Ceramic can be defined as solid compounds that are formed by the application of heat, and sometimes heat and pressure, combination at least one metal and a nonmetallic elemental solid or a nonmetal, comprising of at least two nonmetallic elemental solids, or a comprising of at least two nonmetallic elemental solids and a nonmetal (Barsoum, 1997).
With the advent of the technology and industrial revolution, the technology of ceramics had been improved widely; the ceramics that were not clay or silicate-based depended, but on much more sophisticated raw material, such as binary oxide, carbides, perovskites and even completely synthetic material for which there are not exits naturally. One of the advanced ceramic will be the electro-ceramic.
Electro-ceramic can be defined as the class of ceramic that mainly used in electrical properties. For example ferroelectric, ferrites, piezoelectric and solid electrolytes are the new technology using the electro-ceramic. The properties of the electro-ceramic depend on close control of the structure, composition, ceramic mixture, dopants and dopant distribution (Irvine, 1990).
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Basically electro-ceramic can be divided into two main groups:
Insulators (dielectric) and semiconductors, and
Conductors (ionic and mixed)
While the application of the ceramic in industrial can be divided into two categories:
The use for interconnection and packaging of semiconductor circuit, and
The use of ceramics in circuit components which perform a function in their own right, such as capacitors and sensors. (Whatmore, 1988).
Dielectric material is not a conductor; therefore it is used as an insulator in electronic application. The dielectric properties are different between each material, and the properties are influence by the temperature, frequency of applied field, humidity, crystal structure, and other external forces. Furthermore the respond can be either linear or nonlinear. (Barsoum, 1997)
Dielectric can be divided into four categories:
High permittivity dielectric, ranging 1000 to 25000,
Medium permittivity dielectric ranging 4 to 50,
Low permittivity dielectric between 4-50, and
Ultra-low permittivity dielectric, below 3
1.1 Scope of study
Recent development in microelectronic technology and the rapid development of telecommunication and radar are developed for kilometer-wave to centimeter-wave applications where large quantity of information could be transported with rapid speed. This application is important and play a crucial role in the intelligent transport systems (ITS), future generation of microwave integrated circuit (MIC), ultra high speed wireless LAN and satellite broadcasting. The important characteristics required for dielectric material used in millimeter-wave telecommunication system are:
High quality factor (Q x f) to achieve high selectivity,
Low dielectric constant (Ær) to increase the speed of electronic signal transmission,
Nearly zero temperature coefficient of resonant frequency (τf) for frequency stability. (Sugiyama, 2006)
Figure 1â€‘1: The electromagnetic spectrum (Glenn Research Center)
In this study, the boron trioxide (B2O3) is doped into the pure forsterite ceramic. The objectives of this study are:
To study the dielectric properties effect of boron trioxide toward the forsterite ceramic.
To determine the dielectric constant (Ær), quality factor (Qf) and temperature coefficient of resonance frequency (τf). Of the forsterite ceramic doped with boron trioxide
Dielectric material has played an important role in the wide range of application, in the communication sector such as satellite communication, GPS, and radio transmission. With the revolution in the mobile phone and the satellite communication system microwaves had been as the carrier. Each of the wireless communication technology demands materials have their own specialized requirements. Therefore, the research and development in the material has been become the biggest challenges.
The dielectric resonators (DRs) are an important component in the communication circuit. The current DRs, had replace the traditional microwave cavity resonator due to the factors of cost, dimension, mass, stability, efficiency, tenability, ease of use, ruggedness, and significantly high quality factor compare to traditional microwave cavities. (Sebastian, 2008) DRs able to create and filter frequency in oscillators, amplifiers and tuners. In microwave communication DR filter are used to discriminate between wanted and unwanted signal frequencies from the transmitted and received signals.
High Q minimizes the circuit insertion losses, suppresses the electrical noise in oscillator devices, therefore it is used in highly selective circuit. Density, Ær, Q and τf are all affected by the addition. The higher relative density will result in higher Ær. Furthermore, the properties of microwave ceramics also depend on the purity of the starting material, calcinations temperature and duration, shaping method and sintering temperature and duration. High Qf able to achieve when the ceramic is having the homogeneous microstructure and fine grain. Furthermore, in the microwave integrated circuit (MIC) there been lot of efforts have been made to develop new low as well as high dielectric constant materials for application in electronics industries. For substrate application, low dielectric constant is very important because it yields higher signal propagation velocity through a dielectric medium which is given by:
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Where is the wave length in the dielectric
λ0 is the wavelength in the vacuum
Low dielectric constant also will reduce inductive crosstalk and noise generation in the MIC. Low loss is another is another critical requirement for lightweight portable devices for long battery life. Quality factor Qf will be relating to the efficiency of use of power supplied to the device. (Yiping Guo, 2006)
2.1 Forsterite Crystallography
Forsterite with chemical formula Mg2SiO4 exist in the orthorhombic system with cell parameter a=4.756Çº, b=4.756Çº, c=5.981Çº, and z=4Çº. Çº stand for the angstrom is an unit of length equal to meters. The crystal will be in the form of ( 1 0 0) ( 0 1 0) ( 1 1 0) ( 1 1 1) ( 0 0 1) ( 0 1 1). Forsterite mainly contains with SiO44- anion and combines with 2 Mg2+ cation. The oxygen atom is bonded covalently to the silicon.
Figure 2â€‘1: Crystal form of forsterite (Forsterite Mineral Data)
Figure 2â€‘2: Crystal structure of forsterite (Forsterite Mineral Data)
Forsterite is a low dielectric constant (Ær=6.8), low loss ceramic, reported by (H.Ohsato, 2006). Forsterite (Mg2Sio4) is a member of olivine family, which is an important silicate. Basically, silicate are chemical compounds that containing Silicon (Si), oxygen(O), and one or more metals. Forsterite belongs to the subgroup nesosilicates (single tetrahedrons) whose structure produces stronger bonds and closer packing of ions, this will result a higher density, index of refraction and hardness than chemically similar silicate. Due to its low electrical conductivity, low dielectric constant, chemical stability, refractoriness and excellent insulation properties even in high temperature, therefore it is suitable as the substrate material in electronic. (T.S. Sasikala, 2008)
2.2 Microwave Dielectric Properties and Factors Affecting Them
2.2.1 Permittivity and Dielectric Constant
The dielectric properties of the various materials used in semiconductor fabrication and packaging play an important role in achieving the desired performance of integrated circuits. One important property of a dielectric material is its permittivity. Permittivity (ε) is a measure of the ability of a material to be polarized by an electric field. The dielectric constant (Ær) of a material is the ratio of its permittivity ε to the permittivity of vacuum εo, so Æ = ε/εo. The dielectric constant is therefore also known as the relative permittivity of the material. Since the dielectric constant is just a ratio of two similar quantities, it is dimensionless. Given its definition, the dielectric constant of vacuum is 1. Any material is able to polarize more than vacuum, so the k of a material is always > 1.
A low permittivity will result in a high signal propergation speed. When microwave enter a dielectric material there are slow down by a factor equal to the square root of the permittivity which implies that the wavelength decreases by the same amount and the frequency is unaffected. The ε is related to the refraction index, n by
Where n is the refractive index of the medium
2.2.2 Quality Factor Qf
The dielectric loss tangent (tan δ) of a material denotes the loss of electrical energy due to different physical process such as electrical conduction, dielectric relaxation, dielectric resonance and loss from non-linear process. The total dielectric loss is the sum of intrinsic and extrinsic losses. Intrinsic dielectric losses are the losses in the perfect crystals which depend on the crystal structure, ac field frequency and temperature. On the other hand, the extrinsic losses are associated with imperfections in the crystal lattice such as impurities, grain boundaries, microcrack, microstructure defects, porosity, random crystallite, order-disorder, dislocations, dopant atoms, vacancies etc.
The name quality factors is for the reciprocal of tan δ. The quality factor Qf of a resonator is defined as
Qf is a measure of the power loss of a microwave system. For the microwave resonator, losses are divided into 4 types: (a) dielectric (b) conduction (c) radiation and (d) external.
2.3 Journal Review
2.3.1 Preparation of sample
In this paper, the focus study will be on the forsterite ceramic. Pure forsterite ceramic able to be synthesis and exhibit with high Qf. In the paper reported by (T.Tsunooka, 2003) written that low permittivity and high-Q dielectric ceramic have become important, since ceramic substrate should have a low permittivity for the application of advanced substrate materials needed for microwave integrated circuit (MIC).
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By using high purity raw material powder, the pure forsterite ceramics (Mg2SiO4) are able to synthesis. One of the methods to prepare the pure forsterite ceramic is by using the high purity (99.9%) magnesium oxide powder (MgO) and silicon oxide (SiO2). The chemical were stoichiometrically weighed and were ball milled in the polyethylene bottle using zirconia balls in the distilled water or ethanol as the solvent and grind for 24 hours. The forsterite powder mixture also can be prepared by mixing other chemical power. As reported by (T.S. Sasikala, 2008) the Mg2SiO4 was prepared by mixing stoichiometric amounts of high purity [(MgCO3)4 . Mg(OH)2 . 5H2O] and SiO2 powder. Other than that Mg2SiO4 is also able to synthesis by using high purity chemical such as Mg(OH)2 and SiO4 powder weighed in stoichiometric ratio. (Joshi Sugihara, 2007)
The mixture will become wet slurries, and were dried as to remove the water or the ethanol solvent. Next the mixture was presintered at the temperature between 1150oC for 2 hours (Haikui Zhu, 2010) and 1200oC for 3 hours (K.X. Song, 2008).
After the calcinations, the substituting material for example the rutile (TiO2) was added to the calcined mixture attrition was carried out to reach a homogeneous granulometric distribution within the sample. (T.Tsunooka, 2003) Other than that (Haikui Zhu, 2010) also reported that the sample was prepared by standard double sintering ceramic method that containing two individual phases forsterite and cordierite (Mg2Al4Si5O18) . On the other hand, the paper from (Tomonori Sugiyama, 2006) and (K.X. Song, 2008) the substituting elements were mixed in stoichiometric ratio together with the forsterite.
The powder mixture was added with the polyvinyl alcohol (PVA), normally 1wt% of PVA will be added into the powder mixture. Next the sample powder was pelletized into cylinder compacts of 20mm diameter, 8-12mm thickness, using uniaxial of 30MPa and cold isostatic press (CIP) of 300MPa. The compact were sintered at temperature from 1200oC to 1600oC for 2 hours in air (T.Tsunooka, 2003)
The function of isostatic press is to forms and densifies powdered and cast materials using liquid or gas under extremely high pressure. Unlike mechanical force which compresses a workpiece from one or two sides, isostatic pressure is applied uniformly on all sides of an object, eliminating internal porosity without changing its net shape.
For (Haikui Zhu, 2010) the calcined forsterite powder were re-milled with different content of cordierite powder for 12 hours, mixed with 7wt% polyvinyl alcohol, uniaxially pressed under pressure of 100MPa to obtain a compact with 13mm diameter and thickness of 5-10mm. The compact was than sintered in air at different temperature for 2 hours.
In the experiment done by (Tomonori Sugiyama, 2006) the powder was pressed into cylindrical shape under a uniaxial pressure of 7.84MPa and CIP of 200MPa. The pellets were then sintered at 1400oC for 2 hours in air.
In the (K.X. Song, 2008) paper about the microwave dielectric characteristics of ceramics in Mg2SiO4-Zn2SiO4 system after the powder added with PVA 5wt% and palletized into the compact, it were firstly heated at 600oC in air for 3 hours as to expel the organic binder, and subsequently sintered at 1200oC -1500oC in air for 3 hours. While in the experiment of (T.S. Sasikala, 2008) after making the powder into cylindrical disks of 14mm diameter and about 1mm thickness, the compact were then fired at 600oC for 30 minutes only as to expel the PVA binder before sintering at a temperature in the range 950oC -1500oC.
2.3.2 Review of the current result
In the journal the effect of TiO2 on sinterability and dielectric properties of high-Q forsterite ceramic, (T.Tsunooka, 2003) found out that in X-ray Diffraction (XRD) analysis with TiO2 up to 30 wt.%, the rutile phase disappeared in all sample composite. This is due to the fact that TiO2 additive reacted with the MgO-SiO2 and did not remain as a separate component in the composite. For up to 10 wt.% TiO2 addition, only forsterite exists. The MgTi2O5 will appeared as the amount of TiO2 increased from 10 to 30 wt.%. Furthermore the TiO2 does not exist as a single phase until over 50 wt.%, therefore the sintering procedure needs to be considered in detail.
Figure 2â€‘3: The Relationship between (a) density, (b) dielectric constant and (c) QF sintering temperature of 2MgO.SiO2 modified with TiO2 addition
Figure 2â€‘4: The Relationship between (a) dielectric constant, (b) Qf and (c) the τf and τÆ and TiO2 content of of 2MgO.SiO2
Increase in dielectric constant is cause by the increase in density, and the Q value decrease gradually by addition of TiO2. The grain size increase with 0.5 to 5 wt. % of TiO2 , but when TiO2 additive from 5 to 30 wt. % the grain size will be decrease. Furthermore in order to prepare a pore free and glassy-phase free at the grain boundary, high purity material had to be used. To concluded addition of 0.5-5 wt.% TiO2 increased dielectric constants, did not significantly affect the Q value (Q value 10,000 at 16 GHz),but decreased the sintering temperature by about 100oC, and 1 wt.% of TiO2 exhibited a high Qf value=230,000 GHz with Ær=7.0 and τf=-65ppm/oC
Other than using conventional solid state ceramic route, (Joshi Sugihara, 2007) is using the liquid phase deposition method. Two types of PMMA powders with an average particle size of 5.0 and 1.5μm were used as pore-forming agent. Porosity increase with increasing PMMA content and 5.0μm PMMA particle will result a higher porosity compare with 1.5μm PMMA
Figure 2â€‘5: Variation of porosity by volume fraction of PMMA
Figure 2â€‘6: Variations of (a) amount of deposited TiO2, (b) dielectric constant εr and (c) temperature coefficient of resonant frequency τf (d) quality factor Qf as a function of LPD times
TiO2 weight and Ær increased with increasing number of LPD times. τf value improve from -68 to -46 ppm/oC. but after two times LDP, the τf value of the sample was not improve although the amount of deposited TiO2 increased. This result suggests that the amount of TiO2 filled in porous Mg2SiO4 or the TiO2 react with Mg2SiO4 during the treatment. Therefore filling the open porosity does not improve Qf value. To conclude, a small amount of TiO2 which is about 6.5 wt% has been filled using LPD method. τf value was improved from −68 to −46 ppm/-C by filling TiO2 with several times LPD process. (Joshi Sugihara, 2007)
Normally, the forsterite and its addictive were calcined in air, for example in the paper by (Tomonori Sugiyama, 2006), when the composition x in (1-x)Mg2SiO4-xCa2SiO4 is equal to one or more, it will be sintered in N2 gas. This is because SiO2 and MnO will not react to form the solid solution in air but the MnO will change to Mn2O3. This paper claimed that the largest dielectric polarizability of 6 coordinated of Mn2+ than Mg2+ will cause the Ær to increase, and suggest that in silicates with olivine structure τf maybe shifted to 0ppm/oC by adopting smaller cation.
In this experiment forsterite, Mg2SiO4 was used as base dielectric material. It was prepare by the conventional solid state ceramic route. High purity AR (Analytical Research) grade of chemical is used as to prepare a pore free and glassy-phase free at the grain boundary of the sample. Therefore high purity MgO (99.95%) and SiO2 (99.9%) were taken as the starting material to synthesis the forsterite. By following the equation
2 moles of MgO will react with 1mol of SiO2 and yields 1mol of forsterite (Mg2SiO4). The relative molecular mass (RMM) of MgO, SiO2 and Mg2SiO4 are 40.3044, 60.0843, and 140.6931.
The chemical are stoichiometrically weighed and were ball milled in polyethylene bottle using zirconia balls in distilled water for 24 hours. Next the slurry was dried at 100oC in hot air oven as to remove the water content, and calcined at 1200oC for 3 hours.
In this study, boron trioxide, B2O3 is used as the addictive. It is white, glassy, and solid. It is almost always found as the vitreous (amorphic) form; however, it can be crystallized after extensive annealing.
Figure 3â€‘7: Crystal structure of boron trioxide (Boron Trioxide)
The boron trioxide is become the additive in this study due to the fact of τf showed a tendency to shift more positive value with the decrease of cation ionic radius. In silicates with olivine structure τf maybe shifted to 0ppm/oC by adopting smaller cation as reported by (Tomonori Sugiyama, 2006). Boron is located at the top of group 3 in the periodic table, while the cation (3+) has the radius of 41pm (pico meter;) (Boron Element Facts).
Figure 3â€‘8: Periodic Table (The Periodic Table of Element)
The calcined powder were mixed according to the molar fraction (1-x) Mg2SiO4-xB2O3 and then re-milled for 24 hours. The fine powder mixture were added 1wt% of PVA organic binder, dry and ground well using mortar and pestle. It was then dried and pressed into pellets with dimension of 11mm in diameter and 5mm in thickness under a uni-axial pressure of 7.84MPa and cold isostatics press (CIP) 200MPa. The compacts were sintered at temperature from 1000oC to 1400oC for 2 hours.
The density was measured using Archimedes's method. The phase constitutions were studied using X-ray diffraction analysis (XRD) using Cu Kα radiation. The surface morphology was studied by scanning electron microscopy (SEM) with the back scattering electron images. The dielectric constant and quality factor values at microwave frequencies were measured in the TE011 mode using Hakki-Coleman dielectric resonator method. The temperature coefficient of resonant frequency τf was evaluated in the temperature range between 20oC to 80oC.
3.1 Measurement of Microwave Dielectric Properties (Hakki and Coleman method)
The permittivity of the DR is often measured by using Hakki and Coleman method, in which the disc sample is placed between two mathematically infinite conducting plates, as shown in the figure 3-3
Figure 3â€‘9: Schematic sketch of Courtney setup for measuring the dielectric (Chapter 2. Development of a LTCC dielectric material)
The TE011 mode is normally used to make the measurement. The dielectric puck diameter to height ratio should be about two to get wide mode separation; so that the TE011 mode is not disturbed by other adjacent modes. The resonant wavelength inside the DR and dielectric properties is given by:
Where l is the longitudinal variations of the field along the axis
L is the length of the DR,
D is the diameter of the DR
λ0 is the free space resonant wavelength
Jo (α) and J1 (α) are the Bessel function of first kind of order zero and one
Ko (β) and k1 (β) are the Bessel function of first kind of order zero and one
The parameters α and β depend on the geometry, the characteristic equation is a transcendental equation, therefore a graphical solution or numerical iteration to solve the equation is necessary. α can be obtain by the following equation
The real part of the permittivity can be calculated using the mode chart parameter (α1 and β1), resonant frequency (f) and the dimensions of the dielectric puck can be determine by the equation
S21 is denoted as the maximum amplitude
Where βc is coupling constant
QL is loaded quality factor
Qu is unloaded quality factor
S21 is maximum amplitude
3.1.2 Measurement of loss tangent
The quality factor measure by the Hakki and Coleman will be low since loss occurs due to the conducting plstes and radiation. Therefore, the loss tangent can be calculated using the following formula
Rs is the surface resistance of the conducting plates and is given by
Where σ is the conductivity of the conducting plates.
Qc is quality factor of conduction
μ: permeability for non-magnetic metal ()
Where λ0 is the resonant wavelength
λg =2L/l (l=1,2,3,…)
λg is guiding wavelength of an infinitely long dielectric rod waveguide.
W is the ratio of electric field energy stored outside to inside the rod.
3.1.3 Measurement of Temperature Coefficient of Resonant Frequency (τf)
The temperature coefficient of resonant frequency τf is the parameter which indicates the thermal stability of the resonator. In the microwave resonators device, it requires τf value as close to zero as possible to achieve thermal stability. The relationship is given by
Where τÆ is the temperature coefficient of the permittivity
αL is the linear thermal expansion coefficient of the dielectric material
Furthermore, the τf can also be calculated by plotting the variation of resonant frequency as a function of temperature.
τf is expressed as parts per million per degree Celsius (ppm/0C)