(TiZrHf)P₂O₇: An equimolar multicomponent or high entropy ceramic with good thermal stability and low thermal conductivity

1. Introduction

Metal phosphates MP₂O₇ (M = Ti, Zr, Hf) are widely used as ceramic binder, refractories, catalyst carrier, solid electrolytes and high temperature insulating materials [[1], [2], [3], [4]]. These compounds are electrical insulating with wide bandgaps and have low thermal conductivities due to the anisotropic chemical bonding within their crystal structures [5,6]. They are also predicted as damage tolerant ceramics judged from the low Pugh’s ratio G/B and positive Cauchy pressure (c₁₂-c₄₄) [5,6]. Among them, zirconium pyrophosphate (ZrP₂O₇) is intriguing due to its low density, low thermal expansion coefficient, low thermal conductivity and good sintering-resistance [[7], [8], [9], [10], [11]]. However, the unsatisfied thermal stability is the bottleneck for its high temperature applications. At temperatures higher than 1400 °C, ZrP₂O₇ decomposes to yield β-Zr₂O(PO₄)₂ and P₂O₅, which can be described according to the following equation [11]:

2ZrP₂O₇→β-Zr₂O(PO₄)₂+P₂O₅ (1)

The decomposition results in weight loss and limits its high temperature applications. Thus, it is significant to improve the high temperature phase stability of ZrP₂O₇.

Recently, a new class of ceramics consisting of multi-principal elements in equal molar or near equal molar percent but form a single phase solid solution has attracted much interest, which is referred to as high entropy ceramics (HECs) [[12], [13], [14], [15], [16], [17], [18], [19]].

According to the fundamental thermodynamic equation:

ΔG=ΔH-TΔS (2)

where ΔG is the Gibbs free energy, ΔH is the enthalpy, T is the absolute temperature, and ΔS is the entropy, higher entropy of multicomponent solid solution will lead to lower Gibbs free energy than that of single component phase and better phase stability at high temperatures [20]. This idea opens a new window to improve the high temperature stability of materials, i.e., through forming high entropy solid solutions.

Based on the high entropy effect on the high temperature stability of ceramics, a novel multicomponent equimolar solid solution or high entropy ceramic (HE) (TiZrHf)P₂O₇ was designed and successfully synthesized in this work. The crystal structure parameters, thermal stability and thermal conductivity of this new type of (TiZrHf)P₂O₇ solid solution were investigated. As will be shown in later sections, HE (TiZrHf)P₂O₇ crystallizes in a cubic structure with the space group of Pa-3 and possesses higher thermal stability and lower thermal conductivity than single component phases, which is promising for high temperature insulating applications.

2. Experimental procedure

(TiZrHf)P₂O₇ powders were synthesized as follows: TiO₂, ZrO₂ and HfO₂ powders (99.9% purity; HWRK Chem. co. Ltd, Beijing, China) were mixed in equal molar ratio and ball-milled in ethyl alcohol with agate balls for 6 h and dried at 70 °C for 12 h. A 1:2 molar ratio of the mixed powders and H₃PO₄ (Sinopharm Chemical Reagent Co., Ltd, Beijing, China) were mixed and stirred thoroughly and then heated at 300 °C for 1 h. High temperature synthesis was conducted at 1000 °C for 1 h. The final product was ball-milled with agate balls for 20 h and screened with 300 mesh sieve. For comparison, phase-pure TiP₂O₇, ZrP₂O₇ and HfP₂O₇ powders were also synthesized by the same method.

The phase composition of as-synthesized powders was characterized by an X-ray diffractometer (XRD, D8 Advanced, Bruker, Germany) using Cu K_α radiation (λ = 1.5406 Å) with a step size of 0.02° at a scanning rate of 2°/min. The lattice parameter, atomic positions of as-synthesized (TiZrHf)P₂O₇ solid solution and those of the constituting metal pyrophosphates TiP₂O₇, ZrP₂O₇ and HfP₂O₇ were refined by the Rietveld method utilizing a TOPAS total pattern analysis solutions software (Bruker Corp., Karlsruhe, Germany). To investigate the thermal stability of (TiZrHf)P₂O₇, the as-synthesized (TiZrHf)P₂O₇ powders were heated at 1550 °C for 3 h in air. For comparison, ZrP₂O₇ powders were also heat treated under the same conditions. After heat treatment, the phase compositions of the samples were characterized by XRD.

In order to investigate the thermal conductivity of (TiZrHf)P₂O₇, the preparation of dense bulk (TiZrHf)P₂O₇ compact is essential. Since the sintering of phase-pure pyrophosphate is difficult [10,21], a 2 wt.% mass ratio of MgO was used as sintering additive. The as-synthesized (TiZrHf)P₂O₇ powders and MgO powders were mixed and ball-milled in ethyl alcohol with agate balls for 6 h, and then dried in an oven at 70 °C for 12 h. Green bodies were made by pressing the mixed powders under a uniaxial pressure of 200 MPa in a 13 mm diameter steel die. Bulk (TiZrHf)P₂O₇ compact was prepared by sintering the green body at 1170 °C for 40 min in a muffle furnace. Since the experimental thermal conductivity of single component pyrophosphate has not been reported before, bulk TiP₂O₇, ZrP₂O₇ and HfP₂O₇ compacts were also prepared by pressureless sintering at 1170 °C for 70 min, 1320 °C for 50 min and 1280 °C for 70 min, respectively. A 2 wt.% MgO was also added into the single component pyrophosphates powders as the sintering additive.

The density of as-prepared bulk compacts was measured by Archimedes' method. The theoretical density of MP₂O₇ was calculated from the mass of the atoms in a unit cell and the cell dimensions obtained by the Rietveld refinement of XRD data. Phase identification was conducted by XRD at a scanning rate of 2°/min. The microstructure and element distributions of bulk samples were observed by a scanning electron microscope (SEM, Apollo300, CamScan, Cambridge, UK) equipped with an energy dispersive X-ray spectroscopic system (EDS Inca X-Max 80 T, Oxford, UK).

The thermal diffusivity of bulk MP₂O₇ was measured by laser flash method using a sample with the size of Ø 10 mm × 2 mm. Before the test, the surface of specimens was sprayed with a layer of graphite to prevent heat radiation from penetrating through it. The heat capacity was obtained by Neumann-Kopp rule using the data of their constituent oxides (TiO₂, ZrO₂, HfO₂ and P₂O₅) [[22], [23], [24], [25]]. The thermal conductivity of bulk MP₂O₇ was calculated according to the relationship between thermal diffusivity D_th, heat capacity C_p, density ρ and thermal conductivity κ:

κ=D_th·C_p·ρ (3)

3. Results and discussion

As shown in Fig. 1, the crystal structure of MP₂O₇ (M= Ti, Zr and Hf) is cubic with the space group of Pa-3. The metal atoms (Ti, Zr and Hf) occupy the cation position and form MO₆ octahedrons. Each of the MO₆ octahedron links with six PO₄ groups by the bridge oxygen atoms. The same crystal structure of MP₂O₇ makes them tend to form a solid solution. Fig. 2 shows the experimental XRD pattern of (TiZrHf)P₂O₇ powders synthesized at 1000 °C together with those of TiP₂O₇, ZrP₂O₇ and HfP₂O₇ powders. Clearly, only reflections from single phase solid solution can be detected from as-synthesized (TiZrHf)P₂O₇ powders, which possess the same crystal structure with the single component phases. Thus, it can be concluded that phase-pure (TiZrHf)P₂O₇ solid solution was successfully synthesized. In order to compare the difference of diffraction peaks between the (TiZrHf)P₂O₇ solid solution and the single component phase, the diffraction angle 2θ and corresponding reflections (hkl) of MP₂O₇ (M= Ti, Zr, Hf) and HE (TiZrHf)P₂O₇ obtained from experimental XRD patterns are listed in Table 1. We can see that all the peak positions of HE (TiZrHf)P₂O₇ shift to the high angle direction compared with those of ZrP₂O₇ and HfP₂O₇ but to the low angle direction compared with those of TiP₂O₇. Table 2 summarizes the refined structure parameters of MP₂O₇ (M= Ti, Zr, Hf) and HE (TiZrHf)P₂O₇, including cell dimensions and atom positions obtained from Rietveld refinement. As shown in Table 2, the refined cell dimension of (TiZrHf)P₂O₇ is 8.0979 Å, which is smaller than that of ZrP₂O₇ (8.2528 Å) and HfP₂O₇ (8.2101 Å) but is larger than that of TiP₂O₇ (7.8738 Å). Thus, the theoretical density of MP₂O₇ can be calculated by the refined unit cell parameters, which are 3.02 g/cm³ for TiP₂O₇, 3.13 g/cm³ for ZrP₂O₇, 4.23 g/cm³ for HfP₂O₇ and 3.50 g/cm³ for the (TiZrHf)P₂O₇ solid solution, respectively. The atom positions of (TiZrHf)P₂O₇ are also listed together with those of single component compounds.

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Fig. 1.   Crystal structure of MP₂O₇ (M=Ti, Zr and Hf).

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Fig. 2.   Experimental XRD pattern of (TiZrHf)P₂O₇ powders synthesized at 1000 °C together with those of TiP₂O₇, ZrP₂O₇ and HfP₂O₇ powders.

The thermal stability of (TiZrHf)P₂O₇ solid solution was investigated by annealing experiment in air. Fig. 3 compares the XRD patterns of the HE (TiZrHf)P₂O₇ powders and ZrP₂O₇ powders after annealing at 1550 °C for 3 h. It is seen that second phase β-Zr₂O(PO₄)₂ can be detected from the annealed ZrP₂O₇ powders, which is due to the partial decomposition of ZrP₂O₇ at high temperatures [9]. The HE (TiZrHf)P₂O₇ is, however, stable after annealing at 1550 °C for 3 h and no decomposition product is detectable. Thus, it can be concluded that HE (TiZrHf)P₂O₇ has higher thermal stability than the single component phase, which is beneficial to its high temperature applications. Since a small amount of sintering additive (2 wt.% MgO) is used in all single component MP₂O₇ (M = Ti, Zr, Hf) samples and the (TiZrHf)P₂O₇ solid solution, its effect on the thermal stability of HE (TiZrHf)P₂O₇ can be excluded. High thermal stability of HE (TiZrHf)P₂O₇ is attributed to the high-entropy effect of multicomponent solid solution, which tends to stabilize the high-entropy phases [20].

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Fig. 3.   XRD patterns of the (TiZrHf)P₂O₇ powders and ZrP₂O₇ powders after annealing at 1550 °C for 3 h.

Fig. 4 shows the XRD pattern of bulk HE (TiZrHf)P₂O₇ sample. Clearly, the main crystalline phase is (TiZrHf)P₂O₇ with minimum amount of impurity phase. Fig. 5(a) shows the microstructure of the surface of sintered (TiZrHf)P₂O₇ compact. Only a few pores can be observed at the grain boundaries, indicating that the as-prepared bulk is relative dense, which is consistent with ca 96% relative density. Furthermore, the grains of the bulk sample are fine and uniform. Using the grains collected from Fig. 5(a), it can be found that the grain size has narrow size distribution and the average gain size is 2.4 μm, as shown in Fig. 5(b). For comparison, bulk single component pyrophosphates, i.e., TiP₂O₇, ZrP₂O₇ and HfP₂O₇ with similar relative density were also prepared by pressureless sintering. XRD patterns and SEM images of bulk single component pyrophosphate compacts are not shown here for brevity. Fig. 6 shows the SEM image of the surface of bulk HE (TiZrHf)P₂O₇ sample and the element distributions in the observed area analyzed by EDS. It can be seen that all the metal elements are homogeneously distributed, which indicates that uniform solid solution is formed.

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Fig. 4.   XRD pattern of bulk (TiZrHf)P₂O₇ solid solution sample.

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Fig. 5.   (a) Microstructure of the surface of sintered (TiZrHf)P₂O₇ solid solution compact. (b) Grain size distribution of sintered (TiZrHf)P₂O₇ solid solution compact.

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Fig. 6.   SEM image of the surface of bulk (TiZrHf)P₂O₇ solid solution and the element distributions in the observed area analyzed by EDS.

Table 3 compares the thermal properties and density of (TiZrHf)P₂O₇ together with those of single component pyrophosphates at room temperature. The room temperature thermal diffusivity of HE (TiZrHf)P₂O₇ (0.43 mm²/s) is lower than that of TiP₂O₇ (0.53 mm²/s), ZrP₂O₇ (0.50 mm²/s) and close to that of HfP₂O₇ (0.41 mm²/s). Accordingly, HE (TiZrHf)P₂O₇ exhibits lower thermal conductivity (0.78 W m^-1 K^-1) at room temperature than that of TiP₂O₇ (1.08 W m^-1 K^-1), ZrP₂O₇ (0.92 W m^-1 K^-1) and HfP₂O₇ (0.79 W m^-1 K^-1). Besides, the thermal conductivity of (TiZrHf)P₂O₇ at room temperature is also lower than those of many thermal barrier coating materials, such as BaZrO₃ [26], Y₂SiO₅ [27], Gd₂Zr₂O₇ [28], LaPO₄ [29] and Yb₃Al₅O₁₂ [30]. Low thermal conductivity of (TiZrHf)P₂O₇ can be attributed to two factors: one is the heterogeneous bonds, i.e., strong P—O bonds and weak M—O bonds; the other is the lattice distortion caused by solid solution [9,31]. The heterogeneous bonds and the lattice distortion of HE (TiZrHf)P₂O₇ enhance the phonon scattering and make HE (TiZrHf)P₂O₇ possess low thermal conductivity.

4. Conclusion

In this work, a novel multicomponent equimolar solid solution (TiZrHf)P₂O₇ was designed and successfully synthesized. By means of XRD analysis and Rietveld refinement, it can be found that the as-synthesized (TiZrHf)P₂O₇ is phase-pure solid solution and possesses a cubic crystal structure with a Pa-3 space group, which is the same as that of single component compounds, TiP₂O₇, ZrP₂O₇ and HfP₂O₇. (TiZrHf)P₂O₇ has better thermal stability than ZrP₂O₇ and remains stable after annealing at 1550 °C for 3 h. The thermal conductivity of HE (TiZrHf)P₂O₇ at room temperature is as low as 0.78 W m^-1 K^-1, which is lower than that of TiP₂O₇ (1.08 W m^-1 K^-1), ZrP₂O₇ (0.92 W m^-1 K^-1) and HfP₂O₇ (0.79 W m^-1 K^-1). High thermal stability and low thermal conductivity of HE (TiZrHf)P₂O₇ makes it promising for high temperature thermal insulating applications.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (Nos. 51672064 and U1435206).