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.     

Crystallization of the glassy phase in an Si3N4 material by post-sintering heat treatments

It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si₃N₄ material which was produced by the nitridation pressureless sintering (NPS) method using Y₂O₃ and Al₂O₃ as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment.     

Oxidation resistance of β-Sialon/TiN composites: an ion beam analysis (IBA) study

The oxidation resistance of β-Sialon processed with Y₂O₃ sintering additive and β-Sialon/TiN composites containing 1–10 wt% TiN was studied using ion beam analysis (IBA) techniques, augmented by XRD and SEM measurements. Rutherford backscattering spectrometry was used to monitor the diffusion of Y and Ti in the oxidised samples, and the diffusion of oxygen and nitrogen was observed by particle-induced gamma emission and nuclear reaction analysis. These techniques showed that in the Sialon control sample without TiN, oxygen was the first element to migrate at 1000 °C, followed by Y and N at 1100 °C. At 1200 °C, a N-poor, Y- and O-rich oxidised layer was formed, containing crystalline Y₂Si₂O₇. In the TiN-containing samples, Si, Al, Y and Ti were very mobile even at 1000 °C and the surface nitrogen was depleted by 1250 °C. The combined presence of yttrium aluminium garnet (YAG) and TiN protects the β-Sialon phase by forming an oxygen-rich crystalline barrier layer. The oxidation products of TiN in these composites are TiO₂ and Y₂Ti₂O_7. The details of the oxidation mechanism of the β-Sialon/TiN composites provided by these IBA studies (movement of yttrium and titanium, replacement of nitrogen by oxygen in the glassy yttrium phase and major crystalline and chemical changes in an outer oxidised layer) could not readily have been obtained by any other techniques, and illustrate the value of IBA for oxidation studies of non-oxide ceramics.     

Scaling and intergranular oxidation of an Fe/48 wt% Ni alloy in carbon dioxide at 700 to 1000°C

The oxidation of Nilo 48 has been studied using thermogravimetric, metallographic, and electron probe microanalysis techniques at 700 to 1000° C. After a short period of ill-defined oxidation, the parabolic law was obeyed throughout the exposure period, which varied from 1050 h at 713° C to 50 h at 1000° C. The activation energy for the oxidation reaction was 48±6 kcal/mole. Examination of the external scale indicated that this was single phase and, at 1000° C, its composition corresponded to Ni_xFe^3–xO₄, wherex 0.4. There were also intergranular oxidation and nickel enrichment of the alloy underlying the external scale. After oxidation for only 10 min at 1000° C, the nickel-enriched alloy zone contained 65 wt % Ni. The manganese concentration in the scale was similar to that in the alloy. The results are discussed and compared with those of other workers and it is concluded that the rate-controlling process is the diffusion of iron through the Ni_xFe_3–xO₄ lattice.     

Transient creep behaviour of hot isostatically pressed silicon nitride

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y₂O₃ as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of 1260 kJ mol^–1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of – 1.12 was determined.     

High temperature bending creep of a Sm-α-β Sialon composite

The four-point bending creep behavior of a Sm-- Sialon composite, in which Sm-melilite solid solution (denoted as M) was designed as intergranular phase, was investigated in the temperature range 1260–1350°C and stresses between 85 and 290 MPa. At temperatures less than 1300°C, the stress exponents were measured to be 1.2–1.5, and the creep activation energy was 708 kJ mol^–1, the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300°C, the stress exponents were determined to be 2.3–2.4, and creep activation energy was 507 kJ mol ^–1, the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350°C for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N^3– diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and Sialon phase transformation were analyzed and their effect on creep was evaluated.     

Oxidation behavior and mechanical properties of C/SiC composites with Si-MoSi2 oxidation protection coating

A new kind of oxidation protection coating of Si-MoSi₂ was developed for three dimensional carbon fiber reinforced silicon carbide composites which could be serviced upto 1550 °C. The overall oxidation behavior could be divided into three stages: (i) 500 °C < T < 800 °C, the oxidation mechanism was considered to be controlled by the chemical reaction between carbon and oxygen; (ii) 800 °C < T < 1100 °C, the oxidation of the composite was controlled by the diffusion of oxygen through the micro-cracks, and; (iii) T > 1100 °C, the oxidation of SiC became significant and was controlled by oxygen diffusion through the SiC layer. Microstructural analysis revealed that the oxidation protection coating had a three-layer structure: the out layer is oxidation layer of silica glass, the media layer is Si + MoSi₂ layer, and the inside layer is SiC layer. The coated C/SiC composites exhibited excellent oxidation resistance and thermal shock resistance. After the composites annealed at 1550 °C for 50 h in air and 1550 °C 100 °C thermal shock for 50 times, the flexural strength was maintained by 85% and 80% respectively. The relationship between oxidation weight change and flexural strength revealed the criteria for protection coating was that the maximum point of oxidation weight gain was the failure starting point for oxidation protection coating.     

Investigations of Bi substitution in NdBa2Cu3Oy

Attempts to substitute Bi for Nd in orthorhombic NdBa₂Cu₃O _y , prepared in air or oxygen at about 950°C led instead to formation of Ba₂NdBiO₆, a new cubic compound witha=0.8703 nm. The possibility was then explored of preparing superconducting (Nd_1–x Bi _x )Ba₂Cu₃O _y , by first forming the tetragonal phase at 880–950°C in nitrogen or argon followed by reheating in oxygen or air at 250–500°C in order to insert the additional oxygen required to yield the orthorhombic form while avoiding oxidation of Bi³⁺ to Bi⁵⁺. X-ray diffraction studies, electrical conductivity measurements, and thermogravimetric analysis of products indicate that Bi does not enter the NdBa₂Cu₃O _y , lattice in either the tetragonal or the orthorhombic phase. Ba₂NdBiO₆ clearly forms on reheating in oxygen or air even at low temperatures, and evidence is presented that a poorly crystallized oxygen-deficient form of this compound is already present prior to the reheating.     

Yttria migration in Y-TZP during high-temperature annealing

The microstructural and phase changes occurring during the high temperature (1300 to 1550° CO annealing of Y-TZP were studied using X-ray fluorescence, X-ray diffraction, and TEM. Two processes occurred simultaneously involving the diffusion of yttrium. The Y-TZP partitioned into yttria-rich and yttria-poor phases throughout the material, because the material lies in a two-phase two-phase field of the yttria-zirconia phase diagram. The other process involved the segregation of yttrium to the surface, the extent of which was shown to vary with the state of the surface (ground or polished), annealing temperature, and silica content. Migration of yttrium to the surface caused a significant surface composition change (i.e. from 4.7wt% Y₂O₃ at room temperature to 8.9 wt % Y₂O₃ at 1550°C for 3 h), resulting in a microstructure and phase composition different from the bulk.     

Effects of precursor evaporation temperature on the properties of the yttrium oxide thin films deposited by microwave electron cyclotron resonance plasma assisted metal organic chemical vapor deposition

Yttrium oxide thin films are deposited using indigenously developed metal organic precursor (2,2,6,6-tetra methyl-3,5-hepitane dionate) yttrium, commonly known as Y(thd)₃ (synthesized by ultrasound method). Microwave electron cyclotron resonance plasma assisted metal organic chemical vapor deposition process was used for these depositions. Depositions were carried out at a substrate temperature of 350 °C with argon to oxygen gas flow rates fixed to 1 sccm and 10 sccm respectively throughout the experiments. The precursor evaporation temperature (precursor temperature) was varied over a range of 170-275 °C keeping all other parameters constant. The deposited coatings are characterized by X-ray photoelectron spectroscopy, glancing angle X-ray diffraction and infrared spectroscopy. Thickness and refractive index of the coatings are measured by the spectroscopic ellipsometry. Hardness and elastic modulus of the films are measured by load depth sensing nanoindentation technique.C-Y₂O₃ phase is deposited at lower precursor temperature (170 °C). At higher temperature (220 °C) cubic yttrium oxide is deposited with yttrium hydroxide carbonate as a minor phase. When the temperature of the precursor increased (275 °C) further, hexagonal Y₂O₃ with some multiphase structure including body centered cubic yttria and yttrium silicate is observed in the deposited film. The properties of the films drastically change with these structural transitions. These changes in the film properties are correlated here with the precursor evaporation characteristics obtained at low pressures.     

Estimation of reaction conditions for synthesis of nanosized brookite-type titanium dioxide from aqueous TiOCl<Subscript>2</Subscript> solution

Ti O₂ nanoparticles with a mixture of brookite and rutile phases were prepared from aqueous TiOCl₂ solution at 80–150°C and pure rutile phase at 200°C. The volume fraction of brookite was gradually increased with increase of HCl concentration in the range of about 4.43 M to 6.28 M. The maximum volume fraction of brookite in the as-prepared TiO₂ particles was obtained when oxidation of Ti⁴⁺ to TiO₂ was completed but it was gradually decreased with increase of reaction time. The reaction time for complete oxidation of Ti^{4 +} to TiO₂ was about 15 h at 80°C, about 5 h at 100°C, about 2 h at 120°C, and about 1 h at 150°C, respectively, showing that the kinetics of oxidation is very dependent on the reaction temperature. Brookite phase was not transformed directly to rutile phase but to anatase phase by heat-treatment at about 750°C, which finally converted to rutile phase at 1100°C.     

Discontinuous precipitation of Cr2N in a high nitrogen,chromium-manganese austenitic stainless steel

The main purpose of the present work is to study the effect of a high nitrogen content (1 wt% N), on the microstructural evolution of a Cr-Mn austenitic stainless steel aged over the [400–900 °C] temperature interval. Thermal treatments carried out between 700 and 900 °C lead to the decomposition of the nitrogen supersaturated austenitic matrix by discontinuous precipitation of Cr₂N particles. The microstructural features of the reaction are described and analysed. In the present case, the cellular precipitation of Cr₂N is a peculiar and complex phenomenon which involves two diffusion mechanisms: the diffusion of an interstitial element (nitrogen) and the diffusion of a substitutional one (chromium). The nucleation of the discontinuous precipitation arises from a reduction of the surface energy of the precipitates. Furthermore, the precipitation growth is a non-steady state process, because the reaction is governed at first by the intergranular diffusion of chromium, and then tends to be controlled by its bulk diffusion. Consequently, the features of this discontinuous precipitation do not fit in with the assumptions of usual theories, which have been established for binary substitutional systems that transform in steady state conditions. This discontinuous precipitation brings about a slight hardening. Then, the hardness of the aged samples can be described by an additive relationship between the hardness of the precipitation cells and that of the untransformed matrix. Beside the discontinuous precipitation of Cr₂N, sigma phase forms with significant volume fractions.     

The direct bonding between copper and MgO-doped Si3N4

An application of direct bonding method for copper to silicon nitride (Si₃N₄) joining was investigated. Si₃N₄ was sintered with 5wt% MgO at 1700 ° C for 30 min in nitrogen atmosphere, and oxidized at various temperatures. The bonding was performed at 1075 ° C in nitrogen atmosphere with low oxygen partial pressure. The direct bonding was not achieved for the Si₃N₄ oxidized below 1200 ° C or nonoxidized. During oxidation, magnesium ion added as sintering aids, diffused out to the surface of Si₃N₄ and formed MgSiO₃, which seemed to have an important role in the bonding. Fracture of the bonded specimen under tensile stress took place within the oxide layer of Si₃N₄. The bonding strength was decreased with oxidation temperature and time. Maximum strength was found to be 106 kg cm^–2 for the Si₃N₄ oxidized at 1200 ° C for 1 h.     

Effects of oxidation on intergranular phases in silicon nitride ceramics

The effects of oxidation on changes in the secondary phases of two Si₃N₄ ceramics were investigated by transmission electron microscopy. The Si₃N₄ materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb₂O₃+0.5 vol% Al₂O₃, containing a Yb₂Si₂O₇ crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si₃N₄ material, sintered with 5 wt% Y₂O₃+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y₂Si₂O₇ accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.     

Synthesis of bismuth/strontium/calcium/copper and yttrium/barium/copper oxide systems from malonate precursors

Malonate salts of the above metals dissolved in a glycerol/malonic acid polyester have been found to form amorphous oxide systems at relatively low temperatures. Mixtures of the appropriate stoichiometry rapidly form the corresponding 2212 BSCCO and tetragonal YBCO oxide systems on heating to temperatures in the range 750–850 °C. In the case of YBCO, the formation of crystalline barium carbonate can be largely suppressed by an appropriate thermal programme, but whereas the tetragonal YBCO_6.5 phase can be readily obtained as the sole crystalline phase, oxygen diffusion into this phase is remarkably difficult. This appears to be due to the presence of amorphous intergranular material.     

Preparation of VO2 films by organometallic chemical vapour deposition and dip-coating

Low-pressure organometallic chemical vapour deposition (OMCVD) and dip-coating of VO₂ films using vanadyl tri(isobutoxide) as the starting material were investigated. In OMCVD, discontinuous VO₂ films, which were composed of fine needle crystals, formed under very limited conditions, around 600° C with a flow rate of oxygen gas of 0.2 to 0.5 cm³ sec^–1. However, very uniform and tightly packed VO₂ films were grown by deposition at 300 to 700° C in the absence of oxygen gas and subsequent annealing in nitrogen at 500° C for 2 h. The films exhibited a sharp semiconductor to metal transition at 60 to 70° C, accompanied by a change in the resistivity by four to five orders of magnitude. In dip-coating with two-step heat-treatments (300° C for 1 h in nitrogen and subsequently 500° C for 2 h in nitrogen), of the gel films formed from VO(O-i-Bu)₃-H₂O-i-PrOH system, uniform (0 1 1) oriented VO₂ films were formed. A transition in the electrical conductivity by two to two and a half orders of the magnitude was found to occur around 60° C. Before and after the transition, no distinct variation in the XRD pattern was observed.     

High-temperature oxidation of Fe3Al containing yttrium

The effect of yttrium addition on the oxidation behavior of Fe₃Al alloys was investigated in terms of oxidation rate and oxide adhesion in the temperature range of 800 to 1100 °C. The oxidation rate of the alloys, Fe-14.3 wt% Al and Fe-14.1 wt% Al-0.3 wt% Y, was nearly identical, and the parabolic rate constant as a function of temperature is found to be K _p = 5128 exp[–39506 (cal/mol)/RT] mg²/cm⁴ hr. While the alumina scale formed on the Y-free Fe₃Al alloy was observed to be fragile and spalled easily, the oxide layer formed on the Fe₃Al-Y was protective, dense, and adhesive. Based on the microstructural, morphological, and compositional studies, the adhesion improvement due to the yttrium addition was discussed in terms of growth stress, the formation of pegs and scale growth mechanism.     

Synthesis of nanocrystalline rare earth oxides by glycothermal method

The reaction of yttrium acetate hydrate in 1,2-propanediol at 300 °C yielded a product containing acetate groups and glycol moieties. From this product, Y₂O₃ was directly crystallized at 400 °C without the formation of a carbonate oxide phase. The thus-obtained Y₂O₃ samples had a small crystallite size (2.2 nm) and significantly large surface area (280 m²/g). Other nanocrystalline rare earth (Gd-Yb) oxides were also obtained by this method.     

A novel synthetic route to Y2O3:Tb phosphors by bicontinuous cubic phase process

This study applies a novel approach to prepare the terbium-doped yttrium oxide phosphors (Y₂O₃:Tb³⁺) using the bicontinuous cubic phase (BCP) process. The experimental results show that the prepared precursor powder was amorphous yttrium hydroxide Y_1−xTb_x(OH)₃ with a spherical shape and primary size 30–50 nm. High crystallinity phosphors with body-centered cubic structures were obtained after heat treatment above 700 °C for 4 h. The primary size of the phosphors grew to 100–200 nm, and dense agglomerates with a size below 1 μm were formed during the calcination. The obtained Y₂O₃:Tb³⁺ phosphor had a strong green emitting at 542 nm. The optimum Tb³⁺ concentration was 1 mol% to obtain the highest PL intensity. This study indicates that the calcining temperature of 700 °C needed for high luminescence efficiency in this work is much lower than 1000 °C or above needed for the conventional solid-state method.     

Evolution of the microstructure of undoped and Nb-doped SrTiO3

Undoped and Nb-doped SrTiO₃ specimens with excess titania compositions were prepared by sintering in air at 1420 or 1480 °C. Large grains due to liquid-phase sintering were obtained for undoped specimens containing 0.6 mol % excess titania and fired at 1480 °C. On the other hand uniform fine grains were observed for samples fired at 1420 °C, resulting from grain-growth inhibition due to exsolved TiO₂ second phase. The solubility of excess titania seemed less than 0.2 mol% under our experimental conditions. The microstructural behaviour of Nb-doped SrTiO₃ could be explained well by the Sr-vacancy compensation model. According to this model, the solubility of excess titania in SrTiO₃ increased with Nb₂O₅ dopant concentration. Thus, for specimens which had high excess titania compositions and were sintered at 1480 °C, large grains were observed when the Nb content was low enough to retain sufficient excess titania-forming liquid phase. For specimens having the same compositions and fired at 1420 °C, uniform fine grains were obtained due to grain growth inhibition by the exsolved TiO₂ second phase, when the Nb content was low. If the excess titania was less than the solubility determined by the amount of Nb dopant, Ruddlesden-Popper-type phases were believed to be formed and resulted in poor densification. Although excess titania was the major factor in determining the grain size of the specimens, the niobium dopant enhanced grain growth.