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Synthesis of Ultra-High Temperature Pyrochlore Ceramics for Extreme Conditions

Written By

Radu-Robert Piticescu, Cristina Florentina Ciobota, Mihail Botan and Alexandru Gyorgy Okos

Submitted: 15 September 2024 Reviewed: 18 October 2024 Published: 30 November 2024

DOI: 10.5772/intechopen.1007904

Advanced Ceramic Materials IntechOpen
Advanced Ceramic Materials Emerging Technologies Edited by Amparo Borrell Tomás and Rut Benavente Martínez

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Advanced Ceramic Materials - Emerging Technologies

Amparo Borrell Tomás and Rut Benavente Martínez

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Abstract

Thermal protection systems (TPS) for aerospace industry require the use of coatings made from ultra-high temperature ceramics (UHTCs), a class of materials based on oxides, nitrides, carbides and borides of transitional metals. Yttria-doped zirconia is the actual “gold standard” for aerospace coatings but the high temperature phase transitions limit its use in developing a new generation of TPS. Rare earth zirconia pyrochlores Ln2Zr2O7 (Ln = La, Nd, Sm, Gd, Yb) emerged as future candidates for high temperature coatings due to their high thermal stability on a large temperature range. Here, we review the crystalline structures’ electronic and thermal properties of these materials, the main methods proposed for their synthesis, their advantages and disadvantages. The use of mixed rare earths as dopant for zirconia ceramics is presented further. Finally, the potential applications as thermal barrier coatings (TBCs) are presented.

Keywords

  • ultra-high temperature ceramics
  • pyrochlores
  • rare earth zirconates
  • crystalline structures
  • synthesis
  • TBCs

1. Introduction

Thermal protective systems (TPS) represent the most effective system to protect structural components of spacecraft vehicles from damage caused by high temperature and corrosive/erosive environments. Ultra-high temperature ceramics (UHTCs) are ceramic-based composites consisting of a matrix of transition metal compounds that can be used around 2000°C and above in an oxidizing or corrosive environment. They are promising candidates for high temperature applications as thermal barrier coatings (TBCs) in hypersonic flight, rocket propulsion, atmospheric re-entry, refractory crucibles, plasma-arc electrodes, etc. [1]. An ideal material for TBC should combine high melting temperature, low density, low thermal conductivity and expansion coefficient close to the metal substrate. A comparison between these properties for oxide and non-oxide UHTCs shows that non-oxide ceramics possess the highest melting temperatures but, in most cases, higher densities and thermal conductivities, as can be seen in Figure 1.

Figure 1.

Properties of oxides, carbides, nitrides and borides required for TBC (Tm—melting temperature; d—density, α—thermal expansion coefficient and k—thermal conductivity) [2].

Non-oxide ceramic coatings also have a lower chemical stability at high temperatures in air and oxidizing atmosphere due to their higher oxygen affinity. Another problem to be solved is the adherence of the coating to the substrate, which needs to find the proper metallic bonding having the capacity to form a thermally grown protective Al2O3 oxide layer (TGO) on the metallic substrate and improve the adhesion force by diffusion of metallic component. While for the bonding of oxide layers, alloys based on NiCoCrAlY perform very well, and for carbides, borides and nitrides, the diffusion is low and different bonding methods must be developed.

The “gold standard” material for ceramic coatings is 8 wt.% yttria-stabilized zirconia (8YSZ). However, it has some main drawbacks for use in higher temperature range: its stability at temperature higher than 1200°C decreases due to phase transformation with volume increase and increasing thermal conductivity for long time exposure (generally greater than 100 h) at high temperatures and susceptibility to CMAS (calcium magnesium aluminum silicates) infiltration leading to delamination [3]. Table 1 presents the thermal properties of aluminum oxide (Al2O3) and NiCoCrAlY alloys used as bonding coat and 8YSZ, respectively.

PropertyUnitsAl2O3NiCoCrAlY8YSZ
Melting temperatureK~2323~1863~2873
Thermal conductivityWm−1 K−15.8 (1400 K)320 (293 K)~2.5 (298 K)
Thermal expansion coefficientX10−6 K−19.6 (1273 K)10.7(293-1273 K)10.7 (293-1273 K)

Table 1.

Thermal properties of Al2O3 and NiCoCrAlY alloys and 8YSZ.

There is a continuous search for alternative candidates, motivated by the industry’s demand for more and more performing engines and space components. Numerous candidates have been studied and tested [4]. Here, we discuss the structure, thermal properties and methods for the synthesis of rare earth zirconates with pyrochlore structure as potential materials to improve zirconia coatings in high temperature applications.

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2. Crystalline structure and properties of pyrochlore rare earth zirconates

Rare earth zirconate compounds have gained increasing popularity due to their unique features, which make them ideal for different applications, such as radioactive waste immobilization, thermal barrier coatings and photocatalysis [5]. Ceramic materials with general formula Ln2Zr2O7 (Ln = La, Nd, Sm, Gd, Yb) with an ionic radius ratio ranging from 1.61 to 1.46 crystallize in cubic pyrochlore structure [6] and have a relatively low oxygen diffusion, thermal conductivity in the range 1.1–1.7 Wm−1 K−1, higher thermal stability and chemical inertness, making them to be considered among the most promising candidates for TBCs [4]. Pyrochlore zirconates of Promethium (Pm), Terbium (Tb), Dysprosium (Dy), Holmium (Ho), Erbium (Er), Thulium (Tm), Ytterbium (Yb) and Lutetium (Lu) were until now not reported to exist or to be observed experimentally.

Figure 2 shows the typical crystalline structure of Ln2Zr2O7 pyrochlores crystallized in the cubic Fd-3 m space group. Ln3+ is bonded to eight O2− atoms to form distorted LnO₈ hexagonal bipyramids that share edges with six equivalent LnO₈ hexagonal bipyramids and edges with six equivalent ZrO₆ octahedra. There are two shorter (2.34 Å) and six longer (2.64 Å) Ln-O bond lengths. Zr4+ is bonded to six equivalent O2− atoms to form ZrO₆ octahedra that share corners with six equivalent ZrO₆ octahedra and edges with six equivalent LnO₈ hexagonal bipyramids. The corner-sharing octahedral tilt angles are 50°. All Zr-O bond lengths are 2.11 Å. There are two inequivalent O2− sites. In the first O2− site, O2− is bonded to four equivalent Ln3+atoms to form corner-sharing OLa₄ tetrahedra. In the second O2− site, O2− is bonded in a 4-coordinate geometry to two equivalent Ln3+ and two equivalent Zr4+ atoms.

Figure 2.

Ln2Zr2O7 crystalline structure [7].

Tables 2 and 3 summarize data related to crystalline structure and electronic properties of pyrochlore based on rare earth zirconates.

Ceramic materialSystem/space groupLattice parametersBond lengthDensity
[gcm−3]		Ref.
La2Zr2O7Cubic, Fd-3 ma = 10.81 Å
α = 90°		2 short La-O = 2.34 Å
6 long La-O = 2.64 Å
Zr-O = 2.11 Å		6.02[8, 9, 10, 11, 12, 13, 14, 15]
Cubic, Fd-3 ma = 10.75 Å
α = 90°		La-O = 2.38 Å
6 short Zr-O = 2.28 Å
2 long Zr-O = 2.33 Å		6.12[16, 17]
Ce2Zr2O7Cubic, Fd-3 ma = 10.68 Å
α = 90°		2 short Ce-O = 2.31 Å
6 long Ce-O = 2.47 Å
6 short Zr-O = 2.17 Å
2 long Zr-O = 2.31 Å		6.27[9, 18, 19, 20, 21, 22]
Pr2Zr2O7Cubic, Fd-3 ma = 10.75 Å
α = 90°		2 short Pr-O = 2.33 Å
6 long Pr-O = 2.61 Å
Zr-O = 2.1 Å		6.16[12, 23, 24, 25, 26, 27, 28, 29]
Nd2Zr2O7Cubic, Fd-3 ma = 10.7 Å
α = 90°		2 short Nd-O = 2.32 Å
6 long Nd-O = 2.59 Å
Zr-O = 2.1 Å		6.31[9, 10, 23, 30, 31, 32, 33, 34, 35, 36]
Cubic, Fd-3 ma = 10.69 Å
α = 90°		Nd-O = 2.35 Å
6 short Zr-O = 2.28 Å
2 long Zr-O = 2.31 Å		6.35[37]
Sm2Zr2O7Cubic, Fd-3 ma = 10.62 Å
α = 90°		2 short Sm-O = 2.3 Å
6 long Sm-O = 2.56 Å
Zr-O = 2.09 Å		6.59[9, 12, 38, 39, 40]
Eu2Zr2O7Cubic, Fd-3 ma = 10.62 Å
α = 90°		2 short Eu-O = 2.3 Å
6 long Eu-O = 2.56 Å
Zr-O = 2.09 Å		6.63[41, 42, 43, 44, 45, 46]
Gd2Zr2O7Cubic, Fd-3 ma = 10.54 Å
α = 90°		Gd-O = 2.29 Å
Zr-O = 2.28 Å		6.9[37, 47, 48, 49, 50, 51, 52, 53, 54]

Table 2.

Crystalline structure of pyrochlore based on rare earth zirconates.

Ceramic materialMelting temperature [°C]Predicted formation energy [eV/atom]Band gapMagnetizationRef.
La2Zr2O72300−3.8513.73Non-magnetic[8, 9, 10, 11, 12, 13, 14]
—−3.468Non-magnetic[16, 17]
Ce2Zr2O7> 2000−3.8152.53Ferro-magnetic
2 μB/f.u.		[9]
Pr2Zr2O7—−3.783.46Non-magnetic[12, 23, 24]
Nd2Zr2O7> 2000−3.7973.33Non-magnetic[9, 10, 23, 31, 32]
—−3.43Non-magnetic[37]
Sm2Zr2O72000−3.527Non-magnetic[9, 12, 23, 38]
Eu2Zr2O7—−3.527Ferro-magnetic
12 μB/f.u.		[41, 42]
Gd2Zr2O72570−3.513Ferro-magnetic
14 μB/f.u.		[37, 47, 48]

Table 3.

Electronic properties of pyrochlore based on rare earth zirconates.

The similarity of the cubic structure in terms of lattice parameters and bond lengths is clearly observed. Density is increasing with the atomic number of rare earth element. Melting temperature for all known rare earth zirconates with pyrochlore structure is higher than 2000°C, with a higher value of 2570°C for Gd2Zr2O7. However, research is still needed to have more precise values for the melting point of these compounds.

Pyrochlore materials based on Ln2Zr2O7 (Ln = La, Nd, Pr, Sm, Gd, Yb) have relatively low oxygen diffusion, thermal conductivity in the range 1.1–1.7 W−1 K−1, higher thermal stability and chemical inertness than YSZ, being considered among the most promising candidates for next generation of thermal barrier coating. The absence of phase transitions of these perovskites from room temperature to melting temperature and low thermal conductivities lead to a wider operating temperature range compared to gold standard YSZ.

Thermal conductivity is greatly dependent on temperature, system size, impurities and dislocations. Generally, as the atomic number of a rare earth element increases (from La to Lu), its thermal conductivity decreases. This is frequently linked to enhanced phonon scattering as a result of the higher atomic mass and stronger electron-phonon interaction. Also, thermal conductivity normally decreases with increasing temperature, mainly at higher temperatures, and this is the result of enhanced phonon-phonon scattering.

Yang et al. evaluated thermal conductivity of La2Zr2O7 at temperatures ranging from room temperature to 1200°C. At room temperature, thermal conductivity is 3.2 Wm−1 K−1, drops to 2.1 Wm−1 K−1 at 400°C and then begins to increase to 2.7 Wm−1 K−1 at 1200°C [55]. J. M. Zeerati and coworkers claim that they have achieved perfect agreement with experimental results by utilizing the effective harmonic method and homogeneous nonequilibrium molecular dynamics simulations with machine-learning potentials to calculate the thermal conductivity of candidate materials, such as La2Zr2O7, ZrSiO4 and BaZrO3, at temperatures up to 1500 K. This work conducted a comparison between the thermal conductivity calculated using the homogeneous nonequilibrium molecular dynamics approach and the experimental data for La2Zr2O7. The experimental value was 1.51 Wm−1 K−1 and the simulated value was 1.47 Wm−1 K−1; both the values are very close [4].

Other calculated values for pyrochlore compounds based on rare earth elements are presented by Feng et al. Their findings suggest that Cahill’s approach is more effective for Ln2Zr2O7 molecules. However, for Nd2Zr2O7, both approaches provide significantly lower values than the measured ones [56].

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3. Synthesis of pyrochlore rare earth zirconates

The synthesis of the pyrochlore materials can be done by different methods, such as solid-state reaction and solution methods, such as co-precipitation, combustion method, hydrothermal synthesis and sol-gel method.

3.1 Solid-state reaction method

According to the solid-state reaction method, the rare earth oxide and zirconia powders mixed at the required stoichiometry are dried in an oven to remove moisture and other impurities at around 110–120°C, until a constant weight is reached. The resultant dried mixture is further ball-milled to form a uniform mixture suspension in distilled water and/or absolute ethanol. The resultant colloidal suspension is dried and then thermally treated (calcined) at above 1500°C in a furnace, until the required pyrochlore structure is obtained. To increase the structural uniformity, the ball milling and thermal treatment processes needed to be repeated many times. The general flowsheet of the solid-state synthesis process is presented in Figure 3. Better results are obtained when the final drying step is performed by air spray drying, resulting in powders with better flowability. Zirconia lining and balls are used to avoid impurification with other oxides. The calcination temperatures are in the range 1400–1500°C and milling time 12–72 h [57, 58, 59].

Figure 3.

Schematic flowsheet of the solid-state method.

3.2 Co-precipitation method

In the co-precipitation method, soluble precursors of lanthanum (Ln) and zirconium (Zr) are used as feedstock material, mainly nitrates with high solubility in water. Rare earth oxides (REOs) are generally easily soluble in nitric acid (HNO3) solutions and may be used as raw materials. In the next step, the pH is increased to 9–11 by addition of ammonia (NH3) solution or sodium hydroxide (NaOH). The resulting precipitate formed is separated by filtering and washing to remove the alkaline nitrates formed. The use of ammonia is advantageous because ammonium nitrates are more soluble during washing with distilled water. The washed precipitate is further dried and calcined in the temperature range 1000–1300°C for 1 to 5 hours [60, 61, 62] to obtain the perovskite powders (Figure 4). The process may be described by the reaction

Figure 4.

Schematic flowsheet of the co-precipitation method.

2Ln(NO3)3+2Zr(NO3)4+14NH4OH=Ln2Zr2O7+14NH4NO3+7H2OE1

The use of a fuel source and oxygen source introduced in the co-precipitated flake may produce auto-ignition and eliminate the thermal treatment stage. The fuel chemical agents used are glycine, hydrazine, citric acid or glycerol. However, a post-heating treatment of the fluffy powder is required to remove the impurities formed during combustion. The combustion method improves process efficiency due to low energy consumption and produces pyrochlore powders with uniform morphology and highly porous structure [63].

3.3 Sol-gel method

The sol-gel processes are based on the controlled hydrolysis of organo-metallic compound precursors, with the formation in the first stage of colloidal suspensions (sols) from which the gel is subsequently formed in the presence of chelating agents.

Unlike the hydrolytic processes of precipitation, where the formation of the new phase takes place through a process of inhomogeneous nucleation, in the case of hydrolysis of organo-metallic compounds the nucleation rate can be more easily controlled and the solid phase is formed through a homogeneous nucleation mechanism. Among the organo-metallic compounds used for the synthesis of ultradisperse and nanocrystalline ceramic powders, metal alkoxides are most often mentioned in the literature. Sol destabilization (formation of a colloidal solid containing the fluid component dispersed in a three-dimensional (3D) lattice) can be achieved by dilution with water or hydrolysis catalyzed by acids or bases. Figure 5 presents the schematic flowsheet of the sol-gel process used for rare earth zirconates.

Figure 5.

Schematic flowsheet of the sol-gel method.

La2Zr2O7 pyrochlore was prepared using ZrOCl2*8H2O and La(NO3)3*6H2O, which were dissolved in a mixed water (H2O)/ethanol solution. Citric acid was further added to achieve Zr4+:La3+:citric acid ratio = 1:1:0.2. Formamide and polyethylene glycol were added to the above mixture, which was then stirred at room temperature for 2 h to form a uniform transparent sol solution. The resultant solution was placed into a crucible and aged for 20 h at 80°C, which resulted in gel formation. The resultant gel was dried at 110°C for 5 h and calcinated at 1200°C for 2 h in controlled atmosphere to form homogeneous La2Zr2O7 coatings [64].

3.4 Hydrothermal method

Hydrothermal methods refer to any heterogeneous reaction in the presence of aqueous solvents or mineralizers under high pressure and temperature conditions. In a hydrothermal process, the dissolution and crystallization processes are enhanced to form new solid phases with controlled stoichiometry. The process involves obtaining a mixed solution with desired composition, adjustment of the pH using mineralizer agents, such as ammonia solution or sodium hydroxide, and transfer of the solution into an autoclave to produce the hydrothermal crystallization of the compound that is separated by filtering and dried to obtain the desired powder. The hydrothermal synthesis of Ln2Zr2O7 is described by

2Zr+4+2La+3+14OH−=Ln2Zr2O7+7H2OE2

Figure 6 presents the schematic flowsheet of the hydrothermal process.

Figure 6.

Schematic flowsheet of the hydrothermal process.

The main advantage of the hydrothermal method is the lower temperature of product crystallization, together with faster kinetics and low environmental impact because the process takes place in closed vessels. As an example, Sm2Zr2O7 pyrochlore was obtained from SmO2 and ZrOCl2·8H2O precursors dissolved in nitric acid (HNO3), addition of aqueous ammonia solution to obtain a pH of 5–9, washing and addition of potassium hydroxide (KOH) solution as mineralizer under vigorous stirring and hydrothermal treatment into a Teflon-lined autoclave at 200°C. The powder obtained was filtered, washed and dried in vacuum at 70°C for 18 h [40]. Similarly, La2Zr2O7 was obtained using La and Zr nitrates and NaOH as mineralizer to reach pH = 11, followed by hydrothermal reaction at 200°C for 1 h [65].

Table 4 presents the main advantages and disadvantages of the solid-state reactions, co-precipitation, sol-gel and hydrothermal methods used to obtain rare earth zirconium perovskites and zirconia co-doped with mixed rare earth oxides.

Main parametersSynthesis method
Solid-stateCo-precipitationSol-gelHydrothermal
Process reactionTemperature
Time		pH
Time		pH Temperature
Surfactants		Temperature
Pressure
pH
PrecursorsOxidesSoluble salts/RE oxidesMetallic alkoxidesSoluble salts
NucleationNon-homogeneousNon-homogeneousHomogeneousHomogeneous
DurationHighModerateModerateLow
Particle size/shapeNon-uniform/irregularUniform size/irregular shapesNon-uniform/irregularUniform sizes and shapes
Heat treatmentRequiredRequiredRequiredNo
Kinetic enhancementNopH controlpH and surfactantsMicrowaves

Table 4.

Advantages and disadvantages of perovskite synthesis.

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4. Synthesis of zirconia ceramics doped with mixed rare earth oxides

Modern TBCs have a multilayer structure, with the first layer made by NiCoCrAlY bonding layer forming the alumina TGO, and successive ceramic coatings with the top coating from the ceramic material with the higher melting temperature and thermal stability. Despite the advantages of rare earth zirconates as UHTCs provided by higher melting points and lower thermal conductivity compared to YSZ, their use as intermediate layers is limited due to the chemical, structural and thermal expansion coefficient mismatches. A potential solution to improve the thermal properties of TBC is to use as intermediate layer ZrO2 doped with mixed rare earth oxides (REOs).

In such complex systems, the study of the phase equilibrium diagrams was conducted and the crystalline phases formed with temperature analyzed.

In Piticescu et al. [66], the authors performed a first study on the crystalline phase modifications of hydrothermally synthesized YSZ doped with 4, 6 and 8 wt.% La, Nd, Sm and Gd oxides, respectively, during thermal treatment in the temperature range 400–1400°C and their thermal conductivities.

The phase evolution of powders during thermal treatment in the range 400–1400°C evidenced the progressive formation of different solid solutions by isomorphic substitution of Zr4+ with Ln3+. A cubic ZrO2 solid solution is the major phase and secondary phases with generic formula (Ln0.07Y0.14Zr0.79)O1.90; (Ln0.11Y0.14Zr0.75)O1.88; and (Ln0.14Y0.14Zr0.72)O1.86 were formed depending on the dopant concentration. At temperatures 1200°C and 1400°C, the formation of cubic solid solutions with pyrochlore structures (RE2Zr2O7 where RE = Y and Ln) is observed for all compositions, except for samples co-doped with 4, 6 and 8 wt.% Nd, 6 and 8 wt.% Sm and 8 wt.% Gd, when only the cubic solution (ZrO2)SS-(Ln0.14Y0.14Zr0.72)O1.86 phase was observed.

Low values of the thermal conductivities were measured by the hot plate method at room temperature (Table 5). These values are an order of magnitude lower than the values predicted and measured for rare earth zirconates in Table 6 and closer to thermal conductivity values given in Ref. [67], respectively, 0.7 Wm−1 K−1 for co-precipitated Pr2Zr2O7, 0.4 Wm−1 K−1 for Sm2Zr2O7 and 0.38 Wm−1 K−1 for (Pr, Sm)2Zr2O7, suggesting that both the synthesis method and the complex defect structures induced by co-doping have a beneficial influence on reducing thermal conductivities of perovskites.

SampleThermal conductivity [Wm−1 K−1]Thermal diffusivity [Mm2s−1]Specific heat [MJ*m−3 K−1]
ZrO2-8Y203-6La2O30.3049 ± 0.00790.2603 ± 0.00750.4048 ± 0.0036
ZrO2-8Y203-6Nd2O30.3276 ± 0.00040.2899 ± 0.00180.3502 ± 0.0023
ZrO2-8Y203-6Sm2O30.3923 ± 0.00190.2766 ± 0.00530.3987 ± 0.0060
ZrO2-8Y203-6Gd2O30.3799 ± 0.00120.2799 ± 0.00840.3809 ± 0.0106

Table 5.

Thermal conductivities, thermal diffusivities and specific heat of zirconia co-doped with rare earth oxides by hydrothermal method.

MaterialUnitsDebye modelCahill modelExperimental
La2Zr2O7 (LZO)Wm−1 K−11.21.31.5
Nd2Zr2O7 (NZO)Wm−1 K−11.21.11.6
Sm2Zr2O7 (SZO)Wm−1 K−11.31.51.5
Gd2Zr2O7 (GZO)Wm−1 K−11.21.21.6
Pr2Zr2O7 (PZO)Wm−1 K−11.21.41.5
ZrO2 cubicWm−1 K−12.32.42.5

Table 6.

Predicted and experimental values for thermal conductivities of rare earth zirconates.

In a next step to study more complex systems, nanostructured materials (1-x)ZrO2-x(Ln2O3) and ZrO2 - x(La,Sm,Gd,Yb,Nd)O3 (x = 0.2) encoded as ZrO2-LSGYN were synthesized in a one-step process by hydrothermal method (Figure 7) starting from water-soluble salts of Zr and oxides: ZrCl4 (Merck, New Jersey, USA, p.a. 98%), Sm2O3 (Alfa Aesar, Massachusetts, USA, p.a. >99.9%), Nd2O3 (Alfa Aesar, Massachusetts, USA, p.a. >99.9%), Yb2O3 (Alfa Aesar, Massachusetts, USA, p.a. >99.9%), Gd2O3 (Thermo Scientific, Massachusetts, USA, p.a. >99.9%) and La2O3 (Thermo Scientific, Massachusetts, USA, p.a. >99.9%). Ammonia solution (25 wt.% NH3) was used as a mineralizer to control the solution pH. The amounts of Sm2O3, La2O3, Nd2O3, Gd2O3, Sm2O3 and ZrCl4 were established in agreement with the theoretical molar formula ZrO2- xLn2O3 (x = 0.2, Ln = La, Sm, Gd, Yb, Nd). The final pH was in the range of 9.5–11. The suspension thus obtained was subjected to hydrothermal processing (pressure 100 atm, temperature 200°C and time 2 h). The suspension was introduced into a Teflon-lined vessel of a sealed hydrothermal autoclave reactor (5 L, Berghof Products + Instruments GmbH, Berghof, Germany) endowed with a cooling coil inserted into the autoclave vessel to avoid grain growth. To control the reaction pressure, argon gas was purged inside the autoclave. Powders were sintered in air at 1200°C to obtain homogeneous crystalline structures.

Figure 7.

Schematic flowsheet for hydrothermal synthesis of ZrO2-xLn2O3 (x = 0.2, Ln = La, Sm, Gd, Yb, Nd).

Powders were characterized from the chemical point of view by the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES) technique, using ICP-OES 725 (AGILENT, Santa Clara, California, USA) equipment, according to ASTME E1479-16 standards (Table 7).

CompoundChemical analysis, wt.%
ZrLaSmGdYbNd
(1-x)ZrO2-xSm2O3 x = 0.244.1—35———
(1-x)ZrO2-La2O3 x = 0.240.3634————
(1-x)ZrO2-Gd2O3 x = 0.238.63——37.7——
(1-x)ZrO2-Yb2O3 x = 0.241.4———37.9—
(1-x)ZrO2-Nd2O3 x = 0.243.1————33.5
ZrO2-LSGYN41.87.446.607.858.746.32

Table 7.

Chemical compositions of ZrO2-xLn2O3 (x = 0.2) powders.

Micromeritics® TriStar II Plus (Norcross-Atlanta, USA) using the program for data collecting and calculation was used to analyze the surface proper¬ties of powders.

Powders were characterized from the chemical point of view by the Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES) technique, using ICP-OES 725 (AGILENT, Santa Clara, California, SUA) equipment, according to ASTME E 1479–16 standards.

Micromeritics® TriStar II Plus (Norcross-Atlanta, USA) using the program for data collecting and calculation TriStar II Plus was used to analyze the surface properties of powders.

The chemical compositions of the powders obtained are presented in Table 5 and confirm that the composition is conform with the initially designed one.

Figures 8–11 present the specific surface areas, pore volumes and average pore sizes. The analysis of the surface properties of as-synthesized powders is compared with the analysis of the surface properties of powders thermally treated at 1200°C and aims to optimize the sintering process for further obtaining pellets used for characterization of thermal properties. The combination of grain growth, doping effects and porosity reduction at high temperatures results in a decrease in the surface area of ZrO2 doped with rare earth elements. This phenomenon is crucial in understanding the behavior of these materials in various applications, such as thermal barrier coatings.

Figure 8.

Brunauer-Emmett-Teller (BET) surface area analysis results for ZrO2-LSGYN and ZrO2-LSGYN heat-treated at 1200°C: specific surface area (left), pore volume (center) and pore size (right).

Figure 9.

BET specific surface area analyzed for the five specimens. Left: As obtained 0.8ZrO2-0.2(Ln2O3); right: 0.8ZrO2-0.2(Ln2O3) heat-treated at 1200°C, where Ln = La, Nd, Yb, Sm.

Figure 10.

Pore volumes analyzed for the five specimens. Left: As obtained 0.8ZrO2-0.2(Ln2O3); right: 0.8ZrO2*0.2(Ln2O3) heat-treated at 1200°C, where Ln = La, Nd, Yb, Sm.

Figure 11.

Average pore size analyzed for the five specimens. Left: As obtained 0.8ZrO2-0.2(Ln2O3); right: 0.8ZrO2-0.2(Ln2O3) heat-treated at 1200°C, where Ln = La, Nd, Yb, Sm.

In terms of pore size, ZrO2 samples doped with Sm and Yb oxides treated at 1200°C show that the pore size rises with temperature rather than decreasing, as seen in the other samples. The pore radius in the ZrO2-Nd2O3 and ZrO2-Sm2O3 powders increases when subjected to heat treatment at 1200°C compared to those that were not heat-treated. This can be attributed to various phenomena, including i) the rearrangement of particles during calcination; it is possible that the particles agglomerate differently from their original structure, leading to the formation of larger pores, (ii) crystallite growth, (iii) partial sintering of the particles, (iv) phase transformations that can change the density and structure of the original material, and (v) microstructure development—calcination influences the microstructure of the material, creating a more open porous structure compared to that of uncalcined materials, which is a more compact structure [68].

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5. Obtaining thermal barrier coatings based on rare earth zirconates

The increasing demand for new functional coatings has been a strong incentive for research toward not only understanding the fundamentals and technical aspects of film nucleation and growth, but also developing new deposition techniques that allow for better control of the deposition process. Electron Beam Physical Vapor Deposition (EB-PVD), Powder Flame Spraying, Plasma Thermal Spray and Cold Gas Dynamic Spray Coating are techniques suited for creating different types of TBCs. The EB-PVD process takes place in a high vacuum chamber, ensuring a relatively high deposition rate up to 150 nm/minute. The adhesion of the coating may be improved by controlled heating of the substrate during deposition. Combining these process’s merits, denser coatings could be achieved. The EB-PVD process may be successfully used for selective deposition of multi-layered films based on refractory metal, oxides, carbides, nitrides, etc., for the parts to be used in extreme conditions. A specific aspect of EB-PVD coatings is related to their columnar microstructure determining the behavior of the coating during its service life [69]. The control of substrate temperature on coatings adhesion remains a major issue for optimizing final properties.

Five different TBC systems, YSZ, La2Zr2O7, Gd2Zr2O7, YSZ/, La2Zr2O7 and YSZ/Gd2Zr2O7, were produced and exposed to furnace thermal cyclic oxidation tests. The deposition of protective ceramic top coats was performed with EB-PVD, single-layer coatings having a thickness of approximately 200 μm, while the thickness of each two-layer coating was approximately 100 μm. It was found that TBC double layers have a higher lifetime compared to TBC’s with single layer. As a result of the performed cycling tests, the best performance was exhibited by the YSZ/Gd2Zr2O7 coating system [70].

To prove the effect of the La2Zr2O7 layer deposited on the YSZ layer on the thermal conductivity and oxygen penetration, two systems were exposed to isothermal and thermal cyclic oxidation tests. The two systems, YSZ and YSZ / La2Zr2O7 top coats, were deposited using EB-PVD technique. The stress distribution showed that the YSZ / LZ (lanthanum zirconate) system has a longer life span than the YSZ TBC. Although the oxidation and thermal cyclic tests showed that double-layer YSZ/LZ TBC exhibited better performance than single-layer YSZ TBC, there is no drastic difference between the thermal performances of TBCs [71].

Another study suggested the use of La2Zr2O7 in the design of TBC for its low thermal conductivity. Although YSZ has been used as a top coat for TBC due to its thermal diffusivity, the major disadvantage of its use is due to its limited long-term operating temperature. Thus, two systems of ceramic coatings were obtained Al2O3 /La2Zr2O7/YSZ, functionally graded (a 5-layer system— Al2O3/75%Al2O3 + 25%La2Zr2O7 /25%Al2O3 + 25%YSZ + 50%La2Zr2O7/50%YSZ + 50%La2Zr2O7 /75%YSZ + 25%La2Zr2O7, and a 6-layer system—Al2O3/75%Al2O3 + 25%La2Zr2O7/25%Al2O3 + 25%YSZ + 50%La2Zr2O7/50%YSZ + 50%La2Zr2O7/75%YSZ + 25%La2Zr2O7/100%YSZ) using atmospheric plasma spray (APS) equipment.

The researchers used alumina directly as a starting layer, near the metal substrate, because it acts as an oxygen diffusion barrier that protects the metal substrate from oxidation. La2Zr2O7 powder was prepared using the sol-gel technique. It was found that samples coated with YSZ over 75% YSZ + 25% La2Zr2O7 possess higher resistance to the attack of a Na2SO4 + V2O5 molten mixture up to 50 h of treatment over those coated with 75%YSZ + 25% La2Zr2O7 top coats [72].

Compared to a conventional ceramic double ceramic layers coating (DCLC) of two ceramics linked with different thermal expansion coefficients, a functionally graded coating (FGC) is expected to be an interesting and effective approach to improve TBC performance. To this end, C. Wang and colleagues proposed for comparison two such systems obtained by suspension plasma spraying (SPS): a FGC, with gradual compositional variation along the thickness direction from pure 8YSZ (8 wt% Y2O3-stabilized ZrO2) to pure LZ on the outer surface and a LZ/8ZZ DCLC. Nanosized LZ power (30–50 nm) was prepared by hydrothermal method. The thermal cycling lifetime of optimized La2Zr2O7/8YSZ functionally graded coatings is obviously longer than those of DCLC and LZ SCLC at 1000°C and 1200°C. The thermal cycling times of FGC (67 cycles) is increased by 55% than that of DCLC at 1000°C, and that (12 cycles) is increased by 50% than that of DCLC at 1200°C. The graded coatings reduce the thermal mismatch between two ceramics and integrate them into composite coatings, which help improve the high temperature reliability of TBCs [73].

A comparison was made between YSZ as a conventional ceramic top coating material, Gd2Zr2O7 and YSZ/Gd2Zr2O77 as new generation coating materials with rare earth zirconate content, deposited as ceramic top coatings with EB-PVD method onto the CoNiCrAlY bond coat. These samples were subjected to hot corrosion tests by spreading mixtures of 55% V2O5 and 45% Na2SO4 salt at 5 h intervals at 1000° C. It turned out that double coatings of YSZ/ Gd2Zr2O7 proved to be more resistant to hot corrosion than other single coatings. After the hot corrosion tests, the monoclinic zirconia and YVO4 long rod-shaped crystal structures are formed in YSZ coating, whereas the monoclinic zirconia and GdVO4 crystal structure are formed in coatings with Gd2Zr2O7 and YSZ/Gd2Zr2O7 as a hot corrosion product. Gd2Zr2O7coatings, a good alternative coating material for TBC, have a relatively low fracture strength compared to YSZ, causing extension stresses and early cracking in the deterioration of hot corrosion that occurs at high temperatures. On the other hand, the low thermal conductivity and the high thermal expansion coefficient of the Gd2Zr2O7 coating materials require greater stability at high temperatures [74].

Another YSZ / Gd2Zr2O7 system was developed but this time by the APS technique. Plasma grade flowable 8 wt% YSZ and gadolinium zirconate (GZO) powders were prepared by a single-step co-precipitation technique. Field emission scanning electron microscopy (FESEM) cross-sectional analysis of the YSZ / GZO bilayer coating after corrosion test affirms the effectiveness of the bilayer design in preventing the penetration of corrosive salts to the YSZ layer. Also, YSZ/GZO bilayer TBC exhibited a higher thermal cyclic life (300 cycles) than the single-layer 8YSZ (175 cycles) coatings at 1100° C [75]. Table 8 summarizes the results obtained regarding thermal behavior of different coatings using rare earth zirconates.

Nr.Material for TBCMethodProperties
1YSZ top coatEB-PVD106 ± 10 cycles
Gd2Zr2O7 top coat81 ± 7 cycles
La2Zr2O7 top coatcoat 23 ± 4 cycles
YSZ/Gd2Zr2O7 top coat139 ± 13 cycles
YSZ/La2Zr2O7 top coat131 ± 12 cycles
2YSZEB-PVD
YSZ / La 2Zr2O7
35 layers - Al2O3/75%Al2O3 + 25%La2Zr2O7/25%Al2O3 + 25%YSZ + 50%La2Zr2O7/
50%YSZ + 50%La2Zr2O7/75%YSZ + 25%La2Zr2O7		APSThe samples that have on the last layer YSZ have a higher resistance to the attack of a Na2SO4 + V2O5.
6 layers - Al2O3 / 75%Al2O3 + 25%La2Zr2O7 / 25%Al2O3 + 25%YSZ + 50%La2Zr2O7 /
50%YSZ + 50%La2Zr2O7 /75%YSZ + 25%La2Zr2O7/ 100% YSZ
4La2Zr2O7/ 8YSZ FGC / 8YSZ FGCSPSThe thermal cycling lifetime of FGCis obviously longer than that of
DCLC;
OFCG helps improve the high temperature reliability of TBCs.
5YSZ / Gd2Zr2OEB-PVDIt is more resistant to hot corrosion than other single coatings;
relatively low fracture toughness compared to YSZ
6YSZ/GZOAPSpreventing the penetration of corrosive salts to the YSZ layer;
higher thermal cyclic
life (300 cycles) than the single-layer 8YSZ (175 cycles) coatings at 1100°C

Table 8.

Thermal behavior of different coatings using rare earth zirconates.

In another application, the bonding alloy powders NiCrAlY, ceramic 8YSZ, La2Zr2O7 and Gd2Zr2O7 are fed into the four dense graphite crucibles inserted in water-cooled copper slots of a carousel system that rotates around the electron beam generating source (Figure 12a), with a maximum filling degree of 80% of the crucible’s volume. The slab or bar substrates on which the thermal barrier is deposited are cleaned of traces of organic substances with alcohol in an ultrasonic bath, then mounted in the rotating clamping place located above the crucibles with the material to be deposited (Figure 12b), which can be preheated by means of a mobile device with infrared (IR) lamps (Figure 12c). Close the front sealing door of the installation and start the vacuum pumps. After the vacuum reaches a value of approx. 10−6 torr, the electron cannon is turned on and the deposition of the bonding layer begins by melting and evaporating the NiCrAlY powder from the first crucible, with the help of the electron flux focused on the surface of the powder in the crucible. After consuming, the crucible carousel is rotated and the first ceramic layer is deposited by melting and evaporating the 8YSZ powder in electron flow. The crucible carousel is rotated again and the second ceramic layer is deposited by melting and evaporating the La2Zr2O7 powder in electron beam. The crucible carousel is then rotated and the top ceramic layer is deposited by melting and evaporating the Gd2Zr2O7 top coating layer. After the deposition operation is completed, the vacuum pumps are stopped, the installation is ventilated until the pressure inside reaches atmospheric pressure and the coated substrates in the clamping device are disassembled. Scanning electron microscopy (SEM) analysis is mentioned in the section in Figure 13, highlighting the four layers of the material deposited. The thermal shock resistance check shows that the architecture deposited on the NIMONIC 80 commercial refractory alloy substrate withstands at least 100 heating and cooling cycles at a test temperature of 1200°C without detachment and degradation.

Figure 12.

Combinatorial EB-PVD coating system: (a) electron beam source; (b) device for fixing the substrates to be covered; and (c) mobile IR system for heating the substrates.

Figure 13.

SEM micrograph of the EB-PVD coatings with rare earth perovskite layers.

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6. Conclusions

Ultra-high temperature ceramics are promising candidates for high temperature applications as thermal barrier coatings (TBCs) in hypersonic flight, rocket propulsion, atmospheric re-entry, refractory crucibles, plasma-arc electrodes, etc., to protect structural components of spacecraft vehicles from damage caused by high temperature and corrosive/erosive environments due to their high melting temperatures and low thermal conductivities.

Non-oxide ceramic coatings have highest melting temperatures but lower chemical stability at high temperatures in air and oxidizing atmosphere due to their higher oxygen affinity and lower adhesion forces to the base metallic substrate.

The “gold standard” material for ceramic coatings is yttria-stabilized zirconia (YSZ). However, it has some main drawbacks for use in higher temperature range due to phase transitions at temperatures higher than 1200°C decreasing thermal stability and susceptibility to (calcium magnesium aluminum silicates) infiltration leading to delamination.

Rare earth zirconate compounds with general formula Ln2Zr2O7 (Ln = La, Nd, Sm, Gd, Yb) have unique features, which make them ideal for different applications, such as radioactive waste immobilization, thermal barrier coatings and photocatalysis. Their crystalline cubic pyrochlore structures have a relatively low oxygen diffusion, low thermal conductivity, high thermal stability and chemical inertness, making them among the most promising candidates for TBCs. Melting temperature for all known rare earth zirconates with pyrochlore structure is higher than 2000°C, with higher value 2570°C for Gd2Zr2O7. The thermal conductivity according to modeling and some experimental data is in the range 1.1–1.7 Wm−1 K−1; however, lower values of the thermal conductivities in the range 0.38–0.7 Wm−1 K−1 were reported for co-precipitated rare earth zirconates and 0.31–0.38 for ZrO2-8Y203-6Ln2O3 co-doped powders, suggesting that both the synthesis method and the complex defect structures induced by co-doping have a beneficial influence on reducing thermal conductivities of perovskites. Solid-state reactions, co-precipitation, sol-gel and hydrothermal methods have been used for the synthesis of Ln2Zr2O7, with hydrothermal being the process with lower crystallization temperatures.

Electron Beam Physical Vapor Deposition (EB-PVD), Powder Flame Spraying, Plasma Thermal Spray and Cold Gas Dynamic Spray Coating are the main techniques used for creating different types and architectures of TBCs from rare earth zirconates and zirconia co-doped with mixed rare earth zirconates. In all cases, the bonding of the coatings to the metallic substrate to be thermally protected by ceramic coatings needs the deposition of a NiCoCrAlY thin bonding layer, enabling the formation of a thermal growth alumina layer reducing oxygen diffusion and ensuring the adhesion energy of the ceramic layers to the substrates.

Different architectures containing Al2O3, YSZ, YSZ-co-doped with rare earth oxides and rare earth zirconates were proposed. The parameters defining the TBCs architectures are the number of layers, the kind and sequence of ceramic materials’ and layers’ thicknesses.

With the actual rare earth zirconate materials and configurations used, thermal protection systems were able to support a minimum of 100 heating-cooling cycles.

Fundamental and applied research are further needed to model the thermal and adhesion properties of different rare earth zirconates and oxides co-doped with multiple rare earth oxides, including the development of high entropy oxides based on rare earth oxides.

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Acknowledgments

The authors thank the Romanian Ministry of Research, Innovation and Digitization in the frame of Contract No. PN 23 25 01 01, project “Integration of Combinatorial EB-PVD Technology in Developing Materials for Applications in Energy Co-generation.”

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Radu-Robert Piticescu, Cristina Florentina Ciobota, Mihail Botan and Alexandru Gyorgy Okos

Submitted: 15 September 2024 Reviewed: 18 October 2024 Published: 30 November 2024

© The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 4.0 International License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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