Alkoxide–Hydroxide Route for the Preparation of γ-LiAlO2-Based Ceramics

An alkoxide-hydroxide route has been developed to prepare Li_{4 + x} Al_{4 − 3 x} Si_{2 x} O₈ (0 ≤ x ≤ 0.25) powders by taking into account fundamental aspects of the sol-gel process. This technique allows one to prepare powders which exhibit the β-LiAlO₂ type of structure after drying at 150°C. The β→γ-LiAlO₂ topotactic transformation spreads over a large temperature range (746–839°C for x = 0.125) with no significant dilatometric and enthalpic change. Stoichiometric γ-LiAlO₂-based ceramics with a large variety of uniform microstructures are fabricated by a direct sintering of β-LiAlO₂ powders in the temperature range of 900–1100°C.     

Glass-Free KxBa1−xGa2−xGe2+xO8 Ceramics for Low-Temperature Cofired Ceramics Technology: Synthesis, Phase Transitions, Sintering, and Microwave Dielectric Properties

K _x Ba_{1− x} Ga_{2− x} Ge_{2+ x} O₈ (0.6≤ x ≤1) polycrystalline ceramics are potential materials for glass-free low-temperature cofired ceramics (LTCC) substrates. We have made a comprehensive study of the kinetics of the monoclinic-to-monoclinic P 2₁/ a ⇔ C 2/ m phase transition. The low-temperature-stable P 2₁/ a phase with a high Q × f value was synthesized using a subsolidus method and was well sintered at the LTCC temperature with a H₃BO₃ additive. A good combination of low sintering temperature (910°–920°C), high Q × f values (96 700–104 500 GHz), low permittivities (5.6–6.0), and a small temperature coefficient of resonant frequency (∼−20 ppm/°C) was obtained for ceramics with x =0.67 and 0.9 and with 0.1 wt% of H₃BO₃.     

Microwave Dielectric Properties of CaTiO3-CaAl1/2Nb1/2O3 Ceramics Doped with Li3NbO4

The microwave dielectric properties of CaTi_{1− x} (Al_1/2Nb_1/2) _x O₃ solid solutions (0.3 ≤ x ≤ 0.7) have been investigated. The sintered samples had perovskite structures similar to CaTiO₃. The substitution of Ti⁴⁺ by Al³⁺/Nb⁵⁺ improved the quality factor Q of the sintered specimens. A small addition of Li₃NbO₄ (about 1 wt%) was found to be very effective for lowering sintering temperature of ceramics from 1450–1500° to 1300°C. The composition with x = 0.5 sintered at 1300°C for 5 h revealed excellent dielectric properties, namely, a dielectric constant (ɛ_r) of 48, a Q × f value of 32 100 GHz, and a temperature coefficient of the resonant frequency (τ_f) of −2 ppm/K. Li₃NbO₄ as a sintering additive had no harmful influence on τ_f of ceramics.     

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.     

Preparation of 0.95Bi1/2Na1/2TiO3·0.05BaTiO3 Ceramics by an Aqueous Citrate-Gel Route

This report details development of a route to solution-derived (1− x )Bi_1/2Na_1/2TiO₃· x BaTiO₃ powders. The method developed was the citrate-gel method—an evaporative, aqueous technique. When applied to 0.95Bi_1/2Na_1/2TiO₃·0.05BaTiO₃ (BNBT-5), the method produced perovskite phase powders that readily densified in the temperature range of 1000°C. The grain size of the sintered materials was on the order of 1 μm, and the weak-field dielectric properties at 1 MHz were similar to those reported for conventionally prepared materials sintered at higher temperatures (e.g., 1200°C).     

Densification, Structure, and Thermophysical Properties of Ytterbium–Gadolinium Zirconate Ceramics

(Yb _x Gd_{1− x} )₂Zr₂O₇ (0≤ x ≤1.0) ceramic powders synthesized with the chemical-coprecipitation and calcination method were pressureless-sintered at 1550–1700°C to develop new thermal barrier oxides with a lower thermal conductivity than yttria-stabilized zirconia ceramics. (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics exhibit a defective fluorite-type structure. The linear thermal expansion coefficients of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics increase with increasing temperature from room temperature to 1400°C. The measured thermal conductivity of (Yb _x Gd_{1− x} )₂Zr₂O₇ ceramics first gradually decrease with increasing temperature and then slightly increase above 800°C because of the increased radiation contribution. YbGdZr₂O₇ ceramics have the lowest thermal conductivity among all the composition combinations studied.     

Structural and Dielectric Characterization of Nanocrystalline (Ba, Pb)ZrO3 Developed by Reverse Micellar Synthesis

Nanocrystalline zirconates of barium and lead have been synthesized using a modified reverse micellar route (avoiding alkoxides). The entire solid solution of Ba_{1− x} Pb _x ZrO₃ (0≤x≤1) has been synthesized for the first time. Powder X-ray diffraction studies show the monophasic nature of the powders after heating at 800°C except minor impurities of ZrO₂ (2%–3%) at a higher lead content ( x =0.50 and 0.75). The oxides crystallize in the cubic structure till x =0.25; for higher values, they crystallize in the orthorhombic structure. The particle size obtained from X-ray line-broadening studies and transmission electron microscopic studies is found to be in the range of 20–60 nm for all the oxides obtained after heating at 800^oC. The grain size of the solid solution of Ba_{1− x} Pb _x ZrO₃ (0≤ x ≤1) was found to increase with the lead content. The dielectric constant of the solids corresponding to Ba_{1− x} Pb _x ZrO₃ (0≤ x ≤1) was found to be a maximum at x =0.50. Note that the cubic to orthorhombic transition is also observed between x =0.25 and 0.5. Dielectric properties with respect to variation in frequency and temperature are reported for these nanocrystalline oxides for the first time.     

Microstructure and Microwave Dielectric Properties of (1−x)Ca(Mg1/3Ta2/3)O3/xCaTiO3 Ceramics

Dense (1− x )Ca(Mg_1/3Ta_2/3)O₃/ x CaTiO₃ ceramics (0.1≤ x ≤0.9) were prepared by a solid-state reaction process. The crystal structures and microstructures were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Single-phase solid solutions were obtained in the entire composition range. Long-range 1:2 ordering of B-site cations and oxygen octahedra tilting lead to the monoclinic symmetry with space group P 2₁/ c for x =0.1. For x above 0.1, the long-range ordering was destroyed and the crystal structure became the orthorhombic with space group Pbnm . The microwave dielectric properties showed a strong dependence on the composition and microstructure. The dielectric constant and temperature coefficient of resonant frequency increased nonlinearly as the CaTiO₃ content increased while the Qf values decreased approximately linearly. Good combination of microwave dielectric properties was obtained at x =0.45, where ɛ_r=45.1, Qf =34 800 GHz, and τ_f=17.4 ppm/°C.     

Subsolidus Phase Relationships in the Ga2O3–Al2O3–TiO2 System

Subsolidus phase relationships in the Ga₂O₃–Al₂O₃–TiO₂ system at 1400°C were studied using X-ray diffraction. Phases present in the pseudoternary system include TiO₂ (rutile), Ga_{2−2 x} Al_{2 x} O₃ ( x ≤0.78 β-gallia structure), Al_{2−2 y} Ga_{2 y} O₃ ( y ≤0.12 corundum structure), Ga_{2−2 x} Al_{2 x} TiO₅ (0≤ x ≤1 pseudobrookite structure), and several β-gallia rutile intergrowths that can be expressed as Ga_{4−4 x} Al_{4 x} Ti _{n −4}O_{2 n −2} ( x ≤0.3, 15≤ n ≤33). This study showed no evidence to confirm that aluminum substitution of gallium stabilizes the n =7 β-gallia–rutile intergrowth as has been mentioned in previous work.     

Preparation and Characterization of Nanocrystalline Misch-Metal-Substituted Yttrium Iron Garnet Powder by the Sol–Gel Combustion Process

Nanocrystalline Y_{3− x} MM _x Fe₅O₁₂ powders (MM denotes Misch-metal, x =0.0, 0.25, 0.5, 0.75, and 1.0) were synthesized by a sol–gel combustion method. Magnetic properties and crystalline structures were investigated using X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), and a scanning electron microscope. The XRD patterns showed that the single-phase garnet of Y_{3− x} MM _x Fe₅O₁₂ was formed at x values ≤1.0. The saturation magnetization of powders increased with decreasing MM content and reached the maximum value at Y₃Fe₅O₁₂. The crystallite size of powders calcined at 800°C for 3 h was in the range of 38–53 nm.     

Sintering Behavior, Phase Evolution, and Microwave Dielectric Properties of Bi(Sb1−xTax)O4 Ceramics

Sintering behavior, phase evolution, and microwave dielectric properties of Bi(Sb_{1− x} Ta _x )O₄ ceramics (0.05≤ x ≤0.60) were studied and their relationships were discussed in detail. Phase studies revealed that a pure monoclinic phase could be formed when x ≤0.20 and a pure orthorhombic phase could be obtained when x ≥0.50. As the x value increased from 0.05 to 0.60, the densified temperature of Bi(Sb_{1− x} Ta _x )O₄ ceramics decreased from 1050° to about 960°C whereas the density increased from 8.07 to 8.41 g/cm³. The microwave dielectric constant increased from 20.5 to 34 whereas the Q × f value decreased from 60 000 to 29 000 GHz. In the monoclinic phase region, the temperature coefficients of resonant frequency shifted linearly from −58 to −45 ppm/°C as the x value increased from 0.05 to 0.2 and then remained constant at about −12 ppm/°C when x ≥0.40. The Bi(Sb_{1− x} Ta _x )O₄ ceramics are promising for application of low-temperature cofired ceramics technology.     

Polymorphism of BaTiO3 Acceptor Doped with Mn3+, Fe3+, and Ti3+

The effect of Mn doping on the cubic to hexagonal phase transition temperature in BaTiO₃ has been determined by quenching samples with different Mn contents from a range of temperatures. Under conditions of equilibrating samples in air over the range 1000°–1400°C, cubic solid solutions BaTi_{1− x} Mn _x O_3−δ form over the range 0≤ x ≤0.015(5), whereas hexagonal solid solutions form for x ≥0.02, depending on the temperature. The results are compared with those on doping BaTiO₃ with Fe³⁺ and observations made concerning acceptor doping with Ti³⁺.     

Optimization of Praseodymium-Doped Cerium Pigment Synthesis Temperature

The development of red pigments is of great interest to the ceramic industry. Pr(IV) stabilization in a CeO₂ matrix yields materials with a red color. In this study, the traditional ceramic method involving solid-state reaction was used to prepare pigments in the system Ce_{1− x} Pr _x O_2−δ (0.005 ≤ x ≤ 0.1) from mixtures of rare-earth oxides. The chemical stability of these pigments was then determined in some industrial glazes. The glazing tests indicate that the powder samples calcined at 1200° and1300°C are unstable, whereas those calcined at 1400° and 1500°C are stable. These findings are related to the nonformation of a solid solution, to which the pigmenting power is attributed, in calcinations at temperatures below 1400°C.     

Structure–Property Relationships of BaTi1−2yGayNbyO3 (0≤y≤0.35) Ceramics

BaTi_{1−2 y} Ga _y Nb _y O₃ (BTGN) (0≤ y ≤0.35) powders were synthesized at 1300°C by the conventional solid-state method. Room temperature x-ray diffraction patterns for y ≤0.025 and 0.05≤ y ≤0.30 can be indexed as the tetragonal ( P 4 mm ) and cubic ( Pm     m ) polymorphs of BaTiO₃, respectively, whereas y =0.35 consists of a mixture of the cubic polymorph of BaTiO₃ and an 8H hexagonal-type perovskite ( P 6₃/ mcm ) isostructural with Ba₈Ti₃Nb₄O₂₄. Scanning electron microscopy shows the microstructures of BTGN ceramics ( y ≤0.30) sintered at 1500°C to consist of fine grains (1–3 μm) within a narrow grain size and shape distribution. Room temperature transmission electron microscopy for y ≤0.08 reveals core–shell structures and (111) twins in some grains; however, their relative volume decreases with y . Energy dispersive spectroscopy reveals the cores to be Ga and Nb deficient with respect to y . For y >0.08 there is no evidence of core–shell structures, however, some grains have a high density of dislocations, consistent with chemical inhomogeneity. BTGN ceramics exhibit a diverse range of dielectric behavior in the temperature range 120–450 K and can be subdivided into two groups. 0.025≤ y ≤0.15 display modest ferroelectric relaxor-type behavior, with high room temperature permittivity, ɛ₂₅', (>300 at 10 kHz), whereas 0.25≤ y ≤0.30 are temperature and frequency stable dielectrics with ɛ₂₅'<100 that resonate at microwave frequencies with modest quality factors, Q × f , ∼3720 GHz (at ∼5 GHz) for y =0.30.     

Preparation and Characterization of Samaria-Doped Ceria Electrolyte Materials for Solid Oxide Fuel Cells

The microstructure, thermal expansion, mechanical property, and ionic conductivity of samaria-doped ceria (SDC) prepared by coprecipitation were investigated in this paper. The results revealed that the average particle size ranged from 10.9±0.4 to 13.5±0.5 nm, crystallite dimension varied from 8.6±0.3 to 10.7±0.4 nm, and the specific surface area distribution ranged from 62.6±1.8 to 76.7±2.2 m²/g for SDC powders prepared by coprecipitation. The dependence of lattice parameter, a, versus dopant concentration, x , of Sm³⁺ ion shows that these solid solutions obey Vegard's rule as a ( x )=5.4089+0.10743 x for Ce_{1− x} Sm _x O_{2−1/2 x} . For SDC ceramics sintered at 1500°C for 5 h, the bulk density was over 95% of the theoretical density; the maximum ionic conductivity, σ_800°C=(22.3±1.14) × 10⁻³ S/cm with minimum activation energy, E _a=0.89±0.02 eV, was found in the Ce_0.80Sm_0.20O_1.90 ceramic. A dense Ce_0.8Sm_0.2O_1.9 ceramic with a grain size distribution of 0.5–4 μm can be obtained by controlling the soaking time at 1500°C. When the soaking time was increased, the microhardness of Ce_0.8Sm_0.2O_1.9 ceramic increased, the toughness slightly decreased, which was related to grain growth with the soaking time.     

Influence of Core-Shell Grains on the Internal Stress State and Permittivity Response of Zirconia-Modified Barium Titanate

BaTiO₃ modified with grain-boundary resident ZrO₂ (≤ 2 wt%) results in substantial chemical modification of the microstructure when sintered at 1320°C for 2 h. These modified regions were seen as core-shell grains in the micro-structure and were accompanied by an increase in the internal stress level. The shells had a variable composition (Ba(Ti_{1− x} Zr _x )O₃) and a tetragonal structure. An expansion mismatch between the core and the shell placed the core in compression resulting in a decrease in the lattice parameter and a pseudocubic structure with respect to the unmodified BaTiO₃. The combination of core-shell grains and increased internal stress resulted in an essentially flat permittivity response with temperature, a condition consistent with a distribution of Curie points within the ceramic.     

Crossover of Ba(Ti,Y)O3 Solid Solutions to Ba3Ti2YO8.5–BaTiO3 Composites and their Dielectric Properties

Ceramic samples with the nominal composition (1− x ) BaTiO₃+ x Ba₃Ti₂YO_8.5 ( x =0−0.535) were prepared by the mixed oxide method. X-ray diffraction (XRD) analysis shows that the materials are of single phase with a cubic symmetry as x ≤0.16. The compositions of the solid solutions ( x ≤0.16) can be expressed equivalently as Ba(Ti_{1− y} Y _y )O_3−δ ( y ≤0.122, y = x /(1+2 x )). For x >0.16, the materials are diphasic composites consisting of Ba(Ti_{1− y} Y _y )O₃ ( y =0.122) and Ba₃Ti₂YO_8.5. The microstructure observation by scanning electron microscopy supports the XRD result. The dielectric behavior and phase transitions of the solid solutions ( x ≤0.16) vary with different Y concentrations. The dielectric constant of the composites ( x >0.16) follows approximately the Lichteneker relation in a wide temperature range.     

Fabrication of Al-Added TiN Materials by the Combination of Double Self-Propagating High-Temperature Synthesis and Pulsed Electric-Current Pressure Sintering

Crystalline phases of Al-added TiN, denoted as (Ti_{1− x} Al _x )N _y (0≤ x ≤0.10, 0.8< y <1.0), prepared from a mixture of Ti and Al powders by self-propagating high-temperature synthesis (SHS) in a nitrogen atmosphere, have been investigated. By repeating SHS twice, in the region of 0.0< x ≤0.02 cubic (Ti_{1− x} Al _x )N _y solid solutions, and in the region of 0.02< x ≤0.10 composites consisting of (Ti_{1− x} Al _x )N _y and hexagonal Ti₂AlN were formed. After powder characterization, they were consolidated to dense materials (>97% of theoretical) by pulsed electric-current pressure sintering. With increasing Al addition, the optimum sintering temperatures were lowered, followed by reduction of grain size. Their mechanical properties, that is, three-point bending strength σ_b, Vickers hardness H _v, and fracture toughness K _{I C} were evaluated as a function of Al content.     

Mg–Cu–Zn Ferrites for Multilayer Inductors

Mg–Cu–Zn ferrites can be sintered at T ≤950°C to sufficient density and display adequate permeability profiles for application in multilayer ferrite inductors. The permeability and Curie temperature have to be optimized by proper selection of composition. Ferrites with <50 mol% Fe₂O₃ reveal enhanced densification behavior. Submicrometer powders prepared by fine milling show good sintering activity and density after firing at 900°C. Nano-size ferrite powders prepared by coprecipitation or flame synthesis lead to high density; maximum shrinkage already occurs at T <800°C. The use of Bi₂O₃ as a sintering additive further improves the densification, but also affects the microstructure and, hence, the permeability. A maximum permeability of μ_i=450–500 is obtained.     

Crystal Structure of Zirconia Prepared with Alumina by Coprecipitation

Zirconia was prepared by firing the coprecipitate from ZrOCl₂and AlCl₃mixed aqueous solution with ammonia. When fired above 600°C, the products were fine crystalline tetragonal zirconia of crystallite size <10 nm. In previous studies, the tetragonal phase had been assumed to be a (Zr_{1− x} ⁴⁺Al _x ³⁺)O_{2− x /2}solid solution, where x ≤ 0.25. However, X-ray diffraction pattern simulation and Al K -edge XANES spectroscopy confirmed the present product to be a mixture of t -ZrO₂fine powder with a small amount of δ-Al₂O₃of very low crystallinity, even below the expected compositional range of x ≤ 0.25 in the (Zr_{1− x} ⁴⁺Al _x ³⁺)O_{2− x /2}solid solution.