Na-K Alloys Research Articles

Progress Towards a REBCO Homopolar Motor With the Gallium-Indium-Tin Sliding Contacts

Development of homopolar electric machines is a promising superconducting application. A homopolar motor with a 2G high-temperature superconducting (HTSC) magnetic system and copper disk rotor is designed at the NRC “Kurchatov Institute”. The advantages of such electric machines are considered. The liquid metal sliding contacts based on a gallium-indium-tin alloy are considered as a safe alternative to toxic mercury or NaK alloys. The homopolar motor controlling by a proportional-integral-differentiating (PID) algorithm is simulated and checked experimentally. The results of the preliminary tests of HTSC magnetic system and liquid metal contact are also reported.

Thermodynamics of graphite intercalation binary alloys of Li-Na, Na-K, and Li-K from van der Waals density functionals

Graphite may store lithium or potassium, but not sodium, in its interlayer space under ambient conditions. It is, however, unclear whether binary alkali alloys of Li-Na, Li-K, and Na-K may substitute pure Li or K to form binary alkali alloy-graphite intercalation compounds. We investigate thermodynamics of the binary alloy-graphite intercalation compounds using density functional theory with van der Waals density functionals. We find Li-rich co-intercalation compounds and K-rich ones are associated with negative formation energies, and the Na-K alloy has the broadest domain of co-intercalation (approximately up to 36% Na). Because of convexity of the formation-energy functions, these compounds are metastable and tend to decompose even when formation energies are negative. Na metal is among the decomposition products. Binary Li-K alloys in graphite form segregated phases of LiC6 and KC8, and this allows one to fabricate Li-K mixed-ion batteries using graphite anodes, whereas Li-Na and Na-K alloys are thermodynamically unfavorable. The study highlights the importance of convexity of formation-energy functions in thermodynamics of alloy-graphite intercalation compounds.

Exploring Self-Healing Liquid Na-K Alloy for Dendrite-Free Electrochemical Energy Storage.

The development of high-performance dendrite-free liquid-metal anodes at room temperature is of great importance for the advancement of alkali metal batteries. Herein an intriguing self-healing liquid dendrite-free Na-K alloy, fabricated by a facile room-temperature alloying process, aiming for application in potassium-ion batteries is reported. Through extensive investigation, its self-healing characteristics are rooted upon a thin solid K2 O layer (KOL) coated on the liquid Na-K alloy. The KOL not only acts as a protective layer to prevent the Na-K alloy from making contact with the electrolyte, but also greatly improves the wetting capability and adhesion between the liquid alloy and the carbon matrix (e.g., carbon fiber cloth (CFC)) to form a stable interface. Consequently, the as-prepared CFC/KOL@Na-K alloy anode exhibits prominent electrochemical performance with smaller hysteresis (less than 0.3 V beyond 140 cycles at 0.4 mA cm-2 ), better capacity retention, and higher Coulombic efficiency than the CFC/bare Na-K alloy counterpart. When coupled with a potassium Prussian blue (PPB) cathode, the full cell manifests higher capability retention and improved cycling stability. This research deepens the understanding of self-healing Na-K alloys and opens a new way to achieve high-performance dendrite-free alkali metal anodes for application in rechargeable batteries.

Convection heat transfer of alkali liquid metals and LBE in hexagonal bundles of uniformly heated tubes with helical spacers

High temperature heat exchangers for solar thermal power and alkali liquid metals and lead–bismuth eutectic (LBE) cooled small modular nuclear reactors (SMRs) employ bundles of heated tubes or fuel rods with and without helical spacers for enhancing convection heat transfer. However, applicable Nusselt number correlations are not available and the reported data is relatively scarce. This work reviews, compiles, and correlates the reported experimental data and numerical analysis results for convection heat transfer of sodium, sodium–potassium (NaK) alloys, and lead–bismuth eutectic (LBE) in hexagonal bundles of uniformly heated tubes with helical wire-wrap and finned spacers. The compiled experimental data of fully developed Nusselt number for NaK alloys and LBE is for bundles of 19 and 37 uniformly heated tubes with pitch-to-diameter ratio (P/D) = 1.053–1.2824. It includes 103 Nusselt number values for a wide range of Peclet number (Pe) (9–2560). In addition, the compiled database includes 42 numerical Nusselt number values for liquid sodium and LBE at Pe = 30–1070 in bundles of 7–217 uniformly heated tubes with P/D = 1.194–1.31. The complied database for the bundle average Nusselt Number, Nub,ww, and the Nusselt number in central subchannels, Nuww, is correlated, respectively, as:Nub,ww=[4.387+0.00878(P/D)20.6]+[0.0306-1.647×10-4(P/D)14.03]Pe0.85Nuww=1.515[4.387+0.00878(P/D)20.6]+1.364[0.0306-1.647×10-4(P/D)14.03]Pe0.85Both correlations agree with the majority of the compiled experimental and numerical data to within ±15%. Nuww is consistently lower than Nub,ww, and the difference is independent of P/D, but increases from 38% at low Pe (<100) to as much as 52% at Pe = 1100. Nuww, is also higher than that reported earlier by the authors for bare heated tubes, Nus, with the difference increasing from 42% to 111% with increased Pe from 300 to 1100, and decreased P/D from 1.3 to 1.1.

A review of experimental data and heat transfer correlations for parallel flow of alkali liquid metals and lead-bismuth eutectic in bundles

A comprehensive review is performed of the reported experimental data on convection heat transfer for parallel flows of the alkali liquid metals of sodium, NaK-44, NaK-56, and NaK-78 alloys, and liquid lead-bismuth eutectic (LBE) in bundles of uniformly heated bare tubes in a triangular lattice at Pe=4–3074. The performed experiments by various investigators have employed either a heat exchanger or hexagonal and cylindrical bundles of 7–37 heated tubes with P/D=1.06–1.95. The compiled experimental database of 746 Nusselt number values for Na and various NaK alloys is used to develop the following continuous correlation:Nu=[10.7(P/D)-7.1]+0.024[1-e-10.4((P/D)-1)]Pe0.85For the entire ranges of P/D and Pe values in the experiments, this correlation is in good agreement, within ±15%, with the compiled Nu data. The compiled Nu database for LBE flows in bundles of bare heated tubes with grid spacers, has 205 values for P/D=1.33–1.80 and Pe=211–3049. The present Nu correlation for alkali liquid metals agrees with the LBE data to within ±20%. This agreement suggests that the proposed Nu correlation for alkali liquid metals in bundles of bare tubes may also be used to calculate Nu for LBE.

ChemInform Abstract: Facile and Efficient Exfoliation of Inorganic Layered Materials Using Liquid Alkali Metal Alloys.

AbstractSingle‐ and few‐layer MoS2, WS2, and BN nanosheets are prepared through intercalation and exfoliation using liquid NaK or NaLi alkali metal alloys in DME at room temperature for 24—48 h.

Excess entropy of mixing for binary square-well fluid in the mean spherical approximation: Application to liquid alkali-metal alloys

The expression for the entropy of binary square-well (SW) mixture is derived in the framework of the semi-analytical approach [J. Non-Cryst. Solids 353 (2007) 1798] for the mean spherical approximation. This expression is applied to calculate the concentration dependencies of the excess entropy of mixing for liquid Na-K and Na-Cs alloys at T=373K. It is shown that the SW model allows to achieve a better agreement with experiment than the hard-sphere model with the same values of hard-core parameters.

Surface tension and adsorption of components in the sodium-potassium alloy systems: Effective liquid metal coolants promising in nuclear and space power engineering

The current state of studies on surface tension (ST) of the sodium-potassium binary alloy systems is briefly reviewed. Studies on ST of Na-K alloys prepared in identical conditions are carried out. The ST measurements are performed in the entire concentration range of the alloy compositions which were obtained at a single load of a measuring device with high-purity Na and K starting components. It is shown that there is no minimum in the ST isotherm of the examined system, though it was observed there according to the earlier literature. The potassium component exhibits positive adsorption on the alloy surfaces, which goes through a maximum in alloys with the K content of about 13 at %.

Calculating the thermodynamic parameters of the surface layer of the binary sodium-potassium system

The composition and number of surface monoatomic potassium layers in Na-K alloys are calculated on the basis of theory of the thermodynamic stability of surface layers of metal alloys with respect to their thickness. The surface layer of Na-K alloys is shown to be thermodynamically stable at surface layer thickness of two monoatomic potassium layers in the range of compositions with K in Na content of ∼13–15 at %. Good agreement is obtained using the surface activities of the components in binary metal melts as criteria.

Structure and Inter-Diffusion Coefficients of Liquid Na x K1−x Alloys

Structure and atomic transport properties of liquid Na-K alloys are reported. Inter-diffusion coefficients of liquid NaxK1−x, alloys are calculated using scaling law proposed by Samanta et al. following Dzugutov which express the possible relationship between the excess entropy and diffusion coefficient. The interatomic interactions are described from the individual version of the electron-ion potential proposed by Fiolhais et al. The partial pair distribution functions and structure factors are calculated from the solution of Ornstein-Zernike integral equation with Rogers-Young closure. The evaluation with the composition of static structure and inter-diffusion properties are discussed.

Molecular dynamics simulation study of the interdiffusion properties of liquid Na-K alloys

We use classical molecular dynamics simulations to study the inter-diffusion properties of liquid Na-K alloys versus concentration for several compositions ranging from 10 to 90 atomic percent of Na. The self- and inter-diffusion coefficients are deduced from the successive configurations. The results are compared with the predictions of Darken's law and the validity of this approximation is discussed.

Partial resistivity analysis of the NaK, NaRb and NaCs liquid alloys

The total resistivity for the NaK, NaRb and NaCs alkali alloys was divided into partial resistivities and calculated by means of Zimans’s formula within a simplified and semi-empirical approach. It is shown that the height of the parabolic resistivity behavior versus concentration is related to the scattering potential whereas the position of the maximum depends on the structure of liquid. Further it appears clearly that the leading contribution is due to the sodium as a result of its muffin tin zero enhancement with respect to its value in the pure liquid phase.

Structures of Alkali Metals in Silica Gel Nanopores: New Materials for Chemical Reductions and Hydrogen Production

Alkali metals and their alloys can be protected from spontaneous reaction with dry air by intercalation (with subsequent heating) into the pores of silica gel (SG) at loadings up to 40 wt %. The resulting loose, black powders are convenient materials for chemical reduction of organic compounds and the production of clean hydrogen. The problem addressed in this paper is the nature of the reducing species present in these amorphous materials. The atomic pair distribution function (PDF), which considers both Bragg and diffuse scattering components, was used to examine their structures. Liquid Na-K alloys added to silica gel at room temperature (stage 0) or heated to 150 degrees C (stage I) as well as stage I Na-SG, retain the overall pattern of pure silica gel. Broad oscillations in the PDF show that added alkali metals remain in the pores as nanoscale metal clusters. 23Na MAS NMR studies confirm the presence of Na(0) and demonstrate that Na+ ions are formed as well. The relative amounts of Na(0) and Na(+) depend on both the overall metal loading and the average pore size. The results suggest that ionization occurs near or in the SiO2 walls, with neutral metal present in the larger cavities. The fate of the electrons released by ionization is uncertain, but they may add to the silica gel lattice, or form an "electride-like plasma" near the silica gel walls. A remaining mystery is why the stage I material does not show a melting endotherm of the encapsulated metal and does not react with dry oxygen. Na-SG when heated to 400 degrees C (stage II) yields a dual-phase reaction product that consists of Na(4)Si(4) and Na(2)SiO(3).

Alkali Metals Plus Silica Gel: Powerful Reducing Agents and Convenient Hydrogen Sources

Alkali metals absorbed into silica gel yield three stages of unique loose black powders (M-SG) that are strong reducing agents. All react nearly quantitatively with water to form hydrogen. Liquid Na-K alloys form air-sensitive powders at room temperature that can be converted at 150 degrees C to a form that is sensitive to moisture but not to dry air. Slowly heating sodium and silica gel to 400 degrees C yields a third type that can be handled in ambient air with only slow degradation by atmospheric moisture. These materials eliminate many hazards associated with pure alkali metals and provide easily handled reducing agents and hydrogen sources. They could be used in continuous-flow reactors to reduce and protonate aromatics, dechlorinate alkyl and aryl halides, and desulfurize various compounds.

Reactions of Sodium-Potassium Alloys with Inert Gas Impurities − Potential Hazards after Oxidation

A bibliographical study of the reactions of NaK alloys with their oxides did not explain the related accidents that have occurred in the past, particularly those involving the handling of bubblers installed to remove traces of oxygen and water vapor from blanket gases of LMFR (liquid metal fast reactors). Moreover, it revealed a controversy with respect to the explosive hazard of the NaK alloys + KO2 reaction. A thermodynamic study of the reaction of a NaK alloy with oxygen as well as experimental work on mixtures containing NaK and different oxides (KO2, Na2O, Na2O2) led to the conclusion that violent reactions occurring at ambient temperature, were always linked to the presence of hydrated superoxides or hydroxides. On contact with NaK alloys, these hydrated compounds lead to the formation of hydrogen with a release of heat causing the decomposition of potassium superoxide KO2 and the simultaneous release of oxygen. The oxidation of liquid NaK alloy, by bubbling a mixture of argon and air containing traces of water vapor (H2O < 5 vpm), leads to the formation of mainly KO2, besides the hydrated compounds Na2O2·2H2O, NaOH·H2O and KOH·H2O. The gas follows preferential paths and the product obtained is highly heterogeneous in both composition and hardness (porous parts and dense parts). Hydrated superoxide and hydroxides treated under vacuum result in nearly quantitative dehydration after heating at 100 °C. For storage elements or safety valves, containing NaK that has been submitted to slow surface oxidation and hydration, the oxidized layer contains KO2, Na2O2·nH2O and hydrated hydroxides of sodium and potassium. In order to avoid any hazard due to the contact of the oxides with the residual NaK alloy during transport or under impact, it is necessary to freeze the alloy and to reheat it very slowly. Operations such as draining should also be performed slowly, after having calibrated the tanks (temperature and pressure). Therefore the use of NaK liquid alloy to purify gases is questionable; as preferential paths are established, it remains impossible to predict the period of efficiency of a bubbler. Moreover, the NaK alloy does not chemically transform all traces of water vapor; because some remains present in the form of hydrated products, leading to the formation of wastes with a potential hazard during storage or destruction. In our opinion, this method of purification should be abandoned.

Calculation of Thermodynamic Properties and of Liquidus Line Positions for Na-K Alloys

A model of an ideal associated solution, containing self-associates of different sizes and shapes, was considered. Calculations of the thermodynamic properties of mixing of Na-K liquid alloys and of liquidus line positions were carried out. Possible modifications of the model are discussed.

A molecular dynamics study of the diffusion processes in liquid Na-K alloys

Molecular dynamics simulations are used to investigate the diffusion processes in the liquid metal alloys NacK1-c at three different concentrations. Self-correlation and distinct velocity correlation functions together with the corresponding diffusion coefficients have been calculated and analysed in order to study the phase-separation phenomenon. It is found that Na3K7 and Na5K5 compositions display significant dynamic cross correlations indicating a demixing behaviour for these alloys, whereas Na8K2 alloy is a nearly ideal mixture. The results obtained agree well with the experimental observation of a tendency towards phase separation exhibited by liquid Na-K alloys.

Failure of Linear-Response-Theory for NaK Alloys**The project supported by the University of Texas High Performance Computing Facility, the Welch Foundation (Houston, TX) and the National Science Foundation under Grant No. DMR 9313645

As we previously did for Na, we calculate the Gibbs free energy of liquid and solid using linear-response-theory and molecular dynamics to obtain its melting temperature. The agreement with experiment is even better than that obtained for Na. We then attempted to determine the formation enthalpy of liquid and solid NaK alloys without success and discuss possible reasons for the failure of the method to yield agreement with experiment.

Thermodynamics and structure of liquid binary alloys calculated using an analytic pair potential.

Analytic pair potentials proposed by Pettifor and Ward are developed using the Heine-Abarenkov pseudopotential for Li-Na, Na-K, and Na-Cs liquid binary alloys. The corresponding partial structure factors are calculated using the random-phase approximation. The calculated thermodynamic and structural properties using this real-space formalism are in good agreement with experiments.

Structure and Electrical Resistivity of Liquid Alkali Alloys

Abstract The evaluation of the electrical resistivity of binary liquid alloys requires the true knowledge of the partial structure factors as well as the form factors of the constituent species. In principle partial structure factors are measurable, but the practical difficulties are severe and it is not promising to use the measured data. We, therefore, consider a simple theoretical approach based on hard sphere reference system and the Percus—Yevick equation to compute the partial structure factor. The concentration dependence of the hard sphere parameters are obtained by minimising the Helmholtz free energy of the system following the Gibbs—Bogoliubov variational technique. We have used it to compute the concentration dependence of electrical resistivity of NaK, KRb and NaCs liquid alloys. Our study reveals the like particle correlation contributes positively to the absolute value of electrical resistivity while the unlike particle correlation makes a negative contribution.