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.     

Low-Temperature Sintering and Microwave Dielectric Properties of Anorthite-Based Glass-Ceramics

TiO₂ nucleated anorthite-based glass-ceramics were fabricated from glass powders. After sintering and crystallization heat treatment, various physical properties, including apparent bulk density and water absorption, were examined to evaluate the sintering behavior of anorthite-based glass-ceramic. Results showed that the complete-densification temperature for specimens was as low as 900°C. Sufficient crystallization was achieved by subsequently raising the firing temperature to 950°C, and the dielectric quality factor was promoted to the maximum value. Contents of nucleating agent (TiO₂) played an important role in the dielectric constants. The crystallinity was controlled by raising the firing temperature at a constant heating rate. The degree of crystallization affected the dielectric properties of sintered glass-ceramics. At the resonant frequency of 10 GHz, anorthite glass-ceramics with 5 wt% TiO₂ possessed the lowest permittivity of 8 and exhibited appropriate dielectric properties as compared with those with B₂O₃ and 10 wt% TiO₂.     

Zn‐Cr Co‐Substitution Effect on Structural and Electromagnetic Properties of CuFe2O4 via Oxalate Decomposition Route

Zn‐Cr‐co‐substituted Cu_1‐xZn_xFe_2‐2xCr_2xO₄ ferrites (where x = 0.0–0.5) were prepared via thermal decomposition of oxalate precursors. The thermal decomposition up to ferrites formation was followed by differential thermal analysis–thermogravimetry measurements. Mössbauer technique was used to predict the possible cation distribution of the entire system, and X‐ray diffraction, Fourier transform infrared, and electromagnetic measurements were used for confirmation. The superparamagnetic characteristics estimated via Mössbauer studies, for samples with higher substitution, agreed well with vibrating sample magnetometer, magnetic susceptibility, and conductivity results. All the samples showed semiconducting properties in which conductivity decreases by increasing substitution. The effect of cationic substitution on the entire system was investigated and discussed.     

Enhanced Magnetization of 3 mol% Yttria-Doped Zirconia/Barium Hexaferrite by Post-Plastic Deformation

A 3-mol%-Y₂O₃-doped tetragonal-ZrO₂ polycrystal/BaFe₁₂O₁₉ composite was fabricated by powder metallurgical processes, and its mechanical and magnetic properties were evaluated. Post-plastic deformation (i.e., hot-forging) on the composite improved its magnetic properties—that is, saturation magnetization and coercivity—probably because of the alignment of the hexaferrite platelet particles. Post-plastic deformation may represent a new process to complement conventional slurry-pressing under a magnetic field, for obtaining good magnetic properties in a magnetic-particle-dispersed composite.     

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.     

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.     

Hydrothermal Synthesis and Low-Temperature Sintering of Zinc Gallate Spinel Fine Particles

Nanosized powders of single-phase zinc gallate (ZnGa₂O₄) spinel were hydrothermally synthesized from solutions in the presence of NaOH over the pH range of 1.9 to 7.0 and from solutions above pH 7.0, i.e., the very basic medium (pH of 13.85), by removing the residual ZnO phase by washing with aqueous H₂SO₄ from the precipitate mixtures of zinc gallate spinel particles and ZnO. A very wide compositional range (Zn/2Ga = 0.705–1.157) of zinc gallate spinel solid solutions could be hydrothermally synthesized in the form of nanosized particles from acid and very basic mediums (pH of 2.4–13.85) in the presence of NaOH. These hydrothermally synthesized spinel powders showed good sinterability and almost full densification at 1100°C for 1 h. Dense sintered bodies consisting of single-phase zinc gallate spinel were fabricated at 1100°C using zinc gallate spinel powders having almost stoichiometric composition formed from the solution at pH 9.95 in the presence of aqueous ammonia.     

Low-Temperature Sintering and Dielectric Properties of Ag(Nb,Ta)O3 Composite Ceramics

The sintering behavior and dielectric properties of perovskite Ag(Nb_{1− x} Ta _x )O₃ (0 < x < 1) solid solutions and two-phase composite assemblages were explored. A small amount of CuO (1 wt%) was used for liquid-phase sintering and led to high densification at temperatures <950°C. The temperature coefficient of capacitance, TCC, was adjusted by varying the Nb:Ta ratio within the solid-solution series and by creating composite microstructures. Two-phase assemblages consisting of Ag(Nb_3/4Ta_1/4)O₃ and Ag(Nb_1/4Ta_3/4)O₃ were synthesized to achieve a temperature-stable dielectric material for high-frequency applications. The composite dielectric with CuO addition had an average dielectric constant of 390 and a Q × f factor of 410 GHz at 2 GHz, with a stable TCC (0 to −180 ppm/°C) in the temperature range from −20° to +60°C. In addition, process compatibility with a silver conductor was confirmed by high-frequency ring-resonator measurements and microstructural characterization. The Ag(Nb_{1− x} Ta _x )O₃ solid solutions and composites are promising candidates as embedded capacitors for radio-frequency/microwave applications.     

Formation of BaFe12−xNbxO19 and its high electromagnetic wave absorption properties in millimeter wave frequency range

BaFe_12?xNb_xO₁₉ (BFNO, x=0‐0.6) powders with Nb⁵⁺ substituting for Fe³⁺ were prepared by sol‐gel method. The formation process and electromagnetic (EM) wave absorption properties of the BFNO are investigated in detail. With Nb⁵⁺ content increasing from x=0 to x=0.6, the formation temperature of barium ferrite phase without heat time increases from ~700°C to ~900°C, while the appearance temperature of typical plate grains decreases from ~1300°C to ~1100°C, and the crystallization ability decreases at 600°C‐900°C, while the grain size increases gradually at 1100°C‐1300°C. Increasing sintering temperature and time promote the formation of barium ferrite phase and grain growth in all the samples. The ε′ and ε″ of the sample with x=0.6 sintered at 1300°C for 3 hours reach highest of ~7.9 and ~0.95 over 26.5‐40 GHz. Multiresonance peaks in permeability decrease from 40+ GHz to ~30 GHz with x rising from 0 to 0.6. Ultimately, small RL_min of ~?42 dB, thin d_m of ~0.76 mm, and broad bandwidth of >12 GHz can be exhibited simultaneously around millimeter wave atmospheric window of 35 GHz.     

Influence of Copper(II) Oxide Additions to Zinc Niobate Microwave Ceramics on Sintering Temperature and Dielectric Properties

The effect of CuO additions on the firing temperature of ZnNb₂O₆ ceramics was investigated using dilatometry, transmission electron microscopy, and X-ray diffractometry. A 5 wt% CuO addition to ZnNb₂O₆ ceramics significantly lowered the firing temperature from 1150° to ∼900°C. The presence of a CuO-rich intergranular phase in the specimen was observed and was evidence of the formation of a liquid phase during sintering. The composition of the liquid phase was (ZnCu₂)Nb₂O₈. In particular, the low-fired ZnNb₂O₆ ceramics had good microwave dielectric characteristics— Q × f = 59 500, ɛ_r= 22.1, τ_f=–66 ppm/^oC. These properties were correlated with the formation of a second phase, (ZnCu₂)Nb₂O₈.     

低温烧结CaO-B2O3-SiO2玻璃陶瓷及其性能

在CaO-B2O3-SiO2玻璃粉末中,通过添加助烧剂制备了CaO-B2O3-SiO2(CBS)玻璃陶瓷材料.研究了助烧剂P2O5和ZnO的助烧作用、材料的介电性能和相组成、显微结构.分析认为,P2O5能促进低熔点玻璃相的形成,ZnO则可以提高玻璃相的粘度,扩大烧结温度范围,并防止试样变形,复合添加P2O5和ZnO可成功烧结CBS玻璃粉末;在CBS玻璃陶瓷材料中,包含有β-CaSiO3、α-SiO2和CaB2O4三种晶相,晶粒发育均匀,粒径分布较合理,大小为0.5μm左右;添加质量分数为2%的P2O5和0.5%的ZnO作烧结助剂,可制备10MHz下,εr为6.38,tanδ小于0.002的玻璃陶瓷材料;烧结温度低于900℃,可实现银、铜电极共烧,可用作LTCC材料.     

Investigations of Electrical Properties in Silica/Indium Tin Oxide Two-Layer Film for Effective Electromagnetic Shielding

The effect of zinc ions added to silica film on the electrical and structural properties of a silica/indium tin oxide two-layer film which had been prepared by solution coating for electromagnetic shielding of displays was studied. The volume resistivity of the undoped silica/indium tin oxide film was more than 3 times as high as that of the zinc-doped silica/indium tin oxide film. The addition of divalent cations, zinc ions, to the overcoated layer led volume resistivity of the two-layer film to decrease significantly and also caused a long-term increase in stability. The decrease in volume resistivity was due to the addition of zinc ions that changed the interface ionization and helped to enhance the electrical conductivity in the two-layer film.     

Unusual Elastic and Mechanical Behaviors of Copper Phosphate Glasses with Different Copper Valence States

Elastic and mechanical properties such as Young's modulus E, Poisson's ratio ν, Debye temperature θ_D, Vickers hardness H_v, fracture toughness K_c, and fracture surface energies γ_f of yCuO_x·(100−y)P₂O₅ glasses (y= 45, 50, 55) with different copper valence states, i.e., R(Cu⁺) = Cu⁺/(Cu⁺+ Cu²⁺), at room temperature (humidity 64%) have been examined. The following features have been found: (1) the glass transition temperature (218–434°C), H_v (2.7–4.4 GPa), E (50.6–78.2 GPa), and θ_D (358–434 K) decrease largely with increasing R(Cu⁺); (2) the mean atomic volume, K_c (0.56–1.14 MPa·m^1/2), and γ_f (1.9–11.2 J·m⁻²) tend to increase with increasing R(Cu⁺); (3) 50CuO_x·50P₂O₅ glasses with R(Cu⁺) = 0.42 and 0.55 have a high resistance against crack formation in Vickers indentation tests and no crack is observed in the 45CuO_x·55P₂O₅ glass with R(Cu⁺) = 0.57 under an applied load of about 98 N. The results demonstrate that elastic and mechanical properties of yCuO_x·(100−y)P₂O₅ glasses depend strongly on the copper valence state and the CuO_x/P₂O₅ ratio. The unusal mechanical and elastic properties of copper phosphate glasses are well explained qualitatively by considering unique oxygen coordination and bonding states of Cu⁺ ions, i.e., lower coordination number and more covalent bonding compared with Cu²⁺ ions.     

Thermodynamic Behavior of Copper-Coated Silicon Carbide Particles during Conventional Heating and Spark Plasma Sintering

75Cu·25SiC (vol%) compacts were prepared using copper-coated SiC particles and spark plasma sintering (SPS). The preliminary thermal performance of the coated particles was determined using simultaneous DSC-TG-MS measurement. Characterization of compacts using XRD and SEM techniques was conducted to investigate the physical and chemical changes during the SPS operation. It was found that CuO decomposed at 850° and 500°C during conventional heating and SPS, respectively. Cu₂O facilitated the densification of Cu/SiC composites. The optimized sintering temperature of Cu/SiC composites using SPS was ∼730°C. The compacts showed improved hardness because of the SiC reinforcement.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

Pressureless Sintering and Mechanical and Biological Properties of Fluor-hydroxyapatite Composites with Zirconia

Fluor-hydroxyapatite (FHA) fabricated by a reaction between fluorapatite (FA) and hydroxyapatite (HA) was mixed with ZrO₂ to produce FHA–ZrO₂ composites. When the relative amount of FA to HA increased, the decomposition of the composite was decreased gradually because of the formation of thermally stable FHA solid solutions. With such suppression of decomposition, the FHA–ZrO₂ composites retained fully densified bodies. As a result, significant enhancements in mechanical properties, such as hardness, flexural strength, and fracture toughness, were achieved as the relative amount of FA to HA increased. The highest values in strength and toughness were 220 MPa and 2.5 MPa·m^1/2, respectively, with FHA–40 vol% ZrO₂ composites. In vitro proliferation of osteoblast-like cells (MG63) on the composites showed behavior similar to that observed on pure HA and FHA. Alkaline phosphatase (ALP) activity of the growing cells (HOS) on the composites was slightly down-regulated compared with that on pure HA and FHA at prolonged periods.     

Shrinkage-Free Sintering of Low-Temperature Cofired Ceramics by Loading Dilatometry

Sintering with zero radial shrinkage of low-temperature cofired ceramics (LTCC) was investigated by means of loading dilatometry. The results indicate that constant compressive loads tend to produce either decreased or increased radial shrinkages, and this can vary as a function of density, depending on the magnitude of the applied stresses. Coupling the real-time applied uniaxial load to the radial strain of cylindrical samples afforded a sintering scheme where the radial strain was precisely kept near zero (±0.02%). This approach possibly perfects an existing hot-pressing technique for zero-shrinkage sintering technology of LTCC materials.     

Sintering of Lithium Zinc Ferrites with Alkoxide-Derived Borosilicate Liquid Phase

Lithium zinc ferrites (Li_0.3Zn_0.4Mn_0.05Fe_2.25O₄) of > 97% theoretical density were prepared with alkoxide-derived borosilicate sintering additive. A sol-gel technique was used to incorporate the borosilicate phase prior to sintering. The effects of borosilicate composition and additive quantity on microstructural development and densification are reported. Direct current resistivity, dynamic hysteresis, and microwave property data are discussed in terms of processing parameters and resultant microstructures. The microstructure of the sintered ferrites was analyzed using SEM and TEM. Heterogeneities including regions of liquid-phase coalescence and intragranularly included glass phase were observed in some cases. These microstructural features were shown to have deleterious effects on microwave magnetic properties when compared to those of commercial ferrites of the same nominal composition sintered with Bi₂O₃ additive.     

Enhanced Properties of Tin(IV) Oxide Based Materials by Field-Activated Sintering

The densification of SnO₂ (0.9 mol)–Sb₂O₃ (0.1 mol) solid solution without any additives was studied by conventional and field-activated sintering technique (FAST). FAST sintering achieved a relative density value of 92.4% at 1163 K for 10 min versus 61.3% in conventional sintering at 1273 K for 3 h. An abnormal reduction of the IR transmittance and a semiconductor defect structure with only one donor level in the SnO₂ energy gap were noticed in the FAST-sintered as compared with the conventionally sintered Sn_0.82Sb_0.18O₂ solid solution. A high charge carrier concentration (i.e., electronic conduction) was shown in the FAST-sintered sample by conductivity measurements and the negative values of the Seebeck coefficient.     

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).