Synthesis of Nanocrystalline Electron Emissive Materials with Homogenous Composition in Nanoscale

Abstract

In this research, specific stoichiometric mixture of nanocrystalline barium carbonate, calcium carbonate, and aluminum oxide was fabricated by using the sol-gel combustion method. This mixture is used as emissive materials on impregnated cathodes which make up high power vacuum microwave tubes such as klystrons. Composition uniformity and particle size of emissive materials affect the electron emission properties of impregnated cathodes. In this study, dry gel and heat treated powder at 500 °C are investigated by thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The results show that dry gel combusts at 260 °C , and that during combustion transforms from an amorphous phase to a nanocrystalline phase. The average crystallite size of the powders which is calculated by Debye-Scherrer's method based on the XRD patterns is under 45 nm. The results of SEM and EDX of powder after heat treatment at 500 °C show that the particle size of powder is under 100 nm, and the chemical composition of emissive material is uniform at nanoscale. technique and ball-milling method. They reported that the emission properties of cathode synthesized by sol-gel technique were better than that of cathode produced through ball-milling method. They claimed that small particles and more uniform mixing bring about good emission capability of scandate cathode synthesized by sol-gel technique. Generally, the sol-gel combustion method is a novel technique for the production of crystalline and high purity powders. Sol-gel combustion technique is based on a chemical sol-gel and the subsequent combustion process. During the sol-gel process, an aqueous solution containing salts of the desired metal and an organic fuel form a gel. After the gel obtained, ignition occurs in the gel due to combustion process. The product of this step is voluminous and fluffy with a large surface area. This technique is characterized by inexpensive raw materials, low temperature synthesis, and obtaining nanocrystalline powder with homogenous composition (9). Studies have shown that using emissive materials with uniform composition and crystallite size at nanoscale improve emission performance of cathode. Wang et al. (10) fabricated oxide cathode using nano emissive materials which is produced by liquid-gas method. They reported that the emission current density of oxide cathode used nano emissive materials is significantly higher than that of the conventional oxide cathode under the same condition. The main goals of this work were to synthesize appropriate mixture of emissive materials by sol-gel combustion technique and investigation of crystalline structure and the composition uniformity of synthesized powder.