Micromechanical and thermal behaviour of gel grown pure- and sodium-modified copper tartrate crystals

Results regarding micromechanical characteristics of gel grown pure- and sodium-modified copper tartrate crystals, bearing composition CuC₄H₄O₆·3H₂O, (Cu)_0.77(Na)_0.23C₄H₄O₆·3H₂O and (Cu)_0.65(Na)_0.35C₄H₄O₆·H₂O, as obtained on using indentation induced hardness testing technique are reported. Thermal behaviour of these crystals in the temperature ranging from room temperature (∼25 °C) to about 600 °C is also reported. Pure copper tartrate crystals are found to be thermally more stable than the sodium-modified ones. Dependence of Vickers’ hardness number H_v on load ranging from 0.049 to 2.94 N on two different planes for all the three compositions is analyzed. It is shown that after initial rise in the value of H_v, the same achieves saturation at a load of 0.49 N. Modification of copper tartrate crystal by introducing sodium in its lattice brings about a change in the micromechanical characteristics. The saturation value of H_v decreases with increase in the concentration of sodium ions. The results on (0 0 1) and (1 1 1) planes for both pure and modified copper tartrate crystals suggest hardness anisotropy. Relative difference of hardness between the two planes and yield strength for both pure and modified copper tartrate crystals is worked out. The experimental results are analyzed for applicability of Meyer’s law and Proportional Specimen Resistance Model. It is suggested that the experimental results indicating reverse ISE phenomenon may be explained in terms of the existence of a distorted zone near the crystal-medium interface. The integral method of Coats and Redfern approximation applied to the thermoanalytical data suggests “Random Nucleation Model” for the reaction kinetics of these crystals. Non-isothermal kinetic parameters such as activation energy, frequency factor and order of reaction are calculated.