Dielectric and Magnetic Properties of Low-Temperature-Fired Ferrite–Dielectric Composites

The composition effects on the sintering behavior, microstructure evolution, dielectric, and magnetic properties of BaO·(Nd_0.8Bi_0.2)₂O₃·4TiO₂ (BNBT)+Bi₂O₃–B₂O₃–SiO₂–ZnO (BBSZ) glass–(Ni_0.28Cu_0.12Zn_0.6O)–(Fe₂O₃)_0.99 (NiCuZn ferrite) composites were investigated in developing low-temperature-fired composites for high-frequency electromagnetic interference devices. An X-ray diffractometer, a scanning electron microscope, and a dilatometer were used to examine the BNBT+BBSZ glass powder to NiCuZn ferrite ratio effect on the composites densification and chemical reaction between BNBT and NiCuZn ferrite. The results indicate that these composites can be densified at 950°C with no significant chemical reactions occurring between BNBT and NiCuZn ferrites during sintering. The BNBT+BBSZ glass–NiCuZn ferrite composites sintered at 950°C exhibit excellent dielectric and magnetic properties over a wide frequency range.     

Enhanced Permeability and Dielectric Constant of NiZn Ferrite Synthesized in Nanocrystalline Form by a Combustion Method

The performance parameters of Ni_0.5Zn_0.5Fe₂O₄, synthesized in the nanocrystalline form by an autocombustion method, have been investigated. The sample sintered at 1200°C, with Bi₂O₃ as additive shows a very high value of initial permeability μ' _i of >400 at 1 MHz, with low loss. Similarly, a very high dielectric constant is obtained at lower frequencies. The results show that optimum magnetic and electrical properties can be achieved for the NiZn ferrite nanocrystalline powders synthesized by the present autocombustion method and sintered at a relatively lower temperature of 1200°C when compared with a temperature of 1400°C required for the materials synthesized by the conventional ceramic method.     

High Q Microwave Dielectric Ceramics in (Ni1−x Znx)Nb2O6 System

(Ni_{1− x} Zn _x )Nb₂O₆, 0≤ x ≤1.0, ceramics with >97% density were prepared by a conventional solid-state reaction, followed by sintering at 1200°–1300°C (depending on the value of x ). The XRD patterns of the sintered samples (0≤ x ≤1.0) revealed single-phase formation with a columbite ( Pbcn ) structure. The unit cell volume slightly increased with increasing Zn content ( x ). All the compositions showed high electrical resistivity (ρ_dc=1.6±0.3 × 10¹¹Ω·cm). The microwave (4–5 GHz) dielectric properties of (Ni_{1− x} Zn _x )Nb₂O₆ ceramics exhibited a significant dependence on the Zn content and to some extent on the morphology of the grains. As x was increased from 0 to 1, the average grain size monotonically increased from 7.6 to 21.2 μm and the microwave dielectric constant (ɛ'_r) increased from 23.6 to 26.1, while the quality factors ( Q _u× f ) increased from 18 900 to 103 730 GHz and the temperature coefficient of resonant frequency (τ_f) increased from −62 to −73 ppm/°C. In the present work, we report the highest observed values of Q _u× f =103 730 GHz, and ɛ'_r=26.1 for the ZnNb₂O₆-sintered ceramics.     

Magnetic Properties of Nickel–Zinc Ferrite Toroids Prepared from Nanoparticles

Toroids comprised of silica-coated 10 nm diameter nickel–zinc (Ni–Fe) ferrite nanoparticles (Ni_0.5Zn_0.5Fe₂O₄) have been fabricated by careful control of both the coating process and subsequent densification by viscous sintering. A narrow processing window is identified between a maximum temperature at which the nanoparticles coarsen, losing their super-paramagnetic properties, and a lower temperature required for viscous flow densification. Key to the successful fabrication was drying and cold isostatic pressing of the silica-coated nanoparticles; other routes invariably led to cracking during either drying or sintering. The super-paramagnetic blocking temperature, the coercive field, and remanent magnetization could all be controlled over a wide range by varying the thickness of the silica coating from 1 to 15 nm. The dipole–dipole coupling distance is estimated to be 4 nm. The high-frequency (1–500 MHz) properties were sensitive to the sintering temperature as well as the thickness of the silica coating. Toroids sintered at 1000°C or less exhibited no high-frequency magnetic losses and their permeability decreased with increasing temperature, suggesting that the permeability was controlled by thermally activated magnetization relaxation.     

Sintering of Al2O3–TiC Composite in the Presence of Liquid Phase

The results obtained from the sintering of Al₂O₃–50TiC (in weight percent) composite in the temperature range from 1650° to 1800°C with addition of Y₂O₃ are presented. Densification is accelerated by the formation of liquid at temperatures above 1750°C, and 99% of theoretical density can be achieved by vacuum sintering at 1800°C for 15 min. The liquid presented at the sintering temperature is crystallized to YAG (Y₃Al₅O₁₂) during cooling.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Synthesis of Pure and Manganese-, Nickel-, and Zinc-Doped Ferrite Particles in Water-in-Oil Microemulsions

In recent years, the materials research focuses toward synthesis of finer and finer microstructural features. The unique properties of nanosized particles outweigh their higher production costs. Precipitation in microemulsion is one technique, which promises to produce small particles of controlled size and morphology at reasonable cost. The present study demonstrates the synthesis of nanocrystalline α-Fe₂O₃(hematite), Mn_0.5Zn_0.5Fe₂O_4, and Ni_0.5Zn_0.5Fe₂O₄ particles in a reverse micellar microemulsion system [water–iso-octane–AOT (sodium di-2ethylhexylsulfosuccinate)]. The synthesis of α-Fe₂O₃ is performed to obtain baseline data for the synthesis of Mn_0.5Zn_0.5Fe₂O₄ and Ni_0.5Zn_0.5Fe₂O₄ in the microemulsion system. Nanosized, spherical α-Fe₂O₃, Ni-Zn ferrite, and Mn-Zn ferrite particles (20–80 nm) with very narrow particle size distribution are synthesized. Crystallization of the particles is obtained at temperatures as low as 300°C.     

Processing of Nickel–Zinc Ferrites Via the Citrate Precursor Route for High-Frequency Applications

Nickel–zinc ferrites of various compositions, Ni_{1− x} Zn _x Fe₂O₄ where x =0.2, 0.4, 0.5, and 0.6 investigated in the present work, have been prepared by the citrate precursor method. The complex permittivity (∈'− j ∈") and permeability (μ'− j μ") of the ferrites were measured at X-band (8–12 GHz) microwave frequencies. The dielectric constants or the real part of permittivity, ∈', are observed to lie in the range 5.46–8.95 for ferrites sintered at 1200°C, while those sintered at 1300°C exhibit relatively higher dielectric constants. The permittivity loss, tan δ_∈ (=∈"/∈'), is observed to be of the order of 10⁻²–10⁻³, depending upon the composition and sintering temperature of the ferrite. These losses are lower by at least one to two orders of magnitude compared with those normally reported for ferrites processed by the conventional ceramic method. The presently studied ferrites also exhibit high values of DC-resistivity, 10⁷–10¹¹Ω·cm. The low dielectric losses and high resistivity can be co related to small grain size and better compositional stoichiometry obtained as a result of processing via the citrate route. Magnetic properties such as the Curie temperature, saturation magnetization, initial permeability, and B – H hysteresis parameters of the compositions with x =0.5 and 0.6 are also given.     

Hydrothermal Preparation of Ultrafine Ferrites and Their Sintering

Ultrafine and nearly spherical ferrites such as NiFe₂O₄, ZnFe₂O₄, Ni_xZn_1−xFe₂O₄, Mn_0.5Zn_0.5Fe₂O₄, and CoFe₂O₄ were prepared under mild hydrothermal conditions by precipitating from metal nitrates with aqueous ammonia. These hydrothermal ferrite powders were shown to sinter to almost theoretical density at 1000°C without any sintering aids.     

Characterization of Material for Low-Temperature Sintered Multilayer Ceramic Substrates

The sintering temperature of multilayer ceramic substrates must decrease to 1000° or below to avoid melting the conductors (Pd-Ag, Au, or Cu) during sintering. In this study, SiO₂, CaO, B₂O₃, and MgO were used as additives to Al₂O₃ to decrease the firing temperature by liquid-phase sintering. Compositions with 18.0 and 22.5 wt% B₂O₃ were sintered at around 1000° in an air atmosphere to yield dense ceramics with good properties: relative dielectric contant between 6 to 7 (1 MHz), tan δ≤× 3 × 10⁻⁴ (1 MHz), insulating resistivity > 10¹⁴ω cm, coefficient of thermal expansion ∼ 7.0 × 10⁻⁶/°, and thermal conductivity ∼ 4.1 W/(m · K).     

Synthesis of Y-Doped BaTiO3–(Bi1/2K1/2)TiO3 Lead-Free Positive Temperature Coefficient of Resistivity Ceramics and Their PTC Effects

As a lead-free positive temperature coefficient of resistivity (PTCR) material, (1– x mol%) BaTiO₃– x mol% (Bi_1/2K_1/2) TiO₃– y mol% Y₂O₃–0.5 mol% TiO₂ (BT– x BKT–2 y Y–0.5TiO₂) systems were prepared by the conventional solid-state reaction method. All samples containing <2 mol% BKT sintered in air possessed relatively low room-temperature resistivity (ρ₂₅) and high positive temperature coefficient (PTC) effect. However, when the BKT content exceeded 2 mol%, the sample was not semiconductive after sintering in air. The effects of sintering schedule on the properties of PTCR ceramics were discussed. The results showed that the optimum composition of BT–1BKT–0.2Y–0.5TiO₂, sintered at 1330°C for not-soaking and then fast quenched in air, achieved rather low ρ₂₅ of 28 Ω·cm and a high jump of resistivity (maximum resistivity [ρ_max]/minimum resistivity [ρ_min]) of 4.0 orders of magnitude with T _c about 155°C. The ρ₂₅ of the as-sintered sample could be further reduced to about 10 Ω·cm by annealing in N₂ at 450°C for 30 min, accompanied decrease on the PTC effect.     

Effect of Alkali Metal Oxide Addition on the Sinterability of MgO–B2O3–Al2O3 Glass–Al2O3 Filler Composites

The sintering of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler is terminated due to the crystallization of Al₄B₂O₉ in the glass. The densification of a composite of MgO–B₂O₃–Al₂O₃ glass and Al₂O₃ filler using pressureless sintering was accomplished by lowering the sintering temperature of the composite. The sintering temperature was lowered by the addition of small amounts of alkali metal oxides to the MgO–B₂O₃–Al₂O₃ glass system. The resultant composite has a four-point bending strength of 280 MPa, a coefficient of thermal expansion (RT—200°C) of 4.4 × 10⁻⁶ K⁻¹, a dielectric constant of 6.0 at 1 MHz, porosity of approximately 1%, and moisture resistance.     

Sintering and Properties of β'-Sialon with a Nitrogen-Rich Y2O3 Sintering Aid

The synthesis of dense sintered sialon with external additives selected from the system Y₂O₃–AIN–SiO₂ is reported. The highest density (3.21 g/cm³) was achieved at 1750°C at 90 min of sintering with 5 wt% additive. The degree of sialon substitution increased with the amount of liquid; the YSiO₂N crystalline phase formed concurrently. Strength degradation occurred above 1000°C. The fracture toughness of the material sintered with a lower amount of sintering aid remained relatively unchanged to 1200°C. The material with more additive exhibited decreased toughness above 1000°C.     

BiNbO4-Based Glass–Ceramic Composites for Microwave Applications

The effect of a bespoke glass sintering aid, 0.3Bi₂O₃–0.3Nb₂O₅–0.3B₂O₃–0.1SiO₂ (BN1), developed from the base ceramic composition, BiNbO₄ (BN), on the sinterability, microstructure, and microwave (MW) dielectric properties of BN ceramics has been investigated. Densities >97% theoretical could be achieved at 1020°C for samples with up to 15% BN1 additions. The resulting microstructure was composed of BN laths surrounded by a residual glass phase that contained small fibrous crystals. Some evidence of dissolution of BN crystals was observed. Optimum properties were exhibited for samples with 15 wt% of glass addition sintered for 4 h at 1020°C with a relative permittivity ɛ_r=38, a MW quality factor Q × f ₀=17 353 at 5.6 GHz, and a temperature coefficient of resonant frequency τ_f=−10 ppm/°C. The high Q × f ₀, ɛ_r, and low τ_f, coupled with a relatively low sintering temperature, suggest that the use of bespoke glass sintering aids of this type may have great potential for the fabrication of MW ceramics.     

Rapid Sintering of Stoichiometric Zinc-Modified Lead Magnesium Niobate

A rapid thermal-processing technique was used to densify zinc-modified lead magnesium niobate (PMZN30) of the form Pb(Mg_0.7Zn_0.3)_1/3Nb_2/3O₃. This dielectric composition showed relaxor behavior with a maximum permittivity near room temperature. Rapid densification was observed at temperature. Rapid densification was observed at temperatures of 1100°C and above, whereas lower temperatures showed a significant reduction of densification and electrical properties. A sintering schedule of 4 min at 1100°C with no prevention of PbO volatilization resulted in densification on the order of 90% of theoretical density and a weak field dielectric constant in excess of 14 000. Perovskite grain size was consistently in the range of 1 μm or less. The formation of pyrochlore on sample surfaces showed a direct relation to time and temperature.     

Microwave Dielectric Properties and Chemical Resistance of Low-Temperature-Sintered CaZrB2O6 Ceramics

Dolomite-type borate ceramics consisting of CaZrB₂O₆ were synthesized via a conventional solid-state reaction route; low-temperature sintering was explored using Bi₂O₃–CuO additives of 1–7 wt% for low-temperature co-fired ceramics applications. For several sintering temperatures, the microwave dielectric properties and chemical resistance of the ceramics were investigated. The CaZrB₂O₆ ceramics with 3 wt% Bi₂O₃–CuO addition could be sintered below 925°C, and the microwave dielectric properties of the low-temperature samples were ɛ_r=10.55, Q × f =87,350 GHz, and τ_f=+2 ppm/°C. The chemical resistance test result showed that both CaZrB₂O₆- and Bi₂O₃–CuO-added CaZrB₂O₆ ceramics were durable in basic solution but were degraded in acid solution.     

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.     

Oxidation Behavior of Liquid-Phase Sintered Silicon Carbide with Aluminum Nitride and Rare-Earth Oxides (Re2O3, where Re = Y, Er, Yb)

Silicon carbide (SiC) ceramics have been fabricated by hot-pressing and subsequent annealing under pressure with aluminum nitride (AlN) and rare-earth oxides (Y₂O₃, Er₂O₃, and Yb₂O₃) as sintering additives. The oxidation behavior of the SiC ceramics in air was characterized and compared with that of the SiC ceramics with yttrium–aluminum–garnet (YAG) and Al₂O₃–Y₂O₃–CaO (AYC). All SiC ceramics investigated herein showed a parabolic weight gain with oxidation time at 1400°C. The SiC ceramics sintered with AlN and rare-earth oxides showed superior oxidation resistance to those with YAG and Al₂O₃–Y₂O₃–CaO. SiC ceramics with AlN and Yb₂O₃ showed the best oxidation resistance of 0.4748 mg/cm² after oxidation at 1400°C for 192 h. The minimization of aluminum in the sintering additives was postulated as the prime factor contributing to the superior oxidation resistance of the resulting ceramics. A small cationic radius of rare-earth oxides, dissolution of nitrogen to the intergranular glassy film, and formation of disilicate crystalline phase as an oxidation product could also contribute to the superior oxidation resistance.     

Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics

The low-temperature sintering of lead-based piezoelectric ceramics has been studied. The sintering temperature of lead zirconate titanate (PZT) ceramics could be reduced from ∼ 1250° to ∼960°C by the addition of a small amount of the lower-melting frit, B₂O₃–Bi₂O₃—CdO. It exhibited the following dielectric and piezoelectric properties: K_p= 0.52 to 0.58, Q_m= 1000, ε^T₃₃/ε₀= 800 to 1000, tan δ= 50 × 10⁻⁴, ρ= 7.56 to 7.64 g/cm³. Ceramics with the aid of suitable dopants (CdO, SiO₂, and excess PbO) in the Pb-(Ni_1/3Nb_2/3)O₃—PZT family could be sintered at 860° to 900°C. For these materials, K_p= 0.56 to 0.61, Q_m= 1000, ε^T₃₃/ε₀= 1500 to 2000, tan δ≤ 50 × 10⁻⁴, ρ= 7.80 to 8.03 g/cm³. The microstructure, sintering mechanism, and the effects of various impure additions have been analyzed by means of scanning electron microscopy, scanning transmission electron microscopy, electron probe microanalysis, and X-ray photoelectron spectroscopy.     

Sintering of Si3N4 with the Addition of Rare-Earth Oxides

The effect of rare-earth oxide additives on the densification of silicon nitride by pressureless sintering at 1600° to 1700°C and by gas pressure sintering under 10 MPa of N₂ at 1800° to 2000°C was studied. When a single-component oxide, such as CeO₂, Nd₂O₃, La₂O₃, Sm₂O₃, or Y₂O₃, was used as an additive, the sintering temperature required to reach approximate theoretical density became higher as the melting temperature of the oxide increased. When a mixed oxide additive, such as Y₂O₃–Ln₂O₃ (Ln=Ce, Nd, La, Sm), was used, higher densification was achieved below 2000°C because of a lower liquid formation temperature. The sinterability of silicon nitride ceramics with the addition of rare-earth oxides is discussed in relation to the additive compositions.