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The[Ca24Al28O64]4+:4e- (C12A7:e-) electride composed of densely packed, subnanometer-sized cages. This unique structure makes it possess distinctive applications in fields of electronic emission, superconductor, electrochemical reaction. In this paper, we explore a new method to prepare the bulk of C12A7:e- electride. The following areare systematically studied in this work. 1) the condition of preparing bulk of C12A7:e- electride by solid reaction combining spark plasma sintering and reduction with Ti particles at high temperature, CaCO3 and Al2O3 powders are used as raw materials; 2) the first principle calculations of band structure and density of states of the C12A7:e- electride; 3) the analysis of the electrical transport properties of the C12A7:e- electride. The bulk of C12A7:e- electride is successfully prepared by this method, so the results show that the bulk of C12A7:e- electrode with the electron concentration 1018-1020 cm-3 is synthesized at 1100 ℃ and a vacuum pressure of 10-5 Pa for 10-30 h. In the process of Ti reduction, Ti particles become evaporated and deposit on the surface of C12A7, the free O2- atom in the cages diffuse to the sample surface, the Ti vapor reacts with the O2-, forming a loose TiO_x layer. In order to maintain electrical neutrality, the electrons of the free O2- atom leave from the cages, forming the C12A7:e- electride. In addition, the loose TiO_x layer also provides a channel for the diffusion of the O2- atoms in the cage, ensuring the continuation of the reduction reaction. The calculated band structure and density of states of the bulk C12A7:e- electride show that when electrons replace the O2- atoms in the cage, the Fermi level of C12A7:e- crosses over the cage conduction band (CCB). Thus the free movement of the electron is the main reason for the insulator C12A7 to convert into conductor C12A7:e-. At the same time the electrons near the Fermi level in the cages are easy to jump from the CCB to the frame conduction band (FCB). Combination of the above experimental results suggests that the electrons in cages are easier to escape to vacuum under the action of electric field or thermal field, which is the main reason for low work function of C12A7:e-. This way provides an new approach to the realization of the insulator C12A7 converting into C12A7:e- electride. And the C12A7:e- is a good electronic emission material due to low work function, low working temperature, and highly anti-poisoning ability, so this method of preparing bulk C12A7:e- electride provides a good new way to synthesize a new electronic emission material.

The electron emission properties of lanthanum hexaboride (LaB6) have received much attention because its low work function, low volatility, high brightness, thermal stability and high mechanical strength. However, single crystal LaB6 is an ideal thermionic emission and field emission cathode material, its different crystal surfaces exhibit different emission properties. So far the physical factors of the emission properties of different crystal surfaces of LaB6 single crystal have been rarely reported. In this paper, the density function theory based first-principles calculations are used to analyze the electron density differences, band structures and densities of states of the typical LaB6 (100), (110), (111), (210), (211) and (310) surfaces, and the thermionic emission properties of the high-quality single crystal LaB6 typical surfaces are tested. The theoretical calculation results show that single crystal LaB6 has metal properties, electron emission characteristics and anisotropy of emission performance which are mainly caused by different crystal structures and electronic structures of LaB6 typical surfaces. The densities of La atoms in different surfaces of LaB6 single crystal are different, and a high density of La atoms in a surface is beneficial to its emission performance. The difference between relative positions for the Fermi level of different surfaces has different effect on their emission performance, and a surface with high position of Fermi level against the bottom of conduction band could have small work function and good emission performance. In addition, a surface structure of single crystal LaB6 has a large density of states and a high number of distributions of conduction band near the Fermi level, which are in favor of its electron emission. The (100) surface of single crystal LaB6 with the highest density of La atoms and electronic structure in favor of electron emission could have optimal electron emission performance compared with the remaining crystal surfaces. Thermionic emission test results show that maximum emission current densities of the (100), (110), (111), (210), (211) and (310) surfaces are 42.4, 36.4, 18.4, 32.5, 30.5 and 32.2 A/cm2 at the cathode temperature 1773 K and the voltage 1 kV. The (100) surface of LaB6 single crystal has a maximum emission current density under the same test condition, meaning that this surface has a smallest work function and best emission property compared with the other crystal surface. The thermionic emission test results show that the actual performances are basically accordant with the calculated results, demonstrating that the first principle calculation could provide a good theoretical guidance for studying the electron emission properties of rare earth hexaborides (REB6) and other cathode materials.