Sintering Behaviors, Microstructure, and Microwave Dielectric Properties of CaTiO3–LaAlO3 Ceramics Using CuO/B2O3 Additions

The effect of CuO/B2O3 additions on the sintering behaviors, microstructures, and microwave dielectric properties of 0.95LaAlO3–0.05CaTiO3 ceramics is investigated. It is found that the sintering temperatures are lowered efficiently from 1600 °C to 1350 °C, as 1 wt % CuO, 1 wt % B2O3, and 0.5 wt % CuO +0.5 wt % B2O3 are used as the sintering aids due to the appearance of the liquid phase sintering. The microwave dielectric properties of 0.95LaAlO3–0.05CaTiO3 ceramics with the sintering aid additions are strongly related to the densification and the microstructure of the sintered ceramics. At the sintering temperature of 1300 °C, the 0.95LaAlO3–0.05CaTiO3 ceramic with 0.5 wt % CuO + 0.5 wt % B2O3 addition shows the best dielectric properties, including a dielectric constant (εr) of 21, approximate quality factor (Q × f) of 22,500 GHz, and a temperature coefficient of the resonant frequency (τf) of −3 ppm/°C.


Introduction
Microwave passive communication components can be seen in current communication products, such as in the base station of 5G communication systems or portable mobile phones. Since the size of the microwave passive component determines the dimension of the wireless communication apparatus, the size reduction of the microwave passive components is always an inevitable trend [1]. The microwave materials with high dielectric constants (ε r ), high qualify factor (Q × f) values, and small temperature coefficients of the resonant frequency (τ f ) have been extensively developed in the past 20 years to meet the miniaturization requirement of the circuits [2][3][4]. To further reduce the circuit size, bandpass filters and antenna duplexers are suitable to implement on the multilayered ceramic. In order to achieve energy-saving goals and fabricate the multilayer microwave devices, microwave dielectric materials with low sintering temperatures and low-melting-point conductors are needed to co-fire [5]. However, many commercial dielectric materials used for high-frequency device applications have high sintering temperatures; thus, they are not well matched with low-temperature sintering processes [2,3]. Microwave sintering or cold-sintering technologies are also reported to further reduce the sintering temperature of the dielectric materials [6,7]; however, the industrial production is still needed to develop.

Experimental Procedure
The starting materials were La 2 O 3 , Al 2 O 3 , CaCO 3 , and TiO 2 high-purity oxide powders, which were mixed and ground in distilled water for 12 h in a balling mill with agent balls in relation to the required composition. Mixed powders were dried and then calcined at 1200 • C for 3 h, and then re-milled with different sintering aids for 3 h. In this study, three doping conditions are used to investigate the ability of lowering the sintering temperature, including 1 wt % CuO, 1 wt % B 2 O 3 , and 0.5 wt % CuO + 0.5 wt % B 2 O 3 . After drying, the calcined powder was added with the binder, polyvinyl alcohol (PVA). Pellets with 15 mm diameter that were 5 mm thick were prepared by uniaxial pressing. These pellets were heated at a temperature of 300 • C for 1 h at first to remove the binder. Then, these pellets were sintered at temperatures of 1250-1400 • C for 3 h. Pure 0.95LaAlO 3 -0.05CaTiO 3 ceramics disk without addition were also prepared through the similar procedure and sintered at temperatures of 1500-1600 • C for 3 h for the comparison.
The bulk densities of the sintered pellets were measured by the Archimedes method. The theoretical densities (D) for the sintered 0.95LaAlO 3 -0.05CaTiO 3 ceramics can be obtained as where W 1 and W 2 are the weight percentages of 0.95LaAlO 3 -0.05CaTiO 3 ceramics and sintering aids in combination, respectively. D 1 and D 2 are the densities of 0.95LaAlO 3 -0.05CaTiO 3 ceramic and sintering aid in the mixture, respectively. In generally, Equation (1) is only effective when no interaction happened between the dielectric material and the sintering aid. The crystalline phases of the sintered ceramics were identified by X-ray diffraction patterns (XRD, RIGAKU-2000 X-Ray Diffractometer, Tokyo, Japan). The microstructure of the sintered surface of ceramics was observed by scanning electron microscopy (SEM, JEOL-JSM-6700F, Tokyo, Japan) and energy dispersive spectra (EDS, JEOL-JSM-6700F, Tokyo, Japan).
The dielectric constant ε r and the quality factor Q at microwave frequencies were measured using the Hakki-Coleman dielectric resonator method improved by Courtney [23,24]. The quality factor is further expressed as Q × f, since it is typically constant in the microwave region. A cylindrical dielectric resonator was positioned between two brass plates connected to the measuring system. A HP8757D network analyzer and a HP8350B sweep oscillator were used in the measurement. The temperature coefficient of resonant frequency (τ f ) at the microwave frequency was measured in the temperature range from 20 • C to 80 • C, and estimated by Equation (2), where f 20 and f 80 are the TE 01δ resonant frequency at 20 • C and 80 • C, respectively.

Results and Discussion
Pure 0.95LaAlO 3 -0.05CaTiO 3 ceramics without addition were also prepared and used as the standard reference. The sintered samples without sintering aids have relative theoretical densities of 90% and 95% at the sintering temperature of 1500 • C and 1550 • C, respectively. LaAlO 3 ceramic has approximate dielectric property values of ε r 20, Q × f 65,000 GHz, and τ f −50 ppm/ • C, and CaTiO 3 ceramic has approximate dielectric properties values of ε r 104, Q × f 3600 GHz, and τ f 800 ppm/ • C. The dielectric properties of the 0.95LaAlO 3 -0.05CaTiO 3 ceramic is suggested to follow the well-known mixing rules [3]: where v 1 and v 2 represent the volume fraction of LaAlO 3 and CaTiO 3 , respectively. With the ratio of 0.95LaAlO 3 -0.05CaTiO 3 , it exhibits the approximately dielectric property values of ε r 20-22, Q × f 35,000 GHz, and τ f 1 ppm/ • C. Figure 1 shows the X-ray patterns of 0.95LaAlO 3 -0.05CaTiO 3 ceramics sintered at different temperatures with the addition of (a) 1 wt % CuO, (b) 1 wt % B 2 O 3 , and (c) 0.5 wt % CuO + 0.5 wt % B 2 O 3 . It is observed from Figure 1 that LaAlO 3 is the main crystallize phase of the 0.95LaAlO 3 -0.05CaTiO 3 ceramics. For addition of 1 wt % CuO, as shown in Figure 1a, at the sintering temperature of 1250 • C, LaAlO 3 and the small amount of CaTiO 3 are solid-dissolved together, but the secondary phase CaAl 12 O 19 is still produced. As the sintering temperature rises, the secondary phase gradually disappears. When the sintering temperature is at 1350 • C, the secondary phase completely disappears, and the ceramic is formed as a solid solution.

Microstructure
For additions of 1 wt % B 2 O 3 and 0.5 wt % CuO + 0.5 wt % B 2 O 3 , the crystallize phases of the 0.95LaAlO 3 -0.05CaTiO 3 ceramics with different sintering temperatures show similar results. At the sintering temperature of 1250 • C, many secondary phases such as La 5 Ti 5 O 17 , La 4 Ti 9 O 24 , La 2 Ti 2 O 7 , and CaAl 4 O 7 exist; even LaAlO 3 and CaTiO 3 are solid-dissolved together. However, the secondary phase disappears rapidly as the sintering temperature rises. The secondary phase almost completely disappears, and the ceramics have formed a solid solution completely at the sintering temperature of 1350 • C. Figure 2 shows micrographs of 0.95LaAlO 3 -0.05CaTiO 3 ceramics sintered at different temperature for 3 h with 1 wt % CuO addition. The melting point of the added CuO is as low as about 1026 • C, which is easy to cause a liquid phase sintering for the 0.95LaAlO 3 -0.05CaTiO 3 ceramics. Moreover, the liquid phase effect would be enhanced by the eutectic of CuO-Cu 2 O-TiO 2 (Cu 3 TiO 4 ) at 1070 • C [25].
However, due to an inhomogeneous liquid phase distribution, the growth of crystal grains during high temperature sintering is not homogeneous. At sintering temperatures below 1350 • C, many pores exist. As the sintering temperature rises to 1400 • C, the crystal grain is densely arranged on the ceramic surface. However, the internal structure can be found to have a gap, and it is judged that the dense effect is not good. It is reported that the sintering aids would still exist or evaporate after the liquid phase sintering; thus, EDS analysis is used to identify the elements on the surface of the sintered sample [11]. As shown in Figure 2e, Cu atoms are detected as existing on the grains in the surface after the liquid phase sintering. Figure 3 shows micrographs of 0.95LaAlO 3 -0.05CaTiO 3 ceramics sintered at different temperatures for 3 h with 1 wt % B 2 O 3 additions. When the sintering temperature is 1250 • C, the crystal grains dissolve into small particles. As the sintering temperature increases to 1350 • C, the crystal grains grow and gradually become uniform, which leads to more densification. Liquid phase sintering help the growth of crystal grains, and the surface microstructure will be easily covered by the liquid phase [10]. It is resulted that B 2 O 3 belongs to a low melting point material (about 450 • C) and is a well-known liquid promoter. Moreover, the grain-wetting ability of B 2 O 3 addition is better than that of CuO addition in 0.95LaAlO 3 -0.05CaTiO 3 ceramics. Moreover, as shown in Figure 3e, the B atom is also detected as existing in the surface of the grain after the liquid phase sintering. Figure 4 shows micrographs of 0.95LaAlO 3 -0.05CaTiO 3 ceramics sintered at different temperatures for 3 h with 0.5 wt % CuO + 0.5 wt % B 2 O 3 addition. The CuO and B 2 O 3 added in the experiment are all low-melting materials [13,14]. During the sintering temperatures above 1250 • C, liquid phase sintering is clearly observed, since the crystal grains grow non-homogeneously. When the sintering temperature reaches 1300 • C, the pores have all disappeared. The liquid phase sintering is very obvious, and the complete densification is achieved at the sintering temperature of 1300 • C. It is shown that as the sintering temperature exceeds 1350 • C, an excessive liquid phase is generated, and thus the growth of crystal grains is not homogeneous, and the secondary recrystallization is formed, which would affect the performance of the dielectric characteristics. Secondary recrystallization typically occurs when continuous grain growth is inhibited by the present of impurities or the liquid phase, as shown in Figure 4d. It is also identified that Cu and B atoms are both detected as existing in the surface of the grain after liquid phase sintering, as shown in Figure 4e. τf = (f80 − f20)/(60 × f20) × 10 6 (ppm/°C) (2) where f20 and f80 are the TE01δ resonant frequency at 20 °C and 80 °C, respectively.

Results and Discussion
Pure 0.95LaAlO3-0.05CaTiO3 ceramics without addition were also prepared and used as the standard reference. The sintered samples without sintering aids have relative theoretical densities of 90% and 95% at the sintering temperature of 1500 °C and 1550 °C, respectively. LaAlO3 ceramic has approximate dielectric property values of εr 20, Q × f 65,000 GHz, and τf −50 ppm/°C, and CaTiO3 ceramic has approximate dielectric properties values of εr 104, Q × f 3600 GHz, and τf 800 ppm/°C. The dielectric properties of the 0.95LaAlO3-0.05CaTiO3 ceramic is suggested to follow the wellknown mixing rules [3]: where v1 and v2 represent the volume fraction of LaAlO3 and CaTiO3, respectively. With the ratio of 0.95LaAlO3-0.05CaTiO3, it exhibits the approximately dielectric property values of εr 20-22, Q × f 35,000 GHz, and τf 1 ppm/°C. Figure 1 shows the X-ray patterns of 0.95LaAlO3-0.05CaTiO3 ceramics sintered at different temperatures with the addition of (a) 1 wt % CuO, (b) 1 wt % B2O3, and (c) 0.5 wt % CuO + 0.5 wt % B2O3. It is observed from Figure 1 that LaAlO3 is the main crystallize phase of the 0.95LaAlO3-0.05CaTiO3 ceramics. For addition of 1 wt % CuO, as shown in Figure 1a, at the sintering temperature of 1250 °C, LaAlO3 and the small amount of CaTiO3 are solid-dissolved together, but the secondary phase CaAl12O19 is still produced. As the sintering temperature rises, the secondary phase gradually disappears. When the sintering temperature is at 1350 °C, the secondary phase completely disappears, and the ceramic is formed as a solid solution. For additions of 1 wt % B2O3 and 0.5 wt % CuO + 0.5 wt % B2O3, the crystallize phases of the 0.95LaAlO3-0.05CaTiO3 ceramics with different sintering temperatures show similar results. At the sintering temperature of 1250 °C, many secondary phases such as La5Ti5O17, La4Ti9O24, La2Ti2O7, and CaAl4O7 exist; even LaAlO3 and CaTiO3 are solid-dissolved together. However, the secondary phase disappears rapidly as the sintering temperature rises. The secondary phase almost completely disappears, and the ceramics have formed a solid solution completely at the sintering temperature of 1350 °C. Figure 2 shows micrographs of 0.95LaAlO3-0.05CaTiO3 ceramics sintered at different temperature for 3 h with 1 wt % CuO addition. The melting point of the added CuO is as low as about 1026 °C, which is easy to cause a liquid phase sintering for the 0.95LaAlO3-0.05CaTiO3 ceramics. Moreover, the liquid phase effect would be enhanced by the eutectic of CuO-Cu2O-TiO2 (Cu3TiO4) at 1070 °C [25]. However, due to an inhomogeneous liquid phase distribution, the growth of crystal grains during high temperature sintering is not homogeneous. At sintering temperatures below 1350 °C, many pores exist. As the sintering temperature rises to 1400 °C, the crystal grain is densely  Figure 5 shows the grain size of 0.95LaAlO 3 -0.05CaTiO 3 ceramics as the functions of sintering temperature with different sintering aids. Typically, with the increase of sintering temperature, the surface grain size tends to increase, indicating that high temperature sintering can promote grain growth. It is indicated that the grain size of the ceramics with the addition of CuO is about 0.6-1.2 µm, which is larger than others. However, many pores exist in the sintered ceramics. With B 2 O 3 addition, although the grain is close to dense, the grain growth is still slow due to a small grain size of about 0.6-0.8 µm. With 0.5 wt % CuO + 0.5 wt % B 2 O 3 addition, the surface is also dense, but the grain growth is increased slowly due to many small grain sizes. In the excessive liquid phase at a sintering temperature of 1400 • C, the surface grain size could not be estimated. is judged that the dense effect is not good. It is reported that the sintering aids would still exist or evaporate after the liquid phase sintering; thus, EDS analysis is used to identify the elements on the surface of the sintered sample [11]. As shown in Figure 2e, Cu atoms are detected as existing on the grains in the surface after the liquid phase sintering.  Figure 3 shows micrographs of 0.95LaAlO3-0.05CaTiO3 ceramics sintered at different temperatures for 3 h with 1 wt % B2O3 additions. When the sintering temperature is 1250 °C, the crystal grains dissolve into small particles. As the sintering temperature increases to 1350 °C, the crystal grains grow and gradually become uniform, which leads to more densification. Liquid phase sintering help the growth of crystal grains, and the surface microstructure will be easily covered by the liquid phase [10]. It is resulted that B2O3 belongs to a low melting point material (about 450 °C) and is a well-known liquid promoter. Moreover, the grain-wetting ability of B2O3 addition is better than that of CuO addition in 0.95LaAlO3-0.05CaTiO3 ceramics. Moreover, as shown in Figure 3e, the B atom is also detected as existing in the surface of the grain after the liquid phase sintering.  Figure 6 shows a shrinkage ratio of 0.95LaAlO 3 -0.05CaTiO 3 ceramics sintered at different temperatures with different additions. The shrinkage ratio is defined as the difference of the sample diameter before and after sintering over the sample diameter before sintering. It is found that the shrinkage ratios of 0.95LaAlO 3 -0.05CaTiO 3 ceramics are around 4-6%, 12-17% and 13-18% for the additions of 1 wt % CuO, 1 wt % B 2 O 3 , and 0.5 wt % CuO + 0.5 wt % B 2 O 3 , respectively. Although the grain sizes of the sintered sample added with 1 wt % CuO are larger than those added with 1 wt % B 2 O 3 and 0.5 wt % CuO + 0.5 wt % B 2 O 3 , the shrinkage ratios are the worst due to non-homogeneous grain growth and the existence of many pores. At 1350 • C, the shrinkage ratio of the sintered sample reaches the maximum relative value, showing that B 2 O 3 addition helps the 0.95LaAlO 3 -0.05CaTiO 3 ceramics to shrink and be compact.  Figure 4 shows micrographs of 0.95LaAlO3-0.05CaTiO3 ceramics sintered at different temperatures for 3 h with 0.5 wt % CuO + 0.5 wt % B2O3 addition. The CuO and B2O3 added in the experiment are all low-melting materials [13,14]. During the sintering temperatures above 1250 °C, liquid phase sintering is clearly observed, since the crystal grains grow non-homogeneously. When the sintering temperature reaches 1300 °C, the pores have all disappeared. The liquid phase sintering is very obvious, and the complete densification is achieved at the sintering temperature of 1300 °C. It is shown that as the sintering temperature exceeds 1350 °C, an excessive liquid phase is generated, and thus the growth of crystal grains is not homogeneous, and the secondary recrystallization is formed, which would affect the performance of the dielectric characteristics. Secondary recrystallization typically occurs when continuous grain growth is inhibited by the present of impurities or the liquid phase, as shown in Figure 4d. It is also identified that Cu and B atoms are both detected as existing in the surface of the grain after liquid phase sintering, as shown in Figure  4e. For density measurements, the 0.95LaAlO 3 -0.05CaTiO 3 ceramics without sintering aid addition were also observed as the standard. The 0.95LaAlO 3 -0.05CaTiO 3 ceramic added with 0.5 wt % CuO + 0.5 wt % B 2 O 3 can obtain the measured density of 6.01 g/cm 3 , which is a relative theoretical density of 90%, at the sintering temperature of 1250 • C, and the measured density of 6.05 g/cm 3 , which is a relative theoretical density of 94%, at the sintering temperature above 1300 • C. The 0.95LaAlO 3 -0.05CaTiO 3 ceramics added with 1 wt % B 2 O 3 obtain relative theoretical densities of around 80-90% at the sintering temperature of 1300-1400 • C. However, the 0.95LaAlO 3 -0.05CaTiO 3 ceramic added with 1 wt % CuO have relative theoretical densities less than 90% at a sintering temperature of 1400 • C, which is the same finding as that from the shrinkage. It shows that the addition of B 2 O 3 combined with CuO helps the compaction of 0.95LaAlO 3 -0.05CaTiO 3 ceramics and reduces the sintering temperature by nearly 250 • C, as compared to those without sintering aids.  Figure 5 shows the grain size of 0.95LaAlO3-0.05CaTiO3 ceramics as the functions of sintering temperature with different sintering aids. Typically, with the increase of sintering temperature, the surface grain size tends to increase, indicating that high temperature sintering can promote grain growth. It is indicated that the grain size of the ceramics with the addition of CuO is about 0.6-1.2 μm, which is larger than others. However, many pores exist in the sintered ceramics. With B2O3 addition, although the grain is close to dense, the grain growth is still slow due to a small grain size of about 0.6-0.8 μm. With 0.5 wt % CuO + 0.5 wt % B2O3 addition, the surface is also dense, but the grain growth is increased slowly due to many small grain sizes. In the excessive liquid phase at a sintering temperature of 1400 °C, the surface grain size could not be estimated.  Figure 6 shows a shrinkage ratio of 0.95LaAlO3-0.05CaTiO3 ceramics sintered at different temperatures with different additions. The shrinkage ratio is defined as the difference of the sample diameter before and after sintering over the sample diameter before sintering. It is found that the shrinkage ratios of 0.95LaAlO3-0.05CaTiO3 ceramics are around 4%-6%, 12%-17% and 13%-18% for the additions of 1 wt % CuO, 1 wt % B2O3, and 0.5 wt % CuO + 0.5 wt % B2O3, respectively. Although the additions of 1 wt % CuO, 1 wt % B2O3, and 0.5 wt % CuO + 0.5 wt % B2O3, respectively. Although the grain sizes of the sintered sample added with 1 wt % CuO are larger than those added with 1 wt % B2O3 and 0.5 wt % CuO + 0.5 wt % B2O3, the shrinkage ratios are the worst due to non-homogeneous grain growth and the existence of many pores. At 1350 °C, the shrinkage ratio of the sintered sample reaches the maximum relative value, showing that B2O3 addition helps the 0.95LaAlO3-0.05CaTiO3 ceramics to shrink and be compact. For density measurements, the 0.95LaAlO3-0.05CaTiO3 ceramics without sintering aid addition were also observed as the standard. The 0.95LaAlO3-0.05CaTiO3 ceramic added with 0.5 wt % CuO + 0.5 wt % B2O3 can obtain the measured density of 6.01 g/cm 3 , which is a relative theoretical density of 90%, at the sintering temperature of 1250 °C, and the measured density of 6.05 g/cm 3 , which is a relative theoretical density of 94%, at the sintering temperature above 1300 °C. The 0.95LaAlO3-0.05CaTiO3 ceramics added with 1 wt % B2O3 obtain relative theoretical densities of around 80%-90% at the sintering temperature of 1300-1400 °C. However, the 0.95LaAlO3-0.05CaTiO3 ceramic added  Figure 7 indicates dielectric constants (ε r ) of 0.95LaAlO 3 -0.05CaTiO 3 ceramics as the functions of sintering temperature with different additions. In the sintering range, 0.95LaAlO 3 -0.05CaTiO 3 ceramics added with 0.5 wt % CuO + 0.5 wt % B 2 O 3 have higher dielectric constants of around 19-20.5, which resulted from the full condensation. 0.95LaAlO 3 -0.05CaTiO 3 ceramics added with 1 wt % B 2 O 3 have dielectric constants of around 15.8-18.2 due to the lower densification and the excessive secondary phase formation at lower sintering temperatures. The 0.95LaAlO 3 -0.05CaTiO 3 ceramics added with 1 wt % CuO have low dielectric constants due to the low density and the excess pores with air phase (ε r = 1) [2].

Dielectric Properties
Materials 2020, 13, x FOR PEER REVIEW 9 of 12 with 1 wt % CuO have relative theoretical densities less than 90% at a sintering temperature of 1400 °C, which is the same finding as that from the shrinkage. It shows that the addition of B2O3 combined with CuO helps the compaction of 0.95LaAlO3-0.05CaTiO3 ceramics and reduces the sintering temperature by nearly 250 °C, as compared to those without sintering aids.   Figure 8 indicates the quality factor Q × f values of 0.95LaAlO3-0.05CaTiO3 ceramics as the functions of the sintering temperature with different additions. It is known that the microwave dielectric loss is simultaneously affected by many factors, which are mainly caused not only by densification, pores, and grain sizes/boundaries, and but also by the lattice secondary phases and vibration modes [8]. It seems that the relative theoretical density and pores are important factors in controlling the dielectric loss. As shown in Figure 8, the 0.95LaAlO3-0.05CaTiO3 ceramic added with 0.5 wt % CuO + 0.5 wt % B2O3 has a higher Q × f value of around 22,500 GHz at the sintering temperature of 1300 °C, which is near the Q × f value of the 0.95LaAlO3-0.05CaTiO3 ceramic without sintered aid sintered at 1550 °C. After the sintering temperature exceeds 1350 °C, the Q × f value decreases, which is presumed to be caused by the excessive liquid phase of the ceramic. The 0.95LaAlO3-0.05CaTiO3 ceramics added with 1 wt % B2O3 have Q × f values of around 1000-12,000 GHz, which is affected by the low densification [3]. The 0.95LaAlO3-0.05CaTiO3 ceramics added with 1 wt % CuO have low densities and contain too much pores, resulting in the worst performance of Q  Figure 8 indicates the quality factor Q × f values of 0.95LaAlO 3 -0.05CaTiO 3 ceramics as the functions of the sintering temperature with different additions. It is known that the microwave dielectric loss is simultaneously affected by many factors, which are mainly caused not only by densification, pores, and grain sizes/boundaries, and but also by the lattice secondary phases and vibration modes [8]. It seems that the relative theoretical density and pores are important factors in controlling the dielectric loss. As shown in Figure 8, the 0.95LaAlO 3 -0.05CaTiO 3 ceramic added with 0.5 wt % CuO + 0.5 wt % B 2 O 3 has a higher Q × f value of around 22,500 GHz at the sintering temperature of 1300 • C, which is near the Q × f value of the 0.95LaAlO 3 -0.05CaTiO 3 ceramic without sintered aid sintered at 1550 • C. After the sintering temperature exceeds 1350 • C, the Q × f value decreases, which is presumed to be caused by the excessive liquid phase of the ceramic. The 0.95LaAlO 3 -0.05CaTiO 3 ceramics added with 1 wt % B 2 O 3 have Q × f values of around 1000-12,000 GHz, which is affected by the low densification [3]. The 0.95LaAlO 3 -0.05CaTiO 3 ceramics added with 1 wt % CuO have low densities and contain too much pores, resulting in the worst performance of Q × f values.  Figure 9 indicates τf values of 0.95LaAlO3-0.05CaTiO3 ceramics as the functions of sintering temperature with different additions. The τf values of 0.95LaAlO3-0.05CaTiO3 ceramics added with 1 wt % CuO and 1 wt % B2O3 have a negative value and a positive value, respectively. It is found that the 0.95LaAlO3-0.05CaTiO3 ceramics added with 0.5 wt % CuO + 0.5 wt % B2O3 have an acceptable temperature coefficient of resonant frequency (τf) approaching zero value. Generally, the temperature coefficient of resonant frequency (τf) is dependent on the composition and the existing phase in the ceramics [10]. In this study, the sintered ceramic with CuO addition has a negative temperature coefficient, while that with B2O3 addition has a positive temperature coefficient. Since the ratio of CaTiO3 to LaAlO3 is also appropriate, a system with a temperature coefficient approaching zero can be obtained.   It is found that the 0.95LaAlO 3 -0.05CaTiO 3 ceramics added with 0.5 wt % CuO + 0.5 wt % B 2 O 3 have an acceptable temperature coefficient of resonant frequency (τ f ) approaching zero value. Generally, the temperature coefficient of resonant frequency (τ f ) is dependent on the composition and the existing phase in the ceramics [10]. In this study, the sintered ceramic with CuO addition has a negative temperature coefficient, while that with B 2 O 3 addition has a positive temperature coefficient. Since the ratio of CaTiO 3 to LaAlO 3 is also appropriate, a system with a temperature coefficient approaching zero can be obtained.  Figure 9 indicates τf values of 0.95LaAlO3-0.05CaTiO3 ceramics as the functions of sintering temperature with different additions. The τf values of 0.95LaAlO3-0.05CaTiO3 ceramics added with 1 wt % CuO and 1 wt % B2O3 have a negative value and a positive value, respectively. It is found that the 0.95LaAlO3-0.05CaTiO3 ceramics added with 0.5 wt % CuO + 0.5 wt % B2O3 have an acceptable temperature coefficient of resonant frequency (τf) approaching zero value. Generally, the temperature coefficient of resonant frequency (τf) is dependent on the composition and the existing phase in the ceramics [10]. In this study, the sintered ceramic with CuO addition has a negative temperature coefficient, while that with B2O3 addition has a positive temperature coefficient. Since the ratio of CaTiO3 to LaAlO3 is also appropriate, a system with a temperature coefficient approaching zero can be obtained.

Conclusions
LaAlO 3 is a quite potential microwave dielectric material with a dielectric constant (ε r ) of 23, a quality factor (Q × f) value of 65,000 GHz, and a negative temperature coefficient of temperature (τ f ) of −44 ppm/ • C. For mixing CaTiO 3 ceramic as compensation for the temperature coefficient of resonant frequency, 0.95LaAlO 3 -0.05CaTiO 3 ceramic is used in this study. In order to reduce the sintering temperature, CuO and B 2 O 3 sintering aids for liquid phase sintering are added to the 0.95LaAlO 3 -0.05CaTiO 3 ceramic to help grain growth, to increase the density, as well as to obtain acceptable quality factors of the 0.95LaAlO 3 -0.05CaTiO 3 ceramics.
The effects of the liquid phase sintering aids added to the 0.95LaAlO 3 -0.05CaTiO 3 ceramic are summarized as follows.