Synthesis of Barium Hexaferrite by the Oxidation of a Metallic Barium–Iron Precursor

Tapes of the hard magnetic ferrite, BaFe₁₂O₁₉, were synthesized by the oxidation of metallic barium and iron precursors. Milled mixtures of barium flakes and iron powder (Ba:Fe atomic ratio = 1:12) were sealed within fugitive silver tubes and then compacted and formed into tapes by rolling at room temperature. Oxidation and postoxidation heat treatments were conducted over the temperature range 300°–1250°C (the silver was removed after heat treatment at 900°C). Phase-pure BaFe₁₂O₁₉ tapes were produced after heat treatments at peak temperatures of is greater than or equal to $1080°C. Such tapes exhibited room-temperature magnetization values of is greater than or equal to $65.0 emu/g in an applied field of 16.5 kOe (1.32 × 10⁶ A/m). A coercivity of 3.44 kOe (2.75 × 10⁵ A/m) was achieved in a fine-grained (submicrometer) BaFe₁₂O₁₉ tape that had been annealed for 24 h at a peak temperature of 1080°C.     

Magnetic Properties of Barium Ferrite Formed by Crystallization of a Glass

A glass with composition 0.265 B₂O₃-0.405 BaO-0.33 Fe₂O₃ (mole ratio) was prepared by a fast-quenching technique. When it is heat-treated, this glass exsolves up to ∼45 wt% BaFe₁₂O₁₉ as the only magnetic phase. Magnetic measurements of glasses heated at various temperatures show that superparamagnetic, single-domain, or multidomain magnetic behavior is present, depending on the thermal history. The volume of a typical superparamagnetic particle (calculated from the magnetic data) is equivalent to that of a sphere 47 Å in diameter. The intrinsic coercive forces of two heat-treated glasses were independent of temperature at high levels of H_ci (2600 and 2900 Oe) from 77° to 300°K. Another heat-treated glass has an H_ci of 5350 Oe at 300°K. Apparently, the coherent rotation model of Stoner and Wohlfarth describes the magnetic behavior of BaFe₁₂O₁₉ very well. The single-domain critical size for BaFe₁₂O₁₉ was ∼0.5 μm. An attractive feature of this system is that the BaFe₁₂O₁₉ powder can be recovered from the barium-borate-rich matrix by leaching with a weak acid.     

Preparation and Characterization of Fine-Grained 3Y-TZP/BaFe12O19 In Situ Composites

Composites of BaFe₁₂O₁₉ particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO₃, and γ-Fe₂O₃ powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe₁₂O₁₉ particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe₁₂O₁₉ particles was ∼200-500 nm. Based on the grain size, most of the BaFe₁₂O₁₉ grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.     

Preparation and Magnetic Properties of Barium Hexaferrite Nanoparticles Produced by the Citrate Process

The sol-gel synthesis technique has been used to produce barium ferrite particles. Several methods such as X-ray diffractometry, transmission electron microscopy, and magnetism measurement (VSM) have been used to obtain detailed information on the crystallography and magnetic properties of the precursor and the calcined particles. The correlation of BaFe₁₂O₁₉ formation with the precursor properties and thermal treatment has been studied. The optimum conditions for preparing BaFe₁₂O₁₉ nanoparticles at a low calcining temperature are reported, and homogeneous ultrafine BaFe₁₂O₁₉ with almost-ideal single-domain behavior and coercive force (5950 Oe) and specific magnetization (70 emu/g) values that are similar to the theoretically predicted values is obtained.     

Novel Deposition of Columnar Y3Al5O12 Coatings by Electrostatic Spray-Assisted Vapor Deposition

A novel and cost-effective electrostatic spray-assisted vapor deposition (ESAVD) was used to deposit Y₃Al₅O₁₂ (YAG) coatings. Polycrystalline single-phase Y₃Al₅O₁₂ coatings were synthesized using the ESAVD method in an open atmosphere at 650°C, and then annealed at 700°–900°C for 1 h. The ESAVD process involves the decomposition and chemical reactions of charged aerosol in vapor phase. The low-temperature coating deposition characteristics of the ESAVD process using a suitable sol precursor decreases the reaction and crystallization temperatures for forming Y₃Al₅O₁₂ coatings. The microstructure of the Y₃Al₅O₁₂ coating prepared using the ESAVD method is columnar and such strain-resistance microstructure could be useful for thermal barrier coating applications.     

Preparation of High-Coercivity BaFe12O19

Particles of BaFe₁₂O₁₂ were prepared by chemical coprecipitation; their magnetic properties were studied. A coercive force of 6000 Oe, one of the highest reported for isotropic BaFe₁₂O₁₉, was obtained. X-ray and Moessbauer studies were conducted to examine the mechanism of formation. Superparamagnetic α-Fe₂O₃ was present during synthesis. Ferrites were sintered from these precipitated powders by both the usual method and a hot-press-forging method. The observed magnetic characteristics result from partial orientation of defect-free single-domain coprecipitated powders.     

Glass–Ceramic Composites with Strontium Hexaferrite Ultrafine Particles in the SrO–Fe2O3–B2O3–SiO2 System

Glass samples with nominal compositions SrFe₁₂O₁₉+(12− n )SrB₂O₄+nSrSiO₃, n =3, 6, 9 were prepared by rapid quenching of the melt. Processes of glass devitrification were studied. The samples were annealed at temperatures of 600–900°C, and the resulting glass–ceramics was characterized by XRD, SEM, EDX, and magnetic measurements. SrFe₁₂O₁₉ crystallizes above 700°C and forms nano- and submicron platelet particles with the aspect ratio depending on the thermal treatment conditions. The glass–ceramic samples annealed at 900°C show coercive force values in the range of 422–455 kA/m.     

Strontium Dialuminate SrAl4O7: Synthesis and Stability

Up to now, strontium dialuminate, SrAl₄O₇ (SA₂), could be synthesized only by solidification from the high-temperature liquid state. We describe its synthesis from a spray-dried amorphous precursor, and specify its stability domains. Its kinetics of formation is very low. It can be crystallized in the 900–1000°C temperature range either directly with a low heating rate or via two metastable solid solutions—hexagonal strontium monoaluminate (SrAl₂O₄ (SA)) and γ-alumina—by annealing at 950–1000°C. As the temperature is raised beyond 1100°C, SA₂ becomes metastable, its formation is no longer possible, and the crystallization of Sr₄Al₁₄O₂₅ (S₄A₇) is favored. The latter compound, whose composition is close to that of SA₂, is stable up to 1500°C. At higher temperature it decomposes into SA and SA₂, which in its turn decomposes into SA and SA₆ (SrAl₁₂O₁₉). There is again another stability domain for SA₂, restricted to a narrow temperature scale close to its melting point (∼1800°C). The behaviors at crystallization from amorphous precursors at low temperature and from liquid at very high temperature are symmetrical: low heating or cooling rates produce pure SA₂ while too rapid kinetics result in mixtures of phases.     

The α–β Transformation in Silicon Nitride Single Crystals

Single crystals of α-Si₃N₄ were annealed at 2000°–2150°C. The β phase was detected after annealing at 2150°C only when the crystals were surrounded by MgO·3Al₂O₃ or Y₂O₃ powders. On the other hand, no evidence of the α–β transformation was found when the crystals were annealed without additives. The solution–precipitation mechanism was concluded to be the dominant factor in the α–β transformation of Si₃N₄.     

FTIR Spectra, Lattice Shrinkage, and Magnetic Properties of CoTi-Substituted M-Type Barium Hexaferrite Nanoparticles

Single-phase BaCoTiFe₁₀O₁₉ (BaCoTi-M) nanoparticles were prepared by a modified sol–gel process, using metallic chlorides as starting materials. The physical chemistry process of BaCoTi-M formation, the interdependences between composition, technological conditions, microstructure, and magnetic properties were studied by X-ray diffraction (XRD), Fourier transform-infrared (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). XRD and FTIR results show that BaCoTi-M nanoparticles formed directly from γ-Fe₂O₃, spinel ferrite, and barium salts without the formation of α-Fe₂O₃ and BaFe₂O₄. The lattice shrinkage of BaCoTi-M nanoparticles that occurred on increasing the calcining temperature from 973 to 1173 K under holding for 2 h or on increasing the holding time in the range 0–2 h at 1173 K was discovered by analyzing the dependences of lattice parameters on the heat-treatment conditions. The shrinkage led to a relatively higher concentration of magnetic Fe³⁺ cations in the unit cell, and resulted in an increase of specific saturation magnetization under the corresponding conditions. Microstructural characterization shows that the evolutions of coercivity, remnant magnetization, and squareness ratio depended on the crystal growth and the reduction of structural defect as well as a decrease of grain boundary.     

Preparation of Cobalt- and Fluorine-Doped Barium Hexaferrite by the Glass Synthesis Method

Doped barium hexaferrite (BaFe_12-xCo_xO_19-xF_x) was prepared by the glass synthesis method. The fluorine improves the pouring of the melt and the crystallization of the glass, while the cobalt decreases the coercive force. Addition of Al₂O₃ does not significantly alter the properties of barium hexaferrite, but it increases the crystallization. Doped products are attractive for high-density vertical magnetic recording (1400 < H _c < 2300 Oe, 60 < σ_s < 68 (emu cgs)/g, and σ_r/σ_s≃ 0.5).     

Synthesis of High Magnetoelectric Coefficient Composites Using Annealing and Aging Route

In this paper, aging temperature and time dependence of ferroelectric, ferromagnetic, and magnetoelectric properties of lead zirconate titanate and nickel ferrite composite (1– x ) Pb(Zr_0.52Ti_0.48)O₃– x NiFe_1.9Mn_0.1O₄ (PZT–NFM, x =0.03, 0.05, 0.1) are reported. The magnetoelectric composites of different compositions were fabricated by using a process based on the controlled precipitation route. The processing is a combination of conventional mixed oxide sintering and thermal treatment. The thermal treatment consists of annealing at a high temperature (800°C for 10 h), followed by aging in the range of 300–400°C for 3–15 h. X-ray diffractometry patterns show variation in the amount of spinel phase with different aging temperature and time. Scanning electron microscopy investigation showed that the annealing and aging treatment increases the homogeneity of NFM in the PZT matrix. The magnitude of piezoelectric constant ( d ₃₃), piezoelectric voltage constant ( g ₃₃), and the dielectric constant exhibited significant variation in magnitude with aging time and temperature. Aging at 400°C for 5 h exhibited a maximum magnitude of d ₃₃ and g ₃₃ and minimum magnitude of dielectric constant. The magnitude of the magnetoelectric (ME) voltage coefficient (d E /d H ) for 0.9PZT–0.1NFM samples sintered at 1125 °C was found to be of the order of 78 mV/cm Oe. This magnitude increased to 140 mV/cm Oe after annealing and aging at 300°C for 5 h. This significant enhancement in the magnetoelectric coefficient is probably due to the homogenization and precipitation of the spinel (NFM) phase in the perovskite matrix.     

Effects of Glass Frit Oxides on Crystallization and Zircon Pigment Dissolution in Whiteware Coatings

In experimental whiteware coatings comprised of zircon pigment and multi-oxide silicate glass frits that incorporated ZrO₂, the quantity, size, and morphology of zircon that precipitated during "fast-firing" were mainly dependent upon the ZnO, SrO, and Al₂O₃:alkali levels. In these frits, little or no zircon pigment dissolution occurred, and zircon crystallization was nearly complete by 1000°C. Microstructures of the coatings were consistent over a range of peak firing temperatures from 1000° to 1100°C. Fritted ZnO in these coatings stimulated zircon crystallization and produced high opacity. Replacing ZnO with SrO in the same frits prevented zircon from precipitating and resulted in transparent coatings. In frits without ZrO₂, significant zircon pigment dissolution and crystallization of calcium-based silicates occurred during firing. In these unstable coatings, crystallization and dissolution increased from 1000° to 1100°C and produced a range of microstructures. Raising the Al₂O₃:alkali ratio caused frits with ZrO₂ to precipitate more zircon, and frits without ZrO₂ to form less of the calcium-based silicates and dissolve less pigment.     

Effects of Temperature and Reagent Concentration on the Morphology of Chemically Vapor Deposited β-Ta2O5

Crystalline β-Ta₂O₅ coatings were deposited on hot-isostatically-pressed Si₃N₄ by reacting TaCl₅ with H₂ and CO₂ in the temperature range of 1000°–1300°C and at a pressure of 660 Pa. The Ta₂O₅ coatings generally consisted of wellcoalesced 2–3 μm grains, resulting in the formation of a nonporous coating morphology. However, the presence of microcracks on the as-deposited surface was consistently observed. The surface morphology, texture, and growth rate of the coatings were examined as a function of deposition parameters.     

Phase Equilibria in the Ga2O3In2O3 System

Subsolidus phase relationships in the Ga₂O₃–In₂O₃ system were studied by X-ray diffraction and electron probe microanalysis (EPMA) for the temperature range of 800°–1400°C. The solubility limit of In₂O₃ in the β-gallia structure decreases with increasing temperature from 44.1 ± 0.5 mol% at 1000°C to 41.4 ± 0.5 mol% at 1400°C. The solubility limit of Ga₂O₃ in cubic In₂O₃ increases with temperature from 4.X ± 0.5 mol% at 1000°C to 10.0 ± 0.5 mol% at 1400°C. The previously reported transparent conducting oxide phase in the Ga-In-O system cannot be GaInO₃, which is not stable, but is likely the In-doped β-Ga₂O₃ solid solution.     

Electrical Conductivity and Thermal Expansion of Spinels at Elevated Temperatures

Binary oxide spinels composed of Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, and Zn were synthesized. Electrical conductivity was measured in air at 500°–800°C. Thermal expansion was measured from room temperature to 1000°C. Ferrite spinels have thermal expansion coefficients of 11–12 ppm/K, compared with 7–9 ppm/K for other spinels except Cu–Mn and Co–Mn which show anomalous behavior. The highest electrical conductivity among transition metal spinels was found for MnCo₂O₄ (60 S/cm at 800°C) and Cu_1.3Mn_1.7O₄ (225 S/cm at 750°C).     

Citrate Route to Sn-Doped BaTi4O9 with Microwave Dielectric Properties

Highly reactive and nanometer-sized (30–50 nm) Sn-doped BaTi₄O₉ (BaTi_{4- x} Sn _x O₉; x = 0.0–0.03) powders have been prepared by the citrate-precursor method. The effect of Sn substitution on the crystallization and microwave dielectric properties has also been investigated on the basis of microstructure and crystal structure. Addition of a small amount of SnO₂ resulted in a lowering of the sintering temperature of BaTi₄O₉, and at 1250–1300°C for 2–5 h, dense compounds with a theoretical density up to 99% could be obtained. The Sn-doped BaTi₄O₉ materials were found to have excellent microwave dielectric properties with epsilon_r = 34–37, Q = 8300–8900 at 11 GHz and tau_f = 3.6–16.1 ppm/°C.     

Suppression of Rhombohedral-Phase Appearance and Low-Temperature Sintering of Scandia-Doped Cubic-Zirconia

Inhibition of cubic-rhombohedral phase transformation and low-temperature sintering at 1000°C were achieved for 10-mol%-Sc₂O₃-doped cubic-ZrO₂ by the presence of 1 mol% Bi₂O₃. The powders of 1-mol%-Bi₂O₃–10-mol%-Sc₂O₃-doped ZrO₂ were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral-ZrO₂ phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO₂ sintered at 1200°C was >2 μm because of grain growth in the presence of Bi³⁺. Cubic, stabilized Bi-Sc-doped ZrO₂ sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid-oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate-temperature SOFC.     

Influence of Processing Conditions on the Electrical Properties of CaCu3Ti4O12 Ceramics

The electrical properties of a series of CaCu₃Ti₄O₁₂ ceramics prepared by the mixed oxide route and sintered at 1115°C in air for 1–24 h to produce different ceramic microstructures have been studied by Impedance Spectroscopy. As-fired ceramics are electrically heterogeneous, consisting of semiconducting grains and insulating grain boundaries, and can be modelled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The grain boundary resistance and capacitance values vary as a function of sintering time and correlate with the ceramic microstructure based on the brickwork layer model for electroceramics. The large range of apparent high permittivity values for CaCu₃Ti₄O₁₂ ceramics is therefore attributed to variations in ceramic microstructure. The grain-boundary resistance decreases by three to four orders of magnitude after heat treatment in N₂ at 800°–1000°C but can be recovered to the original value by heat treatment in O₂ at 1000°C. The bulk resistivity decreases from ∼80 to 30 Ω·cm with increasing sintering time but is independent of heat treatment in N₂ or O₂ at 800°–1000°C. The origin of the bulk semiconductivity is discussed and appears to be related to partial decomposition of CaCu₃Ti₄O₁₂ at the high sintering temperatures required to form dense ceramics, and not to oxygen loss.     

Mineralizing Effect of Li2B4O7 and Na2B4O7 on Magnesium Aluminate Spinel Formation

Lithium borate (Li₂B₄O₇) and sodium borate (Na₂B₄O₇) mineralize spinel formation from stoichiometric MgO and Al₂O₃ between 1000° and 1100°C. Mineralization with both compounds is shown to be mediated by B-containing liquids which form glass on cooling. However, the liquid compositions depend on the type of mineralizer and temperature, suggesting that templated grain growth or dissolution–precipitation mechanisms are operating, one dominating over the other under certain conditions. Na₂B₄O₇-mineralized compositions show predominantly templated grain growth at 1000°C, which changes to dissolution–precipitation at 1100°C, whereas Li₂B₄O₇-mineralized compositions show dissolution–precipitation from 1000°C. Li₂B₄O₇ is a stronger mineralizer as spinel formation is complete with 3 wt% Li₂B₄O₇ at 1000°C and with ≥1.5 wt% addition at 1100°C, whereas Na₂B₄O₇-mineralized compositions are found to retain some unreacted corundum even at 1100°C.