Once again, the relation between relative density and permittivity is complying with a mixing rule. Effective permittivity of sintered specimen can be derived from level rule of vacuum and reach to 42,500 GHz without wet milling treatment. If treated to milling in water, the quality factor declines dramatically as more intensive milling energy apply on Ba5Nb4O15 ceramic. It is near to half the original value.
However, XRD diffraction analysis can give an evidence on the degree of ordering structure at high Miller index plane. Considering the analysis results between quality factor and high angle plane of Ba5Nb4O15 is highly related. In previous chapter, the X-ray diffraction pattern of sintered ceramic confirmed the 2θ angle at 119.3° was the indication of ordering plane (0 1 13). The intensity of diffraction peak tends to decrease when milling time increase. Fig 4-15. The lateral plane (0 5 6) oppositely increases as long as milling time increasing. Many literatures state on microwave dielectric materials explain oxygen octahedral tilt is the fundamental essay for contributing lattice ordering82-84. Two adjacent planes owned high indices can be associated to oxygen octahedral tilt indirectly.
Fig. 4-13 The relative density vs. permittivity of Ba5Nb4O15
Fig. 4-14 Permittivity (K) and quality factor (Qxf) for the sintered Ba5Nb4O15 specimens prepared using the powders milled for various times.
Fig. 4-15 High angle diffraction pattern of milled Ba5Nb4O15
4.4 Discussion
4.4.1. Milling behavior of Ba5Nb4O15: Barium dissociated from lattice site
Ba5Nb4O15 calcined powders are easily pulverized through wet mechanical treatment. The initial mean size 2.52um particles can be grinded to 0.44um by the planetary mill. In the beginning, powder distribution is bimodal population and tends to Gaussian normal distribution after grinding.
Deducing energy and particle reduction size can roughly calculate the relation of milling time and particle mean size.
κ× (1/d2-1/d1) ＝ Energy (media mass×rotation speed×milling time) ……… (4.1) κ : Rittinger’s coefficient 1/d1 : before size reduction 1/d2: after size reduction
The equation 4.1 following Rittinger’s law85 is the energy applying on particle. It is proportional to the change of surface area between primary and final reduction particle. Fig. 4-16. The line between milling time and reciprocal of reduction size have linear relevance by fine grinding process.
Mechanical energy obviously changes particle surface area. Through the use of grinding media yttrium stabilized zirconia, less contamination contributed on the milled powders. The zirconia contamination is lower than 500ppm at milling 120min confirmed by XRF analyzer.
Intensive milling apparently induces more surface area of particle86. The new surface of particle can weaken the valence bond of barium atom and tend to react to H2O. Only barium ions were detected quantitatively from filtered water of milled slurry. There are no free niobium ions detected by ICP analyzer. Which certain barium ion in unit cell of lattice structure is easily dissociated? Is it implied to the preferred orientation plane growth? The SAED method by HR-TEM will give the evidence on crystallographic observation. It will be explained next section.
Continuously, the barium ions react to water and convert into barium hydroxide. The carbonic acid from CO2 entering into water neutralized with barium hydroxide to form BaCO3. The evidence on
typical orthorhombic structure of BaCO3 is also verified from X-ray diffraction pattern. SEM morphology of BaCO3 illustrates the recrystallization of precipitated product to accomplish needle-like shape81,87. This study on milling process indicated more weight loss from dissociation of BaCO3 at elevated temperature 1435ºC. The milling time have relatively physical quantity on weight loss of sintered Ba5Nb4O15. It is also accordant with surface area generated by mechanical energy. The weight loss of 0.44um Ba5Nb4O15 is near to 1.0wt.% at sintering 1435ºC. All routes of milled Ba5Nb4O15 results prove the fundamental mechanochemical phenomena which barium ions leach into water through applying mechanical energy.
Fig. 4-16 Rittinger’s Law correlation fitting curve of milled Ba5Nb4O15 powders
4.4.2 Microstructure vs. Composition
To verify the microstructure anisotropic growth may be caused by precipitated BaCO3 which never left out of specimen, an analogical experiment proposed as flow chart in Fig. 4-17. The purpose of experiment confirms anisotropic growth on the preferred orientation plane possibly. Due to BaCO3 precipitate around the milled powders, it could be not homogenously in the powders when water evaporated at drying process. The fine needle shape BaCO3 aggregates to cause composition variation. There is a difference on microstructure of the rich BaCO3 site compared with others. A simple method designed to create the BaCO3 composition gradient on green pellet of un-milled Ba5Nb4O15. Small amount of BaCO3 powders put on surface of Ba5Nb4O15 then co-firing at 1435ºC.
It is an interface of BaO-Ba5Nb4O15 as to be none equilibrium mass transport happened from top of disc. The reference one is green pellet without extra BaCO3 on surface as usual.
Microstructure of the cross section of sintered specimens express grain shape and size are distinct near to the free surface. Fig 4-18 (a)&(b). The grain growth may adopt a preferred direction along with composition gradient. On the top of free surface complies with rich BaCO3 powder like as source of barium rich ingredient. The depth of diffusion path will be decided by composition gradient. When barium rich like as precipitated source from milling process, the sintered specimen feature has the tendency to be high aspect ratio. The elongated grains change aspect ratio from high to low beneath 100um depth. Although, this method roughly described the grain shape and direction may be relevant to preferred orientation property. Leaching barium ions will reentry into lattice site but inhomogeneous composition plays the role of anisotropic growth. The macroscopic investigation still let the proof of preferred orientation grain growth is believable.
Fig. 4-17 Simulated anisotropic growth by composition variation
Fig. 4-18 (a) SEM micrographs for the Ba5Nb4O15 sintered at 1435 °C. The cross section from the top of sintered surface reveal specific growing plane without excess BaCO3
Fig. 4-18 (b) SEM micrographs for the Ba5Nb4O15 sintered at 1435 °C. The cross section from the top of sintered surface reveal specific growing plane with excess BaCO3
4.4.3 Preferred orientation plane by XRD pole figure identification
In addition, the study on anisotropic grain growth of Ba5Nb4O15 reveal to microstructure varied with milling time. The elongated grains present the orientation was always observed in-plane direction. Not only it is derived from the presence of liquid phase assisting coarsening but also the precipitated substance engaged the preferred orientation growth. Firstly, the aspect ratio of Ba5Nb4O15 grain is enhanced by milling energy. Intensive milling time appears to enlarge aspect ratio of grains. More milling time induces the excess surface energy and leaching barium ions assist the lattice diffusion on a certain plane. Tanaka’s88 theoretically investigated on the basis of free energy for material transport proved higher rate of shape change for elongated grains. The derived rate equations are worked on considering lattice diffusion mechanism. It can be carried out the shape change from precipitated BaCO3 with the milled Ba5Nb4O15 powders as the starting none equilibrium system. The rate of shape change is proportion to the surface energy generated from milled Ba5Nb4O15 and inverse proportion to the particle size.
Before the pole figure analysis on specimens, the Laue’s XRD back reflection pattern gives the textural structure information on both samples show disconnected spots at each diffracted ring pattern. Fig 4-19 (a). An interested pattern comparison on 38.3°and 42.7° reversed deviation of intensity after milling treatment. Another approach on this particular 2θ angle 38.3° by phi rocking scan with two dimensional HR-XRD Fig. 4-19 (b), milled specimen has the appearance of intensity spectrum to be preferred orientation plane. Finally, selected planes of (1 0 3) (1 1 0) (0 0 5) (2 0 3) were fully scanned by 2D measurement to be an information of stereographic projection. Fig. 4-19 (c) express the variation of diffracted intensity with respect to the direction of planes. The multipole random distribution reveals its probability of density for each plane, (0 0 5) provides the preferred
Fig. 4-19 (a) Comparison of Laue’s diffraction patterns of sintered specimen milled 120min
Fig. 4-19 (b) Comparison of sintered specimen at 1435°C line scans at 2θ angle at 38.15° (red:
un-milled black: milling 120min)
Fig. 4-19 (c) Pole figure of mill and unmill Ba5Nb4O15 with sintering at 1435°C
4.4.4 TEM lattice plane identification
There is a main difference between X-ray and electron diffraction due to the Ewald spherical radius that is related to wavelength of incident wave. Which means more reflection patterns can be observed by electron diffraction. The interaction by Coulomb force between electron and matter is much stronger than X-ray. This indication of scattering wave can shorter the exposure time to get high intensity. Even though the structure determination is not reliable than XRD data, but tilting the specimen along a direction to investigate the crystallographic orientation can be achieved by Selected Area Electron Diffraction SAED and/or Convergent Beam Electron Diffraction CBED in nm-region area.
Fig. 4-20 Bright field image of un-milled Ba5Nb4O15 specimen at elevated temperature 1350°C The orientation plane is parallel with grain boundary investigated by bright field image of high resolution TEM. The SAED pattern for this region proved the plane index meet the reciprocal distance of (0 0 1) and unit cell of lattice constant in c-axis is 1.181nm. The orthogonal plane meets to (1 0 0) has 0.592nm lattice constant in axis. Another specimen at elevated temperature 1435°C can be found the same orientated plane parallel with grain boundary Fig 4-21. SAED pattern proved the plane direction is perpendicular to grain boundary and growing direction is [1 0 0]. Lattice constant of (0 0 1) is 1.191nm measured from reciprocal distance of SAED pattern. It is little large one according as lattice parameter increase with sintering temperature rule. Fig. 4-22. High resolution image with high magnification illustrates the 5 layers stacking lattice sequentially from observing at zone axis [1 1 0] direction. Fast Fourier Transform obtained the same information on the selected area as SAED pattern. SAED pattern also reveals the 5 spoty intensity repeating the shift type packing status such as sequence of [ACABC]n or(hhccc)n in crystallographic structure89.
The milled Ba5Nb4O15 sample with preferred planes was confirmed the grain by FIB tool. SEM
pictures in Fig 4-23 state the selected area and trapped pores as same as previous study on de-sintering behaviour at high sintering temperature. No doubt, the preferred orientation plane along the grain boundary is (0 0 1) verified by SAED and bright field high resolution image. The two selected sites on one grain show the same orientation plane guide to top surface of specimen. The growth direction on plane (0 0 1) is [1 0 0] which can be imaged to the corner of lattice site.
Leaching barium is split out from the weakest bond at the niobium deficient layer. The vacancy mobility is much fast than lattice site. From macroscopic point of view, composition gradient of barium also enhances the anisotropic growth. The new lattice distance derived from SAED pattern of milled specimen. The d spacing of orientated plane is 1.201nm and axis distance is 0.5915nm which is obviously larger than un-milled specimen. The c-axis is enlarged around 0.8% compared to un-milled one. The lattice distorted by FIB ion higher bombarding energy90.
Fig. 4-20 (a) Bright field image of un-milled specimen at 1350°C
Fig. 4-20 (b) SAED pattern of un-milled specimen at 1350°C
Fig. 4-20 (c) HRTEM Bright field image of un-milled specimen at 1350°C
Fig. 4-21 (a) Bright field image of un-milled specimen at 1435°C
Fig. 4-21 (b) SAED pattern of un-milled specimen at 1435°C
Fig. 4-22 (a) Bright field image of ion milled specimen at 1435°C
Fig. 4-22 (b) SAED pattern of ion milled specimen at 1435°C
Fig. 4-23 (a) FIB preparation for milled 120min specimen at 1435°C
Fig. 4-23 (b) FIB preparation for milled 120min specimen at 1435°C
Fig. 4-24 (a) Bright field image of specimen is milling for 2h and sintered at 1435°C
Fig. 4-24 (b) HRTEM Bright field image of specimen is milling for 2 h and sintered at 1435°C
Fig. 4-24 (c) SAED pattern of specimen is milling for 2 h and sintered at 1435°C
4.4.5 Preferred orientation plane by EBSD identification
EBSD is a very sensitive to crystal orientation tool to identify the preferred orientation plane for most materials. On the other hands, the grain size, boundary misorientation, phase identification and texture structure may be analyzed91-92. The analysis area was selected 200µmX300µm under SEM microscopic observation Fig 4-25. The band contrast map Fig 4-26 derived from Hough transform proved the specimen supporting the image quality factor in the EBSP. A digitized contrast of the Kikuchi band intensity is available for images show the microstructure in a qualitative simulation.
Fig. 4-25 High tilt angle at 70º for EBSD analysis
Data acquiring from HKL software processing to simulate the grain shape and orientation. The inverse pole figure Fig.4-27 based on the band contract configuration verify the plane direction by colour mapping. There is a plane direction in <0 0 1> has possibility density high than others. A well said on the textural orientation direction is from the triangle IPF at Y direction.
Euler map can describe the sample orientation relative to its specific orientation crystal. The contour pole figure in Fig 4-28 shows strongly textural component on {0 0 1} plane.93
Fig. 4-27 IPF mapping of milled specimen
Fig. 4-28 Euler Mapping of milled specimen
4.4.6 Microwave characteristic of milled Ba5Nb4O15
The mechanochemical behavior of a cation-deficient perovskite, Ba5Nb4O15, is investigated in the present study. The maximum quality factor can be achieved without mechanical treatment is higher than 40,000. After milling, the precipitated BaCO3 from water base slurry affect the ceramic to a density decrease, quality factor degradation at a sintering temperature 1435°C. Such excess substance is formed by the leaching of barium ion and reduction with carbonic ion. Through high angle X-ray diffraction verify the degree of ordering structure, (0 1 13) plane of Ba5Nb4O15 crystal structure declined. The quality factor shows a strong dependence on the mechanical treatment in the hexagonal perovskite structure.
In general, the majority variation factors on the permittivity are affected by internal pores or second phase of ceramic extrinsically. More porosity of Ba5Nb4O15 ceramic is produced via exhausting CO2 from precipitated BaCO3. The segregated composition tends to Ba-rich free energy reduce diffusivity at high temperature. The more gradient composition from precipitated BaCO3
effects sintering density. And the presence of sintered density decreased as increasing milling time.
Finally, sintered density determined the fluctuant permittivity of Ba5Nb4O15. It can be comprehended the permittivity value of milled specimens declined from 35 to 30. Less impurity came from milling media zirconia oxide did not affect the composition and temperature coefficient of resonated frequency still kept the same level. Table 4-3.
However, the variation study on quality factor of microwave ceramic strongly depend specific crystalline planes94, especially on the high Miller indices planes which are arranged within short range ordering structure. The Qxf quality factor sustains at highest value over 40,000 without mechanical treatment applied on Ba5Nb4O15. More mechanical treatment time transmits to shear
barium ions leave out the lattice site splitting H2O catalyzed by mechanical energy. The specific plane of Ba5Nb4O15 is hard to fully recover from the same thermal energy at sintering temperature 1435°C. It means the intrinsic defect energy and residual strain energy will reduce the ordering structure. The quality factor declined dramatically one and half of initial value and said lack of the (0 1 13) plane at 119.3° achieve to ordering structure.
High indices plane is always the trace plane to verify the quality factor performance of Ba5Nb4O15 ceramic. It can be achieved to high ordering structure via extreme driving force from temperature、time and pressure. Through mechanical treatment on it, the sintering energy limits the presence of specific ordering plane. The barium ions exit from collapse of (0 0 c) plane and create defect energy. The new interface of BaCO3-Ba5Nb4O15 provides a higher surface energy. Finally, the sintering profile can’t manipulate fully ordering plane as primary degree.
Table 4.3 Temperature coefficient of resonated frequency at different milling time
Milling time /
4.5 Conclusion
The present study demonstrates that the chemical interaction during milling plays an important role on the performance of ceramic products. The mechanical forces applied by the milling media (ZrO2 balls) generate a lot of fresh surface. More barium ion can then be released due to the increase of surface area. Precipitated BaCO3 is formed from the reaction with carbonic acid in water. The orientated microstructure indicates the preferred plane is (0 0 1)H which coincides to anisotropic growth direction. Excessive milling time may bring more barium ions leave out the lattice sites and exhibit the orientated growth. The microwave properties of Ba5Nb4O15 perovskite depend strongly on its density and crystalline structure. Measured permittivity can be predicted by volume ratio between pores and dielectric. The intensity of (0 1 13)H plane indicates the quality factor performance.
To avoid excessive milling time may benefit its microwave performance. Several implications can be drawn from the present study.
1. Chemical interaction during milling plays an important role on the performance of ceramic products. Though the solubility of ceramic in water is usually low, the release of a minute amount of ions during milling may affect the performance significantly.
2. Many barium-containing perovskites are widely used as microwave dielectrics, such as barium titanate, barium strontium titanate, etc. The cation ratio for these dielectrics affects their performance significantly. These barium-containing perovskites many also release barium ion during milling. It is worth noting such chemical aspect of the milling process
Chapter 5: Liquid Phase Sintering of Ba
5Nb
4O
155.1 Introduction
For the applications of LTCC technology95-96, the candidate material Ba5Nb4O15 is a hexagonal perovskite compound ceramic. It has been commercialized with additives (Table 5-1). Many studies on Ba5Nb4O15 material system focused on glass additives to reduce its sintering temperature through the assistance of liquid phase. The co-firing of dielectric and conductive metal was investigated commercialization before action. It was acknowledged that Ba5Nb4O15 was to be sintered at lower temperature by adding a small amount of B2O3, CuO or other kinds of additives97-98. The sintering temperature was dramatically reduced by 400oC by adding liquid phase sintering aids. Low firing Ba5Nb4O15 material can be used to cofire with silver electrode. To verify the silver migration and the degradation on electrical property, the amount of liquid phase is limited99.
Table 5-1 List of LTCC high permittivity microwave dielectric materials for LTCC
5.2 Experimental method
5.2.1 Raw material preparation
The synthesized Ba5Nb4O15 powder with stoichiometric mole ratio 5.0:2.0 was used to prepare.
For CuO 0.5~1.5wt% and/or B2O3 with purity higher than 99.9% are used. The media were 2mm yttrium stabilized zirconia balls. All ingredients mixed with distill water were pulverized by 220rpm planetary mill for 30min. The formulated powders were treated 0.75um.
To investigate co-firing with metal silver, the multilayered samples were prepared by the lab-scaled facilities, including casting machine, printing machine, cold isostatic pressure and cutting machine. A slurry was prepared by mixing with ceramic powders, PVB binder and toluene/alcohol solvent vehicles. The thickness of the green tape was 30um. Pure Ag paste were printed on ceramic foil, then stacked 3 layers in dimension of 3.6 mm length and 1.8 mm width.
To investigate co-firing with metal silver, the multilayered samples were prepared by the lab-scaled facilities, including casting machine, printing machine, cold isostatic pressure and cutting machine. A slurry was prepared by mixing with ceramic powders, PVB binder and toluene/alcohol solvent vehicles. The thickness of the green tape was 30um. Pure Ag paste were printed on ceramic foil, then stacked 3 layers in dimension of 3.6 mm length and 1.8 mm width.