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Forsterite (Mg2SiO4) ceramics were synthesized by the conventional solid state ceramic route. The effect of titanium dioxide as an addition on dielectric properties and microstructure were investigated. The crystal structure and the microstructures of the forsterite were studied by scanning electron microscopic (SEM) techniques and X-ray diffraction (XRD). The dielectric properties of the sintered samples were measured in the range of 1 MHz to 1.8 GHz. By adding 25 wt.% TiO2 a dielectric constant, Ær value of 4.5, dielectric loss tangent, tan δ of 1.75 10-3and Qf value of 500 Hz was recorded when the properties is measured at 900 MHz. Forsterite ceramic with TiO2 indicate a good combination, the properties is even better when it is measure and used in millimeter-wave application..
Keywords: Forsterite ceramic Dielectric properties; Microstructure.
The rapid development of microelectronic, and wireless telecommunication technology such as 3G, 4G, Worldwide Interoperability for Microwave (WIMAX), and Long Term Evolution (LTE). Therefore, dielectric material is playing a crucial role in the future generation of microwave integrated circuit, and ultra high speed wireless communication network. High qualiy factor (QF) to achieve high selectivity, low dielectric constant (Ær) to increase the speed of electronic signal transmission, and low loss tangent (tan δ) as to decrease the dissipation of the electrical energy due to different physical processes is strongly require for microwave application.
Forsterite (Mg2SiO4) having low dielectric constant, and low loss ceramic . Forsterite is a member of olivine family crystal, and forsterite belongs to subgroup nesosilicates (single tetrahedrons) . Forsterite is suitable to used as an electronic material because of low dielectric constant, low conductivity, and chemical stability. Titanium dioxide, TiO2 in natural it is crystalline, and exits in different kind of crystal structure such as rutile, anatase, and brookite. Therefore titanium dioxide is polymorphous. In this study, rutile had been choosing as an additive. Rutile has a tetragonal crystal lattice system with unit cell a = b = 4.587Çº c = 2.953 Çº . In the present paper, the dielectric properties and microstructures of forsterite ceramic, Mg2SiO4 added with rutile TiO2 were investigated.
2. Experimental procedure
Mg2SiO4 was used as the main dielectric material, high purity AR grade MgO and SiO2 were weighed in stoichiometric ratios and mixed and ball milled using zirconia balls in the distilled water as the solvent and grind for 24 hours. The slurry was dried at 100oC in hot air oven as to remove the water content, and presintered at the temperature at 1200oC for 3 hours. After calcinations TiO2 was added to the calcined mixture and a second attrition was carried out according to the molar fraction (1-x) Mg2SiO4-xTiO2. The mixture of powder is being ball-milled for 24 hours and ground well using mortar and pestle. The powder mixture was pelletized into cylindrical compacts of 10 mm in diameter and 1.6 mm in thickness under uni-axial pressure of 200 MPa. Next the green compacts were sintered at temperature 1200oC for 2 hours.
The density was measured by using Archimedes's method. The phase constitutions were studied using X-ray diffraction analysis (Lab X XRD-6000 XRD) using Cu Kα radiation. The surface morphology was studied by scanning electron microscopy (SEM). The dielectric constant, loss tangent, and Qf value were measured by using Agilent 4291B Material Impedance/Material Analyzer. The dielectric properties were evaluated at the frequency range of 1 MHz to 1.8 GHz.
Results and Discussion
In fig. 1 and fig. 2 is the XRD pattern of pure Mg2SiO4 and it composite with 25% TiO2 and 30% TiO2. The XRD pattern indicates the presence of TiO2 in the forsterite ceramic. There was no significant variation spotted between the XRD patterns of 25% rutile and 30% of rutile in the ceramic system. The XRD showed two phase system with a Mg2SiO4 phase, in association with TiO2 phase. Other than that the Mg2SiO4 will partial react with TiO2 and form MgTi2O5. This phase will be increase as the amount of TiO2 increases. Basically the forsterite major phase combined with Mg2TiO4, MgTi2O5, and MgSiO3 secondary phase. MgSiO3 secondary phase, which usually appears in Mg2SiO4 ceramics, could be suppressed by Ti-substitution for Si, while alternative secondary phases such as Mg2TiO4 and MgTi2O5 appeared gradually with increasing the Ti-substitution amount 
Fig. 1. XRD pattern of forsterite ceramic with 25 wt.% of TiO2.
Fig. 2. XRD pattern of forsterite ceramic with 30% of TiO2
In fig. 3 and fig. 4 show the SEM micrograph of forsterite with 25% of TiO2 sintered at 1200oC for 2 hours. The SEM images of forsterite ceramic with 25 wt.% TiO2 and 30% TiO2 ceramic system sintered at 1200oC for 2 hours are demonstrated in Figs 4-3 and 4-4. The grain morphology of forsterite ceramic with titanium dioxide can be group into 2 groups, the shining parts were TiO2, and the dark gray particle was Mg2SiO4. In SEM images, it indicates that the grains of the forsterite and the titanium dioxide are still under growing condition, porous microstructures were observed at 12000C. In order to obtain a bigger grain size, a higher temperature of sintering is required.
Fig. 3. SEM micrograph of forsterite with 25 wt% of TiO2 sintered at 1200oC for 2 hours.
Fig. 3. SEM micrograph of forsterite with 30 wt% of TiO2 sintered at 1200oC for 2 hours
The density of the ceramic is measure by using Archimedes's method. The data was listed in Table 1
Table 1: The density of forsterite with different weight percentage of titanium dioxide
From Table 1 it had shown that the density of the forsterite ceramic is increasing with increasing of TiO2 additive. These results indicate that the particle of forsterite and TiO2 is packed more closely with 30% of TiO2 at the sintering temperature of 12000C.
In fig. 5 it had shown the variation of dielectric constant of forsterite ceramic with 25 wt.% and 30% of rutile, sintered at 1200oC, from 1 MHz to 1.8 GHz. The dielectric constant Ær of forsterite ceramic with 30% of rutile is having a high permittivity than the forsterite ceramic with 25 wt.% of rutile. The dielectric constant of both ceramic with 25 wt.% rutile and 30 wt.% rutile is having a constant value in the region of 10 MHz to 1 GHz.
Fig. 5. Dielectric constant of forsterite ceramic added 25 wt.% and 30 wt.% of rutile, TiO2, sintered at 1200oC
The value of dielectric loss in forsterite ceramic with 25 wt.% of rutile is generally higher than forsterite ceramic with 30 wt.% of rutile. The dielectric loss is gradually decreasing when the frequency is increasing, from 1 MHz to 100 Mhz. Start from 100 MHz onward, the loss tangent of 25 wt.% and 30 wt.% of rutile nearly having the same dielectric loss value, the different is not significant. at the frequency of 900 MHz, forsterite ceramic with 25 wt.% of rutile exhibited tan δ= , while 30 wt.% is having tan δ=
Fig. 6. Dielectric loss of forsterite ceramic added 25% and 30% of TiO2 sintered at 1200oC
The value of dielectric loss is sensitive to humidity. Therefore, the experiment had to be done at air conditioning room, where the humidity of the environment is able to control. In order to obtain a more accurate result, the sample is place at oven and heated at the temperature of 100oC for two minutes to remove is moisture in the sample before testing. Furthermore, dielectric losses will occur when the charge distribute in crystal is deviates from the perfect periodicity . This bring out the fact that forsterite ceramic with 25 wt.% of TiO2 is having the ions distributed more disorderly as compare with 30 wt.% TiO2.
Fig. 7 shows the quality factor of forsterite ceramic with 25 wt.% of TiO2 is higher than the 30 wt% TiO2 throughout the frequency from 1MHz to 1GHz. At 900 MHz forsterite with 25 wt.% of forsterite exhibited Qf =500Hz, while forsterite with 30 wt.% having the value Qf =300Hz. This result can be concluding that the Qf value will be decrease by adding more TiO2. On the other hand in, the value of quality factor both 25 wt.% and 30 wt.% is increasing with increasing of the measure frequency. The Qf value is increasing significantly start from 100 MHz. In the measured Qf is affected by the conductor and radiation losses. These effects can be avoided by using the cavity method in which the sample is placed on a low loss single crystal quartz or Teflon spacer inside the cavity .
Fig. 7. Quality factor of forsterite ceramic added 25% and 30% of TiO2 sintered at 1200oC
Table 2: Ær, tan δ, Qf for forsterite ceramic with 25 wt.% and 30 wt.% of TiO2 measuring at frequency 900 MHz
The dielectric properties of forsterite ceramic with TiO2 additive exhibited low dielectric constant. Addition amount of TiO2 will increase the dielectric constant, due to the increases in density. The dielectric constant is proportional to the density of the ceramic.
Forsterite ceramic with 25 wt.% exhibits Ær = 4.5, while 30 wt.% exhibits Ær = 5.3. Low Ær is desired because of no need for miniaturization and able to reduce the time delay of signal.
Addition of TiO2 does not significantly affect the Qf value, while measuring the frequency range of 1 MHz to 1.8 GHz, but there is a slide decreasing Qf value observed when the amount of TiO2 added is increasing.
The value of dielectric loss in forsterite ceramic with 25 wt.% of rutile is generally higher than forsterite ceramic with 30 wt.% of rutile. Mean while the dielectric loss in the forsterite ceramic is decreasing when the measure frequency is increasing. The dielectric loss of forsterite ceramic with 25 wt.% TiO2 is 1.75 while 30 wt.% is