ZrB2 Nanopowders Solid-State Synthesis: Comparison
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ZrB2 is of particular interest among ultra-high temperature ceramics because it exhibits excellent thermal resistance at high temperature, as well as chemical stability, high hardness, low cost, and good electrical and thermal conductivity, which meet the requirements of high-temperature components of hyper-sonic aircraft in extreme environments. In recent years, the development of ZrB2 powders’ synthesis method has broken through the classification of traditional solid-phase method, liquid-phase method, and gas-phase method, and there is a trend of integration of them. Solid-state synthesis, including the borothermic reduction method, carbothermic reduction method, and metallothermic reduction method, is mostly used because the process is simple and the raw materials are cheap and easily available. 

  • ultra-high temperature ceramics
  • ZrB2 nanopowders
  • solid-state synthesis
  • self-propagating high-temperature synthesis
  • solution-derived precursors
  • plasma technology

1. Introduction

Ultra-high temperature ceramics (UHTCs) mainly include refractory borides, carbides, and nitrides of some transition metals, such as ZrB2, HfB2, ZrC, and TaC, the melting points of which are usually above 3000 °C [1][2][3][4][5][6][7]. UHTCs and their composites have attracted great attention in the past two decades as potential heat-resistance candidates used in hyper-sonic aircraft and high-performance aircrafts [8][9][10][11][12][13][14][15]. Among these UHTCs, borides are considered superior due to their combination of excellent properties, including thermal shock resistance, creep resistance, and thermal conductivity [16][17][18][19]. Among all the borides, ZrB2 and HfB2 exhibit best oxidation resistance at high temperatures, as well as good electrical and thermal conductivity, chemical stability, and high hardness [20][21][22][23][24]. ZrB2 and HfB2 can realize long-time non-ablation in an oxidizing environment above 2000 °C. Furthermore, between these two diborides, ZrB2 has a relatively lower density and lower cost than HfB2, so it is preferred over HfB2 and mostly studied as industrially appealing ultra-high temperature ceramic powders [25].
ZrB2 powders not only act as the basic unit of UHTCs and their composites, but also provide an important way for researchers to improve material properties and explore new properties by way of synthesis design and innovation. To reach the extreme state of UHTCs, it is often necessary to change the original properties of materials, and the most effective methods to change the properties include particle refinement and recombination in the ultra-fine state [26].
Powders with high purity, ultra-fine, and uniform particle size are the basic raw materials for the preparation of advanced performance UHTCs and their composites [27][28]. It has been demonstrated that when the particle size decreases into the nanoscale, the ZrB2 powders exhibit an excellent sintering ability and facilitate the formation of nano-grained materials and C–C composites with improved mechanical properties. Therefore, different methods have been established for the synthesis of ZrB2 nanopowders [29][30][31][32][33][34][35].

2. Solid-State Synthesis

Solid-state synthesis mainly includes direct reaction between elemental boron and zirconium powders, carbothermal reduction reaction, and magnesiothermic reduction reaction. In a direct reaction method, ZrB2 powders are obtained by completing the following reaction [36][37][38]:

Zr + 2B → ZrB2

These two Zreactions are exothermic, and the heat of the reactions promotes the continuous progress of the reaction in turn, which is the reason they are named carbothermal and magnesiothermic reduction reactions. When the heat of a reaction is sufficient, it will successively trigger the adjacent raw material layer to react and subsequently release more heat, so that the reaction automatically propagates in the form of a combustion wave until completion without any other energy supply from outside after initiation. This process is called self-propagation high-temperature synthesis (SHS) + 2B → ZrB2

These two reactions are exothermic, and the heat of the reactions promotes the continuous progress of the reaction in turn, which is the reason they are named carbothermal and magnesiothermic reduction reactions. When the heat of a reaction is sufficient, it will successively trigger the adjacent raw material layer to react and subsequently release more heat, so that the reaction automatically propagates in the form of a combustion wave until completion without any other energy supply from outside after initiation. This process is called self-propagation high-temperature synthesis (SHS)

[39][40][41]

. Adiabatic temperature (T

ad

) is a temperature that the exothermic reaction system can reach in an insulated environment, which is an important parameter to predict a SHS process. Varma A. proposed a thermodynamic criterion for judging the maintenance of a SHS process according to the value of T

ad

: if T

ad

> 1800 K, the SHS can continue; if T

ad

˂ 1500 K, the heat released by the reaction is not enough to make the combustion reaction continue; if 1500 K ˂ T

ad

˂ 1800 K, the system must be provided with additional energy from the outside to continue

[42][43].

.
The common carbothermal reduction systems for synthesizing ZrB2 powders include ZrO2-B4C-C, ZrO2-B2O3-C, and ZrC-B2O3-C as below:
2ZrO2 + B4C + 3C → 2ZrB2 + 4CO(g)
ZrO2 + B2O3 + 5C → ZrB2 + 5CO(g)
ZrC + B2O3 + 2C → ZrB2 + 3CO(g)
The magnesiothermic reduction system of Mg-ZrO2-B2O3 is usually chosen for synthesizing ZrB2 powders as below:
ZrO2 + B2O3 + 5Mg → ZrB2 + 5MgO
The calculated Tad of Equation (5) is 3085 K, which is much higher than 1800 K, so the reaction system of Mg-ZrO2-B2O3 is a typical SHS system, which is the most-used technique for ZrB2 powders both in laboratory and industry [44][45][46].

3. Modified Self-Propagating High-Temperature Synthesis

Different additives were introduced into the SHS system in order to adjust the combustion temperature and isolate ZrB2 particles from each other to get well-dispersed nanoparticles. Zhang W. studied the Mg-ZrO2-B2O3-MgCl2 and Mg-ZrO2-B2O3-NaB4O7 system and Zhang T. studied the Mg-ZrO2-B2O3-MgO system for the preparation of ZrB2 powders by SHS [47][48][49]. Figure 1 shows FESEM images of the ZrB2 powders synthesized without any diluents and the EDS image of point A. Figure 2 shows XRD patterns of ZrB2 powders prepared with different content of NaB4O7.

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Figure 1. FESEM image of ZrB2 powders (a) and EDS of point A (b) [48].
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Figure 2. XRD patterns of ZrB2 powders prepared with different content of NaB4O7 [48].
Results showed that an optimum amount of MgCl2 and NaB4O7 could decrease the grain size of ZrB2 powders effectively, while there was also a significant improvement in ZrB2 purity.
Khanra A. K. synthesized ultrafine ZrB2 powders by SHS from a mixture of H3BO3, ZrO2, and Mg. The experiments were carried out within a tubular furnace and Ar flow was provided continuously into the reaction system. The crystallite sizes of ZrB2 powders calculated using Scherrer formula based on XRD characterization results are given in Table 1. The addition of NaCl to the green mixture as inert diluent decreased the combustion temperature and helped yield ultrafine ZrB2 powders [50]. NaCl melted and vaporized partially in the synthesis process and became coated on ZrB2 particles, inhibiting the nanograin from growing into big particles. NaCl addition also decreased the adiabatic temperature, which might also contribute to the reduction of the grain size.
Table 1. Crystallite size of different synthesized powder samples [50].
NaCl (wt.%) Crystal Size (nm)
0 25
5 20
10 18
15 16
20 13

The other approach for ZrB2 powder production is to use a non-pressed green mixture to avoid sintering and agglomeration at high temperature. Heat and mass transfer become difficult due to the porosity of non-compacted samples, and the reaction has difficulty in automatically propagating without any energy supply from outside after initiation. Therefore, the entire body of the sample needs to be placed in a high temperature environment and heated uniformly. Since the melting point of B2O3 is only 450 °C, the green mixture will become inhomogeneous due to B2O3 loss during the pre-heating process.

It can be seen from Figure 3 that well-dispersed ZrB2 powders were obtained when the green mixture was naturally packed in the crucible, while aggregates appeared in a large area when the packing density increased from 0.66 g/cm3 (a, naturally packed) to 1.32 g/cm3 (b, twice that of the naturally packed).

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Figure 3. SEM picture of the sample obtained using green mixture with packing density of 0.66 g/cm3 (a) and 1.32 g/cm3 (b) [51].
We further developed the method for the synthesis of ZrB2 powders with low oxygen content by a two-step reduction route. ZrB2 powders were synthesized in the first step using Mg-ZrO2-B reaction system as before. Then, in the second step, ZrB2 powders obtained from the first step were mixed with Mg at a mass ratio of ZrB2 to Mg being 10:1 and subjected to heating in the designed furnace again [52]. Figure 4 shows the FESEM images and corresponding particle size distribution of the samples obtained following the first step (a and c) and the second step (b and d). It can be seen that oxygen content decreased after another second synthesis step, while minor change could be observed in the particle size and dispersion of ZrB2 powders.
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Figure 4. SEM images and particle size distribution of samples obtained following the primary synthesis step (a,c) and the second synthesis step (b,d) [52].
A new method for oxygen content calculated based on XRD results for ZrB2 characterization was proposed in this work. Figure 5 shows XRD patterns of samples mixed with the guide Si plate. It was discovered that the lattice constants determined according to XRD patterns were higher than their theoretical values calculated based on the First Principles. The O atom existing as interstitial impurity in the ZrB2 crystal lattice might contribute to the lattice constant changes.
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Figure 5. XRD patterns of samples mixed with the guide Si plate [52].

4. Solution-Derived Carbothermal Synthesis

Synthesis in vacuum is a commonly used improved process. The change of Gibbs’s free energy can be used to determine whether the reaction can happen from thermodynamics. When ∆G < 0, the reaction is spontaneous; when ∆G > 0, the reaction is not spontaneous. The value of Gibbs free energy change in a non-standard state can be calculated based on that of the standard state according to Equation (6):

∆G = ∆G0 + RTlnKp 
When the synthesis of ZrB2 powders is carried out under vacuum, the air pressure in the furnace is supposed to remain below 11 Pa; it can be considered that the partial pressure in the furnace is 11 pa. If one chooses 11 Pa as the partial pressure of CO in the furnace, the minimum reaction temperature (∆G = 0) of Equation (5) is 938 °C, which is much lower than 1509 °C under the standard state [44].
Lv G. prepared ZrB2 powders by a carbon reduction method with zirconia, activated carbon, boron carbide, and boron oxide as main raw materials under vacuum, and obtained ZrB2 powders of high purity, small particle size, and low cost [53]. Figure 6 shows the FESEM images of ZrB2 powders synthesized at 1450 °C for different holding times.
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Figure 6. FESEM images of ZrB2 powders synthesized at 1450 °C for different holding times ((a) 1 h; (b) 1.5 h) [53].
Li M. reported a molten-salt method for the synthesis of ZrB2 powders [54]. Scheme 1 shows a schematic diagram for the preparation of ZrB2 powders. The participation of Na4SiO4 and Na2B4O7 provided a liquid surrounding that could dissolve reactant species and promoted rapid diffusion between them. Molten salt helped shorten the average diffusion distance and lower the reaction temperature.
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Scheme 1. Schematic diagram for the preparation of ZrB2 [55].
Solution combustion synthesis (SCS) is a new approach for the production of ZrB2 powders. Initial reactants are solved in a homogeneous solution and form uniform precursors at the molecular scale. In a SCS process, a huge amount of gas would be generated and make the solid product expanded and porous, which is critical for SCS to synthesize nano-ZrB2 powders [55][56].
Though ZrB2 powders can be synthesized from the precursors prepared by the sol-gel method, the precursors are usually of low effective concentration, poor stability, easy to settle and precipitate, and difficult to store. Therefore, the development of ultra-high temperature ceramics has extended from the sol-gel to the polymer precursor method [57][58][59]. One strategy is to synthesize the polymer with M–O as the main chain by chemical reaction, and then prepare the ultra-high temperature ceramic precursor with the compound containing C (phenolic acid) and B (boric acid). The other strategy is to synthesize a metal organic polymer with Zr–B chemical bonds in the molecules, which can be directly converted into ZrB2 ceramics by cracking.
The gel aging time and calcination had great effects on the product morphology. Figure 7a,b displays the FESEM images of precursor derived from the nascent state gel calcined at 1300 °C and 1550 °C for 2 h, respectively. We can see that hexagonal-prism-like particles emerged calcined at 1550 °C instead of aggregates obtained at low 1300 °C [60].
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Figure 7. FESEM images of precursor derived from the nascent state gel calcined at (a) 1300 °C and (b) 1550 °C for 2 h, respectively. [60].

5. Plasma-Enhanced Exothermic Reaction

Thermal plasma synthesis is the most efficient technique for producing metal and ceramic nanopowders in continuous and scalable way [61][62][63][64][65][66][67].

Generally, plasma synthesis includes physical vapor deposition (PVD) and chemical vapor deposition (CVD). In a PVD process, coarse particles are injected into the plasma flame and gasified at high temperature and nanopowders are deposited after being rapidly cooled under large temperature gradient. Scheme 2 shows the diagram of the formation process of nanopowder in plasma. In a CVD process, more than two reactants are added into the plasma flame together, which are usually gasified at high temperature and react with each other. Ultrafine metal/oxide powders and various nanostructures can be obtained via this instantaneous enhanced reduction/oxidation reaction of active hydrogen or active oxygen provided by thermal plasma.
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Scheme 2. (a) sketch of the RF plasma reactor, (b) temperature thermofluid field in plasma arc and (c) schematic diagram of formation process of nanopowder in plasma [63].
Great progress has also been made for the high temperature ceramic powders such as carbides and nitrides that require high synthesis temperature beyond conventional heating methods in recent years [68][69][70][71]. Generally, gases react directly in the plasma arc or low boiling point raw materials are added into the plasma arc to obtain ultra-fine powder through gasification reaction deposition. Almost all of these kinds of processes are characterized by using gases such as chloride and nitrogen/hydrocarbons or using solid organisms with low boiling point and easy decomposition as raw materials. A multiphase complex system in which more than one high boiling point raw materials participates in the reaction are generally considered not suitable for thermal plasma synthesis. Although the temperature and energy density in the thermal plasma arc area are very high, the gas flow rate in the reactor is very large so the residence time of raw materials in the plasma arc is limited, and the high boiling point raw materials do not easily realize volatilization and continuous gas phase reaction in a short time, which greatly limits its application fields.
SHS is widely used in the synthesis of ultra-high temperature ceramic powders. Typically, a reaction is initiated from a small area and self-propagating throughout the whole body. In the section “Modified self-propagating high-temperature synthesis”, it was concluded that ZrB2 powders could also be produced using a non-pressed green mixture when the reaction system was placed in a high temperature environment and heated uniformly.
We synthesized ZrB2 nano powders successfully in an RF thermal plasma under atmospheric pressure, in which ZrCl4, B, and Mg as raw materials were mixed as the green mixture. The metallothermic reaction was ignited by the high temperature flame when the green mixture was carried by H2 into thermal plasma, and then propagated with the exothermic reaction enthalpies and the external energy supplied by plasma flame. Scheme 3 shows the process flow chart for plasma synthesis of ZrB2 nano powders. Figure 8 shows the TEM image and particle size distribution of the product [72][73].
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Scheme 3. Process flow chart for plasma synthesis of ultrafine ZrB2 powders [73].
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Figure 8. TEM image (a) and particle size distribution (b) of the product [73].

6. Conclusions

In the future, ultra-high temperature ceramics will still develop towards exploring extreme parameters, for which it is often necessary to change the original properties of basic materials, and the most effective methods to change the properties include particle refinement and recombination in the ultra-fine state. Furthermore, physical and chemical problems in powder preparation have a strong correlation with scientific problems in the molding process and thermodynamics and kinetics in the firing process, and solution of these basic problems is the basic guarantee for obtaining material reliability and stability. Therefore, the integration of various synthesis methods, the combination of different material components, and the connection between synthesis and its subsequent application process are the trends of development in the future.

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