Densification, microstructure and mechanical properties of SiO2-cBN composites by spark plasma sintering

SiO₂-cBN composites were consolidated by spark plasma sintering at 1473-1973 K. The effects of cBN content and sintering temperature on the relative density, phase transformation, microstructure and mechanical properties of the SiO₂-cBN composites were investigated. The relative density of the SiO₂-cBN composites increased with increasing SiO₂ content. The phase transformation of cBN to hBN in SiO₂-cBN composites was identified at 1973 K, showing the highest transformation temperature in cBN-containing composites. The SiO₂-20 vol% cBN composites sintered at 1673 K showed the highest hardness and fracture toughness of 12.5 GPa and 1.5 MPa m^1/2, respectively.     

Effects of sintering temperature on structural and electromagnetic properties of MgCuZn ferrite prepared by microwave sintering

Nanocrystalline magnesium–copper–zinc (Mg_0.30Cu_0.20Zn_0.50Fe₂O₄) ferrites were prepared by microwave sintering technique. The effects of the sintering temperature on particle size and magnetic properties were investigated. In this article, optimum sintering temperature required for MgCuZn ferrite system for obtaining good electromagnetic properties, suitable for applications in low temperature co-fired ceramics (LTCC) chip components was studied. The grain size, initial permeability, dielectric constant and saturation magnetisations were found to increase, and dielectric loss was found to decrease with the increasing sintering temperature. Mg–Cu–Zn ferrites with a permeability of μ?=?1110 (at 1?MHz) were fully densified at the standard LTCC sintering temperature of 950°C.     

Magnetoelectric composites of nickel ferrite and lead zirconnate titanate prepared by spark plasma sintering

Magnetoelectric (ME) bulk composites of ferrite and lead zirconnate titanate (PZT) were prepared by spark plasma sintering (SPS) of mechanically mixed ferrites, BaFe₂O₄ or NiFe₂O₄ and a soft lead zirconnate titanate, PZT-5A, powders. The feasibility of retarding possible reactions occurring between the ferrite and lead zirconnate titanate was approved by applying such a dynamic process as SPS. It was further revealed that nickel ferrite and PZT-5A is a more favorable combination that underwent no obvious reactions up to 1050 °C. Efforts were made to optimize the SPS processing parameters in order to produce immiscible composites with high electrical resistivity, low dielectric loss and better magnetoelectric response.     

Enhanced low-field magnetoresistance in LSMO/AZO composites prepared by plasma activated sintering

To improve the low-field magnetoresistance of La_0.7Sr_0.3MnO₃ (LSMO) ceramics, Al (3?wt%)-doped ZnO (AZO) was added to LSMO as the second phase, and (1???x) LSMO/xAZO (x?=?0, 0.1, 0.2, 0.3 and 0.4) composites were first prepared by plasma activated sintering. The effect of the AZO content on the structural, electrical and magnetic properties of the composites was studied. The results of XRD, SEM, EDS and TEM analyses indicated that the LSMO phase coexisted with the AZO phase without any reaction, and that all of the samples had a high relative density (>?96.9%). The magnetization of the (1???x) LSMO/xAZO composites decreased with the increase of the AZO content, while the Curie temperature remained at approximately 360?K. With an increase of the AZO content, both the resistivity and metal-insulator transition temperatures decreased. The resistivity (ρ) – temperature (T) data were well fitted to the equation of $\rho ={\rho }_0+{\rho }_2{T}^2+{\rho }_{4.5}{T}^{4.5}+{\rho }_5{T}^5$ in the low-temperature region, and to the non-adiabatic small polaron hopping model in the high-temperature region. The maximum value of the low-field magnetoresistance at room temperature was obtained for the 0.8LSMO/0.2AZO composite, which reached 16.2%, indicating the addition of AZO effectively enhanced the low-field magnetoresistance of LSMO ceramics.     

Microstructure and magnetic properties of nickel-zinc ferrite ceramics fabricated by spark plasma sintering

Dense nickel-zinc (NiZn) ferrite ceramics were successfully fabricated within tens of seconds via spark plasma sintering. The phase composition and microstructure of the sintered samples were characterized by X-ray diffraction and scanning electron microscopy, respectively. The static magnetic properties at room temperature and Curie temperature of the samples were investigated by vibrating sample magnetometry. The results indicated that the main phase of the sintered samples was Ni_0.75Zn_0.25Fe₂O₄ with spinal structure, and the sintering temperature and heating rate observably affected the microstructure and density, then the magnetic properties of the sample. The Joule heat generated by NiZn ferrite during spark plasma sintering was very important for the rapid preparation of the sample with high density and small grain size. The low sintering temperature and heating rate would be helpful to obtain samples with small grain size, high density, and then good magnetic properties. The samples sintered at 900 °C with the heating rate of 5–10 °C/s were characterized of the relative density above 95%, 4πM_s value beyond 4000 Gs and coercivity below 27.7 Oe.     

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.     

Microwave-assisted solution synthesis,microwave sintering and magnetic properties of cobalt ferrite

A simple, soft, and fast microwave-assisted hydrothermal method was used for the preparation of nanocrystalline cobalt ferrite powders from commercially-available Fe(NO₃)₃?9H₂O, Co(NO₃)₂?6H₂O, ammonium hydroxide, and tetrapropylammonium hydroxide (TPAH). The synthesis was conducted in a sealed-vessel microwave reactor specifically designed for synthetic applications, and the resulting products were characterized by XRD, FE-SEM, TEM, and HR-TEM. After a systematic study of the influence of the microwave variables (temperature, reaction time and nature of the bases), highly crystalline CoFe₂O₄ nanoparticles with a high uniformity in morphology and size, were directly obtained by heating at 130?°C for 20?min using the base TPAH. Dense ceramics of cobalt ferrite were prepared by non-conventional, microwave sintering of synthesized nanopowders at temperatures of 850–900?°C. The magnetic properties of both the nanopowders and the sintered specimens were determined in order to establish their feasibility as a permanent magnet.     

Densification and mechanical properties of boron carbide prepared via spark plasma sintering with cubic boron nitride as an additive

Hexagonal boron nitride (h-BN) can reinforce boron carbide (B₄C) ceramics, but homogeneous dispersion of h-BN is difficult to achieve using conventional methods. Herein, B₄C/h-BN composites were manufactured via the transformation of cubic (c-) BN during spark plasma sintering at 1800 °C. The effects of the c-BN content on the microstructure, densification, and mechanical properties of B₄C/h-BN composites were evaluated. In situ synthesized h-BN platelets were homogeneously dispersed in the B₄C matrix and the growth of B₄C grains was effectively suppressed. Moreover, the c-BN to h-BN phase transformation improved the sinterability of B₄C. The sample with 5 vol.% c-BN exhibited excellent integrated mechanical properties (hardness of 30.5 GPa, bending strength of 470 MPa, and fracture toughness of 3.84 MPa⋅ m^1/2). Higher c-BN contents did not significantly affect the bending strength and fracture toughness but clearly decreased the hardness. The main toughening mechanisms were crack deflection, crack bridging, and pulling out of h-BN.     

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.     

Spark plasma sintering of WC-Ti powder mixtures and properties of obtained composites

Three WC-Ti powder mixtures with 5, 10 and 15 wt% titanium were sintered by the spark plasma sintering technique. The microstructures and phase compositions of the samples were investigated by SEM, STEM, EBSD and XRD. The samples consisted of WC, W₂C and a (W_1-xT_x)C phases when the starting amounts of titanium were 5 and 10 wt%. At the titanium content of 15 wt% the microstructure of the samples included W₂C, (W_1-xT_x)C phases and elemental tungsten. The solubility of WC in TiC with the appearance of the (W_1-xT_x)C phase depended on the stoichiometry of the starting powder composition and sintering temperature. The results of EBSD phase mapping and the XRD investigation are in good agreement with the molar analysis. The best combination of hardness and fracture toughness was achieved with 5 wt% titanium. The appearance of elemental tungsten after sintering the WC-15Ti composition led to a significant reduction in hardness.     

Effect of sintering regimes on the microstructure and magnetic properties of LiTiZn ferrite ceramics

In present work, the influence of sintering regimes on the microstructure, saturation magnetization, density and porosity, the grain size, the Curie point, and the temperature dependence of the initial permeability of LiTiZn ferrite ceramics was investigated. Ceramics was prepared by a standard ceramic technique. The formation of a single-phase cubic spinel structure was confirmed by XRD analysis. The Curie point was determined from both the temperature dependences of the initial permeability and the method of thermogravimetric measurements in a magnetic field. Density/porosity and the grain size, the Curie point and magnetization are sensitive to the sintering regime. The initial permeability of ferrite decreases with sintering temperature (in the range of 1010–1150?°С) and grain size increasing that contradicts the generally accepted Globus and Smith-Wijn theories. A possible reason of such behavior is the formation of intragranular pores growing with the increase in the sintering temperature and inhibiting the domain wall motion inside the grain. These results correspond to the porosity of the investigated ferrite ceramic samples, which grows with sintering temperature increasing.The non-stoichiometry arising due to evaporation of lithium and zinc oxides at temperature above 1010?°C affects the initial permeability. In this work, a qualitative assessment of the defective state of ferrite samples obtained under various sintering regimes was given.     

Structural and magnetic properties of zinc ferrite incorporated in amorphous matrix

Glass ceramics in the (Fe₂O₃)_x·(B₂O₃)_(60−x)·(ZnO)₄₀ (x = 17.5 and 20 mol%) system were prepared by the melt-quench method and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and magnetization measurements. The samples contain a unique magnetic crystalline phase, the zinc ferrite (ZnFe₂O₄), embedded in an amorphous matrix. The ZnFe₂O₄ crystals precipitate during cooling from melting temperature. From the XRD data, the average unit-cell parameter, crystallite size and the quantitative ratio of the crystallographic phases in the samples were evaluated. FTIR data revealed that the BO₃ and BO₄ are the main structural units of these glass ceramics network. FTIR spectra of these samples show features at characteristic vibration frequencies of ZnFe₂O₄. From the magnetization curves it was found that the nanoparticles exhibit ferromagnetic interactions combined with superparamagnetism with a blocking temperature, T_B, which is composition dependent. In all samples hysteresis is present below T_B. The coercive field is dependent on composition and magnetic field being around 0.05μ_B/f.u. for measurements performed in maximum 0.4 T. Finally, the magnetic behavior of iron in this system is discussed.     

Micromechanical mapping of polycrystalline cubic boron nitride composites by means of high-speed nanoindentation: Assessment of microstructural assemblage effects

Polycrystalline cubic boron nitrides (PcBNs) are composites widely used as cutting tool materials due to their exceptional high hardness and wear resistance. Investigation of their micromechanical properties is key for optimizing PcBN’s performance through microstructural design. Within this context, high-speed nanoindentation is proposed and implemented, for three different PcBN grades, to correlate microstructure with local mechanical properties. A total of 40,000 imprints were performed in each grade. The obtained mechanical maps and data sets are statistically treated following two deconvolution approaches: 1D and 2D Gaussian fitting. The use of high-speed nanoindentation is validated not only by the reliable assessment of the intrinsic mechanical properties of cBN particles, binder and interphase region, but also by the successful mirroring of microstructural assemblage within the mechanical maps attained. Comparison of the results determined from 1D and 2D gaussian representations are in satisfactory agreement. Nevertheless, some difficulties and disparity between them arises when involving fine-grained microstructures.     

Toughening mechanism of PPS/Sr ferrite composites by TPU elastomer

In order to improve the impact strength of PPS‐based strontium ferrite composite, the thermoplastic polyurethane (TPU) elastomer was added in the composite as a toughening agent. The composites were obtained by melt‐blending PPS, TPU and strontium ferrites in twin‐screw extruder. The crystalline state, thermal property, surface morphology and impact strength of the composites were investigated by using X‐ray diffraction, differential scanning calorimetry, thermoravimetric analysis, scanning electron microscope and izod impact test. The addition of TPU improves impact strength of PPS‐based strontium ferrite composite. When the addition of TPU increases to 11wt %, the impact strength of Sr‐ferrite/PPS/TPU composite is enhanced by 51.44% compared with the sample without TPU addition, and reaches to 5.77 kJ/m². The occurrence of bonding interaction between PPS and TPU, demonstrated by a series of experiments, changes the structure and impact properties of PPS. Based on the experimental results, a possible mechanism is proposed to explain the improvement of Sr‐ferrite/PPS/TPU composites, which is different from the conventional toughening mechanism by the conformation of elastomers and the suppression of microcracks propagation. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43564.     

Investigation of the structural and mechanical properties of micro-/nano-sized Al2O3 and cBN composites prepared by spark plasma sintering

Alumina-cubic boron nitride (cBN) composites were prepared using the spark plasma sintering (SPS) technique. Alpha-alumina powders with particle sizes of ∼15 µm and ∼150 nm were used as the matrix while cBN particles with and without nickel coating were used as reinforcement agents. The amount of both coated and uncoated cBN reinforcements for each type of matrix was varied between 10 to 30 wt%. The powder materials were sintered at a temperature of 1400 °C under a constant uniaxial pressure of 50 MPa. We studied the effect of the size of the starting alumina powder particles, as well as the effect of the nickel coating, on the phase transformation from cBN to hBN (hexagonal boron nitride) and on the thermo-mechanical properties of the composites. In contrast to micro-sized alumina, utilization of nano-sized alumina as the starting powder was observed to have played a pivotal role in preventing the cBN-to-hBN transformation. The composites prepared using nano-sized alumina reinforced with nickel-coated 30 wt% cBN showed the highest relative density of 99% along with the highest Vickers hardness (H_v2) value of 29 GPa. Because the compositions made with micro-sized alumina underwent the phase transformation from cBN to hBN, their relative densification as well as hardness values were relatively low (20.9–22.8 GPa). However, the nickel coating on the cBN reinforcement particles hindered the cBN-to-hBN transformation in the micro-sized alumina matrix, resulting in improved hardness values of up to 24.64 GPa.     

An oscillatory pressure sintering of zirconia powder: Densification trajectories and mechanical properties

The densification trajectories and mechanical properties of zirconia ceramics obtained by oscillatory pressure sintering (OPS) process were investigated, during the sintering process an oscillatory pressure was applied at three stages. Current results indicated that at intermediate stage the oscillatory pressure revealed a favorable improvement of mechanical properties compared with conventional hot pressing (HP) and pressureless sintering (PS) procedures, while the enhancement was not obvious at initial stage. When the oscillatory pressure was applied at final stage, the OPS specimens exhibited the highest bending strength and hardness of 1455 ± 99MPa and 16.6 ± 0.31GPa compared with the PS and HP specimens. Considering the high elastic modulus and Moiré patterns observed in the OPS specimen, the oscillatory pressure applied at intermediate and final stages was detected to facilitate the sliding of grain boundary, plastic deformation of monolithic grains, the removal of pores and the strengthening of atomic bonds.     

Structural,magnetic and electrical properties of Y1-xScxFeO3 (x= 0, 0.5 & 1) nanoparticles synthesized by the sol-gel method

Nanoparticles exhibiting crystalline and single-phase characteristics with Y_1-xSc_xFeO₃ (x = 0, 0.5 & 1) nanoparticles synthesized by sol-gel method. The sizes of nanocrystals determined by X-Ray Diffractometry (XRD) were obtained between 30 and 39 nm. The shape of nanoparticles and the surface morphology of the samples were carried out using Field Emission Scanning Electron Microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). The magnetic properties of Y_1-xSc_xFeO₃ samples were investigated by Vibrating Sample Magnetometry (VSM) and Field-Cooled (FC) and Zero-Field-Cooled (ZFC) measurements. The results showed antiferromagnetic and paramagnetic behaviors for these prepared perovskites. The electrical properties of the samples were investigated by measuring the polarization-electric loops, dielectric constant changes and dielectric loss through frequency and temperature. These measurements showed the T_N values of the samples to be 628 and 578 K for YFeO₃ and ScFeO₃ nanoparticles, respectively. In addition, diffuse reflectance spectroscopy analysis was used to calculate the band gap energy of the samples based on the Kubelka-Munk function. The achieved values showed the band gap energy of 2.97 and 3.07 eV for YFeO₃ and ScFeO₃.     

Chromium incorporated nanocrystalline cobalt ferrite synthesized by combustion method: Effect of fuel and temperature

Nanocrystalline CoCrFeO₄ samples were prepared by Cost effective and low temperature combustion method using different fuels as glycine, urea and polyvinyl alcohol and the effect of temperature on their physical properties has been studied. The thermal study of the precursor gels was done by using Thermogravimetric and Differential thermal analysis (TG-DTA). Occurrence of the cubic spinel phase for the samples was confirmed by the Rietveld analysis of X-ray diffraction (XRD) data. The refinement results also confirm that the cationic distribution over the tetrahedral and octahedral sites in the spinel lattice of samples is partially inverse. The magnetic studies indicate a superparamagnetic behaviour, showing an increase in the blocking temperatures with the particle size in case of all fuels. All the samples showed ferromagnetic behaviour and the nanoparticles prepared by using glycine as a fuel were found to have largest crystallite size and relatively stronger A–O–B interaction compared to others. Magnetization measurements exhibit non-collinear ferrimagnetic structure for the samples.     

Ultra-low core loss FeSiAl-based soft magnetic composites with ultra-thin MoO3 composite insulating layer

In this work, core/shell structured FeSiAl/MoO₃ (spherical FeSiAl covered by ultra-thin MoO₃ composite insulating layer) soft magnetic composites (SMCs) have been fabricated by a two-step heat treatment process. The influences of ammonium molybdate (AHM) content, first-step annealing temperature and second-step annealing temperature on magnetic, mechanical properties and electrical resistivity (ρ) have been comprehensively investigated. It is shown that the coating integrity and the thickness of MoO₃ nanoparticles layer can be regulated by the content of AHM, leading to the improvement of ρ. Moreover, composite insulating layer with the thickness of 47 ± 8 nm is formed and completely coated on FeSiAl particles with 15 wt% AHM, resulting in the fact that the highest radial crushing strength (K = 62.3 MPa) and lowest core loss (P_cv = 128.8 mW/cm³) is obtained. In addition, P_cv is separated into two components: hysteresis loss component and eddy current loss component. Further studies display that eddy current loss is only half of hysteresis loss. As a result, the FeSiAl SMCs with 47 ± 8 nm MoO₃ composite insulating layer possess the lowest core loss of 128.8 mW/cm³ at 50 mT/100 kHz. The low core loss of the FeSiAl/MoO₃ SMCs with ultra-thin composite insulating layer has a great potential in the fields of conversion and power transmission.     

Densification and microstructure evolution during sintering of silicon under controlled water vapor pressure

Sintering of fine silicon powder was studied under controlled water vapor pressures using the Temperature–Pressure–Sintering Diagram approach. The water vapor pressure surrounding the sample was deduced from thermogravimetric analysis and related to the water content of the incoming gas flux with a simple mass transfer model. The thickness of the silica layer covering silicon particles was then monitored by the water vapor pressure and the microstructure evolution and densification during sintering could be controlled. Stabilizing the silica layer indeed inhibits grain coarsening and allows better densification of the compacts under humidified atmosphere as compared to dry atmosphere.