Effect of Zn Site for Ca Substitution on Optical and Microwave Dielectric Properties of ZnAl2O4 Thin Films by Sol Gel Method
( = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) thin films were prepared by a sol gel method. The XRD patterns displayed the characteristic peaks of (Ca/Zn)Al2O4 with the standard pattern of face-centred cubic (fcc). The addition of Ca decreased the lattice constant from 14.6 nm to 23.2 nm. The optical bandgap of undoped thin film was found to be at 3.84 eV while for doped Ca was observed at 3.50 to 3.73 eV. The substitution of Zn2+ by Ca2+ in ZnAl2O4 thin films was found to increase the crystallite size, grain size, and surface morphology which evidently affect the density and dielectric constant. The thin films were characterized at 20 to 1 MHZ frequency to determine the dielectric constant and unloaded quality factor using LCR spectrometer. It can be observed that specimen using Ca0.25Zn0.75Al2O4 possesses and which is suggested as a candidate material for millimetre-wave applications. Therefore, this ceramic is suggested as a candidate material for GPS patch antennas.
Microwave dielectric ceramics play important roles in the global telecommunications industry with varieties of applications that have been developed. The microwave dielectric ceramic is widely used for applications that operate in the range of microwave band frequencies between 300 MHz and 300 GHz [1, 2] such as wireless fax, cellular phones, global position satellite (GPS), military radar systems, intelligent transport system (ITS), and direct broadcast satellites. Nowadays, these applications have been operating in the range of millimeter wave due to miniaturization of devices especially as a patch antenna for GPS applications. The miniaturization of GPS patch antennas has become a primary issue in these few years.
Many researchers have tried to study the dielectric properties of ZnAl2O4 ceramics such as Wang et al. , Surendran et al. , Wu et al. , and Lei et al. [6–9]. They have found that the ZnAl2O4-based microwave dielectric ceramics have a very extensive prospect research for millimetre-wave applications and GPS antenna in the future with various dielectric properties. The values of the ZnAl2O4 were found in the range of 8.3  to 9.01 , while their unloaded quality factor () for ZnAl2O4 was around 4,590. These ceramics were prepared by conventional solid state  and aqueous gel casting  methods, which involved the high sintering temperature (above 1400°C). The sintering temperature above 1400°C cannot satisfy the requirements for the preparation of modern functional ceramics. Thus, the alternative method should be selected in these exploratory studies due to the fact that ZnAl2O4 has a large number of material types and phase compositions. Moreover, the preparation of ZnAl2O4 ceramic products typically involves heating processes of raw powders material which must undergo special handling in order to control purity, particle size, particle size distribution, and heterogeneity. These factors play important roles in the properties of the finished ZnAl2O4-based microwave dielectric ceramic.
Recently, there are many methods to prepare microwave dielectric ceramics such as solid state mixing reaction of zinc and aluminium oxides [11, 12], coprecipitation , hydrothermal [14, 15], microwave hydrothermal , combustion , and sol gel methods [18, 19]. Among the various methods, the sol gel method is a useful and attractive technique for the preparation of the ZnAl2O4 nanostructure because it offers the advantages of good stoichiometric control and is high homogeneity, producing pure and ultrafine powders and is able to produce thin films at low temperature processing (<900°C) [20, 21]. Furthermore, sol gel is being employed increasingly for the low cost fabrication of ordered high specification materials since its structural and morphological characteristics may be tuned in order to tailor the electrical properties of the material . Therefore, the sol gel method was selected in these exploratory studies of new ZnAl2O4 due to having a large number of material types and phase compositions.
In this study, the effects of substitution of Zn2+ by Ca2+ in the framework of ZnAl2O4 on the structural, morphological, and dielectric properties were investigated. The ( = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30) thin films were prepared by the sol gel method. The effects of the substitution were characterized using X-ray diffraction (XRD), Field-Emission Scanning Electron Microscope (FESEM), Energy-Dispersive X-Ray Spectroscopy (EDX), and ultraviolet visible (UV-Vis) and LCR spectrometer.
2. Experimental Details
Reagent-grade ceramic powders of Zinc acetate dehydrate Zn(O2CCH3)2, aluminium nitrate nonahydrate Al(NO3)3·9H2O, and calcium nitrate Ca(NO3)2 were selected as starting raw materials for zinc, aluminium, and calcium. The different molar ratios of ( = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were prepared by a sol gel method. Initially aluminium nitrate was dissolved in 60 mL absolute ethanol (C2H5OH) as solvent, followed by ethylene glycols (EG) with designated amount (0.5 mL) was introduced as the chelating agent in the solution. Then, calcium nitrate and zinc acetate were added to the solution. After being kept at around 75°C for 1 h, nitric acid (CA) with amount 0.36 mL is utilized for preparing homogeneous solution. The solution was continuously heated at 75°C with constant stirring on a magnetic stirrer for another 1 h until a clear solution had fromed following a chemical reaction. Finally, the solution containing is stirred for 0.5 h at 180°C in order to obtain transparent and stable sol.
Furthermore, the transparent and stable sol of would be prepared on the fluorine tin oxide (FTO) substrate by the spin coating technique at 3500 rpm/min for 35 s and treated at 85°C for 15 min. These coating procedures were repeated 10 times to get a result of good thickness and to remove the organic residuals. This technique was carried out in a clean room facility to avoid contamination of particles and to obtain films with good optical quality. The thin films would be kept drying in the air and subsequently were annealed at 500°C for 1 h in air to eliminate the remaining organic matter.
The thin films were characterized by means of the X-ray diffraction (XRD, Siemens D-500). For Rietveld analysis, the XRD pattern was collected over the range from 20° to 60°, with both selected step scan and step width of 0.0250°. The structural morphology of thin films was then observed by using a Field-Emission Scanning Electron Microscope (FESEM, Zeisz Supra 15 KV) and Energy-Dispersive X-Ray Spectroscopy (EDX), respectively. In the meantime, the optical properties of thin films were examined through recording the absorption spectra using ultra-violet (UV-Vis) double beam spectrophotometer, in the wavelength range of 200–800 nm. The and unloaded quality factor values were measured using an LCR spectrometer (HP 4284A) at frequency range 20 Hz to 1 MHz at room temperature.
3. Results and Discussion
3.1. Structural Analysis
Figure 1 shows XRD patterns of the ( = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) thin films annealed at 500°C for 1 h. It can be seen that the observed diffraction peaks of all samples are corresponding to the standard pattern of face-centred cubic (fcc) ZnAl2O4 with dominant peak (311) at 38.66°. The characteristic peaks of the undoped and (Ca/Zn)Al2O4 nanostructure were noticed according to the JCPDS files 00-005-0669 and 01-087-0426, respectively. All the observed peaks of ZnAl2O4 appearing at 2 = 30.72°, 35.65°, 45.36°, 48.64°, 55.25°, and 58.84° can be indexed as (220), (311), (400), (331), (422), and (511) crystal planes, respectively. The observed diffraction peaks of ZnAl2O4 result are consistent with those reported by Abdullah et al. . The peaks correspond to (Ca/Zn)Al2O4 which appeared at 29.375° (422). This peak appeared clearly when the amount of Ca had been increased more than = 0.15. The XRD analyses suggested that the increasing amount of Ca led to the increasing intensity of the diffraction peaks of (Ca/Zn)Al2O4 . Figure 2 also shows the peaks in the XRD spectra shift to a higher angle implying the formation of (Ca/Zn)Al2O4. The diffraction peaks shifted, perhaps because the ionic radius of Ca2+ (0.99 nm) is larger than Zn2+ (0.74 nm) . The shift of XRD diffraction peaks indicates that the lattice parameter (a) was changed.
The crystallite size and lattice parameter of the can be calculated from the XRD data. The crystallite size was calculated from the dominant peak (311) of XRD diffraction lines based on Scherrer’s equation using the following hkl peak : where is the X-ray wavelength (0.15418 nm) and is the full width at half-maximum intensity of the diffraction line. The crystallite size of ZnAl2O4 is 14.6 nm. The crystallite size increases linearly (15.4 to 23.2 nm) as Ca increases. The increment in crystallite size perhaps was affected by the ionic radii of Zn2+ (0.74 nm)  smaller than Ca2+ (0.99 nm) .
The calculated lattice parameters upon the Ca2+ substitution of Zn2+ were decreased from 8.085 Å to 8.026 Å due to XRD spectra shift to a higher (right) angle. The experimental lattice parameters of ZnAl2O4 (8.085 Å) are in good agreement with the reported value = 8.083 Å to 8.095 Å [16, 27], theoretical values (8.05 Å), and JCPDS files 00-005-0669. Normally, the decreases of lattice parameter are able to increase the density.
3.2. Morphological Analysis
The microstructural changes in ceramics were studied using SEM and EDX to observe the effects of calcium (Ca) content on the morphologies of . The SEM micrographs of thin films are shown in Figure 3. We can see that, in general, all the compositions were well annealed, and the average grain sizes tended to increase. The average grain size of ZnAl2O4 is about 37 nm while for doped Ca was found to increase up to 78 nm. The increase in grain size was observed with the increasing content of Ca. The increases in grain size eliminated the porosity. Thus, we suggested that the porosity was eliminated due to grain growth and in line with the increasing of Ca content. It indicates that the bigger grains are Ca and the smaller are ZnAl2O4. Normally, the increases in ceramics grain size considerably improved on the quality factor and dielectric constant [8, 10, 28].
The atomic force microscopy (AFM) was used to analyze the morphology of the surface (Figure 4) with scanning size of 1 μm 1 μm. The average roughness of ZnAl2O4 thin films is about 14.9 nm. As increases, the average roughness increases linearly from 21.5 ( = 0.05) to 94.7 nm ( = 0.30). The doped samples were identified as rough surface compared to ZnAl2O4. The increment of average roughness and grain size data was parallel with the increment of the crystallite size in XRD data. It is suggested that the increase of surface roughness can promote the increment of dielectric constant ().
3.3. Optical Analysis
The absorption spectra of particles in transmission mode were recorded by dispersing the particles uniformly in liquid paraffin, in the wavelength range of 200–800 nm. For a direct bandgap semiconductor material the absorption coefficient near the band edge is given by  where is the absorption coefficient, is the photon energy, is the energy gap and is constant depending on the type of transition. Equation (2) for any energy can be rearranged and written in the form of From these equations, it is clear when and . The energy gap is determined by plotting against and finding the intercept on the axis by extrapolating the plot to as shown in Figure 5.
The bandgap energy () of ZnAl2O4 was observed at 3.84 eV ( = 0.00). As increases, the optical decreases from 3.73 eV ( = 0.05) to 3.50 eV ( = 0.25). The optical bandgap analyses suggested that the increasing amount of Ca led to the decreasing optical due to the fact that calcium bandgap (3.2 eV)  is smaller than zinc (3.84 eV) [18, 29]. From this experiment, the suggested bandgap of (Ca/Zn)Al2O4 is between 3.50 eV and 3.73 eV which is applicable as for GPS application .
3.4. Microwave Dielectric Properties
The variation of the dielectric constant () of with applied frequency from 20 Hz to 1 MHz is plotted in Figure 6. It can be observed that there is a steady decrease in in applied frequency for each concentration. The dielectric constant () of pure ZnAl2O4 is 8.56, while that for Ca-doped is increased from 8.81 to 10.98. The value of ZnAl2O4 (8.56) is in line with that previously reported . The increment is expected since the dielectric constant of Ca is , which is larger than ZnAl2O4 [6, 10]. It suggested that the combination of ZnAl2O4 with Ca is able to increase the values more than 8.52. Furthermore, the largest values of for doped samples are indicating that the ion polarizability of the ZnAl2O4 is higher than that of the Ca2+. Thus, the dielectric constant () of the ceramics depended on ionic polarizability () of cations and oxygen at microwave frequency . Normally, the changing of values follows the logarithmic mixing rule , as follows: where and are the dielectric constants of phases with volumes and .
Table 1 demonstrates the microwave dielectric properties of thin films annealed at 500°C for 1 h. The densities and dielectric constants increased with increases. Figure 7 illustrates the densities and dielectric constant of the specimens as a function of Ca concentration. With the increase of Ca concentration, the density and dielectric constant were found to increase up to 4.71 g/cm3 and 10.98, respectively. The increases in ceramics density and dielectric constant were due to the increment of grain size (Figure 3). Figure 7 also shows that the variation of the value was consistent with that of density. We suggest that the of ceramics is mainly controlled by apparent density.
Table 1 also shows the unloaded quality factor of the ceramics with different degrees of Ca2+ substitution. The is determined from the materials dielectric loss using LCR spectrometer and was characterized at 20 to 1 MHz frequency using a two-terminal method. This technique is based on the measurement of the dielectric loss constants as a function of frequency of a sample sandwiched between two electrodes . The was found to increase linearly from 4590 ( = 0.00) to 5770 ( = 0.25) and decreased thereafter at = 0.30, by increasing the calcium content as shown in Figure 8. This increase of was attributed to the better density and grain growth of the ceramics with a minor Ca addition. The best obtained was 5770 for Ca content = 0.25. The degraded at = 0.30 may be related mostly to the pores structure and grain growth gradually as shown in Figure 3(g). However, Lei et al.  and Huang et al.  reported that the decrease of is not only mainly caused by the grain size and pores structure but also affected by intrinsic and extrinsic losses such as the secondary phases, impurities, or the lattice defect. The unloaded quality factor of ZnAl2O4 is in line with that previously reported by Surendran et al. . Therefore, we suggest that the relationship between the and the value was consistent with the apparent density and dielectric constant with that of the value. In summary, the minor Zn substitution by Ca improved the microwave dielectric properties of the ZnAl2O4 microwave dielectric ceramics. The best composition was Ca0.25Zn0.75Al2O4 with microwave dielectric properties of = 10.41 and = 5770. Therefore we consider that the CaZnAl2O4 ceramics have a low dielectric constant () and high unloaded quality factor and, in particular, are a suitable candidate material for millimeter-wave applications and GPS patch antenna.
thin films were synthesized by sol-gel method, and their optical and microwave dielectric properties were investigated as a function of Co content. The XRD patterns confirmed that the formation of ZnAl2O4 and CaZnAl2O4 phase was successfully observed. The XRD data showed that diffraction peaks of all samples were corresponding to the standard pattern of face-centred cubic (fcc). The XRD and SEM indicated that the particle size increased with the increases of calcium. Adding Ca2+ by substituting the Zn2+ site with different concentration increased the grain size, crystallite size, density, surface roughness, dielectric constant, and unloaded quality factor. However, the bandgap energy () and lattice parameter of thin films were found to decrease from 3.84 to 3.50 eV and 8.085 to 8.026 Å, respectively, as Ca increased. The specimen with a good combination of and a = 5770 was obtained for Ca0.25Zn0.75Al2O4 specimen. Therefore, the composition of CaZnAl2O4 is considered as suitable material for millimetre-wave application and GPS patch antenna.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This project is carried out in Photonic Technology Laboratory, Institute of Microengineering and Nanoelectronics (IMEN), Universiti Kebangsaan Malaysia, and Department of Physics, Faculty of Science and Technology (FST), Universiti Putra Malaysia (UPM), Malaysia.
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