段維新臺灣大學：材料科學與工程學研究所黃永慶Huang, Yung-ChingYung-ChingHuang2007-11-262018-06-282007-11-262018-06-282006http://ntur.lib.ntu.edu.tw//handle/246246/55221多層陶瓷電容器是重要的被動元件，主要包含鈦酸鋇基之介電質以及金屬內電極。廣泛地使用金屬鎳或銅當成電極材料，主要為了較低電容器的價格。然而這些卑金屬與介電質燒結必須在還原氣氛下，因為於空氣中金屬容易受到氧化。已有許多方法已經被報導指出鈦酸鋇基地能夠保持高的絕緣電阻既使燒結於低氧分壓下。就本研究而言，鎳顆粒均勻地混合在鈦酸鋇粉末中。含鎳的鈦酸鋇陶瓷經由不同燒結條件及氧分壓加以燒結。添加鎳的鈦酸鋇複合材的微結構、緻密化行為、電性、鐵電性及鐵磁性均加以研究。 根據燒結氣氛下的氧分壓，有極少量的鎳離子可能固溶到鈦酸鋇中，取代鈦離子的位置。因此，鎳在鈦酸鋇中當成一個授體。其中鈦酸鋇的正方性及晶粒大小隨著鎳含量之增加而減少。當燒結於氮氣及1330度時，鎳在鈦酸鋇中的固溶量約為0.075-0.1 vol%。另外，鎳在鈦酸鋇的擴散距離係與燒結氧分壓有關。因此，在高還原氣氛下(10-7-10-8Pa)，鎳幾乎不再固溶及擴散於鈦酸鋇基地中。鈦酸鋇緻密化行為的差異受到鎳溶質的影響。鈦酸鋇晶粒大小也受到燒結氣氛影響。鈦酸鋇晶粒隨著氧分壓的降低，先增大，然後減小。 摻雜鎳的鈦酸鋇複合材其電性受到鎳的添加所影響。液相Ba6Ti17O40 及孔洞的出現會降低介電性質。當未摻雜的鈦酸鋇先還原在氧分壓為10-15 Pa，之後再燒結於N2 或 10-2-10-3 Pa時，鈦酸鋇則變為半導化。然而，部分鎳離子固溶於鈦酸鋇中，能改善鈦酸鋇的還原阻力。含鎳的鈦酸鋇於低氧分燒結後，電阻係數仍可高達1010 ohm×m及介電損失低於2 %。製程區間則可利用缺陷化學模式來建立，因而能製備出含鎳金屬之鈦酸鋇絕緣體材料。根據實驗的結果，更能推導出離子導電係數及還原平衡常數。 本實驗也得知，純鈦酸鋇是一個反鐵磁材料，但添加金屬鎳顆粒將會使複合材產生鐵磁性，飽和磁化值隨著鎳含量增加而上升。在鎳含量為35%時，飽和磁化值可以達到 25 emg/g。另一方面，在滲流理論值以下時，可以得到介電常數高於28,800，此現象能與滲流方程式吻合，其中滲流體積分率為0.35。Multilayered ceramic capacitors (MLCCs) composing BaTiO3 based dielectric and metallic inner electrodes are one the most important passive components. The electrodes materials such as Ni or Cu are widely used for reducing the cost of MLCCs. These base-metal electrodes (BME) capacitors have to be fired in a reducing atmosphere to avoid the oxidation of metals. Many approaches have been reported to maintain the insulation resistance of BaTiO3 based dielectrics even when they are sintered in low oxygen partial pressure. In the present study, the Ni particles are mixed with the BaTiO3 powders. The sintering of the Ni/BaTiO3 composites is carried out in different atmosphere with various oxygen partial pressures. The microstructure, densification behaviour, electrical, ferroelectric and ferromagnetic properties of Ni-doped BaTiO3 are measured. Depending on the partial pressure in the sintering atmosphere, a very small amount of Ni ion may dissolve into BaTiO3 to replace Ti ion. The Ni2+ ion thus act as acceptor to BaTiO3. The tetragonality and grain size are thus reduced with the increase of Ni content. The solubility of Ni in BaTiO3 as sintered in N2 at 1330 oC is about 0.075-0.1 vol.% Ni. In addition, the diffuse distance of Ni in BaTiO3 matrix also depends on the oxygen partial pressure in the sintering atmosphere. The solubility of Ni in BaTiO3 is negligible as sintering is carried out in a highly reducing atmosphere (Po2=10-7-10-8Pa). The densification behaviour is affected by the Ni2+ solutes. The grain size of the Ni-doped BaTiO3 is also affected by sintering atmosphere. The grain size of BaTiO3 is increased and then decreased with the decrease of oxygen partial pressure. The electrical properties of Ni-doped BaTiO3 are affected by the Ni addition. The presence of pores and the Ba6Ti17O40 phase degrades the dielectric properties of Ni-doped BaTiO3. The undoped BaTiO3 becomes semiconducting as sintering in Po2=10-15 Pa and then re-oxidized in N2 (Po2=1 Pa) or Po2=10-2-10-3 Pa. However, the solution of Ni2+ into BaTiO3 can improve the reduction resistance of BaTiO3. Electrical resistivity higher than 1010 Ω•m and dissipation factor lower than 2% are thus obtained in the Ni-doped BaTiO3 samples even they are sintered in a relatively low oxygen partial pressure. The process windows for the preparation of insulating BaTiO3-Ni can be built by applying defect models. According the experimental results, the ionic conductivity constant (Ki) and reduction equilibrium constant (KR) can be estimated. The monolithic BaTiO3 is an anti-ferromagnetic material; the addition of metallic Ni particles introduces a ferromagnetic response into BaTiO3. The specific saturation magnetization increases with the increase of Ni content. For the BaTiO3/35%Ni composite, the specific saturation magnetization reaches a value of 25 emu/g. On the other hand, by adding an amount of Ni particles slightly below the percolation limit of the Ni particles can push the dielectric constant to a value as high as 28,800. The phenomena are well fitted by a percolation equation. The value of percolation threshold (Vc) as determined from the experimental data is 0.35.Contents 1. Introduction and Objectives 1 1.1 Introduction 1 1.2 Objectives 2 2. Literature Review 4 2.1 Crystal Structure of BaTiO3 4 2.2 Electrical Properties of BaTiO3 5 2.2.1 Dielectric property 5 2.2.2 Ferroelectric property 10 2.3 The Characterization of Nickel and Nickel Oxide 13 2.4 The Magnetic Property of Nickel 15 2.5 Influence of Ni on BaTiO3 17 2.5.1 Solubility and Diffusion 17 2.5.2 Electrical Properties 19 2.5.3 Ferroelectric Properties 20 2.6 Defect Chemistry of BaTiO3 23 2.6.1 Intrinsic Defects 23 2.6.2 Extrinsic Defects 25 2.6.3 Defect Reaction for Donor-Additives 27 2.6.4 Defect Reaction for Acceptor-Additives 28 2.6.5 Defect Reaction for Self-Compensation 28 2.7 Influence of Oxygen Partial Pressure on Defect Chemistry 29 2.8 Reduction Resistance of BaTiO3 34 3. Experimental Procedures 45 3.1 Raw Materials 45 3.2 Experimental Preparation and Design 45 3.3 Observations of Powders and Particles Size 48 3.4 Phase Identification 48 3.5 Sintering Behaviour 48 3.5.1 Kinetic Study 48 3.5.2 Measurement of Apparent Density 50 3.6 Microstructure Observation 50 3.6.1 Fracture surface 50 3.6.2 Polished Surface 51 3.7 The Solubility 51 3.8 The Electrical Properties 52 3.8.1 Dielectric Constant 52 3.8.2 Insulation Resistivity 53 3.8.3 Ferroelectrical Properties 53 3.9 The Magnetic Properties 53 4. Microstructural and Sintering Characterization 54 4.1 Chemical Composition of Powder 54 4.2 Particle Size Distribution 54 4.3 Morphology Observation 54 4.4 Phase Analysis 54 4.5 Sintering Behaviour 55 5. Solubility Limit and Diffusion Behaviour 63 5.1 Vapor Pressure of Pure Metal 63 5.2 The Lattice Constant Measurement 64 5.3 The EMPA Analysis 66 6. Reduction Resistance and Process Windows 78 6.1 Densification Behaviours and Phase Observation 78 6.2 Electrical Properties 82 6.3 Defect Model and Reduction Resistance 89 7. Functional Properties of BaTiO3/Ni Composites 92 7.1 Dielectric Properties 93 7.2 XRD and Microstructure 96 7.3 Ferroelectric Property 101 7.4 Ferromagnetic Property 105 8. Conclusions and Suggestions for Future Work 107 8.1 Conclusions 107 8.2 Suggestions for Future Work 109 References 110 List of Figures Fig. 2-1 The ideal cubic structure of ferroelectric BaTiO3 illustrated with (a) small cation (Ti4+), and (b) larger cation (Ba2+) in the body-certre cubic form by the other cations respectively [Kingery, 1976]. 7 Fig. 2-2 The sequence of polymorphic phase-transformations upon cooling in BaTiO3 with the corresponding lattice parameters [Jaffee, 1971]. 7 Fig. 2-3 BaO-TiO2 Phase diagram system [Kirby et al., 1991]. 8 Fig. 2-4 Relative dielectric constant of dense BaTiO3 ceramics obtained by different meyhods versue grain size at 70 ºC. HPS: hot-pressure sintering, HIP: pseudoisostatic pressing in a multianvil press, CSM: combined sintering method, SPS: spark plasma sintering [Buscaglia et al., 2006]. 9 Fig. 2-5 Particle size dependence of the a-axis and c-axis for BaTiO3 powders [Wada et al., 2003]. 9 Fig. 2-6 Ion positions in tetragonal BaTiO3 powders [Kingery, 1976]. 12 Fig. 2-7 A typical ferroelectric hystersis loop [Kingery, 1976]. 12 Fig. 2-8 Crystal structure of (a) NiO [Kingery, 1976] and (b) Ni at room temperature. 14 Fig. 2-9 Equilibrium oxygen partial pressure for the oxidation of nickel [Gaskell, 2003]. 14 Fig. 2-10 Schematic magnetization characteristic and hysteresis loop [Valenzuela, 1994]. 16 Fig. 2-11 Crystal symmetry and defect symmetry in Perovskite BaTiO3 structure doped with Ni2+ ions at Ti4+ sites [Zhang and Ren, 2005]. (a) paraelectric state, T >TC, (b) ferroelectric state without Ni2+ dopant, T < TC, (c) ferroelectric state with Ni2+ dopant, T < TC. Large square or rectangle represents crystal symmetry, while small square or rectangle represents defect symmetry. Thick arrow refers to spontaneous polarization, PS, and thin arrow refers to defect polarization, PD. TC is the paraelectric-ferroelectric phase transition temperature. 22 Fig. 2-12 Schematic model of deformed lattice cell forming as elastic dipole [Arlt and Neumann, 1988]. 22 Fig. 2-13 Total electrical conductivity of undoped BaTiO3 against oxygen partial pressure at different temperature [Song et al., 1999]. 30 Fig. 2-14 Effect of increased acceptor content on defect concentrations. Dashed line represents sample with ten times the net acceptor content of sample represented by solid line [Chan et al., 1982]. 37 Fig. 2-15 Tentative energy-level scheme of BaTiO3[Daniels et al., 1974(a)]. 42 Fig. 2-16. The critical oxygen partial pressure, Po20, of undoped BaTiO3 and 0.54 mol% Ni-doped BaTiO3 as a function of temperature. 42 Fig. 2-17. KR over Ki ratio against temperature for acceptor-doped BaTiO3. For the calculation the following values were used from Table 2-4 and =1.7×1020 cm-3. The black line is the best fitted from the boundary of critical oxygen partial pressure for insulator in the present work. 43 Fig. 2-18. Log KR and Log Ki vs reciprocal temperature. The dash lines are the best fitted with data. 43 Fig. 3-1 The flowchart of preparation and testing procedures. 47 Fig. 4-1 Particle size distribution of pure BaTiO3 and Ni-doped BaTiO3 powders: (a)after the calcination in air and (b)after the reduction in Po2=10-15 Pa. 58 Fig. 4-2 TEM micrographs of (a) undoped and (b) 5 vol.% Ni-doped BaTiO3 powder after reduction in Po2=10-15 Pa. 59 Fig. 4-3 The XRD pattern of (a) as-received undoped BaTiO3 powders, (b) as-calcinated 5 vol.% Ni-BaTiO3 powders at 500 oC in air, (c) as-reduced 5 vol.% Ni-BaTiO3 powders at 800 oC in Po2=10-15 Pa. 60 Fig. 4-4 The relative shrinkage curves of Ni-doped BaTiO3 specimens as a function of temperature. The sintering atmosphere is (a) air; (b) N2. 61 Fig. 4-5 The densification rate curves of Ni-doped BaTiO3 specimens as a function of temperature. The sintering atmosphere is (a) air; (b) N2. 62 Fig. 5-1 The vapor pressure of metals as a function of temperature [Kubaschwski et al., 1967]. 69 Fig. 5-2 The lattice constant c/a ratio of Ni-doped BaTiO3 samples as a function of Ni content sintered at 1330 oC in different atmosphere. 69 Fig. 5-3 The SEM micrographs of Ni-doped BaTiO3 samples as a function of Ni content sintered at 1330 oC in different atmosphere. 70 Fig. 5-4 The average size of BaTiO3 grains in Ni-doped BaTiO3 samples sintered at 1330 oC in different atmosphere. The number in the brackets indicates the volume fraction of bimodal structure grains. 71 Fig. 5-5 Typical EPMA measurement results for the 0.5 vol.% Ni doped BaTiO3 after sintering in N2 (Po2=1 Pa). (a)The Ni mapping, (b) the corresponding solubility curve. 72 Fig. 5-6 Typical EPMA measurement results for the 1.0 vol.% Ni doped BaTiO3 after sintering in N2 (Po2=1 Pa). (a)The Ni mapping, (b) the corresponding solubility curve. 73 Fig. 5-7 (a) The SEM of sandwiched BaTiO3 structure after sintering in 1 Pa oxygen partial pressure, (b) The mapping in (a), (c) the corresponding line scan in (a). 74 Fig. 5-8 (a) The SEM of sandwiched BaTiO3 structure after sintering in 10-4-10-5 Pa and re-oxidation treatment in 1 Pa oxygen partial pressure, (b) The mapping in (a), (c) the corresponding line scan in (a). 75 Fig. 5-9 (a) The SEM of sandwiched BaTiO3 structure after sintering in 10-4-10-5 Pa oxygen partial pressure, (b) The mapping in (a), (c) the corresponding line scan in (a). 76 Fig. 5-10 (a) The SEM of sandwiched BaTiO3 structure after sintering in 10-7-10-8 Pa oxygen partial pressure, (b) The mapping in (a), (c) the corresponding line scan in (a). 77 Fig. 6-1 The relative density of Ni-doped BaTiO3 ceramics as a function of Ni content in different sintering condition. 80 Fig. 6-2 The XRD patterns of undoped and 5 vol.% Ni-doped BaTiO3 ceramics after sintering at 1330 oC in different sintering condition. 81 Fig. 6-3 The relative permittivity of Ni-doped BaTiO3 ceramics as a function of Ni content prepared by different sintering conditions. 85 Fig. 6-4 The microstructure of 0.1 vol.% Ni-doped BaTiO3 ceramics after sintering at 1400 oC in air. 86 Fig. 6-5 The dissipation factor of Ni-doped BaTiO3 ceramics as a function of Ni content prepared by different sintering conditions. 87 Fig. 6-6 The electrical resistivity of Ni-doped BaTiO3 ceramics as a function of Ni content prepared by different sintering conditions. 88 Fig. 6-7 The process windows for: (a) undoped BaTiO3 samples (*pre-reduction at 800 oC, then sintered in N2 (Po2=1 Pa)), and (b) Ni-doped BaTiO3. 91 Fig. 7-1 Relative permittivity as function of Ni particle volume content. The lines representing the predictions from Percolation, Maxwell and modified Maxwell laws. 97 Fig. 7-2 Variation of dissipation factor and electrical resistivity with Ni content. 97 Fig. 7-3 XRD patterns of the BaTiO3/Ni composites. 98 Fig. 7-4 Typical micrographs of (a) monolithic BaTiO3 and the composites containing (b) 1 vol.% Ni, (c) 5 vol.% Ni, (d) 35 vol.% Ni, (e) 50 vol.% Ni and (f) nano-Ni cluster.99 Fig. 7-5 Ferroelectric hysteresis loops of monolithic BaTiO3 and BaTiO3/Ni composites. 104 Fig. 7-6 Magnetization versus applied magnetic field of the BaTiO3/Ni composites. 107 List of Tables Table 2-1 Reported values on solubility and diffusion of Ni in BaTiO3. 18 Table 2-2 The formation energies of Schottky and Frenkel defects [Lewis et al., 1985]. 26 Table 2-3 The activation energies of defects in BaTiO3 [Nowotny et al., 1994(g)]. 26 Table 2-4 Comparison of KR and Ki values reported on BaTiO3. 44 Table 3-1 Physical and chemical characteristics of the BaTiO3 powder used in the present study. 46 Table 4-1 Nickel content in the BaTiO3/ Ni powder mixtures 57 Table 7-1 The grain size of BaTiO3 and Ni cluster as a function of Ni content. 100 Table 7-2 The remnant polarization (Pr), saturation polarization(Ps) and internal bias (Eib) of BaTiO3/Ni composites. The internal bias was determined by average the positive and negative coercive field (Eib= 1/2(Ec++Ec-)Ni/BT -1/2(Ec++ Ec-)BT ). 1057743603 bytesapplication/pdfen-US鈦酸鋇多層電容陶瓷鎳氧分壓授體固溶度缺陷微結構晶粒尺寸電阻係數介電損失介電常數鐵磁性滲流BaTiO3MLCCsNioxygen partial pressureacceptorsolubilitydefectmicrostructuregrain sizeelectrical resistivitydielectric lossdielectric constantferromagneticpercolationthesishttp://ntur.lib.ntu.edu.tw/bitstream/246246/55221/1/ntu-95-D91527005-1.pdf