Structural,morphological properties and phase stabilisation criteria of the calcia-zirconia system

Abstract

The phase evolution studies of zirconia, with 4–16 mol-% doping of calcium, have been carried out after sintering the pellets at 1400°C. From the X-ray diffractometer patterns, it is evident that the zirconia exists only in the monoclinic phase. However, Rietveld refinement of calcium-doped zirconia revealed the stabilisation in the monoclinic and cubic phases. With increasing calcium doping, the development of the cubic phase in zirconia is seen and at 16 mol-% doping of calcium, almost fully stabilised cubic phase of zirconia (∼97%) is achieved. The microstructure and elemental analyses of the sintered pellets are done using a field-emission scanning electron microscopy and energy-dispersive spectroscopy, respectively. Raman spectroscopic studies validate the findings of XRD. It is expected that the present study on calcium-doped zirconia opened a new channel for its potential applications in new technology such as oxygen sensors and solid electrolyte.

Keywords

Phase transition Rietveld refinement

Introduction

Ceramics are highly preferred for various kinds of industrial applications because of their fascinating physical properties, such as high mechanical stability, superior chemical stability, high radiation resistance, high melting point, and so on [1 -3]. Ceramics can be broadly classified into oxides and non-oxide categories. The zirconium dioxide (ZrO₂), commonly known as zirconia, is one example of oxide ceramics, which shows versatile physicochemical properties, such as high ionic conductivity, low thermal conductivity, superior chemical stability, high hardness, and prominent optical properties [4 7]. Owing to these fascinating properties, zirconia attracted innumerable researchers in recent years for studies and technological applications, such as oxygen detector, thermal barrier coatings, optical coatings, high-temperature solid oxide fuel cells, and so on [8 13]. Furthermore, zirconia is also a promising contender for inert matrix fuels (IMFs) in nuclear reactors [14,15].

The structural studies of zirconia have shown that it is a polymorphous, which exists in three crystallographic phases (i) monoclinic (m), which has the lowest symmetry structure with thermodynamic stability up to 1170°C; (ii) tetragonal (t), which appears from 1170 to 2370°C; (iii) cubic (c), which exists from 2370 to 2715°C (melting temperature) [4,16 -18]. With improvement in physicochemical properties of zirconia upon phase transition, its high-temperature phases are more preferred for technological applications. Furthermore, pure zirconia is not stabilised in the cubic phase at room temperature because the Zr⁴⁺ ion is too small to sustain the symmetrical structure. Generally, fully stabilised zirconia (FSZ) is obtained by doping the zirconia with cations of lower valence and larger size (relative to Zr⁴⁺) e.g. by incorporating Y³⁺, Ca²⁺, or Mg²⁺ in the ZrO₂ lattice [19]. The doping cations replace the Zr⁴⁺ after high-temperature sintering which causes the oxygen vacancies in the lattice [20].

Yttria-doped zirconia has been studied extensively for transformation in the structural properties of zirconia especially, to stabilise its cubic crystalline phase at room temperature [20 -24]. Gaudon et al. [25] reported that the cubic phase of zirconia can be stabilised at room temperature by doping of 7 mol-% around and above yttria (Y₂O₃) into it. This material is known as yttria-stabilised zirconia (YSZ). Doping by 2–7 mol-% of yttria leads to the stabilisation of the tetragonal phase, i.e. a partially stabilised zirconia, whereas below 2 mol-% of yttria doping, the monoclinic phase is present.

However, limited studies are available on zirconia with calcium doping [26]. Furthermore, the ionic conductivity of calcium-stabilised zirconia is more than that of yttria-stabilised zirconia and it makes zirconia a strong candidate for oxygen sensors and solid electrolytes [27 -29]. Also, the calcium-doped zirconia has toughness similar to YSZ and hence, a significant material for industrial applications [30,31]. Wu and Brook [32] reported the results of conventional sintering in the air at 1560°C for 13 h of partially stabilised zirconia (Ca-PSZ) by doping in the range of 8–20 mol-% of calcium in zirconia. They concluded that the samples with compositions with 12 mol-% of calcium or above had a cubic fluorite-type structure.

In 2008, Nath et al. [33] studied the phase stability relation with a density of only 8 mol-% calcium-doped PSZ and 16 mol-% calcium-doped FSZ. They concluded that the density of material decreases with an increase in phase percentage of c-ZrO₂. Zhang et al. [34] reported a study on the effect of holding time on the phase stability of sintered CaO-doped PSZ. They found that the stability of the tetragonal phase of zirconia in sintered samples is maximum for a holding time of 2 h. The studies reported earlier for calcium-doped zirconia provide only limited information for the relation between phase percentage of cubic zirconia with lattice parameters and density of the material. Furthermore, a complete study of the shrinkage of calcium- doped zirconia on sintering and the atomic structure of calcium stabilised zirconia are not available in the literature which is required to further enhance our knowledge to identify the potential applications of this material. The significance of the present work is to thoroughly investigate the relation of lattice parameters, shrinkage, density, and cubic phase percentage against the different doping percentages of calcium in zirconia. In addition, the crystal structure of calcium-stabilised zirconia has been explored in monoclinic and cubic phases.

In the present work, the structural and morphological studies of zirconia with doping of 4–16 mol-% of calcium at 1400°C are investigated using the XRD, FESEM and RAMAN spectroscopy techniques to understand the role of doping percentage of calcium in zirconia on its phase stabilisation. The phase percentage in synthesised material, thereby optimising the doping percentage of calcium in zirconia to obtain calcia-stabilised zirconia, has been investigated by Rietveld Refinement studies.

Materials and methods

Synthesis of calcium-zirconia pellets

Pellets of calcium-doped zirconia with different composition Ca _x Zr_1–xO_2–x: x = 0.04, 0.08, 0.12, and 0.16, have been synthesised by conventional solid-state reaction technique taking zirconium (IV) oxide (ZrO₂) (∼ 99.999% purity) and calcium oxide (CaO) (∼ 99% purity) as the starting materials. Both the materials in powder form were mixed homogenously in the stoichiometric proportion to get different doping percentages of calcium in zirconia. This mixture was kept at 180°C overnight to remove the moisture content and then, it was ground with an agate mortar and pestle for 5 h and during this, a few drops of acetone were added after a regular interval.

With the help of the hydraulic press and using a 10 mm KBr die set, the pellets of ground powder were prepared subjecting a pressure of 5 MPa on it. The pellets thus formed were 10 mm in diameter and ∼ 2 mm in thickness. Polyvinyl alcohol (PVA) was added as a binding agent to the powder to increase the strength and decrease the chances of a breakdown of the pellets while handling during the sintering. For the solid-state reaction to take place, these pellets were sintered in a furnace at 1400°C for 12 h with heating and cooling rates of 3 and 2 °C min⁻¹, respectively. The pellets of pure zirconia were also synthesised in a similar fashion and sintered at the same temperature to compare the results with calcium-doped zirconia. Table 1 gives the sample code and composition of calcium in zirconia of prepared samples.

Table 1

The sample codes with calcium composition. All samples (except ZrO₂-RT) were sintered at 1400°C.

Sample code	Ca (mol-%)
ZrO₂-RT	0
ZrO₂	0
Ca_0.04Zr_0.96O_1.96	4
Ca_0.08Zr_0.92O_1.92	8
Ca_0.12Zr_0.88O_1.88	12
Ca_0.16Zr_0.84O_1.84	16

Characterisation of samples

For the phase identification, the prepared samples have been analysed using a Bruker D8 advance X-ray Diffractometer (XRD) with CuK_α radiation (λ = 1.54060 Å). The XRD spectra were recorded in the range of 20°–70° with a step size of 0.02° and at a scan speed of 5 s step⁻¹. The Rietveld Refinement studies of the XRD spectra were performed using the TOPAS software program and the crystal structural parameters were determined. For this, all the standard Crystallographic Information File (CIF) was taken from the Crystallography Open Database (COD). The crystal structure of the samples was drawn using the Visualisation for Electronic and Structural Analysis (VESTA) software.

The surface morphology of the samples was recorded using a Field Emission Scanning Electron Microscope (FESEM; JSM 7610F Plus, JEOL). The samples were polished with a silicon carbide paper (mesh size 2000) and cleaned with acetone to remove the surface contamination before the recording of FESEM images. Also, the samples were coated with a 5 nm thin layer of gold using a DII-29030SCTR Smart Coater to prevent charging effects while there was bombardment of the samples with electron beam. Furthermore, the elemental analysis of samples was also carried out using Energy-dispersive X-ray spectroscopy (EDS), which is coupled with FESEM.

A Renishaw In-Via confocal micro-Raman spectrometer was used to measure the Raman spectra of the samples. For excitation, a 488 nm argon-ion laser at room temperature with a maximum beam spot size of 50 μm is available in this set-up. However, we employed an X20 objective lens to get a beam spot size of 2 mm in diameter for recording the spectra of zirconium samples in the range of 30–800 cm⁻¹ with a spectral resolution of 1 cm⁻¹. The Stoke lines were recorded in the backscattering geometry using an edge filter, a diffraction grating (2400 lines mm⁻¹) and a Peltier-cooled charged coupled detector.

Results and discussion

XRD study

Figure 1 shows the XRD spectra of all samples (pellets of as-supplied zirconia powder, pure and doped zirconia sintered at 1400°C). The careful analysis of the XRD spectra of ZrO₂-RT indicates the presence of a monoclinic phase [JCPDS No- 00-036-0420], as verified with the peaks marked by ‘●’ which are in agreement with earlier reported work [35,36]. The XRD peaks with strong intensity appeared at 2θ ≈24.07° , 24.46° 28.19° , 31.48° , 34.19° , 49.27° , and 50.12° belong to the monoclinic phase (Space group: P2₁/c). With increasing the doping percentage of calcium, the monoclinic phase gets mixed with the cubic phase [JCPDS No- 01078-3351], as verified by the peaks marked by ‘*’. The diffraction peaks at 28.19° and 31.48° corresponding to and , which are the characteristic planes of the monoclinic phase, are shown in the zoomed version of the XRD pattern in Figure 1. As the doping percentage of calcium increases, the intensities of these two peaks reduce and a new diffraction peak appears at 30.32°. This new peak is attributed to the (111) plane of the cubic phase [JCPDS No- 01-078-3351] of zirconia [37].

Figure 1

XRD patterns of the samples; ZrO₂-RT, ZrO₂, Ca _x Zr_1–xO_2–x: x = 0.04, 0.08, 0.12, and 0.16. Inset: Enlarged view (2θ: 27.5–32.5°) of XRD patterns.

Garvie and Nicholson [38] proposed the following expression to estimate the percentage of the cubic phase present in ZrO₂, (1)

And C_m= 1–C_C, where C_m and C_C are the phase percentage of the monoclinic and the cubic phase of the material, respectively. The I_m and I_C represent the intensities of the monoclinic and the cubic phase for each diffraction peak. The mixture of phase composition has been calculated for different doping concentrations and depicted in Table 2. It is very evident that the fraction weight % of the cubic phase increases with the doping percentage of calcium in zirconia. In the present studies, the maximum cubic phase weight percentage is 97.58% for Ca_0.16Zr_0.84O_1.84.

Table 2

The crystallite size and cubic phase percentage in the samples.

Sample code	Crystallite size (nm)	Cubical phase weight%
ZrO₂-RT	55.93	0
ZrO₂	54.01	0
Ca_0.04Zr_0.96O_1.96	46.76	38.50
Ca_0.08Zr_0.92O_1.92	58.48	60.03
Ca_0.12Zr_0.88O_1.88	76.75	85.08
Ca_0.16Zr_0.84O_1.84	73.47	97.58

The average crystallite size has been estimated by considering the highest intensity diffraction peak and using Scherrer's equation [39,40], i.e. (2) Here D represents the average crystallite size in nanometers, k is a constant equal to 0.94, λ is the wavelength of the radiation (1.54060 Å for Cu K_α radiation), β is the peak width at the half-maximum intensity and θ is the peak position. The crystallite size of the pure zirconia is 55.93 nm and after sintering it at 1400°C, the size decreases to 54.01 nm due to the broadening of the peak, which may be attributed to an increase in the strain [41]. Furthermore, for the sample Ca_0.08Zr_0.92O_1.92, the XRD peak at 30.32° is of the maximum intensity and therefore, we considered it for estimating crystallite size. A similar approach was used for estimating crystallite size in other samples. The crystallite size and fraction weight % of the cubic phase in the samples are given in Table 2. In doped samples, competition between strain and development of the cubic phase could be a deciding factor for crystallite size. As anticipated, the crystallite size in cubic phase structure is more than that of monoclinic phase structure [42].

The lattice parameters of the sample, ZrO₂-RT, ZrO₂, Ca _x Zr_1–xO_2-x: x = 0.04, 0.08, 0.12, and 0.16 have been calculated by using the following formula for inter-planar spacing of monoclinic crystal: (3)

The five reflections, namely,

, (111), (002), (020), and (200) of the monoclinic phase, have been used to evaluate the lattice parameters. Also, the unit cell volume and the crystal density of the samples are computed using standard formulas [43]. The lattice parameters of the cubic phase of zirconia have been determined from the (111) reflection. The lattice parameters, unit cell volume and crystal density values of all samples are depicted in Table 3. There is a decrease in the crystal density with increasing calcium doping percentage in zirconia. This is quite obvious because, with the incorporation of calcium, the overall molecular weight of the sample is getting reduced. These obtained lattice parameters for pure and doped zirconia samples are consistent with the standard JCPDS file [36 -38].

Table 3

The lattice parameters, the unit cell volume, and the crystal density of the samples.

Sample code	a (Å)	b (Å)	c (Å)	β°	Unit cell volume (Å)³	Crystal density (g cm^-3)	a (Å)	Unit cell volume (Å)³	Crystal density (g cm^-3)
ZrO₂-RT	5.152	5.214	5.321	99.193	141.125	5.799	–	–	–
ZrO₂	5.152	5.214	5.320	99.193	141.125	5.799	–	–	–
Ca_0.04Zr_0.96O_1.96	5.146	5.209	5.314	99.184	140.659	5.691	5.128	134.918	5.933
Ca_0.08Zr_0.92O_1.92	5.152	5.214	5.320	99.193	141.125	5.546	5.132	135.172	5.790
Ca_0.12Zr_0.88O_1.88	5.158	5.220	5.326	99.202	141.592	5.402	5.141	135.944	5.626
Ca_0.16Zr_0.84O_1.84	5.155	5.217	5.323	99.198	141.368	5.284	5.138	135.687	5.505

Analysis through Rietveld refinement

The TOPAS software program was used for Rietveld refinement of XRD patterns to further analyse the samples. The refined XRD patterns of the samples, ZrO₂-RT, ZrO₂, Ca _x Zr_1–xO_2-x: x = 0.04, 0.08, 0.12, and 0.16, are illustrated in Figure 2. The experimental data, the calculated data, the difference between experimental and calculated data, and Bragg's peak are represented by the black close circle, red continuous line, blue continuous line, and the vertical line, respectively. Based on refinement studies, the presence of the cubic phase is reported in the calcium-doped zirconia samples, prepared by solid-state reaction. The cubic (space group Fm m) [44] and the monoclinic (space group P2₁/c) phases have been used as the starting models for refining calcium-doped samples. But, only the monoclinic (space group P2₁/c) [45] phase has been used to refine the samples, ZrO₂-RT and ZrO₂. The lattice parameters, unit cell volume, and crystal density obtained from the refinement results are shown in Table 4. Furthermore, no XRD pattern of calcium is observed in samples during Rietveld refinement of the sintered samples which confirms the substitution of calcium at the appropriate site of the Zr site. The variation in the density of monoclinic and cubic phases for different doping percentages of calcium and its comparison with calculated values in zirconia is shown in Figure 3. From Figure 3, it is observed that the variance between experimental density and theoretical density is very less. The densities decrease with increasing doping and the refinement studies provide a linear dependence. Furthermore, the background and the peak shape were refined using a Chebychev polynomial with six coefficients, and a Fundamental-Parameters function, respectively. The various R factors and phase fraction obtained from the refinement results are listed in Table 5. The observed Rietveld patterns and R-factors (R_p, R_wp, and Goodness of Fit (GOF)) suggest a good agreement between the experimental and calculated data.

Figure 2

Rietveld refined XRD patterns of (a) ZrO₂-RT (b) ZrO₂ (c) Ca_0.04Zr_0.96O_1.96 (d) Ca_0.08Zr_0.92O_1.92 (e) Ca_0.12Zr_0.88O_1.88 (f) Ca_0.16Zr_0.84O_1.84.

Figure 3

Variation in the density of the monoclinic phase and cubic phase with change in doping percentage of calcium in zirconia.

Table 4

Values of lattice parameters, cell volume, and density of samples obtained by Rietveld refinement.

Sample code	Monoclinic phase						Cubic phase
Sample code	a (Å)	b (Å)	c (Å)	β°	Unit cell volume (Å)³	Crystal density (g cm⁻³)	a (Å)	Unit cell volume (Å)³	Crystal density (g cm⁻³)
ZrO₂-RT	5.148	5.211	5.316	99.210	140.770	5.812	–	–	–
ZrO₂	5.154	5.208	5.318	99.198	140.910	5.806	–	–	–
Ca_0.04Zr_0.96O_1.96	5.155	5.210	5.319	99.193	141.020	5.677	5.130	135.006	5.927
Ca_0.08Zr_0.92O_1.92	5.152	5.211	5.318	99.190	140.940	5.553	5.130	135.006	5.797
Ca_0.12Zr_0.88O_1.88	5.152	5.207	5.316	99.189	140.779	5.432	5.130	135.006	5.667
Ca_0.16Zr_0.84O_1.84	5.151	5.199	5.332	99.189	140.959	5.300	5.131	135.085	5.515

Table 5

R-factors and phase fraction obtained from the Rietveld refinement results of the powder XRD patterns.

Sample code	R_p	R_wp	GOF	Phase fraction (%)
Sample code	R_p	R_wp	GOF	m-ZrO₂	c-ZrO₂
ZrO₂-RT	9.10	12.14	1.78	100	–
ZrO₂	13.41	16.53	1.98	100	–
Ca_0.04Zr_0.96O_1.96	14.69	17.96	2.83	77.165	22.835
Ca_0.08Zr_0.92O_1.92	12.86	16.44	2.65	53.698	46.302
Ca_0.12Zr_0.88O_1.88	14.07	17.40	1.80	29.299	70.701
Ca_0.16Zr_0.84O_1.84	11.74	15.91	1.71	3.450	96.550

The crystal structure of the pure and calcium-doped zirconia was drawn using the VESTA [46] software by taking the data files obtained during Rietveld refinement. The monoclinic phase of pure zirconia at RT and, pure and doped zirconia sintered at 1400⁰C, is shown in Figure 4.

Figure 4

The atomic structure of the monoclinic zirconia built using VESTA. (a) ZrO₂-RT (b) ZrO₂ (c) Ca_0.04Zr_0.96O_1.96 (d) Ca_0.08Zr_0.92O_1.92 (e) Ca_0.12Zr_0.88O_1.88 (f) Ca_0.16Zr_0.84O_1.84. The green and red balls are Zr and O atoms, respectively.

The monoclinic phase of zirconia belongs to the space group P2₁/c (# 14) with Zr occupying the position at (0.27231 0.03832 0.20679), and O₁ at (0.07281 0.31591 0.34847), and O₂ at (0.45075 0.75586 0.47220). But, in the cubic phase of zirconia (space group Fm m (#225)), Zr occupies the position at (0 0 0), and O at (0.25 0.25 0.25). When zirconia is doped with calcium, it occupies the position of Zr and creates a vacancy at the position of the O atom. The cubic phase of the calcium-doped zirconia is shown in Figure 5. These atomic structures are in good agreement with the existing structures of zirconia.

Figure 5

The atomic structure of the cubic zirconia built using VESTA. (a) Ca_0.04Zr_0.96O_1.96 (b) Ca_0.08Zr_0.92O_1.92 (c) Ca_0.12Zr_0.88O_1.88 (d) Ca_0.16Zr_0.84O_1.84. The green and red balls are Zr and O atoms, respectively. The blue colour represents the doping of the Ca atom and the light grey in O atoms represents the vacancy of O atoms.

Shrinkage measurement

The shrinkage of the material after sintering affects its product quality and durability due to the change in the dimension of the product [47]. The diameter of all the samples is measured before and after the sintering process to determine the shrinkage. The percentage of the diameter reduction has been calculated by the following Equation (4), (4) where d_o is the diameter before sintering (cm), d_f is the diameter after sintering (cm).

The diameter of the prepared pellet of all samples at room temperature is 10 mm. After sintering to 1400°C, the pellets get compressed due to the presence of voids. The percentage of diameter shrinkage is calculated using Equation (4), which is illustrated in Table 6. It has been observed that pure zirconia has highest shrinkage, whereas the percentage of shrinkage decreases with the increase of calcium doping in zirconia. The result shows that the 16% mol calcium-doped zirconia has least shrinkage. Figure 6 shows the percentage of diameter shrinkage and evolution of the cubic phase (deduced by experimental data and refinement studies) in zirconia as a function of calcium doping. As the cubic phase evolves, the sample shrinkage reduces. The least shrinkage of cubic stabilised zirconia makes it better for industrial applications.

Figure 6

The evolution of the cubic phase and sample shrinkage in zirconia as a function of calcium doping.

FESEM and EDS analysis

The imaging of surface microstructure and the elemental analysis of the samples have been carried out using FESEM and EDS and outputs are shown in Figures 7 and 8, respectively. The FESEM image of pure zirconia (Figure 7(a)) shows a broad grain size distribution and porosity is very much clear. Similar morphology of zirconia has also been reported in the published literature [48,49]. The average grain size is roughly calculated using the ImageJ software [50] (∼0.266 μm). After sintering at 1400°C, the observed morphology of zirconia is highly dense as compared to that of a pre-sintered sample with an average grain size of around 0.677 μm (Figure 7(b)). Similar dense grain size distribution is seen in calcium-doped zirconia after the sintering. The grain size in Ca_0.04Zr_0.96O_1.96 (Figure 7(c)), Ca_0.08Zr_0.92O_1.92 (Figure 7(d)), Ca_0.12Zr_0.88O_1.88 (Figure 7(e)), and Ca_0.16Zr_0.84O_1.84 (Figure 7(f)) is ∼ 0.583, 0.551, 0.620, and 0.586 μm, respectively. The stress developed by the incorporation of calcium and stress relieved during annealing as a statistical process determine the Ostwald ripening [51 -53] and hence, the random grain sizes in the samples.

Figure 7

FESEM image of (a) ZrO₂-RT (b) ZrO₂ (c) Ca_0.04Zr_0.96O_1.96 (d) Ca_0.08Zr_0.92O_1.92 (e) Ca_0.12Zr_0.88O_1.88 (f) Ca_0.16Zr_0.84O_1.84.

Figure 8

EDS elemental images (a) ZrO₂-RT (b) ZrO₂ (c) Ca_0.04Zr_0.96O_1.96 (d) Ca_0.08Zr_0.92O_1.92 (e) Ca_0.12Zr_0.88O_1.88 (f) Ca_0.16Zr_0.84O_1.84.

Table 6

The diameter of samples ZrO₂-RT, ZrO₂, Ca _x Zr_1–xO_2–x: x = 0.00, 0.04, 0.08, 0.12, and 0.16 after sintering and percentage shrinkage. The sample ZrO₂-RT has not been subjected to annealing.

Sample code	Diameter (mm)	Percentage of diameter shrinkage
ZrO₂-RT	10	0
ZrO₂	8.78	12.2
Ca_0.04Zr_0.96O_1.96	8.83	11.7
Ca_0.08Zr_0.92O_1.92	8.89	11.1
Ca_0.12Zr_0.88O_1.88	8.96	10.4
Ca_0.16Zr_0.84O_1.84	9.11	8.9

As carried out by EDS, the elemental analysis of all the samples is shown in Figure 8. Possible signature of Ca, O, and Zr is seen in the images. The atomic % of Ca, O, and Zr as deduced from the peak intensities is compared with the theoretical values calculated from the stoichiometry ratio and is illustrated in Table 7. An increase in the oxygen content in ZrO₂ as compared to ZrO₂-RT is due to the sintering in the open air. Furthermore, the Ca, O, and Zr elements are present almost in a stoichiometric ratio in the calcium-doped pellets. It indicates the complete reaction of the calcium oxides resulting in the occupancy of Ca at Zr sites with expected concentration. Also, Ca is mostly substituted on the Zr site (as supporting by XRD observations as well), when samples are sintered at high temperature.

Table 7

The elemental analysis using EDS measurements. Theoretical values as calculated from the stoichiometry ratio are compared with experimental findings.

Sample	Atomic % (Experimentally)			Atomic % (Theoretically)
Sample	Ca	O	Zr	Ca	O	Zr
ZrO₂-RT	0	67.78	32.22	0	66.67	33.33
ZrO₂	0	68.88	31.12	0	66.67	33.33
Ca_0.04Zr_0.96O_1.96	1.1	68.56	30.34	1.35	66.22	32.43
Ca_0.08Zr_0.92O_1.92	2.5	68.02	29.48	2.74	65.75	31.51
Ca_0.12Zr_0.88O_1.88	3.8	67.43	28.77	4.17	65.28	30.56
Ca_0.16Zr_0.84O_1.84	5.1	66.92	27.98	5.63	64.79	29.58

Raman analysis

To investigate the local structural properties of zirconia after doping of calcium and sintering at 1400°C, Raman spectroscopy of samples was done. Figure 9 shows the Raman spectra of all samples under consideration of the present study. The pure zirconia has shown the complex spectrum in the presence of several vibrational modes, which matches with the monoclinic phase of zirconia [54 -56] of CAS (Chemical Abstracts Service) Registry Number of 1314-23-4. The 14 Raman active bands (105.28, 179.46, 192.00, 223.95, 308.13, 335.55, 349.23, 384.69, 476.75, 503.64, 539.81, 559.83, 616.98, 639.47 cm⁻¹) indicates the presence of monoclinic phase ( ) of zirconia.

Figure 9

RAMAN spectra of samples ZrO₂-RT, ZrO₂, Ca _x Zr_1–xO_2–x: x = 0.04, 0.08, 0.12, and 0.16. Inset: Enlarged view (range: 575–665 cm⁻¹) of RAMAN spectra.

The general shape of the Raman spectrum of the cubic stabilised zirconia depends on the type and content of the incorporated cations. According to the literature data, the yttria-stabilised cubic zirconia presents [57,58] the most prominent band between 607 and 617 cm⁻¹, whereas the material stabilised with calcium oxide shows the same band at a lower wave number, around 600 cm⁻¹ [59]. Raman active bands (157.13, 276.48, 602.40 cm⁻¹) in 16 mol-% calcium-doped zirconia resemble high symmetry cubic phase [60,61] with fluorite structure ( ). The pure zirconia sintered at 1400°C shows a spectroscopic pattern similar to the pure zirconia at room temperature. With 4 and 8 mol-% calcium doping in zirconia, minor changes in the spectrum occur because the monoclinic phase dominates as compared to the cubic phase. The decrease in intensity of the major vibrational modes of the monoclinic phase and an increase in FWHM with increasing calcium content indicate the development of the cubic phase as already verified by XRD results.

A shift of the ∼ 639 cm⁻¹ band (E_g) with increasing doping percentage of calcium represents the stretching of the Zr-O_I bond [61]. As the calcium content is increased, FWHM of the ∼ 616 cm⁻¹ band decreases, and the monoclinic component starts disappearing, and eventually, for 16 mol-%, the calcium-doped zirconia shows the spectrum of the cubic phase. As reported in the XRD analysis for Ca_0.16Zr_0.84O_1.84 after sintering at 1400°C, it shows a fully stabilised cubic phase of zirconia and this conclusion is supported by Raman measurements as well.

Conclusion

From the present investigations of the phase transition in pure and 4–16 mol-% calcium-doped zirconia after sintering at 1400°C, and comparing the structural and morphological evolution with the undoped zirconia sample prepared at room temperature, it is concluded as under:

The XRD analysis shows that the pure zirconia (RT and sintered at 1400^0oC) exists in the monoclinic phase. With calcium doping in zirconia and sintering, the cubic phase starts developing and at 16% mole of calcium, a fully stabilised cubic phase (∼97%) is achieved.

Rietveld Refinement studies support well the experimental XRD data with GOF less than 3. Furthermore, no secondary phase of calcium is found in Rietveld Refinement studies which confirms that Ca is mostly substituted at the Zr site.

Doping with calcium stabilised the cubic phase of zirconia at room temperature because Ca²⁺ ion is larger than Zr⁴⁺ ion. The Raman measurements validate the findings of XRD.

The percentage of diameter shrinkage of pellets of pure zirconia and 16 mol-% calcium-doped zirconia after sintered at 1400^oC is 12.2 and 8.9, respectively. This indicates that with the increase in calcium content in zirconia, the shrinkage in the sample reduces.

The surface morphology as imaged using FESEM shows a high packing fraction of grains in sintered samples as compared to as prepared. Elemental analysis done through EDS is in good agreement with the stoichiometric ratio of the sample.

Major research works published so far show phase stabilisation of zirconia with yttria and, therefore, the present study with calcium incorporation may open a new channel for technological/industrial applications of zirconia.

Footnotes

Acknowledgements

One of the authors, Ankit Kumar is thankful to the Council of Scientific and Industrial Research (CSIR) for providing the Senior Research fellowship under file no. 09/797(0017)/2018-EMR-I.

Disclosure statement

No potential conflict of interest was reported by the author(s).

References

Sommers

, Wang

, Han

, Ceramics and ceramic matrix composites for heat exchangers in advanced thermal systems – a review. Appl Therm Eng. 2010;30(11–12):1277–1291. doi:10.1016/j.applthermaleng.2010.02.018.

Belin

, Martin

, Valenza

, Experimental insight into the radiation resistance of zirconia-based americium ceramics. Inorg Chem. 2009;48(12):5376–5381. doi:10.1021/ic900369b.

Kaur

, Gupta

, Kulriya

, Phase analysis and reduction behaviour of Ce dopant in zirconolite. J Radioanal Nucl Chem. 2019;322(1):183–192. doi:10.1007/s10967-019-06536-3.

Yoshimura

Phase stability of zirconia. Am Ceram Soc Bull. 1988;67(12):1950–1955.

Eichler

, Eisele

, Rödel

Mechanical properties of monoclinic zirconia. J Am Ceram Soc. 2004;87(7):1401–1403. doi:10.1111/j.1151-2916.2004.tb07748.x.

Cao

, Vassen

, Stöver

Ceramic materials for thermal barrier coatings. J Eur Ceram Soc. 2004;24(1):1–10. doi:10.1016/S0955-2219(03)00129-8.

López-Gándara

, Ramos

, Cirera

YSZ-based oxygen sensors and the use of nanomaterials: a review from classical models to current trends. J Sens. 2009;2009:1–15. doi:10.1155/2009/258489.

Wright

, Evans

AG.

Mechanisms governing the performance of thermal barrier coatings. Curr Opin Solid State Mater Sci. 1999;4(3):255–265. doi:10.1016/S1359-0286(99)00024-8.

Dimitrov

, Dushkin

, Petrova

, Oxygen detection using junctions based on thin films of yttria-stabilized zirconia doped with platinum nanoparticles and pure yttria-stabilized zirconia. Sens Actuators A. 2007;137(1):86–95. doi:10.1016/j.sna.2007.02.022.

10.

Manicone

, Iommetti

, Raffaelli

An overview of zirconia ceramics: basic properties and clinical applications. J Dent. 2007;35(11):819–826. doi:10.1016/j.jdent.2007.07.008.

11.

Marrero-López

, Ruiz-Morales

, Peña-Martínez

, Preparation of thin layer materials with macroporous microstructure for SOFC applications. J Solid State Chem. 2008;181(4):685–692. doi:10.1016/j.jssc.2008.01.008.

12.

Sathyaseelan

, Manikandan

, Baskaran

, Studies on structural and optical properties of ZrO₂ nanopowder for opto-electronic applications. J Alloys Compd. 2017;694:556–559. doi:10.1016/j.jallcom.2016.10.002.

13.

Kumar

, Kumar

, Dhaliwal

AS.

Synthesis and Rietveld refinement study of calcium doped zirconia. AIP Conf Proc. 2021;2352(1):020042. doi:10.1063/5.0052559.

14.

Akie

, Muromura

, Takano

, A new fuel material for once-through weapons plutonium burning. Nucl Technol. 1994;107(2):182–192. doi:10.13182/NT107-182.

15.

Lee

, Joung

, Kim

, Inert matrix fuel – a new challenge for material technology in the nuclear fuel cycle. Met Mater Int. 2001;7(2):159–164. doi:10.1007/BF03026954.

16.

Ruh

, Garrett

, Domagala

, The system zirconia-scandia. J Am Ceram Soc. 1977;60(9–10):399–403. doi:10.1111/j.1151-2916.1977.tb15521.x.

17.

Raza

, Cornil

, Oxygen vacancy stabilized zirconia (OVSZ); a joint experimental and theoretical study. Scr Mater. 2016;124:26–29. doi:10.1016/j.scriptamat.2016.06.025.

18.

, Chen

, Peng

, One-step preparation of CaO-doped partially stabilized zirconia from fused zirconia. Ceram Int. 2020;46(5):6484–6490. doi:10.1016/j.ceramint.2019.11.129.

19.

Ruff

, Ebert

, Stephen

Contributions to the ceramics of highly refractory materials: II. System zirconia-lime. Z Anorg Allg Chem. 1929;180(1):215–224.

20.

Kuo

, Shen

, Yen

, Phase transformation behavior of 3 mol% yttria partially-stabilized ZrO2 (3Y–PSZ) precursor powder by an isothermal method. Ceram Int. 2014;40(2):3243–3251. doi:10.1016/j.ceramint.2013.09.112.

21.

Drazin

, Castro

RH.

Phase stability in nanocrystals: a predictive diagram for yttria–zirconia. J Am Ceram Soc. 2015;98(4):1377–1384. doi:10.1111/jace.13504.

22.

Scott

HG.

Phase relationships in the zirconia-yttria system. J Mater Sci. 1975;10(9):1527–1535. doi:10.1007/BF01031853.

23.

Nightingale

, Worner

, Dunne

DP.

Microstructural development during the microwave sintering of yttria—zirconia ceramics. J Am Ceram Soc. 1997;80(2):394–400. doi:10.1111/j.1151-2916.1997.tb02843.x.

24.

Chen

, Khor

, Chan

, Influence of microstructure on the ionic conductivity of yttria-stabilized zirconia electrolyte. Mater Sci Eng A. 2002;335(1–2):246–252. doi:10.1016/S0921-5093(01)01935-9.

25.

Gaudon

, Laberty-Robert

, Ansart

, New chemical process for the preparation of fine powders and thin films of LSMx-YSZ composite oxides. Solid State Sci. 2003;5(10):1377–1383. doi:10.1016/S1293-2558(03)00181-X.

26.

Drazin

, Castro

RH.

Phase stability in calcia-doped zirconia nanocrystals. J Am Ceram Soc. 2016;99(5):1778–1785. doi:10.1111/jace.14151.

27.

Simpson

, Carter

RE.

Oxygen exchange and diffusion in calcia-stabilized zirconia. J Am Ceram Soc. 1966;49(3):139–144. doi:10.1111/j.1151-2916.1966.tb15391.x.

28.

Cales

, Baumard

JF.

Oxygen semipermeability and electronic conductivity in calcia-stabilized zirconia. J Mater Sci. 1982;17(11):3243–3248. doi:10.1007/BF01203490.

29.

Nakamura

, Wagner

JB.

Defect structure, ionic conductivity, and diffusion in calcia-stabilized zirconia. J Electrochem Soc. 1980;127(11):2325–2333. doi:10.1149/1.2129406.

30.

Kelly

, Denry

Stabilized zirconia as a structural ceramic: an overview. Dent Mater. 2008;24(3):289–298. doi:10.1016/j.dental.2007.05.005.

31.

Chevalier

, Gremillard

, Virkar

, The tetragonal-monoclinic transformation in zirconia: lessons learned and future trends. J Am Ceram Soc. 2009;92(9):1901–1920. doi:10.1111/j.1551-2916.2009.03278.x.

32.

, Brook

RJ.

Kinetics of densification in stabilized zirconia. Solid State Ionics. 1984;14(2):123–130. doi:10.1016/0167-2738(84)90086-9.

33.

Nath

, Sinha

, Basu

Microstructure, mechanical and tribological properties of microwave sintered calcia-doped zirconia for biomedical applications. Ceram Int. 2008;34(6):1509–1520. doi:10.1016/j.ceramint.2007.04.021.

34.

Zhang

, Gao

, Kang

, Stability optimisation of CaO-doped partially stabilised zirconia by microwave sintering. Ceram Int. 2019;45(17):23278–23282. doi:10.1016/j.ceramint.2019.08.024.

35.

Raghavendra

, Dasgupta

, Bhaskar

, Synthesis and characterization of Fe-15 wt.% ZrO₂ nanocomposite powders by mechanical milling. Powder Technol. 2016;287:190–200. doi:10.1016/j.powtec.2015.10.003.

36.

Jõgiaas

, Kull

, Seemen

, Optical and mechanical properties of nanolaminates of zirconium and hafnium oxides grown by atomic layer deposition. J Vac Sci Technol A Vac Surf Films. 2020;38(2):022406. doi:10.1116/1.5131563.

37.

Park

, Heo

, Chang

TS.

Dry reforming of methane over Ni-substituted CaZrNiOx catalyst prepared by the homogeneous deposition method. Catal Commun. 2019;120:1–5. doi:10.1016/j.catcom.2018.11.006.

38.

Garvie

, Nicholson

PS.

Phase analysis in zirconia systems. J Am Ceram Soc. 1972;55(6):303–305. doi:10.1111/j.1151-2916.1972.tb11290.x.

39.

Joshi

, Kanchan

, Joshi

, Dielectric relaxation, complex impedance and modulus spectroscopic studies of mix phase rod like cobalt sulfide nanoparticles. Mater Res Bull. 2017;93:63–73. doi:10.1016/j.materresbull.2017.04.013.

40.

Kumar

, Kumar

, Dhaliwal

AS.

Structural studies of zirconia and yttria doped zirconia for analysing it phase stabilization criteria. IOP Conf Ser Mater Sci Eng. 2021;1033(1):012052. doi:10.1088/1757-899X/1033/1/012052.

41.

Cullity

BD.

Elements of X-ray diffraction. London: Addison-Wesley Publishing; 1956.

42.

Lokesha

, Nagabhushana

, Singh

Evidence of luminescence modification with structure of zirconia phases. J Lumin. 2017;192:173–179. doi:10.1016/j.jlumin.2017.06.042.

43.

Hicks

, Mehl

, Gossett

, The AFLOW library of crystallographic prototypes: part 2. Comput Mater Sci. 2019;161:S1–S1011. doi:10.1016/j.commatsci.2018.10.043.

44.

Wang

, Guo

, Liang

, Crystal structure of zirconia by Rietveld refinement. Sci China A Math. 1999;42(1):80. doi:10.1007/BF02872053.

45.

Bhagwat

, Ramaswamy

, Tyagi

, Rietveld refinement study of nanocrystalline copper doped zirconia. Mater Res Bull. 2003;38(13):1713–1724. doi:10.1016/S0025-5408(03)00201-0.

46.

Momma

, Izumi

VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J Appl Crystallogr. 2011;44(6):1272–1276. doi:10.1107/S0021889811038970.

47.

Kamardan

, Zaidi

, Dalimin

, The sintering temperature effect on the shrinkage behavior of cobalt chromium alloy. Amer J Appl Sci. 2010;7(11):1443–1448. doi:10.3844/ajassp.2010.1443.1448.

48.

Wen

, Zhu

, Xie

, A strategy to obtain a high-density and high-strength zirconia ceramic via ceramic injection molding by the modification of oleic acid. Int J Miner Metall Mater. 2017;24(6):718–725. doi:10.1007/s12613-017-1455-9.

49.

Anand

PSP

, Arunachalam

, Vijayaraghavan

Effect of grinding on subsurface modifications of pre-sintered zirconia under different cooling and lubrication conditions. J Mech Behav Biomed Mater. 2018;86:122–130. doi:10.1016/j.jmbbm.2018.06.026.

50.

Rueden

, Schindelin

, Hiner

, ImageJ2: ImageJ for the next generation of scientific image data. BMC Bioinform. 2017;18(1):1–26. doi:10.1186/s12859-017-1934-z.

51.

Dickerson

, Swain

, Heuer

AH.

Microstructural evolution in Ca-PSZ and the room-temperature instability of tetragonal ZrO₂. J Am Ceram Soc. 1987;70(4):214–220. doi:10.1111/j.1151-2916.1987.tb04970.x.

52.

Bateman

, Notis MR.

Coarsening kinetics in Mg (Ca, Ti. Y) – partially-stabilized zirconias. J Am Ceram Soc. 1992;75(6):1566–1569. doi:10.1111/j.1151-2916.1992.tb04226.x.

53.

Guo

, Wang

, Zhao

, Preparation of nano-ZrO₂ powder via a microwave-assisted hydrothermal method. Ceram Int. 2021;47(9):12425–12432. doi:10.1016/j.ceramint.2021.01.099.

54.

Gazzoli

, Mattei

, Valigi

Raman and X-ray investigations of the incorporation of Ca2+ and Cd2+ in the ZrO₂ structure. J Raman Spectrosc. 2007;38(7):824–831.

55.

, Li

UV Raman spectroscopic study on the phase transformation of ZrO₂, Y₂O₃–ZrO₂ and SO₄²⁻/ZrO₂. J Raman Spectrosc. 2002;33(5):301–308. doi:10.1002/jrs.863.

56.

Kwon

, Jang

, Lee

, Investigation of the electrical conductivity of sintered monoclinic zirconia (ZrO₂). Ceram Int. 2017;43(11):8236–8245. doi:10.1016/j.ceramint.2017.03.152.

57.

Cai

, Raptis

, Temperature dependence of Raman scattering in stabilized cubic zirconia. Phys Rev B. 1995;51(1):201. doi:10.1103/PhysRevB.51.201.

58.

Morell

, Katiyar

, Torres

, Raman scattering study of thermally reduced stabilized cubic zirconia. J Appl Phys. 1997;81(6):2830–2834. doi:10.1063/1.363941.

59.

, Tan

, Liu

, Growth and optical properties of thulia-doped cubic yttria stabilized zirconia single crystals. Ceram Int. 2019;45(13):15974–15979. doi:10.1016/j.ceramint.2019.05.106.

60.

Yashima

, Ohtake

, Kakihana

, Determination of tetragonal-cubic phase boundary of Zr1− XRXO2− X2 (R = Nd, Sm, Y, Er and Yb) by Raman scattering. J Phys Chem Solids. 1996;57(1):17–24. doi:10.1016/0022-3697(95)00085-2.

61.

Hemberger

, Wichtner

, Berthold

, Quantification of yttria in stabilized zirconia by Raman spectroscopy. Int J Appl Ceram Technol. 2016;13(1):116–124. doi:10.1111/ijac.12434.