RF Thermal Plasma Synthesis of Ultrafine ZrB2-ZrC Composite Powders

Ultrafine ZrB2-ZrC composite powders were synthesized via a radiofrequency (RF) thermal plasma process. Numerical simulation and thermodynamic analysis were conducted to predict the synthesis process, and experimental work was performed accordingly to demonstrate its feasibility. The as-prepared samples were characterized by XRD, FESEM, particle size analyzer, nitrogen/oxygen analyzer, Hall flowmeter, and the Brunner-Emmet-Teller (BET) measurements. The thermodynamic analysis indicated that ZrB2 was preferentially generated, rather than ZrC, and numerical simulation revealed that the solid raw materials could disperse well in the gaseous reactants, and experimental work showed that free carbon particles were easily removed from the products and the elements of Zr, B, C, and O exhibited a uniform distribution. Finally, ZrB2-ZrC composite powders with a particle size of about 100 nm were obtained, the surface area of which was 32.15 m2/g and the apparent density was 0.57 g/cm3.


Introduction
Borides and carbides of zirconium show a number of excellent properties, such as high melting point, low density, good wear resistance, high thermal and electrical conductivity, and good chemical stability, which makes them attractive candidates in the areas where wear-corrosion-oxidation resistance is demanded, such as high temperature electrodes, molten metal crucibles, thermal protection systems for hypersonic flights, atmospheric re-entry vehicles, rocket propulsion systems, and nose caps [1][2][3][4][5][6][7][8]. However, it is difficult to use single phase material to meet all the necessary requirements demanded by the extreme conditions encountered in aerospace engineering. Therefore, research efforts have now turned to the development of double or multiple phase ceramics [9][10][11][12]. It has been demonstrated that ZrB 2 -ZrC composites possess better properties, such as mechanical strength and oxidation resistance, than those of either individual component [13][14][15][16][17][18]. Sorrell reported that boride-carbide eutectic composites fabricated by a directional solidification method had very good physical and chemical properties [19]. Tsuchida and Yamamoto synthesized a ZrB 2 -ZrC composite by mechanically activating self-propagating high temperature synthesis (SHS) in a Zr/B/C elemental system [20]. a powder-collecting filter, a gas supplier system, and an off-gas exhaust system. The reactor is operated in atmosphere and the pressure is usually adjusted in the range of −500 to −1000 Pa gauge during the operation of experiments. The RF generator (30 kW,4 MHz) was supplied by Tieling High Frequency Equipment Factory, and the reactors were homemade in the Institute of Process Engineering, Chinese Academy of Sciences. The raw materials of ZrCl4, B, and Mg were sifted through a 150 μm sieve and blended in a mechanical mixer. The reactor system was first purged with Ar to remove oxygen and moisture from the reactor, so that a separate inert atmosphere was prepared for the chemical synthesis procedure. A Plasma flame was generated using argon as both the plasma-forming gas and sheath gas. The reactor was heated by the plasma flame for about 5 min until the system reached a steady level, and then the raw materials were injected into the plasma flame in a continuous way with a homemade screw feeder. CH4 and H2 (if necessary) were mixed into the carrier gas and fed axially through the injection probe to the top of the plasma flame. The typical processing parameters for RF thermal plasma in our laboratory are given in Table 1. For the synthesis of the ZrB2 powders, stoichiometric amounts of ZrCl4, B, and Mg were weighed with a 20% excess amount of Mg, in which the molar ratio of Zr/B/Mg/C = 1/2/2.4/0-0.1. For the synthesis of the ZrB2-ZrC composite powders, the mole ratio of Zr/B/Mg/C = 1/1/2.4/0.5. The feeding time was usually 10-20 min, and then the plasma flame was shut down. In order to protect the products from oxidizing at the high temperature, Ar was supplied continuously until the reactor system cooled down. The final products were collected from the powder-collecting filter, and a small The raw materials of ZrCl 4 , B, and Mg were sifted through a 150 µm sieve and blended in a mechanical mixer. The reactor system was first purged with Ar to remove oxygen and moisture from the reactor, so that a separate inert atmosphere was prepared for the chemical synthesis procedure. A Plasma flame was generated using argon as both the plasma-forming gas and sheath gas. The reactor was heated by the plasma flame for about 5 min until the system reached a steady level, and then the raw materials were injected into the plasma flame in a continuous way with a homemade screw feeder. CH 4 and H 2 (if necessary) were mixed into the carrier gas and fed axially through the injection probe to the top of the plasma flame. The typical processing parameters for RF thermal plasma in our laboratory are given in Table 1.  time was usually 10-20 min, and then the plasma flame was shut down. In order to protect the products from oxidizing at the high temperature, Ar was supplied continuously until the reactor system cooled down. The final products were collected from the powder-collecting filter, and a small part of them could be found at the bottom of the collector. The solid products needed further post-treatment with diluted hydrochloric acid. The process flow chart is provided in Figure 2. part of them could be found at the bottom of the collector. The solid products needed further posttreatment with diluted hydrochloric acid. The process flow chart is provided in Figure 2.

Characterization
The crystalline phase of the as-prepared intermediate and final products was characterized by an X-ray diffractometer (XRD, X'pertPRO, Panalytical, CuKα radiation) in a 2θ range from 10° to 90°. Their size and morphology were inspected with field emission scanning electron microscopy (FESEM, JSM-6700 F; JEOL, Tokyo, Japan). The particle size distribution was measured using a LS particle size analyzer (Beckman Coulter LS 13 320, Nyon, Switzerland). The oxygen content was measured by an impulse-thermal conductivity method (Eltra ON-900, NCS, Beijing, China). The carbon content was measured by an infrared C-S measurer (NCS CS-3000, NCS, Beijing, China). The specific surface area was determined using a BET method. The apparent density was examined with the help of a Hall flowmeter.

Thermodynamic Calculation
The plasma synthesis of ZrB2 using ZrCl4, B, and Mg is ruled by the following reactions: Main reactions during the plasma processing can be expressed as below: Zr + C → ZrC The spontaneous direction of a reaction is usually judged by the change of the Gibbs-free energy ΔG. Thermodynamic calculation has been made based on the data given in the literature [35]. Figure  3 shows the free energies of the above reactions as a function of temperature.

Characterization
The crystalline phase of the as-prepared intermediate and final products was characterized by an X-ray diffractometer (XRD, X'pertPRO, Panalytical, Almelo, The Netherlands) in a 2θ range from 10 • to 90 • . Their size and morphology were inspected with field emission scanning electron microscopy (FESEM, JSM-6700 F; JEOL, Tokyo, Japan). The particle size distribution was measured using a LS particle size analyzer (Beckman Coulter LS 13 320, Nyon, Switzerland). The oxygen content was measured by an impulse-thermal conductivity method (Eltra ON-900, NCS, Beijing, China). The carbon content was measured by an infrared C-S measurer (NCS CS-3000, NCS, Beijing, China). The specific surface area was determined using a BET method. The apparent density was examined with the help of a Hall flowmeter.

Thermodynamic Calculation
The plasma synthesis of ZrB 2 using ZrCl 4 , B, and Mg is ruled by the following reactions: Main reactions during the plasma processing can be expressed as below: Zr + 2B → ZrB 2 Zr Nanomaterials 2020, 10, 2497

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The spontaneous direction of a reaction is usually judged by the change of the Gibbs-free energy ∆G. Thermodynamic calculation has been made based on the data given in the literature [35]. Figure 3 shows the free energies of the above reactions as a function of temperature. ΔG value for reaction (1) is much lower than zero at the temperature, and the ΔG value for reaction (2) decreases as the temperature increases and becomes lower than zero at the temperature around 1000 °C, indicating both reactions could take place spontaneously at high temperature. ΔG value for reaction (6) is always lower than that of reaction (7) at a temperature below 3800 °C, indicating that ZrB2 is preferentially generated. Therefore, free C instead of B would exist in the resulting products when C and B in the starting materials excess the stoichiometric ratio.

Numerical Simulation
Fluent software is employed to simulate the system and analyze the flow field and particle trajectories in the reactor as reported in [36]. In the present work, a three-dimensional physical model is applied, and mass-flow-inlet boundary was selected. Figure 4 shows the velocity field. Figure 4a,b shows the carrier gas path lines and Figure 4c,d shows the particle tracks from different points of view. It can be seen that both the carrier gas and particles are well fixed at the central area in the reactor. Their main swelling parts exhibit a similar shape, and the carrier gas expands a bit wider than the particles. These simulation results indicate that the addition of CH4 to the (ZrCl4 + 2B + 2Mg) reaction system can form another well-distributed system. ∆G value for reaction (1) is much lower than zero at the temperature, and the ∆G value for reaction (2) decreases as the temperature increases and becomes lower than zero at the temperature around 1000 • C, indicating both reactions could take place spontaneously at high temperature. ∆G value for reaction (6) is always lower than that of reaction (7) at a temperature below 3800 • C, indicating that ZrB 2 is preferentially generated. Therefore, free C instead of B would exist in the resulting products when C and B in the starting materials excess the stoichiometric ratio.

Numerical Simulation
Fluent software is employed to simulate the system and analyze the flow field and particle trajectories in the reactor as reported in [36]. In the present work, a three-dimensional physical model is applied, and mass-flow-inlet boundary was selected. Figure 4 shows the velocity field. Figure 4a,b shows the carrier gas path lines and Figure 4c,d shows the particle tracks from different points of view. It can be seen that both the carrier gas and particles are well fixed at the central area in the reactor. Their main swelling parts exhibit a similar shape, and the carrier gas expands a bit wider than the particles. These simulation results indicate that the addition of CH 4 to the (ZrCl 4 + 2B + 2Mg) reaction system can form another well-distributed system. is applied, and mass-flow-inlet boundary was selected. Figure 4 shows the velocity field. Figure 4a,b shows the carrier gas path lines and Figure 4c,d shows the particle tracks from different points of view. It can be seen that both the carrier gas and particles are well fixed at the central area in the reactor. Their main swelling parts exhibit a similar shape, and the carrier gas expands a bit wider than the particles. These simulation results indicate that the addition of CH4 to the (ZrCl4 + 2B + 2Mg) reaction system can form another well-distributed system.

Experimental Results
ZrB2 and ZrC can be synthesized using (ZrCl4 + 2B + 2Mg) and (ZrCl4 + CH4) reaction systems, respectively, as reported in our previous work [33,34]. Otherwise, ZrC was also synthesized successfully using (ZrCl4 + C + 2Mg) reaction system, which is similar as the synthesis of ZrB2 reported in [33]. In order to avoid the possible contamination of oxygen residual or leaking in the reaction system for the solid-state synthesis (ZrCl4 + 2B + 2Mg or ZrCl4 + C + 2Mg), hydrogen was mixed into the carrier gas, so that low oxygen content can be achieved.
In the present work, the two reactions (ZrCl4 + 2B + 2Mg) and (ZrCl4 + CH4) coexist in one system, where solid raw materials including ZrCl4, B, and Mg were mixed in advance and CH4 was mixed into the carrier gas which carried the solid raw materials into the plasma flame through the axial injection probe.
The ratio between CH4, B, and ZrCl4 had great effects on the composition of the products. At the beginning of the experimental work, the mixed raw materials for the synthesis of ZrB2 powders were used, in which stoichiometric amounts of ZrCl4, B, and Mg were weighed with a 20% excess amount of Mg. Figure 5 shows the XRD pattern of the sample obtained with the mole ratio of CH4/ZrCl4 of 10%.

Experimental Results
ZrB 2 and ZrC can be synthesized using (ZrCl 4 + 2B + 2Mg) and (ZrCl 4 + CH 4 ) reaction systems, respectively, as reported in our previous work [33,34]. Otherwise, ZrC was also synthesized successfully using (ZrCl 4 + C + 2Mg) reaction system, which is similar as the synthesis of ZrB 2 reported in [33]. In order to avoid the possible contamination of oxygen residual or leaking in the reaction system for the solid-state synthesis (ZrCl 4 + 2B + 2Mg or ZrCl 4 + C + 2Mg), hydrogen was mixed into the carrier gas, so that low oxygen content can be achieved.
In the present work, the two reactions (ZrCl 4 + 2B + 2Mg) and (ZrCl 4 + CH 4 ) coexist in one system, where solid raw materials including ZrCl 4 , B, and Mg were mixed in advance and CH 4 was mixed into the carrier gas which carried the solid raw materials into the plasma flame through the axial injection probe.
The ratio between CH 4 , B, and ZrCl 4 had great effects on the composition of the products. At the beginning of the experimental work, the mixed raw materials for the synthesis of ZrB 2 powders were used, in which stoichiometric amounts of ZrCl 4 , B, and Mg were weighed with a 20% excess amount of Mg. Figure 5 shows the XRD pattern of the sample obtained with the mole ratio of CH 4 /ZrCl 4 of 10%.
In the present work, the two reactions (ZrCl4 + 2B + 2Mg) and (ZrCl4 + CH4) coexist in one system, where solid raw materials including ZrCl4, B, and Mg were mixed in advance and CH4 was mixed into the carrier gas which carried the solid raw materials into the plasma flame through the axial injection probe.
The ratio between CH4, B, and ZrCl4 had great effects on the composition of the products. At the beginning of the experimental work, the mixed raw materials for the synthesis of ZrB2 powders were used, in which stoichiometric amounts of ZrCl4, B, and Mg were weighed with a 20% excess amount of Mg. Figure 5 shows the XRD pattern of the sample obtained with the mole ratio of CH4/ZrCl4 of 10%.   Figure 5. Furthermore, no obvious peaks of ZrC are displayed in the XRD pattern, which is also inconsistent with the conclusion that ZrB 2 will be preferentially generated, from thermodynamic analysis.
In the present synthesis process, with a mole ratio of B/Zr of 2, the element B was sufficient and there was little chance of C combining with Zr. Therefore, excessive CH 4 in the plasma reaction system led to free C in the plasma products. It was observed that there were some black samples floating on the surface of the solution during the post-treatment. The black samples were collected and dried at 110 • C for 120 min. Then, the black samples were confirmed as free C with an infrared C-S measurer, which could be removed from the products easily during the post treatment due to its light weight. Moreover, the C content of the final products measured was quite low. Table 2 displays the carbon contents and oxygen contents of the final products after acid leaching synthesized with a different mole ratio of CH 4 /ZrCl 4 . C content was only 0.32% when the mole ratio of CH 4 /ZrCl 4 was as high as 1/10. In order to demonstrate the separation efficiency of carbon nanoparticles, the sample after acid leaching was filtered and washed without removing the floating black samples on the surface of the solution and characterized using an infrared C-S measurer. The total C content was determined as 1.06%, which was much higher than that of the sample obtained when getting rid of the floating black samples. In consideration of the XRD characterization results, C mainly exists as free carbon instead of combined carbon because no peaks of ZrC are detected in the XRD pattern.
However, the addition of CH 4 to the (ZrCl 4 + 2B + 2Mg) reaction system can help to reduce the oxygen content of the final products. The decomposition products of CH 4 , H 2, and free C would provide Nanomaterials 2020, 10, 2497 8 of 13 a stronger reduction environment and protect the reactants from oxidation. The sealing technique at the ultra-high-temperature region within plasma equipment is still a challenge. The addition of reducing gas to protect the reaction process can really contribute to the synthesis of low oxygen content products.
In order to get ZrB 2 -ZrC composite powders, the mole ratio of B/Zr was set at 1.0, and the mole ratio of C/Zr was set at 0.5. Figure 6 shows the XRD patterns of the sample, in which both diffraction peaks of (001), (100) Carbon and oxygen contents were also measured and are listed in Table 3. Carbon content is 5.29% when the mole ratio of C/B/Zr in the starting materials is set at 1:2:2. The theoretical content of C in ZrC and B in ZrB 2 is 11.6% and 19.1%. Accordingly, the theoretical content of C in the ZrB 2 -ZrC composite powders (the mole ratio of ZrB 2 /ZrC: 1/1) is 5.6%, which is slightly higher than that detected by the impulse-thermal conductivity method. It can be seen that the oxygen content is 3.5%, indicating that some Zr exist as oxides instead of borides or carbides. Therefore, CH 4 was not fully utilized to form ZrC. Part of CH 4 was decomposed into free C and removed from the products during the post-treatment procedure. Carbon and oxygen contents were also measured and are listed in Table 3. Carbon content is 5.29% when the mole ratio of C/B/Zr in the starting materials is set at 1:2:2. The theoretical content of C in ZrC and B in ZrB2 is 11.6% and 19.1%. Accordingly, the theoretical content of C in the ZrB2-ZrC composite powders (the mole ratio of ZrB2/ZrC: 1/1) is 5.6%, which is slightly higher than that detected by the impulse-thermal conductivity method. It can be seen that the oxygen content is 3.5%, indicating that some Zr exist as oxides instead of borides or carbides. Therefore, CH4 was not fully utilized to form ZrC. Part of CH4 was decomposed into free C and removed from the products during the post-treatment procedure. Table 3. Carbon and oxygen contents with different mole ratio of C/B/Zr.

C/B/Zr Carbon Content Oxygen Content
1:2:2 5.29% 3.5% 0.3:2:1 0.32% 3.45% Figure 7a shows a FESEM picture (insertion, magnified 1000 times) and a whole EDS spectrum of the RF thermal plasma synthesized ZrB2-ZrC composite powders when the mole ratio of C/B/Zr in the starting materials was set at 1:2:2. Figure 7b-e is the corresponding maps of the FESEM picture. It can be seen that the concentration of all main elements exhibits similar outlines with the FESEM picture, indicating that Zr, B, C, and O disperse uniformly in the sample. Figure 7f--h shows FESEM pictures of the synthesized ZrB2-ZrC composite powders magnified 5000-40,000 times. The red circles are the magnified regions. The dispersed particles have clear edges and exhibit an average particle size ranging from 100 nm to 500 nm.   Figure 7a shows a FESEM picture (insertion, magnified 1000 times) and a whole EDS spectrum of the RF thermal plasma synthesized ZrB 2 -ZrC composite powders when the mole ratio of C/B/Zr in the starting materials was set at 1:2:2. Figure 7b-e is the corresponding maps of the FESEM picture. It can be seen that the concentration of all main elements exhibits similar outlines with the FESEM picture, indicating that Zr, B, C, and O disperse uniformly in the sample. Figure 7f-h shows FESEM pictures of the synthesized ZrB 2 -ZrC composite powders magnified 5000-40,000 times. The red circles are the magnified regions. The dispersed particles have clear edges and exhibit an average particle size ranging from 100 nm to 500 nm.
The particle size distribution of the powders measured using an LS particle size analyzer (Beckman Coulter LS 13 320, Nyon, Switzerland) is shown in Figure 8. D10, D25, D50, D75, and D90 of the synthesized powder are 0.744 µm, 1.094 µm, 1.630 µm, 2.425 µm, and 3.935 µm, respectively. The mean Nanomaterials 2020, 10, 2497 9 of 13 diameter, the median diameter, and mode diameter are 1.973 µm, 1.630 µm, and 1.593 µm, respectively. The ratio of mean/median is 1.210. It can be seen that the particle size measured by the particle size analyzer is much higher than that detected under FESEM, which may be attributed to most of the particles aggregating together instead of being monodisperse.

C/B/Zr Carbon Content Oxygen Content
1:2:2 5.29% 3.5% 0.3:2:1 0.32% 3.45% Figure 7a shows a FESEM picture (insertion, magnified 1000 times) and a whole EDS spectrum of the RF thermal plasma synthesized ZrB2-ZrC composite powders when the mole ratio of C/B/Zr in the starting materials was set at 1:2:2. Figure 7b-e is the corresponding maps of the FESEM picture. It can be seen that the concentration of all main elements exhibits similar outlines with the FESEM picture, indicating that Zr, B, C, and O disperse uniformly in the sample. Figure 7f--h shows FESEM pictures of the synthesized ZrB2-ZrC composite powders magnified 5000-40,000 times. The red circles are the magnified regions. The dispersed particles have clear edges and exhibit an average particle size ranging from 100 nm to 500 nm. The particle size distribution of the powders measured using an LS particle size analyzer (Beckman Coulter LS 13 320, Nyon, Switzerland) is shown in Figure 8. D10, D25, D50, D75, and D90 of the synthesized powder are 0.744 μm, 1.094 μm, 1.630 μm, 2.425 μm, and 3.935 μm, respectively. The mean diameter, the median diameter, and mode diameter are 1.973 μm, 1.630 μm, and 1.593 μm, respectively. The ratio of mean/median is 1.210. It can be seen that the particle size measured by the particle size analyzer is much higher than that detected under FESEM, which may be attributed to most of the particles aggregating together instead of being monodisperse. The surface area of the synthesized ZrB2-ZrC composite powders calculated by BET was 32.15 m 2 /g, and the apparent density examined with the help of a Hall flowmeter was 0.57 g/cm 3 . Detailed