Nanocrystalline/nanosized manganese substituted nickel ferrites – Ni1−xMnxFe2O4 obtained by ceramic-mechanical milling route

Manganese substituted nickel ferrites, Ni_1−xMn_xFe₂O₄ (x=0, 0.3, 0.5 and 0.7) have been obtained by a combined method, heat treatment and subsequent mechanical milling. The samples were characterised by X-ray diffraction, differential scanning calorimetry and magnetic measurements. The increase of the Mn²⁺ cations amount into the spinel structure leads to a significant expansion of the cubic spinel structure lattice parameter. The crystallite size decreases with increasing milling time up to 120 min, more rapidly for the nickel–manganese ferrites with a large amount of Mn²⁺ cations (x=0.7). After only 15 min of milling the mean crystallites size is less than 25 nm for all synthesised ferrites. The Néel temperature decreases by increasing Mn²⁺ cation amount from 585 °C for x=0 up to 380 °C for x=0.7. The magnetisation of the ferrite increases by introducing more manganese cations into the spinel structure. The magnetisation of the milled samples decreases by increasing milling time for each ratio among Ni and Mn cations and tends to be difficult to saturate, a behaviour assigned to the spin canted effect.     

Room-temperature large magnetocaloric effect and critical behavior in La0.6Dy0.1Sr0.3MnO3

The effect of dysprosium incorporation in La_0.7Sr_0.3MnO₃ perovskite manganite on its magnetic properties, magnetocaloric effect and critical behavior was investigated. The temperature dependent magnetization data exhibit a sharp paramagnetic–ferromagnetic transition at T_C=307 K, which nature has been identified to be a second-order transition by the scaling laws for magnetocaloric effect. The maximum magnetic entropy change and the relative cooling power are found to be, respectively, 8.314 J/kg K and 187 J/kg for a 5 T magnetic field change without a hysteresis loss, making this material a promising candidate for magnetic refrigeration at room temperature. To study the critical behavior of the paramagnetic–ferromagnetic transition, some related critical exponents (β, γ, and δ) have been also calculated. The values of critical exponents indicate that the present phase transition does not belong to the common transition classes but shows some abnormal variation. We suggest that the induced lattice disordering and magnetic disordering due to Dysprosium incorporation are essential reasons for the presence of a large magnetocaloric effect and of an anomalous ferromagnetic phase transition in the present material     

Large magnetocaloric and magnetoresistance effects in metamagnetic Sm0.55(Sr0.5Ca0.5)0.45MnO3 manganite

The metamagnetic transition, magnetocaloric and magnetoresistance effects are investigated in polycrystalline Sm_0.55(Sr_0.5Ca_0.5)_0.45MnO₃ (SSCMO) manganite. A sharp magnetization jump at Curie temperature (T_C) 73.5 K with large thermal hysteresis is observed. Magnetic measurements and Arrott plots analysis indicate that the transition is first order in nature. Under a low magnetic field change of 1 T, the magnetic entropy change exhibits a peak value of $-\Delta S_M^{\max }(T)$=4.01 J/kg K with relative cooling power (RCP) value of 44.1 J/kg in the vicinity of T_C. The mechanism of charge conduction in insulator phase is polaron transport. Large negative magnetoresistance ratio with value of ~99% is obtained within a broad temperature range below metal insulator transition temperature under 1 T magnetic field. These results indicate the potential applications of SSCMO in magnetic refrigeration and spintronics devices.     

Magnetic and magnetocaloric properties of FeMnO3

FeMnO₃, prepared by mechano-synthesis method using high energy planetary ball mill, crystallizes in bixbyite type cubic structure in space group Ia3. Magnetic measurements show that the compound is ferrimagnetic at room temperature and on cooling undergoes antiferromagnetic ordering around, T_o~ 36 K. Magnetization measurements exhibit strong magnetic anisotropy in this system. Change in the magnetic entropy, also known as the magnetocaloric effect, has been observed in FeMnO₃ on subjecting the sample to a changing magnetic field. We found maximum change of magnetic entropy ~1.5 J/kg K, for a field change of 90 kOe at T~2T_o with relative cooling power of 50 J/kg.     

Hydrothermal synthesis and characterization of multiferroic Bi1−xLaxFeO3 crystallites

Bi_1?xLa_xFeO₃ (BLFO, x = 0.0–0.2) crystallites were synthesized by the hydrothermal route. X-ray diffraction results indicate that pure BLFO crystallites could be obtained for x ≤ 0.1, and the phase purity was sensitive to the PH value of precursor solutions. Transmission electron microscope observation reveals that needle like BLFO crystallites were formed for x = 0.1. The coexistence of ferroelectric and magnetic transition is detected by using differential thermal analysis, indicating the multiferroic characteristics of BLFO crystallites. The Néel temperature of BLFO crystallites for x = 0.1 shifts upwards, whereas the Curie temperature shifts downwards, compared with those of BFO crystallites without La substitution. Weak ferromagnetic property of BLFO crystallites was induced and enhanced with increasing the 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.     

Temperature-insensitive dielectric and piezoelectric properties in (1-x)K0.5Na0.5Nb0.997Cu0.0075O3-xSrZrO3 ceramics

Systematic investigation on phase transition, dielectric and piezoelectric properties of (1-x)K_0.5Na_0.5Nb_0.997Cu_0.0075O₃-xSrZrO₃ (x = 0, 0.03, 0.06, 0.09, 0.12, 0.15, abbreviated as KNNC-100xSZ) ceramics was carried out. Due to the coexistence of orthorhombic and tetragonal phase in a wide temperature range, a diffused polymorphic phase transition (PPT) region was achieved in KNNC with x ≥ 0.06. KNNC-12SZ ceramics exhibited high dielectric permittivity (∼1679), low dielectric loss (∼0.02) and small variation (Δe'/ε'_25 °C ≤ 15%) in dielectric permittivity from −78 °C to 237.3 °C. KNNC-6SZ ceramic possessed a high level of unipolar strain (∼0.15%) and maintained a smaller variation of ±12% under the corresponding electric field of 60 kV cm⁻¹ at 10 Hz from 25 °C to 175 °C. d₃₃^*, which was calculated according to the unipolar strain at 60 kV cm⁻¹, was 230 pm V⁻¹ and remained stable below 100 °C. Therefore, our work provided a new promising candidate for temperature-insensitive capacitors and piezoelectric actuators.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

Large relative cooling power of Bi-doped La0.8−xBixSr0.08(Ca0.55Ba0.45)0.12MnO3 (x=0.0, 0.1 and 0.3) perovskites: Magnetic and magnetocaloric properties

We report magnetic and magnetocaloric properties of La_0.8−xBi_xSr_0.08(Ca_0.55Ba_0.45)_0.12MnO₃ (x=0.0, 0.1 and 0.3) perovskite manganites. Polycrystalline samples have been synthesized in air by the sol gel method at a sintering temperature of 1150 °C. Powder X-ray diffraction data show that samples are phase pure and their cell parameters slightly decrease with increase in Bi content. Scanning electron micrographs show that the average particle size increases with increase in Bi content. The temperature dependent magnetization measurements show that the Curie temperatures decrease from 315 K (x=0.0) to 140 K (x=0.3) with increase in Bi content. The isothermal magnetization data is used to estimate the magnetic entropy changes (−$\Delta S_M$) and their calculated values are 1.12 J kg⁻¹ K⁻¹, 1.96 J kg⁻¹ K⁻¹ and 1.62 J kg⁻¹ K⁻¹ for x=0.0, 0.1 and 0.3 samples respectively under an applied magnetic field of 2.0 T. The corresponding values of relative cooling power (RCP) are 90 J kg⁻¹, 180 J kg⁻¹, 136 J kg⁻¹ for x=0.0, 0.1 and 0.3 samples respectively. These results of magnetocaloric effect in our samples suggest that they are promising materials for the magnetic refrigeration applications.     

Structural,magnetic and magnetocaloric properties of CoFe2−xMoxO4 (0.0≤x≤0.3) ferrites

We have investigated structural, magnetic and magnetocaloric properties of CoFe_2-xMo_xO₄ (0.0≤x≤0.3) ferrites. Polycrystalline samples were prepared by the sol gel method and characterized by the powder X-ray diffraction and scanning electron microscopy. X-ray diffraction patterns show that all samples have a cubic spinel structure and the lattice parameter, a, decreases monotonically with increase in Mo concentration. Scanning electron micrographs indicate that most of the particles are in the range of 400–850 nm size. Magnetic measurements, performed by using a cryogen free vibrating sample magnetometer, show that these samples are soft ferromagnets in the measured temperature range. The saturation magnetization, M_s, values of Mo-doped samples are larger than the parent compound with a maximum value of ~106 emu/g for x=0.2 sample. The magnetic entropy change (−ΔS) increases with increase in applied magnetic field and shows a peak in the vicinity of blocking temperature. A maximum value of 0.56 J kg⁻¹ K⁻¹ at 5 T field has been observed for x=0.2 sample.     

Effect of mechanical milling on structural,dielectric and magnetic properties of BaTiO3–Ni0.5Co0.5Fe2O4 multiferroic nanocomposites

The coexistence of ferroelectricity and ferromagnetism has triggered great interest in multiferroic materials. Multiferroic with strong room temperature magnetoelectric (ME) coupling can provide a platform for future technologies. In this paper, we have investigated the effect of mechanical milling on the properties of multiferroic nanocomposites synthesized by mixing barium titanate (BaTiO₃) (BT) and nickel cobalt ferrite (Ni_0.5Co_0.5Fe₂O₄) (NCF). This process has resulted into reliable disposal of a given quantity of NCF nanoparticles in BT grid and composite samples of different particle sizes (<500 nm) have been obtained by varying the duration of ball-milling for 12, 24, and 48 h. The presence of NCF within BT powder has been confirmed by X-ray Diffraction (XRD) and magnetization measurements (MH). Structural analysis was performed by using Reitveld refinement method that shows that the tetragonality of BaTiO₃ structure get reduced in submicron range. Variations in ferroelectric and dielectric properties with reduction in particle size/milling duration have been studied by P-E loop tracer and Impedance analyzer. The dielectric constant value of 400 has been observed for BT-NCF0 that increases to 9.7 K for composite sample ball mill at 48 h whereas remnant polarization increases to 4.2 μC/cm². These composites with high dielectric constant that changes with temperature and particles size find application in energy storage devices, sensor and memory devices.     

The influence of reactive milling on the structure and magnetic properties of nanocrystalline MnFe2O4

Nanocrystalline manganese ferrites (MnFe₂O₄) have been synthesized by direct milling of metallic manganese (Mn) and iron (Fe) powders in distilled water (H₂O). In order to overcome the limitation of wet milling, dry milling procedure has also been utilized to reduce crystallite size. The effects of milling time on the formation and crystallite size of wet milled MnFe₂O₄ nanoparticles have been investigated. It has been observed that single phase 18.4 nm nanocrystalline MnFe₂O₄ is obtained after 24 h milling at 400 rpm. Further milling caused deformation of the structure as well as increased crystallite size. With the aim of reducing the crystallite size of 18.4 nm, MnFe₂O₄ sample dry milling has been implemented for 2 and 4 h at 300 rpm. As a result, the crystallite size has been reduced to 12.4 and 8.7 nm, respectively. Effects of the crystalline sizes on magnetic properties were also investigated. Magnetization results clearly demonstrated that crystallite size has much more effect on the magnetic properties than average particle size.     

High-frequency induction heated sintering of High-energy ball milled TiC0.5N0.5 powders and mechanical properties of the sintered products

Commercial TiC_0.5N_0.5 powders were high-energy ball milled for various durations and consolidated without binder using the high-frequency induction heated sintering method (HFIHS). The effect of milling on the sintering behavior, crystallite size and mechanical properties of TiCN powders were evaluated. A nanostructured dense TiCN compact with a relative density of up to 98% was readily obtained within 3 min. The ball milling effectively refined the crystallite structure of TiCN powders and facilitated the subsequent densification. The sinter-onset temperature was reduced appreciably by the prior milling for 10 h from 1170 °C to 820 °C. Accordingly, the relative density of TiCN compact increased as the milling time increases. The microhardness of sintered TiCN was linearly proportional to the density while its toughness did not show any correlation with the crystalline size or density. It is clearly demonstrated that a quick densification of nano-structured TiCN bulk materials to near theoretical density could be obtained by the combination of HFIHS and the preparatory high-energy ball milling processes.     

Preparation and characterization of K0.5Bi0.5TiO3 particles synthesized by a stirring hydrothermal method

Nanoparticles of potassium bismuth titanate K_0.5Bi_0.5TiO₃ (KBT) with an average particle size of 38 nm were prepared using a stirring hydrothermal method. The pure KBT was obtained in 8 h reaction time instead of 24–48 h for conventional hydrothermal method. X-ray diffraction, Raman spectroscopy and TG analysis were used to check the proportion of hydroxyl group existing into the crude and the calcined KBT. A Hydroxyl group was found to affect the crystallite structure parameters and cell volume. When temperature increases from 25 to 1050 °C, the tetragonal structure presents a c/a ratio which decreases from 1.048 to 1.012. TG analysis and Raman vibration at high frequencies show that c/a is affected by hydroxyl group content below 750 °C and by potassium and bismuth vacancies above this temperature. The ceramic KBT showing a 300 nm size presents an improved ε_r=780 and a dielectric loss tan δ=0.062 at room temperature. Electric conductivity σ_ac was also lowered to 10⁻⁶ (Ω m)⁻¹ with an activation energy change at 673 K from 0.35 to 0.605 eV.     

Structure,magnetocaloric and critical properties of layered La2Sm0.4Sr0.6Mn2O7 perovskite

We report the structure, magnetocaloric effect, and critical phase transition in the manganite La₂Sm_0.4Sr_0.6Mn₂O₇ (LSSMO) synthesized by a sol-gel method. X-ray diffraction together with Rietveld refinement show that the sample crystallizes in a Sr₃Ti₂O₇-type tetragonal structure with a space group of I4/mmm. This compound undergoes a second-order ferromagnetic (FM) to paramagnetic phase transition at T_C=348 K and shows strong FM properties below the T_C. Based on the data of isothermal magnetization measured around the T_C and Maxwell's relation, we calculated the maximum magnetic entropy change (-ΔS_M^max) to be 4.69 J kg⁻¹ K⁻¹ and the relative cooling power to be 233.9 J kg⁻¹ for a μ₀ΔH=5 T magnetic field variation. These results indicate that LSSMO can be considered as a potential candidate material for application in magnetic refrigeration above room temperature. The critical behavior near the T_C was studied through the analysis of the magnetic field dependence of the magnetic entropy change and Widom's scaling relation. The exponent values estimated in this work are fairly close to those theoretically predicted by mean field theory (β=0.5, γ=1.0, and δ=3.0), revealing that long-range FM ordering exists in LSSMO. Scaling law theory also confirms the validity of the deduced critical exponents.     

Influence of milling time on mechanically assisted synthesis of Pb0.91Ca0.1TiO3 powders

Pb_0.91Ca_0.1TiO₃ powders (PCT) were prepared by mechanochemical synthesis from high-energy ball milling process. The influence of milling time on the phase formation, crystal structure, specific surface area, density and powder morphology was observed. We adopted the Rietveld refinement technique to investigate the crystal structure of the PCT powders. Scanning electron microscopy (SEM) analysis revealed that PCT powders milled for 5 h showed a wide distribution of particle agglomerates while milled for 35 h showed a decrease in agglomerates size. Further prolongation of milling time resulted in the agglomerates growth.     

Mechanochemical-molten salt synthesis of α-Al2O3 platelets

The polyaluminium chloride (PACl) precursor was used for a simple and scaled-up mechanochemical-molten salt synthesis of α-Al₂O₃ platelets. PACl, as a low temperature α-Al₂O₃ precursor, was firstly mechanically activated by high-energy ball milling for 5 min, followed by a next 5 min ball milling in the presence of a NaCl–KCl salt mixture. The starting formation temperature of the α-Al₂O₃ phase was 600 °C. In the subsequent annealing in the temperature range of 660–1000 °C, the α-Al₂O₃ phase with a well developed plate-like morphology was obtained. The products were characterized by X-ray powder diffractometry, scanning electron microscopy (SEM), and thermal analysis (DTA, TG) and solution ²⁷Al NMR spectroscopy.     

Dielectric and ferroelectric properties of (1 − x)BaTiO3–xBi0.5Na0.5TiO3 ceramics

Lead-free (1 − x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (x = 0.01, 0.02, 0.05, 0.1, 0.2, 0.3) ferroelectric ceramics were fabricated by the conventional ceramic technique. Sintering was made at 1200 °C for 2–4 h in air atmosphere. The crystal structure was investigated by X-ray diffraction. The dielectric and ferroelectric properties were also studied. Room temperature permittivity was found to decrease as Bi_0.5Na_0.5TiO₃ (BNT) content increases. Only the sample with 0.3 mol BNT was found to have relaxor behaviour. The T_c shifted slightly only for BNT addition lower than 0.1 mol. The highest T_c (about 150 °C) was obtained for 0.2 mol BNT addition. The remanent polarization, P_r, decreases whereas the coercive field, E_c, increases monotonously as the BNT content increases.     

Role of iron in the ferromagnetic-antiferromagnetic boundary of La0.5Ca0.5Mn1-xFexO3+δ (0≤x≤0.5) manganites

Samples of La_0.5Ca_0.5Mn_1−xFe_xO_3+δ (0≤x≤0.5) were synthesized using a solid-state reaction method involving a milling process and thermal treatment up to 1200 °C in an air atmosphere. Samples were characterized structurally with X-ray diffraction analysis and Rietveld refinement with morphology characterization using scanning electron microscopy. Magnetic properties were investigated using a physical property measurement system to obtain zero field cooling and the associated curves to plot hysteresis loops. Our results revealed the interplay between the structural and magnetic properties as Fe ions attached to the crystalline structure. A mechanism based on the substitution of Mn³⁺ and Mn⁴⁺ by low-spin Fe³⁺ and Fe⁴⁺ ions, respectively, was hypothesized to interpret the experimental data. More specifically, the temperature at which the transition from ferromagnetic to paramagnetic occurred increased with increasing Fe content as a result of a greater density of oxygen-mediated ferromagnetic bonds. Conversely, the magnetization weakened because the t_2g electrons were distributed in the respective d orbitals by adopting a low-spin configuration. Such a configuration is preferred as a result of the unit cell distortion in the milling process where the greater ionic radius of the Fe⁴⁺ ions leads to an elongated c-axis tetragonal symmetry and a greater unit cell volume. Finally, low-temperature magnetic behavior revealed the occurrence of a reentrant spin-glass type state within the ferromagnetic matrix favored by a milling-driven structural disorder and the existence of competitive superexchange interactions.     

Comparative study of the structure and microstructure of PAN-based nano- and micro-carbon fibers

The work presents results on the manufacture and comparative assessment of the structure and microstructure parameters of polyacrylonitrile polymer (PAN)-based carbon nano- and micro-fibers. Using the same polymer solution, PAN nano- and microfibers were obtained. The PAN nanofibers were obtained by electrospinning, and microfibers were spun using the conventional solution-spinning method. The PAN-based fiber precursors were annealed to 1000 °C, 2000 °C and to 2800 °C. Using X-ray diffraction and Raman spectroscopy, the structural and microstructural parameters of both types of carbon fibers were examined. The morphology of PAN nanofibers and carbon nanofibers (CNF) were studied by SEM. Both types of ex-PAN carbon fibers (nano and micro) have similar the c-axis spacing (d₀₀₂) values and crystallite sizes after heat treatment to 2000 °C presenting turbostratic structure. HR-TEM images of low temperature CNF show uniform microstructure with the misoriented small carbon crystallites along the fiber axis. The ratio of the integrated intensities of the D and G peaks for carbon nanofibers after heat treatment at 2000 °C was distinctly higher in comparison to carbon microfibers (CF). After additional annealing the fibers to 2800 °C a better structural ordering show CNF. The crystallite sizes (L_c, L_a) in CNF were distinctly higher in comparison to the crystallites in CF. CF consist of two carbon components, whereas CNF contain three carbon components varying in structural and microstructural parameters. One of carbon phases in CNF was found to have the interlayer spacing close to graphite, i.e. d₀₀₂=0.335 nm.