Thermal and Mechanical Properties Optimization of ABO4 Type EuNbO4 By the B-Site Substitution of Ta

Abstract

Ferroelastic ABO₄ type RETaO₄ and RENbO₄ ceramics (where RE stands for rare earth) are being investigated as promising thermal barrier coatings (TBCs), and the mechanical properties of RETaO₄ have been found to be better than those of RENbO₄. In this work, B-site substitution of tantalum (Ta) is used to optimize the thermal and mechanical properties of EuNbO₄ fabricated through a solid-state reaction (SSR). The crystal structure is clarified by means of X-ray diffraction (XRD) and Raman spectroscopy; and the surface microstructure is surveyed via scanning electronic microscope (SEM). The Young's modulus and the thermal expansion coefficient (TEC) of EuNbO₄ are effectively increased; with respective maximum values of 169 GPa and 11.2×10^–6 K^–1 (at 1200 °C). The thermal conductivity is reduced to 1.52 W·K⁻¹·m⁻¹ (at 700 °C), and the thermal radiation resistance is improved. The relationship between the phonon thermal diffusivity and temperature was established in order to determine the intrinsic phonon thermal conductivity by eliminating the thermal radiation effects. The results indicate that the thermal and mechanical properties of EuNbO₄ can be effectually optimized via the B-site substitution of Ta, and that
this proposed material can be applied as a high-temperature structural ceramic in future.

Content

《1. Introduction》

1. Introduction

Ferroelastic rare earth tantalates and niobates (RETaO₄ and RENbO₄, where RE stands for rare earth) are being researched for diverse applications, according to their individual properties [1–5]. The investigative fields of RENbO₄ include protonconducting solid oxide fuel cells, microwave dielectric materials, and shape memory materials [5–8]. The prominent properties of rare earth niobates come from their distinctive crystal structure and the various ligands of niobium (Nb). The crystal structure of RENbO₄ is dominated by Nb, and RENbO₄ undergoes a reversible ferroelastic crystal structure transformation with variation of temperature [2,4,5]. At high temperatures, RENbO₄ is in a tetragonal (t ) phase, which transforms to a monoclinic (m ) phase at room temperature [2,5]. The t–m transformation temperature of RENbO₄ is between 500 and 800 °C, depending on the rare earth elements [2,5]. Usually, an evident change in unit cell volume is detected during crystal structure transformation; however, such change is not found in RENbO₄ and RETaO₄ [2,4,5,8]. Current documents prove that the ferroelastic t–m transformation of RENbO₄ and RETaO₄ is a natural second-order transition; no atomic rearrangement is detected. Therefore, volume variation in RENbO₄ and RETaO₄, that is caused by t–m transformation is neglected.

RETaO₄ exhibits a crystal structure that is analogous to that of RENbO₄. Different crystal structures are found in RETaO₄, which is ascribed to decrease in RE³⁺ ionic radius. RETaO₄ (where RE = Y, Nd–Er) has the m phase, while the rest have the metastable monoclinic (m') phase [1,8]. Furthermore, RETaO₄ exhibits a much higher t–m transition temperature than RENbO₄. For example, the transition temperature of YTaO₄ is about 1430 °C, while it is less than 800 °C for RENbO₄ [2,8]. Ferroelastic toughness is a critical property that allows 6 wt%–8 wt% yttria-stabilized zirconia (6–8YSZ) to be applied as a thermal barrier coating (TBC) [9–12]. However, the working temperature limit of yttria-stabilized zirconia (YSZ) is below 1200 °C because of phase transition, which results in a huge volume change. Much effort has been devoted to optimizing the properties of YSZ, and many materials are being investigated as TBCs [13–19]. Herein, ferroelastic RETaO₄ and RENbO₄ are studied as TBCs with a higher application temperature to replace 6–8YSZ. RETaO₄ possesses better thermal and mechanical properties than RENbO₄, due to the characteristic properties of tantalum (Ta). In addition, the weak bonding strength of RENbO₄ produces an inferior hardness and Young’s modulus, which makes it less useful for application as high-temperature TBCs. Nevertheless, lower centrifugal force will be produced when RENbO₄ is applied as TBCs in comparison with RETaO₄, due to the lower density. To modify the properties of RENbO₄, the B-site substitution of Ta is attempted for EuNbO₄ by applying the atomic weight misfit between Ta and Nb and the difference in bond strength between the Ta–O and Nb–O bonds.

In this paper, EuNb_1–XTa_XO₄ (composition parameter X = 0/6, 1/6, 2/6, 3/6, 4/6) specimens were fabricated via a solid-state reaction (SSR). The crystal structure was clarified by means of X-ray diffraction (XRD) and Raman spectra. The surface grain size, pores, and cracks were surveyed by means of scanning electronic microscope (SEM). The thermal and mechanical properties (i.e., heat capacity, thermal diffusivity and conductivity, thermal radiation resistance, thermal expansion performance, inharmonic lattice vibration strength, and Young’s modulus) were modified by the B-site substitution of Ta. This work stresses that EuNbO₄ ceramics are promising TBCs via further property optimization.

《2. Experimental process》

2. Experimental process

The EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) bulk specimens were synthesized by SSR. Crude substances included Eu₂O₃, Ta₂O₅, and Nb₂O₅ powders and C₂H₅OH (Shanghai Aladdin BioChem Technology Co., Ltd., China). The weighted substance was ball-milled (720 min, 240 r·min^-1 ) within C₂H₅OH. The mixture was kept at 90 °C for 840 min to eliminate C₂H₅OH. The arid mixtures were pressed into a bulk with a radius of 7.5 mm and a thickness of 2 mm. Before sintering, the bulk samples were held at 280 MPa for 8 min, they were then sintered at 1400–1600 °C for 10 h to obtain dense samples.

The crystal structure was confirmed by means of XRD (MiniFlex600, Rigaku Corporation, Japan). Raman spectroscopy was employed to research the change in crystal structure, along with XRD. A confocal spectrometer (Horiba–Jobin Yvon, Horiba, Ltd., USA) was utilized to collect Raman spectra using a He–Ne ion laser (532 nm). SEM (EVO 180, Zeiss, Germany) was employed to survey the superficial morphology, because the grain size, pores, and cracks affected the thermal and mechanical properties.

The longitudinal (V_L) and transverse (V_T) acoustic velocities of EuNb_1–XTa_XO₄ were calculated by determining the transmission interval through an ultrasonic pulser/receiver instrument (UMS– 100, TECLAB, France). Various properties were identified [20]:

The thermal expansion coefficients (TECs) were determined by means of a thermal expansion rate curve. Thermo–mechanical analysis (TMA 402 F3, NETZSCH, Germany) was employed to test the temperature-dependent thermal expansion rate (100–1200 °C). The test was conducted in argon (Ar) gaseous fluid, the specimens were cut to a size of 8 mm × 2 mm × 1 mm to adapt to the sample holder. The heating speed was 5 K·min^-1 . The test time lasted for about 5 h, with only one sample being tested each time.

The bulk specimens were machined into discs with a radius of 3 mm and a thickness of 1 mm to fit the sample holder in order to test the thermal diffusivity (λ). Silver (Ag) and carbon (C) coatings were applied to reduce the thermal radiative conductivity, and ensure to absorption and maximum emissivity. The test was executed under Ar gas protection within a laser flash instrument (LFA 457, NETZSCH, Germany). Three samples were tested each time, and the test lasted for about 12 h. The thermal diffusivity was corrected by means of the ‘‘radiation + pulse” method; three tests were performed at each temperature point and the average value was used. The thermal conductivity (k' ) was determined from the λ, C_P, and ρ as follows [21]:

where the specific heat, C_P, was computed using the Neumann– Kopp principle [22], and ρ is the density. The influence of porosity,, on thermal conductivity was removed as follows [21]:

Debye’s principle was employed to investigate the thermal conduction mechanism. The thermal conduction mechanism was related to the propagation of phonons, as heat is transmitted via the phonons in insulators [23]:

where k is the thermal conductivity of fully dense sample, C_V refers to the specific heat per unit volume, and l refers to the phonon mean free path. The influence of the specific heat on thermal conduction was restricted, as it reached 3k_B (where k_Bis the Boltzmann constant) per atom at high temperatures. Herein, l was obtained:

The phonon mean free path, l, was typically depressed by diverse scattering procedures, indicating that l and k decrease with an increase in the phonon scattering strength.

《3. Results and discussion》

3. Results and discussion

Fig. 1(a) shows that the experimental EuNb_1–XTa_XO₄ XRD peaks are consistent with those of standard PDF#22-1099, and that no peak for the precipitated phase is present. EuNb_1–XTa_XO₄ crystallizes in the m phase; no crystal structure transition was detected with an increase in the Ta content. Fig. 1(b) shows that the main XRD peaks slightly deviate from those on the standard PDF card, which relates to the sintering temperature. The final sintering temperature of EuNbO₄ is 1400 °C; it increases with an increase in Ta content, and is 1600 °C for EuNb_2/6Ta_4/6O₄. Similar roomtemperature Raman peaks were found for EuNb_1–XTa_XO₄, as displayed in Fig. 1(c). The shift and intensity of each Raman vibration mode are connected to the molecular vibration and bond length. No evident peak deviation was observed in the two strongest Raman vibration modes (V₁ and V₂) of EuNb_1–XTa_XO₄. The results of the Raman spectra align with the situation indicated by XRD; that is no phase transition is detected, indicating that each sample crystallizes in the same m phase.

《Fig. 1》

Fig. 1. Phase characterization of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics. (a) XRD, 25° 65°; (b) XRD, 27° 31°; (c) room-temperature Raman spectra (25 °C, 532 nm, 100–900 cm^-1 ).

Fig. 2 shows that the grain size of EuNb_1–XTa_XO₄ is less than 20 μm; the EuNb_2/6Ta_4/6O₄ displays a minimal grain size, which is ascribed to the highest sintering temperature. The final sintering temperature is related to the melting point. The final sintering temperature of EuNbO₄ is 1400 °C, and the substance melts at 1600 °C. The final sintering temperature of EuNb_2/6Ta_4/6O₄ is 1600 °C; the melting point of EuNbO₄ has been increased via the B-site substitution of Ta. A higher melting point implies a higher limit application temperature. The grain boundaries are evident, and the grains bond well with each other. The fine grain size and outstanding combination of grains contribute to produce extraordinary thermal and mechanical properties.

《Fig. 2》

Fig. 2. Typical surface morphology of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics. (a) EuNbO₄; (b) EuNb_5/6Ta_1/6O₄; (c) EuNb_4/6Ta_2/6O₄; (d) EuNb_3/6Ta_3/6O₄; (e) EuNb_{2/ 6}Ta_4/6O₄.

The data presented in Table 1 implies that the B -site substitution of Ta makes a notable difference to the mechanical properties of EuNbO₄. The Young’s modulus of EuNbO₄ is about 76 GPa, so the B -site substitution of Ta has led to an increase in the Young’s modulus. The highest Young’s modulus (169 GPa) is detected in EuNb_2/6Ta_4/6O₄. A similar situation is observed in the bulk modulus, shear modulus, and mean acoustic velocity. The composition dependence of the elastic modulus and acoustic velocity of EuNb_1–XTa_XO₄ is depicted in Fig. 3. When X 3/6, the increase in the elastic modulus and acoustic velocity of EuNb_1–XTa_XO₄ is minute. The Young’s modulus mirrors the bond strength of the chemical bonds. It is clear that the B-site substitution of Ta leads to an increase in the bonding strength. A high Young’s modulus means that EuNb_1–XTa_XO₄ can be directly applied as high-temperature structural ceramics.

《Table 1 》

Table 1 The mean acoustic velocity, elastic modulus (E, B, and G ), Grüneisen parameter (γ), and Poisson’s ratio () of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics.

《Fig. 3》

Fig. 3. Composition-dependent acoustic velocity and elastic modulus of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics. (a) Acoustic velocity; (b) elastic modulus.

The bond strength increases with a decrease in bond length [24]. Fig. 1 implies that the lattice parameters and unit cell volume of EuNb_1–XTa_XO₄ increase with an increase in Ta content, which leads to an increase in bond length. Therefore, it is believed that the Ta–O bond strength is much greater than that of the Nb–O bond, which results in an increase of the Young’s modulus. Greater bond strength leads to a faster phonon propagation speed, which results in an increase of the thermal conductivity, to a certain extent. However, the factors affecting thermal conduction are complex, and will be discussed in detail.

Fig. 4(a) shows that the thermal expansion rate of EuNb_1–XTa_XO₄ rapidly increases with an increase in temperature. At 1200 °C, EuNbO₄ exhibits the lowest thermal expansion rate, while EuNb_3/6Ta_3/6O₄ displays the maximal value. Fig. 4(b) shows that EuNbO₄ exhibits the lowest TEC (10.2 × 10^-6 K^-1 , 1200 °C), and the TEC of EuNbO₄ can be increased by the B-site substitution of Ta. The maximal TEC (11.2 × 10^-6 K^-1 , 1200 °C) was obtained for EuNb_3/6Ta_3/6O₄, this value is much higher than that of 7YSZ (10.0 × 10^-6 K^-1 ) and RE₂Zr₂O₇ (9.0 × 10^-6 K^-1 ) [25–27]. A High TEC will contribute to reducing the thermal stress between topcoat ceramics and substrate alloys during operation, and will prolong the lifetime of the TBC. The crystal structure is relaxed via substitution, leading to an increase in TEC. Nevertheless, the TEC (11.0 × 10^-6 K^-1 ) of EuNb_2/6Ta_4/6O₄ is slightly lower than that of EuNb_3/6Ta_3/6O₄, which can be explained by the dramatic increase in the Young’s modulus. The difference in Young’s modulus between EuNbO₄ and EuNb_3/6Ta_3/6O₄ (26 GPa) is much less than that between EuNb_3/6Ta_3/6O₄ and EuNb_2/6Ta_4/6O₄ (67 GPa). When X 3/6, the increase in the TECs of EuNb_1–XTa_XO₄ is dominated by crystal structure relaxation, as the Young’s modulus variation is minute. The TEC of EuNb_2/6Ta_4/6O₄ is higher than that of EuNbO₄, and lower than that of EuNb_3/6Ta_3/6O₄. The increasing bonding strength will lead to a decrease in the TEC, to some extent, when X 4/6. Thermal expansion of inorganic ceramics stems from inharmonic atomic vibration around the equilibrium position, which is characterized by the Grüneisen parameter. As shown in Table 1, the Grüneisen parameter of EuNbO₄ has been increased by the B -site substitution of Ta, which agrees well with the composition-dependent TEC. Thus, it is believed that the TECs of EuNb_1–XTa_XO₄ are governed by different factors with the variation of Ta content.

《Fig. 4》

Fig. 4. Thermal expansion performance of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics. (a) Thermal expansion rate; (b) TECs.

As shown in Table 2, the specific heat of EuNb_1–XTa_XO₄ increases with an increase in temperature (0.35–0.58 J·K^-1 ·g^-1 , 25–900 °C). Furthermore, the specific heat of EuNb_1–XTa_XO₄ decreases with an increase in Ta concentration. The specific heat decreases with increasing molecular weight, according to the Neumann–Kopp principle. Fig. 5(a) shows that the thermal diffusivity (0.42–1.13 mm² ·s^-1 , 25–900 °C) of EuNb_1–XTa_XO₄ quickly decreases with increase in temperature, the lowest thermal diffusivity (0.42–0.90 mm² ·s^-1 , 25–900 °C) is detected in EuNb_3/6Ta_3/6O₄. Meanwhile, when the temperature is greater than 700 °C, an evident increase in the thermal diffusivity of EuNb_1–XTa_XO₄ (X = 0/6, 1/6) is observed, which is caused by thermal radiation. No obvious increase of thermal diffusivity is detected in EuNb_1–XTa_XO₄ (X = 2/6, 3/6, 4/6), indicating that the B -site substitution of Ta is effective in improving the thermal radiation resistance of EuNbO₄. Fig. 5(b) shows that the thermal conductivity (1.52–3.28 W·K^-1·m^-1 , 25–900 °C) of EuNb_1–XTa_XO₄ decreases with an increase in temperature, and that EuNb_3/6Ta_3/6O₄ exhibits the minimum value (1.52 W·K^-1·m^-1 , 700 °C). The thermal radiation effect causes the thermal conductivity of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6) to increase at high temperatures ( 500 °C). No increase in thermal diffusivity or conductivity is detected for EuNb_2/6Ta_4/6O₄, which is attributed to it having the best thermal radiation resistance.

《Table 2》

Table 2 Temperature-dependent specific heat of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics calculated via the Neumann–Kopp principle.

《Fig. 5》

Fig. 5. Thermal properties of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics. (a) Thermal diffusivity; (b) thermal conductivity; (c) composition dependence of thermal conductivity; (d) phonon mean free path.

Thermal transfer is conducted via phonons—that is, lattice vibration—in insulators [28,29]. During phonon propagation, they are scattered via various processes, including Umklapp phonon– phonon scattering, different point defects scattering, grain boundaries scattering, and the other scattering processes [29–32]. The phonon mean free path (l ), which is restricted by the above processes, consists of different parts [29–32]:

where and are the phonon free paths derived from Umklapp phonon–phonon scattering, point defects scattering, grain boundaries scattering, and other processes, respectively [29–32]. Figs. 1 and 2 indicate that no phase transformation is detected, and that the grain size (microscale) is dozens of times greater than the size of phonon free path (nanoscale). Furthermore, the phonon scattering intensity caused by the grain boundary decreases with increase in temperature. Therefore, grain boundaries scattering can not decide the phonon mean free path. According to the chemical formula of EuNb_1–XTa_XO₄, no vacancy is induced in EuNbO₄ by the B -site substitution of Ta, as both Nb and Ta are pentavalent (+5). The Umklapp scattering degree is reflected via inharmonic lattice vibration. The inharmonic lattice vibration of EuNbO₄ is enhanced by the B -site substitution of Ta, and the lowest value of the Grüneisen parameter is detected in EuNbO₄ (Table 1). As the Grüneisen parameter increases with an increase in temperature, the thermal conductivity decreases with an increase in temperature. Furthermore, point defects are introduced, which are attributed to the atomic weight difference between Nb (92.9 g·mol^-1 ) and Ta (180.9 g·mol^-1 ). The effective ionic radius of Ta⁵⁺ and Nb⁵⁺ with four ligands is equal (0.064 nm); the phonon scattering caused by the ionic radius difference is therefore omitted. Normally, the misfits of atomic weight and ionic radius reach the maximum value when X is 3/6 in the substitution process [25,30,32–34]. Hence, the lowest thermal conductivity of EuNb_1–XTa_XO₄ is detected in EuNb_3/6Ta_3/6O₄. The phonon scattering process sketch map of EuNb_3/6Ta_3/6O₄ is displayed in Fig. 6, in which Ta atoms are the strongest phonon scattering sources. First, the B -site substitution of Ta causes an atomic weight difference, as Ta atoms are much heavier than Nb atoms. Second, the introduction of Ta increases the total unit cell weight and crystal structure complexity. The work by Clarke [35] proves that the thermal conductivity decreases with an increase in unit cell weight and crystal structure complexity. Third, the Ta and Nb atoms are centered by four O atoms to form TaO₄ and NbO₄ tetrahedrons. Phonons are scattered via a cage-like structure to reduce the thermal conductivity, which has been reported in various ceramics [32,36,37]. The complex crystal structure and cage-like structure are important reasons why EuNb_1–XTa_XO₄ exhibits a low thermal conductivity.

《Fig. 6》

Fig. 6. Phonon scattering process sketch map of EuNb_3/6Ta_3/6O₄ ceramics viewed from axis, b and c stand for the rest two axis.

The composition dependence of the thermal conductivity can be clearly observed in Fig. 5(c). At the same temperature, the thermal conductivity of EuNb_1–XTa_XO₄ decreases with an increase in Ta content, and increases slightly when X is 4/6. The phonon mean free path (0.41–1.56 nm, 25–900 °C) of EuNb_1–XTa_XO₄ decreases with an increase in temperature (Fig. 5(d)). The temperature dependence of l and k is analogous. The lowest l (0.41 nm, 900 °C) was detected in EuNb_2/6Ta_4/6O₄. Eq. (10) indicates that l connects to V_M and λ. V_M is temperature dependent; therefore, the temperature dependence of l is determined by the thermal diffusivity. In addition, V_M of EuNb_2/6Ta_4/6O₄ (3022 m·s^-1 ) is much faster than those of the rest of the samples (2246–2393 m·s^-1 ), which results in the lowest l being detected in EuNb_2/6Ta_4/6O₄.

The thermal radiative conductivity occurs at elevated temperature, and results in an increase of the thermal conductivity. The thermal diffusivity and conductivity, as well as the phonon mean free path, of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6) slightly increase at high temperatures. To obtain the intrinsic phonon thermal conductivity of EuNb_1–XTa_XO₄, the thermal radiative conductivity should be removed. In the work of Klemens [30] and Ambegaokar [31], the phonon scattering intensity caused by the point defects and grain boundaries is constant, and the mean phonon free path of the insulator predominantly consists of [33–37]. Therefore, the temperature dependence of the thermal diffusivity is decided by the Umklapp phonon–phonon scattering process. The correlation between the temperature T and of crystalline ceramics is as follows [36,37]:

where is the Debye temperature, h is the Plank’s constant, is the Boltzmann constant, m is the total weight per unit cell, V is the unit cell volume, is a parameter before the exponential, is the revised Debye temperature, and b is a constant set as 2. When the temperature is greater than , is as follows [36,37]:

where C and D are parameters. Fig. 5(d) shows that the relationship between l and T clearly deviates from at elevated temperatures due to the thermal radiation effect. To obtain the intrinsic lattice thermal conductivity of EuNb_1–XTa_XO₄, the intrinsic phonon thermal diffusivity should be determined. Based on the relationship between is determined as follows [36,37]:

Eq. (16) indicates that the reciprocal thermal diffusivity increases with an increase in temperature, when no thermal radiation effect occurs. Fig. 7(a) shows that follows the relationship expressed in Eq. (16) at low temperature. However, when the temperature is greater than 600 °C, deviates from the (dotted lines) relationship. The intrinsic phonon thermal diffusivity of EuNb_1–XTa_XO₄ is corrected.

Fig. 7(a) shows that the intrinsic phonon thermal diffusivity monotonously decreases with increasing temperature. A similar temperature dependence of the intrinsic phonon thermal conductivity and the phonon mean free path is observed in Figs. 7(c) and 7(d). The minimum intrinsic phonon thermal conductivity of EuNb_1–XTa_XO₄ is 1.27 W·K^-1 ·m^-1 (EuNb_3/6Ta_3/6O₄). The variation trend of the thermal conductivity implies that it will decrease further with an increase in temperature, and will approach the theoretical limit value (k_min), which has been derived by Cahill et al. [34–36]:

《Fig. 7》

Fig. 7. Intrinsic thermal properties of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics. (a) Reciprocal thermal diffusivity; (b) intrinsic phonon thermal diffusivity; (c) intrinsic phonon thermal conductivity; (d) intrinsic phonon mean free path.

where n is the atomic number per unit cell. The theoretical minimum thermal conductivity decreases with a decrease in acoustic velocity. As shown in Table 3, k_min of EuNbO₄ is about 0.78 W·K^-1 ·m^-1 , implying that the experimental k of EuNbO₄ can be decreased. The ZrO₂ alloying effects have been applied to reduce the thermal conductivity of rare earth tantalates, and these methods may be effective for EuNbO₄ [38]. Furthermore, the A-site substitution of other rare earth elements (e.g., Gd, Dy, Ho, Yb, Er, and Lu) with a heavier atomic weight can be attempted. As for the thermal radiation effect, dual layer coatings are effective in blocking the thermal radiative conductivity of LaPO₄/La₂Zr₂O₇ ceramics [39]. EuPO₄/EuNbO₄ dual coatings can be used to attempt to reduce the thermal radiative conductivity of EuNbO₄, due to the excellent thermal radiation resistance of rare earth phosphate (REPO₄) [40].

《Table 3》

Table 3 Fitted reciprocal thermal diffusivity () and theoretical minimum thermal conductivity (k_min) of EuNb_1–XTa_XO₄ (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics.

《4. Conclusion》

4. Conclusion

The thermal and mechanical properties of EuNbO₄ synthesized via a SSR have been successfully optimized by the B-site substitution of Ta. The highest TEC reaches 11.2 × 10^-6 K^-1 at 1200 °C (EuNb_3/6Ta_3/6O₄), attributed to crystal structure relaxation and the enhancement of inharmonic lattice vibration strength. The highest Young’s modulus (169 GPa) is detected in EuNb_2/6Ta_4/6O₄, as the Ta–O bond strength is much greater than that of the Nb–O bond. The minimum experimental thermal conductivity (1.52 W·K^-1·m^-1 , 700 °C) is found in EuNb_3/6Ta_3/6O₄, due to the maximum misfit of atomic weight between Ta and Nb. The thermal radiation resistance of EuNb_1–XTa_XO₄ is improved via the B -site substitution of Ta. The theoretical minimum thermal conductivity (0.78 W·K^-1·m^-1 ) of EuNbO₄indicates that the experimental thermal conductivity can be reduced further. It is clear that EuNb_1–XTa_XO₄ exhibits lower thermal conductivity, lower Young’s modulus and greater TECs than the 7YSZ and La₂Zr₂O₇ ceramics. The excellent material properties of EuNb_1–XTa_XO₄ imply that EuNbO₄ is a promising high-temperature TBC.

《Acknowledgements》

Acknowledgements

This research is under the support of the Natural Science Foundation of China (51762028 and 91960103) and the Materials Genome Engineering of Rare and Precious Metal of Yunnan Province (2018ZE019).

《Compliance with ethics guidelines》

Compliance with ethics guidelines

Lin Chen and Jing Feng declare that they have no conflict of interest or financial conflicts to disclose.

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