Proteic Sol Gel Process Research Articles

Magnetic properties and coupled spin‒phonon behavior of M‒type Sr1-xSmxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles

We thoroughly investigated the effects of Sm³⁺ doping on the magnetic, structural, hyperfine, and vibrational properties of Sr1-xSmxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles, which were synthesized using the proteic sol-gel process. The Rietveld refined XRD patterns confirmed the presence of M-type SrFe12O19, alongside a minor fraction of α-Fe2O3. As Sm³⁺ doping increased, the unit cell volume fluctuated, decreasing from 691.73 ų at x = 0 to 690.67 ų at x = 0.10 and slightly rising thereafter. The incorporation of Sm³⁺ induced a conversion of Fe3+ to Fe2+, affecting lattice parameters and crystallinity. At x = 0.10, maximum crystallite size (56.48 nm) and X-ray density (4.83 g/cm³) were attained, alongside decreased microstrain, indicating improved crystallinity and reduced defects. However, at x = 0.15, an increase in microstrain suggested lattice instability. FTIR confirmed that bands in the range 400–600 cm⁻1 might be due to the stretching vibration of oxygen and metal (Fe–O) ions, confirming the formation of hexaferrite. Temperature-dependent Raman spectroscopy studies confirmed the presence of spin-phonon coupling in all samples in the ferrimagnetic transition at ∼725-735 K. Mössbauer spectroscopy elucidated that the substitution of Sr2+ by Sm3+ induced the most pronounced effect on the 12k, 4f2, and 2a sites, corroborating the evolution of the lattice constants and the fact that these three sites have Sr sites in their close vicinity. The room-temperature hysteresis loops exhibited narrow features, indicative of soft magnetic behavior. Saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) varied with Sm3+ doping content (x). Ms initially decreased from 62.62 emu/g and 22.03 emu/g at x = 0.00 to 54.96 emu/g and 19.63 emu/g at x = 0.10, respectively, and then increase to 59.77 emu/g and 21.86 emu/g at x = 0.15. Conversely, Mr and Hc showed a non-monotonic trend with x, suggesting complex interactions between dopant concentration and magnetic properties.

Investigations on the structural and magnetic properties of Ba1-xGdxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles

The structural and magnetic properties of Ba1-xGdxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles prepared using the proteic sol-gel process were investigated. Infrared spectroscopy, Raman spectroscopy, andX-ray powder diffraction measurements with Rietveld refinement analysis confirmed the formation of M-type BaFe12O19, whereas extra peaks of secondary phases were also detected for BaFeO3 (2θ = 28°), GdFeO3 (2θ = 38.4°), and Fe2O3 (2θ = 38.6°). Average crystallite size increases with Gd substitution in the range of 61–63 nm. Scanning electron microscopy revealed that the particle morphology of some samples consisted of a tendency for regular hexagonal platelets with inhomogeneous distribution of average grain size in the range of 250–280 nm. The room temperature magnetic studies revealed that considerable large saturation magnetization (Ms), remanence (Mr), and coercivity (Hc) for the Ba1-xGdxFe12O19 nanoparticles monotonically decreased to a minimum value until x = 0.1. However, it increased again toMs≈ 55.5 emu·g−1, Mr≈ 22.7 emu·g−1, andHc≈ 3.7 kOe for the x = 0.15 sample. The squareness ratio (SQR = Mr/Ms) values were lower than 0.5, illustrating the magnetic multidomain nature and uniaxial anisotropy of the samples. These results were investigated deeply with relation to hyperfine parameters and surface morphology of the samples by 57Fe Mössbauer spectroscopy and magnetic force microscopy (MFM). The hyperfine parameters indicated that the substitution of Gd3+ for Ba2+ induces exchange interactions in 2b-O2-4f2 and 12k-O2-2b, resulting in the higher magnetocrystalline anisotropy of the samples. Additionally, MFM images confirm that grain consisting of distributed smaller grains gives rise to a strip-and helix-like magnetic domain structure.

Development of a β-LiAlSi2O6:Cr-based Ceramic Pigment by Proteic Sol-Gel Process Using Gelatin: Synthesis and Characterization

A novel pigment of β-LiAlSi2O6:Cr ceramic was developed by a partial proteic sol-gel process, using gelatin as a ligand. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and visible and near-infrared diffuse reflectance spectroscopy (vis-NIR DRS) were performed. XRD results confirmed that the crystal structure of the lattice corresponded to β-LiAlSi2O6 (or β‑spodumene), and the addition Cr3+ ions by doping did not interfere in the formation of this crystalline phase. EDS confirmed the homogeneous existence of Cr3+ dopant in β‑LiAlSi2O6 particles. From the vis‑NIR DRS, selective absorption of visible light wavelengths was identified in the bands of 425 nm and 600 nm, resulting in the perception of a yellowish-green color when β-LiAlSi2O6 is doped with Cr3+. CIE-XYZ colorimetric coordinates were generated to characterize the resulting colors. The obtained results demonstrated the viability of β‑LiAlSi2O6:Cr synthesis by a proteic sol‑gel route and its great potential for obtaining a yellowish-green ceramic pigment.

Synthesis of MgB4O7 and Li2B4O7 crystals by proteic sol–gel and Pechini methods

Magnesium-tetraborates and lithium-tetraborates are excellent detectors for ionizing radiation. However, manufacturing these crystalline structures is challenging as it involves multi-step chemical processes and toxic reagents. Over the past decades, alternative production methods for crystals have been studied aiming at lowering production costs and environmental impact and at increasing production yield and reproducibility. Among these is the Pechini method, based on the formation of polymer chains containing uniformly distributed metal ions. Another method is a modified proteic sol–gel process using edible gelatins or coconut water instead of the alkoxides used in conventional processes. In this work, we searched optimal conditions for the synthesis of dosimetry-grade MgB4O7 and Li2B4O7 crystals with the Pechini and the proteic sol–gel methods. The crystal phases were characterized by X-ray diffraction and by thermal analyses. We determined that MgB4O7 can be synthesized either by calcination of the reagents at 900 °C for 2 h with the sol–gel approach, or at just 800 °C for 2 h with the Pechini method. Single-phase Li2B4O7 can be synthesized either by calcination of the reagents at 650 °C for 2 h with the sol–gel approach, or at just 600 °C for 2 h with the Pechini method. Our synthesis times and temperatures are lower than those reported in the literature, which reduces significantly production costs and environmental impact.

Hopkinson effect, structural and magnetic properties of M-type Sm3+-doped SrFe12O19 nanoparticles produced by a proteic sol–gel process

Samarium doped M-type strontium hexaferrites with chemical formula Sr1−xSmxFe12O19 (0≤x≤0.15) (SrM) were synthesized using the proteic sol–gel process. This low cost and environment friendly method yields easily reproducible magnetic powders. The crystal structure and phase purity were studied by X-ray diffraction (XRD). All the XRD patterns have been analyzed by Rietveld refinement technique using the P63/mmc space group which showed crystallite size in the nanoscale. Results of field emission scanning electron microscope show that the grains are regular hexagonal platelets. The saturation magnetization reaches a minimum at x=0.10 and increases at higher concentrations. The coercivity increases with Sm3+ concentration. The remarkable appearance of a Hopkinson peak at the M vs. T curve indicates that particles of the sample are composed of a single domain particles.

Investigation of structural and magnetic properties of nanocrystalline Mn-doped SrFe12O19 prepared by proteic sol–gel process

Nanoparticles of SrFe12-xMnxO19 (x=0.0 and 0.10) were synthesized by a proteic sol–gel process. Thermogravimetric and differential thermal analyses (TG–DTA) indicated the formation of nanocrystalline strontium ferrite phase at a calcination temperature of 1000°C. Structural and microstructural evolutions of the samples were studied by X-ray powder diffraction (XRD) and the Rietveld method. XRD patterns demonstrated that all samples consisted of single-phase M-type strontium hexaferrite. The crystal lattice constant did not change significantly with manganese substitution, ranging from 0.5877(3)nm (x=0.0) to 0.5876(3)nm (x=1.0). In addition, the average crystallite size, which was determined from the Williamson–Hall formula, was about 46.4–52.6nm. Infrared spectroscopy (FT-IR) showed the presence of three principal absorption bands in the frequency ranges around 435–535cm−1 and around 595cm−1, indicating the formation of the hexaferrite. Scanning electron microscopy (SEM) revealed that particles consisted of irregular platelets with sizes from 68 to 204nm. Room-temperature Mössbauer investigations revealed that manganese ions preferentially occupied the 12k, 4f1, 4f2, and 2a sites. Hysteresis loops (M–H) showed that the saturation magnetization, remanence, and coercivity decreased with manganese doping. This effect is discussed in terms of the distribution of metal cations in the tetrahedral and octahedral sites.

Effects of Ca<sup>2+</sup>-Doping on the Crystal Lattice of α-Fe<sub>2</sub>O<sub>3</sub>

Samples of CayFe12-yO19 (0 ≤ y ≤ 1.0) were prepared by a proteic sol–gel process with hematite phase and clusters of M-type calcium hexaferrite. Impedance analysis showed that the resistivity increased with calcium concentration in the 0.0 < y ≤ 0.2 range, but decreased for y > 0.2. The saturation of the electrical resistivity occurred at 7.5 × 106 Ω·cm for Ca0.9Fe11.1O19. The plot of magnetization as a function of the magnetic field showed high values of saturation magnetization (40 emu/g) with low remanence (6.7 emu/g) and coercive field (320 Oe).

Magnetocrystalline Properties of Sr<sub>1.4</sub>Ba<sub>1.6</sub>Co<sub>2</sub>Fe<sub>24</sub>O<sub>41</sub>

Multiferroic Ba1.6Sr1.4Co2Fe24O41 with a Z-type hexagonal structure was obtained through the proteic sol-gel process with a single-phase, combined with thermal treatment at 1200 °C for 2 h. XRD analysis revealed the formation of a single phase, with preferential orientation in the case of a sample in pellet form. Magnetometry measurements revealed a low coercive field characteristic of Z-type hexaferrite, plus an electrical resistivity of approximately 2 × 109 Ω.cm that was considerably higher than expected.

Effect of Substituting Sr<sup>2+</sup> for Gd<sup>3+</sup> on Structural and Magnetoelectric Properties of W-Type Hexaferrite

Sr1-xGdxCo2Fe16O27-δ, (x = 0.05–0.20) was prepared via the proteic sol-gel process. Single-phase W-type hexaferrite doped with gadolinium was synthesized. The ferrite shows ferrimagnetic behavior with high saturation magnetization, low coercive field, low remnant magnetization, and a resistivity greater than 107 Ω·cm. The substitution of Sr2+ for Gd3+ improved the magnetoelectric properties of the ferrite owing to the increase in the concentration of Fe2+ and improvement of electron conduction.

Structural and optical characterizations of Ca2Al2SiO7:Ce3+, Mn2+ nanoparticles produced via a hybrid route

Pure, Ce3+ doped and Ce3+ and Mn2+ co-doped Ca2Al2SiO7 ceramic powders were prepared by two different methodologies which are the proteic sol–gel process and a new hybrid route combining the proteic sol–gel with solid state reaction processes. The second one is an eco-friendly method because it uses natural raw materials in replacement of the metal alkoxides used in the traditional sol–gel routes. X-ray diffraction showed that Ca2Al2SiO7 crystalline phase was obtained for both preparations. Differential thermal analysis indicated that the exothermic event around 850°C, for sample produced by proteic sol–gel method, and around 927°C, for ceramics prepared by hybrid synthesis, can be associated to crystallization of Ca2Al2SiO7. Transmission electron microscope indicates that regular and spherical nanoparticles were obtained with average sizes of about 12nm. The Scherrer’s method was used to determine the average crystallite sizes and it was shown that nanometric crystallites were obtained of about 74nm for samples produced via hybrid route. For all the single phase samples, the crystallite sizes are about the same and that agrees with TEM results. Diffuse optical reflectance measurements were used to estimate the Ca2Al2SiO7 optical band gap and the obtained value is about 6eV, photoluminescence (PL) spectra presented typical emissions of Ce3+ and Mn2+ ions. Upon excitation at 352nm the emission spectra showed a broad band centered at 415nm due to the Ce3+ 4f1→5d1 typical transition. This emission is resonant with Mn2+ excitation and it transfers energy to Mn ions generating a second broad emission band centered at 620nm due to the Mn2+. The PL results were used to obtain, as a fist approach, the Ce3+ energy levels diagram and, using the Tanabe–Sugano diagrams, the transitions due to the Mn2+ were calculated. X-ray excited optical luminescence measurements showed the same emission spectra as the PL emission spectra. Luminescence lifetime decay constants were measured for Ce and Mn co-doped and for Ce doped samples and the results indicate Ca2Al2SiO7:Ce3+, Mn2+ showed quite fast responses with main time constants below 30ns.

Spin–phonon coupling in multiferroic Ba1.6Sr1.4Co2Fe24O41

Multiferroic Ba1.6Sr1.4Co2Fe24O41 with Z-type hexagonal structure was obtained through the proteic sol–gel process with a majoritary phase by thermic treatment at 1200°C for 2h. Raman spectroscopy showed significant abnormalities in the position and intensity of the vibrational modes as a function of increasing temperature. The intensity and Raman shift versus temperature showed three peaks at around 375K, 563K, and 683K. Strong spin–phonon coupling was identified through Raman spectroscopy and is related to three magnetic changes: conical→planar, planar→uniaxial, and ferromagnetic→paramagnetic.

Optical properties of ZnO powder prepared by using a proteic sol-gel process

We have studied the optical properties of ZnO powder synthesized by using a proteic sol-gel process with coconut water as the precursor. The energy dispersive X-ray spectrometer and X-ray diffraction results show high purity of the synthesized ZnO powder. From the low-temperature (12 K) and power-dependent PL spectra, the donor-bound exciton, the acceptor-bound exciton, the donor-to-acceptor pair (DAP), and the phonon-replica of the DAP transition have been observed at 3.38, 3.34, 3.26, and 3.19 eV, respectively. The free exciton emission (∼3.3 eV) is also observed at 300 K in the temperature-dependent PL spectra.

Ferromagnetism in Zn-doped CeO2 Induced by Oxygen Vacancies

Several oxides doped with transition metals can be used in spintronics devices due to their conductive and magnetic properties at room temperature. In this work, samples of Ce1−2xZnxO2−δ were obtained by an alternative proteic sol–gel process for x=0, 0.05 and 0.1 without any spurious phase for x≤0.1. The structural properties of samples were analyzed by X-ray diffraction (XRD) and Raman spectroscopy. Magnetization measurements revealed a ferromagnetic behavior at room temperature probably induced by oxygen vacancies.

Study of the magnetic and structural properties of Mn-, Fe-, and Co-doped ZnO powder

A study of the magnetic and structural properties of Zn1−xMxO powder (where x=0 or 0.01, and M=Mn, Fe or Co) produced by the proteic sol–gel process was undertaken. The sample crystal structure was analyzed by XRD and magnetic measurements were carried out in a SQUID magnetometer. Of the XRD analysis, all samples had hexagonal wurtzite crystal structure with P63mc space group, and no secondary phase was observed. It is observed of the M(H) measures at 2K, that the Co- and Mn-doped ZnO displayed saturation magnetizations (Ms) of approximately 2 and 3.2emu/g, respectively, and no remanence (Mr) was observed, indicating a superparamagnetic behavior in these samples. However, the Fe-doped sample showed a ferromagnetic behavior with Ms∼0.34emu/g, Mr∼0.05emu/g, and coercivity (Hc)∼1090Oe. Already at room temperature, the M(H) measurements reveal a purely paramagnetic behavior for Mn- and Fe-doped ZnO, indicating that the Curie temperature (Tc) is below 300K. However, a weak superparamagnetic behavior was observed in the Co-doped sample, indicating that Tc>300K.

Structural and magnetic study of Fe-doped CeO 2

We report the magnetic and structural properties of Fe-doped cerium oxide, obtained using the proteic sol–gel process. The samples had weak ferromagnetic behavior, with magnetization saturations of 0.05 and 0.10 emu/g for samples with 0.5% and 1% of Fe, respectively. We attribute this ferromagnetic state to the presence of dilute localized magnetic moments inside the semiconductor matrix. In fact, within the accuracy of our measurements, X-ray diffraction data reveal the formation of single-phase structurally isomorphous CeO 2. Rietveld refinement carried out in Fe-doped samples showed a decreasing density of the unit cell. Furthermore, our analysis on the optical absorbance measurements and Raman scattering shows a change in the band gap of 0.18 eV and an increase in the oxygen vacancies, which was confirmed by Raman scattering peaks at 595 and 1176 cm −1.

Nanocrystals of Zn1-xMxO (M = Co, Mn) by proteic sol-gel process

We present in this work a study on the magnetic properties of nanocrystals of Mn- or Co-doped ZnO obtained by a proteic sol-gel process. Analysis of these nanocrystals is accomplished using x-ray diffraction (XRD), the Rietveld refinement through using the General Structure Analysis System (GSAS) program, and magnetic measurements using a vibrating sample magnetometer (VSM). The samples of Zn0.9M0.1O (M = Co or Mn) possess single phase crystals with a wurtzite structure (hexagonal) and a P63mc space group. The Zn0.9Co0.1O and Zn0.9Mn0.1O samples presented, at room temperature, the behavior of a soft ferromagnetic with a coercive field (Hc) of 336 Oe and 1240 Oe, respectively, and a remanent magnetization (Mr) of 5 and 2.5 memu/g, respectively.

Radioluminescence in ZnO

ZnO powders were obtained by proteic sol–gel process using zinc sulfate as the precursor. They have an energy gap of 3.3 eV and display a radioluminescence signal centered in 550 nm, which corresponds to 2.6 eV. This signal has appeared due to defects in the gap (energy sublevels) caused by the distortions in its crystalline structure, which result from the loss of sulfur at high temperature. These powders were characterized by radioluminescence, XRD, SEM, EDS and DTA/TG.

Rietveld refinement of transition metal doped ZnO

Nanocrystals of Zn1−xMxO (M=Mn, Co, or Ni) were grown using proteic sol-gel process, and the crystalline phases were identified by X-ray diffraction and Rietveld refinement. The nanocrystals have hexagonal wurtzite structure, with space group P63mc. The insertion of Mn2+ in the place of Zn2+ provoked an increase in the size of the nanocrystals, and the insertion of Co2+ or Ni2+ caused a reduction in the sizes of the nanocrystals, as compared to pure ZnO. This occurred because these three transition metals have very different ionic radii (Co2+=0.58 A˚, Mn2+=0.66 A˚, Ni2+=0.55 A˚, and Zn2+=0.60 A˚).

SrFe 12O 19 prepared by the proteic sol–gel process

Powders of strontium hexaferrite (SrFe 12O 19) were prepared by the proteic sol–gel process using coconut water as a precursor. X-ray diffraction (XRD) measurement showed the formation of SrFe 12O 19 with a small amount of the hematite for the sample calcined at 1000 °C with Fe/Sr=12. Rietveld refinement disclosed that this sample had 87.56% of the SrFe 12O 19 and 12.44% of Fe 2O 3 and the values for R p, R wp and χ 2 were 4.28%, 5.93% and 1.71, respectively. The magnetic properties were M s=64 emu/g, M r/ M s=0.55 and H c=1.4 kOe for a crystallite size of 57 nm.

Nanocrystals of BaFe 12O 19 obtained by the proteic sol–gel process

Nanocrystals of BaFe 12O 19, with a small amount of the BaFe 2O 4, were obtained by means of the proteic sol–gel process together. The results demonstrated that the magnetic and crystallographic properties are dependent on the calcination temperature and the values of M r/M s are very close to 0.5, which is the expected value by the Stoner–Wohlfarth model. The best calcination temperature was at 900 °C with M s=55.5 emu/g, H c=4.1 kOe and crystallite size 39 nm.