Rare Earth Nano Compounds: Preparation and Thermophysical Characterization

Abstract

Rare Earth Nano Compounds: Preparation and Thermophysical Characterization Rare earth compounds are a big group of functional materials which have varied applications in many fields ranging from Solid Oxide Fuel Cells (SOFCs) to biological labeling/imaging. The newly developed materials and techniques are nontoxic, ultrasensitive, and chemically and physically stable. The main focus of this research work was to attempt to enhance the ionic conductivity of ceria based compounds. Factors like decrease in grain size, doping of trivalent cations and multiple doping are mainly focused to increase the conductivity. Also, Rare earth doped inorganic matrix is synthesized and fluorescence is observed in stabilized fluorophore as bimodal probe for bioimaging. A comparative study for synthesis and characterization of nanocrystalline ceria was done with a range of wet chemical methods including composite mediated hydrothermal method (CMH), co-precipitation method and sol-gel method. The calcination and sintering temperatures were 500 0 C and 750 0 C respectively for all the samples. X-ray diffraction (XRD) confirmed the cubic fluorite structure. Raman spectroscopy seconded the XRD results and characteristic feature of ceria was observed ca. 465 cm -1 . The dc conductivities of the samples were determined in temperature range 200-700 0 C. The highest value obtained was for the sample prepared with CMH method having value 0.345 S-cm -1 at 700 0 C. So, CMH was selected as the synthesis method for the later samples. Further, the synthesis conditions of CMH method were optimized for nanocrystalline samples. The practical parameters were heat treatment time and temperature. The heat treatment temperature during synthesis was held at 180 0 C and 220 0 C whereas treatment time was 45, 70 and 90 minutes. Better values of conductivities were observed for sample with heat treatment time of 45 minutes and heat treatment temperature of 180 0 C. The maximum electrical dc conductivity of the sample was 0.3386 S-cm -1 at 700 0 C in this case. To further enhance the conductivity, the doping of Gd was done in ceria and composition made was Ce 1-x Gd x O δ ; x = 0.1, 0.15, 0.2, 0.25. The fluorite F 2 g band around 465 cm -1 reconfirmed the Gd doped ceria. No peak of Gd 2 O 3 (480 cm -1 ) was observed. DC conductivity was measured in temperature range 300-700 0 C and ac ixconductivity was determined in frequency range 1 kHz to 3MHz at temperatures 300, 400, 500, 600 and 700 0 C. The larger values of conductivities were obtained for Ce 0.75 Gd 0.25 O δ . The jump relaxation model can be used to explain the dc conductivity behavior. By jump of ions to available sites, a hopping motion started thus contributing to dc conductivity. The ̳step‘ ac conductivity in dispersion curves is confirmation of the grain interior and grain boundary conductivities as ionic conduction is dependent on the defect formation due to thermal energies which create vacancies to aid in hopping motion of ions. The maximum conductivity, achieved for Ce 0.75 Gd 0.25 O δ, was 7.4x10 -3 S-cm -1 at 700 0 C. The thermal conductivity values obtained using Advantageous Transient Plane Source (ATPS) method was in low thermal conductivity region. The thermal conduction is dependent on the scattering and mean free path, so the less mean free path and more scattering gave rise to low conductivity values. The effect of multiple doping on conductivity was also studied. La and Nd were co-doped in Gd doped ceria for two samples which showed maximum conductivities in the earlier studies i.e. Ce 0.9 Gd 0.1 O δ and Ce 0.75 Gd 0.25 O δ . Samples with nominal compositions Ce 1-2x Gd x La x O δ and Ce 1-2x Gd x Nd x O δ (x = 0.1, 0.25) were prepared. The Ce-O fluorite breathing mode was observed in Raman spectroscopy to confirm the ceria and doping in ceria. The strong ceria band appeared at ca. 465 cm -1 and weak oxygen vacancy bands appeared ca. 570 and 600 cm -1 . The formation of oxygen vacancies and defects was confirmed through Raman spectroscopy. The jump relaxation model is applicable for dc conductivity and Jonscher power law described the ac conductivity behavior. The maximum dc conductivity achieved was 1.78 S-cm -1 for Ce 0.5 Gd 0.25 Nd 0.25 O δ. The relaxation reorientation peaks can be realized in dielectric constant and dielectric loss plots which shifted toward higher frequencies with increase in temperature. Rare earth hydroxides (R(OH) 3) were synthesized by hydrothermal method and stoichiometric change in composition and morphology was observed. Ce(OH) 3 , La(OH) 3 and Nd(OH) 3 samples were synthesized. XRD confirmed the hexagonal structures of the prepared samples. The crystallite size corresponding to the most intense peaks were 18, 33 and 41 nm for Nd-, La- and Ce- hydroxides. SEM revealed very interesting and fascinating morphologies. Ce(OH) 3 has belts like structures, Nd(OH) 3 has needles like structures and La(OH) 3 has wires like structures. The growth of structures can be ascribed to chemical potential, maintained through precipitating xagent, the pressure inside the vessel, the temperature provided for the hydrothermal treatment and time for hydrothermal treatment. The shape evolution can be explained by Gibbs-Curie-Wulff model which relate the shape evolution with the face energies. When the equilibrium energy is obtained for respective faces the Ostwald ripening is stopped. On heat treatment, the La(OH) 3 first converted into LaOOH at ca. 400 0 C and finally into La 2 O 3 at ca. 600 0 C as observed in DSC plot. The increase of conductivity with temperature is evident from the plots. Nd(OH) 3 achieved maximum conductivity and Ce(OH) 3 acquired minimum among the three possibly due to smaller crystallite sizes in the former case. The smaller grains increase the grain boundaries and charges can pile up on boundaries which increase the conductivity. The corresponding dc conductivity values of Ce(OH) 3 , La(OH) 3 and Nd(OH) 3 were 0.372, 6.648 and 20.369 S-cm -1 , respectively. The fluorescence characteristics of rare earths with intense emissions and stabilized structures were observed with Yb, Er, and Tm doping in F based inorganic matrix NaMnF 3 . Yb has served as sensitizer and Tm and Er were utilized as activators. The synthesis of NaMnF 3 co-doped with Yb;Er/Tm was successfully achieved through solvothermal method. The ethylene glycol (EG) was used as stabilizing agent. Another important feature of this synthesis method was surface functionalization of particles with the synthesis process in a single step. Also, the choice of precursors of Na & F and choice of stabilizing agent (EG) rendered the nanostructures to be rods like. The PEI polymer was used for surface modification. An intense green emission is observed for NaMnF3: Yb, Er, with increase in Yb concentration and for fixed Er at 2 mol%. The observed emission was around 550 nm between levels 4 S 3/2 and 4 I 15/2. Yb20 Mn78 Er2 revealed red emission at 660 nm between levels 4 F 9/2 and 4 I 15/2 which became intense with increase of Er concentration. With Tm as dopant, NEAR IR emission was observed at 800 nm between levels 3 H 4 and 3 H 6 although blue emission was also observed at 480 nm between energy levels 1 G 4 and 3 H 6 . The highest value of conductivity achieved for Ce 0.75 Gd 0.25 O δ made this material a potential candidate as an electrolyte for SOFCs. The low thermal conductivities of R(OH) 3 can be utilized in thermal barrier coatings. The pure red emission from Yb20 Mn78 Er2 and presence of Mn made this material prospective applicant in bimodal bioprobe.