Structures, and Thermophysical Properties Characterizations of (La1-xHox)3NbO7 Solid Solutions as Thermal Barrier Coatings

A sequence of (La1-xHox)3NbO7 solid solutions were fabricated in this work, which were studied as candidate for thermal insulation materials. The lattices were identified via XRD, when SEM and EDS were used to characterize the microstructures and element distributions. The results showed that the highest modulus, hardness, and toughness of (La1-xHox)3NbO7 were 196 GPa, 9.2 GPa, and 1.6 MPa m1/2, respectively, and they accorded with the mechanical property requirements. Also, a low thermal conductivity (1.06 W m−1 K−1) and high thermal expansion coefficients (TECs: 11.3 × 10−6 K−1) were simultaneously realized in (La3/6Ho3/6)3NbO7, at high temperatures. No phase transition was detected up to 1,200°C, which proved their good high-temperature lattice stability. The intense anharmonic lattice vibrations might contribute to the outstanding thermal properties of (La1-xHox)3NbO7 ceramics. The suitable modulus, high hardness, low thermal conductivity, and high TECs of (La1-xHox)3NbO7 solid solutions proclaimed that they were exceptional thermal insulation ceramics.

The excellent thermophysical properties of pyrochlore-/fluoritetype RE 2 Zr 2 O 7 ceramics include low thermal conductivity (1-2 W · m −1 · K −1 , 25-1,000°C), high hardness (9-12 GPa), and good high-temperature phase stability (Wang et al., 2012;Yang et al., 2016;Liu et al., 2019;Wright et al., 2020;Xiang et al., 2021). One shortage of RE 2 Zr 2 O 7 is the inadequate fracture toughness (∼1 MPa m 1/2 ), which stops it from being used as TBCs. Therefore, other oxides possessing pyrochlore-/fluorite-type structures are studied as TBCs. A 3 BO 7 -type rare earth niobates (RE 3 NbO 7 ) have been studied in the past several years, and they have orthorhombic weberite and cubic fluorite lattices, which rely on RE 3+ ionic size (Ma et al., 2008;Zhang et al., 2017;Yang et al., 2019;Chen et al., 2021a;Chen et al., 2021b;Xiang et al., 2021). The crystal structures of RE 3 NbO 7 derive from fluorite A 4 O 8 , when the four A 4+ ions are substituted by three RE 3+ and one Nb 5+ cation, and one oxygen vacancy is produced in the lattice to maintain charge balance (Wang et al., 2012;Chen et al., 2018a;Chen et al., 2021a;Chen et al., 2021b). RE 3 NbO 7 is a weberite lattice when RE 3+ ionic size is longer than 1.03 Å, and others are cubic fluorite with RE 3+ size less than 1.03 Å. There are many similarities between crystal structures of RE 3 NbO 7 and RE 2 Zr 2 O 7 ceramics. The lattices of RE 3 NbO 7 and RE 2 Zr 2 O 7 originate from prototype fluorite A 4 O 8 ; high concentration of oxygen vacancy (12.5%), [REO] polyhedrons, and excellent lattice stability are found in these two-type ceramics. Therefore, RE 3 NbO 7 and RE 2 Zr 2 O 7 may have similar thermophysical properties.
Different structural and thermophysical property features are documented in weberite and fluorite RE 3 NbO 7 oxides, and they display good thermal insulation performance (Chen et al., 2018b;Yang et al., 2019;Chen et al., 2021a;Chen et al., 2021b). The advantages of RE 3 NbO 7 as TBCs include ultralow thermal conductivity, high TECs, and competitive modulus. The features of weberite and fluorite RE 3 NbO 7 have been reported, and the differences of structures and properties between these two types of niobates are distinct. To study the variations of properties of RE 3 NbO 7 along with the changes of lattice structures and to provide good understanding on these series niobates, the relevant study should be performed. The lowest thermal conductivity of weberite niobates is detected in La 3 NbO 7 , and the minimum thermal conductivity of fluorite niobates is found in Ho 3 NbO 7 oxides (Chen et al., 2018b;Chen et al., 2019b;Yang et al., 2019;Chen et al., 2021a;Chen et al., 2021b). Dense (La 1-x Ho x ) 3 NbO 7 solid solutions are fabricated in this work, and their lattices, thermal conductivity, hardness, and other properties are characterized. The variation trends of structures and thermophysical properties of (La 1- x Ho x ) 3 NbO 7 are documented, which enables scholars to study their inner dominated mechanism from a good aspect.

EXPERIMENTS AND METHODS
The bulk samples of (La 1-x Ho x ) 3 NbO 7 (x 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) solid solutions were fabricated via a high-temperature sintering process using La 2 O 3 , Ho 2 O 3 , and Nb 2 O 5 powders (purity beyond 99.9% and a particle size smaller than 20 µm). The RE 2 O 3 and Nb 2 O 5 powders were weighted and then mixed in alcohol via a ball mill at a speed of 300r/min for 10 h, and then they were heated at 90°C for 6 h to remove alcohol. Approximately 1-2 g dry mixtures were taken and compressed via a tablet press under a condition of 300 MPa for 2 min. The green bodies of (La 1-x Ho x ) 3 NbO 7 were finally sintered at 1,600-1700°C for 5-10 h to obtain their dense bulk samples. The lattice structures were identified via XRD (X-ray diffraction), and their surface microstructures were observed via a SEM (scanning electron microscope) and an EDS (energy dispersive spectrometer). The microhardness (H) and toughness (K IC ) were tested by an indentation test (DHV-1000Z-CCD, China), when Young's modulus (E) and mean acoustic velocity (V a ) were obtained by using an ultrasonic pulser/receiver equipment (UMS-100, France). The hardness and toughness of oxides were affected by bonding strengths, which were reflected via Young's modulus and Debye temperature (Ɵ D ) (Anderson, 1963;Schlichting et al., 2001): where h, k B , and m represented Plank's constant, the Boltzmann constant, and the lattice weight, respectively. As for thermal properties, TECs were measured via a TMA (thermal mechanical analysis), and thermal diffusivity (λ) was tested via a LFA to calculate thermal conductivity (k) (Schlichting et al., 2001): where ρ, C P , and ϕ represented density, heat capacity, and porosity, respectively. Porosity was tested via an Archimedes balance, the weight (w 1 ) of sample was measured first, and then the sample was put in water to obtain its weight (w 2 ) under the action of water buoyancy; finally, the sample was taken out from water and wiped up to measure its weight (w 3 ). The porosity was calculated via a formula: ϕ 1−w 1 /(w 3 −w 2 ). Thermal conductivity was calculated via phonons, and the corresponding phonon mean free path (l) was obtained via thermal diffusivity and average acoustic velocity (Kingery, 1955;Anderson, 1963;Kittle, 1996;Schlichting et al., 2001;Qu et al., 2012;Zhao et al., 2016;Wang et al., 2018;Ye et al., 2019): where V a was a constant, and it indicated that the temperature dependence of l relied on λ (Bruls et al., 2005). More details about sample preparations, structures identifications, and properties measurements could be found in our previous articles (Chen et al., 2018b;Chen et al., 2019b;Chen et al., 2021a).

Structures Analysis
In the current work, the variations of properties along with the changes of lattices in (La 1-x Ho x ) 3 NbO 7 oxides are studied. The Frontiers in Materials | www.frontiersin.org July 2021 | Volume 8 | Article 703098 normalized XRD peaks of prepared specimens are depicted in Figure 1, and it is found that three types of lattices are formed. When r RE is 1.060-1.160 Å, the prepared samples (0/6 ≤ x ≤ 4/6) are in an orthorhombic phase with the space group (SG) of Cmcm; when r RE is 1.039 Å, the prepared sample (x 5/6) is in an orthorhombic phase with the SG of C222 1 ; and Ho 3 NbO 7 is a cubic fluorite phase with Fm3m SG. The main XRD peaks shift to the right along with the decline of La content displayed in Figure 1B, which indicates the shrinkage of the lattice. Table 1 exhibits the exact information of lattices of (La 1- x Ho x ) 3 NbO 7 solid solutions. Some certain relationships can be constructed between cubic and orthorhombic (La 1-x Ho x ) 3 NbO 7 lattice parameters. In cubic fluorite Ho 3 NbO 7 , it is found that a c b c c c , when the lattice constants of orthorhombic (La 1-x Ho x ) 3 NbO 7 are derived from the relationships of a o ≈ 2a c , b o ≈ 2 0.5 b c , and c o ≈ 2 0.5 c c . The shrinkages of lattices and increments of unit cell mass lead to their theoretical density increases with the increasing Ho content. Also, Table 1 exhibits that the porosity of each sample is 1-4%, and dense bulk samples are made in this work. The impacts of porosity on thermophysical properties are not taken into account.
The grain sizes of (La 1-x Ho x ) 3 NbO 7 are a micron scale, and most grains have sizes of 1-20 µm shown in Figure 2. The good combinations among neighboring grains and a small grain size may result in good mechanical properties. The sintering temperatures of (La 1-x Ho x ) 3 NbO 7 increase from 1,600 to 1700°C in conjunction with the increasing Ho content. Navrotsky's research showed that the formation enthalpy of RE 3 NbO 7 became more exothermic with the increases of RE 3+ ionic radius, which indicated that the sintering temperature decreased with the increasing RE 3+ ionic size (Mielewczyk and Navrotsky, 2015;Chen et al., 2018b). Furthermore, the crystal structure is affected by RE 3+ ionic radius, and fluorite RE 3 NbO 7 has a smaller grain size than weberite RE 3 NbO 7 . Therefore, the grain size of (La 1-x Ho x ) 3 NbO 7 is affected by the sintering temperatures and crystal structures, and they are dominated by RE 3+ ionic radius. The increments of sintering temperature and order-disorder (weberite-fluorite) transition of crystal structure lead to a decrease of the grain size. Figure 3 exhibits the backscattered electron (BSE) photo and the corresponding element mappings of (La 3/6 Ho 3/6 ) 3 NbO 7 . No precipitated phase is found, and each element is evenly distributed in this sample. The XRD, SEM, and EDS results prove that dense and high-purity (La 1-x Ho x ) 3 NbO 7 samples are made via current methods.
where σ f is the flexural strength and υ is Poisson's ratio. Evidently, comparatively low modulus is necessary for maintaining high hardness and good thermal stress fracture resistance, which is essential for the lifetime of TBCs. Besides, the highest fracture toughness of (La 1-x Ho x ) 3 NbO 7 is lower than that of YSZ (3.5 MPa · m 1/2 ), but it is better than that of La 2 Zr 2 O 7 (1.0 MPa · m 1/2 ) (Schlichting et al., 2001;Yang et al., 2016;Liu et al., 2019). The variations of aforementioned properties are affected by bonding strengths, which are dominated by the following factors.
First, lattice shrinkages caused by the decrements of r RE result in the enhancements of bonding strength as that bonding strengths increase with the decrease of bonding lengths Wang et al., 2018). Second, the Ho-O bonds have higher strengths than La-O bonds, which is reflected by their modulus and Debye temperatures. Third, Figure 3 shows that the grain size of (La 1-x Ho x ) 3 NbO 7 is decreased by the increased Ho content. The decrements of grain size will lead to increases of grain boundary density, fracture toughness, and hardness. Figures 5A,B,D exhibit that thermal diffusivity and conductivity, and phonon mean free path of (La 1-x Ho x ) 3 NbO 7 solid solutions have the similar temperature dependences because they are dominated by phonons. At room temperature, Ho 3 NbO 7 has the lowest thermal diffusivity (0.34 mm 2 /s), and conductivity (0.96 W · m −1 · K −1 ) contributed to its disorder cubic fluorite lattice (Chen et al., 2021a;Yang et al., 2019;Chen et al., 2018b). Amorphous thermal conductivity is found in Ho 3 NbO 7 , when the remaining prepared samples exhibit different temperature dependences of thermal conductivity. The low thermal conductivity originates from the short phonon mean free path shown in Figure 5D. The lowest value of k (1.06 W · m −1 · K −1 ) at 900°C is found in (La 3/6 Ho 3/6 ) 3 NbO 7 , and it has the shortest l (0.29 nm). It is evident that (La 1-x Ho x ) 3 NbO 7 has much lower thermal conductivity (0.96-1.42 W · m −1 · K −1 ) than YSZ FIGURE 4 | Mechanical properties of (La 1-x Ho x ) 3 NbO 7 (x 0/6, 1/6, 2/6, 3/6, 4/6, 5/6, and 6/6) ceramics: (A) Young's modulus, (B) Debye temperature, (C) Vickers hardness, and (D) fracture toughness.

CONCLUSION
Dense (La 1-x Ho x ) 3 NbO 7 oxide bulks have been fabricated in this work, and their structures and thermophysical properties are characterized. The excellent thermophysical properties indicate that the prepared samples are candidate TBCs, and following conclusions are obtained: 1) Dense and high-purity (La 1-x Ho x ) 3 NbO 7 ceramics are obtained by a solid-state sintering process, and phase transitions of Cmcm→C222 1 →Fm3m SG are detected along with the increments of Ho content. The grain sizes are a micron scale, and each element is evenly distributed in these samples. 2) The mechanical properties are affected by their bonding strengths and grain sizes, and the changes of modulus and Debye temperatures prove that their bonding strengths enhance with the increments of Ho content, which is caused by the shortening of bonding length and addition of stiff Ho-O bonds. The highest modulus, hardness, and toughness are 196 GPa, 9.2 GPa, and 1.6 MPa m 1/2 , respectively.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

AUTHOR CONTRIBUTIONS
LC conducted the experiments and wrote this paper, YW and QZ conducted partial experiments on thermal conductivity and thermal expansions, and JF designed the detailed experiments, and discussed and improved this article.

FUNDING
The current work was supported by the National Natural Science Foundation of China (No. 91960103 and 51762028) and Yunnan Province Materials Genome Engineering (No. 2018ZE019).