Effect of Silica on the Thermal Conductivity of Aluminum Nitride

The effect of SiO₂ on the thermal conductivity of aluminum nitride pressureless sintered with 3 wt% Y₂O₃ as a sintering aid was investigated. SiO₂ additions greatly decreased the thermal conductivity of the sintered parts from values of around 160 W/m·K on undoped samples to about 25 W/m·K with 5 wt% SiO₂ added to the green body composition. Microstructural studies, combined with the temperature dependence of the thermal conductivity and lattice parameter measurements, indicated that defect phonon scattering was the mechanism responsible for the decrease in thermal conductivity. SiO₂ can be incorporated in limited solid solution into the AIN lattice, generating AI vacancies for charge compensation in a process not unlike the solution of oxygen in AIN. The mass difference introduced by the vacancies is the main phonon scattering defect. Beyond a concentration threshold of 2%, the SiO₂-induced defects cluster to form SiAION polytypoids derived from the basic 2H AIN structure with stacking sequences that depend on the SiO₂ levels in the sample.     

Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C

Combined oxide additives (Y₂O₃, CaO, La₂O₃, CeO₂, SiO₂, TiO₂, and Fe₂O₃) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂. This lowtemperature material has small grain size (1 to 3 μm).     

Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina

The grain-growth behavior of Al₂O₃ compacts with small contents (≤10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO₂, or CaO + TiO₂. The platelike grains, defined as the abnormal grains larger than 100 μm with an aspect ratio ≥5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na₂O + SiO₂, CaO + SiO₂, SrO + SiO₂, or BaO + SiO₂. However, an addition of MgO to the Al₂O₃ doped with CaO + SiO₂ resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO₂ doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains was explained by an increased interfacial reaction rate due to the codopants. AT the same time the codopants made the basal plane, which appeared as the flat boundaries, the lowest energy plane. The appearance of the platelike grains was favored in compacts with a small grain size and with a narrow size distribution at the onset of abnormal grain growth. Accordingly, the use of starting powders with a small particle size and narrow size distribution, smaller amounts of dopings, and high sintering temperature resulted in an increased number of the platelike grains.     

Evidence of Bilevel Solubility in the Bimodal Microstructure of TiO2-Doped Alumina

A phenomenon of bilevel solubility was observed in a TiO₂-doped (2.0 wt%) alumina with bimodal microstructure sintered in N₂. Surface contributions to dopant signals in individual grains were identified and removed, using a spatially resolved energy dispersive spectroscopy analysis. Two levels of solubility, 1.005 ± 0.166 and 0.504 ± 0.082 mol% of TiO₂ in Al₂O₃, were obtained for anisotropic and equiaxed grains, respectively. No grain size dependence of solubility was found, but segregation of SiO₂ to boundaries with the anisotropic grains was observed. This phenomenon was explained by the incorporation of Ti³⁺ into the Al₂O₃ lattice during the abnormal grain growth caused by SiO₂ liquid under a reducing atmosphere.     

Grain Growth in CaO-Stabilized ZrO2 in the Presence of a Liquid Phase

Calcia-stabilized zirconia, CSZ (7.5 wt% CaO), with impurities of A1₂O₃, SiO₂, MgO, Fe₂O₃, and TiO₂, was sintered in air at 1783 K for times (t) up to 230 h. The microstructure consisted of grains of CSZ with small amounts of pores and a CaO-Al₂O₃-SiO₂ eutectic. Grain diameters grew in proportion to t^1/3. The results are consistent with a mechanism in which grain growth is limited by diffusion of Zr⁴⁺ ions through the liquid eutectic.     

Analytical Electron Microscopic Studies of Doped Dicalcium Silicates

Dicalcium silicates having CaO/SiO₂ molar ratios of 1.8 to 2.2 were sintered at 1450°C for 90 min with or without small quantities of dopants (K₂O or Al₂O₃) and were air quenched. The microstructures of the fired samples were characterized using electron microscopy (SEM and TEM) and associated microanalytical techniques. There was no evidence for the existence of Ca_1.8SiO_3.8 or Ca_2.2SiO_4.2. Amorphous grain-boundary phases were observed between grains and as inclusions within the grains; the amounts decreased as CaO/SiO₂ ratios increased. The compositions of the amorphous phases were always rich in dopants and had a CaO/SiO₂ ratio close to that of wollastonite. High levels of Al₂O₃ were observed to enter the β-Ca₂SiO₄ grains under lime-rich conditions (CaO/SiO₂= 2.2) up to a saturation level of about 3.0 wt%. Some additional crystalline phases were observed to form depending on stoichiometry and dopant level.     

Singular Grain Boundaries in Alumina and Their Roughening Transition

The shapes and structures of grain boundaries formed between the basal (0001) surface of large alumina grains and randomly oriented small alumina grains are shown to depend on the additions of SiO₂, CaO, and MgO. If a sapphire crystal is sintered at 1620°C in contact with high-purity alumina powder, the grain boundaries formed between the (0001) sapphire surface and the small alumina grains are curved and do not show any hill-and-valley structure when observed under transmission electron microscopy (TEM). These observations indicate that the grain boundaries are atomically rough. When 100 ppm (by mole) of SiO₂ and 50 ppm of CaO are added, the (0001) surfaces of the single crystal and the elongated abnormal grains form flat grain boundaries with most of the fine matrix grains as observed at all scales including high-resolution TEM. These grain boundaries, which maintain their flat shape even at the triple junctions, are possible if and only if they are singular corresponding to cusps in the polar plots of the grain boundary energy as a function of the grain boundary normal. When MgO is added to the specimen containing SiO₂ and CaO, the flat (0001) grain boundaries become curved at all scales of observation, indicating that they are atomically rough. The grain boundaries between small matrix grains also become defaceted and hence atomically rough.     

Impurity-Dependent Morphology and Grain Growth in Liquid-Phase-Sintered Alumina

The alumina grains in liquid-phase-sintered (LPS) materials prepared from different commercial sources have a predominantly platelet morphology. Generally, the MgO:(CaO + BaO + Na₂O + K₂O) ratio in the chemical composition controls the morphology in LPS alumina that is 91–94 wt% pure. Within a given range of SiO₂ content (i.e., 4.3–5.2 wt% in the chemical composition), a low MgO:(CaO + BaO + Na₂O + K₂O) ratio (i.e., <1.0) in the LPS compositions favors the formation of elongated grains, whereas ratios of >1.0 result in equiaxed grains. SiO₂ contents outside the 4.3–5.2 wt% range favor the formation of elongated grains. A tendency to form platelike grains is observed for LPS alumina with a purity of 91–94 wt% when both the MgO:(CaO + BaO + Na₂O + K₂O) ratio and the SiO₂ content are relatively low. The sintered density generally increases as the SiO₂ content in the chemical composition decreases.     

Final Sintering of Cr2O3

The effect of oxygen activity on the sintering of high-purity Cr₂O₃ is shown. Theoretical density was approached at the equilibrium O₂ partial pressure needed to maintain the Cr₂O₃ phase ( P _o2=2×10⁻¹² atm). The presence of N₂ in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O₂ activity, correlated with the changing predominant volatile species in the Cr-O system.     

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).     

Singular Grain Boundaries in Alumina Doped with Silica

Pure Al₂O₃ powder compact sintered at 1400°C after adding 100 mol ppm of SiO₂ shows grain boundaries that are flat, even across the triple junctions. TEM observations show that these flat grain boundaries are parallel to the basal planes of the grains on one side. These flat grain boundaries must be singular. At such a low SiO₂ concentration and a low temperature, it is very unlikely that any liquid phase is present at these grain boundaries to cause such flat boundary shapes.     

Microstructural Characterization of Superplastic SiO2-doped TZP with a Small Amount of Oxide Addition

The microstructures of 5 wt% SiO₂-doped TZP, 5 wt% (SiO₂+ 2 wt% MgO)-doped TZP, and 5 wt% (SiO₂+ 2 wt% Al₂O₃)-doped TZP are characterized by high-resolution electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. An amorphous phase is formed at multiple grain junctions but not along the grain-boundary faces in these three materials. A small addition of MgO and Al₂O₃ into the SiO₂ phase results in a marked reduction in tensile ductility of SiO₂-doped TZP. This reduction seems to correlate with segregation of magnesium or aluminum ions at grain boundaries and a resultant change in the chemical bonding state.     

Oxidation State of Chromium in CaO–Al2O3–CrOx–SiO2 Melts under Strongly Reducing Conditions at 1500°C

Equilibrium ratios Cr²⁺/Cr³⁺ of chromium oxide dissolved in CaO–chromium oxide–Al₂O₃–SiO₂ melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions ( p _o2= 10^−9.56 to 10^−12.50 atm). The majority of the chromium is divalent (Cr²⁺) under these conditions and Cr²⁺/Cr³⁺ ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO₂ ratios. In addition, Cr²⁺/Cr³⁺ ratios, at a given CaO/SiO₂ ratio, are relatively unaffected by the amount of Al₂O₃ present.     

Silica and Magnesia Dopant Distributions in Alumina by High-Resolution Scanning Secondary Ion Mass Spectrometry

Trace SiO₂ and MgO additive distributions in sintered alumina have been studied using high-resolution scanning secondary ion mass spectrometry (SIMS). When doped with each additive individually, evidence is seen for both strong silicon segregation to grain boundaries ( C _gb/ C _grain similar/congruent 300) in SiO₂-doped alumina and strong magnesium segregation to grain boundaries ( C _gb/ C _grain similar/congruent 400) in MgO-doped alumina. When codoped with both SiO₂ and MgO, segregation of both ions to grain boundaries is reduced by a factor of 5 or more over single doping. The additive concentrations increase proportionally in the grains, and both dopants become more uniformly distributed throughout the bulk. It is concluded that codoping with these additives increases their mutual bulk solid solubility and decreases their interfacial segregation over single doping. The beneficial effect of MgO additions in controlling microstructure development in alumina and improving corrosion resistance to aqueous HF stems from its ability to redistribute silicon ions from grain boundaries into the bulk.     

Effect of Alumina on Sintering Behavior and Electrical Conductivity of High-Purity Yttria-Stabilized Zirconia

The sintering behavior and electrical conductivity of high-purity 8-mol% Y₂O₃-stabilized ZrO₂ (8YSZ) with Al₂O₃ additions were investigated. The addition of 1 wt% AI₂O₃ to 8YSZ provided dense, sintered samples with 9.1% relative density at 1400°C without a holding time. Addition of 1 wt% SiO₂ enhanced the sinterability of 8YSZ. Na₂O addition of 0.1 wt% remarkably lowered it. Electrical conductivity at 1000°C in air increased slightly with increased Ai₂O₃ content up to 1 wt% and then monotonously decreased. 8YSZ with 1 wt% AI₂O₃ showed the maximum conductivity of 0.16 S/cm at 1000°C.     

Low-Temperature Sintering and High Thermal Conductivity of YLiO2-Doped AlN Ceramics

The present work indicates through thermodynamic considerations that YLiO₂ additive is beneficial for low-temperature sintering of AlN ceramics. Pressureless sintering of commercially available AIN powders with simultaneous additions of YLiO₂ and CaO resulted in materials with high thermal conductivity (170 W·m^–1·K^–1 after sintering at 1600°C for 6 h). It is demonstrated that improvement of thermal conductivity is possible at low firing temperature by use of sintering aids.     

Effect of Alkali Ions on the Crystallization of 3BaO·5SiO2 Glass

In this study we have investigated the effects of alkali additions on the spherulitic crystallization process in 3BaO·5SiO₂ glass. Both Li⁺ and Na⁺ increased the growth rate of the spherulites and reduced the crystallization temperature from that in the base glass. Li⁺ and Na⁺ also reduced the temperature at which the microstructural transformation from spherulites to laths took place. Dielectric loss measurements suggest that the effect of the alkali additions on crystallization temperature is due to their concentration in grain boundaries, thereby lowering the viscosity of the remaining glass, and increasing mobility rates.     

Liquid-Phase Sintering of Aluminum Nitride by Europium Oxide Additives

Europium oxide has been investigated as a sintering aid and dopant for AIN. Pressureless sintering was carried out with 0 to 9 wt% Eu₂O₃ additives, and dense sintered specimens were obtained using 1 to 4 wt% Eu₂O₃. With increasing Eu₂O₃ content, two additional phases were observed in the X-ray diffraction patterns. The lattice parameters a and c of AIN in the wurtzite structure changed slightly and non-monotonically with Eu₂O₃ content and showed their minimum value in a 4 wt% Eu₂O₃ sample. The Raman and photoluminescence spectra of sintered specimens were measured. These experimental results and the sintering mechanism in the system were discussed from the standpoint of the effects of oxygen, europium, and stress.     

Effect of Sintering Aids on Microstructures and PTCR Characteristics of (Sr0.2Ba0.8)TiO3 Ceramics

The effects of liquid-phase sintering aids on the microstructures and PTCR characteristics of (Sr_0.2Ba_0.8)TiO₃ materials have been studied. The grain size of sintered materials monotonically decreases with increasing content of Al₂O₃–SiO₂–TiO₂ (AST). The ultimate PTCR properties with ρ_ht/ρ_rt as great as 10^5.61 are obtained for fine-grain (10-μm) samples, which contain 12.5 mol% AST and were sintered at 1350°C for 1.5 h. The quantity of liquid phase formed due to eutectic reaction between AST and (Sr,Ba)TiO₃ is presumably the prime factor in determining the grain size of samples. The grains grow rapidly at the sintering temperature in the first stage until the liquid phase residing at the grain boundaries reaches certain critical thickness such that the liquid–solid interfacial energy dominates the mechanism of grain growth.     

Oxidation of Silicon Nitride Sintered with Rare-Earth Oxide Additions

The effects of rare-earth oxide additions on the oxidation of sintered Si³N⁴ were examined. Insignificant oxidation occurred at 700o and 1000oC, with no evidence of phase instability. At 1372oC, the oxidation rate was lowest for Y203 and increased for additions of La₂O₃, Sm₂O₃, and CeO₂, in that order. Data obtained from X-ray diffraction, electron microprobe analysis, and scanning electron microscopy indicate that oxidation occurs via diffusion of cationic species from Si₃N₄ grain boundaries.