Preparation of Ta2O5 nanoparticles by using cathode glow discharge electrolysis

Tantalum pentoxide nanoparticles (Ta2O5 NPs) were fabricated by cathode glow discharge electrolysis (CGDE) generated between a needle-like platinum wire cathode and a tantalum foil anode in 6 g L−1 Na2SO4 electrolyte solution containing 5 mL hydrofluoric acid (HF) and 0.075 g cetyltrimethyl ammonium bromide (CTAB). The chemical structure, composition and morphology of the obtained powder were analyzed by using XRD, FT-IR, SEM/EDS, XPS and UV-vis DRS. The results found that Ta2O5 NPs with orthorhombic structure and wide band gap (3.6 eV) are successfully fabricated at 500 V discharge voltage in about 3 h. CTAB as a stabilizing agent can reduce the agglomeration due to forming CTA+ and attaching the surface of the synthetic products. A possible preparation mechanism of Ta2O5 NPs is proposed. Firstly, the tantalum foil anode is oxidized to form a compact Ta2O5 layer. Then, Ta2O5 surface is etched to form soluble [TaF7]2− complexes in the presence of HF. After that, soluble [TaF7]2− complexes can react with H2O to form Ta(OH)5. Finally, Ta(OH)5 is further converted to Ta2O5 from plasma-liquid interface into solution.

Conditions of steady arc ignition in vacuum and hollow cathode operational life

Purpose: The aim of this paper is to determine the conditions of steady arc ignition invacuum and the effect some of the main operating parameters have on the life of tantalumfoilhollow cathodes.Design/methodology/approach: The experiments presented in this paper have beencarried out with equipment for vacuum hollow cathode arc processing similar to whatis used in the industry. In order to find out if steady arc ignition is possible, a two-levelfactorial experiment has been conducted helping to establish the corresponding regressiondependency between the factors examined and the starting parameter values. The evaluationof hollow cathode operational life is based on three criteria: indirectly, considering their erosion[μ]; establishing possible damage on their surface which leads to plasma-forming gas leakageand makes the arc unsteady; taking into account the critical change in the cathode shape.Findings: It has been established that when a multi-stage arc torch is using the necessaryminimum amount of the plasma-forming gas for Ø 3.5-mm-diameter hollow cathodes isQAr=0/3 l/h, and, for Ø 6.0-mm-diameter hollow cathodes, it is QAr=2.4 l/h. It has beenestablished that the operational life of tantalum hollow cathodes can be and even exceed3 hours. Of all the parameters that have been studied, vacuum level has the most negativeeffect. It has been confirmed that tantalum-foil hollow cathodes are suitable mainly forcurrent intensity values of about 120 A.Practical implications: The results of the research allow: guaranteed hollow cathode arcignition regardless of its diameter at the working levels of plasma forming gas; the choiceof operating modes ensures the implementation of processes of varying lengths without theneed for premature replacement of the hollow cathode.Originality/value: This paper presents the results showing the conditions necessary forsteady arc ignition in vacuum with a hollow cathode of the following diameters: Ø 3.5 mmand Ø 6 mm. The effect of some of the main working parameters on the operational life ofhollow cathodes made of tantalum foil is also studied

Morphological and Optical Stability in Growth of Fluorescent SiC on Low Off-Axis Substrates

Fluorescent silicon carbide was grown using the fast sublimation growth process on low off-axis 6H-SiC substrates. In this case, the morphology of the epilayer and the incorporation of dopants are influenced by the Si/C ratio. Differently converted tantalum foils were introduced into the growth cell in order to change vapor phase stochiometry during the growth. Fluorescent SiC grown using fresh and fully converted tantalum foils contained morphological instabilities leading to lower room temperature photoluminescence intensity while an improved morphology and optical stability was achieved with partly converted tantalum foil. This work reflects the importance of considering the use of Ta foil in sublimation epitaxy regarding the morphological and optical stability in fluorescent silicon carbide.