Mats Nygren

Formation of tough interlocking microstructures in silicon nitride ceramics by dynamic ripening.

Ceramics based on Si3N4 have been comprehensively studied and are widely used in structural applications. The development of an interlocking microstructure of elongated grains is vital to ensure that this family of ceramics have good damage tolerance. Until now this has been accomplished by heating the appropriate powder compacts to temperatures above 1,700 °C for extended periods. This procedure involves a necessary step of controlling the size and population of seeds—added ex situ or formed in situ—to ensure selective grain growth. Here we report the very fast (within minutes) in situ formation of a tough interlocking microstructure in Si3N4-based ceramics. The microstructures are obtained by a dynamic ripening mechanism, an anisotropic Ostwald ripening process that results from the rapid heating rate. The resulting microstructures are uniform and reproducible in terms of grain size distribution and mechanical properties, and are easily tailored by manipulating the kinetics. This process is very efficient and opens up new possibilities to optimize mechanical properties and cost-effectively manufacture ceramics.

On the preparation of bio-, nano- and structural ceramics and composites by spark plasma sintering

Abstract Spark plasma sintering (SPS) is a comparatively new technique. It allows very fast heating and cooling rates, very short holding times, and the possibility to obtain fully dense samples at comparatively low sintering temperatures, typically a few hundred degrees lower than in normal hot pressing. During recent years, a wide variety of materials, e.g., ceramics, composites, cermets, metals and alloys, have been successfully compacted by the SPS process. This and other processing techniques that use direct current and electrically conducting pressure dies are briefly described, but the focus of our presentation are on how the kinetics of densification and grain growth can be manipulated by the use of the SPS technique so as to yield various materials that hold significant fundamental and technological interest.

Dense single-phase β-sialon ceramics by glass-encapsulated hot isostatic pressing

Single phaseβ-sialon ceramics, Si6−zAlzOzN8−z, have been prepared from carefully balanced powder mixtures, also taking account of any excess oxygen in the starting materials. Sintering powder compacts in a nitrogen atmosphere (0.1 MPa) at 1800° C or higher transforms the starting mixture into aβ-sialon solid solution atz-values up to about 4.3, but the sintered material has an open porosity. Addition of 1 wt% Y2O3 to the starting mix improved the sintering behaviour somewhat and the density of the sintered compacts reached 95% of the theoretical value. By glass-encapsulated hot isostatic pressing at 1825° C, however, sintered materials of virtually theoretical density could be obtained, with or without the 1 wt% Y2O3 addition. These latter samples have been studied by X-ray diffraction and electron microscopy, and their hardness and indentation fracture toughness have been measured. It was found that the maximum extension of theβ-sialon phase composition at 1825° C and 200 MPa pressure is slightly below 4,z∼ 3.85 and about 4.1 at atmospheric pressure, and that the hexagonal unit cell parameters are linear functions of thez-value. The single-phaseβ-sialon ceramics had no residual glassy grain-boundary phase. The grain shape was equi-axed and the grain size increased from about 1μm at lowz-values to 5μm at highz-values. At lowz-values the hardness at a 98 N load was 1700 and the fracture toughness 3, whereas an increase inz above 1 caused both the hardness and fracture toughness to decrease significantly. Addition of 1 wt % Y2O3 to the starting mix prior to the HIP-sintering gave rise to a small amount of amorphous intergranular phase, changes in grain size and shape, a clear increase in fracture toughness and a moderate decrease in hardness.

Transparent nanocrystalline MgO by rapid and low-temperature spark plasma sintering

We investigated superfast densification of nanocrystalline MgO powders by spark plasma sintering (SPS) between 700 °C and 825 °C under applied pressures of 100and 150 MPa. Fully-dense transparent nanocrystalline MgO with a 52-nm average grain size was fabricated at 800 °C and 150 MPa for 5 min. In-line transmissionsof 40% and 60% were measured compared to MgO single crystal, for the yellowand red wavelengths, respectively. Densification occurs by particles sliding over each other; the nanometric grain size and pores lead to the optical transparency. The light brownish color of the nanocrystalline MgO is due to the oxygen vacancy color centers, originating from the reducing atmosphere of the SPS process.

Origin(s) of the apparent high permittivity in CaCu3Ti4O12 ceramics: clarification on the contributions from internal barrier layer capacitor and sample-electrode contact effects

CaCu3Ti4O12 ceramics with a range of resistivities have been prepared using both conventional sintering and spark plasma sintering. For all cases, the high effective permittivity is associated primarily with an internal barrier layer capacitor mechanism. Additional polarization associated with the electrode-sample interface may appear but its visibility depends on the grain boundary resistivity (Rgb) of the ceramic. If the Rgb is large, the electrode polarization is obscured by sample-related effects; if the Rgb is small, a separate impedance associated with the electrode polarization may be seen. Discrepancies in the literature regarding the magnitude and origin of the high effective permittivity are attributed to a combination of differences in processing conditions, electrode contact material and measuring frequency range.

Ferroelectric properties of dense nanocrystalline BaTiO3 ceramics

Dense BaTiO3 ceramics with 50?nm average grain size obtained by spark plasma sintering were investigated. The dielectric data show a broad ferro?para phase transition with a maximum permittivity of at 390?K and 1?kHz. The local ferroelectric switching behaviour was investigated by piezoresponse force microscopy. Typical piezoelectric hysteresis loops were recorded at different positions of the sample. The present results provide experimental evidence for polarization switching at the local scale, indicating that the critical grain size for the disappearance of ferroelectric behaviour in dense, bulk BaTiO3 nanocrystalline ceramics is below 50?nm.

Electrical and magnetic properties of V1−xWxO2, 0 ≤ x ≤ 0.060

Abstract The semiconductor-metal transition in the phase V 1− x W x O 2 ( 0 ≤ x ≤ 0.060 ) has been studied by magnetic and electrical measurements over the temperature range 80–400 K. An average value of 3 Bohr magnetons per wolfram atom was found for the semiconducting low temperature phase. Electrical conductivity measurements on single crystals gave an activation energy of 0.096 eV in the region 0.006 ≤ x ≤ 0.052 .

Oxidation Kinetics of Nickel Particles: Comparison Between Free Particles and Particles in an Oxide Matrix

Abstract The degradation of selective solar absorbers through oxidation has been studied. We compare the oxidation kinetics of nickel particles of various sizes. Both free particles and particles embedded in an oxide matrix were studied. The oxidation kinetics of polycrystalline nickel nanorods was determined by IR spectroscopy in the temperature range 300–500°C. The particles were oxidized when situated in the porous alumina matrix of an electrochemically deposited solar absorber coating. The oxidation kinetics was compared to that of free nanometer-particles at the same temperature and to micron-sized polycrystalline nickel particles, which were studied by thermogravimetry in a wider temperature range. It was found that the rate constant was markedly lower for the particles in the matrix. Implications for the durability of selectively solar absorbing coatings are discussed.

On the extension of the α-Sialon phase area in yttrium and rare-earth doped systems

Abstract The extension of the α-sialon phase area in Y, Yb, Dy and Nd doped sialon systems has been mapped out on the basis of element analysis of individual α-sialon grains. The α-sialon-forming area expands with decreasing size of the M ion in MxSi12−(m+n)Alm+nOnN16−n. The maximum n value, nmax is 1.0 for M = Nd and 1.2 for the other dopants. The m value varies from 1.0 in all systems studied to a value of 2.75 in the Yb-α-sialon system, indicating that the substitution of AlO units for SiN in α-sialon is more restricted than substitution of AlN for SiN. It is shown that the lattice expansion of the α-sialon phase with increasing content of M ions is due to the substitution of AlN units for SiN, while the lattice parameters are not at all or only very slightly dependent on the ionic radius of the M ion. Based on the elemental analysis of individual α-sialon grains and the lattice parameters obtained, the following empirical relation between the sizes of the a and c axes and the m and n values of the α-sialon phase was obtained: a( A ) = 7.752 + 0.036 m + 0.02 n c( A ) = 5.620 + 0.031 m + 0.04 n

Magnetic, electrical and thermal studies on the V1−xMox02

The monoclinic-to-tetragonal structure transition of oxides V1−xMox02 with 0≤x≤0.20 has been studied by means of DTA, X-ray diffraction, magnetic susceptibility (powder samples) and electrical conductivity (single crystals) measurements within the temperature region 80 K to 400 K. A linear decrease of the transition temperature of 11 K per atom % Mo was observed. The magnetic susceptibility of the low temperature phase was found to be temperature independent paramagnetic for all preparations. Electrical conductivity measurements on the same phase showed crystals with x ⩽ 0.04 to be semiconducting, while a metallic behavior was observed in the region 0.10 ⩽ x ⩽ 0.14.