Preparation of Single-phase High Entropy Carbides by a Modied Citric Acid Complexing Method

: We developed a new method to synthesize single-phase transition metal carbide powders by combining citric acid complexing method and ball-milling dispersion. Ti Ta Nb )C (5TmC-H) and (Zr 0.2 Ti 0.2 Ta 0.2 Nb 0.2 Mo 0.2 )C (5TmC-M) were successfully fabricated by this method using low-cost raw materials. The element and phase composition and microstructures of the obtained carbide powders were investigated. The relationships of synthesis process and temperature with chemical composition were also discussed. (Zr 0.25 Ti 0.25 Ta 0.25 Nb 0.25 )C can be obtained by a one-step process at 1550 °C, while (Zr 0.2 Ti 0.2 Ta 0.2 Nb 0.2 Hf 0.2 )C and (Zr 0.2 Ti 0.2 Ta 0.2 Nb 0.2 Mo 0.2 )C are fabricated by a two-step process of carbothermal reduction followed by solid solution at the temperatures not lower than 1850 °C and 1650 °C. The higher synthesis temperatures of the five-component carbides are attributed to the obvious sluggish diffusion effect induced by the larger lattice distortions. The particle sizes of (Zr 0.25 Ti 0.25 Ta 0.25 Nb 0.25 )C, (Zr 0.2 Ti 0.2 Ta 0.2 Nb 0.2 Hf 0.2 )C and (Zr 0.2 Ti 0.2 Ta 0.2 Nb 0.2 Mo 0.2 )C powders are 118.2±26.1 nm (at 1550 °C), 284.8±73.7 nm (at 1850 °C) and 65.5±13.9 nm (at 1750 °C), respectively.

The synthesis temperature is as high as 1950 °C and the obtained powders have relatively large particle size of approximately 2 µm. Moskovskikh et al. [29] fabricated high-entropy carbide (Hf0.2Ta0.2Ti0.2Nb0.2Zr0.2)C through reactive high-energy ball milling of metal and graphite powders. Although this method is simple and efficient, it is easy to introduce impurities such as oxygen and Fe elements in the synthesis process.
Liu et al. [30] synthesized high-entropy carbide (Nb0.25Ta0.25Mo0.25W0.25)C via the direct reaction between metallic powders and graphite. The synthesis temperature is as low as 1800 °C but the obtained powders have relatively large particle size of 9.0±0.5 µm. Ye et al. [31] successfully fabricated (Zr0.25Ta0.25Nb0.25Ti0.25)C with the particle size of 0.5-2 μm via one-step carbothermal reduction between the oxides and graphite powders at 2200 °C. Feng et al. [32] also prepared (Hf0.2Zr0.2Ti0.2Ta0.2Nb0.2)C via a twostep synthesis process consisting of carbothermal reduction at 1600 °C followed by solid solution reaction at 2000 °C. The synthesis temperatures are also high which are influenced by the activities and particle sizes of the oxide and graphite powders.
Compared with solid-phase methods, the major advantage of liquid-phase method is that the starting reagents can be mixed homogeneously at molecular level in a liquid state. Therefore, the synthesis temperature of the powders is lower and the obtained powders have smaller particle sizes. Li et al. [33] successfully prepared high-entropy (Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)C via liquid precursor approach using transition metal halides and furfuryl alcohol as metal sources and carbon source. The precursor is converted into a carbide mixture via carbothermal reduction at a lower temperature of 1400 °C.
After the following solid solution reaction at 2000 ℃, the obtained single-phase carbide powders are much finer than these derived from solid-phase methods with an average particle of 132±5 nm. However, some of the used transition metal halides, such as HfCl4 and TaCl5, are expensive, which increases the synthesis cost. Du et al. [34] synthesized (Hf0.25Nb0.25Zr0.25Ti0.25)C via polymer-derived ceramic route at 2200 °C using transition metal halides and acetylmethane as metal sources and carbon source. The high-entropy carbide consisted of numerous superfine particles with the average particle size of 800 nm. But the used acetylmethane is poisonous. Sun et al. [35] prepared high entropy carbide powder (Ti0.2Zr0.2Hf0.2Ta0.2Nb0.2)C via liquid polymer precursor route using metal alkoxides and allyl-functional novolac resin as metal sources and carbon source.
The single-phase carbide powders with average crystalline size of 63±6 nm are obtained at 1800 °C. However, the expensive metal halides, such as HfCl4, NbCl5 and TaCl5, were also used to fabricate the corresponding metal alkoxides. Thus, a new low-cost method should be developed to synthesize superfine high entropy carbide powders at lower temperatures.
In this study, we will synthesize (Ti0.25Zr0.25Nb0.25Ta0.25)C (4TmC), (Ti0.2Zr0.2Nb0.2Ta0.2Hf0.2)C (5TmC-H) and (Ti0.2Zr0.2Nb0.2Ta0.2Mo0.2)C (5TmC-M) high-entropy carbide powders by combining citric acid complexing method and ballmilling dispersion, using low-cost raw materials. The element and phase composition and microstructures of the as-synthesized powders will be investigated. The relationships between the synthesis temperatures and chemical composition will also be discussed. Inorganic metal salts were dissolved into deionized water to give out a transparent solution. Then AC, PEG and glucose were added into the transparent solution sequentially. The molar ratio of AC to inorganic metal salts was 1:3 and the mass ratio of AC to PEG was 5:1. The amount of glucose is weighed according to the stoichiometric ratio of the metal oxide and carbon in the following chemical reactions from (1) to (6). Ammonia was used to adjust the pH value of the as-received solution.
After being magnetic stirring at room temperature for 6h, the solution was converted into a translucent sol and metal-citrate chelate complexes were formed in this process.
Then oxide powders were added into the sol followed by ball-milling in a polyethylene jar using ZrO2 milling medium balls for 12h. The well-dispersed suspension was magnetic stirred in a water bath at 70 °C for 10 hours and a brownish wet gel for HEC was obtained. Finally, the gel was dried in an oven at 80 °C.

ZrO2+3C=ZrC+CO
(1) HfO2+3C=HfC+CO (6) The obtained xerogel powders were pressed into disks under a low pressure of 10 MPa in order to improve particle contact for promoting chemical reaction and solid Preparation Technology Co., Ltd., Shenyang, China). The heating rate was 10 °C/min and the specimens were soaked for 2h at the highest temperature. After annealing, the furnace was cooled naturally.
The composition and morphology of the xerogel powders were analyzed by Fourier transform infrared spectroscopy (FT-IR, Vertex70, Burker, Germany) and scanning electron microscopy (SEM, SU8100, Hitachi, Japan). The phase composition of the as-synthesized high entropy carbide powders was analyzed by X-ray diffraction (XRD, D/max2200PC, Japan) using Cu Ka radiation. The XRD patterns of specimens were obtained in the 2θ range from 20° to 90° with a scan speed of 4°/min. The microstructures and morphology of the products were characterized by SEM equipped with Energy-dispersive X-ray spectroscopy (EDS, Elect Super, TÜV Rheinland, USA) and transmission electron microscopy (TEM, FEI Tecnai G2 F20 S-TWIN, FEI, USA).
The particle sizes of the synthesized powders were measured by Nano Measurer software (Version 1.2, Fudan University, China).    The morphology of the as-synthesized powders with an excess of 5% at 1550 °C is analyzed by SEM and TEM, as shown in Fig. 3(a) and (b). The particles are equiaxial with an average size of 118.2±26.1 nm measured by SEM and 71.6±13.3 nm measured by TEM, which is much smaller than the starting Ta2O5 powders (0.8 μm). This result demonstrates that metal oxides are not the reaction centers during carbothermal reduction and solid solution process. Thus, the small particle size of the obtained powders originates from the high reactivity and homogeneity of the precursors and low synthesis temperature, instead of particle sizes of the starting oxide powders. Moreover, 4TmC powders are agglomerate according to the SEM image, but these agglomeration can be broken up into smaller units after ultrasonic dispersion indicated by the TEM image. Thus, superfine high entropy carbide powders with soft agglomeration could be produced by this method. In Addition, element composition of the synthesized powders was investigated by EDS analysis at the point marked by yellow cross in Fig. 3(a). The molar amounts of the present metal elements are nearly equal to each other, which also indicates the formation of single-phase (Zr0.25Ti0.25Ta0.25Nb0.25)C carbide.   According to the EDS point analysis displayed in Fig.5 (c), Zr, Ti, Ta, Nb, and Hf metal elements are detected in the synthesized powders with near equal molar ratio, which is consistent with the composition of these elements in the precursor. This result also reveals that only one kind of high entropy carbide exists in the obtained powders.  The microstructures of 5TmC-M annealed at 1750 °C are shown in Fig. 7. The powders are agglomerated and have an average particle size of 65.5±13.9 nm according to the SEM image ( Fig. 7(a)). All the designed metal elements, i.e., Zr, Ti, Ta, Nb and

Characteristics of liquid-phase precursors
Hf, are detected in the particles by EDS point analysis and their molar amounts are almost equal to each other, which is described in Fig. 7 (c). This result also verifies the formation of single-phase 5TmC-M solid solution. In addition, TEM result also presents the morphology of the synthesized powders, which have regular polygonal shape with the average size of 31.8±7.5 nm as shown in Fig. 7 (b). Thus, the crystal shapes can be well developed with slow grain growth. Where T is the temperature, ∆H and ∆S are the entropy change and enthalpy change, respectively. The multi-component carbide has larger configurational entropy which is beneficial to stabilize the structure at high temperature. However, the enthalpy change of multi-component carbides with different chemical composition will be different, due to the different atomic radius and electronegativity of the metal elements [41]. The differences of atomic radius and electronegativity (δ and ∆χA) in the carbides can be assessed as equation (8) and (9) Where ci is the molar fraction of the i-th metal element, ri and χi are atomic radius and Allen electronegativity of the i-th metal element which are listed in Table 1. Thus, the δ values of 4TmC, 5TmC-H, and 5TmC-M are 3.98%, 4.26% and 4.66%, respectively.
Larger δ value represents large lattice distortion which introduces distortion energy in the system [46]. The ∆χA value of 4TmC, 5TmC-H, and 5TmC-M are 2.56%, 6.57% and 3.84%, respectively. Larger ∆χA value indicates more different physicochemical properties of the metal elements, resulting in reducing the structure stability of carbide solid solution [47]. Thus, the enthalpy changes of these two five-component carbides will be larger than that of the four-component carbide, which will influence the stability of single-phase five-component carbides at low temperatures. In order to investigate the structure stability of single-phase 5TmC-H and 5TmC-M, we annealed these two carbides at 1550 °C for 5h. And their XRD patterns are displayed in Fig. 8   The XRD patterns of (Zr0.25Ti0.25Ta0.25Nb0.25)C samples with different carbon content heated at 1550 and 1450 .  The XRD patterns of (Zr0.2Ti0.2Ta0.2Nb0.2Hf0.2)C samples after heat treatment at 1550 °C, 1650 °C, Figure 5 (a) SEM and (b) TEM images of (Zr0.2Ti0.2Ta0.2Nb0.2Hf0.2)C powders annealed at 1850 , and (b) EDS point analysis of the region in (a) marked by yellow cross.