N. Padmanaban’s research while affiliated with Indian Institute of Technology Kharagpur and other places

Rhodium-substituted CuCr2O4, i.e., CuCr2-xRhxO4, spinel oxides were prepared. X-ray analysis showed that single phase spinels were obtained for x = 0 to 0.8, 1.8, and 2.0. The other samples had an additional Cu2Cr2O4 phase. The temperature variation of electrical resistivity for all the single phase samples except CuRh2O4 was similar to that of CuCr2O4 and with the substitution of Cr (3d transition metal) by Rh (4d transition metal) the conduction process did not change gradually from CuCr2O4 type to CuRh2O4 type.

Rhodium-substituted CuCrâOâ, i.e., CuCr{sub 2-x}RhâOâ, spinel oxides were prepared. X-ray analysis showed that single phase spinels were obtained for x = 0 to 0.8, 1.8, and 2.0. The other samples had an additional CuâCrâOâ phase. The temperature variation of electrical resistivity for all the single phase samples except CuRhâOâ was similar to that of CuCrâOâ and with the substitution of Cr (3d transition metal) by Rh (4d transition metal) the conduction process did not change gradually from CuCrâOâ type to CuRhâOâ type.

Thermoelectric power of the solid solution system CuCr2-xAlxO4 for x = 0-2 was determined between 300 and 673 K. CuCr2O4 shows p-type conduction and for x > 0.08 it changes to n-type. The results show that this is due to the presence of Cu1+tet ions produced during the redistribution of cations in the spinel lattice when aluminum is substituted for chromium ions. In the p-type samples charge hopping is between Cr4+oct and Cr3+oct and in the n-type, between Cu2+tet and Cu1+tet. The samples show n-type or p-type behavior depending on the dominant charge carrier present at a particular temperature.

Thermal studies on aluminium substituted copper chromite spinel oxide, CuCr2−x Alx O4 (x=0.2 tox=0.8) by DSC show that with 20% replacement of Cr by Al, the phase transition temperature of the spinel is lowered but on further increasing the percentage of Al the phase transition temperature becomes higher than that of CuCr2O4. The enthalpy change for phase transition however decreases gradually with increasingx.

Thermoelectric power of the solid solution system CuCr{sub 2-x}AlâOâ for x = 0-2 was determined between 300 and 673 K. CuCrâOâ shows p-type conduction and for x > 0.08 it changes to n-type. The results show that this is due to the presence of Cu{sub tet}{sup 1+} ions produced during the redistribution of cations in the spinel lattice when aluminum is substituted for chromium ions. In the p-type samples charge hopping is between Cr{sub oct}{sup 4+} and Cr{sub oct}{sup 3+} and in the n-type, between Cu{sub tet}{sup 2+} and Cu{sub tet}{sup 1+}. The samples show n-type or p-type behavior depending on the dominant charge carrier present at a particular temperature.