shell’). The dielectric characteristics of the barium titanate-based core-shell grain structure were a superposition of the ferroelectric grain core, paraelectric grain shell [18]. With further addition of Nb₂O₅ dopants, the volume fractions of the shell region increased, leading to the decrease of the dielectric constant.

When a small amount of Nb₂O₅ is doped into the BT matrix, electronic compensation regime appears [19,20,21].

$$N{b}_2{O}_5\to 2N{b}_{Ti}^{\bullet }+2Ti{O}_2+O_0+2e^{\prime }$$

(1)

The insulation resistivity of the sample with 0.1 mol% Nb₂O₅ dopants decreased for the electronic compensation. With the increasing of Nb₂O₅ concentration, the electronic compensation changes into Ti vacancy compensation.

$$2N{b}_2{O}_5\to 4N{b}_{Ti}^{\bullet }+5Ti{O}_2+V_{Ti}^{\prime \prime \prime \prime }$$

(2)

The mechanism is mainly involved with the compensation of Ti vacancies so that the samples with 3.0 mol% Nb₂O₅ can become highly insulating as shown in Table 2 [22].

Figure 4 demonstrates the temperature dependence of dielectric constant and dielectric loss for 0.8BT-0.2BMT with various amount of Nb₂O₅. Double dielectric peaks are observed: one at around 0 °C and a second, smaller, peak at about 130 °C. The curves are flattened by the Nb₂O₅ dopants at the cost of deteriorating dielectric constant. However, when the doped level is above 2.0 mol%, it is difficult to detect the higher permittivity peak. Loss tangent data at room temperature are lower than 5%, and exhibit a decrease with increasing Nb₂O₅ content. Dielectric loss at low temperature is rather high and decreases monotonously with the increasing temperature.

Temperature dependence of capacitance variation rate based on C_{25 °C} for 0.8BT-0.2BMT with various amount of Nb₂O₅ is shown in Figure 5. It can be seen that the temperature-capacitance characteristics have a remarkable improvement with the incorporation of Nb₂O₅, especially at the cold end. At the hot end, the permittivity peak is suppressed with the increase of Nb₂O₅ dopants. The 3.0 mol% Nb₂O₅-doped 0.8BT-0.2BMT ceramic sample is found to satisfy the EIA X9R specification with a permittivity of 764 and dielectric loss of 0.5%. The existence of the double dielectric anomalies over the studied temperature range, which is the characteristic phenomenon of ‘core–shell’ structure in BT ceramics, is beneficial to improving the temperature stability of the dielectric properties [20]. The Nb⁵⁺ will diffuse into the crystal lattice to form the chemically inhomogeneous structure, where the core is pure BT, while the Nb ion mainly concentrated in the shell region. In the ceramics with ‘core–shell’ structure, the double peaks in the temperature dependence of dielectric constant curve can usually be obtained [14]. In the Nb₂O₅ modified 0.8BT-0.2BMT system, the dopant oxides tend to accumulate at the grain boundary and diffuse simultaneously into BT grain. Nevertheless, the diffusion of Bi ions was weakened due to the slow diffusion of Nb⁵⁺ during the sintering process. Thus Bi₂O₃, MgO, and TiO₂ mainly stayed in the shell region which should be responsible for the lower dielectric peak at ~0 °C, while pure BT at cores is attributable to the dielectric peak at ~130 °C, as shown in Figure 4.

4. Conclusions

In this work, we investigated the effect of Nb₂O₅ doping on the dielectric properties of 0.8BT-0.2BMT ceramics. A small amount of Ba₄Ti₁₂O₂₇ secondary phase is observed when Nb₂O₅ doped into 0.8BT-0.2BMT system. The dielectric temperature stability is strongly affected by the Nb₂O₅ dopants. The 3.0 mol% Nb₂O₅-doped 0.8BT-0.2BMT ceramic sample satisfies the requirement of EIA X9R specification with a permittivity of 764. Moreover, it possesses low dielectric loss and high insulation resistance, which ensures the application of this material.