High-temperature stability of nanozirconate-toughed IMF material lanthanum synthesized by an in situ reaction

Abstract

Herein, powders composed of La₂Zr₂O₇ (LZ) and ZrO₂ phases were synthesized by an in situ reaction using a sol-spray pyrolysis method; moreover, 24 mol% LaO_1.5–ZrO₂ (volume ratio = 1 : 1) powders were characterized by XRD, Raman spectroscopy, SEM, and TEM. XRD and Raman results showed that the samples maintained a tetragonal ZrO₂ and a pyrochlore LZ phase from 900 to 1100 °C. The addition of LZ could be helpful in the stabilization of t-ZrO₂ and decreasing the grain size of ZrO₂. The SEM results revealed that the LZ and ZrO₂ phases were homogeneously distributed in the sintered bulk. The HRTEM results suggested that the crystal orientations of the nano-LZ and nano-ZrO₂ phases were accordant; this was in agreement with the characteristics of the coherent boundaries. The fracture toughness of LZ–ZrO₂ was markedly improved by the transformation toughening of the ZrO₂ phase, and a value that was 2.2-fold that of the LZ prepared by a similar technique was achieved.

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1. Introduction

In inert matrix (IM) materials, plutonium is inlaid in a U-free matrix; hence, plutonium can be burnt without breeding any new plutonium by neutron capture in ²³⁸U. The basic criteria for IM materials are as follows: (i) stable irradiation behavior; (ii) good heat transport properties and phase stability at high working temperatures; and (iii) suitable mechanical properties such as elastic constants, fracture toughness and so on.^1–4 La₂Zr₂O₇ (LZ) with a pyrochlore (P) structure is one of the most evaluated potential IM materials because of its good stable irradiation behavior and phase stability at high working temperatures.^5,6

However, the low thermal conductivity and fracture toughness of LZ has limited its applications in a fuel cycle that relies on a reprocessing strategy. The use of higher-thermal-conductivity materials (such as MgO and CeO₂) as a dual phase is a general method to improve the thermal conductivity of the IM materials.^7,8 However, only a few studies have been reported on the toughening of LZ. The incorporation of zirconia (ZrO₂) with high toughness into alumina ceramics to deliberately toughen them has been widely carried out to achieve zirconia-toughened alumina (ZTA) systems.^9,10 Herein, ZrO₂ has been incorporated into LZ to toughen it. Moreover, ZrO₂ has a relatively high thermal conductivity with additional excellent radioactive resistance and good mechanical properties.^11,12 The tetragonal (t)–monoclinic (m) transition of zirconia is believed to easily occur at temperatures less than 1150 °C.¹³ This phase transition accompanying a change in the exterior environment induces a volume dilation of ∼4 vol%. The stresses resulting from the volumetric changes can induce crack openings and thus limit the application of zirconia.¹⁴

Nanosized materials have relatively low activation energy for grain-boundary migration because of the increased surface (interface)/volume ratio as compared to their bulk counterparts.^15,16 For nanozirconia, when the crystallite size is ∼30 nm or less, this phase transition occurs slowly. Moreover, nanocrystalline materials have a superior radiation resistance.¹⁷

The use of second-phase particles causes a grain-boundary motion by drag and can retard the grain growth of the matrix particles. It was reported that less than 1 mol% LaO_1.5 could be dissolved in ZrO₂.^18,19 Based on the phase diagram of the LaO_1.5–ZrO₂ system and the previous study, a pyrochlore-structured LZ phase exists in a wide concentration range of LaO_1.5. The phase composition of the xLaO_1.5 − (1 − x)ZrO₂ (x ≤ 1) system at room temperature consists of LZ and ZrO₂ with the following chemical formula:

(1 − x)LaO_1.5 + xZrO₂ → (1 − x)/2La₂Zr₂O₇ + (2x − 1)ZrO₂ (1)

Soft solution chemistry, such as a sol–gel method, has been used to fabricate lanthanum zirconate and yttria-stabilized zirconia.²⁰ It provides a possible route for the production of nanocrystalline LZ and nanocrystalline ZrO₂ compounds in the nanometer scale. As a result, the grain size of ZrO₂ or LZ in the composite will be smaller than that of the single-phase counterparts. It is suggested that the stabilization of t-ZrO₂ in the LaO_1.5–ZrO₂ system may result from the impact of the LZ phase.

The objectives are to (i) prepare nano–nano LZ–ZrO₂ composite powders by an in situ reaction using a sol-spray pyrolysis method, (ii) measure the toughness of LZ–ZrO₂, and (iii) study the high-temperature stability of LZ–ZrO₂; this provides a method to toughen not only LZ, but also all other pyrochlore IM materials (such as Pu₂Zr₂O₇ (ref. 21) and Nd₂Zr₂O₇ (ref. 8)); this will benefit the emerging area of IM research related to alternate ceramics.

2. Experimental procedures

2.1. Sample preparation

For the preparation of the LZ–ZrO₂ nanocomposite powders, 99.99% purity Zr(NO₃)₄·3H₂O and La(NO₃)₃·6H₂O (Aladdin Chemistry Co., Ltd., Shanghai, China) were used as the starting materials and dissolved in deionized water; the concentration of Zr⁴⁺ and La³⁺ was 0.1 M. In the starting solution, citric acid (C₆H₈O₇·H₂O, 80 g L⁻¹) and polyethylene glycol (PEG, molecular weight = 20 000, 50 g L⁻¹) were used as the chelating agent and the plastering agent, respectively. The solution was stirred using a magnetic stirrer for 30 min to achieve complete dissolution. The resultant solution was then poured into a polytetrafluoroethylene (PTFE) tube and atomized by a stainless steel nozzle. Then, the precursors obtained from the sol-spray pyrolysis were annealed at 900–1200 °C for 6 h in high-purity Al₂O₃ crucibles and cooled down to room temperature (heating and cooling rates = 2 °C min⁻¹). To obtain dense samples, the powders were heated under an applied pressure of 40 MPa to 1450 °C at the constant heating rate of 100 min⁻¹ for 10 min and rapidly cooled down to room temperature. All the bulk samples prepared in this study had a diameter of 15 mm and a height of approximately 5 mm. The bulk samples were polished using a 0.025 μm diamond paste and thermally etched at 1200 °C for 10 h in air. The density of the samples was determined by the Archimedes method.

2.2. Characterization of the sample composition and microstructure

X-ray diffraction (XRD) analysis of the samples was carried out using the X'Pert PRO diffractometer with Cu Kα radiation (λ = 0.15406 nm; PANalytical, Almelo, the Netherlands). The XRD patterns were obtained in the 2θ range of 10–90° at room temperature at the scanning rate of 0.05° s⁻¹ and a step size of 0.033°. The crystallite size was calculated using the Scherrer's formula for the 440, 220 and 111 peaks of La₂Zr₂O₇, t-ZrO₂, and m-ZrO₂, respectively:where D is the crystallite size (nm), λ is the wavelength (nm), β_obs is the FWHM, β_obs is the instrumental broadening parameters (0.05°), and θ is the diffraction angle.

Raman spectra were obtained using the Renishaw InVia Raman spectrometer (Renishaw plc, New Mills, UK). The emission line at 514.5 nm obtained from an Ar⁺ ion laser was used as the excitation source. The samples of the bulk were observed using a field emission scanning electron microscope (FESEM, Apollo 300 FE, Obducat CamScan Ltd., Cambridge, UK) operated at 30 kV. The TEM observation of the samples was conducted using a transmission electron microscope (TEM, JEM-2010, JEOL, Tokyo, Japan).

2.3. Characterization of the sample fracture toughness

A Vickers hardness tester (HV-1000A, Huayin Test Instrument Co., Ltd., Yantai, China) was used to measure the indentation fracture toughness. The indentations were made on polished surfaces with a load (F) of 9.8 N held for 10 s. The fracture toughness (K_IC, MPa m^1/2) for each sample was estimated from a minimum of 9 indents to reduce the experimental uncertainty using the following equation:^22,23where H_V is the hardness, a is half the diagonal length of the Vickers indent (μm), and c is the crack length measured from the indent center (μm). The values of a and c were measured using an SEM instrument.

3. Results and discussion

3.1. Characterization of the LZ–ZrO₂ powders

X-ray diffraction was employed to investigate the crystalline phases of lanthanum zirconate and zirconia. The XRD patterns of xLaO_1.5 − (1 − x)ZrO₂ (x = 0–0.5) powders obtained after annealing the powders at 1200 °C for 6 h are shown in Fig. 1. In the pattern of pure ZrO₂, no clear evidence is found for the presence of the t and c peaks. The shifts of the (−111) and (111) peaks in the pattern of 1 mol% LaO_1.5–ZrO₂ are an indication that LaO_1.5 has been completely doped into the ZrO₂ crystal by substituting La³⁺ for Zr⁴⁺. The presence of the pyrochlore structure is confirmed by the occurrence of the superlattice 311, 331, and 511 peaks.²⁴ The results indicate that the solubility of LaO_1.5 in ZrO₂ is lower than 1 mol%; this in line with that reported in literature.^18,19 The broadening of the m (111) peaks indicated that the grain size of m-ZrO₂ decreased with an increase in the content of LZ. Considering the grain size effect of ZrO₂, the presence of the (101) peak for t-ZrO₂ could be attributed to the grain refinement.

Fig. 1 XRD patterns of the xLaO_1.5 − (1 − x)ZrO₂ (x = 0–50 mol%) powders after annealing at 1200 °C for 6 h in air.

Table 1 summarizes the calculated grain sizes of m-ZrO₂ in xLaO_1.5 − (1 − x)ZrO₂ (x = 0–0.4) powders after annealing at 1200 °C for 6 h. The data showed that the grain size of the m phase gradually decreased with the increasing concentration of LaO_1.5, reaching a value that was approximately equal to the critical grain size of t- to m-ZrO₂ at the addition concentration of 10 and 24 mol%. The (101) peak for t-ZrO₂ significantly increased at the concentration of 24 mol%. The lattice parameters of LZ and ZrO₂ were determined based on the XRD patterns shown in Fig. 1. The volume ratios of LZ and m-ZrO₂ in the samples could be easily calculated using the weight and the number of formula units per elementary cell, which were approximately 1/4, 1/1 and 4/1 for 10, 24 and 40 mol% LaO_1.5–ZrO₂ samples, respectively. The 24 mol% LaO_1.5–ZrO₂ powders were chosen for further characterization.

Table 1 The calculated crystallite sizes and phase composition of m-ZrO₂ in xLaO_1.5 − (1 − x)ZrO₂ (x = 0–0.4) powders after annealing at 1200 °C for 6 h^a

LaO1.5 concentration (mol%)	Phase composition	Crystallite size of m-ZrO2 (nm)
a P – pyrochlore-type La2Zr2O7; m – m-ZrO2; t – t-ZrO2; *small amount.
0	m	38.8
1	m	40.1
3	m + P	32.6
5	m + P+ t*	31.0
8	m + P + t*	28.1
10	m + P + t*	22.5
24	P + m + t	23.4
40	P + t + m*	—

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3.2. Fracture toughness

Fig. 2(a) shows the fracture surface of the 24 mol% LaO_1.5–ZrO₂ spark plasma sintered (SPS) bulk sample after thermal etching at 1100 °C for 2 h. The SEM image reveals that the particle size of the SPS bulk sample is in the range of 50–200 nm. The SEM image reveals that the sintered sample has a dense microstructure, which is very important for the mechanics performance testing. The densities of the sintered LZ and 24 mol% LaO_1.5–ZrO₂ specimens were determined by the Archimedes method to be 97.6% and 95.8% of the theoretical values, respectively.

Fig. 2 (a) The SEM image of the 24 mol% LaO_1.5–ZrO₂ bulk sample. (b) Fracture toughness of the 24 mol% LaO_1.5–ZrO₂ and La₂Zr₂O₇ bulk samples. The data for La₂Zr₂O₇ is obtained from the study reported by Vassen et al.²⁵ Inset shows a Vickers indentation with cracks (polished, SEM, SE) by a 1 kg load of 24 mol% LaO_1.5–ZrO₂ ceramics.

Fig. 2(b) shows the fracture toughness of the 24 mol% LaO_1.5–ZrO₂ sample, and the data have been compared with that of the LZ materials. The experimental results for the LZ samples are in good agreement with those reported in the literature.²⁵ The data showed that the toughness level of the 24 mol% LaO_1.5–ZrO₂ was 120% greater than that of LZ (∼1.0 MP m^1/2). The high fracture toughness of 24 mol% LaO_1.5–ZrO₂ (as compared to that of LZ) can be attributed to the transformation toughening from the martensitic transformation of the t to the m phases.

3.3. High-temperature stability and grain growth

Fig. 3 shows the XRD patterns of ZrO₂ and 24 mol% LaO_1.5–ZrO₂ powders after annealing at 500–1100 °C. Pure ZrO₂ presents clear t peaks at 500 °C. With an increase in the annealing temperature from 600 to 800 °C, pure ZrO₂ undergoes a t to m phase transformation. The grain size of the t phase was below 30 nm (see Table 2), in good agreement with the critical grain size for the t- to m-ZrO₂ transformation.²⁶ Compared with the case of pure ZrO₂, no clear evidence was found for the shift of the t (101) peaks in any of the patterns of 24 mol% LaO_1.5–ZrO₂; this indicated that the solubility of La³⁺ in the ZrO₂ phase was negligible. Table 2 lists the grain sizes of ZrO₂ and 24 mol% LaO_1.5–ZrO₂ powders calculated from the Scherrer's formula on the basis of Fig. 3. For pure ZrO₂, the crystallite size of t-ZrO₂ increased from 9.1 to 26.4 nm as the temperature was increased from 500 to 800 °C, and t-ZrO₂ disappeared above 900 °C. The crystallite sizes of LZ and ZrO₂ in the 24 mol% LaO_1.5–ZrO₂ system gradually increased from 19.5 to 38.1 nm and from 8.7 to 38.5 nm as the annealing temperature was elevated from 900 to 1100 °C. When the addition concentration of LaO_1.5 reached 24 mol%, the critical grain size for the t to m phase transformation increased from ∼20 to ∼40 nm when the corresponding heat treatment temperature was increased from 600 to 1100 °C. The data showed that the addition of LZ could be helpful in decreasing the grain size of ZrO₂.

Fig. 3 XRD patterns of ZrO₂ and 24 mol% LaO_1.5–ZrO₂ powders after annealing at 500–1100 °C for 6 h.

Table 2 The calculated crystallite sizes for ZrO₂ and 24 mol% LaO_1.5–ZrO₂ after annealing at 500–1100 °C for 6 h

Sample (°C)	Crystallite size (nm)
	ZrO2		24 mol% LaO1.5–ZrO2
	t	m	t	P
500	9.1	—	—	—
600	11.2	—	—	—
700	21.3	24.9	—	—
800	26.4	24.8	—	—
900	—	29.9	8.7	19.5
1000	—	42.9	16.9	24.1
1100	—	—	38.5	38.1

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Raman spectroscopy is another useful technique that is generally used to investigate the structures of lanthanum zirconate and zirconia. Fig. 4 shows the Raman spectra of the 24 mol% LaO_1.5–ZrO₂ powders annealed at 800, 900, and 1100 °C and the P–LZ samples.²⁷ The bands at 298, 389, 494, 511, and 744 cm⁻¹ were assigned to the A_1g + E_g + 4F_2g Raman active vibration modes (gray arrows) of the P–LZ sample. The bands corresponding to the A_1g + 3E_g modes (black arrows) of t-ZrO₂ were clearly observed at 273, 463, 600 and 646 cm⁻¹ in the spectra of the 24 mol% LaO_1.5–ZrO₂ powders,²⁸ in good agreement with the XRD results.

Fig. 4 Raman spectra of 24 mol% LaO_1.5–ZrO₂ powders after annealing at 800, 900, and 1100 °C for 6 h, and La₂Zr₂O₇.

Fig. 5(a) and 4(b) show the SEM images of the 24 mol% LaO_1.5–ZrO₂ bulk samples after sintering at 1300 °C for 48 h at different magnifications. Fig. 5(a) reveals that the samples have a homogeneous microstructure. The particle size distribution of the samples is shown in Fig. 5(c), and the observed grain size of the samples is in the range of 0.1–1.4 μm. Fig. 5(b) reveals that the crystal grains of the sample are near spheroidal in shape. The SEM image also shows that the grains of the sample are closely bonded with each other, and the interfaces between grains are legible. In a previous study on the nano–nano composites of lanthanum zirconate and yttria-stabilized zirconia,²⁰ the activation energy for the grain growth of LZ (225 ± 12 kJ mol⁻¹) was much lower than that of 10YSZ (yttria-stabilized zirconia, 382 ± 17 kJ mol⁻¹); this suggested that the first step was the formation of LZ and nuclei, and then, ZrO₂ nucleated on the surface of the LZ crystalline grains during the annealing process. Therefore, the small crystals were the ZrO₂ nuclei, which were evenly distributed on the surface of the LZ nuclei via encircling and insertion.

Fig. 5 (a) and (b) SEM images of 24 mol% LaO_1.5–ZrO₂ bulk samples after sintering at 1300 °C for 48 h, and (c) particle size distribution of the 24 mol% LaO_1.5–ZrO₂ bulk sample obtained from the SEM images.

To rationalize these results, the phase transformation mechanism of ZrO₂ was considered. Pure zirconia has three polymorphs under ordinary pressure, and the monoclinic type is the most frequent structure at room temperature and up to 1170 °C. In our case, pure ZrO₂ and ZrO₂ coexisting in the composite powders possess the t phase at 500 °C and 800–1100 °C, as confirmed by the XRD measurements. The appearance of the high-temperature phase at low temperatures has been discussed, and Parija et al.²⁹ have proposed that the main reason for the stabilization is the crystallite size effect. According to their opinion, the following equation for the radius of the critical crystallite size r_c for the t–m transformation has been proposed:

r_c = −3Δσ/[q(1 − T/T_b) + Δε] (4)

where Δε is the change in the strain energy/unit volume for a particle, q is the heat of the t–m transformation, Δσ is the difference between the surface-free energies of the m and t phases of ZrO₂, T_b is the temperature at which the t–m transformation occurs for ZrO₂ (1170 °C), and T is the temperature at which the crystallites of a given radius r_c undergo the t–m transformation. At room temperature, the following parameter values were used for the calculation of r_c: Δσ = 0.91 J m⁻², q = −2.82 × 10⁸ J m⁻¹,³ T_b = 1443 K, T = 298 K, and Δε = 0.46 × 10⁸ J m⁻¹.³ The value of r_c is ca. 15 nm.

The grain size of pure ZrO₂ presented in Table 1 corresponds well with the calculated r_c. Similar to that of 24 mol% LaO_1.5–ZrO₂, the t-ZrO₂ phase is stable even at the grain size of 38.5 nm. As indicated in eqn (3), the value of r_c depends on Δσ, q, T_b, T, and Δε. In Fig. 2, no peak shift of the t and m phases was observed for all the samples; this indicated that La was not doped into the ZrO₂ crystal. This suggests that the values of Δσ and q are consistent with those of pure ZrO₂. T_b and T are constant; thus, the change in the strain energy may be the main factor for the increased r_c.

In the Raman spectra (Fig. 4), a significant shift (∼14 cm⁻¹) towards lower frequencies for t-ZrO₂ was observed in the range of 600–650 cm⁻¹ with a decrease in the annealing temperature from 1100 to 900 °C. Djurado et al.²⁶ reported that the grain size effect on the Raman spectra was much smaller than the pressure-induced effect, and the maximum shift value was less than 10 cm⁻¹. This suggests that stress exists in the LaO_1.5–ZrO₂ composite samples. Furthermore, the SEM images (Fig. 5(b)) show that small ZrO₂ particles are embedded into the large LZ particles. In other words, the ZrO₂ particles grew at the grain boundary.

To confirm the grain-boundary growth, high-resolution TEM characterization was performed. Fig. 6 shows a TEM image of the 24 mol% LaO_1.5–ZrO₂ powders after annealing at 1000 °C for 6 h. The lattice fringes with 0.620 and 0.315 nm spacings can be assigned to the 111 lattice plane of pyrochlore–LZ (P–LZ) and the 111 lattice plane of t-ZrO₂ phase, respectively. The TEM image shows that the ZrO₂ crystal grows along the surface of the LZ crystal, and the crystal orientation of the two phases is accordant, in line with the characteristics of coherent boundaries. The coherent boundaries with low excess energies are believed to retard the grain growth of the matrix particles because the grain growth of nanosized materials is controlled by the grain-boundary migration. Hence, the stabilization mechanism of the t-ZrO₂ phase of the LZ–ZrO₂ composite powders should be attributed to the grain refinement and lattice stress effects.

Fig. 6 TEM images of 24 mol% LaO_1.5–ZrO₂ powders after annealing at 1000 °C for 6 h.

Based on the abovementioned analysis, the lowest entropy will be selected during the in situ reaction. A possible mechanism for the formation of the LaO_1.5–ZrO₂ nano–nano composite powders has been suggested as follows: the first step is the formation of LZ and nuclei, and then, ZrO₂ nucleates on the surface of LZ crystalline grains during the annealing process.

4. Conclusions

In summary, the addition of LaO_1.5 to ZrO₂ forms a pyrochlore structural La₂Zr₂O₇ phase and induces a tetragonal to monoclinic (t to m) phase transition of ZrO₂. The addition of LZ could be helpful in stabilizing t-ZrO₂ and decreasing the grain size of ZrO₂. The t-ZrO₂ phase in the 24 mol% LaO_1.5–ZrO₂ powders was phase-stable up to a calcination temperature of 1100 °C. Since the solubility of LaO_1.5 in ZrO₂ is lower than 1 mol%, the presence of the t-ZrO₂ phase in the LaO_1.5–ZrO₂ system does not arise from doping stabilization by the substitution of La³⁺ for Zr⁴⁺, but rather from the grain refinement of the LZ phase. The crystal orientation of the nano-LZ and nano-ZrO₂ phases was accordant, in line with the characteristics of the coherent boundaries. The fracture toughness of La₂Zr₂O₇–ZrO₂ has been markedly improved by the transformation toughening of the ZrO₂ phase, reaching a value that is 2.2-fold that of La₂Zr₂O₇ prepared by a similar technique.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Dr Ruishi Xie, Dr Zhaohui Zhou, and Dr Huiping Duan for assistance with XRD, SEM, and TEM. This work was supported by the project of NSFC-11505146 and Defense Industrial Technology Development Program JCKY2016404C001 and Longshan Academic Talent Research Supporting Program of SWUST 18LZX556.

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