Production of cerium hexaboride by molten salt electrolysis

Abstract

The technological advancement is driving the increasing need for next-generation boride materials for a wide range of engineering applications today. Metal borides are notable in material technology due to their outstanding qualities, which make them useful for a variety of applications. These boride compounds are distinguished by their high hardness, high melting temperatures, thermal and chemical stability, superior electrical conductivity, and resistance to corrosion and wear. These properties make metal borides suitable for use in high-temperature environments, surface protection coatings, wear-resistant layers, and cutting tool inserts. Rare-earth borides are particularly valued for their diverse stoichiometry and crystal structures, which contribute to their exceptional mechanical, physical, and electrical properties, gaining substantial attention for engineering applications. Rare earth hexaborides have gained significant interest due to their superconducting properties at low temperatures, valence state changes, and magnetic properties. Furthermore, their chemical resistance in harsh corrosive conditions, high melting points, and high strength make them unique. Due to these remarkable properties, they have been used in a wide range of applications such as electrical devices, including thermoionic cathodes, electron emission materials, and high-temperature protective equipment. CeB6, a rare earth hexaboride, is a very effective electron emitter due to its low work function (≈2.5 eV) and low volatility. The high thermal neutron cross section of element 10B makes it useful in nuclear technology. In literature, variety of methods for producing rare-earth borides were used; however, there are few high-purity production processes appropriate for large-scale industrial applications. Traditional methods such as borothermal reduction, floating zone techniques, optical floating zone methods, flux methods, spark plasma reactive liquid phase sintering, low-temperature synthesis using an autoclave or furnace, self-propagating high-temperature synthesis (SHS), mechanochemical processing, chemical vapor deposition (CVD), physical vapor deposition (PVD), and ball milling are frequently used. The obstacles for conventional methods include the requirement for complex equipment, long preparation and processing periods, high prices, and the production of harmful solid or gas wastes. These difficulties impede the production of consistent and high-purity materials while causing environmental and health risks. However, molten salt electrolysis stands out as an environmentally friendly, cost-effective, and rapid production method. It eliminates the need for expensive equipment and generates no hazardous waste, making it an increasingly popular and innovative option for a variety of engineering applications. This thesis investigates an alternative technique for producing cerium hexaboride (CeB6) by molten salt electrolysis. The CeB6 phase was successfully produced in powder form using an oxide-based electrolyte containing borax (Na2B4O7) and cerium oxide (CeO2). The study investigated several parameters, including as cathode material, temperature, current density, and electrolyte bath composition, to determine how they affected the composition, morphology, and crystallographic structure of the resultant CeB6 powders. The electrolysis process was carried out in a medium-frequency induction furnace using a graphite crucible as the anode and low carbon steel or titanium as the cathode. The cathode material needs to be investigated in order to understand its effect on electrochemical deposition and control cathodic reaction. To investigate this, electrolysis was carried out at 1000 °C for 60 minutes with a current density of 200 mA/cm2, using cathodes made of steel (AISI 1018) and titanium (Grade 2). X-ray diffraction (XRD) analysis of the resultant phases revealed that CeB6 was successfully synthesized when titanium (Grade 2) was utilized, in contrast to unsuccessful attempts with a steel (AISI 1018) cathode. Scanning electron microscopy (SEM) micrographs of cerium hexaboride powders derived from steel cathodes revealed irregular particles with no defined crystal structures. In contrast, titanium powders showed a clearly visible cubic structure of cerium hexaboride, which was compatible with XRD results. The failure to produce cerium hexaboride on a steel cathode was due to boron diffusion into the steel matrix. Steel's cubic crystal lattice allows boron atoms to occupy interstitial spaces, whereas titanium's hexagonal close-packed structure prevents diffusion, maintaining enough boron at the cathode surface for CeB6 production. During temperature-dependent studies, electrolysis was performed for 60 minutes at a constant current density of 200 mA/cm² with an electrolyte containing 10% CeO2 and 90% Na2B4O7. CeB6 was shown to be the major phase during experiments at 800, 900, and 1000 °C. As the temperature increased to 1100 °C, the electrolyte became more aggressive, leading the resulting products to deteriorate. Specifically, at 800 °C, the electrolyte's proximity to its melting point caused considerable viscosity, inhibiting ion and mass transit and resulting in the creation of several secondary phases. As a result, 900 °C was determined to be the optimal synthesis temperature. Notably, the particle size of the powders was irregular, with higher temperatures increasing crystallization and growth and thereby affecting the crystal structure, size, and morphology of cerium hexaboride powders. Below CeB6's critical synthesis temperature (1100 °C), powder grain size increased proportionately with temperature. Experiments were also conducted to produce high-purity CeB6 powders using titanium as the cathode material at different current densities (70, 100, 200, 300, and 500 mA/cm2). In addition to the primary phase of CeB6, CeBO3 was also detected, indicating that production occurs in a thermodynamic mode and is largely unaffected by current density beyond a critical threshold. The experiments demonstrated that it is unnecessary to use high current densities to generate CeB6 powders, as these higher densities negatively affect the cost-efficiency of the process. It was observed that the powder quantity on the cathode increased with rising current density, in accordance with Faraday's law. However, low current densities resulted in limited powder production. The optimal current density for depositing the powder was found to be 200 mA/cm2. The composition of electrolytes must be investigated, particularly in terms of their impact on electrochemical deposition and cathodic reactions. In this study, electrolysis was carried out at 900 °C for 60 minutes with a current density of 200 mA/cm², using different electrolyte compositions (90% Na2B4O7 with 10% CeO2, 95% Na2B4O7 with 5% CeO2, 94% Na2B4O7 with 5% CeO2 and 1% CaF2, and 93% Na2B4O7 with 5% CeO2 and 2% CaF2). Consistent with previous research, it was observed that the presence of CeO2 in the electrolyte aids in the formation of borate phases and enhances the hardness of the resulting glassy phases. Consequently, the CeO2 content was adjusted from 10% to 5% to achieve optimal conditions. Although small amounts of CaF2 used as an activator proved beneficial, higher concentrations were found to be detrimental. Therefore, the CaF2 content was optimized at 1%. The morphology of the produced powders was found to vary with different electrolyte compositions. In this thesis, the deposition of high-purity cerium hexaboride powders on the cathode surface was achieved through the co-deposition of cerium and boron ions. This process utilized cost-effective, straightforward, and stable salts, ensuring no solid waste or gas emissions were produced. The ideal conditions for producing CeB6 were identified as an electrolyte mixture of 94% Na2B4O7, 5% CeO2, and 1% CaF2, with electrolysis conducted at 900 °C, a current density of 200 mA/cm², for 60 minutes, using Grade 2 titanium as the cathode. The resulting powders, containing minor impurities, were subsequently purified using an HCl cleaning procedure. Under these specified electrolysis conditions, it was observed that boron and cerium ions were reduced independently yet concurrently, resulting in the formation of the stoichiometric compound CeB6 on the cathode surface.