Sintering Characteristics and Microwave Dielectric Properties of BaTi₄O₉ Ceramics with CuO–TiO₂ Addition

Abstract

Sintering characteristics, phase evolutions, microstructures, and microwave dielectric properties have been investigated for BaTi₄O₉ ceramics prepared by traditional low temperature sintering using CuO–TiO₂ (CT) additions as aids. The sintering temperature of BaTi₄O₉ ceramics was found to evidently reduce from 1350 °C to about 1100 °C with a very small amount of 0.5 wt% CT addition. When the CT addition increased to beyond 0.5 wt%, however, it was not expected to further lower the sintering temperature. Meantime, the secondary phases of Ba₄Ti₁₃O₃₀, BaTiO₃, and TiO₂ were observed in these BaTi₄O₉-based ceramics when the CT content was beyond 2 wt%. With the introduction of the CT addition, the permittivity (ε) had little enhancement, and the temperature coefficient of the resonant frequency (τ_f) was improved to near zero. The BaTi₄O₉ ceramics with 0.5 wt% CT additions, sintered at 1100 °C, exhibited excellent microwave dielectric properties, such as ε = 36.9, Q × f = 23100 GHz, and τ_f = 2.5 ppm/°C. In addition, the densification mechanism and variations of the microwave dielectric properties have also been discussed with the crystal phase and microstructure’s evolution.

1. Introduction

In the past several decades, great attention has been paid to microwave dielectric ceramics due to their wide applications in mobile and satellite telecommunication systems. Generally, to meet the demands, suitable permittivity (ε), low dielectric loss, and near-zero temperature coefficients of resonate frequencies (τ_f) are required for these microwave dielectric ceramics [1,2,3,4,5,6] For application in base stations or satellite telecommunications, high Qf values, middle permittivity (25 < e < 50), and near zero t_f (−10 ppm/°C < t_f < 10 ppm/°C) are usually required in microwave dielectric ceramics [3]. A series of compounds with middle permittivity, such as BaTi₄O₉, Ba₂Ti₉O₂₀, (Zr,Sn)TiO₄, Ba(Zn_1/3Nb_2/3)O₃, CaTiO₃–NdAlO₃, and CaLa₄Ti₄O₁₅ have been developed for practical applications [3,4,5,6,7]. Recently, some novel dielectric ceramics with middle permittivity have been explored [8,9,10,11]. Among these dielectric ceramics with middle permittivity, TiO₂-rich compounds of BaTi₄O₉ exhibit excellent microwave dielectric properties such as a middle permittivity of 37–39, a high Qf of 21,000–37,000 GHz, and near-zero temperature coefficients of 15 ppm/°C [6,7].

Recently, low temperature co-fired technologies have been developed to meet the demand of the miniaturization of microwave devices. Thus, the sintering temperatures of microwave dielectric ceramics need to match with the internal electrodes of Ag and Ag-Pd with a low melting point, which are required low temperature co-fired ceramics (LTCC). BaTi₄O₉ exhibits excellent microwave dielectric properties such as a middle permittivity of 37–39, a high Qf of 21,000–37,000 GHz, and near-zero temperature coefficients of 15 ppm/°C. However, its sintering temperature is as high as 1350 °C, which is too high for application in the field of LTCC. In addition, lowering the sintering temperature is helpful for energy saving. Therefore, it is interesting to lower the sintering temperature of BaTi₄O₉ microwave dielectric ceramics. For application in the field of LTCC, considering energy savings, two methods are, usually, adopted to reduce the sintering temperatures of dielectric ceramics with high sintering temperatures. One is to prepare the ceramics derived from nano-powder. The other is the introduction of glass or oxides with low melting temperatures to liquid phase sintering. The latter method has often been adopted to lower the sintering temperature of microwave dielectric ceramics [12,13,14,15,16]. In addition, some advanced sintering methods such as cold sintering [17] and microwave sintering [18] have been adopted to prepare functional ceramics at low temperatures. Considering the economic, energy saving, and production efficiencies, liquid phase sintering is the most well-known approach for the low temperature sintering of microwave dielectric ceramics.

Some studies have reported on the low temperature sintering of BaTi₄O₉ ceramics. BaO–ZnO–B₂O₃, La₂O₃–B₂O₃–CaO, B–La–Mg–Ti–O, and B₂O₃–ZnO–La₂O₃ glass and BaCuB₂O₅ and BaB₂O₄ compounds have been adopted as sintering aids to reduce the sintering temperatures of BaTi₄O₉ ceramics [14,15,16,19,20,21]. As shown in

Table 1. Comparison of microwave dielectric properties of BaTi₄O₉ ceramics with different additions.

Addition	e	Qf (GHz)	τf (ppm/°C)	Ts (°C)	Ref.
27.5 wt% BaO-ZnO-B2O3 glass	26.4	27,300	3.3	925	[15]
0.34–4.47 vol% CuB2O4 and BaCuB2O5	36–40	13,000–21,000	20–40	925	[16]
70 wt% B-La-Mg-Ti-O glass	20.49	24,000	145	860	[19]
20 wt% B2O3-ZnO-La2O3 glass	27	~20,000	6.5	900	[20]
50 wt% La2O3-B2O3-CaO glass	26	8000	150	875	[21]
0.5 wt% CuO–TiO2	37.0	23,100	2.5	1100	This work

, sintering temperatures can be significantly lowered from 1350 °C to about 900 °C. For example, after the introduction of 27.5 wt% BaO–ZnO–B₂O₃ glass_, the sintering temperature of BaTi₄O₉–based ceramics is considerably decreased to 925 °C, and excellent microwave dielectric properties such as ε = 26.4, Q × f = 27,300 GHz, and τ_f = 3.3 ppm/°C are achieved [15]. Some examples of glass such as La₂O₃–B₂O₃–CaO and B–La–Mg–Ti–O are even added in at concentrations as high as 50–70 wt% [19,21]. As stated above, although these glasses or compounds can effectively reduce the sintering temperature of BaTi₄O₉ ceramics, high-content aids, for example, 20%, have often been added in order to reduce the sintering temperature. The consequent results are the deterioration of the microwave dielectric properties. One is the degradation of the Qf value, the other is that the temperature coefficient is not zero. Therefore, it is important and urgent to prepare high Qf values and near-zero temperature coefficient BaTi₄O₉ microwave dielectric ceramics with a small amount of sintering aid.

CuO is a common sintering aid for dielectric ceramics [22]. However, CuO, sometimes, does not reduce the sintering temperature enough to meet well the demands of the LTCC. Thus, CuO-based binary sintering aids are investigated. In previous studies, e.g., [23,24,25], once TiO₂ was introduced to CuO, the liquid phase temperature reduced to the eutectic temperature of the CuO–TiO₂ binary system. After the investigation of details via differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis, in conjunction with hot-stage microscopy, the eutectic temperature and composition of the CuO–TiO₂ system was certified as 1010 ± 10 °C and 83 CuO:17 TiO₂, respectively [23]. However, a computed eutectic temperature of 919 °C and a composition of 16.7 mol% TiO₂ at p_O2 = 1 atm have been reported by Lu et al. [24] for the CuO–TiO₂ system, which is shown in Figure 1. Although there is little variation in eutectic temperature and composition in these investigations, the eutectic temperature was low enough to act as a sintering aid. Thus, CuO–TiO₂ may be a prospect sintering aid for microwave dielectric ceramics. In the present work, the eutectic composition of CuO–TiO₂ has been chosen as a sintering aid to lower the sintering temperature of BaTi₄O₉ microwave dielectric ceramics. The sintering behaviors, microstructures, and microwave dielectric properties of BaTi₄O₉ ceramics have been investigated in detail. Additionally, the sintering mechanisms and variations of microwave dielectric properties have been also discussed with the crystal phase and microstructure’s evolution.

2. Materials and Methods

CuO–TiO₂ (CT) additions were prepared according to 83.3 mol% CuO-16.7 mol%TiO₂, using CuO (99.0%) and TiO₂ (99.0%) reagent powder. The weighted powder was ground for 12 h by ball milling in distilled water. Afterwards, the mixtures were dried to become a sintering additive. BaTi₄O₉ powder was synthesized by conventional solid-state reaction methods at a high temperature. According to the formula of BaTi₄O₉, raw materials of BaCO₃ (99.8%) and TiO₂ (99.0%) were weighed and ground in a polyethylene jar for 12 h by ball milling in distilled water. The ground powder was dried at 90 °C for 24 h and then calcined at 1200 °C for 2 h in air to obtain the single phase of the BaTi₄O₉. The calcined BaTi₄O₉ powder with a 0–10 wt% CT addition was reground for 8 h using distilled water. After drying, 5% PVA solution was added as a binder for granulation to the mixed powder. The granulated powder was pressed into pellets with a 13 mm diameter and a 2–6 mm thickness at a pressure of 100 MPa. To remove the PVA binder, the pellets were heated to 600 °C with a heating rate of 5 °C/min and a dwelling time of 2 h. After burning out the binder, the pellets were sintered in air by traditional liquid sintering at the temperature range of 1000–1125 °C with a dwelling time of 3 h to obtain the dense BaTi₄O₉-based ceramics. The heating rate was 5°C/min and cooled with the furnace after dwelling.

Thermogravimetry and differential thermal analysis (TG–DSC) were carried out by a synchronous thermal analyzer (STA 449C, NETZSCH, Germany), with a heating rate of 10 °C/min in an N₂ atmosphere from room temperature to 1150 °C. The bulk densities of the sintered samples were determined by the Archimedes method. The shrinkage (h) of the BaTi₄O₉-based ceramics was calculated from the diameter difference before and after sintering. The equation was h = (r₀ − r₁)/m₀ × 100%, where r₀ and r₁ were the diameters before and after sintering, respectively. The mass loss (d) of the pellets was evaluated by the mass difference before and after sintering, i.e., d = (m₀ − m₁)/m₀ × 100%, where m₀ and m₁ were the masses before and after sintering, respectively. The crystalline phases of the dense BaTi₄O₉-based ceramics were identified by powder X-ray diffraction (XRD, XD−5A, CuK_α, λ = 1.5406 × 10⁻¹⁰ m, Shimadzu, Japan). The polished and thermally etched surfaces of the sintered samples were observed by environmental scanning electron microscopy (ESEM, XL30 ESEM–TMP, Philips, Netherlands). Microwave dielectric properties of sintered samples were measured by the TE_01δ mode [26], using a vector network analyzer (R3767BH, Advantest, Japan). Additionally, the temperature coefficient of the resonant frequency τ_f was calculated from the temperature coefficient of the dielectric constant τ_ε, according to the equation of τ_f = −1/2τ_ε − α, where α was the linear expansion coefficient (~6–10 ppm/°C). τ_ε was measured in the temperature range of 25 to 85 °C using a precise LCR meter (Agilent 4284A, Agilent, Malaysia) at 1 MHz.

3. Results

Figure 2 shows the DSC and TG curves of BaTi₄O₉ powder with 0.5% and 5% CT additions as the function of temperature. As shown in Figure 1, an evident mass loss around 900 °C can be observed for BaTi₄O₉ powder with 0.5% CT, and a corresponding sharp endothermal peak appears in the DSC curve. This endothermal peak can be ascribed to the formation of the CuO–TiO₂ liquid phase, which is in agreement with the eutectic temperature of the CuO–TiO₂ binary reported by Lu et al. [24]. This is about 100 °C lower than the other reported values of eutectic temperatures in CuO–TiO₂ systems [23,25]. Although the CT addition is only 0.5%, the mass loss is as high as about 0.65%, which suggests that some BaTi₄O₉ dissolved into the eutectic liquid phase of the CuO–TiO₂ system. This is helpful for the densification of BaTi₄O₉ ceramics [27]. However, no evident mass loss is observed for the BaTi₄O₉ powder with a 5.0% CT addition when the temperature is beyond 800 °C. This is, evidently, different from that of the BaTi₄O₉ powder with a 0.5% CT addition. Furthermore, the corresponding endothermal peak becomes weak and wide in the DSC curve. The variation between the two DSC and TG curves for BaTi₄O₉ powder with 0.5% and 5% CT additions suggests that reaction occurs in the powders of the BaTi₄O₉ and CuO–TiO₂ addition, which results in the elevating of the liquid phase temperature. This results in no evident mass loss.

After sintering at different temperatures, the mass loss in Figure 3 is for the sintered BaTi₄O₉ ceramics with different contents of CT additions. The larger mass loss is observed for the BaTi₄O₉ ceramics with the higher CT addition. The mass loss is beyond 2.0% for the BaTi₄O₉ ceramics with a 10% CT addition. The mass loss is in the range of 0.55–0.85% for the BaTi₄O₉ ceramics with 0.5–5.0% CT addition. Although the dwelling time is 3 h and the sintering temperature is higher than eutectic temperature of CT, it should be noted that the value of mass loss is lower than the corresponding mass loss at a eutectic temperature of around 900 °C in the TG curve for BaTi₄O₉ powder with a 0.5 wt% CT addition. This is due to the N₂ atmosphere flow during the TG–DSC measurement. In addition, a low mass loss of 0.85% is observed for the BaTi₄O₉ ceramics with a 5.0% CT addition, which is also consistent with the result of the TG–DSC. For every BaTi₄O₉ ceramic with a different addition of CT content, four pellets are measured to evaluate the mass loss before and after sintering. The discrepancy of mass loss is below 0.025%.

The density (r) and shrinkage (h) of BaTi₄O₉ ceramics with various amounts of CT additions are shown in Figure 4. Without CT addition, pure BaTi₄O₉ ceramics are densely sintered at a high temperature of 1350 °C, which is consistent with previous reports [14,15]. After the introduction of a very small amount of CT (0.5%), the dense BaTi₄O₉-based ceramics with the density of about 4.4 g/cm³ can be achieved at 1100 °C, which is evidently lower than the sintering temperature of 1350 °C for pure BaTi₄O₉ ceramics. However, with the increase in the CT addition, despite the density sintering at 1050 °C showing evident enhancements, the sintering temperature for the dense BaTi₄O₉-based ceramics is still around 1100 °C, which has little variation with the CT content. As shown in Figure 4a, similar trends of shrinkage are observed for the BaTi₄O₉-based ceramics with a CT addition. Additionally, the BaTi₄O₉ ceramics with a CT addition exhibit higher shrinkages, which are beyond 15%.

Figure 5 demonstrates XRD patterns of dense BaTi₄O₉ ceramics with different contents of CT addition. According to the XRD patterns, all the diffraction peaks can be ascribed to the BaTi₄O₉ phase (JCPDS File No. 34-0070) for the BaTi₄O₉-based ceramics with a small amount of CT addition (below 2.0%). Otherwise, the secondary phases of BaTiO₃ (JCPDS File No. 05-0626), Ba₄Ti₁₃O₃₀ (JCPDS File No. 35-0750), and TiO₂ (JCPDS File No. 73-2224) are observed when the CT addition is above 2.0%. It has been reported that even a small deviation in BaTi₄O₉ can result in the formation of many stable TiO₂-rich compounds such as Ba₄Ti₁₃O₃₀ and Ba₂Ti₉O₂₀ in the BaO–TiO₂ system [28,29,30]. With increasing CT additions, the intensities of the Ba₄Ti₁₃O₃₀ phase diffraction peaks are evidently enhanced, which indicates that the content of the Ba₄Ti₁₃O₃₀ phase gradually increases. When the CT content further increases to 10%, the Ba₄Ti₁₃O₃₀ phase becomes the major phase. This phase evolution may be due to the introduction of the CT to BaTi₄O₉. Combining the results of the TG–DSC and mass loss suggests that a partial reaction occurs between CT and BaTi₄O₉ during the sintering. When the temperature is elevated to a eutectic temperature of the CuO–TiO₂ system (around 900 °C), the liquid phase of the CT appears during the sintering. The CT liquid phase has two effects. One promotes the densification of BaTi₄O₉-based ceramics. The other induces the decomposition and partial dissolution of BaTi₄O₉ in the liquid phase because the Ti content of Ba₄Ti₁₃O₃₀ is lower than that of BaTi₄O₉. According to the phase evolution, the reaction equation can be depicted as follows,

4 BaTi₄O₉ = Ba₄Ti₁₃O₃₀ + 3 TiO₂

BaTi₄O₉ = BaTiO₃ + 4 TiO₂

The decomposition of BaTi₄O₉ results in more TiO₂, which will increase the TiO₂ content of the CuO–TiO₂ system. Consequently, the composition of the CT addition is away from the eutectic composition, shifts to TiO₂, and ends in the CuO–TiO₂ system. This results in the temperature corresponding to the liquid phase of CuO–TiO₂ system rising and the noncongruent melting occurring, according to the theory of phase diagram. Therefore, a sharp endothermal peak and an evident mass loss are observed in the TG-DSC curves for BaTi₄O₉ powder with a 0.5% CT. However, no strong endothermal peak or evident mass loss are observed in the BaTi₄O₉ powder with a 5% CT addition. Thus, the sintering temperature of the BaTi₄O₉-based ceramics lowers to just around 1100 °C and does not further decrease when the CT addition further increases in the range of 0.5–10.0%. The sintering behaviors are closely related with the phase evolution.

Figure 6 gives SEM images of the dense BaTi₄O₉ ceramics with different amounts of CT additions. The present ceramics exhibit little porosity, which is consistent with the high densities in Figure 4a. After the introduction of the CT addition, the grains of BaTi₄O₉ ceramics are evidently smaller because the lower sintering temperature suppresses the grain growth. Additionally, some white regular grains in the BaTi₄O₉-based ceramics with a 1.0% or more CT addition clearly exist, and this content increases with the increase in the CT addition. Thus, this also certifies that partial reaction occurs between the CT and BaTi₄O₉ during the sintering. These grains correspond to the secondary phases of BaTiO₃, Ba₄Ti₁₃O₃₀, and TiO₂. Additionally, there are a few grains with abnormal grain growths for the BaTi₄O₉-based ceramics with a 0.5% and 1.0% CT, as shown in Figure 5b,c. In addition, as shown in Figure 5e,f, melting phenomena are clearly observed for the BaTi₄O₉-based ceramics with a 5.0% and 10.0% CT. This indicates that more liquid phases are formed in the BaTi₄O₉-based ceramics during sintering.

From the variations in sintering behaviors and structure evolutions, the sintering mechanisms of BaTi₄O₉-based ceramics with CT additions can be concluded as follows. As per the TG–DSC results and the investigation of the CuO–TiO₂ system, the BaTi₄O₉-based ceramics with CT additions exhibit liquid phase sintering. According to the sintering theory of liquid phase sintering, the extensive densification of ceramics is associated with a low liquid solubility in solid combinations and a high solid solubility in the liquid [27]. When the temperature is elevated to the eutectic point of CuO–TiO₂, the liquid phase occurs during the sintering. As stated above, the sintering temperature of BaTi₄O₉ can be reduced from 1350 °C to about 1100°C with a very small amount of CT addition, e.g., 0.5%. In conclusion, BaTi₄O₉ has a high solid solubility in the liquid of CuO–TiO₂. XRD patterns demonstrate that the secondary phases of BaTiO₃, Ba₄Ti₁₃O₃₀, and TiO₂ are formed during sintering, which means that the decomposition of BaTi₄O₉ results in the formation of BaTiO₃/Ba₄Ti₁₃O₃₀ and TiO₂. Once the decomposition occurs, the content of TiO₂ increases. This results in composition shifts to the end of the rich TiO₂ in the CuO–TiO₂ system. Consequently, the liquid temperature rises and noncongruent melting occurs. Therefore, as shown in the TG–DSC curves, no strong endothermal peaks and evident mass losses are observed in the BaTi₄O₉ powder with a 5% CT addition. Thus, sintering temperature has little change with different contents of CT addition. However, more melting phenomena are observed in the SEM images for BaTi₄O₉-based ceramics with higher contents of CT addition.

As a function of CT additions, the microwave dielectric properties of BaTi₄O₉ ceramics are shown in Figure 7. With the increase in CT addition, permittivity keeps around 37.0 in the CT range of 0–1% and then elevates to 41.0 for a 10% CT addition. The permittivity of dielectric ceramics, in general, depends on the density and permittivity of constitution phases. As stated above, all of the BaTi₄O₉-based ceramics with CT additions have high densities. For the BaTi₄O₉-based ceramics with a small amount of CT addition (below 2.0%), only the BaTi₄O₉ phase is observed in the XRD patterns. Otherwise, the secondary phases of BaTiO₃, Ba₄Ti₁₃O₃₀, and TiO₂ with high permittivity are formed. Therefore, the variation in permittivity with CT addition is consistent with the phase evolution of BaTi₄O₉-based ceramics. However, there is a rapid decrease in the Qf values of the BaTi₄O₉ ceramics with a small amount of CT addition. When the CT addition is 10.0%, the Qf value rapidly deteriorates to below 10,000 GHz. This is likely because of the large amount of the BaTiO₃ and Ba₄Ti₁₃O₃₀ secondary phase with a high dielectric loss [16,28,31]. Compared to permittivity, there is an opposite trend for the temperature coefficient of the resonant frequency τ_f. The value of τ_f is around zero in the CT range of 0–1% and rapidly decreases when the CT addition is beyond 1%. Table 1 lists the microwave dielectric properties of some BaTi₄O₉-based ceramics sintered at low temperatures. Although previous studies studied sintering temperatures of only about 900 °C, there are two aspects that should be noted. One is that these investigations adopted larger content additions, such as 20%, which are far above that of the present work. The other is that the BaTi₄O₉-based ceramics exhibit low permittivity or high τ_f values. The excellent microwave dielectric properties of ε = 36.9, Q × f = 23,100 GHz, and τ_f = 2.5 ppm/°C are achieved for the BaTi₄O₉ ceramics sintered at 1100 °C with only a 0.5% CT addition.

4. Conclusions

The effects of the addition of CuO–TiO₂ (CT) on sintering behaviors, phase evolutions, microstructures, and microwave dielectric properties have been investigated for BaTi₄O₉ ceramics. The sintering temperatures of BaTi₄O₉ ceramics were found to evidently reduce from 1350 °C to about 1100 °C with a very small amount of CT addition, e.g., 0.5%. However, the sintering temperatures of BaTi₄O₉-based ceramics did not further decrease with the increase in CT addition. Furthermore, the secondary phases of Ba₄Ti₁₃O₃₀, BaTiO₃, and TiO₂ are observed in these BaTi₄O₉-based ceramics with a 2.0–10.0% CT addition. With a small amount of CT introduction, not only have the sintering behaviors been improved, but, also, the excellent microwave dielectric properties of ε = 36.9, Q × f = 23,100 GHz, and τ_f = 2.5 ppm/°C were achieved for the BaTi₄O₉ ceramics sintered at 1100 °C with a 0.5% CT addition. The variations in microwave dielectric properties and sintering mechanisms have also been discussed with TG–DSC, crystal phases, and microstructure evolutions.