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, EuNb1–X TaX O4 (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 EuNb1–X TaX O4 (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 (VL) and transverse (VT) acoustic velocities of EuNb1–X TaX O4 were calculated by determining the transmission interval through an ultrasonic pulser/receiver instrument (UMS– 100, TECLAB, France). Various properties were identified :
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, CP , and ρ as follows :
where the specific heat, CP , was computed using the Neumann– Kopp principle , and ρ is the density. The influence of porosity,, on thermal conductivity was removed as follows :
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 :
where k is the thermal conductivity of fully dense sample, CV 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 3kB (where kB is 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 EuNb1–X TaX O4 XRD peaks are consistent with those of standard PDF#22-1099, and that no peak for the precipitated phase is present. EuNb1–X TaX O4 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 EuNb2/6Ta4/6O4. Similar roomtemperature Raman peaks were found for EuNb1–X TaX O4, 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 EuNb1–X TaX O4. 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. Phase characterization of EuNb1–X TaX O4 (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 EuNb1–X TaX O4 is less than 20 μm; the EuNb2/6Ta4/6O4 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 EuNb2/6Ta4/6O4 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. Typical surface morphology of EuNb1–X TaX O4 (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics. (a) EuNbO4; (b) EuNb5/6Ta1/6O4; (c) EuNb4/6Ta2/6O4; (d) EuNb3/6Ta3/6O4; (e) EuNb2/ 6Ta4/6O4.
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 EuNb2/6Ta4/6O4. 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 EuNb1–X TaX O4 is depicted in Fig. 3. When X 3/6, the increase in the elastic modulus and acoustic velocity of EuNb1–X TaX O4 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 EuNb1–X TaX O4 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 EuNb1–X TaX O4 (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics.
Fig. 3. Composition-dependent acoustic velocity and elastic modulus of EuNb1–X TaX O4 (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 . Fig. 1 implies that the lattice parameters and unit cell volume of EuNb1–X TaX O4 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 EuNb1–X TaX O4 rapidly increases with an increase in temperature. At 1200 °C, EuNbO4 exhibits the lowest thermal expansion rate, while EuNb3/6Ta3/6O4 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 EuNb3/6Ta3/6O4, 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 EuNb2/6Ta4/6O4 is slightly lower than that of EuNb3/6Ta3/6O4, which can be explained by the dramatic increase in the Young’s modulus. The difference in Young’s modulus between EuNbO4 and EuNb3/6Ta3/6O4 (26 GPa) is much less than that between EuNb3/6Ta3/6O4 and EuNb2/6Ta4/6O4 (67 GPa). When X 3/6, the increase in the TECs of EuNb1–X TaX O4 is dominated by crystal structure relaxation, as the Young’s modulus variation is minute. The TEC of EuNb2/6Ta4/6O4 is higher than that of EuNbO4, and lower than that of EuNb3/6Ta3/6O4. 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 EuNb1–X TaX O4 are governed by different factors with the variation of Ta content.
Fig. 4. Thermal expansion performance of EuNb1–X TaX O4 (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 EuNb1–X TaX O4 increases with an increase in temperature (0.35–0.58 J·K-1 ·g-1 , 25–900 °C). Furthermore, the specific heat of EuNb1–X TaX O4 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 EuNb1–X TaX O4 quickly decreases with increase in temperature, the lowest thermal diffusivity (0.42–0.90 mm2 ·s-1 , 25–900 °C) is detected in EuNb3/6Ta3/6O4. Meanwhile, when the temperature is greater than 700 °C, an evident increase in the thermal diffusivity of EuNb1–X TaX O4 (X = 0/6, 1/6) is observed, which is caused by thermal radiation. No obvious increase of thermal diffusivity is detected in EuNb1–X TaX O4 (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 EuNb1–X TaX O4 decreases with an increase in temperature, and that EuNb3/6Ta3/6O4 exhibits the minimum value (1.52 W·K-1·m-1 , 700 °C). The thermal radiation effect causes the thermal conductivity of EuNb1–X TaX O4 (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 EuNb2/6Ta4/6O4, which is attributed to it having the best thermal radiation resistance.
Table 2 Temperature-dependent specific heat of EuNb1–X TaX O4 (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics calculated via the Neumann–Kopp principle.
Fig. 5. Thermal properties of EuNb1–X TaX O4 (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 EuNb1–X TaX O4, 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 EuNb1–X TaX O4 is detected in EuNb3/6Ta3/6O4. The phonon scattering process sketch map of EuNb3/6Ta3/6O4 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  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 EuNb1–X TaX O4 exhibits a low thermal conductivity.
Fig. 6. Phonon scattering process sketch map of EuNb3/6Ta3/6O4 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 EuNb1–X TaX O4 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 EuNb1–X TaX O4 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 EuNb2/6Ta4/6O4. Eq. (10) indicates that l connects to VM and λ. VM is temperature dependent; therefore, the temperature dependence of l is determined by the thermal diffusivity. In addition, VM of EuNb2/6Ta4/6O4 (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 EuNb2/6Ta4/6O4.
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 EuNb1–X TaX O4 (X = 0/6, 1/6, 2/6, 3/6) slightly increase at high temperatures. To obtain the intrinsic phonon thermal conductivity of EuNb1–X TaX O4, the thermal radiative conductivity should be removed. In the work of Klemens  and Ambegaokar , 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 EuNb1–X TaX O4, 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 EuNb1–X TaX O4 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 EuNb1–X TaX O4 is 1.27 W·K-1 ·m-1 (EuNb3/6Ta3/6O4). 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 (kmin), which has been derived by Cahill et al. [34–36]:
Fig. 7. Intrinsic thermal properties of EuNb1–X TaX O4 (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, kmin 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 . 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 . 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) .
Table 3 Fitted reciprocal thermal diffusivity () and theoretical minimum thermal conductivity (kmin) of EuNb1–X TaX O4 (X = 0/6, 1/6, 2/6, 3/6, 4/6) ceramics.
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 (EuNb3/6Ta3/6O4), attributed to crystal structure relaxation and the enhancement of inharmonic lattice vibration strength. The highest Young’s modulus (169 GPa) is detected in EuNb2/6Ta4/6O4, 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 EuNb3/6Ta3/6O4, due to the maximum misfit of atomic weight between Ta and Nb. The thermal radiation resistance of EuNb1–X TaX O4 is improved via the B -site substitution of Ta. The theoretical minimum thermal conductivity (0.78 W·K-1·m-1 ) of EuNbO4 indicates that the experimental thermal conductivity can be reduced further. It is clear that EuNb1–X TaX O4 exhibits lower thermal conductivity, lower Young’s modulus and greater TECs than the 7YSZ and La2Zr2O7 ceramics. The excellent material properties of EuNb1–X TaX O4 imply that EuNbO4 is a promising high-temperature TBC.
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.