Synthesis and Magneto-mechanical Properties of Ce-TZP/La M-Type Hexaferrite Composite

An in situ composite composed of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP) and La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was synthesized from a powder mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO. The dense Ce-TZP dispersed with platelike La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ crystals as a second phase were formed after sintering from 1250° to 1350°C. The saturation magnetization of the in situ composite Ce-TZP/La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was proportional to the mass fraction of the hexaferrite second phase in Ce-TZP. The coercivity of the composite with a 20 mass% of second phase decreased from 9.14 to 2.52 kOe (from 728 to 201 kA/m) after the pulverization of the composite. The susceptibility (χ) increased by 15%–25% under uniaxial stress on the composite. The change of the susceptibility (Δχ/χ) value increased with decreasing the mass fraction of the second phase in the composite. The Δχ was found to increase linearly with applied stress and abruptly change on cracking, which is expected for the application in fracture sensing of the composite.     

Diffusion of Co2+ and Ni2+ in Magnesium Oxide

Rates of diffusion of Co²⁺ and Ni²⁺ ions in the MgO lattice were determined by a micrographic method. The energies of activation for the transport process were 65 and 36 kcal per mole, respectively. The penetration curve in these systems exhibited a plateau followed by a sudden decrease in concentratior.     

In Situ Formation of Hexaferrite Magnets within a 3Y-TZP Matrix: La2O3–ZnO–Fe2O3 and BaO–Fe2O3 Systems

The in situ formation of magnetoplumbite-type (M-type) hexaferrites within a 3Y-TZP matrix was examined for the La₂O₃–ZnO–Fe₂O₃ and BaO–Fe₂O₃ systems. The formation of barium hexaferrite (Ba-M) was rapid enough at a temperature of 1300°C for 2 h to result in a uniform dispersion of fine Ba-M particles in a tetragonal zirconia polycrystal (TZP) matrix. However, the formation of lanthanum-substituted hexaferrite (La-M) was rather sluggish, despite the existence of a charge-compensating divalent oxide. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ in situ composite possessed good magnetic properties, as well as moderately good mechanical properties.     

Low-Temperature Sintering, Densification, and Properties of Z-type Hexaferrite with Bi2O3 Additives

The effect of the addition of Bi₂O₃ on the densification, low-temperature sintering, and electromagnetic properties of Z-type planar hexaferrite was investigated. The results show that Bi₂O₃ additives can improve the densification and promote low-temperature sintering of Z-type hexaferrite prepared by a solid-state reaction method. The presence of Bi₂O₃ in the grain boundaries and the generation of Fe²⁺ degrade the initial permeability of the samples but make the quality factor and cut-off frequency increase. Various possible mechanisms involved in generating these effects were also discussed.     

Determination of the Pcmn/Ibm2 Phase Boundary at High Temperatures in the System Ca2Fe2O5-Ca2Al2O5

The crystals of Ca₂(Al _x Fe_{1− x} )₂O₅ with 0 ≤ x ≤ 0.4 were examined using powder XRD at temperatures between 25° and 1000°C. The Pcmn to Ibm 2 phase transition was readily detected by the disappearance of the 131 and 151 reflections ( h + k + l odd). Thus, the phase relationship has been determined as combined functions of the temperature and x -value. At 25°C, the space group changed from Pcmn (0 ≤ x ≤ 0.23) to Ibm 2 (0.24 ≤ x ≤ 0.40) across x = 0.235. With increased x -value, the cell dimensions of both phases steadily decreased, showing the nearly complete continuity at that phase boundary. For the crystal with x = 0 (Ca₂Fe₂O₅), the cell dimensions steadily increased during heating to 685°C, at which temperature the phase transition occurred. There was a slight discontinuous decrease in cell dimensions just after the transition. Subsequent heating to 1000°C of the Ibm 2 phase led to a steady thermal expansion along the b -axis and c -axis, while a slight contraction occurred along the a -axis up to 800°C.     

Sintering of (Cu0.25Ni0.25Zn0.50)Fe2O4 Ferrite

A study has been conducted on the sintering of a ceramic ferrite having the composition (Cu_0.25Ni_0.25Zn_0.50)Fe₂O₄. The study analyzes the evolution of ferrite relative to density and microstructure with peak sintering temperature and dwell time at peak temperature. The densification and grain-growth rates are correlated with average grain size, relative density, and temperature. Corresponding rate-controlling diffusion mechanisms are proposed.     

Micromechanical Stresses in SiC-Reinforced Al2O3 Composites

Applying an Eshelby approach, the internal micromechanical stresses within an SiC-inclusion-reinforced (platelet to whisker geometries) polycrystalline alumina matrix composite were calculated. The results are compared to the experimental residual stress measurements of a SiC-whisker-reinforced Al₂O₃ by Predecki, Abuhasan, and Barrett and found to be in excellent agreement. The calculations are then extended to SiC-reinforced composites with polycrystalline mullite, silicon nitride, and cordierite matrices. It is concluded that the internal stresses are significantly influenced by the inclusion geometry as well as the thermoelastic differences between the inclusion and the matrix and also the volume fraction.     

Mechanical Properties of Si2N2O/SiC-Whisker Composites

The mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were studied with emphasis on the effect of matrix composition and of whisker content. The fracture toughness of Si₂N₂O was remarkably improved by 90% with a concomitant 70% strength improvement by addition of SiC whiskers of only 10 vol%. Optimum mechanical and thermal properties of Si₂N₂O/SiC-whisker composites were obtained at an equimolar ratio of Si₃N₄/SiO₂, which is the stoichiometric composition for Si₂N₂O. Additional investigation concerning the Si₂N₂O-matrix/SiC-whisker interface by controlling sintering additives is necessary for further improvement of mechanical and thermal properties of Si₂N₂O/SiC-whisker composites.     

Electric-Field-Induced Crack Growth Behavior in PZT/Al2O3 Composites

Hard lead zirconate titanate (PZT) and PZT/Al₂O₃ composites were prepared and the alternating-electric-field-induced crack growth behavior of a precrack above the coercive field was evaluated via optical and scanning electron microscopy. The crack extension in the 1.0 vol% Al₂O₃ composite was significantly smaller than that in monolithic PZT and the 0.5 vol% Al₂O₃ composite. Secondary-phase Al₂O₃ dispersoids were found both at grain boundaries and within grains in the composites. A large number of dispersoids were observed at the grain boundaries in the 1.0 vol% Al₂O₃ composite. It appears that the Al₂O₃ dispersoids reinforce the grain boundaries of the PZT matrix as well as act as effective pins against microcrack propagation.     

SiC-Whisker-Reinforced Cellular SiO2 Composites

Fabrication of lighweight, cellular ceramic composites by foaming sol–gels is presented. Foams of 20-vol%-SiC-whisker-reinforced SiO₂ can be tailored with relative densities as low as 10%, with either open or closed cell structures. In addition to reducing drying shrinkage and thus gel cracking, whisker reinforcement significantly increases the strength of the composite foams relative to pure silica foams.     

Energetics of the Charge-Coupled Substitution Si4+→ Na++ T3+ in the-Glasses NaTO2–SiO2 (T = Al, Fe, Ga, B)

Heats of solution in molten 2PbO·B₂O₃ at 973 K are reported for glasses x NaT³⁺O₂–(1 – x )SiO₂ for T = Fe, Ga. These measurements, combined with previous data for T = Al, B, give a relative measure of the enthalpy of the charge-coupled substitution Si⁴⁺→ Na⁺+ T³⁺. The heats of solution become more endothermic with increasing x for x → 0.5 and exhibit a maximum near x = 0.5. This indicates an exothermic enthalpy for the substitution and an overall stabilization of the glasses. The degree to which the glasses are stabilized decreases in the order Al > Ga > Fe > B. On the basis of molecular orbital calculations, X-ray scattering, and Raman spectroscopy, it is argued that this trend is primarily due to a decrease in the range of energetically favorable T–O–T bond angles as Al, Ga, Fe, and B are substituted for Si.     

Microstructural Characterization of Sintered MoSi2/SiCP Composites

A MoSi₂/SiC_P composite was synthesized by in situ reactive sintering of a mixture of molybdenum, silicon, and carbon powders. Its microstructural features were studied by X-ray energy dispersive spectroscopy (EDS), conventional transmission electron microscopy (CTEM), and high-resolution transmission electron microscopy (HREM). It was determined that the composite was composed of α-MoSi₂ and β-SiC. There were no specific orientation relationships between the MoSi₂ matrix and SiC_P, because the MoSi₂ and SiC were formed at 1450°C by the reaction of solid Mo and C and liquid Si. The abrupt change occurring in the microstructure of the composite is explained by the presence of an interface between MoSi₂ and SiC_P, where no observable SiO₂ amorphous layer or particles were found. Microtwins and stacking faults were frequently observed in {111} planes of SiC_P.     

Effects of Y2O3 and ZrO2 on the Densification of Si3N4-Zr(Y)O2 Composites

The effects of ZrO₂ and Y₂O₃ on the densification of hotpressed Si₃N₄-Zr(Y)O₂ composites have been studied. High density could not be obtained by the addition of pure or 3-mol%-Y₂O₃-doped ZrO₂ in this composite; however, nearly full density (>97%) could be obtained in Si₃N₄ using 6- and 8-mol%-Y₂O₃-doped ZrO₂. It is concluded that Y₂O₃ diffusing out from the added Zr(Y)O₂ promoted the densification and that ZrO₂ also had some role in the formation of an oxynitride glass.     

Structure Formation in the Combustion Synthesis of Al2O3–TiC Composites

Structure formation in the combustion synthesis of Al₂O₃–TiC composites from TiO₂, Al, and graphite powders was investigated using cylindrical samples and cone-shaped "quenching samples." It is shown that the phases Ti and Ti₃Al exist as intermediates in the combustion synthesis process. Titanium carbide forms in a secondary step through reactions between graphite and liquid Ti or Ti₃Al, then nucleates from a liquid mixture of the three phases Ti, Ti₃Al, and alumina. The nucleated particles grow in the postcombustion stage. Liquid alumina, containing TiC as a dissolved phase, solidifies into corundum grains in the postcombustion stage. Moreover, it is shown that the temperature gradient in the postcombustion stage markedly affects the microstructures of the products. Higher-temperature gradients, typical at the surface of the samples, give rise to the formation of corundum whiskers and TiC agglomerates. In contrast, lower gradients, typical in the center of the samples, lead to the formation of relatively large TiC particles and corundum grains.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.     

Microstructure and Densification of Pressureless-Sintered Al2O3/Si3N4-Whisker Composites

Composite densification was studied by performing slip casting and sintering experiments on an Al₂O₃ matrix and Si₃N₄ whisker system. Even though all the slip-cast powder compacts exhibited high green densities (up to 70% of the theoretical) and narrow pore-size distribution (pore radius around 15 to 30 nm), significant differential densification on a microscopic scale was found due to the existence of local whisker agglomeration. The inhomogeneous whisker distribution resulted in a binary mixture of large and small pores in the sintered composites, in which whisker-associated flaws remained stable even after prolonged sintering. The sintered microstructures showed that the spatial distribution as well as the volume fraction of the Si₃N₄ affect composite densification. Inhomogeneous whisker distribution dominated the complete densification of the composites.     

Properties of (Y)ZrO2–Al2O3 and (Y)ZrO2–Al2O3–(Ti or Si)C Composites

The effect of Al₂O₃ and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al₂O₃–(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al₂O₃ to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO₂ and TiC therefore must have inhibited the t - → m -ZrO₂ martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO₂ and Al₂O₃. In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton.     

Mullite (3Al2O3·2SiO2)–Aluminum Phosphate (AlPO4), Oxide, Fibrous Monolithic Composites

Mullite–AlPO₄ fibrous monolithic composites were fabricated by a co-extrusion technique using ethylene vinyl acetate (EVA) as a binder. Processing routes such as mixing formulation, extrusion sequence, binder removal cycle, pressing, and sintering procedures are described. An effort to make tougher composites was conducted by modifying the microstructures of the composites. Different kinds of monolithic composites were fabricated by changing the number of filaments, and the composition and thickness of interphase layers, and their microstructural and mechanical properties were characterized. To make the interphase more porous and to facilitate debonding and fiber pullout in the composite, graphite was added as a fugitive "space filler" into the interphase material and then removed. A fibrous monolithic composite with a sintered interphase thickness of 5–10 μm and an interphase composition of 50 vol% graphite and 50 vol% AlPO₄ had a three-point bend strength and a work of fracture of 129 ± 2 MPa and 0.86 ± 0.05 kJ/m², respectively. This corresponded to 42% of the strength but 162% of the work of fracture when compared with the values for a single-phase mullite. Two-layer, mixed 50% two-layer:50% three-layer, and three-layer fibrous monoliths were fabricated and their microstructural and mechanical properties were studied. The difference in the sintering behaviors of the two-layer and three-layer composites is described.     

Shape Memory-Like Effect Phenomena in a Ce-TZP/Al2O3 Composite

During an investigation of the role of alumina particles in ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP)¹ we observed that a group of samples (based on a Ce-TZP matrix (8.5 mol% Ceria) containing 15 vol% dispersed alumina particles), exhibited an extended plastic deformation after the first elastic response. This deformation could be retained upon a prompt unloading from a fourpoint bend jig. These deformed (bent) specimens recovered their original straight shape when heated above 200°C, exhibiting features like the shape memory effect (SME). A careful examination of the samples revealed the presence of wedgelike transformation zones intruding the bulk of the specimen inwards from the tensioned side.
In an attempt to increase the degree of bending, other samples of the same composition and microstructure have been placed under a static load and maintained under that condition for ≅15 min. The penetration depth of the transformation zone reached the opposite face of the samples and the monoclinic-phase concentration rose about 50% on both sides. Through this inwards movement, these samples, that were initially bent, showed an elongation of about 0.5% and became straightened. These samples returned to their original size after heating at ≅250°C. To get a deeper insight, samples have been examined by X-ray diffraction (XRD), differential thermal analysis (DTA), and thermodilatometry (TD).     

Processing and Mechanical Properties of Al2O3/Ni3Al Composites with Interpenetrating Network Microstructure

Composites with microstructures of interpenetrating networks were manufactured by gas pressure infiltration of Ni₃Al into porous preforms of aluminum oxide. Composites with Ni₃Al contents of between 15% and 30% by volume were made and evaluated mechanically at temperatures between room temperature and 1000°C. The fracture strength, the fracture toughness, Young's modulus, and the thermal expansion coefficient were measured for each composite and test condition and were correlated with the microstructures of the composites. Composites with low Ni₃Al contents had strengths below 400 MPa, presumably due to microcracking along the interface between the Ni₃Al and the Al₂O₃. The composite with the highest content of Ni₃Al, 30 vol%, had a mean fracture strength of 675 ± 16 MPa, a Weibull modulus of 23.9, and a room-temperature toughness of 9.2 ± 0.5 MPa·m^1/2.