Article ID: 1003-6326(2005)02-0252-05
Optimization of dielectric constant temperature coefficient of pyrochlores containing bismuth
REN Qing-li(任庆利)1, 2, LUO Qiang(罗 强)3, CHEN Shou-tian(陈寿田)2
(1. School of Technical Physics, Xidian University, Xi’an 710071, China;
2. School of Electrical Engineering, Xi’an Jiaotong University, Xi'an 710049, China;
3. The First Department, The Second Artillery Institute of Engineering, Xi’an 710025, China)
Abstract: The Bi2O3-ZnO-Nb2O5(BZN)-based ceramic samples were prepared according to the optimum composition of (Bi3xZn2-3x)(ZnxNb2-x)O7 by solid state reaction. The BZN ceramic structure and the dielectric properties were explored via X-ray diffractometer(XRD), differential thermal analysis(DTA), scanning electron microscope(SEM), and HP4275A impedance analysis. The amphoteric surface active agent with alcohol amine double nature is introduced when the raw materials are mixed and ground. The anatase titania doped BZN-based ceramics was also investigated. Either doping the anatase TiO2 or adding the surface active agent or both of them can widen sintering temperature range which satisfies the zero temperature coefficient (0±30×10-6/℃) of the BZN ceramics. And these BZN ceramics with diphasic pyrochlore structure possess excellent dielectric properties.
Key words: Bi2O3-ZnO-Nb2O5 ceramic; phase transformation; dielectric properties CLC number: TM28
Document code: A
1 INTRODUCTION
Dielectric ceramics are widely used with advances in microelectronic technologies. Bismuth-based pyrochlores exhibit well-known high-frequency dielectric properties for multilayer capacitor. Group of pyrochlore structured compounds is of general formula A2B2O7 (where A is trivalent or bivalent cations and B is quadrivalent and quinquevalent cations). As a member of the pyrochlore family of compound, bismuth zinc niobate is a typical high frequency dielectrics with high permittivity and low dielectric loss, and can be applied as various devices, such as temperature-stable multilayer ceramics capacitor(MLC) and microwave resonator and filter. Bismuth zinc niobate system was reported for the first time by the Chinese engineers in 1970s while wide recognition in world began in 1990s[1-3]. Recently, this system attracts more and more attentions[4-14]. Much work has been performed on the crystal structures and improving dielectric properties, especially enhancing permittivity and reducing dielectric loss. However, the optimization of dielectric constant temperature coefficient of pyrochlores containing bismuth has not been reported. Moreover, because the temperature coefficient of BZN-based ceramics is very sensitive to the change of the sintering temperature, it cannot be used in MLC industry on a large scale[15, 16]. Besides, with the developing trend to get smaller and thinner multilayer devices, one of the major bottlenecks for the practical applications in this low-firing system is how to control the grain growth to gain the fine grain ceramics.
The present paper reports the optimization of dielectric constant temperature coefficient of pyrochlores containing bismuth with composition corresponding to (Bi3xZn2-3x)(ZnxNb2-x)O7 system. The effects of adding the surface active agent and the effects of anatase TiO2 doping on the phase composition, grain size and dielectric properties of BZN-based ceramics are investigated.
2 EXPERIMENTAL
Reagent Bi2O3, ZnO, and Nb2O5 were weighed in a proper mole ratio according to the compositions (Bi3xZn2-3x)(ZnxNb2-x)O7(0.15〈x〈0.85) and mixed in alcohol with agate media for 4h. The slurry was dried and then calcined at 770℃ for 2h and milled again for 4h. The dried powers were then granulated and pressed into pellets. Samples were sintered at 940-1040℃ for 2h. Four kinds of experimental designs were planned: 1) the original BZN samples; 2) BZN samples with adding the surface active agent (abbreviated as SAA); 3) anatase TiO2-doped BZN samples; 4) SAA-added and anatase TiO2-doped BZN samples. It was in the first mixing process of the oxides that the SAA was added. And the SAA is the amphoteric surface active agent with alcohol amine double nature, whose equilibrium value (hydrophilic lipophilic balance (HLB)) is 9.4. Besides, the amount of the SAA added was 1%(mass fraction). And the amount of the anatase TiO2 added was 8.6%(molar fraction).
X-ray diffraction(XRD) patterns were obtained with a Rigaku D/MAX-2400 X-ray diffractometer with CuKα radiation. Differential thermal analysis (DTA) curves were obtained with Thermal Analyzer 2000. And the microstructures of the samples were observed by the JXA-840 scanning electron microscope (SEM). The particle size data of the powder were obtained with Coulter Particle Analyzer. The temperature dependence of the dielectric constant was measured in an automated measurement system consisting of a PC computer, an HP 4275A LCR meter, an HP 6942A multiprogrammer, and a temperature chamber. The temperature range were between -70℃ and 130℃. The measuring frequency was 1MHz. An HP-4140B PA meter was used to measure the resistivity of the samples (about 1mm thick) under 100 V dc voltage for 1min.
3 RESULTS
3.1 Phase composition
Fig.1 and Fig.2 show the XRD patterns of the BZN samples calcined at 770℃ for 2h and sintered from 940℃ to 1040℃ for 2h, respectively. The percentage of cubic pyrochlore phase (PDF 25-443, abbreviated as α phase) or orthorhombic pyrochlore phase (PDF 330-711, abbreviated as β phase) is calculated according to the following equation:
where Iα and Iβ are the strongest diffraction intensities of the α phase and β phase, respectively (Table 1).
Fig.1 XRD patterns of BZN
From the XRD analysis, all the BZN samples are found to have α-β coexisting phase structure. The sintering temperature is dropped by doping the anatase TiO2. And the phase transformation from β phase to α phase is performed with increasing sintering temperature. Either doping the anatase TiO2 or adding the surface active agent or both of them can restrain the phase transition from β phase to α phase.
3.2 Scanning electron micrograph
Fig.3 presents the SEM images of the sintered BZN-based ceramics. Compared with the original BZN ceramic sintered at 1000℃ for 2h, the surface active agent added and anatase TiO2-doped BZN ceramic is of smaller and more homogeneous grains.
3.3 Dielectric properties
The dielectric properties of the sintered BZN-based ceramics are listed in Table 2. The temperature coefficient αc is calculated by the following equation:
where ε1 is the dielectric constant at t1 (here t1 is 20℃), and ε2 is the dielectric constant at t2 (here t2 is -55℃, 85℃, and 125℃, respectively).
Table 1 Phase composition of calcined BZN samples and sintered BZN samples
Fig.2 XRD patterns of sintered BZN ceramic samples
Fig.3 SEM micrographs of BZN-based ceramic samples sintered at 1000℃ for 2h
Table 2 Dielectric properties of BZN ceramic samples
Depending on the zero temperature coefficient (0±30×10-6/℃), the sintering temperature of the BZN ceramics is in the range of 1000-1020℃ for the original samples, 1000-1040℃ for the surface active agent added samples, 960-1000℃ for the TiO2-doped samples, and 940-980℃ for surface active agent added and TiO2-doped samples, respectively.
4 DISCUSSION
In BZN-based diphasic ceramics, the formation of β phase structure may be related to the coupling of 6s2 lone pair electrons of Bi3+ ions[16]. If the sites of the pyrochlore structure are occupied by Bi3+ ions only, the coupling among 6s2 lone pairs might cause distortion of the cell and only β phase occurs. When a certain number of Zn2+ ions enter the A sites with increasing sintering temperature (t>700℃), the coupling of 6s2 lone pair electrons among Bi3+ ions is broken, and α phase appears.
The physical properties of the powders Bi2O3, ZnO, and Nb2O5 are different (ρBi2O3>ρZnO>ρNb2O5, where ρ represents the density; dBi2O3>dZnO>dNb2O5, where d represents the particle size), so the segregation phenomenon appears much easily during the raw material mixing process. In the BZN-based diphasic ceramics with the surface active agent added, the surface active agent wets the powder and forms the adsorbed layer on the surface of the solid particles, which weakens the surface tension of the interface between solid and liquid and enhances the dispersivity of the powders in the liquid media when they are milled. This is beneficial to overcoming the shortcoming of insufficient mixing in conventional ceramic processing. Therefore, Bi2O3, ZnO and Nb2O5 are mixed sufficiently. Moreover, during the following sintering process, the energy consumption is necessary for a certain number of Zn2+ ions entering from B sites to A sites. Because there are more carbohydrate polymers (5% PVA and 1% SAA) which decompose at lower temperature in sintering, the looseness and porosity of the BZN samples are increased. The energy produced by the temperature increasing from 950℃ to 1050℃ is also exhausted by the denseness of the BZN samples. So, there is an approximate terrace at which heat is neither absorbed nor released from 950℃ to 1050℃ in the DTA curve (Fig.4). And there is less transformation from β phase to α phase.
Fig.4 DTA curves of BZN
In anatase TiO2-doped BZN-based diphasic ceramics, the Ti4+(d=0.068nm) is supposed to enter the B sites whose ion radius requirement is about 0.06-0.07nm[17]. So, its composition can be expressed as (Bi3xZn2-3x)(ZnxNb2-x-yTiy)O7. And the stability of BZN-based pyrochlore structure is enhanced because of the more reasonable matching in both ion radius and electronic valence after the anatase TiO2 is doped. In addition, there is a phase transformation from anatase TiO2 to rutile TiO2, whose endothermic value is about 1.256kJ/mol[18]. Thus, there is an approximate terrace at which heat is neither absorbed nor released in the temperature range of 983±60℃ in the DTA curve (Fig.4). During the following sintering process, the process that a certain number of Zn2+ ions in B sites enter the A sites needs energy consumption; at the same time, the TiO2 phase transformation also exhausts the energy. So, there is less transformation from β phase to α phase.
In the sintering process of the anatase TiO2-doped BZN-based diphasic ceramics with surface active agent added, there are mainly four changes needed to absorb the energy: 1) the grains of the α phase and β phase grow; 2) a certain number of Zn2+ ions in B sites enter the A sites so as to complete the phase transformation from β phase to α phase; 3) the denseness of the samples is expected because of the looseness caused by the more amount of the carbohydrate polymers added; 4) the anatase TiO2 transforms to rutile TiO2. Accordingly, the grain growth of the BZN-based diphasic ceramics is controlled (Fig.3(d)), and the phase transformation from β phase to α phase is also restrained.
The temperature coefficient of the BZN ceramics with pure α phase structure is negative (-4×10-4/℃) and that of the BZN ceramics with pure β phase is positive (1.5×10-4/℃)[19]. The temperature coefficient of BZN ceramics with α-β coexisting phases depends on the content ratio of α phase to β phase. The zero temperature coefficient of BZN ceramics can be obtained in a proper content ratio of α phase to β phase. So, the smaller the change of the content ratio of β phase to α phase, the smaller the change of the temperature coefficient with increasing sintering temperature (Table 1 and Table 2).
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Received date: 2004-12-01; Accepted date: 2005-01-18
Correspondence: REN Qing-li, Associate professor, PhD; Tel: +86-29-82551108; E-mail: abcdRenQingli@263.sina.com
(Edited by YANG Bing)
