Introduction

1. Introduction

The microwave dielectric ceramic materials are introduced into wireless communication industry for many decades. It exhibits attractive performance on high permittivity and quality factor.

It can be applied on oscillator, filter or antenna components designed in high frequency circuit^1-3. Additional point of application on endurance issue, those kinds of ceramic material exhibit temperature stability at resonate frequency. It can be near zero variance in extreme climate environment. The microwave dielectric materials were derived from a bunch of oxide ceramics.

They have an advantage of size reduction for microwave components. All of components size depend on the half or quarter-wavelength rule which shorten 1/K^1/2 (K: permittivity) length compared in free space. A novel application in miniaturized components must choose a high permittivity dielectric material which conserve energy with less dissipating at microwave frequency.

Many kinds of high permittivity oxide ceramics have been successfully commercialized. The microwave dielectric materials are usually observed in orthorhombic or complex perovskite structure such as Mg1-xCaxTiO3、BaTi4O9、Ba2(TixSn1-x)9O20、ZrxSn1-xTiO4、Ba(Zn1/3Ta2/3)O3、 Ba6-3xRE8+2xTi18O54. A candidate material in the A5B4O15 (A=Ba, Sr, Ca, Mg, Zn ; B=Nb, Ta) system is hexagonal perovskite structure. One kind of material such as Ba5Nb4O15 ceramics has been investigated for its high permittivity and quality factor in microwave frequency⁴. The merit of Ba5Nb4O15 ceramics is suitable good dielectric constant εr : 38~42 and high quality factor Q×f = 20,000~42,000.

Recently, many researchers approached on low firing Ba5Nb4O15 materials system which introduce glass additives to reduce sintering temperature through liquid phase assistance⁵. The application of technology aims on firing with silver metal together, the LTCC (Low temperature

Co-fired Ceramics), can put in use for passive component at high frequency circuit. Meanwhile studying on co-firing of the dielectric and conductive metal materials is needed. Such as shrinkage mismatch, silver migration and mechanical strength of ceramic body are always of interests for commercialized manufacturers. But the process effects on characteristics of Ba5Nb4O15 material are less reported in the past years. In this sense, studying on high temperature sintering of Ba5Nb4O15

ceramic is a rational solution. The abnormal grain growth and relative density contravene as a function of temperature saying de-sintering phenomena usually illustrated on reports⁶. There is little report on the de-sintering mechanism. However, an approach on measuring ceramic density as a function of temperature and microstructure observation is the basis route of study.

On the other hand, there are many routes to homogenize ingredients, milling process is the basic solid-state route to manufacture ceramic materials powders using intensive mechanical energy.

Regardless minor chemical impurity of raw materials, BaCO3 and Nb2O5 sources are recommended by high purity level. Several technique procedures for ceramic powder preparation, such as milling / drying and calcination process, have been investigated in the past few decades. Eventually, the characteristics of Ba5Nb4O15 strongly relate to effect of process parameters. In our studies, the de-sintering phenomena of Ba5Nb4O15 was observed and the mechanochemical characteristic along milling process. The investigated root cause of such properties can be analyzed quantitively and

Chapter 2: Literature Survey

2.1 Ba5Nb4O15 crystal structure

Most of microwave dielectrics materials with electrophysical properties are oxide ceramic systems⁷. One of them was surveyed on A5B4O15 (A=Ba,Sr,Mg,Ca ; B=Nb,Ta) oxide compounds were the cation deficient hexagonal perovskite structure which vacant existence on B ions lattice site. The formula can be reduced to AB0.8O3 by accommodated charge neutrality between A and B metal cations. In general, the ideal structures can be classified to cubic perovskite structure ABO3

that octahedral corners were shared by BO68-9. Compared to ideal cubic perovskite structure, it can be stacked along principle axes like layer-orientation perovskite structure. The stacking sequence are 1B cation vacancies per 5A cation means 20% of B cation site is occupied by vacancy and 5 layers stacking to be ABO3.

Ba5Nb4O15 compounds is classified to space group P-3m1 that can be close packing in five-layer sequences with shift type arranging the oxygen and barium atoms. Fig. 2-1. Ba5Nb4O15

belongs to cation deficient perovskite structure type that lattice size is a = 5.8065Å and c = 11.8227Å. It is the crystal data collection and refinement from X-ray analysis. These compounds are hexagonal symmetry and has been identified as a layered structure which stacking plane parallel to {111}c (c = cubic). On the corner of Ba5Nb4O15 lattice were structurally shared by NbO6 octahedral and with the barium atom in the center of the cube surrounded by the niobium atoms. According to Goldschmidt’s tolerance factor rule as equation (1)¹⁰, the stability of lattice structure strongly relates to the distance of cation-anion bond at equilibrium state.

t = d(Ba-O) / [2^1/2 ⤫d(Nb-O)] ………. (1)

Corner sharing of NbO6 octahedral can eliminate the repulsion force between cations, meanwhile, the cation-anion distances create stress in the lattice. The tolerance factor can be

identified to lattice structure, relaxation of bond energy will result in twisting of NbO6 octahedral.

For Ba4Nb5O15 the tolerance factor (γBa≈149pm, γNb≈78pm, γO≈120pm) is calculated to t >1, finally niobium ions are tilted out of the octahedral center of NbO6 to form hexagonal type perovskites structure. There is a strong anharmonicity distort the lattice structure from empty octahedral site of NbO6 sub-lattice. The reduction of symmetry can lower energy and distort geometrically by d-shell orbitals from Jahn-Teller theorem⁴².

The cation-deficient perovskite structure is attractive dielectric material applied to the microwave properties. A5B4O15 dielectric ceramics system¹¹ reveal high relative permittivity (εr) with good quality factor (Q) and low temperature coefficient at resonator frequency (τf).

Sreemoolanathan et al.¹² studied hexagonal structure of Ba5Nb4O15 microwave dielectric ceramic proved its εr = 39, Q×f = 24,000 and τf = 78 ppm/°C. The structure of crystal symmetry exhibits five layered close-packed planes with Ba, Nb and O atoms sequentially. There is one layer plane without Nb atom has hexagonal packing which belongs to cation deficient perovskite structure. The microwave permittivity of the Ba5Nb4O15 ceramics can be determined by the polarized phonon anomalies and related to density of ceramic body. The permittivity and quality factor can be varied by substituted atoms such as Sr or Ca atoms in A site to modify the symmetry of crystal. Many researchers on cation partial substitution on A5B54O15 crystal can tailor the microwave dielectric properties and sintering characteristic^13-14. As the composition of cations are strongly related to the stability of resonated frequency following temperature variation,

Fig 2-1. Crystal Structure of Ba5Nb4O15

P-3m1 No.=164 Hexagonal Perovskite a = 0.58065nm

c = 1.18227nm Density: 6.245 g/cm³

2.2 Homogenization of chemical composition with synthesis technology

Many synthetic processes can be applied on making ceramic products with different ingredients.

The merit of process always relies on the product performance and economical benefit for end user.

From the thermo-kinetics point of view, to shorten the synthesis time convert reactant to another substance, the driving force applied on homogenizing ingredients are physical length and temperature under atmosphere. Solid state process always is a simple and favorable technology to synthesis substance as following multistep process from mixing-drying-calcination-pulverization of powders in ceramic industry. Each process step derive from the principal of chemical physics theory will affect the final characteristics of substance. Such as the raw materials properties are chemical formula, particle size and impurity level, the mixing treatment for homogenizing compositions are physical energy of mechanical mixing technology and calcination technology of controlling temperature and atmosphere. The basic phase formation of Ba5Nb4O15 has been published on the Fig. 2-2¹⁵.

Simply statement of solid state process is so-call most of ingredients from powder forms and mixing by mechanical forces assisting with media. The mixed reactants will be homogenized by external force from temperature treatment. Their substances exhibit state of the art characteristics predictably. Most of interests for academic study do not focus on process research of synthesis technology. There are many troubles for industry application such as yield and degrading of characteristics withstanding severe environment. The Ba5Nb4O15 is the candidate of microwave dielectric material but the performance and some physical properties is not the best choice one. Only low fired material system of such kind dielectric material has been commercialized¹⁶.

No matter what kinds of ceramics material is existed, the product is prepared from solid state

There are aqueous or none aqueous conditions can play the role of lubricant and intermedia phase of ingredients engage reaction^17-18. This kind of processing route has been applied for functional ceramics compounds. The reaction temperature can be reduced by activated mechanical energy through chemical reactivity of raw material. The mechanochemical phenomena happened in many oxide ceramics effected on microstructure and physical properties. The mechanical energy can be generated by speed of rotation, prolonging time and media in raw materials. It was clear that crystalline size or particle size would be reduced when increase the mechanical energy. The mechanical energy leads to form a new surface with high strain on particles can be released under the solid state chemical reaction. The solid-state synthesis technology can be improved by this novel mechanochemical treatment to form Ba5Nb4O15 ceramic. The dielectric properties of sintered Ba5Nb4O15 ceramic can be compared to those prepared by different milling time in this study. In this stage, the vehicle plays another role of chemical ingredient to contact new free surface as a reactant.

There is more chemical gradient to enhance energy exchange during these processes contributed by mechanical activation. The purpose of the present study is to verify the mechanochemical effects of Ba5Nb4O15 ceramic materials on microwave dielectric properties and sintering characteristic.

Fig 2-2. Phase Diagram of BaO-Nb2O5

Fig 2-2. Phase Diagram of BaO-Nb2O515

2.3 Densification by liquid phase sintering

In general, the ceramic material sintering technique can be grouped two kinds process, solid state sintering and liquid phase sintering, used to control the powder densification. The purpose of sintering is microstructural control as to be dense body fitting material property or performance.

Kang¹⁹ illustrated the sintering route from phase diagram in Fig. 2-3. There are four kinds of sintering routes based on solid to liquid phase mass transportation. As a thermal dynamic equivalent diagram, the sintering temperature T1-T3 near to phase transformation line of different phases exhibits different sintering mechanism. Basically, the phase diagram of BaO-Nb2O5 oxide ceramic is proposed on attached Fig. 2-2¹⁵ and melting point of Ba5Nb4O15 is near to 1547°C. If sintering temperature of Ba5Nb4O15 under 1342°C confirmed to be solid state sintering but it will occur probably liquid phase sintering near the solidification line of a stoichiometric composition.

Fig 2-3. Illustration of various types of sintering routes¹⁹

Liquid phase sintering (LPS) is a well esatablished method to reduce sintering temperature accomplish powder densification in the presence of a liquid. There are several theories of LPS describe the densification stages such as German and Kingery model^20-22. Obviously microstructure development of sintered dense body gives an evidence of LPS process from identification of interfacial grains. To illustrate the abnormal grain growth phenomena at final stage of grain growth²³, it is controlled by diffusion of atoms and reaction of liquid/solid interface. The description proposed by Lifshitz, Slyozov and Wagner is named LSW theory^24-25. The assumption of LSW theory can’t explain the oriented grain growth. Faceted grains growth is governed by interface reaction that each crystallographic plane has certain diffusion coefficient varies with its orientation.

The behaviors of grain growth can be simulated numerically if it is growing or dissolving at a critical size. The grains growth is simplified as equations derived from two kinds of mechanisms.

Fig. 2-4 (a)~(b)

From the grain growth point of view, it is obviously diffusion or reaction controlled atoms dominate the grain. For grain growth rate on different crystallographic orientations based on the interfacial energy, the low interfacial energies appear slowly growing planes. Usually the simple polyhedral or anisotropic grain shape are belonged to reaction-controlled growth. On the other hand, a fast dissolving planes reveal a high interfacial energy for anisotropic grain growth.

Fig. 2-4 (a) variation of particle growth rate for diffusion-controlled growth¹⁹

.

Fig. 2-4 (b) variation of particle growth rate for reaction-controlled growth¹⁹

In the innovative technology by the presence of liquid phase, many kinds of sintering additives can play the role to enhance chemical reaction or increase diffusivity on the primary powders.

Usually the particular additives were selected from ingredients of phase diagram. All of varied compositions were examined the relative density after a firing procedure. Many researches on additives to lower sintering temperature of Ba5Nb4O15 material have been commercialized by LTCC technology^26-28. The mains liquid phase sintering additives such as B2O3, CuO and glass dopes can reduce the reaction free energy. The sintering temperature is under 900^oC that Ba5Nb4O15 cofire with silver metal. This system occurs to reduce the interfacial energy of chemical composition associate with matrix and solute composition. As a result, there is an activation energy for the total area of boundary to be reduced by solution and precipitation in liquid phase. The grain coarsening in the final stage of sintering accompanied was always verified from microstructure evolution.^29-30

2.4 Polarization of Ba5Nb4O15 dielectric properties under microwave frequency

The behavior of Ba5Nb4O15 microwave dielectric material has the macroscopic polarization which are formed by an inherently permanent dipole orientated dependence with the direction of the external electric field. The polarization is not instantaneous and need time to recovery the storage energy when release the applied field. Ba5Nb4O15 can exhibit a low dissipated energy when dipolar polarization lost the response to electric fields at the highest frequencies.

In the presence of an oscillated electric field, the dielectric materials are polarized to dipole moment in the filed act like clouds charge. A classical dielectric model at different frequency of electric field oscillation can be classified by the dielectric material with four types models. The polarization model is varied by the relationship between the frequency of electric field and dipole moment. When the frequency of electric field is higher, the dipole moment tends to be the model from ionic to electric polarization. The essence of model in physics are summarized to be (a) space charge (Maxwell Wagner effect): Interfacial polarization which build up the charge at the interface (b) dipolar polarization: A permanent dipole result from electric field (c) ionic polarization: relative displacement of nuclei due to separation of positive and negative field (d) electronic polarization:

arises from the realignment of electrons in the specific nuclei³¹. Fig. 2-5. In the electromagnetic spectrum range, microwave frequency extends the carrier wave range from 3MHz to 300GHz which correspond to wavelength between meter to millimeter scale respectively. The electromagnetic radiation wave will interact with condensed matter induce the vibration of lattice. The condensed matter will be polarized by electromagnetic wave like charge carriers are bound to relative displacements between positive and negative field.

Fig 2-5. Models of polarization

Microwave measurement techniques applied for permittivity and dielectric loss properties can be categorized to transmission-reflection and resonance methods^32-34. In generally, the resonance method was classified two types of resonate models in closed cavity and open resonator Fig.2-6.

This resonator model was constructed by conducting metal and lossy geometric structure which can be measured under microwave region. Due to microwave range energy is reduced by the conductive or magnetic material in the space, the incident EM wave propagation nearly reflected and the metallic case shields electromagnetic wave with preventing radiation loss. Both of the dielectric constant and loss factor can be deduced from the resonant frequency under a typical size. For industry application viewpoint, only easy and simple measurement method can be introduced to standardization procedure. Two most commonly used measurement methods are mainly introduced to microwave dielectric material applied to electromagnetic wave frequency under 40GHz.

Fig 2-6. Microwave characteristics measurement methods

2.4.1 Hakki-Coleman method

A general post-resonance technique which was similar to parallel metal plate proposed by B.W Hakki and P.D Coleman in 1960 for measurement of dielectric constant³⁵. The dielectric constant is computed the inductive and capacitive of reactance at resonate TE011 mode derived from cylindrical dielectric sample^36-37 Fig. 2-7. It is complicated calculations for mathematical function which derived a complex Besell’s equation to get the real part of permittivity by computer program. The measurement accuracy is higher that variation error under 0.5% and can be available to high permittivity material. Only one disadvantage of the method was low measurement accuracy of loss factor that the surface resistance of metal plates could affect the conductivity. To overcome the term of conductivity in the derived equation, two dielectric samples with TE011 and TE012 mode must be measured with the same material size. This method has been standardized under the regulation rule of JIS R1627 and IEC-61338-1-3.^38-39

Fig 2-7 (a) Hakki-Coleman resonance technique³⁶

Fig. 2-7 (b) Cylindrical samples for permittivity and quality factor measurement³⁷

2.4.2 Cavity resonate method

Whatever the development of electromagnetic field computation, for simplifying measuring structure, the dielectric resonator is designed as a cylindrical type. The rod sample is placed inside a closed metal cavity. The waveguide reflection resonance mode in the cavity can get a highly accurate measurement for dielectric loss which resultant low conducting loss. Due to the samples suspended inside the metal shields and all resonated radiation wave conserved in the case, the conductor loss will be lower than that of Hakki–Coleman technique.

J. Krupka⁴⁰ proposed a simplified cylindrical metal cavity which restrict size of the sample and height of resonator cavity for measuring the resonate peak with the TE01δ mode. The calculation equations for the dielectric properties are more easily derived than those of other modes. Referring to the cylindrical cavity structure in Fig. 2-8 (a)&(b), both of distance on top and bottom of sample to metal case are nearly equal. The TE01δ mode resonated peak is easily identified from sweeping spectrum but lack of exact solutions of Maxwell’s equation as same in parallel plate structure. The cavity structure family of dielectric resonator for precise measurement can be equipped with Vector Network Analyzer. It is installed specialized calculation software for determination of complex permittivity and quality factor of microwave dielectric material.

However, cavity resonant method can extract TE011 mode by the presence of a sample in the cylindrical cavity as Fig. 2-8. The plate type of dielectric sample cause to electric field perturbation when inserted in the metal cavity. That measurement method was committed to standardization regulation under the JIS R1641, IEC62562 (Plate)

Fig. 2-8 (a) Cylindrical cavity TE01δ resonance technique³⁶

Fig. 2-8 (b) TE011 mode Cavity resonance technique³⁷

Table 2-1Microwave dielectric measurement method accuracy comparison³⁷

2.4.3 Mathematical calculation of permittivity and quality factor

To standardize the permittivity and quality factor measurement method in commercialized regulation, the dielectric resonate method has been published on JIS R1627(1996) and IEC61338-1-3(1999). When electromagnetic wave propagates in the space to conserve the energy, the resonance phenomena occurs at the waves travel at different regions such metal and low-loss materials medium. The shape of resonator represents the boundary of wave function geometrically such as rectangular or cylindrical configurations. In general, a cylindrical type resonator is easily

To standardize the permittivity and quality factor measurement method in commercialized regulation, the dielectric resonate method has been published on JIS R1627(1996) and IEC61338-1-3(1999). When electromagnetic wave propagates in the space to conserve the energy, the resonance phenomena occurs at the waves travel at different regions such metal and low-loss materials medium. The shape of resonator represents the boundary of wave function geometrically such as rectangular or cylindrical configurations. In general, a cylindrical type resonator is easily