Synthesis and characterization of EuB6 by borothermic reduction of Eu2O3

Carbon free high pure europium hexaboride is an urgent need for the fast breeder nuclear reactor program. The properties of EuB6 are highly influenced by the presence of one or more substitutional impurities, particularly, carbon, oxygen and nitrogen. In the present investigation carbon and nitrogen free high pure europium hexaboride was synthesized by borothermic reduction of europium oxide Eu2O3 (using boron as a reducing agent) at relatively low temperature (< 900°C). Glassy B2O3, the by-product of the synthesis process, was leached out completely in slightly acidic warm water from which boron could be recovered by usual electrolysis process. The product obtained was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and chemical analysis. Oxidation behaviour of pressureless sintered EuB6 pellets (> 90 % of theoretical density) was studied at 850°C and revealed that an adherent thin surface of Eu2O3 was formed which prevent further oxidation.


I. Introduction
Europium hexaboride has attracted attention of many scientists, engineers and technicians due to its interesting properties at low temperature [1][2][3].Europium hexaboride is intrinsically semiconductor which orders antiferromagnetically below 6K [4].It is a potential candidate for the control rod in the fast breeder reactor because of high neutron absorption cross-section of both boron and europium atoms for the thermal and especially for the fast neutrons [5].Europium hexaborides dissolve very small amount of carbon which reduces the possibility of the formation of unstable carboborides in moisture media which in turn make this compound more stable under operational conditions at high temperature.Further more it was shown that the EuB 6 neutron absorbing characteristics exceed the similar properties of other boron and europium-containing materials (B 4 C and Eu 2 O 3 ) [6].EuB 6 has been considered in various hard core engineering applications because of its excellent corrosion and oxidation resistance at high temperature [7,8].High tem-perature usability is enhanced due to its high melting temperature (2660°C).
Various methods for the synthesis of rare earth hexaboride have been explained but none of them described synthesis of europium hexaboride in details.Borothermic reduction of europium oxide synthesis process, reported earlier [9,10], required high temperature (1800°C) in which oxygen was removed as boron suboxide (BO).The expansive boron loss in the form of suboxide is the key issue to be resolved for the scaling-up of this process.Electrochemical and solution method of synthesis of EuB 6 were attempted and reported by various scientists and researchers [11][12][13][14].An involvement of the chlorides at high temperature is the associated issue in the electrolysis process.Synthesis of EuB 6 by floating zone technique to prepare a single crystal europium hexaborides was also reported [10] and looks promising.However, the synthesis of the europium hexaborides in large quantity by this process is not cost effective as the substantial materials and power losses are involved throughout the synthesis process.
The present synthesis is based on the hasselfree single step brothermic reduction of europium oxide (Eu 2 O 3 ).The reaction involved in the preparation is shown by the following equation: The above reaction is thermodynamically feasible at room temperature, but kinetically the reaction starts only above 760°C.To fasten the kinetics the temperature employed in the bulk synthesis was 900°C.The B 2 O 3 formed in this synthesis process was in glassy phase and could not be detected in XRD analysis.The formed glassy B 2 O 3 product was leached out in slightly acidic warm water from which boron could be recovered by the usual electrolysis process [15].The synthesized europium hexaborides were free from carbon and nitrogen contamination which enhanced its usability in moisture media.The chemical analysis of the product shows substantially less amount of oxygen (< 100 ppm).

II. Experimental
The materials used in the synthesis process were: i) europium oxide (Eu 2 O 3 ) powder with purity of 99.99% (E-Merck make) and 3 µm median particle diameter and ii) boron powder with purity of 99.9% (with less than 0.01% C, 0.02 % Fe and 0.02% Si) and 40 µm median particle diameter.All these raw materials were oven dried at 100°C under rotary vacuum to remove moisture before use.Particles size distribution and XRD pattern of the raw materials are presented in Figs. 1 and 2, respectively.
Reactants in stoichiometric quantity, according to the reaction (1), were prepared by weighing accurate quantity of individual components and mixed homogeneously.The SEM image of the reactants mixture is illustrated in the Fig. 3 and confirmed the homogeneity of the boron and europium oxide mixture.The obtained reactant mixture was pelletized by uniaxial pressing at pressure of 88 MPa using 12 mm die plunger.The pellet was used for thermal analysis study (TG-DSC Setaram tag 24 Thermoanalyser).For the regular synthesis of EuB 6 , weighed quantity of Eu 2 O 3 and B were mixed homogeneously in a tungsten carbide mortar pestle and then powder mixture was pelletized as described earlier.The pellets were kept in an alumina tray which was further placed inside the controlled atmosphere reaction chamber of silicon carbide furnace.The reaction chamber was purged with argon gas for an hour and then temperature was raised to 900°C at rate of 20°C/min.The argon gas flow rate was kept at 2 l/hours and flow was continued throughout the synthesis process.The temperature of the furnace was measured using chromal-alumel type thermocouple.The furnace was kept at 900°C for about 4 hours to complete the reaction.After the completion of the process, the furnace was cooled down to room temperature.Pellets from the reaction chamber were gently removed and then crushed into the powders using tungsten carbide mortal pestle followed by ball milling.The powder obtained was leached in slightly acidic warm water and the residue was oven dried under rotary vacuum at 100°C.
Phases of the oven dried powder samples were studied by X-ray diffraction (Enel PW 1830 Diffractometer).Particle size distribution of the calcined powder product was studied using the laser light scattering technique (CILAS PSA 10642L Particle size analyser) while the morphological features were probed by scanning electron microscopy (SEM Philips FE1 XL 30).
For the sintering process, EuB 6 powders were compacted in pellets of 12 mm diameter and thickness of ~2 mm, by uniaxial pressing at the pressure of 170 MPa.The sintering of the pellets was done at temperature 1800°C for 4 hours.The sintered density was measured by Archimedes method, while the microstructural features of the sintered specimens were studied using SEM.
Oxidation study of pressureless sintered pellet (>90% of theoretical density) with total surface area of 1.6 cm 2 /g in the form of half circle was conducted in a resistance-heated furnace.All surfaces of the semicircle samples were polished with emery paper and finally with diamond paste up to 3Δ finish.In order to avoid oxidation during heating, the sample was directly inserted into the furnace after the furnace temperature reached 850°C.The sample was oxidized for different time intervals (0.5, 1, 2, 4, 8, 16, 32, and 64 h) at 850°C.The oxidation products were identified by XRD.The morphology and nature of oxide layer formed were analyzed by SEM.

Formation of highly pure EuB 6
Gibbs free energy versus temperature curve for the reactant mixture of Eu 2 O 3 (s) and B (s) with various product combinations were calculated and presented in and EuB 6 as products, is thermodynamically feasible at room temperature, however higher temperature was required for the other reactions.To ascertain the kinetically feasible temperature of the reaction (d), DSC and TG of small portion of the sample were done in the argon atmosphere up to the temperature of 900°C at the heating rate of 8°C/Min.The DSC along with TG curve is illustrated in the Fig. 5.The weight loss of about 0.8% has taken place at the range of 60 to 400°C, which could be attributed to the removal of absorbed water in the capillaries and bonded water.Above 400°C, the sample has achieved mass stability, indicating the completion of the evaporation processes.An interesting observation was the appearance of an exothermic peak at 760°C accompanied with no mass change.The XRD analysis of the product heated at 760°C confirmed presence of EuB 6 .However a few peaks of Eu 2 O 3 phase were also present.
No other crystalline phase was detected in XRD pattern illustrated in Fig. 2.This clearly confirmed the kinetically feasible temperature of the equation ( 1) with glassy phase B 2 O 3 as one of the by-products.
Another experiment was done at 900°C to enhance the kinetics to complete the reaction and confirm the formation of B 2 O 3 .The product obtained at 900°C was dissolved in slightly acidified (5% HCl) warm water.The solution was properly stirred, filtered and residue was oven dried under rotary vacuum.Weight loss of about 15% was observed throughout the process which corresponds to stoichiometric B 2 O 3 formed during the reaction.B 2 O 3 being the glassy phase could not be detected in XRD analysis.Leached product contained less than 100 ppm of oxygen.However, carbon and nitrogen impurities remained below the detectable limit.
The SEM of the leached product EuB 6 is shown in the Fig. 6.It can be seen that the particles of EuB 6 are uniformly distributed which was verified by the distribution of particle size curve shown in Fig. 7.The particle size is seen to be around 4-10 µm indicating the absence of grain growth which is obvious because of low processing temperature.Fine pores of around 2 µm are visible at grain junctions.The small particle size is found to be promising for sintering process.

Oxidation of EuB 6 sintered pellets
The oxidation reaction of europium hexaboride can be presented by the following equation: Free energy curve for the oxidation reaction was calculated and presented in Fig. 8.The free energy curve confirms the thermodynamical feasibility of the oxidation reaction at low temperature.
The percentage weight gain data obtained during the oxidation of dense (> 90 % of theoretical density) EuB 6 pellets at various time intervals are presented in Fig. 9.
It can be seen that the rate of oxidation decreased with time and became zero.After long duration of oxidation (> 16 hours) about 1.2 % weight loss was observed (Fig. 9) that could be due to the evaporation of B 2 O 3 formed during the oxidation.XRD analysis of the oxidized surface shown in Fig. 2

IV. Conclusions
Highly pure carbon free EuB 6 has been prepared at relatively low temperature.Boron from the obtained by-product B 2 O 3 could be easily recovered by commercially available process.Low temperature synthesis prevents the agglomerates formation and keeps particle size very small which is essential for the sintering process.The small particle size favours pressureless sintering.Oxidation study shows the usability of these materials at high temperature because of adherent oxide layer of Eu 2 O 3 formation which prevents further oxidation.

FigureFigure 11 .
Figure 10.SEM Image of oxidized surface of EuB 6 after 64 hours