简介概要

Synthesis and densification of zirconium diboride prepared by carbothermal reduction

来源期刊：Rare Metals2018年第12期

论文作者：Tao Gui Bo Song

文章页码：1076 - 1081

摘要：Using boron powder as additive, the preparation of zirconium diboride（ZrB 2） by carbothermal reduction was investigated. The results show that the carbothermal reduction cannot be completely done until the temperature is more than 1900 ℃. The ZrB2 particles prepared without boron（B） additive at 1900 ℃ for 3 h are rodlike and show a preferential grain growth along [001] direction. B additive changes the heat effect of the raw materials. With B additive, the morphology of ZrB2 particles turns to be regular shape. The average particle size is about 3.6 μm with 2.5 wt% B additives. With more B additive, the shape of particles turns to be round like and the average particle size is decreased to 2.3 μm when 5 wt% B is added. The existence of oxides in grain boundary is a key factor to keep ZrB2 ceramic from deep densification. Using ZrB2 powder prepared with 5 wt% B additives, by controlling carbon content in ZrB2 powder, ZrB2 ceramic with 93%relative density is hot-pressed.

详情信息展示

稀有金属(英文版) 2018,37(12),1076-1081

Synthesis and densification of zirconium diboride prepared by carbothermal reduction

Tao Gui Xing-Ming Wang Lei Yang Yu-Yang Liu Xue Bai Li-Jun Wang Bo Song

Division of Metallurgy and Materials for Rare Metals, General Research Institute for Nonferrous Metals

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing

作者简介：*Xing-Ming Wang,e-mail:mansionwang010@163.com;

收稿日期：7 May 2018

基金：financially supported by the National Natural Science Foundation of China(No. 51674035);

Synthesis and densification of zirconium diboride prepared by carbothermal reduction

Tao Gui Xing-Ming Wang Lei Yang Yu-Yang Liu Xue Bai Li-Jun Wang Bo Song

Division of Metallurgy and Materials for Rare Metals, General Research Institute for Nonferrous Metals

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing

Abstract：

Using boron powder as additive, the preparation of zirconium diboride(ZrB 2) by carbothermal reduction was investigated. The results show that the carbothermal reduction cannot be completely done until the temperature is more than 1900 ℃. The ZrB2 particles prepared without boron(B) additive at 1900 ℃ for 3 h are rodlike and show a preferential grain growth along [001] direction. B additive changes the heat effect of the raw materials. With B additive, the morphology of ZrB2 particles turns to be regular shape. The average particle size is about 3.6 μm with 2.5 wt% B additives. With more B additive, the shape of particles turns to be round like and the average particle size is decreased to 2.3 μm when 5 wt% B is added. The existence of oxides in grain boundary is a key factor to keep ZrB2 ceramic from deep densification. Using ZrB2 powder prepared with 5 wt% B additives, by controlling carbon content in ZrB2 powder, ZrB2 ceramic with 93%relative density is hot-pressed.

Keyword：

Zirconium diboride; Carbothermal reduction; Grain growth; Morphology control; Deep densification;

Received： 7 May 2018

1 Introduction

Zirconium diboride (ZrB₂) is widely used as ultra-hightemperature 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 ZrB₂raw 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 ZrB₂ceramic 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%,1-5μm,China Nuclear JingHuan Zirconium Industry Co.,Ltd),boric acid (H₃BO₃) powder (>99.5 wt%,50μm,Sinopharm Chemical Reagent Co.,Ltd) and graphite (C)powder (>99.99 wt%,5-10μm,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, =-0.8157T+1227.1,where is standard molar Gibbs free energy for chemical reactions and T is temperature.

Boron (B) powder (>99.99 wt%,20μm,YFNANO,Shanghai) was used as an additive with the content of2.5 wt%and 5.0 wt%.ZrB₂ powder was vacuum-hotpressed at the temperature of 1980℃and the pressure of70 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℃,which are contributed to the removal of water absorbed and hydrated.Further,at around 1416℃,both DSC curves show an endothermic peak which is probably resulted by the formation of ZrC or B₂O₂,as shown in Reaction (2)

Fig.1 DSC curves of raw materials under argon flow with a heating rate of 10℃·min^-1

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

Meanwhile,at temperature around 1470℃,DSC curves in Fig.1 show the contrary thermal effect.For the powder without B additive,it is endothermic,but for that with2.5 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℃.It can be seen clearly that ZrB₂ is formed and the carbothermal reduction reacts around 1450℃.When boron is brought into raw materials,boron could be reacted with carbon to form B₄C at temperature around 1470℃and a lot of heat is released at the same time,causing the formation of B₄C as shown in Reaction (4)( =0.0058T-62.948) which is exothermic reaction.

Fig.2 XRD pattern of powder prepared by carbothermal reduction without B additive at 1450℃

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 c arbothermal 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 to1800℃;meanwhile,the phase content of ZrO2 and C is diminished.At the temperature of 1900℃,pure ZrB₂ is obtained.

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℃.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℃.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℃because of the kinetic factors and subsidiary reactions.

Fig.3 XRD patterns of ZrB₂ powder prepared by carbothermal reduction at different temperatures

3.3 Particle size and its morphology control of ZrB2

Figure 4 is SEM image of ZrB₂ prepared by carbothermal reduction at 1900℃for 3 h.It can be seen that the particle is rodlike with the size of about 8μm in length and 2-3μm 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 onedimensional growth [ 31] .

Figure 5 shows SEM images of ZrB₂ powder prepared with 2.5 wt%and 5.0 wt%B additives at 1900℃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 ZrB ₂ powder prepared with 5.0 wt%B is about 2.3μm,which is smaller than 3.6μm of ZrB₂ prepared with 2.5 wt%B.

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 barr_ier 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 nucleiformation.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.

Fig.4 SEM image of ZrB₂ prepared by carbothermal reduction without B additive at 1900℃for 3h

Fig.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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Table 1 ZrB₂ start powder and its hot pressing at 1980℃with70 MPa

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℃and the pressure of 70 MPa.The results are given in Table 1.

As given in Table 1,the relative density of ZrB₂ceramics 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 ZrB2ceramic 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 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)

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] .

Fig.7 TEM image of ZrB₂ ceramic hot-pressed with start powder of No.4

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.

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℃.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 ZrB₂particle size changes from 3.6 to 2.3μm 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 ZrB₂ceramic from getting deep densification.Single-phase ZrB₂ceramic with 93%relative density is obtained by hot pressing using ZrB₂ powder prepared with 5 wt%B additives.