Electrical Conductivities of (CeO2)1−x(Y2O3)x Thin Films

Electrical properties of CeO₂ thin films of different Y₂O₃ dopant concentration as prepared earlier were studied using impedance spectroscopy. The ionic conductivities of the films were found to be dominated by grain boundaries of high conductivity as compared with that of the bulk ceramic of the same dopant concentration sintered at 1500°C. The film grain-boundary conductivities were investigated with regard to grain size, grain-boundary impurity segregation, space charge at grain boundaries, and grain-boundary microstructures. Because of the large grain boundary and surface area in thin films, the impurity concentration is insufficient to form a continuous highly resistive Si-rich glassy phase at grain boundaries, such that the resistivity associated with space-charge layers becomes important. The grain-boundary resistance may originate from oxygen-vacancy-trapping near grain boundaries from space-charge layers. High-resolution transmission electron microscopy coupled with a trans-boundary profile of electron energy loss spectroscopy gives strong credence to the space-charged layers. Since the conductivities of the films were observed to be independent of crystallographic texture, the interface misorientation contribution to the grain-boundary resistance is considered to be negligible with respect to those of the impurity layer and space-charge layers.     

Thermal Expansion of Homogeneous and Gradient Solid Solutions in the System KZr2(PO4)3─KTi2(PO4)3

Low-thermal-expansion ceramics having arbitrary thermal expansion coefficients were synthesized from homogeneous solid solutions in the system KZr₂(PO₄)₃─KTi₂(PO₄)₃ (KZP–KTP). Dense and strong ceramics were fabricated by sintering at 1100° to 1200°C with 2 wt% MgO. The thermal expansion coefficient increased from 0 to +3 × 10⁻⁶/°C with increasing x in KZr_{2 − x}Ti_x (PO₄)₃ (KZTP). In addition, a functionally gradient material with respect to thermal expansion was prepared by forming a series of KZTP solid solutions in a single ceramic body. By heating a pile of KZP and KTP ceramics in contact with each other, KZP and KTP bonded together to form a KZTP gradient solid solution near the interface.     

Sintering of LaFeO3 Ceramics

The sintering of LaFeO₃ has been studied in the temperature interval 1100–1600°C in air. The effect of cation nonstoichiometry on densification, microstructure, and phase composition is emphasized. La₂O₃ was observed to inhibit both sintering and grain growth. In Fe-excess materials, exaggerated grain growth occurred, particularly above 1430°C, where a liquid phase was formed. Postsintering swelling was observed in Fe-excess materials above 1430°C. The swelling mechanism is related to phase equilibria, which are reductive in nature and lead to the evolution of oxygen gas. The density in La-excess materials remains high up to 1600°C, but the ceramics might disintegrate in air.     

Sintering of (Cu0.25Ni0.25Zn0.50)Fe2O4 Ferrite

A study has been conducted on the sintering of a ceramic ferrite having the composition (Cu_0.25Ni_0.25Zn_0.50)Fe₂O₄. The study analyzes the evolution of ferrite relative to density and microstructure with peak sintering temperature and dwell time at peak temperature. The densification and grain-growth rates are correlated with average grain size, relative density, and temperature. Corresponding rate-controlling diffusion mechanisms are proposed.     

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.     

Spark Plasma Sintering of Nano-Sized TiN Prepared from TiO2 by Controlled Hydrolysis of TiCl4 and Ti(O-i-C3H7)4 Solution

Nano-sized TiO₂ powders were prepared by controlled hydrolysis of TiCl₄ and Ti(O-i-C₃H₇)₄ solutions and nitrided in flowing NH₃ gas at 700°–1000°C to form TiN. Nano-sized TiN was densified by spark plasma sintering at 1300°–1600°C to produce TiN ceramics with a relative density of 98% at 1600°C. The microstructure of the etched ceramic surface was observed by SEM, which revealed the formation of uniformly sized 1–2 μm grains in the TiCl₄-derived product and 10–20 μm in the Ti(O-i-C₃H₇)₄-derived TiN. The electric resisitivity and Vickers micro-hardness of the TiN ceramics was also measured.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Low-Temperature Sintering of (Ba0.6Sr0.4)TiO3

An addition of just 0.4 wt% Li₂O to (Ba_0.6Sr_0.4)TiO₃ powder was able to reduce the sintering temperature to ≤900°C and produce ceramics with a relative density of 97%. Small amounts of two secondary phases were formed during this process: Li₂TiO₃ and Ba₂TiO₄. The addition of Li₂O depresses the ferroelectric character of the (Ba_0.6Sr_0.4)TiO₃ and, as a result, reduces the permittivity, improves the temperature coefficient of permittivity, and reduces the dielectric losses. The tunability shows no significant variation with Li₂O concentration and remains between 16.5% and 13.5%. A low-temperature sintering mechanism was proposed. The mechanism involves the intermediate formation of BaCO₃, its melting and the incorporation of Li⁺ into the BST. The sintering mechanism can be characterized as reactive liquid-phase sintering.     

Superconductive Properties of the YBa2Cu4O8 Superconductor

A bulk density of 85% of the theoretical density was achieved by sintering a powder compact of YBa₂Cu₄O₈ (124) at 850°C in flowing oxygen at 1 atm (≅10⁵ Pa). This value is very close to that obtained by the hot isostatic pressure technique (90%). The superconducting properties of the sample were characterized by magnetization and ac susceptibility techniques. The magnetization critical current density at 20 K in zero field was determined to be ∼5 × 10⁴ A/cm², and the superconducting transition temperatures were found to be 77 K for the bulk material and 82 K for the granular phase. The powder X-ray diffraction and ac susceptibility studies revealed the sintered 124 material to be single phase.     

Effects of Heating Rate on the Sintering Behavior and the Dielectric Properties of Ba0.7Sr0.3TiO3 Ceramics Prepared by Boron-Containing Liquid-Phase Sintering

The effects of heating rate on the sintering behavior and the dielectric properties of Ba_0.7Sr_0.3TiO₃ ceramics prepared by boron-containing liquid-phase sintering were investigated. When 0.5 wt% B₂O₃ was added to Ba_0.7Sr_0.3TiO₃, sintering was achieved at ∼1150°C, and the overdoped B₂O₃ did not form an adequate amount of liquid phase or volatilize; it remained in the samples and formed a secondary phase. A transition broadening was observed as the heating rate increased. As the heating rate increased, the Curie temperature increased and the maximum dielectric constant ( k _max) at the Curie temperature decreased. This result is attributable to a decrease in the diffuseness parameter (δ) and the tetragonality ( c / a ).     

Laser Sintering of Bismuth Germanate (Bi4Ge3O12) Ceramics

A solid-state route to synthesize bismuth germanate (Bi₄Ge₃O₁₂, or BGO) ceramics, and the optimum conditions for the laser sintering of these ceramics, were investigated. Special emphasis was placed on the influence of laser power and irradiation time on densification and microstructure evolution. The procedure that was used involved preheating the samples and a laser-irradiation program, which resulted in sintered, crack-free BGO ceramics with good pore shrinkage and a relative density of 95% (±3%).     

Fabrication of TiN/Si3N4 Ceramics by Spark Plasma Sintering of Si3N4 Particles Coated with Nanosized TiN Prepared by Controlled Hydrolysis of Ti(O-i-C3H7)4

TiN-coated Si₃N₄ particles were prepared by depositing TiO₂ on the Si₃N₄ surfaces from Ti(O- i -C₃H₇)₄ solution, the TiO₂ being formed by controlled hydrolysis, then subsequently nitrided with NH₃ gas. A homogeneous TiO₂ coating was achieved by heating a Si₃N₄ suspension containing 1.0 vol% H₂O with the precursor at 40°C. Nitridation successfully produced Si₃N₄ particles coated with 10–20 nm TiN particles. Spark plasma sintering of these TiN/Si₃N₄ particles at 1600°C yielded composite ceramics with a relative density of 96% at 25 vol% TiN and an electrical resistivity of 10⁻³Ω·cm in compositions of 17.5 and 25 vol% TiN/Si₃N₄, making these ceramics suitable for electric discharge machining.     

Negative Thermal Expansion in (HfMg)(WO4)3

A single-phase material (HfMg)(WO₄)₃ with an orthorhombic structure, A₂ (WO₄)₃-type tungstate, has been successfully prepared for the first time by the calcination of HfO₂, MgO, and WO₃, substituting Hf⁴⁺ and Mg²⁺ for A³⁺ cations in A₂(WO₄)₃. The new material shows a negative thermal expansion coefficient of approximately −2 ppm/°C from room temperature to 800°C. The mechanism of negative thermal expansion is assumed to be the same as that of Sc₂(WO₄)₃.     

Processing and Sintering of Ultrafine MgO-ZrO2 and (MgO, Y2O3)-ZrO2 Powders

Chemical coprecipitation was used to produce ultrafine and easily sinterable MgO-stabilized and (MgO, Y₂O₃) stabilized ZrO₂ powders. The sintering behavior is very sensitive to post-precipitation washing because "hard" agglomerates form when the precipitated gels are washed with water, whereas "soft" agglomerates form when they are washed with ethanol. The soft agglomerates pack uniformly, resulting in homogeneous shrinkage of powder compacts to near-theoretical density. The hard agglomerates result in compacts which have regions of localized densification and a signifiint fraction of residual porosity.     

Thermal Expansion of the Low-Temperature Form of BaB2O4

Thermal expansion of the low-temperature form of BaB₂O₄ (β-BaB₂O₄) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L _t = L ₀(1 + At + Bt ²+ Ct ³) are given as follows: a axis, A = 1.535 × 10⁻⁷, B = 6.047 × 10⁻⁹, C = -1.261 × 10⁻¹²; c axis, A = 3.256 × 10⁻⁵, B = 1.341 × 10⁻⁸, C = -1.954 × 10⁻¹²; and cell volume V, A = 3.107 × 10⁻⁵, B = 3.406 × 10⁻⁸, C = -1.197 × 10⁻¹¹. Based on α _t = (d L _t /d t )/ L ₀, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V.     

Alumina Sol-Assisted Sintering of SiC—Al2O3 Composites

The composite sol—gel (CSG) technology has been utilized to process SiC—Al₂O₃ ceramic/ceramic particulate reinforced composites with a high content of SiC (up to 50 vol%). Alumina sol, resulting from hydrolysis of aluminum isopropoxide, has been utilized as a dispersant and sintering additive. Microstructures of the composites (investigated using TEM) show the sol-originating phase present at grain boundaries, in particular at triple junctions, irrespective of the type of grain (i.e., SiC or Al₂O₃). It is hypothesized that the alumina film originating from the alumina sol reacts with SiO₂ film on the surface of SiC grains to form mullite or alumina-rich mullite-glass mixed phase. Effectively, SiC particles interconnect through this phase, facilitating formation of a dense body even at very high SiC content. Comparative sinterability studies were performed on similar SiC—Al₂O₃ compositions free of alumina sol. It appears that in these systems the large fraction of directly contacting SiC—SiC grains prevents full densification of the composite. The microhardness of SiC—Al₂O₃ sol—gel composites has been measured as a function of the content of SiC and sintering temperature. The highest microhardness of 22.9 GPa has been obtained for the composition 50 vol% SiC—50 vol% Al₂O₃, sintered at 1850°C.     

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.     

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.     

Microstructure Tailoring for High Thermal Conductivity of β-Si3N4 Ceramics

β-Si₃N₄ ceramics sintered with Yb₂O₃ and ZrO₂ were fabricated by gas-pressure sintering at 1950°C for 16 h changing the ratio of "fine" and "coarse" high-purity β-Si₃N₄ raw powders, and their microstructures were quantitatively evaluated. It was found that the amount of large grains (greater than a few tens of micrometers) could be drastically reduced by mixing a small amount of "coarse" powder with a "fine" one, while maintaining high thermal conductivity (>140 W·(m·K)⁻¹). Thus, this work demonstrates that it is possible for β-Si₃N₄ ceramics to achieve high thermal conductivity and high strength simultaneously by optimizing the particle size distribution of raw powder.