Thermal barrier materials of Yb₂O₃-ZrO₂ system synthesized by laser excitation

Yb₂O₃ and ZrO₂ (Sinopharm Chemical Reagent Co., Ltd) powders were used as raw materials to prepare compound Yb₂O₃-ZrO₂ ceramics. In the preparation process, raw materials worked as reactants without acidizing or dissolving treatment. Appropriate amounts of these materials were precisely weighted according to molar ratios and then were mixed in ZrO₂ balls milling container for 12 h, the rotating speed of the ball milling was set to 300 rpm. Then, the mixed powders were transferred into a copper vessel with inner diameter of 13 mm. The thickness of powders were 4 mm.

The laser beam was supplied by a multi-mode diode laser system (DILAS, 980 nm, CW, 1200 W), which was irradiated to the mixed powders via a quartz convex lens (f = 30 mm). The solid-state reaction was occurred in atmospheric environment. The spot size of laser beam was 12 mm in diameter and the irradiating time was set to 10 s, afterwards, clumpy resultant was cooled at room temperature naturally. Later, the as-synthesized lump was ground into powder to study its properties.

The crystallographic structure identification of specimens was performed by x-ray diffraction (XRD, Rigaku Ultima VI x-ray diffractometer) with Cu-Kα radiation at room temperature. Transmission electron microscopy (TEM, FEI Tecnai G20) imaging was also performed to study the evolution of the lattice parameters varying with different doping concentrations. Raman spectroscopy (Raman, Brucker RFS100/S) with Argon ion laser radiation at 514 nm was performed. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) was presented to study the alteration of the bonding environment of elements with a monochromatic Al-K_α source. Complex impedance spectroscopy was measured by using a high-precision LCR meter (Agilent 4990 A) which was operated at frequencies ranging from 10 Hz to 10 MHz.

The thermal diffusivities (α) of specimens were examined on TADLF-2800 using a laser-flash apparatus from 25 °C to 1200 °C with an interval of 100 °C. Before the measurement, a disk with 12.7 mm in diameter and 0.8 mm in thickness was carefully formed under the pressure of 200 MPa for 2 min. Both the front and rear surfaces of specimen were coated with a thin film of graphite for thermal absorption of laser beam energy. The density (ρ) was measured based on Archimedes method. The specific heat capacity (C_p) was calculated from heat capacity values of the constituent elements based on the Neumann-Kopp rule [17]. The thermal conductivities (λ) were given by the following equation:

$$\begin{eqnarray}&&\lambda =\alpha \cdot \rho \cdot {C}_{p}.\end{eqnarray} \tag{ 1 }$$

The uncertainty of thermal conductivity was estimated to be 5% with consideration of the uncertainties on the thermal diffusion coefficient (α), density (ρ), and specific heat capacity (C_p). The value of thermal conductivity was corrected by using the following equation [18]:

$$\begin{eqnarray}&&\displaystyle \frac{\lambda }{{\lambda }_{o}}={\rm{1}}-\displaystyle \frac{4}{3}\varphi ,\end{eqnarray} \tag{ 2 }$$

where λ_o is the actual thermal conductivity and φ is the fractional porosity.

Figure 1 shows the mechanism of laser excitation activated self-propagating sintering of Yb₂O₃-ZrO₂ system. Raw materials (Yb₂O₃ and ZrO₂ powders) are mixed adequately and tabled on a copper vessel. A CW diode laser at 980 nm irradiated onto the surface of the raw materials. This mechanism makes good use of Yb³⁺ photon absorption at 976 nm. With 980 nm CW laser irradiating, solid state reaction could be enhanced obviously by near-resonant excitation of Yb³⁺ in self-propagating sintering process [19]. The Yb³⁺ ions in Yb₂O₃ are stimulated to excited level by absorbing laser photons and heats the raw materials simultaneously. The stimulated Yb₂O₃ particles reacted with ZrO₂ to recombine Yb₂O₃-ZrO₂ system nuclei. Moreover, this process enhances laser absorption and energy delivery to heat unreacted powder, leading to self-propagating sintering compounds finally. The technique takes full advantage of Yb³⁺ photon absorption, which significantly lowers the laser power intensity in process of laser sintering.

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We obtained resultants of Yb₂O₃-ZrO₂ systems synthesized by laser excitation, the laser beam power ranges from 100 to 1200 W. As the laser power increases, the clumpy resultant become more and more transparent. When the power reaches 800 W, the almost transparent crystal can be obtained. At higher laser power, however, the clumpy resultant would crack at it cools at room temperature naturally. The as-synthesized lump was ground into powder to study its properties. Figure 2(a) shows the XRD patterns of Yb₂O₃-ZrO₂ system sintered by 12 mm diameter laser beam at 800 W for 10 s. The series of Yb₂O₃-ZrO₂ system were fabricated with different Yb₂O₃ concentrations of 12.5–50 mol% in the raw materials. It can be observed that all Yb₂O₃-ZrO₂ ceramics are single-phase structure without unrelated peaks. Almost every peak of specimens in XRD patterns agrees with standard peak of Yb_0.2Zr_0.8O_1.9 (JCPDS file NO. 78–1309) and Yb₂Zr₂O₇ (JCPDS file number 78–1300) when x is 12.5 and 50, respectively. Yb₂O₃-ZrO₂ system exhibit a defected fluorite-type structure by the ionic radius ratio of r(Yb³⁺)/r(Zr⁴⁺) below 1.26, which is consistent with the XRD of Yb₂O₃ concentrations range of 12.5–50 mol% [20, 21]. These well-defined peaks of all synthesized specimens clearly indicate their good crystallinity. Figure 2(b) shows the XRD patterns of Yb₂O₃-ZrO₂ system ceramics scanning from 58° to 61° in detail. The peak is inclined to a lower degree as increasing x mol% of Yb₂O₃ in Yb₂O₃-ZrO₂ system, which means a distinct increase of lattice parameter. It is in good agreement with Vegard's rule.

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The samples were further characterized by Raman spectroscopy as it is locally sensitive to lattice defects. Figure 2(c) exhibits the Raman spectra of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇. Interestingly, there is a distinct peak at 615 cm⁻¹ of Yb_0.2Zr_0.8O_1.9 which indicates the possibility of huge oxygen vacancies on the surface. Oxygen vacancies are easily formed by disturbed Zr-O bonds and Yb-O bonds for rapid temperature changes, which occurred in the process of drastic laser sintering [22]. According to factor group analysis, Ln₂Zr₂O₇ oxides with pyrochlore structure give six Raman active modes of vibrations, whereas Ln₂Zr₂O₇ almost cannot found Raman peak if it adopts disordered fluorite structure [8]. In principle, Yb₂Zr₂O₇ has disordered fluorite structure. However, there are two obvious peaks observed at 320 and 470 cm⁻¹ as shown in figure 2(c), which means increased Raman activity. The changes of Raman active modes could be attributed to crystal structure transformation from disordered fluorite structure to pyrochlore structure.

Figure 3 clearly demonstrates well-defined crystalline lattice structure of Yb₂O₃-ZrO₂ system ceramics with marked interplanar spacing of the [111]. It can be seen that the spacing shifted slightly from 0.296 to 0.299 nm with the increase of Yb₂O₃ ratios which means crystal lattice parameter increases. It is in accordance with the results of XRD. Respectively, the interplanar spacings of figures 3(a) and (d) are in accord with standard spacing of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇. Accordingly, it can be verified that Yb₂O₃ and ZrO₂ powders generate Yb₂O₃-ZrO₂ ceramics effectively by laser sintering.

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Figure 4(a) represents the XRD patterns of resultants with 12.5 mol%Yb₂O₃ to ZrO₂ powders sintered by laser beam power at 100, 200, 300, 400, 500 and 1200 W for 10 s respectively. At 100 W, there were a bit of Yb_0.2Zr_0.8O_1.9 among those unreactive Yb₂O₃ and m-ZrO₂ after laser sintering. More ingredients were involved in the synthetic process to synthesize Yb_0.2Zr_0.8O_1.9 with higher laser beam power. Nearly pure Yb_0.2Zr_0.8O_1.9 was generated when the laser power was up to 400 W. In addition, in order to investigate the influence of further high laser power on the reaction products, the laser power was increased up to 1200 W with an interval of 200 W. The XRD patterns of resultants sintered at laser power range from 400 W to 1200 W are quite similar to that at 400 W. This also indicates the excellent stabilities of synthesized Yb₂O₃-ZrO₂ system ceramics. Figure 4(b) represents the XRD patterns of resultants with 50 mol%Yb₂O₃:ZrO₂ powders which are sintered at 100, 200, 300, 400, 500 and 1200 W for 10 s. Similarly, a small amount of Yb₂Zr₂O₇ were found among those unreactive Yb₂O₃ and m-ZrO₂ after laser sintering at 100 W. As increasing laser power, Yb₂Zr₂O₇ of increased phase purity was produced. Finally, pure Yb₂Zr₂O₇ was generated when the laser power reached 400 W. The XRD patterns of resultants almost keep unchangeable as laser power worked at the region from 400 W to 1200 W.

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Furthermore, XPS was investigated to study the bonding energy (BE) of O, Zr and Yb peaks on the surface. Figure 5 shows the high resolution XPS spectra of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇ solid solutions synthesized at 800 W, respectively, which is characterized by Yb 4p, Zr 3d and O 1 s profiles. The XPS survey spectra of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇ are shown in figure 5(a).

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Figure 5(b) shows the high resolution Yb 4p XPS spectra of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇. Both of them are resolved into two distinguishable peaks corresponding to 4p3/2 and 4p1/2, which are centered at 332.8 and 346.5 eV respectively. Similarly, as shown in figure 5(c), the 3d_5/2 and 3d_3/2 of Zr were observed at 182.1 and 184.5 eV in Yb_0.2Zr_0.8O_1.9 as well as at 181.9 and 184.3 eV in Yb₂Zr₂O₇. By contrast, the high resolution Zr 3d XPS spectrum of Yb₂Zr₂O₇ shows a slight shift towards lower energy. The Zr⁴⁺ cation contains eight oxygen neighbors in fluorite structure cell; whereas Zr⁴⁺ is coordinated by six oxygen ions in pyrochlore structure, indicating a more stable composition [23]. The BE in synthetic Yb₂Zr₂O₇ is lower than in Yb_0.2Zr_0.8O_1.9, which could be ascribed to crystal structure transformation from disordered fluorite structure to pyrochlore structure. Figure 5(d) shows the high resolution XPS spectra of O 1s electron of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O_7. Interestingly, two peaks of Yb_0.2Zr_0.8O_1.9 are observed at 529.8 and 531.0 eV corresponding to the lattice oxygen and oxygen vacancy, respectively. Similarly, two peaks center at 529.7 and 531.4 eV of Yb₂Zr₂O₇ ceramic corresponding to two different oxygen states respectively. It can be also observed that the amount of the oxygen vacancies in Yb_0.2Zr_0.8O_1.9 is much higher than that in Yb₂Zr₂O₇. The relative amount of the oxygen vacancies is calculated to be 55.3% in Yb_0.2Zr_0.8O_1.9 and 40.2% in Yb₂Zr₂O₇, which is consistent with the huge oxygen vacancies conjecture based on Raman observation.

The variations in thermal diffusivity with temperature for Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇ ceramics are plotted in figure 6(a). The values of thermal diffusivity are the average arithmetic means of three measurements. Distinctly, the thermal diffusivities of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇ ceramic generally decrease with the increase of temperature from 25 °C to 1200 °C. The thermal diffusivity of Yb_0.2Zr_0.8O_1.9 ceramic is in the range of 0.174–0.248 mm²/s from 25 °C to 1200 °C, and the thermal diffusivity of Yb₂Zr₂O₇ ceramic is in the range of 0.145–0.215 mm²/s.

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The thermal conductivities of Yb_0.2Zr_0.8O_1.9 and Yb₂Zr₂O₇ calculated with formula (1) and (2) are plotted as a function of temperature, as shown in figure 6(b). The values in this figure have been corrected to 100% theoretical density. The thermal conductivity of Yb_0.2Zr_0.8O_1.9 gradually decreases with the increase of temperature up to 1200 °C due to the lattice thermal conduction. The thermal conductivity of Yb_0.2Zr_0.8O_1.9 in this study is calculated in the range of 0.677–0.936 W/mK from 25 °C to 1200 °C. Yb₂Zr₂O₇ solid solutions exhibit reduced thermal conductivity over the entire temperature range, within the range of 0.497–0.730 W mK⁻¹ from 25 °C to 1200 °C. The thermal conductivity of Yb₂Zr₂O₇ synthesized by laser sintering is obviously lower than those by conventional solid-state reaction method [24–26]. The much lower thermal conductivity of Yb₂Zr₂O₇ may be ascribed to the scattering of the phonons by oxygen vacancies [14] and the crystal structure transformation from disordered fluorite structure to pyrochlore structure [8].

Figures 6(c) and (d) show the complex impedance spectrum (Z'' versus Z', Nyquist plot) of Yb₂O₃:ZrO₂ with different Yb₂O₃ contents at 25 °C and 500 °C, respectively. The semicircular arc with an intercept on the real axis corresponds to the grain boundary resistance. As shown in figure 6(c), the grain boundary resistance increases when Yb₂O₃ content increases from 12.5 mol% to 50 mol%. This indicates that the defects concentration (i.e., oxygen vacancies) in Yb₂Zr₂O₇ is lower than those in Yb_0.2Zr_0.8O_1.9. When Yb₂O₃ is doped in ZrO₂, Yb³⁺ substitution of Zr⁴⁺ to maintain the electroneutrality of the lattice. At the lower doping amount of Yb₂O₃, the Yb₂O₃-ZrO₂ system has more defects and lattice distortions caused by Yb intervening, which gives rise to oxygen vacancies or even interstitial ions accumulating in the grain boundaries. Thus, the resistances of grain boundaries are larger when Yb₂O₃ content is below 50 mol%. Once Yb increased up to the 1:1 ratio of Yb:Zr and the stable phase Yb₂Zr₂O₇ formed, it can greatly reduce the lattice defects. Therefore, Yb₂Zr₂O₇ further decrease oxygen vacancies due to more Yb³⁺ ions. However, the resistance values still maintain 10⁶ Ω, insulating state. As shown in figure 6(c), the grain boundary resistance decreases for all the Yb₂O₃:ZrO₂ specimens at 500 °C, which reveals increase in the dc conductivity at higher temperature. This could be ascribed to enhanced space charge polarization in ceramics accompanying with temperature rising. The resistance of grain boundary changes abruptly for the samples with different Yb₂O₃ content, whereas the Yb₂Zr₂O₇ still maintains the larger resistance value. This also confirmed that Yb₂Zr₂O₇ has good crystalline structure and there should be oxygen vacancies existing in Yb_0.2Zr_0.8O_1.9.