SYNTHESIS OF 3BaO-2MgO-2NbSYNTHESIS OF 3BaO-2MgO-2Nb22OO55 MICROWAVE DIELECTRIC MICROWAVE DIELECTRIC

CERAMICS BY A REACTION-SINTERING PROCESSCERAMICS BY A REACTION-SINTERING PROCESSYi-Cheng Liou*, Wei-Ting LiYi-Cheng Liou*, Wei-Ting Li  

Department of Electronics Engineering, Kun Shan University, Tainan Hsien 71003, Taiwan, R.O.C.Department of Electronics Engineering, Kun Shan University, Tainan Hsien 71003, Taiwan, R.O.C.

*Corresponding author. ycliou@mail.ksu.edu.tw*Corresponding author. ycliou@mail.ksu.edu.tw

Synthesis of 3BaO-2MgO-2NbSynthesis of 3BaO-2MgO-2Nb22OO55 (BMN) microwave dielectric ceramics by a reaction-sintering process was investigated in this study. (BMN) microwave dielectric ceramics by a reaction-sintering process was investigated in this study.

Without any calcination involved, the mixture of raw materials was pressed and sintered directly. Ba(MgWithout any calcination involved, the mixture of raw materials was pressed and sintered directly. Ba(Mg 1/31/3NbNb2/32/3)O)O33 phase formed as a major phase formed as a major

phase, and the minor phases Baphase, and the minor phases Ba33NbNb55OO1515 and Ba and Ba33NbNb88OO2121 also formed at 1250 also formed at 1250C/2 h sintering. BaC/2 h sintering. Ba33NbNb55OO1515 phase decreased while Ba phase decreased while Ba33NbNb88OO2121

phase increased as sintered at higher temperatures. Increased MgO content in (5-x)BaO-xMgO-2Nbphase increased as sintered at higher temperatures. Increased MgO content in (5-x)BaO-xMgO-2Nb22OO55 could lower the sintering could lower the sintering

temperature from 1450temperature from 1450ooC (x=0) to 1250C (x=0) to 1250ooC (x=2). Some sub-micron (< 0.5 μm) MgO particles were segregated at the surfaces of grains. C (x=2). Some sub-micron (< 0.5 μm) MgO particles were segregated at the surfaces of grains. The The reaction-sintering process has proven a simple and effective method in preparing reaction-sintering process has proven a simple and effective method in preparing 3BaO-2MgO-2Nb3BaO-2MgO-2Nb22OO55 microwave dielectric ceramics. microwave dielectric ceramics.

Fig. 1 shows XRD patterns of BMN ceramics produced using the reaction-sintering process. Ba(Mg1/3Nb2/3)O3 phase formed as a major phase, and the minor phases Ba3Nb5O15 and Ba3Nb8O21 also formed at 1250C/2 h sintering. Ba3Nb5O15 phase decreased while Ba3Nb8O21 phase increased as sintered at higher temperatures. These are different from Ba5Nb4O15 prepared using the reaction-sintering process. The mixture of BaCO3 and Nb2O5 was efficiently transformed into the Ba5Nb4O15 phase even with the calcination stage bypassed. Therefore, (Ba3Mg2)Nb4O15 ceramics with A5B4O15 structure could not be prepared using the reaction-sintering process. On the contrary, Ba3(MgNb2)O9 (i.e. Ba(Mg1/3Nb2/3)O3) with ABO3 structure formed much easier in the pellets. Since the ionic radius of Mg2+ (0.078nm) is much smaller than that of Ba2+ (0.143nm), it would substitute for Nb5+ in stead of Ba2+. (Ba4.5Mg0.5)Nb4O15 phase formed as a major phase in (5-x)BaO-xMgO-2Nb2O5 with x=0.5. While with x=1 the major phase Ba(Mg1/3Nb2/3)O3 formed in our previous study. This implies only less than 10% Ba of the A-site was replaced by Mg in the A5B4O15 structure. The shrinkage results for BMN ceramics are shown in Fig. 2. Values are ranged in 21-24%. 1250oC is high enough for densification. In Fig. 3, the density of BMN ceramics increased with the sintering temperature and saturated above 1270oC. In our study of BaNb4O15, the density increased with the sintering temperature and reached a maximum value 6.13g/cm3 (97.3% of the theoretical value) at 1450C for 4 h and 6 h. Density lower than 5g/cm3 were found in (5-x)BaO-xMgO-2Nb2O5 with x=0.5 even sintered at 1300-1330C. With x=1, density exceeded 5g/cm3 at 1330C sintering. As CuO (1 wt%) was added, the sintering temperature dropped more than 150C and density saturated at 1200C. Increased MgO content in (5-x)BaO-xMgO-2Nb2O5 could lower the sintering temperature from 1450oC (x=0) to 1250oC (x=2 in this study). SEM photographs of as-fired BMN ceramics sintered at 1250-1330oC for 2 h are shown in Fig. 4. Porous pellets were formed at 1250oC sintering, which was in good agreement with the density values in Fig. 3. It can be easily observed that pores decreased as sintering temperature increased. In (5-x)BaO-xMgO-2Nb2O5 with x=0.5, thin and flat (Ba4.5Mg0.5)Nb4O15 grains < 2 μm thick and > 10 μm wide clearly visible. Round Ba(Mg1/3Nb2/3)O3 grains became the major morphology in pellets with x=1. For x=2 in this study, grains of different morphology were observed. Larger grains in Fig. 4 are Ba(Mg1/3Nb2/3)O3 and this matched our finding that Ba(Mg1/3Nb2/3)O3 was the major phase, as shown in the XRD of Fig. 1. Smaller grains (< 2 μm) in pellets sintered at 1250oC are Ba3Nb5O15 and Ba3Nb8O21. The amount of Ba3Nb8O21 increased at higher sintering temperatures. It is also noted that some sub-micron (< 0.5 μm) particles were segregated at the surfaces of grains. These are thought to be MgO particles. The reason caused the MgO segregation was: only one phase (Ba(Mg1/3Nb2/3)O3 or Ba3(MgNb2)O9) formed with the participation of MgO, no participation of MgO in the formation of Ba3Nb5O15 and Ba3Nb8O21. Therefore, the unreacted MgO segregated. More MgO particles segregated at 1300oC and 1330oC due to the high Nb contained phase Ba3Nb8O21 increased which lead to more unreacted MgO remained in the pellets. In (5-x)BaO-xMgO-2Nb2O5 with x=0.5 and 1, there was no MgO segregation in the microstructure morphology.

Fig. 1 XRD patterns of BMN ceramics sintered at (A) 1250oC, (B) 1270oC, (C) 1300oC, and (D) 1330oC for 2 h. (1) Ba(Mg0.333Nb0.667)O3: ICDD PDF # 01-070-9143, (2) Ba3Nb8O21: JCPD # 33-0167, and (3) Ba3Nb5O15: JCPD # 52-0269.

Fig. 4 SEM photographs of as-fired BMN ceramics sintered at (A) 1250oC, (B) 1270oC, (C) 1300oC, and (D) 1330oC for 2 h.

Fig. 3 Density of BMN ceramics sintered at various temperatures and soak time.

Fig. 2 Shrinkage percentage of BMN ceramics sintered at various temperatures and soak time.

Materials and Austceram 2007Materials and Austceram 2007July 4 - 6, 2007, Sydney , AustraliaJuly 4 - 6, 2007, Sydney , Australia