Qunfei Zhou

Interplay of composition, structure, and electron density of states in W-Os cathode materials and relationship with thermionic emission

The presence and composition of W-Os alloys have been found to significantly affect the thermionic emission properties of Os-coated tungsten dispenser cathodes. However, the comprehensive understanding of structure–property relationships needed to design improved tungsten cathodes with larger thermionic emission is still lacking. In this study, composition–structure–property relationships governing thermionic emission from W-Os alloys were investigated using quantum mechanical calculations. Low-energy W-Os atomic configurations at various compositions were determined from first-principles calculations based on density functional theory in combination with cluster expansion calculations. Electronic properties were investigated in terms of the electron density of states. The relative position of the Fermi level with respect to peaks and pseudogaps in the density of states for different W-Os structures can be used to explain, at least in part, observed variations in thermionic emission from Os-coated tungsten dispenser cathodes.The presence and composition of W-Os alloys have been found to significantly affect the thermionic emission properties of Os-coated tungsten dispenser cathodes. However, the comprehensive understanding of structure–property relationships needed to design improved tungsten cathodes with larger thermionic emission is still lacking. In this study, composition–structure–property relationships governing thermionic emission from W-Os alloys were investigated using quantum mechanical calculations. Low-energy W-Os atomic configurations at various compositions were determined from first-principles calculations based on density functional theory in combination with cluster expansion calculations. Electronic properties were investigated in terms of the electron density of states. The relative position of the Fermi level with respect to peaks and pseudogaps in the density of states for different W-Os structures can be used to explain, at least in part, observed variations in thermionic emission from Os-coated tungste...

Quantum mechanical investigation of thermionic emission from Os-coated tungsten dispenser cathodes

Understanding the underlying mechanism of thermionic emission is essential for the design of next-generation high-power vacuum electron devices. Previous research mainly focused on the surface work function but few studies on structure/properties of the bulk cathode materials. In this study, we employ quantum mechanical methods to investigate the structure-property relationships governing thermionic emission of Os-coated tungsten dispenser cathodes. We developed a new model to calculate directly the thermionic current densities by incorporating the full electronic density of states of the cathode materials and the calculated work functions of surfaces with Ba adsorption (as in the case of cathode surfaces during operation). This work shows the relative effects of bulk and surface properties on the thermionic emission of Os-coated tungsten thermionic cathodes. The methods from this work can be used for screening/discovery of next-generation thermionic emitters.

Stable structures and electron density of states of W-Os alloys for dispenser cathodes

Os-W alloys form during the operation of osmium-coated tungsten dispenser cathodes due to the diffusion of tungsten into the coatings. The stable atomic arrangements of Os-W binary alloys with different W concentrations, in both the hcp lattice and bcc lattice, are obtained by combining first-principles calculations based on density functional Theory (DFT) and cluster expansion. The DFT calculations are performed using the Vienna ab initio simulation package (VASP). The density of states of the obtained stable W-Os alloys is obtained by DFT calculations and is used to discuss the electron emission properties of the corresponding Os-W alloys in a tungsten dispenser cathode.