Low temperature synthesis nanoscale Ta4HfC5 powder and characterization of Ta4HfC5 ceramics

Solvothermal treatment was used to synthesize nanoscale 4TaC–HfC (Ta 4 HfC 5 ) powder at a relatively low calcination temperature (1400 °C) and in a short period through material selection and process optimization. High-volume Teon-lined stainless steel autoclave (1000–2000 mL) could be used to prepare powder by using this method. The obtained powder had good size distribution and dispersity. Ta 4 HfC 5 powder could be obtained at a low temperature because of the existence of Ta 2 Hf 6 O 17 during carbothermal reduction.Ta 4 HfC 5 ceramics were consolidated via spark plasma sintering at 2100 °C and at 30, 40, and 50 MPa. Holding time was 5 min. Ceramics had a better densication and smaller grain sizes at a shorter sintering time compared with that of the mechanical ball milling method. The densication behavior of ceramics was analyzed, and the mechanical properties of different samples were investigated. Flexural strength increased from 5.85 ± 0.43 GPa to 10.07 ± 0.61 GPa. Fracture toughness was similar in all the compositions in the range of 2.36–2.56 MPa m 1/2 . This method can be applied to synthesize Ta 4 HfC 5 powder with high purity and uniformity in large quantities. Hence, ceramic sintering and Ta 4 HfC 5 –matrix composite preparation have a broad application prospect.

Raw TaC and HfC powders have been milled to synthesize Ta 4 HfC 5 because of their NaCl-type structure.
However, this method has obvious defects. With mechanical ball milling, powders can be just mixed su ciently, but a single-phase Ta 4 HfC 5 solid solution cannot be obtained because of the high covalent bonds and low self-diffusion coe cients of TaC and HfC [8.9] . Hot pressing or spark plasma sintering should be used to promote the formation of Ta 4 HfC 5 ceramic, not Ta 4 HfC 5 powder. However, the ceramic needs to be broken again and takes a long time to mill to obtain powders. Sintering temperature is normally above 2000 °C, which makes particle sizes nonuniform from several microns to tens of microns [10] . In some works, sintering aids, such as MoSi 2 [11] , B 4 C [12] , and TaB 2 [13] , have been added to reduce sintering temperature and control grain sizes. However, these methods cannot guarantee the purity of Ta 4 HfC 5 . In addition, mechanical ball milling can hardly synthesize super ne powder. These greatly limit the application eld of the Ta-Hf-C system.
Few articles on the preparation of Ta 4 HfC 5 powder via chemical methods have been published. However, there is still possibility of improvement such as reducing calcination temperature, simplifying raw material treatment and reducing drying time [14][15][16] .
This work aimed to synthesize a polymer precursor for Ta 4 HfC 5 powder through material selection and process optimization, which could shorten the preparation period and obtain Ta 4 HfC 5 powder at a lower calcination temperature. Particle size uniformity becomes uncontrollable as the volume of the steel autoclave increases; however, this method could be conducted in 2000 mL Te on-lined stainless steel autoclave to prepare Ta 4 HfC 5 powder with high yield, particle sizes, and dispersity that were similar to those prepared in a 100 mL autoclave. Ta 4 HfC 5 ceramics sintered via spark plasma sintering had a higher relative density at a shorter sintering time compared with that of ceramics sintered through mechanical ball milling method. The mechanical properties of ceramics were investigated, and their densi cation behaviors were analyzed.

Powder preparation
HfCl 4 was dissolved in acetylacetone and added with N-butyl alcohol and a stoichiometric amount of TaCl 5 . After 1 h, a stoichiometric amount of phenolic resin was blended in and stirred for 1-1.5 h. An additional 3 wt.% phenolic resin was added to inhibit the oxidation of the powder surface and avoid oxygen residue during the reaction. The whole process was stirred in an oil bath. After stirring was nished, the mixture was transferred into a Te on-lined stainless steel autoclave and autoclaved at 200°C for 12 h. Then, the mixture was dried at 120 °C to obtain the Ta 4 HfC 5 precursor, which was calcined at 1400 °C for 2 h with a heating rate of 5 °C/min under an argon ow. Then, the furnace was cooled to 400°C at the same rate of 5 °C/min and then naturally cooled to room temperature. The precursors were calcined at 1000 °C, 1100 °C, 1200 °C, 1300 °C, and 1400 °C to study the carbon thermal reduction of Ta 4 HfC 5 precursor at different calcination temperatures.

Spark plasma sintering
Ta 4 HfC 5 powder was sintered through a spark plasma sintering machine. The powder was placed in a 40 mm-diameter graphite die wrapped with graphite foil. The powder was rst loaded to the required uniaxial pressure, and the vacuum was pumped and began to heat up.
The heating rate before 500 °C was 50 °C/min, and the heating rate from 500 °C to 2100 °C was 100°C /min. The uniaxial pressures were 30, 40, and 50 MPa. The holding times were 5 min and cooling with a furnace. The environment in the vacuum was set at a pressure of 2 Pa.

Characterization
X-ray powder diffractometer (XRD, X'PERT PRO MPD, Holland) using CuKa radiation (40 kV, 30 mA) and X-ray photoelectron spectroscopy (XPS, Escalab 250, USA) were employed to analyze the phase composition of the samples. The morphological characteristics of Ta 4 HfC 5 powder and Ta 4 HfC 5 ceramics were investigated with a scanning electron microscope (SEM, HELIOS NanoLab 600i, USA). Fourier-transform infrared (FTIR) spectroscopy was conducted to analyze the chemical structure of These hafnium oxides had two different crystal structures: monoclinic HfO 2 and orthorhombic HfO 2 . No other peaks existed as the temperature increased to 1400 °C. The precursor was completely converted to Ta 4 HfC 5 powder. Orthogonal HfO 2 completely reacted before 1300 °C, but monoclinic HfO 2 needed 1300°C . Fig. 1 XRD patterns of powder calcined from 1000 °C to 1400 °C Ta 4 HfC 5 powder can be prepared at 1400 °C, which below 1600 °C for other works [14,15] . For the Hf-O-C system, the preparation temperature of HfC is generally higher than 1600 °C [17] , which limited the formation of Ta 4 HfC 5 powder at a lower temperature. The precursor is converted to Ta 2 Hf 6 O 17 at 1100 °C instead of independent Ta 2 O 5 and HfO 2 [15] . Converting Ta-O-Hf to Ta-Hf-C took lesser energy than converting Ta-O and Hf-O to Ta-Hf-C.
Ta 4 HfC 5 powder was investigated through SEM analysis as shown in Fig. 2. The powder presented good uniformity and dispersion at lower magni cation (Fig. 2a). The small amount of agglomeration shown in the gures was the agglomeration of the precursor during drying and could be re ned by grinding. Fig.   2(b) presents that the particle size of Ta 4 HfC 5 powder was nanoscale. Figs. 2(c)-(e) show the homogeneous distribution of the different elements as observed via energy-dispersive X-ray spectroscopy, which had over 3000 scan points to ensure the accuracy of the result. Ta and Hf were distributed uniformly. This method solved the problem of the uneven distribution of Ta and Hf when Ta 4 HfC 5 powder was prepared via mechanical ball milling method.  Fig. 3(b) shows the spectrum of Ta4f, which reveals Ta-C bonds at 21.73 eV and 23.63 eV and Ta-O bonds at 24.43 eV and 26.13 eV, respectively. No obvious peaks of tantalum oxide or hafnium oxide were observed in the XRD pattern in Fig. 1; therefore, the Hf-O and Ta-O bonds might be derived from contamination during testing rather than the presence of oxides in the powder itself Hf(AcAc) x , which corresponded to the C=C and C=O stretching vibration in the enol-type structure.
The absorption peak near 1127 cm −1 weakened and disappeared after phenolic resin was added, but peaks appeared at 1164 cm −1 , 1113 cm −1 , and 1051 cm −1 , which were C-O stretching vibrations.
Combined with the spectra of phenolic resin, this peak might be the weak absorption peak caused by Ta-O-C and Hf-O-C.  Table 2 shows the relative density of Ta 4 HfC 5 ceramics. The relative density of the ceramic sintered at 2100 °C for 5 min at 30 MPa was 81.61%. This result was better than the relative density of Ta 4 HfC 5 ceramics sintered through mechanical ball milling at 2050 °C for 20 min at 32 MPa, which was 77.9% [10] .
The relative density increased to 88.60% when uniaxial pressure increased to 50 MPa. Densi cation behavior is discussed in detail later in this article. The grain sizes of the ceramic did not increase too much and still had good uniformity compared with the initial Ta 4 HfC 5 powders. This result was obviously superior to Ta 4 HfC 5 ceramics sintered via mechanical ball milling.  6 shows the sintering curves of Ta 4 HfC 5 ceramics sintered via spark plasma sintering under different uniaxial pressures. All the densi cation rates showed negative growth before 500 °C. The part of gas between particles would expand as temperature increased and would discharge slowly from the gap between mold and punches. Hence, a slower heating rate was applied before 500 °C because a rapid heating rate at the beginning would cause the gas instantly expand and generate local high pressure, which would quickly over ow powder from the gap, even directly destroy the mold.
All the powders started shrinking from 600 °C to 2100 °C because of the partial sintering of ceramics and the exhaustion of gas at a higher temperature. The gas in the pores produced spark discharge, generated ionization, and released a large amount of plasma, thereby promoting sintering densi cation. The maximum shrinking rates as the uniaxial pressure increased were 2.13×10 −3 , 4.45×10 −3 , and 2.35×10 −2 mm/s at 30, 40, and 50 MPa, respectively. The sintering displacement curves of the three ceramics still increased slowly at 2100 °C when sintering temperature was constant. This nding indicated that relative density would continue to increase with the extension of the constant sintering time. If the sintering time was long enough, nearly full-dense Ta 4 HfC 5 ceramics might be theoretically obtained.
In addition, the high-speed moving plasma hit the surface of the particles, which would eliminate the oxides on surface and make it easier to purify and activate. We added additional phenolic resin before to make it possible to remove the oxides, which was bene cial to further increase the densi cation of ceramics.
For the densi cation behavior of Ta 4 HfC 5 ceramics, fully dense Ta 4 HfC 5 ceramic was di cult to sinter at approximately 2000 °C and would need a higher uniaxial pressure because of the numerous covalent bonds and high melting points of the Ta-Hf-C system. Some pores were inevitably left during the shorter sintering time because of low self-diffusion coe cients and small liquid phase lling. Nanoscale particles had a higher sintering activity; however, their relative density was still higher than that sintered through mechanical ball milling under similar conditions.
Although spark plasma sintering had the advantages of the faster heating rate and shorter sintering time, no obvious advantages were observed when sintering Ta-Hf-C ceramics at around 2000 °C. In some works, TaC/TaC-HfC ceramics are sintered without sintering additives, and the relative density is over 90%. However, most works have used hot pressing, which involves longer time, higher temperature, or higher pressure [18,20,21] . However the ultra ne powder is still di cult to obtain full densi cation ceramic via hot pressing [22] . Ta-Hf-C ceramics sintered through spark plasma sintering can also achieve a higher densi cation in a shorter time but will require a higher sintering temperature than hot pressing [10] .

Conclusion
Ta 4 HfC 5 powder was prepared at a lower temperature (1400 °C) and in a shorter period through raw material selection and process optimization. The process was simple and could be used to prepare nanoscale Ta 4 HfC 5 powder in a high-volume autoclave. Particle size distribution was not affected by the change in the size of autoclaves, and the powder had good size distribution and dispersity. The product and structural changes in the precursor in carbon thermal reduction were studied. Ta 2 Hf 6 O 17 was generated at 1100 °C; therefore, the formation of Ta 4 HfC 5 powder at a lower temperature was feasible.
Ta 4 HfC 5 ceramics were sintered via spark plasma sintering and had better densi cation and smaller grain sizes compared with the ceramics sintered through mechanical ball milling method under similar sintering conditions. Figure 1 XRD patterns of powder calcined from 1000 °C to 1400 °C Ta4HfC5 powder can be prepared at 1400 °C, which below 1600 °C for other works [14,15]. For the Hf-O-C system, the preparation temperature of HfC is generally higher than 1600 °C [17], which limited the formation of Ta4HfC5 powder at a lower temperature. The precursor is converted to Ta2Hf6O17 at 1100 °C instead of independent Ta2O5 and HfO2 [15]. Converting Ta-O-Hf to Ta-Hf-C took lesser energy than converting Ta-O and Hf-O to Ta-Hf-C.