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SYNTHESIS AND CHARACTERIZATION OF LEAD- FREE PIEZOELECTRIC (K0.5Na0.5)NbO3 PRODUCED WITH IMPROVED CALCINATION TEMPERATURE

Potassium sodium niobate (KNN) is one of the lead free piezoelectric material that catch the attention of researchers and also those in the industrial field because of its stable piezoelectric responses and environmental friendly composition. The recent development of KNN shows that many methods have been used to synthesize the KNN but the stoichiometric Ka0.5Na0.5NbO3 composition is extremely difficult to consolidate. One of the ways to overcome the problem is to precisely pre-calcine and use milling process for proper homogenization followed by natural sintering process. In this paper, attempt has been made to synthesize the KNN produced at 750 oC calcination temperature. The results of 1060oC sintering temperature for 2 hours show a good composition of KNN with orthorhombic crystal structure where the final sintered pellet can reach to a relative density up to 87.13 %.


INTRODUCTION
Piezoelectric materials based on lead perovskite such as lead zirconium titanate (PZT) and sodium bismuth titanate (NBT) have been widely used for their piezoelectric and ferroelectric properties. The applications of piezoelectric materials include transducers, the devices which convert electrical energy into mechanical strain or vice versa, phonograph cartridges, microphones, speakers, audible alarms, and ultrasonic imaging [1]. Bhalla et al. (2017) describes that PZT ceramics are stiff and brittle variant which are used in many applications such as accelerometers, strain sensors, stress wave emitters and receivers, actuators, vibration sensors and pressure transducers [2,3]. However, the usage of lead-based piezoelectric ceramics is slowly being expelled from the industry owing to concerns regarding their toxicity as they contain more than 60% weight percentage of lead [4,5]. Moreover, Aksel and Jones (2010) add that the lead-based materials are not environmentally friendly due to high lead emission to the air during the materials synthesis [6,7,8]. The fact that the excessive lead release will cause harm to environment is also supported by Dahiyaa and Takura (2014) saying that the environmental issues and cases regarding human health caused by constant inhalation of lead particles were frequently associated with the common use of PZT based materials containing almost 60% weight percentage of lead which being released through lead evaporation to the air during the sintering process [9].
Based on the concerns of lead negative effects on the environment and health, research has been conducted to produce the lead-free based piezoelectric ceramics. Among numerous examples are piezoelectric materials based on alkaline niobate (Na0.5 K0.5) NbO3 abbreviated as KNN, which show the most promising results due to their relatively acceptable piezoelectric as well as ferroelectric properties and being compatible with the environment [10]. Ohbayashi and other researchers also characterized the alkali niobate KNN which exhibits fairly good piezoelectric characteristics and high Curie temperature [11][12][13][14][15][16].
The major problem related to alkaline niobate is during the synthesis and sintering process. According to the Barbara Malic et al. (2008), the stoichiometric (Na0.5 K0.5) NbO3 is extremely difficult to consolidate [17,18]. If the KNN can be properly synthesized and problems regarding obtaining the fine samples are resolved, the material will be useful in many applications such as ultrasonic motors, actuators, inkjet heads and transducers [11]. Ohbayashi (2016) also states that one of the ways to overcome the problems is by hot-press sintering method which is reported to further desrease the crystal grain size, increase the density of the ceramics from 4.25 to 4.46 g/cm 3 , and doubles the piezoelectric constant d33 from 80 to 160 pC/N [11]. The claim also being supported by Birol et al. (2005) by stating that the hot pressing can be an alternative solution to produced fine high densified samples [19]. Birol et al. (2005) again added that, although this method yields high densities and better properties compared to air sintered method, it still needs to carefully investigate and the sintering parameters need to be optimized to result in reproducible and high-quality ceramics [19].
Therefore, in the present work, potassium sodium niobate (KNN) has been synthesized at different temperatures following a reactive sintering process. The microstructural properties of KNN at calcination and sintering stage have been investigated and discussed in detail.

METHODOLOGY
K0.5 Na0.5NbO3 powders were synthesized by a conventional solid state reaction method [20,21]. 0.308 g Sodium carbonate (Na2CO3, 99 % purity, Merck), 0.401 g potassium carbonate (K2CO3, 99 % purity, Merck) and 1.54 g niobium oxide (Nb2O5, 99% purity, Sigma Aldrich, USA) were used as starting precursors. Initially, these three powders were dried separately in an oven at 200 o C for 2 hours [22] and then were weighed for each composition according to a stoichiometric formula as given by: The powders were instantly put into a 50 ml tungsten carbide jar that was initially filled with 40 ml acetone. Retsch PM100 planetary ball mill was used with 25 number of tungsten balls for mixing and wet milling within 3 hours at speed of 300 rpm. The slurry formed after wet milling was sieved and dried in convection oven at 120 °C for 2 hours to remove the acetone solution. The dried slurry was dry milled once again for 30 minutes to break down the hard agglomerates formed from the previous milling process. The powders were re-dried again at 120 °C for 1 hours in order to remove some moisture that was absorbed by the powders during previous milling process since the powders easily absorb moisture.
According to Mahesh and Pamu (2015), the best calcined temperature is 700 o C due to the highest dielectric constant and lower dielectric loss [26]. To increase the homogeneity of the composition, the calcination process in this research was carried out for temperature 650 o C and 750 o C. Both calcinations were carried out for 2 hours and 6 hours respectively in order to determine the best calcination temperature for the earliest formation of KNN compositions. The XRD spectra were obtained using Bruker D8 Advance after calcination of samples to determine the presence of earliest KNN phase as well as choosing the best samples of KNN formation for sintering process. The optimized sample was further analyzed using Hitachi (TM3030) SEM to observe the grain's microstructure [23]. The calcined material was formed into pellets using a custom-made mould and cold pressed with hydraulic press machine at 25 MPa. The pelletized green body of the chosen samples was sintered for 2 hours at different temperatures of 1060 °C and 1100 °C by using the heating rate of 5 °C/min.

X-ray Diffraction (XRD) of Calcined KNN Powders
The calcination was carried out at 650 o C and 750 o C for 2 hours and 6 hours for each temperature.   The KNN peaks can be observed from Figure 1(a) and 1(b) which is indicated by the KNN peak angles. Through comparison between the measured to the data peaks, it can be seen that the area under the peaks are broadened which reduced the sharpness of the graph. The broadened area represent the small size of grains with lower crystallinity, thus lead to the incomplete reaction. Noises can be seen from the graphs which represent the incomplete crystallization of the particles. As we know the good crystallinity will provide the better function of piezoelectric material.
The different colors in XRD pattern show the existence of other phases of KNN compound with different ratio and also non-KNN compounds. Table 1 shows the percentage of each compound existed after the calcination process. The existence of these multiphase of KNN is due to the incomplete reaction of the particles which probably was caused by the insufficient temperature.    For both samples, the other phase of KNN compound with chemical formula of (K0.1Na0.9)NbO3 is detected with 41% and 26.7% peak matching as tetragonal lattice structure. The non-KNN phase such as Sodium Niobate is appeared with the least scores. This indicates that the acetone molecule is not fully removed from the powders after the calcination process. However, the trace of acetone can be removed during the annealing process during sintering later.
The compound of (K0.65Na0.35)NbO3 obtained from all samples has a chemical composition near to the ideal (K0.50Na0.50)NbO3. Therefore, the sample that possessed the most score of this compound is the sample calcined at 750 °C for 2 hours. Hence, for the next step which is sintering process, 750 °C samples for 2 hours and 6 hours duration are chosen.

X-ray Diffraction (XRD) of Sintered KNN Pellets
The chosen calcined powders were cold pressed into pellets and sintered for 2 hours at two different temperatures (1060 °C and 1100 °C). Figures 3(a) and  3(b) show the XRD patterns of sintered KNN pellets at 1060 °C which have been calcined at 750 o C within 2 hours and 6 hours, respectively. While, Figures 4(a)  and 4(b) show the XRD patterns of KNN pellets sintered at 1100 °C which have been calcined at 750 o C for 2 hours and 6 hours, respectively.      Table 3 provides evidence that there is no existence of non-KNN phase for all sintered samples. Based on the results, samples that have been sintered for 1060 °C had chemical formula of (K0.507Na0.537) NbO3 and gives an orthorhombic lattice structure while the samples which have been sintered at 1100 °C had a monoclinic lattice structure. Based on the previous research, the ideal KNN has a chemical composition of (K0.5Na0.5) NbO3 and possess an orthorhombic lattice structure at room temperature (Yang et al., 2018). In crystallography, monoclinic and orthorhombic crystal structures belong to the same system where the orthorhombic has 3 mutually orthogonal axes while the monoclinic has only 2 axes that are perpendicular to each other. Therefore, resulting peaks very slightly shift in angle in XRD analysis. Based on the XRD results in Figure 3(a) and Figure 4(a), there are no new peaks which appeared, but the angle of reflection are very slightly shifted which represent two different crystal structures; orthorhombic and monoclinic. In this case, the samples sintered at 1060 °C have the closest structure with the ideal KNN piezoelectric material which possess similar orthorhombic lattice structure and the chemical composition is near (K0.5Na0.5) NbO3,

Density Measurement for Sintered KNN Pellets
The densities of sintered KNN pellets were calculated by measuring the height and the diameter of the pellet by using caliper. While the mass of the pellets was measured using an electronic beam balance. The percentage of densification can be determined by calculating the density of the pellets, before and after sintering which is tabulated as shown in Table 4.
From the observation, the densification is better at higher sintering temperature (1100 o C). This is due to the faster diffusion processes which homogenized the microstructure [24][25].

Scanning Electron Microscopy (SEM)
The images shown in Figures 5(a) and 5(b) indicate that the starting powders have still not completely reacted with each other. There are spaces between grain clusters which is a sign that the grains have already reacted to each other but at the same time there are still many spaces between the clusters and grains which indicate unreacted KNN grains. On the average, the grain size of the calcined powders is consisting of nano-sized particle which indicates that the particle has a potential to further react with each other which can only be achieved through sintering.  The SEM images that were taken after the sintering show that most of the grains were reacted with each other during the sintering process. The trace of after necking could be observed by noticing the slightly dark area at the smooth surfaces. From the images, it can also be observed that the growth of the grains and densification occurs in most cases, except a few places where porosity can be observed (Figures 6(a) to (d)). For the samples being sintered at 1060 °C, the results can be observed in Figure 6(a) for samples calcined for 2 hours and Figure 6(b) for samples calcined at 6 hours. In Figure 6(a), the signs of the grains reacted and fused with each other could be seen for most of the area in the figure, however there still remains some regions which are not completely sintered with the neighbouring grains. It can be assumed that the main cause is incomplete compaction during cold pressing, which prevents the grains to react with each other.
Besides the calcination time, the sintering temperature is also one of the factors that improves the grain growth of the KNN pellet samples which can be observed in Figure 6(c) for the samples calcined at 750 °C for 2 hours and sintered at 1100 °C.

CONCLUSION
It is observed that 750 C is the correct calcination temperature whereas 6 hours calcination time is the most appropriate as suitable calcination temperature for the solid-state synthesis of KNN. A structural analysis of the sample calcined at 750 °C for 6 hours possessed the most score of KNN compound with monoclinic crystal structure with a few secondary phases. Further sintering at 1060 °C for 2 hours shows the KNN with good stoichiometric composition of (K0.507Na0.537) NbO3 without secondary phase. The crystal structure obtained for this sample is orthorhombic. The sintering of KNN contributed to a higher density of 87.13 % densification. This is also evidenced by the SEM micrography.