Synthesis of Co-Zr doped nanocrystalline strontium hexaferrites by sol-gel auto-combustion route using sucrose as fuel and study of their structural,magnetic and electrical properties

Sol-gel auto-combustion route using sucrose as fuel has been employed to synthesize nanocrystalline particles of SrZr_xCo_xFe_(12−2x)O₁₉ (0.0≤ x ≤1.0). The characterization of these materials has been done by TGA-DTA, FT-IR, XRD and EDS. SEM and TEM techniques have been used to study the structure and morphology. Magnetic properties have been investigated by VSM and Mössbauer spectroscopy (MS). The influence of calcination temperature on morphology and magnetic properties of samples is studied in a wide temperature range of 500–1100 °C. XRD analysis indicates the formation of pure single phase hexagonal ferrites at 900 °C. The crystallite size calculated using Scherrer equation lies in a narrow range of 21–33 nm. The crystallite size is small enough to obtain a suitable signal to noise ratio in high density recording medium. Substitution of Zr and Co for Fe has been found to have a profound effect on the structural, magnetic and electrical properties. Upon substitution saturation magnetization (M_S) first increases from 62.67 emu/g to 64.84 emu/g (up to x=0.4) followed by a decrease to 49.71 emu/g at x=1.0. There is a slow fall in coercivity (H_C) from 5785.74 (x=0.0) to 1796.51 Oe (x=1.0). Dielectric constant, dielectric loss tangent and AC conductivity in the frequency range 20 Hz to 120 MHz have been studied for all the compositions (x=0–1.0). The composition and frequency dependence of these dielectric parameters has been qualitatively explained.     

Properties of NiFe2O4 ceramics from powders obtained by auto-combustion synthesis with different fuels

Nickel ferrite (NiFe₂O₄) powders derived by auto-combustion synthesis using three different fuels (citric acid, glycine and dl-alanine) have been characterized. The sintering behavior of ceramics using these powders has been compared. Oxygen balance (OB) setting for the chemical reaction is found to regulate the combustion reaction rate. A rapid reaction rate and a high flame temperature are achieved with dl alanine fuel yielding single phase NiFe₂O₄ powder in the as-burnt stage, whereas powders derived with citric acid and glycine fuels show poor crystallinity and necessitate post-annealing. The powder particles are largely agglomerated with a non-uniform distribution in shape and size, and the average particle size is estimated in the range ~ 54–71 nm. Powders derived from dl-alanine fuel show better phase purity, smaller crystallite size, larger surface area and superior sintering behavior. Additional Raman modes discerned for dl-alanine derived powder support a 1:1 ordering of Ni²⁺ and Fe³⁺ at the octahedral sites relating to microscopic tetragonal P4₁22 symmetry expected theoretically for the formation of NiFe₂O₄ with inverse spinel structure. Microstructure of sintered ceramics depends on the precursor powders that are used and sintering at 1200 °C is found to be optimum. Citric acid and glycine derived powders yield high saturation magnetization (M_s~47–49 emu/g), but poor dielectric properties, whereas dl-alanine derived powders yield ceramics with high resistivity (~3.4×10⁸ Ω cm), low dielectric loss (tan δ~0.003 at 1 MHz) and high magnetization (46 emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.     

Magnetic and morphological characterization of Bi2Fe4O9 nanoparticles synthesized via a new reverse chemical co-precipitation method

Nano-sized Bi₂Fe₄O₉ (BFO) was successfully synthesized using a new reverse chemical co-precipitation method at different pH values of 8–12. These powders were examined by x-ray diffractometery (XRD), thermogravimetrical differential thermal analysis (TG-DTA), field emission scanning electron microscopy (FESEM) and vibrating sample magnetometery (VSM). The XRD analysis showed the formation of pure phase Bi₂Fe₄O₉ at calcination temperature over 700 ℃. The TG-DTA curves indicated the crystallization temperature of 617 ℃ for the Bi₂Fe₄O₉ sample. The FESEM micrographs revealed a precipitates agglomeration, which is related to the nature of the chemical co-precipitation method and free surface energy of nanoparticles. Furthermore, the particle size of the powder samples increased from 43 to 131 nm as the pH value increased from 8 to 12, respectively. Also, the morphological change from nearly cubic to rod-like shape in the nanoparticles was observed by increasing the pH value. The M-H curves of the as-prepared powders confirmed the antiferromagnetic behavior in all samples. Uncompensated spins from the surface led to the appearance of saturation magnetization in the Bi₂Fe₄O₉ nanoparticles. Besides, a decrease in the particles size resulted in more uncompensated spins, thereby improving the saturation and remnant magnetization from M_s = 0.35 emu/g and M_r = 0.010 emu/g for pH = 12 to M_s = 1.15 emu/g and M_r = 0.042 emu/g for pH = 8. Furthermore, as the pH values increase the coercive fields firstly rise up to 196 Oe for pH = 9 and then decrease to 151 for pH = 12.     

PVA assisted coprecipitation synthesis and characterization of MgFe2O4 nanoparticles

Single phase magnesium ferrite (MgFe₂O₄) nanoparticles were prepared by the coprecipitation method followed by calcination at 700 °C for 1 h. The effects of polyvinyl alcohol (PVA) agent on the structural, microstructure, magnetic properties and AC magnetically induced heating characteristics of MgFe₂O₄ nanoparticles were investigated. The structure and cation distributions investigated by X-ray diffraction method showed single phase MgFe₂O₄ powders had partially inverse spinel structure in which the inversion coefficient increased by adding more PVA. The small particle size and narrow size distribution of the coprecipitated MgFe₂O₄ powders characterized by scanning electron microscopy were achieved using PVA agent. Magnetic properties of MgFe₂O₄ nanoparticles studied by vibrating sample magnetometry showed ferrimagnetic characteristics with the highest saturation magnetization and coercivity of 24.6 emu/g and 17 Oe, respectively. The coprecipitated MgFe₂O₄ nanoparticles assisted by PVA exhibited the lower AC heating temperature of 5.6 °C and specific loss power of 2.4 W/g in comparison with 6.1 °C and 2.7 W/g for the powders coprecipitated without using PVA.     

Effects of excess NaClO4 on phases,size and magnetic properties of Ni–Zn ferrite powders prepared by combustion synthesis

Ni_0.4Zn_0.6Fe₂O₄ powders were prepared by combustion synthesis with different amount of NaClO₄. Phases, particle size and magnetic properties of the powders and annealed powders were systematically investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and vibrating sample magnetism (VSM). The excess content of NaClO₄ offered significant advantages with respect to the size, morphology and magnetic properties of the powders. After annealing, sub-micro ferrite spherical powders with spinel phase in a range of 500–800 nm can be obtained. With the increase of the NaClO₄ content, the saturation magnetization of the powders shows a maximum value at 68.8 emu/g when w=0.4, whereas the coercivity stayed nearly constant. The maximum saturation of annealed powders by combustion synthesis is much higher than the range reported in the literature.     

Effect of surfactant and heat treatment on morphology,surface area and crystallinity in hydroxyapatite nanocrystals

Hydroxyapatite (HA) powders were synthesized by the wet precipitation method, with and without surfactant, under identical processing parameters. These powders were then heat treated at 900 °C for 3 h in air. The detailed characterization of the powders was done by using SEM, dynamic light scattering, nitrogen adsorption, XRD, Raman spectroscopy, and FTIR techniques. The HA phase, identified by well defined PO₄^3? and OH^? ion peaks in Raman and FTIR spectra, was observed in all the powder samples. The addition of surfactant changed the morphology of the particles from spherical to needle/rod-like structure and increased the surface area up to three times (from 33 to 96 m²/g). Also, suppression in the evolution of β-TCP phase was observed along with decrease in the crystal size and crystallinity of the powder due to the addition of surfactant. Synthesized nano-HA crystals were found to have diameters and lengths in the range 10–25 nm and 75–150 nm, respectively. The heat treatment changed the architecture of the particles, increased the crystallinity and reduced the surface area to ≈7 m²/g. However, the relative increase in crystallinity was much higher for the powder synthesized with surfactant. The ratio of the average crystallite size to the crystallinity degree was about 0.53±0.07 for all the powders. The particle size distribution was bimodal and coarser for the powder synthesized without surfactant. The pore size analysis showed transformation of a predominantly mesoporous structure into a meso- plus macroporous one on heat treatment. The intensity of OH^? group peak in Raman spectra was found to be highly sensitive to the crystalline state of the HA powder and may be used to assess crystallinity.     

Thermal behavior of the nonstoichiometric lithium niobate powders synthesized via a combustion method

Lithium niobate (Li_xNb_1?xO_3+δ) powders with various compositions are prepared via combustion synthesis. The thermal properties, crystal structure, and surface morphology of the as-prepared lithium niobate powders are characterized by thermogravimetric and differential thermal analyses (TG/DTA), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM). When the calcination temperature reached 900 °C, the secondary phases Li₃NbO₄ and LiNb₃O₈ were observed. The lithium concentration before 900 °C was 40–43%. The lattice parameters increased slightly with decreasing concentration of lithium ions. When the calcination temperature was higher than 900 °C, the major Li_0.91NbO₃ phase and the minor LiNbO₃ phase coexisted in the nonstoichiometric lithium niobate with 43% lithium content.     

Synthesis,calcination and characterization of Nanosized ceria powders by self-propagating room temperature method

Nanometric ceria powders with fluorite-type structure were obtained by applying self-propagating room temperature method. The obtained powders were subsequently thermally treated (calcined) at different temperatures for different times. Powder properties such as specific surface area, crystallite size, particle size and lattice parameter have been studied. Roentgen diffraction analysis (XRD), BET and Raman scattering measurements were used to characterize the as-obtained (uncalcined) powder as well as powders calcined at different temperatures.It was found that the average diameter of the as-obtained crystallites is in the range of 3–5 nm whereas the specific surface area is about 70 m²/g. The subsequent, 15 min long, calcination of as-obtained powder at different temperatures gradually increased crystallite size up to ～60 nm and reduced specific surface down to 6 m²/g. Raman spectra of synthesized CeO_2?y depicts a strong red shift of active triply degenerate F_2 g mode as well as additional peak at 600 cm^?1. The frequency of F_2 g mode increased while its line width decreased with an increase in calcination temperature. Such a behavior is considered to be the result of particle size increase and agglomeration during the calcination. After the heat treatment at 800 °C crystallite size reached value larger than 50 nm. Second order Raman mode, which originates from intrinsic oxygen vacancies, disappeared after calcination.     

Structural and electrical characterization of (K0.48Na0.52)0.96Li0.04Nb0.85Ta0.15O3 synthesized by spray drying

The synthesis of lead-free ferroelectric materials with composition (K_0.48Na_0.52)_0.96Li_0.04Nb_0.85Ta_0.15O₃ has been achieved by spray drying technique. Pure perovskite phase was obtained after calcination of as-prepared powders at 800 °C for 1 h. Crystallized powders have a particle size average of 100 nm. Well sintered samples were obtained at 1120 and 1130 °C for 2 h in air with densities of 4.58 and 4.49 g/cm³, respectively. We attributed this improvement as the result of sintering process and the very small particle size in powders. Sintered samples have promising piezoelectric parameters, k_p 0.40–0.41, d₃₁ 50–55 pC/N and dielectric losses around 1%.     

Synthesis and magnetic properties of La3+-Co2+ substituted strontium ferrite particles using modified spray pyrolysis–calcination method

The purpose of the research was to improve the intrinsic magnetic properties of strontium ferrite by substituting lanthanum and cobalt for strontium and iron. The salt-assisted ultrasonic spray pyrolysis (SA-USP) following calcination process were used to from La-Co substituted strontium ferrite particles (La_xSr_1-xFe_12-yCo_yO₁₉), and their compositional dependent magnetic properties systemically investigated. All the samples were calcined at 1050 °C for 1 h in an air atmosphere to yield single-phased hexagonal particles several hundred nanometers to microns in size. A saturation magnetization of 70.76 emu/g and a coercivity 7265 Oe were obtained at a composition of La_0.25Sr_0.75Fe_11.75Co_0.25O₁₉. The amount of Co was reduced to obtain an optimized saturation magnetization of 71.40 emu/g and a coercivity of 7572 Oe at a composition of La_0.25Sr_0.75Fe_11.8Co_0.2O₁₉.     

Nano-sized BaSnO3 powder via a precursor route: Comparative study of sintering behaviour and mechanism of fine and coarse-grained powders

Preparation of a very fine BaSnO₃ powder by calcination of a barium tin 1,2-ethanediolato complex precursor and its sintering behaviour are described herein. A rate controlled calcination process to 820 °C leads to a nm-sized BaSnO₃ powder with a specific surface area of S = 15.1 m²/g (d_av. = 55 nm). The powder has a slightly larger cell parameter of a = 412.22(7) pm compared to the single crystal value, which decreases with increasing calcination temperature and reaches the reference value above 1000 °C. The sintering behaviour is compared between fine and coarse-grained BaSnO₃ powders. Corresponding powder compacts of the nano-sized BaSnO₃ achieve a relative density of 90% after sintering at 1600 °C for 1 h and at 1500 °C and a soaking time of 30 h, whereas coarse-grained powder compacts reach only 80% of the relative density at 1650 °C (10 h). Furthermore, the shrinkage mechanisms of fine and coarse-grained powder compacts have been investigated and are discussed.     

Dielectric and magnetic properties of Nickel ferrite ceramics using crystalline powders derived from DL alanine fuel in sol–gel auto-combustion

Dielectric and magnetic properties of NiFe₂O₄ ceramics prepared with powders using DL-alanine fuel in the sol–gel auto combustion technique are studied. DL-alanine fuel yields crystalline as-burnt powders, and when used for ceramic processing yields varying microstructure at different sintering temperatures. The dielectric properties are influenced by the resulting microstructure and the magnetic properties show slight change in saturation magnetization M_s (~44 – 46 emu/g). The coercive fields, dielectric losses and dispersion are reduced considerably at higher sintering temperatures (1200–1300 °C). The influence of changing microstructure is analyzed through dielectric response, complex impedance analysis and electrical modulus spectroscopy in the frequency range (10⁻²–10⁷ Hz) to understand the interactions from the grain and grain boundary phases. Sintering at 1200 °C, is found to be optimum, yields lower losses & reduced dielectric dispersion, and high resistivity (3.4×10⁸ Ω cm).     

Preparation and characterization of LnMgAl11O19 (Ln=La,Nd, Gd) ceramic powders

Lanthanide hexaaluminate powders of LaMgAl₁₁O₁₉ (LMA), NdMgAl₁₁O₁₉ (NMA) and GdMgAl₁₁O₁₉ (GMA) were synthesized via the solid state reaction or sol–gel and calcination method. The LMA and NMA powders synthesized by the sol–gel and calcination method at 1600 °C for 8 h exhibit a single hexaaluminate phase with magnetoplumbite structure; however, the GMA powder synthesized by the sol–gel and calcination method at 1600 °C for 8 h contains both a hexaaluminate phase and a small amount of second phase GdAlO₃ with a perovskite structure. The powders synthesized by the solid state reaction method at 1500 °C for 4 h have a small particle size of 1–3 μm, and a large specific surface area and a good uniformity. The powders synthesized by the sol–gel and calcination method at 1600 °C for 8 h have a particle size of 5–20 μm, and exhibit to a certain extent agglomeration.     

MnFe2O4 bulk,nanoparticles and film: A comparative study of structural and magnetic properties

MnFe₂O₄ bulk sample was synthesized by conventional solid state reaction method, at 1350 °C. Nanoparticles with mean size of 〈D〉_TEM=10.4(±1.1) nm were prepared by thermal decomposition of metal nitrates, at 350 °C. And a film sample was prepared by pulsed laser deposition of bulk ferrite on MgO(100) at substrate temperature of 600 °C. Then a comparative study of the structural and magnetic properties of the samples has been carried out using different measurements. X-ray diffraction pattern of bulk and nanoparticles samples confirmed formation of spinel phase. The film sample showed an epitaxial growth on MgO in (400) direction. Saturation magnetization of nanoparticles at 300 K, M_S=33 emu/g, was comparable with film sample, M_S=38 emu/g, both being ∼2.5 times smaller than that of bulk sample (M_S=82 emu/g). The results showed the importance of surface effects in the film sample and nanoparticles. The obtained zero coercivity of bulk sample at 300 K and the low value of 8 Oe at 5 K is attributed to soft magnetic behavior of the MnFe₂O₄. On the other hand, nanoparticles showed superparamagnetic behavior at 300 K; and blocked state with a large coercivity of 730 Oe at 5 K. The film sample showed non-zero corecivity at both 5 and 300 K which reveals higher magnetic anisotropy of film compared to the bulk ferrite.     

Nanosized barium ferrite powders prepared by spray pyrolysis from citric acid solution

Highly crystalline nanosized barium ferrite (BaFe₁₂O₁₉) powders were prepared by spray pyrolysis from a spray solution containing a high concentration of the metal components. The precursor powders obtained from the spray solution containing citric acid were amorphous with a porous and hollow structure. Purely crystalline and fine BaFe₁₂O₁₉ powders were obtained after post-treatment between 700 and 1000 °C and subsequent mechanical grinding in an agate mortar. The mean sizes of the powders post-treated at 700 and 1000 °C were 125 and 550 nm, respectively. The specific magnetization of the powders prepared from the spray solution containing citric acid was 57 emu/g.     

Investigation of structural,optical, dielectric and magnetic studies of Mn substituted BiFeO3 multiferroics

A series of Mn doped BiFeO₃ with composition BiMn_xFe_1−xO₃ (x = 0.0, 0.025, 0.05, 0.075, 0.1) was synthesized via a citrate precursor method. Structural, morphological, optical, electrical and magnetic properties were investigated by using various measurement techniques. XRD patterns confirmed that the materials possess distorted rhombohedral structure with space group R3c. Average crystallite size was found to be in the range 18–36 nm. A decrease in the value of lattice parameters has been observed due to contraction of unit cell volume with Mn doping. Higher tensile strain for the prepared nanoparticles was observed in Hall-Williamson Plot. Field Emission Scanning Microscopy (FESEM) showed the spherical, uniform, dense nanoparticles in the range 80–200 nm. Reduction in grain size was observed which may be due to suppression of grain growth with Mn doping. FTIR studies reported two strong peaks at 552 cm⁻¹ and 449 cm^-1 which confirmed the pervoskite structure. Dielectric properties were studied by measuring the dielectric constant and loss in the frequency range 1 kHz to 1 MHz. Magnetic hysteresis loop showed the retentivity (M_r) increasing from 0.0514 emu/g of BFO to 0.0931 emu/g of 10% Mn doping. Coercivity was found to increase upto 0.0582 T for 5% Mn doping and then reduced to 0.0344 T for 7.5% Mn doping. Saturation magnetization was observed to increase from 0.6791 emu/g for BFO to 0.8025 emu/g for 7.5% and then reduced to 0.6725 emu/g for 10% Mn doping in BFO. Improvement in dielectric and magnetic properties makes this material as a promising candidate for multifunctional device applications.     

Sr1-xLaxFe12O19 (0.0≤x≤0.5) hexaferrites: Synthesis,characterizations, hyperfine interactions and magneto-optical properties

A simple sol-gel auto combustion process was used to synthesize La³⁺ substituted M-type strontium hexaferrite, Sr_1-xLa_xFe_12-xO₁₉ (0.0≤x≤ 0.5). Structural, magnetic, and optical behavior as a function of La³⁺ substitutions were investigated by Fourier transformed infrared spectroscopy (FT-IR). X-ray powder diffraction (XRD). Scanning electron microscopy (SEM), Mössbauer spectroscopy, vibrating sample magnetometer (VSM), and Diffuse reflectance spectroscopy (DRS). XRD data showed single phase magnetoplumbite structure and Rietveld analysis confirmed P6₃/mmc space group for all the series. The average crystallite size was found to be in the range of 43.2–48.4 nm. The variation in line width, isomer shift, quadrupole splitting, relative area and hyperfine magnetic field values have been determined from ⁵⁷Fe Mössbauer spectroscopy data. The fittings accounted for the Fe²⁺/Fe³⁺ charge compensation mechanism at the 2a site due to replacement of Sr²⁺ by La³⁺. The saturation magnetization (σ_s) decreases from 57,21 to 63,23 emu/g and remnant magnetization (σ_r) decreases from 35.6 emu/g to 28.7 emu/g with increasing La substitution. The decrement is sharper at coercive field (H_c) from maximum value of 5325 to minimum 1825 Oe. Demagnetizing factor (N) is 3 times more for the x=0.3, 0.4, and 0.5. However all samples exhibit ferromagnetic behavior at room temperature. Magnetic anisotropy of Hexaferrites was detected as uniaxial and effective anisotropy constants (K_eff) were between 5.93×10⁵ and 4.76×10⁵ Ergs/g. The high magnitudes of anisotropy fields (H_a) in the range of 13863–15574 Oe reveal that all hexaferrites are magnetic hard materials. Tauc plots were applied to extrapolate the direct optical energy band gap (E_g) of hexaferrites. The E_g values decreased from 1.83 to 1.34 eV with increasing La content.     

Co-precipitation of a Ni–Zn ferrite precursor powder: Effects of heat treatment conditions and deagglomeration on the structure and magnetic properties

A Ni–Zn ferrite precursor powder was synthesized by co-precipitation upon adding ammonia to an aqueous solution of the precursor iron, nickel, and zinc nitrate salts. The powder was calcined at a range of temperatures (200–1200 °C) and the crystalline phase evolution was assessed by X-ray diffraction coupled with Rietveld refinement. Intermediate phases (NiFe₂O₄ and Fe₂O₃) with increasing crystallinity coexisted in the system up to 1000 °C. The required Ni_0.8Zn_0.2Fe₂O₄ phase could only be attained at 1200 °C. The magnetic properties measured using a vibrating sample magnetometer revealed high magnetization saturation level (～59 emu/gm) above 400 °C. The coercivity showed a steady decrease with increasing heat treatment temperature, leading to a change from a hard to soft magnetic state. The BET specific surface area and the SEM morphology were found to be dependent on calcination temperature, atmosphere (air or N₂) and on the milling procedure.     

Microstructure and mechanical properties of ceramics obtained from chemically co-precipitated Al2O3-GdAlO3 nano-powders with eutectic composition

An efficient and flexible chemical co-precipitation method has been used to synthesize nanoscale Al₂O₃-GdAlO₃ powders with eutectic composition. The as-synthesized powders exhibit a highly dispersive and homogeneous distribution with an average particle size of 50 nm. The phase transition in the resulting powders strongly depends upon the calcination temperature. GdAlO₃ undergoes complete crystallization after calcination at 1050 °C, however, the diffraction peaks of α-Al₂O₃ are found at a relatively high calcination temperature of at least 1300 °C. The fully-densified Al₂O₃-GdAlO₃ ceramic with eutectic composition obtained by hot pressing the nanoscale powders at 1500 °C exhibits a room temperature flexural strength of 556 MPa, a Vickers hardness of 17.3 GPa and a fracture toughness of 7.5 MPa m^1/2. The high temperature flexural strength of the as-sintered Al₂O₃-GdAlO₃ ceramic is measured to be 515 MPa after bending tests at 1000 °C.     

Synthesis and characterization of nanometric gadolinia powders by room temperature solid-state displacement reaction and low temperature calcination

Nanometric-sized gadolinia (Gd₂O₃) powders were obtained by applying solid-state displacement reaction at room temperature and low temperature calcination. The XRD analysis revealed that the room temperature product was gadolinium hydroxide, Gd(OH)₃. In order to induce crystallization of Gd₂O₃, the subsequent calcination at 600 ∼ 1200 °C of the room temperature reaction products was studied. Calculation of average crystallite size (D) as well as separation of the effect of crystallite size and strain of nanocrystals was performed on the basic of Williamson-Hall plots. The morphologies of powders calcined at different temperatures were followed by scanning electron microscopy. The pure cubic Gd₂O₃ phase was made at 600 °C which converted to monoclinic Gd₂O₃ phase between 1400° and 1600 °C. High-density (96% of theoretical density) ceramic pellet free of any additives was obtained after pressureless sintering at 1600 °C for 4 h in air, using calcined powder at 600 °C.