Single Rare Earth Oxide Research Articles

Densification Behavior, Microstructural Development and Mechanical Properties of Spark Plasma Sintered α/β-Silicon Nitride Ceramics with Different Rare Earth Oxide Additives

ABSTRACT Monolithic Si3N4 ceramics were fabricated by spark plasma sintering at 1750°C for 15 min under 50 M Pa applied pressure using α-Si3N4 powder with 2 wt% Y2O3, 2 wt% Dy2O3, 2 wt% Eu2O3 and 8 wt% Y2O3 as single rare earth oxide additives. It aimed to investigate the effects of the composition and content of rare earth oxide additives on densification, microstructural development, and mechanical properties of α/β-silicon nitride ceramics via XRD and SEM analyses, temperature and displacement curves obtained through spark plasma sintering and mechanical tests. Addition to 2 wt% Y2O3 as an additive into composition resulted in high relative density (98%), hardness (14.9 GPa), and fracture toughness (9.8 M Pa.m1/2). The use of Dy2O3 instead of Y2O3 decreased β-Si3N4 phase ratio and densification rate. This behavior continued further with Eu2O3 addition. The increment of Y2O3 content to 8 wt% provided the same relative density and fracture toughness (98% and 9.8 M Pa.m1/2), whereas hardness was reduced to 12.6 GPa. As a result, densification was achieved only by Y2O3 addition regardless of additive content, and both high hardness and fracture toughness were obtained using only 2 wt% Y2O3 as a single additive.

The Microstructure of Gradient Composite Bioceramic Coating with Single and Multiple Rare Earth Oxide Fabricated by Wide Band Laser Cladding

To improve bonding strength between Ti alloy and bioceramic coating and to eliminate thermal stress during laser cladding, gradient bioceramic coating materials is designed. The bioceramic coating is fabricated by wide band laser cladding technique. And the effect of adding single and multiple rare earth oxide on microstructure is investigated. The samples are soaked with SBF for 14 days, the experimental results indicate that when the content of single rare earth oxide is 0.6wt.%Y2O3, a large number of bioactivity phase and chrysanthemum shape microstructure can be observed. When the content of single rare earth oxide is 0.2wt.%Sm2O3, a lot of bioactivity phase and coral reef shape microstructure can be observed. When the content of multiple rare earth oxide is 0.3wt.%Sm2O3 and 0.4wt.%Y2O3, a large amount of bioactivity phase and more homogeneous and dense spherical particles can be observed on coating, which demonstrates the microstructure is of better bioactivity.

Study of the formation temperature of mixed LaREO3 (RE ≡ Dy, Ho, Er, Tm, Yb, Lu) and NdGdO3 oxides

Mixed LaREO3 (RE ≡ Dy, Ho, Er, Tm, Yb, Lu) and NdGdO3 oxides were prepared by thermal decomposition of the corresponding co-precipitated mixed oxalates. The decomposition of La/RE and Nd/Gd oxalates was studied by means of differential thermal analysis and thermogravimetric (DTA-TG) measurements; in particular the last step, consisting of the loss of a CO2 molecule from the corresponding oxycarbonate, has been thoroughly investigated, as it is particularly interesting for the study of the formation temperature of mixed rare earth oxides. After the release of CO2, the oxides crystallize in a distorted perovskitic cell or one of the structures typical of rare earth sesquioxides, depending on the cationic size difference and on the average cationic radius. The mixed rare earth oxycarbonate decomposition has been studied in comparison to the decomposition of single rare earth oxycarbonates. A trend of the mixed oxides formation temperature as a function both of the average cationic size and of the cationic sizes difference has been observed and compared to the behaviour of single rare earth oxides.

Emission efficiency optimization of RE 2O 3 doped molybdenum thermionic cathode by application of pattern recognition method

As an alternative for thoriated tungsten thermionic cathodes, molybdenum doped with either a single rare earth oxide such as La 2O 3, Y 2O 3 and Sc 2O 3 or a mixture thereof has been produced by powder metallurgy. It is shown that carbonization can greatly improve the emission properties (i.e. emission capability and stability) of RE 2O 3 doped molybdenum due to the formation of a (metallic) rare earth atomic layer on the surface of the cathode by the reduction reaction of molybdenum carbide and rare earth oxide. Among all the carbonized samples, La 2O 3 and Y 2O 3 co-doped molybdenum cathode showed the best performance in emission. In addition, computer pattern recognition technique has been used to optimize the composition of the material and of the cathode preparation technique. We derive the equation of the emission efficiency as a function of cathode composition and carbonization degree. Based on the projecting coordinates obtained from the equation, the optimum projection region was identified, which can serve as guide for the composition and carbonization degree design.

Study on rare earth oxide-molybdenum cermet cathode materials

The emission property of a new kind of rare earth oxide–molybdenum cermet cathode made by powder metallurgy has been studied in this paper. The experimental results show that adding either single rare earth oxides La2O3, Y2O3, and Gd2O3 or a mixture of these rare earth oxides into molybdenum can improve the secondary emission coefficient of the cathode. After La2O3–Y2O3–Gd2O3–Mo is activated at 1600°C, the maximum secondary emission coefficient can be high to more than 3.0. This kind of cathode also shows good secondary emission stability during 1000h of life experiment and capability of anti-exposure to atmosphere. The zero-field emission density of 0.12A/cm2 is reached at 1470°C.

Effects of rare-earth oxide additions and heat-treatment temperature on the transformation and microstructure of silicon nitride

In this paper, mixtures of Si3N4 powder with different added amounts of single rare-earth oxide Re2O3 (Re = Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) were heat treated at 1600–1800 °C and the effects of Re2O3 on the transformation and the microstructure of Si3N4 were investigated. Compared with the original Si3N4 powder, the transformation ratio of the Si3N4 powders obviously increased and the phase transformation finishing temperatures decreased by about 100 °C when different rare-earth oxides were added. When the heat-treatment temperature was 1700 °C, the transformation ratio of the Si3N4 powder was about 100%. When the heat-treatment temperature was 1600 °C and the amount of Re2O3 addition was 3 mol%, an anomalous transformation ratio occurred. In addition, it was found that the transformation ratio of Si3N4 changed periodically with the increase of the atomic number of the lanthanide when the heat-treatment temperature was 1650 °C or less. The SEM image of the Si3N4 powders obtained at 1700 °C indicated that the heavy lanthanide oxides such as Er2O3 and Yb2O3 were very helpful to develop rod-like β-Si3N4 particles.

The phase transformation behavior of Si 3N 4 with single Re 2O 3 (Re = Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) additive

In this work α-Si 3N 4 powder was heated at 1600–1800 °C for 1.5 h with single rare earth oxide Re 2O 3 (Re=Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb) additives, respectively. The transformation ratio of α-Si 3N 4 to β-Si 3N 4 was determined by X-ray diffraction (XRD) and the behavior of Si 3N 4 transformation was investigated. The results showed that the all of the Re 2O 3 additives could increase obviously the transformation ratio of Si 3N 4 at 1600 °C or more when the amounts were 1 mol%, and the transformation ratio was nearly achieved to 100% at 1700 °C. While the anomalous change of transformation ratio was found when the amount of Re 2O 3 additive was 3 mol% and the heat temperature was 1600 °C. In addition, it was found that the transformation ratio of Si 3N 4 changed periodically with the atomic number of the lanthanide at 1650 °C or less.

Production of rare-earth oxides of high purity

A novel industrial technology for producing rare earth oxides of high purity (>99.99%) by temperature pressurized ion-exchange displacement chromatography is reported. It was concluded from the technological parameters that the concentration of complex agent in the eluent can be several times higher than that used in the classical ion-exchange method if a certain amount of buffer is present; the height equivalent to a theoretical plate (HETP) decreases linearly with increase in the operating temperature, increases slightly with increase in the flow-rate of the eluent and depends on the particle size of the ion exchanger. Rare earth oxides of high purity were manufactured on an industrial scale using a solution of special composition as eluent, and enriched rare earth mixtures or single rare earth oxides with low grade as raw and processed material on a temperature pressurized ion-exchange multi-column operating system containing polytetrafluoroethylene as the inner liner.