Per-Olof Olsson

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.

Formation of an Y/Ce-doped α-sialon phase

Abstract α-β-Sialon materials were prepared with a constant molar addition of Y2O3 + CeO2 sintering aid with varied composition: 0, 25, 50, 75 and 100 mol% Y2O3. The α-sialon grains formed in the microstructure were carefully investigated by analytical and high-resolution electron microscopy. It was found that the α-sialon structure can accept a small amount of cerium to enter the large interstitial cages, if it is stabilized by simultaneous additions of yttrium. Extended crystal defects were found to occur in the α-phase framework; this has not been observed with only yttrium added.

HIP-sinteredβ- and mixedα-β sialons densified with Y2O3 and La2O3 additions

Various fully dense sialon materials sintered by the glass-encapsulated hot isostatic pressing technique were synthesized using Y2O3 and/or La2O3 as sintering aids. Constant molar amounts of the oxide mixtures were added in the ratios Y2O3/La2O3:100/0, 75/25, 50/50, 25/75, 0/100. The samples were sintered at two different temperatures, 1550 and 1825° C. At the lower temperature, unreactedα-Si3N4 was present in the samples in addition toβ-sialon and secondary phases. The samples sintered at 1825° C showed that yttrium but not lanthanum favouredα-sialon formation. The amount of intergranular phase increased by about 50% when Y2O3 was replaced by La2O3. The La-sialon ceramics have as good an indentation fracture toughness as the Y-sialon ceramics, about 5 MPam−1/2, but the Vickers hardness is slightly lower, being 1400 kg mm−2 at a 98 N load.

Cognitive strengths and deficits in schoolchildren with ADHD

Background: Few studies provide detailed analyses of the various aspects of the entire cognitive profile of children with ADHD.

ß-sialon ceramics with TiN particle inclusions

Abstract Fully dense composites of 0–30 wt% discrete TiN particles distributed in a s-sialon matrix of overall composition Si 5·5 Al 0·5 O 0·5 N 7·5 have been prepared by hot isostatic pressing at 1650 and 1750°C. Pressureless sintering at 1775°C gave materials with an open porosity. Typical sizes of the TiN particles were 1–3 μm, and no intergranular glassy phase was observed in the prepared materials. The grain size of s-sialon was below 1 μm in the materials HIPed at 1650°C, and 1–2 μm at 1750°C. The Vickers hardness was fairly constant for the TiN-s-sialon composites with up to 15 wt% TiN added: H v 10 around 17·5 GPa for materials HIPed at 1650° and around 17 GPa at 1750°C, whereas at higher TiN contents the hardness decreased to around 16 GPa. The indentation fracture toughness of the s-sialon ceramic increased approximatively from 3 to 4 MPam 1 2 at an addition of 15 wt% TiN particulates. The fracture toughness could be further increased to 5 MPam 1 2 by addition of small amounts of Y 2 O 3 and A1N to a s-sialon composite with 30 wt% TiN.

Mixed α- and β-(Si-Al-O-N) materials with Yttria and neodymia additions☆

A number of ceramic materials have been fabricated on a semipilot plant scale at different overall compositions in the (Y, Nd)AlON system at 177°C. Constant molar amounts of oxide mixtures of Y2O3: Nd2O3 in the ratios 100:0, 75:25, 50:50, 25:75 or 0:100 have been added. Dense materials were obtained for all compositions except those corresponding to mixed α- and β-(SiAlON) with higher α-(SiAlON) contents and high Nd2O3 contents. At the preparation temperature used in this study, the formation of an α-(NdSiAlON) seems prohibited and, thus, with increasing Nd2O3 content the amount of α-(SiAlON) decreased. The Nd2O3 added mainly formed crystalline intergranular phases such as the N-melilite phase, which increased in amount with increasing Nd2O3 in the starting mix. Hardness and indentation fracture toughness measurements were made and are discussed in relation to the phase composition and the microstructure. Some of the high Nd2O3 content SiAlON materials have as high fracture toughness values as the pure Y2O3 SiAlON materials do.

Pressureless sintering of sialon ceramics with mixed Y2O3-La2O3 additions

Des compositions de Si-Al-O-N ont ete frittes a 1775 o et 1825 o C avec additions de Y 2 O 3 /La 2 O 3 sous plusieurs proportions. On examine les densites et les diverses phases obtenues. Une densification complete est possible si on ajuste les compositions globales vers des teneurs en oxygene elevees. La transformation de phase β-Si 3 N 4 →β-Sialon est complete au bout de 2 h, et les proprietes mecaniques obtenues dans ces conditions sont bonnes

Preparation and crystal structure of U-phase Ln3(Si3 –xAl3 +x)O12 +xN2 –x(x≈ 0.5, Ln = La, Nd)

U-phase Ln3(Si3 –xAl3 +x)O12 +xN2 –x(Ln = La, Nd) occurs as a crystalline phase in rare-earth sialon ceramics formed by devitrification of grain-boundary glasses at 1000–1400 °C. The crystal structure of Nd U-phase has been determined from Cu-Kα X-ray powder diffractometer data and refined by the Rietveld full-profile technique to RF= 0.028. The space group is P321 and the cell dimensions are a= 7.974(1)A, c= 4.873(1)A and V= 268.29 A3. The structure is isomorphic with the La3Ga5GeO14 structure, and exhibits corner-shared layers of (Si,Al)(O,N)4 tetrahedra interconnected by AlO6 octahedra. The rare-earth cations occupy sites between the tetrahedral layers. Transmission electron microscopy and lattice imaging studies support the X-ray structural findings. The structural relationship of the U-phase to other nitrogen-containing ceramic phases is discussed.

Three-dimensional effects in multislice image calculations for oblique cells

Abstract The limitations of using the multislice method for oblique structures are discussed. A way of overcoming the problems is suggested; it includes transforming the original cell to one with two axes nearly perpendicular to the beam, which is parallel to the third axis. Bi2Te3 (rhombohedral cella=10.267A,a=24.5°,Z=1, space group R3m) has been used as a test case. The three-dimensional effects introduced by using slices thinner than the crystallographic repeat have also been investigated.

Defects in β-rhombohedral boron formed during the synthesis of LaB6

Abstract Beta-rhombohedral boron (a = 10.139 A, α = 65.20°, Z = 104.9, space group R 3 m ), formed during the synthesis of LaB6 has been investigated with the use of high-resolution electron microscopy. In a sample of gross composition La0.81B6, β-rhombohedral boron crystals were found. Microanalysis showed that some of these contained minor amounts of La, Si, and O. They were rich in defects of various kinds, such as point defects, line defects, stacking faults, and twinning. Structure models are suggested for some of the defects.