Firse，Introduction

       Rare earth hexaboride cathode material is a substitute for various kinds of cathodes, and is a functional material with excellent electron emission performance. For example, the LaB6 cathode, compared with the general cathode, has many advantages such as low electron escape work, strong ion bombardment resistance, large emission current density, stable performance, good anti-toxicity, long service life, etc. Previously, it was mainly used in atomic energy, aviation industry and other cutting-edge technology departments, such as scanning electron microscopy, electron beam exposure machine, electron probe and other equipment. Now it has been successfully applied in the civil industry such as automobiles, motors, electronics, and instruments, and its share far exceeds the application in the cutting-edge technology sector, and also promotes the progress of production technology and the improvement of economic benefits of related enterprises. The current electron beam technology and equipment, on the one hand, towards the development of microelectron beams, beams are becoming more and more subtle, such as in the semiconductor industry as a non-thermal tool on the wafer anti-corrosion coating exposure, engraving circuit graphics, and manufacturing large-scale integrated circuits; On the other hand, it is developing in the direction of high power. As a radiation source, it is mainly used for large-scale irradiation and disinfection, or as a heat source for melting and drawing large metal ingot, welding ultra-thick parts, large-area coating, etc., so as to promote the development and performance improvement of electron beam and ion beam instruments and equipment.

       Many researchers have devoted their whole life to studying the preparation process, properties, structure and physical and chemical properties of RB6, and have achieved phased results [1]. Among them, rare earth hexaboride, due to the difference of rare earth elements, shows more abundant physical properties. For example, LaB6, which has a very high field emission efficiency, can be used as a superconductor YB6, a valence fluctuation system SmB6, and a narrow-band semiconductor EuB6 with abnormal magnetism and a non-magnetic narrow-band semiconductor YbB6. These rare earth hexaborides have very important applications in many fields.

Second，RB6 structure and performance

       The type structure of the rare earth hexaboride, RB6, is shown in the figure below, in a simple cubic structure of octahedrons, the metal atoms are located in the center of the cube, where the boron atoms are stacked into octahedrons, respectively at each vertex [2].

       RB6 has a cubic crystal structure, as shown in Figure 1. As can be seen from the figure above, each boron atom has five bonded adjacent atoms, that is, the three valence electrons of each boron atom are allocated to five non-polar bonds, and the large metal atoms are surrounded by small boron atoms forming a stable three-dimensional framework. There is no valence bond between the metal atom and the boron atom, so the valence electrons of the metal atom are free, which makes the hexaboride have the electrical conductivity of the metal [3], its resistivity is very low, and the resistivity is proportional to the temperature coefficient. At the same time, although the diameters of various metal atoms and ions are different, the lattice constants of their hexaborides are very similar. The frame of boron is an octahedron, each vertex of the octahedron has an octahedron formed by the frame of boron atoms, and the octahedra are connected with each other by vertices. Because the bond between boron atoms is very tight, this kind of boride has the property of melting resistance, and the melting point is very close.

       Their chemical properties are quite stable, at room temperature only react with aqua regia and nitric acid, generally do not interact with water and oxygen, only at 600~700 ° C they will be oxidized. In vacuum, RB6 and the residual gas interact at high temperatures, and the melting point of the generated compounds is relatively low. At the working temperature, these products are continuously evaporated, exposing the almost pure RB6 low escape work surface to the emission surface, which makes RB6 particularly strong anti-poisoning ability. In addition, the boron frame structure makes the thermal conductivity and ion bombardment resistance of the material good, and has the ability to automatically adjust the evaporation of RB6. When the surface is vacant due to the evaporation of the metal RB6, the internal RB6 atoms will be diffused, so that the boron frame does not evaporate and remains intact, so that the emission surface of the RB6 cathode always remains pure, thus prolonging the service life of RB6.

Third，Synthesis method of RB6

1. Preparation method of RB6 powder

(1) Boron thermal reduction reaction method

This method is to use pure boron to reduce RB6 compounds to prepare RB6 powder, and there are three commonly used equations [4]

 La₂(C0₃)₃+18B=2LaB₆+3B₂0₂+3CO

            2La₂0₃+30B=4LaB₆+3B₂0₂ 

            2La₂S₃+30B=4LaB₆+3B₂S₂

       RB6 with high purity can be prepared by this method, but the disadvantage is that the price of pure boron powder is relatively expensive, and it is not suitable for industrial large-scale production.

(2) molten salt electrolytic synthesis method

       Panwen Shen et al. [5] from Nankai University synthesized single-phase RB6 by electrolysis of ReB03-LiB02-LiF molten salt system at 800℃ and in air. The use of this method requires a low temperature, in contrast, the temperature of boron thermal reduction synthesis of RB6 is as high as 1650℃, and the reaction can be achieved in the air, but the ratio of raw materials is very strict, and low efficiency, the process is also more complex, so it is rarely used.

(3) Self-propagating high temperature synthesis (SHS)

       Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis, is a method that uses the heat release of chemical reactions between substances to make the reaction spontaneous until the end, and synthesizes the required materials in a very short time. The advantages of this method are high synthesis temperature, low energy consumption and fast reaction process, which is suitable for the synthesis of high melting point materials. On the basis of analyzing and synthesizing a large number of literatures and many years of research work, Dr. Zhang Tingan [6] from Northeastern University systematically studied the preparation of LaB6 micropowders by self-propagating metallurgical method. The reaction system is:

La₂0₃+6B₂0₃+21Mg=2LaB₆+21MgO

       The disadvantage of using self-propagating high-temperature synthesis reaction to prepare RB6 powder is that the combustion rate and reaction process are difficult to control, and MgO needs to be removed, so it is still difficult to apply in production.

(4) carbothermal reduction reaction method

       According to the different reactant raw materials, the chemical reaction equation is as follows [7] :

           La₂0₃+3B₄C=2LaB₆+3CO  

           La₂0₃+12B+3C=2LaB₆+3CO

           La₂0₃+6B₂0₃+21C=2LaB₆+21CO

       After the raw material is mixed evenly in a certain proportion, it is pre-pressed, and different synthesis processes are used to prepare RB6 powder according to different reactions in hydrogen or vacuum. The advantages of preparing RB6 powder by carbothermal reduction method are simple and low cost, but the outstanding disadvantages are that the reaction is not complete, the carbon content in the product is relatively high, and the product particles are coarse.

2. Preparation method of RB6 single crystal

       Single crystal RB6 has a wide range of uses, and its service life is longer than that of polycrystalline materials. At present, there are the following four methods that have been studied most at home and abroad [8] : flux method, zone melting method, molten salt electrolysis method and vapor deposition method.

       (1) Flux method

       Flux method is one of the basic methods for preparing RB6 single crystals, including aluminum flux method and rare earth flux method. The two flux methods are similar, except that rare earth elements are used instead of aluminum, as shown in Figure 2 [9]. The aluminum flux method is characterized by simple equipment and process and convenient operation, but the prepared RB6 single crystal size is relatively small, the impurity content is high, the presence of impurity Al cannot be avoided, and the production efficiency is low, and it is only suitable for the production of small needle-like RB6 cathode.

       (2) zone melting method

       Zone melting is one of the most common methods for preparing RB6 single crystals. The quality of RB6 single crystals obtained by zone melting is closely related to the composition of raw materials, the content of impurities, the protective atmosphere used and the growth process. The main heating methods to melt the zone are: electron beam heating, radio frequency heating, arc heating and laser heating. FIG. 2 shows the schematic diagram of LaB6 single crystal prepared by zone-melting method [10]. Zone melting method is characterized by high production efficiency, can prepare large size RB6 single crystal, and can obtain high purity, good quality RB6 single crystal. However, the zone melting method has high requirements on the zone melting equipment, and it is difficult to control the technology in the process of zone melting. At present, only Ukraine has this mature technology for the preparation of RB6 single crystals by zone melting.

       In addition, RB6 single crystal obtained by molten salt electrolysis method has low purity and is easy to contain other impurities, so it is commonly used to prepare small size RB6 single crystal with low purity. The gas phase precipitation method is mainly used for the preparation of single crystal films, and can also improve the electron emission properties of other cathode materials.

3. Preparation method of RB6 polycrystal

       The process of sintering is the densification - grain growth - grain boundary formation of ceramic powder at high temperature. The sintering quality of RB6 polycrystal directly affects its properties. Therefore, the most critical step in the preparation of RB6 polycrystalline bulk materials is the sintering of RB6 polycrystalline powder.

       (1) Hot press sintering

       Hot pressing sintering is one of the most common sintering methods.

       The so-called hot pressing sintering is a sintering method in which a certain external pressure is added at the same time of sintering. Because this method is simple, low cost and mature technology, it has been widely used. However, the sintering temperature required by the process is higher, the sintering time is longer, the energy consumption is large, and the production efficiency is low, and only samples with less complex shapes can be prepared. If the material is sintered densified, it needs to be prepared in a high temperature and high pressure environment, so the grains are easy to grow.

       Gao Ruilan et al. [11] studied the hot pressing sintering of LaB6 polycrystals and concluded that LaB6 polycrystals with a density of 92% and a bending strength of 110MPa could be obtained under the pressure of 50MPa at about 2100℃ and holding time of 2h. This method has begun to be used in electron beam welders and detachable electronic instruments [12].

       (2) Discharge plasma Sintering (SPS)

       SPS is a new material sintering technology. It is the ceramic powder particles directly through the pulse current heating sintering, and in the sintering process to apply a certain pressure to achieve ceramic densification. This method of direct discharge heating has quite high thermal efficiency, and the dispersion distribution of discharge points can achieve uniform heating, so it is easy to prepare dense, uniform and high-quality sintered bodies [13,14].

       Using SPS to prepare high density and fine grain ceramics not only reduces the sintering temperature and increases the density, but also greatly reduces the sintering time and reduces the difficulty of sintering. Figure 2-5 shows the use of SPS advantages to prepare PrB6 and (LaxPrl-x)B6 bulk ceramic materials by sintering. Compared with the traditional sintering method, SPS has the advantages of short sintering time, uniform heating, low sintering temperature, fast heating rate and high production efficiency. The product structure is small and uniform, which can maintain the natural state of raw materials and obtain high density materials. Therefore, the SPS technology has advantages over the conventional method of sintering ceramic materials [15]. In addition, the energy consumed by SPS is only 1/3-1/5 of hot pressing or hot isostatic pressing, so it is still a new technology for energy-saving and environmentally friendly material preparation.

       At present, many first-class universities and scientific research institutions in the world have purchased SPS systems, and in China, Wuhan University of Technology purchased SPS devices for the first time in June 2000, followed by Shanghai Institute of Ceramics, Tsinghua University, Beijing University of Technology and Wuhan University and other schools have purchased SPS devices [16].

Fourth, the application of RB6

       Rare earth hexaboride cathode materials have extremely important applications in many fields, such as the cathode emission material LaB6, which has been put into use, has been widely used in various fields. LaB6 can be made into a high temperature resistant nozzle, which has a wide range of applications in the aerospace field; In the military field, LaB6 single crystal is applied to the cathode of high-power electron tube in radar; In the home appliance industry, it can obtain high brightness and high density current at medium and low temperature, and is used as the kinescope cathode material for plasma ultra-thin TV sets, which has development value [17]. In addition, EuB6 can also be used as a membrane for ion-selective electrodes to determine rare earth ions. RB6 in rare earth borides has the following advantages:

One: The electron escape work is low, and the cathode material with the highest emission current intensity at medium temperature can be obtained, and has constant resistance and good thermal radiation;

Two: ion bombardment resistance is better, can withstand high field strength;

Three: in a certain temperature, the expansion coefficient is close to zero;

Four: The stability in the air is very good, and the surface contamination can be recovered by vacuum heating, and the inert contact with molten metal is good.

       In the RB6 system, LaB6 has the characteristics of high melting point (2713℃), good electrical conductivity, high hardness and good chemical stability. Because of the crystal structure and bonding characteristics of LaB6, it has many special functions, mainly including: low work function (2.66 eV), constant resistance, good thermal radiation resistance; Using LaB6 as the cathode under vacuum or nitrogen atmosphere can obtain a very pure (close to 100%) single boron ion, forming a strong and stable ion source [18], which has broad application prospects in military industry, aerospace and other high-tech fields. At present, LaB6 powder, single crystal and polycrystal materials have been successfully prepared. Internationally, Ukraine and other countries have also produced LaB6 single crystals with excellent performance, mastered the corresponding welding, cutting and other technologies, and developed tube, sheet and wire products, whose comprehensive level has been in the international leading position [19]. In recent years, the research of LaB6 single crystals in Japan has also progressed rapidly, and it has been applied to the cathode of various electron microscopes. In addition, the United States has also launched a series of systematic and comprehensive studies on LaB6 single crystals, and its powder series products have entered the commercialization, and the application of polycrystalline sintering and compaction materials for LaB6 has been studied. Since the 1990s, Chinese researchers have also begun to study and discuss the preparation and application of LaB6.

Fifth，Summary and prospect

       As a mature cathode emission material, LaB6 has been successfully applied in various fields. With the continuous expansion of the application fields of ion beam and electron beam, people put forward higher requirements on the performance of cathode materials, they hope to further improve the electron emission density, extend the service life, reduce the working temperature and so on.

       (1) Although the emission performance of single crystal RB6 is better, but because of its single product preparation is difficult, the size of single crystal is limited and expensive, resulting in its maximum diameter can not meet the requirements of the RB6 cathode size used in the accelerator, so for the occasion of requiring large size and large emission surface, only polycrystalline RB6 products can be used. However, the traditional hot-pressing sintering method to prepare RB6 polycrystals can not meet the performance requirements in practical applications. Therefore, it is very important to obtain high density RB6 bulk samples.

       Discharge plasma sintering is one of the best methods to prepare polycrystalline RB6. SPS has the advantages of fast heating rate, low sintering temperature, short holding time and high sintering pressure, thus producing dense fine-grain materials. This method can not only make the sample compact but also keep the grain in a small size range. It is the most suitable method for preparing massive fine grain materials in powder sintering method.

       (2) As the atomic number of rare earth elements continues to increase, the subgrain boundary of RB6 single crystal continues to decrease, resulting in improved crystal performance. As shown in Table 1-1, in all RB6, except LaB6,CeB6,PrB6 and NdB6, the rare earth elements of other RB6 are extremely volatile under heating conditions, which shortens the service life of the cathode.

       In view of this, researchers are trying to use SPS this advanced sintering method to react and prepare single crystal PrB6 and (LaxPrl-x)B6 polycrystalline block cathode, to study the preparation process, microstructure and properties, in order to achieve the purpose of developing cathode materials with high density and good emission properties.