Oxidation behavior of silicon nitride sintered with Lu2O3 additive

Kyocera SN282 silicon nitride ceramics sintered with 5.35 wt% Lu₂O₃ were oxidized in dry oxygen at 930–1,200 °C. Oxidation of SN282 follows a parabolic rate law. SN282 exhibits significantly lower parabolic rate constants and better oxide morphological stability than silicon nitride containing other sintering additives under similar conditions. The activation energy for oxidation of SN282 is 107 ± 5 kJ/mol K, suggesting inward diffusion of molecular oxygen in the oxide layer as the rate-limiting mechanism.     

Microstructure and properties of silicon nitride ceramics with calcium aluminate additions

Silicon nitride ceramics containing calcium aluminates as sintering aids have been prepared by hot pressing at 1650°C in a nitrogen atmosphere, and the effect of sintering aid content on their microstructure, phase composition, mechanical strength, and air oxidation resistance has been studied. The results demonstrate that the Si₃N₄ ceramic containing 10 wt % calcium aluminates has a uniform distribution of intergranular multicomponent oxide phases and consists of densely packed silicon nitride grains. Owing to this, it offers the maximum mechanical strength (850 MPa) and is stable to air oxidation up to 1300°C.     

High-temperature liquid-phase synthesis and sintering of MoSi<Subscript>2</Subscript> powders

Regularities of synthesis and sintering of molybdenum disilicide are studied. The synthesis is carried out in the combustion mode (self-propagating high-temperature synthesis) with the use of high-caloric mixtures of molybdenum oxide with aluminum and silicon. The cast synthesis product (MoSi₂) is subjected to grinding and subsequent sintering. The studies show that the basic parameters that influence the process of synthesis and its regularities are gas pressure in the reaction vessel and sizes of silicon particles. The sintering process, microstructure, and characteristics of sintered samples may be controlled through varying the sintering temperature and ratios of the ZrO₂ and Y₂O₃ additives.     

Effect of oxide additive in silicon nitride on interfacial structure and strength of silicon nitride joints brazed with aluminium

Silicon nitride ceramics with Y₂O₃ and Al₂O₃ as sintering additives were brazed with aluminium, and the brazed strength and the interfacial structure of the joints were compared with those of the joints made of additive-free silicon nitride ceramics. It is concluded that the additives in silicon nitride ceramics take part in the interfacial reaction, make the reaction layer thicker, and hence increase the brazed strength greatly.     

Effect of various oxide additives on sintering of BaO-SiO2 system glass-ceramics

Glass-ceramics can be produced by sintering and crystallization of a pressed glass powder pellet. Crystallization prior to complete densification results in a porous glass-ceramic. A small quantity of various oxides was added to and melted with 35.7 BaO-64.3 SiO₂ (mol %) glass and the sintering characteristic of the glass powder was evaluated in terms of the density relative to the bulk glass density after heating at a constant rate. Some oxide additives, such as Al₂O₃ and ZrO₂, increased the per cent relative density while others, such as Na₂O, decreased it. The achieved per cent relative density was compared with crystallization characteristics determined by differential thermal analysis (DTA) and viscosity determined by the beam-bending method. The per cent relative density showed a good correlation with the viscosity at the crystallization temperature, the higher per cent relative density being achieved for systems with the lower viscosity at the crystallization temperature.     

Yb2O3-fluxed sintered silicon nitride

Microstructural development and crystallization behaviour of Yb₂O₃-fluxed sintered silicon nitride materials was investigated using CTEM and HREM. The materials contained 5 and 10 vol% Yb₂O₃ as sintering additives. After densification, both compositions were subsequently heat treated to crystallize the residual amorphous secondary phases present at triple-grain regions. In the material doped with 5 vol% Yb₂O₃, only an amorphous secondary phase was observed after sintering, which was about 80% crystalline (Yb₂Si₂O₇) after the post-sintering heat treatment. A metastable phase was formed in the material with 10 vol% additives after sintering, with about 70% crystallinity in the triple-point pockets. Upon postsintering heat treatment, the material could be completely crystallized. During heat treating, the metastable phase combined with the remaining glass to form Yb₂SiO₅ plus Yb₂Si₂O₇ and a small amount of Si₃N₄ which deposited epitaxially on pre-existing Si₃N₄ grains in areas of low-energy within the triple-point pockets. All materials contained thin amorphous films separating the grains. The amorphous intergranular films along grain boundaries (homophase boundaries) revealed excess ytterbium and oxygen. The thickness of the intergranular films was about 1.0 and 2.5 nm for the grain boundaries and the phase boundaries, respectively, independent of additive content and heat-treatment history.     

Study on microstructure and thermal conductivity of Spark Plasma Sintering AlN ceramics

Dense aluminum nitride (AlN) ceramics were prepared by Spark Plasma Sintering with rare-earth oxide and CaF₂ as sintering additives. The effect of sintering additives on the density, phase composition, microstructure and thermal conductivity of AlN ceramics was investigated. The results showed that those sintering additives not only promoted densification through liquid-phase sintering but also improved thermal conductivity by decreasing oxygen impurities. Thermal conductivities of samples sintered with optimum proportion of rare-earth oxide and CaF₂ were higher than those of other samples. During the Spark Plasma Sintering process, the microstructures, especially the content and distribution of secondary phases, played important roles on the thermal conductivity of AlN ceramics.     

Preparation of ZrO2 and ZrO2-Y2O3 coatings on silicon carbide fibers

The formation of ZrO₂ and ZrO₂-Y₂O₃ coatings from hydrous metal oxide sols on Nicalon NLM 202 silicon carbide fibers is investigated in detail. The results indicate that the microstructure of the oxide layer and the surface morphology of the coatings depend on the physicohemical properties of the sol. Kinetic studies of the oxidation of uncoated and coated fibers at different Y₂O₃ contents demonstrate that the oxidation rate of silicon carbide fibers decreases with increasing coating thickness. The effect of oxidation on the phase composition of Nicalon cloth samples coated with ZrO₂ and ZrO₂-Y₂O₃ is examined.     

Effects of amount and type of cation species on the phase formation of single and codoped α-SiAlONs

The phase formation of single phase α-SiAlON ceramics based on α-Si₃N₄ was investigated using different rare earth (RE) or metallic cations to stabilise the α-phase. The effects of sintering temperature, size and type of stabilising cation and the addition of extra liquid forming additives on the phase transformation of α-Si₃N₄ to α-SiAlON were considered. The influence of the chosen parameters on properties such as density and hardness is furthermore discussed for Ca-codoped silicon aluminium oxynitride (SiAlON) ceramics (50% RE cations combined with 50% calcium cations). For smaller and larger RE systems different intermediate phases occur and the dissolution of sintering additives takes place at different temperatures, resulting in different amounts of SiAlON at a given sintering temperature. For smaller cations, the SiAlON formation is favoured below 1,750 °C compared to larger cations. The addition of extra liquid to the starting composition supports the SiAlON formation above the eutectic temperature. Contrary to the RE systems the calcium-doped samples did not show an intermediate phase during sintering. The cation solubility for sintering additives is higher and therefore the amount of SiAlON created below 1,750 °C increased. The final amount of SiAlON at 1,850 °C was about the same for all systems. Mechanical properties are also influenced by the composition of the starting powder. In general, increasing temperature leads to higher density and hardness.     

ZrO2-TiO2 ceramic humidity sensors

Ceramics based on 0.5ZrO₂-0.5TiO₂ were evaluated as humidity sensors. The variation of phase change, microstructure, and conductivity with relative humidity were investigated for base ceramics doped with additives such as CrO_1.5, FeO_1.5 and MgO. It was found that FeO_1.5 enhanced the formation of ZrTiO₄. The addition of MgO and FeO_1.5 increased conductivity and its humidity sensitivity. Open porosity was not affected much by the addition of the three dopants. Nitrogen treatment of sintered ceramics at 900 °C was found to increase conductivities and humidity sensitivity of the ceramics.     

Microstructure and thermal conductivity of spark plasma sintering AlN ceramics

Abstract

Dense aluminium nitride ceramics were prepared by spark plasma sintering at a lower sintering temperature of 1700°C with Y₂O₃, Sm₂O₃ and Dy₂O₃ as sintering additives respectively. The effects of three kinds of sintering additives on the phase composition, microstructure and thermal conductivity of AlN ceramics were investigated. The results showed that those sintering additives not only facilitated the densification via the liquid phase sintering mechanism, but also improved thermal conductivity by decreasing oxygen impurity. Sm₂O₃ could effectively improve thermal conductivity of AlN ceramics compared with Y₂O₃ and Dy₂O₃. Observation by scanning electron microscopy showed that AlN ceramics prepared by spark plasma sintering method manifested quite homogeneous microstructures, but AlN grain sizes and shapes and location of secondary phases varied with the sintering additives. The thermal conductivity of AlN ceramics was mainly affected by the additives through their effects on the growth of AlN grain and the location of secondary phases.     

Selective laser sintering of PZT ceramic powders

Conditions for the selective laser-induced layer sintering of a stoichiometric mixture of PbO, ZrO₂, and TiO₂ powders with the formation of bulk PZT ceramic articles were studied. The laser processing allows articles of the PZT ceramics to be obtained immediately in the course of sintering or upon additional annealing. Data on the microstructure and phase composition of the synthesized PZT ceramics are presented.     

The effect of Bi2O3 on the electrical and mechanical properties of ZrO2-Y2O3 ceramics

ZrO₂-Y₂O₃ ceramics with varying Bi₂O₃ content have been prepared and their microstructure, electrical conductivity and mechanical properties investigated. The sintering rate is strongly increased at low temperatures (1270 to 1420 K) due to the occurrence of a reactive phase the amount of which decreases during the sintering process. The main fluorite phase has an approximately constant composition at 0.85 ZrO₂-0.13 Y₂O₃-0.02 Bi₂O₃ while the amount of second phase depends on overall composition and sintering procedure. The electrical conductivity is also approximately constant but about a factor of five lower than in ZrO₂-Y₂O₃ ceramics. Typical strength and fracture toughness values are 160 MPa and 1.8 MPa m^1/2, respectively. However, the non-reproducibility of the second phase content and morphology seriously influences both mechanical properties.     

An Inhomogeneous Grain Boundary Composition in Silicon Nitride Ceramics as Revealed by 300 kV Field Emission Analytical Electron Microscopy

The chemical compositions of the grain boundary phases of silicon nitride (Si₃N₄) ceramics containing additives of 1 mole% and 10 mole% of an equi-molar mixture of Y₂O₃ and Nd₂O₃ have been studied by 300 kV field emission analytical electron microscopy. The energy dispersive x-ray spectra (EDS) are obtained from both two-grains and triple-grain junctions, where an electron beam of about 0.5 nm in diameter is focused. The thickness of the intergranular thin film is found to be about 1 nm, whose value is almost the same between two samples. The sintering additives are highly enriched at the triple-grain junctions, while they are less concentrated at the two-grain junctions. It is also shown that the additives are distributed inhomogeneously within the triple-grain junctions. Based on the composition analysis among the grain boundaries, an inhomogeneous grain boundary composition model for the Si₃N₄ ceramics is proposed.     

Effect of fluorine impurity on the oxidation of silicon oxynitride ceramics doped with gadolinium oxide

Impurities in raw Si₃N₄ powders remain in intergranular glassy phases in Si₃N₄ and Si₂N₂O ceramics and degrade their high-temperature properties. Fluorine is one of the typical impurities in the raw powders. The oxidation rate of Si₂N₂O ceramics doped with Gd₂O₃ greatly varied with a difference in impurity contents (especially F) of the raw Si₃N₄ powders used. When a high concentration of impurity existed in the intergranular glassy phase, the rate of oxidation was controlled by O^2– diffusion through the glassy phase in the partly oxidized scale and unoxidized body; outward diffusion of Gd³⁺ occurred concurrently. On the other hand, when the impurity contents in the intergranular glassy phase was very low, the diffusion rate of ions (Gd³⁺, O^2–, etc.) in the glassy phase became very low (substantially zero in the oxidation at 1300°C). Only cristobalite (SiO₂) was formed on the surface. The rate of oxidation was controlled by O₂ diffusion through the cristobalite layer, and was very low.     

Characterization of porous silicon nitride/silicon oxynitride composite ceramics produced by sol infiltration

Porous silicon nitride/silicon oxynitride composite ceramics were fabricated by silica sol infiltration of aqueous gelcasting prefabricated Si₃N₄ green compact. Silica was introduced by infiltration to increase the green density of specimens, so suitable properties with low shrinkage of ceramics were achieved during sintering at low temperature. Si₂N₂O was formed through reaction between Si₃N₄ and silica sol at a temperature above 1550 °C. Si₃N₄/Si₂N₂O composite ceramics with a low linear shrinkage of 1.3–5.7%, a superior strength of 95–180 MPa and a moderate dielectric constant of 4.0–5.0 (at 21–39 GHz) were obtained by varying infiltration cycle and sintering temperature.     

Effect of a new additive on mechanical properties of hot-pressed silicon carbide ceramics

The mechanical properties and microstructure of SiC ceramics, hot pressed by simultaneously adding nano-SiC and oxides (MgO+Al₂O₃+Y₂O₃) or nitrate salts (Mg(NO₃)₂+Al(NO₃)₃+Y(NO₃)₃) as additives, were evaluated. The oxide additives system slightly influenced the mechanical properties of the ceramics, while the addition of nano-SiC lead to finer microstructure, and 5 vol.% nano-SiC changed the fracture mode from intergranular type to transgranular type. The ceramics with nitrate salts had fine, equiaxed grains with an average grain size larger than that of the system added oxides, thus inducing lower Viker’s hardness and flexural strength, while the presence of crystalline YAG phase improved the fracture toughness by 54.7%. Also, an observed increase in grain growth—with decreasing weight fraction of liquid and the grounded grain morphology in this system—confirmed a diffusion-controlled growth mechanism. Although the sample with the least amount of additives has the lowest relative density and largest grain size, its flexural strength did not drastically decrease. The influence of nano-SiC on the fracture toughness in the nitrate additive system was negligible.     

Effect of CuO additives on the reversibility of zirconia crystalline phase transitions

We have studied the influence of the differents atmospheres on the sintering of CuO-doped-zirconia ceramics. After an annealing treatment in a reducing atmosphere, we have found nanoparticles of metallic copper in triple points and in the grains of the sintered zirconia. Furthermore, the influence of the intergranular Y₂Cu₂O₅ phase on the tetragonal to monoclinic phase transition and the crystalline structure of zirconia is discussed with respect to results obtained from X-ray diffraction, differential thermal analysis, Raman spectroscopy and High Resolution Transmission Electron Microscopy.     

Coprecipitation-derived mullite and mullite-zirconia composites

Precursor powders of mullite-zirconia (0–40 wt% ZrO₂) were prepared by a hydroxide coprecipitation method and their behaviour during calcination between room temperature and 1500 °C was studied using thermal analysis, X-ray diffraction and electron microscopy. The only crystalline phases present in the precalcined powders were bayerite and gibbsite, and these were stable up to 250 °C. Powders containing ZrO₂ were initially amorphous, but on calcination between 250 and 850 °C produced different crystalline phases at temperatures which depended on the amount of zirconia present. Thus in the case of mullite-40 wt% ZrO₂, zirconia crystallized at about 850 °C and was stable up to 1200 °C, when it reacted with free silica to form zircon (ZrSiO₄). Mullite formed above 1250 °C at the expense of zircon and remained stable at higher temperatures. The oxide powders were very homogeneous, and on sintering produced ceramics with a fine-grained uniform microstructure. The powders were very reactive and could be sintered conventionally to near-theoretical density at 1600–1700 °C without sintering aids. The fracture strength of mullite was about 275 MPa, and this could be improved to 350 MPa by hot isostatic pressing the presintered bodies. Addition of zirconia enhanced the sintering kinetics as well as the fracture strength of mullite.     

Formation of intergranular amorphous films during the microstructural development of liquid phase sintered silicon carbide ceramics

The microstructure of liquid phase sintered SiC ceramics was characterised by means of high resolution transmission electron microscopy (HRTEM). The SiC ceramics were pressureless sintered with the additions of Al₂O₃ and Y₂O₃ at sintering temperatures of 1800 and 1950°C, respectively. At a sintering temperature of 1800°C the microstructure of the SiC ceramics has no crystallised secondary phase and the SiC grains are separated by an intergranular amorphous film. In contrast, in the case of the microstructure of SiC ceramics sintered at 1950°C a clean interface without any amorphous layer between the SiC grains was observed. The secondary phase is crystallised into the Y₃Al₅O₁₂ phase and exhibits a clean interface between the SiC grains. An explanation for the existence or the absence of the intergranular glass films are given by an extended Clarke's model of the force balance of attractive van der Waals forces and repulsive steric forces. The chemical decomposition of the intergranular glass film at elevated temperature was considered.