LaB6 Addition Research Articles

Densification, microstructural evolution and mechanical properties of Si-B-C-N monoliths with LaB6 addition

The mixture of 5 wt% LaB6 and mechanically alloyed Si-B-C-N powders was conventionally ball-milled and then compacted by spark plasma sintering at 1250–2000 °C and 50 MPa. Densification, microstructural evolution and mechanical properties of the as-prepared La/Si-B-C-N monoliths were investigated. At 1360 °C, sintering initiates coupled with rapid densification caused by particle rearrangement and plastic deformation. Grain-boundary sliding and atomic diffusion lead to further densification when sintering continues. Only SiC and La-containing BN(C) phases are detected in La/Si-B-C-N monoliths sintered at temperatures higher than 1900 °C. The monoliths sintered at 2000 °C show a high fracture toughness (4.6 MPa m−1/2) attributed mainly to lamellar La-containing BN(C) grains.

Transition Metal Diborides as Electrode Material for MHD Direct Power Extraction: High-temperature Oxidation of ZrB2-HfB2 Solid Solution with LaB6 Addition

Transition metal borides are being considered for use as potential electrode coating materials in magnetohydrodynamic direct power extraction plants from coal-fired plasma. These electrode materials will be exposed to aggressive service conditions at high temperatures. Therefore, high-temperature oxidation resistance is an important property. Consolidated samples containing an equimolar solid solution of ZrB2-HfB2 with and without the addition of 1.8 mol pct LaB6 were prepared by ball milling of commercial boride material followed by spark plasma sintering. These samples were oxidized at 1773 K (1500 °C) in two different conditions: (1) as-sintered and (2) anodized (10 V in 0.1 M KOH electrolyte). Oxidation studies were carried out in 0.3 × 105 and 0.1 Pa oxygen partial pressures. The anodic oxide layers showed hafnium enrichment on the surface of the samples, whereas the high-temperature oxides showed zirconium enrichment. The anodized samples without LaB6 addition showed about 2.5 times higher oxidation resistance in high-oxygen partial pressures than the as-sintered samples. Addition of LaB6 improved the oxidation resistance in the as-sintered condition by about 30 pct in the high-oxygen partial pressure tests.

Effects of LaB6 addition on arc-jet convectively heated SiC-containing ZrB2-based ultra-high temperature ceramics in high enthalpy supersonic airflows

The performances of a ZrB2–SiC–LaB6 ultra-high temperature ceramic (UHTC) was investigated in high enthalpy supersonic air flow. The UHTC material reached and maintained steady-state radiative surface temperature of 1973 K (monitored by pyrometer) for 5 min, and survived the arc-jet plasma exposure without any optical evidence of mechanical damage. The oxide scale covering externally the sample evolved into different textures. Transient thermal analysis processed via CFD provided a good agreement between the numerical steady-state temperature and the temperature distribution obtained experimentally. The contemporary addition of LaB6 and SiC to ZrB2 had a detrimental effect on the overall oxidation resistance.

The effects of (Ba,Sr,Ca)CO3 or LaB6 addition on the x-ray photoelectron spectroscopy spectra and electrical properties of the MgO thin films in alternating current plasma display panels

MgO thin films are widely used in plasma display panels (PDPs) to protect the dielectric layer, which is composed of PbO2, B2O3, and SiO2 compound, against ion bombardment during discharge. To improve the electrical properties of the MgO thin films, (Ba,Sr,Ca)CO3 or LaB6, which has a lower work function than that of MgO, added to the MgO films. The effects of (Ba,Sr,Ca)CO3 or LaB6 addition on the electrical properties, microstructure, and electronic band structure were investigated. In the case where (Ba,Sr,Ca)CO3 was added, the firing voltage, which is the voltage when the panel is ignited the first time during increasing input voltage, was about 18.4 V lower than that of the conventional MgO films. In the case where LaB6 was added, the firing voltage was also reduced by about 24 V. The luminance and luminous efficiency were also increased. Of particular interest was the valence band spectra changed after adding (Ba,Sr,Ca)CO3 or LaB6. The valence band edge, which is the top of the valence band, was shifted to lower binding states and the width of the valence band was increased. Moreover, the band gap was slightly reduced. Considering the emission mechanism of MgO films in plasma display panels, these results mean that the secondary electrons can be ejected more easily and the ejected electrons have more energy. Therefore, the addition of (Ba,Sr,Ca)CO3 or LaB6 might improve the electrical properties.