Low-calcination temperatures of magnesia partially stabilized zirconia (Mg-PSZ) nanoparticles derived from local zirconium silicates

Partially stabilized zirconia (PSZ) exhibits excellent physical, mechanical, electrical, chemical, thermal, and bioactive properties. Therefore, it is frequently used as a material for thermal barrier coatings, refractories, oxygen-permeating membranes, dental and bone implants. In this study, magnesia-partially stabilized zirconia nanoparticles were successfully prepared from zirconium silicates and MgSO4 assisted with PEG-6000 via a facile templating method. The MgO concentration was varied from 1%–10% in wt% of ZrO2. Zirconium silicates were initially converted to Zr-precursor solution, exhibiting pH 3. Then, the appropriate amount of the Mg-precursor was mixed with the proper amount of the Zr-precursor solution. A 10%(w/v) PEG-6000 solution was added into the PSZ precursor solution at a ratio of the precursor-to-PEG volumes of about 15:1 under stirring and heating, resulting in a very fine white gel. The gel was filtered, dried, and then calcined at elevated temperatures of 600, 800, and 1000 °C. The characteristics of the final product were then evaluated. According to the experimental results, the MgO concentration influences the ZrO2 phase transformation at elevated calcination temperatures. In this study, the lower the MgO dopant concentration added into ZrO2, the more stable the t- ZrO2 phase in PSZ samples at high temperatures. However, the MgO presence is detected as periclase in all samples with a very low peak intensity at elevated calcination temperatures. The obtained PSZ samples consist of nanoparticles and high agglomeration, some of particles exhibit elongated and rod-like shapes. The PEG existence during the PSZ preparation has restrained particle interaction and aggregation of the as-synthesized PSZ samples, leading to PSZ nanoparticles evolution.

Particularly, PSZ comprises of two or more different phases of ZrO 2 . It can be mixtures of the t-, the c-, and the m-ZrO 2 . The stabilized ZrO 2 is usually obtained with the addition of the common dopants such as yttria (Y 2 O 3 ) and magnesia (MgO) into the ZrO 2 lattice [6,19,20]. Yttria-partially stabilized zirconia (Y-PSZ) exhibits more superior properties than magnesia-partially stabilized zirconia (Mg-PSZ). However, it is relatively quite expensive. As comparison to YSZ, previous studies showed that Mg-PSZ revealed promising characteristics in some aspects, namely better mechanical and thermal properties [14], good stability in low-temperature degradation (LTD) of ZrO 2 ceramics [9], and a comparable thermal-expansion coefficient [20]. Hence, Mg-PSZ offers a promising and low-cost sophisticated material for advanced applications.
Many methods have been proposed in the synthesis of Mg-PSZ, including a conventional solid-state reaction [1,28], precipitation and co-precipitation techniques [5,29,30], a polymeric precursor route [31], sol-gel processing [26,[32][33][34], Pechini method [33,35], sugar technique [36], plasma spray synthesis [33,37], electrospinning [38], and microwave heating technique [39]. Nevertheless, most of these methods use high purity grade chemicals such as salt and alkoxide precursors and may require specific design instruments, resulting in high cost and less economic values. For instance, the usage of a local raw material like zircon sand as ZrO 2 precursor, is suggested and shoud reduce the cost in the preparation of Mg-PSZ. Besides, to the the best of our knowledge, there was only one paper that reported the preparation of Mg-PSZ from zirconium silicates. Thus, in other words, the studies on the synthesis of Mg-PSZ using zirconium silicate as the Zr-precursor are still hardly ever reported.
Quadling et al used zircon sand to prepare fused Mg-PSZ by a carbothermal reduction method then followed by melting process with pure magnesia [40]. However, they reported that the final product consisted of some secondary phases in the Mg-PSZ aggregates because of the impurity's interaction of the reduced zircon sand with MgO [40]. In the present paper, we report the preparation of Mg-PSZ nanoparticles from local zircon assisted with PEG-6000 as a polymer template through a facile templating method.
Our previous study successfully synthesized Mg-PSZ nanoparticles from salt precursors via sugar technique [36]. The organic compound was used as a masking template to reduce the tendency toward agglomeration of the as-synthesized particles, leading to the formation of nano-sized Mg-PSZ [36,41]. Another organic compound that can be used as a masking template or a dispersant is an organic polymer, such as polyethylene glycol, for instance. Apart from being a dispersing agent, polyethylene glycol can also be employed as a onedimensional structure directing template [42][43][44]. PEG comprises of the oxyethylene groups ) in its chain structure which is terminated by the hydroxyl groups at the edges. These functional groups facilitate PEG to interact with the other molecules consisting of the hydroxyl groups through the hydrogen bonding. The interaction between PEG and the other molecules such as metal hydroxides would retard the reaction of among metal hydroxide molecules, resulting in the dispersed particles and slower particle growth. This phenomenon is expected leading to the nanoparticle evolution. At high temperatures, the PEG decomposes and turns into volatile matters, leaving zirconia particles in the nanostructures. Thus, the objective of this study is to prepare Mg-PSZ nanoparticles from local zircon (ZrSiO 4 ) assisted with PEG-6000 as a masking polymer via a facile templating method. At first, the local zircon was converted into zirconia precursor. The chemical compositions of both of the materials were analyzed using an XRF method. The zirconia precursor was then used in the synthesis of Mg-PSZ nanoparticles with various doping concentrations. In this study, the calcination temperature applied is defined at lower temperatures of 600°, 800°, and 1000°C. Hence, the present study also aims to evaluate the influence of the dopant concentration on the phase transformation of ZrO 2 of the resulting PSZ at low calcination temperatures. The characteristics of the as-synthesized and the final products were assessed including the infra-red spectrum, mineral phases, and microstructures.

Materials and instruments
Materials used in this study were the local zircon of West Borneo, Na 2 CO 3 , (NH 4 ) 2 CO 3 , a 96% H 2 SO 4 solution, a 10% NH 4 OH solution, carboxyl methyl cellulose (CMC), PEG-6000, and MgSO 4 ·7H 2 O from Merck. Inc. All materials were used without further purification. The laboratory apparatus used in this study was an IKA dual speed mixer model RW 20 DZM with the maximum speed of 2000 rpm, a gas furnace with the maximum temperature of 1300°C, and a Nabertherm electric furnace with the maximum temperature of 1700°C. Meanwhile, the instruments that used for the material characterization were a Thermo ARL 9900 X-ray fluorescence instrument, a Bruker D8 Advance x-ray diffractometer, a Prestige 21 Shimadzu FT-IR Spectrophotometer, a JEOL-JSM-IT300LV SEM, and a JEM-1400 120 kV TEM.

Synthesis of Mg-PSZ nanoparticles from ZrSiO 4 assisted by PEG through a facile precursor-templating method
The synthesis of Mg-PSZ nanoparticles from ZrSiO 4 was initially carried out by destruction of ZrSiO 4 with Na 2 CO 3 via a modified sodium carbonate sintering method [45,46]. All preparation steps of the method were fully adopted in this work, except the hydrolysis of zirconium salt. Nevertheless, hydrolysis of the obtained zirconium salt was conducted at pH 5, resulting in a very fine white gel. Then, the gel was dried in an oven at 100°C . The dried gel was washed with cool and hot water in order to remove the water-soluble impurity of Na 2 SO 4 [44]. Then, the final product was calcined at 1000°C and characterized by the XRF method to evaluate its chemical composition. In addition, the ZrO 2 content in the local zircon-based zirconium hydroxide precursor was approximately about 15%.
The original pH of the prepared zirconium hydroxide gel precursor was pH 5. Before the synthesis of Mg-PSZ using PEG template was carried out, the gel precursor pH was initially adjusted to pH 3. A 96% H 2 SO 4 solution was used to dissolve the gel precursor, resulting in a zirconium salt solution at pH 1. Hereafter, a proper amount of (NH 4 ) 2 CO 3 was added into the precursor salt solution until its pH attaining pH 3. Magnesium salt solution was prepared separately from the gel precursors. Amounts of the MgO dopant were varied from 1 to 10 in wt% of ZrO 2 , namely 1wt% (1PSZ), 5wt% (5PSZ), and 10wt% (10PSZ). The amount of MgSO 4 ·7H 2 O weighed had to produce the desire weights% of MgO. According to the design composition, the appropriate amount of the Mg-precursor solution was then added into the Zr-precursor solution under vigorous stirring, resulting in a homogenous solution. Then, a 10%(w/v) PEG-6000 solution was added into the PSZ precursor solution at a ratio of the precursor-to-PEG volumes (V p /V PEG ) of about 15:1. The solution mixture was stirred under heating at 100°C for 3 h, resulting in two phases which consisted of very fine white gel and a clear solution.
The precursor gel was filtered and dried in an oven at 100°C. The dried gel was washed with cool and hot water, and dried in an oven at 100°C. The as-synthesized PSZ was characterized using the FT-IR spectroscopy and then was calcined at elevated temperatures of 600, 800, and 1000°C. The final product was evaluated including mineral phases and microstructures.

Characterization of the as-synthesized and the calcined PSZ samples
In this study, the chemical composition of local ZrSiO 4 and the prepared ZrO 2 -precursor gel were determined using a Thermo ARL 9900 x-ray fluorescence system. A Prestige 21 Shimadzu FT-IR Spectrophotometer was used to collect typical FT-IR spectra of PEG-6000, the as-synthesized 10PSZ, and the calcined 10PSZ. Meanwhile, particular phases of ZrO 2 in each sample at elevated calcination temperatures were investigated using a Bruker D8 Advance x-ray diffractometer at 40 kV, 40 mA with a Cu/Kα (λ=1.54060 A) radiation source. The diffraction patterns were scanned from 10.00 to 90.00 (2θ) with a step size of 0.020. The typical microstructures of all calcined PSZs were observed using a JEOL-JSM-IT300LV SEM and a JEM-1400 120 kV TEM.

Results and discussion
3.1. Synthesis of PSZ nanoparticles from zircon assisted with PEG-6000 Zirconium hydroxide precursor was synthesized from local zircon of West Borneo using our method in the previous work, it was a modified sodium carbonate sintering method [45,46]. All preparation steps of the method was fully adopted in this work, except the hydrolysis of zirconium salt. In this work, it was carried out at pH 5, resulting very fine white gels of zirconium hydroxides in the solution. However, it comprises of two types hydroxides, such as ZrO(OH) 2 and ZrO(OH) 2 ·XH 2 O [44]. The phenomena that occur during the preparation of the zirconium hydroxide precursor can be briefly explained as follows [45,46]. In the preparation of PSZ, the mixture of gel precursors in reaction (4) was then converted to ZrOSO 4 (pH 1) with the addition of H SO 2 4 (reaction (5)). Then, the pH of the salt precursor was adjusted to 3 with the addition of ( ) NH CO , (6)). The aim of the above precursor conversion is to facilitate the interaction between the zirconia precursor with the PEG template through the hydrogen bonding between both the hydroxyl groups of the precursor and the ether groups of the organic compound [44]. In addition, since the synthesis of PSZ was conducted at pH 3, the polyether of PEG-6000 was hydrolyzed in the existence of sulfuric acid and water, resulting in the degradation of the polymer [44]. The Mg-precursor used in this work was the water-soluble of MgSO 4 , thus producing Mg 2+ ions in the precursor solution. When the PSZ precursor solution was mixed with PEG-6000 under stirring and heating for 3 h, the Zr-precursor reacted with the degraded template to form white gelatinous precipitates of zirconium ethylene glycolates. In a case, Mg 2+ ions were expected to penetrate the gel. Therefore, this phenomenon should describe the formation of the as-synthesized Mg-PSZ compound. However, the PEG existence during the Mg-PSZ synthesis reduces the inclination for agglomeration in the as-synthesized Mg-PSZ particles. During calcination at elevated temperatures, the organic template was decomposed and burnt out as volatile compounds, leaving the residue of Mg-PSZ particles. At the high temperatures over 500°C, magnesium oxide (MgO) reacted with the metathetically formed ZrO 2 to produce a solid solution and stabilized the ZrO 2 in the form of either on the cubic or the tetragonal phases [36,47].    [43][44][45][46][47][48][49][50][51][52][53][54]. According to the elucidation results of the FT-IR spectra on the as-synthesized 10MSZ, some bands of them may overlap each other. For instance, the IR bands with low twin peaks at 1720.50 and 1635.64 cm −1 could be attributed to the hydroxyl groups (-OH) on the surface of the zirconia precursor and the bending vibrations of the CH-groups [46]. However, the band of Mg-O showed up at a finger print area of 617.72 cm −1 , attributing to the stretching vibrations of the Mg-O bonding [49]. This Mg-O band seems to be overlapped with the Zr-O band. Based on figure 1, the intermolecular hydrogen bonding between the Zr-precursor and the organic template in the as-synthesized 10PSZ is not detected in the FT-IR spectrum. The band of the bonding usually appears as a peak shoulder that coincident with the peak of the intramolecular hydrogen bonding [50]. Thus, it can be considered that the ZrO 2 -precursor react with the degraded PEG at the pH synthesis 3, resulting in Zr-(ethylene glycolates) n and releasing water molecules under heating. The degradation of PEG at pH 3 could be confirmed by very low-intensity and weak peaks of the CH 2stretching vibration at 2921.17 and 2870.738 cm −1 . Meanwhile, the common structure of PEG shows a high intensity and strong peak of the CH 2stretching vibration at around 2890 cm −1 as shown by FT-IR spectrum of PEG-6000 in figure 1. This result is in good agreement with the reference [51].

Chemical composition of local zircon and the prepared ZrO 2 -precursor gel
Semi quantitative analysis using an XRF method was conducted to determine the purity of local zircon and the ZrO 2 yield prepared from a local zircon-based zirconium hydroxide precursor after water-washing treatment. Table 2 (the second column) presents the chemical composition of local zircon that used as the zirconia source in this work. The zircon consists of 62.89 wt% ZrO 2 and 32.96 wt% SiO 2 . Meanwhile, the major impurities are Al 2 O 3 and HfO 2 , accounting for 0.942% and 0.856% in weight percent, respectively.
In addition, table 2 (the third right column) shows the ZrO 2 purity obtained after calcination the precursor at 1000°C, containing 91.43 wt% ZrO 2 as the main component. Meanwhile, the impurities are dominated by SiO 2 , Al 2 O 3 , and HfO 2 , accounting for 2.32%, 1.87%, 1.22% in weight percent, respectively. However, the yield percentage of ZrO 2 in this work is relatively higher than in our previous work [46]. The presence of SiO 2 and Al 2 O 3 impurities in the ZrO 2 yield should be produced during the hydrolysis of the sintered products in reaction (2). According to reaction (1), since ZrSiO 4 reacted with Na CO 2 3 at a high temperature, it converted into · sodium zirconates sodium silicates and so did the zircon impurities. For instance, the Al 2 O 3 contaminant also reacted with Na CO 2 3 at a high temperature, resulting in sodium aluminates. Both sodium silicates and sodium aluminates are water-soluble compounds. They are readily hydrolyzed in water, producing fine gelatinous precipitates of hydroxide compounds, such as Si(OH) 4 and Al(OH) 3 . However, although filtration and waterwashing treatments were conducted to separate sodium zirconates from the fine hydroxide contaminants, some of them might attach the surface or infiltrate to the inside of the sintered product. Thus, a few of Si(OH) 4 and Al(OH) 3 fine particles join with sodium zirconates in the acid leaching process (reaction (3)), resulting in acidsoluble compounds. Subsequently, they were solidified along with zirconium salts during hydrolysis process at pH 5, producing gelatinous precipitates. The hydroxides of Si and Al turn into the oxides after calcination of zirconium hydroxides at 1000°C. Therefore, a small amount of them was recorded during the XRF measurement.

XRD investigation on the mineral phase transformation of PSZ at elevated temperatures
Typical phases of ZrO 2 in PSZ samples with three different concentrations of MgO as the dopant were investigated at elevated temperatures from 600°to 1000°C using a Bruker XRD system. Figure 3 However, all PSZ samples show the existence of periclase, magnesium oxide. Its appearance is noticed by a very low peak at a diffraction angle, 2θ, of 43.07°. It is associated with the reflection of the (200) crystal plane of the c-MgO structure. In this study, based on the XRD results in figure 3(a), some of magnesium oxide (MgO) particles reacted with the metathetically formed ZrO 2 to produce a solid solution and stabilized the ZrO 2 in the form of either on the cubic or the tetragonal phases [36,47] and some of them that not reacted with ZrO 2 alter into periclase phase at a high temperature of 600°C. Figure 3(b) presents the XRD analysis results of all PSZ samples calcined at 800°C. According to the XRD patterns of all PSZ samples in figure 3(b), there is no significant change in the mineral phase transformation of both 1PSZ and 5PSZ samples, excluding 10PSZ sample. One small peak is identified at a diffraction angle, 2θ, of 59.57°in the XRD pattern of 10PSZ. It is assigned to the reflection of the (103) crystal plane of the t-ZrO 2 structure. The phenomenon indicates phase transformation from the c-to the t-phases of ZrO 2 in 10PSZ sample during heating at 800°C and then cooling to the ambient temperature. Our findings evidently differ from the report of Duwez et al [55]. They identified the m-ZrO 2 and MgO as the formed phases in the zirconia-magnesia system after firing at temperatures from 600°to 900°C, when using ZrO 2 and MgO as raw materials and the MgO concentrations of about 1-24 wt% [55]. The issue on the different results should be specifically governed by synthesis conditions, such as the type of precursors, template, pH, medium, temperature, and the reaction process. In addition, the XRD analysis results show the existence of MgO phase as periclase still detected in all PSZ samples at 800°C in this study. Figure 3(c) shows the XRD characterization results of all PSZ samples after firing at 1000°C and then cooling to the ambient temperature. All PSZ samples consist of the t-and the m-ZrO 2 in agreement with JCPDS PDF2 No. 791770 and 830944, respectively. The presence of the m-phase in all PSZ samples is indicated by its diffraction main peaks at diffraction angels, 2θ, of 28.41°and 31.48°that associates with the (−111) and (111) crystal planes of the m-ZrO 2 structure, respectively. However, a considerably difference among PSZ samples is noticeably illustrated by 10PSZ sample, showing the m-phase as the major and the dominant phase. The t-and the c-phases of ZrO 2 in 10PSZ sample almost fully transform to the m-ZrO 2 at 1000°C, but no MgO polymorphs are identified in the sample. Similar phenomenon is also observed in 5PSZ sample. At 1000°C, the MgO periclase phase in 5PSZ and 10PSZ samples is assumed starting to merge and to react with ZrO 2 , generating in a solid solution at higher temperatures than 1000°C. By contrast, the t-phase relatively remains stable in 1PSZ sample and the MgO periclase phase is still detected at a very low peak intensity. In completion, based on the experimental results, this study finds that the lower the MgO dopant concentration added into ZrO 2 , the more stable the existing t-ZrO 2 phase in PSZ sample at high temperatures. Nevertheless, further investigation especially on the ZrO 2 phase transformation at the sintering temperature of PSZ samples should be helpful to justify all the phenomena during MgO stabilization of ZrO 2 with the present various dopant concentrations.
In addition, according to the XRF measurement results in table 2, the Zr-precursor that used in the synthesis of PSZ nanoparticles in this study consists of major impurities such as SiO 2 and Al 2 O 3 , accounting for 2.32% and 1.87%, respectively. However, the impurities do not seem to affect the zirconia phase transformation that occurs during the calcination process of the as-synthesized PSZ samples, as according to the XRD results in figure 2. It is probably due to very low content of the impurities, thus they were not identified during the XRD measurements. However, Vasanthavel reported that the existence of SiO 2 in the ZrO 2 -SiO 2 binary system assisted the stabilization of the tetragonal phase of ZrO 2 through the formation of Si-O-Zr bonds between ZrO 2 and SiO 2 [56]. Table 3 presents the average crystallize sizes of any phases formed in PSZ nanoparticles at every calcination temperature using the Scherrer method, assisted with an XRD software. According to table 3, the average crystallite sizes of any phases formed in all PSZ samples are below 50 nm.  Based on figure 2 and the table 3, the tetragonal phase of ZrO 2 is stable in PSZ samples after calcination at 600°and 800°C. Since PSZ samples at those temperatures consist of nanocrystallite (less than 100 nm), the tetragonal phase can be stabilized at room temperature. It is also named as nanoparticle size effect [47,57]. Nanocrystalline powders exhibits very huge specific surface area, thus lowering the surface energy. In addition to the characteristic of nanocrystalline powders, more than 50% of the total atoms are on the surface. Subsequently, a larger number of metal-oxygen bonds are going to frail that lead to desorption of lattice oxygen ions, generating much more oxygen ion vacancies. This phenomenon helps to stabilize the tetragonal phase of ZrO 2 [57,58].

Microstructures of Mg-PSZ
Typical microstructures of all PSZ samples at elevated calcination temperatures were observed by a scanning electron microscope. Figure 3 display the results of SEM analysis in the microstructures of PSZ samples at 600°C. The microstructures of PSZ samples at the temperature show that they comprise of ultrafine particles and high agglomeration. Based on figure 3, the as-synthesized PSZ samples exhibit ultrafine particles below 50 nm in sizes after calcination at 600°C. Since nanoparticles exhibit large surface areas, they usually attract each other forming agglomerate to reduce the interfacial energy of the system. The proposed attraction forces are van der Waals forces that are essentially stronger in the nanoscale [41]. In addition, as according to figure 3, SEM images of all PSZ samples show miscellaneous microstructures. In addition to spherical particles, rod-like particles and elongated agglomerates can be observed in all PSZ samples in figure 3. Those morphologies are noticed by red marks and yellow arrows in SEM images ( figure 3). The existence of the rod-like shape and elongated agglomerate particles should be due to interaction between zirconia precursor and polyethylene glycol during synthesis. In a case, the polymer serves as a one-dimensional directing template [44]. This phenomenon is obviously observed in the 10PSZ sample, more rod-like shape and elongated agglomerate particles can be found. Figure 4 presents the microstructures of all PSZ samples at 800°C, showing ultrafine particles below 50 nm in sizes in general and high agglomeration, as well as all PSZ samples calcined at 600°C. Some of rod-like shape and elongated agglomerate particles also can be found in the microstructures of PSZ samples calcined at 800°C, which are indicated by red marks.
The higher the calcination temperatures applied, the larger PSZ particles produced, as shown in figure 5. Along with the increase of the calcination temperature, the PSZ particles are growing larger, this phenomenon is evidently demonstrated by 10PSZ sample in figure 5. According to the XRD results in figure 2, the tetragonal phase of ZrO 2 in 10PSZ sample almost completely transforms to the monoclinic phase of ZrO 2 at 1000°C. However, this transformation is followed by 3 to 5% volume increase [59], resulting in larger dimension. Thus, 10PSZ sample consisting of the m-ZrO 2 as the dominant or major phase will exhibit larger particles than the other samples consisting of the t-ZrO 2 as the major phase. This phenomenon is obviously shown by SEM images of 10PSZ sample in figure 5, uniform larger particles of the considered m-ZrO 2 can be found in the morphology of 10PSZ sample at 1000°C. Meanwhile, the morphology of 1PSZ sample consisting of the t-ZrO 2 as the major phase demonstrates the finest particles of all PSZ samples, as given by SEM images in figure 5. Figure 6 displays the results of TEM characterization in all PSZ samples after calcination at 800°C, featuring nanoparticles and agglomeration in the microstructures of all PSZ samples. In addition, some marked areas in the TEM images of PSZ samples unveil elongated and rod-like shapes of PSZ nanoparticles, as shown in figure 5. In a case, the presence of PEG template during the synthesis of Mg-PSZ diminishes the inclination of the assynthesized Mg-PSZ particles to agglomerate. PEG 6000 is built up by a lot of ethylene glycol monomers, consisting of ethoxy or oxyethylene structures in its chains and hydroxyl groups at the beginning and at the end of the chain structures. PEG 6000 cleaves at acidic medium and in the presence of a strong acid and water,  generating lower molecular weight PEG, or small units of glycols [44]. However, all types of those degraded PEG contact and attract the molecules of ( ) ZrO OH 2 through the hydrogen bonding. As a result, the degraded PEG molecules cover the surface hydroxyl groups of the ZrO 2 precursor, resulting in reduced particle interaction and aggregation of the as-synthesized PSZ samples. This phenomenon retards the PSZ particle growth during heating at high temperatures, leading to PSZ nanoparticles evolution. The retarding mechanism of the interaction among ZrO(OH) 2 molecules by the degraded PEG during synthesis of PSZ is concisely proposed in figure 7. The Zr-precursor reacted with the degraded template to form very fine white gels of zirconium ethylene/polyene glycolates. Nevertheless, Mg 2+ ions were expected to penetrate the gel, resulting in the assynthesized PSZ compounds. At high temperatures, the organic content of zirconium ethylene/polyene glycolates was burnt out as gases, leaving PSZ particles in the nanostructures.
The TEM observation results in figure 6 confirm the SEM analysis results shown in figure 5, indicating elongated and short rod-like shapes of some nanoparticles in all PSZ samples. The particular microstructures should be generated during the synthesis of PSZ at pH 3. At this point, the PEG degraded at pH 3 behaved as a template as well as a dispersing agent. The degraded PEG reacts with ( ) ZrO OH 2 molecules through the hydrogen bonding between both the hydroxyl groups of the Zr precursor and the degraded PEG. Under heating, the reaction between PEG and the Zr precursor results in zirconium ethylene glycolates and water molecules. In this situation, the structure directing process of ZrO 2 by the degraded PEG occurs, reconciling the template structure [44].

Conclusions
The present study reports the preparation of Mg-PSZ nanoparticles from zirconium silicates and MgSO 4 precursors and assisted with PEG-6000 via a facile templating method. Based on the experimental results in this study, the MgO concentration influence the ZrO 2 phase transformation at elevated calcination temperatures. The lower the MgO dopant concentration added into ZrO 2 , the more stable the t-ZrO 2 phase in PSZ sample at high temperatures. However, the MgO presence is detected as periclase in all samples with a very low peak intensity at elevated calcination temperatures. The obtained PSZ samples consist of nanoparticles and high agglomeration, some particles exhibit elongated and rod-like shapes. The existence of PEG during the PSZ preparation has restrained particle interaction and aggregation of the as-synthesized PSZ samples, leading to PSZ nanoparticles evolution.