Amorphous (a)-${\mathrm{HfO}}_{2}$ is a prototype high dielectric constant insulator with wide technological applications. Using ab initio calculations we show that excess electrons and holes can trap in a-${\mathrm{HfO}}_{2}$ in energetically much deeper polaron states than in the crystalline monoclinic phase. The electrons and holes localize at precursor sites, such as elongated Hf-O bonds or undercoordinated Hf and O atoms, and the polaronic relaxation is amplified by the local disorder of amorphous network. Single electron polarons produce states in the gap at $\ensuremath{\sim}2$ eV below the bottom of the conduction band with average trapping energies of 1.0 eV. Two electrons can form even deeper bipolaron states on the same site. Holes are typically localized on undercoordinated O ions with average trapping energies of 1.4 eV. These results advance our general understanding of charge trapping in amorphous oxides by demonstrating that deep polaron states are inherent and do not require any bond rupture to form precursor sites.

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Deep electron and hole polarons and bipolarons in amorphous oxide

We have studied the Raman spectra of (NdCl2)–(NdCl3) and Ce–(CeCl3) mixtures in eutectic (LiCl)0.58(KCl)0.42 as solvent in the temperature range up to 600 °C. For the highly corrosive samples a windowless cell in connection with a Raman microscope was utilised. To our knowledge the Raman spectra of the solutions of NdCl2, CeCl3 and (most likely) CeCl2 in eut.–LiCl–KCl are shown here for the first time. In accordance with other rare earth halides described in the literature the Raman spectra of the pure trivalent systems are dominated by octahedral LnCl63− species which show a typical broad polarized band centred at 245 cm−1 for NdCl3 and 240 cm−1 for CeCl3, respectively. NdCl2 in (LiCl–KCl)eu shows a complex Raman spectrum consisting of depolarized bands. In mixtures of divalent and trivalent neodymium chlorides both spectra can be observed in parallel and no additional Raman bands appear. Solutions of cerium in CeCl3–(eut.–LiCl–KCl) show temporarily new Raman bands which are presumably due to divalent cerium chloride. These bands disappear in our samples after about 1 h since CeCl2 is not stable under the experimental conditions. Our findings are discussed in the light of the strongly different electronic transport properties in the neodymium and cerium systems.

Raman spectroscopic study of mixed valence neodymium and cerium chloride solutions in eutectic LiCl?KCl melts

Nuclear power remains the most dense and reliable primary form of energy supply worldwide. Electricity generation via fission is also inherently carbon free, with environmental footprints rivalling modern renewable options. However issues arise from the production of highly irradiated spent fuels, with current management options limited to geological repository storage or, more desirably, closing of the fuel cycle by partitioning to recover fissionable species.

Extensive research has been pursued over the past half century in an effort to address an accumulating oxide spent fuel inventory and to circumvent shortcomings of raw uranium supplies. Pyrochemical reprocessing or ‘pyroprocessing’ using molten salt electrolysis for the recovery of desirable spent fuel components is an increasingly sought after solution to the above issues. First developed in the late 1990’s, the FFC Cambridge Process has shown to be a cornerstone in the electrochemical reduction of metal oxides and potentially offers a new iteration of pyroprocessing by introducing a direct conversion of spent oxide fuels to metals. These metals are easier to reprocess and specifically recovered elements can be used directly in advanced civilian nuclear reactors and metallic fuel cycles.

This thesis considers the role that an FFC based process could play in establishing a more sustainable, efficient and safer method for the select recovery of key metals from mixed oxide composites. Using an array of surrogate materials, the appropriation of an effective procedure was investigated in both CaCl2 and LiCl-CaCl2 Eutectic (LCE) molten salts at 810oC and 600oC respectively. At all stages of the process, feeds and electrolysis products have been examined by an assortment of ex-situ analytical tools, primarily SEM, EDS and XRD.
A process engineering approach was taken to designing suitable reactors and cells with the aim of improving operational characteristics, greater electrochemical reduction efficiency and high yields of pure products.
Preparation of electrolyte and feed oxide electrodes (surrogate or spent fuel) was investigated, including unique electrochemical treatment for the two molten salts and precedent for the creation of the oxide electrode via cold pressing or slip casting to kinetically aid optimal reduction. A series of investigations considering the thermodynamic performance of CaCl2 from a standpoint of electronic conduction were carried out, and considerable improvements found via the implementation of a simple cathodic sheath.

Selective partitioning was shown possible by the intended mechanism of partial direct reduction and anodic dissolution in the 2NiO-CeO2 binary. Partitioning of Zr from ZrO2-CeO2, and Ti from TiO2-CeO2 was also achieved, however in both cases it was via the gradual chemical dissolution of partially reduced Ce(III) into the molten salt or phase separation between liquid Ce and solid Zr. Extensive CV experiments were performed to enhance understanding of redox chemistry for each species investigated. CeOCl was found to be the only semi-stable phase of Ce present at potentials between -1.0 V vs. Ag/AgCl and its final reduction potential at approximately -1.95 V in CaCl2 at 810oC.

Active CV experiments using PuO2 and a MOX fuel sample containing 5% PuO2 were initiated, revealing remarkably similar electrochemical behaviour of PuO2 and the CeO2 surrogate. Both PuO2 and the bulk UO2 content MOX could be reduced in CaCl2 and in the lower temperature LCE whilst avoiding any decomposition of the electrolyte. Consequently a route for the direct electrochemical reduction of spent oxides fuels was shown plausible and offers a promising alternative to current pyroprocessing technology, with beneficial implications to the wider materials processing field.

Development of a novel electrochemical pyroprocessing methodology for spent nuclear fuels

In the second chapter, some basic concepts of statistical thermodynamics of liquids and of the nearly free electron model are introduced including quantities such as the pair distribution function g(r), the structure factor S(q), and the dielectric function of electronic screening. This is followed by a more detailed description of intermolecular potentials and microscopic structures of liquid metals, molten salts, and ionic liquids, both their theoretical foundation and experimental determination. A summary of the bulk phase behaviour of Coulombic fluids with emphasis on the liquid range is given. Specific thermodynamic characteristics such as undercooling, the vapour pressures of ionic liquids, or their criticality in binary mixtures are discussed. The mechanisms of electronic and ionic transport are briefly described. In the final section on interfacial characteristics, a few fundamental relations such as the Gibbs adsorption equation and the basic equations of wetting and electrowettting together with the scenario of wetting transitions are introduced. Furthermore, topics such as the stratification at liquid metal/vapour interfaces and charge ordering at electrified liquid/solid interfaces of molten salts and ionic liquids are presented. All sections contain tables with representative numbers for the properties considered.

Liquid Metals, Molten Salts, and Ionic Liquids: Some Basic Properties

Ultrafast dynamics of excess electrons in Na–NaBr and Na–NaI molten solutions at elevated temperatures (T
				= 953–1128 K) were investigated over an extended wavelength range. Modelling the time profiles resulted in two time constants τ1
				=
				(200 ± 40) fs and τ2
				=
				(2.8 ± 0.4) ps for both systems at 1073 K. All transients can be understood in terms of dynamical equilibria between polaron and Drude-type electrons as well as polaron and Drude-type electron forming bipolarons. In agreement with our earlier results for K–KCl melts the fast component is assigned to the relaxation of Drude-type electrons into polarons while the longer component, τ2, represents the time during which Drude-type electrons recombine with polarons leading to bipolarons. In addition, the temperature dependence was studied in Na–NaI: Decreasing the temperature to 953 K resulted in an increase of the time constants to τ1
				=
				(360 ± 50) fs and τ2
				=
				(4.3 ± 0.7) ps, respectively. At temperatures, where the ionic diffusion in Na–NaI melts becomes comparable to Na–NaBr melts, the time constants for the relaxation processes also coincide. The temperature-dependent investigations resulted in an Arrhenius activation energy of (25 ± 5) kJ mol−1 for Na–NaI melts in good agreement with literature data.

Ultrafast dynamics of excess electrons in molten salts: part II. Femtosecond investigations of Na-NaBr and Na-NaI melts.

We have studied the electron spin resonance (ESR) spectra in liquid K–KCl and M–(NaCl/KCl)eut mixtures at different concentrations in salt-rich melts approaching the metal–nonmetal transition region. In both systems F-centre-like characteristics are found. Strongly exchange narrowed signals clearly indicate that fast electron exchange occurs on the picosecond timescale. In contrast, the ESR spectra of a (NdCl2)(NdCl3)–(LiCl/KCl)eut melt are characterized by a large line width of the order of 102 mT which decreases with increasing temperature. In this case, the g-factor and correlation time are consistent with the model of intervalence charge transfer, which is supported by recent conductivity and optical measurements. The different transport mechanisms will be discussed.

Electron spin resonance study of electron localization and dynamics in metal?molten salt solutions: comparison of M?MX and Ln?LnX3 melts (M = alkali metal, Ln = rare earth metal, X = halogen)

W Freyland

Papers

Electron spin resonance study of electron localization and dynamics in metal?molten salt solutions: comparison of M?MX and Ln?LnX3 melts (M = alkali metal, Ln = rare earth metal, X = halogen)