Thermodynamic stability of VO2

in contact with thin metal films

Thermodynamic stability of VO2

in contact with thin metal films

N F Thabezhe (University of Zululand) N F Thabezhe (University of Zululand)

Energy Postgraduate Conference 2013

Introduction• Vanadium Oxide compounds (VO2, V2O3, V2O5,

V6O13 etc ) undergo a first order phase transition if their temperature is raised from below to above their transition temperature.

• This transition is accompanied by variations in their electrical, magnetic and optical transmittance

• VO2 has a transition temperature closer to room temperature

Introduction• At room temperature it is a semiconductor

• At higher temperature (~68C) changes to metallic state.

• Changes from the low temperature monoclinic crystal structure to the high temperature tetragonal-type lattice

Experimental Procedure• VO2 thin films were deposited by means of

an rf-inverted cylindrical magnetron sputtering system on chemically cleaned glass substrates

• The metals (Co, Hf, Ni, Pd and Pt) were deposited on top of VO2 by means of an electron beam evaporator system (shown below) in vacuum of better than 4x10-8 kPa

• Annealing was done in vacuum of better than 2x10-8 kPa for times ranging from 45 min to 1 h.

Results• RBS spectra of

glass/VO2(4500 Å)/Hf(3650 Å) samples as-deposited and annealed for 45 min.

Results• X-ray diffraction

spectra for samples with structure glass/VO2/Hf after being annealed for 45 minutes

Results• RBS spectra of

samples with structure glass/VO2/Pt. RBS shows that there is no reaction between Pt and VO2 even after annealing the samples at 700 oC for 1 hour.

Results

Metal Electronegativity

Parameter (V) ΔHR (kJ(mol at.)-1 Chemical

reaction ? Hf 3.60 -96.85 Yes Co 5.10 +22.30 to +50.50 No Ni 5.20 +12.20 to +78.3 No Pd 5.45 +51.40 to +174.40 No Pt 5.65 + 50.40 to +119.50 No

Conclusion• No interfacial reactions could be detected between

metals ( Co, Ni, Pd, Pt ) and VO2.

• In the case of Hf on VO2 it was found a reaction occurs at about 400 oC producing HfV2 and HfO2.

• In all cases investigated it was found that a reaction happens when the heat of reaction between the reactants is negative and none occurs when it is positive.

• Metals with an electronegativity value of less than 4.9 V were found to react with VO2 while those with a value greater than 4.9 V do not react.

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