Microwave dielectric properties of low-fired CoNb₂O₆ ceramics with B₂O₃ addition

Abstract

Low-fired B₂O₃-doped CoNb₂O₆ microwave dielectric ceramics were synthesized via conventional solid-state method. The effects of B₂O₃ additives on their sintering behavior, phase composition, and microwave dielectric properties were investigated systematically. The addition of B₂O₃ as liquid phase successfully lowered the sintering temperature of CoNb₂O₆ ceramics from 1200 to 1000 °C. The system remained orthorhombic columbite phase at the level of 0.5–1.5 wt% B₂O₃ addition. However, a trace amount of Co₄Nb₂O₉ second phase with major CoNb₂O₆ phase was observed for the 2 wt% B₂O₃-added sample. The microwave dielectric properties were found to strongly correlate with the sintering temperature as well as the amount of B₂O₃ addition. With 1.5 wt% B₂O₃, CoNb₂O₆ ceramics sintered at 1000 °C possessed optimum microwave dielectric properties with an ε _r of 22.4, a high Q × f of 43,979 GHz, and a τ _f of −46.2 ppm/°C.

1 Introduction

With the rapid development of wireless communication industry, low-temperature cofired ceramic (LTCC) technology has been extensively investigated for miniaturization and integration of components [1, 2]. In LTCC devices fabrication, the ceramics are co-fired with highly conductive electrode materials, such as silver (melting point 961 °C) and copper (melting point 1083 °C), to form three-dimensional modules [3]. Therefore, the ceramics are required to have low sintering temperature which can be co-sinterable with the metal electrodes. In addition, high relative permittivity (ε _r  > 20, to allow miniaturization of the component), low dielectric losses (Q > 5000, where Q = 1/tanδ, to improve selectivity at microwave frequencies), and a near zero temperature coefficient of resonant frequency (τ _f  ~ 0 ppm/°C, for temperature stability) are also the key characteristics for practical applications [4, 5]. The optimal balance of these properties is one of the major challenges faced by electronic industry.

The binary ceramic with general formula MNb₂O₆ (M²⁺ = Mg, Ca, Mn, Co, Ni, Zn) was a potential candidate for mechanical filter coatings and electrical applications [6]. Among the niobates investigated, of particular interest was CoNb₂O₆. As to its dielectric properties, the data reported in the literatures were contradictory. According to Lee et al. [7], CoNb₂O₆ had a rather low electrical Q (Q × f = 11,300 GHz), whereas Pullar [8] achieved notably higher Q value: Q × f = 41,700 GHz. Recently, Belous [9] reported its Q × f as high as 82,000 GHz. However, the CoNb₂O₆ ceramics possessed high sintering temperature (>1150 °C) in nearly all cases. As a result, liquid phase flux such as CeO₂, WO₃, V₂O₅ and CuO were introduced to reduce the sintering temperature [10]. Unfortunately, CeO₂ and WO₃ had negative effect on the densification of the niobate. Additions of both V₂O₅ and CuO resulted in Q × f under half that of the pure columbite. So it is of great significance if we can decrease the sintering temperature with desired properties. Considering its low melting point, B₂O₃ was usually selected as an effective sintering aid to develop low-fired dielectric ceramics. For instance, B₂O₃ addition to Zn₂SiO₄ ceramics significantly reduced the sintering temperature to 900 °C without deterioration of the dielectric properties [11]. Besides, Lv and Zuo [12] found the microwave dielectric characteristics of 3 wt% B₂O₃-doped 0.5Ba₃(VO₄)₂−0.5Zn_1.87SiO_3.87 ceramics sintered at 925 °C to be ε _r  = 10, Q × f = 40,800 GHz, and τ _f  = 0.5 ppm/°C. Thus, B₂O₃ was employed to lower the sintering temperature of CoNb₂O₆ ceramics in the present work. Furthermore, the influence of B₂O₃ additions on their sintering behavior, phase composition and microwave dielectric properties was investigated systematically.

2 Experimental procedures

CoNb₂O₆ ceramics were synthesized by conventional solid state reaction route. High-purity oxide powders (>99.9 %) of CoO, Nb₂O₅ and B₂O₃ were adopted as raw chemicals. Stoichiometric amounts of CoO and Nb₂O₅ were mixed with ethanol in polyethylene bottles. Subsequently, the slurries were dried, screened by a 200-mesh sieve, and calcined at 1000 °C for 4 h to obtain CoNb₂O₆. Different weight percentages of B₂O₃ were then added to the calcined powders. After remilling and sieving, the resultant powders with organic binder (5 wt% polyvinyl alcohol) were uniaxially compacted into cylinders of 10 mm in diameter and 6 mm in thickness at a pressure of 150 MPa. Finally, the green bodies were sintered in the temperature range of 900–1300 °C for 4 h. During sintering, both the heating rate and cooling rate were maintained at 3 °C/min.

The relative densities of sintered samples were identified by the common Archimedes method. Crystal structure was undertaken by X-ray diffraction (XRD, Rigaku, DMAX-RB, Japan) with Cu Kα radiation. The microstructure was characterized by scanning electron microscopy (SEM; JSM-6480LV) coupled with EDS spectroscopy. The microwave dielectric properties were evaluated by a network analyzer (HP8720ES, Hewlett-Packard, Santa, Rosa, CA). The dielectric constant was measured according to the Hakki–Coleman post-resonator method [13] by exciting the TE₀₁₁ resonant mode of the DR using the electric probe of an antenna as suggested by Courtney [14]. The unloaded quality factors were measured using the TE_01δ mode in the cavity method [15]. All measurements were made in the frequency range of 4–9 GHz at room temperature. Temperature coefficients of the resonant frequencies of the TE₀₁₁ mode were obtained in the temperature range from 25 to 80 °C. The τ _f values were defined by the following relationship:

$$ \tau_{f} { = }\frac{{f_{2} - \, f_{1} }}{{ \, f_{1} (T_{2} - T_{1} )}} $$

(1)

where f ₁ and f ₂ were the resonant frequency at T ₁ and T ₂, respectively.

3 Results and discussions

The relative densities of B₂O₃-doped CoNb₂O₆ ceramics sintered at various sintering temperatures are offered in Fig. 1. Pure CoNb₂O₆ ceramic was well sintered to approach 95.29 % theoretical density only when the sintering temperature was raised to 1200 °C. When a small amount of B₂O₃ was added, the relative density reached its saturated value at around 1100 °C. But in the case of >1.0 wt% added B₂O₃, the sintering behavior was promoted remarkably. A maximum relative density of about 96.72 % was obtained for the specimen with 1.5 wt% B₂O₃ sintered at 1000 °C. The possible reason for the enhanced sinterability might come from the presence of liquid phase during sintering, since the melting point of B₂O₃ was just 450 °C [16, 17]. In the presence of a liquid phase, particle rearrangement became easier and mass transport by grain-boundary diffusion took place much faster, leading to enhanced densification kinetics. However, further addition of the flux resulted in an apparent decrease in densification.

Fig. 1

Relative densities of CoNb₂O₆ ceramics with different B₂O₃ additions as a function of sintering temperature

Figure 2 presents the XRD patterns of CoNb₂O₆ ceramics with different B₂O₃ additions. It revealed that single CoNb₂O₆ phase (JCPDS NO. 32-0304) was obtained for the B₂O₃-free sample. Similar XRD patterns were detected at the level of 0.5–1.5 wt% B₂O₃ addition. However, when the B₂O₃ content was increased up to 2 wt%, a trace amount of Co₄Nb₂O₉ (JCPDS NO. 38-1457) and Nb₂O₅ (JCPDS NO.26-0885) secondary phases were formed along with the major CoNb₂O₆ phase. The preparation process and sintering condition were all the same for the 1–2 wt% B₂O₃-added samples. Based on this, it might be concluded that high concentration of B₂O₃ favored the phase transformation from CoNb₂O₆ to Co₄Nb₂O₉. To be noted that this transformation was partial, because the amount of second phases was rather small. Columbite CoNb₂O₆ had been reported to crystallize in an orthorhombic α-PbO₂-type structure [18]. Whereas for Co₄Nb₂O₉, it performed hexagonal corundum structure with space group P-3c1 [19]. But in fact both of them were the combination of CoO6 and NbO6 octahedra. A higher B₂O₃ doping level might influence the Nb:Co ratio in the unit cell and, hence the crystallographic distortion. So Co₄Nb₂O₉ was formed through the reaction shown below. Systematic analysis is still needed to verify this explanation.

$$ 4 {\text{CoNb}}_{ 2} {\text{O}}_{ 6} \to {\text{Co}}_{ 4} {\text{Nb}}_{ 2} {\text{O}}_{ 9} + 3{\text{Nb}}_{ 2} {\text{O}}_{ 5} $$

(2)

Fig. 2

XRD patterns of CoNb₂O₆ ceramics added with various amounts of B₂O₃ additions a pure CoNb₂O₆ ceramic, at 1200 °C, b 0.5 wt%, at 1100 °C, c 1 wt%, at 1000 °C, d 1.5 wt%, at 1000 °C, e 2 wt%, at 1000 °C

Figure 3 displays the SEM images of B₂O₃ doped CoNb₂O₆ ceramics sintered at their respective optimum sintering temperatures. The microstructure of un-doped sample was also given here for comparison. After sintering at 1200 °C, pure CoNb₂O₆ ceramic exhibited dense microstructure with closely packed polygonal-shaped grains having 1–8 μm size (Fig. 3a). For the ceramic with 0.5 wt% B₂O₃ addition, some pores were still observed (see Fig. 3b). This might be a result of insufficient liquid phase. The number of pores appeared to decrease with the increase of B₂O₃ content. At a higher B₂O₃ content of 1.5 wt% in Fig. 3d, a fully-densified specimen was obtained with average grain size below 4 μm. On the whole, the average grain size of the B₂O₃-doped samples was found to be a little smaller than that of pure columbite. Thus, B₂O₃ doping, acting as a liquid phase, was believed to facilitate the densification of ceramics but inhibite the grain growth owing to a higher surface energy [20]. Nevertheless, further increasing B₂O₃ content induced considerable de-densification and exaggerated grain growth. From Fig. 3, it was easily to find that all specimens were composed of two kinds of polygon-like grains with different sizes, including the larger one (denoted as A), and the smaller one (denoted as B). Additionally, some elongated grains (denoted as C) existed in grain boundaries after the addition of 2 wt% B₂O₃, as seen in Fig. 3e. The composition of the different grains was qualitatively identified by EDS. The EDS analysis in Fig. 3f depicted that the polygonal-shaped grains were identified as CoNb₂O₆, and the elongated one was Co₄Nb₂O₉. It was also worth noting that no glass phase was found in Fig. 3, most likely due to the evaporation of B₂O₃ in the final stage.

Fig. 3

Scanning electron micrographs of CoNb₂O₆ ceramics with B₂O₃ additions: a pure CoNb₂O₆ ceramic, at 1200 °C, b 0.5 wt%, at 1100 °C, c 1 wt%, at 1000 °C, d 1.5 wt%, at 1000 °C, e 2 wt%, at 1000 °C, and f EDS analysis

The plot of ε _r is given in Fig. 4 as a function of sintering temperatures with various amounts of B₂O₃. As observed in Fig. 4, the ε _r values steadily increased first with increasing sintering temperature and then decreased after reaching the maximum values, exhibiting a similar trend to that of the relative densities. It was not difficult to accept since some of early researches had proved the influence of relative density on the dielectric properties. Higher density for samples meant there were more dipoles per unit volume, which indicated that the ceramics were more liable to be polarized. Hence, the improved densification could be responsible for the increase of ε _r in 0.5–1.5 wt% B₂O₃-doped samples. Nevertheless, the dielectric constants had a trend of decreasing at high B₂O₃-doping levels (2 wt%). This was ascribed to the comparatively lower permittivity of the Co₄Nb₂O₉ (~16.0) compared to that of CoNb₂O₆ (~22.6) [19]. It was still possible that a slight amount of residual liquid phase could degrade the dielectric property.

Fig. 4

Dielectric constants of B₂O₃-added CoNb₂O₆ ceramics as a function of sintering temperature

Figure 5 illustrates the Q × f values of B₂O₃-added CoNb₂O₆ ceramics sintered at different temperatures for 4 h. Generally speaking, microwave dielectric loss included two parts: intrinsic loss and extrinsic loss. The intrinsic loss was mainly caused by lattice variation modes, while the extrinsic loss was dominated by densification, secondary phase, grains sizes, and lattice defects [21]. By increasing the sintering temperature, the Q × f of all compositions increased to the maximum values and declined thereafter. It was also found that the Q × f values of B₂O₃-doped samples were degraded even though the sintering behavior was greatly enhanced. This was because B₂O₃ doping restrained the grain growth, as seen in Fig. 3. The grain boundary was generally the site where dopants/impurities concentrated. As the average grain size became smaller, the total number of grain boundaries per unit volume increased, thereby resulting in higher dielectric loss [22]. With 0.5 wt% B₂O₃ additive, the Q × f of the ceramic sintered at 1100 °C was 27,505 GHz. This low value was due to the relatively inferior microstructure of the CoNb₂O₆ ceramic. The Q × f increased with increasing B₂O₃ concentration and got its maximum value of about 43,979 GHz at 1.5 wt% B₂O₃ addition, possibly depending on the removal of pores. However, further addition of B₂O₃ degraded the Q × f values drastically. This reduction was associated mostly with the presence of second phase Co₄Nb₂O₉ (Q × f~ 5000 GHz) as well as the lattice defects produced in crystal growth [19]. In particular, a higher Q × f value (=82,598 GHz) of pure CoNb₂O₆ ceramic than that reported in Refs. [7–9] was obtained because of the additional screening process.

Fig. 5

Dependence of quality factors on sintering temperature for CoNb₂O₆ ceramics with various B₂O₃ additions

The τ _f values of CoNb₂O₆ ceramics with different B₂O₃ additions are depicted in Fig. 6. It is well known that τ _f largely depends on the composition, the amount of additives, and the second phases present in ceramics [23]. As listed in Fig. 6, the τ _f of un-doped CoNb₂O₆ ceramic exhibited a negative τ _f (~−42.1 ppm/°C). After the incorporation of B₂O₃, it shifted monotonically from −43.6 to −47.6 ppm/°C. This might result from the Co₄Nb₂O₉ s phase having a τ _f of −10 ppm/°C [19]. Moreover, increasing B₂O₃ amount probably led to an increase in the NbO₆ octahedral distortion, which would indirectly decline the τ _f to some extent. Specifically, the CoNb₂O₆ ceramic with 1.5 wt% B₂O₃ sintered at 1000 °C had the optimum dielectric values ε _r  = 22.4, Q × f = 43,979 GHz, and τ _f  = −46.2 ppm/°C, which outperformed the results achieved in [10].

Fig. 6

Temperature coefficients of resonant frequency of CoNb₂O₆ ceramics as a function of B₂O₃ addition

The sintering behavior and microwave dielectric properties of B₂O₃-added CoNb₂O₆ ceramics sintered at their optimum temperatures are summarized in Table 1. There was no doubt that the sintering behavior in the present system was significantly enhanced by B₂O₃ doping. For 2 wt% B₂O₃-added CoNb₂O₆ ceramic, the sintering temperature was lowered to 1000 °C, accompanied by a substantially high Q × f of 43,979 GHz. The mentioned merits made it a very good potential for practical LTCC integration applications.

Table 1 Sintering behavior and microwave dielectric properties of CoNb₂O₆ ceramics with B₂O₃ addition

4 Conclusion

The effects of B₂O₃ addition on both the sintering behavior and microwave dielectric properties of CoNb₂O₆ ceramics had been investigated. The incorporation of B₂O₃ additives considerably decreased the sintering temperature to 1000 °C due to the liquid phase effect. A minor amount of hexagonal Co₄Nb₂O₉ was formed along with the major columbite phase at high B₂O₃ doping levels (2 wt%). Dielectric investigations shown that the addition of B₂O₃ to CoNb₂O₆ ceramics induced a limited degradation in ε _r and Q × f. Typically, 1.5 wt% B₂O₃-doped CoNb₂O₆ ceramic exhibited a well-sintered microstructure with microwave dielectric properties of ε _r  = 22.4, Q × f = 43,979 GHz, and τ _f  = −46.2 ppm/°C at T _s = 1000 °C, making it a promising candidate for LTCC applications.