Richard J. Pulham

Solubilities, and solution and solvation enthalpies, for nitrogen and hydrogen in liquid lithium

The solubilities of nitrogen and of hydrogen in liquid lithium have been determined up to 2.77 mol% N and 5.68 mol% H by electrical resistance methods, and they can be represented, in part, by the equations log10xN = 1.168 −2036T 473 < T < 708 Klog10xH = 1.523 −2308T 523 < T < 775 K where x is the solute mole fraction. The results show that the nitrogen and hydrogen content of lithium can be reduced to 0.08 and 0.03 mol%, respectively, by filtration at 200 °C. Solubilities provide values of the partial molar enthalpies, H(soln), and entropies, S(soln) (with respect to the precipitating phase), of solution of 39.23 kJ mol−1 and 22.35 J K−1 mol−1 (for Li3N), and 44.18 kJ mol−1 and 29.15 J K−1 mol−1 (for LiH). The values of H(soln) are used to derive solvation enthalpies of −3473 and −427 kJ mol−1 for nitride and hydride ions, respectively, in the metal. The hydrogen solubilities augment the liquidus of the Li-LiH phase diagram.

Comparison of chemical reactions in liquid lithium with those in liquid sodium

A comparison is made of the chemical properties of liquid lithium and of liquid sodium which are relevant to the use of the liquid metals in fusion and fission reactors. The liquids dissolve both metals and non-metals: the solubility of transition metals is enhanced by the presence of dissolved non-metals, e.g. nitrogen and oxygen, and this is most obvious in the case of lithium contaminated with nitrogen where the enhanced solubility is accompanied by the existence of stable ternary nitrides, e.g. Li3FeN2. The solubilities of selected non-metals, hydrogen, carbon, nitrogen and oxygen, in the liquid metals are compared, and the fate of salts with polyatomic anions, e.g. NaOH, Li2CO3, is discussed. Chemical reactions are followed in the laboratory, often by pressure and electrical resistivity techniques, and more recently by electrochemical cells. The reactions can be divided arbitrarily into non-metal with non-metal, non-metal with dissolved metal, and non-metal with solid transition metal. Under these headings, the reactions are represented by: 15 Li3N + Li22Si5 → 5 Li5SiN3 + 42 Li in lithium 4Li3N + Li2C2 → 2 Li2NCN + 10 Li in lithium 2“Na3N” + Na2C2 → 2NaCN + 6 Na in sodium 4 Ba2N + BaC2 → 2 BaNCN + 7 Ba in sodium 2 C + 2 Li(or Ba) → Li2C2 (or BaC2 ) in sodium 2 N + 6 Li(or 4Ba) → 2 Li3N (or 2Ba2N) in sodium 2 Li3N + Fe → Li3FeN2 + 3 Li in lithium 5 Li3N + Cr → Li9CrN5 + 6 Li in lithium 3 Na2O + Fe → Na4FeO3 + 2 Na in sodium 2 Na2O + Cr → NaCrO2 + 3 Na in sodium Reasons are suggested for the different behaviour in lithium and in sodium.

Review of the solubility of non-metals in liquid lithium

Abstract Information on the solubility of non-metals in liquid lithium has been critically reviewed. Recommended solubilities are provided for solutions of oxygen and nitrogen. log 10 S (mol% O) = 2.778 −2740/ T (° K ) 523−908 Klog 10 S (mol% N) = 5.088 − 3540/ T (° K ) 523−673 K. No overall solubility trend can be discerned; at temperatures above 400 °C, solubilities decrease in the order: N > H > C > O > F, I > C1 > F and Si > C > Ge. The solubilities are compared with those in sodium. Carbon, nitrogen and hydrogen are more soluble in lithium; oxygen solubility is similar; fluorine and germanium are less soluble in lithium.

Chemical reactions between caesium, tellurium and oxygen with fast breeder reactor cladding alloys: Part III — The effect of oxygen potential on the corrosion by caesium-tellurium mixtures

Abstract The corrosion of the alloys PE16 and M316 by Cs : Te mixtures (1:1, 2:1 and 4:1) has been studied in sealed capsules under partial pressures of O2 set by metal/metal oxide couples at 948 K for 168 h. The alloys suffered severe intergranular corrosion by the 1:1 mixtures and corrosion seemed independent of O2 potential. The 2:1 and 4:1 mixtures produced a matrix (alternate layering) type of corrosion and the depth of corrosion increased with increasing O2 potential. At the highest potential the 4:1 mixture differentiated the two alloys; PE16 suffered a combination of intergranular and matrix attack whereas M316 was penetrated throughout (mainly by Cs/oxygen) intergranularly. Generally Cs, Te, Cr and oxygen were found associated (as a mixture of caesium chromate and chromium telluride), and PE16 was more resistant than M316 steel to corrosion.

Chemical reactions of caesium, tellurium and oxygen with fast breeder reactor cladding alloys: Part I — The corrosion by tellurium

The corrosion of the alloys PE16, M316 and FV448 by liquid Te in sealed capsules has been studied after 168 h at 948 K both with and without a buffer of Mo/MoO2. This buffer sets the thermodynamic oxygen potential at ΔGo2 = −417 kJ mol−1 O2. All three alloys were severely corroded and the extent was in the order M316 >PE16 = FV448. Oxygen diminished the extent of corrosion but did not change the order. All three alloys carried two layers of corrosion products on top of a damaged metal surface. The outer layer contained Fe1.5 Ni1.5 Te2 for PE16 but Fe2.25 Te2 for M316 and FV448. The inner layer contained Cr2Te3 for all three alloys. The damaged metal surface was largely Cr2Te3. The substrate alloys (PE16 and FV448) were penetrated intergranularly by Te, but M316 exhibited an even band of Cr-depleted steel. The results are explained by the diffusion of Te into the alloy and the diffusion of the alloy components in the opposite direction. The corrosion is exacerbated by dissolution of alloy into the liquid Te from which the metal tellurides subsequently precipitate.

Hydrogen in liquid alkali metals

Abstract The chemistry of liquid alkali metal-hydrogen solutions has been surveyed. Solubility data for hydrogen ( Investigations of interactions between hydrogen and non-metals in liquid alkali-metal solutions have shown that, whereas hydrogen and nitrogen act independently in lithium at 420 °C, hydrogen and oxygen interact in sodium at 400 °C according to the equilibrium: O2− + H−⇌ OH− + 2e−. Hydrogen-oxygen interactions in the other alkali metals are also considered and are rationalised in terms of the enthalpy changes of the corresponding solid-state reaction. Furthermore, yttrium has been shown to react, rapidly, with hydrogen dissolved in lithium at a relatively low temperature (400 °C) to form a mixture of a solid solution of hydrogen in yttrium and yttrium dihydride according to the reaction: Li(H) + Y → Li + Y(H) + YH2−x.

Chemical reactions between caesium, tellurium and oxygen with fast breeder reactor cladding alloys

The corrosion of the alloys PE16, M316 and FV448 by liquid caesium in sealed capsules has been studied after 168 h at 948 K in the presence of a buffer of Ni/NiO which set the thermodynamic oxygen potential at ΔGO2 = − 307 kJ mol -1 O2. At this relatively high potential, the caesium is converted to a liquid phase containing ca. 33 mol% O at 948 K, and all three alloys are severely corroded with the degree decreasing in the order M316 > FV448 > PE16. With 316, the corrosion is entirely intergranular causing severe longitudinal disruption between the grains. With PE16, the corrosion is intergranular ahead of layering. With FV448, there is a combination of intergranular and transgranular corrosion on an even front. The chemistry of the corrosion is dominated by the formation of caesium chromate and ferrite.

Depression of the freezing point of lithium by deuterium, and the solubility of deuterium in liquid lithium: A comparison with hydrogen

Abstract The depression of the freezing point of lithium (previously gettered with yttrium at 400°C) caused by deuterium has been determined by thermal analysis. The freezing point drops from 180.490 to 180.415° C at the eutectic composition, 0.013 mol% D. These values are marginally smaller than for hydrogen (depression 0.082 °C at 0.016 mol% H). The solubility of deuterium in the liquid metal has been determined up to 2.63 mol% D by electrical resistance methods and can be represented by the equation log10xd = 2.321−2873/T 549≤ T≤ 724 K, where xD is the mole fraction of D. The solubility provides values of the partial molar enthalpy, H(soln) and entropy, S(soln) (with respect to the precipitating phase) of solution of deuterium of 54.8 kJ mol− and 44.2 J K−1 mol−1, respectively. The value of H(soln) is used to derive a solvation enthalpy of −417 kJ mol−1 for the deuteride ion thereby indicating weaker solvation than for the hydride ion.

Chemical reactions of caesium, tellurium and oxygen with fast breeder reactor cladding alloys: Part IV — The corrosion of ferritic steels

Abstract A study of the corrosion of the steels FV448 and DT2203Y05 by Cs/Te mixtures in sealed capsules containing oxygen buffers at 948 K after 168 h shows that the oxide dispersion strengthened steel is more resistant to corrosion than is FV448 at Cs:Te ratios of 1:1 and 2:1. Both steels generally corrode evenly and show more resistance to the more damaging intergranular penetration than do PE16 and M316 alloys. Corrosion is most severe at 1:1 compositions irrespective of oxygen potential, and the corrosion products are Cs 2 Te + transition metal tellurides. The corrodants Cs 2 Te, Cs 2 Te + Cs and Cs are inert to FV448 in the absence of O 2 but corrosion increases with increasing oxygen potential. At low potentials the dominant corrosion products are caesium chromâtes + Cr 2 Te 3 and these are augmented by CsFeO 2 + FeTe 0.9 at higher potentials. The various types of corrosion are summarised.

Chemical reactions between salts dissolved in liquid lithium: reaction of lithium nitride, Li3N, with dilithium acetylide, Li2C2 , to form the dilithium salt of cyanamide, Li2NCN, in the metal.

The relatively large solubility of Li3N in liquid Li at moderate temperatures, log10 (mol fraction N) = 1.168 – 2036T−1 where 473⩽T ⩽708 and T = (273 + θ°C), and the large increase in the electrical resistivity (7.0 × 10−8ωm/mol%N) that the salt confers on the metal, make it possible to study the chemical reactions of this solute relatively easily and accurately. Thus changes in resistivity show that dissolved Li3N does not react with LiH in Li and no LiNH2 is formed, with Li2C2, however, dissolved Li3N reacts to form the dilithium salt of cyanamide, Li2NCN, which is crystallized and isolated by evaporation of the metal. Li2NCN is tetragonal with a = b = 3.687(3), c = 8.668(5)A, and space group I4/mmm. Each Li+ ion is tetrahedrally coordinated by parallel centrosymmetric NCN−−ions, and each anion is surrounded by eight cations. Unlike most salts which contain heteroatomic anions, this compound is stable towards lithium and has a standard enthalpy of formation at least as negative as −359.5 kj/mol. The changes in resistivity of the metallic solution during reaction show plainly the association between N−−− and C2−− and are not inconsistent with the formation of an intermediate CN4−− species. The implications of the behaviour of Li3N in Li towards LiH and Li2C2 on Li purification and metal corrosion aspects of reactor technology are discussed.