Study On The New Bismuth-based Microwave Dielectric Ceramics With Low Sintering Temperatures

With the rapid development of mobile communication and satellite communication, microwave electronic devices are required to be developed and fabricated for miniaturization and integration. The low temperature co-fired ceramic technology (LTCC) becomes an important fabricating technology that can integrate the passive components within a monolithic bulk module with IC chips mounted on its surface. By this technology, microwave dielectrics are stacked in multilayers and co-fired with internal electrodes, such as Ag, Cu, Au, Al, their alloys etc, in special patterns to fulfill different electrical functions. The microwave dielectric materials used in LTCC field must have a high dielectric permittivity (εr>10), a high Qf value (f=resonant frequency, Q=1/dielectric loss at f, Qf>5000GHz), a near zero temperature coefficient of resonant frequency (TCF≈0ppm/oC), a low sintering temperature (below the melting points of common electrode metals, such as silver, copper, gold, aluminum etc. ) and chemical compatibility with the metal electrodes. Besides that, considering the environment and economic elements, people prefer to explore microwave dielectric ceramics with low price (not contain rare metal oxides) and without toxicity (at least lead free).Bismuth-based dielectric ceramics are well-known as low-fired materials and have been investigated in multilayer capacitors application for many years. According to Shanno'ns study, Bi³⁺ has a polarizability of 6.12(?)³, which could account for the high permittivities in many bismuth based oxides. In this thesis, a series of new bismuth-based oxides prepared by solid state reaction method and high energy ball milling method were explored. Ionic substitution and oxides addition methods were used to lower the sintering temperature, adjust the microwave dielectric properties and their chemical compatibility with metal electrodes. The application of antenna substrate using high permittivity dielectric and the fabrication of multilayers co-fired capacitor using bismuth based oxides were also tried. A series of scientific and engineering results were obtained as follows:1. The influence of V⁵⁺, Cu²⁺, W⁶⁺ and Ta⁵⁺ substitution on the sintering temperature, microwave dielectric properties and phase composition of BiNbO₄ and Bi₃NbO₇ compositions in the Bi₂O₃-Nb₂O₅ binary system prepared by solid state reaction method and high energy ball milling method were studied. A series of BiNbO₄ ceramics sintered at 850⁹60oC withεr≈36⁴5, Qf≈5000²0000GHz and TCF≈0ppm/oC and Bi₃(Nb,Ta)O₇ ceramics sintered at 900⁹90oC withεr≈65⁹5, Qf≈230⁵60GHz and TCF≈-115^-70ppm/oC were obtained. The co-firing between BiNbO₄ ceramics and copper electrode under N₂ atmosphere was first studied. The SEM and EDS results showed that there was no reaction and diffusion between the ceramic and electrode. This makes it possible for BiNbO4 ceramics to be used in LTCC technology. A two element antennas with centre frequency at 3.07 GHz and bandwidth of 34MHz at -10 dB attenuation were obtained using a BiNbO4 ceramic substrate with size 34mm×34mm×1mm. The primary study of BiNbO₄ ceramic in antenna will broaden the application field for microwave ceramics with high permittivity.2. Based on the analysis on a number of Bi(Sb_xNb_yTa_z)O₄ (x+y+z=1) samples, a pseudo-ternary phase diagram of Bi(Sb,Nb,Ta)O₄ system is given below the melting point. It is composed of a monoclinic-phase region (x≥0.78), an orthorhombic-phase region (x≤0.55) and a monoclinic-orthorhombic coexisting phase region (0.55≤x≤0.78). The phase transformation from the monoclinic to orthorhombic structure can be attributed to the decrease of space occupied by the bismuth layers, which is caused by the increase of octahedra volumes when the Nb or Ta amount increases. In orthorhombic phase region, as the sintering temperature increases, the phase transformation to triclinic structure is observed and it is seriously affected by the sintering temperature and the Sb amount in Bi(Sb,Nb,Ta)O₄. Phase transformation fromβ-BiNbO₄ toα-BiNbO₄ in BiNbO₄ bulk samples was first reported and studied. From X-ray diffraction patterns, the transformation fromβtoαphase of BiNbO₄ could be observed by heating the bulk samples ofβ-BiNbO₄ from low temperatures to 700¹030oC. But such a transformation didn't occur in powder samples and in the cooling course. This phenomenon might be related with associated activation of stress and heat energy in the heating course. Differential thermal analysis, shrinkage and dielectric properties as a function of temperature were carried out and all the results confirmed the transformation fromβtoαphase of BiNbO4. In addition, a series of promising candidates for LTCC were explored in Bi(SbxNbyTaz)O4 system: 1, the BiSbO₄ ceramic sintered at 1080oC withεr≈19.3, Qf≈70000GHz, TCF≈-62ppm/oC and chemical compatibility with silver; 2, the 930oC sintered BiSbO4 ceramics with 0.6wt.% ¹.2wt.%B₂O₃-CuO addition withεr≈19.5, Qf≈45400GHz³3700GHz and TCF≈-65ppm/oC; 3, the 960oC sintered Bi(Sb0.6Ta0.4)O₄ ceramic withεr≈27, Qf≈35000GHz and TCF=-12ppm/oC; 4, the 960oC sintered Bi,O₄ ceramic withεr≈34.7, Qf≈16000GHz, and TCF=+16.1ppm/oC.3. The preparation, phase composition, microwave dielectric properties and chemical compatibility with silver and aluminum electrodes were investigated on a series of single phase compounds in the Bi2O3-MoO3 binary system. All materials have ultra low sintering temperatures lower than 820oC. Eight different xBi2O3-(1-x)MoO3 compounds with x value between 0.2≤x≤0.875 were fabricated and the associated microwave dielectric properties were studied. Theβ^Bi₂Mo₂O₉ single phase has a positive temperature coefficient of resonant frequency (TCF) about +31ppm/oC, with a permittivityεr=38 and a Qf=12500GHz at 300K at a frequency of 6.3GHz. Theα^Bi₂Mo₃O₁₂ andγ^Bi2MoO6 compounds both have negative temperature coefficient values of TCF^-215ppmo/C and TCF^-114ppm/oC, with permittivities ofεr=19 and 31, Qf=21800GHz and 16700 GHz at 300K measured at resonant frequencies of 7.6 GHz and 6.4 GHz, respectively. Through sintering the B2iO3-2.2MoO3 at 620oC for 2hrs a composite dielectric containing bothαandβphase can be obtained with a near zero temperature coefficient of frequency TCF=-13ppm/oC and a relative dielectric constantεr=35, and a large Qf¹2000 GHz is also obtained. Owing to the frequent difficulty of thermochemical interactions between low sintering temperature materials and the electrode materials during the cofiring, preliminary investigations are made to determine any major interactions with possible candidate electrode metals, Ag and Al. The results show thatβ^Bi₂Mo₂O₉ andα^Bi₂Mo₃O₁₂ do not react with Al at their sintering temperatures. La and Nd were also used to substitute for the Bi in Bi₂Mo₂O₉. For (Bi_1-xLn_x)₂Mo₂O₉ (Ln=La, Nd) ceramics, as the x value increased, the optimal sintering temperature increased. Substitution of La or Nd could stabilize the monoclinic phase and modify its microwave dielectric properties. The best microwave dielectric properties were obtained in (Bi0.8La0.2)2Mo2O9 ceramic with a permittivity of 32.7, Qf value of 13,490GHz and temperature coefficient about -4.6ppm/oC. From the above results, the low sintering temperature, good microwave dielectric properties, chemical compatibility with Al metal electrode, nontoxicity and price advantage of the Bi2O3-MoO3 binary system, all indicate the potential for a new material system with ultra-low temperature cofirng for multilayer devices application.4. Fabrication of a new kind multlayer co-fired capacitor using Bi₂Mo₂O₉ dielectric and aluminum as internal electrode was studied. The Bi₂Mo₂O₉ powders was calcined at 600oC for 4hrs. The calcined powders were vibratory milled for 24hrs to obtain fine powders for tape-casting use. To obtain fine slurry, the milled Bi₂Mo₂O₉ powders (56wt.%) were added into a solution of MEK (Methyl Ethyl Ketone, 19wt.%), ethanol (19wt.%), and PVB (Polyvinyl Butyral, 6wt.%) mixture and ball-milled for 24hrs. Tape casting was performed on a laboratory-type tape-casting machine with a doctor blade casting head, using 75 microns thick silicone coated mylar (polyethylene terephthalate) as a carrier film. The casting speed was set as 420 cm/min. The cast slurries were dried at room temperature without additional air flow. Aluminum electrode was screen-printed on the tape and laminated before firing. To minimize the warpage of the Bi₂Mo₂O₉-Aluminum MLCCs in this study, we applied an external force during the sintering course. When the internal pressure was too large, the local compressive stresses in MLCC samples could cause the cracks during sintering course. Only when the proper pressure was applied, the warpage could be successfully limited. Scanning electron microscoy, dielectric spectroscopy, dielectric temperature dependence and P-E loop were measured on pellet samples, MLCC samples and monolayer samples. There is no reaction or interdiffusion between electrode layer and ceramic layer. The relative permittivities of multilayer, monolayer, and monolithic Bi₂Mo₂O₉ samples all stabilize at around 39, and the dielectric losses are near 0.001 at 1MHz. Temperature dependence is similar for both monolayer and monolithic Bi₂Mo₂O₉ samples. Energy density of the monolayer Bi₂Mo₂O₉ sample reaches 0.75J/cm³ at 67kV/mm with a thickness of 46μm. This study extends the application of the ultra-low temperature firing Bi₂Mo₂O₉ with Al electrodes.