1 Introduction

Zirconium diboride (ZrB2) is widely used as ultra-high-temperature ceramics (UHTCs), electrode material, thermowell tubes, etc. [1,2,3]. The investigations on preparation and densification of ZrB2 are always hot spots. There are many methods to prepare ZrB2 powder including carbothermal reduction [4, 5], borothermal reduction [6, 7], self-propagating high-temperature synthesis (SHS) [8, 9], solution-based methods [10,11,12], etc. Carbothermal reduction attracts a lot of attention because of its uncomplicated and low-cost raw material [13]. Meanwhile, carbon, one of the raw materials of carbothermal reduction, is helpful for densification of boride ceramics because it can react with oxides existed in grain boundary [14, 15]. However, the particle prepared by carbothermal reduction is in micron size. In addition, the particle has a rod shape which makes it very difficult to be highly densified. Thus, the relative density of single-phase ZrB2 ceramic sintered with ZrB2 raw powder prepared by carbothermal reduction is very difficult to reach more than 90% [16,17,18] unless pretreatment such as attrition milling with raw powder is used or sintering additives are added [19,20,21] or nanometer-grade ZrB2 powder is used [16]. However, the nanometer-grade powder is very costly, and the sintering additives may not be allowed in some applications like the integrated fuel burnable absorber (IFBA) ZrB2 coating on the surface of UO2 pellets in AP1000 reactor [22]. Furthermore, in IFBA ZrB2 coating, boron is enriched with 10B, which makes the preparation highly cost-sensitive. Therefore, it has great significance to develop a new technology to improve particle size and morphology of ZrB2 powder prepared by carbothermal reduction so that highly densified ZrB2 ceramic can be achieved. Combined solgel and carbothermal reduction [10,11,12] can well control the particle size and morphology of ZrB2 powder. Unfortunately, solgel is costly and not easy to be industrialized.

Thus, in this paper, carbothermal reduction was used to prepare ZrB2 powder, and its effect on particle size and morphology was investigated. Also, the performance of densification of ZrB2 powder was discussed.

2 Experimental

Zirconium oxide (ZrO2) powder (> 99.5 wt%, 1–5 μm, China Nuclear JingHuan Zirconium Industry Co., Ltd), boric acid (H3BO3) powder (> 99.5 wt%, 50 μm, Sinopharm Chemical Reagent Co., Ltd) and graphite (C) powder (> 99.99 wt%, 5–10 μm, KAJET, Shanghai) were used as raw materials mixed in a certain proportion. C content in synthesized powder was controlled by adjusting the ratio of C powder in raw material. The carbothermal reaction in Reaction (1) was done with argon protection; for this reaction, \(\Delta_{r} G_{m}^{\varTheta } = - \,0.8157T + 1227.1\), where \(\Delta_{r} G_{m}^{\varTheta }\) is standard molar Gibbs free energy for chemical reactions and T is temperature.

$${\text{ZrO}}_{2} \left( {\text{s}} \right) + {\text{B}}_{2} {\text{O}}_{3} \left( {\text{l}} \right) + 5{\text{C}}\left( {\text{s}} \right) \to {\text{ZrB}}_{2} \left( {\text{s}} \right) + 5{\text{CO}}\left( {\text{g}} \right)$$
(1)

Boron (B) powder (> 99.99 wt%, 20 μm, YFNANO, Shanghai) was used as an additive with the content of 2.5 wt% and 5.0 wt%. ZrB2 powder was vacuum-hot-pressed at the temperature of 1980 °C and the pressure of 70 MPa with the holding time of 4 h.

The thermal effect of the raw materials was detected by thermal gravimetric analyzer (TGA/DSC1, Mettler Toledo). The phase structure was detected by X-ray diffractometer (XRD, D/max 2500, Rigaku, Japan). The particle size and morphology were analyzed by scanning electron microscope (SEM, JSM-6510, JEOL, Japan) equipped with energy-dispersive spectroscopy (EDS) for composition analysis. The relative density of ZrB2 ceramic was analyzed by Archimedes method. The cross sections and grain of ZrB2 ceramic were observed by transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN, FEI, USA).

3 Results and discussion

3.1 DSC analysis of raw materials

Two kinds of raw materials were used in the experiments, with/without boron additive. Figure 1 displays DSC curves of the raw materials with 0 wt% and 2.5 wt% B additives, respectively. It can be seen that both DSC curves show two endothermic peaks at 112 and 163 °C, which are contributed to the removal of water absorbed and hydrated. Further, at around 1416 °C, both DSC curves show an endothermic peak which is probably resulted by the formation of ZrC or B2O2, as shown in Reaction (2) (\(\Delta_{r} G_{m}^{\varTheta } = - \,0.3478T + 578.12\)) and Reaction (3) (\(\Delta_{r} G_{m}^{\varTheta } = - \,0.3304T + 584.49\)), respectively.

$${\text{ZrO}}_{2} \left( {\text{s}} \right) + 3{\text{C}}\left( {\text{s}} \right) = {\text{ZrC}}\left( {\text{s}} \right) + 2{\text{CO}}\left( {\text{g}} \right)$$
(2)
$${\text{B}}_{2} {\text{O}}_{3} \left( {\text{l}} \right) + {\text{C}}\left( {\text{s}} \right) = {\text{B}}_{2} {\text{O}}_{2} \left( {\text{g}} \right) + {\text{CO}}\left( {\text{g}} \right)$$
(3)

Li et al. [23] investigated the preparation of ZrC and reported that at a temperature more than 1127 °C, ZrC can be formed if \(\lg (p_{\text{CO}} /p^{\varTheta } ) = - \,5\) (where pCO is partial pressure of CO and \(p^{\varTheta }\) is standard pressure). Maeda et al. [24] investigated preparation of NbB2 powder by carbothermal reduction and showed that at a temperature higher than 1277 °C, the formation of B2O2 gas in Reaction (3) maybe occurred.

Fig. 1
figure 1

DSC curves of raw materials under argon flow with a heating rate of 10 °C·min−1

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Meanwhile, at temperature around 1470 °C, DSC curves in Fig. 1 show the contrary thermal effect. For the powder without B additive, it is endothermic, but for that with 2.5 wt% B, it is exothermic. Apparently, it reveals that different reactions happen when boron additive is used.

Figure 2 shows XRD pattern of the powder prepared by carbothermal reduction at 1450 °C. It can be seen clearly that ZrB2 is formed and the carbothermal reduction reacts around 1450 °C. When boron is brought into raw materials, boron could be reacted with carbon to form B4C at temperature around 1470 °C and a lot of heat is released at the same time, causing the formation of B4C as shown in Reaction (4) (\(\Delta_{r} G_{m}^{\varTheta } = 0.0058T - 62.948\)) which is exothermic reaction.

Fig. 2
figure 2

XRD pattern of powder prepared by carbothermal reduction without B additive at 1450 °C

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$$4{\text{B}}\left( {\text{s}} \right) + {\text{C}}\left( {\text{s}} \right) \to {\text{B}}_{4} {\text{C}}\left( {\text{s}} \right) $$
(4)

Therefore, although the carbothermal reduction is endothermic without B additive, the overall thermal effect becomes exothermic with 2.5 wt% B additives.

3.2 Effect of temperature on phase structure of carbothermal reaction

Figure 3 shows XRD patterns of ZrB2 powders prepared at different temperatures. It can be seen that with the increase in temperature, the composition tends to be single phase of ZrB2 gradually. The reaction is not completed from 1500 to 1800 °C; meanwhile, the phase content of ZrO2 and C is diminished. At the temperature of 1900 °C, pure ZrB2 is obtained.

Fig. 3
figure 3

XRD patterns of ZrB2 powder prepared by carbothermal reduction at different temperatures

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The similar results are also reported. Khanra et al. [25] prepared ZrB2 using ZrO2–H3BO3–C system and found that B4C and ZrC existed as minor phases with the temperature between 1500 and 1700 °C. Guo and Zhang [26] used ZrO2–B4C–C system to synthesize ZrB2 powder and found that ZrC phase was formed with temperature between 1550 and 1650 °C. These results imply that quite a few subsidiary reactions exist during the process of carbothermal reduction reaction. The carbothermal reduction reaction cannot be completely done until the temperature reaches 1900 °C because of the kinetic factors and subsidiary reactions.

3.3 Particle size and its morphology control of ZrB2

Figure 4 is SEM image of ZrB2 prepared by carbothermal reduction at 1900 °C for 3 h. It can be seen that the particle is rodlike with the size of about 8 μm in length and 2–3 μm in diameter. It illustrates that the oriented grain growth is happened during the preparation. Yang et al. [27] reported that besides crystallographic properties, growth environment also had a great influence on preferential growth along [001] direction to form prism-like ZrB2 particles. On the one hand, according to the periodic bond chain (PBC) theory [28], planes with a higher density of strong bonds (B–B > B–Zr > Zr–Zr) tend to grow faster along its normal direction than other planes in ZrB2 cell [29]. On the other hand, the preferential growth along [001] direction proves that the crystal surface of (001) has lower surface energy which is consistent with the calculation reported by Liu [30]. Moreover, the vaporization of B2O3 possibly reduced the supersaturation of ZrB2 nucleus and led to one-dimensional growth [31].

Fig. 4
figure 4

SEM image of ZrB2 prepared by carbothermal reduction without B additive at 1900 °C for 3 h

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Figure 5 shows SEM images of ZrB2 powder prepared with 2.5 wt% and 5.0 wt% B additives at 1900 °C by carbothermal reduction. It can be seen evidently that the particles are turned into regular shape, and no rodlike particles are found, compared to the particles prepared without B additive as shown in Fig. 4. Further, the mean particle size of ZrB2 powder prepared with 5.0 wt% B is about 2.3 μm, which is smaller than 3.6 μm of ZrB2 prepared with 2.5 wt% B.

Fig. 5
figure 5

SEM images of ZrB2 prepared with a 2.5 wt% B additives and b 5.0 wt% B additives by carbothermal reduction

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As discussed in Sect. 3.1, boron additive could change the heat effect of the overall reactions. With the heat released by B4C formation reaction, the environment of the formation of ZrB2 nucleus is changed. Also, the extra heat source may reduce the reaction barrier of the carbothermal reduction reaction. Thus, with more boron additive, more heat is released, and the reaction rate of carbothermal reduction will be accelerated and lead to more ZrB2 nuclei formation. According to the theory of grain growth, more nuclei will be effective to promote grain refining. Finally, ZrB2 particles become regular shape, and the size will be smaller with more B additives.

3.4 Densification of ZrB2

It is well known that carbon is an effective additive to densify ZrB2 ceramics because it can react with oxides existing in grain boundary of ZrB2. In order to further illustrate the effect, four kinds of ZrB2 powders were prepared with different carbon contents. Vacuum hot pressing was used to sinter ZrB2 ceramic at the temperature of 1980 °C and the pressure of 70 MPa. The results are given in Table 1.

Table 1 ZrB2 start powder and its hot pressing at 1980 °C with 70 MPa
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As given in Table 1, the relative density of ZrB2 ceramics with the start powders prepared without B additive is less than 85%. By contrast, the relative density can be up to 93% when the ceramic was hot-pressed with the start powder prepared with 5.0 wt% B additives, which further illustrates that with boron additive, the sintering property of ZrB2 powder is greatly improved.

Moreover, not only is the relative density of ZrB2 ceramic hot-pressed with the start powder of No. 1 quite low (78%), but also the ceramic is cracked. TEM image (Fig. 6) shows that many holes exist in the ceramic body. And some particles are agglomerated near the boundary as shown in Spot A in Fig. 6. EDS result (Table 2) shows that there are some oxides, such as ZrO2 or B2O3.

Fig. 6
figure 6

TEM image of ZrB2 ceramic hot-pressed with start powder of No. 1

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Table 2 EDS result of hot-pressed ceramic with start powder of No. 1 (Spot A in Fig. 6)
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On the one hand, the presence of surface oxygen reduces the surface energy of the particle and prohibits the diffusion of the particles in the hot-pressing process [32]. On the other hand, the oxide impurities on the surface of the particle form channel between the grain boundaries to keep from further densification [33]. In addition, the residual carbon in synthesized ZrB2 powder can react with oxide impurities on the surface of ZrB2 particles and resolve the aforementioned shortcomings [34,35,36,37,38,39].

Figure 7 shows TEM image of the ceramic hot-pressed with the start powder of No. 4. The relative density is 93%. It can be seen that the particles are packed together closely and no oxides are detected in grain boundary.

Fig. 7
figure 7

TEM image of ZrB2 ceramic hot-pressed with start powder of No. 4

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4 Conclusion

In this study, zirconium diboride (ZrB2) powder was prepared with boron (B) additive by carbothermal reduction. The reaction cannot be completely done until the temperature reaches 1900 °C. The ZrB2 grain shows a preferential growth along [001] direction. B additive can change the heat effect of raw materials and the morphology of final ZrB2 particles. The morphology of final ZrB2 particles is not rodlike; instead, it becomes regular. The final ZrB2 particle size changes from 3.6 to 2.3 μm with the increase in B additives from 2.5 wt% to 5.0 wt%. The existence of oxides in ZrB2 grain boundary is a key factor to keep ZrB2 ceramic from getting deep densification. Single-phase ZrB2 ceramic with 93% relative density is obtained by hot pressing using ZrB2 powder prepared with 5 wt% B additives.