Grain size effect on electrical and reliability characteristics of modified fine-grained BaTiO3 ceramics for MLCCs

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ScienceDirect Journal of the European Ceramic Society 34 (2014) 1733–1739

Grain size effect on electrical and reliability characteristics of modified fine-grained BaTiO3 ceramics for MLCCs Huiling Gong, Xiaohui Wang ∗ , Shaopeng Zhang, Hai Wen, Longtu Li ∗ State Key Lab of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Received 26 October 2013; received in revised form 15 December 2013; accepted 17 December 2013 Available online 1 February 2014

Abstract Fine-grained BaTiO3 -based ceramics of different grain sizes (118–462 nm) with core–shell structures were prepared by a chemical coating method, having good dielectric properties and gentle temperature stability. The grain size effect on the dielectric properties and insulation resistivity of modified fine-grained BaTiO3 ceramics under high temperatures and electric fields were investigated. The DC bias shows a strong effect on the dielectric properties with decreasing grain size. In the finest ceramics, the absolute value of the capacitance stability factor was the smallest, indicating that the modified-BaTiO3 ceramic capacitor with smaller grains had higher reliability under the DC bias voltage. The highly accelerated lifetime test results showed that with decreasing the grain size, samples exhibited higher insulation resistance under elevated temperatures and high voltages. Impedance analysis proved that the finer-grained ceramic with core–shell structure had higher activation energy for both grain and grain boundary, whereas the proportion of ionic conductivity was lower. © 2014 Elsevier Ltd. All rights reserved. Keywords: Modified-BaTiO3 ceramics; Grain size effect; Reliability; Impedance spectroscopy

1. Introduction Today’s technology is gradually developing towards miniaturisation, high portability, networking and multimedia. Simultaneously, multilayer ceramic capacitors (MLCCs), used as passive components, are in high demand, driven by the surface mounting technology and portable device markets. At present, the development trends of MLCCs are high capacity, miniaturisation, thin-layer, low cost, high reliability and high temperature stability.1 In particular, the thickness of the dielectric layers is decreasing to ∼1 ␮m and the number of layers is increasing to meet the requirements for MLCCs being smaller in size but providing large capacitance.2 Thus, the grain sizes of BaTiO3 ceramics should be approximately 100–200 nm to maintain sufficient reliability3 and possess the so-called “core–shell” structure to meet the requirements for temperature-stable characteristics of the capacitance and a reliable performance in MLCCs.4–7 For example, the X7R specification of the Electrical

∗

Corresponding authors. E-mail addresses: [email protected] (X. Wang), [email protected] (L. Li). 0955-2219/$ – see front matter © 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.12.028

Industries Association (EIA) requires less than ±15% change of capacitance from the value at 125 ◦ C, in the range from −55 ◦ C to 125 ◦ C, and the required lifetime of the X7R MLCC is more than 1000 h at 125 ◦ C under an applied DC voltage 1.5 times the specified value. At the same time, a reduction in the dielectric layer thickness directly indicates an increase of the electric field across each layer when the MLCCs are used.8–10 On the other hand, the grain size should be constantly reduced to ensure at least 5–6 grains in each layer to guarantee high reliability. Consequently, under the trend of decreasing grain size, research about the grain size effect on the reliability characteristics and electrical properties of modified fine-grained BaTiO3 is urgently needed for X7R-type MLCC applications. Numerous studies have been conducted on the grain size effect of barium titanate since the first report in the micron range.11 Arlt et al. studied barium titanate ceramics with a grain size range of ∼0.2–100 ␮m and found that the dielectric constant at room temperature did not decrease monotonically with grain size. When the grain size was less than 1 ␮m, there was a sharp decline in the dielectric constant.12 Subsequently, there were several studies on the grain size effect of doped barium titanate.13–21 However, most studies focused on the dielectric properties and microstructure evolution with different

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material systems for various applications. Donnelly and Randall studied the mixed conductions and chemical diffusion in a Pb(Zr0.53 ,Ti0.47 )O3 buried capacitor structure by impedance spectroscopy.22 Yoon et al. discussed the influence of grain size on the impedance spectra and resistance degradation behaviour in acceptor-doped BaTiO3 ceramics with 90 ␮m and 0.8 ␮m.18 Actually, the research report on electrical and reliability characteristics of X7R-type MLCCs with fine grain (<0.5 ␮m) BaTiO3 ceramic has been extremely limited. In this work, dense BaTiO3 ceramics with different grain sizes were obtained by a chemical coating method23,24 and were sintered in a reducing atmosphere. The grain size effects on the electric properties and the reliability of the modified finegrain BaTiO3 ceramics under high temperature and electric field were investigated. The experimental results provide theoretical support for the miniaturisation trend of MLCCs.

2. Experimental procedure Commercial BaTiO3 hydrothermal powders (Guo Teng Co. Ltd., Shandong, China) with different average particle sizes of 80 nm, 200 nm, 300 nm and 450 nm were chosen as starting materials. To induce the core–shell structure in the ceramics, the BaTiO3 powders were modified by a wet chemical coating approach as described previously.24,25 The powders with different particle sizes were separately ball-milled in isopropanol for 18 h to obtain a well-dispersed suspending slurry. Then, the inorganic solution and organic solutions containing the additive elements were added into the slurry to improve the dielectric properties of the BaTiO3 ceramics. For the preparation of the inorganic solution, water-soluble metal salts, including Mn(CH3 COO)2 (99.8%, Beijing Finechemical Co. Ltd., Beijing, China), 0.3 mol%; Mg(NO3 )2 (99.8%, Beijing Finechemical Co. Ltd., Beijing, China), 4 mol%; Ca(NO3 )2 (99.8%, Beijing Finechemical Co. Ltd., Beijing, China), 0.8 mol%; Ho(NO3 )3 (99%, Rare-chem Hi-tech Co. Ltd., Beijing, China), 0.5 mol%; and Y(NO3 )3 (99%, Rare-chem Hi-tech Co. Ltd., Beijing, China), 0.4 mol% were dissolved in deionised water. For the organic solution, 1.5 mol% tetraethylorthosilicate (TEOS, SiO2 ≥28.0%, Beijing Finechemical Co. Ltd., Beijing, China) was dissolved in a mixed solution of ethanol and deionised water (ethanol: water = 3:1, by volume). The molar ratios of these metal salts are relative to the molar content of BaTiO3 . Ammonia solution was then added to adjust the pH value of the mixed slurry to 9.5 for precipitation of the additive elements. Coated BaTiO3 powders were obtained by calcining the dried slurry at 450 ◦ C for 2 h. The coated powders were combined with poly vinyl alcohol (PVA) binders and pressed into green pellets of 10 mm in diameter at 2 MPa. The green pellets were sintered in a reducing atmosphere (H2 /N2 /H2 O, PO2 = 10−13 atm =) by a conventional sintering method (T1 for 2 h, followed by annealing in a weak oxidising atmosphere at Treo = 1050 ◦ C for 150 min). The optimal sintering temperatures (T1 ) for the samples with different starting particle sizes were 1190 ◦ C, 1220 ◦ C, 1230 ◦ C and 1320 ◦ C.

The micrographs of the polished cross-sections after thermal treatment were characterised by SEM (Leo-1530, Germany). The temperature dependence of the dielectric constant and the dielectric loss was measured from −60 ◦ C to 150 ◦ C (heating rate 2 ◦ C/min) at 1 kHz and an oscillation level of 1 Vrms using an impedance analyser (Model HP4192A, Hewlett–Packard Company, Santa Clara, CA, USA) with a thermostat. The insulation resistance (IR) of the samples was measured by a pA metre (Model HP4140B, Hewlett–Packard Company, Santa Clara, CA, USA). The bias dependence of the capacitance was determined by a power device analyser (Model B1505A, Agilent Technologies, Santa Clara, CA, USA) with a maximum voltage of 3000 V. Complex impedance was measured by a frequency response analyser (Solartron SI 1260, Westborough, MA, USA) with a dielectric interface (Solartron 1296, Westborough, MA, USA) over frequencies from 10 mHz to 2 MHz in a temperature range of ∼510–590 ◦ C under a Vrms of 100 mV ac signal. The evaluation of each R (resistance) and C (capacitance) component value for an assumed equivalent circuit was determined by the software Z-View (version 2.3).

3. Results and discussion According to the particle size of the starting BaTiO3 powders (80 nm, 200 nm, 300 nm and 450 nm), the sintered ceramic samples were named as BT08 (∼80 nm), BT20 (∼200 nm), BT30 (∼300 nm) and BT45 (∼450 nm), respectively. Considering the particle size, as well as the surface active energy of the coated powders, different sintering parameters (illustrated in Table 1) were utilised to obtain full dense BaTiO3 ceramics satisfying the EIA X7R specifications. The micrographs of the polished cross-sections after thermal treatment for the fine BaTiO3 ceramic samples are shown in Fig. 1. The average grain sizes were obtained by measuring the Feret’s diameters of more than 150 grains for each sample. The grain sizes of the BaTiO3 ceramics were directly proportional to the particle sizes of the starting powders. The nano-grained ceramic BT08 (∼118 nm) was obtained by a chemical coating method. The grain size distribution was narrow and uniform, demonstrating the advantage of the chemical coating method. The temperature dependence of the dielectric properties and the capacitance change based on the capacitance at 25 ◦ C are described in Fig. 2. Over the whole temperature range, the dielectric constant increases with increasing grain size, in accordance with the results in the submicron BaTiO3 ceramics.12 Inversely, the insulation resistivity of the samples increases with reducing grain size, as shown in Table 1. The temperature capacitance coefficients (TCC) satisfied the EIA X7R specifications for all of the samples. Although the Curie temperature (Tc ) peak can hardly be determined accurately for sample BT08, the ascending trend in the Tc peak intensity could still be confirmed. As the grain size increased from 118 nm to 462 nm, the value of Tc showed a slight increase, in accordance with the results in the submicron BaTiO3 ceramics.12 In addition, the Tc peak can also be observed from the dielectric loss versus temperature curve. The dielectric losses of the three samples with larger grains are

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Table 1 Properties of BaTiO3 -based ceramic samples with different grain sizes at room temperature and sintering conditions. Sample

Sintering temperature

Relative density (%)

Ceramic grain size (nm)

Dielectric constant

Loss

Resistivity (1012  cm)

BT08 BT20 BT30 BT45

1190 ◦ C – 2 h 1220 ◦ C – 2 h 1230 ◦ C – 2 h 1320 ◦ C – 2 h

97.1 96.7 95.6 96.0

118 227 346 462

2043 2321 2578 2713

0.0051 0.0065 0.0063 0.0078

12.18 7.23 4.57 2.05

Fig. 1. Micrographs of polished cross-sections after thermal treatment for the fine BaTiO3 ceramic samples (a) BT08 (average grain size ∼118 nm); (b) BT20 (average grain size ∼227 nm); (c) BT30 (average grain size ∼346 nm); and (d) BT45 (average grain size ∼462 nm).

slightly different below the Tc , whereas the loss of sample BT08 is always lower over the entire temperature range. With the same doping content, the samples with larger grain size had higher Tc peaks, as shown in the TCC curves. This result could be attributed to the large volume fraction of the core in the larger grains, which determines the high temperature end of the TCC curve.25,26 Here, sample BT08 meets the X7R specifications with a dielectric constant of 2100 and a resistivity of 1013  cm. All of these results indicate that the chemical coating method has obvious advantages over the conventional solid state reaction method.24 The capacitance changes versus the DC bias field of the modified BaTiO3 ceramics with different grain sizes are depicted in Fig. 3. The C–V characteristics greatly depended on the grain size. The samples with smaller grain sizes yielded more stable dielectric properties, whereas the capacitance of the samples

with larger grain sizes declined rapidly with the increase of the DC bias field. The capacitance stability coefficient α is defined as α=

C(0) − C(E) × 100% C(0)

(1)

Here, C(0) and C(E) correspond to the capacitance with no bias and with an applied DC bias field, respectively. α describes the extent to which the capacitors suffered from the effect of the DC bias. A positive coefficient means that the capacitance drops with increasing bias field, whereas a negative one stands for the opposite trend. Moreover, a larger absolute value of α means a greater influence of the DC bias on the ceramic samples. The reason why the C–V curve tends to be stable with the reduction of the grain sizes will be analysed in detail in the following section.

Fig. 2. Temperature dependence of the (a) dielectric constants (K–T); (b) temperature coefficients of capacitance (TCC), all satisfied the X7R specification; and (c) dielectric loss.

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Fig. 3. DC bias field dependence of the capacitance variation for the samples with different grain sizes.

According to our previous work, the α of samples with core–shell structures can be separately expressed as follows27,28     βpχVc βpχEc = tanh αc = tanh (2) kTrc kT   2 −1/2 Vs −1/2 3 αs = 1 − 1 + λεs0 (3) = 1 − [1 + λε3s0 Es2 ] rs αr =

  1 qND ε0 Kc 1/2 2 2Vr3

(4)

Here, αc , αs and αr correspond to the coefficients of the grain core, grain shell and grain boundary under DC bias, respectively. Vc , Vs and Vr correspond to the actual voltage applied in the core region, the shell region and the grain boundary region, respectively. rc and rs correspond to the semi-diameter of the cores and the thickness of the shells, respectively. β is the coupling factor, p is the domain polarisation, and χ is the internal stress coefficient, which is introduced to modify the DC bias. k is the Boltzmann constant, and T is the temperature. λ is the phenomenological coefficient, and εs0 is the dielectric constant with no bias field. ND is the density of uncompensated ionised donors, and Kc is the dielectric constants of the compensation regions. With the same dopant content, in the case of various grain sizes, a smaller grain size means smaller cores (rc ), relatively thinner shells (rs ), and a higher grain boundary volume proportion, which means a higher insulation resistance of the grain boundary portion. Here, the conclusions are different from those reported in Park’s work. All different starting particle sizes used the same sintering temperature, and they presumed that the diffusion length of cerium was nearly constant with respect to the grain diameter.29 Thus, Vr increased and Vc and Vs decreased. For simplicity, the local electrical field Ec = Vc /rc and Es = Vs /rs was considered to be constant. The domain polarisation (p) will largely decrease due to the size effect of BaTiO3 .30 Based on Eq. (2), a smaller p results in a smaller αc . Meanwhile, the phenomenological coefficient λ has been reported to be temperature- and grain

Fig. 4. Resistivity of the bulk materials versus time at elevated temperature and DC bias of (a) 200 ◦ C, 1000 V/mm and (b) 260 ◦ C, 1200 V/mm.

size-dependent,31 which means that a smaller grain size will result in a smaller λ value. Therefore, with decreasing grain size, the value of λε3s0 (Vs /rs )2 becomes smaller, and the absolute value of αs decreases. A high Vr produces a small αr according to Eq. (4). The overall α coefficient of MLCC is attributed to the combination of the three parts. Therefore, a smaller α coefficient should be the main reason that is responsible for the good C–V characteristics of smaller-grained BT ceramic samples, according to Eqs. (2)–(4). The time dependence of the insulation resistivity of the finegrained BaTiO3 ceramic samples at high temperature under DC field was investigated. The highly accelerated lifetime test (HALT) was repeated several times to exclude data error. It can be observed from Fig. 4 that the resistivity increases at first and then starts to drop as time passes. It can be concluded that the dropping rate is so slow that there is almost no resistivity degradation, as shown in Fig. 4(a) under 200 ◦ C and 1000 V/mm. However, the resistivity of smaller-grained ceramics is much higher than that of the larger-grained ceramics. When increasing the temperature and the electric field to 260 ◦ C and 1200 V/mm, a significant difference appears among the samples with different grain sizes. The degradation behaviour is critically dependent on the grain size at higher temperature and DC field. Significant resistance degradation was observed for sample BT45 with the shortest life time. From the data, it can

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Fig. 5. Complex impedance data of samples (a) BT08, (b) BT20, (c) BT30, and (d) BT45 at a series of temperatures (the red arrow representing the Warburg impedance). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

be concluded that the resistance degradation behaviour was dramatically improved with decreased grain size. The similar behaviour of the grain size-dependent resistance degradation as above was also reported in the case of acceptor Ni-doped SrTiO3 ceramics.32 Grain boundaries play a major role in the total insulation resistance of capacitors,33,34 which is attributed to the formation of Schottky barriers.3 Yoon et al. considered that the decrease of the applied electric field strength at grain boundaries by the decrease of grain size could increase the potential barrier height, which would result in the retardation of the resistance degradation compared with specimens with larger grain sizes.35 Thus, in the process of long-time use, the MLCCs made of small-grained BaTiO3 ceramics would possess greater reliability. Complex impedance study is an informative and useful characterisation technique in materials research that provides vital information regarding the microstructure of polycrystalline materials, such as grains, grain boundaries and electrode interfaces.36,37 To obtain reliable values for the resistance of grains and grain boundaries with different grain sizes, to understand the electrical conduction mechanisms, and to establish a connection between microstructure and electrical properties, a complex impedance analysis was applied, including measurements of a low-frequency Warburg impedance, which can separate ionic conduction from the total conductivity.38 In the impedance spectra shown in Fig. 5, it was noted that different grain size samples all exhibited heavily distorted large semi-circles at high frequencies and small semi-circles at low frequencies. These features are due to the presence of both grain boundary impedances and mixed conduction, i.e., electronic and ionic conduction. The presence of the low frequency arc in the complex impedance spectra indicates an ionic space charge polarisation.38,39 Such a response occurs due to the particular charged defects, in this case oxygen vacancies, being

blocked at the electrodes, whereas the electronic conduction is reversible.22,39,40 The equivalent circuit model shown in Fig. 6 was employed in this study and was modified based on the model previously reported in Ref.22,40–42 The model was composed of an electronic rail corresponding to the grain and grain boundary impedance in the high frequency region and an ionic rail, including the Warburg impedance (Ws ), in the low frequency region. In the electronic rail, Cg denotes the capacitance related to the domain and dipole reorientation in the gain (the grain core and shell treated as a whole part), Rg is the resistance associated with the grain, CPEgb is the capacitance related to the grain boundary layer (CPE is a constant phase element indicating the departure from ideal Debye-type model), and Rgb denotes the resistance across the grain boundary layer. The component Rs represents a compensation resistor in the equivalent circuit, such as the contributions of the conductor. Here, as is often the case for mixed conductors, the Warburg component dominates the response; therefore, the double layer capacitance, which is due to the ionic polarisation of the electrode, was omitted from the circuit, making the equivalent model more clear and concise.43 An inspection of Fig. 5 illustrates that both the total electronic resistance and the grain-boundary electronic resistance decrease

Fig. 6. A sketch of the effective electrical equivalent circuit model containing electronic and ionic components with the Warburg impedance.

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Fig. 7. Arrhenius plots of ln(R) versus 1000/T. The activation energy is calculated from the slope. (a) Ea denotes the activation energy of the grains with different grain size and (b) Ea denotes the activation energy of the grain boundaries with different grain size.

with decreasing grain. However, there is a distinct increase in the low-frequency Warburg impedance with an increase of grain size, indicating an increasing contribution from ionic conduction. Here, the contribution of ionic conductivity is comparable to the results with ∼3–5% mol of Mg acceptor doping reported by Yoon et al.41,42 Under the conditions of the same doping concentration, smaller-grained samples show much lower ionic conductivity due to the higher proportion of grain boundaries, which are the main obstacle to oxygen vacancy migration.44–47 The fitting results of both the grain and grain boundary are presented in Fig. 7, taking advantage of the classical Arrhenius behaviour in the temperature range. The test results fit the proposed effective electrical equivalent circuit model. As the grain size decreased, the activation energy of the grain bulk is gradually increased, due to the chemically heterogeneous microstructure in a grain, the so-called core–shell structure. Donor and acceptor doped BaTiO3 -based ceramics help to improve the reliability of ceramic capacitors due to the charge compensation mechanism. Simultaneously, the activation energy of the grain boundary increases because of the increasing proportion of the grain boundary. These results agree with the HALT and C–V characteristics for the fine-grained BaTiO3 -based ceramics. 4. Conclusions In this study, modified BaTiO3 ceramics for X7R-type MLCCs with different grain sizes (118–462 nm) were prepared by a chemical coating method and had good dielectric properties and gentle temperature stability, meeting X7R standard applications. In particular, sample BT08, with a grain size of 120 nm, showed an excellent dielectric constant over 2000, with a low dielectric loss of <1% and a high resistivity of 1013  cm. The results of the dielectric properties in finegrained BaTiO3 -based ceramics show the grain size effect on the dielectric constant in accordance with submicron pure barium titanate: with decreasing grain size, the dielectric constants were reduced. The grain size effect of the modified fine-grained BaTiO3 ceramics for X7R-type MLCCs on the dielectric properties and insulation resistance under DC bias field and elevated temperature were studied in detail. The bias characteristics test result

showed that the DC bias field has a strong effect on the dielectric properties with different grain size. In the finest ceramics, the absolute value of the capacitance stability factor was the smallest, which means that modified fine-grain BaTiO3 ceramics with small grains have a greater reliability under a DC bias voltage. The HALT result showed that the insulation resistance of the ceramics samples under high electric field and elevated temperature were greatly improved by decreasing the ceramic grain size. Furthermore, the impedance analysis showed that the smaller-grained BaTiO3 ceramics with core–shell structures had a higher proportion of grain boundaries, leading to a higher activation energy for both bulk and grain boundary. Meanwhile, smaller-grained samples showed much lower ionic conductivity due to the higher proportion of the grain boundary, which is the main obstacle to oxygen vacancy migration. The results provide a theoretical foundation for nano-crystalline ceramics applications in the next generation of ultra-thin layer BME-MLCCs. Acknowledgements The work was supported by the Ministry of Sciences and Technology of China, through the National Basic Research Program of China (973 Program 2009CB623301), the National Natural Science Foundation of China for Creative Research Groups (Grant No. 51221291), the National Natural Science Foundation of China for distinguished young scholars (Grant No. 50625204), and the National Natural Science Foundation of China (Grant No. 51272123), and was also supported by Samsung Electro-Mechanics Co. Ltd. References 1. Pan M-J, Randall CA. A brief introduction to ceramic capacitors. IEEE Electr Insul Mag 2010;26:44–50. 2. Kishi H, Mizuno Y, Chazono H. Base-metal electrode-multilayer ceramic capacitors: past, present and future perspectives. Jpn J Appl Phys 2003;42:1–15. 3. Yang GY, Dickey EC, Randall CA, Barber DE, Pinceloup P, Henderson MA, et al. Oxygen nonstoichiometry and dielectric evolution of BaTiO3 . Part I – improvement of insulation resistance with reoxidation. J Appl Phys 2004;96:7492–9.

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