Urcia Romero, Silvana R.
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Publication Synthesis and study of structural and magnetic properties of ferrite nanocrystals for magnetocaloric applications(2011) Urcia Romero, Silvana R.; Perales Pérez, Oscar J.; College of Arts and Science - Science; Fernández, Félix E.; Jiménez, Héctor; Department of Physics; Gordillo Guerrero, Luis F.Pure and doped CoₓZn₁₋ₓFe₂O₄ (0.5≤x≤1.0) nanocrystals have been synthesized by conventional and modified size-controlled coprecipitation methods. In the latter, the control of the oversaturation in reacting solutions, attained by controlling the flow rate at which the reactants were contacted, promoted heterogeneous nucleation and hence crystal growth. The size- controlled synthesis route allowed the tuning of the maximum magnetization and the coercivity, which increased by a factor of nine. The demagnetization temperature (T𝒹) was also found to be strongly dependent on both, the Co atomic fraction and crystal size and varied between 106 °C and 540 °C. MÖssbauer spectroscopy confirmed the strong influence of the synthesis conditions on the superparamagnetic fraction in the samples. In order to investigate the effect of rare-earth ions on the properties of the ferrite, RE-doped Co₀.₇Zn₀.₃Fe₂O₄, nanoparticles (RE=Gd and Dy ions), were also synthesized by following the above mentioned synthesis approaches. The average crystallite size varied from (9.43±0.13)nm, in the conventional synthesis method, to (18.45±0.11)nm when the ferrite was synthesized at 1mL/min. The maximum magnetization of the ferrite synthesized by the conventional approach decreased from 60emu/g (non-doped ferrite) to 55emu/g when it was doped with 1 at % of Dy; this maximum magnetization went up to 62emu/g when the synthesis was carried out under flow- controlled conditions. The demagnetization temperature went down from 350 ᵒC (non-doped ferrite) to 320 ᵒC for the same content of Dy ions and was further decreased down to 308 ᵒC when the ferrite powders were synthesized under flowrate controlled conditions. Regarding the effect of Gd dopant, estimated T𝒹-values decreased from 350 to 230 °C for Gd-doping levels between 0.00 and 0.05, respectively. The weakening of the Fe-RE super-exchange interaction when Fe³⁺ is substituted by Gd³⁺ (due to screening of the 4𝒻 electrons by the outer electrons), and the paramagnetic character of the Gd³⁺ ions could explain this trend in T𝒹. The results of the experimental design evidenced the strong dependence of the average crystallite size and the coercivity on both, the Gd³⁺ and NaOH concentrations. The variation of the solubility of the hydroxide ferrite precursor would have promoted the spin canting effect and formation of dead magnetic layer on the nanoparticle surface. On a materials application basis, the following remarks summarize our main findings. The highest magnetization was 73 emu/g (x = 0.7) and was attained when the ferrite powders were synthesized by the hydrothermal method. However, neither the coercivity nor the demagnetization temperatures were suitable for applications in the magnetocaloric pump. Although we could decrease T𝒹 by increasing the Zn concentrations (T𝒹 can be as low as 100 °C), it would take place at expenses of the drop in magnetization and the pyromagnetic coefficient. This applicability scenario becomes more promising after doping the ferrite with Gd or Dy species. For 5% doping the maximum magnetization was an acceptable 53 emu/g, while the coercivity reached a low value of 20 Oe, which are in the required range for applications. The lowest value for the demagnetization temperature was 230 °C reached with Gd-doping. This same sample had a pyromagnetic coefficient of 0.59 emu/g-K, which is a significant increase over all other systems evaluated in this work and also the literature. Therefore, from a material application viewpoint in magnetocaloric pumping, the doping of the CoZn ferrite with Gd species will promote both, the drop in the demagnetization temperature (it decreased from 350 ᵒC down to 230 ᵒC) and a remarkable increase of the pyromagnetic coefficient. The moderate magnetization and minimum coercivity of these nanoparticles enable them to be considered a very promising candidate for magnetocaloric pumping systems.Publication Synthesis and characterization of robust porous pillar-layered structure coordination polymers for gas storage and delivery(2023-07-07) Urcia Romero, Silvana R.; Hernández Maldonado, Arturo J.; College of Engineering; Martínez Iñesta, María; Pagán Torres, Yomaira; Suleiman Rosado, David; Department of Chemical Engineering; Vega Olivencia, Carmen A.The CO2 adsorption capacity and structural changes of three isoreticular porous coordination polymers (PCPs), Co2(pzdc)2(bpy)(H2O)n, Zn2(pzdc)2(bpy)(H2O)m, and Ni2(pzdc)2(bpy)(H2O)m (pzdc: pyrazine-2,3-dicarboxylate; bpy: 4,4’-bipyridine), were studied. Co2(pzdc)2(bpy)(H2O)n was characterized by evaluating changes in lattice parameters after CO2 adsorption up to 50 atm using in situ synchrotron X-ray powder diffraction. The effective pore size increased by ~2% with gas adsorption in the 1 to 50 atm pressure range, doubling the adsorption capacity. Hysteretic behavior during CO2 adsorption was observed, indicating structural changes analyzed by in situ synchrotron diffraction using Rietveld refinement. Rotation of carboxylate groups coordinating with the Co(II) metal node caused minor changes in unit cell volume (ΔV ≈ 6 Å3), unlike Cu2(pzdc)2(bpy), where pillar rotations and significant lattice expansion (ΔV ≈ 60 Å3) occurred upon CO2 adsorption. Co, Zn, and Ni-based PCP materials exhibited good thermal stability up to ~250 °C according to high-temperature in situ X-ray diffraction. CO2 adsorption and desorption isotherms at 25 °C for various pressures showed a relationship between hysteretic behavior and pore structural expansion/contraction induced by PCP surface interaction with CO2. Physisorption-level CO2 adsorption was observed, with adsorbent-adsorbate interactions approximately 50% stronger than reported for Cu2(pzdc)2(bpy). The reduction in pore size in the Co-, Zn-, and Ni-PCP samples resulted from changes in coordination of the metallic nodes, forming different crystalline systems. This is attributed to the characteristic ability of these metals, influencing crystal formation and growth kinetics.