Chemical reactions between vanadium oxides and carbon during high energy ball milling

Abstract

It has been well established that high energy ball milling of powder or powder mixtures can signi®cantly accelerate chemical reactions between two solid phases or between a solid phase and a liquid or gaseous phase [1±5], making it possible for the reactions to occur at low temperatures. It appears that the mechanical process of ball milling has little effect on the driving force for the reactions, and that the change of free energy of the system as a result of the reactions can be reasonably estimated by using available thermodynamic data [6]. It is evident that ball milling accelerates the kinetics of chemical reactions through repeated creation of fresh interfaces between reacting phases by dynamic deformation, fracturing and cold welding of the solid particles [7, 8]. If the rate of heat release by the reactions which occur during and=or immediately after each collision event is high enough to heat the powder adjacent to the collision point to a temperature higher than the ignition temperature of a combustion reaction, the combustion reaction occurs during ball milling. Many cases of combustion reactions ignited by high energy ball milling have been reported [e.g., 2, 9±11]. Reduction of one of the vanadium oxides, V2O5, to metallic vanadium by Mg, Al or Ti during high energy ball milling has been studied by Yang and McCormick [12]. It was observed that combustion reactions were ignited within 10 min of ball milling. This shows that high energy ball milling is very effective in activating chemical reactions between vanadium oxides and an active metal. In this letter, we report the ®ndings from an investigation into the chemical reactions between vanadium oxides and carbon activated by high energy ball milling. When metal oxides react with carbon, one of the reaction products is CO, or CO2, which is gaseous and readily separated from the solid reaction products. This means that use of carbon as a reducing agent can eliminate the need for an additional process to separate the reaction products which might be mixed to a very ®ne scale by ball milling. Four stainless steel balls (12.5 mm in diameter) and the powder charge, which is a mixture of 99.7% pure V2O5 powder (average particle size: 50 im) and 99% pure graphite powder (average particle size: 50 im) were placed in a hardened steel vial which was then sealed in a glove-box under an argon atmosphere. The ball milling was performed by shaking the sealed vial at a high speed using a SPEX 8000 mixer=mill. The ratio between the weight of V2O5 and graphite was determined according to the following overall reaction and with 10% of excessive graphite: 2V2O5 ‡ 5C! 4V‡ 5CO2