Eliminating carbon contamination with improved one-step electrochemical preparation of refractory high-entropy alloy TiZrHfNbTa

Creating refractory high-entropy alloys(RHEAs)effectively without impurities is a momentous challenge.Conventional methods for the preparation of RHEAs have exacting requirements for raw material purity and energy consumption.Molten salt electrolytic oxides are applied to the preparation of HEAs by virtue of their low cost and high efficiency.However,the use of graphite anodes in electrolysis will result in the carbon contamination of the products due to the deposition of CO_(3)^(2−)at the cathode.Increasing the reaction temperature accelerates the deoxidation of the oxide,thus reducing the amount of carbide but not eliminating it.Switching HfH_(2)instead of HfO_(2)and using Nb_(2)O_(5)to shield the precursor can effectively remove carbon contamination.However,this leads to a complicated preparation process and energy waste.Herein,we use the solid oxygen ion-conducting membrane(SOM)containing graphite and Sn(named as SOM@C/Sn)anode to separate the carbon from the molten salt and prevent the circulation of CO_(3)^(2−).The pure single-phase TiZrHfNbTa RHEA can be produced in only one-step with the SOM@C/Sn anode,which completely solves the problem of carbon contamination in the molten salt electrolysis preparation of HEAs.This work provides a feasible solution for the preparation of novel complex alloys such as carbon-free RHEAs in a low-cost short process.

Influence of growth rate on the carbon contamination and luminescence of GaN grown on silicon

The unintentional carbon doping concentration of GaN films grown by low pressure metal organic chemical vapor deposition （LP-MOCVD） depends strongly on the growth rate. The concentration of carbon is varied from 2.9 × 1017 to 5.7 × 10^18 cm-3 when the growth rate increases from 2.0 to 7.2 μm/h, as detected by secondary ion mass spectroscopy. It is shown that the presence of N vacancies give rises to high carbon concentration. We show that a reduction of the carbon concentration by one order of magnitude compared to the regular sample with nearly same growth rate can be achieved by operating at an extremely high NH3 partial pressure during growth. The intensity ratios of yellow and blue luminescence to band edge luminescence in the samples are found to depend significantly on carbon concentration. The present results demonstrate direct and quantitative evidence that the carbon related defects are the origin of yellow and blue luminescence.

Enhanced electrodeposition and separation of metallic Cr from soluble K2CrO4 on a liquid Zn cathode

Carbon contamination and the formation of low-valence oxides limit the preparation of refractory metals by molten salt electrolysis.In this paper,a liquid Zn cathode is adopted for the electrochemical reduction of soluble K2CrO4 to metallic Cr in CaCl2-KCl molten salt.It is found that CrO4^2-can be directly electrochemically reduced to Cr via a six-electron-transfer step and low-valence Cr oxides is hardly produced.The reduction rate is obviously increased from 16.7 mgCrh^-1cm^-2 on the solid Mo cathode to58.7 mgCrh-1cm-2on liquid Zn cathode.The electrodeposited Cr is distributed in liquid Zn cathode.Carbon contamination is effectively avoided due to the negligible solubility of carbon in the liquid Zn cathode.Furthermore,Cr can be effectively separated and enriched to the bottom of liquid Zn under supergravity field,realizing the efficient acquisition of metallic Cr and recycling of liquid Zn.The method herein provides a promising route for the preparation of refractory metals with high-purity by molten salt electrolysis.