Search

Submicron Ti2AlC MAX phase powder was synthesized by molten salt shielded synthesis (MS3) using a Ti:Al:C molar ratio of 2:1:0.9 at a process temperature of 1000°C for 5 hours. The synthesized powder presented a mean particle size of ~0.9 µm and a purity of 91 wt. % Ti2AlC, containing 6 wt. % Ti3AlC2. The Ti2AlC powder was sintered by pressureless sintering, achieving a maximal relative density of 90%, hence field‐assisted sintering technology/spark plasma sintering was used to enhance densification. The fine‐grained microstructure was preserved, and phase purity of Ti2AlC was unaltered in the latter case, with a relative density of 98.5%. Oxidation was performed at 1200°C for 50 hours in static air of dense monolithic Ti2AlC with different surface finish, (polished, ground and sandblasted) which resulted in the formation of an approx. 8 µm thin aluminum oxide (Al2O3) layer decorated with titanium dioxide (rutile, TiO2) colonies. Surface quality had no influence on Al2O3 scale thickness, but the amount and size of TiO2 crystals increased with surface roughness. A phenomenon of rumpling of the thermally grown oxide (TGO) was observed and a model to estimate the extent of deformation is proposed. [ABSTRACT FROM AUTHOR]

Micromolding in capillaries has been used to fabricate alumina stripes on smooth (Ra = 0.5 nm) and rough substrates (Ra = 900 nm). Different lateral (10-500 lm) and vertical stripe dimensions (8-27 lm) were used to study the influence of substrate roughness (smooth and rough sapphire) and substrate material (platinum coated and plain sapphire) on sintering behavior. Alumina stripes experience edge delamination during sintering on a rigid substrate independent of substrate roughness. However, enhanced substrate roughness reduced delamination length by half and lowered lateral strains by up to 0.10. Grooves in the rough substrate were found to be responsible for this feature as they act as crack propagation barriers and generate local density minima. Accompanying discrete element simulations revealed a localized triaxial stress state at the grooves of a sinusoidal-shaped substrate interface as the main cause of the density minima. Platinum interlayers also resulted in reduced delamination by 40% in some stripe geometries while density was enhanced by 4% and lateral strains doubled in some geometries. Creep of the metal layer during sintering is thought to be the reason for this seemingly contradictory behavior. [ABSTRACT FROM AUTHOR]

The sintering behavior of alumina containing different amounts of calcium-aluminum-silicate glass as sintering aid was analyzed under AC electric fields between 0 and 150 V/cm. Liquid phase sintering was enhanced by the electric field, and 'flash sintering' behavior depending on the current density and power dissipation within the specimen could be observed. Current flowed only through the liquid phase at high temperature and enhanced the densification process by two effects: Joule heating and athermal response of the viscous liquid under the electric field. Joule heating increased the temperature within the specimen, whereas the applied electric field reduced the viscosity of the liquid phase promoting a more effective matter transport. [ABSTRACT FROM AUTHOR]

Field-assisted sintering technique (FAST) and spark plasma sintering (SPS) apparatuses are based on the same technical principles, but differ in design. A current flows directly through the graphite pressing tool, which acts as heating element in both machines. The main differences are the pressing tool geometry and material, current pulsing and the position of the temperature measurements. Tests under identical conditions (same heating schedule, applied load, and pressing tool, as well as similar pulse patterns) were conducted on both types of set-ups. Three different oxide materials were tested: alumina, 8 mol% yttria-stabilized zirconia (8YSZ) and zinc oxide. [ABSTRACT FROM AUTHOR]

Abstract: A recovery strain after compressive loading was observed for bulk ceramic specimens during sintering. For pure alumina and low temperature co-firing ceramic (LTCC), a transient expansion occurs only in the loading direction after rapid removal of the load. This effect increases with stress magnitude and loading time. [ABSTRACT FROM AUTHOR]

The master sintering curve approach was extended theoretically to the Field-Assisted Sintering Technique (or SPS). Experimental data from constant heating rate testing confirm the general applicability of this framework. An apparent activation energy of ~290 kJ/mol was found for ultrafine alumina below 1200 °C. As alumina densifies by grain boundary diffusion in the present conditions, this low value of the apparent activation energy might be due to thermal gradients in the specimens induced by rapid heating. [ABSTRACT FROM AUTHOR]

The sintering behavior of symmetric laminates has been experimentally measured and modeled. Two cases are distinguished: (i) sintering of a viscous layer between viscous substrates and (ii) sintering of a viscous layer between elastic substrates. A simple model based on a persisting isotropic microstructure allowed computation of developing in-plane stresses. These in conjunction with a viscous Poisson's coefficient were taken to predict out-of-plane stresses and finally densification rate. This approach, however, proved unsuccessful to predict the large observed differences in densification rate between laminates constrained by either green or dense alumina substrates. The discrepancy was found to be due to widely different microstructures developing with different degrees of elastic constraint. Pore area and pore and particle orientation were determined to quantify these microstructural changes. [ABSTRACT FROM AUTHOR]

The sintering behavior of a glass–ceramic (commercial material Ceramtape GC) was characterized using the discontinuous sinter-forging technique. The uniaxial viscosity Ep and the viscous Poisson's ratio np of the composite body were determined as a function of density. Particular attention was paid to microstructural changes in order to quantify the anisotropy induced by load application. Pores and alumina particles were found to be both oriented perpendicular to the applied load direction. Finally, the experimental results were used to predict the value of the hydrostatic sintering stress. [ABSTRACT FROM AUTHOR]

Abstract: Alumina films prepared by tape casting were sintered freely and under geometrical constraint at 1350°C. The effect of film thickness on sintering kinetics and microstructure development was investigated. A decrease in film thickness in the constrained case leads to enhanced retardation of densification and increased orientation of anisometric pores. [Copyright &y& Elsevier]

Alumina thin films were manufactured from aqueous slurries by dip coating. Film thickness as function of substrate withdrawal velocity could be correctly modeled by Landau and Levich's theory. Samples were sintered on a rigid substrate at 1350°C for different isothermal times to achieve relative densities from 84% to 97%. Microstructural analysis of polished cross sections revealed a continuous development of pore alignment, as expected by theoretical considerations. With increasing density pores become more anisometric and orientate along the thickness direction. A further preferential orientation of pores was found in the normal plane, apparently due to the coating process. Constraining conditions had less influence on the size and shape of the grains; they tend to become more equiaxed in the constrained plane, presumably due to the biaxial tensile stress state. [ABSTRACT FROM AUTHOR]

Uniaxial viscosity and sintering stress of pressure filtrated alumina compacts were evaluated from sinter-forging measurements. At a particular density, significantly higher values of sintering stresses are observed compared with specimens made by uniaxial dry pressing followed by cold isostatic pressing. In addition, the uniaxial viscosity at a given density is lower for the pressure-filtrated samples. These differences may be explained by a more homogeneous microstructure and finer pore size and emphasize the importance of green density and packing on the evolution of the constitutive parameters for crystalline materials. [ABSTRACT FROM AUTHOR]

Alumina films were sintered on a rigid substrate to different densities. Stereological measurements suggest that microstructural anisotropy develops during the final stage of sintering, which can be interpreted in terms of the elimination of pore sections rather than anisotropic growth of isolated particle necks. Pore separation seems to be a better measure than grain intercept when compared with phenomenological models. [ABSTRACT FROM AUTHOR]

The chemical compatibility of thin electrolyte layers and their support materials at high temperatures is crucial for the performance in solid oxide cells and membranes. This work describes the chemical interaction between the electrolyte material, BaCe 0.2 Zr 0.7 Y 0.1 O 3 + 0.5 wt% NiO (BCZY), and structural ceramics Al 2 O 3 , 8 mol% yttria stabilized zirconia (8YSZ), TiO 2 , CeO 2 and MgO during sintering. 1:1 wt% powder mixtures of the electrolyte material and structural ceramics were annealed at 1500 °C for 5 h, with the phase composition being determined through XRD analysis at room temperature. Subsequently, the material interaction between BCZY films deposited on the most promising structural ceramic MgO was investigated by SEM and EDS. In particular, the complex sintering requirements to form single-phase BCZY electrolyte layers is troubled upon coating and sintering BCZY on MgO. Hereby, the diffusion of NiO into the MgO support leads to a deficiency of NiO in the BCZY layer, making the solution and precipitation mechanism required to form the perovskitic phase unable to occur. In extreme scenarios, the electrolyte layer depletion of NiO can even cause the decomposition of a single-phase BCZY into BaZrO 3 , Ce-Y fluorite and a BCZY perovskite phase of undefined composition. [ABSTRACT FROM AUTHOR]

Abstract: This study compares hot pressing (HP) and the electric field-assisted sintering technique (FAST) of two different electrically insulating Al2O3 submicron powders with median particle sizes of 150 and 500nm. Sample geometry, heating schedule, applied pressure and atmosphere were identical for both sintering methods. The densification behavior and characterization of the microstructure revealed that FAST sintered samples reached a higher density compared with HP, in particular for the finer powder. It was found that an increase in dwell time was required to reach the same final density by HP. However, analysis of the sintering curves showed that the densification mechanism for both sintering methods was grain boundary diffusion. Increasing the heating rate up to 150Kmin−1 did not modify the densification mechanism. The sintering trajectory showed that the grain size was only dependent on density and was insensitive to the sintering method, in addition to showing a lack of preferential grain orientation. [Copyright &y& Elsevier]

The MAX phase chromium aluminium carbide (Cr 2 AlC) is a potential candidate for high-temperature structural and energy applications due to the unique combination of properties, including excellent oxidation and corrosion resistance in harsh environmental conditions. In this work, the oxidation behaviour of bulk Cr 2 AlC was studied at temperatures of 1000 °C and 1200 °C in humid synthetic air, mimicking a realistic application environment. An α-Al 2 O 3 scale forms at both temperatures, consisting of both inward and outward-growing layers, with a continuous Cr 7 C 3 layer beneath the oxide. A special focus was put on employing atom probe tomography (APT) to analyse the oxide and carbide layers at the nanoscale. Decomposition of Cr 2 AlC to Cr 7 C 3 is the main source (>70%) of aluminium needed for the oxide scale formation, while Al depletion in the underlying MAX phase remains marginal (<1 at%), exhibiting a pattern different from the common high-temperature oxidation of alloys. Al diffusion to the oxide scale proceeds predominantly through grain boundaries (GBs) in Cr 2 AlC and through the bulk in Cr 7 C 3. Ionic diffusion through GBs of the large-grained inner alumina layer seems to be the rate-limiting step for the oxide scale growth, resulting in the apparent parabolic oxidation kinetics. No segregation of Cr, C or any impurities, which could affect ionic transport, was found at these alumina GBs by APT, rendering the results of this study as characteristic of a pure Cr 2 AlC oxidation. [Display omitted] • Protective Al 2 O 3 scale forms mainly due to decomposition of Cr 2 AlC into Cr 7 C 3. • APT shows that Cr 2 AlC tolerates up to 1 at% Al depletion before decomposition. • GB diffusion of O through the inner Al 2 O 3 subscale controls the oxidation rate. • Al diffuses through GBs in Cr 2 AlC and through bulk in Cr 7 C 3, as indicated by APT. • The results represent oxidation of pure Cr 2 AlC unaffected by any contamination. [ABSTRACT FROM AUTHOR]

Abstract: The sintering behaviour of alumina stripes deposited on sapphire substrates by micromolding in capillaries – a soft lithographic method – with lateral dimensions from 10 to 500μm and thicknesses between 7μm and 32μm was studied. Unlike in continuous films, the lateral sintering strain is not negligible, thus reducing the constraint imposed by the substrate. Lateral shrinkage depends on the stripe width and thickness. The degree of constraint exerted on alumina stripes by a rigid sapphire substrate was investigated by comparing the lateral and vertical strains and is found to be dependent on stripe geometry. The formation of a delaminated, highly dense edge zone was observed at the free boundaries. Its influence on overall densification and local density distribution depends on its extension compared to the total film width. A gradient in local density was found that varied both with stripe thickness and width as predicted by finite element and discrete element simulations. [Copyright &y& Elsevier]

The good oxidation resistance of MAX phases up to temperatures around 1200 °C can be compromised for long exposure due to the breakaway of the protective alumina layer. Herein, we unveil a mechanism of breakaway oxidation of the Ti 2 AlC MAX phase, identifying the main trigger and the solutions to avoid it. It is caused by excessive rumpling of the oxide scale on surfaces with arithmetical mean roughness (R a) > 3 µm and constitutes a key factor in subsequent consumption of Ti 2 AlC. First, the oxide scale experienced rumpling due to significant radial stresses generated at the Ti 2 AlC/oxide interface. Second, scale blistering resulted from substantial buckling due to the evolution of in-plane stresses and lateral lengthening. Third, blister collapse and exposure of the underlying Al-depleted Ti 2 AlC surface led to rapid ingress of oxygen and oxide/substrate interface recession. The self-healing ability of Ti 2 AlC has been restrained and breakaway oxidation kinetics following a linear trend have been initiated. Similarly, breakaway oxidation was observed on micro-damaged surfaces. A mixed oxide layer with high porosity mainly composed of rutile titanium dioxide (TiO 2) promptly formed on these surfaces, gradually consuming the base Ti 2 AlC material. [Display omitted] [ABSTRACT FROM AUTHOR]