Microstructure and magnetic properties of Ni0·75Zn0·25Fe2O4 ferrite prepared using an electric current-assisted sintering method

Using a Ni_0·75Zn_0·25Fe₂O₄ nanopowder synthesized by means of a hydrothermal method as a raw material, polycrystalline nickel zinc (NiZn) ferrite ceramics composed of sub-micron grains were successfully prepared via an electric current-assisted sintering method. Temperatures ranging from 800 °C to 950 °C and a dwell time of 20 min were employed. The phase composition and microstructure of the samples were characterized via X-ray diffraction and scanning electron microscopy, respectively. Moreover, the magnetic properties of the samples were investigated using a vibrating sample magnetometer and a ferromagnetic resonance system. The results revealed that each sintered sample was mainly composed of a spinel phase. With increasing sintering temperature, the specific saturation magnetization increased from 71.85 emu/g to 74.58 emu/g, owing mainly to the increase in the relative density and the average grain size of the NiZn ferrites. The coercivity and ferromagnetic resonance linewidth of the ferrite ceramics decreased monotonically with increasing sintering temperature, owing mainly to the magnetostriction coefficient, saturation magnetization, and porosity of the sintered ferrites.     

Effect of sintering temperature on the structural and magnetic properties of MgFe2O4 ceramics prepared by spark plasma sintering

Spark plasma sintering (SPS) is a powerful technique to produce fine grain dense ferrite at low temperature. This work was undertaken to study the effect of sintering temperature on the densification, microstructures and magnetic properties of magnesium ferrite (MgFe₂O₄). MgFe₂O₄ nanoparticles were synthesized via sol–gel self-combustion method. The powders were pressed into pellets which were sintered by spark plasma sintering at 700–900 °C for 5 min under 40 MPa. A densification of 95% of the theoretical density of Mg ferrite was achieved in the spark plasma sintered (SPSed) ceramics. The density, grain size and saturation magnetization of SPSed ceramics were found to increase with an increase in sintering temperature. Infrared (IR) spectra exhibit two important vibration bands of tetrahedral and octahedral metal-oxygen sites. The investigations of microstructures and magnetic properties reveal that the unique sintering mechanism in the SPS process is responsible for the enhancement of magnetic properties of SPSed compacts.     

Characterization of submicrometer-sized NiZn ferrite prepared by spark plasma sintering

High-density submicrometer-sized Ni_0.5Zn_0.5Fe₂O₄ ferrite ceramics were prepared by spark plasma sintering in conjunction with sufficient high energy ball milling. They were evaluated by different characterization techniques such as X-ray diffraction, scanning electron microscopy, and dielectric and magnetic measurements. All samples prepared at sintering temperatures ranging from 850 to 925 °C exhibit a single spinel phase and their relative densities and grain sizes range from 90% to 99% and ~100 nm to ~300 nm, respectively. The dielectric constant increases with decreasing grain size until ~250 nm, and then decreases dramatically with further decreasing grain size. The saturation magnetization increases continuously with increasing grain size/density but the magnetic coercivity decreases. The highest dielectric constant and saturation magnetization at room temperature are approximately 1.0×10⁵ and 84.4 emu/g, respectively, while the lowest magnetic coercivity is only around 15 Oe. These outstanding properties may be associated with high density and uniform microstructure created by spark plasma sintering. Therefore, the spark plasma sintering is a promising technique for fabricating high-quality NiZn ferrites with high saturation magnetization and low coercivity.     

Sintering behavior and electromagnetic properties of Fe-deficient NiZn ferrites

Sintering behavior and electromagnetic properties of Ni_0.5Zn_0.5Fe_2−xO_4−3/2x ferrite (x = 0, 0.4, 0.8) by the sol–gel method are investigated. Fe deficiency in the composition enhances sintering and retards grain growth. The near fully dense Fe-deficient samples could be obtained at a sintering temperature as low as 1120 °C and the highest relative density appears in the x = 0.8 sample sintered at 1150 °C. Second phase zincite ZnO resulting from Fe deficiency plays an important role in spinel NiZn ferrites by acting as a grain growth inhibitor and the grain growth of NiZn ferrite is effectively suppressed. When the sintering temperature is above 1200 °C, extensive grain growth occurs due to the probability of serious volatilization of zinc at high temperatures. The ratio of Ni to Zn of NiZn ferrites increases with increasing Fe deficiency due to the separation of zinc from spinel lattice, which results in the decrease in initial permeability and the increase in Curie temperature and resonant frequency.     

Effect of average grain size and sintered relative density on the imaginary part – µ″ of the complex magnetic permeability of (Cu0.12Ni0.23Zn0.65)Fe2O4 system

The influence of the sintered microstructure on the electromagnetic properties of Cu-doped NiZn ferrites were investigated. Two of the main variables of the thermal cycle have been modified: the maximum sintering temperature and the dwell time at that temperature, or sintering time. The evolution of the imaginary part – µ″ of the complex magnetic permeability was studied as a function of relative density, grain size and amplitude of the grain size distribution of the sintered pieces. The results show that µ″ depends on the sintered microstructure and that there is a limiting value of the average grain size (~20–25 µm) from which the electromagnetic properties of these kinds of materials worsened significantly.     

Spark plasma sintering of SnO2 based varistors

In this research, for the first time, SnO₂-based varistors were fabricated via spark plasma sintering technique (SPS) and the microstructure and electrical properties of these varistors were investigated. Furthermore, the effect of post-annealing temperature in oxygen atmosphere on electrical properties of the SPSed samples was studied. The SPS process was performed at the sintering temperatures of 600, 650, and 700 ᵒC for 15 min with a maximum pressure of 90 MPa under vacuum condition. The SPSed sample which was sintered at 650 ᵒC possessed maximum density of 98% and the ultra-fine-grained microstructure with the mean grain size of 380 nm. Surprisingly, all SPSed samples exhibited Ohmic behavior with very low electric resistances. After post-annealing in oxygen atmosphere, Ohmic to non-Ohmic transition was observed in SPSed samples. The oxygen deficiency during the SPS process was responsible for the Ohmic characteristic of SPSed samples. Post-annealing of SPSed samples in the oxygen atmosphere resulted in Schottky barriers formation through oxygen adsorption at grain boundaries. The sample post-annealed at 1050 ᵒC presented the best non-Ohmic parameters including high breakdown electric field of 4500 V/cm and the nonlinear coefficient of 13. These electrical parameters are comparable with the conventional-sintered samples which was sintered at 1300 ᵒC and its breakdown electric field and nonlinear coefficient were equal to 900 V/cm and 8, respectfully.     

Control of barium ferrite decomposition during spark plasma sintering: Towards nanostructured samples with anisotropic magnetic properties

The sintering of barium ferrite (BaM) nano-sized powders by spark plasma sintering was studied. At the surface of the samples, an iron-rich layer (magnetite) was formed due to the decomposition of BaM and segregation in the secondary phases. To prevent the formation of secondary phases different protection layers between the graphite mould and the sample were used. Their effect on the sample microstructure was studied by X-ray diffraction and scanning electron microscopy. The most suitable protection layer was a highly dense sintered disc of aluminium oxide. Using this dense protection layer, sintered discs of BaM with 82% of theoretical density and grains of 90 ± 50 nm were obtained. A magnetic anisotropy was achieved from the sintering of the BaM particles with the largest shape anisotropy.     

Evolution of phase composition and microstructure of sodium potassium niobate –based ceramic during pressure-less spark plasma sintering and post-annealing

This study was designed to better understand the microstructural and phase evolution of lead-free sodium potassium niobate based piezoceramics with a nominal composition (K_0.5Na_0.5)_0.99Sr_0.005NbO₃ (KNNSr) during pressure-less spark plasma sintering followed by post-annealing in oxygen. The as-sintered samples were dark-coloured and electrically conductive as a result of partial reduction of Nb⁵⁺ to Nb⁴⁺ and formation of oxygen vacancies confirmed by X-ray photoelectron and Raman spectroscopy. The Rietveld refinement analysis showed that the as-sintered samples contained two perovskite phases with monoclinic Pm unit cell and slightly different unit-cell parameters. The microstructure with sub-micrometre-sized grains unambiguously confirmed that rapid heating and short dwell time hindered the grain growth. We found that post-annealing the samples at 950?°C in oxygen led to improvement in functional properties. The samples became white-coloured, the both perovskite unit cells decreased as a result of re-oxidation, while the microstructure remained essentially unchanged. The KNNSr sintered at nominal sintering temperature of 1300?°C for 3?min and post-annealed possessed a relative density of 88% and dielectric and piezoelectric properties similar to those of the conventionally sintered samples. Our findings contribute to the understanding of pressure-less spark plasma sintering of sodium potassium niobate-based materials and suggest that arrested grain growth and minimisation of alkali evaporation not necessarily lead to dense ceramic.     

Spark plasma sintering of combustion-synthesized β-SiAlON powders

In this paper, the sintering behavior of β-Si_6−zAl_zO_zN_8−z (z=1) powder prepared by combustion synthesis (CS) was studied using spark plasma sintering (SPS). The CSed powder was ball milled for various durations from 0.5 to 20 h and was then sintered at different temperatures with heating rates varying from 30 °C/min to 200 °C/min. The effects of ball milling, sintering temperature, and heating rate on sinterability, final microstructure, and mechanical property were investigated. A long period of ball milling reduced the particle size and subsequently accelerated the sintering process. However, the fine powder was easily agglomerated to form secondary particles, which accordingly decreased the densification of the SPS product. The high sintering temperature accelerated the densification process, whereas the high heating rate reduced the grain growth and increased the relative density of the sintered product.     

Magnetic properties and microstructures of a Ni-Zn ferrite ceramics co-doped with V2O5 and MnCO3

The present work investigated the effect of the addition of V₂O₅ and MnCO₃ on the microstructure and magnetic properties of Ni-Zn ferrite ceramic samples prepared by a conventional ceramic sintering method at different temperatures. With this aim, a series of samples were prepared by varying the loadings of V₂O₅-MnCO₃ (un-doped, 0.4–0.1?wt%, 0.1–0.4?wt%, 0.2–0.3?wt%, 0.3–0.2?wt%, and 0.5–0.1?wt%, denoted as sample 1, sample 2, sample 3, sample 4, sample 5, and sample 6, respectively). The initial permeability and power loss of the different Ni-Zn ferrites were investigated with respect to the sintering temperature. The V₂O₅ and MnCO₃ dopants significantly improved the initial permeability and power loss characteristics of the Ni-Zn ferrite at frequencies ≥0.5?MHz. When sintered at 1100?°C, sample 2 showed a maximum initial permeability of 931.23?H/m at a frequency of 1?MHz combined with a minimum power loss of 339.2?kW/m³. Co-doping with V₂O₅ and MnCO₃ also resulted in the sintered samples with larger average grain sizes and higher density, while the sintering temperature of Ni-Zn ferrites was significantly reduced.     

Properties of spark plasma sintered pseudocubic BiFeO3–BaTiO3 ceramics

Perovskite solid solution materials, namely, 0.67BiFeO₃-0.33BaTiO₃, were synthesized by spark plasma sintering method. The effects of the spark plasma sintering temperature on phase purity, microstructure, and electric properties of the as-prepared materials were investigated. The materials could be referred as pseudocubic phases based on the X-ray diffraction patterns. The bulk density first increased and then decreased. The 880 °C-sintered-ceramics had the maximal density and a compact microstructure with grain size of 0.77 ± 0.34 μm. The dielectric constant as a function of temperature exhibited a broad peak. At the optimal spark-plasma-sintering temperature, enhanced ferroelectric properties were observed with a value of P_r ～ 21 μC/cm². This investigation on the spark plasma sintering process confirms it as an efficient approach to prepare outstanding performance BiFeO₃–BaTiO₃ ceramics.     

Spark plasma sintering of Sm2O3-doped aluminum nitride

The high sintering temperature required for aluminum nitride (AlN) at typically 1800 °C, is an impediment to its development as an engineering material. Spark plasma sintering (SPS) of AlN is carried out with samarium oxide (Sm₂O₃) as sintering additive at a sintering temperature as low as 1500–1600 °C. The effect of sintering temperature and SPS cycle on the microstructure and performance of AlN is studied. There appears to be a direct correlation between SPS temperature and number of repeated SPS sintering cycle per sample with the density of the final sintered sample. The addition of Sm₂O₃ as a sintering aid (1 and 3 wt.%) improves the properties and density of AlN noticeably. Thermal conductivity of AlN samples improves with increase in number of SPS cycle (maximum of 2) and sintering temperature (up to 1600 °C). Thermal conductivity is found to be greatly improved with the presence of Sm₂O₃ as sintering additive, with a thermal conductivity value about 118 W m⁻¹ K⁻¹) for the 3 wt.% Sm₂O₃-doped AlN sample SPS at 1500 °C for 3 min. Dielectric constant of the sintered AlN samples is dependent on the relative density of the samples. The number of repeated SPS cycle and sintering aid do not, however, cause significant elevation of the dielectric constant of the final sintered samples. Microstructures of the AlN samples show that, densification of AlN sample is effectively enhanced through increase in the operating SPS temperature and the employment of multiple SPS cycles. Addition of Sm₂O₃ greatly improves the densification of AlN sample while maintaining a fine grain structure. The Sm₂O₃ dopant modifies the microstructures to decidedly faceted AlN grains, resulting in the flattening of AlN–AlN grain contacts.     

Effect of atmosphere and sintering time on the microstructure and mechanical properties at high temperatures of α-SiC sintered with liquid phase Y2O3–Al2O3

The influence that the atmosphere (N₂ or Ar) and sintering time have on microstructure evolution in liquid-phase-sintered α-SiC (LPS-α-SiC) and on its mechanical properties at high temperature was investigated. The microstructure of the samples sintered in N₂ was equiaxed with a grain size of 0.70 μm and a density of 98% of the theoretical value regardless of the sintering time. In contrast, samples sintered in Ar had an elongated-grain microstructure with a density decreasing from 99 to 95% and a grain size increasing from 0.64 to 1.61 μm as the sintering time increased from 1 to 7 h. The mechanical behaviour at 1450 °C showed the samples sintered in nitrogen to be brittle and fail at very low strains, with a fracture stress increasing from 400 to 800 MPa as the sintering time increased. In contrast, the samples sintered in Ar were quasi-ductile with increasing strain to failure as the sintering time increased, and a fracture stress strongly linked to the form and size of the grains. These differences in the mechanical properties of the two materials are discussed in the text. During mechanical tests, a loss of intergranular phase takes place in a region, between 50 and 150 μm thick, close to the surface of the samples—the effect being more important in the samples sintered in Ar.     

Effect of sintering temperature on the synthesis of LiZnMnFe microwave ceramics with controllable electro/magnetic properties

Self-propagating high-temperature (SPHT) assisted method was successfully used to obtain soft magnetic ceramics based on the nanostructured powder of multicomponent lithium ferrite Li_0·45Zn_0·08Mn_0·06Fe_2·41O₄. The influence of various temperature modes of sintering (900–1075 °C) on the microstructure, morphology, phase composition, and magnetic and electromagnetic properties of the initial powder and final ceramic products was analyzed in detail by methods of scanning electron microscopy, transmission electron microscopy, energy-dispersive and atomic absorption spectroscopy, thermogravimetry, calorimetry, X-ray diffractometry, induction magnetometry and measurement of electromagnetic parameters. It was shown that the particle size of the initial powder is 40 nm and depending on the sintering modes, it is possible to obtain lithium-zinc-manganese ferrite ceramics with an average grain size from 0.35 to 2.96 μm and porosity varying from 3.8 to 23.3%. The coercive force, residual induction, and saturation induction of the obtained samples were 51.3–816.6 A/m, 761.40–2131.30 G, and 1234.28–3431.71 G, respectively. It was found that the sample sintered at a temperature of 1075 °C for 8 h demonstrated the most pronounced soft magnetic behavior. The electromagnetic characteristics of the obtained samples also depended on the selected sintering mode and reached their maximum for ceramics sintered at a temperature of 1075 °C (ϵ’ = 11.52; ϵ” = 6.32⸱10⁻³; tan δ_ϵ = 5.49⸱10⁻⁴; ΔH = 280 Oe; ΔH_k = 1.87 Oe).     

Mechanical properties of 2.45 GHz microwave sintered Si3N4–Y2O3–MgO–ZrO2 system

Mechanical properties of 2.45 GHz microwave sintered Si₃N₄–Y₂O₃–MgO–ZrO₂ system have been investigated. Microwave sintered samples exhibited higher hardness compared to conventionally sintered samples. SEM microstructures of microwave sintered samples revealed lower average grain length and width than those of the conventionally sintered samples. Fracture toughness increased with increasing sintering temperature in the case of conventionally sintered samples whereas microwave sintered samples exhibited no variation despite differences in microstructure. The results of present study demonstrated that microwave sintering could influence the microstructure and thereby improve the mechanical properties.     

Microwave dielectric properties of Al2O3 ceramics sintered at low temperature

The sintering behavior, microstructure and microwave dielectric properties of Al₂O₃ ceramics co-doped with 3000ppmCuO₂+6000ppmTiO₂+500ppmMgO (Cu/Ti/Mg) have been investigated. The results show that 1 wt% Cu/Ti/Mg can reduce the sintering temperature of Al₂O₃ ceramics effectively. Samples with relative densities of ≥97% and uniform microstructure can be obtained when sintered at 1150 °C. Higher temperature can further increase the density of the sample, but it inevitably leads to abnormal grain growth. Meanwhile, the investigation results show that the low-firing Al₂O₃ ceramics have good microwave dielectric properties especially high Q × f value. A high Q × f value of 109616 GHz is able to be obtained for the 1150 °C sintered sample. The reason for the low temperature densification, abnormal grain growth behavior and the changing trend of the microwave dielectric properties are discussed in the paper.     

Microstructure and properties of NiAl/TiC composite synthesized by spark plasma sintering of mechanically activated elemental powders

In this study, NiAl/TiC_0.95 composite was synthesized by reactive spark plasma sintering of mechanically activated elemental powders. The microstructure and properties of activated powders and sintered samples were evaluated. The elemental powders were milled after different milling times and as-mixed and 10 h milled powder mixtures were sintered by the reactive spark plasma sintering method. The phase and the microstructure changes were evaluated by x-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy, respectively. The XRD pattern of 0 h milled powder after sintering showed that Ni₃Al, Ni₂Al₃ beside NiAl and TiC_0.75 formed. While after the sintering of 10 h mechanically activated powder, the Ni₃Al and Ni₂Al₃ were eliminated and NiAl remained with TiC_0.95. The nanoindentation result of the SPSed sample showed a hardness of 12.2 ± 0.1 GPa with an elastic modulus of 25.0 ± 0.5 GPa.     

Microstructure and ionic conductivity of Li1.5Al0.5Ge1.5(PO4)3 solid electrolyte prepared by spark plasma sintering

In this paper, the microstructure and ionic conductivity of Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolytes prepared by spark plasma sintering (SPS) were investigated by XRD, SEM, TEM and EIS, respectively. The results showed that as the sintering temperature was increased, both the relative density and the ionic conductivity of the sintered LAGP samples first increased and then decreased, achieving a maximum value of 97% and 2.12 × 10⁻⁴ S cm⁻¹ simultaneously at 700 °C. At the same time, the crystallinity of the sintered samples was improved, while a few impurity phases, such as AlPO₄ and GeO₂, appeared in the samples. It was also found that carbon contamination and oxycarbide gas was be brought in during SPS. Carbon contamination could produce an extra grain boundary impedance to the samples and could be removed by annealing at 500 °C in an air atmosphere. Oxycarbide gas could affect the relative density of the sintered LAGP samples and could be mitigated by choosing a suitable SPS process. Moreover, the shear modulus of the sintered LAGP was measured to be 49.6 GPa, which exceeded the minimum value of 8.5 GPa that was necessary to suppress Li dendrite growth.     

Electrical and Magnetic Properties of NiZn Ferrite Prepared by Conventional and Solar Sintering

The electrical properties of polycrystalline NiZn ferrite, Zn_0.44Ni_0.38Fe_2.18O₄, were investigated by impedance spectroscopy over the frequency and temperature ranges, 5 Hz to 2 MHz and 10–600 K and by magnetic permeability measurements at room temperature. Samples were sintered in either conventional or solar furnaces followed by quenching or slow cooling to ambient temperature. Depending on processing conditions, the room‐temperature electrical resistivity of conventionally sintered samples varied by seven orders of magnitude, from 5 ohm cm for a sample quenched from 1250°C to 10 Mohm cm for a sample quenched from 400°C. These differences were attributed to variations in oxygen content of the ferrite which decreased with increasing quench temperature. Oxygen deficiency led to mixed valence of Fe in the octahedral B sites of the spinel structure and was responsible for high electronic conductivity with low activation energy at low temperatures in oxygen‐deficient samples. By contrast, in oxygen‐stoichiometric samples, Fe on the tetrahedral A sites was believed to be divalent and Fe on the octahedral B sites to be entirely trivalent. Electron hopping between A and B sites had much higher activation energy and dominated the conductivity at high temperature for all samples. Samples sintered in the solar furnace were much more conductive than ones that were slow‐cooled after conventional sintering and this is attributed to the relatively rapid cooling rate after exposure in the solar furnace, which preserved some of the oxygen deficiency present at high temperature. For the same reason, samples that were slow cooled in N₂ were also much more conductive. Solar‐sintered samples with higher density (96%) had higher real permeability than slow‐cooled, conventionally sintered ones (86%) mainly due to a combination of their lower resistivity and higher density. Resistivity seems to have a greater correlation with the imaginary permeability than density has.     

AlMgB14–TiB2 composite materials obtained by self-propagating high-temperature synthesis and spark plasma sintering

In this work, AlMgB₁₄–TiB₂ composite materials were obtained by thermochemical-coupled self-propagating high-temperature synthesis (SHS) and subsequent spark plasma sintering. The mechanism was proposed for the formation of the composite materials in the thermochemical-coupled SHS mode. The phase composition, microstructure, and properties (density and Vickers hardness) of the dense AlMgB₁₄–TiB₂ materials were investigated. At a sintering temperature of 1470 °C, AlMgB₁₄ is decomposed into AlB₁₂ and Mg. The sample sintered at 1470 °C with a holding time of 5 min had a maximum average hardness of 32.1 GPa.