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DOI: 10.1016/j.jmrt.2019.09.058
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Available online 21 October 2019
Crystal growth behavior and phase stability of rare earth oxides (4 mol.% GdO1.5-4 mol.% SmO1.5) doped zirconia nanopowders
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R. Mahendrana,
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mahendran.meta@gmail.com

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, S. Manivannanb, S. Senthil Kumaranc, A. Vallimanalana, M. Muralia, S. Gokul Rajd, S. P. Kumaresh Babua
a Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli, 620015, Tamil Nadu, India
b Department of Mechanical Engineering, Karpagam Academy of Higher Education, Coimbatore, 641021, Tamil Nadu, India
c School of Mechanical Engineering, Department of Manufacturing Engineering, Vellore Institute of Technology, Vellore, 632014, Tamil Nadu, India
d Department of Physics, C. Kandaswami Naidu College For Men (CKNC), Chennai, 600 102, Tamil Nadu, India
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Received 12 December 2018. Accepted 20 September 2019
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Abstract

Nanocrystalline powders of 4mol.% GdO1.5-4mol.% SmO1.5 doped ZrO2 (4Gd4SmSZ) has been synthesized by co-precipitation process. Their phase transformation and crystalline growth behavior was investigated by Thermo gravimetric-Differential Thermal Analysis (TG-DTA), X-ray Diffraction (XRD), Raman spectroscopy and High-Resolution Transmission Electron Microscopy (HR-TEM) after calcinations at different temperatures. The XRD, Raman spectra and HRTEM results confirm the nature of tetragonal zirconia (t-ZrO2). The prepared 4Gd4SmSZ powders, remains in the single metastable tetragonal phase over the whole calcination temperature ranging from 873K to 1273K for 2h. The crystallite size varies from 11.98nm to 18.90nm with increase of temperature from 873K to 1273K. The activation energy of the prepared powders at low temperature is considerably lesser than that at a higher temperature. The annealed 4Gd4SmSZ powders had shown excellent phase stability at 1573K for 100h.

Keywords:
Zirconia
Coprecipitation
Rare earth oxide
Phase transformation
Tetragonal
Activation energy
Full Text
1Introduction

Zirconia (ZrO2) shows good electrical, thermal and mechanical properties and it finds application in oxygen pumps and sensors [1,2], fuel cells [3,4] and thermal barrier coating (TBC) [5,6] applications. Zirconia has three polymorphs;room temperature monoclinic which is stable upto 1400K, intermediate phase is tetragonal(t) and it is stable from 1400K to 2570K and the cubic(c) phase is stable between 2570K and upto their melting point. The high-temperature t phase is the most important one from the standpoint of their structural engineering application where better mechanical properties are expected. However,an inherent drawback is theirpoor stabilityat low temperature. In order to stabilize the t phase at room temperature, stabilizers for ex., MgO, CaO, Y2O3 and other rare-earth oxides can be doped into zirconia. This non-transformable, supersaturated t phase is known as t` favorable for TBC application. When the t phase is sufficiently lean in stabilizers, it undergoes a diffusionless transformation to monoclinic while cooling from 1443K to room temperature. This in turn results in a volume expansion of ˜5%. It is obvious that this volume expansion will be a factor for crack formation on the coatings and eventually spalls from the surface [7]. In recent years, so many researchers concentrate on the addition of rare earth oxides into zirconia to stabilize t or c phase at higher temperatures [8–11]. To produce nanosized powders, several techniques such as co-precipitation [8,12], hydrothermal [13,14], sol-gel methods [15,16] are available. Owing to their simplicity and easy handling, co-precipitation technique is useful for preparing chemically homogeneous,nanosized zirconia powders. In the present work, 4mol % GdO1.5-4mol % SmO1.5-ZrO2 powders were prepared by co-precipitation process at pH 10.9–11.0 and then calcined at various temperatures, 573K and then at 873K to 1273K with an increment of 100K for 2h to investigate their phase transformation and crystal growth behavior. The phase stability of prepared powders at 1573K for 100h is also investigated.

2Experimental work

The starting materials are zirconium oxychloride octahydrate (ZrOCl2.8H2O), gadolinium nitrate hexahydrate (Gd(NO3)3.6H2O) and samarium nitrate hexahydrate (Sm(NO3)3.6H2O). The chemicals were purchased from Alfa-aesar, USA, with high purity of 99.99%. The chemical co-precipitation technique was adopted to prepare 4Gd4SmSZ powders. The respective nitrates and chlorideare taken in a stoichiometric ratio and mixed with deionized water to form a 0.3M solution. The mixture solution was slowly added drop-wise to aqueous ammonia solution which is under stirred condition continuously with the use of magnetic stirrer Careful attention has been given to maintain the solution pH between 10.9–11.0 throughout the entire process. Over a short period of time, white precipitates gradually appear. After that the white precipitates were filtered and collected. Before collecting the precipitates, it was repeatedly washed with de-ionized water and ethanol. The filter precipitates were dried at 100°C for 5h in an oven. Finally the obtained product was ground with the help of agate mortar and pestle. Thereafter, calcination was done on the prepared powders at different temperatures for determining the phase transformation and activation energy. The performance of powders with respect to phase stability was evaluated by annealing them at 1573K for 100h in a box furnace. The powders were heat treated at a rate of 5°C/min and furnace cooling was done. The powders were collected at different intervals to study the phase composition.

3Characterization of the powders

TG-DTAanalysis was performed in nitrogen flow with a heating rate of 10°C/min using simultaneous thermal analyzer (STA 8000, Perkin Elmer). FTIR (Spectrum-II, Perkin Elmer) was done on the as-prepared powders and the calcined samples (873K to 1273K) ranging from 1000 to 4000 cm−1 wave numbers. The structural characterization was carried out on the calcined powders by X-ray diffractometer (XRD - Rigaku Ultima III) with Cu Kα radiation at a wavelength (λ)=0.15406nm). Scanning was done with 2θ range from 20 to 80°. Scherer formula [15] is used to estimate the average crystallite size of t-ZrO2 and is given in eqn. (1),

where, Dt, λ, β and θ are the average crystallite size of t-ZrO2, wavelength, full width at half maximum and diffracted angle. Micro Raman spectroscopy (Labram HR evolution, Horiba Jobin Yvon) was conducted and recorded at room temperature on the calcined powders in the spectrum range of 100 to 700cm−1. The microstructure of the calcined powders was characterizing dusing HR-TEM (JEOL, JEM 2100).

4Results and discussion4.1TG-DTA analysis

Fig. 1 depicts the TG-DTA curve of 4Gd4SmSZ doped zirconia powders. As it can be seen, the weight loss (TG) curve exhibits two weight losses. The first one happens in the temperature range between 300.0K and 421.0K with a weight loss of 16.2%. The endothermic peak at around 340.0K is assigned to the evaporation of absorbed moisture and water molecules present in the dried precipitates. The second weight loss of 7.4% occurs in between 421.0K and 755K, could be related with the decomposition of zirconium hydroxide formed during the precipitation process. After this stage, there was no weight loss displayed in TG curve. The DTA curve, revealed one single exothermic peak near 768.0K, which may account for the formation of crystalline ZrO2. After this, there are no apparent peaks present in DTA curve and no weight loss observed in TG curve.

Fig. 1.

TG-DTA curve of 4Gd4SmSZ powders.

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4.2FTIR analysis of 4Gd4SmSZ powders

Fig. 2 shows the FTIR spectra of the as-prepared and the calcined 4Gd4SmSZ powders at different temperatures. It shows several bands appear in the range between 1000-4000cm−1. The band appearing at 3294cm−1 corresponds to OH stretching and the broadest nature is associated with hydrogen-bonded chains [17,18]. For the as-prepared dried precipitates the 3294cm−1 band appears with higher intensity compared to the calcined powders. When the calcination temperature increases, the intensity of the band decreases and disappears at higher calcination temperatures. The band at 1626cm−1 is another vibration band of water molecule which is arising from HOH bending bonds [19]. The band appearing at 1350cm−1 and 1550cm−1 represents the existence of nitrates and it belongs to the N–O asymmetrical stretches in the N–O bond [20,21].

Fig. 2.

FTIR spectra of 4Gd4SmSZ powders a) as prepared b) 873K c) 973K d) 1073K e) 1173K and f) 1273K.

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4.3XRD pattern of 4Gd4SmSZ powders

The X-ray diffraction pattern of the 4Gd4SmSZ powders is shown in Fig. 3. As can be seen from Fig. 3a (573K), itreveals that the prepared powders are inamorphous condition indicating that crystallization does not take place. From Fig.1 the crystallization of t-ZrO2has occurred well above 700K which agrees with DTA result. From Fig. 3b (873K), the reflection peaks corresponding to tetragonal zirconia (JCPDS: 50–1089) were identified and no other phases have been detected. As the calcination temperature is raised to 973K, it is evident from Fig. 3c, that the prepared 4Sm4Gd-SZ powder remains in a single tetragonal phase. The XRD pattern shown in Fig.3d represents the powder calcined after 1073K for 2h, which results in increase of diffraction peak intensities with corresponding increase in calcination temperature. On raising the calcination temperature to 1173K as shown in Fig. 3e, increase in crystallite size and crystallization occurred which is reflected in peak intensities and a decrease in Full width Half maximum was observed.

Fig. 3.

XRD pattern of 4Gd4SmSZ powders a)573K b) 873K c) 973K d)1073K e) 1173K and f) 1273K.

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. Furthermore, the XRD pattern belonging to 1273K (Fig. 3f) revealed that tetragonal was the dominant phase. It is reported, in a previous study by Hsu et al. [12], that t-ZrO2 was formed between the calcination temperature 873K and 1273K inthe synthesized 3Y-TZP powders. In addition, they reported that intensity of the peak increases throughout the calcination temperature range without any new phase formation. Wang et al. [16] prepared 4mol% yttria stabilized zirconia (4Y-TSZ) nanosized powders using coprecipitation technique and reported only t-ZrO2. Once the powders calcined between 673K and 1273K, the reflection gets increased and they maintained the t-ZrO2 phase over the calcination temperature. The present study agrees with the reports of Hsu et al. [12] and Wang et al. [16].

4.4Raman spectra of 4Gd4SmSZ powders

Raman spectroscopy is very sensitive to local symmetry and it is widely employed to distinguish the different polymorphs of zirconia. The Raman spectra of 4Gd4SmSZ doped zirconia powders are represented in Fig.4. The peaks at 145, 260, 322, 467 and 635cm−1were assigned to the tetragonal mode of zirconia and it becomes stronger as the calcination temperature increases. The present result is further supported by the findings from Kim et al. [22], Qu and K.L. Choy [23] and Niu et al. [24]. The peak at 608cm−1 could be hardly detected at higher temperatures. Moreover it could be pointed that no fingerprints of vibration modes belong to monoclinic polymorph was observed throughout the heat treatment range. This is consistent with the XRD result.

Fig. 4.

Raman spectra of 4Gd4SmSZ powders at various temperatures.

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4.5Crystal growth behavior of 4Gd4SmSZ powders

The crystallite size versus calcination temperature is represented in Fig. 5. From Fig. 5 it is observed that the average crystallite size increases with calcination temperature. When the precipitates were calcined at 873K, the average crystallite size was about 11.97nm and it grows to 12.74nm after calcined at 973K. It attains a value of 13.28nm, when calcined at 1073K. A further increase of temperature to 1173K causes the crystallite size to reach 15.98nm. Finally, it reaches around 18.90nm, for 1273K. These crystallite sizes are lower than the critical size of 30nm, above which tetragonal to monoclinic(m) phase transformation can occur [25,26]. Also it is pointed out by Huang et al. [15] that smaller the particle size more stabilization could be achieved for t phase based on the nanoparticle size effect. Hence in the present study, no m phase transition occurred. XRD and Raman results support this understanding as in (Fig. 3&4).

Fig. 5.

The evolution of crystallite size versus calcination temperature of 4Gd4SmSZ powders.

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The activation energy for crystallization was determined by Arrhenius equation [15], given below in eqn. (2)

where k is the constant value, R denotes the gas constant, Dt is the average crystal size and ΔE is the activation energy for crystallite growth. The lnDt and the reciprocal of the calcination temperature was plotted in Fig. 6. From the slope, the activation energy for the t phase was calculated. The activation energy for the crystal growth, when calcined between 873K–1073K was found as 4.04kJ/mol. and it was 20.04kJ/mol. when calcined between 1073K–1273K. The activation energy was lower than that of 34kJ/mol. [27], 29.2kJ/mol [28]. and 24.79kJ/mol. [29]. The reduction in activation energy in the present study is due to the presence of large vacancies within the nanocrystallites due to the incorporation Gd3+ and Sm3+. Also, it is reported that, an increase in the concentration of oxygen vacancies may occur in nanocrystalline ceramic particle nanocrystallites if their particle size was less than 20nm [30]. Therefore, due to these combined effect, 4Gd4SmSZ nanocrystalline powder experience comparative reduction in the activation energy

Fig. 6.

Relation between ln (Dt) versus 1000/T (K−1).

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4.6Microstructure of the 4Gd4SmSZ powders

SEM micrograph of 4Gd4SmSZ powders calcined at 1073K is shown in Fig. 7a, which reveals the powder particles are in the form of large clusters. EDS measurement was taken on the calcined powders (1073K) which is illustrated in Fig. 7b. It shows the presence of Zr, Sm and Gd constituents and no impurities were observed in the prepared powders which demonstrates the coprecipitation process reliability and the starting material purity.

Fig. 7.

SEM micrograph of a) 4Gd4SmSZ powders calcined at 1073K b) Energy-dispersive spectra showing the corresponding constituents.

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Fig. 8a.shows the HRTEM images of powders calcined at 1073K for 2h. The powders are in agglomerated state. The agglomerated condition is due to the steps involved in the synthesis process. The SAED pattern shown in Fig. 8b, demonstrates that the prepared powders are polycrystalline in nature and was indexed to t-ZrO2 and their planes are indicated in the insert of Fig. 8b.

Fig. 8.

HR-TEM of 4Gd4SmSZ powders a) bright field image b) Selected Area Electron Diffraction pattern.

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4.7Phase stability analysis

Fig. 9 revealed the XRD patterns of the annealed powders at 10h, 50h and 100h, respectively. The low angle diffracted region (26°-36°) was usually made for the identification of m-phase and the use of high angle diffracted (400) region (71°-77°), can provide a distinction between tetragonal and cubic phase. It can be seen from Fig. 9a, that the powder particles exhibit excellent phase stability as the m-phase is absent even after heat treating for 100h. In the (400) region, only tetragonal (004) and (400) peaks do exist and it does not contain any cubic phase. This conclusion is also supported by the deconvolution results, Fig. 10. It was reported that t phase along with c phase coexists after the thermal exposure [9,31–33]. Also, some other reports had shown the existence of multiple tetragonal phases along with or without cubic phase [8,34,35].

Fig. 9.

XRD pattern of 4Gd4SmSZ powders annealed at 1573K a) low angle region b) high angle region.

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Fig. 10.

Deconvoluted (Lorentzian fit) XRD pattern of annealed 4Gd4SmSZ powders at 1573K for 10h b) 100h showing tetragonal phase.

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Hence, deconvolution (Lorentzian fit) of XRD patterns was carried out to identify the phases present after the heat treatment. Fig. 10 displays the deconvolution results, in 71°-77° region of powders annealed for 10h (Fig. 10a) and 100h (Fig. 10b) at 1573K. It clearly shows that, there is no existence of c phase present along with the t phase, which demonstrates that the phase stability of t phase is better. The present result is in line with the report of Jones et al. [36]. When doping ZrO2 with Gd3+ and Sm3+, it produces substitutional defects and creates oxygen vacancies for charge compensation to maintain electroneutrality. Also, it may result in nanostructured defect clusters via stabilizer segregation [8,37], which reduces the concentration of single mobile defects [9]. At higher temperatures, diffusion controlled process is responsible for the decomposition of t` phase to t and c phase and it requires long-range cation diffusion [8,32]. On cooling the stabilizer lean t phase transforms to m phase. However, the presence of defect clusters hinders the atomic mobility and mass transport at a higher temperature. As a result, it suppresses the diffusion of dopants into rich and lean regions, thus maintaining the t phase stability [32].

The obtained results of Raman spectra areshown in Fig. 11. Six distinct characteristic bands were observed at 145, 257, 365, 467, 608 and 642cm−1 and they very well match with tetragonal polymorph. An increase in peak intensity with respect to annealing time was observed. As can be clearly seen, the most characteristic bands 178 and 189cm-1 of monoclinic ZrO2 were not present over the entire period of the experiment. The results of Raman spectroscopy are in good agreement with XRD result (Fig, 9a&b).

Fig. 11.

Raman spectra of annealed 4Gd4SmSZ powders at 1573K for h b) 50h and c) 100h.

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5Conclusions

The ZrO2-4mol.% GdO1.5-4mol.% SmO1.5 powders had retained tetragonal phase when calcined between 873K to 1273K for 2h. The crystallite size increased from 11.97nm to 13.28nm at low temperature (873–1073K) synthesis and from 13.28nm to 18.90nm(1073–1273K) at higher temperature regime. The activation energy for the low temperature (873–1073K) and high temperature (1073–1273K) synthesis were found to be 4.04kJ/mol. and20.04kJ/mol. respectively. HR-TEM confirms the tetragonal nature. The 4Gd4SmSZ powders show better phase stability at 1573K for 100h.

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Journal of Materials Research and Technology

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