Gastight, high-temperature stable sealing between yttria-doped zirconia (ZrO2) and an electrically insulating ceramic joining partner is necessary for a wide range of applications in oxygen sensing and energy conversion. To accomplish this, laser brazing with glass solders is an attractive alternative to existing joining processes. Due to a near-perfect match of its thermal expansion with the one of ZrO2 up to high temperatures (∼800°C) and good thermal and electrical insulation properties, forsterite (Mg2SiO4) is chosen as a candidate joining partner. As the absorptivity of pure forsterite at the used diode laser wavelengths of 808 and 940nm is quite low, increasing the energy absorption by doping forsterite with Fe2O3 appears to be a promising refinement of this technique. The optical properties of the resulting olivine ceramics are evaluated. The results show that Fe2O3-doped forsterite is both suitable as a joining partner for ZrO2 and for tuning its absorptivity.

Partially crystalline precursor derived Si-B-C-N ceramics were investigated at elevated temperatures by compression creep experiments. Load change and constant load experiments at various temperatures in the range of 1300-1500 degrees C yield viscosity data as high as 10(15)-10(16) Pas and the values were determined assuming Newtonian flow. The anelastic behavior was analyzed with the help of Kohlrausch-Williams-Watts equation (KWW) and the fitting parameters are used to describe the relaxation behavior of this family of materials. (c) 2006 Elsevier Ltd. All rights reserved.

Corrosion experiments on a number of rare earth di-silicates containing Y, Yb and Lu have been conducted in flowing air with 30 vol.% H2O at 1500°C. Nominally the corrosion rate is slower by a factor 5 to 10 compared to silica. However, alumina impurity incorporation has a profound influence on the process of corrosion as it masks not only the gravimetric results but also changes the surface phase assemblages to contain rare earth garnets and liquid phases. It is presumed that silica loss occurs under those conditions via the liquid phase and does not necessarily follow the same kinetics as direct silicate corrosion. Furthermore, atmospheric silicon hydroxide saturation effects contribute to the corrosion process. Currently all determined corrosion rates of rare earth silicates are viewed as system specific only.

The synthesis and the phase stability regions of the disilicates of Y, Lu, Yb and Gd have been investigated at temperatures between 1300 and 1600 °C. The mean ionic radius of the rare-earth element ion including Y turns out to be the key parameter to govern the stabilities of polymorph types of both pure disilicates and their solid solutions. Both are predicted correctly by the phase stability diagram of Felsche [1] [J. Felsche, Struct. Bond. 13 (1973) 99–197]. Furthermore, it correlates with the reactivity, with increasing radius a faster reaction is observed at a given temperature. A fast reactivity is assumed to create difficulties in the densification of disilicates with ionic radius exceeding ∼0.88 Å. In the reaction from oxide powders monosilicates are formed in a first step. Disilicates are formed from those initially in the form of low-temperature modifications, which transform in a sequence to high-temperature modifications.

Potential environmental barrier silicates based on rare earth disilicates containing Y, Yb and Lu have been investigated for their corrosive behavior in a gas stream containing water at 1500 degrees C. No currently used test method is unambiguous: silica or silica-forming tubing cause high internal P-Si(OH)4, which should artificially slow down corrosion rates and alumina tubing cause alumina contamination via P-Al(OH)3. We used the latter and report on the details of the interaction. In Y2Si2O7 and a number of solid solutions with a Lu or Yb content up to 50% this contamination resulted in the formation of a melt. Depending on further impurities, particularly Ca, melt formation is accompanied by oxyapatite or monosilicate crystallisation. On cooling rare earth garnets crystallize from the melt. The melt oxyapatite/monosilicate formation does not create a protective effect. The corrosion kinetics is linear; the rates are slower than those of pure silica, but only at a level reflecting reduced silica activity due to dilution by a factor of 2-5. Porosity causes fast initial mass losses. The formation of secondary phases inside the material induces crack formation. A disilicate layer in those systems is unlikely to become an effective environmental barrier for non-oxide systems.For Yb and Lu silicates there are indications for the formation of a rare earth garnet layer during the corrosion process at high temperature, which has protective power. This causes a logarithmic law for the corrosion kinetics: for extended times the mass loss drops asymptotically. Under the assumption that the garnet formation also removes the sink for external alumina, the protection for Si-removal may even become perfect. The total mass change is a balance between Al-input and silica loss, which makes it currently impossible to formulate a quantitative equation for the time dependence of the process. (c) 2006 Elsevier Ltd. All rights reserved.

Antiferroelectric lead zirconate titanate stannate (PZST) ceramics are promising materials for high-strain transducers and actuators. The degradation of the strain excited by an ac field remains largely unknown so far for this family of antiferroelectric ceramics. In this study, the bipolar electric fatigue of antiferroelectric Pb0.97La0.02(Zr0.55Sn0.33Ti0.12)O3 ceramics was investigated. Variations in the strain hysteresis loop and damage in the microstructure of the materials due to the electric cycling were monitored. Higher cycling field yielded a stronger fatigue effect. The material showed an increasingly asymmetric suppression of the strain hysteresis loop and diffuse AFE–FE phase transition with increasing cycle number. A damaged microstructure was observed on the polished surfaces of fatigued samples after acid etching. Electrochemical variations, the pinning of domains, randomly or preferentially orientated, due to the cycling are considered as the main fatigue mechanism of the material.

Abstract The influence of the sintering additive LiYO2 (5– 15 wt.%) on sintering behavior, microstructure and mechanical properties of Si3N4 ceramics was investigated. Since LiYO2 enables densification of Si3N4 at extraordinarily low temperatures, sintering was carried out in the range from 1200– 1700 °C. Densification was found to be enhanced with increasing additive content due to an increasing volume fraction of the liquid. The phase transformation and grain growth occurred through a solution-reprecipitation mechanism, where the precipitation took place preferentially on pre-existing β-Si3N4 nuclei (of which the starting powder already contained 20 wt.%). The indentation fracture toughness increased with both sintering time and additive content as a result of the growth of elongated grains.

Abstract In precursor-derived nanocrystalline Si–B–C–N materials, grain or interphase boundaries constitute a major part of the material. They control various properties of nanostructured solids, e.g., by forming fast diffusion pathways. In such highly covalent materials, where the interfaces between the individual crystallites or domains tend to be broadened and structurally disordered, high-temperature plastic deformation (i.e., the creep behavior) can be used as a probe which is particularly sensitive to processes involving internal interfaces and glass-like features of the microstructure. The focus of the present study is an investigation of the deformation behavior of precursor-derived nanocrystalline Si–B–C–N ceramics in the phase-separated amorphous and nanocrystalline states under inert atmospheres. A concise analysis of creep mechanisms free of any oxidation effects was carried out. Isothermal compression creep experiments at 1400°C and at various loads (50MPa–150MPa) have indicated a stress exponent near to unity hinting diffusion creep, and an activation energy of 0.16 ± 0.03 MJ/mol was derived from temperature change experiments under an iso-stress condition. The mechanisms of deformation are elucidated using a free volume model.

Abstract This paper deals with the densification and phase transformation of Si3N4 with additives of Li-exchanged zeolite during pressureless sintering at significantly reduced temperatures. Dilatometric shrinkage data show that the first liquid forms as low as 1080 °C. Upon sintering at 1500 °C the bulk density increases to more than 95% of the theoretical density without phase transformation from α-S3N4 to β-Si3N4, i. e. the phase transformation lags behind the densification process. Above 1500 °C the secondary phase is completely converted into a glass and the α-to-β transformation takes place. Under these conditions the grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. The results show that a very effective low-temperature sintering additive for silicon nitride can be obtained from Li-exchanged zeolite.

Si–B–C–N ceramics were synthesized from boron modified poly(vinyl)silazanes with the chemical formula (B[C2H4–Si(CH3)NH]3)n. The originally amorphous materials are crystallized at temperatures in the order of 1,800–1,900 °C, which results microstructures with grain sizes significantly below 100 nm. Several parameters of the heat treatment, including temperature, holding time and atmosphere, affect the resulting nanostructures. This and the chemical and phase composition were studied via X-ray diffraction (XRD), transmission electron microscopy (TEM), electron spectroscopic imaging (ESI) and spectrochemical analysis in order to gain an understanding of the mechanisms, which control the crystallization behavior. Ceramic samples were also produced using different particle sizes of the precursor polymer in order to quantify the effect of the varying specific surface on the crystallization behavior.

The sintering behaviour of LPS-SiC and the influence of the size of the rare-earth cations on the secondary phase characteristics were investigated with different rare-earth oxide additions. In all cases, the most important sintering mechanism was found to be the solution-reprecipitation process. This fact was corroborated by TEM and EDS analyses. Post-sintering annealing resulted in devitrification of the secondary phases coupled with anisotropic grain growth due to the phase transformation from β-SiC to α-SiC. Improved fracture toughness of the annealed materials was attributed to crack deflection by the elongated grains. SEM microstructural analyses were performed in order to elucidate structure–property relationships. A comparative study in reference to the additive system Y2O3–AlN demonstrates significantly improved high-temperature properties of the Lu2O3-containing SiC ceramics.

The fatigue behavior of lead zirconate stannate titanate (PZST) ceramics prepared by spark plasma sintering (SPS) was investigated. Polarization and strain hysteresis loops were monitored. The material shows a high resistance to fatigue because of bipolar electric cycling. Both maximum strain and switchable polarization first show a fatigue stage 0 to 10(5) cycles and then a fatigue-free period up to 10(8) cycles. The maximum losses of maximum strain and switchable polarization are 18% and 10% of their initial values, respectively. The dominant fatigue mechanism is assigned to the pinning of domain walls by charged defects.

The impact of different additives on the hydrolysis of AlN powder in aqueous suspensions at room temperature was studied. The results show that citric acid and polyacrylic acid are most effective in chemically protecting AlN against hydrolysis. The protected powder is hydrophilic, which facilitates aqueous processing, and the chemical stability is retained when basic dispersing agents are added. Based on these results, the solid loading of the aqueous slurries was maximized by utilizing bimodal particle size distributions. Combining bimodal powders with the dispersants Dolapix and citric acid, colloidally stable slurries with solid loadings in excess of 50 vol% were obtained.

The fatigue behavior of antiferroelectric lanthanum-doped lead zirconate stannate titanate bulk material induced by bipolar cycling was investigated. Strain and polarization hysteresis loops, acoustic emissions (AEs), and biaxial strength were monitored. The material showed a high resistance to electric fatigue, concerning the losses in maximum strain, switchable polarization, and biaxial strength as well as modifications of AE patterns and microstructure. Fatigue microcracking is weak in the material during cycling. The pinning of antiferroelectric and ferroelectric domains by point defect agglomeration is discussed as the main fatigue mechanism. A diffuse antiferroelectric-ferroelectric phase transition and an asymmetry of the strain hysteresis loop due to the electric cycling are described and explained as a result of the pinning process and offset polarization.

The microstructures of liquid-phase sintered SiC with AIN-Y2O3 additives are systematically investigated by using transmission electron microscopy and analytical electron microscopy. Pure (alpha-SiC as starting powder leads to fine, equiaxied microstructure. Introduction of (alpha-SiC seed crystals into beta-SiC powder accelerates the beta-to-alpha-SiC phase transformation through a solution-precipitation process and promotes anisotropic grain growth, which results in a plate-like microstructure. Core/rim structures were found in both cases as a result of AlN dissolution into the re-precipitated part of SiC grains. This changes the liquid composition during sintering and induces crystallization of Y10Al2Si3O18N4 and Y2O3 in the triple-pockets. Amorphous films were observed to wet both grain boundaries and two-phase interfaces. A low ratio of AIN to Y2O3 in the sintering additive accelerates the devitrification of triple-pockets. Additional annealing can further devitrify the triple-pockets as well as the amorphous GB films, leading to a microstructure with potentially higher creep resistance.

Dense SiC materials from ultrafine starting powders may be obtained with very limited grain growth if uniaxial hot pressing with concurrent lateral flow of the sample is applied. This process, termed sinter forging, allows to decouple the kinetics of densification and grain growth owing to the elimination of the largest and most persistent population of pores by viscous deformation under the action of shear forces.

Oxidation resistance of a liquid-phase-sintered (LPS) silicon carbide (SiC) with Lu2O3–AlN additive system was investigated in air between 1200 and 1500°C, for up to 100h. Oxidation followed parabolic kinetics in the temperature range 1200–1500°C with an activation energy, Qox≈400±50kJ/mol. The material showed high oxidation resistance at and below 1400°C, however, the oxidation resistance degraded significantly at 1500°C. At 1500°C, reaction between the growing oxide layer (mainly SiO2) and the second phase (Lu2O3) produced a low-melting eutectic, resulting in accelerated oxidation. The major oxidation products consisted of SiO2 (α-cristobalite) and lutetium-disilicate (Lu2Si2O7). These oxidation products crystallised from the surface amorphous silicate phase during oxidation. The microstructure of the oxidised surfaces was shown to be dependent on oxidation temperature. The most probably rate-limiting steps were (1) the migration of additive cations along the residual intergranular phase to the interface between the oxide layer and the SiC bulk; and (2) the interfacial reactions between growing oxide layer and Lu2O3.

This paper deals with the densification and phase transformation during pressureless sintering of Si3N4 with LiYO2 as the sintering additive. The dilatometric shrinkage data show that the first Li2O‐ rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α‐Si3N4 to β‐Si3N4 taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α‐Si3N4 to β‐Si3N4 takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li2O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high‐temperature properties despite the low sintering temperature. The results show that the Li2O−Y2O3 system can provide very effective low‐temperature sintering additives for silicon nitride.