Enhancing the injected electron concentration and thermionic emission property in C12A7:e⁻ ceramic via Al site Zr doping

Abstract

Ca 24 Al 28 O 64 ] 4+ (4e − ) (C12A7:e − ) has excellent potential in electric propulsion owing to various advantages, including low work function, low working temperature, and stable chemical properties. However, increasing electron concentration enhances the thermionic emission property of C12A7:e − . This study used a chemical method-spark plasma sintering to synthesize Zr-doped C12A7:e − (C12(A 1 − x Z x )7:e − , where x = 0 to 0.14). The study results show that C12(Al 0.86 Zr 0.14 )7:e − maximum injected electron concentration (n), conductivity (σ), electron mobility (μ n ), and zero-field emission current density (J o ) improved to 2.4 × 10 21 /cm 3 , 1507 S/cm, 5.2 cm 2 /V⋅s, and 2.4 A/cm 2 , respectively. The emission current density of the C12(Al 0.86 Zr 0.14 )7:e − could be maintained at about 2.9 A/cm 2 under 1100 • C, 4000 V, and 600 min. Theoretical calculation results prove that Zr doping broadens the cage conduction band (CCB) of C12A7:e − and reduces the bandgap between the CCB and the frame conduction band. In addition, Zr doping increases the electronic density of states near the Fermi level, and the density of states presents a continuous state. This helps to increase the injected electron concentration and enhance the electrical transport and thermionic emission characteristics of the sample. The calculation results provide theoretical support for the experiment.

Fabrication and electron emission of [Ca24Al24.08Zr3.92O64]4+(4e−) electrides

“…3 This preliminarily indicated that the [Ca 24 Al 24.08 Zr 3.92 O 64 ] 4+ (4e − ) sample with good compactness was successfully synthesized in one step by SPS sintering. 3,5 In addition, the color of the synthesized block is metallic black, which is caused by the transition of electrons in the block. This preliminarily indicates that the injection of high-density free electrons is achieved in the synthetic block.…”

“…[Ca 24 Al 24.08 Zr 3.92 O 64 ] 4+ (4e − ) electrides are a low work functions(∼2.4 eV) material that can store high-density (∼10 21 cm −3 ) free electrons. [1][2][3][4][5][6][7][8][9][10] Its crystal structure is composed of high-density (∼10 21 cm −3 ) subnanometer cage cavities and there are electron transmission channels between cage cavities (as shown in Figure 1). [2][3][4][5][6][7][8] This cage structure with electronic storage enables [Ca 24 Al 24.08 Zr 3.92 O 64 ] 4+ (4e − ) to have good physical and chemical stability, low work function (∼2.4 eV), [16][17][18][19][20][21][22][23][24][25][26][27][28] and outstanding electron emission potential.…”

“…[3][4][5][6][7][8][9]20 and the thruster can work for a long time under microthrust conditions. 3,20 These potential applications of [Ca 24 Al 24.08 Zr 3.92 O 64 ] 4+ (4e − ) have attracted extensive attention. 20 At present, the commonly used synthesis methods of [Ca 24 Al 24.08 Zr 3.92 O 64 ] 4+ (4e − ) include chemical reaction synthesis, graphite crucible reduction, active atmosphere reduction (CO, etc.…”

Identifying different electronic transport mechanisms in nanoporous inorganic electrides – a combined study using Hall measurements and electron paramagnetic resonance spectroscopy

“…Such a consistent description of the transport properties is necessary to evaluate current doping strategies. 26,27 For example, the isovalent substitution of Ca 2+ by Sr 2+ or Bi 2+ is proposed in order to widen the cages and facilitate hopping transport between the cages. 28,29 Another alternative is substituting Al 3+ by Zr 4+ in the cage framework in order to further increase the electron density.…”

“…28,29 Another alternative is substituting Al 3+ by Zr 4+ in the cage framework in order to further increase the electron density. 27 Predictions of the impact of doping on the electronic structures of C12A7 and C12A7 : 2e À and their actual electronic transport properties by ab-initio approaches are extremely difficult. Supplementing ab initio calculations of the electronic structure of doped mayenite or doped mayenite-electride by a comparison of experimental data with empirical transport models, which correlate transport mechanisms with electronic structures (e.g., with the energy levels and the effective densities of states), helps bridging the gap between experimental observations and ab initio predictions.…”