Carsten Schwandt

Electrochemical investigation of lithium intercalation into graphite from molten lithium chloride

Lithium reduction at a graphite electrode in molten lithium chloride was studied at temperatures from 650 to 900 °C using cyclic voltammetry and chronoamperometry. It was found that, during cathodic polarization, lithium intercalation into graphite occurred before deposition of metallic lithium started. This process was confirmed to be rate-controlled by the diffusion of lithium in the graphite. When the cathodic polarization potential was more negative than that for metallic lithium deposition, exfoliation of graphite particles from the electrode surface was observed. This was caused by fast and excessive accumulation of lithium intercalated into the graphite, which produced mechanical stress too high for the graphite matrix to accommodate. The erosion process was abated once the graphite surface was covered by a continuous layer of liquid lithium. These results are of relevance to the mechanism of carbon nanotube and nanoparticle formation by electrochemical synthesis in molten lithium chloride.

The FFC-Cambridge Process for Titanium Metal Winning

The FFC-Cambridge process is a molten salt electrochemical deoxidation method that was invented at the Department of Materials Science and Metallurgy of the University of Cambridge one decade ago. It is a generic technology that allows the direct conversion of metal oxides into the corresponding metals through cathodic polarisation of the oxide in a molten salt electrolyte based on calcium chloride. The process is rather universal in its applicability, and numerous studies on metals, semimetals, alloys and intermetallics have since been performed at the place of its invention and worldwide. The electro-winning of titanium metal is a particularly rewarding target because of the disadvantages of the existing extraction methods. This article summarises the research work performed on the FFC-Cambridge process at the University of Cambridge and its industrial partners with a focus on the electro-winning of titanium metal from titanium dioxide. Topics addressed encompass the invention of the process, early proof-of-concept work, the identification of the reaction pathway, and the investigation and optimisation of the key process parameters. Also discussed are aspects of technology transfer and some of the development work undertaken to date.

The Electrochemical Reduction of Chromium Sesquioxide in Molten Calcium Chloride under Cathodic Potential Control

Electrochemical polarization and reduction experiments are reported which were performed with a three-terminal cell and a molten salt electrolyte consisting of calcium chloride with additions of calcium oxide. Employing a metal cathode, a graphite anode and a pseudo-reference electrode also made from graphite, polarization measurements were carried out with the aim to validate the performance of the pseudo-reference electrode and to assess the stability of the electrolyte. Using a chromium sesquioxide cathode in conjunction with a graphite anode and a graphite pseudo-reference electrode, electrochemical reduction experiments were conducted under potentiostatic control. The key results are: a graphite pseudo-reference electrode has been shown to be appropriate in the present type of molten salt electrochemical experiments that take place on a time scale of many hours; the conversion of chromium oxide into chromium metal has been accomplished under cathodic potential control and in the absence of calcium metal deposition; a significant amount of calcium oxide in the calcium chloride has been found necessary to preclude anodic chlorine formation throughout the entire experiment; a considerable overpotential has been identified at the anode.

Oxide Ion Conduction in Indium-Oxide-Substituted Calcium Zirconate

Indium-oxide-substituted calcium zirconate of the nominal composition CaZr 0.9 In 0.1 O 3-δ is an important perovskite-type high-temperature proton conductor. In the present study, the transition from proton conduction to oxide ion conduction in this material has been investigated experimentally. A specially designed galvanic cell was employed, in which the oxygen chemical potential at the electrodes is determined by appropriate metal/metal-oxide mixtures while the hydrogen chemical potential is fixed by a gas atmosphere of known hydrogen partial pressure. Through cell voltage measurements at different temperatures proton and oxide ion conduction were discriminated, and conditions were identified under which virtually pure oxide ion conduction occurs despite the presence of a significant hydrogen partial pressure. Conductivity measurements allowed the determination of the activation energy of oxide ion conduction. The impact of a variable oxide ion vacancy concentration on the ion conducting properties of the solid electrolyte is discussed.

The detection of hydrogen in molten aluminium

The presence of hydrogen in aluminium poses problems to the foundry and casting industries, because high residual hydrogen contents in molten aluminium cause significant porosity in the solid aluminium after casting. This usually renders the product useless as it may fail mechanically. Therefore, fast, accurate and reliable techniques are required for monitoring the dissolved hydrogen content in molten aluminium, but this particularly harsh environment places considerable restrictions on the equipment that can be used. Several methods are available for the determination of hydrogen in aluminium melts, but they either suffer problems of accuracy, reliability and longevity or are not applicable to industrial environments. It is considered that the most appropriate device for the hydrogen analysis in aluminium melts should be an electrochemical sensor, which employs a proton conducting solid electrolyte in conjunction with a measuring electrode and a suitable reference electrode. The electromotive force of such a cell allows direct calculation of the hydrogen concentration in the melt. However, all the electrochemical sensors reported in the literature thus far exhibit distinct drawbacks. This article discusses the various techniques for the determination of hydrogen in molten aluminium with particular emphasis on the benefits and shortcomings of the existing electrochemical sensors.

Nanowire hydrogen gas sensor employing CMOS micro-hotplate

In this paper we present a novel hydrogen gas sensor comprising a high temperature SOI-MOS micro-hotplate and employing zinc oxide nanowires as the sensing material. The micro-hotplates were fabricated at a commercial SOI foundry followed by a backside deep reactive ion etch (DRIE) at a commercial MEMS foundry. Particular care was taken in designing the heater shape using a systematic parametric approach to achieve excellent temperature uniformity (within 1–2%) as shown by both simulations and experimental infra-red imaging results. Zinc oxide nanowires were grown on these devices and show promising responses to hydrogen with a response (Ra/Rh) of 50 at 100 ppm in argon. The devices possess a low D.C. power consumption of only 16 mW at 300°C and, being CMOS compatible, offer low unit cost in high volumes and full circuit integration. We believe that these devices have potential for application as a sub-

Use of Molten Salt Fluxes and Cathodic Protection for Preventing the Oxidation of Titanium at Elevated Temperatures

1 hydrogen sensor with sub-1mW (pulsed mode) power consumption.

Laser welding studies on Ti-6Al-4V in air in conjunction with cathodic protection

The current study demonstrates that it is possible to protect both solid and liquid titanium and titanium alloys from attack from air by cathodically polarizing the titanium component using an electro-active high-temperature molten salt flux and a moderate polarization potential. The electrolytic cell used comprises a cathode of either solid titanium or liquid titanium alloy, an electrolyte based on molten calcium chloride or fluoride salt, and an anode consisting of an inert oxygen-evolving material such as iridium metal. The new approach renders possible the processing of titanium at elevated temperatures in the presence of oxygen-containing atmospheres.

Oxygen from lunar regolith

Laser beam melting of titanium alloy Ti-6Al-4V was performed with an Yb-fibre laser to produce weld pools under a range of conditions. These included the use of a flux covering of molten CaF2 or a molten mixture of CaF2 and NaF, in an atmosphere of ambient air, either with or without the application of cathodic protection to the Ti-6Al-4V. As anticipated, in the absence of flux, considerable oxidation of the metal occurred. In contrast, in the presence of flux, the degree of oxidation was greatly reduced. In addition, the application of a modest cathodic potential to the flux-covered metal reduced the pick-up of oxygen even further. The results suggest that the use of an appropriate flux combined with cathodic protection when fusion welding titanium and its alloys may offer advantages in circumstances where inert gas shielding is not always feasible, for example, in certain site welding applications.

Preparation of Ta-Nb Alloy Powder by Electro-deoxidation of Ta2O5/Nb2O5 Mixture in a CaCl2-NaCl Eutectic Melt

In the year 2004 NASA declared its mission to prepare for a return of man to the moon as early as 2015 but no later than 2020, while continuing with robotic missions to Mars (NASA 2004). As a long-term goal, it was intended to establish permanent human presence on the moon and eventually send human missions to Mars. Although the future of US space exploration policy is now more uncertain, following a recent review (Augustine Commission 2009) and the cancellation of the Constellation Program (NASA 2010a), it remains true that an extended human presence on the moon is desirable for scientific and economic reasons (e.g., Crawford 2004; Spudis 2005). For this to become possible, significant progress is needed in the field of ‘living off the land’, or in situ resource utilisation (ISRU).