Influence of HfO2 alloying effect on microstructure and thermal conductivity of HoTaO4 ceramics

HfO2 alloying effect has been applied to optimize thermal insulation performance of HoTaO4 ceramics. X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy are employed to decide the crystal structure. Scanning electronic microscopy is utilized to detect the influence of HfO2 alloying effect on microstructure. Current paper indicates that the same numbers of Ta5+ and Ho3+ ions of HoTaO4 are substituted by Hf4+ cations, and it is defined as alloying effect. No crystal structural transition is introduced by HfO2 alloying effect, and circular pores are produced in HoTaO4. HfO2 alloying effect is efficient in decreasing thermal conductivity of HoTaO4 and it is contributed to the differences of ionic radius and atomic weight between Hf4+ ions and host cations (Ta5+ and Ho3+). The least experimental thermal conductivity is 0.8 W·K−1·m−1 at 900 °C, which is detected in 6 and 9 mol%-HfO2 HoTaO4 ceramics. The results imply that HfO2–HoTaO4 ceramics are promising thermal barrier coatings (TBCs) due to their extraordinary thermal insulation performance.


Introduction
* Corresponding author. E-mail: jingfeng@kust.edu.cn Thermal barrier coating (TBC) system, which is applied in multiple gas turbines and aircraft engines, is consist of top coat ceramics, bond coat, and super-alloy substrates [1][2][3][4][5]. As the limit application temperature of present Ni-based alloy is less than 1200 ℃, a great number of researchers are studying top coat ceramics to provide thermal insulation [6][7][8][9]. The main functions of top coat ceramics are to decrease superficial temperature of substrates and increase their application limitations. Current top coat ceramics are yttria stabilized zirconia (YSZ), whose working temperature is less than 1200 ℃. The t-m phase transition is detected in YSZ, and the unit cell volume variation caused by the transition will lead to the failure of coatings [10][11][12]. Nevertheless, no material is able to replace YSZ because the distinctive ferroelasticity produces excellent toughness at elevated temperature [13]. Therefore, much effort has been conducted to perfect the thermal physical performance of YSZ [12][13][14][15]. At the same time, substances exhibiting ferroelasticity are recognized as novel TBCs. Monoclinic ABO 4 -type rare earth tantalates (m-RETaO 4 ) possess the similar ferroelasticity, which engenders extraordinary toughness [16][17][18] (k = 1.5 W·K -1 ·m -1 ), and the typical microstructure has been researched [16,18]. The comprehensive properties of m-RETaO 4 are better than that of YSZ and other candidate TBCs (RE 2 Zr 2 O 7 , RE 3 TaO 7 , REPO 4 , and so on) [19][20][21][22][23]. To promote the material property of rare earth tantalates further, many methods have been tried. For example, TiO 2 and ZrO 2 alloying effects are employed to produce glass-like thermal conductivity; Al 2 O 3 doping is utilized to drop thermal conductivity and enhance thermal expansion coefficients (TECs) [19,24,25]. Among various technologies, alloying effect is considered as the most effective one, as the TECs and thermal conductivity are simultaneously optimized. HoTaO 4 exhibits the best thermal radiation resistance, low Young's modulus, and thermal conductivity, as well as high TECs in the whole series of m-RETaO 4 [18]. TiO 2 and ZrO 2 alloying effects have been tried for other rare earth tantalates, as Hf, Zr, and Ti belong to the same group, HfO 2 alloying effect is tried for HoTaO 4 .
In this paper, HfO 2 alloying HoTaO 4 sample had been composed through a conventional solid-state reaction. X-ray diffraction (XRD) was applied to investigate the crystal structure. Raman spectroscopy was used to appraise the molecule vibration intensity. X-ray photoelectron spectroscopy (XPS) was utilized to ascertain the chemical valence of multiple elements. Scanning electron microscopy (SEM) was employed to research the effect of HfO 2 addition on the microstructure of HoTaO 4 ceramics. Optical properties were thoroughly discussed. Thermal properties (specific heat, thermal diffusivity, and thermal conductivity) of HfO 2 -HoTaO 4 were the emphasis. It is stressed that HfO 2 alloying effect is effective in optimizing thermal insulation performance of HoTaO 4 ceramics.

2 Crystal structure identification and microstructure observation
The lattice structures were distinguished through XRD (Rigaku, MiniFlex600, Japan). The 2-theta degree stretched from 10° to 70°, when the scanning velocity was 6 (°)/min and the step length was 0.02°. Herein, the testing time of each sample XRD pattern continued about 10 min. Archimedes principle was utilized to obtain the bulk density (ρ), and the porosity was computed by the relationship: Raman spectroscopy was more sensitive on distinguishing crystal structure variation than XRD, and it was used to research the vibration variety of unit cell and multiple chemical bonds. Confocal spectrometer (Lab RAM Aramis, Horiba-Jobin Yvon, Edison, NJ, USA) possessing a He-Ne ion laser (514.5 nm) was employed to record the Raman result. XPS was carried out within a PHI 5000 Versaprobe-Ⅱ spectrometer. The specimen was burnished and was wiped to clear up any foreign pollutant. A carbon layer with C1s binding energy of 284.8 eV was coated to eliminate the experimental deviation originating from superficial charging effect.
SEM (JEOL, JSM-7001F, Tokyo, Japan) was used to survey the superficial morphology features including pores, cracks, and grain sizes, which were evidently affected by HfO 2 alloying effect. Before the observation, gold was coated on the surface to increase electrical conductivity; the resulting images with magnification of 500 and 1000 were displayed.

3 Property measurement
The band gap of x mol%-HfO 2 HoTaO 4 was calculated based on reflectance R, which was measured via one UV-vis spectrophotometer (Shimadzu UV-3600plus, Japan). The reflectance curves were obtained, and the band gaps were computed according to the K-M law [26]: where F(R) exhibited a close relationship with the extinction efficiency (φ). Thermal diffusivity (α) experiment was completed in a laser flash instrument (LFA 457, Netzsch, Germany) from 25 to 900 ℃. The experimental values were revised via Radiation+Pulse adjustment mold. The thermal conductivity was calculated based on α, C p , and ρ [27]： Specific heat (C p ) was gained by Neumann-Kopp law [30]. Pores and cracks usually produced an obvious impact on heat propagation; the thermal conductivity (k) of specimen without pores and cracks was [27]: 3 Results and discussion 3. 1 Crystal structure Figure 1(a) displays that XRD peaks of x mol%-HfO 2 HoTaO 4 (x = 0, 3,6,9) are in accordance with the patterns of PDF#24-0478, implying that each sample crystallizes in m phase, and no phase transformation has been detected. According to the ionic radius of Ho 3+ (r = 0.102 nm), Ta 5+ (r = 0.064 nm), and Hf 4+ (r = 0.083 nm) cations, when only Ta 5+ is substituted by Hf 4+ or Hf 4+ occurs as interstitial cation, the crystal structure will be expanded and the XRD peaks shift to left. When Ho 3+ is substituted by Hf 4+ , the crystal structure will be contracted, and the XRD peaks shift to right. Figure 1(b) indicates that the 2-theta position of main XRD peaks of HfO 2 -HoTaO 4 is constant; therefore, it is believed that the same numbers of Ho 3+ and Ta 5+ ions are substituted by Hf 4+ cations [19,24]. A weak peak of HfO 2 is found in 9 mol%-HfO 2 HoTaO 4 , which can be observed more distinctly from Fig. 1(c), proving that HfO 2 alloying content in HoTaO 4 is below 9 mol%. Figure 2(a) displays that the entire series of x mol%-HfO 2 HoTaO 4 ceramics present analogous Raman peaks; no Raman peak position shifting is discovered. The position and intensity of Raman peaks are close to the chemical bonds length and lattice vibration strength. Figures 2(b) and 2(c) imply that no palpable change of crystal structure and bonding length is introduced by HfO 2 alloying effect. Figure 3   indicating that O is combined with metal elements (Ta 5+ , Hf 4+ , Ho 3+ ) to form chemical bonds.

2 Microstructure
The grain size of 0 mol%-HfO 2 HoTaO 4 is small (≤ 20 μm) as displayed in Fig. 4, which is analogous to the former document [18]. In HoTaO 4 , a small quantity of pore and crack is observed, and the relative density is about 95%. However, when HfO 2 is added, a large number of columnar pores are emerged as shown in Figs [27,31,32]. Therefore, the realistic thermal and mechanical properties of HoTaO 4 coatings can be estimated by adding appropriate HfO 2 content to obtain the desirable porosity. Figure 5(a) displays that the reflectance of x mol%-HfO 2 HoTaO 4 increases with increment of wavelength, and periodic decrements of reflectance are presented in some specific positions, which are caused by the Ho 3+ internal 4f 11 transition [33]. Figure 5(b) displays that the absorption decreases with increment of wavelength, which is contrary to the situation of reflectance, and the maximum absorption reaches 63% between 100 and 200 nm of wavelength range. Figure 5(c) displays that the band gaps of x mol%-HfO 2 HoTaO 4 (x = 0, 3, 6) are similar, whereas 9 mol%-HfO 2 HoTaO 4 exhibits narrower band gap than the others, attributed to the existence of second phase HfO 2 . The wide band gap of x mol%-HfO 2 HoTaO 4 proves that they are insulators and heat is carried by phonons (lattice vibration) in these solids. Figure 6(a) displays that the specific heat (0.30-0.39 J·K -1 ·g -1 , 25-900 ℃) of x mol%-HfO 2 HoTaO 4 increases with increment of temperature, contributed to the volume bulge and phonon excitation [17,21]. The introduction of HfO 2 makes little impact on the specific heat of HoTaO 4 . Figure 6(b) displays that thermal diffusivity (0.31-1.37 mm 2 /s, 25-900 ℃) decreases with increment of HfO 2 quantity, and the least value is presented in 6 and 9 mol%-HfO 2 HoTaO 4 . Figure 6(c) displays that thermal conductivity (0.8-3.6 W·K -1 ·m -1 , 25-900 ℃) decreases with increment of temperature. The difference of thermal conductivity k′ among x mol%-HfO 2 HoTaO 4 is much higher than that of the revised thermal conductivity k as shown in Figs. 6(c) and 6(d), because circular pores are effective in scattering phonons and reducing thermal conductivity. The minimum k reaches 1.3 W·K -1 ·m -1 (6 mol%-HfO 2 HoTaO 4 ), which is much less than that of YSZ (2.5-3.0 W·K -1 ·m -1 ), La 2 Zr 2 O 7 (1.8-3.2 W·K -1 ·m -1 ), and other ceramics [29,31,34,35]. The low thermal conductivity of x mol%-HfO 2 HoTaO 4 derives from the complicated crystal structure and point defects engendered by HfO 2 alloying effect. In the crystal structure of HoTaO 4 , each Ta 5+ is surrounded by four O 2ions to form cage-like tetrahedron, which  leads to violent phonons scatter. Hf 4+ has longer ionic radius than Ta 5+ and possesses shorter ionic radius than Ho 4+ . The misfits of ionic radius and atomic weight among Ta 5+ , Ho 3+ , and Hf 4+ induce point defects and enhance phonon scattering intensity [19,36].

4 Thermal conductivity
Figure 6(d) shows that the revised thermal conductivity k (1.3-3.7 W·K -1 ·m -1 , 25-900 ℃) decreases with increment of temperature. The phonon scattering intensity introduced by point defects and grain boundary is constant; the temperature dependent thermal conductivity is governed by inharmonic lattice vibration [37][38][39][40]. Raman characteristic peaks come from the particular vibration of lattice structure and chemical bonds, and the mean full wave at half maximum (FWHM) of Raman peaks can be used to assess anharmonicity of lattice vibration. Figure 7 displays that the anharmonicity of lattice vibration of x mol%-HfO 2 HoTaO 4 increases   with increment of HfO 2 quantity, when the revised k at room temperature displays the opposite HfO 2 composition dependence. HfO 2 alloying effect enhances inharmonic lattice vibration strength and drops thermal conductivity. Figure 8(a) displays that the same amounts of Ta 5+ and Ho 3+ ions are substituted by Hf 4+ cations. As the ionic radius of two Hf 4+ cations is equal to the sum of one Ho 3+ ion and one Ta 5+ ion, no crystal structure expansion or shrinkage is triggered. Figure 8(b) displays that Hf 4+ cations occupying Ta 5+ ionic positions are the most violent phonon scattering sources because of the cage-like tetrahedron and the distinction of ionic radius between Ta and Hf. Phonons can be scattered by Hf 4+ ions to block phonon transportation and result in low thermal conductivity.

Conclusions
HfO 2 alloying effect is successfully employed to decrease thermal conductivity of HoTaO 4 ceramics. HfO 2 alloying content in HoTaO 4 is higher than 6 mol% and is less than 9 mol%, and no crystal structural transition is triggered. The porosity of HoTaO 4 can be regulated by controlling HfO 2 quantity. The wide band gap (~5.0 eV) proves that heat is conducted by phonons in these ceramics. HfO 2 alloying effect is effective in improving thermal insulation performance of HoTaO 4 via enhancing inharmonic lattice vibration, introducing point defects and pores. The least thermal conductivity is 0.8 W·K -1 ·m -1 (900 ℃), which is much lower than that of other TBCs. Current document stresses that HfO 2 alloying HoTaO 4 ceramics are promising TBCs, contributed to the extremely low thermal conductivity.