Oxidation of Copper


Introduction

The aim of this experiment is to determine the kinetics of oxidation of copper in air and to establish the temperature dependence of the rate constant for this process. A simple model of oxidation will be developed in which a number of processes can be rate controlling. Using the experimental results an appropriate physical model of the oxidation process will be selected and the kinetic data related to materials properties.

Experimental method

Samples of copper in the form of thin ~25mm square pieces are provided. Each sample should be “pickled” to remove any pre-existing surface oxide, the surface area dimensions measured and the sample weighed. Make a note of the pickling solution and conditions.

A total of 12 samples are required for each temperature at which the oxidation experiment is to be carried out. Each laboratory section is responsible for determining the oxidation kinetics at 3 or 4 temperatures between 600°C and 900°C.

Section A. 400,500,600,700°C
Section B. 450,55,650,750°C

Samples should be arranged on the wire racks and placed in the furnace. Samples should be removed from the furnace after 5 minutes, then 10 minutes, then every 10 minutes for the first hour and then every 20 minutes, and re-weighed.

Plot a graph of the change in weight per unit area of surface versus time1/2 at the temperature at which you carried out the experiment. Remember that there is a point at zero time (zero weight gain) and that the surface area = 2 x (widthxlength + widthxthickness + lengthxthickness)

Analysis

The thickness of oxide (h) usually varies with time (t) according to a power law



where k is a temperature dependent rate term described by



where Q is the activation energy for the process, R the gas constant and T the absolute temperature. From your measurements of weight gain determine the rate constant at each temperature together with n the rate exponent. Using all the rate constants, k, determined by the class plot a graph that will enable the activation energy to be determined. The graph is Ln(k) vs. 1/T where T is the absolute temperature; the slope of a regression analysis through all the data points (one rate constant per temperature) is -Q/R where Q is the activation energy and R is the gas constant (R=8.3144 J/mol). Does the activation energy correspond to a particular material property?


Determination of Oxide Chemistry

Copper, when oxidized can form either of two oxides, Cu2O and CuO. Collect the powdered oxide from the samples, and use the X-ray diffractometer to identify the oxide. If both oxides are present then estimate the volume fraction of each. If possible determine the lattice parameters. Estimate the maximum thickness of the oxide for your series of specimens.


Written work

This laboratory will be graded based on (a) the presentation of the experimental data (weight gain vs. time) on a properly annotated graph complete with error analysis - you might want to draw this graph by hand given the inadequacies of templegraph! (b) a presentation of the graph used to determine the activation energy - properly annotated - together with an estimate of the precision of your answer; and (c) a written section describing the experimental procedure.

Your write up should consists of the following sections

  1. An experimental procedure
  2. A results section containing the following:-
    • A table showing the wt gain per unit area vs time for all the samples evaluated
    • A graph showing wt gain per unit area vs time or some other measure of time from which the kinetics can be determined- choose 4 sets of data separated by 100°C for inclusion on the graph as examples
    • A table of rate constants vs temperature
    • A graph from which the activation energy can be determined
    • A narrative that ties the results together and explains what is what
    • Identification of the oxide.
    • An appendix containing a spread sheet that clearly shows how you got from the measurements you actually made to the weight gain per unit area for the oxidation temperature that you were personally responsible for.
  3. A discussion section in which the measured value of activation energy, along with the error in activation energy are compared to the published value for the oxide
  4. Compare the measured lattice parameter/crystal structure with that of oxide obtained from the literature.