Al2TiO5-ZrTiO4-ZrO2 Composites: A New Family of Low-Thermal-Expansion Ceramics

The characterization and properties of ceramic composites containing the phases Al₂TiO₅, ZrTiO₄, and ZrO₂ are described. The range of compositions investigated gives very low average thermal expansions (α_24–1000°C as low as −2.0 × 10⁻⁶°C⁻¹) and excellent high-temperature stability. The low thermal expansions are apparently due to a combination of microcracking by the titanate phases and a contractive phase transformation by the ZrO₂. The crystal chemistry and microstructure of the product are processing dependent. Although the composites represent a complex microcracking system, the low thermal expansions and high-temperature stability make them potential candidates for commercial applications requiring thermal shock resistance.     

Effects of B2O3 Addition on the Dielectric and Ferroelectric Properties of Ba0.7Sr0.3TiO3 Ceramics

The effects of B₂O₃ addition on the sintering behavior and the dielectric and ferroelectric properties of Ba_0.7Sr_0.3TiO₃ (BST) ceramics were investigated. The dielectric and ferroelectric properties of a BST sample with 0.5 wt% B₂O₃ sintered at <1150°C were as good as those of undoped BST sintered at 1350°C, and the dielectric loss was better. When >1.0 wt% B₂O₃ was added to BST, the overdoped B₂O₃ did not form a liquid phase or volatilize; it remained in the samples and formed a secondary phase that lowered the sintering behavior and the dielectric and ferroelectric properties of the BST.     

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).     

Electrical Properties of Sr0.7Ba0.3TiO3 Ceramics Doped with Nb2O5, 3Li2O · 2SiO2, and Bi2O3

The electrical properties of Sr_0.5Ba_0.3TiO₃ in the presence of Nb₂O₅ as a donor, 3Li₂O · 2SiO₂ as a sintering agent, and Bi₂O₃ as a dopant have been studied. When the compositions of the ceramics were 1 mol Sr_0.7Ba_0.3TiO₃+ 0.5 mol% Nb₂O₅+ 2 mol% 3Li₂O · 2SiO₂+ 0.2 mol% Bi₂O₃, the ceramics were sintered at 1100°C and exhibited the following characteristics: apparent dielectric constant ɛ, 25000; loss factor tan δ, 2%; insulating resistivity ρ_j, 10¹⁰Ω· cm; variation of dielectric constant with temperature Δɛ/ɛ (−25° to +85°C), +10%, −14%. ɛ and tan δ show only small changes with frequency. The study shows this ceramic can be used in multilayer technology.     

La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Structure and Microwave Dielectric Properties of (Zn1−xCox)TiO3 Ceramics

Dielectric ceramics in the system (Zn_{1− x} Co _x )TiO₃ ( x = 0–1) were synthesized by the solid-state reaction route. The phase distribution, microstructure, and dielectric properties were characterized by using powder X-ray diffraction analysis, electron microscopy, and microwave measurement techniques. Three phase composition regions were identified in the specimens sintered at 1150°C; [spinel + rutile] at 0 ≤ x ≤ 0.5, [spinel + ilmenite + rutile] at 0.5 < x ≤ 0.7, and [ilmenite] phase at 0.7 < x ≤ 1. For the 0 ≤ x ≤ 0.5 region, the amount of Ti-rich precipitates incorporated into the spinel phase decreased with the Co content at 0 ≤ x ≤ 0.5, with a concomitant increase of the rutile phase. The ilmenite phase appeared for high Co content. The microwave dielectric properties depended on the phase composition and volume according to the three phase regions, where the relative amount of rutile to the spinel or ilmenite determined the dielectric properties. The dielectric constant as a function of Co addition was modeled with a Maxwell mixing rule. An optimum phase distribution was determined in this system with dielectric constant of 25, a Q * f 70 000 GHz, and a low temperature coefficient of the resonant frequency.     

Tentative Phase Relationships in the System CaHfTi2O7-Gd2Ti2O7 with up to 15 mol% Additions of Al2TiO5 and MgTi2O5

The phase relationships in the CaHfTi₂O₇-Gd₂Ti₂O₇ (zirconolite-pyrochlore) pseudobinary system were investigated, after heating at 1500°C, because of their importance in the design of pyrochlore-rich titanate ceramics for immobilization of impure surplus plutonium. Up to 15 mol% of MgTi₂O₅ and Al₂TiO₅ were added to CaHfTi₂O₇-Gd₂Ti₂O₇ compositions to elucidate the effects of divalent and trivalent impurities on the phase stability within these systems. From X-ray diffractometry analysis, scanning electron microscopy, and energy dispersive X-ray spectrometry, phase formation and compositional stability limits were evaluated. The main phases observed in these systems were pyrochlore, perovskite, and polytypes of zirconolite. The formation of the 2 M -, 4 M -, and 3 O -zirconolite polytypes was dependent on the amount of aluminum or magnesium present. In the magnesium system, a large area of pyrochlore-only was observed, which indicated that divalent impurities of appropriate ionic size could be readily incorporated in the eightfold site of the pyrochlore. The locations of the tentative phase boundaries are discussed with respect to the chemical composition.     

Microstructure Development in Reactive-Templated Grain Growth of Bi1/2Na1/2TiO3-Based Ceramics: Template and Formulation Effects

"Reactive-templated grain growth" (RTGG) processing of Bi_1/2Na_1/2TiO₃ (BNT)-based ceramics is reported. Molten salt synthesis was used to prepare platelike (∼0.2 μm × 5 μm × 5 μm) Ruddlesden–Popper (Sr₃Ti₂O₇ (ST)) and Aurivillius (BaBi₂Nb₂O₉ (BBN)) phases which were used as "templates" in studies of RTGG with BNT-based matrixes. A "citrate-gel" route was designed to produce intimately mixed, fine-grain matrixes for these studies. The analytical techniques used were powder X-ray diffraction and microstructural examination of dry-pressed and fired compacts. For mixtures templated with BBN, single-phase perovskite readily formed, and an initially heterogeneous microstructure evolved toward a dense assemblage of anisometric, micrometer-scale grains. Perovskite formation was more sluggish in the mixtures templated with ST, and the final sintered microstructure featured larger, porous grains in an equiaxed, micrometer-scale matrix. A qualitative model, which examined the excess constituents in the matrix after formation of stoichiometric ABO₃ perovskite, is proposed to explain the observations. The model predicted an excess of Na₂O and TiO₂ in the matrix in the case of BBN templates and only excess TiO₂ in the case of ST templates. The results indicate that careful examination of matrix and template chemistry could be important in the selection of systems for RTGG processing.     

High-Q Microwave Dielectric Materials Based on the Spinel Mg2TiO4

Composite ceramics based on the spinel Mg₂TiO₄ were prepared by a conventional mixed-oxide route. To achieve the temperature stabilization of the dielectric constant, each of the composites was added with 7 mol% CaTiO₃. The effect of the substitution of isovalent Co for Mg on the microstructure and the microwave dielectric properties of the composite ceramics was also investigated. A maximum Q × f value of around 150–160 THz was obtained for the undoped Mg₂TiO₄, whereas a reduced Q × f value was observed for an increase in the Co concentration in the system (1− x )Mg₂TiO₄− x Co₂TiO₄. Upon doping with 7 mol% CaTiO₃, the Q × f value passed through a maximum with increasing Co concentration. Adding ZnO–B₂O₃ to the composite system based on Co-doped Mg₂TiO₄ resulted in a reduction of the sintering temperature by 150°–200°C without any significant degradation in the Q × f value.     

Reactive-Templated Grain Growth of Bi1/2(Na,K)1/2TiO3: Effects of Formulation on Texture Development

In this paper we report the effects of formulation on texture development for the "reactive-templated grain growth" (RTGG) of Bi_1/2(Na,K)_1/2TiO₃ (BNKT). The solids formulation for BNKT was systematically varied by prereacting to well—defined alkali and bismuth titanates (Na₂Ti₃O₇ (N₂T₃), K₂Ti₂O₅ (K₂T₂), and Bi₂Ti₄O₁₁ (B₂T₄)). Use of these precursors in different BNKT formulations determined that the amount of expansion associated with reacting dry-pressed compacts at 600−800°C could be influenced by formulation. Lotgering factors ( F _{00 l} ) derived from Θ/2Θ X-ray diffraction scans indicated that the formulation route strongly affected the {00 l } texture development in tape-cast and sintered specimens. Prereacting alkali carbonates with TiO₂ to form N₂T₃ and K₂T₂ inhibited texture development in RTGG-processsed BNKT. However, when Bi₂O₃ was prereacted to form B₂T₄, the measured F _{00 l} increased from 0.5 to 0.7.     

Fabrication and Characterization of Pyroelectric Ba0.8Sr0.2TiO3 Thin Films by a Sol-Gel Process

A sol-gel process was used to prepare pyroelectric Ba_0.8Sr_0.2TiO₃ thin films with large columnar grains (100–200 nm in diameter) on Pt/Ti/SiO₂/Si substrates, via using a 0.05 M solution precursor. The relationship between dielectric constant and temperature (ɛ_r- T ) showed two distinctive phase transitions in the Ba_0.8Sr_0.2TiO₃ thin films. Both the remnant polarization and the coercive field decreased as the temperature increased from −73°C to 40°C. Its low dissipation factor (tan δ= 2.6%) and high pyroelectric coefficient ( p = 4.6 × 10⁻⁴ C·(m²·K)⁻¹ at 33°C), together with its good insulating properties, made the prepared Ba_0.8Sr_0.2TiO₃ thin films promising for use in uncooled infrared detectors and thermal imaging applications.     

(Bi1/2Na1/2)TiO3–Ba(Cu1/2W1/2)O3 Lead-Free Piezoelectric Ceramics

(Bi_1/2Na_1/2)TiO₃ with 0–6 mol% Ba(Cu_1/2W_1/2)O₃ (BNT-BCW), a new member of the BNT-based group, has been prepared following the conventional mixed oxide route. The compacted bodies were sintered at 1130°C for 2 h to get dense ceramics. The addition of BCW into BNT ceramics facilitated the poling process because of a reduction in leakage current. 0.995BNT·0.005BCW ceramics exhibit a relatively high piezoelectric constant ( d ₃₃= 80 × 10⁻¹² C/N) and a relatively low dielectric loss (tan δ= 1.5%). Increased amount of BCW was found to increase the dielectric constant and loss of BNT-BCW ceramics and to suppress the grain growth. During sintering, some BCW diffuses into the lattice of BNT to form a solid solution and some remains on the grain boundaries.     

Strain Softening and Hardening during Superplastic-Like Flow in a Fine-Grained MgAl2O4 Spinel Polycrystal

Superplastic-like flow in a fine-grained MgAl₂O₄ polycrystal exhibits strain softening and hardening, which cannot be ascribed to cavity damage and grain growth during deformation, respectively. The softening and hardening can be related to a change in the internal stress, which depends on a decrease and an increase in the density of the intragranular dislocations, respectively, whose motion contributes to the relaxation of stress concentrations exerted through the predominant deformation mechanism of grain-boundary sliding. In these two regions, the rate of deformation is controlled by the continuous recovery of the dislocations limited by lattice diffusion of the oxygen ions.     

Low-Temperature Sintering and Microwave Dielectric Properties of the Zn2SiO4 Ceramics

B₂O₃ was added to nominal composition Zn_1.8SiO_3.8 (ZS) ceramics to decrease their sintering temperature for application to low-temperature cofired ceramic (LTCC) devices. B₂O₃ reacted with SiO₂ to form a liquid phase containing SiO₂ and B₂O₃. The composition and melting temperature of the liquid phase depended on the sintering temperature and the B₂O₃ content. The specimen containing 20.0 mol% of B₂O₃ sintered at 900°C exhibited high microwave dielectric properties of Q × f =53 000 GHz, ɛ _r=5.7, and τ_f=−16 ppm/°C, confirming the promising potential of the B₂O₃-added ZS ceramics as candidate materials for the LTCC devices.     

Synthesis and Characterization of MgAl2O4 Spinel Precursor from a Heterogeneous Alkoxide Solution Containing Fine MgO Powder

A precursor was synthesized from a heterogeneous alkoxide solution that contained fine MgO powder, which allowed the preparation of MgAl₂O₄ spinel powder with high sinterability characteristics. The precursor consisted of a mixture of boehmite (AlO(OH)) and a mixed hydroxide (Mg₄Al₂(OH)₁₄· 3H₂O). The spinel phase formed through two steps: (i) decomposition of the mixed hydroxide at low temperature and (ii) solid-state reaction between MgO and γ-Al₂O₃ at higher temperatures. Dense polycrystalline spinel could be obtained from the calcined powders at sintering temperatures as low as 1400°C.     

Microstructure and Electrical Properties of Chemically Prepared Nb2O5-Doped SrTiO3 Ceramics

Chemically homogeneous SrTiO₃ powders of submicrometer size were obtained by alcohol dehydration and subsequent calcination of citrate/format solutions. Nb₂O₅-doped SrTiO₃ was prepared with various Sr:Ti ratios resulting in an anomalous increase in the dielectric constant ( K 'up to ∼8000) for donor-doped SrO-excess SrTiO₃. No semiconducting behavior was observed for donor-doped TiO₂-excess SrTiO₃ when fired in air. Therefore, a "brick-wall" type of microstructure was formed as a result of the excess SrO, giving rise to anomalously high dielectric constants.     

Effects of Individual Layer Thickness on the Structure and Electrical Properties of Sol–Gel-Derived Ba0.8Sr0.2TiO3 Thin Films

Ba_0.8Sr_0.2TiO₃ thin films were prepared with various individual layer thicknesses using a sol–gel process. The individual layer thickness strongly affected the structure, ferroelectricity, and dielectric properties of the films. The films prepared with an individual layer thickness of 60 nm showed small equiaxed grains, cubic structure, temperature-independent dielectric constant, and no ferroelectricity. The films prepared with an individual layer thickness of 8 nm showed columnar grains, tetragonal structure, good ferroelectricity, and two dielectric peaks in the dielectric constant–temperature curve. The individual layer thickness for layer-by-layer homoepitaxy growth that resulted in columnar grains was <20 nm.     

Electrical Properties of a Bismuth Layer-Structured Ba2Bi4Ti5O18 Single Crystal

Ba₂Bi₄Ti₅O₁₈ single crystals were grown, and their dielectric permittivity, conductivity, and ferroelectricity were investigated along the a -(or b -)axis and the c -axis separately. The dielectric permittivity at 1 MHz along the a -(or b -)axis was 2000 at the Curie temperature (360°C); this value was 8 times greater than that along the c -axis. The dc conductivity was greater along the a -(or b -)axis than that along the c -axis, by one order of magnitude. In regard to the ferroelectricity, the saturated remanent polarization was 120 mC/m² and the saturated coercive field was 3 MV/m along the a -(or b -)axis; values of 8.5 mC/m² and 0.81 MV/m, respectively, were observed along the c -axis. The Ba₂Bi₄Ti₅O₁₈ single crystals had large electrical anisotropies, which were due to the layered structure.     

High-Q Microwave Dielectrics in the (Mg1−xCox)2TiO4 Ceramics

The microwave dielectric properties and the microstructures of (Mg_{1− x} Co _x )₂TiO₄ ceramics prepared by the conventional solid-state route were investigated. Lattice parameters were also measured for specimens with different x . The formation of solid solution (Mg_{1− x} Co _x )₂TiO₄ ( x =0.02–0.1) was confirmed by the X-ray diffraction patterns, energy dispersive X-ray analysis, and the lattice parameters measured. By increasing x from 0 to 0.05, the Q × f of the specimen can be tremendously boosted from 150 000 GHz to a maximum of 286 000 GHz. A fine combination of microwave dielectric properties (ɛ_r∼15.7, Q × f ∼286 000 GHz at 10.4 GHz, τ_f∼−52.5 ppm/°C) was achieved for (Mg_0.95Co_0.05)₂TiO₄ ceramics sintered at 1390°C for 4 h. Ilmenite-structured (Mg_0.95Co_0.05)TiO₃ was detected as a second phase. The presence of the second phase would cause no significant variation in the dielectric properties of the specimen because it possesses compatible properties compared with that of the main phase. In addition, only a small deviation in the dielectric properties was monitored for specimens with x =0.04–0.05 at 1360°–1420°C. It not only provides a wide process window but also ensures an extremely reliable material proposed as a very promising dielectric for low-loss microwave and millimeter wave applications.     

Formation of Nanometric TiB2 from TiO2

Titanium diboride can be produced by ball-milling a mixture of TiO₂, B₂O₃, and Mg metal for between 10 and 15 h. The reaction was found to be completed during the milling with no evidence of residual Mg. The unwanted phase, MgO, was readily removed by leaching in acid. The leached powder obtained after 15 h milling had a particle size of <200 nm and was highly faceted. The particle size decreased to ∼50 nm after 100 h milling and seemed to be relatively monodisperse. Scherrer calculation of the crystallite size showed that the product particles were probably single crystal.