Synthesis of transition metal doped magnesium aluminate nanomaterials and investigation of their properties

Abstract

In the present study, an attempt has been made to improve the electrical, dielectric and thermal properties of magnesium aluminate by doping with the binary mixtures of transition metals: Zn, Cd, Ni and Co and following five series of the doped compounds have been synthesized Mg 1-2x Zn x Ni x Al 2 O 4 , Mg 1-2x Zn x Co x Al 2 O 4 , Mg 1- 2x Zn x Cd x Al 2 O 4 , Mg 1-2x Ni x Co x Al 2 O 4 and Mg 1-2x Ni x Cd x Al 2 O 4 (where x = 0.0-0.5) by a cost effective and an energy efficient urea assisted microwave combustion method. The quantitative analysis shows that the experimental values match with the nominalcompositions and lower Zn content in all the samples is observed due to the volatile nature of Zn. Sharp and high intensity diffraction peaks are obtained with hkl values of (111), (220), (311), (222), (400), (422), (511), (440) (531), (533), (622) and (444) which closely correspond to the standard pattern of spinel MgAl 2 O 4 (ICSD ref. code No. 00-021-1152, a = 8.08 Å, V cell = 528 Å 3 ). The absence of any extra peaks in these patterns indicates that the synthesized samples exhibit a single-phase structure. While in the case of (Zn-Cd)x and the (Ni- Cd)x the pure spinel phase is produced till x = 0.3 while at higher dopant content, the extra peaks in the patterns appear along with the peaks for the spinel phase. The density of the doped samples increases gradually with the increase in the dopant content due to the larger molar mass of the double doped samples compared to the undoped magnesium aluminate The crystallite sizes are found to be in the range of 10-13 nm with the doped samples having remarkably smaller crystallite sizes than the undoped one (47 nm). The bulk density of the doped samples is lower than the undoped ones due to the enhanced porosity. The agglomeration of crystallites produces two types of the regions in electron micrographs, a region of large grains as well as a region of the small grains and the particle size of the samples is found to be within the nano regime. No appreciable changes are seen in the heating and cooling curves in differential thermal analysis (DTA). The samples are found to be thermodynamically stable up to a temperature of 1773 K. The decreasing resistivity with an increase in temperature validates the semiconducting behavior of the samples. However, magnesium aluminate is considered to be a small polaron semiconductor in which energy is required for the mobility of charge carriers. The formation of charged anti- site defects, electron-hole (Al 3+ in A-site) and traps (Mg 2+ in B-site) are responsible for the hopping of electrons in the structure of the ceramic MgAl 2 O 4 material. The resistivity of the doped samples is higher as compared to the pure magnesium aluminate sample. In the transition-metal oxides with incompletely filled 3d shells, the localization of the 3d electrons is responsible for the insulating nature of the doped oxides. In the case of (Zn-Co)x, no d shell electrons are available at Zn 2+ while 3 unpaired electrons are available at Co 2+ (3d 7 ) and this leads to a t 52g e 2g configuration. The large insulating gap is present due to the Coulomb potential difference between e g orbitals which are directed towards the oxygen ions and the t 2g orbitals which are located between the oxygen ions as the crystal field and the exchange splitting energies differ too much. Hence, disfavoring the interionictransitions at TM in (Zn-Co)x which have higher resistivity. While in the case of (Ni-Zn/Co/Cd)x, Ni 2+ (3d 8 ) have all t 2g levels and the two e g levels of parallel spin occupied resulting in t 62g e 2g configuration. The behavior of the insulator type materials under the applied field can be explained on the basis of Maxwell-Wagner type interfacial polarization mechanism. The value of έ decreases with an increase in the applied frequency and becomes eventually constant at higher frequencies. The series comprising (Ni-Cd)x has the highest value of dielectric constant followed by (Zn-Co)x and (Zn-Ni)x while (Ni-Co)x and (Zn-Cd)x has the lowest values. The Cd 2+ have a strong tetrahedral site preference so in (Ni-Cd)x some of Ni 2+ may move from tetrahedral to octahedral sites along with the movement of Al 3+ ions to the tetrahedral sites. The samples have more polarization because of easy exchange of electrons between Ni 2+ at octahedral sites hence have highest values of dielectric constant. While, (Zn-Cd)x have both the TM ions at tetrahedral sites having complete d shells so no electrons are available which can form polarons so have lowest values of dielectric constant. In all the series except (Zn-Ni)x series a dielectric relaxation is observed in some samples in which a maxima is observed at a certain frequency where a maximum loss is taking place. The thermal conductivity of MgAl 2 O 4 is 0.83 W/m.K and it increases with the increase in temperature and its value at 300 K is 1.375 W/m.K. The doped samples have lower thermal conductivities than the pure one due to the contributions of extra electons at TM ions which form polarons hence, polaron-phonon interactions may result in the scattering of phonons causing a reduction in the thermal conductivity in all the transition metal doped compounds. The values of thermal diffusivity lie between 0.0012-0.0014 m 2 /S for all the samples. The value of specific heat Cp for pure magnesium aluminate is 0.29 J/Kg. K. While for the doped samples its value ranges as follows: (Ni-Cd)x; 0.2708-0.0013 J/Kg. K, (Zn-Cd)x; 0.3058-0.3089 J/Kg. K, (Ni-Co)x; 0.2793-0.3060 J/Kg. K, (Zn-Ni)x; 0.3458-0.3612 J/Kg. K and (Zn-Co)x; 0.3087- 0.3458 J/Kg. K.