Effect of Sr 2+ doping on the microwave dielectric properties of BaZrO 3 ceramics

Abstract

In this study, undoped and Sr²⁺ doped BaZrO₃ ceramics were prepared by solid-state reaction using 3 wt-% B₂O₃ as sintering aid. The phase composition and microstructure of BaZrO₃ were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that introducing an appropriate amount of Sr²⁺ makes BaZrO₃ ceramics Q × f value and density increase. In addition, the dielectric constant of BaZrO₃ ceramics increases with the increase of Sr²⁺ addition. With excellent microwave dielectric properties, ϵ_r= 34.17 and Q × f value of 39,070 GHz(@ 7.8 GHz) τ_f = + 140 ppm/°C were obtained in 6 at.-% Sr²⁺ doped BaZrO₃ with 3 wt-% B₂O₃ sintering aid at 1275°C for 5 h. Using this material as the medium, the simulation of GPS antenna by HFSS software shows that this material has the potential to be used as the antenna medium material.

Keywords

Microwave dielectric ceramics antenna barium zirconate

Introduction

The development of modern communication technologies such as mobile communication, satellite communication and global positioning system (GPS) has put forward higher requirements for microwave components such as microwave resonators, filters and microwave substrates. Microwave dielectric ceramics have high dielectric constant and low microwave loss. Low-temperature coefficient has become a key material for manufacturing microwave components [1,2].

BaZrO₃ is a typical perovskite structure material [3], characterised by high thermal conductivity, high mechanical strength, low thermal expansion system, high bandgap width, medium electrical conductivity and corrosion resistance, which is widely used in fuel cells [4–7], luminescent materials [8–10] and refractory materials [11]. However, the extremely high sintering temperature (1600°C) [12] makes it difficult to prepare BaZrO₃ ceramics under conventional experimental conditions. This limits the application of BaZrO₃ ceramics to some extent. Therefore, scientists have tried to reduce the sintering temperature of BaZrO₃ ceramics. Among them, Jong Song Park et al. [13] prepared BaZrO₃ ceramics with a relative density greater than 95% at 1500°C by using CuO as a sintering aid. Peter Babilo et al. [14] reduced the sintering temperature of yttrium-doped BaZrO₃ ceramics to 1300°C by using ZnO as the sintering aid, and at the same time achieved the density of more than 93%. In addition, materials such as NiO, SnO, BaO, MgO and Al₂O₃ have also been used by scholars as sintering aids in the sintering of BaZrO₃ based ceramics, and some achievements have been made [15].

In the report on the dielectric properties of BaZrO₃ ceramics, S. Parida et al. obtained the dielectric properties of ϵ_r= 38.4,Q × f = 5731 GHz, τ_f= 324.35 ppm/°C when sintered at 1670°C by solid phase method [16]. After using Ga³⁺ and Nb⁵⁺ to replace Zr⁴⁺ in BaZrO₃ ceramics, Kolodiazhnyi et al. [17] successfully obtained excellent results of ϵ_r = 36.7, Q × f = 172THz, τ_f = 110 ppm/°C at 1600°C. This result encourages the research on ion substitution direction of BaZrO₃ ceramics. In our previous study, B₂O₃ was used as the sintering aid to reduce the sintering temperature of BaZrO₃ ceramics to 1300°C after adding 3 wt-% B₂O₃.

Patch antennas are widely used in global positioning systems, 5G communications, vehicle navigation, and other systems owing to their small size, ease of fabrication, and strong stability [18,19]. The GPS antenna is prone to phase shift during transmission owing to its long propagation distance, which makes signal decoding difficult. Circular polarisation antenna can enable signal reception from any phase to overcome the problem of phase offset in the signal, and can suppress multipath fading to reduce signal loss during transmission, making it the first choice for GPS antenna applications [20]. Therefore, in this paper, B₂O₃ was used as sintering agent to prepare Ba_1−xSr _x ZrO₃ ceramics, and the effects of Sr²⁺ doping on the microstructure and microwave dielectric properties of BaZrO₃ ceramics were studied. Excellent microwave dielectric properties were obtained by adjusting the content of Sr²⁺. The GPS antenna is designed. The 10 dB bandwidth of the antenna is 80 MHz, the maximum gain is 5.7 dB, and the axis ratio is less than 3 dB in the range of −68° to 68° to achieve circular polarisation.

Experimental procedure

Ba _x Sr_1−xZrO₃(x = 0,0.02,0.04,0.06,0.08,0.1) ceramics were prepared by solid-state reaction. Take ZrO₂ (Country medicine group chemical reagent limited company > 99%), BaCO₃(Country medicine group chemical reagent limited company > 99%) and SrCO₃ (Country medicine group chemical reagent limited company > 99%) as raw materials, weigh them according to massage proportion, then grind them with zirconia balls in deionised water for 4 h, and dry them. The dry powder was calcined in air at 1200°C for 3 h. Then 3 wt-% B₂O₃(Country medicine group chemical reagent limited company > 99%) was added to the pre sintered powder for secondary grinding for 6 h and drying. Add 10 wt-% PVA solution to the powder for granulation, and press the granulated powder into a cylindrical sheet with a diameter of 13 mm and a thickness of 5–8 mm under the pressure of 100 Mp. Finally, the pressed cylinder was held at 450°C for 2 h to remove the binder, and the ceramic samples were obtained at 1250–1350°C for 5 h.

The density was measured by Archimedes method, the micro morphology and sintering state of the sample were observed by using a desktop scanning electron microscope (Phenom ProX), the phase analysis and crystal structure analysis were performed by using an X-ray diffractometer (Bruker D8), and the quality factor and dielectric constant of the sample were tested by using the Hakki Coleman method with an Agilent E8363A vector network analyzer, Calculate the temperature coefficient of the resonant frequency using Formula (1), where f25 and f85 are the resonant frequencies of the sample at 25°C and 85°C, respectively.

τ_{f} = \frac{f_{85} - f_{25}}{60 \times f_{25}} .

(1)

The microstrip patch antenna is simulated, and the Maxwell equation is solved by ANSYS Electronics Desktop software. The initial size of antenna is calculated by formula (2–5). Where

f_{r}

is the central resonant frequency of the design antenna.

ε_{r}

is the dielectric constant of the material.

W_{p}

and

L_{p}

are the initial width and length of the antenna patch, respectively. After the initial size of the antenna is calculated, the model is established in the ANSYS Electronics Desktop software and the desired frequency is obtained by adjusting the length and width. The circularly polarised radiation is realised by cutting the angle of the antenna patch and adjusting the size of the cutting angle.

\begin{matrix} W_{p} = \frac{C}{2 f_{r}} \sqrt{\frac{2}{ε_{r} + 1}} \end{matrix},

(2)

\begin{matrix} L_{p} = \frac{c}{f_{r} \sqrt[]{ε_{e f f}}} - 2 Δ L \end{matrix},

(3)

\begin{matrix} ε_{e_{f f}} = \frac{ε_{r} + 1}{2} + \frac{ε_{r} - 1}{2 \sqrt{1 + \frac{12 h}{W_{p}}}} \end{matrix},

(4)

\begin{matrix} Δ L = 0.412 h \frac{(ε_{e_{f f}} + 0.3) (\frac{W_{p}}{h} + 0.264)}{(ε_{e_{f f}} – 0.258) (\frac{W_{p}}{h} + 0.8)} \end{matrix}

(5)

Results and discussions

Generally speaking, the substitution of Sr²⁺ for Ba²⁺ in the same main group is easy to occur ion substitution to form solid solution [21–24]. In Figure 1 the XRD spectrum of Ba_1−xSr _x ZrO₃ ceramics sintered at 1300°C for 3 h. It can be seen from the figure that all components present BaZrO₃ (Pm-3 m, JCPDS No-74-1299) phase with obvious perovskite structure. In addition, a small number of unknown phases were observed in the XRD spectra of all components, so it is known that these unknown phases were not caused by the introduction of Sr²⁺. It is worth noting that in our previous studies, the introduction of B₂O₃ will produce a similar phase in BaZrO₃ ceramics, which is probably BaZr(BO₃)₂ phase by XRD standard card comparison, and this phase will have some effect on the microwave dielectric properties of BaZrO₃ ceramics. The specific mechanism of the effect is worth further study. The diffraction peak of the second phase does not change significantly with the increase of the amount of Sr²⁺ substitution, so the influence of Sr²⁺ and B₂O₃ on BaZrO₃ is relatively independent. As the amount of Sr²⁺ increases, the diffraction peak shifts slightly from the high angle because the radius of Sr²⁺ ions (1.18 Å) is slightly smaller than that of Ba²⁺ ions (1.35 Å), so Sr²⁺ enters the Ba²⁺ position to form a solid solution and results in a decrease in the lattice constant.

Figure 1

XRD patterns of the BaZrO₃ ceramics with different Sr²⁺ contents sintered at 1300°C for 3 h.

Figure 2(a) shows the curve of BaZrO₃ ceramic density changing with x value. It can be seen from the figure that the density value is basically above 5.7 g/cm³. When the x value is less than 0.06, the density value increases with the increase of x. This is because Sr²⁺ enters into the crystal structure of BaZrO₃ to form solid solution [15], which promotes the sintering of ceramics. At 1325°C, the density reaches the maximum value of 5.825 g/cm³ when the x value is 0.06, After the x value increases to 0.08, the density value begins to decrease by 5.75 g/cm³, which is related to the abnormal growth of grains after the x value increases. Although there is still a certain gap with the relative density of BaZrO₃ ceramics (6.23 g/cm³), the relative density has stabilised at about 95.5%, which indicates that it has been densified and sintered at this time.

Figure 2

Variation curve of density of Ba_1−xSr _x ZrO₃ ceramics sintered at different temperatures with x value (a) bulk density (b) relative density.

In order to better observe the effect of Sr²⁺ on the sintering state of BaZrO₃ ceramics, the natural cross sections of each group of samples sintered at 1300°C were observed by SEM. As shown in Figure 3. It can be seen from the figure that the ceramic grain presents a blocky structure. When x = 0, there are a few pores in the ceramic. With the introduction of Sr²⁺, the pores of the ceramic disappear, and the ceramic sintering is dense. In the report between [25], the introduction of Sr²⁺ will inhibit the growth of BaZrO₃ grains to some extent and agglomeration will occur with the increase of Sr²⁺ content. Although two kinds of grains of the same size appeared with the increase of x value in this study, there was no agglomeration phenomenon, which may be owing to the small amount of Sr²⁺ added in this study and the introduction of sintering additives to promote the rearrangement of grains and weaken the agglomeration phenomenon to a certain extent. When sintered at 1600°C, the average grain size of BaZrO₃ ceramics without doped elements reported in the literature is 1.8–3.2 μm [17], it can be seen from the SEM diagram that most of the grain size in this study is within this range, which indicates that the effect of densification sintering has been achieved at this time.

Figure 3

SEM image the BaZrO₃ ceramics with different Sr²⁺ doping concentrations sintered at 1300°C for 3 h. (a) x = 0; (b) x = 0.02; (c) x = 0.04; (d) x = 0.06; (e) x = 0.08; (f) x = 0.1.

Similar to the change of density value, the dielectric constant of Ba_1−xSr _x ZrO₃ ceramics also increases first and then decreases with the increase of sintering temperature, as shown in Figure 4.

Figure 4

Variation curve of Ba_1−xSr _x ZrO₃ dielectric constant with temperature.

Generally, the main factors affecting the dielectric properties of materials were divided into intrinsic characteristics and extrinsic characteristics. The intrinsic characteristics mainly include ionic polarizability and crystal structure, and the extrinsic characteristics are porosity and second phase.

It can be seen from the figure that the dielectric constant of the ceramics increases first and then decreases with the increase of temperature. This is similar to the change of density, indicating that the dielectric constant is greatly affected by extrinsic characteristics. Although the ionic polarizability of Sr²⁺ (4.25 × 10⁻²⁴ cm³)is lower than that of Ba^2 +(6.40 × 10⁻²⁴ cm³) [26], and the dielectric constant of SrZrO₃ is also low, it can be seen from the samples sintered at the same temperature that the overall dielectric constant of Sr²⁺ is higher than that of the samples added, and the dielectric constant tends to decrease with the increase of the amount of Sr²⁺ added, because one side of the ceramics is still BaZrO₃ ceramics, only a small amount of Sr²⁺ added will not lead to the formation of SrZrO₃ phase, On the other hand, the addition of Sr²⁺ can promote the growth of grains, which will also increase the dielectric constant. Finally, the maximum value of 39.432 is obtained at 1325°C, x = 0.08.

Figure 5(a) shows the curve of Q × f value versus x value of Ba_1−xSr _x ZrO₃ ceramics at different temperatures. It can be seen from the figure that Q × f value increases first and then decreases with the increase of x. On the one hand, the introduction of Sr²⁺ will increase the density of BaZrO₃, reduce the number of pores, reduce the dielectric loss, and increase the Q × f value. Because the quality factor of SrZrO₃ ceramics is low [27] (Q × f = 13,700 GHz), the continued increase of Sr²⁺ will lead to the decrease of the increased Q × f value. On the other hand, the lattice parameters will decrease with the increase of Sr²⁺ concentration [23], which may lead to the increase of dielectric loss.

Figure 5

Curve of Q × f value and temperature coefficient of Ba_1−xSr _x ZrO₃ ceramics changing with x value.

When two materials with different resonant frequency temperature coefficients are compounded, the overall resonant frequency temperature coefficient of the material can be calculated according to Lichtrncker's rule of thumb [28]. Where v₁ and v₂ are the volume fractions of BaZrO₃ and SrZrO₃ respectively, and τ_f1and τ_f2 are the respective temperature coefficients.

τ_{f} = v_{1} τ_{f_{1}} + v_{2} τ_{f_{2}} .

(6)

In Figure 5(b) 1275°C sintered Ba _x Sr_1−xZrO₃ ceramics, the temperature coefficient does not decrease monotonously as expected in the curve of temperature coefficient changing with x value. Although the temperature coefficient of SrZrO₃ ceramics is negative [22], because the XRD spectra of BaZrO₃ and SrZrO₃ are too close to each other, it is difficult to easily judge whether there is SrZrO₃ phase with negative temperature coefficient. Therefore, more research is needed in the future to explain the mechanism of temperature coefficient change. The final material achieves the best dielectric properties at the sintering temperature of 1275°C, x = 0.06: ϵ_r= 34.17, Q × f = 39070 GHz, τ_f = + 140 ppm/°C. The quality factor is much higher than 5731 GHz of BaZrO₃ ceramics mentioned in the literature [16]. The sintering temperature of 1275°C is also lower than 1500–1600°C reported in the literature [12–14,16,17]. Although compared with microwave dielectric ceramics, the temperature coefficient of resonant frequency is still relatively high, but the relatively high quality factor and medium dielectric constant make it possible to use it as a material with positive temperature coefficient of resonant frequency in composite ceramics to adjust the temperature coefficient of the material system with a certain practical prospect.

A GPS patch antenna was designed using self-developed Ba_1−xSr _x ZrO₃ (x = 0.06) ceramics. As shown in Figure 6, the patch antenna is composed of three layers, including the upper radiation layer, the middle layer is a ceramic dielectric layer, and the bottom layer is a grounding layer. We choose the coaxial line as the feed mode of the antenna. In order to achieve circular polarisation of the antenna, it is necessary to adjust the feed position and cut a corner from the radiation layer. After the antenna size is calculated by Matlab software, the HFSS software is used to fine tune the antenna to achieve the best performance. The specific dimensions of the final antenna are shown in Figure 6.

Figure 6

3D Schematic diagram and dimension diagram of GPS antenna.

For the antenna, the matching degree is the premise of the normal operation of the antenna, and the gain is the index used to evaluate the antenna radiation ability. Both are the key parameters of the antenna. Axis ratio can be used to evaluate the polarisation ability of antenna. For this reason, the S11 parameter and the antenna gain are simulated in this paper. In the S parameter diagram in Figure 7(b), it can be seen that the antenna | S11 | is greater than 10 dB in the frequency range of 1.57–1.58 GHz, indicating that good matching can only be generated in this frequency band at this time. It can be seen from the gain curve in Figure 7 that the gain is greater than 3 dB from −48° to 48°, meeting the design requirements. The axial ratio can be used to display the polarisation degree of the antenna. When the axial ratio is less than 3 dB, it means that the length of the major axis and the minor axis in the radiation direction is close to each other, which can be approximately regarded as circularly polarised radiation. In the axial ratio diagram in Figure 7(b), it can be seen that the axial ratio is less than 3 dB from −68° to 68°, which means that the antenna can realise circularly polarised radiation at this time. From the simulation results, it can be seen that Ba_1−xSr _x ZrO₃ is completely suitable for the design of GPS ceramic antenna and is expected to become one of the materials of ceramic antenna.

Figure 7

GPS antenna simulation diagram. (a) antenna gain diagram, (b) antenna axis ratio diagram, (c) antenna S11 curve diagram, (d) antenna model diagram, (e) antenna radiation pattern.

Conclusions

In this paper, BaZrO₃ of 3 wt-% B2O3 is used as the research object to study the effect of Sr²⁺ on the sintering and microwave dielectric properties of BaZrO₃ ceramics. It is found that solid solution is formed in BaZrO₃ ceramics after Sr²⁺ is added, BaZrO₃ ceramics cell shrinks, and Sr²⁺ has a certain role in promoting the sintering of BaZrO³ ceramics to make the dielectric constant and Q × The value of f increases. Excellent dielectric property: ϵ_r= 34.17, Q × f = 39,070 GHz, τ_f = + 140 ppm/°C. Using Ba_0.94Sr_0.06ZrO₃ ceramics as the medium, the simulation of GPS circularly polarised antenna was carried out. The matching is good at 1.58 GHz|S11 |>10 dB, the maximum gain exceeds 5 dB, and the axial ratio between −68° and 68° is less than 3 dB to achieve circular polarisation. This material is expected to be one of the choices of ceramic antenna materials.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

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